1 omo112050 bsc6000 gsm v9 r8c12 cell parameters issue1.00

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Transcript of 1 omo112050 bsc6000 gsm v9 r8c12 cell parameters issue1.00

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Both assignment message and paging message share downlink CCCH channel.

Assignment message can be transmit either by AGCH or PCH, while paging message

can be sent only by PCH.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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P-41

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.

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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.

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P-43

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P-44

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P-45

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.

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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.

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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;

Class13——Public Utilities (e.g. water/gas suppliers);

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.

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P-48

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.

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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).

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P-50

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".

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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.

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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.

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P-55

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.

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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.

3.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.

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P-57

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.

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P-58

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).

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P-60

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.

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P-61

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.

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P-62

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).

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P-63

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.

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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.

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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.

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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.

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P-69

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.

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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.

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P-71

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.

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P-72

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.

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P-73

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.

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P-74

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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.

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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.

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P-79

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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.

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P-81

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.

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P-82

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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.

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P-86

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.

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P-87

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P-88

In order to monitor the communication quality of radio link and conduct power control,

both MS and BTS must have measuring function. But the measurement will encounter

some problems when several independent functions of GSM specification are

combined for use. First, GSM specification allows BCCH frequency to attend

frequency hopping (excluding the BCCH timeslot); secondly, GSM specification

allows to conduct downlink power control for the frequency hopping channel; lastly,

the power of BCCH frequency is not allow to be changed because MS needs to

measure the signal level of the adjacent cells. In the above conditions, the downlink

power control of a channel can only be applicable to a subassembly of the frequency

assembly that the channel is using, that is, the BCCH frequency during the frequency

hopping process cannot be power-controlled. If MS measures the downlink channel

level in ordinary mode, the measurement result involving the BCCH frequency will be

inaccurate for the power control. To reduce the influence of this problem upon power

control, MS is required to deduct the receiving level value obtained from the timeslot

of BCCH carrier when calculating the average value of receiving level during

frequency hopping. To make MS execute the above operation, the parameter “PWRC”

should be set as “yes” in the system.

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P-89

The dual timeslot extended cell combines two timeslots to provide sufficient delay.

Theoretically, a maximum TA value of 219 (63+156) is supported, i.e., the maximum

coverage radius can be 120km theoretically. Dual timeslot extended cell can be

divided into two types: Cell level and TRX level. Versions after 03.1120A (BTS

00.1130A and latter) support TRX level dual timeslot extended cell.

For dual timeslot extended cell, MAX TA should be set as 219, or calls may fail even

there are good signals. For the TRX where the main BCCH/SDCCH/GPRS are

located, the concentric circle (HW_IUO Property) must be set as overlaid subcell, or a

prompt of error is displayed. For dual timeslot TRXs in the dual timeslot extended cell,

only the even timeslots are effective, while the normal TRXs in the dual timeslot

extended cell are the same with the TRXs in normal cell, i.e. eight timeslots are all

effective. The RF hopping of dual timeslot extended cell is similar to that of normal

cell. But if the hopping mode of the dual timeslot extended cell is baseband hopping,

the identical MA group should be applied to TRXs of the same type (either all dual

timeslot TRXs or all normal TRXs), or a prompt of error is displayed.

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P-90

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P-91

When the TCH is idle mode, BTS measures the uplink signal strength and sort the

results into different interference bands and then sends them to BSC. BSC will use

these data as a basic criteria to allocate the channels. Interference can be divided

into six grades according to the interference signal strength. The signal level used to

define the grades is called inference band threshold. Based on these interference

band thresholds, BTS can know which band the current interference belongs to, and

send it to BSC via radio resources indication messages.

In the traffic statistics system, we can register “interference band” in cell

measurement function. Interference band 1 to 5 are supported. The definition of the

interference bands are listed below.

Interference band 1 means the interference intensity is below interference band

threshold 1;

Interference band 2 means the interference intensity is between interference band

threshold 1 and 2;

Interference band 3 means the interference intensity is between interference band

threshold 2 and 3;

Interference band 4 means the interference intensity is between interference band

threshold 3 and 4;

Interference band 5 means the interference intensity is above interference band

threshold 4.

Because interference band threshold 0 is too weak and interference band threshold 5

is too strong, they are not adopted by the traffic statistics system.

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P-92

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P-93

DTX has very limited influence on the conversation quality. But its application has two

advantages: one is that the interference on radio channel is effectively reduced and a

better average conversation quality can be achieved; the other is that DTX can

considerably reduce the power consumption of MS. Therefore DTX is recommended

on the network.

