Orthogonal Frequency Division Multiplexing Downlink Research Articles

New Efficient Timing and Frequency Error Estimation In OFDM

Orthogonal frequency division multiplexing (OFDM) technique is being applied extensively to high data rate digital transmission as it is a bandwidth efficient modulation scheme. In OFDM system, intersymbol interference (ISI) and intercarrier interference (ICI) occur due to synchronization errors. Fast, simple and robust synchronization algorithms are necessary for OFDM systems. In OFDM downlink transmission, every terminal perform synchronization by exploiting reference symbols called training symbols of received frame. In this paper we are presenting very simple method of synchronization which uses training symbol to determine timing and frequency synchronization error. Proposed synchronization techniques result in better performance with respect to all the parameters despite being simple.

Scalable Video Multicast Over Multi-Antenna OFDM Systems

We propose a framework for efficient scalable video multicast over downlink orthogonal frequency-division multiplexing (OFDM) systems with multiple transmit antennas. In conventional video multicast systems, the achievable transmission rate is determined by the user with the worst channel condition, and the system saturates the capacity when the number of users increases. To accommodate the heterogeneous channel conditions and device capabilities of various users, scalable video coding (SVC) encodes video streams into base and enhancement layers. We exploit the advances in multi-antenna OFDM and the layered nature of SVC, and propose a framework for scalable video multicast which guarantees the base layer quality for all users while making best use of limited resource for the enhancement layer of users with good channel conditions. We show that the resource allocation that includes the transmit precoding, subcarrier allocation, and bit and power allocation is a very difficult optimization problem. A low-complexity suboptimal algorithm is proposed which is suitable for practical implementations. Simulation results are provided to show the effectiveness of the proposed solution.

Preamble and Pilot Design with Computationally Efficient Synchronization and Cell Search Algorithms in Cellular OFDM Systems

A computationally efficient initial synchronization technique is introduced in cellular orthogonal frequency division multiplexing downlink systems, which includes symbol timing estimation, frequency offset estimation, frame synchronization, and cell search. Novel preamble and pilot structures are proposed and a simple three-step synchronization scheme is developed based on a variety of practical conditions such as low computational complexity, time-varying frequency selective fading, limited preamble length, low peak-to-average-power ratio, and noncoherent synchronization with no channel estimation. The proposed three-step synchronization is sequentially described as (1) joint timing and frequency synchronization, (2) frame synchronization and preamble pattern identification (ID), and (3) pilot group ID. Simulation results show that the proposed technique nicely works even at low signal-to-interference-and-noise ratio regions.

Capacity analysis and optimal power allocation for coordinated transmission in MIMO-OFDM systems

Coordinated transmission in multicell coordination networks has attracted much attentions in recent years. In the paper, we investigate one open problem in the above area, which is concerned with how to solve the closed-form optimal power allocation scheme of coordinated transmission in downlink orthogonal frequencydivision multiplexing (OFDM) systems with multiple antenna configurations and individual power constraints. A frequency-selective fading multiple-input multiple-output (MIMO) channel is considered. We derive the closed-form solutions of sum-rate capacity and optimal power allocations for coordinated transmission under the diversity case and the multiplexing case by solving the constrained optimization problems. Numerical simulations are performed to verify the derived theoretical conclusions by means of comparing the proposed optimal power allocation (OPA) scheme with the other two typical power allocation schemes. One is the power allocation scheme combining traditional water-filling with equal power (WS-EF PA). The other is equal power allocation (EPA) scheme.

Application of Superposition Coding for Subcarrier and Bit Allocation in Downlink OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is well known for multicarrier modulation technique in which a high data rate stream is divided into different parallel lower data stream. In this paper, we study the margin adaptive - superposition coding (MA-SC) algorithm and rate adaptive - superposition coding (RA-SC) algorithm, where at most two user can share each subcarrier as compared to MA and RA algorithm, where each subcarrier is shared by only single user without SC scheme. We apply SC scheme over MA and RA to achieve maximum system throughput with separate power constraint for real and non real time users, ensuring that QoS requirement for real time and proportional fairness among non real time users is satisfied. The overall computational complexity of proposed modified RA-SC algorithm is same as RA algorithm. In the modified proposed MA-SC, complexity increases in some of the steps due to addition of SC scheme over MA algorithm, but overall complexity remains the same. Monte-Carlo simulations are being carried out to show that the proposed algorithm improves the performance in terms of power required and throughput.

