Page 166 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes
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Caution In reviewing the standard, the technical details and features that differentiate
WiMAX certified equipment from WiFi or other technologies can best be illustrated
by focusing on the two layers addressed in the standard, the physical (PHY) or RF
transmissions and the media access control (MAC) layer design.

10.2.1 WiMAX and the IEEE 802.16a PHY Layer

The first version of the 802.16 standard released addressed Line-of-Sight (LOS) environments
at high frequency bands operating in the 10-66 GHz range, whereas the recently adopted
amendment, the 802.16a standard, is designed for systems operating in bands between 2 GHz
and 11 GHz. The significant difference between these two frequency bands lies in the ability
to support Non-Line-of-Sight (NLOS) operation in the lower frequencies, something that is not
possible in higher bands. Consequently, the 802.16a amendment to the standard opened up
the opportunity for major changes to the PHY layer specifications specifically to address the
needs of the 2-11 GHz bands. This is achieved through the introduction of three new PHY-layer
specifications (a new Single Carrier PHY, a 256 point FFT OFDM PHY, and a 2048 point FFT
OFDMA PHY);major changes to the PHY layer specification as compared to the upper frequency,
as well as significant MAC-layer enhancements. Although multiple PHYs are specified as in the
802.11 suite of standards (few recall that infrared and frequency hopping were and are part of the
base 802.11 standard), the WiMAX Forum has determined that the first interoperable test plans
and eventual certification will support the 256 point FFT OFDM PHY (which is common between
802.16a and ETSI HiperMAN), with the others to be developed as the market requires.
The OFDM signaling format was selected in preference to competing formats such as CDMA
due to its ability to support NLOS performance while maintaining a high level of spectral
efficiency maximizing the use of available spectrum. In the case of CDMA (prevalent in 2G
and 3G standards), the RF bandwidth must be much larger than the data throughput, in order
to maintain processing gain adequate to overcome interference. This is clearly impractical for
broadband wireless below 11 GHz, since for example, data rates up to 70 Mbps would require RF
bandwidths exceeding 200 MHz to deliver comparable processing gains and NLOS performance.
Some of the other PHY layer features of 802.16a that are instrumental in giving this technology
the power to deliver robust performance in a broad range of channel environments are; flexible
channel widths, adaptive burst profiles, forward error correction with concatenated Reed-
Solomon and convolutional encoding, optional AAS (advanced antenna systems) to improve
range/capacity, DFS (dynamic frequency selection)-which helps in minimizing interference,
and STC (space-time coding) to enhance performance in fading environments through spatial
diversity.

10.2.2 IEEE 802.16a MAC Layer

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. While this on the surface sounds like a one line, technical
throwaway statement, it has a huge impact on how the system operates and what services it can
deploy. By starting with a TDMA approach with intelligent scheduling, WiMAX systems will be
able to deliver not only high speed data with SLAs, but latency sensitive services such as voice
and video or database access are also supported.
The standard delivers QoS beyond mere prioritization, a technique that is very limited in
effectiveness as traffic load and the number of subscribers increases. 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.

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