## Archive for the ‘**Research**’ Category

## Unit circle MVDR beamformer for ICASSP 2015

I recently traveled Brisbane, Australia to attend the 41st ICASSP 2015 where I presented the poster on the unit circle MVDR beamformer. The accompanying paper should be available on IEEExplore soon. For now, I have made the pre-print available.

My submission for the upcoming ICASSP 2015 has accepted for presentation (Yay!). Following is the abstract of the accepted paper

The array polynomial is the z-transform of the array weights for a narrowband planewave beamformer using a uniform linear array (ULA). Evaluating the array polynomial on the unit circle in the complex plane yields the beampattern. The locations of the polynomial zeros on the unit circle indicate the nulls of the beampattern. For planewave signals measured with a ULA, the locations of the ensemble MVDR polynomial zeros are constrained on the unit circle. However, sample matrix inversion (SMI) MVDR polynomial zeros generally do not fall on the unit circle. The proposed unit circle MVDR (UC MVDR) projects the zeros of the SMI MVDR polynomial radially on the unit circle. This satisfies the constraint on the zeros of ensemble MVDR polynomial. Numerical simulations show that the UC MVDR beamformer suppresses interferers better than the SMI MVDR and the diagonal loaded MVDR beamformer and also improves the white noise gain (WNG).

## Phase of product of complex-circular Gaussian random variables

What is the distribution of the phase of the product of two independent zero-mean complex-circular Gaussian random variables? I try to answer the question in this IPython notebook.

## Poster session at IEEE/SSP12

My work was presented at the recently concluded IEEE SSP2012 (Michigan). It was a poster titled **Approximate Eigenvalue Distribution of a Cylindrically Isotropic Noise Sample Covariance Matrix**. This work was done in collaboration with my adviser Prof. John R. Buck and Prof. Kathleen E. Wage from GMU.

Unfortunately, due to my internship commitments I wasn’t able to attend the actual conference. My adviser presented the poster on my behalf.

## Exploring USRP N210 on Ubuntu

We acquired a USRP N210 unit from Ettus Research. It was planned to explore MIMO and Radar concepts by implementing simple algorithms on the USRP device. However when we got the device after few months of its release and back then the USRP was still plagued with multiple bugs ( including one in the firmware ). It was no easy task to setup working environment on an Ubuntu system. We came across several problems during installation. I found only one blog detailing the step by step installation ( and workaround ) then http://www.raullen.net/2011/02/20/hello-usrp-n210-how-to-make-usrp-n210-running/. Even with the help from this blog, I was unable to setup the working environment in my Ubuntu machine and unfortunately I had to abandon the project then.

A year later, I have decided I want to get the USRP up and running so I can do some cool stuff besides some abstract mathematics. I set out to install the device on a new system ( Ubuntu 10.04 LTS ). And it turns out that Ettus has done a good job of providing a much more detailed documentation on setting up the N210. Here I have made a list of topic and related links that a newbie may encounter when starting with the N210 USRP or in general N-series USRP from Ettus.

**The safest / easiest way to setup Gnuradio with UHD environment on Ubuntu is to use the build-gnuradio script:**

**N210 has issues with the pre-installed firmware and the FPGA code and hence it needs to be updated before the PC can talk with it. The firmware and FPGA images can be downloaded from here **

## Front panel LEDs

The LEDs on the front panel can be useful in debugging hardware and software issues. The LEDs reveal the following about the state of the device:

**LED A:**transmitting**LED B:**mimo cable link**LED C:**receiving**LED D:**firmware loaded**LED E:**reference lock**LED F:**CPLD loaded

## UASP 2011

I am just back from the Underwater Acoustic Signal Processing Workshop 2011 held at W. Alton Jones Campus, University of Rhode Island. It was a three day conference from 12th – 14th October, 2011.

The chairman of the conference was my advisor Dr. John R. Buck. My lab partner David Hague gave a talk on his Compressed Sensing based active SONAR model inspired by bat’s biosonar capability.

Here are my reflections on the conference:

- The conference began with a reception banquet dinner where G. Clifford Carter was awarded the UASP Award. Apparently it turns out that G. C. Carter invented the Generalized Cross Correlation (GCC) method for time delay estimation. The acronym for this method matches the initials of Carter’s name.
- Although the conference was on underwater signal processing, there were three plenary sessions on underwater autonomy which mainly dealt with robotics and control systems oriented design problems for underwater deployment of autonomous vehicles. I was a bit disappointed to see very less of signal processing. However where were one session each on Array processing , Noise Modeling and Acoustics Communications which were in the ball park of my interest.
- The navy seems to have a huge interest in developing unmanned underwater autonomous vehicles and there a lots of companies and academic laboratories working on this area. I am not particularly interested on the navy’s perspective on this, but as far as I understand the systems development has largely shifted towards being software based design.
- Large fraction of presentations were focused on military (navy) applications or the signal processing problems they were trying to solve were from military applications point of view. The focus on military applications was a bit too much for my liking.
- Certainly there are some civilian applications of the results from these research.
- There were few presentations on Synthetic Aperture SONAR (SAS) and I came to an understanding that synthetic aperture is analogous to taking multiple photographs and stitching them together to form a panorama.
- There was a presentation by Aurther Baggeror on why MFP failed. My perception was that no body was sure why this particular method failed, but they already knew it had died.
- Interesting discussion on Coherence, brought up by Henry Cox.
- It was satisfying to see a large fraction of presentations using real field data for validation of their results. In computer simulations everything works :D.

## Optimum array paper submitted to JASA [Published]

Last Friday I finally submitted the revised paper to JASA. During Spring 2011, I and my adviser wrote a journal paper to publish the findings of my MS research on optimum sensor array design. The first manuscript of the paper was submitted to JASA in May 2011. The paper was partially accepted with few comments from the reviewer.

We were able to reply to all the comments but one. I had an intuitive feel to the solution but did not have a rigorous proof to back my intuition. My adviser always says we should follow our intuition and back it up with a rigorous proof. And over the past week I eventually came up with a mathematical proof and we were able to compose a solid response for the revised submission of the paper.

The paper has been published in JASA Vol 130, Issue 5. The permlink to the paper is http://dx.doi.org/10.1121/1.3644914