Wimax standards and Security The Wimax

Frame Structure

1. 
   Physical Layer
   

When the 802.16 standard was introduced, it had a single carrier (SC) PHY specification to support LOS operations in the 10–66 GHz frequency band. With the 802.16a amendment to the standard, changes to the PHYwere needed to support the 2–11 GHz frequency band. This led to the introduction of a new SC PHY, a 256 point FFT OFDM PHY, and a 2048 point FFT OFDMA PHY.
The 802.16e amendment to the standard provides an enhanced version of OFDMA called scalable OFDMA (SOFDMA) [1].
The SC PHY specification is designed for LOS operation in the 10–66 GHz frequency band. Both TDD and FDD configurations are supported to allow for flexible spectrum usage. The SC PHY is designed for NLOS operation in the 2–11 GHz frequency band and is based on SC technology. The OFDM PHY uses a 256-carrier OFDM and TDMAto provide multiple access to differ- ent subscriber stations. The OFDMA PHY uses a 2048-carrier OFDM design. Multiple access is provided by assigning a subset of the carriers to an indi- vidual subscriber station. The enhancement of the OFDMA PHY, SOFDMA, uses the values 128, 512, 1024, and 2048 to scale the number of subcarriers in a channel [1,5].
The WiMAX Forum decided that the first interoperable tests and certifica- tions for 802.16 devices would support OFDM. While OFDMA can allocate spectrum more efficiently and reduce interference, compared to OFDM it is more complex to install and operate. Therefore, OFDMA is only required for

802.16e certified devices, where it is needed to support mobile customers. The WiMAX Forum has worked with the Korean standard WiBro, which uses SOFDMA to insure the two technologies will be interoperable. Even- tually, SOFDMA will be the PHY layer of choice for indoor and mobile equipment [1].

2. 
   MAC Layer
   

In a WiMAX environment, it can be difficult for subscriber stations to listen to one another. Therefore, the MAC layer was designed to use a flexible frame structure in which the base station schedules subscriber station transmissions in advance. This reduces contention because subscriber stations only need to contend when they access the channel for the first time. The overall effect is increased efficiency, which allows one base station to serve a large number of subscriber stations.

1.