Most wireless networks so far have been designed to maximize spectral efficiency (SE), which can be defined as the
overall system throughput per unit bandwidth in b/s/Hz. However, modern and future green wireless systems need to be
carefully designed so that energy efficiency(EE) is maximized. The EE in general term means that getting the same
task done [e.g., the same throughput and the quality of service (QoS)] with minimal energy. Therefore, we call a wireless
network green when it uses the least amount of power to transmit a given amount of data while satisfying specified
QoS requirements. The EE in b/J is defined as the ratio of the overall system throughput and the total transmit power.
Both instantaneous and average EE can be used depending on the scenario. However, these two design objectives
(SE and EE) may not always align together and quite often conflict with each other. A balance between SE and EE is important for the future green radio designs.