5 Gbit/s real-time processing using π/4-shift DQPSK for bidirectional radio-over-fibre system

Future converged fixed-mobile networks need high-speed radio links in deployment scenarios where fibre is not available or too expensive. In this paper, we present a field-programmable gate array (FPGA)-based real-time transmission system using standard 10G Ethernet interfaces. The system comprises two parallel complex-valued data channels in each direction. Standard FPGAs and low-cost multi-channel analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs) have been used. For enhanced robustness and optimal usage of the power amplifier, π/4-shift differential quaternary phase-shift keying (DQPSK) modulation is used. All digital signal processing routines for synchronization, equalization, forward error correction etc. have been fully implemented and tested. Using a protocol analyzer, error-free bidirectional transmission of Ethernet frames at 5 Gbit/s is verified. Error-vector magnitude (EVM) values below -30 dB indicate that even higher speeds could be realized.