Licensed Assisted Access (LAA) represents a new paradigm for LTE, where unlicensed bands are used in conjunction with licensed bands to increase network capacity. The main challenge of LAA is ensuring that unlicensed bands are shared with WiFi systems in an efficient manner. In this paper, we study a coexistence solution based on channel selection. A centralized channel selection mechanism which increases network capacity of LTE in unlicensed bands is proposed. We also describe the implementation of our scheme using existing LTE signaling mechanisms. Simulation results show that channel selection can improve the performance of both LTE and WiFi by over 100\%.

Positioning is a key aspect for many applications in wireless sensor networks. In order to design practical positioning algorithms, employment of efficient algorithms that maximize the battery lifetime while achieving a high degree of accuracy is crucial. The number of participating anchor nodes and their transmit power have an important impact on the energy consumption of positoning a node. This paper proposes a game theoretical algorithm to optimize resource usage in obtaining location information in a wireless sensor network. The proposed method provides positioning and tracking of nodes using RSS measurements. We use the Geometric Dilution of Precision as an optimization metric for our algorithm, with the aim of minimizing the number and power of anchor nodes that collaborate in positioning, thus saving energy. The algorithm is shown to be a potential game, therefore convergence is guaranteed. A distributed low complexity solution for the implementation is presented. The game is applied to WSN and results show the trade-off between power saving and positioning error.

We study video streaming over a slow-fading wireless channel. In a streaming application, video packets are required to be decoded and displayed in the order they are transmitted as the transmission goes on. This results in per-packet delay constraints, and the resulting channel can be modeled as a physically degraded fading broadcast channel with as many virtual users as the number of packets. In this paper, we study two important quality of user experience (QoE) metrics, namely throughput and interdecoding delay. We introduce several transmission schemes, and compare their throughput and maximum interdecoding delay performances. We also introduce a genie-aided scheme, which provides theoretical bounds on the achievable performance. We observe that adapting the transmission rate at the packet level, i.e., periodically dropping a subset of the packets, leads to a good tradeoff between the throughput and the maximum interdecoding delay. We also show that an approach based on initial buffering leads to an asymptotically vanishing packet loss rate at the expense of a relatively large initial delay. For this scheme, we derive a condition on the buffering time that leads to throughput maximization.

In this paper, the multi-channel performance of large scale deployment of Long Term Evolution (LTE) in Unlicensed (LTE-U) band and WiFi using stochastic geometry is studied. LTE-U and WiFi cell placement is modeled as a Poisson Point Process (PPP), while transmission activity is modeled as hardcore simple sequential inhibition (SSI) point processes, which have been shown to accurately model Carrier sense multiple access (CSMA) access. This stochastic geometry based framework is used to analyze the co-existence of LTE-U and WiFi systems operating in the same set of available channels in unlicensed band. Several interesting results are derived. First, it is shown that fair coexistence can be achieved by LTE-U implementing a Listen-Before-Talk (LBT) protocol. Different variations of LBT are explored to improve the co-existence. Secondly, it is shown that the classical hidden node problem can severely affect performance, and that channel selection mechanisms can significantly alleviate the problem.

In a streaming application video packets are required to be decoded and displayed in the order they are transmitted as the transmission continues. This results in per-packet delay constraints, and in the wireless setting the resulting channel can be modeled as a physically degraded fading broadcast channel with as many virtual users as the number of packets. Two important quality of user experience (QoE) metrics, throughput and inter-decoding delay, are considered jointly, and lower and upper bounds on both metrics are presented.

Streaming transmission over a block fading channel is studied assuming that the transmitter receives a new message at each channel block at a constant rate, which is fixed by an underlying application. A common deadline is assumed for all the messages, at which point the receiver tries to decode as many messages as possible. Various achievable schemes are proposed and compared with an informed transmitter upper bound in terms of average throughput. It is shown that the adaptive joint encoding (aJE) scheme is asymptotically optimal; that is, it achieves the ergodic capacity as the transmission deadline goes to infinity; and it closely follows the upper bound in the case of a finite transmission deadline. On the other hand, in the presence of multiple receivers with different signal-to-noise ratios (SNR), memoryless transmission (MT), generalized time-sharing (gTS) and superposition transmission (ST) schemes are shown to be more robust than the joint encoding (JE) scheme as they have gradual performance degradation with the decreasing SNR.

