Method and system for turbo-coded satellite digital audio broadcasting

• US 6,061,387 A
• Filed: 03/24/1999
• Issued: 05/09/2000
• Est. Priority Date: 08/11/1997
• Status: Expired due to Term

• Current Assignee: Orbital Sciences Corporation (Northrop Grumman Corporation)
• Original Assignee: Orbital Sciences Corporation (Northrop Grumman Corporation)
• Inventors: Yi, Byung Kwan
• Primary Examiner(s): TSE, YOUNG TOI
• Application Number:
  US09/275,081
• Time in Patent Office: 412 Days
• Field of Search: 375/259, 375/260, 375/267, 375/347, 375/130, 375/140, 375/141, 375/142, 375/144, 375/148, 375/150, 370/208, 370/209, 370/320, 370/342, 370/486, 455/3.2, 455/103, 455/132, 455/133, 455/137, 455/427, 455/506, 455/12.1
• US Class Current: 375/142
• CPC Class Codes: H03M 13/2957 Turbo codes and decoding
• CPC Class Codes: H03M 13/3761 using code combining, i.e. using combining of codeword portions which may have been transmitted separately, e.g. Digital Fountain codes, Raptor codes or Luby Transform [LT] codes
• CPC Class Codes: H03M 13/6306 Error control coding in combination with Automatic Repeat reQuest [ARQ] and diversity transmission, e.g. coding schemes for the multiple transmission of the same information or the transmission of incremental redundancy H03M13/3761, H03M13/3769 and H03M13/635 take precedence
• CPC Class Codes: H03M 13/6362 by puncturing
• CPC Class Codes: H04B 1/7115 Constructive combining of multi-path signals, i.e. RAKE receivers
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception
• CPC Class Codes: H04B 7/18523 Satellite systems for providing broadcast service to terrestrial stations, i.e. broadcast satellite service arrangements specially adapted for satellite broadcast receiving H04H40/90; picture transmission via satellite H04N1/00103; television transmission via satellite H04N7/20
• CPC Class Codes: H04H 20/06 among broadcast stations
• CPC Class Codes: H04H 20/74 of satellite networks
• CPC Class Codes: H04H 2201/20 digital audio broadcasting [DAB]
• CPC Class Codes: H04H 40/90 specially adapted for satellite broadcast receiving
• CPC Class Codes: H04J 13/0022 PN, e.g. Kronecker
• CPC Class Codes: H04L 1/0066 Parallel concatenated codes
• CPC Class Codes: H04L 1/0069 Puncturing patterns
• CPC Class Codes: H04L 2001/0096 Channel splitting in point-to-point links
• CPC Class Codes: Y02D 30/70 in wireless communication networks

Abstract

A system and method for broadcasting an audio signal in a turbo-coded satellite digital audio broadcasting system is provided that utilizes the combination of a turbo coding system having code combining and code diversity techniques to lower the power required for transmittal and to transmit at a higher code rate of 1/4 by utilizing the puncturing sequence and a pilot signal assisted orthogonal CDMA; the invention includes an improved receiver system that uses modified RAKE receivers in order to mitigate the Rayleigh multipath fading, shadowing, and temporal blockage and improve performance. The invention further uses a terrestrial gap filler network having a reduced amount of gap fillers.

Claims

1. A method of communicating audio signals using Turbo Coding in a digital audio broadcast (DAB) system, the method comprising the steps of:

encoding each audio signal according to the Turbo Code punctured to a code rate of 1/2 for transmission on a first path to a first satellite in a substantially geosynchronous orbit;

combining a first pilot signal and a predetermined number of encoded audio signals using an orthogonal CDMA (OCDMA) modulator, said OCDMA modulator consisting essentially of a first set of orthogonal Walsh sequences to form a first turbo encoded broadcast signal;

transmitting said first turbo encoded broadcast signal on said first path having said first pilot signal transmitted at a higher power than the encoded broadcast signal;

