Phase 2: Construction of DAS

Before we go into different steps of DAS construction, it’s important that we understand the bill of materials. By understanding the categories of materials and where (i.e., locations) these materials will be installed, construction will become a lot more logical to you.

Note: Construction covered for DAS here is mainly for DAS 2.0 - where DAS has both fiber and coax. DAS 3.0 or all-fiber DAS has minimal coax. So there will be some variations, but most of the basic principles remain same.

Understanding Bill of Materials: 6 common items

When you see a list of materials for DAS for the first time, it can appear complicated. If you understand the 6 broad categories of DAS materials, it will be a lot easier.

1. Electronics: these are the devices that need electrical power to run. Usually they are heavier than the other materials – so I am calling them the heavy stuff.

2. Cable and connectors: these will include coaxial cable and fiber. Most of the coax used in DAS are ½” in diameter.  Similarly, most of the fiber used in DAS are going to be Single Mode Fiber. Since these are long cable, I called them the Long Stuff.

3. Antennas: any type of DAS needs antennas. I am calling them the Ants (from Antennas).

4. Jumpers and Couplers are the Small stuff. Coax and fiber jumpers are usually between 2 ft to 6 ft. We also talked about couplers, which include 2-way or 3-way or 4-way combiners, or different type of couplers – such as hybrid coupler, 15 or 20 dB couplers, duplexers or diplexers.

5. Who is carrying or supporting all of these heavy stuff, long staff, the ants and the small stuff. We call them “Support Structure”. They include but not limited to: 19” racks, ladder rack, cable ladder, mounts & brackets, cable management, J-hook, fiber connector housing, etc.

6. Grounding is for protecting devices from any unwanted electrical surge. All electronics will be grounded through grounding wire and ground bus bar. Installers will make sure that all grounding kits, wire and bus bars are ultimately tied to the building’s existing grounding system.

Understanding construction through locations of materials

We will discuss typical locations of the DAS elements materials.

MDF room or DAS head-end: What is MDF? Short for main distribution frame. If you have a data center in your building or the core of your building network are most likely located at the MDF room of the building.

Signal source and DAS head-end are typically located at the MDF room of the building. DAS electronics closest to signal source are also commonly known as head-end. They may include DAS tray, radio interface units, (RF to Optical Signal) converter and distributor.

Independent Distribution Facility or IDF rooms. Every floor usually have an IDF room. All remotes are typically located at different IDF closets of the building and connected to the head-end assembly via fiber and fiber hub.  You can also use telco room and even electrical or mechanical rooms instead of IDF rooms.

Last but not least, antennas, couplers, combiners – they are usually strategically placed throughout the building. DAS Engineers finds out through engineering analysis the number and locations of the antennas, as well as remotes and even the head-end electronics.

Typical locations of materials in a building

Let’s look at the locations of the materials from another perspective. Here we are showing the cross section of an office building.

1. MDF for Signal source and DAS Head-end electronics: On the first floor, we have MDF and typically signal source and DAS head-end electronics will be located here.

2. IDF for Remotes: Every floor has an IDF closet, so the remotes are located at the IDF closet.

3. Floor area for antennas: Antennas are located out on the office area, commonly in the on ceiling tiles. Couplers and splitters will be installed based on where they are needed, and that can be in IDF or above ceiling tiles in the office area. 

4. Fiber run from MDF to IDF: Fiber will commonly go from the DAS Head-end or MDF to each of the IDF closets where the remotes are located. For all-fiber DAS (DAS 3.0), fiber will go from MDF all the way to the ceiling area where the remotes will be located, but they will still be terminated in the IDF to inject power.

5. Coaxial cable will run from IDF to antennas located out on the floor.

Keep in mind that depending on the building size, shape, and space available, these locations can easily change, but this should give you a general idea how it’s all placed. Variation of these locations are common, but basic principle is same.

At the risk of sounding silly, here is a mnemonic key for remembering the 3 key areas:

M - I – F

MDF - IDF – Floor

(Mouse in Flour)

Construction Activities

We have seen construction from materials perspective. Now let’s dissect it from activity perspective. DAS construction have 5 categories of activities:

1. Installation of Coaxial and Fiber Cable

2. Mounting Electronics & antennas

3. Making interconnections by installing jumpers, couplers, combiners, etc.

4. Grounding, labeling and powering up

5. Running tests

Now… sequence of these activities may vary from project to project, depending on availability of materials, construction crew, etc.

