REPORT. GSM-R and LTE Coexistence study

Transcription

1 REPORT GSM-R and LTE Coexistence study

2 TMALL 0004 Rapport generell v 2.0 Trafikverket Postadress: Röda vägen 1, Borlänge E-post: trafikverket@trafikverket.se Telefon: Dokumenttitel: GSM-R and LTE Coexistence study Författare: Fred Näsström, Bo Bergström, Rolf Folkesson Dokumentdatum: Ärendenummer: TRV2017/66996 Version: 1.1 Kontaktperson: Fred Näsström

3 GSM-R and LTE Coexistence study Table of content 1. SUMMARY INTRODUCTION MEASUREMENTS Equipment used Test Method GSM-R AS VICTIM AND LTE AS INTERFERER Measurements setup Test procedure Explanation of RxQual Test cases Test case 1: GSM-R as victim and a 1,4 MHz LTE interferer and 200 khz guard band Test case 2: GSM-R as victim and a 1,4 MHz LTE interferer and 400 khz guard band Test case 3: GSM-R as victim and a 1,4 MHz LTE interferer and 800kHz guard band Test case 4: GSM-R as victim and a 3 MHz LTE interferer and 200 khz guard band Test case 5: GSM-R as victim and a 3 MHz LTE interferer and 400 khz guard band Test case 6: GSM-R as victim and a 3 MHz LTE interferer and 800 khz guard band Results Table 1: GSM-R as victim 1,4 MHz LTE as interferer Table 2: GSM-R as victim 3 MHz LTE as interferer Summary (LTE as Interferer and GSM-R as victim) LTE AS VICTIM AND GSM-R AS INTERFERER Measurements setup Test procedure Test cases Test case 7: LTE as victim and GSM-R as interferer with 200 khz guard band Test case 8: LTE as victim and GSM-R as interferer with 400 khz guard band Test case 9: LTE as victim and GSM-R as interferer with 800 khz guard band Test case 10: LTE as victim and GSM-R as interferer with 1 MHz guard band

4 5.4. Results Table 3: LTE 1,4 as victim and GSM-R as interferer Summary (GSM-R as interferer and LTE 1,4 MHz as victim) CONCLUSION IMPLEMENTATIONS IN NETWORK, A DISCUSSION... 16

5 1. Summary The measurements in this report show that GSM-R and LTE may be deployed and coexist together with some restrictions involving, among other, limits in maximum differences in power levels between GSM-R and LTE as well as the need for a frequency guard band. 2. Introduction. The very well written LS Telecoms report Coexistence of GSM-R with other Communications Systems ERA SC concludes that it is possible to have coexistence between GSM-R and a coming LTE system. However, the report is mostly theoretical so Trafikverket wanted to verify coexistence possibilities by practical measurements in a test environment. The purpose of the measurements is to show if, under certain conditions, GSM-R and LTE can coexist within the GSM-R frequency bands. (ER UIC band / MHz or the UIC band / MHz) The conditions can be limit in power level differences, introducing guard bands, etc. Particularly power level differences and guard band is studied. This study only looks at the downlink of the system as our results from earlier coexistence measurements between GSM-R and UMTS900 show that the down link is the limiting factor. However, the uplink may need further studies. 3. Measurements 3.1. Equipment used Equipment used: Rohde & Schwarz GSM-R Emulator CMU200, used to simulate a GSM-R base station (SER /035) Rohde & Schwarz LTE Emulator CMW500, used to simulate a LTE base station and as a LTE signal generator (ID K FR) Agilent EB4436B, used as GSM signal generator (SER US ) Funkwerk MESA 23-GTM, Standard GSM-R receiver (SER ) LG G4 LH-815 LTE terminal, Standard hand held LTE Terminal (SER 506KPPB400928) 5

6 Signal level GSM(-R) Carrier 3.2. Test Method The GSM-R and LTE coexistence tests were done in different parts: In test part 1 the GSM-R terminal was the victim and interfered by a 1,4 MHz and later a 3 MHz LTE carrier. With different received power levels, as reported from the GSM-R terminal to the CMU 200, the GSM-R terminal was interfered by a LTE signal with 1,4 MHz bandwidth signal and later with a LTE signal with 3 MHz bandwidth. In test part 2 the LTE terminal was the victim interfered by a GSM-R carrier. With different received power levels, as reported from the LTE terminal to the CMW 500, the LTE terminal was interfered by a GSM-R signal. In this case, the interfering GSM-R level for loss of LTE connection was noted. When GSM-R and LTE shall coexist there is a need to separate the two carriers in frequency. The guard band in these cases was set between 100 khz to 900 khz. See illustration below. Center Frequency LTE Carrier Frequency Guard band kHz Figure 1: Guard band LTE to GSM From the tables the necessary Δp and guard band for the coexistence then can be estimated. In the test when GSM-R was the victim, signal levels between-105 and -65dBm was received by the GSM-R mobile. The GSM-R terminal was then exposed to LTE signals. The difference in GSM-R signal level received by the GSM-R terminal vs. the interfering LTE signal levels sent from the CMW500 was registered.

