OMF010002 Handover-training-20031001-A-1.5

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Handover can maintain the communication continuity of MS which moves across different cells and lower the call drop rate and provide better communication service.

All the network element will be involved in the HO process, including BTS, BSC and MSC.MS--Responsible for measurement report of the serving cell and neighbor cells. BTS--Responsible for monitoring of uplink received level and quality of each served MS, and monitoring of the interference level on its idle traffic channel.BSC--Processing the measurement report and making handover judgment.MSC--Participating in the confirmation of target cell in inter-BSC handover.In dedicated mode, the system performs handover and power control judgment based on measurement reports. The downlink MR are reported to the network via uplink SACCH channel.Downlink--In dedicated mode, MS reports MR via uplink SACCH channel periodically. The report includes the receiving level, received quality, TA, power class and whether DTX is used. At the same time, MS will perform pre-synchronization to neighbor cell defined by the system to obtain BCCH frequencies and BSIC, measure their received signal level and report the six max. neighbor cells with the highest received signal level.Uplink--The uplink measurement report is measured by BTS, including the receiving level and receiving quality from the MS.Both parts are sent by BTS to BSC for further processing at the same time.

There are two values in the measurement report: FULL MR and SUB MR.FULL--Averaging 100 TCH bursts (except the four idle frames in the four 26-multiframes)SUB--Averaging 12 bursts (four SACCH bursts and eight TCH bursts in specific position).SUB should be selected when the Discontinuous Transmission (DTX) function is activated.

SACCH is a bidirectional channel.It sends the downlink measurement report to BTS in uplink SACCH.It sends SYS.INFO. 5 and SYS. INFO. 6 to MS in downlink SACCH.The SACCH measurement report period varies with the channel occupied by MS in dedicated mode.When associated with SDCCH, SACCH measurement period is 470ms, this is because there is an entire SACCH message block in 2 SDCCH 51-multiframes.When associated with TCH, SACCH measurement period is 480ms, this is because there is an entire SACCH message block in 4 TCH 26-multiframesA complete measurement report is formed by four consecutive SACCH bursts. In SDCCH channel the four bursts are transmitted continuously. In TCH channel there is only one SACCH burst in each 26-multiframe, therefore only four 26-multiframe can constitute a complete measurement report.

MR Interpolation:The continuity of measurement report is judged by measurement result number. First-order interpolation is employed in the interpolation algorithm for missing measurement report, i.e., to calculate the missing values (linear) between the measurement values at both ends of the missing measurement reports.

Each measurement report has a serial number. If the serial numbers of received measurement reports are not consecutive, there must be missing measurement reports, and the system will make them up according to the interpolation algorithm.As shown in the diagram, the network receives measurement reports n and n+4, whose serial numbers are not consecutive, so the system will make up for the missing measurement reports n+1, n+2 and n+3 with a certain algorithm.

When the network receives the measurement report, it can not judge the current condition of MS according to just one measurement report due to the signal fluctuation. Therefore, filtering, a more suitable method is applied. Different filters are applied in receiving level, receiving quality and TA of uplink and downlink.When MS fails to handover to a target cell, a penalty will be given on the signal level of this target cell during a so called handover failure penalty time period. That is, when ranking the neighbor cells in the cell list, the corresponding neighbor cell with a failure record within the penalty time will be penalized by cutting certain value on the reported signal level.

In case of BQ emergency handover, the original serving cell will be penalized (called signal level penalty by BQ HO ), during BQ HO penalty time, to avoid MS to be handed back to the original serving cell again within certain time. It is the same for TA handover, i.e. the original serving cell is penalized (signal level penalty by TA HO ) during TA HO penalty time.

Purpose of such penalty: When MS crosses continuously several micro cells, it handover to the umbrella cell with lower hierarchical priority level, to avoid too frequent handovers which will affect the communication quality between those cells. At the same time, penalty is started to avoid handover back to original cell again. A possible example is that a fast moving car is running on a highway.

HW-Overlaid/underlaid HO failure means the failure of handover from the Underlaid to the Overlaid, or from the Overlaid to the Underlaid.RX_LEV(o), RX_LEV(n)-- MS received signal level of the serving cell and the neighbor cell, MSRXMIN(o) and MSRXMIN(n)-- Lowest MS received signal level required by the serving cell and neighbor cell.Pa(o)=MS_TXPWR_MAX(o) P;Pa(n)=MS_TXPWR_MAX(n) P;P =MAX_POWER_OF_MS ;MS_TXPWR_MAX(n)-- Maximum transmission power of mobile phone allowed.MAX_POWER_OF_MS -- Maximum physical transmission power of MS.OFFSET--Minimum access level offsetMAX(0,Pa(o))--The current algorithm takes into consideration only the lowest received power threshold of downlink, uplink is not included. For the uplink: If the physical maximum power of MS exceeds the maximum transmission level required by BSS, then Pa is 0.Otherwise, a compensation value will be added.

The handover algorithm of Huawei divides the whole network into four layers, each of which further divided into 16 levels. The lower the layer is, the higher the handover priority level will be. The Micro Cell layer has the highest priority level.Relevant parameters: Handover [Cell Description Table]Layer of the cellLevel priority

The handover algorithm starts a handover decision every 0.5 second (one measurement report time). The judgment process is described below:First judge whether the emergency handover switch is set to ON in the handover control data table. If not, emergency handover judgment will not be performed.If emergency handover judgment has been enabled, make judgment on TA, BQ and interference and rapid signal level drop of MS and BTS in the current serving cell. The criteria for rapid signal level drop judgment is decided by the parameters of respective filters in the emergency handover data table. Then start load handover judgment.Then execute normal handover judgment: Start edge handover when it reaches the edge handover trigger threshold within the statistics time.Perform hierarchical HO judgment if edge handover is not triggered. That is, within the statistics time, trigger handover when signal level from a neighbor cell with different layers (low) or priorities (low) has reached layer handover threshold. If layer handover is not triggered, trigger PBGT handover when it reaches PBGT threshold (which can only be performed in the same layer and priority).Then execute fast moving handover judgment. Once conditions are met, start handover and penalty.Finally execute overlaid/underlaid handover judgment. If conditions are met, start overlaid/underlaid handover.According to the aforementioned judgments, BSC sends the handover command.Related parameters: Handover-[emergency handover table]TA Thrsh. DL Qual. Thrsh. UL Qual. Thrsh.

This indicates the falling tendency of signal level. Formula:value=K6*MR6+K5*MR5+K4*MR4+K3*MR3+K2*MR2+K1*MR1Since K1=-K6K2=-K5K3=-K4, so: value=K6*(MR6-MR1)+K5*(M5-MR2)+K4*(MR4-MR3). This formula emphasizes the changing tendency of received signal level in a period of time.

Related parameters: Handover-[Emergency handover table]Filter parameters A1~A8Filter parameter B

Related parameters: Handover-[Emergency handover table]UL Qual. Thrsh. for interf. HO (dBm)DL Qual. Thrsh. for interf. HO (dBm)UL RX_LEV Thrsh. for interf. HO (dBm)DL RX_LEV Thrsh. for interf. HO (dBm)

A handover band is defined in load handover, in the range of edge handover threshold ~ edge handover threshold + load handover bandwidth. The handover band itself is divided into multiple equalized handover step sizes, MS (falling within edge handover threshold + N*handover step sizes) are handover to adjacent cells one by one from low to high. Once load of the serving cell decreases (as lower than the load handover start threshold) or load of the adjacent cell increases to a certain extent (as higher than the load handover received threshold), the handover stops. Load handover is an emergency measure, which mainly applicable to abnormal traffic peak in part of the radio network. It should not be used as the major means to solve the traffic congestion problem. If load handover always occurs to some area of a network, TRX re-configuration and network topology re-design is the right way.Related parameters: Handover - [Load handover table]System flux Thrsh. for load HO Load HO Thrsh.Load HO Req. on candidate cellLoad handover bandwidth (dB)Load HO step level (dB) Load HO step period (second)Related parameters: Handover - [Normal handover data table]Edge HO UL RX_LEV Thrsh. Edge HO DL RX_LEV Thrsh. Edge HO watch time (second)Edge HO valid time (second)

In areas with densely distributed cells, the actual radio coverage range has become far larger than the distance between BTS. If MS keeps the conversation within a cell, it will not be effectively handed over to a nearby cell with low transmission power and that will lead to over shooting, thus increasing the interference of radio environment and complicating network planning and optimization. To solve this problem, Huawei Company develops PBGT handover algorithm that is based on path loss.PBGT handover algorithm is intended for the handover based on path loss, in real time, it seeks a cell with lower path loss and meeting certain system requirements, and judges whether its necessary to perform handover.PGBT calculation is described as below:PBGT(n) = ( Min ( MS_TXPWR_MAX,P ) - RX_LEV_DL - PWR_C_D ) - ( Min ( MS_TXPWR_MAX (n),P ) - RX_LEV_NCELL(n) )Meanings of respective parameters are as follows:MS_TXPWR_MAX Max MS transmission power allowed by the serving cellMS_TXPWR_MAX (n) Max MS transmission power allowed by the adjacent cell nRX_LEV_DLMS received power from the serving cellRX_LEV_NCELL(n)MS received power from the adjacent cell nPWR_C_DDifference between the downlink transmission power of the serving cell without power control and the actual downlink transmission power of the serving cellP: MS physical Max. transmission powerRelated parameters: Handover - [Fast-moving handover table]MS Fast-moving watch cells - The total number of cells for judging whether a MS is fast moving or not.MS Fast-moving valid cells - Total N of actual cells that MS fast passes. That is, when N equal to or more than Q cells that MS lately passes are fast passing ones, the fast moving micro cell handover algorithm will be started.MS Fast-moving time Thrsh. - Time threshold (2r/v) specified according to the cell radius (r) and the moving speed (v). If the time used by MS to pass this cell is less than this threshold, it means that MS passes the cell quickly.An illustration of how to define the border between the underlaid and the overlaid.

Related parameters: Handover - [Concentric cell handover table]UO Signal intensity difference (dB) - This parameter represents power compensation for the underlaid .RX_LEV Thrsh. (dBm) - This parameter is used to determine, together with the received level hysteresis, TA threshold, and TA hysteresis, the border between the underlaid and overlaid, which must be larger than the margin handover threshold.RX_LEV hysteresis (dBm) - This parameter is used to determine, together with the received level threshold, TA threshold, and TA hysteresis, the border between the underlaid and the overlaid. TA Thrsh. - One of the factors to determine the border between the underlaid and the overlaid. TA hysteresis- One of the factors to determine the border between the underlaid and the overlaid . UO HO watch time (second) - This kind of handover must satisfy P/N rule.UO HO valid time (second).This section introduces the main handover parameter configuration and the lookup process of handover data in the program.

This table describes mainly the handover algorithm, interval of repeated handover, and switches of various handovers.After the Load HO allowed is disable, the 11th bit -- load adjustment bit-- in the 16bits still works. Therefore, the Load HO Thrsh. and Load Req. on candidate cell in [Load handover table] should still be set with much care.

After the RX_Level_Drop HO allowed is enable, BSC can only make a quick judgment on whether it is rapid signal drop with the help of the original measurement report. Therefore, after the Measurement report preprocessing is enable, the Transfer original measurement report should be set to Yes.

Measurement report preprocessing: Indicates whether the measurement report interpolation, filtering and power control are executed at BTS side.Transfer original MR: It works only when Measurement report preprocessing is turned on. In 15:1 configuration, it should be set to No when there are more than two SDCCH/8, otherwise the link rate may be too low. If the Rx_Level_Drop emergency handover algorithm allowed is enable, it should be set to Yes, otherwise the BSC will not be able to judge whether MS has suffered from rapid signal level drop.Transfer BS/MS power class: It works only when Measurement report preprocessing is enable. If the BS/MS power level is not reported to BSC, the BSC uplink/downlink average measurement results will be affected.This table describes various attributes of a cell, such as layer priorities, layer handover threshold, CGI, BCCH frequency, BSIC and M rule parameter.Ensure the consistency of CGI in [Cell module information table], [BSC cell table] and one of the tables including CGI in MSC.Huawei BSC can not acknowledge cells with the CI FFFF, so the CI value can not be set to FFFF, otherwise no handover will be performed.

There is also an [External cell description table], which is the description of external BSC neighbor cells.When modifying data from [Cell description table] or [External cell description table] online, no matter how many cells data has been modified, send all modules when resetting the entire table, otherwise the handover will be abnormal. The neighbor cells of all cells in the BSC are listed in this table, including respective PBGT handover threshold, inter-cell handover hysteresis and minimum level offset.The module No. and cell No. should be correct, otherwise BSC will send incorrect handover command and thus lead to handover failure.The neighbor cell can not be missed out, otherwise BSC can not find it and thus can not send the handover command.There should not be cells with the same BCCH frequency and BSIC in all neighbor cells of one serving cell, BSC will search for the target cell according to the BCCH frequency and BSIC of a neighbor cell. If BSC can not identify the target cell, it will not send handover command. If BSC sends incorrect handover command, rate of successful handover may be low.

As shown in data configuration, there is not only quality threshold requirement, but also level threshold requirement for interference handover. Only when the level value is higher than a certain threshold, and the quality is worse than a certain threshold, it is regarded as interference. This is the difference between interference handover and BQ handover.

The two parameters are both for judging Rapid signal level drop handover.

After MR preprocessing switch is turned on, the reporting frequency of MR will affect the handover speed.

PBGT handover should satisfy P/N judgment.

Hierarchical handover should satisfy P/N judgment.

This table is valid only when MS Fast-moving HO allowed is turned on. It is usually unused, and is suggested to be applied in special places such as highway.In addition to [Concentric Cell Handover Table], the following should also be configured for cells supporting overlaid/underlaid handover:1. Local office--[Carrier configuration table]: Overlaid/underlaid property is the overlaid or underlaid .2. Handover-[Cell description table]: Cell type is normal cell/concentric cell

Overlaid range:Receiving level>=receiving level threshold + receiving level hysteresis and TA< TA threshold - TA hysteresisUnderlaid range:Receiving level= TA threshold + TA hysteresisWhen TA threshold is 63 and TA hysteresis is 0, the underlaid border is determined completely by the receiving level parameter.When the receiving level threshold is 63 and receiving level hysteresis is 0, the underlaid border is determined completely by TA parameter.

Overlaid/underlaid HO should satisfy P/N rule.This is the feature of overlaid/underlaid handover, i.e. to assign overlaid or underlaid first when assigning voice channel.

Precautions in Incoming to BSC HO optimum layer configuration:1. BCCH TRX should be configured on the underlaid 2. SDCCH should be configured on the underlaid 3. If the receiving level threshold and receiving level hysteresis, or TA threshold and TA hysteresis are configured as 63 and 63, the handover between the underlaid and overlaid will be disabled.

Precautions:1. The module No., cell No. and CGI of all neighbor cells should be found before BSC performs basic cell ranking2. In adjacent cell relation table: If there are cells with the same frequency and BSIC in neighbor cells, BSC will fail to identify the right target cell. As a result, the handover command may be sent incorrectly, thus the handover successful rate will be affected.3. Cell ranking and HO judgment are performed in LAPD board. The handover request message is sent via MPU, therefore the connection between LAPD and MPU is Cell module information table, with CGI as index.4. Once there is no CGI find in the Cell module information table, MPU may regard the target cell as an outer BSC cell. It will send HO-Request to MSC, and count it as an Outgoing-BSC handover request.5. In case of incorrect module No. in Cell module information table, the handover request will be sent to wrong modules and the handover will fail. However, BSC will still count it as an inter-cell handover request.

After receiving the SABM frame reported by MS, BTS will report Establish_IND to BSC, and respond UA frame to MS hereafter. Not until the UA frame is received, MS will send HO-Complete message to the network. SABM and UA, corresponding to each other, are link creation frames of Layer 2 of Um interface. In poor radio environment (weak signal or interference), the network can not receive the SABM frame sent by MS, or MS can not receive UA frame. Either condition will lead to handover failure, and the failure cause value is radio link failure.For synchronous handover in BSC, i.e. When BTS1 and BTS2 is the same BTS, there will be no PHY INFO message. MS reports SABM connection frame right after report of Handover Access message. After receiving the HO-Complete message, BSC will send first HO-Performed message to MSC, and will release the radio channel resource of old BTS. The old BTS1 will respond RF-Channel-ACK to BSC.

The above process include the counter for incoming of target cell and outgoing of original cell.The measurement points of key counter are as follows: 1. Before sending "Channel-Activation" to the target BTS, BSC measures "Request times of in-cell handover in BSC" The number of times of request intra-BSC incoming cell handover and "Request times of out-cell handover in BSC" The number of times of request intra-BSC outgoing cell handover.2. After sending "HO-Command" to MS, BSC measures "Request times of in-cell handover in BSC" The number of times of request intra-BSC incoming cell handover and "Request times of out-cell handover in BSC". The number of times of request intra-BSC outgoing cell handover.3. After receiving "HO-Complete" from MS, BSC measures "Success times of in-cell handover in BSC" The number of times of successful intra-BSC incoming cell handover and "Success times of out-cell handover in BSC". The number of times of successful intra-BSC outgoing cell handover.

Difference between "Internal inter cell handover success rate " and "Cell radio handover success rate " :As viewed from formulas, both numerators are success times of handover, while the denominators are different. While viewed from the measurement points of the counter, Attempted inter cell handovers >= inter cell handovers, so intra BSC handover success rate = inter BSC inter cell handover success rate.

Note: This release is originated from the network side. For the release is originated actively by MS, the signaling process is the same except the three transparent transmission direction mentioned above is reversed.

Note: Signaling process of A interface and Abis interface of inter MSC handover is the same as that of intra MSC handover, only the signaling between two MSC is particular for the inter-MSC handover. As shown in above figure the signaling with "MAP" is of the MAP layer, and signaling of A and Abis interfaces are omitted. After receiving "HO-REQUIRED" of BSC-A (the request message includes CGI of the original cell and target cell), if MSC-A finds that LAC of the target cell doesnt belong to this MSC, MSC-A will query the "REMOT LAC Table (including the LAC and route of the adjacent MSC), and send "Prepare-HO" message to MSC-B according to the route. CGI of the target cell and the indicator of whether to allocate the handover number are included in this Prepare-HO message. According to the received "Prepare-HO" message, if the handover number needs allocating, MSC-B will request the local VLR to allocate the handover number. If VLR has the free handover number, the handover number will be sent to MSC-B through "Send-HO-Report". If no handover number is needed, proceed to the next step.After SCCP link between MSC-B and BSC-B is established, MSC-B sends "HO-REQUEST" to BSC-B. After that, BSC-B activates the target cells channel, and returns "HO-REQUEST ACK" to MSC-B after receiving the channel activation acknowledgement. According to this message, MSC-B sends "Prepare-HO ACK" to MSC-A. MSC-A establishes the route to MSC-B according to the handover number, and sends "Initialize-Address" (IAI) to MSC-B to help the latter to identify which voice channel is reserved for MS. While MSC-B returns "Address-Complete" (ACM) to MSC-A.

After receiving the ACM message, MSC-A sends "HO-Command" to MS. After that MS handover access is performed in the new channel. If its successful, UA frame from BTS-B will be received, and "HO-Complete" will be sent to MSC-B. MSC-B sends "Send-End-Signal" to notify MSC-A to release the original channel. After receiving "Clear-Complete" from BSC-A, MSC-A returns "Sent-End-Signal ACK" to MSC-B, thus finishing the inter MSC handover process.

Description:1. When MSC-B receives handover request for a second time, it will first view whether its CGI belongs to local MSC. If yes, it executes internal MSC handover process, and inform MSC-A after the handover.2. If not, it will not send handover request directly to MSC-B, but send Perform-Subs-HO message to MSC-A, and this message contains MSC-B identification.3. MSC-A finds out in Location cell table that CGI belongs to MSC-B, and then sends Perform-HO message to MSC-B.4. MSC-B applies a new handover number from VLR-B and activates the channel resource. It informs MSC-A Wireless-Channel-ACK, and create a route between MSC-A and MSC-B via IAI and ACM.5. MSC-A returns Subs-HO-ACK message to MSC-B, and MSC-B sends HO-COMD to MS. MS will inform MSC-B after successful access.6.MSC-B sends end signal to MSC-A, and MSC-A sends this signal to MSC-B. MSC-B sends clear command to BSC-B to release the original channel, then remove the physical signal between MSC-A and MSC-B.7. MSC-A sends end signal to MSC-B, and the latter will inform VLR-B to release the handover number.

Exception: If the second handover is back to MSC-A, no handover number is needed. MSC-A will find its BSC to send handover request directly.Note: According to the subsequent handover process, during a continuous conversation through several MSCs, the first MSC created in a conversation must make all other MSCs as the neighbor ones, otherwise the handover may fail.

Note: If there is no handover number available for destination MSC-B, it will return HO-Request-Refuse message to the source MSC-A. There will be no statistics in the destination BSC, but one out-BSC handover request time and one Out-BSC handover failure.