Femtocell Synchronization

7/29/2019 Femtocell Synchronization

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Published by the Femto ForumJune 2010

www.femtoforum.org

FemtocellSynchronization

and Locationa Femto Forum topic brief


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Femtocell Synchronization and Location a Femto Forum topic briefis published by the Femto Forum

June 2010. All rights reserved.

www.femtoforum.org

telephone +44 (0)845 644 5823 ax+44 (0)845 644 5824 email [email protected] PO Box 23 GL11 5WA UK

What is the Femto Forum?

The Femto Forum is the only organisation devoted to promoting emtocell technologyworldwide. It is a not-or-proft membership organisation, with membership open to providerso emtocell technology and to operators with spectrum licences or providing mobile services.The Forum is international, representing more than 120 members rom three continents andall parts o the emtocell industry, including:

l Major operators

l Major inrastructure vendors

l Specialist emtocell vendors

l Vendors o components, subsystems, silicon and sotware necessary to create emtocells

The Femto Forum has three main aims:

l To promote adoption o emtocells by making available inormation to the industry and the

general public;

l To promote the rapid creation o appropriate open standards and interoperability or

emtocells;

l To encourage the development o an active ecosystem o emtocell providers to deliver

ongoing innovation o commercially and technically efcient solutions.

The Femto Forum is technology agnostic and independent. It is not a standards-setting body,but works with standards organisations and regulators worldwide to provide an aggregated

view o the emtocell market.

A ull current list o Femto Forum members and urther inormation is available atwww.femtoforum.org


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Femto Forum: Femtocell Synchronization and Location

Contents

Executive Summary ...................................................................................................................... 2

Introduction .................................................................................................................................. 3

Synchronization and Location Requirements ............................................................................... 4

Challenges to Synchronization and Location Determination ....................................................... 6

Solutions for Synchronization and Location Determination ........................................................ 8

GPS ............................................................................................................................................ 8

Cellular Network Listen ............................................................................................................. 9

TV Broadcast Listen ................................................................................................................ 10

Backhaul Based Solutions ....................................................................................................... 11

Comparison of Solutions ......................................................................................................... 15

Appendix Packet Timing in Detail ............................................................................................ 18


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2010 The Femto Forum page 2 www.femtoforum.org

Executive Summary

Synchronization and location determination are critical functions for the successful operation of femtocells.

While seemingly distinct topics, they are often discussed together since the techniques used to accomplish one

may often also be used to accomplish the other.

In this Topic Brief, we review requirements for synchronization and location derived from cellular standards

and regulatory constraints, point out some of the challenges that must be overcome, and examine the most

common and promising techniques available today. These include GPS, cellular network listen, TV broadcast listen,

and NTP/PTP packet-based solutions. A table is provided, comparing the relative figures of merit of the above

mentioned solutions. It is important to realise that the availability of location and synchronization sources varies

with time and place, and that hybrid combinations of multiple sensors can outperform single-sensor solutions in

many environments. Furthermore, different network operators may have different needs, and should therefore

consider all alternatives before deciding which technology is most suitable for their networks.


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Introduction

Synchronization and location determination are critical functions for successful operation of femtocells.

While seemingly distinct topics, they are often discussed together, since the techniques used to accomplish one

may often also be used to accomplish the other.

There are several techniques available, each having its own set of pros and cons. In this overview paper, we

take a look at some of the most common and promising solutions specifically, the Global Positioning System

(GPS), synchronization via the wired backhaul connection, cellular network listen or sniffing, and TV-GPS timing

and location. These are illustrated in Fig. 1. It is not our intention to promote a specific solution. Rather, we simply

aim to review the main requirements, point out available solutions, and discuss some of the deployment aspects.

Different network operators may have different needs, and would be well advised to carefully consider all options

before deciding which technology is most suitable for their networks.

Fig. 1: Femtocell synchronization and location solutions


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Synchronization and Location Requirements

Requirements for frequency accuracy and timing synchronization are imposed by the various cellular air

interface standards, while requirements for femtocell location determination arise from regulatory constraints and

requirements as well as commercial considerations.

