Antennas are the wires of the wireless world. They are the last stop for a signal on its way from a transmitter and the point of entry for a signal coming home to a receiver. A wireless network can be built with the world’s latest high-performance radio technology and best fiber-optic backbone, but the network will only perform as well as the antennas used to send and receive the signals from the radio. A poor-quality, poor-performing antenna will cripple overall performance and bring the high-performance radios to their knees. Without good antennas, wireless is just less.
However, not all antennas are created equal. The right antenna can make the difference between a wireless system that delivers on expectations and one that falls flat. When designing any wireless system, the antennas must be carefully chosen to fulfill system requirements and the demands of end users. Many additional factors must also be considered, from RF performance to installation requirements to site aesthetics to total cost of ownership.
In this report, we examine a few site antenna solutions, with a primary focus on small cell site antennas including canister antennas and RF lens multibeam antennas. Before we begin that discussion, we examine the differences between macro cells and small cells and what those differences mean when selecting antennas.
Macro Cells and Small Cells
Cell sites can be broadly categorized in two ways: macro cells, the conventional cells that encompass a relatively large area (on the order of miles); and small cells, a more recent approach to wireless coverage that encompasses a much tinier area (less than a mile). Small cells serve to make a given network denser, filling in the gaps between the larger macro cells to extend coverage or increase capacity. Although all small cells accomplish this goal, there are many different categories of small cells depending on their specific function.
One common category of small cells is a distributed antenna system (DAS), which can refer more specifically to indoor and outdoor distributed antenna systems (iDAS and oDAS, respectively). These systems have become indispensable facets of our modern built environment. Distributed antenna systems are found in apartment buildings, offices, airports, train stations, stadiums, hotels, restaurants and many more locations in order to add cellular capacity and ensure sufficient coverage for dense and populous hotspots. DAS solutions can be passive, active, analog, digital or a hybrid of the three and are designed specifically for a given space with known requirements.
Outside of iDAS and oDAS solutions, other categories of small cells complement macro cells more generally in order to densify a network. These types of small cells are often subdivided based on their size and target user, and common terms for these cells include microcell, metrocell, picocell and femtocell, in descending order of area, power level and number of users.
Small cells are becoming increasingly important as wireless frequencies increase — the millimeter-wave (mmWave) frequencies deployed in 5G wireless communications and even higher frequencies slated for future standards cannot propagate as far as the lower-frequency signals common in today’s standards, though they can dramatically improve data throughput. To take advantage of such signals, it is therefore necessary to increase the number of cell sites, with each one of these small cells being closer in proximity to the end user than a conventional macro cell.
Distance between cell sites aside, small cells have several important differences from macro cells that affect the choice of antennas. For one thing, small cell sites are often placed in residential areas, which means they are constrained in both size and aesthetics. The large and looming macro cell towers on the outskirts of a town are not quite as appealing in the middle of a suburb. For this reason, small cell sites are often designed to be unobtrusive, in some cases even disguised. Some of the clever ways that wireless operators disguise small cells are by building them into fake trees, church towers or steeples, and even small decorative streetlight poles. The unsuspecting resident is none the wiser, and nevertheless enjoys the benefits of a robust and dense wireless network. If you have ever seen a fake palm tree with antennas sticking out of it, you have seen one of these small cell sites.
Power is another difference between macro and small cells. Macro cells aim to send signals far and wide for a mile or more, and small cells by design are much more limited in range — from about a mile at the largest to a few tens of feet at the smallest. Thus, the transmit power at each cell site can differ greatly. Even among small cells, the power level can range from 20 watts in an outdoor DAS to less than a tenth of a watt in the smallest femtocell.
For both macro and small cells, it is important to ensure the right amount of coverage for the cell. Too short a range and a cell may not be fully covered; too long a range and you may interfere with a neighbouring cell. It is therefore crucial to understand an antenna’s propagation characteristics and ensure it covers the correct area.
Site Antenna Considerations
Antennas must always contend with trade-offs among size, directionality, gain, interference, frequency and other system characteristics. For both macro and small cells, these trade-offs must be optimized to provide clean, comprehensive coverage.
For one thing, it is important to ensure that antenna beam patterns are directed properly toward the cell. The antennas must encompass the entire cell while not overreaching and interfering with neighbouring cells. Electrical or mechanical beam tilt can ensure that the cell coverage is properly bounded, and techniques such as beam steering can provide further control over the radiation pattern when necessary. To cover the full cell area, there must be multiple independent beams, and the beam patterns must be clean with minimal side lobes, musts have high isolation between beams and must offer an industry-leading sector power ratio, which is a measurement of wasted energy found inside lobes compared with the main coverage beam.
Cell site antennas must also account for the different frequencies and wireless services that may be required in a given cell. Cell providers may own and operate their own cell towers and antennas, or they may share towers with other operators, resulting in several different sets of antennas operating in different parts of the spectrum, both licensed and unlicensed. Frequency bands can encompass wireless standards such as Citizens Broadband Radio Service (CBRS), Wi-Fi, 3G, 4G and, increasingly, 5G. As wireless standards continue to evolve into 6G and further, cell site antenna coverage must keep up as well.
