gsm cell parameters

OMD0540 System Information ISSUE1.1GSM Cell Parameters
Descriptions
Cell Parameters Overview
Cell Parameters include the most of radio network parameters transmitted via Um interface.
Via receiving cell parameters, MS is able to exactly select, access and coordinate with the network through which different kinds of service is provided.
Via reasonably cell parameters, BTS can run in high point and whole system can be effectively utilized to provide more and better services although basing on limited resources.
By reading cell parameters, MS can access the network, perform cell selection and reselection, fully utilize various services provided by the network, and achieve favorable cooperation with the network.
Copyright © 2008 Huawei Technologies Co., Ltd. All rights reserved.
Cell Parameters Overview
Cell Parameters can be sent through two kinds of logical channels
BCCH (in idle mode)
SACCH (in dedicated mode)
Cell parameters can be divided into two parts:
Cell parameters is sent on BCCH, which are used in idle mode.
Cell parameters is sent on SACCH, which are used in dedicated mode.
Descriptions
Introduction to Cell Parameters
Huawei GSM cell parameters:
Other Parameters
Cell Data Mapping in Different Statuses of MS
Power on
Access Network
Network Identity Parameter — CGI
Cell Global Identity (CGI)
Base Station Identity Code (BSIC)
As a global cellular mobile communication system, GSM conducts strict coding for each GSM network in every country, and even every location area, BTS, and cell, so as to ensure that each cell corresponds to a unique number all over the world. The adoption of this coding scheme can achieve the following objectives:
1. Ensure that MS can correctly identify the current network, so that MS can accurately select the network expected by subscribers and operators.
2. Ensure that the network can know the real-time position of MS, so that the network can provide various service requests from the MS.
3. Ensure that the MS can report correct neighbor cells’ information to the network during conversation, so that network can perform handover when necessary to keep continuous conversation for the mobile subscribers.
Network Identity Parameter — CGI
CGI=MCC+MNC+LAC+CI
Once MS receives SYS INFO, it decodes the CGI information, and decides whether it can stay in the cell according to the MCC and MNC indicated by CGI.
At the same time, it judges whether the current location area is changed, so as to decide whether to execute location update. During the location update process, MS will report the new LAI to the network, so that the network can know the LA in which MS is currently located.
As a global cellular mobile communication system, GSM conducts strict coding for each GSM network in every country, and even every location area, BTS, and cell, so as to ensure that each cell corresponds to a unique number all over the world. The adoption of this coding scheme can achieve the following objectives:
1. Ensure that MS can correctly identify the current network, so that MS can accurately select the network expected by subscribers and operators.
2. Ensure that the network can know the real-time position of MS, so that the network can provide various service requests from the MS.
3. Ensure that the MS can report correct neighbor cells’ information to the network during conversation, so that network can perform handover when necessary to keep continuous conversation for the mobile subscribers.
Mobile Country CodeMCC
Definition: MCC consists of 3 decimal numbers. It indicates the home country of the mobile subscriber.
Format: MCC is composed of 3 decimal numbers. The coding range is decimal 000~999.
Location: Cell Attributes
MCC is used in international mobile subscriber identity (IMSI) and location area identity (LAI).
1. LAI. It is periodically transmitted in cell parameters of each cell. MCC indicates the home country of GSM PLMN. MS uses the received information as the important basis for network selection.
2. IMSI of MS. MS’s IMSI also contains MCC. It shows the resident country of the mobile subscriber. When MS logs on the network or applies for a certain service, it must report its IMSI to the network (When TMSI is unavailable.). The network uses the MCC in IMSI to judge whether this subscriber is an international roaming subscriber.
As the unique country identity standard, MCCs are allocated and managed by the International Telecommunication Union (ITU). ITU Default Value E.212 (blue book) stipulated the MCC number for every country. The MCC of China is 460 (decimal). Due to the special meaning of MCC, modification is prohibited once it has been set in the network.
Mobile Network CodeMNC
Definition: MNC is used to uniquely identify a specific GSM PLMN network in a certain country (decided by MCC).
Format: MNC is composed of two decimal numbers. The coding range is decimal 00~999.
MNC is used in international mobile subscriber identity (IMSI) and location area identity (LAI).
LAI. It is periodically transmitted in each cell. Here, MNC indicates the network number of GSM PLMN. MS uses the received information as an important basis for network selection.
IMSI also contains MNC. It shows the home GSM PLMN network of the subscriber. When MS logs on the network or applies for a certain service, it must report IMSI to the network (When TMSI is unavailable.). The network judges whether this subscriber is a roaming subscriber according to the MNC in IMSI, and uses it as one of the important parameters for addressing to subscriber HLR.
If a country has more than one GSM PLMN, different networks must have different MNC. MNC is allocated by relevant telecommunication management department of the country. One operator can have one or more MNC (which regards to the scale provided by the service, usually one operator has one MNC.). Different operators can share the same MNC. Currently, China have two GSM networks, which are operated by China Mobile and China Unicom. Their MNC are 00 and 01 respectively. Due to the special meaning of MNC, modification is prohibited once it has been set in the network.
Location Area CodeLAC
Definition: To locate the location of MS, the whole area covered by each GSM PLMN is divided into different location areas. LAC is used to identify different location areas.
Format: LAI contains LAC, which is composed of two bytes. LAC adopts hexadecimal coding. The available range is from 0001H to FFFEH. The code 0000H and FFFFH cannot be used (please refer to specification GSM0303, 0408, and 1111). One location area can contain one or more cells.
When MS is powered on or LAC of current cell is found to be different from its originally stored Descriptions, MS will inform network of the current location area via location update, and the network uses the LAI for paging. Generally the allocation and coding of LAC is set at the early stage of network construction, and seldom modified during the operation.
The size of location area (LA) is one of key factors in the system. If the LA coverage is too small, the chances for MS to update location increase, and this will increase the signaling load in the system. If the LA coverage is too large, when network conducts paging to the MS, the same paging information will be transmitted in a large number of cells, and this will lead to the heavy load on CCCH. The adjustment of LA size has no unified standard. Operating departments can decide whether to adjust the size according to the currently running network. If the CCCH signaling load is heavy because of too big LA coverage, then reduce the size of LA, and vice versa. It is generally recommended to set the LA as large as possible. The calculation of LA is related with the paging strategies of different manufacturers. If Huawei equipment is employed, it is recommended to set the TRX number within the range of 300 in one location area. In the early stage of network construction, the traffic is not heavy, so the TRX number in one LA can be larger than this value. It is necessary to monitor the PCH load and the increase of traffic in a long term. If necessary, PCH capacity can be increased by adding one extended BCCH channel.
While making LA planning, try to make use of the geographical distribution and behaviors of mobile subscribers to allocate the LA, so as to achieve the objective of reducing the times of location update at the boundary between location areas. Please note that LAC in cell parameters must be in consistent with that in MSC. Otherwise, call setup failure will occur.
Cell IdentityCI
Definition: To uniquely identify each cell in the GSM PLMN, the network operator needs to allocate one code for each cell, which is the cell identity (CI). Cell identity, together with LAI, is used for identity of each cell in the world.
Format: CI is composed of 16 bits, The available range is 0~65535.
Cell Identity (CI) is one part of Cell Global Identity (CGI), transmitted in each cell.
There is generally no restriction for the allocation of CI. Value from 0 to 65535 (decimal) can be obtained. But it should be ensured that one location area cannot have two cells with the same CI.
CI is usually determined in the network design. Except for some special cases, CI value should not be changed during the operation of the system.
Please note that one location area is not permitted to have two or more cells using the same CI. CI on MSC should be the same as that on BSC. Otherwise, MS cannot make calls in this cell.
Network Identity Parameter — BSIC
BSIC=NCC+BCC
In GSM system, each BTS is allocated with a color code, which is called BSIC. MS can identify two cells with the same BCCH by the help of BSIC.
In network planning, make sure that BCCH of neighbor cells are different from the serving cell’s BCCH to reduce the interference.
Practically it is still possible that a same BCCH is re-used in the surrounding cells. For cells using the same BCCH in a relevant near distance, their BSIC must be different so that MS can identify two neighbor cells with same BCCH.
