Previous Section 3.17     Index Index Page     Reports Lab Reports Index

3.18 BER vs Carrier to Noise

The BER for varying C/N threshold levels was plotted for both DTTB systems operating through the test rig transmitters and the link. The plots obtained from this measurement give a visual representation of the error failure characteristic of each of the digital systems.

Due to the different error rate measurement methods in the DTTB systems, we cannot make a direct comparison of the curves. The COFDM BER curve is measured before the reed solomon error correction. If it had been possible to do this test after RS error correction a steeper slope similar to 8-VSB would have resulted. Because of this difference, distinct measurement parameters were required for the two modulation systems during this test.

The measurement was performed as follows:

  1. The DTTB receiver had its signal supplied from the test rig.
  2. A Threshold Carrier to Noise level measurement (Section 3.5) was made to determine the threshold error level due to white noise.
  3. The noise level was then increased above the threshold level by:
    5.0 dB for COFDM and 3.0 dB for 8-VSB
  4. The Bit Error Rate was measured in 109 data samples for accuracy.
  5. The noise level was decreased in the following increments.
    0.5 dB for COFDM and 0.1 dB for 8-VSB
  6. The measurement was repeated (steps 4-6) until the noise level was reduced to the following levels below the original threshold.
    5.0 dB for COFDM and 0.5 dB for 8-VSB.
  7. The data was then plotted.


Figure 3.18.1 - BER vs System C/N Threshold for COFDM V1 Software

Figure 3.18.1 shows the performance of the COFDM system with the various devices tested during the main COFDM laboratory testing phase in April. The Harris and NEC transmitter performance is close to that of the test rig alone. The 600W Harris curve shows some degradation in performance which is probably due to the pre-correction not being optimum at this power level. The link echo signal shows a significant C/N penalty. Variation of the guard interval was found to have no effect on the curves measured. The NEC transmitter operating at low power was used as the source for both the Coax (75W) and Link (3W) echo systems.

In May an upgrade to the system software was supplied by NDS to fix an interleaver problem. The system equalisation was left at version 1.0 however a 0.3 dB increase in the system C/N level was observed.

Figure 3.18.2 shows measurements of the BER vs C/N which were conducted after this upgrade. The original test rig curve is provided for reference. A 0.4 dB change in C/N at the failure point is observed for the coax system direct path, which ideally should match the original test rig measurement. It is noted that when the mixer intermodulation, at the link translator, was fixed the COFDM BER performance degraded slightly. It is thought that this result may be experimental error due to the long period which elapsed between these two measurements and the change in system software.


Figure 3.18.2 - BER vs System C/N Threshold for COFDM V2 Software


Figure 3.18.3 - BER vs System C/N Threshold for 8-VSB

Figure 3.18.3 shows the performance of the 8-VSB system with the various system configurations. A slight degradation in the performance is observed through the coax delay while the degradation due to the link in it’s early state (mixer intermodulation present) is very significant. Once the link system was improved the link shows a similar slope to the transmitted signal with an additional 1 dB C/N margin.

Next Section 3.19     Index Index Page     Reports Lab Reports Index