DAS is common terminology wireless industry professionals use to describe the infrastructure designed to distribute a cellular signal within a certain area, either indoor or outdoor. In most cases, the term references indoor DAS, however there are instances when outdoor DAS is discussed. Obviously, there is a practical need to distinguish between the two. You’ve probably guessed it by now: iDAS stands for indoor DAS and oDAS stands for outdoor DAS.
iDAS is the most common form of DAS infrastructure in use. Here, we will describe the individual aspects of an iDAS system. Remember the fiber backhaul? It’s a fiber optic network run by wireless carriers which connects their central office with remote hubs. As we explained, all signals are transported digitally on this network. Fiber optic cables allow carriers to transmit data long distances with little or no signal loss. Also, various optical technologies are utilized such as DWDM (Dense Wave Division Multiplexing) or CWDM (Coarse Wave Division Multiplexing). This all sounds very complicated, but it’s basically a way to send multiple signals along a single fiber. The actual optical fibers are often maintained by 3rd parties and wireless carriers sometimes have to lease them which can get very expensive. Using as few fibers as possible reduces that cost.
Wireless carriers usually position remote hubs based on demand. Imagine venues like sport complexes, hotels, airports, university campuses, malls and hospitals. These are typical establishments where wireless users congregate in large numbers. Setting up BTS in these venues makes the most economical and strategic sense for carriers. Basically, these BTS are designed to perform analog-to-digital and digital-to-analog signal conversions. BTS have intelligent software built into them and do everything from call processing to frequency hopping. Everything about these BTS are proprietary. You’d have to be a pretty resourceful individual to obtain specifications for these machines. BTS pumps out very high powered RF signals. These signals can be anywhere between 5 and 80 watts depending on the model and manufacturer.
iDAS doesn’t need high RF power, so some processing is done to increase efficiency. This loss of power can be thought of as an “inefficiency wagon”. Carriers use custom attenuators to pad RF power down to a manageable level, somewhere around 0 dBm (roughly equal to 1mW.) They also use RF diplexers to separate various technologies such as AWS, GSM and PCS from one another. (A diplexer is a passive component used to combine or separate two RF frequencies, depending on the direction of the signal.) Then each RF signal goes through a RF duplexer (not to be confused with the diplexer) which further separates the signal as either up or downlink. RF signals originating from a user’s handset are described as uplink and signals coming from a BTS or a donor antenna are known as downlink.
Let’s recap before continuing. The high power analog RF signals from the BTS are attenuated (signal power is reduced), diplexed (signals are combined or separated) and duplexed (signal is separated into its uplink and downlink constituencies). Now it’s finally ready to be fed into a DAS Head-End, which is broadband equipment designed to function as a neutral (multi-carrier, multi-frequency) distribution hub inside buildings. The system loses efficiency at this point (the wagon is rolling) because our processed analog RF signals are converted into optical signals at the DAS Head-End and then get split optically before being distributed over more fiber optic cables to multiple Remote Units (RUs). Each RU does optical-to-RF conversion and re-amplifies the signal before sending it to indoor antennas. Sometimes, multiple RFs are split and signals from one RU get sent to multiple antennas via coaxial cable. This is where RF engineers perform their magic.
Engineers use software like IBwave to calculate RF propagation inside the building in order to distribute signals. They pick and choose from various passive components such as splitters, combiners and directional couplers to balance every antenna equally inside the building. Don’t worry about these different components. Just know that engineers use them to manipulate the signal in order to balance each antenna. This is not an exact science at this point and there is no silver bullet that solves every coverage issue. Buildings are constructed differently and walls are erected at inconvenient spots that block good RF propagation.
If you think installing DAS inside a building sounds complicated, try doing the same thing in a football stadium. Believe us, it’s not an easy task. But nerds do have power and in some cases your happiness depends on them. Suppose your girlfriend wants to update her Facebook status from “Single” to “Engaged” using her mobile phone during an NFL game (because of your impromptu marriage proposal fueled by beer and the adrenaline of seeing your team score the winning touchdown.) Those friendly RF engineers made your special occasion a reality by designing a good DAS system and now your girlfriend is able to announce your engagement to the whole world before you can change your mind. Her phone worked and so did those of the 80,000 other spectators in the same RF jungle you call a stadium.
Usually RF engineers will divide the coverage area inside the stadium into multiple sectors and make sure RF signals propagate properly within a particular sector but don’t experience interference from nearby cell towers and the next DAS sector. Each sector functions independently and has its own DAS equipment. Engineers rely on their RF expertise and industry adopted RF propagation software. Wireless signal handoff occurs when a cellular user moves from one particular sector to another. Everything is controlled by software installed on the BTS. Transfer has to be seamless. Unless of course you like dropped calls and data outage. Similar concerns must be addressed in an oDAS environment, which will be discussed next.
oDAS functions similar to iDAS but requires Remote Radio Heads (RRHs), which are similar to the RUs in iDAS and use relatively high power. You typically see RRHs with 1 to 5 or even 20 Watt output RF power. These RRH nodes are housed in weather-proof outdoor enclosures and placed in public areas to enhance cellular coverage. oDAS deployments are relatively limited in the US because the same areas can be covered by macrocells. Remember macrocells are typically cell towers and owned and operated by turfing vendors and independent tower companies. Wireless carriers don’t own cell towers. Instead, they lease a space on the towers to mount their antennas and amplifiers. Space on a cell tower is prime real estate and carriers pay big money for it.
Another aspect of wireless coverage is emergency mobile deployments. During events like natural disasters, wireless carriers deploy mobile repeater stations (RATs) to provide coverage. There are also COWs. (We’re not sure who makes these names up either.) COW stands for Cell on Wheels and it’s a full-on high powered cell tower mounted on trucks. RAT means Repeater and Trailer. RATs are smaller in capacity compare to COWs, meaning they can’t handle as much signal.
iDAS Head-End equipment is supplied by manufacturers like Commscope, TE Connectivity, Corning/MobileAccess and Solid Technologies in the US. These manufacturers are commonly referred to as DAS OEMs (Original Equipment Manufacturer) or vendors. The DAS market is beginning to mature and has experienced some consolidation during the last 3-5 years. Commscope acquired Andrew, Tyco bought ADC, and MobileAccess became part of Corning. DAS OEMs equipment must be approved by carriers for installation by system integrators at various venues.