Why does more complex modulation lead to higher data rate and PAPR ?

Hi,

We all know that more complex modulation leads to higher data rate and PAPR.

For example,

256 QAM has higher data rate and PAPR than 64 QAM.

But, why ?

Would you please tell me which keywords should I google ?

Thanks a lot~!

One term to look up is "symbol rate".
My simplistic way of looking at it is (for digital transmission) how many bits can you send at the same time.Standard UARTs have a single line and it can be either '0' or '1' so it can send 1 bit at a time. If there are 8 (or 10) bits to be sent to transfer a byte, then the symbol rate is 1/8th of the speed to send each bit.
If you can modulate the signal so that you can send all 8 bits at the same time, then the symbol rate is 8 times faster than 1 bit at a time.
Think of the '256' as representing the number bits being sent at the same time. Therefore the symbol rate is 4x higher than the '64' QAM.
The price you pay is that you generally need more bandwidth to achieve the high symbol rate, and the modulation and demodulation circuits/code gets more complex.
Susan

I fear the second paragraph confuses what's stated correctly in the first. Complex modulation is utilized to achieve a higher data throughput (= bit rate) with the same bandwidth (which has a fixed relation to symbol rate). According to Nyquist theorem, maximum symbol rate is 1/2 the bandwidth. Using complex modulation with more bits per symbol trades data rate against required signal to noise ratio.

Always my problem - a manager's review of me once included "why use 1 word when 20 will do"!
Susan