TCH occupation signaling
1.1
TCH occupation signaling
MSC will send Assignment
Request signaling to BSC after it confirms MS application for TCH.
Upon receiving “Assignment
Request” from MSC, BSC will search for suitable TCHs. If no usable TCHs are
available, BSC will send a “Assignment Failure” message to MSC with the cause
of no radio resource available.
1.2
Definition
of TCH congestion indicator
TCH congestion = TCH congestion
times*100%/ TCH call attempts
Causes of radio network congestion
Main
causes for channel congestion are as follows:
·
High traffic
density, which even exceeds the designed capacity of BTS;
·
Equipment
hardware problem, like lack of usable resources or channel congestion caused by
unstable equipment performance;
·
Problems with
adjacent cells;
·
Unreasonable LAC
planning: if LAC boundary is set at high traffic areas or main transportation
ways, where subscribers are in great number and in frequent movement, LAC
renewal can be very frequent, which will form unreasonable calling modes and
lower system capacity as well;
·
Unreasonable
setting of radio parameters: such as delay of cell reselection, handover
margin, level of outgoing handover trigger, etc., unreasonable setting of these
parameters can result in Ping pong location renewal and Ping pong handover;
·
Burst of high
traffic volume can happen in some areas (such as schools, playgrounds) with
special traffic distribution modes, which exceeds the designed system capacity;
·
Too large
coverage can cause isolated-island effect.
Problem handling procedures
It’s suggested to locate the
problems through checking radio parameters and equipment hardware.
Handling procedures for TCH
congestion are as follows:
1.
Check if the
problem cell and its adjacent cells operate normally, check TCH usability to
locate the unstable equipment. If adjacent cells work abnormally, the problem
cell will have to take part of their traffic besides its own load;
2.
Check MS mobility
to see if the TCH congestion is caused by excess incoming handovers. It it’s
true, we can optimize handover parameters (increase HO Margin) to reduce number
of handovers from adjacent cells to the congested cell, so as to ease the cell
from congestion;
3.
Check setting of
radio parameters: such as delay of cell reselection, handover tolerance limit,
level of outgoing handover trigger, etc., unreasonable setting of these
parameters can result in Pingpong location renewal and Ping pong handover;
4.
Through test of
field strength, analyze if coverage is too large and if isolated-island effect
exists. When isolated-island effect happens to one cell in an area, where
predefined adjacent cells can not be detected, MS will constantly stay with the
serving cell; and normal handovers can not be triggered, in spite of any
changes on signals, and finally call drops will be resulted. To avoid this
case, two methods can be adopted: (1) adjust the antenna of the isolated cell
to eliminate the effect. However, due to the complexity in electric wave
transmission, it takes several tests to abate the effect, and it’s really
difficult to totally eliminate the effect due to high buildings. (2) define new
adjacent cells for the isolated cell. The principle for defining related
parameters is: handovers/LAC renewal from the isolated cell to normal cells has
priority over the reversed ones.
5.
Congestion due to
high traffic density: check if the BTS capacity configuration reaches the max.
If not, expand it with enough TRXs.
Common solutions to TCH congestion
Common solutions to TCH
congestion comprise:
·
Adopt traffic
control in the congested cell, so as to balance traffic load;
·
Open HR, increase
system capacity;
·
Expand TRXs or
split cells, so as to increase sites and increase system capacity.
1.3
Common
methods for controlling traffic volume
1.
Control cell
selection parameters;
2.
Control cell
reselection parameters;
3.
Handovers based
on layered cells;
4.
Control a cell’s
real coverage.
1.3.1
Cell selection parameters
C1 is applied as standard
when MS is selecting cell. It will choose the cell with largest C1 value. According to GSM regulations:
C1=(RXLEV-RXLEV_ACCESS_MIN)-Max(MS_TXPWR_CCH-P,0)
C1=(RXLEV-RXLEV_ACCESS_MIN)-Max(MS_TXPWR_CCH-P,0)
RXLEV: level of MS receive signal;
P: the max receive power of MS;
ACCESS-MIN: the minimum receive level for MS access:
MS-TXPWR-CCH: the allowed max transmitting power for MS access into BCCH; C1 reflects the condition of MS receive level (good/bad), whose value won’t be influenced by network deployment mode.
