Showing papers on "Frequency-division multiplexing published in 2003"

Linear constellation precoding for OFDM with maximum multipath diversity and coding gains

Abstract: Orthogonal frequency-division multiplexing (OFDM) converts a frequency-selective fading channel into parallel flat-fading subchannels, thereby simplifying channel equalization and symbol decoding. However, OFDM's performance suffers from the loss of multipath diversity, and the inability to guarantee symbol detectability when channel nulls occur. We introduce a linear constellation precoded OFDM for wireless transmissions over frequency-selective fading channels. Exploiting the correlation structure of subchannels and choosing system parameters properly, we first perform an optimal subcarrier grouping to divide the set of subchannels into subsets. Within each subset, a linear constellation-specific precoder is then designed to maximize both diversity and coding gains. While greatly reducing the decoding complexity and simplifying the precoder design, subcarrier grouping enables the maximum possible diversity and coding gains. In addition to reduced complexity, the proposed system guarantees symbol detectability regardless of channel nulls, and does not reduce the transmission rate. Analytic evaluation and corroborating simulations reveal its performance merits.

Impact of the propagation environment on the performance of space-frequency coded MIMO-OFDM

Abstract: Previous work on space-frequency coded multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) has been restricted to idealistic propagation conditions. In this paper, using a broadband MIMO channel model taking into account Ricean K-factor, transmit and receive angle spread, and antenna spacing, we study the impact of the propagation environment on the performance of space-frequency coded MIMO-OFDM. For a given space-frequency code, we quantify the achievable diversity order and coding gain as a function of the propagation parameters. We find that while the presence of spatial receive correlation affects all space-frequency codes equally, spatial fading correlation at the transmit array can result in widely varying performance losses. High-rate space-frequency codes such as spatial multiplexing are typically significantly more affected by transmit correlation than low-rate codes such as space-frequency block codes. We show that in the MIMO Ricean case the presence of frequency-selectivity typically results in improved performance compared to the frequency-flat case.

Digital receivers and transmitters using polyphase filter banks for wireless communications

Abstract: Provides a tutorial overview of multichannel wireless digital receivers and the relationships between channel bandwidth, channel separation, and channel sample rate. The overview makes liberal use of figures to support the underlying mathematics. A multichannel digital receiver simultaneously down-converts a set of frequency-division-multiplexed (FDM) channels residing in a single sampled data signal stream. In a similar way, a multichannel digital transmitter simultaneously up-converts a number of baseband signals to assemble a set of FDM channels in a single sampled data signal stream. The polyphase filter bank has become the architecture of choice to efficiently accomplish these tasks. This architecture uses three interacting processes to assemble or to disassemble the channelized signal set. In a receiver, these processes are an input commutator to effect spectral folding or aliasing due to a reduction in sample rate, a polyphase M-path filter to time align the partitioned and resampled time series in each path, and a discrete Fourier transform to phase align and separate the multiple baseband aliases. In a transmitter, these same processes operate in a related manner to alias baseband signals to high order Nyquist zones while increasing the sample rate with the output commutator.

Orthogonal Frequency Division Multiplexing Based on Offset QAM

Abstract: One of the factors determining the performance of wireless Orthogonal Frequency Division Multiplexing (OFDM) systems is time-frequency localization of the transmitter and receiver pulse shaping filters. OFDM based on offset quadrature amplitude modulation (OFDM/OQAM) bypasses a major disadvantage of OFDM based on ordinary QAM, namely the fact that time-frequency well-localized pulse shaping filters are prohibited in the case of critical time-frequency density where spectral efficiency is maximal. In this chapter, we study the problem of pulse shaping filter design for OFDM/OQAM systems and we establish relations between OFDM/OQAM and Wilson and Gabor expansions. We derive general orthogonality conditions for OFDM/OQAM systems and we propose a computationally efficient method for designing time-frequency well-localized OFDM/OQAM pulse shaping filters with arbitrary length and arbitrary overlapping factors. We furthermore introduce biorthogonal frequency division multiplexing based on OQAM (BFDM/OQAM). Finally, design examples are presented to assess the performance of the proposed design algorithm.

BER minimized OFDM systems with channel independent precoders

Abstract: We consider the minimization of uncoded bit error rate (BER) for the orthogonal frequency division multiplexing (OFDM) system with an orthogonal precoder. We analyze the BER performance of precoded OFDM systems with zero forcing and minimum mean squared error (MMSE) receivers. In the case of MMSE receivers, we show that for quadrature phase shift keying (QPSK), there exists a class of optimal precoders that are channel independent. Examples of this class include the discrete Fourier transform (DFT) matrix and the Hadamard matrix. When the precoder is the DFT matrix, the resulting optimal transceiver becomes the single carrier system with cyclic prefix (SC-CP) system. We also show that the worst solution corresponds to the conventional OFDM system; the conventional OFDM system has the largest BER. In the case of zero forcing receivers, the design of optimal transceiver depends on the signal-to-noise ratio (SNR). For higher SNR, solutions of optimal precoders are the same as those of MMSE receivers.

A low-complexity ML channel estimator for OFDM

Abstract: Orthogonal frequency-division multiplexing with cyclic prefix enables low-cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum-likelihood channel estimator. In addition to being optimal (up to the modeling error), this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

Subspace-based blind channel estimation for OFDM by exploiting virtual carriers

Abstract: Reliable channel estimation is indispensable for orthogonal frequency-division multiplexing (OFDM) systems employing coherent detection and adaptive loading in order to achieve high data rate communications. Several options exist in practical OFDM systems-including training symbols, cyclic prefix, virtual carriers, pilot tones, and receiver diversity-to facilitate channel estimation. In this paper, a subspace blind channel estimation method based on exploiting the presence of virtual carriers is proposed for OFDM systems over a time-dispersive channel. The method can be applied to conventional OFDM systems with cyclic prefix as well as OFDM systems with no cyclic prefix. The reduction/elimination of cyclic prefix thereby provides the OFDM systems the potential to achieve higher channel utilization than most previously reported cyclic prefix based estimators. Sufficient channel identifiability condition is developed as well. Comparison with two other recently reported subspace methods is presented via computer simulations to support the effectiveness of the proposed method.

Theoretical analysis and performance of OFDM signals in nonlinear fading channels

Abstract: This paper presents an analytical framework to calculate the average symbol-error rate (SER) of uncoded orthogonal frequency-division multiplexing (OFDM) systems in realistic scenarios impaired by transmitter nonlinearity and frequency-selective fading channels. The results are applicable to cyclically extended OFDM signals characterized by a high number of carriers, which can be modeled as complex Gaussian processes. To avoid intercarrier interference, we also assume that the symbol duration is shorter than the channel coherence time. We derive analytical SER results in Rayleigh and Rice frequency-selective fading channels, for both the nonlinear amplification and the ideal predistortion case. Simulations results demonstrate the validity of the analytical results.

A Spectrally Efficient Frequency Division Multiplexing Based Communication System

Abstract: In this paper we propose a spectrally efficient frequency division multiplexing (FDM) based communications system. This FDM system handles intercarrier frequency separations smaller than those of orthogonal frequency division multiplexing (OFDM) systems. The FDM transmitter consists of a bank of modulators that generate the different information conveying sub-carriers that compose the transmitted FDM signal. The FDM receiver consists of a bank correlators followed by a maximum likelihood (ML) detector. The demodulators extracts a set of sufficient statistics from the received FDM signal and the ML detector uses the sufficient statistics to estimate the transmitted symbols. To alleviate the complexity of ML detection alternative detection methods are examined including linear detectors and genetic algorithms (GA) based detectors. Finally, FDM system fundamental limitations are discussed.

