Modeling of nondirected wireless infrared channels

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and RMS delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion     

Coding and equalization for PPM on wireless infrared channels

We analyze the performance of trellis-coded pulse-position modulation with block decision-feedback equalization (BDFE) and parallel decision-feedback decoding (PDFD) on indoor, wireless infrared channels. We show that the reduced complexities of BDFE and PDFD as compared to maximum-likelihood sequence detection allow for better codes whose increased coding gain more than compensates for the penalty due to suboptimal detection. We quantify these net gains in performance over a range of dispersive channels, indicating where BDFE and PDFD provide the best performance. Finally, we present Monte Carlo simulation results to verify our analysis     

无线激光DH-PIM室内通信系统性能分析

研究了加性高斯白噪声和多径码间串扰下采用双头脉冲间隔调制(DH-PIM)的无线激光室内通信系统的性能,对DH-PIM系统的符号结构、带宽、差错率特性等方面进行了分析,并与OOK,PPM和DPIM调制方式做了比较。     

Propagation modeling of MIMO multipolarized fixed wireless channels

This paper addresses the extension of a stochastic geometry-based scattering model to multipolarized transmissions. The initial approach is based on a geometrical distribution of obstacles derived from known power-delay profiles. Each scattering process is statistically described by a matrix reflection coefficient corresponding to dual-polarization states. Ultimately, the model allows us to simulate the effects of the range on K-factor, delay-spread, Doppler spectrum, channel correlations and capacity, branch power ratio, and cross-polar discrimination. Simulation results are compared with existing measurements at 2.5 GHz. The proposed model is then used to investigate various dual-polarization 2 /spl times/ 2 multiple-input-multiple-output (MIMO) schemes such as 0/spl deg//90/spl deg/ or /spl plusmn/45/spl deg/, as well as to optimize the design of multipolarized MIMO schemes.     

Error modeling schemes for fading channels in wireless communications: A survey

Network system designers need to understand the error performance of wireless mobile channels in order to improve the quality of communications by deploying better modulation and coding schemes, and better network architectures. It is also desirable to have an accurate and thoroughly reproducible error model, which would allow network designers to evaluate a protocol or algorithm and its variations in a controlled and repeatable way. However, the physical properties of radio propagation, and the diversities of error environments in a wireless medium, lead to complexity in modeling the error performance of wireless channels. This article surveys the error modeling methods of fading channels in wireless communications, and provides a novel user-requirement (researchers and designers) based approach to classify the existing wireless error models.     

Iterative water-filling for Gaussian vector multiple-access channels

This paper proposes an efficient numerical algorithm to compute the optimal input distribution that maximizes the sum capacity of a Gaussian multiple-access channel with vector inputs and a vector output. The numerical algorithm has an iterative water-filling interpretation. The algorithm converges from any starting point, and it reaches within 1/2 nats per user per output dimension from the sum capacity after just one iteration. The characterization of sum capacity also allows an upper bound and a lower bound for the entire capacity region to be derived.     

Iterative reduced-search decoding for coded partial-response channels

The full-complexity soft-input/soft-output (SISO) detector based on the BCJR algorithm for coded partial-response channels has a computational complexity growing exponentially with channel memory length. In this letter, we propose a low complexity soft-output channel detector based on the Chase decoding algorithm, which was previously applied to decode turbo product codes. At each iteration, the proposed detector forms a candidate list using all possible combinations of bit patterns in the weakest indices based on tentative hard estimates and a priori information fed back from the outer decoder. To demonstrate the performance/complexity tradeoff of the proposed detector, simulation results over rate-8/9 turbo-coded EPR4 and ME/sup 2/PR4 channels are presented, respectively. It is shown that the proposed detector can significantly reduce the computational complexity with only a small performance loss compared to the BCJR algorithm.     

