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

170 Implementation Issuesin mobile radio channels with fast fading. Blind channel estimation concepts exploitingthe feature that the transmitted data are confined to a finite alphabet set can performchannel estimation from a single OFDM symbol [93]. Modifications of the finite alphabetapproach attempt to reduce the enormous computational effort of these schemes [44].Semi-blind channel estimation takes advantage of pilot symbols that are included in thedata stream. For example, most existing OFDM systems have pilot symbols multiplexedin the data streams such that these symbols in combination with blind algorithms resultin semi-blind schemes with improved estimation accuracy.4.3.9 Channel Estimation in MC-SS Systems4.3.9.1 DownlinkMC-CDMAThe synchronous downlink of MC-CDMA systems is a broadcast scenario where all Kusers can exploit the same pilot symbols within an OFDM frame [42]. The power of thecommon pilot symbols has to be adjusted such that the terminal station with the mostcritical channel conditions is able to estimate the channel. It is therefore possible to realizea pilot symbol scheme with adaptive power adjustment.The performance of an MC-CDMA mobile radio system with two-dimensional channelestimation in the downlink is shown in Figure 4-30. The transmission bandwidth is 2 MHzand the carrier frequency is located at 1.8 GHz. The number of sub-carriers is 512. MMSEequalization is chosen as a low complex detection technique in the terminal station andthe system is fully loaded. The system parameters are given in Table 4-1 and the pilotsymbol grid corresponds to the structure shown in Figure 4-23. The COST 207 channelmodels have been chosen as propagation models. It can be observed that two-dimensionalchannel estimation can handle different propagation scenarios with high velocities up to250 km/h. For the chosen scenario, the SNR degradation compared to perfect channelestimation is in the order of 2 dB.4.3.9.2 UplinkChannel estimation in the uplink requires separate sets of pilot symbols for each userfor the estimation of the user specific channel state information. This leads to a muchhigher overhead in pilot symbols in the uplink compared to the downlink. The increaseof the overhead in the uplink is proportional to the number of users K. Alternatives tothis approach are systems with pre-equalization in the uplink (see Section 2.1.6).MC-CDMAIn synchronous uplinks of MC-CDMA systems, the channel impulse responses of thedifferent uplink channels can be estimated by assigning each user an exclusive set ofpilot symbol positions that the user can exploit for channel estimation. This can either bea one- or two-dimensional channel estimation per user. The channel estimation conceptper user is identical to the concept in the downlink used for all users. In mobile radiochannels with high time and frequency selectivity, the possible number of active userscan be quite small, due to the high number of required pilot symbols.