T-DAB Receiver testingT-DAB Receiver testing

Richard Drinkwater Radiocommunications Agency

IntroductionIntroduction

Complaints received from DAB listeners about poor reception

Is this due to poor service coverage or poorly performing receivers?

Receiver tests performed to establish the sensitivity of a selection of current domestic receivers

TechnologyTechnology

DAB technology– Benefits

• More rugged transmission format• Much less spectrum usage – capable of

carrying 6 quality audio stereo programmes in 1.5MHz against 2.2MHz per programme for complete mainland UK FM coverage

– Drawbacks• More complicated (= expensive) receivers for

the listeners

UK Spectrum Occupancy FM v DABUK Spectrum Occupancy FM v DAB

BBC Radio 2 BBC Radio 3

88.0

90.2

92.4

MHz

2.2MHz 2.2MHz

BBC national radio

1.5MHz

Local radio

1.5MHz

Digital One (National6 stereo programmes + data

1.5MHz

221.

310

222.

810

MHz

223.

186

224.

686

224.

898

226.

398

BS 50248:2001 test criteriaBS 50248:2001 test criteria

BS EN 50248:2001, “Characteristics of DAB receivers”– Minimum power of -81dB(mW) for a BER

of 10-4 at the convolutional decoder output of the receiver (VHF and L-band)

MethodologyMethodology

Objective tests were not possible without connections to internal circuitry

Subjective tests were performed which established the minimum field-strength at which audio impairments (more than 1 click in 5 seconds) were heard

Test set-upTest set-up

GTEM! cell

DABreceiver

X

V volts

Measuring voltage V and height X enables a known field strength (V/m) to be established at the receiver under test

GTEM! cellGTEM! cell

Results on channel 12D Results on channel 12D (229.075 MHz)…(229.075 MHz)…

UEPprotectio

n

Bit ratekb/s

Sampling

ratekb/s

DABRx #1

DABRx #2

DABRx #3

DABRx #4

3 48 24 44.74 31.15 44.80 41.74

3 64 24 43.74 30.15 39.80 41.74

3 96 48 45.74 31.15 46.80 40.74

3 112 48 45.74 32.15 48.80 41.74

3 128 48 44.74 31.15 44.80 42.74

3 160 48 45.74 31.15 46.80 41.74

3 192 48 45.74 33.15 44.80 42.74

……graphicallygraphically

Audio break-up Field Strength

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

48 64 80 96 112 128 144 160 176 192

Audio bit rate

dB

µV/m

DAB # 1 DAB # 2

DAB # 3 DAB # 4

which means…which means…

For a field strength of 45dBµV/m, the receiver input power at 222MHz would be-77dBm if the receiver had a 50Ω input impedance, and a dipole antenna

Tested receivers did not, and the standard is written in antenna port power terms, so it is impossible to equate the tests with the standard

Receivers testedReceivers tested

DAB #1– Miniature personal receiver using earphone

lead as antenna DAB #2

– Portable receiver with rod antenna DAB #3

– Portable stereo unit with rod antenna DAB #4

– Miniature personal receiver using rod antenna

Service planning criteriaService planning criteria

National DAB services– Edge of service area is considered to be when

field strength is 58dB(µV/m) at 10m above ground level for 99% locations, 99% of the time

Field strength at 1.5m agl is very approximately 10dB lower than at 10m agl in open terrain

In a single frequency network, other transmitters within about 70km (246us) add to the signal available to the receiver

Portable equipmentPortable equipment

Inefficient antennasLow antenna heightBody shieldingBuilding shielding and reflections

– Shielding attenuates the signal– Local reflections tend to add to the

signal received

Analysis of test methodAnalysis of test method

Strengths– Straightforward, repeatable method

using GTEM! cellWeaknesses

– Subjective assessment – No multipath reception distortion– Incomparable with BS test method

Some thoughtsSome thoughts

Assuming a field strength at the edge of the service area of approximately 48dBµV/m at 1.5m agl, it would appear that the tested receivers would work in free space conditions. Due to shielding effects there is probably inadequate margin in sensitivity (in 3 out of the 4 tested receivers) for satisfactory reception under everyday usage conditions.

Possible solutionsPossible solutions

Very low power fill in transmitters– Relatively inexpensive solution

• No international clearance required

Improve receiver antenna performance– Particularly the match to input circuitry

ContactsContacts

richard.drinkwater@ra.gsi.gov.uk richard.drinkwater@ofcom.org.uk

RTCGRadiocommunications AgencyWhyteleafe HillWhyteleafeSurreyCR3 0YY +44 (0)20 8655 8300