Showing papers on "Frequency-division multiplexing published in 1998"

Effects of clipping and filtering on the performance of OFDM

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

Robust channel estimation for OFDM systems with rapid dispersive fading channels

Abstract: Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.

A high-efficiency carrier estimator for OFDM communications

Abstract: While multipath induced phase rotations can be dealt with by differential encoding in orthogonal frequency division multiplexing (OFDM) communications, the loss of orthogonality due to the carrier offset must be compensated before discrete Fourier transform (DFT)-based demodulation can be performed. In this letter, we present a high-performance/low-complexity blind carrier offset estimation algorithm by exploiting intrinsic structure information of OFDM signals. The algorithm offers the accuracy of a super resolution subspace method, viz. MUSIC, without involving computationally intensive subspace decompositions.

On the effect of Wiener phase noise in OFDM systems

Abstract: Multicarrier modulation exhibits a significant sensitivity to the phase noise of the oscillator used for frequency down-conversion at the portable receiver. For this reason, it is important to evaluate the impact of the phase noise on the system performance. We present an accurate method to determine the error probability of an orthogonal frequency-division multiplexing (OFDM) system in the presence of phase noise. In particular, four modulation schemes are analyzed and their performances are compared.

Phase noise and sub-carrier spacing effects on the performance of an OFDM communication system

Abstract: This letter analyzes the phase noise effects on an orthogonal frequency division multiplexing (OFDM) signal and its dependence with the sub-carrier spacing. Pilot-based channel estimation, which has been suggested as a means of combating the channel effects, can also correct the phase noise effects under some circumstances, which are investigated.

Intercarrier interference compression in OFDM communication systems by using correlative coding

Abstract: This study considers the use of frequency-domain correlative coding with correlation polynomial F(D)=1-D in OFDM mobile communication systems to compress the intercarrier interference caused by channel frequency errors. A theoretical expression of carrier-to-interference power ratio (CIR) has been derived. It shows that such a simple coding method enhances system CIR by 3.5 dB, without reducing the bandwidth efficiency.

On the optimality of metrics for coarse frame synchronization in OFDM: a comparison

Abstract: Commonly, a repetition preamble is proposed as a burst training sequence to allow frame- and carrier frequency synchronization in digital transmission over unknown, severely dispersive channels. In this paper the preamble is composed of one repeated data-carrying-and therefore random-orthogonal frequency division multiplexing (OFDM) symbol. The current literature suggests several metrics to detect repetition preambles for coarse frame synchronization so we provide a ranking of four different metrics proposed by Chevillat, Maiwald and Ungerboeck (1987), Sandell, van de Beek and Borjesson (1995), Schmidl and Cox (see IEEE Trans. on Commun., vol.45, no.12, p.1613-21, 1997), and Keller and Hanzo (see Proceedings of the International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'96), p.963-67, Taipei, Taiwan, 1996) via a simulative assessment. Furthermore, a probability-based motivation for the metric of Chevillat et al is given.

Performance analysis of Viterbi decoding for 64-DAPSK and 64-QAM modulated OFDM signals

Abstract: The multilevel modulation techniques of 64-quadrature amplitude modulation (QAM) and 64-differential amplitude and phase-shift keying (DAPSK) have been proposed in combination with the orthogonal frequency-division multiplexing (OFDM) scheme for digital terrestrial video broadcasting (DTVB). With this system a data rate of 34 Mb/s can be transmitted over an 8-MHz radio channel. A comparison of these modulation methods in the uncoded case has been presented by Engels and Rohling (see European Trans. Telecommun., vol.6, p.633-40, 1995). The channel coding scheme proposed for DTVB by Schafer (see Proc. Int. Broadcasting Convention, Amsterdam, The Netherlands, p.79-84, 1995) consists of an inner convolutional code concatenated with an outer Reed-Solomon (RS) code. In this paper the performance of the convolutional codes is analyzed for the two modulation techniques. This analysis includes soft decision Viterbi (1971) decoding of the convolutional code. For soft decision decoding of DAPSK modulated signals a new metric is developed.