Spectrum accuracy ensures that the femtocell does not emit signals that interfere with other femtocells or

with the macro-cellular network. Frequency accuracy varies by air interface. Timing or phase synchronization on

the order of a few microseconds is a basic requirement for those cellular standards that require base stations to

operate in synchronous fashion, such as CDMA2000, TD-SCDMA, and the time division duplex (TDD) modes of LTE

and WiMAX.

Table 1 describes the various synchronization and location requirements in more detail.

Table 1: Femtocell synchronization and location requirements

Category Requirement Detailed Requirements

Frequency

stabilitySpectrum accuracy

To maintain frequency alignment with the macro-cellular network,

the femtocells transmit frequency error must be within:

250 parts per billion (ppb) for UMTS i, LTEii, TD-LTEiii forHome BS class (100 ppb for Local Area BSclass)

100 ppb for CDMA2000iv, TD-SCDMAv (250 ppb for HomeBS class expected in Release 10 of TD-SCDMA specifications)

20-40 ppb for WiMAX (depending on modulation and othercriteria)

vi

Timing

synchronizationSynchronization accuracy

To avoid interference and to support call handovers in systems

operating in synchronous fashion, femtocells must be synchronized to

within:

10 s of GPS time for CDMA2000vii 1 s for TDD-WiMAXviii 3 s for TD-SCDMAix 3 s for TD-LTEx


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Location

determination

Emergency caller location

identification

Requirements differ by country. Generally, regulators mandate

identification of the location for both the serving base station and the

terminal. The location accuracy depends on the location technology

utilised. In the US, the FCC E911xi

legislation mandates identification

of the location of the serving cellular base station (Phase 1). Some

interpret this to be a requirement to provide the location of the

femtocell.

Cellular operating license

verification

Femtocells cannot be used in geographic areas for which the cellular

operator does not have a license to operate a wireless network.

Operator control of

customer usage

Operators may wish to restrict the usage of femtocells to certain

geographic areas for a variety of reasons e.g., pricing or service

differentiations, preventing unintended usage, or fraud detection.

Whereas highly stable oven controlled crystal oscillators (OCXO) can meet or exceed the frequency accuracy

requirements above, such oscillators are not a viable option for femtocell applications due to their high cost.

Femtocells therefore require mechanisms to discipline a low-cost oscillator such as a voltage controlled

temperature compensated crystal oscillator (VCTCXO). These mechanisms must be flexible and offer a range of

disciplining options, as they must be able to function in a wide range of scenarios, including indoor use and areas

with limited or no GPS or macro-cellular coverage.


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Challenges to Synchronization and Location Determination

The fundamental challenges of timing and location for femtocells are that: 1) femtocells are often deployed

where coverage is poor; and 2) the service provider has little control over the placement of the femtocell within

the customers residence. Examining these constraints is useful. Poor coverage can be due to a number of

reasons that there is no local macrocell that serves the residence, or that the signal is attenuated or made

unusable either by distance (macrocell is too far away), terrain, or other structures such as buildings, including the

customers residence itself. The placement of the femtocell within the customers residence is driven mostly by

constraints on the local placement of the DSL or cable outlet and on wiring and furniture constraints. Timing and

frequency synchronization, and location determination solutions, which depend on the reliable receipt of RF

signals, must overcome these constraints. Perhaps the most important of these constraints is building

attenuation.

Building attenuation

Fading effects and building attenuation caused by building materials vary greatly by the frequency of the RF

signal. As an example, Table 2 below shows a summary of a NIST study1

of RF attenuation by building materials.

Note the difference in attenuation levels across frequencies for building material, between TV, 900 MHz cellular,

GPS, and WiMAX frequencies. According to the study, for a concrete wall of an apartment building, GPS is

attenuated 6 dB more than TV, 4 dB more than 900 MHz cellular, but 20 dB less than WiMAX frequencies.

1NIST Construction Automation Program Report No. 3, Electromagnetic Signal

Attenuation in Construction Materials,National Institute of Standards and Technology, October 1997.


