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
  • H04L25/03057—Arrangements for removing intersymbol interference operating in the time domain adaptive, i.e. capable of adjustment during data reception with a recursive structure
• • 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
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/60—Receivers
  • H04B10/66—Non-coherent receivers, e.g. using direct detection
  • H04B10/69—Electrical arrangements in the receiver
  • H04B10/695—Arrangements for optimizing the decision element in the receiver, e.g. by using automatic threshold control

Definitions

• Digital communication receivers must sample an analog wavefoim and then reliably detect the sampled data. Signals arriving at a leceivei are typically co ⁇ upted by intersymbol interference (ISI), crosstalk, echo, and other noise In oidei to compensate foi such channel distoitions, communication receivers often employ well-known equalization techniques For example, zeio equalization or decision-feedback equalization (DFE) techniques (or both) are often employed Such equalization techniques are widely-used fbi removing intersymbol interference and to improve the noise margin See, for example, R Gitlin et al , Digital Communication Principles, (Plenum Piess, 1992) and E.A Lee and D G Messerschmitt, Digital Communications, (Kluwei Academic Press, 1988), each incoipoiated by reference heiein Generally, zero equalization techniques equalize the pie-cuisois of the channel impulse iesponse and decision-feedback equalization equalizes the post cursors of the channel impulse
• Spatial DFE 5 eliminates the analog addei operation by sampling the leceived signal using two (oi moie) vertical sliceis that ate offset ftom the common mode voltage The two sliceis aie positioned based on the results of a well-known Least Mean Squaie (LMS) algorithm One slicei is used for transitions from a binary value of 0 and the second slicei is used for transitions from a binary value of 1 The value of the previous detected bit is used to determine which slicer to use foi detection of the current bit.
• LMS Least Mean Squaie
• a number of channels exhibit a non-linear response.
• data transmission is achieved by turning on and off one or more LED sources
• the light source When the light source is on (for example, when transmitting a binary value of 1), energy transmission takes place and the photons are transmitted through an optical fiber
• the signal experiences non-linear distortion due to, for example, chromatic dispersion, polarization mode dispersion (PMD) and modal dispersion in multi-mode fibers
• PMD polarization mode dispersion
• regenerative noise sources distort the transmitted photon energy
• These distortion sources are active along the entire length of the transmission medium
• the signal degradation increases as the transmission distance increases
• the light source is off (for example, when transmitting a binary value of 0)
• no photons aie tiansmitted and the above mentioned distoition does not take place at all.
• the optical eye thus degiades asymmetrically
• a threshold position of a latch employed by a decision-feedback equalizei is determined by constraining input data such that the input data only contains transitions from a first binary value; obtaining a plurality of samples of a single-sided data eye associated with the constrained input data; determining a threshold position of the latch based on the samples; and transforming the determined threshold position to address the non-linearity of the channel
• the constrained input data can compiise (i) transitions from a binary value of 1 to a binary value of 0 or 1; oi (ii) transitions from a binary value of 0 to a binary value of 0 or 1
• the non-linearity may be addressed, for example, using a table or equation that maps measured threshold values to transformed threshold values based on distance In this manner, the latches
• the size of the single -sided data eye can be obtained by analyzing a histogram associated with the single-sided data eye to identify a region having a constant hit count
• the latch is placed approximately in the middle of the single-sided eye
• the latch threshold position is determined for a plurality of single-sided eyes, such as N consecutive uppei and lowei single- sided eyes, and a latch threshold position is selected based on the single-sided eye having the smallest eye amplitude
• FIG. 1 graphically illustrates a number of ideal data eyes associated with a signal
• FIGS 2A through 2D illustrate the distortion that can arise from a channel
• FIG 3 is a block diagram of a transmitter, channel and receiver system that employs equalization techniques
• FIG. 4 is a block diagram of a transmitter, channel and receiver system that employs Spatial DFE
• FIG 5 illustrates an exemplary transition trajectory from an exemplary binary value of 0 to a binary value of 0 or 1;
• FIG 6 illustrates the noise and timing margins for a received signal
• FIG. 7 illustrates the sampling of a signal using a data eye monitor for a tiansition from a binary value of 1 to a binary value of 0 or 1 ;
• FIG. 8 illustrates the sampling of a signal using a data eye monitor for a transition from a binary value of 0 to a binary value of 0 or 1;
• FIG 9 illustrates a histogram indicating an eye opening for a single sided eye in accordance with the present invention
• FIG 10 provides exemplary pseudo code for an illustrative vertical eye search algorithm incorporating features of the present invention
• FIG. 11 illustrates the manner in which the latch positions determined by the vertical eye search algorithm of FIG 10 are used by the spatial DFE of FIG 4;
• FIG 12 illustrates the symmetric placement of DFE latches for the upper and lower DFE eyes;
• FIG 13 illustrates the data eyes as the result of non-linear ISI distortion that may be present in some channels, such as an optical channel
• FIG 14 illustrates the mannei in which the latch positions determined by the veitical eye search algorithm of ⁇ IG 10 aie used by the spatial DfE of FIG 4 in the presence of a non-linear channel
• FIG 15 illustrates an exemplary non-lineai mapping table of FIG 14.
• the present invention provides methods and apparatus foi determining the position of DFE latches in the presence of a non-linear channel
• the non-linearly mapped DFE coefficients generated by the present invention can be used with a classical threshold based DFE structure, as discussed below
• the position for the vertical latches of a DFE is determined based on an evaluation of the incoming data eye
• the exemplary data eye monitor may be implemented, for example, using the techniques described in United States Patent Application Serial No 11/095,178, fried March 31, 2005, entitled “Method and Apparatus for Monitoring a Data Eye in a Clock and Data Recovery System," incorporated by reference herein
• one or more latches associated with the exemplary data eye monitor employ an envelope detection technique to evaluate the amplitude of the signal
• envelope detection techniques see, for example, United States Patent Application Serial No.
• a communication channel typically exhibits a low pass effect on a transmitted signal, causing the opening size of the received data eye to be significantly impaired, with the received data eye often being essentially closed
• Conventional channel compensation techniques attempt to open the received data eye that has been band limited by the low pass channel response.
• a classical DFE implementation places a latch in the middle of the received data eye, which often has a small opening with poor noise and timing margins
• the present invention positions the vertical slicers using a single sided eye
• a single-sided eye also referred to as a DFE eye
• contains only transitions from one binaiy value i.e , only l -> x or 0->x transitions).
• the small data eye is the result of the channel distoitions that tend to close the data eye.
• the present invention recognizes that a laigei DFE eye can be extracted by constraining the data to only contain signal transitions from, for example, a binary value 1 to a binary value of 0 or 1 (referred to as l -> x), and inhibiting any signal transitions from a binary value of 0.
• FIG. 1 graphically illustrates a number of ideal data eyes 110-1 through 110-3 associated with a signal 100.
• each data eye 110 is typically a superposition of a number of individual signals with varying frequency components, in a known manner
• United States Patent Application Serial No. 11/414,522, fried April 28, 2006, entitled “Method And Apparatus For Determining A Position Of A latch Employed For Decision-Feedback Equalization” discloses a technique for determining the amplitude of the received signal 110 based on the relative measurements of fixed and roaming latches. The two latches are used to determine the upper and lower boundaries of the data eye, for transitions from binary 1 and for 1 transitions from binary 0.
• the latches are then positioned between the detected upper and lower boundaries of the data eye for steady state operation
• the latches can be positioned in the middle of the respective upper and lower boundaries of the data eye.
• the data eye monitor can separately measure the received signal 110 along the vertical axis to determine the location of the upper and lower boundaries of the data eye, for both cases of transitions from binary values of 1 and 0 Thereafter, the mid-point between the upper and lower boundaries of the data eye can be established for both cases.
• the latches for both cases can be positioned for steady state operation based on the determined mid-point locations
• FIGS. 2A through 2D further illustrate the distortion that can arise ftom a channel.
• an ideal channel exhibits a delta function 200 as its impulse response.
• FIG. 2B illustrates an exemplary impulse response 210 for 1 a hypothetical channel
• the hypothetical channel may exhibit a response having a magnitude of 1.0 at the primary tap 220
• the hypothetical channel may exhibit a response having a magnitude of 0.5
• FIG, 2C illustrates an ideal clock signal 230 that may be transmitted across a channel.
• 2D illustrates the clock signal 250 that is received over the same channel as the result of channel distortion (after a sample/hold is applied)
• values of +1, +1, -1, -1, +1, +1, -1, -1 are transmitted to generate the clock signal 230.
• the receiver will sample the signal 250 shown in FIG. 2D.
• the +1 that is transmitted in the second time slot will be superimposed with 50% of the +1 that was transmitted in the first time slot
• a value of +1.5 will be measured at the receiver in the second time slot.
• pre-emphasis techniques in the transmitter or equalization techniques in the receiver are employed in a well-known manner so that the signal processed by the receiver looks like the clock signal 230 that was transmitted
• FIG. 3 is a block diagram of a transmitter, channel and receiver system 300 that employs equalization techniques.
• the data is transmitted by a transmitter 310 through a channel 320 after optionally being equalized or filtered through a transmit FIR filter (TXFIR) (not shown).
• TXFIR transmit FIR filter
• the signal may optionally be filtered or equalized by a continuous time feed forward filter 340.
• the feed forward filter 340 performs pre-cursor equalization to compensate for the spillover from future transmitted symbols, in a known manner
• the analog signal out of the feed forward filter 340 is sampled by a data detector 360 that generates data decisions.
• FIG 4 is a block diagiam of a tiansmittei, channel and leceivei system 400 that employs Spatial DFE
• Spatial DPE is a variation of the classical DFE technique shown in FIG 3 that eliminates the analog addei by sampling the leceived signal using two vertical sliceis that aie offset from the common mode voltage
• pie-emphasis techniques 410 aie applied in the tiansmittei befbie the signal is transmitted over a channel 420
• equalization techniques 430 such as zero equalization
• spatial DFE 440 are applied in the leceivei
• a latch positioning system 445 discussed below in conjunction with, for example, FIGS 7 thiough 10, implements a vertical eye search algorithm 1000 (FIG 10), to determine
• FIG 5 illustrates an exemplary transition trajectory foi an exemplary transition from a binary value of 0 to a binaiy value of 0 oi 1 (0->x)
• a tiajectoi ⁇ 510 for example, is associated with a tiansition fiom a binary value of 0 to a 1 (and then followed by anothei 1)
• a trajectory 530 foi example, is associated with a tiansition fiom a binary value of 0 having piioi states 000 to a binaiy value of 1 (followed by a 0)
• a tiajectoiy 540 is associated with a tiansition fiom a binaiy value of 0 having piioi states 000 to a binary value of 0
• the diffeient tiajectoiies aie all associated with a piioi state of 0
• Each tiajectoiy follows a diffeient path
• a single latch 550 must be able to detect whether the cu ⁇ ent data bit is a 0 or a 1, despite the varying paths
• the latch 550 is positioned between the negative iail margin 560 and the amplitude of the lowest expected trajectory 530
• a data eye monitoi can be used to deteimine a location foi the latch 550 used foi the spatial DFE 440
• FIG 6 illustrates the noise and timing margins for a leceived signal 600
• a received signal will typically include transitions 610 from a binary value 1 to a binaiy value of 0 oi 1 (1 -> x), as well as transitions 620 from a binary value 0 to a binary value of 0 oi 1 (0- ⁇ x)
• the present invention recognizes that a larger DFE eye can be extracted by constraining the data to only contain signal transitions from, for example, a binary value 1 to a binary value of 0 or 1 (referred to as l -> x), and inhibiting any signal transitions fiom a binary value of 0 (oi vice versa)
• the size of the classical data eye is approximately associated with the inner circle 670, having an associated timing margin 630 and noise margin 660
• the small size of the data eye 770 with the co ⁇ espond ⁇ ng poor margins 630, 660 makes it very difficult to properly recover the transmitted data
• the distortion that would normally be associated with the 0->x transitions is removed and the resulting upper DFE eye, approximately associated with the outer circle 680, is larger than the small data eye 670 associated with a classical approach (and both sets of transitions)
• the upper DFE eye 680 has an associated timing margin 640 and noise margin 650
• the size of the data eye 680 is significantly increased, and the timing and noise margins 640, 650 are improved
• FIG 7 illustrates the sampling of a signal using a data eye monitor for 1 a transition 730 from an initial state 710 of binary value 1 to a binary value of 0 or a transition 720 from a binary value of 1 to a binary value of 1
• the trajectory 730 associated with the Nyquist frequency and the trajectory 720 associated with the maximum amplitude of the remaining frequencies are shown
• two upper latches LIu and L2u are employed in the exemplary embodiment to determine the amplitudes of the trajectories 720, 730 for the upper DFE data eye and theieby determine a location for the latch(es) used for the spatial DFE 440.
