The digital data have to be transmitted using full width pulse over frequency selective channel with the bandwidth 20 MHz using OFDM technique. The propagation conditions allow considering the sub-channel as a flat fading in the range of 1MHz. The required total bit rate through the entire frequency selective channel is 100 Mbps. Find the appropriate subchannel modulation format (note: the number of constellation points in the modulation mapping scheme should be the power of two)?