Large- and small-signal channel transit time delays in long-channel MOS transistors

Turn-on and turn-off delay times (intrinsic channel transit times) of long-channel MOST's are studied for both large-and small-signal inputs. By developing large-signal dynamic equations in normalized form it is evident that the normalized channel current and voltage distributions are unique and independent of device parameters and applied voltages. Channel transit time delays for both large and small signal are found to be given by a simple analytical expression containing a constant, undefined for large signal but defined explicitly for small signal. Values for the constant are found for large- signal operation in several modes by computer simulation representing the channel as a series of cascaded CCD elements. Those for small signal are found by representing the channel as anRCtransmission line. With the values of the constant determined, the simple analytical expression is shown to accurately predict channel transit time delays regardless of device type, channel length, width, substrate doping, crystal orientation, or effective mobility. It is concluded that the data presented can be used in designing delay lines or low-pass filters employing long-channel devices.     

Adaptive equalization for frequency-selective channels of unknown length

This paper studies adaptive equalization for time-dispersive communication channels whose impulse responses have unknown lengths. This problem is important, because an adaptive equalizer designed for an incorrect channel length is suboptimal; it often estimates an unnecessarily large number of parameters. Some solutions to this problem exist (e.g., attempting to estimate the "channel length" and then switching between different equalizers); however, these are suboptimal owing to the difficulty of correctly identifying the channel length and the risk associated with an incorrect estimation of this length. Indeed, to determine the channel length is effectively a model order selection problem, for which no optimal solution is known. We propose a novel systematic approach to the problem under study, which circumvents the estimation of the channel length. The key idea is to model the channel impulse response via a mixture Gaussian model, which has one component for each possible channel length. The parameters of the mixture model are estimated from a received pilot sequence. We derive the optimal receiver associated with this mixture model, along with some computationally efficient approximations of it. We also devise a receiver, consisting of a bank of soft-output Viterbi algorithms, which can deliver soft decisions. Via numerical simulations, we show that our new method can outperform conventional adaptive Viterbi equalizers that use a fixed or an estimated channel length.     

A Computationally Efficient Multilevel Coding Scheme for ISI Channels

This paper proposes a multilevel coding scheme with linear mapping for intersymbol interference (ISI) channels and derives a low-complexity receiver structure that can achieve the ISI channel capacity. The transmitter superimposes many layers of independent binary antipodal streams to generate a quadrature amplitude modulation (QAM) or Gaussian-like channel input. The receiver performs multistage decoding with decision feedback and interference cancellation. Within each stage is a linear minimum mean-square-error (MMSE) equalizer followed by an error-correcting decoder. The complexity scales linearly with the channel length and the number of layers, and the process is shown to be asymptotically information lossless if a fixed input power is properly distributed over a sufficiently large number of layers. This framework is then extended to achieve the capacity of the ISI channel using a transmitter-side spectral shaping filter that converts a Gaussian input sequence with a white spectrum to one with a water-filling spectrum.      

Blind channel identification: subspace tracking method without rankestimation

Subspace (SS) methods are an effective approach for blind channel identification. However, these methods also have two major disadvantages: 1) They require accurate channel length estimation and/or rank estimation of the correlation matrix, which is difficult with noisy channels, and 2) they require a large amount of computation for the singular value decomposition (SVD), which makes it inconvenient for adaptive implementation. Although many adaptive subspace tracking algorithms can be applied, the computational complexity is still O(m³), where m is the data vector length. In this paper, we introduce new recursive subspace algorithms using ULV updating and successive cancellation techniques. The new algorithms do not need to estimate the rank of the correlation matrix. Furthermore, the channel length can be overestimated initially and be recovered at the end by a successive cancellation procedure, which leads to more convenient implementations. The adaptive algorithm has computations of O(m² ) in each recursion. The new methods can be applied to either the single user or the multiuser cases. Simulations demonstrate their good performance     

Protocols for very high-speed optical fiber local area networks using a passive star topology

This paper deals with the problem of interconnection of many high-speed bursty traffic users via an optical passive star coupler. Each user can tune its laser over a range of wavelengths, thus resulting in a wavelength division multiplexed communication. The total number of wavelengths over which user tunability exists could be much smaller than the number of users. Therefore, some form of random access sharing and packet switching may be necessary. We propose several protocols that require each user to have a tunable receiver. The information on "where" and "when" to tune the laser is confined to a control (setup) channel that users tune to when in idle mode. An interconnection between two users lasting for the length of a data packet is set up on the control channel by the transmitting user who informs the receiving user where to tune in order to receive the data packet. No centralized control or coordination is required among the users. After analyzing each protocol, we present the throughput/ delay versus the offered traffic and the delay versus throughput in a sequence of plots. We show that in typical applications an average throughput of up to 0.95 can be achieved at a reasonable average delay using one of these protocols. In our benchmark examples we present an optical local area network (LAN) with a total throughput of 100- Gbit/s in which every user has access to a 1-Gbit/s data rate and the network can support over 1000 users. The protocols can be used in a) large LAN's that do not require a large capacity, b) small LAN's (1 kin) that require a large capacity and c) large LAN's (tens of kilometers) that require a rather large capacity.     

