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

118 Hybrid Multiple Access SchemesThe transmission signal x (k) is constructed by element-wise multiplication of the compressedvector s (k) with a user-dependent phase vector c (k) of length (L g + L)Q havingthe componentsc (k)l= e −j2πlk/(QL) , l = 0,...,(L g + L)Q − 1. (3.20)The element-wise multiplication of the two vectors s (k) and c (k) ensures that each useris assigned a set of sub-carriers orthogonal to the sub-carrier sets of all other users. Eachsub-carrier set contains Q sub-carriers and the number of active users is restricted toK ≤ L. (3.21)The IFDMA receiver has to perform an equalization to cope with the ISI which is presentwith IFDMA in multi-path channels. For low numbers of Q, the optimum maximumlikelihood sequence estimation can be applied with reasonable complexity whereas forhigher numbers of Q, less complex sub-optimum detection techniques such as linearequalization or decision feedback equalization are required to deal with the ISI.Due to its low PAPR, a practical application of IFDMA can be an uplink where powerefficientterminal stations are required that benefit from the quasi constant envelope andmore complex receivers that have to cope with ISI being part of the base station.3.2.4 Localized DFT-Spread OFDMLocalized DFT-spread OFDM differs from distributed DFT-spread OFDM only in themapping of the sub-carriers. While with distributed DFT-spread OFDM the sub-carriersof the users are distributed over the whole available bandwidth, with localized DFT-spreadOFDM blocks of adjacent sub-carriers are mapped to a user. Figure 3-8 illustrates thedifferences in sub-carrier mapping between both schemes.Localized DFT-spread OFDM has the advantage that flexible channel dependentscheduling in frequency can be implemented. Moreover, it is more robust to frequencyerrors between signals from different users. The disadvantage compared to distributedDFT-spread OFDM is that the frequency diversity achieved per user is reduced. Bothlocalized and distributed DFT-spread OFDM have the same low PAPR. Due to the lowPAPR and the possibility of flexible user scheduling in frequency, localized DFT-spreadOFDM has been chosen as the access technology for the uplink of LTE (see Chapter 5).The receiver for localized DFT-spread OFDM can efficiently be realized by frequencydomain equalization [5].3.3 Multi-Carrier TDMAThe combination of OFDM and TDMA is referred to as MC-TDMA or OFDM-TDMA.Due to its well understood TDMA component, MC-TDMA is part of several high ratestandards; e.g. WLAN-EEE 802.11 a/g and WLL-IEEE 802.16a/ETSI HIPERMAN [4,10, 11] (see Chapter 5).MC-TDMA transmission is done in a frame manner like in a TDMA system. One timeframe within MC-TDMA has K time slots (or bursts), each allocated to one of the K