We discuss the threat that malicious circuitry (a.k.a. hardware Trojan) poses in wireless communications and propose a remedy for mitigating the risk. First, we present and theoretically analyze a stealthy hardware Trojan embedded in the forward error correction (FEC) block of an 802.11a/g transceiver. FEC seeks to shield the transmitted signal against noise and other imperfections. This capability, however, may be exploited by a hardware Trojan to establish a covert communication channel with a knowledgeable rogue receiver. At the same time, the unsuspecting legitimate receiver continues to correctly recover the original message, despite experiencing a slight reduction in signal-to-noise ratio (SNR) and, therefore, remains oblivious to the attack. Next, we implement this hardware Trojan on an experimental setup based on the Wireless Open Access Research Platform (WARP) and we demonstrate (i) attack robustness, i.e., the ability of the rogue receiver to correctly receive the leaked information and (ii) attack inconspicuousness, i.e., imperceptible impact on the legitimate transmission. Lastly, we theoretically analyze and experimentally evaluate a Trojan-agnostic detection mechanism, namely, channel noise profiling, which monitors the noise distribution to identify inconsistencies caused by hardware Trojans, regardless of their implementation details. The effectiveness of channel noise profiling is experimentally assessed using the proposed hardware Trojan under various channel conditions and a different covert Wi-Fi attack previously proposed in the literature.

We investigate coded modulation for full-duplex relay channels, proposing and analyzing a multilevel coding (MLC) framework with capacity approaching performance and practical features. Sufficient conditions are derived under which multilevel coding meets the known achievable rates for decode-and-forward relaying. The effect of a bit additive superposition and the linearity of multilevel code components on the performance of the system are studied. It is shown that linearity of the relay component codes imposes no penalty on rate, however, the linearity of the source-to-relay component codes may impose a performance penalty especially for small modulation constellations. We show that this rate loss occurs because a linearity constraint on codebooks at the source node introduces a new tension between optimality of rate allocation in multilevel coding layers and optimality of source/relay codebook correlations. Motivated by this insight, an alternative modulation labeling is proposed that minimizes the rate loss. The results are extended to multi-antenna relays. Slow fading and fast Rayleigh fading without channel state at the transmitter are also analyzed. The error exponent of the proposed scheme is studied. Finally, the frame- and bit-error rate performance of the proposed scheme is studied via simulations using point-to-point LDPC codes, showing that the proposed MLC relaying has excellent performance.

We propose a defense method capable of detecting hardware Trojans (HTs) in the analog/RF circuitry of wireless local area network (WLAN) transceivers. The proposed method, which is implemented on the receiver (RX) side and cannot be tampered with by the attacker, leverages the channel estimation capabilities present in Orthogonal Frequency Division Multiplexing (OFDM) systems. Specifically, it employs an adaptive approach to robustly isolate possible HT activity from channel and device noise, thereby exposing the Trojan's presence. The adaptive channel estimation (ACE) defense mechanism is put to the test using a HT which is implemented on a printed circuit board (PCB) and mounted on the Wireless Open-Access Research Platform (WARP). This HT, which is introduced through minute modifications in the power amplifier (PA), manipulates the transmission power characteristics of an 802.11a/g transmitter (TX) in order to leak sensitive data, such as the encryption key. Effectiveness of the proposed defense has been verified through experiments conducted in actual channel conditions, namely over-the-air and in the presence of interference.

Unlike the half-duplex relay, the performance of the full-duplex relay is highly sensitive to the correlation between the source and relay codebooks. Linear coding complicates the design of correlated codebooks, for example in multilevel coding (MLC) linear codes at each layer can only have correlation zero or one, leading to a performance penalty that has been characterized in earlier work. In this paper, we propose a new design technique that significantly reduces the correlation penalty of linear codes via intelligent labeling for the modulation. The basic idea is as follows: the chain rule for mutual information, which is the backbone of MLC, is not-unique in two ways: the labeling of modulation constellation as well as the ordering of the chain rule. Our optimization at each level pushes the mutual information terms involving new information (for the relay) or beamforming information to zero or one. In effect this finds a suitable decomposition of overall correlation to a set of binary correlations at individual levels of MLC. Simulations show that point-to-point LDPC codes in combination with the proposed correlation design lead to excellent performance.

Even though there exists a lot of different transmission techniques, the capacity of the general relay channel is still unknown. This paper shows that combining decode-and-forward (DF) with compress-and-forward (CF) can have a performance advantage over each of the two techniques individually. Combining the two techniques goes back to the celebrated work of Cover and El-Gamal [1], more specifically, they gave an achievable rate for the DF-CF combination. In this paper, we re-derive the achievable rate of combining DF with CF in the full-duplex discrete memoryless relay channel. We derive the achievable rate in the AWGN relay channel as well as the constrained constellation AWGN relay channel. We show that even though in the AWGN channel, DF-CF combination does not provide any advantage, under constrained constellation, combining the two techniques has an advantage.

The capacity of the AWGN broadcast channel is achieved by superposition coding, but the superposition of individual coded modulations expands the modulation alphabet and distorts its configuration. Hierarchical modulations, which appear in the literature mostly in the context of unequal error protection, can approach only a few isolated points on the boundary of the broadcast capacity region. This paper studies multilevel coding (MLC) for constellation-constrained broadcast-coded modulation. The conditions under which multilevel codes can achieve the constellation-constrained capacity of the AWGN broadcast channel are derived. We propose a pragmatic multilevel design technique with near-constellation-constrained-capacity performance where the coupling of the superposition inner and outer codes is localized to each bit-level. It is shown that this can be further relaxed to a code coupling on only one bit-level, with a little or no penalty under natural labeling. The rate allocation problem between the bit levels of the two users is studied and a pragmatic method is proposed, again with near-capacity performance. In further pursuit of lower complexity, a hybrid MLC-bit-interleaved-coded modulation is proposed, whose performance is shown to be very close to the boundary of the constellation-constrained capacity region. Simulation results show that good point-to-point LDPC codes produce excellent performance in the proposed coded modulation framework.

The most popular methods for the analysis of LDPC codes, e.g. density evolution and EXIT charts, apply to infinite length ensembles. Finite length analysis is less common, among the notable examples are scaling law analysis and the EXIT band chart. The scaling law is limited to the waterfall region and furthermore is limited to the final error rates and does not give insight into the progression and dynamics of belief propagation decoding. The EXIT band chart was based on empirical considerations and does not enjoy a strong underlying theory. The developments of this paper are motivated by the recent results by Polyanskiy, Poor and Verdu showing that the rate penalty due to finite blocklength depends on the channel dispersion. We propose tracking the dispersion between the transmitted bits and the log-likelihood ratio across the decoder iterations to capture the effect of the codeword length. Based on the proposed analysis, a simple lower bound on the block error probability as a function of the blocklength is derived. The bound is corroborated by simulation.

The capacity of the AWGN broadcast is achieved by superposition coding, but superposition of individual coded modulations does not in general obey a predefined constellation. We propose a coded modulation broadcast with strict channel input modulation constraint via multilevel coding (MLC). For a degraded broadcast channel, conditions of optimality of the multilevel decomposition of a superposition code are derived. Furthermore, a constructive scheme is proposed that simplifies the superposition multilevel code and facilitates the design process, by reducing the inter-dependencies between the levels of the two component codes, and yet achieves rates very close to the constellation constrained capacity. Simulation results on several modulations verify the operation of the proposed coding scheme at or near the constellation constrained capacity.

For the Gaussian broadcast channel, transmission at or near the capacity subject to a specific modulation constraint, has been an essentially open problem. The capacity of the AWGN broadcast is achieved by superposition coding, but superposition of individual coded modulations does not in general obey a predefined constellation. We approach this coded modulation problem with multilevel coding (MLC) which is known to achieve the constellation constrained capacity in the point-to-point channel. In channels involving multiple transmitters or receivers, the application of multilevel coding has received little attention. In this paper, a method is proposed that decomposes superposition coding into binary multilevel superposition coding. The problem of rate allocation through every level for each user is studied and a pragmatic rate allocation procedure is presented. The problem of rate allocation to levels is studied and a pragmatic method for its solution is proposed. The coded modulation with proposed rate allocation achieves rates that are very close to the constellation constrained capacity.

