Title of Invention	A METHOD FOR IMPROVING THE PERFORMANCE OF A RANDOM ACCESS MOBILE COMMUNICATIONS SYSTEM AND A SYSTEM THEREOF
Abstract	A method for processing multiple random access requests is disclosed in which a base station (204) transmits an acquisition indicator signal (A), which indicates that the base station (204) has detected the presence of a random access transmission (P.M). The acquisition indicator (A) can be generated based (212) on the amount of energy received (210) on the random access channel (e.g., as opposed to the correct/incorrect decoding of a random access message). Consequently, the delay between the beginning of the random access transmission (P.M) and the beginning of the acquisition indicator transmissions (A) is significantly shorter than the delay to the beginning of an acknowledgement transmission based on the reception of a correctly decoded random access message. If a mobile station (202) does not receive a positive acquisition indicator (A), it should interrupt the present transmission and start to retransmit the random access burst in the next time slot, while modifying the transmission power level accordingly between the successive retransmissions.

Title of Invention

A METHOD FOR IMPROVING THE PERFORMANCE OF A RANDOM ACCESS MOBILE COMMUNICATIONS SYSTEM AND A SYSTEM THEREOF

Abstract

A method for processing multiple random access requests is disclosed in which a base station (204) transmits an acquisition indicator signal (A), which indicates that the base station (204) has detected the presence of a random access transmission (P.M). The acquisition indicator (A) can be generated based (212) on the amount of energy received (210) on the random access channel (e.g., as opposed to the correct/incorrect decoding of a random access message). Consequently, the delay between the beginning of the random access transmission (P.M) and the beginning of the acquisition indicator transmissions (A) is significantly shorter than the delay to the beginning of an acknowledgement transmission based on the reception of a correctly decoded random access message. If a mobile station (202) does not receive a positive acquisition indicator (A), it should interrupt the present transmission and start to retransmit the random access burst in the next time slot, while modifying the transmission power level accordingly between the successive retransmissions.

Full Text	This application is divided out of the application number IN/PCT/2001, 00405/MUM dated 17/04/2001 CROSS-REFERENCES TO RELATED APPLICATIONS This Application for Patent is related by subject matter to commonly-assigned U.S. Applications for Patent Serial Nos. 08/733,501, 08/847,655, and 09/148,224, filed October 18,1996, April 30,1997, and September 4, 1993, respectively, and Provisional Application Serial No. 60/063,024, filed October 23,1997. The above-cited Applications are useful for illustrating certain important premises and the state of the art for the present Application, and are hereby incorporated by reference herein in their entirety. BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates in general to the mobile telecommunications field and. in particular, to a method for processing multiple random access mobile-originated calls. Description of Related Art The next (so-called "third") generation of mobile communications systems will be required to provide a broad selection of telecommunications services including digital voice, video and data, in packet and channel circuit-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the ability to support faster and more efficient random access is a key requirement in the development of the new generation of mobile communications systems. In other words, the new generation systems will have to use much faster and more flexible random access procedures, in order to increase their access success rates and reduce their access request processing times, A European joint development mobile communications system is referred to as the "Code Division Testbed" (CODIT), In a CODIT-based Code Division Multiple Access (CDMA) system, a mobile station can gain access to abase station by first determining that the RACK is available for use. Then, the mobile station transmits a series of access request preambles (e.g., single 1023 chip symbols) with increasing power levels until the base station detects the access request, As such, the mobile station uses a "power ramping" process that increases the power level of each successive transmitted preamble symbol. As soon as an access request 2 preamble is detected, the base station activates a closed loop power control circuit which functions to control the mobile station's transmitted power level in order to keep the received signal power from the mobile station at a desired level The mobile station then transmits its specific access request data. The base station's receiver despreads and diversity-combines the received signals using, for example, a RAKE receiver or similar type of processing. In many mobile communication systems, a slotted-ALOHA (S-ALOHA) random access scheme is used, For example, systems operating La accordance with the IS-95 standard (ANSI J STD-008) use an S-ALOHA random access scheme. The main difference between the CODIT and IS-95 processes is thai the CODIT process does not use an S-ALOHA random access scheme- Also, another difference is that the IS-95 mobile station transmits a complete random access packet instead of just the preamble. If the base station does not acknowledge the access request, the IS-95 mobile station re-transmits the entire access request packet at a higher power level This process continues until the base station acknowledges tne access request. In the above-cited Applications and theIS-95 CDMA technical specifications, different random access methods baaed on S-ALOHA random access schemes have been described. Essentially (as illustrated in FIGURE 1), using a basic S-ALOHA scheme, there are well-defined instants in time (tune slots) at which random access transmissions are allowed to begin. Typically, a mobile station (user) randomly selects a time slot in which (he transmission of a random access burst (e.g., Ul, U2) is to begin. However, the time slots are not pre-allocated to specific users. Consequently, collisions between the different users random access bursts can occur (e.g., between U3, U4). In e. specific mobile communications system using such an S-ALOHA random access scheme, such as the method disclosed in the above-cited U.S. Application Serial No. 08/733,501 (hereinafter, "the '501 Application"), a mobile station generates and transmits a random access packet. A diagram that illustrates a frame structure for such a random access packet is shown in FIGURE 2, The transmitted random access packet ("access request data frame") or "burst" comprises a preamble {10) and a message part (12).Typically, the preamble does not include user information and is used in the base station receiver primarily to facilitate detection of the presence of the random access burst and derive certain timing information (e.g,, different transmission path delays). Note that, as 3 two mobile stations that have selected the same time slot, me concept of burst "signatures" has been introduced. For example, as described in the 501 Application (see FIGURE 4), the preamble of a random access burst is modulated with a unique signature pattern. Also, the message part is spread with a code associated with the signature pattern used. The signature pattern is randomly selected from a set of patterns that can be, but are not necessarily, orthogonal to each other. Since a collision can occur only between mobile stations' bursts that are using the same signature, the risk of a random access collision is reduced in comparison with other existing schemes. As such, the use of this unique signature pattern feature, as described and claimed in the 501 Application, provides a significantly higher throughput efficiency than prior random access schemes. In the 024 Application, a mobile station transmits a signature on the Q branch within the message part of the burst In preparing for the transmission, the mobile station randomly selects the signature from a set of predetermined signatures. Again, since a collision can occur only between mobile stations' bursts that are using the same signature (the primary advantage of the novel use of signatures in general), the risk of a random access collision is reduced in comparison with other existing schemes. Notably, although the random access systems and methods described in the above-cited Applications have numerous advantages over prior random access schemes, a number of problems still exist that remain to be solved. For example, regardless of the random access method used, a mobile station has to decide just how much random access transmission power to use. Ideally, a mobile station should select a transmission power level such that the random access burst is received at the base station with precisely the power needed for correct decoding of the random access message, However, for numerous reasons, it is virtually impossible to ensure that this will be the case. 4 For example, the power of the received burst as required at the base station is not constant but cart vary (e.g., due to variations in the radio channel characteristics and the speed of the mobile station). As such, these variations are to some extent unpredictable and thus unknown to the mobile station. Also, there can be significant errors in estimating the uplink path-loss. Furthermore, even if a mobile station can determine the "correct" transmission power level to use, because of existing hardware limitations, it is impossible to set the actual transmission power level to precisely the correct value needed. Consequently, for the above-described reasons, there is a significant risk that a random access burst will be received at the base station with too much power. This condition causes excessive interference for other users and thus reduces the capacity of the CDMA system. For the same reasons, there is also arisk that a random access burst will be transmitted with too little power. This condition makes it impossible for the base station to detect and decode the random access burst. In order to reduce the risk of transmitting with too much power, in the afore mentioned IS-95 CDMA system, the initial random access request is transmitted with an additional negative power offset (i.e., with a lower power level than the