What will it take for truly nationwide 5G in the U.S.?

The realization of 5G has required the fusion of multiple radio access technologies (RATs) such as 5G New Radio (NR), 4G LTE, Wi-Fi in various network topologies along with the usage of an array of RF technologies (e.g., mMIMO, FWA, microwave backhaul, etc) and spectrum blocks.

All of these aspects of 5G (and more) have allowed for MNOs to steadily meet 5G key performance indicators (KPIs) as originally specified by IMT-2020 for the various 5G use cases of enhanced mobile broadband (eMBB), ultra-reliable low latency communications (uRLLC), and massive machine-type communications (mMTC). The 3rd generation partnership project (3GPP) -- the major standards organization for 5G -- hit the major milestone of introducing 5G NR back in 2018 in release 15 (Rel-15) with the standardization of the 5G core (5GC).

Where release 16 (Rel-16) broadened use cases of NR targeting new industry verticals such as industrial IoT and vehicle-to-everything (V2X) communications. The world radio communications conference in 2019 (WRC-19) approved the 26 GHz, 28 GHz, 40 Ghz, 50 GHz, and 66 GHz blocks to expand the harmonized spectrum for 5G for ultra-high speed and ultra-low latency communications. Within the U.S., as of April 2020, the FCC took steps to expand unlicensed bandwidths to support the 3GPP 5G unlicensed NR (NR-U) and Wi-Fi 6E by including the 6 GHz (5.925 to 7.125 GHz) spectrum in unlicensed broadband operations.

However, with all the technicalities and standards/specifications around 5G, it is easy to get lost in obscurities. For this reason it can be difficult to discern how true 5G will cover the U.S. landmass. the types of areas in the U.S. and the various RF and backhaul technologies used to best serve these areas.

5G deployment areas and respective site configurations

The classical deployment scenario for 5G has often been centered on “dense urban” (DU) environments with a focus on the installation of a massive amount of outdoor small cells, often with dedicated fiber backhaul to the 5GC. In these DU environments, the backhaul link involves a singular hop from the closest fiber point of presence (PoP) site^[1]. This allows for high data rate (>10 Gbps), low latency communications while meeting the availability/coverage requirements despite the high device density (> 10,000 km²). Urban (U), suburban (SU), semi-rural (SR), and rural (R) environments however, can meet throughput, latency and availability requirements with a macro-cell, microwave backhaul solution. In U and SU areas, capacity (< 5 Gbps) and access requirements can be met with up to two hops at existing hop lengths, from the closest fiber PoP site. For SR and R areas, this number increases to a three-hop distance from the closest fiber PoP site to support up to 3 Gbps link capacities in the 5G mature stage (Table 1).

Table 1.

The urban, rural and low-income divide

It is made apparent that the site distribution and transmission distances grow rapidly from a dense urban environment to rural areas (Figure 1), thus causing closest fiber PoP to be at a further and further distance from the area. This has led to microwave backhaul accounting for over 70% of macro sites as opposed to a fiber optic or copper backhaul connection. This is due to the cost of installing hundreds of miles of fiber via the direct burial methods -- a fiber-optic link is estimated to cost up to $200,000 per km in metropolitan areas^[8]. Microwave backhaul, on the other hand, provides a more flexible connection without the upfront capital expenditures (CAPEX). Mobile Network Operators (MNOs) globally have relied on the 7 GHz to 40 GHz bands for microwave backhaul links. However, V-band (60 GHz) and E-band (70/80 GHz) links are also leveraged to support higher data rates from 10 Gbps to 25 Gbps.

Figure 1. The varying areas 5G must service and their respective backhaul technologies[3]. Image Credit: ITU

Traditionally, rural and under-developed areas of the nation have not had the privilege of a high fidelity connection; let alone access to 5G. More than a quarter of the 29 million rural American households do not have access to basic broadband internet (25 Mbps down). Certain low-income urban neighborhoods face similar circumstances where 1.4 percent are without access to high speed internet while more than a quarter do not have access to a symmetrical high speed connection where both upload and download speeds are the same^[4]. This fundamental coverage issue in the U.S. has caused the FCC to establish the “5G Fund for Rural America” where up to $9 billion is being invested to ensure that rural Americans enjoy high speed, low latency 5G connectivity. While the densification of wireless backhaul is critical in order to best support rural U.S, this connectivity can be augmented with non-terrestrial 5G where satellites can act as a mirror reflecting data from a 5 base station (gNB) to the UE, or, act as a stand-alone gNB fully supporting NR protocols.

Understanding the critical role these backhaul technologies play in delivering 5G

Microwave backhaul and potential of satellites for rural and semi-rural areas

The typical spectrum for wireless backhaul falls below 23 GHz where the Far East and Latin America mostly leverage the 7/8, 15, 18, and 23 GHz bands while Europe relies mostly on the 26 and 38 GHz bands. North America, however, mainly uses the 6 GHz, 11 GHz, 18 GHz, and 23 GHz blocks -- the 7/8 band is a licensed federal band. This illuminated in a map (Figure 2) of the 6 GHz bands for microwave backhaul in the U.S. and a map of the 6/7/8 GHz microwave backhaul infrastructure used in the UK and France.

