Sliding Window Spectrum Sensing for Full-Duplex Cognitive Radios with Low Access-Latency

In a cognitive radio system the failure of secondary user (SU) transceivers to promptly vacate the channel can introduce significant access-latency for primary or high-priority users (PU). In conventional cognitive radio systems, the backoff latency is exacerbated by frame structures that only allow sensing at periodic intervals. Concurrent transmission and sensing using self-interference suppression has been suggested to improve the performance of cognitive radio systems, allowing decisions to be taken at multiple points within the frame. In this paper, we extend this approach by proposing a sliding-window full-duplex model allowing decisions to be taken on a sample-by-sample basis. We also derive the access-latency for both the existing and the proposed schemes. Our results show that the access-latency of the sliding scheme is decreased by a factor of 2.6 compared to the existing slotted full-duplex scheme and by a factor of approximately 16 compared to a half-duplex cognitive radio system. Moreover, the proposed scheme is significantly more resilient to the destructive effects of residual self-interference compared to previous approaches.Comment: Published in IEEE VTC Spring 2016, Nanjing, Chin

Coded LoRa Frame Error Rate Analysis

In this work, we study the coded frame error rate (FER) of LoRa under additive white Gaussian noise (AWGN) and under carrier frequency offset (CFO). To this end, we use existing approximations for the bit error rate (BER) of the LoRa modulation under AWGN and we present a FER analysis that includes the channel coding, interleaving, and Gray mapping of the LoRa physical layer. We also derive the LoRa BER under carrier frequency offset and we present a corresponding FER analysis. We compare the derived frame error rate expressions to Monte Carlo simulations to verify their accuracy

An Open-Source LoRa Physical Layer Prototype on GNU Radio

LoRa is the proprietary physical layer (PHY) of LoRaWAN, which is a popular Internet-of-Things (IoT) protocol enabling low-power devices to communicate over long ranges. A number of reverse engineering attempts have been published in the last few years that helped to reveal many of the LoRa PHY details. In this work, we describe our standard compatible LoRa PHY software-defined radio (SDR) prototype based on GNU Radio. We show how this SDR prototype can be used to develop and evaluate receiver algorithms for LoRa. As an example, we describe the sampling time offset and the carrier frequency offset estimation and compensation blocks. We experimentally evaluate the error rate of LoRa, both for the uncoded and the coded cases, to illustrate that our publicly available open-source implementation is a solid basis for further research.Comment: GNU Radio source code available at: https://tcl.epfl.ch/resources-and-sw/lora-phy

A Maximum-Likelihood-based Multi-User LoRa Receiver Implemented in GNU Radio

LoRa is a popular low-power wide-area network (LPWAN) technology that uses spread-spectrum to achieve long-range connectivity and resilience to noise and interference. For energy efficiency reasons, LoRa adopts a pure ALOHA access scheme, which leads to reduced network throughput due to packet collisions at the gateways. To alleviate this issue, in this paper we analyze and implement a LoRa receiver that is able to decode LoRa packets from two interfering users. Our main contribution is a two-user detector derived in a maximum-likelihood fashion using a detailed interference model. As the complexity of the maximum-likelihood sequence estimation is prohibitive, a complexity-reduction technique is introduced to enable a practical implementation of the proposed two-user detector. This detector has been implemented along with an interference-robust synchronization algorithm on the GNU Radio Software-Defined-Radio (SDR) platform. The SDR implementation shows the effectiveness of the proposed method and also allows its experimental evaluation. Measurements indicate that our detector inherently leverages the time offset between the two colliding users to separate and demodulate their contributions.Comment: 2020 Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, US

Physical Layer Aspects of LoRa and Full-Duplex Wireless Transceivers

Wireless communications are currently faced with two main challenges. The first challenge stems from the enormous number of Internet of Things (IoT) devices that transmit very small amounts of data. The second challenge is the need for ever-increasing data rates required by users of multimedia rich services, as well as the extremely low latency required in emerging applications such as autonomous vehicles and augmented reality. In this thesis we deal with important physical layer (PHY) aspects that have not been analyzed in-depth in the existing literature, and whose study can help to address the aforementioned challenges. Low-power wide-area networks (LPWANs) comprise a big part of the IoT. For energy efficiency reasons, most of LPWAN technologies adopt uncoordinated channel access schemes which result in collisions. This issue becomes more severe as the number of devices increases, putting the scalability of LPWANs at risk as they become interference-limited. To evaluate and support LPWAN scalability, in the first part of this thesis we perform a thorough analysis of the performance of one of the most important LPWAN technologies, namely LoRa. We analyze the LoRa performance in interference scenarios, and we derive expressions, as well as very accurate low-complexity approximations, for the error rate of LoRa for both the uncoded and coded cases, and with carrier frequency offset (CFO). We also propose and analyze the coherent demodulation of LoRa under interference, as a potential receiver improvement in collision scenarios. Finally, we build a standard-compatible LoRa PHY software-defined radio (SDR) prototype based on GNU Radio, which can be used for measurements of LoRa PHY performance. The second part of this thesis focuses on full-duplex radios, which allow simultaneous transmission and reception in the same frequency band, and have been proposed as a possible solution to overcome the capacity bottleneck of high data-rate applications. However, full-duplex transceivers suffer from strong self-interference. Perfect self-interference cancellation is difficult to achieve due to the presence of strong non-linear signal components, which are introduced by hardware imperfections inherent in the transmitter and receiver chains. We propose the digital predistortion of the transmit signal to compensate for the cascade of the transceiver non-linearities and enhance self-interference cancellation. Unfortunately, a residual self-interference component always remains, particularly when operating at realistic transmit powers. To increase the usefulness of full-duplex technology, we examine communication schemes where using full-duplex transceivers can significantly improve the performance in terms of both throughput and latency, even under imperfect self-interference suppression. In particular, we examine the use of full-duplex technology in cognitive radios, and in communication links with asymmetric capacity requirements between the uplink and downlink channels

Coded LoRa Frame Error Rate Analysis

In this work, we study the coded frame error rate (FER) of LoRa under additive white Gaussian noise (AWGN) and under carrier frequency offset (CFO). To this end, we use existing approximations for the bit error rate (BER) of the LoRa modulation under AWGN and we present a FER analysis that includes the channel coding, interleaving, and Gray mapping of the LoRa physical layer. We also derive the LoRa BER under carrier frequency offset and we present a corresponding FER analysis. We compare the derived frame error rate expressions to Monte Carlo simulations to verify their accuracy

A Low-Complexity LoRa Synchronization Algorithm Robust to Sampling Time Offsets

LoRaWAN is nowadays one of the most popular protocols for low-power Internet of Things communications. Although its physical layer, namely LoRa, has been thoroughly studied in the literature, aspects related to the synchronization of LoRa receivers have received little attention so far. The estimation and correction of carrier frequency and sampling time offsets (STOs) are, however, crucial to attain the low sensitivity levels offered by the LoRa spread-spectrum modulation. The goal of this article is to build a low-complexity, yet efficient synchronization algorithm capable of correcting both offsets. To this end, a complete analytical model of a LoRa signal corrupted by these offsets is first derived. Using this model, we propose a new estimator for the STO. We also show that the estimations of the carrier frequency and the STOs cannot be performed independently. Therefore, to avoid a complex joint estimation of both offsets, an iterative low-complexity synchronization algorithm is proposed. To reach a packet error rate of 10(-3), performance evaluations show that the proposed receiver requires only 1 or 2 dB higher signal-to-noise ratio than a theoretical perfectly synchronized receiver, while incurring a very low computational overhead.TC