OFDMA femtocell is a promising technology to improve indoor cellular network coverage cost-effectively. Large-scale deployment of femtocells in the urban area is expected to be realized in the near future. However, inter-femtocell interference significantly limits the achievable throughput of an OFDMA femtocell system, which calls for interference management tailored for femtocell networks. A typical approach to mitigate inter-femtocell interference is known as resource isolation, which aims at assigning non-overlapping resources to interfering femtocells. One of the main challenges for interference mitigation in femtocell networks is that end consumers often install the femtocells. Very limited information about the femtocells is available, making it hard to decipher the inter-femtocell interference. Previous studies either take time to resolve collisions online or adopt a conservative approach to identify interferers. Although the latter approach avoids wasting time on resolving collisions, it may result in resource underutilization. In this paper, we propose an efficient method to identify inter-femtocell interference by analyzing the received patterns observed by mobile stations. We conducted experiments on GNU Radio/USRP to demonstrate that the proposed interference identification method can successfully identify real interferers while excluding non-interfering femtocells from suspect femtocells. Based on the proposed interference identification, we propose a weighted vertex-coloring based resource assignment algorithm to allocate resources with better fairness and higher throughput.

In cognitive radio (CR) networks, control channels are needed for secondary users (SUs) to perform certain handshaking procedures to negotiate communication parameters and to exchange control messages. The process of SUs attempting to meet on a channel is known as rendezvous. Previous studies either use a dedicated control channel or adopt channel-hopping schemes to rendezvous. However, it is not realistic to designate a preselected control channel in licensed spectrum bands and the time consumption for channel hopping grows drastically. Moreover, a requirement of previous studies is that a common channel must be available across the whole network at all SUs. Such a channel might not exist and the SUs cannot form an effective CR network. This paper proposes an efficient approach for guaranteed NC-OFDM-based Control Channel (NCCC) establishment by utilizing subcarrier pulses. Simulation results show that the time needed for establishing control channel is lower than that of channel hopping schemes. Additionally, the proposed approach is able to establish NCCC even if there is no common channel in the CR network. NC-OFDM-based control interfaces improve the control channel establishment rate even when the interface can only access the spectrum bandwidth that is equal to one channel.

The pursuit of high speed wireless communication is pushing wireless networks toward wider channels. Narrowband interference and frequency- selective fading significantly impact the performance of broadband wireless communication. To cope with the harsh channel conditions, orthogonal frequency division multiplexing (OFDM), which divides a band of spectrum into numerous subcarriers that carry data in parallel, is widely adopted. Because subcarriers experience different channel qualities in a wide band of spectrum, different modulation schemes may be used for different subcarriers. This paper exploits the modulation scheme diversity as rich information to assist data transmission. In this paper, we propose encoding each modulation scheme with a bit pattern that occurs frequently when allocating data bits to subcarriers. If the bits to be allocated on a subcarrier match the bit pattern defined on the subcarrier, the receiver is informed by the transmitter through subcarrier nulling. Because the bit pattern usually consists of more bits than that can be represented by a modulation symbol in low-density modulation schemes, the total transmission time is shortened. Further, detecting a null subcarrier is more reliable than decoding a modulation symbol. Therefore, the known bits represented by the selected bit patterns help improve decoding performance. Through experiments on USRP1 we show that the throughput is increased due to shortened transmission time and reduced bit error rates.

To support applications that demand high-speed wireless communication, the fifth generation (5G) Wi-Fi increases the channel bonding from 40 MHz in 802.11n to 80, and even 160 MHz under certain conditions in 802.11ac. However, inefficiency and unfairness issues arise when devices that use different channel widths coexist in a contention domain. In this paper, we propose a dynamic channel bonding (DyB) protocol in which a node is allowed to start a transmission as long as there are some idle narrow channels and it gradually increases the channel width during transmission whenever new narrow channels become available. To enable communication over uncertain channels, a convolution method is introduced to achieve fast spectrum agreement between the transmitter and the receiver. In addition, DyB considers the severe contention in a wide band of spectrum. A compound preamble is designed to make collisions detectable in the frequency domain and a parallel bitwise arbitration is designed to quickly resolve the collisions in the time domain. We implement and evaluate the DyB through both the GNU Radio/USRP platform and ns-2 simulations. Experimental results and simulations show that DyB well addresses the inefficiency and unfairness issues caused by heterogeneous radio coexistence.

