Packet admission control in a direct-sequence spread-spectrum LEOsatellite communications network

The effect of multiple-access interference on the throughput performance of a direct-sequence spread-spectrum low-Earth-orbiting satellite communications network is discussed. To recognize the effect of interference when their sources are either inside or outside the service area of a satellite, we develop a stochastic model for the location of users. We show that the effect of interference on the performance degradation from users with large propagation distance to their connecting satellites is the dominant factor. Hence, to improve the performance of the system, we propose a method in which the transmissions of packets are controlled according to their distances to connecting satellites as well as the traffic distribution     

Transmit selection in spatial multiplexing systems

In this letter we solve the transmit antenna selection problem for a zero forcing spatial multiplexing system with knowledge of the channel statistics at the transmitter. We show through Wishart matrix analysis that the signal-to-noise ratio on the kth stream is a weighted Chi-squared variable with the weight equal to-the kth diagonal entry of the inverted transmit correlation matrix. We use this result to develop selection algorithms for two cases-maximizing ergodic capacity and minimizing the average probability of error. Monte Carlo simulations illustrate potential performance improvements.     

Transmit diversity in 3G CDMA systems

Transmit diversity (TD) is one of the key contributing technologies to defining the ITU endorsed 3G systems W-CDMA and cdma2000. Spatial diversity is introduced into the signal by transmitting through multiple antennas. The antennas are spaced far enough apart that the signals emanating from them can be assumed to undergo independent fading. In addition to diversity gain, antenna gain can also be incorporated through channel state feedback. This leads to the categorization of TD methods into open loop and closed loop methods. Several methods of transmit diversity in the forward link have been either under consideration or adopted for the various 3G standards. This article describes the concept of transmit diversity and explains the features of selected TD techniques     

A rural messaging network using spread ALOHA access with VSAT satellite terminals

This paper underlines the characteristics of a proposed mesh network for messaging applications in C-band using spread ALOHA protocol. A simulation program has been developed for studying the behaviour of large networks employing ALOHA access or its variants. It is a simulation for a large population network consisting of thin traffic small messaging terminals (SMTs), and models an infinite user population which employs fixed length data packets. The program is general and allows for the study of either the mesh or the star connected networks. Certain variations of the classical ALOHA schemes, such as slotted ALOHA (with user selectable slot length) and spread ALOHA, have been studied for network operation. In the case of spread ALOHA access, the program can be run with Barker, pseudorandom, Ping Fai Li or any other spreading sequence. The results of the simulation have confirmed the utility of spread ALOHA as an access protocol for MESCOMNET. It has identified an operating point on the throughput-traffic curve at which spread advantage is available for power reduction of SMTs.     

Simple input control procedure for slotted ALOHA

A simple input control procedure using channel feedback information is presented. The steady state performance characteristics of slotted ALOHA with input control are evaluated. In particular, it is shown that the slotted ALOHA with input control has a unique stable steady state with high throughput.<>     

Transmit power allocation in SVD equalized multicarrier systems

Transmit power allocation and bit loading are important issues in multicarrier transmission via frequency-selective channels. In this contribution, transmit power allocation schemes for BER performance improvements in a wavelet-based multicarrier transmission system with Singular-value decomposition (SVD) equalization are investigated under the constraint of a limited total transmit power. Contrary to other contributions, here the transmit power is not only adapted to the subchannels but rather to each symbol of the SVD equalized data block individually. Based on the Lagrange multiplier method, different solutions are investigated, whereby optimal but complex and suboptimal but efficient schemes are analyzed. In order to obtain numerical results, the schemes are exemplarily applicated to a time-invariant and to different time-variant channels with Rayleigh and Rice distributed path coefficients, respectively.     

Accurate tag estimation for dynamic framed-slotted ALOHA in RFID systems

Dynamic Framed-Slotted ALOHA (DFSA) is one of the most popular algorithms to resolve tag collision in RFID systems. In DFSA, it is widely known that the optimal performance is achieved when the frame size is equal to the number of tags. So, a reader dynamically adjusts the next frame size according to the current number of tags. Thus it is important to estimate the number of tags accurately. In this paper, we propose a novel tag estimation method for DFSA. We compare the performance of the proposed method with those of other existing methods. And, simulation results show that our scheme improves the accuracy of tag estimation and the speed of tag identification.     

Selfish users in energy constrained ALOHA systems with power capture

We consider a slotted ALOHA setting where backlogged, energy-constrained users selfishly select the probability with which they transmit packets. Packets are successfully received, even in case of collision, if the signal to interference plus noise ratio at the access point exceeds some threshold (power capture). The user problem of finding appropriate transmission probabilities is formulated as a static non-cooperative game and the performance limits for stationary and mobile scenarios are determined. The equilibrium analyses show that for stationary scenarios, users with high pathgains share the channel fairly while others never transmit. In the mobile case users utilize a binary strategy where they try to monopolize the channel when their pathgain exceeds some threshold that depends on system parameters (number of users, transmission costs, etc.). Otherwise they shut their transmitters off. Compared to traditional nondiscriminatory distributed multiaccess protocols the operating points achieved by selfish users generally increase sum-utility although this comes at the expense of larger user performance variations.     

Transmit power adaptation for multiuser OFDM systems

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.     

Performance of type-II hybrid ARQ in a shadowed Rician fading LEOsatellite-based spread slotted ALOHA communication channel

Type-II hybrid ARQ is applied to a shadowed Rician fading low earth-orbiting (LEO) satellite-based spread slotted ALOHA communication channel. In particular it is found to be effective in combating heavy shadowing     

A simple and versatile decentralized control for slotted ALOHA,reservation ALOHA, and local area networks

A simple, decentralized control for reducing the delays and stabilizing random-access channels is presented. The control, which is based on a computationally efficient recursive implementation of the minimum mean-squared error (MMSE) predictor of the channel backlog, applies to slotted ALOHA, to reservation ALOHA, and to local area networks (LANs) with carrier-sense multiple access (CSMA) or CSMA with collision detection (CSMA/CD) protocols. The MMSE predictor controller (MMSE-PC) can stabilize the slotted ALOHA for all traffic rates not exceeding e^-1, and it can achieve stable throughput arbitrarily close to one with finite delays in the reservation ALOHA and in LANs. Extensive simulation has shown that the MMSE-PC performs extremely well in all three random-access environments. For the implementation of the MMSE-PC in LANs, synchronization of transmissions is not required but it is required in slotted ALOHA and reservation ALOHA. The MMSE-PC has been implemented in hardware and tested in asynchronous LANs     

Congestion control in wireless links based on selective delivery of erroneous packets

Traditionally, congestion control in packet networks is performed by reducing the transmission rate when congestion is detected, in order to cut down the traffic that overwhelms the capacity of the network. However, if the bottleneck is a wireless link, congestion is often cumulated because of retransmissions derived from bit errors. In this case, it might be beneficial to allow delivery of partly corrupted packets up to the application layer instead of reducing the transmission rate. This would decrease the number of retransmissions in the link layer and therefore relieve congestion, but at the cost of bit errors appearing in the packet payload. In this paper, we study a congestion control mechanism for streaming applications that combines traditional congestion control with selective link layer partial checksumming allowing bit errors in the less sensitive parts of data. We have compared the performance of the proposed mechanism against traditional congestion control with a simulation study. The results show that the proposed approach can improve the overall performance both by increasing the throughput over the wireless and improving the video quality in terms of peak signal-to-noise ratio (PSNR) by up to 8 dB, depending on the error conditions and the content.     

Transmit power allocation for BER performance improvement in multicarrier systems

In a multicarrier system, transmit power allocation over different subchannels is an effective means of improving the performance. We develop the optimal transmit power allocation scheme to improve bit-error rate (BER) performance in a multicarrier system with diversity reception. A simple suboptimal scheme is also derived from the optimal one, and an asymptotic case referred to as the equal-signal-to-noise ratio scheme is discussed. Numerical results show that the optimal and suboptimal power allocation schemes significantly outperform the equal power allocation scheme. The effects of the modulation level, the number of receiving antennas, and the number of subchannels on the BER performance are also investigated.     

Spread slotted ALOHA techniques for mobile and personal satellitecommunication systems

The objective is to develop a general theoretical framework for a class of spread slotted ALOHA (SSA) systems. The contributions include: modeling of a generalized spread slotted ALOHA (SSA) system; derivation of computationally efficient closed form expressions for the SSA system throughput and delay taking into account receiver complexity; and presentation of numerical results to validate the derivation as well as to substantiate the superior performance of the proposed scheme. The results demonstrate that SSA schemes offer significant improvements in terms of throughput, delay, and network stability against excessive loading at very acceptable levels of receiver implementation complexities. Also, SSA is found to be highly robust to errors in the time of arrivals and eliminates the need for a guard time     

The ALOHA systems in shadowed mobile radio channels with slow orfast fading

The influence of receiver capture on the performance of the ALOHA protocol in the presence of shadowing is investigated. The combined effect of Rayleigh fading, log-normal shadowing, and spatial distribution of mobile users is also studied. It is shown that shadowing is similar to fading and near/far phenomena, in that it makes the capture effect possible and provides ALOHA systems with substantial improvements in throughput. It is also confirmed that the superimposed Rayleigh fading, log-normal shadowing, and spatial distribution can further enhance the capture effect, resulting in a ALOHA system with higher throughput. The fast fading effect in ALOHA systems is explored. A very fast fading case is examined which may be considered to be equivalent to interleaving in slow fading. It is found that an ALOHA system under fast fading conditions also benefits from the capture results in higher throughput     

Mechanism of eliminating UDP redundancy control packets in OpenFlow network

Large numbers of redundant control packets produced by connectionless UDP flows may engender serious influence over the performance of the SDN controllers and networks.The endangerment of the redundant control packets for the performance of SDN controllers by testing and modeling was firstly analyzed,and then a basic solution to solve the problem was formed.Therefore,a preinstalling flow-tables & filtering redundant packets (PFFR) mechanism was proposed.By preinstalling flow tables,PFFR limited the initial rate of control packets in UDP flows,and through installing flow tables according to paths and utilizing the redundant packets filtering algorithm,PFFR eliminated redundant packets rapidly.A prototype system based on PFFR was implemented and tested.The experimental results prove that the PFFR mechanism can effectively improve the performance of the controller.     

Slotted ALOHA mobile packet communication systems with multiuserdetection in a base station

A slotted ALOHA mobile packet communication system utilizing the multiuser detection (MD) is proposed for system enhancement. MD can be easily implemented by using an adaptive interference canceler. Employment of MD enables simultaneous detection of several collision packets, which results in system enhancement increasing the throughput and shortening the average delay time. Detailed theoretical analysis and computer simulation with a two-user simultaneous detection in a single-cell environment shows: (1) the throughput increases twice as much as the conventional one and (2) the average delay time decreases by 50%     

Throughput and energy consumption tradeoffs in path-gain-basedconstrained power control in ALOHA networks

Power control has been proposed as a tool to improve the performance of packet radio systems in terms of increased throughput and battery lifetime for mobile terminals. In this paper we study the tradeoff between transmitter dynamic range, average energy consumption and the achievable throughput for a simple path-gain based power control algorithm in a slotted ALOHA radio system. The results show that increasing the throughput significantly by means of transmitter power control requires only a very moderate increase in energy consumption and transmitter dynamic range     

基于小波分组调制的多码多载波跳频通信系统分析

介绍了一种将多码调制、小波分组调制以及跳频技术相结合的高效通信方式——小波分组多码多载波跳频通信系统.推导了系统在Nakagami-m衰落信道下的误码性能,给出了用户数、小波分组类型、小波分组滤波器长度以及跳频点数与系统性能之间的关系.理论分析与数值计算表明:增加小波分组滤波器长度可以有效降低系统误码率,但不同小波分组类型对系统性能的影响不大;跳频技术的引入不仅增加了系统的安全性,同时也可有效降低多址干扰对系统性能的影响,提高了系统的可靠性.