Title of Invention	METHOD AND SYSTEM FOR GENERATING AND RECEIVING A NAVIGATION SIGNAL
Abstract	There is disclosed a method of generating a navigation signal comprising a carrier signal, the method comprising the step of multiplying the carrier signal by at least one subcarrier modulation signal; wherein the at least one subcarrier modulation signal comprises a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2. Also disclosed is a system (1400) for generating the said navigation signal, having a subcarrier generator (1408) for generating at least one subcarrier modulation signal and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier modulation signal, and further a receiver system (1500) having means (1640) with a down converter (1628) and a demodulator (1642) to receive and down convert said navigation signal.

Title of Invention

METHOD AND SYSTEM FOR GENERATING AND RECEIVING A NAVIGATION SIGNAL

Abstract

There is disclosed a method of generating a navigation signal comprising a carrier signal, the method comprising the step of multiplying the carrier signal by at least one subcarrier modulation signal; wherein the at least one subcarrier modulation signal comprises a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2. Also disclosed is a system (1400) for generating the said navigation signal, having a subcarrier generator (1408) for generating at least one subcarrier modulation signal and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier modulation signal, and further a receiver system (1500) having means (1640) with a down converter (1628) and a demodulator (1642) to receive and down convert said navigation signal.

Full Text	METHOD AND SYSTEM FOR GENERATING A NAVIGATION SIGNAL Field of the Invention The invention relates to modulation signals, systems and methods such as, for example, navigation and positioning signals, systems and methods. Background to the invention Satellite Positioning Systems (SPS) rely on the passive measurement of ranging signals broadcast by a number of satellites, or ground-based or airborne equivalents, in a specific constellation or group of constellations. An on-board clock is used to generate a regular and usually continual series of events, often known as 'epochs', whose time of occurrence is coded into a random or pseudo-random code (known as a spreading code). As a consequence of the pseudo-random or random features of the time epoch encoding sequence, the spectrum of the output signal is spread over a frequency range determined by a number of factors including the rate of change of the spreading code elements and the waveform used for the spreading signal. Typically, the spreading waveform is rectangular and has a sine function power spectrum. The ranging signals are modulated onto a carrier signal for transmission to passive receivers. Applications are known that cover land, airborne, marine and space use. Typically, binary phase shift keying is employed to modulate the carrier signal, which, itself, has a constant magnitude. Usually, at least two such signals are modulated onto the same carrier in phase quadrature. The resulting carrier signal retains its constant envelope but has four phase states depending upon the two independent input signals. However, it will be appreciated that two modulating signals do not need to have the same carrier magnitude. It is possible for a constant carrier magnitude of the combined signal to be maintained by appropriate selection of corresponding phases other than π/2 radians. An example of such a satellite positioning system is the Global Positioning System (GPS). Generally, the GPS operates using a number of frequencies such as, for example, L1, L2 and L5, which are centred at 1575.42 MHz, 1227.6 MHz and 1176.45 MHz respectively. Each of these signals is modulated by respective spreading signals. As will be appreciated by those skilled in the art, a Coarse Acquisition (CA) code signal emitted by the GPS Satellite Navigation System is broadcast on the L1 frequency of 1575.42MHz with a spreading code rate (chip rate) of 1.023MHz. The CA has a rectangular spreading waveform and is categorised as BPSK-R1. The GPS signal structure is such that the signal broadcast by the satellites on the L1 frequency has a second component in phase quadrature, which is known as the precision code (P(Y) code) and is made available to authorised users only. The P(Y) signal is BPSK modulated with a spreading code at 10.23MHz with a magnitude which is 3dB lower in signal power than the CA code transmission. Consequently, the Q component has a magnitude which is 0.7071 (-3dB) of the magnitude of the I component. It will be appreciated by those skilled in the art that the phase angles of these states of these signals are ±35.265° in relation to the ±1 axis (phase of the CA code signal as specified in ICD GPS 200C). One skilled in the art also appreciates mat the P code is a function of or is encrypted by the Y code. The Y code is used to encrypt the P code. One skilled in the art appreciates that the L1 signal, containing both I & Q components, and the L2 signal can be represented, for a given satellite, i, as Pi(t) represents the P(Y) ranging code and is a pseudo-random sequence with a chip rate of 10.23 Mcbps. The P code has a period of exactly 1 week, taking values of +1 and -1; Ci(t) represents the CA ranging code and is a 1023 chip Gold code, taking values of +1 and-1; di(t) represents the data message, taking values of+1 and -1. A satellite constellation typically comprises 24 or more satellites often in similar or similarly shaped orbits but in a number of orbital planes. The transmissions from each satellite are on the same nominal carrier frequency in the case of code division access satellites (such as GPS) or on nearby related frequencies such as GLONASS. The satellites transmit different signals to enable each one to be separately selected even though several satellites are simultaneously visible. The signals from each satellite, in a CDMA system like GPS, are distinguished from each one another by means of the different spreading codes and/or differences in the spreading code rates, that is, the pi(t) and Ci(t) sequences. Nevertheless, as will be appreciated from the power spectrum 100 shown in figure 1 there remains significant scope for interference between the signals transmitted by the satellites. Figure 1 shows power spectra 100 for the CA and P(Y) codes. The power spectrum 102 for the CA code has maximum power at the carrier frequency L1 and zeros at multiples of the fundamental frequency, 1.023MHz, of the CA code. For example, it can be appreciated that zeros occur either side of the carrier frequency at ±1.023MHz, ±2.046MHz etc. Similarly, the power spectrum 104 for the P(Y) code has a maximum amplitude centred on the LI and L2 frequencies, with zeros occurring at multiples of ±10.23MHz as is expected with a sine function waveform. It is known to further modulate the ranging codes using a sub-carrier, that is, a further signal is convolved with the P codes and/or CA codes to create Binary Offset Carrier (BOC) modulation as is known within the art see, for example, J. W. Betz, "Binary Offset Carrier Modulation for Radionavigation", Navigation, Vol. 48, pp227-246, Winter 2001-2002. Standard BOC modulation 200 is illustrated in figure 2. Figure 2 illustrates the combination of a portion of a CA code 202 with a subcarrier signal to produce the BOC signal 204 used to modulate a carrier such as, for example, LI. It can be appreciated that the BOC signal is a rectangular square wave and can be represented as, for example, Cj(t)sign(sin(27πfst)), where fs is the frequency of the subcarrier. One skilled in the art understands that BOC(/i/c) denotes Binary Offset Carrier modulation with a subcarrier frequency of fs and a code rate (or chipping rate) of fc. Using binary offset carriers results in the following signal descriptions of the signals emitted from the satellite: Figure 2 also illustrates power spectra for a BPSK-R1 code and pair of BOC signals, that is, BOC(2,1) and BOC(10,5). The first spectrum 202 corresponds to BPSK-R1 code. The second power spectrum 204 corresponds to the BOC(2,1) code and the third power spectrum 206 corresponds to the BOC(10,5) code. It can be appreciated that the side lobes 208 of the BOC(2,1) signal have a relatively large magnitudes. Similarly, the illustrated side lobe 210 of the BOC(10,5) signal has a relatively large magnitude. One skilled in the art appreciates that the energy in the side lobes are a source of interference. It is an object of embodiments of the present invention to at least mitigate the problems of the prior art. Summary of Invention Accordingly, a first aspect of embodiments of the present invention provides a method as claimed in claim 1. A second aspect of embodiments of the present invention provides a signal as claimed in claim 20. A third aspect of embodiments of the present invention provides a system as claimed in claim 28. A fourth aspect of embodiments of the present invention provides a received as claimed in claim 29. A fifth aspect of embodiments provides a computer readable storage as claimed in claim 30. Advantageously, embodiments of the present invention provide significantly more control over the shape of the power spectra of signals, that is, the distribution of energy within those signals. Other aspects of the present invention are described and defined in the claims. Brief Description of the Invention Embodiments of the present invention will now be described, by way of example, only with reference to the accompanying drawings in which: figure 1 shows a power spectrum of a pair of ranging code; figure 2 illustrates power spectra of a ranging code (BPSK-R1) and BOC(10,5) signals; figure 3 illustrates a multi-level sub-carrier; figure 4 illustrates the phase states for at least a pair of multilevel subcarriers according to embodiments of the present invention; figure 5 depicts a power spectrum of a prior art subcarrier and a subcarrier according to embodiments of the present invention; figure 6 illustrates phase states for a subcarrier according to embodiments of the present invention; figure 7 illustrates in phase and quadrature phase subcarriers according to embodiments of the present invention; figure 8 illustrates phase states of a subcarrier according to an embodiment of the present invention; figure 9 shows subcarriers according to embodiments of the present invention; figure 1.0 depicts power spectra of subcarriers according to embodiments of the present invention; figure 11 illustrates subcarriers according to embodiments of the present invention; figure 12 shows an alternative subcarrier waveform according to an embodiment of the present invention; figure 13 illustrates a further alternative waveform according to embodiments of the present invention; figure 14 illustrates, schematically, a transmitter using subcarriers according to embodiments of the present invention; figure 15 illustrates a further embodiment of a transmitter according to the invention; and Figure 16 illustrates a receiver system according to an embodiment of the invention: It will be appreciated that there are preferably restrictions on the combinations of signals, at least one of which is that a constant modulus signal should be maintained. The constraints are (1) that "+1" or "-1" on the in-phase component can only occur in conjunction with "0" on the quadrature phase component and visa versa and (2) "±l/\/2" can only occur on both phases simultaneously. The magnitudes of the in-phase and quadrature phase components of the spreading signals, sc(g(t) or scim(t), can be plotted on an Argand diagram 400 such as is shown in figure 4. The waveforms for the I and Q components are thus built from the following signal element sequences: Any combination of I or Q signal sequences can be chosen from the above set within the constraint of a constant magnitude carrier signal, computed as (I2+Q2)1/2. It will be clear to those skilled in the art, that there are many other equivalent sets of sequences which may be chosen from the set of 5-levels satisfying the criterion of a constant carrier envelope. It can be appreciated that the magnitudes of the subcarriers on the I and Q channels can be thought of as being analogous to the states of an 8-PSK signal. Therefore, such a pair of 5-level subcarrier carrier signals can be thought of as 8 phase subcarrier signals. Figure 5 illustrates the effect of using a stepped or m-level, m>2, subcarrier waveform. Referring to figure 5 there is shown a pair 500 of power spectra. The first power spectrum 502, illustrated using the dotted line, represents the spectrum of a BOC(2,2) subcarrier. It can be appreciated that the energy of the subcarrier is contained within progressively reducing side lobes 504, 506, 508 and 510. The second power spectrum 512 represents the power spectrum of a BOC(2,2) signal that used 8 phase subcarrier signals, that is, 8 phase amplitudes, represented by BOC8(2,2). More generally, BOCm(fsfc) represents an m-phase subcarrier signal of having a frequency of fs and a chipping rate of fc. It can be appreciated that the spectrum 512 of the BOC8(2,2) signal has a number of side lobes 514, 516, 518, 520, 522 and 524. Of those side lobes, it can be seen that the 1st to 4th side lobes are significantly reduced, that is, they comprise significant less energy, as compared to the side lobes of the BOC(2,2) signal spanning the same frequencies. The significant reduction in the 1st to 4 side lobes can be beneficial in situations in which one skilled in the art wishes to use the frequency spectrum spanned by the side lobes for other transmissions. It will be appreciated by those skilled in the art the BOC8(2,2) has significandy improved interference properties as determined using Spectral Separation Coefficients (SSC) and self-SSCs as is well understood by those skilled in the art, that is, the spectral coupling between a reference signal and BOC(2,2) is greater than the spectral coupling between a reference signal and BOC8(2,2). For example, a BOC8(2,2) signal exhibits a 10-12 dB improvement in spectral isolation as compared to a conventional BOC(2,2) signal. Further information on the relationship between SSC and signals according to embodiments of the present invention can be found in, for example, Pratt & Owen; BOC Modulation Waveforms, IoN Proceedings, GPS 2003 Conference, Portland, September 2003, which is incorporated herein by reference for all purposes and filed herewith as in the appendix. Furthermore, embodiments of the present invention utilise the magnitude and duration of the subcarrier to influence, that is, control the energy in harmonics of the resulting modulating waveform. For example, referring still to figure 5, it can be appreciated that additional spectral nulls appear in the BOC8(2,2) spectrum at substantially 6MHz and 10MHz offset from the carrier whereas there are no such nulls in the conventional BOC(2,2) signal. The location of the nulls is influenced by at least one of the magnitude and duration of the steps in the multilevel subcarrier. More specifically, the nulls can be steered to desired locations by changing either of these two elements, that is, the position of the nulls is influenced by these two elements. Appendix A contains an indication of the relationship between the spectra of signals according to embodiments