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Orthogonal frequency division multiple access (OFDMA) is a multi-carrier, multiple access (MA) technique, which is widely adopted in contemporary wireless standards. Single carrier-frequency division multiple access (SC-FDMA) is a modified version of OFDMA which employs single carrier transmission by pre-coding the data symbols using discrete Fourier transform (DFT). However, these systems are highly vulnerable to the adverse effects arising in the channel, carrier frequency offset (CFO) and in-phase/quadrature phase (I/Q) imbalances. In the uplink communication scenario, the signal received at base station is the superposition of signals from all the active users. Even though the adverse effects caused by the communication channel and CFOs are addressed in the related literature extensively, the effect of I/Q-imbalances at the transmitter and the receiver is rarely considered. The effect of I/Q-imbalances will make the equalization process at the base station more complex. It is because the overall effective channel with radio impairments must be included in the system modelling. Hence the receiver processing should contain the equalization for effect of the channel, CFOs and I/Q imbalances. In this paper, we propose a novel technique based on the oblique projection (OP) technique for equalizing the channel, radio imbalances and synchronization errors caused by both transmitter (TX) and receiver (RX) for OFDMA/SC-FDMA uplink systems. We also propose an equalization technique with reduced computational complexity to overcome the adverse effects caused by I/Q imbalances and CFOs. The results of the simulation studies illustrate that the proposed techniques can compensate all the above-mentioned effects and they offer very good performance under both TX and RX I/Q imbalances.

In recent years, the focus of space research and exploration by various countries and international space agencies has been on the return of humans to the moon. Astronauts on lunar missions need to utilize network communication and exchange data. Against this backdrop, it is necessary to consider the performance of communication systems and the extreme conditions of the lunar environment, such as signal attenuation and frequency selection, to ensure the reliability and stability of communication systems. Therefore, providing technical performance adapted to the lunar environment is crucial. In this article, we investigated the applicability of Orthogonal Frequency Division Multiple Access (OFDMA) and Single-Carrier Frequency Division Multiple Access (SC-FDMA) waveforms in the lunar communication environment. Specifically, we used Peak-to-Average Power Ratio (PAPR) and Bit Error Rate (BER) as performance indicators. By studying the impact of different modulation schemes and cyclic prefix lengths on communication performance, we completed the research on waveform adaptability based on lunar channels. Simulation results indicate that the transmission structure we designed can meet the system-level performance requirements of lunar communications. This research provides valuable insights for the design and optimization of communication systems for future lunar missions, paving the way for the seamless integration of advanced ground technologies in extraterrestrial environments.

In this paper, we address the joint estimation of doubly selective channels (DSCs) and carrier frequency offsets (CFOs) in multiple input multiple output orthogonal frequency division multiple access uplink with highly mobile users. Since the channel coefficients are rapidly varying over time and the base station has to perform the estimation task from the received composite signal, the exact solution to this joint estimation problem requires multidimensional search which is computationally intensive. We propose an iterative technique for the joint estimation of DSCs and CFOs based on space alternating generalized expectation maximization algorithm which will decompose the multidimensional optimization to many one dimensional searches. The proposed method works even in the presence of residual timing offsets and it does not require the knowledge of channel statistics at the receiver. Convergence properties of the proposed algorithm in terms of rate matrix is studied and analytically proved that the proposed joint estimation algorithm converges. Simulation studies illustrate that the proposed technique offers good performance even at very high mobile speeds.

In recent years, with the increasing number of terminal connections, high-density deployment scenarios have become important scenarios for future wireless networks. Ultra-high throughput (EHT) in high density deployment scenarios is the technical goal of IEEE 802.11be, the next-generation wireless local area network (WLAN) standard. However, in a high-density deployment scenario, interference suppression between BSSs is serious, which seriously affects the throughput of a WLAN. And the resources available are limited. Therefore, for the next generation WLAN standard, this paper proposes a downlink transmission scheme based on AP coordination and the orthogonal frequency division multiple access (OFDMA) protocol and the non-orthogonal multiple access (NOMA) protocol, named Co-OFDMA–NOMA scheme. The core idea is to transform the interference and suppression relationship between neighboring BSSs into the relationship of mutual coordination and assistance through the Co-OFDMA–NOMA protocol proposed in this paper. Firstly, a downlink transmission scheme named the Co-OFDMA–NOMA protocol is designed. In addition, the protocol and its framework structure have good backward compatibility. Theoretical analysis shows that the proposed Co-OFDMA–NOMA protocol has significant performance gain, and simulation results prove the effectiveness of the scheme.

