Published in IET Communications Received on 10th February 2009 Revised on 12th March 2011 doi: 10.1049/iet-com.2010.0029 ISSN 1751-8628 Novel approach to adjust the step size for closed-loop power control in wireless cellular code division multiple access systems under flat fading H. Saghaei 1,2 B. Seyfe 1 H. Bakhshi 1 R. Bayat 3 1 Department of Electrical Engineering, Shahed University, Tehran, Iran 2 Faculty of Engineering, Research and Science Branch, Islamic Azad University, Tehran, Iran 3 Faculty of Electrical Engineering, Semnan University, Semnan, Iran E-mail: seyfe@shahed.ac.ir Abstract: In this article, we study the power control (PC) process in wireless cellular code division-multiple access systems under flat fading and propose a novel approach to find an optimum step size for closed-loop power control algorithms. In this approach, an optimum step size will be computed from a proposed function. This function depends on system parameters such as, the number of co-channel users, processing gain, the period of PC, Doppler frequency, channel attenuation and the order of diversity. Based on this computation, the mobile station (MS) adjusts its transmit power optimally to decrease interference for other co-channel users. Simulation results for different sets of system parameters show that the proposed algorithm decreases the bit error rate, the outage probability at the base station (BS), and increases the battery life of the MS compared with other values of the step size. The performance of the proposed algorithm is compared with the fixed-step-size power control algorithm and superiority of its performance is confirmed by simulation results. Moreover, the upper and lower bounds of the outage probability and the received signal-to-interference ratio for the proposed algorithm at the BS will be calculated. 1 Introduction The primary goal of cellular radio systems is to provide reliable communication for users irrespective of their location and situation. Third generation of wireless systems are mostly based on the direct sequence-code division multiple access (DS-CDMA) technique, which effectively uses the radio spectrum by means of universal frequency reuse [1]. Since in practice, the signals used in DS-CDMA systems are not completely orthogonal, all users experience co-channel interference from all other users in the system. Thus, the co-channel interference is the dominating factor in CDMA systems and the interference level limits the capacity of such systems [2]. There are several techniques to reduce the co-channel interference. One of them is the power control (PC) method in which the output power of each transmitter is adjusted to reduce the co-channel interference [3]. The necessity of PC in the uplink channel (mobile-to-base) is more serious than that of downlink (base-to-mobile). Because in the uplink channel, the near- far distance problem is inherent, so the multiple access interference becomes a serious problem. Hence, we only consider the uplink PC problem in CDMA systems. To compensate for channel attenuation, mobile station (MS) receives a known signal, that is, the pilot signal, from the base station (BS) in the downlink channel, and adjusts its transmit power proportional to the inverse of the measured value. Since the pilot signal is transmitted at a constant power, the variations of its strength convey sufficient information about the downlink channel attenuation. This is called open-loop power control (OLPC) [4]. The centre frequencies that are allocated to the uplink and downlink transmissions are usually widely separated. As a result, the correlation between the uplink and downlink attenuation variations will be very low. Therefore it is better to use the transmission power updates of the MS, which are based on feedback information from the received signal-to- interference ratio (SIR) at the BS; this forms a closed-loop system for PC [5]. This closed-loop power control (CLPC) (or inner-loop PC) keeps the received uplink signal power level at a specified target value; and if a fast PC is employed, the effects of multipath fading can be compensated [5]. Moreover, the targeted SIR can be varied, because the SIR requirement for a given bit error rate (BER) varies because of change of channel conditions. This is the role of the outer-loop power control (OULPC) [5–7]. PC algorithms can be centralised or distributed [8].A centralised power control (CPC) scheme uses complete information of each user [8], and control measurements are taken for all the users. On the other hand, a distributed power control (DPC) scheme uses only the local information about each user to make a decision [9]. Depending on the type of algorithm, the power control command (PCC) can be in the form of decision feedback (DF) or information feedback (IF) [10]. In the algorithms based on DF, only one bit is needed for the PCC [10], but IET Commun., 2011, Vol. 5, Iss. 11, pp. 1469–1483 1469 doi: 10.1049/iet-com.2010.0029 & The Institution of Engineering and Technology 2011 www.ietdl.org