A Distributed Antenna System is a great way to ensure that your building has wireless coverage at all times. If you are considering a Distributed Antenna System for your building, you should know that there are two types: active and passive. You may want to know the difference between these two systems before deciding on which one to install.

Large-scale distributed-antenna system (L-DAS) with very large number of distributed antennas, possibly up to a few hundred antennas, is considered. A few major issues of the L-DAS, such as high latency, energy consumption, computational complexity, and large feedback (signaling) overhead, are identified. The potential capability of the L-DAS is illuminated in terms of an energy efficiency (EE) throughout the paper. We firstly and generally model the power consumption of an L-DAS, and formulate an EE maximization problem. To tackle two crucial issues, namely the huge computational complexity and large amount of feedback (signaling) information, we propose a channel-gain-based antenna selection (AS) method and an interference-based user clustering (UC) method. The original problem is then split into multiple subproblems by a cluster, and each cluster's precoding and power control are managed in parallel for high EE. Simulation results reveal that i) using all antennas for zero-forcing multiuser multiple-input multiple-output (MU-MIMO) is energy inefficient if there is nonnegligible overhead power consumption on MU-MIMO processing, and ii) increasing the number of antennas does not necessarily result in a high EE. Furthermore, the results validate and underpin the EE merit of the proposed L-DAS complied with the AS, UC, precoding, and power control by comparing with non-clustering L-DAS and colocated antenna systems.

PASSIVE DAS

The design of the distributed antenna system includes antennas connected by a central controller. The controller connects to the base station of a designated wireless carrier network. This is where your system can be passive or active. The passive distributed antenna system can grab cellphone signals from the top of a building’s roof and run those signals to feeder cables to distribute throughout the building.

Passive DAS systems don’t need fiber optic cables in general and consist of simple BDAs (Bi-Directional Amplifier). An example of a passive component used in a DAS system would be a diplexer. It doesn’t require power. The signal just runs through it, much like water runs through a pipe. DAS installations consisting of only passive components are rare. This is because as DAS installations get bigger and more sophisticated, they need to be controlled and monitored remotely. This capability is made possible by active equipment at both remote and Head-End locations connected via fiber. DAS system integrators deploy SNMP-enabled equipment. SNMP allows the system to be accessed and controlled via a GUI (Graphic User Interface) using an IP network.

Many smaller DAS installations can be passive. Let us take an example of a small office in need of enhanced cellular coverage. A system integrator will install a directional antenna pointing to particular carrier’s tower. The signal enters the building via low-loss coaxial cable. A Bi-Directional amplifier known as a BDA is installed to strengthen the signal. Integrators also use passive components such as splitters and directional couplers to distribute RF signals between a BDA and indoor antennas. These installations are relatively inexpensive but require a good degree of RF expertise to ensure proper in-building coverage - Especially when there are multiple carriers and cellular bands (ranges of frequencies) which need to be distributed. Adding public safety UHF (ultra-high frequency) and VHF (very-high frequency) bands to installations will increase the cost and complexity of the deployment even more.