 rmm [n, k] tMTIn, Channel Estimation Hill, k] Figure 2.7 The structure of the MIMO-OFDM system. OFDM offers fully scalable bandwidth to suit varying spectrum allocation and frequency-domain scheduling. Meanwhile, OFDM could be easily combined with other multiple schemes, including FDMA (frequency division multiple access), OFDMA, TDMA, CDMA, and SDMA, in order to support multirate services and to achieve a frequency reuse factor of one. OFDM-CDMA assigns a subset of orthogonal codes to each user; thus, information symbols are spread in either frequency domain or time domain. In OFDMA systems, the signal of each user is transmitted via a set of subcarriers within the duration of several OFDM symbols. In our proposal, the interleaved OFDMA/TDMA is employed in forward link and the localized OFDMA/TDMA in reverse link. OFDM is also very suitable for MIMO transmission. As each subcarrier is flat fading, the MIMO signal processing for the frequency selective fading channel can be performed for each subcarrier, which is substantially simplified. The structure of the V-BLAST-based MIMO-OFDM system can be expressed as in Figure 2.7. 2.2.2.2.5 Modulation Scheme The FuTURE TDD system supports QPSK (quadrature phase shift keying), 16 QAM (quadrature amplitude modulation), and 64 QAM. Even higher-order mod- ulation, such as 128 QAM, is also considered for the downlink. 2.2.2.2.6 MIMO and Transmit Diversity MIMO structure should be adopted in the FuTURE TDD system in order to achieve high data transmission rates. As a research hotspot and potential tech- nique for future communication, MIMO is able to enhance the spectrum efficiency and improve transmission performance by taking advantages of spatial resources. Analysis and simulations prove that MIMO can provide high spectrum efficiency © 2009 by Taylor & Francis Group, LLC Evolution from TD-SCDMA to FuTURE of 20-40 bits/s/Hz. As an important branch, MIMO techniques based on transmit diversity (such as STTC; space-time block code, STBC) can obtain performance gain by repetit