In this letter, we propose an adaptive transmission method for an OFDMA system supporting both band-AMC and diversity modes in a frame, simultaneously. In the proposed method, users are classified into the two groups preferring the band-AMC mode or the diversity mode based on their channel parameters. Then the BS performs resource allocation to maximize the throughput. It is observed that the proposed adaptive transmission method can reduce the feedback overhead with negligible performance loss.

Cooperation can increase the system performance by obtaining the spatial diversity. While most of the present works concentrate on the analysis of the cooperation based on the inter-user channel response and developing a scheme for higher cooperative diversity, in this paper, we focus on practical resource allocation in OFDMA systems. Since the user who uses the same center frequency can not receive the signal when transmitting, this constraint should be considered to apply the cooperation to OFDMA systems. In this paper, we propose the pair-based OFDMA frame structure that overcomes this constraint. Also in this frame structure to achieve the minimum outage probability of system, we select the best partner among the candidate neighbors and allocate the suitable subchannels to bandwidth requested users through a cooperative subchannel allocation (CSA) algorithm. In order to evaluate the proposed resource allocation scheme, we carry out simulations based on IEEE 802.16e. The simulation results show that our proposed algorithm offers smaller outage probability than one based on non-cooperative communications and we get the minimum outage probability when a threshold for selection of candidate neighbors is 10 dB. We analyze that these results can be achieved by helping users located around the edge of the cell.

In the subcarrier and power allocation schemes in Multi-Input Multi-Output and Orthogonal Frequency Division Multiple Access (MIMO-OFDMA) systems, only equal fairness among users has been considered and no scheme for proportional data rate fairness has been considered. In this paper, a subcarrier, bit and power allocation scheme is proposed to maximize the total throughput under the constraints of total power and proportional data rate fairness among users. In the proposed scheme, joint subchannel allocation and adaptive bit loading is firstly performed by using singular value decomposition (SVD) of channel matrix under the constraint of users' data throughput requirements, and then adaptive power loading is applied. Simulation results show that effective performance of the system has been improved as well as each throughput is proportionally distributed among users in MIMO-OFDMA systems.

In this letter, a novel blind CFO estimation algorithm for the uplink of an OFDMA system is proposed. The proposed method exploits the inherent redundant information in OFDMA symbols and does not require additional pilot or preamble overhead. Since it is a post-FFT estimator, it does not use filter banks to separate the desired user's signal from the others in the time domain. Hence, the subcarriers of a certain user are not restricted to be clustered in the frequency domain. Therefore, the proposed estimator can be applied to OFDMA systems with an arbitrary subcarrier assignment over the entire bandwidth, including IEEE 802.16e, to obtain sufficient frequency diversity in a frequency selective fading channel. The proposed method can be efficiently used for continuous tracking of all active users' CFOs only with two FFT windows within a single OFDM symbol. From simulation results, the performance of the proposed scheme is shown better than that uses preamble symbols.

The low complexity tree-structure based user scheduling algorithm is extended into up-link MLD-based multi-user multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing access (OFDMA) wireless systems. The system sum capacity is maximized by careful user selection on a defined tree structure. The calculation load is reduced by selecting the M most possible best branches and sampling in frequency dimension. The performances of the proposed scheduling algorithm are analyzed within three kinds of OFDMA systems and compared with conventional throughput-based algorithm. Both the theoretical analysis and simulation results show that the proposed algorithm obtains better performance with much low complexity.

In this letter, we propose an efficient scheme for combining scheduling and channel allocation functions in multi-channel systems such as an orthogonal frequency division multiple access (OFDMA). In our approach, the scheduling function is embedded in the channel allocation function in an implicit manner, and the implicit scheduler only translates quality-of-service (QoS) requirements into a set of constraints on channel allocation. The channel allocation problem is then formulated as a linear programming (LP) problem, and the optimal solution can be easily obtained through various LP algorithms. Through extensive numerical experiments, it is demonstrated that the proposed scheme can maximize the cell throughput under the given QoS requirements.

The time division duplex cellular system can support various downlink and uplink traffic ratios by setting the downlink and uplink transmission periods appropriately. However, it causes severe co-channel interference problem when some cells are active in the downlink while the others are in the uplink [2]. To mitigate this problem, a resource allocation scheme combined with sectorization is proposed for orthogonal frequency division multiple access. Simulations demonstrate that the proposed scheme improves both spectral efficiency and outage performance compared to the conventional allocation schemes.

