In 5G and beyond networks, the radio communication between a User Equipment (UE) and a base station (gNodeB or gNB), also known as the air interface, is a critical component of network access and connectivity. During the connection establishment procedure, the Radio Resource Control (RRC) layer can be vulnerable to signaling storms, which threaten the availability of the radio access control plane. These attacks may occur when one or more UEs send a large number of connection requests to the gNB, preventing new UEs from establishing connections. In this paper, we investigate the detection of such threats and propose an adaptive threshold-based detection system based on Extreme Value Theory (EVT). The proposed solution is evaluated numerically by applying simulated attack scenarios based on a realistic threat model on top of real-world RRC traffic data from an operator network. We show that, by leveraging features from the RRC layer only, the detection system can not only identify the attacks but also differentiate them from legitimate high-traffic situations. The adaptive threshold calculated using EVT ensures that the system works well under diverse threat scenarios. The results show high accuracy, precision, and recall values (above 93%), and a low detection latency even under complex conditions.

Experimental research in Post-5G involves complex interactions between software, hardware, and protocols. Therefore, it is crucial to develop solutions that allow researchers to conduct their experiments in a reproducible manner. To support this need, the EU SLICES Research Infrastructure (RI) provides a scientific instrument that encompasses all the needs for Post-5G experimental research. The facility is currently being built to enable experimentation with state-of-the-art resources in various fields. SLICES-RI is intent-driven and facilitates the entire lifecycle of thought experiments. This is achieved by enabling reproducible deployment of experiments over the infrastructure using blueprints and by systematically collecting and archiving all outputs through a clear and structured methodology for experimentation. For this demonstration, we focus on the Post-5G part of the facility and will showcase how the entire lifecycle of such an experiment is orchestrated using the tools and functionalities developed. We will showcase blueprints for deploying a cloud-native 5G core and a split 7.2 radio network using open-source software in a fully reproducible manner, with the results being automatically archived and published using the SLICES metadata model. The reproducibility, deployment options, experimenter control capabilities, and access to the collected results will be highlighted.

The development of 6G wireless technologies is rapidly advancing, with the 3rd Generation Partnership Project (3GPP) entering the pre-standardization phase and aiming to deliver the first specifications by 2028. This paper explores the OpenAirInterface (OAI) project, an open-source initiative that plays a crucial role in the evolution of 5G and future 6G networks. OAI provides a comprehensive implementation of 3GPP and O-RAN compliant networks, including Radio Access Network (RAN), Core Network (CN), and software-defined User Equipment (UE) components. This paper details the history and evolution of OAI, its licensing model, and the various projects under its umbrella, such as RAN, the CN, and the Operations, Administration and Maintenance (OAM) projects. It also highlights the development methodology, Continuous Integration/Continuous Delivery (CI/CD) processes, and end-to-end systems powered by OAI. Furthermore, the paper discusses the potential of OAI for 6G research, focusing on spectrum, reflective intelligent surfaces, and Artificial Intelligence (AI)/Machine Learning (ML) integration. The open-source approach of OAI is emphasized as essential for tackling the challenges of 6G, fostering community collaboration, and driving innovation in next-generation wireless technologies.

The fifth generation new radio (5G NR) technology is expected to fulfill reliable and accurate positioning requirements of industry use cases, such as autonomous robots, connected vehicles, and future factories. Starting from Third Generation Partnership Project (3GPP) Release-16, several enhanced positioning solutions are featured in the 5G standards, including the multi-cell round trip time (multi-RTT) method. This work presents a novel framework to estimate the round-trip time (RTT) between a user equipment (UE) and a base station (gNB) in 5G NR. Unlike the existing scheme in the standards, RTT can be estimated without the need to send timing measurements from both the gNB and UE to a central node. The proposed method relies on obtaining multiple coherent uplink wide-band channel measurements at the gNB by circumventing the timing advance control loops and the clock drift. The performance is evaluated through experiments leveraging a real world 5G testbed based on OpenAirInterface (OAI). Under a moderate system bandwidth of 40MHz, the experimental results show meter level range accuracy even in low signal-to-noise ratio (SNR) conditions.

The Open Radio Access Network (Open RAN) vision is based on the three principles of (i) open interfaces; (ii) cloudification; and (iii) automation through closed-loop control. It is a network architecture paradigm embodied and augmented through technical reference specifications of the 3GPP and the O-RAN Alliance. At the centre of Open RAN are open, programmable, and virtualized components, connected to each other through open interfaces that enable closed-loop, data-driven, and intelligent control. For instance, the O-RAN Alliance introduced two RAN Intelligent Controllers (or RICs) that connect through open interfaces to the disaggregated components of the RAN, and implement control loops that run at different time scales.

Innovation and standardization in 5G have brought advancements to every facet of the cellular architecture. This ranges from the introduction of new frequency bands and signaling technologies for the radio access network (RAN), to a core network underpinned by micro-services and network function virtualization (NFV). However, like any emerging technology, the pace of real-world deployments does not instantly match the pace of innovation. To address this discrepancy, one of the key aspects under continuous development is the RAN with the aim of making it more open, adaptive, functional, and easy to manage. In this paper, we highlight the transformative potential of embracingnovel cellular architecturesby transitioning from conventional systems to the progressive principles of Open RAN. This promises to make 6G networks more agile, cost-effective, energy-efficient, and resilient. It opens up a plethora of novel use cases, ranging from ubiquitous support for autonomous devices to cost-effective expansions in regions previously underserved. The principles of Open RAN encompass: (i) a disaggregated architecture with modular and standardized interfaces; (ii) cloudification, programmability and orchestration; and (iii) AI-enabled data-centric closed-loop control and automation. We first discuss the transformative role Open RAN principles have played in the 5G era. Then, we adopt a system-level approach and describe how these Open RAN principles will support 6G RAN and architecture innovation. We qualitatively discuss potential performance gains that Open RAN principles yield for specific 6G use cases. For each principle, we outline the steps that research, development and standardization communities ought to take to make Open RAN principles central to next-generation cellular network designs.

This paper presents a comprehensive analysis and the performance enhancement of short block length channel detection incorporating training information. The current communication systems’ short block length channel detection are assumed to typically consist of least squares channel estimation, followed by quasi-coherent detection. By investigating the receiver structure, specifically the estimator-correlator, we show that the non-coherent term, which is often disregarded in conventional detection metrics, results in significant losses in terms of performance and sensitivity in typical operating regimes of 5G/6G systems. A comparison with the fully non-coherent receiver in multi-antenna configurations reveals substantial losses in low spectral efficiency operating areas. Additionally, we demonstrate that by employing an adaptive DMRS/data power adjustment, it is possible to reduce the performance loss gap which is amenable to a more sensitive quasi-coherent receiver. However, both of the aforementioned ML detection strategies can result in substantial computational complexity when processing long bit length codes. We propose an approach to tackle this challenge by introducing the principle of block/segment coding using First-Order RM Codes which is amenable to low-cost decoding through block-based fast Hadamard transforms. The Block-based FHT has demonstrated to be cost-efficient with regards to decoding time, as it evolves from quadric to quasi-linear complexity with a manageable decline in performance. Additionally, by incorporating an adaptive DMRS/data power adjustment technique, we are able to bridge/reduce the performance gap with respect to the conventional maximum likelihood receiver and attain high sensitivity, leading to a good trade-off between performance and complexity to efficiently handle small payloads.

A science is defined by a set of encyclopedic knowledge related to facts or phenomena following rules or evidenced by experimentally-driven observations. Computer Science and in particular computer networks is a relatively new scientific domain maturing over years and adopting the best practices inherited from more fundamental disciplines. The design of past, present and future networking components and architectures have been assisted, among other methods, by experimentally-driven research and in particular by the deployment of test platforms, usually named as testbeds . However, often experimentally-driven networking research used scattered methodologies, based on ad-hoc, small-sized testbeds , producing hardly repeatable results. We believe that computer networks needs to adopt a more structured methodology, supported by appropriate instruments, to produce credible experimental results supporting radical and incremental innovations. This paper reports lessons learned from the design and operation of test platforms for the scientific community dealing with digital infrastructures. We introduce the SLICES initiative as the outcome of several years of evolution of the concept of a networking test platform transformed into a scientific instrument. We address the challenges, requirements and opportunities that our community is facing to manage the full research-life cycle necessary to support a scientific methodology.

This paper focuses on the problem of separately modulating and jointly estimating two independent continuous-valued parameters sent over a Gaussian multiple-access channel (MAC) under the mean square error (MSE) criterion without bandwidth constraints. To this end, we first improve an existing lower bound on the MSE that is obtained using the parameter modulation-estimation techniques for the single-user additive white Gaussian noise (AWGN) channel. As for the main contribution of this paper, this improved modulation-estimation analysis is generalized to the model of the two-user Gaussian MAC. We present outer bounds to the achievable region in the plane of the MSE's of the two user parameters, which provides a trade-off between the MSE's, where we used zero-rate lower bounds on the error probability of Gaussian channels by Shannon and Polyanskiy et al. Numerical results showed that, the multi-user adaptation of the zero-rate lower bound by Polyanskiy et al. provides a tighter overall lower bound on the MSE pairs than the classical Shannon bound. In addition, we introduced upper bounds on the MSE exponents, namely, the exponential decay rates of these MSE's in the asymptotic regime of long blocks that could make use of any bound on the error exponent of a single-user AWGN channel. The obtained results are numerically evaluated for three different bounds on the reliability function of the Gaussian channel. It is shown that the adaptation of the reliability function by Ashikhmin et al. to the MAC provides a significantly tighter characterization than Shannon's sphere-packing bound and the divergence bound.

Virtualization technology is considered an effective measure to enhance resource utilization and interference management via radio resource abstraction in heterogeneous networks (HetNet). The critical challenge in wireless virtualization is virtual resource allocation on which substantial works have been done. However, most existing researches on virtual resource allocation focus on improving total utility. Different from the existing works, we investigate the dynamic‐aware virtual radio resource allocation in virtualization based HetNet considering utility and fairness. A virtual radio resource management framework is proposed, where the radio resources of different physical networks are virtualized into a virtual resource pool and mobile virtual network operators (MVNOs) compete for virtual resources from the pool to provide service to users. A virtual radio resource allocation algorithm based on biological model is developed, considering system utility, fairness, and dynamics. Simulation results are provided to verify that the proposed virtual resource allocation algorithm not only converges within a few iterations, but also achieves a better trade‐off between total utility and fairness than existing algorithm. Besides, it can also be utilized to analyze the population dynamics of system.

Thorough investigation of the Cloud-RAN (C-RAN) architecture has recently shown that C-RAN can bring advanced cooperated and coordinated processing capabilities as well as the multiplexing gains toward future radio access networks. The baseband processing of each base station instance can now be flexibly split into smaller functional components, that can be placed either at remote radio units (RRUs) or baseband units (BBUs), depending on the available fronthaul (FH) performance. Additionally, with the wide adoption of Ethernet in data centers and core networks, the Radio over Ethernet (RoE) approach is now considered as an off-the-shelf candidate for the FH link. To this end, we propose a unified RRU/BBU architectural framework for C-RAN that can support both a flexible functional split and a FH transport protocol over Ethernet. Furthermore, we experimentally evaluate the main key performance indicators (KPIs) of an operational C-RAN network built based on OpenAirInterface (OAI), a software implementation of LTE/LTE-A systems, under two functional splits and different deployment scenarios.

