Search

Carrier sense multiple access (CSMA) is widely used as a medium access control (MAC) in wireless networks due to its simplicity and distributed nature. This motivated researchers to find CSMA schemes that achieve throughput optimality. In 2008, it has been shown that a simple CSMA-type algorithm is able to achieve optimality in terms of throughput and has been given the name “adaptive” CSMA. Later, new technologies emerged where a prolonged battery life is crucial such as environment and industrial monitoring. This inspired the foundation of new CSMA-based MAC schemes, where links are allowed to transition into a sleep mode to reduce the power consumption. However, the throughput optimality of these schemes was not established. This paper, therefore, aims to find a new CSMA scheme that combines both throughput optimality and energy efficiency by adapting to the throughput and power consumption needs of each link. This is done by controlling operational parameters, such as back-off and sleeping timers, with the aim of optimizing a certain objective function. The resulting CSMA scheme is characterized by being asynchronous, completely distributed and being able to adapt to different power consumption profiles required by each link while still ensuring throughput optimality. The performance gain in terms of energy efficiency compared with the conventional adaptive CSMA scheme is demonstrated through computer simulations.

Future wireless networks will be characterized by users with heterogeneous requirements. Such users can require low-latency or minimum-throughput requirements. In addition, due to the limited-power budget of the mobile devices, a power-efficient scheduling scheme is required by the network. In this work, we cast a stochastic network optimization problem for minimizing the packet drop rate while guaranteeing a minimum throughput and taking into account the limited-power capabilities of the users.

International audience; In this paper, we investigate a network where N links contend for the channel using the well-known carrier sense multiple access scheme. By leveraging the notion of stochastic hybrid systems, we find: 1) a closed-form expression of the average age when links generate packets at will 2) an upperbound of the average age when packets arrive stochastically to each link. This upperbound is shown to be generally tight, and to be equal to the average age in certain scenarios. Armed with these expressions, we formulate the problem of minimizing the average age by calibrating the back-off time of each link. Interestingly, we show that the minimum average age is achieved for the same back-off time in both the sampling and stochastic arrivals scenarios. Then, by analyzing its structure, we convert the formulated optimization problem to an equivalent convex problem that we find its optimal solution. Insights on the interaction between links and numerical implementations of the optimized Carrier Sense Multiple Access (CSMA) scheme in an IEEE 802.11 environment are presented. Next, to further improve the performance of the optimized CSMA scheme, we propose a modification to it by giving each link the freedom to transition to SLEEP mode. The proposed approach provides a way to reduce the burden on the channel when possible. This leads, as will be shown in the paper, to an improvement in the performance of the network. Simulations results are then laid out to highlight the performance gain offered by our approach in comparison to the optimized standard CSMA scheme.

In this work, we combine the two notions of timely delivery of information in order to study their interplay; namely, deadline-constrained packet delivery due to latency constraints and freshness of information at the destination. More specifically, we consider a two-user multiple access setup with random access, in which user 1 is a wireless device with a queue and has external bursty traffic which is deadline-constrained, while user 2 monitors a sensor and transmits status updates to the destination. For this simple, yet meaningful setup, we provide analytical expressions for the throughput and drop probability of user 1, and an analytical expression for the average Age of Information (AoI) of user 2 monitoring the sensor. The relations reveal that there is a trade-off between the average AoI of user 2 and the drop rate of user 1: the lower the average AoI, the higher the drop rate, and vice versa. Simulations corroborate the validity of our theoretical results., Comment: Submitted to ITW

We consider a cognitive shared access scheme consisting of a high priority primary node and a low priority network with $N$ secondary nodes accessing the spectrum. Assuming bursty traffic at the primary node, saturated queues at the secondary nodes, and multipacket reception capabilities at the receivers, we derive analytical expressions of the time average age of information of the primary node and the throughput of the secondary nodes. We formulate two optimization problems, the first aiming to minimize the time average age of information of the primary node subject to an aggregate secondary throughput requirement. The second problem aims to maximize the aggregate secondary throughput of the network subject to a maximum time average staleness constraint. Our results provide guidelines for the design of a multiple access system with multipacket reception capabilities that fulfils both timeliness and throughput requirements.

