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Re: July 2001 President's Letter



>TDM requires high power for a short period of time.

Correct.

>The big question is how much uplink power are we going to assume?

Yes, that is a good question. I'd like to know more about the 23cm
power amplifiers that are currently available. How much peak power can
they produce if the duty cycle is kept low, say to a few percent?  My
link budgets are currently yielding data rates on the order of 300-500
kb/s, so if each burst transmission is, say, 3 kilobits long (enough
to hold about 1/4 sec of encoded voice using the 13kbps GSM vocoder)
then each burst would be 10 milliseconds long at most.

>This determines the bandwidth of the receiver on the satellite and
>how many users can share the uplink.

Yup. On 23 and 13cm we can assume a power-limited channel (as opposed
to a bandwidth-limited one) so that dictates a power-efficient
modulation method like BPSK or QPSK plus forward error correction with
a rate low enough to get good coding gain -- say rate 0.4-0.5.

The transponder bandwidth will then be dictated by that needed to
carry a FEC-coded QPSK signal at the data rate supported by the link
budget. If we assume the link can 500 kbps at rate 0.4, then the
required bandwidth will be on the order of 625 KHz (minimum Nyquist
rate) or more like 1 MHz if we add some excess bandwidth.

Repeated noise is an important issue in any bent-pipe transponder.
This affects system performance in two ways. First, it sets a lower
bound on the noise floor at the receiver. Second, it wastes
transponder power that could go into the signal instead. 

That's why I'm proposing QPSK rather than BPSK; since QPSK requires
half the bandwidth at the same Eb/No, this would halve the transponder
bandwidth for a given data rate and halve the repeated noise power as
well.

Phil

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