Patent Document

PRIOR PROVISIONAL PATENT APPLICATION 
     The present application is a Continuation application of Ser. No. 12/315,993, filed Dec. 9, 2008, now U.S. Pat. No. 7,715,469, which is a Continuation application of Ser. No. 10/970,883, filed Oct. 22, 2004, now U.S. Pat. No. 7,463,677, which is a Continuation application of Ser. No. 09/370,770, filed Aug. 9, 1999, now U.S. Pat. No. 6,823,005, which claims the benefit under 35 USC §119(e) of U.S. Provisional Application No. 60/096,006 filed Aug. 10, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications systems. In particular, the invention concerns a dynamic link adaptation process that offers increased throughput and bandwidth allocation efficiency with particular benefit for wireless data services. 
     BACKGROUND INFORMATION 
     Interest in wireless data communications has grown rapidly in the past few years due to the growth of the Internet. The nature of the data carried over a wireless network is highly determinative of the type of architecture required for efficient and reliable communications. The key to meeting the increasing demand for wireless services is the development of high performance radio systems that take the unique features of the data traffic into account. For example, the architecture of a real time communications system carrying voice and/or video diverges greatly from the design considerations for data communications systems. Compared to voice and other real-time traffic, data traffic usually has a minimum tolerance for transmission errors and a high tolerance of transmission delay. As a result, packet retransmissions are possible and often necessary. For data applications, the techniques of packet switching, dynamic resource assignment and link adaptation are more suitable than conventional techniques such as circuit switching, fixed resource allocation and fixed transmission schemes. 
     The need for re-engineering of communications systems in order to accommodate the needs of data traffic has been recognized. For example, several existing systems, such as Cellular Digital Packet Data (CDPD), Global System for Mobile Communication (GSM) and IS-136 have the capacity to support data services. However, these systems employ circuit switching (except for CDPD) and offer only low data rates. Currently, the data rate of GSM ranges from 2.4 kbps to 9.6 kbps. To enhance the data capability of GSM, a new service called the General Packet Radio Service (GPRS) has been proposed. In addition, the European Telecommunications Standards Institute (ETSI) has standardized a specification entitled Enhanced Data Rates for GSM Evolution (EDGE) as an attractive GSM evolution for providing broadband data services. Both EDGE and IS-136 utilize link adaptation in order to maximize throughput and promote bandwidth efficiency. 
     Link adaptation is a continuous process in which the attributes of each link within a communications system are dynamically updated to maximize throughput (or some other parameter) and efficiently utilize the available bandwidth according to a set of criteria. Typically, a link adaptation scheme consists of a set of modes each incorporating a different modulation/coding scheme or some other link parameter controlling the data rate. Each mode and corresponding modulation/coding scheme has an associated set of performance attributes. For example, the block error rate (BLER) is an important parameter in a link adaptation system. BLER is the probability that a block of bits transmitted from the receiver to the transmitter contains an error after decoding. BLER is a function of the signal-to-interference ratio (SIR) (ratio of signal to interference power) at the receiver such that each mode has a characteristic BLER curve as a function of SIR. Each modulation/coding scheme is also associated with a radio interface rate R, which is the actual rate of information transmission after accounting for the coding overhead. Using the performance attributes BLER and R for each mode, the throughput (measure of the actual bit transmission rate from transmitter to receiver) for each mode can be described as a function of SIR. 
     Link adaptation is accomplished by establishing a set of threshold values for choosing different transmission modes. These threshold values are used to determine the selection of each mode in the adaptation scheme based on some real time performance measure such as SIR. A link adaptation system operates by periodically taking a real time performance measure for each link (e.g., SIR at the receiver), comparing this performance measure with the threshold values for the modes and then selecting the appropriate mode that will maximize throughput. 
     The appropriate link adaptation threshold scheme is crucial to realize performance gains. If the thresholds are too aggressive, i.e., users with poor link quality select higher level modulation/coding schemes, the overall system performance will suffer due to excessive retransmissions. On the other hand, if these thresholds are too conservative, the system performance will also suffer due to low spectrum efficiency, which results in prolonged resource occupancy. 
     Traditional link adaptation schemes use BLER as a basis for establishing the set of adaptation thresholds. Usually the modulation/coding scheme is updated to maintain the BLER at a particular level, (e.g. 10%). Thus, the BLER establishes an acceptable level of error for a communications channel, which is appropriate for real time traffic. On the other hand, data services allow the retransmission of blocks in error at the cost of delay. Therefore, BLER is generally not the only criterion for data services since the ultimate measure for data services is throughput. The throughput depends upon BLER, the transmission rate and the possibility of retransmissions. 
