WiMax air interface / RF physical layer tutorial

- an overview, summary or tutorial about the WiMax physical layer or air interface as defined in IEEE 802.16

The use of WiMax is starting to grow rapidly, and many manufacturers are producing WiMax equipment. One of the areas of particular interest is the physical layer, or air interface as this governs the radio signal that is transmitted and received.

The WiMax, 802.16-2004 standard describes four different RF or air interfaces dependent upon the application envisaged. Of these the one that is intended for non-line of sight applications up to 30 km and for frequencies below 11 GHz is the most widely implemented at the moment. As a result it is often thought of as the WiMax air interface.

Basics of the WiMax air interface

The WiMax RF signal uses OFDM (orthogonal frequency division multiplex) techniques and the signal incorporates 256 carriers in a total signal bandwidth that may range from 1.25 to 20 MHz. Of the 256 carriers possible only 200 are actually used. Some are not used as the frequencies that would be occupied by them are used as a guard band, and the centre frequency carrier is not used because it is very susceptible to RF carrier feed-through.

Note on OFDM: Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. Further information on OFDM can be found on this site under the Cellular telecoms section or by using the Search facility.

The total of 200 carriers used are split between 192 that are used for data payload, and the remaining 8 that are used as pilots. The pilot carriers are always BPSK modulated and the data carriers are BPSK, QPSK, 16 QAM, or 64 QAM.

The WiMax signal bandwidth can be set to a figure between 1.25 and 20 MHz. Regardless of the bandwidth the WiMax signal contains the same 200 carriers. Thus the carrier spacing varies according to the overall bandwidth. To maintain orthogonality between the individual carriers the symbol period must be the reciprocal of the carrier spacing. As a result narrow bandwidth WiMax systems have a longer symbol period. The advantage of a longer symbol period is that this helps overcome problems such as multipath interference that is prevalent on non-line of sight applications. This is a great advantage that WiMax systems posses.

WiMax data structure

Although WiMax can be deployed as TDD (Time Division Duplex), FDD (Frequency Division Duplex) and half duplex FDD, the most common arrangement is the TDD mode. His allows for a greater efficiency in spectrum usage than FDD mode.

Using TDD mode the WiMax base station and the end users transmit on the same frequency, but to enable them not to interfere with each other their transmissions are separated in time. In order to achieve this the base station first transmits a subframe and this is followed by a short gap which is called the Transmit/receive Transition Gap (TTG). After this gap, the users or remote stations are able to transmit their subframes. The timing of these "uplink" subframes needs to be accurately controlled and synchronised so that they do not overlap whatever distance they are from the base station. Once all the uplink subframes have been transmitted, another short gap known as the Receive/transmit Transition Gap (RTG) is left before the basestation transmits again.

There are slight differences between the WiMax subframes transmitted on the uplink and downlink. The downlink subframe begins with a preamble, after which a header is transmitted and this is followed by one or more bursts of data. The modulation within a subframe may change, but it remains the same within an individual burst. Nevertheless it is possible for the modulation type to change from one burst to the next. The first bursts to be transmitted use the more resilient forms of modulation such as BPSK and QPSK. Later bursts may use the less resilient forms of modulation such as 16 QAM and 64 QAM that enable more data to be carried.