Enabling D2D Transmission Mode with Energy Harvesting and Information Transfer in Heterogeneous Networks

. The concept of energy harvesting-assisted relay has been introduced to support the relaying transmission using Device-to-Device (D2D) communications for enhancing communication reliability. Motivated by the recent advance in Heterogeneous Network (HetNet) using relaying techniques, we consider the D2D communication provided by Energy Harvesting (EH) assisted relay where signal is forwarded from a Base Station (BS) to the conventional cellular user (non-D2D user) and D2D user. We ﬁrst derive the outage probability by taking into account the SNR and power allocation parameters, and propose the transmission mode for D2D link as well as non-D2D link. After deriving the outage probability of the D2D-HetNet, we explore the eﬀects of the network parameters on the outage probability and throughput.


Introduction
Today, the need of strengthening the network capacity via the distribution of small cell BSs (e.g., micro, pico, and femto) underlying the common macro cell BSs, especially HetNet which is increasingly growing up.For example, for carrying a massive amount of throughput and bandwidth in satisfying the network quality in order to meet the fast growth of wireless handsets number, HetNet likely affords to accustom because the wireless handsets network capacity appears to achieve the limit.More and more BSs can be supported by HetNet to be distributed in good location to prevent interference.This provided us with a lot of development about network sufficiency and power employing in comparison with macro BSs.Nevertheless, many actual problems have arisen such as load balancing, traffic management, operation and maintenance etc. BSs network scope also has issues like capacity with low power consumption; therefore, wireless energy harvesting plays a very important role for green network for Fifth Generation (5G) world using infrastructures having sufficient and green energy in HetNet [1].When using ambient Radio-Frequency (RF) signals with the new source for energy harvesting, one of the appropriate ways to harvest energy is parallel Wireless Information and Power Transfer (SWIPT) by collecting energy from ambient RF signals [2] and [3].Accordingly, SWIPT helps to make the wireless device which can be obtained higher bandwidth efficiency when considering full-duplex mode in relaying networks [4].However, the process of SWIPT realization has two main difficulties in the large network scope.
The HetNet helps to build up the network capacity through reusing a greater space [5], but this results in the increased interference [6], [7] and [8].When BSs were distributed heavily, HetNet becomes more attractive for efficient RF-SWIPT.Then the gap between the Mobile User (MU) and the BS in HetNet is much closer than in homogeneous macro cell networks.Furthermore, the total interference in HetNet could be an additional energy source with full frequency reuse.SWIPT technique with self-maintaining and inexpensive characteristics employing the interference in Het-Net can increase the spectrum and energy efficiency.
Although the above details are very vital and made a stable infrastructure for promoting new D2D and Ktier network technologies, the effect of wireless power in such networks with D2D capability is less well evaluated.After studying mentioned result in [9], we analyze the design space of future wireless networks about outage performance and energy efficiency for the D2D networks to which energy harvesting enables.On the other hand, in this paper, the system in large-scale HetNet with an energy limited D2D transmitter will be considered.At first, the D2D transmitter collects energy from BSs.Then it performs transporting to the target D2D receiver which accompanies with different levels.

System Model and Protocol Description
We consider a scenario of D2D communications underlaying a cellular network that is shown in Fig. 1, where D 2 intends to exchange information with D 4 in D2D link and D 2 transmit signal from BS to normal cellular user D 3 .In this paper, we consider K-tier communication (i.e.considering two tiers) which is assigned for D2D in the first tier and non-D2D transmission mode in the second tier.

System Model
In this paper, we consider a heterogeneous network, in which the second tier services primary user D 3 by considering the Base Station (BS) wants to transfer information to the primary user D 3 .It is noted that in such case D 3 is assigned as non-D2D user.Meanwhile, the D2D user D 2 wants to transfer its own information which intended for the the other D2D user D 4 .Therefore, D 2 not only forwards to the user in the tier 2 but also delivers the information in the tier 1 simultaneously.As a result, D 2 plays in role of relay node and it required more energy to serve this duty.In particular, the relay D 2 has the ability to scavenge energy from the received signals.Furthermore, the operation of all devices is half-duplex and every device has one signal antenna.In Fig. 2, we present a parameter α 2 1 (0 ≤ α 2 1 ≤ 1) denoting the fraction of the block time allocated for Energy Harvesting (EH) in the block time T /2 with T denoted as block time for signal frame processing, node D 2 is assumed as energy harvesting relaying node which uses Power Splitting-based Relaying (PSR) protocol [2], [3], [4] and [10].We assume that all of the channels are quasi-static fading channels, following Rayleigh fading.Let g 12 , g 23 , g 14 , g 24 denote as the channel coefficients between BS and D 2 , D 2 and D 3 , BS and D 4 , D 2 and D 4 in the concerned block time, respectively.Also, the channels are modeled as follows: g ij ∼ CN (0, Φ ij ) for ij = 1, 2, . . ., 4. In addition, the transmitted power of BS which is equipped a fixed power supply, i.e. power grid P S1 , whereas there is no fixed energy supply for D 2 and it thus needs to harvest energy from the received signals.

The K-Tier Network Structure
In Fig. 1, we show the structure of a two-tier network, in which the first tier includes D2D users D 2 and D 4 , while D 3 is considered as in a conventional cellular user.In this paper, the energy harvesting-assisted device D 2 deploys a fraction of power to transmit the primary information from source BS to the normal cellular user and then uses remaining power to transmit signal for D2D link.It is worth noting that each user only detaches related signal and we concern other signals as the unwanted interferers.

Energy Harvesting and Information Transfer Procedure
In the system model of power splitting-assisted energy harvesting and information transfer protocol includes two phases which are shown in Fig. 2 and it is applied as PSR (Power Splitting-based Relaying) protocol [10].In phase 1, the BS transmits its information to D 2 , energy harvesting of node D 2 and D 4 depends on the EH time of the block time and fraction of information split which is signed by α 1 and α 2 , respectively, while power allocation of the second link (including non-D2D and D2D link) is denoted as . In principle of PSR, P S1 is transferred the harvested signal at the relay D 2 and the total block time denoted by T , from which half of the time, T /2 is occupied for information processing at the first hop of the BS to relay D 2 and the remaining half, T /2 is used for the relay to destination D 3 , D 4 .In particular, the fraction of the received signal power, α 1 P S1 is used for energy harvesting and the remaining received power, (1 − α 1 )P S1 is used for the source to relay information transmission.
In this scenario, the received signals at D 2 can be expressed as: Here, s 1 is the primary signal intended for D 3 in the different tier.It is assumed that , where E {.} is the expectation operator and |.| is the absolute value operator, and noted that distance of each link is installed as simulation results in the next section.
Applying PSR protocol in energy harvesting, the transmitted power at D 2 after energy harvesting is shown as where 0 ≤ η ≤ 1 depicts the energy conversion efficiency and it depends on the rectifier and the energy harvesting.
Similarly, D 4 can be fed by wireless power transfer from the BS.It can be computed the stored energy at the node D 4 is shown as During the phase 2, after energy harvesting, D 2 amplifies 1 − α 2  1 y D2 together with the D2D-assisted information s 2 , then forwards it to D 4 .Interestingly, we assume that the relay D 2 in phase 2 can split its transmitted power into two parts: 3 )P D2 for normal link (non-D2D) and D2D link.Thus, we have the broadcasting information at D 2 is given by where s 2 is the unit-power transmitted information intended for D 4 , n b1 ∼ CN 0, σ 2 b1 denotes the white Gaussian noise introduced by the signal conversion from passband to baseband at D 2 .The amplifier factor G of D 2 is given by At the primary receiver D 3 , we obtained the received signal at destination D 3 , D 4 , respectively, as below where n D3 ∼ CN (0, σ 2 D3 ) and n D4 ∼ CN (0, σ 2 D4 ) represents the additive white Gaussian noise (AWGN) introduced at D 3 and D 4 , respectively.Here, D 4 want to detach to signal s 1 where does not appear interference from the secondary signal s 2 .At D 3 , to decode signal s 1 , the Signal-to-Interferences-plus-Noise Ratio (SINR) of D 3 can be calculated by At the secondary receiver D 4 , the received signal from the source along with the information intended for D 4 can be written as On the other hand, when α 2 = 1, the BS only transfer power to the D2D user.It means that all the harvested power is allocated to harvest energy during all time and no information processing in the link BS-D 4 .This leads to a result that D 3 and D 4 have the primary interference to treat s 1 as interference and after D 3 and D 4 decode the secondary information s 1 .
It is noted that α 2 2 = 1 , it can be computed the SNR expression as below where We assume that the antenna noise power is zero, i.e.
as a result from Eq. ( 8), Eq. ( 11).We can rewrite the received SNR of the node D 3 and D 4 as follows Similarly, SNR at device D 4 can be expressed by Accordingly, from formula of the obtained SNR of the non-D2D user D 3 and the D2D user D 4 , we can calculate the instantaneous rate at D 3 and D 4 as follows

3.
Outage Probability and Throughput Analysis

Outage Probability
The outage probability is defined that the data rate of the D2D user D 3 and the cellular user D 4 falls below the predetermined threshold target rate.Therefore, the outage probability for a given target rate F i is given by where i = D 3 or i = D 4 .Here, we set f i = 2 2Fi −1.F i is the fixed source transmission.According to Eq. ( 14), we have the following propositions. .
The outage probability of D 3 can be expressed as where K 1 (.) is the modified Bessel function of the second kind.Interestingly in this paper, the outage probability of D 4 can be calculated as Proposition 1.
where we use the definition the CDF of N (i.e.F N (n)) can be easily derived through some algebraic manipulation.Finding out, we have P D3 out = F N (f D3 ) .The detailed derivation can be seen in [9].
Because of the secondary interference from the information transmission processing in the secondary network and the receiver antenna, the signal converts from pass-band to baseband at the secondary user D 2 .It is quite easy to obtain from Eq. ( 13) that lim The outage probability the secondary user D 4 can be written as following expression Eq. (10).
In this case, the secondary user can not remove the primary interference from BS when α 2 2 = 1 , yielding lim . As a result, we can calculate the exact expression of P D4 out .Likewise, we have and α 2 2 = 1.This indicates a difficult problem how to choose the factor α 2 3 to satisfy this condition.Note that, when the α 2 3 decreases, P D4 out decreases, but results in P D3 out increasing.Otherwise, if the α 2 3 decreases, P D4 out increases, but results in P D3 out decreasing.

Throughput Analysis
Proposition 1 Derives the outage probability at the D 3 and D 4 , respectively.In this paper, we con- sider both the secondary and primary network.The throughput at D 3 and D 4 is given by where achievable throughput τ m with m is D 3 or D 4 .
Here, the transmitter communicates with fixed rate F m (bps•Hz −1 ).This number illustrates effective communication capability from the source node to the destination node in the block of time T seconds.

Numerical Results
In this paper, we consider outage performance of D2D link and non-D2D link with the help of energy harvesting-assisted relay node D 2 to transfer information to D2D user in tier 1 and non-D2D in tier 2. In this section, we verify the accuracy of the proposed expressions by the simulation to demonstrate the performance of the proposed wireless energy harvesting and information transfer in HetNet.In the simulation, we set the distances of each link which are normalized.Let  In Fig. 3, the analytical results for outage probability are examined.For either D2D user and cellular user, the analytical expressions are confirmed.We observe that the analytical results match well with simulation results for cellular user D 3 .Also, we find that outage performance of each type of link is different due to different power allocation.Besides, the floor value of outage probability remains stable level, especially at high SNR, which verifies our theoretical derivation.In Fig. 4, we compare the result of simulation and exactly analysis at D 3 in case of the increase from 0.3 to 0.5 of α 2 3 , we can see the throughput which will be better when the power allocation coefficient of the second link (including link assigned for non-D2D and D2D link), .i.e.α 2 3 increases to 0.5, we obtain the highest throughput among concerned values.Besides, the simulation and exactly analysis have the same result.
Similarly, the throughput at node D4 is considered in Fig. 5, the throughput is low and it increases very slowly at SNR from 0 dB to 10 dB, but it increases quickly at SNR from 15 dB to 45 dB.Furthermore, when α 2 3 increases, the throughput at node D 4 will increase very slowly.

Conclusion
In this paper, a tractable analytical framework for the evaluation of outage of D2D and non-D2D link in a general K-tier heterogeneous cellular networks has been developed.In particular, we have obtained simple expressions for the non-D2D user in terms of outage probability.Beside that, we have presented some guides to design energy harvesting-based multi-tier cellular networks and evaluate their performance under impacts of power allocation fractions for each kind of D2D link.
The numerical results based on the analysis show that RF energy harvesting can enable technology to power cellular devices.In addition, the simulation results validate the derived expressions, which are efficiently computed numerically.

Fig. 2 :
Fig. 2: Illustration of the key parameters in the power splitting protocol for energy harvesting and information processing at D 2 .
d 1 denote the distance between BS and D 2 , the distance between D 2 and D 3 are denoted by d 2 .Similarly, we define d 3 as the distance between BS and D 4 , and the distance between D 2 and D 4 represented by d 4 , and hence the average channel gains as Φ 1 = 1/d 1 , Φ 2 = 1/d 2 , and Φ 3 = 1/d 3 , Φ 4 = 1/d 4 , for g 12 , g 23 , g 14 and g 24 , respectively.
Sc. degree, and Ph.D. degree from Vietnam National University (VNU-HCMC) in 2003, 2007, and 2013 respectively, all in Communications Engineering.He was a visiting Ph.D. student with Communications Engineering Institute, National Tsing Hua University, Taiwan from 2009 to 2010.Prior to joining Ton Duc Thang University, he was senior engineer at the VinaPhone Mobile Network from 2003 to 2009.Dr. Thuan was recipient of Golden Globe Award from Vietnam Ministry of Science and Technology in 2015.His research interest includes signal processing in wireless communications network, cooperative communications, full-duplex transmission and energy harvesting.Miroslav VOZNAK is an Associate Professor with Department of Telecommunications, VSB-Technical University of Ostrava.He is currently with the VSB-Technical University of Ostrava and he was appointed as an associate professor in 2009.His professional knowledge covers generally Information and Communication technology, in his research, he deals with wireless networks, Voice over IP, security and optimization problems.He is an Advances in Electrical and Electronic Engineering, Mobile, Embedded and Distributed Systems.He was IEEE Senior member and served as Chair of various international conferences.Jaroslav ZDRALEK holds position as an associate professor with Department of Telecommunications, VSB-Technical University of Ostrava, Czech Republic.He received his M.Sc.degree in Computer Science from Slovak Technical University of Bratislava, Slovakia in 1977.He received his Ph.D. degree from VSB-Technical University of Ostrava in 2002, dissertation thesis "Diagnostic system without dismantling of locomotive controller".His research is focused on fault tolerant system and communication technologies.
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