Patent Publication Number: US-11032119-B2

Title: Method and system for combining DFT-transformed OFDM and non-transformed OFDM

Description:
PRIORITY CLAIM INFORMATION 
     This application is a continuation of U.S. patent application Ser. No. 16/273,302 filed Feb. 12, 2019, titled “Method and System for Combining DFT-Transformed OFDM and Non-Transformed OFDM”, by Jianglei Ma, Wen Tong, Ming Jia, Hua Xu, Peiying Zhu, Hang Zhang which is a continuation of 
     U.S. patent application Ser. No. 15/790,678, filed Oct. 23, 2017, titled “Method and System for Combining DFT-Transformed OFDM and Non-Transformed OFDM”, by Jianglei Ma, Wen Tong, Ming Jia, Hua Xu, Peiying Zhu, Hang Zhang (issued as U.S. Pat. No. 10,237,206 on Mar. 19, 2019), which is a continuation of 
     U.S. patent application Ser. No. 15/195,083, filed Jun. 28, 2016, titled “Method and System for Combining DFT-Transformed OFDM and Non-Transformed OFDM”, by Jianglei Ma, Wen Tong, Ming Jia, Hua Xu, Peiying Zhu, Hang Zhang, (issued as U.S. Pat. No. 9,973,365 on May 15, 2018), which is a continuation of 
     U.S. patent application Ser. No. 14/251,629, filed Apr. 13, 2014, titled “Method and System for Combining OFDM and Transformed OFDM”, (issued as U.S. Pat. No. 9,407,487 on Aug. 2, 2016), which is divisional of 
     U.S. patent application Ser. No. 13/047,259, filed Mar. 14, 2011, titled “Method and System for Combining OFDM and Transformed OFDM” (issued as U.S. Pat. No. 8,773,974 on Jul. 8, 2014), which is a divisional of 
     U.S. patent application Ser. No. 11/909,567, filed Sep. 24, 2007 (issued as U.S. Pat. No. 7,929,407 on Apr. 19, 2011), titled “Method and System for Combining OFDM and Transformed OFDM”, which is a U.S. National Stage application of 
     International Application No. PCT/CA2006/000464, filed Mar. 30, 2006, titled “Method and System for Combining OFDM and Transformed OFDM”, which claims the benefit of priority to: 
     U.S. Provisional Application No. 60/674,878, filed Apr. 26, 2005, titled “MIMO-OFDM Air Interface”, and 
     U.S. Provisional Application No. 60/666,548, filed Mar. 30, 2005, titled “MIMO-OFDM Air Interface”. 
     All of the above identified Applications are incorporated by reference in their entireties as though fully and completely set forth herein. 
     The claims in the instant application are different than those of the parent application or other related applications. The Applicant therefore rescinds any disclaimer of claim scope made in the parent application or any predecessor application in relation to the instant application. The Examiner is therefore advised that any such previous disclaimer and the cited references that it was made to avoid, may need to be revisited. Further, any disclaimer made in the instant application should not be read into or against the parent application or other related applications.” 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of wireless communications, more specifically to systems and methods employing orthogonal frequency division multiplexed (OFDM) transmission. 
     BACKGROUND OF THE INVENTION 
     Orthogonal frequency division multiplexing (OFDM) is a particular form of frequency division multiplexing that distributes data over a number of carriers that have a very precise spacing in the frequency domain. The precise spacing and partially overlapping spectra of the carriers provides several benefits such as high spectral efficiency, resiliency to radio frequency interference and lower multi-path distortion. Due to its beneficial properties and superior performance in multi-path fading wireless channels, OFDM has been identified as a useful technique in the area of high data-rate wireless communication, for example wireless metropolitan area networks (MAN). Wireless MAN are networks to be implemented over an air interface for fixed, portable, and mobile broadband access systems. 
     In another type of frequency division multiplexing, rather than using closely spaced frequencies of OFDM, the spectra of adjacent channels are more or less distinct, and bandpass filtering is typically employed to separate channels. This will be referred to as “conventional frequency division multiplexing”. 
     Orthogonal frequency division multiplexing is beneficial in that multiple input multiple output (MIMO) and collaborative MIMO transmission schemes are easily implemented thereon. Furthermore, the use of orthogonal frequency division multiplexing allows for flexible and efficient pilot designs. Also, problems related to noise enhancement can be avoided during signal processing at the receiver. 
     The use of conventional frequency division multiplexing has a lower Peak to Average Power Ratio (PAPR). A disadvantage is that it causes noise enhancement. 
     SUMMARY OF THE INVENTION 
     According to one broad aspect, the invention provides a method comprising, within an available spectral resource: transmitting with OFDM multiplexing on selected sub-carriers and selected OFDM transmission durations; transmitting with T-OFDM (transformed OFDM) multiplexing or direct multiple sub-carrier multiplexing on selected sub-carriers and selected OFDM transmission durations that do not overlap with OFDM transmissions. 
     In some embodiments, transmitting with OFDM multiplexing and transmitting with T-OFDM multiplexing comprises: transmitting a respective signal from each of a plurality of antennas, each signal comprising OFDM multiplexing or T-OFDM multiplexing. In some embodiments, transmitting a respective signal from each of a plurality of antennas, each signal comprising OFDM multiplexing or T-OFDM multiplexing comprises using a distinct frequency resource for each transmitter. 
     In some embodiments, transmitting a respective signal from each of a plurality of antennas comprises transmitting from a plurality of transmitters. 
     In some embodiments, transmitting a respective signal from each of a plurality of antennas comprises transmitting from a single transmitter. 
     In some embodiments, transmitting a respective signal from each of a plurality of antennas, each signal comprising OFDM multiplexing or T-OFDM multiplexing comprises transmitting a respective signal from each of a plurality of transmit antennas using common frequency resources for at least two antennas such that MIMO processing will be required to separate the signals upon receipt. 
     In some embodiments, transmitting a respective signal from each of a plurality of antennas comprises transmitting from a single transmitter. 
     In some embodiments, transmitting a respective signal from each of a plurality of antennas comprises transmitting from a plurality of transmitters so as to implement a virtual MIMO transmission. 
     In some embodiments, the method as summarized above further comprises: mapping symbols to the selected sub-carriers for OFDM multiplexing using a sub-band mapping or a diversity mapping; transforming symbols to be transmitted with T-OFDM to produce transformed symbols and mapping the transformed symbols to the selected sub-carriers for T-OFDM using a sub-band mapping or a diversity mapping. 
     In some embodiments, transmitting from a single transmitter comprises: transmitting with OFDM multiplexing during selected OFDM transmission durations; transmitting T-OFDM multiplexing during OFDM transmission durations distinct from the OFDM transmissions used for OFDM multiplexing. 
     In some embodiments, transmitting with OFDM multiplexing and T-OFDM multiplexing comprises: for each of at least one frequency resource allocation consisting of a plurality of subcarrier frequencies: selecting transmitting using OFDM multiplexing and transmitting using T-OFDM multiplexing. 
     In some embodiments, the at least one frequency resource allocation comprises a plurality of resource allocations each consisting of a respective contiguous set of sub-carriers or a distributed set of sub-carriers. 
     In some embodiments, for each of at least one frequency resource allocation consisting of a plurality of sub-carrier frequencies selecting transmitting using OFDM multiplexing and transmitting using T-OFDM multiplexing comprises: multiplying a respective set of symbols by a selected one of two different transform matrices that result in OFDM multiplexing and T-OFDM multiplexing respectively. 
     In some embodiments, T-OFDM multiplexing comprises multiplying an input set of symbols by an FFT matrix prior to IFFT processing. 
     In some embodiments, the method further comprises transmitting pilots. 
     In some embodiments, the method further comprises transmitting pilots from each antenna on sub-carriers selected from sub-carriers being utilized by that antenna. 
     According to another broad aspect, the invention provides a multiplexing method comprising: selecting one of at least two transform functions to be a selected transform function; performing the selected transform on a set of input symbols to produce a transformed sequence of samples; performing an inverse fast Fourier transform (IFFT) of the transformed sequence of samples to produce a multiplexer output. 
     In some embodiments, the two transform functions consist of an identity matrix and a non-identity matrix. 
     In some embodiments, the non-identity matrix is an FFT matrix, fast Hadamard transform, or a wavelet transform. 
     In some embodiments, a transmitter is adapted to implement the method as summarized above. 
     In some embodiments, a plurality of transmitters is adapted to collectively implement the method as summarized above. 
     In some embodiments, a transmitter is adapted to implement the method as summarized above. 
     According to another broad aspect, the invention provides a method comprising, within an available spectral resource: adaptively switching between transmitting to a receiver with OFDM multiplexing and transmitting to the receiver with T-OFDM or direct multiple carrier multiplexing. 
     In some embodiments, adaptively switching between transmitting to a receiver with OFDM multiplexing and transmitting to the receiver with T-OFDM or direct multiple carrier multiplexing comprises: at a transmitter processing each set of symbols by one of two transform functions one of which results in OFDM multiplexing, the other of which results in T-OFDM multiplexing. 
     In some embodiments, the method further comprises receiving feedback to select which of the two transform functions to use. 
     In some embodiments, the method further comprises selecting between the two transform functions on the basis of one or more of SNR, traffic type, head room in a power amplifier. 
     In some embodiments, a transmitter is adapted to implement the method as summarized above. 
     In some embodiments, the transmitter comprises: a transformer adapted to apply a selected one of the two transform functions to a set of input symbols; an IFFT that receives an output of the transform function. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the attached drawings in which: 
         FIG. 1  contains various block diagrams for implementing a first constituent multiplexing method that is equivalent to conventional OFDM; 
         FIG. 2  contains various block diagrams for implementing a second constituent multiplexing method, transformed OFDM; 
         FIG. 3  contains various block diagrams for implementing a third constituent multiplexing method, direct multiple sub-carrier multiplexing; 
         FIG. 4  is a block diagram of a first example of the co-existence of OFDM and transformed OFDM in transmissions from multiple transmitters; 
         FIG. 5  is a block diagram of a second example of the co-existence of OFDM and transformed OFDM in transmissions a single transmitter; 
         FIG. 6  is a block diagram of a third example of the co-existence of OFDM and transformed OFDM in MIMO transmissions of a single multiple transmitter; 
         FIG. 7  is a block diagram of a fourth example of the co-existence of OFDM and transformed OFDM in MIMO transmissions of multiple transmitters; 
         FIGS. 8 and 9  are block diagrams of OFDM transmitters according to embodiments of the invention; 
         FIG. 10  is a schematic diagram of a transmission frame according to an embodiment of the invention; 
         FIG. 11  is a schematic diagram of a transmission frame according to another embodiment of the invention; 
         FIG. 12  is a schematic diagram of a transmission frame according to a further embodiment of the invention; 
         FIG. 13  is a schematic diagram of a pilot pattern for FDM according to an embodiment of the invention; and 
         FIG. 14  is a block diagram of an OFDM transmitter according to embodiment of the invention. 
         FIG. 15  is a block diagram of a cellular communication system; 
         FIG. 16  is a block diagram of an example base station that might be used to implement some embodiments of the present invention; 
         FIG. 17  is a block diagram of an example wireless terminal that might be used to implement some embodiments of the present invention; 
         FIG. 18  is a block diagram of a logical breakdown of an example OFDM transmitter architecture that might be used to implement some embodiments of the present invention; and 
         FIG. 19  is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
     In OFDM a wideband signal is transmitted on multiple independent parallel narrowband orthogonal carriers. A multicarrier transmitter such as an OFDM transmitter can have a high Peak-to-Average-Power Ratio (PAPR). The OFDM transmitter has a high power amplifier suitable to meet peak power requirements. IFFT (inverse fast Fourier transform) has also been applied to realize orthogonal multiplexing of multiple transmitters. 
     In conventional FDM (frequency division multiplexing) there is no IFFT processing of a signal before transmission and transmission occurs on a single carrier or multiple separated carriers. In addition, modulation schemes used for single carrier transmitters, such as QAM and QPSK have a smaller dynamic range so the PAPR is smaller than for the OFDM transmitter having a similar transmission range. 
     An advantage of an OFDM transmitter over a conventional FDM transmitter is a higher spectral efficiency. A disadvantage of an OFDM transmitter as compared to a conventional FDM transmitter is a higher PAPR resulting in a power amplifier in the OFDM transmitter that has a higher cost. OFDM also allows for advanced MIMO applications, flexible and convenient pilot arrangements, and flexible and efficient sub-channelization. 
     An advantage of a conventional FDM transmitter over an OFDM transmitter is that due to a less noisy output coverage within the cell is better. 
