Forward error correction coding for AM 9kHz and 10kHz in-band on-channel digital audio broadcasting systems

#This invention relates to methods and apparatus for digital audio broadcasting, and more particularly to such methods and apparatus for broadcasting AM compatible in-band on-channel (IBOC) digital audio broadcasting (DAB) signals.

#Digital audio broadcasting (DAB) is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM in-band, on-channel (IBOC) DAB can be transmitted in a hybrid format where a digitally modulated signal coexists with the AM signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. The hybrid format allows existing receivers to continue to receive the AM signal while allowing new IBOC receivers to decode the DAB signal. IBOC DAB requires no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM channel allocation. IBOC DAB promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.

#U.S. Pat. No. 5,588,022 teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. The DAB signal comprises an amplitude modulated radio frequency signal including a first subcarrier modulated by an analog program signal and having a first frequency spectrum, and a plurality of digitally modulated subcarrier signals that are broadcast within a bandwidth that encompasses the first frequency spectrum. The digitally modulated subcarrier signals are modulated by a digital program signal. A first group of the digitally modulated subcarrier signals lies within the first frequency spectrum and is modulated in quadrature with the first subcarrier signal. Second and third groups of the digitally modulated subcarrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first subcarrier signal. U.S. Pat. No. 6,243,424 discloses another embodiment of an AM Digital Audio Broadcasting system.

#FIG. 1 is a schematic representation of partition and subcarrier assignments for a hybrid AM IBOC DAB signal in accordance with this invention.

#FIG. 2 is a functional block diagram of an AM IBOC DAB transmitter constructed in accordance with this invention.

#FIG. 3 is a block diagram illustrating the functionality of forward error correction (FEC) and interleaving for a 20 or 30 kHz AM IBOC system.

#In one aspect, this invention provides a single stream hybrid AM IBOC DAB system having a 20 or 30 kHz bandwidth and 25 and 19 kbps modes. The system broadcasts a waveform including an analog modulated carrier and a plurality of digitally modulated subcarriers in the same channel as the analog modulated carrier. The digital signal is divided into several partitions, which are mapped to groups of the subcarriers. Referring to the drawings, FIG. 1 is a schematic representation of the partition and subcarrier assignments for a hybrid 20 or 30 kHz AM IBOC signal.

#The signal 10 includes an analog modulated carrier signal 12 that is modulated by an analog signal, resulting in an AM host signal generally indicated by line 14. The signal further includes a plurality of subcarriers at evenly spaced positions (numbered −81 to +81, and spanning a bandwidth of about 30 kHz) in the radio channel 16. BPSK modulated timing subcarriers 18 and 20 are located at the subcarrier positions +1 and −1, closest to the analog modulated carrier. A first plurality of subcarrier signals 22 are located at positions 2 through 31 in a first portion 24 of the radio channel and a second plurality of subcarrier signals 26 are located at positions −2 through −31 in a second portion 28 of the radio channel. The first and second pluralities of subcarrier signals are complementary modulated QPSK subcarrier signals and have a power spectral density below the analog modulated carrier by at least a first margin.

#A third plurality of subcarrier signals 30 are located at positions 32 through 36 in a third portion 32 of the radio channel and a fourth plurality of subcarrier signals 34 are located at positions −32 through −36 in a fourth portion 36 of the radio channel. The third and fourth pluralities of subcarrier signals are complementary modulated 16-QAM subcarrier signals and have a power spectral density below the analog modulated carrier by at least a second margin. Complementary modulated subcarriers in the lower sideband groups 26 and 34 are modulated as the mirror image (negative complex conjugate) of corresponding subcarriers in the upper sideband groups 22 and 30.