Abstract:
Methods and apparatus for use in a multi-band OFDM wideband transmission systems are disclosed. A frame of source data is mapped by a transmitter for transmission using a first mapping. The frame of source data is then mapped by the transmitter for retransmission using a second mapping to increase frequency diversity. A receiver may identify source data that experiences fading and communicate the tone/frequency on which the fading occurred to the transmitter so that the transmitter may map the source data that experienced fading during transmission to another tone/frequency for retransmission.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of wireless communication and, more particularly, to enhanced communication systems with improved symbol spreading to improve frequency diversity. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wireless personal area networks (WPANs) provide wireless short-range connectivity for electronic devices such as audio/video devices within a home. The Institute of Electrical and Electronics Engineers (IEEE) 802.15 High Rate Alternative PHY Task Group (TG3a) for WPAN is working to develop a higher speed physical (PHY) layer enhancement to IEEE proposed standard P802.15.3™—Draft Standard for Telecommunications and Information Exchange Between Systems (referred to herein as the proposed IEEE standard). Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) has been proposed for the IEEE standard due to its spectrally efficiency, inherent robustness against narrowband interference, and robustness to multi-path fading, which allows a receiver to capture multi-path energy more efficiently. 
         [0003]      FIG. 1  illustrates the MB-UWB frequency spectrum. In MB-UWB, the UWB frequency spectrum, which covers 7.5 GHz in the 3.1 GHz to 10.6 GHz frequency band, is divided into 13 bands, which each occupy 528 MHz of bandwidth. Each band includes 128 sub-carriers of 4 MHz bandwidth. Information is transmitted using OFDM modulation on each band. MB-UWB may be coded such that information bits are interleaved across various bands to exploit frequency diversity and provide robustness against multi-path interference. MB-OFDM, however, does not offer sufficient frequency diversity for higher code rates. Typical techniques to increase frequency diversity in MB-OFDM systems often have a relatively high level of complexity, which adds to the cost of implementing such techniques. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is embodied in methods, apparatus, and computer program products for transmission of data in multi-band OFDM wideband systems. In accordance with the present invention, a frame of source data is mapped by a transmitter for transmission using a first mapping. The frame of source data is then mapped by the transmitter for retransmission using a second mapping to increase frequency diversity. A receiver may identify source data that experiences fading and communicate the tone/frequency on which the fading occurred to the transmitter so that the transmitter may map the source data that experienced fading during transmission to another tone/frequency for retransmission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings, with like elements having the same reference numerals. When a plurality of similar elements are present, a single reference numeral may be assigned to the plurality of similar elements with a small letter designation referring to specific elements. When referring to the elements collectively or to a non-specific one or more of the elements, the small letter designation may be dropped. The letter “n” may represent a non-specific number of elements. Included in the drawings are the following figures: 
           [0006]      FIG. 1  is a graph of a prior art MB-OFDM frequency spectrum; 
           [0007]      FIG. 2  is a timing diagram depicting an acknowledgment and retransmission example for use in describing an aspect of the present invention; 
           [0008]      FIG. 3  is a block diagram of an exemplary transmitter in accordance with an aspect of the present invention; 
           [0009]      FIG. 4  is a block diagram of an exemplary receiver in accordance with an aspect of the present invention; 
           [0010]      FIG. 5  is a block diagram of an alternative exemplary transmitter in accordance with an aspect of the present invention; 
           [0011]      FIG. 6  is a block diagram of an alternative exemplary transmitter in accordance with an aspect of the present invention; 
           [0012]      FIG. 7  is a block diagram of an alternative exemplary receiver in accordance with an aspect of the present invention; 
           [0013]      FIG. 8  is a timing diagram depicting a symbol retransmission scheme in accordance with an aspect of the present invention; and 
           [0014]      FIG. 9  is a flow chart of exemplary transmission system steps in accordance with an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention is described with reference to the Open Systems Interconnection (OSI) reference model to facilitate description. The OSI reference model sets forth layers present in electronic devices, such as a WPAN compatible electronic devices, to process messages communicated over a network. The OSI reference model includes a physical (PHY) layer, a data-link layer, a network layer, a transport layer, a session layer, a presentation layer, and an application layer. A message originating at a first electronic device for delivery to a second electronic device passes from the application layer of the first electronic device through each layer to the PHY layer, which communicates the message over the network, i.e., a wireless network in a WPAN system. The second electronic device receives the message through its PHY layer and the message is processed through each layer of the second electronic device to retrieve the message from the first electronic device. The data-link layer includes a media access control (MAC) layer and a logical link control layer. 
         [0016]    In an exemplary embodiment, the present invention may be implemented as an enhancement to communication systems in accordance with the proposed IEEE standard. The proposed IEEE standard uses a hybrid automatic repeat request (HARQ) scheme to deal with unreliable channel conditions. The HARQ scheme employs a conventional automatic repeat request (ARQ) scheme together with a forward error correction (FEC) technique. If an error is detected, e.g., through a cyclic redundancy check (CRC), the receiving electronic device (herein receiver) requests that the transmitting electronic device (herein transmitter) resend the erroneously received data packets. 
         [0017]    Receivers may send acknowledgement messages to transmitters to indicate whether received frames are correctly received and/or demodulated. Acknowledgment type is a function of the MAC layer. There are three acknowledgement types defined for a MB-OFDM MAC layer: no acknowledgment (no-ACK), immediate acknowledgement (Imm-ACK), and delayed acknowledgement (Dly_ACK). The type of acknowledgement is indicated by setting an acknowledgment policy field in a broadcast and multicast addressed frame upon transmission. 
         [0018]    A transmitted frame with an acknowledgement policy field set to indicate no acknowledgment (no-ACK) is not acknowledged by the receiver. The transmitter assumes that the transmitted frame is successful for all its local management entities and proceeds to the next frame scheduled for transmission. 
         [0019]    A transmitted frame with an acknowledgement policy field set to indicate immediate acknowledgment (Imm-ACK) is acknowledged by the receiver upon receipt. The receiver may acknowledge receipt of the transmitted frame by transmitting an acknowledgment frame back to the transmitter indicating that the transmitted frame was received. 
         [0020]    A transmitted frame with an acknowledgement policy field set to indicate delayed acknowledgment (Dly-ACK) is acknowledged by the receiver when requested by the transmitter. The receiver may acknowledge receipt of one or more transmitted frames concurrently by transmitting an acknowledgment frame back to the transmitter indicating that those transmitted frames were received. A delayed acknowledgment schedule (e.g., number of frames between acknowledgments) may be set up during negotiations between the transmitter and receiver. If an acknowledgment frame is not received on schedule, or when requested, the last data frame of the burst may be repeated until an acknowledgement is received. The transmitter may send an empty data frame that was not in the original burst, as an alternative to resending the last data frame, as long as the total number of frames, including the empty one, does not exceed a maximum number of frames. The transmitter may not start or resume burst transmissions until an acknowledgement frame is received. The delayed acknowledgement (Dly-ACK) policy is designed to reduce acknowledgement times for burst transmission. 
         [0021]      FIG. 2  is an exemplary timing diagram illustrating implementation of the delayed acknowledgment (Dly-ACK) policy. In  FIG. 2 , M stands for MAC Service Data Unit (MSDU) number and F for Fragment (or frame). Mm-Ff represents Fragment f of MSDUm. When an acknowledgment is expected, but not received during a specified time, the transmitter retransmits the frame (or a new frame if the failed frame&#39;s retransmission limit has been met) after the end of the specified time. Because the transmitter sending the data frame may not correctly receive an acknowledgement, duplicate frames may be sent even though the intended recipient has already received and acknowledged the frame. Retransmitted frames can be assembled in the same burst with other originally transmitted frames in a known manner. 
         [0022]      FIG. 3  depicts a transmitter  300  of a wireless electronic device (herein wireless device), which forms part of a physical layer for the wireless device. The illustrated transmitter  300  includes a scrambler  302 , an FEC encoder  304 , a serial-to-parallel (S/P) converter  306 , an interleaver  308 , a modulator  310 , a pilot/guard/null tone inserter  312 , an inverse fast Fourier transform (IFFT)  314 , a parallel-to-serial (P/S) converter  316 , a frequency hopper  318 , and an antenna  320 . All of these component are controlled by a processor  301 . For the sake of clarity, connections between the processor  301  and the elements of the transmitter  300  are not shown in  FIG. 3 . Suitable components for use within the transmitter  300  will be understood by one of skill in the art from the description herein. 
         [0023]    The scrambler  102  scrambles the source data. In an exemplary embodiment, the scrambler  102  uses a 15-bit Linear Feedback Shift Register (LFSR) to generate a pseudo random binary sequence (PRBS). The scrambler may be initialized with one of four seeds per frame. The seed identifier may be contained in a physical layer header (PHY header) attached to messages for transmission over the network. The 15-bit seed value chosen corresponds to the seed identifier value, which may be set to 00 when the PHY layer is initialized and incremented using a 2-bit rollover counter for each frame that is sent by the PHY layer, i.e., the seeds may be chosen incrementally and circularly. 
         [0024]    The FEC encoder  304  introduces error correction to the source data. The S/P converter  306  coverts the error corrected source data from serial to parallel. Suitable techniques for FEC encoding and S/P conversion will be understood by one of skill in the art from the description herein. 
         [0025]    The interleaver  308  rearranges the data to separate consecutive bits of data. In an exemplary embodiment, a different interleaver pattern is used for the transmission of a frame and each subsequent retransmission of that frame. The interleaving pattern is a function of the number of retransmissions and may be predefined. 
         [0026]    Table 1 sets forth an example illustrating two interleaving patterns on two different transmissions. Data bits are read in sequential order, i.e., 1, 2, 3, . . . , 198, 199, 200. In a first interleaving pattern (Interleaving I), data bits are read out in the following order: 1, 51, 101, 151, 2, 52, 102, 152, . . . , 49, 99, 149, 199, 50, 100, 150, 200. In a second interleaving pattern (Interleaving II), data bits are read out in the following order: 1, 41, 81, 121, 161, 2, 42, 82, 122, 152, . . . , 39, 79, 119, 159, 199, 40, 80, 120, 160, 200. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Interleaving Patterns 
               
