Re: [amsat-bb] WiFi users "may" try AO-40, look out!

[Phil Karn <karn@xxxxxxxxxxxxxxx>]

>With a one watt amp, and our 40 foot dish, we hope to close the link to
>the satellite on one of the channels in the HAM band.

>1)  Are there any TIMING parameters in the implementation of 802.11b that
>are time critical.  That is, the end-to-end turn around time will be on
>the order of a millisecond instead of a microsecond.  WIll this affect
>ack/nak timeouts, etc?

In the "Managed" mode, probably yes. In the "ad-hoc" mode, probably
no.  In the "managed" mode, user terminals acquire and "associate"
with base stations that transmit periodic beacons. This is by far the
most common mode of operation. In the "ad-hoc" mode, user terminals
communicate directly with each other.

>2)  Doppler.  The doppler is insignificant relative to bandwidth, but it
>is changing.  Are the spread spectrum tracking loop filters  fast
>enough (once locked on) to keep moving with changin doppler, or do
>they try to absolutely lock in with a very long time constant.

Consider the modulation types and rates. You're most likely to make
the link work at the lowest speed, 1 Mb/s. At that speed, 802.11b uses
DBPSK, almost certainly with noncoherent detection at the receiver.

There is no FEC, but it is spread with a 11-bit Barker sequence. I.e.,
there is one complete 11-bit Barker sequence per data bit. In an ideal
receiver the spreading is irrelevant to the SNR performance, so you
can analyze the link as if it were an unspread 1Mb/s DBPSK link.

The good news is that DBPSK at this speed would be essentially immune
to Doppler. However, DBPSK without coding requires an Eb/No of about
11 dB for good performance, so at 1Mb/s you'd need a received S/No of
about 71dB-Hz.  You'd need even more if the receiver is less than
ideal. How much do you have? Have you tried to measure the S/No
performance of any actual 802.11b cards?

The ability to use off-shelf hardware is certainly appealing, but you
might be better off to use FEC-coded BPSK at a lower, more manageable
speed. And you wouldn't need anything like a 40' dish.

Phil


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