Figure 2. Usage of 6 GHz and 7/8 GHz bands for microwave backhaul in the US as well as the UK and France[5]. Image Credit: Ericsson

An ongoing discussion is being held around the 6 to 8 GHz bands that are very commonly used for backhaul links with the possibility to expand unlicensed bandwidths into this spectrum space to support applications such as Wi-Fi 6E (IEEE 802.11ax) and the 3GPP 5G NR-U standard for 5G unlicensed networks. As of April 2020, the FCC expanded the unlicensed broadband operations in the 6 GHz (5.925 to 7.125 GHz) spectrum. The expectation is that new Wi-Fi hotspots will integrate this bandwidth and enable faster internet connections while 5G network capacity is also better supported. This allows 5G NR-U deployments to exist bolstering connection speeds in areas where the 6 GHz bands are under-utilized.

Non-terrestrial 5G also has a strong role to play in desolate areas of the U.S. without connectivity. High throughput satellite (HTS) technology and newer, low earth orbiting (LEO) constellations can effectively offer an unimpeded high speed connection to areas previously unconnected with ground terminals. Both of these types of satellites can integrate either transparent or regenerative payload technology. The transparent processor m;irros received signals and transmits an amplified version of the signal over the correct frequency channel. The regenerative payload includes encoding/decoding via either a digital transparent processor (DTP) or a fully regenerative (FR) transponder. This type of satellite has the potential to send/receive NR signals and function as a gNB. Even equipped with a transparent payload a satellite can act similar to microwave backhaul, offering a multi-hop solution from a predetermined fiber PoP. Service ubiquity across the U.S. can be accomplished with satellite-enabled gNBs working in tandem with established satellites to bring 5G to underserved premises, IoT UEs, and even remote factories. This can also be translated to mobile applications where shipping companies, fleet operators, train conductors, and regular travelers with a mobile handset can travel through multiple terrestrial and satellite coverage areas and stay connected. A relatively low latency connection can be better accomplished with the LEO infrastructure due to the shorter distance the signal must travel -- a GEO satellite has a round-trip delay of 280 ms while a LEO satellite can bring that down to 6 ms.

An extensive fiber infrastructure to support dense urban environments

As stated earlier, fiber deployments are critical in supporting outdoor small cells as well as providing adequate backhaul for indoor hotspots. Even macro-cells have shifted from the traditional copper backbone to a fiber optic cable supporting the CPRI protocol for communications between the baseband unit (BBU) and remote radio head (RRH) in 4G base stations (eNBs). The 5G gNB involves multiple functional splits between a centralized unit (CU), one or more distributed units (DUs), with the possibility of a separate radio unit (RU). In this case, the enhanced CPRI protocol is leveraged to enable an intra-PHY split to support NR signals with enhancements such as carrier aggregation MIMO, and coordinated multipoint (CoMP). A small cell, for instance, would have an integrated RU, DU, and CU with a direct fiber connection. A macro-cell however, could have an independent RU, DU, and CU, all located tens of kilometers from one another^[6]. This fiber-based infrastructure ultimately meets extremely high capacity, bandwidth, and data rate requirements of 5G over vast distances, where the underground network of cables supports the massive small cell infrastructure above ground. Table 2 shows sample sample throughput and latency of the various backhaul technologies based upon operator inputs ^[7]. Depending upon the type of deployment and the information exchanged between nodes to meet 5G KPIs, different types of fiber access and wireless backhaul can be leveraged.

Table 2.

Integrated Access Backhaul (IAB)

Initially released in 3GPP Rel-16, integrated access backhaul (IAB) is a concept that leverages already established gNBs to backhaul a signal through a series of hops, often over different frequency bands. This allows the 5G infrastructure to operate similar to wireless backhaul depending upon the cost and availability of the spectrum the gNBs use. For instance, commercial LTE deployments did not proliferate due to the inherently high cost of spectrum in the sub-6 GHz frequencies. However, the dense small deployments in urban areas provide a useful platform for IAB, where the short-range millimeter-wave access calls for small cells to be closely gathered, making them more suitable for self-backhauling. This is expected to be expanded upon in 3GPP Rel-17, offering more detailed use cases and scenarios.

A combination of technologies cooperating to serve the U.S. landmass

The available macro-cells in combinations with the inclusion of hotspots, small cells, and mMIMO macro-cells allow for a high-speed connection in highly-dense regions of the U.S. However, this is only part of the issue, where fiber backhaul to semi-rural and rural areas is not feasible due to its cost. This is where the service ubiquity is accomplished through an extensive wireless backhaul infrastructure along with the support of the non-terrestrial 5G network. The combination of this and inclusion of a multi-RAT network, will allow U.S. citizens to not only enjoy basic connectivity, but a high-speed 5G connection.

References

1. 3GPP, “TR 38.913 V16.0.0: Study on Scenarios and Requirements for Next Generation Access Technologies.” 3GPP, Jul. 2020.
2. ETSI GR mWT 012 V1.1.1 (2018-11)
3. ITU-R, “Evolution of Fixed Services for Wireless Backhaul of IMT 2020/5G.” ITRU-R Workshop Presentation, April. 2019.
4. FCC Form 477 via Purdue University's Center for Regional Development
5. https://www.ericsson.com/en/blog/2020/11/get-your-5g-going-with-microwave-backhaul
6. ITU-T GSTR-TN5G, “Transport network support IMT-2020/5G.” Feb. 2018.
7. ETSI TR 136 932 V16.0.0 (2020-07)
8. H. A. Willebrand and B. S. Ghuman, “Fiber optics without fiber,” IEEE Spectrum, vol. 38, no. 8, pp. 40–45, Aug. 2001.