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Table 2: RF attenuation by building materials

Residential Commercial (Apartment)

Wall

(Lumber or brick-

faced masonry)

Floor

(Lumber,

sheetrock)

Wall

(Concrete)

Floor

(Steel-reinforced

concrete)

500 MHz (UHF TV) 8 dB 16 dB 20 dB 22 dB

900 MHz (Low Band

Cellular)11 dB 22 dB 22 dB 27 dB

1.6 GHz (GPS) 10 dB 20 dB 26 dB 29 dB

3 GHz (WiMAX) 29 dB 59 dB 46 dB 50 dB

Multipath propagation

Indoor operation presents additional challenges beyond signal attenuation. Multipath propagation, due to

reflections off walls, ceilings, and floors, can be a source of errors and interfere with each other, resulting in

significant fading effects. For example, in the case of GPS, receivers offering multipath mitigation processing have

an advantage in these scenarios. However, very often the Line of Sight (LOS) signal is too weak to be received and,

in this case, multipath mitigation is of limited benefit. On the other hand, because the femtocell does not move,

the location data remains constant, enabling better sensitivity as acquisition periods can be longer. Equally

importantly, multipath location errors in a stationary femtocell can be averaged to achieve substantial

improvements in accuracy. The femtocell form factor also allows for more efficient antennas and optimal antenna

orientation, compared to, for example, handheld GPS applications.


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Solutions for Synchronization and Location Determination

In this section we take a look at various individual solutions available for synchronization and location

determination, and then discuss hybrid schemes that combine multiple solutions for improved reliability.

GPS

GPS is the most mature and widely used of several Global Navigation Satellite Systems (GNSS), which also

include GLONASS and Galileo. While location determination is perhaps the most familiar application of GPS, it also

delivers an accurate timing reference, which is needed in cellular systems requiring base stations to operate

synchronously, such as CDMA2000, TD-SCDMA, WiMAX, and TD-LTE. In addition to timing reference, GPS delivers a

quantification of the frequency error, which provides asynchronous systems such as UMTS with the frequency-

disciplining reference they need to satisfy frequency accuracy requirements.

GPS works by multilateration between the receiver and a number of satellites continuously moving across

the sky. Around ten satellites are generally visible at any one time in open sky environments. Each satellite

transmits a signal whose spreading code phase and carrier frequency are known. The receiver operates by

searching for the distinctive waveform from each satellite at a large number of code phase and frequency offsets.

Only when code phase is matched within a microsecond and frequency within a few Hz will the matching

(correlation) process result in a value significantly above the noise.

Once signals from four or more satellites have been detected in this way the receiver locks its tracking loops

to them and makes periodic measurements of carrier frequency and pseudo-range. The latter is calculated as: the

speed of light multiplied by the difference between time of receipt of a signal instant and time of transmission of

that signal instant. The pseudo-ranges constitute four or more independent pieces of information, which is

enough information to solve the 4-dimensional problem of location and time. The four or more carrier frequencies

can also be used to solve the receiver velocity and the frequency error of the reference oscillator.

Knowledge of the receiver clock error can be used to tune that error out and to align a reference pulse (e.g.,

a 1 pulse-per-second 1PPS) with, for example, the GPS second. Accuracy indoors is better than 1 s and is

typically better than 300 ns. Knowledge of the frequency error may be used to discipline the reference oscillator

directly. Frequency accuracy is generally better than 10 ppb and is typically better than 5 ppb.

Conventionally, GPS operates in a self-contained way: the location device operates on its own and all

relevant information must be recovered from the Navigation Message modulated on the satellite signals. This

includes the ephemeris data (i.e., the precise orbital coefficients), the almanac data (a coarse set of orbital

coefficients), satellite clock correction coefficients, ionospheric correction coefficients, and UTC-GPS offset


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correction coefficients. Of these, the ephemeris data is most frequently extracted since it is typically valid for only

four hours and is updated every two hours. Each satellite transmits its own ephemeris data and also almanac data

for the entire constellation. The latter is current for a week and usable for months. It is useful when acquiring

new satellites but is not essential for normal operation.

