**Abstract**

The current demands for extremely high data rate wireless services and the spectrum scarcity at the sub-6 GHz bands are forcefully motivating the use of the millimeter-wave (mmWave) frequencies. MmWave communications are characterized by severe attenuation, sparse-scattering environment, large bandwidth, high penetration loss, beamforming with massive antenna arrays, and possible noiselimited operation. These characteristics imply a major difference with respect to legacy communication technologies, primarily designed for the sub-6 GHz bands, and are posing major design challenges on medium access control (MAC) layer. This book chapter discusses key MAC layer issues at the initial access and mobility management (e.g., synchronization, random access, and handover) as well as resource allocation (interference management, scheduling, and association). The chapter provides an integrated view on MAC layer issues for cellular networks and reviews the main challenges and trade-offs and the state-of-the-art proposals to address them.

**Keywords:** millimeter-wave systems, initial access, resource allocation, mobility management, MAC layer design, 5G

## **1. Introduction**

Increased demands for higher data rates in wireless communication systems, along with new applications such as massive wireless access have motivated enhancing spectral efficiency by using advanced technologies such as full-duplex communications, cognitive and cooperative networking, interference cancelation, and massive multiple-input multiple-output (MIMO). As these enhancements are reaching the fundamental capacity limits, determined by the available spectrum at the sub-6-GHz bands, the millimeter-wave (mmWave) band is becoming an alternative and promising option to support extremely high data rate wireless access [1–3]. The main reason is simple: the aggregated bandwidth of most current (main) commercial wireless systems is <2% of the bandwidth available at the mmWave. Such bandwidth can easily support a Gbps data rate in most use cases.

At the moment, the main use cases of the mmWave spectrum are satellite communications, military applications, point-to-point systems, local multipoint distribution service, and short-range networks [4]. Severe attenuation of the signal at the mmWave band, especially at certain frequency bands such as 60 and 180 GHz (see Figure 1.1 in [5]), had led to a common belief that mmWave communications are suitable either for special applications with special hardware (as mentioned above) or for "whisper radios" for wireless personal area networks (WPANs) with

coverage distances of a few meters [2]. However, recent studies on mmWave mobile networks have convinced academia, industry, and regulatory bodies to repurpose the mmWave band for future wireless (and even mobile) networks.

MmWave communications are particularly attractive for ultra-short range/ high rate communications and gigabit wireless applications such as wireless gigabit Ethernet and uncompressed high-quality video transmission, see **Table 1**.


### **Table 1.**

*Application scenarios for mmWave networks. This table is deduced from ongoing discussions inside IEEE 802.11ay study group. "NS" means not specified yet.*

**85**

terminals.

*MAC Aspects of Millimeter-Wave Cellular Networks DOI: http://dx.doi.org/10.5772/intechopen.89075*

**2.1 The directed mmWave wireless channel**

out affecting the size of their radio chips.

at the MAC layer.

**2. Fundamentals**

The commercial potential of mmWave networks initiated several standardization activities within WPANs and wireless local area networks (WLANs), such as IEEE 802.15.3c [6], IEEE 802.11ad [7], WirelessHD consortium, Wireless Gigabit Alliance (WiGig), and recently IEEE 802.11ay study group on next-generation 60 GHz [8]. The unique hardware requirements and propagation characteristics of mmWave networks lead to many challenges at the physical, medium access control (MAC), and routing layers [9]. These challenges are exacerbated by the potential spectrum heterogeneity, i.e., coexistence with the legacy sub-6-GHz systems. Compared to the sub-6-GHz communications, mmWave systems exhibit orders of magnitude higher path loss and atmospheric absorption, higher penetration loss, very sparse-scattering environments, smaller wavelength, a much higher number of antenna elements, which may result in high antenna gains and possible noiselimited operation. These unique features demand a significant reconsideration in the design principles of the communication architecture and protocols, especially

MmWave communications use frequencies in the range 10–300 GHz. The mmWave systems exhibit high path-loss, high penetration loss, high frequency/ short wavelength, and very large bandwidth. The small wavelength allows for the implementation of massive numbers of antennas in both transmitter and receiver, which boosts the achievable antenna gain with some extra signal processing, with-

The additional antenna gain can almost completely compensate for the higher path-loss of mmWave communications. A byproduct of the directional communication is the new concept of directional spatial channel, i.e., a channel can be established in a specific direction with a range that varies according to the directionality level [9]. Directional communications and vulnerability to obstacles in mmWave

*Deafness* refers to the situation in which a directional communication link cannot be established due to misalignment between the beams of the transmitter and the receiver. To address this problem, we may need a time-consuming procedure of beam training. That is, an operation in which the transmitter and receiver find a beam pair, pointing to each other, which maximizes the link budget. In one hand, the alignment procedure complicates the link establishment phase. On the other hand, it substantially reduces multiuser interference [10], as the receiver listens only to a specific directed mmWave channel. In the extreme case, mmWave networks may operate in a noise-limited regime where multiuser interference is almost completely suppressed and no longer limits the throughput. This is a noticeable change with respect to the conventional interference-limited sub-6 GHz networks. This unique feature makes mmWave suitable for ultra-dense networks (also called massive wireless access) with dense deployments of infrastructure nodes and

*Blockage* refers to a high penetration loss due to obstacles that may not be solved

by increasing the transmission power. Addressing blockage requires utilizing alternative directed mmWave channels that are not blocked. These channels may be provided by reflectors or intermediate relay nodes. In both solutions, overcoming blockage entails the execution of a new time-consuming beam training procedure.

networks have two main consequences: (1) deafness and (2) blockage [9].

*MAC Aspects of Millimeter-Wave Cellular Networks DOI: http://dx.doi.org/10.5772/intechopen.89075*

The commercial potential of mmWave networks initiated several standardization activities within WPANs and wireless local area networks (WLANs), such as IEEE 802.15.3c [6], IEEE 802.11ad [7], WirelessHD consortium, Wireless Gigabit Alliance (WiGig), and recently IEEE 802.11ay study group on next-generation 60 GHz [8].

The unique hardware requirements and propagation characteristics of mmWave networks lead to many challenges at the physical, medium access control (MAC), and routing layers [9]. These challenges are exacerbated by the potential spectrum heterogeneity, i.e., coexistence with the legacy sub-6-GHz systems. Compared to the sub-6-GHz communications, mmWave systems exhibit orders of magnitude higher path loss and atmospheric absorption, higher penetration loss, very sparse-scattering environments, smaller wavelength, a much higher number of antenna elements, which may result in high antenna gains and possible noiselimited operation. These unique features demand a significant reconsideration in the design principles of the communication architecture and protocols, especially at the MAC layer.