In this paper, we examine a system where status updates are generated by a source and are forwarded in a First-Come-First-Served (FCFS) manner to the monitor. We consider the case where the server has other tasks to fulfill, a simple example being relaying the packets of another stream. Due to the server's necessity to go on vacations, the age process of the stream of interest becomes complicated to evaluate. By leveraging specific queuing theory tools, we provide a closed form of the average age for both streams which enables us to optimize the generation rate of packets belonging to each stream to achieve the minimum possible average age. The tools used can be further adopted to provide insights on more general multi-hop scenarios. Numerical results are provided to corroborate the theoretical findings and highlight the interaction between the two streams., 7 figures

Limited spectrum resources and the dramatic growth of high data rate applications have motivated opportunistic spectrum access exploiting the promising concept of cognitive networks. Although this concept has emerged primarily to enhance spectrum utilization and to allow the coexistence of heterogeneous network technologies, the importance of energy consumption imposes additional challenges, because energy consumption and communication performance can be at odds. In this article the approaches for energy efficient spectrum sensing and spectrum handoff, fundamental building blocks of cognitive networks, are investigated. The trade-offs between energy consumption and throughput, under local as well as under cooperative sensing, are characterized. We also discuss the additional factors that need to be investigated to achieve energy efficient cognitive operation under various application requirements.

In cognitive communication, dynamic sensing and opportunistic access enable secondary users to recognize and utilize the white spaces of the licensed bandwidth. Most present efforts focus on designing smarter channel sensing and access algorithms for secondary users, with the aim of optimizing the overall throughput and bandwidth utilization efficiency, under the condition of not interfering with primary users communication. However, as the transmissions of the primary users are inherently random and unpredictable, sensing and sharing spectrum with the primary users inevitably make the cognitive process of the secondary users complex and ineffective. In this paper, a non-uniform bandwidth allocation scheme is proposed that regularizes the primary users bandwidth occupancy pattern. The regularization is not designed to reshape the primary userss traffic, but to improve the sensing efficiency and throughput of the secondary users by optimizing the spectrum allocation. After the description of the new allocation scheme, we demonstrate its performance by theoretic analysis. Then we verify the validity of the non-uniform scheme with numerical simulations under non-fading and fading situations respectively. Through comparisons with the conventional uniform bandwidth allocation scheme, the non-uniform one shows higher sensing efficiency and better spectrum utilization due to lower sensing cost and reduced bandwidth loss. Funding Agencies|China Scholar Council (CSC) [201206155005]; Natural Science Foundation of Guangdong Province (CHINA) [S2011010004773]; NSF [CCF1420651]; ONR [N000141410107]; European FP7 Marie Curie IOF project [329313]

In this work, we study the benefit of partial relay cooperation. We consider a two-node system consisting of one source and one relay node transmitting information to a common destination. The source and the relay have external traffic and in addition, the relay is equipped with a flow controller to regulate the incoming traffic from the source node. The cooperation is performed at the network level. A collision channel with erasures is considered. We provide an exact characterization of the stability region of the system and we also prove that the system with partial cooperation is always better or at least equal to the system without the flow controller., Comment: Submitted for journal publication. arXiv admin note: text overlap with arXiv:1502.01134

Joint pushing and proactive caching holds the promise of significantly increasing the throughput and decreasing the latency of content-centric wireless networks. In this work, we import the pushing and caching mechanism into a traditional packet-oriented network for decreasing the average queueing delay. Since the decreasing relies on extra costs paid for the pushing-caching mechanism, such as the cost for the cache buffer's capacity, extra transmission power for pushing and so on, we mainly derive the average queueing delay under given cost in order to study the relationship between the average queueing delay and the extra cost. In order to do so, a metric named System State is defined to describe the transmitter and user's current buffer states. Based on the System State, a two-dimensional Markov Chain is built to derive the average queueing delay and the average hitting ratio. The results show us how much the average queueing delay can achieve when the cache buffer's capacity which usually has a high cost is given, and then provide us a method to make a tradeoff between the average queueing delay and the cost of the user's buffer size. Simulation results validate our conclusion.

This paper addresses fundamental issues in a shared channel where the users have different priority levels. In particular, we study a two-user cognitive shared channel consisting of a primary (higher-priority) and a secondary user, operating in the cognitive underlay fashion, but in a novel way where interference suffered by the primary user is compensated by requiring the secondary user to cooperatively relay some of the primary's packets. We start by analyzing the case of no node cooperation, where nodes transmit their own packets to their respective destinations. We then extend the analysis to a system in which the secondary node acts as a relay for the primary user, in addition to serving its own packets. Specifically, in the cognitive cooperation case, the secondary node forwards those packets to the primary destination that it receives successfully from the primary source. In such cognitive shared channels, a tradeoff arises in terms of activating the secondary along with the primary so that both transmissions may be successful, but with a lower probability, compared to the case of the secondary node staying idle when the primary user transmits. Results show the benefits of relaying for both the primary as well as the secondary nodes in terms of the stable-throughput region.

