Abstract:
Disclosed is an Orthogonal Frequency Division Multiple Access (OFDMA) based wireless communications system operable to communicate OFDMA type signals over a set of dynamically assigned orthogonal sub-carriers and Code Division Multiple Access (CDMA) type signals over a set of pre-allocated orthogonal sub-carriers. The OFDMA system utilizes pre-allocated orthogonal sub-carriers for CDMA type signal transmission in order to reduce the number of dynamic assignments of orthogonal sub-carriers in a typical OFDMA system. The OFDMA type signals may be signals processed in accordance with well-known OFDMA techniques, whereas the CDMA type signals may be signals processed in accordance with well-known CDMA and OFDMA techniques. The CDMA type signals may also be processed using a pre-coder incorporating a Discrete Fourier Transformer (DFT) matrix or Identity matrix to reduce the Peak-to-Average Power Ratio across the OFDMA system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to wireless communications network and, in particular, to wireless communications network employing orthogonal frequency division multiple access techniques.  
       BACKGROUND  
       [0002]     Orthogonal Frequency Division Multiple Access (OFDMA) has emerged as the leading multiple access technique for next generation wireless communications systems. OFDMA systems are multi-carrier systems in which a bandwidth is divided into a set of orthogonal sub-carriers. The set of orthogonal sub-carriers are further sub-divided into subsets, wherein each subset of orthogonal sub-carriers forms a traffic channel. Each traffic channel can be assigned exclusively to a single user.  
         [0003]      FIG. 1  depicts a transmitter  100  used in an OFDMA system in accordance with the prior art. Transmitter  100  comprises a modulator  110 , a serial-to-parallel (S2P) converter  120 , an Inverse Fast Fourier Transformer (IFFT) module  130 , a cyclic prefix inserter  140 , and a time domain filter  150 . IFFT module  130  includes N ports for receiving modulation symbols. Each of the ports is associated with an orthogonal sub-carrier. IFFT module  130  is operable to use an N×N IFFT matrix to perform an transform operation on its inputs, wherein the entries of the matrix F j,k  are defined as F j,k =e −2πijk/n ,j,k=0, 1, 2, . . . , n−1 and i=√{square root over (−1)}.  
         [0004]     Encoded data symbols are provided as input to modulator  110 . Modulator  110  uses well-known modulation techniques, such as BPSK, QPSK, 8 PSK, 16 QAM and 64 QAM, to convert the encoded data symbols into K modulation symbols S k  which are then provided as input to S2P converter  120 , where K≦&lt;=N. S2P converter  120  outputs parallel streams of modulation symbols which are provided as inputs to one or more ports of IFFT module  130  associated with orthogonal sub-carriers over which the encoded data symbols are to be transmitted. In IFFT module  130 , an inverse fast Fourier transformation is applied to the modulation symbols S k  to produce a block of chips c n , where n=0, . . . , N−1. Cyclic prefix inserter  140  copies the last N cp  chips of the block of N chips and prepends them to the block of N chips producing a prepended block. The prepended set is then filtered through time domain filter  150  and subsequently modulated onto a carrier before being transmitted.  
         [0005]     Compared to its predecessor systems, OFDMA systems enables a more efficient use of bandwidth allocation with increased tolerance to noise and multi-path. OFDMA systems, however, do have several disadvantages. One such disadvantage is that a considerable amount of its forward link capacity is utilized for overhead signaling of reverse link sub-carrier assignments. In OFDMA systems, reverse link sub-carrier assignments are not static. Users are dynamically assigned or reassigned sub-carriers on the reverse link depending on factors such as channel conditions, available resources and type of service. Each assignment and reassignment requires a channel assignment message to be sent over the forward link, wherein the channel assignment indicates the sub-carriers being assigned. Due to this dynamic nature of reverse link channel assignment, the volume of channel assignment messages increase which, in turn, consumes a considerable amount of the forward link capacity.  
         [0006]     One other disadvantage is that OFDMA systems have a high peak-to-average power ratio (PAPR) compared to single carrier systems. When IFFT module  130  performs a transform operation on modulation symbols S k , the result is a block of N chips C n =ΣS k (a)e −i2πjk/N     FFT   , which is a phase weighted sum of modulation symbols S l , . . . S K , wherein S k (a) represents the amplitude of modulation symbol S k . Since each chip c n , is essentially a combination of each of the modulation symbols, the amplitude associated with each chip c n , would be higher compared to its average amplitude over time resulting in a higher PAPR of transmitted waveforms. Multi-carrier systems with higher PAPR require higher rating power amplifiers and have inferior link budgets resulting in coverage limitations, as compared to single carrier systems.  
         [0007]     Accordingly, there exists a need for reducing the amount of overhead signaling on the forward link and lowering the PAPR in OFDMA systems.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is an Orthogonal Frequency Division Multiple Access (OFDMA) based wireless communications system operable to communicate OFDMA type signals over a set of dynamically assigned orthogonal sub-carriers and Code Division Multiple Access (CDMA) type signals over a set of pre-allocated orthogonal sub-carriers. Advantageously, the present invention OFDMA system utilizes pre-allocated orthogonal sub-carriers for CDMA type signal transmission in order to reduce the number of dynamic assignments of orthogonal sub-carriers and overhead signaling associated therewith in a typical OFDMA system. In one embodiment, the OFDMA type signals may be signals generated in accordance with well-known OFDMA techniques, whereas the CDMA type signals may be signals generated in accordance with well-known CDMA and OFDMA techniques. The CDMA type signals may also be processed using a pre-coder incorporating a Discrete Fourier Transformer (DFT) matrix to reduce the Peak-to-Average Power Ratio of transmitted waveforms. In other embodiments, the pre-coder may be bypassed and effectively replaced by an identity matrix, or the pre-coder may incorporate a matrix which depends on the frequency domain channel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
         [0010]      FIG. 1  depicts a transmitter used in an OFDMA system in accordance with the prior art;  
         [0011]      FIG. 2  depicts a bandwidth allocation for use in the OFCDMA system of the present invention; and  
         [0012]      FIG. 3  depicts an schematic diagram of transmitter for use in the wireless communications system of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]     The present invention is an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communications system operable to communicate OFDMA type signals over a set of dynamically assigned orthogonal sub-carriers and Code Division Multiple Access (CDMA) type signals over a set of pre-allocated orthogonal sub-carriers, wherein OFDMA type signals are signals generated in accordance with well-known OFDMA techniques and CDMA type signals are signals generated in accordance with well-known CDMA and OFDMA techniques. Advantageously, CDMA type signals are transmitted over pre-allocated orthogonal sub-carriers and, thus, do not require the dynamic assignment of orthogonal resources (e.g. sub-carriers). Preferably, CDMA type signals are signals associated with users with bursty and periodic traffic patterns.  
         [0014]     The OFDMA system of the present invention is a multi-carrier system in which a bandwidth is divided into a set of orthogonal sub-carriers.  FIG. 2  depicts a bandwidth allocation  200  for use in the OFCDMA system of the present invention. As shown in  FIG. 2 , a bandwidth is divided into a set of orthogonal sub-carriers. The set of orthogonal sub-carriers are categorized into two groups. The first group, referred to herein as OFDMA group, comprises of orthogonal sub-carriers used for the transmission of OFDMA signals. The second group, referred to herein as CDMA group, comprises of orthogonal sub-carriers used for the transmission of CDMA type signals. The OFDMA and CDMA groups include one or more sub-groups referred to herein as OFDMA and CDMA zones, respectively. Each zone includes at least one orthogonal sub-carrier. In one embodiment, the CDMA zones are non-adjacent to each other and equidistant apart from its neighboring CDMA zones. In another embodiment, the CDMA zones can be adjacent to each other. In yet another embodiment, the CDMA zones may occupy the entire bandwidth, i.e., no OFDMA zones.  
         [0015]     A traffic channel comprising of orthogonal sub-carriers in the OFDMA group is referred to herein as an OFDMA traffic channel, whereas a traffic channel comprising of orthogonal sub-carriers in the CDMA group is referred to herein as an CDMA traffic channel. As mentioned earlier, OFDMA type signals are signals generated in accordance with well-known OFDMA techniques, and CDMA type signals are signals generated in accordance with well-known CDMA and OFDMA techniques. In another embodiment, OFDMA type signals may be signals generated in accordance with the well-known Interleaved Frequency Division Multiple Access (IFDMA) technique, or any type of technique for generating signals over a Frequency Division Multiple Access (FDMA) system. Similarly, the CDMA type signals may be generated in accordance with only CDMA techniques, or with CDMA and IFDMA techniques.  
         [0016]      FIG. 3  depicts a schematic diagram of transmitter  300 , in accordance with one embodiment, for use in the wireless communications system of the present invention. Transmitter  300  comprises a first portion  380  for processing CDMA type signals, and a second portion  390  for processing OFDMA type signals. First portion  380  comprises multipliers  305 ,  310 ,  320 ,  325 , summer  325 , serial-to-parallel (S2P) converter  330 , a K pre-coders  335 , Inverse Fast Fourier Transform (IFFT) module  350 , cyclic prefix inserter  360 , and time domain filter  370 . Second portion  390  comprises modulator  340 , S2P converter  345 , IFFT module  350 , cyclic prefix inserter  360  and time domain filter  370 . Pre-coders  335  are operable to use a Discrete Fourier Transform (DFT) matrix or a matrix based on the frequency domain channel to perform a transform operation on its inputs. Each pre-coder  335  has N z  output ports. IFFT module  350  is operable to use an IFFT matrix to perform a transform operation on its inputs. IFFT module  350  has N FFT  input ports, wherein the N FFT  input ports include K×N z  ports associated with orthogonal sub-carriers belonging to CDMA zones, and N FFT -K×N z  input ports associated with orthogonal sub-carriers belonging to OFDMA zones.  
         [0017]     In first portion  380 , pilot symbols and encoded data symbols are provided as inputs into multipliers  305 ,  310 . The pilot and encoded data symbols are spread using spreading codes, such as Walsh codes, with spreading factors N cp  and N cd , respectively. In one embodiment, spreading factor N cp  is equal to N z , which is the number of CDMA zones in the wireless communications system. The spread pilot and data symbols are subsequently scrambled in multipliers  315 ,  320  using a pilot and a data scrambling code, such as Pseudo-random Noise (PN) codes, to produce pilot and data chips, respectively, wherein the scrambling codes have a period N and N&gt;&gt;N cp ,N cd . The scrambling codes may be CDMA zone specific. Additionally, the scrambling codes may have different offsets for the pilot and data branches of first portion  380 . The pilot and data chip streams are code multiplexed in summer  325  to produce a code multiplexed signal, wherein the code multiplexed signal comprises of K×N z  code multiplexed chips. In another embodiment, the pilot and data chip streams are time multiplexed. For purposes of this application, a CDMA type signal may be construed to be the code or time multiplexed chip signal or any signal derived from the code or time multiplexed chip signal.  
         [0018]     The code multiplexed signal is provided as input to S2P converter  330  where it distributes the code multiplexed chips equally among K pre-coders  335 . In one embodiment, the code multiplexed chips may be provided as a block of N z  code multiplexed chips. For example, the first N z  code multiplexed chips are provided as input to the first pre-coder  335 , the next N z  code multiplexed chips are provided as input to the second pre-coder  335 , and so on. In another embodiment, the S2P converter  330  may distribute the code multiplexed chips unevenly among K or less pre-coders, and the block of code multiplexed chips may be a size different from N z .  
         [0019]     Pre-coders  335  use a matrix to perform a transform operation on an input vector in the time domain into a vector in the frequency domain. Note that the input and output vectors of pre-coders  335  comprise of N z  elements or chips. In one embodiment, pre-coders  335  are Discrete Fourier Transformers (DFT) which use a DFT matrix F of size N z  xN z  to transform the input vector comprising of the N z  code multiplexed chips from the time domain to the frequency domain, wherein the entries for matrix F are defined as F j,k  =e −i2πjk/N     z   ,j,k=0,1,2, . . . , n−1 and i=√{square root over (−1)}. If the code multiplexed chips at the input of DFT pre-coder are defined as vector s, where S=[S 1 , S 2 , S 3 , . . . ,S Nz ] T  and T denotes the transpose operation, the output of DFT pre-coder can be defined as vector x, where  
       x   =         1       N   z         ⁢   Fs     =       [       x   1     ,   …   ⁢           ,     x     N   z         ]     T           
 
 and comprises of N z  pre-coded elements or chips. In other embodiments, pre-coders  335  may use an identity matrix to transform the code multiplexed chips into the frequency domain from the time domain. Additionally, pre-coders  335  may use a matrix which is channel sensitive allowing for pre-equalization techniques to be applied to the transformation. 
 
         [0020]     In one embodiment, each of the N z  output ports of the K pre-coders  335  are separately mapped to ports of IFFT  350  associated with orthogonal sub-carriers belonging to CDMA zones. The exact mapping of the N z  output ports to the input ports of IFFT module  350  may be reconfigurable depending on which particular orthogonal sub-carriers the CDMA type signals are to be transmitted.  
         [0021]     In second portion  390 , encoded data symbols are modulated by modulator  340  using well-known modulation techniques, such as BPSK, QPSK, 8PSK, 16QAM and 64QAM, to convert the data symbols into K modulation symbols Sk which are then provided as input to S2P converter  345 , where K≦N. S2P converter  120  outputs parallel streams of modulation symbols which are provided as inputs to one or more ports of IFFT module  130  associated with orthogonal sub-carriers over which the encoded data symbols are to be transmitted.  
         [0022]     In IFFT module  350 , an inverse fast Fourier transformation is applied to the modulation symbols S k  and to pre-coded chips (i.e., output of pre-coder) to produce a block of chips c n , where n=0, . . . , N FFT −1. Cyclic prefix inserter  360  copies the last N cp  chips of the block of N FFT  chips and prepends them to the block of N FFT  chips producing a prepended block. The prepended set is then filtered through time domain filter  150  and subsequently modulated onto a carrier before being transmitted.  
         [0023]     Although the present invention has been described in considerable detail with reference to certain embodiments, other versions are possible. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein.