Clipping distortion mitigation in OFDM systems over fading channels by using DFT-based method

Saeedi et al. have recently proposed a DFT-based method (DBM) for Orthogonal Frequency Division Multiplexing (OFDM) systems to compensate for the performance degradation caused by clipping distortion. They have analyzed the performance of the DBM only in an additive white Gaussian noise channel. But, it is known that the main advantage of an OFDM system is its robustness in fading channels where this system turns a frequency selective channel into a set of parallel flat non-selective fading sub-channels. In this paper, we analyze the performance of the DBM over a frequency selective fading channel with flat fading Rician sub-channels. Our results show that similar to additive white Gaussian noise channel case, the DBM can properly mitigate the clipping distortion in OFDM signals over fading channels. Furthermore, the performance of a Hybrid method that consists of a channel coding method and the DBM is also examined over fading channels.     

加性噪声下网络化多输入离散系统均方可镇定

本文研究了静态信噪比约束下多输入离散系统均方可镇定问题,其中信道资源固定且不可任意分配,信道被建模为加性高斯白噪声.主要目的是借用优化序理论讨论系统可镇定时各子信道的信道容量与系统拓扑熵的关系.本文基本思想是从供需平衡角度讨论可镇定性.具体地,对于通信资源,每个系统输入被视为需求方,而信道被视为供应方.信道的供应资源由其各自的均方容量刻画,网络化系统均方可镇定要求通信资源供求平衡.因信道资源不可配置,人们可以通过调节需求方(一定的传输机制)来满足供给方要求.给出了网络化系统均方可镇定时的一个充分条件和一个必要条件.最后数值算例验证所得结论.     

Variable energy adaptation for asynchronous CDMA communications over slowly fading channels

In this paper, a detailed theoretical analysis of variable energy adaptation in an asynchronous code division multiple access (A-CDMA) system is discussed. Rayleigh and Rician frequency-selective slowly fading channels are considered. The receiver, capable of measuring the received signal energy-to-noise ratio, provides the transmitter with the necessary signal-to-noise ratio measurement to control the transmitter energy through a noise-free feedback channel. System parameters such as fading margin and mean transmitter energy gain are calculated for both Rayleigh and Rician fading channels as a function of the probability of error specification and the probability of unsatisfactory operation.     

Adaptive rate coding using convolutional codes for asynchronous code division multiple access communications over slowly fading channels

This paper presents a method of code rate adaptation using punctured convolutional codes for direct sequence spread spectrum communication systems over slowly fading channels. A blind channel estimation technique is used to estimate the nature of the multi-user channel at the detector (before the decoder). The path gains obtained from the channel estimation technique are used to adapt the code rates. Punctured codes derived from a specific rate 1/2 (M = 4) mother code are used to provide error protection corresponding to the actual channel state. The upper and lower bounds on the bit error probability and the upper bound on the error event probability are derived for hard-decision and soft-decision decoding over Rayleigh and Rician fading channels. The throughput gains obtained using the adaptive scheme and the performances of the punctured codes are studied.     

LQG-like control of scalar systems over communication channels: The role of data losses,delays and SNR limitations

In this work we address the problem of feedback control design for scalar systems in the presence of a non-ideal communication channel, which gives rise to tightly coupled limitations in terms of quantization errors, decoding/computational delays and packet loss affecting the closed loop control performance. We restrict our analysis to the context of LQG-like control, where the estimator and controller gains are forced to be constant, subject to SNR limitations, packet loss, and constant delay, and we derive their impact on the optimal design of the controller parameters. In particular, we show that the stability of the closed loop system depends on a tradeoff among SNR, packet loss probability and delay. Through this analysis we are also able to recover, as special cases, several results already available in the literature that have treated packet loss, quantization error and delay separately. We also show that the estimator and controller cannot be designed independently even if the controller has full knowledge of the packet loss sequence and the control inputs to the plant. In fact the optimal control gain, when accounting for the communication constraints is, in general, different from the optimal gain derived under the classical LQG scenario, which is recaptured when the SNR over the channel goes to infinity.     

语音WSN的QoS路由和准入控制研究

语音无线传感器网络(WSN)数据量大、处理任务复杂,针对其中的服务质量(QoS)保障问题,总结WSN中的QoS路由协议和准入控制的研究进展,认为准入控制可根据网络拓扑结构分为平面准入控制和分簇准入控制,并针对语音无线传感器网络,提出一种将准入控制和QoS路由相结合的方法,从而提高语音传输质量。