Tony S. Pollock

Introducing Space into MIMO Capacity Calculations

The large spectral efficiencies promised for multiple-input multiple-output (MIMO) wireless fading channels are derived under certain conditions which do not fully take into account the spatial aspects of the channel. Spatial correlation, due to limited angular spread or insufficient antenna spacing, significantly reduces the performance of MIMO systems. In this paper we explore the effects of spatially selective channels on the capacity of MIMO systems via a new capacity expression which is more general and realistic than previous expressions. By including spatial information we derive a closed-form expression for ergodic capacity which uses the physics of signal propagation combined with the statistics of the scattering environment. This expression gives the capacity of a MIMO system in terms of antenna placement and scattering environment and leads to valuable insights into the factors determining capacity for a wide range of scattering models.

Characterization of 3D spatial wireless channels

In this paper a novel three dimensional spatial channels model is developed to provide insight into spatial aspects of multiple antenna communication systems. The spherical harmonic representation of wavefields is used to decompose the spatial channel matrix into a product of known and random matrices where the known portion shows the effects of the physical configuration of antenna elements. The model supports any arbitrary antenna array configurations as well as any distribution of scatterers. Possible applications of the model and its usefulness are outlined.

Spatial decomposition of MIMO wireless channels

In this paper a novel decomposition of spatial channels is developed to provide insight into spatial aspects of multiple antenna communication systems. The underlying physics of the free space propagation is used to model the channel in scatterer free regions around the transmitter and the receiver, and the rest of the complex scattering media is represented by a parametric model. The channel matrix is separated into a product of known and random matrices where the known portion shows the effects of the physical configuration of antenna elements. We use the model to show the intrinsic degrees of freedom in a multiantenna system. Potential applications of the model are briefly discussed.

Antenna saturation effects on MIMO capacity

A theoretically derived antenna saturation point is shown to exist for MIMO systems, at which the system suffers a capacity growth decrease from linear to logarithmic with increasing antenna numbers. We show this saturation point increases linearly with the radius of the region containing the receiver antennas and is independent of the number of antennas. Using an alternative formulation of capacity for MIMO systems we derive a closed form capacity expression, which uses the physics of signal propagation combined with statistics of the scattering environment. This expression gives the capacity of a MIMO system in terms of antenna placement and scattering environment and show that the saturation effect is due to spatial correlation between receiver antennas.

Antenna saturation effects on dense array MIMO capacity

We investigate the behaviour of MIMO capacity when the size of the antenna array is constrained. By increasing the number of antennas within a small region in space the antenna array becomes dense and spatial correlation inhibits capacity growth. A theoretically derived antenna saturation point is shown to exist for dense array MIMO systems, at which there is no capacity growth with increasing antenna numbers. We show this saturation point increases linearly with the radius of the region containing the antenna array and is independent of the number of antennas.

Introducing 'space' into space-time MIMO capacity calculations: a new closed form upper bound

We present a new upper bound on capacity for multiple-input multiple-output (MIMO) wireless fading channels, which is more general and realistic than previous capacity expressions. By including spatial information at the antenna arrays, a closed form upper bound on capacity, which uses the physics of signal propagation combined with statistics of the scattering environment, is derived. This expression gave the capacity of MIMO system in terms of antenna placement and scattering environment and lead to valuable insights into the factors determining capacity for a wide range of scattering models.

Limits to multiantenna capacity of spatially selective channels

In this paper we present a new upper bound on the mutual information of MIMO systems. By characterizing the fundamental communication modes between two physical regions, we develop an intrinsic capacity which is independent of antenna array geometries and array signal processing, and depends only on the size of the regions and the statistics of the scattering environment.

Bounds on mutual information of Rayleigh fading channels with Gaussian input

The mutual information of a discrete time Rayleigh fading channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information. We specifically derive a lower bound for the mutual information of this channel when the input distribution is Gaussian. The bound is expressed in terms of the capacity of the corresponding non fading channel and the capacity when the perfect channel state information is known at the receiver

Gaussian inputs: performance limits over non-coherent SISO and MIMO channels

Performance limits of information transfer over a discrete time memoryless Rayleigh fading channel with neither the receiver nor the transmitter knowing the fading coefficients except its statistics is an important problem in information theory. We derive closed form expressions for the mutual information of single input single output (SISO) and multiple input multiple output (MIMO) Rayleigh fading channels for any antenna number at any signal to noise ratio (SNR). Using these expressions, we show that the maximum mutual information of non-coherent Rayleigh fading MIMO channels is achieved with a single transmitter and multiple receivers when the input distribution is Gaussian. We show that the addition of transmit antennas for a fixed number of receivers result in a reduction of mutual information. Furthermore, we argue that the mutual information is bounded by the SNR in both SISO and MIMO systems showing the sub-optimality of Gaussian signalling in non-coherent Rayleigh fading channels. Copyright

Upper bound on non-coherent MIMO channel capacity in Rayleigh fading

Limits of information transfer over a discrete time uncorrelated Rayleigh fading MIMO channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information (CSI) except the fading statistics. We show the capacity supremum with the receive antenna number at any SNR using Lagrange optimisation. Furthermore, we show the asymptotic capacity when the input power is large, and compare with the existing capacity results when the receiver is equipped with large number of antennas