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Accepted for publication in IEEE Proceedings of the Dynamic Spectrum Access Networks (DySPAN2008) Conference, Chicago,
October 14-17, 2008:
Spectrum Pooling for Next Generation Public Safety Radio Systems
William Lehr
Massachusetts Institute of Technology
Cambridge, Massachusetts, USA
Nancy Jesuale
NetCity
Portland, Oregon, USA
Abstract -- The Dynamic Spectrum Access (DSA) research
and development community is maturing technologies that
will enable radios to share RF spectrum much more
intensively. Adoption of DSA technologies by the public
safety community can better align systems with the future of
wireless services more generally and can contribute to
making next generation public safety radio systems more
robust, capable, and flexible.
A critical first step toward a DSA-enabled future is to reform
spectrum management in order to create spectrum pools that
DSA-enabled devices such as Cognitive Radios (CRs) may
make use of under the control of more dynamically flexible
and adaptive prioritization policies than is possible with
legacy technology. Appropriate reform will enable spectrum
portability, facilitating the decoupling of spectrum rights
from the provision of infrastructure.
This paper examines the economic, policy, and market
challenges of enabling spectrum pooling and portability for
public safety radios.
Keywords – spectrum management; economics; policy;
public safety; cognitive radio
I. INTRODUCTION
Dynamic Spectrum Access (DSA) technologies, including
Cognitive Radio (CR) technologies, are in development for the
next generation of commercial, military, industrial and public
safety networks. These technologies hold the promise of more
flexible and adaptive radio architectures, capable of sharing
the RF spectrum much more intensively than is feasible with
today’s currently deployed technologies, regulatory
frameworks, and business models. Such increased sharing is
critical for the continued growth of wireless services in order
to help alleviate growing spectrum scarcity. The
commercialization of DSA technologies represents an
important next step in the evolution of the wireless services
ecosystem. The need for and the opportunities offered by DSA
are especially relevant to the public safety community, which
provides an important test case for the commercialization of
DSA techniques.
The current landscape of wireless networking reflects the
legacy of a world premised on a more limited set of
user/system capabilities and needs, reflecting some
fundamental assumptions about static network architectures
and spectrum allocations. In this world, public safety networks
have traditionally been designed to meet channel capacity and
reliability “standards” that are based on user requirements at
the “worst case” level – that is the capacity and reliability
necessary during an emergency or catastrophe. It is not
assumed that the network will always need these levels of
capacity and reliability during “day to day” operations. But it
is assumed that the network must always have these levels of
capacity and reliability available when needed. Such worst-
case planning implies that significant spectrum and equipment
resources need to be “stockpiled” and remain unused most of
the time. This creates significant artificial spectrum scarcity.
The wireless world is changing. The needs for wireless
systems of all types, and for public safety systems in
particular, have greatly expanded. This increases the costs and
collective infeasibility of continuing worst-case planning and
the wasteful allocation of resources it implies. As we explain,
the radio future, of necessity, will require shifting to more
DSA-friendly modes of spectrum usage. Besides being
inevitable, the transition to DSA will offer many significant
benefits for the public safety community and wireless users
more generally. These benefits will include better mission
responsiveness, expanded capabilities, and ultimately, lower
costs. However, getting to this future will also entail
overcoming important challenges. A number of
complementary innovations are required. These include
further technical developments, public policy reform, and
changing industry and end-user attitudes.
In this paper, we explain why public safety spectrum pooling –
the sharing of public safety spectrum among public safety
users and possibly others – is an important and logical first
step toward the transition to DSA. Even if one assumes that all
of the requisite technology existed today, the DSA world
would be hampered by a lack of appropriate policies and
business models to enable DSA’s safe use. While further
technical research and product development is certainly
needed, our focus here is on the policy and business practice
challenges of developing DSA technologies for use by public
safety systems.
