In the wireless system, the physical medium corresponds to free-space in which the electromagnetic wave propagates. As in any electrical/electronic system, the wireless signals are subjected to corruption by the inevitable presence of noise at the transmission and reception ends as well as along the transmission medium.
Apart from the device/system based electronic noise (such as thermal noise) corrupting the signal, certain characteristics of EM propagation in the mobile environment would also impair the signals under transmission. Such impairments arise from signal fading due to scattering, reflection, and refraction effects that the EM wave may face during propagation and attenuation of EM energy as a result of absorption by rain, snow etc.
With the result, for robust implementation of wireless communication, the receiver technology is being trimmed continuously through available techniques and devices. Thus, all along the passage of information transfer, from the transmitter through the channel to the receiver, there exists a host of noise sources, which may introduce undesirable effects to the signal being transported.
Generally classified as noise sources, these are categorised into two versions in reference to the wireless communication systems. They are:
- Additive noise sources resulting from electronic parts of the transmitter and receiver and due to interference from extraneous electromagnetic effects.
- Multiplicative noise due to nonlinear processes encountered by the electromagnetic wave propagation across the channel.
As indicated above, exclusive to wireless communication systems and the associated electromagnetic wave propagation, the signal along the channel may also suffer a loss in its intensity along the path. It could also be subjected to fading and shadowing arising from the reflection, refraction and diffraction effects encountered by the electromagnetic wave.
These phenomena could significantly influence the signal intercepted at a mobile unit. As the mobile unit traverses its path, the propagation characteristics and interference effects could change dynamically due to the mobility involved. Corresponding considerations find due place in the studies devoted to modern wireless communication systems.
In short, the “genesis” of noisy wireless communication is as follows: “In the beginning of wireless, there was just ‘noise’, and life was simple. Engineers soon called this noise additive white Gaussian noise (AWGN), but even this title was not enough to capture all the idiosyncrasies of a real world RF channel.
Then, engineers, scientists, and mathematicians got involved, and they soon gave names to more subtle effects, such as Rayleigh fading, Ricean fading, impulse noise, cyclostationary noise, and inter-symbol interference, to name just a few of the newer noise-family members. All these noises were in addition to basic channel difficulties of signal attenuation and loss”.
Technological advances and consumer demands are changing the landscape of wireless communications. At present, commercially available wireless mobile communications transmit at rates up to 2 Mbps. At high data rates, radio channels are impaired and need to be reversed at the receiver to realise an acceptable BER. Such impairments, as indicated before, are mainly due to multipath fading.
The trend in the technology is to enhance the bit rate, reduce the multipath fading induced impairments and facilitate mobility enhanced applications including broadband and multimedia services.