IEEE 802.11a

Amendment to the original 802.11a standard was ratified in 1999. The 802.11a standard uses the same basic protocol as the original standard, operates at 5 GHz and uses orthogonal frequency division multiplexing (OFDM) with a maximum allowable data rate of up to 54 Mbit / s, which in practice provides realistically achievable bandwidth about 20 Mbps. The data rate is reduced to 48, 36, 24, 18, 12, 9, and then to 6 Mbps, if required. Initially, 802.11a had 12/13 disjoint channels, 12 of which can be used indoors, and 4/5 of 12 that can be used in point-to-point configurations outdoors. Recently, many countries of the world allow work at frequencies of 5.47-5.725 GHz as a second user, using the sharing method obtained in 802.11h . This will add another 12/13 channels to the total 5 GHz band, which will provide a significant increase in wireless bandwidth, this will allow more than 24 channels in some countries. 802.11a is not compatible with 802.11b, since they operate in separate bands, except when using equipment with the possibility of two bands. Most enterprise-class access points have dual-band connectivity.

The use of the 5 GHz band gives 802.11a a significant advantage, since the 2.4 GHz band is heavily used to such an extent that it can be crowded. The deterioration caused by such conflicts can lead to frequent disconnections and poor service. However, this high-frequency connection also has a slight drawback: the effective overall range of 802.11a is slightly smaller than that of 802.11b / g; 802.11a signals cannot penetrate as far as the signals of 802.11b because they are more easily absorbed by walls and other solid objects in their path and because the path loss in signal strength is proportional to the square of the signal frequency. OFDM , on the other hand, has fundamental advantages when propagating under conditions of large multipath propagation, for example, in an office building, and higher frequencies make it possible to create smaller antennas with higher amplification of the RF system, which eliminates the disadvantage of operating at higher frequencies. The increased number of channels used (4-8 times more in FCC countries) and the almost complete absence of other interfering systems ( microwave ovens , cordless telephones , baby monitors ) provide significant combined 802.11a bandwidth and reliability compared to 802.11b / g.

Different countries have different regulatory support, although the 2003 World Radiocommunication Conference improved coordination of international standards. The 802.11a standard is currently approved by regulations in the United States and Japan , but in other areas such as the European Union , it has had to wait longer for approval. European regulators were considering using the European HIPERLAN standard, but in mid-2002, 802.11a was released for use in Europe. In the United States, a decision by the Federal Communications Commission (FCC) in mid-2003 could open up more options for 802.11a channels.

Devices that support the 802.11a standard began to ship late, lagging behind devices that support 802.11b, due to the complexity of manufacturing components at 5 GHz. The performance of first-generation devices was poor and plagued by problems. When second-generation devices began to arrive, the 802.11a standard did not get wide distribution among consumers, primarily because the less expensive 802.11b standard was already widespread. However, later 802.11a saw a significant penetration into corporate network environments, despite the initial cost disadvantages, especially for enterprises that needed increased bandwidth and reliability compared to networks that only supported 802.11b / g.

With the introduction of less expensive and newer 802.11g products that were backward compatible with 802.11b, the 5 GHz 802.11a bandwidth advantage was eliminated. Manufacturers of 802.11a equipment reacted to the lack of market success by significantly improving implementations (current-generation 802.11a technology has range characteristics that are nearly identical to 802.11b), and made technology that can use more than one range standard.

Dual-band or dual-mode access points and network interface cards (NICs) that can automatically handle a and b / g are currently common in all markets and are very close in price to devices with only b / g.

Of the 52 OFDM subcarriers, 48 ​​are for data transmission, and 4 pilot signals with an interval between carriers of 0.3125 MHz (20 MHz / 64). Each of these subcarriers can be BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 16- QAM (quadrature amplitude modulation) or 64- QAM . The total bandwidth is 20 MHz with a occupied bandwidth of 16.6 MHz. The symbol duration is 4 μs , including a guard interval of 0.8 μs. The actual generation and decoding of orthogonal components is performed in the main frequency band using a digital signal processor (DSP), which is then converted up to 5 GHz in the transmitter. Each of the subcarriers can be represented as a complex number. The signal in the time domain is generated using the inverse fast Fourier transform (IFFT). Accordingly, the receiver down-converts the sampling at a frequency of 20 MHz and performs a fast Fourier transform (FFT) to extract the original coefficients. The advantages of using orthogonal frequency division multiplexing (OFDM) are the reduction in multipath effects during reception and the increased spectral efficiency.

(*) Data transfer rate for channel spacing 20 MHz.

1. Vishnevsky V. M., Lyakhov A. I., Portnoy S. L., Shakhnovich I. L., Broadband wireless information transmission networks. M .: Technosphere, 2005