of the present invention and the magnitude and duration of the steps. Referring to figure 6, there is shown subcarrier states or amplitudes for I and Q signals for a further BOC8 signal, that is, a binary offset carrier having eight states. It can be It will be appreciated that the states 1 to 8 shown in figure 6 are not equidistantly disposed circumferentially. The transitions between states 2&3, 4&5, 6&7, 8&1 are larger in angular step than the transitions between states 1&2, 3&4, 5&6, 7&8. It will be appreciated that when these states are translated into subcarrier amplitudes, the duration of a given amplitude will depend on the duration or dwell time of a corresponding state, that is, the durations for which the subcarrier remains in any given state may no longer be equal unlike the states of figure 4 above. The dwell times are a matter of design choice such as, for example, to minimise the mean square difference between a stepped waveform and a sinusoid. Figure 7a illustrates the subcarriers 700 and 702 corresponding to the states shown in figure 6. It can be appreciated that the durations of or within each state of the subcarriers 700 and 702 are equal. The Q channel subcarrier magnitudes will follow substantially the same pattern as described above but phase shifted by 7t/2 radians. The subcarrier 702 for the Q channel is shown in dotted form in figure 7. It will be appreciated that such subcarriers provide a constant envelope magnitude since (I2+Q2)1/2=l for all amplitude combinations. However, referring to figure 7b, there is shown a pair of subcarriers 704 One skilled in the art will appreciate that the above can be extended to n half cycles of a subcarrier per ranging code chip. It will be appreciated that other phases can be used to describe the subcarriers. For example, phase and amplitude components of 16-PSK can be used to create BOC16 subcarriers having 9 levels, assuming that the first state is at (+1,0). Using m-PSK phase states can be used to produce (m+2)/2 level subcarrier signals. Therefore, setting m=2 gives the conventional BPSK and a two-level subcarrier. Setting m=4 provides a 3 level subcarrier, that is, BOC4 modulation, setting m=8 produces a 5 level subcarrier, that is, BOC8 modulation, setting m=16 produces a 9 level subcarrier, which corresponds to BOC16 modulation. It will be appreciated that several further variations in the assignment of code and data states to the phase locations can be realised. For example, rotation of the states shown in figure 4 by 22.5° leads to a reassignment of angles associated with the states from the angles (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°) to the angles (22.5°, 67.5°, 112.5°, 157.5°, 202.5°, 247.5°, 292.5°, 337.5°). Again, it will be appreciated mat this does not cause a change in the modulus of the spectrum and, again, the required number of amplitude levels reduces from 5 to 4, that is, m-PSK can be used to realise [(m+2)/2-l] amplitudes according to appropriate rotation and alignment of the phase states. The resulting waveforms for the I and Q components are built, in this case, from the following signal element sequences: It should be noted that the I and Q signal element sequences for the cases described above are orthogonal over the duration of one spreading pulse (chip). Clearly, other rotations are possible and will yield orthogonal signal element sets. An alternative way of representing the above is via a state table. Assume that an embodiment of a BOC8 modulation has been realised with equidistant states and the first state has a phase angle of rc/8 radians (22.5°) as shown in figure 8, which corresponds to the above values. The sequence of phase states required for each I and Q ranging code signal component, assuming that the ranging codes transition substantially simultaneously and a desire to maintain a substantially constant output envelope, that is, the states for the subcarriers would be given by Table 1 - Sequence of States for BOC8(x,x) I & Q signal elements It will be appreciated that the subcarrier corresponding to the phase states in Table 1 comprises a half cycle per ranging code chip. Furthermore, the sense of the phasor is clockwise when I and Q are equal and anticlockwise otherwise. It will be apparent that the signal element sequences or state sequences are sections (specifically half cycle sections in the aspect of the invention disclosed above) of a sampled or quantised sinusoid. The concept can, therefore, be extended to include a multiplicity of such samples. Those variants, which appear to be useful, include the cases with samples from a finite number of half cycles, that is, rather than, for example, an I channel value of +1 being represented by the states of 2, 1,8 and 7, it can be represented using some otiier number of states such as, for example, 2, 1, 8, 7, 6, 5, 4, 3, 2, 1, 8, 7 ie by three half cycles of the sample or quantised sinusoid. Table 2 illustrates the phase states for such an embodiment and is based on the phase state diagram of figure 4 for samples but using three half cycles (or an arbitrary number of half cycles) of the sinusoid waveform. The sinusoid or portion or multiple of half cycles thereof is known as the 'basis waveform'. One skilled in the art realises that other basis waveforms can be used such as, for example, a triangular waveform or a set of mutually orthogonal waveforms. One skilled in the art will appreciated mat it is assumed in Tables 1 to 3, that the I and Q chip transitions happen substantially simultaneously and, furthermore, that the I and Q subcarriers take the form of sine and cosine waveforms respectively. However, embodiments can be realised in which me ranging code chip transitions do not occur substantially simultaneously. Furthermore, in circumstances in which the ranging code chip transitions do not occur substantially simultaneously, the subcarriers corresponding to the I and Q ranging code chips can be arranged to take the form of a pair of quantised sine waves. It will seen that there are 4 time samples for each V2 cycle of the waveform. The stepped sinusoidal waveform may be viewed as sub-carrier modulation of the basic spreading waveform. The number of time samples and independent information bearing channels is related to the number of phase states, which me carrier signal has in its representation. Although the examples above have used phase states that are 'powers of 2', embodiments can be realised in which some other number is used. For example, a 6-PSK carrier signal can be used to carry 2 independent information bearing binary channels. In mis case only 3 signal element samples are required per transmitted code chip. One skilled in the art appreciates that the replacement of the stepped sinusoid with a rectangular wave with the duration of each element being equal to a Vi cycle of the sinusoid is well known within the art. As indicated above, it is known as 'Binary Offset Carrier' modulation. There are usually 2 further attributes associated with the BOC description, which relate to the frequency of the code chipping rate and to the frequency of the offset sub-carrier. BOC(2,2) consequently is interpreted as a waveform with a 2.046MHz chipping rate and a 2.046MHz offset sub-carrier. This arrangement has exactly two Vi cycles of the sub-carrier signal for each code element (chip). A further aspect of embodiments of the present invention relates to using a set of subcarriers to modulate ranging codes, with at least one or more, or all, of the subcarriers being multilevel waveforms. One skilled in the art may think of such embodiments as modulation of the subcarrier signal by a further subcarrier signal. The resulting signal transmitted by an ith satellite or system having a carrier frequency of COi, for an additional subcarrier, would have the form: scim(t) and scjm(t) represent first and second subcarrier signals respectively first ranging codes such as, for example, M-codes; and scig(t) and scjg(t) represent first and second subcarrier signal second ranging codes such as, for example, Gold codes. It should be noted that embodiments can be realised in which scim(t) and sc;g(t) are the same or different. Similarly, embodiments can be realised in which scjm(t) and scig(t) are the same or different. Although it is possible to use more than one subcarrier, practical embodiments will typically use 2 subcarriers. Modulation using a pair of subcarriers is known as Double Binary Offset Carrier (DBOC) modulation. Modulation using three subcarriers is known as Triple Binary Offset Carrier (TBOC) modulation and so on such that modulation using a n-tuple of subcarriers is known as N-tuple Binary Offset Carrier (NBOC). As mentioned above, one or more than one of the subcarriers may be stepped, that is, having magnitudes related to respective phase states. As examples of this aspect of the invention, figure 9 illustrates a pair of waveforms 900. In figure 9, as an illustration of the NBOC invention, the subcarrier basis waveforms are assumed to be binary and only a single subcarrier waveform 902 is shown. The time duration in figure 9 is 512 samples and exactly matches the duration of one code element duration (chip). The first subcarrier 902 contains 4 half cycles of a subcarrier per ranging code chip, as illustrated by the dashed waveform. If this was the only sub- carrier component, the modulation would be a BOC(2x,x) type, where x is the frequency of the code rate (chipping rate). However, it can be appreciated that a second sub-carrier (not shown) having 16 half cycles per 512 samples has been used to produce the modulated waveform 904 to be combined with the carrier of the satellite signal. The modulated waveform is shown by the solid curve. As a result of modulation (multiplication) of the two subcarriers, the resulting waveform 904 has phase reversals for the second subcarrier 904 whenever there is a sign reversal in the first subcarrier 902. This is clearly evident in figure 9 at points 906, 908 and 910, where the transitions of the second subcarrier (not shown) would have been opposite. The resulting modulation is denoted Double BOC, or DBOC. In the case of figure 9, the modulation is DBOC(8x,(2x,x)), that is, there are 8 half cycles of the second subcarrier per chip of the ranging code (not shown). The main energy is concentrated around frequencies ±8x from the carrier signal, with a BOC-like double humped spectrum. Referring to figure 10, there is shown a pair of power spectra 1000. A first power spectrum 1002 relates to a DBOC8(16,(2,2)) signal. It will be appreciated that at least one of the first and second subcarriers used to create the DBOC8(16,(2,2)) signal comprised amplitudes derived from 8 corresponding phase states. In the specific embodiment shown, the first subcarrier was the multi-level signal. It will be appreciated mat the nomenclature for representing DBOC modulation or subcarriers is DBOCa(b,c(d,e)), where a and c represent the number of phase states, that is, amplitudes, of the subcarriers having frequencies b and d respectively. The second spectrum 1004 relates to a BOC8(2,2) signal. The spectra shown have been made using a previous aspect of the invention, that is the use of multilevel subcarriers or subcarriers having more than two phase states, in combination with the Double BOC concept. The waveforms for I & Q modulations for the spectrum of figure 10 are shown in figure 11. Referring to figure 11 there is shown a pair 1100 of waveforms. The first pair of waveforms 1102, representing the I channel of the spreading waveform, comprises a stepped or multi-level BOC(2,2) signal 1104, represented by the solid line, and a 16Mhz subcarrier modulated BOC(2,2) signal 1106, represented by the dashed line. It will be appreciated that the 16 MHz subcarrier modulated BOC(2,2) signal has been produced by multiplying the BOC8(2,2), that is, stepped BOC(2,2) signal, by a 16 MHz rectangular waveform (not shown) having amplitudes of +1. The second waveform 1108, representing the Q channel, comprises a quadrature BOC(2,2) signal 1110 together with a 16MHz subcarrier modulated BOC(2,2) signal 1112. It can be appreciated that the first subcarrier 1104 or 1110 is a subcarrier according to an embodiment of the present invention described above whereas the second sub-carrier (not shown) in both cases are conventional binary rectangular waveforms, that is, conventional subcarriers. It can be appreciated that there are regions 1114 of overlap between the two BOC(2,2) subcarriers 1104 and 1110 and their resulting products, that is, 16 MHz subcarrier modulated BOC(2,2) signals 1106 and 1112. In the regions of overlap 1114, the waveforms have the same amplitude profile. An advantage of the embodiments of the signals shown in figure 11 is that the I channel or component has been produced or represents DBOC8 modulation or signal whereas the Q channel has been produced using or represents BOC8 modulation. However, this arrangement still preserves or provides a substantially constant envelope carrier signal to be emitted from the satellite. Embodiments of the present invention have been described with reference to the subcarrier signals being periodic. However, embodiments can be realised in which the subcarrier signal comprises a pseudorandom noise signal. Furthermore, embodiments can be realised in which the shape of the subcarrier takes a form other than a stepped, that is, a multilevel wave or quantised approximation of a sinusoidal waveform. For example, multilevel-pulsed waveforms, multilevel-periodic waveforms or multilevel- aperiodic waveforms, could be used such as the signal shown in figure 12 according to the influence one skilled in the art wishes the resulting modulation to have on the power spectrum of the transmitted signal and/or any appropriate measure of interference such as, for example, SSC or self-SSCs. Referring to figure 13, there is shown a subcarrier waveform 1300 according to a further embodiment of the present invention together with one chip 1302 of a code or otiier waveform such as, for example, another subcarrier. It can be appreciated mat the subcarrier comprises a first portion of a BOC(5,l) waveform, in the 100 ns sections, combined widi portions of a BOC(l,l) waveform, in the 400 ns portions, to produce an overall subcarrier. It will be appreciated tiiat the spectra of the BOC(5,l) waveform will have a peak at 51.023MHz and the BOC(l,l) waveform will have a peak at 11.023MHz. Therefore, one skilled in the art appreciates diat selectively combining the BOC subcarriers allows one skilled in the art to position or relocate the peaks of the overall subcarrier. Again, it can be appreciated diat the subcarrier used, for example, to modulate the ranging codes is derived from more dian one subcarrier. Although the signal described in relation to figure 13 has been derived from BOC(5,l) and BOC(l,l) subcarriers, embodiments can be realised in which otiier combinations of BOC subcarriers are used. In effect, the BOC(5,l) and BOC(l.l) signals have been multiplexed or selectively combined to produce an overall subcarrier signal. It will be appreciated diat other sequences for the subcarriers can be realised according to a desired effect upon the power spectrum of a transmitted signal. For example, a subcarrier can be realised using a pseudorandom sequence as a sub-carrier instead of the stepped modulations. The use of additional sequences