No part of this document may be reproduced or transmitted in any form or by any means without prior written consent of Huawei Technologies Co., Ltd.
Trademarks and Permissions
and other Huawei trademarks are trademarks of Huawei Technologies Co., Ltd.
All other trademarks and trade names mentioned in this document are the property of their respective holders.
Notice
The information in this document is subject to change without notice. Every effort has been made in the preparation of this document to ensure accuracy of the contents, but all statements, information, and recommendations in this document do not constitute the warranty of any kind, express or implied.
1 Introduction to This Document 1.1 Scope This document describes BCCH dense frequency multiplexing of Huawei GBSS, which is also known as tight BCCH. It covers the function of and mechanism regarding BCCH dense frequency multiplexing, including tight BCCH common channel assignment, tight BCCH handover algorithm, and channel assignment for tight BCCH handover.
1.2 Intended Audience It is assumed that users of this document are familiar with GSM basics and have a working knowledge of GSM telecommunication.
This document is intended for:
Personnel working on Huawei GSM products or systems System operators who need a general understanding of this feature
1.3 Change History The change history provides information on the changes in different document versions.
There are two types of changes, which are defined as follows:
Feature change Feature change refers to the change in the BCCH dense frequency multiplexing feature of a specific product version.
Editorial change Editorial change refers to the change in wording or the addition of the information that was not described in the earlier version.
Document Issues The document issues are as follows:
02 (2009-09-30) 01 (2009-06-30)
02(2009-09-30) This is the second commercial release of GBSS9.0.
Compared with issue 01 (2009-06-30) of GBSS9.0, issue 02 (2009-09-30) of GBSS9.0 incorporates the changes described in the following table.
2 Overview In a network where the frequency resources are limited, relatively few frequencies at the FH layer are a capacity bottleneck. Increasing the number of frequencies at the FH layer can improve the system capacity.
In general, the frequencies planned for a network include BCCH frequencies and TCH frequencies. BCCH frequencies and TCH frequencies adopt different multiplexing modes. For example, the BCCH frequencies are multiplexed in 4x3 mode, while the TCH frequencies are multiplexed in 1x3 mode. In a network whose frequencies are limited, if the BCCH uses more frequencies, fewer frequencies are available for the TCHs, and thus the system capacity is smaller.
BCCH dense frequency multiplexing enables the BCCHs to reuse frequencies more tightly to free more frequencies for non-BCCH TRXs, thus increasing the system capacity.
The BCCH dense frequency multiplexing technology is applicable to the network with limited frequency resources. It helps to increase the reusability of BCCH frequencies and reduce the number of frequencies used by the BCCHs. Therefore, more frequencies can be used at the FH layer.
BCCH dense frequency multiplexing has the following advantages:
Reduces the number of frequencies occupied by the BCCHs, and improves the spectrum utilization Increases the number of frequencies available for TCHs and for FH, expands the system capacity without adding new hardware, and saves the costs of adding sites and cells
Assigns the TCHs on BCCH frequencies to only the MSs near the BTS, thus improving the voice quality because of less uplink interference
Reduces random access failures, and improves the access performance
BSS BCCH Dense Frequency Multiplexing 3 Technical Principles
3 Technical Principles When BCCH dense frequency multiplexing is adopted, a cell is classified into different logical layers: TCH layer on the BCCH TRX and FH layer, as shown in Figure 3-1.
The FH layer serves and covers the entire network, including cell edges. The TCH layer on the BCCH frequency, however, covers only the MSs near the BTS to guarantee call access and to reduce interference near the BTS.
Figure 3-1 Logical layers of BCCH dense frequency multiplexing
Call moved to hopping layer if the quality
degrades too much
Limit to move to hopping layer
Limit to move to BCCH
BCCH Frequency
coverage area
A denser frequency multiplexing pattern tends to increase the interference on the BCCH, therefore, proper channel allocation and handover algorithms are required to allocate the TCHs on the BCCH TRX to the MSs near the BTS. In this way, the restriction of multiplexing density on the BCCH TRX is reduced. The BCCH dense frequency multiplexing consists of:
Tight BCCH common channel assignment Tight BCCH handover algorithm Channel assignment for tight BCCH handover
Tight BCCH Common Channel Assignment During an initial access request or a non-tight BCCH handover, if TIGHT BCCH Switch is set to Yes, a TCH is assigned according to the following principles:
If a TCH on non-BCCH TRXs is available, the TCH is preferentially assigned to ensure access performance.
If all the TCHs on non-BCCH TRXs are occupied, the TCH on the BCCH frequency is assigned.
During the assignment, the TCHs on the BCCH TRX are preferentially assigned if the call meets the following conditions:
The uplink receive level is greater than Level Thresh for Assign BCCH Under TBCCH.
The uplink receive quality is smaller than Quality Thresh for Assign BCCH Under TBCCH. The load on the non-BCCH TRXs in the cell is higher than Load Threshold for TIGHT BCCH HO.
Tight BCCH Handover Algorithm When all the TCHs on non-BCCH frequencies are occupied, the signal quality of an MS on the cell edge decreases if the TCH on the BCCH frequency is assigned to the MS. Therefore, if the load on the non-BCCH TRXs exceeds Load Threshold for TIGHT BCCH HO, the MS that is away from the cell edge and whose signal quality is smaller than RX_QUAL Threshold for TIGHT BCCH HO is handed over to the TCH on the BCCH TRX. Thus, the TCHs on the non-BCCH TRX are reserved to ensure the call access performance.
The principle of tight BCCH handover algorithm is as follows: During the period specified by TIGHT BCCH HO Watch Time, if the period when tight BCCH handover conditions are met exceeds TIGHT BCCH HO Valid Time, a handover is initiated. Otherwise, no handover is initiated.
Channel Assignment for Tight BCCH Handover The purpose of tight BCCH handover is to hand over a call near the BTS to the BCCH TRX. Therefore, if the cause of a handover is tight BCCH handover, a TCH on the BCCH TRX is assigned.
BSS BCCH Dense Frequency Multiplexing 4 Engineering Guidelines
4 Engineering Guidelines Recommended Scenarios In general, the 4x3 mode is used in BCCH frequency planning to guarantee high carrier-to-interference ratios (CIRs) between BCCH frequencies. In this mode, the BCCHs occupy 12 frequencies, thus decreasing the number of frequencies available for frequency hopping. In a network where the frequency resources are limited, the number of frequencies available for frequency hopping is a critical capacity factor.
No anti-interference technology such as FH is used on BCCH frequencies. Therefore, if the 3x3 mode is used in BCCH frequency planning, the TCHs on BCCH frequencies have low interference resistance, and the interference in the system is increased. But if a denser frequency multiplexing mode is adopted, the performance of TCHs may decrease badly to an unacceptable level.
Enabling BCCH dense frequency multiplexing on the BSC can solve the above problems.
Impact on Other Features The impact of BCCH dense frequency multiplexing on other features is as follows:
Concentric cells do not support dense BCCH frequency multiplexing. Multiband networks do not support dense BCCH frequency multiplexing.
Impact on System Performance If the load of a cell increases, an MS served by a channel on the non-BCCH TRX may be handed over to a channel on the BCCH TRX, thus increasing the number of handovers in the network.
BSS BCCH Dense Frequency Multiplexing 5 Parameters
5 Parameters This chapter describes the parameters related to BCCH dense frequency multiplexing.
For the meaning of each parameter, see Table 5-1. For the default value, value ranges, and MML commands of each parameter, see Table 5-2.
Table 5-1 Parameter description (1)
Parameter Description
TIGHT BCCH Switch
Whether to enable the BCCH aggressive frequency reuse algorithm.
Level Thresh for Assign BCCH Under TBCCH
During channel assignment, the assignment of channels on the BCCH TRXs depends on the uplink receive level, quality, and non-BCCH load. The TCHs are preferentially assigned on the BCCH TRXs if the following conditions are met: 1. The uplink receive level after filtering is higher than "Level Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 2. The uplink receive quality level after filtering is lower than "Quality Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 3. The non-BCCH load is higher than "Intracell Non Main BCCH Load Threshold of TIGHT BCCH".
Quality Thresh for Assign BCCH Under TBCCH
During channel assignment, the assignment of channels on the BCCH TRXs depends on the uplink receive level, quality, and non-BCCH load. The TCHs are preferentially assigned on the BCCH TRXs if the following conditions are met: 1. The uplink receive level after filtering is higher than "Level Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 2. The uplink receive quality level after filtering is lower than "Quality Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 3. The non-BCCH load is higher than "Intracell Non Main BCCH Load Threshold of TIGHT BCCH".
During channel assignment, the assignment of channels on the BCCH TRXs depends on the uplink receive level, quality, and non-BCCH load. The TCHs are preferentially assigned on the BCCH TRXs if the following conditions are met: 1. The uplink receive level after filtering is higher than "Level Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 2. The uplink receive quality level after filtering is lower than "Quality Thresh. for Assigning Main BCCH Carrier Channel Under TIGHT BCCH". 3. The non-BCCH load is higher than "Intracell Non Main BCCH Load Threshold of TIGHT BCCH".
RX_QUAL Threshold for TIGHT BCCH HO
When an intra-cell TIGHT BCCH handover needs to be performed (handover from the non-BCCH to BCCH), the downlink receive quality must be smaller than the value of this parameter.
TIGHT BCCH HO Watch Time
The P/N criterion must be met for triggering a TIGHTBCCH handover. That is, the TIGHTBCCH handover can be triggered only if P seconds among N seconds meet the triggering conditions. This parameter corresponds to N in the P/N criterion.
TIGHT BCCH HO Valid Time
The P/N criterion must be met for triggering a TIGHT BCCH handover. That is, the TIGHT BCCH handover can be triggered only if P seconds among N seconds meet the triggering conditions. This parameter corresponds to P in the P/N criterion.
Table 5-2 Parameter description (2)
Parameter Default Value
GUI Value Range
Actual Value Range Unit MML
Command Impact
TIGHT BCCH Switch OFF
OFF(Off), ON(On) OFF, ON None
SET GCELLCHMGBASIC (Optional) Cell
Level Thresh for Assign BCCH Under TBCCH 30 0~63 0~63 dB
SET GCELLCHMGAD (Optional) Cell
BSS BCCH Dense Frequency Multiplexing 5 Parameters
