The impact of time asynchronism on the performance of compute-and-forward strategy in relay networks is considered. While the key idea in compute-and-forward is to decode a linear synchronous combination of the transmitted codewords, the relays receive random asynchronous versions of the combinations due to the distributed nature of the network. Over a symbol-asynchronous network, we show that by using a simple equalizer as a part of the signaling scheme, one can transform the asynchronous system into a synchronous one albeit with a smaller channel gain. With frame-asynchronism, we propose to use extra antennas at the relays to efficiently remove the asynchronous delays. By applying a linear filter with a delay-dependent structure before the decoder of compute-and-forward, the achievable rate is maximized at all signal-to-noise-ratios (SNRs).

In a demodulate-and-forward (DF) based cooperative communication system, erroneous relaying of the data leads to degradation in the performance of the destination receiver. However, a maximum likelihood (ML) decoder in the destination can improve the receiver performance. For achieving a diversity gain, the Alamouti space-time block code (STBC) can be used in the DF based cooperative system in a distributed manner. In this paper, we derive an ML decoder of the distributed Alamouti STBC for the DF based cooperative system with two imperfect relaying nodes. We also consider a DF cooperative communication system in which one out of two relays is in outage. A piece-wise linear (PL) decoder for the DF cooperative system with the distributed Alamouti code and one relay in outage is proposed. The PL decoder provides approximately the same performance as that of the ML decoder with reduced decoding complexity. We derive the pairwise error probability (PEP) of the proposed ML decoder with binary phase-shift keying constellation. An optimized transmit power allocation for the relays is performed by minimizing an upper bound of the PEP. It is shown by simulations that the proposed ML decoder enables the DF protocol based cooperative system to outperform the same rate amplify-and-forward protocol based cooperative system when both systems utilize the distributed Alamouti STBC.

In this paper, we derive a maximum-likelihood (ML) decoder for the demodulate-and-forward (DF) based cooperative communication system using Alamouti space-time block code (STBC) in a distributed manner. We also propose a sub-optimal low complexity piece-wise linear (PL) decoder of the distributed Alamouti code in a DF cooperative system in which one relay out of two relays is in outage. The proposed PL decoder does not lead to any significant performance degradation and performs very close to the proposed ML decoder. Moreover, the proposed ML decoder of the DF cooperative system significantly outperforms an amplify-and-forward (AF) based cooperative system when both systems use the same data rate and distributed Alamouti STBC.

The 28 papers in this special issue focus on cooperative networking. The papers can be divided into three categories. The ten papers in the first category focus on relay selection and routing in the network layer. The nine papers in the second category deal with coding and power allocation in the physical layer. The nine papers in the third category study application performance.

The articles in this special issue are devoted to the topic of cooperative networking where individual network nodes can cooperate to achieve desired network performance in a coordinated manner.

One of the serious imperfections affecting OFDM systems is transceiver I/Q imbalance. In this letter, closed-form expressions for the outage probability of beamforming OFDM systems with transmit and receive I/Q imbalances are derived. Moreover, the asymptotic behavior and diversity order of the system is investigated. The analytical results are confirmed by simulations.

In this paper, we explain how to implement differential modulation in multiple access channel (MAC) based uplink two-user multiple-input multiple-output (MIMO) communication system. It is assumed that the users cannot exchange their information and transmit data over uplink MAC channels simultaneously in the same frequency band and without any orthogonal signatures (no CDMA). We derive two types of differential decoders for two-user MIMO MAC system namely 1) Partial differential decoder, 2) Heuristic based differential decoders. The partial differential decoder is able to avoid the knowledge of the channel gains of one user at the receiver for joint decoding of the data of both users. Whereas, the heuristic differential decoders are obtained heuristically and work without any channel knowledge of both users. For the heuristic differential decoder to work properly, the data of one user must be rotated before transmission. An upper bound of the pairwise error probability (PEP) is derived for the proposed scheme. The upper bound of the PEP is minimized for obtaining an optimized value of the rotation angle.

Existing studies on physical layer security often assume the availability of perfect channel state information (CSI) and overlook the importance of channel training needed for obtaining the CSI. In this letter, we discuss how an active eavesdropper can attack the training phase in wireless communication to improve its eavesdropping performance. We derive a new security attack from the pilot contamination phenomenon, which targets at systems using reverse training to obtain the CSI at the transmitter for precoder design. This attack changes the precoder used by the legitimate transmitter in a controlled manner to strengthen the signal reception at the eavesdropper during data transmission. Furthermore, we discuss an efficient use of the transmission energy of an advanced full-duplex eavesdropper to simultaneously achieve a satisfactory eavesdropping performance whilst degrading the detection performance of the legitimate receiver.

In this paper, we derive a maximum-likelihood (ML) decoder of the decode-and-forward (DF) protocol utilizing an arbitrary complex-valued M-point constellation. Existing decoders of the DF based cooperative systems are applicable only to real-valued constellations like BPSK and M-PAM, hence, also to square M2-QAM. We have considered a DF based cooperative system with a single source-destination pair and one relay. The ML decoder is obtained by maximizing the probability density function (p.d.f.) of the received signal at the destination. The proposed ML decoder provides lower decoding complexity than an existing decoder. We also propose an approximate piecewise linear (PL) decoder for M-PSK constellation which performs similar to the proposed ML decoder. The proposed PL decoder provides significant lower decoding complexity than the proposed ML decoder. We analytically derive the approximate symbol error rate of the proposed PL decoder. The DF based cooperative system with the proposed ML and PL decoders outperforms the same rate amplify-and-forward based cooperative communication system from moderate to high signal-to-noise ratio values.

This paper studies training-based transmissions over multiple-input multiple-output (MIMO) fading channels in the presence of jamming. Each transmission block consists of a training phase and a data transmission phase. From an information-theoretic viewpoint, we formulate a max-min problem on the energy allocation between the two phases. The legitimate user of the channel aims to design a robust energy allocation strategy which maximizes its data rate under the worst case scenario assuming that the jammer is able to optimize its jamming energy allocation between the training phase and the data transmission phase. For a fixed training length, we derive an analytical solution to the robust energy allocation. When the training length is allowed to vary, we show that a robust design of the training length is generally larger than the number of transmit antennas and approaches half of the block length at low signal to jamming and noise ratio (SJNR). Our numerical results demonstrate a potential of 20%-40% performance gain by using the proposed robust designs in various scenarios.

The impact of symbol-asynchronism on compute-and-forward strategy in relay networks is considered. The lattice-based coding and decoding scheme presented recently by Nazer and Gastpar is applied over asynchronous distributed networks. While the key idea in compute-and-forward is to decode a linear combination of the transmitted codewords, due to distributed nature of the network, the relays receive random asynchronous versions of the combinations. It is shown that by using a simple equalizer as a part of the signaling scheme, one can transform the system to a synchronous one albeit with a smaller channel gain. The effect of asynchronism on the achievable rates is also studied, which implies a loss compared to the synchronous case. However, the resulting gap vanishes as the signal-to-noise-ratio (SNR) increases.

Relay transmission or cooperative communication is an advanced technique that can improve the performance of data transmission among wireless nodes. However, while the performance (e.g., throughput) of a source node can be improved through cooperation with a number of relays, this improvement comes at the expense of a degraded performance for the relay nodes due to the resources that they dedicate for helping the source node in its transmission. In this paper, we formulate a coalitional game among the wireless nodes that seek to improve their performance by relaying each other's data. The game is classified as a coalition formation game in which the nodes can take individual and distributed decisions to join or split from a given coalition while ensuring that their individual throughput is maximized. A Markov chain model is proposed to investigate the stability of the resulting coalitional structures. Further, we consider the practical case in which the wireless nodes do not have an exact and perfect knowledge of the parameters (e.g., channel quality) in coalition formation. For this scenario, we analyze the stability of the partitions resulting from the proposed coalition formation game under uncertainty. We also define the conditions needed for obtaining the stable and unstable coalitional structures among the nodes that are performing cooperative transmission.

Vehicle-to-roadside (V2R) communications enable vehicular networks to support a wide range of applications for enhancing the efficiency of road transportation. While existing work focused on non-cooperative techniques for V2R communications between vehicles and roadside units (RSUs), this paper investigates novel cooperative strategies among the RSUs in a vehicular network. We propose a scheme whereby, through cooperation, the RSUs in a vehicular network can coordinate the classes of data being transmitted through V2R communication links to the vehicles. This scheme improves the diversity of the information circulating in the network while exploiting the underlying content-sharing vehicle-to-vehicle communication network. We model the problem as a coalition formation game with transferable utility and we propose an algorithm for forming coalitions among the RSUs. For coalition formation, each RSU can take an individual decision to join or leave a coalition, depending on its utility which accounts for the generated revenues and the costs for coalition coordination. We show that the RSUs can self-organize into a Nash-stable partition and adapt this partition to environmental changes. Simulation results show that, depending on different scenarios, coalition formation presents a performance improvement, in terms of the average payoff per RSU, ranging between 20.5% and 33.2%, relative to the non-cooperative case.

Cooperation among wireless nodes has been recently proposed for improving the physical layer (PHY) security of wireless transmission in the presence of multiple eavesdroppers. While existing PHY security literature answered the question “what are the link-level secrecy rate gains from cooperation?”, this paper attempts to answer the question of “how to achieve those gains in a practical decentralized wireless network and in the presence of a cost for information exchange?”. For this purpose, we model the PHY security cooperation problem as a coalitional game with non-transferable utility and propose a distributed algorithm for coalition formation. Using the proposed algorithm, the wireless users can cooperate and self-organize into disjoint independent coalitions, while maximizing their secrecy rate taking into account the costs during information exchange. We analyze the resulting coalitional structures for both decode-and-forward and amplify-and-forward cooperation and study how the users can adapt the network topology to environmental changes such as mobility. Through simulations, we assess the performance of the proposed algorithm and show that, by coalition formation using decode-and-forward, the average secrecy rate per user is increased of up to 25.3 and 24.4% (for a network with 45 users) relative to the non-cooperative and amplify-and-forward cases, respectively.

Due to the limited energy supplies of nodes in many applications such as wireless sensor networks, energy efficiency is crucial for extending the lifetime of these networks. This paper addresses the cooperative transmission for outage-restricted multihop wireless ad hoc networks. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, new multihop cooperative protocol is proposed using the space-time codes for the purpose of energy savings, subject to a required outage probability at the destination. We restrict the cooperation to nodes along a chosen route. Two efficient power allocation schemes are derived, which depend only on the statistics of the channels. Furthermore, three efficient cooperative multihop transmissions are proposed when arbitrary distributed space-time codes are used. The proposed cooperative protocols offer different degrees of energy efficiency, spectral efficiency, complexity, and signalling overhead. Compared to non-cooperative multihop routing, an energy saving of up to 72% is achievable in line networks with 3 relays and an outage probability constraint of 10-3at the destination.

The introduction of relay station (RS) nodes is a key feature in next generation wireless networks such as 3GPP's long term evolution advanced (LTE-Advanced), or the forthcoming IEEE 802.16j WiMAX standard. This paper presents, using game theory, a novel approach for the formation of the tree architecture that connects the RSs and their serving base station in the uplink of the next generation wireless multi-hop systems. Unlike existing literature which mainly focused on performance analysis, we propose a distributed algorithm for studying the structure and dynamics of the network. We formulate a network formation game among the RSs whereby each RS aims to maximize a cross-layer utility function that takes into account the benefit from cooperative transmission, in terms of reduced bit error rate, and the costs in terms of the delay due to multi-hop transmission. For forming the tree structure, a distributed myopic algorithm is devised. Using the proposed algorithm, each RS can individually select the path that connects it to the BS through other RSs while optimizing its utility. We show the convergence of the algorithm into a Nash tree network, and we study how the RSs can adapt the network's topology to environmental changes such as mobility or the deployment of new mobile stations. Simulation results show that the proposed algorithm presents significant gains in terms of average utility per mobile station which is at least 17.1% better relatively to the case with no RSs and reaches up to 40.3% improvement compared to a nearest neighbor algorithm (for a network with 10 RSs). The results also show that the average number of hops does not exceed 3 even for a network with up to 25 RSs.

In this paper, we study multibeam satellite secure communication through physical (PHY) layer security techniques, i.e., joint power control and beamforming. By first assuming that the channel state information (CSI) is available and the beamforming weights are fixed, a novel secure satellite system design is investigated to minimize the transmit power with individual secrecy rate constraints. An iterative algorithm is proposed to obtain an optimized power allocation strategy. Moreover, suboptimal beamforming weights are obtained by completely eliminating the cochannel interference and nulling the eavesdroppers' signal simultaneously. In order to obtain jointly optimized power allocation and a beamforming strategy in some practical cases, e.g., with certain estimation errors of the CSI, we further evaluate the impact of the eavesdropper's CSI on the secure multibeam satellite system design. The convergence of the iterative algorithm is proven under justifiable assumptions. The performance is evaluated by taking into account the impact of the number of antenna elements, number of beams, individual secrecy rate requirement, and CSI. The proposed novel secure multibeam satellite system design can achieve optimized power allocation to ensure the minimum individual secrecy rate requirement. The results show that the joint beamforming scheme is more favorable than the fixed beamforming scheme, especially in the cases of a larger number of satellite antenna elements and higher secrecy rate requirement. Finally, we compare the results under the current satellite air-interface in DVB-S2 and the results under Gaussian inputs.

Distributed space-time block codes (DSTBCs) from complex orthogonal designs (CODs) (both square and nonsquare), coordinate interleaved orthogonal designs (CIODs), and Clifford unitary weight designs (CUWDs) are known to lose their single-symbol ML decodable (SSD) property when used in two-hop wireless relay networks using amplify and forward protocol. For such networks, in this paper, three new classes of high rate, training-symbol embedded (TSE) SSD DSTBCs are constructed: TSE-CODs, TSE-CIODs, and TSE-CUWDs. The proposed codes include the training symbols inside the structure of the code which is shown to be the key point to obtain the SSD property along with the channel estimation capability. TSE-CODs are shown to offer full-diversity for arbitrary complex constellations and the constellations for which TSE-CIODs and TSE-CUWDs offer full-diversity are characterized. It is shown that DSTBCs from nonsquare TSE-CODs provide better rates (in symbols per channel use) when compared to the known SSD DSTBCs for relay networks. Important from the practical point of view, the proposed DSTBCs do not contain any zeros in their codewords and as a result, antennas of the relay nodes do not undergo a sequence of switch on/off transitions within every codeword, and, thus, avoid the antenna switching problem.

Autonomous wireless agents such as unmanned aerial vehicles, mobile base stations, cognitive devices, or self-operating wireless nodes present a great potential for deployment in next-generation wireless networks. While current literature has been mainly focused on the use of agents within robotics or software engineering applications, this paper proposes a novel usage model for self-organizing agents suitable for wireless communication networks. In the proposed model, a number of agents are required to collect data from several arbitrarily located tasks. Each task represents a queue of packets that require collection and subsequent wireless transmission by the agents to a central receiver. The problem is modeled as a hedonic coalition formation game between the agents and the tasks that interact in order to form disjoint coalitions. Each formed coalition is modeled as a polling system consisting of a number of agents, designated as collectors, which move between the different tasks present in the coalition, collect and transmit the packets. Within each coalition, some agents might also take the role of a relay for improving the packet success rate of the transmission. The proposed hedonic coalition formation algorithm allows the tasks and the agents to take distributed decisions to join or leave a coalition, based on the achieved benefit in terms of effective throughput, and the cost in terms of polling system delay. As a result of these decisions, the agents and tasks structure themselves into independent disjoint coalitions which constitute a Nash-stable network partition. Moreover, the proposed coalition formation algorithm allows the agents and tasks to adapt the topology to environmental changes, such as the arrival of new tasks, the removal of existing tasks, or the mobility of the tasks. Simulation results show how the proposed algorithm allows the agents and tasks to self-organize into independent coalitions, while improving the performance, in terms of average player (agent or task) payoff, of at least 30.26 percent (for a network of five agents with up to 25 tasks) relatively to a scheme that allocates nearby tasks equally among agents.

Decode-and-forward (DF) protocol based cooperative communication is vulnerable to the erroneous relaying by the relay. In this paper, we derive a maximum-likelihood (ML) decoder for the DF protocol utilizing arbitrary complex-valued constellations including M-PSK, M-PAM, and M-QAM. A set-up of a single pair of source and destination with one relay is studied. The source and the relay utilize orthogonal uncoded transmissions. The relay performs ML decoding and forwards the decoded symbol to the destination, and it might commit errors in decoding the data. The ML decoder at the destination is obtained by maximizing the probability density function (p.d.f.) of the data received during two orthogonal transmissions at the destination under the assumption that the average probability of error of the source-relay link is known at the destination. The proposed ML decoder is a generalized decoder which is applicable to arbitrary constellations, whereas, one existing DF cooperative decoder is applicable to the real valued constellations like BPSK and M-PAM. One existing decoder is also applicable to M2-QAM constellations. We also derive a low-complexity piecewise linear (PL) decoder for arbitrary complex-valued M-point constellations which performs similar to the ML decoder for all signal-to-noise ratio values. An approximate expression of the symbol error rate (SER) of the PL decoder for M-PSK constellation is derived. By using the approximate SER expressions, it is proved that the proposed ML and PL decoders achieve full diversity of two in the cooperative system studied.

This paper investigates the problem of spectrally efficient operation of a multiuser uplink cognitive radio system in the presence of a single primary link. The secondary system applies opportunistic interference cancelation (OIC) and decodes the primary signal when such an opportunity is created. We derive the achievable rate in the secondary system when OIC is used. This scheme has a practical significance, since it enables rate adaptation without requiring any action from the primary system. The exact expressions for outage probability of the primary user are derived, when the primary system is exposed to interference from secondary users. Moreover, approximated formulas and tight lower and upper bounds for the ergodic sum-rate capacity of the secondary network are found. Next, the power allocation is investigated in the secondary system for maximizing the sum-rate under an outage constraint at the primary system. We formulate the power optimization problem in various scenarios depending on the availability of channel state information and the type of power constraints, and propose a set of simple solutions. Finally, the analytical results are confirmed by simulations, indicating both the accuracy of the analysis, and the fact that the spectral-efficient, low-complexity, flexible, and high-performing cognitive radio can be designed based on the proposed schemes.

