2012 7th International ICST Conference on Communications and Networking in China (CHINACOM) Study on LTE MIMO Schemes for Indoor Scenarios Zhaobiao Lv 1, Jianquan Wang 1, Changling Wang 2, Qingyu Cai 2, Xinzhong Li 1, Jun Yang 1, Youxiang Wang 1, Bin Fan 1, Yu Sheng 1 1. Network Technology Research Center, China Unicom Research Institute Beijing, China 2. Department of Network Construction, China Unicom Group Co., Ltd Beijing, China Email: lvzb7@chinaunicom.cn Abstract With the development of Internet and 3G communication systems, the requirement of data transmission is increasing. Multi-Input-Multi-Output (MIMO) as one of the important techniques to improve the capacity of Long Term Evolution (LTE) systems have been widely discussed. However, how to deploy MIMO for indoor scenarios is still a problem which needs to be solved at present. In this paper, dual-path single-polarized antenna and dual-path dual-polarized antenna solutions are analyzed. According to simulation and test results, under appropriate indoor scenarios, the both solutions have good coverage performance and have 56% improvement in capacity performance compared with traditional single-path singlepolarized antenna solution. MIMO. In the indoor scenarios, higher data rate is required normally. It is necessary to utilize MIMO technique to provide high peak rate. For the traditional 2G/3G distributed antenna systems with single-path single-polarized antenna, it is required to upgrade to multiple antennas. Figure 1. Single-path single-polarized antenna solution Keywords- LTE; Multi-Input-Multi-Output; Indoor; Dual polarized antenna I. INTRODUCTION With the development of the internet and the intensive application of 3G networks, the user's demand for data services is growing sharp. Surveys show that most part of the data services occurred in indoor scenarios. The law of the 3G services development home and abroad shows that 70% of the high-speed data services occurred in indoor scenario, such as video telephony, video streaming, online game and so on. In order to attain high transmit rate for indoor users, the indoor coverage solutions has been discussed, e.g. deploying indoor distribution system, outdoor base station outdoor exposure to indoor, etc. Among these solutions, the deployment of indoor distribution system is deemed as one primary way. The introduction of LTE makes the working frequency band higher than the existing 3G or 2G system. In order to meet the demand of high-speed data services, deep signal coverage in different kinds of buildings need to be solved through indoor distribution system. The MIMO techniques is deemed as one of the most important means to enhance the LTE system capacity. However, most part of the deployed 2G/3G indoor distribution systems are only with single antenna, which cannot support MIMO. In order to upgrade the existing traditional indoor distribution systems to support LTE with MIMO, it is crucial to research the related MIMO solutions, especially the MIMO deployment schemes over single cable. II. ANALYSIS OF LTE INDOOR MIMO TECHNICAL SCHEME Up to now, the deployed 2G/3G indoor distribution systems are configured with single antenna, which cannot support Figure 2. Dual-path single-polarized antenna solution Figure 3. Dual-path dual-polarized antenna solution Based on the structure of the 2G/3G indoor distribution systems, there are general three solutions to configure dual path MIMO, i.e., dual path dual single-polarized antennas, dual path dual-polarized antenna. The detailed performance of the two solutions will be discussed in Section III and IV. III. ANALYSIS OF LTE INDOOR MIMO SOLUTION SIMULATION RESULTS The SCM channel model, which is based on the ray method, can reflect MIMO channel variation characteristics. It can be used for link and system simulations. After comprehensive consideration the simulation complexity and the simulating channel environment accuracy, we can get a conclusion that the channel modeling method based on ray method has more advantages than others. The whole channel models constituted by antenna model, path loss model, 414 978-1-4673-2699-5/12/$31.00 2012 IEEE
correlation model, environmental parameters, generation process of user parameters and channel coefficients. Figure 4. SCM channel model and key parameters Among the channel parameters, antenna correlation, which has relationship with angle spread and antenna array space, is one of the main factors affecting MIMO channel capacity. MIMO channel capacity is different at different angle spread and antenna array space. In the following of this section, the influence of angle spread and antenna array space on channel capacity is analysed based on SCM channel. The corresponding results and conclusions are given. A. Analysis of MIMO simulation results In order to investigate the indoor MIMO performance, comparison are done with the traditional single-path singlepolarized antenna and single-polarized antenna with dualpolarized antenna solutions. The simulation results of the three solutions are given in this section. The Appropriate SNR values in near-point, mid-point and far-point is set according to distance, so the downlink SNR is set to32db,15dband 8dB,since UE s transmit power is lower than enodeb, so the uplink SNR is set to18db,7db and 0dB.Above approximate SNR values may be different from actual ones. Figure 6. Indoor MIMO 1 2 uplink throughput simulation results Figure 5 shows that the system throughput performance of the dual-path single-polarized antenna solution is related to antenna space. Compared to the traditional single-path singlepolarized antenna solution, the downlink aggregated cell throughput is increased by 20% -80% by using the dual-path single-polarized antenna solution with 0.5 wavelength antenna space. The downlink aggregated cell throughput of 4 wavelength dual-path single-polarized antenna solution increases by 58% -80%.While10 wavelength dual-path singlepolarized antenna solution increases by 64%-80%. Compared with the traditional single-path single-polarized antenna solution, the dual-path dual-polarized antenna solution can increase the downlink aggregated cell throughput by 40%-80%. However, there is no improvement for the uplink by dual-path solution. B. Analysis of angle spread influence on channel capacity Different scenarios may have different angle spreads. The antenna correlation is largely related to angle spread. The effect of angle spread to channel capacity in case of fixed antenna space and different channel scenario is investigated in this part. Figure 5. Indoor MIMO 2 2 downlink throughput simulation results Figure 7. Channel average capacity in different angle spread 415
