We consider the Gaussian wiretap channel with amplitude and variance constraints on the channel input. We first show that the entire rate-equivocation region of the Gaussian wiretap channel with an amplitude constraint is obtained by discrete input distributions with finite support. We prove this result by considering the existing single-letter description of the rate-equivocation region, and showing that discrete distributions with finite support exhaust this region. Our result highlights an important difference between the peak power (amplitude) constrained and the average power (variance) constrained cases. Although, in the average power constrained case, both the secrecy capacity and the capacity can be achieved simultaneously, our results show that in the peak power constrained case, in general, there is a tradeoff between the secrecy capacity and the capacity, in the sense that, both may not be achieved simultaneously. We also show that under sufficiently small amplitude constraints the possible tradeoff between the secrecy capacity and the capacity does not exist and they are both achieved by the symmetric binary distribution. Finally, we prove the optimality of discrete input distributions in the presence of an additional variance constraint.

We study the vector Gaussian CEO problem, where there are arbitrary number of agents, each having a noisy observation of a vector Gaussian source. The goal of the agents is to describe the source to a central unit, which wants to reconstruct the source within a given distortion. The rate-distortion region of the vector Gaussian CEO problem is unknown in general. Here, we provide an outer bound for the rate-distortion region of the vector Gaussian CEO problem. We obtain our outer bound by evaluating an outer bound for the multiterminal source coding problem by means of a technique relying on the de Bruijn identity and properties of the Fisher information. Next, we investigate the tightness of our outer bound. Although our outer bound is tight for certain cases, we show that our outer bound does not provide the exact rate-distortion region in general. To this end, we provide an example and show that the rate-distortion region is strictly contained in our outer bound for this example.

We study the multi-receiver wiretap channel (MR-WC) with public and confidential messages. In this channel, there is a transmitter that wishes to communicate with two legitimate users in the presence of an external eavesdropper. The transmitter sends a pair of public and confidential messages to each legitimate user. While there are no secrecy constraints on the public messages, confidential messages need to be transmitted in perfect secrecy. We study the discrete memoryless MR-WC as well as its Gaussian multi-input multi-output (MIMO) counterpart. First, we propose an inner bound for the general, not necessarily degraded, discrete memoryless MR-WC by using Marton's inner bound and rate splitting in conjunction with superposition coding and binning. Second, we specialize this inner bound for the degraded discrete memoryless case. This specialized form of the inner bound can be obtained by using superposition coding and binning only. Next, we obtain an outer bound for the capacity region of the degraded channel, which matches the inner bound for some special cases. Third, we consider the degraded Gaussian MIMO channel, and show that, to evaluate both the inner and outer bounds, considering only jointly Gaussian auxiliary random variables and channel input is sufficient. Similar to the discrete memoryless case, for the Gaussian MIMO case as well, these bounds match for some special cases.

We study the secure lossy transmission of a vector Gaussian source to a legitimate user in the presence of an eavesdropper, where both the legitimate user and the eavesdropper have vector Gaussian side information. The aim of the transmitter is to describe the source to the legitimate user in a way that the legitimate user can reconstruct the source within a certain distortion level while the eavesdropper is kept ignorant of the source as much as possible as measured by the equivocation. We obtain an outer bound for the rate, equivocation and distortion region of this secure lossy transmission problem. This outer bound is tight when the transmission rate constraint is removed. In other words, we obtain the maximum equivocation at the eavesdropper when the legitimate user needs to reconstruct the source within a fixed distortion level while there is no constraint on the transmission rate. This characterization of the maximum equivocation involves two auxiliary random variables. We show that a nontrivial selection for both random variables may be necessary in general. The necessity of two auxiliary random variables also implies that, in general, Wyner-Ziv coding is suboptimal in the presence of an eavesdropper. In addition, we show that, even when there is no rate constraint on the legitimate link, uncoded transmission (deterministic or stochastic) is suboptimal; the presence of an eavesdropper necessitates the use of a coded scheme to attain the maximum equivocation.

We study the vector Gaussian CEO problem, and provide an outer bound for its rate-distortion region. We obtain our outer bound by evaluating an outer bound for the multiterminal source coding problem by means of a technique relying on the de Bruijn identity and the properties of the Fisher information. Next, we address the tightness of our outer bound, and show that our outer bound does not provide the ratedistortion region in general. In particular, we provide a specific example where the rate-distortion region is strictly contained in our outer bound.

We study the Gaussian wiretap channel with an energy harvesting transmitter which does not have a battery to save energy. In the absence of a battery, the necessary transmission energy is maintained by an i.i.d. energy arrival process. We observe that this channel is an instance of the state-dependent wiretap channel with state available only to the transmitter causally, where the state is the available energy at the transmitter. We prove that the entire capacity-equivocation region can be obtained by single-letter Shannon strategies and its boundary is achieved by input distributions with support set of Lebesgue measure zero.

We consider the Gaussian wiretap channel with an amplitude constraint, i.e., a peak power constraint, on the channel input. We show that the entire rate-equivocation region of the Gaussian wiretap channel with an amplitude constraint is obtained by discrete input distributions with finite support. We prove this result by considering the existing single-letter description of the rate-equivocation region, and showing that discrete distributions with finite support exhaust this region. Our result highlights an important difference between the peak power constraint and the average power constraint cases: Although, in the average power constraint case, both the secrecy capacity and the capacity can be achieved simultaneously, our results show that in the peak power constraint case, in general, there is a tradeoff between the secrecy capacity and the capacity, in the sense that, both may not be achieved simultaneously.

We provide an alternative proof for the capacity region of the degraded Gaussian multiple-input multiple-output (MIMO) broadcast channel. Our proof does not use the channel enhancement technique as opposed to the original proof of Weingertan and the alternative proof of Liu et al. Our proof starts with the single-letter description of the capacity region of the degraded broadcast channel, and by using it, obtains a tight (i.e., achievable) outer bound for the capacity region of the degraded Gaussian MIMO broadcast channel, by using two main technical tools. The first one is the generalized de Bruijn identity due to Palomar which provides a connection between the differential entropy and the Fisher information matrix. The second tool we use is an inequality due to Dembo which lower bounds the differential entropy in terms of the Fisher information matrix.

We study the two-user Gaussian multiple-input multiple-output (MIMO) broadcast channel with common and confidential messages. In this channel, the transmitter sends a common message to both users, and a confidential message to each user which needs to be kept perfectly secret from the other user. We obtain the entire capacity region of this channel. We also explore the connections between the capacity region we obtain for the Gaussian MIMO broadcast channel with common and confidential messages and the capacity region of its nonconfidential counterpart, i.e., the Gaussian MIMO broadcast channel with common and private messages, which is not known completely.

We study the degraded compound multi-receiver wiretap channel (DCMRWC). DCMRWC consists of two groups of users and a group of eavesdroppers, where, if we pick an arbitrary user from each group of users and an arbitrary eavesdropper, they satisfy a certain Markov chain. We study two different communication scenarios for this channel. In the first scenario, the transmitter wants to send a confidential message to users in the first (stronger) group and a different confidential message to users in the second (weaker) group, where both messages need to be kept confidential from the eavesdroppers. For this scenario, we assume that there is only one eavesdropper. We obtain the secrecy capacity region for the discrete memoryless channel model, the parallel channel model, and the Gaussian parallel channel model. For the Gaussian multiple-input multiple-output (MIMO) channel model, we obtain the secrecy capacity region when there is only one user in the second group. In the second scenario we study, the transmitter sends a confidential message to users in the first group which needs to be kept confidential from the second group of users and the eavesdroppers. Moreover, the transmitter sends a different confidential message to users in the second group which needs to be kept confidential only from the eavesdroppers. For this scenario, we do not put any restriction on the number of eavesdroppers. As in the first scenario, we obtain the secrecy capacity region for the discrete memoryless channel model, the parallel channel model, and the Gaussian parallel channel model. For the Gaussian MIMO channel model, we establish the secrecy capacity region when there is only one user in the second group.

We study the Gaussian multiple-input multiple-output (MIMO) wiretap channel, which consists of a transmitter, a legitimate user, and an eavesdropper. In this channel, the transmitter sends a common message to both the legitimate user and the eavesdropper. In addition to this common message, the legitimate user receives a private message, which is desired to be kept hidden as much as possible from the eavesdropper. We obtain the entire capacity-equivocation region of the Gaussian MIMO wiretap channel. This region contains all achievable common message, private message, and private message's equivocation (secrecy) rates. In particular, we show the sufficiency of jointly Gaussian auxiliary random variables and channel input to evaluate the existing single-letter description of the capacity-equivocation region due to Csiszar–Korner.

