In the LTE project M, there are two frequency band for LTE, which bandwidth are different. And their downlink central frequencies are 2645MHz (bandwidth 10M) and 875.5MHz (bandwidth 5M). To improve customer experience, the operator required that in the cluster test, UE should mainly camp on the cell with 10MHz bandwidth, the serving cell ratio between 260MHz cell and 850MHz cell should be above 2:3.
With the default parameter, the cluster test was carried out. In the test result, the serving cells of most of the test route are 850MHz cells ,and the ratio of 2600MHz as serving cell is less than 20%, shown in the below figure:
Downlink EARFCN Ratio
Cluster Downlink Throughput
Root cause analysis
1） DT engineer choose some points which are near points (below the antenna), middle point( 1-2KM from site) and far point( above 3KM from site) to do stationary test, the test result is shown below:
UE switch off and on, then download
UE can camp on 2600MHz cell
UE can camp on 2600MHz cell when switch on, but not stable, easy to handover to 850MH z cell
UE always camp on 850MHz
2) DT engineer chose different point as start point to do drive test, the test result are the same, UE are easy to handover to 850MHz cell.
Consider the test result , we need to check the wireless parameters which include handover parameters, power parameters, and so on..
2. Wireless Parameter Checking
1) Power Setting
Because the operator requires that the LTE cell should be configure as full power, so the cells of two frequencies are configured the max power.
2) Re-selection and Re-direction parameters setting are shown below
3) Inter frequency handover parameters setting are shown below:
There are some problems in the parameter setting.
1. In reselection parameters, the cell reselection priority is set, but inter-frequency parameter is the default value, and the priority of the two frequencies is the same, which need to be modified. And set the priority of 2600 Mhz as 7 when the priority of 850MHz is 6.
The threshold of redirection base on measurement and blind redirection are too high, which would result in UE redirection too early from 4G networks to 2G or 3G network.
2. The inter-frequency handover parameters of different frequency are the same, and they are both use A3 event as handover event.
Now all the cells are configured with full power, the bandwidth of 2600MHz is 10M, and the bandwidth of 850MHz is 5M. In FDDLTE system, the transmit power is averagely distributed to all the RBs in the frequency band. With bigger bandwidth, more RBs, the power of each RS signal is smaller. So in the current setting, the RS power of 850MHz cell is bigger than 2600MHz cell.
According to the broadcast characterize of wireless signal, high frequency wave has bigger pathloss than lower frequency wave. So even when the RS power setting of 2600MHz cell and 850MHz cell are the same, the RSRP of 850MHz cell is higher than 2600MHz cells in the same place.
So with current setting, inter-frequency handover is using A3 event as handover event. UE will camp on 2600MHz cell when switched on according to the selection priority setting. But because the power of RS signal of 850MHz carrier is higher, UE will easily handover to 850MHz cells, and will hard to handover to 2600MHz cell again.
Lots of tests prove that even increase A3 offset, hysteresis and time to trigger, UE still cannot easily camp on 2600MHz cell, and still easy to handover to 850MHz cell. These will result in low ratio of 2600MHz as serving cell when do cluster test, and further affect the cluster throughput, lead to worse user experience.
1. In the reselection parameter, set 2600MHz carrier priority as 7, and set 850MHz carrier priority as 6.
2. Modify the inter-frequency handover decision event from A3 event to A4 and A5 event. Higher the ratio of 2600MHz cell as serving cell through use absolute handover threshold.
After parameter modification, inter-frequency handover of 800MHz cell is using A4 event, when the RSRP of neighbor 2600MHz cell is higher than -100dBm, UE will handover to 2600MHz cell.
Inter-frequency handover of 2600MHz cell is using A5 event, when the RSRP of serving cell is lower than -110dBm and RSRP of neighbor 850MHz cell is higher than -100dBm, UE will handover to 850MHz cell.
With this configuration, the low serving ratio of 2600MHz cell problem which is caused by different frequency bandwidth, and big power different of inter-frequency in the same place will be avoid.
The new configuration is shown in the table below:
Inter-frequency handover parameter:
After reselection and handover parameter modification, the serving cell ratio of 2600MHz cell is effectively improved, and the total cluster downlink throughput is improved, too. Test result is shown below:
Serving cell ratio between 2600MHz cell and 850MHz cell:
Cluster downlink throughput:
Summary and Notes
Normally, we use A3 event as handover decision event, it use relative threshold to consider the always change wireless environment to ensure UE can camp on the best serving cell in current position.
But in multi-carriers network, from operator’s perspective, considering user experience, if different carriers have different bandwidth, UE should stay in the wider frequency band to get better network service.
And there will be some scenarios that the RSRP of wider band carrier is low than the carrier in narrow band, but UE can get more RBs, and get better experience. At this time, handover decision event which use absolute event is more suitable for the actual requirement.