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`Page 1 of 13
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`

`
`U.S. Patent
`
`Apr. 8,2003
`
`Sheet 1 of 5
`
`US 6,546,557 B1
`
`140
`
`14d
`
`FIBER NODES
`
`
`
`
`RF AMPLIFIERS
`
`FIG.
`
`1a,
`
`Page2of13
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`Page 2 of 13
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`

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`U.S. Patent
`
`Apr. 8,2003
`
`Sheet 2 of 5
`
`US 6,546,557 B1
`
`.|E”NRw_LLETmm.
`Ic
`
`\\\\\\\\\\\§\\\\\\\\\\\\\\\\\\\\\\MDTl.
`
`mmmw
`
`CT.AI
`
`EVILTA
`
`VIDEO
`SERVER
`
`FIG.
`
`1‘
`
`20
`
`Page 3of13
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`Page 3 of 13
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`

`
`U.S. Patent
`
`Apr. 8, 2003
`
`Sheet 3 of 5
`
`US 6,546,557 B1
`
`79 AM-VSB CHANNELS
`
`256~QAM SIGNALS AT 40 Mb/s
`
`55.25 MHZ
`
`547.25 MHz
`
`FREQUENCY
`
`FIG. 2a
`
`SOURCE
`
`2
`
`55
`
`SPECTRUN0
`ANALYZER
`
`‘
`
`20
`
`
`
`
`I
`|
`
`70
`64/256-QAM
`DEMODULATOR
`
`—4——L—4——L—+—4——L—4——L—
`TRIGGER LINE:
`
`— — — —.TRACEl 3- —I—
`
`TRACE A
`“_|‘—T“'I__I——'I‘_“‘|—"T’_‘T_‘l’—
`I
`I
`I
`I
`I
`I
`I
`I
`I
`
`CENTER 572.500 IIHZ
`#RES BW 3.0 MHz
`
`fvaw 3 MHz
`
`SPAN 0 Hz
`fswp 1oo ILSEC.
`
`Page4of13
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`Page 4 of 13
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`

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`U.S. Patent
`
`Apr. 8,2003
`
`Sheet 4 of 5
`
`US 6,546,557 B1
`
`TE-4
`
`‘H
`
`1E—6
`
`1E—7
`
`1E—8
`
`1E-9
`
`1E-10
`
`256-QAM
`CODED BER
`
`SNR=30 as
`cso= 56.8 dBc
`
`QAN CHANNEL FREQUENCY:
`
`
`
`0.3
`
`100
`30
`10
`3
`1
`INTERLEAVER BURST TOLERANCE (I1 3)
`
`300
`
`FIG. 4
`
`CENTER 569.50 MHz
`#RES BW 30 KHz
`
`#VBW 1 KHz
`
`SPAN 10.00 MHZ
`SWP 1.00 SEC
`
`Page 5of13
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`Page 5 of 13
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`

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`U.S. Patent
`
`Apr. 8, 2003
`
`Sheet 5 of 5
`
`US 6,546,557 B1
`
`1E-4
`
`IE-5
`
`QAM SNR=30dB
`
`256-0AM
`CODED BER ‘E'7
`
`1E—6
`
`1E—8 1E-9
`
` 1E—10
`—65
`-so
`-55
`-50
`
`cso DISTORTION (dBc)
`
`FIG.
`
`6
`
`Page6of13
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`Page 6 of 13
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`

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`US 6,546,557 B1
`
`1
`METHOD AND SYSTEM FOR ENHANCING
`DIGITAL VIDEO TRANSMISSION TO A SET-
`TOP BOX
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to a method and
`system for enhancing digital video transrnission to set-top
`boxes in the presence of burst noise and, more particularly,
`to a method for enhancing the performance of multichannel
`systems providing analog and digital programming to
`subscribers, and most particularly, to a method for enhanc-
`ing the performance of multichannel AM-VSB (amplitute
`modulated vestigial sideband)/QAM (quadrature amplitude
`nodulation) video transmission system.
