RE: PAM-5, what are your BERs ?

[Vivek Telang <vivek@xxxxxxxxxxxxxxx>]

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


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* Vivek Telang
* Cicada Semiconductor Inc.
* 901 MoPac Expressway South
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-----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.