GSM-HRFR Optimization 27

GSM HR/FR Optimization

Course Overview

HR/FR Overview

Channel Type Configuration

Case Analysis

HR/FR Overview

One TCH/F TS on Um interfaced can be divided into two

TCH/H half-rate TSs, provided to MSs that support the

half-rate access to perform the half-rate wireless access.

The half-rate channel combination function refers to

combining the TCH/H TSs of two different TCH/F TSs into

one TCH/F TS by the internal cell handoff. As a result, one

TCH/F TS can be reserved to be provided to MS to

perform the full-rate service, or be dynamically converted

into the PDCH channel to perform the GPRS/Edge data

service.

Frame Structure

The TDMA multi-frame of one voice channel TCH includes 26 frames,

with the duration of 120ms. In the multi-frame, the 13th frame is

SACCH (Slow Associated Control Channel), and the 26th frame is an

idle frame.

Compared with the full-rate encoding, after the system adopts the half-

rate, the frame structure is not changed. Instead, the system only

assigns the odd frames in the multi-frame to one user and assigns the

even frames to another user, so the 26th idle frame becomes the

SACCH of the second user. As a result, one TCH/F service as carried

previously is increased into two TCH/H services, so the channel

capacity is doubled. The negative impact is that the encoding rate of

one user’s channel is decreased, the voice quality is reduced and it

requires the terminal to support the HR function.

Frame Structure

Half-rate Speech Coding

According to the GSM protocol, the half-rate

speech coding is divided into HR (GSM half rate

speech version 1) and AMR HR (GSM half rate

speech version 3).

Between them, the HR speech coding rate is 5.6kbps,

while the AMR HR has 6 coding modes: 7.95kbps,

7.4kbps, 6.7kbps, 5.9kbps, 5.15kbps, 4.75kbps.

However, the Active Codec Set can includes 4 modes

at most.


Compared with the full-rate and enhanced full-rate

speech coding, the half-rate speech coding adopts the

coding mode with higher compression ratio, which

reduces the coding ratio to adapt to the narrower

bandwidth of half-rate channel.

Channel

Type

Coding Algorithm Coding Bit Rate Compressi

on

Ratio

Quality

Score

TCH/FS RPE-LTP-LPC 13kbps 8 3.8

TCH/EFS ACELP 12.2kbps 8.5 4.1

TCH/HS VSELP 5.6kbps 18.4 3.7


The half-rate speech adopts the VSELP coding mode. To

adapt to the half-rate bandwidth, its coding rate is reduced

to 5.6kbps. Compared with the 13kbps of full-rate speech

coding, it is reflected as the decrease of voice quality.

According to MOS (Mean Opinion Score) evaluated from

the 3 speech coding modes, the MOS of half-rate speech

is 0.1 lower than that of the full-rate and 0.4 lower than the

enhanced full-rate, so the introduction of half-rate will lead

to certain decrease of voice quality, but this decrease is

acceptable.


Except for the change of speech coding, to adapt to the half-rate

wireless channel, the channel coding is also changed correspondingly,

including the check bit number, convolution code rate and interleaving

depth. These changes will affect the error-tolerance capability of the

channel.

Check bit number 3Bit, is the same as full-rate, but 15Bit less than the

enhanced full-rate.

Convolution code rate 104/211, is a little higher than half of full-rate or

enhanced full-rate.

Interleaving depth 4, is apparently decreased from the value 8 of full-rate

and enhanced full-rate.

The change of channel coding is comprehensively reflected as the

decrease of the error-tolerance capability of the channel. In the cell

border where the signal is weak and the interference is great, the

channel quality will be decreased and the call drop ratio will be

increased.