This paper presents a practical feasibility study of a novel two-phase three-part-message strategy for half-duplex relaying, which features superposition coding and interference-aware cancellation decoding. Aiming to analyze the performance of the proposed scheme in the non-asymptotic regime, this paper evaluates the spectral efficiency with finite block-length and discrete constellation signaling and compares it with the theoretical performance of Gaussian codes with asymptotically large block-lengths. The performance evaluation is carried out on an LTE simulation test bench. During each transmission phase, the modulation and coding scheme is adapted to the channel link qualities to enhance the overall spectral efficiency. A single-antenna source and relay, and a multi-antenna destination are assumed. The static Gaussian and two frequency selective channel models are considered for the proposed scheme. A spectral efficiency comparison with a baseline scheme (non-cooperative two-hop transmission, i.e., the source-destination link is absent) and with the point-to-point transmission strategy (no relay) is presented. The results confirm that physical-layer cooperation and multi-antennas are critical for performance enhancement in heterogeneous networks. Moreover, they show that physical layer cooperation advantages are within practical reach with existing LTE coded-modulation and interference-mitigation techniques, which are prevalent in modern user-equipment.

This paper considers the problem of jointly estimating two independent continuous-valued parameters sent over a Gaussian multiple-access channel (MAC) subject to the mean square error (MSE) as a fidelity criterion. We generalize the parameter modulation-estimation analysis techniques proposed by Merhav in 2012 to a two-user multiple-access channel model to obtain outer bounds to the achievable region in the plane of the MSE's of the two user parameters, as well as the achievable region of the exponential decay rates of these MSE's in the asymptotic regime of long blocks.

Handover procedure in LTE/LTE-A has been radically evolved when compared to the previous 3GPP standards. In particular, X2 handover is introduced to allow neighboring eNBs to handle the user mobility without the involvement of the core network. While most of the application could considerably benefit from the X2 handover performance improvement, delay breakdown and impact of parameters from the UE perspective are not well investigated. This paper analyzes the performance of the X2 handover from the UE perspective. Furthermore, the impact of the different parameters on the handover decision algorithm is investigated. Preliminary results, obtained from the OpenAirInterface LTE/LTE-A emulation platform, demonstrate that main delay bottleneck resides in the uplink synchronization of the UE to the target eNB.

A low-latency, parameter modulation-estimation feedback protocol for wide-band channels is introduced for both pure line-of-sight and more general fading channels with several degrees of freedom. One round of the protocol consists of a data phase and a control phase and uses noncoherent detection. The asymptotic optimality in energy efficiency of the protocol is analyzed and an upper bound on the distortion level is derived for two rounds. The proposed scheme, as well as known one-way schemes, are compared with classical and very recent lower bounds. Both the lower bounds and performance evaluation of the feedback protocol are extended to a multichannel fading model. The improvement of the feedback protocol over one-shot transmission is shown to be very significant on both line-of-sight and fading channels.

This paper studies networks that consist of N half-duplex relays assisting the communication between a source and a destination. In ISIT'12 Brahma et al. conjectured that in Gaussian half-duplex diamond networks (i.e., without a direct link between the source and the destination, and with N non-interfering relays), an approximately optimal relay scheduling policy (i.e., achieving the cut-set upper bound to within a constant gap uniformly over all channel gains) has at most N + 1 active states (i.e., at most N + 1 out of the 2Npossible relay listen-transmit configurations have a strictly positive probability). Such relay scheduling policies were referred to as simple. In ITW'13, we conjectured that simple approximately optimal relay scheduling policies exist for any Gaussian half-duplex multi-relay network irrespectively of the topology. This paper formally proves this more general version of the conjecture and shows it holds beyond Gaussian noise networks. In particular, for any class of memoryless half-duplex N-relay networks with independent noises and for which independent inputs are approximately optimal in the cut-set upper bound, an approximately optimal simple relay scheduling policy exists. The key step of the proof is to write the minimum of the submodular cut-set function by means of its Lovász extension and use the greedy algorithm for submodular polyhedra to highlight structural properties of the optimal solution. This, together with the saddle-point property of min-max problems and the existence of optimal basic feasible solutions for linear programs, proves the conjecture. As an example, for N-relay Gaussian networks with independent noises, where each node is equipped with multiple antennas and where each antenna can be configured to listen or transmit irrespectively of the others, the existence of an approximately optimal simple relay scheduling policy with at most N + 1 active states, irrespectively of the total number of antennas in the system, is proved.

This paper studies the two-user causal cognitive interference channel (CCIC), where two transmitters aim to communicate independent messages to two different receivers via a common channel. One source, referred to as the cognitive, is capable of overhearing the other source, referred to as the primary, through a noisy in-band link and thus can assist in sending the primary's data. The authors of this paper recently characterized to within a constant gap the capacity of the symmetric Gaussian CCIC in: 1) the strong interference regime and 2) for a subset of the weak interference regime when the cooperation link is larger than a given threshold. This paper characterizes to within a constant gap the capacity for the symmetric Gaussian CCIC in the regime that was still open. To this end, two novel outer bounds of the types 2Rp + Rcand Rp + 2Rcare derived for the class of injective semideterministic CCICs, where the noises at the different source-destination pairs are independent. These outer bounds, as well as an achievable rate region based on Gelfand-Pinsker binning, superposition coding, and simultaneous decoding at the receivers, are then specialized to the Gaussian noise case. It is shown that the novel outer bounds are necessary to characterize the capacity within a constant gap when the cooperation link is weaker than the direct links, that is, in this regime unilateral cooperation leaves some system resources underutilized.

This paper studies the sum-capacity of the Multiple Input Single Output (MISO) Gaussian broadcast channel where K single-antenna users are served by a base station with N antennas, with N <; K. The generalized Degrees-of-Freedom (gDoF) for this system is derived as the solution of a Maximum Weighted Bipartite Matching (MWBM) problem, where, roughly speaking, each of the N transmit antennas is assigned to a different user. The MWBM problem inspires a user selection algorithm where a subset of N out of K users is served. The proposed algorithm runs in polynomial-time (rather than involving an exhaustive search among all possible subsets of size N out of K users) and extends the classical DoF analysis to more realistic wireless channel configurations where users can experience very different channel gains from the base station. Extensive numerical simulations, run in practically relevant Rayleigh fading environments for different numbers of users and of antennas, show that the throughput achieved by serving the set of N users selected by the MWBM-based algorithm is at most N log(K) bits away from an outer bound to the sum-capacity, where in principle all the K users are served. Comparisons with another widely used user scheduling algorithm are also provided.