Other important factors to consider for site antennas are their structural integrity and installation requirements. Antennas high up on macro cell site towers may be exposed to strong winds and other damaging elements, such as ice loading and constant vibration. Together with their radomes, antennas must be resistant to these environmental conditions while remaining light and accessible for installation and maintenance. Similarly, antennas should be concealed, when possible, especially in small cells and in highly populated environments. The appearance of antennas should be customizable based on the surroundings or brand identity of the provider, which may wish to minimize the visual effect or perhaps add a logo to their wireless infrastructure.
A popular type of antenna for small cell sites is called a canister antenna, named for its characteristically slim, cylindrical appearance. The singular name canister antenna is slightly misleading, because canister antennas actually package multiple antennas into a single container. In this way, canister antennas allow for sufficient wireless coverage and capacity while minimizing both visual appearance and installation requirements. Canister antennas are an ideal fit for small cell sites on light poles, power poles, roofs and other existing urban infrastructure.
Because small cells are placed close together and often in noisy RF environments, it is important to ensure that canister antennas are not subject to high amounts of interference. To this end, always look for canister antennas with low passive intermodulation (PIM) interference characteristics. Gammu Nu is a provider of site antenna solutions, including canister antennas, that are specifically designed to minimize both PIM and voltage standing wave ratio (VSWR) losses while maximizing gain and performance. Gammu Nu’s antennas are tested to provide PIM values less than 153 dBc and a VSWR below 1.3:1.
Gamma Nu’s canister antenna portfolio includes 14 distinct antennas across multiple frequency bands, with varying gain. The Pico or MESO canisters antennas can be customized per carrier for their specific frequency range/requirements.
Gamma Nu Street Small Cell Canister Panel Antenna Illustration
The Pico antennas are as slim as (7.9 inches in diameter), small (as short as 23.6 inches in height), lightweight (starting at 12 pounds) and strong (with a rated survival wind speed of 150 miles per hour). The MESO canister antennas are 14 inches to 16 inches in diameter, 0.6 meters to 4 meters in height and strong, with a rated survival wind speed of 150 miles per hour. Some of the company’s canister antennas include remote a electrical tilting (RET) device, allowing operators to remotely adjust down-tilt independently per sector.
Because canister antennas contain a full complement of radiating elements inside a slim and subtle housing, they are an easy and appealing solution for quick small cell deployments. They can provide 360-degree coverage for adding dedicated cell capacity to busy metropolitan locations such as airports, malls and plazas. Small cell canister antennas also can provide an easy way to extend network coverage without requiring the cost and time needed to build a macro cell tower.
RF Lens Multibeam Antennas
For larger cell cites demanding higher gain signals than those available from canister antennas, a type of technology called a radio-frequency (RF) lens may be appropriate. As does a lens in a magnifying glass that bends optical light, an RF lens bends radio waves in such a way as to shape the desired antenna signal. A popular example is the so-called Luneburg lens, a sphere with variable dielectric properties specifically formulated to focus a planar wave to a single point — or, conversely, to collimate a point source into a directional wave front. Because the Luneburg lens is spherically symmetric, multiple antennas can be placed around the surface of such a lens to create multiple independent beams focused in different directions.
Antenna provider MatSing, the global leader in RF lens technology, provides multibeam antennas for a variety of cell site applications. RF lenses can vary significantly in size, with the biggest lenses measuring as many as 5 meters across. Lenses of this size are not practical for the constrained spaces and low profiles of many small cells, but instead are designed to provide exceptional multibeam performance for high-capacity venues such as outdoor concerts, stadiums, arenas and downtown urban cores. For a closer alternative to canister antennas, RF lens technology can be minimized to provide high-performance and high-capacity multibeam antennas in a smaller form factor.
RF lens multibeam antennas such as those provided by MatSing offer several benefits for cell sites. The properties of the lenses allow for multiple independent beams with high isolation among them and low passive intermodulation interference (less than 153 dBc). As with canister antennas, RF multibeam lens antennas enable coverage of multiple bands in one package. MatSing’s multibeam antennas provide the world’s cleanest beam patterns and offer individual beam-tilt adjustment, enabling wireless operators to focus the beam exactly where the coverage is needed. The antennas are light in weight and structurally strong, allowing for easy installation and operation in harsh conditions. However, unlike canister antennas, RF lens multibeam antennas do not provide 360 degrees of coverage in a single package. MatSing’s multibeam base station antennas, for example, provides 120 degrees per band across as many as to seven bands.
The Increasing Importance of Small Cell Site Antennas
As 5G wireless communications continues to gain prominence across the globe, the demand for small cells and the antennas that serve them will only increase. Not only is a smaller cell size better suited to the higher mmWave frequencies employed in 5G, but also the comparatively easy rollout and lower cost of small cells compared to macro cells is becoming increasingly apparent. Small cells provide a convenient means of increasing cell capacity, filling gaps between macro cells and expanding network coverage. To do so effectively, the proper antennas solutions must be deployed.
In this report, we discussed several considerations for proper antenna solutions, from performance characteristics to aesthetics. For small cells, canister antennas and RF lens multibeam antennas are both high-performing options that provide clean, broadband coverage in a single unobtrusive package.
As wireless standards evolve from mmWave into even higher frequencies (the terahertz range is under serious consideration for 6G networks, for example), small cell antennas will become an even more important component of urban infrastructure and, without exaggeration, a crucial enabler of our everyday lives. Cell sites that anticipate this growing importance will be poised to succeed as wireless technology continues to progress.