Research on key technologies for realizing satellite voice communication based on IoT NTN

In recent years, with the rise of commercial aerospace, the cost of satellite manufacturing and launch has been greatly reduced, and the rapid development of spaceborne antenna technology has injected new vitality into the satellite communication industry. At the same time, with the improvement of communication chips and terminal capabilities, the direct connection of public handheld terminals to satellites based on satellite-ground network integration has gradually become a reality. "Mobile phone direct connection to satellite" has a wealth of application scenarios and urgent user needs, especially in the case of rescue, emergency rescue and emergency communication, etc., to provide users with basic communication support, and real-time voice communication capability is a necessary requirement in such application scenarios.

In order to solve the problems of high latency, high path loss and limited capacity of satellite communication system, this paper proposes a voice capability enhancement solution for public handheld terminals based on IoT NTN (Internet of Things terminal access based on non-terrestrial network) technology from the perspective of network optimization.

Needs and status studies

Requirements overview

Satellite communication has the characteristics of wide coverage, strong real-time and strong anti-interference ability, and can provide independent emergency communication services in areas without terrestrial network coverage or when the terrestrial conventional communication infrastructure is damaged. In emergency rescue scenarios, both parties urgently need to establish low-latency and high-reliability real-time interaction in a short time, so the real-time voice capability of satellite communication is indispensable. In the scenario of emergency rescue and disaster relief, the emergency command and dispatch system based on satellite voice communication is highly flexible and is the basis for building a national emergency rescue communication system. In outdoor emergency scenarios, the public can call for help through mobile phones with satellite voice calls.

Compared with traditional mobile satellite communication systems such as GMR (Geostationary Orbit Mobile Radio Interface), the 5G NTN system has natural advantages in terms of standard evolution, industry drive, and satellite-ground integration. However, at present, there are no commercial mobile communication satellite resources in China, which can meet the requirements of NR NTN (5G intelligent terminal access based on non-terrestrial network) under the current standard in terms of bandwidth and link budget.

Analysis of the current situation and feasibility

In recent years, IoT NTN application innovation has developed rapidly at home and abroad. Internationally, Inmarsat and MediaTek have carried out a number of two-way satellite communication tests based on IoT NTN technology, and announced that they will jointly build two-way satellite communication functions for smartphones, Internet of Things devices, cars and other terminals in the future. In China, scientific research institutions, equipment manufacturers, satellite operators and other upstream and downstream enterprises in the industrial chain have jointly carried out multiple satellite-ground convergence communication tests and verifications in chips, terminal modules, network equipment, etc., based on the 3GPP R17 standard, realizing the end-to-end full-link technology integration of IoT NTN. In the future, once satellite voice communication is supported, a ToC emergency communication network based on the NTN industry ecology can be further built, forming a significant business model competitive advantage.

In the process of 5G NTN evolution, in order to meet the voice call needs of users, the industry has carried out research on voice enhancement solutions based on the IoT NTN standard system, and proposed three solutions. Scheme 1 adopts the idea of multi-domain system integration and proposes a voice optimization solution based on the new signaling gateway, which can effectively reduce the signaling interaction between the terminal and the satellite and save satellite resources. Solution 2 is a voice call solution based on Web-RTC (real-time communication from web pages) architecture, which uses custom interfaces to customize voice protocols, which can greatly improve the efficiency of voice signaling interaction. Solution 3 is a voice enhancement solution based on IMS (IP Multimedia System) signaling optimization, which can shorten the interaction delay between terminals and the IMS network and improve the interaction efficiency by streamlining the SIP/SDP (Session Initiation Protocol/Session Description Protocol) process and fields.

According to the research results at the current stage, optimizing the existing ground system and compressing the signaling cost of satellite-ground voice interaction are important foundations for IoT NTN to realize voice services. In order to achieve compatibility with the terrestrial voice communication system, all parties in the industry are committed to innovating voice call solutions based on IMS optimization, carrying out customized research and development on the network and chip side, streamlining the signaling process, and optimizing low-speed voice codec algorithms to better support voice call functions in IoT NTN scenarios.

The key technology of satellite voice communication based on NTN

Network architecture optimization

In view of the network characteristics of the IoT NTN system, in order to meet the needs of users for voice calls in the evolution of satellite communication networks, the existing terrestrial cellular network system and construction scheme can be used as a reference to reduce the signaling cost of satellite-to-ground voice interaction. The existing terrestrial 4G/5G mobile communication network voice service adopts the IMS network architecture, which is optimized in the satellite network to realize the compatibility of satellite and terrestrial voice communication systems.

In terms of signaling interaction, the simplified IMS SIP signaling is considered to be used between the terminal and the IMS network, and the IMS is enhanced to realize the conversion between the simplified SIP and the standard SIP, and the standard SIP signaling is used for interaction between the IMS network and the ground network.

