Re: Wireless link, reliable preamble end detection?

usenet@xxxxxxxxxxxxx wrote:

Hi everyone,

I have a project at the moment that requires a high-reliability one-way
radio link.

I have decided on Viterbi encoding the data and then encapsulating that
with Manchester encoding to keep everything DC balanced.

All that I can code fine it's the preamble, and specifically detecting
when it ends, that I'm having trouble with.

The current format of the preamble is a single repeating byte of
'1010001' chosen because it's asymmetric and I can easily align to
the start of it.
The detection of the start of actual data is giving me trouble; the
channel is very noisy so about every preamble byte in five will have a
single-bit error stopping me from simply waiting for data that 'isn't a
preamble byte'.

The choices as I see them are:

* Unique bytes at the start of real data.
* Wait for two 'non-preamble' bytes to determine preamble end.

Neither of these look like very good solutions and I'm worried that the
preamble may be the weak point in the scheme rather than the forward
error correction side of things.
I can't do anything really fancy as the preamble
stripping/de-Manchestering is being done on a low-end RISC micro and I
can at best examine two bytes in detail before I run out of time and
have to start sampling again.

So does anyone have advice on how to best detect the end of the
preamble?

Jon Starr

Design Engineer
DFx Technology Limited
http://www.dfxtech.co.uk


If the preamble bytes are only off by one bit, how about finding the Hamming difference? If a byte is off by one bit, it's probably still preamble. If the preamble is followed by, say, a 0 byte, it'll be off by 3 bits, or maybe two. Then start the data.

The real problem is that you want high reliability, and the bit error rate is high. Why is it so high?
Can the channel be cleaned up?

Barry
.

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