Synchronization over the air - marubun.co.jp
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Synchronization over the air Albert Yuen
Transcript of Synchronization over the air - marubun.co.jp
Synchronization over the air
Albert Yuen
Agenda• TDD Synchronization Requirement
• Overlapping Coverage Areas
• Synchronization Over The Air
• Measurement Over The Air
• Conclusions
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TDD Synchronization and Overlapping Coverage Areas
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BS 1 BS 1UE 1 UE 1 BS 1
Why do we need synchronization?• TDD networks alternate between upstream and downstream transmissions:
• If synchronization is poor between cells, a neighbouring cell transmission can interfere with UE transmissions:
BS 1 BS 1UE 1 UE 1
Guard period
BS 1
BS 2 BS 2UE 2 UE 2 BS 2BS 2 BS 2UE 2 UE 2
X X
X X
Synchronization error 4
Synchronization Requirement• “The maximum absolute deviation in frame start timing between any pair of cells on the
same frequency that have overlapping coverage areas shall be 3s” *
• This is a phase requirement (i.e. it is relative to the other cell), not a time requirement
• It is normally implemented as a time requirement to a central clock
Phase alignment 3µs
Time alignment 1.5µs Time alignment 1.5µs
Central clock
eNodeB eNodeB
5* 3GPP TS 36.133, section 7.4
Overlapping Coverage
eNodeB eNodeB
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eNodeB eNodeB
Interference due to
poor synchronization
Interference Area
Good synchronizationInterference due to
poor synchronization
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eNodeB
What about small cells?
Small cell
Small cell
Small cell
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• Small cells are often entirely within a macrocell coverage area
• Synchronization errors may cause a significant interference problem
Synchronization Over The Air
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Synchronization over the air• What if you could synchronize one cell from another?
• “Network Listening” cell synchronizes itself to the radio frames coming from a nearby cell that is already synchronized*
• Also known as “radio interface based synchronization” or RIBS
eNodeB
Time Sync(e.g. PTP, GNSS)
Primary, Secondary Synchronization
signals
radio frame
eNodeB
Cell in “network listening” mode
10* 3GPP TR 36.922, section 6.4.2.1,
3GPP TR 36.872, section 6.3
eNodeB
Small cell architectures
Small cell
Small cell
Small cell
• Small cells might obtain synchronization from overlapping macrocell
Time Sync(e.g. PTP, GNSS)
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eNodeB
Daisy-chaining
Small cell
Small cell
Small cell
• Some cells may be outside the macrocell coverage area
• Daisy chain from neighbouring small cell
Time Sync(e.g. PTP, GNSS)
Small cell
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Self-backhaul
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eNodeB
UE
IAB cell
IAB cell
Time Sync(e.g. PTP, GNSS)
IAB cellUE
UE
UE
UE
UE
UEUE
IAB link
IAB link
Daisy-chainedIAB link
• “Integrated Access and Backhaul” (IAB)* – using the cellular link to backhaul traffic to a wide-area basestation
• IAB small cell acts as a UE to the wide-area basestation
* 3GPP TR 38.874
Delay Compensation
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UE slotUE slot BS slotAt BS BS slot BS slot
UE slotUE slot BS slotAt BS BS slot BS slot
• Timing advance alters the UE transmission timing to ensure frames line up at the BS
• Timing advance is a measure of round trip delay, and can be used to compensate BS sync
Timing Advance
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At UE BS frame BS frame
UE frame
UE tx time UE slotUE frame
UE frame
Guard bandBS frame BS frame
UE slotUE slot BS slotAt BS BS slot BS slotUE frame
Prop. delay Prop. delay
BS frame BS frame
UE frame
Prop. delay
late
Timing advance information
Guard band
Prop. delay
Timing advance
UE slotUE frame
UE frame
Active delay measurement• Another method uses active delay measurement using two-way signals:
eNodeB
Time Sync(e.g. PTP, GNSS)
eNodeB
Estimate propagation delay from round trip time
Request to start synchronization procedure
Accept
Signalling T2, T3
T1
T2
T4
T3
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Location-based methods• Timing advance only works if the receiving cell also functions as a UE
• Adds complexity to small cells not using IAB
• Active measurement also requires small cell to include an extra transceiver
• Passive compensation requires configuration of basestation location(latitude, longitude, elevation)• This information is already required for the UE positioning function
• Can be used to calculate distance to serving cell, and compensate for the delay
• 3s is approximately 900m at the speed of light, so tolerance is quite large for TDD
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Measurement Over The Air
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Measuring synchronization over the air• If radio signals are being used for synchronization, you’ll want to measure them, right?
• Need to compensate for distance from eNodeB when calculating time error• Use passive, distance-based compensation
eNodeB
Time Sync(e.g. PTP, GNSS)
Primary, Secondary Synchronization
signals
radio frame
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GPS
Test set
Relative phase measurements• Since phase alignment is the fundamental requirement, measure that too
eNodeB
Time Sync(e.g. PTP, GNSS) GPS
Test set
eNodeB
Relative phase difference
Time Sync(e.g. PTP, GNSS, RIBS)
Need to compensate for distance from each eNodeB when calculating phase alignment 20
Conclusions
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Conclusions• Synchronization over the air is a viable technique for small cells
• The cellular signal itself becomes part of the sync chain
• Measurement over the air verifies the entire synchronization chain• Non-invasive, easy to make measurement
• Doesn’t require physical access to eNodeB site or equipment
• Doesn’t disrupt service while making (or setting up to make) the measurement
• Uses include:• Network design verification
• Installation test
• Troubleshooting
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