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A Chirp Spread Spectrum Modulation Scheme for Robust Power Line Communication Stephen Robson and Manu Haddad, Member, IEEE

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A Chirp Spread Spectrum Modulation Scheme for Robust Power Line Communication Stephen Robson and Manu Haddad, Member, IEEE 1 A Chirp Spread Spectrum Modulation Scheme for Robust Power Line Communication Stephen Robson and Manu Haddad, Member, IEEE Abstract—This paper proposes the use of a LoRa like chirp This paper proposes a PLC modulation scheme based on spread spectrum physical layer as the basis for a new Power Line the Chirp Spread Spectrum (CSS) scheme of the recently stan- Communication modulation scheme suited for low-bandwidth dardised LoRa physical layer [4]. The modification is designed communication. It is shown that robust communication can be established even in channels exhibiting both extreme multipath to combat the two major problems of extreme multipath and interference and low SNR (-40dB), with synchronisation require- low SNR. The former is resolved by subdividing the LoRa ments significantly reduced compared to conventional LoRa. symbol into a reduced set, thereby containing the multipath ATP-EMTP simulations using frequency dependent line and energy into a single symbol. The latter is resolved through the transformer models, and simulations using artificial Rayleigh use of statistical averaging of the modified signal over consec- channels demonstrate the effectiveness of the new scheme in providing load data from LV feeders back to the MV primary utive symbols. This trading off of data rate for performance substation. We further present experimental results based on a makes possible a communication scheme in which many LV Field Programmable Gate Array hardware implementation of feeder monitoring devices can communicate back to a primary the proposed scheme. substation at timescales of several seconds or minutes. Index Terms—PLC, LoRa, LV Monitoring. II. BACKGROUND A. Requirements for Robust Communication on the LV-MV I. INTRODUCTION channel The communication channel linking the LV and MV parts ISTRIBUTION Network Operators (DNOs) already de- of a distribution network is characterised by extreme mul- ploy a wide range of communication technologies to D tipath conditions (σrms = 10’s to 100’s of µs). The main support the move towards smart grids. Advances in the stan- contributing factor to this is not the attenuation of the power dardisation of narrowband Power Line Communication (PLC) line itself, rather it is a result of delayed versions of the solutions such as Prime and G3-PLC, and growing options signal reaching the receiver from many different paths. On in long range wireless communication (i.e. LoRa) have only the MV network, the typical lengths of the line become much added to the options in the last decade. But there remains an larger than the wavelength of the narrowband PLC signal, and unmet requirement for low-cost and robust communication of, lines are terminated by open circuits or transformers with for example, load data from secondary substations. The prob- large reflection coefficients. Therefore, much of the signal lem is amplified by the sheer number of required monitoring energy remains in the power line until it dissipates. Empirical points (typically tens of thousands in a large regional distribu- measurements of the RMS delay spread on MV distribution tion network) and the fact that secondary substations are often networks is in the tens of µs [5]. In contrast, the RMS delay located in rural areas with limited access to conventional wired spread on LV networks is less than 10 µs [6]. Therefore, a or wireless communication infrastructure. robust communication scheme suited to this environment must Previous attempts at providing widescale communication accommodate extremely high RMS delay spreads, far beyond across large distribution networks have tended to focus on what is typical in LV and conventional wireless systems. arXiv:2106.13965v1 [eess.SP] 26 Jun 2021 the use of conventional wired media (i.e. ethernet), wireless When considering cross-network (LV-MV) transmission, for solutions (LoRa, GSM) or PLC. example in a system which relays load information from a Narrowband PLC solutions such as Prime [1] and G3-PLC secondary to a primary substation, a second major problem [2] are now firmly established on Low-Voltage (LV) networks, emerges. Though it has been demonstrated that PLC signals and are often deployed in automatic meter reading (AMR) in the narrowband range (15-500 kHz) can indeed propagate applications and increasingly in support of other smart grid through transformers, the attenuation is large. Empirical mea- services. Over longer distances and across voltage levels, these surements show an average 35 dB attenuation with a high technologies struggle to cope with the increased attenuation degree of frequency selectivity [7]. The SNR penalty imposed and extreme multipath conditions associated with transmission by this scale