We propose two new protocols based on physical layer network coding for collision resolution in multiple access networks. When a collision occurs the receiver decodes the sum of the collided packets and after a number of transmissions, equal to or slightly higher than the number of original packets, it can recover all of them. One of the proposed protocols based on fountain codes can resolve collisions by sending out just one acknowledgement (ACK), thus being particularly suited to networks with large round trip delays such as satellite networks. We carry out a comparison of the average delay achieved by the proposed schemes with other access techniques, and show how the performance can be improved with little coordination at the receiver.

We consider the two-way relay channel with random access for the cases of symmetric and asymmetric channel statistics in the low SNR regime. We propose three different schemes implementing different physical layer techniques for collision recovery and channel adaptation and obtain analytical throughput expressions. We compare the proposed schemes with several benchmarks in order to study their bandwidth gains in practical scenarios.

Two new schemes are proposed for collision resolution in slotted ALOHA networks based on multi-user physical-layer network coding (MU PHY NC). In the proposed random access schemes, a collision of a generic number of packets can be recovered decoding the XOR of the original messages, such that the signal resulting from the collision is exploited rather than being discarded. Two different schemes that differ in terms of the amount of control information that needs to be transmitted from the access point, are studied.

Vehicular networks will enable a high number of services in intelligent transportation systems, and are also expected to improve the availability of wireless services to vehicles. In this paper we address the problem of establishing a vehicular network with high capacity using 802.11 and/or DSRC, and in particular we focus on frequency allocation and power control. We advocate a regular frequency reuse pattern that guarantees SINR requirements of the air interface. Resulting co-channel interference is then reduced using an adaptive power control algorithm. Simulation results for a highway scenario reveal that high capacities are possible with moderate infrastructure access point density.

This paper explores the possibility to use the medium utilization as a metric to be included in Radio Environment Maps (REMs) of WLAN networks. The main objective is to build REMs that go beyond the spectrum use and that can be used to improve deployment and operation of WLANs. Specifically, the paper focuses on, first, exploring the possibility to approximate wireless channel utilization as a spatial random field. Second, the paper proposes an approach to obtain distributed measurements of medium utilization and spatial characterizations of to build utilization maps. And, third, the paper studies the performance of regular spatial interpolation techniques when used to build utilization maps. We have used a WLAN network testbed to support the study for which we obtain and analyze practical maps. Our experimental results show that medium utilization of any location of the area under study can be estimated with average errors of 10\% by only using a limited number of measurement points.

This paper proposes a decentralized interference control scheme for an OFDMA (Orthogonal Frequency Division Multiple Access) cellular scenario where multiple macrocells and femtocells are deployed. In particular, we model the distributed resource allocation problem by means of a potential game, which is demonstrated to converge to a unique and pure Nash equilibrum. In this game, the femto base stations are the players, their actions are the resource blocks and the corresponding power levels to be allocated for downlink transmission, and the utility is designed to guarantee coexistence with the macro system, guaranteeing a fair tradeoff between femto and macro systems' performances. To do that, the utility function does not only consider the capacity of the femtocells, but also the different sources of inter-system interference: macrocell to femtocell, femtocell to femtocell, and femtocell to macrocell. The game is solved by applying a better response dynamic, which first selects the best resource blocks allocation policy for the femtousers, and then selects the most appropriate power levels for each resource block, through the solution of the corresponding convex optimization problem. Simulation results show that the introduction of femtocells in the scenario increases the total system capacity, and that the considered utility function provides an improvement in macro system performance of up to 40%, while reducing the femto capacity only up to 7%, with respect to the well known iterative waterfilling game.