     When a transmitter is a great distance from a receiver it may be desirable to modify the transmitted signal to improve a signal to noise ratio and reduce a possibility of errors at a receiver end. Conversely, when the transmitter and receiver are in close proximity it may be desirable to modify the transmitted signal to employ a high spectral efficiency. 
     In some embodiments, methods and systems are provided in which a signal processing step is added to the technique for processing the signal in the OFDM transmitter to enable the signal to be modified before transmission to meet different operating conditions. In some embodiments, a transmitter is equipped to produce a signal that is selectively modified to result in a conventional OFDM signal or in the form of a conventional FDM signal. 
     In some embodiments, the processing step is a transformation implemented using a transformation function. Depending on the selection of the transformation function for a given implementation, the transformation may produce an output that has transmission characteristics bounded by the transmission characteristics of OFDM and conventional FDM. In some implementations, the processing step may be performed in conjunction with a frequency domain decision feedback equalizer (DFE). 
     In some embodiments, the transmitter is a mobile terminal while in other embodiments, the transmitter is a base station. The transformation may be particularly useful in a mobile terminal as reducing the PAPR has an effect of reducing an amount of power amplification needed for transmitting a signal. Reducing power amplification may lower energy consumption and lower energy consumption results in a longer battery life for battery powered transmitters. 
     Embodiments of the invention provide various mechanisms for combining various constituent multiplexing structures consisting of OFDM, transformed OFDM, and direct multiple subcarrier modulation, either by a single transmitter or multiple transmitters. 
     To begin, three constituent multiplexing structures will be described with reference to  FIGS. 1, 2 and 3 .  FIG. 1  shows a block diagram of a multiplexing structure for generating conventional OFDM channels, generally indicated at  200 . Input symbols  201  are fed to a mapping function  202  the output of which is connected to an IFFT  204 . The purpose of the mapping function is to map the symbols  201  to particular inputs of the IFFT  204 . It is noted that many components might proceed or follow the component shown in  FIG. 1 , such as coding and modulation, interleaving, or RF up conversion, etc. 
     Generally indicated at  210  is another multiplexing structure that produces the same output as that of structure  210 , namely a conventional OFDM signal output. This includes an additional component  206  which is a transform that performs a transformation on the input symbols  201  before the mapping function  202 . However, for an OFDM channel, the transform function is simply equal to an identity matrix, and as such the output of the structure of  210  will be indistinguishable from the output of the structure  200 . 
     A specific example of the mapping function is shown in the structure generally indicated at  212 . Here the mapping function is a sub-band mapping  214  that maps the symbols to a contiguous set of sub-carrier frequencies of the IFFT  204 . 
     Generally indicated at  216  is another example of how the mapping might be performed. In this case, the symbols  201  are input to diversity mapping function  218  which maps the symbols to sub-carriers that are distributed across the sub-carrier frequencies being processed in the IFFT  204 . 
     A second constituent multiplexing structure produces a transformed OFDM signal. Referring now to  FIG. 2 , generally indicated at  219  is a block diagram of a transmitter that generates a transformed OFDM signal. In this case, symbols  201  are input to a transform function  220  the output of which is fed to mapping function  222  the output of which is input to the IFFT  204 . The transform  220  performs a transformation on the input symbols  201 . The transformation is not simply the identity matrix as was the case in structure  210  of  FIG. 1 . Specific examples of the transform function include a fast Fourier transform (implemented in any suitable fashion, for example a DFT), a wavelet transform (such as a Harr wavelet transform) or a fast Hadamard transform (FHT). In some implementations the transform function is represented by an invertible matrix. In some implementations the transform function is represented by an orthonormal matrix. 
     The function of the mapping function  222  and the IFFT  204  is the same as in previous examples. The mapping function  222  maps the outputs of the transform  220  to selected sub-carriers of the IFFT  204 . 
     A first example of the mapping is generally indicated at  221  where the output of the transform  220  is input to a sub-band mapping  224 , the result of which is that a contiguous block of sub-carriers of the IFFT  204  are used. 
     Another example of mapping is shown generally indicated at  223 . In this case, the output of the transform  220  is input to a diversity mapping function  226  which maps the transformed output to a set of sub-carriers that are distributed throughout the OFDM band of the IFFT  204 . 
     It is readily apparent how the channel structure of  FIG. 1  and the channel structure of  FIG. 2  can be implemented using the same physical implementation. In particular, the structure  210  in  FIG. 1  and the structure  219  in  FIG. 2  are identical with the exception of the fact that a different transform is employed. As such, by dynamically selecting the contents of the transform  220 , a channel that is an OFDM channel can be implemented, or a transformed OFDM channel can be implemented. In some embodiments, transmitters are equipped with such a structure to enable them to dynamically select between operating to produce OFDM channels or transformed OFDM channels. 
     Referring now to  FIG. 3 , shown a third constituent channel type that will be referred to as “direct multiple subcarrier multiplexing”, generally indicated at  229 . In this case, input symbols  201  are input to a direct multiple sub-carrier modulator  230  to produce an output. The modulation is direct in the sense that no IFFT or FFT technology is employed, but rather input symbols are multiplied to particular sub-carriers directly. It is referred to as “multiple” sub-carrier modulation because each input symbol is represented in multiple, in some cases all, of the sub-carriers output by the modulator. 
     A very specific example is shown generally indicated at  231 . In this case, the direct multiple sub-carrier modulator consists of a symbol repeater  232  and a complex spreader  234  that multiplies each repeated symbol by a set of complex frequencies to produce an output. It is noted that the output of the direct multiple sub-carrier multiplexing structure  231  is in some instances mathematically equivalent to the output of the structure  223  of  FIG. 2 . In particular, when the transform  220  is an FFT (or equivalent) and the diversity mapping  226  maps the output of the transform  220  to a set of sub-carrier locations that are equally spaced, the output is the same as the output of the structure  231  of  FIG. 3  where the complex spreader multiplies the repeated symbols by individual sub-carrier frequencies that are the same as the spaced sub-carrier inputs to the IFFT  204  of  FIG. 2 . A common feature between the schemes of  FIGS. 2 and 3  is that each input symbol  201  ends up being represented on multiple sub-carrier frequencies; in contrast, the conventional OFDM structure of  FIG. 1  has each symbol  201  appearing on a single sub-carrier. 
     Having defined the three constituent multiplexing structures, various mechanisms for their co-existence are provided. All such mechanisms involve the co-existence of OFDM and one or more of the other multiplexing approaches, be it transformed OFDM or direct multiple sub-carrier multiplexing. In a particular embodiment, OFDM and transformed OFDM are supported. 
     A first example is illustrated in  FIG. 4 . In this example there is a set of transmitters  240 ,  242 ,  244  (only three shown) each of which might for example be a separate mobile station. Each transmitter is equipped with a respective frequency division multiplexer  246 ,  248 ,  250  that implements one or more of the three constituent multiplexer structures of  FIGS. 1, 2 and 3 . Each transmitter has a respective antenna  252 ,  254 ,  256 . With the embodiment of  FIG. 4 , the frequency division multiplexers  246 ,  248 ,  250  each operates using a respective distinct frequency resource or set of sub-carriers. Given an available set of subcarriers, each transmitter is assigned a different subset of subcarriers and having been assigned that set of sub-carriers, one of the three multiplexer structures are described above is implemented. By separating out the sub-carriers in this manner, the different channel structures can co-exist simultaneously. 
     Another example of the co-existence of these channels will now be described with reference to  FIG. 5 . Shown is a single transmitter  260  having multiple frequency division multiplexers  262 ,  264 ,  266  connected to a single antenna  268 . Each frequency division multiplexer  262 ,  264 ,  266  implements one of the three constituent multiplexer structures of  FIGS. 1, 2 and 3 . In this case, the frequency resource is again divided between the different frequency division multiplexers  262 ,  264 ,  266  in a manner similar to the distinct transmitters of  FIG. 4 . Since they are implemented in a single transmitter, if multiple frequency division multiplexers employ IFFT functionality, a single IFFT could be implemented for these in combination. Similarly, mapping could be performed with a single mapper. 
     Referring now to  FIG. 6 , shown is another example of how the different channel structures can be combined. With the example of  FIG. 6 , a transmitter generally indicated at  270  has frequency division multiplexers  272 ,  274 , and output antennas  276 ,  278 . Each frequency division multiplexer  272 ,  274  implements one of the three constituent multiplexer structures of  FIGS. 1, 2 and 3 . In this case, the two frequency division multiplexers  272 ,  274  are not assigned distinct frequency resources as was the case for the example of  FIG. 4 . Rather, they are assigned a common frequency resource, and MIMO (multiple input, multiple output) processing is performed at the receiver to separate these. With the example of  FIG. 6 , the approach generalizes to an arbitrary number of transmit antennas for a given MIMO channel. Furthermore, the details are only shown for a single frequency allocation. The structure  270  can be repeated for multiple different transmitters as in the  FIG. 4  embodiment, or within a single transmitter as in the  FIG. 5  embodiment. 
     In yet another implementation, generally indicated at  FIG. 7 , a structure similar to that of  FIG. 6  is employed, but in which the frequency division multiplexers  272 ,  274  are implemented on separate transmitters  280 ,  282  respectively. In this case, the frequency resources again are common and as such this is referred to as “virtual MIMO”, also referred to as collaborative or co-operative MIMO. A receiver of signals transmitted by the transmitters  280 ,  282  would use MIMO technologies to separate the two transmissions. The structure of  FIG. 7  can be repeated for multiple different frequency resource allocations. 
     With any of the above-described embodiments, the transmitter may operate in an open loop mode in which case there is no feedback input. In other embodiments the transmitter operates in a closed loop mode, for example, a receiver that receives the transmitted signal sends feedback to the transmitter to aid in selecting between the various supported FDM variants, for example by selecting an appropriate transform function. 
     In some embodiments, the receiver transmits a signal that enables the transmitter to determine a distance between the transmitter and receiver or to determine an SNR and an available power margin. The receiver may transmit information that would enable the transmitter to determine a bit error rate (BER), a signal to noise ratio (SNR) or a channel quality estimate. The transmitter may be able to determine whether there is any power head room to increase power for a given user if signal quality needs to be improved. If there is no such room, then an option is to switch to T-OFDM to reduce PAPR so that power can be increased. Based on these determinations the transmitter may select a transform function to modify a coded and modulated signal to improve coverage within the cell by transmitting the transformed signal as transformed OFDM. Conversely, when the transmitter determines from information provided by the receiver that operating conditions are favorable, for example close proximity of the transmitter and receiver or favorable transmission parameters, i.e. BER, SNR the transmitter may select a transform function to modify the transmitted signal to employ a high spectral efficiency capacity by transmitting the transformed signal as OFDM. 
     In some embodiments, the transform function is an FFT. The transform function in the form of an FFT may be more efficiently computed using a DFT (discrete Fourier transform) when the number of samples is a power of 2, but more generally, any approach to computing the FFT can be employed. The size of the DFT depends on the bandwidth assigned for a given user. The DFT approach will be assumed in the following specific example. When the DFT is performed on a single data point (essentially an impulse in the time domain), the result is substantially the same as conventional OFDM, which is distributed over all sub-carriers in the transmission band. When multiple coded and modulated signals are each transformed using a DFT smaller than the size of the IFFT the outputs of the respective DFTs can be mapped to locations in the IFFT input so as to be processed by the IFFT simultaneously for multi-user signal multiplexing. 
     An example of a transform function that includes a DFT is shown below in which the DFT is a two sample point DFT represented by the 2×2 matrix [M] within transform function T. The remainder of the elements in the primary negative sloping diagonal are each equal to one and all other elements in the matrix equal zero. This type of transform could be used to transform two symbol sequences simultaneously in a single transmitter with one sequence being transformed OFDM and the other conventional OFDM. Alternatively, the transform could be viewed as the sum of two transformations performed by separate transmitters. 
     
