Abstract:
A method to increase spectral efficiency in a communication system is described herein. The communication system includes at least one mobile station and is capable of transmitting messages encoded according to a plurality of available modulation coding schemes (“MCSs”). Each available MCS includes a modulation scheme and an effective coding rate. The MCSs are indexed according to increasing complexity. A signal-to-interference ratio (“SINR”) is determined which is sufficient to satisfy a predetermined frame error rate (“FER”). A first MCS and a corresponding amount of transmissions needed to satisfy the predetermined FER at the SINR using the first MCS are determined. The first MCS has a higher effective coding rate than a second MCS. The second MCS sufficiently satisfies the predetermined FER at the SINR in a single transmission. A message encoded according to the first MCS is transmitted through the communication system using hybrid automatic repeat request (“HARQ”).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Submission Under 35 U.S.C. §371 for U.S. National Stage Patent Application of International Application Number: PCT/US2009/041899, filed Apr. 28, 2009 entitled “METHOD AND APPARATUS FOR SPECTRALLY EFFICIENT LINK ADAPTATION USING HARQ IN OFDMA SYSTEMS,” which claims priority to U.S. Provisional Application Ser. No. 61/048,369, filed Apr. 28, 2008, the entirety of both which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and more specifically, to a method and system for improving throughput using spectrally efficient link adaptations with hybrid automatic repeat requests (“HARQ”) in orthogonal frequency division multiple access (“OFDMA”) systems. 
     BACKGROUND OF THE INVENTION 
     The demand for reliable and high data throughput wireless communication networks has never been as great as in the present. While initial consumer and business demand was for wireless communication technologies to support voice communication, this demand has grown both in terms of the sheer volume of users as well as the bandwidth requirements; the latter being the result of demand for wireless broadband data services. These services are provided, for example, by Fourth Generation (“4G”) wireless systems based on 3GPP Long Term Evolution (“LTE”), IEEE 802.16e WiMax, and 3GPP2 Ultra Mobile Broadband (“UMB”), each of which use orthogonal frequency division multiple access (“OFDMA”) technology as the air interface technology. 
     Considering LTE in particular, the main goals include providing peak data rates up to 100 Mbps in the downlink and up to 50 Mbps in the uplink, reduced latency, significantly improved spectrum efficiency, improved system capacity, coverage, and reasonable system &amp; terminal complexity. In order to achieve these goals, several new radio transmission technologies have been proposed. The LTE downlink uses orthogonal frequency division multiplexing (“OFDM”) as an accessing technology, while the LTE uplink uses single carrier frequency division multiple access (“SC-FDMA”). Multiple antennas at the transmitter and receiver increase the data rates as well as achieve diversity gains. By transmitting multiple parallel data streams to single terminal, data rates can be increased significantly. On the other hand, multiple input multiple output (“MIMO”) systems are used for increasing the diversity by transmitting the same symbol on different antennas. Furthermore, as in 3G systems, Adaptive Modulation and Coding (“AMC”) is used in LTE to exploit channel information. 
     In practice, link adaptation (“LA”) in AMC may fail due to inaccuracies in link estimation and the feedback delays in channel quality measurements. To recover from link adaptation errors, hybrid automatic repeat request (“HARQ”) is typically used as a retransmission mechanism. HARQ is used in wireless systems to overcome transmission errors that cannot be corrected using forward error correction (“FEC”). HARQ improves the decoding probability by using information from previous transmissions. Depending on the way the retransmission packets are combined, HARQ systems can be typically classified into two categories namely, Chase combining (“CC”) or incremental redundancy (“IR”). 
     In CC, the basic idea is to send a number of repeats of each coded data packet and allowing the decoder to combine multiple received copies of the packet before decoding. The retransmitted packet is an exact replica of the original packet. In this way, the time diversity gain can be realized. This scheme requires less implementation complexity at the receiver. 
     In IR, instead of sending simple repeats of the entire packet, additional parity information is incrementally transmitted if the decoding fails in the first attempt. Each transmission may or may not be self decodable. If each transmission is self decodable, then it is called partial IR, otherwise full IR. 
