2. A communication system uses OFDM modulation with the following parameters: 128 subcarriers (SCs), modulation formats of 16-QAM, 32-QAM or 64-QAM, and a symbol duration of 50ns. Determine: a) The OFDM symbol rate. b) The gross signal bit rates if all subcarriers use 16-QAM modulation. c) The gross signal bit rates if all subcarriers use 64-QAM modulation. d) The gross signal bit rate if the three modulation formats are allocated as follows. 90 SCs at 16-QAM, 20 SCs at 32QAM, 10 SCs at 64QAM and 8 SCs unused. [1] [2] [2] [3]