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Efficient Design of Chirp Spread Spectrum Modulation for Low IEEE INTERNET OF THINGS JOURNAL, VOL. 6, NO. 6, DECEMBER 2019 9503 Efficient Design of Chirp Spread Spectrum Modulation for Low-Power Wide-Area Networks Tung T. Nguyen , Ha H. Nguyen , Senior Member, IEEE, Robert Barton, and Patrick Grossetete Abstract—LoRa is an abbreviation for low power and long monitoring, etc.), as well as in industrial applications [1], [2]. range and it refers to a communication technology developed for References [1] and [2] review prominent systems and indus- low-power wide-area networks (LPWANs). Based on the prin- try standards for LPWANs, including Sigfox [3], Ingenu [4], ciple of chirp spread spectrum (CSS), LoRa technology is very attractive to provide low bit-rate wireless connections over an DASH [5], and LoRa [6]. This paper is specifically concerned extended communication range and under very low power con- with the physical layer of LoRa because this technology is sumption. While the medium access control (MAC) layer of LoRa gaining tremendous commercial growth in more than 100 specifications is open for developers, the physical layer is not. In countries around the world.1 particular, LoRa modulation and demodulation techniques are Examining the LoRaWAN standard reveals that the standard patented by Semtech and have not been mathematically described in detail. This paper presents novel approaches to modulate mainly describes medium access control (MAC) layer proto- and demodulate LoRa signals with very high implementation cols with a set of network architectures [7], while the rest efficiency, great flexibility, and excellent performance. In partic- of the standard, especially the physical layer (PHY) archi- ular, compared to the commercially available receiver made by tecture, is left for open development. The only requirement Semtech, the proposed design is shown to yield a saving of trans- imposed on LoRa PHY is the use of chirp spread spectrum mitted power from 0.9 to 2.5 dB over the spreading factor (SF) range of 6–12. Moreover, this paper suggests a method to exploit (CSS) as the modulation technique. CSS is known for its the phase information of CSS signals to encode extra information flexibility in providing tradeoffs between reception sensitivity bits, leading to throughput improvement over the conventional and throughput. Spreading factor (SF) is the most important CSS system, for example, by 33%, 25%, 20%, and 17% for SFs parameter in CSS modulation. Increasing SF can significantly of 6, 8, 10, and 12, respectively. extend the communication range, but it comes at the cost of Index Terms—Chirp spread spectrum (CSS), digitally con- a lower transmission rate. BW is another adjustable parame- trolled oscillator (DCO), Internet of Things (IoT), LoRa, orthog- ter. Using a larger BW enhances the communication speed (as onal chirp generator (OCG). expected) and, at the same time, provides better immunity to narrow-band noise and ingress. The LoRa network is expected to exploit the modulation flexibility of CSS to optimize the I. INTRODUCTION network capacity. The evaluation of link performance as well OW-POWER wide-area networks (LPWANs) have as system-level performance of LoRaWAN can be found in [8]. L recently emerged as a promising communication solu- The currently commercialized LoRa PHY solution [9] is tion for many Internet of Things (IoT) applications. LPWANs patented by Semtech [10]. While the design promises reliable are designed to achieve large coverage ranges, extend bat- low-power communication over a long distance, it has poor tery lifetime of end-devices, and reduce the operational spectral efficiency because of two main reasons. cost of traditional cellular networks. By exploiting the sub- 1) The LoRa CSS signal occupies a much larger BW than 1 GHz unlicensed, industrial, scientific and medical (ISM) required for a CSS signal. Shown in [11] as an example, frequency band and sporadically transmitting small packets the 500-kHz CSS signal occupies more than 700 kHz at low data rates, these networks can be operated with very of the configured BW due to the large roll-off regions low reception sensitivities. The long-range and low-power on both sides of the spectrum, i.e., 100 kHz on each properties of LPWANs make these networks an interesting side. The roll-off region creates a gap between channels, candidate for smart sensing technology in civil infrastruc- preventing them from being placed close together. As a tures (such as health monitoring, smart metering, environment result, there is a smaller number of channels that can be used for a given spectrum resource unless the roll-off is Manuscript received April 11, 2019; revised May 23, 2019, June 4, reduced. 