This paper deals with the effect of mesh and cluster-tree topologies (that are defined in 802.15.4/Zigbee or 802.15.4a standards) in the performance of cooperative and range-based localization algorithms. In this type of localization at least three reference or anchor nodes within range are needed for location estimation (two dimensions). Therefore the successful localization depends on the connectivity between nodes. Mesh topology allows complex networks because the connectivity between nodes is high, thus this is an advantage for range-based localization. But the mesh structure is energy-consuming. On the other hand, cluster-tree topology simplifies routing and allows energy saving. But in cluster-tree topology the connectivity between nodes is reduced to parent-children relationships. It affects to the range-based localization because of nodes can not do ranging with all nodes within range. In order to reach a trade-off between energy saving and high connectivity, in this paper we propose two new solutions for the localization in a cluster-tree Wireless Sensor Network.

In this paper we propose network coding to perform multi-hop multicast wireless transmissions. More specifically, we address the problem of reducing redundancy in the completely distributed operation of network coding, so as to increase network throughput. In order to do so, we propose a distributed system where network nodes autonomously make decisions with respect to the packets to encode and forward, with the goal of maximizing the data detection probability and minimizing the overhead of transmitted packets. Our setup consists of a Slotted Aloha network where nodes inject packets at a rate which depends on how useful these packets are to other nodes. We model the selection of transmission probabilities by means of a game theoretic approach, in which nodes reach the desired transmission probabilities as an equilibrium solution to the game. Simulation results show that the proposed scheme outperforms a Slotted Aloha system with optimal uniform retransmission probability.

This paper presents an approach to detect misbehaving nodes in cognitive and cooperative networks. In particular, we propose a cooperative scheme, where cognitive radios are granted access in licensed bands, as long as they compensate primary (licensed) users for the additional interference generated in their bands through cooperation. However, this cooperative transmission can be vulnerable to malicious attacks, which are not easy to detect, since they may affect the error probability experienced by the primary users, but not their signal to noise and interference ratio. We propose a misbehavior detection approach based on measuring and comparing the entropy of data derived by anomalous and normal behaviors. Simulation results show that the probability of detection strongly depends on the transmission power and spectrum handoff rate of the outliers.

This paper presents a Bayesian potential game to model distributed joint power and channel allocation for cognitive radios with incomplete information. We propose a cooperative approach where secondary users (SUs) devote part of their transmission power in licensed channels to relaying primary users' (PUs) messages. In addition, we consider incomplete information in the decision making process, so as to avoid the need for a common control channel (CCC), where users share information. This hypothesis improves the robustness and feasibility of the cognitive radio network supported by the proposed approach. Simulation results show that cooperation benefits both PUs and SUs and that the lack of complete information in the decision process slightly reduces performances in terms of signal to interference and noise ratio (SINR) and outage probability.

This paper studies the benefits that cooperation brings to a cognitive radio network. The proposal in this paper considers that secondary unlicensed users are allowed to opportunistically use the radio spectrum allocated to the primary licensed users, as long as they agree on facilitating the primary user communications by cooperating with them. We refer to this approach as overlay paradigm for cognitive radio and we compare this to the underlay paradigm, according to which cooperation techniques among primary and secondary users are not exploited. To model these schemes we make use of theory of exact potential games. We analyze the convergence properties of the proposed games and we evaluate the outputs in terms of quality of service perceived by both primary and secondary users, outage probability and interference temperature, showing that the overlay paradigm for cognitive radio is a promising framework.

Linear Relaying (LR) is the extension of amplify-and-forward to full-duplex operation, and consists of relay nodes transmitting (on every channel use) a linear combination of previously received signals. From a practical standpoint, it is one of the most profitable and easily deployed forwarding techniques, as only requires buffering and amplifying capabilities. In this paper, we propose to use it at multiple relays in order to enlarge the capacity region of single-antenna multiple-access (MAC) and broadcast channels (BC). Relays are assumed to be fixed, and without own data to transmit. The first part of our contribution is devoted to the MAC. Assuming channel awareness at the users and destination, the achievable rate region with LR is derived, and iterative algorithms are devised to compute the maximum sum-rate (SR) and weighted sum-rate (WSR). From both maximizations, the sources' temporal correlation to attain the boundary points of the region is obtained. In the second part, the BC with LR is studied. Its rate region, achievable through dirty paper coding, is shown to be equal to that of the MAC with a sum-power constraint. As a result, we demonstrate that MACBC duality holds. Making use of duality, iterative algorithms to compute the SR and WSR of the BC are presented.

