What's the Problem?

"Handhelds and cellular phones seem to work, why doesn't amateur packet?"

In order to understand why higher speed amateur packet networks are not more prevalent,  it seems useful to start with Shannon's equation which describes the possible capacity of an information channel. Examining this will lead us to practical ways in which we may improve our amateur networks.

Shannon's Equation

Channel Capacity, bps = B*Log2(S/N+1)

B= information bandwidth, Hz
S = amount of sender power available at the receiver
N = total noise power in the information bandwidth

Noise power may be

• uncorrelated
• internal noise, KTsB, due to system noise temperature
• external natural noise sources, lightning etc
• interference from other manmade sources
• correlated
• distortion products due to transmitter or receiver
• propagation effects, multipath, Doppler

To maximize channel capacity it is necessary to increase B and S and minimize N. There are also implicit modulation/coding and modulation spectral characteristics.

Capacity limitations are due to:

1. Waste

We don't deliver enough signal where we want it, reduces S
We do deliver signal where we don't want it, increases N (multiple user context)

2. Noise

Uncorrelated noise, QRM/QRN  and hardware limitations, increases N and limits (S/N)

Correlated (self-generated ) noise from insufficient bandwidth (inter-symbol interference) and system and multipath induced distortions both increase N and limit (S/N).  Coding, modulation and error correction techniques can mitigate this.

I believe that in the amateur radio context that (1) is usually the primary offender. Specifically, typical amateur radio paths have an extreme amount of excess loss above and beyond that which good paths produce. To appreciate the magnitude of this problem, here is a little quiz just for the shock value.

A practical example from Northern California.

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