We consider digital predistortion (DPD) for linearizing power amplifiers (PAs) of hybrid MIMO systems with subarrays. Designing such a DPD scheme is difficult because one DPD should linearize all PAs of a subarray whose beamforming coefficients are time-varying. We develop an adaptive DPD technique for each subarray based on direct learning method minimizing the sum of squared errors between the input and output signals of PAs. Use of the least mean square (LMS) Newton algorithm is proposed to adaptively control the DPD parameters. It is shown that the proposed DPD can simultaneously linearize multiple PAs in a subarray irrespective of its beamforming coefficients. Computer simulation results demonstrate that the proposed scheme can perform better than an alternative scheme that designs the DPD based on one of the PA outputs and uses the resulting DPD for all PAs of the subarray.

This paper proposes a cooperative wireless relay network consisting of a one-source-one-destination node pair and N relay nodes in three wireless communication environments, i.e., jamming, node geometry, and channel uncertainty. The main objective of this paper is to analytically derive the optimal cooperative amplify-and-forward (AF) wireless relay matrix under three conditions with the relay power constraint based on the minimum mean square error (MMSE) criterion. Using the derived amplifying relay matrix, the MMSE cost function and achievable rate will be investigated numerically and analytically.

A cooperative amplify-and-forward (AF) wireless relay scheme consisting of M sources, N relays, and L destinations all equipped with a single antenna is studied in this paper. The main objective is to design jointly and iteratively the closed-form of minimum mean square error (MMSE)-based source precode and relay amplifying matrices under a jamming environment with transmit power constraints and aggregate power constraints. With the derived optimal source precode and relay amplifying matrices, the jamming influence on system performance with both transmit and aggregate power constraints is examined numerically by using Monte-Carlo simulations.

This paper studies amplify-and-forward (AF) relay strategies for two-way wireless relay networks consisting of two sources with a single antenna and multiple relays with a single antenna. The closed form of relay amplifying matrices (vectors) with the perfect cancelation of self-interference under both no channel uncertainty and channel uncertainty is presented based on the minimum mean square error (MMSE) criterion. With the derived optimal relay amplifying matrices, the MMSE cost function behavior, the relay power usage, and the sum of the achievable rate are analyzed. Additionally, a filter for each destination will be provided to remove the influences of noises at the receivers.

An amplify-and-forward (AF) relay network has been studied extensively, but a closed form of an optimal AF relaying matrix is not available yet for the distributed AF relay network with the destination power constraint (DPC) and the following adverse environments: (a) partial-band noise jamming (PBNJ), (b) node geometry (NG), and (c) channel uncertainty (CU). Hence, this paper presents a closed form of an optimum AF relaying matrix consisting of a one-source-one-destination pair and N relay nodes under such adverse PBNJ, NG, CU, and DPC environments, based on the minimum mean square error (MMSE). By adopting the optimal relay amplifying matrix, the MMSE cost function behaviors are analytically and numerically studied.

This paper studies both cooperative and noncooperative amplify-and-forward (AF) wireless relay strategies with power constraints at the source and at the relays. The main objective is to design jointly and iteratively the closed form of a source scaling factor and a relay amplifying matrix, based on the minimum mean square error (MMSE) criterion under the conditions of both no-channel and channel phase misalignments. With the derived optimal source scaling factors and relay amplifying matrices, impacts on the system performance of both no-channel and channel phase misalignment are investigated by presenting the bit error rate and cost function behavior. Finally, this paper presents the iterative algorithm to solve the constrained Lagrangian optimization problem with a low computational complexity.

This paper addresses a more practical relay network consisting of a mobile source, multiple distributed mobile amplify-and-forward (AF) relays, and a mobile destination node. It considers an appropriate channel model between nodes, which is a cascaded Rayleigh fading channel. The objective of this paper is to determine an optimum AF relay vector (or matrix) using the minimum mean square error (MMSE) criteria for these distributed AF relay networks. The effects of jamming and imperfect channel state information (ICSI) on network performance are included in the analysis. Additionally, by substituting the derived amplifying relay vectors into the MMSE cost functions, it can be predicted which case will yield a better BER performance.

This paper presents both cooperative and noncooperative amplify-and-forward (AF) relay networks consisting of a one-source-one-destination pair and N-relays under receiver power constraint (RPC) at the destination. The optimal AF relay amplifying matrices for the relays by minimizing the mean square error are analytically derived. This paper also shows that the performance of the optimum cooperative relay network is identical to that of the optimum noncooperative network, as long as the same receiver power constraint is applied. The purpose of the RPC is to reduce the interference level for users located in neighboring cells. Finally, using the derived optimal relay amplifying matrices, the achievable rate, the pairwise error probability, and the outage probability are investigated.

This paper proposes amplify-and-forward (AF) relay matrices for a one-source-one-destination pair and cooperative distributed N relay nodes under three adverse wireless relay communication environments: (1) broadband noise jamming (BNJ), (2) node geometry (NG), and (3) channel uncertainty (CU). The main objective of this paper is to analytically derive the minimum mean square error (MMSE)-based optimal relay amplifying matrix. Another objective of this paper is to analyze the MMSE cost function, relay power usage, and signal component power of the received signal at the destination using the derived optimal relay amplifying matrix. Finally, the impacts on these three wireless communication environments for the wireless relay networks will be observed by presenting bit error rate (BER) performance.