• the sampled values (Latch 1 Upper DFE Eye and Latch 2 Upper DFE Eye) are applied to a multiplexer 750 that selects one of the latches, based on the portion of the data eye that is being sampled in accordance with a latch selection control signal.
• the output of the multiplexer 750 is applied to a hit counter 760, discussed further below in conjunction with FIG . 9.
• FIG 8 illustrates the sampling of a signal using a data eye monitor in accoi dance with the present invention for a transition 830 from an initial state 810 of binary value 0 to a binary value of 0 or a transition 820 from a binary value of 0 to a binary value of 1 and then a binary value of 0.
• a transition 830 from an initial state 810 of binary value 0 to a binary value of 0 or a transition 820 from a binary value of 0 to a binary value of 1 and then a binary value of 0.
• the trajectory 820 associated with the Nyquist frequency and the trajectory 830 associated with the minimum amplitude of the remaining frequencies are shown
• two lower latches LH and L2 L are employed in the exemplary embodiment to determine the amplitudes of the trajectories 820, 8.30 for the lowei DFE data eye and thereby determine a location for the latch(es) used for the spatial DFE 440,
• the sampled values (Latch 1 Lower DFE Eye and Latch 2 Lower DFE Eye) are applied to a multiplexer 850 that selects one of the latches, based on the portion of the data eye that is being sampled hi accordance with a latch selection control signal.
• the output of the multiplexer 850 is applied to a hit counter 860, discussed further below in conjunction with FIG 9.
• FIG. 9 illustrates a histogram 920 generated by the hit counters 760, 860 of FIGS . 7 and 8, indicating an eye opening for a single sided eye in accordance with the present invention
• a scope output 910 illustrates the preamplifier output as a function of the unit interval for four consecutive data eyes. For 1 the first two data eyes 914, the output is shown foi all transitions For the second two eyes 918, only transitions fiom a binary value of i to a binary value of 0 or 1 (1 -> x) are shown, in accoi dance with the present invention
• the histogram 920 shows the threshold of the roaming latches LIu, L2u, LU and L2 L , as a function of the eye monitor counts generated by the hit counters 760, 860 As shown in FIG 9, the minimum count occurs when the threshold is at a maximum value (since the entire signal is below the latch) and a maximum count occurs when the threshold is at a minimum value (since the entire signal is above the latch)
• the histogram 920 also contains a region 930 having a constant count, corresponding to the DFE eye opening
• the vertical eye search algorithm 1000 (FIG 10) determines when the histogram 920 is constant, to obtain the range of threshold values when the eye is open.
