Multiple Orthogonal Frequency Division Multiplexing Research Articles

다중안테나 OFDM 멀티캐스트 시스템을 위한 동적 자원할당 알고리즘

최근 차세대 무선 통신 시스템으로 다중 안테나 직교 주파수 다중접속 방식(OFDM)에서 적응적 자원 할당 기술은 사용자들의 QoS를 보장하며 성능을 개선 할 수 있는 중요한 연구 분야로 각광을 받고 있다. 그러나, 현재까지, 대부분의 자원 할당 알고리즘은 유니캐스트 시스템에 국한되어 연구가 진행되고 있다. 따라서, 본 논문에서는 다중 안테나 직교 주파수 다중접속 방식 시스템에서 멀티캐스트 서비스를 위한 동적 자원 할당 기술을 제안하고 이에 대한 전산모의 실험 결과를 통하여 성능 개선 효과를 분석하였다. 실험 분석 결과, 제안하는 동적 자원 할당 기술이 기존의 방식에 비해 우월함을 증명하였다. Multiple antenna orthogonal frequency division multiplexing (OFDM) is a promising technique for the high downlink-capacity in the next generation wireless systems, in which adaptive resource allocation is an important research issue that can significantly improve the performance with guaranteed QoS for users. However, most of the current resource allocation algorithms are limited to unicast system. In this paper, dynamic resource allocation is studied for multiple antenna OFDM based systems with multicast service. In the simulation, the performance of multicast system was compared with that of the unicast system. Numerical results also show that by using the proposed algorithms the system capacity is significantly improved compared with the conventional scheme.

A Novel Channel Estimation Method Using Virtual Pilots in MIMO OFDM Systems

To improve the performance of the optimal pilot sequences over multiple OFDM symbols in fast time-varying channels, this letter proposes a novel channel estimation method using virtual pilot tones in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Assuming that the superimposed virtual pilot tones at the data locations over the specific sub-carriers are transmitted from all transmit antennas, the corresponding virtual received pilot signals at the same locations are obtained from the neighboring real received pilot signals over the same sub-carriers by Wiener filter. Based on the least squares (LS) channel estimation, the channel parameters can be obtained from the combination of the virtual and real received pilot signals over one OFDM symbol. Simulation results show that the proposed channel estimation method greatly outperforms the previous method for the optimal pilot sequences over multiple OFDM symbols in fast time-varying channels, as well as approaches the method for the comb-type optimal pilot sequences in performance.

A low-complexity bandwidth allocation algorithm for frequency-selective multiuser OFDM systems

This paper addresses the problem of efficient bandwidth allocation for multiuser orthogonal frequency division multiplexing (OFDM) systems over frequency-selective channels. For a given coding and medium access scheme and for specific channel gains, the achievable data rates depend on the assignment of the available subchannels to the different users. However, in frequency-selective channels, the various users experience different channel conditions, thus the application of effective bandwidth allocation rules results in enhanced multiuser diversity. This paper presents a computationally efficient solution for dynamic subchannel assignment. The proposed algorithm provides enhanced system performance compared with the fixed time division multiple access (TDMA) solution and also results in proportional data rates according to channel quality of each user. As a case study, we demonstrate the performance of the proposed algorithm in the indoor power line network with multiple OFDM links between a central station and a number of user devices.

Low Pilot Ratio Channel Estimation for OFDM Systems Based on GCE-BEM

Doubly-selective channel estimator for orthogonal frequency division multiplexing(OFDM) systems is proposed in this paper. Based on the generalized complex exponential basis expansion model(GCE-BEM), we describe the timevariant channel with time-invariant coefficients over multiple OFDM blocks. The time variation of the channel destroys the orthogonality between subcarriers, and the resulting channel matrix in the frequency domain is no longer diagonal, but the main interference comes from the near subcarriers. Based on this, we propose a channel estimator with low pilot ratio. We first develop a least-square(LS) estimator under the assumption that only the maximum Doppler frequency and the channel order are known at the receiver, and then verify that the correlation matrix of inter-channel interference(ICI) is a scaled identity matrix based on which we derive an optimal pilot insertion scheme for the LS estimator in the sense of minimum mean square error. The proposed estimator has the advantages of low pilot ratio and robustness against inter-carrier interference.

