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Wireless communication technologies have profoundly impacted the interconnectivity of mobile users and terminals. Nevertheless, the exponential increase in the number of users poses significant challenges, particularly in interference management, which is a major concern in wireless communication. Machine learning (ML) approaches have emerged as powerful tools for solving various problems in this domain. However, existing studies have not fully addressed the problem of interference management for wireless communication using ML techniques. In this paper, we explore the application of recurrent neural network (RNN) approaches to address co-channel interference in wireless communication. Specifically, we investigate the effectiveness of long short-term memory (LSTM), bidirectional LSTM (Bi-LSTM), and gated recurrent unit (GRU) network architectures in two different network settings. The first network comprises 10 connected devices, while the second network involves 20 devices. Our experimental results demonstrate that Bi-LSTM outperforms LSTM and GRU in terms of mean squared error, normalized mean squared error, and sum rate. While LSTM and GRU produce similar results, LSTM exhibits a marginal advantage over GRU. In addition, a combined RNN approach is also studied, and it can provide better results in dense networks. [ABSTRACT FROM AUTHOR]

In this paper, we address scheduling methods for queue stabilization and appropriate power allocation techniques in downlink dense user-centric scalable cell-free multiple-input multiple-output (CF-MIMO) networks. Scheduling is performed by the central processing unit (CPU) scheduler using Lyapunov optimization for queue stabilization. In this process, the drift-plus-penalty is utilized, and the control parameter V serves as the weighting factor for the penalty term. The control parameter V is fixed to achieve queue stabilization. We introduce the dynamic V method, which adaptively selects the control parameter V considering the current queue backlog, arrival rate, and effective rate. The dynamic V method allows flexible scheduling based on traffic conditions, demonstrating its advantages over fixed V scheduling methods. In cases where UEs scheduled with dynamic V exceed the number of antennas at the access point (AP), the semi-orthogonal user selection (SUS) algorithm is employed to reschedule UEs with favorable channel conditions and orthogonality. Dynamic V shows the best queue stabilization performance across all traffic conditions. It shows a 10% degraded throughput performance compared to V = 10,000. Max-min fairness (MMF), sum SE maximization, and fractional power allocation (FPA) are widely considered power allocation methods. However, the power allocation method proposed in this paper, combining FPA and queue-based FPA, achieves up to 60% better queue stabilization performance compared to MMF. It is suitable for systems requiring low latency. [ABSTRACT FROM AUTHOR]

This paper presents a simplified joint synchronization scheme for the integer carrier frequency offset and physical cell identity using a primary synchronization signal (PSS) in 5G new radio (NR) communication systems. We demonstrate the efficiency of our proposed NR-PSS synchronization scheme by deriving its simplified implementation, which exploits the near-zero autocorrelation feature between cyclically shifted NR-PSS symbols. As a figure of merit, we compute the probability of detection failure of the proposed NR-PSS synchronization scheme and validate its accuracy via simulations. To illustrate the benefits and limitations of the proposed NR-PSS synchronization scheme, we compare it with the conventional NR-PSS synchronization scheme, considering factors such as detection performance and computation complexity. Numerical results indicate that, regardless of the channel environment, the proposed NR-PSS synchronization scheme achieves a significant reduction in arithmetic complexity while maintaining the same detection capability as existing NR-PSS synchronization schemes. [ABSTRACT FROM AUTHOR]

Synchronization is a challenging issue in vehicle-to-everything (V2X) cellular communication, especially when V2X devices need to directly communicate with each other outside the network coverage area. By adopting the maximum likelihood principle, we propose joint detection of the sidelink secondary synchronization signal (SL-SSS) and carrier frequency offset (CFO) in a V2X system using 5G new radio sidelink. We formulate an effective joint coherent synchronization scheme for cellular V2X applications by decoupling the estimation of the sidelink identity and CFO, which requires a priori knowledge of channel state information. To verify the feasibility of the proposed detection scheme, we derive a simplified implementation of the proposed SL-SSS detector and a closed-form expression for the detection probability. Simulation results demonstrate that the proposed SL-SSS detector exhibits either comparable performance in terms of detection probability or reduced complexity when compared with conventional SL-SSS detectors. Using the proposed method in the cellular V2X system enables V2X devices to achieve robust synchronization with reduced power consumption during the initial synchronization procedure, while also offering valuable insights for designing a simple, efficient sidelink synchronization receiver. [ABSTRACT FROM AUTHOR]

This paper presents a novel formulation for detecting the secondary synchronization signal in a narrowband Internet of Things communication system. The proposed approach is supported by a noncoherent algorithm that eliminates the need for channel information. A robust joint synchronization scheme is developed by decoupling the estimations of the physical cell identity and the carrier frequency offset. We derive the detection probability of the proposed physical cell identity detector and the mean squared error of the carrier frequency offset estimator, demonstrating their accuracy through simulation results. The performance of the proposed detection scheme is compared with that of existing detection schemes in terms of both estimation accuracy and computational complexity. Experimental results confirm that the proposed synchronization method exhibits superior performance while maintaining relatively lower complexity compared with benchmark methods. [ABSTRACT FROM AUTHOR]

Graph convolutional networks (GCNs) provide an advantage in node classification tasks for graph-related data structures. In this paper, we propose a GCN model for enhancing the performance of node classification tasks. We design a GCN layer by updating the aggregation function using an updated value of the weight coefficient. The adjacency matrix of the input graph and the identity matrix are used to calculate the aggregation function. To validate the proposed model, we performed extensive experimental studies with seven publicly available datasets. The proposed GCN layer achieves comparable results with the state-of-the-art methods. With one single layer, the proposed approach can achieve superior results. [ABSTRACT FROM AUTHOR]

The reconfigurable intelligent surface (RIS) is one of the most innovative and revolutionary technologies for increasing the effectiveness of wireless systems. Deep learning (DL) is a promising method that can enhance system efficacy using powerful tools in RIS-based environments. However, the lack of extensive training of the DL model results in the reduced prediction of feature information and performance failure. Hence, to address the issues, in this paper, a combined DL-based optimal decoding model is proposed to improve the transmission error rate and enhance the overall efficiency of the RIS-assisted multiple-input multiple-output communication system. The proposed DL model is comprised of a 1-dimensional convolutional neural network (1-D CNN) and a gated recurrent unit (GRU) module where the 1-D CNN model is employed for the extraction of features from the received signal with further process over the configuration of different layers. Thereafter, the processed data are used by the GRU module for successively retrieving the transmission signal with a minimal error rate and accelerating the convergence rate. It is initially trained offline using created OFDM data sets, after which it is used online to track the channel and extract the transmitted data. The simulation results show that the proposed network performs better than the other technique that was previously used in terms of bit error rate and symbol error rate. The outcomes of the model demonstrate the suitability of the proposed model for use with the next-generation wireless communication system. [ABSTRACT FROM AUTHOR]

The metaverse is emerging as a next-generation internet paradigm that will enhance human interaction and connectivity. Digital twinning, a fundamental strategy used in the metaverse, allows for the virtualization of real-world items, people, actions, and settings. A virtual world called the metaverse is built on a variety of technologies. Wireless communication is an important part of these technologies. In particular, wireless 6G communication can be essential for the growth of the metaverse. In line with the goal of achieving higher rates in the next-generation wireless network for the metaverse, in this paper, a novel conceptualization of intelligent reflecting surface (IRS)-assisted multiple-input single output-based wireless communication in physical world environments is proposed. More specifically, this paper proposes that in the physical world, the IRS-assisted communication between a communication network and users can be reflected in the metaverse through the virtual world (such as digital avatars and the virtual environment). In the simulations, the bit-error rate and spectral efficiency of the receiver terminal were performed and calculated in the metaverse engine for future consideration. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) has emerged as a promising technology to enhance the spectral efficiency of wireless communication systems. However, if there are many obstacles between the RIS and users, a single RIS may not provide sufficient performance. For this reason, a double RIS-aided communication system is proposed in this paper. However, this system also has a problem: the signal is attenuated three times due to the three channels created by the double RIS. To overcome these attenuations, an active RIS is proposed in this paper. An active RIS is almost the same as a conventional RIS, except for the included amplifier. Comprehensively, the proposed system overcomes various obstacles and attenuations. In this paper, an active RIS is applied to the second RIS. To reduce the power consumption of active elements, a partially active RIS is applied. To optimize the RIS elements, the sum of the covariance matrix is found by using channels related to each RIS, and the right singular vector is exploited using singular value decomposition for the sum of the covariance matrix. Then, the singular value of the sum of the covariance value is checked to determine which element is the active element. Simulation results show that the proposed system has better sum rate performance compared to a single RIS system. Although it has a lower sum rate performance compared to a double RIS with fully active elements, the proposed system will be more attractive in the future because it has much better energy efficiency. [ABSTRACT FROM AUTHOR]

