**4. Resource allocation**

### **4.1 Interference characteristics**

To design a proper hybrid MAC for mmWave networks, the main steps are analyzing the multiuser interference, evaluating performance gain (in terms of throughput/delay) due to various resource allocation protocols, and investigating the signaling and computational complexities of those protocols. Roughly speaking, as the system goes to the noise-limited regime, the required complexity for proper resource allocation and interference avoidance functions at the MAC layer substantially reduces [11, 25, 26]. For instance, in a noise-limited regime, a very simple resource allocation such as activating all links at the same time without any coordination among different links may outperform a complicated independent-set based resource allocation [11]. Instead, pencil-beam operation complicates negotiation among different devices in a network, as control message exchange may require time-consuming antenna alignment (beam-training) procedure to avoid deafness.

The seminal work in [10] shows the existence of a noise-limited regime (also called pseudowired abstraction) in outdoor mmWave mesh networks. However, indoor mmWave WPANs may not be noise-limited, as shown in [11, 25–27]. In particular, the optimal resource allocation policy may need to deactivate some links to handle the non-negligible multiuser interference [11]; the noise power is not always the limiting factor. To have a concrete example, we have simulated an ad hoc network with a random number of mmWave links deployed on a 10 × 10 m2 area, all operating with the same beamwidth at 60 GHz. Each transmitter/receiver is aligned to its own communication link, and they are active with some probability *p* independent of the activity of the other links. We assume 2.5 mW transmission power, 16 dB/Km atmospheric absorption, (on average) one obstacle on every a 4 m2 area, and sector blockage model [28]. We computed and depicted in **Figure 1** the area spectral efficiency (ASE), defined as the network sum-rate divided by the area size,

### **Figure 1.**

*Area spectral efficiency against link (one transmitter-receiver pair) density (m<sup>2</sup> ). Area size is 10 × 10 m<sup>2</sup> . The obstacle density is one every 4 m<sup>2</sup> . Slotted ALOHA provides substantially higher area spectral efficiency, compared to TDMA. These performance gains may improve with the number of links.*

as the performance measure. From this figure, increasing the number of links in the network does not affect ASE of TDMA, which is slightly lower than one packet per slot. The reason is packet loss due to blockage on some links, and this loss almost vanishes when the obstacle density goes to zero. Slotted ALOHA with transmission probability *<sup>p</sup>* <sup>=</sup> 1 provides the highest ASE, which is firstly increasing with the link density and then shows a strictly decreasing behavior due to excessive collision. Using narrower a beamwidth or lower transmission probability alleviates the collision level and improves the ASE at the expense of the alignment-throughput trade-off [11].

The numbers of the figure indicate that, from the perspective of ASE, mmWave networks benefit from dense deployment, yet every link may observe some level of performance degradation when we increase the number of devices in the network [11]. Reference [29] reported similar observations in mmWave cellular networks. Such performance drops imply that the accuracy of the noise-limited assumption to model the actual network behavior reduces with the number of links. The increased directionality level in a mmWave network reduces multiuser interference; however, this reduction may not be enough to take an action (e.g., resource allocation) based on the assumption of being in a noise-limited regime. Consequently, a pseudowired assumption may be detrimental for proper MAC layer design. However, the interference footprint may not be so large that we need to adopt very conservative resource allocation protocols such as time division multiple access (TDMA), which activates only one link at a time, as already adopted by the current mmWave standards [6, 7]. Reference [30] proposed an index to quantify the impact of various components of the interference models and to propose a tractable and accurate interference model for mmWave networks.

### **4.2 Beam-searching and concurrent transmission**

Despite the small interference footprint of mmWave networks, the option of concurrent transmissions scheduling was not included in the existing standards and proposed only recently. The authors of [27] consider the problem of maximizing

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

estimates of the interference terms.

