Orthogonal Frequency Division Multiple Access Transmission Research Articles

Towards a Fair Allocation and Effective Utilization of Resource Units in Multi-User WLANs-based OFDMA technology

To promote the MU (Multi-User) communications in High-Efficiency WLANs (Wireless Local Area Networks), the new IEEE 802.11ax standard introduced the concept of RU (Resource Unit) by means of the OFDMA (Orthogonal Frequency Division Multiple Access) transmission technique. Although the OFDMA MU communications enable multiple users to be served at the same time in both DL (Down-Link) and UL (Up-Link) directions, several challenges remain to be met in order to design a MAC (Medium Access Control) protocol that fairly allocates and effectively utilizes the RUs. The challenges addressed are: (1) define when and how using 802.11ax control frames, (2) establish a fair sending policy of UL needs, (3) adopt a strategy to select a channel structure, and (4) avoid loss in spectral efficiency and timeout BA (Block Acknowledgment). In this paper, we proposed the FAEU-RUs protocol (Fair Allocation and Effective Utilization of RUs) by meeting each of the challenges of OFMDA MU communications. The performance evaluation demonstrates that the FAEU-RUs protocol significantly enhances the overall performance of the network but also the individual performance of each type of traffic (DL and UL) and each category of station.

Uplink Frame Transmission with Functions of Adaptive Triggering and Resource Allocation of OFDMA in Interfering IEEE 802.11ax Wireless LANs

In recent years, wireless LANs (WLANs) are closely deployed which means they interfere with each other. Mobile stations (MSs) like smart phones that connect to such WLANs are also increasing. In such interfering environments, radio interference frequency depends on MS position. In addition, as MSs and their applications become diverse, frame generation rates from MSs are also becoming various. Thus, sufficient frame transmission opportunities should be assigned to MSs regardless of their radio interference frequencies and frame generation rates. One key technology to deal with this issue is uplink orthogonal frequency division multiple access (OFDMA) transmission introduced in IEEE 802.11ax. However, existing works do not consider the differences of the interference frequencies and frame generation rates among MSs in an integrated manner. This paper proposes an uplink frame transmission method for interfering WLAN environments that effectively uses the OFDMA transmission to assign enough transmission opportunities to MSs regardless of their own interference frequencies and frame generation rates, while efficiently using the channel resource. Considering the combined problem, this proposed method allocates resource units (RUs), created by dividing the channel, to MSs. In addition, based on a mathematical analysis of required frame transmission duration, the proposed method flexibly selects the OFDMA transmission or conventional frame transmission with CSMA/CA, which is also not considered in the existing works.

Solving IQ Mismatch Problem for Two-Path Successive Relaying OFDMA Uplink Systems With Direct Conversion Transceivers

In this letter, the properties of two-path successive relaying systems are exploited to develop an iterative decision directed algorithm to estimate in-phase/quadrature-phase mismatch parameters occurring at the transmitters, relays, and destination for the uplink orthogonal frequency-division multiple access transmission. These parameters along with the actual channel taps of different links are treated as a single unknown parameter to be estimated. A space-alternating generalized expectation-maximization tool is employed to compute the maximum-likelihood estimate. The proposed algorithm iterates between data detector and estimator in order to refine the estimation process. Simulation results confirm the effectiveness of the proposed estimator.

