Search

A simple scheme to multiply the repetition rate in an actively mode-locking fiber laser is demonstrated by using a phase modulated optical fiber loop mirror (PMOLM). Numerical simulation shows that the best results can be obtained when the intracavity dispersion is negative and the modulation frequency is an odd harmonic of half of the cavity fundamental frequency.

In this paper, a two stage hybrid interference cancellation and equalization framework is proposed for interference cancellation in the uplink of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The first stage uses time domain equalization to suppress cochannel interference, mitigate asynchronism, and shorten the postequalization channel response to be no longer than the length of the cyclic prefix. The second stage performs low-complexity single tap equalization and detection in the frequency domain. The framework is developed specifically for spatial multiplexing and is applied to multiuser MIMO-OFDM systems with asynchronism between users as well as to single-user MIMO-OFDM systems. Various equalizer design methods are proposed that determine the coefficients directly from the training data and are compared with methods based on channel estimates. The equalizer coefficients and postequalization channel response are found by solving a joint optimization that maximizes the signal to interference-plus-noise ratio (SINR) in the frequency domain. Simulations compare various training-based methods and show the proposed methods provide good bit error rate (BER) and SINR performance in a variety of interference scenarios. Index Terms--Cancellation, interference, MIMO, OFDM, space-time.

Orthogonal frequency division multiplexing (OFDM) is a practical broadband signaling technique for use in multipath fading channels. Over the past ten years, research has shown that power loading, where the power allocations on the OFDM frequency tones are jointly optimized, can improve error rate or capacity performance. The implementation of power loading, however, is dependent on the presence of complete forward link channel knowledge at the transmitter. In systems using frequency division duplexing (FDD), this assumption is unrealistic. In this paper, we propose power loading for OFDM symbols using a limited number of feedback bits sent from the receiver to the transmitter. The power loading vector is designed at the receiver, which is assumed to have perfect knowledge of the forward link channel, and conveyed back to the transmitter over a limited rate feedback channel. To allow for the vector to be represented by a small number of bits, the power loading vector is restricted to lie in a finite set, or codebook, of power loading vectors. This codebook is designed offline and known a priori to both the transmitter and receiver. We present two power allocation selection algorithms that optimize the probability of symbol error and capacity, respectively. Simulation results show that the proposed limited feedback techniques provide performance close to full channel knowledge power loading. Index Terms--Limited feedback, orthogonal frequency division multiplexing, power loading.

An adaptive frequency-domain equalization (FDE) algorithm for implementation in single-carrier (SC) multiple-input multiple-output (MIMO) systems is presented. A novel method of reducing the overhead required to train the proposed equalizer is outlined.

A precoding technique for suppressing multiple-access interference and exploiting multipath and multiantenna in CDMA systems with ultra low-complexity receivers is presented. It is shown that transmitter precoding offers a reasonable alternative to receiver-based multiuser signal processing when minimizing computational complexity at the mobile unit is a priority.

New robust linear receiver techniques for joint space-time decoding and interference rejection in multi-access MIMO systems that use orthogonal space-time block codes and erroneous channel state information (CSI) are developed. The proposed receivers are based on worst-case performance optimization.

A multimode antenna selection, which uses a low-rate feedback channel to improve the error rate performance of spatial multiplexing systems with linear receivers, is proposed. In this technique, both the number substreams and the mapping of substreams to antennas are dynamically adjusted, for a fixed total data rate, to the channel based on limited feedback from the receiver.

Existing multichannel blind restoration techniques assume perfect spatial alignment of channels, correct estimation of blur size, and are prone to noise. An alternating minimization scheme based on a maximum 'a posteriori' estimation with 'a priori' distribution of original images defined by variational integral is developed.

An optimum nonuniform finite impulse response (FIR) transmultiplexer design problem subject to specifications in the frequency domain is considered to minimize the sum of the ripple energy for all the individual filters, subject to the specifications on amplitude and aliasing distortions and to the passband and stopband specifications for the individual filters.

Performance of multichannel lateral insulated gate bipolar transistors (MC-LIGBTs) fabricated in a cost-effective, fully implanted, CDMOS-compatible process in junction isolation technology is reported. It is shown that combining the MC and the SA-segmented N (super +)P/P(super +) [NPN] anode concepts creates a device that exhibits both low on-state and turnoff losses and is best placed for use in power IC applications.

