**2. Fundamentals**

*Wireless Mesh Networks - Security, Architectures and Protocols*

**(s)**

**Usage models Latency** 

Ultra short-range communications

Video streaming in smart homes

coverage distances of a few meters [2]. However, recent studies on mmWave mobile networks have convinced academia, industry, and regulatory bodies to repurpose

MmWave communications are particularly attractive for ultra-short range/ high rate communications and gigabit wireless applications such as wireless gigabit

**Availability Range** 

Augmented reality <0.005 NS <10 20 Interface between a

Data center <0.1 99.99% <5 40 Inter-rack connectivity,

Vehicular networks <0.1 NS <1000 NS Intra- and inter-

Video on-demand <0.01 NS <100 NS Broadcast in crowd

Mobile offloading <0.1 99.99% <100 20 Offload video traffic

Mobile fronthauling <0.035 99.99% <200 20 Wireless connections

Mobile backhauling <0.035 99.99% <1000 20 Small cell backhauling,

*Application scenarios for mmWave networks. This table is deduced from ongoing discussions inside IEEE* 

**(m)**

<1 NS <10 10 Wireless tollgate and

<0.005 NS <5 28 8K video stream

**Rate (Gbps)** **Application scenarios**

kiosks to transfer e-magazine, picture library, 4K movie trailers, 4K movies

between a source device (e.g., set-up box, tablet) and a sink device (e.g., smart TV, split TV), replacement of wired interface

constantly moving high-end wearable devices and its managing device to deliver 3D video

wireless backup connection

car connectivity, intersection collision avoidance, public safety

public places (e.g., classroom, in flight, train, ship, bus, exhibitions)

from cellular interface to the mmWave interface

between remote radio heads and base band unit

mutihop backhauling, inter-building communications

Ethernet and uncompressed high-quality video transmission, see **Table 1**.

the mmWave band for future wireless (and even mobile) networks.

**84**

**Table 1.**

*802.11ay study group. "NS" means not specified yet.*

### **2.1 The directed mmWave wireless channel**

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, without affecting the size of their radio chips.

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

*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 terminals.

*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.

### **2.2 Beam training**

The use of low-complexity and low-power mmWave devices, along with the massive number of antennas, makes traditional digital beamforming based on instantaneous channel state information very expensive. Instead, the existing standards assume the use of analog beamforming and establish an mmWave link using the so-called beam-searching approach. This approach searches among a set of pre-defined beam steering vectors for the transmitter and the receiver (beam training codebook) and selects the best beam pairs [1, 8, 9]. More specifically, current standards suggest a three-stage beam-searching technique to reduce alignment overhead. After a quasi-omnidirectional sweep with very wide beams, a coarsegrained sector-level sweep is performed, followed by a beam-level refinement phase (the highest resolution pattern specified in the codebook). Each level involves an exhaustive search over all possible transmission and reception directions through a sequence of pilot transmissions. The combination of vectors that maximizes the signal-to-noise ratio (SNR) is then selected for the beamforming.

This beam searching process introduces a new *alignment-throughput* trade-off [11]. That is, on the one hand, a narrower beamwidth enhances the beam resolutions, so increases the alignment overhead and leaves less time for data transmission. On the other hand, it provides a higher antenna gain, leading to a higher transmission rate.

One of the main drawbacks of analog beamforming is the lack of multiplexing gain, which is addressed by the hybrid digital/analog beamforming architecture [12]. Efficient beam training for hybrid beamforming is an active field of research.

### **2.3 Control channel**

Many operations such as establishing a communication channel, discovering neighbors, exchanging routing information, and coordinating channel access rely on the exchange of signaling messages on a control channel. The characteristics of mmWave communications introduce fallback and directionality trade-offs, which also appear in mmWave cellular networks [13].

The *fallback* trade-off is the trade-off between sending control messages through an mmWave or a microwave channel. The mmWave channel is subject to blockage, reducing the reliability of the control channel. A dedicated microwave control channel facilitates network synchronization and broadcasting at the expense of [14]. The cost of this approach is higher hardware complexity and energy consumption since an extra transceiver should be tuned on the microwave control channel. Moreover, a microwave control channel cannot be used to estimate the mmWave channel and adopt proper beamforming. Note that realizing a control channel in the mmWave band with omnidirectional transmission/reception may substantially reduce the system performance. The main reason is a mismatch between the ranges at which a high-quality data link can be established (using directional communications) and the range at which control messages can be exchanged [13].

The *directionality* trade-off arises due to the potential of establishing a control channel with multiple antennas. An omnidirectional control channel is subject to a very short range due to the lack of antenna gains, but it diminishes the deafness problem. A directional control channel benefits longer coverage at the expense of extra alignment overhead.

Altogether, we may have two justifiable realizations for physical control channels: (1) omnidirectional-microwave, employed in ECMA 387 [15], and (2) directional-mmWave, employed in IEEE 802.15.3c [6] and IEEE 802.11ad [7].

**87**

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

**3. Initial access and mobility management**

**3.1 Synchronization and cell search**

**3.2 System information**

physical control channels).

**3.3 Random access**

access that should be considered in mmWave cellular networks.

severely limit the performance of mmWave cellular networks.

