Abstract:
Systems and techniques are disclosed relating to communications. The systems and techniques involve transmitting to a remote location a first signal at a first energy level followed by a second signal at a second energy level, determining a target transmission energy level as a function of a target quality parameter at the remote location, and computing the second energy level as a function of the target transmission energy level and the first energy level. It is emphasized that this abstract is provided solely to assist a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
   The present Application for Patent claims priority to Provisional Application No. 60/404,379, entitled “Forward Link Deboosting,” filed Aug. 19, 2002, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 

   BACKGROUND 
   1. Field 
   The present invention relates generally to communications, and more specifically, to deboosting in a communications environment. 
   2. Background 
   Wireless communication systems are designed to allow multiple users to share a common communications medium. One such wireless communications system is a code division multiple access (CDMA) system. The CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, and thereby, spreads the signal over the entire spectrum. The transmitted signals can be separated in the receiver by a demodulation process using a corresponding code to de-spread the desired signal. The undesired signals, whose codes do not match, contribute only to noise. 
   The performance of a CDMA communications system may be enhanced by providing powerful coding methods to facilitate forward error correction (FEC) capability. The coding process provides redundancy that the receiver may use to correct errors. This may be achieved by generating a data packet containing systematic symbols and redundancy symbols from a group of payload bits. The systematic symbols replicate the payload bits. A subpacket containing the systematic symbols and a portion of the redundancy symbols may be initially transmitted to the receiver. If the receiver is able to decode the data packet, the remaining redundancy symbols do not need be transmitted to the receiver. If, on the other hand, the receiver is unable to decode the data packet, a new subpacket with different redundancy symbols may be sent to the receiver on a retransmission. The subpackets may be combined and decoded jointly at the receiver, resulting in a very efficient retransmission, because the energy of the previous transmission is not discarded. This process is known in the art as incremental redundancy. 
   The drawback to incremental redundancy techniques is that the retransmission process consumes valuable resources. Conventional systems employing incremental redundancy have generally been configured to retransmit each subpacket as a stand-alone transmission. That is, the retransmission energy is sufficiently high such that, even if the previous transmissions had not been received, the receiver still has a good chance of being able to decode the retransmission. This is true even when the incremental energy needed to decode the data packet is very small. Accordingly, there is a need for a more efficient and robust methodology for handling retransmissions that takes into account the energy of previous subpacket transmissions derived from the same data packet. 
   SUMMARY 
   In one aspect of the present invention, a method of communications includes transmitting a first signal to a remote location at a first energy level, determining a target transmission energy level as a function of a target quality parameter at the remote location, computing a second energy level as a function of the target transmission energy level and the first energy level, and transmitting a second signal to the remote location at the second energy level. 
   In another aspect of the present invention, a communications apparatus includes a transmitter configured to transmit to a remote location a first signal at a first energy level followed by a second signal at a second energy level, and a processor configured to determine a target transmission energy level as a function of a target quality parameter, at the remote location and compute the second energy level as a function of the target transmission energy level and the first energy level. 
   In yet another aspect of the present invention, a communications apparatus includes means for transmitting to a remote location a first signal at a first energy level followed by a second signal at a second energy level, determining means for determining a target transmission energy level as a function of a target quality parameter at the remote location, and means for computing the second energy level as a function of the target transmission energy level and the first energy level. 
   It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only several embodiments of the invention by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein: 
       FIG. 1  is a conceptual block diagram of an embodiment of a CDMA communications system; 
       FIG. 2  is a functional block diagram of an embodiment of a base station; 
       FIG. 3  is a functional block diagram of an embodiment of a subscriber station; 
       FIG. 4  is a graphical representation illustrating a simplified example of a de-boosting function; and 
       FIG. 5  is a flow diagram illustrating a detailed example of a de-boosting function. 
   

   DETAILED DESCRIPTION 
   The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention can be practiced. The embodiments described throughout this disclosure are intended to serve as an example, instance, or illustration and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention; however, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the present invention. 
