Abstract:
Method and apparatus for transmission of information in multiple access communication system is claim. Information using incremental redundancy is transmitted. A determination is made as to whether reverse link performance drops below a predetermined threshold. Determination of reverse link performance may be done in variety of ways, including use of a filter percentage of ACK erasures, measured error rates on the reverse link control channel, reverse link data channel error rates, and other methods. Upon determination of channel degradation, the access point can decide whether to ignore messages sent from the access terminal to instruct the access terminal to cease transmission.

Description:
BACKGROUND  
       [0001]     I. Field  
         [0002]     The invention relates generally to the field of wireless communications, and more particularly to a method, apparatus, and system for selectively responding to incremental redundancy transmissions in multiple access communication systems.  
         [0003]     II. Background  
         [0004]     In recent years, communication systems&#39; performance and capabilities have continued to improve rapidly in light of several technological advances and improvements with respect to telecommunication network architecture, signal processing, and protocols. In the area of wireless communications, various multiple access standards and protocols have been developed to increase system capacity and accommodate fast-growing user demand. These various multiple access schemes and standards include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiple Access (OFDMA), etc. Generally, in a system which employs TDMA technique, each user is allowed to transmit information in his assigned or allocated time slots whereas an FDMA system allows each user to transmit information on a particular frequency that is assigned to that particular user. A CDMA system, in contrast, is a spread spectrum system which allows different users to transmit information at the same frequency and at the same time by assigning a unique code to each user. In an OFDMA system, a high-rate data stream is split or divided into a number of lower rate data streams which are transmitted simultaneously in parallel over a number of subcarriers (also called subcarrier frequencies herein). Each user in an OFDMA system is provided with a subset of the available subcarriers for transmission of information. The subset of carriers provided to each user in an OFDMA system can be fixed or vary, for example, in the case of Frequency-Hopping OFMDA (FH-OFDMA). Multiple access techniques in TDMA, FDMA, and CDMA are illustrated in  FIG. 1 . As shown in  FIG. 1 , the communication channels in FDMA are separated by frequencies in which a particular channel corresponds to a particular frequency. In a TDMA system, the communication channels are separated by time in which a particular channel corresponds to a particular time slot. In contrast, communication channels in a CDMA system are separated by codes in which a particular channel corresponds to a particular code.  
         [0005]     In wireless systems, it is usually inefficient to guarantee a reliable packet transfer on every single transmission. The inefficiency is particularly pronounced in systems where underlying channel conditions vary drastically from transmission to transmission. For example, in an FH-OFDMA system, there is a wide variation in the received signal-to-noise ratio (SNR) between frames/packets, thus making it difficult and inefficient to guarantee a small frame error rate (FER) for each packet transmission. Such difficulty and in-efficiency also apply to other communication systems which employ orthogonal multiple access techniques including, but are not limited to, TDMA, FDMA, and orthogonal CDMA, etc.  
         [0006]     In such communication systems, a packet retransmission mechanism such as the Automatic Retransmission/Repeat Request (ARQ) scheme may be used to help lessen such inefficiency. However, this is done at the expense of higher packet latency since it takes longer on average for each packet to get through. In general, large packet latency may not be a significant problem for data traffic but could be detrimental to voice traffic or other types of applications that require low latency in transmission of information. Moreover, packet transmission latency is expected to increase as the number of users in the system continues to grow. Thus, to improve system capacity (e.g., based on system throughput or number of users that simultaneously use the system, etc.), transmission latency should be kept low or small.  
         [0007]     The efficiency of early termination in ARQ schemes is based on the reliability of acknowledge (ACK) not-acknowledge (NACK) transmissions. If the error rates of NACKs sent that are interpreted as ACKs becomes too great, than many packet transmissions will be incorrectly terminated prior to success. Further, access terminals may needlessly be sending ACK/NACK messages, thereby causing interference in the system.  
         [0008]     Accordingly, there exists a need for a method, apparatus, and system for reducing overhead in responding to ACK/NACK messages in multiple access systems that employ packet retransmission mechanisms such as ARQ.  
