Title of Invention

METHOD FOR TRANSMITTING DATA IN A WIRELESS NETWORK

Abstract

A system and method for reducing intercarrier interference while transmitting data in a wireless network is disclosed. The system comprises mapping means, dividing means, grouping means, assignment means and interleaving means. The sub-signals of lower bandwidth are interleaved with sub-signals of higher bandwidth. The interleaved sub-signals are orthogonal to each other. The method comprises the following steps: mapping, dividing, grouping, assigning and interleaving.

Full Text

COMPETE AFTER PROVISIONAL LEFT ON 05/09/2006 FORM-2 THE PATENT ACT, 1970 (3.9 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See section 10 and Rule 13) SYSTEM FOR TRANSMITTING DATA IN A WIRELESS NETWORK TATA CONSULTANCY SERVICES LIMITED An Indian Company of Bombay House, 24, Sir Homi Mody Street, Mumbai-400 001, Maharashtra, India. THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. Field of invention: The invention relates to a system for transmitting data in a wireless network. Particularly, the invention relates to a system for transmission of data in the multi-carrier system in a wireless network. More particularly, this invention relates to a system for transmission of data to reduce intercarrier interference (ICI) in the multi-carrier communication system. Description of prior art: OFDM is a scheme of Frequency Division Multiplexing where sub carrier frequencies are orthogonal to each other, such that the null of the spectrum of one sub carrier is at the peak of the other sub carriers. High data rate "OFDM" systems till date exist for Wireless Local Area Networks and also High Speed Data Access in wired network. Other OFDM based wireless systems are Digital Video Broadcasting -Terrestrial, and Digital Audio Broadcasting. The last two are mainly for broadcasting video or audio information. The first two implementations are either single user systems, as in IEEE 802.1 la/g or are multi-user with FDMA as in case of IEEE 802.16a. Most importantly they are not specially configured to support video or audio information. In short each of the systems mentioned above are designed to carry optimally only one type of traffic. Next generation wireless networks will need to support a large number of users and heterogeneous traffic. To support large number of simultaneous users, FDMA with large number of carriers is one of the solutions. With the 2 bandwidth of the spectrum being limited, increasing number of sub-carriers would decrease the sub-carrier spacing. Also, to use the spectrum efficiently, by reducing the overhead of guard interval in OFDM systems, larger symbol duration is desired. In the conventional OFDM system, the data is modulated and demodulated onto individual carriers using an IFFT function in the Transmitter and FFT in the receiver respectively. Typically, the transmitter consists of a single IFFT and a single FFT at the receiver. The basic idea of OFDM transmission is that the available spectrum is divided into N orthogonal subcarriers. The encoded data is mapped onto the subcarriers using an N-point IFFT. The mapper is designed to be used with a bit-loading algorithm. The included constellations are no modulation (ZERO), BPSK, QPSK, 8PSK, 16QAM, and 64QAM. The ZERO constellation does not contain any data, but is included for unused subcarriers. Mapping involves allocating set of data bits to each of these data points. Typically, an orthogonal mapping is a mapping in which the sub carriers are modulated in time orthogonally by IFFT. The IFFT modulates each sub-channel onto a precise orthogonal carrier. The data is then prepared for transmission i.e, serialized and appended with a cyclic prefix. The resulting data is sent to an antenna for transmitter. The input to IFFT is frequency representation signal comprising all the sub carriers. For modulation the IFFT requires all the sub carriers to be equally spaced. In an ideal OFDM system the sub carriers are equally spaced. If the sub carriers are not spaced equally, the modulation does not take place and results in self-interference errors. Errors of synchronization, errors of initial setting, 3 frequency effects and phase effects are collectively called as self interference errors. The self interference errors affect the orthogonality of the sub carriers thereby resulting in intercarrier interference (ICI). The intercarrier interference (ICI) means that the orthogonality between different sub channels in the OFDM signal is destroyed. A larger transmission power for pilot sub-channels results in larger ICIs for information-bearing sub carrier and hence causes implementation difficulties. Since the bandwidth of the spectrum is limited, increasing the number of sub carriers reduces the sub carrier spacing. As the sub carrier spacing reduces, ICI gains prominence in comparison to the additive thermal noise. If the ICI power is more than the thermal power, it results in deterioration of system performance. The lower the ICI power the better is the system performance. Further, intercarrier interference (ICI) the increases BER (bit error rates) and allows low data rates. In such a situation it is very much necessary to improve the system performance by reducing