Wireless networks can be measured by a number of metrics from different perspectives and spectral and energy efficiency are the most important two. Spectral efficiency, defined as the system throughput for unit bandwidth, is a widely accepted metric. For example, the peak value of spectral efficiency is always one key performance indicator of 3GPP evolution. For instance, the target downlink spectral efficiency of 3GPP increases from 0.05 b/s/Hz to 5 b/s/Hz as the system evolves from GSM to long-term evolution (LTE). In contrast, energy efficiency, has previously been ignored by most research efforts and has not been considered by 3GPP as an important performance indicator until very recently. As the green evolution becomes a major trend, energy-efficient wireless networking becomes more and more important. Unfortunately, spectral efficiency and energy efficiency are not always consistent and sometimes conflict with each other. Therefore, how to balance the two metrics needs careful study.

Spectral efficiency

Wireless communications use electromagnetic signals to carry information, and the signals are characterized by their frequency bands. The signals of each user occupies a frequency band, i.e. a set of radio frequencies. If multiple users are communicating over the same frequency simultaneously, they will interfere with each other. Therefore, different wireless access technologies usually use different frequency bands to avoid interfering with each other.