Adaptive OFDM Research Articles

Adaptive OFDM for High Mobility Computer Application

Adaptive OFDM for High Mobility Computer Application

Bit rate comparison of adaptive OFDM and cyclic prefixed single-carrier with DFE

This letter considers two variants of a cyclic prefixed block transmission: (i) orthogonal frequency division multiplexing (OFDM), (ii) cyclic prefixed single-carrier (CPSC) transmission with decision feedback equalization (DFE). We aim at comparing these two schemes in terms of achievable bit rate with adaptive bit loading. It is analytically demonstrated that, for all signal to noise ratio values, OFDM achieves a bit rate greater than or equal to that of DFE-CPSC, independently of whether uniform or optimal power allocation is considered.

Adaptive OFDM synchronization algorithm in frequency selective fading channels

With the development of mobile communication, adaptive algorithms are often chosen in real time process because of their high speed. However, existing adaptive algorithms are not suitable in frequency selective fading channels. In this paper, an improved adaptive synchronization algorithm is proposed for the application under frequency selective fading channels. A novel correlation function is proposed to reduce the effect of intersymbol interference, which only requires coarse channel length estimation. A gate value is also used to combine both the constant and decreasing step size methods; thus, the proposed algorithm is capable of distinguishing between tracking and convergent stages, and then choosing the right step size. Furthermore, an accelerating method is introduced to make the algorithm track faster, i.e. making the algorithm move more steps if it searches in the same direction as the last step. It is shown that the mean square errors of time and frequency offset estimates can be reduced to 0.9243 and 0.003 respectively even when the channel length estimate is not good, which totally satisfies Beek's requirement for synchronization. Simulation also shows that it takes only 10-20 symbols for the proposed algorithm to track the change of channel delay. Compared with the existing adaptive algorithm, the proposed algorithm has a higher accuracy with a higher tracking speed, and the proposed algorithm also has advantages over some nonadaptive algorithms with higher accuracy and lower computational complexity.

Practical multiple‐antenna solutions for enhancing adaptive OFDM performance in mm‐wave WPANs

AbstractThe design of short‐range high‐speed wireless networks for the 60 GHz band requires consideration of factors from a wide range of disciplines. Key system choices that make significant differences to the final cost and performance include the type of antennas, multiple access strategy and channel coding. In this paper we assume a coded orthogonal frequency division multiplexing (OFDM) system providing high‐speed communications between low‐cost terminals over indoor 60 GHz channels that are frequently non‐line‐of‐sight. Under constraints of low transmit power and high path attenuation, we consider two practical options for using multiple low‐cost ‘bond‐wire’ antennas, compared with ideal monopole antennas at each terminal. For specified multiple access and channel coding schemes, we show that a sectorized strategy based on simple 2 element phased arrays can provide substantial benefits in system performance and flexibility. Copyright © 2009 John Wiley & Sons, Ltd.

Target Detection in Clutter Using Adaptive OFDM Radar

We address the problem of detecting a target moving in clutter environment using an orthogonal frequency division multiplexing (OFDM) radar. The broadband OFDM signal provides frequency diversity to improve the performance of the system. First, we develop a parametric model that accounts for the measurements at multiple frequencies including the Doppler shift. Then, we present a statistical detection test and evaluate its performance characteristics. Based on this, we propose an algorithm to adaptively design the parameters for the next transmitting waveform. Numerical examples illustrate our analytical results, demonstrating the achieved performance improvement due to the OFDM signaling method and adaptive waveform design.

Throughput analysis of multi-user OFDMA-systems using imperfect CQI feedback and diversity techniques

In this paper, the throughput of an adaptive multiuser SISO-OFDMA/FDD system with channel quality information (CQI) signalled digitized over a feedback channel to the transmitter is investigated, where the instantaneous signal-to-noise ratio (SNR) of the different subcarriers is used as CQI to exploit multi-user diversity using adaptive subcarrier allocation. The CQI available at the transmitter is assumed to be imperfect due to estimation errors and quantization at the receiver side, time delay and feedback errors. In this paper, a closed form expression of the average throughput of an adaptive multi-user OFDMA system using imperfect CQI and uncoded M-QAM modulation is derived. Furthermore, a closed form expression of the average throughput of an OFDMA system exploiting frequency diversity, which does not require CQI at the transmitter, is presented. Both throughput performances are compared in order to identify the optimal transmission strategy depending on the grade of CQI imperfectness.