`Video signals sent to set-top boxes of cable TV networks
`are often subject to “burst/impulse noise”, originating from
`3eak Composite-Second-Order (CSO),/Composite-'l‘riple-
`Beat
`(CTB) distortions (CSO/CTB) and/or electrical
`interference, leading to both video “blocking” and visually
`degraded areas in the video picture. Recently, there have
`ween many studies on the performance of 64/256-QAM
`channels (as known to those skilled in the art, “quadrature
`amplitude modulation” is a system which greatly increases
`he amount of information which can be carried within a
`given bandwidth; the technique is essentially a combination
`of phase and amplitude modulation, where, for example,
`64-QAM has 8 amplitude states and 8 phase states) in the
`aresence of nonlinear distortions in hybrid analog/digital
`(e.g., AM/QAM) video transmission systems. Most of these
`studies have analyzed the effect of clipping distortion on the
`3it-error-rate (BER) of a QAM channel using CW carriers
`from a multitonc gcncrator. However, a method of climi-
`iating the negative effect of bursty CS0 and CTB
`distortions, on the bit-error-rate (BER) of the QAM channel
`using modulated video carriers, has not been determined.
`In fact, the problems associated with burst noise gener-
`ated errors have been addressed only by using a convolu-
`tional interleaver as part of the error-correction scheme in a
`quadrature amplitude modulation (QAM) modem or
`receiver. In particular, a convolutional interleaver may be
`inserted between the channel encoder and the channel and is
`
`typically characterized by the number of shift registers,
`which is also called the “depth” I (symbols), and by the
`symbol delay increment per register J. Due to memory cost
`and end-to-end delay for the transmitted symbols, in certain
`applications it is advantageous to limit the interleaver (I, J)
`values.
`
`More specifically, robust transmission of 64/256-QAM
`channels over current hybrid fiber/coax cable TV networks
`is achieved with the use of a forward—error—correction (FEC)
`scheme in the QAM modem or receiver. For some digital
`video broadcast (DVB) applications, the FEC scheme con-
`sists of a FEC code, such as Reed-Solomon (R-S) T=8
`(204,188) code, a convolutional interleaver, and a random-
`izer. Interleaving the R-S symbols before transmission, and
`deinterleaving after reception, evenly disperses the burst
`crrors in time, thus cnabling thc burst crrors to bc corrected
`by the FEC in the QAM modem or receiver as if the errors
`were randomly distributed.
`in the article entitled
`As described in greater detail
`“Realization of Optimum Interleavers”, by John L. Ramsey
`(Ramsey, J. L., ‘“‘Realization of Optimum Interleavers”,
`IEEE Trans. Inf. Theory, IT16, 338-345 (1970)), an inter-
`leaver redistributes the channel symbols such that the sym-
`bols from a codeword are mutually separated by somewhat
`
`10
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`15
`
`30
`
`35
`
`40
`
`45
`
`50
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`55
`
`60
`
`65
`
`2
`thus
`more than the length of a typical burst of errors,
`effectively making the channel appear to the decoder as a
`random-error channel. Ramsey further describes four real-
`izations of interleavers that reorder a sequence of symbols
`such that no contiguous sequence of n2 symbols in the
`reordered sequence, contains any pair of symbols that were
`separated by fewer than n, symbols in the original ordering.
`Although the R-S code in the QAM modem cannot
`correct generated CSO/CTB burst errors without an
`interleaver, implementation using a larger interleaver in the
`modem or receiver, to address the problems associated with
`burst noise-generated errors, greatly increases the cost and
`thus, by itself, may not provide the optimal solution.
`SUMMARY OF THE INVENTION
`
`The present invention is therefore directed to the problem
`of reducing the effect of CSO/CTB distortions and burst/
`impulse noise on a transmitted digital signals, such as a
`QAM channel over a cable TV network.
`In a preferred embodiment of the invention, a method to
`improve the performance of a hybrid analog and digital
`video transmission system selects a digital channel map
`based on the relative magnitude and frequency locations of
`nonlinear distortions and thc analog channcl frcqucncy plan.
`In a particular embodiment of the invention, the hybrid
`analog and digital is a multichannel AM-VSB/QAM video
`transmission system. And the digital channel map is selected
`based on the relative magnitude and frequency locations of
`CS0 and CTB distortions and based on the analog channel
`frcqucncy plan. In one particular embodiment, thc rclativc
`magnitude and frequency locations of CS0 and CTB dis-
`tortions are determined based on a type of laser transmitter
`in the video lightwave transmission system, which can be
`either a Directly Modulated (DM) laser transmitter or an
`Externally Modulated (EM) laser transmitter.
