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Prof.Verdu=学术CDMA=低级趣味 (转载)
[版面:电子工程][首篇作者:guvest] , 2010年01月08日23:34:15 ,7845次阅读,62次回复
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guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Fri Jan  8 23:34:27 2010, 美东)

http://blogs.adventnet.com/svembu/2005/12/30/two-philosophies-in-cdma-a-stroll-down-memory-lane/

I have now worked a little over 10 years in the industry, after getting my
PhD. In my very first year of work at Qualcomm, I noticed how even when
speaking about the same subject, namely CDMA, academia and industry were on
totally different planets. When I was in Qualcomm, I co-authored a paper
with Dr. Viterbi, titled Two Different Philosophies in CDMA, A Comparison. I
still stand by the conclusions we reached, though there was a fairly strong
rebuttal from my PhD advisor Prof. Sergio Verdu. His paper is listed in http://www.princeton.edu/~verdu/mud.html and I had located a scanned copy somewhere that I don’t remember now. I will post a link soon. A somewhat related paper by Verdu is http://web.mit.edu/6.933/www/Fall2001/Shannon2.pdf

A very quick summary of the conclusions we reached 10 years ago:

1. Multi-user interference is a major problem in CDMA. Your first line of
defence against it is a powerful error-correcting code. You can potentially
supplement it with non-linear techniques like successive decoding and
successive interference cancellation, but they suffer from parameter
estimation errors, so are not [as of 1995] yet anywhere close to being
practical. Instead of non-linear techniques, our focus in the paper was on
linear techniques common in academia.

2. A second line of defence had been very common in the literature, indeed
the exclusive focus of study in the academic literature [true in 1995],
namely linear dimensional techniques. Many people superficially claimed
these techniques were applicable to commercial Qualcomm CDMA systems then
already in the market. They were not. The fundamental system model the
academic community used to study their interference suppression techniques
differs markedly from Qualcomm CDMA. The systems were philosophically
different, and the academic CDMA model was clearly inferior from an
engineering point of view.

In spite of Verdu’s rebuttal, these conclusions have held up remarkably
well. Today, the error-control code has gotten even more powerful: turbo-
codes, which have caused a revolution. The dimensional techniques have been
progressively abandoned. It vindicates my conviction that the academic
community was barking up the wrong tree, and what they were calling CDMA was
not the same CDMA the world has come to know and love and it was clearly
inferior as an engineering design. Strong words, right?

A recent overview by Jeff Andrews at UT Austin Interference Cancellation for
Cellular Systems: A Contemporary Overview caught my eye, and it referred to
that 10 year old controvery (the reference [18] in the quote below is my
paper and [19] is my advisor’s rebuttal)


Quote:

A controversial paper from QUALCOMM [18] identified at an early stage many
of the fundamental problems with academic research on MUD, particularly with
the widely researched linear projection techniques of the early to mid-
1990s. Although this article made a number of mistakes (as partially
documented in [19]), it may have helped spur more intensive research on
interference cancellation from the late 1990s to the present. We now
summarize the key challenges and historical shortcomings in multi-user
receiver implementation, some of which were identified in [18], but will
revisit some of these issues later when forecasting a bright future for
multi-user reception.

Certainly, the “controversial” part is conceded. I was basically
committing academic patricide, criticizing the very work that made my
advisor famous. But I do stand by the essence of my critique. After all,
Jeff Andrews states that my paper “identified many of the fundamental
problems with academic research on MUD” - and the rebuttal by my advisor
doesn’t really address the central thrust of my paper: namely that there is
a fundamental difference in the mathematical system model studied as CDMA
by the academia and the one built by Qualcomm and the academic model leads
to misleading conclusions. My advisor has been “Dr.CDMA” in academia while
what I consider the “real” CDMA (a fair characterization, because
Qualcomm-is-CDMA, to this day) has been practiced in the industry. It is not
an academic (!) debate: CDMA is now the dominant standard in cellular
communications, and barring a challenge-from-below from Wi-Fi/Wi-Max, it
should rule the roost for a long time.