   In the following detailed description, various aspects of the present invention will be described in the context of a CDMA communications system supporting high speed data applications. While these inventive aspects may be well suited for in use this application, those skilled in the art will readily appreciate that these inventive aspects are likewise applicable for use in various other communication environments. Accordingly, any reference to a CDMA communications system is intended only to illustrate the inventive aspects, with the understanding that such inventive aspects have a wide range of applications. 
     FIG. 1  is a conceptual block diagram of an embodiment of a CDMA communications system with de-boosting capability. A base station controller  102  (BSC) may be used to interface a wireless network  104  to an existing network infrastructure  106 . The network infrastructure  106  may be a packet-switched network, such as the Internet, a corporate intranet, or the like. Alternatively, the network infrastructure  106  may be a circuit-switched network, such as a public switched telephone network (PSTN). The wireless network  104  may be implemented with any number of base stations dispersed throughout a geographic region. The geographic region may be subdivided into smaller regions known as cells with a base station serving each cell. In high traffic applications, the cell may be further divided into sectors with a base station serving each sector. For simplicity, one base station  108  is shown serving an entire sector under control of the BSC  102 . A number of subscriber station  110 A– 110 D operating within the sector may communicate with one another, or access the network  106  through one or more base stations. 
   When power is initially applied to the subscriber station  110 A, it may attempt to establish a wireless connection with the base station  108  using a predetermined access procedure. The access procedure involves the acquisition of a pilot transmitted over a forward link. The forward link refers to transmissions from the base station  108  to a subscriber station, and a reverse link refers to transmissions from a subscriber station to the base station  108 . Once the subscriber station  110 A acquires the pilot, it may access a forward link synchronization channel to acquire broadcast system information, and send a registration request to the base station  108  over the reverse link using an access channel. The base station  108  then forwards the registration request to the BSC  102 . In response, the BSC  102  registers the subscriber station  110 A, and sends a response back to the subscriber station  110 A acknowledging registration. 
   The BSC  102  may then initiate a call from the network  106  to the subscriber station  110  by directing the base station  108  to page the subscriber station  110 A over a paging channel. In response, the subscriber station  108  may transmit signaling messages over the access channel back to the base station  108  indicating that it is ready to receive the call. Alternatively, the subscriber station  110 A may initiate the call by signaling the base station  108  over the access channel. In any event, once a call is initiated, a logical resource connection may be established between the base station  108  and the subscriber station  108 , and the base station  110 A may assign an address to the subscriber station  110 A to identify communications intended for the subscriber station over that connection. The address may be transmitted from the base station  108  to the subscriber station  110 A with the exchange of signaling messages during call set up. A traffic channel may then be established between the base station  108  and the subscriber station  110 A to support the call. A subscriber station with a traffic channel established in said to be an active subscriber station. Depending on the amount of data to be sent between the base station  108  and the subscriber station  110 A, multiple channels may be allocated to the traffic channel. The channel allocations may be based on orthogonal spreading sequences known as Walsh codes. 
   During the call, the subscriber station  110 A may feed back information to the base station  108  relating to the quality of the forward link under current channel conditions. In a manner to be described in greater detail later, the feedback may be used by the base station  108  to limit the transmission power of the forward link to that necessary to achieve a desired quality of service. The feedback may be based on a carrier-to-interference (C/I) ratio computed at the subscriber station  110 A from the forward link pilot by means well known in the art. Based on this feedback, as well as resource availability and user priorities among the various subscriber stations  110 A– 110 D, the base station  108  may schedule the forward link transmission of one or more data packets to the subscriber station  110 A over the traffic channel. 