`In yet a further embodiment,
`the digital channel map
`selected downshifts each QAM channel center frequency to
`reduce the CS0 and CTB distortions.
`In yet a further
`embodiment,
`the selected digital channel map is down-
`loaded to the set-top box.
`In another aspect of the invention, the determination of
`analog frequency plan indicates a harmonic related carrier
`(HRC) channel frequency plan or an interval related carrier
`(IRC) channel frequency plan. If the HRC plan is indicated,
`in accordance with one embodiment, the digital channel map
`selected downshifts each QAM channel center frequency by
`3-MHz relative to the picture carrier frequency. If the IRC
`plan is implemented, again in accordance with one
`embodiment, when the CS0 distortions are dominant, the
`digital channel map selected downshifts each QAM channel
`center frequency by 1.75 -MHz relative to the picture carrier
`frequency; while when the CTB distortions are dominant,
`the digital channel map selected downshifts each QAM
`channel center frequency by 3-MHz relative to the picture
`carricr frcqucncy.
`In yet a further embodiment of the invention, a QAM
`modem in the video transmission system includes an
`interleaver, which may be variable, and the depth of the
`variable interleaver may be adjusted based on the CS0 and
`CTB distortions.
`
`In still a further embodiment of the invention, the he adend
`of a hybrid analog and digital video transmission system, for
`enhancing digital video transmission, includes generating
`means, for generating a composite analog, digital and/or
`data signal, and transferring means, for transferring the
`generated signal. In the system,
`the transferring means
`
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`

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`US 6,546,557 B1
`
`3
`determines the relative magnitude and frequency locations
`of nonlinear distortions, identifies the analog channel fre-
`quency plan, and selects a digital channel map based on the
`determination and identification.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1(a) shows the architecture of a conventional Hybrid
`Fiber/Coax (HFC) cable system; FIG. 1(b) shows the archi-
`ture of a conventional Fiber To The Curb (FTTC) cable
`system.
`FIG. 2(a) shows the channel plan implemented for an
`experimental simulation, FIG. 2(b) illustrates an experimen-
`tal simulation hybrid multichannel AM—VSB/256 QAM
`video transmission system used to test the principles of the
`present invention.
`FIG. 3 shows traces of the average (trace A) and the peak
`(trace B) CSO distortions at 572.5-MHZ from the experi-
`mental simulation setup of FIG. 2.
`FIG. 4 illustrates the measured BER versus the maximum
`interleaver burst tolerance, with and without QAM channel
`frequency offset from the experimental simulation setup of
`FIG. 2.
`
`FIG. 5 shows the RF frequency spectrum, with a bandpass
`filter and without a bandpass filter, where the dominant CSO
`distortions are outside the shifted 569.5 -MHZ 256-QAM
`channel band, illustrating the principles of the present inven-
`tion.
`
`FIG. 6 illustrates the measured 256-QAM Coded BER
`versus the average CSO distortion for (A) no interleaver; (B)
`I=68, J=3 interleaver and (C) I=204, J=1 interleaver, each
`with the QAM channel center frequency downshifted
`3-MHZ,
`in accordance with the principles of the present
`invention, to 569.5-MIIZ.
`
`DETAILED DESCRIPTION
`
`Various braodband network architectures may be used to
`deliver analog/digital video, analog/digital audio, and high
`speed data to cable subcribers. The network architectures
`primarily implemented in the United States are the “Hybrid
`Fiber/Coax (_HFC)” network,
`the “Fiber To The Curb
`(FTTC)” network and the “Fiber To The Home (FTTH)”
`network.
`The conventional HFC architecture is illustrated in FIG.
`
`1(a). As shown, the signals from the master headend 10 to
`the primary (12a, 12b and 12c) and the secondary (1451, 14b,
`14c and 14d) hubs are transmitted over single-mode fiber
`(SMF) using, for example, 1550-nm externally modulated
`(EM) DFB laser transmitters. The composite signal may be,
`for example, a mixture of traditional broadcast analog sig-
`nals with MPEG compressed digital video. At the primary
`and secondary hubs, which may house Synchronous Optical
`Network (SONET) equipment as well as modems, routers,
`and servers for high-speed data, the optical signals may be
`converted to RF signals and then back to optical signals for
`transmission to various fiber nodes (16a, 16b, 16c and 16a)
`using, for example, 1310-mn DFB laser transmitters.
`The coaxial portion of the network architecture illustrated
`in FIG. 1(a), consists of for example, RF amplifiers, taps,
`and coaxial cables, and spans from each fiber node (16a—a)
`to the corresponding subscriber’s home(s), where the digital
`set-top box is placed.
`FIG. 1(b) illustrates a conventional architecture for a
`Fiber-To-The-Curb (FTTC) Network.
`In this network,
`broadcasted analog video signals, interactive digital video
`signals, and high-speed data are transmitted over single-
`
`10
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`60
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`65
`
`4
`mode fiber (SMF) network to a switched digital access
`system 10, which may be located at a central oflice (C0) of
`a telephone company or a headend of a cable TV company.
`The switched access system may be also connected to public
`switched telephone network 20, as shown in FIG. 1(b). The
`combined signal, which consists of video, data, and tele-
`phony signals, is then transmitted via a SMF to a Broadband
`Network Interface 30 11 nit (BNI), which is typically located,
`for example, in the vicinity of a small serving area (20-50
`homes). The BNI may be connected to the Broadband
`Gateway Interface 40 unit (BGI) at the side of the home via
`coaxial cable or twisted cooper wire. The BGI unit distrib-
`utes the video, telephony, and data signals to the appropriate
`information devices as shown in FIG. 1(b). In the Fiber-To-
`The-Home (FTTH) architecture, although not specifically
`illustrated in FIG. 1, the BGI is simply replaced with the
`BN1; and thus the fiber continues to the home rather than
`“the curb”.
`
`While each of the above described architectures vary
`considerably, the inventive concepts described herein are
`applicable to any of the multiple cable system architectures,
`including the HFC system, the FTTC system, and the FTTH
`system, described above.
`For illustrative purposes, an AM-VSB/256-QAM video
`transmission system will specifically be described and dis-
`cussed. However, one skilled in the art will recognize the
`specific embodiments described herein are illustrative pre-
`ferred embodiments, and that, for example,
`the “QAM”
`system could in fact be any known modulation system,
`including QPSK for example.
`In accordance with a preferred embodiment of the present
`invention,
`the effect of burst/impulse noise on a QAM
`channel is significantly reduced by carefully selecting the
`QAM channel frequency plan based on the relative magni-
`tude and frequency locations of the CS0 and CTB distor-
`tions and on the analog channel frequency plan utilized by
`the system.
`First, the relative magnitude of the CS0 distortions as
`compared with the CTB distortions can be determined by the
`particular type of laser transmitter and receiver used by the
`cable operators. Specifically, the relative magnitude of the
`distortions depends on whether the laser transmitter
`is
`Directly Modulated (DM) or Externally Modulated (EM)
`and the type of pre-distortion circuitry. For downstream
`transmission, three types of laser transmitters are typically
`used:
`(1) a 1310-nm DM-DFB laser transmitter,
`(2) a
`1550-nm EM-DFB laser transmitter, and (3) a 1319-nm EM
`YAG laser transmitter. The CSO distortions are typically the
`dominant distortions in the QAM channel when using a
`1310-nm DM-DFB laser transmitter. However,
`the CTB
`distortions are typically the dominant distortions when using
`a 1550-nm EM-DFB laser transmitter. In order to reduce
`equipment cost, cable operators are likely to use either a
`1310-nm DM-DFB laser transmitter or a 1550-nm EM-DFB
`
`lascr transmittcr over a given local network from a single
`vendor. Thus, a determination of which distortions, i.e., the
`CS0 or CTB distortions, are dominant in the QAM channels
`may be made based on the type of laser transmitter.
`In addition, in accordance with a preferred embodiment of
`the invention, the type of cable TV frequency plan utilized
`must be determined. The two most widely used cable TV
`frequency plans are the HRC, harmonically related carrier
`plan, and the IRC, incrementally related carrier plan. In the
`IIRC frequency plan, the picture carrier frequencies in the
`different cable channels are forced to have a strictly propor-
`tional and phase-locked relationship, thereby concentrating
`
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`

`
`US 6,546,557 B1
`
`5
`the distortions at the carrier frequencies. In the HRC system
`the picture carrier frequencies are downshifted 1.25-MHz,
`compared with the corresponding picture carriers in the IRC
`plan. One advantage of the HRC plan is that the CS0 and
`CTB distortion products fall on the picture carrier
`frequencies, and thus, their affect is less disturbing and if
`fact, becomes almost invisible. However, some converters
`and TV sets are incompatible and cannot detune to the new
`carrier frequencies.
