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

38 FundamentalsFrequency hopping (FH) is similar to direct sequence spreading where a code is usedto spread the signal over a much larger bandwidth than that required to transmit thesignal. However, instead of spreading the signal over a continuous bandwidth by mixingthe signal with a code, the signal bandwidth is unchanged and is hopped over a numberof channels, each having the same bandwidth as the transmitted signal. Although at anyinstant the transmit power level in any narrowband region is higher than with DS-CDMA,the signal is present in a particular channel for a very small time period.For detection, the receiver must know the hopping pattern in advance, otherwise it willbe very difficult to detect the signal. It is the function of the PN code to ensure that allfrequencies in the total available bandwidth are optimally used.There are two kinds of frequency hopping [14]: slow frequency hopping (SFH) andfast frequency hopping (FFH). With SFH many symbols are transmitted per hop. FFHmeans that there are many hops per symbol. FFH is more resistant to jamming but it ismore complex to implement since fast frequency synthesizers are required.In order to reduce complexity, a hybrid DS/FH scheme can be considered. Here, thesignal is first spread over a bandwidth as in DS-CDMA and then hopped over a numberof channels, each with a bandwidth equal to the bandwidth of the DS spread signal. Thisallows one to use a much larger bandwidth than with conventional DS spreading by usinglow cost available components. For instance, if we have a 1 GHz spectrum available, aPN code generator producing 10 9 chips/s or hopping achieving 10 9 hops/s might not bepracticable. Alternatively, we could use two code generators: one for spreading the signaland the other for producing the hopping pattern. Both codes could be generated usinglow cost components.1.3.1 Direct Sequence Code Division Multiple AccessThe principle of DS-CDMA is to spread a data symbol with a spreading sequence c (k) (t)of length L,L−1∑c (k) (t) = c (k)lp Tc (t − lT c ), (1.51)l=0assigned to user k, k = 0,...,K − 1, where K is the total number of active users. Therectangular pulse p Tc (t) isequalto1for0≤ t