Abstract:
Method and embodiments in a multipath wireless communication system employing a wireless frame having alternating cyclic prefixes to reduce inter-symbol interference (ISI) are presented herein.

Description:
BACKGROUND 
   In wireless communications systems, multipath is one of many concerns that may affect performance of a wireless communication. Multipath, for instance, may cause signals from a previous symbol to interfere with signals from a subsequent symbol, which is know as inter-symbol interference (ISI). Although traditional techniques have been developed to combat ISI, these techniques significantly reduce the amount of bandwidth that is available to communicate over a wireless medium. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different instances in the description and the figures may indicate similar or identical items. 
       FIG. 1  is an illustration of an environment in an exemplary implementation that is operable to employ techniques to transmit frames wirelessly having alternating cyclic prefixes. 
       FIG. 2  is an illustration of one or more wireless frames in an exemplary implementation having alternating cyclic prefixes. 
       FIG. 3  is a flow diagram depicting a procedure in an exemplary implementation in which data having a plurality of symbols is modulated to be transmitted using alternating cyclic prefixes and demodulated by computing a virtual cyclic prefix for symbols that do not have associated cyclic prefixes. 
       FIG. 4  is a flow diagram depicting a procedure in an exemplary implementation in which symbols are demodulated through estimation of inter-symbol interference (ISI) due to each of a plurality of symbols included in data that is wirelessly transmitted. 
       FIG. 5  is a flow diagram depicting a procedure in an exemplary implementation in which inter-symbol interference is estimated of a symbol having an associated cyclic prefix. 
       FIG. 6  is a flow diagram depicting a procedure in an exemplary implementation in which inter-symbol interference is estimated of a symbol that does not have an associated cyclic prefix. 
   

   DETAILED DESCRIPTION 
   In the following discussion, an exemplary environment is first described that is operable to perform techniques to modulate, transmit, receive and demodulate a frame having alternating cyclic prefixes Exemplary procedures are then described that may be employed in the exemplary environment, as well as in other environments. 
   Exemplary Environment 
     FIG. 1  is an illustration of an environment  100  in an exemplary implementation that is operable to employ alternating cyclic prefix techniques. The illustrated environment  100  includes a client that is communicatively coupled, wirelessly, to a base station  104  that is configured to provide internet access and another client  108 . The client  102  may be configured in a variety of ways, such as a traditional desktop computer as illustrated, as wireless phone, a game console, a personal digital assistant, a laptop as illustrated for the other client  108 , and so on. The client  102 , in portions of the following discussion, may also relate to a person and/or entity that operate the client. In other words, the clients  102  may describe logical clients that include users, software, and/or devices. 
   The client  102  as illustrated includes a processor  110 , memory  112  a display device  114  and a network connection device  116 . Processors are not limited by the materials from which they are formed or the processing mechanisms employed therein. For example, processors may be comprised of semiconductor(s) and/or transistors (e.g., electronic integrated circuits (ICs)). In such a context, processor-executable instructions may be electronically-executable instructions. Although a single memory  112  is shown, a wide variety of types and combinations of memory may be employed; such as random access memory (RAM), hard disk memory, removable medium memory, and other types of computer-readable media. Further, although a display device  114  illustrated as a monitor is shown, the display device may assume a variety of configurations to output data. 
   The network connection device  116  is configured to provide wireless communication through use of a transmitter  118 , receiver  120  and a communication module  122 . The network connection device  116  may be configured to be included in a variety of systems, such as a single or multi-antenna system that may include a variety of types of antennas, such as dipole antennas. The communication module  122  is representative of functionality that is usable to manage wireless communication performed by the network connection device  116 . For example, the communication module  122  may function to modulate signals to be transmitted by the transmitter  118  and to demodulate signals received by the receiver  120 . The network connection device  116 , for instance, may form a wireless network connection with the other client  108  that also has a network connection device  124  having a communication module  126 , transmitter  128  and receiver  130 . Therefore, the client  102  and the other client  108  may communicate back and forth wirelessly through use of the respective network connection devices  116 ,  124  which may be configured in accordance with a variety of protocols and standards. 
   As previously described, however, multipath is one of many concerns that may affect wireless communication performance. For example, multipath may cause previous signals to interfere with “current” signals. Although traditional techniques have been developed to combat multipath, these techniques significantly reduce the amount of bandwidth that is available to communicate over a wireless medium. One such previous technique involved the incorporation of a cyclic prefix with each symbol being wirelessly transmitted, and therefore reduced the amount of bandwidth available over the connection that was consumed by the cyclic prefix. 
   Accordingly, the communication modules  122 ,  126  in the environment  100  of  FIG. 1  may incorporate techniques in which alternating cyclic prefixes are incorporated within wireless frames transmitted and received by the communication modules  122 ,  126 . Therefore, the amount of bandwidth consumed by the cyclic prefixes is reduced, thereby improving the amount of bandwidth available to communicate data as desired by the client yet may still preserve the integrity of the data, further discussion of which may be found in relation to the following figure. 
     FIG. 2  depicts an exemplary implementation of a frame structure  200  having alternating cyclic prefixes. The frame structure  200  includes a plurality of symbols  202 ( 1 ),  202 ( 2 ),  202 ( 3 ),  202 ( 4 ),  202 (S),  202 (S+1). The symbols  202 ( 1 )- 202 (S+1) represent data to be transmitted by or received from a client, such as eight bit representations of characters and so on. Cyclic prefixes are associated with alternating symbols, such that symbol  202 ( 1 ) has an associated cyclic prefix  204 ( 1 ), symbol  202 ( 3 ) has an associated cyclic prefix  204 ( 3 ), symbol  202 (S) has an associated cyclic prefix  204 (S), and so on. 
   As shown in the frame structure of  FIG. 2 , however, symbols  202 ( 2 ),  202 ( 4 ),  202 (S+1) do not have associated cyclic prefixes, which makes additional bandwidth available in the frame structure  200  over traditional techniques. In order to correctly demodulate the symbols, techniques may be applied to account for the lack of a cyclic prefix for each symbol. 
   For example, the communication modules  122 ,  126  may incorporate techniques to estimate inter-symbol interference due to each of the symbols and use the estimates to correctly demodulate the symbols. In effect, these techniques may facilitate a virtual cyclic prefix  206 ( 2 ),  206 ( 4 ) for respective symbols  202 ( 2 ),  202 ( 4 ) that are transmitted without a cyclic prefix. Further discussion of these techniques may be found in relation to the following procedures. 
   Generally, any of the functions described herein can be implemented using software, firmware, hardware (e.g., fixed logic circuitry), manual processing, or a combination of these implementations. The terms “module,” “functionality,” and “logic” as used herein generally represent software, firmware, hardware, or a combination thereof. In the case of a software implementation, for instance, the module, functionality, or logic represents program code that performs specified tasks when executed on a processor (e.g., CPU, CPUs, a processor of a network connection device  116 , and so on). The program code can be stored in one or more computer readable memory devices, e.g., memory within the network connection device  116 . Thus, although the processor  110  and memory  112  are illustrated as “outside” the network connection device  116  in client  102 , the processor  110  and memory  112  and even display device  114  (e.g., indication lights) may be incorporated within the network connection device  116 . The features of the techniques to alternating cyclic prefix in wireless frames described below are platform-independent, meaning that the techniques may be implemented on a variety of commercial computing platforms having a variety of processors. 
   Exemplary Procedures 
   The following discussion describes modulation and demodulation techniques that may be implemented utilizing the previously described systems and devices. Aspects of each of the procedures may be implemented in hardware, firmware, or software, or a combination thereof. The procedures are shown as a set of blocks that specify operations performed by one or more devices and are not necessarily limited to the orders shown for performing the operations by the respective blocks. In portions of the following discussion, reference will be made to the environment  100  of  FIG. 1  and the one or more frames  200  of  FIG. 2 . 
     FIG. 3  depicts a procedure in an exemplary implementation in which data having a plurality of symbols is modulated to be transmitted using alternating cyclic prefixes and demodulated by computing a virtual cyclic prefix for symbols that do not have associated cyclic prefixes. Data having a plurality of symbols is received (block  302 ), such as received by a communication module  122  from an application being executed on the processor  110 , which is storable in memory  112 . 
   A cyclic prefix is formed for alternating symbols (block  304 ). As shown in  FIG. 2 , for instance, a cyclic prefix  204 ( 1 ) may be formed for symbol  202 ( 1 ), a cyclic prefix  204 ( 3 ) formed for symbol  202 ( 3 ), and so on. The cyclic prefixes may be configured in a variety of ways, such as a repetition of data contained within the associated symbol, and so on. 
   The data is then transmitted in one or more frames having the symbols with alternating cyclic prefixes (block  306 ). Thus, as least one of the symbols is not associated with a cyclic prefix, e.g., symbols  202 ( 2 ),  202 ( 4 ). 
   The one or more frames having a cyclic prefix associated with alternating symbols is received (block  308 ). The symbols having the associated cyclic prefixes are demodulated, such as through estimation of inter-symbol interference which is described in greater detail in  FIG. 5 . 
   The symbols which do not having an associated cyclic prefix are demodulated by computing a virtual cyclic prefix for the respective symbols (block  312 ). The virtual cyclic prefix, for instance, may be computed as a part of a previously transmitted symbol, further discussion of which may be found in relation to  FIG. 6 . Therefore, the cyclic prefixes may then be removed and the data output (block  314 ), such as to an application that consumes the data that is executing on the processor of the client. A variety of techniques may be used to demodulate frames having alternating cyclic prefixes, such as through calculation of inter-symbol interference which is described in greater detail in the following example. 
     FIG. 4  depicts a procedure in an exemplary implementation in which symbols are demodulated through estimation of inter-symbol interference (ISI) due to each of a plurality of symbols included in data that is wirelessly transmitted. As previously described, techniques may be employed in a wireless network in which, a cyclic prefix of each alternate symbol in a frame (e.g., an orthogonal frequency division multiplexing (OFDM) frame) is not transmitted, thereby reducing bandwidth loss. A variety of techniques may be used to demodulate frames having symbols which do not have an associated cyclic prefix. 
   In the following exemplary technique, inter-symbol interference (ISI) affecting each symbol may be estimated and the ISI estimates used to correctly demodulate the symbols. In effect, this technique may facilitate a virtual cyclic prefix for symbols transmitted without a cyclic prefix. The alternate symbols having cyclic prefixes preserve channel equalization simplicity, such as used in typical cyclic prefix/orthogonal frequency division multiplexing (CP/OFDM). As should be appreciated, these techniques are not limited to single antenna system and may also be employed by multi-antennal systems. 
   For purposes of the following discussion, the frequency domain vectors of length N are defined as follows:
         X m  is defined as the m th  transmitted symbol;   H represents a diagonal matrix of channel coefficients;   Y m  relates to an “m th ” received symbol;   Y mCP  is defined as the cyclic prefix of the “m th ” received symbol;   Y mISI  represents ISI due to the “m th ” symbol;   where, N is the total number of subcarriers per symbol; and   the estimated value for any vector a is denoted by â.       

