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

RF Issues 207Detection strategyIn the following, we consider the single-user case. In the presence of fading, interferenceand noise, the received signal vector r = (R 0 ,R 1 ,...,R L−1 ) T can be written asr = Hs + Int + n, (4.97)where H is a diagonal matrix of dimension L × L with elements H l,l = a l e jϕ l correspondingto the fading and phase rotation disturbance, s = (S 0 ,S 1 ,...,S L−1 ) T is thetransmitted signal, Int = (Int 0 , Int 1 ,...,Int L−1 ) T is the narrowband interference, andn = (N 0 ,N 1 ,...,N L−1 ) T represents the Gaussian noise. The optimal detection consistsof choosing the best transmitted sequence by minimizing the distance between thereceived sequence and all possible transmitted sequences by using the values a l , ϕ l , Int las channel state information (CSI). Let us denote the possible transmitted sequences as(v (i)0 ,v(i) 1 ,...,v(i) L−1), i = 1, 2. The information bit is detected if we maximize the followingexpression [21]: 2 l =L−1∑l=0H ∗l,l√1 + Int 2 lR l v (i)l, i = 1, 2. (4.98)This corresponds to a conventional correlative detection using the CSI, where the receivedchips are weighted by Hl,l √1 ∗ / + Int 2 l . There are many advantages of such a weighting.The first one is that it is equivalent to soft erasure or soft switching-off of the sub-carrierscontaining interference. The second advantage is that no pre-defined threshold for thedecision of switching-off the interferers is needed, unlike in the classical transform domainnotch-filtering method [57]. Finally, the system does not need extra complexity in termsof hardware, since the FFT operation is already part of the system.If the number of interference tones is Q, the decision process is equivalent to consideringthe case where Q chips are erased at the transmitter as we perform erasing of theinterferers. Therefore, the above expression can be approximated as 2 l ≈L−Q−1∑l=0H ∗l,l√1 + Int 2 lR l v (i)l, i = 1, 2, (4.99)where R l corresponds to the received chips with low interference.Performance evaluationIn Figure 4-66 the performance of an MC-CDMA system in the presence of narrowbandinterference in the case of an uncorrelated Rayleigh fading channel is plotted. OFDMis realized by an inverse FFT with N c = 64 points. The sub-carriers are spaced 20 kHzapart. The useful bandwidth is 1.28 MHz. In all simulations the spreading factor is L = 64where the average received power per chip is normalized to one. Note that the interferencerejection technique, i.e. soft erasing, performs well. The analytical curves are also plottedand match with the simulation results.