To make our case, we first articulate a vision of the radio
future in Section 2, including explaining more fully how
DSA/CR relate to the requirements for next-generation public
safety systems. In Section 3, we examine the role of
government spectrum management policies in creating the
legacy environment and the steps being taken that make it now
reasonable to adopt the spectrum pooling concept as a logical
next step. In Section 4, we examine more fully the concept of
spectrum pooling and summarize the benefits for public safety
![](https://csdnimg.cn/release/download_crawler_static/1221233/bg2.jpg)
from moving toward the DSA/CR future. In Section 5, we
discuss some of the practical next steps that may be taken and
core elements that are needed to implement spectrum pooling,
before turning in Section 6 to address some of the key
challenges impeding adoption of the concept. Section 7
concludes with a discussion of some of the broader issues and
benefits expected from the success of public safety spectrum
pooling and directions for future research.
II. CHANGING ENVIRONMENT FOR PUBLIC
SAFETY RADIOS
As we explain, the future of radio systems of all types –- but
especially those for public safety -- will require much greater
reliance on Dynamic Spectrum Access (DSA) and related
technologies. This is clear from examining trends in
technology and research priorities, market growth, and the
changing mission requirements for public safety professionals.
DSA is necessary to enable new capabilities in the face of
growing demand for spectrum access from all sources.
A. Vision of the Radio Future
While the precise shape of the radio future may be difficult to
discern, certain key aspects appear certain. The radio future
will include lots more wireless of all kinds, greater demand for
mobility and portability and more heterogeneous wireless
networks. These future developments will have concrete
implications for radio network design, including the need for
more broadband capacity, enabling more dynamic and flexible
services and spectrum sharing.
In the following sub-sections, we discuss more fully each of
these developments for the public safety radio future.
1) Lots more wireless of all kinds
There will be many more wireless users, uses, and devices and
lots more wireless traffic than we have today. This means that
demand to share RF spectrum will intensify. There will be
more and richer interactive communication among users
(multi-party) and with devices (computers talking to people
and other computers). These communications will include
exchanging mixed video, voice, and data for services such as
video conferencing, video streaming, distributed collaboration
and resource sharing (files, printers, storage), and cable-free
interconnection. There will be increased use of both remote
(passive) sensing (via satellite imaging) and active sensing
(via RFID or active sensor networks). This means there will be
a greater need for higher bandwidth wireless services
(broadband) and more flexible/dynamic platforms to support
changing user/application needs since the requirements of
applications will differ and the range of applications used by
different users will also vary (over time, by location, and by
type of user).
The same will be true for next generation public safety
systems, especially in light of the heightened awareness of the
need to coordinate across multiple agencies in our post-9/11,
post-Katrina world. First-responders will need lots more
wireless of all kinds to allow them to communicate with the
wider array of agencies and departments with which they will
need to coordinate; to take advantage of the sensor technology
being built into today’s smart buildings and embedded in
transportation grids; to make use of high-resolution satellite
imaging; and to enable their personal smart appliances (e.g.,
biometric monitors embedded in safety gear and clothing) to
communicate with on-site management and possibly hospital
personnel.
2) Greater demand for mobility/portability
A key feature of wireless is that it enables mobility. This
includes the fast mobility required to sustain a conversation in
an automobile going 100km/hour while passing across the
coverage areas of several base stations as well as the slower
mobility of pedestrians moving around the coverage area of a
single base station. It includes the mobility associated with
nomadic uses (WiFi hotspot roaming) and to support
equipment/user moves within an office, across town or state,
or to another country. It includes the flexibility of cable-free
deployment.
1
Moreover, increased mobility often also implies
a need for greater portability with its attendant limitations on
form factor, weight, and power (batteries). While not all
wireless devices need to be portable or have the same mobility
requirements, demand for such portability and mobility (in all
its attendant flavors) will surely increase. And, of course, for
public safety users, mobility and portability are essential
features. First-responders in remote areas or areas where
infrastructure has been destroyed or is otherwise over-
burdened may have no alternative but to bring their
communication capabilities with them.
3) More heterogeneous wireless environment(s)
There will be many types of networks and networking
environments to support all of these wireless uses and users.