**4.3 Association**

the transmit power [9].1

routines in mmWave networks.

the number of scheduled flows such that their quality of service requirement is not violated. A greedy scheduling scheme is proposed, where an additional link is activated in each time slot if it improves the total throughput, i.e., throughput gain from this extra link overweighs the performance drop due to additional interference. Reference [31] proposed a similar greedy heuristic by designing a priority ordering of the links. As long as the signal to interference plus noise ratio at all receivers exceeds a threshold, additional links are activated according to this list. The main issue of those approaches is that they are reactive protocols, i.e., a link has

In proactive protocols, one may need to address the alignment-throughput trade-off while designing concurrent scheduling problem. Because, a narrow beamwidth, besides boosting the link budget, reduces multiuser interference, so increases SINR and thereby achievable transmission rate. However, the price of this rate enhancement is higher alignment overhead per-link and possibly complicated scheduling procedures. Reference [11] addressed this problem by an optimization problem that brings together beam-searching and transmission scheduling using

Association governs the long-term allocation of communication resources among various BSs. Due to high penetration loss, blockage in mmWave networks may be only addressed by re-association or relaying procedures not by increasing

mmWave networks due to the dense deployment of the BSs and limited size of the cells [32]. Relaying techniques can provide a more uniform quality of service by offering robust mmWave connection, load balancing, coverage extension, indooroutdoor coverage, efficient mobility management, and smooth handover operation [1, 9, 13, 32–34]. As shown in, an alternative path through relay nodes can increase the connectivity of a mmWave system by about 100%. Furthermore, the relaying technique can enable high-quality live video streaming over 300 m [34]. Therefore, proper association of clients to BSs and relaying techniques are very important

Developing proper association techniques has been the focus of intense research in the last years [35–40], as it may govern the long-term resource allocation policies of conventional wireless networks [35]. The current mmWave standards use the minimum-distance association, which leads to a simple association metric based on the RSSI [14]. This metric is proved to be suitable for an interference-limited homogenous network, but it may lead to poor use of the available resources in the presence of a non-uniform spatial distribution of clients, non-interference-limited environments, and heterogeneous BSs/relays with a different number of antenna elements and different transmission powers [35]. It may lead to an unbalanced number of clients per BS, drastically reducing the available resources per client in highly populated areas [13] while wasting resources in sparse areas. This poor load balancing indeed decreases network-wide fairness, since overloaded BSs cannot provide their associated clients as much resource as less-loaded BSs. Thus, it is pos-

sible for the clients to associate with farther BSs for better load sharing.

of such reflectors with sufficiently large reflection indices.

Besides the existing association techniques of the current mmWave standards, there are many more solutions for the association and relaying from the literature of microwave networks. In [37], a client association policy is investigated to ensure

<sup>1</sup> As an alternative approach, link establishment via reflectors may address a blockage, given the existence

The association and relaying are particularly important in

to be activated to deduce if it is compatible with other transmissions.

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

the number of scheduled flows such that their quality of service requirement is not violated. A greedy scheduling scheme is proposed, where an additional link is activated in each time slot if it improves the total throughput, i.e., throughput gain from this extra link overweighs the performance drop due to additional interference. Reference [31] proposed a similar greedy heuristic by designing a priority ordering of the links. As long as the signal to interference plus noise ratio at all receivers exceeds a threshold, additional links are activated according to this list. The main issue of those approaches is that they are reactive protocols, i.e., a link has to be activated to deduce if it is compatible with other transmissions.

In proactive protocols, one may need to address the alignment-throughput trade-off while designing concurrent scheduling problem. Because, a narrow beamwidth, besides boosting the link budget, reduces multiuser interference, so increases SINR and thereby achievable transmission rate. However, the price of this rate enhancement is higher alignment overhead per-link and possibly complicated scheduling procedures. Reference [11] addressed this problem by an optimization problem that brings together beam-searching and transmission scheduling using estimates of the interference terms.

