Footprint Part 2 of 6 Transparent Infrastructure
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Footprint Changing the RF cost performance paradigm Further information visit our websites www.summitekinstruments.com www.triasx.com Page 2 - Footprint Two - 1 Introduction Frequency transparency allows multiple RF signals to be overlaid on common infrastructure without degradation or distortion, by providing a linear RF transmission conduit, essential for spread spectrum architectures or any mix of more traditional technologies formerly considered incompatible. Holma and Toskala (WCDMA & UTMS Nokia Finland 2004) believe 1dB loss in CDMA and UTMS receiver sensitivity can mean as much as 11% loss of coverage. Feedback from network operators around the world suggests this may be a conservative estimate, but it is agreed that receiver sensitivity is a key element of network performance. This paper introduces using Passive Intermodulation (PIM) as an efficient and practical way to measure RF system linearity and detect non-linear elements that maybe limiting RF transparency at the BTS. Benefits of Transparency A transparent RF infrastructure should perform as closely as possible to an ideal RF conduit having no loss, perfect RL and complete linearity. Applying equally to both uplink and downlink, the benefits of achieving transparency include: • Maximum coverage and capacity performance. • Integration of merging technologies on a common feed system. • Lower cost of service. • An improved end user subscriber experience with less poor quality or dropped calls, leading to lower churn. In reality system performance is determined by: • Quality and performance of individual RF components used, and • Quality of the component installation. While the individual components will determine the RF performance limits, the system construction and integration quality will determine the RF transparency and whether optimum performance is ever achieved. Achieving Transparency The causes of poor transparency can include: • High Insertion Loss (IL). • High Return Loss (RL), and • In-band and out-of-band Passive Intermodulation (PIM). Traditionally we have had to rely on Return Loss (RL) and Distance to Fault (DTF) measurements/sweeps to determine infrastructure performance. Although these are an excellent measure of electrical ‘match’ in the RF chain, the operator can still be left with performance issues that result in dropped calls and reduce coverage and capacity. Other symptoms of poor transparency are: • Site alarms. • Parasitic noise rise, and • Desensitized or degraded receiver noise floor. Experience suggests RF interference being present as the result of infrastructure build quality or from an outside source. Spectral analysis can help, but often the cause is traced to RF system non-linearity, introduced by poor construction or component deterioration. This can be best confirmed by testing for the presence of PIM. Part 3 of this series of papers deals with PIM in more detail, however in summary it can generate potential receiver interference in proportion to the degree of system non-linearity at a given RF power, and therefore makes an ideal construction Quality Control (QC) performance metric. High Linearity = High Transparency High linearity => Low PIM => High construction quality The value of PIM testing PIM and DTF/RL are different but equally important tests. “Line sweeps”, or a resulting DTF plot and RL measurement are critical tests that cannot replace or be replaced by a PIM measurement. In isolation they present an incomplete analysis of how well the RF interconnection will support receiver performance. It is often found that DTF and RL test results appear acceptable, yet the PIM performance is inferior. RL and DTF sweeps alone can often be unsuccessful in uncovering non linearity faults due to: • The skills and experience needed to configure test equipment and interpret results. • Low RF power used during testing, or • The fault being able to be seen only with a PIM test. A network performance discussion with almost any maintenance team will reveal they are often involved in resolving unexplained receiver noise rise, or unreliable reception of signals from subscribers considered to be within the coverage ‘footprint’. PIM is generally present as the result of the physical condition of the RF infrastructure, and without adequate Transparent Infrastructure – (Part 2 of 6) Frequency transparent RF infrastructure is key to providing cost efficient communications in today’s busy spectrum. This paper explores the concept of transparency, benefits and methodology to confirm it has been achieved. Figure 1.
Transcript of Footprint Part 2 of 6 Transparent Infrastructure
FootprintChanging the RF cost performance paradigm
Further information visit our websiteswww.summitekinstruments.com
www.triasx.com
Page 2 - Footprint Two - 1
Introduction
Frequency transparency allows multiple RF signals to beoverlaid on common infrastructure without degradation ordistortion, by providing a linear RF transmission conduit,essential for spread spectrum architectures or any mix of more traditional technologies formerly consideredincompatible.
Holma and Toskala (WCDMA & UTMS Nokia Finland 2004)believe 1dB loss in CDMA and UTMS receiver sensitivitycan mean as much as 11% loss of coverage. Feedback from network operators around the world suggests thismay be a conservative estimate, but it is agreed thatreceiver sensitivity is a key element of network performance.
This paper introduces using Passive Intermodulation (PIM)as an efficient and practical way to measure RF systemlinearity and detect non-linear elements that maybelimiting RF transparency at the BTS.
Benefits of Transparency
A transparent RF infrastructure should perform as closelyas possible to an ideal RF conduit having no loss, perfectRL and complete linearity.
Applying equally to both uplink and downlink, the benefitsof achieving transparency include:
• Maximum coverage and capacity performance.• Integration of merging technologies on a
common feed system.• Lower cost of service.• An improved end user subscriber experience
with less poor quality or dropped calls, leadingto lower churn.
In reality system performance is determined by:• Quality and performance of individual RF
components used, and• Quality of the component installation.
While the individual components will determine the RFperformance limits, the system construction andintegration quality will determine the RF transparency andwhether optimum performance is ever achieved.
Achieving Transparency
The causes of poor transparency can include:• High Insertion Loss (IL).• High Return Loss (RL), and• In-band and out-of-band Passive
Intermodulation (PIM).