We called the academic CDMA D-CDMA (for “dimensional CDMA”, and you can
also consider it “delusional CDMA” - just kidding!) and the industry CDMA
as R-CDMA (for “random CDMA” or “real CDMA” if you will). D-CDMA system
model assigns signature waveforms (dimensions) to users, and deals with
their correlations at the receiver using a variety of linear and non-linear
algorithms . If the signature waveforms are orthogonal (the ideal situation)
, then D-CDMA is theoretically equivalent to TDMA. D-CDMA system model
simply did not assign any role to error-correcting codes, a significant
omission [see below]. Certainly, no paper by my advisor or collaborators
until that point mentioned error-correcting codes, and it is fair to say
that the D-CDMA model popularized by Dr.CDMA remained by far the most
popular model in academia. I contended then, and stand by it now, that this
is a significant problem.

In contrast to D-CDMA, R-CDMA is a deceptively simple, even simplistic idea:
simply randomize all the users with very long spreading codes, so that
every other user looks like Gaussian noise, then use powerful error-
correcting codes against that noise + noise-like interference. It can be
improved by the well known technique of successive interference cancellation
(which my advisor called a proof technique, and suboptimal in practice, in
his rebuttal but it is certainly more practical - but not very practical yet
! - than many of the algorithms common in the D-CDMA literature).

Qualcomm’s CDMA system, designed based on a very different system model
than what was considered in academia, was already in the market when we
wrote the paper, so the fact that the academics were still researching their
clearly inferior system model that treated demodulation/decoding separately
, illustrates how ostrich-like academia often becomes, and why I decided to
leave. Papers were published, tenures granted, reputations and careers made,
and yet, the basic system model just didn’t make sense, at least to me.

In essence, our critique was not about multi-user demodulation per se but
the incorrect system model adopted by the academic community to analyze “
CDMA”. The D-CDMA model simply took a very statistical detection theoretic
view of the world, while ignoring the fundamental role played by coding
techniques. In any practical (or theoretical!) communication receiver,
statistical detection (matched filtering and other statistical signal
processing like parameter estimation) and decoding work together. Any system
model and analysis technique that does not consider the system holistically
is prone to reach erroneous or at the very least, misleading conclusions.
Yet, the vast D-CDMA academic literature was full of such statistical-
detection-based analysis (they call it “demodulation”, to separate it from
“decoding”). A telling sign is the emphasis on SNR in the D-CDMA
literature, a notion useful in analog parameter estimation, rather than the
more appropriate Eb/No, a notion that is only relevant to digital
communication. In fact, D-CDMA literature, including almost every paper from
my advisor, made the cardinal sin of evaluating digital communications
systems based on the uncoded bit error rate. The effect of that is that
these systems do not display the characteristic “cliff” whereby the bit
error falls steeply when a threshold Eb/No is crossed - that cliff makes
analytical methods intractable, so simulation is all but required. All in
all, my advisor’s rebuttal essentially missed the central point of my paper
, which I believe was reasonably well articulated.

Jeff Andrews provides further confirmation of my critique. To quote from his
overview again (emphasis mine)


Quote:

For asynchronous systems like the cellular uplink with random user timing
offsets, mutually orthogonal codes are not feasible. But again, multi-user
receivers that attempt to project the received users into orthogonal
dimensions do not stand up to careful scrutiny. In particular, most of these
techniques require the construction of a code cross-correlation matrix. But
for the long PN codes generally desired for multi-path properties, this
matrix must usually be recomputed every symbol, which is impractical. Even
more important, it is now well established that strong error correction
codes (ECCs) should be used in the uplink (and the downlink, for that matter
). The spreading gain of these codes is lost on dimensional multi-user
detectors. Furthermore, while the goal of these codes is to lower the
required received signal-to-interference-plus-noise ratio (SINR), this
complicates channel estimation and other aspects of dimensional MUD. Since
linear projection receivers require spreading to be performed by a linear
operation, there is direct competition between ECCs and dimensional (linear)
MUD [24]. However, as discussed next, interference cancelling multi-user
receivers not only do not suffer from this trade-off, but in fact need ECCs
to attain their high performance, a result supported by information theory [
2527].



Most researchers have come to recognize that in a highly frequency-selective
and rapidly changing channel with many users, dimensional separation
techniques at the receiver are not viable. Further, it has been realized
that effective MUD designs must integrate error correction coding/decoding
into the receiver structure, rather than treating coding as a separately
concatenated block. For this reason, successive [3032], parallel [33, 34],
and iterative interference cancellation (turbo MUD) [3538] appear quite
attractive.

A one-line summary of our paper would be precisely that dimensional-
separation techniques at the receiver are not viable. Thank you Jeff, for
making that point! I wonder if Prof. Verdu accepts your conclusion, because
his rebuttal was still optimistic about those techniques. If he does accept
this conclusion, it would be nice to receive an acknowledgement from him,
that, yes, his student got that one right.

