**3. OFDMA and SOFDMA**

To enhance the performance of OFDM flexible extensions have been developed. These include Orthogonal Frequency Division Multiple Access (OFDMA) and Scalable OFDMA (SOFDMA). The OFDMA feature schedules a varying number of subcarriers to each subscriber depending on needs, channel conditions or both. This gives rise to the concept of flexible sub-channelization of the bandwidth. The access domain is further enhanced by multiple access technologies of FDMA and TOMA. SOFDMA on the other hand is used on the transmission end to 'scale' the channel bandwidth. By enabling the adjustment of the FFT size hence the number of carriers to the transmission channel bandwidth, SOFDMA brings scalability to OFDM. OFDMA thus incorporates features of the Code Division Multiple Access (CDMA) by combining OFDMA and allowing lowdata-rate users to transmit continuously at lower power with shorter and constant delay. A similar concept applies in the frequency domain and TOMA where the resources are partitioned in the time-frequency space, and slots are assigned along the OFDM symbol subcarrier indices [8].

Certain frequency selective impacts can be minimized by spreading subcarriers of a user over the entire channel spectrum in addition S/OFDMA can be configured for Adaptive Antenna Systems (AAS) enhancing payload and coverage. OFDMA also comes with better high -performance coding techniques such as Turbo Coding and Low-Density Parity Check (LDPC), enhancing security and NLOS performance increase system gain by use of denser sub-channelization, thereby improving indoor penetration [7]. The sum effect of subchannelization is that the link budget is greatly enhanced by allocating each subscriber one or several portions of the overall bandwidth and NOT the entire channel bandwidth. The immediate benefit of this technique is not hard to discern since the UL-DL budgets can then be easily balanced. With a good scheduling technique a fair throughput trade-off can be optimized at cell edge and overall UL data rate.

An OFDMA transmission is shown in Fig. 6 in the frequency domain, with the Pilot Subcarriers used for channel estimations, the DC subcarrier is the centre frequency, unused, the SS1/2 Data Subcarriers are the user data subcarriers scaled for subscribers 1&2 and the Guard band is to limit ISi and channel decay .