According to the protocol, MS reports BTS with two kinds of measurement report.

One is called full measurement report. It will average the levels and qualities of 100

timeslots in the whole measurement cycle (one measurement cycle involves 4 TCH-

multi-frames except idle frames). The other is called sub measurement. It averages

the levels and qualities of 12 timeslots, including eight “consecutive” TCH bursts and

four SACCH bursts.

According to GSM specification, both BTS and MS should conduct this two kind of

measurement (FULL and SUB) no matter whether the uplink/downlink DTX of the

system is activated or not. Each measurement report of SACCH indicates whether

DTX is active or not. According to this indication, BTS can make the right selection,

either FULL or SUB.

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P-94

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P-95

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P-96

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P-97

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P-98

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P-99

Procedure analysis:

The place is located in a highway section in suburb and is about three

kilometers away from downtown. The terrain structure does not fluctuate

seriously. Theoretically, the signal strength should be about -80dBm, which is

very different from the actual value measured. After conducting frequency

scanning here, we find that cell A, B and C are covering here. Their

frequencies are Fa, Fb, Fc and signal strength is -95, -80dBm, -92dBm

respectively. After checking the data, it is found that cell B is not configured as

cell A’s neighbor, thus an island is formed between cell A and B. When MS

comes from cell A goes towards B, it cannot have cell reselection or handover

to B and this leads to as if the coverage near cell B is not good.

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P-100

Procedure analysis:

This BTS site originally worked normally. The problem arose recently. Since

the subscriber complained that the problem were most serious near BTS A

and B, we decided to check these two BTS first. First we checked the alarm

on BTS A and B, it was normal. Then we checked the traffic statistics data and

found out that the SDCCH requests in cell 2 of BTS A rose from about 3000 in

17th May to about 6000 in 18th May. The obvious change began from 7:00

AM on 18th May. At the same time, SDCCH attempt failures rose suddenly to

more than 3000. It was obviously abnormal. And SDCCH congestion rate in

cell 3 of BTS A was about 30%. But the traffic volume, congestion rate, and

call drop rate of BTS A is normal. No abnormal situation is found in BTS B’s

traffic measurement. After checking the SDCCH traffic measurement of all the

adjacent cell of BTS A, we found a cell in which times of SDCCH attempt

decreased dramatically. The SDCCH statistics result showed the problem

should exist in cell2 and 3 of BTS A.

Where did the 3000 times of SDCCH requests come from? Analyzed from the

failure, we suspected that the data was modified, so we checked the BSC data

operation log. Using MA10 to trace the network access flow of MS, and we

found a lot of LOCATION UPDATE REJECTED messages. Since there was

no record of modification of BSC data, we checked MSC’s related cell

parameters, and found that CGI of cell 2 and 3 of BTS A were modified. All the

three cells in BTS A were using exactly the same CGI. After correcting the CGI

of cell 2 and 3, everything was back to normal.

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P-101

Cause analysis

We check the parameter “RXLEV_ACCESS_MIN” in data management

system, and find out that “RXLEV_ACCESS_MIN” of correlative cells are not

consistent, and some even have great differences. Then in this area, MS is

quite likely to reselect the cell with poor signals. And subscribers will feel the

obvious fluctuation of signals.

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P-102

Cause analysis:

Recently, no modification is conducted on the BSC. The current network only

differs from original network in the number of subscribers. In VLR, there are

more than 4000 local subscribers and 5000 roaming subscribers. Therefore,

the problem might be caused by the sudden increase of subscribers.

The pressure brought by subscriber increase on network is mainly shown in

two aspects: (1) TCH (traffic channel) congestion rate increases; (2) SDCCH

(signaling channel) congestion rate increases. MS maybe cannot make MOC

or MTC, and at the same time cannot conduct location updating successfully.

The direct result of location update failure is MS loses connection with the

network. Check BSC cell parameter data table. It is found that T3212 is set to

2 (unit: 6 minutes), i.e. 12 minutes, and MSC corresponding time is set to 30

minuets. This kind of configuration will cause the result that all activated MS

will originate a periodic location update every 12 minutes. When the number of

subscriber reaches a certain limit, SDCCH will be fully seized. If at this time

some MS originates periodic location update, it will fail because there is no

spare SDCCH available. Thus MS loses the connection with the network.

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P-103