On WiMax MAC layer

History ...
For years, the wildly successful 802.11x or WiFi wireless LAN technology has been used in Broadband Wireless Access (BWA) applications along with a host of proprietary based solutions. Experts reviewed the WLAN technology closely, and it was felt that the overall design and feature set available was not well suited for outdoor BWA applications. The deployment was done with limited capacity in terms of bandwidth and subscribers.

WiMax Introduction
The IEEE conducted a multi-year effort to develop this new standard, culminating in final approval of the 802.16a. In this standard, the technical details and features that differentiate WiMAX certified equipment from WiFi or other technologies is evident from the the physical (PHY) and the media access control (MAC) layer design. Wireless networks are highly dependent on communications channels, and the channels are dynamic, correlated, unreliable, and very expensive. This is why performance will be highly dependent on how well radio resource management supports quality-of-service (QoS) requirements. The three different PHY layers in Mobile WiMAX are single carrier, orthogonalfrequency division multiplexing (OFDM), and OFDMA.

The WiMax Mac
Every wireless network operates fundamentally in a shared medium and as such that requires a mechanism for controlling access by subscriber units to the medium. The 802.16a standard uses a slotted TDMA protocol scheduled by the BTS to allocate capacity to subscribers in a point-to-multipoint network topology. The TDMA approach with intelligent scheduling enables WiMAX systems to deliver not only high speed data with SLAs, but latency sensitive services such as voice and video or database access are also. The MAC layer of Mobile WiMAX provides a medium-independent interface to the PHY layer and is designed to support the wireless PHY layer by focusing on efficient radio resource management. The MAC layer supports both PMP and mesh network modes. The MAC layer in WiMAX certified systems has also been designed to address the harsh physical layer environment where interference, fast fading and other phenomena are prevalent in outdoor operation.

Some Features
Below are listed some of the 802.16a MAC Features
- TDM/TDMA Scheduled Uplink/Downlink frames for efficient bandwidth usage
- Scalablity from 1 to hundreds of subscribers allowing cost effective deployments by supporting enough subs to deliver a robust business case
- Automatic Retransmission request (ARQ) which Improves the end-to-end performance by hiding RFlayer induced errors from upper layer protocols
- Support for adaptive modulation to enable highest data rates allowed by channelconditions, thereby improving system capacity
- Automatic Power control for helping cellular deployments to minimize selfinterference

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