Telecommunications Networking II Topic 12 Wireless LAN Technology

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What are we trying to do? • We are trying to use wireless technologies to physically interconnect devices of various kinds (computers, hubs, printers, personal digital assistants, sensors, entertainment system components, household appliances…. ) • We would like the cost of the wireless components to be << than the cost of the device(s) they are associated with • We would like this to be simple to configure Copyright 2002, S. D. Personick. All Rights Reserved. 2

Possibilities • • UHF 300 MHz - 3 GHz SHF 3 GHz - 30 GHz Optical 300, 000 GHz 1000 MHz => 0. 3 meters: transmitted/re-radiated 10 GHz =>3 cm: transmitted/re-radiated/reflected Optical => ~1 micrometer: absorbed/reflected/scattered Copyright 2002, S. D. Personick. All Rights Reserved. 4

Something to Consider • Delay spreading: Inside a building, re-radiation of r. f. from metallic objects (metal studs, steel building skeleton, file cabinets…. ) leads to delay spreading of the received signals. 30 meters (~98 ft) of path length => 100 ns of delay Copyright 2002, S. D. Personick. All Rights Reserved. 5

Delay Spreading • Delay spreading inside large office buildings and shopping malls can be as large as several hundred nanoseconds. Delay spreading in smaller buildings may be as large as 100 nanoseconds • ~100 nanoseconds of delay spreading limits the achievable symbol rate to ~5 M baud per carrier Copyright 2002, S. D. Personick. All Rights Reserved. 7

IEEE 802. 11 (standard) • Uses r. f. frequency hopping or direct sequence spread spectrum (>10 x spreading ratio) or infrared (light). • R. f. nominal frequency (U. S. ) is 2. 4 GHz (~12. 5 cm wavelength) • 1 -2 Mbps (802. 11); up to 11 Mbps (802. 11 b) • 5 GHz standard under development Copyright 2002, S. D. Personick. All Rights Reserved. 8

A quick calculation • 1 Mbps IEEE 802. 11; required power at the receiver (assume 26 d. B SNR, thermal noise limited operation): k. T x 10**6 x 400 ~ 1. 6 x 10**-9 m. W • NW 660 transmitter : +20 d. Bm= 100 m. W • Link loss budget: (if thermal noise limited) ~110 d. B Copyright 2002, S. D. Personick. All Rights Reserved. 12

Infrared Link • LED emits ~ 1 -10 m. W • Receiver requires ~12, 000 -60, 000 photons per received (on-off modulated) pulse • Photon energy ~ 2 x 10**-19 Joules • Example: 1 Mbps > 1. 2 x 10**-6 m. W • Allowable loss ~60 d. B (maybe less) • Background light adds “shot” noise Copyright 2002, S. D. Personick. All Rights Reserved. 16