Those numbers on cable...

On a recent troubleshooting event, we found that the cables from the public safety and another service provider's antennas were crossed. This created some interesting problems. 


Once we suspected the crossed cable (thanks to Kevin Wysocki), tracing the cables from the roof-top antennas to the head-end equipment became the main challenge - in this particular case, cables from roof-top antennas to lightning protector were all LMR 400, and then from the lightning protector to the equipment were all 1/2" RFS cable. The fact that most of these cables are hidden above the ceiling tiles didn't help either. This is when the numbers printed on the cable comes handy.

Single Point of Failure

I recently (inadvertantly) rebooted a DAS in a courthouse by leaning on the wall. Talk about Hall of Shame...

I was troubleshooting the public safety part of an existing multi-carrier DAS, sitting on the floor, hooked up to the DAS head-end.  After a while got tired, and leaned back on the wall. There was a single plug going into the wall outlet which was connected to 2 UPS units, the whole head-end was connected to these 2 UPS units - basically, this DAS had a single point of failure. While I leaned back, I guess, I wiggled the plug and the power connection went out. Although UPS took over, but the latency in the UPS made the DAS reboot (this reboot shoud not have happened, but it did!). 

Besides the fact that I may be few fries short of a happy meal, I learned a few things:

1. Do NOT to use a plug with its stem sticking out. Touching the stem (especially accidentally) may cause problem.

DAS Flower: an interesting way to understand DAS platform

If you are new to DAS, here is an easy way to understand DAS platforms by solving a very simple puzzle. Based on 3 simple rules below you will have to figure out what will be the connection type in picture 1 between the blue ball (Antenna) and the square box (media converter)?

3 simple rules:

1. RF travels on coaxial cable. Antenna can only receive RF signal.
2. Loss of signal is high in a coaxial cable (signal loss is 50% every 100 ft for a typical ½” cable)
3. Signal loss in fiber is minimal even on a long run (such as over 1000 ft) 

Controlling Uplink ALC for a BDA

Often times, if an indoor BDA (Bi Directional Amplifier) system’s uplink is not properly optimized, it can create a lot of problem at the cell site due to uplink noise issues or excess power. BDAs often try to keep the Uplink (UL) and Downlink (DL) balanced, by keeping same gain in both directions or by a few dB offset. In this post, I am proposing to keep the BDA on its ALC (Automatic Level Control) feature and  limit the UL ALC to a level which can be calculated and then adjusted based on real situations. This discussion is for a CDMA system where power management is an important issue.

Assume that you want your uplink signal to hit the donor (i.e., Cell Site) BTS at -85 dB. Then for the above diagram, a basic link budget becomes,

ALCUL – Indoor side cable & other losses + Donor Antenna Gain – Free Space Path Loss – 
Clutter Loss – Fade Margin + Gain in Cell Site Antenna – Cell Site side cable & other losses = -85

Info-graphics: service affecting components in a DAS

Impact of service diminishes as you move downstream from head-end on a DAS. Need for redundancy, therefore, increases in the opposite direction.

System Map: how can it reduce troubleshooting time?

Close-out package has a lot of benefits. But it should be built around one principal premise:
How much time can it cut down during troubleshooting?

Finding out the source of the problem is a big deal during DAS troubleshooting - especially if you have a short Service Level Agreement (SLA) to resolve issues with a tough customer. Most of the DAS elements being hidden above the ceiling tile, and spread out through a building does not make life easy. A System Map can help... actually quite a bit.

System Map is essentially a spreadsheet (i.e., hardware summary) that allows a technician to quickly trace DAS elements from head-end all the way to floor level where the antennas are installed.

Designing DAS in Pilot Pollution Environment

For CDMA network, an indoor area with Pilot Pollution usually means it's an area with high signal strength, but low signal quality. This is a common phenomenon in high rise buildings in Houston (Houston, we have a problem!)

In CDMA network RSSI (Received Signal Strength Indicator) at a point is the total CDMA power received. So if at any point you are getting a RSSI of -65 dBm on a Sprint phone, that RSSI is the sum of RF Power received at that point point from all the Sprint base stations your handset can see or sniff.

Ec/Io on the other hand is an indicator of signal quality. Ec/Io at any point for a particular server (or base station) is the contribution of power from that base station divided by combined received power at that point from all base stations.

0 + 0 = 3

One of my favorite questions to ask to a new DAS engineer during interview is what is 0 dBm + 0 dBm. Say what? Answer is 3 dBm. And here's how:

0 dBm = 1 milliwatt

So 0 dBm + 0 dBm = 1 milliwatt + 1 milliwatt = 2 milliwatt

2 milliwatt = 10* Log(2) = 3 dBm

Therefore, 0 dBm + 0 dBm = 3 dBm.

Use of Info-graphics for DAS Project Management

I often get a "say-what?" look from a group of people when they come from a diverse background. While working on a healthcare DAS project, I had to give bi-weekly status update to the project stakeholders on the client side who came from diverse background - from IT, to facilities management, nursing department, infectious control, and even doctors. Talking about a DAS project can become little tricky in these situations. After a few of the bumpy sessions, I found that status update through infographics can be very effective.

Infographics is a fancy name for presenting your data visually. If you look around on the web though, infographics is mainly focused on being visually exciting and stimulating; and I don't blame the infographer, because just presenting table of data can be little boring. However, when I am using the graphics, I try to make sure that the background graphics adds value to the update I am presenting. So for example below, you can see how the major elements of the system is connected. Audience is also getting the updates on the different elements of the system. This can be fun, and presentation can be little more interesting than just presenting drab data. Audience from different background can have a better understanding of the system and how it's connected together. I also don't like to use a lot of slides when I am presenting to my clients. If a few slides are getting the point across then I am all for that.

