Channel Coding with Higher modulation

Just had a theoretical question. I'm reading a book on LTE, and I came across an introductory section that talks about using higher modulation with channel coding. I understand that usually using modulation schemes like 16 QAM and 64 QAM will require more Eb/No at the receiver end as compared to QPSK. But then they talk about the higher modulations being more efficient when combined with channel coding, that is, they require a lower Eb/No at the receiver as compared to QPSK. The example the books talks about is as follows:

"As an example, if a bandwidth utilization of close to 2 information bits per modulation symbol is required, QPSK modulation would allow for very limited channel coding (channel-coding rate close to 1). On the other hand, the use of 16QAM modulation would allow for a channel-coding rate of the order of one-half."

I'm confused about the second part of the statement. How do we get a channel-coding rate of one-half? Can anyone shed some light on this or perhaps point me to something I can read to get a better understanding?

• First guess without looking into it: those quadrature amplitude modulation techniques might require higher signal/noise ratio to resolve the different levels, but since they cram more bits in a symbol, you can insert a more advanced encoder before the modulator to lower the required signal/noise ratio for a fixed error rate. Commented Jun 28, 2015 at 0:14
• @MisterMystère I agree, but I'm confused as to how it works. :) They mention in the book that for instance in the 16QAM case and a requirement of 2 bits per modulation symbol, channel coding reduces the data rate, and because of that the bandwidth utilization also decreases to the level of an uncoded QPSK.
– Nick
Commented Jun 28, 2015 at 0:34
• Perhaps 16QAM and variants only become interesting if the signal/noise ratio is sufficiently improved - otherwise heavy encoding is required which may become as big as the actual data. QPSK is a rather legacy modulation now, the sharp improvement of the equipment certainly enabled QAM to develop (by improving the S/N). I think it depends largely on how big your packets are, but I find it hard to believe that the data doubles in size after encoding (I don't have years of experience on that particular topic though). Commented Jun 28, 2015 at 20:22