Consider a wireless transmission from Alice to Bob in the presence of an eavesdropper, Eve. We propose a technique to allow Alice to send a message such that Bob can decode it but Eve can get little information about the message. This scenario is a classic example of the wiretap channel, for which several researchers have begun to develop practical schemes. However, we consider a challenging version in which the channel from Alice to Eve may be as good or better than the channel from Alice to Bob. There have been no practical secrecy coding techniques that have been developed for this scenario. The key to allow secret communication in such a scenario is feedback from Bob to Alice (which Eve can also observe). The scheme we develop uses reliability-based hybrid automatic-repeat-request (RB-HARQ) over a fast fading channel to provide a decoding advantage at Bob. Bob requests information about received symbols that suffer from deep fades. The retransmitted symbols improve Bobpsilas ability to decode the message faster than at Eve, who sees retransmission of bits with random reliabilities. We evaluate the proposed scheme by performance measures such as the probability of successful decoding and the equivocation rate at Eve.
Consider a wireless transmission from Alice to Bob in the presence of an eavesdropper, Eve. We propose a technique to allow Alice to send a message such that Bob can decode it but Eve can get little information about the message. This scenario is a classic example of the wiretap channel, for which several researchers have begun to develop practical schemes. However, we consider a challenging version in which the channel from Alice to Eve may be as good or better than the channel from Alice to Bob. There have been no practical secrecy coding techniques that have been developed for this scenario. The key to allow secret communication in such a scenario is feedback from Bob to Alice (which Eve can also observe). The scheme we develop uses reliability-based hybrid automatic-repeat-request (RB-HARQ) over a fast fading channel to provide a decoding advantage at Bob. Bob requests information about received symbols that suffer from deep fades. The retransmitted symbols improve Bobpsilas ability to decode the message faster than at Eve, who sees retransmission of bits with random reliabilities. We evaluate the proposed scheme by performance measures such as the probability of successful decoding and the equivocation rate at Eve.
In this paper, we study secure hybrid automatic retransmission request (HARQ) over block-fading channels when the channel from a legitimate transmitter (i.e., Alice) to an eavesdropper (i.e., Eve) is less noisy than that to a legitimate receiver (i.e., Bob). In order to have a positive secrecy rate, we exploit the notion of the channel reciprocity in time division duplex mode, where the shared channel state information between Alice and Bob is used as a secret key. The resulting HARQ scheme is referred to as channel-aware secure HARQ (CAS-HARQ) in this paper. In this scheme, in order to keep Eve ignorant of the confidential message from Alice to Bob, deliberative transmissions of random message blocks are considered when the channel to Bob is weak. We derive closed-form expressions for bounds on the probabilities of connection outage and secrecy outage to see reliability and security, respectively, of CAS-HARQ. Based on our analysis, we can see that it is possible to have a positive secrecy rate with certain outage probabilities even if the channel to Eve is less noisy than that to Bob.
Wireless systems are susceptible to eavesdropping as an unwelcome result of the broadcast nature of the wireless medium. Fortunately, the wireless medium also provides endowments including the temporal dispersion, the capability of signal superposition and other properties, to facilitate defending against eavesdropping. The focus of the thesis is to provide schemes for utilizing these properties at the physical layer and their performance analysis from an information-theoretic secrecy perspective. The work is based on Wyner's results on the wire-tap channel, in which the transmission between two legitimate users (Alice and Bob) is eavesdropped upon by Eve. The secrecy level is measured by the equivocation rate at Eve. The system performance is measured by the secrecy capacity. This is the maximum rate at which a message can be sent from Alice to Bob, while constraining the equivocation rate at Eve equal to the communication rate, and thus ensuring that Eve cannot deduce any useful information asymptotically. First, the scenario where a secret message needs to be delivered to the intended receiver within a given delay constraint and without the knowledge of the channel state information (CSI) at the transmitter is considered. In particular, hybrid automatic retransmission request (HARQ) protocols are revisited for a block-fading wire-tap channel via a joint consideration of channel coding, secrecy coding, and retransmission protocols. Two secure HARQ protocols: a repetition time diversity (RTD) scheme with maximal-ratio combining, and an incremental redundancy (INR) scheme based on ratecompatible Wyner secrecy codes are analyzed via an outage-based formulation. The throughput of RTD and INR protocols under probabilistic outage requirements is derived. Asymptotic analysis and numerical computations are given