一种FDD大规模MIMO系统下行信道估计算法

信道估计是大规模多输入多输出(MIMO)系统的关键技术之一。本文针对频分双工(FDD)大规模MIMO正交频分复用(OFDM)系统,研究了下行信道估计问题。通过利用大规模MIMO-OFDM信道在角度-频域中的块稀疏特性,提出了基于块匹配追踪的低复杂度估计算法。另外,针对采用时域正交导频存在估计周期过长,有可能超过系统相干时间的问题,提出了天线分组发送方案,通过牺牲观测数据长度来换取信道估计周期的减少。仿真结果表明,所提算法具有良好的抗噪性能,可以准确找出稀疏向量的非零值位置,并可自适应确定稀疏度。     

差错信道下IEEE 802.11 DCF最优帧长分析及信道自适应策略

分析了差错信道下IEEE 802.11 DCF在饱和状态下的吞吐量以及帧长度对吞吐量的影响.研究表明在一定的误码率（BER）下存在最优的帧长使得系统吞吐量最大,并且该最优值与发送节点的数目无关.在此基础上,提出了一种简单的基于BER区间划分的帧长控制的信道自适应发送策略.仿真表明,与固定帧长的发送策略相比较,使用该策略在差错信道下可以有效地提高吞吐量.     

Near optimum universal belief propagation based decoding oflow-density parity check codes

In this paper, we propose a belief-propagation (BP)-based decoding algorithm which utilizes normalization to improve the accuracy of the soft values delivered by a previously proposed simplified BP-based algorithm. The normalization factors can be obtained not only by simulation, but also, importantly, theoretically. This new BP-based algorithm is much simpler to implement than BP decoding as it requires only additions of the normalized received values and is universal, i.e., the decoding is independent of the channel characteristics. Some simulation results are given, which show this new decoding approach can achieve an error performance very close to that of BP on the additive white Gaussian noise channel, especially for low-density parity check (LDPC) codes whose check sums have large weights. The principle of normalization can also be used to improve the performance of the max-log-MAP algorithm in turbo decoding, and some coding gain can be achieved if the code length is long enough     

Optimal placement of training for frequency-selective block-fading channels

The problem of placing training symbols optimally for orthogonal frequency-division multiplexing (OFDM) and single-carrier systems is considered. The channel is assumed to be quasi-static with a finite impulse response of length (L + 1) samples. Under the assumptions that neither the transmitter nor the receiver knows the channel, and that the receiver forms a minimum mean square error (MMSE) channel estimate based on training symbols only, training is optimized by maximizing a tight lower bound on the ergodic training-based independent and identically distributed (i.i.d.) capacity. For OFDM systems, it is shown that the lower bound is maximized by placing the known symbols periodically in frequency. For single-carrier systems, under the assumption that the training symbols are placed in clusters of length /spl alpha/ /spl ges/ (2L + 1), it is shown that the lower bound is maximized by a family of placement schemes called QPP-/spl alpha/, where QPP stands for quasi-periodic placement. These placement schemes are formed by grouping the known symbols into as many clusters as possible and then placing these clusters periodically in the packet. For both OFDM and single-carrier systems, the optimum energy tradeoff between training and data is also obtained.     

Precoder design and blind identification of MIMO channels

In this paper, blind identification problem based on second-order statistics (SOS) of received signals is investigated for multiple-input multiple-output (MIMO) channels. It is shown that precoders introduced in transmitters can provide certain extra information for blind identification problem and commonly used identifiability conditions on MIMO channels can be relaxed. A design criterion for the precoders in frequency domain is proposed. Two blind identification algorithms based on the knowledge of the precoders and SOS of the received signals are then developed. In these algorithms, due to particular structure of the precoders, the blind identification problem for a MIMO channel is decomposed into several independent single-input multiple-output (SIMO) channel identification problems. The presented algorithms are robust to an overestimated channel order and not sensitive to signal-to-noise ratio (SNR). Compared with the existing results, a larger class of MIMO finite-impulse-response (FIR) channels can be identified by the new methods.     

