Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-04-26
2001-09-18
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C375S362000
Reexamination Certificate
active
06292477
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mobile communication system operating on what is known as the code division multiple access (CDMA) system.
The CDMA system involves multiplexing a plurality of communication channels using spread spectrum codes, each channel being assigned a different spread spectrum code. A given signal to be transmitted is multiplied (i.e., spread) by the spread code assigned to the own channel, and is multiplexed with other similarly spread signals on different channels before being transmitted. At a receiver, the multiplexed signals are multiplied (i.e., despread) by the same spread code so that only the target signal will be extracted correlated on the own channel. The signals on the other channels are perceived merely as noise because these signals with their different spread codes remain uncorrelated. The level of the noise may be sufficiently lowered so as not to disturb the signal reception. The CDMA system is attracting attention as a system fit for drastically improving the efficiency of frequency utilization and has been commercialized in some areas.
Where CDMA communication is implemented using spread codes, some kind of signal modulation (e.g., quadrature phase shift keying or QPSK) precedes the spreading of the signal for transmission. At a receiving point, the despreading of the signal is followed by demodulation. Despreading and demodulation both represent the detection process whereby the transmitted signal is reconstructed. Commonly used detection methods include a coherent detection method based on the PLL (phase locked loop) circuit and a differential detection method. There also exists a recently proposed coherent detection method that utilizes pilot signals.
Where the CDMA system is applied to a mobile communication system adopting the conventional coherent detection method, the bit error rate of data in a mobile station deteriorates if a fading occurs while the station is moving. In a CDMA mobile communication system utilizing the differential detection method, the bit error rate of data in a mobile station can worsen due to the noise on the air transmission channel even if the station is stationary. The pilot signal-based coherent detection method has been proposed for a system to minimize the deterioration of the bit error rate whether the mobile station is in motion or at rest. The method was discussed at the Autumn 1994 Symposium of the Institute of Electronics, Information and Communication Engineers of Japan as disclosed in the IEICE collection of papers B-5 on radio communication systems A and B, p. 306, “Coherent detection for CDMA Mobile Communication Systems” by Yasuo Ohgoshi et al.
Described below is a conventional mobile communication system that uses pilot signals with reference to the above-cited paper supplemented by some details. The description will first center on the down link of the system (i.e., a link from the base station to a mobile station).
FIG. 13
shows a modulation circuit
51
of a base station
1
that transmits data and a first half
52
of the detection circuit of a mobile station
2
. The base station
1
actually transmits signals to a plurality of mobile stations
2
, and
FIG. 13
shows one station as the representative example.
In the modulation circuit
51
(left-hand half of FIG.
13
), data first undergoes QPSK modulation, not shown, to divide into an in-phase signal I and a quadrature signal Q. The signals I and Q are spread (i.e., multiplied) respectively by spread code signals PN
−ID
and PN
−QD
. The two spread code signals are supplied from a spread code generator
91
. The rates of the spread code signals PN
−ID
and PN
QD
(called the chip rates) are used to multiply by k (k: spreading ratio) the pre-spread rates (called the symbol rates) of the signals I and Q so that the latter will attain the chip rates. The signals thus spread pass through a radio frequency quadrature modulator
54
to become mutually perpendicular signals that are transmitted on a radio frequency band from an antenna. A temperature compensated crystal oscillator
61
is provided to furnish the modulator
54
with a carrier C
B
.
The pilot signals will now be described. The transmission circuit is substantially the same as the left-hand half of FIG.
13
and is omitted. An in-phase signal I
P
and a quadrature signal Q
p
of the pilot signals are spread respectively by spread code signals PN
IP
and PN
−QP
. Both spread code signals have the same chip rate as in the case of data. The pilot signals thus spread are subject to radio frequency quadrature modulation by the same carrier C
B
as with data, turning into mutually perpendicular signals transmitted on the same radio frequency band as with data. The pilot signals serve as reference signals for demodulation and are common to all channels utilized.
In the first half
52
(right-hand half of
FIG. 13
) of the detection circuit of the mobile station
2
, the received signals from the antenna (data and the pilot signals) pass through a radio frequency quadrature demodulator
57
to reach a low-pass filter
56
. The low-pass filter
56
removes the radio frequency components from the signals to yield signals S
I
and S
Q
. A crystal oscillator
60
supplies the demodulator
57
with a carrier C
M
. The signals S
I
and S
Q
are composed of the spread signals I and Q (those destined to the own channel as well as to other channels) and of the spread pilot signals I
P
and Q
P
. As such, the signals S
I
and S
Q
include a phase error caused by fading and a frequency error attributable to the precision of the oscillator
60
.
The errors included in the signals S
I
and S
Q
produce a phase difference therein. When the mutually perpendicular pilot signals are plotted in orthogonal coordinates, the received pilot signals are rotated exactly by the phase shift, as shown in FIG.
14
. If the phase shift is represented by &phgr; and the orthogonal coordinates after quadrature demodulation are designated by X
1
and Y
1
, then the coordinate axes X and Y of the received signals are rotated by &phgr; displacing the pilot signals. Consequently, the undisplaced signals i and q that should have resulted with no phase shift become i
1
and q
1
respectively. Such changes are caused by the mixing of one of the two mutually perpendicular signals into the other signal. The phenomenon is expressed by the following formulas:
i
1
=i
cos &phgr;−
q
sin &phgr;
q
1
=q
cos &phgr;+
i
sin &phgr;
The pilot signals are signals that stay constant following the despreading. Generally, i=1 and q=1. The signal changes into i
1
and q
1
permit acquisition of a signal CS with the value cos &phgr; and a signal SN with the value sin &phgr;. With the two signals known, it is possible to correct the phase rotation of the data. Since the data includes the same phase shift, the despread data signals are inversely rotated by &phgr; using the signals CS and SN whereby the initial signals I and Q are correctly reconstructed. Thus the signals CS and SN serve as phase correction signals.
The signals S
I
and S
Q
output by the first half
52
of the detection circuit are subject to despreading and phase correction by the second half of the detection circuit shown in
FIG. 15. A
pilot signal despreading unit
21
in the upper left portion of
FIG. 15
despreads the signals S
I
and S
Q
by use of the spread code signals PN
−IP
and PN
−QP
from a spread code generator
25
, whereby the pilot signals are extracted. The extracted pilot signals are then added and subtracted mutually, becoming a signal CS
C
with a chip rate of cos &phgr; and a signal SN
C
with a chip rate of sin &phgr;. The two signals are converted to the symbol rates by an accumulator
41
and thereby turn into phase correction signals CS
S
and SN
S
of the preliminary stage. The phase correction signals are averaged by an averaging circuit
43
for noise reduction. The averaging provides the phase correction signals CS and SN of the fina
Doi Nobukazu
Ohgoshi Yasuo
Yano Takashi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Ton Dang
Vanderpuye Ken
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