Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-03-06
2001-04-03
Urban, Edward F. (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000
Reexamination Certificate
active
06212399
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunications in general, and, more particularly, to a technique for controlling the power of a signal that is transmitted by a wireless terminal in a wireless telecommunications system.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals
101
-
1
through
101
-
3
) that are situated within a geographic region. The heart of a typical wireless telecommunications system is a wireless switching center (e.g., Wireless Switching Center
120
), which may also be known as a Mobile Switching Center (“MSC”) or Mobile Telephone Switching Office (“MTSO”). Typically, Wireless Switching Center
120
is connected via wirelines (e.g., wirelines
102
-
1
through
102
-
5
) to a plurality of base stations (e.g., base stations
103
-
1
through
103
-
5
) that are dispersed throughout the geographic area serviced by the system and to local and long-distance telephone and data networks (e.g., local-office
130
, local-office
138
and toll-office
140
). Wireless Switching Center
120
is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal
150
), which is connected to the system via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called “cells.”
0
As depicted in
FIG. 1
, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center
120
.
For example, when wireless terminal
101
-
1
desires to communicate with wireless terminal
101
-
2
, wireless terminal
101
-
1
transmits the information-bearing signals to base station
103
-
1
, which relays the signals to Wireless Switching Center
120
via wireline
102
-
1
. Upon receipt of the signals, and with the knowledge that it is intended for wireless terminal
101
-
2
, Wireless Switching Center
120
then returns the signals back to base station
103
-
1
, which relays the signals, via radio, to wireless terminal
101
-
2
.
When wireless terminal
101
-
1
transmits a signal to base station
103
-
1
, two factors dominate the determination of how much power wireless terminal
101
-
1
uses to transmit the signal. The first factor pertains to the quality of the received signal and the second factor pertains to the amount of interference caused by the transmission of the signal.
With regard to the first factor, the quality of the signal as received by base station
103
-
1
is highly correlated to: (1) the amount of power used to transmit the signal and (2) the environmental factors affecting the signal. For example, if wireless terminal
101
-
1
transmits the signal with too little power, then the risk exists that the quality of the received signal will be unacceptable. When the quality of the received signal is unacceptable, base station
103
-
1
is unable to process the signal and there is effectively no communication. As the amount of power that wireless terminal
101
-
1
uses to transmit increases, the signal quality also increases, albeit with diminishing returns.
FIG. 2
depicts a graph that illustrates the relationship between the received signal quality of a signal as a function of the amount of power used to transmit the signal. As is well-known in the prior art, the signal quality can be measured in accordance with a variety of well-known criteria (e.g., signal-to-noise ratio, signal-to-interference ratio, frame error rate, bit error rate, etc.). Furthermore, the amount of power used to transmit a signal can be measured in accordance with a variety of well-known criteria (e.g., absolute power as measured in dBm, average power as measured in dBm, etc.).
Clearly, the first factor mandates that wireless terminal
101
-
1
transmit each signal at at least the minimum power level; otherwise the signal cannot be processed and the utility of the system is undermined. Furthermore, the first factor suggests that wireless terminal
101
-
1
transmit each signal with substantially more power than the minimum to provide a margin of safety.
With regard to the second factor, the extent to which wireless terminal
101
-
1
interferes with the signals of other wireless terminals (e.g., wireless terminals
101
-
2
and
101
-
3
, etc.) is highly correlated to the amount of power used by wireless terminal
101
-
1
to transmit its signals. For example, if wireless terminal
101
-
1
transmits the signal with too much power, then the signals from wireless terminals
101
-
2
and
101
-
3
cannot be received with acceptable signal quality. Therefore, the confluence of the two factors suggests that wireless terminal
101
-
1
should transmit its signals with as much power as necessary to ensure that its signal is received with satisfactory quality, but no more.
FIG. 3
depicts a graph of the interference caused by a wireless terminal as a function of the amount of power used by that wireless terminal to transmit its signals. As is well-known in the prior art, the interference can be measured in a variety of well-known criteria (e.g., signal-to-noise ratio, signal-to-interference ratio, frame error rate, bit error rate, etc.).
In summary, the two factors for determining the amount of power used for transmitting signals oppose each other and a balance must be maintained at all times at each wireless terminal to ensure that its signals are received with satisfactory quality yet do not unnecessarily interfere with any other wireless terminals.
A first technique in the prior art for maintaining that balance is based on: (1) the fact that the quality of the signal is highly correlated to the amount of power used to transmit the signal, and (2) the fact that the signal quality must remain at or above some minimum for the system to have any utility. In accordance with the first technique, base station
103
-
1
continually measures the signal quality of the signals transmitted by wireless terminal
101
-
1
and compares the measured quality against a target quality, called the SIR Target. If the measured quality for the signal is below the SIR Target, the base station sends a message to the wireless terminal directing it to transmit its next signal at an increased power level. In contrast, if the measured quality for the signal is at or above the SIR Target, the base station sends a message to the wireless terminal directing it to transmit its next signal at an decreased power level.
The operation of the first technique is described in detail in the flowchart of FIG.
4
. The first technique begins at step
401
at which base station
103
-
1
establishes a minimum acceptable level of signal quality for the signals received from wireless terminal
101
-
1
. This minimum is called the SIR Target.
At step
402
, base station
103
-
1
receives a signal, S
i−1
from wireless terminal
101
-
1
and compares the quality of the signal against the SIR Target. If at step
403
the measured signal quality is below the SIR Target, control passes to step
404
and a power control signal b
i
is set to +1, which will direct wireless terminal
101
-
1
to transmit its next signal at an increased power level equal to the old power level, P
i−1
, plus a step size, Q. Alternatively, control passes to step
405
and the power control signal b
i
is set to −1, which will direct wireless terminal
101
-
1
to transmit its nex
Kumar Sarath
Nanda Sanjiv
Song Lei
Breyer Wayne S.
DeMont Jason Paul
DeMont & Breyer LLC
Lucent Technologies - Inc.
Perez-Gutierrez Rafael
LandOfFree
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