Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-11-30
2001-07-03
Nguyen, Chau (Department: 2663)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C370S342000
Reexamination Certificate
active
06256338
ABSTRACT:
RELATED APPLICATION(S)
This application is related to a commonly assigned application for a patent titled: Method for Determining Optimum Number of Complex Samples for Coherent Averaging in a Communication System, filed on Nov. 30, 1998, the same day as the filing date of the present application. The related application is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to communication systems, and more particularly, to code division multiple access communication systems.
BACKGROUND OF THE INVENTION
Coherent and non-coherent averaging functions, two well known functions, are performed on a received signal in a receiver portion of a communication system for many different applications. In particular in a code division multiple access communication system, for example, one or more of the averaging functions are performed on a received signal to mitigate some undesired effects of the received signal distortions due to fading and additive noise plus interference. The results of averaging are used to generate a power-delay profile of the channel through which the received signal has propagated. The power-delay profile is then typically used to estimate time delay and amplitude of the received signal to perform demodulation of the received signal in a Rake receiver in the code division multiple access communication system. Moreover, time delay and amplitude of the received signal are used to determine location of a remote communicating unit. For example, time delay and amplitude of the received signal are used in a method disclosed in the U.S. Pat. No. 5,786,791, to Bruckert, assigned to Motorola Inc., assignee of the present invention, and incorporated herein by reference, for determining an angle of arrival of a signal transmitted by a remote communicating unit in a communication system for determining location of the remote communicating unit.
In general, the averaging functions are performed over a limited interval to determine a power-delay profile of the received signal. The phase information is lost in non-coherent averaging as is well known to one ordinary skilled in the art. In contrast, in coherent averaging, the phase information is always preserved. Moreover, advantages of coherent averaging in many different applications in communication systems are well known. As the result of preserving the phase information, more accurate power-delay profile of the received signal than non-coherent averaging at the same signal to noise ratio is produced.
Doppler frequency effects the phase of the received signal among other effects. The Doppler frequency produces phase rotation of the received signal at a proportional rate. Consequently, two samples, namely samples in complex notation, may have 180 degrees phase rotation from each other due to the phase rotation caused by the Doppler frequency. When the complex samples of the received signal have substantial phase differences, the advantage of coherent averaging of the complex samples diminishes which then produces a less accurate power delay-profile of the received signal. If the averaging interval is chosen to be large, as a means to reduce the effect of the Doppler phase rotation, the result of the coherent averaging approaches zero assuming the noise was additive. On the other hand, if the coherent averaging interval is chosen to be small, as a means to reduce the effect of the Doppler phase rotation, the noise variance remains to be large and causes error in the power delay profile of the received signal.
Referring to
FIG. 1
, a block diagram of a power-delay profile generator
100
is shown that may be incorporated in a receiver portion of a code division multiple access (CDMA) communication system. Power-delay profile generator
100
may be incorporated in a searcher element, as commonly referred to by one ordinary skilled in the art, of the receiver portion. Power-delay profile generator
100
receives a code modulated signal
104
at an input of a despreader
102
. Code modulated signal
104
has propagated through a channel before arriving at power-delay profile generator
100
. Despreader
102
despreads code modulated signal
104
using a locally generated replica of the spreading code to produce complex samples
103
of code modulated signal
104
. The operation of despreader
104
is well known by one ordinary skilled in the art. The duration of despreading function in despreader
102
may be equal to many times the chip time of the modulating code, e.g.
256
times the chip duration. One chip time, Tc, in a code division multiple access communication system, is equal to duration of one clock time of the code modulating sequence that is used to code modulate received signal
104
. For example, in a well known code division multiple access communication system operating according to commonly known IS-95 standard, Tc is equal to 1/1.2288 Mcps which is equal to 0.813 micro seconds. Despreader
102
uses pre-assigned code information, and possibly with the use of a sliding correlator, to generate complex samples
103
. Complex samples
103
are input to a coherent averaging block
105
. Coherent averaging block
105
, after receiving a number (N) of complex samples
103
, performs a coherent averaging function over the “N” complex samples
103
to produce a coherently averaged complex sample
106
. The coherent averaging may be performed according to the following:
1
/
N
∑
n
=
1
N
S
(
n
)
where “S(n)” is the received complex sample for each complex sample from n=1 to N. One ordinary skilled in the art may appreciate that coherent averaging may be performed according to the following:
1
/
N
∑
n
=
1
N
S
(
n
)
W
(
n
)
where “W(n)” is a weighting coefficient for received complex sample S(n) for each complex sample from n=1 to N.
The magnitude of coherently averaged complex sample
106
may be squared in a block
107
to produce a coherently averaged real sample
108
. The operation of block
107
may be limited to taking the magnitude of the averaged complex sample
106
to produce averaged real sample
108
as one ordinary skilled in the art may appreciate. Averaged real sample
108
are input to an averaging block
109
. Averaging block
109
, after receiving a number (M) of averaged real samples
108
, performs an averaging function over the number (M) of averaged real samples
108
to produce a power delay sample
110
for generating a power delay profile of the received signal
104
. The averaging in block
109
may be according to the following:
1
/
M
∑
m
=
1
M
Y
(
m
)
where “Y(m)” is averaged real samples
108
for m=1 to M. One ordinary skilled in the art may appreciate that the functions performed in blocks
107
and
109
are in essence in combination equal to a non-coherent averaging function.
According to prior art, an optimum number “N” of complex samples “S(n)” may be determined according to the following:
N=
1/
fD.Ts
where fD is the maximum Doppler frequency experienced by code modulated signal
104
received at power-delay profile generator
100
. The parameter Ts is the despreading duration in despreader
102
. The number (N) of complex samples, when it is based on the maximum Doppler frequency, is least likely to be an optimum number of complex samples for the coherent averaging function in block
105
.
Therefore, there is a need to determine an optimum number of complex samples for performing coherent averaging of code modulated complex signals, and a method for correcting errors in the power-delay profile of the received signal
104
due to Doppler frequency shift and fading.
REFERENCES:
patent: 4945549 (1990-07-01), Simon et al.
patent: 5109390 (1992-04-01), Gilhousen et al.
patent: 5379449 (1995-01-01), Porambo
patent: 5634206 (1997-05-01), Reed et al.
patent: 5712877 (1998-01-01), Ho et al.
patent: 5737327 (1998-04-01), Ling et al.
patent: 5786791 (1998-07-01), Bruckert
patent: 5799011 (1998-08-01), LaRosa et al.
patent: 5809064
Jalloul Louay
Yousef Nabil
Beladi Sayed Hossain
Haas Kenneth A.
Motorola Inc.
Nguyen Chau
Nguyen Phuongchau Ba
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