Industrial Communications Handbook August 2016

cheapest WiFi router has an internal second antenna. Diversity may be spacial , the two antennas suffi- ciently apart to ensure that they are not destructively interfered with at one time. They may occupy the same space, but be differ- ently polarised . (It is unlikely that both vertical and horizontal polarisation will be in destructive mode at the same time.) The diversity may be in frequency , either within or across the 2,45 and 5,8 GHz bands. They can also cleverly use time domain repeti- tion, as a form of time diversity in a rapidly changing environment. Naturally, there is all the above, essentially what IEEE802.11n, MIMO (Multiple Input Multiple Out- put) uses, but noting that it takes computing power either to select or combine the outputs from the an- tennas, which also consume electrical input power. Hence intelligent MIMO devices turn off MIMO unless absolutely required in order to conserve pow- er, especially if battery driven. Another major ‘indoor’, i.e. not ‘outdoor’ problem is non-metallic attenuation. The problem here is the wall, cabinet, chair, passage way, tool chest, etc that gets between the transmitter and receiver. Sadly, a human looks like a lump of water at these frequen- cies, and unfortunately, tends to move about, chang- ing the electromagnetic environment. This challenge is only met by more power, greater antenna gain, repositioning, or adaptive mesh net- working (getting around the obstacle/s). 3.4 Wireless coexistence Radio systems do not exist in isolation. We all share the same ‘ether’. Even systems that operate at different frequencies can still interact by RF swamping of sensitive receiver stages, etc. Of particular interest is narrow-band interference killing wide-band systems. A strong Bluetooth signal often kills WiFi. They operate in the same frequen- cy band, but Bluetooth divides it into 95 channels through which it hops in time, whereas WiFi has only 11 channels (three non-overlapping), much wider, but static in time. Although both spreading and hopping strategies were developed to reduce the possibility of intercept