Opportunistic Spectrum Access: Challenges, Architecture, Protocols

Opportunistic Spectrum Access: Challenges, Architecture, Protocols C. Santivanez, R. Ramanathan (presenter), C. Partridge, R. Krishnan, M. Condell, S. Polit Internetwork Research Department BBN Technologies, Cambridge, MA (ramanath@bbn.com)

Outline • Introduction and motivation • Spectrum Agility • Policy Agility • A simple system for opportunistic spectrum access

Opportunistic Spectrum Access • Basic idea • Sense the spectrum you want to transmit in • Look for “holes” or “opportunities” in time and frequency • Transmit so that you don’t interfere with the licensees WALL PRIMARY 1 PRIMARY 2 PRIMARY 3 FREQUENCY PRIMARY 4 SECONDARY TIME HOLE

Problem Statement Frequency Frequency Frequency Frequency Frequency Time Time Time Time Time • Given • a set of possibly mobile nodes comprising a wireless network • the hole/wall characteristics as a function of time for each node • a set of constraints on the spatial reuse of frequencies • [a set of capacity demands per link] • dynamically control access (xmt/rcv, beam, power etc.) on each frequency such that • network capacity is maximized • constraints are honored • [capacity demands are met]

Why Opportunistic Spectrum Access • Current “static” spectrum allocation strategy is wasteful • Huge opportunities exist in time, frequency and space • Apparent spectrum scarcity • Deployment difficulty • Allocating spectrum for overseas military operations • Using WiFi in different countries in conformance with the country’s policy • Spectrum policy is simply outdated • FCC Spectrum Policy Task Force

OSA in the Wireless Internet: Drivers • Need for more capacity • 2G, 3G, 4G…… • Waveform Diversity • Beamforming, MIMO, UWB, OFDM, Spectrum-adaptive/heteromorphic ….. • Software/Agile/Cognitive Radios • JTRS, Vanu Inc., GNU Radio …… • Secondary markets, FCC Spectrum Policy, …. Current trends point to a need for and capability to do opportunistic access as a key part of next generation Wireless Internet architecture

OSA Challenges • Spectrum Agility • How do we identify holes (opportunities) ? • How can we access those opportunities? • How do we prevent interfering with primary users? • Policy Agility • How do we control access to be in conformance with regulatory policies? • How can we effect a change in node behavior in accordance with policy changes? • How do we support multiple (concurrent) policy authorities

Next ….. • Introduction and motivation • Spectrum Agility • Policy Agility • A simple system for opportunistic spectrum access

Spectrum Agility Components Opportunity Awareness Determine spectrum opportunities and usage constraints, based on sensing, and policy information Opportunity Info, and constraints Usage constraints Opportunity Use Opportunity Allocation Transmit packet in accordance with opportunity assignment, adjust parameters as per constraints Determine how the available opportunities are shared on an instant-by-instant basis Per-packet assignment

Challenge: Opportunity Awareness • Wideband sensing • Problem: Sense energy/ receive signatures over a large portion of the spectrum (e.g. entire U-NII indoor band (5.15 - 5.25 GHz (100 MHz)) • Current wireless receiving technology receives/transmits over much smaller chunks (e.g. each channel in 802.11b is about 22 MHz) • An alternative: Primaries log their current/planned use, secondaries access this database • Opportunity Identification • Problem: Based on sensing, decide whether or not to use a slice of spectrum • Interference occurs at receivers, but you can only sense transmitters • Hidden transmitters • Opportunity Dissemination • Problem: Share opportunities with other nodes to coordinate use of spectrum • Opportunity dissemination can be bandwidth intensive • Chicken-and-egg: Which channel do we use to disseminate opportunities?

Challenges: Opportunity Allocation/Use • Allocation • Problem: Channel access over dynamically changing holes • CSMA/CA: Need to acquire floor over a common channel set • TDMA: Assign time slots over each available frequency • Use • Problem: Implementing the allocation • Waveforms that can use discontiguous portions of the spectrum • Dynamically adapt waveform to required PSD • Adjust beamforming, data rate, modulation, spreading …

Policy Agility Limited or no programmability (e.g. ASICs) FCC Rule Book Hardwired policy Canned behaviors:few/fixed modesof operation Machine-Readable Policies Highly Programmable, fast, low power devices (e.g FPGAs) Dynamic Policy Agile behaviors: numerous modes of operation Policy agility is necessary to exploit the emerging agility of devices and allow in-situ policy-based control of radio behaviors

