WO1998052318A1 - Systeme et procede de transmission de donnees dans la bande de frequence am - Google Patents

Systeme et procede de transmission de donnees dans la bande de frequence am Download PDF

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Publication number
WO1998052318A1
WO1998052318A1 PCT/US1998/010061 US9810061W WO9852318A1 WO 1998052318 A1 WO1998052318 A1 WO 1998052318A1 US 9810061 W US9810061 W US 9810061W WO 9852318 A1 WO9852318 A1 WO 9852318A1
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WO
WIPO (PCT)
Prior art keywords
data
signal
modulation
receiver
subcarrier
Prior art date
Application number
PCT/US1998/010061
Other languages
English (en)
Inventor
Doug Smith
Timothy F. Dyson
Michael A. Dipiano
Original Assignee
Mobile Broadcasting Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from PCT/US1998/004948 external-priority patent/WO1998040966A1/fr
Application filed by Mobile Broadcasting Corporation filed Critical Mobile Broadcasting Corporation
Priority to AU74935/98A priority Critical patent/AU7493598A/en
Publication of WO1998052318A1 publication Critical patent/WO1998052318A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04H—BROADCAST COMMUNICATION
    • H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
    • H04H20/28—Arrangements for simultaneous broadcast of plural pieces of information
    • H04H20/36—Arrangements for simultaneous broadcast of plural pieces of information for AM broadcasts
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04H—BROADCAST COMMUNICATION
    • H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
    • H04H20/44—Arrangements characterised by circuits or components specially adapted for broadcast
    • H04H20/46—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95
    • H04H20/47—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems
    • H04H20/49—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems for AM stereophonic broadcast systems