Duty cycling improves energy efficiency but limits throughput and introduces significant end-to-end delay in wireless sensor networks. In this article, we present a traffic-adaptive synchronous MAC protocol (TAS-MAC), which is a high-throughput, low-delay MAC protocol tailored for low power consumption. It achieves high throughput by adapting time division multiple access (TDMA) to a novel traffic-adaptive allocation mechanism that assigns time slots only to nodes located on active routes. TAS-MAC reduces the end-to-end delay by notifying all nodes on active routes of incoming traffic in advance. These nodes will claim time slots for data transmission and forward a packet through multiple hops in a cycle. The desirable traffic-adaptive feature is achieved by decomposing traffic notification and data-transmission scheduling into two phases, specializing their duties and improving their efficiency, respectively. Simulation results and experiments on TelosB motes demonstrate that the two-phase design significantly improves the throughput of current synchronous MAC protocols and achieves the similar low delay of slot-stealing-assisted TDMA with much lower power consumption.

To improve spectrum utilization, cognitive radio (CR) is introduced to detect and exploit available spectrum resources autonomously. The flexible spectrum use imposes special challenges on broadcast because different CR devices may detect different available spectrum fragments at different locations. The sender and the receivers have to agree on spectrum fragments that will be used for broadcast. There may not exist a common spectrum fragment that is available to all receivers. Most existing work assumes that a device works only in a single channel and thus the sender has to broadcast multiple times in different channels to reach all receivers. The broadcast problem is studied as a channel rendezvous and minimum latency scheduling problem. Recent spectrum-agile designs have enabled a device to utilize partially occupied spectrum. We thus view a wideband channel as an aggregation of multiple narrow channels that can be evaluated independently. A Spectrum Fragment Agile Broadcast (SFAB) scheme is introduced in this paper to support efficient broadcast on fragmented spectrum. It aims at achieving spectrum agreement between the transmitter and the receivers efficiently and maximizing the channel width used for broadcast regardless of the spectrum availability differences at receivers. We validate the effectiveness of SFAB through implementation on the GNU Radio / USRP platform and use ns-2 simulations to evaluate the performance in large deployments.

Event-driven wireless sensor networks (WSNs) usually operate under light traffic load. However, when an event is detected, a large number of packets may be generated. A MAC protocol designed for this kind of WSNs should be able to swiftly adapt to the two conditions. Most WSN MAC protocols are optimized for light traffic for the energy efficiency consideration. In this paper, we propose a novel receiver-centric MAC protocol called RC-MAC that seamlessly integrates duty cycling and receiver-centric scheduling, providing high throughput without sacrificing the energy efficiency. To handle bursty traffic triggered by an event, RC-MAC takes advantage of the underlying data gathering tree structure of WSNs and the multichannel technique supported by current IEEE 802.15.4 RF transceivers to assist scheduling of medium access. The throughput is improved in two phases with receiver-centric medium access scheduling and distributed channel assignment. First, on a data gathering tree, a receiver is able to coordinate the medium access of multiple senders so as to reduce collisions and achieve high throughput. Second, different receivers coordinate their senders in different channels and the throughput is further improved by allowing parallel data gathering. Observing packet processing time on low cost sensor nodes, we design a scheduling pattern that ensures fairness between source nodes without sacrificing the throughput. We evaluate the performance of our RC-MAC through measurements of an implementation in TinyOS on TelosB motes and extensive ns-2 simulations. Compared with contention-based and scheduling-based MAC protocols, we show that the throughput and the fairness under heavy traffic load are significantly improved by the receiver-centric scheduling. Due to the high throughput, the energy efficiency is also improved.

