CELL LOAD SHARING FEATURE AND TRAFFIC OPTIMIZATION CELL LOAD SHARING FEATURE AND TRAFFIC OPTIMIZATION

IN GSM NETWORKIN GSM NETWORK

Vedran Novak

Faculty of Traffic and Transport Engineering, Belgrade

Mentor: Prof. dr Vladanka A}imovi}-Raspopovi}

I INTRODUCTIONI INTRODUCTION

One of the most important features of the cellular telephony systems is constant and rapid growth of the number of subscribers. This process may pose serious problems if growth of mobile subscribers has not been followed by the network capacity growth.

Cellular network can be described as an Erlang’s loss system and allowed congestion level (as a percentage) in Erlang’s loss system is defined by the Grade of Service (GoS) [4, 5]. At the same time, offered traffic in the cellular network is unevenly distributed both in temporal and spatial domain and in spite of this, all network cells must be dimensioned for the case of heavy load, although the average load is significantly lower. With a mechanism to cut the high load peaks, the network can be dimensioned for a higher average load.

Ericsson, the manufacturer of equipment installed in both Serbian cellular networks, 064 Mobilna Telefonija Srbije and 063 Mobtel, offers one possible solution to the problem of traffic congestion [1]. This solution is based on Cell Load Sharing (CLS) feature of Base Station Controller’s (BSC) software. This feature is designed to redistribute traffic load between neighboring cells, but implemented on a larger scale (i.e. all cells in the zone of control) it is possible that its operation could have some effect at traffic congestion. CLS is an optional part of software (operators buy this feature separately from basic software) and it can be turned on or off.

This paper presents an idea (a set of procedures) that could evaluate the effect of CLS feature’s operation at congestion level.

II SHORT TECHNICAL DESCRIPTION OF THE II SHORT TECHNICAL DESCRIPTION OF THE FEATUREFEATURE

Cell load sharing is limited to traffic channels (TCH) in any mode i.e. speech/data or signaling [2, 3]. The logic of the feature CLS is an integrated part of the locating algorithm and consists of the following activities.

• The traffic load in all cells where load sharing is monitored; the load level determines the further activities,

• If a cell has too high load, connections close to a cell border are made to perform handover by recalculating their ranking value in locating,

• The handovers are carried out only if the receiving cells have a low enough load.

The load in every cell is monitored in the BSC. The measure of the load is the amount of idle traffic channels. Two levels (parameters) are relevant for Cell load sharing:

• If the amount of idle traffic channels is equal or decreases below CLSLEVEL (the parameter value is given as the percentage of idle traffic channels in the cell) in a cell, that cell tries to rid itself of some traffic by initiating load sharing handover to neighboring cells,

• If the amount of idle traffic channels is above load CLSACC (also given as a percentage) in a cell that cell is prepared to accept incoming load sharing handovers from other cells.

When the amount of idle traffic channels decreases below CLSLEVEL, new ranking calculations are performed in locating for all connections in a cell. In the recalculations, reduced hysteresis values (KHYST, TRHYST and LHYST) are used. Hysteresis is used to decrease the ranking values for neighboring cells which become somewhat underrated in comparison to the serving cell and in that way ping-pong effect is prevented. If a better neighbour cell is found for any of the connection as a result of this new cell ranking, a load sharing handover is requested for that connection. Successive locating recalculations are done with linear ramping down of the hysteresis with a percentage that is given by the parameter RHYST [3]. The ramping down is performed during a time defined by the parameter CLSRAMP, or until the amount of idle traffic channels increases above CLSLEVEL. The actual hysteresis given at each time is given by

( )

−

−=

CLSRAMP

ttRHYSTHh 0

10021 (dB) (1)

The purpose of ramping down the hysteresis is twofold:

• The mobiles closest to the handover border are selected first,

• The mobiles selected for handover are few at a time; too many lload sharing handovers at the same time might otherwise cause instabilities.

The gain in terms of capacity that can be expected when using the feature CLS depends on how many subscribers that will be situated in the region where they can be subject for the evaluations, i.e. the region defined by the locating hysteresis parameters KHYST (and/or LHYST and TRHYST) and the hysteresis reduction parameter RHYST. After the CLS had been activated, RHYST decreases H value according to Eq.1 i.e. handover margin of the congested cell is “pushed” back towards the actual Base Transciever Station (BTS) site. CLSLEVEL, CLSACC and RHYST represent the feature’s main controlling parameters.

IIIII HYPOTHETICAL SITUATIONI HYPOTHETICAL SITUATION

All the cells belong to the same BSC. Cell shape is approximated for easier calculation of the area of the load sharing region. The author presumed that the central zone is congested (depicted by lighter shade of gray on Figure 1.) while the rest of the cells can accept extra load generated by the load sharing handover. Parameter settings are GoS=2%, RHYST=75% and CLSRAMP=0. Different cells are identified by the BTS sites (site D serves cells D1, D2 and D3). All the cells have 3 Transciever Units (TRU) installed, except cells E2, G1 and D3 that have 2 TRUs.

Observed zone encompasses different traffic density zones .

Figure 1.Figure 1. Hypothetical zone of control

Offered traffic per subscriber is set at 15 mE while the traffic density is tabulated in Table 1. Sites A, B, C, D, E and G are 1.5 km apart from each other except sites F, H and I that are 3 km apart from sites D, E and G.

Table 1.Table 1. The traffic density at different areas of the zone of control

Network cells Sites A, B and C

Cells D3, E2 and G1

Remaining cells

Estimated traffic density

23 E/km2 11 E/km2 5 E/km2

Number of TCH channels per cell

22 14 22

Evaluation method consists of two procedures (Procedure A and Procedure B); each having number of steps. The procedures are based on modified methods of the Transport Problem of the Linear programming.

PROCEDURE APROCEDURE A

STEP 1

During STEP 1, one should calculate the traffic offered to each neighbouring cell in the zone of location – TLI ; the maximum load that uncongested cell can accept before being congested itself – TACCLI ; and the load offered for load sharing handover by congested cell – THLI .

STEP 2

During STEP 2 one should form, tabulate and input initial values into “The Load Sharing Table” (LS Table – Table 2.) in the following manner:

• Column 1 – congested cell markings,

• Column 2 – load THLI – number of entries for each row (congested cell) depends on number of uncongested neighbouring cells,

• Columns 3 through 8 – uncongested cell markings and load TACCLI ; and

• Column 9 – traffic load in congested cell after load sharing had been performed.

Number of columns through 3 to 8 is defined by the congested cell having the most uncongested neighbours while number of rows is defined by number of congested cells.

STEP 3:

After the LS Table had been initially tabulated, traffic load redistribution takes place through following steps: C3-G1, A2-D3, B1-E2, C2-I3, A1-H1, B3-F1, B2-G3, C1-D2, A3-E1, A1-E1, B3-G3 and C2-D2.

Some basic rules were laid to define ground for load sharing:

1. One should observe cells that can hand over its load to a single cell and these receiving cells accept incoming load up to a TACCLI level (if TACCLI < THLI) or whole THLI load (if TACCLI > THLI) (steps C3-G1, A2-D3 and B1-E2).

2. One should observe cells that can accept load from a single congested cell and these uncongested cells accept load in the same manner as defined in the Rule 1. Rule 2 takes control after all the observed cells according to Rule 1 had offloaded its traffic load (steps C2-I3, A1-H1 and B3-F3).

3. Remaining cells are offloaded in the following manner. Cell that can share maximum load is observed and offloaded. If there have been observed more cells that can hand over equal load, the cell that had so far performed minimum number off load sharing handovers is chosen.

Grey colored fields in the LS Table mark cells that had accepted traffic load up to their congestion level, regardless of any neighbouring congested cells that might had not been fully offloaded. After all neighbouring cells had accepted traffic load up to their congestion level, one should increase traffic density in congested zone and start the “Procedure A” all over again. This incremental traffic increase is aimed at finding highest traffic load that the neighbouring cells cooperatively can offload.