This will include Personal Area Networks (PANs), Local Area
Networks (LANs), and Wide Area Networks (WANs).
2
In the
public safety context, these concepts are expressed with
slightly different emphasis. Public safety requires PANs,
Incident Area Networks (IANs), Jurisdictional Networks
(JANs) and WANs.
3
These wireless networks will be
implemented over a wide-range of technologies and
architectures (high and low power, broadband and
narrowband, fixed and mobile, hierarchical and peer-to-peer,
centralized and distributed, planned and ad hoc) under a
1
For example, Bluetooth or UWB to connect stereo components or personal
biometric monitoring sensors mounted on a patient or first-responders hazmat
suit.
2
Wireless technologies for different ranges (from a few inches to thousands of
miles) and use environments have very different requirements that give rise to
specialized and different technologies. While technologies designed for one
purpose may often be used for another (e.g., VoIP over WiFi), there is no
single technology/architecture that is best in all situations.
3
See Section 8, SAFECOM, “Public Safety Statement of Requirements for
Communications & Interoperability,” U.S. Department of Homeland Security,
Volume I, Version 1.2 (October 2006) (“SAFECOM SoR”) (available at:
http://www.safecomprogram.gov/NR/rdonlyres/8930E37C-C672-48BA-
8C1B-83784D855C1E/0/SoR1_v12_10182006.pdf).
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variety of business models (customer-owned and operator-
provided, for-profit and subsidized, single and multi-provider).
The heterogeneity in infrastructure and service models will be
mirrored by (and driven in part by) the heterogeneity in end-
user traffic profiles. We expect user traffic distributions to
become more fat-tailed as new applications/uses become
available and the range of user types expands. For example,
sensors may expand demand for narrowband, delay-tolerant
wireless networking while high-resolution video imaging
(medical imaging) may expand the demand for high-
bandwidth, real-time services. Individual traffic is likely to
become more bursty (i.e., peak-to-average traffic rates will
increase for most users and the range of user types will
expand) as new higher data rate services become available.
4
4) Greater demand for modularity, openness, and
integrated services
Demand will grow for all types of electronic communication
services, both wired and wireless. The growth in wired
services will be synergistic and complementary to the growth
in wireless.
5
An obvious implication of this will be to increase
demand to support seamless integration and interoperability
across wired and wireless services and networks.
Modularity, componentization, standardization, and open
interfaces have helped drive the exponential cost reductions
and productivity improvements that have characterized
information technology for many years. These trends in
hardware and software system design have helped enable mix-
and-match bundling (expanding the product space to augment
aggregate demand), facilitated the realization of global scale
and scope economies, and helped promote competition, with
its attendant benefits in terms of encouraging still greater
efficiency improvements.
These benefits will continue to be felt across the entire
Information and Communications Technology (ICT) value
chain, but increasingly across wireless services and devices,
which traditionally, have been more likely to be single
purpose, closed/proprietary, integrated systems. For public
safety to benefit from these economies of scale, it must adopt
the same advanced technologies as the larger community of
commercial users. The concern with commercial off-the-shelf
technology (COTs) in the public safety environment has
always been, and will continue to be reliability. Commercial
equipment spans the range from consumer-grade solutions
where cost is often more important than reliability to industrial
4
All users will be heavy users some of the time (when they use resource-
intensive applications), and there will be more users who are heavy users
more of the time (and those who are light users more of the time). Examples
of all such profiles/users are easy to suggest.
5
Although in some cases, wired services may be viewed as substitutes for
wireless, overall, we expect the impact of growing wireless and wired services
to be complementary. For example, consider how WiFi routers helped
increase aggregate demand for DSL/cable modem services by making such
services more valuable. And, consider how the expansion of fiber toward the
edges of wired networks expand the capabilities of high-bandwidth, shorter-
range wireless services.
grade solutions, where communications infrastructure are
regarded as mission-critical, essential services. Although
public safety users may have somewhat atypical reliability
requirements (e.g., public safety radios may encounter more
averse environments than in the typical business office), this
does not mean that their reliability needs are best addressed by
building wholly separate, customized solutions. We address
this further below when we discuss the challenges facing
public safety users.