   The forward link transmission generated by the base station  108  may also include a data packet control channel associated with the traffic channel. The packet data control channel, which may be common to all subscriber stations  110 A– 110 D, may be used to carry information packets addressed to individual subscriber stations. Each information packet may be used by the intended subscriber station to receive or decode a corresponding data subpacket carried on its respective traffic channel. The information packet may be synchronized with its corresponding data subpacket or may be offset in time. If the subscriber station  110 A identifies an information packet with its address, the subscriber station  110 A may attempt to decode the corresponding data subpacket. An acknowledgement (ACK) message may be sent from the intended subscriber station  110 A to the base station  108  over a reverse link ACK channel if the intended subscriber station  110 A is able to decode the data packet from the subpacket. If, on the other hand, the data packet is not successfully decoded at the intended subscriber station  110 A, a negative acknowledgement (NAK) message may be sent to the base station over the reverse link ACK channel requesting that the base station  108  transmit another subpacket from the same data packet. 
     FIG. 2  is a functional block diagram of an embodiment of a base station. The base station may be configured to receive data from the BSC (not shown) over a logical resource connection established during call set up. The data may be stored in a queue  206  at the base station, which buffers the data from the BSC (not shown) before transmission to the subscriber station. A processor  208  may be used to release a portion of the data, or payload, from the queue  206  to an encoder  210  at a fixed or variable rate. In CDMA communication systems supporting variable data rates, the C/I ratio fed back from the subscriber station  110  to the base station  108  may be used by the processor  208  to efficiently transmit the forward link traffic at the highest possible data rate under current channel conditions. 
   The encoder  210  may be used to apply an iterative coding process to the payload, such as Turbo coding. The coding process may be used to produce a data packet containing systematic symbols and redundancy symbols from a group of payload bits released by the processor  208  from the queue  206 . The number of payload bits divided by the number of coded symbols produced from the coding process is generally referred to as the coding rate. The lower the coding rate, the higher the coding gain. This coding gain enables the base station  108  to reduce the transmission energy and achieve the same bit error rate (BER) at the subscriber station  110  because of an increase in redundancy. The BER is typically used by those skilled in the art as a design parameter which defines the minimum quality of service. 
   The manner in which the coded symbols are sent to the subscriber station  110  depends on whether the base station is sending an initial transmission or a retransmission of the data packet. If the data packet is queued for initial transmission to the subscriber station  110 , the encoder  210  may package the systematic symbols and a portion of the redundancy symbols into a subpacket. If, on the other hand, the data packet is queued for a retransmission, a new subpacket with different redundancy symbols may be used. In any event, the coded symbols in the subpacket may be provided to a demultiplexer  212  to generate multiple traffic subchannels. In some embodiments, the coded symbols in the subpacket may be interleaved and scrambled using a long pseudo-random noise (PN) sequence before being provided to the demultiplexer  212 . 
   A modulator  214  may be used to map multiple coded symbols from each traffic subchannel to a single modulation symbol in a signal constellation. By mapping multiple coded symbols to a single modulation symbol, improvements in bandwidth efficiency may be achieved. The number of coded symbols that may be represented by a modulation symbol is referred to as the modulation order, and is a function of the modulation scheme. By way of example, QPSK (quadrature phase shift keying) employs a signal constellation wherein two coded symbols may be mapped to a modulation symbol. A higher order modulation scheme, such as 16-QAM, may be used to map four coded symbols to a modulation symbol making it more bandwidth efficient than QPSK. The modulator  214  may be implemented with any modulation scheme known in the art. In at least one embodiment of the base station, the modulation scheme may be programmed by the processor  208 . 
   We can define the transmission format as the combination of the modulation and the set of coded symbols being transmitted. Each format may have different performance, which can be captured by its target energy-per-bit (E b /N t ) target , i.e., the receive energy per bit needed to decode the packet with a given BER. This performance measure is loosely linked to a given receiver implementation but can be derived by means well known in the art. 