Relaying is a key technology to increase capacity in wireless networks. In this paper, the point-to-point AWGN channel with N parallel relays and time-invariant, frequency-flat fading is studied. For it, we derive achievable rates with four coding techniques, namely: decode-and-forward, partial decode-and-forward, compress-and-forward and linear relaying. The first two are based on signal regeneration at the relay nodes and aim at mimicking a transmit antenna array. We study their scaling law for N rarr infin and Rayleigh fading, and show that is lower than log2log (N) due to the source-relays broadcast limitation. In turn, compress-and-forward aims at mimicking a receive antenna array and consists of relay nodes distributedly compressing their signals and transmitting them to destination. We provide its achievable rate considering distributed Wyner-Ziv compression at the relays, and show that it also scales as log2log (N); in this case, due to the relays-destination MAC limitation. Finally, linear relaying is the extension of amplify-and-forward to full-duplex operation. For it we derive the optimum transmitted signal at the source and propose suboptimum linear relaying functions at the relays. All techniques are compared with the max-flow-min-cut upper bound, evaluated for AWGN channels.

This paper studies the uplink channel of a time division multiple access (TDMA) cellular network assisted by wireless relays. Aiming at increasing the networkpsilas transmission rate, we optimally place multiple decode-and-forward (D&F) dedicated relays within every cell. In the first part of the contribution, we consider the relays transmitting Gaussian codewords. Results show that there is almost 39% gain in the transmission rate of each cell with respect to no relaying. In the second part, we analyze the performance impact of finite set modulation at user and relays (i.e. M-PAM) . With such a modulation, the gain is slightly decreased to 27%. Finally, we show that D&F relaying provides large gains in the outage capacity at the edge of the cell, with similar performance for Gaussian and M-PAM modulation.

The achievable rate of a Gaussian multiple relay channel with linear relaying functions is derived here. With linear relaying, relays transmit (on every channel use) a causal linear combination of their past received inputs. In this paper, the optimum temporal covariance of the signal transmitted by the source is derived and linear relaying functions proposed. Results show that linear relaying outperforms all known relaying techniques for relay channels and generates the tightest lower bound on the capacity of the multiple relay channel.

Relaying diversity is a key technology to increase capacity in wireless networks. In this paper, the capacity of a single source-destination wireless channel, aided by a set of N potential relays, is studied. For such a link (analyzed under full- duplex and half-duplex constraint), we derive a capacity upper bound based upon the max-flow-min-cut theorem stated by Cover in [1]. Furthermore, a lower bound is proposed considering the achievable with partial decoding at the relay nodes. To maximize the later, optimum relay selection is carried out. Both bounds are then applied to Gaussian channels with channel knowledge at both transmitter and receiver sides, and two synchronization modes are addressed: the synchronous mode, where relays perform coherent beamforming, and the asynchronous mode. Results show that, surprisingly, partial decoding is more spectrally efficient for the half-duplex scheme, becoming the capacity achieving technique for low number of relays.

The performance of wireless multicasting is negatively affected by multipath fading. To improve reliability, cooperation among the nodes of the network can be used to create spatial diversity. This paper analyzes a two phase, space-time coded, cooperative multicast protocol and investigates two transmission and combination strategies: Maximal Ratio Combining and Incremental Redundancy. Moreover, it addresses two different channel state information (CSI) scenarios: i) no transmit CSI within the network and ii) broadcast transmit CSI at the source.

We consider the multiple access channel (MAC) assisted by N parallel relays that only have buffering and amplifying capabilities. For this channel, we derive the capacity region applying results on MAC with finite memory. Later, the MAC-broadcast (BC) duality is demonstrated using duality arguments for vector channels. Indeed, the capacity region of the Gaussian amplify-based BC with N parallel relays is shown to be the capacity region of the amplify-based MAC with sum power constraint, and to be achievable through dirty paper coding.

