The multi-carrier T-DAB system as adopted by the majority of countries in Europe and also in some countries outside the European continent, has been designed with a bandwidth of about 1.5 MHz. Frequency blocks have been fit in to the 7 MHz VHF channel scheme. A mean rate of about 1.15 Mbit/s is available for the delivery of high quality CD-like sound services in conjunction with text, data and images, for fixed, portable and mobile receivers.
1.2.1 Frequency bands
The Plan to be established by the second session of Regional Radio Conference (RRC-06) should contain assignments and/or allotments for digital broadcasting stations in the following bands:
– Band III (174 to 230 MHz);
– Bands IV and V (470 to 862 MHz).
The European countries after evaluating the other possible options have finally adopted the T-DAB system for Band III.
184.108.40.206 Frequencies for sound channels in the planning area
It is to be noted that whilst the frequency band from 174 to 216 MHz is primarily used for terrestrial analogue television, there are also some T-DAB allotments in this band. The frequency band 216-230 MHz (240 MHz in some countries) is mainly allocated to T-DAB in European countries; nevertheless there is still widespread use of part of this band for television.
Ultimately, a flexible approach will be required as regards the use of T-DAB, or DVB-T, in specific channels in Band III because of the different situations and time-frames all over the planning area, or even within one country. Sharing criteria and clear procedures for both kinds of use are therefore required.
Band III is seen as the optimum solution for a T-DAB band to provide a terrestrial TDAB service.
The band does not suffer from a number of the anomalous propagation characteristics which are a problem in Band I such as sporadic E and F2 layer propagation. Man-made noise is significantly lower in Band III than in Band I, and Band III frequencies are still sufficiently low that the Doppler shift created by moving vehicles at motorway speeds will not create a problem for operation in Mode 1 of the digital system A specification.
This is made possible by a rugged system design that allows seamless and fade-free reception even in highly disruptive conditions, largely dominated by multipath propagation.
It has to be noted that Band II was also considered for T-DAB, but this turned out not to be viable due to the congested situation in many areas.
1.2.3 Location of transmitters
It should be noted that in the case of an SFN the separation distance between transmitters influences the choice of guard interval, which in turn determines the size of the network. The separation distance and the effective height influence the effective radiated power. In the implementation of T-DAB existing transmitting site infrastructures have been used where possible, with the addition of some new supplementary sites. The latter have been adopted in order to fulfil the SFN requirements.
1.3.1 IBOC Overview
The IBOC system was designed for regions where limited spectrum prevents the allocation of new spectrum for digital broadcasting. The IBOC system allows broadcasters to simultaneously transmit an analogue and digital signal without the need for additional spectrum for the digital signal. The IBOC system takes advantage of unused portions of the spectrum on either side of the analogue carrier (as defined by the service frequency allocation “mask”) and implements frequency re-use by including digital carriers in quadrature to the existing analogue carrier. In either case, the analogue signals are in close proximity to the digital signals and great care must be taken to prevent unwanted interference between them.
The IBOC system offers a number of advantages for broadcasters, consumers and regulators. The IBOC system replicates the existing coverage patterns of each radio station thereby retaining the existing economic value of the station. Broadcasters can convert to digital broadcasts with a modest investment and retain the vast majority of their existing physical plant. In addition, the introduction of the digital signal in the existing channel allows the broadcaster to retain the station’s existing dial position. Because the system supports simulcast of the analogue and digital signals, consumers are able to upgrade to digital over an extended period and taking into account normal equipment replacement cycles. Regulators benefit because there is no need for spectrum allocations or licensing of new stations.
The IBOC system offers the following features:
− CD quality audio in the VHF-band and VHF quality audio in the MF band.
− Digital coverage equivalent to existing analogue coverage. In areas where the digital signal is lost, the system automatically blends to the analogue back-up signal to ensure digital coverage is never less than existing analogue coverage.
− Advanced coding technologies and time diversity between the analogue and digital signals ensure a robust signal.
− The VHF system has demonstrated significant robustness in the presence of severe multipath, and the MF system has demonstrated significant robustness in the presence of impulse noise.
− The VHF system offers options for introducing new audio and data services ranging from 1 to
300 kbit/s depending on the mode of operation.
The IBOC system has been tested in North and South America, Europe and Asia. It is currently in operation in approximately 1 800 stations throughout the United States of America. This has added more than 900 new multicast audio streams using existing VHF stations. The system has been used for demonstrations, testing and/or ongoing operations in Brazil, China, France, Indonesia, Mexico, the Philippines, Switzerland, Ukraine, Vietnam.
The IBOC system has been standardized by the National Radio Systems Committee (NRSC), a standards setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association in the United States. The current version of the standard, NRSC-5-B is available from the NRSC at www.nrscstandards.org.
Currently, there are commercially available IBOC receivers in most market segments. OEM receivers are available in the United States as standard equipment or a factory installed option for many major auto manufacturers. More than sixty models of aftermarket automobile receivers, tabletop receivers, home HiFi receivers and car converter products are available from national and local retailers throughout the United States. As the cost of components and the power consumption levels are reduced in the near future, it is anticipated that mobile receivers will become available.
The IBOC system is designed to permit a smooth evolution from current analog modulation to a fully digital system. This system can deliver digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. The system is designed to allow broadcasters to continue to transmit analog MF and VHF simultaneously with new, higher-quality and more robust digital signals, allowing broadcasters and their listeners to convert from analog to digital radio while maintaining each station’s current frequency allocation.
The IBOC system allows a broadcast station to offer multiple services. A service can be thought of as a logical grouping of application data identified by the IBOC system. Services are grouped into one of two categories:
1 Core Services:
a) Main Program Service (both Audio (MPA) and Data (PAD))
b) Station Information Service (SIS)
2 Advanced Application Services (AAS)
The flow of service content through the IBOC broadcast system is as follows:
a) Service content enters the IBOC broadcast system via Service Interfaces;
b) Content is assembled for transport using a specific protocol;
c) It is routed over logical channels via the Channel Multiplex.
It is waveform modulated via the Waveform / Transmission System for over-the-air transmission.
The system employs coding to reduce the sampled audio signal bit rate and baseband signal processing to increase the robustness of the signal in the transmission channel. This allows a high quality audio signal plus ancillary data to be transmitted in band segments and at low levels which do not interfere with the existing analog signals.
220.127.116.11.1 Main Program Service (MPS)
The Main Program Service is a direct extension of traditional analog radio. MPS allows the transmission of existing analog radio-programming in both analog and digital formats. This allows for a smooth transition from analog to digital radio.
Radio receivers that are not IBOC enabled can continue to receive the traditional analog radio signal, while IBOC receivers can receive both digital and analog signals via the same frequency band. In addition to digital audio, MPS includes digital data related to the audio programming. This is also referred to as Program Associated Data (PAD).
18.104.22.168.2 Station Information Service (SIS)
The Station Information Service provides the necessary radio station control and identification information, such as station call sign identification, time and location reference information. SIS can be considered a built-in service that is readily available on all IBOC stations. SIS is a required IBOC service and is provided dedicated bandwidth.
22.214.171.124.3 Supplemental Program Service (SPS)
The Supplemental Program Service allows broadcasters to introduce up to seven new digital audio channels depending on the throughput devoted to the SPS. The SPS includes support for Program Associated Data for each program stream.
126.96.36.199.4 Advanced Application Services (AAS)
AAS is a complete framework in which new applications may be built. In addition to allowing multiple data applications to share the Waveform / Transmission medium, AAS provides a common transport mechanism as well as a unified Application Programming Interface (API). On the transmission side, broadcasters utilize the common AAS interface to insert service(s) into their signal; receiver manufacturers utilize the AAS ‘toolkit’ to efficiently access these new services for the end-user. AAS includes separate audio programming such as reading services and other secondary audio and data services.
The IBOC DSB system uses the HDC codec supplemented by SBR. This delivers high quality “FMlike” stereo audio within the bandwidth constraints imposed on operations below 30 MHz. To further enhance the robustness of the digital audio beyond that provided by FEC and interleaving, special error concealment techniques are employed by the audio codecs to mask the effects of errors in the input bitstream. Furthermore, the audio codec bitstream format provides the flexibility of allowing future enhancements to the basic audio coding techniques.
188.8.131.52 Modulation techniques
The IBOC DSB system uses QAM. QAM has a bandwidth efficiency that is sufficient for transmission of “FMlike” stereo audio quality as well as providing adequate coverage areas in the available bandwidth.
The system also uses a multicarrier approach called OFDM. OFDM is a scheme in which many QAM carriers can be frequencydivision multiplexed in an orthogonal fashion such that there is no interference among the carriers. When combined with FEC coding and interleaving, the digital signal’s robustness is further enhanced. The OFDM structure naturally supports FEC coding techniques that maximize performance in the nonuniform interference environment.
