Free-space optical (FSO) communications will play an important role in the backhaul of future generation of wireless networks in order to support high data rates. Because of narrow beamwidth inherent to an optical signal, acquisition and tracking form an important component of any FSO communication system. In this study, we have analyzed the optimization of received power allocation between tracking and data channels in an FSO receiver. The objective function that is optimized (minimized) are the probability of error and the probability of outage, and the optimization of power allocation is carried out as a function of parameters such as noise power, pointing error variance, pointing error correlation coeﬃcient, and the threshold of outage. We have analyzed the optimization concerning the lognormal and exponentiated Weibull fading scenarios as well. We learn that the optimal power allocation is a function of the received signal-to-noise ratio: a lower signal-to-noise ratio dictates that a higher fraction of received power should be diverted to the tracking channel and vice versa. Index Terms Beam tracking, exponentiated Weibull fading, free-space optical communications, lognormal fading, optimal power allocation, pointing error, probability of error, probability of outage I. I/n.sc/t.sc/r.sc/o.sc/d.sc/u.sc/c.sc/t.sc/i.sc/o.sc/n.sc Free-space optical communications (FSO) is a promising candidate that will play a crucial role in supporting high data rates in the backhaul of future generation of wireless communications [1]. However, 2 due to narrow beamwidth of the optical signal, acquisition and tracking subsystems are required in order to acquire, and thereafter, maintain the link alignment (tracking) with narrow optical beams [2]. In most optical receivers employed in terrestrial FSO links, the received signal energy is divided into beam tracking and data/symbol detection paths or channels [3], [4]. In this regard, a beam splitter is used to split received energy between the two channels. In the tracking channel, an array of detectors is employed in order to track the beam, and a single detector is employed in the data or symbol detection channel. Even though the authors in [5] have proposed that a single array of detectors can be used for both signal detection and beam tracking (thereby doing away with the tracking channel altogether), the single detector scheme may still outperform low complexity combiners—such as an equal gain combiner—that are used to fuse the outputs of each detector in the array /one.sup. However, an array of detector—that employs either a selection combiner or a maximal ratio combiner—may outperform the single detector in terms of probability of error performance if the number of detectors in the array is large enough /two.sup. However, the computational cost of such fusion algorithms can be prohibitively high which, among other factors, depends on the number of detectors in the array [6]. Therefore, depending on the following factors, a single detector receiver based on separate beam tracking and data detection channels would be a more attractive choice as opposed to a detector array that is used for both beam tracking and data detection. 1) Ifthevarianceofthe pointing error isnottoolarge,asingle-detectorsystemcanperformsigniﬁcantly better than an array of detectors (equal gain combiner), and can provide a satisfactory probability of error performance. We deﬁne the pointing error as the Euclidean distance between the beam center and the origin, where the origin is the center of the array in the tracking channel, or the center of the single-detector in the data channel. 2) The rate at which the detector is switched on and oﬀ depends on the junction capacitance of the detector which in turn is a function of the physical size of the detector: A larger detector implies a larger capacitance which in turn limits the rate at which the data can be sensed by the detector [4]. Thus, if an array of detectors is used for simultaneous beam tracking and symbol detection, then a signiﬁcantly large number of smaller detectors may be required to achieve the appropriate surface area in order to satisfactorily track the pointing error of large variance. A large number of /one.supThis is because the equal gain combiner not only adds the signal power from each detector in the suﬃcient statistic, but also the noise power. /two.supThis is an intuitive argument since a maximal ratio combiner will optimally weight the output of each detector in the array. Hence, the output of each detector will be scaled proportionally to the SNR in the said detector before the fusion of all outputs takes place. In this context, a maximal ratio combiner may outperform a single-detector system. 3 detectors in the array would increase the computational complexity manifold with a maximal ratio [6] or a selection combiner. In this scenario, an array with a small number of detectors (large-sized detectors) may only be used in the tracking channel that does not have to have a large bandwidth (since it is not being used for symbol detection), and a single small detector may be employed in the data channel for a faster response. A. Motivations of Current Study In this context, the probability of error of a single-detector receiver is a function of two parameters: pointing error variance and the signal-to-noise ratio in the data detection channel. The variance ofpointing error is a function of the mean-square error performance of the tracking system, which in turn is a function of the signal-to-noise ratio in the tracking channel. In order to maximize the performance of the receiver, it is important to optimally divide received beam power between beam tracking and data detection channels so that there is enough power in the two channels in order to minimize the probability of error. If the power going into the tracking channel is not enough, the tracking performance degrades and the resulting probability of error becomes worse due to a high variance of the resulting pointing error. On the other hand, if more than enough power is channeled into the tracking assembly but not enough in the data detection assembly, the probability of error suﬀers due to low signal-to-noise ratio in the data detection channel. Hence, the power allocation to each channel should be optimized /three.supin order to minimize the probability of error of the receiver. II. L/i.sc/t.sc/e.sc/r.sc/a.sc/t.sc/u.sc/r.sc/e.sc R/e.sc/v.sc/i.sc/e.sc/w.sc /a.sc/n.sc/d.sc M/o.sc/d.sc/e.sc/l.sc A/s.sc/s.sc/u.sc/m.sc/p.sc/t.sc/i.sc/o.sc/n.sc/s.sc