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Optimisation Of Your Deployed Digital PMR NetworksBarry Hack – Technical Specialist – Aeroflex, a Cobham company

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Introduction

• Many digital PMR networks are fully deployed and operational today– DMR, TETRA, P25, NXDN, dPMR– E.g. TETRA systems deployed in over 120 countries !

• Most networks are experiencing issues– Unacceptable call set up times, dropped calls, [perceived] coverage

issues, etc.

• Of course the problem may be a hand-held or vehicle mounted radio– Those issues are not covered in this presentation– One REAL example:

• TETRA radio in trunked mode, 13Hz frequency error• TETRA radio in direct mode, 990Hz frequency error !!!

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Introduction

• Drive testing or static monitoring can be used to evaluate issues– Use dedicated MS terminal based solutions

• E.g. Motorola Scout– Use industry standard test equipment and software

• E.g. Field Test 7 and 3550R, 8150 Radio Air Analyzer, etc.

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Introduction

• So how do we find persistent issues that affect our networks– Engineering teams can spend days and weeks with no success– “Expert consultants” can be employed to ‘optimize’ the network– Radio hardware can be replaced to try and find the issue through

‘substitution’

• Some examples are discussed here and how they might be solved…– All are based on real world scenarios– Note: The networks are ‘anonymized’

for security reasons

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Channel Loading

• Situation– Dangerously long call setup times, even if the traffic channels

appear to be lightly loaded– Up to 10 seconds to place an emergency call !

• Causes– Channel loading not distributed across available channels– Excessive GPS location reports being sent to the control centre,

overloading the control/traffic channels

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Channel Loading (continued)

MCCH/SCCH heavily loaded in relation to other channels during some hours… An additional SCCH would help

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MCCH TCH TCHSCCH TCH TCHSCCH TCH TCHTCH TCH TCH

Time

Carrier 1 Channel No. 3648

Carrier 3Channel No. 3650

Carrier 2Channel No. 3652

Frequency Example

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Call Setup Times

• Situation– Major incidents. Many people press the PTT at the same time,

some randomisation of access burst– Dangerously long call setup times, even if the traffic channels

appear to be lightly loaded

• Causes– Significant number of data collisions resulting in excessive call set

up times.

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Issues with Power Control Path Loss Settings

• Situation– Incorrect transmit power, resulting in interference or loss of

optimised talk time (battery life).– Radio downlink timeout. You can have a strong signal but bad

demod, which forces cell reselection

• Causes– Network “optimisation” of path loss settings have been incorrectly

chosen…

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Security Classes

• Situation– Radios which failed encryption check were allowed on a live police

network in clear mode– Major security risk using non encrypted voice/data

• Causes– The network had been configured to allow this!

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Ensure authentication is enabled !

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Cell Reselection Issues

• Situation– Reports of black spots, dropped calls, etc.– Drive testing and network monitoring could not detect the issue

• Causes– One cell was NOT broadcasting any neighbour cell information

due to bad network configuration– Radios could not move to a better cell and would drop the call

and then rescan and re-register

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Hysteresis/Threshold Not Optimized

• Situation– Excessive loading on the MCCH– Excessive cell reselection attempts

• Causes– Hysteresis parameter not optimised, typically 8dB– If too low or too high, could lead to situations described above

when the radio is blocked by user’s head, for example

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Interference Issues

• Situation– Intermittent network issues– Dropped calls– Failed registrations / location updates

• Causes– Neighbouring country was using some of the same channels

– Need to look for co-channel interference which can be hard to find using ‘normal’ RF analysers …

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Interference Issues (continued)

• Monitor BS sites along a country border. If neighbour uses TETRA as well, then there can be interference if the same frequencies are used. Some countries co-operate on planning, some do not….

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Mismatch in SwMI

• Situation– Conflicting broadcasts from neighbour cell(s)– BS says neighbour cell supports packet data, whereas the actual

neighbour cell says it doesn’t support packet data– Power setting mismatch between serving cell and neighbour cell

• Causes– Incorrect network configuration– Mismatch in SwMI

• SwMI - “Switching and Management Infrastructure”

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Mismatch in SwMI (continued)

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Receiver / Antenna Issues

• Situation– Difficulty in registering radios on to the network– Monitoring systems may suggest this is caused by interference or

poor signal reception– Good signal strength in cell area

• Causes– One of the receive antenna is damaged or broken– Link budget problem– This can be seen as continual MAC_ACCESS requests from the MS

to BS but no response from the BS

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Receiver / Antenna Issues (Continued)

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Frequency Error

• Situation– Radios dropping off network– Radios do not register

• Causes– Faulty GPS on the basestation– User radios operate out of [frequency] spec– Not always picked up by network monitoring tools

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Frequency Error (continued)

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Intermodulation

• Situation– Intermittent radio issues– Radio shows good signal strength from BS but poor comms

• Causes– Not always picked up by network monitoring tools– Degradation in cables, water ingress, loose connections causing

passive intermods– Tx intermods can cause issues with Rx blocking– Example: 3 BS freq with +12.5kHz offset but 8140 scanner shows

4 BS freq with one at -6.25kHz, this is caused by intermod issues with the 3 “real” carriers

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Conclusions

• [Almost] all live networks have issues !– Some are ‘fixed’ by drive testing, network tools, etc.– Engineers diagnose the WRONG causes– Many are unresolved ….

• Most tools cannot find these described issues– Costs time and resources– Puts users at risk

• Consequences– Engineers deployed for long periods of time without resolution of

the problem– Engineers are looking for the wrong cause– Test equipment needs to be ideally suited to the task

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Questions ?

• Barry Hack• Technical Specialist• Aeroflex, a Cobham Company

• +44 7767 391931• barry.hack@aeroflex.com• Skype: baz_aeroflex• www.aeroflex.com/ats

• PMR test products can be found here:• http://ats.aeroflex.com/products/radio-test-sets/land-mobile-radio-lmr

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