With the rapid development of mobile communication technology, it is particularly important to build a high-quality mobile communication system with good coverage, high connection rate and high communication service quality, so that the network can achieve high customer satisfaction under the best resource utilization. When a mobile station moves from the coverage of one base station to the coverage of another base station, how to ensure the continuity of mobile users' calls is an inevitable requirement for all cellular mobile communication systems. An important feature of CDMA system, which is different from other systems, is that it supports three handover types: hard handover, soft handover and softer handover. This function not only enhances the CDMA function, but also maintains the integrity of the call.
Introduction of handover technology in 1 CDMA system
⑴ Hard handover: Hard handover refers to the process of disconnecting from the old community before establishing contact with the new community. Hard handoff includes the following two situations: between different channels in the same MSC; Between different MSCS.
⑵ Soft handover: Soft handover refers to the handover process in which the mobile station keeps connecting with more than two base stations without disconnecting from the original base station before establishing contact with the new base station. The basic principle is as follows: when the mobile station is in the area between adjacent BTS under the control of the same BSC, the mobile station establishes a wireless connection with the target BTS while maintaining the wireless connection with the original BTS, and then releases the wireless connection with the original BTS.
⑶ Soft handover: Soft handover refers to the handover of a mobile station between two sectors with the same frequency in the same cell. Soft handoff occurs when a mobile station moves from one sector to another in a cell. In soft handover, there is no need to switch between base stations, and the handover between sectors is established faster than soft handover.
Microelectronics paper is published in "Realization of System Operation". Three types of switching combinations appear, and there may be soft switching, softer switching and hard switching at the same time. For example, if a mobile station is located in an area where two sectors of one base station and another meet, soft handover and softer handover will occur. If it is at the junction of three base stations, three-way soft handover will happen again. Both soft handoffs are based on the condition that all parties with the same carrier frequency have more capacity. If the same carrier frequency of one of the adjacent base stations has reached full capacity, MSC will let the base station instruct the mobile station to switch to another carrier frequency of the adjacent base station, which is called hard handover.
2 signaling process of hard handover
The general signaling flow of hard handover is shown in figure 1- 1:
A.MS is on the phone.
B. When CDMA co-frequency handover is performed, the mobile station will perform a mobile station assisted handover (MAHO) process to complete signal quality measurement, and the system will not perform steps b and c. If CDMA non-frequency handover and CDMA-to-analog system handover are performed, the system will perform steps b and c to measure signal quality. The serving MSC determines whether to switch to the neighboring MSC according to the internal algorithm, and sends a handover measurement request message (handmreq) to the neighboring MSC.
C. the neighboring MSC will perform the measurement process according to the internal algorithm and return the measurement result to the serving MSC in the handover measurement request message (handmreq) return result.
D. Call the neighboring MSC as the target MSC, and send a device indication message (FACDIR) to the target MSC to instruct the target MSC to start the forward handover process.
E. if there is an IDle traffic channel in the predetermined target cell, the target MSC will increase the segment counter in the charging id parameter by 1 and send a device instruction message return result (facdir) to the serving MSC to start the forward handover process.
F. after the serving MSC receives facdir, the serving MSC sends a handover command to the ms.
G. the target MSC receives the signal of the MS on the allocated traffic channel.
H. the target MSC completes the connection of the relay circuit between the MSC and the service channel. The serving MSC receives the MSONCH message sent by the target MSC and knows that the target MSC has successfully completed the forward handover process.
I. After receiving the message, the serving MSC connects the call to the relay circuit between MSCs, thus completing the whole handover process.
From the signaling process of the above hard handover, we can draw a conclusion that the two MSCs, the handover cell and the handover circuit are several key points in the hard handover process, and the reasons for the failure of the above three key points in the hard handover process may be as follows: (1) the handover direction is unreachable; The handover cell is unavailable; The switch circuit is not available.
3 Hard Handover Fault Handling Case
The following lists several typical failures of hard handoff during network operation. By analyzing the hard handoff process and normal hard handoff data configuration, we can eliminate, locate and handle all kinds of hard handoff failure processes.
(1) The two-way handover from X province to Y province failed, and the result of signaling tracking was that the CIC of the switching circuit failed to obtain. From the point of signaling tracking, it's time to allocate the switching circuit, indicating that there is no problem in the configuration of the switching office and the switching cell, and the failure of the acquisition circuit CIC indicates that there is something wrong with the switching circuit. First, we check the state of the switching circuit in network management. On the X side, the relay status is output blocking, while on the Y side, the circuit status is normal. Both sides take the operation of blocking first and then unlocking, but the fault still exists. Then compare the data configuration of the switching circuits on both sides, including physical location, circuit group number and CIC code, and confirm that it is normal and correct; Finally, check the physical transmission channel, conduct the continuity test, and inform the transmission personnel that the link on the DDF frame has been replaced, and troubleshoot. Check the circuit status of X side and Y side respectively. If they are in normal idle state, dial the hard switch again. Hard handoff successfully completed troubleshooting.
⑵X can't be switched to Y, and the error is an "illegal cell" in signaling tracking, which can be determined as a cell configuration problem of the exchange office. Because X failed to switch to Y, the configuration data of the corresponding Y border cell was checked on the MSC where X was located, and it was found that the configuration error of the relevant cell was due to decimal conversion. After the correction, the field personnel conducted a test at the border and the hard handover was successful.
(3) The bidirectional switch from X to Y fails, prompting a system error. Signaling tracking results show that when switching from X to Y, a device indication message (FACDIR) was sent, but no response was received from the opposite end. If you switch from Y to X and X can't see the switching request message from Y, you can judge that the fault should be in the switching direction, that is, the switching direction on one side is configured incorrectly. It has been determined that it is a local configuration problem. Both parties cooperate to check the local configuration data of both parties, and then test it after modification. Hard handover can be carried out normally.
4 Experience summary
There are many hard handoff problems in CDMA system, but they are all the same. First of all, we should master the principle and signaling flow of hard handover, correctly summarize the fault phenomena, then locate and analyze the key points of the fault in combination with the reality, and understand the different characteristics of various models, and the fault will naturally be solved.
[References]
[1] Steven Zhang, author. Principle and optimization of CDMA switching technology. Machinery industry press.
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