Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-11-02
2003-03-25
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S311000, C370S343000, C370S345000, C370S441000, C370S442000, C370S458000, C370S478000, C370S483000, C370S484000, C370S498000, C370S529000, C370S913000, C375S130000, C375S131000, C375S140000, C455S517000, C455S522000
Reexamination Certificate
active
06539008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to CDMA (Code Division Multiple Access) mobile communication systems, and in particular, to a method for performing high speed power control.
2. Description of the Related Art
Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users. The power of each user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.
The importance of power control in a CDMA system is best illustrated by way of example. Referring to
FIG. 12
, there is illustrated a single cell that has two hypothetical users. Examining the reverse link, which is often the limiting link in CDMA, it is shown that user
2
is much closer to the base station than user
1
. If there is no power control, both users would transmit a fixed amount of power p
t
. If it is assumed, for example, that the difference in distance is such that p
t,2
is 10 times more than p
t,1
then user
1
would be at a great disadvantage. It is apparent that user
2
has a much higher signal-to-noise ratio (SNR), and as such enjoys a great voice quality. Geographical factors can also influence disparities between users. Irrespective of whether the disparity results from distance or geography, this inequity is known as the classic near-far problem. Closed Loop Power control is commonly implemented to overcome the near-far problem and to maximize capacity. Closed Loop Power control is where the transmit power from each user is controlled such that the received power of each user at the base station is equal to one another. When this occurs, a greater number of users can be accommodated by the system.
In closed Loop power control when the received signal strength from a mobile terminal is excessively large, a control signal for reducing the transmit power of the terminal is transmitted from the base station to the terminal. Likewise, when the transmit power of a terminal is small, a control signal for increasing the transmit power of the terminal is transmitted from the base station to the terminal. In so doing, the received signal strength is maintained at an acceptable level in the base station. It is to be appreciated that the method has equal applicability with respect to the reverse link (i.e., signals received by the terminal from the base station).
FIG. 1
is a schematic block diagram of the single cell described in
FIG. 1
comprising a single base station
20
and a single terminal
10
. Closed Loop Power control, as performed in the prior art, will be described with reference to FIG.
1
.
Referring to
FIG. 1
, a signal strength measurer
12
of a terminal
10
measures the signal strength of a forward link channel, and reports the measurement to a power control bit generator
13
. Then, the power control bit generator
13
compares the received signal strength with a reference strength and generates a corresponding power control bit. If the received signal strength is larger than the reference strength value, the power control bit generator
13
generates a power control bit for reducing the transmit signal of the base station
20
. If, however, the received signal strength is smaller than the reference strength value, the power control bit generator
13
generates a power control bit for increasing the transmit signal of the base station
20
. The generated power control bit is inserted into a transmit signal of the terminal and transmitted to the base station
20
. The insertion is performed in a power control bit inserter
14
of the terminal
10
.
A power control bit detector
25
of the base station
20
receives the signal on a reverse link (or forward link) from the terminal
10
, and detects the power control bit from the reverse link. A power control bit processor
26
of the base station
20
processes the detected power control bit to adjust the strength of the transmit signal from the base station
10
(or terminal
10
). Here, the power control bit processor
26
commands a transmit power controller
27
to increase signal strength if the transmit signal strength is to be increased from an analysis of the power control bit, and to decrease the transmit signal strength if vice versa.
The strength of the power-controlled forward link signal is measured by the signal strength measurer
12
of the terminal
10
. Thus, a closed-loop power control for the forward link is achieved.
A controller
11
of the terminal
10
performs other tasks including generation of transmit and receive data bits, which are not shown in
FIG. 1
, while sequentially controlling the closed-loop power control procedure.
Significant factors which impact closed-loop power control include: 1) the rate of power control adjustments; 2) the time required to implement power control through a closed loop; and 3) how to insert a power control bit. These factors require careful consideration as they have great influence on both signal maintenance and service quality.
FIG. 2
illustrates power control as implemented in a conventional IS-95 CDMA system. With a power control unit time selected to be 1.25 msec., power control will be performed 800 times per second. A number of operations are performed in the 1.25 msec interval including; 1) measuring the strength of a receive signal; 2) comparing the receive signal strength with a reference signal strength; and 3) generating and inserting a power control bit for insertion into the forward traffic channel, as shown in FIG.
2
. The insertion position of the generated power control bit, shown in diagram as
213
, is determined by a long-period PN (Pseudo-Random) code within the first two thirds of the 1.25 msec interval. Further, the power control bit is transmitted for a pre-determined time. Thus, power control bit transmit time can be equally distributed when transmitting a power control bit to a plurality of terminals thereby reducing interference, which can be cause by the power control bit, between a transmit signal and other signals.
The conventional IS-95 power control method, however, suffers from certain shortcomings in that the power control speed is limited to 800 Hz and the time delay associated with closed-loop power control ranges from 1.25 to 2.5 msec.
The first problem (i.e., the 800 HMhz speed limitation) with the conventional power control limits both the period and extent of power change which can be controlled. As is well known in the art, the power change period is inversely proportional to the speed of a mobile terminal. That is, as the terminal moves at higher speeds, the power change period becomes shorter and thus power change interval becomes more frequent. As a result, a power control performed at a rate of 800 Hz is not effective for a signal of a mobile terminal moving at high speeds.
In addition, considering the closed-loop power control is based not on a current power strength but on a previous measured power strength, a longer time delay in the closed-loop power control leads to power control being applied to the previous signal. In this case, power control is nullified if power change becomes large. That is, as time delay increases and power change gets larger, information contained within a power control bit which depends on measured signal strength is less accurate because it loses its relation with the current signal strength, thereby increasing a change in signal strength.
Each of the aforementioned shortcomings associated with power control as practiced in the prior art serves to increase the change of signal strength thereby decreasing the service quality. Accordingly, it would be highly advantageous to provide a method for maintaining a high reference signal strength to keep service quality at a predetermined level.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a closed-loop power control method in a CDMA mobile communication system, in whic
Ahn Jae-Min
Kang Hee-Won
Kim Young-Ky
Kong Seung-Hyun
Yie Felix Kwang-Wook
Dilworth & Barrese LLP
Kizou Hassan
Logsdon Joe
Samsung Electronics Co,. Ltd.
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