Robust Plateau Removed Synchronization Techniques for Multiple-antenna OFDM-Based DVB-H Receivers

As a special format of multiple input multiple output (MIMO), multiple-antenna diversity reception can be used to improve the signal quality of the orthogonal frequency division multiplexing (OFDM) based digital video broadcasting for handheld (DVB-H) systems. Under multipath fading channels, synchronization methods proposed for the single input single output (SISO) OFDM systems may be to fail if they are directly applied to the multiple-antenna OFDM-based DVB-H receivers. In this paper, a plateau removed maximum likelihood synchronization scheme is proposed for the multiple-antenna OFDM-based DVB-H receivers. Compared to the conventional maximum likelihood multiple antenna OFDM synchronization scheme, the proposed scheme not only removes the timing ambiguity caused by the plateau problem and has better detection capability but also is robust to different multipath fading channels. System performance studies are used to verify the efficiency of the proposed scheme for the multiple-antenna OFDM-based DVB-H receiver under different fading channel situations.

Peak power reduction scheme base on transform kernel scrambling for OFDM systems

This work presents a novel transform kernel scrambling (TKS) scheme for reducing the peak-to-average transmitter power ratio (PAPR) for Orthogonal Frequency Division Multiplexing (OFDM) symbols. This scrambling scheme suppresses the peak envelope power of OFDM signals due to de-correlation among the original QAM constellation in multiple OFDM symbols via transformation. The performance of the reduction of PAPR for OFDM signals with various unitary transform kernels is also evaluated. Since specific unitary transform kernels in TKS processing block is only used for scrambling, no side information is required. For the ease of presentation, the system performance has been demonstrated by a computer simulation. Simulation results confirm the superiority of the proposed scheme in the PAPR reduction.

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.

Reduced complexity signal detection for OFDM systems with transmit diversity

Orthogonal frequency division multiplexing (OFDM) systems with multiple transmit antennas can exploit space-time block coding on each subchannel for reliable data transmission. Spacetime coded OFDM systems, however, are very sensitive to time variant channels because the channels need to be static over multiple OFDM symbol periods. In this paper, we propose to mitigate the channel variations in the frequency domain using a linear filter in the frequency domain that exploits the sparse structure of the system matrix in the frequency domain. Our approach has reduced complexity compared with alternative approaches based on time domain block-linear filters. Simulation results demonstrate that our proposed frequency domain block-linear filter reduces computational complexity by more than a factor of ten at the cost of small performance degradation, compared with a time domain block- linear filter.

Hybrid Iterative Multiuser Detection for Channel Coded Space Division Multiple Access OFDM Systems

Space division multiple access (SDMA) aided orthogonal frequency division multiplexing (OFDM) systems assisted by efficient multiuser detection (MUD) techniques have recently attracted intensive research interests. The maximum likelihood detection (MLD) arrangement was found to attain the best performance, although this was achieved at the cost of a computational complexity, which increases exponentially both with the number of users and with the number of bits per symbol transmitted by higher order modulation schemes. By contrast, the minimum mean-square error (MMSE) SDMA-MUD exhibits a lower complexity at the cost of a performance loss. Forward error correction (FEC) schemes such as, for example, turbo trellis coded modulation (TTCM), may be efficiently combined with SDMA-OFDM systems for the sake of improving the achievable performance. Genetic algorithm (GA) based multiuser detection techniques have been shown to provide a good performance in MUD-aided code division multiple access (CDMA) systems. In this contribution, a GA-aided MMSE MUD is proposed for employment in a TTCM-assisted SDMA-OFDM system, which is capable of achieving a similar performance to that attained by its optimum MLD-aided counterpart at a significantly lower complexity, especially at high user loads. Moreover, when the proposed biased Q-function based mutation (BQM) assisted iterative GA (IGA) MUD is employed, the GA-aided system's performance can be further improved, for example, by reducing the bit error ratio (BER) measured at 3 dB by about five orders of magnitude in comparison to the TTCM-assisted MMSE-SDMA-OFDM benchmarker system, while still maintaining modest complexity.

Exploiting Multipath Diversity in Multiple Antenna OFDM Systems With Spatially Correlated Channels

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only 0.44/spl lambda/, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.