Scheduling methods for multi-user optical wireless asymmetrically-clipped OFDM

Diffuse optical wireless (DOW) systems have the advantage that they do not require point-to-point siting so one transmitter can communicate with several receivers. In this paper, we investigate multiple access scheduling methods for downlink orthogonal frequency division multiplexing (OFDM) in diffuse optical wire- less networks. Unlike the radio frequency (RF) channel, the DOW channel has low-pass filter characteristics and so requires different scheduling methods than those developed for the RF channel. Multi-user diversity orthogonal frequency division multiple access (OFDMA) systems nominate a cluster of subcarriers with the largest signal-to-noise-ratio for transmission. However, in a DOW channel, most users would choose the lowest frequency clusters of subcarriers. To remedy this problem, we make two proposals. The first is to use a variable cluster size across the subcarriers; the lower frequency clusters will have fewer subcarriers while the higher frequency clusters will have more subcarriers. This will equalize the capacity of the clusters. The second proposal is to randomize a user's cluster selection from a group of clusters satisfying a minimum threshold. Through simulation it is shown that combining these strategies can increase the throughput while ensuring a fair distribution of the available spectrum.

Resource allocation for downlink OFDM system using noisy chaotic neural network

To advance the spectral efficiency further, a dynamic subcarrier allocation technique is investigated using the noisy chaotic neural network (NCNN) in the orthogonal frequency division multiplexing (OFDM) downlink system. By constructing the energy function elaborately, and fully exploiting the multiuser diversity gain, compared to existing schemes the proposed algorithm finds a better solution through the rich neurodynamics (stochastic noise and flexible chaos) of the NCNN.

CSI-Based Resource Allocation for OFDM Downlink Broadband Multimedia Services in Cellular Systems

Using channel state information (CSI) in resource allocation in Orthogonal Frequency Division Multiplexing (OFDM) wireless cellular systems has been recently considered to improve the overall throughput. This paper presents an analytical framework to compare the performance of two resource allocation schemes, using CSI-based and random sub-carrier selection, in terms of their user outage probability and throughput in relation to the system load in frequency-selective fading and interference-limited environments. Analytical and simulation results are in good agreement and indicate that a proper use of CSI can achieve substantially better performance in interference limited cellular systems as compared to the blind approach and thus show the benefits of multi-user diversity.

Scheduling and Resource Allocation for SVC Streaming Over OFDM Downlink Systems

We consider the problem of scheduling and resource allocation for multiuser video streaming over downlink orthogonal frequency division multiplexing (OFDM) channels. The video streams are precoded using the scalable video coding (SVC) scheme that offers both quality and temporal scalabilities. The OFDM technology provides the flexibility of resource allocation in terms of time, frequency, and power. We propose a gradient-based scheduling and resource allocation algorithm, which prioritizes the transmissions of different users by considering video contents, deadline requirements, and transmission history. Simulation results show that the proposed algorithm outperforms the content-blind and deadline-blind algorithms with a gain of as much as 6 dB in terms of average PSNR when the network is congested.

Interference Analysis in Downlink OFDM Considering Imperfect Intercell Synchronization

This paper presents a theoretical performance analysis of downlink orthogonal frequency-division multiplexing (OFDM) cellular radio communication systems. An accurate interference model that takes into account the cochannel and interchannel interference caused by both frequency and timing synchronization errors in adjacent and cochannel cells is developed. The effect of frequency-selective fading on the interfering signals has also accurately been taken into consideration. Based on this model, accurate expressions are given for average bit error rates (BERs) of OFDM-based M-ary quadrature amplitude modulation (QAM). These are extended to include multichannel reception with maximal ratio combining diversity.

Field Experiments on Real-Time 1-Gbps High-Speed Packet Transmission in MIMO-OFDM Broadband Packet Radio Access

This paper presents experimental results in real propagation channel environments of real-time 1-Gbps packet transmission using antenna-dependent adaptive modulation and channel coding (AMC) with 4-by-4 MIMO multiplexing in the downlink Orthogonal Frequency Division Multiplexing (OFDM) radio access. In the experiment, Maximum Likelihood Detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS) is employed to achieve such a high data rate at a lower received signal-to-interference plus background noise power ratio (SINR). The field experiments, which are conducted at the average moving speed of 30km/h, show that real-time packet transmission of greater than 1Gbps in a 100-MHz channel bandwidth (i.e., 10bits/second/Hz) is achieved at the average received SINR of approximately 13.5dB using 16QAM modulation and turbo coding with the coding rate of 8/9. Furthermore, we show that the measured throughput of greater than 1Gbps is achieved at the probability of approximately 98% in a measurement course, where the maximum distance from the cell site was approximately 300m with the respective transmitter and receiver antenna separation of 1.5m and 40cm with the total transmission power of 10W. The results also clarify that the minimum required receiver antenna spacing is approximately 10cm (1.5 carrier wave length) to suppress the loss in the required received SINR at 1-Gbps throughput to within 1dB compared to that assuming the fading correlation between antennas of zero both under non-line-of-sight (NLOS) and line-of-sight (LOS) conditions.

