Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-11-10
2004-03-16
Tran, Congvan (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S438000, C455S423000, C370S318000, C370S320000
Reexamination Certificate
active
06708041
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cellular telephone systems. More specifically, the present invention relates to base station transmit power control during handover situations in a code division multiple access cellular telephone system.
BACKGROUND AND SUMMARY OF THE INVENTION
In a cellular communications system, mobile radio stations communicate over respective radio channels to a base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communications systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway, through one or more switching nodes to the base station(s) which serve(s) the called mobile station. A base station pages the called mobile station and establishes a radio communications channel.
In a Code Division Multiple Access (CDMA) mobile communications system, the information transmitted between a base station and mobile station is modulated by a mathematical code, sometimes called a spreading code, in order to distinguish that information from information associated with other mobile stations or base stations that are using the same radio frequency band. Accordingly, individual radio “channels” are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as
Applications of CDMA and Wireless/Personal Communications,
Garg, Vijay K. et al., Prentice-Hall (1997). Spread spectrum communications permit mobile transmissions to be received at two or more (“diverse”) base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another sector on the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called “diversity handover”. During diversity handover, the signaling and voice information from plural base stations is combined in a common point with decisions being made on the “quality” of the received data. Typically this common point is located at the switching node connected to the base stations. In the opposite direction, voice and signaling information are transmit from plural base stations, and a mobile station combines the results.
CDMA systems provide “soft” and “softer” diversity handover. In soft handover, as the mobile station moves to the edge of a cell, the adjacent cell's base station assigns a transceiver to the call while the current base station continues to handle the call. As a result, the call is handled by both base stations on a make-before-break basis. The soft diversity handover occurs with both base stations handling the call until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. Softer diversity handover occurs when the mobile station is in handover between two different sectors at the same base station.
Because all users in a CDMA communications system transmit information using the same frequency band at the same time, each user's communications interferes with communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station cell boundary. As a result, distant mobile communications are overshadowed and dominated by close-in mobile stations which is why this condition is sometimes referred to as the “near-far effect.”
Therefore, to achieve increased capacity by decreasing unnecessary interference, all mobile-transmitted signals should arrive at a base station with about the same average power irrespective of their distance from the base station. Accordingly, uplink (or reverse) transmit power control (TPC) from the mobile station to the base station is one of the most significant factors in improving the performance and capacity of a CDMA system. In general, the mobile station attempts to control its transmit power based on the signal strength to generate a signal-to-noise (SNR) value (or other suitable measure) of signals received from a base station (open loop transmit power control), and the base station sends transmit power control messages to the mobile station (closed loop power control) with the end goal to control the power received at the base station to within a relatively small tolerance, e.g., 1 dB, for all mobile station transmissions received at that base station.
Downlink (or forward) transmit power control is also important for transmissions from the base station to the mobile station. Specifically, the base station varies its transmit power depending upon downlink transmit power control messages or commands sent by the mobile station. There are several reasons for downlink power control.
One reason for downlink transmit power control is to accommodate the fact that in certain coverage area locations, the downlink channel from base station to mobile station may be unusually poor. An example of such a location is a point where the path loss to one or two neighboring cells is nearly the same as the path loss to the base station communicating with the mobile station. At that location, the total interference is increased several times over the interference experienced by a mobile station at a point relatively close to the base station. An additional reason is that interference from these neighboring cell sites will not fade in unison with the desired signal. Still further, the mobile station may be located where several strong multipath signals arrive resulting in a relatively large interference. In other situations, the mobile station may be located where the signal-to-interference ratio is unusually good. Another reason for downlink power control is to minimize intercell interference caused by unnecessarily high base station transmit power levels. By varying the downlink power level to the minimum value needed to achieve a particular quality, unnecessary interference is avoided. If the downlink power level is fixed at a constant but high level to ensure minimum quality under poor conditions, much of the time the downlink power would be too high thereby causing needless interference. Such interference reduces cell capacity. In all of these cases, it is advantageous for the base station to raise or lower its transmit power to ensure acceptable quality but at the same time reduce to the extent possible interference to other signals.
Because power control in CDMA systems is important, transmit power control adjustments occur very frequently, e.g., every 0.625 milliseconds. In adjusting downlink transmit power, the mobile station is continually measuring the transmit power level received from the base station and determining whether that measured value is higher than a reference value. If so, one or more transmit power control bits having one value are transmit uplink from the mobile station to the base station to decrease the transmit power by a predetermined increment, e.g., 1 dB, down to a minimum transmit power value. On the other hand, when the measured value is lower than the reference value, one or more opposite value transmit power control bits are transmitted uplink to the base station to increase the transmit power by a predetermined increment, e.g., 1 dB, up to a maximum value. This transmit power control begins while uplink and downlink synchronization are being acquired and continues throughout the communication.
A problem arises in coordinating downlink transmit power levels among base stations at diversity handover. One problem is that during a handover, the transmit power command from the mobile station to the base stations involved in the handover may be erroneously received at one or more of the base stations. Another problem is that an offset between the downlink transmit powers of base stations
Andersson Peter B.
Butovitsch Peter
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM (publ)
Tran Congvan
LandOfFree
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