`the first picture carrier frequency is
`In the IRC plan,
`located at 55.2625-MHz with successive picture carriers
`located at 6-MHz apart, up to 1-GHz. The IRC plan elimi-
`nates the detuning problem, however, the CS0 distortions
`are located at 11.25-MHz from the corresponding picture
`carrier frequency, and thus can become visible.
`If the IRC plan is used, and the dominant distorions are
`the CS0 distortions, each QAM channel center frequency
`will be downshifted by 1.75-MHz relative to the picture
`carrier frequency. If however, the dominant distorions are
`the CTB distortions, each QAM channel center frequency
`will be downshifted by 3-MHz relative to the picture carrier
`frequency.
`If the HRC plan is used, each QAM channel center
`frequency will be downshifted by 3-MHz relative to the
`picture carrier frequency regardless of whether the dominant
`distortions are the CS0 or CTB distortions.
`
`In one particular method of providing an offset to the
`QAM carrier frequency, implemented by the assignee Gen-
`eral Instrument, a full frequency plan table is constructed
`from one or more Carrier Definition Table (CDT) structures,
`each defining a starting frequency, number of carriers, and
`frequency spacing for carriers in the group. The specified
`carrier represents the nominal band for all modulation
`methods including AM and QAM. The CDT and a Virtual
`Channel Table (VCT) are broadcasted from the headend, to
`the set-top box at the subscriber, through an Out-Of-Band
`(OOB) channel. The VCT informs the set-top box if the
`tuned channel is MPEG-2, the type of modulation mode
`(QAM or AM), the full frequency plan using the CDT, and
`other parameters.
`the bursty behaviors of the CS0
`As noted above,
`distortions, which are produced by the modulated analog
`video signals at the directly modulated laser transmitter
`(e.g., 1310-nm DM-DFB), are responsible for the Bit-Error-
`Rate (BER) degradation of the 256-QAM channel.
`However, as described in further detail below, in addition to
`offsetting the frequency of the QAM channel relative to the
`dominant distortions,
`in a preferred embodiment of the
`invention, the interleaver depth in a QAM modem may also
`be adjusted so as to combat burst-errors generated by
`time—varying CSO distortions in a hybrid multichannel
`AM-VSB (amplitude modulated vestigial-sideband)/256-
`QAM video lightwave transmission system. In particular, in
`the experimental simulation setup described below, when an
`I=204, J=1 interleaver was used together with QAM channel
`frequency offset with respect
`to the dominant CSO
`distortions,
`the 256-QAM coded bit-error-rate BER was
`rcduccd by more than 500 times as compared with thc case
`where QAM frequency offset was implemented but no
`interleaver was utilized, even in the presence of large CSO
`distortion levels (>—60—dBc).
`Finally, an additional aspect of the invention further
`reduces the CSO/CTB distortions to the cable plant by
`providing a switch, outside the AGC circuitry, which lowers
`the operating point by 3.0-3.4-dB by switching between
`continuous wave (CW) carriers and analog video signals. In
`
`5
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`10
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`15
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`
`6
`the AM CNR and
`in the proposed method,
`particular,
`CSO/CTB distortions are measured with modulated
`AM-VSB video signals and 0-dBm optical power at the fiber
`node receiver. The average RF input
`level
`to the laser
`transmitter, outside the AGC circuitry, is then reduced by
`approximately 3.4-dB and the AM CNR and CSO/CTB
`distortions are once again measured. The results then rep-
`resent an “optimum” operating point for the laser transmitter
`when using modulated video signals.