This paper assumes a practical channel model, called a cascaded Rayleigh fading channel, for a noncooperative distributed mobile relay network, which consists of one mobile source and one mobile destination, and multiple mobile amplify-and-forward (AF) relays. Then, an optimum AF relay amplifying vector (or matrix) is analytically designed using the minimum mean square error (MMSE) criteria and uses it for an efficient mobile relay selection. Power is intentionally not constrained. Instead, this paper presents a scaling factor scheme to meet a target signal-to-noise ratio (SNRTGT) at the destination. This SNRTGTstrategy can implicitly embrace the power constraint problems and be a more practical implementation.

We study benefits of opportunistic routing in a large wireless ad hoc network by examining how the power, delay, and total throughput scale as the number of source-destination pairs increases up to the operating maximum. Our opportunistic routing is novel in a sense that it is massively parallel, i.e., it is performed by many nodes simultaneously to maximize the opportunistic gain while controlling the interuser interference. The scaling behavior of conventional multihop transmission that does not employ opportunistic routing is also examined for comparison. Our main results indicate that our opportunistic routing can exhibit a net improvement in overall power-delay tradeoff over the conventional routing by providing up to a logarithmic boost in the scaling law. Such a gain is possible since the receivers can tolerate more interference due to the increased received signal power provided by the multi user diversity gain, which means that having more simultaneous transmissions is possible.

A new digital predistortion (DPD) technique based on envelope feedback is proposed for linearization of power amplifiers (PAs). Unlike conventional DPD techniques that need frequency down converters (FDCs) in the feedback path to recover the complex envelope of the PA output, the proposed technique does not need an FDC. Instead it employs two envelope detectors, estimating the envelope of the PA output and that of the difference signal between the PA input and the PA output. It is shown that the complex envelope of the PA output can be estimated from the two envelope feedbacks if the sign of the phase distortion of the AM-PM characteristic remains the same for all PA input magnitudes. Simulation results show that the performance of the proposed DPD is comparable to the conventional DPD employing an FDC.

This paper studies minimum mean square error (MMSE)-based full-duplex relay processing matrices and source/destination beamforming vectors under perfect channel state information by imposing constraints on the transmit power of source, relay, both, separately, and in various combinations. The main contribution of this paper is the derivation of a set of relay processing and source/destination beamforming vectors under diverse conditions of transmit power constraints on the source and relay. By comparing the bit error rate (BER) performance of each case, an efficient design of a full-duplex amplify-and-forward relay system is presented.

This paper considers an amplify-and-forward (AF) relay scheme for M-source-M-destination pairs and N relay nodes. Cooperative minimum mean square error (MMSE) strategies for wireless relay networks under both a jamming environment and channel uncertainty with received power constraints at the destination nodes are studied. The main contribution of this paper is the derivation of the MMSE-based amplifying relay matrices (ARMs) under both a jamming environment and channel uncertainty. With the proposed ARMs, the system performance under study is evaluated by observing bit error rate (BER) using Monte Carlo simulations. In addition, it is proven that the proposed ARM is the global optimal ARM.

A new transceiver architecture for overloaded MIMO interference channels is proposed to fix the rate saturation problem of purely linear beamforming in this case. The proposed scheme is based on a mixture of linear beamforming and multi-user detection. It is shown that non-trivial degrees of freedom can be achieved by the proposed mixed scheme properly dividing the interference signals for linear processing and multiuser detection, and the achievable degrees of freedom of the proposed scheme are obtained. Numerical results show that the proposed scheme outperforms linear beamforming in overloaded MIMO interference channels.

A new predistortion technique for dynamic spectrum allocation systems such as cognitive radio (CR) is proposed. The system model considered in this paper occupies a small band at a time, but the center frequency can be changed in the wide range of frequency. In this scenario, the front-end filter may not eliminate the harmonics of the power amplifier (PA) output. The proposed PD reduces the spectral regrowth of the fundamental signal at the carrier frequency (ωο) and removes the harmonics (2ωο, 3ωο, ⋯) at the same time. The proposed PD structure is composed of multiple predistorters (PDs) centered at integer multiples of ωο. The PD at ωοis for removing spectral regrowth of the fundamental signal, and the others are for harmonic reduction. In the proposed PD structure, parameters of PDs are found jointly. Simulation results show that the spectral regrowth can be reduced by 20dB, and the 2nd and 3rd harmonics can be reduced down to -70dB from the power of the fundamental signal.

Based on a linear formulation to interference alignment, an adaptive algorithm for interference-aligning beam tracking in time-varying MIMO interference channels is proposed. It is shown that obtaining the interference-aligning beam vector is equivalent to minimizing a certain Rayleigh quotient, and the conjugate gradient approach is adopted to construct an adaptive algorithm. The convergence and stability of the proposed algorithm are established in static channel case, and numerical results show that the proposed algorithm performs well compared with other existing methods with much less complexity.

In this paper, we consider a class of single-source multicast relay networks. We assume that all outgoing channels of a node in the network to its neighbors are orthogonal while the incoming signals from its neighbors can interfere with each other. We first focus on Gaussian relay networks with interference and find an achievable rate using a lattice coding scheme. We show that the achievable rate of our scheme is within a constant bit gap from the information theoretic cut-set bound, where the constant depends only on the network topology, but not on the transmit power, noise variance, and channel gains. This is similar to a recent result by Avestimehr, Diggavi, and Tse, who showed an approximate capacity characterization for general Gaussian relay networks. However, our achievability uses a structured code instead of a random one. Using the idea used in the Gaussian case, we also consider a linear finite-field symmetric network with interference and characterize its capacity using a linear coding scheme.

This paper analyzes the impact and benefits of infrastructure support in improving the throughput scaling in networks ofnrandomly located wireless nodes. The infrastructure uses multiantenna base stations (BSs), in which the number of BSs and the number of antennas at each BS can scale at arbitrary rates relative ton. Under the model, capacity scaling laws are analyzed for both dense and extended networks. Two BS-based routing schemes are first introduced in this study: an infrastructure-supported single-hop (ISH) routing protocol with multiple-access uplink and broadcast downlink and an infrastructure-supported multihop (IMH) routing protocol. Then, their achievable throughput scalings are analyzed. These schemes are compared against two conventional schemes without BSs: the multihop (MH) transmission and hierarchical cooperation (HC) schemes. It is shown that a linear throughput scaling is achieved in dense networks, as in the case without help of BSs. In contrast, the proposed BS-based routing schemes can, under realistic network conditions, improve the throughput scaling significantly in extended networks. The gain comes from the following advantages of these BS-based protocols. First, more nodes can transmit simultaneously in the proposed scheme than in the MH scheme if the number of BSs and the number of antennas are large enough. Second, by improving the long-distance signal-to-noise ratio (SNR), the received signal power can be larger than that of the HC, enabling a better throughput scaling under extended networks. Furthermore, by deriving the corresponding information-theoretic cut-set upper bounds, it is shown under extended networks that a combination of four schemes IMH, ISH, MH, and HC is order-optimal in all operating regimes.

The problem of interference alignment in time-varying MIMO interference channels is considered. To reduce complexity, an adaptive algorithm for beam vector design is proposed based on our previous work of least squares approach to beam design for interference alignment and matrix perturbation theory. The proposed algorithm calculates interference-aligning beam vectors by additive update of the previous value and reduces complexity significantly. Numerical results are provided to validate the proposed algorithm. It is shown that the proposed adaptive algorithm yields almost the same performance as a non-adaptive method that calculates interference-aligning beam vectors at every time step.

A new partial transmit sequence (PTS) technique for orthogonal frequency division multiplexing (OFDM) signals which does not require side information, is proposed. In the proposed method, the transmitter applies PTS to reduce the peak-to-average power ratio (PAPR) and does not send the side information on the PTS phase factors to the receiver. Based on the OFDM symbol structure having pilot subcarriers, the receiver jointly estimates the channel and PTS phase factors from the pilot subcarriers. The proposed technique inherently causes bit error rate (BER) loss depending on the PTS subgroup size. However, simulation results demonstrate that if the PTS subgroup size is carefully chosen, the proposed method can reduce the PAPR significantly with a negligible BER loss.

In this paper, a Gaussian two-way relay channel, where two source nodes exchange messages with each other through a relay, is considered. We assume that all nodes operate in full-duplex mode and there is no direct channel between the source nodes. We propose an achievable scheme composed of nested lattice codes for the uplink and structured binning for the downlink. Unlike conventional nested lattice codes, our codes utilize two different shaping lattices for source nodes based on a three-stage lattice partition chain, which is a key ingredient for producing the best gap-to-capacity results to date. Specifically, for all channel parameters, the achievable rate region of our scheme is within1/2bit from the capacity region for each user and its sum rate is within log3/2bit from the sum capacity.

We propose a cognitive radio system in fading environments where the secondary sender opportunistically violates the secondary-to-primary (S-P) interference power limit. Assuming a slowly-varying fading interference channel with two-senders and two-receivers, the primary sender adjusts its rate and power depending on its own channel, while ignoring the secondary user. Through an optimization process, the secondary sender's rate and power are determined and the decision is made on whether the secondary sender violates the S-P interference power limit. The primary receiver removes the effect of excessive S-P interference by multiuser decoding (MUD) which jointly decodes the primary and secondary senders' data. Achievable rates of the proposed system are examined by computer simulation. It was observed that the proposed system tends to violate the SP interference power limit when the S-P channel gain is larger than the other channel gains. Remarkably, the proposed system can provide a considerably high data rate for the secondary user even without sacrificing the data rate for the primary user over a wide range of signal-to-noise ratios.

We introduce a multiuser (MU) scheduling method for amplify-and-forward (AF) relay systems, which opportunistically selects both the transmission mode, i.e., either one- or two-hop transmission, and the desired user. A closed-form expression for the average achievable rates is derived under two transmissions with MU scheduling, and its asymptotic solution is also analyzed in the limit of large number N of mobile stations (MSs). Based on the analysis, we perform our two-step scheduling algorithm: the transmission mode selection followed by the user selection that needs partial feedback for instantaneous signal-to- noise ratios (SNRs) to the base station (BS). We also analyze the average SNR condition where the MU diversity gain is fully exploited. In addition, it is examined how to further reduce a quantity of feedback under certain conditions. The proposed scheduling algorithm shows the comparable achievable rates to those of optimal one using full feedback information, while its required feedback information is reduced below half of the optimal one.

