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

Multi-Carrier FDMA 113It should be emphasized that the transmitter and the receiver structure of an OFDMAsystem is quite similar to an OFDM system. The same components, like FFT, channelestimation, equalization and soft channel decoding, can be used for both systems.In order to offer a variety of multi-media services requiring different data rates, theOFDMA scheme needs to be flexible in terms of data rate assignment. This can beachieved by assigning the required number of sub-carriers according to the bandwidthrequest of a given user. This method of assignment is part of an MAC protocol at thebase station.Note that if the number of assigned sub-carriers is an integer power of two, theinverse FFT can be used at the terminal station transmitter, which will be equivalent to aconventional OFDM transmitter. OFDMA is adopted in IEEE802.16e/WiMAX as theaccess scheme for the uplink. This is due to its low power consumption and its highflexibility.3.2.2 OFDMA with Code Division Multiplexing: SS-MC-MAThe extension of OFDMA by code division multiplexing (CDM) results in a multipleaccess scheme referred to as spread spectrum multi-carrier multiple access (SS-MC-MA) [18, 19]. It applies OFDMA for user separation and additionally uses CDM ondata symbols belonging to the same user. The CDM component is introduced in order toachieve additional diversity gains. Like MC-CDMA, SS-MC-MA exploits the advantagesgiven by the combination of the spread spectrum technique and multi-carrier modulation.The SS-MC-MA scheme is similar to the MC-CDMA transmitter with M-Modification.Both transmitters are identical except for the mapping of the user data to the subsystems.In SS-MC-MA systems, one user maps L data symbols to one sub-system, whichthis user exclusively uses for transmission. Different users use different sub-systems inSS-MC-MA systems. In MC-CDMA systems, M data symbols per user are mapped to Mdifferent sub-systems, where each sub-system is shared by different users. The principleof SS-MC-MA is illustrated for a downlink transmitter in Figure 3-4.L data symbolsof user 0...L data symbolsof user K − 1d 0(0)(0)d L−1d 0( K−1)d L−1( K−1)spreaderc (0)...spreaderc (L−1)spreaderc (0)...spreaderc (L−1)+...s (0)s (K−1)+serial-to-parallelconverterserial-to-parallelconverter0...L − 10...frequency interleaving/hopping0...L − 1 N c − 1OFDMxFigure 3-4SS-MC-MA downlink transmitter