We consider the Gaussian multiple-input multiple-output (MIMO) broadcast channel with common and private messages. We obtain an outer bound for the capacity region of this channel. To this end, we show that a parallel Gaussian broadcast channel can be constructed from any given Gaussian MIMO broadcast channel by using the generalized singular value decomposition and a relaxation on the power constraint for the channel input. Due to this relaxation of the power constraint, the capacity region of the constructed parallel channel, which is known, provides an outer bound for the capacity region of the original channel. We show that this outer bound is within a finite gap of the capacity region by comparing it with an achievable rate region that can be obtained either by using dirty-paper coding or by using a variation of the zero-forcing scheme.

We study the secure lossy transmission of a Gaussian vector source to a legitimate user in the presence of an eaves-dropper, where both the legitimate user and the eavesdropper have Gaussian vector side information. The transmitter describes the source to the legitimate user in a way that the legitimate user can reconstruct the source within a certain distortion while the eavesdropper is kept ignorant of the source as much as possible. We obtain an outer bound for the rate, equivocation and distortion region of this secure lossy transmission problem. This outer bound is tight when the transmission rate constraint is removed. In other words, we obtain the maximum equivocation at the eavesdropper when the legitimate user needs to reconstruct the source within a fixed distortion level while there is no constraint on the transmission rate.

We investigate the effects of user cooperation on the secrecy of broadcast channels by considering a cooperative relay broadcast channel. We show that user cooperation can increase the achievable secrecy region. We propose an achievable scheme that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward scheme for relay channels. We derive outer bounds for the rate-equivocation region using auxiliary random variables for single-letterization. Finally, we consider a Gaussian channel and show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation.

In this paper, we consider the Gaussian multiple-input multiple-output (MIMO) multi-receiver wiretap channel in which a transmitter wants to have confidential communication with an arbitrary number of users in the presence of an external eavesdropper. We derive the secrecy capacity region of this channel for the most general case. We first show that even for the single-input single-output (SISO) case, existing converse techniques for the Gaussian scalar broadcast channel cannot be extended to this secrecy context, to emphasize the need for a new proof technique. Our new proof technique makes use of the relationships between the minimum-mean-square-error and the mutual information, and equivalently, the relationships between the Fisher information and the differential entropy. Using the intuition gained from the converse proof of the SISO channel, we first prove the secrecy capacity region of the degraded MIMO channel, in which all receivers have the same number of antennas, and the noise covariance matrices can be arranged according to a positive semi-definite order. We then generalize this result to the aligned case, in which all receivers have the same number of antennas; however, there is no order among the noise covariance matrices. We accomplish this task by using the channel enhancement technique. Finally, we find the secrecy capacity region of the general MIMO channel by using some limiting arguments on the secrecy capacity region of the aligned MIMO channel. We show that the capacity achieving coding scheme is a variant of dirty-paper coding with Gaussian signals.

We obtain the degrees of freedom region of the Gaussian multiple-input multiple-output (MIMO) broadcast channel with common and private messages. We first show that a parallel Gaussian broadcast channel with unmatched sub-channels can be constructed from any given Gaussian MIMO broadcast channel by using the generalized singular value decomposition (GSVD) and a relaxation on the power constraint for the channel input, in a way that the capacity region of the constructed parallel channel provides an outer bound for the capacity region of the original channel. The capacity region of the parallel Gaussian broadcast channel with unmatched sub-channels is known, using which we obtain an explicit outer bound for the degrees of freedom region of the Gaussian MIMO broadcast channel. We finally show that this outer bound for the degrees of freedom region can be attained by the achievable scheme that uses a classical Gaussian coding for the common message and dirty-paper coding (DPC) for the private messages.

