Cooperative Spectrum Sensing

A single cognitive radio may fail to detect a primary transmitter due to shadowing or multipath fading, creating a hidden node. Cooperative sensing solves this by distributing sensors geographically. If one node is in a deep fade, another with a clear line-of-sight can still detect the signal.

• Mechanism: Spatial diversity gain from geographically separated receivers.
• Result: Drastically reduces the probability of missed detection, preventing harmful interference to primary users.

The method of combining individual sensing observations defines the system's performance and overhead. Architectures range from simple decision sharing to complex raw data aggregation.

• Hard Combining: Nodes transmit a local 1-bit decision (signal present/absent). The fusion center applies a logic rule like K-out-of-N or OR rule. Low bandwidth overhead.
• Soft Combining: Nodes transmit full sensing statistics (e.g., energy levels, likelihood ratios). The fusion center uses Equal Gain Combining (EGC) or Maximal Ratio Combining (MRC) for superior sensitivity at the cost of higher control channel bandwidth.

A dedicated, reliable common control channel is essential for sharing sensing data without interfering with primary users. This channel must be robust and low-latency.

• Bandwidth Constraints: Soft combining requires significantly more reporting bandwidth than hard combining.
• Security: The control channel is a vulnerability vector for Spectrum Sensing Data Falsification (SSDF) attacks, where malicious users report false data to corrupt the fusion decision.

The fusion center applies a specific rule to the collected observations to make a final global decision. The choice of rule balances sensitivity and specificity.

• OR Rule: Declares a primary user present if any single node detects it. Maximizes protection for the primary user but increases false alarms.
• AND Rule: Declares a primary user present only if all nodes detect it. Maximizes spectrum reuse opportunity but risks missing the primary user.
• Majority Logic: A compromise where the decision is based on the majority vote, optimizing the trade-off between detection probability and false alarm rate.

Not all cooperating nodes contribute equally. Nodes experiencing deep fading or correlated shadowing can degrade global performance. Intelligent selection is critical.

• Correlation-Aware Clustering: Grouping nodes with uncorrelated observations maximizes spatial diversity gain. Nodes within the same shadowing correlation distance provide redundant information.
• SNR-Based Selection: Excluding nodes with low instantaneous Signal-to-Noise Ratio (SNR) reduces the noise floor in the fusion process, improving overall detection sensitivity.

For cooperative sensing to be effective, all nodes must sense the same spectrum band at the same time. Time synchronization is a non-trivial implementation challenge.

• Slot Alignment: Sensing periods must be aligned to a common reference clock, often derived from GPS or the IEEE 1588 Precision Time Protocol (PTP).
• Phase Mismatch: In soft combining, carrier phase offsets between nodes can cause destructive interference at the fusion center, requiring complex phase compensation algorithms.