TECHNOLOGY > SMALL-CELL SOLUTIONS

SCS site on a street light pole (click image to enlarge)

Mobile communication systems require coverage first over a wide area, then, as users either increase traffic or become concentrated in specific areas, the need arises to bring the radio signals closer to the mobile devices. This is accomplished by distributing a number of radios and their antennas, each covering a much smaller footprint, but able to deliver improved performance to those mobile devices close to them. There are a number of SCS technologies that are used to reach these mobile users.

NextG installs many types of SCS and they all share the common characteristic of being distributed over an area and the mobile communications are fed over fiber from a central control location.

SCS architecture (click image to enlarge)

A distributed antenna system (DAS) SCS is an RF-to-optical-to-RF system. It replaces a local antenna by using electronics that are plugged into the antenna port of a mobile system. The advantage is that it interfaces with the operator's traditional base station equipment at its antenna connector. Electronics next to the mobile base station equipment converts the RF signal to an optical signal, which is carried along a fiber strand until it reaches a remote site. At the remote site, additional DAS electronics convert the signal back to the original RF signal, as in the diagram. The cellular operators with second- and third-generation technology usually use this DAS type of system.

SCS architecture for both outdoor and in-building networks (click image to enlarge)

The optical-driver-to-fiber-to-RF SCS system is becoming popular. The optical output of the mobile base equipment drives light along the optical strands until it reaches the remote site, where, for the first time, the signal becomes a licensed mobile RF signal. This approach avoids one RF conversion step and generally results in a better RF signal. LTE and WiMAX operators often deploy this type of system. The names of SCS system units vary with manufacturer and technology, but examples are repeaters, picocells, and remote radio units.

SCS incorporating Wi-Fi access points in a large network (click image to enlarge)

Another SCS network is a centrally controlled Wi-Fi wireless local area network, where a large number of Wi-Fi Access Points are joined by fiber-fed switches and routers into one large controlled network, as in the figure. These systems have a sophisticated set of protocols for security, monitoring, quality of service, etc. They can support completely isolated private networks, so that multiple mobile operators can share a single infrastructure and air space. As far as smartphone users are concerned, they have attached to their own operator's Wi-Fi network and the users are unaware of the shared network.

CWDM places multiple signals onto a single fiber strand redundant rings shown)(click image to enlarge)

The fiber network in the SCS system can be fairly simple, devoting fiber strands to each SCS equipment site, as in the previous Figures, or it can share many sites on a strand by optical multiplexing, a technique called Coarse Wave Division Multiplexing (CWDM). In a CWDM application, each signal is a different "color" of light, i.e., a different optical wavelength. The optical strands support multiple wavelengths without them interfering with each other. The figure shows each wavelength (symbolized by lambda) as a different color. The wavelengths can be created by different RF frequency bands, as in the Figure, or the CWDM can be used by different operators so that they don't interfere with each other on the fiber strand. The decision to use CWDM is cost driven and usually occurs when adding fiber strands is prohibitively expensive.