Performance analysis of Markov modulated 1-persistent CSMA/CA protocols with exponential backoff scheduling

In this paper, we investigate some cross-layer cooperative strategies for cognitive Time-Division Multiple-Access relay channels with bursty arrivals. The proposed schemes adopt an advanced physical (PHY) layer cooperation and an “intelligent” cognitive network-layer cooperation in order to improve the stable throughput region of the system. In contrast to previously reported work, where relaying is only enabled on periods of source silence, here, we incorporate a Dynamic Decode-and-Forward (DDF) policy which allows relaying assistance also during the source transmission. The enhancement of cognitive relaying with DDF provides more cooperative opportunities which results in faster emptying of the user queues and higher stable throughput compared to the conventional approaches. In addition to this PHY-layer relaying, the cognitive cooperation is supported by an adaptive/non-adaptive superposition scheme which allows the relay node to simultaneously forward packets from different users. We demonstrate that superposition can further increase the transmission opportunities and significantly improve the stable throughput region. The proposed schemes are studied from a networking perspective and their advantages are shown through both theoretical results and computer simulations.

We consider the stability of N-model systems that consist of two customer classes and two server pools. Servers in one of the pools can serve both classes, but those in the other pool can serve only one of the classes. The standard fluid models in general are not sufficient to establish the stability region of these systems under static priority policies. Therefore, we use a novel and a general approach to augment the fluid model equations based on induced Markov chains. Using this new approach, we establish the stability region of these systems under a static priority rule with thresholds when the service and interarrival times have phase-type distributions. We show that, in certain cases, the stability region depends on the distributions of the service and interarrival times (beyond their mean), on the number of servers in the system, and on the threshold value. We also show that it is possible to expand the stability region in these systems by increasing the variability of the service times (without changing their mean) while keeping the other parameters fixed. The extension of our results to parallel server systems and general service time distributions is also discussed.

Energy harvesting (EH) is a promising technology for realizing energy-efficient wireless networks. In this paper, we utilize the ambient RF energy, particularly interference from neighboring transmissions, to replenish the batteries of the EH enabled nodes. However, RF energy harvesting imposes new challenges into the analysis of wireless networks. Our objective in this paper is to investigate the performance of a slotted Aloha random access wireless network consisting of two types of nodes, namely Type I, which has unlimited energy supply and Type II, which is solely powered by an RF energy harvesting circuit. The transmissions of a Type I node are recycled by a Type II node to replenish its battery. We characterize an inner bound on the stable throughput region under half-duplex and full-duplex energy harvesting paradigms as well as for the finite capacity battery case. Additionally, we analyze the case where RF energy harvesting serves as a backup for an unlimited energy source. We present numerical results that validate our analytical results, and demonstrate their utility for the analysis of the exact system.

Currently, most Landsat satellites are deployed in the low earth orbit (LEO) to obtain high resolution data of the Earth surface and atmosphere. However, the return channels of LEO satellites are unstable and discontinuous intrinsically resulting from the high orbital velocity, long revisit interval and limited ranges of ground-based radar receivers. Space-based information networks (SBIN), in which data can be delivered by the cooperative transmission of relay satellites, can greatly expand the spatial transport connection ranges of LEO satellites. While different types of these relay satellites deployed in orbits of different altitudes represent distinctive performances when they are participating in forwarding. In this chapter, we consider the cooperative mechanism of relay satellites deployed in the geosynchronous orbit (GEO) and LEO according to their different transport performances and orbital characteristics. To take full advantage of the transmission resource of different kinds of cooperative relays, we propose a multiple access and bandwidth resource allocation strategy for GEO relay, in which the relay can receive and transmit simultaneously according to channel characteristics of space-based systems. Moreover, a time slot allocation strategy which is based on the slotted time division multiple access is introduced for the system with LEO relays. Based on the queueing theoretic formulation, the stability of the proposed systems and protocols is analyzed and the maximum stable throughput region is derived as well, which provide the guidance for the design of the system optimal control. Simulation results exhibit multiple factors that affect the stable throughput and verify the theoretical analysis.KeywordsSpace-based Information NetworksCooperative CommunicationResource AllocationQueueing TheoryStability Analysis

The impact of capture effect on the IEEE 802.11 networks with different transmission speeds is investigated in this paper. A new Markov chain model considering capture effect for the binary exponential back-off scheme in the MAC layer has been proposed in the first time. Based on the new Markov chain model, a new throughput model is proposed, and then the impact of capture effect on throughput has been analyzed in the condition of different transmission speeds. The performance analysis shows that, in the RTS/CTS scheme, the improvement throughput of high speed networks caused by capture effect is more than that of low speed networks.