### **4.3 Association**

*Wireless Mesh Networks - Security, Architectures and Protocols*

as the performance measure. From this figure, increasing the number of links in the network does not affect ASE of TDMA, which is slightly lower than one packet per slot. The reason is packet loss due to blockage on some links, and this loss almost vanishes when the obstacle density goes to zero. Slotted ALOHA with transmission probability *<sup>p</sup>* <sup>=</sup> 1 provides the highest ASE, which is firstly increasing with the link density and then shows a strictly decreasing behavior due to excessive collision. Using narrower a beamwidth or lower transmission probability alleviates the collision level and improves the ASE at the expense of the alignment-throughput

*). Area size is 10 × 10 m<sup>2</sup>*

*. Slotted ALOHA provides substantially higher area spectral efficiency,* 

*.* 

*Area spectral efficiency against link (one transmitter-receiver pair) density (m<sup>2</sup>*

*compared to TDMA. These performance gains may improve with the number of links.*

The numbers of the figure indicate that, from the perspective of ASE, mmWave networks benefit from dense deployment, yet every link may observe some level of performance degradation when we increase the number of devices in the network [11]. Reference [29] reported similar observations in mmWave cellular networks. Such performance drops imply that the accuracy of the noise-limited assumption to model the actual network behavior reduces with the number of links. The increased directionality level in a mmWave network reduces multiuser interference; however, this reduction may not be enough to take an action (e.g., resource allocation) based on the assumption of being in a noise-limited regime. Consequently, a pseudowired assumption may be detrimental for proper MAC layer design. However, the interference footprint may not be so large that we need to adopt very conservative resource allocation protocols such as time division multiple access (TDMA), which activates only one link at a time, as already adopted by the current mmWave standards [6, 7]. Reference [30] proposed an index to quantify the impact of various components of the interference models and to propose a tractable and accurate

Despite the small interference footprint of mmWave networks, the option of concurrent transmissions scheduling was not included in the existing standards and proposed only recently. The authors of [27] consider the problem of maximizing

**90**

trade-off [11].

**Figure 1.**

*The obstacle density is one every 4 m<sup>2</sup>*

interference model for mmWave networks.

**4.2 Beam-searching and concurrent transmission**

Association governs the long-term allocation of communication resources among various BSs. Due to high penetration loss, blockage in mmWave networks may be only addressed by re-association or relaying procedures not by increasing the transmit power [9].1 The association and relaying are particularly important in mmWave networks due to the dense deployment of the BSs and limited size of the cells [32]. Relaying techniques can provide a more uniform quality of service by offering robust mmWave connection, load balancing, coverage extension, indooroutdoor coverage, efficient mobility management, and smooth handover operation [1, 9, 13, 32–34]. As shown in, an alternative path through relay nodes can increase the connectivity of a mmWave system by about 100%. Furthermore, the relaying technique can enable high-quality live video streaming over 300 m [34]. Therefore, proper association of clients to BSs and relaying techniques are very important routines in mmWave networks.

Developing proper association techniques has been the focus of intense research in the last years [35–40], as it may govern the long-term resource allocation policies of conventional wireless networks [35]. The current mmWave standards use the minimum-distance association, which leads to a simple association metric based on the RSSI [14]. This metric is proved to be suitable for an interference-limited homogenous network, but it may lead to poor use of the available resources in the presence of a non-uniform spatial distribution of clients, non-interference-limited environments, and heterogeneous BSs/relays with a different number of antenna elements and different transmission powers [35]. It may lead to an unbalanced number of clients per BS, drastically reducing the available resources per client in highly populated areas [13] while wasting resources in sparse areas. This poor load balancing indeed decreases network-wide fairness, since overloaded BSs cannot provide their associated clients as much resource as less-loaded BSs. Thus, it is possible for the clients to associate with farther BSs for better load sharing.

