Origin Offset

Origin offset is primarily caused by carrier feedthrough, where the local oscillator signal leaks through the mixer and appears at the output. This occurs due to finite isolation between the LO and RF ports in the mixer stage.

• Results from DC bias voltages in the baseband I and Q paths
• Produces an unmodulated carrier component at the exact center frequency
• Magnitude typically ranges from -25 dBc to -40 dBc relative to the modulated signal
• Varies between devices due to semiconductor doping variations and layout parasitics

The offset manifests as a constant displacement vector that shifts every constellation point by the same amount. This translation is defined by two orthogonal components:

• I-offset: The DC bias in the in-phase path, shifting the constellation horizontally
• Q-offset: The DC bias in the quadrature path, shifting the constellation vertically
• Combined, they form a complex offset vector with magnitude and phase unique to each transmitter
• This vector remains stable over short timeframes but may drift with temperature

Origin offset is often confused with I/Q imbalance, but they are distinct impairments with different root causes and signal signatures.

• Origin offset: Additive error shifting the entire constellation (caused by DC bias)
• I/Q imbalance: Multiplicative error distorting the constellation shape (caused by gain/phase mismatch)
• Origin offset creates a constant displacement regardless of signal amplitude
• I/Q imbalance produces amplitude-dependent distortion that scales with the signal envelope
• Both impairments can coexist and must be separately characterized for accurate fingerprinting

The origin offset creates a spectral spike at the carrier frequency that can be isolated through frequency-domain analysis. This spike corresponds to the unmodulated LO leakage component.

• Detectable using DC-level estimation on demodulated I and Q baseband signals
• Can be measured by computing the mean of the received constellation over many symbols
• Requires carrier synchronization to accurately separate the offset from frequency errors
• Advanced techniques use blind estimation algorithms that operate without known training sequences
• The offset magnitude is typically expressed in dBc or as a percentage of average symbol energy

Origin offset exhibits thermal dependence due to the temperature sensitivity of semiconductor junctions and bias circuits. This characteristic must be accounted for in long-term fingerprinting systems.

• DC offset typically drifts by 0.5-2% per 10°C in commercial transmitters
• Warm-up period of 30-60 seconds required for stabilization after cold start
• Supply voltage variations can modulate the offset magnitude
• Aging effects cause gradual drift over months to years
• Compensation algorithms can track and normalize these variations for robust authentication

Origin offset provides a moderately discriminative feature for device identification, best used in combination with other impairments for high-confidence authentication.

• Advantages: Simple to extract, computationally inexpensive, stable over short sessions
• Limitations: Lower uniqueness than power amplifier non-linearity or phase noise signatures
• Effective for coarse device classification when combined with I/Q imbalance metrics
• Less effective for large populations of identical hardware where offsets may cluster
• Best deployed as part of a multi-feature fingerprinting ensemble for robust physical-layer authentication