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Re: PAM-5, what are your BERs ?


This is an excellent discussion topic. Just so we stay on track I'll act like
the sheepdog for a moment (I need a haircut anyway)

The fastest PAM5 proposal on the table is the one I introduced to the HSSG back
in March '99. There has been a lot of work on this basic proposal since then,
right up to Oscar Agazzi's proposal in January '00 which introduced a subject I
hadn't touched with a 10-foot pole to date: equalization. I thought that I might
be tasting tomatoes if I did. All these proposals observe the same basic
signaling methodology:

10 Gigabits per second data transport

5 Intensity levels per baud

5 Gbaud signaling rate (i.e. 200 ps baud interval)

Intensity probably influences the following number quite a bit, but 5 Gbaud PAM5
translates into a maximum effective signaling rate of 2.5 GHz. Note that this
does NOT correspond to the required bandwidths of the transmitter or receiver
which are generally much higher due to signal rise/fall time requirements, etc.

In summary, encoded PAM5 at 5 Gbaud signaling rate is equal to or less than HALF
the encoded binary signaling rate required to transport 10 Gbps of data.
Therefore, the required signaling frequency is also half or less. 

I don't know of any MultiLevel proposal, PAM5 or otherwise which endorses a 10
Gbaud singaling rate.

Best Regards,

Edward Chang wrote:
> Hi Vivex:
> I believe we agree most of whatever we have been reasoning.  However, the
> details of operations, and the quantitative figures are the areas we did not
> have opportunities to discuss, which does not mean we disagree.
> The single prohibitive task to perform here is to make all these complex
> circuits to perform at 10 GBaud bit rate and 5 GHz clock rate with BER of
> 10^-12 or less.
> As you have already identified that it is questionable the bandwidth of CMOS
> technology and other parts can deliver the performance we need.
> At 10 GBaud, the data pulse waveforms are quite departed from those ideal
> square waveforms, and are quite distorted.  Any normal stray capacitance and
> line inductance of components and pc runs will cause visible distortion to
> the waveforms.  I am not sure if the waveform at the input of a A/D
> converter is same as the one A/D converter is actually digitizing.  The A/D
> converter it self can alter the waveform. It is very hard to use Fourier
> analysis, or a Gaussian pulse approximation to characterize the pulse
> responses, which are quite distorted.  As a result, the equalization result
> may not be as predictable as expected.  It means an incorrect restructuring
> of the original waveform.  It means high BER.
> I just select one example to emphasize the difficulty of a data recovery at
> 10 GBaud.
> The right approach is to keep circuit as simple as possible to avoid
> additional waveform distortion caused by its own circuit.
> Usually the waveform at the TIA out put is very sensitive to the S/N issue,
> especially when the input power is near sensitivity level.  One would
> pre-amplify the incoming signal first before equalizing the waveform.  This
> amplifier better be a real good one, which is allowed to amplify the data
> but not the noise.
> If the waveform is equalized first, I bet, the passive equalizer may
> attenuate the low amplitude signal to case insufficient S/N problem.  It
> means high BER.
> The worst thing: if the eye is closed, we do not know how low is the signal
> amplitude.  It is possible, the signal amplitude of a narrow pulse (high
> frequency pulse) is far below the minimum required optical power at the
> receiver input.
> There will be many new issues we did not have deal before.
> While I agree, theoretically, a closed eye may be recovered depending on the
> S/N ratio, it is highly recommended to adopt a approach that is easier for
> designer to work on it. -- a wider eye the better.
> Regards,
> Edward S. Chang
> NetWorth Technologies, Inc.
> EChang@xxxxxxxxxxxxxxxx
> Tel: (610)292-2870
> Fax: (610)292-2872
> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Vivek Telang
> Sent: Tuesday, February 29, 2000 2:15 PM
> To: stds-802-3-hssg@xxxxxxxx
> Subject: RE: PAM-5, what are your BERs ?
> Hi Ed,
> Good points. Let me see if I can respond to them.
> You're right about the fact that equalization has its limits. These limits
> are well understood, and given the channel and noise characteristics, one
> can easily determine the number of levels that can be supported at a desired
> BER. My point was that this limit is far greater than the one that the open
> eye requirement would lead you to believe.
> The high-pass filter equalizer approach that you describe is not as optimal
> as a minimum-mean-squared-error (mmse) equalizer. It is important to realize
> that the narrow pulse does not simply get attenuated, but is dispersed over
> time. A good mmse equalizer processes numerous samples of the dispersed
> pulse and reconstructs the symbol while trying to minimize the noise energy.
> DFEs are particularly good at doing this.
> Your points about the linear amplifier (bandwidth, non-linearity) are
> correct; however, they are not an issue if the equalization is done
> digitally.
> I can hear you saying, well, if DSP can do all that, what's the catch? The
> catch is that to do any DSP at all, you need an A/D converter up front. This
> A/D needs to run at the symbol rate at least. Also, the A/D adds
> quantization noise that needs to be low enough so as not to be the
> performance-limiting factor. For Oscar's 10G DSP proposal, this means a
> 5Gsps A/D with 6 effective bits of resolution. No mean task in CMOS. Also,
> all the DSP needs to run at 5GHz.
> So in summary, I would reiterate my two comments:
> 1) A closed eye does not necessarily preclude low BERs.
> 2) DSP techniques which would be required to operate under "closed eye"
> conditions are going to be a challenge to implement at 10G bit rates.
> Regards,
> Vivek
> *---------------------------------------
> *
> * Vivek Telang
> * Cicada Semiconductor Inc.
> * 901 MoPac Expressway South
> * Building One, Suite 540
> * Austin, Texas  78746
> *
> * 512-327-3500 x114 voice
> * 512-327-3550      fax
> * vivek@xxxxxxxxxxxxxxx
> *
> *
> *---------------------------------------
> -----Original Message-----
> From:   NetWorthTK@xxxxxxx [SMTP:NetWorthTK@xxxxxxx]
> Sent:   Tuesday, February 29, 2000 12:15 AM
> To:     vivek@xxxxxxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx
> Subject:        Re: PAM-5, what are your BERs ?
> Hi Vivel:
> >From theoretical point of view, you reasoning makes some points.  However,
> from the real implementation point of view,  it is not quite true.  Before
> starting analyzing the frequency response, just ask a question:  "If we can
> simply keep equalizing the receiving signals to bring them back to the
> looking-alike to the original, transmitting signal, why we bother all those
> bandwidth issues?  There must be some limitations to the equalization
> technique.
> The eye closure is caused by the insufficient bandwidth of a receiving path;
> as a result, the narrow pulse (higher frequency pulse) is much more
> attenuated than the wider pulse (lower frequency pulse).  We can cascade a
> high-pass frequency response equalizer to suppers the amplitude of a wide
> pulse, and keep the amplitude of the narrow pulse remain unchanged (but not
> amplified) to open the eye.  However, if the amplitude of a narrow pulse is
> already too small to meet the minimum S/N requirement, the equalizer is
> useless.  Theoretically, a linear amplifier can be added to bring the signal
> amplitude up to meet the minimum S/N requirement.  The linear amplifier will
> need a BW larger than the transmitting signal rise time.  Furthermore, any
> deficiencies in the linearity, will add both timing and amplitude distortion
> to the received data.  The additional distortion is not included in the
> jitter specification; as a result, the link will cause higher BER.
> Especially in a high data rate link, a linear amplifier may cause more
> errors
> than the expected benefit.  In practice, it is impractical to add a linear
> amplifier.
> The right way is to keep eye open at the receiver input.
> Regards,
> Ed Chang
> NetWorth Technologies, Inc.
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