Showing papers on "Frequency-division multiplexing published in 2009"
TL;DR: This paper addresses basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications, including channel estimation and signal detection, time- and frequency-offset estimation and correction, peak-to-average power ratio reduction, and multiple-input-multiple-output (MIMO) techniques.
Abstract: Orthogonal frequency-division multiplexing (OFDM) effectively mitigates intersymbol interference (ISI) caused by the delay spread of wireless channels. Therefore, it has been used in many wireless systems and adopted by various standards. In this paper, we present a comprehensive survey on OFDM for wireless communications. We address basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications, including channel estimation and signal detection, time- and frequency-offset estimation and correction, peak-to-average power ratio reduction, and multiple-input-multiple-output (MIMO) techniques. We also describe the applications of OFDM in current systems and standards.
915 citations
26 Apr 2009
TL;DR: GFDM enables frequency and time domain multi-user scheduling comparable to OFDM and provides an efficient alternative for white space aggregation even in heavily fragmented spectrum regions.
Abstract: This paper presents the GFDM system, a generalized digital multi-carrier transceiver concept. GFDM is based on traditional filter bank multi-branch multi- carrier concepts which are now implemented digitally. Our GFDM approach exhibits some attractive features which are of particular importance for scenarios exhibiting high degrees of spectrum fragmentation. Spectrum fragmentation is a typical technical challenge of digital dividend use cases, exploiting spectrum white spaces in the TV UHF bands which are located in close proximity to allocated spectrum. Specifically, the GFDM features are a lower PAPR compared to OFDM, a ultra-low out-of- band radiation due adjustable Tx-filtering and last but not least a block-based transmission using cyclic prefix insertion and efficient FFT-based equalization. GFDM enables frequency and time domain multi-user scheduling comparable to OFDM and provides an efficient alternative for white space aggregation even in heavily fragmented spectrum regions.
494 citations
TL;DR: In this paper, coherent optical orthogonal frequency division multiplexing (CO-OFDM) techniques for the long-haul transmission of 100-Gb/s-class channels are described.
Abstract: This paper describes coherent optical orthogonal frequency division multiplexing (CO-OFDM) techniques for the long-haul transmission of 100-Gb/s-class channels. First, we discuss the configurations of the transmitter and receiver that implement the optical multiplexing/demultiplexing techniques for high-speed CO-OFDM transmission. Next, we review the no-guard-interval (No-GI) CO-OFDM transmission scheme which utilizes optical multiplexing for OFDM signal generation and the intradyne receiver configuration with digital signal processing (DSP). We examine the transmission characteristics of the proposed scheme, and show that No-GI CO-OFDM offers compact signal spectra and superior performance with regard to tolerance against optical amplifier noise and polarization-mode dispersion (PMD). We then introduce long-haul high-capacity transmission experiments employing No-GI CO-OFDM; 13.4 Tb/s (134 times 111 Gb/s) transmission is successfully demonstrated over 3600 km of ITU-T G.652 single-mode fiber without using optical dispersion compensation.
193 citations
14 Jun 2009
TL;DR: It is shown that it is possible to detect optimally and efficiently FDM signals, with 25% bandwidth gain with respect to analogous OFDM signal parameters, which indicates that the transmission of spectrally efficient non orthogonal F DM signals is tangible.
Abstract: This paper investigates the transmission of Frequency Division Multiplexed (FDM) signals, where carrier orthogonality is intentionally violated in order to increase bandwidth efficiency. In analogy to conventional OFDM, signal generation relies on an Inverse Fractional Fourier Transform (IFRFT) that can be implemented with O(N log2 N) algorithmic complexity. Optimal Maximum Likelihood (ML) detection is overly complex due to the presence of substantial Intercarrier Interference (ICI). Consequently, we investigate an alternative detection mechanism based on the Generalized Sphere Decoding (GSD) algorithm. We examine the bandwidth efficiency and the error performance in Additive White Gaussian Noise (AWGN), for various FDM signal parameters. In particular, we show that it is possible to detect optimally and efficiently FDM signals, with 25% bandwidth gain with respect to analogous OFDM signals. This indicates that the transmission of spectrally efficient non orthogonal FDM signals is tangible.
163 citations
TL;DR: This paper provides a general soft decision SMSE (SDSMSE) framework that extends the original SMSE framework to achieve synergistic CR benefits of overlay and underlay techniques and provides considerable flexibility to design overlay, underlay and hybrid overlay/underlay waveforms that are scenario dependent.
