how does the 802.16 WiMAX standard compare to the 802.11 WLAN standard?

An article in CommsDesign explains:

To start, both are based on orthogonal frequency division multiplexing (OFDM), use multiple pilot tones, and support modulations ranging from BPSK to 64 QAM.

But there are some major differences as well. For instance, rather than a fixed 20-MHz bandwidth with 52 subcarriers as in 802.11, WiMAX systems can use variable bandwidths from 1 to 28 MHz with 256 subcarriers (192 data subcarriers) in either licensed or unlicensed spectrum. The first WiMAX rollouts are expected to use 3.5- and 7-MHz channel bandwidths.

WiMAX supports subchannelization, meaning that instead of transmitting on all 192 data subcarriers, you can transmit on just a subset. In this scenario, by using the same amount of power over fewer carriers, the system achieves greater range.

As WiMAX CPE evolves into in-building devices, it'll be necessary to make up for the power loss incurred when transmitting the signal outside the building. Because CPE is typically limited in power, concentrating the power over fewer subcarriers in the uplink can balance the power in the uplink and downlink, and enable greater range.

While the larger number of subcarriers gives WiMAX an advantage over 802.11, the resulting challenge to the system design is that the subcarriers are spaced more closely together, so there are tighter requirements for phase noise and timing jitter. This translates to a need for higher-performance synthesizers.

WiMAX also uses a variable-length guard interval to improve performance in multi-path environments. The guard interval is a time delay at the beginning of the packet to compensate for multi-path interference. With a very clear channel, the guard interval can be shortened, increasing the throughput. With more subcarriers, and with a variable-length guard interval, a WiMAX system's overall spectral efficiency will be 15 to 40% higher than a WLAN system. For instance, WiMAX achieves a spectral efficiency ranging from 3.1 to 3.8 Mbits/s/MHz, compared to only 2.7 Mbits/s/MHz for 802.11a/b/g (see the table).

Error-vector magnitude (EVM) requirements for 802.11 are specified at -25 dB, which is required to achieve a 10% packet error rate. For 802.16, EVM is held to -31 dB, which is based on a 1% packet error rate. This lower error rate helps contribute to WiMAX's longer range. Also contributing to the longer range is the receiver noise figure, which is more stringent for 802.16. Specifically, 802.11's maximum noise figure is 10 dB, while 802.16 operates at 7 dB.

802.11 only supports time division duplexing (TDD), where transmit and receive (Tx/Rx) functions occur on the same channel, but at different times. In comparison, the 802.16 spec offers more flexibility, supporting TDD, frequency division duplexing (FDD), and half-duplex FDD (H-FDD). FDD uses simultaneous Tx/Rx on different frequencies; H-FDD transmits on different channels at different times. The approach that designers select affects cost, footprint, and design time. For example, an FDD system will cost more because simultaneous Tx/Rx requires two complete radios. However, FDD will allow greater throughput, as bandwidth is dedicated for receive and transmit, and this bandwidth is used simultaneously.

Another significant difference between WiMAX and 802.11 is ranging and transmit dynamic range. In 802.11, the output power is virtually fixed, and systems typically transmit at the same power all the time. However, for WiMAX, a ranging process determines the correct timing offset and power settings. This process ensures that transmissions from each subscriber station arrive at the base station at the proper time and at the same power level.

As a result, the 802.16 standard requires that subscriber stations have a 50-dB transmit dynamic range. This allows systems that are close to the base station to back off their transmit power, while those far away can transmit at maximum power. This is significant because WiMAX supports transmit ranges of several kilometers, and transmitting at maximum power near the base station would be disastrous.