Capacity of Mobile Communication Systems

Background

The development of mobile communication systems for multiple-input multiple-output (MIMO) wireless channels is currently an extremely active area of research. MIMO systems are widely considered to be the most promising avenue for significantly increasing the bandwidth efficiency. By applying multiple antennas at both the transmitter and receiver side, one can obtain a much higher capacity than with single antenna systems. Multiple antennas at a portable device imply that the antenna spacing has to be small. This implies that the signals entering the different antennas will be correlated and the performance will degrade. In general, the number of antennas as well as the propagation conditions determine the capacity of a MIMO system. It is therefore of great importance to develop MIMO channel models for different propagation scenarios to enable the analytical investigation of the channel capacity. MIMO channel models are also important for the optimization, test, and performance evaluation of space-time coding schemes as well as space-time processing techniques in general.

The figure below shows a MIMO system with M and N antennas at the transmitter and the receiver, respectively. The so-called channel matrix is denoted by H=[ h _nm ], where h _nm is the channel coefficient from the m-th transmit antenna to the n-th receive antenna ( n = 1, 2, ..., N and m = 1, 2, ..., M). For example, if the vector x= [ x ₁ ,…, x _M ] ^T is transmitted, then the received vector r = [r ₁ ,…,r _N] ^T is given by

r = Hx + n

where n is a vector with N additive noise components. For a narrowband MIMO channel, the capacity C, in bits/s/Hz, is

where det(∙) is the determinant, I _k is the k × k identity matrix, ρ denotes the signal-to-noise ratio, and (∙) ^H designates the complex transpose operator.

Planned Research

The main task of this project is to study the capacity of MIMO channels in different propagation environments, for example, the one-ring scattering model and the two-ring scattering model. The development of a reference model based on ideal assumptions concerning the propagation environment, is the starting point for the development of an efficient simulation model enabling to study the time variant behaviour of the MIMO channel capacity. This allows a comparison between the statistical average and the time average of the capacity. The simulation model to be developed takes the spatial, the temporal, and the frequency correlation properties of the MIMO channel into account. In many previous works, it has been assumed that the fading signals at the antennas of the receiver are independent, identically distributed. However, as mentioned above the assumption of independence is in general not the case for reasons of insufficient distance between the antenna elements. Thus, the influence of various model parameters on the capacity will be studied. Non-isotropic scattering around the receiver or the transmitter will be taken into account.

Fig. 1: Block diagram of a MIMO system.

Share: