Showing papers on "Frequency-division multiplexing published in 2005"

Cross-layer optimization for OFDM wireless networks-part II: algorithm development

Abstract: We have established a theoretical framework for cross-layer optimization in orthogonal frequency division multiplexing (OFDM) wireless networks. In this paper, we focus on effective and practical algorithms for efficient and fair resource allocation in OFDM wireless networks. We have taken various conditions into account and developed a variety of efficient algorithms, including sorting-search dynamic subcarrier assignment, greedy bit loading, and power allocation, as well as objective aggregation algorithms. We have also modified those algorithms for a certain type of nonconcave utility functions. To further improve performance by exploiting time diversity, a low-pass time filter can be easily incorporated into all of the algorithms. Simulation results have confirmed that the utility-based cross-layer optimization can significantly enhance the system performance and guarantee fairness. The gains come from multiuser diversity, frequency diversity, as well as time diversity. The fairness is automatically achieved by the behavior of marginal utility functions.

A QRD-M/Kalman filter-based detection and channel estimation algorithm for MIMO-OFDM systems

Abstract: The use of multiple transmit/receive antennas forming a multiple-input multiple-output (MIMO) system can significantly enhance channel capacity. This paper considers a V-BLAST-type combination of orthogonal frequency-division multiplexing (OFDM) with MIMO (MIMO-OFDM) for enhanced spectral efficiency and multiuser downlink throughput. A new joint data detection and channel estimation algorithm for MIMO-OFDM is proposed which combines the QRD-M algorithm and Kalman filter. The individual channels between antenna elements are tracked using a Kalman filter, and the QRD-M algorithm uses a limited tree search to approximate the maximum-likelihood detector. A closed-form symbol-error rate, conditioned on a static channel realization, is presented for the M=1 case with QPSK modulation. An adaptive complexity QRD-M algorithm (AC-QRD-M) is also considered which assigns different values of M to each subcarrier according to its estimated received power. A rule for choosing M using subcarrier powers is obtained using a kernel density estimate combined with the Lloyd-Max algorithm.

MMSE and adaptive prediction of time-varying channels for OFDM systems

Abstract: We propose decision-directed channel predictors for orthogonal frequency-division multiplexing (OFDM) communications over time-varying channels Channel prediction is capable of yielding up-to-date channel state information even without regular transmission of pilot symbols It is thus potentially useful for delay-free equalization, antenna combining, space-time decoding, adaptive modulation, adaptive power control, and adaptive transmit antenna diversity We derive a minimum mean-square error channel predictor and its efficient discrete Fourier transform implementation Furthermore, we develop adaptive predictors that do not require any statistical prior knowledge and are able to track nonstationary channel and noise statistics Simulation results involving an OFDM receiver in which channel prediction is used for delay-free equalization demonstrate the excellent performance of the proposed techniques even for fast time-varying channels

Phase noise-tolerant synchronous QPSK/BPSK baseband-type intradyne receiver concept with feedforward carrier recovery

Abstract: Quadrature phase-shift keying (QPSK) is attractive to increase transmission lengths and capacity, especially when it is combined with polarization division multiplex. Baseband processing at the symbol rate allows to keep the required electronic bandwidth low. So far, external cavity lasers seemed to be indispensable for such transmission systems due to linewidth requirements. We propose a feedforward carrier recovery scheme based on regenerative intradyne frequency dividers, i.e., the well-known regenerative frequency divider is extended to process baseband in-phase and quadrature (I and Q) signals. An IF linewidth tolerance of up to 0.001 times the QPSK symbol rate is predicted, 2 decades more than for an optical phase locked loop with a realistic loop delay. This means that commercially available DFB lasers shall suffice for synchronous optical QPSK/BPSK transmission.

Sidelobe suppression in OFDM systems by insertion of cancellation carriers

Abstract: Orthogonal frequency-division multiplexing (OFDM) systems exhibit significant out-of-band radiation caused by high sidelobes of the modulated subcarriers. Existing techniques for reducing this undesirable effect have several drawbacks as they waste the scarce spectral resources or expand the signal in time domain. In this paper, we propose a new technique which overcomes these problems. A few so-called cancellation carriers are inserted on the left and right hand side of the used OFDM spectrum. These special subcarriers are not employed for data transmission, but carry complex weighting factors which are determined such that the sidelobes of transmission signal and cancellation carriers cancel each other. Simulation results show that with the proposed method a significant sidelobe suppression is achieved with only a small loss in bit error rate performance. This loss is due to the fact that a certain amount of the transmission power has to be spent on the cancellation carriers and is not available for data transmission.

Upstream channel bonding in a cable communications system

Abstract: A method for increasing upstream bandwidth per cable modem user in a cable communications system that includes a cable modem termination system (CMTS) and a plurality of cable modems is provided. The method permits a cable modem to transmit data to the CMTS on multiple upstream channels simultaneously using a technique called “channel bonding.” Channel bonding allows smaller bandwidth upstream channels to be bonded together to create a larger bandwidth pipe.

Wavelength division multiplexing in long-haul transoceanic transmission systems

Abstract: Wavelength division multiplexing (WDM) technology used in long-haul transmission systems has steadily progressed over the past few years. Newly installed state-of-the-art transoceanic systems now have terabit per second maximum capacity, while being flexible enough to have an initial deployed capacity at a fraction of the maximum. The steady capacity growth of these long-haul fiber-optic cable systems has resulted from many improvements in WDM transmission techniques and an increased understanding of WDM optical propagation. Important strides have been made in areas of dispersion management, gain equalization, modulation formats, and error-correcting codes that have made possible the demonstration of capacities approaching 4 Tb/s over transoceanic distances in laboratory experiments.

Pilot and data transmission in a quasi-orthogonal single-carrier frequency division multiple access system

Abstract: In a single-carrier frequency division multiple access (SC-FDMA) system that utilizes interleaved FDMA (IFDMA) or localized FDMA, multiple transmitters may transmit their pilots using time division multiplexing (TDM), code division multiplexing (CDM), interleaved frequency division multiplexing (IFDM), or localized frequency division multiplexing (LFDM). The pilots from these transmitters are then orthogonal to one another. A receiver performs the complementary demultiplexing for the pilots sent by the transmitters. The receiver may derive a channel estimate for each transmitter using an MMSE technique or a least-squares technique. The receiver may receive overlapping data transmissions sent on the same time-frequency block by the multiple transmitters and may perform receiver spatial processing with spatial filter matrices to separate these data transmissions. The receiver may derive the spatial filter matrices based on the channel estimates for the transmitters and using zero-forcing, MMSE, or maximal ratio combining technique.

EM-based iterative receiver design with carrier-frequency offset estimation for MIMO OFDM systems

Abstract: In this letter, we study the design of expectation-maximization (EM)-based iterative receivers for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing systems with the presence of carrier-frequency offset (CFO). Motivated by the spirit of maximum-likelihood estimation in the EM algorithm, we first present a pilot-aided CFO estimation scheme that allows fast Fourier transform-based fast implementation. Then this CFO estimation is incorporated into the initialization step of the iterative receiver. Experimental results show the effectiveness of our receiver design in combating CFO.

Downlink resource management for packet transmission in OFDM wireless communication systems

Abstract: In this paper, an optimal downlink resource management scheme for heterogeneous packet transmission in orthogonal frequency-division multiplexing (OFDM) wireless communication systems is proposed. By making use of the channel impulse response and the properties of the OFDM physical layer, a resource management scheme is developed by integrating power distribution, subcarrier allocation, and the generalized processor sharing (GPS) scheduling. The scheme can: 1) maximize system throughput; 2) guarantee the required signal-to-noise ratio for heterogeneous traffic; 3) provide fairness to all the traffic admitted in the system; and 4) satisfy the total transmission power constraint. For practical implementation, a simplified power and subcarrier allocation algorithm, a robust H/sub /spl infin// channel estimation algorithm, and a truncated GPS (TGPS) scheduling scheme are introduced. Simulation results show that the proposed resource management scheme exhibits good throughput performance.

Apparatus and method for simultaneous multiple telephone type services on a single telephone line

Abstract: Apparatus and method for simultaneously providing multiple telephone-type services to any/all POTS-type devices on a single wire pair at a user premises. The present invention provides for the ability to add separately addressable POTS devices on a single service loop. This can be accomplished in at least two ways: first by the use of a multipoint protocol or second by Frequency Division Multiplexing.

Data transmission system in broadband wireless access system using band amc and method thereof

Abstract: Disclosed are a system for transmitting data using a band AMC in a broadband wireless orthogonal frequency multiplexing access system and a method thereof In a broadband wireless communication system, a method for allocating data bursts to a mobile subscriber station according to channel state in a broadband wireless communication system, the method including transmitting allocation information to the mobile subscriber station, the allocation information including information about a sub frequency band containing the data bursts allocated to the mobile subscriber station and information about the number of band AMC subchannels allocated to the mobile subscriber station in the sub frequency band and receiving the data bursts from the mobile subscriber station according to the allocation information

Optical communications based on optical polarization multiplexing and demultiplexing

Abstract: This application describes techniques for optical multiplexing and demultiplexing in optical communication systems based on polarization multiplexing of different signal channels.

Performance analysis of multiband OFDM for UWB communication

Abstract: The frequency-hopping orthogonal frequency-division multiplexing (OFDM) proposal, known as multiband OFDM, is a strong contender for the physical layer IEEE standard for high-rate wireless personal area networks (WPANs) bused on ultra-wideband (UWB) transmission. In this paper we analyze the performance of the multiband OFDM proposal. To this end, we (a) study the channel model developed under IEEE 802.15 for UWB radio from a frequency-domain perspective suited for OFDM transmission, (b) develop and quantify the appropriate information-theoretic performance measures, (c) compare these measures with simulation results for multiband OFDM systems, and (d) consider the influence of practical, imperfect channel estimation on the performance.

OFDM and its application to 4G

Abstract: Summary form only given. Orthogonal frequency-division multiplexing (OFDM) is a bandwidth-efficient signaling scheme for wideband digital communications. The main difference between frequency division multiplexing (FDM) and OFDM is that in OFDM, the spectrum of the individual carriers mutually overlap. Nevertheless, the OFDM carriers exhibit orthogonality on a symbol interval if they are spaced in frequency exactly at the reciprocal of the symbol interval, which can be accomplished by utilizing the discrete Fourier transform (DFT). With the development of modern digital signal processing technology, OFDM has become practical to implement and has been proposed as an efficient modulation scheme for applications ranging from modems, digital audio broadcast, to next-generation high-speed wireless data communications. For example, the high-speed wireless LAN standard IEEE 802.11a is based on OFDM. We discuss receiver design for OFDM systems signaling through unknown frequency-selective fading channels. In particular, we focus on the design of turbo receivers in a number of OFDM systems, including an OFDM system with frequency offset, a space-time block coded OFDM system, and a space-time coded OFDM system based on low-density parity-check (LDPC) codes.

Analysis of power ramping schemes for UTRA-FDD random access channel

Abstract: The random access channel (RACH) in a universal terrestrial radio access-frequency division duplex (UTRA-FDD) system is a contention-based channel mainly used to carry control information from mobile stations (MS) to base stations (BS) The transmission of a random access request contains two steps: preamble transmission and message transmission In preamble transmission, the power ramping technique is used to favor the delayed preambles by stepping up the transmission power after each unsuccessful access In doing so, the success of transmitting a long-delayed preamble is increased due to the power capture effect This paper analyzes the blocking, throughput, and delay performance of preamble transmission under three power ramping schemes with fixed, linear, and geometric step sizes The interference caused by different power ramping schemes is also compared

Intrusion detection system for use on an optical fiber using a translator of transmitted data for optimum monitoring conditions

Abstract: A method and system of intrusion detection system for a multimode fiber optic cable. A light signal is launched into the cable fiber to establish a narrow spectral width, under-filled non-uniform mode field power distribution in the cable. A small portion of the higher order signal modes arriving at the remote end of the cable is sampled by use of a coupler and monitored for transient changes in the mode field power distribution. The power distribution changes with physical disturbance of the cable. When those changes are detected as being characteristic of fiber intrusion, the system activates an alarm. This method can sense and alarm any attempt to access the optical fibers in a fiber optic communication cable. In preferred embodiments, the active signal of a multimode optical fiber is monitored for both signal degradation and transient power disturbance patterns that could indicate fiber damage or physical intrusion. A translator can be provided in an existing optical fiber system in which the data signals are translated in wavelength and/or launch conditions to optimize the monitoring signals in an otherwise non-optimized system.

A framework for investigating radio resource management algorithms in TD-SCDMA systems

Abstract: TDD-CDMA, globally recognized as the basis for Chinese 3G, is designed for symmetrical and asymmetrical 3G mobile services. As a time-division duplexing system, TD-SCDMA works in unpaired frequency allocations, supporting the flexible allocation of uplink/downlink slots in one carrier, and adapting to the different system loads between uplink and downlink. Dynamic time slot allocations make it more spectrally efficient than paired uplink/downlink carriers. Consequently, TD-SCDMA can make full use of the radio spectrum through optimal adaptive radio resource allocation. In order to support peak rates of 5 Mb/s or higher, the TD-SCDMA standard is enhanced via high-speed downlink packet access (HSDPA) mode. In this article the properties and strategies of radio resource management (RRM) are introduced, and the differences in admission control, power control, handover, load control, dynamic channel allocation, and adaptive HSDPA techniques between TDD-CDMA and the conventional FDD-CDMA system are analyzed. Some advanced strategies and architectures for TDD-CDMA RRM algorithms are presented. Since uplink multi-user detection (MUD) and smart antenna (SA) techniques are adopted in TD-SCDMA, the RRM algorithms based on MUD and SA are investigated further. In addition, these advanced RRM algorithms are characterized by an OPNET-based dynamical system-level simulator.

A novel architecture for multilane-free-flow electronic-toll-collection systems in the millimeter-wave range

Abstract: An architecture for simultaneously performing multitarget tracking and multidata communication suitable for millimeter-wave multilane-free-flow electronic-toll-collection (ETC) systems is presented. This architecture combines the idea of frequency multiplexing in communication systems and the technique of target tracking in the pulse-Doppler radar. For target tracking, we make use of pulse ranging by the aid of pulse compression and fine target-direction determination by amplitude comparison to obtain high resolution in the radial direction (downrange), as well as both lateral directions (cross ranges). This architecture can be utilized for both active and passive onboard-unit (OBU) systems. Another important advantage of this architecture is that it can easily identify the passing vehicles not equipped with an OBU. This will activate some subsequent enforcement activities against the violation vehicles.

Novel schemes for local area network emulation in passive optical networks with RF subcarrier multiplexed customer traffic

Abstract: This paper proposes two novel optical layer schemes for intercommunication between customers in a passive optical network (PON). The proposed schemes use radio frequency (RF) subcarrier multiplexed transmission for intercommunication between customers in conjunction with upstream access to the central office (CO) at baseband. One scheme employs a narrowband fiber Bragg grating (FBG) placed close to the star coupler in the feeder fiber of the PON, while the other uses an additional short-length distribution fiber from the star coupler to each customer unit for the redirection of customer traffic. In both schemes, only one optical transmitter is required at each optical network unit (ONU) for the transmission of customer traffic and upstream access traffic. Moreover, downstream bandwidth is not consumed by customer traffic unlike in previously reported techniques. The authors experimentally verify the feasibility of both schemes with 1.25 Gb/s upstream baseband transmission to the CO and 155 Mb/s customer data transmission on the RF carrier. The experimental results obtained from both schemes are compared, and the power budgets are calculated to analyze the scalability of each scheme. Further, the proposed schemes were discussed in terms of upgradability of the transmission bit rates for the upstream access traffic, bandwidth requirements at the customer premises, dispersion tolerance, and stability issues for the practical implementations of the network.

Design and prototype development of MIMO-OFDM for next generation wireless LAN

Abstract: Multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) is regarded as a promising technology enabling higher data rate wireless communications in frequency selective fading channels. This paper proposes the next generation wireless local area network (WLAN) physical (PHY) layer transmission technology using dual-band and MIMO-OFDM schemes and discusses test results of MIMO-OFDM prototype system implemented with field programmable gate arrays (FPGA). This design targets over 200 Mbps of the maximum PHY data rate in 40 MHz bandwidth and compatibility with the legacy IEEE 802.11a. For a cost-effective implementation and improved packet error rate (PER) performance, 2 transmit and 3 receive antennas and 40 MHz channel are used to achieve the desired PER and throughput performance in the 5 GHz band.

Design of dual-band MIMO-OFDM system for next generation wireless LAN

Abstract: Multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) is regarded as a promising technology enabling higher data rate wireless communications in frequency selective fading channels. This paper proposes the next generation wireless local area network (WLAN) physical (PHY) layer transmission technology using dual-band and MIMO-OFDM schemes. It describes a design of the higher data rate WLAN system targeting the maximum speed of 144 Mbps in 20 MHz bandwidth channels and 288 Mbps in 40 MHz. Additionally, the design suggests a new frame structure based on the 802.11a frame format for guaranteeing the compatibility with IEEE 802.11a legacy-OFDM systems and the MIMO channel estimation capability. For a cost-effective implementation and improved error performance, 2 transmit and 3 receive antennas are used in dual-band.

SFDM/CWDM of fiber-optic Fizeau strain sensors

Abstract: In this letter, a novel method for multiplexing a large number of fiber-optic Fizeau sensors using coarse wavelength-division multiplexing (CWDM) and spatial frequency-division multiplexing (SFDM) is proposed and demonstrated. A CWDM device is used to multiplex signals from different sensor channels occupying different wavelength regions divided by the CWDM. More than 10 Fizeau sensors arranged in a single channel can be multiplexed by using SFDM due to the long cavity length of the Fizeau sensor. The experimental results show that a strain accuracy of /spl plusmn/5 /spl mu//spl epsiv/ is achieved. As up to 100 sensors can be multiplexed simultaneously, this approach can improve the multiplexing capacity of Fizeau strain sensors significantly.

A robust fiber-radio architecture for wavelength-division-multiplexing ring-access networks

Abstract: In this paper, we propose a dynamic wavelength-allocation scheme for wavelength-division multiplexing (WDM) fiber-radio ring-access networks. This scheme can be used to improve the use efficiency of wavelengths at the burst traffic load on the fiber-radio networks. Moreover, a novel bidirectional wavelength add-drop multiplexer (B-WADM) is designed on the backboned ring, which can connect the working and standby sub-rings. It can provide the self-protected function that uses just a few optical devices under link failures. Furthermore, we carry out this study through the simulation and analysis for carrier-to-noise ratio (CNR), channel capacity, and spurious free dynamic range (SFDR) of the fiber-radio network. Finally, we set up an experimental network to demonstrate its performance. The experimental results illustrate that this fiber-radio architecture can provide the robust, flexible, and reliable characteristics for large radio terminals.

Quantized multi-mode precoding for spatial multiplexing MIMO-OFDM system

Abstract: Spatial multiplexing with multi-mode precoding can achieve both high capacity and high reliability in multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Multimode precoding uses linear transmit precoding but adapts the number of transmit streams or modes according to the channel conditions. Multi-mode precoding typically requires complete knowledge of the transmit precoding matrices for each subcarrier at the transmitter. In this paper we propose to reduce the feedback requirements by sending back the quantized precoding matrices of a fraction of the subcarriers and obtaining the other precoders using interpolation. Two algorithms are proposed for the interpolation of unitary matrices. Bit error rates simulations demonstrate the performance improvements of the proposed algorithms as a function of the feedback rate.

Time-domain equalizer for OFDM systems based on SINR maximization

Abstract: A new method is proposed in this paper to design the time-domain equalizer for orthogonal frequency division multiplexing systems based on maximizing signal-to-interference-plus-noise ratio (SINR) at the output of the equalizer. The method called Maximum SINR Time-domain Equalization (MSINR-TEQ) is derived based on a new formulation of SINR. Computer simulation and analytical results show the performance of the MSINR-TEQ method and its superiority in comparison with the performance of the much-used method of channel impulse response shortening.

Magnetic sensor array having an analog frequency-division multiplexed output

Abstract: A magnetic sensor array including magnetoresistive sensor elements having outputs combined by frequency division multiplexing (FDM) is provided. Each sensor element provides an input to a mixer which provides a distinct frequency shift. Preferably, time division multiplexing is also used to combine sensor element outputs. Each sensor element is typically in proximity to a corresponding sample. The sensor elements are preferably subarrays having row and column addressable sensor element pixels. This arrangement provides multiple sensor pixels for each sample under test. Multiplexing of sensor element outputs advantageously reduces readout time. A modulated external magnetic field is preferably applied during operation, to reduce the effect of 1/f noise on the sensor element signals. The effect of electromagnetic interference (EMI) induced by the magnetic field on sensor element signals is advantageously reduced by the mixing required for FDM.

Frequency offset estimation for differential OFDM

Abstract: This paper proposes a novel frequency offset (FO) estimator for differential orthogonal frequency division multiplexing. By separating the normalized FO into integral and fractional parts, our FO estimation is carried out in two successive stages. In the first stage, we take advantage of differential coding and the finiteness of signal constellation to derive a channel-independent cost function whose minimization leads to an accurate estimate of the fractional part. It is proved that under certain mild assumptions, the identifiability of the fractional part can be guaranteed with probability one. After compensating the fractional part, the second stage estimates the remaining integral part with the aid of a judiciously designed initial block of differential coding. Combining these two stages, the proposed estimator is shown capable of offering full-range FO estimation without relying on any training or virtual subcarriers. Furthermore, unlike some existing estimators, the proposed estimator is applicable in both frequency-flat and frequency-selective fading channels. Another salient feature of the proposed estimator is that accurate FO estimation can be achieved with two received data blocks only, making it very attractive especially for short-burst communications. The merits of our FO estimator have been confirmed by theoretical analysis as well as corroborating simulations.

Low-complexity joint synchronization of symbol timing and carrier frequency for OFDM systems

Abstract: Timing synchronization for guard-interval-based (GIB) orthogonal frequency multiplexing (OFDM) systems requires a correlator to correlate the last N/sub g/ (length of the guard interval) samples of the received OFDM symbol with their copies ahead. The straightforward implementation of this correlator results in the need to perform N/sub g/ complex additions for each correlator output. In this paper, we present an improved synchronization algorithm for the coarse joint estimation of symbol timing and carrier frequency offset. With the proposed algorithm, the coarse estimator can be implemented with extremely low hardware complexity by replacing the conventional correlator with a l-order low pass filter. Only one complex addition is needed for each output. The probability for frequency offset estimation error is analyzed for ideal channel. Simulation results show that it works reliably over both AWGN and multipath fading channels.

Blind beamforming in frequency division duplex MISO systems based on time reversal mirrors

Abstract: We propose a novel method for both downlink (DL) and uplink (UL) blind channel estimation and beamforming in frequency division duplex (FDD) MISO (multiple-input single-output) antenna systems. The method is related to the concept of time reversal mirrors (TRMs). The TRM is applied so that the subscriber unit (SU) uses the UL channel "doubly", and thus there is no transmission overhead. The method's performance is close to that of optimal beamforming. With four antennas at the base station (BS) there is a gain in the range of 2-3 dB relative to a typical space-time block coder without feedback.