This paper proposes a medium access control (MAC) protocol, termed as CD-MMAC, for wireless networks that utilizes multiple channels and incorporates opportunistic cooperative diversity dynamically to improve its performance. The IEEE 802.11b standard protocol allows the use of multiple channels available at the physical layer but its MAC protocol is designed only for a single channel. The proposed protocol utilizes multiple channels by using single interface and incorporates opportunistic cooperative diversity by using a cross-layer MAC. The new protocol leverages the multi-rate capability of IEEE 802.11b and allows wireless nodes far away from destination node to transmit at a higher rate by using intermediate nodes as a relays. The protocol incorporates opportunistic cooperative diversity by the design of a cross-layer MAC. The protocol improves network throughput and packet delivery ratio significantly and reduces packet delay. The performance improvement is further evaluated by simulation and analysis.

The large number of network nodes and the energy constraints make Wireless Sensor Networks (WSN) one of the most important application fields for Cooperative Diversity. Node cooperation increases the spatial diversity of wireless channels and, thus, reduces the transmitted power. In this paper, we propose a multi-hop WSN with nodes grouped in cooperative clusters that exploits transmit and receive cooperation among cluster nodes. Multi-hop transmission is carried out by concatenating single cluster-to-cluster hops, where every cluster-to-cluster link is defined as a cooperative distributed multiple-input-multiple-output (MIMO) channel. Transmit diversity is exploited through a time-division, decoder-and-forward, relaying scheme based upon two time slots: the Intracluster Slot, used for data sharing within the cluster, and the Intercluster Slot, used for transmission between clusters. At the receiver side, a distributed reception protocol is devised based upon a Selection Diversity algorithm. The proposed multi-hop cooperative WSN is optimally designed for minimum end-to-end outage probability by deriving the optimum time and power allocated on the intracluster and intercluster slots of every single hop, given a per-link energy constraint. A simplified suboptimum resource allocation is also proposed, which performs close to the optimal policy. Results show that the proposed scheme achieves diversity equal to the equivalent MIMO system and significantly reduces energy consumption with respect to. the non-cooperative channel.

Cooperation among users of wireless networks has been lately presented as a suitable technique to obtain reliable communications and capacity gains over fading environments. This paper studies the impact that mutual relaying has over the ergodic capacity of wireless networks when full channel state information (CSI) is available at both receiver and transmitter side, and at the relay nodes. Upper and lower bounds are derived for the ergodic capacity of multiple relay, half-duplex, decode-and-forward Gaussian channels with CSI, assuming Rayleigh flat fading and random relay positions, and a short-term power constraint. Both bounds show spectral efficiency gains growing with the logarithm of the total number of network users, and better performance of cooperative links in the low SNR regime. Likewise, it is shown that the proposed bounds approach the ergodic capacity as SNR diminishes

Direct sequence (DS) and multicarrier (MC) are code-division multiple-access (CDMA) schemes based on single-carrier and MC modulations, respectively. While in DS CDMA, spreading is performed by increasing the transmission rate, or in the time domain, in MC CDMA, spreading is performed using several subcarriers, or in the frequency domain. In this paper, it is shown that MC and DS CDMA are time-frequency dual systems. Their analysis in time- and frequency-dispersive multiple-input multiple-output channels revealed that they can be described equally in terms of dual-channel functions, therefore exhibiting the same performance on dual channels. Furthermore, the duality property is used to derive matched filters for highly time-variant and wide-band channels. Finally, other issues involved in the comparison of DS and MC, namely spreading-sequence design and multiuser detection, are addressed.

The bit error rate of a coded Multiuser OFDM (M-OFDM) system and coded multicarrier CDMA (MC-CDMA) system with multiuser detection is analyzed. The paper focuses on downlink wireless applications, where the channel suffers from frequency-selective fading. We investigate both systems when long time interleaving is possible, though introducing large delays, and when time interleaving is limited. Bit error rate lower bounds and simulation results show that, when the system load is high, M-OFDM performs similar to MC-CDMA while its implementation is less complex as it does not require multiuser detection.