Chowdhury Shahriar

PHY-Layer Resiliency in OFDM Communications: A Tutorial

This tutorial paper addresses the physical layer security concerns and resiliency of Orthogonal Frequency Division Multiplexing (OFDM) communications; the de facto air-interface of most modern wireless broadband standards including 3GPP Long Term Evolution (LTE) and WiMAX. The paper starts with a brief introduction to the OFDM waveform and then reviews the robustness of the existing OFDM waveform in the presence of noise, multipath fading, and interference. The paper then moves on to build comprehensive adversarial models against OFDM waveforms. Robustness of OFDM is first investigated under AWGN noise and noise-like jamming attack scenarios, then under uncorrelated yet colored interferences from modulated sources (both intentional and unintentional). Finally, the paper explores some of the more recent developments in the field of energy efficient correlated jamming attacks that can disrupt communication severely by exploiting the knowledge of the target waveform structure. Potential countermeasures against such jamming attacks are presented, in an attempt to make a robust and resilient OFDM waveform.

Overlapped-MIMO radar waveform design for coexistence with communication systems

This paper explores a collocated overlapped-multiple-input multiple-output (MIMO) antenna architecture and a spectrum sharing algorithm via null space projection (NSP) for radar-communications coexistence. In the overlapped-MIMO architecture, the transmit array of a collocated MIMO radar is partitioned into a number of subarrays that are allowed to overlap. Each of the antenna elements has signals orthogonal to others in the same subarray and to the other subarrays. The proposed architecture not only improves sidelobe suppression to reduce interference to communications system, but also enjoys the advantages of MIMO radar without sacrificing the desirable characteristics such as beampattern and SNR gain. The radar-centric spectrum sharing then projects the radar signal onto the null space of the communications systems interference channel to avoid interference from the radar. Numerical results are presented that show the performance of the proposed design in terms of overall beampattern and sidelobe levels of the radar waveform and finally shows a comparison of the proposed system with existing collocated MIMO radar architectures.

Performance of pilot jamming on MIMO channels with imperfect synchronization

This paper explores effects of pilot based jamming attacks with synchronization mismatches on MIMO-OFDM systems. Both MIMO and OFDM based systems employ known data called pilot tones to estimate the channel frequency response and perform equalization. Jamming pilot tones can potentially disrupt the channel estimation and equalization, thus making the communication impossible. Our earlier research focused on efficient jamming algorithms, such as pilot jamming for OFDM systems and singularity attack for MIMO systems. It has been shown that pilot based jamming is more efficient than broadband attacks on entire signal. However, pilot based jamming may suffer from time and frequency synchronization mismatches that can degrade the efficacy of attacks. This paper presents channel and equalizer model, builds on various pilot based jamming attacks and further examines the cause and effect of mismatches in the jamming attack. Through simulation we demonstrate that singularity jamming achieves 3 dB gain over barrage jamming at 0.4 bit error rate. For 25% and 50% symbol time offset, singularity jamming loses about 0.25 dB and 1 dB gain, respectively, when perfect CSI is available for the jammer. At 0.2 bit error rate, 0.2 and 0.5 normalized frequency offset, singularity jamming loses about 1 dB and 3 dB over synchronized jamming, respectively, when CSI with 20% error is available at the jammer side.

Performance impact of asynchronous off-tone jamming attacks against OFDM

This paper presents power efficient asynchronous off-tone jamming attacks on orthogonal frequency division multiplexing (OFDM). Often wireless communication systems have to operate in environments, which are prone to unknown interference from adversaries such as jamming, which can lead to degradation of performance. This paper begins with presenting existing conventional jamming attacks, such as barrage jamming (BJ), partial band jamming (PBJ), single-tone jamming (STJ), and multi-tone jamming (MTJ) on OFDM and then proposes new power efficient jamming strategies. It is known that signal with frequency offset can affect more spectrum than the occupied bandwidth as the signal energy gets smeared into adjacent spectrum while performing FFT in the OFDM receiver, and thus create inter-channel interference (ICI). This paper builds on this idea, and presents new asynchronous single off-tone and multiple off-tone jamming attacks on OFDM. We first present our channel model, and then undertake an analysis of OFDM systems under off-tone jamming attacks, verifying the idea through simulation. Both analysis and simulation show that off-tone jamming attacks have more adverse effect than jammer aligned with the received signal.