   Channel separation may be achieved by spreading each traffic subchannel with a different Walsh code provided by the processor  208 . A multiplier  216  may be used to multiply each modulation symbol with its respective Walsh code to produce n chips per modulation symbol. The value n is generally referred to as the spreading factor. The channels may then be combined with an adder  218  to produce a forward link traffic channel having a chip rate equal to the modulated symbol rate times the spreading factor. 
   The forward link traffic channel from the adder  218  may be provided to a gain element  220 . A gain signal computed by the processor  208  may be applied to the gain element  220  to control the transmission energy of the forward link traffic channel. The transmission energy of the forward link traffic channel (E c /I or ) is generally expressed as the ratio between the transmission power of the forward link traffic channel to the total power of the base station. The E c /I or  may be computed by determining the target energy-per-chip (E c /N t ) target  required at the subscriber station to meet the minimum quality of service requirements with the selected format, and adding to that value enough energy to overcome the losses associated with the forward link. The (E c /N t ) target  may be computed from the (E b /N t ) target  using the following equation:
 
( E   c   /N   t ) target =( E   b   /N   t ) target (subpacket coding rate)(modulation order)/(spreading factor)  (1)
 
The losses associated with the forward link may be estimated based on the C/I ratio fed back to the base station  108  from the subscriber station  110  by means well known in the art. The E c /I or  required for the transmission may then be computed from the following equation:
 
 E   c   /I   or dB =( E   c   /N   t ) target dB−( C/I  ratio)dB+( E   cp /I or )dB  (2)
 
where E cp /I or  is the ratio between the transmission power of the forward link pilot to the total power of the base station (typically between 10 and 20%).
 
   The forward link traffic channel from the gain element  220  may be provided to a second adder  222  where it may be combined with other overhead channels, such as the packet data control channel. Each overhead channel may be encoded, modulated and spread with its own unique Walsh code to maintain channel separation. The combined signal may then be provided to a multiplier  224  where it may be quadrature spread using short PN codes. The short PN codes are a second layer of coding that is used to isolate one sector from another. The approach allows the reuse of Walsh codes in every sector. The quadrature modulated signal may then be provided to a transceiver  226  for filtering, upconvertion, amplification, and transmission over the forward link via an antenna  228  to the subscriber station. 
     FIG. 3  is a functional block diagram of an embodiment of a subscriber station. The forward link transmission may be received at the subscriber station with an antenna  302  and coupled to a transceiver  304 . The transceiver  304  may be configured to filter and amplify the forward link transmission, and downconvert it to baseband. A demodulator  306  may be used to quadrature demodulate the forward link transmission, and then separate the traffic subschannels through a de-spreading process to extract the packet data control channel. The packet data control channel may be provided to an overhead channel processor  308  to perform various signal processing functions to determine whether it is addressed to the subscriber station. Assuming the overhead channel processor  308  determines that the information packet carried on the packet data control channel is addressed to the subscriber station, the information may be used by the demodulator  306  to de-map the sequence of modulated symbols from each traffic subchannel. The demodulated symbols from the traffic subchannels may then be demultiplexed and provided to a decoder  310  for further processing. In some embodiments, the coded symbols may be descrambled using the long PN code and de-interleaved before being provided to the decoder  310 . In any event, the symbols provided to the decoder  310  may be combined with previous transmissions from the same data packet and jointly decoded using the information from the packet data control channel. An ACK or NAK message may be generated by the decoder  310  to indicate whether the data packet has been successfully decoded. 
   The forward link pilot is typically not encoded, and therefore, may be coupled directly from the demodulator  306  to an estimator  312 . Since the pilot symbol sequence is known, a priori, it can be stored in memory (not shown) at the subscriber station. Based on the demodulated symbols from the forward link pilot and the pilot symbol sequence stored in memory, the estimator  312  may compute a C/I ratio. The C/I ratio computation may be performed by any means known in the art including a mean square error (MSE) algorithm or any other applicable algorithm. 
