Arrangement in a multi-user system

Details Arrangement in a multi-user system Arrangement in a multi-user system

1996-04-01

1998-07-21

Hsu, Alpus H.

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

370343, 455 65, 455 673, H04J 116

Patent

active

057843635

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION



FIELD OF THE INVENTION

The present invention relates to an arrangement in a multi-user system comprising radio channels which are independent and uncorrelated for different users and in which frequencies, due to multi-path propagation, are more or less attenuated for one user and more or less amplified for another user.


DISCUSSION OF THE BACKGROUND

There are time-dispersive radio channels in which signalling occurs in an environment which causes reflections with significant amplitude and long delays. In have delays which are longer than the symbol length. By this is meant that different frequencies in the spectrum, of the signal are dispersed in different ways. In particular, it can be noticed that the different frequencies are attenuated by different amounts. The traditional methods for signalling in such an environment are, among others, different arrangements for achieving diversity, use of equalizers, use of a so-called RAKE receiver or suitable channel coding.
OFDM (Orthogonal Frequency Division Multiplex) is a form of modelling in which a broadband signal is multiplexed on many narrow-band channels, normally several hundred. The narrow-band channels can be packed very tightly since the fact is utilized, that the channels are orthogonal when the channel separation (Hz) is equal to 1/symbol length for rectangular symbols. OFDM is normally implemented with the aid of special circuits which carry out FFT (Fast Fourier Transform) very rapidly.
In OFDM signalling, channel coding is normally used for reducing error probability. OFDM combined with channel coding is called COFDM (Coded Orthogonal Frequency Division Multiplex). Systems which use this form of signalling have in recent years been implemented for different types of broadcasting, that is to say one-way systems for digital radio broadcasting and for high-resolution digital TV. More recently, interest has also begun to be shown for utilizing the good characteristics of OFDM for two-way communication, for example in mobile systems, radio LANs and for point-to-multipoint applications.
In traditional OFDM systems, attempts are usually made to cope with the frequency-selective fading by coding out the errors which arise in the frequencies which are highly attenuated.
Quite generally there is a need to be able to eliminate the fading problem in a simpler manner than, for example, by coding methods. The invention intends to solve this problem, among others. In a system of this type, there is the requirement that it should be possible to hold the frequency attenuation above certain predetermined values for all users in the system. The invention also intends to solve this problem.


SUMMARY OF THE INVENTION

That which can be mainly considered to be characterizing of an arrangement according to the invention is that elements are arranged to sense or measure the attenuation/amplification at the different frequencies and to prevent signal links or information transmission at frequencies which drop below a predetermined lowest permitted or stipulated attenuation.
In one embodiment of the concept of the invention, the multi-user system comprises both uplinks and downlinks. In the uplinks or outstations of a basestation, the elements/basestation measure(s) the attenuation at the different frequencies or, respectively, the different outstations. On the basis of the measurement made, the elements/basestation distribute(s) the frequencies at which the different outstations are to signal or send information. The multi-user system in the embodiment can also include downlinks, that is to say basestation to outstations. Each of the latter outstations or said elements measures the attenuation for each frequency. The result of the measurement is signalled to the basestation/said elements which distribute(s) the frequency.
In a preferred embodiment, an OFDM system is utilized in which an identifiable data symbol for each carrier wave used can be easily extracted. When a respective identifiable data symbol appears, the possibility exists

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