Future inter- and intra-ULSI interconnect systems demand extremely high data rates and bidirectional multiI/O concurrent service, re-configurable computing/processing architecture, and total compatibility with mainstream silicon system-on-chip and system-in-package technologies. These new interconnect schemes facilitate the use of additional multiple access techniques to increase bandwidth and channel concurrency and to reduce channel latency.

An overview of technology options for the physical layer design of next-generation cellular networks to provide high-speed packet-based services is presented. These include air interface design, resource allocation, multiple antenna techniques, duplexing method, and network coordination.

A well-developed niobium based technology was adapted for Superconducting Quantum Interference Devices (SQUIDs) manufacturing in order to be able to fabricate the current sensor and the bolometer on a common substrate. The function of both devices was successfully tested at the operating temperature of 300 mK.

The implication of the variation of the size of the rods is studied by applying coupled mode theory (CMT). CMT can provide a useful insight in the effect of size variations, and unlike other numerical methods such as the finite difference time domain, it neither does nor require excessive computational time.

Multiple-input multiple-output (MIMO) wireless communication systems can offer high data rates through spatial multiplexing or substantial diversity using transmit diversity. In this letter, switching between spatial multiplexing and transmit diversity is proposed as a simple way to improve the diversity performance of spatial multiplexing. In the proposed approach, for a fixed rate, either multiplexing or diversity is chosen based on the instantaneous channel state and the decision is conveyed to the transmitter via a low-rate feedback channel. The minimum Euclidean distance at the receiver is computed for spatial multiplexing and transmit diversity and is used to derive the selection criterion. Additionally, the Demmel condition number of the matrix channel is shown to provide a sufficient condition for multiplexing to outperform diversity. Monte Carlo simulations demonstrate improvement over either multiplexing or diversity individually in terms of bit error rate. Index Terms--Multiple-input multiple-output (MIMO) systems, space-time coding, transmit diversity.

Hadamard multiplexing is a measurement strategy that yields best sensitivity improvements over scanning measurements for signal-independent detector noise. The presence of photon noise degrades the performance of Hadamard multiplexing because of the increase of photon noise by the superposition of multiple signals. I derive the reduction of the sensitivity gain of a Hadamard measurement and an upper limit for the gain of any cyclic multiplexing strategy in the presence of photon noise. This upper limit clearly exceeds the reduced Hadamard gain and can be achieved by multiplexing sequences that differ from Hadamard S sequences but also share some similarities with respect to their autocorrelation. Examples of such sequences are given. As the analysis shows, the presence of photon noise limits the gain of multiplexing strategies to a finite value, which depends on the ratio between photon noise and detector noise and cannot be exceeded by increasing the number of multiplexed channels. In addition, only switching multiplex schemes, which superpose either all the light or no light of individual channels, can achieve the upper limit of the gain. OCIS codes: 230.4110, 350.6980, 120.3940, 270.5290.

The influence of controlled perturbations (slots) in the ridge waveguide of Fabry-Perot lasers emitting around 1550 nm is characterized. The lasers emit with a single wavelength when more than three slots are introduced, in which case the interslot spacing strongly influences the lasing spectrum.

The implementation of Volterra filters of any order P in the form of a multichannel filterbank is discussed with the help of V-vector algebra. On exploitation of a reduced-complexity multichannel structure, the standard least mean squares (LMS) and normalized least mean squares (NLMS) algorithms can be efficiently replaced with affine projection (AP) algorithms.

A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs well in severe near--far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations. Index Terms--Code-division multiple access (CDMA), Doppler effect, interference suppression.

To approach the potential multiple-input multiple-output (MIMO) capacity while optimizing the system bit-error rate (BER) performance, the joint transmit and receive minimum mean squared error (joint Tx/Rx MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams p as well as fixed modulation and coding across these spatial streams. However, the number of spatial streams has been arbitrarily chosen and fixed, which may lead to an inefficient power allocation strategy and a poor BER performance. In this paper, we relax the constraint of fixed number of streams p and optimize this value for the current channel realization, under the constraints of fixed average total transmit power PT and fixed rate R, what we refer to as mode selection. Based on the observation of the existence of a dominant optimal number of streams value for the considered Rayleigh flat-fading MIMO channel model, we further propose an "average" mode selection that avoids the per-channel adaptation through using the latter dominant value for all channel realizations. Finally, we exhibit the significant BER improvement provided by our mode selection over the conventional joint Tx/Rx MMSE design. Such significant improvement is due to the better exploitation of the MIMO spatial diversity and the more efficient power allocation enabled by our mode selection. Index Terms--Joint transmit and receive optimization, Rayleigh flat-fading MIMO channels, selection, spatial multiplexing.