       SUMMARY  
       [0009]     Accordingly, the embodiments discussed herein provide for mechanisms where to minimize situations in which the access point can become out of synchronization with the access terminal. According to one aspect, information using incremental redundancy is transmitted. A determination is made as to whether reverse link performance drops below a predetermined threshold. Determination of reverse link performance may be done in variety of ways, including use of a filter percentage of ACK erasures, measured error rates on the reverse link control channel, reverse link data channel error rates, and other methods. Upon determination of channel degradation, the access point can decide whether to ignore messages sent from the access terminal to instruct the access terminal to cease transmission. By ignoring messages, the access point transitions to a non-incremental redundancy mode. This allows for a graceful transition for access terminals experiencing poor reverse-link quality. By signaling the access terminal to stop sending ACK/NACK messages, interference caused by the ACK/NACK bit transmission is eliminated.  
         [0010]     In another aspect, methods, systems, and apparatuses to determine whether a transmission channel has degraded are described. Data indicative of an acknowledgment (ACK) or indicative of a not-acknowledgment (NACK) is received. The quality of the data received is measured. A determination is made as to whether the channel has degraded as a function of the quality measurement of the data received. This determination may be accomplished by considering the filtered percentage of erasures over a predetermined amount of time. If it is determined that the data received is reliably a NACK, a rapid retransmission of data may be sent. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Various aspects and features of the present invention are disclosed by the following detailed description and references to the accompanying drawings, in which:  
         [0012]      FIG. 1  is a diagram illustrating various channelization schemes in various multiple access systems;  
         [0013]      FIG. 2  is a diagram illustrating packet transmissions with two interlacing packet streams in a dual-channel ARQ system;  
         [0014]      FIG. 3  illustrates a call process in which a NACK-to-ACK error occurs;  
         [0015]      FIG. 4  illustrates a process to respond to recognition of channel degradation;  
         [0016]      FIG. 5  illustrates a graph of erasure detection;  
         [0017]      FIG. 6  illustrates using erasure detection in response to channel degradation;  
         [0018]      FIG. 7  illustrates an incremental redundancy transmission; and  
         [0019]      FIG. 8  illustrates a block diagram of a transmitter and receiver. 
     
    
     DETAILED DESCRIPTION  
       [0020]     In the following detailed description numerous specific details are set forth. However, it is understood that various embodiments of the invention may be practiced without these specific details. It should be appreciated and understood by one skilled in the art that the various embodiments of the invention described below are exemplary and are intended to be illustrative of the invention rather than limiting.  
         [0021]     As described herein, according to one embodiment of the invention, a method is provided to allow efficient user-multiplexing in a multiple access system which employs an incremental redundancy transmission scheme, such as the Automatic Repeat/Retransmission (ARQ) scheme. In the examples that are provided below, while ARQ systems are discussed for the purposes of explanation and illustration, it should be understood and appreciated by one skilled in the art that the teachings of the present invention are not limited to multiple access system with ARQ transmission schemes, but are also equally applicable to other multiple systems which employ different numbers of interlaces for the purposes of providing redundancy.  
         [0022]     The techniques described herein for using multiple modulation schemes for a single packet may be used for various communication systems such as an Orthogonal Frequency Division Multiple Access (OFDMA) system, a Code Division Multiple Access (CDMA) system, a Time Division Multiple Access (TDMA) system, a Frequency Division Multiple Access (FDMA) system, an orthogonal frequency division multiplexing (OFDM)-based system, a single-input single-output (SISO) system, a multiple-input multiple-output (MIMO) system, and so on. These techniques may be used for systems that utilize incremental redundancy (IR) and systems that do not utilize IR (e.g., systems that simply repeats data).  
         [0023]      FIG. 7  illustrates an incremental redundancy transmission between a transmitter and a receiver in a communication system. The timeline for data transmission is partitioned into frames, with each frame having a particular time duration. For the incremental redundancy transmission embodiment shown in  FIG. 7 , the receiver initially estimates the communication channel, selects a “mode” based on the channel condition, and sends the selected mode to the transmitter in frame  0 . Alternatively, the receiver sends back an estimate of the channel quality, and the transmitter selects the mode based on the channel quality estimate. In any case, the mode may indicate the packet size, the code rate, the modulation scheme, and so on, for the packet. The transmitter processes a data packet (Packet  1 ) in accordance with the selected mode, and generates up to T blocks of data symbols for the packet. T is the maximum number of blocks for a given data packet and is greater than one (T&gt;1) for incremental redundency. The first block typically contains sufficient information to allow the receiver to decode the packet under good channel condition. Each subsequent block typically contains additional parity/redundancy information not contained in prior blocks. The transmitter then transmits the first data symbol block (Block  1 ) for Packet  1  in frame  1 . The receiver receives, detects, and decodes the first data symbol block, determines that Packet  1  is decoded in error (i.e., “erased”), and sends back a negative acknowledgment (NAK) in frame  2 . The transmitter receives the NAK and transmits the second data symbol block (Block  2 ) for Packet  1  in frame  3 . The receiver receives and detects Block  2 , decodes Blocks  1  and  2 , determines that Packet  1  is still decoded in error, and sends back another NAK in frame  4 . The block transmission and NAK response may repeat any number of times.  
         [0024]     For the example shown in  FIG. 7 , the transmitter receives a NAK for data symbol block N−1 and transmits data symbol block N (Block N) for Packet  1  in frame n, where N≦T. The receiver receives and detects Block N, decodes Blocks  1  through N, determines that the packet is decoded correctly, and sends back an acknowledgment (ACK) in frame n+1. The receiver also estimates the communication channel, selects a mode for the next data packet, and sends the selected mode to the transmitter in frame n+1. The transmitter receives the ACK for Block N and terminates the transmission of Packet  1 . The transmitter also processes the next data packet (Packet  2 ) in accordance with the selected mode, and transmits the first data symbol block (Block  1 ) for Packet  2  in frame n+2. The processing at the transmitter and receiver continues in the same manner for each data packet transmitted via the communication channel.  
         [0025]     As shown in  FIG. 7 , with incremental redundancy, the transmitter sends each data packet in a series of block transmissions, with each block transmission carrying a portion of the packet. The receiver may attempt to decode the packet after each block transmission based on all blocks received for the packet. The transmitter terminates the transmission of the packet after receiving an ACK indicating successful decoding by the receiver.  
         [0026]     For the example shown in  FIG. 7 , there is a delay of one frame for the ACK/NAK response from the receiver for each block transmission. In general, this delay may be one or multiple frames. To improve channel utilization, multiple data packets may be transmitted in an interlaced manner. For example, data packets for one traffic channel may be transmitted in odd-numbered frames and data packets for another traffic channel may be transmitted in even-numbered frames. More than two traffic channels may also be interlaced, e.g., if the ACK/NAK delay is longer than one frame.  
         [0027]     The system may be designed to support a set of modes, which may also be called rates, packet formats, radio configurations, or some other terminology. Each mode may be associated with a particular code rate or coding scheme, a particular modulation scheme, a particular spectral efficiency, and a particular minimum signal-to-noise-and-interference ratio (SINR) required to achieve a target level of performance, e.g., 1% packet error rate (PER). Spectral efficiency refers to the data rate (or the information bit rate) normalized by the system bandwidth, and is given in units of bits per second per Hertz (bps/Hz). In general, higher SINRs are needed for higher spectral efficiencies. The set of supported modes covers a range of spectral efficiencies, typically in increments that are somewhat evenly spaced. For a given channel condition and received SINR, the mode with the highest spectral efficiency supported by that received SINR may be selected and used for data transmission.  
         [0028]     Spectral efficiency is determined by the code rate and modulation scheme. The code rate is the ratio of the number of input bits into an encoder to the number of code bits generated by the encoder and transmitted. For example, a code rate of 2/9 (or R=2/9) generates nine code bits for every two input bits. A lower code rate (e.g., R=1/4 or 1/5) has more redundancy and thus greater error correction capability. However, more code bits are transmitted for a lower code rate, and spectral efficiency is thus also lower.  
         [0029]     Various modulation schemes may be used for data transmission. Each modulation scheme is associated with a signal constellation that contains M signal points, where M&gt;1. Each signal point is defined by a complex value and is identified by a B-bit binary value, where B≧1 and 2 B =M. For symbol mapping, the code bits to be transmitted are first grouped into sets of B code bits. Each set of B code bits forms a B-bit binary value that is mapped to a specific signal point, which is then transmitted as a modulation symbol for that group of B code bits. Each modulation symbol thus carries information for B code bits. Some commonly used modulation schemes include Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), M-ary Phase Shift Keying (M-PSK), and M-ary Quadrature Amplitude Modulation (M-QAM). The number of code bits per modulation symbol (B) can be given as: B=1 for BPSK, B=2 for QPSK, B=3 for 8-PSK, B=4 for 16-QAM, B=6 for 64-QAM, and so on. B is indicative of the order of a modulation scheme, and more code bits may be sent per modulation symbol for higher order modulation schemes.  
         [0030]      FIG. 8  shows a block diagram of a transmitter  810  and a receiver  850  in a wireless communication system  800  that utilizes IR transmission. At transmitter  810 , a TX data processor  820  receives data packets from a data source  812 . TX data processor  820  processes (e.g., formats, encodes, partitions, interleaves, and modulates) each data packet in accordance with a mode selected for that packet and generates up to T blocks of data symbols for the packet. The selected mode for each data packet may indicate (1) the packet size (i.e., the number of information bits for the packet) and (2) the particular combination of code rate and modulation scheme to use for each data symbol block of that packet. A controller  830  provides various controls to data source  812  and TX data processor  820  for each data packet based on the selected mode as well as the feedback (ACK/NAK) received for the packet, if desired. This process is discussed further with respect to  FIG. 3 . TX data processor  820  provides a stream of data symbol blocks (e.g., one block for each frame), where the blocks for each packet may be interlaced with the blocks for one or more other packets.  
         [0031]     A transmitter unit (TMTR)  822  receives the stream of data symbol blocks from TX data processor  820  and generates a modulated signal. Transmitter unit  822  multiplexes in pilot symbols with the data symbols (e.g., using time, frequency, and/or code division multiplexing) and obtains a stream of transmit symbols. Each transmit symbol may be a data symbol, a pilot symbol, or a null symbol having a signal value of zero. Transmitter unit  822  may perform a form of OFDM modulation if OFDM is used by the system. For example, an OFDMA system employing OFDM schemes may be used. Transmitter unit  822  generates a stream of time-domain samples and further conditions (e.g., converts to analog, frequency upconverts, filters, and amplifies) the sample stream to generate the modulated signal. The modulated signal is then transmitted from an antenna  824  and via a communication channel to receiver  850 .  
         [0032]     At receiver  850 , the transmitted signal is received by an antenna  852 , and the received signal is provided to a receiver unit (RCVR)  854 . Receiver unit  854  conditions, digitizes, and pre-processes (e.g., OFDM demodulates) the received signal to obtain received data symbols and received pilot symbols. Receiver unit  854  provides the received data symbols to a detector  856  and the received pilot symbols to a channel estimator  858 . Channel estimator  858  processes the received pilot symbols and provides channel estimates (e.g., channel gain estimates and SINR estimates) for the communication channel. Detector  856  performs detection on the received data symbols with the channel estimates and provides detected data symbols to an RX data processor  860 . The detected data symbols may be represented by log-likelihood ratios (LLRs) for the code bits used to form the data symbols (as described below) or by other representations. Whenever a new block of detected data symbols is obtained for a given data packet, RX data processor  860  processes (e.g., deinterleaves and decodes) all detected data symbols obtained for that packet and provides a decoded packet to a data sink  862 . RX data processor  860  also checks the decoded packet and provides the packet status, which indicates whether the packet is decoded correctly or in error.  
