1. Field of the Invention
This invention relates to an improved digital signal processing method and apparatus for demultiplexing and multiplexing frequency division multiplexed channels and more particularly, to a lightweight, compact, low power system which synthesizes a polyphased filter, fast Fourier transform architecture suitable for use in applications with a large number of channels.
2. Description of the Prior Art
In the prior art, digital polyphase filters (PPFs) are used in combination with fast Fourier transform (FFT) algorithms to efficiently demultiplex multiple, frequency division multiplexed (FDM) channels. Similarly, the polyphase filters are used in combination with inverse FFT algorithms to multiplex separate signals onto FDM channels. An N branch polyphase filter is realized by factoring (or partitioning) a desired prototype filter's transfer function (or impulse response) into N subfilters. When coupled with an N point FFT (or IFFT) as shown in FIGS. 1 and 2, an N branch PPF can be used to implement a bank of N bandpass, N:1 decimating (or 1:N interpolating) filters for demultiplexing (or multiplexing) up to N FDM channels. The MCD/M thus realized represents the most computationally efficient system known for both demultiplexing and multiplexing large numbers of FDM channels.
The minimum order necessary to synthesize the required polyphase prototype filter is dictated by the ratio of the guard frequency band (separating adjacent FDM channels) to channel center frequency spacing, filter passband gain uniformity, channel cross-talk attenuation requirements, and the number, N, of FDM channels to be either demultiplexed or multiplexed. Given a fixed guard band to channel spacing ratio and fixed filter response requirements, the minimum order of the polyphase prototype filter grows linearly with N.
Cellular, Personal Communication Systems (PCSs) and other emerging telecommunications applications continue to demand increasingly larger numbers of ever more closely packed FDM communications channels. Such systems (e.g., satellite based cellular systems) often place stringent constraints on the power, mass and physical dimensions of the digital MCD/M subsystem designed to generate the constituent FDM communications channels. These constraints necessitate the use of short sample word lengths (e.g., from 8 to 12 bits) and fixed point arithmetic in any practically realizable MCD/M system for these applications. Short sample word lengths and fixed point arithmetic are, however, inherently incompatible with accurate, high order prototype filter realizations. Indeed, in some applications, the indicated (higher order prototype) polyphase filter may not even be physically realizable within the finite word length and fixed point arithmetic dimensions necessary to meet the MCD/M subsystem's power, mass and size constraints.