T-DAB Receiver testing

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T-DAB Receiver testing

• Richard Drinkwater Radiocommunications Agency

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Introduction

• 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

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Technology

• 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

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UK Spectrum Occupancy FM v DAB
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BS 502482001 test criteria

• BS EN 502482001, 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)

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Methodology

• 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

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Test set-up
Measuring voltage V and height X enables a known
field strength (V/m) to be established at the
receiver under test
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GTEM! cell
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Results on channel 12D (229.075 MHz)
UEP protection Bit rate kb/s Sampling rate kb/s DAB Rx 1 DAB Rx 2 DAB Rx 3 DAB Rx 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
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graphically
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which means

• For a field strength of 45dBµV/m, the receiver
  input power at 222MHz would be-77dBm if the
  receiver had a 50O 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

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Receivers 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

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Service 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

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Portable equipment

• Inefficient antennas
• Low antenna height
• Body shielding
• Building shielding and reflections
• Shielding attenuates the signal
• Local reflections tend to add to the signal
  received

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Analysis of test method

• Strengths
• Straightforward, repeatable method using GTEM!
  cell
• Weaknesses
• Subjective assessment
• No multipath reception distortion
• Incomparable with BS test method

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Some 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.

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Possible solutions

• Very low power fill in transmitters
• Relatively inexpensive solution
• No international clearance required
• Improve receiver antenna performance
• Particularly the match to input circuitry

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Contacts

• richard.drinkwater_at_ra.gsi.gov.uk
• richard.drinkwater_at_ofcom.org.uk
• RTCG
• Radiocommunications Agency
• Whyteleafe Hill
• Whyteleafe
• Surrey
• CR3 0YY 44 (0)20 8655 8300