Jan 29

Communication Systems Exam ’12-’13

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Today I had my first exam at PoliTO. When I first came to Torino in September there were also exams going on and they seemed to be pretty organised. People were called by their name to be able to enter the room, they told us you needed to use the terminals in the hallways to print some kind of “entry ticket” which you had to show as your name was called.

Fast forward to today, we could just enter the room where we had exam, everyone was just talking until the professor arrived. People just had to sit at both ends of a row and one in the middle, didn’t matter if your coat and backpack were next to you. Nothing was said about having your wallet or a cellphone in your pocket but to be sure I turned it off and put it away.

The professor came around during the exam, writing down your name on a paper to ‘register’ your attendance. That’s about all the formalities there were today, no papers or name calling at all!

For those who want to know which questions we got on the first exam day of Communication Systems, feel free to download the list of all possible questions: List of questions 2012 2013

The ones asked on the exam were questions 2, 5, 7, 14, 18 and 21.

Jan 15

The Bingham formula

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During my time at the Politecnico di Torino I attended a Communication Systems course which focused on systems such as ADSL/VDSL, DVB-T, LTE and modulation techniques such as PSK, QAM, DMT/OFDM. That being said, the one formula we always used to compute the cardinality of the constellation used was the famous Bingham formula.

When I was looking at the different parameters of this formula I figured I would do a search on Google to see if there was more information to be found. However, a search for “Bingham formula” resulted in not a single useful link. If this is such a famous and used formula, why is there nothing on the searchable web about it? Not even an entry on Wikipedia? This is an honest question, if someone knows please tell me.

What does this formula look like you ask? Here it is:

Bingham Formula

with:
m: the number of bits
1/10: used when solving for a BER of P(e) = 10^-7, 1/14 if you’re using a BER of 10^-10 (e.g. for DVB-T)
Prx: received power
Pn : noise power
Yc/Ys: non-ideality in relation to Shannon + recovery by coding gain
Ym: margin parameters taking other losses into account

When computing the achievable bitrate for a given band, you’d use Rb = Rs * m, where Rs is the symbol rate. Not going in to much detail here as I could spend another three paragraphs on things such as delay spread, guard time and cyclic prefix, just know that once you have solved the equation and have m, you know which QAM you can use. For example, if m = 3, you can use a 2^m = 2^3 = 8-QAM constellation, so that would mean 3 bits per symbol.

The difference with the Shannon formula is that you take into account some non-ideal properties, if you want the ideal bitrate, just take Yc/Ys = 1 and neglect 1/Ym.

Dec 16

MySQL Can’t get hostname for your address

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Error No. 1042
Can’t get hostname for your address

That’s what I got greeted by today. Not sure why as I was connecting to a database that I use almost every day. You’d think it to be a DNS issue, but the url was already resolved to the IP. Anyway, if you get this error and you want to fix it, go to the following folder

C:\ProgramData\MySQL\MySQL Server 5.5\ (or whatever version of MySQL you have installed)

Just add skip-name-resolve option to the [mysqld] section of your my.cnf/my.ini and reboot your MySQL server. That fixed the issue for me.