To achieve reliable transmission in visible light communications (VLC), this paper investigates a new delayed asymmetrically-clipped-optical orthogonal-frequency-division-multiplexing (D-ACO-OFDM) system with protographbased low-density parity-check (P-LDPC) codes. Specifically, we first incorporate the delayed bit-interleaved coded modulation (DBICM) structure into ACO-OFDM systems. Subsequently, based on the reliability differences among labeling bits, we propose a novel constrained delay allocation (CDA) scheme, which effectively optimizes the allocation of delayed time slots for labeling bits, thereby accelerating the update of soft information. Furthermore, a bit-level adaptive mapping is integrated into the proposed delay allocation framework, establishing an enhanced CDA (E-CDA) scheme. We also construct a family of partitioned-protograph-based LDPC (PP-LDPC) codes tailored for the E-CDA-aided D-ACO-OFDM system. In addition, a modified weighted density evolution (MWDE) algorithm is employed to analyze the convergence performance of the proposed PP-LDPC-coded D-ACO-OFDM system with the E-CDA scheme. Numerical results illustrate that our proposed design achieves noticeable performance gains compared with the existing counterparts and demonstrate that it is a competent scheme to meet the high-reliability requirements for VLC applications.
To tackle the challenges of high spectrum demand from massive Internet of Things (IoT) communications, this paper proposes a uncrewed aerial vehicle-assisted cognitive radio (CR) non-orthogonal multiple access (NOMA) transmission scheme. Specifically, a secondary aerial IoT base station concurrently acts as a relay to assist the transmission of primary network with favorable aerial-ground channels, and in return, IoT devices of the secondary network can utilize the primary network’s spectrum resources for data transmission. To reduce the signaling overhead of the secondary network, we adopt a hybrid long-short-packet transmission strategy, with primary users using long-packet transmission and secondary IoT devices utilizing short-packet transmission. Inspired by the notion of CR-NOMA, we propose two power allocation algorithms to respectively avoid affecting the quality-of-service (QoS) and age-of-information (AoI) of the primary network. For rigorous performance evaluation, we derive the theoretical expressions of outage probability, block error rate, and AoI performance for both power allocation algorithms. Moreover, for analytical tractability, we derive the high signal-to-noise-ratio (SNR) approximation expressions for the peak AoI (PAoI) of secondary users (SUs), by which the transmission blocklength of the SUs is optimized to minimize the PAoI performances. Finally, we present simulation results to validate the correctness of the theoretical expressions. The results further demonstrate that the SUs of the AoI-based approach achieves superior performance in the low SNR region, whereas the QoS-based approach is more favorable in the moderate SNR region.
This paper investigates the diversity-multiplexing tradeoff (DMT) for two-user hybrid automatic repeat request with incremental redundancy (HARQ-IR) assisted non-orthogonal multiple access (NOMA) systems employing variable-power (VP) transmission. Since the exact outage probability of the VP-HARQ-IR-NOMA system is analytically intractable, we first derive the diversity order, revealing it to be a piecewise function of the first decoded user’s transmission rate. Building upon this, we derive the asymptotic outage probabilities for both users in the high signal-to-noise ratio (SNR) regime to characterize the diversity gain. Furthermore, to provide deeper insights into performance, we derive the effective multiplexing gain using the DMT analytical framework, highlighting the gains of the proposed system over traditional orthogonal multiple access (OMA) schemes. The DMT analysis also confirms a degradation in diversity performance compared to OMA-based counterparts. Finally, numerical results are provided to validate the analytical derivations.