We study from a cross-layer perspective a cognitive network consisting of one primary user (PU) and one secondary user (SU). In contrast with the oversimplified collision channel model, we assume that simultaneous PU and SU transmissions are successful with some positive probability. We propose and analyze two access schemes at the SU aiming at maximizing its stable throughput while guaranteeing the stability of the PU's queue. These schemes exploit the SU's knowledge of the channel statistics and of the average arrival rate to the PU. In the first scheme, the SU accesses the channel at all slots with fixed probability p^* without sensing. In the second scheme, the SU accesses the channel with probabilities p_1^* and p_2^* when the PU is sensed to be idle and busy, respectively. The analysis shows that if simultaneous PU and SU transmissions are likely to be successful, schemes with no sensing outperform schemes with sensing. Otherwise, schemes with sensing are preferred. Therefore, using complex receivers capable of handling interference, alleviates the need of complex SU transmitters with strong sensing capabilities. We then extend the analysis to the case where the PU has an average delay constraint which is more restrictive than the stability constraint.

In this paper, we investigate the queue stability and throughput of a two-user cognitive radio system with multicast traffic. We study the impact of network-level cooperation, in which one of the nodes can relay the packets of the other user that are not received at the destinations. Under this approach, if a packet transmitted by the primary user is not successfully received by the destination set but is captured by the secondary source, then the secondary user assumes responsibility for completing the transmission of the packet; therefore, the primary releases it from its queue, enabling it to process the next packet. We demonstrate that the stability and throughput regions of this cooperative approach is larger than that of the noncooperative approach, which translates into a benefit for both users of this multicast system. Our system model allows for the possibility of multipacket reception (MPR), and the optimal transmission strategies for different levels of MPR capability are observed in our numerical results. In addition to achieving a larger stability region, our results show that cooperation can result in reduced average delay for both primary and secondary users.

We consider the problem of reliable spectrum sensing and opportunistic access on channels with stochastic traffic in batch processing systems such as network coding (NC). We show how a secondary user (SU) can leverage the structure induced by block-based NC on primary users' (PUs) channels to mitigate the effects of channel sensing errors and improve the detection of idle PU spectrum and the throughput. NC is known to improve the transmission efficiency and therefore when applied on a PU channel, it can extend the spectrum availability for the SUs. We refer to the additional gain of spectrum predictability from NC and show that under possible sensing errors the SU can more reliably detect the idle spectrum if the PUs' channels carry network-coded transmissions even when the channel utilization is fixed. We consider two different objectives at the SU. For quickest detection, the SU applies the Cumulative Summation (CUSUM) algorithm to detect idle slots on a PU channel and further improves the detection capability with the Viterbi algorithm, if the PU spectrum dynamics are known. For throughput maximization, the SU tracks the PU spectrum with the Partially Observable Markov Decision Process (POMDP) approach. Our results show that NC renders the spectrum more predictable, which can be used by the SUs to mitigate the effects of sensing errors and improve the throughput. We validate these results with real radio measurements taken in software-defined radio based wireless network tests.

We investigate the impact of a protocol-level cooperation idea in a wireless multiple-access system. By dynamically and opportunistically exploiting spatial diversity among the $N$ source users, a packet is delivered to the common destination through either a direct link or through cooperative relaying by intermediate source nodes that have a statistically better channel to the destination. The traffic burstiness at the source is taken into account, and the performance metrics of the stable throughput region and delay are evaluated for the case of packet-erasure channels. We consider conflict-free, work-conserving transmission policies as well as plain time-division multiple-access policy. We establish that the stable throughput regions under both classes of cooperative policies are the same, which strictly contain the stable throughput regions achieved without cooperation. Moreover, the optimal policy for minimizing the average delay among the class of all cooperative work-conserving policies is determined. Then, in the case of two users, the closed-form delay expressions are explicitly derived as well. Our results indicate that cooperation can significantly reduce delay for both users.

We study transmission strategies in a multiple-source, multiple-destination wireless network. Each source transmits packets that are intended for a particular destination. However, a transmitted packet can cause interference at other destinations. Our primary performance measure is throughput, which we define to be the average number of packets that are successfully received per intended destination per time slot. The sources are first divided into groups, based on the intended destination of their packets. In our parallel method, each group operates according to its own local protocol (e.g., TDMA), concurrently with and independently of the other groups. Our results show the impact of transmission schedules, channel fading, receiver noise, and other-user interference on network performance. We then show that, for given channel statistics and topology configurations, the network performance can be significantly improved when the groups in the network coordinate their transmissions according to an optimal schedule. Further, in many cases, even the use of randomly generated parallel schedules can provide considerably higher performance than traditional TDMA.

