As known in the art, wireless communications networks are typically formed from a plurality of basestations (also referred to as base transceiver stations, BTSs), each basestation being located in a cell which is a geographical region served by that basestation. User terminals, also referred to as user equipments (UEs), are able to communicate with one or more of the basestations using wireless communications.
Previously many wireless communications networks have used so called “hard handoff” schemes whereby an uplink signal from a UE to a basestation is only actioned at one of the basestations. That is, the uplink signal is only demodulated and decoded at a single “best” cellsite or basestation. Thus, the uplink signal from a UE only results in communication at one of the basestations. As a UE moves, for example, a mobile phone in a car that is travelling, the UE may move between cells and be “handed off” from uplink communication with one basestation to uplink communication with another basestation.
It has also been recognised that uplink signals from one UE are often received by a plurality of different basestations (or sectors at the same basestation) in a wireless communications network, and that by making use of the signals received at two or more of those basestations (or sectors), improved uplink communication is achieved. For example, FIG. 1 is a schematic diagram of a wireless communications network comprising a plurality of basestations 10 each within a tri-sectored cell 11. (In FIG. 1 the sectors are represented by hexagonal shapes.) A plurality of UEs are shown, each represented by a dot 12. Two uplink signal paths from most of these UEs 12 are represented by lines, each to a different basestation 10. The darker of the two lines is intended to represent the uplink signal path giving the stronger signal received at the basestation.
Thus more recently, “soft handoff” schemes have been used and are part of the IS95 standard. In such “soft handoff” schemes the uplink signal is separately demodulated and decoded on a frame-by-frame basis at multiple basestations. The hard-decoded frames (i.e. containing a 1 or 0 in each bit position) from these basestations are then backhauled (communicated in some way) to a common point. This is illustrated schematically in FIG. 2a which shows three basestations 20 each in a tri-sectored cell. A user equipment, in this case a mobile telephone handset 21 is shown with four uplink communication paths indicated by arrows 1 to 4. Arrows 1, 2 and 4 show paths to different basestations whereas arrows 1 and 3 show paths to different sectors of the same basestation. The degree of soft handoff is said to be 4-way because four different uplink paths are used. Each of the basestations demodulates and decodes the uplink signal received from the UE 21. The resulting hard-decoded frames are then backhauled to a common point 22 as indicated by arrows 23, 24, 25.
For example, the common point is typically a selector bank subsystem (SBS) of a base station controller (BSC). A processor at the common point then selects the “best” of the hard-decoded frames received from the different cell sites and discards the rest. For example, the SBS chooses as the single survivor frame (for vocoder decode) one of the backhauled set (if possible) with a correct cyclic redundancy check (CRC) (i.e. a CRC which correctly corresponds to the decoded data). In this way improved uplink communication is achieved (as compared with hard handoff) but at the expense of carrying out extra processing at the basestations, providing and carrying out the backhaul communication and carrying out the additional processing at the common point.
mDiversity Inc. describe a variation on this soft handoff scheme in their U.S. Pat. Nos. 6,349,094 and 6,192,038. They describe a plurality of collectors (similar to basestations) distributed at macro-diverse locations for receiving reverse channel (uplink) signals from users. Each of the collectors processes the uplink signals to yield one or more sequences of data bits as a burst and corresponding initial confidence metrics for each bit. The collectors forward the data bits and confidence metrics to an aggregator or common point. At the aggregator the data for the same user received from the multiple collectors is combined by using the confidence metrics to make a final decision on each bit.
One problem with previous types of soft-handoff methods is that relatively complex processing is required at each basestation in order to demodulate and decode the signals.
It is also desired to further increase uplink capacity as compared with known types of soft-handoff methods.
An object of the present invention is to provide an improved soft-handoff method which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.