tim frost, strategic marketing manager• but ptp devices are not required to insert an accurate t3...
TRANSCRIPT
Synchronising Small Cells
www.calnexsol.com
Tim Frost,Strategic Marketing Manager
• Small Cell Sync Requirements
• Indoor Deployment
• Outdoor Deployment
• Measuring Small Cell Synchronisation
Presentation overview
2Company Confidential
Small Cell Sync Requirements
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
4* Depends on technology and throughput requirements
3GPP have stated it will never require better than ± 1.5µsCompany Confidential
Frequency Sync Delivery Requirement
5
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
Company Confidential
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
Time Sync Delivery Requirement
6Company Confidential
Indoor Deployment
AT&T In-Building Proposal
8Company Confidential
Network Diagram
9Company Confidential
Building LAN
Small CellsPRTC/GM
GNSSSatellite
Rooftop antenna
Building LAN Small Cells
Timing flow
Inter-building connectione.g. microwave
Reference Point 1:Input to network
Ref. Point 2:Input to Link
Ref. Point 3:Output of Link
Ref. Pt. 4:Slave I/P
Ref. Pt. 5:Slave O/P
Ref. Pt. 6:Air interface
• ITU uses “hypothetical reference models” to analyse the noise in a network
• These allow the performance at various points in the network to be quantified, and network limits to be set
• Possible hypothetical reference model for indoor deployment:
Hypothetical Reference Model
10Company Confidential
Distributed BC or TC function
(Probably integrated into the small cell)
PRTC/GM
PTP Unaware
Switch
PTP Slave
PTP Unaware
Switch
PTP Unaware
Switch
PTP Unaware
Switch
PTP Unaware
Switch
PTP Unaware
Switch
PTP Unaware Building LAN PTP Unaware Building LAN
3
-200ns dTE(BC filtering)
±50ns cTE, ±100ns dTE(BC noise generation)
5
±250ns(short term holdover)
Possible Time Error Budget - IndoorNetwork Reference Points
±150ns(small cell)
6
±1.1µs network equipment budget
±1.5µs end-to-end budget
±100ns (PRTC/T-GM)
1
-200ns dTE(slave filtering)
±50ns cTE(slave asymmetry)
4
±450ns dTE(random variation,150ns per switch)
±150ns cTE(50ns per
switch)
±450ns dTE(random variation, 150ns per switch)
2
±150ns cTE(50ns per switch)
Outdoor Deployment
• Small cells often use microwave for last-mile backhaul
• 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
13
Small Cell
Daisy-chained Microwave
Distributed BC or TC function
Aggregation RoutersSmall Cell
PRTC/GM Microwave Last Mile
Distributed BC or TC function
GNSS
Timing flow
Company Confidential
Reference Point 1:Input to network
Ref. Point 2:Input to Chain
Ref. Pt. 4:Slave I/P
Ref. Pt. 5:Slave O/P
Ref. Pt. 6:Air interface
• Possible hypothetical reference model for outdoor deployment:• Assumes up to three PTP unaware aggregation routers
• Up to four daisy-chained microwave segments with BC or TC capability
Hypothetical Reference Model
14
Distributed BC or TC function
1
(Probably integrated into the small cell)
PTP Unaware Aggregation Routers
Distributed BC or TC function
4
PTP Unaware
Switch
PTP Unaware
Switch
PTP Unaware
Switch
PRTC/GM
PTP Slave
Company Confidential
5
±250ns(short term holdover)
Possible Time Error Budget – OutdoorNetwork Reference Points
±150ns(small cell)
6
±1.1µs network equipment budget
±1.5µs end-to-end budget
±100ns (PRTC/T-GM)
1
±450ns dTE(random variation, 150ns per router)
2
±150ns cTE(50ns per router)
-200ns dTE(BC filtering)
4,
±400ns dTE(W BC noise generation,
100ns per segment)
W BC asymmetry, 50ns per segment)
-200ns dTE(slave filtering)
±50ns cTE(slave asymmetry)
Measuring Small Cell Synchronisation
Tests to apply:
• Noise generation (±50ns cTE, ±100ns dTE proposed)
• Noise tolerance (from network limit, possibly 1.1µs max|TE|, 700ns dTE)
Microwave Test Configuration
17
PTP(if present)
PTP-Aware Microwave
Ref. Time & Freq.
Impair
CaptureCapture
Master
Capture
Slave
PTP
Freq., 1pps (if present)
PTP Tester
Apply packet timing impairments
Measure timing outputs (PTP, frequency or 1pps)
Company Confidential
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_Reqmessage.”
• 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
18Company Confidential
Lab Test Configuration
19
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)Company Confidential
PTP Field Tester monitors:• Quality of sync coming from PRTC/GM
• Accuracy of small cell sync (if accurate T3 available)
Field Test Configuration
20
Packet Backhaul NetworkPRTC/GM Small Cell
GNSS
Company Confidential
Timing flow
PTP Field Tester
GNSS
Lab test units
• Provide a sync test output (e.g. 1pps and/or frequency o/p)
• Can be made available on an internal test point if necessary
Field units
• Sync test output not practical for cost reasons
• Make accurate T3 timestamp available to allow monitoring
• Needs to be mandated by carriers to force small cell vendors to make this available
Recommendations for Small Cells
21Company Confidential
INTEGRITYTIME ERROR MEASUREMENTS REQUIRE TRUE PRECISION
Tim Frost
+44 (0) 1506-671-416