Showing papers on "Frequency-division multiplexing published in 1996"

OFDM channel estimation by singular value decomposition

Abstract: We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

A new loading algorithm for discrete multitone transmission

Abstract: A new loading algorithm for discrete multitone transmission is proposed. Thereby rate is not distributed according to channel capacity, but rate and transmit power are assigned to maximize the signal-to-noise ratio in each carrier. Because closed form expressions can be derived the algorithm is of very low complexity, even lower than the loading algorithm recently proposed by Chow et al. (see IEEE Transactions on Communications, no.2/3/4, p.773-5, 1995). Nevertheless achievable performance is higher or at least the same. Results for a typical high rate transmission over twisted pair lines are presented.

Adaptive OFDM for wideband radio channels

Abstract: An OFDM (orthogonal frequency division multiplexing) transmission system is simulated with time-variant transfer functions measured with a wideband channel sounder. The individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used. The simulations show that with adaptive OFDM, the required signal power for an error probability of 10/sup -3/ can be reduced by 5...15 dB compared with fixed OFDM. The fraction of channel capacity which can be achieved with adaptive OFDM depends on the average signal-to-noise ratio and the propagation scenario.

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

Abstract: This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

OFDM codes for peak-to-average power reduction and error correction

Abstract: Orthogonal frequency division multiplexing (OFDM) is a promising way to provide large data rates at reasonable complexity in wireless fading channels. However, a major disadvantage of OFDM is its large peak-to-average power ratio, which significantly decreases the efficiency of the transmitter power amplifier and hence forms a major obstacle to implementing OFDM in portable communication systems. This paper shows the possibility of using complementary codes for both decreasing the peak-to-average power (PAP) ratio and error correction. Set sizes and minimum distance properties of these codes are derived. It is shown that specific subsets of complementary codes have a minimum distance of up to half the code length, while their PAP ratio is only 3 dB. Simulation results demonstrate the viability of using these codes in multipath fading channels. It is currently planned to implement OFDM with complementary codes in the Wireless ATM Network Demonstrator (WAND), a joint European ACTS program.

Carrier frequency acquisition and tracking for OFDM systems

Abstract: We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

Low-overhead, low-complexity [burst] synchronization for OFDM

Abstract: A rapid synchronization method is presented for an OFDM system using either a continuous transmission or a burst operation over a time-varying, fading channel. It will acquire the signal and provide channel estimation upon the receipt of just one training sequence of two symbols in the presence of unknown symbol and frame timing, large carrier and sampling frequency offsets, and very low SNRs, while maintaining low latency and low complexity. It can then track the signal with the same algorithms. Digital TV and wireless LAN are used as examples.

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

Abstract: The performance of two different channel interpolation methods to be used with orthogonal frequency division multiplexing systems are investigated. The considered schemes use constant pilot frequencies for channel response estimation. The interpolation techniques are piecewise-constant and piecewise-linear methods which due to their inherent simplicity are straightforward to implement. The results are given as error probability vs. pilot separation for a channel with exponential type power-delay profile and M-ary quadrature amplitude (MQAM) submodulation with M=4, 16 and 64.

Clustered OFDM with transmitter diversity and coding

Abstract: Multipath delay spread in a radio environment can severely limit the maximum transmission rate. Multicarrier transmission, in particular OFDM, and a single-carrier system with equalization, often proposed as techniques for overcoming these limitations, present practical difficulties which can restrict their application. In a wireless LAN/ATM application, the desire to transmit short packets requires fast start-up, so the potentially long time required to train an equalizer could limit its usefulness. On the other hand, while OFDM requires very little training, it is burdened with a large peak-to-average power ratio which requires the use of highly linear amplifiers. In this paper, we describe a new approach to OFDM where subchannels are clustered into several smaller blocks and transmitted over separate (ideally, independent) antennas. A single receive antenna is used to demodulate the entire OFDM signal. This approach reduces the peak-to-average power ratio, minimizes the receiver training required, and increases the effectiveness of coding across frequencies.

Orthogonal frequency division multiplex synchronisation techniques for wireless local area networks

Abstract: A range of frequency acquisition, frequency tracking, symbol synchronisation and frame synchronisation techniques are proposed and evaluated for employment in 155 Mbit/s, 60 GHz local area networks. The algorithms are based on two correlation functions, exhibit a moderate implementational complexity and high robustness against channel noise.

Performance of an OFDM-TDMA mobile communication system

Abstract: The multicarrier transmission technique (OFDM) is considered in combination with a TDMA/TDD multiple access scheme for a cellular mobile communication system. Performance analysis is presented for the uncoded and coded (TCM) case. It is shown that OFDM systems have a large potential concerning the flexible allocation of the total given bandwidth to different users. This flexibility can be exploited to adapt to the impairments of the radio channel, thus increasing the overall performance. Two methods, namely (1) a simple principle to leave out weak subcarriers, and (2) the individual modulation of each subcarrier (adaptive modulation) are studied (for the up-/downlink case in a frequency selective and time-variant radio channel), and results with respect to throughput and BER performance are discussed.

An optical FDM-based self-healing ring network employing arrayed waveguide grating filters and EDFA's with level equalizers

Abstract: An optical frequency-division multiplexing (OFDM) based self-healing unidirectional ring network is designed and its performance is verified. It employs arrayed waveguide grating add/drop multiplexers (ADMs) and erbium-doped fiber amplifiers (EDFAs) with waveguide level equalizers in each remote node. The route diversity configuration is employed to make the network robust and bit loss-free route switching is performed. The level diagram reproducibility for each OFDM channel is guaranteed by the waveguide-level equalizers. Successful transmission performance confirms the effectiveness of the proposed system.

Performance trade-offs with quasi-linearly amplified OFDM through a two-branch combining technique

Abstract: It is reasonable to expect that the emerging integrated broadband communication technologies, initially intended for optical fibre or cable transmission, will also be available for wireless transmission, within high bit rate radio local area networks. It is currently recognized that multicarrier modulation schemes, also known as OFDM schemes (orthogonal frequency division multiplexing), offer many advantages for signalling at high data rates over time dispersive channels. However, they exhibit strong envelope fluctuations, which implies power amplification difficulties. We evaluate performance trade-offs with a two-branch combining technique for "quasi-linear" power amplification of OFDM signals.

Construction of a multicarrier signal

Abstract: A multicarrier signal to be transmitted to digital receivers, particularly in a non-stationary transmission channel, corresponding to the frequency multiplexing of a plurality of elementary carriers each corresponding to a series of symbols is described, wherein two consecutive symbols are separated by a symbol time τ0. The spacing ξ0 between two adjacent carriers in the signal is equal to half the reciprocal of the symbol time τ0 and each carrier is subjected to spectrum formatting filtering with a bandwidth which must exceed twice the intercarrier spacing ξ0, and selected so that each symbol is highly concentrated in the time and frequency domain. Methods for transmitting and receiving such a signal are also described.

CATV network for transport of radio frequency signals

Abstract: A communication system is coupled to a cable television network wherein cable television signals, data signals and telephonic signals are transmitted on the coaxial cable infrastructure. To facilitate the simultaneous transmission of cable signals, data signals and telephonic signals, a frequency spectrum is defined for a coaxial cable network in which a band of frequencies for carrying telephonic signals is interposed in the cable television signals bands of frequencies. In a coaxial cable system capable of carrying signals up to 1 megahertz in frequency, a total of 5 frequency bands are defined. Within the frequency band for carrying telephonic signals, signals from a PCS system or the like as well as signals, from other telecommunication devices, one system, such as SMS centers, base stations and mobile switching centers may be transmitted. Additionally, television channels may also be transmitted in this frequency band if the beginning of the frequency is at a frequency channel for carrying a cable television channel. A method includes transmitting the various types of signals within the specified frequency bands.