The reconfigurable intelligent surface (RIS) and rate-splitting multiple access (RSMA) are considered as promising technologies for the beyond Fifth-Generation (B5G) and Sixth-Generation (6G) wireless systems by controlling the propagation environment, which attenuates the transmitted signal, and by managing the interference by splitting the user message into common and private messages. Because conventional RIS elements have each impedance connected to the ground, the sum-rate performance improvement of the RIS is limited. Therefore, the new RISs, which have impedance elements connected to each other, have been proposed recently. To be more adaptive to each channel, the optimization of the grouping of the RIS elements is required. Furthermore, since the solution of the optimal rate-splitting (RS) power-splitting ratio is complex, the value should be simply optimized to be more practical in the wireless system. In this paper, the grouping scheme of the RIS elements according to the user scheduling and the solution of the RS power-splitting ratio based on fractional programming (FP) are proposed. The simulation results showed that the proposed RIS-assisted RSMA system achieved a high sum-rate performance compared to the conventional RIS-assisted spatial-division multiple access (SDMA) system. Therefore, the proposed scheme can perform adaptively for the channel and has a flexible interference management. Furthermore, it can be a more suitable technique for B5G and 6G. [ABSTRACT FROM AUTHOR]

A reconfigurable intelligent surface (RIS) has potential for enhancing the performance of wireless communication. A RIS includes cheap passive elements, and the reflecting of signals can be controlled to a specific location of users. In addition, machine learning (ML) techniques are efficient in solving complex problems without explicit programming. Data-driven approaches are efficient in predicting the nature of any problem and can provide a desirable solution. In this paper, we propose a temporal convolutional network (TCN)-based model for RIS-based wireless communication. The proposed model consists of four TCN layers, one fully connected layer, one ReLU layer, and lastly a classification layer. In the input, we provide data in the form of complex numbers to map a specified label under QPSK and BPSK modulation. We consider 2 × 2 and 4 × 4 MIMO communication using one base station and two single-antenna users. We have considered three types of optimizers to evaluate the TCN model. For benchmarking, long short-term memory (LSTM) and without ML are compared. The simulation results are conducted in terms of the bit error rate and symbol error rate which show the effectiveness of the proposed TCN model. [ABSTRACT FROM AUTHOR]

This paper proposes a novel hybrid beamforming and relay selection scheme for spectral efficiency maximization in a non-regenerative multi-relay multi-input multi-output (MIMO) system. The analog beamforming component in the radio-frequency (RF) domain must follow an element-wise constant modulus constraint, which makes the underlying design problem mathematically intractable and therefore, it is quite challenging to obtain the global optimal solution. To address this problem, phase-only precoding/combining matrices are derived by maximizing the end-to-end received signal-to-noise ratio (SNR) under transmit power constraint at the source and each relay node. This task is achieved by decomposing the original complicated optimization problem into two independent components. The first component designs the RF precoder/combiner at source and relay nodes by maximizing the received SNR at relay nodes. While the second component attempts to derive the analog precoder/combiner at relay nodes and destination by maximizing the received SNR at the destination. Digital baseband processing matrices are obtained by deriving the closed-form expression, which minimizes interference among different sub-channels. Finally, the relay selection is made by maximizing the overall SNR from the source to the destination. Computer simulations reveal that the performance of the proposed algorithm is close to its fully digital counterpart and approximately 6% higher than the specified relay-assisted hybrid beamforming techniques. Moreover, the proposed method achieves more than 15% higher performance in a sparse scattering environment when compared with the given relay selection techniques. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) has contributed new unused spectrum in addition to the traditional radio frequency communication and can play a significant role in wireless communication. The adaptation of VLC technology enhances wireless connectivity both in indoor and outdoor environments. Multiple-input multiple-output (MIMO) communication has been an efficient technique for increasing wireless communications system capacity and performance. With the advantages of MIMO techniques, VLC can achieve an additional degree of freedom. In this paper, we systematically perform a survey of the existing work based on MIMO VLC. We categorize the types of different MIMO techniques, and a brief description is given. Different problem-solving approaches are given in the subsequent sections. In addition, machine learning approaches are also discussed in sufficient detail. Finally, we identify the future study direction for MIMO-based communication in VLC. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) has great potential to implement the fifth-generation (5G) requirements of wireless communication. For a NOMA traditional detection method, successive interference cancellation (SIC) plays a vital role at the receiver side for both uplink and downlink transmission. Due to the complex multipath channel environment and prorogation of error problems, the traditional SIC method has a limited performance. To overcome the limitation of traditional detection methods, the deep-learning method has an advantage for the highly efficient tool. In this paper, a deep neural network which has bi-directional long short-term memory (Bi-LSTM) for multiuser uplink channel estimation (CE) and signal detection of the originally transmitted signal is proposed. Unlike the traditional CE schemes, the proposed Bi-LSTM model can directly recover multiuser transmission signals suffering from channel distortion. In the offline training stage, the Bi-LTSM model is trained using simulation data based on channel statistics. Then, the trained model is used to recover the transmitted symbols in the online deployment stage. In the simulation results, the performance of the proposed model is compared with the convolutional neural network model and traditional CE schemes such as MMSE and LS. It is shown that the proposed method provides feasible improvements in performance in terms of symbol-error rate and signal-to-noise ratio, making it suitable for 5G wireless communication and beyond. [ABSTRACT FROM AUTHOR]