the detrimental effect of ICI. Figure 1 depicts time vs frequency plot of a standard OFDM system. A standard OFDM signal is described below: Let T be the symbol duration of the transmitted modulated data symbol in each sub-carrier. Then the sub-carrier spacing is 1/T for the sub-carriers to be orthogonal. The bandwidth of each sub-carrier is equal to 2/T. The bandwidth occupied by each sub-carrier is the same and the symbol duration in each sub-carrier is also the same. The rectangular pulses along the time axis show that all sub-carriers have 4 identical pulse duration for a modulated data symbol and each sub-carrier occupies the same bandwidth. Figure 2 shows the effect of synchronization in a conventional OFDM system. The data stream or user allocated on a particular sub-carrier is based on the performance of the data path on the sub-carrier of interest. The straight line shows the location of frequency sampled because of synchronization errors (Carrier frequency offset, sampling frequency offset and Doppler). The circle BC shows the point in the original signal, which is sampled because of the error. The circles RC show the effect (amplitude or power) of the neighboring sub-carriers on the sub-carrier of interest. As stated before, future wireless networks need to support a large number of simultaneous user with a wide variety of requirements. Requirements will vary in data-rate and sustainable error probability, tolerable latency, etc. This gives us the scope to arrange and design a system to use the situation to our benefit to reduce the ICI effect and improve system performance. (Improving system performance can mean, increasing number sub-carriers, or providing a Signal to Noise Ratio (SNR) advantage, or decreasing the error probability or increasing the system capacity or throughput.) 5 It is an object of the present invention to overcome the disadvantages of the prior art. Another object of the invention is to provide a system for transmission of data to reduce inter carrier interference in the multi-carrier communication system. Summary of the invention: In accordance with this invention there is provided a system for reducing intercarrier interference while transmitting data in a wireless network, said system comprises: (i) at least one mapping means adapted to map transmitted signals; (ii) at least one dividing means for dividing said mapped signals into sub-signals having bandwidth, spacing, symbol duration and amplitude; (iii) at least one grouping means adapted to form groups for sub-signals according to the bandwidth of said sub-signals; (iv) at least one assignment means adapted to assign said divided sub signals into respective groups; (v) at least one interleaving means adapted to interleave at least one lower bandwidth sub-signal in spaces between at least one sub-signal of higher bandwidth. Typically, said grouped sub-signals are orthogonal to each other. Typically, said interleaved sub-signals are orthogonal to each other. Typically, at least one lower bandwidth sub-signal is interleaved between two higher bandwidth signals. Typically, at least one lower bandwidth signal is interleaved between the whole group of higher bandwidth sub-signals. 6 Typically, sub-signals in a group have varying spacing. Typically, sub-signals in a group have varying symbol duration. Typically, sub-signals in a group have varying amplitude. In accordance with another aspect of this invention there is provided a method for reducing intercarrier interference, said method comprising the following steps: (i) mapping the signals; (ii) dividing said mapped signals into respective sub-signals; (iii) forming groups of sub-signals according to the bandwidth of said sub-signals; (iv) assigning said divided sub-signals to respective groups; (v) interleaving said assigned sub-signals in a manner such that at least one sub-signals with lower bandwidth are interleaved in spaces between at least one sub-signals of higher bandwidth. Typically, bandwidth of said interleaved sub-signals is equal. Typically, bandwidth of said interleaved sub-signals is unequal. Several orthogonal sub-carriers are used in the system. To reduce the inter carrier interference adjacent sub-carriers have different symbol durations. The invention envisages a system, which is a deviation from the normal OFDM system such that the first null-to-null bandwidth of each sub-carrier (all sub-carrier frequencies being orthogonal to each other) is not the same for all. Accordingly the pulse duration of symbols in all sub-carriers is not the same. The arrangement of the sub-carriers is such that the wider bandwidth ones are interleaved with sub-carriers with smaller bandwidth. This helps in reducing the effect of ICI. 7 The error probability of the wider sub carriers decreases as compared to a normal OFDM system with such a configuration. Thus the system splits up the available bandwidth into unequal segments, but the sub-carriers are still orthogonal to one another. This generates different groups of sub-carriers, with different performances. These new sub-channel can be distributed to different data streams. Hence the system in accordance with this invention has in its offering an inherent support for different channels with different performance measures with reduced inter carrier interference as compared to a normal OFDM scheme. Brief description of drawings: Figure 1 illustrates the time vs frequency plot of an OFDM system in prior art; Figure 2 illustrates the effect of the synchronization error in the conventional OFDM system; Figure 3 illustrates the time vs frequency plot of an OFDM system according to the present invention; Figure 4 illustrates the effect of the synchronization error in the OFDM system; Figure 5 illustrates the BER vs carrier frequency offset in high data rate system; and Figure 6 illustrates the BER vs carrier frequency offset in low data rate system. 