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

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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

Time Sync Delivery Requirement

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Indoor Deployment

AT&T In-Building Proposal

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Network Diagram

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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

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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

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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

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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

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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)

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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_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

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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)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

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Packet Backhaul NetworkPRTC/GM Small Cell

GNSS

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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

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INTEGRITYTIME ERROR MEASUREMENTS REQUIRE TRUE PRECISION

Tim Frost

tim.frost@calnexsol.com

+44 (0) 1506-671-416