Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-08-13
2002-07-23
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C375S213000
Reexamination Certificate
active
06424644
ABSTRACT:
TECHNICAL DOMAIN
The purpose of this invention is CDMA (Code Division Multiple Access) type digital communication processes with distribution of reference symbols. These processes concern transmission and reception of messages.
The invention has applications in mobile land radiocommunication systems. It may be applied for third generation UMTS and IMT-2000 systems.
STATE OF PRIOR ART
Digital communication systems may be classified into two main categories, depending on whether they use packet transmission or continuous transmission. Time Division Multiple Access (TDMA) systems like GSM and DECT systems use packet transmission. CDMA systems, for example like the IS'95 system, make use of Code Division Multiple Access.
In all these systems, the solution of problems related to knowledge about the characteristics of the propagation channel is facilitated by the addition of reference symbols (or pilot symbols) and/or pilot channels.
TDMA systems are currently limited to the processing of two extreme cases: channels which are very selective in frequency (very wide spreading of delays) but not selective in time (negligible Doppler spread) and channels not selective in frequency but which may be very selective in time. The first case is very frequently encountered in land radio mobile systems like GSM. The second is encountered more frequently in satellite radio mobile systems such as ICO and IRIDIUM.
In the case of TDMA satellite communication systems, the reference symbols are distributed on each data packet for best matching to the channel subject to very fast fading. This distribution of reference symbols was successfully used within the ICO project. For example, a uniform distribution of these symbols is described in FR-A-2 747 870.
On the other hand, in the case of TDMA land communication systems, the reference symbols are grouped in the middle of each data packet so that the pulse response of the multipath channel, which is usually very selective in frequency but is not very or not at all selective in time, can be estimated more simply and more efficiently.
In the CDMA technique, symbols to be transmitted are firstly multiplied by pseudo-random sequences (or codes) which has the effect of spreading their spectrum. On reception, the received signal is unspread by filtering adapted to the sequence used on transmission (or by correlation) and is then demodulated.
This technique enables several users to use the same radiocommunication channel, provided that a particular sequence is assigned to each. This technique is called Code Division Multiple Access (CDMA).
In practice, the CDMA signal includes two components, one component said to be in phase (or real) and conventionally denoted I and one component said to be in phase quadrature (or imaginary) and conventionally denoted Q. Each component is obtained by spreading the symbols spectrum and then by modulation of a carrier that is either in phase with a reference carrier (I component) or in quadrature with the same reference carrier (Q component). On reception, the signal received in two distinct I and Q channels are processed separately, and the restored information is grouped.
A description of this technique is given in the general book by J.G. PROAKIS, entitled “Digital Communications”, 3
rd
edition, MCGRAW-HILL, 1995 (third edition) 1989 (second edition).
Each of the I and Q channels of an CDMA signal comprises information symbols and reference symbols (or pilots) in order to estimate the channel as in TDMA.
TDMA systems also introduce the concept of a power control period (PCP). The power of the signal sent by the transmitter remains constant during each PCP but may vary in different PCPs to overcome slow fading (due to distance and mask effects) and fast fading due to multiple paths when the terminal moves slowly (selectivity in time).
In the case of conventional CDMA systems, the reference symbols are grouped at the beginning of each PCP. These reference symbols enable an estimate of paths at the beginning of each PCP, by correlation. This estimate is then used in the rest of the PCP to optimize demodulation of information symbols from each path and to recombine them in order to make a decision.
For a slow movement of the terminal and therefore low selectivity in time, the paths estimate for a given PCP may be consolidated by weighted or unweighted averaging with the path estimates of a finite number of previous PCPs.
Existing CDMA systems use either a fixed duration PCP or a variable duration PCP to adapt to the speed of fading observed on the channel. In both cases, reference symbols are usually grouped at the beginning of each PCP.
Grouping of reference symbols has the major disadvantage of leading to a channel estimate for a localized time, even if its quality is good due to the number of symbols that contribute to it. It is then very difficult to follow fast variations of fading (high mobile displacement speeds and/or the use of high frequencies). Transmission performances are thus degraded.
The disadvantage of the localization of reference symbols is partially eliminated if the system can use several reference symbols from several PCPs. This solution, which may be considered satisfactory, cannot be used in time slot mode links in which the estimate must be made PCP by PCP.
The use of short PCPs at the same time as conventional demodulators results in a significant reduction in the capacity of the link and therefore of the system, due to the large number of reference symbols transmitted to ensure a good quality of the link.
With the sustained demand for services requiring continuously increasing throughputs, the frequency spreading band of CDMA systems is always increasing. This increase in the band is accompanied by a continuous increase in the number of paths received at the receiver. This increase in the number of paths results in a reduction in the power received per path for a given received power, and therefore a reduction in the quality of the global channel estimate.
Since CDMA systems are inherently limited by multiple access interference, this loss of performance cannot be compensated by an increase in the power. Furthermore, the increase in the number of reference symbols tends to reduce the capacity of the system.
The purpose of this invention is to overcome these disadvantages.
DESCRIPTION OF THE INVENTION
The main purpose of this invention is to improve the performances of CDMA systems by improving the reception quality for a given emitted power and therefore with an unchanged multiple access interference level. This improvement in quality has a number of advantages, including an increase in the capacity and coverage of the CDMA system.
This improvement is obtained by a judicious distribution of reference symbols between the two channels. According to the invention, the reference symbols are distributed in only one of the two components, namely the I component or the Q component. Furthermore, pseudo-random spreading sequences used for one of the components are different from the pseudo-random spreading sequences used for the other component. This means that information related to reference symbols contained in one channel can be retrieved without deserving the other channel, and vice versa.
The use of reference symbols uniformly distributed along a PCP can give better observation of the propagation channel, which can be used advantageously by the receiver to maintain a practically constant connection quality regardless of the speed, even for extremely wide occupied bands. In this receiver, we will only deal with the channel containing reference symbols to restore information about these symbols using unpreading sequences specific to this channel. This channel separation is specific to CDMA and cannot exist in TDMA.
The distribution of reference symbols into one of the two components is one way of observing and accurately following variations in the multipath channel, from end to end, on a given time slot, in an optimized manner. In other words, it is a means of heuristically creating the equivalent
Duponteil Daniel
Siala Mohamed
France Telecom
Jones Prenell
Yao Kwang Bin
LandOfFree
CDMA digital communication processes with distribution of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with CDMA digital communication processes with distribution of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CDMA digital communication processes with distribution of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2859445