Showing papers on "Frequency-division multiplexing published in 2000"

Peak-to-average power ratio reduction of an OFDM signal using partial transmit sequences

Abstract: Orthogonal frequency-division multiplexing (OFDM) is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) has limited its application. Recently, two promising techniques for improving the PAP statistics of an OFDM signal have been proposed: the selective mapping and partial transmit sequence approaches. Here, we present suboptimal strategies for combining partial transmit sequences that achieve similar performance but with reduced complexity.

Prolog to adaptive multicarrier modulation: a convenient framework for time-frequency processing in wireless communications

Abstract: A historical perspective of orthogonal frequency-division multiplexing (OFDM) is given with reference to its literature. Its advantages and disadvantages are reviewed, and its performance is characterized over highly dispersive channels. The effects of both time- and frequency-domain synchronization errors are quantified, and a range of solutions proposed in the recent literature are reviewed. One of the main objectives of this review is to highlight the recent thinking behind adaptive bit allocation and turbo coding in the context of OFDM. This paper concludes with a wide-ranging throughput comparison of the schemes discussed herein under the unified constraint of a fixed target bit error rate of 10/sup -4/.

Pilot-symbol-aided channel estimation for OFDM in wireless systems

Abstract: In this paper, we investigate pilot-symbol-aided parameter estimation for orthogonal frequency division multiplexing (OFDM) systems. We first derive a minimum mean-square error (MMSE) pilot-symbol-aided parameter estimator. Then, we discuss a robust implementation of the pilot-symbol-aided estimator that is insensitive to channel statistics. From the simulation results, the required signal-to-noise ratios (SNRs) for a 10% word error rate (WER) are 6.8 dB and 7.3 dB for the typical urban (TU) channels with 40 Hz and 200 Hz Doppler frequencies, respectively, and they are 8 dB and 8.3 dB for the hilly-terrain (HT) channels with 40 Hz and 200 Hz Doppler frequencies, respectively. Compared with the decision-directed parameter estimator, the pilot-symbol-aided estimator is highly robust to Doppler frequency for dispersive fading channels with noise impairment even though it has some performance degradation for systems with lower Doppler frequencies.

On timing offset estimation for OFDM systems

Abstract: Two timing offset estimation methods for orthogonal frequency division multiplexing (OFDM) systems as modifications to Schmidl and Cox's method (see IEEE Trans. Commun., vol.45, p.1613-21, 1997) are presented. The performances of the timing offset estimators in additive white Gaussian noise channel and intersymbol interference channel are compared in terms of estimator variance obtained by simulation. Both proposed methods have significantly smaller estimator variance in both channel conditions.

OFDM blind carrier offset estimation: ESPRIT

Abstract: In orthogonal frequency-division multiplex (OFDM) communications, the loss of orthogonality due to the carrier-frequency offset must be compensated before discrete Fourier transform-based demodulation can be performed. This paper proposes a new carrier offset estimation technique for OFDM communications over a frequency-selective fading channel. We exploit the intrinsic structure information of OFDM signals to derive a carrier offset estimator that offers the accuracy of a super resolution subspace method, ESPRIT.

Adaptive modulation techniques for duplex OFDM transmission

Abstract: The design tradeoffs of turbo-coded burst-by-burst adaptive orthogonal frequency division multiplex (OFDM) wideband transceivers are analyzed. We demonstrate that upon aiming for a higher throughput a higher proportion of low-quality OFDM subcarriers has to be used for the transmission of inherently vulnerable high-order modem modes, transmitting several bits per subcarrier. Upon invoking turbo coding and adjusting the modem mode switching regime near-error-free performance can be achieved at the cost of a reduced throughput. Various blind modem mode detection techniques have also been investigated and the most complex channel coding trellis-based detection algorithm was found to be the most powerful. Last, the design tradeoffs of spectral pre-equalization have been explored and quantified. We concluded that AOFDM provides a convenient framework for adjusting the required target integrity and throughput both with and without turbo channel coding.

