Transmission system with clock recovery

The invention relates to a transmission system comprising a transmitter and a signal receiver, said receiver comprising correlation means. The invention also relates to a receiver implemented in such a transmission system.

The article entitled "Applications of the digital correlator" by John Eldon published in the journal "Microprocessor and microsystems", Vol 12, No. 4, 1^st May 1988, pages 214-224, describes the use of correlation means for detecting particular timing words and periodicities in a received signal.

The invention consists in particular in applying such teaching to a block-formatted signal transmission system, said blocks being derived from symbols of a constellation and having a redundancy of information, i.e. the same block contains the same information presents a first and a second time. It may, for example, be signals obtained by Orthogonal Frequency Division Multiplexing (OFDM) which consists in splitting a piece of information to be transmitted by distributing it over a large number of elementary channels at low bit rates. A highly selective broadband channel is thus transformed into a large number of non-selective elementary channels. Because the ensemble is a broadband channel, fading during transmission is unlikely to affect the entire channel at the same time. This technique notably makes it possible to reduce intersymbol interference.

Each elementary channel then corresponds to a frequency, the set of frequencies being symmetrically distributed around a central carrier frequency. As at the reception it is difficult to accept the use of selective filters, it is preferred to tolerate an overlap of the spectra but by imposing orthogonality conditions between the frequencies to eliminate the intersymbol interference at the sampling instants. The entire spectrum of an OFDM signal thus tends to a rectangular spectrum.

On reception, the received signals must be demodulated and then decoded to restore the original information. This demodulation uses a local oscillator whose frequency must be slaved to the transmission frequency.

The OFDM signal is organized by frames formed of blocks, some of which are service blocks and other blocks of data. To avoid intersymbol interference, each block contains redundant information. Any block is formed by all the OFDM frequencies implemented by the device, the frequencies being modulated by symbols to be transmitted from a coded modulation, for example a digital modulation or CDM QAM. On transmission, these symbols are encoded at a certain rate that must be found at the reception in order to decode them correctly. The invention therefore proposes to find at the reception the rhythm used on the program.

Document FR 2 639 495 describes a method for broadcasting digital data using an OFDM technique. The method of recovering the synchronization is performed on the frame using two specific blocks which are a zero block at the beginning of the frame, and a wobulation block. This document highlights the interest it has in not synchronizing a clock rate at the binary level as in previous techniques. This document thus opens binary data acquisition windows which it considers to have sufficient precision to decode the binary data.

But by performing a synchronization once per frame, a frame having for example a duration of 20 ms, the accuracy of the synchronization remains insufficient.

The object of the invention is to perform a timing synchronization for a block-formatted signal, themselves containing redundant information, without using synchronization words.

These goals are achieved by performing rate recovery using data received at the rate of the blocks. For this purpose, a transmission system according to the invention is characterized in that:<ul id="ul0001" list-style="dash" compact="compact"><li>said signals being formatted in blocks, said blocks being derived from symbols of a constellation and each comprising information, called identical information, constituted by a part of the useful information transmitted in the block and a copy of this part placed in offset in said block before the emission,</li><li>said correlation means performs a correlation between said identical information to generate a signal said deviation signal whose frequency is representative of the block frequency, said difference signal being supplied to frequency division servocontrol means for controlling a oscillator at the symbol frequency of the received signal.</li></ul>

Thus advantageously the invention uses the redundancy of the data contained in each block to greatly improve the quality of synchronization.

In the particular case of an OFDM signal, with a frame containing 125 blocks, it can be estimated that the accuracy of the rate recovery is improved by a factor of about 125.

Preferably, the correlation means comprise delay means delaying the signals received by a delay separating said identical information, to generate said difference signal.

To subtract from each other the groups of data present at the beginning and end of each block, delay means are used which form a delayed replica of the received signal, a replica which is subtracted from the signal received itself to deliver a control signal tuned to the block rate.

This control signal acts on a phase locked loop for controlling, by frequency division, an oscillator whose oscillation frequency is much higher than the block frequency and which provides the clock of rhythm .

The invention uses certain means which, taken in isolation, are known from the state of the art. It will be noted, for example, that US Pat. No. 3,883,729 describes a correlation method for retrieving the synchronization words contained in a time-division multiplex frame. The described method takes advantage of the fact that these synchronization words are found identically at regular time intervals in the frame. Furthermore, US Pat. No. 3,599,103 describes the use of a delay line, a subtractor and a phase-locked loop for the purpose of stalling in phase the sampling clock of a receiver. an analog transmission system in which the data is coded by phase shift.