184.108.40.206 FEC coding and interleaving
FEC coding and interleaving in the transmission system greatly improve the reliability of the transmitted information by carefully adding redundant information that is used by the receiver to correct errors occurring in the transmission path. Advanced FEC coding techniques have been specifically designed based on detailed interference studies to exploit the nonuniform nature of the interference in these bands. Also, special interleaving techniques have been designed to spread burst errors over time and frequency to assist the FEC decoder in its decisionmaking process.
A major problem confronting systems operating below 30 MHz is the existence of grounded conductive structures that can cause rapid changes in amplitude and phase that are not uniformly distributed across the band. To correct for this, the IBOC DSB system uses equalization techniques to ensure that the phase and amplitude of the OFDM digital carriers are sufficiently maintained to ensure proper recovery of the digital information. The combination of advanced FEC coding, channel equalization, and optimal interleaving techniques allows the IBOC DSB system to deliver reliable reception of digital audio in a mobile environment.
The IBOC DSB system employs time diversity between two independent transmissions of the same audio source to provide robust reception during outages typical of a mobile environment. In the hybrid system the analogue signal serves as the backup signal, while in the alldigital system a separate digital audio stream serves as the backup signal. The IBOC DSB system provides this capability by delaying the backup transmission by a fixed time offset of several seconds relative to the main audio transmission. This delay proves useful for the implementation of a blend function. During tuning, blend allows transition from the instantly acquired backup signal to the main signal after it has been acquired. Once acquired, blend allows transition to the backup signal when the main signal is corrupted. When a signal outage occurs, the receiver blends seamlessly to the backup audio that, by virtue of its time diversity with the main signal, does not experience the same outage.
Digital systems depend on an interleaver to spread errors across time and reduce outages. Generally longer interleavers provide greater robustness at the expense of acquisition time. The blend feature provides a means of quickly acquiring the backup signal upon tuning or reacquisition without compromising full performance.
1.3.4 Operating modes
220.127.116.11 Hybrid MF mode
In the hybrid waveform, the digital signal is transmitted in sidebands on either side of the analogue host signal as well as beneath the analogue host signal as shown in Fig. 3. The power level of each OFDM subcarrier is fixed relative to the main carrier as indicated in Fig. 3. The OFDM carriers, or digital carriers, extend approximately 14.7 kHz from the AM carrier. The digital carriers directly beneath the analogue signal spectrum are modulated in a manner to avoid interference with the analogue signal. These carriers are grouped in pairs, with a pair consisting of two carriers that are equidistant in frequency from the AM carrier. Each pair is termed a complementary pair and the entire group of carriers is called the complementary carriers. For each pair, the modulation applied to one carrier is the negative conjugate of the modulation applied to the other carrier. This places the sum of the carriers in quadrature to the AM carrier, thereby minimizing the interference to the analogue signal when detected by an envelope detector. Placing the complementary carriers in quadrature to the analogue signal also permits demodulation of the complementary carriers in the presence of the high level AM carrier and analogue signal. The price paid for placing the complementary carriers in quadrature with the AM carriers is that the information content on the complementary carriers is only half of that for independent digital carriers.
The hybrid mode is designed for stations operating at MF in areas where it is necessary to provide for a rational transition from analogue to digital. The hybrid mode makes it possible to introduce the digital services without causing harmful interference to the existing host analogue signal.
To maximize the reception of the digital audio, the IBOC DSB system uses a layered codec where the compressed audio is split into two separate information streams: core and enhanced. The core stream provides the basic audio information whereas the enhanced stream provides higher quality and stereo information. The FEC coding and placement of the audio streams on the OFDM carriers is designed to provide a very robust core stream and a less robust enhancement stream. For the hybrid system the core information is placed on highpowered carriers 10 to 15 kHz from the analogue carrier while the enhanced information is placed on the OFDM carriers from 0 to 10 kHz.
To protect the core audio stream from interference and channel impairments the IBOC DSB system uses a form of channel coding with the special ability to puncture the original code in various overlapping partitions (i.e., main, backup, lower sideband and upper sideband). Each of the four overlapping partitions survives independently as a good code. The lower and upper sideband partitions allow the IBOC DSB system to operate even in the presence of a strong interferer on either the lower or upper adjacent, while the main and backup partitions allow the IBOC DSB system to be acquired quickly and be robust to shortterm outages such as those caused by grounded conductive structures.
In the hybrid system the core audio throughput is approximately 20 kbit/s while the enhanced audio throughput adds approximately 16 kbit/s.
18.104.22.168 Alldigital MF mode
The alldigital mode allows for enhanced digital performance after deletion of the existing analogue signal. Broadcasters may choose to implement the alldigital mode in areas where there are no existing analogue stations that need to be protected or after a sufficient period of operations in the hybrid mode for significant penetration of digital receivers in the market place.
