Presented at ISOCC-2022 Session info : Advanced Radio-Frequency Design Techniques Date : Oct 21, 2022 Place : Gangneung, Korea Cite with the shortened URL of https://lnkd.in/gDNAYJJ5 [ URL/DOI of the paper/preprint ] DOI) 10.1109/ISOCC56007.2022.10031373 URL) https://ieeexplore.ieee.org/document/10031373 For Channel Sounding (formerly HADM) of Bluetooth in the slide, see https://lnkd.in/ggmWbnAJ

1. ISOCC2022
Tone-based Measurement
of Excess Group Delay
in Programmable Gain Receiver Chains
for RF Ranging
Ealwan Lee
GCT Semiconductor, Inc.
Oct 21, 2022

2. Outline
• Introduction
• Problem Statement
– Narrow-band RF ranging
• Phase-based distance measurement
– Excess delay in programmable gain receiver chains
• Tone-Based Assessment
– Parameter set for BLE channel sounding application
• Experimental Results
– Applied to Commercial Off-The-Shelf product
• Conclusion
– Seeking for the frugal solution of BLE RF ranging
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3. Introduction
• History of RF ranging
– Wide-band : UWB (802.15.4z)
– Narrow-band
• Early IEEE 802.15.4(WPAN) WG : up to 2013
• Channel Sounding / Bluetooth Low Energy : since 2018
• Distance Measurement
– 2-way active reflection : round-trip-delay, phase
– Ideally, constant resolution in the coverage
• RSSI-based : getting worse for long distance
• Pros/Cons of BLE : Add-on feature of comm.
– data link(main) : made insensitive to channel state
– ranging/sensing : make it sensitive to channel state
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4. Operation of Narrow-band RF Ranging
• Phase difference between two+ tone frequencies
– Exactly the group delay : delay(time) <=> displacement
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𝑑 = 𝑐/4𝜋 ⋅ 𝑓1 − 𝑓2 ⋅ Δ𝜙1,𝑅 − Δ𝜙1,𝐼 − Δ𝜙2,𝑅 + Δ𝜙2,𝐼 Τ
2𝑑 𝑐 ; RTT

5. Effect of Excess Delay
• Long distance -> High gain  -> Bias for long
• Variation in frequency selective channel
– If gain is fixed during the channel sweep, DR of ADC 
– Multi-antenna for diversity -> cost 
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2m
20m (~ 67 ns)
RSSI@2m > RSSI@20m + 20 dB
f f
80 ch 80 ch
20dB
Extra Margin
for Multi-path

6. Key Numbers
• Dynamic Range of Receiver Chains
– Typically > 60 dB
• Practical accuracy target : decimeter* ~ submeter
– 10*~15 cm => TS/4 of UWB (500 MHz band-width)
– Tolerance to delay(time) error : 0.5 ns
• Bluetooth Low Energy(GMSK)
– Channel offset in channel sounding : 80 MHz/(2*40) Ch
– Modulation index : 0.5
– Symbol rate : 1 MHz
– Typical ADC spec :
• ENOB : 7~8 bits
• Sampling rate : 8 Msps
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7. Excess Group Delay
• Programmable Gain Receiver Chain
– Dominant factor : Op-amp in PGA (Range ~ 20 dB)
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8. Excess Delay in Op-Amp
• 2-stage op-amp with Miller compensation.
– Fully differential => Single-ended, gain < 0
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reference delay

9. How to Handle the Excess Delay
• No interest in this matter till now
– No proper terminology on this
• group delay variation : in spectral domain.
– Even a few just focuses on qualitative property[5].
• Do not re-design in panic
– Going to super wide band (x)
• More current consumption
– Adaptive bias adjustment (x)
– Fix gain and use 1x-b ADC (?)
• Pulse-based Measurement
– Limited by dispersion of ...
• Slew-rate
• Filter response
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10. PVT Variation : Partitioning of Delay
• Factory calibration : absolute ref delay
• Power-on calibration : relative volatile part
– temperature, voltage
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Ref point for excess delay
TT+85
SS+25
TT+25
FF+25
3.4 26.6 30.0 ns
2.4 17.7 20.1 ns
3.8 26.9 32.7 ns
4.8 42.8 ns
38.0
Total
P,T
-3 dB 3 dB 9 dB 15 dB 21 dB

11. Tone-based Measurement
• Period = n4 sec
– BLE modulation index = 0.5, symbol rate = 1 Msps.
• Receiver architecture
– Direct conversion, Digital low IF(Mostly fLO = 2MHz)
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12. Estimation Formula
• De-rotate to DC -> Measure the phase difference
• Pre-cal frequency offset between GEN and DUT
– in case 𝑮𝒏 = 𝑮𝒏−𝟏
– Control either GEN or DUT
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𝜟𝝓 = −𝐚𝐭𝐚𝐧
σ𝑮𝒏>𝑮𝒏−𝟏
𝑰𝒏 ⋅ 𝑸𝒏−𝟏 − 𝑸𝒏 ⋅ 𝑰𝒏−𝟏
σ𝑮𝒏>𝑮𝒏−𝟏
𝑰𝒏 ⋅ 𝑰𝒏−𝟏 + 𝑸𝒏 ⋅ 𝑸𝒏−𝟏
, 𝜟𝒕 =
𝜟𝝓
𝟐𝝅
⋅ 𝑻
𝑰𝒏, 𝑸𝒏
VC-TCXO
fc,fs/VEE
Gn
RSSI
pwr

13. 8 9 10 11 12
-2
-1
0
1
2
3
4
ADC ENOB (bits)
Excess
delay
(ns)
Precision for Decimeter Ranging
• ENOB of ADC[8] : 11-bits > Threshold for 0.5 ns
• Spreading gain : 15+3m dB, m=0 for [8]
– (42m) us * 8 Msps
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0 0.5 1 1.5 2 2.5 3 3.5 4
-10
-8
-6
-4
-2
0
2
4
6
8
10
Time(us)
Normalized
1.99 1.992 1.994 1.996 1.998 2 2.002 2.004 2.006 2.008 2.01
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
Time(us)
Normalized
x200
0.5 ns

14. Experimental Results
• Applied to COTS product [8]
– Accumulation of 3-stage PGA excess delays
13/15
Range of
GEN PWR

15. Inter-stage Alignment
• Confine the cumulative error within 0.5 ns
• Signal power level not changed
– Measure the phase difference between stages.
– Same nominal gain but with different gain distribution.
• Correct the cumulative error.
– Re-scale and de-skew in MMSE sense.
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t(g1,0)
t(0,g2)
error
t(g1,0)-t(0,g2)
t(g1,g2)

16. Conclusion
• Accurate narrow-band RF ranging
– Excess Group Delay : One of the blockers
• No need to suppress excess delay in panic
– Alternative : estimation and compensation
• Tone-based assessment
– Basic concept : phase to delay
– Proposal of key parameters
– Correction of cumulative errors
• Reminder
– Attention on the group delay of PGA
– Odds for BLE channel sounding(https://lnkd.in/ggmWbnAJ)
• A thrifty solution to decimeter resolution LBS.
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