Table 2.Table 2. The Load Sharing Table of “Procedure A” for GoS=2% and RHYST=75%

STEP 4

After traffic density increase and a few runs through the “Procedure A”, one should find out that one or more of the congested cells still remain congested, although on a lower level than before load sharing. In that case, one can try to hand over some load to formerly congested cells. Once more, one should run “Procedure A” but this time input values should take into account different traffic load in the congested and their neighbouring cells. Once that all of the neighbouring cells by themselves can not off load congested cells and when STEP 4 of “Procedure A” fails, all the cells in the zone of location should engage the problem.

PROCEDURE BPROCEDURE B

“Procedure B” is the other procedure that employs all of the uncongested cells and is somewhat different than the “Procedure

A”. One run of the “Procedure B” in itself includes two runs of the “Procedure A”. The “Procedure B” can be divided into two phases:

• Phase 1 – uncongested cells perform load sharing in order to offload the neighbouring cells of the congested zone to the maximum level

• Phase 2 – the congested cells perform load sharing handover (this phase acctualy represents single run of the “Procedure A” as described earlier)

Phase 1 can further be divided into several steps:

STEP 1

Identical with the STEP 1 of the “Procedure A” with following exception – instead of the congested cells one takes into account uncongested cells.

STEP 2

This step is also somewhat different from the STEP 2 of the “Procedure A”. The uncongested cells are divided into rings depending on their distance from congested zone (Figure 2.).

Figure 2.Figure 2. The cells of the uncongested zone divided into rings

Figure 2. features different colored cells (cells of the same color have approximately equal distance from congested zone) while arrows orientated the allowed direction of load sharing handover. After cells had been divided into rings, limitations regarding load sharing handover are set. Load sharing handover to cells D3, E2 and G1 (neighbouring cells closest to the congested zone) is allowed only to the congested cells. Finally, the LS Table is formed and one inputs into the Column 1 the uncongested cells that act as congested ones.

STEP 3

After the LS Table had been initially formed, the traffic load have been redistributed through following steps: I1-I2, H3-H1, G3-F2, E1-H3, D2-I1, H2-H3, I3-I1, G1-G3, D3-D2, E2-E1, D3-D1, E2-E3, G1-G2, G3-G2, D2-D1 and F1-F3.

Some basic rules, considering different load sharing subjects, were laid once again:

1. Load sharing towards peripheral rings should have priority.

2. One should observe cells that can hand over its load to a single cell and these receiving cells accept incoming load up

11 22 33 44 55 66

A1A1 1.4796

1.4796

1.5431

H2 H2 1.896

0.4164

E1 E1 2.976

0.0168

0

D3 D3 1.0503

A1 = 13.3996

E

A2A2

1.5431

D3 D3 1.0503

0

A2 = 13.8457

E

A3A3 2.9592

1.5431

E1 E1 2.976

0.0168

E2 E2 1.0503

A3 = 11.9368

E

B1B1

1.5431

E2 E2 1.0503

0

B1 = 13.8457

E

B2B2 2.9592

1.5431

G1 G1 1.0503

G3 G3 2.976

0.0168

B2 = 11.9368

E

B3B3 1.4796

1.4796

1.5431

E2 E2 1.0503

G3 G3 2.976

0.0168

0

F1 F1 1.896

0.4164

B3 = 13.3996

E

C1C1 2.9592

1.5431

D3 D3 1.0503

D2 D2 3.9413

0.0519

C1 = 11.9368

E

C2C2 1.4796

1.4796

1.5431

D2 D2 2.976

0.0168

0

I3 I3 1.896

0.4164

G1 G1 1.0503

C2 = 13.3996

E

C3C3

1.5431

G1 G1 1.0503

0

C3 = 13.8457

E

to a TACCLI level (if TACCLI < THLI) or whole THLI load (if TACCLI > THLI) (steps I1-I2, H3-H1 and G3-F2).

