Enhancing Communications using UAV based Distributed Antenna ArraysHan Yan, Samer Hanna, and Danijela Cabric, UCLA

Conclusions

• UAV based sparse array facilitates MIMO multiplexingwith conventional infrastructure in LOS channel.

• Higher carrier frequency benefits from increaseddegree-of-freedom gain without losing SNR

• Challenges and investigation direction of utilizingswarm array are reviewed.

Introduction

Degree of freedom (DOF) of a wireless MIMO channel depends on• Amount of multipaths (scattering and reflection)• Antenna array aperture (wavelength normalized)

Current MIMO system exclusively relies on multipaths• Indoor environment & dense urban• Line-of-sight (LOS) is “unfavorable” environment• Large aperture is rarely investigated due to space & hardware concerns

Many applications operate in LOS environment• Data backhauling to infrastructures• Air-to-air & air-to-ground communication• Millimeter-wave (mmW) band where non-LOS paths are negligible due to

severe penetration and reflection loss.

Fig (right). Sum SE and SE per eigenmode withvarious number of conventional Tx infrastructure;Top and bottom figures have same color coding.

Challenges & Investigation Directions

References[1] H. Yan, and D. Cabric, “Compressive Initial Synchronization and CellDiscovery for Millimeter-Wave Networks,” to be submitted to IEEE Tran. SignalProcess., Oct. 2018.[2] H. Yan, Samer Hanna, Kevin Balke, R. Gupta, and D. Cabric, “SoftwareDefined Radio Implementation of Carrier and Timing Synchronization forDistributed Array (invited paper),” to be submitted to IEEE AerospaceConference (AeroConf), Oct. 2018.[3] H. Yan, S. Chaudhari and D. Cabric, “Wideband Channel Tracking formmWave MIMO System with Hybrid Beamforming Architecture,” (invitedpaper) in Proc. IEEE International Workshop on Computational Advances inMulti-Sensor Adaptive Processing (CAMSAP), Dec. 2017 .

Distributed processing & data sharing• Centralized array processing, e.g., estimate

channel and forwards to a fusion center, v.s.distributed array processing. e.g., blindeigenspace estimation

Fig. LOS channel tracking for distributedbeamforming applications. Adaptive weightadjustment stabilize beamforming gain whenunavoidable phase drift occur.

Synchronization & self-localization ofdistributed mobile radio [1,2]• Fully wireless intra-group freq. & timing

sync. protocol• Intra-group directional info. facilitates

mmW/THz array for data sharing• Direction info. helps self-localization,

critical step in array geometry optimization

Array geometry optimization• Optimize UAV location for low rank channel

in beamforming and high rank channel inMIMO multiplexing

Error-robust process & channel tracking• Unavoidable location vibration of clock

phase drifting need to be considered.• Our previous work focuses on sparse (LOS)

channel tracking [3]

Reconfigurable Distributed Array Enabling LOS MIMO Multiplexing

Simulation settingsEach color-face representsunique eigenmode

Dashed curves show top andbottom 1% tier of SE (arraygeometry dependent)

Improving degree of freedom in LOS MIMO Channel

How UVA based array help?

Large aperture; few reconfigurable elements

32-Element CriticallySpaced Array

1024-Element Critically Spaced Array

(32x Larger Aperture)

32-Element UniformlySpaced Sparse Array

(32x Larger Aperture)

32-Element Arbitrarily Spaced Sparse Array (32x Larger Aperture)

Large aperture;large elements

Single Wide Main-lobe Narrow (Grating) Lobes

Single Narrow Grating-lobe(w/ controllable spatial emission)

small aperture; few elements

Single Narrow Main-lobe

Large aperture; few elements

Long Distance (>1000 meter or >10000𝜆)

Array Processing

(Decoding 1 Data Stream)

Information Precoding

stream 1

Short Distance (~meter or ~10𝜆)

Array Processing

(Decoding 3 Data Stream)

Array aperture (~10𝜆)

Conventional antenna array (𝝀/𝟐 spaced)• Facilitates beamforming and power gain• Application: beamforming enabled power gain• Fixed access for data backhauling• Antenna array for satellite communication

Microwave Single Element v.s mmW Sub-Array

System Overview

Coherent combining at Rx extends coverage

Coherent combining reduces transmit power

Devices out of coverage zone

Distributed beamforming

UAV array facilitates MIMO multiplexing access and backhauling even in LOS scenarios

Adjustable virtual sub-array architecture

Rank 2 channel Rank 3 channel

Distributed MIMO

Degree of freedom improvement w/

conventional infrastructure

UAV array facilitates range extension for sensor data collection

UAV based swarm array system• Each drone has single microwave

radio or mmW sub-array• Mobility of UAV facilitates array

geometry reconfiguration

Noise concerns: LOS propagation loss• Friis equation – wavelength 𝜆 & antenna gain

DOF concerns: LOS channel rank• Angular resolution of sparse array

Δ𝜙~1/(𝐴/𝜆) where 𝐴 is the aperture size• Capability of resolving 𝑑 spaced antenna:

𝑑 ≈ 𝑅tan Δ𝜙 ~𝜆𝑅/𝐴 where 𝑅 is the linkdistance

For the same channel rank improvement• Short 𝜆 relieves required drone spacing 𝐴• Short 𝜆 supports higher comm. distance 𝑅

Other considerations• High throughput intra-group data sharing• Self-localization via intra-group angle info.

Sub-array gain alleviates or evenreverses propagation loss in mmW.

Rank 2 ChannelRank 1 Channel

Tx Antenna Placement (m) Tx Antenna Placement (m)

Tx Antenna Placement (m)Tx Antenna Placement (m)

Tx Antenna Placement (m)

Fig. Example of the designed grating lobe facilitatesMIMO multiplexing in LOS channel; 32 UAV based Rx and4 Tx antenna at {-1.5,-0.5,0.5,1.5}.

Main results• Distributed UAV swarm array has great improvement in MIMO channel rank• 28GHz suffers from severe pathloss, but UAV w/ 28GHz sub-array facilitates unprecedented

spectral efficiency (SE) in LOS MIMO• Up to 41% additional SE via position optimization

Tx (conventional infrastructure)• Tx array w/ variable antenna size• Constantly spaced Tx array (∀𝑓c)• Tx power 𝑃t=23dBm ∀𝑓𝑐

Rx (UAV swarm)• 10MHz noise BW• 4dB NF• Ideal Rx MIMO processing• 32 drones w/ RMS radius 𝐴=50m

channel• 𝑅=1000m distance• LOS pathloss model (Friis eq.)• Full CSI in TRx; Water-filling

power allocation