Cell reselection is a terminal behavior, usually a user migration initiated by the terminal. The terminal determines whether to initiate the cell reselection process according to the reselection decision criterion according to the current measurement result and the threshold value configured on the network side. Cell reselection is not only an important mobility function of the terminal, but also an important method to achieve mutual compatibility between different networks. Cell reselection is divided into intra-system reselection and cross-system (int-RAT) reselection. As the future evolution direction of TD-SCDMA technology, TD-LTE is inevitable to ensure a smooth transition of network evolution, and dual-mode terminals (such as data cards) supporting TD-SCDMA and TD-LTE. The cross-system cell reselection of such dual-mode terminals is divided into two different processes: TD-SCDMA reselection to TD-LTE and TD-LTE reselection to TD-SCDMA. The former follows the TD-SCDMA reselection rule, while the latter is based on the TD-LTE reselection principle. The cell reselection algorithm has an R criterion and a priority reselection criterion. At present, the cell reselection algorithm of the TD-SCDMA commercial terminal follows the R criterion, and the TD-LTE terminal reselection decision algorithm is the cell reselection based on the priority concept. Through simulation analysis, it is found that the R criterion is not applicable to the TD-SCDMA to TD-LTE cross-system reselection process in the environment of two network co-sites. The results show that TD-SCDMA and TD-LTE dual-mode terminals will generate a "ping-pong effect" if they follow the R re-selection criteria for cross-system cell reselection. This not only makes the user unable to communicate normally, but also greatly increases the signaling burden on the network side. In addition, the R reselection criterion greatly reduces the proportion of dual-mode terminal users residing in the TD-LTE network. Therefore, this paper proposes two solutions in a targeted manner, one is to make partial modifications based on the R criterion, and the other is to introduce priority-based re-election criteria. The simulation data shows that both solutions solve the shortcomings of the R re-election criteria. In comparison, the reselection decision algorithm with the introduction of the priority concept is more flexible and simple in implementation, and has a wider application range, and is more suitable for the coexistence of TD-LTE networks and other multi-standard networks. However, the disadvantage is that the TD-SCDMA network side has a large change, which brings certain difficulties to the network upgrade. Moreover, the probability of measurement by the dual-mode terminal is increased, thereby increasing the power consumption of the terminal hardware. 1.1 Characteristics of TD-SCDMA and TD-LTE reselection criteria The current guidelines for TD-SCDMA commercial terminal reselection are often referred to as R criteria. The criterion is based on the comparison of the signal difference between the target cell and the serving cell. After the terminal triggers the measurement, if the signal value of the target cell is better than the serving cell by a certain threshold, the reselection is triggered. Priority-based reselection criteria are used for cross-system reselection of TD-LTE systems. Simply speaking, after the terminal triggers the measurement, for the high priority target cell, as long as the signal value is better than a certain threshold, a reselection will be triggered. For a low-priority target cell, first, the serving cell signal value is lower than a certain threshold, and the signal value of the target cell is higher than a certain threshold, and the reselection is triggered at this time. It can be seen that the biggest difference between this method and TD-SCDMA is that the reselection criterion of TD-LTE is the comparison of absolute signal values, and the reselection decision of TD-SCDMA is the comparison of the difference between the serving cell and the target cell. 1.2 TD-SCDMA and TD-LTE dual-mode terminal signal reception rules in a common site environment In the cell environment of the common site (TD-SCDMA and TD-LTE), for dual-mode terminals with low static or mobile speed, the received signal code power received separately (RSCP: Received) within a certain period of time. Signal Code Power (received signal code power) and reference signal received power (RSRP: Reference Signal Received Power) obey the fixed linear difference relationship. In general, loss is considered from the perspective of signal coverage, and the effects of fast fading are negligible. Path loss and shadow fading have a major impact on the signal reception of the terminal. The signal power value at the receiving end of the terminal antenna obeys formula (1): Received power = base station effective transmit power + antenna gain - path loss - shadow fading - penetration loss (1) In equation (1): the antenna gain is a function of relative spatial position and frequency, the path loss is a function of distance and frequency, the shadow fading is log-normally distributed, and the penetration loss is related to the physical material and frequency of the building. For the dual-mode terminal, due to the characteristics of the terminal itself, for example, the two communication systems share one hardware unit, share a geographic location, and the TD-SCDMA and TD-LTE occupy frequencies are similar, so the antenna gains are similar. For the path loss model, the Cost231- Hata model for 2 GHz is used, and the path loss of the two is equal. The fading caused by the shadow effect is very similar for the same dual-mode terminal, and the difference is small enough to be negligible. The penetration loss is fixed for the terminal to be stationary in a constant communication environment. Therefore, according to formula (1), it is known that the RSCP and RSRP received by the terminal obey the linear relationship. This conclusion was also confirmed by simulation. When both systems are not optimized for edge users, the full-bandwidth (10MHz) transmit power of TD-LTE is 46 dBm, and the TD-SCDMA full bandwidth (1.28 MHz) transmit power is 33 dBm, dual-mode terminal. The probability distribution of the received signal level values ​​is shown in Figure 1. Figure 1 Dual-mode terminal receiving signal CDF diagram In this base station power configuration, for a stationary dual mode terminal, the signal value relationship received by the different networks is subject to equation (2): RSCP- RSRP=9.2 dB (2) 1.3 Analysis of R re-election criteria in a common site environment
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