Pilot Reduction Using Spatial and Temporal Correlation in Massive MIMO-OFDM Systems

Wireless Personal Communications - In MIMO-OFDM systems, the base station sends pilot signals for channel estimation at receivers. Since the number of pilots is proportional to the number of...     

A Low-BER Clipping Scheme for PAPR Reduction in STBC MIMO-OFDM Systems

Clipping is a simple scheme to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. Further, it can be extended to space-time block coding (STBC) multiple-input multiple-output (MIMO) OFDM systems for the PAPR reduction. In the conventional clipping schemes for STBC MIMO-OFDM systems, the input symbols are first encoded and then clipped. In this paper, a new scheme is proposed, where the clipping operation is performed before space-time block coding. We theoretically prove that the proposed scheme has better bit-error rate (BER) performance while maintaining the same PAPR reduction as the conventional schemes. Additionally, we derive the symbol-error rate (SER) and BER expressions for the new scheme over multipath fading channels. The simulation results show a good match with our analysis.     

PAPR Reduction for Repetition Space-Time-Frequency Coded MIMO-OFDM Systems Using Chu Sequences

Space-time/frequency coding (SFC) can achieve the spatial and multipath diversities for a MIMO-OFDM system by coding across subcarriers, multiple antennas, and/or multiple OFDM sysmbols, where an interesting method to achieve the multipath diversity is repeating across subcarriers proposed by Su et al. While most of the existing space-time/frequency codes do not have the fast ML decoding, a family of space-time-frequency codes with single-symbol ML decoding have been proposed lately by Zhang et al to achieve both full spatial and multipath diversities by using orthogonal space-time block codes (OSTBC) across multiple antennas and OFDM symbols and in the meantime repeating across the subcarriers. In this paper, we first generalize the above OSTBC to linearly transformed quasi OSTBC (QOSTBC) in a straightforward way. The main goal of this paper is to modify the repeating process and adjust their phases so that the peak-to-average power ratio (PAPR) of the OFDM system is reduced. In particular, we propose to use Chu sequences and show that the discrete PAPR can be reduced by Gamma times where Gamma is the times of the repeating across subcarriers for any SFC from the repeating.     

OFDM与MIMO-OFDM系统中PAPR问题研究

正交频分复用(OFDM)和MIMO-OFDM技术都存在高峰均比的问题,大多数方法都是把降低OFDM峰均比的方法直接使用MIMO-OFDM系统,但在与MIMO-OFDM系统的匹配上存在较大问题。分析了OFDM和MIMO-OFDM的系统模型及PAPR,从理论上分析了OFDM和MIMO-OFDM两系统的关系,给出了在MIMO-OFDM系统中降低PAPR需要注意的问题,为OFDM和MIMO-OFDM系统的技术实用化做好理论基础。     

MIMO-OFDM系统中降低峰均功率比技术研究

多输入多输出正交频分复用(M IMO-OFDM)系统已成为新一代高速通信系统研究中的热点,而系统所产生的高峰均功率比(PAPR)信号是限制M IMO-OFDM技术实用化的主要障碍。本文在分类的基础上,对目前主要采用的降低M IMO-OFDM系统PAPR的方法进行介绍,描述了其优缺点及部分改进方案。     

PAPR Reduction in OFDM Systems: Polynomial-Based Compressing and Iterative Expanding

In this paper a companding-based scheme is proposed to reduce the Peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing system. At the transmitter side, a compressing polynomial function is appended to the inverse discrete Fourier transform block; and at the receiver the transmitted signal is retrieved iteratively through combining the discrete Fourier transform block with a reverse expanding function. In the iterative algorithm the Jacobi’s method is used for solving the equations. Also, the general form of the compressing polynomial functions is attained through the use of Daubechies wavelet functions. As an advantage, the proposed method involves less complexity at the transmitter compared to other PAPR reduction methods. Furthermore, it requires less increasing to signal-to-noise ratio for the same bit error rate in comparison with other companding methods. The order of compressing polynomial and the number of iterations for the proposed algorithm at the receiver can be set in accordance with the performance-complexity trade off.     

