Showing papers on "Orthogonal frequency-division multiplexing published in 1997"

Robust frequency and timing synchronization for OFDM

Abstract: A rapid synchronization method is presented for an orthogonal frequency-division multiplexing (OFDM) system using either a continuous transmission or a burst operation over a frequency-selective channel. The presence of a signal can be detected upon the receipt of just one training sequence of two symbols. The start of the frame and the beginning of the symbol can be found, and carrier frequency offsets of many subchannels spacings can be corrected. The algorithms operate near the Cramer-Rao lower bound for the variance of the frequency offset estimate, and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios (SNRs).

ML estimation of time and frequency offset in OFDM systems

Abstract: We present the joint maximum likelihood (ML) symbol-time and carrier-frequency offset estimator in orthogonal frequency-division multiplexing (OFDM) systems. Redundant information contained within the cyclic prefix enables this estimation without additional pilots. Simulations show that the frequency estimator may be used in a tracking mode and the time estimator in an acquisition mode.

OFDM with reduced peak-to-average power ratio by optimum combination of partial transmit sequences

Abstract: The authors propose a very effective and flexible peak power reduction scheme for orthogonal frequency division multiplexing (OFDM) with almost vanishing redundancy. This new method works with arbitrary numbers of subcarriers and unconstrained signal sets. The core of the proposal is to combine partial transmit sequences (PTS) to minimise the peak-to-average power ratio distortion.

Tree-structured generation of orthogonal spreading codes with different lengths for forward link of DS-CDMA mobile radio

Abstract: Tree-structured generation of orthogonal spreading codes with different lengths is presented for orthogonal multiplexing of forward-link code-channels of different data rates in direct sequence code division multiple access DS-CDMA mobile radio. The bit error rate performance under a multi-user environment suffering multipath Rayleigh fading is evaluated by computer simulation.

Synchronous coherent orthogonal frequency division multiplexing system, method, software and device

Abstract: A synchronous coherent orthogonal frequency division multiplexing, SC-OFDM, system, method, software and device include a plurality of SC-OFDM transmitters and a plurality of SC-OFDM receivers. Each of the plurality of SC-OFDM transmitters is synchronized to a reference where the reference is derived from a common source, for transmitting SC-OFDM signals. The plurality of SC-OFDM receivers is arranged to receive the SC-OFDM signals from at least one of the plurality of SC-OFDM transmitters. The SC-OFDM signals each have a cyclic extension, and the SC-OFDM signals from each SC-OFDM transmitter include at least one pilot code in accordance with a predetermined pilot code scheme.

Timing and frequency synchronization of OFDM signals

Abstract: A method and apparatus achieves rapid timing synchronization, carrier frequency synchronization, and sampling rate synchronization of a receiver to an orthogonal frequency division multiplexed (OFDM) signal. The method uses two OFDM training symbols to obtain full synchronization in less than two data frames. A first OFDM training symbol has only even-numbered sub-carriers, and substantially no odd-numbered sub-carriers, an arrangement that results in half-symbol symmetry. A second OFDM training symbol has even-numbered sub-carriers differentially modulated relative to those of the first OFDM training symbol by a predetermined sequence. Synchronization is achieved by computing metrics which utilize the unique properties of these two OFDM training symbols. Timing synchronization is determined by computing a timing metric which recognizes the half-symbol symmetry of the first OPDM training symbol. Carrier frequency offset estimation is performed in using the timing metric as well as a carrier frequency offset metric which peaks at the correct value of carrier frequency offset. Sampling rate offset estimation is performed by evaluating the slope of the locus of points of phase rotation due to sampling rate offset as a function of sub-carrier frequency number.

A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing

Abstract: A method of estimating the channel transfer function is presented for orthogonal frequency division multiplexing (OFDM) mobile communication systems working under time-variant radio channel conditions. The proposed method employs lowpass filtering in a transform domain so that intercarrier interference and additive white Gaussian noise components in the received pilot signals are significantly reduced. The cutoff frequency of the transform-domain filter is dynamically selected by tracking the received pilot signals. The channel transfer function for all the subcarriers is obtained by a high-resolution interpolation realized by zero-padding and DFT/IDFT. The proposed method is applicable for all linear modulation OFDM systems. It is demonstrated with a 16QAM-OFDM system which includes both amplitude and phase modulations.

Effects of clipping and filtering on the performance of OFDM

Abstract: Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication applications. However, an OFDM signal has a large peak-to-mean envelope power ratio (PMEPR), which can result in significant distortion when transmitted through a nonlinear device, such as a transmitter power amplifier. We investigate, through extensive computer simulations, the effects of clipping and filtering on the performance of OFDM, including the power spectral density, the bit error rate, and the PMEPR. Our results show that clipping and filtering is a promising technique for the transmission of OFDM signals using realistic linear amplifiers.

A comparison of peak power reduction schemes for OFDM

Abstract: Two powerful and distortionless peak power reduction schemes for orthogonal frequency division multiplexing (OFDM) are compared. One investigated technique is selected mapping (SLM) where the actual transmit signal is selected from a set of signals and the second scheme utilizes phase rotated partial transmit sequences (PTS) to construct the transmit signal. Both approaches are very flexible as they do not impose any restriction on the modulation applied in the subcarriers or on their number. They both introduce some additional system complexity but nearly vanishing redundancy to achieve markedly improved statistics of the multicarrier transmit signal. The schemes are compared by simulation results with respect to the required system complexity and transmit signal redundancy.

A novel peak power reduction scheme for OFDM

Abstract: An efficient and distortionless scheme for peak power reduction in orthogonal frequency division multiplexing (OFDM) is proposed, which introduces some additional complexity but nearly vanishing redundancy. The approach is very flexible and works with arbitrary numbers of subcarriers and without restriction on the type of modulation applied in them. The core of the approach is the coordination of appropriately phase rotated signal parts to minimize the peak power of the multiplex signal. The improved statistics of peak power in the optimized transmit signal are demonstrated by simulation results. Finally, it is shown that this scheme is close to the theoretical limit curve of redundancy versus minimum peak-to-average power ratio (PAR).

Orthogonal frequency division multiplexing system with dynamically scalable operating parameters and method thereof

Abstract: The scaleable OFDM system according to the principles of the present invention provides increased flexibility and adaptability by providing scaling of the operating parameters and/or characteristics for the OFDM system. For example, control circuitry can scale the bit rate by scaling of the OFDM symbol duration, the number of carriers and/or the number of bits per symbol per carrier. Scaleability permits the scaleable OFDM system to operate in various communications environments requiring various operating parameters and/or characteristics. By scaling the operating parameters and/or characteristics of the OFDM system when control circuitry determines that different operating parameters and/or characteristics are necessary or advantageous, the control circuitry can dynamically change the operating parameters and/or characteristics, thereby providing compatibility or the desired performance. For example, by dynamically scaling the bit rate, widely varying signal bandwidths, delay spread tolerances and signal-to-noise ratio (SNR) requirements can be achieved. As such, a scaleable OFDM system is particularly suitable for application in mobile, wireless communication devices, which support a variety of services, in a variety of environments, indoor as well as outdoor and in radio channels with differing bandwidths.

Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization

Abstract: The aim of the paper is to compare multicarrier and single carrier modulation schemes for radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. In the case of OFDM (orthogonal frequency division multiplexing), the inverse FFT transforms the complex amplitudes of the individual subcarriers at the transmitter into the time domain. At the receiver the inverse operation is carried out. In the case of single carrier modulation, the FFT and its inverse are used at the input and output of the frequency domain equalizer in the receiver. Different single carrier and multicarrier transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In the case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used.

Frame synchronization of OFDM systems in frequency selective fading channels

Abstract: This paper investigates the topic of frame synchronization for systems based on the OFDM principle. After introducing the system model we discuss the task of a frame-synchronization and analyze the effects of a frame offset. From this we derive an appropriate criterion to measure the performance of synchronization algorithms under multipath conditions. We examine two algorithms for frame synchronization: the first algorithm is based on an evaluation of periodic structures. Since it is completely independent of a frequency offset it may be applied for system acquisition and burst synchronization. If the requirements are very high as in terrestrial TV broadcasting a second algorithm is needed to attain sufficient accuracy. The algorithm presented for this purpose is based on the evaluation of the channel estimate, so no further training data is required.

Single chip VLSI implementation of a digital receiver employing orthogonal frequency division multiplexing

Abstract: The invention provides a single chip implementation of a digital receiver for multicarrier signals that are transmitted by orthogonal frequency division multiplexing. Improved channel estimation and correction circuitry are provided. The receiver has highly accurate sampling rate control and frequecy control circuitry. BCH decoding of tps data carriers is achieved with minimal resources with an arrangement that includes a small Galois field multiplier. An improved FFT window synchronization circuit is coupled to the resampling circuit for locating the boundary of the guard interval transmitted with the active frame of the signal. A real-time pipelined FFT processor is operationally associated with the FFT window synchronization circuit and operates with reduced memory requirements.

