GPRS Link AdaptationWhy GPRS Link Adaptation?
Higher bit rates
Low transfer delays
Increased system capacity
GPRS Link Adaptation dynamically selects the most optimal coding scheme depending on the interference situation
CS-1 and CS-2 is supported on UL and will be static
GSM Radio Network Features
Higher Bit Rates
GPRS Link Adaptation increases the bit rate over the radio interface by dynamically selecting the most optimal coding scheme for transmission of data.
Low Transfer Delays
GPRS Link Adaptation minimizes the transfer delay over the radio interface by dynamically selecting the most optimal coding scheme for transmission of data.
Increased Capacity
GPRS Link Adaptation increases the system capacity. More users can be served since it will take them less time to transmit their data.
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The coding schemes in GPRS
Coding Scheme
CS-1
160
8,0
CS-2
240
12,0
CS-3
288
14,4
CS-4
400
20,0
GSM Radio Network Features
The most robust coding scheme (CS-1) is used for transmission of all radio blocks that carry RLC/MAC control messages. Radio blocks can also carry RLC data blocks, and in such case any of the four coding schemes can be used. The coding schemes are summarized the table above.
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RLC throughput, TU 50, ideal FH
0
10
20
0
5
10
15
20
25
30
CS-1
CS-2
CS-3
CS-4
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Radio link quality measurements
The initial coding scheme is set by CHCSDL (NA, CS1, CS2, CS3, CS4)
The MS will send a PACKET DOWNLINK ACK/NACK message to the BSS on request
Downlink Block Error Rate (BLER) is used to select the coding scheme
GSM Radio Network Features
The MS will send a PACKET DOWNLINK ACK/NACK message (see 3GPP Technical Specification 44.060) to the BSS on request. The message includes a bitmap with information of the correctly and incorrectly received RLC data blocks on downlink. The GPRS Link Adaptation algorithm will use the downlink Block Error Rate (BLER) to select the coding scheme.
The BLER is filtered in the PCU before it is used to select the coding scheme. The filtering is as follows:
BLER(n) = (1-k) * BLER(n-1) + k * BLERreported
where k=0.01
Before the filtered BLER is used as input to the GPRS Link Adaptation algorithm it is required that the filter contains the measurements of at least 50 RLC data blocks.
Whenever the suggested coding scheme changes the BLER filter is reset and filled with measurements related only to the new coding scheme.
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Changing coding scheme
A change to a lower coding scheme is performed if the BLER > 10%
GSM Radio Network Features
The coding scheme that offers the best performance varies depending on the radio link quality. CS-1 offers the highest throughput when the radio link quality is poor while CS-4 offers the best performance in good radio conditions. The BLER reflects the performance of the coding scheme in relation to the radio link quality.
In the beginning of a TBF no BLER is available, therefore an initial coding scheme will be used. The initial coding scheme is always set by CHCSDL with value range (NA, CS1, CS2, CS3, CS4). If the value NA has been chosen, the initial coding scheme will instead be set by the parameter CHCODING.
In case the feature Support for EIT Streaming is turned on the decision at what BLER to switch between coding schemes takes the R99 QoS attributes Traffic Class and Transfer Delay into consideration. This approach provides a method to differentiate between bitrate respectively delay sensitive packet flows. A delay sensitive packet flow is identified as such when attribute Traffic Class is Streaming and the value of the attribute Transfer Delay is equal to or lower than 800 ms. Such a packet flow is carried by EIT Streaming TBF.
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Sheet1
From
To
CS-1
CS-2
< 4%
CS-2
CS-3
< 6%
CS-3
CS-4
< 2%
A change to a lower coding scheme is performed if the BLER > 10%.
Table 3 is applicable on EIT Streaming TBFs.
Table 3    Up - and down switching points between coding schemes
From
To
From
To
CS-1
CS-2
< 4%
CS-2
CS-3
< 6%
CS-3
CS-4
< 2%
A change to a lower coding scheme is performed if the BLER > 10%.
Table 3 is applicable on EIT Streaming TBFs.
Up - and down switching points between
coding schemes for EIT Streaming TBFs
From
To
Parameter summary
LA - The parameter is used to activate and deactivate GPRS Link Adaptation per cell. The values are ON and OFF.
CHCODING - This parameter defines the static coding scheme to be used for the uplink with values (CS-1, CS-2). If Link Adaptation is switched OFF, and CHCSDL is NA, then CHCODING will be used also for the downlink.
CHCSDL - The parameter CHCSDL has the value range NA, CS-1, CS-2, CS-3, CS-4 and is set per cell. It is valid for the downlink only. If GPRS Link Adaptation is switched ON in a cell, then CHCSDL will be used as the initial coding scheme.
EITHIGHCS - is used to define the highest possible coding scheme that can be chosen by the Link Adaptation algorithm for GPRS when data is ansferred over a EIT Streaming TBF. The parameter is set per BSC.
Parameters
Sheet1
From
To
CS-1
CS-2
< 4%
CS-2
CS-3
< 6%
CS-3
CS-4
< 2%
A change to a lower coding scheme is performed if the BLER > 10%.
Table 3 is applicable on EIT Streaming TBFs.
Up - and down switching points between
coding schemes for EIT Streaming TBFs
From
To
Parameter name
Default value
Recommended value
Value range
-
Parameter
name
Default
value
coding schemes for EIT Streaming TBFs
FromToRequired BLER
CS-1CS-2< 0.1%
CS-2CS-1> 1%