# Differentiated Reception Modes Based Multiple Access

Authors: Z. Chang; P. Lyu; B. Peng
Journal: Wireless Communications and Mobile Computing
 (2022)

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## Abstract

In recent years, the continuous increase in wireless data services and users’ traffic demand has been imposing great challenges on traditional multiple access control (MAC) methods. Some existing MAC techniques improve the communication system’s spectral efficiency (SE) via signal processing based cochannel interference (CCI) management. However, no interference management (IM) is free, i.e., its realization is based on the consumption of some communication resources, such as power and degree-of-freedom (DoF), which can also be used for the user’s desired data transmission. To lessen the resource cost for IM-based MAC, we exploit interactions among multiple wireless signals to propose a new MAC method, namely, Differentiated Reception Modes based Multiple Access (DRM-MA), in this paper. Under DRM-MA, a central control unit (CCU) is adopted to manage and pair multiple transmitting antennas with their serving receivers (Rxs). The CCU first calculates the phase difference of signals sent from each candidate antenna and perceived by the two receiving antennas of an Rx based on the locations of the transmitting antenna and Rx. Then, the CCU selects and pairs a proper transmitting antenna with each Rx, so that various Rxs can adopt either additive or subtractive reception mode to postprocess the signals received by its two antennas to realize in-phase desired signal construction and inverse-phase interference destruction. DRM-MA can avoid transmission performance loss incurred by signal processing-based IM. Our theoretical analysis and simulation results have shown that DRM-MA can enable concurrent data transmissions of multiple antenna-receiver pairs and output a high system’s SE.


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## Body

## 1. Introduction

With the continuous growth of users’ demand for mobile data services, wireless communication technology has been developing rapidly. Compared to previous communication systems, 5G (the fifth generation) is expected to provide a larger system capacity, higher data rate, lower latency, and more transmission reliability [1]. The Internet of Things (IoT) is a typical application scenario in the 5G era and has been under fast development in recent years, yielding explosive growth of various IoT terminals. It is estimated that by the year 2025 there will be more than 41.6 billion IoT devices connected to the network [2]. The increase of IoT devices and the massive connections of IoT networks impose higher requirements on future wireless communication systems. Due to limited communication resources, efficiently supporting more users with high data transmission quality simultaneously has become a hot topic that is worthy of a thorough investigation.Traditional orthogonal multiple access (OMA) technologies, including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA), allocate various types of communication resources, such as frequencies, time slots, code-words, and spatial subchannels, to multiple users in an orthogonal way to avoid cochannel interference (CCI) among concurrent data transmissions, hence, realizing resource sharing among multiple users [3]. However, the above OMA methods are featured as fixed resource allocation and have low resource utilization. Therefore, due to the limitation of communication resources (especially the spectrum resource) and the rapid increase in the number of wireless users, OMA is facing a great challenge. By dynamically sharing frequency resources to multiple users, ALOHA, carrier sense multiple access (CSMA) [4, 5], and cognitive radio (CR) [6] have been proposed successively, with which the spectrum utilization can be effectively improved. However, such random/opportunistic MACs have collision and transmission failure problems, hence incurring resource waste, to remedy this deficiency, additional cost, and resource consumption (e.g., retransmission and reservation overhead) result.In recent years, nonorthogonal multiple access (NOMA), which is regarded as a promising MAC method that can be applied in 5G, has been invented and attracted a lot of attention. Uplink NOMA [7] allows multiple transmitters (Txs) to transmit to their common receiver (Rx) via the same frequency channel in a non-orthogonal way. The Rx can employ successive interference cancellation (SIC) [8] to mitigate CCI. However, SIC has an error propagation problem [9] which incurs a high bit-error rate (BER) of the subsequently decoded user data, thus limiting its application. By noting that increasing the number of receiving antennas can strengthen Rx’s spatial signal processing capability, and the data carried in multiple concurrent signals can be distinguished and recovered in the spatial domain [10], some researchers incorporate multiantenna with SIC to balance the complexity and BER performance of communication systems [11, 12]. However, due to equipment constraints such as hardware size and complexity, it is impractical to increase the number of receiving antennas without limit, especially for mobile devices. Therefore, some researchers exploit interactions among multiple wireless signals to design multiuser communication schemes. The authors of [13, 14] proposed interference neutralization (IN) in which the desired Tx constructs and sends a neutralizing signal of the same amplitude and opposite phase with respect to the interference perceived by its serving Rx so that the neutralizing signal can counteract the interference at the Rx. Since the power cost for generating the neutralizing signal is high, [15] designed interference steering (IS). By constructing a steering signal at the serving Tx, only the projection of the interference on the desired transmission at the interfered Rx is mitigated, hence, yielding the steered disturbance to be orthogonal to the desired signal.Based on the above discussion, we will propose a novel MAC method, called Differentiated Reception Modes based Multiple Access (DRM-MA) in this paper. By exploiting interactions among wireless signals, DRM-MA lets mobile users adopt either additive or subtractive reception mode based on the phase difference of the signals sent from their serving and interfering antennas and perceived by their two receiving antennas so that the desired signals and the interferences can be constructively and destructively combined at each Rx. In this way, concurrent data transmissions of multiple antenna-receiver pairs are realized. Compared to traditional signal processing-based MAC methods discussed in the previous paragraph, our method does not incur a signal processing burden at either side of the communication link. However, to realize the method, the central control unit (CCU) needs to determine the serving antenna for each Rx, hence incurring some computational complexity.The main contributions of this paper are two-fold:(i)
Proposal of DRM-MA. By exploiting interactions among two wireless signals, an Rx can select the appropriate reception mode according to the phase difference of the signals sent from the antennas of serving Tx and interfering Tx and observed by its receiving antennas, respectively. Then, the desired signal components can be constructively combined while the interferences are neutralized with each other at the Rx(ii)
Development of Antenna Selection Criterion. Aiming at strengthening the desired signal and suppressing the CCI as much as possible, various candidate transmitting antenna sets are calculated, from which the serving antenna for each Rx is then selected, and accordingly, each Rx determines its reception modeThe rest of the paper is organized as follows. Section2 describes the system model while Section 3 details the design of DRM-MA. In Section 4, we evaluate the performance of DRM-MA. Finally, we conclude the paper in Section 5.Throughout this paper, we use the following notations. Let· denote the absolute value of a scalar. argmax· indicates an operation finding the argument that gives the maximum value from a target function.


