Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-03-25
2001-07-10
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C375S148000
Reexamination Certificate
active
06259688
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to an interference canceler and method for reducing interference in a direct sequence, code division multiple access receiver.
DESCRIPTION OF THE RELEVANT ART
Direct sequence, code division multiple access, spread-spectrum communications systems are capacity limited by interference caused by other simultaneous users. This is compounded if adaptive power control is not used, or is used but is not perfect.
Code division multiple access if interference limited. The more users transmitting simultaneously, the higher the bit error rate (BER). Increased capacity requires forward error correction (FEC) coding, which in turn, increases the data rate and limits capacity.
SUMMARY OF THE INVENTION
A general object of the invention is to reduce noise resulting from N−1 interfering signals in a direct sequence, spread-spectrum code division multiple access receiver.
The present invention, as embodied and broadly described herein, provides a spread-spectrum code division multiple access (CDMA) interference canceler for reducing interference in a spread-spectrum CDMA receiver having N channels. Each of the N channels is spread-spectrum processed by a distinct chip-code signal. The chip-code signal, preferably, is derived from a distinct pseudo-noise (PN) sequence, which may be generated from a distinct chip codeword. The interference canceler partially cancels N−1 interfering CDMA channels, and provides a signal-to-noise ratio (SNR) improvement of approximately N/PG, where PG is the processing gain. Processing gain is the ratio of the chip rate divided by the bit rate. By canceling or reducing interference, the SNR primarily may be due to thermal noise, and residual, interference-produced noise. Thus, the SNR may increase, lowering the BER, which reduces the demand for a FEC encoder/decoder.
The interference canceler, for a particular channel, includes a plurality of despreading means, a plurality of spread-spectrum-processing means, subtracting means, and channel-despreading means. Using a plurality of chip-code signals, the plurality of despreading means despreads the spread-spectrum CDMA signals as a plurality of despread signals, respectively. The plurality of spread-spectrum-processing means uses a timed version of the plurality of chip-code signals, for spread-spectrum processing the plurality of despread signals, respectively, with a chip-code signal corresponding to a respective despread signal. The timed version of a chip-code signal may be generated by delaying the chip-code signal from a chip-code-signal generator. Alternatively, a matched filter may detect a particular PN sequence in the spread-spectrum CDMA signal. A chip-code-signal generator may use the detected signal from the matched filter to trigger a timed version of the chip-code signal.
For recovering a particular CDMA channel using an i
th
chip-code signal, the subtracting means subtracts from the spread-spectrum CDMA signal, each of the N−1 spread-spectrum-processed-despread signals, thereby generating a subtracted signal. The N−1 spread-spectrum-processed-despread signals do not include the spread-spectrum-processed-despread signal of the i
th
channel corresponding to the i
th
chip-code signal. The channel-despreading means despreads the subtracted signal from the i
th
chip-code signal.
The present invention also includes a method for reducing interference in a spread-spectrum CDMA receiver having N channels. The method comprises the steps of despreading, using a plurality of chip-code signals, the spread-spectrum CDMA signal as a plurality of despread signals, respectively, spread-spectrum processing, using a timed version of the plurality of chip-code signals, the plurality of despread signals, respectively, with a chip-code signal corresponding to a respective despread signal; subtracting from the spread-spectrum CDMA signal, each of the N−1 spread-spectrum-processed-despread signals, with the N−1 spread-spectrum-processed-despread signals not including a spread-spectrum-processed despread signal of the i
th
channels, thereby generating a subtracted signal; and, despreading the subtracted signal having the i
th
chip-code signal.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
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Y.C. Yoon, R. Kohno, and H. Imai, “A
Kowalski John
Schilling Donald L.
Boakye Alexander O.
InterDigital Technology Corporation
Nguyen Chau
Volpe and Koenig P.C.
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