Importance of PIM in WISP LTE deployments

PIM sweeps are a common thing in the Cellular field.   One of the first questions folks often ask is what is a PIM sweep? If you think of PIM testing as a passive test and line sweeping as an active test that is a good start.  PIM testing looks for problems with things like connectors, cables, and other “layer 1” items.  A PIM test is not a line sweep. Line sweeping measures the signal losses and reflections of the transmission system. this is typically VSWR.  A line sweep is an active test. It can not detect the same things a PIM test can.  Many HAM radio folks are familiar with a line sweep where the reflected power is measure in an antenna system. In a line sweep you deal with reflected power and all that.

What does a PIM test do?

When you do a PIM test typical two high power signals are injected into the antenna line.  You can actually pass a sweep test but not a PIM test.

I won’t go into PIM tests very much because you need high dollar units such as those from Anritsu and Kaelus. These cost 10’s of thousands of dollars new.  Sometimes you can find these used.  However, the next thing you will run into is understanding the output of such a device.  Cell crews go to week long certification classes to become a PIM certified tech from Anritsu and others.

What causes a PIM test to fail?

According to Kaelus the most common problems are:

• Contaminated surfaces or contacts due to dirt, dust, moisture or oxidation.
• Loose mechanical junctions due to inadequate torque, poor alignment or poorly prepared contact surfaces.
• Loose mechanical junctions caused transportation shock or vibration .
• Metal flakes or shavings inside RF connections.
• Poorly prepared RF connections
•Trapped dielectric materials (adhesives, foam, etc.)
•Cracks or distortions at the end of the outer conductor of coaxial cables caused by over tightening the back nut during installation.
• Solid inner conductors distorted in the preparation process causing these to be out of round or tapered over the mating length.
• Hollow inner conductors excessively enlarged or made oval during the preparation process.

Why does cable matter?

Cables do not typically cause PIM, but poorly terminated or damaged cables can and do cause problems.

Cables with Seams can cause issues.  The seam can corrode.  Plated copper, found in cheaper cables, can break away from the aluminum core. This actually allows small amounts of flaking to happen between the connector and the core of the cable.  This will cause PIM issues and is very hard to diagnose. Imagine little flakes inside a connector. You don’t see them until you break open the connector, and even then they may be pretty little flakes.

Cables can change their physical configuration as temperature varies. For instance, sunshine can warm cables, changing their electrical length. A cable that happens to be the right length to cancel out PIM when cool may show strong PIM after changing its length on a warm day, or, it can work the other way around, good when hot and bad when cold. In addition, the physical change in length can make a formerly good connection into a poor one, also generating PIM. Other environmental factors such as water in the connector or cable can be an issue, as with any RF setup.

I think I have PIM issues. What are some indications?

PIM often shows up as poor statistics from the affected antenna. One of the first and most direct indications of PIM can be seen in cells with two receive paths. If the noise floor is not equal between the two paths, the cause is likely PIM generated inside the noisy receive path.

How Do I prevent PIM issues?

Cable quality and connector quality are one of the biggest factors in the PIM quality of a LTE system.  Many WISPs are used to making their own LMR cables and putting on their own connectors.  There is a difference between a low PIM LMR-400 cable and normal LMR-400.  Same for connectors.  One of the recommendations today was to use 1/2” superflex heliax.

The easy recommendation is to buy pre-made cables that have already been PIM certified.  In a typical WISP setup, you do not have lots and lot of components in your setup. Buy already certified components from your distributors that are “Low PIM rated”.

What is VoLTE?

Voice over LTE (VoLTE) allows both Data and voice calls at the same time.  It is also known as “HD Calling” .

Lots of good info here.
https://en.wikipedia.org/wiki/Voice_over_LTE

The advantages
-Simultaneous Voice and Data Calls
-High definition (HD) voice calling

Sprint currently supports VoLTE.  Airspan is supporting VoLTE.

Phones which support VoLTE
https://en.wikipedia.org/wiki/List_of_smartphones_with_HD_Voice_support

Water tower install with mounting frame

We recently headed up a job for a client of installing some RF elements horns, Cambium ePMP, and Baicells LTE for a client.  One of the gems of this job was the frame the client designed for the job.  We can’t take credit for this. We just think it’s cool. Some of these pictures were taken during construction, thus post clean-up.

The frame is truly an example of how WISPs are stepping up their installs to become more standardized and carrier-grade. It costs some money but is worth it in the end.

 

Product of the week: LTE signal booster

Do you have poor cellular signal in your home or office? Maybe a mobile job site? For you tower technicians having one of these in your job trailer might make life simpler.

https://www.ispsupplies.com/SureCall-SC-PolyEZ-DT-72-Kit

The revolutionary SureCall EZ 4G Cell Phone Signal Booster sets up in minutes, delivering unrivaled signal boosting power indoors for all cellular devices. The EZ 4G features cutting-edge technology and an elegant plug-and-play design, boosting voice, text and 4G LTE data signal for all North American carriers- including Verizon, AT&T, T-Mobile and Sprint, without the need to drill holes or mount an antenna outside.

Dropped calls and slow 4G LTE data speeds are history, as the EZ 4G provides clear and consistent coverage for all cellular devices in 1 – 2 rooms, up to 2,000 sq ft. End the frustration of weak and unreliable indoor cell phone signal with the SureCall EZ 4G.

Kit includes:1

  • EZ 4G Booster with built-in antenna (includes removable stand and adhesive tape for easy window mounting).
  • Desktop Antenna rebroadcasts the boosted cellular signal to multiple mobile devices.
  • Coax Cable 50 ft of RG-6 connects the EZ 4G Booster and the Desktop Antenna.

Uplink Frequency Range (MHz): 698-716 / 776-787 / 824-849 / 1850-1915 / 1710-1755 ( G-Block Included )

Downlink Frequency Range (MHz): 728-746 / 746-757 / 869-894 / 1930-1995 / 2110-2155 ( G-Block Included )

Supported Standards: CDMA, WCDMA, GSM, EDGE, HSPA+, EVDO, LTE and all cellular standards

Small cells and network as a Service (NaaS)

In this article, I am going to talk about how WISPs can monetize their networks in the ever-growing hype of 5G. Whether you think 5G is hype, or overblown from a technical aspect, you need to embrace the 5G wave of hype and use it to your advantage.

Many WISPs should be familiar with 5G in terms of how small cells work from a technical, physical, and a philosophical viewpoint. This knowledge is important, as outlined in Small Cells and hybrid networks for WISPs: Part 1, as well as making your network attractive for Network as a Service (NaaS).

Small cell digital signage and cameras

Wireless Service providers, especially ones with active community ties, have a unique advantage over the larger providers such as Verizon and AT&T when it comes to small cells.  Many of the local WISPs have the contacts to be able to put up small cell infrastructure in their coverage areas.   The provider does not have to own any licensed cellular spectrum to do this.  Many WISPs can make a business model with unlicensed (2.4 and 5GHZ) and CBRS band. The big benefit of this is if these providers build this infrastructure in mind of selling space to the larger carriers, then it can be a huge benefit.  The local ISP is now selling its infrastructure. Many ISPs would rather have one client paying $1000 a month and 10 clients paying $100 a month.  With this, you can do both.

How do you do this? In an upcoming podcast, I am going to talk with Tolly Marcus from Airpacket about how WISPs can “up their game” to design and engineer their networks to be in-line with what the larger carriers’ design.  This mindset will focus on the thinking processes  ISPs need to start implementing into their own networks.

One of the things the local provider can start looking at is small cell poles. Companies like Wytec International are implementing the next-generation of smart poles.  These poles tie cellular, CBRS, wifi, iOT, and other technologies in an unobtrusive design. The photo below is from this month’s edition of AGL Magazine.

Inside of a next-generation smart pole

By looking at this pole we can see the many compartments inside. Cities like this design as it covers ugly wires and just kind of blends in.  So, what does this have to do with the WISP? If a WISP were to design and engineer these to take into account the designs the carriers mentioned earlier require then the network can be sold as a service to them. Many factors and things need to be met, but it is doable. Again, the WISP does not have to operate in the Cellular bands in order to put up the pole infrastructure.

WISP puts up these throughout the town or city they can leave options for a carrier or multiple carriers to add their equipment into existing infrastructure.  The local ISP is selling capacity on a purpose-built network they have control over instead of the large carrier rolling over them.  The addition of small cells also opens up additional opportunities for the local ISP which otherwise might go to a 5G carrier.

Some of the opportunities to the local ISP can be
•Cellular Small Cells
•Digital Signage and displays
•Public Wi-Fi
•Survelliance
•IoT networks
•Meter Reading
•Informational Kiosks
These services are just a few of the ones an ISP with local connections can provide many services needed while selling to carriers who are already deploying small cells. 
In closing, if you are an ISP, especially a WISP use the 5G hype to further your business instead of trying to fight it. Adapt what you need to your business model to help provide the next generation of services.  Don’t get hung up on semantics.

Podcast: Cameron Kilton from Alpha and I talk about LTE

Cameron Kilton from Alpha Wireless and i sit down at the 2019 Indiana ISP meeting and talk about LTE and Wireless operators.  Pardon the rough beginning. the room we were in had a flickering light that took me by surprise near the beginning and messed with my rhythm.

Cameron’s Twitter
https://twitter.com/cameronkilton

Alpha Wireless
https://alphawireless.com/