Wireless communication systems are widely known in which subscriber stations communicate wirelessly using cells provided by base stations. As users move around there is a requirement for handover of the users from one cell to another, that is to say from a first base station (source base station) to a second base station (destination base station).
Current, “4G”, wireless communication systems include systems based on the set of standards developed by 3GPP and referred to as LTE or LTE-A, in which the users are referred to as UEs (User Equipments) and the base stations as eNBs (enhanced Node Bs). Mobility management functions in LTE networks for UEs in connected mode handle all necessary steps for handover. These steps include processes that precede the final HO handover decision on the source network side (control and evaluation of UE and eNB measurements), preparation of resources on the target network side, notifying the UE of the new radio resources and finally releasing resources on the source network side. Handover of UEs also involves transferring, from one eNB to another, all the information related to the UE, called its “context”.
A typical handover procedure in LTE networks, taken from 3GPP TS36.300, is illustrated in FIG. 7, showing three phases of handover respectively labelled “Preparation”, “Execution” and “Completion”, and illustrating the signalling which occurs between a UE being handed over, source and target eNBs, a Mobility Management Entity (MME), and a Serving Gateway (S-GW).
Suppose that a connected-mode UE is connected to a Source eNB providing a serving cell, and can receive at least reference signals from a neighbour cell provided by a Target eNB. In a step 1. “Measurement Control”, the UE is triggered to send measurement report by the rules set by system information, specification etc. (see 3GPP TS36.331). In a step 2. “Measurement Reports”, UE performs measurements of attributes of the serving and neighbour cells. Step 3 “HO decision” is for Source eNB to make a decision based on measurement report and RRM information to hand over the UE. Then, (4. “Handover Request”) the Source eNB issues a handover request to the Target eNB, passing necessary information to prepare the handover at the target side. In a step 5. “Admission Control”, admission control may be performed by the Target eNB to determine whether or not it agrees to accept the UE. Then (6. “Handover Request Ack.”) the Target eNB 11 prepares the HO and sends the handover request Ack. to the Source eNB, in which a handover command is included for the Source eNB to forward the command in the form of a message labelled “7. RRC Conn. Reconf.mobilityControlinfo”, to instruct the UE to connect to the target cell.
Several necessary steps are performed on the network side to ensure a lossless user plane path switch, in other words minimum interruption in the data packets being transmitted to or from the UE. These include a step 8. “SN Status Transfer” by which the Source eNB informs the Target eNB of the Sequence Number (SN) up to which it has successfully delivered data packets, in order for the Target eNB to know at which packet to start transmission.
After receiving the handover command, UE performs synchronisation to Target eNB (9. “Synchronization”) and accesses the target cell. The Target eNB responds (10. “UL Allocation+TA for UE”) with an uplink allocation and timing advance. When the UE has successfully accessed the target cell, the UE sends a message (11. “RRC Connection Reconfiguration Complete”) to the Target eNB to confirm the completion of handover.
The subsequent steps 12.-15. in FIG. 7 can be summarised as a user plane path switch, which changes the DL user plane data delivery path from the path: S-GW→Source eNB to: S-GW→Target eNB. Finally, the MME confirms (16. Path Switch Req. Ack) the handover and the Target eNB then sends a message (17. “UE Context Release”) to instruct the Source eNB to release the resources previously allocated to the handed-over UE.
As will be understood, the above handover process is considerably involved and requires a considerable amount of signalling to be exchanged among the UE, eNBs, MME and S-GW. FIG. 7 illustrates a handover of a single UE but the same eNBs may need to hand over multiple UEs at the same time. This is possible if the required control signalling is carried in different radio resources for each UE. Generally, handovers are conducted on a “first come, first served” basis.
Next generation wireless communications systems such as so-called “5G” aim to offer improved services to the user compared to the existing systems. These systems are expected to offer high data rate services for the processing and transmission of a wide range of information, such as voice, video and IP multimedia data.
To meet this expectation, 5G networks will employ “small cells” as an indispensable part of the 5G landscape. Small cells have already been proposed for use in fixed locations in homes, offices and public spaces as an addition to conventional, “macro” cells, for example to fill in coverage holes in the macro cell network and provide localised capacity hotspots. However, 5G networks are also likely to include small cells mounted on moving public transportation vehicles to enhance the coverage, capacity and service quality for the in-vehicle mobile users and to reduce the environmental impact of mobile communication systems by deploying wireless access nodes close to where users are. Such small cells mounted inside vehicles will henceforth be referred to as “moving cells”. Both fixed and moving small cells can be managed in an operator-neutral way (Small Cells as a Service, SCaaS), to avoid unnecessary duplication of hardware.
Taking a bus as an example of a moving public transportation vehicle, user equipments (UEs) inside the bus will connect to an inside base station/access point instead of to outside macro- or small cells, and a transceiver will be placed outside the bus for transmitting/receiving data to/from the macro-cell providing the wireless backhaul to the core network. When stopped at a bus stop, the transceiver may further be able to co-operate with small cells in the vicinity, such as a small cell of a femto base station provided at each bus stop. Thus better signal quality and high data rate can be provided to the in-vehicle UEs to satisfy their needs for various mobile data services including those having rigorous requirements on latency emerging in the 5G era.
There are several issues that need to be addressed for this moving cell scenario, such as the management of handovers that occur when passengers get on/off the bus at bus stops. As will be appreciated from the above discussion of a conventional handover, if many users get on or off a public transportation vehicle at the same time and attempt to perform handover, the result will be a high signalling load on the network and increased latency experienced by the users.