From owner-stds-802-3-hssg@ieee.org Tue Feb 29 23:40 GMT 2000 Received: from gatekeeper.pdd.3com.com (gatekeeper [161.71.169.3]) by isolan.pdd.3com.com (8.9.1b+Sun/8.9.3) with ESMTP id XAA23590; Tue, 29 Feb 2000 23:40:27 GMT Received: from ruebert.ieee.org ([199.172.136.3]) by gatekeeper.pdd.3com.com (Netscape Messaging Server 3.6) with ESMTP id AAA16C6; Tue, 29 Feb 2000 23:38:45 +0000 Received: by ruebert.ieee.org (8.9.3/8.9.3) id RAA05319; Tue, 29 Feb 2000 17:59:07 -0500 (EST) From: "Edward Chang" To: "Vivek Telang" , Subject: RE: PAM-5, what are your BERs ? Date: Tue, 29 Feb 2000 18:05:34 -0500 Message-ID: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2910.0) In-Reply-To: <01BF82B7.070ECDE0@pc24.cicada-semi.com> Importance: Normal X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Sender: owner-stds-802-3-hssg@ieee.org Precedence: bulk X-Resent-To: Multiple Recipients X-Listname: stds-802-3-hssg X-Info: [Un]Subscribe requests to majordomo@majordomo.ieee.org X-Moderator-Address: stds-802-3-hssg-approval@majordomo.ieee.org X-Lines: 176 Status: RO Content-Type: text/plain; charset="us-ascii" Content-Length: 6747 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@NetWorthtech.com Tel: (610)292-2870 Fax: (610)292-2872 -----Original Message----- From: owner-stds-802-3-hssg@ieee.org [mailto:owner-stds-802-3-hssg@ieee.org]On Behalf Of Vivek Telang Sent: Tuesday, February 29, 2000 2:15 PM To: stds-802-3-hssg@ieee.org 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@cicada-semi.com * http://www.cicada-semi.com * *--------------------------------------- -----Original Message----- From: NetWorthTK@aol.com [SMTP:NetWorthTK@aol.com] Sent: Tuesday, February 29, 2000 12:15 AM To: vivek@cicada-semi.com; stds-802-3-hssg@ieee.org 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.