Dimitrios Kaltakis

Information theoretic capacity of cellular multiple access channel with shadow fading

In this paper, we extend the well-known model for the Gaussian Cellular Multiple Access Channel originally presented by Wyner. The first extension to the model incorporates the distance-dependent path loss (maintaining a close relevance to path loss values in real world cellular systems) experienced by the users distributed in a planar cellular array. The density of base stations and hence the cell sizes are variable. In the context of a Hyper-receiver joint decoder, an expression for the information theoretic capacity is obtained assuming a large number of users in each cell. The model is further extended to incorporate the log-normal shadow fading variations, ensuring that the shadowing models are fairly comparable to the free space model. Using these fair models the effect of the shadow fading standard deviation on the information theoretic capacity of the cellular system is quantified. It is observed that a higher standard deviation results in lower capacity if the mean path loss is appropriately adjusted in order to model the mean loss due to the physical obstacles causing the shadow fading. The results validate that larger cell sizes and a higher standard deviation of shadowing (with appropriately adjusted mean path loss) results in lower spectral efficiency.

Information theoretic capacity of Gaussian cellular multiple-access MIMO fading channel

Higher spectral efficiency can be achieved by exploiting the space dimension inherent to any wireless communication system using multiple receiver and multiple transmitter antennas (MIMO). There are several results that provide closed form solutions for acellular system with a single antenna at each base station and each user terminal. Results are also available for the single cell case with MIMO. A cellular system with multiple antennas at the transmitter and the receiver nodes has not been investigated to obtain a closed form solution for the capacity limit. The main information theoretic theorems are not directly applicable to this system because of the form of the channel matrix of such a system. In this paper we extend the well known Wyners model to a MIMO cellular system. It is observed that the achievable rate is bound by an upper limit and lower limit corresponding to two extreme fading conditions: channel with Rayleigh fading and with no fading. The analytical results are verified using Monte Carlo simulations. The analysis provides the insight that for a cellular system, increasing the number of transmitting antennas is not beneficial to increase the achievable rate, and this is reflected in the results obtained.

Uplink Capacity with Correlated Lognormal Shadow Fading

In this paper we study the effect of Base Station (BS) and User Terminal (UT) on the capacity of the Gaussian Cellular Multiple Access Channel. We also build a Monte Carlo simulator using realistic system parameters and distance dependent shadowing correlation estimation models. We are able to properly verify the theoretical analysis and we provide an insight on the maximum achievable capacity for real cellular systems.

Capacity of Cellular Uplink with Multiple Tiers of Users and Path Loss

With the emergence and continuous growth of wireless data services, the value of a wireless network is not only defined by how many users it can support, but also by its ability to deliver higher data rates. Information theoretic capacity of cellular systems with fading is usually estimate.d using models originally inspired by Wyners gaussian cellular multiple access channel (GCMAC). In this paper we extend this model to study the cellular system with users distributed over the cellular coverage area. Based on the distance from the cell-site receiver, users are grouped as tiers, and received signals from each tier are scaled using a distance dependent attenuation factor. The optimum capacity in fading environment is then found by calculating the path-loss for users in each tier using a specific path-loss law and some interesting insights are derived. The results correspond to a more realistic model which boils down to Wyners model with fading, with appropriate substitutions of parameter values. The results are verified using Wyners model with fading and Monte-Carlo simulations. Insights are provided for the real world scenarios.

Interference allowance in clustered joint processing and power allocation

We derive an analytical formula for the sum rate of the uplink of a linear network of cells when clustered joint processing is adopted among the base stations in a generalised fading environment. An inter-cluster interference allowance scheme is considered and various user power allocation profiles are investigated in terms of optimal achievable sum rate to highlight that cell-based power allocation is preferable to cluster-based. The contribution of each base station on the cluster sum rate is investigated and its importance is discussed. Numerical results are produced for a real-world scenario showing how medium density systems are the most viable case for clustered system design by achieving > 80% of the global cooperation capacity.

Fairness and User Rate Distribution in Joint Processing Systems

In this paper we provide a geometric and mathematical model that can be used to evaluate the user rate distribution for a cellular system under the notion of a hyper-receiver incorporating realistic system parameters. Variable cell sizes and different location-based decoding orders are analysed and the uplink user rate distribution of a cellular system in which each transmitted signal experiences a distance dependent path loss, fast fading and slow fading, is derived. Interestingly enough, among the three different decoding orders examined, the forward one provides the best results both in terms of fairness (larger percentage of users get satisfactory rates) and minimum rate. The difference among the decoding orders becomes even more notable as the cell sizes increase. There has also to be noted that none of these decoding orders affect the sum-rate.

Uplink capacity of variable-density cellular system with distributed users and fading

In this paper, we extend the well-known model for Gaussian Cellular Multiple Access Channel originally presented by Wyner and later extended by Somekh et al. with fading. The extension to the model, incorporates the distance dependent path loss (maintaining a close relevance to path loss values in real world cellular systems) experienced by the users distributed in a planar cellular array. The density of base stations and hence the cell sizes are variable. In the context of Hyper-receiver joint decoder, a closed form expression for the information theoretic capacity is obtained assuming large number of users in each cell. The effect of the path loss factor, on the information theoretic capacity of the cellular system, is quantified and it is observed that higher path loss factor results in lower capacity. The results validate that larger cell sizes result in lower spectral efficiency. The closed form formula derived by the mathematical analysis is also validated by Monte Carlo simulations.