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The history of synchronisation in digital cellular networks


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Presented by Prof. Andy Sutton, Principal Network Architect within BT Architecture and Strategy team in the CW (Cambridge Wireless) Heritage SIG (#CWHeritage) event 'Time for Telecoms' on 16 March 2018 at the Science Museum, London.

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The history of synchronisation in digital cellular networks

  1. 1. British Telecommunications plc 2017 The history of synchronisation in digital cellular networks Andy Sutton 16th March 2018
  2. 2. 1 British Telecommunications plc 2017 • GSM radio interface • Nokia DF12 BTS • Base station oscillator modules • Pulse Code Modulation • E1 frame • GSM terrestrial interfaces • Synchronisation Supply Unit • Add some 3G with UMTS and WCDMA • Implications of ATM and SDH • Converged network sync solution • 4G with LTE, LTE-A, LTE-A-Pro • Sync for 4G and beyond… • Summary Contents
  3. 3. © British Telecommunications plc GSM radio (Um) interface (air interface) • TDMA TS = 577uS • TDMA frame = 4.615mS • TS numbers 0 through 7 • Burst duration = 546uS within a 577uS window • This gives a gross Um interface rate of 270.833kbps • Therefore, Each TS = 34.73kbps • Full rate TFC on the Um interface = 22.8kbps • Full rate speech occupies approx. 13.6kbps
  4. 4. © British Telecommunications plc Nokia DF12 GSM (DCS1800) BTS • GSM radio interface requires frequency synchronisation accuracy of 50 parts per billion (ppb) • Frequency of the reference clock • Fref= n x 13 MHz • For all GSM mobiles (any frequency band) • 13000 kHz/48 = 270.833kbps • 48 clock cycle = 1 time bit • 12 clock cycle = 1 quarter bit (time unity in mobiles)
  5. 5. © British Telecommunications plc MCLU • Early BTS equipment typically had standalone high-accuracy oscillators • These standalone oscillators required regular alignment to centre the 13 MHz clock • A highly accurate frequency counter was required for this alignment, typically with a rubidium reference • The actual adjustment was a manual process via a trimming tool • The realignment had to be completed every 6 months on every BTS site Image source: Nokia DF12 BTS training manual
  6. 6. © British Telecommunications plc Rethinking base station frequency sync… • Highly accurate oscillators in base stations were expensive • Regular manual retuning of BTS oscillators was expensive • An alternative solution was required to reduce costs while enhancing network performance • The solution was to use the incoming 2.048Mbps signal as a source of synchronisation • The new “PCM” based sync card could use a lower cost oscillator as it would be constantly disciplined • There is no requirement for manual tuning of oscillators Image source: Nokia DF12 BTS training manual
  7. 7. © British Telecommunications plc BTS synchronisation with 2.048Mbps line reference Image source: Nokia DF12 BTS training manual To guarantee 50 ppb at the air interface the line performance must meet 15ppb
  8. 8. But why is the incoming line synchronous?
  9. 9. © British Telecommunications plc The need for synchronisation in telecommunications • The need for some sort of synchronisation in telecommunications has existed almost as long as telecommunications itself • Synchronisation in the form dominant in the last 50 or so years arose from the introduction of Pulse Code Modulation (PCM) for transmission of voice telephony, and the use of digital switching techniques to establish voice circuits between subscribers as required Image source: BT Archives
  10. 10. © British Telecommunications plc Pulse Code Modulation (PCM) • Pulse Code Modulation (PCM) patented (1937) : turning speech into pulses • In 1937 an English engineer, Alec Reeves, working in Paris for the International Standard Electric Company, patented the Pulse Code Modulation (PCM) transmission system. PCM turns the human voice into electronically coded sequences of digital pulses which are then transmitted and turned back into speech at the far end • This was a visionary concept, underlying the digital systems of today. But Reeves's ideas were well in advance of his time. The techniques he described for coding and decoding signals could not be realised in practical form until suitable components, particularly transistors, became available Marconi Instruments publications from 1976
