Conventional Orthogonal Frequency Division Multiple Access Research Articles

Optimizing the Timeliness of Hybrid OFDMA-NOMA Sensor Networks with Stability Constraints

In this paper, we analyze the timeliness of a multi-user system in terms of the age of information (AoI) and the corresponding stability region in which the packet rates of users lead to finite queue lengths. Specifically, we consider a hybrid OFDMA-NOMA system where the users are partitioned into several groups. While users in each group share the same resource block using non-orthogonal multiple access (NOMA), different groups access the fading channel using orthogonal frequency division multiple access (OFDMA). For this system, we consider three decoding schemes at the service terminals: interfering decoding, which treats signals from other users as interference; serial interference cancellation, which removes signals from other users once they have been decoded; and the enhanced SIC strategy, where the receiver attempts to decode for another user if decoding for a previous user fails. We present the average AoI for each of the three decoding schemes in closed form. Under the constraint of the stable region, we find the minimum AoI of each decoding scheme efficiently. The numerical results show that by optionally choosing the decoding scheme and transmission rate, the hybrid OFDMA-NOMA outperforms conventional OFDMA in terms of both system timeliness and stability.

A Clustered PD-NOMA in an Ultra-Dense Heterogeneous Network with Improved System Capacity and Throughput

In the current era of exponentially growing demand for user connectivity, spectral efficiency (SE), and high throughput, the performance goals have become even more challenging in ultra-dense 5G networks. The conventional orthogonal frequency division multiple access (OFDMA) tech-niques are mature but have not proven sufficient to address the growing user demand for high data rates and increased capacity. Therefore, to achieve an improved throughput in an ultra-dense 5G network with an expanded network capacity, the unified non-orthogonal multiple access (NOMA) technique is considered to be a more promising and effective solution. Throughput can be im-proved by implementing PD-NOMA, as the interference is managed with the successive inter-ference cancellation (SIC) technique, but the issue of increased complexity and capacity with compromised data rate persists. This study implements the clustered PD-NOMA algorithm to enhance user association and network performance by managing the users in clusters with fewer users per cluster with the implementation of the cooperative PD-NOMA within the clusters. In this study, we enhanced the user association in a network and ultimately improved the throughput, sum rate, and system capacity in an ultra-dense heterogeneous network (HetNet). By imple-menting the proposed clustered PD-NOMA scheme, the system throughput has improved by 23% when compared to the unified PD-NOMA scheme and 65% when compared to the OFDMA scheme with a varied number of randomly deployed users, along with an improvement in system capacity of 8% as compared to the unified PD-NOMA and almost 80% as compared to the conventional OFDMA scheme in a randomly deployed ultra-dense multi-tier heterogeneous network. Thus, we improved the network performance with the proposed algorithm and achieved increased capacity, throughput, and sum rate by outperforming the unified PD-NOMA scheme in an ultra-dense heterogeneous network.

Opportunistic non-contiguous OFDMA scheduling framework for future B5G/6G cellular networks

In this paper, we consider the problem of resource allocation within Beyond 5G (B5G) and the envisioned 6G wireless networks with Cognitive Radio (CR) capability. CR technology allows opportunistic and dynamic access to the unused licensed spectrum bands. Unlike the conventional orthogonal frequency division multiple access (OFDMA) schemes that impose spectrum contiguity constraints in assigning the OFDMA resource blocks (RBs), the Dis-contiguous OFDMA (D-OFDMA) approach allows for flexible time–frequency RB scheduling, in which each contending CR user can access adjacent and/or non-adjacent RBs. This can provide a more efficient and flexible spectrum utilization compared to conventional OFDMA schemes. Specifically, we formulate the spectrum assignment and access problem as an optimization problem that attempts to effectively utilize the time–frequency spectrum holes of the D-OFDMA RBs. The main goal is to improve the total network throughput by maximizing the achieved sum-rate of all contending CR users subject to achieving a set of interference and performance requirement constraints. The formulated problem is classified as a binary-linear program (BLP), which is, in general, NP-hard. Thus, we develop a sub-optimal polynomial-time solution based on a relaxing-and-rounding method. Simulation results reveal that the proposed D-OFDMA RB scheduling protocol significantly improves the network throughput while maintaining fairness compared to conventional contiguous-based OFDMA protocols. Specifically, the proposed protocol achieves up to 90% and 150% performance improvements over the contiguous frequency-OFDMA and contiguous time-OFDMA, respectively.

Achievable Rate Upper-Bounds of Uplink Multiuser OTFS Transmissions

In this letter, we study the achievable rates adopting two delay-Doppler (DD) domain multiples access (MA) schemes for uplink orthogonal time frequency space (OTFS) transmissions, namely delay division multiple access (DDMA) and Doppler division multiple access (DoDMA). To shed light on the system performance, we establish the achievable rate upper-bounds for both DDMA and DoDMA in comparisons to that of the conventional orthogonal frequency division multiple access (OFDMA). In particular, we show that both DDMA and DoDMA have more robust signal-to-interference-plus-noise ratio (SINR) performances against channel fluctuations compared to OFDMA and this robustness leads to higher achievable rates for both DDMA and DoDMA. To be more specific, we find that the achievable rate upper-bounds for OTFS depend only on the channel fading coefficients of different paths and the multi-user interference (MUI), while the delay and Doppler shifts also have significant impacts on that of the conventional OFDMA. Our simulation results are consistent with our derivations and demonstrate a noticeable improvement of the achievable rates for both DDMA and DoDMA over OFDMA.

Analysis of user pairing non-orthogonal multiple access network using deep Q-network algorithm for defense applications

Non-orthogonal multiple access (NOMA) networks play an important role in defense communication scenarios. Deep learning (DL)-based solutions are being considered as viable ways to solve the issues in fifth-generation (5G) and beyond 5G (B5G) wireless networks, since they can provide a more realistic solution in the real-world wireless environment. In this work, we consider the deep Q-Network (DQN) algorithm-based user pairing downlink (D/L) NOMA network. We have applied the convex optimization (CO) technique and optimized the sum rate of all the wireless users. First, the near-far (N-F) pairing and near-near and far-far (N-N and F-F) pairing strategies are investigated for the multiple numbers of users, and a closed-form (CF) expression of the achievable rate is derived. After that, the optimal power allocation (OPA) factors are derived using the CO technique. Through simulations, it has been demonstrated that the DQN algorithms perform much better than the deep reinforcement learning (DRL) and conventional orthogonal frequency-division multiple access (OFDMA) schemes. The sum-rate performance significantly increases with OPA factors. The simulation results are in close agreement with the analytical results.

Space Division Multiple Access With Distributed User Grouping for Multi-User MIMO-VLC Systems

To support multiple users in an indoor multiple-input multiple-output visible light communication (MIMO-VLC) system adopting spatial multiplexing, orthogonal frequency division multiple access (OFDMA) is usually adopted, where the overall modulation bandwidth is shared by all the users. In this paper, by fully exploiting the spatial distributions of light-emitting diode (LED) transmitters in the ceiling and end users around the receiving plane, we propose a space division multiple access (SDMA) technique for indoor spatial multiplexing-based MIMO-VLC systems. When applying SDMA, users within the MIMO-VLC system are divided into different user groups (UGs) based on their spatial locations with respect to different LED transmitters. Specifically, each UG can use the overall modulation bandwidth of the system. For efficient implementation of SDMA, two distributed user grouping (DUG) approaches are proposed, including basic DUG and transmit diversity-enhanced DUG (TD-DUG), and a two-step resource allocation algorithm is further designed. The achievable rates of the MIMO-VLC system employing SDMA with both basic DUG and TD-DUG are derived accordingly. To verify the superiority of SDMA over conventional OFDMA, detailed analytical and simulation results are presented. Moreover, a proof-of-concept experiment is conducted to demonstrate the advantage of SDMA in a practical spatial multiplexing-based MIMO-VLC system.