required transmit power level expected), as shown in FIGURE 5. Referring to FIGURE 5, the mobile station then re-transmits the random access burst with a. reduced negative power offset, until the base station acknowledges (ACK) that it has correctly decoded the random access message ("NACK" denotes so acknowledgment message transmitted). Typically, the base station's acknowledgment is based on the calculation of acyclic redundancy check (CRC) over the random access message. However, note that a new estimate of the required transmission power may or may not be calculated for each re-transmission. Consequently, it is only the negative offset that is reduced for each re-transmission. A significant problem that exists with the above-described power ramping approaches is that there is an obvious trade-off between the time delay incurred due to the mobile station re-transmitting the random access bursts until the base Station's acknowledgment message is received, and the amount of interference caused by the random access transmission. As such, with a larger negative initial power offset, on the average, more re-transmissions will be needed before the random access burst is received at the base station with sufficient power. On the other hand, with a smaller initial negative power offset, there is an increased risk that the random access burst will be received at the base station with too much 5 power. On the average, this occurrence will cause more interference for other users. For reasonably large negative power offsets, the delay until the acknowledgement of a correctly decoded random access message is transmitted can be significant, because the base station has to receive an entire random access burst before it can transmit the acknowledgment message. As described in detail below, the present invention successfully resolves the above-described problems. U.S. Pat. No. 5,673,259 to Quick, Jr. generally describes a method and. system for switching between dedicated channels and random access channels based on the bandwidth demand. European Patent Application No. 0,633,671 to Malkamaki generally describes a mobile communication system that reduces the time required to setup a random access request using an array of matched filters each tuned to a signature pattern. WO 98/18280 to Esmailzadeb. generally describes a method in a TDMA mobile communication system wherein the mobile station communicates with the base station using bits that are in the same time slot and mutually orthogonal. This avoids any collision between the communication of information comprising several bits. SUMMARY OF THE INVENTION In accordance with a preferred embodiment of the present invention, a method for processing multiple random access requests is provided whereby a base station transmits an acquisition indicator signal, which indicates that the base station has detected the presence of a random access transmission. For this exemplary embodiment, the acquisition indicator is generated based on the amount of energy received on the random access channel (e.g., as opposed to the correct/incorrect decoding of a random access message). Consequently, the delay between the beginning of the random access transmission and the beginning of the acquisition indicator transmission is significantly shorter than the delay to the beginning of an acknowledgment transmission based on the reception of a correctly decoded random access message. If a mobile station does not receive a positive acquisition indicator, the mobile station should interrupt the present transmission and start to re-transmit the random access burst in the next time slot, while modifying the transmission power level accordingly between the successive re- transmissions. An important technical advantage of the present invention is that significantly fester power ramping can be achieved in an S-ALOHA random access system. Another important technical advantage of the present invention is that with an unchanged initial power offset in an S-ALOHA random access scheme, the random access delay can be significantly reduced, which improves the system performance. Yet another important technical advantage of the present invention is that for the same delay constraints involved, a larger initial power offset can be used for one user in an S-ALOHA random access system, which reduces the risk of excessive interference for other users. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: FIGURE 1 is a diagram that illustrates how collisions between different users random access bursts can occur in an S-ALOHA random access scheme; FIGURE 2 is a diagram that illustrates a frame structure for a random access packet in an S-ALOHA random access scheme; FIGURE 3 is a diagram that illustrates a random access burst that does not include a preamble, FIGURE 4 is a diagram that illustrates a preamble of a random access burst modulated with a unique signature pattern, and a message part spread with a code associated with the signature pattern used; 7 FIGURE 5 is a diagram that illustrates a random access transmission with an initial negative power offset; FIGURE 6 is a block diagram of an exemplary detection section (for one antenna) that can be used in a base station's receiver to detect the presence of a random access transmission from a mobile station, in accordance with a preferred embodiment of the present invention; FIGURE 7 is a diagram that illustrates a mobile station receiving an acquisition indicator signal during an idle period in a random access burst in accordance with the preferred embodiment of the present invention; and FIGURE 8 is a diagram that illustrates a mobile station receiving an acquisition indicator signal in a system where a random access burst has been transmitted without a preamble, in accordance with the preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS The preferred embodiment of the present invention and its advantages are best understood by referring to FlGUREs 1-8 of the drawings, like numerals being used for like and corresponding pans of the various drawings. Essentially, in accordance with a preferred embodiment of the present invention, a method for processing multiple random access requests is provided whereby a base station transmits an acquisition indicator signal, which indicates that the base station has detected the presence of a random access transmission. For this exemplary embodiment, the acquisition indicator is generated based on the amount of energy received (or, possibly, also the interference energy) on the random access channel (e.g., as opposed to the correct/incorrect decoding of a random access message). Consequently, the delay between the beginning of the random access transmission and the beginning of the acquisition indicator transmission is significantly shorter than the delay to the beginning of an acknowledgment signal transmission based on the reception of a correctly decoded random access message, If a mobile station does not receive a positive acquisition indicator, the mobile station should interrupt the present transmission and start to re-transmit the random access burst in the next time slot, while modifying the transmission power level accordingly between the successive re-transmissions. FIGURE 6 is a block diagram of an exemplary detection section (for one antenna) that can be used in a base station's (204) receiver to detect the presence of a random access transmission from a mobile station (202), in accordance with a preferred embodiment of the present invention. The exemplary detection section 200 includes a matched filter 206 (e.g., used during the preamble period) which is tuned (matched) to a preamble's spreading code. For this example, the matched filter is used to detect the presence of the random access burst, despread the preamble part, and feed the despread signal to an appropriate section of an accumulator 208. Since each received preamble can include a unique signature pattern, the accumulator 208 includes one unit tuned to one of the possible signature patterns (1-1) that can be received. The output of each accumulator unit 208(1-1) is coupled to a