  11. 11. © British Telecommunications plc Empress telephone exchange • The GPO embraced the idea of digital transmission enthusiastically. From 1964 it installed more than 7,000 pulse code modulation (PCM) wideband systems on existing copper cables. Four years later it inaugurated the world's first all-digital PCM switching centre in London. • The Empress telephone exchange, near Earl's Court, was opened on September 11, 1968, with an inaugural call from the then Postmaster- General, John Stonehouse, to the Mayor of Hammersmith. • Empress was the first exchange in the world to switch PCM signals from one group of lines to another in digital form. Now Empress solved this problem - and demonstrated that an integrated PCM transmission and switching system was capable of working fully within the existing network of Strowger, Reed-Electronic and Crossbar systems. Image source: BT Archives
  12. 12. © British Telecommunications plc PCM and the 2.048Mbps frame (E1) 1 2 3 4 5 6 7 8 TS0 FAW/NFAW TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10 TS11 TS12 TS13 TS14 TS15 TS16 CAS/CCS TS17 TS18 TS19 TS20 TS21 TS22 TS23 TS24 TS25 TS26 TS27 TS28 TS29 TS30 TS31
  13. 13. © British Telecommunications plc E1 frame used for GSM Abis transmission 1 2 3 4 5 6 7 8 TS0 FAW/NFAW TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10 TS11 TS12 TS13 TS14 TS15 TS16 CAS/CCS TS17 TS18 TS19 TS20 TS21 TS22 TS23 TS24 TS25 TS26 TS27 TS28 TS29 TS30 TS31 1 2 3 4 5 6 7 8 TS0 FAW/NFAW TS1 TRX-1 TRX-1 TRX-1 TRX-1 TS2 TRX-1 TRX-1 TRX-1 TRX-1 TS3 TRX-2 TRX-2 TRX-2 TRX-2 TS4 TRX-2 TRX-2 TRX-2 TRX-2 TS5 TRX-3 TRX-3 TRX-3 TRX-3 TS6 TRX-3 TRX-3 TRX-3 TRX-3 TS7 TRX-4 TRX-4 TRX-4 TRX-4 TS8 TRX-4 TRX-4 TRX-4 TRX-4 TS9 TRX-5 TRX-5 TRX-5 TRX-5 TS10 TRX-5 TRX-5 TRX-5 TRX-5 TS11 TRX-6 TRX-6 TRX-6 TRX-6 TS12 TRX-6 TRX-6 TRX-6 TRX-6 TS13 TRX-7 TRX-7 TRX-7 TRX-7 TS14 TRX-7 TRX-7 TRX-7 TRX-7 TS15 TRX-8 TRX-8 TRX-8 TRX-8 TS16 TRX-8 TRX-8 TRX-8 TRX-8 TS17 TRX-9 TRX-9 TRX-9 TRX-9 TS18 TRX-9 TRX-9 TRX-9 TRX-9 TS19 TRX-10 TRX-10 TRX-10 TRX-10 TS20 TRX-10 TRX-10 TRX-10 TRX-10 TS21 TRX SIG-10 TS22 TRX SIG-9 TS23 TRX SIG-8 TS24 TRX SIG-7 TS25 TRX SIG-6 TS26 TRX SIG-5 TS27 TRX SIG-4 TS28 TRX SIG-3 TS29 TRX SIG-2 TS30 TRX SIG-1 TS31 O&M
  14. 14. © British Telecommunications plc Nokia GSM transmission interfaces DF12 BTS with 64kbps TRX SIG Typical configuration from early to late 1990s Image source: Nokia DF12 BTS training manual
  15. 15. © British Telecommunications plc An E1 frame is synchronous… Nokia DN2 Tellabs DACCS
  16. 16. © British Telecommunications plc Synchronisation signal from PCM E1 frame Although an 8000 sample-per-second tick is the basis of the need for synchronisation, it wouldn’t be found as an interface or a signal for transmission Rather than providing separate transmission for synchronisation, the Primary Rate TDM bit- stream at 2048kbps was adopted (or for local transfer of synchronisation only, a bipolar signal of the same rate) Because of this, short term variations (jitter) will occur and these will accumulate along a chain of nodes in the hierarchy (multiplexing into higher order bit-streams will also add jitter). In longer chains, this introduces a need for a filtering stage, usually realised in the form of a high quality oscillator and a long time constant phase locked loop, to reduce these short term variations to an acceptable level. This filtering function is often needed where one of a small number of incoming transmission paths are to be used as the synchronisation source for a larger number of nodes The requirements to select the source, to filter, and to provide multiple feeds are often met by one piece of equipment, a Synchronisation Supply Unit (SSU) Re-using the TDM bit-stream in this way does, however, create a challenge. Unlike a signal intended only for synchronisation, it will not have a wholly repetitive and predictable pattern of edges from which to recover the reference frequency. Source: The Need for Synchronisation in Telecommunications, Martin Kingston, ITP Journal Vol 10 Part 1, 10 - 13