     Link adaptation decisions for a mobile communication device, i.e., mobile station (“MS”) or base station (“BS”), are based on a reference signal (“RS”) signal to interference ratio (“SINR”) estimate. Conventional link adaptation estimates the SINR from the reference signal measurements and determines the highest MCS that can be supported at a pre-determined FER using 1 HARQ transmission. 
     Link level performance curves show significant signal to noise ratio (“SNR”) gains when using HARQ-IR over HARQ-CC due to the transmission of additional parity information in the former. However, conventional LA schemes are unable to take advantage of these HARQ gains. In a multi-user environment, conventional link adaptation does not provide any spectral efficiency gains using HARQ_IR relative to using HARQ-CC. 
     Therefore, what is needed is a method and apparatus for improving the overall system performance and spectral efficiency of an OFDMA system using link adaptation schemes that can extract gains offered by HARQ. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a method, apparatus and system for improving spectral efficiency in an orthogonal frequency division multiplexing (“OFDM”) communication system. Generally, embodiments of the present invention map a requested modulation coding scheme (“MCS”) to higher order MCS and transmit messages using the higher order MCS in combination with hybrid automatic repeat request with incremental redundancy (“HARQ-IR”). 
     In accordance with one aspect of the present invention, a method is provided for increasing spectral efficiency in a communication system. The communication system includes at least one mobile station and is capable of transmitting messages encoded according to a plurality of available MCSs. Each available MCS includes a modulation scheme and an effective coding rate. The available MCSs are indexed according to increasing complexity. A signal-to-interference ratio (“SINR”) sufficient to satisfy a predetermined frame error rate (“FER”) is determined. A first MCS and a corresponding amount of transmissions needed to satisfy the predetermined FER using the first MCS are determined. The first MCS has a higher effective coding rate than the second MCS. The second MCS sufficiently satisfies the predetermined FER in a single transmission. A message, encoded according to the first MCS is transmitted through the communication system using HARQ. 
     In accordance with another aspect of the present invention, an apparatus is provided for increasing spectral efficiency in a communication system. The communication system includes at least one mobile station and is capable of transmitting messages encoded according to a plurality of available MCSs. Each available MCS has a modulation scheme and an effective coding rate. The available MCSs are indexed according to increasing complexity. The apparatus includes an MCS mapper and a transceiver. The MCS mapper is operable to determine a SINR sufficient to satisfy a predetermined FER. The MCS mapper determines a first MCS and a corresponding amount of transmissions sufficient to satisfy the predetermined FER at the SINR using the first MCS. The first MCS has a higher effective coding rate than a second MCS which is sufficient to satisfy the predetermined frame error rate in a single transmission. The transceiver is communicatively coupled to the MCS mapper. The transceiver is operable to transmit a message encoded according to the first MCS through the communication system using HARQ. 
     In accordance with yet another aspect of the present invention, a communication system includes at least one mobile station and at least one base station. The base station is communicatively coupled to the mobile station. The one base station includes an MCS mapper and a transceiver. The MCS mapper is operable to determine a SINR sufficient to satisfy a predetermined FER. The MCS mapper determines a the MCS and a corresponding amount of transmissions needed to satisfy the predetermined FER using the first MCS. The first MCS has a higher effective coding rate than a second MCS that is sufficient to satisfy the predetermined FER at the SINR in a single transmission. The transceiver is communicatively coupled to the MCS mapper. The transceiver is operable to transmit a message encoded according to the first MCS through the communication system using HARQ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a block diagram of an exemplary OFDMA communication system constructed in accordance with the principles of the present invention; 
         FIG. 2  is a block diagram of an exemplary Long Term Evolution (“LTE”) uplink channel constructed in accordance with the principles of the present invention; 
         FIG. 3  is a table of modulation and coding schemes considered for hybrid automatic repeat request (“HARQ”); 
