Gsmdocument 131119095413-phpapp01

GSM Basic Radio parameters
ZTE University

Objectives
 At the end of this course, you will be able to:
 Understand the meaning of various radio parameters
 Grasp the setting of radio parameters
 State the effect to radio network performance of various
kind of radio parameters

Content
 Network identification parameters
 System control parameters
 Cell selection parameters
 Network function parameters

Roles of identification parameters
 Enable the MS to correctly identify the ID of the current
network
 Enable the network to be real time informed of the correct
geographical location of the MS
 Enable the MS to report correctly the adjacent cell
information during the conversation process

CELL GLOBAL IDENTITY (CGI)
 Cell Global Identity (CGI)
 It is used for identifying individual cells within an LA
3 Digits 2-3 Digits Max 16 Bits
MCC MNC
LAC
Cell Global Identity
Max 16 bits
CI
LAI

ROLES OF CGI
 The CGI information is sent along the system broadcasting
information in every cell.
 When the MS receives the system information, it will
extract the CGI information from it and determines whether
to camp on the cell according to the MCC and MNC
specified by the CGI.
 It judges whether the current location area is changed,
then determines whether to take the location updating
process.

SETTING OF CGI
 MCC（Mobile Country Code）:
 consists of 3 decimal digits, and the value range is the decimal
000 ～ 999.
 MNC（Mobile Network Code）:
 consists of 3 decimal digits, and the value range is the decimal
00 ～ 999.
 LAC（Location Area Code）:
 The range is 1-65535.
 CI（Cell Identity）:
 The range is 0-65535.

BASE STATION IDENTITY CODE (BSIC)
 Base Station Identity Code (BSIC)
 It enables MSs to distinguish between
neighboring base stations
3 Bits 3 Bits
NCC BCC
BSIC
NCC Network/ National Color Code Value Range: 0~7
BCC Base Station Color Code Value Range: 0~7

NCC and BCC ROLES
 NCC:
 In the connection mode (during conversation), the MS
must measure the signals in the adjacent cells and
report the result to the network. As each measurement
report sent by the MS can only contain the contents of
six cells, so it is necessary to control the MS so as to
only report the information of cells factually related to
the cell concerned. The high 3 bits (i.e. NCC) in the
BSIC serve this purpose.
 BCC:
 The BCC is used to identify different BS using the same
BCCH in the same GSMPLMN.

BSIC CONFIGURATION PRINCIPLE
A B C
D E F
 In general, it is required that Cells A, B, C, D, E and
F use different BSIC when they have same BCCH
frequency. When the BSIC resources are not
enough, the cells close to each other may take the
priority to use different BSIC.

ROLES OF BSIC
 Inform the MS the TSC used by the common signaling
channel of the cell.
 As the BSIC takes part in the decoding process of the
random access channel (RACH), it can be used to prevent
the BS from mis-decoding the RACH, sent by the MS to
an adjacent cell, as the access channel of this cell.
 When the MS is in the connection mode (during
conversation), it must measure the BCCH level of adjacent
cells broadcasting by BCCH and report the results to the
BS. In the uplink measurement report, MS must show
BSIC of this carrier it has measured to every frequency
point.

BA LIST (BCCH ADJACENT LIST)
 Adjacent cell BCCH table
 At most 32 adjacent cell
 Carried by BCCH when MS is idle, by SACCH
when MS is dedicated
 The MS will first search carriers from this table
and if none is found it will turns to find any of 30
carriers with highest levels.

RANDOM ACCESS
 Random access is the process that messages
being transmitted on RACH when a MS turns
from “idle” to “dedicate” mode. The main
parameters includes:
 MAXRETRANS
 Tx_Integer
 AC

MAX RETRANS
 When starting the immediate assignment process
(e.g, when MS needs location updating,
originating calls or responding to paging calls), the
MS will transmit the "channel request" message
over the RACH to the network. As the RACH is an
ALOHA channel, in order to enhance the MS
access success rate, the network allows the MS to
transmit multiple channel request messages
before receiving the immediate assignment
message. The numbers of maximum
retransmission (MAX RETRANS) are determined
by the network.

MAX RETRANS
 The MAX RETRANS is often set in the following ways:
For areas (suburbs or rural areas) where the cell radius is more
than 3km and the traffic is smaller, the MAX RETRANS can be
set 11 (i.e. the MAX RETRANS is 7).
For areas (not bustling city blocks) where the cell radius is less
than 3km and the traffic is moderate, the MAX RETRANS can be
set 10（i.e. the MAX RETRANS is 4).
For micro-cellular, it’s recommend that the MAX RETRANS be
set 01（i.e. the MAX RETRANS is 2).
For microcellular areas with very high traffic and cells with
apparent congestion, it’s recommend that the MAX RETRANS
be set 00（i.e. the MAX RETRANS is 1).

Transmission Distribution Timeslots
(Tx_integer)
The Tx_integer parameter is the interval in timeslots at which
the MS continuously sends multiple channel request messages.
The parameter S is an intermediate variable in the access
algorithm, and is to be determined by the Tx_integer parameter
and the combination mode of the CCCH and SDCCH

Format of Tx_Integer
 MS starts the first channel request message : {0, 1, ...,
MAX (Tx_integer, 8)-1}
 The number of timeslots between any two adjacent
channel request messages {S, S+1, ..., S+Tx_integer-1}
 The Tx_integer is a decimal number, which can be 3~12,
14, 16, 20, 25, 32 and 50 (default). The values of the
parameter S are shown as below:
Tx_integer
CCH Combination Mode
CCCH Not Shared with SDCCH CCCH Shared with SDCCH
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

ACCESS CONTROL AC
 The access levels are distributed as follows:
 C 0～C9: ordinary subscribers;
 C11: used for PLMN management;
 C12: used by the security department;
 C13: public utilities （e.g. water, gas）;
 C14: emergency service;
 C15: PLMN staff.

SETTING OF AC
 In the BS installation and commissioning process or in the
process of maintaining or testing some cells, the operator
can set C0～C9 as 0 to forcedly forbid the access of
ordinary subscribers so as to reduce the unnecessary
effects on the installation or maintenance work.
 In some cells with very high traffic, the congestion will
occur in busy hours. For example, the RACH conflict
happens frequently, the AGCH is overloaded and the Abis
interface flow is overloaded. The network operator can set
proper access control parameters（C0～C15）to control
the traffic of some cells.

CCCH_CONF
 The CCCH can be one or more physical channels. The
CCCH and SDCCH can share the same physical channel.
The combination mode of the common control channel in a
cell is determined by the CCCH_CONF
CCCH_CONF
Coding
Meanings
CCCH message
blocks in one
BCCH
0 CCCH use one basic physical channel, not shared with SDCCH 9
1 CCCH use one basic physical channel, shares with SDCCH 3
10 CCCH use two basic physical channels, not shared with SDCCH 18
100 CCCH use three basic physical channels, not shared with SDCCH 27
110 CCCH use 4 basic physical channels, not shared with SDCCH 36
Others Reserved

CCCH_CONF
 The CCCH_CONF is determined by the telecom
operation department according to the traffic
model of a cell.
 If a cell has 1 TRX, we recommend that the CCCH
uses one basic physical channel and shares it with the
SDCCH
 If a cell has 2 ~ 8 TRX, we recommend that the CCCH
uses one basic physical channel but does not share it
with the SDCCH.

AGBLK
 Since the CCCH consists of the access grant
channel (AGCH) and paging channel (PCH), it is
necessary to set how many blocks of the CCCH
information blocks are reserved and dedicated to
the AGCH, the access grant reserve blocks
(AGBLK).
 AGBLK is represented in decimal numerals, and
its value range is:
 CCCH is not combined with SDCCH: 0～7.
 CCCH is combined with SDCCH: 0～2.

AGBLK
 SETTING AND IMPACT OF AGBLK
 The AGBLK setting principle is: given that the AGCH is
not overloaded, try to reduce the parameter as much as
possible to shorten the time when the MS responds to
the paging and improve the quality of service of the
system.
 The recommended value of AGBLK is usually 1 (when
the CCCH is combined with the SDCCH), 2 or 3 (when
the CCCH is not combined with the SDCCH).

BS-PA-MFRMS
 According to the GSM specifications, every mobile
subscriber belongs to a paging group. the MS calculates
the paging group to which it belongs by its own IMSI.
 In an actual network, the MS only "receives“ the contents
in the paging subchannel to which it belongs but ignores
the contents in other paging subchannels. (i.e. DRX
source).
 The BS-PA-MFRMS refers to how many multi-frames are
used as a cycle of a paging subchannel. This parameter in
fact determines how many paging sub-channels are to be
divided from the paging channels of a cell.

BS-PA-MFRMS (2)
 BS-PA-MFRMS is represented in decimal
numerals and its value range is 2～9, its unit is
multiframe （51 frames）, its default value is 2
BS-PA-MFRMS
Multiframes of the same
paging group that cycle
on the paging channel
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9

PERIODIC UPDATING TIMER (T3212)
 The frequency of periodic location update is
controlled via the network and the period length is
determined by the parameter T3212.
 The T3212 is a decimal number, within the range
of 0~255, in the unit of six minutes (1/10 hours).
 If the T3212 is set to 0, it means that the cell
needs no periodical location update.

NCCPERM
 In the connection mode (during the conversation),
the MS will report the measured signals of the
adjacent cells to the BS, but each report may
contain at most 6 adjacent cells.
 Therefore, let the MS only report the information of
the cells that may become the hand-over target
cells.
 The above functions can be fulfilled by limiting the
MS to merely measure the cells whose NCC have
been specified. The NCCPERM lists the NCCs of
cells to be measured by the MS.
 NCCPERM will affect handover

RADIO LINK TIMEOUT (RLT)
 GSM specification stipulates that the MS must have a timer
(S), which is assigned with an initial value at the start of
the conversation, that is, the “downlink radio link timeout”
value.
 Every time the MS fails to decode a correct SACCH
message when it should receive the SACCH, the S is
decreased by 1. On the contrary, every time the MS
receives a correct SACCH message, the S is increased by
2, but the S should not exceed the downlink radio link
timeout value. When the S reaches 0, the MS will report
the downlink radio link failure.
 The radio link timeout is a decimal number, within the
range of 4 ~ 64, at the step of 4, defaulted to 16.

MBCR (1)
 The parameter "multiband indication (MBCR)" is
used to notify the MS that it should report the
multiband adjacent cell contents.
 The value is 0-3

MBCR (2)
0: Based on the signal strength of adjacent cells, the MS reports the
measurement results of 6 adjacent cells whose signals are the strongest,
whose NCC are known and allowed no matter in which band the adjacent
cells lie. The default value is “0”
1: The MS should report the measurement result of one adjacent cell in
each band (not including the band used by the current service area) in the
adjacent table, whose signal is the strongest and whose NCC is already
known and allowed.

MBCR (3)
2: The MS should report the measurement results of two adjacent cells
in each band (not including the band used by the current service area)
in the adjacent table, whose signals are the strongest and whose NCC
are already known and allowed.
3: The MS should report the measurement results of three adjacent cells
in each band (not including the band used by the current service area)
in the adjacent table, whose signals are the strongest and whose NCC
are already known and allowed.

CELL SELECTION C1
 When the MS is turned on, it will try to contact a
public GSM PLMN, so the MS will select a proper
cell and extract from the cell the control channel
parameters and prerequisite system messages.
This selection process is called cell selection.
 The quality of radio channels is an important factor
in cell selection. The GSM Specifications defines
the path loss rule C1. For the so-called proper cell,
C1>0 must be ensured.

CELL SELECTION C1
C1 = RXLEV - RXLEV_ACCESS_MIN
- Max(MS_TXPWR_MAX_CCH - P ,0)
 where:
 RXLEV_ACCESS_MIN is the minimum received level the
MS is allowed to access the network
 MS_TXPWR_MAX_CCH is the maximum power level of
the control channel (when MS sending on RACH);
 RXLEV is average received level;
 P is the maximum TX power of MS;
 MAX（X, Y）＝X; if X Y.
 MAX（X, Y）＝Y; if Y X.

RxLevAccessMin
 The RXLEV_ACCESS_MIN is a decimal number,
within the range of -110dBm ~ -47dBm
 Default value is 0 (-110dBm).
RXLEV_ACCESS_MIN Meaning
-47 dBm > -48 dBm (level 63)
-46 dBm -49 ~ -48 dBm (level 62)
... ...
-108 dBm -109 ~ -108 dBm (level 2)
-109 dBm -110 ~ -109 dBm (level 1)
-110 dBm <-110 dBm (level 0)

Setting and Influence
 For a cell with traffic overload, you can appropriately
increase the RXLEV_ACCESS_MIN
 RXLEV_ACCESS_MIN value cannot be set to too high a
value. Otherwise, “blind areas” will be caused on the
borders of cells.
 It is suggested that the RXLEV_ACCESS_MIN value
should not exceed -90 dBm.

CELL RESELECTION C2
 Cell Reselection (C2) is a process when MS change its
service cell in idle mode.
 When the MS selects a cell it will begin to measure the
signal levels of the BCCH TRX of its adjacent cells (at
most 6)
 When given conditions are met, the MS will move from the
current cell into another one. This process is called cell
reselection.

CELL RESELECTION C2
 When C2 Parameter Indicator (PI) indicates YES，the MS
will get parameters (CRO, TO and PT) , from BCCH, to be
used to calculate C2(channel quality criterion), which serves
as cell reselection norm. The equation is as follows:
C2＝C1＋CRO－H（PT－T）×TO, when PT≠ 31
C2＝C1－CRO , when PT= 31
 Where T is a timer. When a cell is recorded by MS as one
of the six strongest cells, timer starts counting, otherwise, T
is reset to zero.

PARAMETER INDICATOR (PI)
 PI is used to notify the MS whether to use C2 as the cell
reselect parameter and whether the parameters calculating
C2 exist.
 PI consists of 1 bit. “1”means the MS should extract
parameters from the system message broadcasting in the
cell to calculate the C2 value, and use the C2 value as the
standard for cell reselect; “0” means the MS should use
parameter C1 as the standard for cell reselect (equivalent
to C2＝C1）.

CRO, PT AND TO
 The cell reselection initiated by the radio channel quality regards C2
as the standard. C2 is a parameter based on C1 plus some artificial
offset parameters.
 The artificial influence is to encourage the MS to take the priority in
accessing to some cells or prevent it from accessing to others. These
methods are often used to balance the traffic in the network.
 In addition to C1, there are three other factors influencing C2, namely:
CELL_RESELECT_OFFSET (CRO), TEMPORARY_OFFSET (TO)
and PENALTY_TIME (PT).

Format of CRO, PT and TO
 The CRO is a decimal number, in dB, within the range
of 0 ~ 63, meaning 0 ~ 126 dB, at the step of 2 dB.
 The TO is a decimal number, in dB, within the range of
0 ~ 7, meaning 0 ~ 70 dB, at the step of 10 dB, where
70 means infinite.
 The PT is a decimal number, in seconds, within the
range of 0 ~ 31, meaning 20 ~ 620 seconds for 0 ~ 30,
and at the step of 20 seconds. The value of 31 is
reserved to change the direction of effect that the CRO
works on the C2 parameter.

C2 TYPICAL APPLICATIONS
 For cells where the traffic is very heavy or the
channel quality is very low. the PT may be set 31,
making TO invalid, so C2=C1-CRO.
 For cells where the traffic is moderate, the
recommended value for CRO is zero and PT=31,
thus causing C2=C1, i. e. no artificial impact will
be imposed.

C2 TYPICAL APPLICATIONS
 For cells with light traffic, it’s recommended that CRO
be ranged from 0 to 20dB. The greater the CRO, the
more possible the cells will be reselected ,and vice
versa. It’s also suggested that TO is equal or a little
higher than CRO. PT, whose main role is to avoid
frequent cell reselection by MS, is generally
recommended to be set at 20 seconds or 40 seconds.

CELL SELECTION HYSTERESIS (1)
 When a MS reselects a cell, if the old cell and the target
cell are in different locations, then the MS must initiate a
location updating process after cell reselection.
 Due to the fading features of the radio channel, the C2
values of two adjacent cells measured along their borders
will fluctuate greatly.
 MS will frequently conduct the cell reselection, which will
not only increase the network signaling flow and lead to
low efficiency use of radio resources, but reduces the
access success rate of the system, as the MS cannot
respond to paging calls in the location updating process.

CELL SELECTION HYSTERESIS (2)
 To minimize the influence of this issue, the GSM
specifications put forward a parameter called
ReselHysteresis,
 The cell selection hysteresis is represented in
decimal numerals, its unit is dB, its range is 0～14,
its step length is 2dB, and its default value is 4.

CELL RESELECTION PRINCIPLE
 If the MS calculates that the C2 value of an
adjacent cell (Same location area) surpasses the
C2 value of the serving cell and maintains for 5s
or longer, the MS will start cell reselection .
 If the MS detects a cell that is not in the same
location area with the current cell, the calculated
C2 value surpasses the sum of the C2 value of the
current cell and the ReselHysteresis parameter
and if it remains for 5s or longer, the MS will start
the cell reselection .
 The cell reselection caused by C2 should be
originated at least at the interval of 15s.

CELL BAR ACCESS (CBA)
 In the system message broadcasting in each cell, there is a bit
information indicating whether to allow the MS to access to it, which
is called cell bar access (CBA). The parameter CBA is to indicate
whether the cell bar access is set in a cell.
 The CBA bit is a parameter for the network operator to set. Usually
all the cells are allowed to be accessed by MS , so the bit is set
NO. However, in special cases, the telecom operator may want to
assign a certain cells for handover service only, then the bit can be
set YES.

Area A
MS A
BTS B
BTS C
CELL BAR ACCESS (CBA)

CELL BAR QUALIFY (CBQ)
 In areas where the cells overlay with each
other and differ in capacity, traffic and
functions, the telecom operator often hopes
that the MS can have priority in selecting
some cells, that is, the setting of cell priority.
This function is set by way of the parameter
"Cell Bar Qualify" (CBQ).

CELL BAR QUALIFY (CBQ) 2
C1 and C2 States with CBA and CBQ Configurations
CBQ CBA
Cell Selection
Priority
Cell Reselection
State
No No Normal Normal
No Yes Barred Barred
Yes No Low Normal
Yes Yes Low Normal

EXAMPLE OF CBQ SETTING
A B
 For some reasons, the traffic of Cells A and B is apparently higher
than that of other adjacent cells. To balance the traffic in the whole
area, you can set the priority of Cells A and B as low, and set the
priority of the rest cells as normal so that the traffic in the shade
area will be absorbed by adjacent cells. It must be noted that the
result of this setting is that the actual coverage of Cell A and Cell B
is narrowed. However, this is different from reducing the transmitting
power of Cell A and Cell B, the latter may cause blind areas of the
network coverage and the reduction of communication quality.

LIMITn
 According to GSM Specification 05.08, the BTS must
measure the interference levels of the upward links of all
the free channels for the purpose of providing basis for
managing and allocating radio resources.
 Moreover, the BTS should analyze its measured results,
divide the interference levels into 5 grades and report them
to the BSC. The division of the 5 interference grades (i.e.
the so-called interference bands) is set by the operator
through the man-machine interface. The parameter
"Interference band border(LIMITn)” determines the borders
of the 5 interference bands.

Value Range Specified dBm Level
0 <-110 dBm
1 -110 dBm ~ -109 dBm
2 -109 dBm ~ -108 dBm
…
61 -50 dBm ~ -49 dBm
62 -49 dBm ~ -48 dBm
LIMITn
Default: LIMIT1：4 LIMIT2：8 LIMIT3：15 LIMIT4：25
 The division of the interference bands should be favorable in
describing the interference in the system. Generally the default values
are recommended. In the ordinary situations, the free channel
interference level is smaller, so the LIMIT1～4 value should be
smaller. When apparently large interference appears in the system,
you can properly increase the LIMIT1~4 values in order to know the
exact interference.

INTAVE
 Due to the randomness of the radio channel
interference, the BTS must average the measured
uplink interference levels within the specified
period, and this average cycle is determined by
the INTAVE parameter.
 This parameter is a decimal number, in SACCH
multi-frames, within the range of 1 ~ 31.

New Cause Indication (NECI)
 The NECI is a decimal number, within the range of
0 ~ 1, with the meaning described as below:
 When the NECI is 0, it means that the cell does not
support the access of half-rate services.
 When the NECI is 1, it means that the cell supports the
access of half-rate services.

RE-ESTABLISHMENT ENABLE (RE)
 For the drop calls caused by the radio link fault, the MS
can start the call reestablishment process to resume the
conversation, but the network is entitled to determine
whether the call reestablishment is allowed or not.
“0”=Yes, “1”=No.
 In some special circumstances, the drop call may occur
when the MS goes through a blind area during the
conversation. If the call reestablishment is allowed, the
mean drop call rate will be reduced. However, the call
reestablishment process will occupy a longer period of
time, most of the subscribers have hung up before the
reestablishment process is over, as a result, the call
reestablishment failed to achieve its purpose and wasted
many radio resources. We recommend that the call
reestablishment be not allowed in the network except for
some individual cells.

GSM Coverage problem & Solution
ZTE university

Objectives
 To know different kinds of coverage problem, their
causes and solutions.

Contents
 Overview of Coverage Problem
 Main Causes of Coverage Problem & Solutions
 Procedures of Handling Coverage Problem
 Typical Cases

Overview of coverage problem
Weak coverage
Over coverage
No-serving cell coverage
Too small coverage range will cause high
call drop rate and a large number of
customer complaints.
Too large coverage will result in frequent
handovers, and mutual interference as
well, if it’s rather serious, and network
indicators will also be affected.
When cell reselection parameters and
handover scenarios are similar, or there
are 2 or more cells with similar signal
strength ,Pingpong handover is easy to be
caused during calls.

Main causes of weak coverage
too small BTS power
Weak coverage
too low antenna height
too small down-tilt
hardware problem
Obstruction of buildings

Main causes of over coverage
poor antenna
performance
inappropriate down-tilt
too high antenna height

Causes of no-serving cell coverage
unreasonable planning
of antenna parameters
inappropriate type of antenna
too large or too small
carrier transmission power
shrunk coverage caused
by equipment problem
influence of changes
in radio environment
unreasonable setting
of handover parameters
unreasonable setting of
cell reselection parameters
no-serving cell coverage

Procedures of Handling Coverage Problem
Check setting of problem BTS’ radio parameters
Check if strong interference source exists
Check hardware
Check antenna system
Analyze the local geographical environment to
see if site location and type of site are appropriate

Poor coverage at cold storage warehouse
 【Problem description 】
 Subscribers complained about the poor coverage around a cold storage
warehouse of animal foodstuff; it was difficult to detect signal even when
they were not far from the warehouse.
 【Problem analysis】
 According to subscriber’s complaint, we confirmed there was problem with
coverage around the warehouse. We found all radio parameters of the site
were set correct at OMCR. Statistical report showed that idle data of
interference band and UL/DL quality data distribution were normal.
Hardware operated normally, as shown in OMCR warning report.
 Hardware engineers went to the site and checked the system of the BTS,
tested power amplifier's power and VSWR, they were all shown normal.
Connection between equipment was correct. Antenna azimuth and down-tilt
were all set reasonable.
 Through DT on site, network engineers found that the signal strength of
the antenna main lobe was weak, while that of the side lobes was
stronger, so they tentatively confirmed the problem was due to antenna
fault.

Poor coverage at cold storage warehouse
 【Problem handling】
 After the antenna was replaced with a new one, the coverage improved
greatly, so did the speech quality.

Poor coverage of a BTS
 Subscribers complained about weak signal strength around a Food
Bureau (near a BTS).
 【Problem analysis 】
 According to subscriber’s complaint, we confirmed there was
problem with the BTS' coverage. We found all radio parameters of
the site were set correct at OMCR. Statistical report showed that
idle data of interference band and UL/DL quality distribution were
normal. Hardware operated normally, as shown in OMCR warning
report.
 Hardware engineers went to the site and checked the system of the
BTS, tested amplifier's power and VSWR, they were all shown
normal. Connection between equipment was correct. Antenna
azimuth and down-tilt were all set reasonable.
 Through DT on site, network optimization engineers found that the
BTS’ coverage was in normal condition. While the Food Bureau,
where subscribers complained about the signal, was 4km away
from the BTS, and only indoor signal was weak (covered by Cell2).

Coverage shrinking after BTS starts operation
 After Cell3 of a BTS started to operate, its coverage range was
found shrunk. On highway 3km away from the BTS, where the BTS
tower was visible, MS could not detect Cell3’s signal. MS could
receive signal when it’s around the BTS, and the signal level was
about -60dB.
 We checked in radio resource management centre and found
Cell3’s static power class was set 2, which meant its static power
was reduced by 4dB, so we reset it to be 0. The next day, MS on
highway 3km away from the BTS could receive Cell3’s signal, and
its level was -60—70; and the signal level around the BTS was
strong, which was about -40dB.
 we concluded that the cell’s coverage shrinking was caused by
wrong setting of static power control at OMCR.

High handover failure rate due to skip-zone
coverage
 Configuration of a mountain site was S11, and the local network was
single band GSM900. From indicator statistics of the past week, we found
handover success rate of Cell2 under the BTS kept very low, which was
around 80%, while TCH allocation failure rate was completely normal.
 First, we could exclude the possibility of hardware problem and
interference, because there were no TCH assignment failures, which
explained that MS could successfully occupy TCHs assigned to it by BSC;
from DT analysis, we could see when signal level was above -90dbm, no
call drops happened to MS, and speech quality was good, which could
prove that no serious interference existed. Through further analysis, we
found the target cell for handover was a bit far from Cell2; and probably
adjacent cell relations were not set right during assignment planning,
which resulted in isolated-island effect.
 we could make area A and area B become adjacent cells to Cell2; while
Cell2 coverage at A and B was already very weak, so Cell2 should not be
adjacent cell to A and B .
 After adjustment, handover success rate of Cell2 increased greatly, from
80% to 96%.

High handover failure rate due to skip-zone
coverage
Cell1
Cell2

Questions for thinking
 Which parameters can be adjusted to improve
coverage?

GSM/GPRS/EDGE Basic Principles
ZTE University

Objective
 At the end of this course, you will be able to:
 Learn GSM development history
 Learn and master network structure of GSM system and
functions & principles of different portions
 Learn and be familiar with GSM wireless channel and
protocol
 Learn and be familiar with main service call process for
GSM

Content
 Chap.1: GSM Overview
 Chap.2: GSM Network Structure
 Chap.3: Interfaces and Protocols
 Chap.4: GSM Radio Channel
 Chap.5: Basic Service and Signaling Process
 Chap.6: Voice Processing and Key Radio
Technology
 Chap.7: GPRS and EDGE

GSM Overview
 This chapter mainly introduces some basic
information for GSM, including GSM development
history, supported service type, specification, and
system features.
 GSM Basic Concepts
 Services Supported by GSM System
 GSM Specification

GSM Overview
 This section introduces network structure of GSM
system and basic functions of various NEs.
 GSM Area Division Concepts
 GSM composition
 Mobile Switching System (MSS)
 Base Station Subsystem (BSS)
 Operation & Maintenance Subsystem (OMS)
 Mobile Station (MS)
 GSM System Number

GSM Area Division Concepts
Relationship between Areas in GSM

IBM
IBM
BSS MSS
GSM System Composition
MS
PSTN
MS
Other
PLMN
Um
Interfac
e
A
Interf
ace
GSM composition

Mobile Switching System (MSS)
 The MSS consists of such entities as the mobile
switching center (MSC), home location register
(HLR), visitor location register (VLR), equipment
identity register (EIR), authentication center (AUC)
and short message center (SMC).

Base Station Subsystem (BSS)
 BSS serves as a bridge between the NSS and MS.
It performs wireless channel management and
wireless transceiving. The BSS includes the Base
Station Controller (BSC) and Base Transceiver
Station (BTS).

Operation & Maintenance Subsystem (OMS)
The OMS consists of two parts: Operation &
Maintenance Center – System (OMC-S) and OMC-Radio
(OMC-R). The OMC-S serves the NSS, while
the OMC-R serves the BSS.

Mobile Station (MS)
The MS consists of mobile terminals and Subscriber
Identity Module (SIM) card.

GSM System Number
 GSM system number contains:
 Mobile Subscriber ISDN Number (MSISDN)
 International Mobile Subscriber Identity (IMSI)
 Mobile Subscriber Roaming Number (MSRN)
 Handover Number
 Temporary Mobile Subscriber Identification (TMSI)
 Location Area Identification (LAI)

GERAN interfaces
 This chapter introduces GERAN interfaces, User
plane/control plane protocol stack at PS and CS.
Interfaces
 PS-Domain Protocol Stack
 CS-Domain Protocol Stack

PS-Domain Protocol Stack
User plane protocol stack at PS domain

PS-Domain Protocol Stack
Control plane protocol stack at PS
domain

CS-Domain Protocol Stack
User plane protocol stack at CS domain

CS-Domain Protocol Stack
Control plane protocol stack at CS
domain

GSM Working Frequency Band
 This section introduces GSM radio frame, channel
concept, division & function for different channels,
mapping combination mechanism between
channels.
 GSM Working Frequency Band
 Structure of GSM Radio Frame
 Physical Channel and Logical Channel
 System Messages

GSM Working Frequency Band
Currently, the GSM communication system works at
900MHz, extended 900MHz and 1800MHz.
1900MHz band is adopted in some countries.

Structure of GSM Radio Frame
 There are five layers for structure of GSM radio frame, that
is, timeslot, TDMA frame, multiframe, super frame, and
hyper frame.
1 hyper frame = 2048 super frames =2715648 TDMA frame
1 hyper frame = 1326 TDMA frame (6.12s)
(=51 (26 frames) multi-frames or 26 (51 frames) multi-frames
1 (26 frames) multi-frame = 26 TDMA frame (120ms) 1 (51 frames) multi-frame = 51 TDMA frame (3036/13 ms)
TDMA Frame
Hierarchical frame structure in GSM system

Physical Channel and Logical Channel
GSM uses TDMA and FDMA technologies for physical
channel, as shown in the figure below.
Time
Frequency
Frequency
Time

System Messages
System message falls into 12 types: type1, 2, 2bis,
2ter, 3, 4, 5, 5bis, 5ter, 6, 7, 8.

Basic Service and Signaling Process
 This section introduces GSM terminal start,
position register / update, service call and
handover service implementation and signaling
interaction process.
 Mobile subscriber state
 Location Update
 Typical Call and Handover Process
 Basic Signaling Process

Mobile subscriber state
 The mobile subscriber has three states as follows:
 MS starts, network does "Attach" marks on it
 MS shutdowns, separated from network
 MS Busy

Location Update at Same MSC Office
BSC
（2）
（1）
（3）（4）
MSC/VLR
LAI
1
LAI
2
M
S
M
S
Location update between different MSCs
（5）
（2）
（3）（1）
（4）
HLR
MSC/VLR1
MSC/VLR2
M
S
M
S
Location Update

Typical Call and Handover Process
Call process

Typical Call and Handover Process
Handover process

Basic Signaling Process
MS BTS BSC MSC
CH RQD
CH ACT
CH ACT ACK
DR：CH REL
REL IND
RF CH REL
IMM ASS
SABM
CIPH MODE CMD
DISC
Location Update Process of MS
CR：LOC UPD REQ
DT1：CIPH MODE CMD
DT1：CIPH MODE COM
DT1：Clear CMD
RLSD
RLC
RF CH REL ACK
UA
DEACT SACCH
CH REL
DT1：Clear COM
DI：CIPH MODE COM
CIPH MODE COM
ENCRY CMD
CC
EST IND
UA
IMM ASS CMD
CH REQ
DTAP：LOC UPD ACCEPT

MS BTS BSC MSC
CH RQD
CH ACT
CH ACT ACK
DR：CH REL
REL IND
RF CH REL
RF CH REL ACK
IMSI Detach Process
IMM ASS
SABM
DISC
UA
DEACT SACCH
CH REL
CR：IMSI DETACH
CREF
EST IND
UA
IMM ASS CMD
CH REQ

Mobile-Originated Call and Called
Party On-hook Process
MS BTS BSC MSC
CH RQD
CH ACT
CH ACT ACK
DTAP:CM SERV ACCP
PHY CONT CONF
CH ACT
EST IND
RF CH REL
RF CH REL ACK
CR：CM SERV REQ
DT1：CIPH MODE CMD
DT1：CIPH MODE COM
DT1:ASS REQ
DT1：Clear CMD
RLSD
RLC
IMM ASS
SABM
CIPH MODE CMD
SABM
CH REL
DISC
UA
DR：CH REL
DEACT SACCH
REL IND
RF CH REL
RF CH REL ACK
ASS COM
DT1：ASS COM
CH ACT ACK
UA
PHY CONT REQ
DR：ASS CMD
ASS CMD
DT1：Clear COM
DI：CIPH MODE COM
CIPH MODE COM
ENCRY CMD
CC
EST IND
UA
IMM ASS CMD
CH REQ
DTAP:SETUP
DTAP:CALL PROC
DI：ASS COM
DTAP：Alerting
DTAP：Connect
DTAP：Connect ACK
数据流
DTAP：Disconnect
DTAP：Release
DTAP：Release COM

Mobile-Terminated Call and Calling
Party On-hook Process
MS BTS BSC MSC
UDT： PAG CMD PAG PAG REQ
CH RQD
CH ACT
CH ACT ACK
PHY CONT CONF
CH ACT
EST IND
RF CH REL
RF CH REL ACK
CR：PAG RES
DT1：CIPH MODE CMD
DT1：CIPH MODE COM
DT1:ASS REQ
DT1：Clear CMD
RLSD
RLC
IMM ASS
SABM
CIPH MODE CMD
SABM
CH REL
DISC
UA
DR：CH REL
DEACT SACCH
REL IND
RF CH REL
RF CH REL ACK
ASS COM
DT1：ASS COM
CH ACT ACK
UA
PHY CONT REQ
DR：ASS CMD
ASS CMD
DT1：Clear COM
DI：CIPH MODE COM
CIPH MODE COM
ENCRY CMD
CC
EST IND
UA
IMM ASS CMD
CH REQ
DTAP:SETUP
DTAP:CALL CONF
DI：ASS COM
DTAP：Alerting
DTAP：Connect
DTAP：Connect ACK
数据流
DTAP：Disconnect
DTAP：Release
DTAP：Release COM

MS BTS1 BTS2 BSC MSC
DR：HO CMD
RF CH REL
MEAS RES
Inter-cell Handover Process
DT1：HO PERF
HO CMD
CH ACT
MEAS REP
RF CH REL ACK
CH ACT ACK
HO DET
EST IND
DI：HO COM
HO ACCESS
PHY INFO
SABM
UA
HO COM

key radio enhanced technologies
 This section describes basic voice processing for
GSM, and several key radio enhanced
technologies.
 Voice Processing
 Frequency multiplexing
 Adaptive equalizing
 Diversity Receiving
 Discontinuous Transmission (DTX)
 Power Control
 Timing Advance
 Frequency Hopping Technology

Voice Processing
Voice Processing in the GSM System

Frequency multiplexing
Frequency multiplexing is the core concept of the cellular
mobile radio system. In a frequency multiplexing system,
users at different geographical locations (different cells)
can use channels of the same frequency at the same time
(see the figure above).

Adaptive equalizing
Equalizer can do equalizing at frequency domain
and time domain. GSM uses time domain
equalizing, enabling the better performance in
whole system.

Diversity Receiving
Diversity reception technology is commonly used in GSM.
Diversity consists of different forms: Space diversity,
frequency diversity, time diversity and polarity diversity.

Discontinuous Transmission (DTX)
The DTX mode accomplishes two objectives: Lower the total
interference level in the air and save the transmitter power.
Speech Frame Transmission in DTX Mode

Power Control
Power control means to control the actual transmitting power (keep it
as low as possible) of MS or BS in radio propagation, so as to reduce
the power consumption of MS/BS and the interference of the entire
GSM network.
Power Control Process

Timing Advance
In the GSM, the MS requires three intervals between timeslots when
receiving or transmitting signals. See the figure below.
Uplink and Downlink Offset of TCH

Frequency Hopping Technology
Frequency hopping (FH) refers to hopping of the carrier frequency
within a wide frequency band according to a certain sequence.
Basic Structure of FH

section describes evolution of GSM
technologies
 This section describes evolution of GSM
technologies: basic concept, network structure,
radio channel, and basic application of GPRS and
EDGE.
 Definition and Feature
 Inheritance and Evolution
 GPRS Radio Channel
 Radio Link and Media Access Control Flow
 Terminal and Application

Definition and Feature
 The General Packet Radio Service (GPRS) is the
packet data service introduced in GSM Phase2+.
 The GPRS has the following features:
 Seamless connection with IP network
 High rate
 Always online and flow charging
 Mature technology

Definition and Feature
 Enhanced Data Rate for GSM Evolution (EDGE) is a kind
of technology for transition of GSM to 3G.
 The EDGE has the following features:
 EDGE neither changes GSM or GPRS network structure nor
introduces new network element, but only upgrades the BSS.
 EDGE does not change the GSM channel structure, multiframe
structure and coding structure.
 EDGE supports two data transmission modes: packet service (non-real
time service) and circuit switching service (real time service).
 EDGE adopts octal 8PSK modulation technology, supports 303%
of GMSK payload, and provides higher bit rate and spectral
efficiency.
 Compared with GPRS, EDGE adopts new coding mode.

GPRS Radio Channel
 This section introduces GPRS physical channel,
GPRS logic channel, mapping of logical channel
combination in the physical channel, and GPRS
channel coding.

Radio Link and Media Access Control Flow
 This section introduces paging flow, TBF setup
flow, GPRS suspend/resume flow, and TBF
release flow.

Terminal and Application
 The GPRS MSs fall into three categories: Type A,
B, and C.

GSM Handover Problems & Solutions
ZTE university

Objectives
 To master different types of handover and their
signaling flows;
 To master handover statistical signaling point and MR
tasks;
 To know common handover problems and the handling
procedures.

Contents
 Overview of handover
 Flow of handover signaling
 Handover statistics
 Handover problem analysis

Aims of handovers
 Why there are handovers?
 To keep calls going on during movement;
 To improve network service quality;
 To decrease call drop rate;
 To decrease congestion rate.

Handover classification
Inter-MSC
Inter-BSC
Intra-BSC
Intra-cell
Handover
classification

Intra-cell handover
Air A
TC
BTS
BSC
New Channel
Old Channel

Signaling flow of intra-cell handover
MS BTS BSC MSC
1、Measurement Report(SACCH)
2、Measurement Report
3、Channel Activation
4、Channel Activation Ack
5、Assigment Command （FACCH)
6、SABM(FACCH)
8、UA(FACCH)
7、Establish Indication
9、Assigment Complete(FACCH)
10、Receiver Ready(FACCH)
11、HO Performed
12、RF Channel Release
13、RF Channel Release Ack

Inter-cell handover within one BSC
Air A
BTS TC
BTS
BSC
Old Cell / BTS New Cell / BTS

Signaling flow of inter-cell handover within one BSC
MS Old BTS BSC MSC
1、Measurement Report(SACCH)
2、Measurement Report
5、HO Command
7、HO Access(FACCH)
12、UA(FACCH)
13、HO Complete(FACCH)
14、Receiver Ready(FACCH)
16、HO Performed
17、RF Channel Release
18、RF Channel Release Ack
New BTS
6、HO Command(FACCH)
8、HO Detect
9、Physical info(FACCH)
10、SABM(FACCH)
11、Establish Indication
15、HO Complete

Air A
BTS
Old Cell / BTS
New Cell / BTS
BTS
BSC TC
BSC TC
VLR MSC
Inter-BSC handover

Signaling flow of inter-BSC handover
MS Old BTS Old BSC MSC
6、HO Command
13、UA(FACCH)
14、HO ommand
New BTS
10、HO Detect
11、Physical info(FACCH)
12、SABM(FACCH)
New BSC
1、HO_REQ
2、HO_REQ
5、HO_REQ_ACK
7、HO Command
8、HO Command
9、HO Access(FACCH)
15、HO Command
16、HO Command
17、HO Command

Air A
BTS
Old Cell / BTS
New Cell / BTS
BTS
BSC TC
BSC TC
VLR MSC
VLR MSC
Inter-MSC handover

Basic signaling flow of Inter-MSC handover
MS/BSS-A
MSC-A MSC-B
MAP-Prep-Handover req. MAP-Allocate-Handover-Number req.
MAP-Send-Handover-Report req.
A-HO-REQUEST
A-HO-REQUIRED
BSS-B/MS
VLR-B
A-HO-REQUEST-ACK
MAP-Prep-Handover resp.
IAM
MAP-Send-Handover-Report resp.
A-HO-COMMAND ACM
A-HO-DETECT
A-HO-COMPLETE
MAP-Process-Access-Sig req.
A-CLR-CMD/COM MAP-Send-End-Signal req.
ANSWER
RELEASE
End of call
MAP-Send-End-Signal resp.

