Neighbor self-optim izing process design based on X2 in TD-LTE system

ZHANG Gongguo，LIBin

(1 Chongqing University of Posts and Telecommunications，Chongqing 400065，P.R.China;2 Chongqing Information Technology Designing CO.，LTD，Chongqing401121，P.R.China)

1 introduction

TD－LTE is 3G long－term evolution［1］，the system framework includes E－UTRAN(evolved universal terrestrial radio access network)and EPC(evolution packet core network).3GPP(the 3rd generation partnership project)stipulated the series of requirements of LTE system，including downlink peak rate reached 100 Mbit/s，uplink peak rate of up to 50 Mbit/s［2］.

With LTE standardization developing，putting forward SON(self－organized network).SON system improves the performance of the network and reduces operation and artificial cost greatly［3］.Automatic adjacent regions relations play as an important concern by operators for SON.The relationship between adjacent area network planning and optimize the configuration is the focus of the work.To improve the performance of the whole network has an important role.An accurate neighbor relation，can reduce waste of unnecessary switching resources，reduce the adjacent area allocation of improper drop words，and improve throughput，load and measure quality.

2 G and 3G in the neighbor relationship first through the planning software to carry on the forecast，but due to the geographical system data and the building data accuracy limit，it is difficult to reach the adjacent area accurate configuration，cause adjacent regions，such as leakagematch with problems，therefore also needs through the road measure to improve［4］，due to the changing of the geographical environment，the new base station to wait for a reason to need to repeat planning to ensure the accuracy of the relationship between adjacent area

This paper starts from LTE protocol，combining with TD－LTE system characteristics，to analysis the automatic neighbor relation of TD－LTE system，design of the system optimization process，not only to be able to complete the network optimization function，solve the adjacent area with PCI(physical cell identity)conflict and leakage，but also can improve the key performance indicators network.

TD－LTE network，UE(user equipment)measurement does not need the guidance of adjacent regions of the list and adjacent regions list existsmainly in the base station.Measurement and measurement reports in system are based on PCI［5］.TD－LTE system only has504 PCI，means that the PCI is reuse.When switching，switching failure happens because of PCI conflict or confusing.

Adjacent area relationship mainly can be said as Fig.1［6］.

Fig.1 Neighbor relation

You can see OSS(operation support system)and base station connection through the O＆M interface.O＆M monitor adjacent regions list and manage PCI.Figure expounded adjacent regions relations and community logo.Such as district 1 have completed in the neighbor list relationship，because district 2 and 3 do not complete planning accurate list relationship between adjacent area，the 4 as a new village，do not configuration adjacent regions relationship.Below to adjacent area since the optimization algorithm is designed relationship，solving problems in relation to adjacent regions.

2 Process design

TD－LTE system，enhance the UEmeasurement ability，make UE can independent testing measurement configuration and adjacent regions by measuring frequency report parameters such as PCI for eNB，eNB according to judge adjacent regions relationship conditions to increase，delete function.In LTE system，eNB links together through the X2 interface，further enhances the communication ability between eNBs，providingmore conditions for the relationship between adjacent area optimization.This papermain considers with frequency adjacent regions configuration and switching.

The goal of automatic adjacent regions relationship is to be automatic establish and maintain the adjacent area relation table，adjacent area relation table is not broadcast in the system but also not in specialmeasure controlmessage sent，which the problems of the adjacent areawith a leak，adjacent areawithmore，configuration PCIadjacent area［7］conflict，through the UE measurement and X2 interface parameters share with adjacent area automatically adds the leak，the configuration PCI，solve the PCIconflict，and finally establish a perfect relationship in the neighbors.

Adjacent area leakage is the most common problems happened in the first of network product.Because the imprecision of the planning software，it is difficult to achieve a perfect relationship in the neighbors.Adjacent area with leak may lead to UE can't switch to leak village and choose other districtwith switch，lead to switch failure，and even to drop words.PCI conflict is an important question for TD－LTE system adjacent regions relationship.Protocol standard defines with a 504 PCIs，it will happen different village reuse the same ID.If the same adjacent area allocation same PCI，or distribution of existing PCI，will have serious interference，UE even can't distinguish district.

According to the problems of relationship for TDLTE system adjacent area，the paper according to the protocol，rely onmeasuring report and X2 interface parameters on the traditional adjacent regions process improvements，added to the optimization function in the traditional adjacent area process.X2 interface is the internet interface between the base stations.This paper mainly used to process design interface connection establishes news and the X2 to establish connections response news，this article two signaling message contains not only is their own community information，but also including the district of storage area adjacent to the news，through the information of the decision，and successful solves village PCI conflict.The optimization process of the adjacent area is shown as Fig.2.

