Introducing power-shaped advanced resource assignment (PSARA) in fixed broadband wireless access systems Velio Tralli, Riccardo Veronesi, Michele Zorzi DIF - Università di Ferrara - via Saragat, 1 - 44100 Ferrara - Fax:+39-0532-768602 Email: {vtralli,rveronesi,zorzi}@ing.unife.it ABSTRACT In this paper we propose a new technique for assigning radio channels in TDMA-based fixed broadband wireless access systems (FBWA) with full frequency reuse. The proposed technique, named Power-Shaped Advanced Re- source Assignement (PSARA), has the aim of partially organizing the intercell and intersector interference in a cellular system where base stations assign radio re- sources in an uncoordinated fashion. This is done by using an appropriate set of power profiles that limit (or shape) the power transmitted in each slot of the frame. The allocation algorithm assigns time slots to users on the basis of the power required to fulfill a predefined carrier-to-interference ratio, which is suitably estimated. Simulation results for a typical cellular FBWA system show that this technique significantly improves the ca- pacity with respect to a system that assigns resources without power shaping, even with power control. 1 I NTRODUCTION Fixed Broadband Wireless Access (FBWA) systems, including broadband wireless local loops and LMDS, have been recently introduced to provide integrated in- teractive multimedia broadband services, such as video conference, video on-demand, high-speed access to the Internet, to residential and business users. They should be able to provide a large amount of on-demand band- width in a very flexible manner to fixed terminals without the need for laying cables. In a typical FBWA scenario the coverage area is divided into cells having a base sta- tion equipped with sectored antennas posed on a suitable site overlooking the service area; subscriber terminals are usually equipped with directional antennas [1, 2]. For these applications resource management tech- niques play a key role to ensure an efficient use of the radio spectrum and to increase system capacity. To sup- port data and multimedia traffic, data units from different users at the radio interface have to be suitably scheduled in order to share the available radio channels according to the needs of upper layer protocols and applications. In order to achieve efficient use of the radio resources, it is required that scheduling algorithms be channel aware or channel dependent [3]. Different choices are possible when designing re- source allocation and scheduling. A centralized algo- rithm (running on a network unit controlling a set of This work has been supported by Ericsson Lab Italy cells) would be the optimal choice since it has com- plete information and therefore can perform optimal de- cisions. However, it may be complicated and require a large signaling burden, which requires large computing power and/or may result in excessive latency in the de- cisions. A distributed algorithm may be simpler, but it requires a smart management of radio power and band- width with incomplete information to achieve a large sys- tem capacity. This potential inefficiency due to lack of full information is the price to pay for keeping the al- gorithms simple and avoiding single points of failure in the network. A related issue, which also affects how users can simultaneously transmit, is the frequency reuse plan. A system with full reuse greatly simplifies plan- ning needs and allows for easy introduction of additional base stations. The resulting interference, often intolera- ble in classic narrowband cellular systems, is mitigated in FWBA systems by using highly directional antennas (especially at the user terminal) which greatly reduce the amount of power which interferes with other users. Having this view in mind, we propose a resource assignment method, which can be advantageously ap- plied in distributed channel dependent scheduling algo- rithms. The proposed technique, named Power-Shaped Advanced Resource Assignment (PSARA), has the aim of partially organizing the intercell and intersector inter- ference in a cellular system where base stations assign ra- dio resources in an uncoordinated fashion (with the only requirement of frame synchronization). This is done by using an appropriate set of power profiles that limit (or shape) the maximum power transmitted in each slot of the frame. The allocation algorithm assigns time slots to users on the basis of the power required to fulfill a predefined carrier-to-interference ratio, which is suitably estimated. In the literature, several works have appeared which deal with resource management in fixed wireless access systems (i.e., [4, 5, 6, 7]). [4] deals with resource allo- cation in a micro cellular LMDS with OFDM, whereas [5, 6, 7] propose time division schemes for generic fixed wireless. The allocation algorithms in [6, 7] are dis- tributed and take advantage of cell sectorization, taking into account two key points: resource assignment can take advantage of the small number of interferers in the system thanks to the use of directional antennas, thereby avoiding “unfavor- able” time slots (i.e., slots with significant interfer-