7 GSM Δp GSM Δp 1,4 MHz LTE MHz Figure 2: Δp (LTE) In the test when LTE was the victim, RSRP signal levels between-90 and -60 dbm was received by the LTE mobile, and a data connection was established. The LTE terminal was then exposed to interfering GSM-R signals. The difference in LTE RSRP signal level received by the LTE terminal vs. interfering GSM-R signal levels sent from the signal generator Agilent EB4436B was registered. 1,4 MHz LTE MHz Figure 3: Δp (GSM-R) 7

8 GSM Explanation of Δf (guard band) the separation in frequency between LTE and GSM-R. Δf 1,4 MHz LTE MHz Figure 4: Δf

9 4. GSM-R as victim and LTE as interferer Measurements setup At the test a GSM-R base station emulator was connected, via a coax cable and a 30 db directional coupler, to a Funkwerk MESA 23 GSM-R terminal. The GSM-R radio module used in the terminal is a standard GSM-R radio module (not improved). As an interfering signal source a Rohde & Schwarz CMW 500 LTE base station emulator was used in signal generator mode. The LTE signal was injected to the GSM-R terminal via the through port (0 db) at the directional coupler. The GSM-R serving signal was connected to the -30 db port at the directional coupler. LTE Signal Generator (Rhode & Schwarz CMW500) Interferer - 0 db 30 db Directional Coupler (Avitec M166101) GSM-R Terminal (Funkwerk Standard MESA 23) GSM-R Base station emulator (Rohde & Schwarz CMU200) Server Figure 5: -30 db 4.2. Test procedure A call was established between the GSM-R emulator and the GSM-R terminal. The GSM-R received signal level noted (table 1) was the level reported by the GSM-R terminal to the emulator. The signal levels from the GSM-R emulator was set between -105 dbm and -65 dbm in 10 db steps. The interfering LTE signal was injected and set to a low level not to initially cause interference to the GSM-R terminal reporting RxQual =0. The interfering level was then slowly increased until the GSM-R terminal reported RxQual =4, Then the level of the interfering signal was noted in table 1. The test was repeated at -95 dbm, -85 dbm, -75 dbm and -65 dbm. The interfering LTE signal level was then raised until the GSM-R terminal reported RxQual = 4 for each tested GSM-R level. 9

10 4.3. Explanation of RxQual Reporting RxQual is a way for the GSM-R mobile to report the quality of the established connection between the mobile and base station. RxQual value is scaled from 0 (zero) to 7. RxQual = 0 equals excellent connection conditions between mobile and base station. RxQual = 4 equals interfered connection conditions between mobile and base station. RxQual = 7 equals lost/dropped connection conditions between mobile and base station Test cases Test case 1: GSM-R as victim and a 1,4 MHz LTE interferer and 100 khz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 1,4 MHz with the centre frequency 920,3 MHz, giving a guard band GSM-R -> LTE of 100 khz. The results are noted in table Test case 2: GSM-R as victim and a 1,4 MHz LTE interferer and 300 khz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 1,4 MHz with the centre frequency 920,1 MHz, giving a guard band GSM-R -> LTE of 300 khz. The results are noted in table Test case 3: GSM-R as victim and a 1,4 MHz LTE interferer and 700kHz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 1,4 MHz with the centre frequency 919,7 MHz, giving a guard band GSM-R -> LTE of 700 khz. The results are noted in table 1.