Electronic tracking for wireless infrared communications

A high-speed wireless system (/spl ges/100 Mb/s) for indoor infrared (IR) communications via the line of sight is described and feasibility is shown in an experimental demonstrator. A diffuse link is used for connectivity, and tracked directed links are used for high-speed communications. The transmitter is made of a laser diode array in combination with multiple-beam forming optics. For the receiver (Rx), a wide-angle lens, and an avalanche photodiode array are used. For the diffuse link, the signals from all pixels in the array are combined. Pixels are selectively addressed to realize directed links. Fast electronic tracking of a directed link is possible by switching the signal path onto the right pixel in the array. Diffuse link, directed link, position detection, and tracking can be realized with one and the same transceiver hardware. A favorite system design is derived from constraints due to the IR channel, eye safety, lenses, photodetectors, and the overall system complexity. The experimental system shows some key features, namely 155-Mb/s wireless transmission over a distance of nearly 2 m with electronic tracking at an imaging IR Rx. Electronic tracking of IR links, thus, allows both high data rates and high capacity for wireless access in small office and home environments.     

GQR-based models for directional wireless channels

In this paper novel reduced complexity statistical models for the representation of directional wireless channels affected by Rayleigh fading are developed. Their derivation is based on the evaluation of Fourier integrals by means of Gaussian quadrature rules (GQRs). The proposed models represent a communication channel as a tapped-angle and -delay line with non uniform spacing between successive taps. The application of the GQR approach to three different scenarios is also illustrated. Numerical results evidence that these models can provide a simple and accurate stochastic representation of directional fading channels.     

Robust vector quantization for wireless channels

This study focuses on two issues: parametric modeling of the channel and index assignment of codevectors, to design a vector quantizer that achieves high robustness against channel errors. We first formulate the design of a robust zero-redundancy vector quantizer as a combinatorial optimization problem leading to a genetic search for a minimum-distortion index assignment. The performance is further enhanced by the use of the Fritchman (1967) channel model that more closely characterizes the statistical dependencies between error sequences. This study also presents an index assignment algorithm based on the Fritchman model with parameter values estimated using a real-coded genetic algorithm. Simulation results indicate that the global explorative properties of genetic algorithms make them very effective in estimating Fritchman model parameters, and use of this model can match index assignment to expected channel conditions     

Iterative tree search detection for MIMO wireless systems

This paper presents a reduced-complexity soft-input soft-output detection scheme, called iterative tree search detection, for multiple-input multiple-output wireless communication systems employing turbo processing at the receiver. In this scheme, a reduced search space is selected with the aid of the M-algorithm, and QAM signal constellations with block partitionable labels are used in order to make the detection complexity per bit almost independent of the modulation order, as well as asymptotically linear in the number of transmit antennas. Results from computer simulations are presented which demonstrate the capability of the scheme to approach optimal performance at considerably reduced complexity.     

Iterative multiple beamforming algorithm for mimo broadcast channels

We consider multiuser Gaussian broadcast channels with multiple antennas at both transmitter and receivers. An iterative multiple beamforming algorithm is proposed, which is flexible in the antenna configuration and performs well in low to moderate data rates. Its capacity and bit error rate performance are compared with the ones achieved by the traditional zeroforcing method.     

Multispot diffusing configuration for wireless infrared access

In order to combine the advantages and to overcome the drawbacks of a direct line-of-sight or a diffuse configuration for wireless infrared access, a multispot diffusing concept utilizing a holographic spot array generator is presented. Simulation results are presented and compared with those for a pure diffuse configuration in terms of link characteristics, when a single-element or a multibranch composite receiver is employed. The multispot transmitter ensures a more uniform signal power distribution. Improvements of about 2 dBo (optical decibels) can be achieved compared to a Lambertian pattern illumination. The increased power path loss at the edges of the communication cell is accompanied with a decrease in the delay spread resulting in an extension of the coverage range. Utilization of angle diversity detection improves the signal-to-noise ratio by more than 7 dB when selecting the best receiver branch and more than 10.5 dB in the case of maximal-ratio combining. Use of a multibeam transmitter and an angle diversity receiver reduces the likelihood of shadowing of the receiver due to an obstacle standing along the path between the receiver and the transmitter     

Trellis-coded multiple-pulse-position modulation for wireless infrared communications

We present new trellis codes based on multiple-pulse-position modulation (MPPM) for wireless infrared communication. We assume that the receiver uses maximum-likelihood sequence detection to mitigate the effects of channel dispersion, which we model using a first-order lowpass filter. Compared to trellis codes based on PPM, the new codes are less sensitive to multipath dispersion and offer better power efficiency when the desired bit rate is large, compared with the channel bandwidth. For example, when the bit rate equals the bandwidth, trellis-coded (17 2)-MPPM requires 1.4 dB less optical power than trellis-coded 16-PPM having the same constraint length.     

