Millimeter-­Wave  UAV  Communications  and  Channel  Modeling  

Monisha Ghosh  (monisha@uchicago.edu),  Ismail  Guvenc (iguvenc@ncsu.edu),  NadisankaRupasinghe (rprupasi@ncsu.edu) and  Ahmed  S.  Ibrahim  (aibrahim@fiu.edu)

Motivation

• Unmanned   aerial   vehicles   (UAVs)  can   be   used   as  aerial   base   stations   (BSs)  to  deliver  broadband   wireless   connectivity   during,   for  example,   temporary   events   or  after   disasters.

• mmWave communications   allows   high   throughput   delivery   in  hot   spot   scenarios.• mmWave communication,   networking,   and   channel   propagation   characteristics   have   not  been   studied   in   the  context   of   UAVs,  which   constitute   the  main   scope   of  this   project.  

Preliminary   Results  on  Sum  Capacity  (GLOBECOM   2016)

Research  Goals

• Mobility-­based   3D   Beamforming   and   Equalization   Techniques   for  mmWave UAV  Networks  (2017-­2019)

• Multi-­‐Access   and   Multi-­‐Hop   mmWave UAV  Communication   Methods   with   UAV  Mobility  (2016-­2018)

• mmWave Communications   and   Channel   Sounding   Testbed   (channel   sounding   experiments  and   models   in  2017,   multi-­hop   experiments   and   results   in  2018-­2019)

• Capacity   results   in   Globecom 2016

Applications  

mmWave Beamforming   with   Leakage  Minimization

Reconnecting   Disconnected  Backhaul  Network

Equalization  

• Investigate   trade-­off   between   (i)   single-­carrier  mmWave with   Decision   Feedback   Equalization  (DFE)   and   (ii)   single-­carrier   mmWave with   Linear  Equalizers   for   the  mmWave UAV   scenario.

• Past  results   with   simulated   and   real   mmWaveoutdoor   channels   indicate   significant   performance  gains   from  the   use   of  DFEs.  We  will   investigate  gains   in   the  mmWave UAV   channel.

Multiple  Access  for  mmWave

• Investigate   time-­domain,   frequency   domain  and   code-­domain   multiple   access   schemes.

• Beamforming will   reduce   the   frequency  diversity   due   to  multipath,   hence   OFDMA  may  not   be  the   desired   multiple   access   option.

• With  fewer   users   in   a  beam,   and   reduced  frequency   selectivity,   CDMA   becomes   an  attractive   option.

Low  Cost  Channel   Sounding Approaches   using  802.11ad  chipsets

• Since   the  mmW receiver   computes   the   channel   estimates   prior   to  equalization,   one   could  potentially   use   the   values   computed   by  the   chip   itself.  The   problem   however   is   that  most  manufacturers   do   not   make   these   available   as  an   output   in  production   chips:   they   are  available   for  internal   use  only   during   debugging.   (Requires   cooperation   of  chip-­set  manufacturers).

• There   is   an  optional   mode   specified   in   the  standard   whereby   the   receiver   transmits   the  channel   it  just   estimated   to  the  transmitter.   However,   since   this   is  an   optional   mode,   it  is  not  implemented,   yet.  If  this  were   to  be   implemented   in   future   versions,   the  channel   estimates  could   be   retrieved   without   the   need   to  access   he   chip   internals.   (Requires   cooperation   of  chip-­set   manufacturers).

• The   chip-­set   can   be  used   as  a   transmitter   only   and   the   preamble   and   channel   estimation  can   be   implemented   on   a  FPGA.  This  would   require   an   implementation   effort  in  both   the   RF  and   digital   processing,   but  there   is  additional   flexibility   by  doing   so   instead   of  relying   on   the  algorithms   in   the  chip.   For   example,   the   accuracy   of  the  estimate   can   be   improved   by  averaging   over   longer   periods.   (Fairly   intensive   FPGA  development   effort).

Other  Channel   Sounding   Possibilities

• Upconverting lightweight  ultrawideband kits   to  mmWave bands

• Upconverting lightweight   USRPs   to  mmWave bands

• Other   solutions:   Pasternack,   SpaceKLabs,   Virginia   Diodes,   other?

UAV Leakage minimization

UE

InternetGateway

Disconnected

small  

cell  

UAV   (maintains  backhaul  connectivity)

UE

Sum Capacity Over UAV Network

UAV MobilityBased Approach

Fixed UAV

heights

Random UAV

heights

Orthogonal   spreading  codes   within  beams  and   across  clusters

Orthogonal   codes  in  different   beams  minimize  interference  at  edges,   or  can  be  combined

UWB   P410   (Time  Domain   Corp.)

USRPs  (National   Inst.)

Vubiq(Pasternack)

• We  are   investigating  reconnecting  a  disconnected  backhaul  network  using  a  limited  number   of  UAVs  acting  as  flying  relays  among  the  small  cells.  

• We  aim  to  utilize  the  flexibility  of  3-­D  movement   of  the  UAVs   and  guide  them   to  optimal  locations,  which  can  maximize  the  connectivity  of  the  overall  network  (consisting  of  fixed  small  cells  and  flying  UAVs.)

• We consider a mobility based mmWavecommunication network that can densely reuse 3D spatial resources via narrow mmWave beams.

• In one approach, we assume user locations are known; then we only need channel state information from desired user, and we can still minimize interference leakage to other users (no global CSIT is needed).

• With UAV location optimization, this is shown to perform better than random UAV locations with zero-forcing beamforming (which requires global CSIT).