Wireless Connectivity: ISM, WLAN, WMAN, Bluetooth et al. 5

Published:2011/8/18 2:35:00 Author:Amy From:SeekIC

Stefan Tauschek

More bandwidth

This means we need more bandwidth and more chan­nels. The standardization committee satisfied this need in 1999 with the introduction of the 802.1 la standard. It utilizes the 5.8-GHz ISM bond and employs a modulation technique called ’orthogonal frequency division multiplex­ing’ (OFDM). This yields o raw data rate of 54 Mbit/s, although the range is only half as large. The total available bandwidth allocated lo this band is 455 MHz, from 5.150 GHz to 5.725 GHz. Up lo 19 non-over lapping channels are available, allowing the same number of access points to be used without mutual interference. Two years ago, the OFDM modulation technique was also introduced for the 2.4-GHz band via the 802.11g standard (2.4-GHz OFDM PHY). This means there are presently three ’air interfaces’ available - 802.11a, 802.11b and 802.11g - but they are to be replaced by 802.1 1 n in the future. The objective is to further increase the data rate to mare than 300 Mbit/s. This bit of magic is supposed to be achieved by using transmission proto­cols with greater efficiency, compression techniques, and receivers with improved dynamic characteristics (Table 2).



Smart antennas

The Cantenna shows that it’s not that difficult to make antennas with good performance for centimeter-wave sig­nals. Even so, it’s still common practice to use rod antennas with unidirectional characteristics, with the result that most of the emitted RF energy goes unused. That may be a reasonable approach for mobile systems, but in fixed installations the transmission distance con be con­siderably increased by using directional antennas. There are several ’classic’ antenna designs that yield direc­tional characteristics, such as Yagi antennas and dipole arrays. However, a much simpler solution is a waveguide closed at one end, in which a standing wave is gener­ated by suitable mechanical dimensioning. Figure 8 shows (he basic design of such a ’tin<an’ antenna. Here U is the length of the standing wave in the antenna {with the maximum being located at La/4), and Lo is the wave­length of the carrier frequency in free space. The size of the tin and the frequency jointly determine the length of the standing wave. Instructions for building a Cantenna like this and the calculation of L can be found on numerous Internet sites, such as http://www.lumpoint.net/wirelss/cantennahowto.hlml and http://www.saunalahti.fi/elepal/anienno2.hlml. With careful construction, it is certainly possible lo achieve a respectable antenna gain. Measurements show that the range can be increased by at least a factor of two.

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