Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2003-04-25
2004-08-24
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000, C375S346000, C370S342000
Reexamination Certificate
active
06782040
ABSTRACT:
BACKGROUND
The present invention relates generally to signal transmission and reception in a wireless code division multiple access (CDMA) communication system. More specifically, the invention relates to reception of signals to reduce interference in a wireless CDMA communication system.
A prior art CDMA communication system is shown in FIG.
1
. The communication system has a plurality of base stations
20
-
32
. Each base station
20
communicates using spread spectrum CDMA with user equipment (UEs)
34
-
38
within its operating area. Communications from the base station
20
to each UE
34
-
38
are referred to as downlink communications and communications from each UE
34
-
38
to the base station
20
are referred to as uplink communications.
Shown in
FIG. 2
is a simplified CDMA transmitter and receiver. A data signal having a given bandwidth is mixed by a mixer
40
with a pseudo random chip code sequence producing a digital spread spectrum signal for transmission by an antenna
42
. Upon reception at an antenna
44
, the data is reproduced after correlation at a mixer
46
with the same pseudo random chip code sequence used to transmit the data. By using different pseudo random chip code sequences, many data signals use the same channel bandwidth. In particular, a base station
20
will communicate signals to multiple UEs
34
-
38
over the same bandwidth.
For timing synchronization with a receiver, an unmodulated pilot signal is used. The pilot signal allows respective receivers to synchronize with a given transmitter allowing despreading of a data signal at the receiver. In a typical CDMA system, each base station
20
sends a unique pilot signal received by all UEs
34
-
38
within communicating range to synchronize forward link transmissions. Conversely, in some CDMA systems, for example in the B-CDMATM air interface, each UE
34
-
38
transmits a unique assigned pilot signal to synchronize reverse link transmissions.
When a UE
34
-
36
or a base station
20
-
32
is receiving a specific signal, all the other signals within the same bandwidth are noise-like in relation to the specific signal. Increasing the power level of one signal degrades all other signals within the same bandwidth. However, reducing the power level too far results in an undesirable received signal quality. One indicator used to measure the received signal quality is the signal to noise ratio (SNR). At the receiver, the magnitude of the desired received signal is compared to the magnitude of the received noise. The data within a transmitted signal received with a high SNR is readily recovered at the receiver. A low SNR leads to loss of data.
To maintain a desired signal to noise ratio at the minimum transmission power level, most CDMA systems utilize some form of adaptive power control. By minimizing the transmission power, the noise between signals within the same bandwidth is reduced. Accordingly, the maximum number of signals received at the desired signal to noise ratio within the same bandwidth is increased.
Although adaptive power control reduces interference between signals in the same bandwidth, interference still exists limiting the capacity of the system. One technique for increasing the number of signals using the same radio frequency (RF) spectrum is to use sectorization. In sectorization, a base station uses directional antennas to divide the base station's operating area into a number of sectors. As a result, interference between signals in differing sectors is reduced. However, signals within the same bandwidth within the same sector interfere with one another. Additionally, sectorized base stations commonly assign different frequencies to adjoining sectors decreasing the spectral efficiency for a given frequency bandwidth. Accordingly, there exists a need for a system which further improves the signal quality of received signals without increasing transmitter power levels.
SUMMARY
A code division multiple access communication system transmits a pilot and traffic signal over a shared spectrum. The pilot and traffic signal have an associated code. Signals are received over the shared spectrum. The received signals are sampled. The samples are delayed to produce a window. The window has evenly time spaced samples. Each window sample is despread with a pilot code. A weight for each despread pilot code window sample is determined using an adaptive algorithm. Each window sample is despread with a traffic code. Each despread traffic code window sample is weighted with a corresponding weight of the determined weights. The despread traffic code window samples are combined as data of the traffic signal.
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Cheung Gary
Grieco Donald
Mesecher David K.
Reznik Alexander
Ghebretinsae Temesghen
InterDigital Technology Corporation
Volpe and Koenig P.C.
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