In-And-Outs of an Active DAS: Basics, Components, and More.
Large concrete structures. Thousands of cellular devices. There never was a better time for DAS. Amidst the rise of 4G technology and the emergence of 5G, the world is in the midst of a digital revolution. The need to serve wireless systems sans the structural and technological limitations is greater than ever.
No wonder DAS systems are thriving now more than ever before. After all, serving multi-band, multi-technology carriers in larger space is achieved only with the help of minimalistic equipment: Signal source at one end and a distribution system at other. Nevertheless, DAS solutions in itself have evolved into various flavors such as off-air DAS, iDAS, eDAS, oDAS, and hybrid DAS. However, active and passive DAS continues to remain at the top-of-their-games even as they serve as the foundation for all the above variations.
In this edition, we dive deeper into an active DAS and explore its components, attempting an understanding at what makes it the perfect choice indoor wireless solution to enhance capacity and coverage for large-scale public places such as airports and football stadiums.
A DAS can simply be understood as a network of distributed antenna-equipped radio units serving an indoor or outdoor location. In essence, a variety of signal sources connected to a head-end unit which in-turn serves multi-band radio units comprise a DAS's overall skeletal structure. Active gain elements in both the uplink and the downlink direction have earned an active DAS its name. So what are these components and how do they function? Here is an in-depth detail.
5 Components Of Active Distributed Antenna Systems:
As powerful as an active DAS is, the system itself is incapable of producing a standalone signal. These signals are fed from sources such as a service provider feed, Node-Bs, and the modern favorite - small cells. These signal sources reside in the head-end equipment room. Multiple service providers can provide their respective signal feed making it a multi-carrier active distributed antenna system. Only major service providers can provide the feed such as AT&T Wireless, Verizon Wireless, Sprint, T-Mobile, etc. in USA. MVNO's or Mobile Virtual Network Operators would not provide such a signal feed themselves - They would need permission from the main service carriers such as those mentioned above. In Canada, those would be Rogers Wireless, Telus Mobility, Bell Mobility, Freedom Mobile, etc.
Stationed between the signal sources and the head-end unit, the POI tray combines the RF signals from diverse RF sources before it is delivered to the head-end equipment. These signals differ in nature and their abrupt and direct combination at the head-end unit can result in diversity imbalance. Of course, the same can also be achieved by using standalone hybrid combiners, which is the traditional way. But the POI does more.
The downlink signal from RF sources is usually powered at 40 W which is much higher than the average head-end unit can handle. The POI tray configurations attenuate these downlink signals before they are delivered to the head-end unit, thereby preventing excessive power-loss and heating.
In addition to the signal sources, the head-end equipment room houses the head-end unit. On one side, band-specific units called RF modules connect the head-end unit to corresponding uplink and downlink port-pairs on the POI via coaxial cables. These RF modules provide further signal filtration and amplification based on the signal frequencies. Similarly, a series of uplink-downlink port pairs, one for each radio unit, are present on the other end to facilitate zone-wise signal distribution.
Needless to say, the head-end equipment room houses some of the most critical components of an Active DAS. Not only should it be large enough to house the electronic components, it should also be well-equipped to handle their excessive heating and impart cooling thereafter.
Active DAS radio units receive signals from the head-end unit, segregate them based on frequencies, amplify them, and then transmit them to the nearby regions through antennas. These antennas is either mounted on the top of the radio-unit directly or connected to an RF-port. The reverse of this process is carried out in uplink direction. Depending on the transmission power and number of different frequencies supported, different types of radio units are deployed in an active DAS. Radio units are powered by AC or DC supply.
Fiber Optic Cables.
Distributed antenna systems cannot be complete without connecting all components with fiber-optic cables. Thin, flexible, and light-weight, fiber optic cables are perfect for an Active DAS. They don't take up much space and can easily dwell in even the smallest nooks and crannies. Since the optic signal attenuation in fiber optic cables is low, it grants the freedom to place radio units at a considerable distance from the head-end unit without causing signal loss or loss of signal Gain achieved from signal sources such as a service carrier feed. Depending on the type of signal to be transmitted, Single-Mode (SM) or Multi-Mode (MM) fiber optic cables can be used.
Now you know what it takes to power a DAS wireless network in a large airport or a multi-storey shopping complex, allowing you to seamlessly carry on with your wireless communication, regardless of the number of concurrent mobile users and regardless of thick concrete walls that surround such large man-made structures.
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