Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-12-30
2003-10-14
Kincaid, Lester G. (Department: 2685)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000
Reexamination Certificate
active
06633553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) communication system, and in particular, to an apparatus and method for forward power controlling in a receiving device.
2. Description of the Related Art
Power control in a CDMA mobile communication system is described in two ways: forward power control and reverse power control. The forward power control is performed on the transmission power of a base station so that a mobile station can receive a forward traffic channel with a predetermined SIR (Signal to Interference Ratio) from the base station. By the forward power control, the base station increases its transmission power for a mobile station in a bad channel environment and decreases it for a mobile station in a favorable channel environment so that the mobile stations can demodulate data stably. In terms of a system, the forward power control prevents power consumption of a traffic channel transmitted to mobile stations, thereby increasing the capacity of each cell.
Meanwhile, a mobile station is highly susceptible to interference from base stations other than a serving base station when the mobile station is at the boundary of a cell or behind a building. This is termed a corner problem of a forward channel. In this case, the mobile station needs higher reception power than a mobile station near to the serving base station. Therefore, the mobile station notifies the base station of channel status according to the error rate of received frames and the serving base station controls its transmission power based on the received channel status information to allow the mobile station to maintain a predetermined reception power level. This forward power control can be closed-loop power control.
The channel status is checked by measuring the SIR of a signal received at the mobile station. The mobile station can also compare the SIR measurement with a threshold variable depending on the error rate of frames in a demodulator and notify the base station of the comparison result. The threshold may vary in proportion to the frame error rate. The forward power control using the variable threshold corresponding to the frame error rate is called an outer-loop power control.
Meanwhile, according to the radio standard of CDMA-2000, channel status is determined by measuring the ratio of the reception power of a forward channel to interference from other cells, that is, the SIR of a received signal, for forward power control. In this method, a rake receiver in each mobile station measures interference from each path and the strength of a user channel input signal from each path, and compares an SIR with a predetermined threshold.
The interference can be measured using a pilot channel or a sync channel. As an alternative, an approximate interference strength can be measured utilizing orthogonal noise components.
As suggested currently, the reception strength of a forward user channel is measured by measuring the size of the symbols of a power control command directed to a mobile station by a base station for reverse closed-loop power control. In prior art, a mobile station detects the position and size of a power control command symbol in each power control group of a received forward traffic channel signal and squares the symbol size value. However, only a part of a power control group is used for measuring the user channel strength in the conventional technology. Consequently, the forward channel measurement is inaccurate due to lack of symbols for use in the forward user channel measurement. For example, if only three or four power control command symbols among 36 symbols of a 1.25 msec power control group are used in measuring the reception strength of a forward user channel, the measurement is inaccurate in a bad channel environment such as a fading channel. As a result, the mobile station transmits a wrong forward power control command to the base station and then the base station cannot perform a desired forward power control. That is, the base station may increase its high transmission power or decreases its low transmission power based on the wrong forward power control command, resulting in inefficient power use. Traditionally, use of unnecessary high transmission power decreases the entire available cell capacity.
FIG. 1
is a block diagram of a device for measuring the SIR of a forward user channel using reverse power control symbols in a conventional CDMA mobile communication system.
A finger
100
subjects an input signal to PN despreading and orthogonal despreading and outputs the despread, demodulated symbols to a combiner. To demodulate symbols of signals received from different paths, a plurality of fingers
100
are provided. The combiner combines the outputs of the fingers corresponding to the multiple paths and outputs the combined value to a channel decoder.
A despreader
110
in the finger
100
despreads input I- and Q-channels by multiplying them by a PN sequence received from a PN generator
160
. A multiplier
115
extracts orthogonally demodulated traffic channel data by multiplying the output of the despreader
110
by a traffic channel orthogonal code Wt received from an orthogonal code generator (not shown). A symbol accumulator
120
accumulates the output of the multiplier
115
in symbol units.
A multiplier
140
extracts pilot channel data by multiplying the output of the despreader
110
by a pilot channel orthogonal code Wo received from the orthogonal code generator. A symbol accumulator
145
accumulates the output of the multiplier
140
in symbol units.
A multiplier
125
multiplies the output of the symbol accumulator
120
by a complex conjugate of the output of the symbol accumulator
145
. That is, the multiplier
125
multiplies the traffic channel data by the complex conjugate of the strength and phase component of a pilot channel, thereby compensating for the phase component of the traffic channel data. The multiplier
125
outputs the demodulated symbols to the combiner (not shown).
Meanwhile, a power control symbol squarer
130
measures the power levels of reverse power control command symbols inserted in predetermined positions of the orthogonally demodulated channel data received from the symbol accumulator
120
. A symbol accumulator
135
accumulates the output of the power control symbol squarer
130
in symbol units and outputs the strength of the received forward user channel. Here, since two symbols are inserted in each power control group, a frame having a total of 32 reverse power control command symbols can be defined as a corresponding accumulation period.
An interference signal measurer
150
measures the interference strength of the output of the despreader
110
. A divider
155
calculates an SIR by dividing the received forward user channel strength by the interference strength and outputs the SIR to a forward power controller (not shown). The forward power controller sums the SIRs received from the plurality of fingers and determines a forward power control command to be transmitted to the base station according to the frame error rate of the channel decoder (not shown).
In the conventional method, a forward user channel reception strength is obtained by squaring reverse power control command symbols transmitted in each power control group of the forward traffic channel signal.
To solve problems of the conventional method that utilizes only a part of a power control group for measuring the strength of a received forward user channel, the present invention suggests a method for measuring the strength of a received forward user channel that utilizes all of the symbols of the forward user channel. The symbol rate of a frame in received user data changes every 20 ms according to the voice activity of a user. That is, a transmission power level per symbol of each data frame varies depending on symbol repetition at a transmitter. For example, a transmission power per symbol at a frame transmission rate of 4800 bps is a half that at 9600 bps.
Dilworth & Barrese LLP
Kincaid Lester G.
Mehrpour Naghmeh
Samsung Electronics Co,. Ltd.
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