This paper compares scheduling and coding strategies for a multicast version of a classic downlink problem. We consider scheduling strategies where, in each time slot, a scheduler observes the lengths of all queues and the connectivities of all links and can transmit the head-of-the-line packet from a single queue. We juxtapose this to a coding strategy that is simply a form of classical random linear coding. We show that there are configurations for which the stable throughput region of the scheduling strategy is a strict subset of the corresponding throughput region of the coding strategy. This analysis is performed for both time-invariant and time-varying channels. The analysis is also performed both with and without accounting for the impact on throughput of including coding overhead symbols in each encoded packet. Additionally, we compare coding strategies that only code within individual queues against a coding strategy that codes across separate queues. The strategy that codes across queues simply sends packets from all queues to all receivers. As a result, this strategy sends many packets to unnecessary recipients. We show, surprisingly, that there are cases where the strategy that codes across queues can achieve the same throughput region achievable by coding within individual queues.

The problem of joint scheduling and rate control for multicast traffic in wireless networks is considered under the performance objectives of sum throughput maximization and proportional fairness. Our results are also valid for the special cases of unicast and broadcast traffic. First, the problem of maximizing the sum throughput of the network is studied and an optimal scheduling and rate control policy is obtained. Given the combinatorial complexity of providing an optimal policy, a simple, polynomial-time, suboptimal alternative scheme is introduced that restricts the space of scheduling and rate control decisions to operation one at a time or all together. The optimal policy to the problem of maximizing the sum throughput of the network with respect to this restricted action space is found. Next, the objective of proportional fairness is considered. Under this restricted space of actions, the resulting scheduling and rate control policy is explicitly characterized analytically and the effects of the current channel conditions are incorporated into the scheduling decisions. Furthermore, it is shown that the policy under this restricted action space is of threshold type. Finally, our analytical results are verified through a set of numerical experiments.

We study cellular-like wireless networks in which the cells may overlap substantially, and a common channel is used for all cells. Thus, transmissions intended for one destination (or base station) can cause interference at neighboring destinations. We assume the use of a ?collision-channel? model, in which arbitrary communication and interference regions are associated with each destination. The interaction between such cells is best exemplified if the protocol of access in each cell is pure random access, i.e., Slotted Aloha. We derive a mathematical formula for the maximum achievable throughput for multiple-cell networks that satisfy a ?balance? condition, which is related to (but not as stringent as) symmetry. This formula implies that the throughput achieved in a cell is affected only by the degree of overlap with adjacent cells, i.e., a cell's throughput is not affected by transmissions that are outside of its interference region. Moreover, we show that, at the point of maximum throughput, the expected channel traffic is one packet per slot in each cell, an extension of the result obtained many years ago for single-destination networks.

The transmission of packets is considered from one source to multiple receivers over single-hop erasure channels. The objective is to evaluate the stability properties of different transmission schemes with and without network coding. First, the throughput limitation of retransmission schemes is discussed and the stability benefits are shown for randomly coded transmissions, which, however, need not optimize the stable throughput for finite coding field size and finite packet block size. Next, a dynamic scheme is introduced for distributing packets among virtual queues depending on the channel feedback and performing linear network coding based on the instantaneous queue contents. The difference of the maximum stable throughput from the min-cut rate is bounded as function of the order of erasure probabilities depending on the complexity allowed for network coding and queue management. This queue-based network coding scheme can asymptotically optimize the stable throughput to the max-flow min-cut bound, as the erasure probabilities go to zero. This is realized for a finite coding field size without accumulating packet blocks at the source to start network coding. The comparison of random and queue-based dynamic network coding with plain retransmissions opens up new questions regarding the tradeoffs of stable throughput, packet delay, overhead, and complexity.

Obtaining the stable throughput region of a wireless network, and a policy that achieves this throughput, has attracted the interest of the research community in the past years. A major simplifying assumption in this line of research has been to assume that the network control policy has full access to the current channel conditions at every time a decision is made. However, in practice one may only estimate the actual conditions of the wireless channel process, and hence the network control policy can at most have access to an estimate of the channel which can in fact be highly inaccurate. In this work we determine a stationary joint link activation and routing policy based on a weighted version of the "back-pressure" algorithm that maximizes the stable throughput region of time-varying wireless networks with multiple commodities by having access to only a possibly inaccurate estimate of the true channel state. We further show optimality of this policy within a broad class of stationary, non-stationary, and even anticipative policies under certain mild conditions. The only restriction is that policies in this class have no knowledge on the current true channel state, except what is available through its estimate.