to diat of the main spreading code has hitherto been limited to use as a tiered code, which changes state after every complete code repetition interval. The GPS L5 codes are constructed in tiiis manner using Neumann Hoffman sequences of lengtii 10 or 20 to extend a 1ms code (of 10230 chips or elements) to 10ms or 20ms. The use of a subcode chip interval has not previously been considered. A complete sequence (a sub-sequence) has a duration of one code chip, or at most a plurality, of code chips. It fulfills a similar role to the sub- carrier modulation as previously described in that it controls the spectrum of the emissions. One feature of such a sub-sequence is that such sequences may be chosen to be common amongst a satellite constellation or a sub-set of the constellation. One such subset might be a group of ground transmitters providing a local element or augmentation to the space segment of the system. For example, subcarrier amplitudes can be realised that have the sequence -++++—+ in 10 subchip intervals or other sequence of +l's and -l's per ranging code chip or other subcarrier chip according to the desired effect on spectrum of the resulting signal. Examples such as me 7 subchip interval sequences would include ++ , +++—, +-+-+--, and can be chosen to provide similar control over the emitted spectrum. Referring to figure 14, there is shown, schematically, a transmitter 1400 according to an embodiment of the present invention. The transmitter 1400 comprises means 1402, that is, a generator, for generating or selecting the ranging codes for transmission. It will be appreciated by those skilled in the art that such ranging codes may be generated by, for example, shift register implementations. It can be appreciated that the ranging code selection and/or generation means 1402 is illustrated as producing gift) and nij(t). These codes are fed to respective mixers 1404 and 1406. The mixers 1404 and 1406 are arranged to combine the ranging codes with subcarriers according to embodiments of the present invention. Respective subcarrier generators 1408 and 1410 generate the subcarriers. Optionally, a data signal, d((t), is also preferably mixed with the ranging codes and subcarriers. The duration of one bit of the data signal is normally an integer multiple of the code repetition interval. For example, in GPS CA code, it is 20 times the 1 ms code repetition interval, that is, the data rate is 50 bps. The mixed signals 1412 and 1414 are fed to a further pair of mixers 1416 and 1418, where they are mixed with in-phase and quadrature phase signals produced via an oscillator and phase shifter assembly 1420. The further mixed signals 1422 and 1424 are combined, via a combiner 1426, and output for subsequent up conversion by an appropriate up converter 1428. The output from the up converter 1428 is fed to a high-power amplifier 1430 and then filtered by an appropriate filter 1433 for subsequent transmission by, for example, a satellite or other device arranged to emit or transmit the ranging codes. Referring to figure 15, there is shown a schematic representation of a modulation system 1500 according to an embodiment. The system 1500 comprises a ranging code generator 1502 for producing a ranging code. The ranging code is fed to a first lookup table 1504 comprising phase states and a second lookup table 1506 comprising amplitude states. The output of the phase state lookup table 1504 is used to drive a phase modulator 1508, which, in turn, produces a voltage signal to control the phase of a voltage controlled oscillator 1510. The output of the oscillator 1510 is combined, via, a combiner 1512 such as, for example, a gain controlled amplifier or multiplier, with a signal output from the amplitude state table 1506 to produce a subcarrier having me appropriate characteristics. Aldiough the above embodiments have been described with reference to maintaining a substantially constant signal envelope, embodiments are not limited thereto. Embodiments can be realised in which variable modulus signal envelopes are used. It will be appreciated that the constraints described above, which are aimed at preserving unitary magnitude of (I2+Q2)1/2, need not necessary apply. The above embodiments have been described with reference to the I and Q channels having the same chipping rates. However, embodiments are not limited to such arrangements. Embodiments can be realised in which different chipping rates are used. Although embodiments of the present invention have been described with reference to the LI and L2 frequencies, embodiments are not limited to such arrangements. Embodiments can be realised in which other frequencies or frequency bands can be used according to the requirements of the system using the invention. For example, the lower L band (ie E5a and E5b), the middle (ie E6) and upper L-band (ie E2-L1-E1) can also benefit from embodiments of the present invention. It will be appreciated mat such embodiments may use signals having at least three components rather than the two components described above. Furthermore, embodiments of the present invention have been described witii reference to standard BOC. However, one skilled in the art appreciated mat embodiments can also be realised using Alternative BOC. Furthermore, it will be appreciated that embodiments can be realised in which the number of half cycles of a subcarrier per chip of a code can be at least one of odd, even, an integer multiple or a non-integer multiple of the chip, that is, there is a rational number relationship between the number of subcarrier half cycles and the chip duration. Embodiments of the present invention described above have focused on the transmission side of the invention, that is, upon the generation, modulation and transmission of ranging codes combined with a subcarrier or subcarriers. However, one skilled in the art appreciated that a converse system and method are required to receive and process the signals. Once one skilled in the art has designed a system for transmitting such signals, designing an appropriate receiver is merely the converse of the transmit operations. Therefore, embodiments of the present invention also relate to a receiver for processing signals such as those described above. Referring to figure 16, there is shown a receiver 1600 comprising means 1640 to receive and down convert a navigation signal according to embodiments of the present invention. In the embodiment shown the means 1640 to receive and down convert the navigation signal comprises an input filter 1633, an amplifier 1630 and a down converter 1628. The means 1640 to receive and down convert the navigation signal output down converted modulated ranging codes. Also shown is a demodulator 1642 for demodulating the modulated ranging codes using at least one, and preferably two in the illustrated embodiment, m-level subcarrier demodulation signals from respective signal generators 1608 and 1610 to recover at one and preferably two ranging codes ft (0 and w,(0- In embodiments that use I and Q channels there is also provided means 1644 for separating the received signal into I and Q channels. It can be appreciated that the means 1644 for separating the received signal into I and Q channels comprises a pair of signal paths 1622 and 1624 for coupling the output of the down converter 1628 to a pair of mixers or demodulators 1616 and 1618. The mixers or demodulators cooperate with a phase shifter 139 and a local oscillator 1620 to produce an I channel signal 1612 and a Q channel signal 1614. The I channel signal 1612 and the Q channel signal 1614 are fed to a pair of mixers or demodulators 1604 and 1606 that, in conjunction with respective despreading waveforms sc,g(g) and sc>m(t) from respective waveform generators 1608 and 1610, allow recovery of the ranging codes gi(t) and nii(t) 1602. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. WE CLAIM: 1. A method of generating a navigation signal comprising a carrier signal, the method comprising the step of multiplying the carrier signal by at least one subcarrier modulation signal; wherein the at least one subcarrier modulation signal comprises a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2. 2. A method as claimed in claim 1 in which m is selected from at least one of 3, 4, 5, 6, 7, 8 or 9. 3. A method as claimed in any preceding claim in which the at least one subcarrier modulation signal approximates or is derived from a basis waveform. 4. A method as claimed in claim 3 in which the basis waveform is at least one of a sine wave, cosine wave, or triangular waveform. 5. A method as claimed in either of claims 3 and 4 in which the basis waveform is selected according to desired power distribution characteristics of the transmission signal. 6. A method as claimed in any preceding claim in which the at least one subcarrier comprises at least two mutually orthogonal subcarrier modulation signals. 7. A method as claimed in claim 6 in which the at least two subcarriers comprises a pair of subcarriers having a predetermined phase relationship. 8. A method as claimed in any preceding claim in which the at least one subcarrier comprises an in-phase subcarrier and a quadrature phase subcarrier. 9. A method as claimed in claim 8 further comprising the step of determining the respective multiple amplitudes of the in-phase and quadrature phase subcarriers to maintain a substantially constant transmission signal envelope. 10. A method as claimed in any preceding claim further comprising the steps of deriving amplitudes associated with the at least one subcarrier from a plurality of phase states. 11. A method as claimed in claim 10, in which the phase states are equally angularly distributed around a unit circle. 12. A method as claimed in any preceding claim in which durations of the amplitudes of the at least one subcarrier are equal. 13. A method as claimed in any preceding claim in which durations of at least a pair of amplitudes of the at least one subcarrier are different. 14. A method as claimed in any of claims 12 to 13 in which the durations are quantised according to an associated clock signal. 15. A method as claimed in any preceding claim in which at least a pair of subcarriers cooperate to define an associated plurality of phase states resolved according to mutually orthogonal axes. 16. A method as claimed in claim 15 in which the plurality of phase states is associated with respective ranging signals. 17. A method as claimed in either of claims 15 and 16, in which dwell times in at least some of the plurality of phase states are unequal. 18. A method as claimed in any of claims 15 to 17 in which a first group of the phase states have a first dwell and a second group of the phase states have a second dwell time. 19. A method as claimed in any of claims 15 to 18 in which the dwell times are quantised according to a clock. 20. A system (1400) for generating a navigation signal comprising a carrier signal, the system comprising a subcarrier generator (1408) for generating at least one subcarrier modulation signal and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier modulation signal; wherein said at least one subcarrier modulation signal comprises a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2. 21. A system (1400) as claimed in claim 20 in which m is selected from at least one of 3, 4, 5, 6, 7, 8 or 9. 22. A system (1400) as claimed in any of claims 20 to 21 in which the at least one subcarrier modulation signal approximates or is derived from a basis waveform. 23. A system (1400) as claimed in claim 22 in which the basis waveform is at least one of a sine wave, cosine wave, or triangular waveform. 24. A system (1400) as claimed in either of claims 22 and 23 in which the basis waveform is selected according to desired power distribution characteristics of the transmission signal. 25. A system (1400) as claimed in any of claims 20 to 25 in which the at least one subcarrier comprises at least two mutually orthogonal subcarrier modulation signals. 26. A system (1400) as claimed in claim 25 in which the at least two subcarriers comprises a pair of subcarriers having a predetermined phase relationship. 27. A system (1400) as claimed in any of claims 20 to 26 in which the at least one subcarrier comprises an in-phase subcarrier and a quadrature phase subcarrier. 28. A system (1400) as claimed in claim 27 further comprising means for determining the respective multiple amplitudes of the in-phase and quadrature phase subcarriers to maintain a substantially constant transmission signal envelope. 29. A system (1400) as claimed in any of claims 20 to 28 further comprising means for deriving amplitudes associated with the at least one subcarrier from a plurality of phase states. 30. A system (1400) as claimed in claim 29, in which the phase states are equally angularly distributed around a unit circle. 31. A system (1400) as claimed in any of claims 20 to 30 in which durations of the amplitudes of the at least one subcarrier are equal. 32. A system (1400) as claimed in any of claims 20 to 31 in which durations of at least a pair of amplitudes of the at least one subcarrier are different. 33. A system (1400) as claimed in any of claims 31 to 32 in which the durations are quantised according to an associated clock signal. 34. A system (1400) as claimed in any of claims 20 to 33 in which at least a pair of subcarriers cooperate to define an associated plurality of phase states resolved according to mutually orthogonal axes. 35. A system (1400) as claimed in claim 34 in which the plurality of phase states is associated with respective ranging signals. 36. A system (1400) as claimed in either of claims 34 and 35, in which dwell times in at least some of the plurality of phase states are unequal. 37. A system (1400) as claimed in any of claims 34 to 36 in which a first group of the phase states have a first dwell and a second group of the phase states have a second dwell time. 38. A system (1400) as claimed in any of claims 34 to 37 in which the dwell times are quantised according to a clock. 39. A receiver system (1600) comprising means (1640) to receive and down convert a navigation signal; the navigation signal comprising a carrier signal and a ranging code modulated by at least one subcarrier signal; said at least one subcarrier signal comprising a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2; said means (1640) comprising a down converter (1628) for down converting the navigation signal to a baseband signal; a demodulator (1642) for demodulating the baseband signal to extract the ranging code. ABSTRACT METHOD AND SYSTEM FOR GENERATING AND RECEIVING A NAVIGATION SIGNAL There is disclosed a method of generating a navigation signal comprising a carrier signal, the method comprising the step of multiplying the carrier signal by at least one subcarrier modulation signal; wherein the at least one subcarrier modulation signal comprises a number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m > 2. Also disclosed is a system (1400) for generating the said navigation signal, having a subcarrier generator (1408) for generating at least one subcarrier modulation signal and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier modulation signal, and further a receiver system (1500) having means (1640) with a down converter (1628) and a demodulator (1642) to receive and down convert said navigation signal.