   An encoder  314  may be used to perform various signal processing functions on one or more reverse link traffic channels, such as iterative coding and interleaving. The encoded traffic together with the ACK or NAK message from the decoder  310  and the C/I ratio from the estimator  312  may be provided to a modulator  318 . The C/I ratio and the ACK or NAK message may be placed on the appropriate channels, combined with the traffic channels, spread with the long PN code, quadrature modulated with the short PN codes, and provided to the transceiver  304  for upconversion, filtering and amplification before being transmitted over the reverse link through the antenna  302  to the base station. 
   Turning back to  FIG. 2 , the reverse link transmission at the base station may be coupled from the antenna  228  to the transceiver  226  where it may be amplified, filtered and downconverted to baseband before being provided to a demodulator  230 . The demodulator  230  may be configured to perform some of the various functions described earlier in connection with the demodulation process, as well as extract the ACK or NAK message and the C/I ratio from the reverse link transmission. The ACK or NAK message, along with the C/I ratio may be provided to the processor  208  to perform various control and scheduling functions on the forward link. 
   The processor  208  may be used to coordinate the forward link transmissions of the base station to all subscriber stations  110 A– 110 D (see  FIG. 1 ) in its sector. In at least one embodiment of the base station, the processor  208  receives information from the queue  206  indicating the amount of data to be transmitted to each active subscriber station over the forward link. Based on this information, in combination with the C/I ratio, the minimum quality of service requirements and delay constraints, the processor  208  may schedule the forward link transmissions to achieve maximum data throughput while maintaining some form of fairness among multiple subscriber stations. 
   When a forward link transmission is scheduled for the subscriber station, the processor  208  may also select the transmission format for each subpacket. By way of example, the processor  208  may select the data rate, the coding rate and the modulation format based on the quality of the forward link. Depending on the amount of data to be sent, the processor  208  may select the payload size and assign the appropriate number of Walsh channels to support the payload at the selected data rate, coding rate, and modulation format. By way of example, in a relatively distortion free environment with little or no interference, the processor  208  may use relatively few Walsh codes to transmit a large payload at a high data rate and a low coding gain with a 16-QAM modulation format. Conversely, when the quality of the forward link is poor, the processor  208  may use many Walsh channels to transmit a small payload at a low data rate and a high coding gain with a QPSK modulation format. In some embodiments of the base station, the length of the subpacket may also be varied. By way of example, many CDMA communication systems today provide a subpacket transmission over one, two, four or eight 1.25 millisecond (ms) slots. 
   In CDMA communication systems using incremental redundancy, the processor  208  may employ a de-boosting algorithm that takes into account the energy previously transmitted for the same data packet when selecting a retransmission format. With this approach, only the incremental energy needed to meet the minimum quality of service requirements would be transmitted. The energy from the retransmission could then be combined at the subscriber station with the energy from the previous transmissions for the same data packet and decoded jointly. One de-boosting approach takes into account the amount of energy transmitted previously from the same data packet, and reduces the total retransmission energy by this amount. When implementing this approach, a thresholding function should be applied to ensure that the energy-per-symbol (E s /N t ) does not drop below a predetermined level. Alternatively, the total retransmission energy may be reduced by decreasing the coding gain to maintain a sufficiently high E s /N t . 
   A simplified illustration of this latter concept will be explained with reference to  FIG. 4 . Initially, a payload  402  having x bits may be selected and encoded using an iterative coding process. A coding rate of 1/3 may be used to produce a data packet  404  containing 3x symbols. This represents the minimum coding rate, or maximum coding gain, available. Because of the prevailing channel conditions, the initial transmission is made at a coding rate of 1/2 with a subpacket  406  containing twice as many symbols as payload bits, or 2x symbols. The subpacket  406  includes all the systematic symbols and one-half the redundancy symbols from the data packet  404 . 
