Abstract:
A apparatus and method for improved performance in radio telecommunications systems, and in particular multi-carrier code division multiple access (MC-CDMA) networks that employ pulse-shaping filters on the transmit side of a radio link. In order to more accurately transmit a radio signal bearing a symbol sequence, the modulated and spread information stream is upsampled using a technique that involves inserting zeros in the frequency domain. A corresponding downsampling procedure on the receive side permits reconstruction of the transmitted symbols. A new channel estimation algorithm may also be used, and the improved channel estimation advantageously employed to obtain more faithful symbol detection.

Description:
CLAIM OF BENEFIT (35 U.S.C. § 119(e))  
       [0001]     This Application claims the benefit of U.S. Provisional Application No. 60/398,418, filed Jul. 25, 2002, which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to radio telephony, and more specifically to a method and apparatus for using improved sampling and channel estimation techniques to improve the performance of wideband MC-CDMA radio telecommunication systems that use pulse-shaping filters.  
       BACKGROUND OF THE INVENTION  
       [0003]     Radio telephony, generally speaking, involves the use of portable radios for telephone communication by a user through a radio telecommunication network. The network connects a large number of network subscribers with each other and, usually, with subscribers of other networks as well. Connections between calling and called parties are made using a network infrastructure that includes information channels and switching devices to route calls to the appropriate destination. Connected subscribers may engage in voice conversations or exchange text messages, email, or other forms of data.  
         [0004]     In a radio telecommunication network, the only actual radio-frequency connection may be that between the subscribers&#39; radios and the network infrastructure (although this is not necessarily true). Infrastructure nodes are often fixed in location and interconnected using wires, cables, or optical fibers so that they can transfer large amounts of information. The radio connection to the subscribers is important, however, because it gives them mobility. In ideal circumstances, a subscriber can make a call and maintain the connection even when traveling over dozens, or even hundreds of miles.  
         [0005]     In order for such communications to occur, radio telecommunication networks typically employ a large number of fixed base stations spread over a wide geographic area, sometimes referred to as the network coverage area. Each base station employs one or more antennas for communicating with nearby mobile stations, and of course is also connected to the remainder of the infrastructure as well.  
         [0006]      FIG. 1  is a simplified block diagram illustrating selected components of a typical radio telecommunication network  100 . Base stations  105 - 110  are each shown to be connected with an antenna  111 - 116 . Each antenna is intended to handle communications within a selected area, sometimes referred to as a cell. (For this reason the portable subscriber radios used in such a network are often called “cellular” or simply “cell” phones.) For example, in  FIG. 1  cell phones  11 ,  12 , and  13  are shown to be in communication with antennas  111 ,  112 , and  113 , via radio channels  1 ,  2 , and  3 , respectively.  
         [0007]     The broken lines in  FIG. 1  represent cell boundaries. These boundaries do not represent the precise range of their associated antennae, of course, and are not always regular in shape or consistent in size. And although only six cells are delineated, there are typically many more in the network coverage area. Cell phones may and often do move from cell to cell, and their network communications are generally transferred from one network antenna to another though a process called handover.  
         [0008]     Base station controllers (BSCs)  120  and  125  are in communication with, and generally control the operations of base stations  105 - 107  and  108 - 110 , respectively. The base station controllers are in turn connected with a mobile switching center (MSC)  130 , which handles call routing and provides a connection to other network MSCs (not shown) and gateway MSCs such as G-MSC  135 , which may provide a connection to another network. A visitor location register, here VLR  140 , maintains information relating to cell phones in the area services by the associated MSC. (A home location register (HLR) (not shown), may be provided to track the location and other information related to all network subscribers.)  
         [0009]     Note that while cellular telephones have traditionally been used for voice communication, advances in technology have permitted the introduction and growing use of such instruments for other applications including sending of text messages, instant messages, data transfer, and Web surfing. Some have even incorporated functions previously performed by personal digital assistants (PDAs), such as appointment calendaring. For this reason the wide variety of such devices capable of communicating through a radio telecommunication will be referred to simply as “mobile stations”.  
         [0010]     In radio telecommunication networks, the cellular architecture provides a number of advantages. For one, in many networks channelization for individual subscriber or control communications is implemented by assigning different frequencies to each channel. By controlling the range of these communications, assigned frequencies may be reused in non-adjacent cells without creating interference between users. In addition, the close proximity of base stations with which to communicate means that mobile stations can communicate with lower transmission power than if they had to reach distant antennas. Conservation of power is, of course, an important objective of battery-operated devices.  
         [0011]     Frequency channelization in the mobile context is frequently referred to as frequency division multiple access (FDMA). Each mobile station is assigned one or more frequencies within the overall operational bandwidth of communicating with the base station. In some systems, each communication frequency is also divided into time slots, a scheme referred to as time division multiple access (TDMA). In TDMA, each mobile station is assigned one or more of these slots and transmits a portion of its information in turn. Naturally, the slots are of sufficient duration and frequency so that each user perceives their own conversation as continuous.  
         [0012]     Another type of multiple access scheme is called code-division multiple access (CDMA). CDMA operates somewhat differently; rather than divide the available transmission bandwidth into individual channels, many individual transmissions are spread over a frequency band using a spreading code. Transmissions intended for a particular receiver (i.e. mobile station) are spread with spreading code assigned to the mobile station, which decodes only that information intended for it and ignores the differently-coded transmissions intended for others. The number of mobile stations that can operate in a given area is therefore limited by the number of unique encoding sequences available, rather than the number of frequency bands. The operation of a CDMA network is normally performed in accordance with a protocol referred to as IS-95 (interim standard-95) or, increasingly, according to its third generation (3G) successors, such as those sometimes referred to as CDMA2000, 1xEV-DO, and 1xEV-DV, the latter of which provides for the transport of both data and voice information.  
         [0013]     In a more recently developed scheme, the use of CDMA techniques is combined with orthogonal frequency division multiplexing (OFDM). OFDM is a modulation method in which multiple user symbols are transmitted in parallel using a large number of different sub-carriers. These sub-carriers, sometimes called frequency bins, are used to spread transmitted information with respect to frequency rather than time (as with conventional CDMA). This multiple access scheme is sometimes referred to as multi-carrier CDMA (or MC-CDMA).  
         [0014]      FIG. 2  is a simplified block diagram illustrating selected components of a typical MC-CDMA telecommunication system  200 . As depicted in  FIG. 2 , system  200  has a transmit side  205  and a receive side  210 . On the transmit side information, which may be either voice or data for transmission, is first encoded in encoder  215 . The encoded information is passed to modulator  220  for modulation according to one of several modulation schemes such as QPSK or 16 QAM. The modulated symbols are then provided to an MC-CDMA transmitter  225  for transmission over an air interface radio channel  230 .  
         [0015]     The transmitted information is received on the receive side by an MC-CDMA receiver  235 , which processes the information and presents it to detector  240  for symbol detection. Simply stated, detector  240  attempts to faithfully reconstruct the transmitted symbol stream by removing from the received signal the effects of any distortion or noise added in transmission. In part, these undesirable but unavoidable effects are removed, or at least mitigated, by analyzing the quality of certain received symbols called pilot symbols. These pilot symbols are not part of the transmitted user information, but are inserted into it. Their transmitted value is known to the receiver, which can estimate channel effects from the condition in which they are received. The detected symbol stream is then presented to decoder  245  for decoding. The decoded information is stored or provided to a user interface such as a speaker (not shown) so that it may be perceived by the user.  
         [0016]     Another form of interference that may distort transmitted radio communications is referred to as inter-symbol interference (ISI). IS arises largely from the multipath effect, a phenomenon that occurs when a propagating radio signal fans out and encounters, for example, different reflecting surfaces and propagation media creating a number of ‘copies’ of the same signal that may each arrive at the receiver at slightly different times. Transmit filters, such as pulse-shaping filters, that are used to limit the frequency content of the transmitted signal can also introduce ISI.  
         [0017]     Channel equalizers in the receiver are often used to counter ISI induced by the multipath-effect. For ISI caused by the transmit filter, a matched filter may be implemented in the receiver to create an ISI-free composite filter. This approach is not always taken, however. For example, the pulse-shaping filters specified in CDMA standards such as IS-95 and 1xEv-DV are not ISI-free. As a result, the pulse-shaping filters in these systems introduce unabated ISI that degrades the ability of the receive side to accurately estimate the channel and detect the transmitted symbols.  
         [0018]     Needed, therefore, is a way to reduce or eliminate ISI effects in MC-CDMA systems that use pulse-shaping filters in order to improve system performance. The present invention provides just such a solution.  
       SUMMARY OF THE INVENTION  
