This paper focuses on improving the downlink throughput of the base station to train communication link in a high speed train (HST) scenario. First, we provide a theoretical study of the throughput maximization problem in a singlecell MIMO-OFDM train scenario with and without cooperation among carriages. The aim is to give fundamental insight into the problem rather than providing practically realizable algorithms. The theoretical study suggests that it is highly advantageous to exploit the size of the train by increasing the number of antennas, and further allowing the carriages to cooperate. In the practical system level study, we propose two low complexity MIMO-OFDM transmission schemes, which are based on simple antenna selection methods with spatial multiplexing. The main idea is to select the best transmit antennas among different antenna combinations by comparing their estimated throughput performances. The simulation results show that the proposed algorithms outperform LTE-based dynamic rank transmission schemes in terms of throughput and computational load in practical HST scenarios. Unlike the exhaustive search type of dynamic transmission schemes, our simple algorithms are also applicable to large antenna arrays. In conclusion, large antenna arrays with simple antenna selection and spatial multiplexing transmission strategies seem potential solutions to significantly improve the throughput of the base station to train link in HST scenarios.
Integration of the satellite component into the fifth generation (5G) new radio (NR) system is an ongoing activity within the 3GPP standardization body. However, as the 5G NR system is primarily designed for terrestrial applications, its direct usage with satellite communications may raise problems whose solutions affect its underlying design principles and operation. This paper addresses this adaptation and then analyses the suitability of the 5G NR random access procedure for satellite usage. Although many of the already standardized configuration parameters and procedures are flexible enough and can be used without modifications, the timing advance (TA) calculations, its value transmission, as well as associated timers' configuration require reworking. Furthermore, we also observed the guard interval needed by the physical random access signal may be reduced using the so called minimum common delay that has to be transmitted to the terminals.
With the increasing popularity of high speed trains (HSTs) and the traffic forecast for future cellular networks, the need to provide very high data rates using higher frequency bands (HFBs) for train passengers is becoming crucial. In this paper, we present a timer-based beamforming selection algorithm for HST, which exploits the prior knowledge of the position and direction of the HST. A sequential and hierarchical codebook is designed based on the array response vectors and linked to the line-of-sight (LOS) angle-of-arrival/departure base station (BS)-HST link. The effect of velocity feedback errors on the throughput performance was analyzed. The antenna deactivation and the sub-array approaches were considered to mitigate the effect of velocity feedback errors. Evaluation of our proposed beamforming scheme indicates a close performance to the optimal singular value decomposition (SVD) scheme when no velocity feedback error occurs and with the occurrence of velocity feedback errors, the sub-array approach proved to be an efficient way to reduce the effect of the errors.
With increasing popularity of high speed trains and traffic forecast for future cellular networks, the need to provide improved data rates using higher frequency bands (HFBs) for train passengers is becoming crucial. In this paper, we modify the OFDM frame structure for HST, taking into account the increasing sensitivity to speed at HFBs. A lower bound on the SNR/SINR for a given rate for reliable communication was derived considering the physical layer parameters from the OFDM frame. We also analyze different pathloss models in the context of examining the required gain needed to achieve the same performance as with microwave bands. Finally, we present a time-based analogue beamforming selection approach for HST. We observed that, irrespective of the pathloss models used, the required gains are within the same range. For the same SNR/SINR at different frequency bands, the achievable data rate varies with respect to the frequency bands. Our results show the potential of the use of HFBs. However, due to the increased sensitivity of some channel parameters, a maximum frequency band of 38 GHz is suggested. Evaluation of our proposed beamforming scheme indicates a close performance to the optimal SVD scheme with a marginal rate gap of less than 2 b/s/Hz.
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