This letter considers the Hybrid Automatic Repeat Request (HARQ) scheme for Sparse Code Multiple Access (SCMA) systems, where the Message Passing Algorithm (MPA) is adopted in the multiuser detection. In MPA, the reliability can be improved when parts of messages are pre-known. Motivated by this fact, a novel HARQ scheme for SCMA systems is proposed, where some correctly decoded codewords are still retransmitted in the next round of transmission. Analysis and simulation results show that, compared with the traditional HARQ scheme, the reliability of the proposed scheme is enhanced obviously, while the latency increases slightly. Hence, in the proposed scheme, an affordable trade-off between the reliability and latency can be achieved.

In wireless positioning, estimating the location of a receiver is very challenging since the receiver does not transmit signals. The difficulty is to obtain the distance between the anchors and the silent receiver. To deal with the issue, this paper proposes to use the full-duplex relay technique to estimate the anchor-receiver distance. Then, we can obtain the location of the silent receiver. With the proposed method, the location-aware applications are not limited to the conventional transmitter positioning. Instead, they can be extended to the receiver positioning. Simulation results demonstrate the performance of the proposed method.

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Drones raise significant privacy and security threats, by intruding into the airspace of private properties or unauthorized regions. Being able to detect and localize the encroaching drones is essential to build geofencing systems to prevent drone misuse. While most existing approaches focus on detecting and localizing active drones, passive drones that do not emit signals are particularly challenging to localize, without requiring advanced hardware. In this work, we propose a novel, low-cost passive drone localization approach, by leveraging opportunistic environmental RF signals (e.g., LTE or WiFi) that reflect off the target drone, with only a single wireless receiver. We implement a prototype system on a USRP-device based testbed, with standard LTE signals emitted by multiple distributed transmitters, and conduct experiments on top of a campus building to evaluate its performance. We also perform a drone detection range analysis to extrapolate the real-world applicability of our scheme. CCS CONCEPTS• General and reference → Measurement; • Hardware → Digital signal processing; • Security and privacy → Intrusion detection systems; Mobile and wireless security.

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