Sparse Code Multiple Access-Based Edge Computing for IoT Systems

In this paper, a sparse code multiple access (SCMA)-based edge computing scheme is proposed for Internet-of-Things (IoT) systems. The aim of implementing SCMA, which is a nonorthogonal multiple access resource allocation technique, is to improve network connectivity and maximize data rate provision. The proposed edge-IoT system is investigated under different SCMA configurations to explore the various performance aspects such as connectivity, throughput, task completion time, and complexity. First, the problem is formulated as a data rate maximization problem for SCMA-based heterogeneous networks under power constraints. Then, the problem is subdivided into a power allocation problem, which is solved using the water filling approach, and a codebook allocation problem that is solved using a heuristic algorithm. The results show that the SCMA scheme can significantly improve the IoT performance compared to the conventional orthogonal frequency-division multiple access resource allocation scheme in terms of connectivity, throughput, and task completion time provided that SCMA configurations are suitable with IoT processing capabilities to avoid undesired detection latency.

Multiple-Noise-Tolerant CO-OFDMA-PON Uplink Multiple Access Using AM-DAPSK-OFDM With Reflective ONUs

A reflective coherent orthogonal frequency-division multiple-access (OFDMA) passive optical network (PON) based on differential amplitude and phase-shift keying (DAPSK) OFDM is proposed herein. In the conventional OFDMA–PON uplink transmission, each optical network unit (ONU) signal has different channel and phase-noise characteristics. In addition, each ONU has an independent laser source; therefore, the carrier frequency offset (CFO) differs among the ONUs. These multiple noises worsen as the number of ONUs increases and adversely affect the spectral bandwidth and system complexity. The proposed system using reflective-type ONUs avoids multiple CFOs because every ONU shares the same laser source, which is transmitted from an optical line terminator. Moreover, by applying a differential modulation scheme to OFDM, we resolve the multiple channel and phase noises without spectral efficiency loss and system complexity increase. The spectral efficiency is maximized using an adaptive modulation technique. The proposed system is experimentally demonstrated in a 20-km OFDMA–PON uplink transmission link with two reflective ONUs. The results confirm the possibility of uplink multiple access without multiple-noise compensation processes. The optimal spectral efficiency in terms of bandwidth size and number of subcarriers is analyzed.

A New Uplink Multiple Access Based on OFDM With Low PAPR, Low Latency, and High Reliability

This paper proposes a new physical layer architecture based on orthogonal frequency-division multiplexing (OFDM) for uplink multiple access. It aims at high reliability with fixed decoder latency targeting emergent low-latency wireless applications, such as transmission of control information in machine-to-machine (M2M) and device-to-device (D2D) communications. Our proposed system is based on the judicious combination of subcarrier hopping and super-orthogonal convolutional codes for achieving high coding gain and frequency diversity, together with Golay complementary sequences for low peak-to-average power ratio (PAPR) signaling. The low PAPR nature improves the power efficiency at a power amplifier (PA) and thus contributes to the transmission range enhancement. Furthermore, by introducing non-orthogonality among users through the assignment of the same set of subcarriers to multiple users, the overall spectral efficiency can be improved. In order to suppress multiple access interference caused by the non-orthogonal overlap of users, multiuser detection (MUD) is employed. Theoretical expressions of approximate bit error rate for the proposed system with MUD are also developed. Numerical results reveal that our proposed system achieves higher reliability and higher spectral efficiency than the conventional orthogonal frequency-division multiple access (OFDMA)-based system.

Analog-to-Digital Conversion Using Sub-Nyquist Sampling Rate in Flexible Delay-Division Multiplexing OFDMA PONs