Effects of non-homogeneous SINR estimation error statistics on scheduling performance in OFDM downlink systems

This paper investigates the effect of inaccurate SINR estimation on the performance of multi-user scheduling in downlink orthogonal frequency division multiplexing (OFDM) systems. Considering path loss, time and frequency selectivity of the channel, the error statistics of SINR estimation is analyzed. Then, the effect of SINR inaccuracy is evaluated in respect to system throughput and fairness when each user has different error statistics according to channel environments.

Statistical Eigen-Beamforming With Selection Diversity for Spatially Correlated OFDM Downlink

Statistical eigen-beamforming is shown to be effective over a spatially correlated fading channel. In the paper, we investigate the use of the statistical eigen-beamforming with short-term selection for an orthogonal frequency-division multiplexing (OFDM) downlink. The contributions of the paper are as follows: (1) an efficient statistical eigen-beamforming method with limited feedback for OFDM has been proposed; and (2) exact and asymptotic expressions of the average bit error rate for (transmitter side) spatially correlated Rayleigh fading channels are derived. In particular, the asymptotic expression allows us to gain insight to statistical eigen-beamforming with selection diversity in terms of array gain and diversity order.

Double-orthogonal coded space-time-frequency spreading CDMA scheme

In this paper, we propose double-orthogonal coded space-time-frequency spreading code-division multiple-access (DOC-STFS-CDMA) scheme for the downlink orthogonal frequency-division multiplexing (OFDM) systems employing multiple transmit and receive antennas. First, we introduce the so-called STFS-CDMA and the corresponding minimum mean-square error (MMSE) detection algorithms for three scenarios with different user information. Then, we propose a new class of spreading codes, called double-orthogonal code (DOC), to match the MMSE detection algorithms for STFS-CDMA system. With the special double orthogonality, DOC can effectively exploit space, time, and frequency diversity to enhance the performance of MMSE detectors, and provide the well-balanced signal-to-interference-and-noise ratio to different users. Finally, the performance gain of the proposed scheme over conventional schemes is shown by computer simulation.

On the use of OFDM radio interface for satellite digital multimedia broadcasting systems

AbstractThe paper considers the application of Orthogonal Frequency Division Multiplexing (OFDM) techniques in the physical layer of a satellite system devoted to the provision of digital multimedia broadcasting (DMB) services, which are presently the focus of extensive research and development endeavours. In particular, the paper demonstrates that the inherently high spectrum efficiency of OFDM can be retained even in unfavourable propagation channels affected by heavy non‐linear distortion introduced by the on‐board high power amplifier. This encouraging performance is achieved through the proper design and application of coding and pre‐distortion techniques, as described in the paper. Based on these results, the paper considers also the comparison of OFDM and High Speed Downlink Packet Access (HSDPA) techniques. HSDPA is the standardized radio interface for 3G terrestrial mobile networks. Copyright © 2006 John Wiley & Sons, Ltd.

Bit-rate maximization for multiuser OFDM systems

In the downlink of multiuser OFDM (Orthogonal Frequency Division Multiplexing) systems, subcarriers of one OFDM symbol can be allocated to multiple users to enable multiplexing. System performance can be enhanced by appropriate subcarrier allocation and bit-loading. In this paper, while guaranteeing a fixed BER performance, we impose a fixed transmitted power constraint on each user to achieve a fair resource allocation. Based on this condition, we propose an algorithm to maximize the sum-rate of the system. This sum-rate optimization is a discrete optimization problem which is hard to solve by conventional methods. We relax the problem into a continuous convex optimization problem so that the subcarrier allocation to each user can be determined numerically. Using the subcarrier allocation for the continuous problem, bits are then loaded to each user. The effectiveness of the proposed method is substantiated by numerical simulations. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(1): 1–9, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20198