Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-08-13
2002-12-31
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S363000, C370S506000, C370S537000, C370S538000
Reexamination Certificate
active
06501786
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a spread spectrum communication and more particularly to a spread spectrum communication system conforming to a delayed multiplex mode transmission, and a method of communication using the system.
BACKGROUND OF THE INVENTION
Spread spectrum communication systems have recently been developed, and have attracted a great deal of public attention as a new and interesting communication method. A typical modulation system used in conventional data communication systems is a narrow band modulation system which can be realized by relatively small circuitry, but which is weak to interference such as multipath interference and narrow band color noises. Such may occur indoors in offices and workplaces. On the contrary, the spread spectrum communication system is free from the above-mentioned problem since it can spread a data spectrum with a spread code and transmit the spread-spectrum coded data in a wide band.
The spread spectrum communication system, however, has a disadvantage of requiring a widened bandwidth to transmit data at a high speed. For example, a bandwidth of 22 MHz is required to realize spread-spectrum transmission of QPSK (Quadrature Phase Shift Keying) modulated data with 11 chips of spread code at a transmission rate of 2 Mbps. Namely, the bandwidth of 110 MHz is needed to transmit data at 10 Mbps. The fact that radio transmission has a restriction on bandwidth makes it more difficult to realize the high-speed data transmission by the spread-spectrum communication method.
Accordingly, a method for multiplexing spectrum signals with a delay (hereinafter called “Delayed multiplex system”) has been proposed as a method of transmitting data within a limited bandwidth at a high speed, which is described in Japanese Patent Application No. 7-206159.
The use of this method can increase the data transmission rate within a limited bandwidth. For example, the data transmission rate 2 Mbps at 22 MHz (as described before) can be increased to 4 Mbps by multiplexing the data twice and further to 10 Mbps by multiplexing the data five times.
FIG. 6
shows a typical block diagram of a conventional system used for proposed delayed multiplex transmission method. In
FIG. 6
, data generated by a data generating section
10
is differentially coded by a differential coding section
11
and then converted to parallel sets of data to be multiplexed by a series-to-parallel (S/P) converting section
12
. The parallel data sets are transferred to respective multiplying sections (
13
-
1
~
13
-
5
) whereby they are spread by multiplying by a PN code received from a PN generator
14
. Then, the parallel data sets are delayed respectively by the delay elements (
15
-
1
~
15
-
5
).
The delayed parallel data sets are combined by a frequency combining section
17
to form multivalued digital signals which are then modulated with oscillation of an oscillator
18
by a modulator
17
and transmitted through a frequency converter
19
and a power amplifier
20
.
The high-speed data transmission within the limited bandwidth is thus accomplished.
However, the delayed multiplex data transmission still encounters an increase in error rate from interference between multiplexed signals.
Accordingly, the inventor of the present application also proposed a method for improving the correlation and error rate for the delayed multiplex transmission (Japanese Patent Application No. 8-13963).
FIG. 7
shows a typical circuit diagram of a conventional delayed multiplex system by which improvement in error rate has been realized. This example is described below. The transmission system is the same with the system shown before in FIG.
6
. In the receiving system of
FIG. 7
, a received frequency signal is converted to a base band signal by a frequency converting section
21
and correlated by a correlator
23
. The correlation is latched by a latch section
24
at the timing of a correlating spike and, then, is recovered from the deterioration due to auto-correlation by a correlation processing section
25
. This correlation output is distributed by a distributor
26
. The signal is controlled by a latch controller
29
and latched by latch sections
27
and
28
. According to the aforementioned actual example, the signal is latched with 2 chips or 3 chips.
The signal is differentiated by a differential section
30
and discriminated and demodulated by a discriminating section
31
.
FIG. 8
is a typical construction block diagram of the correlating processing section of FIG.
7
. This depicts a five-multiplex case. In
FIG. 8
, there is shown only one line that must be doubled for realizing the system of FIG.
7
.
An input signal is input to shift registers
25
S corresponding to the number of input bits. The registers hold correlation spikes by four before and after a desired demodulation timing spike. An arithmetic unit
25
P with a selecting function carries-out operations on the signals at a timing signal generated by a timing signal generator
25
T which matches the timings of input and output according to signals from a correlation synchronizing circuit (not shown in FIG.
8
).
FIG. 9
is a block diagram showing a typical internal construction of the arithmetic unit with a selecting function, which is shown in FIG.
8
.
The use of the unit of
FIG. 9
can considerably decrease a change in amplitude of a signal due to the influence of an auto-correlation side-lobe, and can therefore make a great improvement in error rate. The operation of this unit is as follows:
As seen in
FIG. 9
, signals A and F are input to a selector
25
P
1
, signals B and G are input to a selector
25
P
2
, signals C and H are input to a selector
25
P
3
and signals D and I are input to a selector
25
P
4
. In this instance, the selectors
25
P
1
-
25
P
4
select signals A, B, C and D respectively. The selected signals A-D are added together by an adder
25
P
5
and then divided by 11 by a divider
25
P
6
. The resultant signal is added to E′ by an adder-subtracter
25
P
9
, then latched by a latching section
25
P
8
and output.
In the example shown, preceding and proceeding correlation signals are used for improvement of the correlation output. When received signals are 5-multiplex signals, each signal is composed of five blocks and five correlatin outputs are used for improvement in correlation. However, five blocks cannot be obtained due to the transmitted data length.
FIG. 10
is a conceptual illustration of an exemplified state of five-multiple data. In the case shown, there is no data in the two last blocks (i.e., the two last symbols are lost) since these signals, (outputs of respective delay elements) before being multiplexed, do not have the data length of a multiple of 5. Consequently, data consisting of 128 symbols is divided into 5 groups, each consisting of 5 blocks of 5 symbols, and thus 3 symbols remain, as shown in FIG.
10
. In this instance, the conventional correlation-improved circuit cannot exert a sufficient effect of improving the correlation because blocks of coded signals are partly omitted. Thus, an error rate for the last three symbols worsens. In packet communication, 1 bit error causes retransmission of data. Therefore, an increase in error rate for above-mentioned symbols decreases the throughput of the packet transmission system.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems, the present invention was made to provide a direct spread spectrum communication system conforming to a delayed multiplex mode transmission and a communication method using the system, which can realize improvement in correlation and improvement in error rate in any data length by applying the above mentioned technical means featuring the present invention.
(1) In view of the above-mentioned problems, the present invention was made to provide a direct spread spectrum communication system conforming to a delayed multiplex mode transmission and a communication method using the system, which can realize improvement in correlation and improvement in error rat
Birch & Stewart Kolasch & Birch, LLP
Chin Stephen
Ha Dac V.
Sharp Kabushiki Kaisha
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