Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-10-23
2003-07-22
Nguyen, Lee (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000, C370S342000
Reexamination Certificate
active
06597923
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of communications. More particularly, the present invention is directed to techniques for dynamically adjusting the power of a transmitter.
BACKGROUND OF THE INVENTION
Communication signals are generally sent by a transmitter across a transmission media and received by a receiver. Some transmission media result in variations in received signal quality over time. For example, in wireless communications transmission signal quality may vary due to other signal interference, physical obstructions, weather, etc. This is particularly true for mobile communications wherein at least one of the transmitter and/or received is moving during a transmission and reception sequence. As a result, various dynamic transmission power control methodologies have been included with the transmitters and/or receivers to ensure adequate transmission signal power (strength) over time so as to adjust for variations in signal quality caused by variation in the transmission media. In addition, the transmission power may be regulated so as to minimize power consumption or cross interference with other transmission signals operating in the same area and/or frequency range.
For example, transmission signal power of mobile communication system transmitters are often dynamically adjusted to achieve as low a power level as possible for a predetermined or desired signal quality and error rate of the received transmission signal. For digital signals the error rate may be referred to as the frame error rate (FER) of the transmitted signal. In essence, the lower the transmission signal power without experiencing unacceptable signal reception error, the higher the user capacity (# of users) of the wireless communication system (e.g., mobile telephone systems). This is because the lower the transmission signals power the less overlap
oise it causes on the adjacent signals in the same or close transmission band. Further, if the transmitter is in a mobile communication device which is powered by one or more batteries, operating at a lower signal power will conserve battery power. In any case, the dynamic power control may include received signal quality monitoring at the receiver, transmission to the original transmitter of a received signal quality information, and signal power adjustment by the original transmitter, which occurs in an iterative manner over transmission time.
Dynamic transmission power control methodologies are typically used for mobile communication devices such as digital mobile telephones that use, for example, code division multiple access (CDMA) signal format. CDMA communication systems typically employ power control on both a forward transmission link (e.g., base station to mobile station) and a reverse transmission link (e.g., mobile station to base station) to guarantee a desired signal quality (quality of service) and to maximize system capacity (by operating at or near a signal power level just sufficient to give a desired signal quality). Achieving such objectives requires controlling the transmitter power level to compensate for time-varying path loss and interference, so that the received power level is approximately equal to a power level corresponding to the desired FER.
One conventional dynamic transmission power control methodology uses a closed loop power control implemented at the receiver for determining the quality of an incoming transmission signal and accordingly returns information on how to adjust the transmission power in an outgoing transmission signal sent to the original transmitter. One conventional closed loop power control system of this method is illustrated in FIG.
1
.
The receiver demodulator unit
105
performs an inner loop power control including estimating a received signal-to-noise ratio (SNR) derived from an incoming transmission signal
130
and compares the estimated SNR with a target SNR
115
value. The target SNR is generated by an outer loop function
140
based on a comparison of an estimated FER with a target FER.
The outer loop
140
estimates the FER and compares such estimate with a target value, typically set on the basis of the required grade of service. The outer loop includes an iterative decoder
106
that decodes information contained in the incoming transmission signal
130
, and has both error correction and detection capabilities. Frame error estimation uses a soft metric N provided by the iterative decoder
106
, which represents the number of iterations it takes the iterative decoder
106
to generate an information frame of acceptable quality. The soft metric N is fed into an error rate threshold detector
107
. Each time a frame erasure is detected, that is, the iterative decoder
106
fails to decode the frame within Nmax iterations, N is set to Nmax+1, the output of the error rate threshold detector
107
is set to 1, and the outer loop increases the target SNR value
115
by an amount &Dgr;
UP
which is a fixed amount of SNR. Each time a good frame is detected; that is, the iterative decoder
106
is able to decode the incoming information frame within Nmax iterations, the output of the error rate threshold detector
107
outputs a 0, and the outer loop
140
decreases the target SNR target value
115
by an amount &Dgr;
DOWN
which is a fixed amount of SNR. Note that typically the outer loop power control step sizes, &Dgr;
UP
and &Dgr;
DOWN
, are a (fixed) function of the target FER.
The &Dgr;
UP
and &Dgr;
DOWN
values are input to a first order accumulator
109
and combined with the previous target SNR
115
. Next, the new target SNR is input to a clipper
110
which is typically used to maintain the SNR target value
115
within a desired range, having a predetermined upper and lower value, within which the output target SNR is limited.
Thus, if the estimated FER exceeds the target FER, then the outer loop increases the value of the SNR target
115
provided to the demodulator and is transmitted to the inner loop power control entity (i.e., the transmitter that originally sent the decoded frame is requested to increase power). If the estimated FER is equal to our less than the target FER, the target SNR value
115
is decreased (i.e., the transmitter that originally sent the decoded frame is requested to decrease power).
A feedback channel exists between the receiver and the transmitter and is provided via the modulator
113
. Based on the comparison of estimated SNR determined by the inner loop of the demodulator
105
and target SNR
115
, power control information is given to the modulator
113
in the power control adjustment signal
135
indicating the magnitude of a power control signal (typically bipolar) and this information is transmitted in the outgoing transmission signal
125
on the feedback channel. If the estimated SNR received is lower than the target SNR
115
, then the polarity of the power control signal transmitted on the feedback channel is set to command the original transmitter to increase its transmit power level. If the estimated SNR received is higher than the target SNR
115
, the polarity of the power control signal transmitted on the feedback channel is set to command the original transmitter to decrease its transmit power level. Note that the power control adjustment signal
135
fed into the modulator is typically multiplexed with user data transmitted on the reverse link from the mobile station to the base station.
However, the conventional power control methodologies suffer from poor tracking and residual variance performance of the closed outer loop power control. The loop parameters, such as the outer loop step size &Dgr;
UP
and &Dgr;
DOWN
and the minimum and maximum target SNR of the clipper
110
are set to achieve the best trade-off between loop variance and loop equivalent bandwidth. Loop variance is a measure of the error reflecting the difference between the target SNR and required SNR that meets target FER. Loop equivalent bandwidth is a measure of how quickly the loop responds to the changes in the
Damnjanovic Aleksandar
Vanghi Vieri
Coats & Bennett P.L.L.C.
Telefonaktiebolaget L.M. Ericsson (publ.)
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