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In this work, we thoroughly analyze the rate-region provided by the asynchronous transmission in multiple access channels (MACs). We derive the corresponding capacity-regions, applicable to a wide range of pulse shaping methods. We analytically prove that asynchronous transmission enlarges the capacity-region of MACs. Although successive interference cancellation (SIC) can achieve the optimal sum-rate for the conventional uplink non-orthogonal multiple access (NOMA) methods, it is unable to achieve the boundary of the capacity-region for the asynchronous transmission. We demonstrate that for the asynchronous transmission, the optimal SIC decoding order to achieve the maximum sum-rate is based on the users' channel strengths. This optimal ordering is in contrast to the conventional uplink NOMA, where various decoding orders can result in the maximum sum-rate. Furthermore, we provide practical transceiver designs to approach the capacity-region. The memory induced by asynchronous transmission enables the use of the trellis-based detection methods which improves the performance. In addition, we propose a transceiver design, based on channel diagonalization to exploit the frequencyselectivity introduced by timing offsets. The proposed transceiver design, joint with the turbo principle, enables us to achieve a rate pair that is not achievable by the synchronous transmission.

In this paper, we show that by investigating inherent time delays between different users in a multiuser scenario, we are able to cancel interference more efficiently. Time asynchrony provides another tool to cancel interference which results in preserving other resources like frequency, time and code. Therefore, we can save the invaluable resource of frequency band and also increase spectral efficiency. A sampling method is presented which results in independent noise samples and obviates the need for the complex process of noise whitening. By taking advantage of this sampling method and its unique structure, we implement maximum-likelihood sequence detection which outperforms synchronous maximum-likelihood detection. We also present successive interference cancellation with hard decision passing which gives rise to a novel forward-backward belief propagation method. Next, the performance of zero forcing detection is analyzed. Simulation results are also presented to verify our analysis.

This letter considers the capacity computations of faster-than-Nyquist (FTN) signaling. It calculates the theoretical capacity of FTN signaling which is obtained by a correlated input. The capacity-achieving power spectral density (PSD) is derived and its superiority over the independent input is shown. The practical issue imposed by the capacity-achieving PSD, i.e., outof-band (OOB) emission, is shown. To solve this issue, an upperbound is introduced for the input PSD to limit the OOB emission. The new optimization problem is solved and the constrained PSD is obtained. The introduced PSD captures the trade-off between the obtained capacity and the OOB emission.

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...

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In this paper, we analyze the benefits of intentionally adding timing mismatch in the downlink transmit beamforming for wireless transmission. Transmit beamforming enables the so-called spacedivision multiple access (SDMA), where multiple spatially separated users are served simultaneously. The optimal beamforming vectors can be found to minimize the average transmit power under each user's Quality-of-Service (QoS) constraint. We show that intentionally adding timing offsets between the transmitted signals can significantly reduce the average transmission power compared with the conventional optimal beamforming method while providing the same QoSs for users. We provide three different methods exploiting the time asynchrony which improve the performance with a computational complexity similar to that of the optimal synchronous beamforming. We derive the expressions for the achievable rates using the proposed methods and then provide efficient algorithms to solve the minimum power optimization. We show analytically and numerically that our proposed methods outperform the conventional optimal transmit beamforming.

In this letter, we propose a trellis-coded nonorthogonal multiple access (NOMA) scheme. The signals for different users are produced by trellis coded modulation (TCM) and then superimposed on different power levels. By interpreting the encoding process via the tensor product of trellises, we introduce a joint detection method based on the Viterbi algorithm. Then, we determine the optimal power allocation between the two users by maximizing the free distance of the tensor product trellis. Finally, we manifest that the trellis-coded NOMA outperforms the uncoded NOMA at high signal-to-noise ratio (SNR).