Network-Controlled Cell-Breathing for Capacity Improvement in Heterogeneous CDMA/TDMA Scenarios J. Pérez-Romero (1) , O. Sallent (1) , R. Agustí (1) , N. García (2) , L.Wang (3) , H.Aghvami (3) (1) Universitat Politècnica de Catalunya (UPC), Barcelona, Spain, E-mail : [jorperez, sallent, ramon] @ tsc.upc.edu (2) Universitat Pompeu Fabra (UPF), Barcelona, Spain, (3) King’s College London (KCL), London, UK Abstract.-This paper proposes a new Common Radio Resource Management (CRRM) methodology which allows increasing the capacity of heterogeneous CDMA/TDMA. This is achieved by controlling the effective cell radius of the CDMA- based system (i.e. a network-controlled cell-breathing) through appropriate initial RAT selection and vertical handover strategies, so that the interference level in the CDMA-based RAT is reduced while at the same time the target coverage area is assured by means of the cooperation of the FDMA/TDMA based RATs. The proposed approach is evaluated by means of system level simulations in a detailed scenario including UTRAN and GERAN as examples of CDMA and FDMA/TDMA-based access technologies. Results reveal that throughput improvements up to 24% can be achieved with respect to other CRRM strategies based on load balancing principles. I. INTRODUCTION In general, cellular wireless systems become interference- limited and, consequently, any engineering technique devoted to either reduce interference or to improve the robustness of the system to bear interference will readily increase network capacity and operator’s revenue. Managing interference has been a recurrent topic of interest for many years and the problem has been coped from many different perspectives. Channel coding, power control or antenna beamforming are only some examples. Conceptually, the ultimate objective of the proposal presented in this paper also pursues a reduction in the interference level in the scenario and, consequently, targets an overall improvement in the radio resources usage. Nevertheless, a different approach is followed, as described in the following. The framework considered in this paper is that of heterogeneous radio access networks (RANs), also known as Beyond 3G (B3G) systems. The heterogeneous network concept is intended to propose a flexible and open architecture for a large variety of wireless access technologies, applications and services with different QoS demands, as well as different protocol stacks. Radio access networks include cellular networks and also other public non- cellular access networks (e.g. WLAN). Cellular networks may in turn be subdivided into different layers (e.g. macro, micro or picocells). In these new scenarios, different Radio Access Technologies (RATs) will coexist and will operate in a coordinated way. Wireless networks differ from each other by air interface technology, cell-size, services, price, access, coverage and ownership. The complementary characteristics offered by the different radio access technologies (RATs) make possible to exploit the trunking gain leading to a higher overall performance than the aggregated performances of the stand- alone networks. Clearly, this potential gain of B3G systems can only turn into reality by means of a proper management of the available radio resources. Common Radio Resource Management (CRRM) refers to the set of functions that are devoted to ensure an efficient and coordinated use of the available radio resources in heterogeneous networks scenarios [1]-[3] . More specifically, CRRM strategies should ensure that the operator’s goals in coverage and QoS are met while providing as high as possible overall capacity (i.e. the sum of the capacities achieved in every single RAN). Within CRRM, the initial RAT selection, i.e. the allocation of connections to specific RANs at session initiation, and the vertical handover (VHO), i.e. the capability to switch on- going connections from one RAN to another, are the key enablers to properly manage the heterogeneous radio access network scenario and become then key CRRM functions [4]. In this context, this paper intends to exploit the different sensitivity that diverse RATs may exhibit to interference so that a smart CRRM follows. In particular, in FDMA/TDMA- based access systems (e.g. GSM/GPRS) there is no intra-cell interference. In turn, inter-cell interference is caused by a single user in every co-channel cell. In contrast, in CDMA- based systems (e.g. UMTS) the intra-cell interference is caused by every single user transmitting in the cell. Furthermore, inter-cell interference is also originated by all simultaneous users in all neighbouring cells, since a complete frequency reuse is considered. Consequently, CDMA systems are much more sensitive to multi-user interference than FDMA/TDMA ones. The underlying idea of the CRRM approach developed in this paper is to take advantage of the coverage overlap that several RANs using different access technologies may provide in a certain service area in order to improve the overall interference pattern generated in the scenario for the CDMA-based systems and, consequently, improving the capacity of the overall heterogeneous scenario. This can be achieved by controlling the effective cell radius of CDMA- based systems (i.e. a controlled cell-breathing effect) through appropriate initial RAT selection and vertical handover approaches, so that the interference level in CDMA-based RATs is reduced while at the same time the target coverage area is assured by means of the cooperation of the FDMA/TDMA-based RATs.