Microwave Engineering Europe - November 2008 - (Page 34) 34 RADIO — E-BAND Fog and clouds One benefit of e-band wireless is that it is essentially unaffected by fog and clouds. Thick fog with a visibility of 50 m (150 feet) has a density of about 0.1 g/m3, which yields an almost negligible attenuation of 0.4 dB/km at e-band frequencies [9]. This almost absence of attenuation is due to the fog and cloud particles being so much Figure 5: North and South America rain zones. smaller than the wavelength of the e-band radio signal (roughly 4 mm, or 0.6 inch). As such, minimal scattering from the fog or cloud’s tiny water particles occurs. Contrast this situation to free space optical (FSO) systems, a high data rate alternative to e-band wireless. Since an FSO optical signal has a wavelength of the same order of magnitude as the small fog and cloud particles, attenuations on the order of 200 dB/km can be experienced with heavy fog in the FSO transmission path. Airborne dust, sand and other small particles Similar to fog and clouds, eband wireless signals are not scattered from particles of much less than 4 mm in the transmission path. This property makes any small airborne particle essentially invisible to e-band wireless systems. Technical attributes of E-band wireless There are a number of additional physical and regulatory-enabled technical characteristics that add to the attractiveness of e-band as useful spectrum for wireless communications. Firstly, the gain of an antenna increases with frequency. Thus it is possible to realize large gains from relatively small antennas at e-band frequencies. Figure 7 shows the variation in gain for a 1 ft (30 cm) parabolic antenna. At the popular 18 GHz common carrier band, such an antenna has about 32.5 dBi of gain. At e-band, an equivalent sized antenna has 44 to 45 dBi of gain. This equates to an extra 24 dB or so of system gain per link – a significant number when one considers that just an additional 6 dB of system gain allows a link to be doubled in length. Therefore, under ideal conditions, a 24 dB improvement in link margin equates to a four-fold improvement in link distance. An alternative comparison is that a 4-foot antenna at 18 GHz has the same gain as a 1-foot antenna at e-band, with obvious benefits for reduced cost, ease of installation and easier planning and zoning. Secondly, in the USA, the FCC permits e-band radios to operate with up to 3 W of output power. This is significantly higher than available at other millimetre wave bands (for example, it is 25 dB higher than the 10 mW limit at 60 GHz). Also the 5 GHz wide e-band channels enable the radio to pass high data rate signals with only low-level modulation schemes (for example, Frequency Shift Keyed (FSK) or BPSK modulation can easily allow 2 Gbps data rates in the 5 GHz channels). The output power in an e-band system is relatively high as the low-order modulation scheme places minimum linearity requirements on the transmitter’s power amplifier (PA), so the PA can be run close to its maximum rated output power. In contrast, a high data rate SDH microwave radio (incidentally offering less than one-sixth the data rate of an e-band radio) has to use 128 or higher modulation to compress the data in the small megahertz-wide channel. Here, power amplifier linearity is of utmost importance, and amplifiers have to be backed off significantly, throttling back output power to many dBs below rated outputs. Together, this high output power and high antenna gain allow e-band radios to operate with very high radiated power (EIRP) and hence overcome the higher rain fading seen at higher frequencies. This enables e-band system performances that are equivalent to the widely-used microwave point-to-point radios. The performance of commercially available e-band wireless systems Figure 8 shows an example of a Millimetre Wave radio system. Note this system’s all-inone design and compact size. To demonstrate how this technology works in practice, the performance of a Millimetre Wave radio for various rain regions across the globe can be considered. For example, it can be shown that in a city such as New York (rain region K), a 2-mile link can provide 99.99 percent weather availability, with an estimated down time of 50 minutes per year. For a drier climate such as Cairo, Egypt, even a 16 km link will be robust enough to achieve better than 99.9 percent weather availability. Summary The 71-76 and 81-86 GHz E-band frequencies are globally available for ultra high capacity point-to-point Figure 6: Asia Pacific rain zones. Microwave Engineering Europe ● November 2008 ● www.mwee.com http://www.mwee.com
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