Traditionally we have had to rely on Return Loss (RL) andDistance to Fault (DTF) measurements/sweeps todetermine infrastructure performance.
Although these are an excellent measure of electrical ‘match’ in the RF chain, the operator can still be left withperformance issues that result in dropped calls and reducecoverage and capacity. Other symptoms of poortransparency are:
• Site alarms.• Parasitic noise rise, and
• Desensitized or degraded receiver noise floor.
Experience suggests RF interference being present as theresult of infrastructure build quality or from an outsidesource. Spectral analysis can help, but often the cause istraced to RF system non-linearity, introduced by poorconstruction or component deterioration. This can be bestconfirmed by testing for the presence of PIM.
Part 3 of this series of papers deals with PIM in moredetail, however in summary it can generate potentialreceiver interference in proportion to the degree of systemnon-linearity at a given RF power, and therefore makes anideal construction Quality Control (QC) performancemetric.
High Linearity = High Transparency
High linearity => Low PIM => High construction quality
The value of PIM testing
PIM and DTF/RL aredifferent but equallyimportant tests.
“Line sweeps”, or a resultingDTF plot and RLmeasurement are criticaltests that cannot replace or bereplaced by a PIMmeasurement. In isolation they present an incompleteanalysis of how well the RF interconnection will supportreceiver performance.
It is often found that DTF and RL test results appearacceptable, yet the PIM performance is inferior. RL andDTF sweeps alone can often be unsuccessful in uncoveringnon linearity faults due to:• The skills and experience needed to configure test
equipment and interpret results.• Low RF power used during testing, or• The fault being able to be seen only with a PIM test.
A network performance discussion with almost anymaintenance team will reveal they are often involved inresolving unexplained receiver noise rise, or unreliablereception of signals from subscribers considered to bewithin the coverage ‘footprint’.
PIM is generally present as the result of the physicalcondition of the RF infrastructure, and without adequate
Transparent Infrastructure – (Part 2 of 6)
Frequency transparent RF infrastructure is key to providing cost efficient communications in today’s busy spectrum. Thispaper explores the concept of transparency, benefits and methodology to confirm it has been achieved.
Figure 1.
FootprintChanging the RF cost performance paradigm
Further information visit our websiteswww.summitekinstruments.com
www.triasx.com
Page 3 - Footprint Two - 2
test equipment, detection can be difficult and timeconsuming with component substitution generally being alast but necessary resort.
Jumper cables and other RF components can be PIMsources, but site reports reveal that the most likely causeof poor BTS sector performance can be traced toconstruction practices. There is a growing pool of evidenceto suggest that a major contributor is badly terminated ordamaged and loose RF connectors, with case studiessupporting this presented in Part 5 of this series.
Analysis of PIM levels in the RF system are the simplest,quickest and most sure way of detecting physicalconditions likely to limit RF transparency. Triasx andSummitek Instruments are jointly promoting a range of portable test sets designed to be used at the BTS for thispurpose.
PIM - A dynamic test
IEC62037 is the international standard for measuring PIM.It provides a block diagram for the test apparatus requiredand defines the test parameters, scope and techniquesthat should be used during the process.
It is important to note the dynamic process (physicaltreatment) that should be applied as a necessary elementof this test process.
PIM is an absolute. The average performance of systemcomponents can not compensate for poor construction of the BTS infrastructure.
Measurement simplicity
Triasx/Summitek test sets have been designed for easyuse in measuring and recording the reflected PIM.
The equipment requires significantly less experience to setup and use than more familiar test sets used to take RLand DTF sweeps. Test levels for RF power, length of thetest, test tone frequencies and pass or fail criteria areeither set at the factory or by a supervising engineerbefore going onsite.
The test set operator need only connect correctly to thedevice or system to be analysed, turn on the RF power,dynamically load or impact the points of interest andrecord the result. It is very much a go no-go test.
The level of expertise required is limited to common work practices pertaining to health and safety, working with RFand general fault diagnosis and correction techniques.
Making a PIM test at the BTS
This is an intrusive test and requires the BTS radio to beisolated from the interconnection RF system. The test setis connected in its place, as shown in Figure 2, with eachfeeder system being tested in turn.
At the discretion of network management the antennamay be replaced by a low PIM cable load to preventunwanted or unauthorized radiation of the RF test power.
Figure 2.
Care must be taken to ensure that all new joints madebetween the test equipment and the system under test,are clean and adequately tightened to a torquerecommended by the component manufacturers in orderto avoid test errors. Once this has been achieved and thetest set configured as required, the test is commenced byswitching on the RF test power.
The PIM level is recorded while the whole system issubjected to a reasonable amount of physical impact andbending force as per IEC62037. This activity will result in apass or fail outcome.
In the event that the system or section under test fails,the test is repeated by breaking the RF interconnectionand moving the cable load forward toward an accessiblemid-point.
Test results are recorded as each PIM source is locatedand corrected.
This process is repeated until the whole RFinterconnection is certified as PIM source free. At thisstage the installation can be regarded as a physicallysound, linear and RF transparent conduit ready for RL andDTF sweeps to carried out and recorded.
Frequency transparent RF infrastructure under pins thecost performance paradigm for BTS now and into thefuture.
In part 3 of this series we will investigate the RFphenomenon of PIM, its causes and prevention.
This paper has been jointly prepared by Tr iasx Pty Ltd (Brisbane,
Australia) and Summitek Instruments (Denver, USA).