Abstract: Recent studies suggest that spectrum congestion is primarily due to inefficient spectrum usage rather than spectrum availability. Dynamic spectrum access (DSA) and cognitive radio (CR) are two techniques being considered to improve spectrum efficiency and utilization. The advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. Within the hierarchical DSA model, spectrum overlay and underlay techniques are employed to enable primary and secondary users to coexist while improving overall spectrum efficiency. As employed here, spectrum overlay exploits unused (white) spectral regions while spectrum underlay exploits underused (gray) spectral regions. In general, underlay approaches use more spectrum than overlay approaches and operate below the noise floor of primary users. Spectrally modulated, spectrally encoded (SMSE) signals, to include orthogonal frequency domain multiplexing (OFDM) and multi-carrier code division multiple access (MC-CDMA), are candidate CR waveforms. The SMSE structure supports and is well suited for CR-based software defined radio (SDR) applications. This paper provides a general soft decision SMSE (SDSMSE) framework that extends the original SMSE framework to achieve synergistic CR benefits of overlay and underlay techniques. This extended framework provides considerable flexibility to design overlay, underlay and hybrid overlay/underlay waveforms that are scenario dependent. Overlay/underlay framework flexibility is demonstrated herein for a family of SMSE signals, including OFDM and MC-CDMA. Analytic derivation of CR error probability for overlay and underlay applications is presented. Simulated performance analysis of overlay, underlay and hybrid overlay/underlay waveforms is also presented and benefits discussed, to include improved spectrum efficiency and channel capacity maximization. Performance analysis of overlay/underlay CR waveform in fading channels will be discussed in Part II of the paper.
157 citations
TL;DR: The background and some of the striking early development of OFDM are described, with explanation of the motivations for using it.
Abstract: Orthogonal frequency-division multiplexing (OFDM) is one of those ideas that had been building for a very long time, and became a practical reality when the appearance of mass market applications coincided with the availability of efficient software and electronic technologies. This article describes the background and some of the striking early development of OFDM, with explanation of the motivations for using it. The author presume a broad definition of OFDM as frequency-division multiplexing (FDM) in which subchannels overlap without interfering. It does not not necessarily require the discrete Fourier transform (DFT) or its fast Fourier transform (FFT) computational method.
154 citations
TL;DR: In this article, a linearly field-modulated, direct-detected virtual single-sideband orthogonal frequency-division multiplexing (VSSB-OFDM) system that employs a tunable frequency gap and an iterative detection technique is proposed as a spectrally efficient format.
Abstract: We analytically and experimentally demonstrate a linearly field-modulated, direct-detected virtual single-sideband orthogonal frequency-division multiplexing (VSSB-OFDM) system that employs a tunable frequency gap and an iterative detection technique. The VSSB-OFDM that uses no frequency gap, which is referred to as the gapless VSSB-OFDM, is proposed as a spectrally efficient format. Compared with the intensity-modulated SSB-OFDM, the gapless VSSB-OFDM saves half the electrical bandwidth (BW), and exhibits better receiving sensitivity and more robust tolerance against fiber chromatic dispersion (CD). Furthermore, by incorporating a tunable frequency gap between the optical carrier and the OFDM data sideband, the calculating burden of the iterative detection is greatly alleviated and the system performance can be flexibly improved within moderate iterations. The width of the optimum frequency gap is found to be ${\sim} {\hbox {0.35}}$ sideband BW, which is reached by trading the levels of signal–signal beat interference and the residual image beat interference. Such a gapped VSSB-OFDM system requires fewer iterations to extract the desired data from the interfered signal and exhibits greater robustness against the carrier-to-signal-power ratio (CSPR) variation, compared with the gapless VSSB-OFDM. In this paper, the analytical model of the proposed gapped VSSB-OFDM system will be addressed. In addition, we also successfully conduct a gapped VSSB-OFDM signal transmission over 1600 km of uncompensated standard single-mode fiber (SSMF) with only ${\sim} {\hbox {3}}$ dB optical SNR (OSNR) penalty, and obtain a significant OSNR sensitivity improvement of ${\sim} {\hbox {8}}$ dB, compared with the gapless VSSB-OFDM, after such a 1600-km fiber link.
150 citations
TL;DR: Theoretical analysis and simulation results for a Rayleigh fading channel show that the proposed algorithm has low computational complexity and good performance in the presence of high normalized Doppler spread.
