Transmitting and receiving method and radio system

Details Transmitting and receiving method and radio system Transmitting and receiving method and radio system

1998-05-04

2001-08-28

Olms, Douglas (Department: 2661)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C370S319000, C370S335000, C370S441000, C375S130000

Reexamination Certificate

active

06282185

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a transmitting and receiving method used in a digital radio system comprising at least one base station and subscriber terminal communicating with one another by transmitting and receiving signals.
The invention also relates to a radio system comprising at least one base station and subscriber terminal with a transmitter and receiver arranged to transmit and receive digital signals.
DESCRIPTION OF THE PRIOR ART
In the CDMA method (Code Division Multiple Access), a narrow-band user data signal is multiplied to a relatively broad band by a spreading code having a considerably higher frequency than the data signal. Upon multiplication, the data signal is spread to the entire band in use. All users use the same frequency band for simultaneous transmission. The objective is to select mutually substantially orthogonal spreading codes, i.e. codes that have minimal mutual correlation.
In a CDMA receiver implemented in a conventional manner, a data signal is returned in the receiver to the original band by multiplying it again with the same spreading code as at the transmission stage. The signals of other users impede the detection of the desired signal by distorting the received signal. This interference, caused by users to each other, is called multiple access interference.
In the OFDMA method (Orthogonal Frequency Division Multiple Access) the spread-coded symbol train of a transmission signal is modulated by subcarriers which are preferably distributed to a wide frequency band. OFDMA modulation is typically carried out by inverse Fourier transformation.
Multipath propagation causes distortion or crosstalk on radio system channels. In the CDMA system this can be detected in that a signal originally spread-coded with orthogonal spreading codes is no longer orthogonal upon arrival at a receiver, there being crosstalk between signals of different channels. In the OFDMA system, subcarriers attenuate frequency-selectively, impairing detection. In the CDMA system, crosstalk is adjusted by using interference cancellation. In the OFDMA system, frequency-selective attenuation is compensated for by error correction.
However, the complexity of all methods eliminating crosstalk generated by multipath propagation increases faster than the amount of information/the bit string to be processed. Consequently, known methods cannot remove this problem.
SUMMARY OF THE INVENTION
It is the object of the present invention to implement a method and a radio system in which the amount of information to be processed can be limited thus allowing better elimination of various disturbances.
This is achieved with the method described in the preamble, characterized in that upon transmission the signals are spread-coded and modulated by subcarriers in accordance with the OFDMA/CDMA methods, and upon reception the signals are demodulated and multiuser-detected by selecting the signals to be detected in the multiuser detection using the subcarriers and consequently reducing the number of signals in the multiuser detection.
The radio system of the invention is characterized in that the transmitter comprises means for spread coding and means for modulating the transmission signals by subcarriers in accordance with the OFDMA/CDMA methods, and the receiver comprises demodulation means arranged to select a detectable signal on the basis of subcarriers, and means for multiuser detection of the received signals.
The method of the invention provides considerable advantages. The method of the invention enables good tolerance of interference caused by channel isolation and signal multipath propagation by means of a simple receiver implementation.


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Youn, et al.,Canadian Conference on Electrical and Computer Engineering, vol. 1, “A Multi-Carrier CDMA System Using a Concatenated Orthogonal/PN Spreading Scheme”, p. 202-205, 1996.
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