A novel bits loading algorithm for adaptive modulation OFDM multimode fiber communication system

A novel bits loading algorithm is proposed for adaptively modulated optical OFDM (AMOOFDM) multimode fiber communication system to decrease the system bit error rate (BER) caused by some OFDM subcarriers located at the deep nulls in the high frequency region of multimode fiber. The simulation results show that the performance of the new algorithm is approximate to the optimal algorithm–Greedy algorithm, but its computational complexity is considerably lower, thus the new algorithm is easy to implement. Comparing to fixed bits loading, adaptive bits loading can reduce the system BER effectively.

Time Dispersion and Delay Spread Estimation for Adaptive OFDM Systems

Time dispersion is one of the most important characteristics of the wireless channel. The knowledge about the time dispersion of a channel can be used to design better systems that can adapt themselves to the changing nature of the transmission medium. In this paper, algorithms to estimate the time dispersion (frequency selectivity) of the channel, which use the frequency-domain channel estimates or the frequency-domain-received signal, are proposed. The proposed algorithms estimate the power delay profile (PDP) from which the two important dispersion parameters, namely root-mean-squared (RMS) delay spread and maximum excess delay of the channel, are estimated. It is shown that synchronization errors bias the performance of the estimators based on the channel correlation, and this bias is removed by obtaining the PDP using the magnitude of the channel estimates. Moreover, the Cramer-Rao lower bound (CRLB) for the estimation of the RMS delay spread is derived, and the performances of the proposed algorithms are compared against this theoretical limit.

Cognitive Radio Dynamic Access Techniques

The ever growing demand for wireless services has placed enormous burden on valuable resources such as spectral bandwidth. This has resulted in a major rethinking in resource allocation policies culminating in an explosion of research activity in the field of Cognitive Radio (CR) towards optimum resource usage. In this tutorial paper the physical layer design and transmission techniques for CR in the context of efficient spectrum utilization are discussed. Spectrum sensing as the key element of CR awareness is described. Orthogonal frequency division multiplexing (OFDM) as a spectrally efficient modulation scheme is discussed and the rationale for its use in the CR system is explained. Spectrum pooling for efficient use of spectrum is studied and the role of adaptive OFDM techniques in this method is highlighted. Wavelet basis function as a replacement of Fourier transform in OFDM is evaluated. MIMO system as an added value to the CR performance is described. Adaptive Waveform and beamforming as alternative techniques in CR are reviewed.

自适应调制的正交频分复用多模光纤通信系统性能分析

自适应调制的正交频分复用多模光纤通信系统性能分析

Downlink Scheduling and Radio Resource Allocation in Adaptive OFDMA Wireless Communication Systems for User-Individual QoS

In this paper, we address the problem of adaptive radio resource allocation (RRA) and packet scheduling in the downlink of a cellular OFDMA system, and propose a downlink multi-carrier proportional fair (MPF) scheduler and its joint with adaptive RRA algorithm to distribute radio resources among multiple users according to their individual QoS requirements. The allocation and scheduling objective is to maximize the total throughput, while at the same time maintaining the fairness among users. The simulation results demonstrate that the methods presented provide for user more explicit fairness relative to RRA algorithm, but the joint scheme achieves the higher sum-rate capacity with flexible parameters setting compared with MPF scheduler.

Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed down-link packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER. Following a basic portrayal of adaptive OFDM, the paper investigates a combined system constituted by a constant-power adaptive modem employing space-time coded diversity techniques in the context of both OFDM and multicarrier code division multiple access (MC-CDMA). The combined system is configured to produce a constant uncoded BER and exhibits virtually error-free performance when a turbo convolutional code is concatenated with a space-time block code. Again, it was found that the advantage of rendering the multiple-input multiple-output (MIMO)-aided modem BbB-adaptive erodes, when the affordable system complexity facilitates the implementation of multiple transmitters and receivers, if it can be ensured that no shadowing and no multiuser interference fluctuation is experienced. By contrast, in the presence of these impairments, only BbB-adaptive transceivers can provide a near-constant BER.

Towards Systems Beyond 3G Based on Adaptive OFDMA Transmission

High data rates, high spectral efficiency, flexibility, and low delays over the air interface will be important features in next-generation wireless systems. The overall challenge will be packet scheduling and adaptive radio transmission for multiple users, via multiple antennas and over frequency-selective wideband channels. This problem needs to be structured to obtain feasible solutions. The basic simplifying assumptions used here are clustering of antennas into cells, orthogonal transmission by use of cyclic-prefix orthogonal frequency-division multiplexing (OFDM) and a time-scale separation view of the total link adaptation, scheduling and intercell coordination problem. Based on these assumptions, we survey techniques that adapt the transmission to the temporal, frequency, and spatial channel properties. We provide a systematic overview of the design problems, such as the dimensioning of the allocated time-frequency resources, the influence of duplexing schemes, adaptation control issues for downlinks and uplinks, timing issues, and their relation to the required performance of channel predictors. Specific design choices are illustrated by recent research within the Swedish Wireless IP program and the EU IST-WINNER project. The presented results indicate that high-performance adaptive OFDM transmission systems are indeed feasible, also for challenging scenarios that involve vehicular velocities, high carrier frequencies, and high bandwidths.

MMSE Noise Plus Interference Power Estimation in Adaptive OFDM Systems

Noise variance and signal-to-noise ratio are important parameters for adaptive orthogonal frequency-division multiplexing (OFDM) systems since they serve as a standard measure of signal quality. Conventional algorithms assume that the noise statistics remain constant over the OFDM frequency band and, thereby, average the instantaneous noise samples to get a single estimate. In reality, noise is often made up of white Gaussian noise, along with correlated colored noise that unevenly affects the OFDM spectrum. This paper proposes a minimum mean square error (MMSE) filtering technique to estimate the noise power that takes into account the variation of the noise statistics across the OFDM subcarrier index, as well as across OFDM symbols. The proposed method provides many local estimates that allow tracking of the variation of noise statistics in frequency and time. The MMSE filter coefficients are obtained from the mean-squared-error expression, which can be calculated using the noise statistics. Evaluation of the performance with computer simulations shows that the proposed method tracks the local statistics of the noise more efficiently than conventional methods.

Adaptive Bit-Interleaved Coded OFDM With Reduced Feedback Information

If the channel is static and is perfectly known to both the transmitter and the receiver, the water-filling technique with adaptive modulation is known to be optimal (Gallager, 1968). However, for orthogonal frequency-division multiplexing (OFDM) systems, this requires intensive traffic overheads for reporting channel state information on all subcarriers to the transmitter. In this paper, we consider an adaptive modulation and coding scheme for bit-interleaved coded OFDM with reduced feedback information satisfying a specified quality of service level. We propose a rate adaptation scheme, which utilizes the estimated bit error rate for supportable transmission rates. In this scheme, a user equipment chooses a modulation and coding scheme (MCS) level, which can provide the maximum spectral efficiency based on one OFDM symbol rather than on all subchannels. Then the user needs to send back only the selected MCS level index. The proposed scheme does not require the water-filling procedure, and the amount of the feedback information reduces to a single integer value irrespective of the number of subcarriers. Simulation results show that the proposed scheme can significantly reduce the system complexity while minimizing the performance loss compared to the optimum water-filling scheme.

