Reduced-complexity Scheme Research Articles

Unfolded coprime bistatic MIMO radar for joint DOD and DOA estimation: from viewpoint on aperture augment for sum coarray

ABSTRACTGenerally, the coprime linear array (CLA) consisting of two interleaved uniform linear subarrays can enlarge the array aperture to attain the better angle estimation performance compared with the uniform linear array (ULA). In this paper, we ulteriorly study the virtual sum coarray of the unfolded coprime (UC) bistatic multiple-input multiple-output (MIMO) radar whose transmitter and receiver array are both unfolded CLA from the viewpoint on the geometry and array aperture. The UC MIMO radar can be exploited to obtain the better joint direction of departure (DOD) and direction of arrival (DOA) estimation performance due to the larger array aperture. Furthermore, we propose an all sum coarray multiple signals classification (ASCA-MUSIC) algorithm for the UC MIMO radar. ASCA-MUSIC can fully exploit all the degrees of freedom (DOFs) in the sum coarray and can obtain the better estimation performance. We also prove that ASCA-MUSIC can avoid the phase ambiguity problem due to the coprime property. In addition, we devise a reduced complexity scheme for ASCA-MUSIC to reduce the high computational complexity and utilize Cramer–Rao Bound (CRB) as a benchmark for the lower bound on the root-mean-square error (RMSE) of unbiased angle estimation. Finally, the numerical simulations verify the effectiveness and superiority of the UC MIMO radar, ASCA-MUSIC and the reduced complexity scheme.

On the Complexity Reduction of the Second-Order Volterra Nonlinear Equalizer for IM/DD Systems

To cope with the various nonlinear signal distortions in intensity-modulation/direct-detection transmission systems, a theoretical analysis on the Volterra nonlinear equalizer (VNLE) is provided, focusing on computational complexity aspects. The analysis yields a simple reduced-complexity scheme for the second-order VNLE (R2-VNLE) based on a performance-complexity tradeoff. An experimental verification is performed with single-sideband 28-GBaud PAM-4 signals, generated by an electroabsorption modulated distributed-feedback laser, over transmission distances of up to 80 km of standard single-mode fiber in the C-band. A comparison of the results for different equalization schemes, including a signal-signal beat interference mitigation technique, shows superior performance for the R2-VNLE.

FPGA Implementation of UFMC Based Baseband Transmitter: Case Study for LTE 10MHz Channelization

Universal filtered multicarrier (UFMC) is a low complexity promising waveform that provides quasi‐orthogonal property among subcarriers. In addition, it can achieve much better out‐of‐band emission performance than orthogonal frequency division multiplexing (OFDM) system. Authors have proposed a hardware platform to implement a UFMC transmitter in this paper. Highly reduced complexity schemes for IFFT, filtering, and spectrum shifting are realized on actual hardware. This helps to achieve overall architecture of the transmitter at the cost of minimal FPGA resource usage. Hence, the overall design uses only 1038 slice registers, 1154 slice LUTs, and 64 multipliers of Xilinx Virtex‐7 XC7VX330t device. A throughput of 773.5 Msamples/sec at an operational frequency of 364 MHz is achieved. This throughput is adequate for processing 50 Physical Resource Blocks (PRB) of LTE 10 MHz channelization in required time. The presented architecture provides a latency of only 2% of one LTE 10MHz channelization symbol due to the implementation of pipelining at different levels. Although the presented hardware design in its current form meets LTE 10MHz channelization throughput requirements, further increase in throughput is possible due to the scalable nature of the architecture. To the best of our knowledge, this work is first ever FPGA solution for UFMC transmitter presented in the literature.

Robust predictive filtering schemes for sub‐band CQI feedback compression in 3GPP LTE systems

The third generation partnership project (3GPP) long-term evolution (LTE) cellular system offers high data rate capabilities by leveraging several techniques including link adaptation and frequency selective scheduling. These techniques rely on accurate channel quality indicator (CQI) feedback reports that are sent by the user equipment (UE) to the evolved node B, a process which results in high signalling overhead. In this study, the authors propose a UE-assisted sub-band feedback compression scheme based on a predictive filtering technique to reduce this signalling overhead. Four schemes based on adaptive filters have been designed, implemented and tested in an LTE system level simulator. Simulation results indicate that the proposed compression scheme has shown efficacy with an overall CQI feedback signalling reduction of up to 92.5% whilst maintaining stable sector throughput, when compared with the standard 3GPP CQI feedback mechanism. Although the proposed scheme exhibits low computational and memory complexity, a reduced-complexity scheme achieving an average computational load reduction of up to 35% is also presented.