To recover the Navigation Message, the received signal strength needs to be high enough to demodulate

the 50 bps Binary Phase Shift Keyed (BPSK) signal. This demands an energy per bit to noise spectral density ratio of

around 10 dB, which corresponds to a carrier-to-noise ratio (C/No) of around 27 dB-Hz, which, in turn, corresponds

to a signal level at the antenna output of around -142dBm. However, slower extraction is achievable down to

around -145 dBm.

In contrast, in an Assisted GPS (A-GPS) enabled femtocell, the small amount of data carried by the satellite

signal is instead supplied as assistance data via a backhaul connection such as DSL or cable. A-GPS removes the

requirement to demodulate the unknown data when processing the signal, allowing the GPS receiver to operate at

a signal level significantly below -145dBm. This ability is crucial for deep indoor operation where signals are subject

to significant attenuation. When a GPS receiver is used to provide time-synchronisation, a signal from at least one

satellite must be received continuously. However, where only occasional location or frequency-adjustments are

required, intermittent operation may be sufficient.

Cellular Network Listen

Another method for acquiring synchronization is to make use of the synchronization signals received from

other cells. All base stations in a cellular network transmit synchronization signals that are used by the mobile

terminals to synchronize to the network. These signals may also be used by femtocells to synchronize to

macrocells that are already synchronized. This is often referred to as network listen or macro sniffing. Note that

the femtocell does not necessarily need to use the same air-interface technology for its transmit and receive

operation as the network it is listening to. For example, an LTE femtocell (HeNB) could derive its timing from a

CDMA2000 base station, which itself is GPS-synchronized.

The availability of such synchronization signals depends on coverage in the macro environment, building

attenuation, and multipath effects. The signal-to-noise ratio (SNR) needed to achieve synchronization is typically

much lower than that needed for regular data coverage, as the femtocell, which is most often stationary, can

integrate synchronization signals over a large time interval.

Network listen is a cost-effective technique as it uses signals that are already present. No additional

infrastructure is needed, and the femtocell only needs a receiver that can perform a correlation on the

synchronization signals. This receiver may simply be the same receiver that the femtocell uses to receive uplink

traffic. In this case, the femtocell may be configured with longer guard period or it may suspend regular traffic

from time to time in order to tune to the desired macro downlink carrier. Alternatively, if a dedicated network

listen receiver is available, it can be tuned to an out-of band downlink carrier on a continuous basis. Either way,


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the functionality is often already integrated into the femtocell as it also uses network listen for other purposes

such as interference and mobility management, and thus incurs no extra cost.

Network listen may also be used for coarse location determination as long as the femtocell can determine

the identity of the surrounding macrocells, whose physical location is perfectly known to the operators

management system. More accurate location may be calculated using triangulation methods.

TV Broadcast Listen

All global standard TV signals include information that can be utilised for precise time and frequency

synchronization and location. The characteristics of broadcast TV signals that make them desirable for use in

location and synchronization are:

TV signals are strong, with broadcast power levels of hundreds of kilowatts or megawatts, providing a linkmargin 50 dB greater than GPS. Outdoor power levels are typically -45 dBm, and ranging has been shown

to be effective with signals down to -125 dBm, a margin of 80 dB. Thus, TV signals can tolerate 50 dB

more attenuation and still be usable as a ranging signal.

TV signals are broadcast at low frequencies, which penetrate buildings well, as seen in Table 1 furtherwidening the link margin.

TV signals are wideband, enabling efficient mitigation of multipath effects. Broadcast analogue anddigital TV standards are either 6 MHz or 8 MHz wide.

TV signals have stable timing and are highly reliable, even during disasters. In the United States,broadcasters have Emergency Alert System obligations.

TV signals are broadcast across every metro area on Earth. New mobile TV networks (DVB-H, T-DMB, ATSC-M/H, and others) are being deployed, and these new

signals can be used in addition to existing broadcast TV networks.

TV-positioning had been shown to be E911-compliant in many metro areas.


