Simpler Throughput Analysis of CDMA/Unslotted ALOHA Radio Networks with Variable Message Length Based on M/M/inf Queueing Model

We propose a new and exact method to analyze the throughput of the DS CDMA unslotted ALOHA system with variable number of fixed-length packets. The proposed scheme requires two-dimensional summation while the previous requires four. Thus the proposed analysis scheme is of less computational complexity. The proposed analysis result is also slightly closer to the simulation result than the previous analysis result is, especially in heavier offered load.     

Performance Analysis of Wireless Networks Over Rayleigh Fading Channel

Employing a cross-layer approach, the explicit relationships between the fading channel characteristics and the significant teletraffic variables as well as the performance metrics in wireless network evaluation are formulated. In particular, the channel holding time, handoff probability, handoff call arrival rate, call blocking probability, call completion probability, and forced termination probability are developed, taking into account the carrier frequency (or equivalently wavelength), maximum Doppler frequency, and fade margin. In addition, the set of formulas are derived with the generalized assumptions for the call holding time and cell residence time. The analytical model has been validated by the simulation with the conventional exponential model and, additionally, the relaxed models for call holding time and cell residence time, e.g., Erlang and hyper-Erlang. The comparison demonstrates that the traditional result without considering the fading channel characteristics leads to substantially overestimated call blocking probability and call completion probability. The methodology presented in this paper provides a feasible manner for the wireless network cross-layer design and optimization.     

An Approximate Analysis of Delay for Fixed and Variable Length Packets in an Unslotted ALOHA Channel

In this paper we compute the mean delay for an unslotted ALOHA random access channel for both fixed and variable length packets. The analysis is based on the concept of a user cycle and obtains steady state results. When the channel is "stable", the results seem quite accurate. The input parameters to the model are the number of users, the mean think time, and mean retransmission time. The model yields total traffic, throughput and delay but only the latter is emphasized here. Because of the steady state nature of the analysis, no information is obtained on stability. The results are verified by simulation.     

Adaptive Detection and Channel Estimation for Dual-Rate DS/CDMA Networks in Frequency-Selective Fading

In a Dual-Rate DS/CDMA system, a primary, ``high rate' network coexists witha secondary one with a slower rate. Demultiplexinghigh-rate users typically entails, at the receiverend, a time-varying processing ifVariable Spreading Length (VSL), Variable Chip Rate (VCR), or Variable ChipRate with Frequency Shift (VCRFS) modulation formats are used, and Rake-Typedemodulation schemes if the chip rate exceeds the channel coherence bandwidth.The realization of fully adaptive receivers thus requires the joint estimationof(a) the interference to be suppressed, whether from other users ofthe high-ratenetwork or from the secondary network, and (b) the channel tap weights.The solutionthat we consider here assumes that,for each user, a pilot signal is transmitted alongwith the information-bearing signal, whether in a time-multiplexed orin a parallel-channel format. We consider both the uplink and the downlink channel and, based on a Minimum-Mean-Square-Error(MMSE) optimality criterion, we have come up with a fully adaptive system,capable on the one handof tracking, through a cyclic Recursive-Least-Squares typealgorithm, the periodically time-varying variations of theinterference subspace andon the other of updating the channel tap weights estimates.A thorough performance assessment is presented, and the convergence dynamicsof the system are derived through computer simulations in order to assessthe impact of the prior uncertainty with regard to the other-users signaturesand thechannel tap weights realizations.     

Throughput performance of slotted DS/CDMA ALOHA with packetcombining over generalised fading channels

A time diversity automatic repeat-request (ARQ) scheme is investigated for a slotted narrowband DS/CDMA wireless data network over a Nakagami fading channel. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA at the expense of a slight increase in implementation complexity     

CDMA无线网络中的突发数据业务延时分析

本文对CDMA无线网络中的突发数据业务的延时性能进行了分析。与以往的工作相比，本文假设数据流量模型为Markov到达过程(而非简单的泊松过程)，并从理论上对系统延时性能进行了分析。本文首先为数据子系统建立了完整的Markov模型，并运用矩阵几何技术求解该Markov链；然后计算数据包延时的概率分布函数(而非简单的平均延时)。仿真结果表明系统建模和延时分析方法是正确的。     

On the Throughput of an ALOHA Channel with Variable Length Packets

In this paper we give a new simple expression for the probability of successful transmission on an infinite population ALOHA channel with variable length packets. Expressions for the throughput and the probability density of the packet length on the channel are derived in a straightforward way along with the best and the worst length densities.     

Adaptive Retransmission Diversity with Packet Combining for Slotted DS/CDMA Packet Radio Networks

A time diversity automatic repeat-request (ARQ) scheme is investigated for slotted random access direct-sequence code-division multiaccess (DS/CDMA ALOHA) wireless packet radio networks on multipath Rayleigh fading channels. The receiver retains and processes all the retransmissions of a single data block (packet) using predetection diversity combining, instead of discarding those which are detected in error. This effectively improves the system throughput and delay characteristics especially at small values of signal-to-noise ratio (SNR) per bit. A simple and practical selection combining rule is proposed, which lends itself to a low-complexity receiver structure and specifically suitable for high data rate transmissions. Owing to the stochastic nature of the multiple access interference, the new maximum output selection diversity (MO/SD) system yields superior performance in comparison to the traditional maximum SNR selection diversity (SNR/SD) model. The bit error rate performance, throughput and the average number of transmissions required to transmit a packet successfully with and without forward error correction (FEC) are evaluated. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA without incurring a substantial penalty in terms of cost or complexity.     

Throughput Analysis of Adaptive Modulated STS-based CDMA System Over Multipath Fading Channel

The throughput enhancement of Space-Time Spreading (STS)-based Code Division Multiple Access (CDMA) system is investigated in this paper. Adaptive Modulation is utilized to improve the data throughput of the system in multipath Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the Bit Error Rate (BER) performance and the output Signal-to-Interference-and-Noise Ratio (SINR) of the RAKE receiver-assisted STS-based CDMA system for QAM data, considering multipath and multiuser interferences. The other contribution of the paper is deriving a new closed form expression for computing the throughput enhancement and the BER performance of the adaptive modulated STS-based CDMA system over multipath fading channel. Simulation and analytical results demonstrate that using adaptive modulation method in this system improves the average throughput by keeping the BER performance at a target level. The novel expressions presented in this paper bring out the facility of evaluation of the proposed system with no need to more calculations and simulations.

Packet Delay in UAV Wireless Networks Under Non-saturated Traffic and Channel Fading Conditions

In this paper, we conduct a statistical analysis for the packet delay in a wireless network of unmanned aerial vehicles (UAVs) under non-saturated traffic and channel fading conditions. Each UAV runs the distributed coordination function of IEEE 802.11 at the medium access control layer, and all UAVs are one-hop neighbors. A pair of UAVs can communicate over the lossy wireless channel of a fixed data rate. A non-saturated traffic condition is used. By modeling each node UAV as a standard queueing system (i.e., $M/M/1$ or $M/G/1$ queue), we derive the mean packet delay under the non-saturated traffic condition. Numerical and simulation results show that the mean packet delay derived based on $M/M/1$ queue is accurate for UAV wireless networks under the non-saturated traffic condition and with an independent packet error rate. It is observed that the mean packet delay increases with either the number of UAVs in the network or the packet generation rate. More important, existing results in the literature, based on the saturated traffic condition (i.e., packets are always supplied for transmission), tend to overestimate by a large amount the mean packet delay for networks with non-saturated traffic. In the second part of this paper, we apply simulation data to analysis of the probability distribution function of the packet delay when the packet error rate equals zero. Using a distribution fitting tool, we observe that the packet delay can be well approximated by the sum of a deterministic delay, which corresponds to the time period during which the UAV senses the medium and is able to perform a successful transmission, and a random delay, which follows a Gamma distribution function.     

Throughput Analysis of Adaptive Slotted-ALOHA CDMA over a Multipath Fading Channel with Capture Effects

This paper aims to investigate the performance of adaptive slotted-ALOHA direct sequence code division multiple access (DS-CDMA) over a multipath fading channel with capture effects. The transmission rate control algorithm has been proposed in response to current traffic conditions. Furthermore, since the performance of S-ALOHA DS CDMA shall decrease in the presence of fading channel environments the capture effect technique has been subsequently proposed. Accordingly, the effectiveness of the throughputs relies on the value of capture probability and the probability of packets being successfully sent. Research indicates that the throughputs of S-ALOHA DS-CDMA (using the following transmission rates: v b/s, 2v b/s, 4v b/s) with capture effects and adaptive S-ALOHA DS-CDMA with capture effects shall increase inverse proportionally to the length of bits (B), the value of different number of paths L _p, as well as capture ratios Q.     

瑞利衰落信道Jakes模型的研究与性能分析

无线衰落信道是无线通信系统的重要组成部分,因此信道建模和仿真对于无线通信系统的开发具有极其重要的作用。文章通过对平坦瑞利(Rayleigh)衰落信道进行仿真建模研究,给出了瑞利衰落信道可以采用Jakes模型进行建模的分析结果。     

Throughput analysis of DS/SSMA unslotted ALOHA system with fixedpacket length

Throughput analysis of direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA with fixed packet length is presented. As the levels of multi-user interference fluctuate during the packet transmission, we calculate the packet error probability and the throughput by considering not only the number of overlapped packets but also the amount of time overlap. On the assumption that packet generation is Poisson, the system can be thought as the queueing system M/D/∞. With Gaussian approximation of multi-user interference, we obtain the throughput as the function of the number of chips in a bit, the packet length, and the offered load of the system. We also analyze the channel load sensing protocol (CLSP), and obtain the optimum threshold of CLSP     

Handoff Performance in Wireless Mobile Networks with Unreliable Fading Channel

Handoff is an indispensable operation in wireless networks to guarantee continuous, effective, and resilient services during a Mobile Station (MS) mobility. Handoff counting, handoff rate, and handoff probability are important metrics to characterize the handoff performance. Handoff counting defines the number of handoff operations during an active call connection. Handoff rate specifies the expected number of handoff operations during an active call, or equivalently, the average handoff counting. Handoff probability refers to the probability that an MS will perform a handoff before call completion. In the literature, the fading channel is not incorporated in deriving these metrics. In addition, the teletraffic parameters are usually simplified into exponentially distributed variables for the sake of analytical tractability. In this paper, we derive the formulas for these metrics over Rayleigh fading. In particular, the results can demonstrate the explicit relationship between the handoff metrics and the physical layer characteristics, e.g., carrier frequency, maximum Doppler frequency, and fade margin. Furthermore, the formulas are developed with the generalized teletraffic parameters. Numerical examples are presented to demonstrate the impact of physical layer on the handoff metrics. The techniques and the results are significant for both deploying practical wireless networks and evaluating the system resilience over an unreliable radio channel.     

平坦瑞利衰落信道中多用户检测器的性能分析

在采用传统信号检测方式的CDMA系统中,多址干扰限制了系统的容量和性能.本文分析了在精确估计信道的情况下平坦瑞利衰落信道中的判决反馈和线性解相关两种多用户检测器的性能,主要分析了二者的误码率性能和抗远近效应的能力,并首次分析了衰落信道中的渐近多用户效率(AME).结果表明,判决反馈多用户检测器的渐近多用户效率优于线性解相关检测器.     

A Flexible Analytical Markov Model for the IEEE 802.15.4 Unslotted Mechanism in Single-Hop Hierarchical Wireless Networks with Hidden Nodes

In this paper, an analytical Markov model (AMM) for the IEEE 802.15.4 unslotted CSMA/CA mechanism in single-hop hierarchical wireless networks (HWN) with hidden nodes is presented. The proposed AMM is flexible enough to operate both in saturation and non-saturation regions. Its prediction accuracy was compared with that of two well known models in the literature as well as with simulation results obtained by using the network simulator (NS)-2. The comparison has indicated that when $r$ , which denotes the maximum frame retransmission attempts, is set to zero and one-tier single-hop HWN with no hidden nodes is considered, the proposed AMM predicts the simulation results with a very small mean absolute percentage error (MAPE). Similarly, when a single-hop HWN with two tiers and hidden nodes is considered, then only one of the two well known models is applicable and the proposed AMM predictions present the smallest MAPE over the simulation results. Lastly, in the case of four-tier single-hop HWN with hidden nodes, only the proposed AMM is applicable and it predicts the simulation results at a respectable level of accuracy. When $r>0$ and the unslotted mechanism continues by incrementing the number of retransmission attempt by one upon channel access failure events, only the proposed AMM can imitate the behavior of the NS-2 IEEE 802.15.4 module.     

Performance Analysis of Cooperative Diversity Wireless Networks over Nakagami-m Fading Channel

This letter analyzes the performance of cooperative diversity wireless networks using amplify-and-forward relaying over independent, non-identical, Nakagami-m fading channels. The error rate and the outage probability are determined using the moment generating function (MGF) of the total signal-to-noise-ratio (SNR) at the destination. Since it is hard to find a closed form for the probability density function (PDF) of the total SNR, we use an approximate value instead. We first derive the PDF and the MGF of the approximate value of the total SNR. Then, the MGF is used to determine the error rate and the outage probability. We also use simulation to verify the analytical results. Results show that the derived error rate and outage probability are tight lower bounds particularly at medium and high SNR     

Performance Comparison of Multiuser Detectors with Channel Estimation for Flat Rayleigh Fading CDMA Channels

Since the conventional detector performs poorly in bandwidth-efficient Code Division Multiple Access (CDMA) systems, several multiuser detectors were proposed recently. In this paper, we compare performance of the decorrelator, the two-stage detector (2S), and the decision-feedback detector (DF) for the Rayleigh flat fading synchronous CDMA channel. First, assuming perfect channel estimation, we show that the 2S and the DF have much lower bit error rate (BER) than the decorrelator, and prove that the ideal DF has unity Asymptotic Multiuser Efficiency (AME). Furthermore, we evaluate the performance of these detectors in the presence of channel mismatch. We model the Rayleigh flat fading channel as the second order Auto Regressive (AR) process, and use the Kalman filter as the channel estimator. The lower bounds on the BER of the 2S and the DF are derived. The analytical results and the simulations show that the estimation error accumulated due to cancellation of other users limits the performance of the 2S and the DF. Therefore, in the presence of channel mismatch, the decorrelator offers comparable or even better performance than more complex decision-feedback and two-stage detectors.     

噪声功率时变的瑞利慢衰落信道有限状态Markov模型

对衰落信道的准确建模对于自适应无线通信、认知无线电等应用中的信道预测具有重要意义。针对噪声功率存在时变特性的无线通信应用环境,提出了一种新的瑞利衰落信道的有限状态Markov模型。通过将接收信号的衰落电平进行离散化处理,建立了衰落电平区间与Markov模型状态之间的一一对应关系,推导了门限电平与状态转移概率和状态分布概率之间的理论关系式,并在此基础上提出了一种易于实现的基于等概率的信道模型。理论分析与仿真结果表明:在噪声功率时变的条件下,已有的基于信噪比的模型失效,而该模型能准确反映信道的衰落特性,最大相对误差小于7%。      

Rayleigh衰落信道下相位误差对相干跳频多址接入MRC分集接收系统的影响

该文研究一种新型混合通信系统,即基于多载波BPSK调制的相干跳频多址接入(FH/CDMA)系统,接收端采用最大比合并(MRC)分集接收。由于采用相位调制,接收端载波相位恢复的精确度直接影响系统性能。假设载波同步通过一阶锁相环电路(PLL)完成,由于载波恢复环路中存在高斯噪声,当系统达到同步稳态时不可避免地会产生稳态相位误差。该文采用Beaulieu级数方法和特征函数方法,得到理想相干和部分相干(存在稳态相位误差)时FH/CDMA系统误码率性能表达式。仿真结果表明,相对于单用户系统,多用户系统误码率受到相位误差影响较小,误码率性能降低的主要因素是多用户干扰。MRC分集接收能提高系统抵抗相位误差和信道衰落的能力。