Design of relay switching to combat an eavesdropper in IoT-NOMA wireless networks

  Citation: Tran, T.-N.; Ho, V.-C.; Vo, T.P.; Tran, K.N.N.; Voznak, M. Design of Relay Switching to Combat an Eavesdropper in IoT-NOMA Wireless Networks. Future Internet 2022,14, 71. https://doi.org/10.3390/fi14030071 Academic Editor: Seong Ki Yoo Received: 24 January 2022 Accepted: 17 February 2022 Published: 24 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). future internet Article Design of Relay Switching to Combat an Eavesdropper in IoT-NOMA Wireless Networks Thanh-Nam Tran 1,2, Van-Cuu Ho 2, Thoai Phu Vo 3, Khanh Ngo Nhu Tran 4and Miroslav Voznak 1,* 1 Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] or [email protected] 2 Faculty of Electronics and Communications, Sai Gon University, No. 273 An Duong Vuong Street, 5th District, Hochiminh 700 00, Vietnam; [email protected] 3 Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, No. 19 Nguyen Huu Tho Street, 7th District, Hochiminh 700 00, Vietnam; [email protected] 4Faculty of Information Technology, Da Lat University, No. 1 Phu Dong Thien Vuong Street, Da Lat 67 0000, Vietnam; [email protected] *Correspondence: miroslav[email protected] Abstract: The requirements of low latency, low cost, less energy consumption, high flexibility, high network capacity, and high data safety are crucial challenges for future Internet of Things (IoT) wireless networks. Motivated by these challenges, this study deals with a novel design of greencooperative IoT network, which employed coupled relays consisting of one IoT relay selected for forwarding signals to multiple IoT devices while another IoT relay transmitted jamming signals to an eavesdropper. For flexibility, all IoT nodes were powered by solar energy enough to sustain themselves, in order to consume less energy. To reach low latency, the study adopted the emerging non-orthogonal multiple access technique to serve multiple IoT devices simultaneously. Furthermore, the study adopted the simultaneous wireless information and power transfer technique which transmits wireless data for information processing and energy for energy harvesting. The study sketched a novel transmission block time period framework which plotted how a signal could travel via an individual IoT model. Maximizing the achievable bit-rate of IoT devices was considered to improve network capacity and data safety as well. Aiming at enhancing secrecy performance, a rest IoT relay played a role as a friendly jammer to transmit a jamming signal to an eavesdropper using energy harvested from the power splitting protocol. The results achieved in this study showed that the proposed model satisfied the requirements of future green IoT wireless networks. Derivatives leading to closed-form expressions are presented and verified by simulation results. The investigated results demonstrated that a friendly jammer based on radio frequency and energy harvesting strongly forces the intercept probability performance of the eavesdropper towards one, while outage probability performance of IoT devices towards zero showed that the signal to noise ratio tends to infinity. Keywords: multi-input-multi-output (MIMO); non-orthogonal multiple access (NOMA); cooperative communications; Internet of Things-relay selection (IoT-RS); energy harvesting (EH); intercept probability 1. Introduction An IoT network deals with massive connections of a variety of objects, i.e., smart things, wearable devices, sensors, etc., through the Internet, for information processing. The massively connected IoT devices are challenging problems because of system performance requirements, i.e., lower latency, higher network capacity, less energy consumption, higher data safety, higher flexibility, higher reliability, etc. In light of emerging techniques, the non-orthogonal multiple access (NOMA) technique was proposed for green IoT network applications, since the NOMA technique may provide massive connections simultaneously using the spectral sharing method. In the last decade, a number of studies on conventional NOMA-assisted IoT networks were published Future Internet 2022,14, 71. https://doi.org/10.3390/fi14030071 https://www.mdpi.com/journal/futureinternet Future Internet 2022,14, 71 2 of 22 by academic researchers and networking providers. The pioneering studies examined outage probability, system throughput, ergodic capacity, and secrecy performance. The achieved results proved that the NOMA technique showed a system performance which outperformed the orthogonal multiple access (OMA) technique. NOMA system served multiple IoT devices by a single radio frequency (RF) transmission block [ 1 – 3 ]. The NOMAIoT network employed superposition coding technique at the transmitter to encode the superimposed signals of multiple IoT devices over the same RF transmission block by allocating power multiplexing [ 4 ]. The initial signals of different IoT devices over the same RF transmission block at the base station (BS) are separated by allocating different power levels. Allocated power levels are based on the channel state information (CSI) of IoT devices [ 5 ]. The BS transmits the superimposed signal to those IoT devices with better CSI at lower transmitting power level. On the contrary, BS allocated high power level to transmit IoT device’s data with poorer CSI [ 6 ]. IoT devices employ successive interference cancellation (SIC) mechanism to decode their own message contained in the superimposed signal. The IoT device decodes the message with strongest power allocation (PA) factor by treating the other messages with lower PA factors and additive white Gaussian noise (AWGN) as interference. The IoT device then repeats SIC until detecting it own message successfully. In the study by [ 7 ], the author examined a dual-hop cooperative network with a multi-branch using all participate relay. Overall, relay selection (RS) strategies are in two categories, i.e., single RS and multi-RS as shown in [ 8 – 13 ]. For clarity, the authors selected relay based on error rate and energy efficiency ratio [ 11 ] or signal to noise ratio (SNR) maximization [ 13 ]. The incremental relay led to the achievement of higher spectral efficiency; however, higher hardware cost and complexity [ 14 ]. Therefore, this study employed a coupled relay as in [ 15 ] which shared its benefits with the multi-relay scheme; however, there was a light incremental hardware cost. This study selects just one relay for forwarding signals, by adopting the decode-and-forward (DF) protocol. Although there have been significant improvements in the benefits of spectral sharing efficiency, the study of the energy efficiency of NOMA-IoT networks is still at the initial stage. Furthermore, simultaneous wireless information and power transfer (SWIPT) the utility of RF to transfer wireless information and energy to the receiver simultaneously has been shown in many studies. Integration of the SWIPT technique in the NOMA system has been studied from the perspective of outage probability (OP) and energy harvesting (EH) performance [ 15 – 18 ]. Unfortunately, due to the utility functions of RF signal and broadcast superimposed signal of NOMA system, the wireless signal propagation from transmitter to receiver may be eavesdropped on by an illegitimate device. More specially, physical layer security (PLS) is another crucial challenge to protect legitimate devices from a potential eavesdropper in IoT wireless networks. In [ 3 ], the EH at receivers may intercept the threat to secure data propagated between legitimate IoT devices. To improve secrecy performance to the low-powered IoT devices, PLS was introduced as an alternative to the ultra-high complex cryptography algorithm [ 19 ]. PLS strategies may enhance the secrecy performance of IoT wireless networks by cooperative IoT relaying [18,20], multi-antenna beamforming [14,21–25], and jamming signal [26–33]. There is a gap for cooperative wireless communication networks to achieve distributed spatial diversity, wider networking coverage, lower energy consumption, and reduced interference [ 14 , 22 ]. Cooperative wireless communication networks were significantly improved by the benefit of RS protocol. RS protocols have been analyzed from classical adaptive diversity combining techniques (e.g., a diversity combiner adds diversified branches until the cumulative output is over a threshold [ 23 , 24 ]). This study also uses RS protocol to propose a new RS strategy enabling for green and secure IoT network. In particular, a friendly jammer is promising as a PLS strategy to improve data safety by adopting jamming signals to poison illegitimate device [ 26 – 32 ]. In [ 30 ], friendly jammer strategies were employed to enhance the secrecy metric to maximize the secrecy outage probability (SOP) for an eavesdropper. In [ 31 ], the OP, of the legitimate devices and the intercept probability (IP) of the eavesdropper was obtained to investigate the impact of the jamming signal. The