Constant-Power Joint-Waterfilling for Coordinated Transmission

It is known that traditional water-filling provides a closed form solution for capacity maximization in frequency-selective block fading channels or multicarrier system with adaptive modulation. This waterfilling solution is derived from a maximum mutual information argument with single transmission point. Motivated by the new technology of coordinated multiple point (CoMP) transmission , a new closed form solution named as joint-waterfilling is derived for a multicarrier system with multiple coordinated transmission point (CTP) [2]. However, like traditional waterfilling, to utilize this joint-waterfilling in practical system, an important issue is the complex transmitter and receiver design, such as variable-rate variable-power MQAM modulation and coding. In this paper we investigate a new constant-power joint-waterfilling scheme for a coordinated transmission system, in which two constant power levels are used across a properly chosen subset of subchannels. A rigorous worst-case performance bound of the constant-power joint-waterfilling is given based on the emph{duality gap} analysis. Furthermore, a low-complexity constant-power adaptation algorithm is also developed. Numerical simulation result shows that the proposed constant-power joint-waterfilling has a negligible performance loss compared with true joint-waterfilling.