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    Contents

    1 Introduction 3

    2 Interface Codes 7

    3 Digital Signal Regeneration 15

    4 Reasons for Bit Errors 23

    Baseband Transmission of DigitalSignals

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    1 Introduction

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    Digital signal devices process the signals as purely binary information, i.e. the signallevel does not change between bits with the same logical state. For this reason,these so-called NRZ-signals (No return to zero) can only be processed together with

    the corresponding clock, which enables the identification of individual bit positions.

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    1 bit

    NRZ signal

    Clock

    Binary information 1 1 1 0 0 0 1 0 1 1 0 0

    Fig. 1 Processing of NRZ signals with the aid of separate clock

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    This clock is not separately transmitted and thus it has to be possible to derive (i.e.regenerate) the clock from the data signal on the receiving side. It is obvious that fora NRZ code this is very complicated, if not virtually impossible. A further

    disadvantage of the NRZ code is that it carries a certain amount of dc-voltage whichexcluded the signals galvanic isolation at the interface (transformer etc.). Due tothese disadvantages, various interface codes have been developed; all of whichcomply with the following requirements:

    l good clock retrieval features

    l no dc-component.

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    2 Interface Codes

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    A suitable interface code has a maximum of transitions between the different signallevels, even for the transmission of lengthy sequences of identical logical states; ithas no dc-component. The survey shows the development of individual codes (fig. 2).

    RZ Code A log. 1 is represented as half-bit with a change of signals levelsfrom Low High Low.

    Advantage: Clock retrieval possible also for adjacent log.1 bits.

    Disadvantage: No clock information for zero sequences, dc-component.

    AMI Code The state log. 1 is represented alternatively as positive or negativesignal level.

    Advantage: Clock retrieval possible also for adjacent log.1 bits,no dc-component.

    Disadvantage: No clock information for zero sequences.

    HDB 3 Code Is derived form the AMI code? Here, four consequent zero bits arereplaced by a 1001 or 0001 combination. This is done in such a waythat the signal receiver detects the mutilation of informationalcontents and cancels it.

    Advantage: Maximum clock information, no dc-component.

    Disadvantage: None

    This code is applied for the device interfaces from 2 Mbit/s up to 34Mbit/s (baseband transmission). The exact coding rules areenumerated in the following.

    CMI Code Due to its easy generation with delay lines and simple gate functionsthe CMI code is suited especially for interfaces with high bitrates.Therefore, this code is standardized for the 140 Mbit/s device inter-faces.

    A further important advantage of the interface code is the possibility it offers to detecttransmission errors by supervising the coding rules. With the HDB3 code, forexample, the receiving of four zero bits would represent the violation of a coding rule,i.e. at least one bit error must have been occurred during transmission.

    The standardization of interface codes only refers to device interfaces. The codes forconductor-bound transmission paths are manufacturer-dependent and are generallyadapted to the requirements of the respective terming unit.

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    Digital Line Codes

    Signal

    Clock

    NRZ

    binary

    RZ

    binary

    AMI

    HDB3

    CMI

    Fig. 2

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    2.0

    1.8

    1.6

    1.4

    1.2

    1.0

    0.8

    0.6

    0.4

    0.2

    0

    2.01.81.61.41.21.00.80.60.40.20

    power

    idensity

    frequency/bitrate

    Binary NRZ

    HDB3

    CMI

    Fig. 3 Amplitude spectrum of various codes

    Fig. 3 shows the amplitude spectrum of various interface codes. For codes without adc-component the maximum energy is within the range of a frequency whichcorresponds to half of the bitrate value. This is obvious when comparing the

    definitions of frequency and bitrate respectively.

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    0 1 1 1 10 0 0 0

    2 bit

    = 1 period

    Fig. 4 Bit sequences 0101....

    The bit sequence represented in fig. 4 shall serve as an example. One signal periodcovers 2 bits and corresponds to the basic wave of the data signal. This wavecontains the greatest amount of energy and has a frequency which equals half of thebitrate value. This is also the frequency that is indicated by a frequency counterconnected to a source of a digital signal.