     Link adaptation systems for data services typically rely upon throughput criteria to select the appropriate adaptation mode. For example, the central technology of EDGE is a link adaptation scheme that dynamically adapts the modulation/coding scheme according to the current link quality to maximize system throughput. EDGE incorporates two different modulation schemes, Offset Quadrature Phase Shift Keying (OQPSK) and Offset 16 Quadrature Amplitude Modulation (O16QAM). Combining these two different modulation schemes with four different coding schemes, EDGE supports a total of eight possible modulation/coding modes. 
     The set of thresholds comprising a link adaptation system is derived from a mathematical model of the wireless environment and the performance attributes for each modulation/coding mode. The choice of an appropriate wireless environment model is critical for establishing the correct link adaptation thresholds. For example, conventional link adaptation schemes such as EDGE are based on a model of a no-retransmission environment that assumes erroneous packets are discarded and do not increase the load in a system (i.e., packets are not retransmitted if lost or damaged in the transmission process). 
     However, retransmissions are in fact necessary for wireless data services and the behavior of a retransmission environment diverges significantly from a no-retransmission environment. In particular, a retransmission environment produces highly complex feedback behavior that can result in system instability and degraded performance. Failure to model this complex behavior and derive a correct set of link adaptation thresholds is a major shortcoming of traditional link adaptation schemes and can result in significantly degraded system performance and instability in the retransmission environment. 
     For example, retransmissions necessarily increase the load on the system, increase interference and lower the SIR. The lowering of the SIR will result in even more retransmissions until either the system reaches a steady state condition if it exists or the system becomes unstable. Thus, neglecting retransmissions significantly underestimates the interference in a wireless communications system and link adaptation schemes designed without considering retransmissions will perform poorly. 
     SUMMARY OF THE INVENTION 
     The present invention implements a method and system for dynamically adapting the modulation and coding scheme for radio links in a wireless communications network based on a retransmission environment model in order to maximize throughput and most efficiently allocate bandwidth resources. The present invention encompasses a refined calculus and methodology for deriving the link adaptation thresholds in a retransmission environment using a complex model and analysis of the retransmission environment. The present invention holds particular application for wireless data communications as opposed to real time data services because it is based on a retransmission model applicable primarily for data services. A critical component of this new link adaptation system is a “no transmission” cutoff mode that is selected for SIR below a base threshold value. This new mode prevents system instability and misallocation of bandwidth in a wireless communication system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the large-scale architecture of a wireless communications system according to one embodiment of the present invention. 
         FIG. 2  depicts the architecture of a link adaptation system consisting of a set N of modulation/coding schemes according to one embodiment of the present invention. 
         FIG. 3  depicts BLER as a function of SIR for the eight transmission modes in EDGE. 
         FIG. 4  depicts a set of curves relating the SIR 0  for the offered traffic to the throughput for each mode using the EDGE modulation/coding architecture according to one embodiment of the present invention. 
         FIG. 5  depicts an example of the determination of a SIR margin for EDGE mode ECS-5 according to one embodiment of the present invention. 
         FIG. 6  depicts the derived throughput characteristics for the EDGE modes using an infinite retransmission model. 
         FIG. 7  depicts the operation of a no-transmission mode (mode 0) according to one embodiment of the present invention. 
         FIG. 8  is a flowchart that depicts a set of steps that may be implemented at a wireless transmitter to utilize a no-transmission mode and perform link adaptation according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is based upon an analysis of link adaptation within a retransmission environment. The present invention departs significantly from traditional link adaptation schemes for wireless data services, which neglect retransmissions and ultimately produce an erroneous link adaptation framework that produces instability, misallocation of bandwidth and poor system performance. 
     The retransmission model underlying the present invention generated two critical discoveries that significantly shaped the present invention. First, the threshold values for a link adaptation system using a retransmission model can be derived from the no-retransmission model thresholds. The retransmission thresholds are obtained by shifting the throughput characteristic curves for the no-retransmission model by an amount relating to the difference between the signal to interference ratio generated by the base offered traffic, SIR 0 , and the resulting signal-to-interference ratio generated due to the base traffic plus retransmissions, SIR. 
     Second, the retransmission model revealed that there should be no transmission at all below a base threshold SIR (referred to herein as the “no-transmission” or “mode 0” threshold). Transmitting below this “no-transmission” threshold produces system instability such that excessive retransmissions result causing unbounded delay and almost zero throughput at the receiver. This instability is a product of the retransmission environment itself and is not analyzed or accounted for in conventional link adaptation systems. The complex analysis and insights underlying the retransmission model are an essential underpinning of the present invention and are outlined below. This analysis was summarized from J. Chuang, X. Qiu, “An Improved Link Adaptation Algorithm and Its Implementation Requirements”, presented at SMG2 EDGE ad hoc on EDGE physical/link layer issues in London, Aug. 12-13, 1998, and “Link Adaptation in Wireless Data Networks for Throughput Maximization Under Retransmissions, AT&amp;T Technical Memorandum, HA6132000-980714-06TM, July 1998, also submitted to IEEE ICC&#39;99, Jun. 6-10, 1999. 
     In one embodiment of the present invention, the retransmission model was derived and analyzed using the modulation/coding schemes outlined in EDGE. However, this analysis would apply to any modulation/coding framework. Thus, link adaptation threshold values in a retransmission environment for any modulation/coding architecture can be derived using the framework outlined herein. Recently, for example, new modulation schemes were proposed, and the methodology outlined herein can be applied to them. Furthermore, this same embodiment relied primarily upon an infinite retransmission model, an assumption that packets would be retransmitted until success. However, the basic analysis presented herein can be used for a retransmission model based upon any arbitrary number of retransmissions (e.g., a one retransmission model or a two retransmissions model). 
       FIG. 1  depicts the large-scale architecture of a wireless communications system according to one embodiment of the present invention. Transmitter  100  communicates with receiver  105  through communications channel  104 . Transmitter  100  contains transceiver  115 , data module  125 , modulator/encoder  110 , controller  122  and antenna  140 . Controller  122  calculates modulation/encoding scheme  150  from quality measure  155  sent from receiver  105  and transmits this information to modulator/encoder  110 . Modulation/encoding scheme  150  is used by modulator/encoder  110  to modulate and encode data retrieved from data module  125 . The modulated/encoded data is sent to transceiver  115  for transmission through antenna  140  onto communications channel  104 . Receiver  105  contains decoder  120 , controller  122 , transceiver  115  and antenna  140 . Transceiver  115  is coupled to antenna  140  and communications channel  104  from which data is received. Data is sent from transceiver  115  to decoder  120 , which is controlled by controller  122 . Decoder  120  outputs decoded data  152  and quality measure  155 , which might for example be the current BLER or SIR at the receiver. Quality measure  155  is transmitted back to transmitter  100  through communications channel  104 . 
       FIG. 2  depicts the architecture of a link adaptation system consisting of a set N of modulation/coding schemes  210  according to one embodiment of the present invention. Each scheme n∈N ( 210 ) is characterized by a set of performance attributes  220  that may include, for example, the radio interference rate R n    225  and BLER n  characteristic  227  where n∈N depicts the particular link adaptation mode  210 . BLER n  characteristic  227  is a function relating the BLER to the SIR at the receiver  105  for each mode  210 . For example,  FIG. 3  depicts BLER as a function of SIR for the eight transmission modes  210  in EDGE. A wireless transmission model  240  is associated with the entire link adaptation scheme and is used to derive a throughput characteristic  250  as a function of SIR for each mode  210 . A threshold level  260  is derived for each mode  210  from the set of throughput characteristics  250  in the link adaptation system. For each mode  210 , the threshold level  260  corresponds to the range of SIR over which that mode  210  produces the highest throughput among all modes  210  in the link adaptation scheme. The set of threshold values  260  dictate the selection of a mode  210  based upon real-time measurement of the SIR at the receiver  105 . 
     Analysis of the Infinite Retransmission Model 
     The wireless environment model  240 , which comprises a mathematical and conceptual framework for the wireless transmission environment, is a critical component in determining the set of threshold values  260  for each mode  210 . The throughput characteristic  250  of each mode  210  is derived from wireless environment model  240  and the performance attributes  220  unique to each mode (i.e. R n  and BLER n  characteristic where n∈N). For example, using a no-retransmission environment model, the throughput S is equal to the probability that a block is transmitted correctly (1-BLER n ) multiplied by the actual data transmission R n  
 