1. Installation of coax and fiber

For installation of coax and fiber, we will have to pull the cables on support structure. Common cable support structure is cable tray – whether new or existing. If cable tray is not available or not feasible then j-hooks will be installed to run the cable. Most of the time these j-hooks will be above the ceiling tiles – so they will not be visible from the ground when DAS is finished.

Once the coax is pulled installers will put on the connectors. For coax, most of the time, installers will use some sort of connectorization tool. Connectorization tool can vary based on the connectors.

Fiber is typically ‘terminated’ or ‘connectorized’ inside a fiber housing. Please see the picture below.

Fiber cable enters the housing at the back of the box. Instead of putting connectors in the field like we do on coax, installers take fiber pig tail and ‘fusion’ splice cable end of the pig tail to the fiber strand we have pulled inside the fiber housing. By ‘fusion’, we mean 2 strands of fiber are fused together. One fiber strand will be from the pig tail and the other one will be from the fiber cable that came inside the housing. Inside the jacket of the fiber cable, there may be several fiber strands – which is also determined during design phase and mentioned in the bill of materials.

As you can see from the picture of the fiber pig tail, one end is open fiber, but the other end is a connector. Connector end of the pig tail is attached to the back of the coupler, and fiber jumper is attached to the front of the coupler to connect to another device.

Fusion is not the only way to attach fiber connector – there are other ways; however, fusion splicing is the most common way to attach a fiber connector.

2. 1 Mounting Antennas

Mounting antennas are easy. We recommend mounting the antennas before pulling the cable. That way installers in the field will know where the coax has to be terminated.

Here is the picture of an antenna installed on the ceiling tile. To install, a hole is made into the ceiling tile. Antenna’s stem goes through the tile. A washer is screwed in to hold the antennas.

Directional antennas are typically mounted on the wall with a mounting bracket. Mounting bracket typically sits on the back of the directional antenna.

2.2 Mounting Electronics

There are several ways to mount electronics. We will discuss 3 typical ways to install them.

First way, Rack mounted or inside a rack. Racks can be 2-post or 4-post. Most of the time head-end DAS electronics will be mounted on 19” racks.

Rack mounted head-end electronics

2nd way, Electronics can also be mounted on H-Frame. When we have large quantities of equipment, we usually mount them on H-Frame. H-frame because it looks like letter H. We use steel poles and Unistrut pieces to fabricate this H-Frame.

RF Signal sources mounted on H-Frame

3rd way of mounting electronics is on the wall. For example, I am showing a remote device here that has been mounted on the wall of an IDF closet. When we mount something on the wall, it’s a good practice to have the electronics placed on a plywood board.

Electronics mounted over a plywood board on a wall inside a telco room

In future, I will have more blog posts on other options we have seen in the field to mount electronics or antenna.

3. Making Interconnections: with coax or fiber jumpers (typical 1’ – 6’)

Interconnections are made with jumpers – whether Coax or Fiber jumpers, and some times copper connections – which is usually for power connections. Jumpers are cables of short length – they can be anywhere from few inches to several feet. Typical coax and fiber jumpers we use range from a foot to 6 feet. Longer jumpers are not unusual either.

There are typically 3 types of interconnections.

1. In-between cards (within the electronics)

2. In-between devices or electronics. Although I am showing connections between 2 electronics in the picture below, in a lot of cases we may connect ½’ cable to a coupler with a coax jumper or coupler to an antenna with a coax jumper.

3. Devices to fiber housing.

In these 3 pictures I am showing the jumpers with the yellow arrow symbol.

Making Interconnections: with coax or fiber jumpers (typical 1’ – 6’)

Making Interconnections on wall mounted electronics

4. Grounding, labeling and powering up

Grounding

Grounding provides a path of least resistance to undesirable electricity to reach the building ground and eventually dissipate in the ground.  “Undesirable electricity” can be electrostatic discharge indoor, or lightning strike from outdoor. In typical DAS installation, following items will be grounded:

1. All electronics

2. Cable outside building (e.g. cable connected to GPS  or donor antenna)

3. All support structure (such as cable tray, 19” racks, etc.)

Basically all metal – except indoor cables, couplers or splitters and antennas.

Let’s look at a simple grounding scheme through a line drawing.

