**5. Conclusions**

*Wireless Mesh Networks - Security, Architectures and Protocols*

**4.4 Spectrum sharing**

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.

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

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.

can further refine the coordination pattern via multiple iterations.

**92**

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 allocation in mmWave networks.