Signaling flow of inter-MSC back-handover
MS/BSS-B
MSC-A MSC-B
MAP-Prep-Sub-Handover req.
A-HO-REQUIRED
BSS-A/MS
VLR-B
MAP-Prep-Sub-Handover resp. A-HO-COMMAND
A-HO-REQUEST-ACK
A-HO-DETECT
A-HO-COMPLETE
MAP-Send-End-Signal resp. A-CLR-CMD/COM
A-HO-REQUEST
Release

Signaling flow of inter-MSC handover to a third MSC
MSC-B
A-HO-REQUIRED
VLR-B
MAP-Prep-Sub-Handover req.
ACM
A-HO-COMMAND
A-HO-DETECT
A-HO-COMPLETE
MSC-A
MS/BSS
MSC-B’ VLR-B’
MAP-Prepare-Handover req.
MAP-Prepare-Handover resp.
MAP-Allocate-Handover-Number req.
MAP-Send-Handover-Report req.
IAM
MAP-Send-Handover-Rep. resp. (1)
MAP-Prep-Sub-Ho resp.
MAP-Process-Access-Signalling req.
MAP-Send-End-Signal req.
Answer
Release
A-CLR-CMD/COM
Release
(end of call)

Basic flow of handover signaling
Inter-cell handover
within BSC
There is no “HO-Request” message for intra-BSC handover; all
information is analyzed within BSC; Once a target cell in the
BSC fulfilling handover conditions is found, send “Channel
activation” message directly;
Inter-BSC handover
within MSC
BSC reports CGI and handover cause of original cell and target
cell to MSC through “HO-Request”;
After MSC finds target cell LAC, it sends “HO-Request” to the
BSC which the target cell belongs to;
Target BSC activates channel in target cell, and executes the
following flow.

Basic flow of handover signaling
Inter-MSC handover
MSC inquires “REMOTLAC sheet” (including LAC and
route address of adjacent MSC);
MSC sends （Prepare-HO） message to the target
MSC-B according to the route address;
According to the （Prepare-HO） message, target
MSC-B requests for Handover number from VLR-B,
then sends “HO-Request” message to BSC-B;
After the target BSC-B receives “HO-Request ACK”, it
sends （Prepare-HO ACK）message to the original
MSC, and executes the following flow.”

Main differences between intra-BSC handover
and inter-BSC handover
MSC participates
or not
CGI is carried
or not
Inter-
BSC
handover
Intra-
BSC
handover
MSC transmits “HO-REQ” message,
and CGI of original cell and target cell
is carried in the message;
As for inter-BSC handover, MSC
participates in it since “HO-Request”;
As for intra-BSC handover, “HO-Performed”
message is sent to MSC
only after the handover is
completed; MSC doesn’t participate
before that;
For intra-BSC handover, CGI isn’t
carried in any message, it’s handled
within BSC.

Flow of handover algorithm
MS BTS BSC MSC
BCCH
frequency
point, BSIC
and level
values of
the six
adjacent
cells (with
strongest
level) and
serving cell;
UL MR
Process of MR
Confirmation of
adjacent cell CGI
Execution of
handover decision
Selection of
target cell
External cell?
Channel activation
HO request
Intra-MSC
handover
Target BSC Target MSC
BA2 sheet
List of cells
under one LAC
HO request
HO request
No
Yes

Common timers at BSC
 T3107
 Suitable for: intra-cell handover
 Start-up: BSC sends “assignment command”
 Stop counting: when “assignment completed” or
“assignment failure” is received;
A1
MS BTS:TRX BSC
CHANNEL ACTIVATE
ASSIGNMENT COMMAND
A2
CHANNEL ACTIVATE ACK
SET T3107
T3107
Timeout

Common timers at BSC
 T3103
 Suitable for: inter-cell handover
 Start-up: BSC sends “handover command”
 Stop counting: when “handover completed” or “handover failure” is
received;
A1
MS Old BTS: BSC
CHANNEL ACT
New BTS
HANDOVER COMMAND
A2
CHANNEL ACT ACK
HANDOVER COMMAND
SET T3103
T3103
Timeout

MR cycle
 MR is sent to BTS in SACCH UL direction;
 When MS is in SDCCH, MR cycle is 470ms/time;
 When MS is in TCH, MR cycle is 480ms/time.
12TCH 1SACCH 12TCH 1 idle
480ms
26 multi-frames
of 4
TCHs

Indicator definition of handover success rate
KPI name Handover success rate
Indicator
definition
（ busy hour number of handover success times /busy hour total
number of handover request times）*100%
V6.20 (C900060098+C900060102+C900060120+C900060094
+C900060096)*100/(C900060097+C900060213+C9000
60214+C900060215+C900060099+C900060100+C900
060101+C900060216+C900060119+C900060093+C900
060095)

Signaling statistical point of handover success
 C900060098  C900060102
BSC BTS
A
 C900060120
HO_COM
BSC-controlled inter-cell incoming handover success
MSC BSC BTS
A
HO_COM
HO_COM
MSC-controlled incoming handover success
A
BSC BTS
ASS_CMD
ASS_COM
Intra-cell handover success
 C900060096
BSC MSC
A
CLEAR_CMD
No. of MSC-controlled outgoing handover success times

Signaling statistical point of handover success
 C900060094
MS
HO_CMD
BTS(Src)
CHL_ACT
BSC
HO_CMD
MEAS_RES
MEAS_RES
SABM
UA
HO_COM
MSC
EST_IND
HO_COM
HO_PERFORM
HO_ACCESS
BTS(Target)
CHL_ACT_ACK
HO DETECT
Phy Info
A
BSC-controlled inter-cell outgoing handover success

Signaling statistical point of handover request
 C900060097
BSC BTS
A
CHL_ACTIV_ACK
 C900060213
BTS( Target) BSC
BSC-controlled inter-cell incoming handover execution
 C900060214
Resource
Available
A
Forced
release
attempt
，
CHANNEL ACT
CHANNEL ACT ACK
Execution of forced release
BTS( Target) BSC
Resource
Available
A
Cell
queuing
，
CHANNEL ACT
CHANNEL ACT ACK
Execution of cell queuing
 C900060215
BTS( Target) BSC
Resource
Available
A
Force
handover
attempt
，
CHANNEL ACT
CHANNEL ACT ACK
Execution of force handover

 C900060099  C900060100
MSC BTS
 C900060101
BSC
A
HO_REQ
HO_REQ_ACK
CHL_ACTIV
CHL_ACTIV_ACK
MSC BSC-controlled incoming handover execution
MSC BSC
BTS
A
HO_REQ
HO_REQ_ACK
Forced release attempt,
resource available
CHL_ACTIV
CHL_ACTIV_ACK
Execution of forced release
 C900060119
MSC BSC
BTS
Cell queuing, resource available
A
HO_REQ
HO_REQ_ACK
CHL_ACTIV
CHL_ACTIV_ACK
Execution of queuing
BTS BSC
A
CHL_ ACTIV_ACK
ASSIGN_CMD
Execution of intra-cell handover

 C900060216  C900060095
BTS
 C900060093
A
MSC
HO_CMD
BSC
HO_CMD
No. of MSC-controlled outgoing handover execution times
BTS( Target) BSC
Resource
available
A
Force
handover
attempt
，
CHANNEL ACT
CHANNEL ACT ACK
Execution of force handover
MS
HO_CMD
BTS(Src)
CHL_ACT
A
BSC
HO_CMD
MEAS_RES
MEAS_RES
SABM
UA
HO_COM
MSC
EST_IND
HO_COM
HO_PERFORM
HO_ACCESS
BTS(Target)
CHL_ACT_ACK
HO DETECT
Phy Info
No. of BSC-controlled inter-cell outgoing handover execution times

Handover-related measurement tasks
Handover
causes
measurement
 Measure the frequency of MS handovers caused by various kinds of
reasons, so as to examine radio environment of a cell;
Common
handover
measurement
 Measure the process of MS handover to inspect handover success or
failure and abnormal situations causing failures, so as to improve the
cell’s radio configuration and observe traffic dispersion, etc.;
Measurement
of adjacent
cell handover
 Measure the number of times of incoming/outgoing handover
attempt/success/failure from/to certain cells, and number of times of
handover caused by different reasons, so as to get the handover
situations of the serving cell and its adjacent cells and to optimize their
radio configurations correspondingly;
Sub cell
statistical
measurement
 Focus on traffic load of the second subcell.

Analysis handover problems
 Analysis of handover problems
 Location method of handover problems

Common handover problems
Common handover
problems
Possible influences
Handover
nonoccurrence
• Result in call drop;
Handover failure • Affect call quality and result in call
drop;
Frequent handover • Affect call quality, and increase
system load;
Handover hysteresis • Affect call quality and result in
call drop;

Discovery of handover problems
Traffic statistics
analysis
Customer complaints
DT/CQT tests
Meters at A interface
TOPN analysis
Abnormal number of handover times
Call drop
Bad coverage
Poor speech quality
Handover problem
Handover to best cell
inhibited
Slow handover
No handover
Handover failure
Frequent handover

Flow of handover problem checking
Too high TCH
handover failure rate
of a cell
Any antenna
problems?
Complete
Check &
eliminate
interference
Eliminate
equipment
faults
Solve
antenna
problems
Is radio
parameter setting
reasonable?
Interference
exists?
Any equipment
faults?
No
Yes
Adjust
parameters
Yes
Yes
Coverage
problem exists?
Improve
coverage
Yes

Location methods of handover problems
 Analyze traffic statistics
 Conduct handover statistics measurement, identify
problem range:
 If just some cells fail to make handovers to the cell, check
handover data, check if co-channel and co-BSIC exist;
 If the cell fails to take handovers from all other cells, check its
data.
 Check warnings: single board malfunction,
transmission and clock malfunctions, etc.;
 Check if radio parameters are set reasonably
 If co-channel or co-BSIC exist among adjacent cells;
 If handover parameters are set reasonably;
 If data configuration of external cells is correct.