OFDM系统中峰平比降低算法研究

提出了一种基于升余弦脉冲峰值抵消的改进方案,在降低系统峰平比的同时避免削波引入的频谱恶化,给出了理论分析及相应的仿真结果与结论。     

PAPR Reduction in OFDM Systems Based on Autoregressive Filtering

Orthogonal frequency division multiplexing (OFDM) has a very high peak-to-average power ratio (PAPR) that causes a severe nonlinear distortion in practical hardware implementation of high power amplifiers (HPA). In this article, a new PAPR reduction method is proposed based on autoregressive (AR) error filtering. This method proposes the use of signal whitening property of error filtering as a preprocessing step to remove the predictable content of stationary stochastic processes which can reduce the autocorrelation of input data sequences and is shown to be a very effective solution for the PAPR problem in OFDM systems. It is shown that the proposed method can achieve a significant reduction in PAPR without degrading the error performance or power spectral levels. It is also shown that the proposed method is applicable to any modulation scheme and can work for any number of subcarriers under both additive white Gaussian noise and wireless Rayleigh fading channel.     

基于Hadamard矩阵变换降低OFDM系统PAPR的新算法

本文研究了一种基于Hadamard变换降低OFDM系统PAPR的新算法,该算法扩展了传统系统的星座点,减小了频域符号间的相关性,其编解码算法简便,易于实现。文中分析了该算法的基本原理,同时仿真结果表明,采用该算法其PAPR可以比传统方法降低3-4dB;并且可以与其它方法联合使用,从而进一步降低PAPR。     

MIMO—OFDM系统中降低峰均功率比技术的研究

多输入多输出正交频分复用（MIMO-OFDM）系统已成为第四代无线通信的关键技术之一,而系统所产生的高峰均功率比（PAPR）信号是限制MIMO-OFDM技术实用化的主要障碍。针对此,对MIMO-OFDM系统中现有的几种降低PAPR的传统及改进算法进行了详细介绍,并描述了改进算法相对于传统算法的主要优点。     

Upper-Lower Bounded-Complexity QRD-M for Spatial Multiplexing MIMO-OFDM Systems

Multiple-input multiple-output (MIMO) technology applied with orthogonal frequency division multiplexing (OFDM) is considered as the ultimate solution to increase channel capacity without any additional spectral resources. At the receiver side, the challenge resides in designing low complexity detection algorithms capable of separating independent streams sent simultaneously from different antennas. In this paper, we introduce an upper-lower bounded-complexity QRD-M algorithm (ULBC QRD-M). In the proposed algorithm we solve the problem of high extreme complexity of the conventional sphere decoding by fixing the upper bound complexity to that of the conventional QRD-M. On the other hand, ULBC QRD-M intelligently cancels all unnecessary hypotheses to achieve very low computational requirements. Analyses and simulation results show that the proposed algorithm achieves the performance of conventional QRD-M with only 26% of the required computations.     

VLM Precoded SLM Technique for PAPR Reduction in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is perhaps the most spectrally efficient, robust transmission technique discovered so far for communication systems, and it also mitigates the problem of multipath environment. High peak-to-average power ratio (PAPR) has always been a major drawback of the OFDM systems. In this article, a new precoding technique has been proposed based on Vandermonde-like matrix (VLM) and selective mapping (SLM) to reduce PAPR in OFDM systems. VLM precoding reduces the autocorrelation of the input sequences while SLM takes an advantage of the fact that the PAPR is very sensitive to phase shifts of the signal. The main advantage of this proposed scheme is to achieve a significant reduction in PAPR without increasing the system complexity. Computer simulations show that, the proposed method outperforms the existing precoding techniques without degrading the error performance of the system.     

Simple Amplitude and Phase Predistortion for PAPR Reduction in OFDM Systems

One of the drawbacks in an OFDM system is the high peak‐to‐average power ratio (PAPR). Among a number of techniques to reduce the high PAPR, simple amplitude predistortion (SAP), a form of active constellation extension, has been proposed to effectively achieve the desired PAPR. In this letter, a novel scheme, simple amplitude and phase predistortion (SAPP), is proposed. In SAP the carriers' amplitude is utilized to combat the peak signal. Each amplitude is amplified according to its degree of contribution as a metric. In addition to amplitude, SAPP also utilizes the phase. Simulation results indicate that the proposed scheme provides better PAPR reduction than SAP.     

Peak Power Reduction in Closed-Loop MIMO-OFDM Systems via Mode Reservation

This letter studies peak-to-average power ratio (PAPR) reduction for orthogonal frequency-division multiplexing (OFDM) signals in eigenbeamformed multiple antenna systems. The weak eigenmodes abandoned by the waterfilling allocation are intentionally filled in order to offset signal peaks in the time domain.Numerical results indicate that this method can significantly increase the efficiency of peak power constrained systems with no modification of the receiver required.     

