5G Railway Wireless Network Architecture And Resource Managements

In the globe,high-speed railway(HSR)has become a significant representative industry of China by using totally domestic technologies,of which the rapid developments also put forward higher requirements for railway wireless communication systems.Currently,GSM-R(Global System for Mobile Communications for Railways),developed from 2G GSM public mobile systems,is adopted by China HSR to wirelessly connect trains and ground control centers.Unfortunately,due to the narrow bandwidth,GSM-R can only support the delivery of basic train control and dispatch services with low data rates.Nevertheless,the increasingly advanced train control system,such as emerging onboard video surveillance devices,urges HSR wireless communication systems to enhance the data transmission capability.Besides,passengers in trains also have a strong desire to get high-quality mobile services during long journeys.How to augment the system capacity to satisfy the high data rate requirements for future train control system and provide wireless access opportunities for passengers becomes a big challenge in improving the image of China HSR.On the other hand,in public mobile networks,GSM has almost been replaced by the more powerful 4G LTE,which is even evolving to next 5G mobile networks to prepare for upcoming unprecedented data storms.As a consequence,it will be more difficult in the future to update products and techniques to support old GSM-R.As a type of dedicated wireless communication systems,HSR wireless networks will also have to follow the evolution pace of the entire wireless communication industry.Firstly,to enhance the system capacity,future 5G public mobile systems will exploit higher frequency bands to gain broadband bandwidth,which also works in future HSR wireless communication networks.However,the path loss is much severer in higher frequency bands compared to widely used lower frequency bands,leading to limited cell coverage and dense network deployments.As a result,the handover frequency will rise,especially for high mobility scenarios,which will aggravate the signaling burdens for both network and user sides.Based on this observation,we propose the HSR C/U-plane decoupled network architecture to physically split the C-plane and U-plane of wireless links into different frequency bands,in which the C-plane is kept on lower frequency bands to guarantee the mobility and coverage performance,while the U-plane is moved to higher frequency bands to gain larger capacity.Under this network architecture,handovers between small cells only involve in some simple U-plane switching signaling,instead of RRC signaling,thereby relieving the network signaling burden.Beside,to ensure the transmission reliability of critical train control information,they can be entirely distributed on lower frequency bands without decoupling.Theoretical and simulation results have shown that the proposed network architecture can guarantee the coverage performance while increasing the transmission capacity.Secondly,we studied the handover problems of wireless communication systems under high mobility scenarios.In the conventional C/U-plane coupled wireless networks,the C-plane and U-plane handovers are executed simultaneously,leading to long handover time.Due to the high mobility,the time a train staying in the handover overlapping area may be shorter than that a handover needs,which will lead to handover failures.Besides,hard handover is applied in LTE networks,i.e.,before establishing connections with target base stations,users firstly need to cut down the current connection with the serving base station,causing transmission interruptions.In HSR scenarios,this kind of interruptions will happen more frequently,heavily impacting the transmission reliability.Nevertheless,in the C/U-plane decoupled networks,the C-plane and U-plane are physically separated into different network nodes.Based on this characteristic,we propose to deploy a small cell within the overlapping area of two adjacent macro cells so as to physically separate the C-plane and U-plane handovers.During macro-cell handovers which are also called C-plane handovers,the signal of U-plane is of high quality and can be kept online without the need to conduct U-plane handovers.Similarly,under this deployment,small-cell handovers,which are also called U-plane handovers,take place in the middle area of macro cells,which guarantees the C-plane connections and avoid C-plane handovers.Moreover,during small-cell handover,without involving time-consuming RRC reestablishment procedures,some simple signaling of U-plane path switching is enough to let target small cells recognize the user requesting U-plane handovers.As the C-plane and U-plane handovers are separated,the handover procedure is greatly accelerated and the handover time is highly reduced.To realize interruption-free