Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03012—Arrangements for removing intersymbol interference operating in the time domain
  • H04L25/03019—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception
  • H04L25/03038—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure
  • H04L25/0305—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a non-recursive structure using blind adaptation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/005—Control of transmission; Equalising
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L2025/0335—Arrangements for removing intersymbol interference characterised by the type of transmission
  • H04L2025/03375—Passband transmission
  • H04L2025/03382—Single of vestigal sideband

Definitions

• the present invention is directed, in general, to timing recovery for wireless receiver systems and, more specifically, to channel equalization to assist in sync based timing recovery for wireless receiver systems.
• Sync based timing recovery schemes generally assume no multipath characteristics for the received signal. When operating over a channel exhibiting long multipath profiles (significant temporal dispersion or significant delay spread), such timing recovery schemes can break down because correlation peaks may become indistinct (that is, either clear correlation peaks are not apparent or false peaks manifest). In effect, the long channel impulse response smears any underlying correlation peaks, making precise peak detection impossible.
• Temporal dispersion in the received signal due to multipath interference is generally corrected by an equalizer in the receiver.
• a simplistic solution to the problem of multipath interference with correlation peaks would be to rectify the temporal dispersion utilizing an equalizer; however, a conventional receiver structure requires both carrier and timing recovery to be operative before equalization may be effectively employed.
• a primary object of the present invention to provide, for use in a wireless receiver, a magnitude only equalizer which equalizes a magnitude of a received wireless signal without regard to phase distortions introduced, and transmits the magnitude equalized signal to a timing recovery loop for improved correlation peak detection in a sync based timing recovery scheme.
• a channel equalizer receiving the output signal from the timing recovery loop equalizes the signal and corrects any phase distortions introduced by the magnitude only equalizer.
• the magnitude only equalizer includes at least one filter utilizing only real coefficients and constrained such that the direct term of the overall filter structure within the magnitude only equalizer is unity.
• Fig. 1 depicts a wireless communications receiver system including a blind magnitude (second order) pre-equalizer for improved channel equalization according to one embodiment of the present invention
• Figs. 2A .and 2B illustrate in further detail feed-forward and feedback, respectively, blind magnitude (second order) pre-equalizers according to various embodiments of the present invention
• Figs. 3A and 3B are comparative simulations illustrating the absence of undesirable artifacts introduced by a minimum output energy magnitude only equalizer according to one embodiment of the present invention.
• Figs. 4 A through 4C are comparative simulations illustrating the improvement of correlation peak detection as a result of utilizing a minimum output energy magnitude only equalizer according to various embodiments of the present invention.
• Fig. 1 depicts a wireless communications receiver system including a blind magnitude (second order) pre-equalizer for improved channel equalization according to one embodiment of the present invention.
• Receiver system 100 includes a receiver 101, which is a digital television (DTV) receiver in the exemplary embodiment, including an input 102 for receiving wireless signals, optionally an output 103, and a demodulator 104.
• DTV digital television
• receiver 101 is intended to receive and demodulate vestigial sideband (VSB) signals with eight discrete levels (8-VSB) according to current Advanced Television Systems Committee (ATSC) st.and.ards.
• VSB vestigial sideband
• ATSC Advanced Television Systems Committee
• receiver 101 may alternatively be any type of receiver for a communications system requiring communications channel equalization and employing sync based timing recovery.
• Receiver 101 may therefore be any audio and/or video communications receiver including a satellite, terrestrial or cable broadcast receiver and/or television, a video cassette recorder (VCR) or digital video recorder (DVR), or a digital versatile disk (DVD) player.
• VCR video cassette recorder
• DVR digital video recorder
• DVD digital versatile disk
• Receiver 101 in the exemplary embodiment includes a timing recovery loop 105 having sequentially connected sample rate converter (SRC) unit 106, carrier recovery (CR) unit 107, and a square root raised cosine (SQRC) filter unit 108, with a timing recovery (TR) unit 109 within the feedback loop.
• a simple forward equalizer (FE) unit 110 may optionally be coupled in series between the timing recovery loop 105 and a signal adder 111 for equalization of the recovered signal based on decision feedback equalizer (DFE) unit 112, with a sync detector (SD) 113 optionally coupled to the equalized output signal from signal adder 111.
• DFE decision feedback equalizer
• SD sync detector
• the received signal is passed through a pre-equalizer (Pre) unit 114 coupled between the input 102 and the timing recovery loop 105.
• Pre pre-equalizer
• sync-based correlation is a second order statistical method, such that full equalization of the channel is not necessary to clean up the correlation signal. Instead, only equalization of the channel in a second order sense is necessary to (at least in principle) gain the full benefits available.
• pre-equalizer 114 employs a blind second order adaptive algorithm to correct the magnitude response, and one class of such algorithms is described in further detail below. Figs.
• FIGS. 2A and 2B illustrate in further detail feed-forward and feedback, respectively, blind magnitude (second order) pre-equalizers according to various embodiments of the present invention.
• the magnitude only equalizers 114 employed for pre-equalization corrects only for magnitude distortions and treats any phase distortion as irrelevant.
• the phase is not unaffected by the magnitude only equalizer 114, which may introduce phase distortions into the signal that generally may be (at least theoretically) characterized, depending upon the algorithm employed.
• an adaptive minimum output energy (MOE) criterion is employed. Further general information regarding use of the adaptive minimum output energy criterion is provided in the related application.
• the minimum output energy criterion is preferably employed with the direct filter response term being constrained to unity, either with the transversal structure of Fig. 2A by fixing the leading coefficient or with the recursive structure of Fig. 2B, which automatically has a direct term of unity in the closed loop.
• pre-equalizer 114 is a purely transversal realization in which the feedback parameter B(z) is zero and the adaptive feed-forward parameter A ⁇ z) introduced by filter component 200 at signal adder 201 is real, with unity gain by filter component 202.
• the feedback parameter B(z) is zero
• the adaptive feed-forward parameter A ⁇ z) introduced by filter component 200 at signal adder 201 is real, with unity gain by filter component 202.
• pre-equalizer 114 is a purely recursive realization in which the feed-forward parameter A(z) is zero and the adaptive feedback parameter B(z) introduced by filter component 203 at signal adder 201 is real, automatically producing a unity direct term due to the closed loop.
• the minimum output energy criterion acts to equalize the minimum phase equivalent of the channel. That is, if the channel is at minimum phase, then the action of the minimum output energy pre-equalizer filter 114 is to fully equalize the channel. When the channel is mixed phase (non-minimum phase), the minimum output energy pre-equalizer filter 114 effectively converts the channel to an all-pass channel defined by the non-minimum phase zeros of the channel.
• Figs. 3 A and 3B are comparative simulations illustrating the absence of undesirable artifacts introduced by a minimum output energy magnitude only equalizer according to one embodiment of the present invention. To be effective, the action of the minimum output energy magnitude only equalizer should be invariant when applied to an all- pass channel. Simulations using the channel:
• Figs. 3 A illustrates the timing recovery correlation response and signal spectrum magnitude for an all-pass channel of a receiver without the minimum output energy magnitude only equalizer
• Fig. 3B illustrates the timing recovery correlation response and signal spectrum magnitude for an all-pass channel of a receiver with the minimum output energy magnitude only equalizer.
• the timing recovery correlation response and signal spectrum magnitude are substantially identical for both cases, corroborating that the minimum output energy magnitude only equalizer is invariant to all- pass factors.
• FIGS. 4 A through 4C are comparative simulations illustrating the improvement of correlation peak detection as a result of utilizing a minimum output energy magnitude only equalizer according to various embodiments of the present invention.