8 Detailed description of the invention: The system in accordance with this invention can now be described as below with reference to the figures 3 to 6 of the accompanying drawings: Let the symbol duration of 'a' number of sub-carriers be Ta, and the symbol duration of 'ft' number of sub-carriers be Tb and so on, such that the total number of sub-carriers is a + b + c + and each group (let there be a total of P groups) has symbol duration Ta, Tb, Tc, ... respectively such that no two symbol durations Ta, Tb, Tc, .. are identical. Accordingly, with reference to statements made above any two type of sub-carriers (can be called the 'a' group, the 'ft' group, or the like), being inversely proportional to the symbol durations, will neither have identical bandwidths. The sub-carriers are so arranged so that the ICI gets reduced. The arrangement of the sub-carriers can be dependent on the system implementation. For example: the situation is to place the sub-carriers with lower bandwidth between the sub-carriers with higher bandwidth. It is important to keep in mind that all the sub-carriers satisfy orthogonality condition, in other words, from a frequency domain view; in an ideal situation without synchronization errors the peak of one sub-carrier is always at the locations where other sub-carriers are null. This reduces the effective ICI. If Ta has the widest spectrum, i.e. the smallest pulse duration, then the pulse duration for the other sub-carriers will be integer multiple of Ta say Mb, Mc, etc. The time-frequency plot explaining this situation is shown in Figure 3. The variations along the time axis show the pulses of the symbol. Each pulse represents one symbol duration. The 9 variations along the frequency axis show the spectrum of the signal. The z-axis indicates the symbol amplitude. Initially the system is implemented with a set of Fast Fourier Transforms (FFT). As many as "n" number of FFT blocks are required, where n is the number of groups into which the sub-carriers are divided for an implementation. Each FFT block has duration Ta or Tb or Tc and so on. But not all the sub-carriers of each FFT block are activated. The sub-carriers are so activated such that orthogonality is maintained between all the frequencies generated by all the FFT blocks in the system. Systems may be implemented with different architectures (only one FFT block, dedicated filters, and the like) as well. Here for example, the implementation of the Transceiver with only one FFT block at the transmitter and at the receiver. In the conventional OFDM system, with one FFT units in the transmitter, the sub carrier spacing, symbol duration and the amplitude (of each sub carrier for M ary PSK) is same for all the sub carriers whereas in the OFDM system according to the present invention, with one or more than one FFT units in the transmitter, the sub carrier spacing, symbol duration and the amplitude (of each sub carrier for M ary PSK) is different for different groups. Figure 4 shows the effect of synchronization error in an OFDM system. The data stream or user allocated on a particular sub-carrier is based on the performance of the data path on the sub-carrier of interest. 10 The straight line shows the location of frequency sampled because of synchronization errors (Carrier frequency offset, sampling frequency offset and Doppler). The circle BC shows the point in the original signal, which is sampled because of the error. The circles RC show the effect (amplitude or power) of the neighboring sub-carriers on the sub-carrier of interest. In the figure 4 the situation is shown for the widest sub-carrier (shortest pulse width) only, since for other sub-carriers, the effect of the neighboring sub-carriers cancels out due to averaging. This is because, smaller the bandwidth of the sub-carrier, larger is the pulse duration. Hence there will be several pulses of the other sub-carriers (with wider bandwidth and shorter pulse duration) during one pulse of the sub-carrier with a smaller bandwidth; on an average, their effect nullifies. Theoretical analysis of OFDM system according to this invention gives a reduction in ICI and hence an improvement in signal to interference ratio as compared to a normal OFDM system. This results in an improved bit error rate performance. BER vs carrier frequency offset plots as shown in Figures 5 and 6 for a 16 QAM (16- quadrature amplitude modulation)for a high data rate system and a low data rate system respectively. In both the plots, the line LI indicates performance of a conventional OFDM system; while the lines L2 trace the performance of the OFDM system according to the present invention with different design parameters (carrier frequency = 5 Ghz, N=256, bandwidth = 20 Mhz and sampling frequency offset = 40 ppm) for high data rate system and a low data rate system. 11 In accordance with the present invention the OFDM system effectively improves the performance of the system. With different channels available that has different data rates and corresponding different error performance, there opens up a scope to support a multitude of applications. The sub-carriers can be bundled in different ways depending upon the requirement of the applications. Several different types of channels can be bundled together to serve a particular program channel with simultaneous heterogeneous contents. High rate channels can be mapped to wider sub-carriers while low rate data streams can be mapped to sub-carriers with lower bandwidth. Also error tolerance of a particular data stream can be considered while designing the mapping, where, data streams requiring better channel can be allocated to sub-carriers with better bit error performance figures. In another application of this invention only the high rate sub-carriers may be grouped (logical grouping; physical adjacency of the sub-carriers is an important factor to reduce the ICI) together to serve high rate data, while low rate channels may be used for low rate channels such as control or signaling. The channels can be grouped freely and the use can be optimized depending on a particular situation. Another important factor is that the OFDM system according to the present invention provides an inherent support to different types of channels with effectively reduced inter carrier interference and improves system performance. Using this OFDM system, leads to P different types of channels. The channel types differ in data rate. For P = 2, we get a low and a high rate channel. These channels can be mapped 12 directly into audio (low rate) and video (high rate) applications. For P>2, a mapping to multi-layer or Multi-Descriptive coding is applicable. The adaptive assignment of the P different types of channels in dependency of the application needs should be considered for possible cross layer optimization. The overall system throughput can be improved. Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention. 13 Claim: 1. A system for reducing intercarrier interference while transmitting data in a wireless network, said system comprising: (i) at least one mapping means adapted to map transmitted signals; (ii) at least one dividing means for dividing said mapped signals into sub-signals having bandwidth, spacing, symbol duration and amplitude; (iii) at least one grouping means adapted to form groups for sub-signals according to the bandwidth of said sub-signals; (iv) at least one assignment means adapted to assign said divided sub signals into respective groups; (v) at least one interleaving means adapted to interleave at least one lower bandwidth sub-signal in spaces between at least one sub-signal of higher bandwidth. 2. A system as claimed in claim 1, wherein said grouped sub-signals are orthogonal to each other. 3. A system as claimed in claim 1, wherein said interleaved sub-signals are orthogonal to each other. 4. A system as claimed in claim 1, wherein at least one lower bandwidth sub-signal is interleaved between two higher bandwidth signals. 5. A system as claimed in claim 1, wherein at least one lower bandwidth signal is interleaved between the whole group of higher bandwidth sub-signals. 6. A system as claimed in claim 1, wherein sub-signals in a group have varying spaces. 7. A system as claimed in claim 1, wherein sub-signals in a group have varying symbol duration. 14 8. A system as claimed in claim 1, wherein sub-signals in a group have varying amplitude. 9. A method for reducing intercarrier interference, said method comprising the following steps: (i) mapping the signals; (ii) dividing said mapped signals into respective sub-signals; (iii) forming groups of sub-signals according to the bandwidth of said sub-signals; (iv) assigning said divided sub-signals to respective groups; (v) interleaving said assigned sub-signals in a manner such that at least one sub-signals with lower bandwidth are interleaved in spaces between at least one sub-signals of higher bandwidth. 10.A method as claimed in claim 9, wherein bandwidth of said interleaved sub-signals is equal. 11 .A method as claimed in claim 9, wherein bandwidth of said interleaved sub-signals is unequal. 12.A system as claimed in claim 1, as herein described with reference to the accompanying drawings. 13.A method as claimed in claim 9, as herein described with reference to the accompanying drawings. Dated this 5th day of September 2006. 15 ABSTRACT: A system and method for reducing intercarrier interference while transmitting data in a wireless network is disclosed. The system comprises mapping means, dividing means, grouping means, assignment means and interleaving means. The sub-signals of lower bandwidth are interleaved with sub-signals of higher bandwidth. The interleaved sub-signals are orthogonal to each other. The method comprises the following steps: mapping, dividing, grouping, assigning and interleaving.