11 Test case 4: GSM-R as victim and a 3 MHz LTE interferer and 100 khz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 3 MHz with the centre frequency 919,5 MHz, giving a guard band GSM-R -> LTE of 100 khz. The results are noted in table Test case 5: GSM-R as victim and a 3 MHz LTE interferer and 300 khz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 3 MHz with the centre frequency 919,3 MHz, giving a guard band GSM-R -> LTE of 300 khz. The results are noted in table Test case 6: GSM-R as victim and a 3 MHz LTE interferer and 700 khz guard band The GSM-R frequency used in the call was set to 921,2 MHz (GSM-R channel 955). The interfering LTE carrier was set to the bandwidth of 3 MHz with the centre frequency 918,9 MHz, giving a guard band GSM-R -> LTE of 700 khz. The results are noted in table Results Table 1: GSM-R as victim 1,4 MHz LTE as interferer Technology Freq MHz Level dbm Level dbm Level dbm Level dbm Level dbm GSM-R freq MHz LTE center freq Guard band f (khz) 100 Delta p (db) GSM-R freq MHz LTE center freq ,5-20,4-17,9 Guard band f (khz) 300 Delta p (db) ,5 54,6 47,1 GSM-R freq MHz LTE center freq ,4-26,4-19,4-16,4-13,6 Guard band f (khz) 700 Delta p (db) 68,6 68,6 65,6 58,6 51,4 Table 1: GSM-R as victim,1.4 MHz LTE as interferer 11

12 Table 2: GSM-R as victim 3 MHz LTE as interferer GSM-R as victim, 3 MHz LTE as interferer Technology Freq Level dbm Level dbm Level dbm Level dbm Level dbm GSM-R freq MHz LTE 919, N/A Guard band f (khz) 100 Delta p (db) N/A GSM-R freq MHz LTE 919, N/A Guard band f (khz) 300 Delta p (db) N/A GSM-R freq MHz LTE RSRP 919, N/A N/A Guard band 700 Delta p (db) N/A N/A Table 2: 4.6. Summary (LTE as Interferer and GSM-R as victim) This summary only concerns the 1,4 MHz LTE as interferer thou this is most likely to be used for the coming railway communications. The power differences between the access systems sets the dynamic range, Δp, of the practical communication. To maintain communication it seems necessary to limit the signal level difference, Δp, between GSM-R and LTE to in the worst case 46 db (guard band = 100 khz) and in the better case 68,6 db (guard band = 700 khz).

13 5. LTE as victim and GSM-R as interferer Due to standards in frequency bands (band 8), the Rohde & Schwarz CMW500 could not use frequencies below 925 MHz in emulator mode. Some measurements had to be done in the public frequency band above 925 MHz. This however, do not affect the results Measurements setup At the test an LTE base station emulator was connected, via a coax cable and a 30 db directional coupler to a LG G4 standard hand held LTE terminal. The terminal was placed in an antenna chamber with about 12 db internal coupling loss. As interfering GSM-R signal source an Agilent EB4436B signal generator was used. The GSM-R signal was injected to the LTE terminal via the -30 db port at the directional coupler. The LTE serving signal was connected to the through port (0 db) at the directional coupler. The total coupling loss from the GSM-R interfering signal to the LTE terminal is - 30 db (directional coupler) plus -12 db (antenna chamber) giving a total coupling loss of -42 db. LTE Base station emulator (Rohde & Schwarz CMW500) GSM Signal Generator (Agilent E4436B) Figure 6 Server Interferer - 0 db 30 db Directional Coupler (Avitec M166101) - 30 db -12 db Coupling loss LTE Terminal (LG G4 LH-H815) 5.2. Test procedure A connection was established between the LTE emulator and the LTE terminal (Standard LG G4 LH-H815). The LTE received signal level noted (table 3) was the level reported by the LTE terminal to the emulator. The signal levels from the LTE emulator was set between -90 dbm and -60 dbm in 10 db steps. The interfering GSM-R signal was injected and set to a low level not to initially cause interference to the LTE terminal still holding the connection. The interfering GSM-R level was then slowly increased until the LTE terminal lost the connection. Then the level of the interfering signal was noted in table 3. 13

14 The test was repeated at -90 dbm, -80 dbm, -70 dbm and -60 dbm. The interfering GSM-R signal level was then raised until the LTE terminal lost the connection for each tested LTE signal level. The LTE bandwidth used was 1,4 MHz Test cases Test case 7: LTE as victim and GSM-R as interferer with 100 khz guard band The LTE frequency used in the connection was 926,0 MHz. The interfering GSM-R signal was set to the centre frequency 926,9 MHz, giving guard band LTE -> GSM-R of 100 khz. The results are noted in table Test case 8: LTE as victim and GSM-R as interferer with 300 khz guard band The LTE frequency used in the connection was 926,0 MHz. The interfering GSM-R signal was set to the centre frequency 927,1 MHz, giving a guard band LTE -> GSM-R of 300 khz. The results are noted in table Test case 9: LTE as victim and GSM-R as interferer with 700 khz guard band The LTE frequency used in the connection was 926,0 MHz. The interfering GSM-R signal was set to the centre frequency 927,5 MHz, giving a guard band LTE -> GSM-R of 700 khz. The results are noted in table Test case 10: LTE as victim and GSM-R as interferer with 900kHz guard band The LTE frequency used in the connection was 926,0 MHz. The interfering GSM-R signal was set to the centre frequency 927,7 MHz, giving a guard band LTE -> GSM-R of 900 khz. The results are noted in table 3.

15 5.4. Results Table 3: LTE 1,4 as victim and GSM-R as interferer Technology Freq Level dbm Level dbm Level dbm Level dbm GSM freq Coupling loss (db) Interfering GSM level (dbm) ,4 MHz LTE RSRP Full Cell BW power -76,8-66,8-56,8-47,8 Guard band f (khz) 100 Delta P (db) GSM freq N/A Coupling loss (db) N/A Interfering GSM level (dbm) N/A 1,4 MHz LTE RSRP Full Cell BW power -76,8-66,8-57,8-47,8 Guard band f (khz) 300 Delta P (db) N/A GSM freq N/A N/A Coupling loss (db) N/A N/A Interfering GSM level (dbm) N/A N/A 1,4 MHz LTE RSRP Full Cell BW power -76,8-66,8-57,8-47,8 Guard band f (khz) 700 Delta P (db) N/A N/A GSM freq N/A N/A N/A Coupling loss (db) -42 N/A N/A N/A Interfering GSM level (dbm) -28 N/A N/A N/A 1,4 MHz LTE RSRP Full Cell BW power -76,8-66,8-57,8-47,8 Guard band f (khz) 900 Delta P (db) 62 N/A N/A N/A Table 3 LTE as victim and GSM as interferer 15

16 5.5. Summary (GSM-R as interferer and LTE 1,4 MHz as victim) This summary only concerns the 1,4 MHz LTE as victim. The power differences between the access systems sets the dynamic range, Δp, of the practical communication as before. To maintain communication it seems necessary to limit the signal level difference, Δp, between LTE and GSM-R to in the worst case 35 db (guard band 100 khz) and in the better case 55 db (guard band = 700 khz). The result shows that a LTE signal is easily interfered by a narrowband signal such as GSM- R. A wideband signal, like LTE, is sensitive to narrowband signal interferers. 6. Conclusion These relatively simple tests only show the conditions in a static test environment with single carriers used. In a live network propagation factors and neighbouring systems may demand other margins. To be able to deploy two different technology systems within the same geographical area there is a need for robustness such as a maximum allowed difference in signal levels between the systems. The measurements show that GSM-R and LTE may coexist together under certain conditions. The LTE technology is the weakest when it comes to withstand interference and therefore the limiting factor. This work shows there is a need for further deeper studies. 7. Implementations in network, a discussion How may this results be interpreted into a real live network, were the GSM-R and LTE technologies shall coexist and cooperate? As the LTE technology is the weakest link, also the technology with the lowest dynamic range, these are the values we need to use in the coexistence. The worst Δp is 35 db and the better is 55 db. What this means in practice is that the received signal levels, both the GSM- R and the LTE from all base stations, at the LTE receiver may not differ more than Δp at any time. If the difference is greater than the above values, the terminal will lose connection. This has impacts in mobile networks, as the worst case of 35 db gives a very little margin for variations.

17 So to keep both signal levels of GSM-R and LTE as close as possible to each other we need to consider the following: Have GSM-R and LTE base stations collocated and use the same type of antenna system with same or similar E.I.R.P. in the same directions, or even better same antenna system for both technologies. This may be difficult because of the different technologies power controls. It is reasonable to assume that the mobile terminal (UE) for both GSM-R and LTE also have to use the same type of antenna system on-board as well as have similar power levels. Understand the relation between LTE bandwidth and power levels. The output power level may change according to bandwidth and service. Frequency adjacent services/bands will affect the Δp and guard band for the serving systems. External filtering on the mobile terminal (UE) will improve the conditions regarding affects from adjacent services. The test were done with only one LTE interferer, as all LTE base stations uses the same frequency, the LTE-interfering level will be higher. As a conclusion this will effect both GSM-R and LTE network design. It might be a need for a changed network design even in the GSM-R network, i.e. more base stations and changes in antenna systems as well as in cell plan. Further studies that needs to be performed: Studies regarding how power control, both for mobile terminals and base stations, for LTE and GSM will affect the situation. Study the Δp and Δf for a BER level of 1,6-3,2% for the LTE connection to compare with the RxQuality=4, which is BER 1,6 3,2 %, for GSM-R. This might give narrower guard band. Study the behaviour in the uplink for both GSM-R and LTE. 17

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