n Fig. 3.30, which shows the baseband structure of a general multicarrier transmitter that is applicable to a variety of multicarrier MA schemes such as OFDMA and SC-FDMA. Blocks of data represented by vector s of size M X 1 are precoded with an MXM precoding matrix P. The M X output vector is then mapped to M out of N inputs of the inverse-DFT block according to the subcarrier mapping N transform matrix L. To overcome the effects of frequency- selective channel fading, a CP of length NCP is appended to beginning of each NX1 block output by the inverse-DFT function. Transmission with different rates among users is available according to each user's requirement, as a different number of Chapter 3 Subcarrier Inverse Cyclic mapping Precoding Fourier (zero insertion) prefix transform insertion Multicarrier (Size N) (M<N) data symbols (N+Ncp)x1 Data block (Mx1Vector) Figure 3.30 General multicarrier transmission scheme [47]. subcarriers and a different modulation and coding schemes can be applied to each user. Let s(n) denote the information symbols which are parsed into data blocks of size M. The ith data block Si can be written as S = [s(iM), s(iM+M-1)]T. Let's further denote by X the Kronecker product, by O the all-zero matrix of size MXN and by I the MXM identity matrix. We assume that the size of the inverse DFT is a multiple of the block size N = MK. The special case of P = I results in OFDMA where the user-specific data blocks are mapped to a subset of M <N subcarriers, which are selected by the user-specific subcarrier mapping matrix L. The vector Ns is fed to the inverse-DFT function. The form of the matrix N might lead to either a localized or a distributed subcarrier mapping as follows: 01 SQNXM=IMXM x Onx1 By assigning different groups of subcarriers to different users, each user's transmit power can be concentrated in a restricted part of the channel bandwidth, resulting in significant coverage enhancement. Different user signals remain orthogonal only if time/frequency syn- chronization is maintained and