Implementation of FDMA requires separate demodulators at the base station for each frequency, whereas OFDMA needs one demodulator followed by a Fast Fourier transform processor. Thus, FDMA is limited to applications with a small number of users [9]. 2.7.1 OFDMA Advantages A lot of R&D hours have been invested in finding the true potential of different multiple access schemes. OFDMA which is the newest of the three (TDMA and CDMA being the other two) has been recognized as the most feasible multiple access technique for broadband data services, as we will see in this section. OFDMA provides a number of advantages over CDMA and TDMA but at the same time they do have some shortcomings as discussed earlier. We will briefly discuss the key advantages of OFDMA over TDMA and CDMA in this section [5, 9, 10]. 2.7.1.1 Scalability On the top of the list is the scalability factor that OFDMA provides over CDMA and TDMA. OFDMA subcarrier structure supports a wide range of bandwidth. The scalability is achieved by adjusting FFT size³ to channel bandwidth while fixing the sub-carrier frequency spacing. For example, in WiMAX, sub-carrier spacing is fixed to 10.94 kHz while FFT size varies (128, 256, 512, 1024, 2048) with the different channel bandwidth sizes (1.25, 2.5, 5, 10, 20 MHz) respectively. One immediate advantage stemming from scalability is the flexibility of deployment. OFDMA systems can be deployed in various frequency band intervals to flexibly address the need for various spectrum allocation and usage model requirements. TDMA and CDMA based systems however, do not provide such flexibility in the traditional sense. For example, CDMA2000 and EV-DO mainly offer 1.25 MHz channel bandwidth while channel bandwidth is fixed to 5 MHz in WCDMA and HSPA systems. 3 FFT size is the length of FFT. Next Generation Mobile Communications Ecosystem 2.7.1.2 Robustness to Multipath In OFDMA systems, subchannels maintain orthogonality in multi-path channels. The number of multi- path components does not limit the performance of the sys