Design of packet-based soft-modem for a band-limited radio channel
The performance of early modems is mainly limited by the hardware; the signal processing of modulation and demodulation are carried out by analog circuitry, which imposed severe restriction on complexity of the algorithms. With the advent of Digital Signal Processors, most of the modem functions are now performed in software. The ability to program the DSP with any communication software makes the modem adapt to more than one communication network. Such a modem is defined as a soft-modem. This work aims at designing an inexpensive soft-modem between two computers separated by a distance of 100-300 meters. An ideal software radio has all the communication blocks of Modulation, Synchronization, Equalization and Channel coding performed by signal processing software running on a DSP rather than a dedicated analog and digital circuitry. The soft-modem, on the other hand, captures most of the features of a software radio but it replaces the expensive DSPs with the processing capabilities of a PC. The IF up/down converters are replaced by FM transceivers. The DAC and ADC on the soundcards synthesize and capture the analog waveforms. The channel is modeled as linear gaussian channel with ISI. A ML joint synchronization and equalization is derived and implemented for a QAM constellation size of 256 points. Various coding methods to achieve high coding gains on this soft-modem are discussed.