Subject: [NEXTEL-1] The Spectrum Policy Task Force Report reflects both an incredible opportunity and an incredible challenge.

From: Marcel <[email-masked]>
Date: Fri, 24 Jan 2003 14:02:18 -0500

Reply Comment to Spectrum Policy Task Force inquiry (Docket 02-135)

by Joel Schindell, Professor of the Practice

MIT Dept of Electrical Engineering
and Computer Science
January 20, 2003

http://gullfoss2.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=65\13404375
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The Spectrum Policy Task Force Report reflects both an incredible opportunity and an
incredible challenge.

The opportunity: The author strongly agrees that the exclusive use model,
while suitable for some of the broadcast operations and limited device
intelligence that characterized early radio applications, leads to very
inefficient spectral utilization. The author also agrees that
microprocessor control and software radio (flexible choice of modulation
type) can greatly improve spectral utilization through use of opportunistic
spectral access.

The challenge: Considerable work is necessary to
(1) characterize existing
spectral usage,
(2) characterize interference susceptibility and explore
ways to reduce susceptibility through improved processing,
(3) develop
means to reduce interference susceptibility by judicious choice of
modulation type, and perhaps most importantly,
(4) develop proactive system
architectures and signalling protocols to facilitate opportunistic spectral
access by allowing a user to indicate that he is being interfered with and
to negotiate solutions to that interference. Note that item 4 is a complex
and almost unexplored area. It is not sufficient for an opportunistic user
to "listen," as is discussed below. Should the units register with a
neutral third party database (maintained by whom?), or should they download
a broadcast database (from whom?), or should they collaboratively negotiate
(using what protocol?) an optimal (by what definition?) spectrum assignment?

Discussion: The Task Force points out (p.28) that "defining interference
measures and setting them at an appropriate level will require the
Commission to have a better grasp of the science involved prior to making
rules and to anticipate better the consequence of different usages. This
means that more engineers should be involved in the rulemaking
process." The author wishes to point out that implementation of this
approach will require studies, both empirical and analytical, to determine
actual spectrum utilization as a function of frequency, geography, and
modulation type. There is also the issue of interference
susceptibility. Susceptibility would range from (1) the interference
vulnerability of conventional modulation schemes (a new system might
utilize a modulation specifically chosen to reduce this vulnerability!), to
(2) the possibility of utilizing modulations which exhibit orthogonality or
"interference resistance" to other modulations which are co-existing in the
spectrum.

The Task Force also points out (p.33) that "The FCC could also allow
'opportunistic' devices to search across licensed spectrum and then to
operate in licensed but unused spectrum without permission of the licensee,
as long as those devices did not cause interference to incumbent licensees
and instantly ceased transmitting whenever a licensee wished to use the
spectrum." This is an excellent idea, but it implicitly requires that the
opportunistic device must have a way of determining the interference that
it is causing. The Task Force Report (p.34) states that "If opportunistic
devices are to be authorized in the future, there will have to be
regulations or protocols to ensure that they listen before they transmit
and that they do not transmit when to do so would cause interference to an
incumbent licensee." However, this is more easily stated than
implemented. One could easily imagine a situation where users A, B and C
are located along a straight line. User B is listening to a transmission
from user A. User C cannot even detect user A, but his "opportunistic"

?transmission might interfere completely with the ability of User B to hear
User A. Even in simple broadcast cases, it can be shown that passive
listening is not sufficient. The question becomes "how does an
opportunistic licensee know that his transmission will not cause
interference to an incumbent licensee?"

Prevention of this type of "jamming" requires the development and
implementation of a signalling infrastructure so that user B [or a
broadcast station] can indicate to user C that he is not free to transmit
(and perhaps they can even "negotiate" a sharing protocol). The
effectiveness of opportunistic systems will depend critically on the manner
in which this interference "reporting" is implemented. How do you build the
genie which controls spectral access? Centralized or
distributed? Adaptive? Interactive? It is not currently clear whether
there is any signalling or network control infrastructure that can meet the
necessary requirements, or even what a well-designed set of requirements
would be. Work needs to be done to (1) clarify the requirements, (2)
provide existence proofs for effective signalling protocols, (3) develop
system configurations that meet the requirements, (4) construct
representative hardware and (5) implement a test program to validate (and
iteratively improve) system performance. This work should be performed
before any regulatory action to permit opportunistic transmission is taken.
The report contemplates (p.35) that "rights of licensed incumbent users
should be limited to some extent to create "easements" for non-interfering
uses below a defined interference temperature." The concept of
interference temperature is a good one, but again, what counts is the
interference at the location that is being interfered with, not at the
location that is doing the transmitting. Again, this requires an active
signalling infrastructure, since a transmitting device at one location must
evaluate the interference that it is generating at another user's location.
Similarly, page 35 states that "certain technologies should probably be
allowed access to spectrum 'holes'," but how is a hole to be defined or
determined? A way is required for a device to determine "permission."
Conclusion: The recommendations are exciting and "outside the box," but it
is important to assess the interference issues, access control
methodologies, potential improvement to be gained, and appropriate policy
structure, that are necessary to implement and regulate this new type of
access. A better understanding of these factors should both precede and
guide the drafting of policy changes.

Respectfully submitted,
Joel Schindall

Professor of the Practice
MIT Department of Electrical Engineering and Computer Science
Cambridge, Massachusetts
January 23, 2003

The author is a member of the MIT Electrical Engineering and Computer
Science faculty. Formerly, the author was Chief Engineer of Globalstar, a
system in which interference detection was required to satisfy a licensing
requirement to avoid interference with radio astronomy sites.

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