The history of synchronisation in digital cellular networks

British Telecommunications plc
2017
The history of synchronisation
in digital cellular networks
Andy Sutton
16th March 2018

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

© 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

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)

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

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

BTS synchronisation with 2.048Mbps line reference
To guarantee 50 ppb at the air interface the line performance must meet 15ppb

But why is the incoming line synchronous?

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

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

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

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

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
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

Nokia GSM
transmission
interfaces
DF12 BTS with
64kbps TRX SIG
Typical
configuration
from early to
late 1990s

An E1 frame is synchronous…
Nokia DN2
Tellabs DACCS

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

Synchronisation Supply Unit (SSU)
• Symmetricom SSU supplied
by Chronos Technologies,
featuring:
• Input ports (bottom)
• input selection, oscillators
and output drivers (middle)
• multiple outputs (top)

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

Converging 2G and 3G backhaul requirements

Converging 2G and 3G backhaul requirements
?

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!

Sync network evolution…

Distributed TN based sync

4G, LTE, LTE-A, LTE-A-Pro
and the road to 5G

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.

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)

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

Additional reading - The ITP Journal, Sync edition
https://www.theitp.org/

ITP Journal papers

The history of synchronisation in digital cellular networks

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