802.11a
Orthogonal Frequency Division Multiplexing (OFDM)
802.11a uses Orthogonal Frequency Division Multiplexing (OFDM) and there are 12 channels
in the 5GHz bands. The lowest 8 channels
currently used lie in the 100MHz bands UNII-1 and UNII-2. In the USA UNII-1 can only be used indoors.
Both UNII-2 and UNII-3 can be used indoors or outdoors. 802.11h adds a further 11 channels plus the use
of UNII-3. Sidebands can cause interference if you use adjacent channels.
OFDM breaks up each 20MHz high speed carrier into 64 subchannels that are about 300KHz wide. 48 of these
subchannels are used for data, while 4 are used for monitoring path shifts, InterCarrier Interference (ICI).
The remaining 12 subchannels are called Zero Subcarriers and these are
located on the sides as frequency guard bands thereby giving 16.5MHz occupied bandwidth.
The central zero is used for DC offset/carrier leak rejection.
Modulation
The modulation used with sub-channels varies depending on the data rates. Data rates up to 24Mbps have to be supported
according to the standard. In the table below 16-QAM means 16-state Quadrature Amplitude Modulation and
64-QAM is 64-state QAM.
| Modulation Technique |
Data rate per subchannel (Kbps) |
Total Data Rate (Mbps) |
| BPSK |
125 |
6 |
| BPSK |
187.5 |
9 |
| QPSK |
250 |
12 |
| QPSK |
375 |
18 |
| 16-QAM |
500 |
24 |
| 16-QAM |
750 |
36 |
| 64-QAM |
1000 |
48 |
| 64-QAM |
1125 |
54 |
The 5GHz frequency bands are divided up as follows:
- UNII-1 - 5.15GHz to 5.25GHz for indoor use only with 6dBi integrated antennae and a maximum power output
of 40mW. This has the channels, 34 (5.170GHz), 36 (5.180GHz), 38 (5.190GHz), 40 (5.200GHz), 42 (5.210GHz), 44 (5.220GHz),
46 (5.230GHz), 48 (5.240GHz).
- UNII-2 - 5.25GHz to 5.35GHz can use removeable antennae and can transmit at up to 200mW.
This has the channels, 52 (5.260GHz), 56 (5.280GHz), 60 (5.300GHz), 64 (5.320GHz).
- UNII-3 - 5.725GHz to 5.825GHz can transmit at up to 1W with 6dBi antennae for point-to-multipoint and 23dBi antennae
for point-to-point. This has the channels, 5.745GHz, 5.765GHz, 5.785GHz, 5.805GHz.
As with 802.11b/g, data rate shifting can occur as the client moves away from the AP. This occurs on a transmission
by transmission basis in the following steps 54Mbps -> 48Mbps -> 36Mbps -> 24Mbps -> 18Mbps -> 12Mbps.
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