Interference: The Hidden Gremlin

Spectrum, the most valuable asset for a mobile service provider, is increasingly crowded and stretched.  New applications and services depend on cleaner and wider RF channels.  No longer just a matter of coverage, spectrum like the rest of the network, is strained for capacity.  Current 4G services add more channels, wider channels, and more aggregation to meet ever faster growing needs of bandwidth.

The RF spectrum is the hardest to manage.  The hidden gremlin is interference, and there’s surprisingly a lot of it almost everywhere. The most talked-about primary cause is unauthorized RF sources – those that should have ceased to transmit, or should have moved to another frequency, misaligned or mistuned radios, spurious generators, and just plain illegal transmissions.

Here are some real-life examples of the impact of interference:

• A 4G service provider found a high degree of VoLTE call drops and capacity halved due to excessive power from a WiMAX service provider and Air Force Radar that were using the same band
• A UMTS service provider found their call drop rate to be more than 3x their acceptable limits, in revenue areas, due to misaligned (and some illegal) repeaters
• An Asian service provider found itself delaying an LTE services launch as it found unauthorized users in the service provider’s allocated bands.

Tracking down the interference source in order to shut it down (or migrate it to another frequency) is an operational necessity.  This process in itself is expertise intensive and time consuming.  Interfering signals are often not polite enough to continue transmitting to allow identification and location.  Furthermore, it is important to get close to the antenna to “see what the antenna sees” to find the interferer.  That means sending a technician up the tower, a practice that takes time, can be expensive, and has safety considerations.  Even with this it can be a hit and miss affair in spite of it all.

The Solution

The Viavi’s CellAdvisor series is the optimal test tool as it makes these critical processes faster, more reliable and safer for service provider and their contractors.

Viavi CellAdvisor’s RFoCPRI/RFoOBSAI measurement allows technicians to make RF measurements on the optical links of a distributed eNodeB.

The optical front haul links carry all the RF uplink and downlink information (IQ) within the CPRI (or OBSAI) frames.  Tapping these links and decoding the frames gives all the RF information in the uplink and downlink paths, including those of interference signals.

What is the advantage?  RFoCPRI measurement allows measuring Downlink and Uplink RF parameters, including RF Spectrum, Spectrogram, RSSI, including MIMO/Conformance testing, right at the BBU, completely avoiding the need to send a technician up the tower.  It reduces risky cell tower climbs, letting technicians test safely from the ground.  In our extensive tests with service providers, RFoCPRI has shown several advantages over traditional RF measurement (spectrum analyzer and antenna combination) techniques:

• Ability to detect interference sources that RF analyzers could not, even right next to the antenna
• Ability to quickly detect intermittent transmissions on the ground; much easier to do than when balancing on a tower
• The ability to find signal corruption due to internal base station RF issues, which showed up as excessive interference (high RSSI) in a service provider’s OSS; traditional RF analyzers could not detect this problem.

Most significant of all, the traditional RF capabilities are there in CellAdvisor as well, right along with RFoCPRI and RFoOBSAI, giving the cell technician the option of choosing whichever method works best in the situation.

Spectrum is an expensive asset, and interference is a significant impediment in effective spectrum utilization.  Viavi CellAdvisor gives the most comprehensive tool set for base station verification and interference detection.

Find out how the CellAdvisor series helps make interference hunting easier.  It is the optimal test tool for installing and maintaining cell sites.