• the simulations were performed to evaluate performance of the timing recovery system of the present invention with an input consisting of an 8-level binary signal that includes four synchronization (SYNC) bits [5, -5, -5, 5] every 832 signals.
• SYNC synchronization
• the generated data was passed through a real channel with additive white Gaussian noise (AWGN) producing a signal-to-noise ratio (SNR) of less than 20 decibels (dB).
• AWGN additive white Gaussian noise
• SNR signal-to-noise ratio
• the channel used for the simulations was a zero dB echo channel with
• This channel is known to be a problem for bandedge timing recovery schemes since the frequency response has a null on one bandedge, and the channel also exhibits a number of nulls throughout the frequency band.
• the signal at the receiver was passed through a minimum output energy magnitude only equalizer filter, then correlated against the SYNC sequence to find the location of those SYNCs in the original data. Simulations were performed using both the feed-forward and feedback structures depicted in Figs. 2A and 2B, and correlation was also performed on an unfiltered version of the received data for comparison.
• Fig. 4A illustrates simulation correlation and magnitude of signal spectrum results for data passed through a 0 dB echo channel and received at the receiver without a minimum output energy magnitude only equalizer (that is, without any magnitude compensation).
• the circles in the correlation plot show where the correlation peaks should be located, while the crosses indicate where correlation peaks were found within a prescribed sample radius (15 in the example shown) of the expected peak and having a magnitude greater than the peak at the expected position (i.e., "false" correlation peaks).
• the crosses thus indicate areas where the timing recovery scheme could have problems in finding the correct peaks, with the channel distortions obscuring clear correlation peaks.
• Figs. 4A illustrates simulation correlation and magnitude of signal spectrum results for data passed through a 0 dB echo channel and received at the receiver without a minimum output energy magnitude only equalizer (that is, without any magnitude compensation).
• the circles in the correlation plot show where the correlation peaks should be located, while the crosses indicate where correlation peaks were found within a prescribed sample radius (15 in the example shown)
• FIGS. 4B and 4C illustrate simulation correlation and magnitude of signal spectrum results for data passed through the 0 dB echo channel and filtered using the feedforward and feedback minimum output energy magnitude only pre-equalizer structures of Figs. 2A and 2B, respectively.
• the correlation peaks are more distinct and the presence of potential false peaks (denoted by crosses) is diminished.
• the present invention enhances timing recovery to overcome problems due to channel temporal (multipath) distortions on sync correlation based timing recovery schemes, particularly as applied to 8-VSB digital television (DTV) transmissions.
• a magnitude only equalizer adapted in a blind fashion using a second-order statistical cost function, may be incorporated into the timing recovery stage of a receiver and operate largely independently of the timing recovery operation.
• Sync correlation based timing recovery schemes may therefore work in situations for which they were not designed (i.e., multipath interference). Furthermore, due to the adaptation speed of the blind adaptive magnitude equalizer, the timing recovery system can converge relatively rapidly, and thereby have the capability of tracking time varying multipath interference.
• machine usable mediums include: nonvolatile, hard-coded type mediums such as read only memories (ROMs) or erasable, electrically programmable read only memories (EEPROMs), recordable type mediums such as floppy disks, hard disk drives and compact disc read only memories (CD-ROMs) or digital versatile discs (DVDs), and transmission type mediums such as digital and analog communication links.
• ROMs read only memories
• EEPROMs electrically programmable read only memories
• CD-ROMs compact disc read only memories
• DVDs digital versatile discs

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Power Engineering (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

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• 2002
  • 2002-06-05 JP JP2003501904A patent/JP2004533180A/ja active Pending
  • 2002-06-05 WO PCT/IB2002/002103 patent/WO2002100057A1/en not_active Application Discontinuation
  • 2002-06-05 CN CNA028112970A patent/CN1513246A/zh active Pending
  • 2002-06-05 KR KR10-2003-7001679A patent/KR20030022362A/ko not_active Application Discontinuation
  • 2002-06-05 EP EP02735743A patent/EP1397894A1/de not_active Withdrawn

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