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

Adaptive Techniques in Multi-Carrier Transmission 191techniques can be based on the measured C/(N + I) or the received average power persymbol or per sub-carrier. These measured data have to be communicated to the transmittervia a return channel, which may be seen as a disadvantage for adaptive techniques.In TDD systems this disadvantage can be reduced, since the channel coefficients aretypically highly correlated between successive uplink and downlink slots and, thus, arealso available at the transmitter. Only if significant interference occurs at the receiver,this has to be communicated to the transmitter via a return channel.4.6.1 Nulling of Weak Sub-CarriersThe most straightforward solution for reducing the effect of noise amplification duringequalization is the technique of nulling weak sub-carriers, which can be applied in anadaptive way. Sub-carriers with the weakest received power are discarded at the transmissionside. However, by using strong channel coding or long spreading codes, the gainobtained by nulling weak sub-carriers is reduced.4.6.2 Adaptive Channel Coding and ModulationAdaptive coding and modulation in conjunction with multi-carrier transmission can beapplied in several ways. The most commonly used method is to adapt channel codingand modulation during each transmit OFDM frame/burst assigned to a given terminalstation [19, 20]. The most efficient coding and modulation will be used for the terminalstation having the highest C/(N + I), where the most robust one will be applied for theterminal station having the worst C/(N + I) (see Figure 4-52). The spectral efficiency ina cellular environment is doubled using this adaptive technique [22].An alternative technique that can be used in multi-carrier transmission is to applythe most efficient modulation for sub-carriers with the highest received power, wherethe most robust modulation is applied for sub-carriers suffering from multipath fading(see Figure 4-53). Furthermore, this technique can be applied in combination with powercontrol to reduce out-of-band emission, where for sub-carriers located at the channelbandwidth border low order modulation with low transmit power and for sub-carriers inthe middle of the bandwidth higher order modulation with higher power can be used.TS 3TS 1 d 3d 1TS 2BSd 2d 3 > d 2 > d 1OFDM symbols up to 64-QAM mod.to TS 1OFDM symbols up to 16-QAM mod.to TS 2OFDM symbols with 4-QAM mod.to TS 3Figure 4-52Adaptive channel coding and modulation per OFDM symbol