Wireless communication is susceptible to eavesdropping attacks because of its broadcast nature. This paper illustrates how interference can be used to counter eavesdropping and assist secrecy. In particular, a wiretap channel with a helping interferer (WT-HI) is considered. Here, a transmitter sends a confidential message to its intended receiver in the presence of a passive eavesdropper and with the help of an independent interferer. The interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending an independent signal. An achievable secrecy rate and several computable outer bounds on the secrecy capacity of the WT-HI are given for both discrete memoryless and Gaussian channels.

A secret-key sharing strategy based on layered broadcast coding is introduced for slow fading channels. In the model considered, Alice wants to share a key with Bob while keeping the key secret from a passive eavesdropper, Eve. Both Alice-Bob and Alice-Eve channels are assumed to undergo slow fading, and perfect channel state information (CSI) is assumed to be known only at the receivers during the transmission. Layered coding facilitates adapting the reliably decoded rate at Bob to the actual channel state without CSI available at Alice. The index of a reliably decoded layer is sent back to Alice via a public and error-free channel, which is exploited by Alice and Bob to generate the secret key. In this paper, the secrecy key rate is derived. In addition, the optimal power distribution over coded layers is characterized. It is shown that layered coding can increase the secrecy key rate significantly compared with single-level coding.

The focus of this paper is an information-theoretic study of retransmission protocols for reliable packet communication under a secrecy constraint. Thehybridautomaticretransmission request (HARQ) protocol is revisited for a block-fading wiretap channel. Here, two legitimate users communicate over a block-fading channel in the presence of a passive eavesdropper who intercepts the transmissions through an independent block-fading channel. In this model, the transmitter obtains a 1-bit ACK/NACK feedback from the legitimate receiver via an error-freepublicchannel. Both reliability and confidentiality of secure HARQ protocols are studied through the joint consideration of channel coding, secrecy coding, and retransmission protocols. In particular, the error and secrecy performance ofrepetition time diversity(RTD) andincremental redundancy(INR) protocols are investigated based on Wyner code sequences. These protocols ensure that the confidential message is decoded successfully by the legitimate receiver and is kept completely secret from the eavesdropper for a set of channel realizations. This paper illustrates that there exists a rate-compatible Wyner code family which ensures a secure INR protocol. Further, it defines theconnection outageandsecrecy outageprobabilities to characterize the tradeoff between the reliability of the legitimate communication link and the confidentiality with respect to the eavesdropper's link. For a given connection/secrecy outage probability pair, an achievable throughput of secure HARQ protocols is derived for block-fading channels. Finally, both asymptotic analysis and numerical calculations demonstrate the benefits of HARQ protocols to throughput and secrecy.

Due to the broadcast nature of the wireless medium, wireless communication is susceptible to adversarial eavesdropping. This paper describes how eavesdropping can potentially be defeated by exploiting the superposition nature of the wireless medium. A Gaussian wire-tap channel with a helping interferer (WTC-HI) is considered in which a transmitter sends confidential messages to its intended receiver in the presence of a passive eavesdropper and with the help of an interferer. The interferer, which does not know the confidential message assists the confidential message transmission by sending a signal that is independent of the transmitted message. An achievable secrecy rate and a Sato-type upper bound on the secrecy capacity are given for the Gaussian WTC-HI. Through numerical analysis, it is found that the upper bound is close to the achievable secrecy rate when the interference is weak for symmetric interference channels, and under more general conditions for asymmetric Gaussian interference channels.

Wireless communication is susceptible to adversarial eavesdropping due to the broadcast nature of the wireless medium. In this paper it is shown how eavesdropping can be alleviated by exploiting the superposition property of the wireless medium. A wiretap channel with a helping interferer (WT-HI), in which a transmitter sends a confidential message to its intended receiver in the presence of a passive eavesdropper, and with the help of an independent interferer, is considered. The interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending independent signals. An achievable secrecy rate for the WT-HI is given. The results show that interference can be exploited to assist secrecy in wireless communications. An important example of the Gaussian case, in which the interferer has a better channel to the intended receiver than to the eavesdropper, is considered. In this situation, the interferer can send a (random) codeword at a rate that ensures that it can be decoded and subtracted from the received signal by the intended receiver but cannot be decoded by the eavesdropper. Hence, only the eavesdropper is interfered with and the secrecy level of the confidential message is increased.

In this correspondence, we show that the uniform throughput capacity of an ultra-wideband (UWB) power-constrained ad hoc network is given by , where is the per-node power constraint, is the fading exponent of radio signal and is the number of nodes randomly distributed inside a disk of unit area. This is a stronger result than the upper bound and the lower bound previously shown by Negi and Rajeswaran. Our proof is simple given the prior work by Gupta and Kumar.

In applications of wireless packet-oriented data networks, a special coding scheme, the hybrid automatic retransmission request (HARQ) exhibits high throughput efficiency by adapting its error correcting code redundancy to channel conditions. Motivated by the increasing importance of secure communication over wireless networks, we investigate secure packet communication based on HARQ over block-fading (BF) channels. More specifically, we consider two legitimate users communicating over a BF channel in the presence of a passive eavesdropper who intercepts the transmission through another independent BF channel. We assume that the transmitter can obtain a 1-bit ACK/NACK feedback from the receiver via a reliable public channel. Under this setting, we consider incremental redundancy (IR) and repetition time diversity (RTD) HARQ schemes based on rate-compatible Wyner secrecy codes from an information theoretic point of view. We study a good Wyner code sequence, with which the legitimate receiver can decode the message and the eavesdropper can be perfectly confused. For a given pair of reliability/secrecy outage probabilities, we derive an achievable secrecy throughput of HARQ protocols for block-fading channels. Finally, we illustrate numerically that HARQ can benefit both throughput and secrecy.

Given a multiple-antenna source and a multiple-antenna destination, a multiple-antenna relay between the source and the destination is desirable under useful circumstances. A non-regenerative multiple-antenna relay, also called non-regenerative MIMO (multi-input multi-output) relay, is designed to optimize the capacity between the source and the destination. Without a direct link between the source and the destination, the optimal canonical coordinates of the relay matrix are first established, and the optimal power allocations along these coordinates are then found. The system capacity with the optimal relay matrix is shown to be significantly higher than those with heuristic relay matrices. When a direct link is present, upper and lower bounds of the optimal system capacity are discussed.