DIMSUMNet: New Directions in Wireless Networking Using CoordinatedDynamic Spectrum Access

(Position Paper for IEEE WoWMoM05, June 2005)

Milind M. Buddhikot Paul Kolodzy Scott Miller Kevin Ryan, Jason EvansLucent Bell Labs Stevens Institute of Technology Lucent Bell Labs Stevens Institute of Technology

mbuddhikot@lucent.com pkolodzy@stevens.edu scm@lucent.com �kryan,jevans�@stevens.edu

Abstract

Recent advances in Software Defined Radio (SDR), wideband spectrum sensing, and environment aware real-timespectrum allocation [13, 10, 4] show promise of enablingthe new paradigm of Dynamic Spectrum Access (DSA) net-works. These networks aim to provide dynamic access tolarge swaths of spectrum which is currently statically parti-tioned and underutilized. Majority of research in this areahas focused on free-for-all, opportunistic methods commonin ad-hoc military applications [10, 12, 13, 15, 16].

In this paper, we argue that a simpler, pragmatic ap-proach that offers coordinated, spatially aggregated spec-trum access via a regional spectrum broker is more attrac-tive in the immediate future. We first introduce two new con-cepts, namely, Coordinated Access Band (CAB) and Statis-tically Multiplexed Access (SMA) to spectrum that form thebasis of our work. We then describe their implementationin the new DIMSUMnet network architecture consisting offour elements: base stations, clients, a Radio Access Net-work Manager (RANMAN) that obtains spectrum leases,and a per-domain spectrum broker that controls spectrumaccess. We also describe DIMSUM-RelayCluster – a multi-hop all-wireless architecture to illustrate application of co-ordinated DSA to fixed wireless access and mesh networks.

1 Introduction

In the United States, the Federal Communications Com-mission (FCC) sets the rules that govern access to spectrum.These rules have lead to reservation of spectrum chunks forspecific purposes; for example, 824-849 MHz, 1.85-1.91GHz, 1.930-1.99 GHz frequency bands are reserved for li-censed cellular and PCS services and require a valid FCClicense, whereas 902-928 MHz, 2.40-2.50 GHz, 5.15-5.35GHz, 5.725-5.825 GHz frequency ranges are reserved asfree-for-all unlicensed bands. This strict, long-term spec-trum allocation (Figure 1) is space and time invariant and

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any changes to it happen under strict FCC control. Regula-tory authorities analogous to FCC exist in other countries,(e.g: Office of Communications (OFOCOM) in UK) andsimilar observations apply equally in their case.

The static partitioning of spectrum has significant oper-ational implications which have been recently brought tolight by extensive spectrum utilization measurements in theUSA and Europe [3]. First, a large part of the radio spec-trum is allocated but barely used in most locations. Severalradio bands allocated for military, government and public-safety use experience negligible utilization. The cellularand PCS bands are however quite well utilized but the uti-lization varies dramatically over time and space.

Often times, technology assumptions that are now anti-quated have served as a basis for the amount of spectrumhistorically reserved for a particular purpose. For example,in case of VHF, UHF bands reserved for television broad-cast in the United States, allocation of 6 MHz per TV chan-nel was based on old analog NTSC system even though bet-ter quality video can be now broadcast with almost 50 %less spectrum per channel. Given the pervasive penetrationof cable-TV, this precious spectrum, though allocated andowned, remains unused in most locations.

On the other hand, unlicensed bands (such as ISM,UNII) which require no spectrum ownership cost, have fu-eled technology innovation, mass market availability of lowcost network and client devices, and rapid network growth.However, often these bands experience significant interfer-

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ence due to uncoordinated, aggressive deployment, leadingto overcrowding and poor network guarantees.

The current spectrum management methods have leftvery little spectrum to allocate both for new services and forexpansion of existing services, leading to an artificial spec-trum scarcity, even though large swath of spectrum remainsunderutilized. In other words, current spectrum usage is ac-cess limited rather than throughput limited [1].

The business implication of current spectrum manage-ment is that it has created purpose built networks which re-quire capital intensive steps of licensing spectrum, deploy-ing network infrastructure and offering end-user services.Often times high costs and long-drawn process for spectrumlicensing have slowed network deployment as evidenced byhuge cost-overruns world-over for 3G networks. This hasled to big player syndrome where only big service providerscan operate networks, stifling competition and fair access.

1.1 Position Statement

We argue that the current spectrum management processmust give way to a new approach that breaks down artificialspectrum access barriers and enables networks and their en-dusers to dynamically access spectrum. In fact, recent tech-nology trends [13, 10, 4] and early policy trends [11] indi-cate long term feasibility of opportunistic, adaptive accessto spectrum, often termed as the new paradigm of DynamicSpectrum Access (DSA). These trends are as follows: (1)Software Defined Radio (SDR): advances in smart, adap-tive antennas, high bandwidth A/D conversion, low poweramplifiers, fast digital signal processors and inexpensive re-configurable hardware will make Software Defined Radio(SDR) a reality as evidenced by new companies such asVanu, Inc., and Sandbridge Technologies [4]. SDRs en-able on–the fly changes to characteristics of radio such aspower, modulation, waveform, and MAC and allow samehardware to be reconfigured for use in different parts ofthe radio spectrum. (2) Wideband Spectrum Sensing: Hard-ware capable of tuning to any part of a large range of fre-quency spectrum (5 MHz to 6 GHz) has been demonstrated[10]. Such spectrum sensing enables real-time measure-ments of spectrum occupancy and inference on underuti-lized portions of the spectrum. Spectrum sensing combinedwith SDR and policy specific functions enables the Adap-tive Cognitive Radio (ACR) that adapts based on its aware-ness of locale and spectrum. This allows spectrum to be uti-lized based on real-time sensing and decision-making. (3)The regulating bodies in the USA [11] and European Union[16] are taking initial steps to alter policy to allow experi-mental networks where spectrum is dynamically managed.