In this paper, a new amplify-and-forward (AF) relay systems using superimposed pilot signals is proposed for frequency selective fading channel environments. In the proposed scheme, the relay superimposes an additional pilot sequence on the received signal from the source, and forwards the combined signal to the destination. The superimposed pilot signal in this way enables the destination node to estimate the channel between the relay and destination (R-D), and this channel information is used to process the received signal at the destination optimally; the noise correlation due to the frequency selective fading in the R-D channel is whitened. The optimal power ratio of the superimposed pilot to the incoming signal at the relay is obtained via simulations, and it is shown under frequency selective channel environments that the proposed scheme yields better bit-error- rate (BER) performance than the conventional method where all relay power is allocated only to the incoming signal.

The problem of optimal sensing and training in a cognitive radio system is considered when the training signal of the primary transmitter is used for both channel estimation at the primary receiver and sensing for the secondary transmitter. First, the optimal operating characteristics of sensing that maximizes the overall system rate for given training is investigated. It is shown that the optimal false alarm probability at the secondary sensor is monotone increasing as the activity of the primary user increases if the sensing ROC curve is concave. When the primary activity factor is unknown, the max-min criterion is applied to optimal sensing strategy and the resulting max-min optimal solution is given by an equalizer rule for any type of sensing ROC curve. The joint optimization of sensing and training has a unique solution and it can be easily found numerically using a gradient ascent algorithm. By optimal design of sensing and training in such a way, the overall system rate can be improved.

We analyze the benefits of infrastructure support in improving the throughput scaling in networks of n randomly located wireless nodes. The infrastructure uses multi-antenna base stations (BSs), in which the number of BSs and the number of antennas at each BS can scale at arbtrary rates relative to n. We introduce two multi-antenna BS-based routing protocols and analyze their throughput scaling laws. Two conventional schemes not using BSs are also shown for comparison. In dense networks, we show that the BS-based routing schemes do not improve the throughput scaling. In contrast, in extended networks, we show what our BS-based routing schemes can, under certain network conditions, improve the throughput scaling significantly.

A pilot emitting amplify-and-forward relay and hop-by-hop beamforming (HBF) scheme are proposed for two-hop relaying systems. The conventional beamforming (CBF) is performed with respect to an overall channel from a source node (SN) to a destination node (DN) through a relay node (RN) while the proposed HBF performs two independent beamformings, from the SN to the RN and from the RN to the DN. For beamforming, the SN and the DN require the channel state information (CSI) from the SN to the RN and from the RN to the DN, respectively. From our analytic and numerical results, it is shown that the bit-error-rate (BER) performance of the proposed HBF is identical to that of CBF with the perfect CSI. However, the proposed HBF can achieve a lower BER than the CBF method since the channel estimation performance of the HBF system is better than that of CBF, and this is verified by deriving and comparing the mean square errors of the channel estimations of HBF and CBF.

In this paper, we analyze the diversity-multiplexing tradeoff (DMT), originally introduced by Zheng and Tse, and outage performance for Rician multiple-input-multiple-output (MIMO) channels. The DMT characteristics of Rayleigh and Rician channels are shown to be identical. In a high signal-to-noise ratio (SNR) regime, the log-log plot of outage probability versus SNR curve for a Rician channel is a shifted version of that for the corresponding Rayleigh channel. The SNR gap between the outage curves of the Rayleigh and Rician channels is derived. The DMT and outage performance are also analyzed for Rician multiple-input-single-output (MISO)/single-input-multiple-output (SIMO) channels over a finite SNR regime. A closed-form expression for the outage probability is derived and the finite SNR DMT characteristic is analyzed. It is observed that the maximum diversity gain can be achieved at some finite SNR-the maximum gain tends to increase linearly with the Rician factor. The finite SNR diversity gain is shown to be a linear function of the finite SNR multiplexing gain. The consistency between the DMTs for finite and infinite SNRs is also shown.

Preamble-based cell identification (CID) schemes are derived for orthogonal frequency division multiplexing (OFDM) systems. They include the optimal schemes based on the Bayesian and the maximum likelihood (ML) approaches, a suboptimal scheme that is a simplification of the ML scheme, and a differential decoding-based scheme that does not require any channel information. The complexities and performances of these CID schemes are examined and compared to existing schemes. The differential decoding-based scheme performs like the suboptimal scheme for most practical channels of interest and outperforms existing schemes, yet it is simpler to implement than the others.

An intercarrier interference (ICI) canceling technique for multiple-input single-output (MISO) orthogonal frequency division multiplexing (OFDM) systems in fast fading channels is proposed. The proposed scheme consists of a linear space-frequency block code (SFBC) at the transmitter and a receive frequency block code (RFBC) at the receiver. The code design is based on the upper bound of pairwise error probability (PEP) which is derived under assumptions and approximations stated. Specifically, the proposed code is designed to minimize the upper bound of the PEP. The simulation results demonstrate that the proposed technique outperforms the conventional methods in fast fading environments.

A peak-to-average power ratio (PAPR) reduction technique for multi-input single-output (MISO) orthogonal frequency division multiplexing (OFDM) systems with beamforming is proposed. This technique consists of adaptive antenna selection and phase rotation. By using the proposed method, a large reduction in PAPR can be obtained at the expense of a small signal-to-noise ratio (SNR) loss. According to the simulation results, the PAPR is reduced by 10% in terms of complementary cumulative distribution function (CCDF) with negligible SNR loss. If a 1 dB SNR loss is allowed, a 33% reduction in PAPR can be achieved.