Say we have electronics mounted on a rack. This will be connected to a rack ground bar via grounding wire. Rack ground bar will be connected to a Master ground bar. ‘Master’ because multiple grounding wire will be connected to this ground bar. Eventually master ground bar will be connected to the building ground system, and the building ground system will be connected to the ground so that any surge of electricity generated around the electronics can eventually dissipate in the ground.

Typical grounding materials for a DAS

In the picture above:

• #1 is typical grounding kit for grounding cable outside the building. This will be applicable when we have a GPS antenna on the roof. Or a donor antenna on the roof for DAS 1.0 system.

• #2 is a typical surge arrestor. These surge arrestors are installed when the cable from the roof top antennas enter the building.

• #3 is a reel of grounding wire and a close-up.

• #4 shows grounding wire is made of copper wire that are highly conductive.

• #5 shows a Master Ground Bar where all the grounding wire from different electronics (or other ground bars) meet

Labeling

Labeling helps to find & understand electronics, connections, etc. So pretty much everything is labeled. Labeling does not contribute to the operation of DAS – but from our experience absolutely critical (for maintenance of DAS). Typical label information will include – but not limited to:

1. Carrier / Operator

2. Name of electronics

3. Frequencies

4. For connections: Source & Destinations for cable or jumper connections

In this collage of sample images below you will see that all electronics, cables (fiber & coax), couplers, antennas, grounding – pretty much everything is labeled.

Labeling

Powering up

Power connections for DAS can be a course by itself. But I will keep it brief and only give you an overview.

Most of the DAS electronics can draw power from regular wall receptacles – in U.S. that is 15-20 amps. Greater and range of power is needed for RF Signal Sources – ranging from regular wall outlet for a femtocell, to 30 amps for small cell to 100 amps for base stations – there is always variations. So let’s look a block diagram to understand a simple power plant which is common for providing power to different cards and electronics of a base station.

1. All power plants will have rectifiers. Rectifiers will convert AC power (coming from building) to DC power.

2. DC power generated from Rectifiers are fed into a DC power distributor.

3. This distributes DC power to all the electronics needed for base station. You are safe to assume majority of the base station electronics run on DC power - -48 Volt DC is very common.

4. DC power is also fed into a battery plant. Batteries are for back-up power – when there is a power outage in the building. Number of batteries will depend on how much back-up power engineers want to have available for the system. They are usually expressed by terms such as - 2 hours of back-up or 4 hours of back-up.

Finally here is a picture of a Ericsson power plant that we have worked with. Note that the rectifiers and DC distributors are stores in the top cabinet. You also see 4 strings of battery with 4 batteries in each string.

Ericsson Power Plant for a Base Station

5. Introduction to Cable Testing

Main purpose of tests during DAS construction are:

1. Finding fault or defect in the system

2. Signal continuity at intended frequencies

3. Reducing loss while signal is traveling through the system

4. Minimizing interference

Note that I said “reducing loss”, not “eliminating loss”. As signal passes through coaxial cable or fiber there will be loss of signal even with a perfect system – however, these loses are calculable, and we want to make sure we get as close to those calculated signal loss as possible and not more than that.

Essentially, every component will get tested by the time DAS is completed, but during the actual construction phase, we test mainly the coaxial and fiber cable and smaller items such as antennas, couplers, connectors, etc.

For coaxial cable we do 2 tests:

1. Sweep test

2. PIM test

For fiber cable, we do following 2 tests:

1. Optical loss test

2. OTDR test

We will get into a little bit more details of these tests. We will focus on the purpose, benefits and limitations of these tests without getting too technical.

For DAS 3.0 or all-fiber systems, coax is minimal (just a few coax jumpers). So for DAS 3.0, you essentially don't need any sweep or PIM tests - just fiber tests.

Sweep Test

Sweep tests have following benefits:

1. Locate any changes in the cable or degradation at the frequency of operation. This can prevent severe system failures.

2. Antennas can be tested at their correct operating frequencies, and make sure they will work.

3. Tests devices such as couplers, splitters or combiners that are on the system and make sure signal can pass through them at desired frequencies.

4. Sweep test generates Return Loss which tells us if the device is a ‘match’ for the system, and conforms to system engineering specifications. Low return loss (typically lower than -15 dB) is good, high return loss (>-15 dB) is bad. Top 4 carriers (AT&T, Sprint, T-Mobile, Verizon) requires return loss to be between -15 dB to -24 dB.