Abstract: In this paper, we consider the case of a high-speed mobile receiver operating in an orthogonal frequency-division multiplexing (OFDM) communication system. We present an iterative algorithm for estimating multipath complex gains with intersubcarrier interference (ICI) mitigation (using comb-type pilots). Each complex gain variation is approximated by a polynomial representation within several OFDM symbols. Assuming knowledge of delay-related information, polynomial coefficients are obtained from time-averaged gain values, which are estimated using the least-square (LS) criterion. The channel matrix is easily computed, and the ICI is reduced by using successive interference suppression (SIS) during data symbol detection. The algorithm's performance is further enhanced by an iterative procedure, performing channel estimation and ICI mitigation at each iteration. Theoretical analysis and simulation results for a Rayleigh fading channel show that the proposed algorithm has low computational complexity and good performance in the presence of high normalized Doppler spread.
141 citations
TL;DR: In this article, a radio frequency (RF)-tone-assisted optical orthogonal frequency-division multiplexing (OFDM) transmission was proposed and experimentally demonstrated, and the optimum value of the important parameter, carrier-to-signal-power ratio (CSPR), was analytically obtained and supported via the experimental results.
Abstract: In this paper, we propose and experimentally demonstrate a radio frequency (RF)-tone-assisted optical orthogonal frequency-division multiplexing (OFDM) transmission. By inserting an RF tone at the edge of the signal band and biasing the Mach-Zehnder modulator (MZM) at the null point, the proposed system has a better sensitivity and chromatic dispersion (CD) tolerance compared to the previous intensity-modulated single-sideband OFDM (SSB-OFDM). We show analytically that the majority of the linear channel impairments, such as the transmitter, CD, optical filtering, and receiver, can be compensated for by a simple zero-forcing equalizer. Besides, the optimum value of the important parameter, carrier-to-signal-power ratio (CSPR), is analytically obtained and supported via the experimental results. We also observe that the relatively worse sensitivity of the previous SSB-OFDM can be attributed to the limited CSPR. We experimentally demonstrate a 10-Gb/s, 8 quadrature-amplitude modulation (QAM) RF-tone-assisted OFDM transmission, and show that our system has a ~ 5-dB better sensitivity compared to the previous intensity-modulated SSB-OFDM and exhibits a negligible transmission penalty after 260-km uncompensated standard single-mode fiber (SSMF).
138 citations
TL;DR: A new linear companding transform (LCT) with more design flexibility than LNST is proposed and computer simulations show that the proposed transform has a better PAPR reduction and bit error rate (BER) performance than LnST with better power spectral density (PSD).
Abstract: A major drawback of orthogonal frequency-division multiplexing (OFDM) signals is their high peak-to-average power ratio (PAPR), which causes serious degradation in performance when a nonlinear power amplifier (PA) is used. Companding transform (CT) is a well-known method to reduce PAPR without restrictions on system parameters such as number of subcarriers, frame format and constellation type. Recently, a linear nonsymmetrical companding transform (LNST) that has better performance than logarithmic-based transforms such as mu-law companding was proposed. In this paper, a new linear companding transform (LCT) with more design flexibility than LNST is proposed. Computer simulations show that the proposed transform has a better PAPR reduction and bit error rate (BER) performance than LNST with better power spectral density (PSD).
127 citations
TL;DR: In this paper, the authors proposed a self-phase modulation (SPM) compensation method that jointly processes two polarization components of a PDM CO-OFDM signal for single-channel and wavelength-division multiplexed (WDM) transmission.
Abstract: We present digital signal processing techniques and algorithms for improving the tolerance of polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexing (CO-OFDM) to fiber nonlinear effects, in both single-channel and wavelength-division multiplexed (WDM) transmission. For single-channel transmission, we discuss a self-phase modulation (SPM) compensation method that jointly processes two polarization components of a PDM CO-OFDM signal. For WDM transmission, we describe a novel OFDM channel estimation scheme that reduces the cross-phase-modulation (XPM) penalty among the WDM channels. The nonlinear tolerance improvements enabled by these signal processing techniques are quantified through numerical simulations for both dispersion-unmanaged and dispersion-managed long-haul optical fiber transmission with 112-Gb/s PDM CO-OFDM wavelength channels.
TL;DR: A 60 GHz broadband picocellular Radio-over-Fiber network architecture that enables seamless connectivity for highly mobile end-users and can provide zero packet loss and call dropping probability values in high-rate wireless services for a broad range of mobile speeds up to 40 m/sec, independently of the fiber link distances is demonstrated.
