BNetzA, Mainz, 25-26June 2012 / M65_30R0_SE24
Date issued:19 June2012
Source:DBNetzAG (supported by UIC)
Status:input document
Subject:WI41“Impact of SRD and RFID applications on radio services and their Co-existence in the frequency bands 870-876 MHz and 915-921 MHz”
Summary:
In accordance with ECC/DEC (04)06 the frequency band873-876 MHz (uplink) paired with 918-921 MHz (downlink) is assigned as extended frequency band (ER-GSM) for railway use. Many European Railways have raised their interest and need to use the ER-GSM frequencies.
In Germany, the ER-GSM band, is already assigned to the GSM-R services as primary users. The use of this spectrum for GSM-R services is required in order to fulfill European and national regulation.
- 873 – 880 MHz is used by the GSM-R uplink; 918 – 925 MHz is used by the GSM-R downlink.
- In the ER-GSM band (873 - 876 MHz and 918 - 921 MHz) secondary usage is allowed if the GSM-R services are not affected.
- Potential secondary users in the band 873 - 876 MHz are SRDs.
- The use of the 918 - 921 MHz by RFIDs is currently discussed in ETSI STF 397.
GSM-R underlies strict regulatory requirements that are partly resulting from European regulation. This regulation reflects the high importance of reliable GSM-R servicesfor the safe and economically efficient operation of railways.
For the 873 – 876 MHz band, rules are required that guarantee a reliable operation of GSM-R without putting unnecessary stringent requirements on the SRD devices and operation. In order to define these rules, models reflecting the specific requirements of GSM-R and an evaluation methodology reflecting the most relevant scenarios are required.
For GSM-R operation the EIRENE specification defines a minimum availability criterion of 95% (several parameters defined for measuring the coverage) for each 100 m section of railway track length. This EIRENE criterion reflects the high sensitivity of railway operations to the availability of GSM-R services.
Within the scenarios discussed in SE 24the interference from SRDs on the GSM-R base stations is the most critical one. For these scenarios a protection criterion needs to be defined reflecting the safety supporting services of GSM-R. The evaluation methodology as well as the evaluation criterion describing the GSM-R needs to reflect the worst case.
To avoid unnecessary burden on the SRD operation the resulting restrictions shall only apply where needed. Any approach averaging uncritical and critical scenarios will either ignore the specific sensitivity of GSM-R services or put unnecessary requirements on SRD operations.
Proposal:
It is recommended to define a two case rule;
First principle approach (cognitive approach):
The SRDs have to take the use of the ER-GSM band into accountand provide protection by technical or administrative measures. This approach requests certain intelligence for SRDs in order to determine the region (frequency or space) where GSM-R cannot be affected and no specific requirements on SRD operation are needed here.
A practical solution is currently under development by ETSI STF 397 with the aim to decode the GSM-R broadcast channel, transmitting the radio channels used in the ER-GSM band.
Second principle approach (non-cognitive approach):
For the case that the SRD is not able to determine via a cognitive feature if GSM-R can be affected or not , generic requirements on SRD operation guaranteeing that GSM-R performance is not critically degraded are required. The SRD transmission have to be scheduled and controlled in a way that the time intervals in which the GSM-R operation is interfered do not extend beyond a maximum interference time Txon and will always be followed by an interference free period of time duration Txoff.
Based on the GSM-R behavior the following parameters can be tolerated:
Txon:<5ms (transmission time)
Txoff: >485ms (transmission free time)
The limits provided above shall be fulfilled in all practical use cases. However, this means have to take the aggregated interference into account, it needs to guarantee the compatibility for those cases where several SRDs operate at the same time and location. The defined criterion may not cover all situations in a real environment, therefore some situations have to be left open and solved on a case by case basis.
Rational for the limit proposal:Within the transmitted GSM control channels the RACH is the most critical one. The RACH is transmitted within a GSM-Burst, 8 Bursts are combined to a frame with a length of 4,612 ms (refered to as 5 ms), the RACH itself is specified according to GSM 05.01(channel organisation) embedded in a 51–frame multi frame with a time period of 235,38 ms. In case of a disturbed RACH it will be retransmitted within the next 51-frame, this leads to a transmit period of 2x 235,38 ms ~ 470 ms to ensure the proper transmission. Cumulative effects caused by several SRD transmitting at the same time and location have to be taken into account. To achieve a safety margin and to avoid any impact on the 4x120 ms used for calculating the QoS-Parameters a Txoff time of 485 ms has to be used.
Additional requirements to be considered
Interference scenarios need to reflect the worst case cumulative effects with respect to GSM-R operational requirements. The derived values representing the sum of all allowed transmissions to be received at the GSM-R BTS side.If the given limits are achieved by a single device, further SRD´s have to take technical measures to prevent GSM-R form being interfered.Further factors have to be taken into account that will limit the probability of interference (e.g. output power, spatial separation, in-/outdoor use).
The aggregation of wanted and unwanted SRD signals is not considered yet, this has to be analyzed in addition.
Compatibility scenario in the 918 – 921 MHz band
The ER-GSM downlink band 918 - 921 MHz is foreseen tobe shared with RFID applications. Within ETSI STF 397 currently a demonstrator is under development which shall be able to show that technical mechanism exists to prevent GSM-R form interference. The solution found within ETSI STF 397 is the usage of GSM-R internal information's transmitted within the Broadcast Channel of GSM-R. This approach is described in the first principle approach.
Suitable conditions to verify the efficiency of the protection mechanism have to be established.
Furthermore the requirements for SRDs have to be optimized in order to be able to protect ER-GSM from harmful interferences.
Background information:
The system of GSM has implemented functions and algorithm to compensate the impact of the radio channel,this functionality shouldn´t be misused to eliminate interferences caused by other radio systems. With the derived values a GSM-R system shall be able to cope with an existing interference without an impact on the quality of service.
Basically a GSM system consist of two types of channel:
- Control channels (carrying information for control and signalling)
- Traffic channels (carrying voice and data)
The RACH channel is used by the mobile to signalize towards the network the need for a dedicated radio resource. The RACH (for group calls NCH named) is also used for the establishment of group call´s (e.g. the railway emergency call) used in the railway environment. This Control Channel is transmitted in a time slot and usually repeated only three times, if this attempt fails no radio resource is available.
Traffic channel:
For a dedicated traffic channel the frame structure is described in the following picture: