Showing papers on "Frequency-division multiplexing published in 2007"

Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems

Abstract: We show, using simulations, that a combination of orthogonal frequency division multiplexing (OFDM) and optical single sideband modulation can be used to compensate for chromatic dispersion in ultralong-haul wavelength-division multiplexed (WDM) systems. OFDM provides a high spectral efficiency, does not require a reverse feedback path for compensation, and has a better sensitivity than nonreturn to zero. This paper provides design rules for 800-4000-km optical-OFDM systems. The effects of WDM channel number and spacing, fiber dispersion, and input power per channel on the received Q are studied using extensive numerical simulations. These effects are summarized as a set of design rules

Base station, communication terminal, transmission method, and reception method

Abstract: A base station includes: means configured to manage frequency blocks; means configured to determine, for each frequency block, scheduling information for assigning one or more resource blocks to a communication terminal being in a good channel state; means configured to generate a control channel including the scheduling information for each frequency block; and means configured to frequency multiplexing control channels within the system frequency band and to transmit it. In addition, the base station transmits the control channel by separating a non-specific control channel to be decoded by a non-specific communication terminal and a specific control channel to be decoded by a communication terminal to which one or more resource blocks are assigned.

Phase Estimation for Coherent Optical OFDM

Abstract: Phase estimation is one of the enabling functionalities in coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) receivers. In this letter, we compare pilot-aided and data-aided phase estimation methods for a CO-OFDM transmission experiment at 8 Gb/s over 1000-km standard single-mode fiber without optical dispersion compensation. We also show that as few as five subcarriers are sufficient for pilot-aided phase estimation

Experimental study on the Gaussian-modulated coherent-state quantum key distribution over standard telecommunication fibers

Abstract: In this paper, we present a fully fiber-based one-way quantum-key-distribution (QKD) system implementing the Gaussian-modulated coherent-state (GMCS) protocol. The system employs a double Mach-Zehnder interferometer (MZI) configuration in which the weak quantum signal and the strong local oscillator (LO) go through the same fiber between Alice and Bob, and are separated into two paths inside Bob's terminal. To suppress the LO leakage into the signal path, which is an important contribution to the excess noise, we implemented a scheme combining polarization and frequency multiplexing, achieving an extinction ratio of 70 dB. To further minimize the system excess noise due to phase drift of the double MZI, we propose that, instead of employing phase feedback control, one simply let Alice remap her data by performing a rotation operation. We further present noise analysis both theoretically and experimentally. Our calculation shows that the combined polarization and frequency multiplexing scheme can achieve better stability in practice than the time-multiplexing scheme, because it allows one to use matched fiber lengths for the signal and the LO paths on both sides of the double MZI, greatly reducing the phase instability caused by unmatched fiber lengths. Our experimental noise analysis quantifies the three main contributions to the excess noise, which will be instructive to future studies of the GMCS QKD systems. Finally, we demonstrate, under the ``realistic model'' in which Eve cannot control the system within Bob's terminal, a secure key rate of $0.3\text{bit}∕\text{pulse}$ over a 5km fiber link. This key rate is about two orders of magnitude higher than that of a practical Bennett-Brassard 1984 protocol QKD system.

An Automatic Digital Modulation Classifier for Measurement on Telecommunication Networks

Abstract: This paper presents a method for the automatic classification of digital modulations without a priori knowledge of the signal parameters. This method can recognize classical single- carrier modulations such as M-ary phase-shift keying, M-ary frequency-shift keying, M-ary amplitude-shift keying, and M-ary quadrature amplitude modulation, as well as orthogonal frequency-division multiplexing modulations such as discrete mul- titone that is used for asymmetric digital subscriber line and very high speed digital subscriber line standards and for power-line carrier transmissions. After identification of the modulation type, the method automatically estimates some parameters characterizing the modulation. To evaluate the method performance, several simulations have been carried out in different operating conditions that should be particularly critical by varying the values of signal- to-noise ratio and the main parameters of each identifiable modulation. To assess the advantages, comparison with other classification methods has been given. To validate the assumption that is made, experimental tests have been performed.

New OFDM peak-to-average power reduction scheme

Abstract: A main drawback of frequency division multiplexing (OFDM) system is the high peak-to-aver-age power ratio (PAPR). High PAPR leads to amplifier nonlinearity, inter-modulation and low inefficiency. As a simplest approach, traditional clipping method can reduce PAPR of OFDM signals, but it causes large unwanted out-of-band radiation. In this paper, a new clipping method is proposed, which can reduce PAPR without out-of-band radiation. It is based on the traditional clipping to generate the error signal. Zero out-of-band radiation is achieved by filtering the error signal. Although the proposed method produces more inband noise, by using noise cancel algorithm, the ideal unclipped signal′s BER performance can be achieved.

Method and apparatus for performing random access in a wireless communication system

Abstract: A method and apparatus for random access in an evolved universal terrestrial radio access (E-UTRA) system are disclosed. For code division multiplexing (CDM), a basic preamble is generated using a constant amplitude zero auto-correlation (CAZAC) sequence. The basic preamble is repeated for M time for generating a random access channel (RACH) preamble. For time division multiplexing/frequency division multiplexing (TDM/FDM), an extended CAZAC sequence is used to generate the basic preamble. Alternatively, a hybrid RACH access period including at least one CDM random access slot and at least one TDM/FDM random access slot may be provided. For synchronized random access, a RACH burst including a preamble part, a message part, and two cyclic prefixes may be generated and transmitted.

Method and apparatus for cell search in an orthogonal wireless communication system

Abstract: Systems and methodologies are described that provide techniques for efficient cell search in a wireless communication system. In one aspect, a frequency reuse pattern can be generated by applying frequency shifts to reference signals transmitted from cells that provide coverage for a NodeB based on cell IDs or cell group IDs for the cells. The frequency shifts applied to reference signals can then be utilized as a basis for multiplexing reference signals from different cells using frequency division multiplexing (FDM) or a combination of FDM and other multiplexing techniques. Other adjustments to reference signals transmitted from respective cells, such as transmit power adjustments, can further be made to improve detection performance.

A Class of Low Complexity PTS Techniques for PAPR Reduction in OFDM Systems

Abstract: This letter presents a class of low complexity partial transmit sequence (LC-PTS) techniques for reducing the peak-to- average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. The basic principle is to analyze and utilize the correlation among the candidate signals generated in PTS, so as to simplify the computational complexity. Furthermore, the proposed technique focuses on simplifying the computation for each candidate signal, instead of reducing the total number of candidate signals such as in (Jayalath and Tellambura, 2000). Thus, it can be easily combined with other simplified techniques. Simulation results show that the new technique can effectively reduce the complexity compared with the conventional PTS scheme.

Radio over fiber link apparatus of time division duplex scheme

Abstract: A radio over fiber link apparatus for transmitting/receiving radio frequency up/downlink signals in a TDD mobile communication system. The radio over fiber link apparatus includes a center site for receiving radio frequency signals from an access point of the mobile communication system. The center site has a first electro-optic converter for converting the radio frequency signals into optical signals, bias control of the first electro-optic converter being performed based on the switching of TDD signals; and a remote for transmitting the radio frequency signals to a mobile communication terminal through an antenna. The remote site has a first photoelectric converter for converting the optical signals transmitted through an optical fiber from the center site into radio frequency signals.

Throughput-Optimal Scheduling in Multichannel Access Point Networks Under Infrequent Channel Measurements

Abstract: We consider the problem of uplink/downlink scheduling in a multichannel wireless access point network where channel states differ across channels as well as users, vary with time, and can be measured only infrequently. We demonstrate that, unlike the infrequent measurement of queue lengths, infrequent measurement of channel states reduce the maximum attainable throughput. We then prove in frequency division multiplexing systems, a dynamic scheduling policy that depends on both the channel rates (averaged over the measurement interval) and the queue lengths, attains the maximum possible throughput. We also generalize the scheduling policy to solve the joint power allocation and scheduling problem in orthogonal frequency division multiplexing systems. In addition, we provide simulation studies that demonstrate the impact of the frequency of channel and queue state measurements on the average delay and attained throughput.

Outdoor Mobile Broadband Access with 802.11

Abstract: The IEEE 802.11 OFDM physical layer was designed primarily for indoor local area networks. Commercially available 802.11 radios suffer greatly reduced performance, even failing completely, when deployed outdoors, where long delay spreads cause self-interference, and vehicular mobility causes fast variations in the radio channel parameters. This article describes an advanced OFDM receiver that overcomes these problems. It works by combining all useful received energy, accounting for inter-symbol interference, and accurately tracking radio channel variations. Complexity and performance advantages arc gained by splitting the processing between the time and frequency domains. Computer simulations show that even for outdoor urban environments at speeds greater than 140 mph, this receiver delivers performance comparable to a non-mobile, indoor system.

Spectral shaping to reduce peak-to-average ratio in wireless communication

Abstract: Techniques for performing spectral shaping to achieve a desired peak-to- average ratio (PAR) are described Spectral shaping may be selectively performed for a single-carrier frequency division multiplexing (SC-FDM) signal based on one or more criteria, eg, in transmit power limited conditions and/or if a modulation scheme with lower PAR is unavailable At least one parameter of a window function or spectral shaping filter may also be adjusted based on at least one characteristic of the SC-FDM signal For example, the roll-off of the spectral shaping filter may be adjusted based on the modulation scheme and/or the number of subcarriers used for the SC-FDM signal A transmitter may perform spectral shaping on modulation symbols, if enabled, to obtain spectrally shaped symbols Spectral shaping may be performed in the frequency domain either within an allocated bandwidth or with bandwidth expansion The SC- FDM signal may be generated based on the spectrally shaped symbols

Joint Frequency Offset and Channel Estimation for OFDM Systems Using Pilot Symbols and Virtual Carriers

Abstract: We consider joint estimation of carrier frequency offset and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) systems with pilot symbols and virtual subcarriers (VCs). We derive the receive-signal correlation structure due to the pilots and VCs, give the evidence of joint multivariate Gaussian distribution of the received samples, and derive an approximate maximum likelihood (ML) frequency offset estimator. We also derive the asymptotic mean-square error (MSE) and an approximate Cramer-Rao bound (CRB) and establish the asymptotic unbiasedness. Without pilots, in high signal-to-noise ratio, our estimator is equivalent to Liu and Tureli's estimator with Nv virtual carriers. When the pilot number (Np) is greater than the channel length L, our estimator acts as a subspace-based estimator with Nv + Np - L virtual carriers. A decision-directed joint ML estimator is derived to iteratively update the estimates of frequency offset, data symbols and CIR. The optimal pilot and virtual carrier placement strategies are also discussed. The resulting decision-directed joint estimator performs within 0.8 dB of the ideal case even when the frequency offset is as large as 20%

Performance Analysis of Multiband OFDM UWB Communications With Application to Range Improvement

Abstract: Ultrawideband (UWB) radio is a promising solution for high-rate wireless communications over short ranges. In this paper, the performance of multiband orthogonal frequency division multiplexing (MB-OFDM) UWB systems is analyzed using the Saleh-Valenzuela channel model in terms of the packet error rate and the transmission range for indoor environments. The performance improvements resulting from the use of linear precoding and multiple antenna techniques are also analyzed and compared. It is shown both by theory and computer simulations that the two methods can effectively enlarge the transmission range of UWB devices.

Orthogonal Wavelet Division Multiplex: An Alternative to OFDM

Abstract: Over the past three decades, the scientific and industrial communities have injected considerable resources into perfecting orthogonal frequency division multiplex (OFDM) - the driving technology behind many applications used today (such as IEEE 802.11g, asymmetric digital subscriber line (ADSL), and digital terrestrial television). In 1997, OFDM was officially sanctioned for use in the European Telecommunications Standards Institute (ETSI) EN 300 744 standard known as digital video broadcasting-terrestrial (DVB-T). As OFDM is such a popular technology, there has been limited research over the years into alternatives. This paper proposes a novel approach to transmit parallel information through a terrestrial channel that could render it a more flexible alternative to OFDM in the future. Investigations have shown that this technique could provide a highly flexible, channel optimized modulation technique based on time-frequency multiplexing and this paper presents the preliminary phase of investigations into this new technique .

Joint Semiblind Frequency Offset and Channel Estimation for Multiuser MIMO–OFDM Uplink

Abstract: A semiblind method is proposed for simultaneously estimating the carrier frequency offsets (CFOs) and channels of an uplink multiuser multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) system. By incorporating the CFOs into the transmitted symbols and channels, the MIMO-OFDM with CFO is remodeled into an MIMO-OFDM without CFO. The known blind method for channel estimation (Zeng and Ng in 2004) (Y. H. Zeng and T. S. Ng, ldquoA semi-blind channel estimation method for multi-user multi-antenna OFDM systems,rdquo IEEE Trans. Signal Process., vol. 52, no. 5, pp. 1419-1429, May 2004.) is then directly used for the remodeled system to obtain the shaped channels with an ambiguity matrix. A pilot OFDM block for each user is then exploited to resolve the CFOs and the ambiguity matrix. Two dedicated pilot designs, periodical and consecutive pilots, are discussed. Based on each pilot design and the estimated shaped channels, two methods are proposed to estimate the CFOs. As a result, based on the second-order statistics (SOS) of the received signal and one pilot OFDM block, the CFOs and channels are found simultaneously. Finally, a fast equalization method is given to recover the signals corrupted by the CFOs.

An Efficient Frequency Offset Estimation Method With a Large Range for Wireless OFDM Systems

Abstract: An efficient frequency offset estimation method is proposed for wireless orthogonal frequency-division multiplexing (OFDM) systems. In the proposed method, the received samples of the training symbol are processed by a novel envelop equalized processing method, and the estimation of the frequency offset in the OFDM systems turns out to be the frequency estimation of a complex tone. A novel frequency estimation method is employed to estimate the frequency offset. The estimation range of the proposed method is as large as the bandwidth of the OFDM signal, and the performance of the proposed method is independent of the structure of the training symbol.

Base station and method in mobile communication system

Abstract: A disclosed base station in a mobile communications system which uses a single-carrier technique for uplink includes a classifying unit which classifies, for each of multiple user apparatuses according to a path loss between the respective user apparatus and the base station, the multiple user apparatuses into two or more groups; and a scheduler which plans allocating of uplink resources to the user apparatuses Uplink signals of each user apparatus that have multiple frequency components lined up in equal frequency intervals are multiplexed using a distributed frequency division multiplexing technique such that they are made mutually orthogonal on a frequency axis, the scheduler plans the resource allocating such that the user apparatuses belonging to different groups utilize different group bands or different time slots, and the group bands are specified by dividing a system band into groups

Reception and Measurement of MIMO-OFDM Signals with a Single Receiver

Abstract: OFDM multi-carrier modulation is expected to be the enabling technology for 4G wireless systems. One of the features that make OFDM the primary choice for 4G is its MIMO compatibility, because MIMO has a very significant potential of enhancing wireless systems for capacity, data rate, and coverage aspects. In this paper, the challenges of MIMO- OFDM measurements are addressed in comparison to SISO, with a special emphasis on WiMAX systems, which employ MIMO- OFDM technology. It is proposed to perform the reception and measurement of MIMO-OFDM signals using a single receiver branch rather than multiple receivers. A complete guide to perform impairment estimation for WiMAX MIMO signals with a single receiver according to the 802.16 standards is also provided.

The Prefix Design and Performance Analysis of DFT-based Overlapped Frequency Division Multiplexing (OvFDM-DFT) System

Abstract: This paper propose the theoretical analysis of the DFT based overlapped frequency division multiplexing (OvFDM)/non-orthogonal frequency division multiplexing(NOFDM) communication system, and investigates the impact of conventional cyclic-prefix and zero-padding cyclic-prefix respectively for the OvFDM-DFT system in the time-variant frequency-selective fading channels The sub-carriers of the DFT based OvFDM/NOFDM communication system are overlapped together and the bandwidth can be larger than the frequency coherence bandwidth, so a kind of implicit frequency diversity will be exploited by the MLSD receiver, which will improve the system performance Theoretical analysis and simulation results show that zero-padding cyclic-prefix has a much better performance than the conventional cyclic-prefix The implicit frequency diversity ability of the OvFDM-DFT symbols is also verified by the simulations

Linear Precoding for Multiuser MIMO-OFDM Systems

Abstract: This paper develops linear preceding schemes for the downlink in multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with multiple data streams per user. We extend an existing multiuser MIMO algorithm, that jointly optimizes the power allocation and the transmit and receive filters, to MIMO- OFDM systems. One extension is to solve the resulting problem of joint power allocation across OFDM subcarriers. This paper also presents efficient methods to reduce the computational load of the algorithm by interpolating the precoding and decoding matrices corresponding to different OFDM subcarriers. The simulations show that the proposed interpolation scheme outperforms previously known schemes, but requires that the precoder for each subcarrier be tailored to the interpolated receiver.

A MIMO Antenna Structure that Combines Transmit Beamforming and Spatial Multiplexing

Abstract: We propose a new closed-loop MIMO (Multiple- Input Multiple-Output) signal processing scheme that combines transmit beamforming and spatial multiplexing in Rayleigh frequency-flat fading channel environment. Diversity gain is achieved by beamforming, and multiplexing gain by spatial multi- plexing. The proposed scheme is particularly useful for downlink communication, where the number of transmit antennas at the base station is greater than the number of receive antennas at the mobile. Beamforming weight vectors are computed at the receiver using the estimated channel matrix and are sent to the transmitter. The computation is based on the mutual information maximization criterion which reduces to a symmetric eigenvalue problem. The BER (Bit Error Rate) performance comparisons with other known schemes are also presented for the cases of the ML (Maximum Likelihood) decoding and the MMSE (Minimum Mean-Squared Error) nulling and ordered-cancellation decod- ing, through extensive Monte-Carlo simulation. The simulation results show that the proposed scheme has lower BER than the other schemes.

A New OFDM Synchronization Symbol for Carrier Frequency Offset Estimation

Abstract: This letter proposes a new data-aided carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) communications suitable for frequency-selective channels. The proposed method is based on the transmission of a specially designed synchronization symbol that generates a particular signal structure between the received observation samples at the receiver. This structure is exploited to derive a closed-form expression of the CFO. The proposed solution offers a wide acquisition range with reduced computational load. Simulations over frequency-selective channels confirm the superiority of the proposed method compared to recent data-aided synchronization algorithms

Apparatus and method of providing relay service in broadband wireless access (BWA) communication system

Abstract: An apparatus and method of providing relay service in a broadband wireless access (BWA) communication are provided. The communication method includes dividing an entire frequency band into a plurality of bands to support a plurality of links; determining subcarrier allocation schemes for the divided bands independently from one another; and communicating using the determined subcarrier allocation schemes by the plurality of the links.When the direct link and the relay link are distinguished through the frequency multiplexing in the multihop relay BWA communication system, the present invention enables the independent channel estimation from the relay link by adopting the dedicated pilot based permutation scheme for the relay link.

Filterbank Multicarrier and Multicarrier CDMA for Cognitive Radio Systems

Abstract: Recent works in cognitive radio (CR) have suggested multicarrier communication based on orthogonal frequency-division multiplexing (OFDM) for CR networks. In this paper, we propose two alternative multicarrier techniques. The first one, filterbank-based OFDM-OQAM (OFDM-offset quadrature amplitude modulation), allows to maximize the secondary users' spectral efficiency by eliminating the need for guard bands. Also, OFDM-OQAM's filterbank spectral estimator has greater dynamic range than the conventional fast fourier transform in OFDM, which further reduces the probability of undesirable collisions between the secondary users (SU) and primary users (PU). However, OFDM-OQAM still requires an SU base station for distributed sensing. For cases where such is not available, or when sensing information cannot be conveyed, we suggest another multicarrier technique, namely multicarrier code division multiple access (MC-CDMA). While MC-CDMA does not achieve comparably high spectral efficiency as OFDM-OQAM, the spreading gain of MC-CDMA results in graceful degradation in the case of collisions between PUs and SUs. Furthermore, compared to direct sequence (DS) CDMA, MC-CDMA allows to exclude narrowband PU interferers locally at the SU receiver, hence improving SU performance. We show that MC-CDMA makes it possible to operate the SU network without distributed sensing and hence no need for a base station infrastructure and/or a signaling channel.

Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems With Virtual Carriers

Abstract: In this paper, we propose adaptive beamforming schemes for orthogonal frequency-division multiplexing (OFDM) communication systems. After applying the discrete-Fourier-transform operation to the output of each antenna, one estimates the signal-plus-interference spatial correlation matrix by selecting the samples at ldquononzero-valuedrdquo subcarriers and the interference-alone spatial correlation matrix by choosing those samples at ldquozero-valuedrdquo subcarriers (also known as virtual carriers). The beamforming weight vector is the ldquolargestrdquo generalized eigenvector of the resulting matrix pencil, wherein the solution is computed in a batch fashion. By using a limited number of samples, the proposed scheme offers competitive performance compared with that of the scheme using optimal beamforming weight vectors that are calculated for each subcarrier under small-angular-spread scenarios. The adaptive solution for calculating the beamforming weight vector is also derived. The proposed schemes are shown to be suitable in scenarios of narrow angular spread with exponential power delay profile. The simulations that are presented here employ the OFDM signal model to illustrate the efficacy of the proposed algorithms.

Performance Evaluation of MB-OFDM Ultra-Wideband Signals over Single mode Fiber

Abstract: A study on multi-band-orthogonal frequency division multiplexing (MB-OFDM) Ultra-Wideband (UWB) signal transmission through an single-mode fiber based intensity modulated laser via Radio over Fiber system is presented. MB-OFDM compliant UWB signals with frequency ranging between 3.1 GHz to 4.8 GHz are generated using the WisAir Development Kit DV9110M. Packet Error Rate measurement is performed to evaluate the system performance based on different optical fiber lengths, data rates, and transmitting UWB power levels. The results of sending UWB signals over RoF show very encouraging results over long distances even in the demanding ultra-wideband environment.

Method and apparatus for allocating frequency resources in a wireless communication system supporting frequency division multiplexing

Abstract: A method for allocating frequency resources to be used for multiple terminals in a Frequency Division Multiplexing (FDM) wireless communication system. The method includes performing at a first transmission time a process of hierarchizing a series of resource units constituting a service frequency band in a plurality of levels, and hierarchically dividing the series of resource units into blocks including at least one consecutive resource unit in each of the levels, and allocating some of the hierarchically divided blocks as frequency resources for each of the terminals; and performing, at a second transmission time, a process of hierarchically hopping the blocks allocated as the frequency resources for each of the terminals so that the blocks each have a different frequency band from a frequency band used at the first transmission time, and allocating the hopped blocks as frequency resources for each of the terminals.