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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
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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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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.