# MAC Protocols for mmWave Communication: A Comparative Survey +
## 1 Introduction  
With the rapid increase in large-volume data sharing and cloud storage, high bandwidth communication is urgently needed. According to the standards issued by the International Telecommunication Union (ITU), a next-generation 5G communication network structure must jointly support the following three services: enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), and ultra-reliable and low-latency communications (URLLC). Here, eMBB refers to the high bandwidth internet connection used in smartphones, mMTC focuses on the narrowband internet access mainly used in sensing and smart cities, and lastly, URLLC aims to achieve the lowest possible delay as low as 1 ms for sensitive applications [1]. Mobile wireless communication has progressed with current modern technologies that are capable of providing high-quality mobile broadband services at end-user data rates of several megabits per second over large areas and to tens of thousands of users. To meet the increasing demand for faster and more reliable wireless networks, mmWave frequency band is a prime choice for a modern 5G communication architecture. Based on the recommendations of the ITU, governments all over the world have sanctioned mmWave bands for commercial use. Although the mmWave bands operate between 30 and 300 GHz, corresponding to the wavelengths from 10 mm to 1 mm, 1.8 and 2.6 GHz bands are also incorporated with 5G [2]. As a result, the mmWave spectrum still has a large-scale unlicensed bandwidth. Due to the fundamental contrasts between mmWave communications and existing microwave-based communication technologies (e.g., 2.4 GHz and 5 GHz), mmWaves, by nature,demonstrate high signal attenuation. This fading channel phenomenon is expected to be frequency-selective [3]. A common solution of this problem is to introduce beamforming. Previously, mmWaves were considered inefficient by the researchers for mobile access networks because of their high vulnerability to shadowing and poor isotropic propagation loss [4]. However, even a small portion of the accessible mmWave spectrum can achieve hundreds of times the data throughput, and capacity realized by the existing cellular spectrum [5]. When facilitating the growing demands of mobile cellular devices, wearable sensors, and the Internet of Things (IoT) with this abundant mmWave spectrum, in next-generation communication systems (beyond 5G), where each of the devices will be incorporated with artificial intelligence (AI), coordination among the connected dense networks will be crucial. Additionally, with billions of connected devices, data transmission will create a long overhead. These issues can be carefully administered at the MAC layer [6], and this could lead to addressing the above-mentioned challenges in terms of access and networking point of view. Furthermore, in 5G networks, neighbor discovery and user coordination are fundamental aspects, which can also be dealt with at the MAC layer [7]. Moreover, the special propagation features and hardware requirements of mmWave systems described in [8; 9] highlight numerous challenges for the MAC, PHY layer, and routing layers [10]. By synchronizing omnidirectional and directional transmissions, MAC layer protocols can significantly increase the overall efficiency of mmWave systems [11]. To circumvent the physical restrictions of mmWaves, and to facilitate the development of a smart communication system, advanced MAC protocols may need to be utilized simultaneously, specific to mmWave bands, and multiple communication layers with different coverage may have to be allowed to coexist [12]. Considering the above, to provide a better grasp of the possible implementation of the currently available MAC protocol and their feasibility for future mmWave wireless systems and pave the route for further development, the contributions of our survey are as follows.