This paper studies the throughput of large-scale decentralized wireless networks with physical layer security constraints. In particular, we are interested in the question of how much throughput needs to be sacrificed for achieving a certain level of security. We consider random networks where the legitimate nodes and the eavesdroppers are distributed according to independent two-dimensional Poisson point processes. The transmission capacity framework is used to characterize the area spectral efficiency of secure transmissions with constraints on both the quality of service (QoS) and the level of security. This framework illustrates the dependence of the network throughput on key system parameters, such as the densities of legitimate nodes and eavesdroppers, as well as the QoS and security constraints. One important finding is that the throughput cost of achieving a moderate level of security is quite low, while throughput must be significantly sacrificed to realize a highly secure network. We also study the use of a secrecy guard zone, which is shown to give a significant improvement on the throughput of networks with high security requirements.

Mobile social networks have been introduced as a new efficient (i.e., minimize resource usage) and effective (i.e., maximize the number of target recipients) way to disseminate content and information to a particular group of mobile users sharing the same interests. In this paper, we investigate how the content providers and the network operator can interact to distribute content in a mobile social network. The objective of each content provider is to minimize the cost pertaining to the time used for distributing the content to all subscribed mobile users as well as the cost due to the price paid to network operator for transferring the content over a wireless connection via a base station. While the content providers can cooperate by establishing coalitions for sharing a wireless connection, the network operator can control the amount of bandwidth of the wireless connection. We introduce a novel coalitional game model, referred to as ``controlled coalitional game'', to investigate the decision makings of the content providers and the network operator. The numerical studies show that, given the allocated bandwidth from network operator, the content providers can self-organize into coalitions while minimizing their individual cost for wireless connection sharing. Also, the results demonstrate that the revenue of the network operator can be maximized when the bandwidth allocation is performed considering the coalitional structure of the content providers.

Delay tolerant networks (DTNs) can be composed of multiple heterogeneous groups (i.e., communities) of nodes. The nodes from these communities can cooperate with each other in order to carry and forward data packets so that the performance (e.g., delay) can be improved. However, this cooperation will incur additional cost on the nodes. In this paper, we first develop an analytical model to investigate the performance gain from cooperation of multiple communities in a DTN. Then, we propose a coalitional game model for analyzing the cooperation decisions of multiple rational communities based on the tradeoff between performance gains and associated costs. As a solution to the proposed game, we determine the stable coalitional structure, i.e., the structure where no community can improve its payoff by changing its cooperation decisions. The proposed analytical and game models will be useful for the performance and cost optimization of multi-community DTNs.

In current peer-to-peer file sharing networks, a large number of peers with heterogeneous connections simultaneously seek to download resources, e.g., files or file fragments, from a common seed at the time these resources become available, which incurs high download delays on the different peers. Unlike existing literature which mainly focused on cooperative strategies for data exchange between different peers after all the peers have already acquired their resources, in this paper, we study the cooperation possibilities among a number of peers seeking to download, concurrently, a number of resources at the time the availability of the resources is initially announced at a common seed. We model the problem as a coalitional game in partition form and we propose an algorithm for coalition formation among the peers. The proposed algorithm enables the peers to take autonomous decisions to join or leave a coalition while minimizing their average download delay. We show that, by using the proposed algorithm, a Nash-stable partition composed of coalitions of peers is formed. Within every coalition, the peers distribute their download requests between the seed and the cooperating partners in a way to minimize the total average delay incurred on the coalition. Analytically, we study the 2-peer scenario and derive the optimal download request distribution policies. Simulation results show that, using the proposed coalition formation game, the peers can improve their average download delay per peer of up to 99.6% compared to the non-cooperative approach for the case with N = 15 peers.

In this paper, we propose a maximum likelihood (ML) decoder for differentially encoded orthogonal spacetime block codes (OSTBCs) based on multiple received data blocks. The proposed ML decoder differs from the existing ML decoders of differential OSTBC as follows: 1) Most existing ML decoders are derived on the basis of two consecutively received data matrices, whereas the proposed decoder takes multiple consecutively received data matrices into account. 2) One existing ML decoder considers multiple consecutively received specific differential OSTBC using two transmit antennas and is applicable to M-PSK constellation only, whereas the proposed ML decoder works with arbitrary differential OSTBCs using arbitrary number of transmit and receive antennas and arbitrary constellations which do not include zero. We have also derived an upper bound of the pairwise error probability (PEP) of the proposed ML decoder for differentially encoded orthogonal STBC (OSTBC) with M-QAM constellation over Rayleigh fading channels.

Recently, a special class of complex designs called Training-Embedded Complex Orthogonal Designs (TE-CODs) has been introduced to construct single-symbol Maximum Likelihood decodable (SSD) distributed space-time block codes (DSTBCs) for two-hop wireless relay networks using the amplify and forward protocol. However, to implement DSTBCs from square TE-CODs, the overhead due to the transmission of training symbols becomes prohibitively large as the number of relays increase. In this paper, we propose TE-Coordinate Interleaved Orthogonal Designs (TE-CIODs) to construct SSD DSTBCs. Exploiting the block diagonal structure of TE-CIODs, we show that the overhead due to the transmission of training symbols to implement DSTBCs from TE-CIODs is smaller than that for TE-CODs. We also show that DSTBCs from TE-CIODs offer higher rate than those from TE-CODs for identical number of relays while maintaining the SSD and full-diversity properties.

Due to the limited energy supplies of nodes, in many applications like wireless sensor networks, energy-efficiency is crucial for extending the networks lifetime. We study the routing problem for multihop wireless ad hoc networks based on cooperative transmission. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, we propose a cooperative multihop routing for the purpose of power savings, constrained on a required bit error rate (BER) at the destination. Two efficient power allocation schemes are proposed, which depend only on the statistics of the channels. In the first scheme, each node needs to know only the local channel statistics, and can be implemented in a distributed manner. The second scheme is a centralized power control strategy, which has a higher energy efficiency, at the expense of more complexity and signaling overhead. It is shown that energy savings of 75% are achievable in line networks with 3 relays for BER = 10-4constraint at the destination.

Distributed Space-Time Block Codes (DSTBCs) from Complex Orthogonal Designs (CODs) (both square and non-square CODs other than the Alamouti design) are known to lose their single-symbol ML decodable (SSD) property when used in two-hop wireless relay networks using the amplify and forward protocol. For such a network, a new class of high rate, training-symbol embedded (TSE) SSD DSTBCs are proposed from TSECODs. The constructed codes include the training symbols within the structure of the code which is shown to be the key point to obtain high rate along with the SSD property. TSE-CODs are shown to offer full-diversity for arbitrary complex constellations. Non-square TSE-CODs are shown to provide better rates (in symbols per channel use) compared to the known SSD DSTBCs for relay networks when the number of relays is less than 10. Importantly, the proposed DSTBCs do not contain zeros in their codewords and as a result, antennas of the relay nodes do not undergo a sequence of switch on and off transitions within every codeword use. Hence, the proposed DSTBCs eliminate the antenna switching problem.

Due to the limited energy supplies of nodes in wireless networks, achieving energy efficiency is crucial for extending the lifetime of these networks. Thus, we study efficient power allocations and transmission protocols for outage-restricted multihop wireless networks based on cooperative transmission. In such multihop networks, a number of nodes, acting as relays, can assist a source node in the transmission of its messages to a single destination. In this paper, several multihop transmission protocols with cooperative routing are proposed. Each of the proposed protocols offers a different rate and energy efficiency. Cooperative routing protocols are introduced using arbitrary distributed space-time codes for the purpose of energy savings, given a required outage probability at the destination. Three efficient cooperative multihop transmissions are proposed, and their corresponding distributed power allocation schemes, which depend only on the statistics of the channels, are also derived. The proposed cooperative protocols offer different degrees of energy efficiency, spectral efficiency, complexity, and signalling overhead. Simulations show that, using the proposed cooperative protocols, substantial energy savings are achievable, compared to non-cooperative multihop routing, in a network having an outage probability constraint.

We consider a wireless relay network with with hybrid-automatic retransmission request (HARQ) and Rayleigh fading channels. In this paper, we analyze the outage probability of the multi-relay delay-limited HARQ system with opportunistic relaying scheme in decode-and-forward mode. A simple and distributed relay selection strategy is proposed for multi-relay HARQ channels. Then, we analyze the performance of the system. We first derive the cumulative density function (CDF) and probability density function (PDF) of the selected relay channels. Then, the CDF and PDF are used to determine the outage probability in the l-th round of HARQ. The packet delay constraint is represented by L, the maximum number of HARQ rounds. Furthermore, closed-form upper-bounds on outage probability are derived, which are used to investigate the diversity order of the system. Based on the derived upper-bound expressions, it is shown that the proposed schemes achieve the full spatial diversity order of N + 1, where N is the number of potential relays. Our analytical results are confirmed by simulation results.

This paper is on the design and performance analysis of practical distributed space-time codes for wireless relay networks with multiple antennas terminals. The amplify-and-forward scheme is used in a way that each relay transmits a scaled version of the linear combination of the received symbols. We propose distributed generalized quasi-orthogonal space-time codes which are distributed among the source antennas and relays, and valid for any number of relays. Assuming M-PSK and M-QAM signals, we derive a formula for the symbol error probability of the investigated scheme over Rayleigh fading channels. For sufficiently large SNR, this paper derives closed-form average SER expression. The simplicity of the asymptotic results provides valuable insights into the performance of cooperative networks and suggests means of optimizing them. Our analytical results have been confirmed by simulation results, using full-rate full-diversity distributed codes.