Full Text

METHOD AND SYSTEM FOR GENERATING A NAVIGATION SIGNAL
Field of the Invention
The invention relates to modulation signals, systems and methods such as, for example,
navigation and positioning signals, systems and methods.
Background to the invention
Satellite Positioning Systems (SPS) rely on the passive measurement of ranging signals
broadcast by a number of satellites, or ground-based or airborne equivalents, in a
specific constellation or group of constellations. An on-board clock is used to generate
a regular and usually continual series of events, often known as 'epochs', whose time of
occurrence is coded into a random or pseudo-random code (known as a spreading code).
As a consequence of the pseudo-random or random features of the time epoch encoding
sequence, the spectrum of the output signal is spread over a frequency range determined
by a number of factors including the rate of change of the spreading code elements and
the waveform used for the spreading signal. Typically, the spreading waveform is
rectangular and has a sine function power spectrum.
The ranging signals are modulated onto a carrier signal for transmission to passive
receivers. Applications are known that cover land, airborne, marine and space use.
Typically, binary phase shift keying is employed to modulate the carrier signal, which,
itself, has a constant magnitude. Usually, at least two such signals are modulated onto
the same carrier in phase quadrature. The resulting carrier signal retains its constant
envelope but has four phase states depending upon the two independent input signals.
However, it will be appreciated that two modulating signals do not need to have the
same carrier magnitude. It is possible for a constant carrier magnitude of the combined
signal to be maintained by appropriate selection of corresponding phases other than π/2
radians.
An example of such a satellite positioning system is the Global Positioning System
(GPS). Generally, the GPS operates using a number of frequencies such as, for
example, L1, L2 and L5, which are centred at 1575.42 MHz, 1227.6 MHz and 1176.45
MHz respectively. Each of these signals is modulated by respective spreading signals.

