Showing papers on "Frequency-division multiplexing published in 2006"

Orthogonal frequency division multiplexing for high-speed optical transmission

Abstract: Optical Orthogonal frequency division multiplexing (OOFDM) is shown to outperform RZ-OOK transmission in high-speed optical communications systems in terms of transmission distance and spectral efficiency. The OOFDM in combination with the subcarrier multiplexing offers a significant improvement in spectral efficiency of at least 2.9 bits/s/Hz.

Optimal training signals for MIMO OFDM channel estimation

Abstract: This paper presents general classes of optimal training signals for the estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations across the transmit antennas are presented and classified as frequency-division multiplexing, time-division multiplexing, code-division multiplexing in the frequency-domain, code-division multiplexing in the time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data-multiplexed schemes.

Subcarrier weighting: a method for sidelobe suppression in OFDM systems

Abstract: In this letter, a method for sidelobe suppression in OFDM systems is proposed and investigated. The proposed method is based on the multiplication of the used subcarriers with subcarrier weights. The subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized according to an optimization algorithm which allows several optimization constraints. As a result, sidelobe suppression by subcarrier weighting reduces OFDM sidelobes by more than 10 dB in the average without requiring the transmission of any side information

Power line enhanced cooperative wireless communications

Abstract: In this paper, we investigate the use of power line communication (PLC) to assist cooperative wireless relaying. We consider a communication scheme that uses the power line to initialize and synchronize wireless amplify-and-forward relays and to broadcast information between the relays. Starting from an analysis of transfer functions and noise measurements of PLC channels in office and residential environments, we propose a power line transmission scheme for the inter-relay-communication and assess the influence of this scheme on wireless relaying. This scheme is based on linear precoded orthogonal frequency-division multiplexing; it is designed to optimally exploit the frequency diversity available on PLC channels. The use of PLC leads to a very flexible way of enhancing wireless communications by plugging in additional relays where they are needed-without additional wiring.

Use of linear programming for dynamic subcarrier and bit allocation in multiuser OFDM

Abstract: An adaptive subcarrier allocation and an adaptive modulation for multiuser orthogonal frequency-division multiplexing (OFDM) are considered. The optimal subcarrier and bit allocation problems, which are previously formulated as nonlinear optimizations, are reformulated into and solved by integer programming (IP). A suboptimal approach that performs subcarrier allocation and bit loading separately is proposed. It is shown that the subcarrier allocation in this approach can be optimized by the linear-programming (LP) relaxation of IP, while the bit loading can be performed in a manner similar to a single-user OFDM. In addition, a heuristic method for solving the LP problem is presented. The LP-based suboptimal and heuristic algorithms are considerably simpler to implement than the optimal IP, plus their performances are close to those of the optimal approach

Random access channel hopping for frequency division multiplexing access systems

Abstract: A method of coordinating resources for repetition of random access bursts performed by a mobile terminal, the method comprising: determining groups of access slots based on parameters from a network, wherein each access slot is defined by any combination of frequency, time and code, and the access slots are organized according to a frequency pattern; transmitting an access burst on an access slot from a chosen group of access slots; and re-transmitting the access burst on the next access slot from the chosen group of access slots.

Pilot transmission and channel estimation for a communication system utilizing frequency division multiplexing

Abstract: A transmitter generates a pilot having a constant time-domain envelope and a flat frequency spectrum based on a polyphase sequence. To generate a pilot IFDMA symbol, a first sequence of pilot symbols is formed based on the polyphase sequence and replicated multiple times to obtain a second sequence of pilot symbols. A phase ramp is applied to the second pilot symbol sequence to obtain a third sequence of output symbols. A cyclic prefix is appended to the third sequence of output symbols to obtain an IFDMA symbol, which is transmitted in the time domain via a communication channel. The pilot symbols may be multiplexed with data symbols using TDM and/or CDM. A pilot LFDMA symbol may also be generated with a polyphase sequence and multiplexed using TDM or CDM. A receiver derives a channel estimate based on received pilot symbols and using minimum mean-square error, least-squares, or some other channel estimation technique.

