requency spacing at 10.94 kHz. Since the resource unit sub-carrier bandwidth and symbol duration is fixed, the impact to higher layers is minimal when scaling the bandwidth. The S-OFDMA parameters are shown in Table 4.1 [4]. 4.3 Advanced Features of Mobile WiMAX Mobile WiMAX provides support features like smart antennas, fractional frequency reuse, multicast and broadcast service that can enhance its performance. These features are briefly described in this section. 4.3.1 Smart Antennas A smart antenna system combines multiple antenna elements with signal processing capabilities to optimize its radiation and/or reception pattern in response to the signal environment. It takes advantage of diversity effect at the source (transmitter), the destination (receiver), or both. Diversity effect involves the transmission and/or reception of multiple radio frequency (RF) waves to increase data speed and reduce the error rate. Smart antennas could include either switched beam smart antennas or adaptive array smart antennas. Switched beams systems use a number of fixed beams at the transmitter site. The receiver selects the beam that provides largest enrichment in the signal and reduction in interference. Adaptive systems include an array of antennas that can change its pattern dynamically to adjust to noise, interference, multipath also according to the mobility of the users. Smart antennas are used to improve the signal quality which in turn increases the coverage and capacity of the systems. Details on smart antennas can be found in [7]. Mobile WiMAX supports multiple antennas systems to enhance its performance. These smart antenna technologies can be classified into Receive and Transmit Diversity and MIMO as shown in Figure 4.7. 4.3.1.1 Receive Diversity Receive Diversity typically involves the use of multiple receive antennas at the base station and it is a common practice in 2G and 3G cellular systems. Receiver diversity (dual antennas) technology can be found in terminals, integrated laptop solutions and PCMCIA cards.