OFDM transmission technique: A strong candidate for next-generation mobile communications

摘要

The orthogonal frequency-division multiplexing (OFDM) transmission technique is used for some broadcast applications (DVB-T, DAB, DRM) and for wireless local loop (WLL) standards (HIPERLAN/2, IEEE 802.11a). However, OFDM has not been used so far in cellular communication networks. The system flexibility and the use of subcarrier-specific adaptive modulation schemes in frequency-selective radio channels are some advantages that make the OFDM transmission technique a strong and technically attractive candidate for the next generation of mobile communications. The general idea of OFDM is to split the total bandwidth into many narrowband sub-channels that are equidistantly distributed on the frequency axis. The sub-channel spectra overlap each other, but the subcarriers are orthogonal and can therefore be separated in the receiver. OFDM can efficiently deal with the interference effects of multipath propagation situations in broadband radio channels. Consequently, the OFDM receiver can also deal with superimposed signals that have been transmitted by several distinct and adjacent base stations (BSs) in a cellular environment. This is the basis for setting up a cellular single-frequency network (SFN), in which all adjacent base stations operate simultaneously in the same frequency band, which leads to a reuse factor of one. The objective of this paper is to describe an OFDM-based system concept, and to discuss all technical details for establishing a cellular single-frequency network, which requires synchronization in time and frequency with sufficient accuracy. In the cellular environment, a flexible frequency-division multiple-access scheme, based on OFDM-FDMA, is developed, and radio-resource management (RRM) employing dynamic channel allocation (DCA) techniques is used. A purely decentralized and self-organized synchronization technique, based on specific test signals, and radio-resource management techniques, based on co-channel interference (CCI) measurements, are developed.