Bitfehlerquotenmessung mittels Software - Bit Error Ratio Measurement by Software

Kurzfassung

The use of Free-space optical communication for telemetry downlinks from satellites in the low Earth orbit (LEO) regime , especially ones using optical on-off keying (OOK), is becoming increasingly popular as it offers very high data rates (DRs). Free-space optical communication is advantageous towards radio frequency (RF), because it is barely limited in spectrum, offers increased security by using a narrow laser beam and is more cost efficient due to lower payload weight and power consumption. To qualify receiver front ends (RFEs), the bit error ratio (BER) is measured, but hardware (HW) test devices have proven to lack the capability to measure high bit error ratios, especially if they have to recover the clock from the received data. During optical LEO downlinks (OLEODLs) no additional clock signal is available and high BERs are expected, rendering BER measurements of the transmission channel using hardware test devices impossible. Another issue of HW-bit error ratio testers (BERTs) is, that their input filter is not adopted to the bandwidth of the tested receiver. This thesis shows the implementation of a software to measure the BER of serial bit streams from sampled waveforms with the goal to overcome these problems. After covering the theoretical background of BERTs and estimating the Doppler shift of data rates during OLEODL, the detailed requirement for this software are presented and the concept of the setup is shown. In addition to the software, that reads different waveform file formats, a bit recovery, a pattern synchronization and an error detector are implemented, which are the main components of a BERT. Two different algorithms for bitrecovery are implemented and their performance when used in the optical RFE testbed is compared. A pseudorandom bit sequence (PRBS) generator is created to synthesize test sequences that can be compared with the received data and a moving average filter is implemented so the sampled signals can be filtered before these are further processed. To use the software in a live scenario like a hardware BERT, the BER measurement software is included in multi-threading structure, which allows the user with a console interface to set test parameters and start and stop the measurements. Finally the software is validated by comparing its results to measurements with the hardware BERT and it is shown, that the software has an advantage regarding the ability to apply digital filters and to recover bits without an additional clock signal. Test of the software were performed by using DRs of 100 Mbit/s and 300 Mbit/s because of the used RFE’s bandwidth limit of 170 MHz, but with the used oscilloscope DRs up to 2.5 Gbit/s would be possible. As data rates of up to 10 Gbit/s are targeted for the next generation of receivers, these high data rates should still be evaluated, which has not yet been done due to a lack of corresponding hardware.