An adaptive modulation algorithm for performance improvement of MIMO ML systems

In this letter, we propose a simplified adaptive modulation algorithm for a multiple input multiple output (MIMO) system with a maximum likelihood (ML) receiver to minimize BER under a fixed data rate. The proposed algorithm has the advantages of good system performance and low computational complexity, when compared to previous algorithms.     

Progressive image transmission over space-time coded OFDM-based MIMO systems with adaptive modulation

This paper considers a progressive image transmission system over wireless channels by combining joint source-channel coding (JSCC), space-time coding, and orthogonal frequency division multiplexing (OFDM). The BER performance of the space-time coded OFDM-based MIMO system based on a newly built broadband MIMO fading model is first evaluated by assuming perfect channel state information at the receiver for coherent detection. Then, for a given average SNR (hence, BER), a fast local search algorithm is applied to optimize the unequal error protection design in JSCC, subjected to fixed total transmitted energy for various constellation sizes. This design allows the measurement of the expected reconstructed image quality. With this end-to-end system performance evaluation, an adaptive modulation scheme is proposed to pick the constellation size that offers the best reconstructed image quality for each average SNR. Simulation results of practical image transmissions confirm the effectiveness of our proposed adaptive modulation scheme.     

A cross layer approach based on queuing and adaptive modulation for MIMO systems

A cross layer approach based on queuing and adaptive modulation for MIMO systems has been presented. To realize the system, an adaptive modulation scheme using MIMO systems combined with finite-length queuing at the data link layer has been designed. A finite state Markov chain for MIMO channels has been constructed to persue the queuing analysis. The packet loss rate, the average throughput, and the average spectral efficiency have been computed. Furthermore, the packet error rate for adaptive modulation to minimize the packet loss rate and maximize the average throughput is extracted. The obtained results present the advantages of using the proposed cross layer architecture.     

采用自适应调制技术的时变MIMO信道的性能分析

本文采用可变速率和功率的MQAM调制方式研究时变情况下多输入多输出（MIMO）系统的性能,推导出了在速率离散分阶的情况下的功率和速率的分配公式.对不同的速率量化算法的仿真实验表明,在不知道信道矩阵概率密度函数的情况下,本文采用的速率量化算法可以有效的提高系统的频谱利用率,同时大大的减小了计算的复杂性.     

大规模MIMO系统中的一种自适应SIC检测算法

与传统系统相比,大规模多入多出（MIMO）系统能更加有效地提高频谱效率。利用传统的最小均方误差（MMSE）信号检测算法求解大规模MIMO系统,虽然检测结果接近最优,但是矩阵的求逆运算导致计算的复杂度非常高。提出了一种自适应排序干扰消除（SIC）检测算法,在逐次超松弛（SOR）迭代运算的基础上,通过干扰消除降低待检测矩阵的维度。通过仿真分析,得出所提算法的复杂度低于Jacobi、SOR检测算法,且在迭代次数较少的情况下,算法的误码率（BER）性能明显优于SOR检测算法。     

一种MIMO中继系统的自适应自干扰消除方法

为节省频率资源,全双工中继一般采用相同的频率接收和发射信号。由于收发天线之间无法充分隔离,接收天线容易受到自身发射天线的回波干扰。针对宽带全双工多输入多输出( MIMO)中继的自干扰问题,提出了一种基于梯度下降自适应算法的自干扰消除方法。该方法利用中继反馈的已知信号进行自干扰信道估计,并产生一个对自干扰信号的估计信号,从而在接收端将干扰抑制。仿真结果表明,该方法在自适应滤波器的跟踪性能、收敛分布和不同 MIMO 配置下的均方误差( MSE)性能等方面均取得良好效果。     

Block mapping spatial modulation scheme for MIMO systems

In this paper,a block mapping spatial modulation(BMSM)scheme is proposed to increase the transmit rate of multiple-input multiple-output(MIMO)wireless communication systems.In the BMSM scheme,the information to be transmitted is mapped into different combinations of transmit antenna indices and distinct constellation symbols at each time instant.Multiple transmit antennas are activated,which is different from spatial modulation(SM)and generalised spatial modulation(GSM)techniques,and also the information bi...     

