 2.6. 2.2.2.2.4 OFDM Modulation OFDM is an attractive method of transmitting high-rate information over highly dispersive mobile radio channels by dividing the serial input data stream into a number of parallel streams and transmitting these low-rate parallel streams simul- taneously. As unquestionable proof of its maturity, OFDM was standardized as the European digital audio broadcast (DAB) as well as the digital video broadcast (DVB) scheme. It was also selected as the high-performance local area network (HIPERLAN) transmission technique as well as part of the IEEE802.11 wireless local area network (WLAN) standard. Furthermore, it has become the E3G mobile radio standard. OFDM converts a frequency-selective channel into a parallel collection of fre- quency-flat subchannels. The subcarriers have the minimum frequency separation © 2009 by Taylor & Francis Group, LLC Evolution from TD-SCDMA to FuTURE Frequency (subcarrier) Subcarrier group 1 Subcarrier group 2 Subcarrier group 3 User 1 User 2 Figure 2.5 Two-dimensional resource allocation. required to maintain orthogonality of their corresponding time domain wave- forms, yet the signal spectra corresponding to the different subcarriers overlap in frequency. Hence, the available bandwidth is used very efficiently. OFDM is robust against multipath fading because the intersymbol interference (ISI) is com- pletely eliminated by introducing a CP or a guard interval of each OFDM symbol. Meanwhile, the intercarrier interference (ICI) is also avoided, which is SO crucial in high data rate transmission. Moreover, the CP enables the receiver to use fast signal processing transforms such as a fast Fourier transform (FFT) for OFDM implementation, thus dramatically reducing the complexity. SDMA/TDMA SDMA/FDMA FDMA/TDMA FDMA/ TDMA/SDMA Figure 2.6 Flexible combinations of multiple access schemes. C 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Radio Channel [n, k] r [n, k] t1[n, k] t2[n, k] r2[n,k t2[n, k b[n, k] b[n, k] MTIn, k]