OpenBTS( - Meetupfiles.meetup.com/18094742/Harvind_Samra_OpenBTS.pdfOpenBTS Architecture GSM HandsetGSM HandsetGSM HandsetGSM Handsets Digital Radio GSM Stack (L1, L2) Hybrid GSM/SIP

OpenBTS  

A  so,ware-­‐defined  mobile  network    

What  is  OpenBTS?  

“The  OpenBTS  so,ware  is  a  Linux  applicaAon  that  uses  a  so,ware-­‐defined  radio  to  present  a  standard  3GPP  air  interface  to  user  devices,  while  simultaneously  presenAng  those  devices  as  SIP  endpoints  to  the  Internet.  This  forms  the  basis  of  a  new  type  of  wireless  network  which  promises  to  expand  coverage  to  unserved  and  underserved  markets  while  unleashing  a  plaLorm  for  telecom  innovaAon.”  

What  is  OpenBTS?  –  in  English  

•  It’s  the  21st  century!  •  We  have  the  internet,  we  have  VoIP  and  SIP,  and…  

•  We  have  SDRs,  and  we  don’t  have  to  go  broke  to  get  one  

•  We  have  servers  and  open-­‐source  high-­‐performance  so,ware.  

•  Networks  can  be  soooo  much  simpler  to  build  

How  does  it  work?  

OpenBTS.org

OpenBTS turns this...

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How  does  it  work?  

How  does  it  work?  

OpenBTS.org

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How  does  it  work?  

OpenBTS.org

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How  did  it  start?    

How  did  it  start?    

•  Project  publicly  launched  in  2008  •  Voice  only,  USRP1  only  •  30,000  page  hits  first  few  days  •  By  2011:  – Custom  SDR  and  hardware  – Added  SMS  – Started  development  on  GPRS  and  UMTS  

The  first  incarnaAon  

A  closer  look…  

OpenBTS Architecture

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GSM Stack(L1, L2)

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GSM  Layer  1  structure  

An  even  closer  look…  

Using  an  SDR  

•  ModulaAon  is  GMSK  (MSK  w/  Gaussian  filter  pulse  shaping)  

•  Easily  approximated  as  a  BPSK  signal  with  frequency  shi,  and  pulse  shaping  

•  Makes  processing  much  simpler,  parAcularly  the  demodulator  

•  Our  approach:  Keep  dependency  on  radio  as  lifle  as  possible!!!    

•  Want  radio  to  do  as  lifle  as  possible.  

OpenBTS  and  the  SDR  •  Two  separate  process:  

OpenBTS  and  Transceiver  •  OpenBTS:  L1.5  and  up  •  Transceiver:  L1  and  

below  

•  MulA-­‐threaded  approach  to  perform  easily  parallelizable  operaAons  

•  Receive  stream  is  the  BTS  clock.Sets  the  Aming  for  enAre  BTS.  

•  Bursts  for  transmission  need  to  get  to  radio  on  Ame,  especially  over  USB.  

OpenBTS-­‐UMTS  and  the  SDR  •  RadioInterface  

abstracts  radio(s)  from  Transceiver  

•  Keep  radio  simple:    

•  Interpolate,  Decimate  

•  Tune  TX,  RX  frequencies  

•  Tune  TX  and  RX  gains  

•  Calibrate  VCTCXO  •  Minimize  amount  

of  on-­‐radio  DSP  

GSM  signal  processing  

•  Per-­‐ARFCN  demodulaAon  is  fairly  straighLorward  

•  Higher  layers  indicated  Ameslot  has  access  burst  or  normal  burst  

•  Correlate  midamble  sequences  to  obtain  channel  esAmate  

•  Apply  phase/amplitude  compensaAon  or  decision-­‐feedback  equalizaAon  if  warranted  

•  Pass  so,-­‐outputs  up  to  Viterbi  decoder    

Moving  to  3G  

•  GSM  w/  a  vastly  different  physical  layer  and  access  scheme.  (I’m  oversimplifying  a  bit…)  

•  No  more  TDMA  and  GMSK,  its  WCDMA  •  SAll  have  frames  that  of  10,  20,  40,  80  ms  •  Each  frame  contains  15  slots.  •  QPSK  modulaAon.    Data  rate  is  3.84MHz.  •  Root  raised  cosine  filter  w/  0.22  rolloff  factor  

What  is  spreading/scrambling?  