Effect of carrier phase noise and frequency offset on the performance of multicarrier CDMA systems

Abstract: Kaiser (1995) showed that OFDM-CDMA outperforms DS-CDMA in radio fading channels in terms of spectral efficiency, under the assumption of ideal carrier recovery and chip synchronization. However, residual carrier frequency offset and phase noise degrade the system performance significantly if suitable countermeasures are not taken. In this work we consider the downlink of a cellular multicarrier CDMA system. In particular, we evaluate the impact of the carrier frequency offset and Wiener phase noise on the system performance considering typical indoor radio channels. Moreover, a frequency detector is used to reduce the impairment produced by the carrier frequency offset. Results are given in terms of BER obtained by means of simulations and analytical computations.

An application of MMSE predistortion to OFDM systems

Abstract: Orthogonal frequency division multiplexing (OFDM) particularly suffers from the presence of nonlinearities since the signal amplitude is Rayleigh distributed. The degradation introduced by the nonlinear amplifier in the transmitter can be significantly reduced by using an analog cubic predistorter minimum mean square error (MMSE) predistorter proposed by the authors (see ibid., vol.43, no.12, p.2966, 1995).

Operation and maintenance for an all-optical transport network

Abstract: For the enhancement of the existing electrical transport network a new one based on optical frequency division multiplexing (or wavelength division multiplexing) will be added to the existing one. This article focuses on the system/network supervision and operation and maintenance, which are of great importance for the success of the optical transport network. For a better understanding of these topics, it is necessary to derive a layered network model for the optical transport network and to make a workable definition of the term "network transparency." The proposed operation and maintenance concept deals with the identification of its related functions, the supervision of the optical regenerators, the maintenance signals required for failure localization, and the difficulty of defining a suitable mechanism for performance monitoring in transparent networks. Appropriate realization aspects are discussed too. Possible solutions for failure detectors are described. For transferring the individual maintenance signals between the associated network elements, a communication channel with a suitable structure is proposed.

On the sensitivity of multicarrier transmission over multipath channels to phase noise and frequency offset

Abstract: The sensitivity of multicarrier (MC) transmission over a multipath channel, to the phase impairment is addressed. The degradation of the performance of the system with the BPSK and QPSK signaling due to the variance of phase error, carrier frequency offset, delay spread of the multipath channel as well as the number of carriers is investigated. The results can be used in the design and evaluation of realistic nonideal multicarrier systems for high speed communications over multipath mobile/indoor radio channels.

Method and circuit arrangement for improving carrier separation for the transmission of ofdm signals

Abstract: In the OFDM methods, a plurality of modulated carriers are transmitted by frequency multiplexing, which results in an almost rectangular spectrum owing to the high number of carriers. In order to separate the carriers from one another again in the receiver, a fast Fourier transform is carried out such that each carrier can be cleanly separated from the others when the carriers are exactly orthogonal relative to one another. However, the orthogonality of the carriers can be disturbed by various causes. Furthermore, in the receiver the useful signal has to be separated from the undesired neighbouring channel signals by analog or digital filtration. In order to improve carrier and channel separation, the selectivity of FFT filtration can be increased by increasing the number of FFT components. However, this normally leads to an undesirably high increase in computing steps. The claimed configuration of the time window used within the context of the FTT and oversampling before the FFT render the calculation of some of the coefficients superfluous.

OFDM in bandpass nonlinearity

Abstract: The behavior of orthogonal frequency division multiplexing (OFDM) signals in bandpass nonlinearity is presented. In particular, in-band bit-error-rate (BER) degradation and induced adjacent channel interference, as a result of amplitude limiting or clipping, are analyzed. In the presence of both nonlinear distortion and additive Gaussian noise, optimized output power back-off is provided to balance the requirements of the minimum BER and the tolerance of adjacent channel interference for a given OFDM system.

