Small-Cell Installation in Transportation Infrastructure

Published by Alice Seaborn on Wednesday, 16 June, 2021

Commissioned by the Illinois Department of Transportation (IDOT) in the Fall of 2019, this research paper examined the intersection of small-cell 5G antenna with the transportation infrastructure that is required to host them. My advisor for this project was Dr. Suruz Miah at Bradley University and together we learned how municipalities engage and interface with emerging cellular technologies. Emphasis was placed on how Departments of Transportation and the cellular service providers with which they do business, assess and mitigate novel risks that could be introduced to the populace through the installation of 5G technologies.

Please take note that this blog post is a simplified and compressed summary of the research and is not documented to IEEE publication standards. Please consider this blog post as a synopsis of my larger completed work.

Table of Contents

  1. Abstract
  2. Introduction
  3. Small Cell Overview
  4. Safety
  5. Location Density
  6. Legal Capital Challenges
  7. Conclusion
  8. References

Abstract

The need for cities to update their cellular infrastructure looms as user equipment demands greater bandwidth than ever before. With each successive update in cellular technology, we have unlocked more agile and creative computing projects. Concordantly, the development of fifth-generation (5G) cellular infrastructure will enable users to leverage the faster communication speeds to empower the next generation of their product lines. Unlike 4G-LTE, which relies on macro-towers and base-stations to communicate with user-equipment, 5G networks require a distributed network of communication devices. Operating with less power consumption and smaller ranges of communication, these small-cell antennas (SCs) can service a smaller group of users with faster speeds of communication.

Introduction

The transmission of data throughout an urban environment has become vital to the success of modern cities. Previous generations of cellular infrastructure can no longer keep pace with the growing demand for communication (data transmission) bandwidth and throughput. The macro-towers that have been serving fourth-generation-long term evolution (4G-LTE) networks cannot connect to the rapidly growing number of devices that each user is simultaneously utilizing. Previously, user-equipment (UE) consisted of cell-phones and a few users owned multiple instances of their user-equipment. The contemporary data-landscape, however, has shifted beneath our feet with demands for mobile devices, such as cell-phones, smart-watches, laptops, and vehicles for each individual user to be connected to the internet. Consequently, connection issues for city-dwellers have become apparent and the frustration against poor connections has become palpable. To remedy this issue, 5G technology offers greater coverage and bandwidth to provide connections to the many devices utilizing the cellular network.

The next generation of cellular technology, 5G, represents a paradigm shift for cellular infrastructure. Illustrated in the figure below, SCs act as a high-speed intermediary between the user-equipment (UE) and the macro-tower. The UE connects directly to the nearest SC tower to send and receive data. This signal is processed and sent through fiber-optic cable, known as back-haul, to a macro-tower which then forwards the signal through back-haul to the carrier's switching office or to cellular servers which connect to the internet to fulfill the user's request. These SCs offer higher bandwidth in part because they serve a smaller audience and at a higher cellular frequency than existing systems. Existing macro-towers must serve a wide area of coverage and therefore serve bandwidth to a greater population. The frequency of a cellular signal is proportional to the bandwidth that it provided and is inversely proportional to the area of coverage. Consequently, macro-towers provide 4G and 4G-LTE coverage at lower frequencies (2GHz to 8GHz) to a broader area. Small-cells, on the other hand, provide superior 5G coverage at higher frequencies of up to 28GHz according to the FCC in their official 5G standards. This strategy necessitates a higher density of short-range antennas which require installation posts, structures standing approximately 15 feet tall or higher, in order to provide optimal coverage and in order to comply with existing electromagnetic frequency (EMF) exposure regulations.

5G-general

The structures mentioned above take the form of monopoles, which elevate the SC antenna and host the back-haul at the base of the structure, or existing infrastructure items such as a light-poles or traffic lights. These infrastructure elements fall within the public domain as they are installed within public walk-ways or intersections. Consequently, although the SC equipment is owned and operated by the cellular carrier, the installation sites themselves are under the control of the local Department of Transportation (DOT). In order to install a new SC to develop a 5G network, carriers must submit an application to the local DOT authority. It is the responsibility of the DOT to thoroughly investigate if the SC request complies with federal and local regulations with respect to the zoning and safety of the radio-frequency equipment. In addition to safety regulations, many DOTs have implemented regulations which determine if an SC would disturb the aesthetics of the surrounding environment. Once these requirements have been satisfied, the carrier will proceed with the installation according to the terms agreed upon with the DOT.

Previous modes of cellular communication relied upon macro-tower installations which were fewer and farther between than the requirements of 5G small-cell poles. Consequently, the increased urban density required by SCs has inspired many cities to develop online databases where potential SC locations can be zoned and tracked in real-time. These databases are typically publicly available, thus increasing the transparency of the 5G roll-out with the general public. Despite this transparency, however, many municipalities have faced legal concerns from the public. As outlined in legal and capital challenges, these arguments claim that the installation of small-cell antennas represents a violation of the reasonable accommodations requested by electrically sensitive people under the Americans with Disabilities Act (ADA). Although no such legal argument has come to fruition, many DOTs have researched the potential legal and capital liabilities that such claims represent. Although 5G technology poses several technological and policy challenges, the benefits afforded to 5G capable cities is significant.

Small Cell Overview

Small-cell antennas are much smaller compared to their 4G counterparts. The average outdoor small-cell weights approximately 1kg whereas previous solutions to communications infrastructure weighted about ten times as much. The most common description of SC antennas is that they are as large as the size of a pizza box. However, many SC antennas are no larger than the UE that manufacturers are servicing. For instance, the Huber Suhner SENCITY Omni-S antenna shown in the figure below. The weight of the Huber Suhner SENCITY Omni-S antenna is about 0.32kg and it encompasses approximately 550cm^3.

sencity-omni-s

The operating principle of SCs is quite simple, as illustrated below. An electrical signal that encodes information in the form of data packets is passed through the antenna which, in turn, induces an electro-magnetic field. The signal is then decoded by the UE and analyzed within the context of information. For example, a macro-tower may convert information into binary data packets which are sent through fiber-optic cables to an SC antenna where the information is converted into electro-magnetic waves to be sent through 5G frequencies to UE where it is disseminated back into audio and displayed to the end-user. Using Field Programmable Gate Arrays (FPGAs) and high-resolution digital-to-analog converters (DACs) and analog-to-digital converters (ADC), this communications process can take as little as several milliseconds.

sc-processing

Receiving information from the network is as simple as reversing the information flow. First data optical converter reads optical data into binary and the MUX splits the signal into their respective signal paths. Each signal is then processed by the FPGA before being converted from digital to analog (DAC) and sent through a series of filters before being broadcast through the SC antenna. This process, similarly, occurs in a matter of a few milliseconds.

For cellular devices to communicate with each other and with the internet, a series of data processing and communications steps must intervene to secure the connection and perform the requested tasks. For cellular networks, this challenge has traditionally involved macro-towers and Evolved Node B (eNodeB) base-stations, which connect directly with the UE and executes the task of passing their information to cellular servers that route the information to and from its intended destination. These eNodeB units process the antennas signals and pass the information along the cellular network. The figure below shows the 4G-LTE cellular connection process between a macro-tower and a UE. Macro-towers are typically installed on very tall structures such as metal lattice towers or the rooftops of buildings. Consequently, the average distance between UE and macro towers is higher than the proposed SC solution.

4G-system

The issue with macro-towers is that although they are the most cost efficient connection solution, they cannot compete with the added bandwidth afforded by SC. Rather than allowing users to connect directly to the macro-tower, the SC antenna connects to the UE and acts as a very high speed intermediary capable of connection to multiple devices on multiple bandwidths simultaneously. Note in the diagram below how the 5G cellular connection process flows between a macro-tower and UE through back-haul. Macro-towers communicate with the UE under their area of coverage and send the information that they receive to the cellular servers through fiber-optic cable known as back-haul. This fiber-optic cable represents the fastest and most reliable method of delivering information. More specifically, once the SC has received information from or for the UE, it sends the data through fiber-optic back-haul to the nearest macro-tower where the data is then sent to the network servers. The advantage of SCs in advancing cellular connection speeds is two fold: higher frequency bands offering greater bandwidths and better urban coverage densities by using many shorter range devices. Together, these advantages allow for cellular networks to improve services while leveraging existing infrastructure.

