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TEC-ELEC48-06 · Lesson 11 · 12.5 clock hours · theory

Cellular and Distributed Antenna Systems (DAS) Overview

Objective. Students explain how a cellular network is organized into cells, describe the purpose and architecture of a passive and an active distributed antenna system (DAS), and interpret a simple DAS splitter/coupler power-distribution diagram.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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How a Cellular Network Is Organized

A cellular network divides a geographic area into smaller coverage zones called cells. Each cell is served by a base transceiver station (BTS), which is the radio equipment — antennas, transmitters, and receivers — physically located at the tower or rooftop site. The BTS communicates over the air with mobile devices using assigned radio frequency (RF) channels.

A base station controller (BSC) manages one or more BTSs. It handles channel assignment, handoff decisions (moving an active call from one cell to another as the user moves), and power control. Think of the BSC as the supervisor that keeps neighboring BTSs coordinated.

Above the BSC sits the core network — the packet-switching and call-routing infrastructure that connects mobile users to each other and to the public internet. The core network handles authentication, billing records, and routing of voice and data traffic.

The link that carries traffic between the BTS (or its equivalent in newer architectures) and the core network is called the backhaul link. Backhaul can be fiber, licensed microwave, or leased copper circuits. Backhaul capacity and latency directly affect call quality and data throughput.

Frequency Reuse

Because radio spectrum is finite, the same frequencies are reused in non-adjacent cells. Cells that reuse the same frequencies are spaced far enough apart that interference stays below acceptable thresholds. This frequency reuse pattern is what allows a network operator to serve thousands of simultaneous users across a large area with a limited block of licensed spectrum. The smaller the cells (microcells, picocells), the more times frequencies can be reused per square mile, and the greater the network capacity.

Why Distributed Antenna Systems Exist

Outdoor cellular signals do not penetrate thick concrete and steel structures well. Basements, parking garages, hospitals, stadiums, and high-rise office towers can have weak or no in-building coverage. A distributed antenna system (DAS) solves this by bringing the RF signal inside the building and distributing it through a network of small antennas placed where users actually are. The result is consistent, strong coverage without the interference that a single high-power indoor antenna would cause.

DAS installations are a major part of modern in-building wireless work, and technicians who understand DAS architecture are in demand in both the commercial construction and public-safety communications markets.

Passive DAS Architecture

A passive DAS distributes RF signal entirely through passive components — coaxial cable, splitters, and directional couplers — with no active amplification between the signal source and the remote antennas.

Passive DAS is cost-effective for small- to medium-sized buildings. Because there are no powered components in the distribution path, it is simple to maintain. The limitation is distance and loss: coaxial cable attenuates signal, and every splitter and coupler adds more loss, so coverage area is limited without amplifiers.

Active (Fiber-Based) DAS Architecture

An active DAS converts the RF signal to a digital or analog signal and transports it over fiber-optic cable to remote units placed throughout the building. Each remote unit (RU) converts the signal back to RF and amplifies it before feeding a local antenna.

Key nodes in an active DAS:

Active DAS can cover very large or complex buildings because fiber has negligible signal loss over long runs, and each RU includes a calibrated amplifier. The trade-off is higher equipment cost, more complex commissioning, and the need for power at every remote unit location.

Safety Notes

RF energy at transmitter output levels can cause tissue heating. Never connect a transmitting signal source to an open antenna port — always terminate unused ports with the correct load. When working near active antennas, observe the manufacturer's minimum safe distance. Follow your employer's RF safety program and applicable OSHA guidelines.

When installing coaxial trunk cables in cable trays or conduit, apply lockout/tagout (LOTO) procedures before working in electrical rooms where the BDA or active DAS equipment is powered. Wear appropriate PPE — safety glasses and gloves — when handling connectors and pulling cable.

Fiber-optic work requires laser safety awareness: active DAS fiber carries infrared laser light invisible to the naked eye. Never look into a fiber end face or connector without first verifying the link is dark. Use a fiber optical power meter or visual fault locator only as trained.

Key terms10

Base Transceiver Station (BTS)The radio equipment at a cell site — antennas, transmitters, and receivers — that communicates over the air with mobile devices.
Backhaul linkThe transmission path (fiber, microwave, or other medium) connecting a cell site to the core network; its capacity affects call quality and data throughput.
Frequency reuseThe practice of assigning the same radio frequencies to geographically separated, non-adjacent cells so that limited spectrum can serve many simultaneous users.
Distributed Antenna System (DAS)A network of spatially separated small antennas connected to a common signal source, used to provide uniform in-building or in-venue wireless coverage.
SplitterA passive component that divides RF power equally among two or more output ports, introducing both the theoretical split loss and a small additional insertion loss.
Directional couplerA passive component that diverts a specified fraction of RF power from the main trunk to a tap port, characterized by its coupling value (dB) and through-path insertion loss (dB).
Passive DASA DAS that distributes RF signal using only passive components — coaxial cable, splitters, and couplers — with no active amplification in the distribution path.
Active DASA DAS that converts RF to an optical or digital format, transports it over fiber to powered remote units, and reconverts and amplifies the signal at each remote location.
Remote unit (RU)The powered node in an active DAS installed at a floor or zone location that receives the signal from the master unit and drives local antennas after amplification.
Insertion lossThe signal power lost as RF passes through a component (cable, connector, coupler through-path, or splitter) expressed in decibels; it reduces the level available at the next point.

