Multiplex communications – Channel assignment techniques – Combining or distributing information via code word channels...
Reexamination Certificate
1998-08-07
2003-11-11
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Channel assignment techniques
Combining or distributing information via code word channels...
C370S535000, C375S148000, C375S347000, C375S349000
Reexamination Certificate
active
06647022
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to interference canceling, and more particularly, to interference canceling for use in a code division multiple access (CDMA) cellular telephone system.
2. Description of the Related Art
Code Division Multiple Access (CDMA) is a digital multiple access method specified by the Telecommunications Industry Association (TIA) as IS-95. CDMA is a form of spread-spectrum. The basic principle of spread-spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much larger than that required for simple point-to-point communication at the same data rate. CDMA receivers separate communications channels using pseudo-random modulation that is applied and removed in a digital domain, not on the basis of frequency. Multiple users occupy the same frequency band. This universal frequency reuse is crucial to the very high spectral efficiency that is characteristic of CDMA.
CDMA systems are interference limited, which means that the interference caused by other users is the critical factor which prevents even more users from being added. Therefore, controlling the impact of interference is a key concern in systems which use CDMA.
Current systems, such as IS-95, derive their interference reduction from two sources, random spreading codes and power control. Random spreading ensures the interference looks like noise, not a signal and power control attempts to keep the interference power at a minimum.
A completely different approach to improving systems which use CDMA is to actively cancel interference, rather than having it contribute to noise. Such approaches improve CDMA system capacity by at least 50%. Two types of known interference cancellers are “parallel” and “successive” interference cancellers, where successive interference cancellers are generally considered to provide better performance.
In both, interference signals are subtracted out from the desired signal. The difference is primarily the order in which the subtraction is performed. In a parallel canceller, all subtractions are done at the same instant. For example, a parallel canceller includes a number of parallel processors. Each parallel processor includes a detector, a regenerator, a subtractor, and a delay circuit. Each parallel processor receives the same input signal. If there are K users in the system, then each branch of the parallel canceller receives K input signals. Each parallel branch of the parallel canceller “picks out” one user signal from the total and subtracts that one user signal.
Successive cancellers differ from parallel cancellers in that cancellations are not done at the same instant, but rather sequentially. In particular, the first serial processor in the successive canceller receives K signals, the second serial processor receives only K−1, then K−2, and so on. By the time the signal gets to the second serial processor, one signal has been cancelled. If the cancellations are done correctly, then each successive serial processor down the chain receives a signal with a better signal-to-noise ratio.
As shown in
FIG. 1
, the structure of a conventional successive interference canceller
10
includes several identical processors
12
,
14
,
16
, in series. Each serial processor
12
,
14
,
16
includes a detector
20
which produces a data bit estimate
34
, a regenerator
24
which receives the data bit estimate
34
, and produces a waveform
26
, a delay circuit
28
which delays the input signal to processor
12
to produce a delayed signal
30
, and a subtractor
32
which subtracts the waveform
26
output from the regenerator
24
from the delayed signal
30
output from the delay circuit
28
to produce a signal including K−1 users
35
.
Each serial processor
12
,
14
,
16
in the “pipeline” architecture illustrated in
FIG. 1
can be implemented as a separate microprocessor or as a separate digital signal processor circuit. The “pipeline” architecture is used to divide up the problem because of the limited speed of each microprocessor/digital signal processor circuit by assigning each of the K users to a separate serial processor
12
,
14
,
16
, which can be implemented separately from the other serial processors, and hence run quickly. Each serial processor
12
,
14
,
16
produces a data bit estimate
34
,
36
,
38
for one of the K users.
There are several problems with the conventional successive interference canceller illustrated in FIG.
1
. One is the delay for a particular user. The data for the Kth user is not detected until K symbol times after receipt, which is often unacceptably long. The second problem is a “reordering” problem. To achieve the best performance from a successive canceller, the K users must be assigned to particular places in the hierarchical chain, depending on the power of the user's signal. This order must change from time to time as the K users move about and their signal strengths vary. An order controller
40
is required to control the detector
20
and the regenerator
24
by assigning spreading codes, parameters, etc., to each serial processor
12
,
14
,
16
, and this ordering must change in real time since the ordering reflects the receive power of each user.
This reordering causes a special problem for the conventional successive interference canceller architecture. To illustrate this problem, assume a receiver is running with an established order of users. Due to signal strength variations, a third user (user
3
), should be placed first (the current location of user
1
), and user
1
should be moved to the previous location of user
3
. If the order is switched immediately, the data “in the pipeline” that is being held in between positions will be adversely affected. In this example, the data at stage
2
has already canceled user
1
. However, when reaching stage
3
, the architecture now cancels user
1
again, and user
3
is never canceled.
SUMMARY OF THE INVENTION
The present invention solves this problem by providing a successive interference canceller which includes a number of parallel processors. Rather than being assigned a particular user, each of the parallel processors is assigned a time segment which includes data for all users. Each processor receives an assigned time segment and assigns an order to each of the K user signals independent of the other processors. The outputs of the N processors are then multiplexed together to form a data signal for all K users in time sequence. This architecture does not result in an unacceptable delay for any one of the users and does not have the same reordering problem found in the conventional successive interference canceller architecture, since each processor is permitted to assign its own order independently.
REFERENCES:
patent: 4527020 (1985-07-01), Ito
patent: 5341395 (1994-08-01), Bi
patent: 5463660 (1995-10-01), Fukasawa et al.
patent: 5493307 (1996-02-01), Tsujimoto
patent: 5553062 (1996-09-01), Schilling et al.
patent: 5579304 (1996-11-01), Sugimoto et al.
patent: 5644592 (1997-07-01), Divsalar et al.
patent: 5687162 (1997-11-01), Yoshida et al.
patent: 5790588 (1998-08-01), Fukawa et al.
patent: 5982825 (1999-11-01), Tsujimoto
patent: 6067333 (2000-05-01), Kim et al.
patent: 6157685 (2000-12-01), Tanaka et al.
R. Kohno et al., “Combination of an Adaptive Array Antenna and a Canceller of Interference for Direct-Sequence Spread-Spectrum Multiple-Access System,” IEEE Journal on Selected Areas in Communications, vol. 8, No. 4, May 1990, pp. 675-681.
Hsu Alpus H.
Lucent Technologies - Inc.
LandOfFree
Interference canceller does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interference canceller, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interference canceller will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3161171