For time division duplexing (TDD) systems, the physical channel in the air is reciprocal for uplink (UL) and downlink (DL) within the channel coherence time. However when the transceivers' radio frequency (RF) hardware is taken into consideration, TDD channel reciprocity no longer holds because of the non-symmetric characteristics of RF transmit and receive chains. Relative calibration has been proposed to compensate this hardware impairment with a multiplicative matrix. In this paper we perform hardware measurements on this calibration matrix which gives a direct insight on the physical phenomenon of TDD transceivers. Especially, we inspect the assumption that this calibration matrix is diagonal, which is widely adopted in literature but has never been verified by experiments. This work can be regarded as an experimental base for TDD calibration or for theoretical analysis of non-perfect channel reciprocity of TDD systems.

We describe a channel sounding measurement campaign for cellular broadband wireless communications with high speed trains that was carried out in the context of the project CORRIDOR. The campaign combines MIMO and carrier aggregation to achieve very high throughputs. We compare two different scenarios, the first one reflects a cellular deployment, where the base station is about 1km away from the railway line. The second scenario corresponds to a railway deployed network, where the base station is located directly next the railway line. We present the general parameters of the measurement campaign and some results of Power Delay Profiles and Doppler Spectra and their evolution over time. Finally we present a simple channel model that captures the main effects observed in the measurements.

This paper presents a practical implementation of a novel three-message transmission strategy for the Gaussian half-duplex relay channel based on Turbo code superposition encoding and interference-aware successive interference cancellation. The impact of finite block-length and discrete input constellations on the Block Error Rate (BLER) performance is evaluated through extensive simulations on an LTE simulation test bench and compared to the theoretical performance of asymptotically large block-length Gaussian codes. For the practically relevant BLER value of 10-2and by varying the direct source-destination link strength, the maximum spectral efficiency gap between theory and the presented implementation is found to be of 0.458 bits/dim when the strength of the source-destination and relay-destination links is the same and of 0.681 bits/dim when the relay-destination link is 5 dB stronger than the source-destination link. These values indicate that practical implementations of high-performing HD relay techniques for future Heterogeneous Network deployments are within reach. A comparison with a baseline strategy without direct source-destination transmission, as currently proposed in the LTE standard for relay scenarios, shows superior performances of the proposed scheme. In particular, the rate gain is of a factor of 2 when the strength of the source-destination and relay-destination links is the same and of a factor of 1.2 when the relay-destination link is 5 dB stronger than the source-destination link, thereby highlighting the critical importance of physical-layer cooperation in broadband wireless systems.

This paper considers a Gaussian network where N half-duplex multiple-antenna relays assist the communication between a source and a destination. A novel antenna switching policy is proposed, where each relays' antenna can be configured to either receive or transmit independently of the others. The rate achieved by noisy network coding is shown to be to within a constant gap from the cut-set bound, where the gap only depends on the total number of antennas in the system. Moreover, the optimal number of different relay antenna configurations needed to attain the constant gap is proved to be at most N + 1, that is, it only depends on the number of relays but not on the total number of antennas. Such a relay scheduling policy is referred to as simple. Through an example, it is shown that independently switching the antennas at the relays not only achieves in general strictly higher rates compared to using the antennas for the same purpose, but can actually provide a strictly larger pre-log factor. This implies that in broadband wireless networks with half-duplex multiple-antenna relays, the relay antennas should be dynamically configured to either transmit of receive depending on the channel conditions.

In ISIT2012 Brahma, Özgür and Fragouli conjectured that in a half-duplex diamond relay network (a Gaussian noise network without a direct source-destination link and with N non-interfering relays) an approximately optimal relay scheduling (achieving the cut-set upper bound to within a constant gap uniformly over all channel gains) exists with at most N + 1 active states (only N + 1 out of the 2Npossible relay listen-transmit configurations have a strictly positive probability). Such relay scheduling policies are said to be simple. In ITW2013 we conjectured that simple relay policies are optimal for any half-duplex Gaussian multi-relay network, that is, simple schedules are not a consequence of the diamond network's sparse topology. In this paper we formally prove the conjecture beyond Gaussian networks. In particular, for any memoryless half-duplex N-relay network for which the cut-set bound is approximately optimal to within a constant gap under some conditions (satisfied for example by Gaussian networks), an optimal schedule exists with at most N + 1 active states. The key step of our proof is to write the minimum of a submodular function by means of its Lovász extension and use the greedy algorithm for submodular polyhedra to highlight structural properties of the optimal solution. This, together with the saddle-point property of min-max problems and the existence of optimal basic feasible solutions in linear programs, proves the claim.

Commoditization and virtualization of wireless networks are changing the economics of mobile networks to help network providers (e.g., MNO, MVNO) move from proprietary and bespoke hardware and software platforms toward an open, cost-effective, and flexible cellular ecosystem. In addition, rich and innovative local services can be efficiently created through cloudification by leveraging the existing infrastructure. In this work, we present RANaaS, which is a cloudified radio access network delivered as a service. RANaaS provides the service life-cycle of an on-demand, elastic, and pay as you go 3GPP RAN instantiated on top of the cloud infrastructure. We demonstrate an example of real-time cloudified LTE network deployment using the OpenAirInterface LTE implementation and OpenStack running on commodity hardware as well as the flexibility and performance of the platform developed.