Security is an important aspect for the future wireless networks. Since the number of network nodes increases constantly, and, in addition, the networks are decentralized and ad-hoc in nature, it becomes more challenging to apply the current cryptographic methods that require exchange of keys between the communicating parties. Information-theoretic secrecy is an emerging security field that explores the possibility of achieving perfect secrecy data transmission between the intended network nodes, while possible malicious nodes (also called eavesdroppers) are kept ignorant of the transmitted information. In other words, that is the ability of the physical layer to provide security (beside the reliability) of the transmitted (broadcasted) data. For that reason, it is often called physical layer security. In this paper we observe how a simple cooperative network with one relay node can improve the physical layer security by decreasing the area in which the eavesdropper can reside and listen to the information transmitted to the destination. This region is called vulnerability region and we provide its characterization. Under certain conditions, the vulnerability region vanishes, which makes our wireless system perfectly secure for any position of the eavesdropper within the wireless network.

In vehicular-to-roadside (V2R) communications the bandwidth from roadside units (RSUs) can be shared among the vehicular users in order to improve the resource utilization and reduce the costs of bandwidth reservation. We formulate a coalitional game model to analyze the situation in which multiple vehicular users can cooperate for sharing the bandwidth from serving RSUs. First, we consider a \emph{rational coalition formation} approach in which each vehicular user is self-interested, and, hence, decides to join the coalition which maximizes its individual utility. For this approach, we propose a dynamic model based on Markov chain which allows to obtain a stable coalitional structure. Further, for implementation of rational coalition formation, we propose a distributed algorithm based on well-defined merge and split mechanisms. Then, we consider the optimal coalition formation process in which the coalitions are formed so that the social welfare of all vehicular users is maximized. The performance evaluation shows that optimal coalition formation yields a higher utility than rational coalition formation due to the group-interest of all vehicular users. Also, both optimal and rational coalition formation achieve a significantly higher utility than the case without bandwidth sharing (non-cooperative case).

In order to maintain a conflict-free environment among licensed primary users (PUs) and unlicensed secondary users (SUs) in cognitive radio networks, providing frequency and geographical information through control channels, such as the cognitive pilot channel (CPC), has been recently proposed. While existing literature focused on the type of information that these control channels need to carry, this paper investigates the problem of gathering this information cooperatively, among a network of secondary base stations (SBSs). In this regard, given a cognitive network where every SBS can only have accurate knowledge on a small number of different primary users (PUs) or channels, each SBS can cooperate with neighboring SBSs in order to improve its view of the spectrum, i.e., learn about new PUs that can subsequently be used by its served SUs. We model the problem as a hedonic coalition formation game among the SBSs and we propose an algorithm for forming the coalitions. Using the proposed algorithm, each SBS can take an individual decision to join or leave a coalition while maximizing its overall potential utility, which accounts for the tradeoff between the benefit from learning new channels through coalition members and the cost from receiving inaccurate information. Simulation results show that the proposed algorithm yields a performance advantage, in terms of the average payoff per SBS reaching up to 165% relative to the non-cooperative case for a large network with 27 SBSs.

An important transmitter adaptation technique used in multiple-input multiple-output (MIMO) communication systems is precoding based on the dominant right-singular vectors of the channel matrix. These vectors are typically estimated at the receiver, quantized, and fed-back to the transmitter via a low-rate feedback channel. We present a method for designing an efficient low-rate quantizer for the dominant right singular vectors of a MIMO channel matrix, which exploits the memory in the slow-varying channel fading process. A memory-based (recursive) quantizer is considered which quantizes the time-trajectory of points representing the channel subs-spaces on a Grassmannian manifold. The quantizer codebook is optimized with respect to an objective functions which can be directly related to MIMO channel capacity. The main contribution of this paper is the derivation of a stochastic gradient-based algorithm for codebook optimization. Numerical results obtained with channel simulations are presented, which demonstrate the MIMO channel capacity improvement achieved by using the codebooks optimized as proposed in this paper.

Estimating channel state information (CSI) in the fast fading conditions is very challenging. In this study, a simple structure for cooperative diversity in decode-and-forward mode is examined, in which the transmitter, the relay and the receiver do not know the CSI. In this scheme, transmission of information is done in a two-phase process. In the first phase, differential modulated signals are radiated from the source. After decoding the received signal in the relay, the source and the relay collectively send information using differential space–time codes. A closed-form optimum power allocation in the sense of minimising pairwise error probability (PEP) for high signal-to-noise ratio (SNR) scenarios has been obtained. An interesting property of the optimum point is that it is independent of the channel statistics and the position of the relay, which improves its feasibility in the future communication systems. Numerical simulations verify the analytical results.

We derive a decoder for differential data of two multiple access channel (MAC) based uplink users. Both users utilize differential modulation for transmitting their data. It is assumed that the users cannot exchange their information. In addition, it is assumed that the users transmit data over multiple access uplink channels simultaneously in the same frequency band and without any orthogonal signatures (no CDMA). Their transmissions are assumed to be perfectly synchronized in time, phase, and frequency. The decoder is obtained by maximizing the joint probability distribution function (p.d.f.) of consecutively received data samples. We first obtain a partially differential decoder which avoids the knowledge of channel of one user at the base-station. Then we derive a heuristic differential decoder which completely avoids the knowledge of the channels of both users. For the heuristic differential decoder to work properly, both users must use rotated constellations. Optimized values of rotation angles for different M-PSK constellations are numerically obtained. It is further shown by simulations that the proposed differential decoders perform better than a same rate existing TDMA based two-user transmission scheme.

A linear precoder is designed to minimize an upper bound of the pairwise error probability (PEP) when using arbitrary space-time block codes (STBC) over correlated Ricean fading channels. The proposed precoder design differs from the previously proposed precoders as follows: 1) Existing precoders are applicable when there exists only transmit correlation and no receive correlations, however, the proposed precoder is applicable to correlated Ricean MIMO channels with an invertible correlation matrix. 2) We minimize the upper bound of the PEP of non-orthogonal STBC for obtaining a precoder over correlated Ricean channels, whereas, one existing precoder minimizes an upper bound of PEP for orthogonal STBC (OSTBC). The proposed precoder outperforms two existing precoders for non-orthogonal STBC especially for highly correlated channels.

In this paper, we propose a TDMA based simple transmission scheme, which overcomes the effect of the delays caused by the poor synchronization of the relaying nodes over Ricean channels. The proposed scheme is able to provide an optimized coding gain in unsynchronized cooperative networks as compared to the existing delay tolerant distributed space-time block codes.

The presence of carrier offsets in the multiple-input multiple-output (MIMO) channels is an important practical and theoretical problem. Double-differential coding is a technique which allows the receiver to decode the data without any channel or carrier offset knowledge. We propose a double-differential (DD) coding scheme which is applicable to square orthogonal space-time block codes (OSTBC) using M-PSK constellation. The main advantages of our proposed DD coding scheme are: 1) The previously proposed DD codes are applicable only to the specific class of space-time block codes which follow the diagonal unitary group property, whereas our DD coding is applicable to any square OSTBC. 2) We propose a suboptimal decoder which preserves the linear decoding property of the OSTBC. 3) A theoretical analysis is performed to find a pairwise error probability (PEP) upper bound of the proposed double-differential orthogonal space-time block codes (DDOSTBC). 4) In order to improve the performance of DDOSTBC over the arbitrarily correlated Rayleigh channels we propose a precoder which minimizes an upper bound of the PEP. The proposed DDOSTBC are able to achieve higher coding gain than the similar rate existing DD coding scheme. In addition, the proposed precoded DDOSTBC achieves performance gain for correlated channels as compared to the unprecoded DDOSTBC.

We propose a wireless access scheme which is based on a channel reservation sharing method for a group of mobile users. This proposed scheme exploits the mobility diversity of the mobile users in order to reduce the cost of wireless access. Another aspect of the proposed scheme is contention resolution among mobile users belonging to the same group in order to access the reserved channel while they are at the same location. A game theoretic model is developed for this wireless access scheme through which the rational mobile users can minimize the cost of wireless access while satisfying their quality-of-service (QoS) requirements (e.g., packet loss rate and average packet waiting time). The proposed game model consists of two interrelated formulations: a coalitional game for channel reservation and a stochastic game for channel access. The stable coalitional structure and equilibrium channel access policy are obtained from this game model.

Current multibeam satellite systems perform centralized power and carrier allocation assuming a multi-frequency time division multiple access (MF-TDMA) scheme with fixed regular frequency reuse. This approach is not optimal when individual SINR constraints are to be met. In this paper, we formulate the problem of joint power and carrier allocation allowing non-regular frequency reuse. This setting will increase the interference levels but in turn will allow a more efficient use of the power increasing the spectral efficiency. We focus on a distributed formulation of the problem and compare it with the conventional power balancing solution. Numerical results show that the centralized power balancing approach performs better for high SINR constraints, balancing the co-channel interference among all the users and beams. Conversely, the decentralized approach optimizes the satellite terminals' self-interest deteriorating SINR of high loaded beams resulting in worse performance for highly loaded beams.

Due to the limited energy supplies of nodes in many applications like wireless sensor networks, energy efficiency is crucial for extending the lifetime of these networks. This paper addresses the routing problem for outage-restricted multihop wireless ad hoc networks based on cooperative transmission. The source node wants to transmit messages to a single destination. Other nodes in the network may operate as relay nodes. In this paper, a new cooperative routing protocol is introduced using the Alamouti space-time code for the purpose of energy savings, given a required outage probability at the destination. Two efficient power allocation schemes are derived, which depend only on the statistics of the channels. In the first scheme, each node needs to know only the local channel statistics, and can be implemented in a distributed manner. In the second scheme, a centralized power control strategy is proposed, which has a higher energy efficiency, at the expense of more complexity and signalling overhead. Compared to non-cooperative multihop routing, an energy saving of 80% is achievable in line networks with 3 relays and an outage probability constraint of 10-3at the destination.

In this paper, we propose a game theoretic approach to tackle the problem of the distributed formation of the hierarchical network architecture that connects the nodes in the uplink of a wireless multi-hop network. Unlike existing literature which focused on the performance assessment of hierarchical multi-hop networks given an existing topology, this paper investigates the problem of the formation of this topology among a number of nodes that seek to send data in the uplink to a central base station through multihop. We model the problem as a hierarchical network formation game and we divide the network into different hierarchy levels, whereby the nodes belonging to the same level engage in a noncooperative Nash game for selecting their next hop. As a solution to the game, we propose a novel equilibrium concept, the hierarchical Nash equilibrium, for a sequence of multi-stage Nash games, which can be found by backward induction analytically. For finding this equilibrium, we propose a distributed myopic dynamics algorithm, based on fictitious play, in which each node computes the mixed strategies that maximize its utility which represents the probability of successful transmission over the multi-hop communication path in the presence of interference. Simulation results show that the proposed algorithm presents significant gains in terms of average achieved expected utility per user up to 125.6% relative to a nearest neighbor algorithm.

In this paper, we propose a game theoretical approach to tackle the problem of the distributed formation of the uplink tree structure among the relay stations (RSs) and their serving base station (BS) in an IEEE 802.16j WiMAX network. Unlike existing literature, which focused on the performance assessment of the network in the presence of the RSs, we investigate the topology and dynamics of the tree structure in the uplink of an 802.16j network. We model the problem as a network formation game, where each RS aims to maximize its utility that accounts for the gains from cooperation in terms of bit error rate (BER) and the delay costs resulting from multi-hop transmission. The proposed utility model is based on the concept of the R-factor which is a parameter suitable for assessing the performance of VoIP services. For forming the tree structure, we propose a distributed myopic best response dynamics in which each RS can autonomously choose the path that connects it to the BS through other relays while optimizing its utility. Using the proposed dynamics, the RSs can self-organize into the tree structure, and adapt this topology to environmental changes such as mobility while converging to a Nash tree network. Simulation results show that the proposed algorithm presents significant gains in terms of average achieved MS utility reaching up to 42.57% compared to the star topology where all RSs are directly connected to the BS, and up to 44.78% compared to the case with no RSs.

Collaborative spectrum sensing among secondary users (SUs) in cognitive networks is shown to yield a significant performance improvement. However, there exists an inherent trade off between the gains in terms of probability of detection of the primary user (PU) and the costs in terms of false alarm probability. In this paper, we study the impact of this trade off on the topology and the dynamics of a network of SUs seeking to reduce the interference on the PU through collaborative sensing. Moreover, while existing literature mainly focused on centralized solutions for collaborative sensing, we propose distributed collaboration strategies through game theory. We model the problem as a non-transferable coalitional game, and propose a distributed algorithm for coalition formation through simple merge and split rules. Through the proposed algorithm, SUs can autonomously collaborate and self-organize into disjoint independent coalitions, while maximizing their detection probability taking into account the cooperation costs (in terms of false alarm). We study the stability of the resulting network structure, and show that a maximum number of SUs per formed coalition exists for the proposed utility model. Simulation results show that the proposed algorithm allows a reduction of up to 86.6% of the average missing probability per SU (probability of missing the detection of the PU) relative to the non-cooperative case, while maintaining a certain false alarm level. In addition, through simulations, we compare the performance of the proposed distributed solution with respect to an optimal centralized solution that minimizes the average missing probability per SU. Finally, the results also show how the proposed algorithm autonomously adapts the network topology to environmental changes such as mobility.

In this paper, we study how to improve the performance of a decode-and-forward protocol based cooperative system over the delay constrained channels. We propose a simple transmission scheme, which makes the cooperative system tolerant of the delays caused by the poor synchronization of the relaying nodes. The proposed scheme is able to provide an improved coding gain in unsynchronized cooperative network as compared to the existing delay tolerant distributed space-time block codes.

In this paper, we propose an idea on how game and information theoretic results can be combined to analyze the performance of wireless cooperative networks. More precisely, we consider a four node wireless network, where the transmit nodes help each other acting as relays during the periods in which they do not transmit their own information. In order to help the other node, each node has to use a part of its available power to relay the signal of the other transmitter. The network is modeled as a non-cooperative game in which each player (node) maximizes its own utility function (information rate). The goal of the game designer (network provider) is to maximize the objective function (in this case the sum rate) in order to get better network efficiency. Here, we analyze the so called equilibrium efficiency, as the ratio between the objective function at the worst Nash equilibrium and the optimal objective function. Using game theoretical language, it is the price of anarchy of the proposed game. In this scenario, the Nash equilibrium is achieved by selfish (non-cooperative) behavior between the players. In other words, in order to maximize its own utility function each node chooses a strategy to use its available power only for itself, and not helping the other node. Earlier, we derived an upper bound for the worst case equilibrium efficiency and in this paper we present a lower bound. From the comparisons, we conclude that for path loss coefficients that are of practical importance the proposed bounds are tight. Our results show that the worst case equilibrium efficiency for the proposed simple network is very small (below 10%). Hence, there is a large possibility for improvements if the network nodes are encouraged to cooperate by designing certain mechanisms.

In this paper, we consider the multiple input multiple output (MIMO) quasi static channel. Our objective is to study the power allocation (over the transmit antennas) problem where not only the performance with respect to (w.r.t.) the transmission reliability but also the cost in terms of the consumed power is accounted for. We first review the existing results w.r.t energy efficiency functions (benefit per cost) which focus mainly on the single input single output (SISO) case and then propose several extensions to the MIMO case. Then, we introduce a new energy efficiency metric based on the outage probability. We conjecture that there is a non-trivial solution to the proposed optimization problem. Several special cases are thoroughly analyzed and simulation results will be provided to sustain the conducted analysis.

Physical layer security is an emerging security area that explores possibilities of achieving perfect secrecy data transmission between the intended network nodes, while possible malicious nodes that eavesdrop the communication obtain zero information. The so-called secrecy rate can be improved using friendly jammers that introduce extra interference to the eavesdroppers. Here, we investigate the interaction between the multiple source-destination links and a friendly jammer who assists by ''masking" the eavesdropper. In order to obtain a distributed solution, one possibility is to introduce a distributed auction theoretic approach. The auction is defined such that the source-destination links provide bids for the jammer to interfere the eavesdropper, therefore increasing their secrecy capacities. We propose a distributed auction using the share auction and iteratively updating the bids. To compare with the performances, we construct a centralized solution and a VCG auction, which cannot be implemented in practice. Our analysis and simulation results show the effectiveness of friendly jamming and convergence of the proposed scheme. The distributed game solution is shown to have similar performances to those of the centralized ones.

We consider a wireless relay network with multiple antennas terminals over Rayleigh fading channels, and apply distributed space-time coding (DSTC) in amplify-and-forward (A&F) mode. It turns out that, combined with power allocation in the relays, A&F DSTC results in an opportunistic relaying scheme, in which the best relay is selected to retransmit the source's space-time coded signal. Next, assuming M-PSK or M-QAM modulations, we analyze the performance of the cooperative diversity wireless networks using A&F opportunistic relaying with the multiple-antennas source and destination. We first derive the probability density function (PDF) of the received SNR at the destination. Then, the PDF is used to determine the symbol error rate in Rayleigh fading channels. Then, we derived closed-form approximations for SER in high SNR scenario, from which we find the diversity order of system Rmin{Ns, Nd}, where R, Ns, and Nd are the number of the relays, the source antennas, and the destination antennas, respectively. Simulation results show that the proposed system obtain 2 dB gain in SNR over DSTC for BER 10-5, when R = 2, Ns= 2, Nd= 2.

Physical layer security is an emerging security field that explores the possibilities of achieving perfect secrecy data transmission between the intended network nodes, while possible malicious nodes that eavesdrop the communication obtain zero information. In this paper, we observe how node cooperation improves the physical layer security of a simple wireless network by reducing the surface of the geographical area in which the malicious nodes can listen to the transmitted data from the source to the destination. Our analysis and simulation results show a dramatic improvement even by cooperation with only one relay node. The improvement gets better by adding more cooperating nodes. We also observe that if cooperating nodes are closer to the line that connects the source and the destination node, the region in which the malicious node can profit from the eavesdropping gets smaller.