authors proposed an algorithm to pair source and destination using Future Internet 2022,14, 71 3 of 22 matching theory with a particular jammer [ 32 ]. In contrast, this study adopts the SWIPT protocol. There are two major SWIPT techniques, i.e., time switching (TS) and power slitting (PS). In [ 34 ], the authors investigated the wireless-powered cooperative networks with TS and PS protocols. Moreover, Yang et al. [ 16 ] investigated the impact of PA factors on the NOMA network with SWIPT. By different scope, this paper investigates the impact of the PS factor on the system performance of IoT networks. Inspired by the above-mentioned studies, this paper employed the cooperation of a coupled IoT relay, on one hand, to improve OP performance and PLS performance for a green IoT network. To reach these aims, some work was undertaken, which are also the contributions of this paper, such as: (i) This study designed a green-and-cooperative IoT wireless network, where IoT relays and IoT devices are powered by solar and communicate using RF. (ii) To prolong IoT network lifetime, this study adopted SWIPT for EH at coupled relays by applying PS protocol. In particular, the study optimized OP performance of legitimate IoT devices by PS factor optimization in the first-half transmission block time period. (iii) In the second-half transmission block time period, the EH at the rest IoT relay intercepts the confidential information being exchanged between legitimate IoT devices. For clarity, the study proposed a selected IoT relay for forwarding signal to legitimate IoT devices using EH while another IoT relay for transmitting jamming signals to illegitimate device using EH as well. In this way, the study reached IP performance at an illegitimate device tending to one. The rest of the present paper is structured as follows: Section 2sketches and describes the design of IoT wireless communication models, and follows this by making formulations. Section 3 presents an analysis based on the proposed IoT model and presented the algorithms using for RS strategy and OP investigations. Section 4presents the analysis and Monte Carlo simulation results. Section 5concludes the achieved results of the study. 2. System Model and Formulation Inspired by the study by [ 30 ], this study sketches a green-cooperative IoT wireless network, as shown in Figure 1consisting of an IoT hub S , a coupled IoT relay, and the number of N IoT devices. Assuming all IoT nodes have been solar-powered enough for their operation and equipped with a single antenna. The study employed coupled IoT relays R1 and R2 to assist IoT devices. The coupled IoT relays adopted the DF protocol for forwarding superimposed signals to IoT devices. To prolong IoT network online time, the study employed the SWIPT technique with the PS protocol to transmit simultaneously information for information processing and energy for EH at coupled IoT relays. Furthermore, the study proposed an RS strategy to select the relay for the forwarding signal. The selected relay used RF-EH for forwarding the signal instead of its own energy. However, the IoT network as shown in Figure 1had an eavesdropper E that eavesdropped the IoT relay R1 or R2 by wiretapping channel hR1,E for odd transmission block or hR2,E for even transmission block since the broadcast function of RF. Therefore, the study proposed the rest IoT relay to transmit the jamming signals to the eavesdropper Eusing RF-EH instead of its own energy. Table 1shows a comparison of the works in this study, in contrast to previous studies. Please note that the IoT devices Dn for n∈N had distances from IoT hub Sas ordered dS,D1<. . . <dS,DN. Future Internet 2022,14, 71 4 of 22 Figure 1. The green-cooperative IoT wireless network. Table 1. Comparison to the other studies on secrecy performance. Studies Number of Relays RS Number of Devices SWIPT Jamming Signal [21] 0 no 2 no no [18] 1 no 2 yes no [25] 0 no Kno yes [20] 1 no 2 no yes [33] 1 no 1 no yes [35]Kyes 1 yes no This study 2 yes Nyes yes Figure 2. Power resource allocation with PS factor λand PA factors αifor i={1, . . . , N}. 2.1. Signal Transmission Block Time Period Observing Figure 1, the IoT network needs two time slots to complete a signal propagation from the transmitter (IoT hub S ) to the receivers (IoT device Dn for n={N, . . . , 1} ) through coupled IoT relays R1 and R2 . For clarity, the study designed a novel transmission block time period diagram as shown in Figure 2. By adopting the SWIPT technique (PS protocol), the IoT hub S transmits including simultaneous energy by power domain λRrP for EH and superimposed information by remaining power domain (1−λRr)P for information processing at the IoT relay Rr , where r={1, 2} , λRr is PS factor and P is power domain at IoT hub S . By the RS strategy as shown in Algorithm 1, only the selected IoT Future Internet 2022,14, 71 5 of 22 relay is used for forwarding signal to legitimate IoT devices while the other IoT relay is used for transmitting the jamming signal to eavesdropper E . In the second transmission block time, Algorithm 1switches the selected relay, where variable f lag is zero and repeats the signal transmission period. Algorithm 1 Algorithm for switching relay selection Input: R1←0; R2←1; time_period ←random(1 : 9); selected_relay ←random(R1,R2); f lag ←time_period; Output: The selected relay forwarded legitimate signals while the other relay transmitted jamming signals. 1: while true do 2: if f lag!=0then 3: Function_Information_Processing(selected_relay); 4: Function_Forwarding_Signal(selected_relay); 5: Function_Jamming_Signal(!selected_relay); 6: f lag ←f lag −1; 7: else 8: f lag ←time_period; 9: selected_relay ←!selected_relay; 10: end if 11: end while 2.2. Relay Selection Strategy This study proposed the IoT hub S controlled whole private IoT network. It means that the IoT hub S pre-defined the cooperative script and synchronized the cooperative script to the coupled IoT relays and N devices. Please note that the cooperative script is re-configurable to achieve flexible cooperation. Algorithm 1selects which IoT relay ( R1 or R2 ) is being selected among coupled relays. For clarity, we first assigned values zero and one for coupled IoT relays R1 and R2 , respectively. The study initialized time_period to hold the number of times an IoT relay would be selected for information processing and forwarding legitimate signals to legitimate IoT devices. Variable f lag is used for counting whether the selected IoT relay forwarded signals enough number of transmission block time period or not. We had a while loop of to ensure the IoT network operated in real-time. We had an if condition to check variable f lag. There are two cases: (i) If variable f lag is non-zero, it means that an IoT relay has been selected. The selected IoT relay has to process legitimate information and then forward legitimate information after, while the non-selected IoT relay has to transmit a jamming signal. We counted down variable f lag. (ii) If variable f lag is zero, it means that the selected IoT relay finishing its obligation. Algorithm 1swaps obligations between IoT relays and resets variable f lag. This assumes that the coupled relays own the cooperative script and activated synchronization. 2.3. Formulations To keep the following simple, the study assumed that variables in Algorithm 1were assigned initial values as time_period = 1 and selected_relay =R1 without losing general properties of Algorithm 1. It is worth noticing that we have sequential block time period T=nT(1),T(2), . . . , +∞o∈Z+ . Since assuming time_period = 1 and selected_relay =R1 , Future Internet 2022,14, 71 6 of 22 we split the set T into two subsets, where T(odd)=nT(1),T(3), . . .o∈Z(odd) for odd transmission block and T(even)=nT(2),T(4), . . .o∈Z(even)for even transmission block. The EH at the receiver was provided as [ 36 ] (Equation (6)) for the PS protocol. From the aforementioned expressions, the EH at the coupled IoT relays in the first phase of the first time slot T(odd) 1∈T(odd)or T(even) 1∈T(even), is expressed as follows: EHT(odd) 1 S,R1=ηλR1PσS,R1, (1) EHT(odd) 1 S,R2=ηλR1PσS,R2, (2) EHT(even) 1 S,R1=ηλR2PσS,R1, (3) EHT(even) 1 S,R2=ηλR2PσS,R2, (4) where λR1 and λR2 are the PS factors (0 <λR1< 1 in the odd transmission block and 0 <λR2< 1 in the even transmission block), P is power domain at IoT hub S , η is collect factor for 0 ≤η≤ 1, and expected channel gains σS,R1 and σS,R2 are given by σS,R1=EnhS,R12oand σS,R2=EnhS,R22o, respectively. Furthermore, in the second phase of the first time slot T(odd) 1∈T(odd) or T(even) 1∈ T(even) , in terms of NOMA benefit, the IoT hub S encodes the messages xi for i={N, . . . , 1} of IoT devices and superimposes the messages into the signal by superposition coding and sharing the remaining power domain ( 1 −λR1)P for T(odd) 1 or ( 1 −λR2)P for T(even) 1 with different PA factors αi . Then, the received signals at the IoT relays R1 and R2 express as follows: yT(odd) 1 R1=hS,R1q1−λR1P 1 ∑ i=N √αixi+nR1, (5) yT(odd) 1 R2=hS,R2q1−λR1P 1 ∑ i=N √αixi+nR2, (6) yT(even) 1 R1=hS,R1q1−λR2P 1 ∑ i=N √αixi+nR1, (7) yT(even) 1 R2=hS,R2q1−λR2P 1 ∑ i=N √αixi+nR2, (8) where nR1 and nR2 are AWGN, i.e., nR1∼CN(0, N0) and nR2∼CN(0, N0) , at IoT relays R1 and R2 , respectively. Moreover, hS,Rr=gd−ε 2 S,Rr is channel coefficient from IoT hub S to IoT relay Rr ( r={1, 2} ), g denoted Rayleigh distribution from IoT hub to coupled IoT relays with g∼CN(0, 1) , ε is path-loss exponent, dS,R1 and dS,R2 are distances from IoT hub S to IoT relays R1 and R2 , respectively. Assuming IoT relays are closed to each other (dS,R1=dS,R2). From the system model as shown in Figure 1, the devices D1 and DN are nearest and farthest from the IoT hub S , respectively. In the concept of NOMA, the farthest device DN was allocated the biggest PA factor since the farthest device DN owned poorest CSI. Inspired by studies [ 21 , 37 ], the PA factor αi for i={N, . . . , 1} carried the message xiis given as follows: Future Internet 2022,14, 71 7 of 22 αi=i,N ∑ n=1 n, (9) s.t. αN>. . . >α1, (10) αN+. . . +α1=1. (11) In another term of NOMA theory, the SIC mechanism was adopted at coupled IoT relays. Please note that the message xN owned the biggest PA factor αN . The message xN thereby is prioritized SIC first by treating the other messages xj={x1, . . . , xN−1} and AWGN nRr as interference. The SIC mechanism repeats until successfully decoded the last message x1since it was allocated the smallest PA factor α1. Therefore, signal to interference plus noise ratio (SINR) obtained at the IoT relay R1 in odd transmission block time period T(odd) , when R1 decoded the message xi(i={N, . . . , 1} ) by treating the messages xj={x1, . . . , xi−1}and AWGN nR1as interference, as follows: γT(odd) 1 R1−xi i={N,...,1} =1−λR1hS,R12αiρ 1−λR1hS,R12ρi−1 ∑ j=1 αj+1 , s.t. i>1, (12) =1−λR1hS,R12αiρ, s.t. i=1, (13) where SNR ρ=P N0. Similarly, SINR obtained at the IoT relay R2 in even transmission block time period T(even) , when R2 decoded the message xi ( i={N, . . . , 1} ) by treating the messages xj= {x1, . . . , xi−1}and AWGN nR2as interference, as follows: γT(even) 1 R2−xi i={N,...,1} =1−λR2hS,R22αiρ 1−λR2hS,R22ρi−1 ∑ j=1 αj+1 , s.t. i>1, (14) =1−λR2hS,R22αiρ, s.t. i=1. (15) An instantaneous bit-rate threshold is achievable after the coupled IoT relays R1and R2 decoded the message xi in the received signal given by (5) for odd transmission block time period T(odd)and (8) for even transmission block time period T(even)as follows: RT(odd) 1 R1−xi i={N,...,1} =1 2log2 1+γT(odd) 1 R1−xi i={N,...,1} , (16) RT(even) 1 R2−xi i={N,...,1} =1 2log2 1+γT(even) 1 R2−xi i={N,...,1} . (17) Minimum of instantaneous bit-rates at relay R1 in odd transmission block time period T(odd)and relay R2in even transmission block time period T(even)express as follows: min RT(odd) 1 R1=min i={N,...,1}RT(odd) 1 R1−xi, (18) min RT(even) 1 R2=min i={N,...,1}RT(even) 1 R2−xi. (19) In the second time slot, the IoT relays R1 for T(odd) 2 and R2 for T(even) 2 retrieves, reencodes and forwards the superimposed signals to the IoT devices. There are two feature Future Internet 2022,14, 71 8 of 22 forwarding protocols (i.e., amplify-and-forward (AF) and DF). This study employed the DF protocol at IoT relays R1 and R1 to ensure that R1 and R1 received and decoded the messages successfully. After receiving the signals as given by (5) for odd transmission block and (8) for even transmission block, the IoT relays R1 and R2 decoded the messages of devices and retrieved the messages and then forwarded them to the devices using the EH as given by (1) for odd transmission block T(odd) and (4) for even transmission block T(even). The received signal at IoT device Dnfor n∈Nis expressed as follows: yT(odd) 2 Dn=hR1,Dn 1 ∑ i=NrαiEHT(odd) 1 S,R1xi+nDn, (20) yT(even) 2 Dn=hR2,Dn 1 ∑ i=NrαiEHT(even) 1 S,R2xi+nDn, (21) where nDnis AWGN at the terminal device Dn, i.e., nDn∼CN(0, N0). IoT device Dn also adopted the SIC mechanism to decode its own message xn . By substituting (1) into (20) and (4) into (21) , we obtain SINR at device Dn for n∈N when the device Dn, respectively, decoded the data symbol xi, where i={N, . . . , n}, as follows: γT(odd) 2 Dn−xi i={N,...,n} i>1 =hR1,Dn2αiηλR1ρσS,R1 hR1,Dn2ηλR1ρσS,R1 i−1 ∑ j=1 αj+1 , (22) i=1 =hR1,Dn2αiηλR1ρSσS,R1, (23) where expression (22) is used for IoT device Dn ( n> 1 and n≤N ) while IoT device D1 has to adopt, respectively, expressions (22) first and then (23) to decode its own message