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Synchronizing Small Cells
www.calnexsol.com
Tommy Cook, CEO
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• Small Cell Sync Requirements
• Deployment Types
• Measuring Small Cell Synchronization
Presentation overview
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Diverse landscape, multiple architectures, multiple technologies
Where do you start?
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Input limits (network i/f)
Output limits (RF i/f)
C-RAN
LAN DSL
Microwave
NTP
PTP
Macrocell sniff GPS
CoMP eICIC
TDD
FDD
DAS
RRU
LTE
WiFi
Cellular offload
Mid-chain (e.g. at BC)
FTS
PTS
APTS
SyncE Metrics
Testing Small Cells Synchronization
Sync performance
Network architecture
Technology
Sync method
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Small Cell Sync Requirements
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Frequency requirements:
• Carrier small cell: 100ppb
• Residential small cell: 250ppb
Time requirements:
• LTE FDD with no co-ordination: no requirement
• LTE TDD with no overlapping coverage: no requirement
• LTE TDD with overlapping coverage: ± 1.5µs
• LTE FDD or TDD with eICIC or CoMP: ± 0.5 – 5s *
• E911 OTDOA (Observed Time Difference of Arrival): ± 0.1µs
“Air Interface” Sync Requirements
5 * Depends on technology and throughput requirements 3GPP have stated it will never require better than ± 1.5µs
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Frequency Sync Delivery Requirement
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G.8261.1-Y.1361.1(12)_F04
MTIE
18 sμ
9 sμ
0.05 0.2 32 64 1125
16 ppb
Observationinterval (s)τ
Figure 4/G.8261.1 – Output wander network limit for case 3 based on [ITU-T G.823]
562.5
32ppb
Modified limit for Carrier small cell
100ppb
180
Modified limit for Residential small cell
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Air interface requirements, less:
• 150ns for basestation internal tolerances
• 250ns for short term holdover (e.g. to allow reference switch)
Example for LTE TDD:
• ±1.5s at the air interface
• ±1.1s at the network interface
Example for LTE CoMP (tightest tolerance):
• ±500ns at the air interface
• ±350ns at the network interface (no reference switching)
Time Sync Delivery Requirement
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• CRAN Architecture • Baseband units co-located, fronthaul network used to distribute
baseband signals to remote radio units
• Common interface standards: CPRI, OBSAI
• NGFI: Next Generation Fronthaul Interface • Evolution of fronthaul to use Ethernet with PTP/SyncE time
synchronization
• Required for future “5G” radio requirements
• Frame Time Alignment Requirements between RRUs: • Inter-band carrier aggregation: 1325ns frame time alignment
• Intra-band carrier aggregation: 155ns frame time alignment
• MIMO and TX Diversity: 65ns frame time alignment
Coming Soon…
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Deployment Types
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AT&T In-Building Proposal
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In-Building Network Diagram
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Building LAN
Small Cells PRTC/GM
GNSS Satellite
Rooftop antenna
Building LAN Small Cells
Inter-building connection
Timing flow
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Urban canyons, microwave backhaul • Poor GNSS environment, multipath reflections and restricted sky view
• Backhaul typically NLOS or LOS microwave
Street deployment
Pictures from Telefonica wi-fi small cell trial, London, summer 2012
Small cell
Microwave backhaul unit
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• Microwave equipment often includes PTP support, e.g. BC or TC
• What performance standards must the BC or TC function meet?
Small Cell Microwave Backhaul
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Small Cell
Daisy-chained Microwave
Distributed BC or TC function
Aggregation Routers
Small Cell
PRTC/GM Microwave Last Mile
Distributed BC or TC function
GNSS
Timing flow
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• Centralizes Baseband Units (BBUs) in a “baseband hotel”
• BBUs all co-located, simplifying sync for eICIC and CoMP
• Requires accurate latency measurement of “fronthaul” network to synchronize RRUs
C-RAN Architecture
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Core IP/MPLS network
Remote Radio Units
“Fronthaul” network
(CPRI, OBSAI)
“Baseband Hotel”
Gateway Router
PRTC/GM
Timing flow
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Measuring Small Cell Synchronization
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• What should the performance spec be for a distributed BC/TC function (e.g. microwave unit)?
• Current BC spec (G.8273.2) is for a single network element
• Proposal for a “distributed BC/TC”: • Based on sum of two G.8273.2 BCs
• G.8171.1 HRM can remain unchanged; distributed BC uses two nodes
Distributed BC and TC
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Distributed BC or TC function
PTP timing in PTP timing out
Microwave link
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Tests to apply:
• Noise generation (±100ns cTE, 50ns p-p dTE proposed)
• Noise tolerance (from network limit, possibly 1.1µs max|TE|, 500ns p-p dTE)
• Noise transfer (tests filtering capability)
Microwave Test Configuration
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PTP (if present)
PTP-Aware Microwave
Ref. Time & Freq.
Impair
Capture Capture
Master
Capture
Slave
PTP
Freq., 1pps (if present)
PTP Tester
Apply packet timing impairments
Measure timing outputs (PTP, frequency or 1pps)
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1. At the sync interface output • BUT most small cells don’t have extra outputs such as sync interfaces
• Possibly a test point is accessible in the lab, but not in a field environment
2. At the network interface • Probe PTP flow at network interface, then the originTimestamp in the
Delay_Req message (T3) gives a direct readout of internal clock state
• BUT PTP devices are not required to insert an accurate T3 value
• IEEE1588-2008, Clause 11.3.2: “The originTimestamp shall be set to 0 or an estimate no worse than ±1 s of the egress time of the Delay_Req message.”
• Requires on-the-fly hardware timestamp insertion, which is only mandated for one-step PTP master
3. At the air interface • Monitor the RF output signal, decode frames for frequency and alignment
How to measure small cell sync
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Lab Test Configuration
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Ref. Time & Freq.
Impair
Capture
Master
Capture
PTP Freq., 1pps (if present)
Small Cell
PTP Tester
Apply packet timing impairments
Measure timing outputs (PTP T3, frequency or 1pps)
Tests to apply: • Noise generation and tolerance for frequency (from G.8263)
• Noise generation and tolerance for time (to be determined)
• Noise transfer (tests filtering capability)
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PTP Field Tester monitors: • Quality of sync coming from PRTC/GM
• Accuracy of small cell sync (if accurate T3 available)
Field Test Configuration
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Packet Backhaul Network PRTC/GM Small Cell
GNSS
PTP Field Tester
GNSS
Timing flow
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PTP Field Tester monitors: • Frequency and time sync of RF output
• Non-invasive method; no disconnection required to insert tap
Over-the-air Sync Test Configuration
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Packet Backhaul Network PRTC/GM Small Cell
GNSS
Timing flow
PTP Field Tester
GNSS
Monitor RF output
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INTEGRITY TIME ERROR MEASUREMENTS REQUIRE TRUE PRECISION
Tommy Cook
+44 (0) 1506-671-416