Radio communication system employing spectral reuse transceivers

Abstract: A spectral reuse transceiver-based communication system conducts communications between a master site and a plurality of remote sites using a selected portion of a communication bandwidth containing a plurality of sub-bandwidth channels. Each remote site transceiver monitors the communication bandwidth for activity on the sub-bandwidth channels, and informs a master site transceiver which sub-bandwidth communication channels are absent communication activity and therefore constitute clear channels. The master site transceiver compiles an aggregate list of clear channels from all the remote sites and then broadcasts the aggregate list to the remote sites. The master site and a remote site then conduct communications therebetween by frequency-hopping and/or orthogonal frequency multiplexing among the clear channels using an a priori known PN sequence.

High-throughput, high-performance OFDM via pseudo-orthogonal carrier interferometry spreading codes

Abstract: The paper introduces to orthogonal frequency-division multiplexing (OFDM) systems a novel pseudo-orthogonal carrier interferometry spreading code which spreads each parallel data stream over all the OFDM carriers. Pseudo-orthogonal carrier interferometry (PO-CI) spreading codes are carefully selected to introduce the following benefits to OFDM: up to 2N parallel data streams can be coded onto N carriers, with little degradation in performance; when rate 1/2 channel coding is applied in addition to PO-CI spreading codes, the resulting binary phase-shift keying OFDM systems demonstrate the performance of coded OFDM and the throughput of uncoded OFDM; PO-CI codes are carefully selected to spread in a manner which eliminates the peak-to-average power ratio problems characteristic of traditional OFDM.

Wideband: multimedia unplugged

Abstract: Promising to eliminate just about all of a home's signal cables, ultrawideband very-low-power wireless technology, when finally standardized, will be a carrier-based system most likely incorporating frequency hopping and orthogonal frequency-division multiplexing (OFDM). Its purpose: to replace almost every data cable in the home, even the ones going in and out of the television set, a job that requires moving hundreds of megabits of data per second. That' s faster than all but the speediest of wired networks. The speed is achieved, however, over distances of only 10 meters or so. Ultrawideband (UWB) handles anything from high-speed streams carrying real-time HDTV programs to images downloaded from a digital camera to low-speed timing signals for keeping clocks accurate.

Co-channel interference cancellation for space-time coded OFDM systems

Abstract: Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on null-steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.

System for providing dynamic service using optical sub-carrier multiplexing type multi-channel access and method of controlling the same

Abstract: Disclosed herein is a system for providing dynamic service using optical sub-carrier multiplexing type multi-channel access and method of controlling the same. The system includes a plurality of optical sub-carrier multiplexing modems, an optical gigabit-Ethernet switch and a plurality of wavelength division multiplexing couplers. The optical sub-carrier multiplexing modems transmit user traffic using multiple optical sub-carrier channel control. The optical gigabit-Ethernet switch is connected to an access network upstream and to the optical sub-carrier multiplexing modems downstream to manage a plurality of wavelengths and a plurality of optical sub-carrier channels according to the wavelengths. The wavelength division multiplexing couplers connect the plurality of optical sub-carrier multiplexing modems with the optical gigabit-Ethernet switch, perform aggregation and branching of a certain wavelength and transmit it.

A MIMO system with backward compatibility for OFDM based WLANs

Abstract: Orthogonal frequency-division multiplexing (OFDM) has been selected as the basis for the new IEEE 802.11a standard for high-speed wireless local area networks (WLANs). We consider doubling the data rate of the IEEE 802.11a system by using a multi-input multi-output (MIMO) system with two transmit and two receive antennas. We propose a preamble design for this MIMO system that is backward compatible with its single-input single-output (SISO) counterpart as specified by the IEEE 802.11a standard. Based on this preamble design, we devise a sequential method for the estimation of the carrier frequency offset (CFO), symbol timing, and channel response. We also provide a simple soft-detector to obtain the soft-information for the Viterbi decoder. Both the sequential parameter estimation method and the soft-detector are ideally suited for real-time implementations. The effectiveness of our methods is demonstrated via numerical examples.

Adaptive duplexer for multiband transreceiver

Abstract: This paper proposes new adaptive duplexer architecture for multiband systems. It reduces the need for multiple switched duplexers. The propose new technique is based on two step isolation processes: (1) a wide-band circulator (20 dB isolation); and (2) an active double loop canvelling technique that leads to two nulls at the desired frequency and the transmitted frequency. This solution is based on multiband direct conversion receiver. In a direct conversion receiver a simple synchronous detector at the duplex offset frequency provides a convenient feedback signal. Using this approach it was possible to achieve 66 dB total isolation between the transmit signal and the receive signal and 37 dB total transmitter noise cancellation at the receiver band. Adaptive duplexing eliminates many external components in multiband transceiver front end. The described scheme is simple, uses a relatively low frequency error signals and has low computational complexity.

Constrained least squares detector for OFDM/SDMA-based wireless networks

Abstract: The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth which necessitates a high spectral efficiency. A combined orthogonal frequency division multiplexing (OFDM)/spatial division multiple access (SDMA) approach can efficiently tackle both obstacles and paves the way for cheap, high-capacity wireless indoor networks. The channel distortion due to multipath propagation is efficiently mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. However, to keep the cost of an indoor wireless network comparable to its wired counterpart's cost, low-complexity SDMA processors with good performance are of special interest. In this paper, we propose a new multiuser SDMA detector which is designed for constant modulus signals. This constrained least squares (CLS) receiver, which deterministically exploits the constant modulus nature of the subcarrier modulation to achieve better separation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receiver. Additionally, since the CLS detector relies on reliable channel knowledge at the receiver, we propose a strategy for estimating the multiple input multiple output (MIMO) channels. Simulations for a Hiperlan II-based case-study show that the CLS detector significantly outperforms the ZF detector and comes close to the performance of the MMSE detector for QPSK. For higher order M-PSK, the CLS detector outperforms the MMSF detector. Furthermore, the estimation complexity for the CLS detector is substantially lower than that for the MMSE detector which additionally requires estimation of the noise power.

Channel modeling and associated inter-carrier interference equalization for OFDM systems with high Doppler spread

Abstract: We address the problem of OFDM transmission over a time-varying, frequency-selective channel with high Doppler spread. This creates situations where the channel significantly evolves over the time span of one OFDM symbol. We analyze the CP-OFDM transmission mechanism, and the impairments due to channel variations, using a decomposition of these variations over a base of sinusoid functions sampling the Doppler spectrum at subcarrier frequencies. On the one hand, this leads to a fairly parsimonious parameterization of the time-varying channel impulse response. On the other hand we show that, considering a whole CP-OFDM symbol, this leads to a duality between equalization of the delay spread of a time-varying channel in the time domain, and the equalization of the Doppler spread of a frequency selective channel in the frequency domain. Using this duality, we show that equalization in the frequency domain can benefit from all known time domain equalization methods.

Channel estimation algorithms for broadband MIMO-OFDM sparse channel

Abstract: In the broadband communication, channel impulse response usually exhibits sparse behavior (i.e.. many nearly zero taps). This paper considers the channel estimation of the sparse channel for broadband multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) systems. Three algorithms for MIMO-OFDM sparse channel estimation are presented and compared: least square channel estimation (LSCE), constraint least square channel estimation (CLSCE) with ideal delay estimation, matching pursuit based channel estimation (MPCE). Mean square error (MSE) performances an also analyzed. Simulation results show that MPCK has better performance than LSCE and is very close to CLSCE under the high SNR, and also MPCE does not require the a priori of channel. Using the MP-based channel estimator, the detection performance is nearly optimal.

Maximum-likelihood frame timing instant and frequency offset estimation for OFDM communication over a fast Rayleigh-fading channel

Abstract: A novel maximum-likelihood (ML) technique for frame timing instant and frequency offset estimation in an orthogonal frequency-division multiplexing system is investigated. By taking advantage of the cyclic prefix within the guard interval, the proposed technique can accurately estimate the frame timing instant and frequency offset with no assistance from pilot symbols. The proposed technique also takes fast Rayleigh fading into consideration for the purpose of wireless applications. Furthermore, several combining techniques are also investigated in order to improve the performance of the proposed estimators. Extensive simulation experiments show that the proposed techniques can effectively achieve lower estimation errors in frame timing and frequency offset estimation.

Diversity coding for subcarrier multiplexing on multimode fibers

Abstract: The bandwidth-distance product limitations of multimode fiber caused by intermodal dispersion may be overcome by using subcarrier multiplexing. Since there are deep nulls in the frequency response within the frequency band of interest, it is necessary either to avoid these frequencies adaptively or to use an appropriate coding technique. We consider the case of using a training sequence to identify the subcarriers to be discarded, and also diversity coding in which the information may be recovered in the presence of the loss of a fixed number of subcarriers. Simulation results indicate that diversity coding, which is less complex than the use of training sequences, provides very good performance over a wide range of channel conditions.

Robust channel estimation in MIMO-OFDM systems

Abstract: To combat strong multitone narrowband interference (MTNBI), a robust channel estimation method, based on the M-estimation method, is proposed for channel estimation in multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) systems. Simulation results show that the new channel estimation method in the presence of MTNBI is a promising approach.

Two-dimensional differential demodulation for OFDM

Abstract: The possibility of combining differential demodulators in time direction and frequency direction for orthogonal frequency-division multiplexing (OFDM) frames is investigated. The proposed algorithm uses not only the direction in which the information is modulated, but also joins it with other symbols in time or frequency direction of the OFDM frame, resulting in an additional second direction that is differentially demodulated as well. In the special case that is investigated here, the second direction does not carry any modulated information. Nevertheless, it is shown that it can be used for the two-dimensional demodulation process without changing the transmitter. Therefore, the proposed algorithm is applicable for existing digital transmission systems, for example, digital audio broadcasting, as well as new systems.

Worst-case design for optimal channel equalization in filterbank transceivers

Abstract: A worst-case approach is adopted to tackle optimal channel equalization for multirate filterbank transceivers, which are widely used in data communication networks, such as discrete wavelet multitone (DWMT) systems in digital subscriber lines (DSL), orthogonal frequency multiplexing division (OFMD) in frequency division multiple access (FDMA) systems, applicable to direct-sequence/spread-spectrum (DS/SS) code division multiple access (CDMA) networks. It is assumed that the observation noise is nonwhite with bounded power-norm or root-mean-squared (RMS) value. Our objective is to design the optimal receiving filterbanks that not only achieve the zero-forcing (ZF) condition or channel equalization but also minimize the RMS error between the transmitted symbols and the received symbols in the presence of the worst-case nonwhite noise. All ZF receiving filterbanks will be parameterized, and optimal design for channel equalization will be converted into an equivalent optimal H/sub /spl infin// filtering problem for the augmented receiving filterbanks with RMS error preserved. Our main results cover computation of the optimal RMS error achievable for the worst-case noise and an explicit design algorithm for suboptimal channel equalizers with the performance index arbitrarily close to the optimal one. A simulation example is used to illustrate the proposed optimal channel equalization algorithm.

Throughput and spectral efficiency analysis in 3G FDD WCDMA cellular systems

Abstract: Multimedia applications require from wireless communications service providers to support service mix traffic (voice, data, and real time audio/video) to the end wireless subscriber. Wideband code division multiple access (WCDMA) has been chosen as a basic radio access technology for the Universal Mobile Telecommunications System/ International Mobile Telecommunications-2000 (UMTS/IMT- 2000) to support multimedia services. Throughput and spectral efficiency are fundamental parameters in capacity planning for 3G cellular network deployments. In this paper, we investigate the downlink (DL) aggregate user average throughput and spectral efficiency performance of the standard universal mobile telecommunications system terrestrial radio access WCDMA frequency-division duplex (UTRA WCDMA FDD) system for different scenarios of user population density, coverage area and frequency reuse factor in a cellular environment. A standard 19-basestation cellular architecture is used in our analysis. Our analysis reveals that the aggregate user throughput performance is fluctuating as a result of changes in cell size, channel bandwidth, frequency reuse and subscribers density. Therefore, very careful network capacity planning is needed in order to operate the WCDMA FDD networks in a cost-effective manner.

A frequency-domain SQUID multiplexer for arrays of transition-edge superconducting sensors

Abstract: We describe the development of a frequency-domain multiplexer (MUX) to read out arrays of superconducting transition-edge sensors (TES). Fabrication of large-format arrays of these sensors is becoming practical; however, reading out each sensor in the array is a major instrumental challenge. Frequency-domain multiplexing can greatly simplify the instrumentation of large arrays by reducing the number of SQUID's (superconducting quantum interference devices) and wires to the low temperature stages. Each sensor is AC biased at a different frequency, ranging from 380 kHz to 1 MHz. Each sensor signal amplitude-modulates its respective AC bias frequency. An LC filter associated with each sensor suppresses Johnson noise from the other sensors. The signals are combined at a current summing node and measured by a single SQUID. The individual signals from each sensor are then lock-in detected by room temperature electronics. Test chips with fully lithographed LC filters for up to 32 channels have been designed and fabricated. The capacitance and inductance values have been measured and are close to the design goals. We discuss the basic principles of frequency-domain multiplexing, the design and testing of the test chips, and the implementation of a practical system.

OFDM-CPM signals for wireless communications

Abstract: A class of orthogonal frequency division multiplexingcontinuous phase modulation (OFDM-CPM) signals is introduced in which the binary data sequence is mapped to complex symbols using the concept of correlated phase states of a CPM signal. A multiple-symbol-observation receiver is used to decode the received sequence, and an investigation of bit error rate over typical wireless multipath channels with additive white Gaussian noise is presented. The performance of a variety of OFDM-CPM signals is presented and analyzed. Performance is a function of parameter and the observation interval, both of which are at the disposal of the system designer. It is shown that OFDM-CPM is a promising signalling scheme in multipath fading channels. Results for multi- and asymmetric OFDM-CPM signals are also presented. The peak-to-average power ratio (PAPR) performance of these signals is also presented, and an algorithm to reduce PAPR is proposed that is based on a multi-amplitude CPM constellation. It is shown through numerical simulations that the proposed algorithmreduces the PAPR of a 128-carrier OFDM-CPM signal by more than 4 dB.

Deterministic perturbation gradient approximation for transmission subspace tracking in FDD-CDMA

Abstract: Multiple antenna transmission promises to bring substantial capacity benefits to wireless communication systems. Knowledge of the channel can be used to further improve the communication. In frequency division duplex channels, such knowledge can be obtained using feedback. The feedback bandwidth is limited, hence efficient feedback of such channel information to the transmitter is required. This paper proposes one such method based on subspace tracking principles. A perturbation vectors is used to approximate the gradient of the cost function. Simulation results show good performance when there is temporal or spatial correlation in the channel.

System capacity from UMTS smart antenna concepts

Abstract: Exploitation of the spatial dimension is a significant issue for third generation wireless networks. The main benefits expected are emission reduction and system capacity enhancements. There are several approaches to exploit the spatial dimension of the mobile channel. Concepts suggested are higher order sectorization (6 sector sites), fixed beam switching concepts (a number of fixed beams covers the area of a sector) and adaptive beamforming (user specific antenna patterns are evaluated). This paper documents the results of a system level investigation considering different antenna concepts when applying power control in UTRA FDD (UMTS terrestrial radio access, frequency division duplex) downlink. Furthermore, load control algorithms based on power rise evaluation and user elimination applying a stepwise removal algorithm are also taken into account. Simulations have been carried out for different data services utilizing dedicated channels. The investigations were accomplished within 3GPP-UMTS boundary conditions.

Multitributary OFDM optical transmitter using carrier-suppressed optical single-sideband modulation

Abstract: A multitributary optical transmitter is reported using carrier-suppressed OSSB modulation. A number of OFDM-channels can be generated by a single optical modulator. Duobinary modulation has also been demonstrated in this transmitter using RF signal processing.