| IEEE 802.11a |
IEEE 802.11a
- an overview or tutorial about the 802.11a the new Wi-Fi standard providing
data rates of 54 Mbps at 5 GHz
The 802.11a standard is alphabetically the first of the
variety of 802.11 standards that are in widespread use today. Although 802.11a
was ratified at the same time as 802.11b, it never caught on in the same way
despite the fact that it offered a much higher data transfer rate. The reason
for this was that it operated in the 5 GHz ISM band rather than the 2.4 GHz
band, and this made chips more expensive. 802.11 was also, possibly ahead of its
time. With the introduction of wireless LAN technology, people were happier to
settle for any connection, and even one with a lower speed. Nevertheless 802.11
did achieve a significant amount of use and it also forced up the speed of other
802.11 technologies running at 2.4 GHz.
802.11a specification
802.11a boasts an impressive performance. It is able to transfer data with raw
data rates up to 54 Mbps, and has a good range, although not when operating at
its full data rate.
| Parameter |
Value |
| Date of standard approval |
July 1999 |
| Maximum data rate (Mbps) |
54 |
| Typical data rate (Mbps) |
25 |
| Typical range indoors (Metres)
|
~30 |
| Modulation |
OFDM |
| RF Band (GHz) |
5 |
| Number of spatial streams |
1 |
| Channel width (MHz) |
20 |
Summary of 802.11 Wi-Fi Standards
The 802.11a standard uses basic 802.11 concepts as its base,
and it operates within the 5GHz Industrial, Scientific and Medical (ISM) band
enabling it to be used worldwide in a licence free band. The modulation is
Orthogonal Frequency Division Multiplexing (OFDM) to enable it to transfer raw
data at a maximum rate of 54 Mbps, although a more realistic practical level is
in the region of the mid 20 Mbps region. The data rate can be reduced to 48, 36,
24, 18, 12, 9 then 6 Mbit/s if required. 802.11a has 12 non-overlapping
channels, 8 dedicated to indoor and 4 to point to point.
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.
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802.11a RF signal
The OFDM signal used for 802.11 comprises 52 subcarriers. Of these 48 are used
for the data transmission and four are sued as pilot subcarriers. The separation
between the individual subcarriers is 0.3125 MHz. This results from the fact
that the 20 MHz bandwidth is divided by 64. Although only 52 subcarriers are
used, occupying a total of 16.6 MHz, the remaining space is used as a guard band
between the different channels.
A variety of forms of modulation can be used on each of the
802.11a subcarriers. BPSK, QPSK, 16-QAM, and 64 QAM can be used as the
conditions permit. For each set data rate there is a corresponding form of
modulation that is used. Within the signal itself the symbol duration is 4
microseconds, and there is a guard interval of 0.8 microseconds.
| Data rate (Mbps) |
Modulation |
Coding rate |
| 6 |
BPSK |
1/2 |
| 9 |
BPSK |
3/4 |
| 12 |
QPSK |
1/2 |
| 18 |
QPSK |
3/4 |
| 24 |
16-QAM |
1/2 |
| 36 |
16-QAM |
3/4 |
| 48 |
64-QAM |
1/2 |
| 54 |
64-QAM |
3/4 |
As with many data transmission systems, the generation of the
signal is performed using digital signal processing techniques and a baseband
signal is generated. This is then upconverted to the final frequency. Similarly
for signal reception, the incoming 802.11a signal is converted down to baseband
and converted to its digital format after which it can be processed digitally.
Although the use of OFDM for a mass produced systems such as
802.11a may appear to be particularly complicated, it offers many advantages.
The use of OFDM provides a significant reduction in the problems iof
interference caused by multipath effects. The use of OFDM also ensures that
there is efficient use of the radio spectrum.
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