Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-03-23
2004-04-06
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S355000, C375S365000, C370S509000
Reexamination Certificate
active
06717996
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a digital signal timing synchronization process. It is used in radio transmission systems and more particularly in code distribution multiple access (CDMA) systems.
PRIOR ART
The principles of a digital communication and the link existing between baseband signals and carrier frequency signals are known and described e.g. in the work by John G. PROAKIS entitled “Digital Communications”, McGraw Hill International Editions.
FIG. 1
is the circuit diagram of a radio digital transmission chain.
In the transmission chain E, an original digital signal
10
A which it is wished to transmit undergoes a preprocessing in a circuit
10
. This preprocessing can involve numerous scrambling, interleaving or coding operations, which will not be dealt with in the remainder of the description. The circuit
10
delivers a sequence
10
B of digital signals a(k), in which k designates the rank of the symbol. The symbol a(k) is in general a complex number represented by a pair of real values. The frequency of the symbols a(k) is designated Hs and the corresponding period Ts, with Ts=1/Hs.
On the basis of the sequence a(k), a shaping device
20
develops the baseband analog signal
20
A to be transmitted and designated b(t), in which t is the time variable. The signal b(t) is a complex signal represented by two real quadrature signals b
I
(t) and b
Q
(t). The baseband signal
20
A is converted to a carrier frequency by a radio transmitter
30
, which incorporates various means, namely a modulator, frequency conversions, filters, local oscillators, amplifiers and an antenna, but to which no further reference will be made hereinafter. It is merely assumed that the transmitter performs a linear, mathematical operation with respect to the baseband signal. The transmitted radio signal
30
A is propagated to the receiver, whilst undergoing different types of degradations.
In the reception chain R, the radio signal received
40
A is firstly processed by a radio receiver
40
, which incorporates different devices, namely an antenna, frequency conversion means, filters, local oscillators and amplifiers, but to which no further reference will be made hereinafter. The receiver
40
delivers an analog baseband signal
40
B, which is designated r(t). The signal r(t) is a complex signal represented by two real quadrature signals r
I
(t) and r
Q
(t). On the basis of the signal r(t) a detection device
50
develops a set of symbols or digital samples
50
A. The set of detected samples
50
A constitutes a more or less faithful image of the sequence of symbols a(k). The detected samples
50
A undergo a postprocessing in a circuit
60
. This postprocessing comprises various operations corresponding to the preprocessing operations
10
of the transmission chain E and delivers the restored signal
60
A.
The development of the set of detected samples
50
A assumes as precisely known the value of the period Ts of the timing of the sequence a(k) and its phase relative to the baseband signal r(t). A synchronization device
70
, by means of the signals
50
B which it exchanges with the detection device
50
, estimates the timing of the signals received and communicates the result of this estimate to the detection device. Certain detection processes known as coherent demodulation also require the knowledge of the phase of the carrier frequency of the radio signals received. This knowledge is not envisaged here and demodulation can be both coherent and non-coherent. It is merely assumed that the beat frequency between the carrier frequency used in the receiver and the real carrier frequency is low compared with the timing Hs of the symbols a(k).
The functional partitioning which has been made is to a certain extent arbitrary and certain operations can overlap. However, it is assumed that there is effectively a received baseband signal
40
B or an equivalent representation of this signal in the form of digital samples.
The present invention essentially relates to the synchronization operation carried out in the reception chain.
Synchronization is linked with the shaping of the baseband signal to be transmitted b(t) and to the corresponding detection conditions. It is here assumed that the shaping corresponds to the linear mathematical operation:
b
(
t
)
=
∑
k
a
(
k
)
·
h
(
t
-
kTs
)
in which
∑
k
represents a summation on all the symbols a(k) and in which h(t) designates a real or complex function of the time t.
An important case is that of the direct sequence spreading, where
h
(
t
)
=
∑
n
=
0
N
-
1
α
(
n
)
·
g
(
t
-
nTc
)
.
In this expression, ∝(n) is a family of previously defined, complex or real numbers independent of the value of the symbols a(k) which it is wished to transmit. The numbers ∝(n) are known as chips in terminology widely used in this field. With each rank k are associated N successive chips ∝(n) numbered n=0 to n=N−1. The chips are delivered with a period Tc=Ts/N and the corresponding timing is designated Hc. The number N of chips per symbol is called the spread factor. The function g(t) is a real or complex function independent of the rank k and the number n. It is called the “shaping function” of the chip. The direct sequence spread CDMA transmission systems allocate to each user a particular family of chips ∝(n), the different families of chips being chosen so as to reduce interference between users.
Although the invention does not apply directly to the actual detection, it is necessary to take account thereof. In the case of a transmission channel which does not deform the signal, but merely superimposes thereon an independent interference signal called Gaussian white noise, the optimum detection is obtained by the matched filtering method or an equivalent method. This method consists of applying the baseband signal received r(t) to a transfer function filter h*(−t) in order to obtain a signal s(t):
s
(
t
)=
h
*(−
t
)*
r
(
t
)
where * represents the complex conjugation operation when it is placed at the exponent, or the convolution operation when it is placed at midheight. Strictly speaking, matched filtering is carried out with the aid of a transfer function filter h*(Tr−t), where Tr is a fixed delay or lag chosen in such a way that the function h*(Tr−t) is causal with respect to the variable t. This delay corresponds to the time necessary to complete the calculation of s(t) on the basis of r(t) for a given time t, but plays no part in the following explanations. For reasons of simplicity, it is assumed to be zero hereinafter. The values of the signal s(t) at appropriately chosen times constitute the set of detected samples
50
A or would make it possible to develop said set with the aid of supplementary operations.
In the case of direct sequence spreading, matched filtering is broken down into a filtering matched to the shape of the chip:
s
c
(
t
)=
g
*(−
t
)*
r
(
t
)
and a filtering matched to the chip sequence:
s
(
t
)
=
∑
n
=
0
N
-
1
α
*
(
n
)
·
s
c
(
t
+
nTc
)
where the function s
c
(t) is used as a calculation intermediate. Filtering matched to the chip sequence is called despreading.
Numerous methods exist for the recovery of the timing or clock on the basis of the baseband output signal of the matched filter. Certain make use of a global approach where the carrier frequency phase, timing phase and symbols are jointly estimated. From the practical standpoint it is often simpler to separately estimate the phase of the timing or clock. In general terms, timing recovery requires a derivation with respect to the time of the baseband output signal of the matched filter, in order to reveal signal transitions. Among possible processes, certain bring together a differentiator and a phase locked loop, whereas others effect a non-linear operation followed by a filtering and a zero passage detector of the signal,
Du Reau Philippe
Duponteil Daniel
Yuan-Wu Julie
France Telecom
Tran Khai
Tran Khanh Cong
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