Narrowband Internet of Things (NB-IoT) is one of the low-power wide-area network technologies that aim to support enormous connection, deep coverage, low power consumption, and low cost. Therefore, low cost of implementation and maintenance is one of the key challenges of NB-IoT terminals. This paper presents a low-complexity formulation for narrowband secondary synchronization signal (NSSS) detection in the NB-IoT system, supported by a coherent algorithm that requires a priori knowledge of the channel. By exploiting a symmetric conjugate feature of the NSSS sequence, a joint physical cell ID and radio frame number detection method with low complexity is proposed for coherent detection. The probability of erroneous detection of the presented NSSS detection method is computed, and the analytical model is verified by means of simulation. Numerical experiments will demonstrate that the proposed detection scheme remarkably reduces the computational complexity with almost the same detection ability compared to the existing detection scheme. [ABSTRACT FROM AUTHOR]

An intelligent reflecting surface (IRS) is a programmable device that can be used to control electromagnetic waves propagation by changing the electric and magnetic properties of its surface. Therefore, IRS is considered a smart technology for the sixth generation (6G) of communication networks. In addition, machine learning (ML) techniques are now widely adopted in wireless communication as the computation power of devices has increased. As it is an emerging topic, we provide a comprehensive overview of the state-of-the-art on ML, especially on deep learning (DL)-based IRS-enhanced communication. We focus on their operating principles, channel estimation (CE), and the applications of machine learning to IRS-enhanced wireless networks. In addition, we systematically survey existing designs for IRS-enhanced wireless networks. Furthermore, we identify major issues and research opportunities associated with the integration of IRS and other emerging technologies for applications to next-generation wireless communication. [ABSTRACT FROM AUTHOR]

A cell-free massive multiple input multiple output (MIMO) system is an attractive network model that is in the spotlight in 5G and future communication systems. Despite numerous advantages, the cell-free massive MIMO system has a problem in that it is difficult to operate in reality due to its vast amount of calculation. The user-centric cell-free massive MIMO model has a more feasible and scalable benefit than the cell-free massive MIMO model. However, this model has the disadvantage that as the number of users in the area increases, there are users who do not receive the service. In this paper, the proposed scheme creates connections for unserved users under a user-centric scheme without additional access point (AP) installation and disconnection for existing users. A downlink user-centric cell-free massive MIMO system model in which the APs are connected to the central processing unit (CPU) and the APs and users are geographically distributed is considered. First, the downlink spectral efficiency formula is derived and applied to the user-centric cell-free massive MIMO system. Then, the proposed scheme and power control algorithm are applied to the derived formula. The simulation results show that the unserved users within the area disappear by using the proposed scheme, while the bit error rate (BER) performance and sum rate improve compared to the existing scheme. In addition, it is shown that the proposed scheme works well even with a very large number of users in the area, and a significant service performance improvement for the worst 10% of users and the overall improvement of per-user throughput for the bottom 70% of users are ensured. [ABSTRACT FROM AUTHOR]

After coarse synchronization in orthogonal frequency division multiplexing (OFDM) systems, there might still be a residual frequency offset (RFO) and a sampling clock frequency offset (SFO), which seriously degrade the performance of the systems. This paper suggests a simple way of estimating the SFO in OFDM-based digital radio mondiale (DRM) system. To demonstrate the efficiency of the proposed SFO estimator, comparisons are made with other existing estimators in terms of mean square error (MSE) performance, estimation range, and complexity. Index Terms--Digital radio mondiale (DRM), OFDM, sampling frequency offset, synchronization.

