RF Field Notes
Elevated RTWP with a clean site is not a mystery — it is a search problem. Here is the sequence a field team follows, and why each step exists.
The symptom that starts it
An external interference investigation almost always begins the same way: uplink RTWP is elevated on one or more sectors, subscribers are reporting poor performance in the uplink direction, and the site itself checks out. Sweeps pass. PIM tests pass. Alarms are clear. The radio has been swapped and nothing changed.
At that point the problem is no longer on the site. It is in the RF environment around the site, and finding it means leaving the compound.
Step one: work the data before you mobilise
The cheapest hour of an interference hunt is the one spent in front of the RTWP trends. What you are looking for is structure:
- Which sectors are affected, and which are not. A single sector points toward a bearing. All sectors elevated equally points toward something at or very near the site.
- Does it correlate across neighbouring sites? If two or three sites see it on the sectors that face a common area, you already have a rough location before anyone drives anywhere.
- What is the time pattern? Business hours only, overnight only, continuous, or intermittent bursts. A source that runs 09:00–17:00 on weekdays behaves like equipment somebody switches on.
- When did it start? Correlate against site work, nearby construction, or a new tenant in a nearby building.
A team that mobilises without this is searching blind, and the search area is the whole sector footprint.
Step two: characterise the signal
On site, the first job is to see the interferer on a spectrum analyser and record what it looks like. Centre frequency, occupied bandwidth, duty cycle, whether it sweeps or sits still, whether it keys on and off.
This matters because the signature narrows the candidate list dramatically. A narrow constant carrier, a wideband noise-like signal, and a pulsed emission are three different sources with three different likely owners. It also gives the team something unambiguous to track: once you know what you are hunting, you stop chasing every rise in the noise floor.
Step three: take bearings, plural
Direction finding with a handheld directional antenna is simple in principle: rotate, find the peak, record the heading. In practice, reflections off buildings, terrain and structures mean a single bearing can point confidently at nothing at all.
The discipline is to take bearings from several separated vantage points and look for agreement. Two bearings intersect somewhere; three bearings that intersect in the same place are a location. Bearings that do not converge usually mean multipath, and the answer is to move further and try again rather than to trust the strongest single reading.
Step four: close on the source
Triangulation gets you to a building or a block. Getting to the device means working at street level with an attenuator on the front end, because the analyser will be saturated long before you reach it. From there it is a walk-down: signal strength rises, you follow it, and eventually you are standing in front of the thing.
In practice the usual suspects are a small set. Failed or misconfigured signal boosters and repeaters. Unlicensed amplifiers installed by a well-meaning building owner. Damaged or faulty customer equipment radiating out of band. Occasionally something industrial with a failed enclosure or filter.
Step five: mitigation, and the part nobody enjoys
If the device is accessible and the owner is cooperative, the fix can happen the same day: power it down, replace it, filter it, or reorient it. Frequently the owner has no idea their equipment is causing a problem and is glad to resolve it.
When the source is not accessible, or the owner will not cooperate, the field work is still the deliverable. A documented case — signature, bearings, location, photographs, dates and times — is what allows escalation to the property owner or, where appropriate, to the regulator. An undocumented hunt cannot be escalated.
Step six: prove it
The last step is the one most often skipped. Re-run the same sweeps from the same vantage points, and compare RTWP against the pre-work baseline over a comparable period.
Without that comparison, the claim is that the interference was removed. With it, the improvement is a measurement. That distinction is the whole reason to do the work carefully: it is what allows the customer’s engineering team to close the ticket rather than watch it for another month.
What a good investigation leaves behind
- Pre- and post-mitigation spectrum plots, from the same locations
- RTWP trends before and after, across all affected sites
- Bearings with vantage points and headings, on a map
- The located source, with coordinates and photographs where accessible
- What was done, by whom, when — and what remains open