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Synchronization in OFDM systems

Time synchronization is a key problem in OFDM, to avoid ISI between closely spaced carriers. Add to this the fact that wireless terminals are at varying distances from the base-station and there is no natural 'slotting' mechanism as is used in TDMA systems. Support for synchronization is built into the Wimax specification through the mechanism of ranging and cyclic prefixes, which we shall describe further here.

The basic techniques that are used in OFDM system design have been described in synchronization and will not be repeated here. In this article, we shall review the specific challenges for an OFDM environment, preamble/postamble design in the Wimax specification and how synchronization can actually be implemented.

The principal problem in OFDM systems is that we are using a multitude of narrowband carriers simultaneously. Since each carrier is narrowband, the OFDM symbol time is very large. Carrier spacing is deliberately reduced by placing them just sufficiently apart so as to align them each on the other's spectral null. However, due to the very large symbol time, it is easily possible that the timing offset is fractional to the symbol timing - this is often not handled by standard timing recovery algorithms. As shown in [Mostofi06], the effect of timing offset on SIR is also potentially assymetric, depending on the direction of the error. Timing error in the direction of the CP causes phase error after FFT, whereas timing error away from the CP results in ICI and ISI. Simulation results shown in [Athaudage02] show severe system degradation even for QPSK modulated systems, with a BER of at least 0.0001.