5G Voice Technology: VoLTE & VoNR

Voice in 5G

• As radio access evolves from GSM to 5G, technology adoption is key.
• Quality and performance must match or exceed legacy systems before 5G commercial deployment.
• Technology maturity can take time.
• End-to-end aspects of voice should be considered.
• GSM uses circuit switched tech for the initial voice application.
• WCDMA provides voice and data simultaneously, introducing new HD voice codecs.
• VoLTE enables voice and superior data connectivity, faster call setup time, HD voice from start, and EVS codec introduction.
• VoNR allows access to large bandwidth and voice, EVS codec from start and prepares for ultra-reliable services using packet switched technology.

Voice Migration to 5GS

• 3GPP 5G architecture has 5 options for radio network and packet core.
• Early NR adopters prefer Option 3.
• Early 5GC adopters prefer Option 2.
• 5G evolution is not led by voice, but voice support is crucial in deployed 5G architectures/evolution steps.
• All phones, mostly MBB devices, require a voice service in 5G.
• IMS telephony, known as VoLTE in 4G, also functions in 5G.
• Both VoLTE and CS Fallback are supported in Option 3.
• The voice solution for 5GS needs to be tightly linked to existing 4G VoLTE deployment initially.

Voice in EN-DC (option 3)

• The typical coverage scenario for VoLTE has option 3 deployments.
• There is initially spotty NR coverage that overlaps with LTE coverage (NSA).
• Supporting VoLTE involves using wide-area LTE coverage.
• The VoLTE network may collaborate with the underlying 2G/3G network, like single radio Voice call continuity (SRVCC), SMS over 4G NAS, and CSFB.
• E-UTRAN is enhanced to include NR support or E-UTRA-NR Dual Connectivity (EN-DC).
• Legacy 4G VoLTE continues using IMS over RAN that is partially or fully upgraded to EN-DC.
• There are no architectural changes or impact on IMS.
• All VoLTE principles from 4G are applicable.
• PDN connections to IMS APN with default EPS bearer used for SIP signaling (QCI=5) and dedicated EPS bearer used for Voice media (QCI=1).
• Recommends Bearers for SIP signaling (QCI=5) and Voice media (QCI=1) in IMS PDN served by LTE.
• Internet PDN default bearer (QCI=9) is mapped to a split bearer for LTE and NR combination.
• Voice KPIs are better (Voice flow retainability).
• VoLTE leverages reuse of features, optimization, and tuning.
• As early NR deployments on high/mid bands with limited coverage are unsuitable for Voice.

Voice over NR

• The 5GC supports NR, LTE & Wi-Fi as well as NR CP & UP. Emergency and regulatory requirements should be considered. Voice can be made on LTE & NR.
• UE establishes PDU session with session type=IP for corresponding DNN.
• IMS support includes SSC mode 1.
• P-CSCF discovery occurs during PDU session establishment.
• Possible establish one QoS flow with QFI for signaling.
• All associated media flows (RTP & RTCP) supports single media component carried within same QoS flow.
• Interface to PCF is available for policy control.

Voice over NR (Option 2)

• NR coverage is connected to 5GC, however, there is no support of native voice.
• EPC supports VoLTE and controls wide-area LTE coverage.
• The VoLTE network may interwork with 2G/3G (SRVCC support).
• Either VoLTE or CS network handles emergency services.
• When setting up the phone at call, it's connected to NR and EPS fallback to E-UTRAN performed.
• When establishing voice flow bearer with QoS over NR: Rejects QoS flow setup, EPS fallback in progress
• Initiate transfer of all PDU sessions from 5GS to EPS.
• There are two standardized methods to move UE to EPS. 1. Release with redirect 2. IRAT with inter-system handover
• Seamless voice service provided using IRAT HO between 4G and 5G by tight interworking 5GC/EPC.
• CS interworking from EPC 4G is for 5G RAN with 5GC.Surrounding 4G with EPC provides IMS voice/emergency service.

Communication Services & Network Slicing in 5GC

• SMF & UPF support S5 interface.
• Helps IP flows to use same SMF + UPF, regardless 5G UE on NR or LTE which maintains the IP address.
• This is done by mapping 5GS & EPS QoS (5QI & QCI) which is single policy control point for 4G/5G combined PCF/PCRF.
• N26 connects MME & AMF and allows UE context transfer.
• IRAT with inter-system handover can meet KPI of max 300 ms voice gap with EPS Fallback, but N26 limits call setup time to meet KPI.
• N26 resides between MME and AMF for UE context transfer.
• A common function for SMF and PGW-C, mapping between 5QI and QCI is found. Specifically a 5G UPF is selected for 5G capable UEs, both by MME and 5G SMF.

Voice on integrated WLAN

• EPC integrated WLAN supports IMS services and required handover cases.
• Handover support is between Wi-Fi & LTE or Wi-Fi & NR, use "EPC integrated WLAN" (evolve).
• There is no urgent need for "5GC integrated WLAN".

5G voice evolution Steps

• Dual Connectivity (EN-DC): LTE for voice and NR as data booster.
• EPS Fallback: NR for data, but falling back to LTE (for voice and data) when making or receiving calls with 2 variants such as Release with Redirect, Handover. The EPC and 5GC also have N26 interworking and IP address preservation in 5GC.
• Voice over NR: NR dimensioned for voice coverage is available. There is a seamless handover between 5G and 4G (and WiFi), without handover/fallback from 5G to CS.

Charging in 5G Core

• Charging uses Nchf, a service-based interface as SMF triggers converged offline and online charging. This can enable PCF trigger spending limit control as CHF act as CDF & generate CDRs for all 5G NF.
• CHF service is exposed through Nchf. OCS, CCS, CDF, CGF functions are used in the logical charging architecture.