Showing papers on "Frequency-division multiplexing published in 1994"

A technique for orthogonal frequency division multiplexing frequency offset correction

Abstract: This paper discusses the effects of frequency offset on the performance of orthogonal frequency division multiplexing (OFDM) digital communications. The main problem with frequency offset is that it introduces interference among the multiplicity of carriers in the OFDM signal. It is shown, and confirmed by simulation, that to maintain signal-to-interference ratios of 20 dB or greater for the OFDM carriers, offset is limited to 4% or less of the intercarrier spacing. Next, the paper describes a technique to estimate frequency offset using a repeated data symbol. A maximum likelihood estimation (MLE) algorithm is derived and its performance computed and compared with simulation results. Since the intercarrier interference energy and signal energy both contribute coherently to the estimate, the algorithm generates extremely accurate estimates even when the offset is far too great to demodulate the data values. Also, the estimation error depends only on total symbol energy so it is insensitive to channel spreading and frequency selective fading. A strategy is described for initial acquisition in the event of uncertainty in the initial offset that exceeds 1/2 the carrier spacing, the limit of the MLE algorithm. >

Frequency synchronization algorithms for OFDM systems suitable for communication over frequency selective fading channels

Abstract: In this paper, the problem of carrier synchronization of OFDM systems in the presence of a substantial frequency offset is considered. New frequency estimation algorithms for the data aided (DA) mode are presented. The resulting two stage structure is able to cope with frequency offsets in the order of multiples of the spacing between subchannels. Key features of the novel scheme-which are presented in terms of estimation error variances, the required amount of training symbols and the computational load-ensure high speed synchronization with negligible decoder performance degradation at a low implementation effort. >

Frequency-domain equalization of mobile radio and terrestrial broadcast channels

Abstract: For mobile radio and terrestrial broadcast applications, we compare orthogonal frequency-division multiplexing (OFDM) and single-carrier transmission with frequency-domain equalization. With respect to our earlier results, we include channel coding and frequency-domain interleaving which are necessary for OFDM systems on multipath fading channels characterized by deep notches in the signal spectrum. Our results indicate that a single-carrier system with a frequency-domain equalizer achieves similar performance to coded-OFDM, while alleviating the carrier synchronization and nonlinear distortion problems inherent to this technique.

Reduction of four-wave mixing crosstalk in WDM systems using unequally spaced channels

Abstract: Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM is presented. It is shown that suitable unequal channel separations can be found for which no four-wave mixing product term is superimposed on any of the transmitted channels. This is obtained at the expense of some expansion of the system bandwidth. Simulations are presented to show the effectiveness of this technique in a 10-channel, 10-Gb/s per channel, system. >

An analysis of orthogonal frequency-division multiplexing for mobile radio applications

Abstract: Using static and time-varying channel impulse responses, the authors analyze the performance of orthogonal frequency-division multiplexing (OFDM) on multipath fading channels. They show that, provided it employs a frequency domain equalizer, single-carrier transmission substantially outperforms OFDM signalling, a result that contradicts the usual claims that OFDM signalling is more resistant to multipath fading than is standard single-carrier transmission. This finding is supported by computer simulation results and analytical arguments related to the decision process. The implication of the results is that not only OFDM signalling increases the system sensitivity to nonlinear distortion and carrier synchronization errors, but it also turns out to offer no performance advantage on fading radio channels. >

Optimal finite duration pulses for OFDM

Abstract: This paper presents pulses of finite duration for use in orthogonal frequency division multiplexing (OFDM) using offset QAM modulation. In such a system the pulses will overlap in both frequency and time without intersymbol or interchannel interference. The pulses are generated by an optimization procedure which minimizes the out-of-band energy under the constraints of zero intersymbol and interchannel interference. Low out-of-band energy is important to achieve high bandwidth efficiency, especially when the number of channels is small. The optimization procedure leads to an integral equation which is solved by expanding the pulse in truncated prolate spheroidal wave functions. Examples of optimized pulses and their spectrum are presented for lengths of 2 and 4 symbol intervals.