soft handovers,we adopt Co MP and Bi-casting technologies to establish another U-plane connection in advance to continue the data transmissions even when the original U-plane connection is cut down.Simulation results have shown that the proposed scheme can realize soft and fast handovers.Thirdly,the scheduler configurations and resource allocations in MAC layers under C/U-plane decoupled HSR wireless communication systems are investigated.In multi-user public mobile networks,to harmoniously share radio resources,a centralized scheduling scheme is applied,in which both the uplink and downlink resource managements are controlled by base stations.In other words,when users have uplink data to transmit,they have to report it to base stations and wait for the arrangements from base stations to decide how to send these data,which obviously enlarge the uplink transmission delay.Nevertheless,in HSR,to guarantee the operation safety,a blocking section of about 10 km,which is much larger than the coverage of a base station,is restricted between two successive trains.That is to say,apart from the seldom train crossing and station staying scenarios,there is at most one train within the coverage of a base station at a given time.Based on this observation,to enhance the usage flexibility of uplink resources,we propose to add an additional MAC scheduler on mobile relays to authorize the train side to self-manage the uplink resources.Furthermore,based on the optimization theory,a new resource allocation scheme is proposed to enhance the spectrum efficiency of HSR wireless networks under the constraint of transmission reliability.Simulation results have demonstrated that the proposed scheduler configuration scheme can reduce the time consumption of uplink transmissions and the resource allocation scheme can enhance the spectrum efficiency.Then,we present a low-latency HARQ scheme for C/U-plane decoupled HSR wireless networks.Through retransmissions to ensure transmission reliability,the HARQ technique plays an important role in LTE networks.However,in C/U-plane decoupled HSR wireless networks,the C-plane signaling,including HARQ acknowledgments,is transmitted on relatively high-quality lower frequency bands,while the U-plane data are carried by higher frequency bands.In other words,the ACK/NACK message and corresponding acknowledged data of an HARQ process are in a macro cell and a small cell,respectively,not in the same network node.Therefore,macro cells need to forward the received HARQ acknowledgments to small cells via X3 to instruct small cells what to do next.In reality,there is forwarding delay in X3 interfaces.Consequently,the time interval between two adjacent(re)transmissions of an HARQ process in C/U-plane decoupled HSR wireless networks is enlarged.To address this challenging problem,we propose a low-latency collaborative HARQ scheme on the basis of the diversity transmission theory.Specifically,we develop a new collaborative transmission framework where the possible spare resources on lower frequency bands of macro cells,excluding those used by C-plane transmissions,can be utilized to help small cells relay erroneously received data.Simulation results have revealed that the proposed scheme can improve the success probability of retransmissions and reduce the retransmission delay.Lastly,a new method to appropriately evaluate the transmission reliability of C/U-plane decoupled HSR networks is proposed.When designing the C/U-plane decoupled network architecture,the C-plane is kept at relatively reliable lower frequency bands to guarantee its transmission reliability.Nevertheless,due to the lack of ability in reflecting the importance of C-plane,the metric of conventional outage probability cannot properly evaluate the transmission reliability of the C/U-plane decoupled architecture whose primary design consideration is exactly that the C-plane has much greater effects on the transmission reliability thereby being kept at dependable lower frequency bands.To solve this problem,a novel indicator named unreliability factor(URF)is proposed.To highlight the effect of C-plane,the value of URF is directly set to 1 if the C-plane signal quality is lower than a threshold.Otherwise,the URF value is determined by the U-plane outage probability.This evaluation method also conforms to the reality that when the C-plane BER is higher than a threshold,the U-plane data cannot be correctly decoded even if the U-plane data are reliably transmitted.Acceding to the conditional independence theory,the closed-form expression of the reliability metric under the proposed evaluation method is derived.Theoretical and simulation results have demonstrated that the proposed evaluation method precisely highlights the effects of C-plane on entire transmission process and thereby is more appropriate to assess the performance of the C/U-plane decoupled network architecture.