基于频谱效率的低复杂度自适应MIMO系统

在一定的误比特率(BER)下,使用自适应MQAM调制可以最大化MIMO系统的频谱效率。通过不同的算法可以得到自适应调制MIMO系统的离散速率频谱效率(DRSE)。为了进一步提高频谱效率,提出了一种基于DRSE,在不同的算法之间进行切换的低复杂度自适应机制。研究了在两种2×2信道下即:独立同分布(i.i.d)的平坦瑞利衰落信道和空间相关的瑞利衰落信道,自适应MIMO系统在正交空时块编码(OSTBC)和空间复用(SM)之间切换的自适应机制。仿真结果表明,通过选择一种最优的调制模式及发射模式的组合方式,此切换算法可以使系统的频谱效率得到有效提高,同时只增加有限的反馈信息,降低了系统复杂度。     

Distributed adaptive algorithms for large dimensional MIMO systems

An algorithm for multi-input multi-output (MIMO) adaptive filtering is introduced that distributes the adaptive computation over a set of linearly connected computational modules. Each module has an input and an output and transmits data to and receives data from its nearest neighbor. A gradient-based algorithm for adapting the parameters in each module to minimize the global mean-squared error is derived using principles of back propagation. The performance surface is explored to understand the characteristics of the adaptive algorithm. The minimum mean-squared error is a many to one function of the parameters; therefore, upper bounds on each parameter are used to prevent excessive parameter drift and ensure stability with fixed step sizes. Guidelines for choosing the LMS algorithm step sizes and initial conditions are developed. Several examples illustrate the performance of the algorithm     

OFDM系统中自适应调制技术的研究

研究了正交频分得用OFDM系统中基于子载波组的自适应调制技术,提出了一种新的OFDM自适应调制策略,该策略可在上行链路发生错误时,要求重新传送信道信息,使发射机仍能根据信道的实际情况正确选择合适的调制制式,达到降低误比特率,提高系统性能的目的.     

MIMO spread spectrum GFDM communication systems with index modulation

In the fourth-generation communication system (4G), orthogonal frequency division multiplexing (OFDM) is widely used. However, due to the cyclic prefix (CP) being added, the low spectrum efficiency and high out-of-band radiation (OOB) cause OFDM not to be suitable for the beyond fifth-generation (B5G) communication system. Therefore, to meet the rigorous demands in B5G for the Internet of Things (IoT) and massive machine-type communications (MMTC), generalized frequency division multiplexing (GFDM) technology has received significant attention. On the other hand, index modulation (IM) can provide flexible settings of different parameters to achieve the ideal system goals. Therefore, this paper considers the combination of spread spectrum (SS) and IM under the framework of GFDM, called SS-IM-GFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional M-ary modulated symbols, which increase the spectrum efficiency and improve the bit error rate performance. Furthermore, a low-complexity maximum likelihood (ML) detector is proposed to recover the transmitted signal effectively at the receiver. Finally, the space-time block code (STBC) is employed to construct the multi-input multi-output SS-GFDM-IM (MIMO SS-GFDM-IM) system to improve the system's reliability. The simulation results show that the proposed scheme is superior to the existing systems.     

Adaptive modulation for SVD-based MIMO systems with outdated CSI

siderably more to be robust against CSI imperfection.     

Fuzzy adaptive partial parallel multistage detection for MIMO systems

Vertical Bell Laboratories Layered Space-Time (V-BLAST) detection followed by a full parallel interference cancellation (full PIC) or a constant-weight partial parallel interference cancellation (CW-PPIC) scheme has been proposed. However, its performance degrades when the received power of the antenna is unbalanced and/or the system is heavily loaded. Therefore, V-BLAST is proposed, followed by fuzzy adaptive partial parallel multistage detection for MIMO systems. The simulation results show that the error probability of this proposed method is better than that of V-BLAST followed by full PIC or CW-PPIC schemes     

Blind adaptive receiver for uplink multiuser massive MIMO systems

Herein, we consider uplink multiuser massive multiple‐input multiple‐output systems when multiple users transmit information symbols to a base station (BS) by applying simple space‐time block coding (STBC). At the BS receiver, two detection filters for each user are used to detect the STBC information symbols. One of these filters is for odd‐indexed symbols and the other for even‐indexed symbols. Using constrained output variance metric minimization, we first derive a special relation between the closed‐form optimal solutions for the two detection filters. Then, using the derived special relation, we propose a new blind adaptive algorithm for implementing the minimum output variance‐based optimal filters. In the proposed adaptive algorithm, filter weight vectors are updated only in the region satisfying the special relation. Through a theoretical analysis of the convergence speed and a computer simulation, we demonstrate that the proposed scheme exhibits faster convergence speed and lower steady‐state bit error rate than the conventional scheme.     

Multifunctional reconfigurable MEMS integrated antennas for adaptive MIMO systems

Multi-input multi-output systems with associated technologies such as smart antennas and adaptive coding and modulation techniques enhance channel capacity, diversity, and robustness of wireless communications as has been proven by many recent research results both in theory and experiments. This article focuses on the antenna aspect of MIMO systems. In particular, we emphasize the important role of the reconfigurable antenna and its links with space-time coding techniques that can be employed for further exploitation of the theoretical performance of MIMO wireless systems. The advantages of the reconfigurable antenna compared to the traditional smart antenna are discussed. Establishment of reconfigurable antennas requires novel radio frequency microelectromechanical systems technology, which has recently been developed in the authors' group. We briefly introduce this technology with emphasis on its distinct advantages over existing silicon-based MEMS technologies for reconfigurable antennas. A reconfigurable antenna design that can change its operating frequency and radiation/polarization characteristics is described. Finally, we present the experimental and theoretical results from impedance and radiation performance characterization for different antenna configurations.     

