Interference factor formula for hexagonal networks

Two frameworks for the study of cellular networks are considered: the tra-ditional hexagonal model and the fluid model. Both models leads to compa-rable results for the interference factor as a function of the distance to the BS. If we want to go further in the comparison of both models, in particular with the computation of outage probabilities, we need however to be more accurate.

The aim of this section is to provide an alternative formula for f that better matches the simulated figures in an hexagonal network. Note that this result is not needed if network designers use the new framework proposed in this thesis. An accurate fitting of analytical and simulated curves shows that f should simply be multiplied by an affine function ofηto match with Monte Carlo simulations in an hexagonal network. The expression (3.3) can then be re-written as follows:

f_hexa(r) = (1 +A_hexa(η))2πρBSr^η

η−2 (2R_c−r)^2−η, (6.11) where A_hexa(η) = 0.15η+ 0.68 is a corrective term obtained by least-square fitting. Note that for an accurate fitting of the analytical formulas presented in this section to the Monte Carlo simulations performed in an hexagonal network, µ_f should be multiplied by (1 +A_hexa(η)), σ_f by (1 +A_hexa(η))² and the expressions (3.3) replaced by (6.11).

6.3.4 Results

Figures 6.2 and 6.3 show the kind of results we are able to obtain instan-taneously thanks to the simple formulas derived for voice service (γ_u^∗ =

−16 dB). Analytical formulas are compared to Monte Carlo simulations in an hexagonal cellular network. As a matter of fact, Eq.6.11 is used. Figure 6.6 shows the global outage probabilities as a function of the number of MS per cell for various values of the path-loss exponent η. It allows us to easily find the capacity of the network at any given maximum percentage of outage.

For example, the outage probability when there are 12 users per cell is about 10% with η = 3.5. Figure 6.7 shows, as an example, the capacity with 2%

outage as a function ofη.

Figure 6.8 shows the spatial outage probability as a function of the dis-tance to the BS for η = 3 and for various number of MS per cell. Given that there are already n, these curves give the probability that a new user, initiating a new call at a given distance, implies an outage. As an example, a new user in a cell with already 16 on-going calls, will cause outage with probability 0.17 at 900 m from the BS and with probability 0.05 at 650 m from the BS.

With this result, an operator would be able to admit or reject new con-nections according to the location of the entering MS. Thus, this allows a finer admission control than with the global outage probability.

6.4 Concluding remarks

In this chapter, we showed the simplicity of the fluid model allows a spatial integration off leading to closed-form formula for the global outage proba-bility and for the spatial outage probaproba-bility. Monte Carlo simulations done with a hexagonal network show some differences with the fluid model results, due to the high sensititivy of the error functionQto the mean and standard deviation of f. As no approach is better to modelize the reality of a network, we can say that the observed differences are inherent to the modelization process. However, to have a fluid model as close as possible to a hexagonal one, wefitted the fluid model to the hexagonal one as a function of it. The proposed framework is a powerful tool to study admission control in CDMA networks and design fine algorithms taking into account the distance to the BS.

6.4. CONCLUDING REMARKS 149

Figure 6.6: Global outage probability as a function of the number of MS per cell and for path-loss exponents η = 2.7, 3.5 and 4, simulation (solid lines) and analysis (dotted lines).

Figure 6.7: Capacity with 2% outage as a function of the path-loss exponent η, simulations (solid lines) and analysis (dotted lines) are compared.

Figure 6.8: Spatial outage probability as a function of the distance to the BS for various number of users per cell and forη = 3.

Refinements of the fluid model

151

We developed a fluid model considering a radio network as a continnum of base stations. One of the hypothesis on which the calculation of the inter-ference factor lays is a radial and deterministic pathloss which only depends on the distance between the transmitter and the receiver.

In Chapter 7, we propose a first refinement by considering a pathgain also de-pending on the antenna gain. This last approach enables to analyze networks with sectored cells. As an application, a comparison between sectorisation and densification is proposed.

In Chapter 8, we propose a second refinement by considering the shadowing, in our analysis.

And in Chapter 9, we analyze the uplink in term of fluid model.

Chapter 7

Fluid Model for Sectored Networks

This chapter proposes a refinement of the fluid model by considering sectored networks. As an application, we analyze the densification and the sectoriza-tion of CDMA networks, in mono and multi-service cases. This chapter is based on the article [Kel07].

7.1 Introduction

Among the solutions to answer an increasing traffic in a CDMA network, a provider has the possibilities to install base stations in new sites (see Section 4.5) or to sectorize it i.e to replace the existing BS with directional antenna BS in the same place as the existing ones. To analyse the advantages and drawbacks of each kind of solution, a provider generates simulations with simulators tools. These last ones need to create an environment and to set the network’s parameters. They do not give instantaneous results, may last an important time, and moreover, a great number of simulations are generally required.

We generalize the fluid approach developed for omni-directional base sta-155

tions networks to sectorised ones. We first establish the expression of the interference factor for a sectorized network, with three sectors per site. We validate this approach comparing it to a numerical computed network. It becomes possible to analyse and compare instantaneously different solutions with the aim to adapt the network, or a given zone of the network, to an increasing traffic demand. As an application of this model, we analyze the densification and the sectorisation and propose a comparison of these means as solutions to an increasing traffic. We show, this model enables to analyse the mobile admission in CDMA networks. We end by generalizing our model for aq-sectored network, with q≥1.

Our fluid model gives results close to the ones obtained by planning tools (see remark Section 7.7.1) which take into account a real environment.