3. Remaining cells are offloaded in the following manner. The cell that can share maximum load is observed and offloaded. If there have been observed more cells that can hand over equal load, the cell that had so far performed minimum number off load sharing handovers is chosen. When and if all the cells belonging to one ring are full, offloading cells in nearer ring should take place. One should notice here that some of the cells had not handed over all of its load as direct consequence of acting upon the Rule 1 of this procedure (steps E1-H3 through I3-I1). The cells from the nearest ring hand over their maximum load upon their turn (steps G1-G3 through G1-G2).

4. Once that the nearest ring cells have handed over their load, remaining cells can take part in load sharing. The cell that can share maximum load is, once again, observed and offloaded. If there have been observed more cells that can hand over equal load, the cell that had so far performed minimum number off load sharing handovers is chosen.

The Rule 1 ensures that enough spare capacity will be left over to accept traffic load from the nearest ring cells. If while acting upon the Rule 1 and while offloading load into the most distant ring, one can start offloading by handing over the equal load from several candidates, the first cell to perform handover should be chosen randomly.

Once the load in the uncongested zone had been shared, the Phase 1 is over. The Phase 2 actually represents second run of the “Procedure A” without any changes to previously set rules for redistribution. The LS Table is tabulated again and input values for the table regarding the uncongested cells should be taken from final setup of the LS Table from the Phase 1.

0

1

2

3

4

5

6

7

8

23 24 25 26 27 28

TRAFFIC DENSITYTRAFFIC DENSITY

GO

S (

%)

GO

S (

%)

CLS OFF

A1

A2

A3

B1

B2

B3

C1

C2

C3

Graph 1.Graph 1. Grade of Service and the traffic density

At a certain level of the traffic load, one should find out that neither procedure could relieve traffic congestion. At this point, Cell load sharing’s congestion relieving maximum had been reached.

Graph 1. depicts GoS (%) for each of the congested cells depending on traffic density whether the feature is turned on or off (CLS OFF data series).

IV CONCLUSIONIV CONCLUSION

The procedures presented in this paper have been based upon the idea and the description of the Cell load sharing operation, for the reason that its operation had not been possible to test in reality since both Serbian cellular networks do not operate this feature and some of operation details represent Ericsson’s trade secret. Operating this feature can provide that Quality of Service remains unaffected during peak hours or until the network capacity is expanded provided that the rate of subscriber growth is rather slow. Nevertheless, the CLS feature cannot help in case of the largely undercapacitated network since it had not been designed for that task in the first place, but it could help to provide maximum utilization of the network. The efficiency limits are defined both by setting range of key parameters and the actual network capacity, and are also affected by the size and number of congested and uncongested cells.

Finally, it can be said that the Cell load sharing feature could help to relieve congested network but does not represent the solution that could permanently solve the capacity deficit in the cellular network.

REFERENCESREFERENCES:: [1] CME 20/CME 40 System Survey, Ericsson Radio Systems

AB, Stockholm, 1997. [2] GSM Cell Planning Principles, Ericsson Radio Systems AB,

Stockholm 2000. [3] Cell Load Sharing,User description, Ericsson Radio Systems

AB, Stockholm 2000. [4] Svetozar V. Vukadinovi}: Masovno opslu`ivanje, Nau~na

knjiga, Beograd 1988. [5] Dejan Su~evi}: Primeri primene matemati~kih metoda u PTT

saobra}aju, Saobra}ajni fakultet, Beograd 1996.

Abstract:Abstract: The network cells in mobile telephony systems must be dimensioned for the case of heavy load, although the average load is significantly lower. The unevenly distributed traffic, both in temporal and spatial domain, causes the traffic congestion. Cell Load Sharing (CLS) feature of Base Station Controller’s (BSC) software is designed to redistribute traffic load between neighbouring cells and is one of the possible solutions of traffic congestion problem.

This paper presents an idea (a set of procedures) that could evaluate the effect of CLS feature’s operation at congestion level.