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)

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.

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:

Table : Relations among CBQ, CBA, cell selection priority and cell reselection condition

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:

Table Static power class

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