Besides the existing association techniques of the current mmWave standards, there are many more solutions for the association and relaying from the literature of microwave networks. In [37], a client association policy is investigated to ensure

<sup>1</sup> As an alternative approach, link establishment via reflectors may address a blockage, given the existence of such reflectors with sufficiently large reflection indices.

network-wide max-min fair bandwidth allocation to the clients in WLANs. In the seminal work of [36], a joint association and resource allocation problem is formulated for a heterogeneous cellular network to ensure network-wide fairness, by a distributed solution algorithm. These association procedures are highly sub-optimal for mmWave networks due to frequent handovers of mmWave networks and small interference footprint. Reducing the overhead of frequent reassociation, together with the natural need of load balancing among the BSs, justifies that a client in mmWave networks may be advantageously served by a farther but less-loaded and easy-to-find BS [13]. Robustness of the association to random blockage should be improved to reduce the number, and thereby the overhead/delay, of reassociation and to provide a seamless handover [9, 13]. Reference [41] addressed the association problem in 60 GHz mmWave communications. However, it did not consider relays, a vital part of mmWave networks, which substantially increases the difficulty of the association and relaying problem. This problem has been addressed in [42] where the authors showed that the optimal relay selection improves the load-balancing throughout the network and affects heavily the ability of a terminal to reach a farther BS. Moreover, [22] proposed an adaptive reassociation mechanism for timevarying mmWave networks, wherein the previous association solution is used as a proper initial guess to solve a new network-wide association optimization problem.

### **4.4 Spectrum sharing**

Spectrum sharing between multiple operators was recently proposed as a way to allow more efficient use of the spectrum in mmWave networks. Preliminary studies have shown that the specific features of mmWave frequencies, including the propagation characteristics and narrow beam operations, facilitate spectrum sharing in the mmWave bands. Reference [43] proposed a mechanism to let two different IEEE 802.11ad access points transmit over the same time/frequency resources. To realize this mechanism, the authors introduced a new signaling report, which is broadcast by each access point to establish an interference database that facilitates scheduling decisions. A similar approach was proposed in [44] for mmWave cellular systems, with both centralized and distributed coordination among operators. In the centralized case, a new architectural entity determines the links that cannot be concurrently activated, based on the reports of the interference powers. In the decentralized case, the victim network sends a message to the interfering network. The two networks can further refine the coordination pattern via multiple iterations.

Reference [45] investigated the feasibility of sharing the mmWave spectrum between the device-to-device/cellular and access/backhaul networks and proposed a new MAC layer in order to regulate concurrent transmissions in a centralized manner. Given the sporadic presence of strong interference in mmWave networks, reference [13] showed the need for only on-demand inter-cell interference coordination as opposed to often heavy coordination requirements of spectrum sharing at the sub-6-GHz bands. Reference [46] investigated the feasibility of spectrum sharing in mmWave cellular networks and showed that, under certain conditions such as idealized antenna pattern, spectrum sharing may be beneficial even without any coordination in the entire network. Reference [47] showed that infrastructure sharing in mmWave cellular networks is also beneficial and its gain is almost identical to that of spectrum sharing. Reference [48] discussed the architectures and protocols required to make spectrum sharing work in practical mmWave cellular networks and provided preliminary results regarding the importance of coordination. Reference [49] studied the performance of a hybrid spectrum scheme in which exclusive access is used at frequencies in the 20–30 GHz range while spectrum sharing (or even unlicensed spectrum) is used at frequencies around 70 GHz.

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**Author details**

Hossein S. Ghadikolaei

KTH Royal Institute of Technology, Stockholm, Sweden

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: hshokri@kth.se

provided the original work is properly cited.

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

allocation in mmWave networks.

**Acknowledgements**

2018-00820.

This chapter summarized main characteristics of mmWave systems, including severe attenuation, sparse-scattering environment, huge bandwidth, blockage and deafness, and possible noise-limited operation. We discussed initial access and mobility management (e.g., synchronization, random access, and handover), characterized interference footprint and reviewed existing solutions for resource

This work was supported in part by the Swedish Research Council project

**5. Conclusions**
