Radio Within Reach:
Digital Satellite Broadcasting Gets a Good Reception

Michael Rack, International Datacasting Corporation (IDC)

In some developing countries there is just one radio station for every two million people. Compare that with one station for every 30,000 in the developed world and you begin to see the reasoning behind an ambitious project to bring radio programming to some of earth's most remote regions.

And this won't be just any programming. The WorldSpace project, the brainchild of WorldSpace chairman and CEO Noah Samara, is creating a whole new global medium - direct digital satellite radio broadcasting. This new medium will bring CD-quality music, news, and sports of every sort imaginable, not to mention educational and informational programming, to almost five billion people in the developing world. That's five billion people, each of whom may be a jazz, gospel, or dance music fan, and not even know it yet.

This new medium will bring CD-quality music, news, sports, and educational programming to almost five billion people.

Music to their ears

Central to the WorldSpace system will be three geostationary satellites orbiting 35,000 km above Africa, Asia, and the Americas, each supplying more than 80 channels of audio programming, text, data, software, and images directly to millions of portable receivers. Each WorldSpace satellite has three downlink spot beams that cover 14 million square km each, so when all three satellites are launched (by the end of 1999), Africa, the Middle East, Asia, Latin America, and the Caribbean will have access to quality sound broadcasts - in many cases, for the first time.

In addition to an unprecedented variety of music, WorldSpace listeners will have access to health advisories, disaster warnings, and public service announcements.

Understandably, WorldSpace has attracted some of the world's top broadcasters, who are eager to tap into huge audiences in previously inaccessible corners of the globe. CNN International, for example, has signed a contract with WorldSpace to provide its 24-hour pan-European audio programming on all three beams of the satellite serving Africa. Other broadcasters that have signed on include Bloomberg, the World Radio Network, the Kenya Broadcasting Corporation, the National Broadcasting Authority of Ghana, South Africa's Kaya-FM, the Egyptian Radio and Television Union, and many Internet providers.

Part of WorldSpace's transmissions have been reserved for educational programming from institutions like the World Health Organization, unesco, and others. So, in addition to an unprecedented variety of music, WorldSpace listeners will now have regular access to health advisories, disaster warnings, and other public service announcements.

Over a dozen manufacturing and technology partners including Alcatel, Arianespace, Matra Marconi Space, STMicroelectronics, and IDC, are working together to accomplish the WorldSpace project, a vision that's been almost ten years in the making. IDC, in partnership with SED Systems, is playing an important role in this brand-new medium by supplying QNX-based equipment used to uplink audio, data, and multimedia material to the satellites.

WorldSpace at a glance

In each region, broadcasters can provide program feeds to the satellite through either of two different types of satellite uplink stations - processed feeder link stations (P-FLS) or transparent feeder link stations (T-FLS). (See Figure 1.)

On the upside

A P-FLS accepts analog or digital audio inputs or data inputs from broadcasters and encodes them into prime rate channels (16 kbit/s information channels that are the basic building blocks of WorldSpace broadcasting). In a P-FLS, each prime rate channel is modulated onto a single-channel- per-carrier FDMA, or frequency division multiple access carrier, and is uplinked to the satellite. Often, larger broadcasters will own their P-FLS equipment making it convenient to uplink directly from their studios.

A T-FLS also accepts analog and digital audio inputs or data inputs from broadcasters. However, more processing is done on the ground at this type of station, therefore a T-FLS uplinks prime rate channels to the satellite in the fully processed form of three time division multiplexed (TDM) beams. Smaller, local broadcasters which are often less able to afford their own feeder link station equipment, send their programming over fiber or land lines to the regional T-FLS for processing and uplinking to the satellite.

On the downside

Each satellite can receive up to 288 FDMA-encapsulated prime rate channels from multiple P-FLSs, which it further processes into three downlink TDM carriers. Likewise, each satellite can also receive up to 288 TDM-encapsulated prime rate channels provided by the regional T-FLS's three TDM carriers. Since the beams received from T-FLSs have been processed on the ground and are already in TDM form, the signal requires no further processing by the satellite. The satellite allocates and reorganizes the prime rate channels from P-FLSs into three TDM L-band carriers (downlink beams) that are polarized either left or right. Prime rate channels from FDMA carriers (originating from P-FLSs) are assigned dedicated time slots in the three processed TDM downlink carriers by the WorldSpace Mission Control Center. For broadcasters to reach users in areas covered by all three spot beams, prime rate channels for a particular broadcast must occupy a slot in more than one of a satellite's downlink beams.

Getting an earful

To receive digital sound and data transmissions directly from WorldSpace satellites, listeners will need a new type of portable receiver specially designed to pick up digital signals. Although WorldSpace has determined basic guidelines for the receivers, manufacturing partners like Hitachi, JVC, Panasonic, and Sanyo have each designed their own innovative versions of the units. All of the receivers, however, have the Starman chipset in common. The chipset, manufactured by STMicroelectronics and Micronas Intermetall, consists of three micro-integrated circuits that process WorldSpace satellite transmissions. The receivers contain data ports that can be used to receive multimedia programming, and some models offer standard AM and FM reception, as well as shortwave.

Feeder link stations - the inside story

IDC enters the WorldSpace project at the level of the feeder link stations - P-FLSs and T-FLSs. The work done at these stations ensures that the broadcasters' signals are transmitted to the satellites in a form that's compatible with WorldSpace protocols.

Processed Feeder Link Stations

A P-FLS consists of two elements that can be located either together or separately: the baseband subsystem (BBS), which is supplied by IDC, and the RF subsystem (see Figure 2). The BBS itself is modular and is further broken down into two elements: studio side and uplink side.

To begin the process of sending a broadcaster's signal to the satellite, the MPEG encoder module inputs an analog or digital audio input, digitizes it, and compresses it to an MPEG 2.5 Layer 3 digital format. Data inputs are received by Data I/F modules and are processed by the system.

Both audio and data inputs are multiplexed by broadcast channel multiplexers (BC Mux) into broadcast channels with control, ID, and addressing information, as required. The output of the broadcast channel multiplexers is transferred over a high-speed bus to the broadcast channel transport interface (BCTI I/F) module, which then converts the broadcast channels into a proprietary serial transmission format to conform to the WorldSpace BCTI network standard. The BCTI I/F is designed to use standard telecom facilities and is compatible with transmission formats used worldwide.

The BCTI I/F is then received by a prime rate channel (PRC) controller module on the uplink side that re-identifies the original broadcast channels and reformats them for modulation as individual prime rate channels. Prime rate channel modulators then modulate these as 38 kbit/s FDM signals at an IF frequency of 140MHz. From there, the signals are fed to the RF subsystem for transmission to the WorldSpace satellite.

Transparent Feeder Link Stations

What happens inside a T-FLS is virtually identical to that of a P-FLS right up until the BCTI I/F sends the signal to the PRC controller. There, the BCTI I/F passes the signals to what's known as the beam technical center (BTC). (See Figure 3.) The BTC is itself comprised of two main subsystems: broadcast channel controllers (BCC) and time division multiplexer controllers (TDMC). The controllers communicate with one another over two LANs: one TCP/IP (for monitor and control messages), and one UDP/IP (for broadcast channel data). BCCs and TDMCs have similar software architectures. Each are comprised of six processes:

Data manager - the data manager is responsible for processing broadcast channel data over the TCP/IP LAN and interfacing with the custom hardware (i.e. BCTI demultiplexer cards in BCCs, and TDM generators in TDM controllers).

Command processor - the command processor interfaces with the monitor and control systems and the user interfaces over the LAN. It's responsible for parsing command syntax, receiving and validating configuration information and status requests, passing configuration information on to the data manager, requesting status updates from the data manager, and providing the appropriate responses to the monitor and control systems. The command processor is the main process of each application and spawns all other processes based on its configuration.

TCP/IP server - the TCP/IP server is responsible for maintaining IP connections with the various monitor and control systems. It uses multiplexed I/O to receive commands over TCP/IP and to forward them to the command processor via QNX messaging.

Logger - the logger maintains the event log file and stores messages provided by other processes in the system.

Watchdog - the watchdog monitors the health of the system by periodically checking registered processes.

User interface - the user interface provides a mechanism for defining and graphically representing the system configuration and status. It communicates with the command processor through the IP stack and uses the same protocol as the monitor and control systems.

Gathering tools

After IDC signed on with WorldSpace, the company was faced with developing uplink equipment for the first direct digital satellite radio broadcast system ever - and had just under a year to do it.

Since WorldSpace is based entirely on PC architecture, one decision - which platform to use - was already made. But the race was still on to find an OS to support the BBS system. Key criteria were, of course, realtime performance and reliability, but memory utilization, TCP/IP support, and availability of development tools were definite issues as well.

QNX was chosen after a tradeoff analysis with many of today's popular operating systems because of its performance, particularly in the area of interrupt latency and Fast Ethernet performance.

PC architecture

For the BBS, IDC uses industrial-grade rack-mount PCs containing backplanes of up to 20 slots in combinations of ISA, PCI, and PICMG. A typical PC in this system contains one or more custom IDC cards, a Teknor Pentium 233 CPU with onboard NIC, and a Corman NIC. Proprietary UDP/IP broadcast channel data is transmitted on the 100Mbit LAN connected to the Teknor NIC; TCP/IP monitor and control data is transmitted over the LAN connected to the Corman NIC. Drivers for the NICs were already available through either QSSL or Corman, so we only had to write custom drivers for our custom EISA cards: the BCTI demultiplexer and the TDM generator. All of the BBS processes are under the control of the BBS controllers, which are Pentium CPUs running QNX 4 and BBS software. To maintain a very high degree of availability, feeder link stations in the WorldSpace broadcast satellite network are normally installed as fully redundant. A second complete system, designated the standby thread, uses the same audio and data inputs, performs the same processing, and generates the same outputs as the main thread for uplinking to the satellite. Control of the main and standby thread switching is carried out by the LMCS (supplied by another company), which on detection of a possible error or an out-of-specification condition in the main thread, switches automatically to the standby configuration.

Off the launch pad

With the successful launch of WorldSpace's first satellite, Afristar, on October 28, 1998, the project is now well on its way to its goal of bringing radio broadcasting to some of the most remote villages in the world. Afristar will bring service to all of Africa. WorldSpace's other two satellites, Asiastar and Ameristar, are scheduled for launch in 1999 to provide service to Asia and Latin America. When all phases of the world's first direct satellite-based digital radio broadcast system are in place, WorldSpace will reach 120 countries on three continents - ultimately enriching the lives of over 80% of the earth's population.