In terms of voice media stream interaction, in order to realize low-speed voice media stream transmission, low-speed voice codecs are deployed in terminals and IMS network gateways. Among them, the voice stream adopts low-speed voice codec interaction between the terminal and the IMS voice gateway, and the conversion between low-speed voice coding and standard voice coding is realized at the IMS voice gateway.

The UP plane of the core network bears the voice solution

The data transmission on the wireless side of IoT NTN supports CP (control plane) mode and UP (user plane) mode. The CP mode supports data transmission through NAS signaling, which carries less data and is suitable for IoT burst packet services, while the UP mode is suitable for continuous data services by establishing a normal data wireless bearer DRB (data wireless bearer).

In the IoT NTN system, the UP mode supports only two DRBs and cannot carry data and voice services at the same time. For mobile satellite communications, voice is an important business. In this example, the three types of bearers (data bearer, voice signaling bearer, and voice media stream bearer) implemented on the UP plane can be automatically switched between different services, that is, different bearers are updated for users as services change.

Consider enhancing the base station by setting network trigger conditions so that the base station releases data bearers and establishes voice bearers under certain conditions. In this process, the terminal receives a bearer release triggered by the network, which may cause the terminal to initiate the TAU (Tracking Area Update) process, resulting in signaling consumption, so it is also necessary to partially enhance the terminal. When a user stops a voice call, the voice bearer is released, and the network should have the capability to fall back to the data bearer.

SIP Lite Solution

1. Analysis of IMS SIP simplification principles

Due to the limited air interface resources of the NTN network, the standard IMS SIP signaling messages are lengthy and the endpoint call setup time is long, resulting in a lower voice establishment success rate. Optimizing the voice establishment signaling interaction process and shortening the call setup time with a small signaling length is the main method for IoTNTN to achieve voice service support capabilities. In order to promote the interconnection with the 5G NTN network architecture and smoothly migrate to future voice services, the voice network architecture optimized based on IMS should be given priority.

Specific signaling optimization can be based on but not limited to the following principles: first, the SIP header name is encoded in abbreviated format, and the header name without abbreviation is not defined in SIP; second, some parameters of SIP messages sent by UE to IMS can be omitted at the air interface and then added by IMS network functions; third, the IPv6 address encoding of SIP/SDP messages sent by UE uses a simplified format; and fourth, some voices (according to the priority or real-time situation of resources) omit signaling interaction steps.

In addition to the above-mentioned signaling reduction, an enhanced voice signaling gateway device is introduced into the terrestrial network to support satellite communication voice codec negotiation and conversion, and the difference in voice media flow between UE and IMS networks is shielded in the SIP message reduction scheme.

Since NTN network bandwidth resources are limited and latency is high, SIP signaling needs to adapt to the performance characteristics of NTN network to simplify the complexity of signaling interaction and reduce latency. In order to simplify SIP messages based on the IMS network, some network functions between the terminal and the IMS need to be enhanced to shield the message differentiation between the terminal and the standard IMS network by shielding SIP signaling, so that the IMS network can improve the network interaction efficiency while ensuring the call experience of end users.

2. Latency analysis

The transmission delay of IoT NTN voice communication includes terminal vocoder delay, baseband delay, satellite air interface delay, base station protocol stack delay, core network to IMS user plane delay, IMS user plane voice codec conversion delay, large network processing delay, and large network terminal processing delay. Due to the limited air interface bandwidth (which cannot be frequently packaged) and the long-distance transmission of high-orbit satellites, the uplink delay is concentrated in the vocoder module and satellite air interface transmission, accounting for about 80% of the overall delay, and the downlink delay is concentrated in the satellite air interface transmission and IMS voice conversion process, accounting for about 80% of the overall delay.

The analysis shows that packet compression, vocoder optimization, and codec optimization can effectively reduce the transmission delay of voice packets in satellite access scenarios and improve the user experience of NTN network voice calls.

Summary and outlook

Based on the preliminary research and standardization process of 3GPP, this paper proposes the IoTNTN converged networking architecture and system optimization direction that can support voice communication, and comprehensively considers the network characteristics of satellite resource limitation and high latency, and proposes a solution for the simplification and optimization of voice interaction signaling, so as to reduce the consumption of satellite-ground voice interaction signaling on satellite-ground resources and ensure the quality of voice services in satellite access scenarios. However, there are still some key problems to be studied, such as the relatively high air interface delay of satellite communication, which needs to be further adapted and optimized to enhance the function and performance of network elements, and the Doppler frequency shift caused by dynamic changes of satellites affects the quality and stability of voice signals, and the stability of voice services needs to be improved through the enhancement of algorithms and codec capabilities.

Driven by technological innovation, user needs, and network evolution, "virtual and real symbiosis, immersion and multi-sensory, human-like intelligence, and intelligent connection of all things" will become the development vision of real-time communication in the future. As an important technology for space-ground integration and ubiquitous connection in the future, NTN network has broad application prospects and practical significance, and is an important cornerstone for realizing ubiquitous real-time, multi-dimensional connection and high-quality communication services.