Special Articles on Mobile Satellite Communications Systems (1)
Overview of WIDESTAR III System and Services

Mobile Satellite Communications Services LTE over GEO Satellite Data Acceleration

Kenji Kamogawa, Hiroki Oyabu and Masahiro Inoue
Network Department

Motohiro Tanno
Device-Tech Development Department

Tomoyuki Otani
Core Network Design Department

Abstract
WIDESTAR III, which was launched nationwide in October 2023, is a pioneering mobile satellite communications service based on the LTE over GEO satellite system that covers all of Japan and its exclusive economic zones with 64 beams using the new generation geostationary communications satellite N-STAR e. To meet the social mission and needs of mobile satellite communications, WIDESTAR III services must provide faster data transmission speeds and greater convenience, in addition to the succession of the existing WIDESTAR II services. Therefore, we have developed new core network, satellite base station equipment, satellite mobile stations, and an application for connecting to satellite mobile stations that meet these requirements, enabling economical operations and provision of a variety of satellite communications services with higher speeds.

01. Introduction

  • Under the “WIDESTAR” brand, NTT DOCOMO has been ...

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    Under the “WIDESTAR” brand, NTT DOCOMO has been providing mobile satellite communications services by integrating the mobile satellite communications services launched in March 1996 [1] and the satellite packet communications services launched in March 2000 [2]. In April 2010, the service was succeeded and expanded as “WIDESTAR II” [3], which supports higher speeds, to provide mobile satellite communications services that handle the usage scenarios of customers. Since the start of service, WIDESTAR has been an important means of communications for ships to ensure distress and safety communications while at sea as a general communications facility of the Global Maritime Distress and Safety System (GMDSS)*1. On land, this service is used by local governments for disaster countermeasures and coverage of cell phone dead zones, and is increasing in importance as a means of communications in Business Continuity Planning (BCP) during disasters [4].

    In October 2023, NTT DOCOMO launched the new mobile satellite communications services, WIDESTAR III [5], simultaneously across Japan. This service system was developed to realize a satellite communications network with higher speed and capacity to cope with increased traffic volume during disasters, to improve user convenience, and to meet changing social needs and user demands for more severe and prolonged disasters due to climate change caused by global warming. WIDESTAR III is the world's first commercialization of the LTE over Geostationary Orbit (GEO) satellite system, enabled by customizing the LTE system [6], the international standard for cellular systems.

    By effectively utilizing the communications performance of the LTE Radio Access Technology and the satellite relay capability of the N-STAR e [7] digital high-throughput communications satellite equipped with an ultra-large deployable antenna, this system achieves more than six times the number of simultaneous connections (voice call equivalent) of WIDESTAR II and the world's fastest downlink data transmission speed (up to 3 Mbps, approximately eight times faster than WIDESTAR II) in mobile satellite communications systems using frequencies in the 3 GHz band or lower. In addition, the service provides a multi-call*2 service and Short Message Service (SMS), which were not available in the past, as well as a real-time fax service, and by developing an application that connects to a satellite mobile station (hereinafter referred to as the “mobile station”), a Wi-Fi connected smartphone can be used as a handset, which greatly improves user convenience.

    This article provides an overview of the development of the WIDESTAR III mobile satellite communications system and service.

    In these special articles, this and other articles describe technological details of mobile stations [8], satellite base station equipment [9], and the core network*3 developed for WIDESTAR III.

    1. GMDSS: A world-wide system for safety at sea.
    2. Multi-call: The processing of multiple calls, such as when a voice call arrives during packet communications.
    3. Core network: A network comprising switching equipment, subscription information management equipment, etc. A mobile terminal communicates with the core network via a radio access network.

    • 02. Overview of WIDESTAR III

  • WIDESTAR III uses the N-STAR e geostationary communications ...

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    An image of the WIDESTAR III service is shown in Figure 1.

    WIDESTAR III uses the N-STAR e geostationary communications satellite equipped with a next-generation large antenna to form a service area covering the whole of Japan and the seas around Japan with 64 beams with radii of approximately 150 km to 400 km, compared to the same service area formed by the conventional service with four beams with radii of approximately 600 km. The specifications of N-STAR e and N-STAR d used for WIDESTAR II are shown in Table 1. N-STAR e is equipped with an ultra-large 18-meter-diameter antenna and has relay capabilities for high-speed, large-capacity data communications, as well as the ability to change the relay frequency bands and beams in orbit through digital beamforming*4. The feeder link*5 between the base earth station*6 (hereafter referred to as the “base station”) and the satellite uses the C-band*7 (6/4 GHz band), while the service link*8 between the mobile station and the satellite uses the S-band*9 (2.6/2.5 GHz band), which is resistant to rainfall attenuation.