 S   n   0   =R   n (1−BLER n (SIR 0 ))  (1)
 
where SIR 0  is the signal-to-interference ratio for the base offered traffic of the system without taking into account any retransmissions. Based upon the no-retransmission environment model  240  as codified in equation (1) and the BLER/SIR relationship depicted in  FIG. 3 ,  FIG. 4  depicts a set curves relating the SIR 0  for the offered traffic to the throughput for each mode  210  using the EDGE modulation/coding architecture.
 
     However, the curves depicted in  FIG. 4  are erroneous in a retransmission environment (such as that required for data services). In fact, using such a link adaptation scheme in a retransmission environment will actually reduce system performance and result in instability in the system. For example, in EDGE, in the range of SIR for which ECS-6 is chosen, the average BLER is higher than 65%, meaning that 65% of packets require retransmission. As a result of this BLER, the load in the system and the interference in the system will be increased substantially. The increase of interference will further lower the SIR and cause even more retransmissions until either the system reaches the steady state if it exists, or the system becomes unstable resulting in a throughput of zero. 
     The realization that the traditional no-retransmission model  240  could not adequately capture the behavior of the retransmission environment led to a complex and detailed analysis of an infinite retransmission environment underlying the present invention. To develop a conceptual and mathematical model to account for infinite retransmissions required analysis of the traffic load in a communications system operating in a retransmission environment. ρ 0  represents the average offered traffic in the communications system neglecting retransmissions. However, the actual load in a transmission system will be higher, represented by ρ, the amount of traffic in the system including base offered traffic and retransmission traffic. 
     Thus, the total load considering retransmissions ρ will be the offered load ρ 0  plus the amount of traffic generated by retransmissions. p n  represents the probability of using a particular modulation/coding mode n∈N, where Σ n∈N p n =1. For the first retransmission, the additional traffic will be the offered traffic ρ 0  multiplied by the probability of choosing mode n (n∈N)  210  multiplied by the BLER for mode n  210  summed over all modes n (n∈N)  210 . The same relationship will apply for the second retransmission except that the BLER term will be of second order due to the two retransmissions. If a user does not change the modulation/coding scheme until the current packet is successfully transmitted, in the steady state, the load in the transmission system under the assumption of infinite retransmissions is given generally by: 
     
       
         
           
             
               
                 
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     Simplification Yields: 
                     ∑     n   ∈   N       ⁢           ⁢         ρ   0     ⁢     p   n         1   -     BLER   n                 (   3   )               
where BLER n  is a function of SIR.
 