Multicarrier transmission technique in cellular mobile communications systems

Abstract: The well known orthogonal frequency division multiplexing (OFDM) multicarrier transmission technique is tailored for a broadband cellular mobile communications system. A performance analysis in multipath fading radio channels is presented and the average bit error rate (BER) is calculated in dependence of the signal-to-noise ratio for bad urban and hilly terrain channel models, following the COST-207 report. Pros and cons for OFDM and code division multiple access (CDMA) techniques are discussed on the basis system robustness, receiver complexity and bandwidth efficiency. >

Polarization control method for suppressing polarization mode dispersion influence in optical transmission systems

Abstract: Recent progress in long-span optical repeater systems indicates that the polarization-mode dispersion (PMD) influence has become one of the main degradation factors in high speed and wideband systems. This paper discusses polarization-control methods for suppressing the PMD influence for both the coherent FDM system and the IM-DD optical repeater system. A principal-state transmission method, which can avoid PMD influence in coherent FDM common polarization control, has been proposed. In this scheme, FDM light is launched at one of the principal states in the fiber using two polarization controllers, located at both ends of the fiber. The feasibility of this scheme was confirmed through 2.5-Gb/s CPFSK, three-channel FDM transmission experiments. For a 150-km long fiber, a 700-GHz optical bandwidth can be used with the principal-state transmission method. This bandwidth is about three times wider than that for conventional common polarization control in a 150-km long fiber. The principal-state transmission method has been modified to apply to a long-span optical-repeater transmission system which includes optical isolators. In this method, small frequency modulation was added to the signal light to search for the principal state for the total transmission line. The modified method can avoid inter-symbol interference (ISI) degradation due to accumulated PMD in long-span optical fibers and optical components. Power-penalty-free operations were successfully demonstrated with up to one bit time PMD value in 5-Gb/s IM-DD, 1000-km transmission experiments. This method is expected to apply to a long-span undersea optical-repeater transmission system. >

High data rate satellite communication system

Abstract: A satellite-based communication system providing high rate transfer of data between linked terminals. The system comprises a plurality of user terminals (VSAT's) that are linked by and that communicate with each other by way of a satellite-based relay system. A network control center provides configuration signals that control the satellite relay system and coordinate linking of terminals to each other. Frequency division multiplexing uplinks are used from the terminals and the network control center to the satellite relay system. Time division multiplexing are used on downlinks from the satellite relay system to the terminals and the network control center. Each user terminal comprises a data compression and decompression circuit, a transceiver, and an antenna for transmitting and receiving data to and from the satellite relay system. The satellite relay system comprises a satellite, a wide area antenna, a plurality of receive antennas, a plurality of transmit antennas, and a signal processor. The wide area antenna is provided for communicating control signals between the user terminals and the network control center. The plurality of receive antennas operate in a first frequency band and produce a first plurality of beams that cover a service area. The plurality of receive antennas receive time domain modulated data from the user terminals. The plurality of transmit antennas operate in a second frequency band and produce a second plurality of beams that cover the service area. The plurality of transmit antennas transmit frequency domain modulated data to the user terminals. The signal processor demodulates the time domain modulated data received from a user terminal, routes the demodulated data so that it is transmitted to a second user terminal, remodulates the data to provide frequency domain modulated data, and transmits the frequency domain modulated data to the second user terminal. Video cameras and monitors are coupled to the user terminals to provide for video teleconferencing.

A system for test of multiaccess methods based on OFDM

Abstract: There are several methods developed to deal with time dispersion on a radio channel. One of the methods, OFDM ("orthogonal frequency division multiplexing"), has only been used for broadcasting of digital radio and television and has not previously been used in a multiaccess environment. As OFDM seems to be an attractive alternative to CDMA and TDMA, the authors investigate some methods to make it usable in a multiaccess system. In the downlinks (base to mobile) a transmission scheme, similar to that used in an ordinary broadcasting system, could be used. For the uplinks however, one can not expect the perfect synchronization needed to get the orthogonal carriers necessary for normal OFDM signalling. The authors present a hardware radio system which currently is being developed to test different approaches for multiaccess OFDM. The system will provide a reconfigure tool for the implementation of different synchronization methods, user multiplexing approaches and detection strategies. The system will be built around VME based SUN Sparc stations. The signal processing will be performed by dedicated FFT processors, general purpose digital signal processors and standard ICs for other time critical tasks. >

