With the seamless coverage of large areas, satellite-based broadcasting to mobile users has become more and more popular in recent years. Digital Radio Mondiale (DRM) also offers large coverage areas and, therefore, these systems are sometimes considered as competing technologies. However, with each system’s benefits, the two approaches can actually complement one another in certain scenarios.


DRM is an openly standardized radio broadcasting system [1], which was developed to replace analog radio broadcasting in the AM and FM/VHF bands. For the design of the technology, the following requirements played an important role:

• Minor impact to overall frequency planning. Especially for short wave, international frequency coordination is required. To allow for a smooth transition from analogue to digital, the DRM channel allocation must be compatible to existing frequency regulations. The DRM system was designed as a narrowband system, with each broadcast frequency assigned to a specific broadcaster. Typically, one audio program together with some data is transmitted per carrier. At the beginning, the design was focused on broadcast frequencies below 30MHz. Later, an extension to carrier frequencies above 30MHz was included, adding a new parameter configuration set. This extension is called “DRM+”, whereas the original parameter sets of the DRM standard are referred to as “DRM30”. An overview of the frequency range covered by the standard is given in Figure 1.
• High audio quality. DRM is based on the audio coding standard MPEG-4 HE-AAC v2. To achieve FM-like stereo audio quality in the AM bands, requires bitrates in the range of 24kbps. 24kbps bitrates are feasible within one frequency slot. Using DRM+ or several frequencies would also allow higher bitrates.
• Data features. Essentially any kind of data can be transmitted over a digital system. Applications using protocols optimized for narrowband systems such as Journaline (official ETSI standard [2]) can be combined with audio broadcasting. To achieve the required bitrate on a short wave channel bandwidth of 10kHz for example, a modulation scheme with high spectrum efficiency is required. For DRM30, OFDM modulation using QAM16 and QAM64 signal constellations fulfills this demand. The DRM standard offers a high flexibility to trade off between transmission robustness and data capacity, and can be adapted to different constraints or propagation challenges. Table 1 gives an overview of the key parameters of the operation modes.

Satellite-Based Systems
In the past years, powerful satellites have become available allowing for full direct reception of the satellite signal with very small antennas. Typically, the antennas are similar in size to those of GPS receivers. This makes the systems attractive for mobile reception, which is possible with portable and even handheld devices. SDARS (Satellite Digital Audio Radio Service), sold commercially as XM Satellite Radio and Sirius Satellite Radio, is a good example. SDARS was introduced in 2001 and has since become a popular service in the United States. More than one hundred audio programs are assembled within a broadband multiplex, offering the customer a high variety of content. The user can choose between different music formats, talk, news and sports channels. In addition, the audio broadcasting is combined with data services.

The SDARS system was the first hybrid system to use satellite broadcast combined with complementary ground components (CGC, also called ATC = Ancillary Terrestrial Component). In the meantime, open standards such as DVB-SH (DVB standards for Satellite to Handheld [4]) or ESDR (ETSI standard for Satellite Digital Radio [5]) have been released and successfully tested in the field. Deployments of these systems are in planning phase and pilot installations are already on-air.

The concept behind the hybrid system is depicted in Figure 2. The satellite provides the core coverage. With technologies such as time diversity and/or spatial diversity, high service availability can be achieved by only using the satellite signal. However, in dense urban areas and for indoor reception, the satellite does not typically provide sufficient field strength. For such areas, operators install terrestrial repeaters to improve the QoS.

Coverage Characteristics
Satellite-based systems offer within the footprint of the beam, a nearly constant field strength over the complete coverage area. For frequency bands such as L- (1.5 GHz) or S-band (2 to 3GHz) the atmosphere and weather have a minor impact on propagation conditions. Therefore, services can operate 24 hours a day, seven days a week without interruption. In particular, for geostationary satellites the reception conditions are very stable, allowing, for example, installation of accessories to ensure access to the signals in areas without repeaters and no direct satellite reception. For example, a user can install detachable antennas or micro repeaters in places well-positioned to receive the satellite signal, such as a south-facing window.

There are three notable types of system deployments:

• Nomadic application/“Cooperative users only.” Satellite systems operate at low field strength. For line-of-sight (LOS) reception, sufficient margin is provided, and to support Non-LOS, more margin is required. This makes the system very expensive or reduces the capacity. A much higher bitrate can be offered if the customer is willing to cooperate and does not expect coverage for bad reception conditions. If the customer is willing to look for a good antenna position such as a south-facing window, the overall network costs can be reduced. In addition, mobile reception in open environments is feasible, and this type of network is typical for two-way communication. An example is the Inmarsat BGAN service. Using small portable antennas placed in a good position allows high bitrates at acceptable cost.
• The network targets mainly mobile reception in the car. To achieve high QoS for mobile reception in areas where obstacles block the LOS signal, it is necessary to use countermeasures. Technologies such as time diversity/time interleaving are typical methods. Rural and suburban areas can be served without any terrestrial repeater. In urban areas, the critical issues are traffic jams, and time diversity fails for these scenarios. In areas with Non-LOS (narrow streets, tunnels, etc) and stop-and-go traffic, repeaters ensure an outage free reception.
Full coverage (including indoor and handheld). Without repeaters, very high satellite power is required, or the capacity (number of programs) provided per satellite must be low. Using repeaters is a good trade-off. The satellite segment is designed to serve rural and suburban areas, and repeaters are installed in areas where the satellite is not sufficient or indoor reception is expected.