As shown in the Fig.2，the algorithm process first judge target area PCIwhether in the neighborhoods list according tomeasure report content.If did not exist，can judge for leakage matches the situation，but at the moment，and can't tell whether there are PCI conflict，through the core nets and unknown base station setting up X2 interface connection，and through the X2 interface information for PCI conflict and solve［8］judgment.

Fig.2 Adjacent area self－optimizing process

3 Process simulation

3.1 Simulation scene

In the simulation algorithm，usingwraparound village layout，a total of 19 cells，the initial community PCI value is randomy selected from 1－20 in order to test automatic adjacent regions algorithm.

When the PCI conflict and confuse causing the is－sue that need tomodify village PCIvalue，the PCI list value configurated by O＆M ranges of 0－30.

Simulation Parameters are shown in Tab.1.

Tab.1 Simulation parameters configuration

3.2 Simulation results

The initial scenario setting all eNodeB adjacent regions list is empty，now simulating the handover success rate，the simulation results shown as Fig.3.

Fig.3 Handover success rate

Handover success rate is an important indexwhich is using for evaluate algorithm of automatic relationship adjacent area.

The simulation result shows，when there has no configuration of adjacent regions relations，unable to successfully launch switch，so the handover success rate at the beginning is 0.But with the trigger of the algorithm，UE launches switching and report themeasure report，eNodeB choice of the top quality village PCI value，compared with their own list of PCI value and the provision of PCI value，if it is not existing in the list of adjacent areas，so report to the O＆M that adding the PCI value to the list of adjacent area.

Curve c only has a adjacent area algorithms with leakage processing，at the first15 s before the simulation，the curve rise significantly，with the adding of leaking adjacent areas perfectly，because of unable to solve the PCIcollision problems，handover success rate rise smoothly and even decline occasionally，PCI conflicting and confusing become the key factor to restrict the success of switching.

Curve a shown with solving the simulation results of the algorithm which is combining the leakage processing and PCI conflict processing eNodeB added PCI value and get the target area IP address through the core nets at the same time，and complete X2 interface connection with the target area，and transfer the adjacent area information，and compare with their own neighborhoods and through the X2 interface access list in the neighbor PCI list，if it is over but there is not existing PCIconfusion，if there are PCIconfusion，first of all，according to the distributed PCI schememodify one of a village PCI value，eventually improve adding of the relationswith neighboring areas and PCI conflict and confusion solution.

Compared with simulation results of curve a and c which can be seen that in the top 15 smain to completed the adjacent area leakage，PCI conflicts influence the handover success rate，but leak with factors more evident，with adjacent regions with the perfection of the leak case，add PCI conflict resolution algorithm can solve the blindness of switch，and compared with the curve c，switching performance has been improved a lot after 15 s.

Adjacent regions list update simulation results are shown in Fig.4.

The simulation results can be seen，with the UE continuedmoving，UE will continue to report themeasured area PCI，the current base station will be added to the position village PCIadjacent area list，will eventually add the 84 leak neighboring areas adjacent area simulated into neighbor list.When the user is small，launch switching timeswill also less，can’t report the more PCI to source eNodeB，with the increasing of the time，switching frequencymeasurement is continuously increasing，and reported more PCIvalues to the source eNodeB，according to the algorithm will add to the unknown PCI adjacent area list，will eventually as UE measurement report finished adjacent regions list automatic updates.The simulation results show that this algorithm can be done automatically relationship between adjacent area planning and optimization，reduce the traditional network adjacent area of artificial cost planning，has certain significance.

Fig.4 NRT update cumulative number

4 Conclusions

This paper analyzes the deficiency of the traditional adjacent area optimization，and based on this，improve and design the automatic adjacent regions relationship accord with TD－LTE.On the basis of switching signal process to solve the leak with neighboring areas，with more with PCI conflict and confuse the issue.The simulation results indicate that the algorithm can be perfect adjacent regions list update，solve the adjacent area unreasonable allocation problem，and can improve the success rate of switching.

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ZHANG Gongguo(张功国)(1979－)，male，the engineer，the wireless communication engineeringmaster，mainly engaged in wireless mobile communication network research.E－mail:zhanggongguo@cqcyit.com.

LIBin(李彬)(1977－)，male，the engineer，the communication engineeringmaster，mainly engaged in the researches and application of new communication technology.E－mail:libin@cqcyit.com.

(编辑:魏琴芳)