## 2. System Model

We consider a downlink communication scenario consisting of a central control unit (CCU) and multiple distributedly located transmitting antennas under CCU’s control. The antennas are uniformly deployed in the area ofV×H. As Figure 1 shows, the antenna whose y- and x-coordinates are v and h, respectively, is denoted as Txvh (v∈1,2,⋯,V,h∈1,2,⋯,H). Txvh can also be regarded as a single-antenna transmitter. The transmit power of each antenna is PT. For simplicity, we plot two Rxs, say Rxm and Rxk, in the communication environment. Each Rx is equipped with NR=2 antennas. The two antennas of Rxm are denoted as m1 and m2, while Rxk’s antennas are k1 and k2. Let gm1vh and gm2vh represent the channel fading coefficients between Txvh and Rxm’s two antennas. Similarly, gk1vh and gk2vh are the fading coefficients between Txvh and Rxk’s antennas, respectively. We adopt free-space propagation model [16], hence, the power of received signal at antenna κ (κ∈m1,m2,k1,k2) from Txvh can be computed as Pκvh=PTGvhGκλ2/Γ4πlκvh2 where λ represents the signal’s wavelength. Gvh and Gκ are the gains of the transmitting antenna Txvh and receiving antenna κ. Γ is the path-loss factor. lκvh denotes the distance from Txvh to κ. We assume that CCU can accurately obtain the channel coefficients from all transmitting antennas to each receiving antenna of an Rx. By exploiting channel reciprocity [17], the uplink and downlink can have the same channel coefficients.Figure 1
System model.We employφκvh to represent the phase of the signal sent from Txvh and perceived by receiving antenna κ. To reduce signaling overhead, we let Rxℓ (ℓ∈m,k) only feed back to CCU the midpoint coordinate, i.e., Cℓ, of the line segment ℓ1ℓ2¯ between Rxℓ’s two receiving antennas, and the phase angle θ of ℓ1ℓ2¯ with respect to the horizontal axis. In Figure 2, taking Rxm as an example, CCU can calculate the coordinates of Rxm’s two receiving antennas, i.e., m1 and m2, according to Cm, θ, and antenna spacing d (we assume the distance between Rx’s two antennas is available to CCU). As Figure 2 shows, we denote the coordinate of the midpoint of line segment m1m2¯ as Cmαm,βm, then, the x- and y-coordinates of m1 can be calculated as αm1=αm+1/2dcosθ and βm1=βm+1/2dsinθ, respectively. Similarly, the coordinates of receiving antenna m2 are αm2=αm−1/2dcosθ and βm2=βm−1/2dsinθ.Figure 2
Geometrical illustration ofRxm’s two receiving antennas, and m1m2¯’s midpoint Cm and phase angle θ.


## 3. Design of DRM-MA

This section details the design of Differentiated Receive Mode-based Multiple Access (DRM-MA). We will first present the basic principle of DRM-MA and then give the criterion based on which the transmitting antennas serving multiple Rxs are selected and paired with the Rxs; accordingly, and each Rx determines its reception mode.


### 3.1. Basic Design of DRM-MA

For clarity, we take two Rxs as an example to present the principle of DRM-MA. As Figure1 shows, we assume that the serving antennas for Rxm and Rxk have been determined (the antenna selection method will be given in the next subsection). Without loss of generality, we let antennas Txvmhm and Txvkhk send desired signals to Rxm and Rxk, respectively. It should be noticed that Txvmhm causes interference to Rxk and vice versa. Therefore, we also call Txvmhm and Txvkhk permissive interfering antennas of Rxk and Rxm, respectively.We usexm and xk to denote the desired data symbols of Rxm and Rxk. Both Txvmhm and Txvkhk send one desired signal to their intended Rx. Then, the mixed signals perceived by antennas m1 and m2 of Rxm, denoted as ym1 and ym2, respectively, can be expressed as
(1)ym1=gm1vmhmxm+gm1vkhkxk+nm1,ym2=gm2vmhmxm+gm2vkhkxk+nm2,where gκτ (τ∈vmhm,vkhk and κ∈m1,m2) denote the fading coefficient of the channel from Txτ to Rxm’s antenna κ. The first term on the right-hand side (RHS) of each subequation in Eq. (1) represents for the desired signal from Txvmhm, while the second term denotes the interference from Txvkhk. The third term is Additive White Gaussian Noise (AWGN) whose element has zero-mean and variance σn2.The complex channel coefficientgκτ can be expressed as [18]
(2)gκτ=gκτejφκτ,wheregκτ denotes the amplitude fading, and φκτ∈0,2π is the phase offset yielded by the channel.Substituting Eq. (2) into Eq. (1), we can get
(3)ym1=gm1vmhmxmejφm1vmhm+gm1vkhkxkejφm1vkhk+nm1,ym2=gm2vmhmxmejφm2vmhm+gm2vkhkxkejφm2vkhk+nm2.From Eq. (3), we can obtain the phase difference of the desired signal components perceived by Rxm’s two antennas as Δφmvmhm=φm1vmhm−φm2vmhm. If Δφmvmhmmod2π=π (mod· represents modulo operation) holds, Rxm can simply subtract one antenna’s received signal from the other, so that the in-phase superposition of the desired signal is realized. Otherwise, if Δφmvmhmmod2π=0 holds, Rxm can add the received signals of its two antennas to achieve the in-phase desired signal combination. Meanwhile, m1 and m2 also receive interference from Txvkhk (see the second terms on the RHS of Eq. (3)). If the phase difference of the two interfering components satisfies Δφmvkhkmod2π=π, Rxm should add up the two interferences to mitigate their influence. Otherwise, if Δφmvkhkmod2π=0 holds, Rxm should subtract one disturbance from the other to realize interference cancellation. Likewise, Rxk can realize desired signal construction and interference destruction based on the phase difference of two desired/interfering signal components at its antennas. Therefore, in the use of DRM-MA, CCU first calculates the phase difference of signals sent from each candidate antenna and perceived by the two receiving antennas of an Rx based on the locations of the transmitting antenna and Rx, then determines the Rx’s reception mode according to the phase difference.Upon employing various reception modes, i.e., additive or subtractive, each Rx realizes both desired signal strengthening and interference cancellation. Without loss of generality, we takeΔφmvmhmmod2π=π and Δφmvkhkmod2π=0 as an example, then, Rxm can adopt subtractive mode to get the following equation.
(4)ym=gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm⏟In−phasedesiredsignalconstruction.xm+gm1vkhkejφm1vkhk−gm2vkhkejφm2vkhk⏟Inverse−phaseinterferencedestruction.xk+nm.Sincegm1vmhmejφm1vmhm and gm2vmhmejφm2vmhm are known to Rxm, Rxm can adopt coefficient gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm−1 to postprocess ym to obtain the estimated desired signal as
(5)y^m=gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm−1ym.Meanwhile, the phase difference of the desired and interfering signal sent fromTxvmhm and Txvkhk, respectively, and received by Rxk’s two antennas k1 and k2 should satisfy Δφkvkhkmod2π=0 and Δφkvmhmmod2π=π. In such a case, Rxk can employ additive mode to get the following equation.
(6)yk=gk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk⏟In−phasedesiredsignalconstruction.xk+gk1vmhmejφk1vmhm+gk2vmhmejφk2vmhm⏟Inverse−phaseinterferencedestruction.xm+nk.Then, by adoptinggk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk−1 to postprocess yk, Rxk can get the estimated signal as
(7)y^k=gk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk−1yk.We can see from Eqs. (4) and (6) that employing additive and subtractive mode, respectively, Rxm and Rxk can postprocess the received signals of their antennas. In this way, the desired signal is strengthened while the disturbance is suppressed, thus realizing DRM-MA.


### 3.2. Design of Antenna Selection Criterion

In the previous subsection, we have presented the basic idea of DRM-MA under the assumption that serving antennas for Rxs have been determined. In this subsection, we will still take two Rxs as an example to design the serving antenna selection criterion.To select the proper antenna to serve an Rx, sayRxm, CCU needs to calculate the coordinates of m1 and m2 based on the information of Cm, d, and θ and then compute the phase of the signal sent from each candidate antenna Txvh (v∈1,2,⋯,V,h∈1,2,⋯,H) and perceived by Rxm’s receiving antenna κ (κ∈m1,m2) as
(8)φκvh=2lκvhπλ=2lκvhπfc,where f denotes the frequency of the transmitted signal and λ is the signal’s wavelength. c represents the speed of light. lκvh is the distance from Txvh to Rxm’s antenna κ.Then, we can get the phase differenceΔφmvh of the signals sent from Txvh and received by Rxm’s two antennas as
(9)Δφmvh=φm1vh−φm2vh=2πflm1vh−lm2vhc.Since we employ the free-space propagation model, in what follows, we only consider the influence of the signal’s propagation onφκvh and Δφmvh. Without loss of generality, we take Rxm as an example. If Δφmvhmod2π=π holds, we store Txvh in a transmitting antenna set Ωom. Otherwise, if Δφmvhmod2π=0 holds, Txvh is added to transmitting antenna set Ωem. When an antenna in sets Ωom and Ωem is selected to serve Rxm, Rxm needs to adopt additive and subtractive reception modes accordingly. Similarly, transmitting antenna sets Ωok and Ωek for Rxk can be obtained.Next, we calculate intersections ofRxm’s sets Ωom and Ωem and Rxk’s Ωok and Ωek, respectively, to obtain four candidate transmitting antenna sets, i.e., Roomk=Ωom⋂Ωok, Roemk=Ωom⋂Ωek, Reomk=Ωem⋂Ωok, and Reemk=Ωem⋂Ωek, as given in Table 1. Accordingly, four differentiated reception modes of the two Rxs can be determined. Taking candidate antenna set Roomk as an example, the subscript oo indicates that the phase difference of signals sent from each antenna in set Roomk and perceived by the two receiving antennas of both Rxm and Rxk is odd times of π. That is, when an antenna in Roomk serves Rxm or Rxk, either Rxm or Rxk should adopt the subtractive reception mode to strengthen its desired signal. As for Reomk, its subscript eo indicates that the phase difference of the signals sent from Reomk’s antenna and observed by Rxm’s and Rxk’s two antennas is even and odd times of π, respectively. Therefore, when an antenna in Reomk is selected to serve Rxm or Rxk, Rxm should adopt additive mode, while Rxk should use subtractive, to strengthen their desired signal. As Table 1 shows, Rxm and Rxk can adopt either identical reception modes (cases II and III) or different modes (cases I and IV) in terms of their serving transmitting antenna sets.Table 1
Determining candidate serving antenna set (SAS) andRx’s reception mode (RM) under K=2.
CaseSAS forRxmSAS forRxkRxm’s RMRxk’s RMIRoomkReemkSubtractiveAdditiveIIRoemkReomkSubtractiveSubtractiveIIIReomkRoemkAdditiveAdditiveIVReemkRoomkAdditiveSubtractiveIn practice, when selecting a serving antenna for an Rx, not only does the desired signal at the intended Rx need to be strong but also the interference to other unintended Rxs should be as small as possible. In what follows, we will present the serving antenna selection criterion on the premise that the candidate serving antenna sets have been determined.Without loss of generality, we take the case I in Table1 as an example. When a candidate Txvh∈Roomk serves Rxm and interferes with Rxk, Rxm adopts subtractive mode to subtract the desired signal perceived by one antenna from the other. Then, we can have•
(10)ΔAmvh=gm1vhejφm1vh−gm2vhejφm2vh.As forRxk, it employs additive reception mode to alleviate the interference sent from Txvh and received by its two antennas. We can get
(11)ΣAkvh=gk1vhejφk1vh+gk2vhejφk2vh.Then, we should select theTxvh that can maximize ΔAmvh, denoted as Txvmhm, as Rxm’s serving antenna; that is, Txvmhm=argmaxTxvh∈RoomkΔAmvh. However, by taking the interference from Txvh to Rxk into account, we should employ ΣAkvh−1 as another factor affecting Rxm’s serving antenna selection. That is, since ΣAkvh indicates residual interference at Rxk, we should use an antenna yielding as small ΣAkvh as possible to serve Rxm. Based on the above discussion, to take both desired signal strengthening at Rxm and interference cancellation at Rxk into consideration, an antenna outputting the largest μmkvh=ξΔAmvh+1−ξΣAkvh−1 where ξ∈0,1 is a weight coefficient and should be selected to serve Rxm. Specifically, in this example, we determine Rxm’s serving antenna according to Txvmhm=argmaxTxvh∈Roomkμmkvh. If one prefers a better performance of Rxm, a larger ξ should be adopted; otherwise, if one wants the interference at Rxk to be small, a smaller ξ should be used.In the previous design, we simply assume that the phase difference of signal components perceived by an Rx’s two antennas is either odd or even times ofπ, based on which the candidate serving antenna sets can be determined. However, in practice, the phase difference is usually not exactly the odd or even times of π. In such a situation, we need to introduce a tolerance coefficient ε to relax the requirement of the phase difference of signals at Rx’s two antennas. Otherwise, too strict a phase difference requirement may yield an empty candidate serving antenna set, hence incurring the unavailability of DRM-MA.Based on the above discussion, we adoptΔφmvhmod2π=π±ε and Δφmvhmod2π=±ε instead of Δφmvhmod2π=π and Δφmvhmod2π=0, respectively, as the criterion to determine the candidate serving antenna sets. In this way, we can ensure that the candidate serving antenna set is not empty by using a proper ε. However, this will incur nonideal in-phase construction of desired signal and inverse-phase interference destruction at Rx. To solve this problem, Rx can perform phase compensation according to the phase difference of the signals perceived by its two antennas [19], and then accurate in-phase desired signal superposition and inverse-phase interference cancellation can be realized. The stronger the phase compensation capability of the Rx is, the larger ε can be used.It should be noticed that although phase compensation can yield the phase difference of the signal components perceived by Rx’s two antennas to be exactly odd times or even times ofπ, the signal components’ amplitudes may not be the same, hence incurring residual interference at Rx. Fortunately, since the size of Rx and its antenna spacing are small, the difference in propagation distance from the interfering antenna to Rx’s two receiving antennas is limited. Therefore, the influence of residual interference is negligible. For space limit, we omit the detailed discussion about the residual interference in this paper.


