IEEE 802.11a

IEEE 802.11a was one of the first Wi-Fi standards to be launched - it provided the capability for raw data speeds of up to 54 Mbps at 5 GHz.


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The IEEE 802.11a standard is the first standard in the IEEE 802.11 series. It defines a WiFi format for providing wireless connectivity in the 5 GHz ISM band to give raw data speeds of up to 54Mbps.

Although, alphabetically it is the first standard in the 802.11 series, t was released at the same time as IEEE 802.11b which was aimed at connectivity using the 2.4 GHz ISM band.

Using the technology of the time, IEEE 802.11a was more costly and a little more difficult to implement as it operated at 5 GHz rather than 2.4 GHz and as a result it was less widely used.

802.11a specification

802.11a boasted an impressive level of performance. It was able to transfer data with raw data rates up to 54 Mbps and at the time it was thought to have a good range, although it could not provide the maximum data rate at its extremes.

Summary of 802.11 Wi-Fi Standards
 
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

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 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.

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.


The adoption of the IEEE 802.11a standard was less than that of 802.11b as a result of the higher frequencies involved. Although technology has moved on significantly since then the 5GHz requirement for 802.11a meant that it was considerably less widely deployed than the “b” version which operated at 2.4 GHz. Also Wi-Fi hotspots tended to focus less on 802.11a. Despite this Wi-Fi as a whole moved forwards considerably and 802.11a was used, but less than other versions.



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