We study the Gaussian multiple-input multiple-output (MIMO) broadcast channel with common and private messages. We first obtain an outer bound for the capacity region of the two-user discrete memoryless broadcast channel with common and private messages. We next show that if jointly Gaussian random variables are sufficient to evaluate this outer bound for the Gaussian MIMO broadcast channel, the dirty-paper coding (DPC) region is the capacity region of the Gaussian MIMO broadcast channel with common and private messages. However, we can evaluate only a loosened version of this outer bound, which yields the result that extending the DPC region in the common message rate direction by a fixed amount is an outer bound for the capacity region of the Gaussian MIMO broadcast channel with common and private messages. However, this fixed amount, i.e., the gap, is not finite for all channels. We derive the necessary and sufficient conditions for this gap to be finite.

We study the two-user Gaussian multiple-input multiple-output (MIMO) broadcast channel with common and confidential messages. In this channel, the transmitter sends a common message to both users, and a confidential message to each user which is kept perfectly secret from the other user. We obtain the entire capacity region of this channel. We also explore the connections between the capacity region we obtained for the Gaussian MIMO broadcast channel with common and confidential messages and the capacity region of its non-confidential counterpart, i.e., the Gaussian MIMO broadcast channel with common and private messages, which is not known completely.

We study the Gaussian multiple-input multiple-output (MIMO) wiretap channel, which consists of a transmitter, a legitimate user, and an eavesdropper. In this channel, the transmitter sends a common message to both the legitimate user and the eavesdropper. Moreover, the legitimate user receives a private message, which is desired to be kept hidden as much as possible from the eavesdropper. We obtain the entire capacity-equivocation region of the Gaussian MIMO wiretap channel. This region contains all achievable common message, private message, and private message's equivocation rates. This capacity-equivocation region is equal to the capacity region of a Gaussian MIMO broadcast channel where the transmitter sends a common message to both the legitimate user and the eavesdropper, a public message to the legitimate user on which there is no secrecy constraint, and a confidential message to the legitimate user which needs to kept perfectly secret from the eavesdropper.

We consider the digital fingerprinting (FP) problem and model it as a multiuser communications problem and develop a detection theoretic framework. In the general case, colluders apply uniform linear averaging followed by additive colored Gaussian noise. For each user, the receiver computes the correlation between the attacked signal and a linear-transformed version of that user's fingerprint, and performs thresholding (focused detection). Assuming independent colluders with potentially unequal priors, we derive generic exact bit-error probability (BEP) expressions, together with tight bounds, for arbitrary FP codes. Then, we specialize our results to orthogonal, simplex and Gaussian codes in the presence of additive white Gaussian noise; under mean squared error distortion constraints on the embedder and the colluders, we analytically quantify the optimal detection rule, the resulting minimum BEP and its asymptotic behavior, the collusion resistance, and the error exponent for the aforementioned codes, and compare their performances. We show that the minimum BEP expressions for these codes obey the same functional form and that they can be ordered as simplex, orthogonal, and Gaussian in terms of increasing BEP.

In this paper, two novel iterative chip-level synchronization methods, one blind and the other data-aided, are proposed for chip-interleaved time-hopping (TH) ultra-wideband signals that are subject to intersymbol interference (ISI) and multiple access interference (MAI). Both algorithms are soft-input, soft-output devices which receive the a posteriori probability of each chip from the minimum mean-square error (MMSE) filter that suppresses both ISI and MAI. Using this soft information and the structure of TH codes, the blind synchronizer produces the probabilities of time delays which are fed into the channel decoder. In addition to the knowledge of TH codes and the soft information coming from the MMSE filter, the data-aided synchronizer uses a training sequence in the generation of the probabilities of time delays. Although the channel decoder for chip-interleaved systems is rather simple, here it is redesigned to incorporate the soft information. The resulting receivers perform as well as iterative equalization with perfect timing information, and catch the single-user performance at moderate signal-to-noise ratios.

We consider the Gaussian multiple-input multiple-output (MIMO) multi-receiver wiretap channel, and derive the secrecy capacity region of this channel for the most general case. We first prove the secrecy capacity region of the degraded MIMO channel, in which all receivers have the same number of antennas, and the noise covariance matrices exhibit a positive semi-definite order. We then generalize this result to the aligned case, in which all receivers have the same number of antennas, however there is no order among the noise covariance matrices. We accomplish this task by using the channel enhancement technique. Finally, we find the secrecy capacity region of the general MIMO channel by using some limiting arguments on the secrecy capacity region of the aligned MIMO channel. We show that a variant of dirty-paper coding with Gaussian signals is optimal.

