concentric cells

8/12/2019 Concentric Cells

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MCD FF0004E3.DOC v 3 3DC 21144 0004 TQZZA 2/9

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1. REFERENCES

GSM 04.08 version 5.4.0 Mobile Radio Interface Layer 3 Specification.GSM 08.58 version 5.3.0 BSC-BTS interface Layer 3 Specification

ACRONYMS

BSC : Base Station ControllerBSS : Base Station SubsystemBTS : Base Transceiver StationEDR : External Directed RetryETSI : European Telecommunication Standards InstituteHO : HandoverIDR : Internal Directed RetryMS : Mobile StationMSC : Mobile Switching CenterOMC-R : Operation and Maintenance Center : Radio partSDCCH : Stand-alone Dedicated Control ChannelTCH : Traffic ChannelTRX : Transceiver


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2. INTRODUCTION

This document deals with the operation of the Concentric Cells feature in the BSS release B6.2.

The present functional feature description is a complement of the description of the features40 14 02, 15 50 30, 15 50 40 and 15 50 50 provided within the document GSM900/GSM1800 BSSRelease B6.2 Feature Description.

The scope of the present document is hereafter detailed :

In section 3, an abstract shows the benefits brought by the feature,

In section 4, the use cases of concentric cells are presented,

In section 5, the initialization of calls within concentric cells is described,

In section 6, how intracell and intercell handovers are performed is shown,

In section 7, the configuration aspect is addressed.


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3. ABSTRACT

With the booming success of mobile communications, the GSM networks experience large amountof traffic with a limited frequency spectrum available. They must therefore handle more efficiently theavailable scarce radio resources.

Transforming macrocells into concentric cells is a solution which will bring the operator two mainadvantages :

- increase the quality of the network without reducing the capacity with the same number ofTRXs,

- increase the capacity while keeping the same quality if additional TRX are used. For example,transforming 3 TRX-cells into cells with 1 TRX for the inner zone and 3 TRXs for the outerzone can bring a 30% capacity increase.

4. USE OF CONCENTRIC CELLS

The concentric cell consists in splitting the cell area of a regular cell into an inner zone and an outerzone which can be operated differently. To each zone is attached a certain number of TRXs. Theouter zone contains at least one TRX bearing all control channels including BCCH.

The zone is a geographical concept which does not necessarily match a radio coverage. This meansthe MS which are closer to the base station use preferably the inner zone TRXs whereas the MSsituated in the outer zone communicate on the pertaining TRXs.

The concentric cell increases capacity by using spectral resources more efficiently. It takesadvantage of its definition in two zones to apply a frequency plan with a smaller frequency reusepattern for the inner zone TRXs.

The smaller cluster size will most of time enable to add one additional TRX in the inner zone withoutthe need for extra frequencies. The result is an overall capacity increase which can reach 25 percent

on a cell with four TRXs.

One of the advantages of the concentric cell implementation in two zones is the possibility to be usedin parallel with frequency hopping. The hopping can be introduced on all channels in the inner zoneto minimize the impact of smaller reuse cluster size and induced interference without impactingcontrol channels borne on outer zone TRX. As a result, the capacity increase can be obtained whilemaintaining the same level of interference.

Concentric cells appear to be a cost-effective solution for solving congestion problems on hot spots.


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It will be efficient if cell radius is not yet too small in order that the inner zone, whose area is typicallyabout one half of total cell area, can catch a good proportion of the overall cell traffic. Typical cellradius limit is about 1 km.

It will therefore mainly be used in the early densification steps of a network as a temporary low-costsolution prior to deploying a more sophisticated capacity enhancement solution in a subsequentphase such as cell splitting.

Practically, the operator will see the following short term benefits :

- save money because it is a low cost solution,- save money because it allows to delay temporarily investment in new sites if traffic growth is

moderate,- increase revenues by decreasing blocking rate on congested cells and thus call requests not

served.

5. INITIATION OF CALLS

An SDCCH connection is always allocated to the outer zone.

Then, in order to assign from the start a TCH in the zone corresponding to the MS location,information on level measured on SDCCH will be used : if there is a too high level on the uplink anddownlink outer zone, the MS will be assigned a TRX of the inner zone. It can be noted that thiscriteria will also be used for interzone handover detection (see section 6).

The MS will also benefit from the quality check performed by this handover detection which consistsin checking that the interference level created by neighbour cells on this frequency is weak. Thismakes sure the MS will get a good quality of communication. The interference levels are taken intoaccount only for neighbour cells in the same layer and the same frequency band as the serving cell.This check is further detailed in section 6.2.

6. HANDOVERS

6.1 Outgoing Handovers

For initiation of an intercell handover between a concentric cell (inner and outer zone) and thedefined adjacent cell, the same handover criteria and handover strategies hold true as for non-concentric cells.


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6.2 Incoming Handovers

In case of incoming intercell handover on SDCCH a channel of the outer zone of the concentric cellis always assigned to the MS.

In case of incoming intercell handover on TCH, the BSS will optimize the allocation and the MS willbe handed over in the zone corresponding to its location.

This improves the traffic handling of the BSS especially on tri-sectored sites. Indeed, direct handoverfrom the inner zone of one sector towards the inner zone of a second sector will be possible withoutintermediate outer zone transfer. This will reduce the overall number of handovers in concentric cellsand additionally in this example it suppresses the risk of loosing the call if the outer zone is saturated.

This optimization corresponds to the feature 15 50 40. It will be performed each time the twofollowing conditions are fulfilled :

- the parameter EN_BETTER_ZONE_HO is set to enable,- the target concentric cell belongs to the same BSC as old serving cell (this means it is an

intercell intra-BSC handover).

In the other cases, the MS will be handed over to the outer zone.

6.3 Intracell Handover

In case of concentric cell environment, the intracell handover can be of two types : intrazone orinterzone. It can also be triggered upon two kinds of configuration : emergency handover or a betterzone handover.

6.3.1 Emergency Intracell Handover due to Interference

The emergency handovers are mostly triggered either for cause : too high interference level on theuplink or too high interference level on the downlink. In this case, it becomes necessary to changethe channel assigned to the call for another channel in the same cell, on which the measured

interference level is the smallest possible.

In concentric cells, this strong interference is very likely to happen mostly when the MS is in the innerzone as the maximum capacity in concentric cells can only be achieved when the inner zones arelarge enough.

In this case it is relevant to change of TRX (non hopping case). This is achieved with the feature 1550 30 (see corresponding feature description).


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As a result, when the MS is in the inner zone, the target channel will be selected according to thefollowing order of preference :

- it belongs to another TRX,- it belongs to same TRX.

The channel may or may not change of zone. If it changes of zone, the intracell handover is namedinterzone handover.

When the MS is in the outer zone, the preference will be :

- another TRX within the outer zone,- same TRX.

When frequency hopping is used on the channel, this will be taken into account for the evaluation ofthe interference level and the handover decision. This will be reflected by the parameterOFFSET_HOPPING_HO defined on a per cell basis at the OMC-R.

6.3.2 Emergency Intracell Handover due to Level

Two emergency handovers are specific to concentric cells. The first one is triggered for cause toolow level on the uplink, inner zone and the second one too low level on the downlink, inner zone.

These handovers will be triggered when the MS is handled by an inner zone TRX when one of thefollowing conditions is fulfilled :

- the inner zone uplink level is low,- the inner zone downlink level is low.

In the first case, the MS has actually reached its maximum allowed power in the inner zone (seecorresponding parameter in section 7) and the received level by the BTS is still too low.

In the second case, the BTS has actually reached its maximum allowed power in the inner zone (see

corresponding parameter in section 7) and the received level by the MS is still too low.

In both cases, a handover towards the outer zone is triggered.


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6.3.3 Betterzone Intracell Handover

Betterzone handovers can also take place inside the concentric cell. They enable to handle the MSbetween the two zones as efficiently as possible. They are all based on level considerations.

The better zone handover consists in changing of zone within the concentric cell because the otherzone is more suitable to handle the MS. This is not an emergency handover in the way the MS cankeep going on same channel but it is not the most suitable one.

This handover applies on an MS handled by an outer zone TRX while entering the inner zone : it cango on with this TRX but it is better to switch to an inner zone TRX to maximize traffic in the innerzone. Therefore, when the outer zone level in uplink and downlink are both too high, an interzonehandover from an outer zone TCH towards an inner zone TCH is triggered. This is available withfeature 15 50 20.

The better zone handovers will be enabled/disabled through the parameter EN_BETTER_ZONE_HOwhich is introduced also with the feature 15 50 40 (see section 6.2).

The feature 15 50 50 will make sure that the MS is safely switched from the outer zone into the innerzone and keeps a good quality of communication. With this feature, each decision to transfer a call toa frequency of the inner zone is dependent on the condition that the interference level created byneighbour cells on this frequency is weak enough.

Therefore, the handover detection will take into account the levels received from the neighbour cellswhose BCCH frequency corresponds to one of the inner zone frequencies. The levels will becompared to the thresholds NEIGHBOUR_RXLEV(0,n) which will be set in advance by the operatoron a couple of cells basis. This flexible tuning will allow a case by case tuning of the acceptableinterference.

The interference levels will also be taken into account only for neighbour cells in the same layer andthe same frequency band as the serving cell.

If the inner zone is congested, the MS will stay in the outer zone.


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