Interference Temperature

Unlike traditional noise floor limits, interference temperature sets an aggregate upper bound on the total emissions from all secondary transmitters within a primary receiver's band. This cap is measured in degrees Kelvin, representing the equivalent temperature of the total RF power spectral density. The model accounts for the sum of the original thermal noise floor and the tolerable additional interference, creating a hard limit that no combination of secondary users may exceed.

Interference temperature limits are not globally uniform; they are defined for a specific geographic location and frequency band at a given time. A primary receiver's protected contour is mapped, and the interference temperature is enforced at that precise spatial boundary. This allows for dynamic, location-based spectrum reuse where secondary transmitters far from the primary receiver may operate at higher power than those nearby, maximizing spatial efficiency without violating the aggregate cap.

The concept was formally introduced by the Federal Communications Commission (FCC) in its 2002 Spectrum Policy Task Force Report. It was proposed as a shift from command-and-control licensing to a more flexible, interference-based management paradigm. The goal was to quantify and manage the 'noise floor' actively, enabling underlay spectrum sharing where secondary devices could transmit continuously, provided the total RF energy at any primary receiver remained below the established interference temperature threshold.

Interference temperature is the core regulatory mechanism enabling underlay spectrum sharing. In this model, secondary users do not wait for a channel to be vacant (as in interweave access). Instead, they transmit simultaneously with primary users by spreading their signal power, often using ultra-wideband (UWB) or code-division multiple access (CDMA) techniques, such that their contribution to the interference temperature at the primary receiver remains negligible. This requires precise, real-time power control.

A significant practical challenge is the direct measurement of interference temperature at a primary receiver without cooperation from the primary system. Proposals involve deploying a network of distributed sensing nodes that construct a real-time radio environment map (REM). These sensors estimate the cumulative interference at the protected contour using spatial interpolation. Enforcement relies on cognitive radios dynamically adjusting their transmit power based on a policy engine that calculates the permissible interference margin from the sensed data.

Interference temperature is mathematically related to the receiver's noise figure. The total effective noise temperature is T_total = T_antenna + T_system + T_interference, where T_system is derived from the receiver's noise figure. The interference temperature limit effectively caps T_interference, ensuring the total noise power spectral density (N0 = k * T_total, where k is Boltzmann's constant) does not degrade the primary receiver's minimum detectable signal below its operational threshold.

A coexistence technique where secondary users transmit simultaneously with primary users by spreading their signal power below the interference temperature limit. This approach treats interference as a cumulative resource rather than a binary avoid/detect condition.

• Typically employs Ultra-Wideband (UWB) or Direct Sequence Spread Spectrum (DSSS)
• Requires precise power control to stay below the regulatory cap
• Enables continuous secondary access without spectrum sensing

The classic opportunistic access model where secondary users identify and exploit temporal or spatial spectrum holes. Unlike underlay sharing, interweave requires that primary users are confirmed absent before transmission begins.

• Relies on continuous spectrum sensing to detect white spaces
• Must vacate immediately upon primary user return (spectrum handoff)
• Does not contribute to the cumulative interference temperature during operation

The foundational awareness mechanism that measures the electromagnetic environment to detect primary user presence. Sensing accuracy directly determines whether secondary transmissions will exceed the interference temperature threshold.

• Matched filter detection: Optimal when primary signal characteristics are known
• Energy detection: Simple but struggles below the noise floor
• Cyclostationary feature detection: Exploits periodic signal properties for robust classification

A security threat where a malicious actor mimics the signal characteristics of a licensed primary user to illegitimately reserve spectrum. This attack exploits the interference temperature model by falsely signaling that the cumulative interference budget has been exhausted.

• Denies legitimate secondary users access to available spectrum
• Mitigated through RF fingerprinting and location verification
• Undermines the trust assumptions of dynamic spectrum access protocols

An automated frequency coordination engine that dynamically manages spectrum assignments in the CBRS band. The SAS enforces interference protection criteria by calculating aggregate interference levels and allocating channels across three tiers:

• Incumbent Access: Federal radar and satellite systems (highest priority)
• Priority Access: Licensed users with interference protection guarantees
• General Authorized Access: Opportunistic use constrained by interference temperature limits

A channel access mechanism requiring a transmitter to perform a Clear Channel Assessment (CCA) and verify the absence of other transmissions before initiating its own. LBT operationalizes interference temperature constraints at the device level.

• Widely used in Wi-Fi and License-Assisted Access (LAA) for LTE/5G
• Employs energy detection thresholds to determine channel occupancy
• Includes random backoff periods to prevent collisions among competing devices