In this work, we demonstrate and validate a novel architecture for next generation cellular networks that enables collaborative forwarding at Layer 2 among adjacent eNBs with the aid of enhanced user equipment (UE) devices, that act voluntarily as packet forwarders. We introduce an evolved-UE (eUE) which is capable of operating simultaneously over multiples eNBs in order to enable reliable multi-hop operation through relaying and to achieve low-latency communication through efficient L2/MAC forwarding. For the demonstration and the evaluation of this architecture, we used the OpenAirInterface emulation platform to implement it, and also to evaluate its performance. The obtained results show that, the proposed architecture achieves significant reduction in latency (up to 16.94%) and improvement on packet loss rate (up to 59.25%), as the number of the employed eUEs increases with increasing BLER up to 20%. Moreover, the proposed architecture enables eUEs to increase the aggregated data rate in downlink by exploiting data connection to multiple eNBs.

In this paper, we propose a novel architecture for next generation cellular networks that enables collaborative forwarding at Layer 2 among adjacent eNBs with the aid of enhanced user equipment (UE) devices, that act voluntarily as packet forwarders. Therefore, legacy UEs are leveraged as active network elements being capable of operating simultaneously over multiple base stations (eNBs). To this end, we introduce an evolved-UE (eUE) in order to enable reliable multi-hop operation through relaying and to achieve low-latency communication through efficient L2/MAC forwarding. Through extensive experimentation with OpenAirInterface emulation platform, we evaluated the performance and also validated the feasibility of the proposed architecture. Our results show that, in certain use cases corresponding to public safety and moving/small cell scenarios, the proposed architecture achieves significant reduction in latency (up to 16.94%) and improvement on packet loss rate (up to 59.25%), as the number of the employed eUEs increases with increasing BLER up to 20%. Moreover, the proposed architecture enables eUEs to increase the aggregated data rate in downlink by exploiting data connection to multiple eNBs at the expense of extra power consumption, which calls for the appropriate incentives to enable such a cooperation.

TV White Spaces (TVWS) technology allows wireless devices to opportunistically use locally-available TV channels enabled by a geolocation database. The UK regulator Ofcom has initiated a pilot of TVWS technology in the UK. This paper concerns a large- scale series of trials under that pilot. The purposes are to test aspects of white space technology, including the white space device and geolocation database interactions, the validity of the channel availability/powers calculations by the database and associated interference effects on primary services, and the performances of the white space devices, among others. An additional key purpose is to perform research investigations such as on aggregation of TVWS resources with conventional resources and also aggregation solely within TVWS, secondary coexistence issues and means to mitigate such issues, and primary coexistence issues under challenging deployment geometries, among others. This paper provides an update on the trials, giving an overview of their objectives and characteristics, some aspects that have been covered, and some early results and observations.

This work considers distributed sensing and transmission of sporadic random samples. A new lower-bound is presented on the reconstruction error of a common vector imperfectly measured by a network of sensors. The noisy correlated observations of the source vector are transmitted with finite energy to a single receiver via an additive white Gaussian noise asynchronous multiple-access channel (MAC). Transmission makes use of a perfect causal feedback link to the encoder connected to each sensor. Asymptotic upper-bounds on the distortion are provided for a retransmission protocol which is inspired by the classical scheme of Yamamoto and Itoh and extended to a more general network scenario. Additionally, we introduce lower-bounds on the reconstruction error for individual estimators of the noisy observations themselves. Both the upper and lower-bounds show that collaboration can be achieved through energy accumulation under certain circumstances. To investigate the practical performance of the proposed protocol we provide a numerical evaluation of the upper-bounds in the non-asymptotic energy regime using low-order quantization in the sensors. It is shown that an increase in the size of the network brings benefit in terms of performance, but that the gain in terms of energy efficiency diminishes quickly at finite energies due to a non-coherent combining loss.

In this paper, we consider low-complexity detection of coded spatial data streams with uniform power and non-uniform rate distribution in a single-user MIMO system. The receiver decodes these different streams as if facing a multiple access channel (MAC). Conventional receiver solutions for such schemes are based on successive interference cancellation (SIC) by employing a linear minimum mean square error (MMSE) successive stripping detector, where the optimality is nonetheless constrained to Gaussian codebooks. As a remedy, this paper introduces a novel near-optimal low-complexity max-log-MAP demodulator for a 2×nrsystem (nris the number of receive antennas) which reduces the complexity of detection from O(|χ|2) to O(|χ|1), where |χ| indicates the size of the signal set. In the sequel, we extend the proposed low-complexity demodulation scheme to higher-dimensional MIMO systems via a hybrid detector, where significant complexity saving is realized at the expense of slight performance degradation.

This paper considers the two-user Gaussian Causal Cognitive Interference Channel (GCCIC), which consists of two source-destination pairs that share the same channel and where one full-duplex cognitive source can causally learn the message of the primary source through a noisy link. The GCCIC is an interference channel with unilateral source cooperation that models practical cognitive radio networks. Different achievable strategies are shown to be at most a finite number of bits away from an outer bound for a set of the channel parameters that, roughly speaking, excludes the case of weak interference at both receivers.

This paper studies the two-user interference channel with unilateral source cooperation, which consists of two source-destination pairs that share the same channel and where one full-duplex source can overhear the other source through a noisy in-band link. Novel outer bounds of the type 2R1+ R2and R1+ 2R2are developed for the class of injective semi-deterministic channels with independent noises at the different source-destination pairs. The bounds are then specialized to the Gaussian noise case. Interesting insights are provided about when these types of bounds are active, or in other words, when unilateral cooperation is too weak and leaves some system resources underutilized.

LTE 4G cellular networks are gradually being adopted by all major operators in the world and are expected to rule the cellular landscape at least for the current decade. They will also form the starting point for further progress beyond the current generation of mobile cellular networks to chalk a path towards fifth generation mobile networks. The lack of open cellular ecosystem has limited applied research in this field within the boundaries of vendor and operator R&D groups. Furthermore, several new approaches and technologies are being considered as potential elements making up such a future mobile network, including cloudification of radio network, radio network programability and APIs following SDN principles, native support of machine-type communication, and massive MIMO. Research on these technologies requires realistic and flexible experimentation platforms that offer a wide range of experimentation modes from real-world experimentation to controlled and scalable evaluations while at the same time retaining backward compatibility with current generation systems.