By the way, I used Microsoft Excel and Paint for this presentation. Approximate time to put together the graphics was 20 minutes. Once I have the base graphics prepared status update boxes take just a few minutes to update.

Blue and White Coax for MIMO

Depending on the type of DAS platform (i.e., SOLiD, Andrew, MobileAccess) you are deploying, if it's a MIMO DAS you may have to pull parallel cable. For those who are not familiar with MIMO deployment, you will have to pull two sets of coaxial cable, splitters/couplers to a pair of antennas at the same location only 4-6 ft apart. In the field, this may become little complicated, because you don't want the parallel cables to get crossed. Installers usually mark the tip of the cables before pulling so that they can differentiate the cable sets.

One idea might be using cables of different color. So for example, you can pull a set of blue coax, and white coax as parallel network. I am strictly looking at it from the perspective of reducing confusion; however, logistically this may become a little complex - especially if the two colors of coax are produced by different manufacturers, etc.  

Cut me a slack...

During the DAS deployment phase, it's not unusual for cable pullers to leave some slack at the points where connectors will be installed. They do that with the thinking that having a little slack is much, much better than coming short on cable run at the point of, say, splitters or couplers. I had been in a project recently where the crew chief picked up a few crew from the local Union Hall to pull cables, and to make things simple and straightforward asked them to leave 5 ft slack at every connector point. This ended up costing us 15 ft of wasted cable per 2-way splitter or directional coupler (3 ports, 5 ft per port, a total of 15 ft). Waste doubled because this was a MIMO system. This waste is in addition to the waste we usually get from reaching the end of the spool. So when the crew chief asked for additional cable, we started suspecting the scale of the design first. We got more puzzled when we found that design was done according to the right scale. It took us almost over a week before we could figure out the principal cause.

A slackin' Situation

For this particular MIMO project, we had close to 376 antennas, 33 remotes, and 306 splitters/couplers. Just for the 5 ft slack at each connection point, the waste came out to be about 6,635 ft. From materials perspective, best thing to do is to connectorize as you go. In that case though, you may have to have crew who pulls cable and also knows how to put connectors on.

Walking-Coffin

We often use low tech thingy-ma-jiggy to build hi-tech network. One such device is - as the builder Jeremy Tesson fondly calls it - Walking-Coffin. Walking-Coffin may look like a transport vehicle for Fred Flintstone, but this was used to pull 2 sets of coax simultaneously for a neutral host MIMO DAS, where 2 parallel coax were going to 2 different antennas at a single location.

I also liked the way Jeremy accesorized the walking-coffin - a front end basket to carry around hardware and light tools, a hook to hang a dust pan. Cool!

lightRadio: a new promise for DAS

Base Station on the palm of your hand

Here is a paradigm shifting technology from Alcatel-Lucent (ALU)… lightRadio. A Base Station that fits in the palm of your hand. It’s generating bold, disruptive titles such as: “The tiny cube that could save wireless”, “lightRadio shoves a base station in a box”, “End of Mobile Towers?” – and they are all legitimate claims or predictions.
Even bigger news is that is not a concept anymore.  Telefonica, whose customers are in 25 countries across Europe and Latin America, announced the first live test of the 4G network over lightRadio, and the cube was supporting 1,000 users at the last Mobile World Congress of 2012 in Barcelona in February.

I am illustrating some basic configuration block diagram of lightRadio cube based on the information provided by ALU.

QR Code in DAS: is it feasible?

QR Code is an interesting new technology. Invented in Japan, QR is short for Quick Response. They can be read quickly by a smart phone by using QR reading apps. They can store more information than a standard bar code; such as URL links, Latitude and Longitude, text (upto 250 characters) etc. Another advantage over bar code is that you won't need a separate hand-held scanner, by using QR reading apps on your smart phone you will be able to read them.

Here is an example...

This is a QR Code for a text phrase, "Relax! It's On Air". I used a free QR generator at Kaywa. For reading this QR I use a free Android app "Qr Barcode Scanner" on my Smasung Galaxy S2. There are other free QR Generators as well as readers available. [To learn more about this technology, you can read Marc Lyne's article or check out the USA Today article, "What's a QR code and why should you care?".]

Can we use QR code for DAS industry?
We are exploring a few options in a current DAS project in a large healthcare campus expected to be completed in a month.

Using Video Clip as a part of DAS survey, design or closeout package

A picture is worth a thousand words; and video explains whatever words you didn't understand from those thousand words. Well no need to talk about the power of video - we all know that.

Video can always be a powerful addition to DAS package - whether it's survey, design or engineering, or close-out package. Sometimes you can look at multiple pictures, and still may not be able to figure out how it all comes together. It's not feasible for the DAS Design Engineer to survey all the sites - video can be the 2nd best thing. During my site walks I try to take video which may be hard for me to explain to the engineers. These can be especially effective if you are working on some sort of manufacturing plant.

A few weeks ago when I was working with one of our designer (Mr. Mir) on a large underground parking garage DAS, it was becoming hard to explain just through pictures, how the large I-Beams in the ceilings will have big impact on running the coaxial cable. Floor plans did not show these I-beams, and pictures were not sufficient. After seeing various pictures, when I pulled out the video on my phone the designer immediately understood what I was trying to explain. The short video clip basically brought it all together. You can see the video here: