| WiMax air interface / RF physical layer tutorial |
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.
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