Improving the performance of a slotted ALOHA packet radio networkwith an adaptive array

The use of an adaptive antenna array as a means of improving the performance of a slotted ALOHA packet radio network is presented. An adaptive array creates a strong capture effect at a packet radio terminal by automatically steering the receiver antenna pattern toward one packet and nulling other contending packets in a slot. A special code preamble and randomized arrival times within each slot allow the adaptive array to lock onto one packet in each slot. The throughput and delay performance of a network with an adaptive array are computed by applying the standard Markov chain analysis of slotted ALOHA. It is shown that throughput levels comparable to carrier sense multiple access (CSMA) are attainable with an adaptive array without the need for stations to be able to hear each other. The performance depends primarily on the number of adaptive array nulls, the array resolution, and the length of the randomization interval within each slot     

Throughput and packet error probability of cellularfrequency-hopped spread-spectrum radio networks

The authors characterize multiple-access interference for cellular mobile networks, in which users are assumed to be Poisson-distributed in the plane and use frequency-hopped spread-spectrum signaling with a transmitter-oriented assignment of frequency-hopping patterns. Exact expressions for the bit error probabilities are derived for binary coherently demodulated systems without coding. Approximations for the packet-error probability are derived for coherent and noncoherent systems and these approximations are applied when forward-error-control coding is used. In all cases, the effects of varying interference power are accurately taken into account according to some propagation law. Numerical results are given in terms of bit-error probability for the exact case and throughput for the approximate analyses. Comparisons are made with previously derived bounds, and it is shown that these tend to be very pessimistic     

Combinations of an adaptive array antenna and a canceller ofinterference for direct-sequence spread-spectrum multiple-access system

In the realization of code-division multiple access based on a spread-spectrum communication system, i.e. spread-spectrum multiple access (SSMA), reduction of cochannel interference is an important problem. An adaptive array antenna system is proposed that includes a cancellor of cochannel interference, which can improve performance by a combination of temporal and spatial filtering. While the adaptive array suppresses interference sources with arrival angles different from those of the desired user, the adaptive digital filter-canceller rejects those whose arrival angles are the same as those of the desired user. The proposed system can achieve stable acquisition and low error rate of demodulated data even in a heavy-interference channel where a conventional array antenna system cannot achieve satisfactory acquisition     

Microstrip adaptive array antenna

A planar adaptive array of three microstrip patch antennas consisting of a central active microstrip antenna and two symmetrical lateral passive antennas is used to steer the beam. An electronic beam scanning of ±20° can be achieved with the control of reactive loads located into the lateral patches     

Tamed adaptive antenna array

The adaptive array under directionaily constrained minimization of power (DCMP) algorithm is improved by adding a "pseudonoise." It is effective to protect the desired signal from cancellation or distortion in such cases as 1) where a coherent interference is incident, or 2) where the desired signal direction for the constraint contains some pointing error, or 3) when the desired signal is broad band, The optimum amount of pseudonoise to be added is also discussed and its formula is given. This system is named "tamed adaptive antenna" since its killing capability is somewhat moderated so as not to hurt the desired signal.     

An adaptive array in a spread-spectrum communication system

This paper describes the integration of an LMS adaptive array into a pseudonoise (PN) coded biphase modulated communication system. The paper explains how these systems my be combined and presents a systems overview of the interaction between the adaptive array and the signaling waveform. An experimental system is described, and typical performance results are presented. The hybrid system requires only very modest spectrum-spreading ratios, such as 5:1, for the adaptive array to null interference. The combined system provides the interference protection of the adaptive array during the code timing acquisition phase as well as after code lockup.     

A protocol for adaptive transmission in direct-sequence spread-spectrum packet radio networks

Wireless communication channels may change greatly from one transmission to the next, due to variations in propagation loss and interference. The use of fixed transmission parameters for such channels results in wasted energy when channel conditions are good. Adaptation of the power, code rate, and symbol rate reduces energy consumption and interference caused to other systems. Such adaptation requires information about the characteristics of the channel, which is more difficult to obtain in a packet radio network (PRN) or other mobile ad hoc network than in a typical cellular communication system. We develop methods for providing partial information about the channel state from three statistics that are derived by different subsystems in the receiving terminals of a direct-sequence spread-spectrum PRN. We present and evaluate a protocol that uses this information to adapt the transmission parameters in response to changes in interference and propagation conditions in the network. The performance of the new adaptive-transmission protocol is compared with a system with fixed transmission parameters and with an adaptive protocol that is furnished with perfect knowledge of the channel state at the completion of each transmission.     

Throughput increase in packet forwarding engines using adaptive block-selection scheme

We propose a new approach for using the block-selection scheme to increase the search throughput in a multi-block TCAM-based packet forwarding engine. While the existing methods try to counter and forcibly balance the inherent bias of the Internet traffic, our method takes advantage of it hence improving flexibility of table management and scalability towards high rates of change in traffic bias. This approach also offers higher throughput than the current art.     

窄波束全向接收的自适应天线阵研究

为了实现对短波频段全空域低信噪比信号的良好接收,本文提出了一种窄波束全向接收的知适应天线阵。文中首先简要回顾了自适应天线阵的基本理论,并针地短波波段频程较宽的特点,提出了一种接收信号频率决定阵元间距的阵元间距动态可变组阵方案,文末通过计算机仿真比较了该方案实现的窄波束全向接收自适应天线阵和常规短波天线阵的性能。     

Planar antenna array control with genetic algorithms and adaptive array theory

Planar arrays are successfully employed in radar and communication systems in order to allow three-dimensional-scanning. In the framework of real-time control of antenna arrays, starting from the Applebaum theory, this paper presents an approach based on a customized genetic algorithm aimed at adaptively eliminating interfering signals. Successful applications are presented in order to evaluate the effectiveness of the proposed approach in term of performances and computational complexity.     

An adaptive antenna array under directional constraint

The concept of the adaptive system working on the principle of minimizing the output power under the constrained response to specified directions is introduced. Theoretical analysis and computer simulation experiments on this directional constraint system are also presented. By comparison with the computer experiment, the differential equation approach is shown to be useful to predict the behavior of the system. The effect of error in setting the constraint direction is investigated, and it is demonstrated that the system is not very sensitive to such error. This margin can be made even greater by adopting the double directional constraints system. This system is also analyzed theoretically and experimented by computer simulation. History of output power components during adaptation is shown, which enables the comprehension of the behavior of the system.     

Throughput of strongly interfering slow frequency-hopping networks

With the increasing use of short-range wireless devices for high-data-rate communication in the shared frequency bands, the level of interference can be expected to increase. The ability to estimate the performance degradation of these devices due to increasing interference is, therefore, important. In this paper, the throughput of devices that perform frequency hops after each transmitted packet in order to achieve diversity is investigated. The system model allows for an analysis of systems where packets of variable durations are used, and the throughput derivation is based on the assumption that collisions result in a total loss of the data in the colliding packets. The resulting expression for the throughput is given as a function of the number of frequency channels used for frequency hopping, the number of interfering networks, the durations of the packet types available, and the probability of networks selecting a certain packet type for transmission. An approximation of the exact expression for the throughput is also derived, and the results are applied to an example system consisting of Bluetooth piconets.     

Optimum beamforming for pre-FFT OFDM adaptive antenna array

It is well known that orthogonal frequency-division multiplexing (OFDM) is robust to frequency-selective fading in wireless channels due to the exploitation of a guard interval that is inserted at the beginning of each OFDM symbol. However, once delayed signals beyond the guard interval are introduced in a channel with a large delay spread, intersymbol interference causes a severe degradation in the transmission performance. In this paper, we propose a novel pre-fast Fourier transform (FFT) OFDM adaptive antenna array, which requires only one FFT processor at a receiver, for suppressing such delayed signals. We derive the optimum weight set for beamformers based on the maximum signal-to-noise-and-interference power ratio (Max-SNIR) and the minimum mean square error (mmse) criteria, respectively. In addition, we propose a novel mmse-criterion-based commutative optimization scheme, which is more robust to the estimation error of the channel state information. Furthermore, we show the equivalence between the Max-SNIR-criterion-based scheme and the proposed commutative optimization scheme. Computer simulation results show its good performance even in channels where directions of arrival of arriving waves are randomly determined.     