    Figure 1 WIDESTAR III service image, Table 1 N-STAR satellite specifications
    1. Beamforming: A technique to increase/decrease antenna gain in a specific direction by forming a directional pattern on the antenna through control of the amplitude and phase of multiple antennas. In this article, beamforming is achieved by digital control.
    2. Feeder link: The radio communications path between a base earth station and a communications satellite in a satellite communications system.
    3. Base earth station: A radio base station for wireless communications with a communications satellite in a satellite communications system. In this article, this consists of a C-band antenna and satellite base station equipment.
    4. C-band: A name for the frequency band from 4 to 8 GHz. WIDESTAR III uses the 6/4 GHz frequency bands.
    5. Service link: The radio communications path between a mobile station and a communications satellite in a satellite communications system.
    6. S-band: A name for the frequency band from 2 to 4 GHz. WIDESTAR III uses the 2.6/2.5 GHz frequency bands.

    • 03. Requirements

  • 3.1 Service Requirements

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    WIDESTAR III must continue to offer the basic services provided by the existing WIDESTAR II, such as voice calls, data communications, and faxes, while improving user convenience and providing solutions to satisfy user demands. Further, system capacity requires improvement to allow for the lower data communications fees necessary to expand solutions.

    3.2 Network Requirements

    To reduce the burden of development and operation for economical systems, general-purpose technology has been actively applied to each WIDESTAR III equipment. To smoothly implement the gradual system migration from WIDESTAR II, both systems must be able to be accommodated on the same communications satellite by frequency division multiplexing*10 [10], and there must be no radio interference between the systems. In addition, the system capacity must be larger than WIDESTAR II (more than 10,000 simultaneous connections (voice call equivalent)) to accommodate increased traffic due to the severity and duration of disasters and the diversity of disaster relief efforts, and radio frequency settings must be flexibly changed to accommodate traffic demand in disaster areas.

    1. Frequency division multiplexing: A multiple access method that divides frequencies and allocates a radio channel to each mobile station.

    • 04. System

  • The schematic configuration of the WIDESTAR III mobile satellite ...

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    The schematic configuration of the WIDESTAR III mobile satellite communications system (hereinafter referred to as the “WIDESTAR III system”) is shown in Figure 2.

    For the radio system, we aimed to develop and put into practical use the world's first LTE over GEO satellite system based on the LTE radio communication protocol, using the existing base station C-band antenna equipment, the newly introduced N-STAR e, and newly developed satellite base station equipment and mobile stations. The core network system was upgraded from conventional 3G-based proprietary equipment to equipment based on a common architecture with the LTE service system used in the cellular system. The aim was to develop an economical system by adding and customizing only the functions necessary for mobile satellite communications to these pieces of equipment.

    As satellite links, the feeder link uses right- and left-handed circularly polarized waves*11 of 6,345 to 6,425 MHz uplink and 4,120 to 4,200 MHz downlink, while the service link uses right-handed circularly polarized waves of 2,655 to 2,690 MHz uplink and 2,500 to 2,535 MHz downlink.

    Figure 2 WIDESTAR III system configuration

    4.1 Main Features

    1) Network Reliability

    To ensure high reliability of equipment, both the satellite base stations and core network in the WIDESTAR III system consist of cards*12 and servers in redundant configurations. In addition, the satellite base station equipment and core network each follow a site diversity*13 network configuration with two sites located far apart, so that services can continue even if one of the stations fails or is damaged. At each base station site, the sun transit phenomenon*14 occurs twice a year, in spring and fall, when the communications satellite and the sun appear to overlap, which may temporarily degrade communications quality. In addition, if radio wave transmission from the base station at one site to the satellite is halted due to station construction or other reasons, system capacity is reduced by half. However, using the digital processing function of N-STAR e to change the allocation of frequency resources to one of the base stations online makes it possible to minimize the impact on services.

    2) Load Balancing

    Load balancing of base station equipment and the core network is necessary for stable system operations. To distribute the communications load between the two base stations, mobile stations must be able to use information from the base station equipment to timely select the cell of the base station equipment with the lower load. To distribute the communications load of the core network at the two sites, distributed selection of core network is directed by the base station equipment for mobile stations accessing the network.

    3) Satellite Orbit Information

    N-STAR e used for WIDESTAR III operates in an inclined geostationary orbit at a maximum tilt angle of +/-7 degrees to form a figure 8 in the north-south direction, which always causes variations in propagation delay of satellites between base station equipment and mobile stations. To adjust the base station equipment reception timing of the uplink signal from the mobile station to always stay within a certain range, the base station equipment periodically obtains N-STAR e orbit information from the satellite control system and notifies mobile stations of the transmission timing through system control signals. Since satellite orbit information contains information over a significant period of time, there will be no service impacts even if the link between the base station equipment and satellite control system is temporarily broken.

    4.2 Main Component Network

    1) Mobile Station

    NTT DOCOMO has developed two types of the WIDESTAR III mobile station: a portable type and a stationary type. While achieving the same or better continuous call connection and standby times with battery operation as conventional WIDESTAR II portable type terminal, the LTE chipset implementation functions were customized for mobile satellite communications systems, resulting in faster up and down link data transmission speeds. The portable model is equipped with a Wi-Fi router function so that a smartphone can be used as the handset for voice services and other applications using a connection application we developed for installation on smartphones.

    2) Satellite Base Station Equipment

    The satellite base station equipment consists of a satellite Radio Frequency transmitting and receiving Equipment (RFE) and a Satellite Access Controller (SAC).

    RFE's main roles are frequency conversion and gain*15 adjustment for transmitting and receiving to/from N-STAR e with C-band radio signals, which were achieved by combining general-purpose radio equipment units.

    The main roles of SAC are communications processing, such as access control processing of mobile stations and transmission/conversion processing of user data, which are performed in conjunction with the core network, and equipment maintenance processing, such as monitoring alarms and traffic collection for equipment components. These were realized by installing customized LTE protocol applications on general-purpose hardware and servers.

    Monitoring of base station equipment can be controlled via a Web monitoring screen running on a general-purpose PC deployed at the operations center. Interface monitoring with satellite control system can also be performed at the same time.

    3) The Core Network

    The core network is based on architecture common with LTE service systems used with cellular systems, enabling the provision of more advanced voice and data communications services compared to the existing WIDESTAR II. In addition, signal compression technology was applied as a satellite-specific function, and various timer values were customized to counter propagation delay and improve bandwidth efficiency.

    Between the base station equipment and the core network, voice calls are communicated as ITU-T G.711*16 voice data converted by SAC with codec*17 and fax calls are communicated as data using the ITU-T T.30 protocol*18 under the routing control common to cellular. To control packet pack services*19, the minimum speed was changed to match the transmission rate of the satellite radio link.

    The core network is built separately from the cellular accommodation facilities to ensure high reliability unaffected by the cellular congestion*20 that can be expected in the event of a large-scale disaster.

    1. Right- and left-handed circularly polarized waves: A form of radio wave propagation in which the electric field (magnetic field) rotates circularly (right- or left-handed rotation) in the direction of the radio wave propagation.
    2. Cards: Component panels and boards of power amplifiers, frequency converters, and modulation and demodulation processing functions.
    3. Site diversity: A method of installing base stations at a greater distance from each other to reduce the effects of rainfall attenuation between base stations and communications satellites.
    4. Sun transit phenomenon: A phenomenon in which the base station antenna receives solar noise when receiving radio signals from a satellite due to the overlap of the satellite and the sun, resulting in reduced communications quality. Occurs twice a year in spring and fall.
    5. Gain: The power increase ratio of amplifier input power to output power.
    6. ITU-T G.711: A standard specification for 64 kbps fixed bit rate voice encoding defined by International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and used for transmitting voice signals in a fixed telephone network.
    7. Codec: Technology for coding and decoding data such as audio signals.
    8. ITU-T T.30 protocol: A transmission control protocol standardized by ITU-T for G3 fax communications over switched telephone networks and Integrated Services Digital Networks (ISDN).
    9. Packet pack service: A data service menu in WIDESTAR III. If the amount of available data is exceeded, the communications speed will be throttled to maximum of 1 kbps.
    10. Congestion: A state where communications requests are concentrated in a short time and exceed the processing capabilities of the network, thereby obstructing communications.

    • 05. Services Provided

  • Services offered through WIDESTAR III are listed in ...

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    Services offered through WIDESTAR III are listed in Table 2. Services provided by WIDESTAR III have been selected and discarded in consideration of demand and differentiation from other telecommunications services, even among those offered under WIDESTAR II.

    We have made efforts to enhance WIDESTAR III data communications services to meet the needs during disasters. In addition to inheriting the existing direct connection services, which provide Peer to Peer (P2P)*21 data communications between mobile stations, a best-effort type packet communications service has achieved faster speeds of up to 1 Mbps uplink and 1.5 Mbps downlink. Furthermore, using carrier aggregation*22 enables the world's fastest downlink speed of 3 Mbps with mobile satellite communications systems in the 3 GHz band or lower.

    User convenience is also enhanced by newly offering SMS and multi-call services and by enabling smartphones to be used as handsets instead of mobile stations through a Wi-Fi connection application for smartphone linkage.

    The main services are as follows:

    1) Voice Call Service

    For voice call services, the 4 kbps AMBE+2TM codec [11], which is used in global mobile satellite communications services and has a low rate and excellent speech quality, was adopted to significantly increase the number of simultaneously connectable voice calls compared to WIDESTAR II while achieving voice call quality equivalent or better than WIDESTAR II. For voice calls to and from the fixed network, voice call services are provided by converting the codec to ITU-T G.711μ-law*23 codec at the base station equipment and transferring and exchanging it with the core network, while voice calls between WIDESTAR III mobile stations are made without conversion.

    2) Data Communications

    In data communications, WIDESTAR III provides best-effort packet communications services with asymmetric uplink and downlink transmission speeds. The customized LTE radio access technology achieves a maximum uplink speed of 1 Mbps (approximately seven times that of WIDESTAR II) and a maximum downlink speed of 1.5 Mbps (approximately four times that of WIDESTAR II) by (1) increasing the carrier bandwidth (from the 300 kHz of WIDESTAR II to 1.2 MHz) and (2) using higher-level modulation schemes (shift from π/4 shift Quadrature Phase Shift Keying (QPSK)*24 to 16 Quadrature Amplitude Modulation (16QAM)*25 and 64QAM*26). With carrier aggregation, the downlink transmission speed can be doubled to a maximum of 3 Mbps. This exceeds the speed offered by the Inmarsat system [12] in the L-band*27 (1.6/1.5 GHz) and provides the world's fastest data communications in mobile satellite communications services in the 3 GHz band or lower.

    3) Fax Service

    In the conventional WIDESTAR II fax service, a fax adapter connected to the mobile station converts G3 fax to e-mail, and data communications between the mobile station and the core network section is performed using the ITU-T T.37 protocol*28, which means that the sender cannot confirm delivery in real time. By adopting the ITU-T T.38 protocol*29 for the mobile station to base station equipment section and the ITU-T T.30 protocol for the base station equipment to core network section, G3 fax equipment can be connected to WIDESTAR II and WIDESTAR III mobile stations and fixed networks simply by connecting the equipment to the mobile stations to enable real-time fax services.

    Table 2 Services provided with WIDESTAR III
    1. P2P: Communications in which multiple computers exchange information with each other on an equal footing, as opposed to server-client communications. In this article, it means that mobile stations exchange information with each other on an equal footing.
    2. Carrier aggregation: A technology that uses multiple carriers to transmit and receive simultaneously to achieve wide bandwidth and high-speed transmission.
    3. ITU-T G.711μ-law: A compression method for speech coding standardized by ITU-T. Compresses 14-bit uncompressed Pulse Code Modulation (PCM) data to 8 bits.
    4. QPSK: A digital modulation method that allows transmission of 2 bits of information at the same time by assigning one value to each of four phases.
    5. 16QAM: A digital modulation method that allows transmission of 4 bits information simultaneously by assigning one value to each of 16 different combinations of amplitude and phase.
    6. 64QAM: A digital modulation method that enables the simultaneous transmission of 6 bits of information by assigning one value to each of 64 different combinations of phase and amplitude.
    7. L-band: A name for the frequency band from 0.5 to 2 GHz.
    8. ITU-T T.37 protocol: A transmission control protocol standardized by ITU-T for G3 fax communications via e-mail over IP networks.
    9. ITU-T T.38 protocol: A transmission control protocol standardized by ITU-T for real-time G3 fax communications over IP networks.

    • 06. Conclusion

  • This article has provided an overview of the advanced WIDESTAR III ...

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    This article has provided an overview of the advanced WIDESTAR III mobile communications satellite system and services, which began service in October 2023. WIDESTAR III, services based on the world's first LTE over GEO satellite system, are expected to meet a wide variety of customer needs through faster data transmission and usability with a connection application, in addition to serving as a conventional means of communications during emergencies and disasters. We plan to expand data communications use by improving service quality and introducing solutions based on customer feedback, while promoting migration from WIDESTAR II. Toward the Beyond 5G/6G era, Non-Terrestrial Networks (NTN)*30 that link satellite and terrestrial communications systems are demanded, and the WIDESTAR III system is expected to become more valuable as a GEO system.

    1. NTN: A network that extends the communications area to diverse locations including the air, sea, and space using non-terrestrial media such as satellites and High-Altitude Platform Stations (HAPS) without limiting the coverage area to land.

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