We consider the performance of orthogonal and non-orthogonal space-time block codes (STBCs) over arbitrarily correlated Rayleigh fading multiple-input multiple-output (MIMO) channels. A pairwise error probability (PEP) bound of arbitrary STBC is derived. Based on this PEP bound, we design a precoder to improve the performance of the orthogonal and non-orthogonal STBC over arbitrarily correlated channels.

Cooperation between wireless network nodes is a promising technique for improving the physical layer security of wireless transmission, in terms of secrecy capacity, in the presence of multiple eavesdroppers. While existing physical layer security literature answered the question “what are the link-level secrecy capacity gains from cooperation?”, this paper attempts to answer the question of “how to achieve those gains in a practical decentralized wireless network and in the presence of a secrecy capacity cost for information exchange?”. For this purpose, we model the physical layer security cooperation problem as a coalitional game with non-transferable utility and propose a distributed algorithm for coalition formation. Through the proposed algorithm, the wireless users can autonomously cooperate and self-organize into disjoint independent coalitions, while maximizing their secrecy capacity taking into account the security costs during information exchange. We analyze the resulting coalitional structures, discuss their properties, and study how the users can self-adapt the network topology to environmental changes such as mobility. Simulation results show that the proposed algorithm allows the users to cooperate and self-organize while improving the average secrecy capacity per user up to 25.32% relative to the non-cooperative case.

Joint transmitter and receiver design problem for frequency-selective, time-invariant fading channels are studied. The authors first propose a simple approximate maximum likelihood serial decision-feedback equaliser (A-ML-SDFE) through DFE, a Gaussian approximation, a pre-whitening filter and a matched filter. Secondly, assuming full channel knowledge available at the transmitter side, the authors perform pre-equalisation in a downlink scenario by moving the decision-feedback part of the A-ML-SDFE to the transmitter. The proposed A-ML-SDFE achieves much better performance than linear minimum mean square error (MMSE) and MMSE-DFE with a lower complexity. The pre-equaliser further improves the system performance at low signal-to-noise ratio with a reduced receiver complexity.

This letter analyzes the performance of single relay selection cooperative wireless networks usingamplify-andforwardrelaying. The network channels are modeled as independent, nonidentical, Rician distributed coefficients. We derive approximate formulas for the symbol error rate (SER) of the opportunistic relaying cooperative network. We first derive the PDF of the approximate value of the total SNR. Then, assumingM-PSK orM-QAM modulations, the PDF is used to determine the SER. For sufficiently large SNR, this letter derives the close-form average SER. The simplicity of the asymptotic results provides valuable insights into the performance of cooperative networks and suggests means of optimizing them. We also use simulation to verify the analytical results. Results show that the derived error rates are tight bounds particularly at medium and high SNR.

In this paper, we propose a maximum likelihood (ML) decoder for differentially encoded full-rank square nonorthogonal space-time block codes (STBCs) using unitary or non-unitary signal constellations, which is also applicable to full-ranked orthogonal STBC (OSTBC). As the receiver is jointly optimized with respect to the channel and the unknown data, it does not require any knowledge of channel power, signal power, or noise power to decode the signal, and the decision is purely based on two consecutively received data blocks. We analyze the effect of channel correlation on the performance of the proposed system in Rayleigh fading channels. Assuming a general correlation model, an upper bound of the pair-wise error probability (PEP) of the differential OSTBCs is derived. An approximate bound of the PEP for the differential nonorthogonal STBCs is also derived. We propose a precoder designing criterion for differential STBC over arbitrarily correlated Rayleigh channels. Precoding improves the system performance over the correlated Rayleigh MIMO channels. Our precoded differential codes differ from the previously proposed precoder designs for differential OSTBC in the following ways: 1) We propose a precoder design for arbitrarily correlated Rayleigh channels, whereas the previous work considers only for transmit correlation. 2) The previous work is only applicable to the OSTBCs with PSK constellations, whereas our precoder is applicable to any type of full-rank square STBCs with unitary and non-unitary signal constellations.

Cooperative communications via distributed space-time codes has been recently proposed as a way to form virtual multiple-antennas that provide dramatic gains in slow fading wireless environments. In this paper, we consider the design of practical distributed space-time codes for wireless relay networks using the amplify-and-forward (AF) scheme, where each relay transmits a scaled version of the linear combinations of the received symbols and their complex conjugate. We employ GABBA codes, which are systematically constructed, orthogonally decodable, full-rate, full-diversity space-time block codes, in a distributed fashion. Our scheme is valid for any number of relays with linear orthogonal decoding in the destination, which make it feasible to employ large numbers of potential relays to improve the diversity order. We generalize the distributed space-time codes in AF mode when the source-destination link contributes in both phases of the transmission. Assuming MPSK or M-QAM constellations and maximum likelihood (ML) detection, we derive an approximate formula for the symbol error probability of the investigated scheme in Rayleigh fading channels. The analytical results are confirmed by simulations, indicating both the accuracy of the analysis, and the fact that low-complexity, flexible, and high-performing distributed space-time block codes can be designed based on GABBA codes.

Cooperation in wireless networks allows single antenna devices to improve their performance by forming virtual multiple antenna systems. However, performing a distributed and fair cooperation constitutes a major challenge. In this work, we model cooperation in wireless networks through a game theoretical algorithm derived from a novel concept from coalitional game theory. A simple and distributed merge-and-split algorithm is constructed to form coalition groups among single antenna devices and to allow them to maximize their utilities in terms of rate while accounting for the cost of cooperation in terms of power. The proposed algorithm enables the users to self-organize into independent disjoint coalitions and the resulting clustered network structure is characterized through novel stability notions. In addition, we prove the convergence of the algorithm and we investigate how the network structure changes when different fairness criteria are chosen for apportioning the coalition worth among its members. Simulation results show that the proposed algorithm can improve the individual user's payoff up to 40.42% as well as efficiently cope with the mobility of the distributed users.

In this paper, distributed double-differential coding is proposed to avoid the problem of carrier offsets in amplify and forward protocol based cooperative network with two relays. A double-differential orthogonal-space-time block code is transmitted by the relays in a distributed manner without channel knowledge. We derive a low complexity linear decoder which does not require the channel and carrier offset knowledge. A pairwise error probability (PEP) analysis is also conducted and we found an upper bound of the PEP for the proposed system. In addition, an optimized power allocation is proposed to improve the SER performance of the system.

This paper investigates the problem of the formation of an uplink tree structure among the IEEE 802.16J network's relay stations (RSs) and their serving base station (MR-BS). We model the problem as a network formation game in which the RSs want to form a directed tree graph to improve their utility, in terms of the packet success rate (PSR), by using multi-hop cooperative transmission while accounting for a link maintenance cost. In this game, the relay stations engage in bilateral negotiations which result in a contractual agreement to form a directed link between each pair. For network dynamics, we propose an algorithm based on the local best responses of the RSs that converges to a local Nash network. Moreover, the proposed dynamics algorithm allows the RSs to autonomously adapt the network topology to changes in the environment due to mobility or to the presence of heterogeneous traffic. Simulation results show how the RSs can self-organize in a tree structure while improving the network's overall PSR up to 19.7% and 17.3% compared, respectively, to the cases where no RSs exist and where the RSs are directly connected to the MR-BS.

Communications over multiple-input multiple-output (MIMO) channels with carrier offsets is an important practical and theoretical problem. Double-differential coding is a technique, which allows the receiver to decode the data without any channel or carrier offset knowledge. We propose a double-differential (DD) coding scheme which is applicable to any square orthogonal space-time block codes (OSTBC) using M-PSK constellation. The main advantages of the proposed DD coding scheme are: 1) The previously proposed DD codes are applicable only to the specific class of space-time block codes which follow the diagonal unitary group property, whereas our DD coding is applicable to any square OSTBC. 2) We propose a suboptimal decoder which preserves the linear decoding property of the OSTBC. We derive an upper bound of the pairwise error probability (PEP) of the proposed double-differential orthogonal space-time block codes (DDOSTBCs). The proposed DDOSTBC is able to achieve better performance than the similar rate existing DD coding scheme. In addition, the proposed DDOSTBC outperforms the conventional training based system.

This paper proposes an affine projection adaptive filtering algorithm incorporating a data selection strategy based on the set-membership concept along with a partial update technique. The resulting algorithm is flexible in the sense that it allows more general tradeoff between speed of convergence and misadjustment while constraining the overall computational complexity. Simulation experiments in a typical echo cancellation environment confirm the effectiveness of the proposed algorithm.

In this paper, we consider the joint transmitter and receiver design problem based on a novel approximate maximum likelihood decision feedback equalizer (A-ML-DFE) over frequency-selective, time- invariant fading channels. By applying decision feedback equalization, Gaussian approximation, a pre-whitening filter, and a matched filter, the proposed A-ML-DFE can realize single symbol detection. The proposed scheme achieves near-optimal performance with a complexity lower than the linear MMSE and the MMSE-DFE. By assuming full channel knowledge, we perform pre-equalization and precoding in a downlink scenario at the transmitter side. The pre-equalizer can achieve perfect decision feedback and thus, provide better performance at low signal- to-noise ratio (SNR) with a reduced receiver complexity. The precoder designed to minimize the analytical symbol error rate (SER) of this system can further improve the performance.

