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

BERDiversity Techniques for Multi-Carrier Transmission 31710 −110 −21 Tx, 1 Rx2 Tx, 1 Rx1 Tx, 2 Rx2 Tx, 2 Rx10 −310 −410 −52 4 6 8 10 12 14 16E b /N 0 in dBFigure 6-19f D = 10 HzPerformance of DVB-T with CDD and MRC: 4-QAM; rate 1/2; 2k FFT; indoor;Transmit and receive diversity techniques can easily be combined. Performanceimprovements with CDD in the transmitter and MRC in the receiver are shown fora DVB-T transmission in Figure 6-19 [8]. The chosen DVB-T parameters are 4-QAM,code rate 1/2, and 2k FFT in an indoor environment with maximum Doppler frequencyof 10 Hz. A single antenna system is given as reference.Further performance improvements can be obtained by combining transmit and receivediversity techniques with beamforming [10]. Beamforming reduces interference withina propagation environment and can efficiently cancel interference. Since the channel ofeach beam has a small delay spread, the channel appears nearly flat. The diversity techniquespresented in Sections 6.3.1 and 6.3.2 can artificially introduce frequency- andtime-selectivity and, thus, improve the performance.6.3.4 Space–Frequency Block Codes (SFBC)Transmit antenna diversity in the form of space–time block codes exploits time and spacediversity and achieves a maximum diversity gain for two transmit antennas without rateloss. They have to be applied under the assumption that the channel coefficients remainconstant for two subsequent symbol durations in order to guarantee the diversity gain.This is a tough precondition in OFDM systems, where the OFDM symbol duration T s isN c times the duration of a serial data symbols T d . To overcome the necessity of using twosuccessive OFDM symbols for coding, symbols belonging together can be sent on different