TCP throughput and fairness performance in presence of delay spikes in wireless networks

Wireless networks are likely to experience delay spikes exceeding several times the typical round‐trip‐time figures, which can cause spurious timeouts that lead to unnecessary retransmissions and reduction of the TCP sender's transmission rate, and thus, the throughput of the TCP is degraded. This paper presents some research results on the effect of delay spikes caused by handover on TCP performance by using three different mobility models. It is shown that the throughput of TCP connection over a single bottleneck link is decreased in the presence of delay spikes significantly. Furthermore, it is shown that the fairness feature of TCP is also severely affected in the presence of delay spikes. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of TCP-like congestion control on the throughput of multicast groups

We study the impact of random queueing delays stemming from traffic variability on the performance of a multicast session. With a simple analytical model, we analyze the throughput degradation within a multicast (one-to-many) tree under TCP-like congestion and flow control. We use the (max,plus) formalism together with methods based on stochastic comparison (association and convex ordering) and on the theory of extremes to prove various properties of the throughput. We first prove that the throughput predicted by a deterministic model is systematically optimistic. In the presence of light-tailed random delays, we show that the throughput decreases according to the inverse of the logarithm of the number of receivers. We find analytically an upper and a lower bound for the throughput degradation. Within these bounds, we characterize the degradation which is obtained for various tree topologies. In particular, we observe that a class of trees commonly found in IP multicast sessions is significantly more sensitive to traffic variability than other topologies.     

Bounds on the delay distribution of window random-access algorithms

A method for analyzing the delay distribution of window random-access algorithms is presented. The window size is allowed to vary during the operation of the algorithm. It is shown that the quantities of interest in the computation of the delay distribution can be related to the solution of appropriate infinite systems of linear equations. Once the constants and the coefficients of the unknowns of the system are determined, bounds on the solution can be developed by applying previously developed methodologies. The method is applied to the delay distribution analysis of the Capetanakis window random-access algorithm and the part-and-try algorithm, both under binary feedback     

Global stability of Internet congestion controllers with heterogeneous delays

In this paper, we study the problem of designing globally stable, scalable congestion control algorithms for the Internet. Prior work has primarily used linear stability as the criterion for such a design. Global stability has been studied only for single node, single source problems. Here, we obtain conditions for a general topology network accessed by sources with heterogeneous delays. We obtain a sufficient condition for global stability in terms of the increase/decrease parameters of the congestion control algorithm and the price functions used at the links.     

Bounds on recovery times of decision feedback equalizers

New upper and lower bounds on the mean recovery times of decision feedback equalizers are derived. The analysis uses an approach based on writing difference equations for conditional, state dependent, mean recovery times. This method is simple and leads to simple expressions for the bounds. Two different recovery times are investigated; the channel error recovery time and the startup, or reset, error recovery time. Quantitative relationships between the two recovery times are established. Both noiseless and noisy equalizer operating conditions are examined. The new upper bounds are several orders of magnitude smaller than previous upper bounds in some practical cases     

Bounds on the throughput gain of network coding in unicast and multicast wireless networks

Gupta and Kumar established that the per node throughput of ad hoc networks with multi-pair unicast traffic scales with an increasing number of nodes n as lambda(n) = ominus(1/radic(n log n)), thus indicating that performance does not scale well. However, Gupta and Kumar did not consider network coding and wireless broadcasting, which recent works suggest have the potential to significantly improve throughput. Here, we establish bounds on the improvement provided by such techniques. For random networks of any dimension under either the protocol or physical model that were introduced by Gupta and Kumar, we show that network coding and broadcasting lead to at most a constant factor improvement in per node throughput. For the protocol model, we provide bounds on this factor. We also establish bounds on the throughput benefit of network coding and broadcasting for multiple source multicast in random networks. Finally, for an arbitrary network deployment, we show that the coding benefit ratio is at most O(log n) for both the protocol and physical communication models. These results give guidance on the application space of network coding, and, more generally, indicate the difficulty in improving the scaling behavior of wireless networks without modification of the physical layer.     