The majority of the research on DSA networks, [10, 12,13, 15, 16] fostered by on-going ambitious programs suchas DARPA XG [2], has focused on free-for-all, opportunis-tic spectrum access for peer-to-peer ad-hoc communica-tion, typically targeting military applications. A majorityof these approaches resort to spectral bandwidth brokering

at the individual node level. However, the complexity in theprotocols and the sensing and agility requirements at theindividual radio in this case is quite high. Also, its initialutility is limited to homogeneous radio systems[10]. We ar-gue that this is the most ambitious form of DSA. We see

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Coordinated DSA with FixedWireless Access And Cellular

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Figure 2. Range of options for DSA

a range of options exist for DSA as illustrated in Figure 2.On this range, the current static allocation represents theleft most point, whereas the fully-distributed DARPA XGmodel represents the rightmost point. In between exist whatwe call as Coordinated Dynamic Spectrum Access Networkswherein the access to the spectrum in a region is controlled,coordinated by a centralized entity called Spectrum Broker.As one goes from left to right on the range indicated, thecomplexity in terms of heterogeneity of networks, amountof spectrum accessed and rapidness of spectrum access in-creases.

Figure 3 illustrates our model of coordinated DSA for in-frastructure based multi-provider cellular, fixed-wireless ac-cess and mesh networks. Here the service providers and/orusers of these networks do not apriori own any spectrum;instead they obtain time bound rights from a regional spec-trum broker to part of the spectrum and configure it tooffer the network service. Realization of this model re-quires new technologies in the form of coordination, sig-naling protocols, network elements and client devices. We

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assert that our model has several impacts: (1) analogousto Dense Wavelength Division Multiplex (DWDM) opticalnetworks wherein activating (lighting) a new wavelength in-creases bit transport capability of deployed fiber, dynam-ically adding spectrum enables new kind of wireless net-works with bandwidth-on-demand capabilities. (2) Our “as-when-where-needed” spectrum leasing model can enablecomplete unbundling of service provider market. This willenable new kind of providers and businesses and potentiallyreduce costs in network deployment and services. (3) Fromthe enduser perspective, this improves spectrum access effi-ciency and enables true wireless broadband.

In the following sections, we make a case for above as-sertions.

1.2 Outline

The rest of this paper is organized as follows: Section2 provides the overview of the three main ideas, namelyCoordinated Access Band (CAB), Statistically MultiplexedAccess (SMA) to spectrum, and DIMSUMnet. Section3 describes the DIMSUMnet architecture in detail. Sec-tion 4 outlines the DIMSUM-RelayCluster architecture thatbrings benefits of coordinated DSA to FWA and mesh net-works. Finally, Section 5 summarizes the conclusions ofthis paper.

2 Overview of Our Approach

In the following, we provide an overview of our threemain ideas for coordinated dynamic spectrum access.

2.1 Coordinated Access Band (CAB)

PredefinedSpectrumInformationchannels

Spectrumshared byTransportService

X GHZY GHz

CellularBand

CAB LMDSCABMMDSCAB

Figure 4. Coordinated Access Band (CAB)

We introduce the concept of Coordinated Access Band(CAB), illustrated in Figure 4, which is a contiguous chunkof spectrum reserved by regulating authorities such as FCCfor controlled dynamic access. Multiple parts of the radiospectrum can be allocated as CAB spectrum. The seman-tics of the CAB spectrum usage is not apriori specified. For

a geographical region, allocation of various parts of CABspectrum to individual networks or users is controlled bya spectrum broker. As such, the spectrum broker perma-nently owns the CAB spectrum and only grants a timeboundlease to the requesters. The lease conditions may specifyadditional parameters such as extent of spatial region forspectrum, maximum power, and exclusive or non-exclusivenature of the lease. The compliant use of CAB spectrum re-quires that the “lessee” entity meet power budget constraintsall the time and also, “return the spectrum” to the broker atthe end of the lease. Clearly, measuring and verifying com-pliance is a challenge but an infrastructure based approachmakes it feasible.

The CAB band resembles a licensed band in that thespectrum lease is a short-duration license. The key differ-ences are: (1) current licenses signify long-term (if not per-manent) and sole ownership. (2) CAB leases are awardedby an automated machine-driven protocol. This contraststhe slow, regulatory legalistic process for licenses.

The current unlicensed ISM, UNII bands differ fromCAB; there is no “lease” required to use them and noco-ordination mandated. The work on spectrum etiquetteservers for unlicensed band devices [14] attempts to pro-vide CAB like coordination which cannot be mandated andenforced. Also, any use of this spectrum is not priced andtherefore, is not a revenue stream.

There are two possible models for CAB usage. In thesimplest model (CAB-M1), requests for CAB spectrum canbe generated only by the network operators. In the morecomplex model (CAB-M2), the enduser Mobile Node (MN)devices (PDAs, laptops, PCs) participate in the spectrumleasing process and request spectrum for communicationwith peer enduser devices or with network elements suchas base stations.

Within a single CAB, certain fixed frequencies are re-served as SPectrum Information (SPI) channels. In caseof simple CAB-M1 model, SPI channels are unidirec-tional from the BS to MNs, whereas in case of CAB-M2model, they are bi-directional between BS and MN. The SPIchannels are analogous to Common Spectrum CoordinationChannel (CSCC) proposed in [14] which aims at develop-ing a spectrum etiquette protocol for usage of unlicensedspectrum.

One key question to answer is what part of the spectrumis classified as CAB. We suggest that the CAB bands be co-located with current cellular and PCS bands. Also, unusedbroadcast TV channels or highly underutilized public safetybands can be designated CAB bands. The current cellularproviders will continue to own their licensed spectrum andoperate their existing networks unaffected. However, theycan deploy or use new networks that dynamically obtain andconfigure spectrum from CAB bands. When their existingnetwork experiences overload situation, the CAB band net-work which appears as yet another cellular network can addnew capacity. This can improve cellular data and voice ser-vices and translate into cost savings for cellular providers,

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who otherwise need to split existing cells into smaller cellsand install new base stations.

When CAB bands are located adjacent to existing LMDSand MMDS frequency bands, they can help compensatefor channel impairments and load surges, and enable en-hanced broadband access and backhaul services common inthe fixed wireless access networks.

Advantage of co-locating the CAB with existing cellu-lar, PCS, LMDS, and MMDS bands is that changes to theradio components in the client and network elements can beimplemented using existing technologies as the frequencyrange covered over which reconfigurable radio needs to op-erate is not very large. Over time, as the CAB networks andwideband radio electronics and frontends mature, increas-ingly larger portion of spectrum can be converted to CABspectrum, thus allowing gradual relaxation of static spec-trum partitioning and wide spread use of dynamic spectrumaccess.

2.2 Statistical Multiplexing of Spectrum Access inCAB

The concept of CAB improves the spectrum access ef-ficiency and fairness, whereas the concept of statisticallymultiplexed access is aimed at improving the spectrum uti-lization in the CAB.

Consider current multi-provider cellular networks. Theclassic cellular base station site provisioning providesa match between available bandwidth and the coveragearea/user density product for all times of operations for asingle provider. It performs a worst-case analysis at the in-dividual site-level. Figure 5 illustrates the spectral usage ata location (x,y), where two base stations of Network ServiceProvider (NSP) A and C and one base station of NSP B op-erate. It shows the spectral usage due to individual providersignals and their aggregate as waveforms varying over time.If we consider time period ���, ���, the spectral utilizationof provider B peaks whereas the provider A and C use muchless spectrum. Clearly, even though the per-provider usageat (x, y) varies dramatically, the current cell-site analysiswill allocate a peak spectral bandwidth of P units per site.Therefore, in our example, 3P units will be used instead ofactual instantaneous aggregate usage across all providers.This situation is analogous to using a transmission chan-nel with constant capacity of R bps for a variable data ratestreams whose rate varies over [0..R] bps. Although the cur-rent site provisioning optimizes the number of base stations,it actually is sub optimal in spectral bandwidth utilization.In this case the most efficient use of the spectrum is eitherto 1) have a single service provider use all spectrum in a lo-cation or 2) have uniform spectral usage in space and time.Neither condition is a feasible option.

Now consider the scenario where the spectrum utiliza-tion requirements can be aggregated across multiple serviceproviders and across multiple cells. If the spectral band-width requirements of providers are spatially and tempo-

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rally uncorrelated, the aggregate demand will be less thantotal of peak demands. As such, the most efficient use ofthe spectrum is to have the aggregate use at a particularregion to be constant. The aggregation effect can signifi-cantly reduce the required spectrum necessary for a regionwhile maintaining the independence and competitive busi-ness characteristics. We call this mode of operation as sta-tistically multiplexed access to spectrum.

In above discussion, we did not distinguish the kind ofservice a NSP provides. Clearly, services such as emer-gency response, public safety, telemetry, cellular data andvoice, fixed wireless access and mesh networks have dif-ferent temporal and spatial use characteristics. This sug-gests sharing of spectrum bands among these services hasstrong potential for statistical multiplexing gain. In fact,even among providers of homogeneous services such as cel-lular, significant lack of correlation may exists over smallerspatial and temporal scales. Therefore, opportunities mayexist for statistical sharing even within such single-use spec-trum bands.

This concept can be extended to all dynamic spatial foot-prints of base stations in dense configurations inclusive ofsmart antennas. Such a configuration would be closer tocreating uniform usage patterns across these aggregate re-gions and thus a more efficient spectral use. It also allowsan NSP to use more spectrum and dynamically add capac-ity during peak loads and offer better quality-of-service toendusers.

2.3 DIMSUMnet: An Implementation Architec-ture for Coordinated DSA

The acronym DIMSUMnet stands for Dynamic Intel-ligent Management of Spectrum for Ubiquitous Mobile-access network and is inspired from the model on whichpopular Chinese eateries operate. The DIMSUMnet imple-ments statistically multiplexed coordinated access to spec-trum in the CAB band. It uses a centralized, regional net-work level brokering mechanism that aims to significantlyimprove spectrum utilization while reducing the complex-ity and the agility requirements of the deployed system. Inthe following, we describe the DIMSUMnet architecture indetail.

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3 DIMSUMnet Architecture

The main components of the DIMSUMnet cellular archi-tecture (Figure 6) are: (1) a Spectrum Information and Man-agement (SPIM) broker, (2) a radio access network (RAN)consisting of new type of base stations, (3) a RAN manager(RANMAN), and (4) new intelligent enduser devices. DIM-SUMnet employs two new control protocols: (1) SPEctrumLease (SPEL) protocol involving three entities: the SPIMbroker, the RANMAN and the BS, and (2) a SPectrum In-formation Channel (SPIC) protocol between the BS and theenduser devices over the SPI channels in the CAB band.

In the simplest CAB-M1 mode of DIMSUMnet opera-tion, the enduser devices do not participate in the spectrumleasing (recall Model CAB-M1 in Section 2.1). In this case,the basic operation is as follows: When a base station (BS)in the RAN boots, it registers with its designated RAN-MAN. The RANMAN negotiates a lease with the SPIMbroker for an appropriate amount of spectrum. If the lease issuccessfully obtained, the RANMAN configures the leasedspectrum in the base station, which in turns configures itsdevices to offer the application services for voice and data.This secure three party interaction resulting in negotiationand instantiation of spectrum lease forms the core of theSPEL protocol. The BS broadcasts or multicasts the spec-trum information snapshots received from the RANMAN tothe clients. The clients use the information therein to selectthe application services. This information dissemination isthe core function of SPIC protocol.

In the advanced CAB-M2 mode of operation, the SPIchannels are bidirectional and the SPIC protocol allows MNto request BS to acquire spectrum and on-the-fly configurea traffic channel between them for a certain time duration.Such capability is useful when enduser would benefit fromextra bandwidth, for example, when it wants to transfershort burst of large data (files, video). The BS aggregatessuch demands from MNs and forwards them to the SPIMserver via the RANMAN.

In the following, we describe the components in greaterdetail.

3.1 SPIM Spectrum Broker

The SPIM server manages CAB spectrum rights andpropagation of information about those rights to any en-tity interested in using the CAB spectrum in a given geo-graphical region �. It manages all dimensions of the CABspectrum, namely frequency, time, space (location, direc-tion), signal (polarization, coding/modulation) and power.The basic tenet of DIMSUMnet spectrum usage is that anyuse of CAB spectrum not approved by SPIM server is anon-compliant usage.

The SPIM server maintains a complete topographicalmap of the region � which records position of all base sta-tions and approximate extent of cell coverage associated

Internet

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DIMSUMRANMAN

DIMSUMIP CoreNetwork

DIMSUMnet: Dynamic IntelligentManagement of Spectrum forUbiquitous Mobile networks

MN

DIMSUMIP BS

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DIMSUMIP BS

DIMSUMIP BS

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Figure 6. DIMSUMnet architecture

with each BS. For every cell, it maintains a spectrum snap-shot [15] that records: (1) spectrum used for SPI channelsand (2) a spectrum allocation map (SAM). Each SAM en-try records spectrum parameters such as (a) start and endof spectrum band, (b) network service provider (NSP) it isallocated to, (c) the current waveform or network accessmethod (e.g: GSM) used in the spectrum, (d) time dura-tion of lease, (e) maximum transmission power allowed, and(f) optionally, information about interference due to thermalnoise and other sources such as secondary users. The SAMchanges with time as the various parameters in SAM entrieschange. For example, when the SPIM server de-allocates aspectrum, the corresponding SAM entry is removed.

Note that in a deployed DIMSUMnet network, giventhe critical nature of spectrum resource, instead of a sin-gle SPIM server per region, multiple SPIM servers must re-dundantly maintain consistent information. In this case, ifone of the SPIM servers fails, one of the remaining serverscan still satisfy spectrum lease and information requests ina region. We call this mode of operation where a cluster ofSPIM server provides redundant oversight for a region R asthe SPIM Overlay. We need to examine a scalable mech-anisms such as application-layer multicast and distributedhash tables (DHTs) to minimize overhead of frequent anddeterministic dissemination of spectrum snapshot informa-tion in the overlay. Also, a loose interaction among SPIMservers serving overlapping regions may be necessary tominimize interference.

The overhead of spectrum brokering increases as thenumber of cells in a region increase. This scenario wouldbe common when a region is composed of a large numberof micro-cells (each with range of 100s of feet) and pico-cells (each with range of 10s of feet). It is unrealistic toexpect the base stations in such cells to acquire spectrumleases from a remote SPIM server. Also, in most cases,these base stations are inexpensive, simple devices that sup-port a fixed radio technology (analogous to 802.11 WLANor Bluetooth operating in CAB band) and may not supportspectrum information channels. In this case, a hierarchy of

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SPIM servers, where servers in the lower levels in the hi-erarchy are responsible for smaller geographical areas, willbe required.

3.2 RANMAN

The Radio Access Network Manager (RANMAN) is anew network element, which controls spectrum leases forseveral DIMSUM base stations. It is aware of the staticcharacteristics of the base stations, specifically hardwareand software capabilities of ACRs such as the supportedradio frequency range, signal processing and various wave-form (CDMA, OFDM, specific modulation etc.) capabili-ties, maximum power. It also keeps track of their dynamiccharacteristics such as current load, power usage, availablepower etc. Based on location specific policies, base stationcharacteristics, and geographical coverage requirements forservice provider, it estimates the amount of spectrum re-quired to meet and sends bids to the SPIM server. If thespectrum leases are granted, it sends commands to the basestations to configure their ACRs as per the service providerspecific MAC and radio protocol and thus, activates radioaccess channels. It periodically renews existing leases orterminates them. It may renegotiate new spectrum due toreasons such as price changes, increased interference in ex-isting band, and increased load reported by the base sta-tions.

3.3 DIMSUM Base Station

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Figure 7. DIMSUM-BS and DIMSUM-Client

Each DIMSUM-Base Station (DIMSUM-BS) (Figure 7)contains multiple instances of ACRs out of which a subsetare reserved for SPI channels and others are used to provideaccess channels for bit transport service for enduser traffic.Each BS can be either static or mobile and uses one of theACRs periodically configured as a GPS receiver to know itsexact location.

Each BS contains two daemons that implement protocolsrelated to the spectrum management:

� Spectrum-Info daemon: This daemon communicateswith the domain SPIM server to obtain spectrum-mapsnapshots relevant for its location and broadcasts itover the reserved SPI channels. It also collects infor-mation such as number of endusers using each chunkof spectrum, per user information such as geo-location,SNR for the current wireless access channel etc. andpropagates all or aggregates of this information to theSPIM server.

� Spectrum-Lease Daemon: This daemon communi-cates to the Radio Access Network (RAN) manager,to obtain spectrum leases and commands to configureits ACR devices. The commands specify the frequencybands, power and type of waveform to be used. For ex-ample, the command may specify using max power of30 watts, a carrier frequency of� � �����GHz, band-width of� � ���� MHz with CDMA Direct SequenceSpread Spectrum (DSSS) waveform. If the ACRs aresuccessfully configured, the daemon registers with theRANMAN its use of the spectrum chunk and the asso-ciated parameters. In the event spectrum lease expiresand the RANMAN does not send a lease renewal no-tification, it disables ACRs and notifies SPIM serverof the de-allocation of spectrum chunk. It also noti-fies the Spectrum-Info daemon of this event to ensurespectrum snapshot propagated to enduser is appropri-ately adjusted.

We envisage DIMSUM-BS to be an IP-aware base sta-tion which implements various protocols, namely: mi-cro/macro mobility protocols (e.g: Mobile IP, MobileNATanchor node[8]), address management (e.g: DHCP, NAT),AAA protocols (e.g: RADIUS), and QoS support (e.g: Diff-serv packet labeling, class-based QoS).

3.4 DIMSUM Client Device

The DIMSUM client device contains at least two logicalor physical instances of ACR devices, one used as a con-trol channel interface and the other as a data interface .In simplest (CAB-M1) mode of operation, it uses the con-trol channel ACR to scan SPI channels in the CAB bandto obtain spectrum snapshots broadcast by the DIMSUM-BSs in the region. These snapshots enable client softwareto obtain information on availability of application servicesin different parts of the spectrum, specifications of the net-work providers offering these services, layer-1/2 specifica-tions such as modulation, MAC, and other information suchas current load, interference levels etc. Based on this in-formation, its own QoS needs, ACR waveform capabilities,and power and location constraints, the client decides on theparts of the spectrum and the application service to use and

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configures one or more the data interface ACRs with appro-priate radio characteristics. Multiple ACRs concurrently ac-cessing transport in different parts of spectrum enable clientto dynamically adjust to variable bandwidth requirements ofits applications.

Note that rudimentary forms of a subset of above con-cepts operating over slow timescales are already used in thecurrent cellular networks. The client devices for these net-works maintain a Preferred Roaming List (PRL) whichis an ordered list of tuples �System ID (SID), Network ID(NID), Radio Frequency f �, where SID, NID uniquely char-acterize the provider base stations that use frequency � .When the device detects deteriorating radio signal, it con-sults PRL to decide which carrier frequencies to scan tofind service offered by its preferred provider or its roam-ing partners. The PRL list can be downloaded to the phonedynamically over the air interface using signaling chan-nels [5]. This concept has been successfully employedto achieve global roaming across multi-technology, multi-provider networks [7]. However, in the current network,PRL changes and therefore, such downloads are very in-frequent. On the contrary, in DIMSUMnet, the spectrumsnapshots, which are somewhat analogous to PRLs, maychange frequently (e.g.: every few minutes) due to changesin spectrum allocation.

Note that the application service client is accessing maybe re-mapped to another part of the spectrum with poten-tially different characteristics. In this case, the client mustdetect this event and reconfigure its data interfaces to con-tinue its network and transport protocol connections. Theclient may also pro-actively reconfigure its data interface torespond to events such as increased interference, loss of sig-nal, reduced service price, need for increased data rates anddata rate degradation due to congestion or mobility. Duringsuch a change, the client software must support session con-tinuity to ensure seamless enduser experience. Availabil-ity of multiple ACR devices allows concurrent detection ofspectrum snapshot changes or other detrimental events andreconfiguration of data interfaces.

In the advanced CAB-M2 mode of operation where theSPI channels are bidirectional, the client device may alsouse a reverse link in the SPI channel to convey to the bases-tation changes in its bandwidth requirements and requestadditional spectrum to be exclusively configured for its traf-fic channels to the BS. The DIMSUM client may optionallycontain a spectrum-sensing component, which periodicallymeasures observed power spectral density in a broad rangeof CAB spectrum or in frequency bands adjacent to currentcarrier frequency. It communicates both these data and theiraggregates periodically to the base station via the SPI chan-nels.

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Figure 8. Unbundling the service providermarket

3.5 Deployment Models: Shared vs. Non-sharedBase Station

In the existing cellular networks, large NSPs (e.g: Veri-zon) license spectrum from FCC, deploy their radio accessnetworks and sell voice and data services to the endusers.However, this model requires large capital investments andtherefore, has led to non-competitive environment that sti-fles innovation and introduction of new technologies andservices. A new form of Mobile Virtual Network Opera-tors (MVNO) (e.g.: Virgin Mobile) that do not own anyspectrum and infrastructure but offer application servicesby leasing services from large providers are becoming pop-ular [13]. However, their growth is still controlled by theprimary providers. DIMSUMnet enables new deploymentmodels (Figure 8), specifically shared base station modeland non-shared base station model, which show promiseof creating new kind of providers. Common to both thesemodels are the regional or national spectrum providers thatoperate the SPIM server overlays and spectrum brokeringservices.

Shared base station model In this model, a new formof infrastructure providers called the RAN providers ownand operate the base stations and the RANMAN but do notown any spectrum. They lease CAB spectrum from thespectrum-providers as an when needed. The application-service providers (ASPs) that offer specific application ser-vices (e.g: voice, data) to endusers are the clients ofthe RAN providers and are essentially MVNOs in theCAB spectrum [13]. As an example, the fcc-spectrum-borker.nj.gov can be a spectrum broker network in NewJersey, which has three RAN provider clients central-nj.ran.com, north-nj.ran.com, sounth-nj.ran.com whichown and operate radio access networks in the CAB bandin northern, central and southern regions of state of NewJersey.

The ASPs only maintain the enduser authentication,billing and roaming agreement databases and do not con-cern with infrastructure operations and spectrum leasing.

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Their services are offered in different parts of spectrum atdifferent times depending on which part of CAB spectrumis leased. If multiple RAN providers operate in a region, anASP may use one or more of those to offer its services andafford additional service resiliency. This however requiresthat the RAN providers bid and use non-overlapping fre-quencies chunks for the same service. In this case, the sameASP’s service will be simultaneously available in differentparts of the spectrum increasing potential for spectrum uti-lization and per-user data rates.

This approach also requires mechanisms and policiesfor appropriate allocation and isolation of base-station re-sources such as the ACR devices, power budget for radioamplifiers, IP backhaul bandwidth, and transmit power usedfor each service provider.

Non-shared base station model In this model, theprovider that offers the application services also owns andoperates the base stations and leases the spectrum requiredto operate the service. Therefore, each service provider hasits own RANMAN and base stations. This model differsfrom the current cellular model in that the provider stilldoes not statically own any spectrum but leases it dynam-ically. Offering redundant physical coverage in this modelrequires the provider to deploy multiple base stations in thesame area and therefore it can be capital intensive.

In both the models, the current cellular providers cancontinue to hold their existing licenses and use their exist-ing infrastructure much the same way. Any new capacitythey add in the CAB bands on-demand is accessible to theenhanced enduser devices via the PRL updates in the spec-trum snapshots.

3.6 Spectrum Allocation

Allocation Algorithm

SpectrumAllocationPolicies

SpaceRepresentation

DataStructure

NetworkState

D0 , D1 , D3 . OFR0 , OFR1 , OFR3 .

Network State- Utilization

- Current Load- Intference

temprature

Demand:

1) Type: Advanced vs. Online 6) Amount of spectrum2) Provider ID 7) Preferred location in spectrum3) RANDMAN ID 8) Duration of lease4) BS ID 9) Primary vs. Secondary lease5) BS Location 10) <Optional price >

Offer

Amount of spectrumDuration of leasePower to useOffered price

Figure 9. Spectrum allocation model

The spectrum allocation algorithms and policies focus onanswering three critical questions: (1) How much spectrumcan a provider get, if any?. (2) How long can a requesterhold the leased spectrum? and (3) How can a provider wina spectrum bid? At any point in time, the answers to thesequestions (i.e. rules of the resource allocation) are dynamic,varying over time and location. It is the system’s responsi-

bility to determine, state, update, and enforce these rules assystem conditions change.

In our model for spectrum allocation (Figure 9), weassume that the spectrum broker receives a series of de-mands �� at different time instances. Each demand hasa type of reservation which can be either advanced whichallows a RAN manager to reserve spectrum much in ad-vance of its actual use or online instantaneous in whichcase the reserved spectrum is immediately used. Other rel-evant demand parameters are various infrastructure identi-fiers (BS, RANMAN, provider ID, BS location) and leasedetails (amount of spectrum, spectrum location, primaryvs. secondary status, duration). The output of the spec-trum allocation is an offer ���� that is forwarded to therequesting RANMAN. The offer contains amount of allo-cated spectrum, its location, amount of maximum transmitpower, duration of the lease and a price. We advocate a softstate model for spectrum allocation, where each lease is fora fixed duration and after the duration expires the lease mustbe renewed.

A(2)B

C(2)

A(1)C(1)

Region R

Region R1

f1 f2 f3f4

f5

Figure 10. Spectrum allocation and regionalinterference

The knowledge of the clients’ radio environments in agiven region can greatly aid the smarter decisions on spec-trum allocation. Figure 10 shows a case where users in aregion R use the services of several base stations in thatregion, which are operating in different parts of the CABspectrum. These users may experience adjacent channelinterference caused by uplink transmitters using an adja-cent spectrum portion. If the SPIM server allows multipleoperators in the same band operate at different power lev-els, significant variability in interference levels may be ob-served. However, if end-users periodically collect data oninterference observed in the entire CAB or adjacent bandsof spectrum they are using, this information can be propa-gated back to SPIM server via the SPI channels. The SPIMserver can then construct a dynamic map of radio usage andobserved interference in the region, and use that informa-tion for more efficient spectrum allocation. For example, ifthe interference in a subset of the region (Region R1, Fig-ure 10) exceeds the tolerable level, it may request the RANmanager operating the interfering BS to reduce its transmis-sion power, thus reducing the co-channel interference. Al-ternately, it may ask its BS to reassign a different part of thespectrum to the users in Region R1 who are experiencing

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intolerable interference.The spectrum pricing may work in two modes: (1) auc-

tion mode in which each demand (bid) has an associatedprice and the winning bid decides the final price. (2) mer-chant mode in which the price is entirely decided by theallocation procedure. We believe that utility pricing tech-niques used in current electric power markets may be rele-vant to spectrum allocation. A technique of specific interestis the modified price index cap. In this method, electric util-ity tariffs (or bids) rise with inflation (CPI), and are simul-taneously forced to decrease by a pre-determined variable,which is based on each company’s relative inefficiency. Inthe application of this approach to spectrum management,the relative efficiency of a service provider can be a dynamicvariable that is based on current provider specific and totalspectrum utilization.

Different policy methods also should play role in thespectrum allocation decisions. Following is a short list ofdifferent policy methods of interest:Pure CAB vs. (Partial CAB+Cellular): In the pure CABmode of operation, the entire band is dynamically allo-cated. This increases complexity of the client devices andsignaling and potential of service disruption. However, itincreases competition and access efficiency. An alternatemodel where part of the CAB is converted to long-livedbase licenses can improve service quality. The providersthat already hold this base license or a license in the exist-ing cellular or PCS bands should not be given first right todynamically shared part of CAB spectrum.Cost of CAB spectrum access vs. owned spectrum:The CAB spectrum should be cheaper per time period thanowned spectrum. A mechanism is necessary to assure thatproviders do not abuse the cheaper dynamic spectrum orhoard resources. To this end, a provider must prove that itneeds the spectrum when it makes a bid for the CAB band.One way to do this is to use periodic utilization reports fromthe provider for its existing leases to prove that they are suf-ficiently utilized to warrant new allocations.CLEC vs. ILEC status of the lease request: The Com-petitive Local Exchange Carriers (CLECs) are the emergingservice providers that compete with large Incumbent LocalExchange Carriers (ILECs) to provide network access. Toencourage increased competition, spectrum allocation maygive greater weight to demands from CLECs.Effect of spectrum allocation history on bids: The spec-trum lease history should affect allocation decisions. Forexample, if a service provider is granted spectrum in a giventime period, will it be granted new leases (either renewalof existing lease or completely new spectrum)?. A policyneeds to be devised that can trade off fair access to spec-trum vs. the application service continuity. Disregardingthe history and allocating the spectrum randomly can resultin rapid re-mapping of application services in different partsof the spectrum, leading to frequent service handoffs (evenin absence of enduser mobility) and in the worst case leadto service disruption. On the contrary, sticky allocations

that favor providers with already allocated spectrum reduceaccess fairness.Access Fairness and Cost Incentive: Pricing must assurefairness and feasibility of such a system. Specifically, theservice providers with the most money cannot influence orgain a competitive advantage through monetary disparitywith smaller operators.

Clearly, the spectrum allocation algorithm must formal-ize the policy considerations and combine them with themathematical formulations of network states and radio en-vironment. Our ongoing work aims to design, analyze,and implement candidate spectrum allocation algorithmsfor auction and merchant modes of operations.

4 DIMSUM-RelayCluster

Gateway

Relay

Relay

BS

Relay

AP

Relay

AP

3G

802.16

802.16

Relay

AP802.11g

802.11b

Relay

BS

802.11b

802.16a

3G

Internet

802.16a

802.16a

Control, Management,

Configuration

SPIM Client

RAN Manager

Cluster Manager

Figure 11. DIMSUM-RelayCluster

The DIMSUM-RelayCluster architecture brings the ben-efits of dynamic spectrum to Fixed Wireless Access andmesh networks.

