A topic is involved here: the cell handover during a mobile phone call.
Cell Handover (Hand Over): In a wireless communication system, when a mobile station moves from one cell (referring to the coverage of a base station or base station) to another cell, channel switching is required to keep mobile users' uninterrupted communication.
It means that when the mobile station moves from the coverage area of one base station to the coverage area of another base station during a call, or the call quality is reduced due to external interference, it must change the original voice channel and switch to a new idle voice channel Go up to continue the process of holding the call.
Handover is a very important technology in mobile communication systems. Failure to handover will cause call drops and affect the quality of network operation.
Cell handover can be roughly divided into two categories: hard handover and soft handover.
(1) Hard handover: Break up before the ex, then talk about a new relationship
The hard handover process is:
When the UE moves from the source cell to the target cell, because the carrier frequency of the source cell and the target cell are different, the UE will disconnect from the source cell channel after entering the target cell. After switching the carrier frequency, re-synchronize the new cell, contact the target cell, establish a new channel, start the uplink and downlink data communication under the new frequency, and finally complete the handover from the source cell to the target cell.
Obviously, there is a pause interruption period in the hard handover during the handover process. The hard handover is a process of "release and establishment", which is characterized by "disconnect first, then switch." And at any time, the UE only connects to one cell. But this "disconnection" time is very short, users may not be able to feel it.
Hard handover is a handover mode that occurs between cells of different frequencies .
The 2G GSM FDMA (Frequency Division Multiple Access, Frequency Division Multiple Access ) is a hard handover.
(2) Soft handover: Step on two boats, talk about a new relationship first, and then break up with the ex
Soft handover is a handover mode that occurs between different cells of the same frequency .
The soft handover process is:
Since the handover is carried out between cells of the same frequency, when the UE moves to the coverage junction area of multiple cells, the UE can keep in touch with multiple cells at the same time because of the same frequency, receive the channel quality reports of these cells, and communicate with the system. The specified threshold is compared, and the cell corresponding to the best value is selected as the target cell for handover. At this time, the UE cuts off the connection channel with the source cell to complete the UE handover from the source cell to the target cell.
Therefore, soft handover is a process of "establishing, comparing, and releasing", characterized by "switching first, then disconnecting." At any moment in the handover state, the UE can connect to multiple cells at the same time.
The code division multiple access system of 3G CDMA d is a kind of soft handover.