             
          
           
               
                 Interleaving I 
                 Interleaving II 
               
               
                   
               
             
          
           
               
                  1 
                 51 
                 101 
                 151 
                 1 
                 41 
                 81 
                 121 
                 161 
               
               
                  2 
                 52 
                 102 
                 152 
                 2 
                 42 
                 82 
                 122 
                 162 
               
               
                  3 
                 53 
                 103 
                 153 
                 3 
                 43 
                 83 
                 123 
                 163 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 48 
                 98 
                 148 
                 198 
                 38 
                 78 
                 118 
                 158 
                 198 
               
               
                 49 
                 99 
                 149 
                 199 
                 39 
                 79 
                 119 
                 159 
                 199 
               
               
                 50 
                 100  
                 150 
                 200 
                 40 
                 80 
                 120 
                 160 
                 200 
               
               
                   
               
             
          
         
       
     
         [0027]    The modulator  310  spreads symbols over multiple tones and applies OFDM modulation. In an exemplary embodiment, the modulator  310  is a dual-carrier modulator (DCM) that spreads each symbol over two tones using an operation such as shown in equation 1: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 n 
                               
                             
                           
                           
                             
                               
                                 y 
                                 
                                   n 
                                   + 
                                   50 
                                 
                               
                             
                           
                         
                         ] 
                       
                       = 
                       
                         
                           
                             1 
                             
                               10 
                             
                           
                           [ 
                           
                             
                               
                                 2 
                               
                               
                                 1 
                               
                             
                             
                               
                                 1 
                               
                               
                                 
                                   - 
                                   2 
                                 
                               
                             
                           
                           ] 
                         
                          
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     x 
                                     
                                       a 
                                        
                                       
                                         ( 
                                         n 
                                         ) 
                                       
                                     
                                   
                                   + 
                                   
                                     
                                       j 
                                        
                                       x 
                                     
                                     
                                       
                                         a 
                                          
                                         
                                           ( 
                                           n 
                                           ) 
                                         
                                       
                                       + 
                                       50 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     x 
                                     
                                       
                                         a 
                                          
                                         
                                           ( 
                                           n 
                                           ) 
                                         
                                       
                                       + 
                                       1 
                                     
                                   
                                   + 
                                   
                                     
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                                        
                                       x 
                                     
                                     
                                       
                                         a 
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                                           ( 
                                           n 
                                           ) 
                                         
                                       
                                       + 
                                       51 
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                     , 
                     
                       n 
                       = 
                       0 
                     
                     , 
                     1 
                     , 
                     2 
                     , 
                     … 
                      
