Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-03-04
2001-05-08
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06229840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diversity circuit, more specifically to a diversity circuit for antenna diversity of a radio communication system employing a spread spectrum system, such as code division multiple access (CDMA).
2. Description of the Related Art
In a conventional combining diversity circuit, a set of an antenna, receiver, and demodulator is needed for each of the diversity paths. Signals from each diversity path are combined at a final stage of signal processing by using the maximal ratio combining technique, where finally the signals from each diversity path are combined into one composite signal.
FIG. 2
illustrates an example of the foregoing conventional circuit.
FIG. 2
is a typical example of an antenna combining diversity circuit. Signals received by each of two antennas
201
,
202
are respectively demodulated into base band signals by receivers
203
,
204
. The demodulated outputs of receivers
203
,
204
are respectively converted into digital signals by A/D converters
205
,
206
. The digitally converted signals are respectively demodulated by digital demodulators
207
,
208
, where finally the two demodulated signals are added, or combined by a combiner
209
. The output from the combiner
209
is decoded by a decoder
210
to reproduce transmitted data.
In the conventional combining diversity circuit having dual branches, a set of an antenna, receiver, A/D converter and digital demodulator is needed for each of the two diversity paths, which doubles the scale of the circuit compared to a case without applying the diversity. Naturally, increased scale of the circuit will increase the power consumption, cost, and size of the unit, which makes it difficult to use the diversity circuit for a portable telephone, for example.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a diversity circuit in which the scale of circuit is reduced, and thereby the size, power consumption, and cost of the unit can be reduced.
In order to accomplish the foregoing object and other objects, a diversity circuit for receiving a spread spectrum modulated wave signal according to the invention includes a plurality of antennas, a plurality of delay circuits that each give different delay times to a plurality of radio-frequency signals received by a plurality of the antennas, combining means for adding outputs of a plurality of the delay circuits, demodulation means for demodulating an output from the combining means, A/D conversion means for converting a demodulated result by the demodulation means into a digital signal, and digital signal processing means for processing an output from the A/D conversion means to decode.
In the foregoing diversity circuit, the signals received by a plurality of the antennas may be amplified by a plurality of amplifiers, and thereafter passed through the delay circuits, or, further converted into signals of intermediate frequencies by frequency conversion means, passed through the delay circuits, and thereafter combined and demodulated.
Further, in case of two antennas, only one delay circuit is needed, and the diversity circuit may be formed such that a radio-frequency wave signal received by one antenna is not processed as it is and only the signal received by the other one antenna is delayed by the delay circuit, and thereafter the two signals are combined.
Further, in the diversity circuit, the digital signal processing means may apply a Rake reception system that gives de-spreading codes to each of signals having different delay times and combines the results, and a combined result may be decoded by decode means.
Preferably, differences in the delay times of the delay circuits are sufficiently larger than differences in transmission times by multipaths produced on separate transmission paths.
In the spread spectrum communication system according to the invention, it is possible to lower the correlation between signals shifted in time base and de-spreading codes by properly selecting spreading codes. Therefore, the signals shifted in time base can be identified as signals passed through different communication lines, and can be separated as such. The present invention exploits the foregoing theory.
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Oguchi Takuro, Space Diversity System, Patent Abstracts of Japan, vol. 005, No. 030 (E-047), 2/81 & JP 55 156433 (Fujitsu Ltd)., 12/80.
K. Fujimoto et al., “Mobile Antenna Systems Handbook”, 1994, p. 78-85.
A. J. Viterbi., “CDMA-Principles of Spread Spectrum Communication”, Addison-Wesley Wireless Communication Series, Jun. 1995, p. 87-96.
D. P. Whipple, “North American Cellular CDMA”, Hewlett-Packard Journal, Dec. 1993, p. 90-97.
Corrielus Jean B.
Foley & Lardner
NEC Corporation
Pham Chi
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