 pairs by knowing the transmitted symbols and calculating the crosstalk coupling gains then predistorting the transmitted symbol stream on the wanted channel to eliminate the crosstalk. Microwave vendors are, however, keen to point out that whatever you might do to copper it can suffer the same fate as fibre and be taken out by accident when roads are dug up and can take several days to restore and make the case that a combination of backhaul provides the best guarantee of availability. The microwave economic case is, however, dependent on the link budget including the fade margin (see weather events above). Link budgets are determined by the system gain that is a composite of the transmitted output power and TX/RX antenna gains minus the receiver threshold. This is set as the minimum received power required to support a bit error rate of less than one in 1012. The system gain for a commercial backhaul radio will be of the order of between 160 and 200 dB. This implies that a range of up to 3 km is practical, but not more. This in turn implies these systems can only be used in urban or suburban applications with systems in the lower bands being better suited to rural applications. Excepting that caveat Ericsson provide an example of a radio that supports 1.25 Gbps in both directions effectively an optical rate interface with a latency of less than a microsecond, transmitting in the higher band (81 to 86 GHz) and receiving in the lower band (71 to 76 GHz) on 5 GHz channels using differential binary phase shift keying (DPSK). The system gain is 162 dB. This would support 1.5-km hop lengths in heavy rain. The point-to-point microwave market is served both by Tier 1 vendors including Erics- son, NSN and Huawei and more specialist vendors such as Bridgewave, Proxim, Aviat and Ceragon. The Tier 1 vendors have an advantage in that they have greater visibility to the future form factor of mobile broadband base stations, but essentially there should be room for both. Making Telecoms Work Chain Figure 13.1 Network topologie