5) Broadband needed
While many of the future wireless services – including
traditional voice and low-bit-rate data services do not require
high-bandwidth channels – many of the newer services will.
This will include bandwidth hungry applications such as high-
resolution video streaming or video conferencing. Even
narrower-band services such as voice-over-IP (VoIP) or other
overlay services may need access to a broadband channel.
This represents a new challenge for mobile networks and for
the current spectrum management regime.
Traditional spectrum management has sliced spectrum into
narrow frequency bands, especially for the beachfront lower-
frequency spectrum below 3GHz. While advances in
modulation techniques continue, the biggest benefits in
accelerating data rates are likely to come from spreading
signals over wider-frequency bands (spread spectrum). This is
especially true for legacy public safety radio systems in which
narrow banding and dedicated channel assignments have left
the public safety spectrum overly fragmented. DSA
technologies may enable the bonding of multiple narrowband
channels and facilitate dynamic service relocation to meet
broadband “on demand” needs.
6) More dynamic, flexible, and interoperable radios
needed
The increased heterogeneity of uses, users, and environments
means that radio systems will need to become more flexible
and dynamic as well as more reliable. This flexibility and
reliability is needed to support the heterogeneous usage
models. Users do not want to carry separate radios for each of
their diverse application tasks. Flexibility and dynamic
adaptability are also needed to help support end-to-end
interoperability and reliability in the heterogeneous
networking environment expected in the future. Additionally,
increased flexibility and dynamic capabilities are needed to
support end-user customizability and are in keeping with the
trends toward modularity, competition, and open systems
discussed above.
One key direction for expanding the capabilities of wireless
systems is to better enable federated, ad hoc, mesh networking
to support end-to-end interoperability across diverse
users/applications/networks, to support roaming, and to
expand coverage.
6
These capabilities, which are of special
6
Federated networking refers to the ability to traverse heterogeneous network
architectures owned by others. Ad-hoc networking refers to the ability for
devices to communicate without network infrastructure. Mesh networking is a
![](https://csdnimg.cn/release/download_crawler_static/1221233/bg4.jpg)
importance to public safety radios, are largely missing today.
They can also provide so-called “infrastructure-less” or
“carrier-free” networking. For example, public safety users in
a community may find themselves in a location where there is
no public safety or operator infrastructure (e.g., they are in a
remote locale or the infrastructure has been destroyed). Ad-
hoc networking will be a key innovation on the reliability
front. If devices can rely on the ability to self-form networks,
without power or transmission infrastructure, a key concern
about today’s network reliability and resiliency (that the tower
will go down) is moot.
Finally, the rapid pace of innovation and the need to
communicate between legacy and new systems further
accentuates the need for flexibility and interoperability.
7) Spectrum sharing needs to increase
The overall growth in wireless of all flavors and the need to
support broadband and heterogeneous usage implies that
spectrum will be increasingly scarce. Dedicating spectrum to a
specific use or radio technology and “worst case” provisioning
typical in public safety architectures will be harder to sustain.
Users and uses will need to share spectrum more intensively.
There will be many drivers for increasing spectrum sharing.
First, there are demand-side drivers propelling wireless
services toward greater spectrum sharing. For example,
increased demand for broadband services and more dynamic
services make it less feasible to dedicate spectrum to a single
use/user in advance of an actual need.
Second, users seeking seamless interoperability are going to
expect their services to roam across diverse devices, wireless
platforms, and from wireless to wired networks. From a
commercial perspective, operators are going to want to offer
services that support a customer experience that is
independent of the physical layer (as much as possible). In the
public safety realm, responders will need to access
applications that are familiar whether they are in their home
area or not, and whether they are mobile, or in the
stationhouse. This drive toward increased interoperability and
independence between infrastructure, services, and
applications/uses will be accentuated by industry restructuring
(through mergers and acquisitions) in the carrier community
that will change the spectrum resources available to the
operator and the networks that need to be integrated and
supported.
