> the only limit is Shannon's limit, but that's only concerned with data rate.
But that limit, the capacity, is also the maximum possible limit of information transfer on the channel. Modulation schemes don't increase it or change it, they just push the achievable information rate of the system closer to the capacity.
Modulation is definitely the wrong word in this context. I was thinking about channel coding and compression algorithms that increase the efficiency of the system without requiring additional capacity.
But that limit, the capacity, is also the maximum possible limit of information transfer on the channel. Modulation schemes don't increase it or change it, they just push the achievable information rate of the system closer to the capacity.