t-dab receiver testing
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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
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