In the programming and the design of the mobile communication systems, the management and the control of the radio resource occupies the central status, it has an important influence to overall performance of the system. Handoff technology is an important part in the function of the radio resource management, it directly affects the capacity and the quality of service of the system, simultaneously it is also the particular function and the essential characteristic in a cellular system.  After the user equipment connects with the network in initial cell ,it may leave the range—serving of the cell because of each kind of reasons. The handoff process in mobile communication system means cut the connection of initial cell and connect with other cells,it can make communication without interrupting. In the system of WCDMA and CDMA2000,it support hard handoff、soft handoff . In the system of TD-SCDMA,it support hard handoff、relay handoff. Rapid and exact handoff can guarantee the integrity of calls effectively.

   The optimum design for handoff process is very important to all cellular communication systems. From the aspect of the network efficiency, when the user equipment is not in the suitable serving-cell to carry on the communication,it will not only affect the quality of oneself ,simultaneously it will also increase the burden of the network, even worse ,it may increase the disturbance to other users.  Mobile users should use the optimized communication link in the network and connect the corresponding basic state.

This article first introduced some basic concept of handoff technology ,then it introduced handoff technology in different systems. For TD-SCDMA is the only standard belong to China,and it will be used extensively in our country possibly. Therefore this article emphasis in the introducing of relay handoff in TD-SCDMA , and understand establishment of basic parameter in handoff ,contacting with simulation in practical work.