encoding each interleaved audio signal according to the Turbo Code punctured to a rate of 1/2 for transmission on a second path to a second satellite;

combining a second pilot signal and the interleaved encoded audio signals using another OCDMA modulator, said another OCDMA modulator consisting essentially of a second set of orthogonal Walsh sequences;

transmitting said second turbo encoded broadcast signal on said second path having said second pilot signal transmitted at a higher power than the encoded broadcast signals;

at a receiver;

receiving said first and second turbo encoded broadcast signals;

demodulating a selected audio channel from said first and second turbo encoded broadcast signals using orthogonal Walsh sequences using separate RAKE receivers so as to optimally combine audio signals for the selected audio channel from said first and second turbo encoded signals received from said first and second paths.

2. The method of claim 1 further including the step of:

transmitting said first and second turbo encoded broadcast signals to a network of gap fillers.

3. The method of claim 2 further including the step of:

delaying the transmission of said first and second turbo encoded broadcast signals so as to compensate for signal delay through said first and second paths to a network of satellites.

4. The method of claim 3 is further including, at the receiver, the steps of:

demultiplexing said first and second turbo encoded broadcast signals so as to separate systematic audio signals from multiplexed systematic sequences and parity sequences; and

combining a code diversity of each of said first and second turbo encoded broadcast signals, each of said first and second turbo encoded broadcast signals having a code rate of 1/2, whereby said code diversity and said combining of said first and second turbo encoded broadcast signals result in a code rate of 1/4.

5. The method of claim 4 further including the steps of:

at the receiver;

receiving said first and second turbo encoded broadcast signals on an antenna;

obtaining said first and second pilot signals including information indicative of a rate of transmission of said orthogonal Walsh sequences of said received first and second turbo encoded broadcast signals;

demultiplexing each of said received turbo encoded broadcast signals into separate first and second systemic audio signals and first and second parity sequences;

supplying the first parity sequence and the first systemic audio signal to a first MAP decoder;

supplying a decoded output signal of said first MAP decoder to an interleaver;

combining an interleaved decoded output signal from said interleaver with said second parity sequence to form a second output signal;

supplying said second output signal and said second systemic audio signal to a second MAP decoder so as to output a decoded digital audio signal thereby deinterleaving the audio signals; and

providing the audio signals to an output device.

6. A transmission system comprising:

long pseudo noise (PN) sequence means for generating a first PN sequence for a first PN channel and a second PN sequence for a second PN channel;

a plurality of audio encoder means, each successively sampling and encoding input analog audio data into packetized digital audio data, said audio encoder means having inputs for a predetermined number of audio channels, each of said audio channels being coupled to said audio encoder means;

a plurality of turbo encoder means, each coupled to one of said audio encoder means for encoding said packetized digital audio data, each of said turbo encoder means encoding digital audio data according to a Turbo Code encoding process, each of said turbo encoder mean encoding said digital audio data into a first turbo encoded signal and a second turbo encoded signal for each of said audio channels;

modulator means for modulating said first PN sequence, said second PN sequence, said first turbo encoded signal and said second turbo encoded signal according to orthogonal Walsh sequences, said modulator means modulating said first PN sequence into a first pilot signal by multiplying said first PN sequence by a Walsh code W₀, said modulator means modulating said second PN sequence into a second pilot signal by multiplying said first PN sequence by a Walsh code W₃₂, said modulator means modulating said first turbo encoded signal for each of said audio channels by multiplying said first turbo encoded signal on each of said channels by a Walsh code W₁, W₂, . . . W₃₁, respectively, said modulator means modulating said second turbo encoded signal by multiplying said second turbo encoded signal for each of said audio channels by a Walsh code W₃₃, W₃₄ . . . , W₆₃, respectively;

means for combining said first pilot signal spread by Walsh code W₀ and said first turbo encoded signals spread by said Walsh codes W₁ . . . W₃₁ to form a first turbo encoded broadcast signal, and for combining said second pilot signal spread by said Walsh code W₃₂ and said second turbo encoded signal spread by said Walsh codes W₃₃ . . . W₆₃ to a second turbo encoded broadcast signal, said first and second pilot signals are transmitted at a higher power than the turbo encoded broadcast signals; and

means for transmitting said combined first pilot signal and said first turbo encoded broadcast signal on a first transmission path, and for transmitting said combined second pilot signal and said second turbo encoded broadcast signal on a second transmission path.