Worked examples4

1. A passive DAS signal source outputs +20 dBm into a two-way splitter. The splitter has a manufacturer-rated insertion loss of 3.5 dB per port (this value already includes the power division). What is the signal level at each output port of the splitter?
  1. Write down the input level: +20 dBm.
  2. A two-way splitter applies 3.5 dB of loss to each output port.
  3. Output level = Input level − Splitter loss = 20 dBm − 3.5 dB = 16.5 dBm.
Answer: +16.5 dBm at each output port of the two-way splitter.
2. Signal enters a directional coupler at +16.5 dBm. The coupler has a coupling value of 10 dB and a through-path insertion loss of 1.0 dB. What is the signal level at (a) the tap port and (b) the through (main-line) output?
  1. The coupling value tells you how much lower the tap port is relative to the input: Tap level = Input − Coupling value = 16.5 dBm − 10 dB = 6.5 dBm.
  2. The through-path loses only the insertion loss: Through level = Input − Insertion loss = 16.5 dBm − 1.0 dB = 15.5 dBm.
Answer: Tap port: +6.5 dBm. Through (main-line) output: +15.5 dBm.
3. Continuing down the main trunk: the +15.5 dBm through-port output travels 30 meters of 1/2-inch coaxial cable with an attenuation of 0.20 dB/m at the operating frequency. It then hits a second directional coupler (10 dB coupling, 1.0 dB insertion loss). What is the level at the tap port of the second coupler?
  1. Calculate cable loss: 30 m × 0.20 dB/m = 6.0 dB.
  2. Level after cable: 15.5 dBm − 6.0 dB = 9.5 dBm entering the second coupler.
  3. Tap port level = 9.5 dBm − 10 dB = −0.5 dBm.
Answer: The tap port of the second coupler is at −0.5 dBm.
4. A remote antenna on a tap port requires a minimum drive level of −5 dBm for adequate coverage. The tap port in the previous example delivers −0.5 dBm. Is the level adequate, and what is the margin?
  1. Compare tap level to minimum requirement: −0.5 dBm vs. −5 dBm minimum.
  2. Margin = Actual level − Minimum required = (−0.5 dBm) − (−5 dBm) = 4.5 dB.
  3. Because the actual level is above the minimum (less negative), the link has positive margin and coverage is adequate.
Answer: The level is adequate with a 4.5 dB margin above the minimum required drive level.

The math in this lesson1

Learners subtract splitter, coupler and insertion losses in dB to find signal levels at DAS ports.
A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Study a cellular network architecture diagram: students annotate and describe the roles of the base transceiver station, base station controller, core network, and backhaul link in their own words, then discuss how frequency reuse enables capacity. · Examine a passive DAS one-line diagram for a multi-floor building showing the signal source, coaxial trunk, splitters, directional couplers, and remote antennas; students calculate the signal level at each tap point using the component insertion-loss and coupling-loss values provided. · Compare passive DAS and active (fiber-based) DAS architectures in a structured two-column notes exercise: list at least four advantages and limitations of each based on the provided reference materials.
Materials: Cellular architecture diagram handout, passive DAS one-line diagram, splitter and coupler loss tables, active DAS architecture reference sheet, calculators
Check: DAS power-level calculations at four tap points scored; all four must be within ±1 dB of instructor answer key to pass. Architecture comparison notes reviewed for completeness (four valid entries per column required).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ETA CET — Communications and RF Systems
1. Which network element is responsible for managing handoff decisions and channel assignments between multiple base transceiver stations?
ETA CET — Communications and RF Systems
2. Frequency reuse in a cellular network primarily enables which of the following?
BICSI Installer — Wireless and DAS Systems
3. In a passive DAS, what is the primary function of a directional coupler?
ETA CET — RF Circuit Analysis
4. A signal source outputs +22 dBm into a two-way splitter rated at 3.5 dB insertion loss per port. What is the signal level at each output port?
ETA CET — RF Circuit Analysis
5. A directional coupler receives +18 dBm. Its coupling value is 15 dB and its through-path insertion loss is 1 dB. What is the signal level at the tap port?
BICSI Installer — Wireless and DAS Systems
6. Which of the following is the most significant advantage of an active (fiber-based) DAS over a passive DAS for a large, multi-building campus?
ETA CET — Safety and Work Practices
7. When performing maintenance on a powered active DAS master unit in an electrical room, what is the correct first safety step before opening the equipment enclosure?
ETA FOI/FOT — Fiber Optic Safety
8. Why should a technician never look directly into the end face of an active fiber connector on a DAS fiber trunk?
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