LT码的性能分析及仿真

LT码（Luby Transform Codes）属于喷泉码的其中一种,它是一类基于图的线性纠删码,其应用的信道环境是删除信道（BEC,Binary Erasure Channel）。这里主要介绍在二元删除信道模型下,LT码的4种度分布设计方法及,分析了短码长和长码长情况下的LT码在不同信道删除概率情况下的性能及比较。研究结果表明：在删除概率非常大的情况下,只要译码端接收到足够多的包都能以高概率译出原始数据包,只是产生的冗余度大,这里的结果将为系统选择不同参数的LT码提供参考。     

A capacitance method to determine channel lengths for conventional and LDD MOSFET's

A simple method for determining the channel length and in situ gate-oxide thickness of MOSFETs is described. The method is based on the linear relationship between the intrinsic gate capacitance and effective channel length. Measurements from two gate biases on devices of different channel lengths are sufficient to obtain a full characterization. In contrast to the channel-resistance method, the accuracy of the capacitance method is independent of the source-drain and contact series resistance. It can, therefore, be used for conventional as well as lightly-doped drain (LDD) devices. Channel length and gate-oxide thickness determined by this method are given for conventional and LDD MOSFET's. For conventional MOSFET's, the new method agrees with the traditional effective length measurements to better than 0.1 µm.     

一种不受信道阶数估计影响的SIMO直接盲均衡算法

针对单输入多输出（Single-Input-Multiple-Output, SIMO）模型提出一种完全不需要信道阶数估计的直接盲均衡算法。文章利用接收数据的截短协方差矩阵和信号子空间的关系设计一种零延迟均衡器,并通过信道矩阵和均衡器系数的合响应特性克服了算法相位偏转的问题,最后得到一种对信道阶数估计鲁棒并且没有相位偏转的盲均衡算法。该算法不同于一般子空间类算法,不需要直接对接收信号的协方差矩阵进行信号子空间和噪声子空间的分解,因此对信道阶数估计具有很强的鲁棒性。文章给出了算法的Batch实现过程,同时为更好适应一般时变信道环境和实现实时处理的要求,通过递归迭代得到算法的自适应实现过程。仿真实验表明该算法几乎不受信道阶数过估计或欠估计的影响,同时该算法具有良好的均方误差（Mean Square Error, MSE）和误符号率SER（Symbol Error Rate, SER）性能,并且具有很快的收敛速度。      

基于理想自相关训练序列的双选信道估计方法

高速移动下的无线宽带通信要经历时间和频率双选择性衰落,近些年来双选信道的估计越来越受到关注.该文利用复指数基扩展模型(BEM),提出一种基于理想自相关训练序列的双选信道估计方法,其训练序列的能量均匀分布,能够消除峰均比的问题.文中分析并证明了具有理想自相关特性等间隔放置的训练序列,能够使信道估计的MSE最小化.仿真表明与传统的zero-padding方式信道估计相比,在获得相同信道估计性能的情况下,采用该文信道估计方法的系统的峰均比有很大改善.     

Reduction of channel hot-electron-generated substrate current insub-150-nm channel length Si MOSFET's

Channel hot-electron-generated substrate currents were measured in MOSFET devices with channel lengths down to 0.09 μm, and a family of characteristic plots of substrate current, normalized to drain current, I_SUB/I_D, rather than (V _DS-V_DSAT)^-1 was obtained. For channel lengths greater than 0.5 μm, the characteristics are independent of channel length. For channel lengths in the range of 0.15 μm, the characteristics are independent of channel length. For channel lengths in the range of 0.15 μm, the normalized substrate current at constant V_DS increases with decreasing channel length. However, as the channel length is decreased below 0.15 μm, a decrease of the normalized substrate current is observed. The decrease is larger at 77 K than at 300 K. This decrease accompanies the onset of electron velocity overshoot over a large portion of the channel. It is suggested that the decrease is due either to a decrease of carrier energy because energy relaxation and transit times become comparable, to a relative decrease of the carrier population in the channel, or to both     

Optimal Joint Detection/Estimation in Fading Channels With Polynomial Complexity

The problem of sequence detection in frequency-nonselective/time-selective fading channels, when channel state information (CSI) is not available at the transmitter and receiver, is considered in this paper. The traditional belief is that exact maximum-likelihood sequence detection (MLSD) of an uncoded sequence over this channel has exponential complexity in the channel coherence time. Thus, for slowly varying channels, i.e., channels having coherence time on the order of the sequence length, the complexity appears to be exponential in the sequence length. In the first part of this work, it is shown that exact MLSD can be computed with only polynomial worst case complexity in the sequence length regardless of the operating signal-to-noise ratio (SNR) for equal-energy signal constellations. By establishing a relationship between the aforementioned complexity and the rank of the correlation matrix of the fading process, an understanding of how complexity of the optimal MLSD receiver varies as the channel dynamics change is provided. In the second part of this paper, the problem of decoding turbo-like codes in frequency-nonselective/time-selective fading channels without receiver CSI is examined. Using arguments similar to the ones used for the MLSD case, it is shown that the exact symbol-by-symbol soft-decision metrics (SbSSDMs) implied by the min-sum algorithm can be evaluated with polynomial worst case complexity in the sequence length regardless of SNR for equal-energy signal constellations. Finally, by simplifying some key steps in the polynomial-complexity algorithm, a family of fast, approximate algorithms is derived, which yield near-optimal performance     

一种改进的快变信道展开模型

 在高速移动正交频分复用系统（OFDM）中,信道的快速变化引起载波间干扰,降低系统性能,且使信道的准确估计变得更为困难. 由于在接收端待估计的信道参数多于接收信号样点数,信道估计方程无确定解. 为了解决这一问题,通常将快变信道展开为基函数叠加的近似表达形式,信道估计问题变成对展开系数的估计,待估计的参数大大减少. 本文分析并比较了常用的基展开模型的特点,指出了它们的不足,并在此基础上提出一种改进的快变信道展开模型,该模型的核心思想是在过采样基础上通过基线补偿减小吉布斯效应的影响,从而减小展开误差. 该模型性能与信道的统计性质无关,且在展开基的变化赶不上实际信道变化速度时,展开误差不会明显增大,优于CE-BEM, GCE-BEM, KL-MEM等模型,可用于高速移动OFDM系统的快变信道估计.     

Clustered OFDM with channel estimation for high rate wireless data

Clustered OFDM can provide in-band diversity gain for wideband wireless communication systems. It is a promising technique for high rate packet data access over wideband mobile wireless channels. Due to the smaller size of each cluster for clustered OFDM than for classical (nonclustered) OFDM, edge effects can be very large. In this letter, we present new transforms for channel estimators in clustered OFDM systems. The new transforms are independent of the channel delay profiles and can effectively mitigate the edge effects. It is shown by computer simulation that the performance of clustered OFDM with the estimator using the new transforms is very close to the performance with the optimum estimator that is determined by the individual channel delay profile. Furthermore, clustered OFDM with the new estimator is almost as good as classical OFDM with transmit diversity     

MOSFET channel length: extraction and interpretation

This paper focuses on MOSFET channel length: its definition, extraction, and physical interpretation. After a brief review of the objectives of channel length extraction and previous extraction methods, the principle and the algorithm of the latest “shift and ratio” (S&R) method are described. The S&R method allows the channel mobility to be an arbitrary function of gate voltage and, at the same time, provides a way to determine the threshold voltage of short-channel devices independent of their parasitic resistances. Accurate and consistent results are obtained from nMOSFET and pMOSFET data down to 0.05 μm channel length. By applying the S&R method to model generated current-voltage (I-V) curves, it is shown that the extracted channel length should be interpreted in terms of the injection points where the MOSFET current spreads from the surface layer into the bulk source-drain region. This implies significant degradation of short-channel effects (SCE's) if the lateral source-drain doping gradient is not abrupt enough. Several remaining issues, including errors due to channel-length-dependent mobilities, difficulties with lightly-doped drain (LDD) MOSFET's, and interpretation of capacitance-voltage (C-V) extracted channel lengths, are discussed     

Middleman covert channel establishment based on MORE routing protocol using network coding in ad hoc networks

Covert channels have been recently the subject of the study in both creation and countermeasure aspects. There are many different ways to embed the covert data in network standards and protocols, especially in wireless networks. MORE (MAC‐independent opportunistic routing) is an opportunistic routing protocol which uses networks coding to enhance routing performance by reducing the repetitions. This protocol can be a suitable medium for covert channel establishment. A middleman covert channel establishment method is proposed in this paper over MORE routing protocol and with the use of network coding. Hidden data are transferred through packet's payload bytes. Covert sender manipulates coding mechanism by calculating packets' coefficients instead of random selection. The proposed covert channel provides the average throughput of 218 and 231 bps, using two different data length approaches which is relatively a good comparing to the previous network layer covert channels. The proposed covert channel is also a covert storage channel and cannot be removed or restricted. Effect of different network characteristics on the proposed method's capacity and security is investigated by a simulation study, and the results are discussed.