  17. 17. © British Telecommunications plc Synchronisation Supply Unit (SSU) • Symmetricom SSU supplied by Chronos Technologies, featuring: • Input ports (bottom) • input selection, oscillators and output drivers (middle) • multiple outputs (top)
  18. 18. Let’s add some 3G UMTS…
  19. 19. © British Telecommunications plc 3G UMTS with WCDMA and ATM • Direct Sequence Code Division Multiple Access • Operates in the 2.1 GHz band • FDD & TDD modes of operation • TDD hasn’t been used in UK • 5 MHz channel spacing with 200 kHz channel raster • 190MHz duplex spacing (FDD mode) • CDMA is a spread spectrum technique • The rate of spreading is referred to as chip rate rather than bit rate – 3GPP WCDMA chip rate = 3.84Mcps • FDD synchronisation requirements are the same as GSM, 50ppb at the air interface
  20. 20. © British Telecommunications plc Converging 2G and 3G backhaul requirements
  21. 21. © British Telecommunications plc Converging 2G and 3G backhaul requirements ?
  22. 22. © British Telecommunications plc ATM aggregation Lucent PSAX 2300 (illustrated) deployed at BSC sites which became TNs, PSAX 4500 deployed on core sites immediately prior to RNC BALUN panels STM-1 MSP cards in slots 1 and 2 Fibre optic cables to be connected to STM-1 MSP cards Stratum cards CPU2 cards 2 x 21 E1 MS cards in slots 9 and 10 2 x 21 E1 IMA cards in slots 13 and 14 This breaks the simple E1 sync path!
  23. 23. © British Telecommunications plc Sync network evolution…
  24. 24. © British Telecommunications plc Sync network evolution…
  25. 25. © British Telecommunications plc Distributed TN based sync
  26. 26. 4G, LTE, LTE-A, LTE-A-Pro and the road to 5G
  27. 27. © British Telecommunications plc 4G LTE, all IP with Carrier Ethernet • True IP based mobile broadband technology • Flexible OFDM based radio interface • Channel bandwidths from 1.4 MHz to 20 MHz • All transmission interfaces are Carrier Ethernet, no TDM! • Base stations evolve to support multiple RATs • Spectrum technology neutrality and new frequency bands • Support for TDD, CoMP, co-channel small cells, broadcast and multicast etc.
  28. 28. © British Telecommunications plc Mobile network sync for 4G and beyond… • ITU-T Synchronous Ethernet (SyncE) – Physical layer sync reference for frequency synchronisation only • IEEE 1588-2008 Precision Time Protocol (PTP), also known as 1588 v2 – Layer 2.5 message based sync reference for frequency synchronisation and/or time of day (ToD) +/-1.5us (+/-1.1us) – ToD generally referred to as phase sync – Traceable to a source of UTC via a new node known as PRTC (Primary Reference Time Clock)
  29. 29. 28 British Telecommunications plc 2017 Summary • Frequency synchronisation is as essential to the operation of state of the art 4G and 5G networks as it was to 2G and 3G • Advanced radio features and modes of operation require the addition of phase synchronisation to ensure time alignment between adjacent cell sites • Synchronisation networks must be robust and resilient as they underpin network operation and performance • SyncE and PTP are important network synchronisation technologies to enable our mobile future • 5G will drive extensive use of PTP based phase sync and likely many more GNSS deployments in support of TDD operation • As radio access frequency bands increase to deliver ever higher peak and average data rates with lower latency and better performance, phase sync becomes ever more critical as an integral component of the E2E solution
  30. 30. 29 British Telecommunications plc 2017 Additional reading - The ITP Journal, Sync edition
  31. 31. 30 British Telecommunications plc 2017 ITP Journal papers
  32. 32. 31 British Telecommunications plc 2017 ITP Journal papers
  33. 33. Thank You! Any questions? 3