Safety

The protection of its citizenry is the chief priority of a municipality. Unlike bridges and railroads, where the structural integrity of a creation is tangible and considered common to the public, electronic infrastructure operates in an invisible domain which attracts the imaginations of the public to form uninformed conclusions. It is utterly vital that the safety of small-cells be thoroughly explored and communicated to the public to ensure the healthy operation of this ambitious community project.

The FCC sets guidelines and requirements for cellular network providers and municipal entities to protect the public safety from exposure to harmful levels of radio-frequency emissions (RFE). Using recommendations from the National Council on Radiation Protection and Measurements (NCRP), American National Standards Institute (ANSI) and Institute of Electrical and Electronics Engineers (IEEE), the FCC determines what RFE are safe for the general public. Specifically, the FCC sets regulations for the maximum permissible exposure (MPE) of the public to RFE for communications sites. To install new communications equipment, MPE reports must be filed with the regionally appropriate DOT. These documents specify what specific values and circumstances must be met for an installation to be deemed safe for the public.

For cellular transmitters, the FCC recommends the MPE level of no more than 580 uw/cm^2 See the FCC guidlines in the full report. The maximum exposure levels for RFE are many times greater than the RFE measurements near cell-towers. Additionally, these guidelines reflect an MPE which is below the threshold for harm. Ergo, cellular towers are unlikely to achieve this level of emissions and persons exposed to the maximum threshold of emission are still not likely to experience harmful effects. To quote their guidelines below:

"Calculations corresponding to a 'worst-case' situation (all transmitters operating simultaneously and continuously at the maximum licensed power) show that, in order to be exposed to RF levels near the FCC's guidelines, an individual would essentially have to remain in the main transmitting beam and within a few feet of the antenna for several minutes or longer. Thus, the possibility that a member of the general public could be exposed to RF levels in excess of the FCC guidelines is extremely remote."

The scenario of exposure impacts the measurements of RFE for a given exposure site. Workers and the general public are often within the line-of-sight of multiple RF beams simultaneously. The illustration below depicts a single-source exposure, wherein the user is exposed to signals with two antennas installed at different heights. The user depicted in this scenario is exposed to both signals simultaneously. Consequently, the combined emissions of these sites must be considered when determining MPE compliance. Given that SC technology requires a greater density of installations than previous communications technology, the multiple-exposure scenario shown below will become more common for MPE assessments throughout 5G deployment.

exposure-scenario

The public has battled with its understanding of cellular technology since its inception. Their negative reception to the radio and television were motivated by a misunderstanding of how electromagnetic waves interact with the human body and advances in cellular infrastructure will inspire similar passions. However, a large body of research has been conducted on this topic and a reasoned analysis of the evidence suggests that RFE, while hazardous at extremely high amplitudes over long period of time, does not represent a significant threat to the public after taking into account the existing regulations.

In 2009, Anders Ahlbom and other researchers for the International Commission for Non-Ionizing Radiation Protection's (ICNIRP) Standing Committee on Epidemiology investigated the relationship between RFE from cell phones and the risk of developing tumors. Their report surveys the existing studies into statistical relationships between cell-phone use and the developing of tumors among users. They begin by explaining that the emergence of commercially available digital technology in the 1990s, which involve the use of low-power microchips and integrated circuits, represented a significant departure from previous analog phones with higher effective radiated power levels. This advancement led to more efficient uses of the public air-waves and significantly decreased the RFE of cell-phones. Although the frequency bands used by UE has increased over the decades, the power levels of these technologies have diminished significantly.

In a similar study initiated by the World Health Organization's (WHO) International Agency for Research on Cancer (IARC) in the year 2000, the controlled effects of RFE on four types of tissue cancers were examined. This controlled study spans thirteen countries and includes more samples than any other known study as of the date of its eventual publication in 2010. After an extensive analysis of their data, researchers at IARC found no significant increase in the risk of brain cancer as a result of heavy, moderate, or light cell phone use.

