802.11

[Bob Bruhns]

Hi Alex,

I used to work for a company that made a wireless voice communications system using 802.11 LAN cards.  It's been a few years now, so I could be slightly off on the exact numbers, but here goes.

Their system in those days was single-frequency TDMA at 11 megabits per second raw bit rate using PN-SS using the card manufacturer's proprietary 20 bit PN code, which was truncated from a 22 bit code and not great.  11 megs / 20 = 550 KB/S encoded bit rate.  This 550K was broken into 7 time slots of about 833 uS each (about 456 bit times), one for the repeater and six for the portable units.  Speech was encoded at 32 KB/S.  The round-robin took about 5.8 mS.  5.8 mS worth of 32 KB/S (about 166 bits) were compacted into the 833 uS windows, of which some was guard-band and over half was sync time.  This permitted perceived full-duplex voice communications with about 11 mS delay round trip.  Some users noticed the delay as echo, but it wasn't bad, and with tight headphones you didn't notice it at all.  The repeaters ran 100mW and the portables ran 50 mW.

Lots of trouble with multipath in that system, supposed to be caused primarily by poor receiver bit sync acquisition and the truncation of the PN code, which crippled its correlation properties and degraded code sync acquisition.  Fortunately for them, that 802.11 card was discontinued.  Hopefully they went with a better one since.

  Bob, WA3WDR