       [0019]     In one aspect, the present invention is an improved apparatus for use in a radio telecommunication system including a transmitter and a receiver, the transmitter including pulse-shaping filter having a known frequency response and an orthogonal frequency division multiplexing (OFDM) modulator for taking an inverse fast Fourier transform (IFFT) of the symbol stream to be transmitted after the stream has been divided into sub-streams and zeros inserted between the sub-streams. Preferably the stream is divided into two sub-streams of equal length and the zeros are inserted as high-frequency components. The apparatus may further include a receiver to receive a transmitted radio signal, take a fast Fourier transform (FFT), and remove the previously inserted zeros. The apparatus may also apply the known pulse-shaping filter frequency response in a least squares analysis to determine a maximum likelihood estimate of the transmission channel frequency response.  
         [0020]     In another aspect, the present invention is a receiver for receiving a time-domain signal transmitted by a MC-CDMA transmitter that upsamples a symbol stream and inserts zeros in the frequency domain before converting the symbol stream into the time domain and mapping the symbols into frequency bins for transmission, the receiver including an OFDM demodulator that removes the zeros from the symbol stream after converting it back to the frequency domain.  
         [0021]     In yet another aspect, the present invention is a method of processing an MC-CDMA signal including the steps of encoding the information, modulating the encoded signal onto a carrier, dividing the modulated signal into a plurality of streams and spreading each stream with a spreading code, summing the spread streams into a symbol sequence of length N b , dividing the symbol sequence into to a plurality of portions, inserting zeros between each separate portion, taking an IFFT of the resulting stream to obtain a time-domain signal, providing the time-domain signal to a pulse-shaping filter, and transmitting the signal. The method may further include the steps of receiving the transmitted signal and removing the previously-inserted zeros. Finally, the method may also include the step of applying the known pulse-shaping filter frequency response in a least squares analysis to determine a maximum likelihood estimate of the transmission channel frequency response, and the step of applying the channel estimate thus determined in a detector to detect the transmitted symbol stream. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     For a more complete understanding of the present invention, and the advantages thereof, reference is made to the following drawings in the detailed description below:  
         [0023]      FIG. 1  is functional block diagram illustrating the relationship of selected components of a typical CDMA telecommunication network, such as one that might advantageously employ the system and method of the present invention.  
         [0024]      FIG. 2  is a simplified block diagram illustrating selected components of a typical MC-CDMA telecommunication system.  
         [0025]      FIG. 3  is a flow diagram illustrating a method of radio transmission in an MC-CDMA radio telecommunication system.  
         [0026]      FIG. 4  is a functional block diagram illustrating selected components of a radio telecommunication system operable according to an embodiment of the present invention.  
         [0027]      FIG. 5  is a flow diagram illustrating a method of channel estimation according to an embodiment of the present invention.  
         [0028]      FIG. 6  is a flow diagram a method of processing and transmitting a radio signal according to an embodiment of the present invention.  
         [0029]      FIG. 7  is a flow diagram illustrating a method of receiving and processing a radio signal according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]      FIGS. 1 through 7 , discussed herein, and the various embodiments used to describe the present invention are by way of illustration only, and should not be construed to limit the scope of the invention. Those skilled in the art will understand the principles of the present invention may be implemented in any similar radio-communication device, in addition to those specifically discussed herein.  
         [0031]     The present invention is directed to a system and method for communication in a radio telecommunication network, and is of particular advantage when applied to a multi-carrier code division multiple access (MC-CDMA) system that includes a pulse-shaping filter on the transmit side.  FIG. 3  is a flow diagram illustrating a relevant portion of the process of signal transmission in such a system. At START, it is assumed that the information to be transmitted has been encoded and modulated. The modulated symbols are then spread (step  305 ) using an appropriate spreading code and provided to the OFDM modulator. (While one is discussed here, there may be and often is more than one symbol stream.) The OFDM modulator converts the signal to a time-domain signal (step  310 ) by taking an inverse fast Fourier transform (IFFT) and provides this signal to a pulse-shaping filter. After pulse-shaping (step  315 ), the signal is transmitted (step  320 ) over an air interface.  
         [0032]     At the receiver, the transmitted signal is received (step  325 ) and converted from the time domain to the frequency domain (step  330 ) in a demodulator applying a fast Fourier transform (FFT). A channel estimate is made (step  335 ), and the demodulated signal is provided to a detector for symbol detection (step  340 ). (Note that the timing and regularity of channel estimation may vary with system design.)  
