Showing papers on "Orthogonal 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. >

Multi-Carrier CDMA in Indoor Wireless Radio Networks

Abstract: This paper examines a novel digital modulation/multiple access technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) where each data symbol is transmitted at multiple narrowband subcarriers. Each subcarrier is encoded with a phase offset of 0 or π based on a spreading code. Analytical results are presented on the performance of this modulation scheme in an indoor wireless multipath radio channel. Introduction This paper examines the performance of a new spread spectrum transmission method called “MCCDMA” in an indoor wireless environment. MC-CDMA may be a suitable modulation technique in the indoor environment where the dispersive character of indoor propagation [1] allows for the exploitation of this technique. With MC-CDMA, each data symbol is transmitted over N narrowband subcarriers where each subcarrier is encoded with a 0 or π phase offset. If the number of and spacing between subcarriers is appropriately chosen, it is unlikely that all of the subcarriers will be located in a deep fade and consequently frequency diversity is achieved. As an MC-CDMA signal is composed of N narrowband subcarrier signals [2] each with a symbol duration, Tb, much larger than the delay spread, Td, an MC-CDMA signal will not experience significant intersymbol interference (ISI). Multiple access is achieved with different users transmitting at the same set of subcarriers but with spreading codes that are orthogonal to the codes of other users. Basic Principles of MC-CDMA The generation of an MC-CDMA signal can be described as follows. As shown in Fig. 1, a single data symbol is replicated intoN parallel copies. Each branch of the parallel stream is multiplied by one chip of a spreading code of length N and then binary phase-shift keying (BPSK) modulated to a subcarrier spaced apart from its neighboring subcarriers by F/Tb Hz whereF is an integer number. The transmitted signal consists of the sum of the outputs of these branches. For F = 1, this scheme is similar to performing Orthogonal Frequency Division Multiplexing (OFDM) [3] on a Direct-Sequence spread-spectrum signal [4]. Recently, there has been a growing interest on idea of combining OFDM and DS-CDMA [5] [6] [7] [8] [9]. Modern DSP methods make the implementation of MC-CDMA feasible and attractive. With F = 1, the transmit bandwidth is minimized. However, larger values of F may be desired to further increase the transmit bandwidth, i.e., to achieve a larger frequency diversity gain, without increasing the complexity in signal processing due to large spreading factors,N. The transmitted signal corresponding to the kth data bit of themth user ( am[k] ) is (1) sm t ( ) cm i [ ] am k [ ] 2πfct 2πi F Tb t + ( ) Tb t kTb − ( ) cos i 0 = N 1 − ∑ = cm i [ ] 1 1 , − { } ∈

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.

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.

Method and apparatus for frame synchronization in mobile OFDM data communication

Abstract: A method and apparatus are disclosed for achieving symbol (frame) synchronization of digital data in an OFDM channel such as an OFDM/FM radio link. The method and apparatus are suitable for use in a pure ALOHA environment because synchronization is achieved on a frame-by-frame basis. The required bandwidth overhead is less than 10 %. The bit-error-rate performance achievable with this technique is within 1.5 dB of the performance assuming ideal synchronization. The method and apparatus provide a three-stage synchronization process. First the onset of an ODFM frame is detected. Second, coarse synchronization is achieved by sampling the received signal, and measuring the correlation between the received signal and a reference signal. Coarse synchronization provides synchronization to within ±1/2 sample period. The correlation is preferably carried out in the frequency domain after carrying out a Fast Fourier Tranform on the sampled signal date. Third, synchronization is achieved by calculating the time-shift between the coarse synchronization point and the actual synchronization point and using the calculated time shift to determine the phase correction to apply to each data carrying sub-carrier. Finally the transmitted data is recovered by decoding the information obtained about the phase and amplitude of the data-carrying sub-carriers.

The BER performance of OFDM systems using non-synchronized sampling

Abstract: In fully digital receivers, carrier and timing information is derived from samples of the (anti-aliasing-filtered) received continuous-time signal. In case of synchronized sampling, this information is used to align the sampling clock of the receiver with the remote transmit clock. In nonsynchronized sampling systems, the sampling at the receiver is performed by means of a fixed free-running clock, and additional post-processing is necessary to perform timing correction in the digital domain. We investigate the effect of non-synchronized sampling on the BER performance of OFDM systems. We calculate the BER degradation caused by a given frequency offset between receiver and transmitter clock, as compared with the case of ideal sampling. The obtained results are compared with the performance of synchronized sampling systems.

Method and apparatus for synchronization in digital transmission systems of the ofdm type

Abstract: The invention relates to a method and apparatus for synchronization of transmitters and receivers in digital transmission systems of the OFDM type. The system uses FFT technique to carry out the modulation and demodulation procedures. According to the invention, the transmitter sends synchronization frames with known frequencies and phase positions and with known time intervals in certain time slots. The receiver carries out a series of time-shifted FFT operations over the time position where the synchronization frame is calculated to be. For each operation, a cross-correlation is carried out in the frequency plane of the output signal with the known frequency function of the synchronization frame. The correlation maximum is detected, and this determines the time slot which contains the synchronization frame, whereupon this is used as time base for the following data frames.

Multi-user spread spectrum communication system

Abstract: Multi-user communications system which combines means (12, 22) (12B, 22B) implementing the technique of spread spectrum coding SSC with means (14n, 24n) (14Bn, 24Bm) implementing the technique of orthogonal frequency division multiplexing OFDM. This makes it possible to enhance the performance of the SSC system by simplifying certain data acquisition and processing mechanisms. One favoured use is in systems for telephone communication between mobile stations (U1, U2) and base stations (B). Application: telephone communications system.

Controlled equalization of multi-carrier CDMA in an indoor Rician fading channel

Abstract: In this paper, a novel digital modulation technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) is analyzed. With MC-CDMA, each data symbol is transmitted at multiple subcarriers with each subcarrier modulated by "1" or "-1" based on a spreading code. Analytical results are presented on the performance of this modulation/multiple access scheme in the downlink of an indoor wireless Rician fading channel. The performance of a controlled equalization technique that attempts to restore the orthogonality between users is evaluated. >

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. >

Method for the transmission of reference signals in an OFDM system

Abstract: Various methods, such as OFDM, QPSK and QAM modulation, are used for the terrestrial transmission of digital radio signals. One of the main problems with such systems is synchronization when a receiver is switched on or tuned into another channel. The invention calls for the power of the signal during part of the length of the synchronization symbol to be fixed at zero or almost zero. During another part of the time, a modulation can be utilized which differs from the modulation used in the rest of the signal and which includes at least one sequence with optimun auto-correlation characteristics, e.g. a number of CAZAC sequences or a bit sequence modulated on a centrally positioned carrier, the interval between the bits in the sequence or sequences being equal to the time intervals used in scanning the OFDM symbols or a multiple of these time intervals, or half the effective length of the symbol is used with only every other carrier. As a result, only a single symbol is necessary for synchronisation. In a receiver, rough synchronisation is carried out at zero power using the part-symbol, followed by processing of the received signal component of the synchronisation symbol and follow-up control of the oscillator.

Method of transmitting a data stream, transmitter and receiver

Abstract: The invention relates to a method of transmitting data by modulating a frequency multiplex of carriers at a relatively low symbol rate, such as in OFDM (orthogonal frequency division multiplex). According to the inventive method, intersymbol interference is deliberately introduced at the transmitter side to reduce cross-talk between data of different carriers at the receiver side, such as to simplify an equalizing device in a receiver. The intersymbol interference at the transmitter side is introduced after the frequency multiplex modulation has taken place, by repeating groups of output symbols produced by an IFFT transformer and subsequently shaping these repeated groups by means of symbol-by-symbol scaling with coefficients constituting a window function. Subsequent shaped and repeated groups of these output symbols mutually overlap.

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. >

Narrow-band interference rejection in orthogonal multi-carrier spread-spectrum communications

Abstract: In order to combat strong narrow band interference, different measures called notch-filtering in the time domain (based on LMS algorithm) and in the transform domain (based on FFT) with spread-spectrum (SS) have been analyzed. Another interesting approach is based on orthogonal multi-carrier transmission called the orthogonal-frequency-division-multiple-access (OFDM) technique. This technique permits to achieve a high spectral efficiency and to combat the frequency selective fading and the interference in the channel by switching-off some sub-carriers. The aim of the article is to study the performance of the combination of a spread-spectrum system with OFDM in the presence of narrow-band interference. This combination allows one to use the spectrum in an efficient way, to exploit frequency diversity, to increase the robustness against fading and interference and to retain many advantages of a spread-spectrum system. The interference is considered as narrow-band multi-tone sine interference. A method for evaluating both interference and fading process based on frequency domain analysis is studied. The estimated interference and fading process is used for weighting each received chip before despreading. The results showed that the combination of spread-spectrum/OFDM in the presence of multi-tone narrow-band interference in a frequency/time selective fading channel is a promising approach. >

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. >

Orthogonal frequency division multiplex modulation apparatus and orthogonal frequency division multiplex demodulation apparatus

Abstract: An orthogonal frequency division multiplex (OFDM) modulation apparatus which can transmit just one of single side bands by a simple circuit and an OFDM demodulation apparatus superior in the ratio of the signal power to noise power ratio, wherein a serial/parallel converter converts the input signal from the serial to parallel format, inputs the results to the 1 to N-1 stage of an inverse discrete Fourier transformation (IDFT) circuit, and inputs a fixed value to the other stages (0 stage and N stage to 2N-1 stage). The IDFT circuit is a 2N input IDFT circuit which performs IDFT processing on the input signal converted to the parallel format by the serial/parallel converter and inputs the real number portion of the transformation result to a parallel/serial converter and the imaginary number portion to a parallel/serial converter. The imaginary number portion of the output signal of the IDFT circuit becomes the Hilbert transformation of the real number portion. These output signals are combined by modulation by two orthogonal carrier wave signals of the same frequency so as to take out just a single side band signal of the demodulated signal.

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. >

OFDM receiver with compensation for differential delays

Abstract: Receiver for receiving a transmission signal comprising N frequency multiplexed data modulated carriers, having a good comprise between insensitivity to multipath reception and receiver complexity, and which may enable a reduction in transmission bandwidth and transmitter power. In such a receiver modulation signals of the carriers are supplied by a frequency multiplex demodulator to a symbol detection device via an equalizer device. The equalizer device compensates distortion in the modulation signals supplied thereto, such that the symbol detection device may detect the transmitted data with reduced error probability. When the transmission signal reaches the receiver with different delay times, this distortion is mainly a kind of cross-talk between the modulation of adjacent carriers. Therefore substantial compensation is achieved by combining the modulation signal of a carrier with signals representative of the data of the adjacent carriers. In the receiver these signals may be obtained from the multiplex signal path between the frequency multiplex demodulator output and the output of the symbol detection device.

Analysis of a concatenated coding scheme for a discrete multitone modulation system

Abstract: Discrete multitone (DMT) modulation partitions a band-limited transmission channel into a number of independent, parallel subchannels and maximizes performance by sending more bits over those subchannels with large signal-to-noise ratios. The combination of DMT modulation with powerful coding techniques results in a high-performance transceiver that may be used in a variety of high-speed applications. This study investigates the performance of a concatenated coding scheme consisting of an inner trellis code and outer Reed-Solomon code, when applied to a DMT system. Theoretical coding gain projections are derived for several band-limited channels that are representative of the asymmetric digital subscriber line (ADSL) application. Some laboratory test results obtained from a DMT prototype for ADSL are also presented. >

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.

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. >

Digital signal transmission method

Abstract: PURPOSE: To reduce the reception error by multiplexing a signal transmitted by the SS transmission method and an OFDM digital signal in a broad band including a channel for the transmission of the digital signal. CONSTITUTION: Information with high importance is sent by using the SS(spread spectrum) transmission method and other information is sent by using the OFDM (orthogonal frequency division multiplex) transmission method through a channel of the same frequency band. That is, the transmission information is classified by an information classification circuit 8 depending on the importance of information and the important information is sent by using the SS transmission methods 2, 5 and the other information is sent by using the OFDM transmission methods 3, 7. For example, as to video information coded by the MPEG II, the information classification circuit 8 classifies the information into G.O.P header information with high importance and other information. Then the G.O.P header information with high importance is sent by the SS transmission methods 2, 5 and the other information is sent by using the OFDM transmission methods 3, 7. An adder 4 multiplexes signals modulated by modulators 2, 3 on a same channel. COPYRIGHT: (C)1995,JPO

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. >

Compensation for local oscillator errors in an ofdm receiver

Abstract: A receiver for orthogonal frequency division multiplexed (OFDM) signals, includes means (14) for calculating the (discrete) Fourier transform of the received signal, and means (20) for calculating the phase error due to local oscillator errors.

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 >