QRZ! Ham Radio 1/QRZ Ham Radio Callsign Database - December 1993.iso/ucsd/arrl/arrl_tar.z/arrl_tar/spectrum.Z/spectrum

Text File | 1988-09-07 | 9.6 KB | 244 lines

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Digital Radio Networks and Spectrum Management

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Paul A. Flaherty, N9FZX

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Computer Systems Laboratory \fIand\fR
Space, Telecommunications, and Radioscience Laboratory
Department of Electrical Engineering, Stanford University
ERL 408A,  Stanford, CA 94305




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Abstract
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    Spectrum Management is a vital part of amateur radio.  Questions
of where to place services in the available spectrum continue
to plague frequency coordinators.  This paper contends that
multiaccess radio systems should be allocated in
the spectrum below one GigaHertz, and that monoaccess or link
oriented systems be placed above that frequency.



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Introduction
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    Electromagnetic Spectrum is a scarce, sometimes renewable resource.
Much of the research in radioscience today is devoted to spectrum - efficient
methods of communication, including such mechanisms as amplitude - compandored
sideband telephony, and minimal shift keying data transmission.  Only recently,
however, has research touched on the area of spectrum reuse, and the impact
of position within the radio spectrum considered.

    Propagation characteristics of certain bands make those spectra
valuable to classes of users.  Ionospheric propagation below 30 MHz makes
the High Frequency bands valuable to the world community.  Small component
size and portability are important to mobile users, and so the Very High
and Ultra High bands play an important part in mobile communications.

    Beyond these characteristics, however, little can be generalized
about the appropriate spectra for certain classes of applicants.  It is not
readily apparent that one band should be preferred for multiaccess
applications, and another for link - oriented systems.

    Packet Radio is considered to be a spectrally efficient mechanism
for digital communications.  Using time - division techniques, several
users may share spectrum without interference, if certain traffic characteristics
hold, and if the network load is limited.  Techniques for time - sharing
spectrum abound, but all require some degree of omnidirectionality in
the transmission or reception system, which is characteristic of all
all multiaccess networks.

    Using packet switching techniques, it is possible to construct a
link - oriented, or monoaccess network, which is functionally equivalent
to a multiaccess network.  This duality can be exploited for networks with
fixed or portable stations.

    In a hierarchal networking architecture, the Terminal Network is
usually defined as that hierarchy or subnet which connects to end users.
The telephone local loop plant, and radio repeaters are two examples of
terminal networks.  This paper is primarily concerned with terminal networks,
although many of the principles may apply elsewhere.

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Synthesis
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    The forward gain of a parabolic reflector antenna is given as:

.EQ C {Gain}
G = eta pi sup 2 d sup 2 f sup 2 over C sup 2
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    It is of no small consequence that the gain of a reasonably sized
antenna increases dramatically with frequency; many digital satellite services
exist explicitly because of this fact.

    For the purposes of discussion, a "reasonably sized" antenna is
considered to be unity, or one meter in diameter, for terrestrial
applications.  "Reasonable size" is often a matter of community tastes
and economics; however, the one meter size covers a large portion of
of the contingencies.  Thus, the gain of reasonably sized antenna is:

.EQ C {Normal}
G sub 0 = eta pi sup 2 f sup 2 over C sup 2
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    The half power beamwidth of a typical parabolic reflector is:

.EQ C {Degrees}
A = 139 over sqrt G
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    Digital modulation schemes may be divided into two classes: orthogonal
modulation techniques, such as phase shift keying, and antipodal modulation,
such as amplitude or frequency shift keying.  In order to add another bit
per symbol in  a constant - bandwidth channel, an increase in the signal -
to - noise ratio of 3 db is required for orthogonal modulation, and 6 db for
antipodal systems.

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Frequency Division Tradeoff
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    The Frequency Division Tradeoff between multiaccess and monoaccess
networks arises out of the increase in signal - to - noise ratio that
occurs with the use of directional radiators.  With the increase comes
the ability to either multiply the bit rate, or divide the bandwidth to
obtain equivalent service.  Because antenna gain is tied integrally with
frequency, the ability to fraction the bandwidth increases frequency, until
a point is reached where each node occupies its own channel.  The transition
from a multiaccess network to its monoaccess dual occurs at a certain
Critical Frequency, which is determined in turn by channel access
technique, and network size.

