Multicarrier modulation receiver using remodulation

Abstract: A Fourier transform circuit (21) in a receiver (2) performs the Fourier transform on a orthogonal frequency division multiplexing signal received through a transmission channel (3) for each symbol to sequentially reproduce a received reference vector sequence and a received vector sequence. An interleave circuit (22) interleaves elements of the received reference vector sequence and the received vector sequence. A memory (23) stores the received reference vector sequence. A vector modulation circuit (24) modulates the received reference vector sequence by respective signal point vectors representing each signal point to generate a modulation vector sequence for each symbol. A metric generation circuit (25) obtains a difference between the received vector sequence and the modulation vector sequence to generate the difference as a branch metric sequence. A trellis decode circuit (26) reproduces a data sequence on the basis of the branch metric sequence.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to methods of receiving an orthogonal frequency division multiplexing signal and receivers thereof, and more particularly, to a method of receiving an orthogonal frequency division multiplexing signal and a receiver thereof for receiving through a transmission channel the orthogonal frequency division multiplexing signal formed by multiplexing a plurality of modulating waves orthogonal to each other for each symbol having a predetermined length, and maximum-likelihood-decoding a data sequence transmitted by a transmitting side. Description of the Background Art In recent years, communications using an orthogonal frequency division multiplexing (hereinafter referred to as OFDM) signal have been attracting attentions in mobile digital sound broadcasting, digital terrestrial television broadcasting etc. This is due to the fact that the OFDM signal can transmit a large amount of data at a high speed, its characteristic is hardly degraded by a reflected wave even if there is no waveform equalizer, and its signal waveform is in a shape close to a random noise, whereby other services are not easily subjected to radio interference. A transmission system using the OFDM signal is disclosed in "A Digital Modulation Method for Terrestrial Digital Broadcasting Systems Using a Trellis Coded OFDM under Multipath Conditions" written by Masafumi Saito and Shigeki Moriyama of NHK Science and Technical Research Laboratories, Transactions of Television Society Vol. 47, No. 10, pp.1374 to 1382, October 20, 1993, where it is discussed that trellis coding modulation is combined with the OFDM signal for improving a bit error rate characteristic. US-A-5,274,629 (Helard, J.F. et al.) discloses a method for the broadcasting of digital data, notably for radio broadcasting and high-bit rate towards mobile receivers, with time-frequency interlacing and coherent demodulation. More precisely, the invention disclosed is aimed at providing a method of digital broadcasting with time-frequency interlacing enabling a coherent demodulation, in facilitating the extraction of phase and amplitude references. EP-A-0 373 277 (Liethoudt, T. et al.) discloses a system for modulation and demodulation of a plurality of frequencies using a trellis code as modulation code. The system comprises an encoder, a discrete inverse Fourier-transformer and a D/A converter at the transmitter side, and an A/D converter, a discrete Fourier-transformer an a decoder at the receiver side. Further, selection of the modulation code in view of a reliable transmission is discussed. "Trellis coded orthogonal frequency division multiplexing for digital video transmission", Proceedings of the Global Telecommunications Conference, IEEE, vol. -, December 2, 1991, XP000332782, ISBN: 0-87942-697-7) by Helard, J.F. et al. discloses the combination of orthogonal frequency division multiplexing and trellis coding for digital video transmissions. Further, coherent detection of a coded orthogonal frequency division multiplexing signal in the Rayleigh fading channel and quality criteria for code selection in the Rayleigh fading channel are discussed. Fig. 2 is a block circuit diagram showing the whole structure of a conventional system disclosed in the above prior art. In Fig. 2, a transmitter 100 and a receiver 200 are connected through a wire or radio transmission channel 300. The receiver 200 includes a Fourier transform circuit 210, an interleave circuit 220, a differential demodulation circuit 230, a metric generation circuit 250, and a trellis decode circuit 260. The differential demodulation circuit 230 includes a memory 231 and a division circuit 232. Operation of the system in Fig. 2 is subsequently described. The transmitter 100 trellis-codes a data sequence to be transmitted to the receiver 200 so as to distribute the data sequence to any one of a plurality of signal points with a predetermined digital modulation scheme (16QAM, QPSK, for example), to generate a modulation vector M x (i, f) (i: an arbitrary symbol number, f: a frequency of a carrier which the modulation vector M x is allotted to). Next, the transmitter 100 multiplies the modulation vector M x (i, f) by a transmitted reference vector X(p, f) to generate a transmitted vector X(i, f)=M x (i, f)·X(p, f) of the frequency f of the symbol i. The transmitted reference vector X(p, f) is a transmitted vector transmitted by a symbol p (which is regularly transmitted) precedent to the symbol i, and represents references of amplitude and phase of the modulation vector M x (i, f). Next, the transmitter 100 applies an interleave operation which interleaves elements of the transmitted vector sequence. The interleave operation has also been applied to the transmitted reference vector sequence. Next, the transmitter 100 allots the transmitted vector X(i, f) of the whole frequency for each symbol to each of a plurality of carriers (528, for example) and applies the inverse Fourier transform to generate an OFDM signal x(t) of the symbol i in the time domain, and transmits the signal to the transmission channel 300. When a transmission characteristic including multi-paths of the transmission channel 300 is taken as h(t) and a noise added in the transmission channel 300 is taken as n(t), an OFDM signal y(t) received by the receiver 200 is y(t)=x(t)*h(t)+n(t). * represents a convolution operation. The Fourier transform circuit 210 in the receiver 200 applies the Fourier transform to the OFDM signal y(t) in the time domain to generate a received vector sequence Y(i, f) (Y(i, f)=X(i, f)· H(i, f)+N(i, f)) in the frequency domain corresponding to the transmitted vector X(i, f). The Fourier transform circuit 210 also applies the Fourier transform to the symbol p to generate a received reference vector sequence Y(p, f) (Y(p, f)=X(p, f).H(p, f)+N(p, f)) corresponding to the transmitted reference vector X(p, f). H(p, f) and H(i, f) are obtained by converting the transmission characteristic h(t) of the transmission channel 300 in the frequency domain. N(p, f) and N(i, f) are obtained by converting the noise n(t) in the frequency domain. The interleave circuit 220 interleaves orders of the elements of the received reference vector sequence Y(p, f) and the received vector sequence Y(i, f) outputted from the Fourier transform circuit 210 to output the sequences. This is to prevent an excess of error correction capability of the trellis decode circuit 260 by randomizing at the receiver 200 a burst-like degradation of transmission characteristic added at the transmission channel 300 through interleaving the elements. Although outputs from the Fourier transform circuit 210 and the interleave circuit 220 are different in the orders of their elements, the outputs from the interleave circuit 220 are also represented as the received reference vector sequence Y(p, f) and the received vector sequence Y(i, f) which are outputs of the Fourier transform circuit 210, as far as generality is not impaired. The memory 231 in the differential demodulation circuit 230 holds in a prescribed period the received reference vector sequence Y(p, f) of the symbol p outputted from the interleave circuit 220, and outputs the received reference vector sequence Y(p, f) when the symbol i is received. For eliminating an influence by the transmission characteristic of the transmission channel 300, the division circuit 232 differential-demodulates by dividing the received vector sequence Y(i, f) by the received reference vector Y(p, f) to generate a demodulation vector sequence My(i, f) represented in the following equation (1). My ( i , f )= Y ( i , f )/ Y ( p , f ) = { X ( i , f )· H ( i , f )+ N ( i , f )} { X ( p , f )· H ( p , f )+ N ( p , f )} The characteristic H(i, f) of the transmission channel 300 is taken to have a small varied in time. Therefore, the characteristics at the time of the symbol i and the symbol p are taken as the same, and it is taken that H(i, f)=H(p, f)=H(f). Since generality is not impaired when the reference vector is taken as X(p, f)=1 and the noise N(i, f) is normalized with X(p, f), when X(p, f)=1, X(i, f)=Mx(i, f). Therefore, the demodulation vector sequence My(i, f) of the equation (1) is represented as the following equation (2). My ( i , f )= { Mx ( i , f )· H ( f ) +N ( i , f )} { H ( f )+ N ( p , f )} = Mx ( i , f )+ { N ( i , f )- Mx ( i , f )· N ( p , f )} { H ( f )+ N ( p , f )} When a difference between the modulation vector sequence Mx(i, f) and the demodulation vector sequence My (i, f) is taken as an error vector sequence E(i, f), the error vector sequence E(i, f) is represented as the following equation (3). E ( i , f )= My ( i , f )- Mx ( i , f ) = { N ( i , f )- Mx ( i , f )· N ( p , f )} { H ( f )+ N ( p , f )} The metric generation circuit 250 obtains for each branch of trellis coding a difference between the demodulation vector sequence My(i, f) outputted from the division circuit 232 and a signal point vector M(k) (k is a natural number not more than m (k=1,...,m) and m is a number of modulation scheme (in case of 16QAM, m=16)) which represents each signal point, to output the difference as a branch metric sequence BM(i, f, k) shown in the following equation (4). BM ( i , f , k )= My ( i , f )- M ( k ) = Mx ( i , f )- M ( k )+ E ( i , f ) The trellis decode circuit 260 performs trellis decoding on the basis of the branch metric sequence BM(i, f, k) outputted from the metric generation circuit 250 to reproduce decoded information most likelihood to the data sequence. In the transmission channel, as multi-paths may have an effect to cause fading and the transmission characteristic of some frequency may be almost 0. In the receiver 200 in Fig. 2, when {H(f)+N(p, f)} is close to 0, since the error vector sequence E(i, f) becomes an excessively large value by the equation (3) because differential demodulation includes a division in {H(f)+N(p, f)}, a demodulation vector in which noise elements are amplified is outputted. Therefore, there has conventionally been a problem that a large error occurs in the branch metric sequence BM(i, f, k) to degrade decoding performance of trellis decoding. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a method of receiving an orthogonal frequency division multiplexing signal and a receiver thereof which can prevent increase of errors of a branch metric even if a transmission characteristic of a transmission channel is degraded. The present invention has the following characteristics to attain the above object. A first aspect of the present invention is directed to a method of receiving an orthogonal frequency division multiplexing signal as defined in independent Claim 1. A second aspect of the present invention is directed to a receiver for receiving an orthogonal frequency division multiplexing signal as defined in independent Claim 3. According to the first and second aspects, since the received reference vector sequence is modulated with a signal point vector representing each signal point to generate the modulate vector sequence for each symbol, and the difference between the received vector sequence and the modulation vector sequence is obtained and its difference is generated as the branch metric sequence, no large error occurs in the branch metric sequence even if the transmission characteristic of the transmission channel is degraded, thereby preventing degradation of trellis demodulation capability. In the first and second aspects, orders of elements of the received vector sequence and the received reference vector sequence may be interleaved. Since this randomizes the burst-like degradation of transmission characteristic, demodulation capability of trellis demodulation is exerted without exceeding error correcting ability. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block circuit diagram showing the whole structure of an example of an embodiment of the present invention. Fig. 2 is a block circuit diagram showing the whole structure of a conventional system. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is subsequently described referring to the drawings. Fig. 1 is a block circuit diagram showing the whole structure of an example of an embodiment of the present invention. In Fig. 1, a transmitter 1 and a receiver 2 are connected through a wire or radio transmission channel 3. The receiver 2 includes a Fourier transform circuit 21, an interleave circuit 22, a memory 23, a vector modulation circuit 24, a metric generation circuit 25 and a trellis decode circuit 26. Operation of the system in Fig. 1 is subsequently described. The transmitter 1 trellis-codes a data sequence to be transmitted to the receiver 2 so as to distribute the data sequence to any one of a plurality of signal points with a prescribed digital modulation scheme (16QAM, QPSK, for example), to generate a modulation vector M x (i, f) (i: an arbitrary symbol number, f: a frequency of a carrier which the modulation vector M x is allotted to). Next, the transmitter 1 multiplies the modulation vector M x (i, f) by a transmitted reference vector X(p, f) to generate a transmitted vector X(i, f)=M x (i, f)·X(p, f). The transmitted reference vector X(p, f) is a transmitted vector transmitted by a symbol p (which is regularly transmitted) precedent to the symbol i, and represents references of amplitude and phase of the modulation vector M x (i, f). Next, the transmitter 1 applies an interleave operation which interleaves elements of the transmitted vector sequence. The interleave operation has also been applied to the transmitted reference vector sequence. Next, the transmitter 1 allots the transmitted vector X(i, f) of the whole frequency for each symbol to each of a plurality of carriers (528, for example) and applies the inverse Fourier transform to generate an OFDM signal x(t) of the symbol i in the time domain. Next, the transmitter 1 performs a frequency conversion on the OFDM signal x(t) from a base band to a transmission band of the transmission channel 3 to transmit the frequency-converted OFDM signal x(t) to the transmission channel 3. When a transmission characteristic including multi-paths of the transmission channel 3 is taken as h(t) and a noise added in the transmission channel 3 is taken as n(t), an OFDM signal y(t) received by the receiver 2 is y(t)=x(t)*h(t)+n(t). * represents a convolution operation. The receiver 2 performs a frequency conversion on the received OFDM signal y(t) from the transmission band of the transmission channel 3 to the base band. The frequency converted OFDM signal y(t) is inputted into the Fourier transform circuit 21. The Fourier transform circuit 21 performs the Fourier transform on the OFDM signal y(t) in the time domain to generate a received vector sequence Y(i, f) (Y(i, f) =X(i, f)·H(i, f)+N(i, f)) in the frequency domain corresponding to the transmitted vector X(i, f). The Fourier transform circuit 21 also performs the Fourier transform on the symbol p to generate a received reference vector sequence Y(p, f) (Y(p, f)=X(p, f)·H(p, f)+N(p, f)) corresponding to a transmitted reference vector X(p, f). H(p, f) and H(i, f) are obtained by converting the characteristic h(t) of the transmission channel 3 in the frequency domain. N(p, f) and N(i, f) are obtained by converting the noise n(t) in the frequency domain. The interleave circuit 22 interleaves orders of the elements of the received reference vector sequence Y(p, f) and the received vector sequence Y(i, f) outputted from the Fourier transform circuit 21 to output the sequences. This is to prevent an excess of error correction capability of the trellis decode circuit 26 by randomizing at the receiver 2 a burst-like degradation of transmission characteristic added at the transmission channel 3 through interleaving the elements, whereby decoding performance of trellis decoding can be exerted without the excess of error correction capability. Although outputs from the Fourier transform circuit 21 and the interleave circuit 22 are different in orders of their elements, the outputs from the interleave circuit 22 are also represented as the received reference vector sequence Y(p, f) and the received vector sequence Y(i, f) which are outputs of the Fourier transform circuit 21, as far as generality is not impaired. The memory 23 holds in a prescribed period the received reference vector sequence Y(p, f) of the symbol p outputted from the interleave circuit 22, and outputs the received reference vector sequence Y(p, f) when the symbol i is received. The vector modulation circuit 24 modulates the received reference vector sequence Y(p, f) for each branch of trellis decoding by a signal point vector M(k) (k is a natural number not more than m, m is a number in a modulation scheme) to output a modulation vector sequence Y(p, f)·M(k). The metric generation circuit 25 obtains a difference between the received vector sequence Y(i, f) and the modulation vector sequence Y(p, f)·M(k) to output the difference as a branch metric sequence BM(i, f, k) shown in the following equation (5). BM ( i , f , k )= Y ( i , f )- Y ( p , f )· M ( k ) = X ( i , f )· H ( i , f )+ N ( i , f )- M ( k )· X ( p , f )· H ( p , f )- M ( k )· N ( p , f ) The characteristic H(i, f) of the transmission channel 3 is taken to have a small varied in time. Therefore, the characteristics at the time of the symbol i and the symbol p are taken as the same, and it is taken that H(i, f)=H(p, f)=H(f). Since the generality is not impaired when the reference vector is taken as X(p, f)=1 and the noise N(i, f) is normalized with X(p, f), the branch metric sequence BM(i, f, k) in the equation (5) is represented as the following equation (6). BM ( i , f , k )= Mx ( i , f )· H ( f )+ N ( i , f )- M ( k )· H ( f )- M ( k )· N ( p , f ) ={ Mx ( i , f )- M ( k )}· H ( f )+ N ( i , f )- M ( k )· N ( p , f ) The trellis decode circuit 26 performs trellis decoding on the basis of the branch metric sequence BM(i, f, k) outputted from the metric generation circuit 25 to output decoded information most likelihood to the data sequence. The received vector sequence Y(i, f) is obtained by adding the effects of the characteristic of the transmission channel 3 and the noise to the transmitted vector X(i, f). The transmitted vector X(i, f) is obtained by multiplying the modulation vector Mx(i, f) distributed to any one of a plurality of the signal points by the transmitted reference vector X(p, f). On the other hand, the modulation vector sequence Y(p, f)·M(k) is obtained by multiplying the received reference vector sequence Y(p, f), which is obtained by adding the effects of the characteristic of the transmission channel 3 and the noise to the transmitted reference vector X (p, f), by the signal point vector M(k) representing each signal point. In the branch metric BM(i, f, k) of the equation (6), the terms caused by the noise are {N(i, f)-M(k)·N(p, f)}. Accordingly, even if the characteristic of the transmission channel 3 is degraded by fading etc., noise elements are not amplified, which is unlike the conventional art, an increase of the error included in the branch metric can be prevented and decoding performance is not degraded in the trellis decode circuit 26. This is due to the fact that the receiver 200 in Fig. 2 of the conventional art uses division on the demodulation part, while the receiver 2 in Fig. 1 of the present invention does not use division. While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.

Claims: A method of receiving an orthogonal frequency division multiplexing signal through a transmission channel and maximum-likelihood-decoding a received data sequence, said signal consisting of a multiplexed plurality of modulating waves orthogonal to each other at a predetermined period, such that the orthogonal frequency division multiplexing signal contains a symbol which is generated each time said modulating waves are modulated and has a predetermined length, each of said modulating waves consisting of a multiplexed generated transmitted vector sequence, said generated transmitted vector sequence being a result of modulating a transmitted reference vector sequence with a transmitted modulation vector sequence, said transmitted reference vector sequence consisting of a transmitted vector sequence transmitted with a symbol precedent to said generated symbol, and representing references for amplitude and phase of said transmitted modulation vector sequence, said transmitted modulation vector sequence consisting of said data sequence which is trellis-coded so as to distribute said data sequence to any one of a plurality of signal points with a prescribed modulation scheme, and the method comprising: a Fourier transform step for performing a Fourier transform on the orthogonal frequency division multiplexing signal received through said transmission channel (3) for each symbol to reproduce a received vector sequence in a frequency domain corresponding to said transmitted vector sequence; a storing step for storing a received reference vector sequence in the frequency domain corresponding to said transmitted reference vector sequence among the received vector sequences reproduced by said Fourier transform step; and a trellis decoding step for reproducing the data sequence on the basis of a branch metric sequence generated by a metric generating step, characterized in that the method comprises: a vector modulating step for modulating the received reference vector sequence stored by said storing step with respective signal point vectors representing each of said signal points to generate a modulation vector sequence for each symbol; and a metric generating step for obtaining a difference between the received vector sequence generated by said Fourier transform step and the modulation vector sequence generated by said vector modulating step to generate the difference as said branch metric sequence. The method of receiving the orthogonal frequency division multiplexing signal according to claim 1, wherein orders of elements of said transmitted vector sequence and said transmitted reference vector sequence are interleaved, and said sequences are transmitted, the method further comprising an interleave step correspondingly interleaving orders of elements of the received vector sequence and the received reference vector sequence generated by said Fourier transform step. A receiver for receiving an orthogonal frequency division multiplexing signal received through a transmission channel and for maximum-likelihood-decoding a received data sequence, said signal consisting of a multiplexed plurality of modulating waves orthogonal to each other at a predetermined period, such that the orthogonal frequency division multiplexing signal contains a symbol which is generated each time said modulating waves are modulated and has a predetermined length, each of said modulating waves consisting of a multiplexed generated transmitted vector sequence, said generated transmitted vector sequence being a result of modulating a transmitted reference vector sequence with a transmitted modulation vector sequence, said transmitted reference vector sequence consisting of a transmitted vector sequence transmitted with a symbol precedent to said generated symbol, and representing references for amplitude and phase of said transmitted modulation vector sequence, said transmitted modulation vector sequence consisting of said data sequence which is trellis-coded so as to distribute said data sequence to any one of a plurality of signal points with a prescribed modulation scheme, and the receiver comprising: Fourier transform means (21) for performing a Fourier transform on the orthogonal frequency division multiplexing signal received through said transmission channel (3) for each symbol to reproduce a received vector sequence in a frequency domain corresponding to said transmitted vector sequence; storing means (23) for storing a received reference vector sequence in the frequency domain corresponding to said transmitted reference vector sequence among the received vector sequences reproduced by said Fourier transform means (21); and trellis decoding means (26) for reproducing the data sequence on the basis of a branch metric sequence generated by a metric generating means (25), characterized in that the receiver comprises: vector modulating means (24) for modulating the received reference vector sequence stored by said storing means (23) with respective signal point vectors representing each of said signal points and for generating a demodulation vector sequence for each symbol; and metric generating means (25) for obtaining a difference between the received vector sequence generated by said Fourier transform means (21) and the modulation vector sequence generated by said vector modulating means (24) and for generating the difference as said branch metric sequence. The receiver for receiving the orthogonal frequency division multiplexing signal according to claim 3, wherein orders of elements of said transmitted vector sequence and said transmitted reference vector sequence are interleaved, and said sequences are transmitted, the receiver further comprising interleave means (22) for correspondingly interleaving orders of elements of the received vector sequence and the received reference vector sequence generated by said Fourier transform means (21).

Inventors: HARADA YASUO, HAYASHINO HIROSHI, KIMURA TOMOHIRO, UNO YASUHIRO, HARADA, YASUO, HAYASHINO, HIROSHI, KIMURA, TOMOHIRO, UNO, YASUHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO. LTD.
Application year: 1996
Main classification: H04L5
IPC codes: H04L1, H04J11, H04L27