The reason for the love affair with statistical detection, to the exclusion
of decoding, is the ivy-league academic quest for “closed form”
mathematical results, at the expense of any kind of real world relevance. If
your PhD doesn’t have the requisite number of Lemmas and Theorems, forget
it - that seems to be the attitude. Turbo codes left leading coding
theorists in the dust, with a radically simple and practical approach, which
essentially achieves Shannon capacity (and that was only “proved” through
simulations, no theorems, leading to enormous scepticism about the work
initially). My advisor, a closet mathematician, always loved the closed form
, theorem proof approach himself, and essentially prohibited me from doing
any kind of “simulation work” to get a PhD. To be perfectly fair to him,
that fit very well (too well!) with my own IIT Brahmin attitude about not
getting hands dirty with something as mundane as writing code, an attitude I
got out of after I had an epiphany that my entire PhD was useless
abstractions piled on top of useless abstractions. That was what led me to
abrubtly end my academic career. Looking back after 10 years, that was the
single best career decision I made in my life.

When I entered Qualcomm I realized what a valuable tool the computer is to
study real world systems, and how limiting the quest for “closed form” can
be. In fact, my introduction to the world of software arose from modeling
and simulating communication receivers, and I ended up embracing software as
a career eventually. I read somewhere that Wolfram quit academia and went
into software for what sounded to me (may be I imagined it!) like similar
reasons: the Wolfram thesis asserts that the universe is equivalent to a
Turing Machine and computer programs are the best tools to study the
universe. Wolfram seems to deemphasize the theorem-proof methodology himself
- in fact, he asserts in his book that most interesting questions are
logically undecidable (i.e unprovable). I agree.

This is not to criticize mathematics; indeed far from it. Simple
analytically tractable models (i.e the closed form again!) are illuminating,
but they cannot substitute for more realistic models that can so often only
be simulated. In particular, the holistic demodulation/decoding system
cannot really be handled in any closed form solution I know of. Often,
simulation can and does lead to insights into theory and can open
theoretical doors that we may not suspect existed.

The reason I am writing this is to set the record straight. I have long ago
moved on, and have transferred my affections to software. CDMA is like a
long-lost old flame for me - it still excites some passion, but I am happily
married to software now ;-)

With the benefit of hindsight, read our paper, and read my advisor’s
rebuttal. Then read Andrews’ recent survey. See if the points we made about
the flawed system model that is D-CDMA resonate, 10 years later. As for the
“mistakes” pointed out by my advisor? Just one sample would suffice. His
rebuttal [see section 2.5 in his paper] quotes the “misconception 2.20″ (
not specifically attributed to our paper, though the implication is there)
that “Multi-user Detection is Successive Cancellation”. We didn’t say
that - what we actually said was that if you want real multi-user detection,
there is always successive cancellation, a technique well-known and
something Dr. Viterbi, my co-author was well-aware of, and had written about
long ago in the CDMA (R-CDMA) context. There are still numerous practical
difficulties with it, having to do with the fact that various parameters
have to be estimated in a noisy changing environment. Guess what? Andrews
optimism for MUD is now based on this same technique, dubbed as “
misconception” by my advisor. Deja vu all over again …

Here is a pure speculation/suggested PhD thesis: in statistical parameter
estimation, there is the Cramer-Rao bound (it is a sort of the analog-
equivalent of Shannon capacity, which is a digital construct). It basically
says that your accuracy of estimation is limited by the variance of the
noise. I have a strong suspicion [I am willing to stand corrected if I see
some good results] that the regime of signal to noise you operate in CDMA
makes any kind of accurate parameter estimation, which is needed for almost
all of these multi-user techniques, including successive cancellation, quite
difficult. There is an interplay and tension between bit rate maximization
(digital communication) and accurate parameter estimation (analog
communication, you assume that the channel is “communicating” those
parameters to you!) that I have long wondered about, but done absolutely
nothing more than wonder about. Note that if the parameters to be estimated
don’t change much with time, you can average over a long interval, so you
can do OK. But the real world is not so nice.

Basically after a long detour into inapplicable techniques arising from a
faulty system model, academia is finally getting to a point that some of us
in the industry reached 10 years ago. Welcome to the real world, guys.

--

※ 修改:·guvest 于 Jan  8 23:37:29 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]


 
StevenLow
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发信人: StevenLow (CrossLayer), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Fri Jan  8 23:57:56 2010, 美东)

http://www.ee.princeton.edu/people/alumni/iss/?id=77bc6af36de29b9fcc86e64ae1273c10

哥们挺牛啊。
【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: http://blogs.adventnet.com/svembu/2005/12/30/two-philosophies-in-cdma-a-stroll-down-memory-lane/
: I have now worked a little over 10 years in the industry, after getting my
: PhD. In my very first year of work at Qualcomm, I noticed how even when
: speaking about the same subject, namely CDMA, academia and industry were
on
: totally different planets. When I was in Qualcomm, I co-authored a paper
: with Dr. Viterbi, titled Two Different Philosophies in CDMA, A Comparison.
I
:  still stand by the conclusions we reached, though there was a fairly
strong
:  rebuttal from my PhD advisor Prof. Sergio Verdu. His paper is listed in http://www.princeton.edu/~verdu/mud.html and I had located a scanned copy somewhere that I don’t remember now. I will post a link soon. A somewhat related paper by Verdu is htt
: A very quick summary of the conclusions we reached 10 years ago:
: 1. Multi-user interference is a major problem in CDMA. Your first line of
: ...................


--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 71.62.]

 
guvest
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我的博客
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sat Jan  9 00:01:31 2010, 美东)

刚google了下,
这厮开了个软件公司,08年销售额大概7千万刀.
我估计它每月利润最少100万刀.确实牛.

(他雇了600印度人,6个美国人,而且他说在印度他不要大学生.
所以人力成本可以忽略不计......)

【 在 StevenLow (CrossLayer) 的大作中提到: 】
: http://www.ee.princeton.edu/people/alumni/iss/?id=77bc6af36de29b9fcc86e64ae1273c10
: 哥们挺牛啊。
: on
:  I
: strong



--

※ 修改:·guvest 于 Jan  9 00:03:15 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]




 
StevenLow
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发信人: StevenLow (CrossLayer), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sat Jan  9 00:04:07 2010, 美东)

你嚼着他跟verdu谁牛逼。
【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: 刚google了下,
: 这厮开了个软件公司,08年销售额大概7千万刀.
: 我估计它每月利润最少100万刀.确实牛.
: (他雇了600印度人,6个美国人,而且他说在印度他不要大学生.
: 所以人力成本可以忽略不计......)



--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 71.62.]

 
guvest
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[回复] [回信给作者] [本篇全文] [本讨论区] [修改] [删除] [转寄] [转贴] [收藏] [举报] [ 5 ]

发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sat Jan  9 00:13:25 2010, 美东)

我猜,

如果你
(1)给学校贡献1000万刀利润. (也就是说你5年拿了大概2000万刀funding)
(2)有点垃圾文章.(这个比较容易,找一组博士后就行了)
应该能混个princeton tenure.

verdu这类人我不太懂他们玩的是什么.没啥资格谈.咱就瞎说一下.
我有问题:
(1).做数学,能做过数学家吗? 尤其是,能做过苏联人么?
(2).做工程,那维特比都出马写文章说你有基本的错误了?

所以说上面那哥们说verdu引领低级趣味,这话还真不好反驳.

也许有些人积累的多了,最终能做出来双赢的东西.
毕竟,kalman filter,fft,etc很多最常用的算法都是做理论的搞出来的.
不过我觉得verdu这年纪够呛了.

btw:IMO,普林斯顿的李教授肯定比verdu牛,同是牛教授.
据说前段时间他的公司卖了几个B.

【 在 StevenLow (CrossLayer) 的大作中提到: 】
你嚼着他跟verdu谁牛逼。
【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: 刚google了下,
: 这厮开了个软件公司,08年销售额大概7千万刀.
: 我估计它每月利润最少100万刀.确实牛.
: (他雇了600印度人,6个美国人,而且他说在印度他不要大学生.
: 所以人力成本可以忽略不计......)



--




--

※ 修改:·guvest 于 Jan  9 01:52:27 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]








 
dreamland
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发信人: dreamland (呓犹未尽), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 18:20:31 2010, 美东)

很有意思的文章。能跟verdu学习,跟viterbi共事,实在是幸事;然后两者皆抛,去做软件,很有魄力。

有一点深有同感:理论的假设条件很重要,假设条件能够简化问题,让复杂问题能够被
简化从而能被分析;同时简单的假设条件也限制了理论的应用范围。




【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: http://blogs.adventnet.com/svembu/2005/12/30/two-philosophies-in-cdma-a-stroll-down-memory-lane/
: I have now worked a little over 10 years in the industry, after getting my
: PhD. In my very first year of work at Qualcomm, I noticed how even when
: speaking about the same subject, namely CDMA, academia and industry were
on
: totally different planets. When I was in Qualcomm, I co-authored a paper
: with Dr. Viterbi, titled Two Different Philosophies in CDMA, A Comparison.
I
:  still stand by the conclusions we reached, though there was a fairly
strong
:  rebuttal from my PhD advisor Prof. Sergio Verdu. His paper is listed in http://www.princeton.edu/~verdu/mud.html and I had located a scanned copy somewhere that I don’t remember now. I will post a link soon. A somewhat related paper by Verdu is http://web.mit.edu/6.933/www/Fall2001/Shannon2.pdf
: A very quick summary of the conclusions we reached 10 years ago:
: 1. Multi-user interference is a major problem in CDMA. Your first line of
: ...................





--

※ 修改:·dreamland 於 Jan 10 18:22:14 2010 修改本文·[FROM: 24.4.]
※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 24.4.]

 
guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 18:59:15 2010, 美东)

IMO,简化问题去做理论。还不如用仿真和试验去研究。
有些作理论的价值观成问题。EE,物理,一切工科都应该并重理论和实验(含仿真)。

【 在 dreamland (呓犹未尽) 的大作中提到: 】
: 很有意思的文章。能跟verdu学习,跟viterbi共事,实在是幸事;然后两者皆抛,去
做软件,很有魄力。
: 有一点深有同感:理论的假设条件很重要,假设条件能够简化问题,让复杂问题能够被
: 简化从而能被分析;同时简单的假设条件也限制了理论的应用范围。
: on
:  I
: strong



--

※ 修改:·guvest 于 Jan 10 19:01:55 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]




 
Genghis
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发信人: Genghis (CC), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 19:16:00 2010, 美东)

现在学术界已经过分重视仿真了,可以随便去翻翻那些IEEE Trans看看
所以没有什么新的有指导意义的理论出现
Shannon在这个时代是很难存活下去的。。。

【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: IMO,简化问题去做理论。还不如用仿真和试验去研究。
: 有些作理论的价值观成问题。EE,物理,一切工科都应该并重理论和实验(含仿真)。
: 做软件,很有魄力。



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※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 68.48.]

 
guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 20:32:38 2010, 美东)

IMO,两个apparoach都要有,不然很快一个方向就死了.

【 在 Genghis (CC) 的大作中提到: 】
: 现在学术界已经过分重视仿真了,可以随便去翻翻那些IEEE Trans看看
                     这可能是因为真正有意义的试验和仿真掌握在公司手里.
             就是学界,仿真和试验里面放点错误数据阻止别人复制也是常事.
             前面有位说维特比后期文章不行.但我想如果他愿意透漏技术,
             那能发无数好文章.
: 所以没有什么新的有指导意义的理论出现
: Shannon在这个时代是很难存活下去的。。。



--

※ 修改:·guvest 于 Jan 10 20:35:51 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]


 
rainbowrain
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发信人: rainbowrain (温柔乡里人), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 20:36:17 2010, 美东)

Prof.Verdu=学术CDMA=低级趣味

我看人家并没有这个意思,什么“低级趣味”是你的理解吧。这个人对Verdu是有不满
,但是并不存在讽刺的意味。就像你那个帖子说的一样,这只是2种方法的不同,他教
授强迫他走数学公式路线。

另外一个问题是模型问题。我觉得搞理论的人,能提出一个自己的模型,能有一套理论
就很不错了,至于这个模型是不是有商业价值,实用不实用,往往不是搞理论的人能够
掌控的。搞理论就是要剔除很多现实的限制,而这些限制也许很多年以后就不再是限制
,也可能永远得不到解决而导致这个理论没有应用价值。这些是东西是很难能够事先预
料到的。

