Multiplex communications – Channel assignment techniques – Combining or distributing information via code word channels...
Patent
1995-08-29
1998-11-03
Ton, Dang
Multiplex communications
Channel assignment techniques
Combining or distributing information via code word channels...
375205, H04J 1300
Patent
active
058319840
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a reception method to be used in a CDMA system, in which signals of several users are detected simultaneously from a received signal, an adaptive nonlinear decision rule is utilized for detection and parameters of a detector are supervised on the basis of the received signal to correspond better to signal states of the received signal.
CDMA is a multiuser system based on spread-spectrum technique, the application of which system to cellular radio systems has started recently, besides the previous FDMA and TDMA systems. CDMA has several advantages compared to the previous methods, such as simplicity of frequency planning and spectrum efficiency.
In the CDMA method, a narrow-band data signal of a user is multiplied to a relatively broad band by a spreading code having a considerably broader band than the data signal. Bandwidths used in known test systems are for instance, 1,25 MHz, 10 MHz and 25 MHz. In connection with the multiplication, the data signal spreads to the whole band to be used. All users transmit simultaneously by using the same frequency band. Each connection between a base station and a mobile station uses its own spreading code and the signals of the users can be separated from each other in receivers on the basis of each user's spreading code. The purpose is to select the spreading codes in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
Correlators in a CDMA receiver implemented in a conventional manner are synchronized with a desired signal, which is detected on the basis of a spreading code. The data signal is returned to the original band in the receiver by remultiplying it by the same spreading code as was used for spreading the original narrow band signal at the transmission stage. In an ideal case, the signals multiplied by some other spreading code do not correlate and do not return to the narrow band. Thus, they appear as noise with respect to the desired signal. Accordingly, the aim is to detect a desired user's signal from among several interfering signals. In practice, spreading codes are not decorrelatable and other users' signals make the detection of the desired signal more difficult by distorting the received signal nonlinearly. This interference caused by the users to each other is called a multiuser interference.
The single-user detection method described above is not optimal, because it ignores in connection with detection the information included in other users' signals. Additionally, conventional detection is not capable of correcting nonlinearities, which are partially caused by nonorthogonal spreading codes and a distortion of a signal on a radio path. An optimum receiver considers the information included in the signals of all users so that the signals may be detected optimally by using the Viterbi algorithm, for instance. An advantage of this detection method is that bit error ratio curves of the receiver resemble a situation of the single-user CDMA system with no multiuser interferences occurring. No near-far problem exists, for instance. The term near-far problem refers to a situation in which a transmitter close to a receiver covers with its transmission the transmitters located farther away. The most serious deficiency of the Viterbi algorithm is that the computational intensity required increases exponentially with an increasing number of users. For instance, a ten-user system having a bit rate of 100 kbit/s using QPSK modulation would require 105 millions of measurements to be made per second for a computation of the likelihood function. In practice, this prevents implementation of the optimum receiver.
An optimum receiver can, however, be approximated by different methods. As prior art are known different kinds of methods for simultaneous multiuser detection. To the best known methods belong ones using a linear multiuser detector, decorrelating detector or multistage detector. These methods are described in more detail in the references Varanasi, Aazhang; Multistag
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Ton Dang
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