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Low Power Wide Area Networks (LPWANs) are an emerging paradigm for low-cost and long-range connection of sensor networks. While most technologies focus on sub-GHz frequencies for their low path loss, the 2.4 GHz band is also an interesting choice due to its high bandwidth and global availability. In this paper, we report our results from a measurement campaign performed in Karlsuhe, Germany, where we investigate the interference background in different scenarios typical for Smart City applications. Using our software implementation of the IEEE 802.15.4 LECIM DSSS PHY and the recorded noise and interference, we also quantify its impact on the performance of a practical LPWAN waveform in terms of packet error rate and effectively available bandwidth at different received signal strengths. Even though it turns out that link budget calculations based on AWGN might be too optimistic, promising results are presented for a two-way single-channel network with up to two retransmissions.

We present the design of a low-complexity air interface for the uplink in Low Power Wide Area Networks (LPWANs) based on a recently published Differential Spatial Modulation scheme for two transmit antennas achieving full transmit diversity (FD-DSM). Extending the authors' work, we derive an optimized assignment from bits to space-time matrices comparable to Gray coding, which improves the bit error rate by up to 1 dB. Furthermore, FD-DSM's superiority over single-antenna differential PSK schemes as used in current LPWAN technologies is evaluated in extensive simulations and confirmed in uncoded and coded scenarios, where a convolutional code with soft-input Viterbi decoding is employed. It is shown that existing systems are outperformed by at least 5 dB for packet error rates below 1%. Our system utilizes the DSSS technique to achieve the enormous processing gains required to enable links over many kilometers. We propose the insertion of time gaps between chip sequences to allow for the rise and fall time of conventional, spectrally-efficient pulse shaping filters and to thus facilitate antenna switching without spectral broadening, a challenging issue in Spatial Modulation (SM) systems. The resulting rate loss is negligible but allows the realization of practical SM systems with the theoretical minimum of a single RF chain.

Industrial environments often exhibit harsh propagation conditions for wireless transmissions. This paper investigates the influence of different interference characteristics on the performance of IEEE 802.15.4 using the OQPSK and Chirp Spread Spectrum (CSS) PHY. Bit error rates are determined for AWGN, Rayleigh fading and self-interference scenarios. GNU Radio, a Software Defined Radio framework, serves as simulation environment. In addition to the implementation of the CSS transmitter and receiver, a low-complexity synchronization algorithm, which only consists of a correlator and a second order Phase Locked Loop (PLL), and its limitations are presented. Furthermore, MAC layer simulations are performed for beacon-enabled (slotted) as well as nonbeacon- enabled (unslotted) mode to make a statement about achievable throughput and latency in the presence of interference. It shows that the CSS PHY clearly surpasses the OQPSK PHY in throughput. Moreover, the simulation results are confirmed by the derivation of the theoretical maximum throughput for the investigated PHY layers.