hchenji
Joined 12 years ago
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Member since
Nov 30
Recent submissions
Comment · 8 years ago
very interesting!! Do you have a source for this? Would like to read up on it
Comment · 9 years ago
Yes. See Haar Basis on page 4 of http://www.fourierandwavelets.org/FSP_v1.1_2014.pdf
Comment · 9 years ago
Lectures and video from http://www.cse.wustl.edu/~jain/cse574-16/index.html
Book: https://www.pearsonhighered.com/program/Beard-Wireless-Commu...
Comment · 9 years ago
In my experience WiFi clients at 2.4GHz never end up using the high MCS indices whenever they transmit (whenever the channel is clear). This has something to do with the fact that the CCA thresholds are fairly high, and that high SNR (30-50dB) is required for activating the high MCS indices[1]. I don't think these SNRs are achievable in a typical setting.
It is true that MAC backoffs are also a contributing factor to the throughput being low. The CCA assessment procedure detects both wifi preambles and non-WiFi interference (pure energy detection), which is why I say interference is modeled as noise. CCA does not for example, have some intelligent coexistence algorithm for dealing with zigbee or LTE-U or other ISM traffic.
[1]: http://www.revolutionwifi.net/revolutionwifi/2014/09/wi-fi-s...
Comment · 9 years ago
See my comment https://news.ycombinator.com/item?id=13793876 .
I suggest reading Andrea Goldsmith's or Pramod Vishwanath's book on Wireless Communication. If not, Intro to Communication Systems by Madhow is recommended.
Comment · 9 years ago
Reducing transmit power reduces interference, but it also reduces capacity since the received power (and hence the SNR at the receiver) is reduced! The Shannon limit on channel capacity dictates exactly how much bit rate (C=B*log2(1+SNR)) is achievable over a channel. SNR is a fundamental ingredient of this equation.
Comment · 9 years ago
802.11ax won't mean anything unless you reduce interference (see my reply below). Without high SNR, all these new efficiency techniques won't get you anything. Shannon capacity is still what limits performance.
Comment · 9 years ago
The article misses out on explaining WHY we cannot get the promised bitrates. The answer is Shannon's limit on channel capacity, which mandates that you pay in either bandwidth or high power or low noise (SNR) to get higher capacity. Now these WiFi devices have internal rate adaptation algorithms that choose a particular modulation and coding scheme (MCS) index based on the measured SNR. A higher MCS index means better higher bits per symbol (modulation) and lower overhead code rates, and more antennas (spatial streams), which is how you get the xx Gbps bandwidth advertised on the box. List of MCS indices: http://mcsindex.com/
In today's devices, interference is considered as noise, which means that SNR simply drops to a point where the higher MCS indices are not chosen at all. So, even though the device is capable of the advertised XX Gbps bitrate, the SNR isn't high enough to switch to those higher rates.
Comment · 9 years ago
No, since wired networks are full duplex and are their own collision domains. No need to do carrier sensing at all.
Comment · 9 years ago
You mean like 3G/4G/LTE?
Comment · 9 years ago
You can also do channel bonding on 5G and use 40MHz channels. Almost double the capacity!
Comment · 10 years ago
Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.
— Tanenbaum, Andrew S. (1989). Computer Networks. New Jersey: Prentice-Hall. p. 57. ISBN 0-13-166836-6.
Comment · 10 years ago
What would be more interesting is the internals of numpy. What is it that makes numpy 'blazing fast'?
Instead, there is only high school level math in the article, with dot products and matrices and so on.
Comment · 10 years ago
Then why not Dijkstra + source routing?
Comment · 10 years ago
Use a Pseudoinverse or a decomposition instead. A great book to read is Matrix Computations by Golub/van Loan.