This paper considers a broadband cellular orthogonal frequency-division multiple-access (OFDMA) system with relay nodes operating in decode-and-forward and half-duplex mode. Two transmit resource allocation problems for sum-rate maximization are formulated for such a system. The first one is the optimization of subcarrier allocation with predetermined power assignment for each subcarrier, and the other is the joint optimization of power and subcarrier allocation. Since these problems can't be solved easily in direct forms, we make continuous relaxation and solve the dual problems using a subgradient method. Numerical results show that a remarkable increment in sum-rate is achieved, with the aid of relay nodes and sophisticated resource allocation, compared to a system without relay nodes.

A limited feedback-based transmit beamforming technique for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is investigated in time-varying channels. The performance of the system is significantly degraded by outdated feedback information even when the channel varies slowly. To compensate for the impairment in time-varying channels, the optimal transmit beamforming vector for a future channel, which maximizes the expected effective channel gain, is derived by applying the autoregressive (AR) model to the channels. These are obtained at the receiver. Following this, schemes for the selection of beamforming vectors are proposed to reduce the feedback amount. These can effectively reduce the amount of feedback information by utilizing both the frequency and time correlation of transmit beamforming vectors. Simulation results show that the proposed techniques outperform existing schemes in terms of the bit error rate (BER) performance with the same amount of feedback.

A simple algorithm to select user-pair and allocate frequency band is proposed for multiuser-MIMO OFDMA systems. For sake of comparison, various resource allocation algorithms, such as max-throughput, best-fit, first-fit, and random-fit algorithm, are examined with regard to the system throughput, computational complexity, and fairness among the users. Computer simulation results show that the first-fit algorithm can achieve a large reduction of computation for resource allocation as well as good fairness among users, with only a small reduction of throughput.

A new polynomial predistortion technique for linearizing nonlinear power amplifier is introduced. The proposed predistorter is based on direct learning, whereas most other techniques are based on the indirect learning architecture. The resulting structure of the proposed method is simpler than the existing ones. Computer simulation shows that the proposed method is robust to the initial conditions and attains comparable performance to other methods based on the indirect learning architecture under normal conditions.

Motivated by joint cooperative diversity and scheduling (JCDS) in frequency-flat fading relay systems, this paper proposes JCDS for frequency-selective fading relay systems with an OFDMA technique. While JCDS for frequency-flat fading relay systems uses time-varying phase rotation at the relays, JCDS in this paper uses fixed cyclic delay times instead. This technique can be considered as an extension of cyclic delay diversity in a conventional MIMO-OFDM system to a multiuser relay system in order to gain both multipath diversity and multiuser diversity. It is shown that JCDS using fixed cyclic delay has significant gains in aggregate throughput while per-link throughput can be maintained when there are enough relays. The effect of delay spread to the required number of relays and to the throughput performance are shown by simulation.

Rate-t/k (tlesk) intercarrier interference (ICI) canceling codes for orthogonal frequency division multiplexing (OFDM) systems are proposed. The capacity lower bounds of OFDM systems employing these codes are derived for time-varying frequency-selective channels. The optimal codes maximizing these bounds are designed numerically. The simulation results indicate that the optimal codes can provide both higher capacity lower bounds and lower bit error rates (BERs) than the existing ICI canceling codes

Design of space-time codes that scale with the number of transmit antennas is a difficult problem. In this paper, we introduce a new family of space-time trellis codes (STTC) that can be applied to any arbitrary number of transmit antennas. This family is constructed by utilizing QPSK STTCs as component codes to construct STTCs with larger constellation size. Unlike the design of existing STTC, the search space in our design does not grow exponentially with the constellation size or the number of transmit antennas. Additionally, we propose a practical approach to reduce the computational complexity of our proposed scheme using interference mitigation techniques. Simulation results compare the performance of our approach with that of space-time block codes for the case of two transmit antennas and several different number of receive antennas, a spectral efficiency of 4 bits/s/Hz, and slow Rayleigh fading channels.

The rate-t/k (t

In this paper, we first generalize a recent work of Zheng and Tse on diversity-multiplexing tradeoff (DMT) for some rank-deficient channels (poor scattering). We show that rank deficiency lowers DMT curves from that of i.i.d. Rayleigh fading channels. We show an interesting observation that suggests a fractional diversity gain may be possible at integer multiplexing gains. As the scattering becomes rich, the DMT approaches that of the i.i.d. Rayleigh fading channels. We next focus on spatially correlated multiple-input multiple-output (MIMO) channels. We show that spatial correlation does not change the DMT but still degrades the outage performance and analyze such a degradation at high SNR

Space time convolutional codes (STCCs) are an effective way to combine transmit diversity with coding. The computational complexity of designing STCCs generally increases exponentially with the constellation size of the transmitted symbols. In this paper, we first present an innovative approach to design STCCs with high spectral efficiencies by utilizing QPSK STCCs as component codes and consequently. Unlike existing techniques, the search space does not grow exponentially with the constellation size. Then, we present two approaches to reduce the computational complexity of our proposed scheme. This scheme is applicable to cases with any number of transmit antennas without any requirement to change the encoder design. Simulation results evaluate the performance of our approach for the case of two transmit antennas and several different number of receive antennas, a spectral efficiency of 4 bits/s/Hz, and slow Rayleigh fading channels.

