On the Subject of Spectrum Sharing

Dr. Stewart D. Personick

Introduction

In this white paper it is argued that spectrum sharing by unrelated
parties, using methods other than traditional frequency division
multiplexing, presents difficult engineering challenges and difficult
business/regulatory challenges that are far greater than those
associated with traditional frequency division multiplexing... even
though, theoretically, new methods of spectrum sharing, such as spread
spectrum methods, are mathematically equivalent to frequency division
multiplexing.

Traditional Frequency Division Multiplexing

Traditional sharing of the spectrum by using frequency division
multiplexing... i.e., different users utilize different, non-overlapping
bands of frequencies, is relatively easy to implement (from an
engineering perspective) and manage (from a business and regulatory
perspective) because it is technically and economically feasible to
implement band-limiting filters that have very high levels of blocking
of unwanted frequencies (e.g., easily greater that 70 dB of attenuation
of adjacent frequencies), and that have very high dynamic range (i.e.,
they will not have problems separating very strong signals that are well
outside of the "passband" from very weak signals inside of the
passband\*). It is the very nature of frequency division multiplexing
(the use of sinusoidal "basis vectors") to be compatible with frequency
separation methods based upon passive components (inductors, capacitors,
mechanical filters, crystal filters....) that have these desirable
properties. One can implement filters using modern digital signal
processing methods... and this is often done in modern radio
transmitters and receivers. However, the imperfections of
analog-to-digital (A/D), and digital-to-analog (D/A) converters, limit
the dynamic range of "digital filters". Thus, to ensure that systems
using non-overlapping frequencies bands do not cause harmful
interference, transmitters and receivers often supplement digital
filters with traditional analog filters (in the front end and 1^st^ i.f.
stages of the receiver, and in the final stages of the i.f. chains, and
the r.f. stages of transmitters).

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\*Since radio frequency signals experience a very wide range of
attenuation values as they propagate through a real environment... and
since radio frequency receivers can function properly with received
signal levels that are very tiny (e.g., -110 dBm received signal level,
compared to the greater than 0 dBm level of the transmitted signal) it
is very important that the receiver be able to separate a very strong
interfering signal from a weak desired signal.

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It is relatively easy to determine, with simple, inexpensive, and
easy-to-understand measurement equipment, whether or not a given
transmitter is emitting signals outside of the band of frequencies that
it is authorized to be using; and whether or not those spurious
emissions are within prescribed limits set by the FCC. It is relatively
easy to determine, with simple, inexpensive, and easy-to-understand
measurement equipment, whether or not a given receiver is overly
susceptible to harm caused by signals that are outside of the band of
frequencies it is intended to be sensitive to.

Combining Traditional Frequency Division Multiplexing with Spread
Spectrum Techniques

Case 1. Spread Spectrum Techniques Employed by Unique Administrative
Entities within Non-Overlapping Frequency Bands

It is well known that 2^nd^ generation cellular systems employ modern
spread spectrum techniques. Even the consumer name, "CDMA", captures one
of these methods. \[One can also consider TDMA as a spread spectrum
method... but some experts might not think of it as such\].

What is less well known, even among some academics teaching courses on
modern telecommunications technologies, is that 2^nd^ generation
cellular systems, operated by different administrative entities (e.g.,
Verizon, Cingular, AT&T), do not operate in overlapping frequency bands.
In CDMA systems, the overall (aggregate) band of spectrum available for
use by cellular operators (e.g., 20 MHz) is divided into
[non-overlapping bands]{.underline} of frequencies that are 1.25 MHz
wide. Within a given 1.25 MHz band, the associated system operator
(e.g., Verizon) may employ spread spectrum techniques to increase the
effective utilization of that band (i.e., number of customers that can
be simultaneously served in a given geographical area)... but no other
cellular systems, within the same geographical area, operate in that
band.

Naturally, when a single administrative entity operates a system or
network that employs spread spectrum techniques... that administrative
entity is highly motivated to resolve any harmful interference problems
that arise as the result of the signals being produced and the equipment
being employed within his own system. It makes no business sense for the
chief engineer of that administrative entity to be pointing his finger
at himself. Therefore, the administrative entity will quickly take
whatever steps are needed, at its expense, to resolve its own harmful
interference problems. It may seek technical support and subsequent
compensation from the suppliers of its equipment... but litigation, if
any, is an after-the-fact (i.e., subsequent to resolution of the harmful
interference) matter.

Since different administrative entities use non-overlapping frequency
bands (traditional frequency division multiplexing) it is relatively
easy to determine whether or not a system operator is generating
spurious signals (outside of his assigned frequency band) that are in
excess of what is permitted under FCC rules. It is relatively easy to
design cost-effective transmitting equipment in such a way as to ensure
that such spurious emissions do not occur; and it is relatively easy to
design cost-effective receiving equipment to be sufficiently insensitive
to strong signals on frequencies outside of the band of frequencies that
it is intended to be receiving.

Case 2. Multiple Administrative Entities Employing Spread Spectrum
Techniques to Share Overlapping Bands of Frequencies

It is well known that the FCC has designated certain bands of
frequencies as "unlicensed bands". In those bands, various spread
spectrum techniques are employed to enable uncoordinated (e.g.,
administratively unrelated) users to share the available spectrum in a
shared geographic area. Examples of applications of technologies that
use these unlicensed bands are: 802.11x local area networks, cordless
telephones, wireless point-to-point links, etc.

To the extent that users of the unlicensed band manage to avoid, or are
willing to tolerate operational problems caused by excessive
interference (reduced throughput, erratic operation, the need to "find a
good spot" to operate their equipment), and to the extent that users
manage to cooperate to alleviate interference problems that do occur...
sharing of a band of frequencies using spread spectrum methods is a very
cost effective and administratively easy approach.

However uncoordinated sharing of a band of frequencies by
administratively unrelated users is likely to result in significant,
difficult to resolve conflicts among users who do not share a spirit of
cooperation because:

As in any other radio frequency wireless methodology, the systems must
be designed to accommodate a very large amount of attenuation between a
transmitter (output \~ 0 dBm) and a receiver (minimum required signal
typically -80 dBm or less, depending upon the data rate), and thus one
has to design the system to accommodate a very large interfering signal
(from a nearby transmitter) while receiving a very weak desired signal.

However, because of the technical limitations of receivers and
transmitters which employ spread spectrum techniques (e.g., limited
transmitter output stage linearity, limited dynamic range/linearity of
the receiver front end, limited accuracy of A/D converters that are
employed at i.f. or baseband), a combination of engineering design
errors, component variations (a bad A/D converter, a bad r.f. or i.f.
amplifier stage), and uncooperative operational approaches (I don't have
to outrun the bear, I just have to outrun you) will lead to
difficult-to-diagnose, technically complex (extremely hard for a
technical expert, not to mention a regulator or a juror, to understand)
cases of harmful interference.

Conclusion

While spread spectrum techniques offer the possibility of more efficient
use of available spectrum, and also the operational benefit of
uncoordinated spectrum sharing... the FCC should be concerned about the
serious practical difficulties of employing pure spread spectrum
techniques (as opposed to combinations of spread spectrum and frequency
division multiplexing) among administratively unrelated users.