Channel Estimation

Transmission medium, or channel, is one of the major components in a digital communications system.  The transmitted signal must pass through channel reach the receiver.  Channel is also where where the impairments to signal including, distortions and Additive noise and interference.  To achieve best possible performance, it is necessary for a communications to have the knowledge of the channel characteristics as precise as possible.  Thus, one important task of receiver design and implementation is to accurately estimate various parameters of the communication channel.  It is not exaggerating to say that accurate estimation of channel characteristics is the key to combat the introduced impairments in order to achieve best possible system performance.

In a wireless communication system, channel estimation is even more important.  It is because due to the nature of the wireless communication channel, it is always time varying.  It’s characteristics can change rapidly.  Thus, it is impossible to track the magnitude and phase of the channel by using classical means used in wireline communication systems, e.g., phase locked loop.  In that sense, channel estimation is also an integrated part of synchronization.

Recently, using self-interference cancellation to achieve full-duplex communication in wireless systems has attracted attention in the wireless research community and industry.  The level of achievable cancellation of the linear component of the interference is totally determined by the accuracy of the interference channel estimation.  Thus, it is essential to understand the limit of the channel estimation in the design and implementation of this technology.

In this presentation, these aspects of channel estimation and their theoretical foundations are discussed.

The presentation can be viewed or downloaded from here.

Self interference cancellation for wireless communications

In past a few years, the concept of transmitting and receiving data in the same band at the same time has attracted attention to the engineers and researchers in the wireless communication area.  While the idea of cancelling interference from the Tx signal of the same unit is relatively new in the wireless communication field, it is one of the key technologies that has been known, implemented and successfully commercialized in wireline communication area for over 35 years.

Considering the similarity between the echo cancellation for wireline and self-interference cancellation for wireless, I feel it may be worthwhile to share my understanding and experiences gained from my work on research and development of echo cancellation technology in mid-80s.   Thus, I wrote a paper to compare these two technologies for the Information Theory and Applications Workshop sponsored by UCSD held in February 2014.

This paper provides overviews on the these two technologies and discussed their commonalities and differences.  On one hand, the echo and self-interference cancellers are essentially the same in principle.  In the paper, the common factors that impacts echo/self-interference cancellers’ performance are analyzed.  It is shown that the main factors limiting its performance are the excess MSE, echo-channel estimation error, and various nonlinear effects. By establishing a linearized model of the channel estimator, we derive expressions of the errors at the echo canceller output as functions of the estimator’s time constant, the bandwidth of channel variation, the receiver SNR and the echo level.  The optimal adaptation time constant are considered.  The analysis shows that the channel fading may limit the achievable cancellation, and cannot be ignored even if the channel fading frequencies are low.

On the other hand, there are differences between them.  The non-linearity is usually more severe for wireless transmitter and special care should be taken to obtain better performance.  Moreover, for wireless systems with high channel attenuation, it would be difficult to reduce the self-interference to necessary level for achieving reliable communication.  On the positive side, better isolation between the transmitted and received signals may be easier to achieve for wireless communications if separate Tx and Rx antennas can be used in the same unit.

The in-band full-duplex wireless communications may be useful for certain applications.  For example, it is most suitable for applications when the channel attenuation is not too high and better Tx/Rx signal isolation can be achieved, e.g., for point to point systems and/or on the base station side.  It will be a challenge for its applications in systems with high propagation loss and in complex environments, such as cellular communication systems.  In any case, it will be a stretch to interpret it as a universal solution to double spectrum efficiency of wireless communications, in my opinion.

My experiences and expertise are in the echo cancellation modem area.  As for the wireless self-interference cancellation, what stated in the paper are from reading online publications and discussions with other people working in the area.  Thus, my knowledge may not be complete, especially for the later.  Moreover, since this paper is not peer reviewed, it  and, may contain errors and mistakes.  Your feedback, comments and critiques will be greatly appreciated.

The copy of the conference paper can be accessed from here.

