Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2002-09-05
2004-02-17
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S457000, C342S453000
Reexamination Certificate
active
06693591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for co-operative radio direction-finding in transmission for signals comprising a reference signal.
The invention can be applied especially to the case of signals comprising a synchronization signal or reference signal, consisting of a sequence of known symbols defined by the transmission standard used within the transmission system.
For example, the GSM (Global System for Mobile communications), consisting of several reference signals, has this characteristic. In such a context, a reception system such as a base station may perform a selective direction-finding operation and associate a direction of arrival with a synchronization sequence. The techniques of selective direction-finding can be used to isolate one transmitter from the others on the basis of knowledge of its reference signal. It is thus possible to carry out a selective direction-finding operation for the transmitter, thus reducing the risk of ambiguity.
2. Description of the Prior Art
The patent FR 2 764 074 describes a method for carrying out a direction-finding operation in the case of single-user operation and for linear modulations or waveforms that may be approximated in terms of linear modulations. In this patent, linearly modulated digital signals s
e
(t) are sent. The symbols a
1
, a
2
, a
3
, etc., associated with the signal, are transmitted at the period T
s
and then filtered by a FIR (Finite Impulse Response) filter with a response h(t).
Thus, the following is the relationship between the transmitted signal s
e
(t) and the symbols a
n
:
s
e
(
t
)
=
∑
n
h
(
t
-
nT
s
)
a
n
(
1
)
where h(t) corresponds to the shaping filter.
FIG. 1
gives a diagrammatic view of the principle of the method.
The example given in this patent relates chiefly to the GSM standard in which the signal sent is a GMSK (Gaussian Minimum Shift Keying) phase modulation such that:
s
e
(
t
)
=
exp
{
j
∑
n
Φ
(
t
-
nT
s
)
a
n
}
(
2
)
where a
n
corresponds to a symbol sent at a rate of T
s
samples per symbol.
In order that the GMSK standard may form part of the linear modulations, the authors of this patent have made the following approximation:
s
e
(
t
)
≈
∑
n
C
0
(
t
-
nT
s
)
j
n
b
n
with
b
n
=
∏
k
=
-
∞
n
a
k
(
3
)
The linearization is obtained, for example, by applying the method described in reference of Pierre Andre Laurent, “Exact and approximate construction of digital phase modulations by superposition of modulated pulse (AMP)” in IEEE Trans. on Communications Vol 34 (1986) pp 150-160.
In the sequence of symbols {a
n
}, there is a sequence of symbols {s
k
} such that a
p+k−1
={s
k
} for 1≦k≦K. The sequence of symbols {s
k
} is defined by the transmission standard and is called a learning sequence. In particular, the GSM standard comprises eight different sequences formed by K=26 symbols.
The signal s
e
(t) travels in transit towards the n
th
antenna of a reception system through an FIR linear filter propagation channel C(t). Under these conditions, there is a relationship of linear filtering between the comb of symbols s(t) (such that s
e
(t)=h(t)*s(t) where <<*>> designates the convolution product) and the sensor signals x
n
(t), illustrated in FIG.
2
.
Given that C(t)=[C
1
(t) . . . C
N
(t)]
T
, the relationship between the transmitted symbols a
n
and the received signal
x
(t) is expressed in the form:
x
_
(
t
)
=
C
(
t
)
*
s
e
(
t
)
=
(
C
*
h
(
t
)
)
*
s
(
t
)
=
∑
n
G
(
t
-
nT
s
)
a
n
+
b
_
0
(
t
)
(
4
)
where
for t=nT
s
, s(t)=a
n
and for t≠nT
s
, s(t)=0.
G(t)=C(t)*h(t) is a N×1-sized vector and
b
(t) is the vector formed by the background noise of the N receivers.
The method disclosed in the patent FR 2 764 074 is subdivided into three steps:
1
st
step: Digitization of the signal received
x
(t) at the sampling period T
e
. Since the input of the system is formed by symbols {a
n
}, T
e
must be chosen so that T
s
=P T
e
where P is an integer. Thus, if we write G
p−nP
=G((p−nP)T
e
) the relationship (4) becomes:
x
_
(
pT
e
)
=
∑
n
G
p
-
nP
a
n
+
b
_
0
(
p
T
e
)
(
5
)
In the description of the patent, the authors write P=2. The fact of choosing the sampling period T
e
as a function of the symbol period T
s
is a is heavy constraint. Generally, the equipment cannot be used to obtain T
s
=P T
e
(P integer) directly. The sampling is done at T′
e
and, thereafter, to have a sampling at T
e
, it is necessary to carry out digital processing operations which resample the signal at T
e
. In the GSM standard, the signals sent verify the relationship 1/T
s
=270 kHz. To meet the conditions of the Shannon theorem at reception, the system receives them at 1/T
e
=500 kHz and T
s
/T
e
=500/270 is in fact a non-integer.
2
nd
step: Synchronization procedure: the goal is to determine the instant t
0
=pT
e
of appearance of the learning sequence {s
k
} such that a
p+k−1
={s
k
} for 1≦k≦K:
x
_
(
(
k
+
p
)
T
e
)
=
∑
n
G
k
-
nP
s
n
+
b
_
0
(
(
k
+
p
)
T
e
)
(
6
)
To this end, it is considered that the symbol comb s(n T
e
) is the input signal and it is sought to maximize the following synchronization criterion in terms of &tgr;:
Cri
(
τ
)
=
r
^
xs
(
τ
)
H
R
xx
-
1
(
τ
)
r
^
xs
(
τ
)
With
r
^
xs
(
τ
)
=
1
KP
∑
k
=
1
KP
x
_
(
k
T
e
+
τ
)
s
(
k
T
e
)
*
and
R
xx
(
τ
)
=
1
KP
∑
k
=
1
KP
x
_
(
k
T
e
+
τ
)
x
_
(
k
T
e
+
τ
)
H
(
7
)
K: number of symbols of the learning sequence
and s(n T
e
)=s
kP
for n=kP and s(n T
e
)=0 for n≠kP and where
x
H
designates the transposition and conjugate of
x
.
3
rd
step: The signals
x
(t+t
k
) and the symbol comb filters s(n T
e
) associated with a source are used in order to isolate this source and carry out a direction-finding operation on a single source at the instant t
0
=p T
e
. With the use of the received signal
x
(t+t
k
) and a single symbol comb filter s(n T
e
), the method is said to be a single-user method.
While the prior art method performs well, it nevertheless has certain drawbacks. It can be applied to waveforms that can be linearized or that can be approximated in such a form, and in single-user contexts.
The method according to the invention relies on a novel approach that consists especially in exploiting the signal or reference sequence containing symbols proper to a standard used or to a given format for the signal, the received signal and the knowledge of the instance of arrival of the different reference sequences, to determine the coordinates of a source from a measurement of its incidence, this operation being commonly known as direction-finding.
In the description, the terms “transmitters”, “users” or “useful transmitters” designate one and the same object.
SUMMARY OF THE INVENTION
The invention relates to a method of co-operative radio direction-finding in transmission in a system of radio direction-finding for a radiocommunications system comprising one or more transmission sources or transmitters, the transmitted signal comprising a reference sequence formed by a sequence of symbols {s
k
}, the receiver comprising at least one array of several sensors coupled to a radio direction-finder. The invention is characterized in that it comprises at least the following steps:
a)
Delaveau François
Ferreol Anne
Blum Theodore M.
Thales
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