Machine Understandable Policies OSA node Machine-readable Policies Spectrum Policies OSA node Internet Policy Repository

Benefits of policy agility • Adaptation to policies changing over time • Allows development of technology in advance of policies • Adaptation to policies changing over geography • E.g., use a new smart card when in a new country • Self-checking policies • Implications of policy interactions can be worked out in advance • Consistency checking at the logical level • Sub-policy management • Secondary markets: Overarching policy, sub-policy – allow primary user to develop sub-policies for secondary users • Radio system capabilities based policies • E.g. two different policies depending on whether a radio system can detect certain waveforms or not

Challenges: Policy Agility • A language for policy expression • Inheritance, Reification (rules about rules), Inference (derivable rules), Extensibility, Scalability, Declarative • An ontology for policies • What are the “primitive” objects underlying policies? • What are the inter-relationships between the primitive objects • Reasoning about policies • Conformance: Is this usage permitted by policy? • Constraints: What is the range of parameters for this particular usage? • Searches: Here are my specs, my needs. Give me the usage. • BBN has developed an initial ontology and a policy language based on OWL (Semantic Web Language)

A policy-aware OSA system Rest of talk System Policy Regulatory Policy System Strategy Reasoner OSA capable Device Ask for transmission opportunities Device Configuration Return transmission opportunities Sense Policy Conformance Reasoner Transmit Ask for usage validity Allow/Deny

Spectrum Agility: Components To PCR To SSR Information Flow Upper Layer Control Flow XG Opportunity API Sense Interface Kernel ND-HIP Opportunity Identification Nbr list, nbr HIAs OSA Adaptation Layer My HIA • Waveform • selection • Idle Channel Selection - ND - HIP - RMAC My IDLE CHANNEL, my HIA, pathloss FFT XG Transceiver API TRANSCEIVER

Sense Interface 1 1 0 0 0 1 0 0 1 1 • Hole Information Array (HIA) Frequency (KHz)

IDLE Channel Selection Idle Channel My_HIA 1 1 0 0 0 1 0 0 1 1 Nbr_1_HIA 1 0 0 0 0 1 1 0 0 1 Nbr_2_HIA 0 1 1 0 0 0 1 0 1 1 Nbr_3_HIA 1 1 0 0 0 1 0 1 1 1 • IDLE Channel (freq., rate, waveform) is not the XG Coordination Channel • While node listen to IDLE Channel MUST be reachable by all 1-hop neighbors.

IDLE Channel Selection (contd.) Point to point opportunity to Nbr 1 My_HIA 1 1 0 0 0 1 0 0 1 1 Nbr_1_HIA 1 0 0 0 0 1 1 0 0 1 My_HIA 1 1 0 0 0 1 0 0 1 1 Nbr_2_HIA 0 1 1 0 0 0 1 0 1 1 Point to Point opportunity to Nbr 2 • Communication with neighbors may then switch to a more favorable channel. • E.g. RTSs need to be sent using a nbr IDLE channel, while DATA packet may be sent using one of the channels below:

Spectrum Utilization BW Occupation (x 100 KHz) Time Zoom BW Occupation (x 100 KHz) Time

Concluding Remarks • Opportunistic Spectrum Access de-bunks the “spectrum scarcity” myth and a promising technology for Wireless Internet • Realizing true opportunistic spectrum access requires solving not only the spectrum agility problem but also the policy agility problem • There is plenty of “low hanging fruit” that can be harvested with a relatively simple set of mechanisms • We presented one such system, that incorporates both spectrum and policy agility • Showed an order of magnitude gain in capacity • Future research to target “higher hanging fruit” • Dealing with rapidly changing hole information in a large network • Constructing a coordination channel “on the fly” • Theoretical capacity bounds of OSA

Resources • Much of this work was based on the DARPA XG Program • Phase I: BBN project “Medium Access Control for XG (X-MAC)” • Phase 2: BBN project “XG Architecture and Protocols (XAP)” • Documents • “The XG Vision”, version 2.0, http://www.ir.bbn.com/projects/xmac/vision.html • “The XG Policy Language Framework”, version 1.0, http://www.ir.bbn.com/projects/xmac/vision.html • Web sites • http://www.ir.bbn.com/projects/xmac/index.html • http://www.darpa.mil/ATO/programs/xg/index.htm

Opportunistic Spectrum Access: Challenges, Architecture, Protocols