On Usefulness of Multipath Diversity for Multiple Antenna OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) technology hold much promise in realizing broadband wireless systems which are both power efficient and bandwidth efficient. This review article focuses on MIMO-OFDM wireless systems, and the usefulness of multipath diversity in such systems are addressed. With increasing data rate requirements for wireless access, the technology challenges of the physical layer designers are moving towards wideband wireless communications. One of the key issue to be confronted in such systems is the frequency selective fading of the wireless channel due to multipath propagation. Orthogonal frequency division multiplexing helps in transforming this frequency selective channel into multiple narrowband flat sub-channels which facilitate a computationally efficient equalization. Usually multipath delay spread is considered as an annoyance factor. However, such channels are rich in multipath diversity, which could potentially be exploited to yield performance benefits. Using simple frequency domain processing in OFDM systems with closely spaced antennas, this multipath diversity could be exploited to achieve performance close to that of space diversity systems using well separated antennas. With such closely spaced antennas, it appears as though there is sampling in the spatial domain. Linear dispersion codes are capacity achieving space-time codes designed by optimizing the information-theoretic capacity function. Multipath diversity available as frequency diversity could be exploited by linear dispersion codes when the dispersion is over OFDM sub-channels in frequency and space. In this article, we also review such space-frequency linear dispersion codes which are particularly useful at low rate and for low complexity systems.

Pilot-Based Channel Estimation for OFDM Systems by Tracking the Delay-Subspace

In orthogonal frequency division multiplexing (OFDM) systems over fast-varying fading channels, channel estimation and tracking is generally carried out by transmitting known pilot symbols in given positions of the frequency-time grid. The traditional approach consists of two steps. First, the least-squares (LS) estimate is obtained over the pilot subcarriers. Then, this preliminary estimate is interpolated/smoothed over the entire frequency-time grid. In this paper, we propose to add an intermediate step, whose purpose is to increase the accuracy of the estimate over the pilot subcarriers. The presented techniques are based on the observation that the wireless radio channel can be parametrized as a combination of paths, each characterized by a delay and a complex amplitude. The amplitudes show fast temporal variations due to the mobility of terminals while the delays (and their associated delay-subspace) are almost constant over a large number of OFDM symbols. We propose to track the delay-subspace by a subspace tracking algorithm and the amplitudes by the least mean square algorithm (or modifications of the latter). The approach can be extended to multiple input multiple output OFDM or multicarrier code-division multiple-access systems. Analytical results and simulations prove the relevant benefits of the novel structure.

Optimal training design for MIMO OFDM systems in mobile wireless channels

This paper describes a least squares (LS) channel estimation scheme for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems based on pilot tones. We first compute the mean square error (MSE) of the LS channel estimate. We then derive optimal pilot sequences and optimal placement of the pilot tones with respect to this MSE. It is shown that the optimal pilot sequences are equipowered, equispaced, and phase shift orthogonal. To reduce the training overhead, an LS channel estimation scheme over multiple OFDM symbols is also discussed. Moreover, to enhance channel estimation, a recursive LS (RLS) algorithm is proposed, for which we derive the optimal forgetting or tracking factor. This factor is found to be a function of both the noise variance and the channel Doppler spread. Through simulations, it is shown that the optimal pilot sequences derived in this paper outperform both the orthogonal and random pilot sequences. It is also shown that a considerable gain in signal-to-noise ratio (SNR) can be obtained by using the RLS algorithm, especially in slowly time-varying channels.

Pilot-Symbol-Assisted Channel Estimation for Space-Time Coded OFDM Systems

Space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to provide an efficient means of achieving near optimal diversity gain in frequency-selective fading channels. For these systems, knowledge of the channel parameters is required at the receivers for diversity combining and decoding. In this paper, we propose a low complexity, bandwidth efficient, pilot-symbol-assisted (PSA) channel estimator for multiple transmitter OFDM systems. The pilot symbols are constructed to be nonoverlapping in frequency to allow simultaneous sounding of the multiple channels. The time-varying channel responses are tracked by interpolating a set of estimates obtained through periodically transmitted pilot symbols. Simulations are used to verify the effectiveness of the propsed estimator and to examine its limitations. It is also shown decision-directed minimum mean square error MMSE channel estimator for OFDM transmitter diversity systems.

Three non-pilot based time- and frequency estimators for OFDM

Time-domain maximum-likelihood (ML) estimators of time and frequency offsets are derived for three orthogonal frequency division multiplexing (OFDM) signal models: a pulse-shaped one-shot OFDM signal, a stream of multiple OFDM signals and an OFDM signal in a dispersive channel environment. We then develop structures to simplify their implementation. Simulation results show the relative performance and strengths of each of these three estimators.