Design Tool for Spectrum Sensing of Cognitive Radio 5/15/2009 Jihoon Park
Spectrum Allocation Problem Few spectrum slots are left to allocate. Allocated spectrum is idle most time. FCC allows use of allocated spectrum under following regulatory constraints Minimum signal-to-noise ratio () Minimum probability of detection (PD,req) Maximum detection delay (DD)
Idle Spectrum Detection :detection (quiet) time Freq. Detection delay (DD) :data transmission Primary User Primary User Bandwidth open for use by the CR Available Idle Bandwidth (Bav) Detected idle Bandwidth (Beff) Time Quiet time (TQT) Available data transmission time Cognitive Radios (CR) can use spectrum allocated to a primary users (PU) as long as it is idle i.e. unused by the PU A key function is “detecting” if allocated spectrum is idle Design Objective: Optimize detection efficiency Maximize amount of idle bandwidth detected Minimize quiet time (increases data transmission time) Minimize cost of a design of the spectrum sensing
Spectrum Detection Efficiency Detection Efficiency depends on MAC, PHY, radio, and control channel parameters Sensing Radio: (radio bandwidth) Sensing PHY: TS , PFA Sensing MAC: , : parameters determined by how to utilize bandwidth for detection process (Cooperation scale: #users who report the sensing results to cooperatively make a decision if a channel is idle) Control channel: TR , (how to utilize bandwidth for reporting operation) Traditional techniques optimize individual layers Sub-optimal detection of idle bandwidth Defined cross layer parameters Fu : Bandwidth utilization factor TQT : Quiet time TD : Detection time Defined cross layer efficiency model
Spectrum sensing design tool Input Cross Layer Parameters Design Parameters Regulatory specs Minimum signal-to-noise ratio () Minimum probability of detection (PD,req) Maximum detection delay (DD) Network specs Number of Cognitive Radio users (N) Number of channels of primary users (nc) Bandwidth of a PU channel (b) Channel Model (CR PU) Fu , TQT , TD Control Chn. ( , TR ) Sensing MAC (Cooperation scale (), , ) Sensing PHY (TS, PFA) Sensing Radio (BW ()) * Output Sensing PHY Parameters Optimal sensing time (TS) Optimal false alarm probability (PFA) Performance Optimized detection efficiency(D)
System Architecture Dedicated control channel / collaborative sensing Fu = 1 , TQT = aTS , TD = aTS + bTR a and b are determined by the parameters of the individual layers. we focus on the effect of the following two parameters: Bandwidth of the sensing radio (Implementation cost) Cooperation scale (MAC complexity or power efficiency) Freq. : Sensing Ded. CC TS TR : Reporting Sub-chn 1 : Dedicated CC Sub-chn 2 Sub-chn 3 : Data channel Sub-chn 4 TQT TQT Time TD DD TD
Trade-off between radio bandwidth and cooperation scale =0.7 (cooperation of 7 users) =0.34 (6.8 MHz) Param. Values PD,req 0.9 -12 dB DD 2 sec b 200 kHz nc 100 N 10 =0.6 (cooperation of 6 users) =0.5 (10 MHz) There are points at which high performance can be achieved with the low cost of radio and the low complexity of cooperation (or low power consumption)
Conclusion The proposed design tool makes it possible for a designer to choose the design configuration of the best trade-off between implementation cost and power consumption. For given network specs and regulatory specs, there exist design configurations which achieve high performance without wasting excessive cost and power. The wideband architecture(high cost) does not perform better than the narrowband architecture (low cost) if many users cooperate for detection.