### 3.3. Extended Design of DRM-MA

In previous subsections, we simply assume two Rxs for clarity. In this subsection, we will extend DRM-MA to a multi-Rx situation to show its scalability.We let the number of Rxs be three, i.e.,Rxm, Rxk, and Rxs are in the communication scenario. First, CCU calculates the phase difference of the signals sent from each Txvh and perceived by the Rxs’ receiving antennas. Then, an antenna set suitable for serving each Rx under additive and subtractive reception modes can be obtained. We denote the set of serving antennas for Rxϖ under additive and subtractive modes as Ωeϖ and Ωoϖ, respectively, where ϖ∈m,k,s. Then, we select one set from the serving antenna sets of Rxm, Rxk, and Rxs in turn and calculate the intersection of the three selected sets, so that eight cases of candidate serving antenna sets can be obtained as Rooomks=Ωom⋂Ωok⋂Ωos, Rooemks=Ωom⋂Ωok⋂Ωes, Roeomks=Ωom⋂Ωek⋂Ωos, Roeemks=Ωom⋂Ωek⋂Ωes, Reoomks=Ωem⋂Ωok⋂Ωos, Reoemks=Ωem⋂Ωok⋂Ωes, Reeomks=Ωem⋂Ωek⋂Ωos, and Reeemks=Ωem⋂Ωek⋂Ωes. As Table 2 shows, the above eight sets correspond to eight combinations of the three Rxs’ reception modes. Taking Reeemks as an example, its subscript is eee, this indicates that when an antenna in set Reeemks serves Rxϖ (ϖ∈m,k,s), the phase difference of the signals perceived by Rxϖ’s two antennas is even times of π, thus Rxϖ should adopt subtractive reception mode.Table 2
Determining candidate serving antenna set (SAS) andRx’s reception mode (RM) under K=3.
CaseSAS forRxmSAS forRxkSAS forRxsRxm’s RMRxk’s RMRxs’s RMIRooomksReeomksReoemksSubtractiveAdditiveAdditiveIIRooemksReeemksReoomksSubtractiveAdditiveSubtractiveIIIRoeomksReoomksReeemksSubtractiveSubtractiveAdditiveIVRoeemksReoemksReeomksSubtractiveSubtractiveSubtractiveVReoomksRoeomksRooemksAdditiveAdditiveAdditiveVIReoemksRoeemksRooomksAdditiveAdditiveSubtractiveVIIReeomksRooomksRoeemksAdditiveSubtractiveAdditiveVIIIReeemksRooomksRoeomksAdditiveSubtractiveSubtractiveBased on the various combinations of the serving antenna sets, the reception modes of multiple Rxs can be determined. Since there is always no cooperation among multiple Rxs, Rxs’ reception modes should be determined by the CCU and then informed to each Rx. We further present the extension of DRM-MA to the case ofK (K>2) Rx. First, we index all Rxs from 1 to K. For simplicity, we replace the subscripts o and e of the candidate serving antenna sets with binary numbers 0 and 1, respectively. In this way, the string composed of o and e can be equivalent to a binary code. Provided with K Rxs, DRM-MA first calculates Ωeϖ and Ωoϖ where ϖ∈1,2,⋯,K for each Rx, based on which 2K cases of candidate serving antenna sets, denoted as Rb1⋯bK1⋯K where b1⋯bK can be either o0 or e1, can be obtained. Next, we can select any one of the 2K candidate antenna sets (e.g., case I in Table 2) and mark it as the serving antenna set for Rx1 (e.g., as for case I in Table 2, Rooomks whose subscript is 000 serves Rxm). Then, we select the serving antenna set for Rx2 (e.g., as for case I in Table 2, Reeomks whose subscript is 110 serves Rxk); the 1st and 2nd bits of Rx2’s serving antenna set’s subscript should be opposite to those of Rx1’s, while the rest bits of the two Rxs’ serving antenna sets’ subscripts are the same. As for Rx3 (under K=3, according to Table 2, Reoemks whose subscript is 101 serves Rxs); the 1st and 3rd bits of Rx3’s serving antenna set’s subscript should be opposite to those of Rx1’s, while the remaining bits of the two Rxs’ sets’ subscripts are the same. As for RxK, the 1st and Kth bits of its serving antenna set need to be opposite to those Rx1’s, while the remaining bits of the two Rxs’s sets’ subscripts are the same.Based on the above process,2K combinations of serving antenna sets for K Rxs can be obtained. Then, an Rx, say Rxk^ (k^∈1,⋯,K) can determine its reception mode in terms of the k∧th bit of its serving antenna set’s subscript. Specifically, if the k∧th bit is o0, Rxk^ should adopt subtractive mode; otherwise, for e1, additive reception mode should be used. We take case I in Table 2 as an example, the first user Rxm adopts subtractive mode according to the 1st bit of Rooomks’s subscript. Similarly, the second user Rxk employs additive mode based on the 2nd bit of Reeomks’s subscript. As for the last user Rxs, its uses additive mode according to the 3rd bit of Reoemks)’s subscript.Based on the above descriptions, DRM-MA can be applied to the communication scenario withK Rxs.


## 3.1. Basic Design of DRM-MA

For clarity, we take two Rxs as an example to present the principle of DRM-MA. As Figure1 shows, we assume that the serving antennas for Rxm and Rxk have been determined (the antenna selection method will be given in the next subsection). Without loss of generality, we let antennas Txvmhm and Txvkhk send desired signals to Rxm and Rxk, respectively. It should be noticed that Txvmhm causes interference to Rxk and vice versa. Therefore, we also call Txvmhm and Txvkhk permissive interfering antennas of Rxk and Rxm, respectively.We usexm and xk to denote the desired data symbols of Rxm and Rxk. Both Txvmhm and Txvkhk send one desired signal to their intended Rx. Then, the mixed signals perceived by antennas m1 and m2 of Rxm, denoted as ym1 and ym2, respectively, can be expressed as
(1)ym1=gm1vmhmxm+gm1vkhkxk+nm1,ym2=gm2vmhmxm+gm2vkhkxk+nm2,where gκτ (τ∈vmhm,vkhk and κ∈m1,m2) denote the fading coefficient of the channel from Txτ to Rxm’s antenna κ. The first term on the right-hand side (RHS) of each subequation in Eq. (1) represents for the desired signal from Txvmhm, while the second term denotes the interference from Txvkhk. The third term is Additive White Gaussian Noise (AWGN) whose element has zero-mean and variance σn2.The complex channel coefficientgκτ can be expressed as [18]
(2)gκτ=gκτejφκτ,wheregκτ denotes the amplitude fading, and φκτ∈0,2π is the phase offset yielded by the channel.Substituting Eq. (2) into Eq. (1), we can get
(3)ym1=gm1vmhmxmejφm1vmhm+gm1vkhkxkejφm1vkhk+nm1,ym2=gm2vmhmxmejφm2vmhm+gm2vkhkxkejφm2vkhk+nm2.From Eq. (3), we can obtain the phase difference of the desired signal components perceived by Rxm’s two antennas as Δφmvmhm=φm1vmhm−φm2vmhm. If Δφmvmhmmod2π=π (mod· represents modulo operation) holds, Rxm can simply subtract one antenna’s received signal from the other, so that the in-phase superposition of the desired signal is realized. Otherwise, if Δφmvmhmmod2π=0 holds, Rxm can add the received signals of its two antennas to achieve the in-phase desired signal combination. Meanwhile, m1 and m2 also receive interference from Txvkhk (see the second terms on the RHS of Eq. (3)). If the phase difference of the two interfering components satisfies Δφmvkhkmod2π=π, Rxm should add up the two interferences to mitigate their influence. Otherwise, if Δφmvkhkmod2π=0 holds, Rxm should subtract one disturbance from the other to realize interference cancellation. Likewise, Rxk can realize desired signal construction and interference destruction based on the phase difference of two desired/interfering signal components at its antennas. Therefore, in the use of DRM-MA, CCU first calculates the phase difference of signals sent from each candidate antenna and perceived by the two receiving antennas of an Rx based on the locations of the transmitting antenna and Rx, then determines the Rx’s reception mode according to the phase difference.Upon employing various reception modes, i.e., additive or subtractive, each Rx realizes both desired signal strengthening and interference cancellation. Without loss of generality, we takeΔφmvmhmmod2π=π and Δφmvkhkmod2π=0 as an example, then, Rxm can adopt subtractive mode to get the following equation.
(4)ym=gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm⏟In−phasedesiredsignalconstruction.xm+gm1vkhkejφm1vkhk−gm2vkhkejφm2vkhk⏟Inverse−phaseinterferencedestruction.xk+nm.Sincegm1vmhmejφm1vmhm and gm2vmhmejφm2vmhm are known to Rxm, Rxm can adopt coefficient gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm−1 to postprocess ym to obtain the estimated desired signal as
(5)y^m=gm1vmhmejφm1vmhm−gm2vmhmejφm2vmhm−1ym.Meanwhile, the phase difference of the desired and interfering signal sent fromTxvmhm and Txvkhk, respectively, and received by Rxk’s two antennas k1 and k2 should satisfy Δφkvkhkmod2π=0 and Δφkvmhmmod2π=π. In such a case, Rxk can employ additive mode to get the following equation.
(6)yk=gk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk⏟In−phasedesiredsignalconstruction.xk+gk1vmhmejφk1vmhm+gk2vmhmejφk2vmhm⏟Inverse−phaseinterferencedestruction.xm+nk.Then, by adoptinggk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk−1 to postprocess yk, Rxk can get the estimated signal as
(7)y^k=gk1vkhkejφk1vkhk+gk2vkhkejφk2vkhk−1yk.We can see from Eqs. (4) and (6) that employing additive and subtractive mode, respectively, Rxm and Rxk can postprocess the received signals of their antennas. In this way, the desired signal is strengthened while the disturbance is suppressed, thus realizing DRM-MA.


## 3.2. Design of Antenna Selection Criterion

In the previous subsection, we have presented the basic idea of DRM-MA under the assumption that serving antennas for Rxs have been determined. In this subsection, we will still take two Rxs as an example to design the serving antenna selection criterion.To select the proper antenna to serve an Rx, sayRxm, CCU needs to calculate the coordinates of m1 and m2 based on the information of Cm, d, and θ and then compute the phase of the signal sent from each candidate antenna Txvh (v∈1,2,⋯,V,h∈1,2,⋯,H) and perceived by Rxm’s receiving antenna κ (κ∈m1,m2) as
(8)φκvh=2lκvhπλ=2lκvhπfc,where f denotes the frequency of the transmitted signal and λ is the signal’s wavelength. c represents the speed of light. lκvh is the distance from Txvh to Rxm’s antenna κ.Then, we can get the phase differenceΔφmvh of the signals sent from Txvh and received by Rxm’s two antennas as
(9)Δφmvh=φm1vh−φm2vh=2πflm1vh−lm2vhc.Since we employ the free-space propagation model, in what follows, we only consider the influence of the signal’s propagation onφκvh and Δφmvh. Without loss of generality, we take Rxm as an example. If Δφmvhmod2π=π holds, we store Txvh in a transmitting antenna set Ωom. Otherwise, if Δφmvhmod2π=0 holds, Txvh is added to transmitting antenna set Ωem. When an antenna in sets Ωom and Ωem is selected to serve Rxm, Rxm needs to adopt additive and subtractive reception modes accordingly. Similarly, transmitting antenna sets Ωok and Ωek for Rxk can be obtained.Next, we calculate intersections ofRxm’s sets Ωom and Ωem and Rxk’s Ωok and Ωek, respectively, to obtain four candidate transmitting antenna sets, i.e., Roomk=Ωom⋂Ωok, Roemk=Ωom⋂Ωek, Reomk=Ωem⋂Ωok, and Reemk=Ωem⋂Ωek, as given in Table 1. Accordingly, four differentiated reception modes of the two Rxs can be determined. Taking candidate antenna set Roomk as an example, the subscript oo indicates that the phase difference of signals sent from each antenna in set Roomk and perceived by the two receiving antennas of both Rxm and Rxk is odd times of π. That is, when an antenna in Roomk serves Rxm or Rxk, either Rxm or Rxk should adopt the subtractive reception mode to strengthen its desired signal. As for Reomk, its subscript eo indicates that the phase difference of the signals sent from Reomk’s antenna and observed by Rxm’s and Rxk’s two antennas is even and odd times of π, respectively. Therefore, when an antenna in Reomk is selected to serve Rxm or Rxk, Rxm should adopt additive mode, while Rxk should use subtractive, to strengthen their desired signal. As Table 1 shows, Rxm and Rxk can adopt either identical reception modes (cases II and III) or different modes (cases I and IV) in terms of their serving transmitting antenna sets.Table 1
Determining candidate serving antenna set (SAS) andRx’s reception mode (RM) under K=2.
CaseSAS forRxmSAS forRxkRxm’s RMRxk’s RMIRoomkReemkSubtractiveAdditiveIIRoemkReomkSubtractiveSubtractiveIIIReomkRoemkAdditiveAdditiveIVReemkRoomkAdditiveSubtractiveIn practice, when selecting a serving antenna for an Rx, not only does the desired signal at the intended Rx need to be strong but also the interference to other unintended Rxs should be as small as possible. In what follows, we will present the serving antenna selection criterion on the premise that the candidate serving antenna sets have been determined.Without loss of generality, we take the case I in Table1 as an example. When a candidate Txvh∈Roomk serves Rxm and interferes with Rxk, Rxm adopts subtractive mode to subtract the desired signal perceived by one antenna from the other. Then, we can have•
(10)ΔAmvh=gm1vhejφm1vh−gm2vhejφm2vh.As forRxk, it employs additive reception mode to alleviate the interference sent from Txvh and received by its two antennas. We can get
(11)ΣAkvh=gk1vhejφk1vh+gk2vhejφk2vh.Then, we should select theTxvh that can maximize ΔAmvh, denoted as Txvmhm, as Rxm’s serving antenna; that is, Txvmhm=argmaxTxvh∈RoomkΔAmvh. However, by taking the interference from Txvh to Rxk into account, we should employ ΣAkvh−1 as another factor affecting Rxm’s serving antenna selection. That is, since ΣAkvh indicates residual interference at Rxk, we should use an antenna yielding as small ΣAkvh as possible to serve Rxm. Based on the above discussion, to take both desired signal strengthening at Rxm and interference cancellation at Rxk into consideration, an antenna outputting the largest μmkvh=ξΔAmvh+1−ξΣAkvh−1 where ξ∈0,1 is a weight coefficient and should be selected to serve Rxm. Specifically, in this example, we determine Rxm’s serving antenna according to Txvmhm=argmaxTxvh∈Roomkμmkvh. If one prefers a better performance of Rxm, a larger ξ should be adopted; otherwise, if one wants the interference at Rxk to be small, a smaller ξ should be used.In the previous design, we simply assume that the phase difference of signal components perceived by an Rx’s two antennas is either odd or even times ofπ, based on which the candidate serving antenna sets can be determined. However, in practice, the phase difference is usually not exactly the odd or even times of π. In such a situation, we need to introduce a tolerance coefficient ε to relax the requirement of the phase difference of signals at Rx’s two antennas. Otherwise, too strict a phase difference requirement may yield an empty candidate serving antenna set, hence incurring the unavailability of DRM-MA.Based on the above discussion, we adoptΔφmvhmod2π=π±ε and Δφmvhmod2π=±ε instead of Δφmvhmod2π=π and Δφmvhmod2π=0, respectively, as the criterion to determine the candidate serving antenna sets. In this way, we can ensure that the candidate serving antenna set is not empty by using a proper ε. However, this will incur nonideal in-phase construction of desired signal and inverse-phase interference destruction at Rx. To solve this problem, Rx can perform phase compensation according to the phase difference of the signals perceived by its two antennas [19], and then accurate in-phase desired signal superposition and inverse-phase interference cancellation can be realized. The stronger the phase compensation capability of the Rx is, the larger ε can be used.It should be noticed that although phase compensation can yield the phase difference of the signal components perceived by Rx’s two antennas to be exactly odd times or even times ofπ, the signal components’ amplitudes may not be the same, hence incurring residual interference at Rx. Fortunately, since the size of Rx and its antenna spacing are small, the difference in propagation distance from the interfering antenna to Rx’s two receiving antennas is limited. Therefore, the influence of residual interference is negligible. For space limit, we omit the detailed discussion about the residual interference in this paper.