This paper details a delay-division multiplexing (DDM) scheme for orthogonal frequency-division multiple access (OFDMA) passive optical networks (PONs). Unlike detection in a conventional OFDMA PON, the detection of OFDM signals in the proposed scheme involves an analog-to-digital converter (ADC) operating at a sampling rate below the Nyquist rate for each optical network unit (ONU). Based on preallocated relative time delays among the ONUs, preprocessed signals sent from an optical line terminal can be detected as different downstream signals following spectral aliasing caused by ADCs operating at a sub-Nyquist sampling rate. It should be noted that the proposed scheme reduces the requirement of sampling rate, but not the analog bandwidth of an ADC, such that all sent subcarriers can be retained before spectral aliasing. Thus, the proposed DDM scheme can reduce computational complexity and power consumption in the demodulation process, leading to an improvement in cost efficiency. In experiments, we demonstrate that each ONU with an ADC operating at $1/2\hbox{--}1/32$ of the Nyquist sampling rate is able to receive $1/2\hbox{--}1/32$ of the downstream data, with an insignificant performance penalty. Furthermore, in the DDM scheme, the sampling instant of an ADC is a key parameter in the selection of required data. This paper also proposes a method for estimating the adjustments required in the sampling instant at ONUs. The proposed method is based on the use of deviations in the channel response, which provides the ability to identify with a high degree of precision the adjustments required for an ADC with sub-Nyquist sampling rate. Estimation error associated with the necessary adjustments has only negligible effects on performance, thereby confirming the reliability of the proposed method.

Development of Efficient Resource Allocation Algorithm in Chunk Based OFDMA System

The emerging demand for diverse data applications in next generation wireless networks entails both high data rate wireless connections and intelligent multiuser scheduling designs. The orthogonal frequency division multiple access based system is capable of delivering high speed data rate and can operate in a multipath environment. OFDMA based system dividing an entire channel into many orthogonal narrow band subcarriers. Due to this, it is useful to eliminate inter symbol interferences which is a limit of total available data rates. In this paper, investigation about resource allocation problem for the chunk based Orthogonal Frequency Division Multiple Access (OFDMA) wireless multicast systems is done. In this paper, it is expected that the Base Station (BS) has multiple antennas in a Distributed Antenna System (DAS). The allocation unit is a group of contiguous subcarriers (chunk) in conventional OFDMA systems. The aim of this investigation is to develop an efficient resource allocation algorithm to maximize the total throughput and minimize the average outage probability over a chunk with respect to average Bit Error Rate (BER and total available power).

Resource Allocation for Dynamic Intra-Cell Subcarrier Reuse in Cooperative OFDMA Wireless Networks

Resource reuse schemes in relay-enhanced cooperative orthogonal frequency-division multiple access (OFDMA) networks have been well studied in the literature from an interference mitigation perspective and with cell-partitioning being the basis. In this paper however, we study dynamic intra-cell subcarrier reuse in cooperative OFDMA networks in which the users are allowed to share any subcarrier in the relay links provided that the resultant intra-cell interference is managed; an option not available in non-cooperative networks. Specifically, when the target data-rates of the users are not reachable in a conventional cooperative OFDMA network, dynamic intra-cell subcarrier reuse enables the users to achieve their target data-rates, even when the number of users surpasses the number of subcarriers. We formally define the problem of maximizing the system sum-rate subject to per node power and target data-rate constraints in a cooperative OFDMA single-cell network in which subcarriers are allowed to be reused in the relay links. After analyzing the problem in the dual domain and deriving the performance bounds, we propose a suboptimal resource allocation algorithm where subcarriers and links (relay and/or direct) are assigned to the users. Simulation results show that by employing our resource allocation algorithm, the system sum-rate and the outage ratio are significantly improved, specially in an overpopulated network.

음영 지역을 위한 다중 중계기 기반의 주파수 공간 블록 부호화 OFDMA 전송 시스템

음영 지역 해소 및 셀 커버리지 확대를 위하여 중계기를 활용하는 이동 통신 시스템이 널리 연구되고 있다. 본 논문에서는 중계기를 통해 다중 경로가 확보된 음영 지역에서 주파수 공간 블록 부호를 적용한 OFDMA(orthogonal frequency division multiple access) 전송 시스템을 제안한다. 다중 중계기에서 복원된 전송 신호에 대하여 주파수 공간 블록 부호를 적용하여 재전송함으로서 음영 지역 내 단말기의 수신 성능을 크게 향상시킬 수 있다. 실험을 통해, 제안된 주파수 공간 블록 부호화 OFDMA 전송 시스템의 성능이 기존의 단일 경로 OFDMA 전송 시스템의 성능보다 우수함을 확인한다. In order to cope with shadow areas and to extend cell coverage, relay-assisted wireless communication systems have been widely studied. In this paper, we propose a space-frequency(SF) block coded orthogonal frequency division multiple access(OFDMA) transmission system in a relaying multi-path shadow area. The receiving performance of mobile station in a shadow area can be improved by applying SF block code to the recovered signals of multiple relays before re-transmitting them. The simulation result shows that the proposed SF block coded OFDMA transmission system considerably outperforms the conventional single-path OFDMA transmission system.

