The digital front-end of software radio terminals

of software radio is the expansion of digital signal processing toward the antenna, and thus to regions where analog signal processing has been dominant so far. It is straightforward to realize that the hardware platform is a most prominent enabling component of a software radio terminal. Of special interest is the very part of the terminal where analog signal processing is replaced by digital signal processing, namely the digital front-end. Having its naming derived from radio frequency (RF) front-end and digital signal processing, the digital front-end is a part of the receiver realizing front-end functionalities digitally that were formerly realized by means of analog signal processing (i.e., downconversion and channel filtering). A main reason for replacing analog with digital signal processing is the possibility to " softly " reconfigure the system, thereby enabling the implementation of different air interfaces on a given hardware platform. Although baseband processing has been realized digitally for some time, parameters related to the digital front-end such as channel spacing/band-width and carrier frequency could not be changed by means of software. Therefore, such terminals, even though often built with high-performance signal processors, are sentenced to " hardwired-looking " terminals. Apparently a standard design of mainly digital filters, the digital front-end turns out to be one of the most power-and time-critical functionalities of the software radio terminal. This is due to the combination of large bandwidth and high dynamic range of the signals to be processed. Consequently, the digital signals have large word lengths and high sample rates. Besides increasing power consumption, high sample rates make the use of time-shared hardware infeasible. Despite these facts, we show below that even under strong constraints software radio is feasible today, specifically the digital front-end as one of the most critical parts. Generally, the receiver is regarded as the more demanding design problem. Therefore, and since all considerations are principally valid for both the transmitter and receiver of a software radio terminal, we shall stick to the receiver in the following. Starting with a description of receiver architectures and important signal characteristics, the digital front-end of a software radio receiver, with its main functionalities, channeliza-tion and sample rate conversion, is the object of this article. The architecture of the ultimate software radio receiver [1] with a minimum of analog components (analog front-end) is given in Fig. 1. Since all channelization tasks (discussed later) are performed digitally, the analog front-end and …