As will be appreciated by those skilled in the art, a Coarse Acquisition (CA) code signal
emitted by the GPS Satellite Navigation System is broadcast on the L1 frequency of
1575.42MHz with a spreading code rate (chip rate) of 1.023MHz. The CA has a
rectangular spreading waveform and is categorised as BPSK-R1. The GPS signal
structure is such that the signal broadcast by the satellites on the L1 frequency has a
second component in phase quadrature, which is known as the precision code (P(Y)
code) and is made available to authorised users only. The P(Y) signal is BPSK
modulated with a spreading code at 10.23MHz with a magnitude which is 3dB lower in
signal power than the CA code transmission. Consequently, the Q component has a
magnitude which is 0.7071 (-3dB) of the magnitude of the I component. It will be
appreciated by those skilled in the art that the phase angles of these states of these
signals are ±35.265° in relation to the ±1 axis (phase of the CA code signal as specified
in ICD GPS 200C). One skilled in the art also appreciates mat the P code is a function
of or is encrypted by the Y code. The Y code is used to encrypt the P code. One skilled
in the art appreciates that the L1 signal, containing both I & Q components, and the L2
signal can be represented, for a given satellite, i, as

Pi(t) represents the P(Y) ranging code and is a pseudo-random sequence with a chip rate
of 10.23 Mcbps. The P code has a period of exactly 1 week, taking values of +1 and -1;
Ci(t) represents the CA ranging code and is a 1023 chip Gold code, taking values of +1
and-1;
di(t) represents the data message, taking values of+1 and -1.

A satellite constellation typically comprises 24 or more satellites often in similar or
similarly shaped orbits but in a number of orbital planes. The transmissions from each
satellite are on the same nominal carrier frequency in the case of code division access
satellites (such as GPS) or on nearby related frequencies such as GLONASS. The
satellites transmit different signals to enable each one to be separately selected even
though several satellites are simultaneously visible.
The signals from each satellite, in a CDMA system like GPS, are distinguished from
each one another by means of the different spreading codes and/or differences in the
spreading code rates, that is, the pi(t) and Ci(t) sequences. Nevertheless, as will be
appreciated from the power spectrum 100 shown in figure 1 there remains significant
scope for interference between the signals transmitted by the satellites. Figure 1 shows
power spectra 100 for the CA and P(Y) codes. The power spectrum 102 for the CA
code has maximum power at the carrier frequency L1 and zeros at multiples of the
fundamental frequency, 1.023MHz, of the CA code. For example, it can be appreciated
that zeros occur either side of the carrier frequency at ±1.023MHz, ±2.046MHz etc.
Similarly, the power spectrum 104 for the P(Y) code has a maximum amplitude centred
on the LI and L2 frequencies, with zeros occurring at multiples of ±10.23MHz as is
expected with a sine function waveform.
It is known to further modulate the ranging codes using a sub-carrier, that is, a further
signal is convolved with the P codes and/or CA codes to create Binary Offset Carrier
(BOC) modulation as is known within the art see, for example, J. W. Betz, "Binary
Offset Carrier Modulation for Radionavigation", Navigation, Vol. 48, pp227-246,
Winter 2001-2002. Standard BOC modulation 200 is illustrated in figure 2. Figure 2
illustrates the combination of a portion of a CA code 202 with a subcarrier signal to
produce the BOC signal 204 used to modulate a carrier such as, for example, LI. It can
be appreciated that the BOC signal is a rectangular square wave and can be represented
as, for example, Cj(t)*sign(sin(27πfst)), where fs is the frequency of the subcarrier. One
skilled in the art understands that BOC(/i/c) denotes Binary Offset Carrier modulation
with a subcarrier frequency of fs and a code rate (or chipping rate) of fc. Using binary
offset carriers results in the following signal descriptions of the signals emitted from the
satellite:

Figure 2 also illustrates power spectra for a BPSK-R1 code and pair of BOC signals,
that is, BOC(2,1) and BOC(10,5). The first spectrum 202 corresponds to BPSK-R1
code. The second power spectrum 204 corresponds to the BOC(2,1) code and the third
power spectrum 206 corresponds to the BOC(10,5) code. It can be appreciated that the
side lobes 208 of the BOC(2,1) signal have a relatively large magnitudes. Similarly, the
illustrated side lobe 210 of the BOC(10,5) signal has a relatively large magnitude. One
skilled in the art appreciates that the energy in the side lobes are a source of
interference.
It is an object of embodiments of the present invention to at least mitigate the problems
of the prior art.
Summary of Invention
Accordingly, a first aspect of embodiments of the present invention provides a method
as claimed in claim 1.
A second aspect of embodiments of the present invention provides a signal as claimed
in claim 20.

A third aspect of embodiments of the present invention provides a system as claimed in
claim 28.
A fourth aspect of embodiments of the present invention provides a received as claimed
in claim 29.
A fifth aspect of embodiments provides a computer readable storage as claimed in claim
30.
Advantageously, embodiments of the present invention provide significantly more
control over the shape of the power spectra of signals, that is, the distribution of energy
within those signals.
Other aspects of the present invention are described and defined in the claims.
Brief Description of the Invention
Embodiments of the present invention will now be described, by way of example, only
with reference to the accompanying drawings in which:
figure 1 shows a power spectrum of a pair of ranging code;
figure 2 illustrates power spectra of a ranging code (BPSK-R1) and BOC(10,5) signals;
figure 3 illustrates a multi-level sub-carrier;
figure 4 illustrates the phase states for at least a pair of multilevel subcarriers according
to embodiments of the present invention;
figure 5 depicts a power spectrum of a prior art subcarrier and a subcarrier according to
embodiments of the present invention;
figure 6 illustrates phase states for a subcarrier according to embodiments of the present
invention;

figure 7 illustrates in phase and quadrature phase subcarriers according to embodiments of
the present invention;
figure 8 illustrates phase states of a subcarrier according to an embodiment of the present
invention;
figure 9 shows subcarriers according to embodiments of the present invention;
figure 1.0 depicts power spectra of subcarriers according to embodiments of the present
invention;
figure 11 illustrates subcarriers according to embodiments of the present invention;
figure 12 shows an alternative subcarrier waveform according to an embodiment of the
present invention;
figure 13 illustrates a further alternative waveform according to embodiments of the present
invention;
figure 14 illustrates, schematically, a transmitter using subcarriers according to
embodiments of the present invention;
figure 15 illustrates a further embodiment of a transmitter according to the invention; and
Figure 16 illustrates a receiver system according to an embodiment of the invention:

It will be appreciated that there are preferably restrictions on the combinations of
signals, at least one of which is that a constant modulus signal should be maintained.
The constraints are (1) that "+1" or "-1" on the in-phase component can only occur in
conjunction with "0" on the quadrature phase
component and visa versa and (2) "±l/\/2" can only occur on both phases
simultaneously. The magnitudes of the in-phase and quadrature phase components of
the spreading signals, sc(g(t) or scim(t), can be plotted on an Argand diagram 400 such as
is shown in figure 4. The waveforms for the I and Q components are thus built from the
following signal element sequences:

Any combination of I or Q signal sequences can be chosen from the above set within
the constraint of a constant magnitude carrier signal, computed as (I2+Q2)1/2. It will be
clear to those skilled in the art, that there are many other equivalent sets of sequences
which may be chosen from the set of 5-levels satisfying the criterion of a constant
carrier envelope. It can be appreciated that the magnitudes of the subcarriers on the I
and Q channels can be thought of as being analogous to the states of an 8-PSK signal.
Therefore, such a pair of 5-level subcarrier carrier signals can be thought of as 8 phase
subcarrier signals.
Figure 5 illustrates the effect of using a stepped or m-level, m>2, subcarrier waveform.
Referring to figure 5 there is shown a pair 500 of power spectra. The first power
spectrum 502, illustrated using the dotted line, represents the spectrum of a BOC(2,2)
subcarrier. It can be appreciated that the energy of the subcarrier is contained within
progressively reducing side lobes 504, 506, 508 and 510. The second power spectrum
512 represents the power spectrum of a BOC(2,2) signal that used 8 phase subcarrier
signals, that is, 8 phase amplitudes, represented by BOC8(2,2). More generally,
BOCm(fsfc) represents an m-phase subcarrier signal of having a frequency of fs and a

chipping rate of fc. It can be appreciated that the spectrum 512 of the BOC8(2,2) signal
has a number of side lobes 514, 516, 518, 520, 522 and 524. Of those side lobes, it can
be seen that the 1st to 4th side lobes are significantly reduced, that is, they comprise
significant less energy, as compared to the side lobes of the BOC(2,2) signal spanning
the same frequencies. The significant reduction in the 1st to 4* side lobes can be
beneficial in situations in which one skilled in the art wishes to use the frequency
spectrum spanned by the side lobes for other transmissions.
It will be appreciated by those skilled in the art the BOC8(2,2) has significandy
improved interference properties as determined using Spectral Separation Coefficients
(SSC) and self-SSCs as is well understood by those skilled in the art, that is, the spectral
coupling between a reference signal and BOC(2,2) is greater than the spectral coupling
between a reference signal and BOC8(2,2). For example, a BOC8(2,2) signal exhibits a
10-12 dB improvement in spectral isolation as compared to a conventional BOC(2,2)
signal. Further information on the relationship between SSC and signals according to
embodiments of the present invention can be found in, for example, Pratt & Owen;
BOC Modulation Waveforms, IoN Proceedings, GPS 2003 Conference, Portland,
September 2003, which is incorporated herein by reference for all purposes and filed
herewith as in the appendix.
Furthermore, embodiments of the present invention utilise the magnitude and duration
of the subcarrier to influence, that is, control the energy in harmonics of the resulting
modulating waveform. For example, referring still to figure 5, it can be appreciated that
additional spectral nulls appear in the BOC8(2,2) spectrum at substantially 6MHz and
10MHz offset from the carrier whereas there are no such nulls in the conventional
BOC(2,2) signal. The location of the nulls is influenced by at least one of the
magnitude and duration of the steps in the multilevel subcarrier. More specifically, the
nulls can be steered to desired locations by changing either of these two elements, that
is, the position of the nulls is influenced by these two elements. Appendix A contains
an indication of the relationship between the spectra of signals according to
embodiments of the present invention and the magnitude and duration of the steps.
Referring to figure 6, there is shown subcarrier states or amplitudes for I and Q signals
for a further BOC8 signal, that is, a binary offset carrier having eight states. It can be

It will be appreciated that the states 1 to 8 shown in figure 6 are not equidistantly
disposed circumferentially. The transitions between states 2&3, 4&5, 6&7, 8&1 are
larger in angular step than the transitions between states 1&2, 3&4, 5&6, 7&8. It will
be appreciated that when these states are translated into subcarrier amplitudes, the
duration of a given amplitude will depend on the duration or dwell time of a
corresponding state, that is, the durations for which the subcarrier remains in any
given state may no longer be equal unlike the states of figure 4 above. The dwell times
are a matter of design choice such as, for example, to minimise the mean square
difference between a stepped waveform and a sinusoid. Figure 7a illustrates the
subcarriers 700 and 702 corresponding to the states shown in figure 6. It can be
appreciated that the durations of or within each state of the subcarriers 700 and 702 are
equal. The Q channel subcarrier magnitudes will follow substantially the same pattern
as described above but phase shifted by 7t/2 radians. The subcarrier 702 for the Q
channel is shown in dotted form in figure 7. It will be appreciated that such subcarriers
provide a constant envelope magnitude since (I2+Q2)1/2=l for all amplitude
combinations. However, referring to figure 7b, there is shown a pair of subcarriers 704

One skilled in the art will appreciate that the above can be extended to n half cycles of a
subcarrier per ranging code chip.
It will be appreciated that other phases can be used to describe the subcarriers. For
example, phase and amplitude components of 16-PSK can be used to create BOC16
subcarriers having 9 levels, assuming that the first state is at (+1,0). Using m-PSK

phase states can be used to produce (m+2)/2 level subcarrier signals. Therefore, setting
m=2 gives the conventional BPSK and a two-level subcarrier. Setting m=4 provides a 3
level subcarrier, that is, BOC4 modulation, setting m=8 produces a 5 level subcarrier,
that is, BOC8 modulation, setting m=16 produces a 9 level subcarrier, which
corresponds to BOC16 modulation.
It will be appreciated that several further variations in the assignment of code and data
states to the phase locations can be realised. For example, rotation of the states shown
in figure 4 by 22.5° leads to a reassignment of angles associated with the states from the
angles (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°) to the angles (22.5°, 67.5°, 112.5°,
157.5°, 202.5°, 247.5°, 292.5°, 337.5°). Again, it will be appreciated mat this does not
cause a change in the modulus of the spectrum and, again, the required number of
amplitude levels reduces from 5 to 4, that is, m-PSK can be used to realise [(m+2)/2-l]
amplitudes according to appropriate rotation and alignment of the phase states. The
resulting waveforms for the I and Q components are built, in this case, from the
following signal element sequences:

It should be noted that the I and Q signal element sequences for the cases described
above are orthogonal over the duration of one spreading pulse (chip). Clearly, other
rotations are possible and will yield orthogonal signal element sets.
An alternative way of representing the above is via a state table. Assume that an
embodiment of a BOC8 modulation has been realised with equidistant states and the
first state has a phase angle of rc/8 radians (22.5°) as shown in figure 8, which

corresponds to the above values. The sequence of phase states required for each I and Q
ranging code signal component, assuming that the ranging codes transition substantially
simultaneously and a desire to maintain a substantially constant output envelope, that is,
the states for the subcarriers would be given by

Table 1 - Sequence of States for BOC8(x,x) I & Q signal elements
It will be appreciated that the subcarrier corresponding to the phase states in Table 1
comprises a half cycle per ranging code chip. Furthermore, the sense of the phasor is
clockwise when I and Q are equal and anticlockwise otherwise. It will be apparent that
the signal element sequences or state sequences are sections (specifically half cycle
sections in the aspect of the invention disclosed above) of a sampled or quantised
sinusoid. The concept can, therefore, be extended to include a multiplicity of such
samples. Those variants, which appear to be useful, include the cases with samples
from a finite number of half cycles, that is, rather than, for example, an I channel value
of +1 being represented by the states of 2, 1,8 and 7, it can be represented using some
otiier number of states such as, for example, 2, 1, 8, 7, 6, 5, 4, 3, 2, 1, 8, 7 ie by three
half cycles of the sample or quantised sinusoid. Table 2 illustrates the phase states for
such an embodiment and is based on the phase state diagram of figure 4 for samples but
using three half cycles (or an arbitrary number of half cycles) of the sinusoid waveform.
The sinusoid or portion or multiple of half cycles thereof is known as the 'basis
waveform'. One skilled in the art realises that other basis waveforms can be used such
as, for example, a triangular waveform or a set of mutually orthogonal waveforms.

One skilled in the art will appreciated mat it is assumed in Tables 1 to 3, that the I and Q
chip transitions happen substantially simultaneously and, furthermore, that the I and Q
subcarriers take the form of sine and cosine waveforms respectively. However,
embodiments can be realised in which me ranging code chip transitions do not occur
substantially simultaneously. Furthermore, in circumstances in which the ranging code
chip transitions do not occur substantially simultaneously, the subcarriers corresponding
to the I and Q ranging code chips can be arranged to take the form of a pair of quantised
sine waves.
It will seen that there are 4 time samples for each V2 cycle of the waveform. The
stepped sinusoidal waveform may be viewed as sub-carrier modulation of the basic
spreading waveform. The number of time samples and independent information
bearing channels is related to the number of phase states, which me carrier signal has in
its representation. Although the examples above have used phase states that are
'powers of 2', embodiments can be realised in which some other number is used. For
example, a 6-PSK carrier signal can be used to carry 2 independent information bearing
binary channels. In mis case only 3 signal element samples are required per transmitted
code chip.

One skilled in the art appreciates that the replacement of the stepped sinusoid with a
rectangular wave with the duration of each element being equal to a Vi cycle of the
sinusoid is well known within the art. As indicated above, it is known as 'Binary Offset
Carrier' modulation. There are usually 2 further attributes associated with the BOC
description, which relate to the frequency of the code chipping rate and to the frequency
of the offset sub-carrier. BOC(2,2) consequently is interpreted as a waveform with a
2.046MHz chipping rate and a 2.046MHz offset sub-carrier. This arrangement has
exactly two Vi cycles of the sub-carrier signal for each code element (chip).
A further aspect of embodiments of the present invention relates to using a set of
subcarriers to modulate ranging codes, with at least one or more, or all, of the
subcarriers being multilevel waveforms. One skilled in the art may think of such
embodiments as modulation of the subcarrier signal by a further subcarrier signal. The
resulting signal transmitted by an ith satellite or system having a carrier frequency of COi,
for an additional subcarrier, would have the form:

scim(t) and scjm(t) represent first and second subcarrier signals respectively first ranging
codes such as, for example, M-codes; and
scig(t) and scjg(t) represent first and second subcarrier signal second ranging codes such
as, for example, Gold codes.
It should be noted that embodiments can be realised in which scim(t) and sc;g(t) are the
same or different. Similarly, embodiments can be realised in which scjm(t) and scig(t)
are the same or different.