    As an example, consider a terminal network of eight nodes, using a
Carrier Sense - Multiple Access, and frequency shift keying, running at a
rate of 19.2 Kbps.  Assuming the best case for CSMA (no hidden nodes), the
best aggregate throughput we can expect from such a network is about
10.6 Kbps.

    The dual of this network is a set of eight links connected to a
packet switch.  Again assuming the best case for CSMA, each user has access
to a 19.2 Kbps data rate.  We wish to accomplish this transition using
equivalent power and bandwidth; therefore, we require an eightfold increase
in the aggregate bit rate.  Assuming the use of n-ary frequency shift keying,
this in turn requires an increase of 42 db in the signal - to - noise ratio.
Such an increase can be obtained by a pair of one meter aperture antennas,
operating at 1.5 GHz, using a 55% efficient feed.  The aggregate throughput for
this network is 153.6 Kbps, in the same bandwidth.

    In general, for a large class of terminal networks, the Critical
Frequency lies around one GigaHertz.  The extent of the tradeoff is limited
in practice by packet switching speeds, and the extensibility of multilevel
modulation schemes.

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Space Division Tradeoff
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    The propagation characteristics of radio limit the spatial dimensions
of any network.  However, it is often the case that the network itself
covers far less territory than the radio spectra used to service it.  This is
particularly true with multiaccess networks which require omnidirectional
radiators.

    Radio propagation models are somewhat involved; the more exacting
models have been implemented as computer simulations by researchers.
However, even a cursory analysis reveals that spectrum reuse is much more
practical at higher frequencies.  In particular, path loss increases as the
square of the frequency, as does antenna gain (which results from a narrower
beamwidth).  Wave polarity separation also increases accordingly.  In general,
it should be possible to model the multiaccess - monoaccess tradeoff, using
the available computer tools.

    As an example, consider the CSMA network mentioned earlier.  The
farthest node is at a distance \fBR\fR from the hub.  In order to preclude the
"hidden station" problem, stations on the circle described by \fBR\fR must have
enough power for range \fB2R\fR.  In the limit, as the number of stations grows,
the area covered by the radio network becomes four times as large as the area
of the physical network.  The monoaccess dual is no larger than physical
network area at some Critical Frequency, and can indeed be considerably
smaller.

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Towards a Spectrum Efficiency Quotient
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    Clearly, a combination of three separation techniques (spatial,
spectral, and polar) can yield a spectrally efficient monoaccess network
at higher frequencies.  At lower frequencies, however, the multiaccess model
predominates.

    The term "spectrally efficient" has been used to describe multiaccess
networks, without specificity.  What is needed is a "figure of merit" to
describe a radio network, and compare it with other alternatives.  Propagation
characteristics of the spectrum below one GigaHertz lend themselves to
applications requiring a high degree of mobility and portability.  For fixed
or semiportable operation, however, a monoaccess network provides a spectrally
efficient alternative, when operated above the Critical Frequency.

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Summary
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    The spectral efficiency of monoaccess and multiaccess networks varies
with the frequency used.  The exact calculation of the Critical Frequency
of the tradeoff is currently the subject of research.  However, in general,
multiaccess networks tend to be more spectrally efficient below one GigaHertz,
and monoaccess networks predominate above.

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Implications for the Amateur Service
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    Coordination between different types of services in the Amateur
Service at frequencies above 30 MHz has been accomplished fairly haphazardly
and ad hoc.  With the advent of packet radio, it has been difficult in major
metropolitan areas to coordinate use of spectrum.  Repeater links have
been traditionally placed in bands close to repeaters, because of the
availability of equipment, and economy.

    Ultimately, some changes need to be made in bandplans for the
Amateur Service.  In particular, it is recommended that stations in
Auxiliary Service (as defined in Part 97.86) should be relocated to
frequencies above one GigaHertz.  Terrestrial digital links, used to
interconnect multiaccess networks, should also be placed in the microwave
region.  In turn, multiaccess digital networks should be placed in the
Amateur VHF and UHF allocations.



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References
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Wozencraft and Jacobs, \fIPrinciples of Communications Engineering\fR,
1965, John Wiley and Sons, New York.  ISBN 0-471-96240-6

William Stallings, \fIData And Computer Communications\fR, 1985,
Macmillan Publishing, New York.  ISBN 0-02-415440-7