Semi-Persistent Scheduling

When the physical downlink control channel (PDCCH) results in insufficient system capacity, this function can improve VoLTE user capacity by reducing the PDCCH overhead, or can increase data throughput with the unchanged number of VoLTE users. 

When voice services are dynamically scheduled, time-frequency resources or Multimedia Communication Services (MCS) are updated on the PDCCH every 20 ms, which consumes a large number of PDCCH resources. Huawei provides the VoIP semi-persistent scheduling feature for periodically transmitted small packet VoIP services. Before entering a talk spurt state, the eNodeB only allocates fixed resources to the UE through a PDCCH message once and does not need to allocate resources again before ending a talk spurt or releasing resources. In this way, PDCCH resources are saved. When setting up the QCI1 DRB, the eNodeB configures the semi-persistent scheduling parameters in the RRC Connection Reconfiguration message for a UE that supports the semi-persistent scheduling feature. As long as a UE meets the requirements for activating semi-persistent scheduling, the eNodeB instructs the UE to enable this function in the uplink or downlink through a PDCCH Order. For details about the PDCCH Order format for activating semi-persistent scheduling, see section 9.2 in 3GPP TS 36.213 (V12.3.0). 

The period of semi-persistent scheduling can only be set to 20 ms and is applicable only for QCI1 and PTT QCI services. 

Uplink Delay-Based Dynamic Scheduling

When a system has a large number of VoLTE users, VoLTE performance at the cell center is compromised to improve VoLTE performance at the cell edge. In this way, the QoS satisfaction rate of voice users is improved throughout the entire system. When using the uplink delay-based dynamic scheduling function, the eNodeB prioritizes voice packets based on their waiting times and sorts scheduling priorities, achieving a balanced scheduling sequence. This helps improve voice quality, especially for cell edge users with poor channel quality. With a high amount of voice services, uplink delay-based dynamic scheduling improves the satisfaction rate of voice users.  

AMR

VoLTE uses the Adaptive Multirate (AMR) voice encoding and decoding mode. AMR is an optimized audio data compression mode for speech coding and is now widely used on GSM and UMTS networks. The AMR encoding mode includes the adaptive multirate AMR-WB and the adaptive multirate narrowband (AMR-NB). The following shows the eight voice coding rates of AMR-NB and nine voice coding rates of AMR-WB: 

AMR-NB: 12.2 kbit/s, 10.2 kbit/s, 7.95 kbit/s, 7.4 kbit/s, 6.7 kbit/s, 5.9 kbit/s, 5.15 kbit/s, and 4.75 kbit/s

AMR-WB: 23.85 kbit/s, 23.05 kbit/s, 19.85 kbit/s, 18.25 kbit/s, 15.85 kbit/s, 14.25 kbit/s, 12.65 kbit/s, 8.85 kbit/s, and 6.6 kbit/s

The AMR voice encoding and decoding mode is usually used for VoLTE services. The voice   service model is shown in the figure below. A specific AMR voice encoding scheme is selected through UE negotiations. The IMS is an optional entity in this process. This negotiation process is visible to the eNodeB. 

Voice services exist in the following two states:

Talk spurts

A talk spurt refers to a period when a terminal sends voice data packets in the uplink or receives them in the downlink. Voice data packets are sent every 20 ms. The size of voice data packets depends on the current coding rate. 

Silent periods

A silent period refers to a period when a terminal sends a silence insertion descriptor (SID) frame in the uplink or receives one or in the downlink. SID frames are sent every 160 ms. The length of an SID frame is 56 bits for the AMR speech coding rate.

The following lists the key differences between talk spurts and silent periods: 

The size of voice packets is greater than SID frames. 

Intervals between adjacent voice packets and adjacent SID frames are noticeably different. 

Based on the above differences, the eNodeB can determine whether the voice service is in a talk spurt or in a silent period.

ROHC

Robust Header Compression (ROHC) is a mechanism dedicated to solving high bit error rates (BERs) and long loopback time on radio links by compressing the headers of data packets. The ROHC function helps reduce the header load, packet loss, and interaction response time. In addition, the ROHC reduces voice data packet size and PRB load System capacity can be increased without sufficient PRBs. ROHC compresses RTP/UDP/IP headers of voice packets and uses fewer fragmented packets to efficiently ensure the correct transmission of voice data packets and increase the cell edge coverage of voice services. 

ROHC uses different header compression algorithms for data flow based on different protocols. The ROHC compression efficiency varies based on the ROHC operating mode and changes of dynamic domains of packet headers at the application layer. Therefore, compressed packets appear in a variety of sizes. Packet headers can be compressed up to one byte, effectively reducing the size of voice data packets. 

TTI Bundling

By using TTI bundling, a data block is transmitted over four consecutive TTIs which are treated as one resource unit to achieve HARQ combining gains in the uplink. TTI bundling reduces retransmission and the round trip time (RTT) while leveraging the combined HARQs. 

At the cell edge, TTI bundling improves the PUSCH coverage by about 1 dB with poor channel quality and limited transmit power TTI bundling is designed to improve the uplink quality for voice users with limited transmit power at the cell edge. Gains can be observed when certain requirements for voice quality are met, such as MOS is 3.

After the TTI bundling function is enabled, the eNodeB automatically determines whether to use TTI bundling based on channel conditions. After the UE uses the TTI bundling, the eNodeB determines the number of PRBs and selects MCS based on channel quality and the amount of data to be transmitted. 

For carriers, VoLTE deployment ushers in a new era of mobile broadband voice service evolution. VoLTE will eventually help carriers increase wireless spectral efficiency and reduce network costs. For voice services, LTE spectral efficiency is much higher than that of the traditional standard. For example, LTE spectral efficiency is more than four times higher than that of GSM. 

In addition, VoLTE provides far better user experience than traditional CS voice services. Firstly, the introduction of encoding and decoding technologies for HD voice and HD video improve communication quality. Secondly, VoLTE greatly shortens call connection duration, with test results showing call connection duration shortened by more than half of CS voice. Thirdly, seamless integration with the Rich Communication Suite (RCS) results in diversified services.