The modulation efficiency in bit/s is the gross bit rate (including any error-correcting code) divided by the bandwidth.
For example, in a cellular telephone network with frequency reuse, spectrum spreading and forward error correction reduce the spectral efficiency in (bit/s)/Hz but substantially lower the required signal-to-noise ratio in comparison to non-spread spectrum techniques.
As discussed below, a more relevant measure for wireless networks would be system spectral efficiency in bit/s/Hz per unit area.
However, in closed communication links such as telephone lines and cable TV networks, and in noise-limited wireless communication system where co-channel interference is not a factor, the largest link spectral efficiency that can be supported by the available SNR is generally used.
It is a measure of the quantity of users or services that can be simultaneously supported by a limited radio frequency bandwidth in a defined geographic area.
[1] It may for example be defined as the maximum aggregated throughput or goodput, i.e. summed over all users in the system, divided by the channel bandwidth and by the covered area or number of base station sites.
If it is defined as a measure of the maximum goodput, retransmissions due to co-channel interference and collisions are excluded.
The system spectral efficiency of a cellular network may also be expressed as the maximum number of simultaneous phone calls per area unit over 1 MHz frequency spectrum in E/MHz per cell, E/MHz per sector, E/MHz per site, or (E/MHz)/m2.
Examples of predicted numerical spectral efficiency values of some common communication systems can be found in the table below.