•  MulAply  each  bit  by  a  pseudo-­‐noise  sequence  of  N  bits.  

•  Spreads  signal  power  over  a  bandwidth  of  that  is  N  Ames  larger.  Signal  looks  like  noise.    

•  MulAple  signals  can  share  same  bandwidth.    Use  different  sequences  (or  codes)for  spreading.  

•  N  is  called  the  spreading  factor.  •  Scrambling  is  simply  XOR  of  post-­‐spread  stream  with  another  pseudo-­‐noise  sequence.  

3G  Physical  Layer  

•  ModulaAon  is  simple:  – Convert  slot’s  worth  of  bits  to  QPSK  – Spread  bits  by  factor  of  4-­‐512  to  match  3.84MHz  – Scramble  bits  and  transmit  

•  Downlink:    Users  disAnguished  by  spreading  codes,  all  share  same  scrambling  code  

•  Uplink:  Users  disAnguished  by  scrambling  codes,  all  share  same  spreading  code  

3G  Physical  Layer  

3G  Physical  Layer  

•  Handsets  close  to  NodeB  can  drown  out  handsets  that  are  very  far  away.  

•  Called  the  near/far  problem.    Classic  issue  w/  CDMA    

•  Power  control  the  handsets.    Measure  power  and  signal  quality  (w/  SDR)  of  each  handset.      

•  Turn  down  power  if  handset  is  above  target,  turn  up  power  if  handset  is  below  target.  

•  Consider  other  factors:  data  rate,  QoS,  etc.  

What  is  OpenBTS  now?  •  St.  Pierre  &  Miquelon  

–  Commercially  launched  Nov  2014  

–  2nd  Operator  in  the  town  of  St.  Pierre,  pop.  6,000  

–  6-­‐node  network  using  50W  OpenCell  base  staAons  at  900MHz  serving  an  urban  area  of  25  km2  

–  Voice,  Text,  Fill-­‐in  Data  (where  WiFi  not  available)  

–  Deployed  by  a  Cisco  CerAfied  Network  Administrator  (CCNA)  

What  is  OpenBTS  now?  

6/5/15   Range  Networks   24  

What  is  OpenBTS  now?  

St.$Pierre$IP$Backhaul$Network$

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PSTN$(Rest$of$World)$$

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What  is  OpenBTS  now?  

• Research  StaAon  in  AntarcAca  • Connects  3  labs  •  Satellite  (IP)  link  to  connect  with  the  rest  of  the  world  

What  is  OpenBTS  now?  • Base  staAon  in  a  helicopter  • Rescue  hikers  using  their  cellphone  as  a  beacon  

 “We  can  now  complete  the  mission  in  the  amount  of  3me  it  used  to  take  to  launch  the  mission”  

 -­‐Baldvin  Hansson    Director  of  Technology,  Rogg  

What  does  the  future  hold?  

•  More  and  more  tracAon  amongst  operators  and  equipment  providers  

•  One  of  the  last  industries  that  use  closed,  proprietary  customized  hardware  soluAons  

•  Much  more  awareness  of  so,ware-­‐defined  radios  and  so,ware-­‐defined  networking.  

•  Time  for  commodiAzed  hardware!  •  SDRs  are  a  crucial  cornerstone  of  this  evoluAon  

Thank  you!  

•  Project  site:  openbts.org  –  Join  the  mailing  list  – Download  the  so,ware  – Download  design  files  of  first-­‐gen  SDR  – Contribute  back  changes  

•  Email  me:  [email protected]