The superheterodyne principle has been employed for
more than 80 years in the design and realization of radio
receivers. These receivers are basically analog in nature.
The received signal is first processed by an analog RF front end.
The amplified signal is then downconverted into the IF by means of an
analog mixer. At the IF stage, only the components of the signal
at desired frequencies are retained and the others are filtered
out. Finally, the IF signal is further downconverted into the
baseband frequency. One can use an analog-to-digital converter to
convert the analog baseband signal into the digital domain.
Digital signal processing is then used to process the signal in order
to estimate the transmitted information data sequence. Advantages
of receiver implementation based on the aforementioned principle are
that the system architecture is proven, the RF/IF analog technology is
mature, and the requirements on analog-to-digital converters and
digital signal processors are relatively undemanding. However,
one of the greatest disadvantages of analog receivers is their lack of
flexibility. One receiver is designed for one communication
system. As more and more mobile communication standards emerge
and coexist around the world, a multimode radio receiver would
certainly be a significant advantage. One approach for achieving
multimode capability is to pack several discrete receivers for
different standards, but this approach greatly increases the weight and
size of the resultant composite receiver.
Fundamentally, a software radio uses an
analog-to-digital converter to digitize the received signal when the
signal frequency is still at the RF band or has been downconverted to
the intermediate frequency (IF) band, and processes the digitized
signal in the discrete-time domain by programmable processors.
different communication standards, different programs (in the form of
software) are loaded into the processors to demodulate the received
signal. Multimode capability is therefore supported without the
need to duplicate hardware.
radio is a broad topic
and covers a large number of research areas.
It can be analyzed from many different perspectives, including
network level, terminal level, device level, hardware implementation
and software aspects. In this study, only the signal processing
in the receiver design were considered. In particular,
bandpass sampling, decimation filtering, channelization, and fractional
sampling-rate conversion were emphasized, all of which being important
components in the implementation of software radios.
Yik-Chung Wu, Tung-Sang Ng and
Kun-Wah Yip, “Chapter 7 --
Software-Radio: A Prospective Technology for the Future Broadband
Communication Systems,” in Advances in 3G Enhanced Technologies for
Wireless Communications, edited by J. Wang and T. S. Ng, Artech House,
2002. (This book has been
translated into chinese language)
Yik-Chung Wu and Tung-Sang Ng,
“New Implementation of a GMSK
Demodulator in Linear Software Radio Receiver,” Proceedings of the 11th
IEEE International Symposium on Personal, Indoor and Mobile Radio
Communications (PIMRC2000), pp. 1049-1053, Sept., 2000.