   On retransmission, there is no need to send more symbols than required to achieve the minimum coding rate at the receiver. Accordingly, a second subpacket  408  should be constructed from the data packet  404  with no more than x symbols (i.e, the remaining redundancy symbols). However, the total retransmission energy may be computed at the combined coding rate for both subpackets as seen by the receiver. In other words, the second subpacket  408  is not transmitted as a stand alone transmission at an extremely high energy level to support a coding rate of 1. Instead, the total retransmission energy may be computed based on the total number of symbols from both subpacket transmissions. In this case, the transmission of the second subpacket  408  with x symbols will result in 3x symbols being combined at the receiver  410 . Accordingly, the total retransmission energy for the second subpacket  408  may be computed at a 1/3 coding rate. 
     FIG. 5  is a flow chart illustrating a detailed example of a de-boosting operation performed by the processor  208  (see  FIG. 2 ). In the following example, the processor may be used to control the transmission to the subscriber station of a first signal at a first energy level followed by a second signal at a second energy level. The second energy level may be computed by the processor as a function of a target transmission energy level and the first transmission energy level. The target transmission level may be computed by the processor as a function of a target quality parameter at the subscriber station. Alternatively, these de-boosting functions may be performed by multiple processors each performing one or more of the de-booting functions 
   The processor may be implemented in electronic hardware, computer software, or a combination of the two. The processor may be implemented with a general or specific purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic, discrete gate or transistor logic, discrete hardware components, any combination thereof, or any other equivalent or non-equivalent structure designed to perform one or more of the de-boosting functions. 
   Referring to  FIG. 5 , the processor may determine, in step  502 , whether certain resources should be used to support an initial transmission of a new data packet or a retransmission of a data packet previously queued based on the scheduling decisions made earlier. If the processor determines that an initial transmission of a new data packet should be made, then in step  504 , the processor may compute the number of coded symbols and the coding rate that can be supported by every transmission format. The various transmission formats should include every possible permutation of payload size, data rate, modulation order, number of Walsh channels-per-subpacket, and number of slots-per-subpacket that the base station can support. The number of coded symbols may be computed from the following equation:
 
No. of Coded Symbols= W   Traffic (Chip Rate)/ W   Total (Modulation Order)  (3)
 
where W Traffic  is the number of Walsh codes assigned to the transmission format; and
 
   W Total  is the total number of forward link Walsh codes available to the base station. 
   The coding rate may be computed by dividing the number of payload bits by the number of coded symbols for each transmission format. If the computed coding rate is lower than the minimum coding rate, the minimum coding rate should be used. 
   In step  506 , each transmission format may be mapped to the associated target E b /N t . Based on the target E b /N t , the transmission energy E c /I or  needed to support each transmission format may be computed, in step  508 , from equations (1) and (2). If the computed E c /I or  exceeds the total available power for any transmission format, then that transmission format may be eliminated as a possible choice to support the initial transmission in step  510 . The transmission format with the highest data rate whose E c /I or  is within the available power range for the forward link traffic channel may be selected in step  512 . A subpacket may then be transmitted over the forward link, in step  514 , using the selected transmission format. 
   In step  516 , the processor awaits a response from the initial transmission. If a NAK message is not received, either because an ACK message is received or no response is received within a predetermined time, the processor may loop back to step  502 . Conversely, if a NAK message is received, the parameters for the initial transmission format may be recorded at the base station, in step  518 , for later use during retransmission. Specifically, the recorded parameters may include the number of coded symbols transmitted as well as the transmission energy E c /I or . The processor may then loop back to step  502 . 