Pilot assisted channel estimation for OFDM in mobile cellular systems

Abstract: The use of pilot symbols for channel estimation introduces overhead and it is thus desirable to keep the number of pilot symbols to a minimum. The number of needed pilot symbols for a desired bit error rate and Doppler frequency is highly dependent on the pilot pattern used in orthogonal frequency division multiplexed, OFDM, systems. Five different pilot patterns are analysed by means of resulting bit error rate, which is derived from channel statistics. Rearrangement of the pilot pattern enables a reduction in the number of needed pilot symbols up to a factor 10, still retaining the same performance. The analysis is general and can be used for performance analysis and design of pilot patterns for any OFDM system.

A maximum rate loading algorithm for discrete multitone modulation systems

Abstract: A novel approach to loading for discrete multitone modulation (DMT) systems is proposed. This algorithm assigns energy to different subchannels in order to maximize the data rate for a given margin while previous algorithms proposed are aimed mainly at maximizing the margin at a target data rate. Two implementations are suggested, both with finite granularity: the first one leads to the optimal waterfilling solution, and the second results in a suboptimal but slightly less complex flat-energy distribution. The algorithm is extended to the case of a rate adaptive system with both a target guaranteed fixed data rate service and a variable one. Simulation results are presented for a Rayleigh fading channel with additive white Gaussian noise.

Peak-to-mean power control and error correction for OFDM transmission using Golay sequences and Reed-Muller codes

Abstract: A coding scheme for OFDM transmission is proposed, exploiting a previously unrecognised connection between pairs of Golay complementary sequences and second-order Reed-Muller codes. The scheme solves the notorious problem of power control in OFDM systems by maintaining a peak-to-mean envelope power ratio of at most 3 dB while allowing simple encoding and decoding at high code rates for binary, quaternary or higher-phase signalling together with good error correction.

Performance comparison of different multiple access schemes for the downlink of an OFDM communication system

Abstract: A comparison of different multiple access schemes for the downlink of an OFDM communication system is performed The access methods OFDM-TDMA, OFDM-FDMA, and OFDM-CDMA are described and compared with regard to flexibility and computation complexity Further on, the performance of the different approaches is quantitatively analysed, comparing the resulting BER for the given OFDM system in a time-variant and frequency-selective radio channel

A high efficiency carrier estimator for OFDM communications

Abstract: In OFDM communications, the loss of orthogonality due to carrier offset must be compensated before DFT-based demodulation can be performed. We present two blind carrier offset estimation algorithms by exploiting the intrinsic structure information of OFDM signals. Both algorithms offer the accuracy of super resolution subspace methods, viz., MUSIC and ESPRIT, without involving computationally intensive subspace decompositions. Adaptation rules for implementation of one of the proposed algorithm are provided. Also derived is the Cramer-Rao bound of the carrier offset estimates with which the performance of the new algorithms is evaluated.

Transmission system and method employing peak cancellation to reduce the peak-to-average power ratio

Abstract: The present invention teaches a system and method to reduce the peak-to-average power ratio of systems transmitting parallel channels. Examples of such systems are Orthogonal Frequency Division Multiplex (OFDM) and Orthogonal Code Division Multiplex (OCDM), also known as multi-code CDMA. Both systems have the disadvantage of a large peak-to-average power (PAP) ratio, which degrades the efficiency of the power amplifier of the transmitter. The present invention reduces the PAP problem by canceling the large signal peaks through subtraction of an appropriate reference function. That is, a time shifted and scaled reference function is subtracted from the sampled signal, such that each subtracted reference function reduces the peak power of at least one signal sample. In accordance with the present invention, an appropriate reference function with approximately or exactly the same bandwidth as the transmitted signal is selected. In this way it is assured that the peak power reduction does not cause any out-of-band interference. One example of a suitable reference signal for use in the present invention is a sinc function. The present invention also describes a variety of systems for implementation of the peak reduction technique. In accordance with another embodiment of the present invention, forward error correction coding is applied across several OFDM/OCDM symbols, in order to correct for symbols with low average power. In case of packet transmission, a different data scrambling is applied for each retransmission, in order to ensure that the PAP ratios and error probabilities are uncorrelated for initial packets and retransmissions.

