The use of WiMAX as a wireless broadband technology is established with a variety of manufacturers are producing WiMAX equipment. One of the areas of particular interest is the WiMAX RF 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 multiples of 128 carriers in a total signal bandwidth that may range from 1.25 to 20 MHz.
Note on OFDM:
Orthogonal Frequency Division Multiplex, OFDM is a form of signal format that uses a large number of close spaced carriers that are each modulated with low rate data stream. The close spaced signals would normally be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is shared across all the carriers and this provides resilience against selective fading from multi-path effects.
Read more about OFDM, Orthogonal Frequency Division Multiplexing.
The WiMAX signal bandwidth can be set to a figure between 1.25 and 20 MHz. 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 possess.
More advanced versions including 802.16e utilise MIMO (Multiple Input Multiple Output) and as a result they support multiple antennas. The use of these techniques provides potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency.
Note on MIMO:
MIMO is a form of antenna technology that uses multiple antennas to enable signals travelling via different paths as a result of reflections, etc., to be separated and their capability used to improve the data throughput and / or the signal to noise ratio, thereby improving system performance.
Read more about MIMO technology
WiMAX adaptive modulation and coding
WiMAX modulation and coding is adaptive, enabling it to vary these parameters according to prevailing conditions. WiMAx modulation and coding can be changed on a burst by burst basis per link. To determine the required WiMAX modulation and coding scheme the channel quality feedback indicator is used. The mobile can provide the base station with feedback on the downlink channel quality and for the uplink, the base station can estimate the channel quality, based on the received signal quality.
|Modulation||BPSK, QPSK, 16 QAM, 64 QAM; BPSK optional for OFDMA-PHY||BPSK, QPSK, 16 QAM; 64 QAM optional|
|Coding|| Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6|
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC, RS-Codes for OFDM-PHY
|Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6|
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC
WiMAX physical layer data rates
One of the key performance factors of any wireless broadband system is the data rates that can be achieved. As WiMAX is particularly flexible in terms of channel bandwidth, modulation and also the coding scheme, these can significantly vary the data rates that can be achieved.
A summary of the different modulation access / modulation technologies and oversampling rates is given in the table below:
|Channel Bandwidth (MHz)|
|Physical layer modulation / access mode||128 OFDMA||256 OFDM||512 OFDMA||1024 OFDMA|
The table below gives a summary of the physical later data rates that may be achieved using different WiMAX modulation, coding and channel bandwidths.
|Physical layer data rate (kbps)|
& code rate
|2016||614||7526||2611||10 080||2611||20 160||5376|
|2268||691||8467||2938||11 340||2938||22 680||6048|
|2520||768||9408||3264||12 600||3264||25 200||6720|
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.
Using this RF interface, WiMAX is able to provide a very effective form of wireless brioadband sysstem that can be sued in many areas.
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