respective threshold detection unit 210(1-1). The accumulator unit 208 accumulates the energy received over the duration of the preamble. During the preamble period, if a threshold detection unit 210(1-1) detects an input signal that exceeds a predetermined detection threshold, that threshold detection unit outputs a signal. This output signal (indicating detection of sufficient energy from a received random access burst) is coupled to a respective acquisition indicator generator circuit 212(1-1), which outputs an acquisition indicator signal (A) for transmission by the base station. For the case where a burst is transmitted without a preamble, the matched filter 206 in FIGURE 6 is matched to the spreading code used on the control part of the burst (i.e., where a signature is located). However, in this case, the accumulation performed, by the accumulator 208(1-1) occurs for a specified period of Lime (e.g., just enough time to provide a good estimate, whether or not the base station has received a random access burst. Notably, the present invention provides a solution that is applicable for those cases where the random access burst both, does or does not include a preamble. Specifically a s illustrated by the uplink and downlink transmission diagram shown in the embodiment of FIGURE 7, in those cases where a preamble is used, if the idle period in the burst between the preamble (P) and message part is sufficiently large, a mobile station can receive an acquisition indicator (A) transmitted by the base station during that idle period. However, in accordance with an underlying principle of this exemplary embodiment, the mobile station will not transmit the message part (Mt) of the random access burst until an acquisition indicator (A1) is received (no acquisition indicator transmission is denoted by "NA"). Instead of transmitting the message part of the burst if no acquisition indicator is received (e.g.,NA1, NA2, the mobile station will continue to transmit a new preamble (e.g., P2, P3). As illustrated by the uplink and downlink transmission, diagram shown in FIGURE 8, in those cases where a preamble is not used in a random access burst (or, for example, the idle period between the preamble and message part is too short in duration), a mobile station will receive the base station's transmitted acquisition indicator (A;) during the mobile station's transmission of a message part (M3) of the burst. However, in accordance with the principles of this exemplary embodiment, if 9 no acquisition indicator is received (e.g., NA,, NA2) at a predetermined instant of tune, the mobile station will interrupt its transmission of the message part (M1, M2 of the random access burst, and re-transmit the random access burst in the next time slot until an acquisition indicator (A1) is received. In a different aspect of the present invention, for those cases where signatures are used in the random access scheme, each acquisition indicator transmitted by a base station can indicate reception of a corresponding signature (transmitted from a mobile station). Alternatively, a plurality of signatures can share one acquisition indicator In this case, the base station's transmission of the acquisition indicator indicates that at least one of the corresponding signatures (transmitted from a mobile station) has been received. In another aspect of the present invention, a mobile station can also select (randomly or non-randomly) a new signature and/or a new RACH for each burst re-transmission (until an acquisition indicator is received), A base station can transmit an acquisition indicator signal on a downlink physical channel. Such a physical channel can be dedicated and used only for transmissions of acquisition indicator signals or, for example, the acquisition indicator signals can be rune-multiplexed with other signals on one physical channel or on a plurality of different physical channels. As such, a physical channel used for transmission of an acquisition indicator signal can be either orthogonal or non- orthogonal to other downlink physical channels used by the mobile communication system. In another aspect of the present invention, a base station can transmit an acquisition indicator as a type of "on-off " signal. In other words, the base station transmits the signal only if the base station has detected a random access burst, and does not transmit the signal if a random access burst has not been detected. For example, the base station can transmit acquisition indicator signals as different orthogonal cods words for different signatures. In that case, the base station's transmission of a specific code word would indicate the base station's acquisition of a random access signal with the corresponding signature. Alternatively , a plurality of signatures can share one acquisition indicator. In This case, the base station's transmission of the acquisition indicator indicates that at least one of the corresponding signatures has been received. Although a preferred embodiment of the method and apparatus of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims. 10 WE CLAIM: 1. A base station (204) for use fn a mobile communication system, the base station comprising means for detecting a presence of a random access request transmitted by a mobile station (202), generating an indicator signal indicating said presence of said random access request, and transmitting said indicator signal (NA1. NA 2, A) before completion of decoding of said random access request. 2. A base station as claimed in claim 1, wherein said random access request includes a preamble modulated with a signature pattern randomly selected from a set of patterns and wherein said indicator signal comprises an acquisition indicator signal associated with the selected signature. 3. A base station as claimed in any one of claims 1-2, wherein said indicator signal is associated with a unique signature. 4. A base station as claimed in any one of claims 1-2. wherein said indicator signal is associated with a set of unique signatures 5. A method in a mobile station (202) for performing random access in a mobile communication system, the method comprising the steps of: transmitting a preamble part of a random access request, said preamble being modulated with a signature pattern randomly selected from a set of patterns; receiving an indicator signal (NA1, NA2, A) associated with the selected signature and indicating that a base station has detected a presence of said random access request prior to decoding of the random access request; transmitting a message part of the random access request in response to receiving said indicator signal. 6. A method as claimed in claim 5, wherein said indicator signal is associated with a unique signature. 7. A method as claimed in claim 5, wherein said indicator signal is associated with a plurality of unique signatures 11 -2- 8. A method as claimed in claim 5, wherein the method includes retransmitting the preamble if no indicator signal is received by a predetermined instant of time. Dated this 16th day of January 2006 A method for processing multiple random access requests is disclosed in which a base station (204) transmits an acquisition indicator signal (A), which indicates that the base station (204) has detected the presence of a random access transmission (P.M). The acquisition indicator (A) can be generated based (212) on the amount of energy received (210) on the random access channel (e.g., as opposed to the correct/incorrect decoding of a random access message). Consequently, the delay between the beginning of the random access transmission (P.M) and the beginning of the acquisition indicator transmissions (A) is significantly shorter than the delay to the beginning of an acknowledgement transmission based on the reception of a correctly decoded random access message. If a mobile station (202) does not receive a positive acquisition indicator (A), it should interrupt the present transmission and start to retransmit the random access burst in the next time slot, while modifying the transmission power level accordingly between the successive retransmissions.