Compensation of nonlinear distortions for orthogonal multicarrier schemes using predistortion

Abstract: In the Digital Terrestrial Television Broadcasting European project, the orthogonal frequency division multiplexing (OFDM) technique is seen as a good candidate. In this paper, characterization of the effects of a nonlinear channel over the OFDM signal is performed for 16-QAM and 64-QAM signal constellations. As expected, the attained performance is poor when compared to a single carrier scheme. To improve the performance low-complexity predistortion techniques are applied. It is shown that a predistorted OFDM system achieves almost the same performance as a single carrier scheme without predistortion.

16 QAM modulation with orthogonal frequency division multiplexing in a Rayleigh-fading environment

Abstract: We introduce a method for tracking a Rayleigh-fading channel with Orthogonal Frequency Division multiplexing (OFDM) so that multi-amplitude bit rate schemes such as 16 QAM may be used in a wireless channel. In addition, we derive a distribution to predict the symbol error rate of the modulation scheme. The modulation scheme is applied to an indoor wireless system operating at a rate of 25 Mb/s. The probability of error derived from simulation shows good agreement with the theoretically predicted probability of error. We keep a fairly large bit rate by using few training symbols; in simulation, the error propagation accounts for a slight increase in symbol error rate, but is not catastrophic due to the use of rotationally-invariant 16 QAM constellations. >

The behavior of orthogonal frequency division multiplexing signals in an amplitude limiting channel

Abstract: This paper presents an investigation of orthogonal frequency division multiplexing (OFDM) signal distortion in an amplitude limited channel. In the presence of both limiter distortion and channel noise, the back-off can be optimized to give maximum signal-to-noise ratio for a given system. >

A new approach to motor condition monitoring in induction motor drives

Abstract: A new method for condition monitoring of an electrical machine is proposed. The method uses the power leads to the machine itself as the communication link between the sending station located within the machine and the receiving station located remotely outside the machine. The required communication circuit to realize data transmission in both the cases of the ordinary 60 Hz power line and the special PWM inveter-fed power line is then constructed. The communication circuit uses an asynchronous serial communication protocol and an FSK modulation for realizing frequency multiplexing in the power line. An on-line winding temperature monitoring system for an inverter-fed induction machine is constructed using this power line communication link

Efficient design methodology for microwave frequency multiplexers using infinite-array prototype circuits

Abstract: Frequency multiplexers of the manifold type, in which individual channel filters connect to a main trunk line without the use of isolating directional circuit components, are noted for their compactness, achieved through controlled signal interactions among channel circuits. A major drawback often associated with manifold designs is the potentially large number of network variables that must be handled simultaneously. The new multiplexer design approach being presented utilizes infinite-array prototype circuits based on logarithmic-periodic principles which, in turn, allow a significant reduction in the simultaneous-variable count. The technique is not confined to manifold architectures and can accommodate both contiguous and noncontiguous channels with a wide variety of frequency band allocations. The versatility of the approach is illustrated by two experimental contiguous-band five-channel multiplexer circuits that operate at C- and X-band frequencies, with one circuit designed for equal fractional bandwidths of 20%, and the other for constant absolute bandwidths of 800 MHz. These examples are believed to represent the first practical and successful utilizations of logarithmic periodicity in microwave multiport network design. >

Equalization of orthogonal frequency division multiplexing signals

Abstract: An equalizer is generally a transversal filter which can be used either at the passband or the baseband. Adaptive equalizers adapt filter coefficients to cancel the channel response. We study two different frequency domain adaptive equalizers for orthogonal frequency division multiplexing systems (OFDM). After introducing some theoretical aspects we derive a lower bound for the signal-to-noise ratio reduction due to equalization. Comparisons between two different equalizer structures, the least-mean square (LMS) and proportional equalizers, are presented and tested by simulation in a terrestrial TV channel with strong microreflections.

