Download Interference filters for an increasingly complex

White paper
Interference filters for an increasingly complex
RF environment
Steve Musgrave
Applications engineering manager, RF conditioning products and solutions
www.commscope.com
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Contents
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Interference in an evolving wireless landscape
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Politics and public safety compound the problem
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Effects of spectrum reallocation
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Interference filtering
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Growing need for custom interference mitigation solutions
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Developing relationships to control interference 5
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What do widespread deployment of LTE, aggressive frequency re-use and the rise in mobile operator
co-locations have in common? All three are popular strategies being used by mobile operators to help address
their immediate need for more capacity. However, these tactics also share another critical characteristic: they
significantly increase the potential for signal interference.
Amid rising demand for wireless capacity, the airwaves are becoming overcrowded and the availability of usable
wireless spectrum is shrinking. The importance of controlling interference has never been greater.
At the same time, mobile networks are growing more complex—and the variables contributing to interference
are increasing. As a result, it is becoming more difficult to adequately address the problem using standard
off-the-shelf filters.
This paper provides an overview of the various major contributors to system interference and demonstrates
why the issue is expected to grow worse in the near future. It also describes the process involved in
developing a custom interference mitigation solution in order to help mobile operators evaluate potential
interference solution providers.
Interference: causes and effects
Interference is the sum of all signals that are neither noise nor the
wanted signal. Effects include limited range, dropped calls or low data
rates, especially at the cell’s edge.
Interference takes one of two forms: cochannel interference (CCI)
is crosstalk from two different radio transmitters using the same
frequency, while adjacent channel interference (ACI) is caused by
power from a signal in an adjacent channel.
CCI is typically the result of frequency re-use, whereas ACI results from
imperfect receiver filters that allow nearby frequencies to leak through.1
Some of the common causes of interference are:
• Overlapping bands in the same network
• Co-located carriers on adjacent frequencies
• Unrelated networks on adjacent frequencies
• Cross-border providers with conflicting regulations
Growth of co-located sites: In markets where the capacity need
is greatest, mobile operators are quickly running out of space to
deploy additional infrastructure. This has stimulated an increase of
co-location agreements in which carriers share cell sites, antenna
placements and even RF path components. One result is a greater
potential that spurious transmitter emissions from one carrier will
fall outside their transmit band and within the receive band of a colocated carrier.
Frequency conflicts in emerging markets: In developing wireless
markets such as north and central Africa and the Middle East, the
problem of CCI and ACI is intensifying. This is due, in part, to how
some networks are being deployed. In Africa, Europe, the Middle
East and Asia, most of the wireless operators use the 900 MHz
GSM band. Many operators, however, have deployed services in
the 850 MHz band. This creates a situation in which both bands
must co-exist with minimal separation, resulting in high levels
of ACI. The same situation exists where 1900 MHz services are
operating in proximity to 2100 MHz UMTS networks.
Interference in an evolving wireless landscape Politics and public safety compound
the problem
In many cases, some of the very tactics operators are using in hopes
of increasing their network capacity, profitability and performance
are driving co-channel interference (CCI) and adjacent channel
interference (ACI) higher and higher.
Use of LTE: With high-sensitivity, low-noise requirements, LTE is
especially susceptible to interference. As of September 2014, more
than 330 operators have commercially launched LTE networks and
services in 112 countries.2
Spectrum overcrowding: Both CCI and ACI are exacerbated as more
spectrum is released in order to supply capacity-starved networks.
Traditionally, mobile operators have employed guard bands in order
to separate and protect their frequency bands from out-of-band
emissions. With the value of spectrum steadily increasing, operators
are driven to pare these guard bands or abandon them altogether in
order to squeeze more capacity out of their allocated spectrum.
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With the amount of available spectrum shrinking, wireless carriers
often operate at frequencies that are extremely close to those of
noncellular services. In Europe, for example, the recently released
2600 MHz band opens the potential for interference to radar
systems such as those used for air traffic control, weather forecasting
and air defense. The same is true of the lower end of the 800 MHz
spectrum in Europe, which operates in close proximity to digital
terrestrial television broadcasters.
For operators whose networks are close to national borders, the
problems created by CCI and ACI are compounded. In 2004, a large
North American wireless operator faced a court battle because its
transmissions in the 800 MHz spectrum were allegedly interfering
with the regional public safety network used by over 200 local
agencies. To solve the problem, the operator and the Federal
Communications Commission agreed to a transition plan, which
included a reconfiguration of the 800 MHz spectrum.3 Complicating
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the issue was the fact that the carrier’s network shared a border with
another country. Therefore, any change in frequency usage had to be
approved by the neighboring government as well.
