Best sum-throughput evaluation of cooperative downlink transmission nonorthogonal multiple access system

ABSTRACT


INTRODUCTION
To increase the system performance regarding the next gen of wireless network communication technologies, nonorthogonal multiple access (NOMA) has recently appeared as a possible strategy [1]- [4].The fundamental idea behind NOMA is that, unlike traditional orthogonal multiple access (OMA) systems, There is user multiplexing in the power domain [1].To illustrate, in a two-user NOMA system, a base station (BS) converses with both users at once while one of them has a worse channel status and is typically placed at the center of cell or close to the BS.Data signals from the two users are mounted at the BS with differing power distributions, with a higher power allocation coefficient for cell-edge users compared to cell-center users.The superposed signal is split at the receiver section using the successive interference cancellation (SIC) methodology [2].According to previous research [3], under certain conditions, NOMA can outperform OMA and increase throughput by 34.2 at user of the cell edge.Cell center and edge users collaborate and gain by being awarded greater bandwidth when NOMA is used; thus, the spectral efficiency can be greatly increased [4].
According to the current literature, the functioning of NOMA poses the question of rate of data fairness between users at the cell center and edge.The reality that users of cell-edge frequently experience  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol.14, No. 1, February 2024: 509-519 510 lower rates than cell-center users makes user throughput fairness a crucial problem [5].Furthermore, as stated in a previous study [6], if the user of cell edge wants a maximum throughput similar to that of the user of cell center, the power allocation coefficient of the user of cell center should be near to zero.A large portion of the transmitted power is allotted to users of cell edge; thus, it might negatively affect the quality of service of cell-center users.In addition, it could jeopardize the cell-edge users' capacity to receive signals reliably [7].Therefore, solving the user throughput fairness problem while enhancing the cell-edge user's reception dependability is crucial in the NOMA context.
Using cooperative relaying transmissions is one way to approach this fairness problem while ensuring the efficiency and dependability of the cell-edge user.Previous authors [8] presented a cooperative NOMA transmitting technique wherein cell-center users (with improved channel characteristics) take advantage of historical data already present in the NOMA scheme to increase the reception dependability of cell-edge users (with weak connections to a BS).According to those results [8], cooperative NOMA transmissions improve the outage probability (OP) compared with traditional OMA and noncooperative NOMA systems.Zhang et al. [9] demonstrated how cooperative relaying transmissions, in which the cellcenter user serves as a relay, may greatly increase the sum rate of NOMA systems.However, a sizable performance disparity still exists between cell-center and cell-edge users.
Another study [10] examined a two user NOMA system where the best user of cell center is chosen to act as a relay to serve a cell-edge user operate better during outages.The problem of how cell-center users equitably spend their energy is posed because they must analyze and transfer their data from the cell-edge users, even if cooperative relaying transmissions are a acceptable approach to overcome the stated problems in NOMA schemes.A novel wireless multiple access technique that is effective in terms of energy and spectrum, the cooperative simultaneous wireless information and power transfer (SWIPT) NOMA protocol, was presented [11] by merging cooperative NOMA and SWIPT.In particular, the study [11] revealed that, in contrast to the traditional cooperative NOMA, the adoption of SWIPT allows the users of cell center to selfpower and has no impact on the benefits of variety for users of cell edge.
Time division multiple access and NOMA are wireless protocols used in the downlink scenario [12], whereas in the uplink scenario, NOMA with timeshare is taken into account.Multiple-input, single-output (MISO) system applications of NOMA have also been researched in the literature [13], [14].Further, it was investigated how the concept of quasi-degradation affected MISO-NOMA downlink communication [13].A quality-of-service system model was also researched [14] while taking a two-user MISO-NOMA system into account with two objective interference levels.Furthermore, An energy-saving and low-complexity transmission technique for a BS with numerous antennas is called transmit antenna selection (TAS) [15], [16].Particularly, TAS systems might be a valuable compromise between the variety advantage and installation expense [17].Nguyen et al. [18] created user timetables and antenna selection methods to increase the throughput of multiple-input multiple-output-NOMA networks, in which various selected antennae split users into pairs.Networks with NOMA downlink energy harvesting (EH) multiple antennas were examined using a TAS system [19].A specialized EH amplify-and-forward relay facilitates the transmission from the sender to receiver, where NOMA is conducted at the relay because the authors expected direct linkages between a source and destination to be prohibited.In this study, a two-user cooperative MISO-NOMA system is considered, in which a cell-center user serves as a relay to facilitate communication from a BS to a cell-edge user.The proposed techniques increase energy efficiency for the cell-center user while enhancing the cell-edge user's performance.To achieve this, we provide cooperative transmission networks where the BS uses TAS criteria, the user of cell center uses SWIPT and the decodeand-forward relaying approach, and the cell-edge user uses selection merging.The following is a summary of the paper's key contributions.The full article follows the standard structure: introduction, system model, outage performance analysis, performance analysis for the optimal sum throughput, numerical results and discussion, and conclusions.

SYSTEM MODEL
In Figure 1, we take into account a CDT-NOMA downlink transmission which a BS, represented with S, simultaneously interacts with a user of cell center (user ) and a cell-edge user (user ) using a two-user NOMA scheme.Each user has a single antenna, whereas the BS has L antennae.We define ℎ  as the channel fading coefficient from antenna j,  = {1, . . ., } to a user K, where  ∈ {, }, and Rayleigh block flat fading is present in every wireless channel in the system, where a complicated independent Gaussian random variable with zero mean and variance  , can be used to model ℎ  .Furthermore, we let   and   denote the downconverter at user  and the additive white Gaussian noise at the receiving antenna, respectively, with variance   2 and zero mean   2 .Thus, the channel gain |ℎ  | indicates the average gain of the channel equal to  (  / 0 )  , where   describes the separation between two nodes,  represents the path-loss exponent, the reference distance is specified by  0 , and  is the average attenuation of the signal power at  0 .Relay  acts as a hybrid TS/PS EH relay in the presented approach (see [20]- [24] and the references therein for additional information on hybrid TS/PS receivers).Three subblocks of block time  are separated.Relay  initially harvests energy in the first subblock with an  duration time, where 0 ≤  < 1 specifies the amount of block time used for EH.Relay  concurrently uses a portion of the received power denoted by  for PS and a portion denoted by (1 − ) for EH.The remaining portion (denoted by 0 ≤  < 1) is used for data decoding.Relay  uses all captured energy to execute its relaying activity in the final subblock with a (1 − )/2 duration.Figure 2 displays the hybrid TS/PS's timing structure for cooperative relaying communication.We assume the hybrid TS/PS SWIPT provides a thorough analysis and creates a generic analytical model for SWIPT design.The hybrid receiver offers broad options for the design process.If TS or PS is not required, then  (or ) is set to zero.Given that TS and PS have equal status, it may not matter which phase is completed first [22].Furthermore, because the data decoding and relaying procedures are sequential, considering TS first makes the temporal structure in Figure 2 more logical.The downlink case of the proposed approach is performed in two stages: the EH and direct data transfer phases, which are required in the first and second subblocks, and the cooperative relaying transfer phase, which is necessary for the third subblock, following the period of the hybrid TS/PS EH protocol in Figure 2. The second and third subblocks have identical lengths.
Furthermore, throughout the final subblock, or the cooperative relaying transfer phase,  remains silent while relay  transmits to user .The same frequency is used for direct and relaying communications.Therefore, the cell-edge subscriber faces cochannel interference due to the continued broadcast by the BS if relay  continues to relay messages to user .Therefore, the direct and relaying transmissions are implemented in two distinct submodules to stop such cochannel interference.The literature has widely  ISSN: 2088-8708 Int J Elec & Comp Eng, Vol.14, No. 1, February 2024: 509-519 512 accepted using two independent subblocks, where a relay communicates to a sender while the BS is silent (e.g., [25]- [30]).

First phase: energy harvesting
We assume that BS antenna  transmits the data.Based on the NOMA concept, the purpose is to send messages   and   for relay  and user , respectively, combined as √    + √    and transmitted with antenna was chosen at the start of the 1'st subblock period, where   and   specify the power allocation coefficients for relay  and user , respectively.Utilizing the NOMA concept, we presume that |ℎ  | 2 > |ℎ  | 2 , 0 <   <   , and   +   = 1.

Relay 𝑿
The observation that the antenna for user  can be expressed as (1): where ℎ  ∼ (0,   ) and   ∼ (0,   2 ).The overall harvested energy at relay  connected to antenna ii may be stated as follows using the hybrid TS/PS EH technique: where  represents the efficiency of energy conversion scaled from 0 to 1, the channel gain between relay  and antenna j represented by |ℎ  | 2 for data decoding (DD), The signal that was received at relay  is calculated as (3): where   ∼ (0,   2 ).The SIC receiver at relay  initially decodes   based on the NOMA principle, the next step deducts this fraction from the signal that arrives to get the intended information.(i.e.,   ) [31].To decode this, the received signal to interference plus noise ratio at relay  is When relay  is used to decode   , the received signal-to-noise ratio (SNR) is represented as (5): (5)