Course Overview

HR/FR Overview


Case Analysis


Parameter Name Value Range & Unit Default

Dynamic TS Yes/No Yes

Combination

types of TS

channel

TCH/F

TCH/F

TCH/H(0,1)+FACCH/H(0,1)+SACCH/TH(0,1)

TCH/H(0,0)+FACCH/H(0,1)+SACCH/TH(0,1)+TC

H/H(0,1)

SDCCH/8+SACCH/C8

FCCH+SCH+BCCH+CCCH

BCCH+SDCCH/4

BCCH+CCCH

BCCH+SDCCH/4+CBCH

SDCCH+CBCH

PBCCH+ PCCCH+PDTCH+PACCH+PTCCH

PCCCH+PDTCH+PACCH+PTCCH

PDTCH+PACCH+PTCCH

Related Parameters for Dynamic HR Conversion

动态HR转换

Level Parameter Name Abbreviation Value

Range &

Unit

Default

BSC Support dynamic HR or not DynaHREnable Yes/No No

Threshold of applying for half rate HRThs 1~100, % 50

Threshold of AMR dynamic HR

conversion

AmrHRThs 1~100, % 50

Threshold of HR TS percentage HRTsPercentage 1~100, % 50

Cell Support cell dynamic Hr parameter or not UseCellDynHRPara Yes/No No

Support dynamic HR or not DynaHREnable Yes/No No

Threshold of applying for half rate HRThs 0~100, % 50

Threshold of AMR dynamic HR

conversion

AMRHRThs 0~100, % 50

Threshold of HR TS percentage HRTsPercentage 0~100, % 50

Channel Assignment

When MS applies for resources to BSS in the

wireless network, the BSS network refers from

the following three aspects, to decide the

wireless channel type for this access.

The capability of support channels on the MS

The restriction of MSC upon the channel rate and type

of the user’s access channel

The consideration of BSS about the resource

assignment in this cell

MS Capability of Supporting Channels

GSM MS can support the following types of speech

versions:

GSM full rate speech version 1（FR）

GSM full rate speech version 2 （EFR）

GSM full rate speech version 3 （AMR FR）

GSM half rate speech version 1（HR）

GSM half rate speech version 3 （AMRHR）

The MS capability of supporting the speech version is

based on the Bearer Capability of Setup (calling party) or

Call Confirmed (called party).

Restrictions for MS Distribution

When MS is applying for network resources, MSC restricts the channel rate and type according to the MS access capability and user’s access level, to provide BSC with the available channel range that this MS can choose from.

For TCH channel, there are the following two situations: The channel of one rate is specified, so BSC must assign channels

according to this requirement.

Choose form full rate or half rate. It can be one rate preferable or no preferable relationship between the two rates. In this situation, BSC adjusts the sequence of assigning the channels according to its own setting.

BSC Parameter Setting

When the cell has enabled the dynamic half0-rate, if the

occupancy ratio of the cell channel exceeds the set

threshold, BSC starts to assign the half-rate channel.

On V6.20.100e or later versions, the option of forced full-

rate preferable is enhanced.

Besides, considering that although AMR-HR adopts the

half-rate channel, it can still provide better voice quality

and anti-interference capability. Therefore, when deciding

the policy of assigning channels, we also need to consider

whether MS supports AMR-HR.


BSC Parameter

Setting

Result

Do not change the

channel selection mode

Use the channel rate specified by MSC

Only one rate is

allowed.

If the rate allowed by BSC is the same of MSC, this

rate is chosen. Otherwise, the assignment fails.

Two rates are allowed,

and one rate is

preferable.

Use the rate specified by MSC


If there is no change, BSC chooses the channel strictly

according to the channel range assigned by MSC.

When the cell has configured the preferable or allowed

rate, if it conflicts with the preferable channel range

distributed by MSC, it takes the set cell-level parameter

preferably. For example, if the channel ranged specified by

MSC is half-rate channel preferable, but the cell parameter

set within BSC is to assign the full-rate channel only, BSC

assigns the channel according to the rule of assigning the

full-rate channel only.

If there is no intersection area between the channel

preferable range distributed by MSC and the cell

parameter specified by BSC, this channel application fails.

Impact of Half-rate Function Upon GSM Network

Impact of HR upon the coverage range: There is no impact upon the network wireless coverage by enabling

the half-rate function.

HR impacts the network indexes.

Call drop ratio: The half-rate function basically does not affect the call drop ratio.

Blocking ratio: After the half-rate function is enabled, the blocking ratio is greatly decreased within a short period. On the long term, the increase of traffic and attempted calls cause the blocking ratio to increase. In this situation, we need to consider expanding the whole core network resources.

Uplink/Downlink quality: The half-rate function basically does not affect the uplink/downlink quality.

Call completion ratio: When the half-rate function is first enabled, the call completion ration will apparently increase, and the core network resources will become the bottleneck of call completion ratio.


