Reliable broadband wireless communication for high speed trains using baseband cloud

We propose a novel high-speed train communication using baseband cloud (C-HSTC) system framework for providing continuous broadband services to highly mobile users. This framework is featured with a new virtualized single cell design which mitigates the impact of conventional handover failures and guarantees continuous communication services. Through exploiting the baseband units (BBU) cloud and the ful frequent frequency reuse in the virtualized single cell, we also proposed a highly efficient joint transmit beamforming algorithm targeting at compensating the inter-carrier interference (ICI) caused by severe Doppler frequency shift due to mobility. Numerical analysis shows that the new architecture and corresponding algorithms are suitable for high-speed train communication and can provide a continuous data rate of more than 100 megabits per second (Mbps) for passengers at a speed of 450 kilometers per hour (kmph). This would help to achieve satisfactory mobile broadband services for high speed train passengers. Introduction Mobile broadband communications have been extensively developed for terrestrial transportation to respond to the increasing traffic of online multimedia, gaming, mobile application downloading. However, without changes, those mobile communication technologies may not be suitable for the needs of data-intensive communications for high speed train passengers, since the relevant moving speeds are much higher and more challenging for communication designs, for example, the China Railways High-speed (CRH) train between Beijing and Shanghai at a speed of 350 kilometers per hour (kmph). In recent years, numerous efforts have been made on adapting the conventional Global System for Mobile Communications (GSM) framework to High-Speed Train Communication (HSTC) which gives birth to the railway specific GSM standard, GSM-R. In the high speed train scenario, although GSM-R has been so successful in voice communications [1], it cannot support high-rate broadband data services, such as online video or gaming, for a train at a speed up to 500 kmph. This paper presents a framework on high data rate communications for passengers on high-speed train. Our * Correspondence: [email protected] Bell Laboratories China, Room D400, Building 3, No. 388, Ningqiao Road, Pudong New District, Shanghai 201206, People’s Republic of China Full list of author information is available at the end of the article © 2012 Luo et al.; licensee Springer. This is an Attribution License (http://creativecommons.or in any medium, provided the original work is p proposed scheme is capable of providing continuous broadband service for users on high mobility by reducing the signal degradation caused by frequent handover failures and high Doppler frequency spread. Previous work Two major challenges have been identified for providing mobile high-rate data services to passengers on the highspeed train: 1) too frequent handover which causes a high call drop rate and quick battery drain; and 2) poor link quality due to high Doppler spread. To reduce the call drop rate, conventional GSM-R based HSTC systems require base stations to be densely deployed along railway tracks, and adjacent cells to have a large overlapped area so as to guarantee sufficient handover time [2]. Full functional base stations can be replaced by distributed antenna systems (DAS), radio-over-fiber (RoF) systems [3,4], or remote radio heads (RRH) [5] for cost saving and network topology simplification. A radio-over-fiber (RoF) [3] based solution for broadband railway communication consists of several radio access units (RAU) located along the rail tracks, and an optical ring network interconnects them. The radiofrequency (RF) band signals transmitted by a base station are firstly carried by an optical fiber to the RAUs, and then transmitted into the air. Commonly, each RAU has its fixed radio frequency. A range of issues related to Open Access article distributed under the terms of the Creative Commons g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction roperly cited. Luo et al. EURASIP Journal on Wireless Communications and Networking 2012, 2012:285 Page 2 of 12 http://jwcn.eurasipjournals.com/content/2012/1/285 RoF in HSTC systems including the handover [4], coverage efficiency [6], multi-mode multi-band accessing [7], and field measurement and analysis [8] have been addressed in the literatures. The major challenge of the RoF based HSTC system is that it requires frequent band switching and complex optical routing which restricts the length of the coverage of a base station. Remote radio head (RRH) based solution as shown in Figure 1 is a more favorable evolution. RRHs have been widely deployed in cellular systems, and the RRH based HSTC solution relies on a fiber network to convey signals along the tracks, similar to the RoF system, but the signals running on the fiber, i.e. a Common Public Radio Interface (CPRI), is not RF-band signals but baseband I/Q data symbols so as to avoid RF-band signal degradation due to long-distance propagation. Theoretically, the length of a CPRI link is unlimited, while in practice, it is only constrained by the timing errors accumulated along the propagation [9]. RRH-based HSTC systems have been presented in [5]. The analysis in [2] based on a conventional GSM-R system shows that the handover process in such a system requires multiple seconds to complete, thus the coverage efficiency (effective coverage area per radio head) can become prohibitively low with the increase of train speed. With the introduction of Long Term Evolution (LTE) systems to high-speed train communications [10], the handover delay could be significantly reduced to less than 1 second [11]. Due to the limited uplink resource and inadequate uplink scheduling, the LTE system may still suffer from frequent handover failures caused by unreliable uplink measurement reports at high mobility. Furthermore, for a train with hundreds of passengers, a RRH1 RRH2 eNB1 X.2 Wireless Cor Figure 1 Conventional HSTC system based on RRHs. large amount of handover requests in a short time may easily leads to signaling congestions. To address this issue, there are numerous interests in adopting LTE-based mobile relays dedicated for high speed train scenario [12] where user traffics are aggregated by a mobile relay fixed on the top of a train carriage and then communicated to the track side base stations (BS). The system only needs to maintain one high-mobility link between the mobile relay and the track-side BS, e.g. the mobile backhaul, instead of hundreds of individual access links. Mobile relay can reduce the possibility of signaling congestions, but it still requires a large overlapped cell boundary area for smooth handovers. Doppler spread is another major challenge for realizing wireless broadband high speed train communications. Especially for HSTC system based on orthogonal frequency division multiplexing (OFDM), the fast channel variations within one OFDM block destroy the orthogonality between subcarriers resulting in inter-carrier interference (ICI) proportional to the Doppler frequency. In [13,14], the authors established a compact model of ICI channel with a scalable parameterization, and proposed new receivers capable of ICI cancellation based on the ICI channel estimation. The authors of [15] proposed a time-domain windowing operation in the transmitter and receiver to form nulls around a subcarrier in frequency domain so as to reduce the ICI. Recently, Doppler compensation via exploiting multiple antennas has attracted a lot of attention. [16] proposed to combine the received signals from each antenna with regard to the ICI power levels, while [17] tried to form virtual stationary antennas by using spatial interpolation to mitigate Doppler spread.