Inter-femtocell interference significantly limits the achievable throughput of an OFDMA femtocell system, which calls for interference management tailored for femtocell net-works. A typical approach to mitigate inter-femtocell interference is known as resource isolation, which aims at assigning nonoverlapping resources to interfering femtocells. One of the main challenges for interference mitigation in femtocell networks is that the femtocells are often installed by end-consumers without any pre-planning. Very limited information about the femtocells is available, making it hard to decipher the inter-femtocell interference. In this paper, we propose an efficient method to identify inter-femtocell interference by analyzing the received patterns observed by mobile stations. We conducted experiments to demonstrate that the proposed interference identification method can successfully identify real interferers while excluding non-interfering femtocells from suspicious interferers. Based on the proposed interference identification, we propose a weighted vertex-coloring based resource assignment algorithm to allocate resources with better fairness and achieve higher throughput.

Cognitive radio appears as a promising technology to allocate wireless spectrum between licensed and unlicensed users in an efficient way. When unlicensed users opportunistically utilize spectrum holes, prediction models that infer the availability of spectrum holes can help to improve the spectrum extraction rate and reduce the collision rate. In this paper, a spectrum occupancy prediction model based on Partial Periodic Pattern Mining (PPPM) is introduced. The mining aims at identifying frequent spectrum occupancy patterns that are hidden in the spectrum usage of a channel. The mined frequent patterns are then used to predict future channel states (i.e., busy or idle). Based on the prediction, unlicensed users are able to utilize spectrum holes aggressively without introducing significant interference to licensed users. PPPM outperforms traditional Frequent Pattern Mining (FPM) by considering real patterns that do not repeat perfectly due to noise, sensing errors, and irregular behaviors. Using real-world Wi-Fi and personal communication service (PCS) activities, we show a significant reduction on miss rate in channel state prediction. With the proposed prediction mechanism, the performance of Dynamic Spectrum Access (DSA) is substantially improved. Further, we extend the three-state PPPM to an N-state PPPM to predict the duration of high/low utilization in a channel. The frequent patterns of channel utilization duration are critical in optimizing channel switch strategies. The high prediction accuracy is validated with data collected in the paging bands.

To support applications that demand high-speed wireless communication, the ongoing standardization of the next generation Wi-Fi increases the channel bonding from 40 MHz in 802.11n to 80, and even 160 MHz under certain conditions in 802.11ac. However, inefficiency and unfairness issues arise when devices that use different channel widths coexist in a contention domain. In this paper, we propose a dynamic channel bonding (DyB) protocol in which a node is allowed to start a transmission as long as there are some idle narrow channels and it gradually increases channel width during transmission whenever new narrow channels become available. A challenge is the communication over uncertain channels. To enable fast spectrum agreement between transmitter and receiver, a partial spectrum correlation method is introduced. In addition, DyB considers the severe contention in a wide band of spectrum. A compound preamble is designed to make collisions detectable in the frequency domain and a parallel bitwise arbitration is used to quickly resolve the collisions in the time domain. We implemented and evaluated the DyB through both the GNU Radio/USRP platform and ns-2 simulations. Experimental results and simulations show that DyB can well address the inefficiency and unfairness issues caused by heterogeneous radio coexistence.