Presented in this paper is a study of the capacity evaluation of various multiuser MIMO schemes in cellular environments. The throughputs per user of the generalized zero-forcing with rank adaptation and vector perturbation schemes are compared with the capacity bound of the Gaussian MIMO broadcast channel, obtained by dirty paper coding under proportional fairness scheduling. The average cell throughputs of these schemes are also compared. From these comparisons, this study provides vital information for applying multiuser MIMO schemes in multicell environments

This paper proposes a block diagonalization (BD) method, which nullifies the multiuser interference and, hence, increases the sum capacity in a multiuser multiple-input multiple-output (MU-MIMO) downlink system. The proposed BD scheme employs all users' receive processing as well as channel information. In addition, this paper proposes three adaptive mode selection schemes for the proposed BD: namely, an exhaustive mode search (EMS); an up-tree-based mode search (UTMS); and a down-tree-based mode search (DTMS). The EMS is a greedy search, whereas UTMS and DTMS are simplified searches that reduce the EMS computations significantly but have almost the same performance as the EMS. Simulation results show that the proposed BD method with the proposed adaptive mode selection schemes can increase the sum capacity significantly.

This paper proposes a technique that jointly compensates the amplifier nonlinearity and quadrature modulation (QM) errors. The proposed method is derived based on the indirect learning architecture, which has been originally developed for the predistortion (PD). The proposed compensator updates the parameter vector based on the recursive least squares (RLS) criterion. The advantage of the proposed adaptive technique is demonstrated through computer simulation.

In this paper, we employ the propagator method (PM) to find the direction of arrival angles (DOAs) from the incident sources without any eigendecomposition. This can reduce complexity when compared to eigensubspace methods such as forward/backward spatial smoothing, which requires eigenvalue decomposition (EVD). Also, we apply our proposed algorithm in the situation when the incident sources are coherent. Our proposed algorithm can be applied for more practical situations when the unknown covariance noise matrix is in a complex symmetric Toeplitz form, whereas in Prasad's model the unknown noise is in a real symmetric Toeplitz form. Moreover, the proposed algorithm, when compared with Prasad's model does, has three advantages: (1) it does not require any EVD to find the DOAs, whereas Prasad's model does, (2) our proposed algorithm requires the number of sensors M to be larger than the number of sources K, i.e., M>K, but Prasad's method requires M>2K, and (3) the proposed method can be applied in the situation when the sources are coherent, but Prasad's method cannot. Our proposed method is based on a covariance matrix difference between the forward/backward spatial smoothing for covariance matrices of the received data and the Hermition of the backward spatial smoothing only. This difference is introduced to eliminate noise components from the array structure. The numerical results verify that the proposed method gives better performance and less computation than forward/backward spatial smoothing.

A constrained minimum mean square error (CMMSE)-RAKE receiver for multipath fading channels is developed by extending the CMMSE receiver for flat fading channels. Based on the observation that interpath interference causes a bias of the channel estimator in , a receiver that can remove such a bias is proposed, plus a closed-form expression of the bit-error rate of the receiver is derived. Computer simulation is used to demonstrate that the proposed receiver can outperform existing RAKE receivers.

The constrained minimum mean square error (CMMSE)-RAKE receivers for multipath fading channels are analyzed. In particular, it is shown that for known channels the output signal-to-interference and noise ratio (SINR) of the CMMSE-RAKE receiver with a single constraint (Type 1) is greater than or equal to that of the CMMSE-RAKE with multiple constraints (Type 2). For Type 1 CMMSE-RAKE receiver, a maximum likelihood (ML) channel estimate is derived. Computer simulation results indicate that CMMSE receivers improve the bit error rate (BER) as compared with the conventional RAKE and currently available adaptive MMSE receivers, and that Type 1 performs the best.

This paper deals with carrier frequency estimation for transmissions with antenna diversity. Joint maximum likelihood estimates (MLE) of channel and frequency offset are derived and periodic space-time training sequences that can simplify the implementation of the ML frequency estimate, while minimizing the mean square error (MSE) of the estimate are designed. Statistical analysis indicates that the MLE is unbiased and almost achieves the Cramer-Rao bound (CRB). Simulation shows that MLE with optimal training sequences is preferable to one with arbitrary training sequences in mobile communications.

A new frame synchronization technique, which is robust to carrier frequency and phase errors, is proposed for M-ary PSK systems. This technique is derived through modification of the procedure used for obtaining the maximum-likelihood (ML) rule in the paper by Gansman et al. (1997). The proposed rule is based on an operation called a double correlation which evaluates a correlation after properly multiplying the received signal with a sync pattern. It was shown through computer simulation that the proposed rule generally outperformed the existing rules when a frequency offset existed.

This paper suggests the use of a prefilter before baud-rate timing recovery (Mueller and Muller 1976). It is shown that the timing phase of the baud-rate timing synchronizer can be set to a desirable timing phase, at which point a digital equalizer performs best, by proper prefiltering. The procedure for the optimal design of such a prefilter is developed under the assumption that the channel impulse response and desirable timing phase for equalization are known. Application of the proposed scheme to the gigabit Ethernet system demonstrated that prefiltering can improve receiver performance.

Adaptive linear minimum mean square error (LMMSE) receivers for frequency selective fading channels are developed by modifying the constrained LMMSE receiver of Kim, Jeong and Choi (see IEEE Trans. Commun., vol.48, p.1793-96, 2000). In particular, additional constraints regarding interpath interference are imposed on the constrained optimization proposed by Kim et al., and simple adaptation rules are derived by converting the constrained MMSE problem into an unconstrained optimization. It is shown that the instantaneous SINR of the proposed receiver is always higher than that of the RAKE receiver. Computer simulation results indicate that the proposed receivers can perform better than the existing RAKE and adaptive LMMSE receivers in multipath fading environments.