         FIG. 4  is a graph illustrating link level performance curves for a system using modulation and coding scheme (“MCS”) index  6 , QPSK with R=3/4; 
         FIG. 5  is a flow chart of an exemplary link adaptation process according to the principles of the present invention; 
         FIG. 6  is a flowchart of an exemplary static maximum effective spectral efficiency mapping (“MESEM-S”) process according to the principles of the present invention; 
         FIG. 7  is a graph illustrating link level performance curves for a system using MCS index  11 ,  16 -QAM with R=5/6; 
         FIG. 8  is a flowchart of an exemplary dynamic maximum effective spectral efficiency mapping (“MESEM-D”) process according to the principles of the present invention; and 
         FIG. 9  is a graph illustrating MCS distribution using conventional Chase combining (“CC”) compared to dynamic maximum effective spectral efficiency mapping (“MESEM-D”) in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As an initial matter, while certain embodiments are discussed in the context of wireless networks operating in accordance with the 3rd Generation Partnership Project (“3GPP”) evolution, e.g., Long Term Evolution (“LTE”) standard, etc., the invention is not limited in this regard and may be applicable to other broadband networks including those operating in accordance with other orthogonal frequency division multiplexing (“OFDM”)-based systems including WiMAX (IEEE 802.16) and Ultra-Mobile Broadband (“UMB”), etc. Similarly, the present invention is not limited solely to OFDM-based systems and can be implemented in accordance with other system technologies, e.g., code division multiple access (“CDMA”), single carrier frequency division multiple access (“SC-FDMA”), etc. 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to improving the overall system performance and spectral efficiency of a communication system using link adaptation schemes. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     One embodiment of the present invention advantageously increases the spectral efficiency of the system by mapping a requested lower level modulation and coding scheme (“MCS”) to a higher level MCS and implementing hybrid automatic repeat request (“HARQ”) with incremental redundancy (“IR”). Overall system throughput may be increased over twenty percent (20%) by applying the principles of the present invention. 
     Referring now to the drawing figures in which like reference designators refer to like elements, there is shown in  FIG. 1 , an exemplary orthogonal frequency division multiplexing (“OFDM”) communication system  10  is provided in accordance with the principles of the present invention. Communication system  10  includes at least one base station (“BS”)  12  communicating with a plurality of mobile stations (“MS”)  14   a ,  14   b  (referenced collectively as mobile station  14 ). Although only one base station  12  and two mobile stations  14  are shown in  FIG. 1  for illustrative purposes, it should be noted that communication system  10  may include any number of base stations  12  and mobile stations  14 . 
     According to one embodiment, mobile station  14  may include a wide range of portable electronic devices, including but not limited to mobile phones, personal data assistants (“PDA”) and similar devices, which use the various communication technologies such as LTE, advanced mobile phone system (“AMPS”), time division multiple access (“TDMA”), CDMA, global system for mobile communications (“GSM”), general packet radio service (“GPRS”), 1× evolution-data optimized (abbreviated as “EV-DO” or “1xEV-DO”) and universal mobile telecommunications system (“UMTS”). The mobile station  14  also includes the hardware and software suitable to support the control plane functions needed to engage in wireless communication with base station  12 . Such hardware can include a receiver, transmitter, central processing unit, storage in the form of volatile and nonvolatile memory, and input/output devices, among other hardware. 
     Base station  12  transmits information to the mobile stations  14  using a downlink channel  16   a ,  16   b  (referred to collectively herein as downlink channel  16 ). In a similar manner, mobile stations  14  transmit information to the base station  12  using an uplink channel  18   a ,  18   b  (referred to collectively herein as uplink channel  18 ). The base station  12  may also include an MCS mapper  20 , which determines an alternative MCS for use with HARQ-IR. Available MCSs may include Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation. Operation of the MCS mapper  20  is discussed in greater detail below. Alternatively or additionally, the MCS mapper  20  may reside in mobile station  14 . 
