Code division multiple access transmitter and receiver

Details Code division multiple access transmitter and receiver Code division multiple access transmitter and receiver

1995-06-05

1998-05-05

Olms, Douglas W.

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

H04B 7216

Patent

active

057486239

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a transmitter and a receiver used in mobile communications based on a code division multiple access (CDMA) system.


BACKGROUND ART

The following three systems are typical access systems in mobile communications:
(1) SCPC/FDMA (Single Channel Per Carrier/Frequency Division Multiple Access) system.
(2) TDMA/FDMA (Time Division Multiple Access/Frequency Division Multiple Access) system.
(3) CDMA system.
In the SCPC/FDMA system, a user occupies a channel associated with a carrier. In TDMA/FDMA system, a carrier is time-divided and each time slot is assigned to a user. In these systems, a base station communicates with a mobile station through an assigned frequency, or an assigned frequency and time slot.
On the other hand, in the CDMA system, the output signal of primary modulation such as QPSK is converted into a wideband signal by secondary modulation using a spreading code, and is transmitted. Since many users share the same carrier, and individual users are identified by spreading codes, this system is referred to as a spread spectrum multiple access system. The CDMA system is further classified into a direct sequence (DS) system and a frequency hopping (FH) system. The direct sequence system is characterized in that the primary modulated output signal is spread by using a high rate spreading code. On the other hand, the frequency hopping system resolves one symbol into elements called chips, and translates the individual chips into signals of different center frequencies at a high rate. Since the FH system is difficult to implement in the state of the art, the DS system is usually used.
At a spread spectrum RF receiving end, a received signal is demodulated in the order opposite to that at the transmitting end. Specifically, secondary demodulation which converts the wide-band received signal into a narrow-band signal by despeading using a spreading code is followed by primary demodulation which recovers source information symbols by synchronous detection or delay detection. The despreading at the receiving end is carried out by detecting correlation between the received signal and the spreading code corresponding to the desired signal wave. Thus, the signal that has been spread by the spreading code is despread.
In the DS system, each bit (symbol) of binary information is represented by a code sequence consisting of so-called chips with an interval much shorter than the interval of the bit, and the number of chips per symbol is called a processing gain. This is because the bandwidth of the transmitted signal is expanded by a factor of the processing gain. Denoting the processing gain as PG, 2.sup.PG PN (Pseudo-Noise) code sequences are generated, and they are the candidates of the spreading codes. However, not all the PN codes can be used as the spreading code because of some correlations between them: Only a limited number of the PN codes having small correlations can be used as the spreading codes. As a result, the actual capacity in terms of the number of users per carrier is reduced to a fraction of the processing gain. Thus, a plurality of carriers must be used in a CDMA cell of a large capacity in terms of the number of subscribers. In other words, both the spreading code and the carrier frequency must be designated to transmit and receive a desired signal.
FIG. 1 is a block diagram showing a conventional transmitter, in which both a spreading code and a carrier must be designated. In this figure, a transmitted information signal applied to an input terminal 11 is BPSK, QPSK, or GMSK (Gaussian filtered Minimum Shift Keying) modulated by a primary modulator 12, and then supplied to a secondary modulator 13 as an I-channel signal and a Q-channel signal (in the BPSK, the in-phase signal I and the quadrature signal Q are the same). On the other hand, a spreading code generator 14 generates the spreading code associated with a desired received signal, and supplies it to the secondary modulator 13. The spreading code generator 14 has a memory circuit for

REFERENCES:
patent: 4630283 (1986-12-01), Schiff
patent: 5267260 (1993-11-01), Lee
patent: 5271034 (1993-12-01), Abaunza
patent: 5471497 (1995-11-01), Zehavi
patent: 5488629 (1996-01-01), Takahashi et al.

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