CAPACITY BOUNDS ANALYSIS OF SWITCHED BEAM SMART ANTENNAS UNDER CDMA ISMAT A. ALDMOUR; KHALID AL-BEGAIN AYMEN I. ZREIKAT MoCoNet, School of Computing, University of Glamorgan Mu'tah University, Pontypridd CF37-1DL, UK (61710), P.O. Box 7, Mu'tah-Karak, Jordan ialdmour@glam.ac.uk ; kbegain@glam.ac.uk aymen@mutah.edu.jo KAMAL ALAMEH COMPS, Edith Cowan University, Joondalup, Australia k.alameh@ecu.edu.au ABSTRACT In CDMA concept, interference from other users within the same cell and from neighboring cells limits the maximum admissible connections below which the required quality of service requirements are met. In determining cell capacity a certain level of interference from other cells is usually tolerated for the omnidirectional antenna at the base station and thus the ratio of the total interference from other cells to interference generated within the same cell is an important de- sign parameter. The adoption of smart antennas at the base station allows for increased tolerance to interference levels from within the cell and from neighboring cells and therefore permitting higher capacities. Interference capacity bounds represent a critical issue for power allocation, resource management and other schemes. In this paper, an effective inter- ference ratio that depends on the omnidirectional ratio and antenna parameters is defined and used to analyze and quan- tify the interference levels and capacity bounds on the uplink for a single service under CDMA when utilizing a switched beam smart antenna. Capacity gains over the omnidirectional antenna are derived and presented graphically and their relationship to the allowable interference levels is discussed. Keywords: Capacity Bounds, CDMA, Smart Antennas, Interference. Ι. INTRODUCTION Code Division Multiple Access (CDMA) was designed to optimize spectrum utilization by allowing all users to share the common allotted frequency band (Viterbi 1995). However, the continuous growth in demand for more services, higher throughputs and more connec- tions calls for new approaches to provide higher ca- pacities (WWRF 2001). The default omnidirectional transmission method spreads the electromagnetic en- ergy of the signal over large regions of space. The in- tended mobile station actually receives a very small portion of the transmitted energy accompanied by much interference from all other sources. Thus, one of the promising frontiers to enhance capacity in mobile communications systems is the adoption of antennas that have directional properties whereby the antenna is capable of concentrating energy in the direction of the intended other part of the communication link, whilst simultaneously suppressing energy levels in all other directions. This is especially beneficial to CDMA sys- tems in which interference generated by other users in the cell and in neighboring cells limits the system ca- pacity (Rappaport 2002). This is called the interference capacity limit and forms an upper limit on capacity whereby actual capacity will be lower than this limit due to other reasons including code shortages, limited power in the base station on the downlink and limited power of the mobile station on the uplink (Holma and Toskala 2002). Therefore, the upper interference limits on capacity is critical in all coding strategies, power control and power sharing and scheduling techniques in CDMA communications systems. Smart antennas (Godora 2004) (Bao 2002) represent a broad variety of antennas that differ in performance and in transceiver complexity, including switched beam antennas, steered beam antennas, adaptive array antennas, and multiple-input-multiple-output links. In this paper we analyze interference and capacity bounds for CDMA systems utilizing switched beam antennas that represent the simplest of the aforementioned an- tennas. Switched beam smart antennas are character- ized as having a number of main beams that focus in on individual end-users. Switched beam systems have a fixed number of predefined antenna patterns that are selected according to incoming signal directions. The ability of such antennas to concentrate power in a cer- tain direction provides a directional gain whilst sup- pressing interference along the non-active beams. The presence of side lobes contributes to the accumulation of noise along these non-active beam directions and hence reduces the SNR of the desired signal.