BSIC is transmitted on Synchronous Channel (SCH) of each cell. Its functions are as below:
1. If MS have read SCH, it is considered as being synchronous with that cell. However, to correctly read the information on the downlink common signaling channel, MS must get the TSC (Training Sequent Code) that is adopted by the common signaling channel. According to GSM specification, TS (Training Sequent) has eight fixed formats, which are represented by TSC ranged 0~7 respectively. TSC number adopted by common signaling channel of each cell is just the BCC of the cell. So one of the functions of BSIC is to inform MS of the TSC adopted by the common signaling channel of the cell.
2. Since BSIC attends the coding process of information bits in random access burst, it can be used to prevent the BTS from accepting a RACH transmitted from MS in a neighbor cell as the access signal from the MS of the serving cell.
3. When MS is in dedicated mode, it must measure the BCCH level of the neighbor cells and report it to BTS according to BA2 that is sent on SACCH, including their respective BSIC. In special circumstance, when there are two or more cells using the same BCCH in the neighbor cells, BSC can use BSIC to distinguish these cells and avoid wrong handover or even handover failure.
4. MS must measure the BCCH signals of neighbor cells in dedicated mode, and report the results to the network. Since MS sends measurement report which contain the Descriptions of a maximum of 6 neighbor cells each time, it is necessary to control MS to report only the cells which have neighbor relationships with the serving cell. The NCC is used for the above purpose. Network operators can use parameter “ NCC Permitted” to control MS to report the neighbor cells with NCC permitted in the serving cell only.
Network Color CodeNCC
Definition: NCC is a part of BSIC. MS uses it to distinguish adjacent BTS that belong to different GSM PLMN.
Format: NCC is composed of 3 bits, with the range of 0 to 7.
NCC and BCC together form the base station identity code (BSIC), transmitted on synchronous channel of each cell.
In many cases, different GSM PLMNs have the identical coverage in many But their network planning are independent from each other. To ensure that adjacent BTSs have different BSICs, it is generally regulated that adjacent GSM PLMN select different NCC.
Adjacent or close cells with the same BCCH frequency must have different BSIC. Special attention should be paid to the configuration of cells in boundary areas.
BTS Color CodeBCC
Definition: BCC is a part of BSIC. For its function, please refer to above sections.
Format: BCC is composed of 3 bits. The available range is 0~7.
BSIC includes BCC and NCC which is transmitted on SCH. BCC is a part of BSIC, used to identify different cell with the same BCCH in the same GSM system. According to the requirements of GSM specification, TSC of BCCH in each cell should be the same with BCC of the cell. Generally this consistency must be ensured by manufacturers. Adjacent or close cells using the same BCCH must have different BSIC, otherwise, inter cell handover might be unsuccessful.
BCC planning has three solutions. All of them have taken distance principle into consideration to avoid collisions of adjacent cells with the same BCCH and same BSIC.
1. Based on the existing BCC set, select one of the BCC that has been used by other cell, ensure at the same time that BCC selected will not cause BSIC/BCCH collision with adjacent cells. The advantage of this solution is that it can ensure BCC be evenly distributed in the whole network. However, if done manually, this solution is time-consuming and troublesome, we can use automatic distribution tools.
2. When defining BCC, try to assign the value from 0. When causes BSIC/BCCH collision, expand the value range. The advantage is that the number of BCC used is kept to the smallest. So when adding a new BTS, in order to avoid the BSIC/BCCH collision, a new BCC can be selected without modifying the BCC of original cells around.
3. Allocate BCC according to its reuse model. That is to use the same BCC within one cluster. It means that adjacent cells cannot use the same BCCH with the service cell. This solution is frequently used, and also the simplest one.
Idle Mode Parameters
Period of Periodic Location Update(T3212)
Neighbor Cell Description (BA Table)
There are a lot of parameters in GSM system, they are usually transmitted to MS from BTS via Um interface. It aims to maintain favorable cooperation between MS and BTS. On the other hand, the values of these parameters directly affect the traffic load and signaling flow of each part of the system. Therefore, proper configuration of these parameters is important to the favorable and stable operation of the system. The following will elaborate on the definitions, value ranges, and effects on the system of these system control parameters.
ATT
Default ValueYes
Description: It is Attach-detach allowed. It is used to inform MS whether IMSI attach-detach is allowed in this cell. If it is set to “Yes”, the network will not process the connection to the called mobile subscriber when MS is power-off. Thus network processing time and radio resources are saved. Otherwise the network will process the connection even though the MS has been powered off.
Location: Cell Attributes/Idle Mode/Basic Idle Parameters
Detach process (IMSI) refers to the process that MS informs the network that it is shifting from working state to non-working state (usually a power-off process), or the SIM card is being taken out from MS. Upon receiving the notice from MS, the network knows that the IMSI subscriber is in non-working state. Therefore, if the MS is called, the call connection will be implemented.
IMSI attach process is opposite to detach process. It is the process that MS informs the network it has entered the service area (usually a power-on process) or SIM card has been inserted into MS. After entering service state again, MS will test whether the current location area (LAI) is the same with the latest LAI recorded in MS. If yes, MS will start IMSI attach process. Otherwise MS will start location update process, upon receiving the location update or IMSI attach process, the network will indicate that this IMSI subscriber is in working state.
Note that ATT configuration of different cells in the same LAI must be the same. It is because IMSI detach process will be started when MS is power-off in the cell with ATT set as yes. The network will record that this subscriber is in non-working state and reject all the called connection requests to this subscriber. When MS is power-on again, if it is in the same LAI as it was power-off (thus the LAI update process will not be started) but in another cell, and ATT of the cell is set as no, then the MS will not start IMSI attach process. In this case, this subscriber can not be called normally until the MS starts the location update process.
CCCH Conf
Value Range: 1 Compounding CCCH , 1 Non-Compounding CCCH, 2 Non-Compounding CCCHs, 3 Non-compounding CCCHs, 4 Non-compounding CCCHs.
Default Value: When there is one TRX in the cell, one combined CCCH is recommended (in a system with few paging messages in location area). For others, it is configured according to the number of TRX in the cell.
Description: It is Common Control Channel Configuration. CCCH configuration determines the capacity of PCH, AGCH and RACH. This parameter can be automatically configured by system according to the TRX channel configuration.
In GSM system, the downlink common control channel mainly includes Access Granted Channel (AGCH) and Paging Channel (PCH). It serves to send the access granted (immediate assignment) and paging messages. CCCH is shared. According to the configuration of traffic channel and traffic model, CCCH can be carried by either one or multiple physical channels. Moreover, CCCH and SDCCH can share one physical channel. The MS needs to know how the CCCH(s) is/are configured, so that it can find and select one to listen to. The CCCH Conf is just used to tell the MS about this matter.
When CCCH is a physical channel which combined with SDCCH, the capacity of CCCH is the lowest. When CCCH is a physical channel which is not combined with SDCCH, the capacity is higher. For other cases, the more the physical channels are used as CCCH, the higher the CCCH capacity is.
Configuration of CCCH Conf is specified according to the traffic model. This model is closely related to the cell location and environment. According to experiences, when TRX quantity in the cell is 1 or 2, it is recommended to use a combined CCCH as the common control channel. When TRX quantity in the cell is 3 or 4, it is recommended to use a non-combined CCCH as the common control channel.
Currently CCCH can be configured according to actual traffic load. If the paging load is very heavy, the paging traffic of cell should be distributed via multiple CCCH physical channels other way. Special attention should be paid to PCH in CCCH. Generally PCH capacities of various cells under one LAC must be the same.
BS_AG_BLKS_RES
Value Range: 0~7 (Non-Compounding CCCHs), 0~2 (1 combined CCCH)
Unit: Block
Default Value: 2 (Non-combined CCCH), 1(1 combined CCCH)
Description: It is also called Access Granted Blocks Reserved. It is the number of CCCH channel message blocks that are reserved in one multi-frame for access granted channels (AGCH).