Analysis of diversity schemes employing antenna selection in the presence of channel estimation errors and feedback delay

The main objective of this paper is to provide an extensive and complete examination on the effect of practical impairments such as channel estimation errors (CEEs) and feedback delay (FD) on the performance of diversity schemes over Nakagami‐m fading channels. Under erroneous channel estimation and outdated feedback cases, statistical expressions and several performance metrics related to the post‐processing signal‐to‐noise ratio (SNR) are derived for four different diversity schemes: transmit antenna selection (TAS)/orthogonal space–time block coding, TAS/maximal‐ratio transmission (MRT), MRT/receive antenna selection (RAS), and joint transmit and RAS. Exact analytical expressions for outage probability and average error rates of M‐ary modulations are derived in order to provide insightful perspectives on the capacity and error performance of diversity schemes that experience both CEE and FD. The asymptotic diversity order of the investigated diversity schemes are derived via a high‐SNR approximation approach. In order to assess the real‐world performance of the investigated diversity schemes and to observe their robustness or sensitivities in practical imperfections, various configurations are considered together with several performance comparisons. Also, Monte Carlo simulations are performed in order to validate the theoretical results. Copyright © 2014 John Wiley & Sons, Ltd.     

Bounds on maximum throughput for digital communications withfinite-precision and amplitude constraints

The problem of finding the maximum achievable data rate over a linear time-invariant channel is considered under constraints different from those typically assumed. The limiting factor is taken to be the accuracy with which the receiver can measure the channel output. More precisely, the following problem is considered. Given a channel with known impulse response h(t), a transmitter with an output amplitude constraint, and a receiver that can distinguish between two signals only if they are separated in amplitude at some time t ₀ by at least some small positive constant d, what is the maximum number of messages, N_max, that can be transmitted in a given time interval [0,T]? Lower bounds on N_max can be easily computed by constructing a particular set of inputs to the channel. The main result is an upper bound on N_max for arbitrary h(t). The upper bound depends on the spread of h(t), which is the maximum range of values the channel output may take at some time t₀>0 given that the output takes on a particular value α at time t=0. Numerical results are shown for different impulse responses, including two simulated telephone subscriber loop impulse responses     

Bounds on the probability of delay for queued vehicles waiting to merge

The delay encountered by vehicles waiting to merge is considered. Tight upper bounds on the probability of delay are derived based on a "gap acceptance" model. The general approach is developed, and specific results of some interest are presented.     

Effect of feedback delay on tracking-loop frequency acquisition

A general method for calculating the effect of feedback delay on second-order tracking-loop frequency-acquisition capability is presented. The method uses a power-series expansion of the delay differential equation and is extendable to discontinuous systems whenever a Fourier-series approximation discontinuity can be made.     

On the stability of MIMO EWMA run-to-run controllers with metrology delay

As production requirements in semiconductor manufacturing continue to escalate, it is rarely possible to perform quality measurements on a product before subsequent process operations are performed. This delay between manufacturing and metrology coupled with inaccurate process models can lead to closed-loop instabilities. This paper investigates the effect of metrology delay on the closed-loop stability of a multiple input-multiple output exponentially weighted moving average run-to-run controller. The generalized Routh-Hurwitz stability criteria are used to derive the necessary and sufficient conditions for stability. A sufficient condition for stability is then derived for systems with any length of metrology delay. This condition states that if all of the eigenvalues of a model-mismatch matrix fall inside a circle with unit radius and centered at {1,0} on the complex plane, then the closed-loop system is stable.     

Performance of TCP congestion control with explicit rate feedback

We consider a modification of TCP congestion control in which the congestion window is adapted to explicit bottleneck rate feedback; we call this RATCP (Rate Adaptive TCP). Our goal in this paper is to study and compare the performance of RATCP and TCP in various network scenarios with a view to understanding the possibilities and limits of providing better feedback to TCP than just implicit feedback via packet loss. To understand the dynamics of rate feedback and window control, we develop and analyze a model for a long-lived RATCP (and TCP) session that gets a time-varying rate on a bottleneck link. We also conduct experiments on a Linux based test-bed to study issues such as fairness, random losses, and randomly arriving short file transfers. We find that the analysis matches well with the results from the test-bed. For large file transfers, under low background load, ideal fair rate feedback improves the performance of TCP by 15%-20%. For small randomly arriving file transfers, though RATCP performs only slightly better than TCP it reduces losses and variability of throughputs across sessions. RATCP distinguishes between congestion and corruption losses, and ensures fairness for sessions with different round trip times sharing the bottleneck link. We believe that rate feedback mechanisms can be implemented using distributed flow control and recently proposed REM in which case, ECN bit itself can be used to provide the rate feedback.     