A new data-aided frequency estimator is introduced for phase-shift keying signals transmitted over frequency-selective fading channels. This estimator is developed based on a maximum likelihood criterion. It assumes the use of a special class of pilots, called near-i.i.d. (independent identically distributed) sequences, with impulsive fourth-order moments. With the help of such pilots, the proposed method can estimate frequency offsets without the need for channel information. The pilots of GSM and IS-136 mobile communication systems have been observed as being near-i.i.d., and statistical analysis indicates that the proposed estimate is almost unbiased if the pilot is near-i.i.d. The advantage of the proposed estimator over conventional methods is demonstrated via computer simulation.

A new data-aided frequency estimator for frequency-selective fading channels is introduced. The proposed estimator is developed based on a least squares (LS) error criterion and can estimate frequency offsets without the need for channel information. Statistical analysis indicates that the resulting estimate is unbiased and tends to approach the Cramer-Rao lower bound (CRLB). Simulation shows that the proposed LS method is preferable to existing techniques in mobile communications. The application of the LS estimator to systems with transmitter antenna diversity is also considered. In particular, it is demonstrated that the LS method can be successfully applied to third-generation wireless communication systems.

A frequency offset compensation method for OFDM-FDMA, that can correct offsets after the DFT via circular convolution, is proposed as an alternative to the direct method that corrects frequency offsets by multiplying the complex exponents of the offset estimates before the DFT. In contrast to the direct method whose complexity increases proportional to the number of users, the computational load of the proposed scheme decreases as the number of users increases. It is shown that the proposed method is simpler to implement than the direct method when the number of users is greater than two. Furthermore, the former can outperform the latter, because a frequency offset compensation for one user after the DFT does not affect the data of other users. Computer simulation results demonstrate the advantage of the proposed method.

A double-dwell acquisition system that can simultaneously operate search and verification processes, by employing two correlation blocks, is proposed. The search block continuously looks for threshold-crossing instants, and at those instants, the verification block is reset and reinitiated. The threshold is time varying and updated depending on the partial correlation. Computer simulation results indicate that the proposed technique can perform better than existing double-dwell systems which alternate between search and verification modes.

This paper describes the MetaCore system which is an application-specific instruction-set processor (ASIP) development system targeted for digital signal processor (DSP) applications. The goal of the MetaCore system is to offer an efficient design methodology meeting specifications given as a combination of performance, cost, and design turnaround time. The MetaCore system consists of two major design stages: design exploration and design generation. In the design exploration stage, MetaCore system accepts a set of benchmark programs and structural/behavioral specifications for the target processor and estimates the hardware cost and performance for each hardware configuration being explored. Once a hardware configuration and instruction set are chosen, the system helps generate the target processor design in the form of hardware description language (HDL) along with the application program development tools such as C compiler, assembler, and instruction set simulator. The effectiveness of the MetaCore system was verified with a successful design of MDSP-II, a programmable DSP processor targeted for mobile communication.

A new frame synchronization technique, which is robust to carrier frequency and phase errors, is proposed for M-ary PSK systems. This technique is derived through some modification of the procedure for obtaining the maximum likelihood (ML) rule in Gansmann et al. (1997). The proposed rule is based on an operation called double correlation, which evaluates the correlation after properly multiplying the received signal with a sync pattern. It is shown through computer simulation that the proposed rule outperforms the existing ML rule when the frequency offset is greater than about 0.02/T.

Based on the fact that the effective bandwidth of regenerated RF signals of high density digital versatile disk (DVD) systems is about 1/4 T where T is the symbol period, we propose a DVD receiver that can recover source data with rate 1/T after 1/2 T sampling. This receiver employs an interpolator that increases the data rate to 1/T. It is shown that the proposed receiver can perform like conventional receivers with a sampling rate 1/T at the expense of a minor increase in computation. Since the proposed receiver can recover the data at twice of the A/D convertor rate, it should be very useful for high-speed data recovery.

Interpolated second-order polynomials (ISOPs) are proposed to design efficient cascaded integrator-comb (CIC)-based decimation filters for a programmable downconverter. It is shown that some simple ISOPs can effectively reduce the passband droop caused by CIC filtering with little degradation in aliasing attenuation. In addition, ISOPs are shown to be useful for simplifying halfband filters that usually follow CIC filtering. As a result, a modified halfband filter (MHBF) is introduced which is simpler than conventional halfband filters. The proposed decimation filter for programmable downconverter is a cascade of a CIC filter, an ISOP, MHBFs, and a programmable finite impulse response filter. A procedure for designing the decimation filter is developed. In particular, an optimization technique that simultaneously designs the ISOP and programmable FIR filters is presented. Design examples demonstrate that the proposed method leads to more efficient programmable downconverters than existing ones.

This paper describes the MetaCore system which is an ASIP (Application-Specific Instruction set Processor) development system targeted for DSP applications. The goal of MetaCore system is to offer an efficient design methodology meeting specifications given as a combination of performance, cost and design turnaround time.

