GBC_005_E1_0 GSM Handover and Power ControlCourse Objectives: Understand GSM system handover types and causes

Grasp common handover algorithms and parameters

Understand basic concepts of GSM system power control

Grasp common settings of power control parametersContents

11GSM Handover Principles

11.1Overview

11.2Handover Types

41.3Implementation Methods

41.3.1Cell Layer Configuration

41.3.2Measurement Report Preprocessing

51.3.3Destination Cell Selection

61.3.4Destination Cell Sorting

71.3.5Handover Penalty Strategies

81.4Basic Handover Algorithms and Parameters

81.4.1Uplink/Downlink Handover due to Interference

91.4.2Relevant Parameters

111.4.3Uplink/Downlink Handover due to Quality

111.4.4Relevant Parameters

131.4.5Uplink/Downlink Handover due to Level

131.4.6Relevant Parameters

151.4.7Better Cell (PBGT)

172Power Control

172.1Overview

182.2Power Control Process

192.3Rapid Power Control

202.4Power Control Parameters

202.4.1PcUlInclLevThs, PcUlInclLevP, PcUlInclLevN

212.4.2PcDlInclLevThs, PcDlInclLevP, PcDlInclLevN

222.4.3PcUlRedLevThs, PcUlRedLevP, PcUlRedLevN

232.4.4PcDlRedLevThs, PcDlRedLevP, PcDlRedLevN

232.4.5PcUlInclQualThs, PcUlInclQualP, PcUlInclQualN

242.4.6PcDlInclQualThs, PcDlInclQualP, PcDlInclQualN

252.4.7PcUlRedQualThs, PcUlRedQualP, PcUlRedQualN

262.4.8PcDlRedQualThs, PcDlRedQualP, PcDlRedQualN

1 GSM Handover Principles1.1 OverviewHandover is a very important function of the cellular mobile system.In GSM cellular system, the multiplexing technology for radio frequency resource is fully adopted to realize the coverage by several cells. Thus the concept of cross-cell handover is introduced. Handover enables a user to keep continuous conversation during the process of passing through different cells. Handover also adjusts the traffic of cells. Moreover, handover is implemented without being noticed by users, and does not require users involvement.The following are some of the handover causes: Signal strength is too weak Signal quality is too poor Signal interference is too large Mobile user is far away from the base station Uplink level degrades suddenly Macro-micro handover There is a more appropriate cell1.2 Handover TypesZXG10 series products are designed with advanced ideas, realizing various types of effective handover, increasing handover speed, and reducing handover failure ratio. They also combine with many new technologies to increase the network capacity and service quality. 1.Handover types

(1)According to the two cells involved before and after handover, ZXG10BSC (V2) supports four handover types: Intra-cell handoverThe handover is completed by the BSC to which the cell belongs. Intra-BSC inter-cell handoverThe two cells before and after handover are different cells under the same BSC.The handover does not require MSC and is completed by BSC. Intra-MSC inter-BSC handoverThe two cells before and after handover are under different BSCs, and the two BSCs are controlled by one MSC. The handover is completed by MSC and the two BSCs. Inter-MSC handoverThe two cells before and after handover are under different MSCs.The handover is completed by the two MSCs and two BSCs to which the two cells belong.(2)According to how MS establishes connection with the destination cell, ZXG10-BSC (V2) supports three handover types: Synchronous handoverMS uses the same Time Advance (TA) in the destination cell and the source cell. The synchronous handover is fast, and usually occurs inside a cell or between two sectors of the same site. Asynchronous handover

MS does not know the TA used in the destination cell. The asynchronous handover is slow, and is adopted if none of the two cells synchronizes with BSC. Pseudo-synchronous handover

MS can calculate the TA used in the destination cell. The pseudo-asynchronous handover is fast, and is adopted if both the cells synchronize with BSC.2.Special handover functions of ZXG10-BSC (V2)

