Abstract:
A preprocessing apparatus and method for a transmitter and a receiver to prevent system performance degradation caused by Inter-Channel Interference (ICI) or inter-symbol interference (ISI) in an OFDM system. In the OFDM system using N carriers having different frequency bands and K carriers of the N carriers designated as redundant carriers, the transmitter renders ICI-causing parts to be zeroes in a data frame, when a cyclic prefix is not longer than a channel impulse response. Therefore, the ICI is prevented. The receiver eliminates the ISI involved in a current frame using an interference estimated from a previous data frame. Therefore, noise spread caused by the use of redundant carriers is prevented and system complexity is remarkably reduced.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Transmitting/Receiving Apparatus and Method in an Orthogonal Frequency Division Multiplexing System Using Insufficient Cyclic Prefix” filed in the Korean Intellectual Property Office on Jul. 8, 2003 and assigned Serial No. 2003-46201, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) system, and in particular, to a preprocessing apparatus and method in a transmitter/receiver, which prevents system performance from being degraded due to ICI (Inter-Channel Interference) or ISI (Inter-Symbol Interference).  
         [0004]     2. Description of the Related Art  
         [0005]     OFDM is a multi-carrier modulation scheme that splits data over a number of carriers and transmits the split data in parallel at a low rate so that frequency-selective fading or narrowband interference is mitigated, as opposed to conventional single carrier schemes that transmit data at high rate. In OFDM, spectra of sub-carriers overlap with orthogonality, which increases spectral efficiency. A transmission signal is modulated by IFFT (Inverse Fast Fourier Transform) and a received signal is demodulated by FFT (Fast Fourier Transform). Thus, a digital modulator and demodulator can be implemented efficiently. A major benefit from this configuration is that a receiver is easily implemented using an equalizer that requires more than one complex multiplication per carrier.  
         [0006]     In OFDM, interference caused by multi-path fading is mitigated by increasing a symbol period in proportion to a number of sub-channels. In this mitigation method, a guard interval is inserted every predetermined number of symbols. The last symbols of a frame comprising as many symbols as carriers are duplicated in the guard interval. These symbols are termed a cyclic prefix (CP). The CP must be longer than a channel impulse response (CIR) representing the channel characteristics of a radio channel.  
         [0007]     If the CIR is longer than the CP, then ICI and ISI degrade system performance. The length of the CP is given preliminarily in an actual system. In view of the random property of an air interface, the CIR may have a longer period than the CP during transmission. The transmission over a radio channel is characterized by frequency selectivity.  
         [0008]     In general, if a channel gain is 0 (null) or a frequency band having a bad SNR (Signal to Noise Ratio) exists because of significant interference, the frequency band is excluded from data transmission. Techniques like AMC (A daptive Modulation and Coding), which are useful in a nomadic or indoor office environment, leave the frequency band as redundancy. In this context, a scheme of suppressing ICI and ISI caused by an insufficient CP length using the redundant frequency band has been proposed. This technique is termed FEQ-DMT (Frequency-domain Equalizer-Discrete Multi-tone Modulation).  
         [0009]     For explanation purposes, herein below, the carriers delivering user data will be called data carriers and all other carriers will be called redundant carriers in an OFDM system.  
         [0010]     An FEQ-DMT transmitter transmits a null signal over redundant carriers. In other words, it transmits no signals over the redundant carriers. An FEQ-DMT receiver then extracts interference signals from signals over the redundant carriers and subtracts the extracted interference signals from signals over data carriers, thereby eliminating the effects of the interference.  
         [0011]      FIG. 1  is a block diagram of a conventional FEQ-DMT receiver using a redundant frequency band. It is assumed that the total number of carriers is N, including M data carriers and K redundant carriers. Therefore, N=M+K.  
         [0012]     Referring to  FIG. 1 , an FEQ-DMT receiver  100  comprises a serial to parallel (S/P) converter  110 , a CP remover  120 , an FFT  130 , a redundant carrier sorter  140 , a Q filter  150 , adders  160 - 1  to  160 -M, and a 1-tap FEQ (Frequency Equalizer)  170 . The S/P converter  110  collects a predetermined number of received serial symbols and outputs them in parallel. The CP remover  120  removes CPs from the symbols. The FFT  130  demodulates the output of the CP remover  120  by FFT and outputs N frequency components, Y[0], Y[1], . . . , Y[N−2], Y[N−1] corresponding to N frequencies. The redundant carrier sorter  140  separates data carrier components Y[p 1 ], . . . , Y[p K ] and redundant carrier components Y[u 1 ], . . . , Y[u M ] from the N components.  
         [0013]     The Q filter  150  extracts interference components I[u 1 ], . . . , I[u M ], which are caused by the data carriers using the redundant carrier components, by a known interference extraction technique. The interference components cover ICI and ISI. The adders  160 - 1  to  160 -M eliminate the influence of the interference by subtracting the interference components I[u], . . . , I[u M ] from the data carrier components Y[p 1 ], . . . , Y[p K ]. The 1-tap FEQ  170  equalizes the outputs of the adders  160 - 1  to  160 -M. Although not illustrated in  FIG. 1 , a decider is further included in the FEQ-DMT receiver  100 , for detecting the original data symbols from the equalized data.  
         [0014]     One of the shortcomings of the FEQ-DMT receiver as illustrated in  FIG. 1  is the complexity of the Q filter. Another shortcoming is that noise in the signals over the redundant carriers is added to signals over the data carriers in the Q filter, resulting in noise enhancement. Because the redundant carriers usually have a bad SNR, noise over the redundant carriers spreads to the data carriers, thereby yielding even worse system performance.  
       SUMMARY OF THE INVENTION  
       [0015]     Therefore, an object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an object of the present invention is to provide a transmitting/receiving apparatus and method for eliminating an influence of interference, while reducing system complexity in an OFDM system using an insufficient CP.  
         [0016]     Another object of the present invention is to provide a transmitting/receiving apparatus and method for preventing ICI caused by an insufficient CP using a redundant frequency band in an OFDM system.  
         [0017]     A further object of the present invention is to provide a transmitting/receiving apparatus and method for preventing ISI caused by an insufficient CP using a redundant frequency band in an OFDM system.  
         [0018]     The above objects are achieved by providing a preprocessing apparatus and method for a transmitter and a receiver to prevent system performance degradation caused by ICI or ISI in an OFDM system.  
         [0019]     According to one aspect of the present invention, in a transmitting apparatus of an OFDM system in which N carriers having different frequency bands are used and K ones of the N carriers are set as redundant carriers, a P filter receives (N−K) data symbols and generates K virtual data symbols. An IFFT, having N input taps corresponding to the N carriers, receives the (N−K) data symbols at (N−K) taps corresponding to data carriers other than the redundant carriers, receives the K virtual data symbols at K taps corresponding to the redundant carriers, inverse-fast-Fourier-transforms the data symbols and the virtual data symbols, and outputs an IFFT data frame. Here, the virtual data symbols are set such that values causing ICI in the data frame become zeroes.  
         [0020]     According to another aspect of the present invention, in a receiving apparatus in an OFDM system using N carriers having different frequency bands, a CP remover receives a data frame and removes a CP of a predetermined length from before the data frame, and an FFT fast-Fourier-transforms the output of the CP remover and outputs N frequency components corresponding to the N carriers. A plurality of adders are used to subtract an interference estimated from a previous data frame from the frequency components. A 1-tap FEQ equalizes the outputs of the adders. A decider detects original data symbols from the equalized data. An interference estimator estimates interference from the detected data symbols and provides the interference estimate to the adders for a next data frame period. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0022]      FIG. 1  is a block diagram of a conventional FEQ-DMT receiver using a redundant frequency band;  
         [0023]      FIGS. 2A and 2B  illustrate the structures of OFDM data frames, referred to for describing the present invention;  
         [0024]      FIG. 3  is a block diagram of a transmitter in an OFDM system according to an embodiment of the present invention;  
         [0025]      FIG. 4  illustrates extraction of L-1:L−L cp  data symbols by use of a matrix A;  
         [0026]      FIG. 5  illustrates sorting redundant carriers among a plurality of carriers;  
         [0027]      FIG. 6  is a block diagram illustrates a receiver when a CP is not used according to an embodiment of the present invention;  
         [0028]      FIG. 7  is a block diagram illustrates a receiver when a CP is used according to another embodiment of the present invention;  
         [0029]      FIG. 8  is a graph illustrating a static channel environment having a fractional power distribution; and  
         [0030]      FIG. 9  is a graph illustrating results of a simulation comparing performance according to the present invention with FEQ-DMT performance. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0032]     As indicated above, the present invention, among other things, is intended to prevent system performance degradation caused by ICI and ISI involved in a number of carriers.  
         [0033]      FIG. 2A  illustrates a structure of an OFDM data frame with a longer CP than a CIR. As illustrated in  FIG. 2A , an nth frame is X n   1:N  and the length of a CP is L cp  Here, N is both the length of a transmission frame and the number of entire carriers. A copy of the last L cp  of the N samples is attached as cyclic CP before the nth frame. The length of a CIR is L. If L cp  is equal to or greater than L, because a longer CP than a CIR is interposed between frames, the CP prevents ICI and ISI.  
         [0034]      FIG. 2B  illustrates the structure of an OFDM frame with a CIR that is longer than a CP. Here, L is greater than L cp (L−L cp ) more CP samples are needed to cope with the CIR. In an nth frame, there is sample data of the previous frame, X n−1   N−Lcp+1:N  in place of expected CP sample data, that is, sample data of the current frame, X n   N−L+1:N−L+Lcp . The expected but not transmitted CP samples X n   N−L+1:N−L+Lcp  and the unexpected samples of the previous frame X n−1   N−Lcp+1:N  cause ICI and ISI, respectively.  
         [0035]     The ICI and ISI can be prevented by transmitting 0s at those sample positions. Specifically, a transmitter transmits 0s at particular data positions over redundant carriers of a frequency band. The number of necessary redundant carriers is proportional to the number of necessary zero symbols.  
         [0036]     As illustrated in  FIG. 2B , however, because symbols causing the ICI and ISI, respectively, are arranged at different positions, the number of redundant carriers required to prevent both the ICI and ISI is two times greater than that of the redundant carriers required to prevent either of them. An increase in the number of redundant carriers causes a decrease in frequency efficiency. Based on the idea that a receiver easily eliminates the ISI if channel characteristics can be estimated, the transmitter prevents the ICI and the receiver eliminates the ISI in accordance with the present invention. Accordingly, system configuration is simplified and the effects of the ICI and ISI are minimized without decreasing frequency efficiency.  
         [0037]      FIG. 3  is a block diagram of a transmitter in an OFDM system according to an embodiment of the present invention. As above, it is assumed herein below that the number of entire carriers is N, including M data carriers and K redundant carriers. Therefore, N=M+K.  
         [0038]     Referring to  FIG. 3 , a transmitter  200  includes a P filter  210 , an IFFT (Inverse Fast Fourier Transformer)  220 , a parallel to serial (P/S) converter  230 , and a CP adder  240 . Other known components of the transmitter  200 , for example, a modulator, a spreader, and an RF (Radio Frequency) module are not shown.  
         [0039]     The P filter  210  generates virtual data symbols V 1:K  to be transmitted over redundant carriers using (N−K) input user data symbols U 1:N−K . The virtual data symbols V 1:K  have values that render positions causing the ICI on a time axis to be zeroes. In the case illustrated in  FIG. 2B , the ICI-causing positions are N−L+1:N−L+L cp . The operation of the P filter  210  will be described later in more detail.  
         [0040]     The (N−K) data symbols, U 1:N−K  are applied to input taps corresponding to used carriers among the input taps of the IFFT  210 , and the K virtual data symbols V 1:K  are applied to the other input taps of the OFFT  220  corresponding to unused carriers, that is, redundant carriers. The IFFT  220  modulates the input data symbols by IFFT. The P/S converter  230  serializes the output of the IFFT  220 , and the CP adder  240  adds a CP of length L cp  before the serial data, prior to transmission on a radio channel.  
         [0041]     As described above, N is the total number of carriers, K is the number of redundant carriers, L is the length of a CIR, and L cp  is the length of a CP. More accurately, L is an estimated maximum length of the CIR. A receiver measures CIR characteristics and feeds back the measurement to a transmitter. The transmitter then estimates the maximum length of a CIR sample based on the measurement.  
         [0042]     If U Q   1:N−K  represents user data symbols and V Q   1:K  represents virtual data symbols, 
 
 V   1:K   n   =PU   1:N−K   n   (1) 
 
 where P denotes the filtering tap coefficients of the P filter  210 , which can be expressed as a K×(N−K) matrix. The data symbols and the virtual data symbols are applied to the input of the IFFT  220 . The input of the IFFT  220  can be represented as X I:N , where, 
 
 X   1:N   n   =S   1   U   1:N−K   n   +S   0   V   1:K   n   (2) 
 
 and S 0  is an N×K matrix that assigns V 1:K  to redundant carriers, and S 1  is an N×(N−K) matrix that assigns U 1:N−K  to data carriers. The output of the IFFT  220  is expressed as 
 
 X   1:N   n   =W   N   *X   1:N   n   (3) 
 
 where the FFT output x n   1:N  is the time-domain value of X n   1:N , W N  is an N-point FFT function, and (.)* is the conjugate. 
 
         [0046]     As illustrated in  FIG. 2B , samples associated with the ICI in the data frame are x n   N−L+1:N−Lcp , which are extracted from x n   1:N  by 
 
 x   N−L+1:N−L     cp     n   =Ax   1:N   n   (4) 
 
 where A is an (L−L cp )×N matrix for extracting L−L cp :L−1 data symbols. A is defined as 
 
 A=[ 0 (L−L     cp     )×(N−L)   I   (L−L     cp     ) 0 (L−L     cp     )×L     cp   ]  (5) 
 
 where 0 is a zero matrix and I is an identity matrix. Therefore, A is a combination of a (L−L cp )×(N−L) zero matrix, an (L−L cp )×(L−L cp ) identity matrix, and a (L−L cp )×L cp  zero matrix. 
 
         [0049]      FIG. 4  illustrates extraction of L−L cp :L−1 data symbols by the matrix A. As illustrated in  FIG. 4 , x N−L+1:N−L+L     cp    is extracted by multiplying the matrix A by an (N×1) data symbol matrix, x.  
         [0050]      FIG. 5  illustrates an example of carriers classified by S 0  and S 1  when N=4 and first and third carriers are designated as redundant. As illustrated in  FIG. 5 , S 0  is a 4×2 matrix having the first and third rows of a 4×4 I matrix, and S 1  is a 4×2 matrix having the other rows of the I matrix. Virtual data symbols V 1  and V 2  are assigned to first and third positions, while data symbols U 1 and U   2  are assigned to second and fourth positions, according to these matrices.  
         [0051]     The filtering coefficient P of the P filter  210  must be set such that x N−L+1:N−Lcp  is 0. By substituting Eq. (1) and Eq. (3) for Eq. (4), then, 
 
x N−L+N−L     cp     n   =AW   N *( S   1   U   1:N−K   n   +S   0   PU   1:N−K   n )=0 (L−L     cp     )×1   (6) 
 
 which can be reduced to 
 
 P=− ( AW   N   *S   0 ) −1   AW   N   *S   1   (7) 
 
         [0053]     As described above, the transmitter transmits the virtual data symbols over the redundant carriers by use of the P filter  210 , thereby preventing the ICI even when the CP is shorter than the CIR. A special case of the CP being shorter than the CIR may occur in which the CP is not used. Referring to  FIG. 2 , in this case, the data symbols x n   N−L++1:N  of the current frame cause the ICI, and the data symbols x n−1   N−L++1:N  of the previous frame cause the ISI. That is, the ICI and ISI arise from the same position in one frame.  
         [0054]     When the CP is not used, prevention of the ICI leads to prevention of the ISI. Thus, the transmitter prevents both the ICI and ISI by inserting virtual data symbols. In summary, in the presence of the CP, the ISI occurs in a received signal, while in the absence of the CP, no ISI is generated in the received signal in the transmitter of the present invention. Therefore, when the CP is not used, that is, the CP adder  245  is omitted in the transmitter illustrated in  FIG. 3 , a receiver being the counterpart of the transmitter does not need an additional component for eliminating the interference.  
         [0055]      FIG. 6  is a block diagram illustrating a receiver when a CP is not used according to an embodiment of the present invention. The receiver is configured so as to eliminate ISI in a received signal. As illustrated in  FIG. 6 , the receiver is applicable to both cases where virtual data symbols are transmitted at redundant frequencies, and where zero symbols are transmitted over redundant carriers.  
         [0056]     Referring to  FIG. 6 , a receiver  300  comprises an S/P converter  310 , an FFT  320 , a redundant carrier sorter  330 , a 1-tap FEQ  340 , and a decider  350 . The S/P converter  310  collects received symbols in one frame and outputs them in parallel. The FFT  320  demodulates the parallel symbols by FFT and outputs N frequency components, Y[0], Y[1], . . . , Y[N−2], Y[N−1] corresponding to N frequencies. The redundant carrier sorter  330  separates the N frequency components into data carrier components Y[p 1 ], . . . , Y[p K ] and redundant carrier components Y[u 1 ], . . . , Y[u M ] from the N components.  
         [0057]     The redundant carrier components are neglected because they are used to prevent interference that affects the data carriers. The 1-tap FEQ  340  equalizes the data carrier components, which are interference-free due to the redundant carrier components. The decider  350  detects the original data symbols from the equalized data.  
         [0058]     When a CP is used, that is, when the length of the CP is not 0 and less than the length of a CIR, ISI is generated during transmission. Therefore, the receiver needs an additional component for removing the ISI.  
         [0059]      FIG. 7  is a block diagram illustrating a receiver when a CP is used according to another embodiment of the present invention. The receiver is configured so as to eliminate ISI in a received signal. Further the receiver is applicable to both cases where virtual data symbols are transmitted at redundant frequencies, and where zero symbols are transmitted over redundant carriers.  
         [0060]     Referring to  FIG. 7 , a receiver  400  comprises an S/P converter  410 , a CP remover  420 , an FFT  430 , a redundant carrier sorter  440 , adders  450 - 1  to  450 -M, a 1-tap FEQ  460 , a decider  470 , a delay (D)  480 , and an H ISI  filter  490 . As compared to the receiver  300  illustrated in  FIG. 6 , the receiver  400  further includes the CP remover  420 , the adders  450 - 1  to  450 -M, the delay  480 , and the H ISI  filter  490 .  
         [0061]     The S/P converter  410  collects received serial symbols in one frame and outputs them in parallel. The CP remover  420  removes CPs from the symbols. The FFT  430  demodulates the output of the CP remover  420  by FFT and outputs N frequency components, Y[0], Y[1], . . . , Y[N−2], Y[N−1] corresponding to N frequencies. The redundant carrier sorter  440  separates the N frequency components into data carrier components Y[p 1 ], . . . , Y[p K ] and redundant carrier components Y[u 1 ], . . . , Y[u M ]. The redundant carrier components are neglected because they are used to prevent ICI.  
         [0062]     The adders  450 - 1  to  450 -M subtract interference estimated from data symbols of a previous frame from the data carrier components involving ISI, thereby eliminating the influence of the ISI. The 1-tap FEQ  460  equalizes the data carrier components and the decider  470  detects the original data from the equalized data.  
         [0063]     The delay  480  delays the detected data by one frame period so that the delayed data can be used to estimate interference for the next frame. The H ISI  filter  490  functions as an interference estimator for computing an interference estimate from the delayed data by a known interference extraction technique. The interference estimate is provided to the adders  450 - 1  to  450 -M, to eliminate interference from the next frame.  
         [0064]     If a CIR of length L is represented as [c 0 , c 1 , . . . , C L ], the filtering coefficient of the H ISI  filter  490  for eliminating the ISI is determined by  
                 H   ISI     =       S   1   T     ⁢     W   N     ⁢     C   ISI     ⁢       W   N   *     ⁡     (       S   1     +       S   0     ⁢   P       )           ,     
     ⁢       C   ISI     =     [           0       (     L   -     L   cp       )     ×     (     N   -   L   +     L   cp       )               C   t               0       (     N   -   L   +     L   cp       )     ×     (     N   -   L   +     L   cp       )               0       (     N   -   L   +     L   cp       )     ×     (     L   -     L   cp       )               ]       ,     
     ⁢       C   t     =     [           c   L           c     L   -   1           ⋯         c       L   cp     +   1               0         c   L         ⋯         c       L   cp     +   2               ⋯       ⋰       ⋰       ⋯           0       ⋯       0         c   L           ]       ,           (   8   )             
 
 where (.) T  is the transpose. 
 
         [0066]     The receiver having the above-described configuration eliminates interference from the current frame using an ISI estimate detected from the previous frame, which obviates the need for redundancy in the frequency band. While the receivers  300  and  400  are illustrated with the redundant carrier sorters  330  and  440  in  FIGS. 6 and 7 , without redundant carriers, that is, if all carriers are assigned for data transmission, the redundant carrier sorters  330  and  440  are unnecessary.  
         [0067]     The conventional FEQ-DMT requires (N−K)×K complex multiplications in a receiver to implement a Q filter. However, because a received signal contains values at decimal places to the right of the decimal point, the complex multiplications require complex hardware or software. However, the inventive transmitter  200  can be implemented more easily with shift registers by use of a P filter having an integer input. Similarly, the inventive receiver  400  can be implemented without multiplications because it estimates an interference using an integer input. Therefore, a more simplified configuration is possible for the receiver.  
         [0068]     The performance of the transmission/reception technology according to the present invention will be compared below with the conventional technology, especially FEQ-DMT. The filtering coefficient of a Q filter used in the FEQ-DMT is expressed as 
 
 Q=−S   1   T   W   N   A   red ( S   0   T   W   N   A   red ) −1   (9) 
 
 where W N  is an N-point FFT matrix, S 0  is an N×K matrix that assigns V 1:K  to redundant carriers, and S 1  is an N×(N−K) matrix that assigns U 1:N−K  to data carriers, as stated before. A red  is an (L−L cp )×N matrix for extracting 1:L−L cp  data symbols, expressed as 
 
 A   red   =[I   (L−L     cp   )0 (L−L     cp     )×(N−L+L     cp     ) ]  (10) 
 
         [0070]     To verify the performance of the present invention, the performance of the FEQ-DMT system and the system performance according to the present invention were simulated under a static channel environment showing a fractional power distribution, as illustrated in  FIG. 8 . For the simulation, 128 carriers, a 5-MHz bandwidth, and AWGN (Additive White Gaussian Noise) having an SNR of 30 dB were assumed. It was also assumed that the sum of transmission signal powers is constant and data carriers have uniform transmission power. The simulation result was represented in terms of an effective data rate under the condition of an error rate of 10 −7  and a margin of 6 dB. In the simulation, a CIR corresponding to two samples in length was compensated for by using carriers having too poor an SNR for bit loading together with a CP in the FEQ-DMT and the present invention.  
         [0071]      FIG. 9  illustrates the results of the simulation. Reference numeral  10  denotes the transmission/reception structure using a CP according to the present invention, reference numeral  20  denotes an FEQ-DMT reception structure using the CP, and reference numeral  30  denotes a transmission/reception structure using the CP without additional component for interference elimination. It is noted from  FIG. 9  that the FEQ-DMT using the CP and the present invention using the CP exhibit better performance than the use of only the CP. Accordingly, it is concluded that the transmission/reception scheme of the present invention minimizes performance degradation caused by a CIR that is longer than a CP.  
         [0072]     As described above, the present invention reduces system complexity remarkably without deepening system deterioration caused by the use of redundant carriers when a CIR that is longer than a CP appears. The inventive transmission/reception scheme is easily implemented by integer input-based multiplications and prevents noise spread caused by the redundant carriers.  
         [0073]     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.