Wireless networks are evolving continuously and expected to provide seamless experience for multiple real-time internet applications. Quality-of-service is one of the major component associated with user experience. In this paper, we have considered the provisioning of desired QoS to mobile users that are capable of running multiple internet applications in parallel. For this purpose, a three-step iterative downlink scheduler is proposed for resource management at per-userper-service level. The scheduler performs sorting in multiple iterations on the basis of three weights. In the first iteration, the scheduler performs sorting based on the throughput weight. The second iteration latency weight and followed by buffer weight in the third iteration. The allocation of resources is done to satisfy the promised QoS to all the services of every user. A comparison is carried out with traditional scheduling algorithms in terms of system throughput, fairness index and percentage of satisfied guaranteed bit-rate users. Results show that the proposed algorithm outperforms existing schemes and the performance is more closer to theoretical system throughput.

Mobile devices have evolved remarkably over the last decade and are now being utilized to access much broader range of internet applications. Moreover, their capability to simultaneously run many applications has significantly transformed the traffic characteristics of mobile networks. Quality of service (QoS) is a fundamental component associated with these applications and network should be able to support multiple QoS requests from the same user at same time. This requires complex buffer management and simultaneous scheduling of resources to multiple users with multiple services. In this paper, we propose a framework with pre-processor for MAC-layer scheduler including two-dimensional buffer management (users × services) that enable more efficient allocation of resources to users running multiple internet applications in parallel. The framework will enhance the performance of existing scheduling algorithms by increasing the resolution of scheduling. A comparative analysis of traditional scheduling algorithms is provided to show the gains of proposed framework.

This paper studies the two-user Gaussian interference channel with half-duplex causal cognition. This channel model consists of two source-destination pairs sharing a common wireless channel. One of the sources, referred to as the cognitive, overhears the other source, referred to as the primary, through a noisy link and can therefore assist in sending the primary's data. Due to practical constraints, the cognitive source is assumed to work in half-duplex mode, that is, it cannot simultaneously transmit and receive. This model is more relevant for practical cognitive radio systems than the classical information theoretic cognitive channel model, where the cognitive source is assumed to have a non-causal knowledge of the primary's message. Different network topologies are considered, corresponding to different interference scenarios: (i) the interference-symmetric scenario, where both destinations are in the coverage area of the two sources and hence experience interference, and (ii) the interference-asymmetric scenario, where one destination does not suffer from interference. For each topology the sum-rate performance is studied by first deriving the generalized Degrees of Freedom (gDoF), or "sum-capacity pre-log" in the high-SNR regime, and then showing relatively simple coding schemes that achieve a sum-rate upper bound to within a constant number of bits for any SNR. Finally, the gDoF of the channel is compared to that of the non-cooperative interference channel and to that of the non-causal cognitive channel to identify the parameter regimes where half-duplex causal cognition is useless in practice or attains its ideal ultimate limit, respectively.

This paper considers the two-user Gaussian causal cognitive interference channel (GCCIC), which consists of two source-destination pairs that share the same channel and where one full-duplex cognitive source can causally learn the message of the primary source through a noisy link. The GCCIC is an interference channel with unilateral source cooperation that better models practical cognitive radio networks than the commonly used model which assumes that one source has perfect noncausal knowledge of the other source's message. First, the sum-capacity of the symmetric GCCIC is determined to within a constant gap. Then, the insights gained from the study of the symmetric GCCIC are extended to more general cases. In particular, the whole capacity region of the Gaussian Z-channel, i.e., when there is no interference from the primary user, and of the Gaussian S-channel, i.e., when there is no interference from the secondary user, are both characterized to within 2 bits. The fully connected general, i.e., no-symmetric, GCCIC is also considered and its capacity region is characterized to within 2 bits when, roughly speaking, the interference is not weak at both receivers. The parameter regimes where the GCCIC is equivalent, in terms of generalized degrees-of-freedom, to the noncooperative interference channel (i.e., unilateral causal cooperation is not useful), to the non-causal cognitive interference channel (i.e., causal cooperation attains the ultimate limit of cognitive radio technology), and to bilateral source cooperation are identified. These comparisons shed light into the parameter regimes and network topologies that in practice might provide an unbounded throughput gain compared to currently available (non cognitive) technologies.

This paper considers the Gaussian half-duplex relay channel (G-HD-RC): a channel model where a source transmits a message to a destination with the help of a relay that cannot transmit and receive at the same time. It is shown that the cut-set upper bound on the capacity can be achieved to within a constant gap, regardless of the actual value of the channel parameters, by either partial-decode-and-forward or compress-and-forward. The performance of these coding strategies is evaluated with both random and deterministic switch at the relay. Numerical evaluations show that the actual gap is less than what analytically obtained, and that random switch achieves higher rates than deterministic switch. As a result of this analysis, the generalized degrees-of-freedom of the G-HD-RC is exactly characterized for this channel. In order to get insights into practical schemes for the G-HD-RC that are less complex than partial-decode-and-forward or compress-and-forward, the exact capacity of the linear deterministic approximation (LDA) of the G-HD-RC at high signal-to-noise-ratio is determined. It is shown that random switch and correlated nonuniform inputs bits are optimal for the LDA. It is then demonstrated that deterministic switch is to within one bit from the capacity. This latter scheme is translated into a coding strategy for the original G-HD-RC and its optimality to within a constant gap is proved. The gap attained by this scheme is larger than that of partial-decode-and-forward, thereby pointing to an interesting practical tradeoff between gap to capacity and complexity.

This paper considers a Gaussian relay network where a source transmits a message to a destination with the help of N half-duplex relays. The information theoretic cut-set upper bound to the capacity is shown to be achieved to within 1.96(N+2) bits by noisy network coding, thereby reducing the previously known gap. This gap is obtained as a special case of a more general constant gap result for Gaussian half-duplex multicast networks. It is then shown that the generalized degrees-of-freedom of this network is the solution of a linear program, where the coefficients of the linear inequality constraints are proved to be the solution of several linear programs referred as the assignment problem in graph theory, for which efficient numerical algorithms exist. The optimal schedule, that is, the optimal value of the 2Npossible transmit-receive configuration states for the relays, is investigated and known results for diamond networks are extended to general relay networks. It is shown, for the case of N=2 relays, that only N+1=3 out of the 2N=4 possible states have a strictly positive probability and suffice to characterize the capacity to within a constant gap. Extensive experimental results show that, for a general N -relay network with N≤8 , the optimal schedule has at most N+1 states with a strictly positive probability. As an extension of a conjecture presented for diamond networks, it is conjectured that this result holds for any half-duplex relay network and any number of relays. Finally, a network with N=2 relays is studied in detail to illustrate the channel conditions under which selecting the best relay is not optimal, and to highlight the nature of the rate gain due to multiple relays.

LTE systems do not suffer from intra-cell interference, but they are affected by interference coming from adjacent cells. However, most of the research on resource allocation and repetition protocols has not paid attention to the interference case. In this paper, we consider the problem of dynamic resource allocation for IR-HARQ schemes under the presence of interference. We consider resource allocation by means of rate and physical dimensions adaptation in each HARQ round. We provide a mathematical framework that can be applied for the analysis of heterogeneous networks. Rather than performing extensive simulations, we take an information theoretic approach to derive analytical expressions that represent the long-term throughput of the network and we consider distributed resource allocation policies. Our policies are applicable for both the uplink and downlink channels.

This paper considers the causal cognitive interference channel that consists of two full-duplex transmitter-receiver pairs sharing the same channel, where one transmitter can causally learn the message of the other transmitter through a noisy link. This channel models unilateral source cooperation. The work focuses on the generalized degrees-of-freedom of the symmetric, i.e. the two interfering links and the two direct links have the same strength, sum-capacity for the Gaussian noise channel. It is shown through evaluation of various achievable schemes that known sum-rate upper-bounds are achievable to within a constant gap regardless of the strength of the channel parameters. The achievable schemes are quite simple in the sense that only superposition coding is used, while it is shown that more complex schemes using binning can achieve a smaller gap.

This paper considers the Gaussian relay channel where the relay node operates in half-duplex mode. The exact capacity of the linear deterministic approximation of the Gaussian channel at high SNR is derived first. This result is then used to inspire an achievable scheme valid for any SNR in the original channel. The scheme is quite simple: it uses successive decoding and does not incur in the typical delay of backward decoding. The achievable rate is then showed to be at most 3 bits away from the cut-set upper bound, which allows to analytically determine the generalized Degrees-of-Freedom of the channel. A closed form expression for the gDoF-optimal fraction of time the relay node transmits is found as well.

This paper studies the Gaussian half duplex relay channel, where the relay node can not transmit and receive at the same time. The main contribution lies in showing that both Partial-Decode-Forward and Compress-Forward achieve the CutSet upper bound to within a constant gap regardless of the channel parameters. This provides a closed form characterization of the Generalized Degrees-of-Freedom (gDoF) of the channel, which for certain channel parameters is strictly smaller than the gDoF of the full duplex channel. Half duplex channels can convey information through the random switch between the receive and retransmit phases; this work shows numerically that random switch achieves larger rates compared to deterministic switch, which is usually considered in the literature.

This paper studies the sum-capacity of the Gaussian half-duplex causal cognitive interference channel, a channel model with two transmitter-receiver pairs where a (cognitive) source cooperates with the other (primary) source in sending data through a shared channel. In contrast to the classical cognitive radio model, here the cognitive source can not transmit and receive at the same time and must causally learn the primary message through a noisy channel. Achievable strategies are developed and shown to match known upper bounds on the symmetric sum-capacity of this channel to within a constant gap for all values of channel parameters. In the process, the generalized degrees of freedom of the channel is characterized.

This work considers distributed sensing and transmission of sporadic random samples. Lower bounds are derived for the reconstruction error of a single normally or uniformly-distributed vector imperfectly measured by a network of sensors and transmitted with finite energy to a common receiver via an additive white Gaussian noise asynchronous multiple-access channel. Transmission makes use of a perfect causal feedback link to the encoder connected to each sensor. A retransmission protocol inspired by the classical scheme in [1] applied to the transmission of single and bi-variate analog samples analyzed in [2] and [3] is extended to the more general network scenario, for which asymptotic upper-bounds on the reconstruction error are provided. Both the upper and lower-bounds show that collaboration can be achieved through energy accumulation under certain circumstances.

This paper studies a Gaussian relay network, where the relays can either transmit or receive at any given time, but not both. Known upper (cut-set) and lower (noisy network coding) bounds on the capacity of a memoryless full-duplex relay network are specialized to the half-duplex case and shown to be to within a constant gap of one another. For fairly broad range of relay network sizes, the derived gap is smaller than what is known in the literature, and it can be further reduced for more structured networks such as diamond networks. It is shown that the asymptotically optimal duration of the listen and transmit phases for the relays can be obtained by solving a linear program; the coefficients of the linear constraints of this linear program are the solution of certain `assignment problems' for which efficient numerical routines are available; this gives a general interesting connection between the high SNR approximation of the capacity of a MIMO channel and the `assignment problem' in graph theory. Finally, some results available for diamond networks are extended to general networks. For a general relay network with 2 relays, it is proved that, out of the 4 possible listen/transmit states, at most 3 have a strictly positive probability. Numerical results for a network with K - 2 <; 9 relays show that at most K-1 states have a strictly positive probability, which is conjectured to be true for any number of relays.

In this paper, we analyze the downlink performance of a multi-user system consisting of mobile users with wide range of velocity. We categorize these users into different groups on the basis of their velocity range and study the impact of feedback delay on each group for closed-loop transmit beamforming. Based on this analysis, we propose an adaptive feedback-scheduling algorithm to minimize the effect of feedback delay on the performance of the system. We derive generalized performance measuring expressions for the proposed algorithm and carry out simulations to validate the analysis.

