Cellular or mobile radio access networks (RAN) have become critical infrastructure that ensures voice and data connectivity exists wherever you are. At least once a day, people glance at their smartphone to make sure a signal is present by the number of bars shown on the screen and there is a certain amount of comfort in knowing that the more bars shown means the phone is well connected to the internet and to the world. But what if it does not? Because it really does not.
Often forgotten behind the buzzwords of 4G and 5G is the transport layer that connects the radio to the mobile core. This transport layer, referred to as mobile backhaul (MBH), is one of the critical components of a cellular network that ensures a great connection between the phone and the internet. Without it, the “bars” on the phone screens are meaningless.
TWO OPTIONS EQUATE TO TWO CHOICES, NOT ONE
Operators, like any other company in a competitive market, will increase their market share and revenue while enjoying higher profits if they are first to market and offer a superior product experience. This principle drove the rapid deployment of 4G and is driving the rapid deployment of 5G. Therefore, when operators begin to roll out 5G networks, each one will face the same challenge: obtaining full population coverage in the shortest time possible to ensure a great customer experience because anything less tarnishes the company brand. To create the population and geographic coverage, operators need to deploy 5G in a multitude of environments: urban, suburban and rural areas. In each of these locations, the biggest concern will be getting the backhaul in place because, without a good MBH network, the user experience will suffer.
Most people assume that the best MBH technology is the one that uses fiber and therefore, all MBH should be done with a fiber system. This is true in theory, but in reality, the use of a fiber system is limited by the availability and cost of fiber. The alternative is to use a wireless technology for MBH such as point-to-point (PTP) or point-to-multipoint (PTMP) microwave transmission systems, a technology that is not reliant on a physical cable and can be installed in days rather than months. Hence, there are and always will be two choices for MBH: fiber and microwave. Table 1 shows examples of the different types of MBH networks, along with use cases, advantages and disadvantages.
On a global basis, the use of fiber systems exceeds microwave systems, but in many major regions of the world, including Europe, Latin America and Asia Pacific (excluding China) microwave systems are used more often than fiber systems because fiber is too costly to install or impractical to use. Therefore, microwave systems are needed for operators to achieve full 5G coverage, or stated another way, full 5G coverage cannot be achieved without microwave backhaul.
Microwave Transmission Evolution
Before there were fiber optics, there was microwave transmission. As early as 1930, microwave transmission was used in communication networks. During this early stage, the microwave application was for the transport of voice across short distances. In time, operators installed microwave systems across continents to deliver tens of thousands of voice circuits to connect cities along one coast to those on the other.
Since those early days, microwave transmission technology has evolved, changing to meet the needs of every new generation. When cellular technology emerged, microwave transmission was used to transport hundreds of voice circuits; when 2G arrived, it was used to transmit kilobits of voice traffic and with 4G, hundreds of megabits of data. Over this period, networks shifted away from carrying lots of voice traffic towards transmitting an ever-increasing amount of data, requiring microwave equipment to grow from carrying analog voice to carrying large amounts of packets. Some details on this microwave transmission evolution are shown in Figure 1.
The newest microwave transmission equipment designed to meet the requirements of 5G backhaul has all the latest features, including packet switching, carrier aggregation and software-defined networking. These technologies increase the link capacity, improve performance and reduce operating costs. Dell’Oro Group estimates that more than one-third of all 5G cell sites will need a PTP microwave system for backhaul in the next five years. Also, while not a new technology, 5G is driving the use of E-Band spectrum (70 and 80 GHz).
E-Band spectrum for microwave transmission has been available for decades. While this spectrum offers higher throughput speeds, it initially targeted campus networking applications due to its shorter spans and higher costs. Its use steadily shifted to MBH when the 4G standards required backhaul links to have speeds as high as 1 Gbps.
The interest in E-Band continued to grow with 5G since the requirements laid out were for higher backhaul links and lower latencies. In time, new E-Band systems were released that could transmit up to 20 Gbps in a single box across urban and suburban spans stretching multiple kilometers. With these new products, prices also declined with the use of lower-cost chipsets. Today, the average price premium for an E-Band system that can transmit 10x more bandwidth than a microwave system operating at a standard frequency of less than 40 GHz is only 60 percent. Dell’Oro Group estimates that about 80 percent of new E-Band radios are purchased for use in MBH applications.
Another benefit of using E-Band is that these frequencies in the 70 and 80 GHz range are license-free or lightly licensed in a number of countries, lowering the spectrum licensing costs compared to those required for standard microwave frequencies. The biggest problem with using E-Band spectrum for PTP microwave is the high amount of signal degradation caused by rain, wind and heat. Rain fade occurs with all microwave signals, but it is worse at higher frequencies, and E-Band is at the highest frequencies currently used for backhaul. Wind and heat create alignment issues since E-Band signals have a very small beam angle and the alignment between two radios can be easily disturbed by high winds and temperatures if the microwave radio is not mounted on a stable structure.
However, manufacturers have been steadily addressing these issues in a few ways. The first is to introduce multi-band systems that combine a traditional radio with an E-Band radio on a single link. This new solution strengthens the signal quality across the two radios and ensures that, in the event of high rainfall, at a minimum, the data will traverse across the traditional frequency band. The second way that manufacturers ameliorate these problems is to increase the E-Band radio power to both extend the signal reach and improve link availability. The last technology advancement, which is more mechanical, addresses the issue caused by high winds that cause the tower holding the E-Band radio to sway. To compensate for the shifting angle between two radios, manufacturers add active antenna alignment systems to constantly realign the two radios. This feature is also important when changes in temperature cause the towers to expand and contract, which also affects the radio alignment.
On the roadmap to address the growing demand for higher capacities and the needs of 6G, the development of microwave systems using new spectrum is underway. The new spectrum includes W-Band (92 to 114.5 GHz) and D-Band (141 to 174.8 GHz). Links using both bands are expected to be ready when 6G rolls out.
5G DRIVES MICROWAVE DEMAND
5G deployments began in 2018 and ramped up significantly in 2019. Most of these early deployments occurred in fiber-rich countries like China, Japan, South Korea and the U.S. They leveraged the fiber network put in place for 4G and the focus of the operators was on population coverage at existing customer locations. During the initial 5G rollout, there was little need for additional backhaul systems and when a new system was required, a fiber system was often chosen. Looking forward to the next phase, operators that started off using fiber are expanding their 5G network to territories where fiber is sparse and they will need to use microwave systems to save both time and money.
The rollout of 5G is still at an early stage when looking beyond the fiber-rich countries to regions that typically rely on microwave transmission for MBH, such as Europe, Latin America and Southeast Asia. These countries have just started to roll out 5G networks. Therefore, while microwave systems were not initially used when 5G first began rolling out in 2018, its share of cell sites is expected to rapidly increase to over 35 percent. Dell’Oro Group projects that over the next five years, $8 billion of PTP microwave systems will be purchased for 5G MBH. Among the technology segments comprising the microwave transmission market, E-Band systems are predicted to have the highest growth over the next five years due to MBH, growing at a compounded annual growth rate of over 20 percent. Dell’Oro Group’s latest forecast for E-Band radio shipments is shown in Figure 2.
The transport layer is a critical component of a 5G network and can oftentimes define whether the user experience is satisfactory or not. It is also the most challenging to roll out if fiber is the only option since it can take years to install. Hence, operators continue to need microwave systems each time a new mobile RAN is deployed and for every generational upgrade. This is why, at the end of the day, 5G needs microwave in backhaul.