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,

Contents 1.Introduction 2.System model 3.New FHSS transmission for cognitive radios 4.Demodulation and Performance analysis 5.Simulations 6.Conclusions

1.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

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)

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

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

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

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

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

Spectrum slot selection Each chip is to be transmitted via a channel of i th segment F i Transmitters and receivers use a common binary sequence c n to determine channel selectability in this segment

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 f i,j1 if u i,j1 =1 Secondary receiver use f i,j2 if w i,j2 =1

Successful transmission→ Tx and Rx selected the same channel, i.e., j 1 =j 2

Illustration of multiple CR transmissions using our scheme

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

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

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

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

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

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

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

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

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