In wireless networks, throughput optimization is an essential performance objective that cannot be adequately characterized by a single criterion (such as the minimum transmitted or sum-delivered throughput) and should be specified over all source-destination pairs as a rate region. For a simple and yet fundamental model of tandem networks, a cross-layer optimization framework is formulated to derive the maximum throughput region for saturated multicast traffic. The contents of network flows are specified through network coding (or plain routing) in network layer and the throughput rates are jointly optimized in medium access control layer over fixed set of conflict-free transmission schedules (or optimized over transmission probabilities in random access). If the network model incorporates bursty sources and allows packet queues to empty, the objective is to specify the stability region as the set of maximum throughput rates that can be sustained with finite packet delay. Dynamic queue management strategies are used to expand the stability region toward the maximum throughput region. Network coding improves throughput rates over plain routing and achieves the largest gains for broadcast communication and intermediate network sizes. Throughput optimization imposes fundamental tradeoffs with transmission and processing energy costs such that the throughput-optimal operation is not necessarily energy efficient.

In this paper we consider the two-user broadcast channel and we characterize its stable throughout region. We start the analysis by providing the stability region for the general case without any specific considerations on transmission and reception mechanisms. We also provide conditions for the stable throughput region to be convex. Subsequently, we consider the case where the transmitter uses superposition coding and we consider two special cases for the receivers. The first one is when both receivers treat interference as noise. The second is when the user with a better channel uses successive decoding and the other receiver treats interference as noise., Comment: Submitted for journal publication

This paper addresses network coding in wireless networks in conjunction with medium access control (MAC). It is known that coding over wired networks enables connections with rates that cannot be achieved by routing. However, the properties of wireless networks (e.g., omnidirectional transmissions, destructive interference, single transceiver per node, finite energy) modify the formulation of time-varying network coding in a way that reflects strong interactions with underlying MAC protocols and deviates from the classical approach used in wired network coding. To perform network coding over conflict-free transmission schedules, predetermined network realizations are separately activated by a time-division mechanism and the content of network flows is derived through network coding to optimize performance measures such as achievable throughput and energy costs. A systematic method is presented to construct linear wireless network codes and interactions with MAC schedules are discussed under wireless assumptions. Network coding is also extended to operate with arbitrary (random or scheduled access based) MAC protocols. Alternatively, conflict-free transmission schedules are jointly constructed with network codes by decomposing wireless networks into subtrees and employing graph coloring on simplified subtree graphs. Finally, network coding and plain routing are compared in terms of throughput, energy and delay performance under different MAC solutions.

In this paper, a novel cognitive multiple-access strategy in the presence of a cooperating relay is proposed. Exploiting an important phenomenon in wireless networks, source burstiness, the cognitive relay utilizes the periods of silence of the terminals to enable cooperation. Therefore, no extra channel resources are allocated for cooperation and the system encounters no bandwidth losses. Two protocols are developed to implement the proposed multiple-access strategy. The maximum stable throughput region and the delay performance of the proposed protocols are characterized. The results reveal that the proposed protocols provide significant performance gains over conventional relaying strategies such as selection and incremental relaying, specially at high spectral efficiency regimes. The rationale is that the lossless bandwidth property of the proposed protocols results in a graceful degradation in the maximum stable throughput with increasing the required rate of communication. On the other hand, conventional relaying strategies suffer from catastrophic performance degradation because of their inherent bandwidth inefficiency that results from allocating specific channel resources for cooperation at the relay. The analysis reveals that the throughput region of the proposed strategy is a subset of its maximum stable throughput region, which is different from random access, where both regions are conjectured to be identical.

In cognitive communication, dynamic sensing and opportunistic accessing enable secondary users to recognize and utilize the white spaces of the licensed bandwidth. Most present efforts focus on designing smarter channel sensing and access algorithms for secondary users to optimize the overall throughput and bandwidth utilization efficiency, without interfering with primary users' communication. However, the transmission of the primary users are basically random and unpredictable, which usually makes the cognitive process complex and ineffective. In this paper, a non-uniform bandwidth allocation scheme is proposed, in order to regularize primary users' bandwidth occupancy, which can in turn improve the sensing efficiency and throughput of the secondary users. The performance benefits are demonstrated analytically and verified by numerical simulations. In comparison to the conventional uniform bandwidth allocation scheme, the non-uniform scheme shows a higher sensing efficiency and spectrum utilization due to less bandwidth loss and lower sensing cost.