         [0033]      FIG. 4  is a simplified block diagram illustrating an exemplary system  400  for sending information over an air interface using MC-CDMA in accordance with an embodiment of the present invention. The portion of the Figure above the broken line represents a transmitter  401 , such as one that might be found in a telecommunication network base station, and below is illustrated a receiver  451  for example one operating in a mobile station. The broken line itself represents a multipath channel over the air interface of the radio telecommunication network.  
         [0034]     In transmitter  401 , serial-to-parallel (S/P) converter  405  splits the modulated symbol streams (of all K users) into K blocks of J streams (S 0,0  to S K-1,J-1 ). Each of these streams s is spread by multiplication with a Walsh-Hadamard code (c 0  to c j-1 ), and then presented to a summer ( 410   0  . . .  410   k  . . .  410   K−1 ), which sums the streams associated with each block 0 to K−1 into a single spread stream (S 0  to S K−1 ). The spread streams S k  are then passed through S/P converters  415   0  . . .  415   k  . . .  415   K−1  before being presented to interleaver  420  for block interleaving. The output of the interleaver  420  is labeled X, which represents a symbol stream of length N b . OFDM modulator (IFFT)  425  is coupled to interleaver  420  and maps the interleaved signal into frequency bins (sub-carriers), and may add a cyclic prefix.  
         [0035]     In accordance with the present invention, prior to taking an IFFT of the symbol stream X (here X 0  . . . X k  . . . X K−1 ) OFDM modulator  425  first separates the stream into two streams (of length N b /2 each). Between the two halves a group of N b (N s−1 ) zeros at a high frequency are inserted, creating a new vector of size N b N s . This means that the IFFT will be larger (that is, of size N b N s ) but the resulting time-domain signal will have the desired sampling rate N s  without having modified the frequency content of the signal. The resulting time-domain signal is then passed through a pulse-shaping filter  430  and transmitted over a radio channel using antenna  435 .  
         [0036]     Receiver  451  includes the antenna  453  for receiving the transmitted radio signal. The received signal may first be passed through a matched band-pass receive filter  455  to suppress out-of-band noise and interference. Note that under certain conditions, the matched filter may be unnecessary, as discussed below. The filtered signal is then passed through an OFDM demodulator (FFT)  460  and demodulated into frequency-domain signal Z k  (signals of other blocks may be present as well, but for simplicity only one is shown). In accordance with the present invention after taking the FFT (of size N b N s ), the previously added high-frequency components (zeros) are removed and the two haves of the symbol stream are rejoined to form a single stream of length N b . Deinterleaver  465  deinterleaves signal Z k  and is coupled to parallel-to-serial (P/S) converter  470   k , which creates a bit stream Y k  (again, there may be one associated with each block, even though only one stream is shown in  FIG. 4 ). A detector  475   k  generates soft or hard decision outputs for each original symbol or bit stream (Ŝ k,0  to Ŝ K,J−1 ).  
         [0037]     As mentioned above, the present invention involves a new upsampling and downsampling technique, the advantages of which will now be described in greater detail. In accordance with an embodiment of the present invention, the OFDM modulator  425  input vector X (see  FIG. 4 ) is divided into two parts of equal length N b /2 by inserting between them N b (N s−1 ) zeros at high frequency to form a vector of length N b N s . OFDM modulator  425  then applies an IFFT of size N b N s  to obtain a time-domain signal that is then passed to pulse-shaping filter  430  for processing prior to transmission. Note that using this method the time-domain signal achieves the desired sampling rate of Ns samples per chip without modifying the frequency content of the signal.  
         [0038]     In the receiver  451 , the OFDM demodulator  460  receives the time domain signal and applies an FFT of size N b N s  before removing the previously-inserted N b (N s−1 ) high-frequency components. A frequency domain signal of length N b  comprising the two low-frequency parts of the symbol stream may then be de-interleaved and further processed.  
         [0039]     Note that removing the previously-inserted high-frequency components is, in effect, a lowpass filtering, and for this reason may eliminate the need for a separate receive filter (such as filter  455  shown in  FIG. 4 ), so long as the bandwidth of the received time-domain signal is not greater than N s /T c  Hz.  
         [0040]     In accordance with the present invention, the performance of the receiver is also enhanced by an improved channel estimation technique. As mentioned above, channel estimation may be performed by evaluating the condition of received pilot symbols.  FIG. 5  is a flow diagram illustrating a method  500  of channel estimation according to an embodiment of the present invention. At START, it is assumed that an MC-CDMA telecommunication system such as that illustrated in  FIG. 4  has been provided. It is also assumed that the transmission channel is a multi-path channel of L c  samples in length, and that at least L c  pilot symbols have been equally spaced though the transmitted symbol stream. In this embodiment, the pilot symbols are inserted in the frequency domain.  
         [0041]     First, the pilot symbols are collected from deinterleaved symbol stream Y to form vector Y p  (step  505 ). In similar fashion, the vectors H cp  and            p  and the matrix H tp  are formed (step  510 ) from the corresponding elements of H c ,          , and H t  respectively (corresponding to the channel, the additive noise, and the transmit pulse-shaping filter). These are related as follows: 
 
 Y   p   =H   tp   H   cp   +N   p  
 
         [0042]     If W is a matrix consisting of the first L c  columns of the OFDM demodulator FFT matrix, then H c  is a diagonal matrix: 
 
 H   c   =diag ( Wh   c ) 
 
 and if W p  is a matrix considering only those rows including transmitted pilot signals: 
 
H cp =W p h c . 
 
 then: 
 
 Y   p   =H   tp   W   p   h   c   +N   p . 
 
         [0045]     A maximum-likelihood (ML) value for the channel impulse response h c  is then estimated (step  515 ). To obtain the ML estimate for h c , the following log-likelihood function is maximized: 
 
 L ( h   c )=ln  p ( Y   p   |h   c )= A−B||Y   p   −H   tp   W   p   h   c || 2 , 
 
 which since A and B are constant scalar quantities (with B&gt;0), is equivalent to the following optimization:  
           h   ^     c     =         arg   ⁢           ⁢   min       h   c       ⁢              Y   p     -       H   tp     ⁢     W   p     ⁢     h   c              2           
 
         [0047]     This optimizing problem may be evaluated as a least squares (LS) problem using the method of singular value decomposition (SVD). If the SVD of H tp W p  is given by H tp W p =USV H , then: 
 
ĥ c =VS −1 U H Y p . 
 
         [0048]     The channel estimate according to the present invention may then be obtained by taking the FFT of ĥ c  (step  520 ) as follows: 
 
 Ĥ   c   =diag ( Wĥ   c )= diag ( WVS   −1   U   H   Y   p ) 
 
 Finally, since W, H t , V, S, and U are known, a matrix L=WVS −1 U H  can be calculated (step  525 ) and the channel estimation for each OFDM symbol may be expressed as Ĥ c =diag(LY p ). The method of the present invention thereby advantageously applies the known transmit pulse-shaping filter frequency response. 
 
         [0050]     As mentioned above, in accordance with an embodiment of the present invention the channel estimate is applied in detector  475   k . Having calculated the channel estimate separately, it is now combined with the (known) frequency response of the transmit pulse-shaping filter H t  to form a composite channel matrix          =H c H t . If the composite channel effect related to the k-th transmit block is represented as            k , and the corresponding additive noise is            k , then the input to the detector  475   k  may be represented as: 
 
 Y   k   =H   k   S   k   +N   k . 
 
 From these new parameters the transmitted symbol stream may be reconstructed by application of a variety of methods, for example by applying a conventional matched-filter detector, or by using a maximum likelihood detector. 
 
         [0052]      FIG. 6  is a flow chart illustrating a method  600  of transmitting a radio signal according to an embodiment of the present invention. At START, it is presumed that a transmitter such as transmitter  401  shown in  FIG. 4  has been provided. The method begins when information to be transmitted is encoded (step  605 ). The encoded information is then modulated (step  610 ) according to any one of several existing schemes. The modulated symbols are then divided into K blocks of J streams each (step  615 ). Each stream within a block is spread with a unique spreading code (step  620 ), generally a Walsh-Hadamard code, and summed into a single stream (step  625 ). This symbol stream is then re-divided into parallel paths (step  630 ) in a serial-to-parallel converter for interleaving (step  635 ) with the streams of the other blocks.  
         [0053]     In accordance with the present invention, the interleaved output X (in this illustration X 0  through X K−1 ) is divided into two streams (step  640 ) each of length N b /2. Then, N b (N s −1) zeros are inserted between the two parts (step  645 ), forming a new vector of length N b N s . These signals are then mapped into frequency bins (step  650 ) using an OFDM modulator that takes an inverse fast Fourier transform (IFFT) of size N b N s  to obtain a time-domain signal of the same length.  
         [0054]     The result is a time-domain signal of sampling rate N, that was obtained without modifying the signal&#39;s frequency content. The time-domain signal is then presented to a pulse-shaping filter (step  655 ) and then transmitted (step  660 ) over a radio channel.  
         [0055]      FIG. 7  is a flow chart illustrating a method  700  of receiving a radio signal according to an embodiment of the present invention. At START, it is presumed that a receiver such as receiver  451  shown in  FIG. 4  has been provided, and that a signal according to the present invention has been transmitted. First the transmitted time-domain radio signal is received (step  705 ). In order to recover N b  symbols at the output of the OFDM demodulator, a fast Fourier transform (FFT) of size N b N s  is applied to the received signal (step  710 ), converting the signal back to the frequency domain, and then the middle N b (N S -1) high-frequency symbols are discarded (step  715 ).  
         [0056]     The N b  symbols output from the OFDM demodulator (represented by the vector Z in  FIG. 4 ), are then provided to a deinterleaver for deinterleaving (step  720 ), and the originally transmitted blocks are reconstructed (step  725 ). In order to reproduce the transmitted symbol streams, a channel estimation is performed (step  730 ), and using the estimated channel, each block is presented to a detector for symbol detection (step  735 ).  
         [0057]     The foregoing description therefore provides an improved system and method for transmitting information in a MC-CDMA telecommunication system that reduces or eliminates the ISI due to typically used (non-ideal) pulses-shaping filters in the MC-CDMA transmitters. An improved channel estimation technique is also provided, and the improved estimate is advantageously applied in symbol detection.  
         [0058]     The preferred descriptions are of preferred examples for implementing the invention, and the scope of the invention should not necessarily be limited by this description. Rather, the scope of the present invention is defined by the following claims.