   Reference will now be made again to  FIG. 2 , if “M” is the number of transmitted symbols in a given frame, then each symbol transmitted with a cyclic prefix is received as expressed by the following equation:
 
 Y   m   =Y   mISI   +Y′   m   =HX   m  where,  m =1, 3, . . . M,   Equation (1)
 
In the above equation, Y′ m  represents a symbol without ISI, H is the channel as previously described and X m  is the symbol. It should be noted that H is rendered diagonally by the use of cyclic prefix.
 
   If X 1  is known (e.g., by preamble or another known sequence) then a diagonal matrix of channel coefficients H can be estimated (block  402 ) from the following equation: 
   
     
       
         
           
             
               
                 
                   H 
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                     Y 
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                     X 
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                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   2 
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   Using channel estimate Ĥ it is possible to estimate symbols transmitted with cyclic prefixes for each of the alternate frames (block  404 ), e.g., m=1, 3, . . . M, as shown by the following equation: 
   
     
       
         
           
             
               
                 
                   
                     X 
                     ^ 
                   
                   m 
                 
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                     Y 
                     m 
                   
                   
                     H 
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                 Equation 
                 ⁢ 
                 
                     
                 
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                   ( 
                   3 
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   For symbols transmitted without a cyclic prefix (e.g., the “other” alternate symbols, such as where m=2, 4, . . . , M−1), the symbols may be estimated as follows:
 
 Y   m   =Y   (m−1)ISI   +Y′   m    Equation (4)
 
In the above equation, Y (m−1)ISI  is the ISI from the previous symbol (block  406 ).
 
