Iterative rake receiver and corresponding reception process

Details Iterative rake receiver and corresponding reception process Iterative rake receiver and corresponding reception process

1999-08-13

2004-01-06

Kizou, Hassan (Department: 2662)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C370S334000, C370S441000, C370S479000

Reexamination Certificate

active

06674740

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The purpose of this invention is an iterative rake receiver and a corresponding reception process. It is used in radiocommunications applications and more particularly in the Code Division Multiple Access (CDMA) technique. It may be applied to the system defined by the IS'95 standard and in third generation UMTS and IMT-2000 systems.
2. Discussion of the Background
In the CDMA technique, information symbols that are to be transmitted do not directly modulate a carrier, but are firstly multiplied by pseudo-random sequences (or codes) which have the effect of spreading their spectrum. On reception, the received signal is unspread by filtering adapted to the sequence used in transmission (or by correlation) and is then demodulated.
This technique enables several users to use the same radiocommunications channel, provided that a particular sequence is assigned to each.
A multipath channel is generally used, in the sense that the radio-electric wave propagates along several different paths between the place of transmission and the place of reception. Therefore, the receiver does not receive a single signal for each transmitted information symbol, but it receives several more or less delayed and more or less modified copies. In order to reliably restore the transmitted information, the largest possible number of these copies has to be taken into account and they have to be recomposed in the receiver.
One way of doing this was to design a special receiver, called a rake receiver, in the sense that it “rakes” information at different instants symbolizing the teeth of the rake. This type of receiver separates signals corresponding to different paths and includes several “teeth” or “branches” or “pins”, each of which processes one of these signals. The signal is unspread and demodulated in each tooth. The next step is to recompose all signals in an adder.
A rake receiver was initially described by R. PRICE and P. E. GREEN in an article entitled “A Communication Technique for Multipath Channels” published in the “Proceedings of the IRE”, vol 46, March 1958, pp 555-570.
There is also a description of this receiver in the general book by J. G. PROAKIS entitled “Digital Communications”, 3
rd
edition, MCGRAW-HILL, 1995, (third edition) 1989, (second edition).
FIG. 1
attached diagrammatically shows a receiver of this type. As shown, this receiver comprises a main input E, a filter
10
with a width adapted to the signal spreading band, L means
12
0
, . . . ,
12
l
, . . . ,
12
L−1
of restoring L signals unspread in frequency corresponding to L paths (these means usually include a filter adapted to one of the pseudo-random sequences used in transmission or a correlator and means of searching for signal peaks), L means
14
0
, . . . ,
14
l
, . . . ,
14
L−1
of estimating the characteristics of the L paths used by the various signals, L demodulation means
16
0
, . . . ,
16
l
, . . . ,
16
L−1
combining the unspread signals and estimate of paths, an adder
18
adding the N contributions output by the N demodulators, and finally a decision circuit
20
that outputs transmitted or reference symbols on a general output S in order to test the communication.
After a strong demand from services requiring ever increasing throughputs, the frequency spreading band of CDMA systems is continuously being increased. This increase in the band is accompanied by a continuous increase in the number of paths received at the receiver. For a given received power, this increase in the number of paths tends to reduce the power received per path and therefore to reduce the global quality of the channel estimate. Consequently, a constructive combination of contributions from these paths to the receiver is rarely guaranteed and can result in a severe loss of transmission quality.
Since CDMA systems are inherently limited by multiple access interference, this loss of performance cannot be compensated by increasing the power. Furthermore, a solution involving an increase in the number of pilot symbols would reduce the system capacity.
The purpose of this invention is to correct this disadvantage.
SUMMARY OF THE INVENTION
The main purpose of this invention is to increase the performances of CDMA systems by improving the reception quality for a given transmitted power, and therefore with an unchanged level of multiple access interference. This quality improvement can increase the capacity and coverage of the CDMA system. This improvement is obtained by optimizing operation of the receiver in the classical case of slow fading, but also in the more difficult case of very fast fading.
Another purpose of the invention is to facilitate the construction of terminals by making them much less sensitive to inaccuracies in the local oscillator used to transpose the received signal in the base band.
CDMA systems introduce the concept of a power control period (PCP). The power of the signal emitted by the transmitter remains constant during each of these periods but it can vary from one power control period to the next in order to compensate for slow fading (due to the distance and mask effects), and fast fading due to multipath effects when the terminal is moving slowly. Furthermore, control symbols are used in addition to symbols transporting information. The invention can reduce the relative number and/or power of these control symbols, for an equal reception quality. This objective is achieved by taking optimum account of control symbols for an arbitrary number of consecutive power control periods in the channel estimate. It is also achieved by taking account of control symbols (if any) included in the pilot channels of some CDMA systems in the optimal channel estimate, in the absence of matching antennas. It is also achieved by taking account of control symbols (if any) assigned to other users in the down link in the estimate, still in the absence of matching antennas. Finally, it is achieved by taking optimum account in the estimate of this channel of all or some of the data symbols for these power control periods which are obviously more numerous and often contain more energy than the control symbols. The reduction in the number and/or the power of the control symbols is a way of consolidating the encoding of useful data and increasing the proportion of the transmitted power allocated to these data.
With the invention, the optimum channel estimate can also take account of control symbols multiplexed in time and control symbols multiplexed on in-phase and/or in-quadrature components of the modulated signal.
All power control period symbols can be used with the invention. Thus, shifts in the local oscillator frequency can be monitored and corrected, even if control symbols are grouped together.
According to the invention, block by block processing is carried out every time that the received signal corresponding to a given number of power control periods is available. Like a conventional rake receiver, it always begins by unspreading the signals corresponding to significant paths selected for the final combination. It then makes an approximate estimate of the multipath channel by using control symbols associated with the received block only. This estimate is a way of characterizing the variation in the phase and the amplitude of each path during the block to be processed, for each symbol in the block, symbol by symbol. The receiver according to the invention demodulates and then combines the estimated path contributions and outputs a sample (or weighted output) for each data symbol contained in the block.
In the case of a conventional rake receiver, these weighted outputs are used directly to detect and decode transmitted data symbols. These outputs have a certain reliability with respect to values taken on by the data symbols sent during a block. In the case of the receiver according to the invention, they may be used in addition to the control symbols, to provide an improved estimate of.each received path. This improved estimate of the multipat

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