Transmission modulator for cable television upstream data transmission

Abstract: A cable television upstream data transmission system provides a plurality of uniquely assignable time slots for use by subscriber terminals in upstream data transmission. The unique assignment of time slots provides guaranteed access for each subscriber terminal during its time slot. The width and number of time slots are controlled by a pair of programmable values which may be downloaded from the cable system headend to accommodate dynamic changes of the cable system. A transmission modulator used within the subscriber terminal produces an upstream transmission carrier having a frequency locked to a common network reference signal such as the system symbol clock. The frequency of the carrier signal produced by the transmission modulator is programmably controlled by a plurality of program values which are selected to provide frequency and phase lock of the modulator's carrier to the symbol clock network reference signal.

Orthogonal frequency division multiplexing with BFSK modulation in frequency selective Rayleigh and Rician fading channels

Abstract: The performance of noncoherent reception with orthogonal frequency division multiplexing (OFDM) and binary frequency-shift-keying (BFSK) modulation over multipath fading channels with noise is investigated. This analysis demonstrates that it is possible to transmit information in selective channels with no symbol interference. Expressions of the bit error probability (BEP) are derived in the context of frequency selective Rayleigh and frequency selective Rician fading channels with and without convolutional coding. >

Performance of convolutionally coded CDMA/OFDM in a frequency-time selective fading channel and its near-far resistance

Abstract: The OFDM technique is an interesting approach in mobile communications in order to achieve a high spectral efficiency and to combat the frequency selectivity of the channel. Another interesting technique is a CDMA system with Rake-receiver. The main advantages of CDMA are well known, but its capacity is limited by multiuser interference. The aim of this study is to analyze the performance of a convolutionally-coded CDMA system combined with OFDM in a frequency/time selective fading channel, taking into account the near-far problem. This combination allows one to perform a maximum-likelihood detection (MLD), to use the available spectrum in an efficient way, to exploit frequency diversity and time diversity (provided by channel coding), and to retain many advantages of a CDMA system with a simpler hardware realization. An example for a mobile communication system using convolutionally-coded CDMA/OFDM with Walsh-Hadamard code-spreading for the downlink (base-mobile) is studied. The performance of a MLD is examined by taking into account the near-far problem. It is shown that the MLD is very robust to the near-far problem. It is also shown that by using BPSK modulation, in a 1.28 MHz bandwidth, one can transmit 64 active users at rate of about 10.34 kbit/sec. >

OFDM with guard interval and sub-channel equalization in a 2-resolution transmission scheme for digital television broadcasting

Abstract: The interest in a new digital terrestrial TV system, that allows improved quality and receiver portability, leads to the choice of a modulation system suitable for operation in a very hostile environment (interferences, multipath). A very effective solution, already experimented with digital audio broadcasting, is the orthogonal frequency division multiplexing (OFDM) technique with a guard interval. Unluckily such an interval grows from a few ten microseconds, sufficient for typical urban propagation environment, up to 600 microseconds, when the emerging single frequency network (SFN) is considered. A long guard interval (as required in a SFN) results in a excessive prolongation of the symbol period (T) and thus in bandwidth efficiency loss. In addition, the use of a too long symbol period is limited by the effects of time selectivity of the channel and by technological limitations. In this light, the use is suggested of a shorter guard interval and a low complexity equaliser at the output of each subcarrier to reduce the error produced by the echoes exceeding the guard interval duration. A multiresolution (MR) transmission scheme has been adopted in order to eliminate the sharp threshold effect in the fringes of the broadcast area. The system performances of a 2-resolution OFDM system have been evaluated by computer simulation on a frequency-selective channel for Rice and Rayleigh propagation environment. >

Design and test of a multicarrier transmission system on the shallow water acoustic channel

Abstract: On the shallow water acoustic channel, the transmission systems are exposed to some severe working constraints link to, on the one hand, the channel's non-stationarity and on the other hand, the presence of multiple paths In order to solve these problems we suggest a multicarrier transmission system based on the OFDM principle which implementation by means of DFT is quite simple The performance of such a transmission was evaluated from experiments carried out in the ocean and led to satisfying results >

Hybrid channel coding for multiresolution HDTV terrestrial broadcasting

Abstract: This paper describes a new HDTV system that applies joint multiresolution (MR) source and channel coding to efficiently use the available radio spectrum. Hybrid analog/digital MR channel modulation provides the benefits of digital source coding and the more efficient spectrum usage of analog transmission. Nonuniform spacing of the digital signaling levels provides MR delivery of the digital components while spread-spectrum processing permits MR delivery of the analog components. Error correction coding and OFDM channel modulation deliver the MR service in widely varying channel conditions. Simulation results demonstrate performance in various regions of the service area. >

Demonstration of WDM survivable unidirectional ring network using tunable channel dropping receivers

Abstract: An experimental optical WDM reconfigurable ring network is proposed. It is composed of three nodes incorporating tunable channel dropping receivers (TCDRs) and EDFAs. The reconfiguration and the protection of the ring are computer-controlled. The authors present first experimental results of a fully connected testbed demonstrator.

Visual communication system in an apartment house using only twisted paired cable

Abstract: The paper presents a visual communication system for an apartment house, constructed of the Home Bus and a network, which can be used in an apartment complex. This system uses only twisted paired cable, and offers a variety of services. >

Implementation of a high performance wireless LAN

Abstract: A system is presented that implements an indoor microwave LAN that approaches the performance of fibre optic LANs. The goal of the system is to produce a bit rate of 150 Mbit/s with a bit error rate of 10/sup -12/ in a 30 m cell. The high bit rate is achieved by combining hundreds of low speed signals with orthogonal frequency division multiplexing (OFDM) so that no channel equalization is required. The low bit error rate is achieved through a combination of antenna diversity and coding. Results from a prototype network are presented. The paper also discusses the physical layer of the network and begins with a brief description of the difficulties of sending high speed data through a multipath radio channel. >

Multicarrier COFDM scheme in high bitrate radio local area networks

Abstract: After a short description of the principles of a multicarrier COFDM (coded orthogonal frequency division multiplex) scheme some simulation results concerning bit/packet error rates gained with three different characteristic multipath channels are presented and discussed. Aspects of COFDM design principles and complexity of the modulation and demodulation part of the system, as well as effects of the non-linearity of amplifiers will be considered.

Decision-directed maximum-likelihood estimation of OFDM frame synchronisation offset

Abstract: The authors propose a method to estimate the synchronisation offset for orthogonal frequency division multiplexing (OFDM) frame alignment without resort to pilot tones. A decision-directed maximum-likelihood estimation of frame synchronisation offset is derived, and the performance of the proposed scheme is confirmed by computer simulation for QAM systems.

FM multiplex broadcasting system "DARC"

Abstract: NHK has developed a new FM multiplex broadcasting system called DARC (data radio channel). DARC can provide new services such as general information, traffic information, radio paging and differential GPS data by multiplexing digital signals with existing FM stereo broadcasting signals. Data transmitted by DARC can be received correctly even when mobile, therefore, it is expected that traffic information by DARC will be broadcast in Japan. Three levels of traffic information service have been considered. For safety reasons, control data for easy operation can be thought out. >

Requirements of timing and frequency synchronizations for multi-user OFDM on satellite mobile channel

Abstract: The performance of multi-user orthogonal frequency division multiplexing (OFDM) with raised cosine time domain pulse shaping is studied on the satellite mobile channel (SMC). The relationship between the requirement of the synchronization and the roll-off rate (/spl beta/) of the pulse shaping is explored. In Gaussian and typical Rician (K=10) channels multi-user OFDM with /spl beta/=0.2 requires timing and carrier frequency synchronization accuracies of 4% and 2%, respectively, in order to avoid severe degradation due to multi-user adjacent channel interference, while on Rayleigh channels these values must be better than 2% and 1%.