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

2Outline Introduction and motivation Spectrum Agility Policy Agility A simple system for opportunistic spectrum access

3 Opportunistic Spectrum Access TIME FREQUENCY PRIMARY 1 PRIMARY 2 PRIMARY 3 PRIMARY 4 HOLE SECONDARY WALL 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

4 Problem Statement 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] Time Frequency Time Frequency Time Frequency Time Frequency Time Frequency

5 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operations –Using WiFi in different countries in conformance with the country’s policy Spectrum policy is simply outdated –FCC Spectrum Policy Task Force

6 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

7 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

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

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

10 Challenge: Opportunity Awareness Wideband sensing –Problem: Sense energy/ receive signatures over a large portion of the spectrum (e.g. entire U-NII indoor band ( GHz (100 MHz)) –Current wireless receiving technology receives/transmits over much smaller chunks (e.g. each channel in b 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?

11 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 …

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

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

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

15 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

16 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)

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

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

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

20 Sense Interface Hole Information Array (HIA) Frequency (KHz)

21 IDLE Channel Selection My_HIA Nbr_1_HIA Nbr_2_HIA Nbr_3_HIA Idle Channel –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.

22 –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: My_HIA Nbr_1_HIA Nbr_2_HIA My_HIA Point to Point opportunity to Nbr 2 Point to point opportunity to Nbr 1 IDLE Channel Selection (contd.)

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

24 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

25Resources 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, –“The XG Policy Language Framework”, version 1.0, Web sites – –