Underlay Spectrum Sharing is a concurrent transmission paradigm where secondary users (SUs) are permitted to transmit simultaneously with primary users (PUs) in the same frequency band, provided the aggregate interference generated by the SUs at any primary receiver remains strictly below a predefined interference temperature limit. This contrasts with opportunistic interweave models that require spectrum holes; underlay access exploits the interference tolerance of primary receivers to achieve continuous, low-power connectivity without disrupting incumbent operations.
Glossary
Underlay Spectrum Sharing
A concurrent transmission paradigm where secondary users transmit simultaneously with primary users, provided their interference is strictly constrained below a defined interference temperature limit.
CONCURRENT TRANSMISSION PARADIGM
What is Underlay Spectrum Sharing?
A spectrum access technique where secondary users transmit simultaneously with primary users, constrained by a strict interference temperature limit to ensure coexistence.
The core mechanism relies on wideband spread-spectrum techniques, such as direct-sequence spread spectrum or ultra-wideband, to spread secondary transmission power over a very wide bandwidth, effectively appearing as a negligible noise floor increase to narrowband primary receivers. The Federal Communications Commission introduced the interference temperature concept to quantify this permissible noise rise, shifting regulatory thinking from rigid exclusion zones to dynamic, measurement-based interference management. This model is foundational for dense IoT deployments and device-to-device communications within 5G and beyond.
CONCURRENT TRANSMISSION PARADIGM
Key Features of Underlay Spectrum Sharing
Underlay spectrum sharing enables secondary users to transmit simultaneously with primary licensees by strictly constraining interference below a defined interference temperature limit. This approach maximizes spectral efficiency without requiring spectrum holes or temporal vacancies.
01
Interference Temperature Limit
The foundational regulatory metric that defines the maximum permissible interference at a primary receiver. Unlike traditional noise-floor limits, the interference temperature model caps the aggregate RF energy from all secondary transmitters at the primary receiver's antenna.
- Measurement: Expressed in Kelvin, representing the equivalent temperature of radiated power per unit bandwidth
- Enforcement: Secondary users must dynamically adjust transmit power to stay below this cap
- Aggregation challenge: Multiple underlay transmitters compound interference, requiring coordinated power control
- FCC precedent: First proposed in the 2002 Spectrum Policy Task Force report as a paradigm shift from command-and-control licensing
2002
FCC Task Force Proposal Year
02
Wideband Spread Spectrum Foundation
Underlay sharing relies on direct-sequence spread spectrum (DSSS) or ultra-wideband (UWB) techniques to spread secondary transmission power across a bandwidth far wider than the information rate. This deliberate spectral dilution keeps power spectral density below the noise floor of primary narrowband receivers.
- Processing gain: The ratio of spread bandwidth to data bandwidth determines interference resilience
- UWB underlay: FCC Part 15 rules permit UWB operation from 3.1–10.6 GHz at -41.3 dBm/MHz EIRP
- CDMA heritage: Code-division multiple access pioneered underlay principles in 3G cellular
- Despreading at receiver: Correlator recovers the original narrowband signal from below the noise floor
-41.3 dBm/MHz
FCC UWB Emission Limit
04
Primary Exclusive Region
A Primary Exclusive Region (PER) is a geographically defined zone around each primary receiver where secondary transmitters are prohibited or severely restricted. This spatial guard zone ensures the interference temperature limit is never violated regardless of secondary user density.
- Radius calculation: Derived from primary receiver sensitivity, secondary max transmit power, and propagation model
- Dynamic PER: Radius adapts in real-time based on measured interference levels and secondary user distribution
- Outage probability constraint: PER designed to guarantee primary outage probability below a target threshold (e.g., 1%)
- Cognitive beacon: Primary receivers may broadcast a beacon defining their PER boundary for secondary user awareness
05
Spectrum Mask Compliance
Secondary transmitters must conform to a strict spectral emission mask that limits out-of-band emissions and adjacent channel leakage. This mask is more stringent than typical licensed transmitters to protect primary receivers operating on adjacent frequencies.
- Adjacent channel leakage ratio (ACLR): Typically requires >45 dB suppression in adjacent channels
- Pulse shaping: Root-raised cosine and Gaussian filters minimize spectral splatter
- Non-linear distortion mitigation: Digital pre-distortion compensates for power amplifier non-linearities that cause spectral regrowth
- Regulatory certification: Devices undergo rigorous testing per ETSI EN 301 893 or FCC Part 15 before underlay operation is permitted
06
Coexistence with Primary ARQ/HARQ
Underlay sharing must account for the primary network's Automatic Repeat reQuest (ARQ) and Hybrid ARQ retransmission protocols. Intermittent secondary interference can corrupt individual primary packets, triggering retransmissions that degrade primary throughput even if the interference temperature limit is nominally satisfied.
- Bursty interference impact: Even low-average-power underlay can cause clustered packet losses during primary transmission bursts
- Cross-layer design: Secondary MAC protocols should sense primary ARQ feedback and yield during retransmission windows
- Effective throughput metric: Primary performance measured by goodput (successfully delivered packets) rather than raw SINR
- Rateless code adaptation: Secondary users can employ fountain codes that adapt transmission duration based on primary activity patterns
UNDERLAY SPECTRUM SHARING
Frequently Asked Questions
Explore the core concepts of underlay spectrum sharing, a concurrent transmission paradigm where secondary users operate beneath the interference tolerance of primary licensees.