         [0033]     A controller  870  receives the channel estimates from channel estimator  258  and the packet status from RX data processor  860 . Controller  870  selects a mode for the next data packet to be transmitted to receiver  850  based on the channel estimates. Controller  870  also assembles feedback information, which may include the selected mode for the next packet, an ACK or a NAK for the packet just decoded, and so on. The feedback information is processed by a TX data processor  882 , further conditioned by a transmitter unit  884 , and transmitted via antenna  852  to transmitter  810 .  
         [0034]     At transmitter  810 , the transmitted signal from receiver  850  is received by antenna  824 , conditioned by a receiver unit  242 , and further processed by an RX data processor  844  to recover the feedback information sent by receiver  850 . Controller  830  obtains the received feedback information, uses the ACK/NAK to control the IR transmission of the packet being sent to receiver  850 , and uses the selected mode to process the next data packet to send to receiver  850 .  
         [0035]     Controllers  830  and  870  direct the operation at transmitter  810  and receiver  850 , respectively. Memory units  832  and  872  provide storage for program codes and data used by controllers  830  and  870 , respectively.  
         [0036]      FIG. 3  illustrates a call process  300  in which a NACK-to-ACK error occurs. Specifically,  FIG. 3  illustrates signals sent between access terminal  304  and access point  308 . A call is established and various control signals are sent  312  from the access terminal  304  to the access point  308 . These control signals include CQI, which is an indication of forward link channel quality, a REQUEST CHANNEL which indicates the initial request of a channel, and an ACK/NACK bit, which is an indication as to whether a transmission was properly received or not received.  
         [0037]     An “access terminal” refers to a device providing voice and/or data connectivity to a user. An access terminal may be connected to a computing device such as a laptop computer or desktop computer, or it may be a self contained device such as a personal digital assistant. An access terminal can also be called a subscriber station, subscriber unit, mobile station, wireless device, mobile, remote station, remote terminal, user terminal, user agent, or user equipment. A subscriber station may be a cellular telephone, PCS telephone, a cordless telephone, a Session Initiation Protocol (SIP) phone, a wireless local loop (WLL) station, a personal digital assistant (PDA), a handheld device having wireless connection capability, or other processing device connected to a wireless modem.  
         [0038]     An “access point” refers to a device in an access network that communicates over the air-interface, through one or more sectors, with the access terminals or other access points. The access point acts as a router between the access terminal and the rest of the access network, which may include an IP network, by converting received air-interface frames to IP packets. Access points also coordinate the management of attributes for the air interface. An access point may be a base station, sectors of a base station, and/or a combination of a base transceiver station (BTS) and a base station controller (BSC).  
         [0039]     Upon receipt of ACKs, data is transmitted  316  from the access point  308  to access terminal  304 . In response to successful data transmissions, access terminal  304  responds with an ACK  320  to the access point  308 . In response to receiving an ACK, the access point  308  then transmits the next data packet of interest  324  to the access terminal  304 . If access terminal  304  did not properly decode the data  1 A transmitted, a NACK message  328  is sent. However, due to channel degradation, an error  332  may occur such that the NACK message  328  is actually interpreted to be an ACK message  336  as received by access point  308 .  
         [0040]     Access point  308 , assuming the access terminal  304  properly decoded data packet  1 A (as illustrated by step  324 ), transmits data ( 340 ) to access terminal  304 . Access terminal  304 , by virtue of sending NACK  328 , is expecting a retransmission of data  1 B, the next set of incremental redundancy bits for data  1 A ( 324 ), but instead receives data  2 A ( 340 ). This causes the access terminal  304  to become out of sync with access point  308 . When the access terminal  304  and access point  308  become out of sync, then various recovery mechanisms are needed to get the access terminal  304  and access point  308  back into synchronization. This process is cumbersome and usually results in many data packers being lost. In any event, this situation is one to be avoided.  
