Superposition Coding Scheme Research Articles

A Study on Improved Sum Rate of Cross-Correlated SC NOMA toward 6G URLLC

Since recently only an auto-correlated superposition coding (SC) scheme for non-orthogonal multiple access(NOMA) has been investigated, this paper proposes a cross-correlated SC scheme for NOMA. First, we derive the closed-form expression of the sum rate of the proposed cross-correlated SC scheme. Then, numerical analyses demonstrate that the sum rate of the proposed cross-correlated SC scheme is larger than that of the conventional auto-correlated SC scheme. We also show that for the stronger channel gain user, the signal-to-noise ratio (SNR) gain of the proposed cross-correlated SC scheme is about 15, compared with the conventional auto-correlated SC scheme. As a result, the proposed cross-correlated SC scheme could be a promising technology for 6G ultra-reliable low-latency communications (URLLC).

Superposition Coding for Downlink Underwater Acoustic OFDM

Superposition coding (SC) is a nonorthogonal scheme for downlink communications, in which all users are allowed to use the full degrees of freedom of the channel and successive interference cancellation (SIC) is adopted for user decoding. Combining SC with orthogonal frequency-division multiplexing (OFDM) modulation, the optimal resource allocation depends on the perfect channel state information (CSI) of all users at the transmitter, which is hard to obtain for underwater acoustic (UWA) channels. In this paper, we propose a practical OFDM-modulated SC scheme for downlink UWA communications, where the transmitter splits the power between two users based on statistical CSI. The expressions to characterize the boundary of the ergodic rate region achievable by the proposed scheme over long codewords are presented first, followed by the analysis of outage probability when coding is applied within one OFDM block. Then we examine the performance of SC in an OFDM-modulated system with practical coding and modulation pairs. Simulation results show that the OFDM-modulated SC scheme outperforms the orthogonal frequency-division multiple access (OFDMA) in performance of both block error rate (BLER) and spectral efficiencies under different data rate pairs. Recorded data from both medium-range and short-range sea tests verify that channel statistics are stable over a long period of time and can be used to assist resource allocation for the proposed scheme. Compared with OFDMA, considerable increase of spectral efficiencies has been found for SC based on experimental data when both users with disparate channels have nonzero data rates.

Superposition Coding Based User Combining Schemes for Non-Orthogonal Scheduling in a Wireless Relay System

We consider a wireless system where multiple users are served in Downlink (DL) by one Base Station (BS) and one Relay Station (RS). Previous research has shown that, combining two users' messages via Superposition Coding (SC) could enhance their achievable rate and fairness, in both two-user and multi-user systems. In this work, we propose two novel SC schemes referred as Relayed/Direct (RD-SC) and Relayed/Relayed SC (RR-SC) schemes, that combine the messages to a pair of Relayed/Direct users or Relayed/Relayed users into three or four SC layers, where the number of SC layers is equal to the number of available links. For each scheme, power allocation under sum-rate maximization is analyzed, enabling to derive the optimal power ratios to each SC layer numerically. Then, we design the non-orthogonal Max Rate-Two User-SC (MR-TU-SC) Scheduler and Proportional Fair-Two User-SC (PF-TU-SC) Scheduler where a pair of selected users are allocated simultaneously based on RD-SC and RR-SC schemes. The simulation results show that the proposed schedulers significantly outperform the conventional orthogonal schedulers where a single user is allocated on a unit resource block, in terms of system throughput, fairness and outage, while approaching the upper bound performance.

Performance of Superposition Coding for Downlink Coordinated Two-Point System

In this correspondence, we consider a downlink coordinated two-point (DCTP) system, where two users communicate with two base stations. A superposition-coding (SC)-based transmission scheme is proposed to improve its error performance. A joint interference-cancellation and maximum-ratio-combining (IC-MRC) detection algorithm is proposed for the SC scheme, which reduces the error probability of the system compared with the classic SC (CSC) detector. Closed-form expressions of the bit error probability (BEP) of the proposed scheme are derived. Under the same power and time constraint, a signal-to-noise ratio (SNR) gain is obtained when the SC scheme is employed. Tight upper bounds of the BEP are derived to provide insight. Optimal power allocation is also derived to optimize the system performance in terms of the BEP.

Transmission of Visual Data over Wireless Fading Channel in Real-Time Systems Based on Superposition Coding Scheme

Real-time visual applications are among the most important requirements in the next generation wireless communication systems. In these applications, the transmitted data comprise different layers with different importance levels based on their influence on the reception quality. Furthermore, the real-time transmission nature of these applications makes them sensitive to data losses and transmission delay. To address these issues, an efficient superposition adaptive modulation and coding system, for the optimal system performance, is proposed in this paper. The proposed system switches its modulation and coding scheme adaptively to select the suitable modulation order and coding rate that best match with the instantaneous channel condition. The channel state information is estimated in receiver and fed back to transmitter. In such method, better performances in both data rate and bit error rate (BER) can be attained. Here, the source data are divided into different priority layers with different importance. Each layer bit stream is sent with specific error protection level against channel corruption. The highest error protection level is assigned to the highest priority layer, and vice versa. The modulated bit streams of all layers are then superimposed together and transmitted via Rayleigh fading channel. At the receiver side, a specific multi-stage decoding receiver is used to reconstruct the source data which demodulates the layers in the order of their priorities. Simulation results show that the proposed system provides up to 18 dB SNR and 46 % data rate gains, respectively, compared to the traditional BPSK scheme at BER of 10−4.

Approximate Performance Analysis for Linear Codes in Superposition Schemes over Gaussian Broadcast Channels

Unequal error-protection schemes obtained by means of two-level superposition coding are considered. Their performance over Gaussian broadcast channels (GBCs) is investigated with optimal maximum-likelihood decoding as well as with a suboptimal decoding strategy based on interference cancellation. We focus on GBCs without fading and, assuming that linear codes are used, we evaluate, for both decoding strategies, analytical approximations of the word-error rate based on a suitable application of the union bound. As in the case of turbo codes and turbo-coded modulations in schemes without superposition, the derivation of the approximations exploits the concept of uniform interleaving. The analytical expressions obtained are in excellent agreement with the simulation results, and thus provide a useful tool for analysis and design of practical superposition-coding schemes. Unlike the existing design tools, which rely on the assumption of infinite-length superposition codes, the proposed approach allows us to study the effectiveness of finite-length coding schemes with known distance spectrum.

A Simple Superposition Coding Scheme for Optimizing Resource Allocation in Downlink OFDMA Systems

In this paper, we propose to apply a simple superposition coding strategy for downlink of OFDMA systems. The novelty of this paper consists on allowing at most two users to share the same subchannel. The main idea is to consider the subchannels allocated to the users with the weakest link, and allow these subchannels to be shared by some potential users who can transmit some number of bits with only a small amount of power. To decrease the overhead of the proposed OFDMA system, we restrict to use a predetermined superposition encoding|decoding scheme. We address the problem of resource allocation, which consists on finding the optimal subchannel assignment in the OFDMA system. A low complexity algorithm, denoted Share Specific Subcarrier Allocation (SSSA) is then proposed. It offers a fairness allocation among users. This can be done by taking into account all user's buffer states information. Simulation results confirm that the proposed technique outperforms the classical algorithms in terms of total throughput and dropping probability.