Symbol Timing Estimation with Multi-h CPM Signals

A new symbol timing estimation algorithm for Multi-h CPM signals is proposed. It is an extension to an existing non-data aided symbol timing estimator that is only for Single-h CPM signals. A reduced-complexity scheme of proposed algorithm is introduced with negligible performance losses. Performance in AWGN channel is assessed by computer simulation, and the simulation results show that the proposed algorithm is suitable for full response and partial response formats. Meanwhile, it has a good performance and is not sensitive to the carrier frequency

A New Beamforming Design for Multicast Systems

In this paper, we study a transmit beamforming technique for multiple-input–multiple-output (MIMO) downlink multicast systems. Assuming that channel state information (CSI) is available at a transmitter, we try to maximize the lowest transmit data rate among users for a given transmit power level. Since finding the optimal beamforming vector requires prohibitively high computational complexity, we propose a reduced complexity scheme by applying greedy vector search and a simple power-allocation (PA) algorithm. Simulation results show that the proposed scheme outperforms the conventional semidefinite relaxation (SDR) technique and provides a performance quite close to the optimal performance with much reduced complexity.

A reduced complexity scheme for carrier frequency synchronization in uplink 802.16e OFDMA

Synchronization of the carrier frequency offset (CFO) in the uplink of OFDMA systems is a complex task because each user presents a different CFO. It is performed in two stages: estimation and compensation. Although a few existing schemes already achieve good performance in terms of bit error rate (BER), all of them perform huge amounts of operations, specially during estimation, thus leading to very costly, if not impractical, hardware implementations. In addition, if estimation is performed in the time domain, a preamble is required. Since the IEEE 802.16e standard for OFDMA has no preamble at the beginning of the uplink frame, optional midambles must be used at the cost of reducing the system throughput. This article proposes a combined CFO estimation and compensation technique that uses the pilot subcarriers of the 802.16e frame. It achieves similar BER performance than previous approaches without any throughput penalty, while reducing the number of required computations by several orders of magnitude. This reduction is the result of a simpler estimation algorithm and two techniques that improve its accuracy: an iterative process integrated with the compensation stage and the delayed correction of the phase shifts.

A reduced-complexity scheme using message passing for location tracking

Abstract This article presents a low-complexity and high-accuracy algorithm using message-passing approach to reduce the computational load of the traditional tracking algorithm for location estimation. In the proposed tracking scheme, a state space model for the location-estimation problem can be divided into many mutual-interaction local constraints based on the inherent message-passing features of factor graphs. During each iteration cycle, the message with reliability information is passed efficiently with an adaptive weighted technique and the error propagation law, and then the message-passing approach based on prediction-correction recursion is to simplify the implementation of the Bayesian filtering approach for location-estimation and tracking systems. As compared with a traditional tracking scheme based on Kalman filtering (KF) algorithms derived from Bayesian dynamic model, the analytic result and the numerical simulations show that the proposed forward and one-step backward tracking approach not only can achieve an accurate location very close to the traditional KF tracking scheme, but also has a lower computational complexity.

Power Allocation Schemes in OFDM-Based Femtocell Networks

Orthogonal frequency division multiplex-based femtocell networks in downlink with two-tier interference are considered in this paper. In order to satisfy the quality of service of macrocell users, a capacity maximization problem with the total and the upper power constraints by allocating power to subcarriers for a user in femtocell systems becomes important. In this paper, the optimal power allocation is derived by using the Lagrangian technique. Based on the analysis of the Karush-Kuhn-Tucker conditions, subcarriers can be classified into two sets with different power allocation strategies according to the upper power constraint. One set of subcarriers is allocated with upper power, and the other set of subcarriers is processed by using the waterfilling approach. A direct linear search scheme is presented to achieve the optimal performance by finding the threshold over the channel states of all subcarriers to determine the two sets. In order to reduce the computational load, a reduced complexity scheme is designed for the optimal solution by utilizing the relationship of the allocated power in the two sets of subcarriers. Unlike the schemes in iterative manners, a fast power allocation scheme with a near-optimal performance is also developed. Simulation results reveal that the proposed schemes outperform the existing scheme. The efficacy of the proposed schemes is demonstrated in the simulations.

Reduced-complexity scheme using alpha–beta filtering for location tracking

This study presents an efficient location tracking algorithm to reduce the computational complexity of the conventional Kalman filtering (KF) algorithm. In the proposed training and tracking scheme, the authors replace the decision mode of the KF algorithm with an alpha–beta (α–β) algorithm to avoid repeatedly calculating the Kalman gain. After the mode with α–β – tracking, the exact information of the state and measurement noise parameters used in the KF algorithm is not required. Using the inherent fixed-coefficient feature of α–β filtering, the location information between the prediction phase and correction phase is efficiently cycled, thus simplifying implementation of the KF approach. Under a stationary environment, numerical simulations show that the proposed training and tracking approach not only can achieve the location accuracy close to the KF scheme but has much lower computational complexity.

