Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-01-05
2002-07-09
Urban, Edward F. (Department: 2685)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S504000, C455S069000
Reexamination Certificate
active
06418321
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a transmission power controller for controlling the transmission power of a mobile station comprising part of a radio communication system. More specifically, it relates to a transmission power controller for use with a code division multiple access (CDMA) communication system.
2. Description of the Background Art
The CDMA communication system has been standardized by the Telecommunications Industry Association TIA) for use in North America and has adapted spread spectrum communication technology. In this system, since a plurality of mobile stations share the same frequency band, signals transmitted by several such stations tend to interfere with each other resulting in degradation of the quality of the signal transmitted by each of the mobile stations. To overcome this problem, a closed-loop power control operation is employed which adjusts the transmission power of each mobile station. Specifically, power control information is transmitted from a base station to each mobile station to minimize interference between the mobile stations.
Referring to the block diagram of
FIG. 2
, the following is an explanation of the power control operation of a conventional CDMA communication system comprising a base station
100
, a mobile station
200
, and a base station controller
300
.
The base station
100
includes a demodulator
101
, a signal-to-noise (S/N) power measuring circuit
102
, a power up/down discriminator
103
, a power control information multiplexer
104
and a modulator
105
. The demodulator
101
demodulates a received signal transmitted from the mobile station
200
via a receiving amplifier
106
, and the S/N power measuring circuit
102
measures the received power level of the received signal provided by the demodulator
101
.
The power up/down discriminator
103
determines whether the received power level is in a proper or improper status by comparing the received power level with a threshold level provided by a threshold set-up circuit
303
of the base station controller
300
and outputs a power control signal. The threshold level is determined by calculating an error rate of the received signal. For example, when the received power level exceeds the threshold level, the power up/down discriminator
103
outputs a power control bit having a logical value such as “−1”, which commands a decrease in the transmission power of the mobile station
200
. In contrast, when the received power level does not exceed the threshold level, the power up/down discriminator
103
outputs a power control bit having a logical value such as “+1”, which commands an increase in the transmission power of the mobile station
200
. The power control information multiplexer
104
inserts the power control bit into the transmission signal to be transmitted, the transmission signal is modulated to a spread-spectrum signal for the CDMA communication system by the modulator
105
, and the signal is transmitted to an antenna (not shown) via an amplifier
107
.
Each mobile station
200
comprises an amplifier
200
a
, a demodulator
201
, a power control information extracting circuit
202
, a decoder
203
, an up/down decision circuit
204
, a transmission amplifier
205
, and a transmission circuit
206
.
The demodulator
201
demodulates the spread-spectrum signal transmitted from the base station
100
via the receiving amplifier
200
a
to a baseband signal. The power control information extracting circuit
202
extracts the power control bit from the demodulated received signal. The decoder
203
decodes the demodulated received signal to a voice or data signal. The transmission amplifier
205
is a variable gain amplifier which change its gain as a function of the command signal provided by the up/down decision circuit
204
.
The up/down decision circuit
204
is an accumulator which sums the signals received from the power control information extracting circuit
202
during a series of signals received from the base station
100
. For example, if five signals, +1, +1, −1, +1, +1 are received from the poser control information extracting circuit
202
, the up/down decision circuit
204
outputs these command signals, +
1
, +
2
, +
1
, +
2
, +
3
, sequentially to the transmission amplifier
205
. In response, the transmission amplifier
205
would increase the transmission power by 3dB. In the conventional CDMA communication system, the power control operation is periodically performed every 1.25 micro-seconds. As a result, the mobile station
200
frequently controls the transmission power in accordance with the power control bit.
The base station controller
300
comprises a decoder
301
, an error rate measuring circuit
302
, a threshold set-up circuit
303
and a coder
304
. The decoder
301
decodes the received demodulated signal, provided by the demodulated
101
of the base station
100
, to a voice or data signal. The voice or data signal is supplied to the error rate measuring circuit
302
which measures the error rate of the voice or data signal. The threshold set-up circuit
303
determines a threshold level, based on the error rate, which is the transmission power of the mobile station
200
to satisfy the predetermined quality of speech or data signal. The signal threshold level is supplied to the power up/down discriminator
103
of the base station
100
. The coder
304
codes a voice or data signal to be transmitted to the mobile station
200
.
FIG. 3
is a graph having an abscissa divided into
73
divisions each designating an instant T(n), defined as a sample time, at which the base station
100
transmits a “1” or a “+1” signal to the mobile station
200
where it is received by the power control information extracting circuit
202
and inputted to the up/down decision circuit
204
. The ordinate of
FIG. 3
shows variations in the power transmitted by the mobile station
200
to the base station
201
, in decibels, at each instant of time T(n). The graph of
FIG. 3
illustrates the operation of a typical CDMA communication system in which the power transmitted by the mobile station
200
changes in 1 dB increments at the following sample times: (1) At sample times T(
1
-
10
), the transmission power alternates between 1 dB and 0 dB (2) At sample times T(
20
-
53
), the transmission power alternates between 10 dB and 11 dB; and (3) At sample times T(
60
-
73
), the transmission power alternates between 4dB and 5dB.
During the sample times T(
11
-
21
), the base station
100
transmits a series of only “+1” signals and therefore the power transmitted by the mobile station
200
gradually increases to 11dB, and during the sample times T(
53
-
60
), the base station
100
transmits a series of only “−1” signals and therefore the power transmitted by the mobile station
200
gradually decreases to 4dB.
It is desirable to keep a fixed transmission power level at the mobile station
200
if the transmission power level matches the threshold level. However, in the conventional CDMA communication system, as shown in
FIG. 3
, even when the transmission power level matches the threshold level, the mobile station is forced to change transmission power because the base station
100
always sends a transmission power control bit that commands either an increase or a decrease in the transmission power. That is, the power being transmitted by the mobile station
200
is always being changed. As a result, the signal received by the base station
100
does not have a constant received power level.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a transmission power controller that is capable of suppressing deviation in the transmission power at the mobile station. It is an another object of the invention to provide a transmission power controller that is capable of not only suppressing such deviations but also capable of providing a quick response to change in the transmission power when t
Craver Charles
Oki Electric Industry Co. Ltd.
Sartori Michael A.
Urban Edward F.
Venable
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