x1. Similarly, the device Dn in the even transmission block also adopted the SIC mechanism to decode its own message xn . SINR obtained at device Dn for n∈N when the device Dn, respectively, decoded the data symbol xi, where i={N, . . . , n}, as follows: γT(even) 2 Dn−xi i={N,...,n} i>1 =hR2,Dn2αiηλR2ρσS,R2 hR2,Dn2ηλR2ρσS,R2 i−1 ∑ j=1 αj+1 , (24) i=1 =hR2,Dn2αiηλR2ρσS,R2, (25) where expression (24) is used for IoT device Dn ( n> 1 and n≤N ) while IoT device D1 has to adopt, respectively, expressions (24) first and then (25) to decode its own message x1. The instantaneous bit-rate threshold is reachable when the IoT device Dn decodes the data symbols xi={xN, . . . , xn} in odd and even transmission blocks, respectively, as follows: RT(odd) 2 Dn−xi i={N,...,n} =1 2log2 1+γT(odd) 2 Dn−xi i={N,...,n} , (26) RT(even) 2 Dn−xi i={N,...,n} =1 2log2 1+γT(even) 2 Dn−xi i={N,...,n} . (27) Future Internet 2022,14, 71 9 of 22 The minimum of the achievable bit-rate threshold reached at device Dn ( n∈N ) in odd and even transmission blocks, respectively, as follows: min RT(odd) 2 Dn=min i={N,...,n}RT(odd) 2 Dn−xi, (28) min RT(even) 2 Dn=min i={N,...,n}RT(even) 2 Dn−xi. (29) It is the presence of the eavesdropper E as shown in Figure 1. The eavesdropper E eavesdrops the IoT devices by wiretapping channel hR1,E for odd transmission block time period T(odd) or wiretapping channel hR2,E for even transmission block time period T(even) . To combat eavesdropper E , this study adopted the rest relay as a friendly jammer. Please note that the selected IoT relay and the rest IoT relay in the odd transmission block time period T(odd) are IoT relays R1 and R2 , respectively. This study proposes the IoT relay R2 uses the EH as given by (2) to transmit jamming signals to the eavesdropper E . Simultaneously, the eavesdropper E receives the wiretapping signal from the IoT relay R1 and the jamming signal from the rest IoT relay R2 in the second time slot T(odd) 2 . In contrast, the selected IoT relay and the rest IoT relay in the even transmission block time period T(even) are IoT relays R2 and R1 , respectively. Similarly, the IoT relay R2 forwards signal using EH as given by (4) while IoT relay R1 uses the EH as given by (3) to transmit the jamming signal to eavesdropper E . The eavesdropper E receives simultaneously the wiretapping signal from the IoT relay R2 and the jamming signal from the rest IoT relay R1 in the second time slot T(even) 2. In odd or even transmission blocks, the received signal at eavesdropper E is expressed as follows: yT(odd) 2 E=hR1,E 1 ∑ i=NrαiEHT(odd) 1 S,R1xi | {z } Wiretapping signal +hR2,ErδEHT(odd) 1 S,R2˜ x | {z } Jamming signal +nE |{z} AWGN , (30) yT(even) 2 E=hR2,E 1 ∑ i=NrαiEHT(even) 1 S,R2xi | {z } Wiretapping signal +hR1,ErδEHT(even) 1 S,R1˜ x | {z } Jamming signal +nE |{z} AWGN , (31) where nE is AWGN at eavesdropper E with nE∼CN(0, N0) , δ is status of jamming signal. If δ= 0, the rest IoT relay did not transmit the jamming signal. If δ= 1, the rest IoT relay transmitted the jamming signal fully by EH as (2) or (4). From expression (30) for odd transmission block time period, eavesdropper E adopted SIC to detect IoT devices’ message xi={xN, . . . , x1} . SINR obtained at the eavesdropper E, after the eavesdropper Edecoded the data symbol xi(i={N, . . . , 1}) as follows: γT(odd) 2 E−xi i={N,...,1} i>1 =hR1,E2αiηλR1ρσS,R1 ηλR1ρ hR1,E2σS,R1 i−1 ∑ j=1 αj+hR2,E2σS,R2!+1 , (32) i=1 =hR1,E2αiηλR1ρσS,R1 hR2,E2ηλR1ρσS,R2+1. (33) Future Internet 2022,14, 71 16 of 22 The system throughput results in Figure 5a,b tended to pre-defined bit-rate threshold R when SNR ρ→∞ . It is important to notice that IoT devices’ throughput performance reached in Figure 5b is higher IoT devices’ throughput performance reached in Figure 5a since IoT devices’ bit-rate threshold R= 0.499 b/s/Hz is greater than fixed IoT devices’ bit-rate threshold R=0.1 b/s/Hz. 0 5 10 15 20 25 30 35 40 45 50 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 (a) 0 5 10 15 20 25 30 35 40 45 50 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 (b) Figure 5. Comparisons of system throughput at IoT devices with ( a ) fixed PS factor λRr= 0.4 and fixed bit-rate threshold R=0.1, (b) optimized bit-rate threshold and PS factors. To verify the potential threat affecting the information propagated on the proposed IoT network, the study investigated the probability that an eavesdropper successfully decoded information from the wiretapping signal as shown in Figure 6. Unfortunately, the results obtained plotted that eavesdropper E has a better probability result than legitimate IoT devices decoding their own proprietary information. In other words, the results shown in Figure 6are better than those shown in Figure 4a. It is worth explaining cause eavesdropper E has better probability results in decoding information than legitimate IoT devices. Please note that the expression (52) becomes (45), where δ= 0. In addition, the distance from the eavesdropper E to the IoT relays ( dRr,E= 4 m) is less than the distance from the IoT devices to the IoT relays ( dRr,D1= 5 m, dRr,D2= 7 m, dRr,D3= 12 m). As a consequence, eavesdropper E has channel gain ( σRr,E= 0.0625) that is greater than the channel gains possessed by IoT devices ( σRr,D1= 0.04, σRr,D2= 0.0204, σRr,D3= 0.0069). Thus, we obtained probability results that eavesdropper E can successfully decode the information better than IoT devices can. In other words IPE→Dn<OPDn , where δ= 0 and dRr,E<dRr,Dn. In contrast, the major aim of this study is to improve the security of an individual IoT wireless network as shown in Figure 1. There are two cases. In the first case, transmission script is unknown at eavesdropper E . Eavesdropper E randomly selected a received signal (30) or (31) for SIC. In the second case, the transmission script is well-known at eavesdropper E , eavesdropper E selected signal as (30) for SIC in odd transmission block and then signal as (31) for SIC in even transmission block. Figure 7a shows the potential threat when eavesdropper E is able to decode messages of legitimate devices from the wiretapping signal. For example, the eavesdropper E had IPE→Dn(t)≈ 0.2 probability to decode message x3 successfully at SNR ρ= 30 dB. However, IP performance reached IPE→Dn(t)≈ 1 approximately at SNR ρ= 30 dB. In particular, the eavesdropper E had just IPE→Dn(t)≈ 0.9986 even ρ→∞ since the impact of the friendly jamming signal as shown in Figure 7b. Future Internet 2022,14, 71 17 of 22 0 5 10 15 20 25 30 35 40 10-4 10-3 10-2 10-1 100 Figure 6. IP performance of cooperative NOMA-IoT network without jamming signal. 0 5 10 15 20 25 30 35 40 45 50 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 (a) 0 5 10 15 20 25 30 35 40 45 50 0.9988 0.999 0.9992 0.9994 0.9996 0.9998 1 (b) Figure 7. IP performance of cooperative NOMA-IoT network with jamming signal, ( a ) fixed PS factor λRr=0.4 and fixed bit-rate threshold R=0.1, (b) optimized bit-rate threshold and PS factor. 5. Conclusions Future IoT network requirements have accelerated information propagation rapidly and safely. To satisfy these major aims, the study employed the emerging cooperative NOMA technique and proposed some new solutions, i.e., RS strategy and RF-EH-enabling for forwarding signal and transmitting jamming signal. On one hand, IoT devices’ threshold is defined to maximize throughput performance. On the other hand, the PS factor is also defined to optimize OP and IP as well. Based on the practical model, analysis and simulation results, the study conducted the propositions may provide high network capacity and high reliability to toward next IoT networks. The study is still opening some future tasks such as how Nakagami-mdistributions affect OP performance, or how secrecy performance is impacted if eavesdropper equipped multi-antennas. Author Contributions: T.-N.T., V.-C.H. and T.P.V. delivered main ideas for discussion; V.-C.H. and T.P.V. collected and provided theoretical knowledge about wireless communication techniques to T.-N.T. for analyzing. T.-N.T. and K.N.N.T. designed a system model and formulated its parameters and performance characteristics. T.-N.T. and K.N.N.T. programmed simulations to validate results of the model while V.-C.H. and T.P.V. prepared the manuscript by MS Word. T.-N.T. organized and re-wrote the paper in LaTeX template. M.V. formed initial research ideas and directions, provided Future Internet 2022,14, 71 18 of 22 supervisions, critically reviewed the organization of the paper and served as a corresponding author. All authors have read and agreed to the published version of the manuscript. Funding: This research received funding from the Ministry of Education, Youth and Sports within the grant reg. No. SP2021/25 and partially within the Large Infrastructures for Research, Experimental Development and Innovations project reg. No. LM2018140. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Appendix A. Proof of Theorem 1 By substituting (1), (4) into (5), (8), respectively, we obtain the received signals at IoT relays R1 and R2 in odd and even transmission blocks, respectively. By applying SIC, we obtain an instantaneous bit-rate threshold at coupled IoT relays as shown in (16) and (17). By substituting expression (16) or (17) into expression (40) and after the algebraic transformation, we obtain OP at coupled IoT relays in closed-form by applying PDF over Rayleigh distribution as follows: OPRr(t)=1−Pr|hS,Rr|2≥γ (1−λRr)βρ  =1− ∞ Zγ (1−λRr)βρ 1 σS,Rr exp−x σS,Rrdx =1−exp−γ (1−λRr)βρσS,Rr. (A1) In Lemma 1, this study assumed that IoT hub S has been equipped with the number of antennas AS and coupled IoT relays have been equipped with the number of antennas AR . By employing TAS protocol at IoT hub S , SC protocol at coupled IoT relay as in [ 15 ] and applying CDF as given by [ 21 ] (Equation (57)), we obtain OP at coupled IoT relays for MIMO scenario as follows: OPRr(t)=1−Prmax [AS×AR]n|HS,Rr|2o≥γ (1−λRr)βρ  = A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)! ∞ Zγ (1−λRr)βρ 1 σS,Rr exp−x σS,Rrdx = A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)!exp−ψγ (1−λRr)βρσS,Rr, (A2) where A=ASAR. Appendix B. Proof of Theorem 2 From (44), OP at IoT device Dnis rewritten as follows: OPDn(t)=max           1−Prmin i={N,...,n}RT(t) 1 Rr−xi≥ R | {z } Zn , 1 −Prmin i={N,...,n}RT(t) 2 Dn−xi≥ R | {z } Qn            . (A3) Future Internet 2022,14, 71 19 of 22 After the algebraic transformation, expression Zn in (A3) could be obtained by applying PDF given by [21] (Equation (55)) for i>1 or [21] (Equation (56)) for i=1 as follows: Zn=1−Pr|hS,Rr|2≥γ (1−λRr)βnρσS,Rr =1− ∞ Zγ (1−λRr)βnρ 1 σS,Rr exp−x σS,Rrdx =1−exp−γ (1−λRr)βnρσS,Rr, (A4) s.t. βn=min i={N,...,n}{βi}, (A5) where βiin expression (A5) is given by (42). By substituting (26) for t=odd or (27) for t=even into (A3) and applying PDF given by [ 21 ] (Equation (55)) for i> 1 or [ 21 ] (Equation (56)) for i= 1, expression Qn in (A3) could be obtained as follows: Qn=1−Pr|hRr,Dn|2≥γ ηλRrβnρσS,Rr =1− ∞ Zγ ηλRrβnρσS,Rr 1 σRr,Dn exp−x σRr,Dndx =1−exp−γ ηλRrβnρσS,RrσRr,Dn, (A6) where βnin (A6) is given by (A5). From results presented in study [ 18 ], the OP result at IoT device referred to OP result at either IoT relays or IoT device. Therefore, expression (A3) could be rewritten as follows: OPDn(t)=max{Zn,Qn} =1−exp−γ Ωn, (A7) s.t. vn=min i={N,...,n}{(1−λRr)βiρσS,Rr}, (A8) ωn=min i={N,...,n}{βiηλRrρσS,RrσRr,Dn}, (A9) Ωn=min{vn,ωn}. (A10) In Lemma 2, IoT hub S , relays and devices have been equipped multiple antennas with AS,ARand AD, respectively. Therefore, expressions Znand Qnin (A3) are rewritten as follows: Zn=1−Prmax [AS×AR]n|HS,Rr|2o≥γ (1−λRr)βnρ =1−     1− A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)! ∞ Zγ (1−λRr)βnρ 1 σS,Rr exp−x σS,Rrdx     = A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)!exp−ψγ (1−λRr)βnρσS,Rr, (A11) Future Internet 2022,14, 71 20 of 22 Qn=1−Prmax [AR×AD]n|HRr,Dn|2o≥γ ηλRrβnρσS,Rr =1−   1− A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)! ∞ Zγ ηλRrβnρσS,Rr 1 σRr,Dn exp−x σRr,Dndx    = A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)!exp−ψγ ηλRrβnρσS,RrσRr,Dn. (A12) By substituting (A11) and (A12) into (A3) , we obtain OP at device Dn in MIMO scenario as follows: OPDn(t)=max{Zn,Qn} = A ∑ ψ=0 (−1)ψA! ψ!(A−ψ)!exp−ψγ Ωn. (A13) where Ωn is given by (A10) and A=ASAR or A=ARAD if max{Zn,Qn}=Zn or max{Zn,Qn}=Qn, respectively. Appendix C. Proof of Theorem 3 From expression (49), IP at eavesdropper Eis rewritten as follows: IPE→Dn(t)=1−Prminmin i={N,...,n}RT(t) 1 Rr−xi, min i={N,...,n}RT(t) 2 E−xi≥ R. (A14) After the algebraic transformation, we obtain IP at eavesdropper Eas follows: IPE→Dn(t)=1−min i={N,...,n}Pr|hS,Rr|2≥γ (1−λRr)βnρ, Pr  |hRr,E|2≥γδηλRrρσS,Rr|hR¯ r,E|2+1 ηλRrρσS,Rrβn,|hR¯ r,E|2≥0     =1−min         ∞ Zγ (1−λRr)βnρσS,Rr 1 σS,Rr exp−x σS,Rrdx, ∞ Z0 ∞ Z γ(δηλRrρσS,Rry+1) ηλRrρσS,Rrβn 1 σRr,EσR¯ r,E exp−x σRr,E−y σR¯ r,Edxdy           =1−min         exp−γ (1−λRr)βnρσS,Rr | {z } K ,βnσS,RrσRr,E βnσS,RrσRr,E+δγσS,R¯ rσR¯ r,E | {z } U exp−γ ηλRrρβnσS,RrσRr,E | {z } V          .(A15) Observing expression (A15) , IP at eavesdropper E referred to IPE→Dn(t)= 1 − min{K,UV}= 1 −UV since the impact of the jamming signal. IP at eavesdropper E then tends to floor IPE→Dn(t)= 1 −min{K,UV}= 1 −U since V→ 1 when SNR ρ→∞ . The major aim of this study is to improve secrecy performance for IoT network. It means that expression U in (A15) needs to tend to zero ( U→ 0). 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