Baseband Volterra filters for implementing carrier based nonlinearities

Abstract: A diagonal coordinate representation for Volterra filters is developed and exploited to derive efficient Volterra filter implementations for processing carrier based input signals. In the diagonal coordinate representation, the output is expressed as a sum of linear filters applied to modified input signals. Hence, linear filtering methods are employed to implement the nonlinear filter on a baseband version of the input. Downsampling is then used to reduce computational complexity. The same approach is employed to develop efficient implementations for processing continuous-time carrier-based signals, pulse amplitude-modulated signals, and frequency division multiplexed input signals.

Adaptive antennas for OFDM

Abstract: This paper explores the combination of adaptive antennas and orthogonal frequency division multiplexing (OFDM) for operation in a faded delay-spread channel. We consider the performance of the well-known sample matrix inversion (SMI) algorithm for controlling an adaptive antenna in a pilot-symbol-assisted OFDM system. Several strategies are considered for deploying SMI with different levels of tracking capability over an OFDM time-frequency slot. We propose a modification to the SMI method which incorporates the concept of pilot-interpolation into the computation of the sample covariance matrix. This modified SMI algorithm is shown to provide performance superior to the other variations of SMI under consideration.

System for distribution of satellite signals from separate multiple satellites on a single cable line

Abstract: A system for distributing separate multiple satellite communication services signals to receivers at a local earth site on a single cable line. A dual satellite antenna receives the signals from two separate satellites, each of which can correspond to a respective satellite communication service. The received satellite signals are processed into two separate frequency bands. A frequency converter using frequency division multiplexing converts at least one of the received frequency bands so as to position both of the frequency bands adjacent to each other. A summer receives the adjacent frequency bands and distributes them on a single cable line to receivers so that the receivers have access to the separate multiple satellite communication service signals on a single cable line. A demultiplexer coupled to the adjacent frequency bands from the summer enables the plurality of receivers at the output of the demultiplexer lines to obtain simultaneous access to both satellite communication services distributed over the single cable line to the receiver.

OFDM performance in amplifier nonlinearity

Abstract: The activities of the current European RACE and ACTS projects have led to an increasing interest in OFDM (orthogonal frequency division multiplexing) as a means of combating impulsive noise and multipath effects and making fuller use of the available bandwidth of the system. This paper analyses the performance of OFDM signals in amplifier nonlinearity. In particular, bit error rate (BER) degradation as a result of amplitude limiting or clipping are analysed. In the presence of both nonlinear distortion and additive Gaussian noise, optimized output power back off is provided to balance the requirements of minimum BER and power amplifier efficiency. For this purpose, an OFDM system has been built using the SPW (Signal Processing Worksystem) simulator.

Highly spectrum efficient OFDM/PDM wireless networks by using optical SSB modulation

Abstract: Optical frequency division multiplexing (OFDM) wireless access network using optical single-sideband modulation is proposed, which allows to deliver different wireless signals from a central station to each base station over an optical fiber network. The optical single-sideband (SSB) OFDM can achieve the highest spectrum efficiency of 0.25 b/s/Hz. A 0.25 b/s/Hz has been experimentally verified by the error-free transport of two-channel 4.98 Gb/s BPSK signal on 10 GHz wireless carrier with the frequency channel interval of 20 GHz at 1550 nm over 50 km-long standard single-mode fiber link. Furthermore, it has been also experimentally confirmed that by introducing the polarization division multiplexing (PDM), the ultimate spectrum efficiency of 0.5 b/s/Hz can be achieved by the hybrid OFDM/PDM.

A comparative performance evaluation of DMT (OFDM) and DWMT (DSBMT) based DSL communications systems for single and multitone interference

Abstract: Multicarrier modulation techniques have been proposed for digital subscriber line (DSL) applications. The performance of DMT (OFDM) and DWMT (DSBMT) techniques for single and multitone interference are investigated. It is shown that a DMT system is sensitive to the location of narrow band interference. The DMT technique needs additional narrow band interference canceller before the forward FFT transform for performance improvements. In the DSBMT technique, due to a limited spectral overlap between its subcarriers, single (multi)-tone interference could affect only a few subchannels which correspond to these interferences. The DSBMT has a superior performance than DMT.

Filtered decision feedback channel estimation for OFDM-based DTV terrestrial broadcasting system

Abstract: Orthogonal frequency division multiplexing (OFDM) is one of the transmission techniques used for digital television (DTV) terrestrial broadcasting. A high quality channel estimator with a low training overhead is the key to the successful delivery of DTV services which require high spectrum efficiency and robustness to strong and dynamic ghosts. Robustness to multipath distortion is especially important in a single frequency emission environment. This paper presents an application of a filtered decision feedback channel estimator for OFDM-based DTV systems using high order QAM modulations. The implementation and the performance of the channel estimator are discussed. Computer simulations were conducted to evaluate the performance of the channel estimator. The channel estimation loss is about 1.2 dB from the ideal case where the channel is assumed to be known by the receiver. For a given multipath spread, the loss can be further reduced by increasing the FFT size. The FFT size is however subject to an upper limit imposed by the Doppler spread.

The effect of carrier phase jitter on the performance of orthogonal frequency-division multiple-access systems

Abstract: We investigate the sensitivity to carrier phase jitter of an orthogonal frequency-division multiple-access (OFDMA) system. When all OFDMA carriers have the same power level and jitter spectrum, the degradation caused by the jitter is shown to be equal to the degradation of an OFDM system. Also, traditional FDMA is found to be slightly more robust than OFDMA.

Hardware nonlinearities in digital TV broadcasting using OFDM modulation

Abstract: FFT-based coded orthogonal frequency division multiplexing (COFDM) is one of the techniques for digital TV broadcasting over multipath fading channels. A FFT-based OFDM signal is subject to various hardware nonlinearities in both the transmitter and receiver. Hardware nonlinearities not only affect the in-band performance of an FFT-based OFDM system but also may affect the system performance of an adjacent channel signal because of regenerated sidelobes of the transmitted signal. The paper investigates the in-band and out-of-band behaviour of a 64QAM-OFDM system under various nonlinear devices. It is shown that the inherent signal clipping in the IFFT processors with a limited word length reduces the required RF amplifier output backoff (OBO) where adjacent channel interference is the limiting factor. For a 0.25% clipping rate, an additional 2 dB OBO is required for the COFDM signal to achieve the same level of adjacent channel interference as for the single carrier system. The loss in SNR due to signal clipping is negligible in a coded OFDM system.

Indoor and outdoor frequency measurements for MM-waves in the range of 60 GHz

Abstract: Using a spectrum analyzer and synthesized signal generator, several measurements for the frequency-domain characterization of the radio channel in the 60 GHz band were performed. In the experiments, a carrier wave (CW) was swept with a constant amplitude across the 100 MHz band, centered around 59.9 GHz. The channel frequency response was measured at the receiver side. The results can be used for the design of an OFDM (orthogonal frequency division multiplex) system. The measurement environments were a corridor and a big college room in a high-rise office building (indoor) and a parking site and a grassfield at the side of that building (outdoor). The spectrum samples were taken in each of the environments. From such samples, it is possible to calculate the Rice parameter k and the path loss coefficients. It was necessary to the develop an appropriate measurement analysis, stemming from the fact that the spectrum analyzer gives no information on the phase, only amplitude. Measurement analysis method and results are explained.

Simultaneous broadcasting of analog FM and digital audio signals by means of adaptive precanceling techniques

Abstract: A method for broadcasting digital audio simultaneously with existing analog frequency modulation (FM) radio is presented. The method is based on precomputing the response of the host analog FM signal at the digital receiver and precanceling it at the transmitter. As a result, the digital transmission is free from interference from analog FM. We select the rate and power level of the digital transmission in a manner that the interference the digital data incur on the analog FM signal remains at acceptably low levels. The digital transmission is based on adaptive orthogonal frequency-division multiplexing (OFDM) (adaptive multicarrier). The frequencies and number of carriers of the digital multicarrier modem are judiciously selected in a time-varying fashion so as to cause a negligible distortion in a standard receiver for analog FM. Simulations based on conservative nonoptimized signal design indicate that data rates up to about 130 kb/s inside the 200-kHz FM channel are achievable for acoustic test signals. We present a number of numerical examples where the average digital data use up to 50% of the 200-kHz power spectrum with digital signal power levels 25-35 dB below the analog signal. Due to the resulting variable-rate digital transmission, a control channel is required. A method of precanceling with multiple orthogonal direct-sequence spread-spectrum schemes is also presented.

Matched filter bound optimization for multiuser downlink transmit beamforming

Abstract: This paper is devoted to the optimization of the matched filter bounds (MFB) of different co-channel users, using adaptive antenna arrays at base stations for downlink transmit beamforming in cellular mobile communication systems. We mainly consider time division multiple access (TDMA) frequency division duplexing (FDD) based systems. Note that in the case of time division duplexing (TDD), under terrain assumptions the downlink channel can be assumed to be practically the same as the uplink channel. On the contrary, when using FDD, the downlink channel can not be directly observed and estimated. That makes FDD based systems most difficult to deal with, although the proposed criteria and methods are general and suitable for both FDD and TDD. Problem formulations are provided for both spatial division multiple access (SDMA) and non-SDMA spectrum reuse techniques. Novel analytical solutions and algorithms are derived, implementation issues are discussed and simulations are provided in order to compare different approaches.

Broadband transmission using OFDM: system performance and receiver complexity

Abstract: This paper investigates the system performance of coherent OFDM transmission over frequency selective fading channels taking into account the constraints imposed by the receiver complexity. The effects of synchronization errors in the receiver are analyzed. In a properly designed inner receiver, these errors lead to extra noise. Specifying the limits for this additional noise allows one to derive the requirements for each component and thus to design and optimize the inner receiver independently from the outer receiver. The resulting receiver structure and the synchronization algorithms applied are introduced. Particular attention is paid to the channel estimation unit since its accuracy is most essential for system performance. A short analysis of the complexity of the algorithms allows one to determine the limits of the single components. System simulations including inner and outer receiver are used to evaluate the ideal vs. the realizable performance of the receiver and thus show how it is bounded by receiver complexity.

Method for dynamically allocating carriers in a wireless packet network, with reuse of carriers

Abstract: We disclose a method of dynamic channel assignment for wireless transmission systems that employ time or frequency multiplexing, or both time and frequency multiplexing. The invention is specifically addressed to the problem of avoiding interference in the channels of such systems. In a broad aspect, the invention involves partitioning base stations of a network into non-interfering sets. Channels are allocated to the non-interfering sets according to need. Stages of channel reallocation take place periodically. The reallocation takes place through coordinated activity by the base stations. That is, the channel reallocation is carried out in response to information that is exchanged between base stations, or it is centrally directed by the network in response to information passed to the network by the base stations.

Blind carrier synchronization and channel identification for OFDM communications

Abstract: In OFDM communications, the loss of orthogonality due to carrier offset must be compensated before DFT-based demodulation can be performed. We present a high accuracy blind carrier offset estimation algorithm and a blind channel equalizer which exploit the intrinsic structure information of OFDM signals. The latter method allows the receiver to perform coherent demodulation in changing environments without the overhead required for additional pilots.

Linear two dimensional pilot assisted channel estimation for OFDM systems

Abstract: Orthogonal Frequency Division Multiplexing ( OFDM ) is considered an effective technique in high speed digital transmission due to its spectral efficiency, its robustness in different multipath propagation environments and its ability to combat intersymbol interference ( ISI ). Coherent modulation combined with OFDM gives an optimum performance but needs a powerful channel estimator. In this paper we present a channel estimation method based on pilot symbols inserted both in time and frequency domains, and a linear interpolation algorithm. This linear interpolation algorithm has the advantage of minimising the system complexity and processing delay while giving a good approximation to the real mobile channel. The performance of the linear pilot aided estimation algorithm is compared to coherent modulation with perfect channel estimation as well as to differential modulation.

An OFDM all digital in-band-on-channel (IBOC) AM and FM radio solution using the PAC encoder

Abstract: The advances in digital communications and compression algorithms have made more efficient and more robust transmission schemes possible. Radio broadcast systems have not fully utilized these advances to their benefit. All digital robust radio broadcast systems for the AM and the FM band are proposed. The proposed systems are based on orthogonal frequency division multiplexing (OFDM) technology in conjunction with PAC for both the AM and the FM bands. The Perceptual Audio Coder (PAC) developed by Bell Laboratories compresses audio signals very efficiently with CD-like quality at 96 kbps and stereo FM-like quality at 48 kbps. These are rates achievable with 200 kHz and 30 kHz bandwidths available per FM and AM station respectively. No new spectrum is required since the digital signals are transmitted within the current allocated FCC masks. In an FM channel, a wide-band data subchannel can be provided in addition to the 96 kbps error protected PAC audio information.

An adaptive diversity receiver for OFDM in fading channels

Abstract: Interest in OFDM has renewed with the standardization of OFDM for digital audio broadcasting in Europe. In this paper, we consider an adaptive, diversity receiver for OFDM signals in a Rayleigh fading channel. The diversity receiver has L branches with each branch receiving the signal from L independently fading diversity channels. We model the fading process as a vector auto-regressive process and use the Kalman filter to obtain the MMSE optimum channel estimates for each branch. The channel estimates and the signals are combined using the maximal ratio combining rule to obtain the decision variables. We analyze the performance of this receiver and provide a numerical example to highlight the advantage of using diversity.

Wavelength division multiplexing optical transmission system

Abstract: In a wavelength division multiplexing optical transmission system having an optical fiber transmission line (224) whose zero-dispersion wavelength is around 1550 nm, the wavelengths of at least two signal lights among a plurality of wavelength-multiplexed signal lights lie in a range between 1450 nm and 1530 nm or between 1570 nm and 1650 nm.

A comparison of the performance of linearly equalized single carrier and coded OFDM over frequency selective fading channels using the random coding technique

Abstract: Different attempts have been made to compare the performance of coded orthogonal frequency division multiplexing (COFDM) and a corresponding single carrier system employing a linear equalizer often by comparing the average bit error rates. The random coding technique is used to analyze the performance of these systems for signaling over piecewise constant channels. The probabilities that the cutoff rates are less than a desired rate are calculated and compared. It is shown that for low SNRs and low code rates, the cutoff rates of both systems are approximately the same. With an increasing SNR the cutoff rate for the linearly equalized system, however, converges faster to its maximum value than the cutoff rate for the COFDM system. Simulation results for the outage probability are presented for the two-ray Rayleigh fading channel with QPSK as the fundamental modulation technique. For code rates close to one or for uncoded transmission, the linearly equalized single carrier system substantially outperforms the COFDM system. For low to medium code rates COFDM and a linearly equalized single carrier system perform approximately identically.

Wireless telecommunications system architecture supporting block radio technology

Abstract: A wireless telecommunications system architecture is disclosed that is capable of supporting a centralized block radio architecture and of distributing the information-bearing signals from the block radio to geographically-dispersed radio heads using relatively low-cost cable (e.g., twisted-pair, etc.). In accordance with the illustrative embodiment of the present invention, a radio multiplexes, modulates, and channel codes one or more information-bearing signals and upconverts them, not to radio frequency, but to an intermediate frequency that can be transmitted over a low-cost cable. Co-located with each radio head is an upconverter that upconverts the intermediate frequency signal to radio frequency in preparation for radiation by an antenna.