As the carrier marketplace changes, the public safety
landscape will change with it. Public safety users will expect
(justifiably!) to have available enhanced applications which
are (at least) as capable, simple to use, and offer the same sort
of ubiquitous, plug-and-play, 24/7 availability across multiple
vendor platforms as will be available to mass market
consumers.
specific type of ad-hoc networking wherein devices form networks by routing
traffic from and to other devices nearby.
Third, on the supply-side, spectrum sharing will contribute to
efforts to minimize network provisioning costs in the face of
increasing traffic burstiness, fat-tailed usage patterns, and the
need to support multimedia traffic. These forces will
encourage commercial and public safety network operators to
adopt technologies for sharing spectrum resources more
intensively.
Fourth and finally, regulatory policies for spectrum
management are being reformed. Restrictions that precluded
more intensive spectrum sharing are being removed. For
example, new allocations of unlicensed spectrum and more
flexible licensing frameworks (e.g., technology neutral,
tradable licenses) are being adopted in a number of countries.
The unprecedented award of a national license for public
safety broadband spectrum to a non-profit trust in the US is
another example of policy reform that enhances prospects for
sharing.
7
B. DSA, Cognitive Radio technologies help make wireless
future feasible
At the same time that user requirements for wireless services
are increasing and the policy environment is becoming more
favorable to spectrum sharing, the capabilities of wireless
technology have expanded significantly. Advances in antenna
design, signal processing, software/cognitive radio, and new
networking technologies (e.g., mesh and ad hoc networking)
are making it increasingly feasible to support the diverse array
of wireless services and usage scenarios suggested by the
future described above.
These technologies significantly expand the capabilities of
wireless systems to support more rigorous application
requirements such as higher data rates and better signal
resolution in more adverse environments (e.g., with lower
power, lower signal-to-noise ratios, and more congestion) and
at lower cost.
8
These technologies also make it increasingly
viable to implement systems that are more dynamic and
responsive to their local environments, including allowing
those devices to be frequency agile.
Collectively we refer to these as Dynamic Spectrum Access
(DSA) technologies. These allow radios to traverse one or
more frequency bands across time, geography, and users/uses.
While such sharing may be enabled by a single network
operator over spectrum resources under a single party’s
control (e.g., when public safety networks trunk frequencies
and share them among multiple entities), more generally, such
technology may enable spectrum sharing across multiple
providers’ infrastructure and users. This includes
infrastructure-less mesh or ad hoc networking. Such sharing
could utilize unlicensed spectrum or share exclusively licensed
spectrum resources (i.e., spectrum pooling – as we shall
7
We refer to the recent award of the 700 MHz public safety broadband
spectrum license to the Public Safety Spectrum Trust (PSST). This policy is
discussed more fully in subsequent sections of this paper.
8
At this early stage of development, DSA/CR technology solutions are likely
to be more expensive than legacy solutions when used for legacy applications,
at least in the near term.
![](https://csdnimg.cn/release/download_crawler_static/1221233/bg5.jpg)
discuss further below). While the unlicensed management
model provides only limited interference protection, the
spectrum pooling model is explicitly designed to provide
robust interference protection.
9
Cognitive radio (CR) captures the flavor of these advances: a
CR is capable of sensing its local radio environment and
negotiating modifications to its “waveform” (modulation
scheme, power-level, or frequency/channel access behavior) in
real-time with other CRs, subject to “policy constraints” (e.g.,
that may limit the range of waveforms allowed). The policy
constraints are enforced by the radio’s policy engine. Policies
may include authorization to transmit in specific locations and
frequencies at specific times, or may include access protocol
constraints (e.g., listen-before-talk). These policies may be
static and hard-coded into the radio, downloaded from a
database, or may be dynamic and subject to updating in real-
time in communication with a network operator or other CRs.
DSA/CR devices typically require location awareness
capability in order to support the policy engine and because
interference is a local phenomenon occurring at a receiver’s
location. This may be implemented via some sort of GPS
technology (e.g., terrestrial or satellite). Finally, CRs are
inherently multi-band radios, allowing the radio to transmit or
receive in a wider range of frequencies than might be used in
any specific communication environment. This allows CRs to
opportunistically make use of unused spectrum and facilitates
their interoperability with legacy radio systems.
While significant technical work remains to be done in
academic research and commercial product development
laboratories to field a commercially viable CR, prototypes
already exist and many aspects of the technology are already
embedded and working at scale in commercial systems. In this
paper, we do not focus on the technical developments that still
must be made, but rather on the policy innovations that are
required to make commercialization viable. In Table 1, we
summarize how DSA/CR technology will aid in realizing the
DSA future.
C. Next Generation Public Safety radios need to enable the
wireless future
As already noted, the same forces that are shaping the future
for commercial wireless apply with even stronger force to
public safety wireless systems.
First, public safety first-responders are more likely than most
other users of ICT to require mobile, wireless access. For
many folks, a wired alternative may be less convenient, but
may still be feasible. In many first-responder scenarios, the
only option is wireless.
9
A key difference between unlicensed spectrum access and spectrum pooling
is that the former does not prioritize or limit user access. With unlicensed
spectrum, any device that complies with the technical access requirements
may use the spectrum, and users are not protected from interference due to
congestion or from other compliant uses. With spectrum pooling, the range of
users may be restricted and priorities granted to enable interference protection.
Assurances of such protection are likely to be essential in gaining public
safety user trust and acceptance of the pooling concept, as we discuss further
below.
Second, first-responders who are dealing with life-and-death
situations are perceived generally as having a higher social
welfare value than commercial or leisure uses, and thus
meriting higher priority in the event of competition for
resources.
Third, first-responders may be more likely to deal with
adverse environments. For example, in the event of a natural
disaster or forest fire, first-responders may need to operate
where there is little physical infrastructure (remote areas) or
the infrastructure has been destroyed (Katrina, inside a
burning tunnel underground). This increases their need for
flexible, adaptive systems (e.g., capable of supporting ad hoc
or mesh networking in the absence of other supporting
infrastructure). First-responders are likely to suffer from
localized congestion: disasters typically happen in specific
places and at specific times. The demand for all wireless
services by all first-responders are likely to be concentrated in
time and place, increasing the peak-provisioning problem.
10
While demand is likely to be positively correlated, pooling
will enable users to take advantage of such multiplexing
opportunities as exist and offer greater flexibility in
prioritizing whatever rationing needs to occur.
Fourth, first-responder systems have traditionally been locally
provisioned and subject to the vagaries of public funding. As
such, these are often the users whose budget constraints and
whose ability to fund dynamic capacity adjustments are most
challenged.
11
As we discuss further below, this helps explain
why public safety network operators are especially cost-
sensitive and risk-adverse. Spectrum pooling will facilitate
cost sharing and more dynamic capacity planning.
Fifth, while the general need for rich interactive multimedia
entertainment services remains suspect (e.g., mobile
television), there are many compelling first-responder
applications promised by the wireless future. This includes
much better capabilities for situation awareness (e.g., on-site
medical video, remote/local sensing data sharing), interactive
communications (e.g., video conferencing, robust telephony
for adverse environments
12
), and interoperability support (e.g.,
to support inter-agency/department communications, roaming
for mutual aid support). Specific demand scenarios are
discussed further below.
Sixth, in the post-9/11, post-Katrina world, there is a
heightened awareness of the challenges that first-responders
and other public safety providers need to be prepared for, and
the role for advanced wireless services such as those discussed
above.
10
This is another reason why traditional worst-case provisioning is cost
prohibitive in the public safety future.
11
Oversight of public funding and non-profit status impose bureaucratic
constraints on expenditures and budgets that make it difficult for public safety
network operators to rapidly scale or adjust their capacity, and typically also
impose tight constraints on overall spending.
12
This includes voice conferencing in infrastructureless environments
(underground, remote areas, where traditional infrastructure has been
destroyed) and noisy environments (high interference).
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