Definitions

  • This invention relates to a system and method for transmitting digital information concurrent with a co-existing monophonic amplitude modulated (AM) signal and permitting the user at the receiver to choose various options as to delaying output of the data and as to the output format.
  • AM monophonic amplitude modulated
  • the Kahn method is similarly restricted to comparatively low data rates due to these same concerns and the placement of the auxiliary carrier at the AM Broadcast carrier frequency spacing.
  • FCC Federal Communications Commission
  • auxiliary modulation bandwidth must be controlled to remain within the region of the "mixed highs" of the AM broadcast. This leads to practical limitation of the modulation bandwidth of the auxiliary information to less than 2 Khz.
  • Audio interference with the data signal is also a mutual-interference problem. Extreme negative modulation of the carrier envelope by the primary (e.g., voice) transmission reduces the available carrier energy to the receiver. This appears as a virtual fade of the received signal without propagation effects being present. In Kahn, the data modulation rate is varied in direct proportion to the audio signal level. Thus in periods of weak carrier small or no data are present so interference is controlled. This approach also reduces the effective data rate of the method.
  • Kahn As pointed out in the discussion of Kahn above, most prior art uses angle modulation to carry auxiliary data.
  • the prior art in Kahn is substantially a linearly superposed additional amplitude modulated carrier, albeit one in phase quadrature with the AM Carrier. Being an additive additional signal, it carries most of the disadvantages of amplitude modulation and the current state-of-the-art known as In-band, On-channel (IBOC) modulation.
  • IBOC In-band, On-channel
  • the current invention overcomes many of the above noted problems of the prior art and thereby achieves higher data rates by using strictly non-linear modulation of the AM Carrier coupled with high efficiency subcarrier modulation made possible by accurate synchronization and timing recovery.
  • the current invention further includes a receiver for picking up and manipulating signals in this modulated AM frequency and for providing the user various options as to outputting the data. These options comprise storing the received data for later output and outputting the data in audio voice format as well as text/image display format.
  • the present invention enables concurrent data and main program transmission with a minimum of interference at substantially higher data rates than in the prior art. This higher efficiency is the result of maximizing the available baseband bandwidth and controlling the phase modulation index in order to minimize mutual interference.
  • the invention relates to many aspects of the transmission and reception of encoded digital information.
  • a Data Inserter which encodes, packetizes, channel encodes and subcarrier modulates the data signal into a quadrature phase modulated baseband format.
  • a RF Exciter (or modified AM Stereo Exciter) then uses quadrature modulation to effect the constant envelope phase modulation of the AM Broadcast carrier with the baseband signal including embedded synchronization and identification mechanisms.
  • the information signal in one embodiment, is Error Correction coded, m-ary DPSK modulated onto an orthogonal set of 128 subcarrier signals using Gray Coding.
  • the symboling rate is 9500/128 symbols per second per subcarrier.
  • the current invention organizes all data into self-contained transmission packets of finite extent. This allows continuous re-acquisition and re- synchronization of the received signal fostering rapid recovery when severe fading or loss of signal occur. Packet synchronization and symbol timing recovery can be accomplished as described in US Patent 5,220,584 wherein phase transitions of a reference signal are detected and synchronized to. In the preferred embodiment, these issues are addressed by incorporating a uniquely formulated spread spectrum sequence into each transmission packet. This design relies on the sequence having an autocorrelation function with high peak-to-sidelobe ratio. Thus, matched filtering to this common wide bandwidth sequence allows large processing gain so that highly reliable packet detection and resynchronization is assured.
  • the invention allows the flexibility to "gear-shift" the baseband digital modulation density to match the outgoing data rate to the available channel.
  • modulation type can be changed to lower the data rate and thereby improve the reception quality as measured in terms of the received bit error rate (BER).
  • the current invention encodes the modulation type as part of the transmitted packet immediately following the synchronization signal. Both of these signals are designed as spread spectrum waveforms which allow highly sensitive recovery and detection at the receiver through the use of properly designed matched filter receivers. Such filters can be designed and implemented in many alternative ways by those skilled in the art.
  • the current invention uses angle modulation of the AM Carrier frequency to convey the information bearing signal.
  • AM Stereo Modulation is not employed at the time of auxiliary data transmission.
  • the carrier phase is available for auxiliary data modulation.
  • the current invention phase modulates the AM Carrier and controls both the spectrum of the resulting sidebands and the modulation index so as to minimize interference with the concurrent AM Broadcast. Since phase modulation of the AM Carrier is orthogonal to any amplitude modulation, envelope detection receivers are largely unaffected by the presence of the auxiliary data.