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Frequency and Timing Accuracy

The excellent stability of digital TV pilots, having a median variance (over both short 10-second and longer

1-day intervals) of 6 ppb, is the source of the precise frequency stability provided by TV. By observing the pilot

signals at the TV-timing client and comparing the measurements with those of independent timing reference

monitors, TV signals can be used to discipline the local oscillator. Further, precise knowledge of frequency and

time, as determined by TV signals, is used to dramatically improve GPS signal acquisition time. Timing accuracy for

hybrid TV-GPS indoors is better than 1 s and is typically better than 300 ns. Knowledge of the frequency error is

used to discipline the reference oscillator directly. Frequency accuracy is generally better than 10 ppb and is

typically better than 5 ppb. This frequency accuracy is typically achieved within 10 seconds, even in very

challenging environments.

Backhaul Based Solutions

For synchronization using the wired backhaul, protocols such as Network Time Protocol (NTP), and Precision

Time Protocol (PTP) (also known as IEEE 1588) are available. In principle, both of these protocols can be used to

provide frequency and time synchronization throughout packet networks. The primary problems that must be

overcome by backhaul based solutions over a wide area network are:

Variability in time transfer latency (jitter) due to network latency created by hubs, switches, cables, andother hardware that reside between the clocks;

latency associated with the processing of timing packets; time uncertainty introduced by asymmetry; and the cost of server and network load.

It has been established that the need to synchronize a femtocell to a reliable frequency, and in many cases

time reference, cannot be avoided. From the integrators perspective, the ideal synchronization technology would

be reliable, fast, inexpensive and universally available. Wired network packet-based solutions have been adopted

as, at least, the fall-back solution in the current generation of femtocells because they meet an important subset

of these desires, notably the universal availability we can take it for granted that a femtocell enjoys a network

connection.

Historically NTP has been used to deliver approximate time across the internet from a few servers to a very

large number of clients. Short packet exchanges used for synchronization occur every few days and clocks are

synchronised to within 100 ms or so. PTP has been used in industrial settings on local area networks and in the

core of telecommunications networks to deliver microsecond synchronization. Synchronizing femtocells is a new

and unique application of packet-based timing.


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PTP and NTP operate in a fundamentally identical way, relying on a short exchange of time-stamped packets

to estimate the time at the client. Frequency is derived from a rate-of-change-of-time observation at the client

using time measurements spread over some duration.

Packet Timing over Home Broadband Access

Packet-based timing over home broadband access presents some challenges. In the absence of access to a

Network Timing Reference (e.g., the DSL symbol rate clock or Synchronous Ethernet) a packet-based scheme

provides no direct estimate of frequency error. Frequency error between local and reference clocks is measured by

quantifying the difference in apparent elapsed time over some duration. To put this in perspective, a typical 100

ppb measurement resolution requires a time measurement accurate to 100 s over a duration of 1,000 seconds

(around 17 minutes). In this context, the impact of network Packet Delay Variation (PDV) and its effect on

acquisition time is apparent from Fig. 1. The challenge for the measurement algorithm is to assess the slope of the

distribution of points within 100 s on the Y-axis (half a division) over 1,000s on the X-axis.

Fig. 2: Plot of individual time offset measurements over 83 minutes via a typical home DSL broadband

connection

To some extent, higher packet rates can be used to mitigate the effects of PDV observed at the femtocell

client and can certainly be used to discipline an inexpensive reference oscillator. However, at least in early

deployments, we cannot assume that timing servers will be accessible at any points in the network other than at

the gateways, which typically serve 20,000 100,000 clients. Operators will want to distribute the cost of these

dedicated servers across a large number of femtocells, restricting the rate at which any one femtocell can poll the

server. Note that this restriction applies equally regardless of the packet protocol (i.e., NTP or PTP) or customary

deployment and usage model. For example, an operator may choose to deploy PTP servers at a density more

typical of NTP infrastructure to extend the lifetime of the investment. (As a rule of thumb, a redundant pair of

servers per gateway might support 5-20 polls per minute from each femtocell.)

Fundamentally, any packet-based scheme delivers to the client an estimate of time at intervals determined

by the packet rate and subject to jitter according to the network Packet Delay Variation. The frequency offset of

the local oscillator (usually a TCXO at the femtocell) can then be estimated via a statistical treatment of the

individual measurements of time. At its crudest, the statistical treatment could be nothing more than a simple


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average. The limiting factor in the frequency measurement accuracy is then determined by the stability of the local

baseline for the time measurements i.e., the stability of the local TCXO. With some sophistication in the

statistical processing and a local oscillator with 100 ppb stability, low packet rate (5-20 polls per minute) solutions

can maintain absolute frequency accuracy of the order of tens of ppb in a home environment. Access and backhaul

networks introduce packet delay distortions that go beyond variable queuing delays, including but not limited to

path diversity and route reconfiguration.

