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.4.1. Wired backhaul networks technologies

Although wired backhaul solutions ensure reliability with large capacity and bandwidth, low latency, the cost of wired connections is highly dependent on the offered capacity as well as the distance.
In addition to that highly reliable wired backhaul connectivity is usually not important for the small cells that are typically serving a relatively reduced traffic load compared to a macro site, even in locations hard to be reached by optical fiber or copper cable.
3.4.1.1. Copper cables

In the past, the leased copper lines dominated the backhaul solutions, as they provided the sufficient capacity per cell site to handle 2G traffic including voice and short message service. Recently, the required backhaul capacity has significantly increased due to the increasing number of mobile subscribers, the availability of mobile high-speed data services and significant growth in the number of deployed base station sites. In addition to the limited capacity, the price of copper increases linearly with capacity, thus it is not a cost efficient choice for 5G backhaul network. The alternative to copper cables for mobile backhaul is optical fiber that can provide almost unlimited capacity. In this regard, there are two standard Plesiochronous Digital Hierarchy (PDH) hierarchies the T-carriers (T1, T2, T3 and T4) and E-carriers (E1, E2, E3, E4 and E5). T1 links operate with 1.544 Mbit/s while E1 connections operate with 2.048 Mbit/s. Currently, copper cables are being replaced by optical fibers due to their higher rates/bandwidth and low latency. 15

3.4.1.2. Optical Fiber

The optical fiber is the best backhauling medium to offer high capacity meeting all the expectation for traffic increasing thanks to the high bandwidth they offered. But despite its almost unlimited bandwidth, reliability, low latency, low transmission loss, jitter and immunity to electromagnetic interference, in many countries fiber penetration in the backhaul network has been relatively slow compared to microwave solutions, due to the high installation cost and impracticality of laying fiber in difficult terrain (mountains, forests, deserts, jungles…). Deploying a new fiber require a lot of civil works as digging trenches in roads requires permits, traffic management and, once the ducts have been laid, reconstruction of the road by backfilling the hole then reinstating the surface, so owning a fiber is a significantly expensive capital expenditure (CAPEX) option. It is also estimated that leased lines currently account for roughly 15% of the network operating expenditure (OPEX). Optical fiber can travel for long distances, because light propagates through the fiber with much lower attenuation compared to electrical cables. This allows long distances to be divided with few repeaters. The main fiber access options include GPON (gigabit passive optical network), carrier Ethernet and point?to?point (PTP) fiber. 15

3.4.2. Wireless backhaul networks technologies

By contrast, wireless backhaul is more cost effective and quick to deploy. In addition, capacities supported by microwave continue to grow, securing its place in future wireless backhauling networks. For less populated rural areas, where the cost to lay fiber can be prohibitive, wireless backhaul will be the only suitable solution. In addition, because radio propagates over the air faster than light travels through fiber, wireless backhaul can achieve lower latency than fiber. Therefore, if wireless backhaul can achieve high data rates comparable to fiber capacity, it will be a cost effective and a very attractive solution for remote communities with broadband services. It can also serve as, or be part of, an ultra-low latency network for low latency applications. Reasons include the possibility of frequency reuse, easy operation and maintenance and increased flexibility of backhaul resource allocation. There are several wireless backhauling solutions exist such as TVWS (800 MHz), sub-6 GHz (licensed and unlicensed), microwave spectrum between 6 GHz and 42 GHz, millimeter wave spectrum between 60 and 90 GHz and FSO (free space optical) spectrum within the laser spectrum. Figure 3.5 shows Microwave and millimeter wave radio frequency spectrum.

Figure 3.5: Wireless Backhaul radio frequency spectrum. 21

3.4.2.3. Traditional microwave (6 – 42 GHz):

Microwave backhauling has been the primary solution for cost-effective wireless mobile backhaul infrastructure worldwide. Microwave backhaul networks tend to operate within the 6-42 GHz licensed band, are deployed using FDD with paired channels, since low frequencies are less sensitive to rain, these bands will continue to be used for long hop distances. The microwave spectrum is divided into specific frequency bands administered by national regulators in individual countries. The licensing fee is determined by formulas depending on the transmission data rate, the required bandwidth, or both.
A point-to-pint microwave link between two base stations located at fixed points composed of four main elements: a transmitter, a receiver, antennas, and transmission lines (see Fig. 3.6). The transmitter produces the signal that carries the information to be communicated. It generates the microwave energy using the selected frequency and configured transmission power, and modulates it with the input signal. From the transmitter, the signal is sent to a directional antenna through the IF cable. On the transmitting end, the antenna emits the signal from the IF cable into free space. At the receiver site, an antenna directed towards the transmitting station collects the signal energy and feeds it into the IF cable for processing by the receiver. The microwave directional antennas characteristic of concentrating the received signal allows communication over long distances. Finally, the receiver extracts information from the signal by detection and demodulation.
Since the bandwidth of each microwave channel is narrow (7 to 56MHz), the data rates of traditional microwave links are relatively low, it can provide up to 1 Gbps. The introduction of wider channels such as 112 MHz has started, which, together with new spectrum-efficient technologies such as Polarization multiplexing, Line-of-sight MIMO, higher modulation and Multi-layer header compression will further boost capacity. Traditional licensed LOS microwave point-to-point links have played important roles in backhaul for 3G and 4G, but the demands of 5G will quickly deplete its capacity. The 5G capacity requirements will push the backhaul spectrum toward higher frequency bands (millimeter wave bands). 17