Multifrequency complementary phase-coded radar signal

Abstract: A multifrequency radar signal is considered It uses M subcarriers simultaneously The subcarriers are phase modulated by M different sequences that constitute a complementary set Such a set can be constructed, for example, from the M cyclic shifts of a perfect phase-coded sequence of length M (eg P4) The subcarriers are separated by the inverse of the duration of a phase element t/sub b/, yielding orthogonal frequency division multiplexing (OFDM), well known in communications A single pulse of such a signal exhibits a thumbtack ambiguity function with delay resolution of t/sub b//M The power spectrum is relatively flat, with width of M/t/sub b/ The signal can be constructed by power combining M fixed-amplitude signals The resulting signal, however, is of variable amplitude The peak-to-mean envelope power ratio can be maintained below 2 A train of complementary pulses and a weight function along the frequency axis are useful for further sidelobe reduction

Space-time code design in OFDM systems

Abstract: We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system in frequency-selective fading channels. By analyzing the pairwise error probability (PEP), we show that STC-OFDM systems can potentially provide a diversity order as the product of the number of transmitter antennas, the number of receiver antennas and the frequency selectivity order, and that the large effective length and the ideal interleaving are two most important principles in designing STCs for OFDM systems. Following these principles, we propose a new class of trellis-structured STCs. Compared with the conventional space-time trellis codes, our proposed STC's significantly improve the performance by efficiently exploiting both the spatial diversity and the frequency-selective-fading diversity.

Carrier frequency offset compensation for uplink of OFDM-FDMA systems

Abstract: A frequency offset compensation method for OFDM-FDMA, that can correct offsets after the DFT via circular convolution, is proposed as an alternative to the direct method that corrects frequency offsets by multiplying the complex exponents of the offset estimates before the DFT. In contrast to the direct method whose complexity increases proportional to the number of users, the computational load of the proposed scheme decreases as the number of users increases. It is shown that the proposed method is simpler to implement than the direct method when the number of users is greater than two. Furthermore, the former can outperform the latter, because a frequency offset compensation for one user after the DFT does not affect the data of other users. Computer simulation results demonstrate the advantage of the proposed method.

Frequency ambiguity resolution in OFDM systems

Abstract: In orthogonal frequency division multiplexing systems, the carrier-frequency offset can be divided into two parts: (1) an integer one-multiple of the subcarrier spacing 1/T and (2) a fractional one-less than 1/2T in amplitude. Some schemes proposed in the literature can only recover the fractional part. We derive two algorithms for estimating the integer part. They are based on the observation of two consecutive OFDM symbols. The first algorithm exploits pilot symbols multiplexed with the data, the other is blind.

Frequency-offset synchronization and channel estimation for OFDM-based transmission

Abstract: This letter is concerned with a frequency offset estimation technique for OFDM based transmission systems. The frequency estimation technique utilizes a repetitive signal structure inside of a OFDM symbol which is used to enlarge the range and increase the accuracy of offset estimation. Also, an averaged decision-directed channel estimation (ADDCE) technique suitable for burst data is proposed.

A subspace method for blind channel identification in OFDM systems

Abstract: It has been shown that cyclostationarity in the received signal allows the receiver to blindly identify the channel impulse response using only second-order statistics. In orthogonal frequency-division multiplexing (OFDM) systems, cyclostationarity is embedded at the transmitter due to cyclic prefix. In this paper, a subspace approach based on second-order statistics is proposed for blind channel identification in OFDM systems. We derive a sufficient condition that guarantees all the channels to be identifiable no matter what their zero locations are. Computer simulations demonstrate the superior performance of the proposed algorithm over methods reported earlier in the literature.

An efficient channel estimation technique for OFDM systems with transmitter diversity

Abstract: We propose an efficient channel estimation technique for orthogonal frequency-division multiplexing (OFDM) systems with transmitter diversity. The proposed technique estimates uniquely all channel frequency responses needed in space-time coded OFDM systems using "comb-type" training symbols. The computational complexity of the proposed technique is reduced dramatically, compared with the previous minimum mean-squared error (MMSE) technique, due to the all the processing being done in the frequency-domain. Also, several other techniques for mitigating the random noise effect and tracking channel variation are discussed to further improve the performance of the proposed approach. The performances of the proposed approach are demonstrated by computer simulation for mobile wireless channels.

Time-variant distortions in OFDM

Abstract: We analyze the degradation of orthogonal frequency division multiplexing (OFDM) signals from time-variant distortions like carrier and sampling frequency offset, Doppler spread and oscillator phase noise. We propose a novel analytical framework in the discrete-time domain based on the FFT demodulation of a single OFDM symbol and unify previous methods with some new results. Exact definitions of the signal-to-subcarrier-interference ratio (SIR) and the effective signal-to-noise ratio (SNR) are given and evaluated for a HIPERLAN/2 system.

Transmitter redundancy for blind estimation and equalization of time- and frequency-selective channels

Abstract: Joint mitigation of time- and frequency-selective fading is an important and challenging problem in mobile communications. Relying on transmitter-induced redundancy, we propose novel channel estimation and symbol recovery approaches for blind identification and equalization of time- and frequency-selective channels, where the time variation is modeled deterministically by a basis expansion. The resulting statistical algorithm enables the usage of a single antenna, dispenses with channel disparity conditions of existing approaches, and allows channel order overestimation. In addition, new deterministic algorithms for generalized OFDM systems are introduced that produce reliable estimates with few data points at high SNR's. Simulations illustrate the approaches developed.

Multiplexing of signals using superframes

Abstract: In this paper we intend to present the concept of superframes and its use, primarily, in multiplexing techniques. The signal are supposed band limited and three multiplexing schemes are considered: time division multiple access, frequency division multiple access and frequency hoping multiple access (FHMA). The first two schemes give rise to tight superframes, whereas for FHMA, the associated superframes are more complex. For some such superframes the dual superframe is obtained in closed form. An example of a FHMA scheme is also presented.

Making use of a two stage multiplexing scheme in a WDM network

Abstract: The paper explores the implications of using a two stage optical multiplexing scheme in optical WDM nodes. This allows to keep bands of adjacent wavelengths intact, thereby reducing the cost of multiplexing, increasing the number of hops and simplifying network management.

Optical communications networks utilizing frequency division multiplexing

Abstract: A frenquency division multiplexing (FDM) node used in optical communications networks provides add-drop multiplexing (ADM) functionality between optical high-speed channels and electrical low-speed channels. The FDM node includes a high-speed system and an ADM crosspoint. The high-speed system converts between an optical high-speed channel and its constituent electrical low-speed channels through the use of frequency division multiplexing and preferably also QAM modulation. The ADM crosspoint couples incoming low-speed channels to outgoing low-speed channels, thus implementing the ADM functionality for the FDM node.

A systematic approach to detecting OFDM signals in a fading channel

Abstract: We derive the maximum a posteriori probability (MAP) receiver for orthogonal frequency-division multiplexed signals in a fading channel. As the complexity of the MAP receiver is high, we obtain a low-complexity, suboptimal receiver and evaluate its performance.

A spreading sequence allocation procedure for MC-CDMA transmission systems

Abstract: A novel spreading sequence allocation procedure for multi-carrier code division multiple access (MC-CDMA) systems is proposed and investigated. This new technique, which relies on an analytical evaluation of the multiple access interference (MAI) mitigates the interference between different users by optimizing the spreading sequence selection within a given spreading sequence family. For low-loaded transmissions over different realistic frequency correlated channels, it is shown that this new selection procedure outperforms systems where no specific allocation rule is employed. Furthermore, this technique affects the emitted signals and its performance improvement is observed whatever the detection technique used at the receiver side. Thus, this technique improves the capacity of MC-CDMA systems.

Modeling of transoceanic fiber-optic WDM communication systems

Abstract: The rapid growth of long-haul wavelength-division multiplexing (WDM) fiber-optic telecommunications challenges lightwave system designers to increase the transmission capacity per fiber pair, while reducing the time to market. This makes it essential to have fast and accurate computer modeling tools to aid the systems design. There is a natural engineering tradeoff between simulation speed and accuracy; fast approximations tend to be inaccurate, while the exact treatment of the physical processes affecting transmission are nearly impossible to capture in a time-efficient algorithm. The proof of a successful tool development lies in the comparison of simulation results to transmission system measurements. In this paper, we discuss a new simulation technique based on careful evaluation of the key physical effects that produce system impairments. We show that this new approach can produce fast and accurate simulations of long-haul WDM transmission systems.

OFDM system channel estimation using time-domain training sequence for mobile reception of digital terrestrial broadcasting

Abstract: A channel estimation technique using time-domain training sequence is investigated. It is demonstrated that this technique can estimate the channel status more accurately, under low C/N situations, than the conventional frequency domain technique using in-band pilots. A novel channel estimation and compensation scheme for a fast fading channel is developed for an OFDM system using time-domain training sequence. It is compared with the frequency domain pilot channel estimation scheme.

Adaptive co-channel interference (CCI) cancellation for OFDM communication systems

Abstract: This paper proposes a new adaptive co-channel interference (CCI) canceller for an orthogonal frequency division multiplexing (OFDM) scheme. It employs a parametric CCI cancellation scheme, least-mean-squares (LMS) maximum-likelihood estimation (MLE), which simultaneously estimates time-varying channels and transmitted symbols both for desired and CCI signals. Computer simulations were performed on interference canceling characteristics of the proposed CCI canceller both under static and Rayleigh fading conditions, wherein a single CCI is considered. Computer simulations confirm that the proposed canceller attains a BER of 1.5/spl times/10/sup -2/ at a signal-to-interference ratio of -10 dB in Rayleigh fading condition with the maximum Doppler frequency of 52 Hz and average E/sub b//N/sub 0/ of 30 dB.

Fast burst synchronisation technique for OFDM-WLAN systems

Abstract: An MMSE (minimum mean square error) based joint timing and frequency synchronisation algorithm utilising a simplified frame structure is proposed for fast burst synchronisation in orthogonal frequency division multiplexing (OFDM) systems. A single synchronisation symbol is exploited in the time domain to give, first, an estimate of coarse frame starting position and fine frequency offset, and later, in the frequency domain, the multiple of subcarrier spacing frequency offset using a reference symbol power adjustment (RPA) technique. The power of the start symbol, which also acts as a reference symbol in differential modulation, is varied linearly to determine the multiple subcarrier frequency offset. The performance of the proposed frame is confirmed by simulation for a differential quaternary phase shift keying (DQPSK) system in a multipath fading environment.

On the achievable information rate with peak-power-limited orthogonal frequency-division multiplexing

Abstract: This paper addresses the problem of determining the statistical distribution of the peak-to-average power ratio (PAR) of orthogonal frequency-division multiplexing (OFDM) transmit signals. In practice, transmission systems are amplitude-limited, thus the large peak values of OFDM signals are clipped and nonlinear distortion follows. In order to avoid the resulting degradations, i.e., spectral widening and increased bit-error rate, a systematic construction of OFDM signals without large peaks is desired. The paper provides new results on the PAR distribution and hence on the achievable information rate with OFDM signals that are peak-limited. In particular, it is shown that in the case of phase-shift-keying (PSK) subcarrier modulation, the frequently applied model of Gaussian data coefficients is not appropriate. By solving an integral equation, new analytic expressions for the PAR and the achievable information rate are derived. Comparison with simulation results confirms that the new model is accurate for all ranges of practical interest.

High speed wireless OFDM transceiver modem

Abstract: An apparatus for receiving and/or transmitting signals using orthogonal frequency division multiplexing, said apparatus being adapted for outputting a first output signal, comprising: a time synchronisation circuit, being adapted for determination of control information from a first signal, said first signal comprising at least of a first part being a non-orthogonal frequency division multiplexing signal and a second part being an orthogonal frequency division multiplexing signal, said determination of control information exploiting said first part of said first signal; said time synchronisation circuit, further being adapted for converting said second part of said first signal into a second signal, being in time domain representation; a transformation circuit at least converting said second signal into a third signal, being in frequency domain representation; a first frequency domain circuit, at least converting said third signal into said first output signal; and said second signal and said second part of said first signal being orthogonal frequency division multiplexing signals.

Frequency division multiplexed wireline communication for ultrasound probe

Abstract: Received echo data are transferred from an ultrasound probe to the signal processing circuitry of an ultrasound imaging system by use of a reduced number of multiple coaxial cables. The system operates by frequency division multiplexing of the analog output signals produced by multiple individual array elements in the probe onto a single coaxial cable, which connects the probe to the central host computer or console for conventional signal processing and imaging.

System and method for performing soft handoff between frequency division duplex and time division duplex communication systems

Abstract: A system and method for controlling information transmission and communication handoff between frequency division multiplexing (FDD) and time division multiplexing (TDD) communication systems using any multiple access scheme is provided. The mobile unit transmits information via a current one of the FDD and TDD communication systems while it moves toward the other, or targeted one of the FDD and TDD communication systems. A pilot search signal is generated by the targeted communication system, where the pilot search signal corresponds to a transmission range of the targeted communication system. A communication handoff is initiated from the current communication system to the targeted communication system when the mobile unit recognizes a predetermined threshold level of the pilot search signal. Concurrent communications are synchronized and temporarily maintained between the mobile unit and both the current and targeted communications systems using an increased data rate for each of the communications links. When the handoff is complete, transmission between the mobile unit and the current communication system is terminated, while communication between the mobile unit and the targeted communication system is maintained when the handoff is complete.