This article describes the various backhaul technologies available for 5G networks, with a focus on E-band radio links and how they support the continued deployment of 5G networks around the world. We will conduct a technical analysis of the system requirements necessary for E-band technology. We then mapped the results into the physical radio design while gaining insight into the millimeter wave (mmW) signal chain.
5G network topology
With the successful advancement of 4G Long Term Evolution (LTE) technology, 5G networks have begun to be deployed on a large scale around the world. Figure 1 illustrates the topology of a 5G network to help us clearly understand the radio network from access to backhaul. The topology depicts four scenarios, each with a separate connection back to the core network.
User equipment (UE) such as mobile phones and 5G wireless internet will access the network by connecting to a base station (gNodeB) in the Next Generation Radio Access Network (NG-RAN). In Figure 1, we represent gNodeB as macro cells, small cells, 5G mmW access points, and repeaters. Macro and small cells cover the frequency range (FR) from 410 MHz to 7.125 GHz (FR1). The 5G mmW solution covers the frequency range from 24.25 GHz to 52.6 GHz (FR2). Macro cells have a larger coverage radius, whereas small cells are more numerous than macro cells and are easier to deploy, but have a smaller coverage radius. Small cells are used to handle traffic in areas with dense subscribers, as well as to expand network capacity or coverage more efficiently without adding macro cells. 5G mmW is the latest generation of technology that can meet the demands of higher network capacity and support new user experiences, such as live sports events where fans can watch replays on their mobile devices. There are also instances of NG-RAN devices that can operate in the FR1 and FR2 bands, such as massive MIMO radios, small cells, femtocells, picocells, etc.
Figure 1.5G network topology, including backhaul
Figure 2. The Evolution of RAN
Backhaul (also known as backhaul) or mobile backhaul refers to the transport network that connects the core network (CN) and the radio access network (gNodeB in 5G). As cell site density increases, mobile and fixed wireless backhaul become increasingly important due to the need for high-capacity links to connect the core network. According to the Ericsson Microwave Outlook 2022 report, urban cellular sites will require 5 Gbps to 20 Gbps of backhaul capacity per site by 2025. In Figure 1, we show the wireless backhaul as microwave (μW) and E-band (mmW) radios. E-band radios can be co-located with μW radios or can be used as a higher data bandwidth alternative to μW radios. While 5G presents new business opportunities, mobile operators are under increasing pressure to quickly deliver (time-to-market) high-capacity, low-latency, reliable, scalable, cost-optimized backhaul links in urban or rural areas.
What is the difference between backhaul, midhaul and prequel?
In 5G RAN, baseband unit (BBU) functions are divided into distributed units (DUs) and centralized units (CUs). How operators choose to place these devices depends on the available fronthaul interfaces and link transport technologies, and how much processing can be done at the edge with low latency compared to a more centralized approach. Figure 2 illustrates the architectural evolution of the radio access network. Postbacks are a core part of every solution.
u Cellular Site RAN: In a traditional configuration, the Radio Unit (RU) and BBU functions are located at the cellular site. A separate backhaul link is connected to the core network.
u Centralized RAN (low-level splitting): This mode allows a portion of the network to be concentrated at the edge site, which provides virtualization benefits (vBBU). Processing power will be delegated to edge sites, where there is only a physical layer, reducing complexity. However, fronthaul links are now required to transfer large amounts of data between the RU and the centralized BBU. This is sometimes referred to as a low-level split.
u Decoupled RAN (Advanced Splitting): The RU and DU can be co-located at the cellular site or separately. This model not only provides virtualization benefits (vBBU) but also improves cost efficiency. CNs are located at the edge locations alone. This is called advanced splitting:
u RU and DU are co-located at the cellular site, while the CN is located at the edge site. This means that a midhaul link is required to connect the remote CN (edge site) to the RU + DU (cellular site).
u RU, DU, and CN are placed separately.
Both centralized and disaggregated RAN models support hardware and software implementations from multiple vendors, which should be cost-effective for network deployments. Devices must be interoperable (RU, DU, CU), allowing for a mix and match of solutions from different vendors to increase efficiency. This is the core ethos of the Open RAN (O-RAN) Alliance. Previously, device providers' interface solutions were proprietary and not interoperable with devices from other vendors.
In addition, these links are evolving as operators deploy fronthaul and midhaul links in both centralized and discrete RAN configurations. If no fiber is available and/or the cost of installing fiber is prohibitive, or fiber is not a viable option for deployment in the short term, then the E-band can provide a great solution.
It is important to note that there is a fundamental difference between 4G and 5G: in 5G NR, the traditional EPC (Evolved Packet Core) runs on dedicated hardware, usually located near base stations or cell towers, resulting in splitting. This allows the functions to run on commercial off-the-shelf (COTS) hardware. As a result, the core network of 5G is actually more fragmented as functions move to the edge. See Figure 3. Core network functions can now be co-located at the edge, enabling faster communication and lower user latency. It also supports network slicing, which is the creation of virtual networks for specific application needs. For example, one slice can provide high-speed broadband, while another slice can provide machine-to-machine connectivity for the Internet of Things. In addition, this edge cloud architecture supports edge computing. As a result, networks can set up small data centers close to the edge to support video streaming of the same content, rather than laboriously backhauling data from a central location. In general, this 5G architecture is more efficient and flexible in configuring network access, hardware, functions, and backhaul.
Figure 3.5G network slicing
What backhaul solutions are currently available?
Fiber backhaul is the highest capacity option available to mobile network operators (MNOs). It is the mainstream small cell backhaul technology currently in use because fiber is available in many densely populated urban/indoor areas, and small cells are used in these areas to increase coverage/capacity. Fiber has a capacity of up to 1.6 Tbps (160 signals × 10 Gbps per signal). Fiber is the highest capacity option for MNOs. However, fiber deployment has problems such as high cost, difficult procurement, complex planning approval, and time-consuming. According to the GMSA, the cost of deploying fiber is about $70,000 per kilometer. Capex and deployment time have always been factors hindering continued growth. It is important to note that μW/mmW backhaul and fiber are complementary solutions that coexist in the network. Wireless and fiber optics provide operators with alternative backhaul technologies. The ideal backhaul solution needs to consider many factors, including deployment time, federal/state and city permits, right-of-way, data bandwidth requirements, terrain, and total cost of ownership.
μW and mmW backhaul are currently the mainstream backhaul technologies for macro cells, accounting for about 50% of macro cell backhaul links.
μW licensed band technology is powerful, easy to deploy, and relatively inexpensive (no need to destroy city streets or dig trenches). It covers frequencies from 6 GHz to 42 GHz, and these bands are ideal for medium and long-haul links, with a range of up to 25 km.
The use of mmW backhaul technology in the V-band (57 GHz to 66 GHz) and E-band (76 GHz/86 GHz) has been ongoing for many years. However, the V-band suffers from severe oxygen absorption, with large signal attenuation occurring at 60 GHz. In addition, countries have different regulations on the use of this frequency band. Some countries use some of their spectrum licences for backhaul, while others leave it for license-free use. Europe and the United States are the regions where license-free use is allowed, and rules are being developed to reduce the probability of interference in different configurations. However, the V-band remains unreliable in providing high-quality backhaul. Its use is expected to be primarily license-free short-range indoor and outdoor coverage solutions (WiGig). The E-band provides a solution with wider bandwidth and lower signal attenuation, enabling high-availability links.
In 4G networks, mmW backhaul technology is not fully utilized due to the available bandwidth capacity and is only used in certain scenarios, so most wireless backhaul is implemented using the licensed μW band (6 GHz to 42 GHz). With the explosion and densification of 5G networks, the situation has changed, and backhaul capabilities of 10 Gbps or more are now required.
So, what are the core advantages of using the E-band, and how does it compare to fiber and μW? The E-band offers two 5 GHz spectrum bands: 71 GHz to 76 GHz and 81 GHz to 86 GHz. These frequency bands are subdivided into multiple 250 MHz channels. A major advantage of spectrum allocation is that it can be used for time division duplex or frequency division duplex links. Capacity is also not an issue, as the maximum amount of data that can be transferred is greater than 60 Gbps1 in a licensed E-band point-to-point link. The E-band is also expected to be used in point-to-multipoint systems, which will further increase the available backhaul data bandwidth. Compared to traditional μW radios, the channel capacity is significantly increased. Due to frequency availability issues, traditional μW radio links have a capacity of only about 2.4 Gbps. In addition, E-band antennas concentrate electromagnetic energy in a very narrow beam of energy (e.g., a divergence angle of only 1 degree), making it possible to construct high-gain (45 dBi) radio equipment with a small form factor (30 cm antenna diameter) that is ideal for covert placement on buildings or towers. Even if the RF transmit power is not high, the E-band can typically support link lengths of up to 3 km2. Table 1 compares several commonly used backhaul techniques.
Table 1.Comparison of backhaul technologies
Copper is a traditional technology that uses the T1/E1 protocol. Copper can't easily scale to provide the bandwidth needed for 4G, let alone 5G. It's still an option for indoor small cells and public spaces, but operators have begun to abandon the technology. Satellites are not widely used compared to fiber optics or μW/mmW due to limited data rates and latency is an issue because geostationary satellites are in very high Earth orbits. Low-Earth orbit (LEO) satellites have improved latency and may play an increasing role, but the specifics remain uncertain. The main advantage of satellites is to connect rural areas where there are no alternatives available. With the exception of a very small number of emerging markets, Wi-Fi is not a widely used backhaul technology. These bands are license-free, so the growing number of access points can cause interference, and limited coverage is also a problem.
