WiMAX RF physical layer, & modulation

- an overview, summary or tutorial about the WiMAX RF physical layer with the use of WiMAX, OFDM, WiMAX MIMO and modulation

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 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 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 other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..

Click on the link for an OFDM tutorial


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.

WiMAX MIMO

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:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

Click on the link for a MIMO tutorial


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.


Parameter Downlink Uplink
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 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)
Attribute 1.25 3.5 5 10
Physical layer modulation / access mode 128 OFDMA 256 OFDM 512 OFDMA 1024 OFDMA
Oversampling 28/25 8/7 28/25 28/25

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)
Channel B/W 1.25 3.5 5 10
Modulation
& code rate
Downlink Uplink Downlink Uplink Downlink Uplink Downlink Uplink
BPSK
1/2
-- -- 946 326 -- -- -- --
QPSK
1/2
504 154 1882 653 2520 653 5040 1344
QPSK
3/4
756 230 2822 979 3870 979 7560 2016
16QAM
1/2
1008 307 3763 1306 5040 1306 10 080 2688
16QAM
3/4
1512 461 5645 1958 7560 1958 15 120 4032
64QAM
1/2
1512 461 5645 1958 7560 1958 15 120 4032
64QAM
2/3
2016 614 7526 2611 10 080 2611 20 160 5376
64QAM
3/4
2268 691 8467 2938 11 340 2938 22 680 6048
64QAM
5/6
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.

By Ian Poole


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