Discrete Wavelet Multitone Modulation for ADSL & Equalization Techniques

Abstract

The reliable delivery of information over severe fading wireless or wired channels is a major challenge in communication systems. At the heart of every communication system is the physical layer, consisting of a transmitter, a channel and a receiver. A transmitter maps the input digital information into a waveform suitable for transmission over the channel. The communication channel distorts the transmitted waveform. One of the many sources of signal distortion is the presence of multipath in the communication channel. Due to the effect of the multipath signal propagation, inter-symbol interference (ISI) occurs in the received waveform. Moreover, the transmitted signal gets distorted due to the effect of various kinds of interference and noise, as it propagates through the channel. ISI and the channel noise distort the amplitude and phase of the transmitted signal, which lead to erroneous bit detection at the receiver. It is desirable for a good communication system that its receiver is able to retrieve the digital information from the received waveform, even in the presence of channel impairments such as, multipath effect and noise. Orthogonal Frequency Division Multiplexing (OFDM) is a Multi-Carrier Modulation (MCM) technique that enables high data rate transmission and is robust against ISI (Saltzberg, 1967), (Weinstein and Ebert, 1971), (Hirosaki, 1981). It is a form of frequency division multiplexing (FDM), where data is transmitted in several narrowband streams at various carrier frequencies. The sub-carriers in an OFDM system are orthogonal under ideal propagation conditions. By dividing the input bit-stream into multiple and parallel bitstreams, the objective is to lower the data rate in each sub-channel as compared to the total data rate and also to make sub-channel bandwidth lower than the coherence bandwidth of the communication channel. Therefore, each sub-channel will experience flat-fading and will have small ISI. Hence an OFDM system requires simplified equalization techniques, to mitigate the inter-symbol interference. The ISI can be completely eliminated in OFDM transceivers by utilizing the principle of cyclic prefixing (CP). Therefore, high data rate communication systems prefer to apply multicarrier modulation techniques. OFDM has been standardized for many digital communication systems, including ADSL, the 802.11a and 802.11g Wireless LAN standards, Digital audio broadcasting including EUREKA 147