Simple-relay aided resource allocation (SRARA) schemes are incorporated with throughput guarantee scheduling (TGS) in IEEE 802.16 type time division duplex--orthogonal frequency division multiple access (TDD-OFDMA) downlink in order to enhance service coverage, where the amount of resources at each relay is limited due to either its available power which is much smaller than base station (BS) power or the required overhead. The performance of SRARA schemes is evaluated with both proportional fair (PF) and TGS schedulers at 64 kbps and 128 kbps user throughput requirements when total RS power is set to 500 mW or 1 W. For SRARA with RSs of relatively lower power (500 mW), schemes that put total power into only one subchannel offer larger coverage than when both subchnnels are used with equal power allocation, while the RS with evenly power-allocated two subchannels could provide larger coverage gain for a relatively higher power (1 W). Depending upon the target throughputs it is shown which of the relay scheme or scheduler design would play more important role in improving coverage. In a lower target (64 kbps), more improvement comes from relay scheme rather than scheduler design. For a relatively higher level (128 kbps), it comes from scheduler design rather than relay due to the fact that simple relay can't help using strictly limited amount of resources.

In orthogonal frequency-division multiplex access (OFDMA) downlink systems, the carrier-frequency offset (CFO) between the multiple transceivers introduces inter-carrier interference (ICI). In this letter, we propose an iterative precoding scheme to suppress the ICI due to CFO. This scheme is applied at the transmitter, and can jointly cancel the ICI for all the receivers. Moreover, by the studies of the convergence behavior of the iterations, a sufficient condition for the convergence is presented. The theoretical analysis and simulation results both show that this iterative scheme is equivalent to the zero-forcing (ZF) scheme in function, but with much lower complexity.

This paper proposes pilot-based channel quality reporting for orthogonal frequency division multiple access/time division duplex (OFDMA/TDD) systems with cochannel interference. In the proposed method, a terminal reports his channel quality in multiple subbands to base station (BS) using channel reciprocity of TDD systems. The terminal transmits uplink pilot signals in the subbands with different transmit power which is inversely proportional to the subband-based interference power. The BS can obtain knowledge of the terminal's received signal-to-interference-plus-noise power ratio on subband basis, measuring the pilot signal power. In performance evaluation, accuracy of channel quality reporting and amount of uplink signalling are examined. From numerical results, it is found that the proposed method becomes effective as the number of subbands and terminals for channel quality reporting increases.

We propose an effective subcarrier allocation scheme for multiuser orthogonal frequency division multiple access (OFDMA) system in the downlink transmission with low computational complexity. In the proposed scheme, by taking multiple attributes of a user's channel, such as carrier gain decrease rate and variation from the mean channel gain of the system, to determine a rank for the user, subcarriers are then allocated depending on the individual user's rank. Different channel characteristics are used to better understand a user's need for subcarriers and hence determine a priority for the user. We also adopt an attribute weighing scheme to enhance the performance of the proposed scheme. The scheme is computationally efficient, since it avoids using iterations for the algorithm convergence and also common water-filling calculations that become more complex with increasing system parameters. Low complexity is achieved by allocating subcarriers to users depending on their determined rank. Our proposed scheme is simulated in comparison with other mathematically efficient subcarrier allocation schemes as well as with a conventional greedy allocation scheme. It is shown that the proposed method demonstrates competitive results with the simulated schemes.

In previous literature on adaptive transmission in multiuser OFDMA systems, only uncoded case or capacity (coded with infinite length of codeword) has been considered. In this paper, an adaptive transmission algorithm for coded OFDMA systems with practical codeword lengths is investigated. Also, in order to keep the feedback overhead within a practical range, a two-step partial CQI scheme is adopted, which has both better performance and reduced feedback overhead compared to conventional partial CQI schemes. By allowing a long codeword block across all allocated sub-bands with appropriate power and modulation order allocation rather than using short codeword blocks to each sub-band, high coding gain can be obtained, which leads to performance improvement.

In orthogonal frequency-division multiplex access (OFDMA) uplink, the carrier-frequency offsets (CFOs) between the multiple transmitters and the receiver introduce inter-carrier interference (ICI) and severely degrade the performance. In this paper, based on the perfect estimation of each user's CFO, we propose two low-complexity iterative algorithms to cancel ICI due to CFOs, which are denoted as the basic algorithm and the improved algorithm with decision-feedback equalization (DFE), respectively. For the basic one, two theorems are proposed that yield a sufficient condition for the convergence of iterations. Moreover, the interference-power-evolution (IPE) charts are proposed to evaluate the convergence behavior of this interference cancellation algorithm. Motivated by the IPE chart, the procedure of DFE is introduced into the iterations, which is the basic idea of the improved algorithm. For this improved algorithm, the error-propagation effect are analyzed and suppressed by an efficient stopping criterion. From IPE charts and simulation results, it can be easily observed that the basic algorithm has the same capability of ICI cancellation as the linear optimal minimum mean square error (MMSE) method, but offers lower complexity, while the improved algorithm with DFE outperforms the MMSE method in terms of the bit-error rate (BER) performance.