【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: http://blogs.adventnet.com/svembu/2005/12/30/two-philosophies-in-cdma-a-stroll-down-memory-lane/
: I have now worked a little over 10 years in the industry, after getting my
: PhD. In my very first year of work at Qualcomm, I noticed how even when
: speaking about the same subject, namely CDMA, academia and industry were
on
: totally different planets. When I was in Qualcomm, I co-authored a paper
: with Dr. Viterbi, titled Two Different Philosophies in CDMA, A Comparison.
I
:  still stand by the conclusions we reached, though there was a fairly
strong
:  rebuttal from my PhD advisor Prof. Sergio Verdu. His paper is listed in http://www.princeton.edu/~verdu/mud.html and I had located a scanned copy somewhere that I don’t remember now. I will post a link soon. A somewhat related paper by Verdu is http://web.mit.edu/6.933/www/Fall2001/Shannon2.pdf
: A very quick summary of the conclusions we reached 10 years ago:
: 1. Multi-user interference is a major problem in CDMA. Your first line of
: ...................



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guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 20:46:13 2010, 美东)

"
what they were calling CDMA was not the same CDMA the world has come to know
and love and it was clearly inferior as an engineering design
"
"
so the fact that the academics were still researching their clearly inferior
system model that treated demodulation/decoding separately, illustrates how
ostrich-like academia often becomes
"
inferior翻译成低级不合适?

【 在 rainbowrain (温柔乡里人) 的大作中提到: 】
: Prof.Verdu=学术CDMA=低级趣味
: 我看人家并没有这个意思,什么“低级趣味”是你的理解吧。这个人对Verdu是有不满
: ,但是并不存在讽刺的意味。就像你那个帖子说的一样,这只是2种方法的不同,他教
: 授强迫他走数学公式路线。
: 另外一个问题是模型问题。我觉得搞理论的人,能提出一个自己的模型,能有一套理论
: 就很不错了,至于这个模型是不是有商业价值,实用不实用,往往不是搞理论的人能够
: 掌控的。搞理论就是要剔除很多现实的限制,而这些限制也许很多年以后就不再是限制
: ,也可能永远得不到解决而导致这个理论没有应用价值。这些是东西是很难能够事先预
: 料到的。
: on
: ...................


--

※ 修改:·guvest 于 Jan 10 20:51:25 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]


 
rainbowrain
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发信人: rainbowrain (温柔乡里人), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 20:50:58 2010, 美东)

所以我说了,这个是你的理解。


【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: "
: what they were calling CDMA was not the same CDMA the world has come to
know
:  and love and it was clearly inferior as an engineering design
: "
: inferior翻译成低级没问题吧?



--

※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 173.2.]

 
guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 20:53:36 2010, 美东)

那你觉得,他说的inferior是什么意思.

"so the fact that the academics were still researching their clearly
inferior system model that treated demodulation/decoding separately,
illustrates how ostrich-like academia often becomes"


【 在 rainbowrain (温柔乡里人) 的大作中提到: 】
所以我说了,这个是你的理解。


【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: "
: what they were calling CDMA was not the same CDMA the world has come to
know
:  and love and it was clearly inferior as an engineering design
: "
: inferior翻译成低级没问题吧?



--

※ 修改:·guvest 于 Jan 10 20:56:40 修改本文·[FROM: 66.25.]
※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 173.2.]



--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]

 
rushier
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发信人: rushier (走来走去), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 21:00:32 2010, 美东)

学术界的研究不是很容易知道“real problem",有局限也很正常,也不能苛求研究都
那么实用。我倒觉得上面的分歧只是style不一样。有的人喜欢抽象,有的人喜欢实际
。作研究当然尽量study on good problem,但是能这样的人很少,也很难。

大批量的灌水就不正常了。记得David Tse书的前言里提到:好的理论应该不是越来越
繁杂,应该是用一个unified framework,看起来很简单。原话不记得了。



【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: IMO,两个apparoach都要有,不然很快一个方向就死了.
:                      这可能是因为真正有意义的试验和仿真掌握在公司手里.
:              就是学界,仿真和试验里面放点错误数据阻止别人复制也是常事.
:              前面有位说维特比后期文章不行.但我想如果他愿意透漏技术,
:              那能发无数好文章.



--

※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 199.106.]

 
guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 21:06:04 2010, 美东)

我倒觉得大规模的关税也是贡献.这是我前面写的:

"
但是,全局的看,IMO,人类知识的主要部分就是这么来的。
换句话说,基本的方法是:死路全探出来之后,才知道哪里是活路。
没有垃圾文章的冲刷,真正有价值的问题和方向是不会展示出来的。

"
即使在bbs上,不灌水,不讨论,也无法提高自己的认识,对吧?