Location methods of handover problems
 Interference checking
 DT analysis
 Signaling analysis: Um interface、Abis interface 、 A interface;
 Hardware checking: like DCU, transceiver, clock generator, RF
connection lines between boards;
 Antenna system checking

Analysis of handover problems
 Coverage & interference
 Antenna system
 BTS software & hardware
 transmission
 BSC software & hardware
 A interface malfunction
 Busy target cell
 Connection & adaptation to equipment from different suppliers

Coverage & interference
 Coverage:
 Poor coverage: due to influence from forest, complex
landforms, houses, indoor coverage, etc.;
 Isolated site: no adjacent cells around;
 Skip-zone coverage: no adjacent cells available due to
isolated-island effect;
 Interference:
 It makes MS unable to access in UL, or DL signal
receiving problem will be resulted.

Handover nonoccurance due to isolated-island
effect
adjacent cell N1
adjacent cell N2
Adjacent cell N3
Non-adjacent
cell
Non-adjacent
cell
Non-adjacent
cell
Serving cell
Handover can’t
happen due to
lack of adjacent
cells.
Skip-zone
coverage leads to
isolated island.

Antenna system problems
 Too large VSWR
 Reversed installation of antenna
 Non-standard antenna installation
 Unreasonable azimuth, down-tilt
 Below-standard antenna insulation
 Twisted cables, loosened connectors and wrong
connections;

BTS software/hardware
 Problems about :
 Single board
 Clock generator malfunction
 Internal communication cable malfunction
 BTS software malfunction

Transmission and BSC problems
 Transmission fault
 Unstable transmission
 Too high transmission error rate
 BSC hardware/software malfunctions
 Clock generator malfunction: unconformity among clocks in
different BTSs due to clock generator malfunction;
 Problem about single board
 Wrong data configuration
 Unreasonable setting of handover threshold
 CGI, BCCH and BSIC values in “external cell data sheet” do not
match up to those in the corresponding BSC;
 Wrong BSC signaling point in “list of cell under a LAC” in MSC; co-channel&
co-BSIC adjacent cells exist.

A interface malfunction
 A interface malfunction
 Abnormal handover due to lack of link resource, abnormal calls;
 Busy target cell
 Abnormal handover due to lack of link resource, abnormal calls;
 handover between equipment from different suppliers
 Difference in signaling at interface A and interface E between ZTE
and other suppliers’ equipment, causing non-recognition or non-support
problem, including speech version, handover code and
addressing mode (CGI or LAI) etc., which will result in handover
failure.

Typical case 1- frequency interference
 Problem description:
 The data in performance report shows that Cell 1 under
a BTS suffers from low handover success rate.
 Problem analysis
 Examine the problem cell, discover that 2 cells under a
BTS co-channel and co-BSIC, and close to each other,
which results in low handover success rate in the cell.
 Problem handling
 After adjustment of frequency point, handover success
rate obviously increases, and number of handover times
reduces.

Typical case 1- frequency interference
Changes of HO indicators before & after Frequency point adjustment
180
150
120
90
60
30
0
9-4 9-5 9-6 9-7 9-8 9-9 9-10 9-11
Number of HO Req./number of HO success
120%
100%
80%
60%
40%
20%
0%
HO success rate
切No.换 o请f 求HOR总eq次. 数切换成功总次数H切O 换su成cce功ss率(%)
rate
No. of HOsuccess

Typical case 2- clock malfunction
 Problem description
 For a newly-commissioned BTS, handover nonoccurrence appears
during DT: the MS occupies a channel in cell A; during DT from cell
A to cell B, cell B can’t be observed in the adjacent cell list, and it
doesn’t start normal handovers.
 It’s a common network problem that handover nonoccurrence
appears in many cells;
 It’s a newly-commissioned BTS; handover parameters are as
default in the system;
 Check adjacent cells relation, no problem found;
 Observe from test MS, find out that adjacent cell frequency
appears in the adjacent cell, but BSIC can’t be decoded.
 Since adjacent cell is searched through BA2 table during a call, and
BA2 relies on BCCH and BSIC to confirm an adjacent cell, when the
adjacent cell’s BSIC is unobtainable, BSC is unable to locate it, thus
handover won’t be started.

 Process of MS decodeing on DL channel
 decode FCCH decode SCH（SCH comprises MS frame
synchronous information and BSIC.
 MS can show adjacent cell frequency point, but not BSIC. It’s
suspected that adjacent cell’s SCH information can’t be decoded
by MS due to clock or transmission fault.
 Check clock and transmission
 BTS adopts network clock
 BSC traces superior clock
 MSC traces superior GPS clock through long-distance satellite link
 The long-distance satellite link is found unstable, which leads to
high error rate on the meter, and warning of clock deterioration
appears on MSC.

 Problem handling
 Decide that it’s handover problem
caused by poor clock quality.
 Bring new GPS clock device and
adopt the local one, thoroughly
solve clock malfunction.
 Problem of handover
nonoccurrence is solved.
 Experience conclusion
 If no high accuracy clock
available, clock in BTS can be
used; calibration of each BTS
must be made by using
frequency meter and LMT to
ensure that frequency deviation
meets precision requirement.

Typical case 3-HO parameter setting problem
 Problem description
 During DT at a BTS, we find slow handover problem is
common (>10S), which affects speech quality and even
causes call drops.
 Problem: level of cell 2 is higher than that of cell 3 by
20dB, total handover time is 15s.

Typical case 3-HO parameter setting problem
 Problem analysis and handling
 Slow handover seriously affects network quality. Make adjustment of
handover parameters accordingly:
 Change adjacent cell handover threshold to improve timeliness of
handover trigger;
 Adjust the whole network’s handover window to be 2, so as to
accelerate handover speed;
 Adjust the whole network’s handover preprocess to 2, so as to
accelerate handover speed.
Parameter Before
adjustment
After adjustment
Level threshold
(HOMARGINRXLEV)
30 28
Quality threshold
(HOMARGINRXQUAL)
30 26
 Result
 Test after adjustment shows that handover time is reduced to 5s; the slow
handover problem is solved and speech quality is improve.

 Please simply illustrate effects on handover due to
changing T3103、T3107.
 Suggestions on parameter settings of handovers on
highway.

GSM Network Interference &
Solutions
ZTE university

Training goals
 To know the classification of interference;
 To master the analytical methods of interference
problem;
 To master the flow of handling interference problem;
 To know the analytical tool of interference problem;
 To be able to handle common interference problems.

Contents
 GSM Frequency Allocation
 Phenomena & Classification of Interference
 Flow of Handling Interference Problem
 Analytical Methods of Interference Problem
 Typical Cases

GSM Frequency Allocation
Frequenc
y band
UL
frequency
DL
frequency
Duplex
interval
Band
width
Carrier
frequenc
y interval
EGSM+G
SM900
880MHz
~915MHz
925MHz~9
60MHz
45MHz 35MHz 200kHz
DCS1800
1710MHz~1
785MHz
1805MHz~
1880MHz
95MHz 75MHz 200kHz

Phenomena of Interference
Call drop
Unable to
establish calls
On-and-off
speech
Metallic noise
Poor
speech
quality
Phenomena

Classification of Interference
Internal interference
Internal interference refers to unreasonable frequency planning
and equipment hardware faults, which could lead to decrease in
network service quality.
External interference
External interference refers to unknown signal source out of the
network, whose existence could seriously disturb the network’s
signals and lead to decrease in service quality.
UL interference
DL interference

Internal Interference _Causes
Unreasonable frequency planning
Equipment faults
Skip-zone coverage
Internal
interference

Internal Interference
_due to unreasonable frequency planning
 Unreasonable frequency planning :
 Frequency and adjacent cell relation may be set
unreasonable in network planning because of planning
tools or human mistakes .
 Interference will be reflected in too large DL_RxQuality,
MS unable to access into network, poor speech quality,
and call drop.

Internal Interference
_due to unreasonable frequency planning
 Check and confirm problem:
 Use planning tool to check if co-channel exists; co-channel
is easy to be detected if it does exist.
 As for cells in boundary areas, we can block co-channel
cells in the network; meanwhile, make tracing
test with DT devices at areas with emergence of large
DL_RxQuality. If co-channel interference does exist, the
DL_RxQuality value shall become smaller after the
blocking of co-channel cells, thus we can adjust the
cell’s frequencies to eliminate the interference.

Internal Interference _due to skip-zone
coverage
 Interference caused by skip-zone coverage
 If the actual cell coverage greatly exceeds requirement,
interference will be increased.
 Incorrect setting of engineering and network
parameters may lead to skip-zone coverage.

Internal Interference _due to skip-zone
coverage
 Unreasonable setting of engineering parameters:
 Wrong antenna type, down-tilt and azimuth may result
in over large cell coverage, which exceeds actual
coverage need;
 Unreasonable setting of network parameters:
 Network parameters include: minimum access level,
BTS transmission power, MS max transmission power,
handover thresholds, etc..Improper setting of these
parameters will result in skip-zone coverage problem
and interference as well.

Internal Interference _ due to equipment
fault
 Interference caused by equipment fault:
 Radio fault of BTS is mainly caused by defective UL
unit parts.

External Interference
 Definition:
 External interference refers to other interferences caused by
external factors, but not due to equipment fault or unreasonable
frequency planning.
 Common external interferences:
 due to wide-band repeater;
 due to CDMA system (trailing signal);
 due to signal jammer;
 Characteristic:
 It’s hard to detect this kind of interference without
specific devices.

Flow of Handling Interference Problem
Confirm
interference
range
Check
frequency,
change
frequency
points
Complete
Poor speech
quality due
to
interference
Check and
change
TRX
Check
external
interference
One cell
Check
VSWR/antenna/divider/dupl
exer
Interference
exists
One
TRX
Interference
exists
Interference
exists
Any new sites? If thorough change
of frequency parameters taken
recently?
Several
cells

Analytical Methods of Interference
Problem
Analytical
Methods of
Interference
Problem
Statistical
analysis of
network
performance
indicators
Analysis of
parameter
checking
Investigation
of hardware
fault
Drive Test
and Dialing
Test
External
interference
test

Analytical Methods of Interference Problem
- Statistical analysis of network performance
indicators
Statistical analysis of network performance indicators
Statistics of interference band : When TCHs are in idle status, UL
noise/interference is constantly being measured BTS, and the
measurement result will be analyzed, and interference level will be
sent to BSC in 6 levels. 。
Statistics of handover due to UL/DL interference : We can judge
whether interference exists through statistics of handover caused by
UL/DL interference.
Collection of UL/DL RQ samples during speeches : RxQual is an
indicator to reflect speech quality, which is based on error rate and
falls into 8 grades (0～7).

