We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system.

There are several methods developed to deal with time dispersion on a radio channel. One of the methods, OFDM ("orthogonal frequency division multiplexing"), has only been used for broadcasting of digital radio and television and has not previously been used in a multiaccess environment. As OFDM seems to be an attractive alternative to CDMA and TDMA, the authors investigate some methods to make it usable in a multiaccess system. In the downlinks (base to mobile) a transmission scheme, similar to that used in an ordinary broadcasting system, could be used. For the uplinks however, one can not expect the perfect synchronization needed to get the orthogonal carriers necessary for normal OFDM signalling. The authors present a hardware radio system which currently is being developed to test different approaches for multiaccess OFDM. The system will provide a reconfigure tool for the implementation of different synchronization methods, user multiplexing approaches and detection strategies. The system will be built around VME based SUN Sparc stations. The signal processing will be performed by dedicated FFT processors, general purpose digital signal processors and standard ICs for other time critical tasks.>