     Referring now to  FIG. 2 , a block diagram for an exemplary uplink channel  18  provided. Uplink channel  18  is established by a mobile station  14  having a transmitter  22  with a single transmit antenna  24  and a base station  12  having a receiver  26  that uses two receive antennas  28   a ,  28   b . The transmitter  22  uses a turbo convolutional code (“TCC”) encoder  30  to encode data from the controller (not shown) with generator polynomials in octal form. The TCC encoder  30  is followed by a random interleaver  32  and a constellation mapper  34  followed by an SC-FDMA modulator  36  to map coded bits to modulated symbols. The modulated symbols are then transformed from the frequency domain to the time domain by an inverse fast Fourier transform (“IFFT”)  38  and a cyclic-prefix (“CP”)  40  is added before the resulting SC-FDMA signal is transmitted through the antenna  24 . 
     At the receiver  26  side, the inverse process is followed. Two receive antennas  28   a ,  28   b  receive SC-FDMA signals. A CP remover  42   a ,  42   b  strips the CP from the signal, which is then converted from the time domain to the frequency domain via a fast Fourier transform  44   a ,  44   b . A minimum mean square error (“MMSE”) detector  46  performs frequency domain equalization and is followed by IFFT  47  to recover time domain symbol. This is followed by random de-interleaver  48  and a turbo decoder  50 . The de-interleaver  48  is used to compensate for the interleaving operation used at the transmitter  22 . Decoded data from the turbo decoder  50  is sent on to the base station  12  controller (not shown) for further processing. 
     Referring now to  FIG. 3 , a table 52 is provided showing the modulation coding schemes and effective coding rates per transmission for use with HARQ-IR. The data packets are generated from an R=1/3 turbo code. In table 52, Tx denotes transmission iteration. The parity bits are punctured such that different code rates can be generated. The effective code rate decreases with each transmission for IR, while for CC, the effective code rate remains same in successive retransmissions as there is no new information. For example, MCS index  11  (16 QAM) has an effective code rate of 5/6 on the first transmission which includes very few parity bits. On the second transmission, the effective code rate decreases to 5/12 as some new parity bits are included. By the third transmission, all the remaining parity bits are transmitted and the effective rate is back to 1/3. It should be noted that for IR, once all the parity is transmitted, the process repeats and essentially becomes CC. 
       FIG. 4  illustrates an exemplary link level curve  54  for an AWGN channel for MCS index  6  (QPSK with R=3/4). As can be seen from link level curve  54 , a SINR of approximately 4.5 dB is required to achieve a 10% FER on the first transmission using QPSK with R=3/4. Systems using HARQ can use the same MCS for lower SINR conditions by allowing for subsequent re-transmissions. As shown in  FIG. 4 , the needed SINR for a 10% FER drops to less than 0.5 dB on the second transmission using HARQ-IR. 
     Referring now to  FIG. 5 , an exemplary operational flowchart is provided that describes steps to increase the spectral efficiency of link adaptation algorithms that use HARQ-IR. The process begins by determining the modulation coding scheme (MCS old ) needed to transmit a message meeting a predetermined 10% frame error rate (“FER”) in a single transmission (step S 102 ). Generally, the MS measures the SINR of its surrounding environment and requests a certain MCS. The needed MCS is generally determined by comparing the received SINR to a set of predetermined link level curves, such as those provided in  FIGS. 4 and 7 . 
     The MCS mapper  20  changes the requested MCS to a new MCS (MCS new ) having a higher coding rate (step S 104 ) and the message is transmitted using MCS new  and HARQ-IR (step S 106 ). Embodiments of the present invention may use one of three methods to determine MCS new , as described in detail below. 
     One embodiment of the present invention uses a selective aggressive mapping (“SAM”) mechanism to map the requested MCS to the highest coding rate available for the selected modulation scheme. In other words, for the modulation coding schemes of table 52 (See  FIG. 3 ), MCS index  3  (QPSK with R=1/3) maps to MCS index  6 , MCS index  7  (16-QAM with R=3/7) maps to MCS index  11  (16-QAM with R=5/6), and so on. Table 1 shows the MCS mappings resulting from SAM for table 52 from  FIG. 3 . An exception to the general rule is made for MCS indices  1  and  2  to protect against any errors occurring as a result of the rate increase as mobile devices requesting these MCS schemes are already experiencing poor SINR conditions. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Old MCS 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
               