Underlay spectrum sharing is a concurrent transmission paradigm that allows secondary users (SUs) to transmit simultaneously with primary users (PUs) in the same frequency band, provided the aggregate interference generated by the SUs remains strictly below a regulatory-defined interference temperature limit at the primary receiver. Unlike opportunistic interweave models that wait for spectrum holes, underlay systems enforce a strict interference cap using ultra-wideband (UWB) or spread spectrum techniques. The mechanism works by spreading the secondary signal's power over a bandwidth so wide that its power spectral density falls below the noise floor of the primary receiver, effectively making the secondary transmission invisible to the primary system. This is mathematically governed by the constraint I_agg ≤ k * T * B, where I_agg is the aggregate interference, k is Boltzmann's constant, T is the interference temperature limit, and B is the bandwidth. The critical engineering challenge is maintaining this interference mask in real-time as channel conditions and primary user activity fluctuate.
SPECTRUM ACCESS PARADIGMS
Underlay vs. Overlay Spectrum Sharing
A technical comparison of the two primary dynamic spectrum sharing paradigms, contrasting their interference management philosophies, operational constraints, and deployment complexity.
| Feature | Underlay Sharing | Overlay Sharing | Interweave Sharing |
|---|---|---|---|
Core Principle | Concurrent transmission constrained by an interference temperature limit | Secondary users opportunistically access spectrum holes in time, frequency, or space | Secondary users sense and occupy only vacant licensed bands, vacating immediately upon primary user return |
Primary User Protection Mechanism | Strict transmit power control to maintain interference below a predefined threshold | Spectrum sensing and immediate channel vacation | Spectrum sensing, geolocation database lookup, and channel evacuation |
Secondary User Transmission Opportunity | Continuous, but at very low power | Bursty and opportunistic; only when spectrum is idle | Bursty and opportunistic; requires silent periods for sensing |
Interference to Primary User | Controlled, low-level, and constant | Theoretically zero, but subject to sensing errors and hidden node problems | Theoretically zero, with strict regulatory database enforcement |
Spectral Efficiency Gain | Moderate; limited by the interference temperature constraint | High; exploits temporal and spatial white spaces | High; maximizes utilization of vacant spectrum |
Implementation Complexity | High; requires precise real-time power control and channel estimation | Medium; requires reliable spectrum sensing and fast handoff mechanisms | Low to Medium; relies on a centralized geolocation database for authorization |
Regulatory Framework Example | Ultra-Wideband (UWB) under FCC Part 15 | TV White Spaces (TVWS) with geolocation database | Citizens Broadband Radio Service (CBRS) with Spectrum Access System (SAS) |
Suitable for High-Power Secondary Services |
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Real-World Applications of Underlay Sharing
Underlay spectrum sharing transitions from theory to practice in environments where strict interference temperature limits can be enforced, enabling concurrent primary and secondary transmissions.
01
Ultra-Wideband (UWB) Indoor Localization
UWB systems operate as secondary underlays beneath licensed GPS and cellular bands by spreading power across gigahertz of bandwidth. Power spectral density is kept below the noise floor of primary receivers, enabling precise real-time location services in industrial IoT without causing harmful interference.
< 10 cm
Localization Accuracy
-41.3 dBm/MHz
FCC Emission Limit
02
Device-to-Device (D2D) Communications in 5G
3GPP Release 12+ specifies D2D underlay where proximal handsets communicate directly while reusing uplink resources of distant cellular users. Power control algorithms dynamically cap D2D transmit power to maintain the interference temperature at the base station receiver below a critical threshold.
50%+
Spectral Efficiency Gain
3GPP Rel-12
Standardization
03
Cognitive Machine-to-Machine (M2M) Networks
Massive sensor deployments in smart grids operate as secondary users beneath TV White Space primary signals. Spectrum sensing combined with geolocation databases allows M2M nodes to transmit concurrently with broadcast TV, constrained by strict out-of-band emission masks to protect incumbent receivers.
470-698 MHz
TVWS Frequency Range
< 40 mW
Typical TX Power
04
Satellite-Terrestrial Integrated Networks
Low-power terrestrial base stations underlay incumbent satellite downlinks in C-band by exploiting the spatial separation between satellite earth stations and terrestrial users. Advanced beamforming and null steering create spatial notches toward satellite receivers, maintaining the interference temperature constraint.
3.7-4.2 GHz
C-Band Downlink
-109 dBm/m²
ITU PFD Limit
05
Military Cognitive Radio Coexistence
Tactical radios operate as secondary underlays beneath civilian LTE networks during joint operations. Cyclostationary feature detection identifies primary user signals, while adaptive power control ensures secondary transmissions remain below the noise floor of commercial base stations, preserving both operational security and civilian QoS.
< 0.1%
Primary Outage Probability
MIL-STD-188
Compliance Standard
06
LoRaWAN Underlay in ISM Bands
LoRa devices using chirp spread spectrum inherently operate as underlay systems in the crowded 868/915 MHz ISM bands. Their processing gain allows successful demodulation even when signals are received below the noise floor, enabling coexistence with primary short-range devices without explicit coordination.
-137 dBm
LoRa Sensitivity
SF7-SF12
Spreading Factors
About the author
Prasad Kumkar
CEO & MD, Inference Systems
Prasad Kumkar is the CEO & MD of Inference Systems and writes about AI systems architecture, LLM infrastructure, model serving, evaluation, and production deployment. Over 5+ years, he has worked across computer vision models, L5 autonomous vehicle systems, and LLM research, with a focus on taking complex AI ideas into real-world engineering systems.
His work and writing cover AI systems, large language models, AI agents, multimodal systems, autonomous systems, inference optimization, RAG, evaluation, and production AI engineering.
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