ACCESS-MIN: the minimum receive level for MS access:
MS-TXPWR-CCH: the allowed max transmitting power for MS access into BCCH; C1 reflects the condition of MS receive level (good/bad), whose value won’t be influenced by network deployment mode.
Usually, priority of all
cells should be set “Normal”, i.e. CBQ=0. In some cases, like microcell
application, dual-band network, multi-layer network, etc., operators may
favorably want MS to access into certain type of cells, we can set priority of
these cells as “Normal” and that of other cells as “Low”, or in some high
traffic areas we can set cells’ priority as “Low” to reduce their load. CBQ has
no influence on selection but cell reselection. CBQ and C2 should be used
coordinately in optimization. In order to make dual-band cell phones access
into 1800M system,
we can set CBQ and CBA values to make a difference in priority of DCS1800 and
GSM900 networks, so that 1800M
network will be chosen preferably (cell’s priority won’t affect cell
reselection). The relations among CBQ, CBA, cell selection priority and cell
reselection condition are shown below:
CellBarQualify
|
CellBarAccess
|
Cell selection priority
|
Cell reselection condition
|
0
|
0
|
Normal
|
Normal
|
0
|
1
|
Barred
|
Barred
|
1
|
0
|
Low
|
Normal
|
1
|
1
|
Low
|
Normal
|
In order to make MS choose 1800M network, we can set 1800M cell with Normal priority, its CBQ=0,
CBA=0; set 900M cell
with Low priority, its CBQ = 1, CBA = 0.
1.3.2
Cell reselection parameters
In accordance to GSM
standards, when cell selection is to be carried out, MS will order adjacent
cells according to their C2 values and check which one fulfills the conditions
for MS residing in the cell; if conditions are fulfilled, MS will reside in the
cell. Cell reselection is based on its algorithm C2, which is shown bellow:
·
C2 = C1 + CRO – TO ´ H(PT – T), when PT ¹ 31,
·
C2 = C1 – CRO, when PT=31;
CRO = CELL_RESELECT_OFFSET;
TO = TEMPORARY_OFFSET;
PT = PENALTY_TIME.
According to C2 standard, in
order to reduce cell reselection in dual-band network, we can set CRO of
DCS1800 cell a large value to make C2 in
DCS1800 larger than that in GSM 900, so as to keep MS residing in DCS1800
cells. During cell reselection, if we need some idle cells to share some
traffic load with those with high traffic volume, we can increase their CRO;
conversely, when some cells suffer from high congestion rate, we can set PT=31,
reduce value of C2 in the
serving cell, thus “push” away some traffic volume and reduce TCH load. We must
note that CRO can not be set over 20dB.
Example:
Suppose an area is covered by
two cells simultaneously (GSM900 cell and DCS1800 cell), and the two cells’
access priority is the same, CRO of DCS1800 cell=20, CRO of GSM900 cell=0, PT
and TO of the two cells are 0, strength of MS receiving signal from GSM900 cell
is -68dBm, that from DCS1800 is -78dBm, and their minimum access level is the
same, -104dBm. Then C1(900)=-68-(-104)=36, C1(1800)=-78-(-104)=26. MS selects GSM900 cell when it’s
powered on. After a while, in cell reselection, MS will resides in DCS1800
cell, because C2(900)=-68-(-104)+0-0=36,
C2(1800)=-78-(-104)+20-0=46.
1.3.3
Handover based on layers
From the perspective of
multi-layered cells, effective traffic control and traffic balance can also be
realized through planning layers and setting relevant parameters in dual-band
network. Among the current ZTE system equipment, the layer-related and most
commonly used handover algorithms comprise PBGT handover, traffic handover,
macro-micro handover. Traffic control in dual-band network can be reached
through these handover algorithms, which are simply described as follows:
l PBGT handover
Through setting PBGTHoLayer
and NCellLayer, we can control whether the handover can be carried out among
undefined layer, same layer different frequency band, upper layer, and lower
layer, thus we can reach flexible control over traffic distribution. For
specific parameters, please refer to relevant technical guidebooks.
l Traffic handover
Through setting parameters: layer
priority-TrafficHoLayrCtl (same layer, upper layer, lower layer), frequency
band TrafficHoFreqCtl and NCellLayer, we can contol the layer and frequency
band for target cell of traffic handover, and traffic distribution can be
controlled flexibly as well.