The footprint of the satellite beam defines the core coverage area. Figure 3 shows the footprint of the SDARS systems. The service targets the United States, but the signal also can be received in neighboring countries and many islands around North and Central America. If required, the coverage area also can be reduced by using spot beam satellites. For Europe it may be worthwhile to consider linguistic zones and use several spot beams instead of a global beam (Figure 4). In addition, combinations of global beams and spot beam coverage may be attractive.

DRM for the AM bands (‘DRM30’ parameter configurations) mainly distinguishes between two propagation paths:

• Ground-wave (typically long wave and medium wave daytime)
• Sky-wave (typically short wave and medium wave nighttime)

Ground-wave propagation essentially describes the field strength as a function of the distance to the transmitter. The coverage area depends highly on the available transmission power. Ground-wave propagation is typical for local/regional broadcasting. Figure 5 shows a medium wave propagation example. During the day, coverage is mainly defined by the ground-wave, while during the night, interference caused by stations far away and sky-wave propagation causes interference and reduces the effective coverage area. In this case DRM could be dynamically reconfigured to use a more robust transmission mode. See Figure 5.

For wide area coverage, the sky-wave propagation becomes relevant. Signals are reflected in the ionosphere and on the ground, and may approach the receiver after several hops. Sky-wave propagation allows for beaming the signal toward regions far away from the transmitter, thus international SW broadcasts can span multiple continents. On the other hand, such effects as sun spot activity must be taken into account for frequency planning purposes. A 24-hour service may partially require frequency or transmitter site diversity. If one carrier frequency is not suitable, an alternative resource is selected. The DRM standard includes many hooks for automatic receiver hand-over to alternative frequencies and can adapt the modulation robustness parameters if the propagation scenarios change. Generally, it is essential to carefully plan a DRM network and take into account the time variant propagation conditions.

Terminal + Receive Antenna Considerations
The size of the antenna is a key issue for many terminals and car installations. The antenna size and the wavelength are correlated. For L- and S-band frequencies the wavelength l is in the range of 12 (S-band) to 20cm (L-band). A simple l/4 monopole antenna would already be matched to the transmit frequency. For these frequency ranges, very small antennas or antennas integrated into the handheld are typical. For a MF frequency of 800kHz, the wavelength is 375m, thus alternative antenna concepts are required. A typical antenna is a ferrite antenna, which makes the antenna compact while maintaining a good antenna gain. The drawback for such an antenna is the bandwidth, as the antenna design defines the useful frequency range.

The Service Concept
The DRM system was designed to use the same overall infrastructure concept as traditional AM and FM – one broadcast frequency operated by one broadcaster. Typically, one transmitter broadcasts the radio program together with complementary audio programs and data services. Besides a one-by-one replacement or extension of analogue stations by a digital system, DRM also offers alternative network concepts such as single frequency networks to extend the coverage area of a transmitter and to allow for the use of lower transmitter power per site. Program diversity to target individual listener groups is provided by a multitude of broadcasters and radio stations.


The satellite-based systems follow a different concept. A multiplex offers in a single set many (typically 30 -100 or even more) programs usually combined with various data applications. The program formats can be coordinated to ensure that each program to a certain extent offers a unique content. The user has a choice between different genres or talk channels addressing dedicated subjects or selected interest groups. However, offering a high variety of programs also has drawbacks. To be commercially successful, services offering many programs as a package should also target large coverage areas to reach a sufficient number of listeners per program. Using one carrier frequency for a multiplex of many programs also allows the parallel reception of many sub channels. This is especially attractive for data services and future radio services based on forward and store concepts. Innovative user interfaces offering functions such as time shift (repeat stored data, record a program while listing to another program, etc.) allows attractive combinations of listening to favorite music and getting up-to-date information via real-time broadcasting.

Conclusions
Today, AM and FM broadcast is characterized by many small stations partly targeting only local areas. The DRM standard, with its DRM30 and DRM+ configurations, is a powerful technology enabling these traditional radio stations to benefit from the advantages of digital broadcast. Significantly higher audio quality and additional listener benefits are offered together with more reliable coverage or lower required transmit power.

Satellite-based systems target rather a new type of service and offer a broadband service including a multiplex with many data streams. The broadband concept allows transmission of a bouquet of audio programs combined with attractive wideband data applications, and even video services can be added.

The two system approaches — DRM as the terrestrial individual-station digital radio standard and satellite-based broadband multiplex services — should therefore be considered as complementary and not as direct competitors. It even may be partly feasible to combine the systems. If large coverage areas have to be combined with local information, a combined solution may become attractive. Core technologies like the audio coding schemes and, increasingly, data applications are very similar. Audio broadcasting today benefits from combinations of attractive music, up-to-date information for all users and information channels focusing on special interest groups.

References
[1] ETSI ES 201 980 (DRM system specification)
[2] ETSI TS 102 979 (Journaline specification) which can also be found online at: www.worlddab.org/introduction_to_digital_broadcasting/applications_list/journaline
[3] Digital Radio Mondiale (DRM) - A Broadcaster’s Guide, June 2010, available on www.drm.org
[4] EN302583, TS 102585 (DVB-SH standard)
[5] TS 102550, TS102551-1, TS102255-2 (ESDR standard)