Mini Series on Synchronizatoin

Synchronization is one of the most important area in applied digital communications.  Design and development engineers probably spent more time in the final stages of communication system and modem development on synchronization than in others.  This is because synchronization subsystems has most close interaction with the physical conditions and thus need dealing with very complex environment.  However, on the other hand, at least in my opinions, synchronization has less coverage in text books then others.  Engineers working in this area need to search for various books and papers to find the useful information.

The objective of this mini-series of presentations is to summarizing my experience obtained from working in the synchronization area during the past 30 years.  Hopefully, it can serve as an introduction for new communication system engineers to speed up their learning curve.  I also hope these presentations will also be helpful to more experienced communication system engineers in the areas that they didn’t have chance to get into.

The mini-series consists of the presentations:  Initial Acquisition, Frequency Synchronization and Phase-Locked Loops, and Timing Synchronization.  The slides can be downloaded from the following links: Initial Acquisition, Frequency Synchronization and Timing Synchronization.

Tutorial on MIMO

Fundamentals of MIMO Technology and Spatial Multiplexing

Fuyun Ling

MIMO Technology has attracted a lot attention in the digital communication field during the past 15 years.  It is because MIMO could greatly increase the spectrum efficiency of digital transmission under right circumstances as the other technologies cannot.  In this presentation, we will first give a brief review of the history of MIMO.  Its distinctive feature, i.e., realization of Spatial Multiplexing, will be presented.  Then we will discuss how it increases the spectrum efficiency, under what conditions and when it cannot.  The examples of MIMO technologies used in practical systems, including LTE and WIFI will be presented.  The emphasis of the talk will be on the realization and practical implementation aspects of MIMO technology in transmitters and receivers. 

Even though the terminology MIMO has been used in very broad sense, this talk will focus on its distinctive characteristics: realization of spatial multiplexing in a single user MIMO system.  Other aspects of the MIMO technology, such as multi-user communication and achieving diversity with MIMO technology will also be briefly mentioned.

Click here to download the presentation

Tutorial on OFDM

Fundamentals of OFDM Communication Technology

Abstract

OFDM is probably one of the most talked about technologies in digital communications during the recent years. It is a kind of multi-carrier communication technique and has advantages over single carrier communication techniques in given communication environments. On the other hand, it also has shortcomings in other cases. In order to select the right communication technology for a given application, it is important to understand the advantages and disadvantages for each communication techniques.

In this tutorial, we will present and discuss the basic aspects of this technology and its comparison with signal carrier communications.  In the tutorial, we first review the history of the development of the OFDM technology. The main advantages of the OFDM technology comparing to its single carrier counter parts will be summarized. The basics of most popularly form of OFDM is then presented in detail. Various component of the OFDM waveform, such as cyclic prefix and guard-carriers, and their functions are then presented.  The practical considerations of transmitter implementations to generate the OFDM waveforms are also discussed.

The second part of the tutorial is focused on impact of channel impairments to the performance of an OFDM communications system. These conditions include multipath channel, frequency offset and Doppler effects, sampling frequency offset and etc. Based on the analytical results presented, we discuss the design parameter selection criteria for the optimization of an OFDM system based on given channel conditions.

In the final part of the tutorial, we present the theory and practical considerations of the key receiver components including channel estimation, carrier frequency offset compensation, optimal FFT window placement (timing) and initial timing and frequency acquisition.

Click here to download the presentation

Introduction

This is my blog page on Topics in Digital Communications and other subjects that I am interested.  The main purpose of this blog is to establish a convenient reference point for the slide decks and other notes that I created for myself and others.

Based on my work in the digital communications area for over 30 years, I feel that I have accumulated quite some experience and knowledge in this area that may be useful for others.  I would like to point out that these presentations and notes are not peer reviewed and are very likely to contain mistakes.  Please feel free to point out if there’s anything you feel are incorrect and/or need improvement.  However, you will be responsible for any action that you are taking by using the materials that you read from my publications.  You can distribute the materials referenced here freely without alteration.  Hope you will feel them helpful.

Your Truly

Fuyun Ling (aka. Twinclouds)