We consider the fading broadcast channel from a secrecy point of view. In this channel, each user views the other user as an eavesdropper, and wants to keep its information as secret from the other user as possible. First, we consider a more general channel model which consists of L independent sub-channels, where in each sub-channel, one of the users' channel is less noisy with respect to the other user. Since the user which has the less noisy observation can be different in each sub-channel, the overall channel is not less noisy for any one of the users. We establish the secrecy capacity region of this channel for the case where the transmitter sends a common message to both users and an individual confidential message to each user. This channel model encompasses the sub-class of channels, where in each sub-channel, one of the users' observation is degraded with respect to the other user. The parallel Gaussian broadcast channel belongs to this sub-class. In the Gaussian case, we identify the optimum input distribution, which is Gaussian, and the optimum power allocation corresponding to each point on the boundary of the secrecy capacity region. Finally, noting that the fading Gaussian broadcast channel is equivalent to a parallel Gaussian broadcast channel from an ergodic capacity perspective, we explicitly evaluate the ergodic secrecy capacity region of the fading broadcast channel.

We consider the Gaussian multi-receiver wiretap channel and evaluate its secrecy capacity region. This evaluation requires the identification of underlying auxiliary random variables. For this purpose, we first visit the converse proof of the scalar Gaussian broadcast channel, and show that this proof cannot be extended to this secrecy context. The failure of this extension comes from the insufficiency of the entropy-power inequality to resolve the ambiguity regarding the auxiliary random variables. Instead, we provide two converse proofs. The first one uses the alternative representation of the mutual information as an integration of the minimum-mean-square-error (MMSE) along with the properties of the MMSE. The second one uses the relationship between the differential entropy and the Fisher information via the de Bruin identity along with the properties of the Fisher information.

We investigate the effects of user cooperation on the secrecy of broadcast channels by considering a cooperative relay broadcast channel. We show that user cooperation can increase the achievable secrecy region. We propose an achievable scheme that combines Martonpsilas coding scheme for broadcast channels and Cover and El Gamalpsilas compress-and-forward scheme for relay channels. We derive outer bounds for the rate-equivocation region using auxiliary random variables for single-letterization. Finally, we consider a Gaussian channel and show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation.

Ultra-wideband (UWB) communication is envisaged to be deployed in indoor environments, where the noise distribution is decidedly non-Gaussian. A critical challenge for impulse radio UWB is the synchronization of nanosecond-long pulses. In this paper, we propose two acquisition schemes which are robust to uncertainties in the noise distribution and which operate directly on samples taken at symbol-rate. The only channel state information that the algorithms need can be obtained by estimation of an aggregate channel gain, avoiding channel-related complications. Following Huber's M-estimates for the Gaussian mixture noise model, the proposed robust acquisition systems outperform their traditional counterparts designed according to the Gaussian noise assumption. Necessary modifications for operation under multiple access interference are also introduced. Theoretical and simulation-based performance evaluations that reflect the asymptotic variance, normalized mean-square error and the bit error rates demonstrate the gains offered by the robust procedures.

Ultra-wideband (UWB) communications is envisaged to be deployed in indoor environments, where the noise distribution is decidedly non-Gaussian. A critical challenge for UWB is the synchronization of nanosecond-long pulses. In this paper, we propose a signal acquisition scheme that is robust to uncertainties in the noise distribution. Following Huber's M-estimates for the Gaussian mixture noise model, the successive sampling-based robust acquisition system outperforms Gaussian-optimal maximum likelihood approach. We present performance evaluation based on the normalized mean-square error and the bit-error rate.

Ultra-wideband (UWB) communications is envisaged to be deployed in indoor environments, where the noise distribution is decidedly non-Gaussian. A critical challenge for UWB is the synchronization of nanosecond-long pulses. In this paper, we propose an averaged template-based signal acquisition scheme that is robust to uncertainties in the noise distribution. Following Huber's M-estimates for the Gaussian mixture noise model, the averaged template-based robust acquisition system outperforms its traditional counterparts which are designed with Gaussian noise in mind. The authors present performance evaluation based on the asymptotic variance, normalized mean-square error, as well as the bit error rate.