As downlink CCCH includes both AGCH and PCH, it is necessary to set the number of blocks, which are reserved for AGCH among CCCH message blocks. To let MS know such configuration information, the cell parameter of each cell includes a configuration parameter, which is the number of access granted blocks reserved (BS_AG_BLKS_RES). This parameter actually assigns the proportion of AGCH and PCH on CCCH. It affects the time of MS’s response to the paging.
The network operator can adjust this parameter to balance the traffic of AGCH and PCH by referring to the following principles:
1. Principle for BS_AG_BLKS_RES: make this parameter as small as possible without causing overload of AGCH, so as to increase the capability of paging and improve the system performance.
2. Generally it is recommended to select 1 (when CCCH Conf is 1 combined CCCH), 2 or 3 (when CCCH Conf is one of other values) for BS_AG_BLKS_RES.
3. During operation, observe the statistics of AGCH overload and adjust BS_AG_BLKS_RES properly.
Note: In Huawei system, when AGCH has been all occupied, if PCH is free, it can be used to send the immediate assignment command. If AGCH blocks reserved is set as 0, the immediate assignment would be sent only when there is free PCH channel. Therefore, a fixed capacity reserved for AGCH is necessary.
BS_PA_MFRAMES
Unit: Multi-frame period (51 frames)
Default Value: 2 Multi-frame period
Description: It is Paging Channel Multi-frames. It defines the number of multi-frames used as a cycle of paging sub-channels.
This parameter specifies the number of paging sub-channels that are assigned in a cell. In the network, MS only monitors the paging sub-channel it belongs and ignores the Description of the others. When this parameter is set larger, there will be more paging sub-channels in the cell and accordingly there will be less MS in each paging sub-channel. Therefore, the bearing capability of PCH will be more (theoretically the capacity of each PCH does not increase, but the buffer that buffers paging message in each BTS is increased, which makes the sending of paging messages more even in the time domain), and the lifetime of MS battery will be longer. The value of this parameter should be as small as possible under the condition that the overload on PCH does not occur. In the operation, the PCH load should be measured regularly and the value of this parameter should be adjusted properly according to the PCH load. In a location area, paging is sent in all the cells. Therefore, all cells in the same location area should have the same or nearly the same PCH capacity (number of paging sub-channels). In the area where the PCH bears a medium or large load, it is suggested to be set as 6 or 7 (6 or 7 multi-frames are used as a cycle of paging). For the area with a small load, it is set as 4 or 5. Besides, it is often set as 2.
Note:
1. One CCCH block (four consecutive CCCH timeslots) can bear the information of two IMSI pagings or four TMSI pagings or two AGCH immediate assignments.
2. In idle mode MS camps in a cell. The DSC is initialized to the integer part of 90/N (N is BS_PA_MFARMES, with the value range: 2~9). when MS can successfully decode the message on paging sub-channel, DSC will increase by 1, but it will not exceed initially value. If decoding fails, DSC will decrease by 4. If DSC<=0, the downlink signaling link fails, resulting in cell reselection.
Paging Times
Default Value: 4
Description: This parameter is used for BTS to determine whether paging is resent. Together with the paging times configured in MSC, they jointly control the paging resend times. The total paging times are approximate to be the multiplication of the two.
Location: Cell Attributes/Call Control/Basic Call Control Parameters
The MSC paging resend strategy is as follows:
1) Huawei MSC6.0 can resend paging message at most 4 times, and the resend intervals are 3 seconds, 3 seconds, 2 seconds, and 2 seconds respectively.
2) Within 2 seconds after the last paging message is sent, i.e., 12 seconds after the first paging message was sent, if there is no paging response from MS, MSC will regard it as timeout.
Period of Periodic Location Update(T3212)
Value Range: 0~255
Unit: 6 minutes
Default Value: 20
Description: It is the Periodic Location Update Timer. It defines the interval of periodic location update.
MS will make location update when detecting the change of location. Besides, MS will make periodic location update controlled by parameter T3212. Once MS read T3212 from cell parameter, it will store it in SIM card. When the time reaches T3212 value, the location update process will be triggered. The shorter the period is, the better the performance is. But it will bring more signaling load for system. On setting of this parameter, the processing capability of MSC and BSC, the flux of A interface, Abis interface and Um interface, the flux of HLR and VLR should be considered. Generally this parameter is set as a larger value for urban area and smaller for suburb, countryside or the place with poor coverage.
Large T3212( 16 hours 20 hours) is recommended for the area with heavy traffic, and small T3212 (3 hours, 2 hours) for the area with normal traffic. For the area where the traffic exceeds the system capacity, it is recommended to set T3212 as 0 (no periodic location update). To set the value of T3212 properly, it’s necessary to conduct long-term measurement on the processing capability and flux of each entity in the system. If any overload occurs, increase the value T3212.
Note that this value should be smaller than the period by which the network queries the IMSI attached subscriber. Otherwise, the following situation occurs: When MS has not done any operation in a certain time, and it is not yet the time for periodic location update, the network will set IMSI flag of MS as detached, because its query result shows that MS has not done any operation. Thus, the network will not process the paging of this MS. So, before MS initiates another round of periodic location update, once there is a call to the MS, the network will voice the calling party that the called MS is has been powered off. As usual, the T3212 is set smaller than one third of the MSC’s check time.
When MS reselects a cell in a different location area, it will make a non-periodic location update and reset T3212 in the new cell. If it reselects in the same location area, then the timer value will be remainder of the original one divided by the new T3212.
Definition of Location Update
Inform the system the LA (Location Area) where the MS is to be paged.
LA 1
LA 2
Condition of Location Update
LA 2
LA 3
LA 1
Procedure of Location Update
Abnormal Occasion
The subscriber you dialed has been powered off
LA 3
LA 2
LA 1
Reason1 of Abnormal Occasion
Reason2 of Abnormal Occasion
Application of T3212
Even if T3212 setting is less than the system (MSC) query time, the system will still sometimes voice “The subscriber you dialed is powered off”.
A
B
“Ping-pong Reselect”
Let us make the following assumption. The system query time is set to 1 hour, T3212 value of cell A is set to 4 (0.4 hour), and T3212 value of the adjacent cell B is set to 8 (0.8 hour), and they are in the same location area. MS reselects B when the periodic location update status in A is 3/4, and the MS periodic location update status in B changes to 3/8. If MS stays in B for some time and the location update status reaches 7/8, then MS reselects A. At this time, it can be seen that in cell A, MS reselection status changes to “7/4”, i.e. 3/4. If MS reselects to B at this time, the status will change to 3/8 instead of 7/8. The above analysis shows that if the above case occurs (the probability is high), though T3212 values of both cells are smaller than the system query time, the MS’s frequent cell reselection leads to the final equivalent time is greater than system query time. Thus the subscriber will be considered as a power-off subscriber within certain time even it is in normal idle mode.
Neighbor Cell Description
There are table BA1 and table BA2.
Table BA1 describes BCCH frequencies of the adjacent cells to be measured when the MS is in idle mode.
Location: Cell Attributes/Idle Mode/Advanced
Table BA2 describes BCCH frequencies of the adjacent cells to be measured when the MS is in dedicated mode.
Location: Cell Attributes/Handover data/Advanced
MS keeps on measuring the BCCH signal level of the serving cell and the neighbor cells. In order to know the adjacent cells, neighbor cell description information will be broadcast periodically in cell parameter of each cell. This information lists the BCCH of all neighbor cells. MS extracts the information from cell parameter and use it as basis for neighbor cell measurement.
For GSM network, the neighbor relationship between cells is accomplished when designing the network topology. During the network construction, the neighbor cell relationship must be configured in accordance with the topology design that has been planned. Moreover, after the commission of network, neighbor relationship should be modified according to the data of drive test and traffic measurement. When the network’s architecture is changed (e.g. adding BTSs or changing the network frequency configuration.), the network operator must strictly follow the changed-cell-neighbor-relationship, re-set and verify it. Improper neighbor cell description is usually one of the main reasons of call drop. Besides, since the actual network topology structure is often greatly different from the theoretical calculation result, and network is in ever-changing environment, the network operator must configure the neighbor cell description according to the actual situation.
Application of Neighbor Cell Description
Theoretical Neighbor Cells
Actual Neighbor Cells
B
C
A
D
B
C
A
D
In theoretical calculated neighbor cell relationship, cell A and cell C are not adjacent cells. Assume that one MS moves from cell A to cell C during the conversation, theoretically, MS needs twice of inter-cell handovers. Assume that the interference in cell D is rather serious, call drop is may occur during this period. But in fact, the coverage of A, B, C, and D is not the case as the theory. A and C have overlapping coverage. If A and C are regarded as adjacent cells here, that is to say, add the BCCH of C and A respectively to the neighbor cell description of A and C, then when MS passes from A to C, only one handover happens. What’s more, call drop could be avoided because of the good quality of cell C.
Application of Neighbor Cell Description
B
C
A
A
Part of the signals from cell A leaks out and covers some areas far away from this cell. It is overshooting. If MS is in dedicated mode in the shady area and moves from this area towards B and C, since there are no BCCH of cell B and C in cell A’s neighbor cell description, call drop is unavoidable. If the antenna of BTS is located too high, or the transmitting power is too large, overshooting will occur. BTSs built at the early stage of GSM construction usually have this problem, because coverage is the major purpose at that time and the antenna height is very high. The best solution for this phenomenon is to adjust the location and downtilt of the antenna, or to adjust the transmitting power of the BTS to eliminate the BTS’s over-covered area. In real situation, it is hard or even impossible to change the location of antenna. So one more simple and applicable method is to add BCCH of B and C to the neighbor cell description of cell A (no need to add A’s BCCH to B and C). But it must be ensured that there are no cells which are all neighbor cells of cell A and using the same frequency and same BSIC with cell B and C. Generally, this method is not recommended.
Call Control Parameters
MS MAX Retrans
Emergent Call Disable
There are a lot of parameters in GSM system, they are usually transmitted to MS from BTS via Um interface. It aims to maintain favorable cooperation between MS and BTS. On the other hand, the values of these parameters directly affect the traffic load and signaling flow of each part of the system. Therefore, proper configuration of these parameters is important to the favorable and stable operation of the system. The following will elaborate on the definitions, value ranges, and effects on the system of these system control parameters.
MS MAX Retrans
Unit: Time(s)
Default Value: 4 Times
Description: It is one of the random access control parameters. MS MAX Retrans is the upper limit of times that MS is allowed to send “Channel Request” in one immediate assignment procedure.
After initiating immediate assignment process, MS keeps monitoring messages on BCCH and CCCH group it belongs to. If the network does not send Immediate Assignment or Immediate Assignment Extend message, MS will resend the channel request message at a certain time interval. The larger this parameter is, the higher the call setup success rate is, but also the heavier the load of RACH and SDCCH is.
When MS initiates immediate assignment, it will send the “channel request” message to the network via RACH. As RACH is an ALOHA channel, the network is incapable of controlling the access time of MS. Thus in heavy traffic spot, it is unavoidable that several MSs may simultaneously make access request and cause collision which will lead to two results: one is when one request signal level is obviously higher than the others’ access signals, the access request with higher level will be handled; the other is the network can recognize none of them due to mutual-interference. As the traffic is increasing, access request loss due to collision will increase, too. To make sure that the system can correctly receive the access request and increase the access success rate, the network allows MS to send several channel requests before receiving an immediate assignment message so as to achieve a higher access success probability. MS will return to idle mode if it fails to receive an immediate assignment command after the MAX Retrans exceeds. Once MS sends a channel request, it will start timer T3120 and wait on the downlink CCCH. When T3120 times out and RACH resend times are not more than “MAX Retrans.”, MS will resend channel request message (containing one new random reference), and restart T3120 with a random value. When T3120 is times out and “MAX Retrans” is reached, MS will start T3126. If MS still fails to receive a response from the network after T3126 times out, it will give up the access. If MS receives the access rejection, it will stop T3120 and start T3122. Within T3122, no new access attempt will be allowed.
Recommendation:
Set to 7 for areas with low traffic (suburban or rural area) and the cell radius more than 3 km.
Set to 4 for areas with ordinary traffic (non-busy area in the city) and the cell radius is less than 3km.
Set to 2 for micro-cell
Set to 1 for micro-cell with heavy traffic or with obvious congestion.
Set to 4 or 7 for satellite transmission BTS.
TX-integer
Value Range: 3~12, 14, 16, 20, 25, 32, 50
Unit: RACH Timeslot (equals to a TDMA frame, 4.615ms)
Default Value: 32
Description: Used to calculate the number of timeslots in the interval between multiple channel requests sent by MS.
It is set to reduce the collisions on RACH. It mainly affects the execution efficiency of the immediate assignment process. The value of this parameter is related to CCCH configuration mode, both of them together determine the parameter S (see the next page). The MS sends the first random access burst at a random TS in the set {0, 1, …, MAX(T, 8)--1}. And the TS number between any two adjacent channel request messages is a random value in the set {S, S+1, …, S+T-1}.
Generally, parameter T+S should be as small as possible (in order to shorten the access time of MS), but AGCH and SDCCH must not be overloaded. If AGCH or SDCCH of the cell is overloaded, then parameter T can be changed to make parameter S larger, until AGCH or SDCCH of the cell is not overloaded.
When the RACH collisions is serious, value T should be large. When the number of RACH collisions is small, value T should be as small as possible.
Calculation of S
3,
8,
14,
50
55
41
4,
9,
16
76
52
5,
10,
20
109
58
6,
11,
25
163
86
7,
12,
32
217
115
When T becomes larger, the interval range between channel request messages sent by different MSs will increase and RACH collisions will be reduced. When value S becomes larger, the interval between channel request messages sent by the same MS will increase, collisions on RACH will be reduced and the availability of AGCH and SDCCH will increase. But the increase of either will prolong the access duration of MS, resulting in deterioration of access performance of the entire network. Generally, value T should be selected to make S as small as possible (in order to shorten MS access time), but AGCH and SDCCH must not be overloaded.
RACH Min. Access Level
Value Range: -121~-47
Default Value: -115
Description: The parameter affects the access of an MS. That is to say, the BTS determines the threshold of the level for the random access of the MS. When the receive level of the RACH burst is lower than the threshold, the BTS regards this access as an invalid access and no decoding is performed. Only when the receive level for the burst timeslot of the random access is greater than the threshold, the BTS regards that this timeslot has an access request.
Location: Cell Attributes/Call Control/Advanced/Access Control
To avoid MS being unable to set up call even it is in the coverage area, consideration should be given to BTS sensitivity and MS RXLEV_ACCESS_MIN during the setting of this parameter.
A too small value for this parameter makes MSs easily to access but the call drop rate may rise.
A too big value for this parameter may cause some MSs unable to make calls.
Radio Link Timeout
Unit: SACCH period (480ms)
Default Value: 52
Description: This parameter is used for MS to decide downlink disconnection in case of SACCH decoding failures.
Once assigned with a dedicated channel, MS will start counter S. From then on, S will decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when decoded correctly. When S decreases to 0, there will be a radio link failure. This allows either re-establishment or release of the connection. If the value of this parameter is too small, the radio link will easily get failed which will result in call drops. If it is too large, MS will not release for a long time which will lower the availability of resources (this parameter functions for the downlink).
For area with little traffic (remote area), it is recommended to be between 52~64.
For area with light traffic and large coverage(suburb or countryside), it is recommended to be between 36~48.
For area with heavy traffic (urban), it is recommended to be between 20~32.
For the area with very heavy traffic (area covered by microcell), it is recommended to be between 4~16.
For the cell with obvious coverage hole or the area where the call drops is serious during movement, this parameter can be increased appropriately in order to increase the possibility to resume the conversation.
Note: Radio link timeout is the parameter used to judge the downlink failure. Likewise, the uplink will be monitored at BTS, either based on the uplink SACCH error or based on the receiving level and quality of the uplink.
Application of Radio Link Timeout
Impact of radio link timeout
A
B
P
Q
Poor coverage
If cell A and B are adjacent to each other, assume that one MS moves from point P to point Q during a conversation, usually an outgoing cell handover will occur. If the value of parameter “radio link timeout” is too small and the quality of signal at the edge of cells A and B is poor, the radio link will time out before the handover occurs, thus resulting in call drops.
Radio Link Timeout
To increase the robustness of the SACCH channel, to improve the AMR network coverage performance, and to lower the call drop rate, in practice the AMR FR/HR RLT should be set higher.
Normal Call
AMR FR
AMR HR
Default Value
Parameter
64
52
Because the robustness of the AMR voice frames is not at the same level with that of SACCH frames, the GSM specification only improves the robustness of the AMR voice frames but not that of the SACCH frames. Therefore, the actual coverage capability of AMR is determined by the coverage capability of the SACCH.
In application, the data——[RLT] and [SACCH Multi-Frames] should be set to higher values for AMR channels to increase the robustness of the SACCH. Thus the network coverage performance of AMR is improved and the call drop rate is reduced.
"Robustness" indicates the anti-interference capability. Protocols in 3GPP R6 version provide several new technologies to enhance the robustness of the SACCH. The problem about low robustness of the SACCH is expected to be solved with the evolution of technologies.
SACCH Multi-Frames
Default Value: 31
Description: It is a timer used to determine whether the uplink radio link connection fails. BSS judges the uplink radio link failure according to uplink SACCH BER.
Every time BTS fails to decode the MR sent from MS, this timer decreases by 1;
Every time BTS succeeds to decode the MR, this timer increases by 2.
When this timer reaches 0, BTS judges that the uplink radio connection fails, then BTS sends a radio connection failure message to BSC.
This parameter and the radio link timeout (RLT) are used to judge the uplink/downlink radio connection failure.
SACCH Multi-Frames
To increase the robustness of the SACCH channel, to improve the AMR network coverage performance, and to lower the call drop rate, in practice the AMR FR/HR SACCH Multi-Frames should be set higher.
Normal Call
AMR HR
Default Value
NCC Permitted
Default Value: 11111111
Description: Network Color Code. It lists NCC that need be measured by MS. If a neighbor cell’s NCC is permitted, MS will report MRs of it to the network.
In working status, MS needs to measure adjacent cells BCCH signals and report them to network. But each report can only include a maximum of six adjacent cells. Thus it is necessary to make MS only report the potential target cells for handover, instead of reporting all according to the signal level (usually MS does not report the signals of cells from other GSM PLMN). The above function can be implemented by making MS only measure the cells whose NCC are selected. Parameter “NCC Permitted” lists the NCCs of the cells that the MS needs to measure.
BSIC is transmitted continuously on SCH of each cell and the higher three bits of BSIC are NCC. MS only needs to compare the measured NCC of the adjacent cell with parameter NCC Allowed. If it is allowed, MS will report it to BTS, otherwise it will discard the measurement result.
Note: Improper setting of this parameter will lead to lots of call drops.
Common Access Control Class
Description: One of the parameters of random access control information. It is used for load control of ordinary subscribers, to permit or forbid the network access of some common level users. “1” stands for forbidden and “0” for permitted.
In some special case, the operator expects to prohibit all or part of the MSs from sending access requests or paging response. For example, emergency status occurs or a serious fault occurs to a GSM PLMN. Therefore, GSM specification 0211 requires to assign an access level for each common GSM subscriber. The common access level is divided into level 0~9, which is stored in the SIM card of MS, and has nothing to do with access priority.
Some cells with extremely heavy traffic may be congested during busy hour, resulting in a large number of RACH collisions, AGCH overload, Abis interface overload, etc. GSM specifications provide a variety of ways for dealing with the overload and congestion, but most of them will lower the availability of equipment resources. The network operator can set the access control parameter (C0-C9) properly to control the traffic inside the cell. For example, when the cell suffers traffic overload or congestion, Ci can be set as 0 to prohibit MS with this access level from accessing this cell (change of Ci will not affect MS in dedicated mode), thus reducing the traffic of the cell. To solve this problem, values of C0-C9 in the cell can be changed periodically. For example, at intervals of five minutes, alternatively allow the access of MS with odd access levels and those with even access levels.
For example, 1000000000 indicates to allow the access of subscribers with the levels other than 0. During installation and commissioning of BTS or during maintenance test for some cells, they all can be set to “1” to prohibit the access of subscribers.
Special Access Control Class
Description: It is used for load control, permitting or forbidding the network access of some special level users. “1” stands for forbidden and “0” for permitted.
For some special subscribers, GSM specifications have reserved five special access levels 11-15, which usually have higher access priority. A special subscriber can have one or multiple access levels (between 11 and 15) at the same time, which are also stored in the SIM card of the subscriber.
Class15——PLMN Staff;
Class14——Emergency Services;
Class12——Security Services;
Class11——For PLMN Use.
For subscribers with the access levels 0~9, their access right is also applicable to the home PLMN and visit PLMN. For subscribers with the access levels 11-15, their access right is only applicable to the home PLMN. For subscribers with the access levels 12, 13 and 14, their access right is applicable to the area of the country to which the PLMN belongs.
Subscribers with the access level 11~15 have a higher access priority than those with the access levels 0~9.
The access level control parameter consists of 16 bits: C0-C15, which respectively corresponds to 15 access levels in bit mapping mode (C10 is used for permitting emergency call). When a bit is 1, it indicates not to allow MS with the corresponding level to access then cell. Otherwise it indicates to allow the access.
MBR
Default Value: 0
Description: Multi-Band Reporting. It is used to inform MS to report the adjacent cells in a controllable way.
When the value is “0”, MS will report measurement results of six strongest adjacent cells no matter which band they are in. When it is “X(X<=3)”, MS will report measurement results of X strongest adjacent cells in each band other than the serving band, and the remaining positions are used to report the strongest ones in the serving band. If there are still some position left, MS will use them to report the left strongest cells no matter what band they are. If there are no special requirements on different bands and the traffics in the various bands are basically the same, “0” is recommended. When the traffics on various bands are obviously different from each other and MS is expected to enter a band preferably, “3” is preferred. When traffics on various bands are slightly different from each other, “1” or “2” is recommended.
In the initial stage of dual-band network, the traffic of GSM1800 system is very light, usually dual-band MS are expected to work on this band preferably. Therefore, the priority of GSM1800 cells for HO should be higher than that of GSM900 cells, and “3” is recommended for MBR.
Note: In the single-band system, the MBR parameter does not exist.
Application of MBR
GSM1800
GSM900
S
S is a GSM900 cell, cells A~H are adjacent to cell S. Of them, A and B are GSM1800 cells and others are GSM900 cells. The above diagram shows the influences of different MBR parameters as follows:
1)When MBR = 0, MS will report six adjacent cells with strongest signals without considering the bands, the report result is: C, E, B, H, F, D.
2)When MBR = 1, the result is: C, B, E, H, F, D.
3)When MBR = 2, the result is: C, E, B, A, H, F.
4)When MBR = 3, the result is: C, E, H, B, A, F (3 GSM1800 cells should be reported. But there are only 2 currently, so 2 GSM1800 cells are reported. For the rest, GSM900 cells will be reported).
ECSC
Default Value: No
Description: Early Classmark Sending Control. It indicates if an MS in the cell is allowed to use early Classmark sending.
Yes: The MS reports Classmark3 to the network immediately after link setup.
No: The MS is forbidden to report its Classmark3 to network initiatively.
In GSM network, MS’s service capability, supported band, power capability, encryption capability and so on are described by classmarks. There are three classmarks which are classmark1, classmark2 and classmark3. The network can know the MS’s capability by checking the classmarks of the MS. After receiving the class mark enquiry message, MS will send classmark change message to the network as soon as possible. CM3 (Classmark 3) includes the information about MS power, multiband and/or multislot capability. To perform handover between different bands, the power level must be described correctly. In the process of paging and sending of the BA2 information between different bands, the CM3 message must be known.
Note:
1) ECSC is invalid for single-band MS. For dual-band MS, when ECSC is not used, after the MS sends EST IND , MSC will still send the CLASSMARK REQUEST message, and MS will response with the CLASSMARK UPDATE message, and other functions are not affected. For the dual-band MS, when this parameter is set to Yes, the connection time between different MS will be obviously shortened.
2) When the encryption function is enable, The parameter must be set to "Yes".