  • the information bearing signal phase modulates the AM Carrier by employing quadrature amplitude modulation to effect the trigonometric double angle identity. This embodiment allows the data signal to utilize existing AM Stereo Exciters after minor modification, to provide carrier modulation.
  • a receiver and data demodulator which make maximum advantage of the angle modulated nature of the data signal.
  • the receiver performs phase-locked loop (PLL) tracking of the carrier/IF to persist through carrier fades, self-synchronize to the transmitted subcarrier and recover symbol timing in order to synchronously demodulate the auxiliary data.
  • PLL phase-locked loop
  • the receiver is equipped with the means for permitting the user to choose from a variety of output modes, to include delaying outputting of data and of outputting the data in audio or visual formats. Further, the system permits the user to select these options in a variety of ways, to include selection by means of a keypad, touch screen, or speech input.
  • FIG. 1A is a schematic diagram of the auxiliary data insertion and modulation process according to the preferred embodiment of the invention.
  • FIG. 1 B is a schematic diagram of a conventional C-QUAM Stereo Exciter.
  • FIG. 2 is a graphical presentation of the data encoding and modulation structures used for digital modulation.
  • FIG. 3A is a schematic diagram of the invention's use of multiple orthogonal subcarrier modulation method.
  • FIG. 3B depicts the preferred embodiment's efficient method of digitally generated orthogonal subcarrier modulation.
  • FIG. 4 is a graphic diagram of the invention's phase modulated signal spectrum as produced in the preferred embodiment.
  • FIG. 5 depicts the preferred embodiment's transmission packet structure incorporating timing recovery and phase reference information.
  • FIG. 6 is a schematic of the preferred embodiment of the quadrature modulator arrangement to constant envelope phase modulate a carrier signal
  • FIG. 7 is a schematic diagram of the radio receiver design according to the preferred embodiment of the invention.
  • FIG. 8 is a diagram of the process of digital demodulation in the invention's preferred embodiment.
  • FIG. 9 is a depiction of the complex demodulator used in the preferred embodiment of the invention.
  • FIG. 10 is a schematic representation of the method of frequency division demultiplex used in the preferred embodiment of the receiver-demodulator aspect of the invention.
  • FIG. 11 is a graphic illustration of byte level de-interleaver in the preferred embodiment.
  • FIG. 12A is a schematic diagram of the receiver system having audio voice output of the received data.
  • FIG. 12B is a schematic diagram of the receiver system in the preferred invention having alternative voice or visual output of the received data.
  • FIG. 1A The preferred embodiment of data signal subcarrier modulation and carrier phase modulation is illustrated in FIG. 1A.
  • Baseband modulation is accomplished by first applying an 10 Error Correction Encoding of the incoming information source.
  • This error correction encoding is optional and may be omitted in alternative embodiments.
  • the preferred method is an efficient block code such as a Reed- Solomon code for which methods are well known to those skilled in the art. Redundantly encoded data is next 12 Byte Interleaved using a block interleave based on the packet structure.
  • Each transmission packet depending on the selected modulation type 16, consists of a fixed number of encoded data blocks.
  • FIG 2 illustrates the data structures as the data is organized and processed for packet transmission.
  • M encoded data blocks 72 used to form a transmission packet, is shown in Fig. 2 along with the associated data bytes.
  • Byte interleaving 74 re-organizes the encoded data in preparation for digital modulation 14.
  • Modulation results in a set 78 of K Data Frames, each containing N channel coded symbols.
  • the channel coding is differentially encoded m-ary PSK, so a phase reference frame 79 is included at the head of the data.
  • Packet framing 20 concatenates the unique Synchronization sequence 18, the Modulation type identification sequence 16 and the assembled data frames to form a transmission packet.
  • Data frames are generated at a sample rate equivalent to four times the symbol rate. In one embodiment, as illustrated in the example in FIG. 2, each data frame is generated from 128 16-PSK symbols.
  • FIG. 3A depicts the Digital Modulation 14 in more detail.
  • interleaved bytes are mapped 80 to the appropriate symbol (Trellis Coded, PSK, etc.) and these symbols are loaded into a serial-to-parallel buffer 82 to form the basis of a data frame.
  • Each parallel symbol is integrated 84 with the corresponding symbol in the immediately previous frame.
  • Each symbol of a frame 86 modulates one of N orthogonal tonals which are subsequently summed 88 to form the baseband data frame time waveform.
  • the digital modulation is a form of frequency division multiplex.
  • Baseband frame samples are interpolated 94 to the required output rate and then a guard band is appended.
  • This guard band circularly extends each data frame by a time span sufficient to support uncorrupted decoding.
  • the guard band is equal to the expected multipath differential delay. In the current invention, multipath effects are not manifest in a linear manner.
