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

Multi-Carrier FDMA 1073.2 Multi-Carrier FDMAThe concept of the combination of spread spectrum and frequency hopping with multicarriertransmission opened the door for alternative hybrid multiple access solutions suchas: OFDMA [28], OFDMA with CDM (SS-MC-MA) [18], distributed DFT-spread OFDM(Interleaved FDMA) [35], and localized DFT-spread OFDM. All of these schemes arediscussed in the following.3.2.1 Orthogonal Frequency Division Multiple Access (OFDMA)3.2.1.1 Basic PrincipleOrthogonal frequency division multiple access (OFDMA) consists of assigning one orseveral sub-carrier frequencies to each user (terminal station) with the constraint that thesub-carrier spacing is equal to the OFDM frequency spacing 1/T s (see References [28]and [30] to [32]).To introduce the basic principle of OFDMA we will make the following assumptions:– One sub-carrier is assigned per user (the generalization for several sub-carriers peruser is straightforward).– In addition, the only source of disturbance is AWGN.The signal of user k,k = 0, 1,...,K –1,whereK = N c ,hastheforms (k) (t) = Re{d (k) (t)e j2πf kt e j2πf ct }, (3.1)withf k = k T s(3.2)and f c representing the carrier frequency. Furthermore, we assume that the frequency f k ispermanently assigned to user k, although in practice a frequency assignment could be madeupon request. Therefore, an OFDMA system with, for example, N c = 1024 sub-carriersand adaptive sub-carrier allocation is able to handle up to thousands of simultaneoususers.In the following, we consider a permanent channel assignment scheme in which thenumber of sub-carriers is equal to the number of users. Under this assumption the modulatorof the terminal station of user k has the form of an unfiltered modulator with arectangular pulse (e.g. unfiltered QPSK) and carrier f k + f c . The transmitted data symbolsare given byd (k) (t) =+∞∑i=−∞d (k)irect(t − iT s ), (3.3)