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    HDB3-Coding rules

    (Third-Order-High-Density-Bipolar-Code)

    The HDB3-code is a modified version of the AMI-code. Binary signals or AMI-codesignals may contain lengthy 0 sequences, which hinder the clock retrieval in theregenerative repeaters along digital transmission paths. The HDB3 code enables theelimination of 0 sequences with more than 3 zeros.

    1. If there are more than 4 consecutive 0-signal elements, the fourth 0-signalelement shall be replaced by a V-signal element (=1-signal element. A V-signalelement causes a Violation of the AMI-rule.

    2. If between the V-signal element, inserted according to the conditions specifiedabove (rule 1), and the preceding V-signal element there is an even number of

    1-signal elements, then the first of four 0-signal elements shall be replaced byan A-signal element (=1-signal element). The polarity of the A-signal elementcomplies with the AMI-rule. The last of four 0-signal elements becomes again a

    V-signal element (A00V). In this the A- and V-signal elements have the samepolarity.

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    Binary

    HDB3

    Binary

    HDB3

    0 0 1 1 1 0 0 0 0 1

    V

    previous V-bit

    0

    rule 1 appliesrule 2 does not apply

    0 0 1 1 0 0 0 0 0 1

    previous V-bit

    0

    rule 1 and 2 apply

    V

    V

    A V

    Fig. 5 Conversion of binary signals into HDB3-signals

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    3 Digital Signal Regeneration

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    The digital signal regeneration is one of the advantages of the digital transmissiontechnique. Theoretically, it enables the signals to be transmitted via an unlimiteddistance without any quality loss.

    During transmission, a digital signal is attenuated and distorted; which results in areduction of the signal /noise ratio. The regeneration process has the task ofcanceling such distortions and regenerating the originally sent signal from theactually received signal. That is why every interface on the receiving side is followedby a regenerator.

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    TX REG+

    attenuation +

    interference

    signal source transmission path regenerator

    regenerated

    signal

    regenerated

    signal

    received

    signal

    transmitted

    signal

    Fig. 6 Principle of digital signal regeneration

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    Four basic function blocks are necessary for the digital signal regeneration:

    lAmplification block (balancing of attenuation losses)

    l

    Clock retrieval blocklAmplitude decision block

    l Time decision block.

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    AGC AD TD regener.

    data signalReceived

    signal

    distorted

    and

    attentuated

    PLL

    CR

    regenerated

    clock

    AGC : Automatic gain controlled amplifierAD : Amplitude decisionTD : Time decisionCR : Clock retrieval

    Fig. 7 Block diagram of a digital signal regenerator

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    These four functions are represented in next figure.

    l The receiving signal is fed into a controlled amplifier (AGC) which keeps theamplitude of the outgoing signal at a constant value over a wide range of incoming

    amplitudes. Thus, the attenuation of the transmission path is balanced.

    l The constant output level is a precondition for the functioning of the amplitudedecision block (AD) which follows. This AD decides on the basis of an internalthreshold value whether the level of incoming signal is above or below thisthreshold. Accordingly, a signal with the level Log. 1 or Log.0 is emitted at theoutput. The output signal thus consists of pulses, the width of which only dependson the period during which the output signal exceeds the decision threshold.

    l The time decision block (TD) has the task of generating signal pulses withconstant width. For this, it requires the regenerated receive signal clock whichsamples the output signal of the amplitude decision block. If, at the time ofsampling the signal has a level of Log. 1, the time decision block emits a pulse

    with constant width. Thus, incoming pulses of any width are turned into pulsescorresponding exactly to the bit width of the transmitted signal. The time decisionprocess is the final stage of regeneration.

    l The clock retrieval CR block is in charge of regenerating the transmitted signalclock from the receive signal clock. In order to effect this function, a phase lockedloop (PLL) is employed, basically consisting of a voltage-controlled oscillator

    whose frequency can be changed by a control-voltage.

