I/Q Origin Offset

I/Q Origin Offset appears as a constant DC voltage superimposed on the baseband I and Q signals before upconversion. This results from transistor mismatches in the differential pairs of the modulator's local oscillator (LO) path, causing carrier leakage. The offset shifts the entire constellation diagram away from the origin, creating a measurable carrier feedthrough component that is unique to each device due to microscopic manufacturing variances.

The offset causes a rigid translation of the entire symbol constellation. For ideal QPSK, the four points shift uniformly from their nominal positions. Key observable effects include:

• Error Vector Magnitude (EVM) degradation proportional to offset magnitude
• Asymmetric distortion in amplitude-phase plots
• A deterministic, non-zero mean in the I and Q sample distributions This translation is stable over time and independent of the transmitted data sequence.

Also known as Local Oscillator (LO) Leakage, this impairment injects an unmodulated carrier tone at the center frequency. The leakage path originates from:

• Finite isolation between the LO port and the RF output
• DC offsets in the baseband amplifier stages
• Self-mixing effects in the mixer core The leaked carrier power relative to the modulated signal power is a quantifiable, device-specific metric often expressed in dBc.

Origin offset is estimated by computing the long-term statistical mean of the received I and Q samples after synchronization. The process involves:

• Coherent averaging over thousands of symbols to suppress noise
• Compensation for channel rotation before offset calculation
• Separation of transmitter offset from receiver DC bias using differential techniques The resulting I_offset and Q_offset pair forms a 2D feature vector for fingerprinting classifiers.

I/Q Origin Offset provides strong anti-spoofing characteristics because:

• It is an unintentional artifact of the analog silicon, not a programmable parameter
• Exact replication requires physical access to the target device and nanometer-scale fabrication control
• The offset exhibits temperature-dependent drift patterns unique to each component
• Attempting to inject a compensating DC bias at the adversary's transmitter introduces its own measurable distortion signature

While both are modulator impairments, origin offset and I/Q imbalance are distinct phenomena:

• Origin Offset: Additive DC bias shifting the constellation center; modeled as c(t) = c_I + j*c_Q
• I/Q Gain Imbalance: Multiplicative amplitude mismatch between I and Q branches
• I/Q Quadrature Error: Phase error deviating from the ideal 90-degree separation These impairments are statistically independent and provide complementary fingerprinting features when extracted jointly.