This paper studies finite-terminal random multiple access over the standard multipacket reception (MPR) channel. We characterize the relations among the throughput region of random multiple access, the capacity region of multiple access without code synchronization, and the stability region of ALOHA protocol. In the first part of the paper, we show that if the MPR channel is standard, the throughput region of random multiple access is coordinate convex. We then study the information capacity region of multiple access without code synchronization and feedback. Inner and outer bounds to the capacity region are derived. We show that both the inner and the outer bounds converge asymptotically to the throughput region. In the second part of the paper, we study the stability region of finite-terminal ALOHA multiple access. For a class of packet arrival distributions, we demonstrate that the stationary distribution of the queues possesses positive and strong positive correlation properties, which consequently yield an outer bound to the stability region. We also show the major challenge in obtaining the closure of the stability region is due to the lack of sensitivity analysis results with respect to the transmission probabilities. Particularly, if a conjectured "sensitivity monotonicity" property held for the stationary distribution of the queues, then equivalence between the closure of the stability region and the throughput region follows as a direct consequence, irrespective of the packet arrival distributions.

In this paper, we address the crosslayer problem of joint medium access control (MAC) and routing in ad hoc wireless networks from the novel perspective of stochastic dynamic games. As a starting point to understand the efficient modes of wireless network operation, we look at the basic problem of multi-hop communication over the simple topology of a single relay channel. A stochastic game is formulated for transmitter and relay nodes competing over collision channels to deliver packets to a common destination node using alternative paths. We rely on a reward mechanism to stimulate cooperation for packet forwarding and evaluate the conflicting multiple access and routing strategies of direct communication and relaying through a detailed foray into the questions of cooperation incentives, throughput, delay and energy-efficiency. Under the separate models of selfish and cooperative network operation, we study the interactions among the equilibrium strategies and present a detailed performance analysis based on multiple system parameters that involve the packet arrival rates, throughput rewards, delay and energy costs.

This paper extends our earlier results. We assume that the receiver has the capability of capturing multiple packets so long as the signal-to-interference-plus-noise ratio (SINR) of each packet is above a designed threshold T throughout its transmission period. We prove that, compared with a multiple-power-level system, the single-power-level system in which all nodes transmit at the maximum allowable power level achieves optimal throughput, under a condition that T exceeds the value 3.33. Given a minimum throughput requirement, under the same condition on T, the single-power-level system also achieves the maximum average packet capture probability as well as the optimum energy usage efficiency. If the multiple-power-level systems are constrained such that higher power levels always have shorter packet lengths, then the above results hold for T greater than 2.

Traditional voice and video-oriented networks such as the cellular and satellite networks are being increasingly used to carry data traffic. We endeavour to compare the downlink broadcast performance of the two architectures against each other on the basis of energy consumption, end-to-end delay and maximum stable throughput. The architectures are modelled as systems of Geo/G/1 queues. Queuing theory arguments and then sample-path based comparisons are used to show that the satellite architecture while being more energy-efficient has a higher delay and a lower maximum throughput. The variation of energy and delay with the total number of receiver nodes is also studied. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper, we introduce a cross-layer design framework to the multiple access problem in contention-based wireless ad hoc networks. The motivation for this study is twofold, limiting multiuser interference to increase single-hop throughput and reducing power consumption to prolong battery life. We focus on next neighbor transmissions where nodes are required to send information packets to their respective receivers subject to a constraint on the signal-to-interference-and-noise ratio. The multiple access problem is solved via two alternating phases, namely scheduling and power control. The scheduling algorithm is essential to coordinate the transmissions of independent users in order to eliminate strong levels of interference (e.g., self-interference) that cannot be overcome by power control. On the other hand, power control is executed in a distributed fashion to determine the admissible power vector, if one exists, that can be used by the scheduled users to satisfy their single-hop transmission requirements. This is done for two types of networks, namely time-division multiple-access (TDMA) and TDMA/code-division multiple-access wireless ad hoc networks.