This research allows the secure surveillance approach for the Internet of Things (IoT) methodology to be developed by integrating wireless signalling and image encryption strategy. Since the Cloud Service Telco (CST) is a semi-trusted body in cloud services, user data is encrypted before uploading to a cloud server for data protection from disclosure. The flexibility of encrypted data sharing is essential for cloud storage users. This study investigates the Discrete Wavelet Transform (DWT) technique with modified Huffman compression and Elliptic Curve Cryptography (ECC). It encrypts and decrypts the data and enhances industrial security surveillance in transmission. It uses the wireless network’s next generation (5G or 6G) as uplink Single Carrier Frequency Division Multiple Access (SC-FDMA) strategies via the IoT. This study presented a novel approach to proposing hardware architecture for a secure web camera integrated with the Atmel in the mega AVR family (ATMEGA) microcontroller, suitable for IoT applications. The experimental results confirm the proposed model’s efficacy compared with existing robustness and security analysis algorithms. These systems are also used by implementing industry-standard protocols using IoTs to monitor industrial applications. The proposed framework can also minimise bandwidth, transmission cost, storage space, tracking data, and decisions about abnormal events such as potential fraud and extinguisher detection in surveilling applications.

Pseudorange bias handling is essential in satellite positioning, navigation and timing (PNT) services and ionospheric modeling. Differential code bias (DCB) is commonly utilized and parameterized as the differential form of the pseudorange bias. To overcome the challenges of the inflexible and inconvenient extendable DCB calibration for the multi-frequency observations with multiple signal modulations, the pseudorange observable-specific signal bias (OSB) is used alternatively as the observation-specific individual bias representation. The study estimates the pseudorange OSBs of all the observation channels with two rigorous and modified multi-frequency approaches: multi-frequency modified carrier-to-code leveling (MFMCCL) and multi-frequency modified precise point positioning (MFMPPP) approaches. Both two approaches extract the slant ionospheric observables by considering the time-variant characteristic of the receiver pseudorange bias, after that estimating the pseudorange OSBs with certain reliability. After solving the various linear combinations of the pseudorange OSBs by two approaches, including the slant ionospheric observables and satellite-plus-receiver (SPR) pseudorange biases, all possible multi-frequency pseudorange OSBs are estimated simultaneously. Both two approaches are discussed for the GNSS observations with the code division multiple access (CDMA) and frequency division multiple access (FDMA) models. The two methods are validated with one-month data to estimate 31 types of pseudorange OSBs for the GPS, BDS, GLONASS and Galileo systems. Compared with the Chinese Academy of Sciences (CAS) pseudorange OSB product, the results indicated that both two proposed approaches can estimate the reliable multi-frequency multi-GNSS pseudorange OSBs with high stability and consistency.

To address the problems of network congestion and spectrum resources shortage in multi-user large-scale scenarios, this paper proposes a twice random access OFDMA-NOMA-RA protocol combining the advantages of orthogonal frequency division multiple access (OFDMA) and non-orthogonal multiple access (NOMA). The idea of this protocol is that OFMDA is used to divide the entire frequency field into multiple orthogonal resource units (RUs), and NOMA is used on each RU to enable more users to access the channel and improve spectrum efficiency. Based on the protocol designed in this paper, in the case of imperfect successive interference cancellation (SIC), the probability of successful competition subchannels and the outage probability are derived for two scenarios: Users occupy the subchannel individually and users share the subchannel. Moreover, when two users share the channel, the decoding order of the users and the corresponding probabilities are considered. Then, the system throughput is obtained. To achieve better outage performance in the system, the optimal power allocation algorithm is proposed in this paper, which enables the optimal power allocation strategy to be obtained. Numerical results show that the larger the imperfect SIC coefficient, the worse the outage performance of weak users. Compared with pure OFDMA and NOMA, OFDMA-NOMA-RA always maintains an advantage when the imperfect SIC coefficient is less than a specific value.

