I have optical OFDM baseband TX and RX implemented in FPGA. The design is tested using one FPGA board in loopback connection from DAC to ADC via coaxial cable. It works fine, the BER is about 1.10^-8 for QAM-16 modulation. The ADC output is captured, and looks like this:
The amplitude of the OFDM signal is reduced, so the synchronizer (using training symbol) fails to detect the start of symbol.
- DAC sampling rate: 50 MHz, resolution 14-bit
- Analog LPF (after DAC): 50 MHz
- Analog LPF (before ADC): 50 MHz
- ADC sampling rate: 50 MHz, resolution 14-bit
- What is the mistake in design, is it because of the ADC sampling rate, which is equal to the OFDM bandwidth?
- When sampling OFDM signal, should I use ADC sampling rate larger than 2x bandwidth (nyquist)?