THE STRUCTURE AND GENERATION OF ROBUST WAVEFORMS FOR FM IN-BAND ON-CHANNEL DIGITAL BROADCASTING Paul J. Peyla iBiquity Digital Corporation 8865 Stanford Blvd. 20 Independence Blvd. Columbia, MD 21045 Warren, NJ 07058 iBiquity Digital Corporation has developed a IBOC SERVICES AND PROTOCOLS digital broadcasting solution that permits a In order to provide broadcaster flexibility and smooth evolution from current analog FM radio enhance the listening experience, iBiquity’s FM to a fully digital in-band on-channel (IBOC) IBOC system supports a variety of digital system. The system delivers digital audio and program services. These include a main program data services to mobile, portable, and fixed service (MPS), personal data service (PDS), receivers from terrestrial transmitters in the station identification service (SIS), and auxiliary existing VHF radio band. Broadcasters may application service (AAS). continue to transmit analog FM simultaneously with the new, higher-quality and more robust The MPS delivers existing programming digital signals. This approach allows formats in digital audio, along with digital data broadcasters to convert from analog to digital that directly correlates with the audio radio while maintaining their current frequency programming. Whereas the MPS broadcasts a allocations. This paper describes the structure, traditional audio program to listeners, the PDS generation, and inherent flexibility of the enables listeners to select on-demand data transmitted FM IBOC waveforms. services, thereby providing personalized, user- valued information. The SIS provides the control and identification information required to allow INTRODUCTION the listener to search and select IBOC digital radio stations and their supporting services. iBiquity’s FM IBOC solution affords Finally, the AAS allows a virtually unlimited broadcasters the ability to tailor their digital number of custom and specialized IBOC digital audio broadcasts to meet their own specific radio applications to co-exist concurrently. needs. During the transition period to digital, Auxiliary applications can be added at any time each station will have the opportunity to convert in the future. at its own pace – beginning with a hybrid analog/digital waveform, and eventually turning Simultaneous support of these services is off the analog and broadcasting an all digital provided via the layered protocol stack signal. illustrated in Figure 1. Source material (audio or data) moves down the protocol stack from layer To support a wide variety of program formats 5 to layer 1 at the transmitter, is broadcast over and broadcaster requirements, the FM IBOC the air, and is passed back up the protocol stack system was designed with a high degree of from layer 1 to layer 5 at the receiver. flexibility. Each waveform – whether hybrid or all digital – can be configured in a number of At the transmitter, layer 5 receives audio or ways by judiciously adjusting the throughput, data program content from the broadcaster. latency, and robustness of the audio and data Layer 4 provides content-specific source program content as it is converted into an IBOC encoding (such as audio compression), as well as waveform. station identification and control capabilities. Layer 3 ensures robust and efficient transfer of This paper describes the structure and layer 4 data, and layer 2 provides limited error generation of the FM IBOC waveforms, and detection, addressing, and multiplexing. presents the various configurations from which a broadcaster can choose in order to transmit digital audio or data in a manner that best supports his or her particular programming needs. 1 Layer 5 (Application) Station Identification Main Program Service Service Others TRANSMITTED WAVEFORMS AND L5 MPS L5 MPS Audio Data SIS Data (PDS,AAS) SPECTRA Layer 4 (Encoding) The iBiquity FM IBOC design provides a L4 MPS L4 MPS L4 SIS Audio Data Data flexible means of transitioning to a digital Coding Formatting Formatting broadcast system by providing three new Layer 3 (Transport) waveform types: hybrid, extended hybrid, and all L3 MPS L3 SIS digital. The hybrid and extended hybrid types retain the analog FM signal, while the all digital Layer 2 (Service Mux) type does not. L2 Service Multiplexer In all waveforms, the digital signal is modulated using orthogonal frequency division Layer 1 (Physical) multiplexing (OFDM). In a single-carrier digital L1 Physical modulation scheme, the digital symbols are transmitted serially, with the spectrum of each symbol occupying the entire channel bandwidth during its appointed signaling interval. Figure 1 IBOC Protocol Stack Conversely, OFDM is a parallel modulation scheme in which the data stream simultaneously Layer 1 receives the formatted content from modulates a large number of orthogonal layer 2 and creates an FM IBOC waveform for subcarriers. Instead of a single, wideband carrier over-the-air transmission in the VHF band. Since at a high signaling rate, OFDM employs a large most of the signal processing required to number of narrowband subcarriers that are generate an FM IBOC waveform occurs in layer simultaneously transmitted at a much lower 1, it is the focus of this paper. composite symbol rate. The long symbol times Formatted program content is received from of OFDM provide superior robustness in the layer 2 in discrete transfer frames via multiple presence of multipath fading and interference.1 logical channels. A transfer frame is an ordered OFDM is also inherently flexible, readily collection of bits originating in layer 2, grouped allowing the mapping of specific logical for processing through a logical channel. A channels to different groups of subcarriers. logical channel is simply a signal path that The following sections describe the conducts transfer frames from layer 2 through transmitted spectrum for each of the three digital layer 1 with a specified grade of service. The waveform types. Each spectrum is divided into service mode defines the active logical channels several sidebands, which represent various and their associated transmission characteristics. OFDM subcarrier groupings. All spectra are This paper describes the FM IBOC system by illustrated at baseband, with an upper and lower introducing the following concepts: sideband centered around 0 Hz. • transmitted waveforms and spectra – Frequency partitions and spectral conventions describes the spectral structure of the The OFDM subcarriers are assembled into broadcast IBOC waveforms frequency partitions. Each frequency partition is • system configuration – describes the comprised of eighteen data subcarriers and one means by which a broadcaster can tailor reference subcarrier, as shown in Figure 2 a particular IBOC waveform to meet his (ordering A) and Figure 3 (ordering B). The or her specific needs position of the reference subcarrier (ordering A • logical channels – describes the or B) varies with the location of the frequency 2 conduits which carry the formatted partition within the spectrum. program content through layer 1, and For each frequency partition, data subcarriers their intended use d1 through d18 convey digital program content, • functional components – describes the while the reference subcarrier conveys system layer 1 processes used to convert logical control. OFDM subcarriers are numbered from 0 channels of formatted program content at the center frequency to ±546 at either end of to an FM IBOC waveform the channel frequency allocation. 2 Reference Subcarrier Reference 18 Data Subcarriers d1 d2 d3 Figure 2 Frequencyd Partition4 – Ordering A d5 d6 d7 d8 Frequency d9 d10 d11 d12 d13 d14 d15 18 Data Subcarriers d1 d16 d2 d17 d3 d18 within Besideseach frequency the reference partition, subcarriers depending resident on d4 the service mode, up to five additional reference d5 subcarriers are inserted into the spectrum at d6 d7 subcarrier numbers Figure 3 Frequency Partitiond8 – Ordering B Reference Subcarrier The overall effect is a regular distribution of d9 Frequency reference subcarriers throughout the spectrum. d10 d11 d12 subcarrier is assigned a unique identification d13 For notational convenience, each reference d14 number between 0 and 60. All lower sideband d15 reference subcarriers are shown in Figure− 4. All d16 upper sideband reference subcarriers are shown546, d17 in Figure 5. The figures indicate the relationship d18 − Reference between reference subcarrier numbers and 279, 0, 279, and 546. OFDM subcarrier numbers. -546 -527 Lower Primary Sideband -508 subsectionsEach spectrumshows the described subcarrier in number the remaining and -489 center frequency of certain key OFDM 0 -470 subcarriers. The center frequency of a subcarrier 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 17 18 19 20 21 22 23 24 25 26 27 28 29 -451 is calculated by multiplying the subcarrier -432 number by the OFDM subcarrier spacing -413 363.373 Hz. The center of subcarrier 0 is located -394 at 0 Hz. In this context, center frequency is -375 relative to the radio frequency (RF) allocated -356 channel. -337 Figure 4 Lower Sideband Reference Subcarrier-318 Spectral Mapping -299 Lower Secondary Sideband 0 -280 19 Upper Secondary Sideband -279 38 30 -266 57 31 32 33 34 35 36 -247 Reference Subcarrier Numbers 76 -228 95 -209 114 -190 ∆ 133 -171 f 152 ≈ -152 171 37 -133 190 38 39 40 -114 209 -95 228 Figure 5 Upper Sideband Reference Subcarrier Spectral Mapping -76 41 42 43 44 4546247 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Upper Primary Sideband -57 266 -38 280 279 -19 299 318 OFDM 337 Subcarrier Reference Subcarrier Numbers 356 Numbers 375 Frequency 394 413 432 451 470 3 489 508 527 546 SubcarrierOFDM Numbers Frequency Hybrid waveform In the hybrid waveform, the digital signal is The power spectral density of each OFDM transmitted in primary main (PM) sidebands on subcarrier in the PM sideband, relative to the either side of the analog FM signal, as shown in host analog power, is given in Table 1. A value Figure 6. The analog signal may be monophonic of 0 dB would produce a digital subcarrier whose or stereo, and may include SCA channels.