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper presents our investigation into reducing the multiple access interference (MAI) introduced by symbol timing misalignment in an OFDMA uplink system. To combat the MAI, we provide a new OFDMA symbol frame employing a simple symbol repetition coupled with a cyclic time shift for typical OFDMA uplink scenarios. Theoretical analysis and computer simulations are used to assess the performance of the proposed OFDMA uplink system. It is found that this scheme can decrease the probability of destroying the orthogonality among the users and provide the MAI-free reception. Index Terms--Broadcasting, cyclic time shift, frequency division multiaccess, interference suppression, MAI, OFDMA uplink, symbol timing misalignment, synchronization, uplink broadcasting.

Orthogonal frequency division multiplexing (OFDM) systems are highly sensitive to carrier frequency offset and symbol timing error. This paper deals with estimation method of integer frequency offset (IFO) without the aid of pilot symbols. The proposed IFO estimator exploits two consecutive identical OFDM data symbols with only change of phase. In order to improve the accuracy of the IFO estimator, receive diversity is adopted. To demonstrate the efficiency of the proposed IFO estimators, comparisons are made with other existing estimators in terms of error performance, estimation range, and complexity. Index Terms--Integer frequency offset, OFDM, receive diversity, synchronization.

In this letter, a constant-amplitude biorthogonal multicode (CABM) modulation is proposed. The proposed modulation is a biorthogonal modulation scheme that employs a constant-amplitude transmission of multicode signals by introducing a simple constant-amplitude coding scheme with redundant bits. The proposed CABM modulation can provide bandwidth efficiency up to the level (i.e., 9/16) of constant-amplitude orthogonal multicode (CAOM) modulation. We prove that our encoding scheme proposed in this letter provides this constant amplitude property. The bit-error rate (BER) performance expressions and the union bounds of the proposed CABM modulation and demodulation pairs are derived analytically. The results of the computer simulation and theoretical analysis performed show that the BER performance and the hardware complexity of the proposed CABM modulation are significantly better than those of a CAOM modulation. Index Terms--Bandwidth efficiency, bit-error rate (BER), constant-amplitude biorthogonal multicode (CABM) modulation, constant-amplitude orthogonal multicode (CAOM) modulation, union bound.

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper deals with our investigations into improving the performance and reducing the complexity of a digital multimedia broadcasting (DMB) system with a return channel when OFDMA is used as an access scheme. To alleviate a multiple access interference (MAI) introduced by a symbol timing misalignment, the OFDMA-based DMB system adopts frequency diversity and cyclic suffix (CS) at the transmitter, namely FD-OFDMA DMB system with CS. When the system is fully loaded, in addition to low complexity at the transmitter, the performance of the FD-OFDMA DMB system with the CS comes close to that of a single-user FD-OFDMA system at the cost of a small loss of throughput. Index Terms--Cyclic suffix, digital multimedia broadcasting (DMB), frequency diversity, multiple access interference (MAI), orthogonal frequency-division multiple access (OFDMA).

In this paper, we apply multi-antenna scheme to DAB system for high-rate transmission. In the MIMO-DAB system with a number of transmitting and receiving antennas increases the transmission rate efficiently with low multiplication operations. For channel estimation in the M1MO-DAB system, we propose a new PRS structure and evaluate the channel estimation performance. The proposed PRS structure can estimate multi-channel information based on the semi-blind processing. The performance of proposed PRS is compared with that of conventional training sequence employing Tarokh's transmission matrix. And also we investigate the performance limitation due to the imperfect channel estimation in a MIMO-DAB receiver. It has been shown that the MIMO-DAB system with multi-antennas can achieve the high-rate transmission for multimedia broadcasting and the performance of MIMO-DAB system is impaired by imperfect channel information. Index Terms--Channel estimation, channel estimation error, DAB, effective SIR, MIMO, MIMO-DAB, OFDM.

In this paper, an orthogonal frequency-division multiplexing (OFDM)-code-division multiple-access (CDMA) system employing various diversity schemes is considered as a possible candidate for broadband wireless access network applications. With an emphasis on a preamble design for multichannel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in the space-time block-coded (STBC) OFDM-CDMA systems, respectively. Also, for a simple construction of the OFDM-CDMA system with the low peak-to-average power ratio (PAR) and reduced transmitter complexity, we investigate a simple PAR reduction scheme and its combination with both decimated selective mapping (SLM) and partial transmit sequence (PTS) methods. Taking into account the false side information introduced by the PAR reduction scheme, the effects of diversity techniques and PAR on the performance of the STBC OFDM-CDMA system are investigated and the maximum achievable diversity gain for a two-path Rayleigh-fading environment is evaluated. From the presented results, we find that the OFDM-CDMA signal with the low PAR is easily constructed by a simple phase transform and its combination version with the SLM and PTS approaches can be implemented to be less complex by decimating the OFDM samples. At the same time, the OFDM-CDMA system applying a space-time-frequency diversity with two and four transmit antennas can give the diversity of D = 4 and D = 8 for the multiuser case with half user capacity, respectively, even if encountered with the false side information. Index Terms--Broadband wireless access network, channel estimation, diversity, multichannel separation, orthogonal frequency-division multiplexing (OFDM)-code-division multiple access (CDMA), peak-to-average power ratio (PAR), preamble, space-time block code (STBC).

