Power Control in Semi-Persistent Scheduling

This section describes voice service power control policies when semi-persistent scheduling is used for VoLTE. For details about power control, see Power Control Feature Parameter

Description.

Power Control in Uplink Semi-Persistent Scheduling

When semi-persistent scheduling is used for VoLTE in the uplink, closed-loop power control for the physical uplink shared channel (PUSCH) can be enabled or disabled by setting the

CloseLoopSpsSwitch option of the CellAlgoSwitch.UlPcAlgoSwitch parameter.

l If the CloseLoopSpsSwitch option is selected, the eNodeB adjusts transmit power for the PUSCH based on the measured IBLER of voice services.

l If the CloseLoopSpsSwitch option is deselected, the eNodeB uses open-loop (not closed-loop) power control for the PUSCH.

Power Control in Downlink Semi-Persistent Scheduling

When semi-persistent scheduling is used for VoLTE in the downlink, power control for the PDSCH can be enabled or disabled by setting the PdschSpsPcSwitch option of the

CellAlgoSwitch.DlPcAlgoSwitch parameter.

l If the PdschSpsPcSwitch option is selected, the eNodeB periodically adjusts the PDSCH transmit power for UEs based on the measured IBLER.

l If the PdschSpsPcSwitch option is deselected, power control for the PDSCH in semi- persistent scheduling is not used. Instead, the eNodeB transmit power is evenly shared by each RB.

4.1.2 ROHC

This section describes how the optional feature LOFD-001017 RObust Header Compression (ROHC) works for VoLTE. For details about this feature, see ROHC Feature Parameter

Description.

ROHC provides an efficient header compression mechanism for data packets transmitted on radio links to solve the problems of high bit error rates (BERs) and long round trip time (RTT). ROHC helps reduce header overheads, lower the packet loss rate, and shorten response time.

In the current version, ROHC is used to compress the headers of only voice packets (QCI of 1 and PTT QCI services), as shown in Figure 4-4. ROHC reduces the packet size and physical resource block (PRB) overheads. When PRBs are insufficient, ROHC helps increase system capacity.

Figure 4-4 ROHC for VoLTE

After deploying VoLTE, operators can enable or disable ROHC by setting the

different profiles for data streams compliant with different protocols. Profiles define the compression modes for streams with different types of protocol headers. Voice services use profiles 0x0001 and 0x0002.

The ROHC compression efficiency varies with the ROHC operating mode and variations in the dynamic part of packet headers at the application layer. A header can be compressed to a size as small as 1 byte, which efficiently reduces the voice packet size.

4.2 Coverage Improvement

Operators can enable the following features to improve voice service coverage in poor coverage scenarios:

l TTI Bundling l ROHC

l Uplink RLC segmentation enhancement

4.2.1 TTI Bundling

This section describes the principles of the optional feature LOFD-001048 TTI Bundling and how this feature works for VoLTE.

4.2.1.1 Overview

TTI bundling enables a data block to be transmitted in four consecutive TTIs, which are bound together and treated as the same resource. Different HARQ redundancy versions of the same data block are transmitted in different TTIs. TTI bundling makes full use of HARQ combining gains and reduces the number of retransmissions and RTT.

When the UE's channel quality is poor and transmit power is limited, TTI bundling increases the cell edge coverage of the PUSCH by about 1 dB. The gains produced by this feature can be observed when voice quality is maintained at a certain level, for example, when the mean opinion score (MOS) is 3.

The TtiBundlingSwitch option of the CellAlgoSwitch.UlSchSwitch parameter controls whether to enable TTI bundling. When this option is selected, the eNodeB determines whether to activate TTI bundling based on the channel quality. After activating TTI bundling, the eNodeB determines the number of PRBs and selects an MCS based on the channel quality and the amount of data to be transmitted.

According to section 8.6.1 "Modulation order and redundancy version determination" in 3GPP TS 36.213 V10.1.0, when TTI bundling is enabled, the resource allocation size is restricted to a maximum of three PRBs and the modulation scheme must be QPSK. Therefore, the selected MCS index cannot be greater than 10. After TTI bundling is enabled, the

maximum available TBS is as large as 504 bits. Voice services are delay-sensitive. If higher- layer data is not transmitted within the specified delay budget, voice quality deteriorates. To prevent this, TTI bundling is disabled when a G.711-defined high speech codec rate is used.

4.2.1.2 Principles

Entry into the TTI Bundling State

In eRAN8.1, the CELLULSCHALGO.TtiBundlingTriggerStrategy parameter is introduced. VoLTE Feature Parameter Description 4 Enhanced VoLTE Features

l When the TtiBundlingTriggerStrategy parameter is set to

SERVICE_VOIP(SERVICE_VOIP), TTI bundling applies to only VoLTE. Under this

parameter setting, the conditions for entering the TTI bundling state are as follows: – The TtiBundlingSwitch of the eNodeB is turned on.

– The UE supports TTI bundling.

– The UE has only one QCI 1 dedicated bearer and stays in the talk spurts state. In addition, the UE does not have data to transmit on the default bearer.

– The UL power of the UE is limited, and the number of PRBs is less than or equal to 3.

– The measured SINR is less than the target SINR for multiple consecutive times. The number of consecutive times is specified by the

CellUlschAlgo.StatisticNumThdForTtibTrig.

If the UE meets all these conditions, the eNodeB sends the UE an RRC Connection Reconfiguration message, instructing the UE to enter the TTI bundling state. l When the TtiBundlingTriggerStrategy parameter is set to

SERVICE_MULTIAPP(SERVICE_MULTIAPP), TTI bundling can apply to VoLTE

or a combination of VoLTE and data. Under this parameter setting, the conditions for entering the TTI bundling state are as follows:

– The TtiBundlingSwitch of the eNodeB is turned on. – The UE supports TTI bundling.

– The UE has a QCI 1 dedicated bearer.

– The UL power of the UE is limited, and the number of PRBs is less than or equal to 3.

– The measured SINR is less than the target SINR for multiple consecutive times. The number of consecutive times is specified by the

CellUlschAlgo.StatisticNumThdForTtibTrig.

If the UE meets all these conditions, the eNodeB sends the UE an RRC Connection Reconfiguration message, instructing the UE to enter the TTI bundling state. The processing in versions earlier than eRAN8.1 is the same as that when the

TtiBundlingTriggerStrategy parameter is set to SERVICE_VOIP(SERVICE_VOIP) in

eRAN8.1.

Data Block Transmission

For the UE in the TTI bundling state, the eNodeB determines the number of PRBs and MCS based on channel quality and the amount of data to transmit. Then, the eNodeB transmits data blocks.

As shown in Figure 4-5 , the UE transmits identical data within four consecutive TTIs in a bundle and performs HARQ retransmission also within four TTIs in a bundle. The

retransmission operates in synchronous non-adaptive mode. The HARQ retransmission interval is changed from 8 TTIs (Normal HARQ RTT) to 16 TTIs (Bundle HARQ RTT). Take the transmission of a data block as an example. Assume that the UE transmits the data block in a bundle of TTIs, among which the last TTI is numbered N. The eNodeB sends an ACK or NACK as feedback to the UE in the (N + 4)_th TTI. Based on the feedback, the UE determines whether a retransmission is required. If it is required, the UE retransmits the data block in the (N + 13)_th through (N + 16)_th TTIs.

When the UE is in the TTI bundling state, the maximum number of uplink HARQ

retransmissions is specified by the CellUlschAlgo.TtiBundlingHarqMaxTxNum parameter.

Figure 4-5 TTI bundling

In the TTI bundling state, the number of RLC segments of a voice packet cannot be greater than the value specified by the CellUlschAlgo.TtiBundlingRlcMaxSegNum. The number is 4 in Figure 4-6.

Figure 4-6 Collaboration between TTI bundling and RLC segmentation

When the UE is located at the cell edge, RLC segmentation in collaboration with TTI bundling produces fewer RLC segments than pure RLC segmentation, reducing PDCCH overheads.

Exit from TTI Bundling

When the measured SINR is greater than the sum of the target SINR and the

CellUlschAlgo.HystToExitTtiBundling parameter value for multiple consecutive times, the

eNodeB instructs the UE to exit the TTI bundling state through an RRC Connection Reconfiguration message. The number of consecutive times is specified by the

StatisticNumThdForTtibExit parameter.

The eNodeB does not instruct the UE to exit the TTI bundling state even when the UE has data to transmit on the default bearer, needs to set up a new dedicated bearer, or stops the voice service (QCI 1). The eNodeB instructs the UE to exit the TTI bundling state when the UE meets the exit conditions, experiences handover or service drop, or needs to reestablish a new connection.

4.2.2 ROHC

This section describes how the optional feature LOFD-001017 RObust Header Compression (ROHC) works for VoLTE. For details about this feature, see ROHC Feature Parameter

Description.

ROHC can compress the RTP, UDP, or IP header of a voice packet, thereby reducing the size of the entire packet. ROHC results in a higher probability of correctly transmitting voice packets with fewer segments and enhances the edge coverage for voice services.