State-owned telecommunications company TOT's 2300-megahertz spectrum is not responsible for interfering with the signalling system of the Bangkok Mass Transit System (BTS), according to an initial report by the telecom regulator Tuesday evening.

The latest inspection by the National Broadcasting and Telecommunications Commission (NBTC) also found that Total Access Communication (DTAC), which operates its 4G service under TOT's partnership contract on the 2310-2370MHz bandwidth, is not responsible for delays experienced by commuters using the BTS Skytrain since early Monday morning.

The telecom regulator Tuesday morning sent two signalling test vehicles to verify the train route, following rumours that the 2300MHz spectrum caused the interference. The BTS issued a post on its website on Monday claiming bandwidth from the mobile phone operator was interrupting train-signalling, leading to this week's delays.

Even though the BTS did not name the mobile phone operator, it is believed to have been referring to DTAC, the biggest operator using the closest bandwidth.

Tuesday's inspection discovered that the DTAC-operated bandwidth was too far from the 2400MHz spectrum range used by the BTS's train-signalling system.

To settle the matter, the NBTC summoned all related parties to a meeting on Wednesday to discuss the matter and defuse speculation about who was at fault and why.

The three parties were due to try and thrash out a resolution today: The BTS currently owns the 2300-MHz spectrum; TOT and DTAC operate the 2300MHz bandwidth.

"There may be several factors generating problems for the BTS's system and the train control system, not only the spectrum," a well-informed source at the NBTC told the Bangkok Post. The source said the BTS has been using an unlicensed frequency for its signalling system via the 2,400MHz bandwidth for the past decade.

"The unlicensed 2400MHz range is now used by many networks as a WiFi hot spot so those who attend the meeting need to come up with a solution to stave off potential problems in the future," the source said.

The BTS said on Monday the delays were being caused by radio signals transmitted from high buildings near the train route that were interfering with its signalling system.

The comments were reposted on various social media platforms, drawing wide criticism of how the BTS's operations were being managed as well as the operations of the DTAC spectrum pending a decision by the NBTC on how the spectrum allocation will be regulated and managed.

Suebsak Suebpakdee, a telecom researcher at Bangkok University, agreed with the NBTC's findings.

He said it is unlikely the 2300MHz spectrum operated by DTAC was responsible for interfering with the BTS's signalling system.

He said the BTS has had its operations upset in the past and several other factors could be to blame, and suggested that blaming "spectrum interference" was not very helpful.

"No matter where the problem lies, the train service providers need to learn their lesson and figure out what to do to avoid any repeat in the future," he said.