Background

The use of mobile broadband technologies UMTS/HSPA and LTE in the 800 and 900 MHz band will cause interference to the GSM-R system which operates in a band adjacent to the public 900 MHz band. In order to avoid the potential detrimental effects of this 900 MHz interference to the GSM-R system, Omnitele has investigated several interference mitigation measures concerning public 900 MHz signals for a European railway company.

Solution

Omnitele has carried out a technical review of measures to both limit the signal levels of public mobile UMTS and LTE technologies in the 900 MHz band and the level of unwanted out-of-band emission of UMTS and LTE technologies in the 900 MHz band.

Omnitele has corroborated that the findings of CEPT/ECC reports on this matter are incorrect and that the norms identified by UIC form a more realistic basis for 900 MHz interference mitigation.

Omnitele’s conclusion was that 900 MHz interference to the GSM-R system is within bounds with the norms proposed by Omnitele, and faulty operation of the GSM-R network due to 900 MHz interference is averted. Furthermore, Omnitele’s proposal was not unnecessarily restrictive for mobile broadband operation in the public 900 MHz band.

Alternative technical measures to mitigate the 900 MHz interference were investigated. It was noted by Omnitele that with these alternative measures, it would be mandatory to define maximum values for the public operator power levels. The application of an individual or a combination of alternative measures would have implications for either the design or the operation of the GSM-R network. The use of filters in GSM-R cab radios was reviewed as a technical possibility but its application was prohibited by European legislation.

Interference due to spurious emission from
GSM/UMTS/TE900 public systems

GSM-R receiver blocking due to intermodulation products caused by non-linear receive characteristics in GSM-R receivers

Customer benefits

Review of all available solutions

Review of all interference mitigation solutions:

• Increase of GSM-R fieldstrength
• Build more GSM-R base stations
• Collocatie of GSM-R and public 900 MHz public base stations
• Reduce the transmit power of public 900 MHz basestations
• Change antenna patterns and configurations
• Introduce redundance
• Improve performance of GSM-R cab radios, introduce band filters
• Implement GSM-R repeaters
• Introduce 900 MHz transmit filters for public mobile operators
• Introduce frequency hopping in the GSM -R network
• Downlink Power Control for publieke 900 MHz networks  Frequency coordination  between the GSM-R network and public mobile networks  

Verification of solution

• Omnitele carried out field measurements on railways and railway stations to verify research findings

Input to standardisation

• Based the Omnitele report and findings, the railway operator was able to enter into discussion with the regulator on implementation of suggested solutions. The report also served as input for European level standardisation workgroups

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