Settings for relevant
parameters:
u Open traffic handover;
u Traffic handover threshold can be
set70;
u Level threshold for traffic handover (TrafficLevThs)can be set
0dB;
u Frequency control value (TrafficHoFreqCtl) can be set 0.
l Macro-micro handover
Macro-micro handover is to
handover the MS moving with slow speed from macro cell layer to micro cell
layer. The micro cell mentioned here is just a concept in logic. In this
example, DCS1800 cell can be regarded as micro cell, and the macro-micro handover
can only be carried out to adjacent cells on lower layer.
Relevant parameters:
u Set layer relations and set DCS1800
cell layer “Lower”;
u Open macro-micro handover function;
u Macro-micro handover threshold (MacroMicroHoThs) can be: -90~-80dBm
Counter for Macro-micro
handover threshold(MacroMicroHoN): 2~4
1.3.4
Control of cell coverage
The main reason for some
cells suffering from congestion is unreasonable planning or non-standard
installation work, which causes long coverage and large serving area to cells
and makes the cells absorb too much traffic volume, thus cell congestion is
inevitably formed.
Common methods for locating
cells with congestion due to over coverage are as follows:
·
Evaluate cell coverage through DT, analyze and
find out if over coverage exists;
·
From TA distribution report at OMCR, get the
distribution of the cell’s main traffic TA; combining planning data, analyze
and find out that over coverage exists.
There are two main methods
for controlling cell coverage and eliminating over coverage problem.
·
Adjust antenna down-tilt and antenna height;
As for antenna
down-tilt, it’s 6-10°in dense urban area, 4-6°in urban area, 2-6°in suburb,
0-4°in villages. When adjusting antenna down-tilt, we must take into
consideration factors like the distance to neighboring cells, landforms. If
it’s necessary, we can also use DT to get the down-tilt for best coverage.
·
Adjust TRX static output power
Usually adjustment of
TRX static output power can help achieve coverage control, but in order not to
affect indoor coverage, it’s recommended that this method be applied only after
adjustment in antenna fails to solve the problem completely. Note that power
class of all TRXs in the cell must be adjusted to be unanimous during
adjustment of TRX static output power, or UL-DL unbalance will be resulted.
Currently, TRX static
power class can be adjusted at OMCR. Its 7 classes are listed in Table 5-2:
static
power class
|
Actual maximum output power
|
static RF power step
|
Pn
|
0
|
Maximum output
power
|
1
|
Maximum output
power – 2dB
|
2
|
Maximum output
power – 4dB
|
3
|
Maximum output
power – 6dB
|
4
|
Maximum output
power – 8dB
|
5
|
Maximum output
power – 10dB
|
6
|
Maximum output
power – 12dB
|
1.3.5
Principles of network expansion
u Principles for GSM900/1800 TRX expansion
First we need to calculate
and obtain GSM900/1800 frequency resource and the max configuration plan, which
can be reached basing on frequency planning scheme. Compare the max
configuration with that required by actual traffic need to see if the max
traffic can be fulfilled.
l When the actual configuration need is under the max
configuration plan, we can consider carrying out expansion.
l When the actual configuration need is beyond the plan,
we can consider carrying out cell split or adding new sites.
u Conditions for cell split
Cell-split is aimed at
macro-cells;
Macro-cells of single
frequency band;
In
the BTS, which the macro-cell belongs to, there is only one busy cell; the cell
can be split;
Pay attention to adjustment
of antenna parameters during cell-split.
u Conditions for setting up new GSM900 BTS
If the traffic need still can
not be satisfied when the TRX is expanded to the max allowed, new BTSs need to
be set up;
The average distance between
the BTS and those around >400m ,
and number of TRX configured in the BTSs around doesn’t reach the max allowed
for GSM900, in this case, new GSM900 BTSs can be set up.
u Conditions for setting up GSM1800 BTS
If the traffic need still can
not be satisfied when the TRX is expanded to the max allowed, new BTSs need to
be set up;
The average distance between
the BTS and those around >400m ;
and number of TRX configured in the BTSs around has reached the max allowed for
GSM900, new GSM900 BTSs would make the frequency interference out of control.
In this case, we can set up new GSM1800 BTSs, and make them co-site with those
of GSM900 to absorb some traffic.