IEEE 802.16a standard does not require line- of-sight transmission. It allows use of lower frequencies (2 to 11 GHz). IEEE 802.16a can achieve a range of 50 km and data rates of over 70 Mbps.

IEEE 802.16a provides a new physical layer standard with some modifications to the the 802.16 MAC layer. Compared to the higher frequencies, the 2-to- 11-GHz range offers the opportunity to reach many more customers less expensively, though at generally lower data rates. This suggests that such services will be oriented toward individual homes or small- to medium-sized enterprises. Design of the 2-to-11-GHz physical layer is driven by the need for non-line-of- sight (NLOS) operation. This is essential to support residential applications since rooftops may be too low for a clear sight line to a BS antenna, possibly due to obstruction by trees. Therefore, significant multipath propagation must be expected. Furthermore, outdoor mounted antennas are expensive due to both hardware and installation costs.

The 802.16a standard specifies that systems implement one of three air interface specifications, each of which provides for interoperability:

• WirelessMAN-SCa: This uses a single-carrier modulation format.

• WirelessMAN-OFDM: This uses orthogonal frequency division multiplexing with a 256-point transform. Access is by TDMA.

• WirelessMAN-OFDMA: This uses orthogonal frequency division multiple access with a 2048-point transform. In this system, multiple access is provided by addressing a subset of the multiple carriers to individual receivers.

IEEE 802.16a supports both TDD and FDD modes of operation, along with a range of channel bandwidths.

Fig: Data Rates Achieved at Various 802.16a Bandwidths: