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. For more information on OFDM click here

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.