The problem of jointly controlling the data rates and transmit powers of users, so as to maximize throughput in cellular direct-sequence code-division multiple-access (CDMA) networks is addressed. The multicode (MC)-CDMA system, where the processing gain of each code is fixed and high-rate users use multiple codes for transmission, and the variable gain (VG)-CDMA architectures are considered. The throughout maximization problem is formulated as a classical optimization problem, modeling the constraints arising from the data rate requirements and power budgets. Optimal strategies to maximize throughput in both systems are derived. While the MC-CDMA system has a simple optimal rate-power allocation algorithm, the VG-CDMA system has a more complex solution.

The effect of full-duplex cooperative relaying in a random access multiuser network is investigated here. First, we model the self-interference incurred due to full-duplex operation, assuming multi-packet reception capabilities for both the relay and the destination node. Traffic at the source nodes is considered saturated and the cooperative relay, which does not have packets of its own, stores a source packet that it receives successfully in its queue when the transmission to the destination has failed. We obtain analytical expressions for key performance metrics at the relay, such as arrival and service rates, stability conditions, and average queue length, as functions of the transmission probabilities, the self interference coefficient, and the links' outage probabilities. Furthermore, we study the impact of the relay node and the self-interference coefficient on the per-user and aggregate throughput, and the average delay per packet. We show that perfect self-interference cancelation plays a crucial role when the SINR threshold is small, since it may result to worse performance in throughput and delay comparing with the half-duplex case. This is because perfect self-interference cancelation can cause an unstable queue at the relay under some conditions., Accepted for publication in the IEEE Transactions on Wireless Communications

Multiple-power-level ALOHA has been proposed to take advantage of the capture phenomenon in order to improve the throughput of a multiple random access system. We study the effect of the use of multiple transmission power levels and of the corresponding packet lengths on the system throughput and energy efficiency. We prove that the single-power-level system in which all transmit at the maximum allowable power level achieves both optimal throughput and energy usage efficiency under a condition on the decodability threshold value.

In this work, we analyze the energy efficiency of a wireless link, while considering spectrum sharing, and cooperation. Insightful results are obtained using a simple network model, with two source-destination pairs. We assume that one source-destination pair has higher priority to use the spectrum, while the other pair is allowed to access the channel under power constraints. The proposed cooperative scheme, named Cooperative Underlay, allows simultaneous transmissions of both high- and low-priority users, and the low-priority user may be requested to act as a relay. We investigate the trade-off between energy efficiency and throughput for the high-priority user, with and without cooperation, and demonstrate the improvement in energy efficiency due to the use of cooperation. Additionally, we study the case that the relay node can transmit and receive simultaneously, and discuss the effect of self-interference in the energy efficiency, for different degrees of accuracy in interference cancellation.

In this paper, we investigate the effect of channel estimation on the performance of a secondary network in a cognitive radio system. We focus on estimating the sensing-channel from the primary source to the secondary source which helps in determining the reliability of the sensing decision. The channel is estimated opportunistically when the secondary source senses the primary source to be active. We show the improvement in the performance of the secondary system compared to the system with no channel estimation. We also compare the performance to the system in which the channel is estimated accurately at every time slot. We consider the performance criterion to be the consumed energy by the secondary system constrained by a required average data transmission rate for the secondary system and an allowable average failure probability for the primary system.

Priority based link-bandwidth partitioning is required to support wireless multimedia services, having diverse QoS (delay, throughput) requirements, in mobile ad hoc networks with multimedia nodes. A new class of service disciplines, termed “batch and prioritize” or BP admission control (AC), is proposed. The BP algorithms use the delay tolerance of applications to batch requests in time slots. Bandwidth assignment is made either at the end of the slot, or during the slot, on a priority basis. Analytical and simulation models are developed to quantify the performance of the BP schemes. The results are compared with those obtained for a First-Come-First-Served (FCFS) service discipline. The class of BP schemes trade off the delay and loss tolerance of applications to improve the net carried traffic on the link. Further, such schemes enable an easy implementation for adaptive prioritization, where the degree of precedence given to an application varies with offered load and the link capacity.

In error controlled packet reception, a packet is received only if its error probability can be kept below a predetermined level. Error probability control is achieved by posing a minimum signal to noise ratio (SNR) threshold with corresponding packet internal coding scheme, which upper-bounds the packet data rate. We first consider packet transmission over a single-user wireless fading channel with additive Gaussian noise. We derive the optimal SNR threshold that maximizes the communication throughput. We show under a set of generous conditions that the optimal SNR threshold in the low-SNR regime is proportional to the transmit power; the ratio depends neither on the packet internal coding scheme nor on the pre-determined error probability level. The result is then extended to packet multicasting where common information is transmitted to a group of receivers over fading channels.