  • the guard band is still used in the invention however to counteract the symbol smearing effects of timing jitter in the receiver/demodulator.
  • An inverse fast-Fourier Transform (IFFT) 108 performs the subcarrier modulation and summation of the differentially encoded symbols of buffer 106.
  • 104 represents a unit delay in feedback of the integrated symbol buffer. Therefore, T is the reciprocal of the orthogonal subcarrier spacing, or the time span of the N input symbols.
  • the maximum signaling rate is therefore (N/T) symbols-per-second. Assuming m-bit symbols, the maximum transmission rate allowed is m(N/T) bits-per-second.
  • the need for the guard band limits the actual rate to (T/(T+g)) times the maximum possible rate, where 'g' is the length in seconds of the guard band inserted.
  • the preferred embodiment is the most efficient modulation method for several reasons.
  • the IFFT is well known to those skilled in the art to be computationally optimal for the process employed.
  • the resulting time series will represent the sin(Nx)/sin(x) interpolated series having the interpolation ratio (L/N).
  • L a highly composite value (e.g., power of 2, 3 or 4 or product of primes) results in maximum efficiency.
  • FIG. 4 depicts the spectrum of the composite phase modulated AM Carrier.
  • the IFFT facilitates the selective nulling of the frequency region immediately surrounding the carrier frequency.
  • the N subcarrier tones may be selected to be any subset of the first half of the L tonal positions of the IFFT input.
  • the input symbols for a data frame are aligned in the input buffer at frequency "bins" 3 through 130.
  • the first 100 Hertz to either side of the AM Carrier are therefore nulled in the sidebands generated by the phase modulation.
  • FIG. 5 shows the final transmission packet structure in the invention's preferred embodiment.
  • 1.2 RF Carrier Modulation As depicted in FIG 1A, successive transmission packets are concatenated and filtered to the required bandwidth ( ⁇ 10,000 Hz in the preferred embodiment) using a complex-valued, half-band, low pass finite-impulse response (FIR) filter 22. To those skilled in the art such filters are well known to have linear phase response and permit efficient implementations. Continuously filtered transmission packet data samples (at 38,043.478 Hz sampling rate in the preferred embodiment) are quadrature phase modulated by taking the 28 SIN and 24 COS of each sample and then converting the two orthogonal sequences to analog form using a dual, synchronous analog-to-digital converter 26.
  • FIR finite-impulse response
  • FIG. 6 is a schematic of the quadrature modulation circuit that results from this modification to the AX-10 Exciter.
  • This modification assumes differential inputs from the Data Inserter 52 in the preferred embodiment, wherein, I and l-not are input to amplifiers 320 and 322 respectively. Q and Q-not are input to associated amplifiers 324 and 326, respectively.
  • 320 and 322 incorporate 11 K potentiometers for gain balancing between the in-phase and quadrature channels to control modulation phase error in the IF Carrier.
  • Clocking source 328 clocks at a 50% duty cycle at the chosen IF frequency (nominally 250khz in the preferred embodiment).
  • Filter 332 serves to remove the unwanted odd order harmonics and sums
  • FIG. 7 provides a simplified schematic of the invention's phase modulation radio receiver.
  • the receiver is designed to operate in the AM Broadcast band in the preferred embodiment and as such builds upon the available C-QUAM integrated 5 circuit technology.
  • Motorola part numbers MC13030, MC145157-2 from the C-QUAM Radio chip set are utilized.
  • FIG. 7 is one approach of those equivalent others that could be arranged by those skilled in the art.
  • the radio is electronically tuned to the desired broadcast frequency by a PLL circuit used to adjust the voltage controlled oscillator 408 to the 0 required mixing frequency, F RF + 10.7Mhz, used in mixer 404.
  • Phase Detector/Comparator 412 provides an output signal proportional to the phase difference of the reference tuning setting and the gain adjusted VCO 408 output signal.
  • Loop filter 410 is set to allow relatively quick response frequency adjustment.
  • Mixer 404 output is amplified into the Bandpass Filter 416 and Limiter 5 418 combination to produce the constant envelope phase modulation.
  • Reference Oscillator 432 provides the phase referencing signal for the second stage down conversion and for the Demodulator Anaiog-to-digital conversion clock.
  • the limiter 418 output is mixed with the PLL 436 controlled secondary IF signal. This signal is used to phase lock the second IF output to the
  • PLL 436 is also used to track minor variations in the carrier frequency, hence the reference to carrier-follower design.
  • IF Filter 422 is designed to allow a bandpass of not more than 28,000 Hz centered at the nominal IF frequency, 76,086.95 Hz or one-fourth the conversion clock phase reference frequency. Since the nominal spectral bandwidth of the significant part of the phase
  • 35 modulated IF signal is approximately 2.2 times the baud rate of the digital modulation (i.e., about 21,250 Hz), this IF is a reasonable compromise between noise rejection, adjacent channel rejection and carrier tracking.
  • Final phase demodulation occurs by computing the angle associated with the phasor represented by the in-phase and quadrature phase complex demodulator output.
  • the arctangent computation 444 implemented by a quadrant based successive approximation/table look-up procedure.
  • the arctangent look-up table is designed to limit the maximum angular error to less than one degree of phase.