自适应MIMO系统中的支持选择功能的四单元天线

天线选择是自适应MIMO系统中实现可重构多天线的一种有效方法.提出了一种支持选择功能的四单元天线.通过在馈电网络中增加PIN二极管,该天线具备选择任意一组天线单元的功能,同时未被选择的天线单元终端加载匹配的集总负载以维持方向图的稳定.测量结果表明:在任意一种天线单元组合下,该天线都可以覆盖UMTS频段并保持较低的互耦;同时天线单元的方向图具有稳定性.     

An adaptive partial parallel multistage detection for MIMO systems

The full parallel multistage detection for multiple-input multiple-output systems has less processing delay and better performance than the vertical Bell Laboratories layered space-time detection scheme. However, due to the disadvantage of full parallel interference cancellation, its performance degrades when the received power of the substreams at one receiving antenna is unbalanced and/or when the system is heavily loaded. Therefore, we introduce adaptive partial weighting coefficients into the parallel multistage detection. The weighting coefficients are evaluated using the average decision correct probabilities in the previous stage. The simulation results show that the uncoded error probability of the proposed method is lower than that of the recently proposed full parallel multistage detection scheme with perfect or imperfect channel estimation.     

On the performance of adaptive modulation in cellular systems

Adaptive modulation techniques have the potential to substantially increase the spectrum efficiency and to provide different levels of service to users, both of which are considered important for third-generation cellular systems. In this work, we propose a general framework to quantify the potential gains of such techniques. Specifically, we study the throughput performance gain that may be achieved by combining adaptive modulation and power control. Our results show that: (1) using adaptive modulation even without any power control provides a significant throughput advantage over using signal-to-interference-plus-noise ratio (SINR) balancing power control and (2) combining adaptive modulation and a suitable power control scheme leads to a significantly higher throughput as compared to no power control or using SINR-balancing power control. The first observation is especially important from an implementation point of view. Adjusting the modulation level without changing the transmission power requires far fewer measurements and feedback as compared to the SINR-balancing power control or the optimal power control. Hence, it is significantly easier to implement. Although presented in the context of adaptive modulation, the results also apply to other variable rate transmission techniques, e.g., rate adaptive coding schemes, coded modulation schemes, etc. This work provides valuable insight into the performance of variable rate transmission techniques in multi-user environments     

Performance enhancement of adaptive orthogonal modulation in wireless CDMA systems

Recent research in wireless code-division multiple-access systems has shown that adaptive rate/power control can considerably increase network throughput relative to systems that use only power or rate control. In this paper, we consider joint power/rate optimization in the context of orthogonal modulation (OM) and investigate the additional performance gains achieved through adaptation of the OM order. We show that such adaptation can significantly increase network throughput, while simultaneously reducing the per-bit energy consumption relative to fixed-order modulation systems. The optimization is carried out under two different objective functions: minimizing the maximum service time and maximizing the sum of user rates. For the first objective function, we prove that the optimization problem can be formulated as a generalized geometric program (GGP). We then show how this GGP can be transformed into a nonlinear convex program, which can be solved optimally and efficiently. For the second objective function, we obtain a lower bound on the performance gain of adaptive OM (AOM) over fixed-modulation systems. Numerical results indicate that relative to an optimal joint rate/power control fixed-order modulation scheme, the proposed AOM scheme achieves significant throughput and energy gains.     

On spectral efficiency of low-complexity adaptive MIMO systems in Rayleigh fading channel

Adaptive MQAM modulation is used to maximize spectral efficiency of Multiple-Input Multiple-Output (MIMO) systems while keeping bit error rate (BER) under a target level. Closed-form expressions of the average spectral efficiency, coined as discrete-rate spectral efficiency (DRSE), are derived for adaptive modulation MIMO systems using different algorithms. To further enhance the spectral efficiency, a low complexity adaptation scheme is suggested to switch across different algorithms based on the DRSE. In the current letter, we investigate the adaptation scheme that switches between Orthogonal Space-Time Block Codes (OSTBC) and spatial multiplexing with zero-forcing (ZF) detection for MIMO systems with two transmit antennas. Two types of operating environment are considered: flat Rayleigh fading channel without spatial correlation and spatially correlated Rayleigh fading channel with transmit correlation.