## 3.3. Extended Design of DRM-MA

In previous subsections, we simply assume two Rxs for clarity. In this subsection, we will extend DRM-MA to a multi-Rx situation to show its scalability.We let the number of Rxs be three, i.e.,Rxm, Rxk, and Rxs are in the communication scenario. First, CCU calculates the phase difference of the signals sent from each Txvh and perceived by the Rxs’ receiving antennas. Then, an antenna set suitable for serving each Rx under additive and subtractive reception modes can be obtained. We denote the set of serving antennas for Rxϖ under additive and subtractive modes as Ωeϖ and Ωoϖ, respectively, where ϖ∈m,k,s. Then, we select one set from the serving antenna sets of Rxm, Rxk, and Rxs in turn and calculate the intersection of the three selected sets, so that eight cases of candidate serving antenna sets can be obtained as Rooomks=Ωom⋂Ωok⋂Ωos, Rooemks=Ωom⋂Ωok⋂Ωes, Roeomks=Ωom⋂Ωek⋂Ωos, Roeemks=Ωom⋂Ωek⋂Ωes, Reoomks=Ωem⋂Ωok⋂Ωos, Reoemks=Ωem⋂Ωok⋂Ωes, Reeomks=Ωem⋂Ωek⋂Ωos, and Reeemks=Ωem⋂Ωek⋂Ωes. As Table 2 shows, the above eight sets correspond to eight combinations of the three Rxs’ reception modes. Taking Reeemks as an example, its subscript is eee, this indicates that when an antenna in set Reeemks serves Rxϖ (ϖ∈m,k,s), the phase difference of the signals perceived by Rxϖ’s two antennas is even times of π, thus Rxϖ should adopt subtractive reception mode.Table 2
Determining candidate serving antenna set (SAS) andRx’s reception mode (RM) under K=3.
CaseSAS forRxmSAS forRxkSAS forRxsRxm’s RMRxk’s RMRxs’s RMIRooomksReeomksReoemksSubtractiveAdditiveAdditiveIIRooemksReeemksReoomksSubtractiveAdditiveSubtractiveIIIRoeomksReoomksReeemksSubtractiveSubtractiveAdditiveIVRoeemksReoemksReeomksSubtractiveSubtractiveSubtractiveVReoomksRoeomksRooemksAdditiveAdditiveAdditiveVIReoemksRoeemksRooomksAdditiveAdditiveSubtractiveVIIReeomksRooomksRoeemksAdditiveSubtractiveAdditiveVIIIReeemksRooomksRoeomksAdditiveSubtractiveSubtractiveBased on the various combinations of the serving antenna sets, the reception modes of multiple Rxs can be determined. Since there is always no cooperation among multiple Rxs, Rxs’ reception modes should be determined by the CCU and then informed to each Rx. We further present the extension of DRM-MA to the case ofK (K>2) Rx. First, we index all Rxs from 1 to K. For simplicity, we replace the subscripts o and e of the candidate serving antenna sets with binary numbers 0 and 1, respectively. In this way, the string composed of o and e can be equivalent to a binary code. Provided with K Rxs, DRM-MA first calculates Ωeϖ and Ωoϖ where ϖ∈1,2,⋯,K for each Rx, based on which 2K cases of candidate serving antenna sets, denoted as Rb1⋯bK1⋯K where b1⋯bK can be either o0 or e1, can be obtained. Next, we can select any one of the 2K candidate antenna sets (e.g., case I in Table 2) and mark it as the serving antenna set for Rx1 (e.g., as for case I in Table 2, Rooomks whose subscript is 000 serves Rxm). Then, we select the serving antenna set for Rx2 (e.g., as for case I in Table 2, Reeomks whose subscript is 110 serves Rxk); the 1st and 2nd bits of Rx2’s serving antenna set’s subscript should be opposite to those of Rx1’s, while the rest bits of the two Rxs’ serving antenna sets’ subscripts are the same. As for Rx3 (under K=3, according to Table 2, Reoemks whose subscript is 101 serves Rxs); the 1st and 3rd bits of Rx3’s serving antenna set’s subscript should be opposite to those of Rx1’s, while the remaining bits of the two Rxs’ sets’ subscripts are the same. As for RxK, the 1st and Kth bits of its serving antenna set need to be opposite to those Rx1’s, while the remaining bits of the two Rxs’s sets’ subscripts are the same.Based on the above process,2K combinations of serving antenna sets for K Rxs can be obtained. Then, an Rx, say Rxk^ (k^∈1,⋯,K) can determine its reception mode in terms of the k∧th bit of its serving antenna set’s subscript. Specifically, if the k∧th bit is o0, Rxk^ should adopt subtractive mode; otherwise, for e1, additive reception mode should be used. We take case I in Table 2 as an example, the first user Rxm adopts subtractive mode according to the 1st bit of Rooomks’s subscript. Similarly, the second user Rxk employs additive mode based on the 2nd bit of Reeomks’s subscript. As for the last user Rxs, its uses additive mode according to the 3rd bit of Reoemks)’s subscript.Based on the above descriptions, DRM-MA can be applied to the communication scenario withK Rxs.


## 4. Evaluations

In this section, we use MATLAB to evaluate the performance of the proposed DRM-MA. We consider a communication scenario of10m×10m, in which multiple antennas, denoted as Txvh (v∈1,2,⋯,V,h∈1,2,⋯,H), are uniformly distributed. The transmit power of Txvh is PT. The carrier frequency is 2.4 GHz. Two Rxs, denoted as Rxm and Rxk, are arbitrarily located in the communication area and equipped with two antennas. The distance between Rx’s two receiving antennas, d, is 0.2 m. We employ the free space propagation model as given in Section 2. The signal power sent from Txvh and received by antenna κ (κ∈m1,m2,k1,k2) is Pκvh=PTGvhGκλ2/Γ4πlκvh2 where Gvh=1, Gκ=1, and Γ=1. lκvh (measured in meter) is the distance from Txvh to antenna κ. We adopt the serving antenna selection weight ξ∈0,1 and define the signal-to-noise ratio (SNR) as γ¯=10lgγdB where γ=PT/σn2 and σn2 represents for the noise power. In determining transmitting antenna sets Ωom and Ωem, we take Txvh in the range of ±ε near odd and even times of π into account. To be specific, we define two phase intervals, ℤo=π−ε,π+ε and ℤe=0,ε⋃2π−ε,2π. Then, for example, if Δφmvhmod2π∈ℤe (or Δφmvhmod2π∈ℤo) holds, Txvh can serve Rxm and Rxm should employ additive (or subtractive) reception mode.Figure3 simulates the distribution of candidate serving antennas for Rxm and Rxk in the communication scenario under various εs. As the figure shows, the coordinates of the midpoints of m1m2¯ and k1k2¯ are set to be Cm4.9,4.5 and Ck6,6; and accordingly, the antennas’ coordinates are m14.8,4.5, m25,4.5, k16−3/20,6.05, and k26+3/20,5.95, respectively. As for Txvh in the red area, on one hand, it yields signals’ phase difference at Rxm satisfying Δφmvhmod2π∈ℤe, thus is added to set Ωem; on the other hand, the signals’ phase difference at Rxk satisfies Δφkvhmod2π∈ℤo, and hence Txvh belongs to set Ωok. Therefore, we add Txvh to the candidate serving antenna set Reomk=Ωem⋂Ωok. Then, if Txvh in Reomk transmits to Rxm, Rxm should employ additive reception mode; if Txvh in Reomk serves Rxk, Rxk should adopt subtractive mode. Similarly, as for Txvh in the dark blue area, on one hand, it yields signals’ phase difference at Rxm satisfying Δφmvhmod2π∈ℤo, thus is added to set Ωom; on the other hand, the signals’ phase difference at Rxk satisfies Δφkvhmod2π∈ℤe, and hence, Txvh belongs to set Ωek. Therefore, we add Txvh to the candidate serving antenna set Roemk=Ωom⋂Ωek. Accordingly, if Txvh in Roemk transmits to Rxm, Rxm should employ subtractive reception mode; if Txvh in Roemk serves Rxk, Rxk should adopt additive mode.Figure 3
Distribution of candidate serving antennas forRxm and Rxk under various εs.
(a)
ε=π/10(b)
ε=π/18As forTxvh in the light blue area, it yields signals’ phase difference at Rxm’s and Rxk’s two receiving antennas satisfying Δφmvhmod2π∈ℤe and Δφkvhmod2π∈ℤe, respectively. Thus, Txvh belongs to Ωem and Ωek, yielding candidate serving antenna set Reemk=Ωem⋂Ωek. Accordingly, if Txvh in Reemk serves Rxm or Rxk, either Rx should employ additive reception mode. Similarly, as for Txvh in the magenta area, it yields signals’ phase difference at Rxm’s and Rxk’s two receiving antennas satisfying Δφmvhmod2π∈ℤo and Δφkvhmod2π∈ℤo, respectively. Thus, Txvh belongs to Ωom and Ωok, yielding candidate serving antenna set Roomk=Ωom⋂Ωok. Accordingly, if Txvh in Roomk serves Rxm or Rxk, either Rx should employ subtractive reception mode.As Figure3 shows, under a small ε, the ranges of phase intervals ℤo and ℤe become small too, hence yielding reduced areas and decreased the number of candidate serving antennas for Rxm and Rxk. Given the strong enough phase compensation capability of the Rx, a large ε can be adopted, then, the ranges of phase intervals ℤo and ℤe are enlarged, yielding more candidate serving antennas for Rxm and Rxk. In what follows, we will evaluate DRM-MA’s SE performance and compare it with zero-forcing (ZF) reception. Since DRM-MA employs a single transmitting antenna for each Rx’s data transmission, Tx-side array signal processing methods cannot be used in our communication scenario. However, as the Rx is equipped with 2 antennas, the Rx-side array processing such as ZF is taken into account. In the following simulation, we assume 2 Rxs in the communication area and 16 candidate serving antennas are involved in sets Roomk and Reemk, respectively. We let Txvmhm in Roomk and Txvkhk in Reemk serve Rxm and Rxk. Accordingly, Rxm and Rxk adopt subtractive and additive reception mode, respectively. Without loss of generality, we assume that Txvmhm in Roomk serves Rxm, and Txvkhk in Reemk serves Rxk. Then, according to Eqs. (10) and (11), we can compute SE of Rxm and Rxk as rm=log21+PTΔAmvmhm2/PTΔAmvkhk2+σn2 and rk=log21+PTΣAkvkhk2/PTΣAkvmhm2+σn2, respectively.Figure4 plots the variation of two Rxs’ sum SE with γ¯ under cardRoomk=cardReemk=16 where card· denotes the number of elements in a set, and ξ∈0.1,0.4,0.7,1.0. As the figure shows, the system’s average SE grows with the increase of γ¯. Under fixed γ¯, DRM-MA’s system SE increases as ξ grows. This is because when ξ is large, a serving transmitting antenna that can yield a high desired data transmission rate is preferred (see Section 3.2). Moreover, since the residual interference is negligible in our system settings when applying DRM-MA, it is better to focus on selecting a good serving antenna for each Rx rather than avoiding residual interference to unintended Rxs. Therefore, we can see from the figure that under ξ=1, DRM-MA outputs the highest system’s SE.Figure 4
Variation of system’s average SE withγ¯ under various ξs.Figure5 plots the variation of two Rxs’ sum SE with γ¯ under DRM-MA and ZF reception. We set cardRoomk=cardReemk=16 and ξ=1. To make the simulation results more convincing, both DRM-MA and ZF are divided into two versions for comparison. The method that employs antenna selection given in Section 3.2 is called optimal selection. As its counterpart, method that randomly chooses a serving antenna from Roomk and Reemk for Rxm and Rxk is called random selection.Figure 5
Comparison of DRM-MA and ZF reception.As Figure5 shows, the SE of DRM-MA (Optimal selection) outputs the highest system’s SE. DRM-MA (random selection) ranks second. Then comes ZF (optimal selection). ZF (random selection) yields the lowest system’s SE. This is because the optimal selection chooses the best serving antenna that yields the strongest desired signal at its intended Rx; as a comparison, random selection randomly selects an antenna from sets Roomk and Reemk to serve Rxm and Rxk, respectively. Moreover, as abovementioned, the residual interference is so small that can be neglected. Therefore, optimal selection excels random selection in the system’s SE. Given the fixed antenna selection strategy, DRM-MA outputs higher SE than ZF. This is because, under DRM-MA, each Rx can realize desired signal construction and interference destruction simultaneously via serving antenna selection and reception mode adaptation; and there is no desired signal power loss in the use of DRM-MA. However, as a comparison, ZF causes desired signal’s power loss while suppressing the interference [24]. Therefore, DRM-MA is advantageous over ZF in SE.


## 5. Conclusion

In this paper, we have proposed a novel MAC method called DRM-MA. Based on the phase difference of signals sent from each candidate transmitting antenna and perceived by the two receiving antennas of multiple Rxs, proper serving antennas are selected and paired with the Rxs. Then, each Rx adopts either additive or subtractive reception mode to postprocess the signals received by its two antennas, to realize in-phase desired signal construction and inverse-phase interference destruction simultaneously. In this way, multiple concurrent data transmissions are realized. Our simulation results have shown that DRM-MA can effectively strengthen the desired signal and suppress CCI among coexisting antenna-receiver pairs, hence outputting a high system’s SE.



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*Source: 5328007-2022-10-11.xml*