Underlay Spectrum Access

The foundational regulatory metric that defines the maximum allowable interference at a primary receiver. Unlike traditional noise floor limits, interference temperature accounts for the cumulative RF energy from all secondary transmitters within a primary's coverage area. A secondary user must constrain its transmit power so that the additional interference, measured in Kelvin, does not exceed this pre-defined threshold. This ensures the primary link's signal-to-interference-plus-noise ratio (SINR) remains above its operational minimum.

To operate below the interference temperature, secondary users often employ spread spectrum techniques such as Direct Sequence Spread Spectrum (DSSS) or Ultra-Wideband (UWB) signaling. By spreading transmission power over a bandwidth far wider than the information rate, the power spectral density (PSD) at any given frequency falls below the noise floor of a narrowband primary receiver. This allows the secondary user to transmit continuously without the primary receiver detecting a distinct interfering signal.

The viability of underlay access depends entirely on closed-loop power control. A secondary transmitter must dynamically adjust its output power based on real-time estimates of path loss to the nearest primary receiver. This requires precise channel state information (CSI) or geolocation databases. Overestimating the path loss leads to harmful interference; underestimating it unnecessarily constrains secondary throughput. This creates a critical feedback loop between the secondary link and the protected primary infrastructure.

Due to the severe transmit power constraints, underlay access is inherently limited to short-range communication. The secondary user's coverage radius is a function of the interference temperature limit and the distance to the primary receiver. Typical applications include femtocells, device-to-device (D2D) communication, and personal area networks where the secondary transmitter is physically close to its intended receiver and far from the primary infrastructure, minimizing the required transmit power.

Unlike overlay or interweave cognitive radio, underlay access does not require the secondary user to detect spectrum holes or primary user activity. The secondary transmitter operates continuously, relying solely on the interference temperature constraint to protect the primary link. This eliminates the hidden node problem and sensing-throughput tradeoff inherent in opportunistic access. The trade-off is a permanent bandwidth or power penalty, as the secondary user can never exploit periods of primary silence to increase its own throughput.

The theoretical capacity of an underlay secondary link is governed by the interference temperature constraint. Shannon's channel capacity formula is modified to include a peak or average interference power limit at a third-party receiver. This leads to distinct capacity scaling laws, such as the log-squared behavior in fading channels, where secondary capacity grows logarithmically with the interference constraint. In practice, this means the secondary user trades peak data rate for the privilege of guaranteed, continuous access.

The regulatory metric defining the maximum permissible interference at a primary receiver. Underlay access requires secondary transmitters to constrain their power so that the cumulative RF energy at the primary receiver remains below this threshold.

• Measured in Kelvin or dBm per unit bandwidth
• Represents the noise floor elevation a primary receiver can tolerate
• Differs from traditional noise limits by accounting for aggregate interference from multiple secondary users
• Defined by the FCC Spectrum Policy Task Force as a more dynamic alternative to fixed power limits

A physical layer technique where secondary signals are deliberately spread over a bandwidth far wider than the information rate, reducing power spectral density below the noise floor of primary receivers.

• Direct Sequence Spread Spectrum (DSSS): Multiplies data with a high-rate pseudo-noise code
• Frequency Hopping Spread Spectrum (FHSS): Rapidly switches carrier across many frequencies
• Processing gain allows secondary users to operate at negative SNR relative to primary signals
• Originally developed for military anti-jamming; repurposed for civilian spectrum sharing

A specific underlay implementation using extremely short pulses (sub-nanosecond) to occupy several gigahertz of spectrum simultaneously. UWB devices operate under strict FCC Part 15 emission masks.

• Power spectral density limited to -41.3 dBm/MHz
• Operates in 3.1-10.6 GHz band overlapping with existing services
• Enables high-precision indoor positioning and short-range high-data-rate links
• The canonical commercial example of regulatory-approved underlay spectrum access

The adaptive transmission power algorithm that ensures secondary users remain below the interference temperature limit at all primary receivers. This is the central control problem in underlay systems.

• Requires estimation of channel gain to primary receivers
• Often formulated as a convex optimization problem maximizing secondary throughput subject to interference constraints
• Reinforcement learning agents can learn optimal power policies without explicit channel models
• Must account for shadow fading and multi-user interference among secondary transmitters

The third major cognitive radio paradigm alongside underlay and overlay. Interweave access relies on opportunistic spectrum hole exploitation rather than concurrent transmission.

• Secondary users must perform spectrum sensing to detect idle bands
• No simultaneous primary-secondary transmission occurs
• Vacates channel immediately upon primary user return (spectrum mobility)
• Fundamentally different from underlay: avoids interference rather than managing it below a threshold
• Also known as Dynamic Spectrum Access (DSA) or opportunistic access