Although it is possible to use more than one subcarrier, practical embodiments will
typically use 2 subcarriers. Modulation using a pair of subcarriers is known as Double
Binary Offset Carrier (DBOC) modulation. Modulation using three subcarriers is
known as Triple Binary Offset Carrier (TBOC) modulation and so on such that
modulation using a n-tuple of subcarriers is known as N-tuple Binary Offset Carrier
(NBOC). As mentioned above, one or more than one of the subcarriers may be stepped,
that is, having magnitudes related to respective phase states.
As examples of this aspect of the invention, figure 9 illustrates a pair of waveforms 900.
In figure 9, as an illustration of the NBOC invention, the subcarrier basis waveforms are
assumed to be binary and only a single subcarrier waveform 902 is shown. The time
duration in figure 9 is 512 samples and exactly matches the duration of one code
element duration (chip). The first subcarrier 902 contains 4 half cycles of a subcarrier
per ranging code chip, as illustrated by the dashed waveform. If this was the only sub-
carrier component, the modulation would be a BOC(2x,x) type, where x is the
frequency of the code rate (chipping rate). However, it can be appreciated that a second
sub-carrier (not shown) having 16 half cycles per 512 samples has been used to produce
the modulated waveform 904 to be combined with the carrier of the satellite signal. The
modulated waveform is shown by the solid curve. As a result of modulation
(multiplication) of the two subcarriers, the resulting waveform 904 has phase reversals
for the second subcarrier 904 whenever there is a sign reversal in the first subcarrier
902. This is clearly evident in figure 9 at points 906, 908 and 910, where the transitions
of the second subcarrier (not shown) would have been opposite. The resulting
modulation is denoted Double BOC, or DBOC. In the case of figure 9, the modulation
is DBOC(8x,(2x,x)), that is, there are 8 half cycles of the second subcarrier per chip of
the ranging code (not shown). The main energy is concentrated around frequencies ±8x
from the carrier signal, with a BOC-like double humped spectrum.
Referring to figure 10, there is shown a pair of power spectra 1000. A first power
spectrum 1002 relates to a DBOC8(16,(2,2)) signal. It will be appreciated that at least

one of the first and second subcarriers used to create the DBOC8(16,(2,2)) signal
comprised amplitudes derived from 8 corresponding phase states. In the specific
embodiment shown, the first subcarrier was the multi-level signal. It will be
appreciated mat the nomenclature for representing DBOC modulation or subcarriers is
DBOCa(b,c(d,e)), where a and c represent the number of phase states, that is,
amplitudes, of the subcarriers having frequencies b and d respectively. The second
spectrum 1004 relates to a BOC8(2,2) signal. The spectra shown have been made using
a previous aspect of the invention, that is the use of multilevel subcarriers or subcarriers
having more than two phase states, in combination with the Double BOC concept. The
waveforms for I & Q modulations for the spectrum of figure 10 are shown in figure 11.
Referring to figure 11 there is shown a pair 1100 of waveforms. The first pair of
waveforms 1102, representing the I channel of the spreading waveform, comprises a
stepped or multi-level BOC(2,2) signal 1104, represented by the solid line, and a 16Mhz
subcarrier modulated BOC(2,2) signal 1106, represented by the dashed line. It will be
appreciated that the 16 MHz subcarrier modulated BOC(2,2) signal has been produced
by multiplying the BOC8(2,2), that is, stepped BOC(2,2) signal, by a 16 MHz
rectangular waveform (not shown) having amplitudes of +1. The second waveform
1108, representing the Q channel, comprises a quadrature BOC(2,2) signal 1110
together with a 16MHz subcarrier modulated BOC(2,2) signal 1112. It can be
appreciated that the first subcarrier 1104 or 1110 is a subcarrier according to an
embodiment of the present invention described above whereas the second sub-carrier
(not shown) in both cases are conventional binary rectangular waveforms, that is,
conventional subcarriers. It can be appreciated that there are regions 1114 of overlap
between the two BOC(2,2) subcarriers 1104 and 1110 and their resulting products, that
is, 16 MHz subcarrier modulated BOC(2,2) signals 1106 and 1112. In the regions of
overlap 1114, the waveforms have the same amplitude profile.
An advantage of the embodiments of the signals shown in figure 11 is that the I channel
or component has been produced or represents DBOC8 modulation or signal whereas
the Q channel has been produced using or represents BOC8 modulation. However, this
arrangement still preserves or provides a substantially constant envelope carrier signal
to be emitted from the satellite.

Embodiments of the present invention have been described with reference to the
subcarrier signals being periodic. However, embodiments can be realised in which the
subcarrier signal comprises a pseudorandom noise signal. Furthermore, embodiments
can be realised in which the shape of the subcarrier takes a form other than a stepped,
that is, a multilevel wave or quantised approximation of a sinusoidal waveform. For
example, multilevel-pulsed waveforms, multilevel-periodic waveforms or multilevel-
aperiodic waveforms, could be used such as the signal shown in figure 12 according to
the influence one skilled in the art wishes the resulting modulation to have on the power
spectrum of the transmitted signal and/or any appropriate measure of interference such
as, for example, SSC or self-SSCs.
Referring to figure 13, there is shown a subcarrier waveform 1300 according to a further
embodiment of the present invention together with one chip 1302 of a code or otiier
waveform such as, for example, another subcarrier. It can be appreciated mat the
subcarrier comprises a first portion of a BOC(5,l) waveform, in the 100 ns sections,
combined widi portions of a BOC(l,l) waveform, in the 400 ns portions, to produce an
overall subcarrier. It will be appreciated tiiat the spectra of the BOC(5,l) waveform
will have a peak at 5*1.023MHz and the BOC(l,l) waveform will have a peak at
1*1.023MHz. Therefore, one skilled in the art appreciates diat selectively combining
the BOC subcarriers allows one skilled in the art to position or relocate the peaks of the
overall subcarrier. Again, it can be appreciated diat the subcarrier used, for example, to
modulate the ranging codes is derived from more dian one subcarrier. Although the
signal described in relation to figure 13 has been derived from BOC(5,l) and BOC(l,l)
subcarriers, embodiments can be realised in which otiier combinations of BOC
subcarriers are used. In effect, the BOC(5,l) and BOC(l.l) signals have been
multiplexed or selectively combined to produce an overall subcarrier signal. It will be
appreciated diat other sequences for the subcarriers can be realised according to a
desired effect upon the power spectrum of a transmitted signal. For example, a
subcarrier can be realised using a pseudorandom sequence as a sub-carrier instead of the
stepped modulations. The use of additional sequences to diat of the main spreading
code has hitherto been limited to use as a tiered code, which changes state after every
complete code repetition interval. The GPS L5 codes are constructed in tiiis manner
using Neumann Hoffman sequences of lengtii 10 or 20 to extend a 1ms code (of 10230
chips or elements) to 10ms or 20ms. The use of a subcode chip interval has not

previously been considered. A complete sequence (a sub-sequence) has a duration of
one code chip, or at most a plurality, of code chips. It fulfills a similar role to the sub-
carrier modulation as previously described in that it controls the spectrum of the
emissions. One feature of such a sub-sequence is that such sequences may be chosen to
be common amongst a satellite constellation or a sub-set of the constellation. One such
subset might be a group of ground transmitters providing a local element or
augmentation to the space segment of the system. For example, subcarrier amplitudes
can be realised that have the sequence -++++—+ in 10 subchip intervals or other
sequence of +l's and -l's per ranging code chip or other subcarrier chip according to
the desired effect on spectrum of the resulting signal. Examples such as me 7 subchip
interval sequences would include ++ , +++—, +-+-+--, and can be chosen to
provide similar control over the emitted spectrum.
Referring to figure 14, there is shown, schematically, a transmitter 1400 according to an
embodiment of the present invention. The transmitter 1400 comprises means 1402, that
is, a generator, for generating or selecting the ranging codes for transmission. It will be
appreciated by those skilled in the art that such ranging codes may be generated by, for
example, shift register implementations. It can be appreciated that the ranging code
selection and/or generation means 1402 is illustrated as producing gift) and nij(t). These
codes are fed to respective mixers 1404 and 1406. The mixers 1404 and 1406 are
arranged to combine the ranging codes with subcarriers according to embodiments of
the present invention. Respective subcarrier generators 1408 and 1410 generate the
subcarriers. Optionally, a data signal, d((t), is also preferably mixed with the ranging
codes and subcarriers. The duration of one bit of the data signal is normally an integer
multiple of the code repetition interval. For example, in GPS CA code, it is 20 times the
1 ms code repetition interval, that is, the data rate is 50 bps. The mixed signals 1412
and 1414 are fed to a further pair of mixers 1416 and 1418, where they are mixed with
in-phase and quadrature phase signals produced via an oscillator and phase shifter
assembly 1420. The further mixed signals 1422 and 1424 are combined, via a combiner
1426, and output for subsequent up conversion by an appropriate up converter 1428.
The output from the up converter 1428 is fed to a high-power amplifier 1430 and then
filtered by an appropriate filter 1433 for subsequent transmission by, for example, a
satellite or other device arranged to emit or transmit the ranging codes.

Referring to figure 15, there is shown a schematic representation of a modulation
system 1500 according to an embodiment. The system 1500 comprises a ranging code
generator 1502 for producing a ranging code. The ranging code is fed to a first lookup
table 1504 comprising phase states and a second lookup table 1506 comprising
amplitude states. The output of the phase state lookup table 1504 is used to drive a
phase modulator 1508, which, in turn, produces a voltage signal to control the phase of
a voltage controlled oscillator 1510. The output of the oscillator 1510 is combined, via,
a combiner 1512 such as, for example, a gain controlled amplifier or multiplier, with a
signal output from the amplitude state table 1506 to produce a subcarrier having me
appropriate characteristics.
Aldiough the above embodiments have been described with reference to maintaining a
substantially constant signal envelope, embodiments are not limited thereto.
Embodiments can be realised in which variable modulus signal envelopes are used. It
will be appreciated that the constraints described above, which are aimed at preserving
unitary magnitude of (I2+Q2)1/2, need not necessary apply.
The above embodiments have been described with reference to the I and Q channels
having the same chipping rates. However, embodiments are not limited to such
arrangements. Embodiments can be realised in which different chipping rates are used.
Although embodiments of the present invention have been described with reference to
the LI and L2 frequencies, embodiments are not limited to such arrangements.
Embodiments can be realised in which other frequencies or frequency bands can be
used according to the requirements of the system using the invention. For example, the
lower L band (ie E5a and E5b), the middle (ie E6) and upper L-band (ie E2-L1-E1) can
also benefit from embodiments of the present invention. It will be appreciated mat such
embodiments may use signals having at least three components rather than the two
components described above.
Furthermore, embodiments of the present invention have been described witii reference
to standard BOC. However, one skilled in the art appreciated mat embodiments can
also be realised using Alternative BOC.