These and other aspects of the invention will be apparent and elucidated from the embodiments described hereinafter.

The invention will be better understood with the aid of the following figures given as non-limiting examples which represent:<ul id="ul0002" list-style="none" compact="compact"><li>Figure 1: a timing diagram indicating the generation of a gap signal at the block rate.</li><li>Figure 2: a diagram of a rhythm recovery device according to the invention arranged in a receiver.</li></ul>

The following description is developed in the particular case of an OFDM signal, but it also applies to other block-formatted signals for which each block contains redundant information.

The OFDM technique consists in frequency multiplexing several orthogonal carriers modulated by the symbols. An OFDM symbol can be written:<maths id="math0001"><img file="EP0608024B1_D0001.tif" /></maths> for<maths id="math0002"><math display="block"><mrow><mtext mathvariant="italic">j</mtext><mtext>. </mtext><msub><mrow><mtext mathvariant="italic">T '</mtext></mrow><mrow><mtext mathvariant="italic">s</mtext></mrow></msub><mtext> &lt; </mtext><mtext mathvariant="italic">t</mtext><mtext> &lt; (</mtext><mtext mathvariant="italic">j</mtext><mtext>+1) </mtext><msub><mrow><mtext mathvariant="italic">T '</mtext></mrow><mrow><mtext mathvariant="italic">s</mtext></mrow></msub></mrow></math><img file="EP0608024B1_D0002.tif" /></maths> with<maths id="math0003"><math display="block"><mrow><msub><mrow><mtext>φ</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>(</mtext><mtext mathvariant="italic">t</mtext><mtext>) = </mtext><msup><mrow><mtext mathvariant="italic">e</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><msup><mrow><mtext>​</mtext></mrow><mrow><mtext mathvariant="italic">iπkt</mtext></mrow></msup><msup><mrow><mtext>​</mtext></mrow><mrow><msub><mrow><mtext mathvariant="italic">/ T</mtext></mrow><mrow><mtext mathvariant="italic">s</mtext></mrow></msub></mrow></msup><msub><mrow><mtext mathvariant="italic"> for jT '</mtext></mrow><mrow><mtext mathvariant="italic">s</mtext></mrow></msub><mtext> ≤ </mtext><mtext mathvariant="italic">k</mtext><mtext> ≤ (</mtext><mtext mathvariant="italic">j</mtext><mtext>+1) </mtext><msub><mrow><mtext mathvariant="italic">T '</mtext></mrow><mrow><mtext mathvariant="italic">s</mtext></mrow></msub></mrow></math><img file="EP0608024B1_D0003.tif" /></maths> or :<ul id="ul0003" list-style="none" compact="compact"><li>T '_S : total duration of an OFDM symbol, T '_S = T_S + Δ.</li><li>R_e : real part of a complex number.</li><li>k: index of orthogonal carriers.</li><li>T_S : useful life of an OFDM symbol.</li><li>Δ: guard interval.</li><li>N: maximum number of carriers.</li><li>f_o : arbitrary frequency.</li><li>j: OFDM symbol index.</li></ul>

So between the moments jT '_S and (j + 1) T '_S, an OFDM signal consists of a block of complex symbols x_k, each x symbol_k modulating an orthogonal carrier 0 ≤ k ≤ N-1.

To avoid the problem of spectrum overlap and to facilitate filtering on reception, the sum corresponding to equation (1) is carried out on N_u carriers where N_u is the number of useful carriers (N_u &lt;N).

To perform the OFDM modulation, ie to form the signal s (t) of equation 1, a modulator is used which performs a calculation of a fast inverse Fourier transform (FFT^-1). For this we choose for N a number of the form 2^x where x is an integer. Other control blocks for transmission are also inserted.

The chosen parameters are, for example, the following:<ul id="ul0004" list-style="none" compact="compact"><li>T '_S = 160 μs, T_S = 128 μs and Δ = 32 μs,</li><li>N = 1024 carriers, N_u = 900 carriers.</li></ul>

The main purpose of the guard interval Δ is to absorb echoes from the multipath channel and having delays of less than Δ. During the guard interval (which is preferably equal to one quarter of the useful life), a signal identical to that of a part of the useful life is transmitted.

The choice of N_u = 900 comes from the fact that the band around each carrier is 1 / T_S = 7.81 KHz, it takes 900 carriers to have an effective transmitted signal band of about 7 MHz (the exact bandwidth being 7.031 MHz).

