Naive beamforming for multi-element antenna GNSS receiver

1. Naive beamforming for multi-element antenna
GNSS receiver
Ilya V. Korogodin
NRU “Moscow Power Engineering Institute”
PIERS, Toyama, Japan, 2018

2. Regular GNSS antenna
• Several SVs
• SVs move
• User moves
⇒
Omnidirectional
antennas for
regular receivers

3. Controlled Reception Pattern Antenna (CRPA)
Directional properties are
useful:
• Increase singal-to-noise
• Supress multipath
• Reject interferences
• Detect spoofing
CRPA is a great solution

4. Telecommunication also needs BF
• Smaller wavelength means smaller captured energy at antenna
3GHz → 30GHz gives 20dB extra path loss due to aperture
• Larger bandwidth means higher noise power and lower SNR
50MHz → 500MHz bandwidth gives 10dB extra noise power
*Illustrations of Robert W. Heath Jr.

5. Conventional BF scheme
Antenna
model
Attitude
Calibration
coe¡cients
Desired
direction
...
...
Regular beamforming for n-th SV
Correlator
Processing for s-th GNSS signal
... Locking
Loops
• BF adds weighted antenna elements signals
• The weights are predicted by an antenna model
• Transparent for a consequence baseband processing

6. Antenna model for weights calculation
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Antenna
model
Attitude
Calibration
coe¡cients
Desired
direction
,,1iny
• An antenna model predicts phase differences for the certain
direction
• It allows to allign signals
• We can add the signals together in-coherent and increase SNR

7. Practical Issues
Huston, we have three problems:
• We need elements RP
calibration
• We need antenna’s attitude
• We need a terrible
bi-directional interface
between CRPA and receiver
~50 signals!!!

8. Naive BF idea
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If it’s so annoying to predict the phase differences,
let’s just measure them!

9. Naive BF scheme
...
...
Multi-input correlator
DPLL
...
...
Naive beamforming
...
Locking
Loops
Processing for s-th GNSS signal
• Correlators for each antenna element
• Baseband is extended by difference phase locking loop (DPLL)
• DPLL isn’t influenced by TCXO → Tight BW → Low noise,
improved sustainability

10. Antenna array
LNA
LNA
LNA
LNA
LNA
LNA
LNA
Gain 30 dB
Kn 1.4 dB
RG58A/U
~ 25 meters
~ 12 dB loss
L1 ~20cm wavelength
7-element convex antenna
30 degrees inclination
Analog interface

11. Frontend
Multi-input frontend
ADC
ADC
ADC
...
FSTCXO
Clk
Heterodyne
GNSS receiver (Xilinx ZC702 Board)
s-th channel
Multi-input
correlator
Acquisition engine
DPLL
BF LL
Rinex
• Custom seven-input frontend
• Common geterodyne and TCXO
• fif ∼ 10MHz, fs = 125MHz

12. Baseband
Multi-input frontend
ADC
ADC
ADC
...
FS
Clk
Heterodyne
GNSS receiver (Xilinx ZC702 Board)
s-th channel
Multi-input
correlator
Acquisition engine
DPLL
BF LL
Rinex
• Xilinx SoC Zynq
(ARM+FPGA) (Z702 board)
• CoreZh GNSS receiver
project:
• Correlators, acquisition,
desers, ... (PL: Verilog)
• Loops, syncs, navigation,
interfaces, ... (PS: C++)

13. Multi-input correlator
...
...
Multi-input correlator channel
Code Gen Carrier Gen
...
Regular correlator channel
Code Gen Carrier Gen
Regular correlator channel
Code Gen Carrier Gen
...
• Correlators is quite heavy, they consume a lot of ASIC
area/FPGA cells
• Code/carrier generators, timescales are duplicated, it’s
overspending
• Special multi-input correlator 7 regular correlators:
it consumes ∼ 150% of the regular one

14. Full mockup scheme
Multi-input frontend
ADC
ADC
ADC
...
FSTCXO
Clk
Heterodyne
GNSS receiver (Xilinx ZC702 Board)
s-th channel
Multi-input
correlator
Acquisition engine
DPLL
BF LL
Rinex
+ a laptop for control and data logging
+ matlab scripts for log processing

15. It works!
...
...
DPLL
• Phase differences RMS is
about 0.1 degree
• Beam direction err 0.1
degree

16. Performance gain
0 50 100 150
Time, min
30
35
40
45
50
55
60
SNR,dBHz
Focused
One channel
BF ON
Signal LOSS
• About 7-8 dB of SNR gain
• The SNR gain increases
range/phase estimation
accuracy (about x2)
• Both for GPS and GLONASS
46 48 50 52 54
SNR, dBHz
0
0.5
1
1.5
2
2.5
Pseudorangenoise,m
GPS L1C/A #15
Cramer-Rao Bound
Before BF
After BF
46 48 50 52 54 56 58
SNR, dBHz
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Pseudophasenoise,mm
GLONASS L1OF #3
Before BF
Cramer-Rao Bound
After BF

17. Conclusion
The naive beamforming approach and experimental results are
presented
Pros:
• No antenna RPs calibration
• It doesnt’t requere attitude estimations
• Simple interface btwn the antenna and receiver
Cons:
• About 50% correlator ASIC/FPGA logic overspending
• It can be less sustainable to a multipath propagation
Combine conventional and naive BFs to reduce − and unite +

18. Contacts
Thank you for your attention!
Dr. Ilya V. Korogodin
Moscow Power Engineering Institute
e-mail: korogodiniv@gmail.com
site: srns.ru