                     
                         
                     
                     , 
                     49 
                   
                    
                   
                     
 
                   
                    
                   where 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     a 
                      
                     
                       ( 
                       n 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             n 
                           
                         
                         
                           
                             
                               n 
                               = 
                               0 
                             
                             , 
                             1 
                             , 
                             2 
                             , 
                             … 
                              
                             
                                 
                             
                             , 
                             24 
                           
                         
                       
                       
                         
                           
                             
                               2 
                                
                               
                                   
                               
                                
                               n 
                             
                             + 
                             50 
                           
                         
                         
                           
                             
                               n 
                               = 
                               25 
                             
                             , 
                             26 
                             , 
                             … 
                              
                             
                                 
                             
                             , 
                             49 
                           
                         
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
         [0028]    The block of complex symbols {y n } is then further modulated using an OFDM modulation scheme, such as quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK). Where a QPSK modulation is used, a four or five bit analog-to-digital converter (ADC) may offer satisfactory performance due to the simplicity of this modulation scheme. A four or five bit ADC simplifies Fourier transform implementation and facilitates the development of lower power wireless devices. In addition, QPSK modulation enables the description of channel distortion as a phase rotation on each carrier, which can be handled through the use of simple one-tap equalizers. 
         [0029]    Table 2 illustrates a dual carrier modulation operation for modulating input bits {x n } to generate output symbols {y n }. In Table 2, four bits are mapped to each symbol and each bit is mapped to two different symbol/tones. For example, bit 1 is modulated onto symbol/tone 1 and 51 along with bits 2, 51, and 52. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 DCM Operation 
               
             
          
           
               
                 Output 
                   
                   
               
               
                 (symbol) 
                 Input (bits) 
               
               
                   
               
             
          
           
               
                  1 
                 1 
                 2 
                 51 
                 52 
               
               
                  2 
                 3 
                 4 
                 53 
                 54 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 24 
                 47 
                 48 
                 97 
                 98 
               
               
                 25 
                 49 
                 50 
                 99 
                 100 
               
               
                 26 
                 101 
                 102 
                 151 
                 152 
               
               
                 27 
                 103 
                 104 
                 153 
                 154 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 49 
                 147 
                 148 
                 197 
                 198 
               
               
                 50 
                 149 
                 150 
                 199 
                 200 
               
               
                 51 
                 1 
                 2 
                 51 
                 52 
               
               
                 52 
                 3 
                 4 
                 53 
                 54 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 74 
                 47 
                 48 
                 97 
                 98 
               
               
                 75 
                 49 
                 50 
                 99 
                 100 
               
               
                 76 
                 101 
                 102 
                 151 
                 152 
               
               
                 77 
                 103 
                 104 
                 153 
                 154 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 99 
                 147 
                 148 
                 197 
                 198 
               
               
                 100  
                 149 
                 150 
                 199 
                 200 
               
               
                   
               
             
          
         
       
     