System information includes cell configurations such as frequency band, downlink and uplink bandwidth, cell identity, random access procedure, and the number of transmit and receive antennas. In LTE, the so-called master and system information blocks embed system information. They are transmitted in the physical broadcast dedicated channel and the physical downlink shared channel, respectively. While dedicated control channels can be established with omnidirectional communications, a UE still needs to decode a directional shared channel to extract system information in an mmWave cellular network. Consequently, it will become subject to the fallback and directionality trade-offs. This is a fundamental MAC layer challenge, which is not present in the legacy microwave cellular networks, as all their rendezvous signaling are done in the single antenna mode (omnidirectional

At the very beginning, a UE has no reserved channel to communicate with the BS(s). In this case, it sends a channel reservation request using either contentionfree or contention-based channel access schemes. In the contention-free approach, the network broadcasts multiple access signals that uniquely poll individual UEs to avoid potential collisions. Upon decoding a signal, each UE knows its uplink parameters including analog or digital beamformer, random access preamble, and allocated resource for transmission of the preamble. Embedding all this information a priori is a challenging task due to the lack of spatial synchronization at the very beginning. The contention-based approach is another strategy to send channel access requests wherein requests may be dropped due to potential collision (in the case of simultaneous transmissions in the same cell) or not be received (in the case of blockage or deafness). The comprehensive analysis of the next chapter shows that small to modest size mmWave networks operating with a simple

Initial access and mobility management are fundamental MAC layer functions that specify how user equipment (UE) should connect to the network and preserve its connectivity. In this section, we highlight important design aspects of initial

In the long-term evolution (LTE) systems, the so-called primary and secondary synchronization signals enable acquiring time-frequency domain synchronization during the cell search phase. Current cellular networks use beamforming only *after* omnidirectional synchronization and cell search procedure. However, as pointed out in [16], performing cell search on an omnidirectional physical control channel while having antenna gain in data transmission causes a mismatch between the ranges at which a link with reasonable data rate can be established and the range at which a broadcast synchronization signal along with cell identity can be detected. At a normal free-space propagation environment at 28 GHz, the data range can be at least four times larger than the synchronization range with only 30 dBi combined antenna gains. Such a huge mismatch in the ranges of the control and data plane can *Wireless Mesh Networks - Security, Architectures and Protocols*

The use of low-complexity and low-power mmWave devices, along with the massive number of antennas, makes traditional digital beamforming based on instantaneous channel state information very expensive. Instead, the existing standards assume the use of analog beamforming and establish an mmWave link using the so-called beam-searching approach. This approach searches among a set of pre-defined beam steering vectors for the transmitter and the receiver (beam training codebook) and selects the best beam pairs [1, 8, 9]. More specifically, current standards suggest a three-stage beam-searching technique to reduce alignment overhead. After a quasi-omnidirectional sweep with very wide beams, a coarsegrained sector-level sweep is performed, followed by a beam-level refinement phase (the highest resolution pattern specified in the codebook). Each level involves an exhaustive search over all possible transmission and reception directions through a sequence of pilot transmissions. The combination of vectors that maximizes the

This beam searching process introduces a new *alignment-throughput* trade-off [11]. That is, on the one hand, a narrower beamwidth enhances the beam resolutions, so increases the alignment overhead and leaves less time for data transmission. On the other hand, it provides a higher antenna gain, leading to a higher

One of the main drawbacks of analog beamforming is the lack of multiplexing gain, which is addressed by the hybrid digital/analog beamforming architecture [12]. Efficient beam training for hybrid beamforming is an active field of research.

Many operations such as establishing a communication channel, discovering neighbors, exchanging routing information, and coordinating channel access rely on the exchange of signaling messages on a control channel. The characteristics of mmWave communications introduce fallback and directionality trade-offs, which

The *fallback* trade-off is the trade-off between sending control messages through an mmWave or a microwave channel. The mmWave channel is subject to blockage, reducing the reliability of the control channel. A dedicated microwave control channel facilitates network synchronization and broadcasting at the expense of [14]. The cost of this approach is higher hardware complexity and energy consumption since an extra transceiver should be tuned on the microwave control channel. Moreover, a microwave control channel cannot be used to estimate the mmWave channel and adopt proper beamforming. Note that realizing a control channel in the mmWave band with omnidirectional transmission/reception may substantially reduce the system performance. The main reason is a mismatch between the ranges at which a high-quality data link can be established (using directional communica-

tions) and the range at which control messages can be exchanged [13].

The *directionality* trade-off arises due to the potential of establishing a control channel with multiple antennas. An omnidirectional control channel is subject to a very short range due to the lack of antenna gains, but it diminishes the deafness problem. A directional control channel benefits longer coverage at the expense of

Altogether, we may have two justifiable realizations for physical control channels: (1) omnidirectional-microwave, employed in ECMA 387 [15], and (2) directional-mmWave, employed in IEEE 802.15.3c [6] and IEEE 802.11ad [7].

signal-to-noise ratio (SNR) is then selected for the beamforming.

**2.2 Beam training**

transmission rate.

**2.3 Control channel**

also appear in mmWave cellular networks [13].

**86**

extra alignment overhead.