Since the underlying process is in essence a measurement of slope, a good estimate of absolute time at the

femtocell is unnecessary to achieving a good estimate of frequency. All that can be said of absolute time at the

client is that it lies between the limits of round-trip delay measured using the packet protocol. Other than for

exceptional links, however, correct absolute time is usually within a few milliseconds of the centre of this range.

Ultimately, packet-based synchronisation methods on domestic broadband cannot be relied upon for microsecond

timing without overcoming the limitations of the software or part-software implementations of modem, Network

Address Translation and firewall functions in the subscribers home equipment.

Good implementations of standard packet timing protocols (construction and exchange of the messages on

the wire) are readily available. On a lightweight client platform with potentially significant latency in software

processing, the application of local time stamps requires some care and, ideally, hardware support to avoid

unnecessary aggravation of the PDV problem. While the standard implementations of NTP and PTP include default

statistical treatments of the resulting time measurements, they do not provide sufficient performance to

adequately discipline a femtocell oscillator. To date, proprietary designs are used for this part of successful

solutions.

A more detailed description of PTP can be found in the Appendix.

Infrastructure Requirement

There is no significant fundamental difference between the backhaul data rate required to support PTP and

NTP clients delivering the same level of performance. In simple deployments with a single master (at a gateway,

for example) serving clients directly, infrastructure costs will be comparable for comparable packet rates and

performance. However, PTP introduces the important enhancement of support for boundary clocks, which may be

placed at intermediate points in the network for example, at local exchange DSLAMs. Where a boundary clock

can be placed close to the subscriber and outside any part of the network subject to contention, a very much

higher packet rate can be supported for each client with very much less apparent jitter. This will certainly allow the

cost of the timing solution at the client to be reduced; the effect on infrastructure cost will depend on how it is

shared and any additional costs of access.

In the long term it seems likely that PTP boundary clocks will exist widely at local exchanges (and cable

head-ends). Their availability for femtocell synchronization will then be subject mainly to the commercial

relationship between the operator of the local exchange and the mobile operator. This is especially relevant where

a secure time source is required e.g., for certificate validation purposes.

In some deployments (typically over cable) it may be possible to operate at typical PTP packet rates of many

per second end-to-end across the access network (client to gateway) and make full use of the client BOM cost


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savings. This approach comes at the cost of increased investment in server hardware at the gateway and monthly

traffic burdens that run into many GByte. For deployments currently intended for third-party DSL access networks

target packet rates are in the order of several per minute per femtocell, rather than several per second.

Network Timing Reference (NTR)

For deployments on friendly networks where the femtocell and broadband modem are packaged together,

in principle it is possible to use the Network Timing Reference for synchronization. The NTR is derived from a

frequency reference at the local exchange or head-end and used to generate DSL symbol-rate timing and a range

of frequencies embedded in cable distribution. Where it is available, use of the NTR is very efficient (potentially

free and very fast) but relies on the femtocell including the broadband modem and the local-exchange operators

cooperation.

Location Determination

Coarse location may be derived from the IP address associated with the DSL or cable connection. This

assumes that the femtocell or the management system has access to a database linking IP address to physical

location. This database could possibly be owned by the broadband service provider, as opposed to the femtocell

operator.


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Comparison of Solutions

A comparison of the different component technologies discussed above is given in Table 3. As is evident

from the table, each technology has its pros and cons. Furthermore, the availability and performance of

synchronization sources varies with time and location. It is therefore beneficial for a femtocell to have access to

multiple sources and possess the ability to dynamically switch from one to another depending on which source is

the best available, or at least have access to fallback options should the primary source become unavailable.

Ideally, the femtocells synchronization subsystem includes a control application that can request and monitor

reliability information from the various sources e.g., GPS, TV broadcast/macro-cellular network listen, or

PTP/NTP and determine the best source at any given time.

Hybrid combinations of multiple sensors can outperform single-sensor solutions in many environments. Forexample, packet-based synchronization coupled with network listen as secondary source appears to be quite

common in currently deployed UMTS femtocells. As another example, consider a hybrid solution where TV

broadcast listen and GPS are combined to provide location and synchronization, leveraging the benefits of both

component technologies. Where line-of-sight GPS signals are readily available, the location and timing will be

dominated by GPS; where few satellites can be seen, the system will function as a hybrid, and where GPS is

completely unavailable, TV signals will be used alone. As all inputs are integrated, the system seamlessly finds and

uses the best signals for each unique customer deployment. By measuring the timing of TV and GPS signals from

three or more macrocell towers or satellites, TV+GPS can compute ranges to those points and then can compute

the location of the femtocell device even in very challenging indoor environments. TV and GPS are highly

complementary in their availability in dense urban areas where there are large buildings and very challenging

indoor settings, TV geometry is generally excellent, and in remote areas where there are few TV towers, urban

canyons and large multi-story steel-reinforced concrete buildings are rare. TV signals frequency accuracy of a few

parts per billion can also be used to greatly enhance GPS component performance, thereby improving the overall

solution.

In the event that no sources are available, the femtocell may either free-run its oscillator i.e., run its

oscillator without disciplining or shut down operation, depending on operator policy. In some scenarios, the

former may be acceptable, at least until frequency and timing drift reach levels that are beyond the tolerance of

the attached user terminals.


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Table 3: Comparisons of component technologies

GPS Network Listen TV Broadcast Listen NTP/PTP

Assumptions

about operating

conditions

Assisted GPS; 4

satellites in view

during acquisition; at

least one satellite in

view while tracking;

see notes for

description of indoor

scenarios

Microsecond-level

timing accuracy

requires estimation of

location using signals

from three

synchronized

macrocells (e.g.,

CDMA2000 AFLT)

Timing and frequency

can be acquired and

maintained with a

single DTV signal.

Position determination

requires three or more

towers in view. TV

signals are usable 40dB

below the level

required for TV

viewing.

All implementations

are as hybrid TV-GPS,

described below

Statistical treatment of

time measurements

performed over large

number of samples

Frequency

Excellent accuracy:

10ppb; often better

than 5 ppb

Excellent accuracy:

50 ppb

Excellent accuracy:

10 ppb; often better

than 5 ppb

Moderate accuracy over

typical broadband:

150 ppb in 20 minutes, or

50 ppb in 2 hours

Time to

Frequency Lock

1-30 seconds

(outdoors);

1-60 seconds (Indoor

Scenario 1);


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Location Excellent accuracy Moderate accuracy Excellent accuracy

Coarse location possible

via IP address to physical

address mapping

database

Time to Location

Fix

See Time toFrequency Lock

above

A few seconds 3 minutes or less A few seconds

Miscellaneous

GPS signals notalways available

indoors

Insignificantbackhaul

bandwidth

requirementwith A-GPS

Dedicatedreceiver needed

A-GPS serverinfrastructure

preferable

Macro signals notalways available

indoors

No backhaulbandwidth

requirement

Re-uses availablefemto receiver

No assistanceinfrastructure

needed

TV signals notalways available in

remote areas

Insignificantbackhaul

bandwidth

requirement

Dedicated receiverneeded

Timing referencemonitor

infrastructure

needed for

unsynchronized TV

networks; not

required for

synchronized

networks

Backhaul alwaysavailable by

definition

Moderate backhaulbandwidth

requirement

(


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Appendix Packet Timing in Detail

Precision Time Protocol Version 2 also known as IEEE 1588-2008 employs a client/server

architecture to provide time from PTP servers to PTP clients residing in, for example, femtocell access points

distributed throughout the network. Based on the distribution of time and the use of oscillator-disciplining

software at the femtocell, precise timing and frequency synchronization may in principle be achieved. PTP

servers are either grandmaster clocks or boundary clocks. Grandmaster clocks are the primary reference

sources for all other PTP elements within their network domain, while boundary clocks act as intermediary

masters with their own stable references between grandmaster clocks and their clients, thus reducing the

number of hops and resulting delay variation between the master and the client and reducing thevolume of traffic that has to be passed through the network to the grand masters. PTP achieves accurate

distribution of time over the network through the exchange of time-stamped packets between the server

and its clients. The sequence of PTP packets used to transfer time from the master to the client is shown in

