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Incremental Redundancy
Last updated: December 2, 2008
This section is only applicable to the EGPRS lab application.
Support for incremental redundancy in the test set allows you to verify a mobile station's implementation of Type II hybrid ARQ (incremental redundancy), as well as stress the mobile station's memory and processor. The Wireless Protocol Advisor (WPA) software (see
Protocol Logging
) also provides full tracing of the current modulation and coding scheme and the puncturing scheme.
Downlink TBF
By default, the test set sends 1/3 convolutionally encoded RLC data blocks using puncturing scheme 1 (PS1). When incremental redundancy is enabled, the test set begins transmitting RLC data blocks using PS1 and retransmits blocks using the next puncturing scheme based either on Negative Acknowledgments (NACKs) received from the mobile station for specific RLC block numbers or, if there is no new data to send, for RLC blocks that have not yet been acknowledged or not acknowledged. For additional retransmissions, the test set continues to cycle through the available puncturing schemes for the selected modulation and coding scheme (MCS) until the blocks are acknowledged.
The mobile station must be able to store the original received bits in memory for failed block transmissions, combine those bits with retransmissions of the same block using different puncturing schemes, and decode the combined bits until the decode is successful.
You can force the mobile station to NACK RLC blocks by defining some downlink blocks to be corrupted intentionally by the test set. See
Downlink Corruption
for more information.
Handling of the MS_OUT_OF_MEMORY indication
When the mobile station no longer has enough memory to perform incremental redundancy, it indicates this by setting the MS_OUT_OF_MEMORY field in the EGPRS Packet Downlink Ack/Nack message. In response to this indication, the test set stops sending new blocks to the mobile station and only retransmits blocks that have already been NACKed by the mobile station. Only when the mobile station sends an indication that enough memory has been freed up for normal operation does the test set begin transmitting new blocks.
Uplink TBF
The test set does not allow you to change the state of incremental redundancy for uplink TBFs. This situation exists because the test set does not yet support link adaptation to MCS below MCS-5. This would require splitting RLC blocks in half for transmission and then resegmenting the split blocks by the receiver. By ordering the mobile station to not resegment retransmitted RLC data blocks in RLC MAC control messages, the test set is turning on incremental redundancy for uplink TBFs, as specified in 3GPP TS 04.60, Section 9.3.2.1.
Forcing Uplink Retransmissions
You can observe the use of incremental redundancy on uplink TBFs by setting up the test set to intentionally NACK a percentage of correctly received uplink blocks. Incorrectly received blocks are always NACKed and do not affect the specified percentage of good blocks to be NACKed.
The value of the NACK Good Blocks percentage is not the exact percentage of correctly received blocks that will be NACKed. This value is used to evaluate the probability of any individual block begin NACKed.
Operating Considerations for NACK Good Blocks
The operating mode must be Active Cell and the serving cell must be EGPRS. However, the forced uplink retransmission parameter can be changed in other operating modes, but does not affect the test set's operation until the operating mode is changed to Active Cell.
The forced uplink retransmission parameter is applicable in all connection types.
Window Size
This section is only applicable to the EGPRS lab application. The Window Size for GPRS operation is fixed to 64 bits. According to 3GPP TS 51.010, section 14.18.7, one of the conditions that Incremental Redundancy requires for the MS to achieve a long-term throughput of 20 kbps per time slot measured between the LLC an RLC/MAC layer is that the transmit window size be set to Maximum.
The window size parameters set the RLC window size on the downlink for EGPRS. The window size can be set to the maximum allowable value for the number of downlink slots or the minimum window size for all timeslots. The maximum and minimum allowed window sizes can be found in 3GPP TS 04.60, Table 9.1.9.2.1.
The test set allows you to set the window size for different Data Connection Types. The parameters available in the test set are:
The window size specified by these parameters will be used with the next TBF that is established, not with the current TBF if one is in progress.
These parameters are applied to both uplink and downlink TBFs.
When the window size is set to "Maximum", it is not possible to reconfigure the multi-slot configuration to one which reduces the number of active uplink or downlink slots while the TBF is in progress. This restriction is implemented to conform to 3GPP TS 04.60, Section 9.1.9.2 which specifies that the window size may be changed to a larger size during a TBF, but not to a smaller one.
If the multi-slot configuration is changed while a TBF is in progress, the test set stores this value and uses it the next time a TBF is established.
How Do I Configure the Window Size?
This procedure is only applicable to the EGPRS lab application. (The Window Size for GPRS operation is fixed to 64 blocks.) This procedure describes how you manually configure the RLC/MAC Window Size parameter for different Data Connection Types in the test set.
-
Press
CALL SETUP
.
-
Press the left
More
key.
-
Press
Protocol Control
(
F4
).
-
Press
RLC/MAC
(
F2
).
-
Press the left
More
key.
-
Press
Window Size
(
F3
). This displays the
Window Size Parameters
menu.
-
Use the knob to highlight, select and change the value of the parameters listed in this menu.
To configure the Window Size via the GPIB interface, use the command appropriate for your Data Connection Type. See
Window Size
for more information.
Support for the Incremental Redundancy Performance Specification
The test set currently allows you to configure all the fields in the protocol stack according to the Method of Test for Incremental Redundancy Performance defined in 3GPP TS 51.010 Section 14.18.7. It is currently necessary to use a method external to the test set to evaluate the throughput between the LLC and RLC/MAC layers.
Parameters used for Configuring Incremental Redundancy
Operating Considerations
The operating mode must be Active Cell and the serving cell must be EGPRS. However, parameters associated with Incremental Redundancy can be changed in other operating modes, but do not effect the test set's operation until the operating mode is changed to Active Cell.
The Incremental Redundancy State parameter is only applicable when the connection type is set to Auto.
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