【 在 rushier (走来走去) 的大作中提到: 】
: 学术界的研究不是很容易知道“real problem",有局限也很正常,也不能苛求研究都
: 那么实用。我倒觉得上面的分歧只是style不一样。有的人喜欢抽象,有的人喜欢实际
: 。作研究当然尽量study on good problem,但是能这样的人很少,也很难。
: 大批量的灌水就不正常了。记得David Tse书的前言里提到:好的理论应该不是越来越
: 繁杂,应该是用一个unified framework,看起来很简单。原话不记得了。



--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]

 
Genghis
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发信人: Genghis (CC), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 21:16:22 2010, 美东)

同意
unified framework是最重要的
这也是为什么Shannon如此的伟大
他的model是如此的简单
这些基础性的东西,本身扎根于现实问题,但往往不被现实的人所理解
Shannon的理论也是花了20年才被工业界理解,用来真正指导通信系统的实现的


【 在 rushier (走来走去) 的大作中提到: 】
: 学术界的研究不是很容易知道“real problem",有局限也很正常,也不能苛求研究都
: 那么实用。我倒觉得上面的分歧只是style不一样。有的人喜欢抽象,有的人喜欢实际
: 。作研究当然尽量study on good problem,但是能这样的人很少,也很难。
: 大批量的灌水就不正常了。记得David Tse书的前言里提到:好的理论应该不是越来越
: 繁杂,应该是用一个unified framework,看起来很简单。原话不记得了。



--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 68.48.]

 
rushier
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发信人: rushier (走来走去), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 21:27:06 2010, 美东)

你非要说灌水有益处,我也无话可说。
但发文章和在mitbbs上发帖不是一回事吧?
【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: 我倒觉得大规模的关税也是贡献.这是我前面写的:
: "
: 但是,全局的看,IMO,人类知识的主要部分就是这么来的。
: 换句话说,基本的方法是:死路全探出来之后,才知道哪里是活路。
: 没有垃圾文章的冲刷,真正有价值的问题和方向是不会展示出来的。
: "
: 即使在bbs上,不灌水,不讨论,也无法提高自己的认识,对吧?



--

※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 199.106.]

 
rainbowrain
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发信人: rainbowrain (温柔乡里人), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 21:56:50 2010, 美东)

说以Tse搞了一个deterministic model,呵呵。

【 在 rushier (走来走去) 的大作中提到: 】
: 学术界的研究不是很容易知道“real problem",有局限也很正常,也不能苛求研究都
: 那么实用。我倒觉得上面的分歧只是style不一样。有的人喜欢抽象,有的人喜欢实际
: 。作研究当然尽量study on good problem,但是能这样的人很少,也很难。
: 大批量的灌水就不正常了。记得David Tse书的前言里提到:好的理论应该不是越来越
: 繁杂,应该是用一个unified framework,看起来很简单。原话不记得了。



--

※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 173.2.]

 
guvest
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发信人: guvest (我爱你老婆Anna), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 22:15:39 2010, 美东)

对个人来讲,关税可能没什么用。

但是很多情况下,不灌水确实是不知道什么模型是对的。
反正我觉得,拿钱拿人堆是个发展学术的好方法。

比如说著名的庞家来猜想,前后总共出了6个菲尔兹奖。
但据我观察,拓扑学也是灌了20年左右的水,才发现这是核心问题。

【 在 rushier (走来走去) 的大作中提到: 】
: 你非要说灌水有益处,我也无话可说。
: 但发文章和在mitbbs上发帖不是一回事吧?



--

※ 修改:·guvest 于 Jan 10 22:19:27 修改本文·[FROM: 66.25.]
※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 66.25.]


 
Genghis
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发信人: Genghis (CC), 信区: EE
标  题: Re: Prof.Verdu=学术CDMA=低级趣味 (转载)
发信站: BBS 未名空间站 (Sun Jan 10 22:24:10 2010, 美东)

多发文章和发垃圾文章是2个概念


【 在 guvest (我爱你老婆Anna) 的大作中提到: 】
: 对个人来讲,关税可能没什么用。
: 但是很多情况下,不灌水确实是不知道什么模型是对的。
: 反正我觉得,拿钱拿人堆是个发展学术的好方法。
: 比如说著名的庞家来猜想,前后总共出了6个菲尔兹奖。
: 但据我观察,拓扑学也是灌了20年左右的水,才发现这是核心问题。



--

※ 来源:·BBS 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 68.48.]

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