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

Diversity Techniques for Multi-Carrier Transmission 315108time-variant PDDD/PDSCD with interleavingSCD without interleavinggain in dB64201 2 3 4MFigure 6-16 Spatial transmit diversity gain over the number of antennas M: k = 2; F 1 = 100 Hzfor time-variant phase diversity; indoor; BER = 3 × 10 −4The BER performance of the presented spatial transmit diversity concepts is shownin Figure 6-17 for an indoor environment with two transmit antennas. Simulation resultsare shown for coded OFDM and OFDM-CDM systems. The performance of the OFDMsystem with one transmit antenna is given as a reference. The corresponding simulationresults for an outdoor environment are shown in Figure 6-18. It can be observed that timevariantphase diversity outperforms the other investigated transmit diversity schemes andthat phase diversity and delay diversity are superior to sub-carrier diversity in the indoorenvironment. Moreover, the performance can be improved by up to 2 dB with an additionalCDM component.Finally, some general statements should be made about spatial transmit diversity concepts.The disadvantages of spatial transmit diversity concepts are that multiple transmitchains and antennas are required, increasing the system complexity. Moreover, accurateoscillators are required in the transmitters, such that the sub-carrier patterns at the individualtransmit antennas fit together and ICI can be avoided. As long as this is donein the base station, e.g. in a broadcasting system or in the downlink of a mobile radiosystem, the additional complexity is reasonable. Nevertheless, this complexity can alsobe justified in a mobile transmitter.The clear advantage of the presented spatial transmit and receive diversity conceptsis that significant performance improvements of several dB can be achieved in criticalpropagation scenarios. The results presented in this chapters for DD and PD are alsovalid for CDD. Moreover, the presented CDD technique is standard-compliant and canbe applied in already standardized systems such as DAB, DVB-T, or IEEE 802.11a.