Furthermore, it will be appreciated that embodiments can be realised in which the
number of half cycles of a subcarrier per chip of a code can be at least one of odd, even,
an integer multiple or a non-integer multiple of the chip, that is, there is a rational
number relationship between the number of subcarrier half cycles and the chip duration.
Embodiments of the present invention described above have focused on the
transmission side of the invention, that is, upon the generation, modulation and
transmission of ranging codes combined with a subcarrier or subcarriers. However, one
skilled in the art appreciated that a converse system and method are required to receive
and process the signals. Once one skilled in the art has designed a system for
transmitting such signals, designing an appropriate receiver is merely the converse of
the transmit operations. Therefore, embodiments of the present invention also relate to
a receiver for processing signals such as those described above.
Referring to figure 16, there is shown a receiver 1600 comprising means 1640 to receive
and down convert a navigation signal according to embodiments of the present
invention. In the embodiment shown the means 1640 to receive and down convert the
navigation signal comprises an input filter 1633, an amplifier 1630 and a down
converter 1628. The means 1640 to receive and down convert the navigation signal
output down converted modulated ranging codes. Also shown is a demodulator 1642
for demodulating the modulated ranging codes using at least one, and preferably two in
the illustrated embodiment, m-level subcarrier demodulation signals from respective
signal generators 1608 and 1610 to recover at one and preferably two ranging codes
ft (0 and w,(0-
In embodiments that use I and Q channels there is also provided means 1644 for
separating the received signal into I and Q channels. It can be appreciated that the
means 1644 for separating the received signal into I and Q channels comprises a pair of
signal paths 1622 and 1624 for coupling the output of the down converter 1628 to a pair
of mixers or demodulators 1616 and 1618. The mixers or demodulators cooperate with
a phase shifter 139 and a local oscillator 1620 to produce an I channel signal 1612 and a
Q channel signal 1614.
The I channel signal 1612 and the Q channel signal 1614 are fed to a pair of mixers or
demodulators 1604 and 1606 that, in conjunction with respective despreading
waveforms sc,g(g) and sc>m(t) from respective waveform generators 1608 and 1610,
allow recovery of the ranging codes gi(t) and nii(t) 1602.

All of the features disclosed in this specification (including any accompanying claims,
abstract and drawings), and/or all of the steps of any method or process so disclosed,
may be combined in any combination, except combinations where at least some of such
features and/or steps are mutually exclusive.
Each feature disclosed in this specification (including any accompanying claims,
abstract and drawings) may be replaced by alternative features serving the same,
equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly
stated otherwise, each feature disclosed is one example only of a generic series of
equivalent or similar features.

WE CLAIM:
1. A method of generating a navigation signal comprising a carrier signal, the method comprising
the step of multiplying the carrier signal by at least one subcarrier modulation signal; wherein
the at least one subcarrier modulation signal comprises a number, m, of discrete amplitude
levels derived from or associated with an m-ary phase constellation, where m > 2.
2. A method as claimed in claim 1 in which m is selected from at least one of 3, 4, 5, 6, 7, 8 or 9.
3. A method as claimed in any preceding claim in which the at least one subcarrier modulation signal
approximates or is derived from a basis waveform.
4. A method as claimed in claim 3 in which the basis waveform is at least one of a sine wave, cosine wave,
or triangular waveform.
5. A method as claimed in either of claims 3 and 4 in which the basis waveform is selected according to
desired power distribution characteristics of the transmission signal.
6. A method as claimed in any preceding claim in which the at least one subcarrier comprises at least two
mutually orthogonal subcarrier modulation signals.
7. A method as claimed in claim 6 in which the at least two subcarriers comprises a pair of subcarriers
having a predetermined phase relationship.
8. A method as claimed in any preceding claim in which the at least one subcarrier comprises an in-phase
subcarrier and a quadrature phase subcarrier.
9. A method as claimed in claim 8 further comprising the step of determining the respective multiple
amplitudes of the in-phase and quadrature phase subcarriers to maintain a substantially constant
transmission signal envelope.
10. A method as claimed in any preceding claim further comprising the steps of deriving amplitudes
associated with the at least one subcarrier from a plurality of phase states.
11. A method as claimed in claim 10, in which the phase states are equally angularly distributed around a
unit circle.

12. A method as claimed in any preceding claim in which durations of the amplitudes of the at least one
subcarrier are equal.
13. A method as claimed in any preceding claim in which durations of at least a pair of amplitudes of the at
least one subcarrier are different.
14. A method as claimed in any of claims 12 to 13 in which the durations are quantised according to an
associated clock signal.
15. A method as claimed in any preceding claim in which at least a pair of subcarriers cooperate to define an
associated plurality of phase states resolved according to mutually orthogonal axes.
16. A method as claimed in claim 15 in which the plurality of phase states is associated with respective
ranging signals.
17. A method as claimed in either of claims 15 and 16, in which dwell times in at least some of the plurality
of phase states are unequal.
18. A method as claimed in any of claims 15 to 17 in which a first group of the phase states have a first
dwell and a second group of the phase states have a second dwell time.
19. A method as claimed in any of claims 15 to 18 in which the dwell times are quantised according to a
clock.
20. A system (1400) for generating a navigation signal comprising a carrier signal, the system
comprising a subcarrier generator (1408) for generating at least one subcarrier modulation
signal and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier
modulation signal; wherein said at least one subcarrier modulation signal comprises a number,
m, of discrete amplitude levels derived from or associated with an m-ary phase constellation,
where m > 2.
21. A system (1400) as claimed in claim 20 in which m is selected from at least one of 3, 4, 5, 6, 7, 8 or 9.
22. A system (1400) as claimed in any of claims 20 to 21 in which the at least one subcarrier modulation
signal approximates or is derived from a basis waveform.

23. A system (1400) as claimed in claim 22 in which the basis waveform is at least one of a sine wave,
cosine wave, or triangular waveform.
24. A system (1400) as claimed in either of claims 22 and 23 in which the basis waveform is selected
according to desired power distribution characteristics of the transmission signal.
25. A system (1400) as claimed in any of claims 20 to 25 in which the at least one subcarrier comprises at
least two mutually orthogonal subcarrier modulation signals.
26. A system (1400) as claimed in claim 25 in which the at least two subcarriers comprises a pair of
subcarriers having a predetermined phase relationship.
27. A system (1400) as claimed in any of claims 20 to 26 in which the at least one subcarrier comprises an
in-phase subcarrier and a quadrature phase subcarrier.
28. A system (1400) as claimed in claim 27 further comprising means for determining the respective
multiple amplitudes of the in-phase and quadrature phase subcarriers to maintain a substantially constant
transmission signal envelope.
29. A system (1400) as claimed in any of claims 20 to 28 further comprising means for deriving amplitudes
associated with the at least one subcarrier from a plurality of phase states.
30. A system (1400) as claimed in claim 29, in which the phase states are equally angularly distributed
around a unit circle.
31. A system (1400) as claimed in any of claims 20 to 30 in which durations of the amplitudes of the at least
one subcarrier are equal.
32. A system (1400) as claimed in any of claims 20 to 31 in which durations of at least a pair of amplitudes
of the at least one subcarrier are different.
33. A system (1400) as claimed in any of claims 31 to 32 in which the durations are quantised according to
an associated clock signal.
34. A system (1400) as claimed in any of claims 20 to 33 in which at least a pair of subcarriers cooperate to
define an associated plurality of phase states resolved according to mutually orthogonal axes.

35. A system (1400) as claimed in claim 34 in which the plurality of phase states is associated with
respective ranging signals.
36. A system (1400) as claimed in either of claims 34 and 35, in which dwell times in at least some of the
plurality of phase states are unequal.
37. A system (1400) as claimed in any of claims 34 to 36 in which a first group of the phase states have a
first dwell and a second group of the phase states have a second dwell time.
38. A system (1400) as claimed in any of claims 34 to 37 in which the dwell times are quantised according
to a clock.
39. A receiver system (1600) comprising
means (1640) to receive and down convert a navigation signal; the navigation signal comprising a carrier signal
and a ranging code modulated by at least one subcarrier signal; said at least one subcarrier signal comprising a
number, m, of discrete amplitude levels derived from or associated with an m-ary phase constellation, where m >
2; said means (1640) comprising
a down converter (1628) for down converting the navigation signal to a baseband signal;
a demodulator (1642) for demodulating the baseband signal to extract the ranging code.

ABSTRACT

METHOD AND SYSTEM FOR GENERATING AND RECEIVING
A NAVIGATION SIGNAL
There is disclosed a method of generating a navigation signal comprising a carrier signal, the
method comprising the step of multiplying the carrier signal by at least one subcarrier
modulation signal; wherein the at least one subcarrier modulation signal comprises a number,
m, of discrete amplitude levels derived from or associated with an m-ary phase constellation,
where m > 2. Also disclosed is a system (1400) for generating the said navigation signal,
having a subcarrier generator (1408) for generating at least one subcarrier modulation signal
and a mixer (1404) for multiplying the carrier signal by said at least one subcarrier modulation
signal, and further a receiver system (1500) having means (1640) with a down converter (1628)
and a demodulator (1642) to receive and down convert said navigation signal.

METHOD AND SYSTEM FOR GENERATING AND RECEIVING A NAVIGATION SIGNAL

Documents:

Inventors:

PCT Conventions: