1. Field of the Invention
The present invention relates to discrete multitone modulation (DMT), and more specifically, to the generation of cyclic prefixes in a DMT modulation transmission. The DMT modulation is for example used by standards ADSL and ADSL-lite.
2. Discussion of the Related Art
In a DMT modulation, data coded in the form of complex frequency coefficients are, on the transmit side, translated into time samples by inverse fast Fourier transform (IFFT).
An IFFT of the group of coefficients Ai.ejφi is formed by the sum of the sections of sinusoidal carriers obtained by IFFT of each of coefficients Ai.ejφi for i included between 1 and N, this sum being called a “symbol”. The IFFT of N coefficients provides a symbol Dt formed of a succession of N complex digital samples S1 to SN. It should be noted that the shape of the symbol Dt shown is not realistic, but aims at simplifying the under-standing of the present description.
The time samples obtained by IFFT are converted into analog to be transmitted, for example, by a telephone line. On the receive side, the analog signal of the line is converted into digital, and the resulting samples are converted into complex frequency coefficients by fast Fourier transform (FFT).
To suppress a number of problems due to interference between symbols appearing upon transmission of the symbols, a “cyclic prefix” (or guard interval) is interposed before each symbol. The cyclic prefix is the reproduction at the beginning of a symbol of the last samples of this symbol.
At a time t1, IFFT circuit 12 provides a first sample S1 of symbol Dt, and memory 14 is controlled in the write mode to store this sample and the following. Multiplexer 16 is switched to select the output of memory 14, which provides a sample of a preceding symbol. This configuration of circuit 10 remains unchanged until a time tN−τ.
At time tN−τ, memory 14 has ended providing the samples of the preceding symbol and it contains the samples of the current symbol Dt, to the last sample preceding the cyclic prefix. IFFT circuit 12 starts providing the prefix samples, which samples, designated as SI to SN, continue being stored in memory 14. Meanwhile, multiplexer 16 is switched so that it transmits these prefix samples SI to SN. This configuration of circuit 10 remains unchanged until a time tN.
At time tN+1, IFFT circuit 12 is stopped, memory 14 contains the entire current symbol Dt and the prefix has just been transmitted. Multiplexer 16 is switched again to transmit the samples S1 to SN provided by memory 14, that is, symbol Dt.
At a time tN+τ+1, IFFT circuit 12 is reactivated and it starts providing the samples of the next sample. Time tN+τ+1 corresponds for the next symbol to previously-described time t1.
This configuration of circuit 10 remains unchanged until a time 2tN when symbol Dt will have been transmitted after its cyclic prefix.
Time 2tN+1 corresponds for the next symbol to previously-described time tN−τ.
A major disadvantage of circuit 10 is that the introduction of the cyclic prefix results in a delay tN (of N samples) in the transmission of symbol Dt. In some applications, such as telephone communications or other real time communications, the introduction of such a delay is not acceptable.
Besides, in prior art circuit 10, since the number N of samples may be high, memory 14 may have a large size.
An object of the present invention is to provide a cyclic prefix generation circuit that introduces a particularly low transmission delay.
Another object of the present invention is to provide such a circuit that uses a memory of reduced size.
To achieve these objects, the present invention provides a circuit for generating a cyclic prefix of a symbol comprised of a sequence of time samples, said prefix being the reproduction of the last samples of the symbol at the beginning of the symbol, the symbol being obtained by inverse Fourier transform of complex coefficients corresponding to respective frequencies, including means for shifting the phase of each complex coefficient by a value proportional to its frequency, a memory for storing the samples of the beginning of the symbol, and means for copying at the end of the symbol the stored samples.
According to an embodiment of the present invention, the means for shifting the phase of the complex coefficients include a multiplier connected to multiply each complex coefficient by a complex value having a unity norm and a phase proportional to the frequency associated with each coefficient.
According to an embodiment of the present invention, the memory is of FIFO type.
According to an embodiment of the present invention, the means for copying the stored samples include a multiplexer, a first input and a second input of which are respectively connected to the input and to the output of the memory.
The present invention further aims at a method for generating a cyclic prefix of a time symbol, said prefix being the reproduction of the last samples of the symbol at the beginning of the symbol, the symbol being obtained by inverse Fourier transform of complex coefficients corresponding to respective frequencies, that includes the steps of shifting the phase of each complex coefficient by a value proportional to the frequency with which it is associated, storing the samples of the beginning of the symbol, and copying the stored samples at the end of the symbol.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
The present invention provides circularly shifting the samples of a symbol to which a cyclic prefix is desired to be added, this so that the last samples forming the symbol before shifting are at the beginning of the symbol after shifting, and thus directly form the prefix. By transmitting the symbol thus shifted, the prefix is first transmitted, followed by a portion of the symbol which only needs be completed by the prefix to restore the symbol. Thus, a delay equal to the prefix only is introduced in the transmission and it is sufficient to only store the prefix to be able to retransmit it to complete the symbol.
The circular shifting of the symbol must correspond to a same circular shifting of all the sinusoids that form the symbol. For this purpose, each complex frequency coefficient is multiplied by a complex factor causing a time shift, corresponding to the desired circular shift.
According to the present invention, the N complex coefficients A1.ejφ1 to AN.ejφN are phase-shifted so that the corresponding sinusoid sections are all circularly shifted by a same value, or by the same number τ of samples. For this purpose, each coefficient Ai.ejφi is multiplied by a coefficient ejKiτ, where Ki is 2πfi. Thus, symbol Dt′ formed of the sum of the sinusoid sections corresponding to coefficients Ai.ejφi.ejKiτ, where i varies from 1 to N, corresponds to the preceding symbol Dt having undergone a circular shifting by τ samples.
Coefficients ejKiτ are predetermined, and they can for example be stored in a ROM.
As seen in relation with
At a time t1, IFFT circuit 12 provides the first sample S1′ of symbol Dt′, and memory 24 is controlled in the write mode to store the samples generated by the IFFT circuit. Multiplexer 16 is switched to select the output of IFFT circuit 12. This configuration of circuit 10 remains unchanged until a time tτ; it enables storing samples S1′ to SJ′ in memory 24 and providing at the output of multiplexer 16 the cyclic prefix, formed by samples S1′ to SJ′.
At time tτ+1, memory 24, which has just stored samples S1′ to SJ′, is deactivated. The position of multiplexer 16 is not modified, and this configuration of circuit 10 is maintained until a time tN. Multiplexer 16 provides in this interval samples SJ+1′ to SN′ of symbol Dt′, which correspond to previously described samples S1 to SI.
At time tN+1, IFFT circuit 12 is stopped, memory 24 is controlled in the read mode to provide the first sample S1′ that it contains, and multiplexer 16 is switched to select the output of memory 24. This configuration of circuit 10 remains unchanged until a time tN+τ. In this interval, multiplexer 16 successively provides samples S1′ to SJ′ read from memory 24, which correspond to above mentioned samples SI+1 to SN.
At time tN+τ+1, IFFT circuit 12 is reactivated to provide the samples of the next symbol and the cycle just described is resumed as at time t1.
The present invention enables generating the cyclic prefix of a symbol by only delaying the symbol by duration τ of the prefix. This is a time gain of tN−τ with respect to prior art, which is particularly valuable in the case of real time transmissions.
Further, memory 24 used according to the present invention is of reduced size, since it is used to only store the samples forming the prefix.
Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.
Number | Date | Country | Kind |
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99/01062 | Jan 1999 | FR | national |
This application is a continuation of U.S. patent application Ser. No. 09/491,685, filed Jan. 26, 2000, now pending, which application is incorporated herein by reference in its entirety.
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Number | Date | Country | |
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Number | Date | Country | |
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Parent | 09491685 | Jan 2000 | US |
Child | 10761708 | US |