Synchronizing Small Cells

www.calnexsol.com

Tommy Cook, CEO

• Small Cell Sync Requirements

• Deployment Types

• Measuring Small Cell Synchronization

Presentation overview

2

Diverse landscape, multiple architectures, multiple technologies

Where do you start?

3

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

Small Cell Sync Requirements

4

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

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

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

11

Building LAN

Small Cells PRTC/GM

GNSS Satellite

Rooftop antenna

Building LAN Small Cells

Inter-building connection

Timing flow

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

12

• 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 Routers

Small Cell

PRTC/GM Microwave Last Mile

Distributed BC or TC function

GNSS

Timing flow

• 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

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

16

Distributed BC or TC function

PTP timing in PTP timing out

Microwave link

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)

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

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)

• Noise transfer (tests filtering capability)

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

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

21

Packet Backhaul Network PRTC/GM Small Cell

GNSS

Timing flow

PTP Field Tester

GNSS

Monitor RF output

INTEGRITY TIME ERROR MEASUREMENTS REQUIRE TRUE PRECISION

Tommy Cook

tommy.cook@calnexsol.com

+44 (0) 1506-671-416