A novel space-time multipath (STM) coded multi-antenna transmissions over frequency-selective Rayleigh fading channels is designed. Designs are found to rely on digital phase sweeping or block circular delay operations and apply to both single-and multi-carrier systems.

A low-cost, stochastic gradient implementation of the Spatially Preprocessed, Speech Distortion Weighted, Multichannel Wiener Filter (SP-SDW-MWF) is developed. Experimental results show that the proposed frequency-domain stochastic gradient algorithm preserves the benefit of the exact SP-SDW-MWF over the Quadratic Inequality Constraint-Generalized Sidelobe Canceller (QIC-GSC).

A new method of combined source-channel coding for the scalar quantization of a discrete memoryless source is presented, which takes advantage of the reliability information produced by a soft-output channel decoder. Numerical results are presented for a memoryless Gaussian source in conjunction with turbo code showing up to 1-dB improvement in the end-to-end distortion with respect to a traditional channel optimized scalar quantizer. The results include a Gaussian source designed using closed-form expression without the need for a training sequence, as well as image pixels using a training sequence. Furthermore, certain issues related to the effect of the channel mismatch and spectral efficiency of the system are studied. It is shown that the increase in distortion due to a channel mismatch can be substantially reduced by using an adaptive receiver. Index Terms--Quantization, soft-output decoding, turbo code.

Wavelet packets have good properties such as orthogonality and multirate flexibility, and have resulted in a number of works for its applications to code-division multiple-access (CDMA) communications. In this paper we investigate the use of a special set of the wavelet packets, i.e., the waveforms formed from a full binary wavelet packet tree, in multicarrier CDMA systems. In the conventional multicarrier CDMA systems, a simple frequency-domain combining receiver is commonly used to achieve frequency diversity. To combat multipath channel effects, guard intervals are inserted between consecutive symbols. On the other hand, wavelet packets have the property of localization in both time and frequency domains. A novel receiver design utilizing this property to achieve both frequency and time diversity is proposed and evaluated in this paper. This receiver eliminates the need for guard intervals. Simulations are given to support the system and receiver design. This analysis can be extended to the more general wavelet packet basis waveform sets. Index Terms--Code-division multiple access (CDMA), detection, multicarrier, wavelet.

Multicarrier direct-sequence code-division multiple-access (MC-CDMA) overlay has been proposed to be used for ultra-wide-band (UWB) communications. Interference reduction and interference suppression are the key issues for sharing the spectrum in harmony between the established narrow-band systems and the overlaid UWB system. In this paper, investigation is carried out on the use of compromising measures incorporated to the MC-CDMA overlay to meet these goals, as well as on their impacts to the involving parties. At the transmitter, interference reduction to the established narrow-band systems is done by using notch filters. A multipath Nakagami fading channel is assumed. At the receiver, the interference suppression from those narrow-band systems is fulfilled by minimum mean square error (MMSE) detection technique. Numerical results show that precombining MMSE with selective-maximal combining provides the UWB system with much better performance than the receiver made up of notch filter in cascade with code correlator. Index Terms--Minimum mean square error (MMSE) adaptive rake, multicarrier code-division multiple-access (CDMA), Nakagami fading, notch filter, ultra-wide-band communications.

We investigate the effective temporal diversity property of a multiantenna system employing the phase sweeping transmit diversity (PSTD) scheme in a slow fading channel. We analyze how the number of transmit antennas used for phase sweeping and the sweeping frequency spacing affect the normalized time mean-square covariance (NTMSV) value which characterizes the temporal diversity of a channel. It is revealed that, in some situations, given the maximum sweeping frequency, there is an optimal antenna number that maximizes the temporal diversity, and the NTMSV can be used to find the optimal antenna number. Although the temporal diversity provided by PSTD is at the expense of the spatial diversity, we show by analyzing the fundamental tradeoff between the spatial diversity and the temporal diversity that the overall diversity gain can be maintained when applying the PSTD technique. Numerical results show that, with low complexity receivers, the coded PSTD multiple antenna system is a promising candidate to exploit the possible diversity in slow fading channels. Index Terms--Channel coding, diversity, multi-input multi-output (MIMO) channels, phase sweeping, slow fading.

