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

172 Implementation IssuesT s /Kh (0) h (1) h (2) h (K − 1)T stFigure 4-31MC-CDMA uplink channel estimationSection 4.3.3. Alternatively, in a synchronous uplink, a single-carrier modulated midamblecan be inserted between OFDM symbols of an OFDM frame, such that classical singlecarriermulti-user channel estimation concepts can be applied [4].Another approach is to use differential modulation in an MC-CDMA uplink to overcomethe problem with channel estimation. Using chip- and symbol-level differential encodingfor the uplink has been investigated in Reference [85]. The results show that the capacityis limited to moderate system loads and that the performance of this approach stronglydepends on the propagation environment.SS-MC-MAIn the following, the performance of an SS-MC-MA mobile radio system with pilot symbolaided channel estimation in the uplink is presented. The SS-MC-MA system transmitson 256 sub-carriers. The transmission bandwidth is 2 MHz and the carrier frequency islocated at 1.8 GHz. The channel estimation in the SS-MC-MA system is based on filteringin the time direction. An OFDM frame consists of 31 OFDM symbols as illustrated inFigure 4-32.Each user exclusively transmits on a subset of eight sub-carriers. The spreading isperformed with Walsh–Hadamard codes of length L = 8. The receiver applies maximumlikelihood detection. Convolutional codes of rate 1/2 and memory 6 are used for channelcoding.The BER versus the SNR for an SS-MC-MA system in the uplink is shown inFigure 4-33. The COST 207 channel models, each with a different velocity of the terminalstation, are considered. It should be noted that the performance of the SS-MC-MA systemis independent of the number of active users due to the avoidance of multiple access interference;i.e. the performance presented is valid for any system load, assuming that ICIcan be neglected. It can be observed that one-dimensional channel estimation can handledifferent propagation scenarios with high velocities of 250 km/h. One-dimensional uplinkchannel estimation requires additional overhead compared to two-dimensional channelestimation for the downlink, which results in higher SNR degradation (of about 1 dB) dueto the required power for additional pilot symbols.MC-DS-CDMADue to the close relationship of MC-DS-CDMA and DS-CDMA, a channel estimationtechnique design for DS-CDMA can be applied in the same way to MC-DS-CDMA.