Low power wide area (LPWA) networks are attracting a lot of attention primarily because of their ability to offer affordable connectivity to the low-power devices distributed over very large geographical areas. In realizing the vision of the Internet of Things, LPWA technologies complement and sometimes supersede the conventional cellular and short range wireless technologies in performance for various emerging smart city and machine-to-machine applications. This review paper presents the design goals and the techniques, which different LPWA technologies exploit to offer wide-area coverage to low-power devices at the expense of low data rates. We survey several emerging LPWA technologies and the standardization activities carried out by different standards development organizations (e.g., IEEE, IETF, 3GPP, ETSI) as well as the industrial consortia built around individual LPWA technologies (e.g., LoRa Alliance, Weightless-SIG, and Dash7 alliance). We further note that LPWA technologies adopt similar approaches, thus sharing similar limitations and challenges. This paper expands on these research challenges and identifies potential directions to address them. While the proprietary LPWA technologies are already hitting the market with large nationwide roll-outs, this paper encourages an active engagement of the research community in solving problems that will shape the connectivity of tens of billions of devices in the next decade.

Low Power Wide Area Networks: An Overview

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies.

/pdf/optimum-receiver-design-for-wireless-broad-band-systems-15owvi935e.pdf

Optimum receiver design for wireless broad-band systems using OFDM. I

This paper deals with the estimation of the channel impulse response (CIR) in orthogonal frequency division multiplexed (OFDM) systems. In particular, we focus on two pilot-aided schemes: the maximum likelihood estimator (MLE) and the Bayesian minimum mean square error estimator (MMSEE). The advantage of the former is that it is simpler to implement as it needs no information on the channel statistics. On the other hand, the MMSEE is expected to have better performance as it exploits prior information about the channel. Theoretical analysis and computer simulations are used in the comparisons. At SNR values of practical interest, the two schemes are found to exhibit nearly equal performance, provided that the number of pilot tones is sufficiently greater than the CIRs length. Otherwise, the MMSEE is superior. In any case, the MMSEE is more complex to implement.

A comparison of pilot-aided channel estimation methods for OFDM systems

Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.

/pdf/multicarrier-communication-over-underwater-acoustic-channels-yq791qq0ef.pdf

Multicarrier Communication Over Underwater Acoustic Channels With Nonuniform Doppler Shifts

From the Publisher:
Multi-carrier modulation, in particular orthogonal frequency division multiplexing (OFDM), has been successfully applied to a wide variety of digital communications applications for several years. Although OFDM has been chosen as the physical layer standard for a diversity of important systems, the theory, algorithms, and implementation techniques remain subjects of current interest. 

This book is intended to be a concise summary of the present state of the art of the theory and practice of OFDM technology. This book offers a unified presentation of OFDM theory and high speed and wireless applications. In particular, ADSL, wireless LAN, and digital broadcasting technologies are explained. 

It is hoped that this book will prove valuable both to developers of such systems, and to researchers and graduate students involved in analysis of digital communications, and will remain a valuable summary of the technology, providing an understanding of new advances as well as the present core technology.

/pdf/multi-carrier-digital-communications-theory-and-applications-15x93hdfw3.pdf

Multi-Carrier Digital Communications: Theory and Applications of OFDM

The performance of two different channel interpolation methods to be used with orthogonal frequency division multiplexing systems are investigated. The considered schemes use constant pilot frequencies for channel response estimation. The interpolation techniques are piecewise-constant and piecewise-linear methods which due to their inherent simplicity are straightforward to implement. The results are given as error probability vs. pilot separation for a channel with exponential type power-delay profile and M-ary quadrature amplitude (MQAM) submodulation with M=4, 16 and 64.

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

In this paper we study the feasibility of IEEE 802.11ah radio technology for internet of things (IoT) and machine to machine (M2M) use cases. Devices in internet of things and machine to machine networks are foreseen to communicate wirelessly. Therefore, it is of foremost importance to have a reliable, low-energy wireless technology available that may be used anywhere and anytime without draining the device battery and could achieve satisfactory coverage and data rate. Here we study the link budget, achievable data rate and packet size design of the IEEE 802.11ah radio technology in different channel scenarios. System design and simulation results show that when using this technology the system parameters can be selected to satisfy in most of the cases a reliable communication link while providing better coverage and relatively comparable throughput when compared to other existing solutions like ZigBee.

https://www.ie.u-ryukyu.ac.jp/~wada/system15/fesibility_study_80211ah.pdf

Feasibility study of IΕΕΕ 802.11ah radio technology for IoT and M2M use cases

This paper addresses the analysis and mitigation of the signal distortion caused by oscillator phase noise (PN) in OFDM communications systems. Two new PN mitigation techniques are proposed, especially targeted for reducing the intercarrier interference (ICI) effects due to PN. The first proposed method is a fairly simple one, stemming from the idea of linearly interpolating between two consecutive common phase error (CPE) estimates to obtain a linearized estimate of the time-varying phase characteristics. The second technique, in turn, is an extension to the existing state-of-the-art ICI estimation methods. Here the idea is to use an additional interpolation stage to improve the phase estimation performance around the boundaries of two consecutive OFDM symbols. The paper also verifies the performance improvement of these new PN estimation techniques by comparing them to the existing state-of-the-art techniques using extensive computer simulations. To emphasize practicality, the simulations are carried out in 3GPP-LTE downlink -like system context, covering both additive white Gaussian noise (AWGN) and extended ITU-R Vehicular A multipath channel types.

Phase noise modelling and mitigation techniques in ofdm communications systems

This paper presents an investigation of orthogonal frequency division multiplexing (OFDM) signal distortion in an amplitude limited channel. In the presence of both limiter distortion and channel noise, the back-off can be optimized to give maximum signal-to-noise ratio for a given system. >

The behavior of orthogonal frequency division multiplexing signals in an amplitude limiting channel

An equalizer is generally a transversal filter which can be used either at the passband or the baseband. Adaptive equalizers adapt filter coefficients to cancel the channel response. We study two different frequency domain adaptive equalizers for orthogonal frequency division multiplexing systems (OFDM). After introducing some theoretical aspects we derive a lower bound for the signal-to-noise ratio reduction due to equalization. Comparisons between two different equalizer structures, the least-mean square (LMS) and proportional equalizers, are presented and tested by simulation in a terrestrial TV channel with strong microreflections.

J. Rinne

Papers

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

Feasibility study of IΕΕΕ 802.11ah radio technology for IoT and M2M use cases

Phase noise modelling and mitigation techniques in ofdm communications systems

The behavior of orthogonal frequency division multiplexing signals in an amplitude limiting channel

Equalization of orthogonal frequency division multiplexing signals