Spectral Efficiency

Spectral efficiency is a measure of the performance of channel coding methods. It refers to the ability of a given channel encoding method to utilize bandwidth efficiently. It is defined as the average number of bits per unit of time (bit rate) that can be transmitted per unit of bandwidth (bits per second per Hertz). The term channel encoding refers to the procedure of mapping a bit stream to an analog signal,which can be transferred through a physical channel and later, after reception, decoded to yield the original bit stream, as shown in Fig. 1. The received signal may be distorted in various ways and corrupted by noise and other interference on the channel. This can cause the bit stream at the output of the channel decoder to be different from the bit stream at the input of the channel encoder. The ratio of erroneously received bits to all transmitted bits is called the BER and is used as a measure of performance. A small BER is acceptable in practical systems. The value of acceptable BER varies from 10 10 to 10 1 depending on the system concerned. The transmitted analog signal occupies a certain bandwidth (frequency range), which depends on the bit rate and on the way that the binary sequence is mapped to the continuous signal, i.e., on the channel encoding method. The ratio of bit rate to bandwidth is called spectral efficiency. Spectral efficiency cannot be made arbitrarily large. The maximal spectral efficiency depends on the power of the transmitted signal, on the acceptable BER, and on the characteristics of the channel (distortion, noise, interference). The coding method that achieves maximal spectral efficiency on a given channel is considered optimal for this channel.