2012年12月15日星期六

DFTS-OFDM for uplink transmission in LTE (ZZ)

Today multi-carrier transmission and OFDM are means to allow for very high overall transmission bandwidth while still being robust to signal corruption due to radio-channel frequency selectivity. However, a drawback of OFDM modulation, as well as any kind of multi-carrier transmission, is the large variations in the instantaneous power of the transmitted signal. Such power variations imply a reduced power-amplifier efficiency and higher power-amplifier cost. This is especially critical for the uplink, due to the high importance of low mobile-terminal power consumption and cost. Several methods have been proposed on how to reduce the large power variations of an OFDM signal. However, most of these methods have limitations in terms of to what extent the power variations can be reduced. Furthermore, most of the methods also imply a significant computational complexity and/or a reduced link performance. Thus, there is an interest to consider also wider-band single-carrier transmission as an alternative to multi-carrier transmission, especially for the uplink, i.e. for mobile-terminal transmission. One of such single-carrier transmission scheme can be implemented using DFT-spread OFDM

DFT-spread OFDM (DFTS-OFDM) is a transmission scheme that can combine the desired properties for uplink transmission i.e. : 
• Small variations in the instantaneous power of the transmitted signal (‘single carrier’ property). 
• Possibility for low-complexity high-quality equalization in the frequency domain. 
• Possibility for FDMA with flexible bandwidth assignment. 

Due to these properties, DFTS-OFDM has been selected as the uplink transmission scheme for LTE, which is the long-term 3G evolution.

DFTS-OFDM signal generation
PAR distribution for OFDM and DFTS-OFDM, respectively.

More detail can be accessed from here:
http://wirelesscafe.wordpress.com/2009/04/07/what-is-single-carrier-fdma-sc-fdma/

2012年12月3日星期一

How to calculate/plot confidence interval

http://people.richland.edu/james/lecture/m170/ch08-int.html

2012年10月8日星期一

Matlab "Out of Memory"

If you are using 32 bits Window XP, Vista and Window 7, the default memory loaded in Matlab is 2GB.

You can try to extend it by using the following:

1) Run the command in administrator:
2) > BCDEdit /set increaseuserva 3072

Restart the computer and matlab.

2012年9月4日星期二

Channel estimation in MIMO-OFDM

Wireless LAN Comes of Age:Understanding theIEEE 802.11n Amendment
by Thomas Paul and Tokunbo Ogunfunmi

published in IEEE circuits and systems magazine
Tutorial for WLAN design (including 802.11n, 802.11ac)

Check Section III. 2.5


Also, some tutorial on least square estimation techniques, which is used to derive the results in channel estimation.

http://www.stat.math.ethz.ch/~geer/bsa199_o.pdf

2012年8月17日星期五

Write Your Own Module in GNU Radio ZZ


Burn Firmware and FPGA


To connect USRP2 to an external clock, you need to change the firmware and FPGA code on the SD Card
    1. Insert the SD card into the card reader
    2. Use "dmesg" command to check which interface is in use. The following is an example output in my machine, where the SC card is installed in /dev/sdc
    3. $ dmesg
      sd 4:0:0:1: [sdc] 3878912 512-byte logical blocks: (1.98 GB/1.84 GiB)
      sd 4:0:0:1: [sdc] Assuming drive cache: write through
      sd 4:0:0:1: [sdc] Assuming drive cache: write through
      sdc: unknown partition table
    4. Run$ cd gnuratio/usrp2/firmware/
      $ sudo ./u2_flash_tool --dev=/dev/XXXX -t s/w txrx.bin -w
      $ sudo ./u2_flash_tool --dev=/dev/XXXX -t fpga u2_rev3-20100603.bin -w 
      Be careful: /dev/XXXX has to be replaced with the device for the SD card reader.
    5. Insert the SD card to USRP2 and reboot USRP2. If your SD card is upgraded successfully, you should see all 6 LEDs flash, and LED 'D' and 'F' will remain on. If not, the card is not upgraded properly.

NOTE: Note that these firmware and FPGA codes work only when the external clock is in use. Otherwise, the USRPs behavior is unpredictable.



http://nms.citi.sinica.edu.tw/n+/software.html




Write Your Own Module


To create your own python module, you will need to write the python script (.py files), C library (.cc files), and the swig file (.i file). The simplest way to create a new module is to copy the existing module and modify it to what you want. We suggest modifying the 'mimo_ofdm_siso' module because it provides the most basic functionality so that you can build your own functions on top of it. In the following, we use modifying 'mimo_ofdm_siso' as the example to demonstrate how to create a new module, called 'mimo_ofdm_new'.
    1. Create python files$ cd gr-mimo/python/
      $ cp mimo_usrp2_ofdm_receiver_siso.py mimo_usrp2_ofdm_receiver_new.py
      $ ln -sf mimo_usrp2_ofdm_receiver_new.py mimo_usrp2_ofdm_receiver.py 
      Edit 'mimo_usrp2_ofdm_receiver_new.py' and replace all strings "_siso" with "_new". You might need to make other modifications if you want to create a module with a different number of input ports and output ports.
    2. Create C library$ cd ../lib/
      $ cp mimo_ofdm_siso.cc mimo_ofdm_new.cc
      $ cp mimo_ofdm_siso.h mimo_ofdm_new.h
      Edit 'mimo_ofdm_new.cc' and 'mimo_ofdm_new.h', and replace all strings "_siso" and "_SISO" to "_new" and "_NEW", respectively. Modify the makefile 'Makefile.am'. Add "mimo_ofdm_new.h" under the tag "modinclude_HEADERS =", and add "mimo_ofdm_new.cc" under the tag "libgnuradio_mimo_la_SOURCES =".
    3. Create .i file$ cd ../swig/
      $ cp mimo_ofdm_siso.i mimo_ofdm_new.i
      Edit 'mimo_ofdm_new.i', and replace all strings "_siso" with "_new".

      Add the following two lines in 'mimo.i'.#include "mimo_ofdm_new.h"
      %include "mimo_ofdm_new.i"
      Modify the makefile 'Makefile.am'. Add "mimo_ofdm_new.i" under the tag "mimo_swiginclude_headers =".
    4. Re-build$ cd ../
      $ make; sudo make install
    5. Create the new transmitter code$ cd gnuradio/usrp2/host/apps/
      $ cp mimo_siso_tx1.cc mimo_new.cc
      Add "mimo_new.cc" in 'Makefile.am', and reinstall$ make; sudo make install
    6. Run the example
      In the receiver,$ cd gr-mimo/python
      $ sudo ./mimo_usrp2_rx.py -f 2.45G -d 64 -e eth0 --rx_ant_num 1 -v
      In the transmitter,$ cd source/
      $ sudo mimo_new -f 2.45G -i 64 -e eth1 -v

2012年7月27日星期五

SC-FDMA vs OFDMA


SC-FDMA has drawn great attention as an attractive alternative to OFDMA, especially in the uplink communications where lower PAPR greatly benefits the mobile terminal in terms of transmit power efficiency and terminal costs. It has been adopted as the uplink multiple access scheme in 3GPP Long Term Evolution (LTE), or Evolved UTRA.[3][4][5]
The performance of SC-FDMA, in relation to OFDMA has been the subject of various studies.[6][7][8] Although the performance gap is not much, SC-FDMA's additional advantage of low PAPR makes it a favorite especially for uplink wireless transmission in future mobile communication systems where transmitter power efficiency is of paramount importance.

Different transmitters (users) are assigned different Fourier coefficients. This assignment is carried out in the mapping and demapping blocks. The receiver side includes one demapping block, one IDFT block and one detection block for each user signal to be received.
SC-FDMA.svg

2012年6月29日星期五

millimeter wave propagation study

Dr. Theodore (Ted) S. Rappaport


Latest research in millimeter wave cellular propagation and broadband communications using adaptive antennas – 5th Generation Cellular


http://faculty.poly.edu/~tsr/

2012年5月28日星期一

port usrp1 code to usrp2 (zz)


from "The SDR blog"
This is by no means a complete list, and I will be updating it as I find more of them…
  • import usrp2
  • usrp.source_c() becomes usrp2.source_32fc(“eth0″, “”)
  • adc_rate is 100MS/s instead of 64MS/s, multiply decimation with ~1.5
  • set_decim_rate() becomes set_decim()
  • remove all “subdev”-related code
    subdev.gain_range() becomes usrp2.gain_range()
    subdev.set_gain() becomes usrp2.set_gain()
  • usrp.tune() becomes usrp2.set_center_freq()
////////////////////////////////////////////////////////////////////////////////////////

2012年5月25日星期五

Free e-Books (ZZ)


Free e-Books

NOW Foundations and Trends® Books

Information and Communication Theory

Networking Theory

Game Theory

Probability, Random Processes, and Queueing Theory

Optimization

Control Theory

Graph Theory

Machine Learning

Algorithms & Complexity

  • S. Dasgupta, C. H. Papadimitriou, and U.V. Vazirani, Algorithms

Misc.

2012年4月9日星期一

Apple IOS development

Apple development: (1) Learn Object-C  (2) IPad&Iphone Tutorial

2012年4月6日星期五

A Wireless Road Around Data Traffic Jams (zz)



http://www.nytimes.com/2012/01/15/business/a-wireless-way-around-data-center-traffic-jams.html?_r=1&partner=yahoofinance#h[]
THE vast data centers that process information for the Facebooks and Amazons of the Web work at a brisk clip. But even so, they can’t always keep up.
Microsoft
At Microsoft Research in Redmond, Wash., 60 GHz wireless links are atop server racks. The links are intended to speed data flow at crunch times.
During sudden bursts of activity, bottlenecks occur as traffic moves among dense clusters of servers. Typically, the servers are stacked one on top of another in rack after rack — and are connected by switches, routers and cables.
To better handle the congestion, researchers are testing a shortcut that doesn’t involve costly rewiring. They are experimenting with wireless links, mounted atop the server racks, to supply extra bandwidth for moving data along at crunch times.
Researchers in the field, as well as data center administrators, initially were skeptical about the idea of applying wireless technologies inside data centers, which have stringent requirements for reliability and security, saysVictor Bahl, director of the mobile computing research center at Microsoft Research in Redmond, Wash. His group began trying the links to supplement wired systems three years ago.
“That skepticism is normal,” Dr. Bahl says. “Wireless links have always been considered flaky.”
Wireless performance, as anyone who has lost a cellphone connection can attest, can be affected by many things, from a door opening or a microwave oven starting to changes in the weather.
But a data center is a highly predictable environment, with controlled temperatures and infrequent movement of people and equipment. That minimizes fluctuations in the quality of wireless links. “You set up these racks of servers and walk away,” Dr. Bahl says. “The environment stays the same.”
The Microsoft team forged ahead with the project, building and testing a system with tiny directional antennas at the top of each rack to send and receive data. A central controller monitors traffic patterns, finds network bottlenecks, configures the antennas and turns on the wireless links when more bandwidth is required, says Daniel Halperin, a Ph.D. candidate at the University of Washington, who worked on the project as an intern at Microsoft. Signals go out on a horizontal plane and are steered right or left. The design sped up traffic by at least 45 percent in 95 percent of the cases tested, Mr. Halperin says.
The wireless signal that the Microsoft group uses to carry multigigabits of data per second between racks isn’t the familiar Wi-Fi of coffee-shop hot spots. That type of signal spreads out over an entire room so that many people can open their laptops and go to work.
The wireless signal for the data centers is a narrow beam of extremely high-frequency radio waves in a part of the spectrum known as the 60 gigahertz band. The band is already used in home entertainment, replacing bulky cables that connect high-definition televisions to Blu-ray players, for example. Hardware for this bandwidth, which is available worldwide for unlicensed use, is likely to drop in cost as the band gains more use, Dr. Bahl says.
At the University of California, Santa Barbara, Heather Zheng, an associate professor in the computer science department, and colleagues are also designing multigigabit wireless links to augment wired data center networks. The links in the system can travel horizontally, but they can also leave one rack, bounce off a reflective ceiling at an angle, then move to another rack.
“When we bounce signals off the ceiling, we take advantage of much more bandwidth than we could if we restricted ourselves to a horizontal plane,” says Amin Vahdat, a professor at the University of California, San Diego, who specializes in large-scale data center architecture and deployment and is working with Dr. Zheng on the project. Dr. Vahdat, who is on sabbatical leave from the university, is also a principal engineer at Google.
Craig Mathias, a principal at the Farpoint Group, an advisory firm specializing in wireless communications and mobile computing, agrees that the 60 GHz band is suitable for indoor, high-bandwidth use in information technology, but he maintains that wired systems are best for connecting servers.
The wired connections used in a data center are highly reliable, so “why introduce variability in a mission-critical situation?” he asks.
MARK THIELE, executive vice president for data center technology at Switch Communications, a Las Vegas builder and operator of data centers, says he thinks wireless technology offers a promising solution to congestion. “If it deals effectively with bottlenecks, a wireless connection could eliminate hundreds of thousands of dollars in required network connectivity,” he says.
Much work is still ahead. “You’ll have to prove its security,” he says, “and you’ll have to prove that 100 times out of 100 times” no information is lost.