Advancements in wireless communication techniques have increased the wireless physical layer (PHY) data rates by hundreds of times in a dozen years. The high PHY data rates, however, have not been translated to commensurate throughput gains due to overheads incurred by medium access control (MAC) and PHY convergence procedure. At high PHY data rates, the time used for collision avoidance (CA) at MAC layer and the time used for PHY convergence procedure can easily exceed the time used for transmission of an actual data frame. Recent work intends to reduce the CA overhead by reducing the backoff time slot size. However, the method introduces more collisions in presence of hidden terminals because the tiny backoff slots can no longer de-synchronize hidden terminals, leading to persistent collisions among hidden terminals. As collision detection (CD) in wireless communication became feasible recently, some protocols migrate random backoff from the time domain to the frequency domain, but they fail to address the introduced high collision probability. We investigate the practical issues of CD in the frequency domain and introduce a binary mapping scheme to reduce the collision probability. Based on the binary mapping, a bitwise arbitration (BA) mechanism is devised to grant only one transmitter the permission to initiate data transmission in a contention. With the low collision probability achieved in a short bounded arbitration phase, the throughput is significantly improved by our proposed WiFi-BA. Because collisions are unlikely to happen, unfairness caused by capture effect of radios is also reduced. The bitwise arbitration mechanism can further be set to let high priority messages get through unimpeded, making WiFi-BA suitable for real time prioritized communication. We validate the effectiveness of WiFi-BA through implementation on FPGA of USRP E110. Performance evaluation demonstrates that WiFi-BA is more efficient than current Wi-Fi solutions.

To support high data rate applications such as multimedia streaming, the ongoing standardization of the next generation Wi-Fi increases the channel bonding from 40 MHz in 802.11n to 80, and even 160 MHz under certain conditions in 802.11ac. However, inefficiency and unfairness issues arise when devices that use different channel widths coexist in a contention domain. A device with channel bonding has to wait until all bonded channels to be idle to commence a transmission while narrow channel interferers have more channel access opportunities. To address the inefficiency and unfairness issues in channel bonding, we propose an adaptive channel bonding (ACB) protocol in which a node is allowed to initiate a transmission as long as a narrow channel is available and gradually increase channel width during transmission whenever a new narrow channel becomes available. ACB aggregates all available narrow channels as one wide channel, removing the need of setting guard bands between contiguous narrow channels. A challenge in the design is the communication over uncertain channels. To enable fast spectrum agreement between transmitter and receiver, a partial spectrum correlation method is introduced. ACB also considers the severe contention in a wide band of spectrum. When new channels become available, multiple nodes may contend for them. A compound preamble is designed to make collisions detectable in the frequency domain and a parallel bitwise arbitration mechanism is introduced to quickly resolve the collisions in the time domain. We implemented and evaluated the ACB through the GNU Radio/USRP platform. Experimental results show that ACB can well address the inefficiency and unfairness issues caused by heterogeneous radio coexistence.

Duty cycling improves energy efficiency but limits throughput and introduces significant end-to-end delay in wireless sensor networks. In this paper, we present a traffic-adaptive synchronous MAC protocol (TAS-MAC), which is a high throughput low delay MAC protocol tailored for low power consumption. It achieves high throughput by using Time Division Multiple Access (TDMA) with a novel traffic-adaptive allocation mechanism that assigns time slots only to nodes located on active routes. TAS-MAC reduces the end-to-end delay by notifying all nodes on active routes of incoming traffic in advance. These nodes will claim time slots for data transmission and forward a packet through multiple hops in a cycle. The desirable traffic-adaptive feature is achieved by decomposing traffic notification and data transmission scheduling into two phases, specializing their duties and improving their efficiency respectively. Simulation results and tests on TelosB motes demonstrate that the two-phase design significantly improves the throughput of current synchronous MAC protocols and achieves the similar low delay of slot stealing assisted TDMA with much lower power consumption.

Cognitive radio appears as a promising technology to allocate wireless spectrum between licensed and unlicensed users. Predictive methods for inferring the availability of spectrum holes can help to reduce collision and improve spectrum extraction. This paper introduces a Partial Periodic Pattern Mining (PPPM) algorithm to identify frequent spectrum occupancy patterns that are hidden in the spectrum usage of a channel. The mined frequent patterns are then used to predict future channel states (i.e., busy or idle). PPPM outperforms traditional Frequent Pattern Mining (FPM) by considering real patterns that do not repeat perfectly. Using real life network activities, we show a significant reduction on miss rate in channel state prediction.