`FIG. 2(b) provides an experimental simulation setup, of a
`hybrid multichannel AM-VSB/256-QAM video lightwave
`transmission system,
`that was used to test tl1e proposed
`method. As shown in the channel plan of FIG. 2(a), the
`channel plan for the simulated system consisted of seventy-
`nine AM-VSB (amplitude modulated vestigial sideband)
`broadcast TV video channels 10, from a cable TV headend,
`which were combined with two 256-QAM digital channels
`(MPEG sources 20 and 24, and QAM modulators 22 and
`26), operating at 40.5 Mb/s (for compressed digital video
`and high speed data services) at a RF frequency of 571.25-
`MHz and 643.25—MHz,
`to directly—modulate a 1310—nm
`DFB laser transmitter 40. Once again it is noted that the
`channel plan shown in FIG. 2(a) was used for a simulation
`and is exemplary only and could in fact consist of any analog
`and/or digital requirements including digital audio.
`A broadband white Gaussian noise source 30 was also
`used in the QAM link for BER-SNR measurements. After
`transmission through 10.6-km of a standard single-mode-
`fiber, the combined signal was detected at 0-dBm received
`optical power at the cable TV receiver 50. The CNR, CTB
`and CS0 of the AM band were measured on a spectrum
`analyzer 60. The 256-QAM signals were down-converted to
`IF frequency, demodulated by 64/256-QAM demodulator
`70, and then fed to an error-detector 80. The peak modulated
`video power to the average QAM channel power ratio was
`5.6 dB.
`The measured AM-VSB CNR at the cable TV receiver
`
`was 51.8 dB with an average CSO distortion of -56.8-dBc
`and CTB distortion of -60-dBc in the 256-QAM band. As
`expected, tl1e CSO distortion at 572.5-MHz was the domi-
`nant non-linear distortion in the QAM channel band. The
`1310-nm DFB laser transmitter was operating at AM modu-
`lation index of 3.5% per channel with a clipping index
`y=4-10‘4.
`FIG. 3 shows typical 100-ys time-traces of the average
`(trace A) and the peak (trace B) CSO distortion at 572.5-
`MHz on a spectrum analyzer (SA) in a zero span mode. The
`SA trigger was setup to start the sweep for any event of the
`peak CSO distortion, which is above the solid line. The line
`represents the threshold level for impulses with higher
`amplitudes that would degrade the coded BER.
`The observed bursty behavior of the CS0 distortion,
`which has a non-Gaussian statistics, can be explained by the
`fact
`that
`the peak envelope power of modulated video
`signals (unlike CW carriers) can vary by as much as 18-dB,
`depending on the picture content. Specifically, the synchro-
`nization pulses of the modulated video signals may tempo-
`rarily align with cach othcr, causing thc corrcsponding vidco
`carriers to be at their maximum power at the same time,
`resulting in increased CSO/CTB distortions. Thus, the use of
`CW carriers to simulate modulated video carriers does not
`
`accurately represent the time-dependence of the peak CSO/
`CTB distortions.
`
`the use of CW carriers from a multitone
`In addition,
`generator to simulate modulated video carriers also affects
`the laser clipping distortion. In particular,
`the Automatic
`
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`

`
`US 6,546,557 B1
`
`7
`Gain Control (AGC) circuitry in the directly modulated
`(DM) DFB laser transmitter maintains the average RF input
`power at a fixed level. Under these conditions, when CW’
`carriers are used, the AGC circuitry reduces the AM modu-
`lation index by a factor of \/[3, and the CNR is lowered by
`a factor of [3 (approximately 3.4-dB), as compared with
`using modulatcd vidco carricrs, resulting in rcduccd CSO/
`CTB distortions.
`
`In order to combat the generated burst errors, a variable
`interleaver in the QAM modern, with depth I up to 204
`symbols, and J=204,T symbols, was used with R-S T=8
`(204,188) code. The maximum burst length that can be
`corrected by the variable interleaver and deinterleaver com-
`bination is given by:
`
`N.’
`_(J-1-+1)-T_|1—
`Rs
`
`(1)
`
`where I and J are the interleaver parameters, N=204 symbols
`is the R-S block size, R5=5.056 Mbaud is the transmitted
`symbol rate, and T=8 symbols.
`FIG. 4 shows the mcasurcd 256-QAM codcd BER versus
`the maximum interleaver burst tolerance ‘U (us) for QAM
`channel center frequencies of 571.25-MHZ (solid line) and
`569.5—MHz (dotted line). The 256-QAM SNR was set to
`30-dB, corresponding to a coded BER of 1.5~10‘9 with no
`AM-VSB channel loading (CSO=—56.8 dBc). Since the
`interleaver was used with I~J=204 symbols,
`increasing I
`according to the above equation results in increasing the
`burst tolerance as shown in FIG. 4. For an I=204, J=1
`interleaver, a nearly four times reduction in the 256-QAM
`BER was obtained by the QAM channel frequency offset
`alone.