E-mail: novelties@nytimes.com.

2012年3月12日星期一

Data center network traffic pattern

Empirical study by Theophilus A. Benson


pages.cs.wisc.edu/~tbenson/papers/imc192.pdf

2012年3月6日星期二

Theory of Seismic Imaging

Theory of Seismic Imaging [pdf]
http://samizdat.mines.edu/imaging/


Links To Geosciences Freeware/Shareware


2012年3月5日星期一

7 Common Interview Questions That Trip Up Job Hunters(ZZ)

7 Common Interview Questions That Trip Up Job Hunters

February 22, 2012 RSS Feed Print

No matter how long you spend rehearsing answers to all possible interview questions, it’s likely that you’ll end up having to answer some very obscure and tough interview questions a t some point in your career. Many employers ask nontraditional, creative questions at the first stage of the interview process to see how well you can think on your feet. Some can be trick questions, while others are designed to highlight some desirable characteristics for the position. Whatever the case may be, you need to be prepared to handle any type of question when you’re sitting in the hot seat.

Here are just seven of the most common interview questionsthat trip up even the most prepared job seeker:

[See 50 Ways to Improve Your Finances in 2012.]

1. If you could change one thing about your last position, what would it be?

This can be a tricky one. Very tricky. It opens the doors to sharing too much information about your previous job experience and casting your former employer (or coworkers) in a negative light. Instead of looking this as an opportunity to bash your former company, talk about some non-people problems you struggled with–not having your own office, working with outdated computers, a lack of fast internet speeds, and other challenges you faced are a few points you could discuss.

2. Tell me about yourself.

As friendly as this request sounds, don’t drop your guard just yet. Keep things professional by focusing on your career, your recent job experiences, your educational background, and anything else work-related. Remember that this is not an invitation to talk about your family, your hobbies, or your pet peeves–that discussion can wait until you actually become an employee.

[See 5 Things That Turn Employers Off During the Interview.]

3. What’s the biggest risk you’ve ever taken?

This can be a loaded question. Is this the opportune time to tell your potential boss about your recent skydiving adventure? Not exactly. Employers asking this question are looking for those who have taken calculated risks or overcome an obstacle by plowing through. They want to know if you have the grit and tenacity to handle adversity. Provide an example of a situation where you encountered a major setback and pushed yourself to explore a new direction–and succeeded.

4. Are you good at networking?

Employers typically ask this question if the position will involve a lot of “face time” with prospects and clients, require you to travel frequently, or is a sales position. You need to prove you that you are comfortable in social situations and have a knack for networking and socializing. If these aren’t your strong points, remember that they’re skills you can work on. Don’t forget to highlight your offline and online networking skills–how many professional contacts do you have on LinkedIn or even on Twitter? Do you communicate with these people often? Don’t overlook the many hidden personal networks that could provide some job-search benefits.

5. What’s your definition of success?

This is a challenging one for many a job hunter because success can be subjective. You might think that success means landing this job and working your way up in the company, but your employer wants to know why you would be successful with the company. What would you be doing on a day to day basis that would lead to success? You might talk about how you maintain a strong work ethic, work with feedback from your boss and peers, and maintain a high level of integrity at all times.

[See How to Kill a Job Offer at the Last Minute.]

6. What are your salary requirements?

The Money Talk can make many interviewees uncomfortable—especially women—so make sure you’re well-prepared for this one. Remember that the hiring manager and employer have most likely already had the money discussion and decided what the salary for this position will be; few will be in a position to negotiate beyond a given range. Your safest bet is to explain you are looking for the best fit job-wise, and not necessarily focused on the money. Let the employer make an offer first and see how it compares to your most recent position. You may be able to negotiate a higher amount if you don’t receive a reasonable offer because of your most recent salary or work experience, or by proving you can provide more than any other applicant.

7. What’s your biggest weakness?

Don’t say “I’m a perfectionist”. This is a cliché answer that your interviewer has probably heard more than a dozen times. Talk about characteristics or attributes that you are working on improving. For example, you could say that you don’t have strong public speaking or presentation skills but plan on taking a class to improve yourself in this area. Or, you don’t have strong networking skills but are planning to join (or have already joined) a local networking club or group to improve your skills in that area. Showing your employer that you are working towards improvement can help you earn some extra points with this tricky question.

Sabah Karimi is a top Yahoo contributor and a writer for WiseBread.com, where you can find answers to the most common interview questions.