capacity result curves nearly coincide to each other when the space is 4 wavelength and 10 wavelength. IV. ANALYSIS OF TEST RESULTS FOR THE PROPOSED SCHEMES Coverage and capacity performance of each proposed scheme are tested and compared with traditional single-path single-polarized antenna solution with the same environment and bandwidth (20 M) configuration. Performance of each scheme is analyzed according to test results. A. Test results of coverage Figure 8. Channel outage capacity in different angle spread Figure 8 shows channel outage capacity results with different angle spread under the condition where antenna space is 0.5λ and SNR is set to be 10dB. It is obviously that in specificed interruption probability the smaller angle spread with stronger antenna correlation can get the smaller outage capacity. In addition, the average channel interrupt capacity is equal to the capacity when the outage probability is 50%. It is correspond with the average channel capacity when SNR equals to 10dB shown in Figure 7.. These simulation results indicate that channel capacity is different with different angle spread when the space of different antennas is fixed. The smaller angular spread with greater antenna correlation will induce the smaller channel capacity, vice versa. C. Analysis of antenna space influence on channel capacity In order to investigate the relationship between antenna space and channel capacity, 2 2 MIMO system is studied in this paper. Figure 10. RSRP test results Figure 11. SINR test results Figure 9. Channel capacity in different scenarios and antenna space Comparing the results of channel capacity for two scenarios, we can conclude that the greater capacity will be gained by extending the space between different antennas when angle spread is small. However, when two antennas are closed to unrelated, channel capacity gain will reach its limit. So the Figure 12. Downlink application layer rate test results From Figure 10 to Figure 12, we can conclude that three schemes, traditional single-path single-polarized antenna solution, dual-path single-polarized antenna solution and 416
dual-path dual-polarized antenna solution all have good performance of coverage, almost above 90% of indoor area can meet the requirements of RSRP and SINR when the user throughput is more than 10 Mbps. antenna solution. Only when the distance between two singlepolarized antennas is big enough the we can get a good performance for indoor MIMO solutions. B. Test results of capacity Figure 15. Test results of single UE downlink peak rate in near-point in Figure 13. Indoor MIMO solutions downlink throughput test results Figure 16. Test results of single UE downlink peak rate in mid-point in Figure 14. Indoor MIMO solutions uplink throughput test results From above two figures, we can find that the average downlink throughput of single-path solution is 47.7 Mbps, while dual-path solution is 74.6 Mbps. The average throughput of MIMO solution is 56% higher than that of single-path solution. In near-point the capacity increase is highest, but in far-point it is not obvious because of poor channel condition, and even decline. We also can find that the average uplink throughput of single-path solution is 33.5Mbps, while due-path solution is 34.8Mbps. The difference is within 4%, which can be considered as same for two schemes by considering the test error. When the antennas are uncorrelated, the difference of capacity is small by using two single-polarized antennas and one dual-polarized antenna. It is difficult to measure the difference of performance between single-polarized antenna and dual-polarized antenna in the existing test environment, but it s need to verify the difference between two scenarios in more rigorous environment. C. Analysis of antenna space influence on system capacity with dual-path single-polarized antenna solution The Correlation between two single-polarized antennas is closely related to system capacity in dual-path single-polarized Figure 17. Test results of single UE downlink peak rate in far-point in From above three figures, we can see that the download peak rate is affected by distance between two single-polarized antennas. In enclosed scene, download peak throughput will increase by extending the distance between two antennas. When the distance is set to 4 wavelength or more, the influence of distance extention is less to download throughput. When it is set to 10 wavelength, the throughput reaches its peak point. In open scene, download peak throughput increases with extention of the distance between two antennas. When the distance is less than 10 wavelength, the influence of distance extention on throughput is obvious. When the distance is 10 wavelength, the throughput reaches its peak value. 417
V. CONCLUSION The performance results of two indoor MIMO schemes and various factors which may impact them are analyzed in this paper through simulation and testing, which may provide some reference basis for future MIMO technique application in indoor system. REFERENCES [1] 3GPP TS 36.211: Evolved Universal Terrestrial Radio Access (E- UTRA); Physical Channels and Modulation (Release 10). [2] J. Duplicy and B. Badic, MU-MIMO in LTE Systems in EURASIP Journal on Wireless Communications and Networking. [3] 3GPP TS 36.201: Evolved Universal Terrestrial Radio Access (E- UTRA); Physical Layer General Description(Release 10) [4] 3GPP TS 36.212: Evolved Universal Terrestrial Radio Access (E- UTRA); Multiplexing and channel coding (Release 10) [5] 3GPP TS 36.213: Evolved Universal Terrestrial Radio Access (E- UTRA); Physical layer procedures (Release 10) [6] 3GPP TS 36.143: Evolved Universal Terrestrial Radio Access (E- UTRA);FDDrepeater conformance testing(release 8) [7] 3GPP TS 36.101: Evolved Universal Terrestrial Radio Access (E- UTRA); User Equipment (UE) radio transmission and reception (Release 10) [8] 3GPP TS 36.106: Evolved Universal Terrestrial Radio Access (E- UTRA); FDD repeater radio transmission and reception; (Release 8) 418