Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Synchronization 139The performance of any synchronization and channel estimation algorithm is determinedby the following parameters:– Minimum SNR under which the operation of synchronization is guaranteed– Acquisition time and acquisition range (e.g. maximum tolerable deviation range oftiming offset, local oscillator frequency)– Overhead in terms of reduced data rate or power excess– Complexity, regarding implementation aspects and– Robustness and accuracy in the presence of multi-path and interference disturbances.In a wireless cellular system with a point to multi-point topology, the base station actsas a central control of the available resources among several terminal stations. Signaltransmission from the base station towards the terminal station in the downlink isoften done in a continuous manner. However, the uplink transmission from the terminalstation towards the base station might be different and can be performed in a burstymanner.In case of a continuous downlink transmission, both acquisition and tracking algorithmsfor synchronization can be applied [24], where all fine adjustments to counteracttime-dependent variations (e.g. local oscillator frequency offset, Doppler, timing drift,common phase error) are carried out in the tracking mode. Furthermore, in the case ofa continuous transmission, non-pilot-aided algorithms (blind synchronization) might beconsidered.However, the situation is different for a bursty transmission. All synchronization parametersfor each burst have to be derived with required accuracy within the limited timeduration. Two ways exist to achieve simple and accurate burst synchronization:– enough reference and pilot symbols are appended to each burst or– the terminal station is synchronized to the downlink, where the base station willcontinuously broadcast synchronization signaling to all terminal stations.The first solution requires a significant amount of overhead, which leads to a considerableloss in uplink spectral efficiency. The second solution is widely adopted in burst transmission.Here all terminal stations synchronize their transmit frequency and clock to thereceived base station signal. The time-advance variation (moving vehicle) between theterminal station and the base station can be adjusted through a closed loop by transmittingregular ranging messages individually from the base station to each terminal station.Hence, the burst receiver at the base station does not need to regenerate the terminalstation clock and carrier frequency; it only has to estimate the channel. Note that in FDDthe uplink carrier frequency has only to be shifted.In time- and frequency-synchronous multi-carrier transmission the receiver at the basestation needs to detect the start position of an OFDM symbol or frame and to estimate thechannel state information from some known pilot symbols inserted in each OFDM symbol.If the coherence time of the channel exceeds an OFDM symbol, the channel estimationcan estimate the time variation as well. This strategy, which will be considered in thefollowing, simplifies a burst receiver.