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Calnex Solutions
calnexsol.com
Tim Frost
Synchronization for Fronthaul and O-RAN Timing ArchitecturesWSTS, March 2021
Fronthaul Evolution
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Centralized RAN
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Disaggregated Network
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Reasons for disaggregation
• Trend towards virtualization• Generic “white box” server hardware
• Add capacity and services easily
• Software-based services leading to faster customer turn-on and more bespoke services
• Less hardware out in the field• Data centers safer and more secure
• More radio units required to meet 5G capacity• Small, simpler, lower-cost units can be deployed
• Allows high-density deployments
• New standards required from bodies such as O-RAN Alliance and Telecom Infra Project• O-RAN Alliance mission statement is “Transforming the Radio Access Networks Industry Towards
Open, Intelligent, Virtualized and Fully Interoperable RAN”
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O-CU
O-DU
O-RU
gNodeB
O-RAN gNodeB (basestation) Protocol Stack
6
RRC
PDCP
MAC
Upper PHY
Lower PHY
RF
RLC
SDAP
calnexsol.com
O-CU
O-RU
O-RAN 5G Functional Splits
7
RRC
PDCP
MAC
Upper PHY
Lower PHY
RF
RLC
SDAP
FronthaulMidhaul
O-CU
O-RU
RRC
PDCP
SDAP
Lower PHY
RF
O-CU
O-RU
MAC
Upper PHY
RLC
Upper Layer Split
Lower Layer Split
O-DU
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Time and Latency
Sensitive Functions
Time and Latency
Insensitive Functions
Frame Timing and RF functions
Synchronization Requirements
8
O-DUO-RU
O-DUO-RU
O-RU
O-CU
O-RU
260ns TAE(Category B, e.g. intra-band CA in FR1)
260ns TAE(Category B, e.g. intra-band CA in FR1)
3µs CPSA(Category C,TDD radios)
Core
No critical sync requirement No hard limit;
around 3 - 5µs Hard limit; within ±1.5µs
Radio frame timing
Latency management
calnexsol.comCentral PRTC
O-DUO-RU
O-RUO-CU 130ns TAE
(Category A, e.g. intra-band CA in FR2)
Fronthaul Latency
• O-DU estimates one-way latency to schedule traffic on the O-RU
• Relative time error between the O-DU and O-RU causes error in that latency estimate, and must be accounted for in O-RU receive buffer depth
• No hard limit on O-DU time error, but somewhere in the range of 3 to 5µs is reasonable9
O-DU O-RU
Maximum Latency 100µs
O-RU Receive Buffer Depth
Fronthaul Network
Minimum Network Latency (fixed) Packet Delay VariationO-RU Time Error
O-DU Time Error
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ORAN LLS* Sync Architectures
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Configuration C1:
O-DU O-RUDirect Ethernet link carrying PTP/SyncE
GNSS
PRTC/T-GM Sync flow
Configuration C3:
GNSS
O-RU
PRTC/T-GM Sync flow
O-DU O-RUFronthaul network
Configuration C2:
O-DU O-RUFronthaul network
PRTC/T-GM Sync flow
GNSS
*LLS: “Lower Layer Split” – in effect, fronthaul
Configuration C4:
GNSS
O-RU
PRTC/T-GM
Sync flow
O-DU Fronthaul network
PRTC/T-GM
GNSS
Sync flow
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In detail: O-RAN C2 fronthaul configuration
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Fronthaul network
(if local PRTC/TGM)
Sync network
PRTC/T-GM
O-DU
O-RU
O-RU
TAE req’t3µs (Cat C)
260ns (Cat B)130ns (Cat A)
Reference Point C
Max|TEL| < 1.1µs(G.8271.1 or
G.8271.2)
Network |TE|(95 to 140ns*)
Relative |TE|(60 to 190ns*)
*Figures from O-RAN Table 9-3
GNSS
Sub-millihertz low-pass filter
< 15ppb frequency error
< 50ppb frequency error
< 36ppb frequency error
Network |TE|(max|TE| < 1µs,
dTEL+H < 63ns p-p*)
In detail : O-RAN C3 fronthaul configuration
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Fronthaul network
(if local PRTC/TGM)
Sync network
PRTC/T-GM
O-DU
O-RU
O-RU
TAE req’t3µs (Cat C)
260ns (Cat B)130ns (Cat A)
GNSS GNSS
*Figures from O-RAN Table 9-4
Relative |TE|(60 to 190ns*)
Network |TE|(max|TE| < 1µs,
Network |TE|(not defined)
Reference point C
Max|TEL| < 1.1µs(G.8271.1 or
G.8271.2)
No filtering
< 50ppb frequency error
Calnex Solutions
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Insight and Innovationcalnexsol.com
Tim Frost – Strategic Technology Manager