3) M900/M1800 hybrid cells sharing BCCH are advised to be configured as "yes", and M1800 cells in dual-band network are advised to be configured as “yes".
Definition of Classmark
In GSM system, the abilities that MS supports, such as service ability, band ability, power ability, encryption ability, is described by the classmark. BSC decodes the information according to the requirement of network and transfer to the network.
There are 3 kinds of classmark:Classmark1Classmark2Classmark3.
Network knows about abilities of MS either via inquiring for MS’s CLASSMARK or via requiring MS to report CLASSMARK3 immediately by itself after link setup.
Classmark Update Flow
For dual-band MS, after the SCCP connection is established and before receiving message "CM Service Accepted" there will be a class mark update flow.
CR message reports the information of Classmrk 2. And in the class mark update flow, MS will report Classmark 3 information which is related to the dual-band capability.
Classmark Change
Classmark Enquiry
Class mark update flow.
For dual-band MS, after the SCCP connection is established and before receiving message “CM service Accepted”, there will be a classmark update flow. Classmark is originated by MSC. MSC will send one “Classmark Request” message, then BSC will convert this message into “Classmark Enquiry”. This is classmark request. Then MS will send one “Classmark Change”, and convert it into “Classmark Update” at BSS. MSC can get the MS Classmark 3 via this message. Here the “Classmark Update” is to make MSC get MS Classmark 3. CR message reports the information of Classmark 2. And in the classmark update flow, MS will report Classmark 3 information which is related to the dual-band capability. Therefore, MSC needs classmark update to get the MS Classmark 3.
Information about Classmark 2/3
Emergent Call Disable
Default Value: No
Description: Emergence Call Disable. When EC Disable is set to “No”, it means emergency call is permitted. Otherwise it is prohibited.
For MS with common access control class 0~9, when “Emergent Call Disable” is “Yes”, it indicates not to allow an emergency call.
For MS with access levels 11~15, the emergency call will not be allowed only when the corresponding access control class bit is set to “0” and “Emergent Call Disable” is set to “Yes” at the same time.
Cell Selection/Reselection Parameters
CRH ( Cell Reselection Hysteresis)
When MS is powered on, it will try to find a GSM PLMN. MS will select an appropriate cell and read cell parameter. This process is called cell selection. The “appropriate cell” is restricted by many factors. For example, whether this cell belongs to the selected network (under manual network selection mode), whether the cell is barred, the cell selection priority of the cell, whether the access level of MS is prohibited by the cell, whether the quality of radio channel meets the requirement of communication, etc. Among them, the quality of radio channel is one important factor of cell selection. GSM specification stipulates one parameter called path loss principle C1. The appropriate cell must ensure the C1>0. C1 is obtained by the calculation of receiving level and the cell selection parameters.
After MS selects cell, MS will settle in the selected cell. At the same time it begins to measure the signal level of BCCH of neighbor cells. It records six neighbor cells with the strongest signals (refresh at least every 60s), extracts various cell parameter and control information of each neighbor cell. (MS must conduct data block decoding for all the six strongest BCCH of neighbor cells within 5 minutes, including parameters affecting cell reselection. When MS regards a new neighbor BCCH as one of the six strongest BCCHs, it will conduct data block decoding for this new BCCH at least every 30s). Moreover, MS must check one of the six strongest at least every 30s. If BSIC has any changes, it will be regarded as a new BCCH and data decoding will be conducted again. During this process, MS will not stop monitoring PCH. When a certain condition is satisfied, MS will move from current cell to anther cell. This process is called cell reselection. The condition includes many factors, and they are all related to the quality of radio channels. When a neighbor cell’s radio channel quality is better than current cell’s, cell reselection occurs. The channel quality standard for cell reselection is C2. C2 is obtained by calculation of the receiving level and a number of parameters.
Cell Selection Process
Stored list cell selection
In fact, the process might be different for different MSs
If the SIM card of MS has not stored any BCCH (usually it is a new SIM card), it will search all the 124 RF channels and 374 more GSM1800 channels for dual-band MS, and measures the receiving signal level of each. The whole process lasts about 3s to 5s. During this period, MS obtains at least 5 measurement samples from each RF channel. Then MS tunes to the carrier with strongest receiving level, and judge whether it is BCCH (by searching for FCCH). If yes, MS tries to decode SCH and makes itself synchronous with that BCCH, then the MS reads cell parameter on it. If MS can correctly read the cell parameter and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell. Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still can’t find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority. If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card. If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode). Meanwhile, the MS keeps on monitoring all RF channels.
Note:
1. When MS access level is prohibited by this cell, cell selection algorithm is not affected. If the condition is fulfilled, MS will still try to reside in this cell.
2. MS belongs to the PLMN selected, but is prohibited from access, or C1<0, then MS will obtain the BA table from this cell and search those BCCHs according to this BA.
When MS is powered off, it will store some BCCH carrier information. When MS is powered on, it will first search the BCCHs which have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BA table of this cell and try these BCCHs. If it still cannot pass, MS will start the cell selection process without BCCH list.
Stored List Cell Selection
Normal cell selection
No
Yes
No
Yes
When MS is powered off, it will store some BCCH carrier information. When MS is powered on, it will first search the BCCHs which have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BA table of this cell and try these BCCHs. If it still cannot pass, MS will start the cell selection process without BCCH list.
Normal Cell Selection
Search all frequency
Come into idle mode
Right PLMN&C1>0& priority is Normal ?
Select the next frequency with the higher Rx_level
YES
No
YES
Try the cell of other PLMN allowed in the SIM card
Stay in a cell(RX-LEV is strongest, C1>0 , cell priority is not prohibited) without considering the PLMN
All frequency not satisfy
No
Search all frequency
Judge whether it is BCCH
If MS can correctly read the cell parameter and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell.
Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still can’t find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority.
If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card.
If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode).
Cell Selection Parameter
RXLEV_ACCESS_MIN: Minimum receiving level of MS permitted to access
MS_TXPWR_MAX_CCH: Maximum transmit power level of MSs
P: Maximum Physical supported transmitting power of MS.
RXLEV_ACCESS_MIN
Default Value: 8
Description: It means the minimum receive signal level required for MS to access a cell.
To prevent MS from accessing the system when the receiving signal level is very low (this will make unsatisfactory communication quality and waste the radio resources of the network), GSM specifications require that the MS’s receiving level must be greater than a threshold when it needs to register in the network. The threshold is the RXLEV_ACCESS_MIN.
For some cells with high traffic, this parameter can be increased appropriately to lower the values of C1 and C2 of this cell. Accordingly the effective coverage range of the cell will be reduced. But the value of RXLEV_ACCESS_MIN should not be too big, otherwise coverage hole (with regard to idle mode MS) will be created at the edge of the cell. When this method is used to balance the traffic, the value of RXLEV_ACCESS_MIN is recommended to be no more than 20.
Except for the areas with densely distributed BTS and good coverage, generally it is not recommended to use RXLEV_ACCESS_MIN to adjust the traffic of the cell. For isolated BTS or BTS with poor coverage, this value should be set properly, otherwise the call drop rate may increase and QoS will be affected.
MS_TXPWR_MAX_CCH
Default Value: 5 (900MHz cell),0 (1800/1900MHz cell)
Description: This parameter determines the maximum transmit power level of the MS when it begins to access a cell and has not yet received power control command.
Location: Cell Attributes/Other Attributes/Advanced/Public Channel Control
During MS’s communication with BTS, its transmitting power is controlled by the network via power control command. This command is transmit in SACCH (There are two head bytes. One is power control byte, the other is time advance). MS must get the power control head from the downlink SACCH, and output the power as is indicated in the in the power control header. If the MS cannot support the power level in the power control header, it will use the nearest power level it supports.
Since SACCH is associated signaling channel, it must be combined with other channels, such as SDCCH or TCH. Therefore, the control of MS’s power begins after MS receives SACCH. While the power level used by MS before it receives SACCH (power used when RACH is sent) is determined by the “MS TXPWR_MAX_CCH “ (maximum power level of control channel).
This parameter will affect cell selection and cell reselection.
