Network densification will be an integral part of deploying 5G architecture that promises vastly increased data rates, from megabits per second (Mbps) to gigabits per second (Gbps), and ultra-reliable lower latency, from tens of milliseconds to milliseconds. The 4G radio access network (RAN) is roughly 10x denser than the 3G network, and that densification is predicted to continue through 2022 before new 5G equipment takes over the growth trend. Macro cell towers carried the bulk of 4G mobile traffic, with small cells deployed where the capacity is needed most - close to the consumer. It is predicted that 5G networks will need to be 10x denser than 4G networks, a 100x increase over 3G. 5G densification will be accomplished in space, time and frequency.
Mobile network operators (MNO) have invested billions of dollars to buy different frequency bands within the same geographical areas, and they want to maximize their investments by using carrier aggregation to increase capacity. This necessitates using three, four or five different licensed bands at the same time, and they may use MIMO technology for additional capacity. All these requirements multiply the amount of RF hardware at a site. Excellent RF performance, with low loss, low passive intermodulation (PIM) and high inter-band isolation must be maintained, as the demands of 4G LTE-Advanced already require it. There is a cost associated with meeting all of these requirements. These sometimes conflicting factors are difficult to design into the components; nonetheless, new products have been able to solve the challenges and constraints of today’s deployments. Solutions for tomorrow’s rollouts will take advantage of these new techniques to satisfy the demands of more bands and configurations.
Outdoor small cells come in many different shapes, sizes and configurations. In this article, a small cell is defined as a single geographic site and can be made up of radios, antennas and other equipment. They can differ from city to city, even street corner to street corner, depending on the requirements of the site, municipal jurisdiction, MNO or subscriber population and mobility in the area. They can support multiple frequency bands, multiple sectors and multiple operators within a common structure. Each of these requirements brings unique challenges to the design and deployment of small cells at the scale required for 4G expansion and future 5G networks.
The challenge of location means that small cells must be put in the available space, both horizontally and vertically, which may not be ideal. Small cells can be located on dedicated poles, roof tops, inside street furniture and on existing utility poles (see Figure 1). In New York City, for example, two of the poles at an intersection are reserved for public safety and traffic control, which limits the physical space available for small cells. What is possible really depends on the restrictions within each municipality. Additionally, neighborhood residents will not accept an eyesore to get better service, so pleasing concealment is vital. Compact and adaptable components are critical to successfully deploying outdoor small cells.
In a neutral host small cell, a third party finances the small cell and rents access to the MNOs. A neutral host small cell can have two or more different network operators, each using multiple frequency bands. With each MNO using multi-band carrier aggregation, it is not uncommon to see 12 or more frequency bands within a single small cell. In this crowded RF environment, signal performance is critical, typically requiring the use of a multi-band combiner with minimal insertion loss and maximum inter-band isolation.
The small cell components must be physically small and offer the necessary RF performance. If the small cell equipment is too large, mounting the cell at the required location may not be possible. Every cubic inch of space within the enclosure is a premium, making component size and dimensions a critical design factor. If the small cell’s physical size is small, more options will be available for locating the cell. This presents more options for network engineers, as they design the network architecture; however, it presents a larger challenge to the equipment vendors. Network equipment vendors must continually innovate and optimize designs to fit within the physical constraints and achieve the desired RF performance for the small cell marketplace.
Yet another consideration for small cell equipment is hardiness against the elements. The products must work across a large temperature range, from sub-zero temperatures away from the equator to scorching summers closer to it. They must be designed with dust ingress protection (IP) for desert climates and prevent corrosion in humid, salty coastal areas. The temperature specifications, IP or National Electrical Manufacturers Association ratings and salt/fog compliance are important factors to select the right equipment.
For in-building coverage, distributed RAN (D-RAN) is a cost-effective way to meet wireless coverage and capacity needs in venues like stadiums, hospitals, office buildings and hotels. If small cells were deployed everywhere coverage is required, the cost would be very high and the system would be well over the capacity needed. D-RAN uses a small network of passive components with low power radios as the signal source. D-RAN is generally both a neutral host and multi-band. In the D-RAN architecture, a point of interface (POI) has several ports for combining, with multiple outputs for distribution. The POI allows for efficient combining and is a cost-effective solution for in-building designs, as the coverage and capacity can be optimized simultaneously.
D-RAN has some of the same design constraints as outdoor small cells. Small size of the components is critical to the ability to deploy the equipment where it needs to be placed, not just in a convenient location. But RF performance is still critical - if the network does not have the necessary RF performance, it is not able to do its fundamental job of wireless connectivity.
As the industry begins its foray into the 5G era, small cells need to be future proof. In only the last three years, just for 4G, the large U.S. MNOs have each increased spectrum usage by 100 MHz or more. Typical commercial bands now extend from 600 to 3800 MHz. Additionally, the RAN has begun to include unlicensed spectrum features for LTE-LAA, up to 5925 MHz. Over the next decade, the increase in spectrum usage will be in the thousands of MHz. Ultra-wideband RF components that span several GHz of bandwidth, to cover the licensed and unlicensed sub-6 GHz range, provide the flexibility to adapt to existing and potential spectrum for future use. 5G will require even more spectrum below 6 GHz.
Flexibility to adapt to these changing spectrum requirements helps reduce the total cost for the MNOs to continuously upgrade their networks. Small cells are expensive to deploy and upgrade, especially if upgrades must be approved by the municipality. Deploying future proof technology can dramatically reduce the cost and time to deploy. D-RAN solutions must also be flexible to adapt for various use cases in stadiums, offices, warehouses and other locations. The more flexible the solution, the more likely it is to actually get deployed. Flexibility also extends to configuration, i.e., two sector, multiple bands, etc. Small cells and D-RAN will not just be single sector, single band deployments; the limited locations are too valuable for that. Compact, high quality, flexible products that do not sacrifice RF performance are indispensable.
Microlab has been focusing its R&D on small cell components, developing rugged, ultra-wideband and compact components. Each of the product categories offers frequency coverage options from 350 to 5925 MHz for TETRA, commercial wireless, CBRS, LTE-LAA and future 5G bands. These products have multiple mounting configurations that allow system integrators the flexibility to adapt to each site’s unique requirements. Many of Microlab’s products are designed to cover −40°C to +75°C, and the salt/fog series (see Figure 2) complies with Telcordia GR-3108-CORE paragraph 6.2, Salt Fog Exposure, as Class 4 products for 30 days, defined by ASTM-B117. These products are hard anodized, resulting in an even harder and more durable coating. They come with an IP68 rating, which means they are protected against the effects of immersion in water under pressure for prolonged periods.
For small cell and D-RAN deployments, Microlab’s MCC Series™ is a modular POI solution (see Figure 3). Designed to fit any operator or neutral host provider, the series offers a modular solution that can accommodate any wireless communications band up to 6 GHz and can be adapted for any site with any band or carrier configuration. This one-size-fits-all platform was designed as a future proof solution, enabling easy upgrades and reconfigurations as capacity and bandwidth requirements evolve. The custom, bolt-on design supports fast and easy installation, with guaranteed end-to-end performance of the passive components.
For 5G networks, RF performance is even more critical, since 5G essentially maximizes the spectral efficiency (bps/Hz) of the LTE waveform to deliver ultra-reliable and low-latency communication and greater mobile broadband bandwidth. To provide these capabilities, the RAN ecosystem must perform.
5G will not be able to meet its performance goals without cell densification. Actually, hyper-densification is required to deliver the promise of 5G. So the industry must be able to deploy high quality small cells, for use indoors and outdoors, in a cost-effective and adaptable manner.