Key VoLTE Technologies
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.