Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-23
2003-11-25
Legree, Tracy (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S438000, C455S440000, C455S442000, C455S069000, C455S522000, C370S332000
Reexamination Certificate
active
06654608
ABSTRACT:
BACKGROUND
The present invention relates to the use of power control techniques in cellular radio telephone communication systems, and more particularly, to methods and systems related to power control for base stations.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
FIG. 1
illustrates an example of a conventional cellular radio communication system
100
. The radio communication system
100
includes a plurality of radio base stations
170
a-n
connected to a plurality of corresponding antennas
130
a-n
. The radio base stations
170
a-n
in conjunction with the antennas
130
a-n
communicate with a plurality of mobile stations (e.g. stations
120
a
,
120
b
and
120
m
) within a plurality of cells
110
a-n
. Communication from a base station to a mobile station is referred to as the downlink, whereas communication from a mobile station to the base station is referred to as the uplink.
The base stations are connected to a mobile switching center (MSC)
150
. Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile station from one cell to another. The MSC, in turn, can be connected to a public switched telephone network
160
, which services various communication devices
180
a
,
180
b
and
180
c.
Power control techniques have been implemented in radiocommunication systems to ensure reliable reception of a signal at each mobile station, i.e., to provide a signal to interference ratio (SIR) above a prescribed threshold for each mobile station. To improve the reception of a mobile station whose SIR drops below this threshold, the energy of the signal may be increased to appropriate levels. However, increasing the energy associated with one mobile station increases the interference associated with other nearby mobile stations which may be located in the same cell or in nearby cells. As such, the radio communication system must strike a balance between the requirements of all mobile stations, located in the same cell and in nearby cells, sharing the same common channel and adjacent channels. A steady state condition is reached when the SIR requirements for all mobile stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state condition may be achieved by using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high power levels increases interference experienced at each mobile receiver, and limits the number of signals which may be successfully communicated on the common channel and on adjacent channels (e.g. reduces system capacity).
This technique for controlling transmit power in radiocommunication systems is commonly referred to as a fast power control loop. The initial SIR target is established based upon a desired quality of service (QoS) for a particular connection or service type. For non-orthogonal channels, the actual SIR values experienced by a particular mobile station or base station can be expressed as:
SIR
=
Mean
power
of
received
signal
Sum
of
the
mean
powers
of
all
interfering
signals
(
1
)
The SIR is measured by the receiving party and is used for determining which power control command is sent to the transmitting party.
A slow power control loop can then be used to adjust the SIR target value on an ongoing basis. For example, the mobile station can measure the quality of the signals received from the base station using, for example, known bit error rate (BER) or frame error rate (FER) techniques. Based upon the received signal quality, which may fluctuate during the course of a connection between the base station and a mobile station, the slow power control loop can adjust the SIR target that the fast power control loop uses to adjust the base station's transmitted power. Similar techniques can be used to control uplink transmit power.
FIG. 2
illustrates the physical relationship between the base stations of two cells A and B and their relative transmit power levels. In a conventional wireless communication system, base station
210
A transmits signals to mobile stations which are located within cell A, as defined by the cell border
250
. The cell border
250
can be a point where the strength of the signals transmitted at full power from base station
210
A equals the strength of the signals transmitted at full power from base station
210
B.
When a mobile station which is currently communicating with base station
210
A moves over the cell border
250
into cell B, the mobile station will continue to communicate with base station
210
A until the mobile station crosses hysteresis boundary
220
B. The area in cell B from cell border
250
to hysteresis boundary
220
B is known in the art as the hysteresis zone. The hysteresis zone is used by the wireless communication system to avoid a “ping-pong” handoff effect, i.e., a mobile station which continuously hands-off based solely upon which base station is providing a greater signal strength at a particular instant of time. Accordingly, the mobile station will continue to communicate with base station
210
A until there is a more significant change in the relative strength of the signals transmitted from base stations
210
A and
210
B. Typically the hysteresis zone is set such that there is approximately a 3-5 dB difference in the signal strengths transmitted from base station
210
A and base station
210
B. Although
FIG. 2
illustrates the hysteresis zone as a uniform area surrounding the cell border
250
, one skilled in the art will recognize that hysteresis is typically implemented in wireless communication systems by adding a predetermined signal strength value to the signal strength of the current connection. Hence, the actual area of the hysteresis zone will depend upon signal propagation conditions.
Now that the concept of hysteresis has been explained, a brief overview of a conventional handoff from base station
210
A to base station
210
B is presented below. When a mobile station in communication with base station
210
A is moving towards base station
210
B, the mobile station maintains communication with base station
210
A until the mobile station crosses over the end of the hysteresis zone at hysteresis boundary
220
B. When the mobile station crosses hysteresis boundary
220
B, the mobile station is handed off to base station
210
B. In typical wireless communication systems, since the base station does not know how far the mobile station is from the base station, the new base station begins transmitting signals to the mobile station at full power. For example, when a mobile station is handed off from base station
210
A to base station
210
B, base station
210
B begins transmitting signals to the mobile station at full power such that the signals transmitted from base station
210
B provide an acceptable quality signal out to the end of the hysteresis zone at hysteresis boundary
220
A. However, as described above, mobile stations are typically located inside the hysteresis zone at handoff, not at the far boundary of the hysteresis zone. Accordingly, transmitting signals to a mobile station which has just undergone handoff at full power unnecessarily increases the interference in cell A and other nearby cells (not shown).
A second situation where a base station unnecessarily transmits to a mobile station at full power is during call set-up. Call set-up occurs when a mobile
Monell Gunnar
Östman Thomas
Legree Tracy
Telefonaktiebolaget LM Ericsson (publ)
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