Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-06-29
2004-11-09
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C375S152000, C370S335000
Reexamination Certificate
active
06816542
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a direct sequence CDMA receiver, and more particularly to a receiver of the CDMA (Code Division Multiple Access) system that performs despreading demodulation using a spreading code that is contained in the received signal.
2. Description of the Related Art
CDMA is a method of transmitting by spreading information data into a broadband signal by means of a spreading code having a spread rate (=number of chips/symbols) pg. On the receiving side, the leading position of the spreading code contained within the received signal must be detected and despreading demodulation must be performed using the spreading code that is synchronized with this leading position.
To achieve synchronization on the receiving side, correlation values with the spreading code are found using a portion having a known data sequence, such as pilot symbols or a preamble data sequence that is periodically inserted, and timings are found for which these correlation values are high.
FIG. 1
shows the configuration of a receiver. In the receiver in
FIG. 1
, a signal received by an antenna (not shown) is demodulated to convert to a baseband signal, and which is sampled at twice the chip rate by A/D (analog/digital) converter
41
. Correlation values between the spreading code and the sampled received data are then found by matched filter
42
.
In delay profile production unit
43
, delay profiles are sought from the correlation values of the sampling interval of received data obtained by matched filter
42
. In timing detector
44
, prescribed locations are selected in descending order from the largest value among the correlation values of this delay profile, and timing signals are generated based on the timings of each of the selected values. Then, in despreader
45
-
47
, the received data are despread using a spreading code that is synchronized with these timing signals, and data are obtained through RAKE/diversity mixing at mixer
48
.
Referring now to
FIG. 2
b
, matched filter
42
comprises received signal shift register
51
, spreading code register
52
, multipliers
53
, and adder
54
.
Received signal shift register
51
receives and shifts the received signal. Spreading code register
52
sets a spreading code sequence of the same bit length as this received signal shift register
51
. Multipliers
53
multiply the values of received signal shift register
51
and spreading code register
52
. Adder
54
adds the output signals of multipliers
53
.
As shown in
FIG. 2
a
, the received signal sequence is constituted by a known portion (pilot portion) and a data portion that is not know, and is sampled at, for example, twice the chip period of the spreading code, and through A/D conversion, becomes a digital value between −1.0-+1.0. In this case, received signal shift register
51
shifts the received signal each ½ chip period in accordance with the sampling period of the received signal.
The output signals of each stage of this received signal shift register
51
and each bit of the spreading code sequence set by spreading code register
52
are multiplied by multipliers
53
for each shift, and the results of multiplication are added in adder
54
to produce the correlation values. If the size of these correlation values is arranged on the time axis as shown in
FIG. 3
, a delay profile is obtained in which the correlation values increase at &tgr;0, &tgr;1, and &tgr;2.
The delay profile varies with fading or movement of a mobile terminal and has a plurality of maximum values, and the timing of which is extracted by timing detector
44
. If the delay profile is represented as shown in
FIG. 3
, the correlation values in the delay profile are selected in descending order of size and the timing at which each of these correlation values was obtained is reported to despreaders
45
-
47
by means of respectively different timing signals.
Japanese Patent Laid-open No. 178386/98 and Japanese Patent Laid-open No. 336072/98 each disclose an example of the aforementioned CDMA receiver.
In a CDMA receiver of the aforementioned prior art, the spreading code and received data must be reliably and accurately synchronized, and it is well known that reception characteristics deteriorate drastically with a variance in synchronization of just ¼ chip. The deterioration is particularly noticeable in the case of a low spreading rate, such as in high-speed data transfer.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide a direct sequence CDMA receiver that can accurately acquire synchronization of despreading timing without increasing the sampling rate of the A/D conversion of received data, and to thereby improve reception characteristics.
The direct sequence CDMA receiver according to the present invention comprises an analog/digital conversion means, a matched filter, and a delay profile production means.
The analog/digital conversion means samples, at twice the chip rate, a baseband signal obtained by demodulating a signal received by way of an antenna. The matched filter acquires correlation values of the received data sampled by the analog/digital conversion means and a spreading code while time-shifting said received data. The delay profile production means interpolates correlation values found by the matched filter, and finds the delay profiles from the interpolated correlation values. That is, when establishing synchronization on the receiving side, correlation values are found between a spreading code composed of 1/−1 data and then received data, and delay profile is produced using the correlation values of the sampling period. The timings of peaks in the correlation values in the delay profile are then obtained to realize synchronization between the received data and the spreading code. The accuracy of synchronization is therefore dependent on the sampling interval of the received data.
In contrast, the direct sequence CDMA receiver according to the present invention inputs a signal received by an antenna and converted to a baseband signal, and samples the input at twice the chip rate in an A/D converter. Thereafter, correlation values with the received data are then found by means of the matched filter using the 1/−1 spreading code and are interpolated by means of, for example, a filter in the delay profile production means having an interpolation function. These interpolated data are then used to provide delay profiles as a continuous characteristic. As a result, the timing of peaks in the delay profiles can also be obtained with higher accuracy in a timing detector irrespective of a sampling period.
In addition, the correlation values in a delay profile are selected in descending order of size, the timings at which these correlation values were obtained are extracted, and respectively different timing signals are supplied to despreader. The despreaders are not limited to three in number, and may one or more.
In the despreaders, the spreading code is interpolated to synchronize with these timing signals, and the received data are despread using the interpolated values. The despread signals are RAKE/diversity mixed in a mixer to obtain data.
Synchronization of despread timing can thus be accurately achieved without increasing the A/D conversion sampling rate of the received data, thereby improving reception characteristics.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.
REFERENCES:
patent: 5982763 (1999-11-01), Sato
patent: 5999560 (1999-12-01), Ono
patent: 6275521 (2001-08-01), Uno
patent: 6370184 (2002-04-01), Hellberg
patent: 6507605 (2003-01-01), Fukumoto et al.
patent: 6567482 (2003-05-01), Popovic′
patent: 0 654 677 (1994-11-01), None
patent: 0 820 156 (1997-07-01), None
patent: 5-7130 (1993-01-01), None
patent: 758669 (1995-03-0
Bayard Emmanuel
Dickstein , Shapiro, Morin & Oshinsky, LLP
Ghulamali Qutub
NEC Corporation
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