Microwave Engineering Europe - November 2008 - (Page 33) RADIO — E-BAND 33 Figure 4: Europe, Middle East and Africa rain zones. introduced in 2005 and the first commercial e-band radios were installed soon after. Wireless regulators in Europe quickly followed the US lead. In 2005, the European Conference for Postal and Telecommunications Administrations (CEPT) released a European-wide band plan similar to the USA [4]. In 2006, the European Telecommunications Standards Institute (ETSI) released technical rules for equipment operating in the 71-76 and 8186 GHz bands [5]. These were consistent with European EU rules to allow e-band wireless equipment to be commercially used in Europe. Many parts of the world have now followed the US and European leads, and have opened up the e-band frequencies for high capacity point-to-point wireless, enabling gigabit-speed transmission in the millimetre wave bands. The e-band frequency allocation The e-band frequency allocation consists of the two unchannelized bands of 71-76 GHz and 81-86 GHz, as shown in figure 1. This allocation is significant for two main reasons. Firstly, the combined 10 GHz of spectrum is significantly larger than any other frequency allocation. Together this is over 50-times larger than the entire spectrum allocated in the USA for all generations, technologies and flavors of cellular services, and much larger than all the widely-used microwave communication bands. The availability of such a large swath of spectrum enables a whole new generation of wireless transmission to be realized. Secondly, the e-band allocation, divided into two paired 5 GHz channels, is not further partitioned, as is the case in the lower frequency microwave bands. In the USA, the FCC slices each common carrier microwave band into channels of no more than 50 MHz. This channel size ultimately limits the amount of data that can be squeezed into the channel. With 5 GHz channels at e-band (100-times the size of even the largest microwave band and larger than the wide 60 GHz and 90 GHz allocations) significantly more data can be carried by each signal. The e-band spectrum allocation is enough to transmit a gigabit of data (1 Gbps or GigE) with simple modulation schemes such as Binary Phase Shift Keying (BPSK). With more spectrally efficient modulations, full duplex data rates of 10 Gbps (OC-192, STM-64 or 10GigE) can be realized. Since there is no need to compress the data into small frequency channels, systems can be realized with relatively simple architectures. Radio equipment can take advantage of low-order modulation modems, non-linear power amplifiers, low-cost diplexers, direct-conversion receivers, and many more relatively non-complex wireless building blocks. This reduces system cost and complexity whilst increasing reliability and overall radio performance. E-band wireless propagation Wireless propagation at e-band frequencies is well understood. Characteristics are only slightly different to those at the widely-used, lower-frequency microwave bands, enabling transmission distances of many miles to be realized. The atmospheric attenuation of radio waves varies significantly with frequency. Its variability has been well characterized [6] and is shown in figure 2. At the microwave frequency bands of up to 38 GHz, the attenuation due to the atmosphere at sea level is low at 0.3 dB/km or less. A small peak is seen at 23 GHz, followed by a large peak at 60 GHz, corresponding to absorption by water vapor and oxygen molecules, respectively. This effect at 60 GHz in particular (where absorption increases to 15 dB/ km) significantly limits radio transmission distance at this frequency. Above 100 GHz, numerous other molecular absorption effects occur, limiting the effectiveness of radio transmissions. A clear atmospheric window can be seen in the spectrum from around 70 GHz to 100 GHz. In this area, low atmospheric attenuation around 0.5 dB per km occurs, close to that of the popular microwave frequencies, and very favorable for radio transmission. For this reason, e-band wireless systems can transmit high data rate signals over many miles under clear conditions. Weather and other effects on e-band The physical properties of high frequency radio transmission in the presence of various weather conditions are well understood. With proven models of worldwide weather characteristics available, link distances of several miles can confidently be realized over most of the globe. Rain As with any radio transmission above about 10 GHz, rain attenuation will place natural limits on link distances. As shown in figure 3, millimetre wave transmissions can experience significant rain attenuations in the presence of rain [7]. “Heavy” rainfall at the rate of 25 mm/hour (1” per hour) yields just over 10 dB/km attenuation at e-band frequencies; this increases to 30 dB/km for 100 mm/hour (4” per hour) “tropical” rain. These values of attenuation are used in link planning to determine the maximum link length allowed to overcome rain events. Global rain patterns have been studied and characterized over many years. The ITU and other bodies publish models derived from decades of rain data from around the world [8]. Models are available to predict rain intensities and annual rainfall at those intensities, to enable link designers to engineer radio links to overcome even the worst weather, or to yield acceptable levels of rain outage on longer links. Figures 4, 5, and 6 show ITU rain data for various geographic areas. Microwave Engineering ● November 2008 ● www.mwee.com http://www.mwee.com
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