Wimax standards and Security The Wimax

802.16 Time Division Multiple Access

In this section, we describe the 802.16 mesh TDMA MAC technology and the research problems posed by this technology. First, we describe the orthog- onal frequency division multiplexing (OFDM) technology at the physical layer, which provides equal-duration time division multiplexing (TDM) slots required for TDMA. We then summarize research activities toward alterna- tive technologies that can provide TDM timing for 802.16 MAC. Second, we describe how TDM slots are grouped into frames and how transmissions are scheduled with logical TDMA channels. The 802.16 standard speci- fies scheduling algorithms for the logical channels used for mesh control messages. We outline the current research into the performance of those algorithms. The scheduling algorithms for data channels are left entirely to the implementation of the standard. We summarize the research proposing TDMA scheduling algorithms for 802.16 networks. We conclude the section with a description of how nodes are assigned their initial TDMA band- width when they enter the network, which resolves a practical problem often ignored in research.

1. 
   802.16 Physical Layer
   

IEEE 802.16 is a TDMA-based MAC protocol built on a TDM physical layer. In TDM physical layers, the time is divided into time slots of equal length, and during each time slot, a block of bytes is transmitted. IEEE 802.16 uses OFDM to implement the TDM physical layer. OFDM transforms blocks of bits


TABLE 8.1
Comparison of 802.11a and 802.16 Raw Data Rates
Raw Bitrate (Megabits/Second)

Modulation