Location information-assisted opportunistic beamforming in LTE system for high-speed railway

Communication systems of high-speed railway have inherent disadvantages and advantages. The estimation error of direction-of-arrival (DOA) of desired users caused by channel feedback delay and high moving speed make the common beamforming technique of smart antennas work abnormally. Instead, the opportunistic beamforming (OBF) with dumb antennas which does not need channel feedback is proper for high-speed railway. As the system has linear topology, regular movement as well as predictable location and speed information, the OBF can be improved to adapt to this scenario. In this article, conventional OBF is first introduced to the communication systems of high-speed railway. By multiplying random complex on each transmit antenna, the channel fluctuation dynamic range is extended so that multiuser diversity can be exploited. And then location information is employed to assist the conventional one. Communication systems in high-speed railway have the advantage of predictable location and speed information. The improved OBF can perform closer to the performance of coherent beamforming. Numerical analysis and simulation results show that both two algorithms can improve the system performance significantly. Introduction As the industry of high-speed railway develops very fast, people pay a lot of attention to the reliability and efficiency of communication systems for high-speed railway. High mobility of onboard mobile stations brings about Doppler Effect and frequent handovers which are sources of hindrance to high performance communications. However, it has some special characteristics such as linear topology of cells, so the movement is regular and location as well as speed information is predictable [1]. If these features are properly made use of, they can compensate for the disadvantages to some extent, even bring about some gains. Several advanced and novel technologies of LTE can be applied to communications for high-speed railway, since GSM for Railway will evolve to LTE for Railway (LTE-R) [2]. Multi-antenna and smart antenna techniques are widely used to solve the problems existing in high-speed railway [3]. Since most parts of the carriage * Correspondence: [email protected] Institute of Mobile Communications, Southwest Jiaotong University, Chengdu 610031, China © 2012 Cheng and Fang; licensee Springer. Th Commons Attribution License (http://creativeco reproduction in any medium, provided the orig of high-speed trains are made of thick metal materials, the direct communication link between users and base stations (eNodeB) will bare the penetration loss of 20–30 dB [4]. Usually, convergence antennas named onboard transceivers will be deployed on top of the carriage to play the role of mobile relay stations. Passengers will communicate through WiFi and Femtocell which exist in the carriage [5]. We also notice that, for the number of passengers on high-speed train is large and there is a growing demand on data traffic, multiple onboard transceivers can be deployed on the top of carriages as convergence point theoretically. On one hand, they can increase the chance to trigger the handover when the train goes through the overlapping region of neighboring cells, then the handover success probability can be improved by this means. On the other hand, the multiuser diversity or selection diversity [6] can be exploited to increase the system performance. That is, at any time, the system resource being allocated to the user with best channel condition will maximize the system throughput. Figure 1 illustrates the onboard transceivers on the top of carriages. To obtain multiuser diversity gain, the fluctuation of users’ channels should have big is is an Open Access article distributed under the terms of the Creative mmons.org/licenses/by/2.0), which permits unrestricted use, distribution, and inal work is properly cited. Figure 1 Illustration of the onboard transceivers in communication system of high-speed railway. Cheng and Fang EURASIP Journal on Wireless Communications and Networking 2012, 2012:210 Page 2 of 7 http://jis.eurasipjournals.com/content/2012/1/210 dynamic range. In the scenario of high-speed railway, the onboard transceivers usually have strong line-of-sight path to eNodeB except when the train goes through tunnels or mountainous areas. According to the analysis of [7] which states that little scattering results in small dynamic range of channel fluctuations, this is not good for multiuser diversity. To compensate for this deficiency, opportunistic beamforming (OBF), as one of the supporting technologies, is employed to change the dynamic range of channel magnitude by multiplying a complex weight on each antenna to make effective use of multiuser diversity. “Opportunistic” here means the probability that there exists a user whose instantaneous channel gains are close to matching the current powers and phases allocated at the transmit antennas. Beamforming implies that multiple antennas are used to form the transmission or reception beam and increase the signalto-noise ratio (SNR) at the receiver [8]. The sum capacity can be achieved if beamforming with dirty-paper coding is used at the eNodeB with perfect channel state information to transmit signals to multiple users [9,10]. The common beamforming technique we refer to belongs to smart antenna techniques in which desired users’ direction-of-arrival (DOA) is estimated from the channel condition feedback, then the direction of main lobe of the antenna is steered to the desired users [11]. By using this technique, the quality of received signal is improved without interfering undesired users. However, though the smart antennas can adjust the elevation, beam width, and azimuth angle, the changing is very slow [12]. Since the speed of the train is at least above 350 km/h and the estimation error of DOA which are caused by feedback delay is large, beamforming technique of smart antennas cannot achieve expected effect. Instead, OBF is simply realized with dumb antennas by multiplied a complex vector of which the magnitude and phase both are random variables. Simultaneously, there is no need to track the channel vectors of users. Only SNR should be reported to eNodeBs and the user with highest SNR will be scheduled. When there are enough users in the system, multiuser diversity gain will be obtained. Certainly, the amount of OBF overhead is much smaller than that of smart antenna systems. However, compared with omni-directional antenna system, it costs more overhead and complexity to achieve better performance. In the communication system of high-speed railway, location information is predictable. With the help of location information and length of the train, the range of DOAs of onboard transceivers can be calculated. This angle range can be used to make the generation of random complex weight vector more precise to extend the dynamic range of channel magnitude. In this article, OBF algorithm is applied to the scenario of high-speed railway under the assumption of equal spacing onboard transceivers deployed on the top of carriages. Simulation results show that conventional OBF can bring some performance gain. With the help of location information, the performance can be improved further. So far as we know, no similar correlated study has been reported on the OBF for high-speed railway as well as its location information assisted version. The rest of this article is organized as follows. Section 2 introduces the system and signal models of this article; Section 3 describes the principles of OBF and its application to high-speed railway simply; Section 4 analyzes how to improve OBF for high-speed railway with known location information; Section 5 contains simulation results and performance analysis; Section 6 concludes the article.