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IEEE Military Communications Conference (MILCOM)

Opportunistic Power Control for Physical Layer Security

Kennedy A. Lee, John R. Barry

Published Oct 28, 2024• 0 citations
Publisher

Abstract

We introduce opportunistic power control (OPC) as a physical-layer security technique to enable wireless communications with secrecy in fading channels, enabling secure communication in the presence of an eavesdropper. OPC is a scheme where the transmitter only transmits when the channel gain is above a certain threshold, and a larger modulation alphabet than necessary is employed to maintain a target spectral efficiency. When the transmitter does transmit, it transmits with less power due to the higher channel gain. This effect reduces the signal power received by the eavesdropper, inducing decoding errors for the eavesdropper and enhancing the secrecy. Hence, the secrecy improves as the threshold increases. However, since the transmitter does not always transmit, buffers are necessary at the transmitter and receiver to smooth out gaps in the transmission, introducing delay. A higher threshold results in a longer delay before that threshold is met. Due to the direct relationship between the threshold and the delay, there exists a tradeoff between secrecy and delay. We obtain a closed-form expression for the average achievable secrecy rate with OPC under Rayleigh fading and optimize over the target SNR to obtain the maximum average achievable secrecy rate. We then present numerical results to quantify the incurred delay. We find that with OPC, the secrecy improves with increasing delay, reaching the aforementioned maximum, beyond which, there is no benefit in secrecy nor in delay. We also find that communication with a positive average achievable secrecy rate is possible even when the eavesdropper channel is better than the main channel on average, and that for the maximum average achievable secrecy rate, there is an inverse relationship between the target spectral efficiency and the secrecy, and a direct relationship between the target spectral efficiency and the delay.

DOI
10.1109/milcom61039.2024.10773817

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We introduce opportunistic power control (OPC) as a physical-layer security technique to enable wireless communications with secrecy in fading channels, enabling secure communication in the presence of an eavesdropper. OPC is a scheme where the transmitter only transmits when the channel gain is above a certain threshold, and a larger modulation alphabet than necessary is employed to maintain a target spectral efficiency. When the transmitter does transmit, it transmits with less power due to the higher channel gain. This effect reduces the signal power received by the eavesdropper, inducing decoding errors for the eavesdropper and enhancing the secrecy. Hence, the secrecy improves as the threshold increases. However, since the transmitter does not always transmit, buffers are necessary at the transmitter and receiver to smooth out gaps in the transmission, introducing delay. A higher threshold results in a longer delay before that threshold is met. Due to the direct relationship between the threshold and the delay, there exists a tradeoff between secrecy and delay. We obtain a closed-form expression for the average achievable secrecy rate with OPC under Rayleigh fading and optimize over the target SNR to obtain the maximum average achievable secrecy rate. We then present numerical results to quantify the incurred delay. We find that with OPC, the secrecy improves with increasing delay, reaching the aforementioned maximum, beyond which, there is no benefit in secrecy nor in delay. We also find that communication with a positive average achievable secrecy rate is possible even when the eavesdropper channel is better than the main channel on average, and that for the maximum average achievable secrecy rate, there is an inverse relationship between the target spectral efficiency and the secrecy, and a direct relationship between the target spectral efficiency and the delay.
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We introduce an opportunistic scheduling to enhance the physical layer security with transmit antenna selection (TAS) in multiuser environment. We consider a wireless communication system composed of a single transmitter and multiple legitimate users in the presence of several eavesdroppers with each node having multiple antennas under quasi-static Rayleigh fading channel. The transmitter selects the best transmitting antenna and the best user to maximize signal-to-noise ratio (SNR) at the selected user. The user and eavesdropper can employ either selection combining (SC) or maximal ratio combining (MRC) to combine the received signals. New closed-form expressions for probability of positive secrecy and outage are derived. Moreover, asymptotic analysis reveals the outage diversity gain and array gain for the proposed scheme. The impact of number of users, eavesdroppers, and antennas on secrecy performance are clearly demonstrated with mathematical analysis and numerical results.
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We consider a single-user, single-hop wireless communication system, in which data packets arrive at a data queue to be transmitted to a receiver over a block fading channel, privately from an eavesdropper. We assume that the eavesdropper listens to the transmitter over another independently fading channel and that the transmitter only has knowledge of the distribution of the eavesdropper's channel. We propose a joint secrecy rate, transmission, and admission controller based on a simple index policy that only relies on the distribution of the eavesdropper's channel rate. Given any arrival sample path, we show that our controller achieves the maximum possible data admission rate, while keeping the data queue stable as well as meeting an upper bound on the rate of secrecy outage, i.e., the fraction of data packets that are in part or fully decodable by the eavesdropper. While the solution is not unique, i.e., there are other schemes that can achieve the aforementioned performance, we show that our scheme also achieves a low queuing delay for the data packets enqueued at the data queue by striking the correct balance between direct secrecy encoding for data bits and secret key generation and utilization. To obtain this result, our transmission controller makes use of the secret key queue to smooth out the variations in the achievable secrecy rate of the associated fading wiretap channel.
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