2019, and June 30, 2019; accepted July 6, 2019. Date of publication July 17, 2019; date of current version December 11, 2019. This work was 2) The generated chirps in LoRa CSS appear to be supported by the Natural Sciences and Engineering Research Council of nonorthogonal [12], which causes performance degra- Canada/Cisco Industrial Research Chair in Low-Power Wireless Access for dation when compared to the conventional orthogonal Sensor Networks. (Corresponding author: Ha H. Nguyen.) T. T. Nguyen and H. H. Nguyen are with the Department of Electrical and frequency-shift keying (FSK) system [13]. Computer Engineering, University of Saskatchewan, Saskatoon, SK S7J 4E3, To guarantee orthogonality among the chirps, there are Canada (e-mail: [email protected]; [email protected]). proposals that employ the discrete Fresnel transform, i.e., R. Barton is with Cisco Systems Canada, Vancouver, BC, Canada. P. Grossetete is with Cisco Internet of Things Business Unit, Paris, France. Digital Object Identifier 10.1109/JIOT.2019.2929496 1https://lora-alliance.org/ 2327-4662 c 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 9504 IEEE INTERNET OF THINGS JOURNAL, VOL. 6, NO. 6, DECEMBER 2019 orthogonal chirp-division multiplexing [14], or a hybrid com- II. CHIRP SPREAD SPECTRUM SYSTEM bination of FSK and CSS [13]. Unfortunately, hardware A. CSS Modulation resource to implement the discrete Fresnel transform is costly The CSS modulation used in LoRa converts each data sym- and thus is not suitable for low-cost, low-power applica- bol into a chirp, which is a signal whose frequency linearly tions expected for LoRa technology. The hybrid FSK and increases or decreases over time. A chirp is also called a sweep CSS solution significantly increases the occupied BW in signal and one CSS symbol sweeps through the BW once. exchange for orthogonal signaling, thus the spectral effi- When the instantaneous frequency of a CSS signal reaches the ciency is not improved. Although Semtech claims to have a highest, it will wrap over and start from the lowest frequency. design that produces orthogonal CSS chirps [15], [16], the SF is the most important parameter of the CSS system. The underlying method is not publicly known since the trans- CSS modulation order is defined as M = 2SF, which means mitter employs a look-up table (LUT) to store samples of that each CSS symbol carries SF bits. At baseband, each CSS the chirp. symbol contains M complex samples, which are sent out at Although the LUT design appears to be simple, the flexibil- a rate equal to the BW of the signal. Thus the CSS sym- ity requirement of CSS modulation would lead to more expen- bol duration is given as T = (M/BW) (seconds). Then the sive transceivers, since extra hardware resource is required to sym chirp rate, i.e., the rate at which the frequency of a CSS signal support multiple SF and BW settings. Generally, the size of changes over time, can be defined as an LUT increases exponentially with SF. While this might not pose a serious problem for SF values from 6 to 12 as sup- BW BW2 μ = = (Hz/s). (1) ported by available commercial LoRa transceivers, the LUT T M design will become exceedingly costly when higher SF values sym are desired. CSS modulation produces different chirps based on the Against the above background, the first part of this paper basic chirp. The basic chirp is a chirp that starts at the low- (Section II) presents an overview of CSS modulation and est frequency, i.e., −BW/2, sweeps through the entire BW, discusses important aspects such as orthogonality of CSS and then stops at the highest frequency, i.e., BW/2. As such, signaling, theoretical bit-error-rate (BER) evaluation for both the basic chirp at baseband is centered at zero frequency and coherent and noncoherent detection, continuous phase criterion defined by the following continuous-time waveform: and spectra of CSS signals. The property of phase continuity ⎧ ⎫ ⎪ ⎪ is discussed in Section III. Section IV presents an efficient ⎨⎪ ⎬⎪ design that allows supporting multiple SFs and BWs at very μt BW x0(t) = exp j 2π − t , 0 ≤ t ≤ Tsym (2) low cost. In particular, for a digital communication system, ⎪ 2 2 ⎪ ⎩⎪ ⎭⎪ data symbol modulation is generally done at baseband in a φ (t) complex plane consisting of in-phase and quadrature signals. 0 In fact, this is a must for the LUT design since it is most where φ0(t) is the phase function of the basic chirp. The efficient to store samples at the lowest rate. Then the up- instantaneous frequency of the signal at time t is the phase conversion process is applied to both in-phase and quadrature slope of the signal at that moment. That slope can be obtained signals individually using pairs of filters and digital-to-analog by taking the derivative of the phase function over time, i.e., converters (DACs), i.e., quadrature up-converters. The up- ∂φ (t) BW converter could be sophisticated since it must be able to 0 = 2π μt − (radians/s) (3) support all LoRa BW options, typically from 7.8 to 500 kHz.
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