         [0030]    In an alternative exemplary embodiment, a multi-carrier technique is utilized where each symbol is multiplied by each element of a vector of length N elements (where N is greater than 2 and each vector element is associated with one tone) to produce N vectors. Thus, each symbol is spread over N tones. The N vectors can be transmitted simultaneously, as they are orthogonal. Thus, the data rate is not changed by the spreading operation. 
         [0031]    An advantage of this alternative embodiment is that frequency diversity is achieved up to diversity order N. Spreading a symbol with a spreading code, however, may change the profile of the energy level for the tones. Since UWB systems have a strict emission mask to avoid interference to other existing wireless systems operating in the same spectrum, the signal level of some tones may exceed the emission mask. Reducing the transmission level of these tones reduces/eliminates orthogonolity of the code and decreasing transmission levels of all tones reduces coverage range. A minimum mean squared error (MMSE) equalizer may be used to restore orthogonality. 
         [0032]    The pilot/guard/null tone inserter  312  inserts pilot, guard, and null tones into the data. The IFFT  314  transforms the modulated data from the frequency domain to the time domain. The P/S converter  316  converts the data from parallel to serial for transmission. The frequency hopper  318  processes the serial data for transmission from the antenna  320 . The frequency hopper  318  may include a digital-to-analog converter (DAC) for converting digital data to analog for transmission. Alternatively, digital to analog conversion may be performed at other locations within the transmitter  300 . Suitable techniques for pilot/guard/null tone insertion, IFFT transformation, parallel to serial conversion, and frequency hopping for use with the present invention will be understood by one of skill in the art from the description herein. 
         [0033]      FIG. 4  depicts a receiver  400  of a wireless device, which forms part of a physical layer for the wireless device. The illustrated receiver  400  includes another antenna  402 , a frequency de-hopper  404 , a S/P converter  406 , a fast Fourier transform (FFT)  408 , a Zero-Forcing Equalizer (ZFEQ)  410 , a pilot/guard/null tone remover  412 , a demodulator  414 , a de-interleaver  416 , a P/S converter  418 , an FEC decoder  420 , and a de-scrambler  422 . All of these component are controlled by a processor  401 . For the sake of clarity, connections between the processor  401  and the elements of the receiver  400  are not shown in  FIG. 4 . Suitable components for use within the receiver  400  will be understood by one of skill in the art from the description herein. 
         [0034]    The frequency de-hopper  404  follows the frequency hopping used by the transmitter  300  to receive a signal transmitted by the transmitter  300  ( FIG. 3 ) via the antenna  402 . The S/P converter  406  converts the received signal from serial to parallel for processing. The FFT converter  408  converts the signal from the time domain to the frequency domain. The ZFEQ  410  equalizes the signal to minimize any inter symbol interference (ISI) attributable to the modulation performed by the modulator  310  ( FIG. 3 ) in the transmitter  300 . The pilot/guard/null tone remover  412  removes pilot, guard, and null tones. The demodulator  414  reverses the modulation introduced by the modulator  310  ( FIG. 3 ). The de-interleaver  416  reverses the interleaving introduced by the interleaver  308  ( FIG. 3 ). The P/S converter  418  converts the signal from parallel to serial. The FEC decoder  420  decodes the signal. The de-scrambler  422  reverses the scrambling introduced by the scrambler  302  ( FIG. 3 ). The frequency de-hopper  404  may include an analog-to-digital converter (ADC) for converting received analog signals to digital signals. Alternatively, analog to digital conversion may be performed at other locations within the receiver  400 . 
         [0035]      FIG. 5  depicts an exemplary transmitter  500 . The transmitter  500  is similar to the transmitter  300  described above with reference to  FIG. 3  with the exception that a mapper  502  is inserted between the interleaver  308  and the modulator  310 . When positioned before the modulator  310 , the mapper  502  may remap the input bits being supplied to the modulator  310 . Operation of the mapper  502  is described in detail below. Component common to the two transmitters  300  and  500  are identically numbered and are not described in further detail. The mapper  502  may be integrated with the interleaver  308  with multiple interleavers  308  being employed, i.e., one for each mapping. Each time a frame is transmitted, a different mapping may be used for that frame. Transmitter and receiver mappings for a particular frame may be synchronized through the use of a retransmission number associated with that frame. 
         [0036]      FIG. 6  depicts an alternative exemplary transmitter  600  that is similar to the transmitter  500  of  FIG. 5  with the exception that the mapper  502  is positioned between the modulator  310  and the pilot/guard/null tone inserter  312 . When positioned after the modulator  310 , the mapper  502  may remap the output symbols generated by the modulator  310 . Those of skill in the art will understand that the mapper  502  may be inserted within the transmitter at other positions from the description herein. 
         [0037]      FIG. 7  depicts an exemplary receiver  700  that is configured for use with the exemplary transmitters  500  and  600  of  FIGS. 5 and 6 , respectively. The exemplary receiver  700  is similar to the receiver  400  of  FIG. 4  with the exception that one or more of the components within the receiver are repeated to handle different mappings introduced by the mapper  502  ( FIG. 5 ). Each group of repeated components is designated with a small letter designation from a-n. The group selected for processing a particular frame may be indicated through the use of a retransmission number associated with that frame. 
         [0038]    The mapper  502  ( FIGS. 5 and 6 ) may map a symbol {y n } to {z n } in accordance with equation 2: 
         [0000]    
       
         
           
             
               
                 
                   
                     z 
                     n 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               
                                 
                                   y 
                                   n 
                                 
                               
                               
                                 transmission 
                               
                             
                             
                               
                                 
                                   y 
                                   
                                     n 
                                     + 
                                     
                                       iM 
                                        
                                       
                                         ( 
                                         
                                           mod 
                                            
                                           
                                             ( 
                                             N 
                                             ) 
                                           
                                         
                                         ) 
                                       
                                     
                                   
                                 
                               
                               
                                 retransmission 
                               
                             
                           
                            
                           
                               
                           
                            
                           n 
                         
                         = 
                         1 
                       
                       , 
                       2 
                       , 
                       … 
                        
                       
                           
                       
                       , 
                       N 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where i is the number of retransmission, M is the offset of a start symbol and N is the total number of symbols in a frame.  FIG. 8  illustrates this mapping scheme for a transmission and three retransmissions.  FIG. 8  shows that for a total of four transmissions, each symbol {y n } is mapped onto four different tones for transmission. Therefore, without increasing signal processing complexity, a spreading gain of four is achieved. 
         [0039]    In the above embodiments, spreading does not take channel characteristics into consideration. Thus, symbols on tones with deep fade may be retransmitted on tones with deep fade again. In an alternative exemplary embodiment, the receiver notifies the transmitter of the best and worst tones. The offset of a next retransmission of the frame is then selected such that the symbols on the worst tone in a previous transmission are mapped to the best tone in a subsequent transmission. In an exemplary embodiment, the receiver records an average level of each symbol. The offset of a next retransmission of the frame is then selected to map the symbols with the lowest reception level onto the best tone in the next retransmission. 
         [0040]    The mapping described above is relatively easy to implement and enables easy synchronization between transmitters and receivers. Symbols experiencing the deepest fade, however, may not be mapped to tones with the least fade in the next retransmission. After retransmissions, some symbols may still experience less energy reception than other symbols at the receiver. The energy from multiple transmission of a symbol can be combined to improve the signal to noise ratio of the symbol. 
         [0041]    The goal of synchronization in this alternative exemplary embodiment is for receivers to inform transmitters of channel conditions and suggest symbol to sub-carrier mapping for the next retransmission. To simplify synchronization implementation, tones can be divided into a few categories based on the energy level of received signals on the tones. Only those symbols falling into the lowest levels may be specified for remapping to other tones. Other unspecified symbols may be mapped in order, e.g., sequentially, to the remaining tones. 
         [0042]    Tone remapping may be achieved by two bit-mapped tables representing current symbol-to-tone mapping and next symbol-to-tone mapping, shown in Table 3. In Table 3, a ‘1’ in the second row represents the tones in the category of lowest signal reception level and a ‘1’ in the third row represents the tones for use in the next transmission of the above symbols. For example, symbols on tones 2, 4, and 5 in the current transmission (shown in the second row) may be retransmitted on tones 3, 6, and 8 (shown in the third row). Other unspecified symbols in the current transmission are arranged in order onto those unspecified tones in the next retransmission, i.e., symbols 1, 3, 6, 7, and 8 may be sent on tones 1, 2, 4, 5, and 7. 
         [0000]                                                                                                          TABLE 3                   Symbol-to-Tone Mapping                Index of tones                1   2   3   4   5   6   7   8                        Tone usage of current Tx   0   1   0   1   1   0   0   0       Tone usage of next Tx   0   0   1   0   0   1   0   1                    
The 128 tones utilize 128 bits, or 16 bytes, for a current symbol-to-tone mapping and 128 bits, or 16 bytes, for a next symbol-to-tone mapping. Thus, 32 total bytes are used. The receiver may send notification of the reception after a burst of frames. This notification may include the mapping bytes.
 
