Interweave Spectrum Sharing

The defining constraint of interweave sharing is the absolute priority of the licensed incumbent. Secondary users (SUs) must vacate a channel within a predefined channel evacuation time upon detecting a returning primary user (PU).

• Interference Avoidance: The goal is not interference management but complete avoidance; SUs must remain transparent to the PU receiver.
• Detection Sensitivity: Requires sensing algorithms capable of detecting PU signals at very low signal-to-noise ratios (SNR), often below -20 dB, to overcome the hidden node problem.
• Regulatory Compliance: This model directly maps to regulatory frameworks like the FCC's Interference Temperature concept and the operation of TV White Spaces devices.

Interweave access relies entirely on the accurate, real-time identification of spectrum holes—gaps in the frequency, time, or geographic domain where no primary user is actively transmitting.

• Temporal Holes: Silent periods between PU transmission bursts, exploited using time-division sensing.
• Spatial Holes: Geographic areas outside the PU's protected contour where secondary transmission is permissible, often verified via a geo-location database.
• Frequency Holes: Unused sub-carriers within a PU's wider allocated band, detectable through high-resolution spectral analysis.

Unlike overlay or underlay models, interweave access mandates a continuous sense-before-talk cycle. A secondary transmitter cannot assume a channel remains free; it must periodically cease transmission to re-assess the spectrum.

• Quiet Periods: MAC-layer protocols enforce synchronized silent intervals where all SUs in a network pause to sense, preventing self-interference during detection.
• Sensing-Throughput Trade-off: A fundamental engineering tension exists: longer sensing durations improve PU detection probability but reduce the time available for SU data transmission, directly impacting network throughput.

When a primary user is detected, the secondary user must execute a spectrum handoff—a seamless transition to another identified spectrum hole to maintain session continuity without dropping the connection.

• Proactive Handoff: An optimal strategy where SUs maintain a ranked backup channel list based on predictive occupancy models, minimizing switching latency.
• Reactive Handoff: Triggered only upon PU detection, requiring an immediate, on-the-fly search for a new vacant channel, which introduces higher latency.
• Connection Integrity: The handoff mechanism must preserve upper-layer protocol states to prevent TCP timeouts or application-layer failures during the transition.

To combat the hidden node problem—where a single sensor fails to detect a PU due to shadowing or multipath fading—interweave networks often employ cooperative sensing. Multiple spatially distributed nodes share their local observations with a fusion center.

• Hard Combining: Nodes report binary decisions (PU present/absent); the fusion center applies logic rules like OR, AND, or K-out-of-N voting.
• Soft Combining: Nodes forward raw energy measurements or likelihood ratios, allowing the fusion center to perform more sensitive statistical tests at the cost of higher backhaul overhead.
• Spatial Diversity Gain: Cooperation dramatically reduces the probability of missed detection, enabling more aggressive spatial reuse of spectrum holes.

The foundational awareness mechanism for interweave sharing. A cognitive radio must continuously monitor the electromagnetic environment to detect spectrum holes—gaps in time, frequency, or space where the primary user is absent.

• Matched filter detection: Optimal when primary signal characteristics are known
• Energy detection: Suboptimal but requires no prior signal knowledge
• Cyclostationary feature detection: Distinguishes signals from noise using periodicity

Sensing accuracy directly determines the probability of harmful interference.

A critical failure mode in interweave systems where a secondary transmitter cannot detect a primary transmitter due to physical obstructions or distance, yet its transmission still interferes with a nearby primary receiver.

• Occurs when sensing is performed locally rather than cooperatively
• Results in false negatives—declaring a channel vacant when it is occupied
• Mitigated through cooperative spectrum sensing with spatially distributed nodes

This is the primary reason regulatory bodies mandate conservative detection thresholds.

The process by which a secondary user vacates a channel immediately upon detecting a returning primary user and seamlessly transitions to an alternative spectrum hole.

• Reactive handoff: Triggered by primary user detection; requires rapid channel switching
• Proactive handoff: Pre-computes backup channels using spectrum mobility prediction
• Target channel selection must minimize handoff latency to prevent session disruption

Effective handoff protocols maintain quality of service for secondary users while guaranteeing primary protection.

A denial-of-service attack where a malicious actor mimics the signal characteristics of a legitimate primary user to trick secondary radios into vacating a channel, thereby reserving spectrum for the attacker's exclusive use.

• Exploits the fundamental interweave rule: secondary users must yield to primaries
• Countered through RF fingerprinting—identifying unique hardware imperfections in transmitters
• Location verification techniques can also detect emulators by triangulating signal origin

PUEA represents the most significant security vulnerability in cognitive radio networks.

A channel access protocol requiring a transmitter to perform a clear channel assessment (CCA) before initiating any transmission. If energy above a defined threshold is detected, the device must defer.

• Core mechanism in Wi-Fi (CSMA/CA) and LTE-U/LAA
• Uses random backoff periods to avoid collisions when multiple devices contend
• Differs from pure interweave: LBT is a polite coexistence rule, not a primary-protection mandate

LBT is the dominant etiquette protocol for unlicensed spectrum sharing worldwide.

A regulatory metric proposed by the FCC that defines the maximum tolerable interference level at a primary receiver. Rather than requiring absolute primary user absence, this model permits secondary transmissions as long as the cumulative RF energy stays below the threshold.

• Measured in Kelvin, representing equivalent noise power at the receiver antenna
• Enables underlay sharing—concurrent primary and secondary operation
• Never fully adopted due to challenges in real-time measurement at the receiver

The concept bridges interweave and underlay paradigms by quantifying acceptable interference.