Signal-to-interference-plus-noise ratio

Mathematical definition

The definition of SINR is usually defined for a particular receiver (or user). In particular, for a receiver located at some point x in space (usually, on the plane), then its corresponding SINR given by

${\displaystyle \mathrm{S}\mathrm{I}\mathrm{N}\mathrm{R}(x)=\frac{P}{I+N}}$

where P is the power of the incoming signal of interest, I is the interference power of the other (interfering) signals in the network, and N is some noise term, which may be a constant or random. Like other ratios in electronic engineering and related fields, the SINR is often expressed in decibels or dB.

Propagation model

To develop a mathematical model for estimating the SINR, a suitable mathematical model is needed to represent the propagation of the incoming signal and the interfering signals. A common model approach is to assume the propagation model consists of a random component and non-random (or deterministic) component.

The deterministic component seeks to capture how a signal decays or attenuates as it travels a medium such as air, which is done by introducing a path-loss or attenuation function. A common choice for the path-loss function is a simple power-law. For example, if a signal travels from point x to point y, then it decays by a factor given by the path-loss function

${\displaystyle \ell (|x-y|)=|x-y{|}^{\alpha }}$,

where the path-loss exponent α>2, and |x-y| denotes the distance between point y of the user and the signal source at point x. Although this model suffers from a singularity (when x=y), its simple nature results in it often being used due to the relatively tractable models it gives. Exponential functions are sometimes used to model fast decaying signals.

The random component of the model entails representing multipath fading of the signal, which is caused by signals colliding with and reflecting off various obstacles such as buildings. This is incorporated into the model by introducing a random variable with some probability distribution. The probability distribution is chosen depending on the type of fading model and include Rayleigh, Rician, log-normal shadow (or shadowing), and Nakagami.

SINR model

The propagation model leads to a model for the SINR. Consider a collection of ${\displaystyle n}$ base stations located at points ${\displaystyle x_1}$ to ${\displaystyle x_n}$ in the plane or 3D space. Then for a user located at, say ${\displaystyle x=0}$, then the SINR for a signal coming from base station, say, ${\displaystyle x_i}$, is given by

${\displaystyle \mathrm{S}\mathrm{I}\mathrm{N}\mathrm{R}(x_i)=\frac{\frac{F_i}{\ell (|x_i|)}}{\sum _{j\ne i}\left[\frac{F_j}{\ell (|x_j|)}\right]+N}}$,

where ${\displaystyle F_i}$ are fading random variables of some distribution. Under the simple power-law path-loss model becomes

${\displaystyle \mathrm{S}\mathrm{I}\mathrm{N}\mathrm{R}(x_i)=\frac{\frac{F_i}{|x_i{|}^{\alpha }}}{\sum _{j\ne i}\frac{F_j}{|x_j{|}^{\alpha }}+N}}$.

Stochastic geometry models

In wireless networks, the factors that contribute to the SINR are often random (or appear random) including the signal propagation and the positioning of network transmitters and receivers. Consequently, in recent years this has motivated research in developing tractable stochastic geometry models in order to estimate the SINR in wireless networks. The related field of continuum percolation theory has also been used to derive bounds on the SINR in wireless networks.