Inadequate cell phone service in residences is a common problem. While this issue is usually due to a weak signal from the local carrier, the situation is often exacerbated by the use of modern construction materials within the building. Concrete, metal and low-E glass (low-emissivity glass), especially in LEED-certified buildings, can prevent cellular signals from penetrating the exterior, resulting in speech that is, at best, choppy or distorted.
Network engineers have developed a number of solutions to address these communication problems. Whether due to a weak off-air signal, or the attenuation of the signal by modern building materials, there are a variety of options to improve cell signals and decrease dropped calls. The most effective of these solutions involve the use of passive and active distributed antenna systems (DAS).
While these technologies are a welcome response to a frustrating problem, the evolution of cell system equipment has resulted in an increasingly complex array of options. This overview is intended to serve as a brief guide to the most common solutions used to address poor cell service in a residential environment.
DISTRIBUTED ANTENNA SYSTEM: THE BASICS
A distributed antenna system, commonly referred to as a DAS, is a network of antennas or radio units that sends and receives cell signals on a carrier’s licensed frequencies, thereby improving voice and data connectivity for end-users.
At its core, an in-building DAS has two basic elements – the Signal Source and the Distribution System:
SIGNAL SOURCE
A distributed antenna system is designed to distribute a cell signal from one or more carriers. This signal is usually fed to the distribution system from a source that is independent of the system itself. Whether 4G LTE or 5G, there are typically three sources of external signal that are used as an input to the DAS. These sources include an off-air signal (fed from an antenna installed on the roof), an on-site BTS (Base Transceiver Station), or an enterprise small cell.
DISTRIBUTION SYSTEM
Once a cellular signal has been obtained from one of the three common sources, it must be distributed throughout the structure. There are four main types of distribution systems: Passive, Active, Hybrid and Digital.
While each of these cell systems distribute a signal in a different way, the objective is to provide both adequate coverage and capacity (the ability of the system to support an adequate number of users simultaneously).
In residential applications, the vast majority of installations are based on the first three of these systems - passive, active and hybrid. While highly sophisticated, digital DAS equipment uses a system architecture that currently renders it cost prohibitive in these environments. In a digital DAS, each carrier's signal is converted into a binary signal before being combined and distributed over fiber optic or Ethernet cable. This conversion from analog to digital requires significantly more internal processing, which makes digital DAS systems considerably more expensive than their analog counterparts.
SIGNAL SOURCES: 3 VARIANTS
The signal source (or sources) for an in-building cell system is one of the most important factors in determining the overall success of the system. Regardless of the size or sophistication of an in-building cell distribution system, it's always limited by the performance of the signal supplying the network. The three main signal sources for DAS equipment includes Off-Air, Small Cell and Base Transceiver Station (BTS).
OFF-AIR SIGNAL
When a distributed antenna system uses an Off-Air signal, it relies on a donor antenna installed at the roofline (or another optimal location) to receive and transmit signals from the cell carrier. If this signal is reasonably strong and clean (i.e. without significant noise), the DAS will offer an excellent means of distributing the signal throughout the residence and overcoming any obstacles presented by a building’s exterior.
Unfortunately, this scenario - one where the user has strong outside signal, but a weak inside one - is rarely the case. In most applications, the problem arises from an outside signal that is simply too weak and/or noisy to be useful. While the installation of a highly directional antenna can improve the signal-to-noise ratio from the input source, in many cases the carrier signal is simply inadequate to serve as an satisfactory input to the distribution system.
SMALL CELL SIGNAL
Small cells are an umbrella term for many different types of equipment with similar but slightly different properties. There are three main types of small cells including femtocells, picocells, and microcells. Each type of cellular device caters to different capabilities in bandwidth and user load.
In residential and small enterprise applications, femtocells are the most common devices, being used to extend coverage and provide backup during periods of heavy congestion. These types of small cells typically take the form of a low-cost radio access point with low radio frequency (RF) power output, footprint and range. They can be deployed indoors or outdoors, and in licensed, shared or unlicensed spectrum. These devices establish a communication link with the carrier over a broadband connection.
Small cells can be effectively networked together to provide limited, but seamless, coverage within a given space. Alternatively, a single small cell may be deployed as an input source to a distribution system that extends coverage beyond the range typically provided by one or more small cell devices.
BASE TRANCEIVER STATION (BTS)
A BTS (sometimes referred to as an eNodeB) is the fundamental radio equipment commonly used at outdoor cell phone towers to generate the cellular signal. The BTS is connected to the primary or “core” network of a wireless service provider by a fiber optic connection.
These base station transceivers can be provided by a specific carrier and installed at a client’s site to serve as an input to a distributed antenna system. In some cases, large venues may even have multiple BTS systems serving as inputs to their distributed antenna equipment, enabling the system to provide coverage from multiple carriers.
This type signal source is more robust than small cells, providing both superior power and capacity. Unfortunately, the deployment of this type of system input is not without its drawbacks. DAS systems that utilize a BTS signal source typically take longer to deploy, are significantly more expensive, and require both design and carrier approval. Moreover, each carrier must run their own fiber connection to site to establish BTS connectivity, a process that typically costs a minimum of $50,000 for each BTS deployment.
DISTRIBUTION SYSTEMS: 4 APPROACHES
Whichever signal source a system uses, a distributed antenna system needs to amplify, distribute and rebroadcast it through the residence. There are four main types of signal distribution technology: Passive, Active, Hybrid and Digital.
PASSIVE DAS
A passive distributed antenna system is referred to as such because it utilizes a series of “passive” components intended to distribute a cellular signal. System components are regarded as passive when they require no electricity, and instead rely on solely a combination of coaxial cable, splitters, and antennas.