     Using a first order approximation, assuming that the total interference I is a linear function of the load ρ, the interference can be described as: 
                   I   =       ∑     n   ∈   N       ⁢           ⁢         I   0     ⁢     p   n         1   -     BLER   n                   (   4   )               
where I 0  is the interference at the receiver  105  if erroneous packets are discarded (i.e., no retransmissions). Therefore, in the steady state, the SIR at a particular link is:
 
                   SIR   =       SIR   0         ∑     n   ∈   N       ⁢           ⁢       p   n       1   -     BLER   n                     (   5   )               
where SIR 0  is the SIR at a link receiver  105  without considering retransmissions.
 
     Expressed in dB, the SIR at the receiver  105  is: 
     
       
         
           
             
               
                 
                   
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     Thus, the SIR at the receiver  105  is the SIR of the offered traffic (i.e. without retransmissions) plus an additional factor C(ρ) (herein referred to as the “SIR margin”) corresponding to a reduction in SIR at each receiver link  105  due to retransmissions
 
SIR|db=SIR 0 |dB+ C (ρ)  (7)
 
where:
 
     
       
         
           
             
               
                 
                   
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     Relating equation (1) to the preceding analysis, in the steady state, the throughput using the infinite retransmission model  240  is:
 
 S   n   s   =R   n (1−BLER n (SIR 0   +C (ρ))  (9)
 
     According to one embodiment of the present invention, the determination of C(ρ) was simplified by making the assumption that all users in the system use the same modulation/coding scheme n (nÅN)  210 , i.e., p n =1, when the SIR margin of mode n (n∈N)  210  is considered. Without this assumption, evaluation of C(ρ) proved to be highly complex since C(ρ) is a function of both {p n } and {BLER n } where {p n ) is a function of the offered load ρ 0  and many other parameters such as the propagation environment. Using this analysis, the determination of the SIR margin was greatly simplified since C(ρ) is reduced to a function of BLER n  alone which itself is a function of SIR (see  FIG. 3 ). Predictions based upon this assumption have corresponded very closely with measured experimental results. Thus, using the assumption that interaction between different modes can be decoupled, the SIR at each receiver link  105  is:
 
SIR|dB=SIR 0 |dB+10 log(1−BLER n ) where  (10)
 
 C (ρ)=10 log(1−BLER n )  (11)
 
Method for Deriving Threshold Values for Infinite Retransmission Model
 
     Assuming that there is a well defined BLER n  characteristic for a given mode  210  and provided with SIR 0 , SIR and C(ρ) can be obtained analytically by solving equation (10). According to one embodiment of the present invention, the following steps describe a method to evaluate C(ρ):
     1. For different values of SIR 0 , the curves y=SIR 0 +C(ρ) and y=SIR are plotted as a function of SIR.   2. For a given value of SIR 0 , the intersection of the curves y=SIR 0 +C(ρ) and y=SIR yields the SIR that satisfies equation (10). The SIR margin can then be calculated as:
 
 C (ρ)=SIR−SIR 0 .
   3. If y=SIR 0 +C(ρ) and y=SIR do not intersect for a given SIR 0 , then there is no SIR that satisfies Equation (10) and the system is not stable under this offered load.   