C1 = RLA_C - RXLEV_ACCESS_MIN - MAX((MS_TXPWR_MAX_CCH - P), 0)
RLA_C: mean receiving level of MS
RXLEV_ACCESS_MIN: minimum receiving level of MS permitted to access
P: maximum physical supported transmitting power of MS.
CBA
Description: Cell_Bar_Access, worked together with CBQ to set the priority status of the cell in idle mode for cell selection and reselection.
The network operator can set the cell access is permitted or not by the parameter. Usually all cells allow MS to access, thus it is set as “No”. But in the special cases, the operator may want a cell to be used for handover service only, which can be realized by setting the parameter as “Yes” (CBQ should be “No” in this case).
CBQ
Default Value: No
Description: Cell_Bar_Qualify. CBQ only affects the cell selection, but is not related to cell reselection. It works with CBA to define the access priorities of cells.
For the area overlapped by cells, the operator often wants MS to preferably select certain cell during cell selection according to the cell capacity, traffic and cell functions, i.e. setting the cell priority. This function can be implemented by setting parameter “cell bar quality”. It works with parameters “cell bar access” together to determine the cell selection and cell reselection priority of the cell.
Usually the priorities of all cells should be set as “Normal”. But in some special cases such as micro-cell and dual-band network, the operator may expect MS to preferably enter the cells of a certain type. In this case, the network operator can set the priority of this type of cell as “Normal” while setting the other cells as “Low”. MS will select the cell with lower priority only when there is no appropriate cell with the priority as “Normal”. During the network optimization by means of cell priority, it is necessary to note that CBQ only influences the cell selection. Therefore, in order to achieve the target, C2 (cell reselection parameter) must be taken into consideration.
Application of CBA and CBQ
CBA
CBQ
Application of CBQ
The traffic of cell A and B is heavy. Set these two cells with CBA=“Yes”, CBQ=“No”.
A
B
Each circle in the diagram indicates a cell. For some causes, the traffic in cell A and that of cell B are obviously higher than those of the adjacent cells. To make the traffic of the entire area distributed on average, set the priorities of cell A and cell B as “Prohibited” and those of other cells as “Normal”. In this way, the services in the shadow areas in the diagram will be shared by the adjacent cells. It must be pointed out that this setting will reduce the actual coverage areas of cell A and cell B, which is different from decreasing the transmitting powers of cell A and cell B.
Application of CBA
B is a micro cell. Set B to “Normal” and A to “Low”.
B
A
Assume that micro-cell B and macro-cell A together cover an area. In order to make micro-cell B share more traffic of macro-cell A, the priority of cell B can be set as “Normal” and that of cell A as “Low”. Thus in the coverage area of cell B, MS will select cell B as long as the level of cell B reaches the RXLEV_ACCESS_MIN, no matter cell B has a lower signal level than cell A’s or not. And then reselection parameters can be set appropriately to make MS not to reselect cell A.
Cell Reselection Process
Cell reselection for cells in the same location area
If the C2 value of the target cell is higher than that of the serving cell for longer than 5 seconds, a cell reselection process will be performed and the MS tunes to the new cell.
Cell reselection for cells in different location areas
If the C2 value of the target cell is higher than that of the serving cell by at least the value of CRH for longer than 5 seconds, a location update process and the cell reselection process will be performed.
Two consecutive cell reselections caused by C2 have a time interval of 15 seconds. In other words to say, if because of C2 a MS reselected to a cell, then the MS cannot reselect to another cell by the cause of C2 within 15 seconds.
If current serving cell is prohibited, or down link fails, or C1 is less than 0 continuously for 5s, cell reselection will also be triggered.
MS starts a cell reselection if the access times exceed the MAX retrans.
Cell Reselection Parameter
C2=C1+CRO-TO*H(PT-T) When PT is not equal to 31
C2=C1-CRO When PT is equal to 31
For neighbor cell:
For serving cell:
H(PT-T) = 0
C2=C1+CRO-TO*H(PT-T) When PT is not equal to 31:
C2=C1-CRO When PT is equal to 31.
Wherein:
1. Function H(x)=0, when x<0; H(x)=1, when x>0.
2. T is a timer with initial value 0. When a certain cell becomes one of the six strongest neighbor cells, T corresponding to this cell begins to count. When the cell is out of the six strongest neighbor cells, the corresponding timer is reset.
3. CRO is used to revise the C2 intentionally.
4. The function of TO is to reduce the value of C2 from T begins to T reaches the stipulated PENALTY_TIME.
5. PT is the time that TO functions on C2. But if PT=31, the C2 formula is changed as C2=C1-CRO.
CRO
Value Range: 0~63( the corresponding level value: 0~126dB, stepped every 2dB).
Unit: 2dB
Default Value: 0
Description: Cell Reselect Offset. It is a parameter in C2 calculation to give an intentional modification in MS cell reselection.
After cell selection, MS will reselect another better cell in idle mode. It is C2 parameter that determines cell reselection. The principle for MS reselection is: select the cell with the maximum C2 as the serving cell. C2 is determined by the following factors:
C2=C1+CRO-TO*H(PT-T) (PT <31)
C2=C1-CRO (PT = 31)
H(x)=0 if x<0
H(x)=1 if x≥0
As shown above, C1 indicates the quality of radio channel. The larger C1 is, the better the channel is. C2 value is based on C1, through CRO, C2 of various cells can be adjusted. Thus C2 value can be calculated according to CRO, TO and PT in order to prefer selecting the cell in reselection process. That is, in dual-band network,several parameters that influence C2 value can be set to make C2 value of GSM1800 larger than that of GSM900. Therefore, even though the signal strength of GSM1800 cell is weaker than that of GSM900 cell, MS can still reside in GSM1800 by the aid of these parameters. Besides CRO, there are another two parameters influencing C2:TEMPORARY_OFFSET(TO) and PENALTY_TIME(PT).
CRO is a artificial modification on C2. Reasonable setting of this parameter can reduce handover times and realize assignment to a better cell. Usually it is not set as larger than 25dB. Generally the cells with the same priority in the network have basically the same CRO. Setting of this parameter only affects MS of GSM Phase II and above.
TO
Value Range: 0~7 (the corresponding value is 0~60dB, 7 corresponds to “Infinite”).
Default Value: 0
Description: Cell Reselect Temporary Offset. It is a parameter in C2 criterion to give a temporary modification within PT time.
Location: Cell Attributes/Idle Mode/Advanced/Idle Parameter
TO indicates the temporary modification on C2. Temporary means that it functions for C2 only within a duration which is determined by PT parameter. Setting of this parameter only affects MS of GSM Phase II and above.
PT
Value Range: 0~30(the corresponding time is 20~620s), 31(reserved for changing the effect of CRO on C2)
Unit: Second
Default Value: 0
Description: Cell Reselect Penalty Time. It is used to ensure the safety and validity of cell reselection because it helps to avoid frequent cell reselection.
Location: Cell Attributes/Idle Mode/Advanced/Idle Parameter
If the communication in a cell is affected due to very heavy traffic or some other reasons, this cell should not be the preferable cell that MS works in (a repulse should be made for this cell). In this case, PT can be set as 31, which causes TO invalid and C2=C1 – CRO. Therefore, C2 value of this cell is decreased. MS will reselect this cell with little possibility. Besides, the network operator can set CRO according to the repulse degree to this cell. The higher the repulse degree, the larger CRO.
For the cell with very low traffic, MS should prefer to work in this cell. In this case, CRO is recommended to be between 0-20dB. It can be set according to the preference degree to this cell. The higher the preference degree and the larger CRO. Generally TO is recommended to be the same as or a little more than CRO. PT is mainly used to prevent MS’s too frequent cell reselections. Generally it is recommended to be 0 (20s) or 1 (40s).
For the cell with medium traffic, generally CRO is recommended to be 0 and PT be 31 as a result of C2=C1.
Setting of PT can effectively prevent the fast moving MS from accessing the micro-cell. This parameter can be set according to the size of micro-cell. And it is recommended to be 20s for the ordinary micro-cells. When PT is set as 31, it is used to change the direction of CRO.
PI
Default Value: Yes
Description: Cell Reselect Parameters Indication, sent on the broadcast channel of the cell. It is an indication to the existence of “Cell Reselect Offset(CRO)”, “Temporary Offset(TO)” and “Penalty Time(PT)”.