These systems begin by amplifying a signal from the "donor" antenna – usually located on the roof of the building – and rebroadcasting it throughout the building via a series of inside antennas on the interior walls (panel antennas) or ceilings (dome antennas). The donor signal is usually amplified by a bi-directional repeater, which distributes the signal through this array of passive components.
These components are typically used indoors and intended for smaller areas. A passive DAS is regarded as the simplest option in terms of design and is therefore more cost effective. It is often recommended for buildings shrouded in metal, concrete, or thick masonry, which make it difficult for RF signals to penetrate and, as a result, tend to block signals and cause cellular dead spots.
A passive DAS conforms to FCC regulations and generally does not require outside regulatory approval. Since most systems are pre-approved, and because of the low infrastructure requirements for installation, a passive DAS can be installed in a matter of days or weeks.
Pros
Next to zero ongoing maintenance, passive DAS devices very rarely fail and consequently they are the most durable solutions.
Easier to add on additional RF bands for future upgrades as passive DAS components can typically support the full range of frequencies used by cellular technologies - typically 700-3,800MHz.
These systems are cost-effective to deploy and require minimal maintenance.
Cons
Coaxial cable distribution can be space-intensive on vertical risers which may not have enough capacity to support a Passive DAS. Coaxial cable can often be cumbersome to install.
The distances a Passive DAS can provide coverage from the signal source location can be limited due to RF power loss through the system and may not be sufficient for very large buildings.
A Passive DAS is essentially a ‘dumb’ solution when it comes to monitoring. If things do go wrong, it can be harder to know where or even if something has happened until a customer complains.
ACTIVE DAS
Active DAS equipment uses components that require a power source at various points in the system. This type of system utilizes a Master Unit that accepts signals from one or more carriers, and converts these signals into a format that enables them to be distributed over Ethernet or fiber optic cable to various locations throughout the building, facility, or outdoor space.
Once converted, an active DAS transmits the digital signal from the master unit to remote radio units (RRUs) that convert the signal back to an RF signal. Unlike their Passive DAS counterparts, Active DAS deployments minimize the use of coaxial cable used to distribute signal. In fact, in some cases there is no coaxial at all, resulting in a system that consists solely of a master unit and remote radio units.
Pros
An active system offers the best results due to the highest degree of flexibility.
Most active systems possess the ability to significantly expand through the addition of system modules and fiber-based components.
There is virtually no limit to the length of individual cable runs due to an architecture in which signal is distributed without any signal loss.
These types of systems are ideal for very large buildings or campus environments.
Cons
Most active systems are significantly more expensive than passive systems.
Many active systems require carrier approval before being deployed.
The installation of these systems often involves fiber optic backbones and carrier equipment, resulting in more complex architecture.
Lengthy deployment.
HYBRID DAS
A hybrid DAS combines the characteristics of both passive and active systems. In a hybrid system, the remote radio units are separate from the antennas, allowing the system to use a combination of fiber optic cable and coaxial cable to distribute signal throughout a building. Because this configuration requires fewer RRUs, a hybrid DAS is usually less expensive than an active DAS.
A typical hybrid DAS configuration in a residential environment might include an RRU installed on each floor that converts the digital signal to analog RF. The analog RF signal is then connected to multiple antennas on that floor with coaxial cable. This architecture also permits the use of a fiber-based RRU installed in a separate structure that extends the overall system to multiple external buildings.
Pros
These systems are typically less expensive than Active DAS.
A Hybrid DAS offers better results than Passive DAS though a combination of superior technology and scalability.
Incorporating copper and fiber into the architecture allows for long cable runs.
Carrier approvals are not always required when deploying a hybrid system.
It is possible to add additional capacity to the system.
Cons
Hybrid systems are typically more expensive than a Passive DAS.
Carrier approval may be needed in some cases, depending on the manufacturer and application.
Installation is more complicated than a Passive DAS
DIGITAL DAS
While various systems use Ethernet over fiber optic cable to transport the RF signals, a digital DAS provides an end-to-end conversion of the signal into a digital medium. Digital distributed antenna systems are relatively new under the Common Public Radio Interface (CPRI) specification, using a Base Band Unit (BBU) to connect directly to a master unit without analog-to-digital conversions. These systems remain an emerging technology and do not presently offer any useful solutions for smaller applications.
SUMMARY
As the use of mobile wireless devices has become nearly ubiquitous, indoor wireless service has become a necessity. Commonly referred to as the “Fourth Utility,” indoor cellular service is a must-have for homeowners, guests, security personnel and first responders.
Despite the importance of high-quality cell service, many of the highest-end homes suffer from poor cell service. This is often due a combination of site location and building structure. External wireless radio signals often fail to pass through a building's exterior walls and reflective low-E windows, resulting in inadequate coverage within the home. Additionally, when part of a residence is built below ground, it’s virtually guaranteed that cell service will not be available on that floor of the house.
Given the aforementioned combination of potential cell signal sources and distribution systems, there are a number of options available to resolve this problem. This process associated with deploying a cell system typically begins with a site survey and involves the following elements:
Project Scope – Create a scope of work and project specification.
Pre-Installation Signal Testing – Test a building or site with industry-calibrated equipment to gain an understanding of the signal strength, targeting only areas needing enhancement to keep costs to a minimum.
System Design – Establish a preliminary design based on client requirements.
Cabling Specification - Provide appropriate cable specification and equipment space requirements.
Equipment Procurement – Equipment is prepared and tested by technicians.
Equipment Installation – System is deployed and tested for complete coverage.
Post-Installation Data Collection and Processing – follow up with client to ensure the cellular DAS meets or exceeds the design requirements and customer expectations.