Cross-Layer Adaptive Resource Management for Wireless Packet Networks With OFDM Signaling

Adaptive resource management has become a key technique for the next-generation wireless systems to provide desired services at appropriate QoS. Most of the current resource management algorithms are confined to a single layer of the network protocol stack, which leads to an inferior performance. In this paper, we propose a joint MAC-PHY layer resource management algorithm for wireless OFDM networks. The algorithm aims to maximize the system power efficiency given that the fulfillment of the QoS as well as the fairness is guaranteed. We formulate the integrated design of packet scheduling, subcarrier allocation, and power control into a constrained optimization problem. The formulation defines the interaction and cooperation between different components of the algorithm as well as the information exchange between the PHY and MAC layers. A reduced-complexity scheme that solves the optimization problem in polynomial time is then developed based on the special structure of the problem. Analytical and numerical results will show that the proposed algorithm is able to guarantee the same degree of QoS and fairness as the fair queueing systems do in wired channels. Meanwhile, the power efficiency and system performance are significantly improved compared to traditional systems where resources are allocated based on a strict layering architecture

Interference Cancellation with DFE in Frequency Domain for OFDM Systems with Insufficient CP

In OFDM systems, employing a cyclic prefix (CP) as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), however it reduces the transmission efficiency of the system, especially for some channels with a very long delay spread. In this paper, we consider the OFDM system with insufficient CP, much more efficient than conventional OFDM systems. First, we present the system mathematical model and give the ISI and ICI analysis. Then the signal-to-interference power ratio (SIR) performance is presented. To reduce the ISI and ICI due to the insufficient CP, we develop a minimum-mean-square-error decision feedback equalizer (MMSE_DFE). Based on the MMSE criterion, the optimum feedforward and feedback filter coefficients are derived. For time-varying channel, to avoid brute force matrix inversion in conventional schemes, we propose an adaptive LMS based solution to update the filtering coefficients by tracing the channel variation. Since the high complexity of MMSE-DFE, a reduced complexity scheme, ordered successive partial interference cancellation DFE (OSPIC-DFE), is developed. From the performance comparison between the MMSE-DFE and the OSPIC-DFE, we see that the latter is very near to the former. Finally the simulation shows these proposed methods are highly effective in combating ISI and ICI with low complexity.

Capacity of MIMO systems with antenna selection

We consider the capacity of multiple-input multiple-output systems with reduced complexity. One link-end uses all available antennas, while the other chooses the L out of N antennas that maximize capacity. We derive an upper bound on the capacity that can be expressed as the sum of the logarithms of ordered chi-square-distributed variables. This bound is then evaluated analytically and compared to the results obtained by Monte Carlo simulations. Our results show that the achieved capacity is close to the capacity of a full-complexity system provided that L is at least as large as the number of antennas at the other link-end. For example, for L = 3, N = 8 antennas at the receiver and three antennas at the transmitter, the capacity of the reduced-complexity scheme is 20 bits/s/Hz compared to 23 bits/s/Hz of a full-complexity scheme. We also present a suboptimum antenna subset selection algorithm that has a complexity of N/sup 2/ compared to the optimum algorithm with a complexity of (N/sub L/).

Performance of iterative multiuser receivers for turbo-coded asynchronous DS-CDMA systems over flat-fading channels

The performance of iterative multiuser receivers in a turbo-coded asynchronous direct-sequence code-division multiple-access system over a fully interleaved Rayleigh flat-fading channel is presented. The full-complexity soft-in/soft-out (SISO) multiuser detector based on the logarithmic maximum a posteriori probability (log-MAP) algorithm has a computational complexity growing exponentially with the number of users. It is shown that the computational complexity can be significantly reduced by replacing the MAP multiuser detector in the receiver with suboptimal SISO detectors. A novel hybrid reduced-state algorithm that effectively combines the advantages of the T-algorithm and the channel memory reduction algorithm is proposed. The computational complexity of the proposed detector is determined by the reduced channel memory length and can be further reduced adaptively based on channel conditions. Simulation results show that a substantial reduction in complexity can be obtained at the expense of slight degradation in performance compared with the full-complexity MAP detector. The proposed algorithm is compared with other reduced-complexity schemes and the superiority of the proposed algorithm in terms of performance/complexity tradeoffs is demonstrated.

Multichannel Estimation and Equalization Algorithm for Asynchronous Uplink DS/CDMA

This paper addresses the problem of channel estimation and equalization in asynchronous uplink DS/CDMA. We propose a complete receiver structure which contains a multichannel estimator suitable for time-varying channels, a MMSE equalizer and a propagation delay estimator. The multichannel estimator is a stochastic gradient algorithm which does not require any knowledge about the propagation delays. It estimates a matrix which contains a linear combination of the impulse response of each channel and the propagation delays. This estimate is used to derive a MMSE equalizer. In the despreading stage knowledge on the integer part (chip period multiple) of the propagation delays is needed. Two reduced complexity schemes for propagation delay estimation are presented. The channel estimator convergence is studied in stationary conditions while the overall algorithm performance is demonstrated by using simulations in time-varying channel conditions.

Delay performances of standard and modified CEBUS schemes

The performance of the CEBUS, a local area network proposed by the Electronics Industries Association for a smart house is analyzed under varying network loads and traffic patterns. The performances of reduced complexity versions of the CEBUS scheme are compared with that of the standard CEBUS. A message's delay in a CEBUS network, at a given network bandwidth, is dependent not only on its priority but also but also on the traffic priority pattern in the network. The deleterious effects of assigning too much traffic to the highest priority are quantified by simulation analysis. It was also observed that the performance of a reduced complexity scheme was comparable to that of the standard CEBUS protocol. >