How does a wireless E-band link transmit data wirelessly?
The E-band uses traditional digital modulation coding, e.g. from BSPK to 1024 QAM. But what are the factors that limit link distance?
u Inclement weather: Rain, fog, sleet, and snow can attenuate signal strength in unpredictable ways, resulting in a decrease in the level of signal received by the receiver, which in turn reduces the signal-to-noise ratio (SNR). It is important to note that the E-band radio link can use adaptive modulation when experiencing rain decay. This means that the link can be switched to less complex modulation to prevent data loss. By reducing capacity during this time, the connectivity of the high-availability data link is maintained. At rainfall levels of up to 100 mm/hr, Analog Devices' System-on-Package (SiP) solutions ensure 99.999% availability for 1 km links.
u Baseband capability: When operating at E-band frequency, the baseband unit becomes the bottleneck of data throughput. A typical BBU supports 10 Gbps of data throughput, while the available spectrum can support more than 60 Gbps of data throughput. ADI E-band SiPs will support modulation orders up to 1024 QAM.
Phase noise of the LO: Phase noise limits the modulation order. LO jitter results in a lower signal-to-noise ratio (SNR) as noise is superimposed on the target signal to be upconverted/downconverted. Analog Devices offers best-in-class wideband external phase-locked loop/voltage-controlled oscillator (PLL/VCO) sources, as well as E-band on-chip LO path multipliers and amplifiers.
Table 2 shows the expected bit efficiency and SNR requirements for the multiple modulations supported by the E-band technology.
Figure 4. E-Band Radio Unit System Diagram (Blue = ADI Solution)
Table 2. E-band technology supports digital modulation coding and SNR
Are E-band radios more difficult to design than μW radios?
Surprisingly, E-band radios can take advantage of a large portion of current μW radio baseband card designs, including modem cores, processors, memory modules, clock recovery/generation, synchronous 1588 circuitry, and lower frequency analog front ends. This makes it easier for μW radio vendors to transition to the E-band domain. See Figure 4. E-band front-end modules, duplexers, and antennas are the new design blocks required to convert μW radios to E-band radios.
There is no doubt that the 76 GHz/86 GHz design may seem intimidating, as the mmW design is more complex compared to lower frequency RF or even μW. As shown in Figure 4, waveguide conversion is now integrated as part of ADI's E-band SiP to minimize radio frequency (RF) loss from the antenna and convert to higher frequency signals. ADI SiP eliminates chip, bonding, and epoxy assembly. ADI SiPs can be assembled using standard die assembly equipment. E-band SiP makes radio assembly similar to μW radio assembly.
E-band link budgets can be challenging due to 131 dB of free space loss and 17 dB/km and 31 dB/km of rain attenuation at 1 km4 (for 99.99% and 99.999% availability, respectively)5. Designers must carefully consider requirements such as gain, transmit power, noise figure, and IP3 to meet the backhaul requirements of 5G network operators.
Analog Devices has a deep heritage in μW and mmW backhaul technologies. We have developed E-band devices to address many of these design and assembly challenges, making it easier for more designers to develop E-band products.
E-band – the next important option for 5G backhaul needs
This article highlights the ability of the E-band to provide higher bandwidth for 5G networks, expanding backhaul options. It is an excellent complement to fiber optics and provides operators with greater flexibility in planning, deploying, and balancing centralized and disaggregated RAN solutions.
Analog Devices has developed surface-mount, highly integrated SiPs with baseband inputs or outputs, as well as integrated waveguide outputs or inputs, eliminating much of the heavy lifting associated with E-band front-end designs. Designers no longer need to worry about chip handling and can take advantage of Analog Devices' E-band packaging technology solutions. Analog Devices is committed to advancing this market by providing more easy-to-use technology for more RF/μW and mmW designers. Part 2 will delve into the E-band link budget and the technical details of ADI's E-band SiP family.
author
Andy Boyce is a Systems Architect at Analog Devices, Inc., where he develops signal chains and system solutions. He has been designing RF and microwave products for wired, wireless, and defense systems for over 30 years. Andy holds a bachelor's degree in electrical engineering from the University of Massachusetts Lowell and a master's degree in finance from Bentley University.
Donal McCarthy is the Director of Marketing and Business Development for Analog Devices, Inc. in Microwave Communications (Cork, Ireland). He holds a Bachelor of Business Administration from College Cork, an MBA from Boston College, and a Degree in Marketing from Irish Institute of Management, Dublin. Donal has held a number of positions, including MACOM Design Engineer, Hittite Field Sales Engineer and Marketing Positions, and Analog Devices Marketing Manager and Director Positions.