   In order to use Ĥ to equalize the m th  received symbol Y m  to obtain an estimate of X m , the effects of a cyclic prefix for a symbol that has been transmitted without one may be introduced. As the cyclic prefix shields a symbol from the ISI due to the previous symbol, the ISI from the previous (m−1) th  symbol Y (m−1)ISI  is removed from Y m . Further, the cyclic prefix by itself adds ISI to its corresponding symbol. As a result, the m th  symbol cyclic prefix ISI Y mISI  is added to Y m . Y (m−1)ISI  and Y mISI  may be estimated using knowledge of the estimated channel coefficients and the already equalized symbols that were transmitted with a cyclic prefix. Hence, by removing the previous symbol ISI and introducing the cyclic prefix ISI, a “virtual cyclic prefix” is introduced for a symbol that has been transmitted without one (block  408 ). As a result:
 
 Y   m   +Y   mISI   −Y   m−1)ISI   =Y′   m   +Y   mISI   =HX   m    (from equation (1))
 
This implies the following:
 
                   X   m     =         Y   m     +     Y   mISI     -     Y       (     m   -   1     )     ⁢   ISI         H             Equation   ⁢           ⁢     (   5   )                 
By estimating and using the ISI components Ŷ mISI  and Ŷ (m−1)ISI  in Equation (5), X m  can be estimated:
 
                     X   ^     m     =         Y   m     +       Y   ^     mISI     -       Y   ^         (     m   -   1     )     ⁢   ISI           H   ^               Equation   ⁢           ⁢     (   6   )                 
As a result, the channel matrix is once again rendered diagonal allowing equalization for those symbols transmitted without a cyclic prefix. In order to estimate Y (m−1)ISI  and Y mISI  in this implementation, however, {circumflex over (X)} m−1  and {circumflex over (X)} m+1  are first obtained. Hence, if the “mth” symbol X m  has been transmitted without a cyclic prefix then X m−1  and X m+1  (which are both transmitted with cyclic prefixes) are demodulated before X m  by Equation (3).
 
     FIG. 5  depicts a procedure in an exemplary implementation in which inter-symbol interference is estimated of a symbol having an associated cyclic prefix. Time domain channel estimate samples ĥ are obtained by taking the Inverse Discrete Fourier Transform (IDFT) of a channel estimate Ĥ (block  502 ) where k is a frequency domain sample, n is a time domain index, with L representing a length of the channel (most significant time domain channel taps), as expressed in the following equation:
   ĥ ( n )=Σ k=0   N−1   Ĥ ( k ) e   j2πnk/N  where,  n =0, 1 , . . . L− 1   Equation (7) 
   After estimating X m  from the channel estimate samples (e.g., by equation (3)) (block  504 ), the time domain coefficients of the “mth” symbol x m  are obtained from frequency domain samples of the “mth” symbol estimate (block  506 ), which may be expressed as follows:
 
 {circumflex over (x)}   m ( n )=Σ k=0   N−1   {circumflex over (X)}   m ( k ) e   j2πnk/N  where,  n =0, 1 , . . . N− 1   Equation (8)
 
   A number of samples in a previous symbol responsible for inter-symbol interference are computed (block  508 ). For example, the last L samples in the symbol responsible for ISI (represented as x′ m ) may be computed using the following equation:
 
 x′   m ( n )= {circumflex over (x)}   m ( n+N−L ) where,  n =0, 1 , . . . L− 1   Equation (9)
 
In the above equation, “N” is, a length of the symbol and “L” is a length of a channel.
 
   The number of samples in the previous symbol responsible for inter-symbol interference is convolved with a time domain channel estimate to obtain a time domain channel affected signal. (block  510 ). Continuing with the previous example, the last L samples in the symbol responsible for ISI x′ m  is convolved with the time domain channel estimate ĥ to obtain a time domain channel affected signal, which may be expressed as follows:
 
 y′   m ( n )=Σ p=0   L−1   x′   m ( p ) ĥ ( n−p ) where,  n =0, 1, . . . 2 L− 1   Equation (10)
 
   A time domain ISI introduced is calculated from the time domain channel affected signal (block  512 ), such as through use of the following exemplary equation: 
   