Emulated CP jamming and nulling attacks on SC-FDMA and two novel countermeasures

Single-Carrier Frequency Division Multiple Access (SC-FDMA) uses Cyclic Prefix (CP) to mitigate inter-symbol interference (ISI) and inter-channel interference (ICI). CP ensures that the convolution of the channel impulse response with the modulated symbols has the form of a circular convolution. This results in simple one-tap equalization in the receiver by removing ICI. These crucial roles played by the CP make SC-FDMA particularly vulnerable to jamming or nulling attacks through CP. These attacks are effective if the CP is disrupted before the symbol interacts with the channel. The attacks that happen to jam the CP after the symbol is already convolved with the channel, reduce to no-jamming scenarios in their effectiveness. But CP disruption ensuring the symbol is not already distorted by channel is practically infeasible. Consequently, we have designed the jammers so that they emulate the effects of CP jamming and nulling in frequency-selective static channel. We have also proposed two novel countermeasures. Simulations are performed to validate the analytical predictions. The results reflect the fact that CP attacks are particularly suitable for power-constrained jammers in high SNR regime. The newly proposed anti-jamming techniques prove to be very effective in restoring the system.

Physical-layer security challenges of DSA-enabled TD-LTE

The Time-Division mode for Long Term Evolution (TD-LTE) is viewed by many as the first waveform that will be enabled by Dynamic Spectrum Access (DSA) and see widespread adoption in both the commercial and public safety arena. The Third Generation Partnership Project (3GPP) is developing standards for DSA-enabling TD-LTE. In this paper we take an initial look at the Evolved Universal Terrestrial Radio Access Network (EUTRAN), the air interface for TD-LTE, and seek to identify how a DSA-enabled version of TD-LTE could be efficiently jammed by an adversary. Robust communications in the face of hostile jammers will be a necessary component as DSA-TD-LTE is being to be deployed by public safety users. We focus on physical-layer features such as the OFDM and MIMO, and how DSA could potentially be implemented. Then we lay out a roadmap for future research and development that ensures that the waveform can be ruggedized in an interoperable way.

An Anti-jam Communications Technique via Spatial Hiding Precoding

This work describes a new method of designing an anti-jam communications system. The key insight is that many of the concepts learned from interference alignment (IA) in multiple antenna (MIMO) systems can be applied to the field of antijam and robust communications in the presence of intentional jammers. It provides a new fundamental technique of hiding a signal, similar to the mature techniques of frequency hopping or direct sequence spread spectrum. This paper considers a realistic jamming scenario and provides a precoding technique that allows a pair of two-antenna transceivers to communicate while being jammed by a malicious single-antenna node. The precoder uses a small amount of channel information to shape the signal to appear orthogonal to the jammer at the intended receiver. The novelty introduced in this paper is both the application of interference alignment (IA) to anti-jamming communications and a specific method that allows these IA techniques to be practically employed.

Performance Impact of Imperfect CSI on Spatial Hiding Anti-jam Communications

This paper explores the effect of imperfect channel state information (CSI) on spatial hiding anti-jam (AJ) communications technique. Recently a fundamental technique of hiding a signal is proposed that provides a precoding technique that allows a pair of two-antenna transceivers to communicate while being jammed intentionally by a malicious single-antenna node. The precoder uses a small amount of CSI to shape the signal to appear orthogonal to the jammer at the intended receiver. However, performance of the scheme depends on the quality of two estimated parameters -- transceiver and jamming CSI. In practice, CSI estimation is always subject to error due to the presence of noise, statistical characteristics of wireless channel, and limitations of hardware. By modeling the CSI estimation error as independent complex Gaussian random variables, analytical model for post-processing residual interference is derived in closed-form for both transceiver and jamming CSI estimation error. In addition, a lower bound for jammers interference as a function of jamming CSI estimation error is derived. Through simulation we demonstrate the effect of imperfect CSI on spatial hiding anti-jam and validate the analytical model as well.