###### Abstract  
Millimeter Wave (mmWave) communication is considered as an enabling technology for the next generation of cellular networks because it offers much larger bandwidth and higher data rate than the current lower-frequency cellular systems to satisfy the exponential growth of mobile data demand. High gain directional antennas are needed to overcome high propagation losses in mmWave bands. However, the reliance on highly directional antennas will result in a more complicated initial cell search procedure since both base station and mobile device have to look for each other over a large space to establish the link. This paper focuses on analyzing the performance of the directional cell search procedure where the base stations periodically transmit signals in a set of optimal directional patterns to scan the coverage area. The mobile terminals detect the signals from the base station using the Generalized Likelihood Ratio Test (GLRT). The results show that with an appropriate scanning scheme, the use of directional antennas can outperform their omnidirectional counterparts in terms of signal detection performance as well as total time required. was detected through brick pillars throughout the mmWave signals, yet signals through a hollow plasterboard wall resulted in a penetration loss ranging between 5.4 dB and 8.1 dB. To overcome these loss mechanisms, higher gain, i.e., directional antennas, are necessary. Since the wavelengths at the mmWave frequencies can be orders of magnitude smaller than the frequency band of the current state of art, and thanks to the advances in materials and low-power CMOS RF circuitry, it is possible to pack a large number of antenna elements in compact transceiver systems [12; 13; 14; 15; 16; 17; 18]. Therefore, it is feasible to integrate highly directional antennas in base station (BS) and user equipment (UE) systems to enhance the quality of mmWave communications. While the use of highly directional antennas in mmWave cellular networks addresses the loss issues, another challenge emerges due to the complication of the initial cell search procedure [19; 20], i.e., a mobile device needs to discover an available base station and establish initial communications. Although current cellular networks such as 3 GPP LTE/4G have been using beamforming techniques to enhance the quality of communication links, they are still based on using omnidirectional transmissions or transmissions with fixed antenna patterns for the initial cell search process [20; 21]. This is not the case in mmWave cellular networks because it is essential to use high gain antennas for propagation loss compensation. Although the use of omnidirectional antennas simplifies the cell search problem, they may default to establishing connection only with the closest UEs even though communications may be feasible with other UEs that may reside further away using a high-gain directional antenna. Such UEs cannot be detected by the omni-directional antenna. The initial cell search problem was recently investigated in [21], where the authors established a Generalized Likelihood Ratio Test (GLRT) at the mmWave receiver to assess the performance of signal detection. Although the idea of using GLRT for signal detection is not new for traditional cellular networks, it is novel in [21] that the authors applied the concept of GLRT in a mmWave network where directional array at BS was used to scan for the coverage region at random directions in time. The authors in [21] showed that omnidirectional transmissions from the BS generally have better performance than random directional scanning over a given scan time. In [22; 23; 24], the authors proposed and analyzed the performance of the initial access problem using an exhaustive method (the BS performs a brute-force sequential beam searching) and an iterative method (minimize the region of interest in the first stage and focus only on this region in the second stage). It was shown in these works that each method has its own advantages and drawbacks.