Thanks all for help and guidance.
I have analysed and found following things….
1. Increasing Sequence length does not make the BER graph smooth (you can yourself observe it in attached file.)
2. Making iterations too high or low also does not effect the BER graph smooth. I think logically it does not make sense.We can have even more than 100 iterations for getting very precise and smooth graph.
3. What I really found working is just disabled “Generate random seed” in components properties which then gave smooth BER graph.
4. One important Thing: If you make multiple iterations for a single set-up without sweeping values you will see different BER value for each iterations and this was making the BER graph fluctuating and this issue is resolved by disabling “Generate random seed” in components properties.
Thanks