A Channel Sounder for Low Frequency Sub-Surface Radio
Paths
Low frequency sub-surface radio is used for communications with miners and
cavers. The signal attenuation is dependent on frequency and on the electrical
conductivity of the ground. In addition, the optimum frequency for
communications is dependent on depth below the surface; and on the degree of
noise and co-channel interference. An adaptive communications system, under
development now, makes use of a channel-sounding technique by transmitting a
wideband signal and deriving the signal/noise ratio (SNR) as a function of
frequency. The communications system can thereby alter its transmission
frequency, and other parameters, to optimise SNR under different conditions.
The same principles are relevant to applications in sub-surface radiolocation,
archaeology and geophysics. The channel-sounding technique makes use of a
modified pseudo-random binary sequence, which is cross-correlated with its
inverse at the receiver, and which uses a code-locked loop to maintain
synchronism with the transmitter. The transmitting antenna for such a system is
essentially an induction loop, and the extreme wideband nature of the system
requires this to be operated untuned. The resultant low efficiency is countered
by using a suitable binary sequence at the transmitter and signal-averaging
techniques at the receiver.
My work on a low-frequency through-rock channel sounder has been described
in several papers. References and some PDFs are given here. Copies of other
papers are available. The channel sounder is described in detail in my
Sub-Surface Communications Book, which is based
on my PhD thesis. At the heart of the Channel Sounder is a novel method using
'inverse sequences'. A paper, based on a chapter of my PhD thesis, and having
the title Sequence Design for System Identification, is in the process
of being submitted for publication. The draft abstract reads...
System identification using a periodic
pseudorandom sequence is well-established. The procedure relies on the perfect
(impulsive) nature of the auto-correlation function of the test sequence. It is
possible to use a non-ideal test sequence provided that the input-output
cross-correlation is carried out between the applied sequence and its inverse
reference. A simple algorithm is presented for calculating the inverse of an
arbitrary sequence under the operation of cross-correlation, and a number of
observations of the behaviour of inverse sequences are discussed. An example of
the use of inverse sequences is given for a code-locked loop with a reduced
aquisition time.
Please enquire to
david(at)caves...etc.
for further information.
The above material is summarised in the following published book
chapter...
- Gibson, D. and M. Darnell (2002). A Channel Sounder for Low-Frequency
Sub-Surface Radio Paths. In Advanced Signal Processing for Communication
Systems. ed. T. Wysocki, M. Darnell and B. Honary. Dordrecht: Kluwer
Academic Publishers. ISBN 1 4020 7202 3.
The above book chapter draws on material from two earlier conference
presentations...
- Gibson, D. & M. Darnell (1999), Adaptive Digital Communications
for Sub-Surface Radio Paths, Proceedings of 5th International Symposium
on Digital Signal Processing for Communications Systems (DSPCS '99),
Perth-Australia, 237-244.
Download PDF (347K)
- Gibson, D. & M. Darnell (2002), A Channel Sounder for Low
Frequency Sub-Surface Radio Paths, Proceedings of 6th International
Symposium on Digital Signal Processing for Communications Systems, Sydney:
University of Wollongong, 49-54, ISBN 0 86418 791 2.
In an application to measure subterranean
radio propagation, a wide-band low-frequency channel sounder makes use of a
modified pseudo-random binary sequence, which is cross-correlated with its
inverse at the receiver, and which uses a code-locked loop to maintain
synchronism with the transmitter. The transmitting antenna for such a system is
essentially an induction loop, and the extreme wideband nature of the system
requires this to be operated untuned. The resultant low efficiency is countered
by using a suitable binary sequence at the transmitter and signal-averaging
techniques at the receiver.
Download PDF (267K)
The above material also appeared, in a slightly different form, in the
non-peer-reviewed journals of the BCRA's
Cave Radio & Electronics Group...
- Gibson, David (1999), A Channel Sounder for Sub-Surface
Communications. Part 1 - Adaptive Digital Communications for Sub-Surface Radio
Paths. CREGJ 38, pp23-28, Dec 1999
Download
PDF (386K)
- Gibson, David (2002), A Channel Sounder for Sub-Surface
Communications. Part 3 - Spectrum Survey: Preliminary Results. CREGJ
49, pp14-22, Sept. 2002
Download
PDF (783K)
A photograph of one of the boards was given in...
- Gibson, David (2001), Using a Scanner as a Digital Camera.
CREGJ 44, p27, June 2001.
Download
PDF (362K)
The material was also presented with less technical detail in:
- Gibson, D. (2001), A Low Frequency Channel Sounder For Sub-Surface
Communications, Radio Solutions Conference Proceedings 2001, Low Power
Radio Association (LPRA). www.lpra.org
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