With the development of various new technologies, some special handover functions are added to ZXG10-BSC (V2): Concentric circle handover Network capacity can be increased by using special network planning methods, of which the concentric circle technology is most commonly used.The concentric circle means that a common cell is divided into two regions: exterior layer and interior layer. The exterior layer covers traditional micro cells, usually adopting the 43 multiplexing mode. The interior layer covers the area near the site and adopts more aggressive multiplexing mode such as 23 or 13. The exterior layer and the interior layer share the site address and the same antenna system. They also use the same BCCH, and the BCCH must belong to the exterior layer.There are several types of concentric circle technologies. ZXG10-BSC (V2) adopts a highly effective C/I-based concentric circle technology, with which specific handover strategies are designed, and the network capacity is increased by more than 30%. Micro-cell handoverAnother method to increase network capacity is the micro-cell technology. It is also an effective way to solve network coverage.The micro-cell and the macro-cell constitute the multi-layer network. In other words, the large continuous coverage is realized by the macro-cell, forming the top layer of the multi-layer network; while the micro cell is used to realize continuous small-area coverage which is overlapped on the micro-cell, forming the bottom layer of the multi-layer network. The micro-cell mainly serves low-speed mobile users. For high-speed mobile users, services are provided by the macro-cell, avoiding call drops that are caused by too frequent handover or handover failure due to insufficient time.ZXG10-BSC (V2) tests MSs moving speed relative to the site through software and then performs the speed-based micro-cell handover. Dual-frequency handover

Network capacity can also be increased by forming the dual-frequency network through adding 1800 MHz (or 1900 MHz) layer. It can solve the problem of insufficient 900 MHz frequency points.Considering that the capacity of 1800 MHz (or 1900 MHz) layer is not fully used, make 1800 MHz (or 1900 MHz) cells absorb traffic as much as possible during handover.ZXG10-BSC (V2) can manage 900 MHz cells and 1800 MHz (1900 MHz) cells simultaneously. In addition to enhancing 1800 MHz (or 1900 MHz) cells traffic absorbability by modifying common cell parameters, it can also set special priorities for handover from 900 MHz cell to 1800 MHz (or 1900 MHz) cell.1.3 Implementation Methods1.3.1 Cell Layer ConfigurationThe concept of relative layer is adopted in cell hierarchy. For each service cell, the adjacent cell can be configured as undefined layer, upper-layer, co-layer, and lower-layer.During the handover process, the cell priority should be considered when sorting candidate cells. Three factors determine the sequence of candidate cells: priority, traffic, and radio condition. Priority and traffic have more influences on the sorting, and radio condition is considered only in cases that the first two factors influences are the same.1.3.2 Measurement Report PreprocessingThe measurement report provides original data for handover decision. ZXG10-BSC (V2) adopts the rolling average method, which can have different weights to realize smooth handover.The rolling average method has the following features: The number of measurement reports must reach the average window size before calculating the average value. If DTX is enabled, the accuracy of the level and quality value in measurement report will decrease. Thus when performing the weighted average calculation, the weight of the measurement report with DTX must be different from that of the measurement report without using DTX. The fixed weight of the measurement report when DTX is enabled is 1. The weight of the measurement report when DTX is disabled can be configured as 1, 2, or 3; if the weight is configured as 1, the measurement report is no different from that when DTX is enabled. The number of measurement reports that are allowed to be lost is ZeroAllowed at most. If the number of lost measurement reports is too large, the queue resets, and those lost measurement reports are taken as of measurement value 0 (i.e. -110 dBm), which are not used in the average calculation. For example, suppose the No. (K-1) measurement report is lost and the average window size is 8, then the average value = 1/7 (RXLEV_NCELL(K) + 0 + RXLEV_NCELL (K-2) + ... + RXLEV_NCELL (K-7)). After power control is performed, implement power compensation for relevant handover decision.1.3.3 Destination Cell SelectionAfter a comparison succeeds, that is, after BSC decides to perform handover, the destination cell is selected according to different handover causes.ZXG10-BSC (V2) can find the most appropriate destination cell according to specific handover causes. For intra-cell handover, ZXG10-BSC (V2) specifies the type of the TRX where the new channel is located according to the handover cause. The TRX types include macro-cell common TRX, macro-cell special TRX, and other TRX in micro-cell.For the cause of HO_NEARTOFAR in the extended cell, the type of the TRX where the new channel is located is the extended carrier. For the cause of HO_FARTONEAR in the extended cell, the type of the TRX where the new channel is located is the common carrier.For inter-cell handover, the destination cell is selected according to the following formulas: Selection rule 1:AvRxLevNCell(n) > RXLEV_MIN(n) + MAX(0,(MS_TXPWR_MAX(n)- P(n))) Selection rule 2:PBGT(n) > HO_MARGIN(n) Selection rule 3:AvRxLevNCell(n) > avRxLevDL + HO_MARGIN_QUAL(n) Selection rule 4:

AvRxLevNCell(n) > avRxLevDL + HO_MARGIN_LEVEL(n)ParameterMeaning

RXLEV_MIN(N)The minimum level required to handover-in the adjacent cell

PBGT(N)Power budget of the adjacent cell

H0_MARGIN(N)Power budget threshold for handover-in the adjacent cell

HO_MARGIN_QUAL(N)Level threshold for handover-in the adjacent cell

HO_MARGIN_LEVEL(N)BER threshold for handover-in the adjacent cell

MS_TXPWR_MAX(n)The maximum MS power allowed in the adjacent cell

P(n)MS power in the adjacent cell

avRxLevDLThe average value of MSs downlink strength

AvRxLevNcell (N)The average value of the adjacent cells downlink strength

Selection rule 1 must be satisfied, that is, the average level of the handover-in adjacent cell must be larger than the minimum handover-in level. Selection rule 2 is used if the handover cause is better cell. Selection rule 3 is used if the handover cause is uplink/downlink quality. Selection rule 4 is used if the handover cause is uplink/downlink strength.Except for the case of rapid fading, the destination cell can be decided if the selection rule and the hierarchical relationship between the destination cell and the service cell are decided. After being processed by the sorting module, the sorted cell list is generated. If destination cells contain cells of different layers, concatenate the several cell lists according to the generating sequence to get the final result. 1.3.4 Destination Cell SortingIf more than one adjacent cell is found, these adjacent cells should be sorted. After the sorting is completed, attempt handover according to the sorted list.The sorting strategy of ZXG10-BSC (V2) is based on priorities and penalties, improving the handover success ratio and controlling the handover flow.The sorting rule of adjacent cell list is as follows:Sort cells according to their dynamic priorities first. If the dynamic priorities of two cells are the same, then sort the two cells according to their power budgets. In destination cells, the extended cells priority is lower.The dynamic priority depends on the cells static priority and the cells resource ratio. The cells static priority has eight levels: 0 ~ 7, and the larger the level, the higher the priority. The cells static priority, which can be set according to the traffic statistics, mainly depends on the cells geographical position. For example, the micro cell in a building and its adjacent cells which are on the same floor are assigned with higher priorities, while its other adjacent cells on different floors are assigned with lower priorities. In this way, it guarantees that handover is performed on the same floor, which decreases interference and improves call quality.The cells resource ratio refers to the percentage of idle TCHs in total TCHs, with a range of 0 ~ 100. During the handover process, MS only concerns the handover-in cells TCHs. The higher the percentage of available TCHs is, the lighter the cells load is, which indicates a higher handover success ratio. The sorting flow has the following features: For the speed-based handover, it is cross-layer handover, thus cells of the same layer must be removed first. For the interference-based handover, distinguish different carrier groups in the cell and handle them respectively.Adjust the handover candidate cells according to the load: (within the same BSC) adjust candidate cells priorities according to their load, which influences the destination cell selection and dynamically balancing traffic.1.3.5 Handover Penalty StrategiesAdopting penalty strategies after handover failure occurs can effectively avoid repeated failures and increase the handover success ratio. Inter-cell handover (including BSC-controlled and MSC-controlled)If handover fails, then during the next handover attempt, manually decrease the destination cells downlink level by an offset of PenaltyLevOffset. After doing that, if the penalty cell still ranks first (for example, the cell is the only destination cell, or the cells level is much higher than that of other cells), then perform handover to the cell again. When performing offset penalty for the destination cell, the counter PenaltyCount is enabled and set as 1. The counter increments when handover fails, the offset level increases by PenaltyLevOffset at the same time. When the number of handover failures reaches 3 (the maximum attempt times) and the cell is still in the penalty period, the cell is filtered. The previous penalty scheme is applied and handover is not attempted towards the cell. In this way, repeated handover attempts can be avoided and the handover success ratio will not be influenced. Intra-cell handoverIf a user performs intra-cell handover repeatedly, it indicates that the user is located where interference is serious and can not find appropriate channel. In this case, the user should be prohibited to perform handover within a certain period of time. The judgment method is as follows:

If handover occurs again during the timer TMaxIHos interval, it indicates that the previous handover does not have effect on interference, the counter IHoCount increments, and TMaxIHo restarts. If handover occurs after TMaxIHos interval expires, it indicates that the previous handover is effective, IHoCounts value is cleared. If IHoCounts value reaches MaxIHo, it indicates that it is unnecessary to continue the handover attempt within a certain period of time, and intra-cell handover penalty strategy due to interference can be adopted, that is, the intra-cell handover attempt due to interference should not be implemented any more.1.4 Basic Handover Algorithms and Parameters1.4.1 Uplink/Downlink Handover due to InterferenceThe handover is caused by: Poor uplink/downlink receiving quality

High level MS entering predefined interference areaIn the interference area, the higher the level is, the easier it is to find a channel with less interference. Therefore, the intra-cell handover standard is not unified for all calls in the cell. In other words, if the level is high, the intra-cell handover can be performed even if RQ is low; if the level is low, the intra-cell handover is performed only if RQ is high. In this way, call quality is guaranteed, call drop rate decreases, and ineffective handovers are avoided.1.4.2 Relevant Parameters1.4.2.1 IntraHoUlLevThs, IntraHoUlLevP, IntraHoUlLevN

DescriptionAccording to GSM specifications, handover decision is performed after a series of average values are obtained. Uplink co-frequency interference is one of the handover causes. The judgment process is as follows:If the uplink quality handover conditions are satisfied, and P of the latest N average values of uplink signal strength are larger than relevant thresholds, then handover is performed. The handover is due to too strong uplink co-frequency interference. IntraHoUlLevThs: defines relevant threshold values IntraHoUlLevN: defines relevant N values IntraHoUlLevP: defines relevant P values

Usually, an intra-cell handover is performed if the handover condition is satisfied. Values1 IntraHoUlLevP IntraHoUlLevN 32IntraHoUlLevThsCorresponding Level Value (dBm)

0< -110

1-110 ~ -109

2-109 ~ -108

61-50 ~ -49

62-49 ~ -48

63> -48

Settings

Usually, the value of IntraHoUlLevThs must be larger than the threshold value (PcUlRedLevThs in table R_POC) that causes uplink power control (decrease), to avoid unnecessary intra-cell handover. The default value can be 30 (i.e. -81 dBm ~ -80 dBm). The default value of P can be 3 and the default value of N can be 4. Reference

GSM05.08 A.3.2.21.4.2.2 IntraHoDlLevThs, IntraHoDlLevP, IntraHoDlLevN

DescriptionAccording to GSM specifications, handover decision is performed after a series of average values are obtained. Downlink co-frequency interference is one of the handover causes. The judgment process is as follows:

If the downlink quality handover conditions are satisfied, and P of the latest N average values of downlink signal strength are larger than relevant thresholds, then handover is performed. The handover is due to too strong downlink (co-frequency) interference.

IntraHoDlLevThs: defines relevant threshold values

IntraHoDlLevN: defines relevant N values IntraHoDlLevP: defines relevant P values

Usually, an intra-cell handover is performed if the handover condition is satisfied. Values1 IntraHoDlLevP IntraHoDlLevN 32

IntraHoDlLevThsCorresponding Level Value (dBm)

0< -110

1-110 ~ -109

2-109 ~ -108

61-50 ~ -49

62-49 ~ -48

63> -48

Settings

Usually, the value of IntraHoDlLevThs must be less than (or equal to) the threshold value (PcDlRedLevThs in table R_POC) that causes downlink power control (decrease), to avoid unnecessary intra-cell handover. The default value can be 30 (i.e. -81 dBm ~ -80 dBm). The default value of P can be 3 and the default value of N can be 4. Reference

GSM05.08 A.3.2.2 NED2.71.4.3 Uplink/Downlink Handover due to QualityThe handover is caused by poor uplink/downlink receiving quality.

If the receiving quality is so poor that exceeds the predefined value, the handover is triggered to improve the call quality.1.4.4 Relevant Parameters1.4.4.1 HoUlQualThs, HoUlQualP, HoUlQualN

Description

According to GSM specifications, handover decision is performed after a series of average values are obtained. Uplink receiving quality is one of the handover causes. The judgment process is as follows:

If P of the latest N average values of uplink signal quality are larger than relevant thresholds, then handover is performed. The handover is due to too poor uplink signal quality.

HoUlQualThs: defines relevant threshold values

HoUlQualN: defines relevant N values HoUlQualP: defines relevant P values

Values1 HoUlQualP HoUlQualN 32

HoUlQualThsCorresponding Quality GradeMeaning

00BER