A combining of MIMO signal processing with OFDM is regarded as a promising solution of enhancing the performance of next generation wireless system. Therefore, in this paper, an OFDM-based wireless system employing layered space-time architecture is considered for a high-rate transmission. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in MIMO-OFDM system. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be executed based on semi-blind processing in the case of more than two transmitting antennas. And also we evaluate the PAR performance in the MIMO-OFDM system using the SLM and PTS approaches. The investigated SLM and PTS schemes for MIMO-OFDM signals select the transmitted sequence with lowest average PAR over all transmitting antennas and retrieve the side information very accurately at the expense of a slight degradation of the PAR performance. The low probability of false side information can improve the overall detection performance of the MIMO-OFDM system with erroneous side information compared to the ordinary SLM and PTS approaches, respectively. Also, we provide closed form of the average BER performance in MIMO-OFDM system using analytic approach. Index Terms--Average BER, BLAST, channel estimation, MIMO, OFDM, PAR.

In this paper, the effect of an imperfect channel estimation and carrier frequency offset on the performance of a space-time block coded (STBC) OFDM system is investigated. For the performance evaluation, the average bit error rate (BER) expression is derived in the presence of the imperfect channel information including both the inherent estimation error and the imperfect windowing error derived in a mismatched channel separation. From presented results, the inherent channel estimation error is a dominating term in the system performance, compared to other parameter imperfections. Index Terms--Channel estimation error, frequency offset, OFDM, STBC, windowing.

This paper investigates the peak-to-average power ratio (PAR) reduction scheme employing a simple symbol transform in OFDM-CDMA systems. This approach is very simple because of no additional complexity and works without restriction on the allocation of spreading code, maintaining an original transmission efficiency. Simulation results show that the investigated scheme gives the PAR reduction gain of 2 ~ 3 dB compared to the original OFDM and OFDM-CDMA signals, and can provide the further PAR reduction by combining the selective mapping (SLM) and partial transmit sequence (PTS) schemes, which are less complex compared to the ordinary SLM and PTS approaches, respectively. Index Terms--OFDM-CDMA, partial transmit sequence (PTS), peak-to-average power ratio (PAR), selective mapping (SLM), symbol transform.

In this paper, an OFDM-CDMA system employing various diversity schemes is considered for a possible candidate of broadband wireless access networks and broadcasting applications. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in OFDM-based multiple-antenna transmission systems. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be applied to the HIPERLAN/2 and IEEE 802.11a standards in the case of more than two transmit antennas. Also, an effect of diversity techniques on the performance of OFDM-CDMA based broadband wireless access networks is investigated and the maximum achievable diversity gain for a two-path Rayleigh fading environment is evaluated. Simulation results show that the OFDM-CDMA system applying a space-time-frequency diversity with a full-rate full diversity code can give the diversity of D = 4 and D = 8 for both multi-user cases of maximum user and half user capacities, respectively. Index Terms--Broadband wireless access network, channel estimation, diversity, multi-channel separation, OFDM-CDMA.

In this article, a complexity-effective secondary synchronization signal (SSS) detection scheme is presented for fifth-generation (5G) new radio (NR) communication systems, which uses a gold sequence for SSS generation by multiplying two different m-sequences. By taking advantage of good autocorrelation property between cyclic-shifted versions of m-sequence, the search space of SSS hypothesis testing is reduced by decoupling the detection of two m-sequences. The combination of such a sequential strategy with maximum likelihood detection criterion facilitates low-complexity and reliable SSS detection in the 5G NR system. Both numerical and simulation results are presented to justify the effectiveness of the proposed detection scheme. It is demonstrated from presented results that the proposed SSS detector achieves a considerable complexity reduction while incurring slight performance loss, compared to existing SSS detectors. [ABSTRACT FROM AUTHOR]