7. The transmission system of claim 6 wherein said first transmission path is to a first satellite and to a gap filler.

8. The transmission system of claim 7 wherein said second transmission path is to a second satellite and a gap filler.

9. A receiver for a Digital Audio Broadcasting (DAB) system, comprising:

an antenna for receiving radio frequency (RF) signals consisting of spread spectrum turbo encoded broadcast signals;

an amplifier coupled to said antenna for amplifying said RF signals;

down converter means coupled to said amplifier for converting the amplified RF signals to a predetermined frequency determined by a local oscillator, said down converter means multiplying the amplified RF signals by a sinusoidal wave form generated by said local oscillator;

analog-to-digital (A/D) converter means coupled to said down converter means for converting from analog to a digitized signal for said received turbo encoded broadcast signals;

demodulator means for demodulating said turbo encoded broadcast signals supplied by said A/D converter means, said demodulator means including dual synchronization means, each of said synchronization means detecting at least one pilot sequence by multiplying said turbo encoded broadcast signals by a first Walsh sequence W₀ to form a first estimation of a first bit and packet epoch time RX₁ for a turbo encoded broadcast signal transmitted on a first signal path A₁ and by multiplying said received turbo encoded broadcast signals by a second Walsh sequence W₃₂ to form a second estimation of a second bit and packet epoch time RX₂ of a turbo encoded broadcast signal transmitted on a second signal path A₂,

dual RAKE receiver means, each of said dual RAKE receiver means being coupled to said A/D converter means for receiving the digitized turbo encoded broadcast signals, said dual RAKE receiver means further receiving input from said demodulator means concerning said bits and packet epoch times from respective said synchronization means in the form of output signals RX₁ and RX₂,

dual channel selection means for selecting a desired channel from one of a predetermined number of channels, said channel selection means providing a selected one of Walsh Code Sequence W_i and W_i+32 to each respective one of the dual RAKE receiver means;

turbo decoder means for decoding turbo encoded broadcast signals of the selected channel in the output signals RX₁ and RX₂ according to a Turbo Code, said turbo decoder means comprising;

demultiplexing means for demultiplexing two sequences from each of the output signals RX₁ and RX₂ from said dual RAKE receiver means,

code diversity combiner means for combining said the sequences from said demultiplexer means into the sequences X₁, Y₁, X₂, Y₂, and

an iterative decoding means for decoding noise-contaminated sequences X₁, Y₁, X₂, Y₂ into a packetized error-free audio data signal d_k, said iterative decoding means comprising a first MAP decoder, a packet combiner means for combining output of the first MAP decoder and an output of said code diversity combiner means representative of said X₂ sequence;

audio decoder means for decoding said packetized error-free audio data signal d_k into an analog audio signal; and

output means for outputting said analog audio signal to a user.

10. The receiver of claim 9 wherein said antenna is a hemispherical coverage antenna.

11. The receiver of claim 9 wherein said predetermined frequency is a base band frequency.

12. The receiver of claim 9 wherein said predetermined frequency is an intermediate frequency.

13. The receiver of claim 12 wherein said packet combiner means combines an output signal Λ

₁ received from said first MAP decoder via an interleaver and the sequence X₂ received from said code diversity combiner means, said packet combiner means adding outputs of each of a log-likelihood ratio of the output signal Λ

₁ and of a log-likelihood ratio of said sequence X₂ whereby said packet combiner means provides two independent estimations of said sequence X₂.

14. The receiver of claim 13 wherein said iterative decoding means further includes a second MAP decoder, said second MAP decoder decoding an output signal Λ

_combined and Y₂ sequences.