Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-10-08
2002-07-02
Bost, Dwayne (Department: 2681)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C370S350000, C375S134000, C375S137000, C375S149000, C375S340000, C375S341000, C375S364000, C375S365000, C375S366000, C375S367000, C375S368000
Reexamination Certificate
active
06414951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system for accurately detecting short codes in a communication environment which includes continuous wave interference.
2. Description of Prior Art
With the dramatic increase in the use of wireless telecommunication systems in the past decade, the limited portion of the RF spectrum available for use by such systems has become a critical resource. Wireless communications systems employing CDMA techniques provide an efficient use of the available spectrum by accommodating more users than time division multiple access (TDMA) and frequency division multiple access (FDMA) systems.
In a CDMA system, the same portion of the frequency spectrum is used for communication by all subscriber units. Typically, for each geographical area, a single base station serves a plurality of subscriber units. The baseband data signal of each subscriber unit is multiplied by a pseudo-random code sequence, called the spreading code, which has a much higher transmission rate than the data. Thus, the subscriber signal is spread over the entire available bandwidth. Individual subscriber unit communications are discriminated by assigning a unique spreading code to each communication link. At times it is also useful in a CDMA system to transmit codes which are of shorter length than the usual spreading code.
It is known in the art of CDMA communication systems to use a sequential probability ratio test (SPRT) detection method to detect the transmission of a short code. However, in the presence of continuous wave (CW) interference, the use of known SPRT detection methods can result in a large number of false short code detections. These false detections degrade system performance by delaying the detection of valid short codes.
A background noise estimation is required for the SPRT detection method. The background noise estimation is typically performed by applying a long pseudo-random spreading code to a RAKE despreader. The output of the RAKE despreader has a probability distribution function, (PDF). Referring to
FIG. 1A
, curve
1
shows a typical PDF background for noise which is calculated using a long pseudo-random spreading code where there is no CW interference. Curve
3
shows a typical PDF in the presence of a valid detected signal. However, when CW interference is present during the transmission of short codes, the background noise PDF is a curve like
2
, which is shifted away from curve
1
and which appears similar to the PDF for a valid detected signal, curve
3
. The noise estimate becomes skewed because the short code, which is not completely random is applied to the RAKE and it begins to correlate with the repetitive CW interference. Accordingly, as curve
2
shifts further toward curve
3
due to the presence of CW interference, the SPRT detection method will falsely detect invalid noise as a valid signal.
Referring to
FIG. 1B
, there is shown a block diagram of a prior art short code detector system
10
. The short code detector system
10
is typically located in a base station for detecting short codes received from a subscriber unit. A signal containing short codes, continuous wave interference and other forms of background noise is applied to the short code detector system
10
by way of the detector input line
12
, and is received by a detector input block
14
. The detector input block
14
includes a RAKE demodulator having M different phases. The RAKE demodulator operates on the input signal by combining it with the short pilot code. The pilot code is a pseudorandom code which is generated locally by the base station and transmitted by subscribers initiating a call setup.
A first output signal of the detector input block
14
is applied to a detection block
16
of the detector system
10
. The detection block
16
contains a SPRT detection method. The output signal of the detection block
16
appears on a decision line
20
. The signal of the decision line
20
represents a decision by the SPRT detection method of detection block
16
whether a short code is present in the signal received by the input block
14
.
A second output signal of the input block
14
is applied to a noise estimator, which is comprised of a separate RAKE demodulator (AUX RAKE) which uses a long pseudorandom code in combination with the input signal to perform a background noise estimation. The result of the background noise estimation performed in block
18
is a PDF which is applied to the SPRT detection method of detection block
16
.
Referring now to
FIG. 2
, there is shown prior art short code detection method
40
. The detection method
40
is used to detect the presence of short codes transmitted in a wireless communication system. For example, the short code detection method
40
is suitable for operation within the detection block
16
of the short code detector system
10
to detect the presence of short codes in the input signal of the input line
12
.
Execution of the short code detection method
40
begins at the start terminal
42
and proceeds to step
44
where one of the M different phases of the RAKE
14
is selected. The short code detection method
40
proceeds to step
46
where a background noise estimate, performed by the AUX RAKE, (in the noise estimator
18
of FIG.
1
B), is updated. The signal is applied by the noise estimator
18
to the detection block
16
. At step
50
, a sample of the signal from the selected phase of the input line
12
as received by the input block
14
is applied to the detection block
16
for computation according to the short code detection method
40
.
Referring now to
FIG. 3A
, there is shown a graphical representation
70
of the operation of the short code detection method
40
. An acceptance threshold
74
and a rejection threshold
76
are set forth within along with two likelihood ratios
80
,
84
. A likelihood ratio is a decision variable that is well known to those skilled in the art. It is useful when determining the presence of a signal in a communication system. The likelihood ratios
80
,
84
have starting values approximately midway between the thresholds
74
,
76
. They are repeatedly adjusted by the short code detection method
40
for comparison with thresholds
74
,
76
in order to determine the presence of short codes.
Although, the starting values of the likelihood ratios
80
,
84
are approximately midway between the thresholds
74
,
76
, adjustments are made to the likelihood ratios
80
,
84
which can be positive or negative as determined by the calculations of the detection method
40
. As the likelihood ratio of a phase increases and moves in the direction of the acceptance threshold
74
, there is an increasing level of confidence that a short code is present. When a likelihood ratio crosses the acceptance threshold
74
the level of confidence is sufficient to determine that a short code is present in the phase. As the likelihood ratio decreases and moves in the direction of the rejection threshold
76
, there is an increasing level of confidence that a short code is not present in the phase. When a likelihood ratio crosses the rejection threshold
76
, the level of confidence is sufficient to determine that no short code is present.
Returning to
FIG. 2
the likelihood ratio of the current phase is updated at step
54
. It will be understood by those skilled in the art that such a likelihood ratio is calculated for each of the M different phases of the RAKE. The likelihood ratio of the current phase is calculated in view of the background estimate of step
46
and the input sample taken at step
50
.
At step
56
, a determination is made whether the likelihood ratios of all M phases are below the rejection threshold
76
. If even one of the likelihood ratios is above the rejection threshold
76
it is possible that a short code is present in the received transmission. In this case, execution of short code dete
Jacques Alexander M.
Ozluturk Faith M.
Bost Dwayne
InterDigital Technology Corporation
Lahjouji Ahmed
Volpe and Koenig P.C.
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