IMPROVED SYNTHESIS OF TRANSMIT DIGITAL & RECEIVE ANALOG PULSE SHAPING FILTERS

Abstract

Finite impulse response filters are often employed in voice-grade modems to ensure both Nyquist and matched filtering. For high data-rate applications, however, we note that the realization of the receive pulse shaping filter in digital form is very costly. On the other hand, a digital transmit filter can be efficiently implemented using a ROM-based technique. The combination of a digital transmit pulse shaping filter and an analog receive front-end filter therefore offers potential application in high speed data transmission systems. In this thesis, we propose two techniques to design these two filters. The first method optimizes both the transmit and receive filters for achieving a joint minimization of noise, adjacent channel interference and intersymbol interference, subject to a fixed transmit power. In the second method, an additional penalty function is introduced, so that a Nyquist pulse shape with good robustness to timing jitters is achieved. Our analysis shows that both methods provide improved performance when compared to the truncation technique, a method commonly used in practice. The second phase of the project involves the design and implementation of a 5 GHz, 8 Mbit/s radio modem for indoor applications. In this work, we adopt a differential phase-shift keying scheme to avoid carrier synchronization at the receiver. In the absence of adaptive equalization and error-control coding, we observe that line-of-sight transmission gives an acceptable BER in the region of 10-4 to 10-8; this improves with the use of a larger excess bandwidth for the pulse shaping. In non line-of-sight transmission, however, a larger delay spread results in a more significant intersymbol interference at the receiver. We note that pulse shaping alone is insufficient to combat the frequency-selective fading at 8 Mbit/s, and it should be employed as part of the other techniques, such as adaptive equalization or multi-carrier transmission.