**3.4 Mobility management and handover**

Pencil-beam operations of mmWave systems suppress the interference at the price of more challenging mobility management and handover tasks. Vulnerability to random obstacles, UE mobility, and loss of precise beamforming information may trigger frequent handovers if only the received signal strength indicator (RSSI) is used as a reassociation metric [13]. Every handover may entail a beam-training overhead. In the presence of frequent handovers, the transmitter and receiver may remain most of the time in the beam-training phase rather than the data transmission phase.

To alleviate extra handovers due to random blockage, we can adopt the following association options in mmWave networks:

**89**

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

facilitate handover events.

**4. Resource allocation**

**4.1 Interference characteristics**

In the first approach, a client adopts multi-beam transmissions toward several BSs (relays) at the same time to establish multiple paths, either at the same band [13] or different frequency bands [19]. This approach provides seamless handover, continuous connectivity, and blockage robustness. However, we may observe an SNR loss for each beam (when a transmitter uses a fixed total power budget), higher signaling and computational complexities for beamforming, and more complicated resource management and relay selection. Cell-free access is a very similar approach wherein a processor unit coordinates the communication among multiple BSs and UEs, without associating a UE to a particular BS [20]. To enable this mode, we may need to have digital or hybrid beamforming [21]. To reduce the computational and signaling overhead of the beamforming with many antenna elements, current mmWave standards adopt analog beamforming, avoiding the realization of multiple parallel connectivity. Instead, a client may be associated with several BSs (relays) with several paths and establishes a data channel using only one of these paths, while using the others as backups. This single sequential connectivity scenario, as reported in [22], is standard-compliant and mitigates disadvantages of the multiple parallel connectivity scenario. Moreover, recent works exploited sparse scattering properties of mmWave channels to model the spatial and temporal correlation of the mmWave channels between a stationary BS and a mobile UE [23, 24]. Using this model, they have proposed efficient beam-tracking approaches to predict and

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

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

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,

area, all

area,


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

*Wireless Mesh Networks - Security, Architectures and Protocols*

wasteful contention-free ones [17].

transmission;

random delay;

random access.

sion phase.

contention-based protocol (slotted ALOHA) experience a very small collision probability. Moreover, narrower transmission and reception beams reduce the contention level, making contention-based procedures more justifiable than complex and

In LTE, a UE triggers a timer after sending a contention-based channel access preamble, and upon receiving no response from the base station (BS), it retransmits the preamble after a random waiting (backoff) time or/and with increased transmission power. As we have discussed, the deafness problem of mmWave communications cannot be efficiently addressed by an increased transmission power or a backoff time. In fact, a UE may undergo multiple subsequent backoff executions in the deafness condition, resulting in an unnecessarily prolonged backoff time [18]. To alleviate this problem, [18] introduces a novel collision-notification (CN) signal at the MAC layer to distinguish packet drops due to a collision to those due to deafness and blockage. It builds on the following observation: if the received signal has enough energy but it is not decodable, the receiver declares a collision event. Whereas, if the received signal is not decodable due to lack of energy, the receiver declares a deafness-or-blockage event. During the beam-searching phase, if a BS receives energy from a direction that is not decodable due to collisions, it sends back a CN message in that direction. After transmitting a preamble and depending on

the received control signal, a UE will take one of the following actions:

direction or adjusts the transmission beamwidth.

**3.4 Mobility management and handover**

association options in mmWave networks:

• Multiple parallel connectivity, and

• Single sequential connectivity.

1.A reservation grant is received before the timeout: the UE starts its

2.A CN message is received before the timeout: the UE assumes a contention in that spatial direction, starts the backoff procedure, and retransmit after a

3.No signal is received before the timeout: the UE assumes that there is a deafness-or-blockage event in that directed spatial channel, investigates another

Action three avoids unnecessary backoff procedures in the case of deafnessor-blockage events, substantially improving the performance of contention-based

Pencil-beam operations of mmWave systems suppress the interference at the price of more challenging mobility management and handover tasks. Vulnerability to random obstacles, UE mobility, and loss of precise beamforming information may trigger frequent handovers if only the received signal strength indicator (RSSI) is used as a reassociation metric [13]. Every handover may entail a beam-training overhead. In the presence of frequent handovers, the transmitter and receiver may remain most of the time in the beam-training phase rather than the data transmis-

To alleviate extra handovers due to random blockage, we can adopt the following

**88**

In the first approach, a client adopts multi-beam transmissions toward several BSs (relays) at the same time to establish multiple paths, either at the same band [13] or different frequency bands [19]. This approach provides seamless handover, continuous connectivity, and blockage robustness. However, we may observe an SNR loss for each beam (when a transmitter uses a fixed total power budget), higher signaling and computational complexities for beamforming, and more complicated resource management and relay selection. Cell-free access is a very similar approach wherein a processor unit coordinates the communication among multiple BSs and UEs, without associating a UE to a particular BS [20]. To enable this mode, we may need to have digital or hybrid beamforming [21]. To reduce the computational and signaling overhead of the beamforming with many antenna elements, current mmWave standards adopt analog beamforming, avoiding the realization of multiple parallel connectivity. Instead, a client may be associated with several BSs (relays) with several paths and establishes a data channel using only one of these paths, while using the others as backups. This single sequential connectivity scenario, as reported in [22], is standard-compliant and mitigates disadvantages of the multiple parallel connectivity scenario. Moreover, recent works exploited sparse scattering properties of mmWave channels to model the spatial and temporal correlation of the mmWave channels between a stationary BS and a mobile UE [23, 24]. Using this model, they have proposed efficient beam-tracking approaches to predict and facilitate handover events.