• FIG 10 provides exemplary pseudo code for an illustrative vertical eye search algorithm 1000 incorporating features of the present invention
• the illustrative vertical eye search algorithm 1000 is employed during a training mode to determine the position of the latches that used by the Spatial DFE 440 in the normal operating (steady state) mode
• the exemplary vertical eye search algorithm 1000 contains a first measurement section 1010 for measuring the upper DFE data eye, a second measurement section 1020 for measuring the upper DFE data eye, and an analysis section 1030 for analyzing the count data to establish the latch positions for the upper and lower DFE data eyes
• the latches are positioned in the middle of the constant count region 930 for each of the upper and lower DFE data eyes
• FIG 11 illustrates the manner in which the latch positions determined by the vertical eye search algorithm 1000 of FIG 10 are used by the spatial DFE 440 of FIG 4
• the received signal is amplified by a preamplifier 1110 and then analyzed on a scope 1120
• the latch positioning system 445 of FIG 4 analyzes the received data eyes during a training mode using the single-sided eye techniques disclosed herein, in accordance with the vertical eye search algorithm 1000, to determine the latch positions (i e , threshold values)
• the threshold values are applied to the latch array 1130 for steady state operation
• FIG 12 illustrates the symmetric placement of DFE latches 1210, 1220 for the upper and lower DFE eyes, respectively
• the upper and lower DFE latches 1210, 1220 aie placed symmetrically about the zero crossing
• the upper and lower DFE latches 1210, 1220 are equally spaced from a threshold of 0
• the latch positioning system 445 could determine a latch position for one of the upper and lower DTE, data eyes, and position the second latch to ensure symmetry about the zero crossing
• a further variation of the present invention provides improved immunity to the data pattern sensitivity by considering one or more unit intervals, each corresponding to a data eye having upper and lower DFE data eyes, and positioning the latches based on the worst case (i e , the smallest eye) Thus, the system is assumed to be symmetric and the smallest eye is used for setting the latches for both upper and lower DFE eyes
• FIG 13 illustrates the data eyes as the result of non-linear ISI distortion that may be present in some channels, such as an optical channel
• many channels such as optical channels, exhibit a non-linear response
• data transmission is achieved by turning on and off one or more LED sources
• the light source When the light source is on (for example, when transmitting a binary value of 1), energy transmission takes place and the photons are transmitted through an optical fiber
• the signal experiences non-linear distortion due to, for example, chromatic dispersion, polarization mode dispersion (PMD) and modal dispersion in multi-mode fibers
• PMD polarization mode dispersion
• regenerative noise sources distort the transmitted photon energy
• FIG 14 illustrates the manner in which the latch positions determined by the vertical eye search algorithm 1000 of FIG 10 are used by the spatial DFE 440 of FIG 4 in the presence of a non-linear channel
• the non-linear Iy mapped DFE coefficients generated by the present invention can be used with a classical threshold based DFE structure, as shown in FIG 14 As shown in FIG 14, the received
• the table 1500 contains a plurality of records, each associated with a different threshold For each measured threshold, the table 1500 indicates a transformed threshold for various optical lengths, such as the distances shown in FIG 13
• an equation may be implemented to characterize the non- linear channel and map the measured threshold values to transformed threshold values It is also noted that a channel model can be employed instead of actual measurements.
• the single-sided eye can be observed, for example, on an oscilloscope and the single- sided eye can be skewed to address the non-linearity
• a plurality of identical die are typically formed in a repeated pattern on a surface of the wafer
• Each die includes a device described herein, and may include other structures or ciicuits
• the individual die aie cut oi diced fiom the wafer, then packaged as an hitegiated circuit
• One skilled in the ait would know how to dice wafers and package die to pioduce integiated chcuits Integrated ciicuits so manufactured aie consideiedpait of this invention
• vaiious functions may be implemented in the digital domain as piocessing steps in a software piogiam, in hardware by circuit elements oi state machines, or in combination of both software and hardware
• Such softwaie may be employed in, for example, a digital signal processoi, microcontroller, oi geneial-purpose computer
• Such hardware and software may be embodied within circuits implemented within an integrated circuit
• the functions of the present invention can be embodied in the form of methods and apparatuses for practicing those methods
• One or more aspects of the present invention can be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted ovei some tiansmission medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention
• the program code segments combine with the processor to provide a device that operates analogously to specific logic ciicuits

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

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