SOURCEFORGE » RALF » 2016 Channel emulation for satellite communications

h1. Performance analysis and results

Several tests are carried out in order to evaluate the performance of the developed channel emulator. Firstly, it must be verified that the required Eb/No level is achieved at the demodulator output. Secondly, the time delay of transmitted packets must be measured to guarantee compliance with the specified delay. 

h2. Noise measurements

Consider the test bench shown in Figure 1 in order to measure Eb/No level during the transmission. The hub modem sends a QPSK modulated signal to the station modem through the satellite channel represented by the VST. Then, the channel-applied signal is received at the demodulator input of the station modem where the Eb/No measurement is performed. The carrier-to-noise power can be obtained in two different ways by using the monitoring tools of the modem or the spectrum analyzer. 

p=. !{width:50%}Noise_test_bench.png!

*Figure 1. Test bench used to measure Eb/No*

A first approach consist of using the monitoring tools included in the modem configuration interface. Figure 2 shows the measured carrier-to-noise power over 2 minutes for 0 dB, 5 dB, 10 dB, 15 dB, 20 dB and 50 dB. As can be observed the obtained values are approximately the same as the selected Eb/No values in the front panel of the emulator. It can be noticed that for 0 dB, the demodulator is not capable to demodulate the QPSK signal and recover the received IP packets as shown in Figure 3 where the IP throughput for 5 dB and 0 dB are plotted.

Furthermore, as can be seen in Figures 2(e) and 2(f), the obtained carrier-to-noise value is around 16.5 dB while the required Eb/No values are 20 dB and 50 dB. Therefore, it means that there is an upper bound in the Eb/No that can be generated by the channel emulator. Since the modems are connected through an active device, the noise power level is increased according to the VST noise figure and, therefore, the carrier-to-noise power is degraded. 

p=. !{width:60%}Modem_carrier_to_noise_ratio.png!

*Figure 2. Eb/No measurements obtained from the modem interface. Selected Eb/No value:(a) 0 dB;(b) 5 dB;(c) 10 dB;(d) 15 dB;(e) 20 dB and (f) 50 dB*

p=. !{width:70%}Ip_throughput_for_5dBand0dB.png!

*Figure 3.  IP throughput for 5 dB and 0 dB*

In order to analyze the impact of the channel emulator on the noise power level, the following measurements are performed: the carrier-to-noise power ratio when the modems are directly connected between them and the carrier-to-noise power when the VST is included in the transmission chain but without performing any signal processing, i.e., adding noise or including time delay.

As can be seen in Figure 4(a), the carrier-to-noise ratio is around 22 dB when the VST is not connected, which means that the channel emulator is constrained to produce a maximum Eb/No value below 22 dB. On the other hand, even if the channel emulator is not adding noise to the QPSK signal, the carrier-to-noise is reduced by 5.5 dB as can be observed in Figure 4(b). Therefore, the channel emulator is limited to deliver a signal whose Eb/No value is below 16.5 dB.  

p=. !{width:70%}Degradation_introduced_by_the_emulator.png!

*Figure 4. Eb/No degradation introduced by the emulator*

In addition, it can be seen how the transmitted signal is affected by the emulator channel by using the modem interface which also provides information about the power in dBm of the received signal. The received power level when the channels are directly connected is equal to -44 dBm (Figure 5(a)) while it is equal to -62 dBm (Figure 5(b)) when the VST is included in the transmission chain without performing any signal processing. In Figure 5(c), it can be observed the load effect of the spectrum analyzer on the received power, the fact of disconnecting the spectrum analyzer from the test bench generates an increase of 6 dB in the received power.

p=. !{width:50%}Load_effect.png!

*Figure 5. Output power degradation introduced by the emulator*

p=. !{width:50%}Input_demodulator_spectrum.png!

*Figure 6. Signal-to-noise power measurements carried out with the spectrum analyzer*

p=. !{width:50%}Degradation_in_the_output_level.png!

*Figure 7. Signal power degradation due to the emulator*

h2. Delay measurements

p=. !{width:50%}Delay_test_bench_2.png!

*Figure 8. Test bench used to measure a reference round trip time*

p=. !{width:50%}Delay_test_bench.png!

*Figure 9. Test bench used to measure the total round trip time*

p=. !{width:50%} Total_delay_without_emulator.png!

*Figure 10. Round trip time measurements using the ping command (10 packets of 1B and 1500B, respectively) when the emulator is not connected*

p=. !{width:50%}Delay050100150200_1bytes.png!

*Figure 11. Round trip time measurements using the ping command (10 packets of 1B) when the emulator is connected for different delays values*

p=. !{width:50%}Delay050100150200_1500bytes.png!

*Figure 12. Round trip time measurements using the ping command (10 packets of 1500B) when the emulator is connected for different delays values*