30-gb/s signal transmission over 40-km directly modulated DFB-laser-based single-mode-fiber links without optical amplification and dispersion compensation

Abstract: Based on a recently proposed novel optical-signal-modulation technique of adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), numerical simulations of the transmission performance of AMOOFDM signals are undertaken in directly modulated DFB laser (DML)-based single-mode-fiber (SMF) links without optical amplification and dispersion compensation. It is shown that a 30-Gb/s transmission over a 40-km SMF with a loss margin of greater than 4.5 dB is feasible in the aforementioned simple configuration using intensity modulation and direct detection (IMDD). In addition, the DFB-laser frequency chirp and the transmission-link loss are identified to be the key factors limiting the maximum achievable transmission performance of the technique. The first factor is dominant for transmission distances of 80 km. It is also observed that fibers of different types demonstrate similar transmission performances, on which fiber nonlinear effects are negligible.

Multiplexing radiography using a carbon nanotube based x-ray source

Abstract: Speed and temporal resolution are critical for tomographic imaging of objects in rapid motion. Current x-ray scanners record images sequentially in the time domain. The serial approach limits their performance and demands increasingly high x-ray peak power and gantry speed. We have developed a multipixel carbon nanotube based field emission x-ray source that produces spatially and temporally modulated radiations. Using this device we show the feasibility of multiplexing radiography that enables simultaneous collection of multiple projection images using frequency multiplexing. A drastic increase of the speed and reduction of the x-ray peak power are achieved without compromising the imaging quality.

Performance analysis of optimized millimeter-wave fiber radio links

Abstract: We present a comprehensive performance analysis of several optimized fiber radio distribution schemes for millimeter-wave radio services. The analysis includes the noise and nonlinear characteristics of the transmitter (Tx)-receiver (Rx) pair integrated with the analog optical link in the downlink transmission of a given wavelength without optical amplification. Investigations are focused on four configurations of optimized fiber radio links, which were derived by considering the best performing possible Tx-Rx configuration and specifications of commercially available devices to support multichannel subcarrier multiplexed transmission. It was found that the nonlinear characteristics of the Mach-Zehnder modulator are the major source of performance degradation of the fiber radio links. A comparison of RF-over-fiber and IF-over-fiber transport schemes also shows that RF-over-fiber can yield 3-dB improvement in performance compared to IF-over-fiber techniques.

Random access dimensioning methods and procedures for frequency division multiplexing access systems

Abstract: A method of determining random access resources performed by a mobile terminal, the method comprising: receiving information on available random access resources from a network; deciding how to derive the random access resources to be allowed based on default values or information received from the network; measuring received signal quality of at least one of a cell to be accessed and a neighboring cell; and deriving the allowed random access resources based on the deciding and the measuring.

Performance analysis of uncoded and coded OFDM broadband transmission over low voltage power-line channels with impulsive noise

Abstract: Broadband communications for indoor power-line networks with impulsive noise using orthogonal frequency division multiplexing (OFDM) is considered. From earlier investigations, it is known that this channel suffers from multipath fading and frequency selectivity along with man-made impulsive bursty noise. Nevertheless, the calculated channel capacity limit promises very high data rates over this channel. Enhancement techniques, such as coding can help an OFDM system to tackle impulsive noise burst. In this paper, the bit error rate (BER) performance of the OFDM system under impulsive noise and frequency fading is theoretically analyzed and closed form expression for the performance is derived. Two different impulsive noise environment models are studied and it is confirmed that both models represent the same environment. Furthermore, a theoretical upper bound on the performance of coded OFDM system is obtained, given perfect interleaving. The effect of the interleaver length on coding performance is also studied. Simulations show that the upper bound is quite tight for the case of employing a longer interleaver. The effect of interleaver size on the system performance is studied, as well

Downlink transmission of broadband OFCDM Systems-part II: effect of Doppler shift

Abstract: The orthogonal frequency and code division multiplexing (OFCDM) system with 2-D spreading (time- and frequency-domain spreading) is becoming a promising candidate for future broadband wireless communication systems. OFCDM is more attractive than orthogonal frequency-division multiplexing (OFDM) both by introducing frequency-domain spreading for frequency diversity provision and time-domain spreading for flexible data rate provision. To provide high-speed mobile services, multicode transmission is employed in conjunction with OFCDM. In a Gaussian or flat-fading channel, multicode channels are orthogonal. However, in a realistic wireless channel, the orthogonality no longer maintains. Thus, multicode interference (MCI) is caused. This paper focuses on the investigation of the effect of Doppler shift on the downlink transmission of high-speed mobile OFCDM systems. A practical channel estimation algorithm based on a code-multiplexed pilot channel is employed to track the variations of fading channels. Hybrid MCI cancellation and minimum mean-square error (MMSE) detection proposed by the authors is employed as an efficient way to eliminate the MCI in the frequency domain. The system performance is analytically studied with imperfect channel estimation to show how it is affected by parameters such as the window size in the channel estimation, Doppler shift, the number of stages of the hybrid detection, the power ratio of pilot to data channels, spreading factor, and so on

On the Achievable Sum Rate for MIMO Interference Channels

Abstract: In this correspondence, we study some information theoretical characteristics of vector Gaussian interference channels. Resorting to the superposition code technique, a lower bound of the sum capacity for the vector Gaussian interference channel is obtained. Alternatively, orthogonal transmission via frequency division multiplexing is considered and we establish the concavity of sum rate as the bandwidth allocation factor for the vector channel case. Numerical examples indicate that the achievable sum rate via the superposition code compares favorably with orthogonal transmission: the lower bound obtained via the superposition code dominates the best achievable sum rate through orthogonal transmission. This improvement holds for all interference power levels, a sharp contrast to that of the scalar counterpart

Support vector machines for robust channel estimation in OFDM

Abstract: A new support vector machine (SVM) algorithm for coherent robust demodulation in orthogonal frequency-division multiplexing (OFDM) systems is proposed. We present a complex regression SVM formulation specifically adapted to a pilots-based OFDM signal. This novel proposal provides a simpler scheme than an SVM classification method. The feasibility of our approach is substantiated by computer simulation results obtained for IEEE 802.16 broadband fixed wireless channel models. These experiments allow to scrutinize the performance of the OFDM-SVM system and the suitability of the epsiv-Huber cost function, in the presence of non-Gaussian impulse noise interfering with OFDM pilot symbols

Optimizing power efficiency of OFDM using quantized channel state information

Abstract: Emerging applications involving low-cost wireless sensor networks motivate well optimization of orthogonal frequency-division multiplexing (OFDM) in the power-limited regime. To this end, the present paper develops loading algorithms to minimize transmit-power under rate and error probability constraints, using three types of channel state information at the transmitter (CSIT): deterministic (per channel realization) for slow fading links, statistical (channel mean) for fast fading links, and quantized (Q), whereby a limited number of bits are fed back from the transmitter to the receiver. Along with optimal bit and power loading schemes, quantizer designs and reduced complexity alternatives with low feedback overhead are developed to obtain a suite of Q-CSIT-based OFDM transceivers with desirable complexity versus power-consumption tradeoffs. Numerical examples corroborate the analytical claims and reveal that significant power savings result even with a few bits of Q-CSIT

Channel Estimation for MIMO-OFDM Systems by Modal Analysis/Filtering

Abstract: In this paper, we investigate the benefits of exploiting the a priori information about the structure of the multipath channel on the performance of channel estimation for multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems. We first approach this problem from the point of view of estimation theory by computing a lower bound on the estimation error and studying its properties. Then, based on the insight obtained from the analysis, efficient channel estimators are designed that perform close to the derived limit. The proposed channel estimators compute the long-term features of the multipath channel model through a subspace tracking algorithm by identifying the invariant (over multiple OFDM symbols) space/time modes of the channel (modal analysis). On the other hand, the fast-varying fading amplitudes are tracked by using least-squares techniques that exploit temporal correlation of the fading process (modal filtering). The analytic treatment is complemented by thorough numerical investigation in order to validate the performance of the proposed techniques. MIMO-OFDM with bit-interleaved coded modulation and MIMO-turbo equalization is selected as a benchmark for performance evaluation in terms of bit-error rate.