As shown in Fig. 4, the principal difference between the hybrid mode and the alldigital mode is deletion of the analogue signal and the increase in power of the carriers that were previously under the analogue signal. The additional power in the alldigital waveform increases robustness, and the “stepped” waveform is optimized for performance under strong adjacent channel interference.
The same layered codec and FEC methods, with identical rates (i.e. ~20 kbit/s for the core audio and
~16 kbit/s for the enhanced audio), are used in the alldigital system as is used in the hybrid system. This simplifies the design of a receiver having to support both systems.
22.214.171.124 Hybrid VHF mode
The digital signal is transmitted in sidebands on either side of the analogue FM signal. Each sideband is comprised of ten frequency partitions, which are allocated among subcarriers 356 through 545, or 356 through –545. Subcarriers 546 and 546, also included in the sidebands, are additional reference subcarriers. The amplitude of the subcarrier within the sidebands is uniformly scaled by an amplitude scale factor.
Spectrum of the hybrid waveform–service mode
(The level of the digital subcarriers is such that the total power of these carriers
is 20 dB below the nominal power of the FM analogue carrier)
126.96.36.199 All Digital VHF mode
The All Digital waveform is constructed by removing the analogue signal, fully expanding the bandwidth of the primary digital sidebands, and adding lower-power secondary sidebands in the spectrum vacated by the analogue signal. The spectrum of the All Digital waveform is shown in Fig. 6.
Spectrum of the all digital waveform
(The level of the digital subcarriers is such that the total power of these carriers is no more than 10 dB
below the nominal power of the FM analogue carrier that it replaces)
1.3.5 Generation of the signal
188.8.131.52 Transmission Subsystems
A basic block diagram representation of the system is shown in Fig. 7. It represents the IBOC digital radio system as three major subsystems.
The Audio subsystem performs the source coding and compression of the sampled digitized Main Program Service (MPS) audio program material. “Source coding and compression” refers to the bit rate reduction methods, also known as data compression, appropriate for application to the audio digital data stream. In hybrid modes the MPS audio is also analog modulated directly onto the carrier for reception by conventional analog receivers. Several categories of data may also be transmitted on the digital signal including station identification, messages related to the audio program material, and general data services.
184.108.40.206.2 Transport and Service Multiplex
“Transport and service multiplex” refers to the means of dividing the digital data stream into “packets” of information, the means of uniquely identifying each packet or packet type (data or audio), and the appropriate methods of multiplexing audio data stream packets and data stream packets into a single information stream. The transport protocols have been developed specifically to support data and audio transmission in the MF and VHF radio bands.
IBOC digital radio broadcasting model
220.127.116.11.3 RF/Transmission System
“RF/Transmission” refers to channel coding and modulation. The channel coder takes the multiplexed bit stream and applies coding and interleaving that can be used by the receiver to reconstruct the data from the received signal which, because of transmission impairments, may not accurately represent the transmitted signal. The processed bit stream is modulated onto the OFDM subcarriers which are transformed to time domain pulses, concatenated, and up-converted to the VHF band.
RF/Transmission function in context of overall system
1.3.6 Reception of the signal
A functional block diagram of an MF IBOC receiver is presented in Fig. 9. The signal is received by a conventional RF front end and converted to IF, in a manner similar to existing analogue receivers. Unlike typical analogue receivers, however, the signal is filtered, A/D converted at IF, and digitally down converted to baseband inphase and quadrature signal components. The hybrid signal is then split into analogue and DSB components. The analogue component is then demodulated to produce a digitally sampled audio signal. The DSB signal is synchronized and demodulated into symbols. These symbols are deframed for subsequent deinterleaving and FEC decoding. The resulting bit stream is processed by the audio decoder to produce the digital stereo DSB output. This DSB audio signal is delayed by the same amount of time as the analogue signal was delayed at the transmitter. The audio blend function blends the digital signal to the analogue signal if the digital signal is corrupted and is also used to quickly acquire the signal during tuning or reacquisition.
Noise blanking is an integral part of the IBOC receiver and is used to improve digital and analogue reception. Receivers use tuned circuits to filter out adjacent channels and intermodulation products. These tuned circuits tend to “ring”, or stretch out short pulses into longer interruptions. A noise blanker senses the impulse and turns off the RF stages for the short duration of the pulse, effectively limiting the effects on the analogue “listenability,” of ringing. Short pulses have a minimal effect on the digital data stream and increases “listenability of the analogue signal” (see Note 1).
NOTE 1 – The data paths and the noise blanker circuit are not shown for simplicity.