Detecting Primary Receivers for Cognitive Radio Applications Ben Wild, Kannan Ramchandran UC Berkeley Dept. of Electrical Engineering and Computer Science 11/9/2005 Work funded by NSF ITR 3R Spectrum Sharing grant

Why Detect Receivers? FCC Interference temperature proposal not well defined without knowing where receiver located. Interference happens at the receiver!. Many Applications.. we will discuss re-use of TV band for unlicensed use in this talk.

Detection of Transmitters TV decodability region  Shadowed CR could mistakenly use channel  CR must be able to detect weaker signals, making the no- talk zone larger. CR

Detecting Receivers Allow secondary users in licensed bands if they can guarantee that they do not interfere with primary receivers. Ch.5 Ch.7 Ch.6 Ch.4 Ch.3 Ch.5 Ch.9 Ch. 3,4,5,6,7 shadower

Detecting “Passive” Receivers Legacy receivers must be able to function interference free. All RF receivers leak local oscillator out of antenna. Solution: Low cost sensors to detect leakage. Reverse LO leakage

LO Leakage versus Model Year Figure 2: TV LO Leakage Power versus Year [3]

TV Detector Vans Used in Britain since 1920s

Cognisensor Optimal to connect sensor directly between receiver and antenna. Increases LO power coupled to sensor, reduces interference. Allows sensor to reuse receive antenna for transmission. cognisensor Cognisensor, close up Antenna input Receiver Input Directional Coupler LO Detector Transmitter Duplexer CR

Detector Architectures cos(  o t)+n(t) IF PLL Envelope Detector Decision cos((  0 +  )t+  ) Non-Coherent cos(  o t)+n(t) LPF PLL cos(  0 t) Coherent Decision v(t)

Detection Time (coherent) For -120dbm coupled LO leakage powerFor probability of error

Experimental Results TV Tuner Amplifiers Mixer Frequency Synthesizer IF filter Amplifier Envelope Detector Non-coherent detector prototype

System Considerations How to best combine information from primary receivers with information from transmitters? How do CRs collaborate to switch to unused channel when channel becomes occupied? How to make the system scalable to many CRs?

Interference Avoidance Primary receivers communicate channel usage information over a control channel to any secondary user that is within its interference radius. Secondary user must switch to unused channel fast enough to cause negligible interference. Interference radius Ch. 7 Ch. 13 Ch. 2 CR Available channels UHF 2-6,8-13 UHF 2-12

Interference Avoidance Interference radius Ch. 2 Ch. 13 Ch. 3 CR Available channels UHF 3-13 UHF 2-12 Request Ch.3-13 Approve Ch.3 Secondary user must switch to unused channel fast enough to cause negligible interference.

Applicability Television receivers. Cable headend receivers. Wireless microphone receivers. Public safety receivers. Many other types of receivers.

Concerns with Approach Who will pay for them?  Cost (TV):  ($5/each)(30M) = ($150M), projected Wi-Fi sales in 2009  $8B (probably much larger once TV band is opened). Government will spend > $1B on D/A conversion boxes. Will sensor impact receiver reception quality?  Negligible 0.5dB decrease in SNR. Can recover by increasing transmit power by  10%. How often do batteries of sensors need to be replaced?   10 years. Assuming 1km range, 25% on, 1 D battery.

Conclusions and Future Work Detecting primary receivers solves the hidden node problem, and could be more efficient than detecting transmitters alone. Showed with this approach, interference temperature could become simple to estimate, is not ill-defined anymore. Demonstrated prototype sensor node to detect primary receiver channel usage. Future Work: Analyze scalability issues. Build system around this concept.

References [1] A. Sahai, N. Hoven, and R. Tandra, “Some Fundamental Limits on Cognitive Radio” Allerton Conference on Communication, Control, and Computing, October [2] N. Hoven, "On the feasibility of Cognitive Radio”, Master's thesis, University of California at Berkeley, Berkeley, CA, [3] Weiss, S. Merrill, Weller, Robert D., Driscoll Sean D. “New measurements and predictions of UHF television receiver local oscillator radiation interference” [online]. Available: h-e.com/pdfs/rw_bts03.pdf

Backup Slides

Spectrum Allocation vs. Usage  Perceived Scarcity vs. Actual Utilization