Co-channel interference canceller using CMA adaptive array antenna

A co-channel interference canceller that uses a constant modulus algorithm (CMA) adaptive array antenna is presented. In this canceller, when a CMA adaptive array antenna captures a co-channel interference signal, a replica of this signal is generated and then eliminated from a received signal, thus a desired signal can be obtained. Computer simulation results show that the proposed canceller can effectively reduce co-channel interference     

A lens antenna array with adaptive optical processing

This paper presents an X band smart antenna array in which adaptive processing of the received signals is performed by dynamic holographic optical circuitry. The optical circuitry adaptively extracts the principal component of the received signal space, that is the strongest first-order independent temporal component of the ensemble of received signals. The adaptive receiver system can be used, for example, to mitigate multipath interference effects and can separate one received signal from another even though their power spectra may entirely overlap. A prototype two-channel system is designed to fit in a standard-size briefcase and consume less than 50 W of power. The input to the system are modulated waves with a carrier in X band and the output is an electronic demodulated signal. Three major components of this system are described in detail: (1) the quasi-optical lens antenna array front end with angle-of-arrival preprocessing and downconversion, (2) the two-channel electrooptic modulation and optical carrier suppression stage, and (3) the smart optical processor (auto-tuning filter). Component and end-to-end system measurements give quantitative indicators for the usefulness of optical processing in wireless communications     

An adaptive ARQ scheme with packet combining for time varyingchannels

The dependence of the efficiency of hybrid type-II ARQ schemes on the packet size in the context of a simple packet combining scheme is discussed. A simple algorithm for adopting the optimum packet size according to the channel bit error rate (BER) is presented. Also, a very simple method of estimating the channel BER is provided     

天线罩对自适应阵列天线性能的影响分析

建立了可受变化干扰源激发的阵列天线和天线罩系统分析模型,分别通过阵列天线接收到的干扰源的辐射功率以及阵列榆入矩阵的相关矩阵特征值分布两种方案,对天线罩的影响效应进行了评估.结果表明,天线罩的存在对阵列天线的自适应性能明显降低.采用物理光学法分析了天线罩存在时天线阵的方向图,并利用闪烁瓣的观点对其结论进行了解释.     

抗干扰自适应天线阵波束形成零陷算法

针对传统的功率倒置算法应用在卫星信号接收时,不能在有用信号方向上形成主波束,不具有提高信噪比的功能,提出一种新的波束形成零陷算法,提高了该功能。通过软件仿真验证,结果表明提出的新算法克服了功率倒置算法在有用信号方向不能形成主波束的不足,不仅能在干扰方向形成更深的＂零陷＂,而且具有更高的输出信噪比。     

Throughput performance of SAW-ARQ protocol with adaptive packetlength in mobile packet data transmission

In a noncellular or large cell-size mobile radio communication system, log-normal shadowing as well as Rayleigh fading becomes the predominant source of system degradation. This paper proposes an efficient stop and wait automatic repeat request (SAW-ARQ) protocol with adaptive packet length to provide reliable mobile data packet transmission. The adaptive SAW-ARQ protocol controls the transmitting packet length according to the time-varying channel condition estimated with the number of ACK (acknowledgment packet) and NACK (negative-acknowledgment packet). Computer simulation results show that the proposed protocol can achieve high throughput and reduce the number of retransmission effectively for slow and fast Rayleigh fading/log-normal shadowing conditions