Question

How to calculated bandwidth of qpsk from sampling rate?

Answer 1

By bandwidth, do you mean RF channel width? Because in WiFi, that's always 20 MHz, or multiples of 20 MHz if your WiFi supports carrier aggregation.Since you mention specifically 54 Mb/s, and 2.4 GHz, it sounds like you're talking about 802.11g. That would not use carrier aggregation, so the answer is 20 MHz channel width.To achieve 54 Mb/s, 802.11g would use the highest OFDM constellation, in this case 64-QAM. So what you might be able to calculate is the total symbol rate, given that each symbol carries 6 bits worth of data. But the channel width is predetermined.