Download Enhancing small cell performance with synchronised backhaul

Enhancing small cell performance
with synchronised backhaul
The increasing importance of network synchronisation
Timing synchronisation is fundamental to wireless network performance. Every individual radio transmitter throughout
the network, effectively every cell, needs to be tightly aligned with its neighbours. This doesn’t just benefit seamless
handover as we move between cells, but reduces inter-cell interference, significantly increasing the total network
throughput and capacity.
Three major trends are increasing its importance today:
Densification of the network, adding many more shorter range and lower power small cells.
LTE-Advanced (sometimes called 4.5G) added features such as eICIC, ABS and CoMP which require phase synchronisation in addition to frequency sync used for earlier LTE releases.
TD-LTE, already popular in China and India, is becoming available for use elsewhere and is particularly intolerant
of jitter and delay variation.
Different sources of synchronisation
Network architects usually specify at least two alternative synchronisation sources to ensure network resilience and high
availability. These include:
Timing source
Advantages
Drawbacks
GPS/GNSS
GPS inherently provides accurate phase timing
of 30 nanoseconds or better. The introduction of
other satellite constellations (GLONASS, BeiDou
and Galileo) will increase the usefulness and
resilience of this system.
Weak signals can be much more difficult to
discern at street level in urban canyons with
poor views of the sky. Potentially prone to
jamming and multi-path.
Network Listen
Slaving from nearby macrocells.
Could cause entire area outage if macrocell
goes offline. Isn’t suitable for small cells in
coverage holes.
Backhaul
Embedding synchronisation through wireline
and/or wireless backhaul using a combination of
Synchronous Ethernet and IEEE 1588 v2.
Requires an end-to-end implementation.
Susceptible to failure in any link throughout the
chain. The small cell backhaul network can be
independent of macro network with unreliable
CoS and high PDV.
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Timing and sync within the wireless backhaul mesh
Wireless backhaul also requires accurate timing and sync to operate efficiently.
Each backhaul node can be affected by the same issues for resilience between timing
sources as do small cells. No single source is good enough on its own and the system
must cater for a wide range of potential outages and use cases.
CCS Metnet self-organising wireless backhaul inherently incorporates accurate timing and synchronisation as a fundamental
requirement for its own operation. Each node automatically switches to the most suitable synchronisation source available.
The same three choices are available as for small cells: GPS, network listen slaved from another backhaul node or from an
external wireline source.
Metnet passes this synchronisation timing signal over dedicated radio registration channels rather than bearer traffic
channels, avoiding delay variation. Mesh topology, where most Metnet nodes have more than one diverse route back to
the network, further adds to the high resilience of the backhaul. Switching between alternative routes is instantaneous and
should not result in synchronisation loss.
The Metnet system can operate with in a range of scenarios with a variety of sync options. Inputs can include local
nodes locked to GPS, local nodes locked to neighbouring GPS via the proprietary distributed GPS feature, or external
synchronisation from the core networks i.e. frequency (SyncE) and phase and time (PTP 1588v2).
Outputs to the small cells include:
SyncE (G.8261/G.8262/G.8264) either generated
directly from the Metnet GPS or from a core
network device carried across the mesh.
1588v2 PTP T-GM PRTC/Master Clock
(G.8271/G.8272)generated directly from the
Metnet GPS.
Urban canyon deployment in China. No GPS available.
Sync from neigbouring nodes.
1588v2 PTP T-TC (G.8273) from the core
network boundary clock or grandmaster and
propagated across the Metnet mesh.
Synchronisation as a service
Capitalising on the highly resilient and multi-sourced timing and synchronisation capabilities built into Metnet, CCS has
developed proprietary mechanisms to transmit timing and sync for small cells. Standard interfaces are provided for each
small cell. This greatly simplifies the timing architecture by providing a robust and reliable timing source, connected through
the same Ethernet cable used for backhaul and power.
Each small cell receives a reliable SyncE and 1588v2 signal and does not have to deal with multiple sync sources or prioritise
between them. Metnet seamlessly determines the best source and provides it in the same format.
While any timing source is available anywhere in the Metnet (GPS received at any node or from external SyncE/PTP source),
then all small cells can remain synchronised.
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Metnet flexible synchronisation options
Macro
GPS
GPS
No GPS
GPS
ne
et
M
tm
w
ro
ic
e
av
SyncE Master / Slave
1588 T-GM/T-TC
k
lin
Metnet timing
recovered
from peers
Core network
Metnet node
locked to GPS
SyncE / 1588 slave
and T-TC pass through
Metnet node
locked to GPS
delivering SyncE
and T-TC to small cell
Metnet node
recovering timing
delivering SyncE
and T-GM to small cell
Metnet node
locked to GPS
delivering SyncE
and T-GM to small cell
Implications for small cell
deployment
Over the top synchronisation
also supported
Identifying and arranging use of suitable sites for urban
small cells is complex and difficult. Technical issues include
ensuring the site is close enough to traffic hotspots,
ensuring a power source, site security and access, and
suitable backhaul. Commercial issues involve not just the
site rental but planning/zoning permits, maintenance access
(e.g. without closing off the street) and other costs.
Some network architects may prefer to use their own
centralised master clock sources. The robust timing
architecture of the Metnet also supports seamless carriage
of SyncE and 1588v2 with low packet delay variation and
Removing the technical constraints to have a good view of
the sky for GPS reception increases the number of potential
small cell sites that could be used. Mesh backhaul further
relaxes selection criteria by catering for temporary line of
sight disruptions and longer term ad-hoc network evolution.
A typical scenario is that one Metnet node with a clear view
of the sky can be used as the primary timing source for
the entire mesh backhaul and small cell network, including
those located indoors or without adequate GPS reception.
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high resilience meeting the G.8262 and G.8273 standards.
Best of both worlds
CCS has designed a high performance, robust and resilient
timing and synchronisation solution as an inherent part
of the Metnet mesh design. This capability can be used
to provide a very accurate and low jitter timing signal to
adjacent small cells.
Alternatively, for those who prefer to retain separation of
backhaul and timing functions, these services can be passed
through transparently.
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