       FIG. 5  depicts an example of the determination of C(ρ) using the above steps for EDGE mode ECS-5 ( 210 ). Points of intersection ( 510 ) of the line y=SIR ( 530 ) and y=SIR 0 +C(ρ) ( 540 ) represent stable solutions for the infinite retransmission model  240 . C(ρ) can be calculated by finding the difference between SIR and SIR 0  at any of these intersection points. For instance, following the steps outlined above, based upon the data in  FIG. 5 , C(ρ) is approximately −2 dB for SIR 0 =6 dB. 
     Once C(ρ) is calculated according to the above-mentioned steps, it is possible to calculate the threshold values  260  for a link adaptation system based upon an infinite transmission model  240  by simply shifting the throughput characteristic curves  250  derived for the no-retransmission model  240  (e.g.,  FIG. 4 ). This is evident from Equation (9) which is of the same form as Equation (1) except for the additional term C(ρ), the amount by which SIR is reduced due to retransmissions (C(ρ)=SIR−SIR 0 ). Therefore, the thresholds for the no-retransmission model  240  should be increased approximately by −C(ρ) in order to obtain the thresholds for the infinite transmission model  240 . 
       FIG. 6  depicts the derived throughput characteristics for the EDGE modes  210  using the infinite retransmission model  240  as outlined herein. The threshold values for this infinite retransmission model are obtained by finding the mode  210  that produces the highest throughput over the entire SIR range. As described earlier, other retransmission models such as one-retransmission or two- retransmissions can be analyzed using a similar framework. See J. Chuang, X. Qiu, “An Improved Link Adaptation Algorithm and Its Implementation Requirements”, presented at SMG2 EDGE ad hoc on EDGE physical/link layer issues in London, Aug. 12-13, 1998, and “Link Adaptation in Wireless Data Networks for Throughput Maximization Under Retransmissions”, AT&amp;T Technical Memorandum, HA6132000-980714-06TM, July 1998, also submitted to IEEE ICC&#39;99, Jun. 6-10, 1999. 
     Analysis of the infinite retransmission model produced a further critical discovery that in a retransmission environment there exists a cutoff SIR 0 , below which there should be no transmissions at a transmitter  100 . If a transmitter  100  is operating with SIR below this cutoff threshold, transmitting will result in system instability, close to zero throughput and waste of bandwidth resources. For example, an examination of  FIG. 5  reveals that there is no stable solution for Equation (9) if SIR 0  is below approximately 4 dB. This is apparent by noting that none of the curves y=SIR 0 +C(ρ) ( 540 ) below SIR 0 =4 dB (marked with ‘x’) intersect the line y=SIR ( 530 ). Because SIR is a function of SIR 0 , this means that there is a minimum SIR threshold  260  below which system behavior will become unstable. For example, for the 4 dB value of SIR 0  from  FIG. 5 , the corresponding minimum SIR threshold  260  was determined to be approximately 9 dB (see  FIG. 6  ( 610 )). 
     The discovery of this minimum SIR threshold  260  led to a new no-transmission mode (or mode 0) for link adaptation systems. This mode 0 (cutoff threshold) is the SIR level at a link receiver  105  below which transmission should cease at the corresponding transmitter  100 . If transmissions continued below the mode 0 threshold  260 , system instability and near zero throughput would result at the link receiver  105 . Thus, transmitting below mode 0 wastes bandwidth and system resources and produces near zero throughput. This no-transmission mode is different from conventional admission control, which is performed only once upon admitting a user. Mode 0 is part of the continuous link adaptation process. 
       FIG. 7  depicts the operation of a no-transmission mode (mode 0) according to one embodiment of the present invention. At time  710 , the SIR at the link receiver  105  exceeds the cutoff threshold. Thus, at time  710 , the corresponding transmitter  100  is transmitting using the appropriate mode X  210  for the current SIR in the link adaptation system. At time  720 , the SIR at link receiver  105  falls below the cutoff threshold and the transmitter  100  enters mode 0 ending transmission. At time  730 , however, the SIR at link receiver  105  again exceeds the mode 0 cutoff threshold and the transmitter  100  begins transmitting using the appropriate mode Y  210  for the current SIR. 
       FIG. 8  is a flowchart that depicts a set of steps that may be implemented at a wireless transmitter to utilize a no-transmission mode and perform link adaptation according to one embodiment of the present invention. In step  805 , the procedure is initiated. In step  820 , a signal quality value is measured at a receiver. The signal quality value may be a SIR, BLER or any other value corresponding to the suitability of the signal for reception. In step  830 , the signal quality value is compared to a no-transmission threshold value. If the signal quality value is less than the no-transmission threshold (‘yes’ branch of step  830 ), the receiver ceases transmission to the receiver (step  840 ). Otherwise (‘no’ branch of step  830 ), link adaptation is performed. In particular, in step  850  a best link adaptation mode is selected (e.g., a mode that maximizes some performance measure such as throughput). In step  860 , a modulation and/or coding scheme is adjusted at the transmitter to conform to the best link adaptation mode selected in step  850 . The procedure ends in step  870 .

Technology Category: 5