OFDM Air-Interface Design for Multimedia Communications

The aim of this dissertation is the investigation of the key issues encountered in the development of wideband radio air-interfaces. Orthogonal frequency-division multiplexing (OFDM) is considered as the enabling technology for transmitting data at extremely high rates over time-dispersive radio channels. OFDM is a transmission scheme, which splits up the data stream, sending the data symbols simultaneously at a drastically reduced symbol rate over a set of parallel sub-carriers. The first part of this thesis deals with the modeling of the time-dispersive and frequency-selective radio channel, utilizing second order Gaussian stochastic processes. A novel channel measurement technique is developed, in which the RMS delay spread of the channel is estimated from the level-crossing rate of the frequency-selective channel transfer function. This method enables the empirical channel characterization utilizing simplified non-coherent measurements of the received power versus frequency. Air-interface and multiple access scheme of an OFDM-based communications system are proposed and investigated in part two of this work. Cumulative data rates up to 155 Mbit/s are reached under optimum channel conditions, in indoor and short range outdoor scenarios at low mobility (pedestrian speed). Wireless LANs (local area networks) are a typical application for the system. Synchronization and channel estimation algorithms are developed and evaluated, utilizing a known training symbol, which is periodically transmitted in the beginning of the fixed frame structure. It has been concluded that robust and efficient synchronization and channel estimation schemes — critical tasks for an OFDM receiver — are enabled by this training symbol, at the cost of a very small overhead. Detailed topics in synchronization include the analysis of a fine timing-offset estimation algorithm over multipath channels, and the analysis of the impact of DC-offsets and carrier feed-through on a popular frequency-synchronization scheme. A remedy is found for the latter issue. For the up-link, pre-equalization is suggested in a time-division duplexing (TDD) scheme to pre-compensate for the frequency-selectivity of the radio channel and thereby to simplify the data detection at the base station. Synchronization is mostly done by the mobiles, which implies a distribution of the high complexity involved. Concepts are presented for keeping the power of the up-link signal “constant” and for estimating the remaining synchronization-offsets. The main signal processing algorithms for the OFDM transceivers have been implemented and validated on a DSP-based experimental platform, which operates in realtime, however, at drastically downscaled data rate. Forward error correction coding is an essential part of OFDM schemes, because frequency-diversity is exploited by spreading the coded data symbols over the “large” signal bandwidth. The performance of coded OFDM systems is evaluated, indicating that increased system bandwidth and channel delay spread (the latter under certain constraints) lead to enhanced performance. A novel antenna diversity technique is proposed, which can improve the performance at low computational complexity, if the system bandwidth and/or the channel?s delay spread are small. Generally, it has been concluded that the OFDM scheme is an efficient and robust method for transmitting data at very high rates. However, some critical hardware issues, as for instance the linearity of amplifiers and the phase noise of local oscillators, have to be solved.