Showing papers on "Frequency-division multiplexing published in 1999"

Optimum receiver design for wireless broad-band systems using OFDM. I

Abstract: Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies.

An equalization technique for orthogonal frequency-division multiplexing systems in time-variant multipath channels

Abstract: A loss of subchannel orthogonality due to time-variant multipath channels in orthogonal frequency division multiplexing (OFDM) systems leads to interchannel interference (ICI) which increases the error floor in proportion to the Doppler frequency. A simple frequency-domain equalization technique which can compensate for the effect of ICI in a multipath fading channel is proposed. In this technique, the equalization of the received OFDM signal is achieved by using the assumption that the channel impulse response (CIR) varies in a linear fashion during a block period and by compensating for the ICI terms that significantly affect the bit-error rate (BER) performance.

A real-time sub-carrier allocation scheme for multiple access downlink OFDM transmission

Abstract: In this paper, we propose sub-carrier allocation algorithms for a multiple access scheme in downlink OFDM transmission. Knowing the channel characteristics of all the users at the base station, the sub-carrier allocation algorithm assigns sub-carriers to the users in a way that the total transmit power is minimized. An optimal algorithm is presented to provide the best sub-carrier assignment. The complexity of the optimal solution renders it impractical for real-time application. To deal with the frequently changing channel characteristic of a fast time-varying multipath fading channel, a heuristic algorithm based on constructive assignment and iterative improvement is proposed which can give out a valid solution in real time. Experimental results show that the performance of this real-time algorithm is close to that of the optimal allocation.

Time and frequency synchronization for OFDM using PN-sequence preambles

Abstract: Fast and reliable time and frequency synchronization is crucial for packet-based orthogonal frequency division multiplex (OFDM) systems. In this paper we compare and analyze preambles for OFDM systems based on repeated OFDM data symbols or repeated short pseudonoise sequences (PN-sequences). We make an analytical evaluation for AWGN channels and simulate the performance in oneand two-tap Rayleigh fading channels. The PN-based preamble gives better detection properties in terms of lower false detection probability and lower probability of missing the synchronization signal. The PN-based preamble has low peak-to-average power ratio and it makes it possible to use an ADC with one-bit quantization in stand-by mode. For frequency offset estimation both preambles give similar performance, but the PN-based preamble allows for a greater reduction in stand-by mode power consumption.

A novel subblock partition scheme for partial transmit sequence OFDM

Abstract: In general, there has been a trade-off between performance of peak-to-average power ratio (PAPR) reduction and computational complexity in partial transmit sequence (PTS) orthogonal frequency division multiplexing (OFDM). In this paper, a novel subblock partition scheme (SPS) for PTS OFDM is proposed and analyzed. In this scheme, signals assigned randomly in partial subbands are duplicated and concatenated repetitively to generate each subblock. The proposed scheme, therefore, has a form of concatenation of pseudo-random and interleaved SPS. As results of simulations, the proposed scheme shows almost same PAPR reduction performance as compared to the conventional pseudo-random SPS which has been known to have the best performance. However, computational complexity can be reduced extensively. Hence, the proposed scheme may be considered to be more suitable than the conventional ones for application in high speed transmission systems such as digital terrestrial broadcasting.

Broad-band OFDM radio transmission for multimedia applications : Video Transmission for Mobile Multimedia Applications

Abstract: Future multimedia services will require the transmission of very high data rates over broad-band radio channels. In order to provide these services to mobile users, an appropriate transmission technique has to cope with frequency-selective and time-variant radio channels. The computation complexity for an equalizer increases in a frequency-selective radio channel for high data-rate applications. Furthermore, the overhead for channel estimation procedures increases in time-variant channels. To overcome these drawbacks orthogonal frequency division multiplexing (OFDM) has been considered for broad-band applications in many publications. The objective of this paper is to describe the large potential, the flexibility and adaptivity of the OFDM transmission technique in frequency-selective and time-variant radio channels. In this paper several technical aspects of OFDM transmission systems are discussed, especially the topics of differential modulation for which we compare different demodulation methods and channel coding with soft decision decoding. For higher level differential modulation, multilevel coding is taken into consideration.

High data rate ethernet transport facility over digital subscriber lines

Abstract: A facility transport system for transporting high speed Ethernet data over digital subscriber lines. The system, referred to as 100BaseS, is capable of transmitting 100 Mbps Ethernet over existing copper infrastructure up to distances of approximately 400 meters. The system achieves bit rates from 25 to 100 Mbps in increments of 25 Mbps with each 25 Mbps increment utilizing a separate copper wire pair. Each pair used provides a bidirectional 25 Mbps link with four copper wire pair connections providing 4×25 Mbps downstream channels and 4×25 Mbps upstream channels. The system utilizes framing circuitry to adapt the 100BaseT input data signal to up to four separate output signals. A DSL Ethernet Port card couples the modem to each twisted pair used. Each DSL Ethernet Port card comprises modem transmitter and receiver circuitry for sending and receiving 100BaseS signals onto twisted pair wires. The system utilizes QAM in combination with frequency division multiplexing (FDM) to separate downstream channels from upstream channels and to separate both the downstream and the upstream channels from POTS and ISDN signals.

A low-complexity frame synchronization and frequency offset compensation scheme for OFDM systems over fading channels

Abstract: This paper presents a fast low-complexity synchronization scheme for orthogonal frequency division multiplexing (OFDM) systems over fading channels. By utilizing the guard interval in OFDM signals, the frame synchronization and the frequency offset estimation are considered simultaneously. The implementation can be simplified by only using the sign bits of the in-phase and the quadrature components of the received OFDM signal for frame synchronization and frequency offset compensation. A frequency-offset independent frame synchronization algorithm is derived, and a low-complexity frequency offset estimator based on the synchronized correlator output is presented in this paper. Due to the subcarrier ambiguity in the guard-interval-based (GIB) frequency detector, the maximum correctable frequency range is limited to /spl plusmn/1/2 of the subcarrier spacing. In this paper, we also present a new frequency acquisition scheme that can solve the subcarrier ambiguity problem and extend the frequency acquisition range to nearly a half of the useful OFDM signal bandwidth.

Performance analysis of a coded OFDM system in time-varying multipath Rayleigh fading channels

Abstract: We analyze the combined influence of the interchannel and intersymbol interferences, which result from the time variation and delay spread of mobile channels, on the performance of an orthogonal frequency-division multiplexing (OFDM) system. Both analysis and simulation results are presented for uncoded 16-QAM. We also investigate the performance of a Reed-Solomon (RS) coded 16-QAM OFDM system when the number of subcarriers varies: it is observed that there is an optimum number of subcarriers that minimizes the post decoding symbol error probability of the RS code for each channel state.

Design of pulse shaping OFDM/OQAM systems for high data-rate transmission over wireless channels

Abstract: Orthogonal frequency division multiplexing (OFDM) is a promising technique for high data-rate transmission over wireless channels. In general, wireless channels are time-frequency dispersive. The performance of wireless OFDM therefore depends critically on the time-frequency localization of the pulse shaping filter used. It has been pointed out in Haas (1996) that OFDM systems based on offset QAM (OFDM/OQAM) bypass a major disadvantage of OFDM schemes based on ordinary QAM, namely the fact that well-localized pulse shaping filters are prohibited in the case of a critical time-frequency grid where spectral efficiency is maximal. In this paper, we derive general orthogonality conditions for OFDM/OQAM systems and we propose efficient (FFT-based) design procedures for time-frequency well-localized OFDM/OQAM pulse shaping filters with arbitrary length and arbitrary overlapping factors. Finally, we present design examples.

Parallel combinatory OFDM signaling

Abstract: A parallel combinatory-orthogonal frequency division multiplexing (PC-OFDM) system is proposed and analyzed The proposed system selects at each symbol interval a subset of the available subcarriers, and the selected subcarriers are modulated by points from an M-PSK signal constellation, PC-OFDM systems can be designed to have lower peak-to-average power ratio (PAPR), higher bandwidth efficiency, and lower bit error probability on Gaussian channels compared to ordinary OFDM systems A bit mapping procedure using the Johnson (1993) association, together with a position algorithm for the PSK symbols, is proposed Good analytical approximations of the BER for PC-OFDM systems are derived for AWGN and Ricean fading channels, and extensive simulation results are presented

Effects of time selective multipath fading on OFDM systems for broadband mobile applications

Abstract: The effects of time selective multipath fading are investigated for orthogonal frequency-division multiplexing (OFDM) systems in broadband mobile applications. Although in a multipath environment an OFDM system is very robust against frequency selective fading, it is very sensitive to time selective fading characteristics of the mobile channel, causing inter-carrier-interference and degrading system performance. To achieve a signal-to-interference ratio of 20 dB, the OFDM symbol duration must be less than 8% of the channel coherence time.

Sensitivity of the MC-CDMA access scheme to carrier phase noise and frequency offset

Abstract: The combination of multicarrier (MC) modulation and code-division multiple access (CDMA) is seen as a very promising technique for the development of high-capacity wireless local communications. On the other hand, this scheme is very sensitive to the signal distortion generated by the imperfect frequency down conversion at the receiver due to the local oscillator phase noise and frequency offset. In this work, we consider the MC-CDMA architecture that was independently proposed in Yee et al. (1993) and Falsafi et al. (1996), and, in particular, we evaluate the impact of both the carrier frequency offset and the phase noise on the system performance. The results are given in terms of symbol error rate (SER) obtained by means of an analytical approach for the additive white Gaussian noise (AWGN) channel. However, the procedure can be applied to the performance evaluation in the presence of a multipath fading channel as part of a semianalytical approach. This approach offers a realistic means of performance evaluation when the target error probability values are of the order of 10/sup -6/ or lower.

Adaptive interference suppression in multiuser wireless OFDM systems using antenna arrays

Abstract: This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin.

System and method for effecting information transmission and soft handoff between frequency division duplex and time division duplex communications systems

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

OFDM time and frequency synchronization by spread spectrum pilot technique

Abstract: Correct time and frequency synchronization is important for the performance of orthogonal frequency division multiplex (OFDM) systems. We evaluate a system where a pseudonoise sequence (PN-sequence) is used for estimation of these synchronization parameters. The PN-sequence is superimposed on the OFDM signal. The system is evaluated by means of the variance of the frequency estimation error and the probability for correct timing synchronization. Both theoretical and simulated results are presented. The proposed technique is very flexible and works also at low signal to noise ratios. The frequency offset range has been significantly increased compared to conventional OFDM synchronizer schemes.

Synchronization for systems with antenna diversity

Abstract: This paper deals with the synchronization of systems with frequency domain signal processing and antenna diversity. The results can be applied for both, single carrier transmission with frequency domain equalization (FDE) and orthogonal frequency division multiplexing (OFDM). For the considered systems, carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. A synchronization concept for systems with antenna diversity is presented which uses one pilot FFT symbol per transmission frame. The pilot FFT symbol consists of two periods of a chirp signal.

Multiplexing of fiber Bragg grating sensors using a FMCW technique

Abstract: We report on the use of a frequency-domain reflectometry technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in the frequency-domain and demodulated using a tunable optical filter. A three FBG sensor system is experimentally demonstrated. The potential of the technique for multiplexing a large number of FBG sensors is discussed.

Efficient design of pulse-shaping filters for OFDM systems

Abstract: Orthogonal frequency division multiplexing (OFDM) has recently become a popular technique for high-data-rate transmission over wireless channels. Due to the time-frequency dispersion caused by the channel, the performance of OFDM systems depends critically on the time-frequency localization of the pulse shaping filters. In this paper, weshow how the recent duality and biorthogonality theory"2 developed in the context of Weyl-Heisenberg frames can be used to devise simple and efficient design procedures for well-localized OFDM pulse shaping filters. We consider OFDM systems employing time-frequency guard regions and OFDM systems based on offset QAM. We proposeFFT-based design methods for pulse shaping filters with arbitrary length and arbitrary overlapping factors. Finally, we present some design examples. Keywords: OFDM, pulse shaping filter, Weyl-Heisenberg frames 1. INTRODUCTION Orthogonal frequency division multiplexing (OFDM)3'° is employed in current telecommunications standards such

Comparison of multicarrier DS-CDMA broadcast systems in a multipath fading channel

Abstract: The performance of a multicarrier direct sequence code-division multiple-access (CDMA) system employed in the forward link of a cellular system operating over a Rayleigh fading channel is analyzed and compared to the performance of both single-carrier CDMA and hybrid multicarrier CDMA/frequency division multiplexing systems. A RAKE receiver is provided for each subcarrier. We compare the performance of all three systems for various multipath intensity profiles. It is found that for a service requiring high quality and a small number of users, the multicarrier system is the best, but for a service requiring low quality and a large number of users, the hybrid system can support more users than the others. Also, for the case when nonorthogonal codes are used, the multiple-access interference in different resolvable paths are correlated. In that case, to maximize the signal-to-noise ratio in a correlated interference environment; maximal-ratio combining (MRC) is not optimal. However, we found that there is not much difference between the optimum combining and the conventional MRC.

Delivery of digital video and other multimedia services (>1 Gb/s bandwidth) in passband above the 155 Mb/s baseband services on a FTTx full service access network

Abstract: For low cost passive optical access networks (PON), we have studied the downstream delivery of an additional >1 Gb/s in the passband above the 155 Mb/s baseband services. Both signals are transmitted on the same fiber using a single transmitter and a single receiver within the power budgets of ITU-T G983. Although our study has used broadcast DSS digital video services for the passband that delivers about 1.2 Gb/s, the passband bandwidth can be used for any broadcast or switched multimedia services, i.e., digital video, data, and audio. For switched services, the baseband upstream will be the return path. Furthermore, the passband can be used to provide a dedicated bandwidth to any subscriber. The passband approach offers a graceful upgrade of a 155 Mb/s baseband system, and is compatible with baseband only optical network unit (ONU). We multiplexed a 155.52 Mb/s baseband signal with quadrature phase shift keying (QPSK) modulated passband digital video channels in the 270-1450 MHz range with minimal degradation. We used an additional 860 MHz carrier modulated with 8 Mb/s using QPSK as a passband test-signal. An optical-to-electrical (O/E) receiver using a PIN diode satisfies the need for class B operation as defined in ITU-T G983 (-28 dBm receiver sensitivity for a 10/sup -10/ bit error rate) with /spl sim/1.5 dBo/sup 1/ margin if we consider the proposed technique as an upgrade of the baseband 155 Mb/s to 622 Mb/s. Otherwise it only meets the specifications of class B (-30 dBm receiver sensitivity). However, an APD based receiver satisfies the power budget needs of ITU-T G983 class C operation (-33 dBm receiver sensitivity) with /spl sim/5.5 dBo margin for both baseband and passband with forward error correction (7/8 convolution and (96,110) Reed-Solomon coding). The available optical power margin with an APD receiver may be large enough that it can also allow use of 16-QAM in place of QPSK in class C mode or 64 QAM in class B mode.

A wavelength-selectable laser system using cavity resonance frequency, especially useful for fiber optic communication and wavelength division multiplexing

Abstract: A wavelength-selectable laser system (10) is achieved by addressing the suitable mode-locked frequency of a multi-length laser cavity formed by multiple wavelength-selective mirrors (20) such as fiber gratings. This is useful for fiber optic communication and WDM (wavelength division multiplexing) networks where there is incorporated an information encoder/modulator (22) of the light in the sending side and a detector in the receiving side, which are matched to the laser system.

Performance of multiresolution OFDM on frequency-selective fading channels

Abstract: Several multiresolution modulation (MRM) schemes with orthogonal frequency division multiplexing (OFDM) are examined. The effect of interchannel interference (ICI) caused by channel time variation and carrier frequency offset is considered. The performances of the multiresolution schemes for both uncoded and trellis-coded systems are evaluated in terms of their bit error rate (BER), degree of prioritization, and decoding complexity. Finally, applications to high-definition television (HDTV) broadcasting are considered, where a concatenated coding scheme is employed to provide the required performance levels.

Ofdm-cdma system communication equipment

Abstract: PROBLEM TO BE SOLVED: To prevent the error rate characteristic of a demodulation signal from deteriorating without lowering transmission efficiency. SOLUTION: An adding part multiplexes a signal 1 to a signal n diffused by each diffusing part and a 1st known signal. An IFFT part performs frequency division multiplexing frequency by allocating a multiple signal converted into a signal of a plurality of series by an S/P converter to a subcarrier 1 to a subcarrier k and allocating a 2nd known signal to a pilot carrier. A residual phase error detecting part detects a residual phase error from the 2nd known signal transmitted by the pilot carrier. The residual phase error detecting part detects a residual phase error from the 1st known signal transmitted by each subcarrier. An averaging part equalizes the residual phase error detected by each residual phase error detecting part. Each phase compensating part applies phase compensation processing to a demodulation signal by using the averaged residual phase error. COPYRIGHT: (C)2001,JPO

Efficient method for carrier offset correction in OFDM system

Abstract: We present a simple approach to estimate and correct the carrier offset in an orthogonal frequency division multiplexing (OFDM) system. The approach leads to a computationally and statistically efficient estimator for the carrier offset. Computer simulations verify that the estimation accuracy is comparable to the Cramer-Rao bound (CRB). We demonstrated that by incorporating the estimated carrier offset (obtained using reasonable frames of OFDM data) in the demodulation process, the bit error rate (BER) can approach that of the ideal OFDM system with no carrier offset.

Ofdm-ds-cdma communication system

Abstract: PROBLEM TO BE SOLVED: To reduce deterioration in an error rate characteristics of an OFDM- DS-CDMA communication system. SOLUTION: Adder sections 102a1, 102am multiplex k/2-sets of spread information signals to generate a multiplexed signal. The adder sections 102a1-102am-1 multiplex k-sets of spread information signals to generate a multiplexed signal. An IFFT section 103 assigns the multiplex signal from the adder section 102a1 to a subcarrier 1 whose characteristics is deteriorated and the multiplex signal from the adder section 102am to a subcarrier m whose characteristics is deteriorated respectively and assigns the multiplex signal from the adder sections 102a1-102am-1 to subcarriers 2-m-1 with excellent characteristics respectively to conduct frequency division multiplexing. COPYRIGHT: (C)2001,JPO

Unified distributed voice and data local area networking

Abstract: A local networking system, offers integrated communication services to multiple local voice stations and data terminals via a common transmission medium. This system, utilizing Frequency Division Multiplexing (FDM) signals as carrier, transports information with CSMA/CD protocol. Voice transmissions are organized in traditional TDMA format with fixed frame length and dynamically assigned time slots to avoid interference caused by other devices in the system. Data services operate in traditional CSMA/CD format, but restricted to specified blocks of time intervals. The voice service quality is guaranteed because the required channels and bandwidths, up to the designed maximum, are always available when needed. The grade of data service may be impacted by voice traffic volume which is automatically managed by data terminal's inherent design with CSMA/CD protocol. This unified digital multiplexing system is applicable to transmission media with restricted channels. For example, even with very limited transmission bandwidth, a Dual Area Network (DAN) for integrated voice and data transmissions still can be set up on power or telephone line. On the other hand, this technique can maximize channel utilization of any given bandwidth in a wide band transmission facility, such as radio wave, coaxial cable or infrared system.

Communication, transmission and receiving method and apparatus using orthogonal frequency division multiplexing modulation system

Abstract: A communication method in an orthogonal frequency division multiplexing modulation communication system includes a transmission apparatus in which a frequency bandwidth of a signal modulated in accordance with an orthogonal frequency division multiplexing modulation system is changed and the modulated signal is transmitted and a receiving apparatus in which the modulated signal is received and a frequency bandwidth for demodulation of the received signal is changed to follow the change of the frequency bandwidth of the modulated signal.

Electrical add-drop multiplexing for optical communications networks utilizing frequency division multiplexing

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

Spatial and temporal equalization for broadband wireless indoor networks at millimeter waves

Abstract: The combined use of adaptive antennas and decision feedback equalization (DFE) is analyzed in a realistic propagation scenario at millimeter waves, taking the direction of arrivals (DOA's) of the received paths into account. The joint antennas and DFE scheme, with one forward filter for each antenna and a single feedback filter (FBF), can be viewed as a spatial and temporal DFE (ST-DFE). The performance of this solution is compared with the cascade of adaptive antenna used for beamforming and DFE. It is found that ST-DFE achieves better performance since it combines the beamforming capability of the antenna array with the equalization properties of the DFE, with great advantages especially when rays arrive from similar angles. The mean square error (MSE) is analytically derived for infinitely long filters in a quasi-static environment with multiple rays having different DOAs, and compared (for the two-path model) with simulation results assuming filters with a small number of taps. Finally, service availability through coverage evaluation is developed and compared with that of a coded-orthogonal frequency division multiplexing (C-OFDM) system.