Spectrum Management Methodology for WCDMA Systems Encompassing Uplink and Downlink J. Nasreddine, J. Pérez-Romero, O. Sallent, R. Agustí Dept. of Signal Theory and Communications, Universitat Politècnica de Catalunya (UPC) Barcelona, Spain [jnassred, sallent, jorperez, ramon]@tsc.upc.edu Abstract— In this paper a new spectrum management methodology for WCDMA systems is proposed based on the concept of coupling matrix, which is able to capture inter-cell interactions. The proposed methodology takes into account the fact that uplink and downlink frequency carriers are jointly allocated and optimizes simultaneously the two link directions. Simulation results for symmetric and asymmetric services show the efficiency of the methodology in increasing capacity and reducing transmitted power. Keywords- Coupling matrix; Spectrum Management; WCDMA I. INTRODUCTION The tremendous increase in mobile user density and the pervasive non-homogeneity in both spatial and temporal traffic distribution in cellular systems (e.g. urban hotspots and rural environment, daytime and nighttime traffic distributions, etc.) require an evolution in the spectrum allocation vision for WCDMA systems. Traffic increase, together with non- homogeneity, leads to the presence of islands of saturated cells although the overall system may not be congested from a global point of view. This is due to the fact that some cells have high interactions leading to high inter-cell interference in addition to the difference in cell loads. In the presence of several frequencies (i.e. typically two or three), which is the case of currently deployed WCDMA systems, frequency carriers could be distributed in smart way in order to prevent such cells of sharing the same frequency. Spatial non-homogeneity is currently faced by deploying micro-cells in high traffic areas, leading to Hierarchical Cell Structures (HCS) [1]-[3]. In that sense, it is usual to operate the different cell layers (i.e. macro-cell and micro-cell layers) with different carrier frequencies, although depending on the specific interference levels, this condition may be broken. For instance, scenarios breaking the HCS in WCDMA systems were investigated in [4] and it has been shown that in some cases it is more suitable to share a frequency by both layers. Nevertheless, the spectrum management problem neither has to be limited to HCS nor it has to be presumed that it has an a priori guide on a suitable solution, since it is strongly dependant on interference and traffic distribution along the deployed scenario. Therefore, spectrum management methodologies based on system characteristics can be of particular interest. In this context, we propose a heuristic spectrum management algorithm based on the concept of coupling matrix proposed in [5][6] for uplink and which is able to capture the interaction among the different cells from both macroscopic and microscopic points of view. In addition to the formulation of the coupling matrix for downlink, we propose in this paper a spectrum management methodology to distribute the available WCDMA frequency carriers among cells in a smart way. This paper assumes that the amount of traffic per cell is such that only one WCDMA carrier per cell is required, although the proposed methodology could be extended to the general case in which more than one carrier per cell is required. The paper is organized as follows. In Section II, we introduce the coupling matrix for both uplink and downlink. In Section III, we present the spectrum management methodology. In Section IV, we describe the simulation model and provide some illustrative results in different scenarios. Finally, we conclude with relevant remarks and future work. II. COUPLING MATRIX Let us consider a WCDMA system with K base stations and F frequencies. The set of all base stations is called Λ = {j: j ∈ [1, K]} and the set of all frequencies is called Φ = {f: f ∈ [1, F]}. The number of users connected to base station j is n j . In the following, two coupling matrices are developed for downlink and uplink respectively. A. Downlink In the downlink, the E b /N 0 requirement for the i-th user of the j-th cell, denoted as user i j , can be expressed as: , , , , 0, , j j j j j j j Ti i i j b o Tj Ti i D i D i j P L E N P P N L χ α ×Θ   =     -   + + ×       (1) where , j Ti P is the power devoted to the user i j , j i χ represents the inter-cell interference experienced by this user, j i Θ is the spreading factor including the modulation and code rate, , j i j L is the pathloss between mobile i j and base station j, α D is the orthogonality factor in downlink (α D = 0 for perfect orthogonality and 1 for non-orthogonality), N 0,D is the downlink background noise power, and P T,j is the total