       
         
           
             T 
             = 
             
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                         1 
                         , 
                         1 
                       
                     
                   
                   
                     
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     After the coded and modulated output is multiplied by such a transform, the transformed output is then input to the IFFT function as before. Different transform functions can be selected for different users. 
     In some implementations each coded and modulated input is transformed with a particular transform function. In some implementations the coded and modulated inputs can be collectively transformed by a single transform function as in the above example matrix. The number of coded and modulated signals is implementation specific depending on a desired number of users per transmitter. 
     In some embodiments, the manner by which the overall bandwidth is subdivided between the different constituent multiplexer types used is updated dynamically, for example every scheduling period. 
     In some embodiments of the invention a transform function is a parameterized transform function. The parameterized transform function may be for example a parameterized orthonormal matrix, for example a Teoplitz matrix as shown below, where the variables a and b are constant values along negative-sloping diagonals. 
     
       
         
           
             T 
             = 
             
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     In some embodiments the parameterized transform function is used for partial response signaling (PRS) in which a controlled amount of inter-symbol interference (ISI) is permitted. As the amount of ISI is known, the effect of the ISI can be compensated. 
     In some embodiments of the invention the transform function is generated as a function of a desired performance criterion. For a given transform function, a function is created, namely Q(T). In some embodiments the function Q(T) is used in combination with the transform function to optimize particular performance criteria, for example minimum mean square error (MMSE) or minimum bit error rate (BER) at a receiver output. 
     In some embodiments the transform function may be generated by a relation between the transform function T and the function of transform function T, Q(T) as min∥TQ(T)−I∥ 2 . 
     In some embodiments the function Q(T) has matrix algebraic form representation. In some embodiments the function Q(T) is a minimization procedure, for example a Viterbi trellis search. 
     In some embodiments, to an embodiment of the invention: T=Q(H), where H is the channel matrix. Transform T may be used for channel decomposition, where (.) H  represents the Hermitian:
 
 H=UΛV Q ( H )= U   H  
 
This can be employed to provide channel pre-equalization and pre-distortion transmission.
 
     In some embodiments, the generation of the transform function may include the generation of a family of transform functions. 
     In some implementations of the invention the transform function is selected to maximize the receiver output signal to noise ratio. In some implementations of the invention the transform function is selected to minimize the PAPR. In some embodiments the transmitter selects the parameters of the transform function based on several criteria, such as minimizing the PAPR and/or minimizing the MMSE. 
     In some embodiments of the invention the receiver may provide feedback to the transmitter to facilitate the selection of the transform function. If the receiver is aware of which transform function is being used by the transmitter the receiver is able to receive and detect the transformed sequence of samples when they arrive at the receiver. In some embodiments of the invention, when the receiver is not involved in selection of the transform function, the transmitter sends an identification to the receiver of the particular transform function used in generating the output transmitted by the OFDM transmitter. 
     In some embodiments the transmitter dynamically and/or adaptively optimizes the transform function for particular operating conditions. In some embodiments of the invention, there is provided a mechanism that allows adaptive and dynamic optimization of the transform function to optimize both transmitter and receiver performance. 
     In some embodiments of the invention the use of an appropriate transform function reduces the PAPR for low power devices. 
     Various specific examples will now be described with reference to  FIGS. 8 to 14 . 
     Referring now to  FIG. 8  shown is an example of a single input single output (SISO) OFDM transmitter, generally indicated at  600  that is enabled to perform the transform on a sequence of samples in accordance with an embodiment of the invention. The sequence of samples is provided to coding and modulation logic  610 . An output sequence of symbols of the coding and modulation logic  610  is provided to transform function  620 . An output of the transform function  620  is provided to an IFFT function  630 . The output of the IFFT function  630  is then provided to an antenna  640  for transmission. 
       FIG. 9  shows an example of a multiple input multiple output (MIMO) OFDM transmitter, generally indicated at  650  that is enabled to perform the transform in accordance with an embodiment of the invention. In the MIMO OFDM transmitter  650  two portions of a bit stream a single user (not shown) are provided to respective coding and modulation function  660 ,  661 . Respective outputs of the coding and modulation functions  660 ,  661  are provided to transform  680 . The output of the transform  680  is input to a space-time coding function  670  that produces a respective output for each of two IFFTs  690 ,  691 . The outputs of the IFFTs  690 , 691  are output on antennas  695 ,  696 . With the example of  FIG. 9 , the transform  680  contains a sub-matrix for each of the two inputs from coding and modulation. If the sub-matrix is an identity matrix, then OFDM is used for that input. If the sub-matrix is a non-identity transformation, then transformed OFDM is used for that input. 
     The example of  FIG. 9  also includes space-time coding function  670  to achieve spatial diversity. Any of the multi-antenna transmitter embodiments may be further supplemented with space-time coding to provide for spatial diversity. 
     Except for the transform function  620 ,  680 , the components of the OFDM transmitters of  FIGS. 8 and 9  can be defined in an implementation specific manner as would be understood by one skilled in the art. In a specific example, these can be configured to operate in a similar manner to various components of the transmitter described below with reference to  FIG. 18 . 
       FIG. 9  illustrates a two antenna, single user implementation, but it is to be understood that there may be any number of antennas and corresponding coding and modulation blocks. 
     The transform function  620 , 680  in  FIGS. 8 and 9  are each shown to have an additional input indicated by  625  and  685 , respectively. In some embodiments these respective inputs are used to provide feedback to the transform function  620 , 680  for selection of the transform function in a dynamic fashion. 
     The examples of  FIGS. 8 and 9  only implement the constituent multiplexers of  FIGS. 1 and 2  (OFDM and T-OFDM), but not the constituent multiplexer of  FIG. 3  (direct multiple subcarrier modulation). Other permutations are possible. 
     A mathematical representation of processing performed by the OFDM transmitter can express each block of  FIGS. 8 and 9  as a matrix. The particular transform is selected to enable the OFDM transmitter to transmit the sequence of samples as ODFM or transformed OFDM on a per user basis. 
     If the entire content is to be transmitted using OFDM, then the transform function T is selected to be the identity matrix shown below, in which elements along a primary negative sloping diagonal are each equal to one and all other elements in the matrix equal zero. 
     
       
         
           
             T 
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                     1 
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     0 
                   
                 
                 
                   
                     0 
                   
                   
                     1 
                   
                   
                     … 
                   
                   
                     0 
                   
                   
                     0 
                   
                 
                 
                   
                     ⋮ 
                   
                   
                     ⋮ 
                   
                   
                     ⋱ 
                   
                   
                     ⋮ 
                   
                   
                     ⋮ 
                   
                 
                 
                   
                     0 
                   
                   
                     0 
                   
                   
                     … 
                   
                   
                     1 
                   
                   
                     0 
                   
                 
                 
                   
                     0 
                   
                   
                     0 
                   
                   
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                     0 
                   
                   
                     1 
                   
                 
               
               ] 
             
           
         
       
     
     Using the identity matrix as the transform function results in an output from the OFDM transmitter being that of a conventional OFDM signal. Using the identity matrix is substantially the same as if no transform were performed in the OFDM transmitter. 
     For the  FIG. 8  example, the user of the identity matrix for the transform T  620  results in the single coded and modulated output being transmitted using the entire set of OFDM sub-carriers or an assigned sub-set thereof, be that a sub-band set of sub-carriers or a diversity set of sub-carriers. For the  FIG. 9  example, this results in the two coded and modulated outputs sharing a set of OFDM sub-carriers on two antennas. 
     In the opposite extreme, the transform function can be selected to be a fast Fourier transform (FFT) performed on the entire sequence of input symbols. When the FFT has an equal number of samples used by the IFFT functions following the transform, the OFDM transmitter output is a single carrier conventional FDM transmission. In the particular case where the number of samples in the FFT and that used by the IFFT functions is the same the FFT and the IFFT are opposite transforms. The result of performing the two transforms is substantially the same as if neither of the two transforms were performed on the sequence of samples. 
     For the  FIG. 8  example, the user of a full sized FFT for the transform T  620  results in the single coded and modulated output being transmitted as if it were a single carrier transmission.  FIG. 9  differs from  FIG. 8  in that the output is a MIMO transmission. 
     For single user implementations, the mode change can be based on feedback from the receiver, or be made autonomously at the transmitter. In some embodiments, the mode is adaptively changed and/or the transform function is selected as a function of one or more of: 
     SNR—this is really a function of the distance between the receiver and the transmitter; 
     traffic type—e.g. control channel may need a better channel/performance; 
     head room at the power amplifier in the transmitter as discussed previously. 
     Referring now to  FIG. 10 , shown is an example frame  800  structure in which during a given OFDM symbol duration, only one of the multiplexing structures are employed, but within a frame consisting of a set of such OFDM durations multiple multiplexing structures are employed. The frame structure has a two dimensional appearance which is represented as collection of adjacent columns  810 , 820 . The vertical direction represents a transmission band in the frequency domain and the horizontal direction represents time slots in the time domain. In the example of  FIG. 10 , the frame  800  is divided into OFDM time slots  810  that are used for OFDM, and time slots  820  that are used for T-OFDM. A similar frame (not shown) could be implemented to divide the frame into OFDM time slots and slots containing direct multiple sub-carrier multiplexing and/or slots containing conventional FDM signals. 
     The size of the transmission band, the duration of the time slots and the number of time slots in the frame are all implementation specific parameters. The assignment of the time slots to the various constituent multiplexing types is also implementation specific, can take any desired arrangement. In some embodiments, the arrangement of time slots can vary from frame to frame. In other embodiments consecutive frames may transmit the same arrangement of time slots.  FIG. 10  can be implemented using some of the transmitter structures described previously such as the structures of  FIGS. 8 and 9 . 
       FIG. 11  shows an example frame  900  used in accordance with some embodiments of the invention. The frame has a two dimensional orientation similar to  FIG. 10  in which time is represented in the horizontal direction and frequency in the vertical direction. In  FIG. 11 , in each time slot, the transmission band is segmented into logical clusters or sub-bands  910 ,  920 . In the illustrated example, each cluster consists of contiguous sub-carriers. The clusters are shown to be of two different types, that is OFDM clusters  910  and T-OFDM clusters  920 . In some embodiments, the clusters include one or more logical sub-channels, where each sub-channel is a designated grouping of active sub-carriers. An OFDM cluster is a grouping of sub-carriers that each provide a narrowband frequency component of a transmitted signal in a particular time slot. A T-OFDM cluster is a grouping of sub-carriers that collectively provide a wideband frequency sub-band representation of a transmitted signal in a particular time slot. 
     There are shown to be four clusters in each time slot of the example of  FIG. 11 , but the number of clusters in the transmission band and the number of active sub-carriers in each cluster are implementation specific parameters. Furthermore, the size of the transmission band, the duration of the time slots and the number of time slots in the frame are all implementation specific parameters as well. The assignment of the clusters to OFDM and T-OFDM is also implementation specific, where the assignment of OFDM and T-OFDM clusters can take any desired arrangement. In some embodiments, the arrangement of clusters can vary from frame to frame. In other embodiments consecutive frames may transmit the same arrangement of clusters. In the example of  FIG. 11 , the two dimensional resource is divided into regions that are used for OFDM, and regions used for transformed OFDM (TOFDM). A similar frame (not shown) could be implemented to subdivide the frame into OFDM regions and regions containing direct multiple sub-carrier multiplexing and/or regions containing conventional FDM signals. 
       FIG. 11  can be implemented by a multi-carrier OFDM transmitter adapted to transmit both OFDM and transformed OFDM. Examples of such OFDM transmitters are shown in  FIGS. 8 and 9  and described above. 
     In  FIG. 12 , an example frame  1000  is shown for implementing a time and frequency multiplexing scheme in accordance with some embodiments of the invention. The frame  1000  is segmented into zones of different size and shape in both time and frequency. Examples of OFDM zones are indicated by  1010 ,  1025 ,  1030 ,  1045 ,  1055 ,  1060  and T-OFDM zones are indicated by  1015 ,  1020 ,  1035 ,  1040 ,  1050 ,  1065 . 
     The number of zones and manner in which the zones are distributed in the frame are implementation specific parameters. Furthermore, the size of the transmission band, the duration of the time slots and the number of time slots in the frame are all implementation specific parameters as well. The assignment of zones to OFDM and T-OFDM is also implementation specific, where the assignment of OFDM and T-OFDM zones can take any desired arrangement. In some embodiments, the arrangement of zones can vary from frame to frame. In other embodiments consecutive frames may transmit the same arrangement of zones. In the example of  FIG. 12 , the two dimensional resource is divided into regions that are used for OFDM, and regions used for transformed OFDM (TOFDM). A similar frame (not shown) could be implemented to subdivide the frame into OFDM regions and regions containing direct multiple sub-carrier multiplexing and/or regions containing conventional FDM signals. 
       FIG. 12  can be implemented by a multi-carrier OFDM transmitter adapted to transmit both OFDM and T-OFDM. Examples of such OFDM transmitters are shown in  FIGS. 8 and 9  and described above. 
       FIG. 13  shows an example pattern of a pilot design that can be used with some embodiments of the invention. In  FIG. 13 , time is shown in the vertical direction and frequency is shown in the horizontal direction. Each circle in the pattern represents content of a particular sub-carrier transmitted at a particular time. A horizontal row of such circles represents the sub-carriers transmitted in one or more symbols in a particular time slot. A vertical column represents the contents transmitted on a given scheduled sub-carrier over time. There are a finite number of sub-carriers in the frequency direction. It is to be understood that the number of sub-carriers in a symbol is a design parameter and that  FIG. 11  is to be considered to give only one example of a particular size of a symbol. 
     In  FIG. 13  pilots are generally indicated by  1110 . Each sub-carrier in a first time slot transmits a pilot and each sub-carrier in a sixth scheduled time slot transmits a pilot. Data, generally indicated by  1120 , is transmitted on each subcarrier of second to fifth time slots. The data portion of the frame is transmitted using any of the mechanisms described above, so as to allow both OFDM and T-OFDM, to coexist. A particular pilot pattern has been shown in which every first time slot and every fifth time slot thereafter is used for pilots. More generally, many different approaches to inserting pilots can be employed that may insert the pilots in the middle of frames, the end of frames, or in a scattered manner to name a few specific examples. 
     In some implementations pilots are inserted in the frequency domain in particular sequences to modulate pilot subcarriers to reduce PAPR. Pilots and data may be transmitted by different OFDM symbols. In some implementations, distributed pilots are used for T-OFDM such that the same frequency indexes are used for pilots and data for each sequence of samples from different users transmitting in the transmission band. In some implementations sub-band based pilots are used for T-OFDM such that same frequency indexes are used for pilots and data for each user transmitting in the transmission band. 
     In some embodiments, a pilot is implemented as a time domain training sequence. This can be transmitted during a reference symbol at the beginning, middle or end of frame for example. The reference symbols can include a training sequence from a single receiver or training sequences from multiple receivers. In some implementations, the training sequence is selected to be a sequence with a low PAPR. 
     Reference to  FIG. 14  will now be made in describing an arrangement for an OFDM transmitter  1200  in accordance with an embodiment of the invention. A sequence of symbols for each of a plurality of users is applied to a respective processing block to calculate a FFT for that sequence of symbols, the FFTs being specific examples of transform functions. Symbols  1210  for a first user (not shown) are provided to a first FFT processing block  1212 , symbols  1220  for a second user (not shown) are provided to a second FFT processing block  1222  and symbols  1230  for a third user (not shown) are provided to a third FFT processing block  1232 . Outputs of each of the three processing blocks  1212 ,  1222 ,  1232  are provided to a mapping function  1250 . An output of the mapping function  1250  is provided to an N sample size IFFT processing block  1260 . An output of the IFFT processing block  1260  is provided to block  1270  in which a cyclic prefix is incorporated into the output of the IFFT processing block  1260 . 
     The FFT processing blocks  1212 ,  1222 ,  1232  are shown to have FFT sample sizes of M 1 , M 2  and MN, respectively. In some embodiments the sample sizes are the same (M 1 =M 2 =MN) in all three FFT processing blocks  1212 ,  1222 ,  1232 . In some embodiments the sample sizes are different for the three FFT processing blocks  1212 ,  1222 ,  1232 . Equivalently, the three FFT processing blocks  1212 ,  1222 ,  1232  could be implemented as a single large matrix containing three sub-matrices. 
     Only three FFT processing blocks  1212 ,  1222 ,  1232  are shown in the example of  FIG. 14 . However, it is to be understood that the number of FFT processing blocks is an implementation specific parameter. 
     In operation, each FFT processing block  1212 ,  1222 ,  1232  performs the respective sample size FFT on the sequence of symbols with which it is provided. The mapping function  1250  applies a mapping to the FFT processed sequence of symbols of each user depending on whether sub-band transformed OFDM or diversity transformed OFDM is being employed. For diversity transformed OFDM, the mapping function  1250  distributes the FFT processed sequence of symbols for each sequence to sub-carriers across the transmission band, such that two sequences are not mapped to the same sub-carrier. For sub-band transformed OFDM, the mapping function  1250  maps the FFT processed sequence of symbols for each sequence within a grouping of contiguous sub-carriers in transmission band, such that two sequences are not mapped to the same grouping of sub-carriers. The output of the mapping function  1250  is provided to the N sample size IFFT processing block  1260  and the IFFT transforms a frequency spectrum of the transmission band into a time sequence of symbols. The block  1270  incorporates the cyclic prefix into the time sequence of symbols prior to transmission. 
     In some embodiments, a sum of the multiple M sample size FFT outputs of the FFT processing blocks  1212 ,  1222 ,  1232  equals a number of samples N, which is the same as the number of samples in the N sample size IFFT. In other embodiments the sum of the multiple M sample size FFT outputs of the FFT processing blocks  1212 ,  1222 ,  1232  does not equal the number of samples N. In some embodiments the number of samples is expanded or padded to equal N samples before being applied to the N sample size IFFT processing block  1260 . 
     In some implementations the OFDM transmitter includes additional processing elements between the FFT processing blocks  1212 ,  1222 ,  1232  and the IFFT processing block  1260 . An example of an additional processing element is a sample size expander to match the number of samples of the FFT processing blocks  1212 ,  1222 ,  1232  to the number of samples of the N sample size IFFT processing block  1260 . Another example of an additional processing element is a pulse shaping element to further reduce the PAPR of the transmission. 
     In some embodiments of the invention access air interface selection may be based on the PA (power amplifier) backoff room. 
       FIGS. 15 to 19  provide context for the above embodiments. Shown are specific examples of known implementations for OFDM transmitters. Many of the features shown in these figures may be included in systems that implement one or more of the constituent multiplexing structures described above. 
     For the purposes of providing context for embodiments of the invention for use in a communication system,  FIG. 15  shows a base station controller (BSC)  10  which controls wireless communications within multiple cells  12 , which cells are served by corresponding base stations (BS)  14 . In general, each base station  14  facilitates communications using OFDM with mobile and/or wireless terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. 
     A high level overview of the mobile terminals  16  and base stations  14  upon which aspects of the present invention may be implemented. With reference to  FIG. 16 , a base station  14  is illustrated. The base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , multiple antennas  28 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  16  (illustrated in  FIG. 15 ). A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  22  processes the digitized received signal to extract the information or data symbols conveyed in the received signal. This processing may comprise demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 . 
     On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  28  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the base station and the mobile terminal. 
     With reference to  FIG. 17 , a mobile terminal  16  configured according to one embodiment of the present invention is illustrated. Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , multiple antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  34  processes the digitized received signal to extract the information or data symbols conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, the baseband processor  34  receives digitized data, which may represent voice, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station. 
     In operation, OFDM may be used for the uplink and or the downlink transmission between the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with “n”&gt;=1 transmit antennas  28 , and each mobile terminal  16  is equipped with “m”&gt;=1 receive antennas  40 . Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labeled only for clarity. 
     With reference to  FIG. 18 , a logical OFDM transmission architecture will be described. Initially, the base station controller  10  will send data to be transmitted to various mobile terminals  16  to the base station  14 . The base station  14  may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI for each mobile terminal  16  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
     Scheduled data  44 , which is a stream of symbols, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  16 . Again, the channel coding for a particular mobile terminal  16  is based on the CQI. In some implementations, the channel encoder logic  50  uses known Turbo encoding techniques. The encoded data is then processed by rate matching logic  52  to compensate for the data expansion associated with encoding. 
     Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . In some embodiments, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation may be chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
     At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic  60 , which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal  16 . The STC encoder logic  60  will process the incoming symbols and provide “n” outputs corresponding to the number of transmit antennas  28  for the base station  14 . The control system  20  and/or baseband processor  22  as described above with respect to  FIG. 2  will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the “n” outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal  16 . 
     For the present example, assume the base station  14  has two antennas  28  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic  60  is sent to a corresponding IFFT function  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT functions  62  will operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT functions  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
     Reference is now made to  FIG. 19  to illustrate reception of the transmitted signals by a mobile terminal  16 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC)  74  to control the gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
     Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time domain to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. The synchronization logic  76  may include frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
     At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the subcarriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM subcarriers in a known pattern in both time and frequency. Examples of scattering of pilot symbols among available sub-carriers over a given time and frequency plot in an OFDM environment are found in PCT Patent Application No. PCT/CA2005/000387 filed Mar. 15, 2005 assigned to the same assignee of the present application. Continuing with  FIG. 19 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
     The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols. 
     The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The deinterleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then deinterleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for de-scrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
     In parallel to recovering the data  116 , a CQI, or at least information sufficient to create a CQI at the base station  14 , is determined and transmitted to the base station  14 . As noted above, the CQI may be a function of the carrier-to-interference ratio (CR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. The channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each subcarrier throughout the OFDM frequency band being used to transmit data. 
       FIGS. 15 to 19  each provide a specific example of a communication system or elements of a communication system that could be used to implement embodiments of the invention. It is to be understood that embodiments of the invention can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.