Statistical Transceiver Designs with ICI Reduction for MIMO-OFDM Systems

In this paper, transceiver designs that take into account inter-carrier interferences (ICI) in the framework of game theory for multiple-input multiple-output orthogonal frequency division multiplexing systems are presented. With statistical channel state information at the transmitter, two transceivers: minimal mean-squared-error equalizer and minimal mean-squared-error decision-feedback equalizer are designed. The transceiver designs with the objective of minimizing the expectation of detection mean squared error become a complicated strategic non-cooperative game problem. In the paper, heuristic algorithms based on the best reply dynamic in game theory are proposed, whose convergence to Nash equilibrium can be numerically verified. Compared with traditional transceivers which ignore ICI effect by assuming perfect orthogonality between subcarriers, our designs require a moderate increase in design complexity. Nevertheless, the proposed transceivers have the same implementation complexity as the traditional counterparts. Numerical results verify that the proposed transceivers are more robust to ICI effect from the perspective of bit error rate than the traditional ones.     

Joint ICI Cancellation and PAPR Reduction in OFDM Systems Without Side Information

Intercarrier interference (ICI) self-cancellation, new ICI self-cancellation and conjugate cancellation schemes have been proposed in the literature to mitigate the effect of ICI. In this paper we have performed the mathematical analysis of PAPR performances for ICI self-cancellation, new ICI self-cancellation and ICI conjugate cancellation schemes and it is found that PAPR performance of these schemes are either very close to or poorer than the standard OFDM signal, which necessitates the requirement of PAPR reduction. After realizing the need of PAPR reduction in ICI cancellation schemes, we have proposed a joint scheme to reduce ICI and PAPR simultaneously. In this paper, we have proposed a multipoint partial transmit sequence (PTS) scheme, to improve the PAPR performance of ICI cancellation schemes. The proposed multipoint PTS based PAPR reduction scheme is coupled with ICI cancellation schemes in such a way that CIR performance of these schemes after coupling remains unchanged and no SI is required at the receiver to recover the original data signal. A comparison of CIR and PAPR performances for ICI cancellation schemes with and without PAPR reduction is also presented in this paper. The analytical results of CIR and PAPR performances for conventional ICI cancellation and joint ICI cancellation and PAPR reduction confirm the outperformance of the proposed scheme. We have also evaluated the SER performance of the joint schemes over additive white Gaussian noise and fading channels and presented a comparison with other existing schemes.     

PTS with Non-Uniform Phase Factors for PAPR Reduction in OFDM Systems

Partial transmit sequence (PTS) is one of the effective and uncomplicated PAPR reduction methods for OFDM systems. The phase factors applied in the weighting sequences are assumed to be uniformly distributed within [0, 2pi). By carefully investigating the phase distribution of OFDM signals, we proposed a modified PTS scheme outperforming the conventional PTS scheme in PAPR reduction performance. The process of the proposed scheme is the same as the conventional PTS, except for applying the new non-uniform phase factors. The required side information is the same as the conventional PTS applying uniform phase factors.     

一种降低MIMO-OFDM信号峰平功率比的改进算法

子块逐次变换法（SST）是MIMO—OFDM系统中降低信号PAPR的一种有效方法,充分利用空域自由度。但是,SST没有考虑子块在频域的自由度。针对这一问题,文章提出了一种改进的SST算法（Improved SST,ISST）,不仅在空域上子块做与SST相同的变换,而且在频域上同一天线上的子块也逐次旋转,既充分利用空域上所有天线的自由度,又利用频域自由度。仿真结果表明,提出方法的PAPR降低效果要明显优于SST,而且M越大优势越明显。     

Spatial Transmission Mode Switching in Multiuser MIMO-OFDM Systems With User Fairness

Multiantenna radio systems allow accessing the channel in diversity or spatial multiplexing (SMUX) mode. Adequate switching between these modes according to current channel conditions was shown to yield significant performance improvements while requiring little feedback from the receiving side. We present a transmission concept for the downlink of a multiuser multiple-input–multiple-output orthogonal frequency-division-multiplexing (MU-MIMO-OFDM) system aiming at high user rates with limited feedback demands. An extended score-based scheduling (SB) approach ensures fair-resource allocation to the users, whereas transmission mode switching is used to guarantee high user rates. The degree of fairness of the scheduler can be adapted by adequately configuring a weighting function for the scores. Comparison with single-mode schemes reveals substantial throughput gains of the adaptive switching concept. Furthermore, targeting maximum throughput, we show that a considerable proportion of the capacity of the MIMO broadcast channel (BC) can be achieved with a comparatively low amount of required feedback.