We propose a new interference cancellation scheme called the symbol-by-symbol based adaptive interference canceller (SAIC). The SAIC adaptively estimates and removes both multiple access interference (MAI) and intersymbol interference (ISI) at the output of the RAKE receiver. It requires neither respreading nor the evaluation of partial cross-correlation. In addition, its complexity is independent of the number of fingers of the RAKE receiver. Consequently, the SAIC is considerably simpler to implement than existing techniques. Computer simulation results demonstrate that the SAIC can perform much better than a conventional RAKE receiver.

A new baseband signal processing method for compensating frequency shift in differential phase-shift keying (DPSK) systems is introduced. This method consists of a signal processor for the initial acquisition of frequency shift and a simple adaptive equalizer for tracking. The former is based on the observation that the effect of frequency shift can be eliminated by using boundary values of the received baseband signal between symbol periods. The latter is a single tap equalizer employing the least mean-square (LMS) adaptation algorithm. Computer simulation results demonstrate that the proposed frequency shift compensation technique outperforms existing methods, and works well for a wide range of frequency shift.

An approach to the derivation of variable loop gain sequences of dual-loop digital phase-locked loop (DPLL) is developed based on some modifications of the Kalman filtering formulation. It is shown that optimal loop gain sequences which are independent of measurement noise statistics can be obtained under a deterministic source model. Computer simulation results demonstrate that the adaptive dual-loop DPLL designed by using the proposed method is more robust to noise variations than the adaptive DPLL of Driessen (see ibid., vol.47, p.673-75, 1994).

The cyclotomic polynomial (CP) prefilter design problem is formulated as an optimization problem with linear objective functions by applying the logarithm to the transfer function of the CP prefilter. Then this problem is solved by mixed integer linear programming (MILP). Design examples demonstrate that this method leads to more efficient cascaded FIR prefilter-equalizers than existing methods.

The cyclotomic polynomial (CP) prefilter design problem is formulated as an optimization problem with linear objective functions by applying logarithms to the transfer function of the CP prefilter. This problem is then solved by mixed integer linear programming (MILP). Design examples demonstrate that this method leads to more efficient cascaded finite impulse response (FIR) prefilter-equalizers than existing methods.

New formulas for McClellan (1973) transform parameters for the design of 2-D zero-phase FIR fan filters are optimally derived under the integral squared error (ISE) criterion. By imposing the constraint that F(0,0)=cos w/sub c/, where F(/spl middot/) is the McClellan transform and w/sub c/ is the cutoff frequency of the one-dimensional (1-D) prototype filter, the ISE is directly minimized without modifying it and, as a consequence, closed-form formulas for the McClellan transform parameters are obtained. It is shown that these formulas lead to a very efficient design for 2-D zero-phase FIR fan filters.

A new buffer-control policy for intrafield coding of video signals is presented. This method employs two statistical bit rate predictors-feedback and feedforward. It uses a feedback predictor in stationary portions of an image sequence and a feedforward predictor at scene changes. It is shown that this buffer-control policy is reasonably simple to implement and can effectively control output bit rate even at scene changes.

An optimization problem for designing a non-uniformly spaced, linear phase FIR filter with minimal complexity is formulated, and solved by mixed integer linear programming (MILP). Design examples illustrate that the proposed method is useful for designing a wide range of filter types, and can outperform the subset selection-based design methods.

A class of nonlinear digital filters, called the threshold Boolean filter (TBF), is introduced. The TBF is defined by a Boolean function on the binary domain and is a natural extension of stack filters. In this paper, the TBF is further extended to a larger class of filters, called the extended TBF (ETBF), which encompasses linear FIR and linear combination of order statistic (LOS) filters as well as the TBF. The optimal design of the TBF and ETBF under the mean square error (MSE) criterion is investigated. It is shown that the optimization of the TBF can be formulated as a quadratic zero-one programming, and that the optimization of the ETBF as a classical quadratic problem. Thus, linear FIR and median-type nonlinear filters can be analyzed and designed in a unified framework of the ETBF. The ETBF has been applied to enhance noisy images. The results show that the ETBF outperforms the TBF and the FIR Wiener filter.>

A prefilter-equalizer structure for cascade form 2PFIR filtering is proposed. This structure consists of a prefilter made up of cascaded cyclotomic polynomial (CP) filters, and an FIR equalizer having signed powers-of-two coefficients. It is shown that this prefilter-equalizer is often easier to design, and can perform better than direct and cascade form 2PFIR filters.>

The Winsorizing smoother (W smoother), which is a center weighted median (CWM) filter giving more weight only to the central value of each window, is studied. This filter can preserve image details while suppressing additive white and/or impulsive-type noise. The statistical properties of the W smoother are analyzed. It is shown that the W smoother can outperform the median filter, while its implementation is almost as simple as median filtering. Some relationships between W smoothers and other median-type filters, such as the weighted median filter and the multi-stage median filter, are derived.>

Median filtering can be viewed as operation that selects a sample from each window close to the last output. This observation results in a new edge preserving smoother called the last output reference (LOR) filter. The LOR filter is similar in function to the median or recursive median filters, but has additional advantages particularly in suppressing impulses. It has been shown that repeated applications of LOR filtering produces a sequence that is invariant to subsequent passes through the same filter. Also, it has been proven that any sequence can be converted to a locally monotone sequence by using a combination of "forward" and"backward" LOR filters. When the LOR filter is applied to an actual noisy image, it is shown to perform well.