Maximizing the stable throughput of heterogeneous nodes under airtime fairness in a CSMA environment

This paper studies the performance of Non-persistent CSMA protocols using K-Exponential Backoff collision resolution algorithms. We model it as a multi-queue single-server system with infinite buffer size and consider the service time distribution of Head-of-line packets as a Markov Chain to get the network throughput as well as the stability. The stable region of retransmission factor q is illustrated in examples of Exponential Backoff (K=∞). For throughput stability, we show the stabilized throughput can be obtained in networks even for infinite nodes.

Standard power control techniques are not deployable in Aloha networks due to lack of central controlling entity and/or inefficiency of such algorithms in large networks with bursty traffic. To handle this problem in practice, simple transmission power selection algorithms are used for ranging and/or combating harsh channel conditions. In such algorithms, the transmission power is steadily increased by an amount called power step, until the packet is successfully transmitted. Noting that ranging is the major concern of this approach, small power steps are ideal for its operation. However, as we will show in this paper, using small power steps with this algorithm causes a throughput collapse in large networks, when capture effect is considered in the analysis. In the asymptotic case, the throughput of this algorithm will be less than the throughput of the Binary Exponential Backoff (BEB) scheme which only uses a single power level. To show these results, we will present an asymptotic analysis of this algorithm with small power steps in an ideal communication channel where BEB is used as the retransmission scheme. In order to strengthen the results, we will prove the existence of better choices for power steps by extending the analysis for large power steps and we will show that properly chosen large power steps guarantee higher throughputs and require approximately the same average powers as small power steps. Our analysis and simulation results show that small power steps should be avoided with the mentioned power selection algorithm except probably for ranging and once the ranging process is complete, larger power steps should be used to exploit the capture effect.

In wireless personal area network, a channel is shared by a number of nodes. Therefore packet collision may take place and it degrades the throughput performance. To improve the throughput of slotted CSMA/CA in IEEE 802.15.4, we propose knowledge-based exponential backoff (KEB) scheme for enhancing channel utilization solely based on the locally available channel state information. In addition, we propose an analytical model of slotted channel access mechanism with finite retry and derive the theoretical throughput limit. In simulation experiments, we show that the existing MAC scheme, binary exponential backoff (BEB) scheme, operates very far from the theoretical limits due to increased time for negotiating channel access. Also performance results indicate that KEB shows significant improvement in throughput performance over BEB with the knowledge of the network status.

Existing packet-mode schedulers will result in long waiting time for short control packets in IP networks. To overcome this problem, this paper proposes a packet-mode practical scheduling algorithm called short-packets-first (SPF). With uniform Poisson arrival process and low to medium offered load, we prove that SPF can reduce the average packet waiting time for overall packets by greatly lowering the average packet waiting time for short packets. Moreover, simulations under a real traffic model demonstrate that SPF performs very well compared with other packet-mode and cell-mode scheduling algorithms, especially for short packets under heavy offered load..

A stochastic scheduling algorithm for precedence constrained tasks on Grid

This work investigates a remote status updating system where the transmission process is modeled as a multi-stream M/G/1/1 non-preemptive system. We derive the closed-form expression of the average AoI of each stream in a heterogeneous case, where the distributions of service time are different for streams. To obtain more insights, we apply the results in a homogeneous system, where the service time distributions are identical, and find that preemption of packets would not always lead to the reduction of AoI, especially when the variance coefficient of the service time is small. We further optimize the generation rate to minimize the sum of average AoI. The results in heterogeneous cases show that given the same average service time for all streams, a higher generation rate should be allocated to the stream with a small service time variance. For the homogeneous cases with different AoI urgency weights for each stream, a higher generation rate should be reserved for the stream with more urgent AoI requirements for timeliness improvement. Besides, a lower bound on sum of average AoI is also provided, which only depends on the service rate and the number of data streams in homogeneous systems. Numerical results validate our theoretical analysis.

Performance implications of very large service-time variances

It is well known that queues with exponentially distributed service times have the smallest Shannon capacity among all single-server queues with the same service rate. In this paper, we study the capacity of timing channels in which the service time distributions have bounded support, i.e., Bounded Service Timing Channels (BSTC). We derive an upper bound and two lower bounds on the capacity of such timing channels. The tightness of these bounds is investigated analytically as well as via simulations. We find that the uniform BSTC serves a role for BSTCs that is similar to what the exponential service timing channel does for the case of timing channels with unbounded service time distributions. That is, when the length of the support interval is small, the uniform BSTC has the smallest capacity among all BSTCs.

Algorithm for computing the queue length distribution at various time epochs in DMAP/G(1, a, b)/1/N queue with batch-size-dependent service time