Full Text

This application is divided out of the application
number IN/PCT/2001, 00405/MUM dated 17/04/2001
CROSS-REFERENCES TO RELATED APPLICATIONS
This Application for Patent is related by subject matter to commonly-assigned
U.S. Applications for Patent Serial Nos. 08/733,501, 08/847,655, and 09/148,224, filed
October 18,1996, April 30,1997, and September 4, 1993, respectively, and Provisional
Application Serial No. 60/063,024, filed October 23,1997. The above-cited
Applications are useful for illustrating certain important premises and the state of the art
for the present Application, and are hereby incorporated by reference herein in their
entirety.
BACKGROUND OF THE INVENTION
Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field
and. in particular, to a method for processing multiple random access mobile-originated
calls.
Description of Related Art
The next (so-called "third") generation of mobile communications systems will
be required to provide a broad selection of telecommunications services including
digital voice, video and data, in packet and channel circuit-switched modes. As a result,
the number of calls being made is expected to increase significantly, which will result in
much higher traffic density on random access channels (RACHs). Unfortunately, this
higher traffic density will also result in increased collisions and access failures.
Consequently, the ability to support faster and more efficient random access is a key
requirement in the development of the new generation of mobile communications
systems. In other words, the new generation systems will have to use much faster and
more flexible random access procedures, in order to increase their access success rates
and reduce their access request processing times,
A European joint development mobile communications system is referred
to as the "Code Division Testbed" (CODIT), In a CODIT-based Code Division
Multiple Access (CDMA) system, a mobile station can gain access to abase station
by first determining that the RACK is available for use. Then, the mobile station
transmits a series of access request preambles (e.g., single 1023 chip symbols) with
increasing power levels until the base station detects the access request, As such,
the mobile station uses a "power ramping" process that increases the power level of
each successive transmitted preamble symbol. As soon as an access request
2

preamble is detected, the base station activates a closed loop power control circuit
which functions to control the mobile station's transmitted power level in order to
keep the received signal power from the mobile station at a desired level The
mobile station then transmits its specific access request data. The base station's
receiver despreads and diversity-combines the received signals using, for example,
a RAKE receiver or similar type of processing.
In many mobile communication systems, a slotted-ALOHA (S-ALOHA)
random access scheme is used, For example, systems operating La accordance with
the IS-95 standard (ANSI J STD-008) use an S-ALOHA random access scheme.
The main difference between the CODIT and IS-95 processes is thai the CODIT
process does not use an S-ALOHA random access scheme- Also, another
difference is that the IS-95 mobile station transmits a complete random access
packet instead of just the preamble. If the base station does not acknowledge the
access request, the IS-95 mobile station re-transmits the entire access request
packet at a higher power level This process continues until the base station
acknowledges tne access request.
In the above-cited Applications and theIS-95 CDMA technical
specifications, different random access methods baaed on S-ALOHA random access
schemes have been described. Essentially (as illustrated in FIGURE 1), using a
basic S-ALOHA scheme, there are well-defined instants in time (tune slots) at which
random access transmissions are allowed to begin. Typically, a mobile station (user)
randomly selects a time slot in which (he transmission of a random access burst (e.g.,
Ul, U2) is to begin. However, the time slots are not pre-allocated to specific users.
Consequently, collisions between the different users random access bursts can occur
(e.g., between U3, U4).
In e. specific mobile communications system using such an S-ALOHA
random access scheme, such as the method disclosed in the above-cited U.S.
Application Serial No. 08/733,501 (hereinafter, "the '501 Application"), a mobile
station generates and transmits a random access packet. A diagram that illustrates a
frame structure for such a random access packet is shown in FIGURE 2, The
transmitted random access packet ("access request data frame") or "burst"
comprises a preamble {10) and a message part (12).Typically, the preamble does
not include user information and is used in the base station receiver primarily to
facilitate detection of the presence of the random access burst and derive certain
timing information (e.g,, different transmission path delays). Note that, as
3

