Time Diversity Feature

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GSM BSS

Transmit DiversityFeature Parameter Description

Copyright Huawei Technologies Co., Ltd. 2011. All rights reserved.

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Contents

1 Introduction1.1 Scope

1.2 Intended Audience

1.3 Change History

2 Overview of Transmit Diversity3 Technical Description

3.1 Transmit Diversity

3.2 Dynamic Transmit Diversity

4 Parameters5 Counters6 Glossary7 Reference Documents
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1Introduction

1.1 Scope

This document describes transmit diversity and dynamic transmit diversity.

1.2 Intended AudienceThis document is intended for:

Personnel who need to understand Transmit DiversityPersonnel who work with Huawei products

1.3 Change History

This section provides information on the changes in different document versions.

There are two types of changes, which are defined as follows:

Feature change: refers to the change in the transmit diversity feature of a specific product version.

Editorial change: refers to the change in wording or the addition of the information that was notdescribed in the earlier version.

Document Issues

The document issues are as follows:

01 (2011-03-31)Draft A (2011-01-15)01 (2011-03-31)

This is the first release of GBSS13.0.

Compared with issue draft A (2011-01-15) of GBSS13.0, issue 01 (2011-03-31) of GBSS13.0 has nochange.

Draft A (2011-01-15)

This is the draft release of GBSS13.0.

Compared with issue 01 (2010-06-30) of GBSS12.0, issue Draft A (2011-01-15) of GBSS13.0 has nochange.


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2Overview of TransmitDiversity

In the radio propagation environment, transmit signals may be adversely affected by buildings andother obstacles. Transmit signals arrive at the receiving end in various ways, such as perpendicularincidence, reflections, and scattering. As the MS moves around, Rayleigh fading occurs because thetransmit signals traveling through various paths are combined. Rayleigh fading leads to rapid

fluctuation of the signal amplitude. Therefore, the signal quality is severely affected. Shadow fadingand Rayleigh fading severely degrade the quality of received signals. The two types of fading are themajor factors that result in the vulnerable radio link of the mobile communications.

Transmit diversity can help to improve the quality of the signals received by the MS and reduce theeffect of multipath fading. When Transmit Diversity is enabled, one baseband signal is transmittedthrough two RF channels. The combined signals in the multipath transmission are optimized. Thus,the impact of Rayleigh fading on the MS is reduced.

In transmit diversity mode, the baseband signal of an MS is transmitted on two RF channels. Asthere is no correlation between the two antennas, the problems of dramatic change of signal leveland QoS decrease due to multipath propagation are solved, thus improving the downlink coverage.Transmit Diversity improves the downlink signal quality, thus meeting the demanding requirements

for radio communications in certain cases.

In dynamic transmit diversity mode, the corresponding timeslots of the two TRXs in one double-transceiver unit are set to transmit diversity, and other timeslots are set to non-transmit diversity.

After the timeslots in transmit diversity mode are released, they can be set to non-transmit diversity.Dynamic Transmit Diversity makes full use of idle timeslots and improves the signal strength in areaswith weak coverage, such as at cell edges, indoors, or in cars. Based on actual network conditions,adjustments can be made to achieve the balance between capacity and coverage. The resourceutilization is increased and good quality services can be provided if required.

The gain of Transmit Diversity is high in scenarios where multipath effects are evident. In scenarioswhere few obstacles exist (such as grassland and sea), Transmit Diversity may result ininsignificant diversitygain.


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3Technical Description

3.1 Transmit Diversity

The two TRXs of the double-transceiver unit transmit the data of the same user, but there is delay orphase difference between the two transmit channels. When the output power of the two TRXs

remains unchanged, 3 dB of transmit gain is achieved as the power superposes over the Uminterface and doubles. In addition, a maximum of 1.5 dB of diversity gain can be achieved in variousmultipath environment. Therefore, when Transmit Diversity is enabled, 3 dB to 4.5 dB of downlinkgain can be achieved. (GBFD-115902 Transmit Diversity)

Figure 3-1shows the working principles of Transmit Diversity enabled in a double-transceiver unit.The signal from TRX0 is divided into two signals by the divider. Then, the two signals enter RFunits TX1 andTX2 respectively. The signals are amplified by the RF units and then transmitted to theradio channels by the antenna.

Figure 3-1 Working principles of Transmit Diversity

3.2 Dynamic Transmit Diversity

Dynamic Transmit Diversity mainly applies to concentric cells. If the downlink coverage in theoverlaid subcell is restricted, continuous coverage is impossible, and the capacity of the underlaidsubcell is low. Thus, the underlaid subcell may be congested, whereas the overlaid subcell is idle. Intransmit diversity mode, only one TRX in a double-transceiver unit is used, and the TRX utilization islow.

Dynamic Transmit Diversity is timeslot-based. Normally, two TRXs in a double-transceiver unit work

independently. The two channels with the same timeslot number on the two TRXs can work as achannel group if required. That is, the two channels can be converted into a channel group, and thechannel group into two independent channels. This balances cost, capacity, and coverage. (GBFD-118101 Dynamic Transmit Diversity)

When a double-transceiver unit works in dynamic transmit diversity mode, the channels with the same timeslotnumber on the two TRXs in one double-transceiver unit form a transmit diversity channel group. The two channelsmust be TCHs (TCHF or TCHH). The channel with a smaller TRX number is the primary channel, and the channel
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with a greater TRX number is the secondary channel. The primary channel and the secondary channel are thechannels with the same timeslot number on one double-transceiver unit.

BCCHs, SDCCHs, PDCHs, and CBCHs cannot form a channel group. They can only be used as independentchannels.

During channel assignment, the BSC searches for the optimal channels. After dynamicTransmit Diversity is enabled, the BSC determines whether the optimal channels can form a

transmit diversity channel group if the transmit diversity channel group should be assigned. If atransmit diversity channel group can be formed, the BSC blocks the secondary channel. Then, theBSC assigns the channel group to the call. If the optimal channels cannot form a channel group, theBSC regards the channels as independent channels. If the optimal channels can form a channelgroup but one of them is occupied by a call, the BSC attempts to hand over the call to anotherchannel. After the channel turns idle, a channel group is formed and then assigned to the call.

In dynamic transmit diversity mode, the downlink-limited handover decision algorithm is added todetermine whether the downlink signal strength is limited. If the downlink signal strength is limitedand the serving cell supports dynamic transmit diversity, the call is handed over to an availablechannel group. If one of the two channels with the same timeslot number on one double-transceiverunit is occupied by the call, the call on the other channel should be handed over to another idlechannel.

When the downlink signal strength is increased to a certain level, the transmit diversity channel is notrequired. The BSC divides the channel group into two independent channels and maintains the callon the primary channel. At the same time, the BSC releases the secondary channel to increase thecapacity of the cell.

After dynamic Transmit Diversity is enabled, the BSC reserves most possible channel pairs that can form channelgroups when assigning common independent channels.

With dynamic Transmit Diversity enabled in the overlaid subcell, the coverage of the overlaid subcellis expanded through the handover of calls in the vicinity of the edge to the overlaid subcell. Thishelps to balance the traffic between the overlaid subcell and that in the underlaid subcell.

Dynamic Transmit Diversity is a software solution. That is, the hardware connections of the BTS indynamic transmit diversity mode are the same as those in transmit diversity mode, and only thesoftware needs to be upgraded.

Specify theDYNPBTSUPPORTEDparameter to determine whether the cell supportsdynamic TX diversity and dynamic PBT
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4Parameters

Table 4-1 Parameter description

Parameter ID NE MML Command Description

DYNPBTSUPPORTED

BSC6900 SETGCELLCHMGBASIC(Optional)Meaning:Whether to enable thecell to support dynamictransmitdiversity or dynamic PBT

GUI ValueRange:NOTSUPPORT(NotSupport),DDIVERSITY(DDIVERSITY),DPBT(DPBT)

Actual ValueRange:NOTSUPPORT,DDIVERSITY, DPBT

Unit:None

Default Value:NOTSUPPORT
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5Counters

There are no specific counters associated with this feature.


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6Glossary

For the acronyms, abbreviations, terms, and definitions, see theGlossary.
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7Reference Documents

[1] BSC6900 Feature List[2] BSC6900 Optional Feature Description[3] BSC6900 GSM Parameter Reference[4] BSC6900 GSM MML Command Reference

Time Diversity Feature

Documents

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