Multiple antennas are used to an essential part in order to support high link quality in wireless communication systems. The diversity techniques with multiple antennas provide better reliability than existing systems. But these techniques generally require more than one antenna at the transmitter or receiver. So, the system with multiple antennas has several limitations including size, cost, hardware complexity, and antenna space. In this letter, we propose a cooperative transmission technique that uses space time delay code in OFDMA uplink system; DVB-RCT. The proposed technique overcomes these limitations and gives the diversity gain of cooperative transmission in a concept of virtual MIMO with single transmit antenna.

In this paper, we investigate an efficient user selection and sub-band allocation algorithm in which each user transmits two-step partial CQI to reduce the amount of feedback in multi-user downlink OFDMA systems. Simulation results show that we can greatly reduce the feedback rate at the expense of negligible performance degradation compared to the full CQI feedback schemes or that we can greatly improve the performance with slightly reduced feedback rate compared to conventional partial CQI feedback schemes.

This paper proposes new resource management schemes for multiple data streams in an orthogonal frequency and space division multiplex access (OFSDMA) system using Radio-on-Fiber (RoF) ubiquitous antennas. The proposed schemes classify the services into some classes in which the number of sub-carriers is dynamically assigned according to the requested data size. The computer simulation results show that the proposed schemes improve the number of users satisfying the required bit error rate (BER) level as well as the average throughput and also show that the RoF ubiquitous antennas can improve system capacity.

We propose a simple initial frame timing acquisition algorithm for cellular OFDMA systems. The proposed algorithm utilizes the 9 dB boost in preamble power set by the IEEE 802.16e standard. Simulation results show that the proposed algorithm succeeds in acquiring the starting point of a frame under not only single cell but also multi-cell environments, while the conventional autocorrelation-based method fails under multi-cell environment.

It is well known that the diversity gain attained by DCA (Dynamic Channel Allocation) is generally very high over OFDM (Orthogonal Frequency Division Multiplexing)-based broadband networks. This paper introduces a numerical approach for measuring the performance gain afforded by DCA. In the mathematical analysis, the property of order statistics is adopted to derive the upper bound of the expected throughput via the use of DCA. In the simulation, it was possible to achieve a gain of 5 dB by exploiting multi-user and spectral diversities when the number of users is 16 and the total number of subcarriers is 256.

This letter describes two low complexity receiver structures over a multi-broadcast channel of an orthogonal frequency division multiple access (OFDMA) multi-user system. The first is a one-group occupied receiver structure, whose complexity is much lower than that of a conventional OFDMA receiver structure. The other one, a multi-group occupied receiver structure, exploits multiple groups for one user, by which users' down-link data rate can be adaptively controlled by a base station (BS). Unlike unchangeable complexity of an OFDMA receiver structure that performs full-size of a fast Fourier transform (FFT) operation although only a few subcarriers are taken, its complexity linearly increases with the number of occupied subcarrier groups. The proposed receiver structures can meet the possible high-rate demand in the down-link and will become one of the strong candidates in next generation mobile communication systems.

We have been investigating an orthogonal frequency division multiple access (OFDMA) based cellular system that is called "dynamic parameter controlled orthogonal frequency and time division multiple access (DPC-OF/TDMA)" for the development of beyond third generation (B3G) mobile communication systems. Moreover, we have already proposed a time alignment control (TAC) to compensate propagation delays that induce a multiple-access interference (MAI) in the uplink OFDMA. However, that TAC includes a large amount of computations. This means that it is quite difficult for the OFDMA systems to implement TAC into volume-limited hardware devices such as field programmable gate array (FPGA). Thus, we propose a new complexity-reduced TAC (CRTAC) in this paper. CRTAC can be implemented into such devices easily. In this paper, we show some computer simulation results, and then evaluate the error rate performances of DPC-OF/TDMA employing CRTAC. Moreover, we also show the benefit of the reasonable level of the implementation complexity made by CRTAC.