Frequency and Quadrature-Amplitude Modulation for Downlink Cellular OFDMA Networks

The distribution of the intercell interference (ICI) in conventional cellular networks employing orthogonal frequency-division multiple-access (OFDMA) with quadrature-amplitude modulation (QAM) tends to approach a Gaussian distribution when all available subcarriers in each cell are fully loaded. Recently, it has been also shown that the worst-case distribution of the ICI as additive noise in wireless networks with respect to the channel capacity is Gaussian. Thus, the channel capacity in cellular networks is expected to be further enhanced when the ICI could be designed properly so that it has a non-Gaussian distribution. This observation motivates us to propose, in this paper, a downlink cellular OFDMA network employing a modulation scheme called frequency and QAM (FQAM). We also derive maximum-likelihood metrics for the binary or non-binary error-correcting codes employed in the proposed network and propose their practical sub-optimal versions. Numerical results demonstrate that the distribution of the ICI in the proposed network deviates far from the Gaussian distribution. As a result, the transmission rates for the cell-edge users in the proposed network are significantly improved. In addition, the measurement results using practically implemented FQAM-based OFDMA systems verify that the transmission rates for the cell-edge users can dramatically increase, compared with the conventional QAM-based OFDMA network.

A cross-layer switching of OFDMA and MU-MIMO for future WLAN systems

We propose a cross-layer switching method of orthogonal frequency division multiple access (OFDMA) and multiuser multiple input multiple output (MU-MIMO) for the future wireless local area network systems. The proposed method, employed on the medium access control layer, switches between OFDMA and MU-MIMO as the transmission method after processing by using physical layer information such as an overhead of channel state information feedback, station number, and data length in order to improve the transmission efficiency. Simulation results show that the proposed method achieves higher total throughput than conventional OFDMA or MU-MIMO where switching is not performed.

Chunk‐based and fairness‐based resource allocation in multicast distributed MISO‐OFDMA systems

SUMMARYThe resource allocation problem for the downlink of orthogonal frequency‐division multiple access (OFDMA) wireless multicast systems is investigated. It is assumed that the base station consists of multiple antennas in a distributed antenna system (DAS), whereas each user is equipped with a single antenna. The multicasting technology is able to support several groups of users with flexible quality of service (QoS) requirements. The general mathematical formulation is provided, but achieving the optimal solution has a high computational cost. In our approach, the allocation unit is not the subcarrier, as in conventional OFDMA systems, but a set of contiguous subcarriers, which is called ‘chunk’. For practical implementation, a suboptimal but efficient algorithm is proposed in order to maximize the sum of the maximum attainable data rates of multicast groups of users, subject to total available power and proportional maximum attainable data rate constraints among multicast groups of users. Simulation and complexity analyses are provided to support the benefits of chunk‐based resource allocation to multicast OFDMA DASs, supporting that the proposed algorithm can be applied to latest‐generation wireless systems that provide QoS guarantees. Copyright © 2013 John Wiley & Sons, Ltd.

Chunk-Based Resource Allocation in Multicast MISO-OFDMA with Average BER Constraint

The resource allocation problem for the downlink of Orthogonal Frequency Division Multiple Access (OFDMA) wireless multicast systems is investigated. It is assumed that the Base Station (BS) consists of multiple antennas in a Distributed Antenna System (DAS), whereas each user is equipped with a single antenna. The allocation unit is not the subcarrier, as in conventional OFDMA systems, but a set of contiguous subcarriers (chunk). The aim of the proposed efficient algorithm is to maximize the total throughput subject to constraints on total available power and average Bit Error Rate (BER) over a chunk. Simulation and complexity analysis are provided to support the benefits of chunk-based resource allocation to multicast Multiple Input Single Output (MISO)-OFDMA DAS systems.

On the PAPR Reduction in LTE-Advanced: Precoding-Based SLM–LOFDMA Uplink Systems

Peak-to-average power ratio (PAPR) reduction is still a major challenge in the orthogonal frequency division multiple-access (OFDMA) uplink systems. The sub-carrier mapping in OFDMA can be done in two modes, clustered or localized mode and distributed mode. The localized mode is more appropriate for the practical implementations than the distributed mode due to its low sensitivity against the imperfect power control, phase noise and inter-carrier interference. In this paper, we present two precoding-based PAPR reduction techniques: Zadoff–chu and generalized chirp-like precoding-based techniques for localized OFDMA uplink systems, respectively, for the upcoming 3 GPP long-term evolution-advanced standard. In addition, the traditional selected mapping (SLM) PAPR reduction technique is implemented with the proposed systems to reduce more PAPR. At clip rate of 10−3, computer simulation results show that the precoding-based PAPR reduction techniques can reduce the PAPR up to 6.5 dB and precoding-based SLM techniques can further reduce the PAPR up to 4.4 dB, respectively, with user sub-carriers M = 16 and system sub-carriers N = 64, for the QPSK modulation. The symbol-error rate performance of the proposed systems is also better than the conventional OFDMA uplink systems and the WHT-based systems.

Zadoff–Chu Matrix Transform Precoding-Based Orthogonal Frequency Division Multiple Access Uplink Systems: A Peak-to-Average Power Ratio Performance

High peak-to-average power ratio (PAPR) is one of the major drawbacks in orthogonal frequency division multiple access (OFDMA) systems. In this paper, we present two new Zadoff–Chu matrix transform (ZCMT) precoding-based OFDMA uplink systems, ZCMT precoding-based distributed OFDMA uplink system and ZCMT precoding-based localized OFDMA uplink system with reduced PAPR. The proposed systems are based on precoding the constellation symbols with the ZCMT precoder before subcarrier mapping to keep the PAPR low. The PAPR of the proposed systems is analyzed with root-raised-cosine pulse shaping to keep out of band radiation low and to meet the transmission spectrum mask requirement. Simulation results show that the proposed systems have better PAPR than the Walsh–Hadamard transform (WHT)precoded OFDMA uplink systems and the conventional OFDMA uplink systems.

On the peak-to-average power ratio reduction in mobile WiMAX: A discrete cosine transform matrix precoding based random-interleaved orthogonal frequency division multiple access uplink system

Mobile worldwide interoperability for microwave access (Mobile WiMAX) is a broadband wireless solution that enables the convergence of mobile and fixed broadband networks through a common wide area radio-access (RA) technology and flexible network architecture. Since January 2007, the IEEE 802.16 working group (WG) has been developing a new amendment of the IEEE 802.16 standard i.e. IEEE 802.16m as an advanced air interface to meet the requirements of ITU-R/IMT-Advanced for 4G systems. The mobile WiMAX air interface adopts orthogonal frequency division multiple access (OFDMA) as multiple access technique for its uplink (UL) and downlink (DL) to improve the multipath performance. All OFDMA based networks, including mobile WiMAX experiences the problem of high peak-to-average power ratio (PAPR). This paper presents: Discrete-Cosine transform matrix (DCTM) precoding based random-interleaved OFDMA uplink system and selecting mapping (SLM) based DCTM precoded random-interleaved OFDMA uplink system respectively, for PAPR reduction in mobile WiMAX systems. PAPR of the proposed systems is analyzed with the root-raised-cosine (RRC) pulse shaping to keep out of band radiation low and to meet the transmission spectrum mask requirement. Simulation results show that, the proposed systems have low PAPR than the Walsh-Hadamard transform (WHT) precoded random-interleaved OFDMA uplink systems and the conventional random-interleaved OFDMA uplink systems. Symbol-error-rate (SER) performance of the proposed system is also better than the conventional random-interleaved OFDMA uplink systems and at par with WHT based random-interleaved OFDMA uplink systems. Good improvement in PAPR and SER offered by the proposed systems can notably reduce the cost and complexity of the transmitter.