GPS-quality Network Timing for

TD-LTE and LTE-Advanced

Martin Nuss – CTO, Vitesse Semiconductor

LTE World Summit, Barcelona

May 2012

Agenda

Synchronization requirements for TD-LTE and

LTE-Advanced (LTE-A)

GPS challenges for LTE small-cell deployments

Network solutions for GPS-quality network timing for LTE

and LTE-Advanced

2

Small (Pico/Femto) Cells Integral to LTE

Small cells are the

most obvious solution

to the spectrum/

bandwidth problem

Small cells allow

spatial re-use of the

same spectrum

But small cell backhaul

and timing delivery are

main challenges

3

Traditional Base Station Timing Architectures

Traditionally timing (frequency)

derived from E1/T1 operator

connections to the base station

For Time-of-Day (phase) GPS

receiver at each base station most

commonly deployed

With 4G/LTE, this timing model

is challenged:

E1/T1 replaced by Ethernet, requiring

new network timing solutions

Many more (small) cell sites, most of

them with poor GPS visibility

Other concerns about GPS (ease of

jamming, US-controlled, etc..)

4

E1/T1

GPS Rx

Ethernet

E1/T1

All Ethernet

Primary

Reference

Clock

Ethernet Network Timing:

1) SyncE – Frequency only

2) IEEE 1588 – Frequency and

Phase (Time-of-Day)

Outdoor Small Cells Will Require Network Timing

5

GPS satellite visibility is poor in urban corridors Network timing required

Fiber-to-the-lamppost not common Timing over Microwave essential

LTE Indoor Coverage and Timing Challenges

Common problem for LTE

will be indoor coverage, in

particular in the higher

frequency bands

Predominant access

technologies to buildings

(VDSL, FTTx) have problems

delivering good timing

Challenge: How to provide

accurate timing to LTE

femtocells inside building?

6

Timing Gets Even Tougher for TD-LTE and LTE-A

7

TDD Standards generally require tight

Phase/Time-of-Day synchronization

LTE-Advanced puts additional burden

on synchronization

For network timing distribution,

synchronization errors accumulate in

every network element

Timing Accuracy Requirements for 3G/4G

Technology Frequency Accuracy Phase Accuracy

GSM ≤ ±50 ppb N/A

UMTS FDD ≤ ±50 ppb N/A

UMTS TDD ≤ ±50 ppb ≤ 2.5 µsec

3GPP2 CDMA200 ≤ ±50 ppb ≤ 3 µsec

TD-SCDMA ≤ ±50 ppb ≤ 3 µsec

LTE Long-term cumulative error ≤ 1.5 µsec

LTE-Advanced/MIMO ≤ 0.5 µsec between neighboring towers

Intro to Timing Through Packets – IEEE 1588

8

IEEE 1588v2 Precision Timing

Protocol estimates the time

at the slave by calculating the

propagation delay between

master/slave via a series of

time-stamped messages

Traditional Frequency Synchronization

Packets can be used for synchronization

if time stamped by a Master Clock

Impairments are short-term jitter and

long-term phase stability (wander)

Impairments are packet delay variations

through the network (jitter) and long-term

time stability (wander)

Packet networks can have large packet delay variations,

introducing timing inaccuracies

Master

Clock Slave

Network

Solutions to Packet Delay Variations in IEEE1588

9

PDV Filtering

in Software

Packet Delay Variation (PDV) algorithm on subset of packets

that have experienced the least delay

Usually requires reserving the highest priority queue for

1588 packets

1588

Transparent

Clocks

Network elements perform time stamping at the port/PHY-level

only and update time stamps on egress

Boundary clocks can be sprinkled throughout network to create

1588 domains

Each network element engages in deriving Time-of-Day using

the 1588 protocol, and updates the time stamps before sending

out to next network element

Very similar to BITS/SyncE model

1588 Boundary

Clocks in

Every Network

Element

Network Timing Cost Comparison

10

Performance

Co

st

PDV Filtering

– Needs DPLL, OCXO, CPU

in every node

– Difficult to support multiple

timing domains, MPLS-LSR

Boundary Clocks

+ Do not need DPLL, OCXO,

CPU in every node

+ Supports multiple timing

domains, MPLS-LSR

Transparent Clocks

Transparent clocks are the most cost effective and least

disruptive way to provide nanosecond-accurate timing

+ Low implementation cost

(Software only)

– Phase accuracy usually not

good enough for TD-LTE

TCs Easily Support Multi-Operator Environments

11

With transparent

clocks, transport

operators can offer

a synchronization

service to multiple

service providers

Transparent clocks

improve 1588 packet

time accuracy 1000x

from microseconds

to nanoseconds!

0.5 us

-0.5 us

0

No TC With TC

TC TC TC Grand

Master Slave

TC TC TC Grand

Master Slave

Synchronization Service: Transparent Clock

Operator 1 PRC

Operator 1

Base Station

Transport

Operator UNI

UNI Operator 1

Controller /

Gateway

Operator 2

Controller /

Gateway

Operator 2

Base Station UNI

UNI

UNI-C UNI-C

UNI-N UNI-N

Synchronous Ethernet Domain

Transport Operator Frequency

(TOD not required)

Network Solutions to LTE/LTE-A Timing Problems

Femtocell Indoor Synchronization

Provide 1588 network timing through

the access network, or

GPS antenna on top of building generates 1588

packets for time distribution inside the building

12

Urban Picocell Synchronization

Provide 1588 Network Timing to the picocell

Transparent Clocks over Microwave

and Millimeter-wave links can easily

meet TD-LTE and LTE-A requirements

Summary

TD-LTE and LTE-Advanced require GPS-grade

timing/synchronization

Use of GPS is often not possible – both indoor and outdoor

IEEE1588 network timing is maturing and being implemented by all

major equipment manufacturers

Transparent clocks are the most cost effective and least disruptive

way to provide nanosecond-accurate timing

13

Vitesse is leading the way with TD-LTE/LTE-A ready technologies.

Learn more at www.vitesse.com/ce/ce_timing.php

Thank You

Martin Nuss – CTO, Vitesse Semiconductor

nuss@vitesse.com