Many existing healthcare facilities still rely on the legacy Wi-Fi 5 (IEEE 802.11ac) standard, which is based on Orthogonal Frequency-Division Multiplexing (OFDM). OFDM supports single-user-per-channel access, leading to increased contention, higher latency, jitter, and packet loss under dense device deployments commonly found in clinical environments. This study presents a quantitative performance evaluation of Wi-Fi 5 and Wi-Fi 6/7 by comparing the effectiveness of OFDM with Orthogonal Frequency-Division Multiple Access (OFDMA) and Target Wake Time (TWT) in a simulated dense IoMT environment. Simulations were conducted using Network Simulator 3 (NS-3), and relevant Quality of Service (QoS) metrics. The results demonstrated that OFDMA reduces average network delay by up to approximately 37%, improves throughput by approximately 20%, and reduces packet loss ratio by up to 85% compared to OFDM under high-density operations, while exhibiting marginally improved jitter performance (approximately 2%). In addition, the use of TWT achieved substantial reductions in device power consumption of up to approximately 90%, at the cost of reduced aggregate throughput of up to approximately 75% under high station densities. These results demonstrated that Wi-Fi 6/7 technologies can offer significant advantages in terms of QoS and energy efficiency over legacy Wi-Fi 5 for dense IoMT environments.

Accurate estimation of atmospheric delays and reliable ambiguity resolution (AR) are major challenges in implementing network real-time kinematic (NRTK) positioning technology. Previous studies on NRTK positioning have focused on using a double-differenced (DD) model, which restricts the flexibility of interpolated atmospheric delays and lacks a rigorous strategy for GLONASS AR due to the frequency-division multiple access (FDMA) regime. In this contribution, the ionosphere-weighted single-differenced RTK (IW-SD-RTK) model is proposed to obtain more accurate and flexible interpolation of SD atmospheric delays. Then, the influence of the short-term variation in the receiver hardware biases on the estimation of SD ionospheric delays is analyzed by comparing the common clock (CC) and decoupled clock (DC) IW-SD-RTK models, where the receiver hardware biases are treated as constant in the CC-IW-SD-RTK model and as white noise in the DC-IW-SD-RTK model. Furthermore, to improve the compatibility and interoperability of code division multiple access (CDMA) and FDMA in NRTK positioning, a novel integer-estimable (IE) FDMA model is employed for GLONASS AR. The results demonstrate that the DC-IW-SD-RTK model obtains more accurate and stable ionospheric delays than the CC-IW-SD-RTK model, and the DC-IW-SD-RTK model also outperforms the CC-IW-SD-RTK model in NRTK user positioning performance. Additionally, compared to standalone GPS, the incorporation of GPS and GLONASS observations improves the ADOP by approximately 42%, 45%, and 49% and the user 3-dimensional positioning precision by approximately 10%, 34%, and 19% for small-, medium-, and large-scale networks, respectively. The mean time to first fix (TTFF) is also improved by 36%.

This paper aims at improving the system throughput of orthogonal frequency division multiple access small cell networks. Different with traditional schemes that neglect the cooperation among small cells, a scheme named as resource block exclusion-based power control (RBEBPC) is proposed by sharing the interference correlated information. RBEBPC consists of two steps that are iteratively conducted. First, based on current power allocation results, partial system resource blocks are excluded by playing the formulated cooperative coalition formation games. Second, the transmission power of each small cell is determined by solving a modified throughput maximization problem after the resource block exclusion. As the generated interference is constrained in the second step, part of the small cells transmit without full power. Thereby, the overall system interference keeps non-increasing after RBEBPC is adopted. Simulation results indicate that about 15 % system throughput gain and 13 % power saving gain are obtained compared to traditional iterative water filling scheme.