Blocks at the output of a channel encoder are transmitted in frames. Thus a frame groups several time-multiplexed OFDM blocks. An OFDM block may contain data or may be a particular block serving for synchronization (frame, timing, carrier) or serving as a reference block for differential modulation.

An example of a frame structure is given by:<tables id="tabl0001" num="0001"><img file="EP0608024B1_D0004.tif" /></tables>

The frame contains 125 OFDM blocks and has a duration T_f of 20 ms:<ul id="ul0005" list-style="dash" compact="compact"><li>The first block is the null block during which nothing is transmitted (x_k = 0, k = 0, N-1). It is used to synchronize the beginning of the frame.</li><li>The second is the AFC (Frequency Automatic Control) block used for frequency synchronization of the local oscillator of the receiver relative to that of the transmitter.</li><li>The third is the wobulation block defined by:<maths id="math0004"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">x</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext> = </mtext><msqrt><mtext>2</mtext></msqrt><mtext></mtext><mtext mathvariant="italic">e</mtext><msup><mrow><mtext>​</mtext></mrow><mrow><msub><mrow><mtext mathvariant="italic">jφ</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub></mrow></msup><msub><mrow><mtext>, φ</mtext></mrow><mrow><mtext mathvariant="italic">k</mtext></mrow></msub><mtext>= π </mtext><mfrac><mrow><msup><mrow><mtext mathvariant="italic">k</mtext></mrow><mrow><mtext mathvariant="italic">2</mtext></mrow></msup><mtext></mtext></mrow><mrow><mtext mathvariant="italic">NOT</mtext></mrow></mfrac><mtext> + π / 4</mtext></mrow></math><img file="EP0608024B1_D0005.tif" /></maths></li></ul>

The wobulation block is used as a reference block for differential coding and also for estimating the impulse response of the channel in order to precisely synchronize the start of the frame.<ul id="ul0006" list-style="dash" compact="compact"><li>The fourth and fifth blocks are additional blocks that can be used to transmit service data.</li><li>Finally, we have the 120 OFDM blocks of data.</li></ul>

A frame contains 100 codewords generated by the channel encoder.

The invention takes advantage of the existence of a guard interval in each block of an OFDM frame. Figure 1 shows two consecutive blocks B1 and B2. The following explanation applies to all blocks. Blocks B1 / B2 consist of a guard interval of duration Δ followed by a useful interval of duration T_S which contains useful data. In a transmitted block, the data that appears at the end of the useful interval T_S are copied at the beginning of the block before being transmitted on the channel. We therefore observe that in each block identical information appears at the beginning and at the end of the block. In Figure 1, the received signal is the signal r (t). This signal r (t) is delayed by delay delay means such that the delayed signal start information r_d(t) is in phase with the end information of the undelayed signal r (t). For the same block B1, these two pieces of information are identical. By performing a subtraction operation of these two signals, a signal e (t) is obtained which is formed of a time interval Δ where e (t) is zero preceded by a time interval T_S where he is non-zero. In FIG. 1, the signal e (t) is represented in the form of slots, but in reality this signal has a more complex and sinusoidal shape, having a frequency that is fundamentally equal to the block frequency. It is therefore possible by filtering to isolate this block frequency.

FIG. 2 represents a diagram of a device according to the invention that makes it possible to isolate the block frequency and to slave an oscillator oscillating at the rhythm frequency of the initial symbols of the coded modulation. Means 11 make it possible to generate the difference signal e (t) at the block frequency. The means 11 comprise tuned filtering means 14. The signal r (t) is delayed in delay means 12 to provide the signal r_d(T). The signals r (t) and r_d(t) are subtracted from one another in subtraction means 13 and output the difference signal e (t). This difference signal e (t) is previously filtered in a bandpass filter 14 tuned to the block frequency used on transmission.

The difference signal enters a phase lock loop. It is intended to slave a VCO-type local oscillator 16 which oscillates in free mode on a frequency close to the symbol rate frequency of the coded modulation. The loop 15 is formed of a comparator 20, a low-pass filter 22 and a frequency divider 24. For example, the frequency of the signal e (t) may be equal to 12.5 KHz and the frequency of the oscillator is close to 16 MHz. The output signal of the filter 22 (12.5 KHz) slaves the frequency of the oscillator 16. To reduce the frequency of the oscillator signal at a frequency of 12.5 KHz for the loop 15 to operate, it is necessary to insert a frequency divider 24. In the example cited, it is necessary to perform a division by a factor of 1280. When the locking is done, the output of the oscillator delivers the clock rate Hr.