Cooperative Asynchronous Non-Orthogonal Multiple Access With Power Minimization Under QoS Constraints

Abstract

Recent studies have demonstrated the superiority of non-orthogonal multiple access (NOMA) over orthogonal multiple access (OMA) in cooperative communication networks. In this paper, we propose a novel half-duplex cooperative asynchronous NOMA (C-ANOMA) framework with user relaying, where a timing mismatch is intentionally added in the broadcast signal. We derive the expressions for the individual throughputs of the strong user (acts as relay) which employs the block-wise successive interference cancellation (SIC) and the weak user which combines the symbol-asynchronous signal with the interference-free signal. We analytically prove that in the C-ANOMA systems with a sufficiently large frame length, the strong user attains the same throughput to decode its own message while both users can achieve a higher throughput to decode the weak user's message compared with those in the cooperative NOMA (C-NOMA) systems. Besides, we obtain the optimal timing mismatch when the frame length goes to infinity. Furthermore, to exploit the trade-off between the power consumption of base station and that of the relay user, we solve a weighted sum power minimization problem under quality of services (QoS) constraints. Numerical results show that the C-ANOMA system can consume less power compared with the C-NOMA system to satisfy the same QoS requirements. Index TermsNon-orthogonal multiple access, asynchronous transmission, cooperative communication, interference cancellation, power control.Non-orthogonal multiple access (NOMA) has been regarded as one of the key technologies for the next generation wireless communications [1]. Compared with the conventional orthogonal multiple access (OMA), NOMA can provide massive connectivity and high spectral efficiency [2].The key rationale behind NOMA is to allow users to share non-orthogonal wireless resources, e.g., frequency, time, and code. For multiuser detection, the superposition coding and the successive interference cancellation (SIC) are employed at the transmitter and receiver, respectively.Cooperative communication is an effective approach to exploit spatial diversity available through cooperating terminals' relaying signals for one another [3][4][5]. Cooperative relaying network with NOMA has been extensively studied in the literature, e.g., [6][7][8]. It has been shown that the cooperative NOMA (C-NOMA) systems outperform the cooperative OMA systems in terms of the spectral efficiency [6] and the outage probability [7]. Instead of dedicated relay nodes, users can also be adopted as relays in a cooperative network. A key feature of NOMA is that users with better channel conditions have prior information about the messages of other users. Ding et al. [9] proposed a C-NOMA scheme to fully exploit the prior knowledge at the strong user, where the users could cooperate with each other via short-range communication channels. Yue et al. [10] compared different operation modes of the relay user in a C-NOMA system. The half-duplex relay user receives and transmits in separate time slots w...

Downlink Asynchronous Non-Orthogonal Multiple Access Systems with Imperfect Channel Information

“…We assume that H 1 ∼ Exp(0.5) and H 2 ∼ Exp(1). We set the total transmit power of the BS P = 10 and τ = 0.5 since it has been proved in [2,3] that τ = 0.5 is the asymptotically optimal value to maximize the user throughput. For comparison, we employ the uniform quantizer proposed in [8] where the maximum quantization level L is derived by solving L = 1 λ∆ log 1 ∆ , λ is the parameter of the exponential distribution, and ∆ is the quantization bin width.…”

“…At the receiver, the composite signal is sampled at iT and (i+τ )T after matched filtering. This sampling method is called "oversampling" and the details have already been presented in [2,3]. For the sake of brevity, we omit it in this work.…”

“…If τ = 0, ANOMA degrades to NOMA and the received samples at iT and (i + τ )T are identical. The number of received samples is doubled in ANOMA (τ = 0) compared with NOMA, which then results in sampling diversity [2][3][4].…”

“…Note that NOMA can be considered as a special case of ANOMA, simply by setting τ = 0 in (2). It has been shown that R weak in ANOMA is higher than that in NOMA while R strong is the same for ANOMA and NOMA [3]. In this letter, we assume that CSI is perfectly estimated by users and fed back to the BS via an error and delay-free link.…”

“…Recently, a novel scheme called asynchronous NOMA (ANOMA) has been proposed to further improve the performance of NOMA, for example, in the uplink system [2] and the cooperative network [3]. In ANOMA, the intentional introduction of timing mismatch at the transmitter and the oversampling technique at the receiver result in the sampling diversity [2][3][4]. It has been demonstrated that ANOMA outperforms NOMA in terms of the throughput performance, the power consumption, etc.…”