In this paper, we consider the effect of carrier offsets in arbitrarily correlated flat-fading multiple-input multiple-output (MIMO) Rayleigh channels. We propose a double-differential coding for square orthogonal space-time block code (OSTBC) with non-unitary constellations over flat-fading MIMO channels with carrier offsets. In order to improve the performance of the proposed double-differential orthogonal space-time block code (DDOSTBC) over arbitrarily correlated Rayleigh channel we introduce a full memoryless precoder matrix in the transmitter. For the proposed precoded DDOSTBC, a pair-wise error performance (PEP) analysis is conducted. Then we propose a precoder design based on the PEP upper bound. The proposed DDOSTBC differs from the previously proposed DDOSTBC by: 1) The previously proposed DDOSTBC is aplicable to only a specific class of OSTBC which follows diagonal group property, whereas, the proposed DDOSTBC is applicable to any square OSTBC. 2) The previously proposed DDOSTBC can work withM-PSK constelation only, whereas, the proposed DDOSTBC is applicable toM-QAM,M-PAM, orM-PSK constellations. The proposed DDOSTBC provides higher performance gain as compared to the same rate previously proposed DDOSTBC. In addition, the proposed precoded DDOSTBC is able to perform better than the unprecoded DDOSTBC over arbitrarily correlated Rayleigh channels.

This paper presents a novel adaptive reduced-rank multi-input-multi-output (MIMO) linear equalization structure based on joint iterative optimization of adaptive filters. The proposed reduced-rank linear equalization structure consists of a joint iterative optimization of two equalization stages, namely, a projection matrix that performs dimensionality reduction and a reduced-rank linear equalization filter that retrieves the desired transmitted symbol. The novel linear reduced-rank structure is responsible for cancelling the inter- antenna interference caused by the associated data streams and exploiting the available degrees of freedom at the antenna-array receiver. We describe least squares (LS) expressions for the design of the projection matrix and the reduced-rank filter along with computationally efficient recursive least squares (RLS) adaptive estimation algorithms. Simulations for a MIMO linear equalization application show that the proposed scheme outperforms the state-of-the-art reduced-rank and the conventional estimation algorithms at about the same complexity.

The minimum power allocation strategy for the repetition-based amplify-and-forward (AF) relaying scheme given a required symbol error rate (SER) at the destination is first derived. We consider the scenario where one source and multiple partners cooperate to transmit messages to the destination. The power allocation strategy that demands the minimum transmit power of all cooperating partners is described and analyzed. However, this power minimization strategy does not necessarily maximize the lifetime of battery-limited systems. Then, we propose two other AF cooperative schemes to exploit the statistical channel state information (CSI), the residual battery energy, and the quality-of-service (QoS) requirement. It is shown that the network lifetime can be extended considerably by taking all these three factors into account.

In this paper, we propose a novel and simple approximate maximum likelihood detector (A-ML-D) for single input and single output (SISO) systems over frequency-selective fading channels based on Tomlinson-Harashima pre-equalizer. By assuming full channel state information (CSI) at the transmitter side, the pre-equalizer can remove some inter-symbol interference (ISI) at the transmitter. At the receiver, we implement a Gaussian approximation, a pre-whitening filter and a matched filter to realize a set of parallel SISO schemes, and thus, facilitate single symbol detection. The proposed scheme can obtain full multi-path diversity and achieve near-optimal performance with a complexity lower than linear MMSE and MMSE-DFE. Analytical symbol error rate (SER) is derived to further justify the proposed detector.

In this paper, we propose double-differential (DD) modulation for an amplify-and-forward protocol based on cooperative communication over Nakagami-m fading channels. The proposed scheme is able to achieve performance gain in the presence of random carrier offsets and without channel knowledge at relay or destination. The proposed scheme reflects its utility for the Nakagami-m type flat fading channels with carrier offsets, where the conventional single differential scheme breaks down. In addition, the proposed scheme outperforms the conventional direct transmission double-differential system.

This paper presents a novel adaptive reduced- rank multi-input-multi-output (MIMO) decision feedback equalization structure based on joint iterative optimization of adaptive estimators. The novel reduced-rank equalization structure consists of a joint iterative optimization of two equalization stages, namely, a projection matrix that performs dimensionality reduction and a reduced-rank estimator that retrieves the desired transmitted symbol. The proposed reduced- rank structure is followed by a decision feedback scheme that is responsible for cancelling the inter-antenna interference caused by the associated data streams. We describe least squares (LS) expressions for the design of the projection matrix and the reduced-rank estimator along with computationally efficient recursive least squares (RLS) adaptive estimation algorithms. Simulations for a MIMO equalization application show that the proposed scheme outperforms the state-of-the-art reduced-rank and the conventional estimation algorithms at about the same complexity.

In this contribution, the performance of a multiuser system is analyzed in the context of frequency selective fading channels. Using game theoretic tools, a useful framework is provided in order to determine the optimal power allocation when users know only their own channel (while perfect channel state information is assumed at the base station). This scenario illustrates the case of decentralized schemes, where limited information on the network is available at the terminal. Various receivers are considered, namely the matched filter, the MMSE filter and the optimum filter. The goal of this paper is to extend previous work, and to derive simple expressions for the non-cooperative Nash equilibrium as the number of mobiles becomes large and the spreading length increases. To that end two asymptotic methodologies are combined. The first is asymptotic random matrix theory which allows us to obtain explicit expressions of the impact of all other mobiles on any given tagged mobile. The second is the theory of non-atomic games which computes good approximations of the Nash equilibrium as the number of mobiles grows.

Abstract — This paper derives exact expressions for the symbol error rate (SER) of orthogonal space-time block codes over a spatially correlated multiple-input multiple-output (MIMO) channel, in which the signal propagation suffers from a keyhole effect. A correlated double Rayleigh fading keyhole channel is assumed and easy to evaluate expressions are presented for multi-level phase shift keying (M-PSK), pulse amplitude modulation (M-PAM), and quadrature amplitude modulation (M-QAM). These expressions are verified by estimating the SER via the simulation of the MIMO system. The given expressions are then used to quantify the performance improvements attainable with minimum SER linear precoding over a correlated keyhole channel. I.

A systematic theory is introduced for finding the derivatives of complex-valued matrix functions with respect to a complex-valued matrix variable and the complex conjugate of this variable. In the framework introduced, the differential of the complex-valued matrix function is used to identify the derivatives of this function. Matrix differentiation results are developed for use in signal processing and communications applications. Several other examples are given.

Estimating channel state information (CSI) in the fast fading conditions is very challenging. In this paper, a simple structure for cooperative diversity in decode-and-forward mode is examined, in which the transmitter, the relay and the receiver do not require to know CSI. In this scheme, transmission of information is done in a two phase process. In the first phase, differential modulated signals are radiated from the source. After decoding the received signal in the relay side, the source and the relay collectively send information using differential space-time codes. The optimum power allocation in a sense of minimizing pairwise error probability (PEP) in high SNR scenarios has been obtained using an analytical method. An interesting property of the optimum point is that it is independent of the channel statistics and the position of the relay, which improves its feasibility in the future communication systems. Numerical simulations verify the analytical results.

We present symbol error rate (SER) expressions for the double differential (DD) modulation with MPSK constellations (DDMPSK) over AWGN, Rayleigh, and Ricean fading channels with carrier offsets in a single-input single-output (SISO) system. A closed-form expression is derived for SER in double differential binary phase shift keying (DDBPSK) over AWGN channels with carrier offset. Next, we derive a closed-form solution for the SER of DDBPSK and lower and upper bounds over the SER for DDMPSK signals over Rayleigh fading channels with carrier offset. A closed-form SER formulation for DDBPSK over Ricean fading channel with carrier offset is also presented.

The idea of space-time coding, devised for multiple-antenna systems, can be applied to communication over a wireless relay network. This paper is on the design of practical distributed space-time codes for wireless relay networks, in which uses amplify-and-forward (AF) scheme in a way that each relay transmits a scaled version of linear combination of the received symbols. We use the recently presented GABBA codes, which are systematically constructed, orthogonally decodable, full-rate, full-diversity space-time block codes, in a distributed fashion. Our scheme is generalized to any number of relays with linear orthogonal decoding in the destination, which make it feasible to employ large number of potential relays to improve diversity order. Assuming BPSK modulation and maximum likelihood (ML) detection, we derive a formula for the bit error probability of the investigated scheme in Rayleigh fading channels. Our analytical results have been confirmed by simulation results, using full-rate, full-diversity distributed codes.

A memoryless precoder is designed for orthogonal space-time block codes (OSTBCs) for multiple-input multiple-output (MIMO) channels exhibiting joint transmit-receive correlation. Unlike most previous similar works which concentrate on transmit correlation only and pair-wise error probability (PEP) metrics. 1) The precoder is designed to minimize the exact symbol error rate (SER) as function of the channel correlation coefficients, which are fed back to the transmitter. 2) The correlation is arbitrary as it may or may not follow the so-called Kronecker structure. 3) The proposed method can handle general propagation settings including those arising from a cooperative macro-diversity (multi-base) scenario. We present two algorithms. The first is suboptimal, but provides a simple closed-form precoder that handles the case of uncorrelated transmitters, correlated receivers. The second is a fast-converging numerical optimization of the exact SEE which covers the general case. Finally, a number of novel properties of the minimum SER precoder are derived

Cooperative diversity is a rapidly emerging topic for wireless communications, with ad hoc and hybrid/relay networks as two main applications so far. In this paper, we investigate the cooperative diversity concept for MIMO multicell networks, where the processing must be optimized to account for the variability of the channel conditions across the cooperative devices. This can be done via distributed preceding and is realistically based on channel statistics (average gains, correlations, etc.). We give a new approach to the previously coined equal diversity spread principle, through minimization of an approximated SER expression. Next, we focus on a low-complexity approach to minimizing a PEP-based performance measure. Gains are evaluated in a multicell scenario with collaborating base stations.