User 𝒀
User  can execute EH or remain silent in the first subblock and only decode information in the second subblock.Compared to relay , user  can decode the data signal because user  has a greater transmit power allocation; Consequently, interference from relay 's data transmission might be regarded as noise [32].To decode   using the received SNR at user , the observed data may be represented as (6): where ℎ  ∼ (0,   ),   ∼ (0,   2 ),   ∼ (0,   2 ), and

Second phase: direct data-decoded transmission
The following definition can be used to describe the transmit power of relay  in the second phase, assuming that the relaying mechanism is powered entirely by the energy captured during the initial phase, as in [22], [23].User  uses the selection combining (SC) approach to merge two signals: the direct signal from the BS and the relaying signal from relay .Thus, the possible SNR for both signals received combined at user  can be written as ( 8 The signal received by user F might be described as follows using the decode-and-forward relaying protocol: where ℎ  ∼ (0,   ), and the re-encoded form of   is represented by the symbol  ̂.The observed SNR at user  to detect   sent by relay  can be expressed from ( 8) and ( 9) as ( 10), ( 11)

Proposed transmit antenna selection criteria
The suggested TAS technique is carried out by the signaling and channel state information (CSI) assessment system prior to data transfer.We presume the necessary CSI for each scheme is available [33], [34].The ability of   to be decoded at relay  determines whether the cooperative relaying operation succeeds.In light of this, the end-to-end SNR at user  can be expressed in (12).This approach selects an antenna from  antennae to maximize the instantaneous transmission rate for user .The outcome of the scheme selection can be specified formally (14): When user  connects to antenna , the instantaneous transmission rate is represented as ( 13) * =  1≤≤ {    , {  ,   }}.

OUTAGE PERFORMANCE ANALYSIS
The OP is a probability that the information rate will go under the required threshold rate for data [35].Assuming  ℎ, and  ℎ, indicate the desired data rates in bits/s/Hz for relay X and Y, respectively: .
Function (, , ) is described in (21).The suggested scheme's OP for relay X and user Y can be stated as:

Outage probability of relay 𝑿
When the SIC process is unable to correctly decode the message   or when   is correctly decoded but   is not, outage events happen at user .Thus, the OP of relay  may be written as ( 15) where  1 ≜ 2 2 ℎ, −1 and  2 ≜ 2 2 ℎ, −1 represent the corresponding thresholds of SNR for messages successfully decoded   and   .The OP of relay  for the closed-form equation can be expressed as ( 16) The OP of relay X may be written from ( 4), (5) .
Therefore, the OP closed-form equation of relay  may be obtained using specific algebraic methods using certain algebraic operations to examine the relative connections between  1 ,  2 ,  , as presented in (13).

Outage probability for user 𝒀
Message   for user  is decoded at user Y and in the SIC process at relay , as observed in the first phase of the system model.The SIC process connected to   is already considered in the formulation of   2 in (11).Consequently, the user Y's OP could be expressed as ( 18) where the threshold of SNR for successfully message decoded   is denoted by  2 ≜ 2 2 ℎ, − 1.The following is an approximate closed form expression of user Y's OP: where the SNR threshold for successfully decoding message   is represented by   ,  ≜ 2 2 ℎ, /(1−) .

PERFORMANCE ANALYSIS FOR THE OPTIMAL SUM THROUGHPUT
At this stage, we perform an optimum evaluation of the sum throughput, or , of the NOMA approaches explored for downlink case.We specifically offer a way to determine the optimal value of , indicated by   , which maximizes the sum throughput of the network, which is possible to written as (21).
We describe the issue under consideration as an unrestricted optimization problem, defined as ( 22) where (): (): (0,1) ⟶  + , and  + stands for the collection of positive real numbers.To make the analysis of  as easy as possible to simplify (), we assume that (1 − )  ≈   because the noise power that an antenna introduces is low.For notational simplicity, we let   =   /(  +   ) and   =   /(  +   ).Therefore, () can be represented as ( 23) where j=1 if As observed, () has a very complex representation, making it difficult to perform an optimum analysis on this function.We use the gradient descent approach [12] in this study to address the specified issue.In particular, we propose the following simple yet effective technique for locating the ideal PS coefficient.We write the optimal structure of this PS coefficient as   .The goal is to create a minimizing sequence:  0 ,  1 , . . .  , . . .∈ domain g with (  ) ⟶ (  ) as  ⟶ ∞, where, where  indicates the iteration number,   denotes the step length, and   represents a search direction step.We selected a search vector which is the negative gradient of the function using the gradient decrease approach   = −  , resulting in  +1 <   .Until the halting requirement is met, the algorithm continues to execute (i.e., || +1 || 2 ≤ , where  identifies a stopping threshold and ||.|| 2 implies  2 − ).In equation (19), the gradient of the desired function may be represented as in technique 1, which provides more information about the proposed algorithm: The proposed technique allows for off-line optimization depending on the system characteristics obtained through the estimate procedure for CSI (and in advance of the data transfer).

RESULTS AND DISCUSSION
This part presents the representative numerical results to check the designed evaluation and show the reachable performance improvement of the ideal SWIPT-NOMA system in contrast to traditional OMA or noncooperative NOMA systems and the PS-based SWIPT relay [36].During the simulation setup, considering that the source, N=X and F=Y users constitute a line network [37]- [40].We also assumed the following: − The antenna noise power density is -100 dBm/Hz, and the bandwidth is 1 MHz, − The information-processing noise power density is -90 dBm/Hz, − The selected desired data rate is 1 bit/s/Hz, and user  () (power allocation coefficient) value is 0.1, − User 's power allocation coefficient is1-PX, − S is located 10 meters away from user , − User  and S are 3 meters apart, − User  and  are separated by dSY-dSX, − The path-loss exponent is 3, − The path loss at the reference distance is -30 dB, and − The EH process has a 0.70 energy conversion efficiency.
We define the OP's of users  and  as a function of the transmit power PS (dB) and the PS coefficient, respectively, of the source in Figures 3 and 4.These two figures illustrate a clear agreement between the analytical and simulation results, demonstrating the accuracy of the established methodology.Additionally, even in low SNR conditions, such as when PS is minimal, the approximate OP for user  remains close to its actual value.In traditional noncooperative NOMA systems, the cell-center user beats the cell-edge user, as presented in Figures 3 and 4. We can still enhance the cell-edge user's OP by applying a SWIPT-cooperative relaying transfer.Therefore, the OP for user  has decreased and is inferior to that for user  for a specific value of PS.The reality that user  acts as an RF EH relay may be a factor.Specifically, some of the power it receives is used to transmit information, lowering the received SNR of user .
Figure 5 presents the outcome of the proposed methodology for locating the ideal PS coefficient.As observed, the sum throughput is a curved function regarding the PS coefficient.Additionally, the objective function's relaxed value is quite close to its real value.We compared the performance of the traditional OMA, noncooperative NOMA, and optimal SWIPT-NOMA systems in Figure 6.In the beginning considered the combined throughput (bits/s/Hz) of the described three systems.The benefit of NOMA with respect to throughput enhancement is confirmed by the fact that the ideal SWIPT-NOMA and noncooperative NOMA systems produce a superior sum throughput compared with the traditional OMA system.Unexpectedly, the possible sum throughputs of the noncooperative NOMA and the best SWIPT-NOMA are comparable.Only one-half of a block period is spent using the NOMA transmission when SWIPT-based relaying is used in the SWIPT-NOMA system.The BS transmits NOMA ISSN: 2088-8708  Best sum-throughput evaluation of cooperative downlink transmission nonorthogonal … (Ahmad Albdairat) 517 data for the full block time in a noncooperative NOMA system.Due to the calculated values, we can conclude that the sum throughput of the two-user NOMA system under consideration is not jeopardized by the SWIPT-based relaying transmission by user  to aid user .In contrast, the throughput of user  in the best SWIPT-NOMA system is greater than that of the cell-edge user in the noncooperative NOMA system, as illustrated in Figure 6.It implies that the throughput of cell-edge users in NOMA systems is increased using cooperative relaying transmissions.
Figure 6.Comparisons of the OMA, noncooperative NOMA, and proposed NOMA systems for performance

CONCLUSION
This research investigated the OP and sum throughput of the cooperative PS-based SWIPT two-user NOMA system.We applied the tight closed-form approximation expression for the OP of the cell-edge user and the closed-form expression for the cell-center user.To discover the ideal PS coefficient value that maximizes the sum throughput of the system under consideration, we suggested an approach employing the gradient descent technique.According to the numerical findings, using a cooperative SWIPT relaying transmission with an ideal PS coefficient can increase throughput for cell-edge users without endangering the sum throughput of two-user NOMA systems.Thus, cooperative SWIPT relaying transmissions may be considered a long-term fix for the problems of performance equity between cell-center and cell-edge users and energy usage equity for cell-center users.

Figure 2 .
Figure 2. Timing framework of the hybrid TS/PS in cooperative relaying transmission Best sum-throughput evaluation of cooperative downlink transmission nonorthogonal … (Ahmad Albdairat)

Figure 3 . 3 Figure 4 .
Figure 3. Outage probability for users  and , following the signal strength at the sender when ρ=0.3