An OFDM scheme with a half complexity

Abstract: The paper deals with an OFDM (orthogonal frequency division multiplexing) system based on filter-bank architecture. The known implementation uses a DFT (discrete Fourier transform) processor and a polyphase network (PPN). Even if it is based on complex components, in the final step it operates the real part extraction of the incoming signal. This leads to redundant operations in the DFT processor and in the PPN. Specifically, for the transmission of N complex symbol sequences at a given rate ½F, an N-point DFT processor and an N-branch PPN, both working at the rate F, are required. This implementation can be improved with a complexity reduction by a factor of two. In fact, in the paper an architecture is presented based on an N-point DFT processor and N-branch PPN at the rate F, for the transmission of 2N (in place of N) complex symbol sequences at the rate ½F

Simulated Imaging Performance ofUWB SARBased on OFDM

Abstract: Thispaperconcerns initial simulation study ofanimaging radarsystememploying frequency division multiplexing asmeansofgenerating radarpulses. General architecture ofOFDM communication system withrealistic parameters wasadopted andthenmodified tobeusedfor synthetic aperture radarmodeling. Arbitrary target function consisting ofseveral point scatterers wasusedtotest system's ability toproduce imagery. Attheconclusion ofthestudyit wasfoundthatUWB OFDM imaging radardoes, indeed, presentcertain benefits, suchas dynamicspectrum allocation, anti-jamming potential through pulsediversity, possibility fordualuseasacommunication system - while alsohaving a potential toproduce high-resolution target images onparwithother pulse-based UWB radars. IndexTerms- Frequency division multiplexing, radar imaging, simulation, spreadspectrumradar,synthetic aperture radar.

100-gb/s transmission using orthogonal frequency-division multiplexing

Abstract: The possibility of 100-Gb/s transmission over 25-GHz bandwidth using orthogonal frequency-division multiplexing (OFDM) is demonstrated It is shown that 100-Gb/s transmission over 3840 km can be achieved using single-sideband quadrature-phase-shift keying OFDM transmission and low-density parity-check codes

A cross-Layer diversity technique for multicarrier OFDM multimedia networks

Abstract: Diversity can be used to combat multipath fading and improve the performance of wireless multimedia communication systems. In this work, by considering transmission of an embedded bitstream over an orthogonal frequency division multiplexing (OFDM) system in a slowly varying Rayleigh faded environment, we develop a cross-layer diversity technique which takes advantage of both multiple description coding and frequency diversity techniques. More specifically, assuming a frequency-selective channel, we study the packet loss behavior of an OFDM system and construct multiple independent descriptions using an FEC-based strategy. We provide some analysis of this cross-layer approach and demonstrate its superior performance using the set partitioning in hierarchical trees image coder.

Multiuser bit loading for multicarrier systems

Abstract: A multiuser bit-loading problem is investigated in multicarrier communication systems. Assuming knowledge of all the channel gains, we propose a multiuser bit-loading algorithm that attempts to minimize the total power to transmit a target rate-sum of all users. It is shown that this algorithm is optimal when the interference among users is nonexistent or strong. The simulation results show that the proposed algorithm finds a near-optimal solution for very-high-speed digital subscriber line systems

A general SFN structure with transmit diversity for TDS-OFDM system

Abstract: In the digital television terrestrial broadcasting, the Single Frequency Network (SFN) has much advantage. SFN can serve an arbitrary large area with the same information broadcasted at the same frequency, resulting in the potential diversity gain. To improve the transmission performance of the Time Domain Synchronous-Orthogonal Frequency Division Multiplexing (TDS-OFDM) system in SFN, a general SFN structure with transmit diversity is introduced, theoretically analyzed and computer simulated. The proposed SFN structure is flexible to set up. Simulations show that this method can greatly improve the reliability of the signal transmission over the frequency selective fading channels and is suitable for the TDS-OFDM system in SFN.

