Design Concepts for Multimedia Satellite System

Shao-dong Feng, Hong-peng Zhu, Guang-xia Li Satellite Communication Lab, Institute of Communication Engineering

PLA University of Science &Technology Nan Jing, China

fengshaodong1981@163.com

Abstract—Our government’s whitepaper “China Spaceflight from 2006”pointed out that the large GEO multimedia satellites and broadband multimedia technology should be developed urgently. Satellites system can provide a high quality broadband communication services for subscribers especially in the internet access and disaster recovery. The development of satellite communication is reviewed; some design issues of Satellite system have been discussed. Based on the analysis and discussion a novel design concept for Multimedia Satellite System is proposed from internet point of view. A possible deign scheme has also been proposed which adopt the DVB-S2 and DVB-RCS from ETSI.

Keywords-Multimedia;satellite ; communication; broadband;

I. INTRODUCTION

The last few years have witnessed an explosive growth in the development and deployment of multimedia applications that transmit and receive over the Internet.

Multimedia applications such as video, VoIP, web browsing, teleconference, interactive games and distance learning etc seem to be heard everyday. The service requirements of these applications differ from the traditional application such as the telephony and television. Some of them are delay sensitive and others are not.

Multimedia is the combined presentation of several sources of data, notably text, audio and pictures (moving or still).It has many types and calls for different QOS. These different service requirements suggest that the traditional network may not be suited for the multimedia applications. So a novel system framework is urgently needed to support them.

Satellite communications distinguished by several characteristics such as global coverage, bandwidth on demand, flexibility, multicast, and broadband capability is an important part of the Global Information Infrastructure(GII)[1].During the earthquake in Si Chuan province thousands of satellite equipments have been used which show the powerful ability in disaster recovery and broadband access. It is undoubted that the multimedia will be the dominant service in the future. As an indispensable network, how could the satellite System evolves to better support the multimedia applications?

Multimedia is so diversiform but there is one thing in common: all of the multimedia traffic is based on IP.IP has been viewed as the next generation network platform and

widely supported by both service providers and network vendors.

Multimedia Satellite System can be defined as a high speed satellite network aimed at transfer and presentation of multimedia data, audio and picture (moving or still) based on IP.

Multimedia Satellite System can and will be used for numerous applications. Firstly, coarse division may be:

Broadband Access to the Global Information Infrastructure (e.g. Internet access);

Closed, global or regional, broadband communication systems. (e.g. LAN expansion).

Some typical multimedia services for satellite are:

Internet access and web browsing;

IP trunking

Electronic file transfer

Email and other electronic message services

IPTV

DTH&DBS

VoD(Video on demand)

Audio on demand

World-wide radio and music distribution

Books on demand. Local publishing and printing

Switched broadcast services and interactive TV

Teleconference

Distance learning

In recent years cost effective solutions for Multimedia Satellite Systems are being developed for multimedia applications. These systems require high quality of service, affordable prices and good matching of customer demand with the satellite system capacity for successful and profitable business operation. A number of regional access networks e.g. Wildblue, IPStar, SPACEWAY3 provide broadband Internet access as an alternative to cable, DSL, and wireless. Forward and return channels fully for such two-way IP platforms,

This work is supported by the National Natural Science Foundation of China (60472051))

978-1-4244-2251-7/08/$25.00 ©2008 IEEE

TDMA based standard technologies such as Digital Video Broadcasting – Return Channel via Satellite (DVB-RCS), and Data Over Cable Service Interface Specification (DOCSIS) are also very popular. Standardization activities [2, 3] for Multimedia Satellite Systems and QoS architectures [4] defined are carried out by ITU, ETSI, and IETF.

The Internet is definitely now a driving factor the development of Multimedia Satellite System not only in the market but also in the technology. The TCP/IP reference model has a great impact on the design concept of the satellite system. So history and design issues of satellite communication will be reviewed in part and a possible design will be discussed in part .

II. SATELLITE COMMUNICATIONINFRASTRUCTURE DEVELOPMENT

The Space Age began in 1957 with the U.S.S.R.’s launch of the first artificial satellite, called Sputnik, which transmitted telemetry information for 21 days. This achievement was followed in 1958 by the American artificial satellite Score, which was used to broadcast President Eisenhower’s Christmas message. The first geostationary satellite, called Syncom, was launched in 1963. The race for space exploitation for commercial and civil purposes thus truly started.

Till now satellite communication has developed for almost 50 years. It can be generally divided into three phases: the analog satellite communication, the digital satellite communication and multimedia satellite communication.

A. Analog Satellite Communication The first series of commercial geostationary satellites

(Intelsat and Molnya) was inaugurated in 1965.The analog signal is adopted and the main service is analog telephone. The system is designed according to their services. So system designers should consider every aspects of the system including the signal wave pattern, the signal transmission and the switching method etc. During the analog satellite communication period FDM/FM/FDMA was always adopted to multiplex several circuits together.

B. Digital Satellite Communication With the development of the PCM, ADPCM etc, the digital

services appeared and the multiple access and multiplexing technology also moved from FDM/FDMA to TDM/FDMA. IDR (Intermediate Data Rate) equipments provided by Intelsat had data transmission rate between 64kbps to 44.763Mbps. ADPCM/TDM/QPSK/FDMAor ADPCM/TDM/QPSK/MCPC was adopted. By using the digital techniques such as DCME (Digital Circuit Multiplication Equipment), DSI (Digital Speech Interpolation) space resources can be saved greatly. The digital satellite network can have a direct connection with the terrestrial network through E1, T1 etc. Later the high speed burst modulation techniques made the earth station can share the satellite transponder from FDMA to TDMA. In FDMA satellite system the carriers should be arranged carefully to avoid intermodulation while in TDMA system one carrier can

Figure 1. Digital Satellite Communication

occupy the whole transponder so the transponder can operate at the saturation point without the effect of intermodulation

.Although it has great progress compared with analog satellite communication, the services still limited to voice and data, as illustrated by Fig. 1.The satellite resources assignment is Fixed Assignment (FA).

C. Multimedia Satellite Communication The development of computer and network make the IP

multimedia application available. The multimedia combine text, audio, picture, video together in IP platform and make the people around the world to enjoy the service through internet. Different multimedia has different features and QOS requirements. The traffic can be divided into CBR, UBR, rt-VBR, nrt-VBR, ABR according to the ATM Forum. The Internet depends on different transfer media (fibre, coaxial-cable, cellular, satellite).In order to connect the different network around the world the ISO has defined an OSI/RM model. Although the model was not adopted by the manufacturers for its complexity. It helps the other model TCP/IP become the most popular reference model around the world. Multimedia Satellite System has many application scenarios as the following [5]:

1) Satellites as LAN Inter-connection When satellites are used for LAN inter-connection, a

typical requirement is the ability to offer symmetrical transmission capabilities. A company that wants to extent their LAN to another location will be able to use satellites between e.g. a location in Europe and the far east. Satellites will then typically be an alternative (or supplement) to terrestrial LAN interconnection, such as Ethernet.

2) Satellites as Broadband User Access When satellites are used for broadband user access, as for

multimedia Internet applications, typical transmission rate requirements are asymmetric. More data is sent to the user than from the user. In these cases, an uplink with lower capacity, maybe in the order of tens to hundreds of kbps can be acceptable. Downlink rates, on the other hand, must be able to carry video, at a few Mbps, or moving images with HDTV quality, at a few tens of Mbps.

3) Satellites for Broadband Service Providers

Application

Transportation

Network

Data Link

Physical

Application

Transportation

Network

Data Link

Physical

Satellite Data Link

Satellite Physical

Satellite Data Link

Satellite Physical

Satellite Data Link

Satellite Physical

SAP SAP

Satellite Independent Layers

Satellite Dependent Layers

Figure 2. Protocol Stack for Multimedia Satellite System

When considering the service providers, anything from one to hundreds of gateways may be required. Each gateway generally has from one to a few uplink carriers, and from one (broadband) to potentially thousands of (TDMA) downlinks. If the asymmetric users with individual traffic dominate the traffic pattern, then the gateways will in general uplink more traffic than they downlink.

In some sense, it is the internet that drives satellite system from “digital” to “multimedia”. It still causes a revolution in the satellite system design .The most distinct one is the system design issues changes from the “telecommunication system design” to “bearer system design”.

From the TCP/IP standpoint, a novel description for Multimedia Satellite System from layered approach is presented (shown in Fig. 2) .In a Multimedia Satellite System, the Physical Layer and Data Link Layer are satellite dependent which need to change with different system design, the Network Layer, Transportation Layer, Application Layer are satellite independent which don’t change with the media they rely on.

The Service Access Point (SAP) defines a formal interface between the satellite independent upper layers and the satellite dependent lower layers. The satellite dependent layer provides bearer service to satellite independent layer through SAP.

The Internet Engineering Task Force (IETF) work on providing QoS over IP resulted in the development of 2 different models starting in the mid-1990s: Integrated services (Intserv) in 1994 and Differentiated services (Diffserv) in 1998.The IP QOS can be achieved though SAP by mapping into Satellite Dependent Layers. Therefore the Multimedia Satellite System designers can focus their attention on the Satellite Dependent Layers design and the SAP optimization.

III. MULTIMEDIA SATELLITE SYSTEM DESIGN From part we can find that the Multimedia Satellite

System design is composed of three parts (shown in Fig. 3): Satellite Data Link Layer, Satellite Physical Layer and SAP.

A. Satellite Data Link Layer Design The Data Link Layer defines the format of the units of data

exchanged between the nodes at the ends of the link as well as the actions taken by these nodes when sending and receiving data unites. The units of data exchanged by a Data Link Layer protocol are called frames.

Multiple Access RA/DA/FAMultiplex TDM/ATM/DVB

Network

Transport

Application

Satellite Data Link

Satellite Physical

Encapsulate frame Multiple Access Flow Control Error Detection Mode Full duplex, Half duplex

Coding and Modulation Power Control Synchronization Equalization Diversity

Satellite Independent Layer

Satellite Dependent Layer

Main Services:

Main Services:

SAP

Figure 3. Layered Description for Multimedia Satellite System

The services provided by the Data Link Layer (shown in Fig 3) include encapsulate frame, multiple access, flow control, operation mode, error detection and correction. A specific Data Link Layer protocol may adopt some of these functions. For example, the error detection and retransmission are seldom used in satellite link for its long transmission delay.

The services provided at the Data Link Layer depend on the specific link-layer protocol that is employed over the link .That is from transmitting node, over the link, to receiving node. In satellite system there are two distinct nodes: satellites and earth station .So there are two distinct links: uplink from earth station to satellites and downlink from satellites to earth station. The specific protocol used in uplink is called Multiple Access and the protocol used in the downlink is often called Channel Multiplexing.

In traditional satellite system, multiple access protocols are always FDMA, TDMA and CDMA. These are fixed access method aim at circuit service. In Multimedia Satellite System more flexible multiple access protocols needed to support the high dynamic traffic .A new access scheme called Multi-Frequency Time Division Multiple Access (MF-TDMA) is suitable. MF-TDMA allows a group of earth station to share a set of carrier frequencies, each of which is divided into time-slots. The system will allocate to each earth station a series of bursts, each defined by a frequency, a bandwidth, a start time and duration. There are two kinds of MF-TDMA: Fixed MF-TDMA and Dynamic MF-TDMA. In Fixed MF-TDMA, the bandwidth and duration of successive traffic slots used by an earth station is fixed. Dynamic-Slot MF-TDMA can vary the bandwidth and duration of successive slots allocated to an earth station.

The standards for ATM were first developed in the mid 1980s.It provide a complete networking solution to support integrated service and can be adopted to multiplexing the satellite channel (shown in Fig. 4).

B. Satellite Physical Layer Design While the job of the data link layer is to move entire frames

from one node to another. The job of the physical layer is to move the individual “bits” within the frame from one node to the next. The protocols depend on the transmission media. In order to ensure the reliable transmission. Physical layer Protocol for Satellite communication contains channel coding, modulation, power control, synchronization, Equalization etc.

Multiple Access MF TDMA

Multiplexing ATDM

PHY

MF TDMA ATDM

Figure 4. Data Link Layer design for Multimdia Satellite System

Till now there are many excellent schemes for high speed satellite transmission. One of the possible ways is to adopt the ETSI standards DVB-RCS as the uplink transmission protocol and DVB-S2 as the downlink transmission protocol (shown in Fig. 5), DVB-S2 is another landmark activity after the 3rd generation wireless standard where the best of coding technology can be applied on a world-wide scale commercially. The selected codes based on concatenation of LDPC and BCH codes exceed the original capacity by 35% to 40%.

Selecting the roll-off factor and combining different code rates and modulation techniques, DVB-S2 system can operate at C/N ratios from -2.4dB to 16dB in an AWGN channel, Fig. 6[6] shows the required C/N versus spectrum efficiency. Compared with DVB-S and DVB-DSNG under the same conditions there is a capacity increase of about 30% and the distant from Shannon limit ranges from 0.7 to 1.2dB.

DVB RCS DVB S2

DLC

DVB RCS Signal

DVB S2 Signal

Figure 5. Physical Layer design for Multimedia Satellite System

Figure 6. Required C/N versus spectrum efficiency

IV. CONCLUSTION Although there are still many problems to realize a

Multimedia Satellite as it needs the development of the satellite platform ,payloads, launch vehicles and the advanced signal processing ,high speed modulation, on board switching technology etc. It can be an important driving force for the expansion of the internet and the convergence of the television broadcast network, telecommunication network and computer network.

REFERENCES

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[2] ETSI. Satellite Earth Stations and Systems (SES);Broadband Satellite Multimedia; Overview of BSM families[S]. 2003.

[3] ETSI. Satellite Earth Stations and Systems (SES); Broadband Satellite Multimedia; Services and Architectures[S]. 2002.

[4] ETSI. Satellite Earth Stations and Systems (SES); Broadband Satellite Multimedia; IP over Satellite[S]. 2002.

[5] ETSI. Satellite Earth Stations and Systems (SES); Broadband satellite multimedia; Part 1: Survey on standardization objectives[S]. 1998.

[6] ETSI. Digital Video Broadcasting (DVB);Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications[S]. 2005.