Effects of spectrum reallocation
In today’s wireless environment, spectrum planning is unable to
keep up with the rapid deployment of new technologies; therefore,
conflicts between adjacent bands are inevitable. It is even possible
for an operator to create ACI issues within its own network.
U P LI N K
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DOWNL INK
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Lower 700 MHz band
Figure 1: The lower and upper 700 MHz frequency is allocated in eight bands.
Source: http://www.cse.wustl.edu/~jain/cse574-08/ftp/700mhz/#sec3_1
In 2009, parts of the 700 MHz spectrum in North America were
reallocated from television broadcasts to wireless networks. As
shown in figure 1, the lower 700 MHz frequency band consists of
eight blocks. Blocks A, B and C are all paired with the lower three
blocks carrying uplink traffic; the upper three carry downlink traffic.
In between are the unpaired D and E blocks.
Prior to March 2011, the D and E blocks were licensed to a mobile
television provider. When that service was discontinued, the D and E
block licenses were sold to a national telecom provider who planned to
use the extra 12 MHz of bandwidth for LTE downlink traffic. This created
a potential interference issue with the uplink traffic in the adjacent C
block, whose licensee happened to be the same telecom provider.
The issue of ACI is common in instances where spectrum blocks are
reallocated or are sold at different times. In cases where adjacent
countries allocate their spectrum differently, wireless networks in
proximity to the border are also vulnerable to increased interference.
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Interference filtering
Interference mitigation filters play a critical role in controlling CCI and
ACI. The filters are typically deployed as standalone solutions anywhere
in the RF path but can also be integrated into antenna-line products
such as tower mounted amplifiers and combiners.
There are several basic types of RF filters and microwave filters. Bandpass filters are active or passive circuits that pass signals from a specific
frequency band and reject signals from out-of-band frequencies.
Band-reject filters, sometimes referred to as band-stop or notch filters,
are tuned circuits that prevent the passage of signals within a specified
frequency band. Low-pass filters allow low frequencies to pass while
rejecting high frequencies. High pass filters pass high frequencies and
reject low frequencies.
Filter material and construction varies from ceramic, cavity, and stripline
filters to crystal, SAW and adjustable filters.
Clearing the air in Europe
For one European mobile operator, the adjacent channel interference
in their 900 MHz frequency band wasn’t just annoying. At –60
to –70 dBm, the noise from a competitor’s 850 MHz service was
threatening customer satisfaction and revenues. The operator
turned to CommScope, and we took it from there. Working quickly,
CommScope engineers developed two custom interference filters,
one for the victim’s radios and another for the offender’s. With
interference levels back to an acceptable –105 dBm, the operator
was back in business, supported by a partner they could depend on
as they continued to grow.
“It is an unavoidable fact that wireless communications
systems must coexist in extremely complicated signal
environments. These environments are comprised of
multiple operating wireless networks ranging from mobile
communication services to specialized mobile radio and
paging/broadcast systems. At the same time, wireless local
area networks (WLANs) and digital video broadcasting
are introducing new technologies and signal sources that
further threaten to disrupt wireless communications
service.”
Application Note: Fundamentals of Interference in Wireless Networks;
Anritsu; July 6, 2011
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Frequency
pass
block
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Figure 2: Band-pass filters (L) and band-reject filters (R) work by either allowing
signals of a specified frequency to pass through or rejecting signals of specified
frequency ranges.
Source: www.instructables.com/id/Passive-Filter-Circuits/
Growing need for custom interference
mitigation solutions
In some cases, CCI and ACI can be effectively addressed by using a
standard filter—either a standalone passive component located within
the RF path or a filter that has been integrated into another component
such as a tower-mounted amplifier.
In many cases, however, an off-the-shelf solution is incapable of
reducing the noise level below the operator’s specified levels. This is due
to the complex nature of today’s wireless RF systems and the multitude
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of variables that must be managed. Each cell within the network has its
own RF fingerprint based on the transmission environment, performance
specifications and link budgets. The specific filter type and design
needed will vary depending on the application, RF environment and
performance specifications required. For example, the requirements of
a high-powered BTS system typically dictate the use of cavity filters—
either air coaxial or dielectric resonator filters.
Many times, therefore, controlling CCI and ACI requires a well planned
and individually designed approach—one that balances network
throughput with minimal noise, and cost concerns with timely
deployment. Anecdotally, from CommScope’s experience, approximately
half of the interference situations encountered require a customdesigned solution.
For most manufacturers of interference mitigation solutions, the
engineering challenges and inhouse resources required to bring a custom
solution to market are simply too great. This poses a significant problem
for the mobile operator. Understanding what is involved in developing
and supplying an effective custom interference solution will help the
mobile operator evaluate and select the best solutions provider.