This paper proposes the extrinsic information transfer (EXIT) method for the analysis of belief propagation in community detection on random graphs, specifically under the degree correlated stochastic block model. Belief propagation in community detection has been studied under density evolution; this work for the first time brings EXIT analysis to community detection on random graphs, which has certain advantages that are well documented in the parallel context of error control coding. We show using simulations that in the case of equally-sized communities, when the probability of connectivity in the communities are different, there is only one intersection point on the EXIT curves, hence belief propagation is optimal. When the probability of connectivity in the communities are the same, we show that belief propagation is equivalent to random guessing and the EXIT curves intersect at the trivial zero-zero point. For the roughly equal-sized communities, we show that there is always only one intersection point on the EXIT curves, suggesting that belief propagation is optimal. Finally, for the communities with disparate size, we show that there are multiple intersection points, hence belief propagation is likely to be sub-optimal.

The overwhelming majority of the large literature on coding for the relay channel has been dedicated to the low-SNR regime (binary signaling) and the half- duplex scenario. For non-binary full-duplex signaling, important challenges remain in the simultaneous design of coded modulation at the source and relay. Part of the difficulty is to design the codes with the requisite correlation prescribed by theory. This paper proposes a systematic code design methodology for the relay channel via multilevel coding (MLC). The main difficulty was that a straightforward extension of the chain rule MLC decomposition, as was previously used in the point to-point MLC, would result in many dependencies between the coded layers of the source and relay. A key contribution of this paper is showing that MLC joint source/relay code design can be implemented via standard binary code design procedures. Specifically, we outline a design process where the correlation between the source and relay codes are implemented purely via pairwise correlations between the binary codes of source and relay at each layer, without any other inter-layer correlations. Moreover, a specific design example is provided via a simple XOR operation on two conventionally designed codes. Numerical results show excellent performance for the proposed scheme.

It is known that multi-level coding (MLC) with multi-stage decoding (MSD) approaches the point-to-point constellation constrained channel capacity subject to appropriate choice of code rates at each level. MLC/MSD in the context of multi-node networks has not been previously investigated, and is the subject of this paper. It is shown that MLC/MSD can approach the MAC capacity. Issues related to achieving non-corner points of the capacity region are discussed. A MLC-based superposition scheme for the degraded broadcast channel is discussed and an achievable region outlined. The half-duplex orthogonal degraded relay channel is studied under MLC/MSD. The achievable rate is calculated under two conditions: First we consider end-to-end layer-by-layer decoding, where each layer is decoded at the relay and destination only with help from previous layers. Then, we remove this constraint via a parity-forwarding approach at the relay, where the destination performs two multi-stage decoding operations: one corresponding to the relay-destination link and then one corresponding to the source-destination link. The achievable rate under parity forwarding is shown to be superior.

In this paper, a platform for measuring the impulse response of wireless multiple-input-multiple-output (MIMO) vehicle-to-vehicle (V2V) channels is presented. The platform is developed using Rice University Wireless Open-Access Research Platform (WARP) boards and utilized to perform an experimental study of V2V channels through conducting field measurements. A full characterization of MIMO-V2V channels is given via extracting the channel parameters from the acquired measurements. These parameters include the power-delay profile, the Doppler spectrum, and the MIMO transmit and receive correlation matrices of the channel. Numerical simulations are utilized to examine the effect of the channel parameters on the bit error rate (BER) performance of an orthogonal frequency division multiplexing (OFDM) based system.

Space frequency block coding with orthogonal frequency division multiplexing (SFBC-OFDM) suffers from the effect of inter-carrier interference (ICI) in doubly-selective communication channels. In this paper, a scheme is proposed in which windowing is applied to the received signal to reduce the effect of ICI to a limited number of neighboring sub-carriers. The sub-carriers holding the SFBC components of each codeword are separated by a number of sub-carriers larger than the ICI range, and hence, they do not interfere with each other. In order to preserve the structure of the SFBC, the separation between the codeword components is also selected within the coherence bandwidth of the channel. As a result, the diversity gain of the SFBC is preserved. A decision feedback equalizer is proposed to estimate the transmitted data symbols with low complexity. Simulation results are presented showing the ability of the proposed scheme to significantly improve the performance of SFBC-OFDM and preserve its diversity gain.