Timing synchronization in a receiver employing orthogonal frequency division multiplexing

Abstract: A method and apparatus are disclosed for determining the boundaries of guard intervals of data symbols being received in a coded orthogonal frequency division multiplexed signal. Temporal samples separated by an interval of an active interval of a data symbol are associated in pairs, and difference signals obtained. The dispersion of a first comparison block of difference signals is determined, and compared to the dispersion of a second comparison block of difference signals, the second comparison block being displaced from the first comparison block by n samples. An F ratio is calculated for the dispersions of the two blocks. F ratios are iterated in a succession of comparison blocks, and a signal representing the F ratios subjected to peak detection. The peaks represent the boundaries of the symbol's guard interval. This information is utilized in synchronizing an FFT window for subsequent signal reconstruction.

M-sequences for OFDM peak-to-average power ratio reduction and error correction

Abstract: A block coding scheme using the maximum-length shift-register sequences (m-sequences) is suggested for orthogonal frequency division multiplexing (OFDM). The scheme, with a very simple implementation, can significantly reduce the peak-to-average power ratio of OFDM signals and provide error-correcting capability at the same time.

DVB-T: the COFDM-based system for terrestrial television

Abstract: In early 1996, the European DVB (Digital Video Broadcasting) Project finalised what is now called the 'common 2k/8k specification' for terrestrial television transmission. This specification is an important cornerstone of digital video broadcasting in Europe and numerous other countries in the world. Called DVB-T, it is based on the channel-coding algorithms the DVB Project had earlier devised for digital television on cable and satellite. The modulation scheme that is used for DVB-T is OFDM (orthogonal frequency division multiplexing), which when used in conjunction with channel coding is known as COFDM. Two variants are described, one employing 1705 individual carriers in an 8 MHz channel ('2k variant'), the other employing 6817 carriers ('8k variant'). Each individual carrier can be modulated either by QPSK, 16-QAM, 64-QAM or hierarchical modulation.

Multi-carrier spread-spectrum

Abstract: I. General Issues of Multi-Carrier Spread-Spectrum. Multi-Carrier Spread-Spectrum: An Attractive Special Case of General Multiuser/Multisubchannel Transmission Methods J. Lindner Overview of the Results About Multi-Tone CDMA Detection L. Vandendorpe. Comparison of Multiple Access Schemes for an OFDM Downlink System H. Rohling, et al. Link/System Level Performance of an OFDM/CDMA Downlink Configuration J.P. Castro, et al. II. Applications of Multi-Carrier Spread-Spectrum. Performance Analysis of a New Multi-Code and Multi-Carrier Hybrid Transmission Scheme for Future Broadband Mobile Communication Systems H. Harada, R. Prasad. A Spread-Spectrum Multi-Carrier Multiple-Access System for Mobile Communications S. Kaiser, K. Fazel. Orthogonal Frequency-Division Multiple-Access with Frequency Hopping and Diversity H. Sari. M-ary Orthogonal Modulation for MC-CDMA Systems in Indoor Wireless Radio Networks A. Dekorsy, K.-D. Kammeyer. Comparison of WLAN Multi-Carrier DS-SS Physical Layer Configurations Measured Indoor Environment P. Banelli, et al. III. Coding and Detection for MC-SS. Multi-Carrier CDMA Using Convolutional Coding and Interference Cancellation over Fading Channels J.J. Maxey, R.F. Ormondroyd. An Approach for a Multi-Carrier Spread-Spectrum System with RAKE Receiver A. Nahler, G.P. Fettweis. Multi-Carrier Modulated Orthogonal Code-Division Multiple-Access: D.Th. Magill. Interleaved FDMA - A New Spread-Spectrum Multiple-Access Scheme U. Sorger, et al. Aspects on Wideband Multi-Carrier Communication J.E.M. Nilsson. Equalization and Coding for Extended MC-CDMA over Time and Frequency Selective Channels J. Egle, et al. Detection Method for MC-CDMA Based on a Recurrent Neural Network Structure W.G. Teich, et al. IV. Synchronisation and Channel Estimation. Sensitivity of OFDM/CDMA to Carrier Phase Jitter H. Steendam, M. Moeneclaey. Time Domain Uplink Channel Estimation in Multi-Carrier CDMA Mobile Radio System Concepts B. Steiner. Subspace-Based Joint Time-Delay and Frequency-Shift Estimation in Multi-Tone Code Division Multiple Access (MT-CDMA) Systems M. Eric, et al. Pilot-Symbol-Aided Channel Estimation in Time and Frequency P. Hoeher, et al. A Family of Extended Gaussian Functions with a Nearly Optimal Localization Property Ch. Roche, P. Siohan. On the Duality of Multi-Carrier Spread-Spectrum and Single-Carrier Transmission K. Brueninghaus, H. Rohling. Mismatched Multi-Carrier Complementary Spread-Spectrum Radar and Sonar Systems I.S. Simic, et al. V. MC-SS for the Uplink of a Cellular System. An OFDM/SFH-CDMA Transmission Scheme for the Uplink L. Tomba, W. Krzymien. Uplink Spectral Efficiency of Multi-Carrier Joint Detection Code-Division Multiple-Access Based Cellular Radio Systems F. Berens, et al. Simulation of a DSSS/MCM System in a Doppler Spread Channel T.B. Welch, R.E. Ziemer. On the Performance of Asynchronous Multi-Carrier CDMA H. Ochiai, H. Imai. Performance Comparison B.