Robust Adaptive OFDM with Diversity for Time-Varying Channels

The performance of an orthogonal frequency-division multiplexing (OFDM) system can be significantly increased by using adaptive modulation and transmit diversity. An accurate estimate of the channel, however, is required at the transmitter to realize this benefit. Due to the time-varying nature of the channel, this estimate may be outdated by the time it is used for detection. This results in a mismatch between the actual channel and its estimate as seen by the transmitter. In this paper, we investigate adaptive OFDM with transmit and receive diversities, and evaluate the detrimental effects of this channel mismatch. We also describe a robust scheme based on using past estimates of the channel. We show that the effects of the mismatch can be significantly reduced with a combination of diversity and multiple channel estimates. In addition, to reduce the amount of feedback, the subband approach is introduced where a common channel estimate for a number of subcarriers is fedback to the transmitter, and the effect of this method on the achievable rate is analyzed.

Design of a Channel-Aware OFDM Transceiver

The transmission performance of an OFDM system can significantly be improved by exploiting the channel characteristics. In this paper, we consider the design of a channel-aware OFDM transceiver whose parameters are adjusted in response to the change of channel condition. To this end, we first estimate the channel state information (CSI), such as the signal to interference power ratio, low order moments of Doppler spectrum and power-delay profile of the channel. The proposed CSI estimator can estimate these CSI parameters altogether in a unique manner by exploiting the autocorrelation properties of the channel impulse response (CIR). Then, we design a CIR estimator and adaptive OFDM modulator that adjust their parameters according to the estimated CSI. Finally, we verify the performance of the proposed OFDM transceiver by computer simulation

Multi-user adaptive orthogonal frequency-division multiplexing system for indoor wireless optical communications

An adaptive orthogonal frequency-division multiplexing (OFDM) system is proposed for multi-user communications over indoor wireless optical channels. The designed system uses multi-user least-squares detection techniques applied to space-division multiple access and OFDM schemes, in conjunction with angle-diversity reception. The system, which does not present an excessive increase in complexity with respect to the previous schemes, can support high bit rates for multiple users, beyond 100 Mbits/s. It also mitigates the channel fluctuations induced when either the space distribution or the number of emitters and receivers varies. The performance of the new proposed scheme is compared with that of a non-adaptive multi-user system and an adaptive single-user system, both described in the previous works, when they face similar environmental situations. The obtained results show a significant enhancement with respect to both the previous multi-user system and the adaptive single-user one, since the new scheme allows adaptively managing the system throughput on a multi-user environment.

Unified models for adaptive OFDM systems when QAM or PSK modulation is applied

AbstractIn this paper, the solutions of the major bit‐loading problems that are found in the literature, are unified into simple equations (models). These equations give an insight of the affection of the channel over a multicarrier system. Moreover, the models allow the transition from the solution of one loading problem to the solution of the others. The models are dependent on the modulation type that is used. We examine the quadrature amplitude modulation (QAM) modulated orthogonal frequency division multiplexing (OFDM), and the phase shift keying (PSK)‐modulated OFDM systems. For each modulation type, the continuous throughput model is extracted. The continuous throughput model refers to a system that its subcarriers can be loaded with non‐integer number of bits. The non‐integer bit distribution can then be rounded to integers, using a rounding method. The models except from the calculation of the optimised quantities (total data rate, total power and symbol error rate) without the need of running a loading algorithm to its extent, provide a means to manipulate a whole multicarrier system in a way similar to single‐carrier system. Copyright © 2007 John Wiley & Sons, Ltd.

Blind Modulation Classification Algorithm for Adaptive OFDM Systems

Adaptive modulation is an efficient method to increase the spectral efficiency of OFDM based high-speed wireless data transmission systems in multipath channel. Blind modulation classification schemes play an important role in adaptive modulation systems, eliminating the need for transmitting modulation information, thereby increasing spectral efficiency. In this paper, a novel blind modulation classification algorithm is derived from the finite alphabet property of information symbols and the equivalent parallel model of OFDM systems. The performances of the proposed algorithm and M2M4P algorithm [1] are tested and compared using Monte-Carlo simulations. It is found that, the novel algorithm yields performance better than that of M2M4P algorithm and with much less complexity.