Fig. 3. The master clock periodically sends a Sync message to the client. A time stamping unit marks the

exact time t1 the Sync message is sent, and a Follow-up message containing t1 is immediately sent to the

client. The client clocks time stamping unit stamps the arrival of the Sync message (t2), and compares the

arrival time to the departure time provided in the Follow-up message and is then able to estimate the

packet processing latency and adjust its local clock accordingly. Network latency is estimated by measuring

the round trip delay between server and client. The client sends a Delay Request message (at t3) to the

server, which issues a Delay Response message containing the arrival time t4 of the Delay Request message,

enabling the client to estimate the clock offset as follows:

This computation is based on the assumption that the packet delays in both directions are the same.

Assuming the packet propagation times are about the same, the main source of errors is the queuing delay

that the packets experience at the routers and switches in the network. To minimize this impact, hardware

time stamping is used as shown in Fig. 4. A hardware time stamping unit residing between the Ethernet

MAC and PHY transceiver issues a time stamp when an outgoing or incoming IEEE 1588 packet is observed,

precisely marking the time of departure or arrival of the packet.

If the packet delays in both directions are different, the estimated clock offset will be affected by an

error which will result in a time synchronization error. Asymmetry in the packet delays can occur for several

reasons. The first is packet queuing latencies. In consumer grade backhauls such as DSL or cable links,

uplink packets may suffer from significantly longer delays than downlink packets. Even with carrier grade

Ethernet backhaul, it is possible to have significantly more traffic in one direction than the other, which may

cause larger delays in one direction than the other. Finally, there are additional delays not captured by the


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hardware time stamping process. Physical layer encoding/modulation and decoding/demodulation occur

after and before the time stamping, respectively. Asymmetries in these times would create a time offset

even in the absence of any packet delay asymmetry. However, if these delays are known, they can be

calibrated out.

Master clock time Slave clock time

t1

t2

t3

t4

Data at slave

t2

t1, t2t1, t2 , t3

t1, t2 , t3 , t4

PTP Protocol Entity Local Clock

Transport Layer

Network Layer

Data Layer

Physical LayerPrecise Time Stamp

Generator

Fig. 3: PTP packet flow Fig. 4: PTP Hardware time stamping

It should also be noted that the packet delay drawback can be mitigated by enabling PTP at

intermediate network nodes. If all the intermediate network nodes on the route are PTP enabled then the

queuing delays and the upper layer processing time at these nodes can be taken into account. For the

accuracy and reliability required for femtocell applications, the use of multiple master clocks and boundary

clocks is likely needed.


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i3GPP TS 25.104 Base Station (BS) Radio Transmission and Reception, September 2008.

ii3GPP TS 36.104 Evolved Universal Terrestrial Radio Access (E-UTRA); Base Station (BS) Radio Transmission and

Reception, December 2009.iii

Same as ii) above.iv

3GPP2 C.S0002-A v6.0, Physical Layer Standard for cdma2000 Spread Spectrum Systems Release A, February

2002, and 3GPP2 C.S0024-B v2.0 cdma2000 High Rate Packet Data Air Interface Specification, March 2007.v

3GPP TS 25.105 Base Station (BS) Radio Transmission and Reception (TDD), May 2009.vi

IEEE Std 802.16-2009, IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for

Broadband Wireless Access Systems, May 2009.vii

Same as iv) above.viii

Same as vi) above.ix 3GPP TS 25.123 Requirements for Support of Radio Resource Management (TDD), December 2008.x

3GPP TS 36.133 Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for Support of Radio

Resource Management, September 2008.xi

Federal Communications Commission (FCC) OET Bulletin No. 71, Guidelines for Testing and Verifying the

Accuracy of Wireless E911 Location Systems.

Documents

Femtocell Synchronization