A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR UNCOORDINATED COGNITIVE RADIOS

1. A FREQUENCY HOPPING SPREAD SPECTRUM TRANSMISSION SCHEME FOR UNCOORDINATED COGNITIVE RADIOS Xiaohua (Edward) Li and Juite Hwu Department of Electrical and Computer Engineering State University of New York at Binghamton {xli, jhuw1}@binghamton.edu http://ucesp.ws.binghamton.edu/~xli

2. Contents • Introduction • System model • New FHSS transmission for cognitive radios • Demodulation and Performance analysis • Simulations • Conclusions

3. Introduction • Cognitive radios (CRs) • Detect and utilize spectrum white spaces • Should avoid interfering primary users • A major issue: “Chicken-and-Egg Problem” • CRs are initially not synchronized (e.g., in picking spectrum) for transmission • Transmission is required to negotiate such synchronization • Our goal • Develop a transmission scheme for uncoordinated CRs, tolerable to spectrum/channel uncertainty and spectrum sensing errors

4. Introduction (cont’) • Basic idea: Frequency-hopping over uncertain spectrum slots • CR transmitters and receivers hop over available spectrum slots • Hopping pattern determined by: • Spreading codes (shared) • Spectrum detection results (independent) • Channel selection rules (shared)

5. Introduction (cont’) • Assumptions • CR transmitters and receivers do have • Some common spreading codes • A common channel selection rule • Common procedure of adapting transmission parameters, such as symbol rate, modulations, etc • CR transmitters and receivers do not • have common spectrum white space information

6. 2. System model • Spectrum slots for frequency hopping • Divide the spectrum into I segments • Divide each segment into J frequency bands • Each band is a basic slot for frequency hopping, which we call “channel” • CR transmitters and receivers know slot structure, but do not know which slot is available in each time

7. 2. System Model • Major problem • A channel may be available to a transmitter but unavailable to a receiver • Define parameters:

8. 2. System Model Segmentation-based spectrum detection: When the CR transmits in a channel, it also collects information about the channels of next segment.

9. 3. New FHSS transmission • Spreading • To transmit a sequence • Each symbol spreaded into M chips • This procedure is identical to CR transmitters and receivers

10. Spectrum slot selection • Each chip is to be transmitted via a channel of ith segment Fi • Transmitters and receivers use a common binary sequence cn to determine channel selectability in this segment

11. Channel selection rule • There may be many channels selectable in each segment • Each CR Tx or Rx needs to select one channel to transmit or receive • Distributed channel selection means Tx and Rx may choose different channels  synchronization problem • Smart channel selection rule can alleviate this problem • A simple rule: choose the first available channel of this segment • Secondary transmitter use fi,j1 if ui,j1=1 • Secondary receiver use fi,j2 if wi,j2=1

12. Successful transmission→ Tx and Rx selected the same channel, i.e., j1=j2

13. Illustration of multiple CR transmissions using our scheme

14. FHSS/MFSK demodulation Vector symbol model for FHSS/MFSK signals 4. FHSS demodulation and performance analysis

15. FHSS/MFSK received signal model Baseband channel matrix Frequency slot synchronization indicator function

16. Element-wise description Coherent: Maximum Likelihood detection • Demodulations: coherent demodulation • Demodulations: non-coherent demodulation

17. 4. FHSS demodulation and performance analysis • Performance analysis • Major issue: Tx and Rx may use difference frequency slots  channel mismatch • SNR for coherent demodulation

18. Performance is limited by the correctness of frequency-selection • Assume mismatch probability pd be the probability that there is mismatch in the first j channels • With our simple channel selection rule

19. Average channel mismatch probability • For every M transmissions, number of correct matches

20. 5. Simulations Spreading gain M=40 Symbol amount K=100 Segments I=20 J=100 channels/segment

21. 5. Simulation Mismatch pd≒0.1 Symbol amount K=100 Segments I=20 J=100 channels/segment

22. 6. Conclusions • Developed an FHSS-FSK transmission scheme for uncoordinated cognitive radios • Tolerate spectrum sensing errors • No need of coordination assumptions • Use FHSS spreading gain to combat spectrum sensing errors and to avoid interfering primary users • Resolve the “chicken-and-egg” problem: provide a way for CRs to initiate communications in uncertain spectrum • Simulations demonstrate reliable performance even in large spectrum sensing errors