Transactions Papers - Space-time-frequency characterization of MIMO wireless channels

Abstract: In this paper, we characterize the cross-correlation function (CCF) between the time-frequency transfer functions (TF)s of two sub-channels of a multiple-input multiple-output (MIMO) wireless fading channel. The TF of each sub-channel consists of a number of multi-path components caused by propagation of a transmitted signal in random scattering media. The proposed CCF is expressed in terms of several environmental parameters (such as the moment generating function (MGF) of the delay profile (DP) and the pathloss exponent). It is a summation of two terms: the first term is due to the autocorrelation of multi-path components while the second term is due to the cross-correlation of multi-path components. Each term is a product of several correlation functions. Each of these correlation functions represents different dependencies of the wireless channel in terms of time, carrier frequency and the position of the antenna elements around both the transmitter and the receiver site. Interestingly, the last two terms of these functions are (n/2)-order (or n-order) integrations of the MGF of the DP, evaluated at two carrier frequencies (or the difference between carrier frequencies), where n is the pathloss exponent of the environment

Diversity analysis for frequency-selective MIMO-OFDM systems with general spatial and temporal correlation model

Abstract: In this paper, the effect of a general spatial and temporal fading correlation structure on the performance of coded multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems is studied. The analysis handles an arbitrary joint transmit-receive spatial correlation model, including the non-Kronecker model. An upper bound on the maximum achievable diversity order for frequency-selective MIMO-OFDM systems with general temporal and spatial correlation is derived. Furthermore, a space-time-frequency code design that can achieve the upper bound for any arbitrarily correlated channel scenario is provided. The general framework of the analysis includes space-frequency (SF)-coded systems as a special case. For the SF-coded MIMO-OFDM system, it is shown that any SF code designed to achieve full diversity in the independent fading channel can achieve full diversity in an arbitrary spatially correlated channel. The derived analytical results are consistent with those in the existing literature for special correlation structures. Extensive simulation results are provided to confirm the theoretical analysis.

Methods and systems for ofdm multiple zone partitioning

Abstract: Aspects of the invention include methods and devices for inserting data and pilot symbols into Orthogonal Frequency Division Multiplexing (OFDM) frames having a time domain and a frequency domain. A method involves inserting in at least one zone of a first type a two dimensional array of data and pilot symbols in time and frequency and inserting in at least one zone of a second type a two dimensional array of data and pilot symbols in time and frequency. In some implementations the zone of the first type comprises common pilot symbols that can be detected by all receivers receiving the OFDM frame. In some implementations the zone of the second type comprises dedicated pilot symbols that are only detectable by a receiver that is aware of pre-processing used to encode the dedicated pilot symbols. According to various embodiments of the invention, the zones of the first and second types can be partitioned in the OFDM frame based on time division multiplexing (TDM), frequency division multiplexing (FDM) or combined TDM/FDM. Examples of types of transmissions that could be transmitted in different zones of a single OFDM frame are OFDM multiple input multiple output (MIMO) transmissions and OFDM beam forming transmissions.

Pilot-Assisted Time-Varying OFDM Channel Estimation Based on Multiple OFDM Symbols

Abstract: In this paper, we deal with channel estimation for Orthogonal Frequency-Division Multiplexing (OFDM) systems. The channels are assumed to be Time-Varying (TV) and approximated by a Basis Expansion Model (BEM). Due to the time-variation, the resulting channel matrix in the frequency domain is no longer diagonal, but can be approximated as banded. Based on this band approximation, we propose a channel estimator that can combat both the additive noise and the out-of-band interference. Compared to our previous work, the proposed channel estimator can span multiple OFDM symbol intervals such that more time-correlation information can be explored to improve the estimation accuracy.

Adaptive quantization method and apparatus for an OFDM receiver

Abstract: A quadrature frequency division multiplexing (“OFDM”) wireless receiver, including methods and devices for adaptive quantization of OFDM signals according to modulation and coding schemes and sub-carrier frequency responses, is provided. Efficient quantization may be utilized to reduce the large dynamic range of signals to achieve circuit simplification and chip area reduction. In one embodiment, a quantization circuit includes a quantization selector to select quantization thresholds according to modulation and coding schemes and sub-carrier frequency responses, and a non-uniform quantizer to reduce input dynamic range so that an output is represented by fewer bits than an input.