High-Performance Handover And Multi-Antenna Technologies Under High Mobility

Now in the world, the majority countries have set the speed limit at 120 kilometres or 40 miles per hour on motorways. The high-speed mobility refers to the speed of more than 200 kilometres per hour (km/h) on the gound. In general, such a speed can only be achieved by the high-speed railway (HSR). The HSR all over the world has achieved remarkable achievements in recent years:the operating speeds of 200-350km/h and trial speed of 574km/h. The safe and efficient operation system of HSR is indispensable to the support of modern wire, wireless communication and information processing technology. The railway wireless communication can be divided into two main categories:the specifical wireless communication system for railway dispatching and controlling and the broadband wireless communication system for passengers. The existing GSM for Railway (GSM-R) network is mainly built on the second-generation Global System for Mobile Communications (GSM), whose data rate can only meet the needs of railway-specific wireless dispatch and control operation, but fail to satisfy the requirement of the future intelligent train control, the broadband mobile communication access and other value-added service demands of passengers. In order to avoid the high-speed train becoming the’ information island’, the broadband wireless communication must provide broadband services and applications for users on HSR. Thus, in this dissertation, we will focus on the high-performance wireless transmission technologies under high mobility, including how to realize the highly reliable, fast handover and design the multi-antenna schemes with high capacity and low bit-error rate.In high-speed railway scenario, moving fast would cause some special problems, such as frequent handover, postponed handover and low-reliability handover. Traditional Coordinated Multiple Point (CoMP) is used to efficiently mitigate inter-cell interference and hence to improve the system frequency spectral efficiency. For railway scenario, the Train Control Information, such as train’s velocity and exact location, can be shared by the railway mobile communication system and the CoMP transmission technology and dual vehicle stations coordination mechanism can be applied to improve the traditional hard handover performance. The scheme improves the quality of received signal and offers a reliable communication between train and ground eNodeBs, providing a possibility that the train can receive signals from the adjacent base stations simultaneously and obtain diversity gain when it moves through the overlapping areas. The proposed scheme can also decrease the outage probability remarkably during handover and hence guarantee the reliability of train to ground communication.Under high-speed movement, another special problem about handover is that, in overlapping areas, the handover signaling (i.e. handover measurement and decision) is of higher priority than Hybrid Automatic Repeat reQuest (HARQ) retransmission so that the HARQ retransmission will be canceled when conflicts happen. In this dissertation, we construct a model of HARQ retransmission under high-speed scenarios and propose a position assisted coordinate HARQ scheme in LTE system. When the handover is completed, the UE sends the ACK of the last continuous packet to ensure lossless handover and avoid the UE to retain already-received packets in order to wait for a potential retransmission. The results of the analysis show that our proposal can reduce the VoIP PER in high speed railway scenario, and improve the system reliability, as well as VoIP capacity.How to design the multi-antenna scheme is one of the key issues in high-speed railway scenario. In this dissertation, the capacity of a multiple-input-multiple-output (MIMO) channel with N transmit and receive antennas for highspeed railways (HSRs) is analyzed based on the 3-D modeling of the line of sight (LOS). The MIMO system utilizes a uniform linear antenna array. Instead of increasing the number of antennas or simply changing the parameters of the antenna array, such as separation and geometry, the capacity gain can be obtained by adjusting the weights of multiantenna array groups, because there are few scatterers in strong LOS environments. On the other hand, it is hard to obtain the array gain of MIMO beamforming for HSRs because of drastic changes in the receiving angle when the train travels across E-UTRAN Node B. Without changing the antenna design of Long-Term Evolution (LTE) systems, this paper proposes a multiple-group multiple-antenna (MGMA) scheme that makes the columns of such an MIMO channel orthogonal by adjusting the weights among MGMA arrays, and the stable capacity gain can be obtained. The value of weights depends on the practical network topologies of the railway wireless communication system. However, the reasonable scope of group number N is less than 6. In selecting N, one important consideration is the tradeoff between practical benefit and cost of implementation.High mobility leads to the irreducible BER and its performance cannot be improved simply by increasing the signal-to-noise ratio (SNR). Under the signal model of Differential modulation and demodulation, two multi-antenna diversity receptions (Linear Combiner and Statistic Combiner) are proposed, and then we conclude the effective diversity reception scheme of train-to-ground wireless broadband communication for high-speed railway scenarios. Mathematical analysis and simulation results indicate that by increasing the diversity number the proposals can reduce the error floor because of high mobility. The performance of Linear Combiner is much better than that of Statistic Combiner, but the receiver complexity of Linear Combiner is also higher than that of Statistic Combiner. In term of practical application, the tradeoff between performance and complexity must be made according to the Quality of Service (QoS) requirements.Lastly, this paper provides a detailed analysis of Open-Loop (OL) and Closed-Loop (CL) Multiple Input Multiple Output (MIMO) scheme based on Orthogonal Space-time Block Code (OSTBC), and lays emphasis on how the velocity influences the OSTBC in high-speed railway scenario. When the train moves at a fantastic speed, the fast time-varying channel destroys the orthogonality of OSTBC reducing the diversity gain which would severely degrade the Bit Error Rate (BER) performance. Our paper proposes the Trellis Orthogonal Reconstruction Algorithm (TORA), in which the intra-code orthogonality is reconstructed through givens transformation and the transmit beamforming direction can be changed as well. In high-speed railway scenario, our proposal obtains not only the array gain by transmit beamforming, but also the same diversity gain as the train is motionless. The simulation shows that the TORA improves BER performance of MIMO schemes based on OSTBC for mobile communication system in high-speed railway.