Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1998-11-24
2001-06-12
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S529000, C375S347000
Reexamination Certificate
active
06246698
ABSTRACT:
This invention relates to a method of communication and a transmitter and receiver system for the terrestrial radio-frequency (RF) broadcast and reception of digital data, preferably including stereo high-fidelity music, to accomplish the goal of digital audio broadcast (DAB). The digital signal generated by the DAB transmitter exists together with the conventional analog frequency-modulation (FM) signal in the commercial broadcast FM-band (87.9 MHZ to 107.9 MHZ in the United States) and is typically emitted from the same antenna system. The analog FM signal and the digital signal are unrelated and are received independently. The digital signal is generated to occupy the upper and lower sideband frequency regions which surround the analog FM signal. The spectrum of the digital signal is substantially contained within the predetermined RF emission mask. Redundant data is transmitted in both sidebands simultaneously or with diversity delay so that the loss (i.e. unreliable determination) of the source information in only one of the two sidebands due to multipath or adjacent station interference does not substantially disrupt continuous reception of the digital signal.
CLAIM TO COPYRIGHT IN REFERENCE TO MICROFICHE APPENDIX
A portion of the disclosure of this patent application contains material which is subject to copyright protection. The copyright owner has no objection to the reproduction by anyone of the patent document as it appears in the Patent and Trademark Office files or records, but otherwise reserves all copyrights whatsoever. Software for carrying out some of the methods and systems described herein has been filed with the United States Patent and Trademark Office herewith in the form of a microfiche appendix including numerous frames, one of which being a title frame. This software may be included as part of a chip or disc according to certain embodiments of this invention. The microfiche appendix is entitled IN-BAND ON-CHANNEL DIGITAL BROADCASTING METHOD AND SYSTEM and includes two (2) microfiche and one hundred six (106) frames.
BACKGROUND OF THE INVENTION
The goal of digital audio broadcast (DAB) is to provide a robust method and system for the simplex transmission and reception of high-quality audio (music and/or speech) and ancillary digital data by radio-frequency (RF) signals. The RF signals are generated in a transmitter system, emitted for free-space atmosphere propagation, and received by one or a plurality of receiver systems, which may be mobile. The transmitter may be space-borne or terrestrially located or a combination thereof (i.e. space-borne with terrestrial re-broadcasting). In general, terrestrial transmission is required for adequate mobile receiver performance in areas with dense natural and/or man-made structures because satellite reception typically requires line-of-sight (LOS) propagation due to large propagation path signal loss.
The RF signal for DAB represents digital (bit) information which is encoded in the signal generated in the transmitter system by a modulation method. Unlike conventional analog FM modulation, the information represented by the digital signal (e.g. digitized speech, music, and/or data) is typically unrelated to the characteristics of the transmitted RF signal. A primary goal of DAB is to eventually supplant the existing commercial analog radio broadcast network (i.e. AM-band and FM-bands). Since the primary function of the existing AM-band and FM-band is to provide audio services in the form of music and/or speech, it is presumed that a significant fraction of the encoded digital data represents one or a plurality of digital audio signals.
In many circumstances, the quality of the digital audio signal recovered in the DAB RF signal receiver is expected to be improved when compared to the audio signal recovered from the conventional analog frequency-modulated (FM) signal. Typically, the digital audio signal has i) a higher signal-to-noise (SNR) ratio, ii) a wider (i.e. larger) audio bandwidth, and iii) improved stereo separation (i.e. spatial fidelity) when compared to conventional FM-band reception. For example, conventional analog FM-band broadcast signals have a recovered audio bandwidth of about 15 kHz. While 15 kHz audio bandwidth is substantially greater the audio bandwidth received for commercial AM-band radio broadcast (i.e. 535 kHz-1705 kHz), which is typically less than 10 kHz, 15 kHz is less than the bandwidth of pre-recorded music on the compact-disc (CD) format, which is about 20 kHz.
The improved audio bandwidth advantage of DAB when compared to conventional analog FM-band reception is generally desirable. However, the most significant advantage of DAB is the greater immunity, determined at the receiver, of the DAB signal when compared to the analog FM-band signal to various forms of distortion and interference. The presence of distortion and interference in the recovery of the analog FM-band signal may cause various undesirable artifacts in the audio signal determined at the receiver, for example, static noise, hiss and hum, and clicks. In certain circumstances, the quality of the recovered signal is limited by the effects of background noise. Background noise may be generated by galactic, atmospheric, thermal, and man-made sources (e.g. ignition-noise), for example. Background noise is typically distinguished from distortion which is i) related to the propagation characteristics of the RF signal, for example, multipath, and ii) interference caused by other RF broadcast sources. Background noise is particularly noticeable in the recovered analog FM-band signal when there are pauses or quiet passages in the audio program. In general, the process of analog FM modulation typically improves the robustness of the recovered signal when compared to other analog modulation methods (e.g. amplitude modulation as used in the AM broadcast band) by causing “processing gain” by increasing the occupied bandwidth of the signal. However, the benefits of the analog FM processing gain are substantially eliminated when the source audio signal is silent or quiet.
Reception of the FM-band signal may be significantly disturbed by the effect of multipath propagation. The deleterious effects of multipath are a result of the relatively high RF carrier frequencies, which are characteristic of the FM-band frequency range, and the use of omnidirectional receiving antennas, especially for radio receivers in vehicles. Multipath propagation results from the presence of specular and/or diffuse reflectors in or about the propagation path between the transmitter and the receiver. As a result, multiple signals with varying delay, phase, amplitude, and frequency are received, these signals corresponding to different propagation paths. In general, the deleterious effects of multipath are: i) attenuation of the received RF signal due to destructive coherent interference between paths, ii) dispersion in the received RF signal due to the frequency selective characteristic of multipath, and iii) intersymbol interference between adjacent signal baud intervals. Multipath is typically mathematically modeled as a deterministic linear sum or stochastic function of the transmitted signal and reflections, with background noise modeled independently in the summation.
In addition to background noise and multipath, interference in the received signal may be caused by the presence of other RF signal sources with similar frequencies, including other FM-band transmitters. In circumstances when there are many transmitters, for example, in large urban areas, inter-station interference due to other transmitters operating at the same or similar RF frequencies may the primary cause of signal degradation, except for weak-signal conditions at substantial distances from the transmitter or unusual reception circumstances (e.g. signal shielding in a tunnel).
Because broadcast transmitters may emit high-power RF signals, their operation is often dictated by rules and regulations enacted by governmental agencies. Although the rules are intended to prevent interferenc
Digital Radio Express, Inc.
Hyun Soon-Dong
Liniak, Berenato, Longacre & White LLC
Nguyen Chau
LandOfFree
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