IIR Filter

An IIR high-pass filter is a recursive digital filter that attenuates low-frequency components including DC,

while allowing higher-frequency components to pass.

Its frequency response is determined by feedback coefficients,

which control the cutoff behavior and the degree of low-frequency suppression.

 

 

 

 

 

1-1. IIR Filter before Range FFT

It mainly serves as a signal pre-processing stage, removing DC offset and low-frequency noise rather than directly altering range estimation.

Its direct impact on target detection performance is limited, as it does not alter underlying range or velocity information

but instead conditions the input signal to improve stability, dynamic range, and SNR, with only negligible effects near zero-range components.

Removes DC and low-frequency components in fast-time I/Q signals.
Improves dynamic range of the Range FFT input.
Stabilizes range peak detection.
Has negligible impact on range resolution and bias.

 

 

 

1-2. Example cases

The b value is tuned to control the input dynamic range of the Range FFT and to stabilize range peak detection.

 

System Environment	Condition	a	b	Key Criteria
HW System	Large ADC offset, LO leakage	-1	0.99 ~ 0.999	Strong DC component → aggressive suppression
	Typical system condition	-1	0.98 ~ 0.995	Basic DC removal with minimal signal distortion
	Distortion-sensitive, micro-signal important	-1	0.95 ~ 0.98	Preserve low-frequency components, avoid over-filtering
Reflection Environment	Strong static reflection (wall/floor)	-1	0.99 ~ 0.997	Suppress low-frequency components (including static reflections)
	Typical environment (weak reflection)	-1	0.97 ~ 0.99	Stable DC removal under moderate reflection
	Low clutter, clean environment	-1	0.95 ~ 0.98	Minimal filtering to preserve signal integrity

 

 

 

 

2-1.  IIR Filter is before Doppler FFT

The filter operates along the slow-time axis and directly shapes the Doppler spectrum,

making its tuning critical depending on which motion-related signals (targets) need to be preserved or suppressed.

It suppresses low-frequency components corresponding to static or slowly varying signals.

These low-frequency components correspond to near-zero Doppler (i.e., low or zero velocity targets).

 

 

Removes static-target clutter (0-Doppler components) in slow-time signals.
Flattens the Doppler background spectrum.
Stabilizes CFAR thresholding and reduces false alarms.
Reduces sensitivity to low-Doppler targets and may affect velocity estimation.

 

 

 

2-2. Example Cases

The b parameter is adjusted according to the motion detection criteria.

 

Target Category	Detailed State (Scenario)	a	b (Coefficient)	Effective Detectable Speed Range
Human(Low-speed)	Micro-movement(Heartbeat/Breathing)	-1	0.995 ~ 0.999	Ultra-low speed (≤ 0.1 m/s) pass-through, very high precision
	Static Presence(Keyboard Typing)	-1	0.968 ~ 0.984	Extraction of subtle movements from a stationary person
	Walking & Direction Change	-1	0.921 ~ 0.953	Optimized clutter removal (tree leaves, static object residuals) during normal walking (~1 m/s)
	Vigorous Motion(Gestures)	-1	0.875 ~ 0.906	Reduced influence of low-frequency residual components during fast motion such as arm swinging
Vehicle(High-speed)	Urban Slow Driving(Stop & Go)	-1	0.843 ~ 0.875	Detection of initial acceleration of stop-and-go vehicles
	City Driving(Normal)	-1	0.750 ~ 0.812	Significant suppression of noise from surrounding buildings and roadside trees
	High-speed Driving(Highway)	-1	0.625 ~ 0.750	Strong attenuation of all low-frequency components except high-speed targets