DIMSUM-RelayClusters consists of two new networkelements (Figure 11): the DIMSUM-Relay and theDIMSUM-Gateway. The relay nodes contain multiple ACRdevices, which can be operated in two modes: access in-terface and packet-relay interface. The access interfacesare configured to operate as legacy 802.11 or 3G interfacesand are used by the end-user devices for network access.The gateway element contains multiple ACRs, which canbe operated in two modes: packet-relay interface and in-ternet backhaul interface that connects the gateway to theInternet. Note that the backhauls can be wired interfacessuch as Ethernet and in that case they are not ACR de-vices. The packet-relay interfaces are adaptive radio linksconstructed using dynamic spectrum access in the CABspectrum. These links may be configured as 802.16 longrange FWA links [6] or short-range 802.11 a/g links. Theyare used to construct a self-configuring, secure, power andbandwidth adaptive multi-hop packet routing backbone that

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forwards end-user traffic received on access interfaces tothe Internet gateway nodes. The management entity calledClusterManager either co-located with a gateway node orimplemented as a separate element, implements the RANmanager and SPIM Client functionality. It also implementsthe SPI daemon, which broadcasts the spectrum snapshotson SPI channels on its relay interfaces.

When a relay node boots, it listens to the SPI channels todiscover the spectrum snapshots and the presence of relayelements in the certain parts of the CAB. For example, thesnapshot may show that part of spectrum is used to oper-ate 802.16 links (dotted lines in Figure 13) and also providethe current network load. The relay node may configureits ACR to those channels and using Point-to-Multipoint orMesh mode of 802.16 [6] join the existing links in the re-lay network. In this case, the relay interface on the gatewaynode serves as the 802.16 base station. The relay node regis-ters its ACR capabilities with the ClusterManager and alsoperiodically forwards to it information such as its visibleneighbors, observed network load and interference levels.The cluster manager thus knows the complete topology ofthe relay network and the links used in the forwarding in-frastructure. If it detects that certain links are getting over-loaded, based on its knowledge of relay ACRs, it may de-cide to configure additional links (shown as dashed lines in(Figure 11). Specifically, it may setup additional point-to-point link between pairs of nodes or a point-to-multipointor mesh link among subset of nodes. For this setup it ne-gotiates spectrum on behalf of the relay nodes using SPELprotocol and then send RAN manager commands to relaysto reconfigure their ACRs.

The ability to add bandwidth-on-demand to relay infras-tructure makes it possible to alleviate throughput degra-dations due to congestion, channel impairment and loadsurges. For example, under heavy load condition when traf-fic in access interfaces of all relay is high, a large amountof traffic directed to the gateway nodes creates a funnel-ing effect where links closer to the gateway experience con-gestion. In DIMSUM-RelayCluster, such congestion is de-tected and compensated by adding capacity on links leadingto the gateway.

Clearly, the gateway and relay nodes can employ newadvanced antennas, dynamic spectrum allocation, andscheduling to improve relay throughput without affectingthe client devices in the access networks. The key designproblems in this architecture are as follows:

� Gateway discovery and secure registration proto-col: A newly deployed relay node or a node that re-boots must be able to auto-discover the set of gatewaynodes in its vicinity based on its own selection criteriasuch as QoS along forwarding paths, capacity of up-links at the gateway, current load in the network etc.Once gateways are discovered, relays must perform asecure registration.

� Auto-configuration: Multiple relays and gateways

must auto-configure themselves and converge to a sta-ble packet-forwarding infrastructure that makes appro-priate QoS guarantees. Auto-configuration involvesautomatic spectrum and power level selection, neigh-bor discovery, and configuration of relay forwardingvia negotiation of operational characteristics such asaddressing schemes, mobility, and forwarding seman-tics. In the event of node failures (due to power loss),interference, and frequency reassignment, the networkmust reconfigure itself rapidly.

5 Conclusion

Dynamic Spectrum Managed networks will be a dis-ruptive force that will lead to new form of wireless net-works. In this paper, we argued a case for coordinated,real-time dynamic spectrum access instead of opportunis-tic,uncoordinated methods common in ad-hoc military ap-plications. We introduced the concepts of Coordinated Ac-cess Band (CAB) and Statistically Multiplexed Access tospectrum. We described the DIMSUMnet architecture thatimplements these concepts for cellular networks using acentralized regional spectrum broker. We elaborated onnew control plane technologies in the form of RAN man-ager and associated spectrum leasing and information pro-tocols and new data plane technologies in the form of DSAaware reconfigurable base stations and client devices. Wealso highlighted issues in deployment models and spectrumallocation. We also presented the DIMSUM-RelayClusterfor fixed wireless and multihop all-wireless infrastructure.The potential impact of our schemes is that they can lowercapital expenditures for spectrum acquisition and therebyincrease competition and rapid introduction of new cellularand FWA services.

References

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[6] IEEE P802.16-REVd/D4 IEEE Standard for Localand metropolitan area networks Part 16, May 2004.

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[7] Proceedings of Enhanced Preferred Roaming List(PRL) Workshop, February 18-19, 2004, La Jolla, CA.

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