In this paper, a general analytical framework is developed for calculating the symbol error rate of two-dimensional (2-D) signaling in Ricean fading with imperfect channel estimation. We show that in the presence of channel estimation errors, the symbol error rate of arbitrary 2-D signaling in Ricean fading can be expressed as a two-fold proper integral with finite integration limits, which is suitable for numerical evaluation. This new analysis is applicable to any channel estimation scheme where the estimated and the actual channel gains are jointly complex-Gaussian. System parameters related to specific channel estimation schemes are optimized and the effect of Doppler frequency shift in the channel line-of-sight component on the error performance is investigated using our analysis. Index Terms--Imperfect channel estimation, minimum mean square error (MMSE) estimation, pilot symbol assisted modulation (PSAM), Ricean fading, symbol error rate (SER), two-dimensional (2-D) signaling.

This paper studies adaptive equalization for time-dispersive communication channels whose impulse responses have unknown lengths. This problem is important, because an adaptive equalizer designed for an incorrect channel length is suboptimal; it often estimates an unnecessarily large number of parameters. Some solutions to this problem exist (e.g., attempting to estimate the 'channel length' and then switching between different equalizers); however, these are suboptimal owing to the difficulty of correctly identifying the channel length and the risk associated with an incorrect estimation of this length. Indeed, to determine the channel length is effectively a model order selection problem, for which no optimal solution is known. We propose a novel systematic approach to the problem under study, which circumvents the estimation of the channel length. The key idea is to model the channel impulse response via a mixture Gaussian model, which has one component for each possible channel length. The parameters of the mixture model are estimated from a received pilot sequence. We derive the optimal receiver associated with this mixture model, along with some computationally efficient approximations of it. We also devise a receiver, consisting of a bank of soft-output Viterbi algorithms, which can deliver soft decisions. Via numerical simulations, we show that our new method can outperform conventional adaptive Viterbi equalizers that use a fixed or an estimated channel length. Index Terms--Adaptive equalization, mixture model, multimodel, soft-output Viterbi algorithm (SOVA).

The optimal decoding scheme for asynchronous code-division multiple-access (CDMA) systems that employ convolutional codes results in a prohibitive computational complexity. To reduce the computational complexity, an iterative receiver structure was proposed for decoding multiuser data in a convolutional coded CDMA system. At each iteration, extrinsic information is exchanged between a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders. A direct implementation of the optimal SISO multiuser detector, however, has exponential computational complexity in terms of the number of users which is still prohibitive for channels with a medium to large number of users. This paper presents a low-complexity SISO multiuser detector using the decision-feedback scheme, of which tentative hard decisions are made and fed back to the SISO multiuser from the previous decoding output. In the proposed scheme, the log-likelihood ratios (LLR) as well as the tentative hard decisions of code bits are fed back from the SISO decoders. The hard decisions are used to constrain the trellis of the SISO multiuser detector and the LLRs are used to provide a priori information on the code bits. The detector provides good performance/complexity tradeoffs. The computational complexity of the detector can be set to be as low as linear in the number of users. Simulations show that the performance of the low-complexity SISO multiuser detector approaches that of the single-user system for moderate to high signal-to-noise ratios even for a large number of users. Index Terms--Code-division multiple access, convolutional codes, multiuser detector.

The rake receiver performance of a direct-sequence code-division multiple-access (DS-CDMA) system can significantly be degraded unless the channel condition is properly estimated. Since the channel condition is time-varying and location-dependent, it is desirable to employ an adaptive channel estimator (ACE) that can properly adjust the bandwidth of the channel estimation filter (CEF) according to the channel condition such as the maximum Doppler frequency. To this end, we propose low-complexity ACE and signal-to-interference power ratio (SIR) estimator for wide-band CDMA downlink systems with and without the use of transmit antenna diversity. The proposed schemes adjust the bandwidth of the CEF by exploiting the correlation characteristics of the received pilot signal. The analytic design is verified by computer simulation. Simulation results show that the proposed ACE and SIR estimator can significantly improve the link performance for a wide range of channel condition, particularly when the channel condition is poor. Index Terms--Channel estimation, power control, signal-to-interference power ratio (SIR) measurement, wide-band code-division multiple access (WCDMA).

Few ultrawide-band (UWB) models directly fit original baseband UWB data to simultaneously account for their correlation structure, non-Gaussianity, and nonstationarity. The difficulty arises from the fact that no relevant result is available, even in multivariate statistical analysis. It also arises from the attempt to pursue the details of the mechanism that is imagined to be responsible for the generation of UWB data. The consequence is the modeling complexity, which makes it difficult to handle the received signal correlation on the basis of a single realization. Accordingly, various partial characterization is used in the literature instead by virtue of second-order statistics (such as power delay profile), nonparametric characteristics (such as zero-crossing rate), or their combination. In this paper, we take a different philosophy, which believes that the information in the received UWB data itself, as long as fully exploited, plus some simple physical intuition should suffice for the model identification and its parameter estimation. A received UWB signal is decomposed into three factor processes and each is parsimoniously parameterized. The application of the new model to data regeneration and receiver design is illustrated by using the real UWB data acquired by Intel and the TimeDomain Corporation. Index Terms--Correlated baseband ultrawide-band (UWB)-signal models, generation of correlated non-Gaussian UWB data, modeling UWB signals.

