Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-08-19
2002-08-13
Marcelo, Melvin (Department: 2733)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C455S522000
Reexamination Certificate
active
06434130
ABSTRACT:
The present invention relates to a switching control method and apparatus for a wireless telecommunication system and, in particular, but not exclusively for a cellular telecommunications system.
The use of code division multiple access (CDMA) is currently being proposed for the next generation of cellular telecommunication networks. CDMA uses a digital spread spectrum multiple access technique which it is believed will allow the volume of traffic supported by a cellular telecommunications network to be increased. One of the problems of using CDMA techniques is that of power control. With CDMA systems, it is desired that all the signals reaching a given base transceiver station from all the mobile stations in the cell associated with that base transceiver station have the same level. This permits the number of simultaneous calls which can be supported at the same time to be maximised. In particular, if the level of the received signals from the mobile stations are of the same level, the signal to interference ratio of each signal received at the base transceiver station is minimised. If the signal from a given mobile station is received by the base transceiver station with too low a power level, the bit error rate will be too high to provide a reasonable quality of communication. On the other hand, if the level of the signal received from a given mobile station is too high, interference with the other mobile stations sharing the same channel is increased so that some of these other mobile stations may not achieve an acceptable quality of communication with the respective base transceiver station.
In one method which has been proposed, open loop power control and/or closed loop power control is used to determine the level at which a signal is to be transmitted from a mobile station. In open loop power control, the mobile station determines a value for the power of the signal to be transmitted to the base transceiver station based on the level of the signal received at that mobile station from the base transceiver station. Thus, the strength of the signal received by the mobile station from the base transceiver station is used by the mobile station to adjust the power of its own transmission. In other words, the stronger the signal received by the mobile station, the lower the power used by the mobile station to transmit to the base transceiver station.
In closed loop power control, the base transceiver station measures the power level of a signal received from a given mobile station. This measured power level is compared with a desired power level. Based on this comparison, the base transceiver station will send to the mobile station a power adjustment signal indicating to the mobile station the power level at which the mobile station should transmit signals to the base transceiver station.
In methods which use a combination of the closed loop and open loop power values to arrive at the desired power level, the power adjustment signal provided by the base transceiver station and the open loop estimate arrived at by the mobile station itself are both used to obtain the final value for the power level of the signal to be transmitted by the mobile station to the base transceiver station.
If the power control method uses closed-loop power control with short adjustment intervals, the control mechanism can react to fast-fading. Fast-fading is caused by destructive interference between different reflected paths of the same radio signal. It is highly frequency-selective, and therefore the fast-fading process between different radio frequencies is independent. Thus fading might occur at one frequency and not at another, even if the same paths are used. In frequency division duplexed (FDD) systems, where uplink (the link from mobile station to base station) and downlink (the link from base station to mobile station) transmissions are transmitted on different frequencies, closed-loop power control is required to be able to take the difference between link directions into account.
It is also proposed that some cellular telecommunication systems will use hard handover. In particular, it is believed that hard handover may be simpler to implement in practice with certain proposed applications of CDMA. Handover or handoff is when a mobile station stops communication with one base transceiver station and instead communicates with a different base transceiver station. Typically this occurs when a mobile station moves from one cell into another cell. Hard handover or handoff occurs when the mobile station is only in communication with one base station at a time. In other words, the connection with the first base transceiver station is severed before the connection with the second base transceiver station is made. This contrasts with soft handover or handoff where a mobile station can be in communication, at the same time, with several transceiver stations during the transferring process.
In known handoff or handover methods, the mobile station measures the received strength of reference signals transmitted by the base transceiver stations. The mobile station then sends a report back to the base transceiver station with which it is in communication reporting the signal levels of the reference signal received at the mobile station from that base transceiver station as well as the adjacent base transceiver stations from which it was able to receive the reference signals. Based on this report, the base transceiver station in combination with a mobile switching centre, will decide whether or not the mobile station should remain in communication with the current base transceiver station or whether it should be switched to another base transceiver station.
However, this method of achieving handover or handoff has the disadvantage that only the downlink signal (i.e. the signal from the base transceiver station transmitted to the mobile station) is taken into account. The frequency of the downlink signal is generally different from that used in the uplink signal (i.e. the signal transmitted from the mobile station to the base transceiver station). Accordingly, if there is fading in the uplink signal, but not in the downlink signal, handover will not occur, even if handover is in fact appropriate. Instead, the power control method outlined hereinbefore will try to compensate for the fading in the uplink signal by increasing the transmission power of the mobile station. This can cause unnecessary interference with mobile stations in neighbouring cells or indeed other mobile stations contained in the same cell.
It is therefore an aim of embodiments of the present invention to provide a method and apparatus which is able to reduce or at least mitigate the problems described hereinbefore.
According to one aspect of the present invention, there is provided a switching method for a wireless telecommunication system comprising at least one first station and at least one second station, said method comprising the steps of determining at a first station the power level of a signal received from a second station, said first and second stations being in communication; providing a first value for the power level of the signal to be transmitted from the first station to the second station based on said determined power level of the signal received from said second station; determining at the second station the power level of the signal received from said first station; providing a second value for the power level of the signal to be transmitted from the first station to the second station based on said determined power level of the signal received from the first station; comparing said first and second values and based on said comparison determining if said second station is to be switched.
This method is advantageous in that the comparison of the two values takes into account the path between the first station and the second station and the path between the second station and the first station. In other words, if the behaviour of the path in an uplink direction is quite different from that in a downlink direction, it
Rinne Mikko
Soininen Pekka
Marcelo Melvin
Nokia Mobile Phones Limited
Perman & Green LLP
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