Angle diversity for nondirected wireless infrared communication

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared (IR) communications systems. Multiple transmitter beams and multiple narrow field-of view receivers reduce the path loss, multipath distortion, and background noise of the channel, which leads to improved range. We also discuss practical considerations for multielement angle diversity systems, including channel characterization and suboptimal detection techniques. Maximal-ratio combining provides nearly optimal performance up to 100 Mb/s for the angle diversity systems considered. The design and performance of a prototype angle diversity IR communication system are discussed. The prototype can maintain 70 Mb/s at a P_e of 10^-9 over a 4-m range     

一种CDMA循环分级式自适应Rake接收机

提出了一种新颖的循环分级式自适应Rake接收机(IM),与目前研究较多的LMMSE算法相比,二者复杂度相当,但IM在接收性能上更为优越,而且避免了LMMSE算法的收敛慢、收敛步长不确定等问题。同时,系统对于辅助向量部分采用了新的约束条件,并提出了一种循环分级优化系统的简化结构,每一级系统参数都只需进行简单的循环迭代计算即可决定,与AV方法相比,其性能相当,但系统的复杂性明显降低。     

Iterative power control based admission control for wireless networks

This paper studies transmission power control algorithms for cellular networks. One of the challenges in commonly used iterative mechanisms to achieve this is to identify if the iteration will converge since convergence indicates feasibility of transmit power allocation under prevailing network conditions. The convergence criterion should also be simple to calculate given the time constraints in a real-time wireless network. Towards this goal, this paper derives simple sufficient conditions for convergence of an iterative power control algorithm using existing bounds from matrix theory. With the help of suitable numerical examples, it is shown that the allocated transmit powers of the nodes converge when sufficient conditions are satisfied, and diverge when they are not satisfied. This forms the basis for an efficient link data-rate based admission control mechanism for wireless networks. The mechanism considers parameters such as signal strength requirement, link datarate requirement, and number of nodes in the system. Simulation based analysis shows that existing links are able to maintain their desired datarates despite the addition of new wireless links.     

Stable scheduling policies for fading wireless channels

We study the problem of stable scheduling for a class of wireless networks. The goal is to stabilize the queues holding information to be transmitted over a fading channel. Few assumptions are made on the arrival process statistics other than the assumption that their mean values lie within the capacity region and that they satisfy a version of the law of large numbers. We prove that, for any mean arrival rate that lies in the capacity region, the queues will be stable under our policy. Moreover, we show that it is easy to incorporate imperfect queue length information and other approximations that can simplify the implementation of our policy.     

Adaptive modulation systems for predicted wireless channels

When adaptive modulation is used to counter short-term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction will improve the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. We study the optimum design of an adaptive modulation scheme based on uncoded M-quadrature amplitude modulation, assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize the spectral efficiency, subject to average power and bit-error rate constraints. The key issues studied here are how a known prediction error variance will affect the optimized transmission properties, such as the signal-to-noise ratio (SNR) boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. This investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular, but efficient, channel power predictor. Optimum solutions for the rate and transmit power are derived, based on the predicted SNR and the prediction error variance.     

Soft quasi-maximum-likelihood detection for multiple-antenna wireless channels

The paper addresses soft maximum-likelihood (ML) detection for multiple-antenna wireless communication channels. We propose a soft quasi-ML detector that maximizes the log-likelihood function by deploying a semi-definite relaxation (SDR). Given perfect channel state information at the receiver, the quasi-ML SDR detector closely approximates the performance of the optimal ML detector in both coded and uncoded multiple-input, multiple-output (MIMO) channels with quadrature phase-shift keying (QPSK) modulation and frequency-flat Rayleigh fading. The complexity of the quasi-ML SDR detector is much less than that of the optimal ML detector, thus offering more favorable performance/complexity characteristics. In contrast to the existing sphere decoder, the new quasi-ML detector enjoys guaranteed polynomial worst-case complexity. The two detectors exhibit quite comparable performance in a variety of ergodic QPSK MIMO channels, but the complexity of the quasi-ML detector scales better with increasing number of transmit and receive antennas, especially in the region of low signal-to-noise ratio (SNR).