Cognitive radio appears as a promising technology to allocate wireless spectrum between licensed and unlicensed users in an efficient way. The availability of spectrum holes vastly affects the throughput and delay of unlicensed users. Predictive methods for inferring the availability of spectrum holes can help to improve spectrum extraction rate and reduce collision rate. In this paper, a spectrum occupancy prediction model based on Partial Periodic Pattern Mining (PPPM) is introduced. The mining aims to identify frequent spectrum occupancy patterns that are hidden in the spectrum usage of a channel. The mined frequent patterns are then used to predict future channel states (i.e., busy or idle). Based on the prediction, unlicensed users will be able to make use of spectrum holes efficiently without introducing significant interference to licensed users. PPPM outperforms traditional Frequent Pattern Mining (FPM) by considering real patterns that do not repeat perfectly due to noise, sensing errors, and irregular behaviors. Using real life network activities we show a significant reduction on miss rate in channel state prediction. With the proposed prediction mechanism, the performance of Dynamic Spectrum Access (DSA) is substantially improved.

Greedy forwarding is a simple yet efficient technique employed by many routing protocols. It is ideal to realize point-to-point routing in wireless sensor networks because packets can be delivered by only maintaining a small set of neighbors' information regardless of network size. It has been successfully employed by geographic routing, which assumes that a packet can be moved closer to the destination in the network topology if it is forwarded geographically closer to the destination in the physical space. This assumption, however, may lead packets to the local minimum where no neighbors of the sender are closer to the destination or low-quality routes that comprise long distance hops of low packet reception ratio. To address the local minimum problem, we propose a topology aware routing (TAR) protocol that efficiently encodes a network topology into a low-dimensional virtual coordinate space where hop distances between pairwise nodes are preserved. Based on precise hop distance comparison, TAR can assist greedy forwarding to find the right neighbor that is one hop closer to the destination and achieve high success ratio of packet delivery without location information. Further, we improve the routing quality by embedding a network topology based on the metric of expected transmission count (ETX). ETX embedding accurately encodes both a network's topological structure and channel quality to nodes' small size virtual coordinates, which helps greedy forwarding to guide a packet along the optimal path that has the fewest number of transmissions. We evaluate our approaches through both simulations and experiments, showing that routing performance are improved in terms of routing success ratio and routing cost.

In this paper, we propose a new opportunistic multicast protocol to improve multicast throughput in Wireless Mesh Networks (WMN). It builds upon opportunistic routing (OR) strategies that have been designed to improve unicast throughput in wireless networks. The key concept in our multicast protocol is a tree backbone. Our tree backbone protocol represents a tradeoff between traditional structured multicast protocols where a complete multicast tree is constructed and unstructured protocols where multicast is treated as a collection of unicasts. Tree backbone selects multiple nodes as intermediate nodes. Each pair of upstream and downstream nodes may be multiple hops away, and packet delivery between them takes advantage of OR. For single-rate WMNs, we show that constructing an efficient tree backbone that minimizes the number of transmissions is NP-hard, and we devise one effective heuristic algorithm for it. For multi-rate WMNs, we investigate the inherent rate-distance tradeoff and propose a Euclidean opportunistic multicast protocol by devising a Euclidean tree backbone as well as an efficient rate selection scheme to minimize the number of transmissions. In our simulations, our tree backbone multicast protocols outperform both the completely structured traditional multicast protocols and the completely unstructured unicast-based protocols augmented with OR in both throughput and delay.

Wireless sensor networks usually operate under light traffic loads. However, when an event is detected, a large volume of data may be generated and delivered to the sink. The demand for simultaneous data transmission may cause severe channel collision and thus decrease communication throughput in contention-based medium access control (MAC) protocols. In this paper, we introduce a novel receiver-centric data transmission paradigm, which takes advantage of the tree structure that is naturally formed in data collection of a sensor network to assist scheduling of channel access. On the tree structure, a receiver is able to coordinate its multiple senders' channel access so as to reduce channel contention and consequently improve communication throughput. The protocol seamlessly integrates scheduling with contention-based medium access control. In addition, to ensure reliable data transmission, we propose a sequence-based lost packet recovery scheme in a hop-by-hop recovery pattern, which could further improve communication throughput by reducing control overhead. We present the performance of our receiver-centric MAC protocol through measurements of an implementation in TinyOS on TelosB motes and extend the evaluation through ns-2 simulations. Compared with B-MAC and RI-MAC, we show the benefits of improving throughput and fairness through receiver-centric scheduling under heavy traffic loads.

In this paper, a cooperative localization algorithm is proposed that considers the existence of obstacles in mobility-assisted wireless sensor networks (WSNs). In this scheme, a mobile anchor (MA) node cooperates with static sensor nodes and moves actively to refine location performance. The localization accuracy of the proposed algorithm can be improved further by changing the transmission range of mobile anchor node. The algorithm takes advantage of cooperation between MAs and static sensors while, at the same time, taking into account the relay node availability to make the best use of beacon signals. For achieving high localization accuracy and coverage, a novel convex position estimation algorithm is proposed, which can effectively solve the localization problem when infeasible points occur because of the effects of radio irregularity and obstacles. This method is the only range-free based convex method to solve the localization problem when the feasible set of localization inequalities is empty. Simulation results demonstrate the effectiveness of this algorithm.

Limitation on available resources is a major challenge in wireless sensor networks. Due to high rates of unexpected node/link failures, robust data delivery through multiple hops also becomes a critical issue. In this article we present a state-free gradient-based forwarding (SGF) protocol to address these challenges. Nodes running SGF do not maintain states of neighbors or network topology and thus can scale to very large networks. Without using routing tables, SGF builds a cost field called gradient that provides each node the direction to forward data. The maintenance of gradient is purely driven by data transmissions and hence incurs little overhead. To adapt to transient channel variations and topology changes, the forwarder of a routing node is selected opportunistically among multiple candidate nodes through a distributed contention process. Simulation results show that SGF achieves significant energy savings and outperforms several existing data forwarding protocols in terms of packet delivery ratio and end-to-end delay.

Minimizing energy consumption is an important challenge in wireless sensor networks. The proposal of reactive on-demand philosophy where routes are established only when required is a major step. But it still needs to keep information about routes. The constantly changing topology, however, makes the routing state upkeep difficult. Thus multi-path routing is usually used for robust data delivery. But it is inefficient in terms of energy and bandwidth consumption especially for the slightly larger data packets such as in image/video dissemination. In this paper, we present a gradient-based routing protocol which works without knowledge of the existence of any neighboring nodes hence manifests the property of robust performance and eliminates the latency as well as the overhead of state upkeep. The data forwarding is based on the gradient -a cost field which provides each sensor node the minimum energy cost path to forward sensing data hence the energy consumption is further improved. Simulation studies show that our protocol is more energy-efficient than comparable work like GRAB and is as robust as its multi-path delivery.

In this paper, we propose a range-free cooperative localization algorithm for mobile sensor networks by combining hop distance measurements and particle filtering. In the hop distance measurement step, a differential error correction scheme is devised to reduce the positioning error accumulated over multiple hops. A backoff-based broadcast mechanism is also introduced in our localization algorithm. It efficiently suppresses redundant broadcasts and reduces message overhead. The proposed localization method has fast converges with small location estimation error. We verify your algorithm in various scenarios and compare it with conventional localization methods. Simulation results show that our proposal is superior to the state-of-the-art localization algorithms for mobile sensor networks.

Wireless sensor networks (WSNs) have been proposed for a multitude of location-dependent applications. To stamp the collected data and facilitate communication protocols, it is necessary to identify the location of each sensor. In this paper, we discuss the performance of a novel received signal strength indicator (RSSI) positioning scheme, which uses a generalized geometrical location algorithm to achieve an accurate estimation based on mean received signal strength measurements. In order to improve the network performance and address limitations of static WSNs position estimation, mobile sensors are utilized effectively and an attractive movement strategy with mobile elements is designed. The effectiveness of our approach is validated and compared with the traditional RSSI method by extensive simulations.