The following outlines the steps in developing a custom interference
filtering solution.
Identify the interference issues: In some cases, the cell or cell sector is
the interferer, while, in others, it is the victim. Operators with sufficient
expertise and resources can not only determine if they are the interferer or
victim—and for which bands—they can also provide detailed performance
specifications the filtering solution must meet. Many operators may simply
realize they are not achieving the link performance needed. In these cases,
they look to their filter provider for an accurate assessment.
Establish the design parameters: There are a wide variety of RF
performance and network characteristics that determine the
optimum filter composition and design. Key variables include:
• Power handling capabilities
• Required rejection value
• Guard band width
• Acceptable insertion loss
• Passive intermodulation (PIM)
• Dimensional constraints
Although an acceptable solution must meet the minimum performance
specifications for all variables, it is not possible to optimize all variables
with a single solution. Therefore, achieving an acceptable result
involves a series of tradeoffs. The mobile operator, with the help of the
solutions provider, must prioritize which variables are most important.
Run trials on any acceptable off-the-shelf solution: As previously
mentioned, in some cases, the filter provider may have a solution that
has already been developed that meets the performance specifications
for the application. Obviously, that is preferable to developing a
customized solution. If a potentially successful solution is on hand,
it must be tested to determine how well it meets the identified
performance specifications. It is possible that a suitable solution can
be found by adapting existing filters to achieve the desired results.
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Design and integration: Once the design goals have been
established, engineers can begin to design the filter. Sophisticated
software and powerful computers are employed to synthesize,
analyze, and optimize the filter response. The key variables during
the design phase include pass-band, cut-off frequency, ripple band,
transition band, stop-band, number of poles, roll-off, phase shift
and impedance. The design must also take into consideration where
in the RF path the filter will be located. If it is to be deployed as
a standalone solution or integrated into any other antenna line
functions, such as combiners or tower-mounted amplifiers, this
must be accounted for in the initial design.
Feasibility study: The feasibility study analyzes the requirements
needed to produce the final interference mitigation filter as designed.
It takes into account the customer’s current known needs and, if
possible, anticipates future demand as well. During this phase, the
solutions provider will also be asked to develop a production schedule
based on their current engineering and production capabilities.
To this point, the filter solution exists only as a product concept.
This presents a challenge for the network operator, as their decision
regarding which of the competing filter designs best meets their need
is typically made at this point. This is due to the cost incurred by the
filter manufacturer in going from concept to prototype.
The network revenues that can be lost due to an interference issue
can be significant; therefore, resolving the issue as quickly as possible
is important. In the rush to solve the problem, however, the network
operator must be confident that the theoretical solution selected will
translate into a physical product that meets the operator’s specifications.
Prototype development: The first time the network operator is able to
fully vet the physical solution is after the prototype is completed and has
been delivered. Modifications may be made to the filter at this point, but
at the expense of a prompt deployment. This emphasizes the importance
of thoroughly vetting each potential design as well as the capabilities and
track records of competing filter providers prior to settling on a solution.
Production scale-up: Two critical aspects of production scale-up are
speed and quality. As mentioned, interference exacts a daily cost on
a mobile network. Therefore, the filter provider must be able to go
from prototype to production manufacturing as quickly as possible.
At the same time, there must be sufficient quality control and testing
throughout the manufacturing process to ensure each custom filter
solution will meet the required specifications. As the mobile operator
is vetting the various providers, typically during the feasibility phase,
they should check to make sure adequate production and distribution
capabilities are in place to meet their needs during production scale-up.
Developing relationships to
control interference
As modulation schemes continue to grow more sophisticated and
the RF path grows more complicated, mobile networks will become
more sensitive to the effects of CCI and ACI. In some cases, the
interference can be adequately managed using standard, off-the-shelf
filter solutions or those that have been slightly modified. But, as more
variables are introduced into the RF environment, the less effective
these standard filter solutions will be overall.
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The design, development and production of a custom filter solution is a
multi-phase process that depends, in part, on the operator and solutions
provider understanding each other’s processes, capabilities and, most
importantly, expectations. A strong working relationship between operator
and filter provider will result in a proactive and cost-effective interference
mitigation strategy and more profitable network performance.
 What Are Interference And Its Source, Effect And Types In GSM; Telecom
Techniques Guide; January 15, 2013
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 GSA: Operators have commercially launched 331 LTE networks; Fierce
Wireless Europe; September 18, 2014
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 Official: Years to fix public safety interference; San Diego Union-Tribune;
July 13, 2004
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