Spread spectrum communication transmitter and receiver, and...

Details Spread spectrum communication transmitter and receiver, and... Spread spectrum communication transmitter and receiver, and...

1997-03-19

2002-04-30

Olms, Douglas (Department: 2661)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S342000

Reexamination Certificate

active

06381233

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) mobile communication system and method using a spread spectrum communication system. Further, the present invention is concerned with a spread spectrum communication transmitter and receiver used for such a CDMA mobile communication system.
2. Description of the Related Art
FIG. 1
is a block diagram of a base station transmitter used in a CDMA mobile communication system using a conventional spread spectrum communication system, which is typically described in the IS/95 that is a standard system in the U.S. Telecommunications Industry Association/Electronic Industries Association (TIA/EIA).
FIG. 2
is a block diagram of a mobile station receiver in the CDMA mobile communication system.
The transmitter shown in
FIG. 1
can simultaneously communicate with n mobile stations where n is an integer. More particularly, the transmitter includes traffic channel transmit units
31
1
,
31
2
, . . . , and
31
n
, which respectively communicate with the first, second, . . . , and nth mobile stations. Each of the traffic channel transmit units
31
1
through
31
n
includes an information modulator
2
and a spread spectrum modulator
5
. The information modulator
2
of each traffic channel modulates transmit data (information)
4
by a BPSK, QPSK or another modulation method. The modulated transmit data is applied to the spread spectrum modulator
5
. The spread spectrum modulators
5
of the traffic channel transmit units
31
1
through
31
n
generate respective spreading codes (PN codes). The spread spectrum modulator
5
of each traffic channel spread the spectrum of the modulated transmit data from the information modulator
2
.
The transmitter shown in
FIG. 1
has a pilot channel transmit unit
30
. The mobile receivers discriminate the base stations from each other by referring to the pilot channel. The pilot channel transmit unit
30
includes a pilot data generator
1
, an information modulator
2
and a spread spectrum modulator
3
. The information modulator
2
modulates pilot data generated by the pilot data generator
1
by the BPSK, QPSK or another modulation method. The spread spectrum modulator
3
spreads the spectrum of the modulated pilot data by using a spreading code specifically used for the pilot channel and different from the spreading codes used for the traffic channels. The pilot signal thus generated can be arbitrary data which can be known in the base stations and the mobile receivers. For example, data consisting of only binary ones or binary zeros can be used as the pilot data.
The output signals of the traffic channel transmit units
31
1
through
31
n
and the pilot channel transmit unit
30
are combined so that the pilot channel and the traffic channels are simultaneously transmitted in a given frequency band. Then, the combined radio signal is transmitted via an antenna.
FIG. 3
shows a relation between the pilot and traffic channels with respect to time. As shown in
FIG. 3
, the pilot signal is always transmitted without any interval. In this regard, the pilot signal is a continuous signal.
Referring to
FIG. 2
, the mobile receiver used in the conventional CDMA mobile communication system includes a pilot channel receive unit
34
, and a traffic channel receive unit
35
. The pilot channel receive unit
34
includes a despreader
8
, a path detector
11
and a hand-over controller
19
. The traffic channel receive unit
35
includes despreaders
9
and
10
, a RAKE combiner
12
, an information demodulator
13
, and a level measuring unit
14
for controlling a transmit power.
The despreader
8
performs a despreading process on the received signal by using the spreading code for the pilot channel. The despreaders
9
and
10
perform a despreading process on the received signal by using the spreading code allocated to the receiver shown in
FIG. 2
at the transmitter. The path detector
11
detects multiple paths from the pilot signal. The hand-over controller
19
performs a hand-over control by using the results of the multipath detection obtained by the path detector
11
. The output signal of the path detector
11
is also used as a timing signal used for the despreading process carried out by the despreaders
9
and
10
. The RAKE combiner
12
performs a RAKE process on the despread signals from the despreaders
9
and
10
. The information demodulator
13
demodulates the output signal of the RAKE combiner
12
to thereby generate the original information. The level measuring unit
14
performs a level measuring operation for controlling the transmit power.
FIG. 4
shows a cell structure of the CDMA mobile communication system having the above transmitter and receiver. There are illustrated first, second, third and fourth base stations
21
,
22
,
23
and
24
, which cover service areas (cells)
26
,
27
,
28
and
29
, respectively. All the base stations
21
through
24
have transmitters as shown in
FIG. 1. A
reference number
25
indicates a mobile receiver (station) having the structure shown in FIG.
2
. The mobile station
25
is located within the cell
26
covered by the base station
21
, and can communicate with the base station
21
.
FIG. 5
is a timing chart of timings at which the base stations
21
through
24
respectively transmit the pilot signal. In the conventional CDMA mobile communication system, all the base stations
21
through
24
employ the same spreading code for spreading the pilot data. The period of the spreading code used to spread the pilot data is sufficiently longer than one symbol time of information (data). As shown in
FIG. 5
, the base stations
21
through
24
transmit the same spreading code for the pilot channel with respective inherent offset times equal to a time t′. That is, the starting points of the spreading codes used in the base stations
21
through
25
are offset by the time t′.
The mobile station
25
shown in
FIG. 4
receives the pilot signals from the base stations
21
,
22
,
23
and
24
. Usually, the pilot signal from the base station
21
closet to the mobile station
25
has the strongest level. The despreader
8
of the pilot channel receive unit
34
shown in
FIG. 2
performs the despreading process on the received signal by using the same spreading code as used in the transmitter.
FIG. 6A
shows a correlation between the spreading code for the pilot channel and the pilot signal transmitted by the base station
21
and received by the mobile station
25
. Similarly,
FIGS. 6B
,
6
C and
6
D show correlations with the pilot signals transmitted by the base stations
22
,
23
and
24
and received by the mobile station
25
. Peaks
201
through
204
respectively shown in
FIGS. 6A through 6D
indicate timing synchronization points in the pilot channels of the base stations
21
through
24
. Variations in the waveforms other than the peaks
201
through
204
shown in
FIGS. 6A through 6D
result from a self-correlation of the spreading code for the pilot channel. These variations in the waveforms are noise components for the mobile station
25
(receiver).
The mobile station
25
shown in
FIG. 4
receives the signals of the pilot channels transmitted by the base stations
21
through
24
in such a state that the signals are superimposed. Hence, the output signal of the despreader
8
of the pilot channel receive unit
34
has a formation in which the four waveforms shown in
FIGS. 6A through 6D
are superimposed. It should be noted that the correlations shown in
FIGS. 6A through 6D
are not affected by multipath fading or Rayleigh fading.
The path detector
11
shown in
FIG. 2
detects the greatest peak in the output signal of the despreader
8
(the greatest peak in the superimposed correlation waveform). In the case of
FIG. 4
, the mobile station
25
is located within the cell
26
of the base station
21
. Hence, the propagation distance between the base station
21
and the mobile station
25
is shorter than the propa

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