What does a sweep tester or gear do?

Sweep Gear measures the signal losses and reflections of the transmission system.  A sweep gear sends signals with variable frequencies (starting from lowest to highest) from one end of the cable to the other end, and finds the performance or faults down the transmission line. Sweep gears are available from various different manufacturers.

Sweep gear shown here is a Site Master™ Handheld Cable & Antenna Analyzer (S331L). Manufacturer is Anritsu.

Sweep gear for coaxial cable test

Use gear that is approved by all of the Operators. So for example, if you want to bring the signal source from 4 major carriers such as Sprint, AT&T, T-Mobile, Verizon, please make sure that their engineering group approves the sweep gear to be used. You integrator should be able to find this out.

If problems show up during this test, there is a good likelihood that those problems will affect the end user. A poorly matched antenna will generate high return loss which means that good part of the signal will not be available for transmission and will instead go back to the transmitter. This extra energy returned to the transmitter will not only distort the signal but will also affect the efficiency of the transmitted power and the corresponding coverage area.

PIM Test: indicator of construction quality

An on-site PIM test is a comprehensive measure of construction quality. PIM shows up as a set of unwanted signals created by the mixing of two or more strong RF signals due to damaged DAS parts such as connectors, cables, couplers, splitters, duplexers, and antennas. PIM lowers the reliability, capacity and data rate of cellular systems.

PIM Test for DAS

In the plot shown above, 2 red lines – at frequencies f1 and f2 are coming from signal source – so they are desirable. If they are strong enough – say 10 watts or higher – it can produce signals in other frequencies which are undesirable because they can start interfering with signals at f1 and f2 frequencies. These are intermod signals and shown with blue bars. The formulas above the blue lines show at which frequencies these undesirable signals may come up. PIM is measured in dBc unit. dBc stands for decibels relative to the carrier.

Operators typically considers -153 dBc or lower as the passing PIM test.

LTE systems can be pretty sensitive of these, and we should keep the system out of PIM.

It is possible to have a PIM test pass while Sweep Test fails, or PIM test fail while Sweep test passes. Sweep tests and PIM testing are different, but both are very important. Important best practice to remember here is that: Sweep coax before PIM test.

PIM test gears are available from a few manufacturers. We are showing here 2 popular units in the market from Anritsu and Kaelus.

Make sure that the integrators using PIM gears are approved by the carriers.

Prevention of PIM:

What are some ways to prevent PIM in a DAS system?

1. Use LOW PIM rated components for DAS.

2. Cleaning: Keep connectors clean to minimize PIM.

3. Inspection: When the connectors are apart, inspect them for physical damage. 

4. Torque: Torque literally means twisting force. Torque wrenches are used to tighten the connectors on the cable. Tighten connectors to the proper value. Proper torque on the connector is both required and will help minimize PIM.

Here are 2 torque wrenches from 2 different manufacturers.

Fiber Tests

As we mentioned before, there are 2 types of tests done (OTDR & Optical Loss Test) on fiber cable during DAS construction.

OTDR:

OTDR stands for Optical Time-Domain Reflectometry.

• OTDR measures the overall length of the fiber.

• It finds out the localized stress points or points where signal will have problem, for example, if you have any kink in the line - it will show up in OTDR results.

• It also ‘estimates’ the loss or attenuation of signal strength over the length of the fiber. Note that I said estimate, not accurately measure. Optical Loss Test does that.

If you are not dying to know the length of the fiber, I say, Optical Loss test is good enough for DAS fiber. OTDR is mainly a requirement from the carriers.

Here, we are showing an OTDR tester from Fluke, but there are quite a few OTDR testers in the market.

Again, OTDR is mainly a requirement from Operators. So use gear that is approved by all of the Operators.

Optical Loss Test

Optical Loss Test is the only test that indicates how the fibers will perform in the network under actual power conditions.

With this test set we measure the difference between a known quantity of power input and a measured quantity of power output. We need a laser source and power meter to conduct this test. Laser source feeds a known quantity of power input into one end of the fiber, and the power meter measures the output of the optical signal on the other end and reports the loss over the fiber.

When the proper instruments are used for this test, the quality of fibers, connectors and connector adapters in the link is ensured.

In the next post, we will get into further testing and commissioning of the newly built DAS.

Related Posts:

Commissioning & Integration