Multiplex communications – Generalized orthogonal or special mathematical techniques – Plural diverse modulation techniques
Patent
1996-07-26
1998-01-06
Chin, Wellington
Multiplex communications
Generalized orthogonal or special mathematical techniques
Plural diverse modulation techniques
370210, 370342, 370441, 375201, 375206, H04J 1300
Patent
active
057062757
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a data transmission method in a system wherein the CDMA multiple access method is utilized and wherein each data signal to be transmitted is multiplied by a pseudorandom code of a certain length, having a bit rate that is higher than that of the data signal to be transmitted.
CDMA (Code Division Multiple Access) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) methods. CDMA has several advantages over the prior methods, for example spectral efficiency, the simplicity of frequency planning and traffic capacity.
In the CDMA method, the narrow-band data signal of the user is conventionally multiplied to a relatively wide band of a traffic channel by a spreading code having a considerably broader band than the data signal. In known cellular network test systems, traffic channel bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band, or traffic channel, simultaneously. A separate spreading code is used over each connection between a base station and a subscriber terminal, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of each connection.
Correlators provided in the conventional CDMA receivers are synchronized with a desired signal, which they recognize on the basis of the spreading code in the signal. The data signal is restored in the receiver to the original band by multiplying it again by the same spreading code as during the transmitting stage. Signals multiplied by some other spreading code during the transmitting stage do not correlate in an ideal case with the spreading code used in the receiver, and they are therefore not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that the codes used in each system cell are mutually orthogonal, i.e. they do not correlate with each other.
The most time-consuming process in a CDMA transceiver implemented in known manners is correlation, and, simultaneously, the most expensive component is the correlator situated in the receiver. In the correlator, a received signal is compared bit by bit with a known spreading code, and the comparison produces a correlation value.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a data transmission method which utilizes CDMA and in which the use of correlation can be replaced with a faster and more effective method.
Instead, the signal multiplied by the pseudorandom code is further modulated by a group of waveforms the number of which equals the number of bits in the pseudorandom code, the frequency of each waveform being included in the frequency domain given.
The invention also relates to a transmitter comprising means for multiplying each data signal to be transmitted by a pseudorandom code of a certain length, having a bit rate that is higher than that of the data signal to be transmitted. The transmitter according to the invention comprises means for modulating the signal multiplied by the pseudorandom code by a group of waveforms the number of which equals the number of bits in the pseudorandom code, the frequency of each waveform being included in the frequency domain given.
The invention also relates to a receiver comprising means for converting a received analog signal into a digital form, and means for generating a desired pseudorandom code. The receiver according to the invention comprises means for subjecting the desired pseudorandom code to an inverse Fourier transform, and means for multiplying the digitized received signal by the converted pseudorandom code, and means for subjecting the multiplied signal to a Four
REFERENCES:
patent: 3808412 (1974-04-01), Smith
patent: 4943975 (1990-07-01), Kurihara et al.
patent: 5008899 (1991-04-01), Yamamoto
patent: 5175743 (1992-12-01), Crespo et al.
Chin Wellington
Nokia Telecommunications Oy
Vu Huy D.
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