- Statistical analysis of network performance
indicators
 Corresponding relation between RxQual
and Ber

- Analysis of parameter checking
Check
parameters
related to
transmitting
power
Check antenna
engineering
parameters
Check frequency
planning
parameters
Check
parameters
related to skip-zone
coverage
Parameter
checking

- Checking hardware fault
Checking hardware fault
OMCR warning analysis
Checking latent equipment fault

- Checking latent equipment fault
Block the two
input ways of
TRX, observe
UL
interference
band; if it’s 0,
it’s proved
that TRX
hasn’t
brought UL
interference.
Input the two
stimulations
of TRX
without
connecting
them to
power
amplifier,
observe UL
interference
band; if it’s
0, it means
external
interference
doesn’t exist.
If serious UL
interference exists
even though there
is no stimulation
imposed on
power amplifier,
disconnect rack
top feeder cables,
if the interference
disappears, we
can infer that the
problem is caused
by external
factors.
Disconnect the
rack top feeder
cables, and
observe UL
interference
band; if the
interference
isn’t fading at
all, then we can
conclude that
the problem is
with the divider
unit.

- Drive Test and Call Quality Test
 Drive Test and Call Quality Test
 Drive test can effectively detect the location
and degree of interference, which is
convenient for analyzing the cause of
interference.
 In CQT, we can actually feel the speech
quality at areas being interfered, and we can
see call quality class on the test phone.

 DT parameters:
 C/I: co-channel carrier-to-interference ratio
25
20
15
10
5
0
0 1 2 3 4 5 6 7
C/I[dB]
RxQual 0 1 2 3 4 5 6 7
C/I[dB] 23 19 17 15 13 11 8 4

 DT parameters:
 SQI：SPEECH QUALITY INDEX is the comprehensive
description of BER, FER and HANDOVER EVENT by TEMS.

- Test of external interference
 Confirm external interference with
SITEMASTER :
 Test of UL interference;
 Connect the input port of frequency-sweep
generator to the output port of divider to increase
the degree of sensitivity, as shown in the figure.

 Confirm external interference with SITEMASTER :
 persistent strong level exists within the bandwidth of
20MHz, we can conclude that serious UL interference
exists.

 Confirm external interference with YBT250:
 Make UL interference analysis of GSM 900M UL frequency
band with frequency scanning meter-NetTek Analyzer(TEK
company). The model we usually use is YBT250.
 Connection method of YBT250:
 One is to use its own test antenna ;
 One is to obtain interference information through connection to
the output port of divider.

 Connection method using YBT250 to test UL
interference:
Antenna
CDU
YBT 250
Feeder

 Wave graph of UL interference tested by YBT250:
 This output is the average value of the test results of
one minute, which shows the frequency and
strength of interference. Persistent observation is
needed to confirm if the interference continues.

 Time scatter graph of UL interference tested by YBT250:
 TEK frequency scanning meter features in three
dimensional recording of time, frequency and signal.The
vertical bold red lines in the graph represent the time
duration, signal level strength and frequency .
vertical
axis=time
Colour
spectrum
=strengt
h
horizontal
axis=frequency

Typical case 1: Problem description
Since March 2005, an operator has received a lot of
complaints about poor speech quality; sometimes calls
even couldn’t be setup; the caller could hear the
counterpart, but could not be heard.

Typical case 1: Problem analysis
At the
beginning we
thought it was
caused by
poor signal.
After on-site
test, we found
it wasn’t
coverage
problem.
When the level
tested by MS was
-85dbm, UL call
problem
occurred, which
was displayed as
on-and-off
speech, silence,
metallic noise
and current noise,
so we concluded
that the problem
was caused by
interference.
Performanc
e statistics
at OMCR
showed that
the rank of
idle channel
interference
band was
high.
Confirmed the
problem was
caused by
interference

Typical case 1: Problem handling process—
STEP1
Test UL interference with YBT250 connected to CDU. CDMA wave
form was strong when wave filter wasn’t used, the peak value reached
about -35dbm (average about -60dbm), which was close to GSM UL
wave band and could cause UL interference to GSM network.

STEP1
In the three dimensional graph of interference tested by YBT250, the
CDMA wave form was strong and the wave form of GSM background
noise on the right was high in a long period of time.

STEP2
Use CDMA wave filter to eliminate CDMA
interference.
Antenna
Common
CDU
YBT 250
Feeder
CDMA wave
filter

STEP2
When CDMA wave filter was adopted, CDMA wave
form was obviously weakened, but it was still strong at
some certain point; the background noise in GSM
frequency band was also reduced.

STEP2
Because of CDMA wave filter, the UL interference in GSM
frequency band reduced greatly.

STEP3
With the aim to eliminate CDMA interference, adopt IRCDU
+CDMA wave filter.
CDMA wave Antenna
filter
YBT 250
IR CDU

STEP3
Adoption of IRCDU＋CDMA wave filter can effectively
filter CDMA waves to below -104dbm. This kind of filtering
effect can help completely avoid CDMA network interfering
GSM UL network.

STEP3
Adoption of IRCDU＋CDMA wave filter can eliminate
CDMA wave form to a great extent; during the test period,
CDMA interference was almost eliminated.

Typical case 1: Summary
The interference source was from CDMA system.
Through comparisons of tests above, we can see after
IRCDU+CDMA wave filter was used, call quality
obviously improved.

 How is interference resulted from
wrong setting of transmitting power-related
parameters?
 What is the flow of checking external
interference?

SDCCH Assignment Analysis
& Solutions
Zte university

Contents
 Overview
 Analysis of signaling and counters related to
immediate assignment
 Radio parameters
 Instructions on checking of SDCCH assignment
failure
 Typical cases on SDCCH assignment

Definition of SDCCH
 SDCCH: the Standalone Dedicated Control
Channel is used to transmit information like
channel assignment, which falls into the following
two types:
 SDCCH/8: the standalone dedicated control channel;
 SDCCH/4: the SDCCH that is combined with CCCH.
 In brief, the following processes shall be taken into
consideration in the process of occupying SDCCH:
 Location update, periodical location update;
 IMSI attach/detach
 Call setup
 SMS

Signaling flow of immediate assignment

Counters related to SDCCH assignment &
corresponding signaling messages V3
C900060242
Number of
SDCCH
assignment
success
 Function:
 After BSC sends out the immediate assignment message (IMM_ASS),
this counter counts the number of successful MS accesses to the
corresponding SDCCH.
 Sampling:
 when BSC receives the correct EST_IND or the message of assignment
complete.
C900060243
Number of
SDCCH
assignment
failure
 Function:
 After BSC sends out the immediate assignment message (IMM_ASS),
this counter counts the number of failed MS accesses to the allocated
SDCCH.
 Sampling:
 when BSC receives the wrong EST_IND, or when T3101 expires.

SDCCH assignment success rate
KPI SDCCH assignment success rate
Definition Number of successful SDCCH assignments*100/(Number of successful
SDCCH assignments + Number of failed SDCCH assignments)
Counter
formula
V2 C11644*100%/( C11644+ C11645)
V3 V6.2
C900060242*100%/(C900060242+C900060243)

Difference: Random access success rate
 Definition: Number of successful random
accesses / Number of random access
requests*100%
 Number of random access requests
 Definition: MS applies for a channel in the idle mode.
 Trigger point: it counts the message of CHANNEL
REQUIRED received by BSC from MS. (A1)
 Number of successful random accesses
 Definition: BSC successfully assigns a dedicated
channel for MS.
 Trigger point: it counts the message of IMMEDLATE
ASSIGNMENT sent from BSC to MS. (A2)

Analysis of Channel Request cause
 Establishment Cause

 Establishment Cause (continued)

 Summary on Establishment Cause
 Emergency call
 Call re-establishment
 Paging response（MTC）
 Mobile originating call（MOC）
 Location update （LOC）
 Other access causes
 One-step access
 LMU service
 MBMS service

Channel Required
 Request Reference
 RA（Random access reference）: it continues to use the Cause and
Random Reference in the Channel Request.
 Byte 3 and 4 (T1, T2, T3): receive the frame number(42432) of the
burst pulse.

Channel Required
 Access Delay
The estimated TA
 Physical Context
including Rxlev_UL

Immediate Assignment
Page Mode = same as before
Packet Response Type and Dedicated mode or TBF
Downlink assignment to mobile in Ready state: no meaning
TBF or dedicated mode: this message assigns a dedicated mode resource
PR Type: immediate assignment procedure for RR connection establishment
Channel Description
Type = SDCCH/8[0]
Timeslot Number: 1
Training Sequence Code: 0h
ARFCN: 104
Request Reference
Random Access:
Establish Cause: E0h = Originating call and TCH/F is needed, or originating call
and the network does not set NECI bit to 1
Random Reference: 12h
N32: 13h; N51: 1Fh; N26: 0Dh
Timing Advance: 1 = 0,6 km
Mobile allocation

Establish Indication
T represents the sub-channel number.

 Information on layer3:
 CM SERVICE REQUEST
 LOCATION UPDATING REQUEST
 IMSI DETACH
 PAGING RESPONSE
 CM RE-ESTABLISHMENT REQUEST
 NOTIFICATION RESPONSE
 IMMEDIATE SETUP
 RR INITIALISATION REQUEST

 CM SERVICE REQUEST
 Originate call
 Emergency call (Access statistics show that emergency
call is not included in MOC )
 SMS
 Supplementary service
 Group call establishment
 Voice broadcast call

Access counters
 Basic measurement
Counter Number Counter name
C900060001 Number of MTC access requests
C900060002 Number of MTC access successes
C900060131 Number of CM SERVICE REQ of MOC
C900060136 Number of MOC access requests
C900060137 Number of accesses due to paging response
C900060236 Number of MOC access successes

Access counters
 Radio access measurement (I)
Counter Number Counter name
C901110001 Number of invalid access requests
C901110003 Number of successful process for MOC access
C901110006 Number of successful process for MTC access
C901110008 Number of call re-establishment access requests
C901110009
Number of successful process for call re-establishment
access
C901110010 Number of call re-establishment access success
C901110011 Number of emergency call access requests
C901110012
Number of successful process for emergency call
access
C901110013 Number of emergency call access success
C901110014 Number of LOC access requests
C901110015 Number of successful process for LOC access
C901110016 Number of LOC access success
C901110017 Number of access requests due to other causes
C901110018
Number of successful process for other causes’
access
C901110019 Number of access success of other causes

Access counters
 Radio access measurement (II)
C901110020 Number of LMU Establishment access requests
C901110021
Number of successful process for LMU Establishment
access
C901110022 Number of LMU Establishment access success
C901110023 Number of accesses due to location update
C901110024 Number of accesses due to CM SERVICE REQ
C901110026
Number of Emergency Call (CM SERVICE REQ)
accesses
C901110027 Number of SMS (CM SERVICE REQ ) accesses
C901110028
Number of supplementary service (CM SERVICE REQ)
accesses
C901110029
Number of accesses for LCS (CM SERVICE REQ )
accesses
C901110031 Number of accesses due to call re-establishment
C901110032 Number of accesses due to IMSI de-activation
C901110033 Number of accesses due to other causes

TxInteger
 Before response to the previous “channel request”
is received, MS waits for a period of time at
random and sends the request again after
expiration. TxInteger is to decide the random
waiting time.
 The interval (number of timeslots) from MS originating
the immediate assignment to the transmission of the
first “channel request” message is a random number
among { 0,1,…,Max（T,8）-1 }.
 The interval (number of timeslots) between two
consecutive “channel request” is a random number
among {S,S+1,…,S+T-1}.

