construction of a digital beamforming system as an fm...

N T SConstruction of digital beamforming system

as an FM receiver with USRP devices

Xuan Liu, Yuhui Li

Chair of

Communication Systems

Construction of a Digital Beamforming System as an FM Receiver with USRP Devices

Xuan Liu, Yuhui Li

p.1

Overview



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Motivation and Objectives

Theoretical Background

Measurement and Result Analysis

Conclusion and Further Works

p.2

Motivation



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Communication Systemsp.3

Objective

MIMO Cable Ethernet Cable

FM Music signal 2

FM Music signal1

Music 1Music 2



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NI USRP

NI USRP N2920 Block Diagram[r]



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[r] http://zone.ni.com/reference/en-XX/help/373380B-01/usrphelp/2920_block_diagram/

p.4


NI USRP N200



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NI USRP WBX Daughterboard



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• Noise figure: 5 dB

• Output power: up to 100 mW

• Frequency Range: 50 MHz-2.2 GHz

• Bandwidth: 40 MHz

• 2 quadrature frontends: (1 transmit, 1 receive)

• Full-duplex operation

• 180 degree phase ambiguity

p.6


Radiation Pattern



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General radiation pattern in a polar diagram

• Main lobe

• Side lobes

• Back lobe

• Nulls

p.7


Radiation Pattern



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Omnidirectional radiation pattern Patch antenna radiation pattern

p.8


Array Antenna



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Δx = 𝑑

2·cos𝜃 ∙

2𝜋

𝜆

𝐸 = const · 𝑓 𝜃, 𝜙 · [𝐼(-𝑑2)𝑒−j𝑘

d

2cos𝜃 + 𝐼(+

𝑑

2)𝑒j𝑘

𝑑

2cos𝜃]

With 𝑓(𝜃, 𝜙) is element pattern factor, 𝐼(±𝑑

2) is current phasor of element 1 and 2

p.9


Digital Beamforming



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Array propagation vector 𝛖: 𝛖 = 1 𝑒jк𝑑 sin 𝛼 ⋯⋯𝑒jк(𝑁−1)𝑑 sin 𝛼

Complex weight of 𝑛𝑡ℎantenna: 𝑤𝑛 = 𝑎𝑛𝑒j𝑛𝜑

The radiation pattern 𝑆 𝛼 : 𝑆 𝛼 = 𝑛=1𝑁 𝑤𝑛 · υ = 𝑛=0

𝑁−1𝑎𝑛 · 𝑒j(к𝑛𝑑 sin 𝛼) · 𝑒j𝑛𝜑

𝑆 𝛼 reaches its maximum response N·𝑎𝑛 when 𝜑 = −к𝑑sin𝛼

p.10


Beamforming Receivers Model



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Problem: WBX daughterboard may have 180° phase ambiguity !

p.11


Beamforming Receivers Model



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Solution: Calibration• Take advantage of the switch leakage inside of USRP

• Distribute a single calibration signal to Rx2 port of each USRPs with matched length cables

• Assign different frequencies to the DUT and calibration signal

• Applying two bandpass filters in LabVIEW to separate DUT and calibration signal

p.12


Which Antenna



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VS

• Omni-directional antenna

• Flexible operating frequency

• Moderate radiation pattern

• Fixed operating frequency

VERT2450 vertical antenna Patch antenna

p.13


FM Transmitters Model



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FM Modulation Frequency Upconversion

p.14


Capture EffectQuestion: What is the sufficient power level difference for the capture effect to work properly?

Conclusion: 2 dB power level difference is sufficient for receiving one music signal without significant noise and interference



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Tuning the power level

p.15


Software for beam pattern measurement

It verifies whether the DUT signal

can be received and calibrated correctly

Saving the acquired raw DUT and

calibration data separately

Perform the calibration to DUT

data and export as spreadsheet file



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Beam pattern measurement setup

Inside of an RF anechoic chamber (-90°) Inside of a lab room (0°)



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1st Fresnel Zone


Beam pattern inside RF chamber

Conclusion: The beam can be successfully steered from -10° to 10°



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10° 0° -10°


Beam pattern inside lab room

Conclusion: The beamforming performance is dramatically degraded, poor beam pattern and formation of large sidelobes



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10° 0° -10°


Multipath Propagation

Constructive and destructive interferences

Phase shift



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60 70 80 90 100 110 120 130 140 150 160 1700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Distance between transmit and receive antenna in cmN

orm

alized a

mplitu

de o

f th

e r

eceiv

ed s

ignal

In lab room

In freespace

p.20

Conclusion and Further works

Conclusion

Futher works

Expand the Array size to achieve beam pattern with high Gain and narrow beamwidth

Adaptive beamforming algorithm can be employed instead of simple conventional beamforming algorithm



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USRP#1

USRP#2

FM Tx#1

FM Tx#2

Time/Phase

Synchronization

Construction of two

FM music signals

Beamforming in RF

chamber

Phase alignment

LabVIEW software for:

· Beam pattern measurement

· Toggling the demodulation

between two FM music signals

which transmitted at same

frequency

p.21

Thank you for your attention!

Any Questions?



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construction of a digital beamforming system as an fm...

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