Transmissionsystem with clock recovery.

The invention relates to a transmission system comprising a transmitter for transmitting a transmission signal formatted by blocks originating from symbols of a constellation, the same block containing the same information present a first and a second time, and a receiver for receiving said transmission. signal.

The invention also relates to a receiver implemented in such a transmission system.

In general, the invention relates to signals formatted by blocks for which said blocks can have a great complexity with redundancy of information. It may, for example, be signals obtained by multiplexed distribution of N orthogonal frequencies (OFDM: Orthogonal Frequency Division Multiplexing) which consists in splitting information to be transmitted by distributing it over a large number of elementary channels at low bit rate. A highly selective broadband channel is thus transformed into a large number of elementary non-selective channels. Since the assembly constitutes a broadband channel, it is unlikely that fading during transmission will affect the entire channel simultaneously. 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 reception it is difficult to accept the use of selective filters, it is preferable to tolerate overlapping of the spectra but by imposing conditions of orthogonality between the frequencies to eliminate the intersymbol interference at the sampling instants. The whole spectrum of an OFDM signal thus tends towards a rectangular spectrum.

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

The OFDM signal is organized by frames made up of blocks, some of which are service blocks and others of data blocks. 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 originating from a coded modulation, for example a digital MDP or MAQ modulation. On transmission, these symbols are coded at a certain rate which must be found on reception in order to be able to decode them correctly. The invention therefore proposes to find at the reception the rhythm used for the transmission.

Document FR 2 639 495 is known, which describes a method of broadcasting digital data using an OFDM technique. The synchronization recovery process is carried out on the frame using two specific blocks which are a zero block at the start of the frame, and a wobulation block. This document highlights the advantage which it presents of not synchronizing a rhythm clock at the binary level as in the prior techniques. This document thus opens binary data acquisition windows which it considers to have sufficient precision to decode the binary data.

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

The object of the invention is thus to perform timing synchronization with better precision than in the prior techniques for a signal formatted by blocks themselves containing redundant information.

This goal is achieved by performing rhythm recovery using data received at the rate of the blocks. For this, the transmission system is remarkable in that the receiver includes:<ul id="ul0001" list-style="none"><li>first means for generating a local clock of symbol rate, said clock having a first frequency,</li><li>second means for generating a difference signal tuned to a second frequency, the difference signal being obtained by correlation, between the information present the first time and that present the second time in the same block,</li><li>- third means for slaving the first frequency to the second frequency.</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 timing recovery is improved by a factor of around 125.

Preferably, the second means carry out the correlation by operating a subtraction between the information present the first time and the information present the second time.

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

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

These different aspects of the invention and others will be apparent and elucidated from the embodiments described below.

The invention will be better understood using the following figures given by way of nonlimiting examples which represent:<ul id="ul0002" list-style="none"><li>Figure 1: a timing diagram indicating the generation of a deviation 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 description which follows is developed in the particular case of an OFDM signal, but it also applies to other signals formatted by blocks for which each block contains redundant information.

The OFDM technique consists in multiplexing in frequency several orthogonal carriers modulated by the symbols. An OFDM symbol can be written be written:<maths id="math0001"><img file="EP0608024A1_D0001.tif" /></maths>for<maths id="math0002"><img file="EP0608024A1_D0002.tif" /></maths>with<maths id="math0003"><img file="EP0608024A1_D0003.tif" /></maths>or :<ul id="ul0003" list-style="none"><li>You_s : total duration of an OFDM symbol, T '_s = T_s + A.</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>A: guard interval.</li><li>N: maximum number of carriers.</li><li>f_o : arbitrary frequency.</li><li>j: index of the OFDM symbol.</li></ul>

So between the moments jT '_s and (j + 1) T '_S, an OFDM signal consists of a block of complex symbols _Xk, each symbol x_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 carry out OFDM modulation, that is to say 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 2x where x is an integer. Other control blocks intended for transmission are also inserted.

The parameters chosen are, for example, the following:<ul id="ul0004" list-style="none"><li>You_s = 160 µs, T_s = 128 µs and A = 32 µs,</li><li>N = 1024 carriers, N_u = 900 carriers.</li></ul>

The main role of guard interval A is to absorb echoes from the multipath channel and with delays less than Δ. During the guard interval (which is preferably equal to a quarter of the useful life), a signal identical to that of 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, 900 carriers are required to have an effective band of the transmitted signal of approximately 7 MHz (the exact bandwidth being 7.031 MHz).

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

An example of a frame structure is given by:<tables id="tabl0001" num="0001"><img file="EP0608024A1_D0004.tif" /></tables>

The frame contains 125 OFDM blocks and has a duration T_f 20 ms:<ul id="ul0005" list-style="none"><li>- The first block is the null block during which nothing is transmitted (_Xk = 0, k = 0, N-1). It is used to synchronize the start of the frame.</li><li>- The second is the AFC (automatic frequency control) block used for frequency synchronization of the local oscillator of the receiver with respect to that of the transmitter.</li><li>- The third is the wobulation block defined by:<maths id="math0004"><img file="EP0608024A1_D0005.tif" /></maths></li></ul>

The wobulation block is used as a reference block for differential coding and also to estimate the impulse response of the channel in order to precisely synchronize the start of the frame.<ul id="ul0006" list-style="none"><li>- The fourth and fifth blocks are additional blocks which can be used to transmit service data.</li><li>- Finally, we have the 120 OFDM data blocks.</li></ul>

A frame contains 100 code words generated by the channel encoder.

The invention takes advantage of the existence of a guard interval in each block of an OFDM frame. FIG. 1 represents two consecutive blocks B1 and B2. The following explanation applies to all blocks. The blocks B1 / B2 are formed by a guard interval of duration A followed by a useful interval of duration T_s which contains useful data. In an emitted block, the data which appears at the end of the useful interval T_s are copied at the beginning of the block before their transmission on the channel. We therefore observe that in each block identical information appears at the beginning and at the end of the block. In FIG. 1, the signal received is the signal r (t). This signal r (t) is delayed by delay delay means such as the information of the start of the delayed signal r_d(t) is in phase with the end information of the non-delayed signal r (t). For the same block B1, these two pieces of information are identical. By performing an operation of subtraction of these two signals, one obtains a signal e (t) which is formed by an interval of duration A where e (t) is zero preceded by an interval of duration T_s where it is not 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 more sinusoidal shape, having a fundamental frequency 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 which makes it possible to isolate the block frequency and to control a oscillator which oscillates 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 supply the signal r_d(t). The signals r (t) and r_d(t) are subtracted from each other in subtraction means 13 and deliver the deviation signal e (t). This difference signal e (t) is previously filtered in a bandpass filter 14 tuned to the block frequency used for transmission.

The deviation signal enters a phase control loop 15. It is intended to control a local oscillator 16 of the VCO type which oscillates in free mode on a frequency close to the rhythm frequency of the symbols 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) can be equal to 12.5 KHz and the frequency of the oscillator be close to 16 MHz. The output signal of the filter 22 (12.5 KHz) controls the frequency of the oscillator 16. To reduce the frequency of the oscillator signal to a frequency of 12.5 KHz so that the loop 15 can work, it is necessary 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 performed, the output of the oscillator delivers the rhythm clock Hr.