Tighter low bound of achievable rate in downlink multiuser MIMO with imperfect CSIT

One of the challenges to implementing downlink multiuser multiple-input multiple-output in practical systems is to overcome the detrimental impact of the imperfect channel state information at transmitter (CSIT). Researchers have derived a low bound to describe the imperfect CSIT´s impact on the achievable rate. However, this low bound is not tight, especially when the quality of CSIT is poor. A tighter low bound using the exponential integral is proposed. A strict proof is given and the simulation results verify the tightness.