This parameter is to inform MS whether C2 is used as the standard for cell reselection. The minimum interval between cell reselections caused by C2 parameter is 15s to avoid too frequent cell reselection.
If PI=Yes, C2 is used for cell reselection standard; if PI=No, C1 is used for cell reselection.
CRH
Unit: dB
Default Value: 6
Description: Cell Reselection Hysteresis. It is the parameter used when cell reselection happens between two location areas.
This parameter aims to prevent the frequent location update that may increase the network signaling flow and to reduce the possibility of paging message loss. If the value of this parameter is too small, the location update will have “ping-pong” effect and the signaling load on SDCCH will increase. Moreover, the call setup successful rate of the system will become lower because MS will not respond paging during location update. When it is too large, the cell where MS resides for a long time may not be the best when MS enters a new location area. When MS reselects a cell from a different location, MS will start a location update. Due to the fading of radio channel, C2 values of two cells at the their edges will fluctuate, which causes MS to reselect frequently. To reduce the influence, GSM specifications define a parameter called cell reselection hysteresis. It is required that MS start cell reselection only when C2 value of the adjacent cell (in a different location area) is greater than that of serving cell and their difference is greater than the value of reselection hysteresis.
Appropriate cell reselection hysteresis is important for network optimization. Usually it is recommended to be 8~10dB and can be adjusted in the following cases:
When the traffic of an area is very heavy and the signaling overload often occurs, it is recommended to increase the value of cell reselection hysteresis of the adjacent cells belonging to different LACs.
When the overlapping coverage of the adjacent cells belonging to different LAC are wide, it is recommended to increase the value of cell reselection hysteresis.
Other Parameters
Min Power Level for Direct Try
Max Resend Times of Phy. Info.
T3105
PWRC
Call Reestablishment Forbidden
Description: Informing MS whether call reestablishment is allowed or not.
In case of call drop, MS can start call re-establish process to resume the conversation. The network has the right to decide whether to re-establish or not. This function is achieved by setting “Call Reestablishment Forbidden”.
In some special cases (e.g. the cell has a dead zone at a fixed position), call drop will occur if MS passes through the dead zone during the conversation. If call re-establishment is permitted, the dropped call be resumed. However, call re-establishment takes relatively long time. Most of the subscribers have already released manually before the re-establishment is completed. Therefore, the re-establish of call not only fails to achieve the target, but also waste a lot of radio resources. So it is recommended that except for some special cells, it is not permitted to re-establish the call on the network.
TCH Immediate Assignment
Default Value : No
Description: Option "Yes" means that TCH channel can be immediately assigned as signaling and traffic channel when SDCCH has no available resource. The option "No" means that only SDCCH can be assigned.
Immediate Assignment Optimization
Default Value : No
Description: This parameter is mainly used for satellite transmission, i.e., to reduce the impact of satellite transmission delay.
Yes: when the Transmission Mode of the BTS is Satellite transmission
No: when the Transmission Mode of the BTS is Terrestrial transmission
Location: Cell Attributes/Call Control/Advanced/Call Control
This parameter is used to concurrently deliver the channel activation message and immediate assignment command message to quicken the processing of signaling. Thus the response speed of the network is ensured.
This function is used in satellite transmission mode. In satellite transmission mode, the Immediate Assignment Opt parameter must be set to Yes to minimize the effect of the satellite transmission on time delay. In terrestrial transmission mode, the default value of this parameter is No.
Signaling Flow of Immediate Assignment Optimization
MS
BTS
BSC
MSC
Channel_req
Channel_Required
Channel_Active
Channel_Active_Ack
Direct Retry
Default Value : Yes
Description: This parameter indicates the whether the directed retry on the TCH is allowed. If this parameter is set to Yes, the handover procedure is used and the MS is handed over to the neighboring cell.
In TCH assignment process, if there is no TCH available, hand over can take place instead of assignment, and this is called direct retry.
Direct retry is not a main measure to solve traffic congestion. Instead, it is only an emergency operation during traffic peak in local radio network.
If direct retry takes place frequently in a certain part of the radio network, it is necessary to adjust BTS TRX configuration and even the network layout.
Assignment Cell Load Judge Enable
Value Range: Enable, Disable
Default Value: Disable
Description: When the Assignment Cell Load Judge Enabled is set to Yes, the direct try flow is allowed if the following two conditions are met:
The cell supports the direct retry function.
The load of the cell is larger than or equal to the Cell Direct Try Forbidden Threshold.
In TCH assignment process, if there is no TCH available, hand over can take place instead of assignment, and this is called direct retry. Direct retry is not a main measure to solve traffic congestion. Instead, it is only an emergency operation during traffic peak in local radio network. If direct retry takes place frequently in a certain part of the radio network, it is necessary to adjust BTS TRX configuration and even the network layout.
Cell Direct Try Forbidden Threshold
Value Range: 0~100(%)
Default Value: 50
Description: If the parameter Assignment Cell Load Judge Enabled is set to Yes, the direct retry process is started and channels are assigned when the cell supports direct retry and the load of the current cell is equal to or greater than the value of Cell Direct Try Forbidden Threshold during channel assignment.
Location: Cell Attributes/Other Attributes/Advanced/Other Parameters
Directed Retry Load Access Threshold
Value Range: 0~100(%)
Default Value : 85
Description: This parameter is used to adjust the candidate target cells during direct retry.
When target cells are selected during direct retry, only the cells whose loads are smaller than or equal to the Directed Retry Load Access Threshold are selected as candidate target cells.
Min Power Level for Direct Try
Value Range: 0~63 (-110dBm~-47dBm)
Default Value :16
Description: This parameter indicates the selection of candidate cell for direct try. If the receiving level of the cell exceeds the value of this parameter, this cell can be selected as candidate target cells.
Location: Cell Attributes/ Handover Data/Advanced/HO Control
Max Resend Times of Phy. Info.
Range Value :1~255
Description: This parameter specifies the maximum times Ny1 for re-sending physical information.
If the number of resending times exceeds Ny1 and BTS still has not received any correct SAMB frame from MS, BTS will send BSC the connection failure message and handover failure message.
After BSC receives the messages, it will release the newly assigned dedicated channel and stop timer T3105.
Location: Cell Attributes/Handover Data/Advanced/Cell HO Data
During the asynchronous handover, MS constantly sends the handover access Burst to BTS. When BTS detects the Burst, BTS send physical information to the MS on the main DCCH/FACCH, and starts timer T3105. At the same time, it sends the MSG_ABIS_HO_DETECT message to BSC. The physical information contains related information of different physical layers so as to guarantee the correct access of MS. If the timer T3105 times out before receiving the SAMB frame from MS, BTS re-sends physical information to MS.
T3105
Default Value : 7
Description: When BTS sends physical information to the MS, the BTS starts the timer T3105. If the timer T3105 times out before receive the SAMB frame from MS, BTS re-sends physical information to MS and restarts the timer T3105.
Location: Cell Attributes/Handover Data/Advanced/Cell HO Data
When BTS sends physical information to the MS, the BTS starts the timer T3105. If the timer T3105 times out before receive the SAMB frame from MS, BTS re-sends physical information to MS and restarts the timer T3105. The maximum times for re-sending physical information is Ny1.
The physical information is sent through FACCH, which is sent every four TDMA frames and takes 18 ms. If the value of T3105 is smaller than or equal to 18 ms, then BTS should re-send the physical information to the MS when the timer T3105 times out for the first time. This timeout is meaningless if the physical information sent at the previous time is not sent on the FACCH because the time is shorter than a FACCH period. Considering the previous factors, 20 ms is the reasonable minimum value for this parameter. At present, the default value of this parameter is 70 ms.
Intra-BSC Handover Signaling Flow
PWRC
Default Value: Yes
Description: It is used to indicate MS whether to delete received level value obtained from the timeslots of BCCH when MS measures receiving level during base band hopping mode.
When it is set to “No”, it means the influence of receiving level on BCCH is in consideration.
If it is set to “Yes”, process it in the way mentioned above.
Location:

gsm cell parameters

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