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

Spatial Pre-Coding for Multi-Carrier Transmission 32110 010 −1OFDM; M = 1OFDM; M = 2OFDM-CDM; M = 1OFDM-CDM; M = 210 −2BER10 −310 −410 −5012345E b /N 0 in dB678910Figure 6-23 BER versus SNR for different OFDM schemes: code rate R = 2/3The BER performance of various OFDM-based diversity schemes with and withoutSFBC is shown in Figure 6-23. The channel code rate is R = 2/3. OFDM-CDM appliessoft interference cancellation with one iteration at the receiver.In Figure 6-24, the gain, i.e. SNR reduction, with SFBC OFDM-CDM compared toSFBC OFDM is shown versus the channel code rate for a BER of 10 −5 . Additionally,OFDM-CDM without SFBC is compared to SFBC OFDM. In the case of OFDM-CDM,soft interference cancellation with one iteration is applied. It can be observed that the gainsdue to CDM increase with an increasing code rate. This result shows that the weaker thechannel code, the more diversity can be exploited by CDM.6.4 Spatial Pre-Coding for Multi-Carrier TransmissionSpatial pre-coding schemes exploit knowledge about the channel state information at thetransmitter. The signals at the multiple transmit antennas are pre-coded such that theyimprove the performance at the receiver. The channel knowledge available at the transmitteris typically provided by a feedback channel from the receiver to the transmitter.The spatial pre-coding schemes described in this section differ in the amount of channelknowledge required at the transmitter. An optimized system design has to find the appropriatetrade-off between the amount of channel knowledge required and the performance.With increased channel knowledge the overhead for channel estimation and the feedbackchannel increase.