Collector signal delay in the presence of velocity overshoot

It is pointed out that the well-known expression equating the collector signal delay to one-half of the transit time through the base-collector depletion region is incorrect in the presence of a nonconstant carrier velocity, as occurs in the case of velocity overshoot. The correct expression yields a smaller signal delay than the conventional estimate for typical situations, further emphasizing the benefit of velocity overshoot in bipolar devices     

Effect of feedback delay error on adaptive digital predistortion

The effect of feedback delay error on adaptive digital predistortion is analysed, i.e. a linearisation technique for RF power amplifiers is developed. A method for calculating achievable adjacent channel interference level with certain delay error is presented. The method and results can be utilised in estimating the required accuracy for delay compensation     

On the design of feedback controllers to achieve prescribed initial value constraints

The problem of designing a stabilizing compensator for a control system to achieve prescribed initial value constraints ⁽ⁱ⁾(0⁺)=y_i is considered. Indeed, modulo certain technical conditions, such a compensator exists if and only if y_i=0;i= 0,1,...,rp +rt –2; whererp is the relative degree of the plant andrt is the relative degree of the system input. This theorem is derived and a complete parameterization of the set of compensators that achieve the prescribed design constraints is formulated.This research was supported in part by NSF Grant No. 921106.     

Transfer-function matrix of multivariable systems with cascade and feedback controllers

Simple expressions relating the closed-loop transfer function matrix to the cascade and feedback controller matrices are derived for a multivariable system. An important result is deduced which can simplify some of the design procedures of linear multivariable systems.     

Computer-aided design of suboptimal feedback controllers using optimal low-order models

The fully automated design of optimal or near-optimal feedback controllers is considered for least-pth and minimax cost functions. Solutions are obtained by approximating to the desired optimal output by a rational function. Optimal results for a 2nd-order model and suboptimal responses for the corresponding 7th-order linear system are used to illustrate the method.     

A definition of pulse delay in the presence of distortion

It is proposed that pulse delay, in the presence of distortion, be defined as ∫TX(T) dT/∫X(T) dT where X(T) = ∫|m1(t - T)|²|m2(t)|²dt is the cross correlation of the squares of the magnitudes of the transmitted and received pulse modulation. In the absence of the distortion, the definition reduces to the group delay.     

反馈延迟对光纤陀螺振动误差特性的影响

振动环境导致光纤陀螺引入较大的扰动。由于非线性因素,陀螺跟踪扰动的能力严重影响着其误差特性,而反馈延迟的存在使得跟踪性能变得更加复杂。因此,分析反馈延迟对于解决陀螺振动问题极其重要。首先,分析了非线性因素作用时不同反馈延迟对陀螺闭环跟踪能力和扰动稳定性的影响;然后在线性控制模型的基础上加入陀螺非线性环节,保证稳定裕度不变,仿真分析了不同反馈延迟下陀螺的跟踪性能以及输出。仿真及实验结果表明,减小反馈延迟,提高系统的跟踪扰动能力,可以减小非线性误差,改善振动环境下陀螺的输出性能。     

Effect of delay on uncertainty feedback systems (Corresp.)

Recent work on feedback communications systems is predicated on negligible delay in the forward and the feedback channels. The effect of delay on binary feedback systems of the type considered by Turin and Horstein is studied. Both sequential and nonsequential detection at the receiver are considered. It is shown that if the round-trip delay is small relative to the detection time at the receiver, uncertainty feedback offers considerable advantage. However, for the class of signals considered, the presence of delay precludes operation at channel capacity with zero probability of error.