Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-06-30
2001-01-09
Bost, Dwayne D. (Department: 2744)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S239100, C455S069000
Reexamination Certificate
active
06173188
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power control in cellular communication systems and, more particularly, to a method for performing forward power control in a Code Division Multiple Access (hereinafter “CDMA”) cellular communication system.
DESCRIPTION OF THE RELATED ART
A CDMA cellular communication system consists of numerous Base station Transceiver Subsystems (BTS) which provide service to mobile stations, and also a Base Station Controller (BSC), a Base Station Manager system (BSM), a Mobile Switching Center (MSC) and a Location Registering System.
The area in which each base station provides cellular service is referred to as a cell. A cell is divided into several sectors. In ascending order, the coverage of a cell is expanded into a BTS area, a BSC area and a MSC area.
As is known, cellular communication systems consist of forward and reverse communication channels. A forward communication channel is a channel that is formed from a base station to a mobile station, and includes a pilot channel, an optional synchronization channel, an optional paging channel, and several forward traffic channels for transmitting voice or data. A reverse communication channel is a channel that is formed from a mobile station to a base station, and includes a primary traffic channel, a secondary traffic channel and a signaling channel.
In a CDMA system, many subscribers access the same frequency band at the same time. Therefore, in order to increase subscriber capacity, the signal power of interference signals should be minimized.
Thus, one way to increase subscriber capacity is through power control. There are two types of power control, forward power control and reverse power control. Reverse power control refers to a method for equalizing the power transmitted to a base station from all its corresponding mobile stations. Reverse power control may be required when interference is present as a result of signals or noise originating from other base stations. Further, the near-far problem and cell-outside interference can be minimized through reverse power control.
On the other hand, forward power control, which is performed in a transceiver selector bank of a base station, refers to a method for increasing or decreasing the power transmitted from a base station to a mobile station in order to maintain regular speech quality for all channels. The method is generally performed when, during a conversation, a mobile station moves toward cell boundaries where multipath propagation phenomena, noise or interference are severe. Further, forward power control can generally reduce interference signals resulting from “neighboring” cells in the radio environment.
A mobile station reports a Frame Error Rate (FER) to a base station every reporting cycle or whenever the FER is less than a threshold value. The FER is the number of bad (i.e., error) frames. The mobile station reports the FER, which is indicative of the forward link quality, to the base station in a pilot measurement report message. The base station, after receiving the pilot measurement report message from the mobile station, controls its transmitting power by adjusting the transmission gain of the forward link.
FIG. 1
is a signal flow diagram illustrating a method for performing forward power control according to the prior art. As stated above, the conventional method is performed by a transceiver selector bank in a base station. According to the method, a mobile station receives/transmits messages from/to a call control processor through a channel element and a transceiver selector bank.
The call control processor sends a call assign message to the transceiver selector bank, which then prepares to establish a traffic channel by exchanging time synchronization data with the channel element. The mobile station then informs the channel element of the start of a traffic channel by transmitting null traffic. Once the traffic channel has started, the transceiver selector bank initializes forward power control (step
110
).
Upon receiving a mobile acquisition message from the channel element, the transceiver selector bank waits for a pilot measurement report message to be sent from the mobile station (step
120
). The transceiver selector bank reads a pilot measurement report message transmitted by the mobile station and computes if the FER is less than a threshold value (step
130
). The FER is based on the number of error frames and the number of total frames received by a mobile station.
FIG. 2
is a waveform diagram illustrating the forward link transmission (TX) gain of the power transmitted from a base station to a mobile station according to the prior art method for performing forward power control. A base station receiving a pilot measurement report message performs forward power control by adjusting the forward link transmission gain (forward gain) by specific values over specific time intervals, as described below. The base station continues to control power until the next pilot measurement report message is received. Consequently, the actual power control cycle, which is the time interval within which the forward gain of the transmitted power is adjusted corresponding to a single pilot measurement report message, is the time corresponding to one traffic frame. A traffic frame is typically 20 ms in duration. Generally, any change in the forward gain resulting from power control is within the range of ±6 dB of the nominal power.
The base station receives the pilot measurement report message and, beginning at the nominal forward link transmission gain (nominal gain), reduces the forward gain by a value “slow down delta” over a time interval “slow down time”. The forward gain is reduced as many times as possible in the “slow down mode” until a “fast down mode” can be entered. That is, the gain-reduction is accelerated by the base station unless the mobile station reports that the quality of the forward link is poor. If the forward link quality is not poor, then the forward gain is reduced by a value “fast down delta” over a time interval “fast down time”.
Upon receiving the pilot measurement report message from the mobile station, the base station utilizes the frame error rate contained therein to determine the forward power control. As such, the forward gain is increased up to a value “small up delta” when the frame error rate is less than a predetermined FER threshold, or up to a value “big up delta” when the frame error rate is greater than the predetermined FER threshold. The increasing step of the forward gain is repeated whenever a pilot measurement report message is received.
According to the prior art method for performing forward power control, a base station receives a pilot measurement report message from a mobile station either on a cyclic basis or whenever the number of error frames is greater than a threshold value. When the forward link quality is degenerating due to a change in the radio environment, the forward gain is increased in proportion to the reported FER. Therefore, the reporting cycle of a mobile station has to become short enough to perform the forward power control timely when radio environment dynamically changes.
Generally, satellite and microwave communications systems transmit directly from a sending antenna to a receiving antenna utilizing a direct wave. In contrast, in conventional mobile communication systems, either a reflected, diffracted or scattered wave is utilized. This is due to the very short length of the antenna of the mobile station, which is generally lower than neighboring obstacles such as buildings. Disadvantageously, the use of reflected, diffracted or scattered waves results in reduced signal power and significant interference.
Therefore, the range of power control to be implemented (i.e., change in gain) and the time interval in which such implementation is to occur, should be decided only after thoroughly analyzing the signal intensity change within the frequency band used for power control. Additionally, such range and time interval changes need to be
Bost Dwayne D.
Craver Charles
Dilworth & Barrese LLP.
Samsung Electronics Co,. Ltd.
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