Content for  TS 45.022  Word version:  16.0.0

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1  ScopeWord‑p. 7

The present document gives examples for the Radio sub-system link control to be implemented in the Base Station System (BSS) and Mobile Switching Centre (MSC) of the GSM and DCS 1 800 systems in case hierarchical cell structures are employed.
Unless otherwise specified, references to GSM also include DCS 1 800, and multiband systems if operated by a single operator.

2  References

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
GSM TS 03.22 (ETS 300 930): "Functions related to Mobile Station (MS) in idle mode and group receive mode".
GSM TR 03.30 (ETR 364): "Radio network planning aspects".
TS 45.008: "Radio subsystem link control".
GSM TR 01.04 (ETR 350): "Abbreviations and acronyms".

3  Abbreviations

Abbreviations used in the present document are listed in 3GPP TR 01.04 [4].

4  General

ETS 300 911 (GSM 05.08 [3]) specifies the radio sub system link control implemented in the Mobile Station (MS), Base Station System (BSS) and Mobile Switching Centre (MSC) of the GSM and DCS 1 800 systems of the European digital cellular telecommunications system (Phase 2).
The present document gives several examples of how the basic handover and RF power control algorithm as contained in (informative) Annex A to ETS 300 911 [3] can be enhanced to cope with the requirements on the radio subsystem link control in hierarchical networks.
A hierarchical network is a network consisting of multiple layers of cells, allowing for an increased traffic capacity and performance compared to a single layer network.
The radio sub-system link control aspects that are addressed are as follows:
  • Handover;
  • RF Power control.

5  Hierarchical networksWord‑p. 8

5.1  General

In a hierarchical, or microcellular network, traffic is supported on multiple layers of cells. Typically, a network operator could implement a layer consisting of microcells as a second layer in his existing network consisting of large or small cells. The addition of this second layer would improve the capacity and coverage of his network.
In the present document the following naming convention is used for the different layers. For a network consisting of three layers the layer using the biggest cells is the "upper layer", followed by the "middle layer", and then the "lower layer" which has the smallest cells. For a network consisting of two layers, only "upper layer" and "lower layer" are used.
The intention in a hierarchical network is to use the radio link control procedures to handle the majority of the traffic in the lower layer, i.e. the smallest cells, as this will limit interference and therefore improve the frequency reuse.
However, a part of the traffic cannot always efficiently be handled in the lower layer. Examples are cases where the MS is moving fast (relative to the cell range), or where the coverage is insufficient, or where a cell to make a handover to on the same level may not be available fast enough (going around corners, entering/leaving buildings).

5.2  Cell types

GSM 03.30 [2] distinguishes between three kinds of cells: large cells, small cells and micro cells. The main difference between these kinds lies in the cell range, the antenna installation site, and the propagation model applying:

5.2.1  Large cells

In large cells the base station antenna is installed above the maximum height of the surrounding roof tops; the path loss is determined mainly by diffraction and scattering at roof tops in the vicinity of the mobile i.e. the main rays propagate above the roof tops; the cell radius is minimally 1 km and normally exceeds 3 km. Hata's model and its extension up to 2 000 MHz (COST 231-Hata model) can be used to calculate the path loss in such cells (GSM 03.30 [2] annex B).

5.2.2  Small cells

For small cell coverage the antenna is sited above the median but below the maximum height of the surrounding roof tops and so therefore the path loss is determined by the same mechanisms as stated in subclause 5.2.1. However large and small cells differ in terms of maximum range and for small cells the maximum range is typically less than 1 3 km. In the case of small cells with a radius of less than 1 km the Hata model cannot be used.
The COST 231-Walfish-Ikegami model (see GSM 03.30 [2] annex B) gives the best approximation to the path loss experienced when small cells with a radius of less than 5 km are implemented in urban environments. It can therefore be used to estimate the BTS ERP required in order to provide a particular cell radius (typically in the range 200 m - 3 km).

5.2.3  Microcells

COST 231 defines a microcell as being a cell in which the base station antenna is mounted generally below roof top level. Wave propagation is determined by diffraction and scattering around buildings i.e. the main rays propagate in street canyons. COST 231 proposes an experimental model for microcell propagation when a free line of sight exists in a street canyon (see GSM 03.30 [2]).
The propagation loss in microcells increases sharply as the receiver moves out of line of sight, for example, around a street corner. This can be taken into account by adding 20 dB to the propagation loss per corner, up to two or three corners (the propagation being more of a guided type in this case). Beyond, the complete COST231-Walfish-Ikegami model as presented in annex B of GSM 03.30 [2] should be used.
Microcells have a radius in the region of 200 to 300 metres and therefore exhibit different usage patterns from large and small cells.

6  Idle mode proceduresWord‑p. 9

GSM 03.22 [1] outlines how idle mode operation shall be implemented. Further details are given in Technical Specifications GSM 04.08 and GSM 05.08 [3].
A useful feature for hierarchical networks is that cell prioritization, for Phase 2 MS, can be achieved during cell reselection by the use of the reselection parameters optionally broadcast on the BCCH. Cells are reselected on the basis of a parameter called C2 and the C2 value for each cell is given a positive or negative offset (CELL_RESELECT_OFFSET) to encourage or discourage MSs to reselect that cell. A full range of positive and negative offsets is provided to allow the incorporation of this feature into already operational networks.
The parameters used to calculate C2 are as follows:
    When the MS places the cell on the list of the strongest carriers as specified in GSM 05.08 [3], it starts a timer which expires after the PENALTY_TIME. This timer will be reset when the cell is taken off the list. For the duration of this timer, C2 is given a negative offset. This will tend to prevent fast moving MSs from selecting the cell.
    This is the amount of the negative offset described in (ii) above. An infinite value can be applied, but a number of finite values are also possible.
The permitted values of these parameters and the way in which they are combined to calculate C2 are defined in GSM 05.08 [3].

7  Examples of handover and RF power control algorithms.

7.1  General

In the following annexes four examples of handover and power control algorithms are presented. All of these are considered sufficient to allow successful implementation in hierarchical or microcellular networks. None of these solutions is mandatory.
The "Description of algorithm" of each annex, contains a text as provided by the authors of the algorithm. Any discussion on the algorithms is contained in a separate clause, "Discussion of algorithm".

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