`
`As illustrated in FIG. 4, at a burst duration of 30-ys, the
`256-QAM coded BER slowly decreases as the interleaver
`depth is increased since the average burst length is larger
`than the maximum burst tolerance of the interleaver and
`deinterleaver combination. However, as the maximum inter-
`leaver burst tolerance becomes significantly larger than a
`burst duration of about 30-us (~3X), most of the generated
`burst errors are effectively corrected by the interleaver,
`resulting in a steeper reduction in the coded BER. In fact, the
`256-QAM coded BER was reduced by two orders of mag-
`nitude in comparison to the “BER without implementation
`of an interleaver” case.
`
`The results illustrated in FIG. 4 also indicate that using an
`I=68, J=3 interleaver with QAM channel frequency offset
`will provide the same coded BER as a three times longer
`interleaver (I=204, J=1) without QAM channel frequency
`offset. From implementation cost point of view, it can be
`significantly less expensive to keep the depth and the symbol
`delay (I, J) of the convolutional interleaver as short as
`possible, as it requires the addition of Static Random Access
`Memory (SRAM) in the QAM receiver.
`In addition, by shifting the QAM channel center fre-
`quency down to 569.5 -MHZ, FIG. 5 shows that the dominant
`CSO distortions at 572.5-MHZ and at 566.5-MHZ (trace 2)
`are now located outside the downshifted QAM channel
`band. The other nonlinear distortions in the downshifted
`
`QAM band, namely, the CS0 distortion at 570-MHz and the
`CTB distortion at 571.25-MHZ, have smaller magnitudes
`relative to the QAM carrier. If the CTB distortion becomes
`the dominant distortion in the QAM band, then a 3-MHZ
`downshift of the QAM channel frequency to 5 68.25 -MHZ is
`required to improve the 256-QAM coded BER.
`Those skilled in the art will appreciate that the proposed
`fixed QAM channel frequency offset method can also be
`
`8
`applied to multiple contiguous digital (e.g., QAM) channels,
`where the dominant non-linear (e.g., CSO or CTB) distor-
`tions will fall in, the small gaps between the digital channels.
`Finally, FIG. 6 provides an illustration which indicates
`that an I=204, J=1 interleaver in a QAM receiver will also
`work as well at higher CSO distortion levels. To obtain the
`mcasurcmcnts illustrated in FIG. 6,
`the AM modulation
`index per channel at the laser transmitter was changed while
`keeping the power ratio between the QAM and the AM-VSB
`channels constant. Specifically, FIG. 6 shows the measured
`256-QAM coded BER, with the QAM channel at 571.25-
`MHZ, versus the average CSO distortion at a frequency of
`572.5-MI-I7 for (A) no interleaver, (B) I=68, J=3 interleaver,
`and (C) downshiftcd QAM channel to 569.5-MHZ with 30
`dB SNR and I=204, J=1 interleaver. The I=68, J=3 inter-
`leaver (trace B) works best at CSO distortion levels less than
`—60—dBc. At higher CSO distortion levels however, the I=68,
`J=3 interleaver is overwhelmed with burst errors, resulting
`in a BER performance that approaches the no interleaver
`case. As illustrated by trace C, the I=204, J=1 interleaver,
`together with tl1e QAM channel frequency offset method,
`provides robust transmission, even in the presence of large
`CSO distortions (2-55 dBc).
`As is clcarly shown, by frcqucncy offsctting a QAM
`channel relative to the dominant bursty CSO distortions, and
`implementing a variable interleaver (i.e., in this case, an
`I=204 J=1 interleaver), nearly error—free transmission was
`achieved, even in the presence of large CSO distortions
`(>—55-dBc) in a hybrid AM-VSB/256-QAM video light-
`wave transmission system. Accordingly, based on (1) the
`magnitude and frequency locations of the CS0 and CTB
`distortions in the cable TV network and on (2) the analog
`channel frequency plan, the selection of the proper inter-
`leaver depth as well as the proper shifting of QAM channel
`frequency plan may be made.