Abstract: We demonstrate a 60 GHz broadband picocellular Radio-over-Fiber network architecture that enables seamless connectivity for highly mobile end-users. Its seamless communication capabilities arise by the supported handover scheme that relies on a novel Moving Extended Cell (MEC) concept. MEC exploits user-centric virtual groups of adjacent cells that transmit the same data content to the user and utilizes a switch mechanism for restructuring the virtual multi-cell area according to the user's mobility pattern, so that a virtual antenna group moves together with the mobile user. We present the theoretical formulation for MEC and show that it can provide zero packet loss and call dropping probability values in high-rate wireless services for a broad range of mobile speeds up to 40 m/sec, independently of the fiber link distances. We also demonstrate the physical layer network architecture and switch mechanism both for a RoF network with a single 60 GHz radio frequency (RF) over each wavelength, as well as for a RoF configuration supporting simultaneous multi-RF channel transmission over each optical wavelength. The performance of the multi-RF-over-lambda network implementation is evaluated via simulations showing successful 100 Mb/s radio signal transmission over fiber links longer than 30 km. To this end, MEC can enable seamless connectivity and bandwidth guarantees in 60 GHz picocellular RoF networks being also capable of serving multiple users over the same wavelength in a RF frequency-division-multiplexed (FDM) approach.
Patent•
09 Apr 2009
TL;DR: In this paper, a class of bandwidth reduction techniques are used to develop a broad class of modulation types collectively called SSB-FM, which can be used to construct communication systems that provide bandwidth-normalized performance gains of 10 dB or more when compared to popular prior art modulation methods.
Abstract: A class of bandwidth reduction techniques are used develop a broad class of modulation types collectively called SSB-FM. These signals can be used to construct communication systems that provide bandwidth-normalized performance gains of 10 dB or more when compared to popular prior art modulation methods. An aspect of the invention involves mapping trellis paths in a complex signal space onto corresponding real-valued trellis signals with desirable spectral properties. The invention can be used map continuous phase modulated (CPM) signals onto simpler amplitude-modulated trellis signals having double the channel capacity of prior art CPM signals. Multi-amplitude signaling and frequency division multiplexing may also be incorporated to further accommodate more information per symbol.
TL;DR: In this article, the authors present an overview of DMT over SI-POF and demonstrate how DMT can be used to improve transmission rate in such IM/DD systems.
Abstract: The use of standard 1-mm core-diameter step-index plastic optical fiber (SI-POF) has so far been mainly limited to distances of up to 100 m and bit-rates in the order of 100 Mbit/s. By use of digital signal processing, transmission performance of such optical links can be improved. Among the different technical solutions proposed, a promising one is based on the use of discrete multitone (DMT) modulation, directly applied to intensity-modulated, direct detection (IM/DD) SI-POF links. This paper presents an overview of DMT over SI-POF and demonstrates how DMT can be used to improve transmission rate in such IM/DD systems. The achievable capacity of an SI-POF channel is first analyzed theoretically and then validated by experimental results. Additionally, first experimental demonstrations of a real-time DMT over SI-POF system are presented and discussed.
01 Jan 2009
TL;DR: Among the different communication technologies, optical communications generally has the edge over baseband electronic or RF transmission systems whenever high aggregate bit rates and/or long transmission distances are involved.
Abstract: Digital electronics and optical transport The rapid transition from analog to digital systems over the past ~50 years has enabled universal processing of all kinds of information, fundamentally without loss of quality [1]. Breakthroughs in digital semiconductor technologies and their enormous ability to scale [2] have enabled cost-effective mass-production of richly functional yet highly reliable and power-efficient microchips that are found in virtually any electronic device today, from high-end internet routers to low-end consumer electronics. Closely coupled to the generation, processing, and storage of digital information is the need for data transport, ranging from short on-chip [3] and board-level [4,5] data buses all the way to long-haul transport networks spanning the globe [6,7] and to deep-space probes collecting scientific data [8], cf. Fig. 1 [5,10]. Each of these very different applications brings its own set of technical challenges, which can be addressed using electronic, radio-frequency (RF), or optical communication systems. Among the different communication technologies, optical communications generally has the edge over baseband electronic or RF transmission systems whenever high aggregate bit rates and/or long transmission distances are involved. Both advantages are deeply rooted in physics: First, the high optical carrier frequencies allow for high-capacity systems at small relative bandwidths. For example, a mere 2.5% bandwidth at a carrier frequency of 193 THz (1.55 μm wavelength) opens up a 5-THz chunk of continuous communication bandwidth. Such “narrow-band” systems are much easier to design than systems with a large relative bandwidth. Second, transmission losses at optical frequencies are usually very small compared to baseband electronic or RF technologies. Today’s optical telecommunication fibers exhibit losses of less than 0.2 dB/km; the loss of typical coaxial cables supporting ~1 GHz of bandwidth is 2 to 3 orders of magnitude higher. In free-space systems optical beams have much smaller divergence angles than in the microwave regime, at the expense of significantly exacerbated antenna pointing requirements, though. The narrow beam width favorably translates into the system’s link budget, in particular in space-based systems where atmospheric absorption is less of a problem. Apart from the above two major advantages, other considerations sometimes come into play, such as the unregulated spectrum in the optical regime or the absence of electromagnetic interference.
TL;DR: The instrument design for multidimensional position and orientation measurement based on the simultaneous position measurement of multiple modulated sources using frequency-domain-multiplexed (FDM) PSDs is described.
Abstract: Position-sensitive detectors (PSDs), or lateral-effect photodiodes, are commonly used for high-speed, high-resolution optical position measurement. This paper describes the instrument design for multidimensional position and orientation measurement based on the simultaneous position measurement of multiple modulated sources using frequency-domain-multiplexed (FDM) PSDs. The important advantages of this optical configuration in comparison with laser/mirror combinations are that it has a large angular measurement range and allows the use of a probe that is small in comparison with the measurement volume. We review PSD characteristics and quantitative resolution limits, consider the lock-in amplifier measurement system as a communication link, discuss the application of FDM to PSDs, and make comparisons with time-domain techniques. We consider the phase-sensitive detector as a multirate DSP problem, explore parallels with Fourier spectral estimation and filter banks, discuss how to choose the modulation frequencies and sample rates that maximize channel isolation under design constraints, and describe efficient digital implementation. We also discuss hardware design considerations, sensor calibration, probe construction and calibration, and 3-D measurement by triangulation using two sensors. As an example, we characterize the resolution, speed, and accuracy of an instrument that measures the position and orientation of a 10 mm times 5 mm probe in 5 degrees of freedom (DOF) over a 30-mm cube with 4-mum peak-to-peak resolution at 1-kHz sampling.
TL;DR: Channel estimation in the presence of frequency offsets is developed for cooperative orthogonal frequency-division multiplexing (OFDM) systems and the optimal power allocation ratio between the source and the relays to minimize the pairwise error probability (PEP).
Abstract: Channel estimation in the presence of frequency offsets is developed for cooperative orthogonal frequency-division multiplexing (OFDM) systems. A two-time-slot cooperative channel estimation protocol is proposed. The source broadcasts the training sequence to the relays and the destination (first time slot), and the relays retransmit the training sequence (second time slot). Pilot designs for amplify-and-forward (AF) and decode-and-forward (DF) relays are derived. These designs eliminate interrelay interference (IRI), which occurs due to the simultaneous relay retransmissions, and minimize the mean square error (MSE). Consequently, the number of AF and DF relays is constrained to be less than lfloorN/(2L - 1)rfloor and lfloorN/Lrfloor, respectively, where N is the total number of subcarriers, L is the channel order, and lfloorarfloor is the maximum integer part of alpha. The pairwise error probability (PEP) of orthogonal space-time coding in cooperative OFDM due to both frequency offset and channel-estimation errors is also evaluated. The optimal power allocation ratio between the source and the relays to minimize the PEP is derived for AF and DF relays. When L < 16, DF relays outperform AF relays in terms of PEP. With L = 4 and 16 active relays, the gap is 9 dB for a frequency offset error variance of 10-3, and this gap increases to about 11.3 dB when the variance increases to 10-2.
TL;DR: In this article, the authors investigate the transmission performance of a novel modulation based on a recently proposed technique of adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) for next-generation passive optical network (NG-PON).
Abstract: We experimentally investigate the transmission performance of a novel modulation based on a recently proposed technique of adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) for next-generation passive optical network (NG-PON). This signal was generated by direct modulation of cost-effective and low-bandwidth commercially available distributed-feedback and vertical-cavity surface-emitting lasers. Using the Levin-Campello adaptive bit-loading algorithm for channel capacity optimization over AMOOFDM modulation to reach maximum transmission performance, we experimentally demonstrated that high bit rate can be transmitted in optical access network without the need for chromatic dispersion compensation and optical amplification on the link.
TL;DR: In this article, the feasibility of 1 Tb/s per channel CO-OFDM transmission is investigated and the performance difference between single channel and wavelength division multiplexing (WDM) transmission is small.
Abstract: Coherent optical frequency-division multiplexing (CO-OFDM) is one of the promising pathways toward future ultrahigh capacity transparent optical networks. In this paper, numerical simulation is carried out to investigate the feasibility of 1 Tb/s per channel CO-OFDM transmission. We find that, for 1 Tb/s CO-OFDM signal, the performance difference between single channel and wavelength division multiplexing (WDM) transmission is small. The maximum Q is 13.8 and 13.2 dB respectively for single channel and WDM transmission. We also investigate the CO-OFDM performance on the upgrade of 10-Gb/s to 100-Gb/s based DWDM systems with 50-GHz channel spacing to 100-Gb/s systems. It is shown that due to the high spectral efficiency and resilience to dispersion, for 100-Gb/s CO-OFDM signals, only 1.3 dB Q penalty is observed for 10 GHz laser frequency detuning. A comparison of CO-OFDM system performance under different data rate of 10.7 Gb/s, 42.8 Gb/s, 107 Gb/s and 1.07 Tb/s with and without the impact of dispersion compensation fiber is also presented. We find that the optimum fiber launch power increases almost linearly with the increase of data rate. 7 dB optimum launch power difference is observed between 107 Gb/s and 1.07 Tb/s CO-OFDM systems.
TL;DR: Three different colorless optical networking unit (ONU) architectures, electroabsorption modulator-based, reflective semiconductor optical amplifier-based and injection-locked Fabry-Perot laser diode-based ONUs, are tested and compared and the possibility of using OFDM for signal remodulation in LR-PONs is studied for the first time.
Abstract: The passive optical network (PON) using orthogonal frequency-division multiplexing (OFDM) is a subject of many research works recently. The OFDM signal is especially good for long reach (LR)-PON due to its high tolerance to chromatic dispersion and high spectral efficiency. We study, for the first time, the possibility of using OFDM for signal remodulation in LR-PONs. Three different colorless optical networking unit (ONU) architectures, electroabsorption modulator-based, reflective semiconductor optical amplifier-based and injection-locked Fabry-Perot laser diode-based ONUs, are tested and compared. Error-free operations are achieved in 100-km fiber transmission without dispersion compensation.
TL;DR: A blind symbol synchronization algorithm is presented for orthogonal frequency-division multiplexing (OFDM) systems, and a new timing function based on the redundancy of the cyclic prefix (CP) is introduced, proving that the maximum of this function necessarily points to the correct timing offset, irrespective of channel conditions when the signal-to-noise ratio is high.
Abstract: In this paper, a blind symbol synchronization algorithm is presented for orthogonal frequency-division multiplexing (OFDM) systems, and a new timing function based on the redundancy of the cyclic prefix (CP) is introduced. It proves that the maximum of this function necessarily points to the correct timing offset, irrespective of channel conditions when the signal-to-noise ratio is high. Using the timing function, the timing offset is estimated through a searching algorithm. Channel power profile and channel length information are unnecessary. Simulation results show that the proposed algorithm is robust and outperforms the existing CP-based algorithms, particularly in frequency-selective fading channels.
TL;DR: The combination of a multihop relay system and orthogonal frequency-division multiplexing (OFDM) modulation is a promising way to increase the capacity and coverage area and a suboptimal scheme with less complexity is proposed, which can also be used to better understand the effects of power allocation.
Abstract: The combination of a multihop relay system and orthogonal frequency-division multiplexing (OFDM) modulation is a promising way to increase the capacity and coverage area. For the OFDM two-hop relay system with separate power constraints, joint subcarrier matching and power allocation is considered in this paper, which uses the ldquodecode-and-forwardrdquo relay strategy. The aforementioned problem can be formulated as a mixed binary integer programming problem, which is prohibitive when trying to find the global optimum. By separating the subcarrier matching and the power allocation, the optimal scheme, i.e., the optimal joint subcarrier matching and power allocation, is presented in this paper. After that, a suboptimal scheme with less complexity is also proposed, which can also be used to better understand the effects of power allocation. Simulation results show that the capacity of the optimal scheme is almost equivalent to the upper bound of the system capacity, and the capacity of the suboptimal scheme is close to that of the optimal scheme. In addition, simulation results also show that the one-to-one subcarrier matching is almost optimal, although it simplifies the system architecture.
TL;DR: This paper introduces array codes into the PLC environment and study the channel's performance, which is evaluated in terms of the bit-error rate for different Eb/N0 values via computer simulations.
Abstract: Power-line communications (PLC) is a field that has raised a lot of research in the past years. In this paper, we introduce array codes into the PLC environment and we study the channel's performance. In particular, generalized array codes (GAC), as well as row and column array codes (RAC) are applied. We examine their performance by obtaining three different code rates from each category. Therefore, the (8, 4, 4), (12, 7, 4) and (16, 11, 4) GAC codes as well as the (9, 4, 4), (12, 6, 4), and (15, 8, 4) RAC codes are used, meaning that we obtain code rates of (1/2), (7/12), (11/16), and (4/9), (6/12), (8/15) respectively. In addition, for reasons of completeness, convolutional codes are also being applied under the same channel conditions. Moreover, we suggest a hybrid coding technique, which combines the (8, 4, 4) GAC and the (15, 8, 4) RAC code in order to meet the requirements of the PLC time-varying channel and improve its performance. Concerning the system's design, we take into consideration Zimmermann's model for the PLC channel. We apply Middleton's model for the channel's background and impulsive noise, while we also introduce a novel way of estimating the system's impulsive noise. Finally, the well-known transmission technique of orthogonal frequency-division multiplexing is used. The channel's performance is evaluated in terms of the bit-error rate for different Eb/N0 values via computer simulations.
TL;DR: A channel-estimation approach for orthogonal frequency-division multiplexing (OFDM) systems using a superimposed training (ST) scheme, in which the pilot symbols are superimposed onto the data streams prior to transmission, shows that the ST-based channel estimation schemes have a slightly poorer performance but yield higher system capacity.
Abstract: Channel estimation in multipath environments is typically performed using the pilot-symbol-assisted modulation (PSAM) scheme. However, the traditional PSAM scheme requires the use of dedicated pilot subcarriers and therefore leads to a reduction in the bandwidth utilization. Accordingly, this paper investigates a channel-estimation approach for orthogonal frequency-division multiplexing (OFDM) systems using a superimposed training (ST) scheme, in which the pilot symbols are superimposed onto the data streams prior to transmission. By using equally spaced pilot symbols of equal power and assuming that the number of pilots is larger than the channel order, it is shown that the channel-estimation performance is independent of the number of pilots used. The optimal ratio of the pilot symbol power to the total transmission power is analyzed to maximize the lower bound of the channel capacity. Overall, the current results show that the ST-based channel estimation schemes have a slightly poorer performance than the PSAM scheme but yield higher system capacity.
TL;DR: It is shown that the proposed overcomplete W-H frame transmission system with a lower-order constellation is more robust against the TF dispersive channels than the incomplete Riesz basis system employing a higher- order constellation with the same spectral efficiency.
Abstract: The issue of multicarrier transmission with high spectral efficiency and low interference over time-frequency (TF) dispersive channels is addressed in this paper. Specifically, we propose a digital signal transmission scheme employing overcomplete Weyl-Heisenberg (W-H) frames as modulation pulses. The proposed W-H frame transmission scheme, which relaxes the conventional orthogonality/biorthogonality constraints for perfect transmultiplexing, can be viewed as a generalization of the orthogonal frequency division multiplexing (OFDM) and nonorthogonal (biorthogonal) frequency division multiplexing (NOFDM). With this new transmission scheme, one can not only achieve higher spectral efficiency effectively but also design the pulse shape more flexibly to combat the impact of the propagation channel. In order to optimally mitigate the intersymbol interference (ISI) and interchannel interference (ICI) caused by the wireless channel, the selections of the Gaussian pulse shape and T-F grid parameters are addressed jointly from the viewpoint of minimum energy perturbation for several typical channel scattering functions. Simulation results are presented to study the performance of the proposed system. It is shown that the proposed overcomplete W-H frame transmission system with a lower-order constellation is more robust against the TF dispersive channels than the incomplete Riesz basis system employing a higher-order constellation with the same spectral efficiency.
TL;DR: The false-urgency issue that was caused by variations in available system resources is effectively avoided by a proposed rate-requirement calculation mechanism based on the status of the packets in queue and system resource availability.
Abstract: We study the resource-allocation problem in a multiuser orthogonal frequency-division multiplexing (OFDM)-based cognitive radio (CR) system using a cross-layer approach. The goal is to provide satisfactory quality of service (QoS) to both real-time and non-real-time applications, despite the rapid variations in available resources caused by the activities of the primary users. The dynamic nature of the available spectrum gives rise to two resource allocation issues: 1) problem feasibility and 2) false urgency. To solve the problem-feasibility issue, which arises when resources are insufficient to meet all user QoS requirements, we adopt a goal-programming approach. The false-urgency issue that was caused by variations in available system resources is effectively avoided by a proposed rate-requirement calculation mechanism based on the status of the packets in queue and system resource availability. Simulation results show that the proposed cross-layer resource-allocation algorithm for CR systems performs better than existing algorithms that were designed for multiuser OFDM systems.
Patent•
13 May 2009TL;DR: In this article, the first and second local oscillator frequencies are selected such that intermediate-frequencies formed by the frequency conversion are mirror-symmetrical relative to the sampling frequency of the A/D converter.
Abstract: An arrangement for transmitting magnetic resonance signals, with a transmission link that connects a local coil with a receiver, has a first channel of the local coil with a first single antenna to acquire a first magnetic resonance signal, as well as a first mixer connected with the first single antenna. The first mixer forms an intermediate-frequency first signal from the supplied first magnetic resonance signal. A second channel of the local coil has a second single antenna to acquire a second magnetic resonance signal, as well as a second mixer connected with the second single antenna. The second mixer forms an intermediate-frequency second signal from the supplied second magnetic resonance signal. The local coil has a device for signal combination that, by frequency multiplexing, that combines the intermediate-frequency first signal of the first channel and the intermediate-frequency second signal of the second channel so that it arrives at the receiver via the transmission path. The receiver has an A/D converter at which one of the transmitted intermediate-frequency signals of an associated channel arrives in order to be sampled with a sampling frequency for digitization. For frequency conversion, a first local oscillator frequency is connected at the first mixer and a second local oscillator frequency is connected at the second mixer. the first and second local oscillator frequencies are selected such that intermediate-frequencies formed by the frequency conversion are mirror-symmetrical relative to the sampling frequency of the A/D converter.
TL;DR: In this paper, the performance degradation of orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) radio signals caused by transmission along dispersive single-mode fiber is assessed theoretically and by numerical simulation.
Abstract: The performance degradation of orthogonal frequency-division multiplexing (OFDM) ultra-wideband (UWB) radio signals caused by transmission along dispersive single-mode fiber is assessed theoretically and by numerical simulation. It is shown that the performance degradation of OFDM-UWB signals caused by fiber dispersion is due to the power fading induced on the subcarrier showing stronger fading. Results show that the power fading description based on a small signal analysis provides good estimates of the optical signal-to-noise ratio degradation for OFDM-UWB signals with modulation index (defined as the ratio between the root-mean-square voltage of the OFDM-UWB signal and the bias voltage of the Mach-Zehnder modulator) values up to 18%. This is a pessimistic value for some OFDM-UWB carrier frequencies.
20 Dec 2009
TL;DR: Simulation results show that the EFFR scheme is more flexible and robust than the SFR scheme, and can gain substantial improvements in terms of both, the overall cell capacity as well as the cell-edge user performance.
Abstract: Inter-cell interference (ICI) mitigation is always a big challenge issue in cellular systems. In this work we propose an Enhanced Fractional Frequency Reuse (EFFR) scheme combined with power allocation and an interference-aware reuse mechanism to achieve not only ICI limitation at cell edge but also enhancement of overall cell capacity in orthogonal frequency division multiple access (OFDMA) based communication networks. The EFFR scheme divides the whole available bandwidth into a Primary Segment and a Secondary Segment. The exclusive reuse-3 subchannels in the Primary Segment will be prior used by cell-edge users with higher transmission power, whereas the remaining subchannels are all reuse-1 subchannels allowing to be used with lower power. In addition, the resources in the Secondary Segment will be occupied by means of signal-to-interference-ratio (SINR) estimation. We implement the proposed EFFR scheme in a system-level simulator and compare its performance with the well-known Soft Frequency Reuse (SFR) scheme and the classical reuse-1 scheme. In order to investigate the impact on the performance by power allocation, schemes are simulated with various power masks, and using a scenario with surrounding cells up to 2nd-tier. Simulation results show that the EFFR scheme is more flexible and robust than the SFR scheme, and can gain substantial improvements in terms of both, the overall cell capacity as well as the cell-edge user performance.
TL;DR: In this paper, the minimum number of bits required in the digital-to-analog converter (D/A) is computed for the quadrature MZ with digital predistortion and hard clipping.
Abstract: Orthogonal frequency-division multiplexing (OFDM) has a high peak-to-average ratio (PAR), which can result in low optical power efficiency when modulated through a Mach-Zehnder (MZ) modulator. In addition, the nonlinear characteristic of the MZ can cause significant distortion on the OFDM signal, leading to in-band intermodulation products between subcarriers. We show that a quadrature MZ with digital predistortion and hard clipping is able to overcome the previous impairments. We consider quantization noise and compute the minimum number of bits required in the digital-to-analog converter (D/A). Finally, we discuss a dual-drive MZ as a simpler alternative for the OFDM modulator, but our results show that it requires a higher oversampling ratio to achieve the same performance as the quadrature MZ.