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DAS System | Distributed Antenna System basics

This tutorial covers DAS antenna system basics. It mentions distributed Antenna System block diagram and DAS system types with difference between them. It mentions advantages of DAS over single antenna system and over smart cells.

As we know, normally cellular coverage has distance limitation. This is mainly due to path loss from the transmitter to the receivers. Moreover it is very difficult to provide cellular connectivity to users inside building, hotels due to complex channel environment. In order to provide efficient cellular connectivity inside such places DAS system is needed.

DAS system is a network of spatially separated antennas connected to a common source via transport medium which provides wireless service within geographic area or structure.

As the name suggests DAS stands for Distributed Antenna System. Unlike single antenna system, DAS uses multiple antennas to cover region inside the premises. Following are the DAS applications. The system requires single RF source which provides the RF signal input to be distributed inside the building for better signal coverage to the users inside without much channel interference.

DAS Applications

• Hospitality- Hotels, resorts, conference centers
• Public Venues- Convention Centers, stadiums, airports
• Government & Education-Govt. offices/municipality, university campuses
• Hospitals- Medium/larger hospitals more than 100 beds
• Mixed Use- Malls, mix of residential, entertainment, and retail spaces

DAS System Types

There are three types of DAS Antenna systems namely passive, active and hybrid depending upon the components used in the system. Let us understand basic difference between these systems.

Passive DAS system

Figure-1 depicts passive DAS system. As shown it consists of bi-directional amplifier feeding multiple antennas via coaxial cable.

As shown the system uses tap splitters or power splitters which divides the power from one main source line to multiple lines as required. These splitted ports are connected with coax cables to interface with multiple antennas as needed as per finalized layout. Before choosing the coax. cable take following specifications into considerations.

Table below mentions attenuation (or loss per 100 m) in dB for different types of coaxial cables.

Coax Cable Type Loss @ 900MHz Loss @ 1800MHz Loss @ 2100MHz
1/4 inch 13 19 20
1/2 inch 7 10 11
7/8 inch 4 6 6.5
1 and 1/4 inch 3 4.4 4.6
1 and 5/8 inch 2.4 3.7 3.8

The advantages of passive DAS are:
• simple but time consuming to design• can be installed in harse environment• components from different manufacturers can be used.

The disadvantages of passive DAS are:
• high losses will degrade data performance and hence active DAS is preferred in this situation • No check for errors occurred in the system • Not flexible for upgradation

active DAS system

Figure-2 depicts active DAS system. As shown it consists of active electronics at each passive end and transport later composed of fiber, coax cable or CAT-5/6e cabling. Active DAS system is further divided into pure active and one with optical active DAS system.

Figure depicts the active DAS system composed of Master Unit, Expansion Units and Remote Units. The functions of each are explained below.

Main Unit (Master Unit) connects to low power base station or repeater. The Main Unit (MU) distributes the signal to rest of the system via expansion units (EU). Usually MU will be connected to the EU by optical fibers. MU also monitors the performance of entire DAS antenna system. It is possible to access MU remotely using modem or over TCP-IP protocols.

Expansion units are typically distributed throughout the building premises. It takes the signal from MU and distributes to all remote units(RUs). EUs are placed in central cable raisers or in IT X-connect rooms. EU also feed DC power needed for RUs.

Remote Units are placed very close to antenna in order to keep passive losses to the minimum and improve the radio link performance. RU is connected to EU with thin coax or CAT5 cable or similar thin cables which makes it quick to install compare to rigid passive coax cable used in passive DAS system.

optical transmit receiver in DAS System

Figure-3 mentions analog configuration of Active DAS system. In this system, analog RF signal is modulated over light and transmitted using optical fiber cable to the other end. At the other end signal is first demodulated and then amplified and connected to antennas for radiation. In digital configuration of active DAS system, analog RF Signal is converted to digital signal first. Then signal is passed to optical transmitter before being transmitted through optical cable to the optical receiver. At optical receiver signal is converted back to digital form from modulated light form. The demodulated signal is passed to DAC and converted back to analog form. This analog signal is amplified and radiated using antenna.

Hybrid DAS: It consists of fiber in the riser(i.e. vertical pipe) and coax cable in the horizontal runs with active electronics. It is a combination of both passive and active DAS systems. Hence it will have hybrid remote unit and power splitters/taps are used to distribute the main signal to multiple antennas.

Benefits of DAS (Distributed Antenna System)

Following are the major benefits of DAS system due to which it is considered over smart cells.

• Coverage: DAS architecture provides coverage in areas that cannot be effectively addressed with traditional sites.

• Capacity: DAS can closely align capacity to actual market requirements, managing available radio resources.

• Spectrum: DAS uses available frequency spectrum efficiently through multiple low-power transmission points.

• Interference: DAS reduces interference through low radiation centers and lower output power.

• Data: DAS provides better data throughput given signal strength and proximity of transmission points to user equipment.

• Scalability: DAS is a scalable network that can meet future capacity requirements, or additional carriers, by adding additional nodes.

• Adaptability: DAS can respond to market dynamics, equipment architecture changes and new technologies.

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