To get a reliable BER, we need a long sequence length. But a long sequence length is very time consuming. I usually use a 65536 bit sequence and run 5 iterations with a random seed in the PRBS generator, then average the 5 BERs. For samples per bit, since we are emulating the analog signal, ideally this value should be as large as possible. However, if you are using our new DSP component, according to the Nyquist theory, 2 samples/symbol is enough to recover the signal. Therefore the minimum samples per bit is 2*bit per symbol. Bit per symbol depends on the modulation format (QPSK: 2, 16QAM: 4, BPSK: 1).