         [0041]     Accordingly, the embodiments discussed herein provide for mechanisms where to minimize situations in which the access point can become out of synchronization with the access terminal. Specifically, various ways are used to determine channel degradation such that NACK to ACK errors are avoided.  FIG. 4  illustrates such a process  400  to respond to recognition of channel degradation. A call is established and various control data is transmitted  404  between the access terminal and access point. The access point then determines whether the channel has degraded  408 .  
         [0042]     The access point can determine this channel degradation in a variety of ways. For example, since there is a correlation between the reverse link performance and the NACK to ACK error rate, the access point is able to determine when the error rate is likely to be high. An example of reverse link performance measures include the reverse link control channel pilot or received power over noise. Another example is the measured error rates on the reverse link control channel when known control values are sent provides information to access point to determine channel degradation. In another embodiment, the number of higher layer NACK messages, such as from the RLP layer, during a window of time indicating that the physical layer ACK&#39;ed were not successfully received is used as a measure of channel degradation. Also, reverse link data channel error rates or reverse link reported power control parameters are also can give indications of access point to access terminal channel degradation.  
         [0043]     In another embodiment, the filtered percentage of ACK erasures maybe used as a reverse link performance measurement. This is discussed in more detail below with respect to  FIGS. 5 and 6 .  
         [0044]     Upon recognition of channel degradation, the access point may stop responding  412  to the ACK messages sent from the access terminal, and then switch  416  to transmission of data not using incremental redundancy. Alternatively, access point may request  420  the access terminal to stop sending ACK messages. This has the added benefit of minimizing noise in the system as viewed by one or more access points.  
         [0045]      FIG. 5  illustrates the concept of erasure detection. Erasure detection is typically utilized in the CQI channel and can give the indication of channel degradation. Erasure detection involves the establishment of an erased region, defined by lines  504  and  508  in  FIG. 5 . Data bits received within the erased region  512  are indicative of a lack of confidence as to whether the bit sent is a “0” or a “1”. If the bit received corresponds to being received in region  520 , there is a high degree of confidence that the data received is a “1”. If the bit received corresponds to being received in region  516 , there is a high degree of confidence that the data received is a “0”.  
         [0046]     This process is also described in  FIG. 6 , which illustrates using erasure detection in response to channel degradation  600 . Data is received at the ACK channel  604 . The data is sent both to an ACK decode element  608  and quality measurement element  612 . The ACK decode element  608  attempts to decode whether the bit received is an ACK bit or a NACK bit, in accordance with the scheme illustrated in  FIG. 5 . More specifically, the ACK decode element  608  determines whether the bit received is in region  520 , and therefore in fact an ACK bit, or not an ACK bit, and therefore is either in the erased region  512  or in the NACK region  516 .  
         [0047]     The data received on the ACK channel is also sent to the quality measurement element  612 . The quality measurement element  612  considers the filtered percentage of erasures over time. If the filtered percentage of erasures is above or low a certain threshold, quality measurement  612  declares whether the ACK channel is good or bad  616 . Thus, quality measurement element  612  distinguishes between a bit received in region  520  (a “1”), region  516  (a “0”), or in the erased region  512 . If the bit received is in region  516  or  520 , there is a high degree of confidence that the bit received is a true reading. Accordingly, quality measurement element  612  can emit a “good channel” indicator. Conversely, if there is a lower level of confidence that the bit received is a true reading. Accordingly, quality measurement element  612  can emit a “bad channel” indicator.  
         [0048]     The various aspects and features of the present invention have been described above with regard to specific embodiments. As used herein, the terms ‘comprises,’ ‘comprising,’ or any other variations thereof, are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment.  
         [0049]     As examples, the various illustrative logical blocks, flowcharts, windows, and steps described in connection with the embodiments disclosed herein may be implemented or performed in hardware or software with an application-specific integrated circuit (ASIC), a programmable logic device, discrete gate or transistor logic, discrete hardware components, such as, e.g., registers and FIFO, a processor executing a set of firmware instructions, any conventional programmable software and a processor, or any combination thereof. The processor may advantageously be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The software could reside in RAM memory, flash memory, ROM memory, registers, hard disk, a removable disk, or any other form of storage medium known in the art.  
         [0050]     While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.