sing RLSS R-DPICH Insert in All Hop Patterns Modulation Tiles Symbols with R-ODCH Sum of (B), (C), Inverse Overlap- Upconversion (D) and (E) Fourier Windowing and-Add on each Transform Operation Operation subcarrier Operation Figure 6.10 Structure of the Uplink OFDMA and CDMA. Reproduced with permission from C 2008 IEEE [4] Note: The details of OFDMA channels as shown in Figure 6.10 can be found in [4]. FLSS = Forward Link Serving Sector; RLSS = Reverser Link Serving Sector. as often as every OFDM symbol and is different for different sectors. The CDMA channels consist of one or more CDMA sub-segments where each sub-segment consists of 128 contiguous sub-carriers. All CDMA sub-segments are hopped only among the CDMA hopping zones. The structure of uplink OFDMA and CDMA is shown in Figure 6.10. As shown, the sequences of NFFT complex modulations symbols from both CDMA and OFDMA segments are converted into a complex baseband waveform using an Inverse FFT operation, a window operation and an overlap operation. The resultant waveform is then unconverted and amplified to be transmitted towards the access network. 6.5.2 Wideband Mode Key Features This section will highlight some key features of the IEEE 802.20 standard. Next Generation Mobile Communications Ecosystem 6.5.2.1 Multi-Antenna Techniques The air interface specifics a number of multi-antenna techniques to improve the end user experience. The air interface supports two modes of operation for the Lower MAC sublayer and physical layer namely Single-input-single-output (SISO) mode and Multiple-input-multiple-output (MIMO) mode. The MIMO mode further supports both multiple codeword (MCW)4 and single codeword (SCW) operations. According to 802.20 specifications all MBWA terminals should support SISO mode. A MIMO-capable terminal can support either the MIMO SCW or MIMO MCW. The MIMO mode requires at least two antennas at the base station and at least two antennas at the access terminal [4]. Beamforming in the downlink increases user data rates by focusing the trans