     
       
         
           
             
               
                 
                   
                     
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                     mISI 
                   
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                   11 
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   The frequency domain coefficients of the ISI due to a symbol transmitted with a cyclic prefix are obtained (block  514 ), such as by taking N point Discrete Fourier Transform of the time domain ISI calculated in equation (11) as follows:
 
 Ŷ   mISI ( k )=Σ n=0   N−1   ŷ   mISI ( n ) e   j2πnk/N  where,  k =0, 1 , . . . N− 1   Equation (12)
 
In this way, inter-symbol interference is estimated of a symbol having an associated cyclic prefix, which may then be used to estimate inter-symbol interference for symbols that do not have a cyclic prefix, an example of which is shown in the following procedure.
 
     FIG. 6  depicts a procedure in an exemplary implementation in which inter-symbol interference is estimated of a symbol that does not have an associated cyclic prefix. For example, a client may receive a first symbol, a second symbol that does not have a cyclic prefix and a third symbol having an associated cyclic prefix (block  602 ). The first and third symbols, for instance, may have cyclic prefixes, whereas the second does not and therefore the cyclic prefixes are “alternating”. For example, if the mth symbol is transmitted without a cyclic prefix, then the (m+1)th symbol will have a cyclic prefix. Therefore, the third symbol having the associated cyclic prefix is estimated (block  604 ). In this case, {circumflex over (X)} m+1 =Y m+1 /Ĥ may be computed using equation (3) before equalization of Y m , which may be represented as follows:
   Y   (m+1)CP   =Y   mISI   +Y′   (m+1)CP    Equation (13) 
   The cyclic prefix of the received (m+1)th symbol (i.e., the third symbol having the cyclic prefix) is estimated without inter-symbol interference from the mth (i.e., second) symbol (block  606 ). For example, if N CP  is the length of the cyclic prefix, then x′ (m+1)CP  may be defined as the last N CP  samples in the (m+1)th symbol estimate which form the cyclic prefix. Therefore, the following equation may be used to compute cyclic prefix samples without inter-symbol interference from equation (8) above:
 
 {circumflex over (x)}   (m+1)CP ( n )= {circumflex over (x)}   M+1 ( n+N−N   CP ) where,  n =0, 1, . . .  N   CP −1   Equation (14)
 
   A channel affected cyclic prefix in time domain is computed from the cyclic prefix estimate without inter-symbol interference (block  608 ), which may be represented as follows:
 
 ŷ′   (m+1)CP ( n )=Σ p=0   L−1   {circumflex over (x)}′   (m+1)CP ( p ) ĥ ( n−p ) where,  n =0, 1 , . . . N   CP −1   Equation (15)
 
   By taking an “N” point Discrete Fourier Transform of the channel affected cyclic prefix in the time domain, the frequency domain coefficients of the cyclic prefix of the (m+1)th symbol (i.e., the third symbol) are obtained which has not been affected by ISI (block  610 ), as shown in the following expression:
 
 Ŷ′   (m+1)CP ( k )=Σ n=0   N−1   ŷ′   (m+1)CP ( n ) e   −j2πnk/N  where,  k =0, 1 , . . . N− 1   Equation (16)
 
   The ISI due to the mth symbol (i.e., the second symbol) may then be estimated from the estimation of the third symbol having the associated cyclic prefix (from equation (13)) and the frequency domain coefficients of the cyclic prefix of the third symbol (from equation (16)) (block  612 ), which may be represented as follows:
 
 Ŷ   mISI   =Y   (m+1)CP   −Ŷ′   (m+1)CP    Equation (17)
 
   The estimate of X m  may then be obtained by using the estimate of the ISI due to the mth symbol (the second symbol from equation (17)) and the estimate of the frequency domain coefficients of the ISI due to a symbol transmitted with a cyclic prefix, which may be thought of as the “m−1” symbol (i.e., the first symbol) and may be computed through equation (12) as previously described (block  614 ), such as through substitution in equation (6). Thus, in this way a virtual cyclic prefix for the second symbol may be computed, thereby reducing the effects of multipath on the second symbol even though the second symbol does not have a traditionally associated cyclic prefix. As previously described, it should be readily apparent that these techniques are also applicable to multi-antenna systems. 
   CONCLUSION 
   Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claimed invention.