TxInteger
T(Number of
timeslots
Of TxInteger)
S
(CCCH is NOT
combined with
SDCCH)
S
(CCCH is
combined with
SDCCH)
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
TxInteger Number of
timeslots (T)
0 3
1 4
2 5
3 6
4 7
5 8
6 9
7 10
8 11
9 12
10 14
11 16
12 20
13 25
14 32
15 50

MaxRetrans
 Because RACH is a ALOHA channel, in order to
improve MS access success rate, the network
allows MS to send several Channel Request
messages before it receives the Immediate Assign
message. The max number of Channel Requests
sent by MS is decided by MaxRetrans.
MaxRetrans Max number of retransmission
0 1
1 2
2 4
3 7

TaAllowed
 It represents the max TA allowed for access to the
cell.
 It is used to filter out fake accesses.

RachAccessMin
 New parameter for iBSC 6.20.100e
 Used to filter out fake access, but not
recommended because it will affect the paging
performance.

Contents
 Overview
 Analysis of signaling and counters related to
immediate assignment
 Radio parameters
 Instructions on checking of SDCCH
assignment failure
 Typical cases on SDCCH assignment

Explanation on common causes of SDCCH
assignment failure
 MS frequently originates location update due to
poor downlink quality;
 Improper setting of Tx-Integer;
 High SD assignment failure rate due to LAPD
delay
 Co-channel/co-BSIC interference
 Uplink interference
 Overshooting

Improper setting of Tx-Integer
 The default of Tx-Integer is 14, which is also the
max value.
 Usually, the one-way signaling transmission delay
at Abis interface is 60ms~100ms; there should be
a delay of about 240ms from MS originates
Channel Request till it receives Immediate Assign.
 When the transmission link delay is long, while
TxInteger is set with a small value, it will result in
MS sending too many access requests. However,
MS only responds to the first Immediate Assign it
receives.

Improper setting of Tx-Integer
 Flow chart of repeated assignment failure
Channel Request
Channel Required
M S
Channel Active
Channel Active Ack
Imm Assign(OK)
Imm Assign Cmd
B T S
Channel Request(Re-Send）
TxInteger
Lapd
Delay
Channel Required
Channel Active
Channel Active Ack
Imm Assign Cmd
Imm Assign(Fail)
MS change
to SDCCH
B S C

LAPD delay
 Possible causes of LAPD delay
 Application of LAPD 1:4 multiplexing will lead to the situation that
several BCCH TRXs are multiplexed on one LAPD, which will
cause heavy flow on the LAPD and hence delay.
 Heavy flow on LAPD leads to delay. For example, improper LAC
division will lead to large amount of paging and hence LAPD flow
control.
 Transmission equipment fault leads to loss of messages on LAPD
or long LAPD delay. These phenomena are often accompanied
with SDCCH assignment failure.
 The transmission equipment’s own delay, such as the delay
caused by satellite transmission at Abis interface.
 Impact of PS service: PS service is more sensitive to network delay.
Any LAPD delay will leads to re-transmission of PS service
message, which increases the flow on LAPD and causes longer
LAPD delay, then a malicious circle will be resulted.

Co-channel & co-BSIC
 Two cells have same BCCH and same BSIC
 The Channel Request sent by MS is received by two
cells and they assign SDCCH at the same time, but MS
can only accept one SDCCH, therefore, one of the two
cells will inevitably experience SDCCH assignment
failure.
 For RACH coding, first add in 6bit color code, which is
obtained through taking mod2 of 6bit BSIC and 6bit
parity checking code. Therefore, co-BCCH and co-BSIC
may cause the BTS to incorrectly decode MS access
bursts to other sites, which will lead to SDCCH
assignment failure

Co-channel & co-BSIC
 Two cells have same BSIC and the TCH Arfcn of one cell
is same as the BCCH Arfcn in the other cell.
 The handover access request occurring on the TCH timeslot is
received as Channel Request by the other cell, which thereafter
performs assignment. This certainly leads to SDCCH assignment
failure.
 It’s stipulated in protocols that the MS-started handover access
information and the random access request share the same format,
which is AB frame; the difference is that the handover access
information content (RA) in one handover started by MS is the
same, and the FN is in consecution.
 Signaling related to this problem displays that the RA is the same,
TA is in consistence and FN in consecution. It’s confirmed that all
the large amount and consecutive Channel Requests are fake
accesses caused by handovers between co-channel cells.

Overshooting
 If the coverage of cell is too large, the DL Rxqual at the cell
margin will be poor. In this case, BTS can receive Channel
Request sent by MS, but MS can not receive Immediate
Assign sent by BTS, for BTS is more sensitive than MS,
 If the coverage of cell is too large, the cell may share
channel and BSIC with the cell which is far away.
 Solution to overshooting:
 Adjust the engineering parameters of antenna to limit the cell
coverage.
 TA_allowed can effectively decrease SDCCH assignment failures
caused by overshooting. The side effect it brings is that the distant
MS is not able to access network. Therefore, the threshold of
TA_allowed shall be set a bit higher than the cell’s actual coverage.
Besides, we should take into account the transmission distance of
repeater when calculating the cell coverage range.

Uplink Interference --- Fake Access
 BTS receiving sensitivity is -112dbm~-125dbm. If the random access signal
strength received by BTS is lower than BTS sensitivity, it usually is confirmed to
be interference. The interference can be decoded as random access, which is
called as fake access, and will definitely lead to SDCCH assignment failure.
 Another feature of fake access is that TA is larger than that needed for the actual
coverage range.
 Solution: TA_allowed
Note:
① RachAccessMin is not recommended to use
② As for TA-allowed, the corresponding name used by Nortel is RNDACCTIMADVTHRESHOLD,
whose description is as follows: adjust the parameter according to the cell’s actual coverage range.
Fake RACH request can be filtered out through setting proper threshold, therefore unnecessary
SDCCH assignment can be avoided. Test results prove that if TA-allowed is set 35Km for cells with
small coverage radius, fake RACH (the system demodulate the noise into RACH pulse by mistake)
accounts for almost 30% of all RACH requests. After rndAccTimAdvThreshold is changed to 2, fake
RACH is totally filtered out.

Frequent location update started by MS
 If MS needs to make location update, while the
radio environment is poor, it will retransmit
Channel Request with the cause of location
update again and again, but it can never receive
Immediate Assign message.
 The frequent location update will cause
fluctuations in SDCCH assignment indicators.

Frequent location update started by MS
Number of SDCCH
assignment
successes
Number of SDCCH
assignment failures
SDCCH assignment
success rate
Number of
MOC access
requests
Number of
MOC access
successes
Number of
MTC access
requests
Number of
MTC access
successes
Number of
SDCCH
occupation
attempts (for
assignment)
(MOC+MT
C)
assignment
success rate
(MOC+MT
C)
proportion
Reference
indicators

Troubleshooting instructions
 Check TxInteger of the problem cell, along with LAPD
delay observed from signaling.
 Check whether the LAPD link of BCCH TRX in the problem
cell is multiplexed with that of other cells.
 Check whether any of the adjacent cells have same Arfcn
and BSIC with the problem cell.
 Check whether the value of counter “number of access
attempts due to other causes” is big. If so, and the counter
“number of access successes due to other causes” is zero,
it is possible that “handover access” on other TCH TRXs
are decoded as “channel request” by the problem cell.
 Error Report with Channel Number 0x88 is available in the
mplog file.

Troubleshooting instructions
 Check SDCCH allocation KPIs and transmission
alarms.
 If SDCCH &TCH assignment indicators are all bad,
the problem shall be related to radio environment.
 Analyze signaling and check if Channel Request
with large TA, if so, fake access exist and
TA_allowed restriction can be used.

LAPD delay—Case 1: Large amount of
paging
 Problem description: It’s found through performance
analysis that ZTE BSC3 has low SD assignment success
rate, which is only about 60% on late busy hours.
 Problem analysis:
 It’s observed that all the cells are experiencing high SD assignment
failure rate, so impact from radio parameters is excluded.
 Indicators of other BSCs are normal; the SD assignment success
rate is low in only BSC3 and the Siemens BSC, both of which are
under MSC7.
 The paging success rate in MSC7 is also very low; as the traffic
volume increases, the amount of paging increases as well.

LAPD delay—Case 1: Large amount of
paging
 Adjustment measure:
 Add one LAC under MSC7. After the adjustment, the SD assignment
success rate of BSC3 returns to normal, reaching above 95％.
100%
90%
80%
70%
60%
50%
100000
80000
60000
40000
20000
0
BSC3 SDCCH指配成功率对比
3月10日3月11日3月12日3月13日3月14日3月15日
SDCCH指配成功次数SDCCH指配失败次数SD指配成功率

LAPD delay—Case 2: Satellite transmission
 Problem description: 4BTSs are under BSC01, but
belong to different peripheral modules. The SD
assignment failure rate of the 4BTSs reaches as
high as 50%.
 The time stamp shows that it takes an average of
0.58s to successfully activate a channel.

LAPD delay—Case 2: Satellite transmission
 How to confirm that two Channel Requests are
started by the same call attempt?
 They should have the same Establish Cause;
 The same Access Delay;
 The frame number interval corresponds to the setting of
TxInteger:
 Calculation formula: FN＝T1*26*51+((T3-T2)mod 26)*51+T3

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