Re: choosing the modulation technique

On Tue, 01 Apr 2008 15:47:05 -0500, "barry_b" <bbuternowsky@xxxxxxxxx>
wrote:

On Apr 1, 3:38 pm, "barry_b" <bbuternow...@xxxxxxxxx> wrote:
Can I get some feedback on my thought process below for choosing a
modulation technique?

I am in the process of choosing the modulation and coding for the
following system:
- frequency band: 2 - 20 Hz
- bit rate: configurable 2 - 16 bps
- transmitter: battery powered
- receiver: PC based (i.e. lots of processing power)
- channel: some burst noise
- adjacent signals/channels: none
- traffic: 1 direction, single user, bursty, asynchronous
- data in payload: 1 to 150 bits

I was considering either bpsk, qpsk, msk or 4-cpfsk. The transmitter
can
accomodate psk or fsk.

I ranked the modulations as follows:
- bspk/qpsk: good error rate performance curve
- msk: smaller bandwith and same BER as bpsk; thus I can use a
narrower
filter and get better performance
- 4-cpfsk (mod index = 0.5): better BER curve than bpsk and msk
(according
to Matlab)

Given that 4-cpfsk has a better BER curve, does it make sense to use
4-CPFSK even for low bit rates? I.e which
is better:
1) bspk/qpsk
2) msk
3) 4-cpfsk (mod index = 0.5)
4) bspk with spreading
5) msk with spreading

From what I know, spreading doesn't provide any coding gain, but only
provides immunity to frequency drop-outs
frequency jamming, thus 3) should be the best. Correct?

Given the short payload, I don't think convolutional coding is
applicable.
I will be using a reed-solomon
code.

In the receiver, I will be implementing a coherent detector.

thanks,
barry

What is the channel?


The channel is:
- transmitter and receiver are not moving
- wireless
- signal is coupled to the ground with antennas
- ground acts as a low pass filter which has acceptable attenuation below
20 Hz

I'll second Vladimir's input on the noise, and also that phase noise
becomes very problematic at such low bit rates. So phase-modulated
signals may not be the best choice for such low rates.

Since it's wireless, what's the carrier frequency? Expected range?
Indoor or outdoor or mixed?





Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
.

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