The present invention relates generally to a wireless communication technology, and more particularly to collaborative beam forming of base transceiver stations (BTS's) for reducing interference and enhancing signal strength in a wireless multi-cell network.
A cellular wireless network comprises a plurality of cells, each of which is deployed with at least one BTS for transmitting and receiving signals to and from mobile stations (MS's) within the cell. Conventionally, signals transmitted and received in the network are carried by radio frequencies modulated according to various techniques, such as frequency division multiple access (FDMA), time division multiples access (TDMA), code division multiple access (CDMA), orthogonal frequency division multiplexing (OFDM), and Orthogonal Frequency Division Multiple Access (OFDMA). In order to increase the capacity of the network, neighboring cells are usually assigned with various frequency bands that do not overlap for signal transmission and reception. One typical scheme of interference reduction is called frequency reuse where the same frequency band is used for multiple cells that do not neighbor with each other. A frequency reuse factor is defined as the rate at which the same frequency band can be used in the network. For example, a frequency reuse factor of seven means that seven different frequency bands are employed for a cellular wireless network to differentiate signal transmission and reception in neighboring cells.
One challenge facing the conventional cellular wireless network is interference that occurs when signals from a cell, ether a BTS or an MS, spills over to its neighboring cells. This limits signal capacity and coverage, thereby degrading the performance of the network. Moreover, the severity of interference depends on the frequency reuse factor employed by the network. The smaller the reuse factor, the more serious the interference.
Another challenge is the signal strength degradation due to radio wave propagation. For example, when a MS is distant from a BTS, or the communication link between the MS and the BTS is blocked by certain objects, the signal received by the MS or BTS can be very weak, thereby causing reliability issues for signal transmission.
As such, what is needed is a scheme that reduces the interference and enhances the signal strength for a cellular wireless network.