   In the event that the processor determines, in step  502 , that certain resources should be used to support a retransmission, the processor may identify the data packet queued for retransmission in step  520 . The processor may then compute, in step  522 , the energy expected to have been received by the subscriber station for all previous transmissions from the same data packet. This may be achieved by computing the accumulated energy-per-chip (E c /N t ) accumulated  received by the subscriber station from the E c /I or , recorded for each transmission of such data packet. The computation of the accumulated received energy may take into account potential de-mapping losses based on the received energy-per-symbol (E s /N t ) accumulated  during the associated transmission. De-mapping losses generally occur in the case of higher order modulation (e.g., 8 PSK or 16 QAM) due to the fact that the combining of multiple transmissions occurs at the coded symbol level and may be estimated by means well known in the art for a given receiver implementation. The accumulated (E s /N t ) accumulated  can be computed from the following equation:
 
( E   s   /N   t ) accumulated =( E   c   /N   t ) accumulated /(spreading factor)  (5)
 
   Once the de-mapping losses are computed, the accumulated energy-per-symbol (E s /N t ) accumulated  may be adjusted by the de-mapping losses. The accumulated energy-per-chip (E c /N t ) accumulated  may then be computed from the adjusted (E s /N t ) accumulated . 
   The processor may use the C/I estimation at the time of transmission and assume a completely stable forward link wireless channel to estimate the received energy. If C/I information measured closer in time to the transmission event becomes available, this may be used as an estimate of the link quality. For fast fading channels, an average of the forward link quality may be a more appropriate measure. Either way, a correction factor may be added to account for errors in the C/I ratio. If a correction factor is used, its value may be dependent on the velocity of the subscriber station if that can be determined. 
   Next, the processor may compute a target transmission energy level for each retransmission format. This may be achieved by first computing the overall coding rate for each retransmission format in step  524 . Note that the number of possible retransmission formats should be significantly reduced because the payload size is fixed by the initial transmission. The overall coding rate for each possible retransmission format is based on the number of coded symbols supported by that retransmission format plus the accumulated number of coded symbols received by the subscriber station for the same data packet. The accumulated number of symbols may be determined from the recorded parameters of the previous transmissions. Once the overall coding rates are computed, then for each retransmission format, its respective overall coding rate may be mapped, in step  526 , to a target transmission energy-per-bit (E b /N t ) target  as a function of the BER by means well known in the art, and converted to a target transmission energy-per-chip (E c /N t ) target . 
   The accumulated energy (E c /N t ) accumulated  received by the subscriber station (computed in step  522 ) may then be subtracted from the target transmission energy-per-chip (E c /N t ) target  for each possible retransmission format in step  528 . The resulting computation yields a retransmission energy-per-chip (E c /N t ) retransmission  for each possible retransmission format. The (E c /N t ) retransmission  for each possible retransmission format may be adjusted, in step  530 , for incremental redundancy losses associated with the joint decoding of symbols received from multiple transmissions. Since the performance of forward error correction codes in general, and Turbo codes in particular, is affected by the distribution energy across the symbols, it may be prudent to apply some decoding losses. The decoding losses would be a function of the overall coding rate and the signal-to-noise ration (SNR) distribution across the data packet, and may be computed by means well known in the art. In the case of higher order modulations, each (E c /N t ) retransmission  may be further adjusted, in step  532 , for de-mapping losses corresponding to the expected received energy-per-symbol E s /N t  for the retransmission. In any event, the retransmission energy E c /I or  needed to support each retransmission format may then be computed, in step  534 , from its respective (E c /N t ) retransmission  in accordance with equation (2). Next, retransmission formats having a computed E c /I or  that exceeds the available power may be eliminated in step  536 . The retransmission format with the highest data rate whose E c /I or , is within the available power range for the forward link traffic channel may be selected in step  538 . A subpacket may then be transmitted over the forward link, in step  540 , using the selected retransmission format. 
   In step  542 , the processor awaits a response from the retransmission. If a NAK message is not received, either because an ACK message is received or no response is received within a predetermined time, the processor may loop back to step  502 . Conversely, if a NAK message is received, the parameters for the selected retransmission format may be recorded at the base station, in step  544 , for later use during a subsequent retransmission. The processor may then loop back to step  502 . 
   The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
   The methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. A storage medium may be coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a subscriber station. In the alternative, the processor and the storage medium may reside as discrete components in a subscriber station. 
   The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.