Patent Application
20100150249
The present invention generally relates to communications systems and, more particularly, to wireless systems, e.g., terrestrial broadcast, cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.
The ATSC DTV (Advanced Television Systems Committee Digital Television) system (e.g., see, United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, Document A/53, Sep. 16, 1995 and “Guide to the Use of the ATSC Digital Television Standard”, Document A/54, Oct. 4, 1995) offers about 19 Mbits/sec (millions of bits per second) for transmission of an MPEG2-compressed HDTV (high definition TV) signal (MPEG2 refers to Moving Picture Expert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1)). As such, around four to six TV channels can be supported in a single physical transmission channel (PTC) without congestion. Additionally, excess bandwidth remains within this transport stream to provide for additional services. In fact, due to improvements in both MPEG2 encoding and the introduction of advanced codec (coder/decoder) technology (such as H.264 or VC1), even more additional spare capacity is becoming available in a PTC.
However, the ATSC DTV system was designed for fixed reception and performs poorly in a mobile environment due to fading and Doppler effects that can easily cause signal loss for a period of a second or more at the receiver. In this regard, there has been strong interest in developing an ATSC DTV system for mobile and handheld (M/H) devices while maintaining backward compatibility with the existing ATSC DTV system.
One way to improve performance in a mobile environment is to use time diversity techniques combined with forward error correction (FEC). Some examples of forward error correction are block codes (e.g., Reed-Solomon, BCH), convolution codes, low-parity check codes (LDPC) and turbo codes. Time interleaving can be accomplished either using block or convolution interleaving techniques. FEC, when used in combination with interleavers, vastly improves communication performance over fading channels. Unfortunately, these systems generally incur a time delay that is proportional to the time diversity. As such, an unfortunate side effect of such time diversity techniques in the context of a mobile TV system is that a user would see this delay in the form of long channel change times when switching channels, which may be highly objectionable to the user. As such, the designer of a mobile TV system is forced to tradeoff fast channel change against time diversity for and fade protection. Increasing the performance in one area generally means a decrease in the performance in another area.
However, we have realized that both time diversitý for fade protection and rapid channel change can be achieved if a set of requirements is imposed on both the broadcast and terminal device. In particular, StaggerCasting (a form of time diversity protection) is used in accordance with the principles of the invention to provide protection to a wireless transmission stream from fades without incurring any channel change delay.
In accordance with the principles of the invention, a receiver receives a channel comprising at least one encoded stream and an error correcting stream, wherein the encoded stream is staggered with respect to the error correcting stream; decodes the received encoded stream for providing content; corrects the received encoded stream using the received error correcting stream upon detecting errors in the received encoded stream; and when a different channel is selected, decodes a received encoded stream of the different channel for providing content even though for an initial period of time equal to a time delay errors in the received encoded stream of the different channel are not correctable by the received error correcting stream of the different channel; wherein the encoded stream of the different channel is delayed with respect to the error correcting stream of the different channel by the time delay.
In an illustrative embodiment of the invention, an Advanced Television Systems Committee Digital Television (ATSC DTV) mobile, or handheld, device comprises a receiver for receiving a digital multiplex that includes a mobile DTV channel, which is transmitted in StaggerCast form. In particular, the receiver receives a StaggerCast signal comprising an encoded stream for conveying the content for a selected program, e.g., the video and audio, and an error correcting stream, e.g., FEC blocks. With respect to the StaggerCasting, the encoded stream is delayed with respect to the error correcting stream by a time delay. Illustratively, all StaggerCast signals have the same time delay. The receiver decodes the received encoded stream for providing content for the selected program and, if errors are detected in the received encoded stream, uses the received error correcting stream to attempt to correct the errors. However, when the uses changes programs, or channels, to a different StaggerCast stream, the receiver decodes a received encoded stream of the different StaggerCast stream for providing content even though for an initial period of time equal to the time delay errors in the received encoded stream of the different StaggerCast stream are not correctable by the received error correcting stream of the different StaggerCast stream.
In view of the above, and as will be apparent from reading the detailed description, other embodiments and features are also possible and fall within the principles of the invention.
Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. For example, other than the inventive concept, familiarity with Discrete Multitone (DMT) transmission (also referred to as Orthogonal Frequency Division Multiplexing (OFDM) or Coded Orthogonal Frequency Division Multiplexing (COFDM)) is assumed and not described herein. Also, familiarity with television broadcasting, receivers and video encoding is assumed and is not described in detail herein. For example, other than the inventive concept, familiarity with current and proposed recommendations for television (TV) standards such as NTSC (National Television Systems Committee), PAL (Phase Alternation Lines), SECAM (SEquential Couleur Avec Memoire), ATSC (Advanced Television Systems Committee), Digital Video Broadcasting (DVB), Digital Video Broadcasting-Terrestrial (DVB-T) (e.g., see ETSI EN 300 744 V1.4.1 (2001-01), Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television, DVB-H and the Chinese Digital Television System (GB) 20600-2006 (Digital Multimedia Broadcasting—Terrestrial/Handheld (DMB-T/H)) is assumed. Further information on ATSC broadcast signals can be found in the following ATSC standards: Digital Television Standard (A/53), Revision C, including Amendment No. 1 and Corrigendum No. 1, Doc. A/53C; and Recommended Practice: Guide to the Use of the ATSC Digital Television Standard (A/54). Likewise, other than the inventive concept, other transmission concepts such as eight-level vestigial sideband (8-VSB), Quadrature Amplitude Modulation (QAM), and receiver components such as a radio-frequency (RF) front-end (such as a low noise block, tuners, down converters, etc.), demodulators, correlators, leak integrators and squarers is assumed. Further, other than the inventive concept, familiarity with protocols such as the File Delivery over Unidirectional Transport (FLUTE) protocol, Asynchronous Layered Coding (ALC) protocol, Internet protocol (IP) and Internet Protocol Encapsulator (IPE), is assumed and not described herein. Similarly, other than the inventive concept, formatting and encoding methods (such as Moving Picture Expert Group (MPEG)-2 Systems Standard (ISO/IEC 13818-1)) for generating transport bit streams are well-known and not described herein. It should also be noted that the inventive concept may be implemented using conventional programming techniques, which, as such, will not be described herein. Finally, like-numbers on the figures represent similar elements.
To see how a receiver enjoys the benefits of redundancy without inducing additional delay in changing channels in accordance with the principles of the invention refer again to
In the above-described example, the time diversity is represented by the time delay TD. In accordance with the principles of the invention, after a channel change the receiver processes data without the benefit of time diverse FEC for this same interval of time. The time delay TD can be tuned in order to provide an appropriate trade-off. While it is assumed that all StaggerCast streams have the same time delay, the inventive concept is not so limited and the time delays can vary between different StaggerCast streams. For example, one StaggerCast stream may have a first time delay, TD1, while a second StaggerCast stream may have a different second time delay TD2. In such cases, it is assumed that the receiver receives associated program and system information indicating the appropriate time delay for a received StaggerCast signal. In fact, the delay on the same channel, TD, itself may not be fixed and can vary. In the case of a varying delay, the value can be bounded, e.g., 0<TD≦TDmax. A variable delay might be required if variable bit rate (VBR) content is conveyed over a constant bit rate (CBR) channel, or CBR content is conveyed over a VBR channel. In this case a sequence number found in an RTP (Real-Time Protocol) specific field can be used to by the receiver for re-aligning, or re-synchronizing, the FEC stream and the base stream in the receiver.
Thus, and in accordance with the principles of the invention, a receiver receives a channel comprising at least one encoded stream and an error correcting stream, wherein the encoded stream is staggered with respect to the error correcting stream; decodes the received encoded stream for providing content; corrects the received encoded stream using the received error correcting stream upon detecting errors in the received encoded stream; and when a different channel is selected, decodes a received encoded stream of the different channel for providing content even though for an initial period of time equal to a time delay errors in the received encoded stream of the different channel are not correctable by the received error correcting stream of the different channel; wherein the encoded stream of the different channel is delayed with respect to the error correcting stream of the different channel by the time delay.
Turning now to
The elements shown in
Both full stream 11 and FEC stream 12 are applied to mux 115, which multiplexes the two logical channels (full stream 11 and FEC stream 12) to provide a multiplexed stream 116 for application to modulator 120. An example of multiplexed stream 116 is shown in
Referring now to
It should be noted that in general, it is preferable not to use a time interleaver with significant delay for the full stream 11. However, if even better fading performance is desired, time interleaving can be used on the FEC stream 12. This does not add to the overall channel delay experienced by the receiver. In addition, although the example above was illustrated with a simple rate ½ FEC repetition code, a much more sophisticated code could be used. For example, a long code could be used to provide the ability to recreate even a completely lost base datagram. A simple example of this is a ¾ FEC code that operates on 2 blocks from the above diagram shown in
Referring now to
Turning now to receiver 205, an illustrative portion of receiver 205 in accordance with the principles of the invention is shown in
Receiver 205 comprises demodulator 305, demultiplexer (demux) 310, delay buffer 315 and FEC decoder 320. Only those portions relevant to the inventive concept are shown. As noted above, receiver 205 receives a broadcast signal 204 (e.g., via an antenna (not shown)) for processing. Broadcast signal 204 is downconverted by front-end processing (not shown) to provide received signal 304. The latter is demodulated by demodulator 305, which provides demodulated signal 306 (a stream of symbols) to demux 310. Demux 310 performs the inverse function of mux 115 of transmitter 100 and separates out the full stream from the FEC stream. In particular, demux 310 provides full stream 311, which corresponds to the received version of full stream 11, and also provides FEC stream 312, which corresponds to the received version of FEC stream 12. The latter is delayed in time by delay buffer 315 to provide delayed FEC stream 316. Delay buffer 315 provides a corresponding time delay of TD to realign in time the full stream with the FEC stream. FEC decoder 320 receives both the delayed FEC stream 31 and the full stream 311 for providing output signal 321. The latter is processed by other circuitry (not shown) of receiver 205 as represented by ellipses 325 to recover therefrom, e.g., the video signal 206.