               
                   
                   
               
             
          
           
               
                 New 
                 1 
                 2 
                 6 
                 6 
                 6 
                 6 
                 11 
                 11 
                 11 
                 11 
                 11 
                 14 
                 14 
                 14 
               
               
                 MCS 
               
               
                   
               
             
          
         
       
     
     Referring now to  FIG. 6 , an exemplary operational flowchart is provided that describes steps to increase the spectral efficiency of link adaptation algorithms that use HARQ-IR using a static maximum effective spectral efficiency mapping (“MESEM-S”). For the following process, “i” denotes the MCS index as table 52 (See  FIG. 3 ). The process begins with the first MCS listed in table 52, i.e., i=1 (step S 108 ). Using the link level curves for MCS i , the SINR required to achieve a 10% FER in the first transmission is determined (step S 110 ). Generally, the MS requests a certain MCS level and the BS “reverse engineers” the required SINR (denoted as SINR i ) from the link level curves. A second index, denoted as “j”, is used to compare higher MCS schemes (MCS j ). Beginning at one level higher than MCS i , i.e., j=i+1 (step S 112 ), the number of transmissions “n” needed by MCS j  to achieve the same FER as obtained by MCS i  (step S 114 ). If the desired FER level cannot be reached within a predetermined maximum number of retransmissions, n max  (step S 115 ), then the current MCS j  is not a suitable candidate for further consideration and is discarded (step S 116 ). In one embodiment, n max =6. 
     An exemplary link level graph for MCS index  11 ,  16 -QAM with R=5/6, is provided in  FIG. 7 . As can be seen by comparing  FIG. 4  to  FIG. 7 , higher coding rates require higher SINR rates to achieve the same FER rate. For example, as shown in  FIG. 4 , an SINR vale of 4.5 dB is required for a 10% FER on the first transmission using MCS 6 , but this value is increased to about 9.2 dB using MCS 11 . However, using two transmissions, MCS 11  needs only about 3.4 dB to achieve the same results. 
     If the number of transmissions “n” needed by MCS j  to achieve the desired FER rate is less than or equal to n max , the MCS mapper  20  calculates an effective spectral efficiency (ESE j ) value for MCS j  (step S 116 ) according to the equation: 
                   ESE   =       code   ⁢           ⁢   rate   ×   modulation   ⁢           ⁢   factor       number   ⁢           ⁢   of   ⁢           ⁢   transmission   ⁢           ⁢   attempts               (   1   )               
where the modulation factor is number of bits per M-ary QAM constellation, e.g., 2 bits for QPSK, 4 bits for 16-QAM and 6 bits for 64-QAM. If an ESE has not been determined for all the higher possible MCS schemes for the link adaptation, i.e. j&lt;j max  (step S 118 ), then j is incremented (step S 120 ) and an ESE j  is determined for all values of j (steps S 114  thru S 117 ).
 
     For example, assuming MCS i =5 (i.e. QPSK with R=2/3), then assuming 2 transmissions are required for MCS 6  to achieve a 10% FER, then 
               ESE   6     =         2   *     3   /   4       2     =     0.75   .             
Table 2 illustrates all ESE values generated for all values of j&gt;5. In this case, the required SINR=3.5 dB. With MCS  12 - 14 , even 6 transmissions are not sufficient to provide 10% FER at 3.5 dB. So MCS  12 ,  13  and  14  are not suitable candidates for further ESE comparison.
 
                                                                           TABLE 2               MCS   6   7   8   9   10   11                                Retx.ct   2   2   3   3   3   4       ESE   0.75   0.858   0.666667   0.296296   1   0.833                    
The MCS scheme having the highest ESE j  is selected as the new MCS scheme (MCS new ) (step S 122 ) and mapped to MCS i  (step S 124 ). As can be seen from Table 2, MCS 10  has the highest ESE when determining a new MCS for MCS 5 , thus MCS 5  is mapped to MCS 10 .
 
