Qasim Chaudhari (@qasim_chaudhari)

My main area of interest is signal processing for wireless communication systems. I write about it at

There and Back Again: Time of Flight Ranging between Two Wireless Nodes

Qasim Chaudhari October 23, 20175 comments

With the growth in the Internet of Things (IoT) products, the number of applications requiring an estimate of range between two wireless nodes in indoor channels is growing very quickly as well. Therefore, localization is becoming a red hot market today and will remain so in the coming years.

One question that is perplexing is that many companies now a days are offering cm level accurate solutions using RF signals. The conventional wireless nodes usually implement synchronization...

A Beginner's Guide to OFDM

Qasim Chaudhari May 1, 20176 comments

In the recent past, high data rate wireless communications is often considered synonymous to an Orthogonal Frequency Division Multiplexing (OFDM) system. OFDM is a special case of multi-carrier communication as opposed to a conventional single-carrier system. 

The concepts on which OFDM is based are so simple that almost everyone in the wireless community is a technical expert in this subject. However, I have always felt an absence of a really simple guide on how OFDM works which can...

Minimum Shift Keying (MSK) - A Tutorial

Qasim Chaudhari January 25, 201717 comments

Minimum Shift Keying (MSK) is one of the most spectrally efficient modulation schemes available. Due to its constant envelope, it is resilient to non-linear distortion and was therefore chosen as the modulation technique for the GSM cell phone standard.

MSK is a special case of Continuous-Phase Frequency Shift Keying (CPFSK) which is a special case of a general class of modulation schemes known as Continuous-Phase Modulation (CPM). It is worth noting that CPM (and hence CPFSK) is a...

Some Thoughts on Sampling

Qasim Chaudhari November 15, 20162 comments

Some time ago, I came across an interesting problem. In the explanation of sampling process, a representation of impulse sampling shown in Figure 1 below is illustrated in almost every textbook on DSP and communications. The question is: how is it possible that during sampling, the frequency axis gets scaled by $1/T_s$ -- a very large number? For an ADC operating at 10 MHz for example, the amplitude of the desired spectrum and spectral replicas is $10^7$! I thought that there must be...

For this, you have to differentiate between the mechanisms of positive/negative band edge filters versus even/odd band edge filters. If you consider even and odd...
That is correct but the frequency domain derivative is discontinuous at the edge and taking an iDFT would produce an impracticably long impulse response.

Re: Phase Locked Loop Books in a time of DSP

Reply posted 4 years ago (03/09/2019)
Hi Tim,   Here is one paper which can help you.Simultaneous clock phase and frequency offset estimation, K.E. Scott ; E.B. OlaszIEEE Trans on Comms. 1995   The...

Re: SC FDMA Channel Equalization

Reply posted 4 years ago (03/07/2019)
Remember that the channel here is being estimated in frequency domain with only a few pilot subcarriers, leaving room for getting the intermediate results from interpolation....
Hi Michael  In a few steps, it is derived as follows.1. Maximum likelihood implies taking the derivative w.r.t. the CFO (Carrier Frequency Offset).2. Through some...

Re: A Sinusoid with Missing Periods

Reply posted 5 years ago (02/15/2018)
Great suggestions all. I will work on these pointers and see how it goes.

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