Fundamental properties of the EE-SE relationship

Energy efficiency (EE) has been recognized as another important metric as well as spectral efficiency (SE). This paper investigates the fundamental properties of the EE-SE relationship. The point-to-point MIMO transmission link is considered with different channel state information (CSI) conditions. Our analysis proves that the slope of EE-SE curves only depends on the circuit power and bandwidth when the SE approaches zero while it depends on the multiplexing gain when the SE approaches infinity. Reducing circuit power is helpful to improve EE in the low SE region while introducing more antennas benefits the EE in the high SE region. We also study the properties of EE-optimal points. It is found that reducing circuit power brings exponential EE increasing with linear SE loss at the EE-optimal points. More multiplexing gain will alleviate the speed of EE reduction with the increase of SE. More diversity gain brings a fixed EE gain.