Optimization Design of CDMA system soft switching parameters and impact on the system (1)

1 Overview

The CDMA system supports multiple types of switching, including hard switching, soft switching, and soft switching. The purpose of improving the switching process and adjusting the switching parameters is to enhance the CDMA function while maintaining call integrity. Hardware switching is a time discrete event. It occurs when a call is switched from one cell to another or from one carrier to another. Soft switchover is a state where multiple base stations support one call at the same time. Hardware switching events must be short-lived. On the contrary, mobile devices are often in soft switching for a long call period. All access technologies have hardware switching, such as AMPS, TDMA, GSM, and CDMA. Soft switching is unique to CDMA systems.

2. Brief description of soft switchover process

The IS-95 system specifies soft switching: the soft switching process starts from the Mobile Station, and it must continuously measure the signal strength of the system's Pilot) channel. In order to effectively search the pilot channel, the pilot channel in the IS-95A system is divided into active set), candidate set), neighbor set) and the remaining set. The activity set consists of a pilot that has sufficient signal strength and is supporting mobile call requests. The candidate set is composed of a pilot that can support mobile call requests; A neighboring set is composed of a pilot that does not belong to an activity set or candidate set but may participate in soft switching. For example, these cells may be in a known adjacent area ); the remainder is composed of the residential band guides that belong to the CDMA system but are not included in the other three groups. When the strength of a guide in the adjacent set or remaining set measured by the Mobile Station exceeds the threshold T_ADD for the Guide; or the intensity of a pilot in the Candidate Set exceeds 0.5 * T_COMPdB of any pilot intensity in the active set) T_COMP adds a comparison threshold for the pilot ); or, if the pilot frequency in the activity set is lower than the T_DROP threshold of the pilot frequency discard and the duration reaches the T_TDROP threshold of the pilot frequency discard timer, the Mobile Station will send the "pilot Frequency Intensity Measurement message" to the base station ", report the results of the pilot search and the status of switching the drop timer. At the same time, the relative time interval between the Pilot Channel and the time benchmark of the mobile station should also be reported in the "pilot intensity measurement message. The Base Station Subsystem (BSS) sends a "Switch indication message" to the new forward business channel assigned to the mobile station) to respond to the "Frequency guide strength measurement message ". In addition, the "Switch indicator message" is also used to identify the pilot frequency removed from the active set. The mobile station will stop using the pilot frequency removed from the active set and send the "Switch completion message" concurrently ".

Under normal circumstances, the number of mobile stations in the switching status should be controlled at around 30% of the total number of system users. When the system resources are insufficient, that is, when the forward business channel is overdraft, user access to a residential area may be poor, such as delay or failure ). Without changing the existing network devices, you can reduce the percentage of soft switchover to release the business channel to provide normal communication.

3. Soft Switch parameter settings and their impact on the system

(1) The setting is too small:

◆ Generate a large number of pilot measurement messages, notify the base station, and prepare for switchover, occupying a large amount of resources of the Mobile Station and the base station;

◆ Unnecessary switching is added and more TCH (Traffic Channel) is used to enable soft switching for more mobile devices, especially as T_ADD decreases, the proportion of switching between three cells will increase;

◆ The power of the mobile station to reach the remote community is too low, or even ignored, and the meaning of diversity reception is lost, resulting in a waste of TCH.

(2) The setting is too large:

◆ Poor voice quality, even resulting in disconnection, reducing the coverage of the base station;

◆ The switchover delay is caused;

◆ The transmission power of the mobile station near the base station boundary is too high, resulting in a large amount of power consumption and other adverse consequences.

Select T_ADD as appropriate, so that most of the soft switches in the system are two cell switches, and the distance between the mobile station and the two cells is roughly the same. This will increase the average transmit power of the Mobile Station, reduces system interference.

When the value of the T_ADD threshold is greater than-11dB, the switching ratio decreases rapidly. There is a maximum value in the range of T_ADD. As T_ADD decreases, more mobile stations enter the switching status of the two communities, the curve increases. When T_ADD is further reduced, more three-cell switches are generated, and the ratio of the Two-cell switches is reduced.