`Applicant notes that while the detailed description of the
`invention corresponding to FIGS. 2-6 is related only to a
`preferred embodiment of a multichannel AM-VSB/256
`QAM video lightwave transmission system, several different
`transmission systems are used internationally, and the inven-
`tive concepts described herein are equally applicable to all
`hybrid analog/digital video transmission systems.
`In addition, it will be appreciated that modifications and
`variations of the present invention are covered by the above
`teachings and within the purview of the appended claims
`without departing from the spirit and intended scope of the
`invention.
`What is claimed is:
`
`10
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`15
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`30
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`35
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`40
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`45
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`50
`
`1. Amethod to improve the performance of hybrid analog
`and digital video transmission systems comprising the step
`of:
`
`55
`
`60
`
`65
`
`determining the relative magnitude and frequency loca-
`tions of nonlinear distortions;
`identifying an analog channel frequency plan used in said
`transmission system; and
`selecting a digital channel r11ap based on said determining
`step and said identifying step,
`wherein a frequency band of a first digital channel is
`selected in said selecting step such that dominant linear
`distortions fall outside said frequency band.
`2. A method in accordance with claim 1, wherein the
`method is applied to multiple contiguous digital channels,
`and wherein the nonlinear distortions fall in gaps between
`the multiple contiguous digital channels.
`3. A method in accordance with claim 1, further compris-
`ing the step of downloading the selected digital channel map
`to the set-top box.
`
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`US 6,546,557 B1
`
`9
`4. A method in accordance with claim 1, wherein the
`digital channel map selected in said selecting step down-
`shifts each channel center frequency to reduce the nonlinear
`distortions.
`5. A method in accordance with claim 1, wherein the
`hybrid analog and digital video transmission system is a
`multichannel AM-VSB/QAM system.
`6. A method in accordance with claim 5, wherein the
`nonlinear distortions determined in said determining step are
`CS0 and CTB distortions.
`7. A method in accordance with claim 6, wherein the
`digital channel map selected in said selecting step down-
`shifts each QAM channel center frequency to reduce the
`magnitude of the CS0 and CTB distortions.
`8. A method in accordance with claim 6, wherein said
`determining step determines the relative magnitude and
`frequency locations of CS0 and CTB distortions based on a
`type of laser transmitter in the video transmission system.
`9. A method in accordance with claim 8, wherein the laser
`transmitter of the video transmission system is a Directly
`Modulated (DM) laser transmitter.
`10. A method in accordance with claim 8, wherein the
`laser transmitter of the video transmission system is an
`Externally Modulated (EM) laser transmitter.
`11. A method according to claim 5, wherein a QAM
`modem in said video transmission system includes an inter-
`leaver.
`12. Amethod according to claim 11, wherein the depth of
`the interleaver is variable.
`
`13. A method according to claim 12, wherein the depth of
`the interleaver may be adjusted based on the CS0 and CTB
`distortions determined in said determining step.
`14. A method according to claim 12, wherein increasing
`the depth of the interleaver reduces the magnitude of the
`CS0 and CTB distortions.
`15. A method in accordance with claim 1, wherein said
`identifying step identifies a harmonic related carrier (HRC)
`channel frequency plan.
`16. A method in accordance with claim 15, wherein the
`digital channel map selected downshifts each QAM channel
`center frequency by 3-MHZ relative to the picture carrier
`frequency.
`17. A method in accordance with claim 1, wherein said
`identifying step identifies an interval related carrier (IRC)
`channel frequency plan.
`18. A method in accordance with claim 17, wherein when
`said determining step determines that the CS0 distortions
`are dominant, the digital channel map selected downshifts
`each QAM channel center frequency by 1.75-MHz relative
`to the picture carrier frequency.
`19. A method in accordance with claim 17, wherein when
`said determining step determines that the CTB distortions
`are dominant, the digital channel map selected downshifts
`each QAM channel center frequency by 3-MHZ relative to
`the picture carrier frequency.
`20. A method according to claim 1, wherein continuous
`wave (CW) carriers are used to simulate modulated video
`carriers.
`
`21. A method according to claim 20, wherein Automatic
`Gain Circuitry (AGC) maintains an average RF input power
`at a fixed level.
`
`22. A method according to claim 21, wherein a laser
`transmitter of the video lightwave transmission system is a
`Directly Modulated (DM) laser tran