An analysis of two-dimensional pilot-symbol assisted modulation for OFDM

Abstract: In this paper we analyze two-dimensional (2D) pilot-symbol assisted modulation (PSAM) for wireless orthogonal frequency-division multiplexing (OFDM). 2D-PSAM has been suggested for wireless OFDM by several authors and is included in a preliminary draft of the standard for European digital video broadcast. We generalize the analysis of single-carrier PSAM to the 2D time-frequency lattice of OFDM. We analyze 2D-PSAM for two different channel estimators: one with good performance and high complexity and one suboptimal with low complexity. We verify that a good rule of thumb is to place the pilots at least as twice as close, in time and frequency, as required by the 2D sampling theorem.

An adaptive data predistorter for compensation of nonlinear distortion in OFDM systems

Abstract: In this paper, an adaptive data predistortion technique which can compensate for the nonlinear effect of a TWT high-power amplifier (HPA) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. It is shown that the convergence time and MSE of an adaptive data predistorter can be significantly reduced by using a broadcasting technique and by designing appropriate training signals.

OFDM burst frequency synchronization by single carrier training data

Abstract: We propose a burst frequency synchronization procedure which is based on the usage of single-carrier training data and orthogonal frequency division multiplexing (OFDM) payload modulation. The payload modulation format is similar to the one that is used in the DAB standard, whereas training data are chosen as simple CAZAC sequences. It is shown that the resulting modulation and transmission scheme is suitable for burst transmission and single-burst demodulation. Performance degradation due to synchronization errors is shown to be small.

A New Symbol Timing Recovery Algorithm For OFDM Systems

Abstract: A new, patent-pending, symbol timing recovery algorithm for OFDM based systems is described for which supplementary pilot signals are not required. The proposed algorithm is composed of a coarse timing acquisition stage which does not require prior carrier synchronization and a fine symbol time tracking stage which requires (fine) carrier frequency synchronization so that the residual frequency offset is a multiple of the subcarrier spacing. However, the results of the coarse timing acquisition provides information required for the fine carrier frequency synchronization thus making the algorithm self-contained. Simulation results show that the proposed symbol timing recovery algorithm gives satisfactory performance for E/sub b//N/sub 0/ as low as 0dB.

Analysis of intertone and interblock interference in OFDM when the length of the cyclic prefix is shorter than the length of the impulse response of the channel

Abstract: Orthogonal frequency division multiplexing (OFDM) is a modulation technique that resists the effects of a multipath channel when provided an ample cyclic prefix. No intersymbol interference is caused in channels whose impulse responses are shorter than the cyclic prefix length. However interference arises when this is not the case. This paper describes the degradation of the system under this scenario. In particular, simulations describing a suburban hilly environment are done for several cyclic prefix lengths.

Minimization of the Intermodulation Distortion of a Nonlinearly Amplified OFDM Signal

Abstract: In the course of development of the HIPERLAN-Standard OFDM (Orthogonal Frequency Division Multiplexing) was a possible candidate for the modulation scheme. An advantage of OFDM is that it can be implemented simply and that it is suitable for the transmission of high data rates in the mobile radio channel. Furthermore, it is insensitive against disturbances caused by multipath propagation. However, the OFDM signal shows a nonconstant envelope that leads to unwanted out-of-band radiation, in case the transmitter amplifier has a nonlinear characteristic. In this paper a method for the reduction of the fluctuation of the envelope of an OFDM signal is presented. Combined with a predistortion to linearize the characteristic of the transmitter amplifier an efficient method for the reduction of the out-of-band radiation results so that OFDM could prove to be a suitable modulation scheme for future mobile telecommunication applications.