Electrical computers: arithmetic processing and calculating – Electrical analog calculating computer – Particular function performed
Reexamination Certificate
1999-10-07
2003-09-30
Mai, Tan V. (Department: 2124)
Electrical computers: arithmetic processing and calculating
Electrical analog calculating computer
Particular function performed
C375S343000
Reexamination Certificate
active
06629121
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field to Which the Invention Pertains
The present invention relates to a surface acoustic wave-matched filter for demodulation use in a DS (Direct Sequence)-SS (Spread Spectrum) communication system, which is applicable to any of all radio communication systems such as a fixed satellite communication system, a mobile satellite communication system, a fixed land radio communication system, a land mobile communication system, a radio LAN system and a private radio communication system, or any of all wire communication systems which transmit information over any of lines, such as optical fibers, coaxial cables or similar wires. The invention also pertains to a differential detector which uses the matched filter to demodulate spread spectrum signals in case of employing a quadrature phase shift keying (QPSK) system as a data signal modulating technique.
2. Prior Art
In a conventional DS-SS communication system, the bandwidth of a transmission signal is spread normally by multiplying a phase-modulated data signal by a pseudo noise code (PN code), or by phase-modulating an data signal multiplied by the PN code. To demodulate spread spectrum signals, it is customary in prior art to use two kinds of demodulation technique: (1) a demodulation system of a despreading scheme and (2) a demodulation system using a matched filter. In the systems, the despreading demodulation system (1) requires the receiving side to recover a timing clock from the received signal for despreading it. In a communication system for use indoors, such as a radio LAN, however, the communication channel becomes a multi-path fading channel, making the timing clock recovery or the carrier recovery very difficult and the required receiver construction very complex.
On the other hand, the demodulation system (2) using the matched filter could be implemented by two methods, i.e. a method of demodulating the spread spectrum signal by a surface acoustic wave filter (hereinafter referred to simply as a SAW filter) in the intermediate-frequency (IF) band, and a method of demodulating the spread spectrum signal by a digital signal processing technique after converting a received analog signal by an A/D converter to a discrete value. Now, a description will focus on a scheme using the SAW device.
The SAW filter is usually formed by two or more kinds of transducers such as input and output transducers deposited on a surface of a piezoelectric substrate. The input transducer generates a surface acoustic wave by the excitation of the substrate surface according to an applied voltage thereto of an electric signal. On the other hand, the output transducer outputs the voltage level of an electric signal converted from the surface acoustic wave generated by the input transducer. To demodulate the spread spectrum signal by the SAW filter, it is necessary that the weighting of the output transducer forming the SAW filter be so preset as to match with the PN code for spectrum spreading use. A SAW filter of the type, which has its output transducer thus associated so as to establish a correlation with the PN code thereto is called a SAW-matched filter.
When supplied with the spread spectrum signal, the SAW-matched filter outputs a phase-modulated signal having an information component as a pulse-like peak waveform of the correlation value when each of the code signals weighted for respective output transducers and the PN code of the input spread spectrum signal are completely in-phase with each other over the entire output transducer structure. Since this peak waveform of the correlation value is provided at each symbol period of the information signal, that is, each cycle period of the PN code, it is easily feasible to establish synchronization with the symbol period at the receiving side.
Next, it is necessary to make a decision on the data of the phase-modulated signal which is the output signal from the SAW-matched filter; a detection system in this case could be implemented by either of koherent and differential detection schemes. In general, the coherent detection scheme involves carrier recovery from the received signal but encounters much difficulty in the carrier recovery from the output signal of the SAW-matched filter since it does not have a continuous waveform but periodically has a pulse waveform of a short duration. For this reason, when the SAW-matched filter is used as a demodulating means for the spread spectrum communication system, it is customary to employ the differential detection scheme which does not require the carrier recovery. A description will be given below of examples of a transmitter and a receiver of a conventional spread spectrum communication system which uses a conventional SAW-matched filter and the differential detection scheme.
Examples of a transmitter and a receiver which implement the conventional spread spectrum communication system are depicted in
FIGS. 3 and 4
, respectively. Incidentally, the phase-modulating system used is a quadrature phase shift keying (QPSK) system.
In
FIG. 3
, reference numeral
21
denotes an input data train generator which generates a binary-coded information signal;
23
denotes a differential encoder which differentially encodes an input data sequence
22
of the binary-coded information signal to provide a differentially-encoded baseband information signal
24
;
25
denotes a spread spectrum modulator which spreads the spectrum of the differentially-encoded baseband information signal
24
;
26
denotes a multiplier which performs a binary multiplication of the differentially-encoded baseband information signal
24
and a pseudo-noise (hereinafter referred to as PN) code
28
from a PN code generator
27
to provide a baseband-spread spectrum signal
29
;
27
denotes the PN generator which generates a PN code sequence
28
for spreading the spectrum of the differentially-encoded baseband information signal
24
;
30
denotes a quadrature phase-shift keying (hereinafter referred to as QPSK) modulator which QPSK-modulates a baseband-spread spectrum signal
29
;
31
denotes a multiplier which performs a multiplication of the baseband-spread spectrum signal
29
and a quadrature phase-shift keyed-signal (hereinafter referred to as a QPSK-signal)
33
from a QPSK signal generator
32
to provide a baseband spread spectrum/QPSK signal
34
;
35
denotes a modulator which modulates the baseband spread spectrum/QPSK signal
34
from the QPSK modulator
30
to the radio frequency (RF) band signal by a carrier signal of a frequency f
c
which is output from a local oscillator
36
to provide an RF-band spread spectrum/QPSK signal
38
;
39
denotes a bandpass filter which extracts from the RF-band spread spectrum/QPSK signal
38
frequency components necessary for transmission to provide a band-limited RF-band spread spectrum QFSK signal
40
;
41
denotes a power amplifier which amplifies the output signal of the bandpass filter
39
; and
43
denotes a transmitting antenna which radiates a power-amplified RF-band spread spectrum/QPSK transmitting signal
42
.
In
FIG. 4
, there is depicted an example of a receiver which demodulates the RF-band spread spectrum/QPSK signal sent from the transmitter of FIG.
3
. In
FIG. 4
, reference numeral
51
denotes a receiving antenna for receiving the RF-band spread spectrum/QPSK signal transmitted from the transmitting side;
52
denotes a bandpass filter for extracting from the received signal only frequency components necessary for the demodulation thereof;
54
denotes a frequency converter for converting the output signal
53
of the bandpass filter
52
to an intermediate-frequency (IF) band signal;
55
denotes a local oscillator for generating an RF-local signal
56
of a frequency f
L
for frequency-conversion use;
58
denotes a bandpass filter for extracting only an IF signal
59
of a frequency f
O
from a frequency converted signal
57
;
60
denotes a surface acoustic wave-matched filter (hereinafter referred to as a SAW-matched filter) for extracting a QPSK-signal
61
f
Ishikawa Hiroyasu
Shinonaga Hideyuki
Lachenbach Siegel, LLP
Mai Tan V.
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