Covert communications, or covert channels, are commonly exploited to establish a data exfiltration channel from an insider on a trusted network to a malicious receiver outside the network without using normal communication of the network. It is because the malicious receiver is an unauthorized user of the communication network and so he cannot communicate with any entity in the network. In this study, we construct a new covert wireless unidirectional communication mechanism in an IEEE 802.11 environment. Our covert communication is based on a covert timing channel exploiting the beacon interval of a given commercial-like AP. Because the wireless covert channel we proposed can be implemented only with firmware modification to the WLAN MAC protocol, it is very suitable for application in a real public AP environment. In order to dramatically reduce the chance of covert signals being detected by others, a new and simple covert data encoding scheme, called ping-pong covert timing channel (PPCTC), is proposed, and we show that the covertness of the PPCTC is excellent compared to the previous timing-based covert channels. Although this wireless covert communication is unidirectional communication, since PPCTC has recovery characteristics against consecutive 2-bit errors, stable communication is guaranteed. Furthermore, a covert frame structure is presented for providing the confidentiality and integrity of the information transmitted via our covert channel. To the best of our knowledge, this is the first attempt.

This paper presents the structure of terminal ASIC chip with low speed, low power, and small characteristics for satellite IoT transmission. The external connection with the sensor, flash external, and tuner will be made centering on the ASIC chip. Three transmission waveforms, such as the LoRa, I-LoRa, and LoRa-E, will be used to send the collected data to LEO satellite. As a result, the data collected from the sensor is transmitted to the ASIC chip through the sensor interface, and the ASIC chip modulates the collected data into an uplink signal and sends it to LEO satellite through the tuner.

This paper discusses ranging for IEEE 802.15.4a impulse-radio ultra-wideband (IR-UWB) system, especially targeting implementation. The essential capabilities for ranging are the ranging counter and determination of the arrival time of a signal at the receiver, which are implemented in our full-digital coherent IR-UWB system. The ranging measurements verify our design for ranging.

Erasure selection for errors-and-erasures decoding algorithms is investigated and analyzed, where declaring erasures provides additional gain for hard decision decoding. To derive an analytical criterion for optimal erasure selection, we first investigate erasure effects on decoding performance, by which a metric function for erasure selection is derived. From the derived metric, a suboptimal threshold is defined, which can be used for errors and erasures decoding of large code-distance channel codes. Moreover, application to M-ary (M>2) PAM constellation is also described. For verification of the proposed erasure threshold, simulation results with popular channel codes are included.

The conventional pilot-aided orthogonal frequency division multiplexing (OFDM) systems use the fixed number of pilots to estimate the channel. In this paper, we propose a new channel estimation scheme that exploits reliable decision-feeback symbols in the OFDM systems. The reliable symbols having low symbol error rate can be used to refine the channel estimate again. The resultant algorithm gives a more accurate channel estimate and lower bit error rate (BER) than the conventional method. The analysis of Cramér Rao lower bound (CRLB) and simulation results verify our claims that the enhanced channel estimation and lower BER can be achieved.

This paper considers the problem of synchronization in frequency hopping systems. A simple digital signal processing block along with the existing RF circuits forms a phase detector that generates a signal corresponding to the phase difference between the hop timings of the local and received signals. The two hop-timing acquisition algorithms proposed in this paper - based on energy detection - utilize the phase detector and result in low-complexity implementations. In the first one, a simple cost function relying on the power sum definition estimates the hop timing. This estimator allows for only a limited acquisition range. Based on this study, we propose a hop-timing estimator with a full acquisition range which still maintains low-complexity. Furthermore, this paper analyzes the optimal threshold to detect the signal existence based on the maximum likelihood detection rule.

We design pilot-symbol-assisted modulation for carrier frequency offset (CFO) and channel estimation in orthogonal frequency-division multiplexing transmissions over multi-input multi-output frequency-selective fading channels. The CFO and channel-estimation tasks rely on null-subcarrier and nonzero pilot symbols that we insert and hop from block to block. Because we separate CFO and channel estimation from symbol detection, the novel training patterns lead to further decoupled CFO and channel estimators. The performance of our algorithms is investigated analytically, and then compared with an existing approach by simulations.

We aim at increasing the throughput of the hybrid automatic retransmission request (HARQ) protocol in space-time (ST) coded multi-antenna transmission systems. By utilizing reliability information at the decoder, we obtain an improved probability of successful decoding, which enhances the overall system throughput at low complexity. Simulations and analytical results demonstrate the performance of our scheme in AWGN and fading multi-input multi-output (MIMO) channels.

We design pilot symbol assisted modulation for carrier frequency offset (CFO) and channel estimation in orthogonal frequency division multiplexing (OFDM) transmissions over multiinput multioutput (MIMO) frequency-selective fading channels. By separating CFO and channel estimation from symbol detection, the novel training patterns lead to low-complexity CFO and channel estimators. The performance of our algorithms is investigated analytically, and then compared with an existing approach by simulations.