Using OFDMA for MU-MIMO User Selection in 802.11ax-Based Wi-Fi Networks

Two innovative multi-user (MU) communication mechanisms, multi-user multi-input multi-output (MU-MIMO) and orthogonal frequency division multiple access (OFDMA), have recently attracted much attention as key technologies to boost the capabilities of 802.11 Wi-Fi systems. The method of MU transmission in Wi-Fi systems still suffers from severe problems with channel state information (CSI) feedback overhead, however, and this precludes obtaining much higher MU-MIMO gains through user selection. Although in 802.11ax-based Wi-Fi systems the methods used for uplink OFDMA transmission and downlink MU-MIMO transmission bear a great deal of resemblance to each other, in the sense that both are initiated by the AP, this similarity in protocol is not appropriately utilized for close collaboration between the two technologies. This paper proposes MUSE, a novel multi-user transmission scheme for 802.11ax networks. By exploiting uplink OFDMA transmissions for MU-MIMO user selection, MUSE takes advantage of both OFDMA and MU-MIMO functionalities. In particular, the AP estimates and gathers multiple CSI values from the uplink OFDMA frame, and this channel information is then used to find the optimal downlink MU-MIMO receiver group to maximize the system utility. To realize MUSE, a new OFDMA resource allocation algorithm and frame structure are developed. Extensive MATLAB simulation results show that MUSE significantly improves the network throughput, even in dense network scenarios, and also works effectively in co-existence with legacy nodes.

Zadoff-Chu sequence-based upstream ranging in OFDMA-PON mobile radio access networks

Orthogonal frequency division multiple access (OFDMA) uplink in a passive optical network (PON) requires the delay alignment for OFDMA symbols from remotely distributed optical network units (ONUs). In this paper, we experimentally demonstrate and analyze the performance of a Zadoff-Chu (ZC) sequence-based upstream ranging scheme in an intensity modulation/direct detection (IM/DD)-based OFDMA-PON. The experimental results show that the proposed scheme can achieve upstream synchronization with only marginal inter-carrier interference (ICI) and requires no additional bandwidths in a typical OFDMA transmission with cyclic prefix (CP).

Dynamic Broadband Spectrum Refarming for OFDMA Cellular Systems

Conventional spectrum refarming (SR) techniques assemble the legacy services to partial licensed spectrum, and free up the remained spectrum for operating other communication systems. In this paper, we propose a new broadband SR framework in which the orthogonal-frequency division multiple access (OFDMA) system dynamically shares the code division multiple access (CDMA) spectrum, which contains multiple bands in a concurrent manner. The interference margin provided by the downlink isometric random precoded CDMA system is first derived. To protect all the CDMA users, interference constraints that regulate the OFDMA transmission are then formulated. We formulate the joint CDMA load planning and OFDMA resource allocation problem for the SR system. By applying primal decomposition, it is shown that the higher-level problem is non-convex in general. We first propose an efficient algorithm to find the myopic optimal solution by investigating the derivative property of the higher-level problem. Through enhancing the original constrains, the higher-level problem is shown to be convex and solved with another efficient algorithm. Simulation results are provided to evaluate the SR performance, illustrating the significance of the CDMA load planning on the OFDMA throughput, the advantage of the proposed SR model over conventional SR, and the protection to the CDMA system.

An Energy Efficient Multiuser Uplink Transmission Scheme in the Next Generation WLAN for Internet of Things

This paper proposes a novel and energy efficient Internet of Things (IoT) communication scheme for next generation Wireless Local Area Network (WLAN). There are a couple of crucial requirements among unique IoT requirements: a large number of communication devices must be supported, and they must have low power consumption. Since sensor-type IoT devices, which are expected to be one of the major types of IoT devices, normally generate uplink traffic rather than downlink traffic, an energy efficient multiuser uplink transmission scheme is a crucial feature of IoT communication. In the next generation WLAN, IEEE 802.11ax, Orthogonal Frequency Division Multiple Access (OFDMA) is adopted to support a greater number of devices. However, uplink OFDMA procedures that consider the unique IoT requirements have not been fully considered in the next generation WLANs. A random access-based WLAN uplink OFDMA transmission scheme is proposed in this paper, and analytical modeling of the proposed scheme is provided. The proposed random access-based WLAN uplink OFDMA transmission scheme is able to dynamically adjust the number of contending users by uniquely applying congestion status in a very simple and distributed manner. Our numerical analysis and extensive simulation corroborate the fact that the proposed scheme is able to support a greater number of IoT devices with less power consumption.

