Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1997-03-13
2001-05-15
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06233272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to spread spectrum communication receivers, and more particularly to a spread spectrum communication receiver which is suitable for mobile communications and is capable of demodulating received signals which have been distorted during propagations of multiple paths.
2. Description of the Related Art
Recently, there has been considerable activity in the research and development of a communication system using a spread spectrum communication system and a CDMA (Code Division Multiple Access) communication system which is a multiple access system utilizing the spread spectrum communication system. These communication systems are not liable to be affected by interference, disturbance and multipath propagations. A so-called RAKE receiver which combines multipath transmissions can be used in the above system, so that diversity can be obtained. Hence, the above systems are attractive as applications to mobile communications.
A spread spectrum communication receiver including a RAKE receiver will be described.
FIG. 1
is a block diagram of such a conventional spread spectrum communication receiver. The receiver includes demodulator units
1
,
2
and
3
, a symbol combiner
5
, a transmission path state estimating unit
6
, and a decision unit
7
. The demodulator units
1
,
2
and
3
perform respective demodulation processes on a received signal
11
in mutually independent phases. The symbol combiner
5
pulls the demodulated signals from the demodulation units
1
,
2
and
3
in phase, and them combines the demodulated signals. The transmission path state estimating unit
6
estimates the states of multiple transmission paths (main lobes), and supplies the demodulator units
1
,
2
and
3
with phase information indicative of the phases of the multiple paths obtained by the estimating unit
6
.
FIG. 2A
shows demodulation timings of the demodulator units
1
through
3
.
FIG. 2A
shows transmission (propagation) paths (main lobes)
13
,
14
and
15
and a correlation therebetween. The receive power levels of the signals respectively propagated through the paths
13
,
14
and
15
have respective delay times.
The operation of the spread spectrum communication receiver shown in
FIG. 1
will be described below.
The transmission path state estimating unit
6
estimates the states of the transmission paths from the received signal
11
. More particularly, the estimating unit
6
measures the correlation levels in the phases of spreading codes respectively used in the demodulator units
1
,
2
and
3
, and thus obtains the receive power levels of the signals propagated through the paths
13
,
14
and
15
. The information concerning the phases and receive power levels is applied to the demodulator units
1
,
2
and
3
as information
12
.
The demodulator units
1
through
3
perform the demodulation processes in synchronism with the phases of the paths
13
,
14
and
15
estimated by the estimating unit
6
. The receiver shown in
FIG. 1
has three demodulator units
1
,
2
and
3
, which thus perform the demodulation processes on the received signals in synchronism with the phases of the paths
13
,
14
and
15
, respectively. The demodulated signals output by the demodulator units
1
,
2
and
3
are combined together in a maximum ratio fashion. A decision on the resultant combined signal is made by the decision unit
7
, which then outputs the demodulated data.
The transmission path state estimating unit
6
provides the demodulator units
2
and
3
with relative phase difference information concerning the paths
14
and
15
with respect to the path
13
. Hence, the demodulation processes of the demodulators
2
and
3
can be pulled in phase with the signals propagated through the paths
14
and
15
.
However, the above conventional spread spectrum communication receiver has the following disadvantages, which will be encountered when there is a signal which is propagated through a path having a delay time longer than the symbol interval and which overlaps with a signal of the next symbol propagated through a path having a relatively small delay time.
FIG. 2B
shows a path
16
located in the next symbol interval. The path
16
has a delay time longer than the symbol interval. The path
16
shown in
FIG. 2B
does not overlap with any path located in the next symbol interval. In this case, the spread spectrum communication receiver can demodulate the symbol from the signals propagated through the paths
13
,
14
and
15
and the symbol from the signals propagated through paths
17
,
18
and
19
.
FIG. 2C
shows that the path
16
overlaps with the path
18
in the next symbol interval. The conventional receiver does not discriminate the signals respectively propagated through the paths
16
and
18
from each other. Hence, the signal propagated through the path
16
functions as noise when the signal propagated through the path
18
is demodulated in the next symbol interval. This degrades the reception performance of the receiver.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a spread spectrum communication receiver in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a spread spectrum communication receiver capable of performing the demodulation process without a degradation of the reception performance of the receiver even if paths located in different symbol intervals overlap with each other.
The above objects of the present invention are achieved by a spread spectrum communication receiver comprising: a transmission path state estimating unit which estimates states of paths from a received signal; demodulator units independently performing demodulation processes on signals propagated through the respective paths; a symbol combiner combining output signals of the demodulator units; a decision unit which is directly or indirectly coupled to the symbol combiner and makes a decision on an output signal of the symbol combiner; and an unnecessary component eliminating unit eliminating a component of a first symbol which is an unnecessary component that is propagated through one of the paths and overlaps with a received component of a second symbol to be demodulated, so that the unnecessary component has been eliminated from the output signal of the decision unit.
The above spread spectrum communication receiver may be configured so that the unnecessary component eliminating unit comprises: an unnecessary component demodulator unit demodulating the unnecessary component by referring to estimated states of the paths and the output signal of the decision unit; and an erasing unit subtracting an output signal of the unnecessary component demodulator unit from the output signal of the symbol combiner.
The above spread spectrum communication receiver may be configured so that the unnecessary component eliminating unit comprises: a receive quality measuring unit measuring a receive quality by using an output signal of an erasing unit and the output signal of the decision unit; a control unit generating a replica of the unnecessary component by using the output signal of the decision unit and the receive quality measured by the receive quality measuring unit; and the above erasing unit subtracting the replica generated by the control unit from the output signal of the symbol combiner.
The above spread spectrum communication receiver may be configured so that the unnecessary component eliminating unit comprises: an unnecessary component demodulator unit demodulating the unnecessary component by referring to estimated states of the paths and the output signal of the decision unit; a receive quality measuring unit measuring a receive quality by using an output signal of an erasing unit and the output signal of the decision unit; a control unit generating a replica of the unnecessary component by using the output signal of the decision unit and the receive quality measured by the receive quality meas
Burd Kevin M
Ladas & Parry
Pham Chi
YRP Mobile Telecommunications Key Technology Research Laboratori
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