Power control with effective Eb/N0

Details Power control with effective Eb/N0 Power control with effective Eb/N0

1999-09-07

2003-05-27

Appiah, Charles N. (Department: 2682)

Telecommunications

Transmitter and receiver at separate stations

Plural transmitters or receivers

C455S069000, C370S318000, C370S332000

Reexamination Certificate

active

06571104

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to wireless communications systems and, in particular, to power control in wireless communications systems.
BACKGROUND OF THE INVENTION
Wireless communications systems use power control to improve system performance and increase system capacity. Power control involves tracking the fading of communication channels. In order to compensate for the fading, power control uses the tracked fading to manage the power level at which signals are transmitted from base stations to mobile terminals and from mobile terminals to base stations. One type of wireless communication system uses Code Division Multiple Access (CDMA) techniques.
In CDMA communication systems, digital information is encoded in an expanded bandwidth format, and multiple signals are transmitted simultaneously within the same frequency band. The number of signals that can be transmitted simultaneously is limited by the interference they cause each other. Typically, the larger the signal's transmit power the more interference it causes other signals. Thus, reducing the power of the signals increases the capacity of the wireless communication system. However, reducing the power of a signal increases the number of errors in that signal when it is received and decoded by the receiver. A goal of power control is to keep the power level as close as possible to a level that allows the maximum capacity while keeping the number of errors in the signal at an acceptable level.
As shown in
FIG. 1
, when a call is set up in a CDMA wireless communications system, base station
10
and mobile terminal
20
communicate over forward link
30
and reverse link
40
. Forward link
30
includes communication channels for transmitting signals from the base station to the mobile terminal, and reverse link
40
includes communication channels for transmitting signals from the mobile terminal to the base station. Base station
10
transmits certain types of control information to mobile terminal
20
over a communication channel, referred to herein as a forward control channel, also known in the art as a forward overhead channel. Forward control channels include the pilot, paging, and synchronization channels. Base station
10
transmits voice or data, and certain types of control information over a communication channel, referred to herein as a forward traffic channel. Mobile terminal
20
transmits certain types of control information to base station
10
over a communication channel, referred to herein as a reverse control channel, and it transmits voice or data over a communication channel, referred to herein as a reverse traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. Frames are typically 20-millisecond (ms) in length. The signals transmitted over the control channels are referred to herein as control signals, and the signals transmitted over the traffic channels are referred to herein as traffic signals. Forward traffic frames are frames transmitted over the forward traffic channel, and reverse traffic frames are frames transmitted over the reverse traffic channel. Each forward and reverse traffic frame includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC).
Power control varies the power output of base station
10
and mobile terminal
20
to maintain a constant frame error rate at both the base station and the mobile terminal. A frame error occurs when one or more uncorrectable bit errors occur in a frame. The frame error rate is the number of frame errors divided by the total number of frames observed. A desired frame error rate is selected to minimize power and therefore optimize capacity without compromising signal quality. If the frame error rate exceeds the desired frame error rate, the usefulness of the signal is reduced and the power level is increased to decrease the number of frame errors. If the frame error rate is below the desired frame error rate, the power level exceeds the optimum power level, and the power level is reduced.
In CDMA 2000 wireless communications systems, the power control information in updated at an 800 Hz rate on both the forward and reverse links. Each frame includes sixteen 1.25 ms time intervals, referred to herein as power control groups. Power control information, referred to herein as a power-control bit, is sent once every power control group, or every 1.25 ms.
In CDMA 2000 communication systems, power control on the reverse link is implemented using outer loop
50
and inner loop
60
. Outer loop
50
adjusts a targeted signal-to-noise ratio for the reverse link, where the targeted signal-to-noise ratio is chosen to produce a desired frame error rate. Inner loop
60
keeps the signal-to-noise ratio on the reverse link as close as possible to the targeted signal-to-noise ratio. Signal-to-noise ratios are often expressed as the ratio E
b
/N
0
, where E
b
is the energy per information bit and N
0
is the power spectral density of the interference seen by the receiver.
Outer loop
50
of base station
10
determines targeted E
b
/N
0
70
using a desired frame error rate, which is typically 1%, but can be increased or decreased depending on the desired system performance. In outer loop
50
, base station
10
checks the CRC of each reverse traffic frame to determine whether the reverse traffic frame contains an error. If there is an error in the reverse traffic frame, targeted E
b
/N
0
70
is increased by one up step size. If there is no error in the reverse traffic frame, targeted E
b
/N
0
70
is decreased by one down step size. The down step size is typically much smaller than the up step size. For example, in a typical system, the down step size is about 0.01 dB, and the up step size is about 1 dB. The ratio of the down step size to the up step size is set equal to the desired frame error rate. For example, 0.01 dB/1 dB=1%, so that in steady state, the targeted E
b
/N
0
70
settles at a value close to the E
b
/N
0
needed to achieve the desired frame error rate.
In inner loop
60
, targeted E
b
/N
0
70
is compared to the received signal's E
b
/N
0
80
at the end of every power control group. The base station measures and averages the energy per information bit for the power control group, and it measures and averages the noise and interference of the signal for the power control group. The ratio of these two averages is power-control-group (pcg) E
b
/N
0
80
. Although, the pcg E
b
/N
0
80
can be measured in any way that obtains an accurate measurement. When pcg E
b
/N
0
80
is smaller than targeted E
b
/N
0
70
, base station
10
sends a power-control bit on forward link
30
indicating that mobile terminal
20
should increase the power of reverse link
40
by a fixed amount. When pcg E
b
/N
0
80
is larger than targeted E
b
/N
0
70
, base station
10
sends the power-control bit on forward link
30
indicating that mobile terminal
20
should decrease the power of reverse link
40
by a fixed amount.
In some conventional CDMA wireless communications systems, the reverse-link power control is identical to the reverse-link power control for CDMA 2000 wireless communications systems described above. In other conventional CDMA systems, the reverse-link power control is slightly different. In the latter systems, instead of measuring the E
b
/N
0
for every power control group, the base station measures a different energy measurement closely related to the E
b
/N
0
and uses this energy measurement instead of E
b
/N
0
.
The forward-link power control in some CDMA 2000 systems also works similarly to the reverse-link power control in CDMA 2000 systems described above. In outer loop
110
, mobile terminal
20
determines targeted E
b
/N
0
120
using a desired frame error rate, which is typically 1%, but can be increased or decreased depending on the desired system performance. In outer loop
110
, mobile terminal
10
checks the CRC of each forward traffic frame to determine whether the for

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