Multi-carrier symbol synchronization

Details Multi-carrier symbol synchronization Multi-carrier symbol synchronization

1998-11-03

2002-10-01

Pham, Chi (Department: 2631)

Pulse or digital communications

Synchronizers

Reexamination Certificate

active

06459744

ABSTRACT:

BACKGROUND OF THE INVENTION
The field of the invention is that of the transmission of multicarrier digital signals, that is to say signals implementing a plurality of multicarriers transmitted simultaneously and modulated each by distinct data elements. More specifically, the invention relates to the time synchronization of receivers of such signals.
Multicarrier signals are generally called frequency division multiplex (FDM) signals. A particular example of these signals to which the invention can be applied in particular is that of OFDM (orthogonal frequency division multiplex) signals.
An OFDM signal is used for example in the digital broadcasting system described especially in the French patent FR 86 06922 filed on Jul. 2, 1986 and in M. Alard and R. Lassalle,
Principes de modulation et de codage canal en radiodiffusion numérique vers les mobiles
(“Principles of modulation and channel encoding in digital radio broadcasting towards mobile units”), Revue de l'UER, No. 224, August 1986, pp. 168-190, known as the COFDM (coded orthogonal frequency division multiplex) system.
This COFDM system has been developed in the context of the European DAB (digital audio broadcasting) project. It is also being put forward for standardization for the terrestrial broadcasting of digital television (the DVB-T standard in particular). More generally, the COFDM system can be used to transmit any type of digital signals (or analog signals that are sampled but not necessarily quantified).
This system is based on the joint use of a channel encoding device and an orthogonal frequency division multiplex method of modulation. This system is particularly suited to the broadcasting of digital signals at a high bit rate (some megabits per second) in channels that are assigned multiple paths whose characteristics vary in time (for example in the case of mobile reception in an urban environment).
The modulation method proper makes it possible to overcome problems related to the frequency selectivity of the channel. It consists in redistributing the information to be transmitted over a large number of carriers juxtaposed and modulated at a low bit rate. A system for the interlacing of information to be transmitted is associated with the encoding method in such a way that the maximum statistical independence of the samples is ensured at the input of the decoder.
The time synchronization of a COFDM receiver consists of the determining, in the frame of the OFDM signal received, of the location of the useful part of each symbol (constituted by a guard interval and a useful part) for the application thereto of the FFT window enabling the selection of the useful part of each symbol. This information of time synchronization is also used for the feedback control of the clock of the receiver in order to implement the rate recovery device.
This time synchronization function of the receiver can be generally subdivided into a rough time synchronization (during the acquisition period) and a fine time synchronization.
According to a known technique, implemented especially in the DAB digital broadcasting program, the time synchronization can be based on special symbols designed for this purpose, generally placed at the beginning of a frame.
In this case, each frame advantageously starts with at least two special symbols, S
1
and S
2
, used for the synchronization. It then comprises a certain number of useful symbols, each comprising a plurality of modulated orthogonal carriers.
The symbol S
1
is a zero symbol enabling firstly the performance of a rough synchronization. The symbol S
2
is a second synchronization symbol formed by a non-modulated multiplex of all the carrier frequencies with a substantially constant envelope. This enables a more precise recomputation of the synchronization by analysis of the pulse response of the channel. The role and the mode of preparing these symbols S
1
and S
2
are described in the patent FR 88 15216 filed on Nov. 18, 1988 on behalf of the present Applicants.
The symbol S
2
is also known as the CAZAC symbol and the TFPC symbol in other embodiments.
The idea is now being envisaged of producing COFDM signals that do not have such special symbols dedicated to time synchronization. This is especially the case with digital television signals under standardization.
Other synchronization methods should therefore be developed. Thus, there is a known technique, called the guard interval correlation technique which enables the performance of a rough synchronization.
The guard interval of an OFDM symbol consists of the repetition of the samples of the end of said OFDM symbol. The method consists of the computation of the correlation between the samples constituting the guard interval and the samples of the end of the symbol in order to extract a correlation “peak” therefrom.
After temporal filtering, this correlation “peak” can then be used as a synchronization pulse to determine the length of the OFDM symbol and of the guard interval &Dgr;, and hence the beginning of the FFT window. This operation is performed before the demodulation FFT on the COFDM signal received in the temporal domain.
If x(t) designates the COFDM signal received in the temporal domain, the measurement of the correlation at the instant t=T
n
can be given by the following expression:
Γ
x
⁡
(
T
n
)
=
∑
t
=
T
n
T
n
+
T
i
⁢
&LeftBracketingBar;
x
⁡
(
t
)
·
x
*
⁡
(
t
-
t
s
)
&RightBracketingBar;
where * signifies the “conjugate” of a complete number and | | signifies “the modulus” of a complex number.
The measurement of the correlation is done on blocks with a length T
i
equal to or smaller than the length of the guard interval &Dgr;. Should the receiver not have a priori knowledge of the length of the guard interval (there are provided, in certain systems, variable sizes depending on the application), the measurement of the correlation may be done at the outset on blocks of a length equal to the minimum length of the guard interval.
Under ideal conditions where there is no noise, no multiple paths and no co-channel interference, the correlation “peak” (or “pulse”) obtained may be exploited to generate the “rough” time synchronization.
For example,
FIG. 1
shows the measurement of the correlation obtained after temporal filtering in the case of a noise-affected ideal transmission (
11
) and a noiseless ideal transmission (
12
) with a pulse response h(t)
13
of the channel with only one path
14
.
This information can also be used for the fine time synchronization: by measuring the distance 15 between two successive correlation “peaks”, it is possible to deduce the length T
s
=T
s
+&Dgr; of an OFDM symbol and therefore the length of the guard interval &Dgr;.
By contrast, in the presence of major echoes or a high level of interference, the correlation peak obtained is highly deformed and is more or less spread as a function of the spread of the echoes.
FIG. 2
shows the measurement of the correlation obtained after temporal filtering in the case of a noise-affected transmission (
21
) and a noiseless transmission (
22
) which however is characterized by a pulse response h(t)
23
of the channel having two paths
24
1
and
24
2
spaced out by the length of the guard interval &Dgr; and received with identical power.
The correlation peak
25
may then be exploited to determine the length of an OFDM symbol and generate the rough time synchronization but its precision is insufficient for the deduction therefrom of a fine temporal synchronization.
Indeed, when the reception conditions evolve in time, which is the case in portable and mobile reception, the form of the measurement of the correlation of the guard interval is highly fluctuating. A fine time synchronization generated solely from this information, even if this information is temporally filtered, will then be affected by a substantial amount of jitter.
Furthermore, the measurement of the correlation of the guard interval will be greatly polluted in the presence of intersymbol interference due t

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