two mobile stations that have selected the same time slot, me concept of burst
"signatures" has been introduced. For example, as described in the 501
Application (see FIGURE 4), the preamble of a random access burst is modulated
with a unique signature pattern. Also, the message part is spread with a code
associated with the signature pattern used. The signature pattern is randomly
selected from a set of patterns that can be, but are not necessarily, orthogonal to each
other. Since a collision can occur only between mobile stations' bursts that are
using the same signature, the risk of a random access collision is reduced in
comparison with other existing schemes. As such, the use of this unique signature
pattern feature, as described and claimed in the 501 Application, provides a
significantly higher throughput efficiency than prior random access schemes.
In the 024 Application, a mobile station transmits a signature on the Q
branch within the message part of the burst In preparing for the transmission, the
mobile station randomly selects the signature from a set of predetermined
signatures. Again, since a collision can occur only between mobile stations' bursts
that are using the same signature (the primary advantage of the novel use of
signatures in general), the risk of a random access collision is reduced in comparison
with other existing schemes.
Notably, although the random access systems and methods described in the
above-cited Applications have numerous advantages over prior random access
schemes, a number of problems still exist that remain to be solved. For example,
regardless of the random access method used, a mobile station has to decide just
how much random access transmission power to use. Ideally, a mobile station
should select a transmission power level such that the random access burst is
received at the base station with precisely the power needed for correct decoding of
the random access message, However, for numerous reasons, it is virtually
impossible to ensure that this will be the case.
4

For example, the power of the received burst as required at the base station
is not constant but cart vary (e.g., due to variations in the radio channel
characteristics and the speed of the mobile station). As such, these variations are to
some extent unpredictable and thus unknown to the mobile station. Also, there can
be significant errors in estimating the uplink path-loss. Furthermore, even if a
mobile station can determine the "correct" transmission power level to use, because
of existing hardware limitations, it is impossible to set the actual transmission
power level to precisely the correct value needed.
Consequently, for the above-described reasons, there is a significant risk
that a random access burst will be received at the base station with too much
power. This condition causes excessive interference for other users and thus reduces
the capacity of the CDMA system. For the same reasons, there is also arisk that a
random access burst will be transmitted with too little power. This condition makes
it impossible for the base station to detect and decode the random access burst.
In order to reduce the risk of transmitting with too much power, in the afore
mentioned IS-95 CDMA system, the initial random access request is transmitted
with an additional negative power offset (i.e., with a lower power level than the
required transmit power level expected), as shown in FIGURE 5. Referring to
FIGURE 5, the mobile station then re-transmits the random access burst with a.
reduced negative power offset, until the base station acknowledges (ACK) that it has
correctly decoded the random access message ("NACK" denotes so
acknowledgment message transmitted). Typically, the base station's
acknowledgment is based on the calculation of acyclic redundancy check (CRC)
over the random access message. However, note that a new estimate of the required
transmission power may or may not be calculated for each re-transmission.
Consequently, it is only the negative offset that is reduced for each re-transmission.
A significant problem that exists with the above-described power ramping
approaches is that there is an obvious trade-off between the time delay incurred due
to the mobile station re-transmitting the random access bursts until the base
Station's acknowledgment message is received, and the amount of interference
caused by the random access transmission. As such, with a larger negative initial
power offset, on the average, more re-transmissions will be needed before the
random access burst is received at the base station with sufficient power. On the
other hand, with a smaller initial negative power offset, there is an increased risk
that the random access burst will be received at the base station with too much
5

power. On the average, this occurrence will cause more interference for other
users. For reasonably large negative power offsets, the delay until the
acknowledgement of a correctly decoded random access message is transmitted can
be significant, because the base station has to receive an entire random access burst
before it can transmit the acknowledgment message. As described in detail below,
the present invention successfully resolves the above-described problems.
U.S. Pat. No. 5,673,259 to Quick, Jr. generally describes a method
and. system for switching between dedicated channels and random access channels
based on the bandwidth demand.
European Patent Application No. 0,633,671 to Malkamaki generally
describes a mobile communication system that reduces the time required to setup a
random access request using an array of matched filters each tuned to a signature
pattern.
WO 98/18280 to Esmailzadeb. generally describes a method in a TDMA
mobile communication system wherein the mobile station communicates with the
base station using bits that are in the same time slot and mutually orthogonal. This
avoids any collision between the communication of information comprising several
bits.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, a
method for processing multiple random access requests is provided whereby a base

station transmits an acquisition indicator signal, which indicates that the base
station has detected the presence of a random access transmission. For this
exemplary embodiment, the acquisition indicator is generated based on the amount
of energy received on the random access channel (e.g., as opposed to the
correct/incorrect decoding of a random access message). Consequently, the delay
between the beginning of the random access transmission and the beginning of the
acquisition indicator transmission is significantly shorter than the delay to the
beginning of an acknowledgment transmission based on the reception of a correctly
decoded random access message. If a mobile station does not receive a positive
acquisition indicator, the mobile station should interrupt the present transmission
and start to re-transmit the random access burst in the next time slot, while
modifying the transmission power level accordingly between the successive re-
transmissions.
An important technical advantage of the present invention is that
significantly fester power ramping can be achieved in an S-ALOHA random access
system.
Another important technical advantage of the present invention is that with
an unchanged initial power offset in an S-ALOHA random access scheme, the
random access delay can be significantly reduced, which improves the system
performance.
Yet another important technical advantage of the present invention is that
for the same delay constraints involved, a larger initial power offset can be used for
one user in an S-ALOHA random access system, which reduces the risk of
excessive interference for other users.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the method and apparatus of the present
invention may be had by reference to the following detailed description when taken
in conjunction with the accompanying drawings wherein:
FIGURE 1 is a diagram that illustrates how collisions between different
users random access bursts can occur in an S-ALOHA random access scheme;
FIGURE 2 is a diagram that illustrates a frame structure for a random
access packet in an S-ALOHA random access scheme;
FIGURE 3 is a diagram that illustrates a random access burst that does not
include a preamble,
FIGURE 4 is a diagram that illustrates a preamble of a random access burst
modulated with a unique signature pattern, and a message part spread with a code
associated with the signature pattern used;
7

FIGURE 5 is a diagram that illustrates a random access transmission with
an initial negative power offset;
FIGURE 6 is a block diagram of an exemplary detection section (for one
antenna) that can be used in a base station's receiver to detect the presence of a
random access transmission from a mobile station, in accordance with a preferred
embodiment of the present invention;
FIGURE 7 is a diagram that illustrates a mobile station receiving an
acquisition indicator signal during an idle period in a random access burst in
accordance with the preferred embodiment of the present invention; and
FIGURE 8 is a diagram that illustrates a mobile station receiving an
acquisition indicator signal in a system where a random access burst has been
transmitted without a preamble, in accordance with the preferred embodiment of
the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the present invention and its advantages are
best understood by referring to FlGUREs 1-8 of the drawings, like numerals being
used for like and corresponding pans of the various drawings.
Essentially, in accordance with a preferred embodiment of the present
invention, a method for processing multiple random access requests is provided
whereby a base station transmits an acquisition indicator signal, which indicates that
the base station has detected the presence of a random access transmission. For this
exemplary embodiment, the acquisition indicator is generated based on the amount
of energy received (or, possibly, also the interference energy) on the random access
channel (e.g., as opposed to the correct/incorrect decoding of a random access
message). Consequently, the delay between the beginning of the random access
transmission and the beginning of the acquisition indicator transmission is
significantly shorter than the delay to the beginning of an acknowledgment signal
transmission based on the reception of a correctly decoded random access message,
If a mobile station does not receive a positive acquisition indicator, the mobile
station should interrupt the present transmission and start to re-transmit the random
access burst in the next time slot, while modifying the transmission power level
accordingly between the successive re-transmissions.
FIGURE 6 is a block diagram of an exemplary detection section (for one
antenna) that can be used in a base station's (204) receiver to detect the presence of a
random access transmission from a mobile station (202), in accordance with a
preferred embodiment of the present invention. The exemplary detection section 200
includes a matched filter 206 (e.g., used during the preamble period) which is tuned

(matched) to a preamble's spreading code. For this example, the matched filter is
used to detect the presence of the random access burst, despread the preamble part,
and feed the despread signal to an appropriate section of an accumulator 208. Since
each received preamble can include a unique signature pattern, the accumulator 208
includes one unit tuned to one of the possible signature patterns (1-1) that can be
received. The output of each accumulator unit 208(1-1) is coupled to a respective
threshold detection unit 210(1-1). The accumulator unit 208 accumulates the energy
received over the duration of the preamble.
During the preamble period, if a threshold detection unit 210(1-1) detects an
input signal that exceeds a predetermined detection threshold, that threshold
detection unit outputs a signal. This output signal (indicating detection of sufficient
energy from a received random access burst) is coupled to a respective acquisition
indicator generator circuit 212(1-1), which outputs an acquisition indicator signal (A)
for transmission by the base station.
For the case where a burst is transmitted without a preamble, the matched
filter 206 in FIGURE 6 is matched to the spreading code used on the control part
of the burst (i.e., where a signature is located). However, in this case, the
accumulation performed, by the accumulator 208(1-1) occurs for a specified period of
Lime (e.g., just enough time to provide a good estimate, whether or not the base
station has received a random access burst.
Notably, the present invention provides a solution that is applicable for those
cases where the random access burst both, does or does not include a preamble.
Specifically a s illustrated by the uplink and downlink transmission diagram shown
in the embodiment of FIGURE 7, in those cases where a preamble is used, if the idle
period in the burst between the preamble (P) and message part is sufficiently large, a
mobile station can receive an acquisition indicator (A) transmitted by the base station
during that idle period. However, in accordance with an underlying principle of this
exemplary embodiment, the mobile station will not transmit the message part (Mt) of
the random access burst until an acquisition indicator (A1) is received (no acquisition
indicator transmission is denoted by "NA"). Instead of transmitting the message part
of the burst if no acquisition indicator is received (e.g.,NA1, NA2, the mobile station
will continue to transmit a new preamble (e.g., P2, P3).
As illustrated by the uplink and downlink transmission, diagram shown in
FIGURE 8, in those cases where a preamble is not used in a random access burst (or,
for example, the idle period between the preamble and message part is too short in
duration), a mobile station will receive the base station's transmitted acquisition
indicator (A;) during the mobile station's transmission of a message part (M3) of the
burst. However, in accordance with the principles of this exemplary embodiment, if
9