Figure 3.6: Microwave link components

3.4.2.4. Millimeter wave:

The millimeter wave is regarded as a promising candidate for 5G mobile backhauling in dense urban areas. Millimetre Wave is a wireless enabled technology for multi Gb/s data transmission communication systems. It refers to wave lengths 1 to 10 mm where is corresponding to frequencies in the range 30-300 GHz. The explosive developments in circuit technologies have led to mmWave now being considered a viable option, and indeed foreseen as shaping next?generation wireless backhaul. The mmWave link can be used in line of sight (LOS) and non-line of sight (NLOS) applications. 18
IEEE 802.11ad standard was developed in 2014 for outdoor backhaul. It specifies the physical layer and MAC layer in the frequencies above 40 Gbps and support wireless transmission with multi Gb/s but with limited range. Current research has shown that point-to-point systems using mmWave either in V-band (57-64GHz) or E-Band (71–76 GHz and 81–86 GHz) can achieve higher data rate (up to 10 Gbit/s). The V- and E-Band spectra were regulated or were being considered for regulation for the deployment of communication systems by most countries and region in the wold. 19
Millimeter wave communications suffer from huge propagation loss compared with other lower frequencies communication system. The rain attenuation and the atmospheric and molecular absorption characteristics of millimetre wave propagation limit the range of millimetre wave communications 18, which will be discussed in details in chapter 4. However, this may be advantage for densely deployed small cells because of the reduced interference from adjacent links and, in turn, increased the probability of frequency reuse.
The millimetre wave in V-Band (57-64GHz) has been proposed in 2009 which allows very high data rate over 2 Gb/s. The advantages of using this band include inference migration, security and QOS is it an unlicensed band. However, V-Band suffers from high atmospheric attenuation due to oxygen absorption (around 15 dB/km) and limitation of the transmitted power (< 0.5 W) seriously limiting radio transmission distances. 20
The millimeter wave in E-Band (71–76 GHz and 81–86 GHz) is favourable for high data rate due to small atmospheric attenuation 0.5 dB/km. E-band is the highest allocated frequency band by the Federal Communications Commission (FCC) in 2003 in wireless history.
Furthermore, the E-Band technology offers many advantages over the wireless communication technologies such as low cost of construction, quick deployment, flexibility, high reliability and security. Moreover, the E-Band can operate with up to 3W of output power and high focused signals. Thanks to the small wavelength of E-band signals, it is possible to achieve high antenna gains at E-band frequencies. The high antenna gain and high transmission power allow E-band to overcome the high propagation loss and high rain fading experienced at E-band frequencies providing an opportunity for higher data rates with long range.
The E-band frequency allocation consists of the two 5 GHz frequency channels (71–76 GHz and 81–86 GHz), which is larger than any other microwave frequency band, enabling a new generation and high capacity of wireless transmission to be introduced. E-band frequency spectrum is divided into a pair of 5 GHz channels. A single pair 5 GHz channel at E-band is around 100 times the size of the current largest microwave frequency channel. The advantages of E-band wireless communications include the large, uncongested, inexpensive spectrum with total 10 GHz of available bandwidth which enables very high data rates beyond 10 Gbps. These advantages make the E-band links ideally suited for short range (0-3 km) wireless communications. E-band can be used also as link protection in the case of fiber breakage, the service can be restored temporary more rapidly than restoring the original fiber link. 18
To promote E-band usage, the national wireless link regulators and administrators in many countries have introduced “light licensing” for managing implementation this band, providing the E-band with an attractive alternative to existing licensed traditional frequency bands. The “light licensing” policy “first come first served” allows the E-band licenses to be applied for a little time and at low cost per year, significantly faster and cheaper than those for traditional microwave bands. The “light licensing” policy comes from the unique characteristics of the E-band. First, since there are few E-band services currently. Second, the high frequencies at E-band allow the systems to adopt highly directional antennas and communicate via highly focused transmissions, leading to dense configuration of communication links without interference concerns and thus a high degree of frequency reuse. Third, the E-band frequencies are configured as a single pair of 5 GHz channels, which makes the frequency planning/coordination unnecessary and the related interference analysis significantly simplified. Thus, the E-band administration and cost of license are highly reduced.
Looking to the future, wireless communication has an interest in the use of frequencies above 100 GHz, as they will enable capacities in the 40 Gb/s range over hop distances of about a kilometer.

Figure 3.7: A summary of wireless backhaul technologies in 5G 9
3.6 Conclusion

In brief, the main challenges face Mobile backhauling, is how to achieve high data rate to meet the evolution of mobile communication network requirements, the backhaul link distance and how to achieve ultra-low latency communications between end users across the backhaul networks. Optical fiber is considered an appropriate medium to carry transport network traffic. However, in some deployment scenarios, it is an expensive option. While millimeter wave connectivity is a cost-effective solution for 5G backhaul, which is able to meet the capacity demand. However, it is sensitive to weather conditions. Wireless based solutions have emerged as an alternative way to facilitate transport network for 5G. This is obvious that, wireless backhaul/fronthaul can be much more flexible and agile compared to optical networks.