Showing papers on "Frequency-division multiplexing published in 2002"

Frequency domain equalization for single-carrier broadband wireless systems

Abstract: Broadband wireless access systems deployed in residential and business environments are likely to face hostile radio propagation environments, with multipath delay spread extending over tens or hundreds of bit intervals. Orthogonal frequency-division multiplex (OFDM) is a recognized multicarrier solution to combat the effects of such multipath conditions. This article surveys frequency domain equalization (FDE) applied to single-carrier (SC) modulation solutions. SC radio modems with frequency domain equalization have similar performance, efficiency, and low signal processing complexity advantages as OFDM, and in addition are less sensitive than OFDM to RF impairments such as power amplifier nonlinearities. We discuss similarities and differences of SC and OFDM systems and coexistence possibilities, and present examples of SC-FDE performance capabilities.

Simplified channel estimation for OFDM systems with multiple transmit antennas

Abstract: Multiple transmit-and-receive antennas can be used in orthogonal frequency division multiplexing (OFDM) systems to improve communication quality and capacity. In this paper, we present two techniques to improve the performance and reduce the complexity of channel parameter estimation: optimum training-sequence design and simplified channel estimation. The optimal training sequences not only simplify the initial channel estimation, but also attain the best estimation performance. The simplified channel estimation significantly reduces the complexity of the channel estimation at the expense of a negligible performance degradation. The effectiveness of the new techniques is demonstrated through the simulation of an OFDM system with two-transmit and two-receive antennas. The space-time coding with 240 information bits per codeword is used for transmit diversity. From the simulation, the required signal-to-noise ratio is only about 9 dB for a 10% word error rate for a channel with the typical urban- or hilly-terrain delay profile and a 40-Hz Doppler frequency.

On the comparison between OFDM and single carrier modulation with a DFE using a frequency-domain feedforward filter

Abstract: Most comparisons between single carrier and multicarrier modulations assume frequency-domain linear equalization of the channel. We propose a new frequency-domain decision feedback equalizer (FD-DFE) for single carrier modulation, which makes use of a data block transmission format similar to that of the orthogonal frequency-division multiplexing with cyclic prefix (OFDM). The scheme is a nonadaptive DFE where the feedforward part is implemented in the frequency domain, while feedback signal is generated by time-domain filtering. Through simulations in a HIPERLAN-2 scenario, we show that FD-DFE yields a capacity very close to that of OFDM. This result is also confirmed by analytical derivations for a particular case. Furthermore, when no channel loading is considered, FD-DFE performs closely to OFDM for the same averaged frame error rate in a coded transmission. Design methods of the FD-DFE are investigated and a reduced complexity technique is developed, with the result that FD-DFE and OFDM have a similar computational complexity in signal processing.

M-QAM-OFDM system performance in the presence of a nonlinear amplifier and phase noise

Abstract: The orthogonal frequency-division multiplexing (OFDM) modulation format has been proposed in Europe as the standard for broadcasting both audio and television digital signals and for wide-band wireless communication systems (e.g., HIPERLAN II). The performance of the OFDM scheme is severely affected by the nonlinearity of the high-power amplifier at the transmitter end and by the phase noise of the oscillators. In this paper, we investigate the joint effects induced on the OFDM signal by the amplifier nonlinearity and by the phase noise. An accurate statistical description of each contribution to the signal distortion is provided. Theoretical results show perfect agreement with those obtained by simulation and they can be used to derive the OFDM system performance, without the need to run extensive simulations.

LDPC-based space-time coded OFDM systems over correlated fading channels: Performance analysis and receiver design

Abstract: We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels.

Subspace-based (semi-) blind channel estimation for block precoded space-time OFDM

Abstract: Space time coding has by now been well documented as an attractive means of achieving high data rate transmissions with diversity and coding gains, provided that the underlying propagation channels can be accounted for. We rely on redundant linear precoding to develop a (semi-)blind channel estimation algorithm for space time (ST) orthogonal frequency division multiplexing (OFDM) transmissions with Alamouti's (see IEEE J. Select. Areas Commun., vol.16, p.1451-58, Oct. 1998) block code applied on each subcarrier. We establish that multichannel identifiability is guaranteed up to one or two scalar ambiguities, regardless of the channel zero locations and the underlying signal constellations, when distinct or identical precoders are employed for even and odd indexed symbol blocks. With known pilots inserted either before or after precoding, we resolve the residual scalar ambiguities and show that distinct precoders require half the number of pilots than identical precoders to achieve the same channel estimation accuracy. Simulation results confirm our theoretical analysis and illustrate that the proposed semi-blind algorithm is capable of tracking slow channel variations and improving the overall system performance relative to competing differential ST alternatives.

OFDM code-division multiplexing in fading channels

Abstract: In this paper, orthogonal frequency-division multiplexing-code-division multiplexing (OFDM-CDM) is presented and investigated as alternative to conventional OFDM for high rate data transmission. An additional multipath diversity gain can be obtained with OFDM-CDM by spreading each data symbol in frequency and time. The optimum reliability information for the Viterbi (1979) decoder is derived for OFDM-CDM systems, and the tradeoff between spreading and channel coding in OFDM systems is presented. By using efficient interference cancellation or joint detection techniques, it can be shown that OFDM-CDM outperforms conventional OFDM with respect to bit error rate (BER) performance and bandwidth efficiency.

Blind channel identification and equalization in OFDM-based multiantenna systems

Abstract: Wireless systems employing multiple antennas at the transmitter and the receiver have been shown to have the potential of achieving extraordinary bit rates. Orthogonal frequency division multiplexing (OFDM) significantly reduces the receiver complexity in multiantenna broadband systems. We introduce an algorithm for blind channel identification and equalization in OFDM-based multiantenna systems. Our approach uses second-order cyclostationary statistics, employs antenna precoding, and yields unique channel estimates (up to a phase rotation for each transmit antenna). Furthermore, it requires only an upper bound on the channel order, it does not impose restrictions on channel zeros, and it exhibits low sensitivity to stationary noise. We present simulation results demonstrating the channel estimator and the corresponding multichannel equalizer performance.

System for transporting multiple radio frequency signals of a multiple input, multiple output wireless communication system to/from a central processing base station

Abstract: The system includes, at least first and second antennas receiving first and second radio frequency signals, and a multiplexing system converting the first and second radio frequency signals to first and second optical signals and multiplexing the first and second optical signals for transmission over the optical fiber. A central processing base station is adapted for connection to the optical fiber. The central processing base station demultiplexes the first and second optical signals from the optical fiber, converts the first and second optical signals into the first and second radio frequency signals, and processes the first and second radio frequency signals to obtain information signals. In another embodiment, a conductor such as a coaxial cable replaces the optical fiber, and the multiplexer multiplexes the first and second radio frequency signals onto the conductor. In a further embodiment, the central processing base station uses the same techniques to send signals to an access point for transmission.

Low-complexity blind carrier frequency recovery for OFDM signals over frequency-selective radio channels

Abstract: This paper introduces a blind (ie, data independent) algorithm for carrier frequency offset recovery in an orthogonal frequency-division multiplexing (OFDM) receiver operating over frequency-selective fading channels The main idea behind this algorithm is to exploit the time-frequency-domain exchange inherent to the modulation scheme Due to this feature, a carrier frequency offset has a similar impact on OFDM as a clock timing offset has in a quadrature amplitude modulation (QAM) system The scheme we propose is a variant of Oerder-Meyr's (1988) feedforward clock recovery Its performance is assessed by simulation, and the results are compared to those obtained from Van de Beek-Sandell-Borjesson's (1997) frequency synchronizer, which bears comparable complexity The new scheme is shown to outperform the latter over frequency-selective fading channels, notably at medium to high signal-to-noise ratios We also evaluated the efficiency of two different (time domain and frequency domain) offset correction strategies embedded in a particular OFDM receiver

The cyclic prefix of OFDM/DMT - an analysis

Abstract: We address the impact of a too short cyclic prefix on multicarrier systems such as orthogonal frequency division multiplexing (OFDM) and discrete multitone (DMT). The main result is that the intersymbol interference (ISI) and intercarrier interference (ICI) may be spectrally concentrated and analytical expressions showing this are given. A practical implication is, e.g., that the cyclic prefix in some xDSL systems can be surprisingly short, as shown in one example of ADSL transmission.

Minimum BER power loading for OFDM in fading channel

Abstract: A novel approach to power loading for orthogonal frequency-division multiplexing (OFDM) systems is proposed. The corresponding algorithm minimizes the aggregate bit-error rate, but appears to be rather complicated. Consequently, a much simpler, quasi-optimal loading algorithm is developed. The performance of various power loading strategies known in the literature are compared with the proposed quasi-optimal algorithm in the Rayleigh frequency-selective fading channel.

Differential space-time coding for frequency-selective channels

Abstract: We introduce two space-time transmission schemes which allow full-rate and full-diversity noncoherent communications using two transmit antennas over fading frequency-selective channels. The first scheme operates in the frequency domain where it combines differential Alamouti (seeIEEE J. Select. Areas Commun., vol.16, p.1451-58, Nov. 1998) space-time block-coding (STBC) with OFDM. The second scheme operates in the time domain and employs differential time-reversal STBC to guarantee blind channel identifiability without the need for temporal oversampling or multiple receive antennas.

Maximum likelihood estimation of OFDM carrier frequency offset

Abstract: We develop a maximum likelihood estimate for orthogonal frequency division multiplexing (OFDM) carrier frequency offset in the presence of virtual carriers. It is found that the resulting estimate has an identical form to that of a previously proposed blind OFDM carrier frequency offset estimate by Liu and Tureli (see IEEE Commun. Lett., vol.2, p.104-106, 1998). Insights are provided as to why these two algorithms are equivalent.

Optical orthogonal frequency division multiplexing using frequency/time domain filtering for high spectral efficiency up to 1 bit/s/Hz

Abstract: Summary form only given. We have proposed a novel optical orthogonal frequency division multiplexing technique that can overcome the spectral efficiency limitation of the conventional WDM system. This scheme permits substantial overlapping of the spectrum and can achieve the spectral efficiency up to 1 bit/s/Hz in principle. For demultiplexing, we used a newly developed optical discrete Fourier transformer (DFT) instead of electrical digital processing, which is impossible to apply in the optical frequency range. The optical DFT was realized by using a set of delay lines, a phase shifter and a coupler in the frequency domain and bit synchronization and an optical gate in the time domain. In experimental demonstration of this scheme, error-free operation was obtained with a 0.8 bit/s/Hz of spectral efficiency.

Performance analysis of equalized OFDM systems in Rayleigh fading

Abstract: Channel estimation is usually needed to compensate for the amplitude and phase distortions associated with a received orthogonal frequency-division multiplexing (OFDM) waveform. This paper presents a systematic approach for analyzing the bit-error probability (BEP) of equalized OFDM signals in Rayleigh fading. Closed-form expressions for BEP performance of various signal constellations [phase-shift keying (PSK), differential phase-shift keying (DPSK), quaternary phase-shift keying (QPSK)] are provided for receivers that use a linear pilot-assisted channel estimate. We also derive the optimal linear channel estimates that yield the minimum BEP and show that some previous known results are special cases of our general formulae. The results obtained here can be applied to evaluate the performance of equalized single-carrier narrowband systems as well.

Allocation of bit streams for communication over multi-carrier frequency-division multiplexing (fdm)

Abstract: An architecture for providing high-speed access over frequency-division multiplexed (FDM) channels allows transmission of ethernet frames and/or other data across a cable transmission network (105) or other form of FDM transport The architecture involves downstream and upstream FDM multiplexing techniques to allow contemporaneous, parallel communications across a plurality of frequency channels Also, the modulation indices of various upstream frequency channels may be different, but a plurality of upstream channels may be used to carry a single data flow generally in parallel The upstream data flow is fragmented into blocks and formed into superframes to allow transmission over at least one upstream frequency channels When a plurality of upstream frequency channels are utilized, the superframes facilitate the possibility of having different modulation indices on the plurality of frequency channels

A vision-based DSP embedded navigation sensor

Abstract: Spacecraft missions such as spacecraft docking and formation flying require high-precision relative position and attitude data. Deep space missions require the use of alternative technologies. One such technology is the vision-based navigation (VISNAV) sensor system developed at Texas A&M University. VISNAV comprises an electro-optical sensor combined with light sources or beacons. This patented sensor has an analog detector in the focal plane with a rise time of a few microseconds. Accuracies better than one part in 2000 of the field of view have been obtained. Simultaneous activation of beacons with frequency division multiplexing is given as part of the VISNAV sensor system. The synchronous demodulation process uses digital heterodyning and decimating filter banks on a low-power fixed point digital signal processor, which improves the accuracy of the sensor measurements and the reliability of the system. This paper also presents an optimal and computationally efficient six-degree-of-freedom estimation algorithm using a new measurement model based on the attitude representation of modified Rodrigues parameters.

High-resolution operation of frequency-multiplexed transition-edge photon sensors

Abstract: We present the multiplexed readout of two gamma-ray microcalorimeters made from transition-edge sensors. We use a frequency-domain multiplexing scheme in which each sensor is biased at an identifying frequency. We show that the energy resolution of the sensors is unaffected by multiplexing, and that crosstalk between the sensors is negligible. Our results indicate the feasibility of multiplexing 30 sensors or more to one readout line.

A sequential Monte Carlo blind receiver for OFDM systems in frequency-selective fading channels

Abstract: We consider the application of the sequential Monte Carlo (SMC) methodology to the problem of blind symbol detection in a wireless orthogonal frequency-division multiplexing (OFDM) system over a frequency-selective fading channel. Bayesian inference of the unknown data symbols in the presence of an unknown multipath fading channel is made only from the observations over one OFDM symbol duration. A novel blind SMC detector built on the techniques of importance sampling and resampling is developed for differentially encoded OFDM systems. The performance of different schemes of delayed-weight estimation methods is studied. Furthermore, being soft-input and soft-output in nature, the proposed SMC detector is employed as the first-stage demodulator in a turbo receiver for a coded OFDM system. Such a turbo receiver successively improves the receiver performance by iteratively exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel decoder. Finally, the performance of the proposed sequential Monte Carlo receiver is demonstrated through computer simulations.

An interference suppressing OFDM system for wireless communications

Abstract: The orthogonal frequency division multiplexed (OFDM) system introduced here, termed interference suppressing OFDM (IS-OFDM), has the capabilities of suppressing narrowband interference in wideband wireless applications. The IS-OFDM encodes each transmitted symbol in all frequency bins. Each frequency bin then "contains" all transmitted symbols which are distinguished and separated from each other by orthogonal Hadamard sequences. The IS-OFDM can provide a point-to-point wireless link without spreading the incoming data rate. In addition, the IS-OFDM has all the advantages of the ordinary OFDM, which is shown to be a special case of the IS-OFDM. We present the IS-OFDM system design and evaluate its performance in the presence of narrowband interference and AWGN channel.

Performance analysis of adaptive interleaving for OFDM systems

Abstract: We proposed a novel interleaving technique for orthogonal frequency division multiplexing (OFDM), namely adaptive interleaving, which can break the bursty channel errors more effectively than traditional block interleaving. The technique rearranges the symbols according to instantaneous channel state information of the OFDM subcarriers so as to reduce or minimize the bit error rate (BER) of each OFDM frame. It is well suited to OFDM systems because the channel state information (CSI) values of the whole frame could be estimated at once when transmitted symbols are framed in the frequency dimension. Extensive simulations show that the proposed scheme can greatly improve the performance of the coded modulation systems utilizing block interleaving. Furthermore, we show that the adaptive interleaving out performs any other static interleaving schemes, even in the fast fading channel (with independent fading between symbols). We derived a semi-analytical bound for the BER of the adaptive interleaving scheme under correlated Rayleigh fading channels. Furthermore, we discussed the transmitter-receiver (interleaving pattern) synchronization problem.

SQUID-based readout schemes for microcalorimeter arrays

Abstract: Microcalorimeter arrays with superconducting phase-transition thermometers are an attractive alternative for construction of imaging X-ray spectrometers. The low source impedance and low operating temperatures make dc SQUIDs good candidates for their preamplifiers. In large arrays, the circuit complexity as well as heat leakage through wiring make some sort of a multiplexing scheme desirable. Some circuit topologies for time-domain and frequency-domain multiplexing schemes are compared. One particular frequency-domain multiplexing circuit, being developed for the XEUS space mission, is reviewed in more detail. The design issues related with dynamic range, bandwidth and noise folding are discussed.

Overview of high performance fiber optic sensing

Abstract: Summary form only given For the purposes here, a high performance sensing application is defined as one that simultaneously requires high sensitivity (1-100 f/spl epsiv/), wide bandwidth (1-100 kHz), and large dynamic range (>120 dB) High performance fiber optic sensing applications include tactical and surveillance grade underwater acoustics, high sensitivity acceleration, and acoustic and seismic sensing for oil exploration Traditionally, high performance fiber optic sensor systems have relied on interferometric measurement techniques to extract the strain information from the fiber optic transducer Distributed Bragg reflector (DBR) fiber laser based systems with interferometric readout approach the sensitivity of interferometric approaches, but have seen little use outside the laboratory The performance of DFB fiber laser sensors can rival interferometric approaches, but there are questions about the multiplexing the lasers and the packaging requirements for stable operation While interferometric based approaches still offer the highest performance levels with proven multiplexing gain, DFB fiber laser sensors could fill some application requirements in the near future

Light wavelength-multiplexing systems

Abstract: A wavelength-division multiplexing optical communication system for wavelength-division multiplexing a plurality of optical signals having different wavelengths and transmitting a wavelength-division multiplexed signal via an optical fiber transmission line. The wavelength-division multiplexing system includes a multiplexing unit that wavelength-multiplexes a plurality of optical signals having different wavelengths. A storing unit stores information regarding an addition or subtraction of an optical signal to be wavelength-multiplexed, into a predetermined time slot of a frame signal that is composed of a plurality of time slots. The wavelength-division multiplexing system outputs a monitor/control frame signal, based upon the stored information. A transmitting device transmits the monitor/control signal as an optical signal having a wavelength different from the wavelengths of the multiplexed optical signals.

Partial transmit sequence arid selected mapping schemes to reduce ICI in OFDM systems

Abstract: Orthogonal frequency division multiplexing (OFDM) is sensitive to the carrier frequency offset (CFO). We introduce the peak interference-to-carrier ratio (PICR) to measure the resulting intercarrier interference (ICI). This paper shows that PICR can be reduced by selected mapping (SLM) and partial transmit sequence (PTS) approaches. These schemes are analyzed theoretically and their performances are evaluated by simulation.

Multi-carrier frequency-division multiplexing (FDM) architecture for high speed digital service

Abstract: An architecture for providing high-speed access over frequency-division multiplexed (FDM) channels allows transmission of ethernet frames and/or other data across a cable transmission network or other form of FDM transport. The architecture involves downstream and upstream FDM multiplexing techniques to allow contemporaneous, parallel communications across a plurality of frequency channels. Furthermore, the architecture allows a central concentrator to support a plurality of remote devices that each have guaranteed bandwidth through connection-oriented allocations of bi-directional data flows. The upstream and downstream bandwidth allocation can support symmetrical bandwidth as well as asymmetrical bandwidth in either direction. The architecture generally can be used to support connection-oriented physical layer connectivity between a remote device and the central concentrator. Furthermore, the architecture may be integrated into other higher level devices such as, but not limited to, bridges, switches, routers, and/or gateways. The architecture generally may peacefully coexist with other services commonly-found in cable distribution networks.

Radio transmission apparatus and radio communication method

Abstract: A radio transmission apparatus capable of further improving the frequency use efficiency and the transmission speed while maintaining the communication quality. In this apparatus, switching is performed according to the transmission path condition between the followings: transmitting different information (transmission signal A ≠ transmission signal B) with an identical frequency from a plurality of antennas (106, 116) (space multiplexing), transmitting different information (transmission signal A ≠ transmission signal B) with different frequencies from the plurality of antennas (106, 116) (frequency multiplexing), transmitting identical information (transmission signal A = transmission signal B) with an identical frequency from the plurality of antennas (106, 116) (space diversity), and transmitting identical information (transmission signal A = transmission signal B) with different frequencies from the plurality of antennas (106, 116) (frequency diversity).

Performance analysis of closed-loop transmit diversity in the presence of feedback errors

Abstract: Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennas have been included in 3GPP WCDMA release 4. System capacity can be increased from that of open-loop modes if the transmitter is equipped with additional side information of the downlink channel. In a frequency division duplex system, this means that the receiver has to provide the information through a feedback mechanism. We study the performance loss caused by feedback errors using the bit error probability as a performance measure. It is shown that the asymptotic diversity of closed-loop schemes is reduced to one in the presence of feedback errors, i.e., the performance is similar to a single transmit antenna system with some SNR gain. The proof is given in a very general manner and it is valid for a large variety of feedback schemes. Two example schemes, selection and cophasing algorithms, are discussed in more detail to verify the results.

Multi-user perspectives in UWB communications networks

Abstract: Ultra-wideband technology provides robust high data rate networks that can be used to meet the needs of wireless personal area network (WPAN) applications. An essential part of this design is the selection of a multiplexing technique to provide multiple physical channels (piconets) from the single shared UWB band. This paper provides an overview of multiplexing techniques and suggests ways in which they can be applied to UWB networks. Relative advantages and challenges of each technique are identified in order to understand the suitabilitity of each for use in a multi-piconet WPAN system.