Referring briefly back to
After the time delay, TD, FEC decoder 320 can attempt to correct any detected errors in full stream 311 by using the corresponding error correcting data in FEC stream 316 in providing output signal 321.
Referring now to
In accordance with the principles of the invention, there are a number of interesting variations. For example, less bits can be dedicated to the FEC encoding and the FEC can be combined with a more powerful code that has the ability to correct for longer blocks to achieve superior performance with no additional bandwidth or delay requirements. One example of this is a block code with the ability to control errors that are distributed over multiple blocks such as a convolutional code, turbo code, LDPC code. Another example of this is interleaving the error correcting stream with a long interleaving delay without incurring additional delay. This is illustrated in
In addition, SVC (scalable video coding) can be used to encode the full stream. In SVC, there is typically an SVC base layer and at least one SVC enhancement layer. The SVC base layer provides a basic level of video resolution, e.g., standard definition, while any SVC enhancement layers increase the video resolution, e.g., high definition. In the context of this invention, the SVC enhanced layer can be broadcast without any StaggerCasting protection, and StaggerCasting of error correcting data, e.g., FEC data, can be provided only to the SVC base layer. This provides for a fallback video signal to be available with very high reliability without unnecessary increase in the bit rate.
This is further illustrated in accordance with the principles of the invention in transmitter 600 of
The elements shown in
Turning now to receiver 205, an illustrative portion of receiver 205 in accordance with the principles of the invention for use in SVC is shown in
Receiver 205 comprises demodulator 305, demultiplexer (demux) 710, delay buffer 315 and FEC decoder 720. Only those portions relevant to the inventive concept are shown. As noted above, receiver 205 receives a broadcast signal 204 (e.g., via an antenna (not shown)) for processing. Broadcast signal 204 is downconverted by front-end processing (not shown) to provide received signal 304. The latter is demodulated by demodulator 305, which provides demodulated signal 306 (a stream of symbols) to demux 710. Demux 710 performs the inverse function of mux 620 of transmitter 600 and separates out the full stream from the FEC stream. In particular, demux 710 provides a full stream, as represented by a received base layer stream 711 and an enhancement layer stream 712, which corresponds to the received version of full stream 11, and also provides FEC stream 312, which corresponds to the received version of FEC stream 12. The latter is delayed in time by delay buffer 315 to provide delayed FEC stream 316. Delay buffer 315 provides a corresponding time delay of TD to realign in time the full stream with the FEC stream. FEC decoder 720 receives both the delayed FEC stream 31 and base layer stream 711 for providing output signal 721. The base layer now represented by output signal 721 and the enhancement layer stream 712 are processed by other circuitry (not shown) of receiver 205 as represented by ellipses 725 to recover therefrom, e.g., the video signal 206.
At receiver startup, or just after selecting a channel, the delay buffer 315 in receiver 205 is flushed, i.e., empty, for the period of time equal to TD. As such, in this initial period after a channel change, FEC decoder 720 does not have any FEC data for protecting the base layer stream, so it merely passes through the unprotected base layer stream 711 to its output, i.e., as output signal 721. After the time delay, TD, FEC decoder 720 can attempt to correct any detected errors in base layer stream 711 by using the corresponding error correcting data in FEC stream 316 in providing output signal 321. The method shown in
It should also be noted that inventive concept equally applies to the transmission of audio as the encoded stream. As such, the apparatus and methods described above in accordance with the principles of the invention also apply to compressed audio both non-scalable and scalable for implementing fast channel change. For example, in terms of audio, device 205 of
As described above, and in accordance with the principles of the invention, StaggerCasting is used to provide protection to a wireless transmission stream from fades without incurring any channel change delay. It should be noted that although the inventive concept was described in the context of blocks, e.g., blocks “A”, “B” and “C” of
In view of the above, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be embodied in one or more integrated circuits (ICs). Similarly, although shown as separate elements, any or all of the elements may be implemented in a stored-program-controlled processor, e.g., a digital signal processor, which executes associated software, e.g., corresponding to one or more of the steps shown in, e.g.,
This application claims the benefit of U.S. Provisional Application No. 60/966,431, filed Aug. 28, 2007.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2008/010117 | 8/26/2008 | WO | 00 | 2/16/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60966431 | Aug 2007 | US |