The Price of Self-sustainability for Block Transmission Systems

In this work, the self-sustainability of block transmission systems is analyzed. In particular, orthogonal frequency division multiple access (OFDMA) is taken as a reference, due to its popularity and rather simple signal model. More precisely, a generalized variant of this scheme in which the transmitted signal is obtained as the sum of an OFDMA and a cognitive interference alignment (CIA) component, acting as an energy bearer, is considered. In this scenario, the self-sustainability of the transmission is made possible by the flexibility of the adopted strategy and the introduction of a novel energy harvesting OFDMA receiver. Both the feasibility conditions for the self-sustainability and the optimal power allocation to maximize the effectiveness of the energy transfer performed through the CIA signal are derived. Numerical results show that full self-sustainability can be achieved for several system configurations and channel statistics. However, this comes at the cost of a rate penalty with respect to a standard classic OFDMA transmission, which is termed the price of self-sustainability . A study of the relationship between the performance of both the energy and the information transfer is carried out. A CP size that minimizes the price of self-sustainability can be found for all the considered configurations.

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.

Matrix columns allocation problems

Orthogonal Frequency Division Multiple Access (OFDMA) transmission technique is gaining popularity as a preferred technique in the emerging broadband wireless access standards. Motivated by the OFDMA transmission technique we define the following problem: Let M be a matrix (over R ) of size a × b . Given a vector of non-negative integers C → = 〈 c 1 , c 2 , … , c b 〉 such that ∑ c j = a , we would like to allocate a cells in M such that (i) in each row of M there is a single allocation, and (ii) for each element c i ∈ C → there is a unique column in M which contains exactly c i allocations. Our goal is to find an allocation with minimal value, that is, the sum of all the a cells of M which were allocated is minimal. The nature of the suggested new problem is investigated in this paper. Efficient algorithms are suggested for some interesting cases. For other cases of the problem, NP-hardness proofs are given followed by inapproximability results.

Adaptive Method of Slot and Bit Allocation for OFDMA Transmission Systems

This paper proposes the Orthogonal Frequency Division Multiple Access (OFDMA) transmission scheme with an adaptive slot and bit allocation method which can provide the broadband wireless LAN (WLAN) systems. From the fact that the radio links between different users and a base station experience the independent multi-path fading, this paper proposes a priority-based adaptive slot and bit allocation method in conjunction with an allocation adjustment method. The proposed method can achieve the higher efficient usage of frequency resource than those for the conventional fixed slot assignment with adaptive bit allocation method and the fixed bit assignment with adaptive slot allocation method. This paper presents various computer simulation results to evaluate the proposed method and demonstrates that the proposed method can achieve the higher frequency resource utilization with less computation complexity in the severe frequency selective multi-path fading channels.

Adaptive Method of Slot and Bit Allocation for OFDMA Transmission Systems

This paper proposes the Orthogonal Frequency Division Multiple Access (OFDMA) transmission scheme with an adaptive slot and bit allocation method which can provide the broadband wireless LAN (WLAN) systems. From the fact that the radio links between different users and a base station experience the independent multi-path fading, this paper proposes a priority-based adaptive slot and bit allocation method in conjunction with an allocation adjustment method. The proposed method can achieve the higher efficient usage of frequency resource than those for the conventional fixed slot assignment with adaptive bit allocation method and the fixed bit assignment with adaptive slot allocation method. This paper presents various computer simulation results to evaluate the proposed method and demonstrates that the proposed method can achieve the higher frequency resource utilization with less computation complexity in the severe frequency selective multi-path fading channels.

Performance Analysis of an OFDMA Transmission System in a Multicell Environment

The paper deals with design and performance analysis of orthogonal frequency-division multiple-access (OFDMA)-based downlink cellular wireless communications. Due to a high degree of user mobility, the base station is assumed to have only a statistical knowledge of the users' channels. Relying on the ergodic capacities connected to the user rates, a subcarrier and power allocation that minimizes the total transmitted power is proposed. The allocation strategy requires only the knowledge of the channel statistics and the rate requirements for all users. An extension and a performance analysis of this allocation algorithm in a multicell environment working with a frequency reuse factor equal to one is also conducted. A condition for the multicell network to be able to satisfy all rate requirements is derived