no acquisition indicator is received (e.g., NA,, NA2) at a predetermined instant of
tune, the mobile station will interrupt its transmission of the message part (M1, M2
of the random access burst, and re-transmit the random access burst in the next time
slot until an acquisition indicator (A1) is received.
In a different aspect of the present invention, for those cases where signatures
are used in the random access scheme, each acquisition indicator transmitted by a
base station can indicate reception of a corresponding signature (transmitted from a
mobile station). Alternatively, a plurality of signatures can share one acquisition
indicator In this case, the base station's transmission of the acquisition indicator
indicates that at least one of the corresponding signatures (transmitted from a
mobile station) has been received. In another aspect of the present invention, a
mobile station can also select (randomly or non-randomly) a new signature and/or a
new RACH for each burst re-transmission (until an acquisition indicator is received),
A base station can transmit an acquisition indicator signal on a downlink
physical channel. Such a physical channel can be dedicated and used only for
transmissions of acquisition indicator signals or, for example, the acquisition
indicator signals can be rune-multiplexed with other signals on one physical channel
or on a plurality of different physical channels. As such, a physical channel used for
transmission of an acquisition indicator signal can be either orthogonal or non-
orthogonal to other downlink physical channels used by the mobile communication
system.
In another aspect of the present invention, a base station can transmit an
acquisition indicator as a type of "on-off " signal. In other words, the base station
transmits the signal only if the base station has detected a random access burst, and
does not transmit the signal if a random access burst has not been detected. For
example, the base station can transmit acquisition indicator signals as different
orthogonal cods words for different signatures. In that case, the base station's
transmission of a specific code word would indicate the base station's acquisition of
a random access signal with the corresponding signature. Alternatively , a plurality
of signatures can share one acquisition indicator. In This case, the base station's
transmission of the acquisition indicator indicates that at least one of the
corresponding signatures has been received.
Although a preferred embodiment of the method and apparatus of the present
invention has been illustrated in the accompanying Drawings and described in the
foregoing Detailed Description, it will be understood that the invention is not limited
to the embodiment disclosed, but is capable of numerous rearrangements,
modifications and substitutions without departing from the spirit of the invention as
set forth and defined by the following claims.
10

WE CLAIM:
1. A base station (204) for use fn a mobile communication system, the base station
comprising means for detecting a presence of a random access request transmitted by a
mobile station (202), generating an indicator signal indicating said presence of said
random access request, and transmitting said indicator signal (NA1. NA 2, A) before
completion of decoding of said random access request.
2. A base station as claimed in claim 1, wherein said random access request includes a
preamble modulated with a signature pattern randomly selected from a set of patterns
and wherein said indicator signal comprises an acquisition indicator signal associated
with the selected signature.
3. A base station as claimed in any one of claims 1-2, wherein said indicator signal is
associated with a unique signature.
4. A base station as claimed in any one of claims 1-2. wherein said indicator signal is
associated with a set of unique signatures
5. A method in a mobile station (202) for performing random access in a mobile
communication system, the method comprising the steps of:
transmitting a preamble part of a random access request, said preamble being
modulated with a signature pattern randomly selected from a set of patterns;
receiving an indicator signal (NA1, NA2, A) associated with the selected signature and
indicating that a base station has detected a presence of said random access request
prior to decoding of the random access request;
transmitting a message part of the random access request in response to receiving said
indicator signal.
6. A method as claimed in claim 5, wherein said indicator signal is associated with a unique
signature.
7. A method as claimed in claim 5, wherein said indicator signal is associated with a
plurality of unique signatures
11

-2-
8. A method as claimed in claim 5, wherein the method includes retransmitting the
preamble if no indicator signal is received by a predetermined instant of time.
Dated this 16th day of January 2006

A method for processing multiple random access requests is disclosed in which a base
station (204) transmits an acquisition indicator signal (A), which indicates that the
base station (204) has detected the presence of a random access transmission (P.M).
The acquisition indicator (A) can be generated based (212) on the amount of energy
received (210) on the random access channel (e.g., as opposed to the correct/incorrect
decoding of a random access message). Consequently, the delay between the beginning
of the random access transmission (P.M) and the beginning of the acquisition indicator
transmissions (A) is significantly shorter than the delay to the beginning of an
acknowledgement transmission based on the reception of a correctly decoded random
access message. If a mobile station (202) does not receive a positive acquisition
indicator (A), it should interrupt the present transmission and start to retransmit the
random access burst in the next time slot, while modifying the transmission power
level accordingly between the successive retransmissions.

A METHOD FOR IMPROVING THE PERFORMANCE OF A RANDOM ACCESS MOBILE COMMUNICATIONS SYSTEM AND A SYSTEM THEREOF

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