On Partially Overlapping Coexistence for Dynamic Spectrum Access in Cognitive Radio

Transcription

1 On Partially Overlapping Coexistence for Dynamic Spectrum Access in Cognitive Radio Ebrahim Bedeer, Mohamed Marey, Octavia Dobre, and Kareem Baddour Memorial University of Newfoundland, St. John s, NL, Canada Communications Research Centre, Ottawa, ON, Canada {e.bedeer, mmarey, June 10, / 20

2 Outline Introduction 1 Introduction / 20

3 Problem Increasing demand of bandwidth to support new services with limited spectrum resources. Solution Cognitive Radio (CR) technology. Approach 1 Cognitive Users (CUs) access Primary Users (PUs) spectrum holes (temporally or spatially). Approach 2 Spectral sculpting of CUs to allow partial frequency overlap (coexistence) with PUs. 3 / 20

4 Attractive modulation candidate for CUs: OFDM due to its flexibility and adaptivity. Power PU CU interference CU Frequency PU interference Partially overlapping coexistence concept. 4 / 20

5 Study the coexistence of CUs (OFDM) and PUs (NB and OFDM, respectively). Find the minimum frequency separation between coexisting systems to meet a target BER. Investigate techniques (windowing, nulling subcarriers) to increase minimum frequency separation. Explore the effect of windowing and nulling subcarriers on the OFDM CU PAPR and spectral efficiency. 5 / 20

6 OFDM (CU & PU) s OFDM (t) = 1 To n= k Ω an k ei2πf k(t nt o) w(t nt o ) NB PU s NB (t) = k= b kp(t kt ξ) e j2πfct 6 / 20

7 Simulation Setup Introduction OFDM BW OFDM = 1.25 MHz N = 128 F = khz T u = µsec T cp = 25.6 µsec Modulation: QPSK Channel: AWGN - frequency selective channel NB BW NB = 15 khz Roll-off factor, α = 0.35 Modulation: QPSK Channel: AWGN - frequency flat channel F n = fc 0.5 BW OFDM F 7 / 20

8 First Coexistence Scenario OFDM CU NB PU. Estimated PSD (db) Frequency Normalized to OFDM BW 8 / 20

9 AWGN channel Introduction SIR = 0 db SIR = 10 db SIR 10 2 BER Fn NB PU BER as a function of F n in AWGN channel at E b N o = 10 db. 9 / 20

10 AWGN channel Introduction 10 2 SIR = 24 db SIR = 44 db SIR 10 3 BER Fn OFDM CU BER as a function of F n in AWGN channel at E b N o = 10 db. 10 / 20

11 Fading channel Introduction Fn = 12 Fn = 20 bound 10 2 BER Fading bound SIR (db) NB PU BER as a function of SIR in fading channel at E b N o = 35 db. 11 / 20

12 Second Coexistence Scenario 10 0 OFDM CU OFDM PU. Estimated PSD (db) Frequency Normalized to OFDM BW 12 / 20

13 AWGN channel Introduction 10 2 SIR = 0 db SIR = 10 db SIR 10 3 BER Fn OFDM PU BER as a function of F n in AWGN channel at E b N o = 10 db. 13 / 20

14 AWGN channel Introduction 10 2 Fn = 10 Fn = BER Number of subcarriers OFDM PU BER as a function of OFDM CU number of subcarriers in AWGN channel at SIR = 0 db and E b N o = 10 db. 14 / 20

15 Windowing Introduction BER 10 3 Fn = 0 Fn = β NB PU BER as a function of the raised cosine window roll-off factor β in AWGN channel at SIR = 0 db and E b N o = 10 db. 15 / 20

16 Nulling Introduction 10 0 Fn = 0 Fn = BER Number of nulled subcarriers NB PU BER as a function of the OFDM CU number of nulled subcarriers in AWGN channel at SIR = 0 db and E b N o = 10 db. 16 / 20

17 Effect of windowing and nulling on OFDM CU PAPR P(PAPR > PAPRo) No nulling, No windowing nulling (3 nulled subcarriers) windowing (β = 0.15) PAPRo (db) OFDM CU PAPR for β = 0.15 and 3 nulled subcarriers, respectively. 17 / 20

18 Effect of windowing and nulling on spectral efficiency ζ = mn u/(t s (1 + β)) N F Windowing } β = 0.15 = ζ = (bits/sec)/hz N u = 128 Nulling } β = 0 N u = 125 = ζ = (bits/sec)/hz 18 / 20

19 Coexistence between OFDM CU, and NB PU and OFDM PU systems is considered. Minimum frequency separation to meet a target BER is found to be a function of SIR and channel conditions. Minimum frequency separation can be improved by windowing or nulling subcarriers; however, this reduces spectral efficiency. Balancing trade-offs between spectral efficiency and minimum frequency separation for the coexistence scenarios. 19 / 20

20 Questions? 20 / 20