    By adequate evaluation of the receiving signal it is now possible to reach a controlvoltage which can set the oscillator to the exact clock frequency value of thetransmitting signal.

    The following examples show a regenerator for HDB3 signals, as well as the signalshape between individual function blocks.

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    1

    2

    3

    4

    5

    6

    7

    8

    receiving signal

    after

    amplification

    after amplitude

    decision

    retrieved clock

    after time

    decision

    after addition

    PLL

    12

    3

    4

    5

    6

    7

    8

    Fig. 8 Regeneration of HDB3 signals

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    4 Reasons for Bit Errors

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    The decisive quality criterium for the transmission of digital signals is the so-called biterror rate (BER). This BER represents the proportion of bits which have beenmutilated (i.e. incorrectly recorded) during transmission, to the total amount of bits

    transmitted within a certain interval. The BER directly influences the quality of thetransmitted services (e.g. voice channels, data channels, video signals). Twosignificant BER are explained exemplary in the following:

    l BER = 10-6

    This BER virtually cannot be perceived in a voice channel. For the transmission ofdata channels, however, this value represents the maximum acceptable limit. Thetransmission system is in a state of "degraded quality", which is indicated by adegradation alarm (low priority) on the devices involved. The transmission pathremains, nevertheless, in operation.

    l BER = 10-3

    This BER causes a strong interference noise in a voice channel. The operatingstate is judged to be of "unacceptable quality", which is signaled by the devicesinvolved by the emission of a failure alarm (high priority). The transmission pathgoes out of operation.

    How do bit errors arise?

    In the previous section it was mentioned that digital signals can be regenerated asrequested, i.e. a transmission without quality reduction is possible. This statement is,

    however, only partially true, i.e. whenever the impairment of the transmission signalsis within limits which still permit the regeneration at the receiving side. The reasonsfor the formation of bit errors are

    l low signal/noise ration

    ljitter

    l intersymbol interference

    Low signal/noise ratio

    Noise amplitudes which influence the amplitude decision process are superimposed

    to the originally sent signal.

    The superimposed interference peaks lead to an incorrect signal interpretation at thereceiving end. Reasons for a low S/N-ratio are:

    3. too strong signal attenuation during transmission

    4. external interference during transmission.

    For transmission in cable sections (especially optical fiber) both reasons can belargely eliminated by careful planning.

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    Decision treshold

    Fig. 9 Low S/N-ratio

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    TT2510EU01AL_0126

    Jitter

    Due to jitter, the transitions between signal levels log. 0 and log. 1 do not take place

    at periodically recurring points in time (characteristically moments) as for undisturbedsignals, which means that the transitions oscillate around the characteristicallymoments.

    Jitter is characterized by jitter amplitude (unit intervals UI) and jitter frequency.

    One UI means that, because of deviation from the characteristically moments, thesignal edges are within a range equal to the width of 1 bit.

    The jitter frequency is the number of oscillations around the characteristically momentper one second. Jitter influences the time decision process in the regenerator andcauses bit errors for high jitter amplitudes and frequency.

    Jitter arises in the devices used for signal transmission. (I.e. in regenerators anddemultiplexers = systematical jitter), or on the transmission path due to externalinfluences (non-systematic jitter).

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    TT2510EU01AL_0127

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    signal without jitter

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    edge (+T/2)

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    edge (-T/2)characteristical

    moment

    T

    T

    Fig. 10 Representation of an Unit Interval (UI)

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    Intersymbol interference.

    Is caused by a discrepancy between the bandwidth of the transmission path and the

    bandwidth required for the digital signal. This leads to a bit extension, so that there isan overlap of bits which follow each other. Thus, bit errors occur, the reasons of

    which can be traced back to the impairment of amplitude decision process. Forconductor-bound transmission of digital signals this effect can be excluded byadequate planning. For transmission on radio paths this effect is of fundamentalimportance as the frequency response of the transmission path can change due toatmospherically influence.