In this work, we study the impact of a relay node to a network with a finite number of users-sources and a destination node. We assume that the users have saturated queues and the relay node does not have packets of its own; we have random access of the medium and the time is slotted. The relay node stores a source packet that it receives successfully in its queue when the transmission to the destination node has failed. The relay and the destination nodes have multi-packet reception capabilities. We obtain analytical equations for the characteristics of the relay's queue such as average queue length, stability conditions etc. We also study the throughput per user and the aggregate throughput for the network., Comment: Accepted for publication in Elsevier Computer Communications. arXiv admin note: substantial text overlap with arXiv:1105.0452

In this work, we investigate the queue stability of a two-user cognitive radio system with multicast traffic. We study the impact of network-level cooperation, in which one of the nodes can relay the packets of the other user that are not received at the destinations. Under this approach, if a packet transmitted by the primary user is not successfully received by the destination set but is captured by the secondary source, then the secondary user assumes responsibility for completing the transmission of the packet; therefore, the primary releases it from its queue, enabling it to process the next packet. We demonstrate that the stability region of this cooperative approach is larger than that of the noncooperative approach, which translates into a benefit for both users of this multicast system. Our system model allows for the possibility of multipacket reception, and the optimal transmission strategies for different levels of multipacket reception capability are observed in our numerical results.

We present a loop-free, distributed routing protocol for mobile packet radio networks. The protocol is intended for use in networks where the rate of topological change is not so fast as to make “flooding” the only possible routing method, but not so slow as to make one of the existing protocols for a nearly-static topology applicable. The routing algorithm adapts asynchronously in a distributed fashion to arbitrary changes in topology in the absence of global topological knowledge. The protocol's uniqueness stems from its ability to maintain source-initiated, loop-free multipath routing only to desired destinations with minimal overhead in a randomly varying topology. The protocol's performance, measured in terms of end-to-end packet delay and throughput, is compared with that of pure flooding and an alternative algorithm which is well-suited to the high-rate topological change environment envisioned here. For each protocol, emphasis is placed on examining how these performance measures vary as a function of the rate of topological changes, network topology, and message traffic level. The results indicate the new protocol generally outperforms the alternative protocol at all rates of change for heavy traffic conditions, whereas the opposite is true for light traffic. Both protocols significantly outperform flooding for all rates of change except at ultra-high rates where all algorithms collapse. The network topology, whether dense or sparsely connected, is not seen to be a major factor in the relative performance of the algorithms.

In this work, we model a random access multicast network with cooperative relaying and analyze its throughput. The network consists of two source nodes and two destination nodes, where both source nodes generate data that needs to be transmitted to both destination nodes. We study the possibility that one of the sources can also act as a cooperative relay by transmitting some of the packets originating at the other source node. It is assumed that the destination nodes are equipped with multiuser detectors, and hence are capable of successfully decoding transmissions from multiple source nodes simultaneously (multipacket reception). We derive the achievable service rates for packets originating at each node in a system with cooperative relaying, and thereby compute the overall throughput region. Further analysis of the system (stability region) is reserved for future work.

In this paper, we evaluate the benefits of using one user of a two-user random access system to relay traffic of the other user.

We study, from a network layer perspective, the effect of an Ad-Hoc secondary network with N nodes randomly accessing the spectrum licensed to a primary node during the idle slots of the primary user. If the sensing is perfect, then the secondary nodes do not interfere with the primary node and hence do not affect its stable throughput. In case of imperfect sensing, it is shown that if the primary user's arrival rate is less than some calculated finite value, cognitive nodes can employ any transmission power or probabilities without affecting the primary user's stability; otherwise, the secondary nodes should control their transmission parameters to reduce the interference on the primary. It is also shown that in contrast with the primary's maximum stable throughput which strictly decreases with increased sensing errors, the throughput of the secondary nodes might increase with sensing errors as more transmission opportunities become available to them. Finally, we explore the use of the secondary nodes as relays of the primary node's traffic to compensate for the interference they might cause. We introduce a relaying protocol based on distributed space-time coding that forces all the secondary nodes that are able to decode a primary's unsuccessful packet to relay that packet whenever the primary is idle. In this case, for appropriate modulation scheme and under perfect sensing, it is shown that the more secondary nodes in the system, the better for the primary user in terms of his stable throughput. Meanwhile, the secondary nodes might benefit from relaying by having access to a larger number of idle slots due to the increase of the service rate of the primary. For the case of a single secondary node, the proposed relaying protocol guarantees that either both the primary and the secondary benefit from relaying or none of them does., 37 Pages with 7 figures