  • a critical aspect of data recovery is to accurately determine when a transmission packet has arrived and to synchronize to the data framing structure, as indicated in FIG. 2.
  • the invention facilitates packet detection and data synchronization by incorporating a known polyphase sequence in every transmission packet. This sequence is optimally detected in the receiver- demodulator by applying matched filter 448 to the phase demodulator output data.
  • the matched filter (MF) output is sent to threshold detector 450 which compares the local maximum MF output power to the surrounding RMS power over the processing block.
  • the comparison 446 implements the maximum likelihood test for sequence presence at fixed probability of false detection.
  • the preferred embodiment utilizes a sequence with a bandwidth-time product of 128 (21 dB processing gain).
  • Symbol decode 464 is effected in each embodiment according to the digital modulation format employed at the modulator.
  • the decoding operation in all cases maps the differentially decoded phasors representing soft- symbols to a corresponding bit pattern representing the transmitted encoded binary data. Methods of accomplishing this are varied and well-known to those skilled in the art.
  • Frame recovery 466 assembles the decoded bits from each of the N symbols per data frame every T r seconds. Because the modulation type is encoded in each packet header, symbol decode 464 and frame recovery 466 will produce a variable amount of binary data according the bandwidth efficiency used at the transmitter.
  • the table below codifies the relationship between modulation density, in bits-per-symbol (bpsy), and the number of Reed-Solomon blocks per transmission packet assuming a fixed packet length of 27 data frames, as in the preferred embodiment: TABLE 1 bpsy bit/frame bytes/pkt blks/pkt
  • Frame recovery aggregates decoded symbols into bytes. Bytes are organized into blocks of 72 and arranged in the de-interleave buffer in accordance with TABLE 1.
  • M is the appropriate value from Table 1.
  • M is the appropriate value from Table 1.
  • consecutive encoder symbols i.e., bytes
  • This mapping is defined as follows: Define the parameters: m > minimum effective code length (odd and relatively prime to 72 and M)
  • Error correction 470 performs Reed-Solomon error detection and correction processing is applied to the de-interleaved transmission packet buffer 514.
  • Reed- Solomon (RS) decoding is well-known and has several efficient alternative available implementations, as known by those skilled in the art.
  • coding blocks are organized around 50 byte data blocks. Accordingly, the preferred embodiment performs RS Decoding of the RS(72,50) coded blocks using the well known Berlekamp-Massey and Forney Algorithms.
  • the preferred embodiment of the invention uses a table driven methodology based on 8-bit symbols in the Golais field associated with the primitive polynomial:
  • the output from RS Decoding results in 50 byte blocks in the preferred embodiment. Therefore, 300m bytes per transmission packet are delivered to the intended application by the invention. This equates to a payload rate of 6126J6m bits-per-second or a conservative minimum of 6126 bps.
  • Table 2 provides the generator for the preferred embodiment. Table 2
  • Fig.12A describes one embodiment of this control function of the invention.
  • This control signal allows the user to select from all available options. For instance, the user can opt to receive information in real-time or may store data in memory for later "off-line" retrieval. Further, the user can opt to view the data as textual/image or may listen to the data via an audio output 630. If the data is textual originally, it can be sent as text and voice synthesized at the receiver by speech synthesizer/decoder 626 in order to provide an audio output
  • the text image display 634 may be any display that is well known in the art but the preferred embodiment would be a liquid crystal display (LCD).
  • the display would provide the user with a visual of the available system options.
  • the visual display could comprise a menu driven interface requiring the user to select a number corresponding to a particular selection.
  • the display could also incorporate touch screen technology in which the user need only touch the area of the screen corresponding to the desired selection in order to trigger a control signal.
  • the audio output or speaker 630 may also include an adjustable volume control (not shown) in order to allow the user to raise or lower the volume of a broadcast dependent on the current environment.
  • the subcarrier broadcast signals may be encrypted at the transmission end of the system in order to ensure that only those subscribing to a particular service would be able to convert received signals into a usable format, be it text/image or audio.
  • a usable format be it text/image or audio.
  • one local broadcaster may use the subcarrier of a local AM radio station to insert 24 hour sports scores and updates for users who subscribe and pay a fee for such a service. In this case the broadcaster would encrypt the data to prevent unauthorized users access to the information. Similarly, other content providers could do the same.
  • the receiver would then be outfitted with a decryption device to allow authorized users to access data for which they are paying.
  • the present invention permits the individual receiving the transmitted data to elect several options for outputting the data to include delaying the output and choosing an image or speech output format.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

