Post on 26-Oct-2015
description
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.
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.
Half-rate Speech Coding
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
Half-rate Speech Coding
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.
Half-rate Speech Coding
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.
Channel Type Configuration
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
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
BSC Parameter Setting
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.
Impact of Half-rate Function Upon GSM Network
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.
Impact of Half-rate Function Upon GSM Network
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.
Applicable Range for Enabling the Half-rate
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.
Applicable Range for Enabling the Half-rate
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.
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.