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

206 Implementation IssuesFor the downlink, the simulation results given in Reference [66] show that for differentnumbers of active users and a given spreading factor the PAPR in the case ofWalsh–Hadamard codes decreases as the number of users increases, while for other codes(e.g. Golay codes) it increases. For instance, for a spreading factor of L = 16, in the caseof Walsh–Hadamard codes the PAPR is equal to 7.5 and for Golay codes it is about 26.The simulation results given in Reference [66] confirm that for the downlink the bestchoice, also for the minimization of multiple access interference, are Walsh–Hadamardcodes.4.7.3 Narrowband Interference Rejection in MC-CDMAThe spread spectrum technique in combination with rake receivers is an interestingapproach to remove the effects of interference and multi-path propagation [87]. In orderto combat strong narrowband interference different techniques of notch filtering in thetime domain (based on the LMS algorithm) and in the transform domain (based on theFFT) have been analyzed that provide promising results for spread spectrum systems [57].However, in a frequency- and time-selective fading channel with coherent detection, itshigh performance is no longer guaranteed without perfect knowledge about the impulseresponse of the channel and without performing an optimum detection.Another interesting approach is based on the MC-CDMA technique [21]. The interferencecan be considered as narrowband multitone sine interference. A method forevaluating both interference and the fading process based on frequency domain analysisis studied in the following. The estimated interference and fading process is used forweighting each received chip before de-spreading.The narrowband interference can be modeled as consisting of a number of Q continuoussine wave tones [46]Q−1∑Int(t) = A m cos(2πf m t + φ m ), (4.96)m=0where A m is the amplitude of the interferer at frequency f m and φ m is a randomphase.4.7.3.1 Interference EstimationThe interference estimation in multi-carrier systems can be based on the transmission of anull symbol, i.e. one OFDM symbol with a non-modulated signal (see Section 4.2.1). Atthe receiver side this null symbol contains only interference and noise. The power of theinterference Int l in each sub-carrier l can be estimated by performing an FFT operationon this null symbol. The estimated interference power is used for weighting the receivedsub-carriers.However, interference can also be detected without the use of a null symbol. It can bedone, for instance, by performing an envelope detection of the received signal after theFFT operation. Of course, this method is not accurate, since it suffers from the presenceof fading and ICI.