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From: John on 1 Apr 2008 21:27
On Apr 1, 4:47 pm, "barry_b" <bbuternow...(a)gmail.com> wrote:
> >On Apr 1, 3:38 pm, "barry_b" <bbuternow...(a)gmail.com> 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

You might consider using subaudible tones, like radios use for
squelch. Look up CTCSS. The idea is to define a set of tones, say 16,
and map each group of four bits to a tone. The receiver would need to
detect what tone is present to decide the four bits.

John
From: barry_b on 1 Apr 2008 23:43
The channel frequency band is fixed at 2-20 Hz (maybe up to 30 Hz). The
transmitter works in this range and cannot be changed. We have a working
system that uses bpsk at 12 Hz (bit rate is 4 bps). I am looking into
improving the performance of the system, thus I am wondering if other
modulation techniques (msk, 4-cpfsk, etc) may provide a better error rate.
The system works outdoors for up to 2 km.

>
>You might consider using subaudible tones, like radios use for
>squelch. Look up CTCSS. The idea is to define a set of tones, say 16,
>and map each group of four bits to a tone. The receiver would need to
>detect what tone is present to decide the four bits.
>
>John
>
From: Randy Yates on 1 Apr 2008 23:54
"barry_b" <bbuternowsky(a)gmail.com> writes:

> The channel frequency band is fixed at 2-20 Hz (maybe up to 30 Hz). The
> transmitter works in this range and cannot be changed. We have a working
> system that uses bpsk at 12 Hz (bit rate is 4 bps). I am looking into
> improving the performance of the system, thus I am wondering if other
> modulation techniques (msk, 4-cpfsk, etc) may provide a better error rate.
> The system works outdoors for up to 2 km.

Barry,

For bandwidth-limited systems, you may want to investigate Trellis-coded
modulation. See, e.g., [proakiscomm].

--Randy

@BOOK{proakiscomm,
title = "{Digital Communications}",
author = "John~G.~Proakis",
publisher = "McGraw-Hill",
edition = "fourth",
year = "2001"}

--
% Randy Yates % "With time with what you've learned,
%% Fuquay-Varina, NC % they'll kiss the ground you walk
%%% 919-577-9882 % upon."
%%%% <yates(a)ieee.org> % '21st Century Man', *Time*, ELO
http://www.digitalsignallabs.com
From: Eric Jacobsen on 2 Apr 2008 01:04
On Tue, 01 Apr 2008 22:43:41 -0500, "barry_b" <bbuternowsky(a)gmail.com>
wrote:

>The channel frequency band is fixed at 2-20 Hz (maybe up to 30 Hz). The
>transmitter works in this range and cannot be changed. We have a working
>system that uses bpsk at 12 Hz (bit rate is 4 bps). I am looking into
>improving the performance of the system, thus I am wondering if other
>modulation techniques (msk, 4-cpfsk, etc) may provide a better error rate.
>The system works outdoors for up to 2 km.

Is that your RF frequency or just the signal bandwidth? If that's
the rf frequency your antenna is kinda big, no? That's what the
military used to use for submarine communication since it penetrates
sea water reasonably well.

FWIW, PSK/QAM provides the best power efficiency among common
modulation types, which is why they're used so often. If you have
BPSK working and the phase noise is manageable, then bumping up to
QPSK shouldn't be too big of an issue if you have 3dB of link margin
to use up. Even if you don't, putting coding on top of it when you
move from BPSK to QPSK will help, but your packets are so small that
you're not going to be able to get a lot of coding gain.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
From: cb135 on 2 Apr 2008 09:16
<snipped.>
> 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.
>
<snipped>

Phase noise is problematic at low bit rates? How are the two
related? Not enough pilot symbols available to estimate the phase?
What else am I missing?

col
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