The basestations used by the providers of current day multiple channel wireless communication services, such as cellular mobile telephone (CMT) and personal communication systems (PCS), typically designate signal processing equipment for each single receiver channel. This is probably a result of the fact that each basestation is configured to provide communication capability for only a limited predetermined number of channels in the overall frequency spectrum that is available to the service provider.
A typical basestation may thus contain several racks of equipment which house multiple sets of receiver and transmitter signal processing components that service a prescribed subset of the available channels. For example, in an IS-136 TDMA cellular system, a typical basestation may service a pre-selected number of RF channels, such as 12, simultaneously supporting a total number of 36 mobile units, of the total number, such as 416, of the RF channels available to the service provider.
Wireless service providers would prefer, however, to employ equipment that would be more flexible, both in terms of where it can be located, as well as in the extent of the available bandwidth coverage provided by a particular transceiver site. This is particularly true where relatively large, secure, and protective structures for multiple racks of equipment are not necessarily available or cost effective. Additionally, service providers desire equipment that can accommodate subscriber growth with features making more efficient use of the available RF spectrum, such as for PCS applications.
One way to resolve this difficulty is to implement a basestation transceiver using a high speed analog-to-digital (A/D) converter and equipment which makes use of efficient digital filtering algorithms such as the Fast Fourier Transform (FFT) to separate the incoming signal energy into multiple baseband channels. On the transmit side, this implementation includes an inverse FFT processing combiner which outputs a combined signal representative of the contents of the baseband signal provided to it.
U.S. Pat. No. 5,940,384 assigned to the same assignee as the present invention and hereby incorporated by reference describes a method of flexibly allocating modulators and demodulators (in the form of digital signal processors or DSPs) to ones of these baseband channels as additional resources are needed, for example, during times of high message traffic. By making the basestation's implementation of call processing resources modular, the basestation can initially be configured to support a limited number of channels. Then, as the demand for services grows, additional channels can be supported by the addition of additional DSPs. The DSPs allow a change or expansion in the type of service, for example, into one of several air interface standards such as code division multiple access (CDMA) as well as time division multiple access (TDMA).
To ensure non-interfering coverage among dispersed basestations, each basestation uses only a subset of the available RF channels, so that mutual interference among any of the channels of the network is reduced. To further reduce interference, frequency hopping can be used. Frequency hopping can significantly reduce the average interference on a given RF channel compared to statically tuned channels. With reduced interference, higher frequency reuse is possible allowing more efficient use of the available RF spectrum, thus enabling higher capacity within the network.