Impacts of HR upon GSM network resources:

The half-rate function in fact greatly increases the number of TCH.

According to the number of half-rate channels in dynamic half-rate

setting and the threshold in dynamic half-rate setting, the theoretic

number of TCH will increase by 0% ~ 100%. In fact, as certain MS

does not support HR, TCH can be increased by about 60%.

Therefore, to enable the half-rate function, we need to consider

expanding the BSC capacity. The bottleneck of BSC capacity is the

Erl number that can be supported, because the half-rate function

causes the increase of TCH and thus increases the Erl number. As

a result, BSC needs to make expansion accordingly. Besides, the

half-rate function allows more users to be accessed at the wireless

side, leading to the result that the resources on the MSS side might

be insufficient, so the corresponding expansion is necessary.


User experiences: Though according to MOS, the MOS of half-rate speech is a little

lower than the full-rate and especially enhanced full-rate speech, this decrease basically does not affect the understandability of speech.

According to the testing in the existing network, in places with better wireless coverage, during a normal call, the user basically cannot feel the decrease of voice quality caused by half-rate. In places with week wireless coverage where the voice quality itself is poor, the user can experience the apparent decrease of voice quality.

Besides, the half-rate coding is a coding mode with higher compression rate, which is proposed for the speech in particular. It has relatively great impacts upon the sounds that has richer spectrums such as music, so when transmitting this type of sound, its voice quality is apparently decreased according to the enhanced full-rate.

Applicable Range for Enabling the Half-rate

Function

Application of HR in areas with burst traffic

The most effective application of half-rate function is to deal with

the rapidly-increasing service requirements, such as emergency

communication in the stadium, campus and large-scale assembly.

These areas have the outstanding features that the traffic is busy

periodically or suddenly, such as the traffic increase during a

competition in the stadium or during the break time on campus.

They cause impacts upon the network.

Under these situations, it is most appropriate to use the dynamic

half-rate function. Under the normal situation with low traffic, the

channel is in the full-rate status; when the burst services suddenly

increase, it is automatically converted into half-rate, which both

relieves the congestion and reduces the waste caused by the

expansion that the carrier usually adopts.


Function

Application of HR in areas with concentrated traffic

In the dense urban area such as downtown, airport, station and

plaza, the traffic is highly concentrated. With the rapid development

of the city and the network users, there will be frequently expansion

requirements in these areas.

To avoid the frequent network adjustment, we can adopt the half-

rate mode to temporarily relieve the pressure of pressure. Before

the next expansion, we can enable the dynamic or static half-rate

function at proper time to deal with the urgent capacity increase,

which, together with the long-term planning and expansion, can

provide the carrier with a flexible option of expansion. At the same

time, the half-rate function is also one network expansion plan in

the dense urban areas where the frequency resources are

restricted and the site type cannot be expanded.


Function

Application of HR in areas with low-end users: In the areas with low-end users, such as distant suburban areas,

usually the carrier also provides the coverage because of network integrity and brand competitiveness. However, the population in these areas is not dense, so the input/output ratio of the carrier is very low.

Because the low-end users in these areas usually do not have very high requirements, and they only need to make calls, we can adopt the half-rate mode. By combining some wide-coverage technology, the static or dynamic half-rate function can satisfy the requirements of making calls in an open area, so it provides a low-cost coverage mode.

Besides, the low-end traffic area also has many unexpected traffic burst, such as the capacity change caused by convenes or population shift. In this case, the half-rate function can serve as a solution.

Course Overview

HR/FR Overview


Case Analysis

Case: The traffic congestion increased after HR was

enabled.

Problem symptom: After xx carrier enabled HR, when certain congestion cell

configured the HR channel, the congestion was not relieved.

Problem Analysis: After analyzing the performance report, we found that:

There were normal occupations on the TCH/H channel.

The maximum occupation number of TCH/F = the maximum number configured.

It indicates that the TCH/F channel was occupied.

Signaling analysis:

The proportion of supported HR on MS is much less than normal cells. This problem existed because it is a BSC on the border network.

Problem processing: We modified the channel assignment policy into HR preferable,

so that the speech services used the HR channel first.

GSM-HRFR Optimization 27

Documents

Transcript of GSM-HRFR Optimization 27