         [0043]    The various aspects of the present invention provide a mechanism to utilize packet retransmission with symbol spreading to achieve further spreading without increasing implementation complexity. The basic concept is to utilize different mapping of bits to tones in transmissions and subsequent retransmissions so that each bit can be transmitted on different tones in each transmission to increase spreading in frequency. The scheme can be used in multi-carrier wireless communication systems to improve frequency diversity by improving symbol/bit spreading. 
         [0044]      FIG. 9  depicts a flow chart  900  of exemplary steps for transmitting a frame of source data over a plurality of tones/frequencies in accordance with an aspect of the present invention. The steps will be described with reference to the transmitters  500 / 600  depicted in  FIGS. 5 and 6  and the receiver  700  depicted in  FIG. 7 . At block  902 , a scrambler  302  scrambles the source data, a FEC encoder  304  introduces forward error correction to the source data, and an S/P converter  306  converts the source data from serial to parallel. 
         [0045]    At block  904 , an interleaver  308  interleaves the bits within the source data. In an exemplary embodiment, the interleaver  308  interleaves the bits within the frame of source data using a first interleave pattern for source data being transmitted for the first time and interleaves the bits within the frame of source data using a second interleave pattern for source data being retransmitted. 
         [0046]    At block  906 , a modulator  310  generates symbols from the bits within the frame of source data and modulates each symbol onto at least two of the tones such that each bit is modulated onto at least two different tones. In an exemplary embodiment, the modulator  310  multiples each symbol by each element of a vector having three or more elements, wherein each element is associated with a different tone. 
         [0047]    At block  908 , a mapper  502  maps the frame of source data for transmission using a first mapping and maps the frame of source data for retransmission using a second mapping that is different from the first mapping to increase frequency diversity. Subsequent retransmissions of the source data may be mapped using mappings that are different from the first and second mapping (e.g., a second retransmission may be mapped using a third mapping that is different from the first and second mappings) to further increase frequency diversity. The mappings for the retransmitted frames may be based on feedback received from the receiver  700 . For example, the receiver may notify the transmitter  500 / 600  of bits/symbols on tones/frequencies experiencing deep fade in a transmission or retransmission. The processor  301  within the transmitter  500 / 600  may then select a mapping for a first retransmission or subsequent retransmission such that the bits/symbols are mapped to tones/frequencies that are not experiencing deep fade. 
         [0048]    In an exemplary embodiment, the mapper  502  is positioned after the modulator  310  such as depicted in  FIG. 6 . In accordance with this embodiment, the mapper  502  maps that source data on a symbol-by-symbol basis. In an alternative exemplary embodiment, the mapper  502  is positioned before the modulator  310  such as depicted in  FIG. 5 . In accordance with this embodiment, since the bits have not yet been converted to symbols by the modulator  310 , the mapper  502  maps the source data on a bit-by-bit basis. Also, in accordance with this embodiment, the step set forth in block  908  would be performed between the steps set forth in blocks  904  and  906 . 
         [0049]    At block  910 , the pilot/guard/null tone inserter  312  inserts pilot, guard, and null tones, the IFFT  314  converts the source data from the frequency domain to the time domain, a P/S converter  316  converts the source data from parallel to serial, and a frequency hopper  318  processes the serial data for transmission. At block  912 , the transmitter  500 / 600  transmits the source data from the antenna  320 . 
         [0050]    At block  914 , the receiver  700  receives the transmitted source data at one or more other antennas  402 . At block  916 , the receiver  700  processes the received source data to reverse the modulation, mapping, and interleaving introduced by the transmitter  500 / 600 . In an exemplary embodiment, the receiver  700  includes a frequency de-hopper  404 , a S/P converter  406 , a FFT  408 , a ZFEQ  410 , a pilot/guard/null tone remover  412 , a demodulator  414 , a de-interleaver  416 , and a P/S  418  corresponding to each mapping used by the transmitter  500 / 600  to map the source data. 
         [0051]    At block  918 , the processor  401  within the receiver optionally identifies bits/symbols on tones with deep fade and sends at notification to the transmitter (e.g., during acknowledgement) notifying the transmitter of the tones with deep fade so that the transmitter may remap the bits/symbols to tones without deep fade. 
         [0052]    At block  920 , the FEC decoder  420  performs error correction and a descrambler  422  descrambles the source data to recover the original source data. In an exemplary embodiment, the source data from multiple transmissions is combined to improve the signal to noise ratio of the transmitted source data. The process is then repeated for one or more retransmissions as indicated by dashed line  950 . 
         [0053]    Although the invention has been described in terms of interleavers  308 , de-interleavers  416 , mappers  502 , modulators  310 , and demodulators  414 , the invention may be implemented in software on a computer (not shown). In this embodiment, one or more of the functions of the various components may be implemented in software that controls the computer. This software may be embodied in a computer readable carrier, for example, a magnetic or optical disk, a memory-card or an audio frequency, radio-frequency, or optical carrier wave. 
         [0054]    Further, although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.