The effect of carrier frequency offset on the performance of band limited single carrier and OFDM signals

Abstract: In this contribution, the bit error rate (BER) degradation caused by a carrier frequency offset between the transmitter and receiver carrier oscillator is investigated for single carrier (SC) and multi carrier (OFDM) signaling. For both modulation formats a carrier frequency offset results in a rotation and an attenuation of the transmitted symbols. In addition, this impairment gives rise to inter-symbol interference (ISI) and destroys the orthogonality between the carriers of the multi carrier signal which yields inter-carrier interference (ICI). The analysis shows that for SC transmission the BER degradation is dominated by the reduction of the useful signal component, provided that the carrier frequency offset is small with respect to the SC symbol rate. For OFDM transmission, a comparison of the different interferences reveals that the BER degradation is determined by the ICI.

Integrated-optic versus microoptic devices for fiber-optic telecommunication systems: a comparison

Abstract: We assess the relative merits and prospects of integrated-optics in comparison with microoptic and fiber-based techniques. Firstly, the market for fiber-optic components for telecommunications is analyzed. Secondly, the technological issues which make integrated optics quite different from integrated electronics will be discussed. Thirdly, a specific comparison between the potential of the competing optical technologies will be made for the polarization-diversity hybrid and the optical wavelength (de)multiplexer, two optical devices used in networks that are based on optical frequency division multiplexing (OFDM).

Clustered-OFDM transmitter implementation

Abstract: An elegant means by which high-speed burst wireless transmission can be accomplished with small amounts of overhead is through a novel technique referred to as clustered-OFDM. By using OFDM modulation with a long symbol interval, this scheme overcomes the complex and costly equalization requirements associated with single carrier systems. Moreover, the need for highly linear power amplifiers typically required in OFDM systems is alleviated through the use of multiple transmit antennas combined with nonlinear coding. The clustering technique also leads to a natural implementation of transmit diversity. This paper reports on the implementation of a clustered-OFDM transmitter which was constructed using off-the-shelf components in a relatively short period of time. Despite the limitations associated with the prototyped system, it can deliver 7.5 Mbps and it is expected that this data rate could be easily tripled with existing technology. In addition to providing an introduction to clustered-OFDM, we describe the various architectural trade-offs which were made in order to reduce the hardware complexity of the boards as well as some experimental results showing the operation of the transmitter.

Optimal detection when combining OFDM-CDMA with convolutional and turbo channel coding

Abstract: Orthogonal frequency division multiplexing (OFDM)-CDMA is an interesting multiple access technique for future mobile radio systems. Especially in the down-link OFDM-CDMA enables low complexity mobile receivers since OFDM can prevent intersymbol interference (ISI), and thus reduce the complexity of a RAKE receiver in a multipath channel. In this paper an OFDM-CDMA system combined with convolutional and turbo channel coding is investigated for the down-link. We derive the optimal soft value-the so called log-likelihood ratio-for the Viterbi decoding algorithm, applied in the OFDM-CDMA system. Based on this derivation we present the performance of an OFDM-CDMA system with channel coding. The investigations are carried out for conventional detection with minimum mean square error (MMSE) equalization and for joint detection with maximum likelihood sequence estimation (MLSE). The various combinations between detection techniques and decoding schemes enable a comparison between achievable system performance and necessary system complexity.

Performance evaluation of OFDM transmission with conventional and two-branch combining power amplification schemes

Abstract: It is recognized that OFDM schemes (orthogonal frequency division multiplexing) offer many advantages for signalling at high data rates over time dispersive channels. However, they exhibit strong envelope fluctuations, which is considered to be an important drawback over many single-carrier schemes, since the inherently high peak-to-mean envelope power ratio leads to heavy power amplification requirements. We present a new approach to the performance evaluation of OFDM transmission with nonlinear power amplification. This approach can be employed for both conventional, single-branch, amplification and two-branch combining amplification, whenever a high number of subcarriers is assumed, and relies on the decomposition of the power amplified signals into uncorrelated "useful" and "self-interference" components. A simple analytical method is presented for evaluating BER performances, within the proposed approach, for a selected OFDM/QAM multichannel system. The analytical results are in good agreement with simulation results and allow simple optimization of the operating points for the power amplification within the system, as well as simple comparisons of the several amplification schemes.

Multiplexed optical fiber sensors using a single Fabry-Perot resonator for phase modulation

Abstract: This paper describes the use of different optical return paths in a single Fabry-Perot phase modulator to perform coherence division multiplexing, and a combination of coherence and frequency division multiplexing of Fabry-Perot interferometric optical fiber sensors. A path-matched differential interferometry (PMDI) using low finesse Fabry-Perot sensors, one moderate finesse Fabry-Perot read-out resonator, and a broadband light source consisting of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) is used to illustrate the idea. The first, second and third order of multiple paths in a single Fabry-Perot read-out resonator are used to path-match three Fabry-Perot sensors. These three orders of path-matches provide a single, double and triple depth of modulation (phase-change) when the read-out is used as an optical phase modulator, and therefore provide a single, double and triple carrier frequency when serrodyne demodulation is used. The use of a single moderate finesse read-out Fabry-Perot resonator to simultaneously path-match coherence multiplexed sensors reduces the complexity of the optical system. Experiments and modeling are used to show the existence of the first, second, and third order of interference path-match conditions, and to determine the read-out configuration that enhances the visibility of the higher order path-match conditions. Using the multiple return paths in a single PZT-based Fabry-Perot read-out interferometer and serrodyne demodulation, several multiplexing schemes are investigated, and their relative merits discussed.

60 GHz millimeter-wave generation and transport using stabilized mode-locked laser diode with optical frequency DEMUX switch

Abstract: A novel optical generation and transport technique of optical frequency division multiplexing (OFDM) mm-wave signals is proposed. A number of mm-wave signals can be simultaneously generated by using a coherent multi-frequency light source and transported over an optical fiber network. The OFDM signals are demultiplexed and distributed to individual microcell zones. Each pair of lightwaves is converted into a mm-wave at the base station by optical self-heterodyning. The key device to the coherent multi-frequency light source is a stabilized mode-locked laser diode (MLLD) combined with a special class of optical frequency demultiplexing (DEMUX) switch. 60 GHz mm-wave generation and the transport over a 40 km-long optical fiber is successfully demonstrated. The 3 dB linewidth of the generated mm-wave as narrow as 0.3 kHz shows an excellent frequency stability of the light source.

Performance of the multi-stage detector for a MC-CDMA system in a Rayleigh fading channel

Abstract: Multi-carrier (MC) CDMA is a multiple access scheme that combines multi-carrier modulation (or OFDM) with direct sequence spread spectrum (DS-SS), and allows for efficient utilization of the bandwidth, resistance against frequency selectivity inherent in broadband mobile radio channels, and efficient implementation using the FFT. Since the complexity of the optimum maximum likelihood detector (MLD) grows exponentially with the number of users, simpler suboptimum detectors are considered in practical applications. We analyze the performance of a new form of the multi-stage detector using the threshold orthogonality restoring correlation (TORC) detector at the initial stage, in a frequency selective, slowly fading Rayleigh channel. Analytical and simulation results are presented for the initial stage (TORC detector) and the first stage of the multi-stage detector, which show that with only one stage significant performance enhancement can be achieved.

CATV-based wireless communications scheme

Abstract: Radio frequency (RF) mobile telephone communications signals are converted to cable television (CATV) frequencies for transmission over existing CATV infrastructure. In the upstream path (i.e., from one or more mobile telephones to a base station), multiple receivers (called remote converters) receive and convert RF signals to cable signals. The cable signals from different receivers are combined by a remote cable node and transmitted over tie cable network to tie base station. In order to reduce noise in the combined cable signal, only those remote converters that receive RF signals of sufficient power level will contribute cable signals to the combined signal. This scheme enables the communications network to be configured with a greater number of relatively inexpensive, low-power remote converters for each remote cable node while still maintaining signal integrity.