This paper explores the idea of cooperative spatial multiplexing for use in MIMO multicell networks. We imagine applying this cooperation for several multiple antenna access-points to jointly transmit streams towards multiple single-antenna user terminals to neighbouring cells. We make the setting more realistic by introducing a constraint on the hybrid channel state information (HCSI), assuming that each transmitter has full CSI for its own channel, but only statistical information about other transmitters' channels. Each cooperating transmitter then makes guesses about the behaviour of the other transmitters, using the statistical CSI. We show two of several possible transmission strategies under this setting, and include simple optimization at the receiver to improve performance. Comparisons are made with fully cooperative (full CSI) and non-cooperative schemes. Simulation results show a substantial cooperation gain despite the lack of instantaneous information

In this paper, we investigate the cooperative diversity concept for use in MIMO multi-cell networks. We show that, in such networks, cooperative diversity processing must be optimized to account for the variability of channel conditions across the cooperative devices. This can be done via distributed precoding and, in mobile networks, it is based realistically on channel statistics. The cooperative MIMO correlation matrix admits a special structure which is used to optimize the precoder. We investigate algorithms for exact error-rate and low-complexity approximated optimization. Gains are evaluated in multi-cell scenarios with collaborating base stations.

Block equalizer structures formulated under minimum-mean-square-error (MMSE) criteria are known results which perform MMSE power allocation via eigenvector decomposition of certain defining channel dependent matrices. In a practical channel-estimate based scenario, however, performing such eigenvector decomposition is a formidable task. This paper proposes an alternative suboptimal power allocation scheme where a Cholesky decomposition is used for the problem parametrization. The proposed solution shows to exhibit small degradation in BER while offering efficient and numerically reliable methods for obtaining the equalizer matrices. We restrict our approach to zero-order and interblock-interference (IBI) free structures, and verify the performance of the proposed schemes via simulations

A memoryless linear precoder is designed for orthogonal space-time block codes (OSTBC) for improved performance over block-fading flat correlated Rayleigh fading multiple-input multiple-output (MIMO) channels. Original features of the proposed technique include 1) the precoder can handle both transmit and receive correlation, and 2) the precoder handles any arbitrary joint correlation structure, including the so-called Kronecker (non-Kronecker) correlation models. The precoder is designed to minimize a symbol error-based metric as function of the joint slowly-varying channel correlation coefficients, which are supposed to be known to the transmitter. Several useful properties of the optimal precoder are given, evidencing the impact of receive correlation on transmitter optimization in certain situations. An iterative fast-converging numerical optimization algorithm is proposed. Monte Carlo simulations over fading channels are used to validate our claims.

The problem of multi-user receiver design in direct sequence single antenna-code division multiple access (DS-CDMA) uplink networks is studied over multipath channels. An exact expression for the bit error rate (BER) is derived and an algorithm is proposed for finding the finite impulse response (FIR) receiver filters such that the exact BER of the active users is minimized. The algorithm performance is found for scenarios with different channel qualities, receiver filter lengths, near-far effects, and channel mismatch. The proposed FIR receiver structure has significant better BER with respect to Eb/N0and near-far resistance than the corresponding minimum mean square error (MSE) filters

In orthogonal frequency-division multiplexing (OFDM) systems, an error in the receiver demodulation frequency causes large errors in the detected signal values, which increases the probability of bit errors. Therefore it is important to minimize this offset. In this paper, we propose an interpolation-based high-precision frequency acquisition method for OFDM systems. Its precision is shown to be higher than that of existing methods and it is applicable for acquisition purposes as only one OFDM training symbol block is required and it has no inherent limit for the carrier frequency offset acquisition range, and it is independent of a carrier phase offset that may occur

The problem of designing an optimal prefilter transform for finite impulse response (FIR) multiple-input multiple-output (MIMO) communication systems is addressed. The bit error rate (BER) is minimized under a power constraint. Binary signaling is assumed. The transmitter requires knowledge of the channel transfer function coefficients and the receiver transform. It is shown that the proposed prefiltering transform outperforms three previously known prefilter transform methods in terms of BER versus channel quality. The proposed prefilter transform is applicable to downlink communication systems where the base station can estimate the channel while receiving data from the mobile station.

This paper presents a closed-form linear precoder for a MIMO spatial multiplexing (SM) system in the presence of transmit correlation and a Ricean component. Existing SM (V-BLAST and similar schemes), based upon channel matrix inversion, rely on the linear independence of antenna channel responses for stream separation and suffer considerably from high levels of fading correlation and/or dominating ill-conditioned line-of-sight channel components. We propose a simple algorithm that adjusts the transmitted constellation through power weighting and phase shifts that can be interpreted in some extreme cases as a higher order constellation design scheme. We obtain a rate-preserving MIMO multiplexing scheme that can operate smoothly at any degree of transmit correlation and any type of LOS channel component.

A memoryless precoder is designed for orthogonal space-time block codes for multiple-input multiple-output channels exhibiting joint transmit-receive correlation. Unlike most previous similar work which concentrated on transmit correlation only and pair-wise error probability (PEP) metrics, the precoder is designed to minimize the exact symbol error rate (SER) as a function of the channel correlation coefficients, which are fed back to the transmitter, and the correlation may or may not follow the so-called Kronecker structure. The proposed method can handle general propagation settings including those arising from a cooperative macro-diversity (multi-base) scenario. We present two algorithms. The first is suboptimal, but provides a simple closed-form precoder that handles the case of uncorrelated transmitters, correlated receivers. The second is a fast-converging numerical optimization which covers the general case. The results show the SER based precoder has small gains over the PEP based precoder for moderate signal-to-noise ratios (SNR). Several properties of the minimum SER precoder are given.

The problem of uplink communications is considered where multiple mobile terminals equipped with multiple antennas are communicating with one base station also having multiple antennas. A theory for jointly optimizing the transmitter and receiver finite impulse response (FIR) multiple-input multiple-output (MIMO) filters is developed. The signals from the individual users are assumed to be uncorrelated with each other and the additive channel noise is uncorrelated with the original signals. The FIR MIMO channel filters are assumed to be known for both the mobile terminals and the base station. All the input signals to the FIR MIMO channels are assumed to be power-constrained. For given FIR MIMO channels with maximum allowable average input powers, the transmitter and receiver FIR MIMO filters are jointly optimized such that the mean square error (MSE) between the desired and reconstructed signals is minimized. An iterative numerical optimization algorithm is proposed. Numerical simulation results show that the proposed method has substantially better performance than a code division multiple access (CDMA) based system using minimum MSE receiver filters.

A generalized maximum likelihood symbol synchronization scheme is proposed. It enables use of variable fractional-delay (interpolation) filters when the desired sampling rate conversion factor is non-integer or time varying. Furthermore, both IIR and FIR type variable filter structures can be used in efficient configurations. The paper includes both performance analysis and discussion of implementation complexity.

A theory for jointly optimizing nonuniform analysis and synthesis FIR filter banks with arbitrary filter lengths and an arbitrary delay through the filter bank is developed. The FIR subband coder is optimized with respect to the minimum mean square error between the output and the input signals under a bit constraint. The subband quantizers are modeled as additive noise sources. Theoretical comparisons are made against a well-known 5-3 wavelets used in a tree-structure. The proposed filter banks, which are both rate- and source-dependent, have a better distortion rate performance. Equations for finding jointly optimized analysis and synthesis filter banks under a power constraint are also presented.

The performance of subband image coders depends on the proper choice of filter banks. It is therefore desirable to find algorithms for optimizing the filter banks. A theory for finding jointly optimized analysis and synthesis filter banks with arbitrary filter lengths and arbitrary delay through the filter bank is proposed. Theoretical results are included. Comparisons are made against the well known 5/3, 9/7, and 10/18 wavelets. The proposed filter banks, which are both rare and source dependent, have a better distortion rate performance for all cases.

A subband coder structure is fully optimized with respect to the minimum block mean squared error between the output and the input signals under a bit constraint. The analysis filter bank structure generates maximally decimated and equal bandwidth subbands. The subband quantizers are modeled as additive noise sources. To simplify the optimization an optimal multiple-input multiple-output system is first derived. Illustrative examples showing the system performance as well as filter transfer functions are given. The performance results are compared to the rate distortion curves.

A method for coding a source modeled by an infinite Gaussian mixture distribution is proposed. The source is first split into N classes. The samples of each class are then quantized by an infinite-level uniform threshold quantizer followed by an entropy coder designed for each class. The problem of joint optimization of this system's rate distortion performance is first solved theoretically, assuming an exponential mixing density. A comparison to a system optimal for high rates, using one common quantizer for all classes, showed that for a fixed distortion the rate was reduced by 11-12% at low rates for a fixed N=5. A subband image coder, using the optimum theoretical parameter values was simulated. The resulting coder has high performance and low complexity.