Frame aggregation (FA) significantly enhances throughput by frame header reduction and payload compression. However, FA affects the performance of latency and deadline adherence due to aggregation delays. In this paper, we investigate time-sensitive FA scheduling to ensure low latency and low delay violation probability while maintaining high throughput. We formulate the complex FA scheduling problem by constraint programming (CP), which considers variable FA sizes and queue availability in FA scheduling and yields near-optimal solutions. To address the NP-hard problem imposed by CP, we propose a composite dispatching cost - genetic algorithm (CDC-GA) for FA scheduling. According to our theoretical analysis, the proposed CDC encourages more aggressive aggregation when frame header overhead is high, thereby optimizing for both throughput and time-centric performance. The GA then refines the initial schedule using a proposed ternary chromosome encoding, which comprehensively captures all necessary scheduling decisions. The proposed CDC-GA approach accommodates various FA structures (e.g., fixed/variable length, with/without data compression), and outperforms the existing approaches, both conventional and learning-based, by adeptly handling variable FA sizes and integrating the impact of FA on time-sensitive performance. Simulation results show that the proposed CDC-GA significantly outperforms the existing approaches, improving throughput by 77% and reducing average latency by 36%.

Short frame structure and its optimization plays an important role in ultra reliable and low latency communication (URLLC). We investigate and compare the latency and throughput performances of the independent encoding (IE) and joint encoding (JE) frame structures for heterogeneous multi-device URLLC in the finite block length regime. There is a counter-intuitive finding that, despite a longer frame, IE enables a much lower average latency and higher reliability than JE, thanks to lower queuing latency, while JE achieves higher throughput with lower traffic heterogeneity, thanks to less channel dispersion. It is also shown that traffic heterogeneity has less adverse effects on the performance of the IE frame structure, and can even help reduce its average latency with the shortest block length first (SBF) scheduling rule proposed. We also provide an intensive analysis of the trade-off between pilot power and pilot overhead, with near-optimal pilot power and block length derived in closed form. Low-complexity joint pilot power, pilot length and block length optimization algorithms are proposed for IE and JE frame structures. Numerical results verify the effectiveness of the proposed algorithms, and also show that pilot power optimization plays a significant role in enhancing throughput at low to medium SNR.

We consider a multiple-input single-output (MISO) optical wireless communications (OWC) system with highly correlated channels, causing bit error rate (BER) performance degradation. Because of intensity modulation and direct detection (IM/DD), the transmitted signals are real-valued and non-negative, which limits the utilization of precoding in phase domain. We employ direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) modulation to design a group of phase rotation (PR) factors in frequency domain for OWC systems, robust against high channel correlations. The proposed PR-based precoding has a number of advantages over the power factor-based design in the literature: i) no change of transmission power on each LED; ii) no signal-to-noise ratio (SNR) and no BER degradation on transmitted signals of all LEDs. We formulate an optimization problem to obtain the optimal PR factors by maximizing the minimum pairwise Euclidean distances between all candidate signals. The optimization problem is non-convex, requiring multi-dimensional exhaustive searches. In order to reduce the complexity, we propose three low-complexity PR-based precoding approaches which provide BER performances better than the power factor-based designs in the literature, and are close to their own analytical results derived, respectively. The proposed approaches are validated via a built testbed, producing comparable performance between measured and simulated BERs.