of attenuation renders existing narrowband PLC through transformers and Medium Voltage (MV) networks [3]. technologies unusable. The situation is further exacerbated by New technologies are required in this space. regulatory limits on transmit power on power lines. Therefore, reliable inter-transformer communication is only possible with S. Robson and A. Haddad are with the Advanced High Voltage Engi- communication schemes that can work at low SNRs. neering Research Centre (AHIVE) , Cardiff University, UK. e-mail: rob- [email protected] The dual problem of extreme multipath and high attenu- Manuscript received June 19, 2021; revised:. ation makes the design of a communication system for this 2 channel extremely challenging. Recently, the emerging LoRa Channel Impulse Response standard was proposed for PLC communication [8], and then for time disemmination [9][10]. LoRa has several favourable Impulse response appears as “reversed” properties, including excellent receiver sensitivity and low in the LoRa correlator output power. However, in its raw form, it performs poorly in severe multipath. Here, we exploit the unique properties of LoRa, with a few key modifications, for robust performance in the low SNR and high multipath regime. LoRa Correlator Output, y k-5 k-4 k-3 k-2 k-1 k k+1 2SF B. The LoRa physical layer Fig. 1. The correlator output resulting from transmission in a multipath channel. The mathematical basis underpinning the LoRa physical layer has been studied extensively in several recent works [11][12][13]. LoRa transmits symbols as frequency shifted delay spreads of several tens of µs have been recorded. In 1 this case, the multipath energy will smear across samples and chirps. With a bandwidth of B = T , the transmitted symbol, wk is defined as: will be more appropriately modelled as frequency selecting fading. A basic model of the situation can be constructed r Es j2π·(k+n) mod 2SF · n in which delayed versions of the transmitted symbol arrive w (nT ) = e 2SF (1) k 2SF at the receiver. Due to the unique way LoRa is modulated - The above equation describes a series of n = effectively as time-shifted versions of a base chirp - a delayed 0; 1; 2 ::: 2SF −1 consecutive samples forming a LoRa symbol. version of a transmitted symbol will be demodulated as if it SF 2 f7; 8 :::; 12g is the so called Spreading Factor, which belonged to an adjacent symbol, as shown in Eqn. 4, where i = k − 1 : : : k − x α : : : α determines the number of transmitted samples per LoRa index are proportionate to 2 x, α symbol. k 2 0; 1; 2 ::: 2SF −1 is the transmitted symbol and where is the impulse response of the channel. This is shown graphically in Fig. 1. Es is the symbol energy. It has been shown in [11] that SF the 2 basis functions are orthogonal allowing a sufficiently p r 8 α E + φ i = k synchronised receiver to demodulate using correlation. If k > 1 s i >p is the received symbol corrupted by Additive White Gaussian SF −1 > α2Es + φi i = k − 1 ∗ 2 > Noise (AWGN), φi, and w is the complex conjugate of X <. i r (nT ) · w∗(nT ) = . (4) k k i . symbol (i.e. that corresponding to the transmitted symbol), >p y k n=0 > α E + φ i = k − x the correlator output will exhibit a peak at index . > x s i :> φi elsewhere 2SF −1 (p X ∗ Es + φi i = k Therefore, the channel impulse response can be mapped out rk(nT ) · wi (nT ) = (2) φi i 6= k from the correlator output. In standard LoRa modulation, the n=0 presence of strong multipath interference in which the signal p yk = arg max(jδk;i Es + φij) (3) arrives by one or more indirect paths presents a problem for the LoRa demodulator because strong correlation peaks caused by It is demonstrated in [11] how the more computationally these paths will compete against the true transmitted symbol, efficient method removes the need to perform the full cor- raising the SER. relation over all 2SF basis functions. Indeed, the method used by LoRa requires only the multiplication of rk with the III. DESCRIPTION OF THE PROPOSED METHOD complex conjugate of the base down chirp (a process known as A. Enhancement for Robustness in Extreme Multipath dechirping). The dechirped signal comprises a pure frequency tone which is proportionate to k, so an FFT and find max It is interesting to note that the correlator output of the LoRa routine completes the demodulation process. demodulator mimics the channel impulse response. This was Equation 2 shows that correct demodulation will take place noted in [14], which exploits the regularity of the channel p impulse response across subsequent LoRa symbols through when Es + φk exceeds the maximum value of φ across all correlations. Although there may be significant distance the use of cross-correlation. This method would be particularly p interesting in PLC applications because the channel is fixed. between the PDFs of φ and Es + φk, it is actually the PDF of the maximum of φ per symbol that is of interest in Symbol However, the performance is similar to that of conventional Error Rate (SER) calculations.
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