Nonetheless, the FCC has established guidelines for public safety and mandate the carriers conduct simulations of RF emmision patterns to verify that the installation of a new antenna will not violate existing standards. These reports compare the general scenario of exposure when taken to extremes against the FCC's maximum permissible exposure (MPE) limit. To elaborate, the report assumes that safety signs are universally obeyed by pedestrians and that pedestrians flow evenly through the walkway, creating a model for standard exposure. After these parameters are controlled, the communication equipment is taken to its extremes of RFE. The equipment is assumed to broadcast its signal at all times throughout the day at maximum power. Measurements of RFE are taken across space and the emissions field is determined for the installation.

Impartial third-parties conduct these measurements by testing the RFE at regular intervals of distance from the base of the antenna for each individual frequency band for which the antenna operates. As was discussed previously, individuals may be exposed to multiple fields of RFE simultaneously and the individual quantities of exposure from each emitter are added to form the total quantity of radiation exposure. This is no less true for antennas which emit frequencies of radiation on separate bands of operation. Each emitter's normalized MPE while operating at full power throughout the measurement are combined to form a total MPE estimate. If MPE estimates are within FCC limits for walkways and expected paths of pedestrian travel then the installation is approved.

mpe-heat-map

Protocols have been established, tested, and studied to ensure that the public is not exposed to harmful quantities of radiation. In case, however, members of the public are exposed to greater levels of MPE than permitted, the federal limitations are set such that the MPE limit is below the threshold for legitimate harm. Consequently, the existing regulations work to over-protect the public from equipment. Now that the nature of electromagnetic radiation (EMR) has been explored and solutions to the problem of safety have been explained, the locations that DOTs and cities determine for SC installation can be explored. In the next chapter, maps of SC locations will be evaluated for density as well as location preferences and policies will be explored which accelerate the deployment of SC technology in America's most advanced cities.

Location Density

The location of small cell (SC) installations is determined through a combination of technical and legal considerations. For instance, there is a technical component to the placements of SCs with respect to their performance in an ultra-dense network (UDN) as well as their coverage with respect to the surrounding urban environment. Additionally, there are safety concerns generated both through public input as well as through FCC regulations governing public and professional electromagnetic frequency (EMF) exposure. Consequently, the manner in which SC sites are determined is a multivariate problem.

It is important to distinguish the different contexts for determining SC sites. Individual locations are subject to MPE reviews, aesthetic concerns, and structural requirements. This section covers how these constraints motivate DOTs to favor certain installation types over others.

As a logical consequence of MPE limitations, SC sites are required to distance themselves from public walkways without compromising cellular coverage. These distances vary depending on the context of the installation as well as the frequency bands and effective radiated power (ERP) of the antenna in question. Generally, municipalities prefer to put approximately 20 feet of distance between the SC and public places of travel such as sidewalks. To accommodate this request, many cities have utilized mono-poles and rooftops as well as taking advantage of existing infrastructure such as light poles and power lines.

Setting aside the technical and safety challenges facing SC distribution, many cities are concerned that the SC sites will compromise the aesthetics of the city. Consequently, many municipalities have drafted their own guidelines for SC roll-out and include preferred designs to standardize the look of new structures as well as to govern the use of existing infrastructure such as street lights which are now being re-purposed to include the role of hosting antennas in addition to luminaire.

Recall that SCs serve a smaller radius of users with a larger bandwidth. Consequently, more SCs must be installed to provide cellular service to an equivalent area of coverage. However, if the network becomes too dense then the signals can destructively interfere with each other, resulting in lowered area spectral efficiency (ASE). Carriers are challenged to balance the theoretical needs of their 5G networks with the local installation constraints facing each individual SC placement.

It is common to expect cellular coverage to improve after installing more macro-towers. While this mode of operation is generally valid for 4G networks, interference still occurs between macro-towers and this effect is amplified for SC networks. Due to the high bandwidth of SCs and the increased number of base-stations (BSs) per unit area, SCs are liable to compete with each other and destructively interfere with other 5G signals. Destructive interference occurs when the amplitude of a signal cancels out the amplitude of another signal of the same frequency. This signal behavior destroys the cellular information while the packets are in transit from the antenna to the UE and forces the antenna to send duplicates of the information again (SYN) until the signal is received (ACK) properly, thus reducing the efficiency of the network.

ase-crawl

Many municipalities have determined what city infrastructure is available to support SC antennas and which locations would be ideal for a 5G roll-out. Expressed geographically, these databases have been published through ArcGIS and made available to both carriers as well as the general public. In the cities of Los Angeles and Boston, a database was created with the locations of SC antenna installations throughout the city. This database enabled carriers to determine where SCs ought to be installed based on the density of existing installations. Denver and San Jose developed a more robust solution to SC location databases by zoning where SC installations ought to be installed and then by adding additional layers to their database such as specific DOT jurisdictions and the locations of municipal wi-fi and camera poles. This allows for carriers to more acutely contextualize SC locations by understanding how 5G will merge with the city's existing infrastructure. By aggregating a database of preferred SC installation sites for the carriers, DOTs can efficiently cooperate with industrial actors.

Legal Capital Challenges

When pursuing its 5G ambitions, the City of San Diego asked the public for input into how and if 5G technology ought to be implemented in their city. Despite the many technological advantages that small-cell (SC) antennas offer the city, a large volume of the public input was focused on eliminating SCs to protect the public health. Despite these concerns, federal law prohibits SD-DOT from regulating SC placement based on environmental effects caused principally by RF emissions.

Specifically, some comments asserted that the installation of SCs represented a discrimination against electrically-sensitive people in violation of the Americans with Disabilities Act (ADA). This electrical sensitivity, known as Idiopathic Environmental Intolerance attributed to Electromagnetic Fields (IEI-EMF), is so far medically unexplained. Subjects claiming to have IEI-EMF reported their symptoms to researchers when tested with sham and active electromagnetic field (EMF) exposures under double-blind conditions. Despite strong beliefs in the test subjects that their irritations were caused by exposure to WiFi, no relationship was found between EMF exposure and IEI-EMF symptoms. However, a relationship was found between belief of exposure and severity of symptoms. Although these studies do not reject IEI-EMF claims outright, the World Health Organization (WHO) supports the consensus among scientists that there is no scientific relationship between electrical sensitivity and EMF exposure. Nevertheless, municipalities must be cognizant of the political sensitivity of this subject matter.

Conclusion

Despite the concerns expressed by the public, the scientific community is resolute in its consensus that electromagnetic field (EMF) emissions, such as those experienced when exposed to an SC signal path, do not pose a significant threat to the public. Numerous epidemiological studies into the the effects of EMF exposure have not found a significant relationship between EMF exposure and the development of cancerous tumors. The Federal Communications Commission (FCC) has developed guidelines for exposure which require carriers to verify that a proposed SC site does not expose the public to more EMF than the maximum permitted exposure (MPE) set by the FCC in conjunction with other scientific authorities.

Provided that these MPE guidelines are respected, many municipalities still screen SC proposals to ensure that they do not negatively affect the aesthetics of the surrounding environment. Many cities have developed specific guidelines for carriers to ensure that antennas are deployed in organized housings or disguised in other structures to avoid disturbing the aesthetics of the city. Once these guidelines are met, cities develop databases which provide the locations of existing SC sites as well as the locations of suitable sites for future SC installations. These databases provide transparency for the city and efficiency for the carriers and greatly improve the 5G rollout.

After reaching out to the public, some municipalities have faced threats of legal suit against the city due to discrimination against electrically sensitive people. These suits rely on the protections afforded by the Americans with Disabilities Act (ADA). No legal precedent, however, supports electrically sensitive people as a protected disability class and the medical community has yet to find a scientific explanation for the symptoms experienced by electrically sensitive people. These cities have opted to proceed with their 5G ambitions while researching the possible litigious liabilities.

The benefits of SC technology are broad and as of yet mostly unexplored. Increases in affordable bandwidth for the general public will support the development of new products and software and will provide jobs those charged with installing and maintaining this new network. Notwithstanding the challenges faced by DOTs to deploy SCs, many systems have been implemented to support DOTs in the success of their 5G projects.

References

See the full published paper for the most accurate and updated reference material.