Abstract

Procédé et système permettant de transmettre des informations numériques en même temps qu'un signal modulé en amplitude (AM) monophonique coexistant. Ledit système permet à n'importe quelle transmission AM, en particulier les diffusions radio AM commerciales, de comporter un signal de données à modulation orthogonale sans interférence significative avec la diffusion AM en cours. Les données sont ensuite démodulées au niveau du récepteur (700) du système. Ledit récepteur (700) permet des options utilisateur concernant le report de la sortie des données et le choix des sorties audio (630) ou visuelles (624). Ledit système permet également le cryptage des données avant la transmission pour permettre ainsi uniquement aux utilisateurs autorisés l'accès aux données du côté récepteur.
PCT/US1998/010061 1997-03-14 1998-05-13 Systeme et procede de transmission de donnees dans la bande de frequence am WO1998052318A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU74935/98A AU7493598A (en) 1997-05-13 1998-05-13 System and method of data transmission in the am frequency spectrum

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
US4071597P 1997-03-14 1997-03-14
US4638097P 1997-05-13 1997-05-13
US60/046,380 1997-05-13
USPCT/US98/04948 1998-03-13
PCT/US1998/004948 WO1998040966A1 (fr) 1997-03-14 1998-03-13 Systeme et procede de diffusion a grande vitesse de donnees numeriques en meme temps qu'une diffusion modulee en amplitude

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WO1998052318A1 true WO1998052318A1 (fr) 1998-11-19

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Cited By (2)

* Cited by examiner, † Cited by third party
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GB2352062A (en) * 1999-02-12 2001-01-17 John Christian Doughty Nissen Computing device for seeking and displaying information
US7581110B1 (en) 1999-08-25 2009-08-25 Nokia Corporation Key distribution for encrypted broadcast data using minimal system bandwidth

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US3959592A (en) * 1972-12-21 1976-05-25 Gretag Aktiengesellschaft Method and apparatus for transmitting and receiving electrical speech signals transmitted in ciphered or coded form
US4841571A (en) * 1982-12-22 1989-06-20 Nec Corporation Privacy signal transmission system
US4920567A (en) * 1986-07-03 1990-04-24 Motorola, Inc. Secure telephone terminal
US5003593A (en) * 1989-06-05 1991-03-26 Motorola, Inc. Teleconferencing method for a secure key management system
US5341427A (en) * 1991-10-16 1994-08-23 Motorola, Inc. Algorithm independent cryptographic key management apparatus
US5465300A (en) * 1993-12-27 1995-11-07 Motorola, Inc. Secure communication setup method

Patent Citations (6)

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Publication number Priority date Publication date Assignee Title
US3959592A (en) * 1972-12-21 1976-05-25 Gretag Aktiengesellschaft Method and apparatus for transmitting and receiving electrical speech signals transmitted in ciphered or coded form
US4841571A (en) * 1982-12-22 1989-06-20 Nec Corporation Privacy signal transmission system
US4920567A (en) * 1986-07-03 1990-04-24 Motorola, Inc. Secure telephone terminal
US5003593A (en) * 1989-06-05 1991-03-26 Motorola, Inc. Teleconferencing method for a secure key management system
US5341427A (en) * 1991-10-16 1994-08-23 Motorola, Inc. Algorithm independent cryptographic key management apparatus
US5465300A (en) * 1993-12-27 1995-11-07 Motorola, Inc. Secure communication setup method

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2352062A (en) * 1999-02-12 2001-01-17 John Christian Doughty Nissen Computing device for seeking and displaying information
US7581110B1 (en) 1999-08-25 2009-08-25 Nokia Corporation Key distribution for encrypted broadcast data using minimal system bandwidth

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