Application Note

PTC Testing using Anritsu

LMR Master

™

S412E

Background

dio manufacturers have already started to ship their new ITC-R Positive Train Control (PTC) radios, and the

first PTC digital radio communications systems are now being deployed and tested in the field. Information is

beginning to be gathered on how these new PTC signals propagate, and an understanding of PTC system

performance is beginning to emerge. Advanced propagation prediction software has been used to estimate

coverage, and verification work is under way using the first deployed base stations to measure and confirm

effective RF propagation.

PTC radio systems project commissioning teams from several different railroads are using their own individual

approaches, with no final testing procedures defined yet. The focus on PTC RF testing has been mostly on

verifying through RSSI (Received Signal Strength Indication) values that the coverage for each base station is

similar to what the propagation software predicted. Typically, previously installed base station radios are

used to transmit and receive PTC signals, and sometimes locomotive and/or wayside radios are also used in

the field to verify coverage to and from the base stations.

This document describes and proposes an improved PTC field testing methodology which will be more

precise, time saving, and cost effective. The improved methodology uses features already available in

instruments such as the Anritsu S412E LMR Master™, and different configurations of instruments and PTC

radios. Anritsu’s LMR Master includes optional support for ITC-R PTC testing with a PTC Signal Generator, PTC

Signal Analyzer, Spectrum Analyzer, and the ability to coverage map RSSI, BER, and Error Vector Magnitude

(EVM) for PTC systems.

This document consists of four sections: Testing Parameters, Field Testing, Comments and Suggestions, and

Test Bench pictures.

Testing Parameters – Section 1.0

This section is focused on the parameters being measured during PTC wireless path testing in the field,

what is normally being measured and how it can be improved. This section also describes the PTC testing

features available on the Anritsu LMR Master. Several recommendations and testing tips are offered to

improve the quality and accuracy of the testing and to fully utilize the features of the PTC Analyzer/Generator

of the LMR Master.

Current field testing methodologies may address one or more of the following:

• Coverage,currentlymeasuredinRSSI

• Variablenoisefloorandde-sensinginsomeareas

• On-channelinterferencefromotherradios

• Near-channelinterferencefromotherradios

• Intermodulationproductsfromotherradiosonseveralfrequencies

• Randomnoisewithdifferentpatternsfrommultiplesources

• Multipathfadingcausedbyreflectionsonstructures,terrainandlargevehicles

• Otherinterferenceincludingradars,powerlinesandbroadcastingsystems

ThemainparametercurrentlymeasuredisRSSI.Oneormoreoftheotherparameterslistedabovemay

also be measured and recorded, but as of today there is no process in place to correlate how one or more

interfering parameters might affect RSSI.

Field Testing – Section 2.0

This section examines methodologies currently employed for PTC wireless path field testing, and provides

several recommendations to change and improve the entire testing process, making it much faster and

significantly reducing the cost of field testing work. Some semi-automated testing methods are proposed

and described.

2.1 RSSI Testing (Current Testing Method)

RSSI provides a signal level resulting from the sum of multiple components that include the original

signal received directly from the transmitter, other multipath signals produced by one or more

reflections/bounces of the original signal on structures, terrain or other physical objects, plus any other

signals from other radios, or products from different types of interference such as intermodulation or

equipment noise, plus the ambient RF noise floor, which in densely populated areas may exceed –95 dBm.

An RSSI value can be estimated/extrapolated taking into consideration all the external factors, but it

would not be very accurate or useful to determine how the PTC wireless path will be operating in the

real world. There could be a section of 60 miles of track with a noise floor below –120 dBm, and almost

perfect coverage of every foot of track with RSSI levels between –75 and –90 dBm, and there could still

be large holes in coverage caused by multipath and other factors that degrade the PTC wireless path

performance.

RSSI measurements only provide a small, one-dimensional view of the overall system performance. RSSI

cannot discriminate between desired signals and undesired RF intereference, thus testing which relies

solely on RSSI can lead to false consclusions, poor system performance, and added expense to diagnose

and resolve failures. RSSI can be used as a secondary source for reference, but the primary testing has to

be done using a more robust, precise testing method measuring the actual packet success performance

of the PTC wireless path under test. This performance varies and so even with an adequate RSSI level

there could be communications problems in the PTC wireless path.