19
Synchronisation over the air Tim Frost, ITSF 2017
Synchronisation over the air
Tim Frost, ITSF 2017
Agenda• TDD Synchronisation Requirement
• Overlapping Coverage Areas
• Synchronisation Over The Air
• Measurement Over The Air
• Conclusions
TDD Synchronisation and Overlapping Coverage Areas
3
BS 1 BS 1UE 1 UE 1 BS 1
Why do we need synchronisation?• TDD networks alternate between upstream and downstream transmissions:
• If synchronisation 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
Synchronisation error 4
Synchronisation 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
6
eNodeB eNodeB
Interference due to
poor synchronisation
Interference Area
Good synchronisation
Interference due to
poor synchronisation
7
eNodeB
What about small cells?
Small cell
Small cell
Small cell
8
• Small cells are often entirely within a macrocell coverage area
• Synchronisation errors may cause a significant interference problem
Synchronisation Over The Air
9
Synchronisation over the air• What if you could synchronise one cell from another?
• “Network Listening” cell synchronises itself to the radio frames coming from a nearby cell that is already synchronised
• Also known as “radio interface based synchronisation” or RIBS
eNodeB
Time Sync(e.g. PTP, GNSS)
Primary Synchronisation
signal
radio frame
eNodeB
Cell in “network listening” mode
10
eNodeB
Small cell architectures
Small cell
Small cell
Small cell
• Small cells might obtain synchronisation from overlapping macrocell
Time Sync(e.g. PTP, GNSS)
11
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
12
What about delay?• Several methods proposed to compensate for delay between the transmitting and
receiving cells
• For example, one method includes two-way signals:
eNodeB
Time Sync(e.g. PTP, GNSS)
eNodeB
Estimate propagation delay from round trip time
Request to start synchronisation procedure
Accept
Signalling T2, T3
T1
T2
T4
T3
13
Measurement Over The Air
14
Measuring synchronisation over the air• If radio signals are being used for synchronisation, you’ll want to measure them, right?
• Need to compensate for distance from eNodeB when calculating time error
eNodeB
Time Sync(e.g. PTP, GNSS)
Primary Synchronisation
signal
radio frame
GPS
Test set
15
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 16
Conclusions
17
Conclusions• Synchronisation 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 synchronisation chain
• Uses include:• Network design verification
• Installation test
• Troubleshooting
