Communication system performance: achieving the ultimate information-theoretic limits?

The role of information theory in communications is considered, the former giving the ultimate limits and sometimes showing how to approach them. A survey of the development of telephone channel modems, where the capacity is practically achieved, is presented. As a contrast, an account of multi-antenna systems is included, where a long journey to achieve capacity has just begun. A very important characteristic of multi-antenna systems is stressed: the capacity increases almost linearly with respect to the transmitted power as the number of antennas increases, keeping the corresponding bandwidth constant.