     This process is repeated for each index i value of the link adaptation. In other words, if an MSC new  has not been determined for all the MCS schemes for the link adaptation, i.e. i&lt;i max  (step S 126 ), then i is incremented (step S 128 ) and the process repeated until all the MCS indices are covered. Simulated results using MESEM-S are shown below in Table 3. In Table 3, old MCS corresponds to the MCS with conventional LA scheme and new MCS corresponds to the MSC selected after MESEM-S mapping. It should be kept in mind that in this scheme the table is generated only once and is not altered after that. Hence for practical applications, a pre-determined table is computed and stored based on the turbo interleaver length. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Old MCS 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
               
               
                   
                   
               
             
          
           
               
                 New 
                 1 
                 5 
                 6 
                 10 
                 10 
                 11 
                 10 
                 11 
                 14 
                 14 
                 14 
                 13 
                 14 
                 14 
               
               
                 MCS 
               
               
                   
               
             
          
         
       
     
     Referring now to  FIG. 8 , an exemplary operational flowchart is provided that describes steps to increase the spectral efficiency of link adaptation algorithms that use HARQ-IR using a dynamic maximum effective spectral efficiency mapping (“MESEM-D”). In this scheme, the mapping is based on instantaneous received SINR. The environmental SINR est  is estimated by the MS or the BS (step S 128 ). The index value indicating the number of intended transmission, denoted as “r”, is set equal to 1 (step S 130 ). Using the link level curves, the highest MCS index value MCS r  that can satisfy satisfy the required FER as SINR est  is determined (step S 132 ). The MCS mapper  20  calculates a corresponding ESE r  for MCS r  (step S 134 ) according to Eq. (1). If an ESE has not been determined for all additional transmission indices r, i.e. r&lt;r max  (step S 136 ), then r is incremented (step S 138 ) and an ESE r  is determined for all values of r (steps S 132  and S 134 ). It should be observed that this algorithm determines the MCS that maximizes the ESE based on a search as opposed to a static table mapping. This process may be repeated each time an MS estimates the SINR. 
     Simulation results, as provided in Table 4, indicate a significant gain in average sector throughput using the embodiments of the present invention, i.e. SAM, MESEM-S and MESEM-D, as compared to conventional link adaptation processes, e.g., Chase and IR. It should be noted that even though link level curves indicate potential gains using IR, there is no gain in sector throughput in using IR as compared to CC with the conventional LA scheme. Using the embodiments of the present invention, it can be seen that huge gains in throughput may be achieved as compared to CC techniques. It can be seen that MESEM-D outperforms all the remaining schemes in average sector throughput as well as FER outage. It should also be noted that MESEM-D provides additional gain as compared to MESEM-S or SAM due to the exhaustive search based on current SINR, while MESEM-S maps the conventional MCS to a more statically spectrally efficient MCS. Even though SAM it not based on environmentally measured values, an almost 18% in throughput may be achieved. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Average Sector 
                 % FER 
                   
               
               
                   
                 HARQ Scheme 
                 Throughput (Kbps) 
                 outage 
                 % Gain 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Chase-Baseline 
                 6832 
                 0.51 
                 — 
               
               
                   
                 IR-Baseline 
                 6855 
                 0.8 
                  0.34 
               
               
                   
                 MESEM-S 
                 8233 
                 0.8 
                 20.36 
               
               
                   
                 MESEM-D 
                 8373 
                 0.7 
                 22.56 
               
               
                   
                 SAM 
                 8023 
                 0.8 
                 17.43 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 9  provides a graph  58  illustrating the MCS distribution for conventional link adaptation and MESEM-D. It can be observed that at the lowest MCS scheme, the distribution is the same for both. However, all the lower MCS schemes for MESEM-D, i.e. index&gt;1, are mapped to the more spectrally efficient MCS schemes. It can be observed that because of this MCS mapping, significant gains in throughput are achieved, as displayed in Table 4. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.