An accurate propagation channel model is crucial for evaluating the performance of a communication system. A propagation channel can be described by a Markov model with a finite number of states, each of which is considered to be quasi-stationary over a short period. This work proposes a two-layer multistate Markov model. Instead of a large Markov transition matrix used in a conventional single-layer Markov model, two small Markov transition matrices are employed by a two-layer Markov model to reduce the computational complexity of the model without increasing the memory requirements. The proposed approach characterizes the multiplicative processes of a propagation channel as shadowing and fast fading. Each type of fading is considered as several channel states and each of the states corresponds to a specific mixed Rayleigh-lognormal distribution. Numerical results reveal that the statistical properties of the simulated data are quite close to those obtained from the measurements; indeed, the proposed two-layer Markov model is more accurate and less complex, and requires less memory than the single-layer Markov model. Furthermore, the proposed two-layer Markov model enables the fading statistics and error probability performance of a quadrature phase-shift keying modulation scheme in a typical urban Taipei environment to be more accurately predicted. Besides, it can easily be applied to similar environmental scenarios. Index Terms--Markov model, propagation channel modeling.

An efficient method for generating correlated Nakagami-m fading envelope samples is presented; this method is applicable for arbitrary values of the fading parameter m. The new method is compared to other methods used to generate Nakagami-m random variates. An accurate approximation to the inverse Nakagami-m cumulative distribution function, valid for all values of m, is derived. Uncertainties regarding the autocorrelation of the Nakagami-m fading process are discussed. The fading envelope autocorrelation is determined by simulation and asymptotic analysis. Index Terms--Fading channels, probability, simulation, stochastic processes.

The ratio between the discrete and continuous maxima of signals used in multicarrier communications is analyzed. An exact expression, up to an additive term, is obtained for the considered ratio when the number of uniformly distributed samples is equal to the number of carriers. A more accurate upper estimate on the continuous maximum from discrete samples of the signal and its derivative is suggested. Finally, a tight upper bound is given for the value of the continuous maximum via discrete values of samples when the number of samples is greater than the number of carriers. Index Terms--Multicarrier communications, orthogonal frequency-division multiplexing (OFDM), peak-to-average power ratio (PAPR) reduction.

High peak-to-average power ratio (PAPR) of the transmitted signal is a major drawback of orthogonal frequency division multiplexing (OFDM). In this paper, we propose a modified selective mapping (SLM) technique for PAPR reduction of coded OFDM signal. In this technique, we embed the phase sequence, which is used to lower the PAPR of the data block, in the check symbols of the coded OFDM data block. It is shown that we can achieve both PAPR reduction from the SLM technique as well as error performance improvement from the channel coding with no loss in data rate from the transmission of side information. In addition, approximate expression for the complementary cumulative distribution function (CCDF) of the PAPR of the modified SLM technique is derived and compared with the simulation results.

Thin-film filters used for dense wavelength division multiplexing (DWDM) applications are processed by a variety of deposition techniques, including ion-beam sputtering. Ion-beam sputtering produces high-quality coatings and provides flexibility of coating materials. However, DWDM filters consisting of oxide films that are reactively deposited by ion-beam sputtering, as in most sputter techniques, typically exhibit high levels of compressive stress. This affects the optical characteristics of the filters. Details of the filter passband characteristics and wave-front distortion illustrate the influence of the stress. Spatial variation of the stress on the filter surface causes the filter center wavelength to have spatial variation, and it causes the filter to have an asymmetric passband characteristic. OCIS codes: 060.0060, 060.1810, 310.6860, 310.1860, 310.6870.

While rapid variations of the fading channel cause intercarrier interference (ICI) in orthogonal frequency-division multiplexing (OFDM), thereby degrading its performance considerably, they also introduce temporal diversity, which can be exploited to improve the performance. In this paper, we first derive a matched-filter bound (MFB) for OFDM transmissions over doubly selective Rayleigh fading channels, which benchmarks the best possible performance if ICI is completely canceled without noise enhancement. We then derive universal performance bounds which show that the time-varying channel causes most of the symbol energy to be distributed over a few subcarriers, and that the ICI power on a subcarrier mainly comes from several neighboring subcarriers. Based on this fact, we develop low-complexity minimum mean-square error and decision-feedback equalizer (DFE) receivers for ICI suppression. Simulations show that the DFE receiver can collect significant gains of ICI-impaired OFDM with affordable complexity. In the relatively low Doppler frequency region, bit-error rate of the DFE receiver is close to the MFB. Index Terms--Doubly selective Rayleigh fading, interference suppression, orthogonal frequency-division multiplexing (OFDM).

Subcarrier multiplexing by chaotic multitone modulation is examined and discussed.

It is shown that the analog representation and DFT-based OFDM systems are only equivalent in special cases.

The linear precoder is determined which minimizes the bit error rate (BER) at moderate-to-high signal-to-noise ratios (SNR) for block transmission systems with zero-forcing (ZF) and threshold detection.

Hghly boron-doped silicon dioxide is used to prepare optical waveguides with low birefringence

Noise performance of a SQUID multiplexer ( MUX) is examined and discussed.

Research is described having to do with gamma-ray transition-edge sensors (TES) in LC resonant circuits.

This paper examines the asymptotic (M [right arrow] [infinity]) performance of M-ary frequency-shift keying (M-FSK) in multichannels, or multiple frequency-nonselective, slowly fading channels, with coding, side information, and diversity reception. In particular, Reed-Solomon (RS) coding is considered in conjunction with the ratio-threshold test (RTT), which generates side information regarding the reliability of received symbols. The asymptotic performance of orthogonal signaling in multichannels with maximal ratio combining (MRC), postdetection equal gain combining (EGC), hybrid selection combining (H-SC), and selection combining (SC) is derived for an arbitrary statistical fading model and diversity order. The derivations reveal that coherent and noncoherent implementations of diversity combining schemes yield the same performance asymptotically. In addition, the asymptotic results are evaluated assuming a Nakagami-m fading model, and the effect of fading severity, diversity order, code rate, and side information upon the performance of the various diversity combiners is investigated. The minimum signal-to-noise ratio (SNR) required to achieve arbitrarily reliable or error-free communication, as well as the associated optimal RS code rate, are determined for various cases. Index Terms--Asymptotic performance, diversity combining, Nakagami-m fading, orthogonal modulation, Reed-Solomon coding.

The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited availability of bandwidth which necessitates a high spectral efficiency. A combined orthogonal-frequency division multiplexing/spatial-diversity multiple access (OFDM/SDMA) approach can effectively tackle both obstacles. The channel distortion due to multipath propagation is easily mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. In order to keep the network's cost acceptable, simplified SDMA processors are preferred over the exponentially complex optimal maximum-likelihood processors. However, these simplified processors perform significantly worse than the optimal ones in terms of average bit error rate (BER). In this paper, we show that by adapting the constellation sizes applied on the individual subcarriers to the channel conditions, the performance of OFDM/SDMA processors can be significantly enhanced. In the uplink, we derive simple closed-form equations for the optimal constellation sizes for both a simple linear minimum mean-square error (MMSE) detector and for a nonlinear MMSE decision feedback equalizer (DFE) detector. Furthermore, for each detector, we introduce a simplified loading algorithm which lowers the computational and signaling complexity substantially at a small performance penalty. In the downlink, we study the dual precoders of the uplink detectors, respectively, the linear MMSE precoder and the nonlinear Tomlinson-Harashima (TH) MMSE-based precoder. For both precoders, we derive expressions for the optimal and the simplified constellation sizes. Additionally, we show that in time-division duplexing systems, the constellation distribution of a set of dual detectors/precoders is identical for up- and downlink, which effectively halves the computational complexity of adaptive loading. In the fully loaded uplink, the proposed adaptive loading algorithm results in a gain of 9 dB for a BER = [10.sup.-3] for the linear MMSE detector and a gain of 4.5 dB for the nonlinear MMSE-DFE detector. In the fully loaded downlink, a gain of 6.3 dB is achieved for the MMSE precoder and 5.5 dB for the TH-MMSE precoder. Index Terms--Adaptive antennas, adaptive loading, orthogonal-frequency division multiplexing (OFDM), spatial-diversity multiple access (SDMA).

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

The channel coding problem in orthogonal frequency-division multiplexing (OFDM) is re-examined with the aim of making better use of exploitable frequency diversity of the transformed multipath channel.