to demonstrate both the benefits of HARQ protocols and the tradeoff between reliability and confidentiality in delay-constrained communications. Second, the effects of interference on information-theoretic secret communication is considered. More specifically, the wiretap channel with a helping interferer (WT-HI) is proposed and studied. Here, an interferer, which does not know the confidential message, helps in ensuring the secrecy of the message by sending independent signals. Achievable secrecy rates and upper bounds on the secrecy capacity are given for both discrete memoryless and Gaussian channels. Interference has conventionally been regarded as a harmful phenomenon that leads to decreasing the rate and the reliability performance of wireless systems. In this work, it is shown that interference can be exploited to assist wireless secrecy. Lastly, the problem of secret key generation over fading channels, in which there exists an additional authenticated, public and error-free feedback channel from Bob to Alice, is considered. A key generation scheme consisting of a communication phase and a key-generation phase is presented. The communication phase is based on layered broadcast coding, which adapts the decoded rate at Bob to the actual channel state without requiring CSI available at Alice. The key-generation phase is based on the messages decoded by Bob after the communication phase and the public feedback. The achievable secrecy key rate and the optimal power distribution over coded layers for maximizing the secrecy key rate are characterized. Theoretical and numerical results show that the broadcast approach outperforms the single-level-coding based approach significantly, and hence demonstrate the role of introducing self-interference in facilitating secret-key generation over slowly fading channels when transmit CSI is not available.
In our previous work, we have shown that hybrid ARQ (HARQ) can be used to achieve secret communication over a fast fading channel. This is a physical-layer technique that can be used in conjunction with conventional cryptography to provide further protection from an eavesdropper. The key to our previous work was to use reliability-based HARQ to improve the decoder performance at the desired user much faster than at an eaves-dropper. In this paper, we propose a new HARQ approach that provides better secrecy performance by using the most-reliable received bits at the desired user to obfuscate information transmitted in the HARQ process. In addition, we present several other performance improvements, including transmitting incremental redundancy instead of repeating information and using bit-interleaved coded modulation with higher-order modulation. Performance measures such as information rate and fractional equivocation rate are provided to demonstrate that the obfuscated incremental-redundancy HARQ protocol is a promising candidate for secret sharing.
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.
We consider a scenario where agent Alice aims at transmitting a message to agent Bob, while keeping it secret from a third eavesdropper agent Eve. The transmissions occur on block-fading channels, and Alice only has a statistical channel state information (CSI) on both Bob and Eve channels. In this paper, we add secrecy features to hybrid automatic repeat request (HARQ) in what becomes a secure HARQ (S-HARQ) scheme. In particular, Alice encodes the secret message and splits the codeword into blocks. Then, she transmits one block at a time and after each transmission Bob feeds back an acknowledge (ACK) (not acknowledge (NACK)) packet, indicating he was (was not) able to decode the secret message. Upon an ACK Alice stops transmission, while upon a NACK Alice re-encodes the secret message into a new codeword and transmits it block by block, waiting for a new feedback. The process is iterated until either Bob decodes the message or a maximum number of codewords has been transmitted. We characterize the set of channels for which there exists a sequence of codes that ensure both vanishing error probability to Bob and vanishing rate of information leakage to Eve in the limit of infinitely long codewords. We also analyze the secrecy and decodability outage probabilities, providing closed form expressions for the case of additive white Gaussian noise channels with Rayleigh fading. The tradeoff between the number of blocks per codeword and the maximum number of codewords to be transmitted is also discussed.
We propose a hybrid automatic repeat request (HARQ) scheme with quantized channel state information (CSI) feedback for secure transmission from agent Alice to agent Bob over block fading channels. Bob feeds back to Alice a one-bit information on his CSI and she decides whether to transmit at a fixed rate or stay idle. If Bob fails to decode the secret message, the feedback, encoding and transmission processes are iterated. In order to prevent information leakage to a passive eavesdropper agent Eve, a different wiretap code is used at each iteration. We derive the secrecy outage probability and the number of bits per channel use that are reliably delivered to Bob while not being leaked to Eve. Such an opportunistic use of the channel leads to the reduction of the number of transmissions, with a) a higher secure throughput and b) a reduction of energy consumption (per transmitted bit).