Documents:

1074-mum-2005-abstract(5-9-2006).pdf

1074-mum-2005-abstract.pdf

1074-MUM-2005-CLAIMS(AMENDED)-(2-4-2013).pdf

1074-MUM-2005-CLAIMS(AMENDED)-(4-4-2014).pdf

1074-MUM-2005-CLAIMS(MARKED COPY)-(4-4-2014).pdf

1074-mum-2005-claims.pdf

1074-mum-2005-correspondence(22-4-2008).pdf

1074-MUM-2005-CORRESPONDENCE(30-5-2014).pdf

1074-mum-2005-correspondence-received-050905.pdf

1074-mum-2005-description (complete).pdf

1074-mum-2005-description(provisional)-(5-9-2005).pdf

1074-MUM-2005-DRAWING(2-4-2013).pdf

1074-MUM-2005-DRAWING(4-4-2014).pdf

1074-mum-2005-drawing(5-9-2005).pdf

1074-mum-2005-drawings (complete).pdf

1074-mum-2005-drawings (provisional).pdf

1074-MUM-2005-FORM 1(2-4-2013).pdf

1074-MUM-2005-FORM 1(30-5-2014).pdf

1074-MUM-2005-FORM 13(2-4-2013).pdf

1074-MUM-2005-FORM 13(4-4-2014).pdf

1074-mum-2005-form 18(22-4-2008).pdf

1074-mum-2005-form 2(provisional)-(5-9-2005).pdf

1074-MUM-2005-FORM 2(TITLE PAGE)-(2-4-2013).pdf

1074-MUM-2005-FORM 2(TITLE PAGE)-(30-5-2014).pdf

1074-mum-2005-form 2(title page)-(5-9-2006).pdf

1074-mum-2005-form 2(title page)-(provisional)-(5-9-2005).pdf

1074-mum-2005-form 26(5-5-2005).pdf

1074-MUM-2005-FORM 5(2-4-2013).pdf

1074-mum-2005-form-1.pdf

1074-mum-2005-form-2 (complete).pdf

1074-mum-2005-form-2 (provisional).pdf

1074-mum-2005-form-26.pdf

1074-mum-2005-form-3.pdf

1074-mum-2005-form-5.pdf

1074-MUM-2005-MARKED COPY(30-5-2014).pdf

1074-MUM-2005-REPLY TO EXAMINATION REPORT(2-4-2013).pdf

1074-MUM-2005-REPLY TO HEARING(4-4-2014).pdf

1074-MUM-2005-SPECIFICATION(AMENDED)-(2-4-2013).pdf