Multi-user MIMO communication can provide significant gains by exploiting spatial multiplexing. However, it requires better feedback to provide accurate channel state information at the transmitter (CSIT) for minimizing the multiuser interference. 3GPP LTE provides support for MU-MIMO, but it is not sufficient to extract sizable gains. In this paper, our primary goal is to efficiently exploit the system's resources for MU-MIMO in LTE. In the existing 3GPP LTE standard (Rel. 8), we observed that the transmission mode 5 (TM5) dedicated for MU-MIMO utilizes wideband feedback method for providing channel directional information/ precoding matrix indicator (CDI/PMI). The standard supports finer granularity feedback i.e. sub-band feedback method, but it's not utilized for MU-MIMO and restricted only to SU-MIMO. Therefore in this work, we propose to exploit the sub-band feedback for providing more frequent update of PMI. However, in order to support this feedback method, we need to propose a new downlink control information (DCI) format for TM5 that will contain additional fields in comparison to the existing DCI format 1D. Furthermore, to extract optimal performance at the system level, we also propose a MAC-layer scheduling algorithm that deals with resource management on sub-band basis. Utilizing these proposed methods, we show considerable gains in MU-MIMO for 3GPP LTE at the system level.

In this paper, we propose a dynamic resource allocation method to enable efficient and low-latency machine type communications (MTC) in LTE/LTE-A with the contention based random access (CBA) scheme [9]. In the proposed method, we firstly estimate the probabilities of events caused by a CBA transmission and then calculate the latency with the measured resource unit. We increase the amount of CBA resources until the estimated latency satisfies the application QoS requirement. The simulation results demonstrate that with the proposed resource allocation method for CBA, the uplink channel access latency has been drastically reduced and that it always guarantees the latency requirements. Furthermore, the achievable latency is significantly reduced when compared to the regular scheduling and the standard random access scheme.

Software Defined Radio platforms are a flexible and cost efficient solution to deal with the increasing number of today's wireless communication standards. One interesting use case can be found in the automotive industry where the IEEE 802.11p standard enables Car-to-Car and Car-to-Infrastructure communication. In the context of this paper we present a physical layer implementation of the 802.11p receiver for the OpenAirInterface ExpressMIMO platform. Our results show that a real-time processing is already possible for most of the modulation schemes when applying a centralized control flow. The results are further extended by recommendations of further design improvements and the derivation of general guidelines for further standard deployment on the platform.

Maddah-Ali and Tse have recently shown that delayed transmitter channel state information (CSIT) can still be useful in increasing the degrees-of-freedom (DoF) over the MIMO broadcast channel. This was achieved by constructing a scheme that, in the presence of two transmit antennas, of two single-antenna receivers, and of CSIT that is delayed by one coherence time, manages to provide each user with 2/3 DoF, improving upon the 1/2 DoF corresponding to no CSIT. This same scheme though, as well as all subsequent schemes pertinent schemes, achieve DoF gains by suppressing the inherent diversity of the broadcast parallel channel. The current work proposes a novel broadcast scheme which, over the above described setting of the delayed CSIT broadcast channel, employs a form of interference alignment to achieve both full DoF as well as full diversity.

We present a method to integrate the Quantize-Map-Forward (QMF) relaying scheme [1] into the standard LTE operation, for a two-relay diamond network configuration. Our approach implements QMF using mainly existing LTE modules and functionalities, and results in minimal changes in the standard link-layer LTE operation. In particular, the destination operation is only affected in that we adapt the log-likelihood ratio (LLR) calculations at the decoder input to take into account the existence of relays; thus, the decoding complexity and operations (apart the LLR calculations) are not modified. We report extensive performance evaluations of our scheme using the OpenAirInterface (OAI) link-level simulation tools.

To enable the efficient and low latency machine-type communications (MTC) over long term evolution (LTE), a contention based access (CBA) method is proposed. With CBA, UEs transmit packets on the randomly selected resource without having any UE specific scheduled resources. To address the problem of collision caused by CBA in high traffic load, eNB exploits the MU-MIMO detection technique to decode radio network temporary identifier (RNTI) of the collided UEs and use this information to perform a regular scheduling in subsequent subframe. Detailed low layer signaling enhancement to implement CBA technique in current LTE specification (Rel. 10) is also presented. Simulation results demonstrate that the CBA significanlty outperforms the existing uplink channel access methods.

Most of the recent wireless communication systems are interference limited rather than noise limited. In the case of a very strong interferer the conventional assumption of the interference as Gaussian is extremely suboptimal. However optimal (capacity achieving) receivers utilize some prior knowledge about the interference to reach optimality. The link abstraction for such receiver structures is not studied well. We investigate how the conventional mutual information based link abstraction technique can be extended for the accurate and efficient link performance modeling for low complexity optimal receivers. So, in this paper we propose a mutual information based link abstraction methodology of an optimal, low complexity interference aware receiver for multi-user MIMO in the frame work of LTE. For the sake of comparison we performed abstraction of interference aware receiver with Exponential Effective SINR Mapping (EESM) method as well. We show with the help of results that our proposed method outperforms the EESM and provides the system level with more accurate link quality metric.

In the next generation cellular systems, such as LTE-A (Release 10 and beyond), relay node (RN) deployment has been adopted due to its potentials in enlarging coverage and increasing system throughput, even with primitive relaying functionalities. For example, in LTE-A Release 10 only Type-I (non-transparent) RNs are considered wherein no cooperative transmission to the Donor evolved-NodeBs (DeNBs) is allowed. In this paper, we would like to add more functionalities to the RNs and see the advantages of using cooperative relaying, i.e., Type-II RNs. In particular, we study an interference relay channel (IRC) consisting of two single-antenna transmitter-receiver pairs and a shared multiple-antenna RN, which is exploited in a way that interferer's signal components at each receiver node are eliminated. Specifically, at the RN a transmit filtering is performed such that the compound received signal at each user equipment (UE) has a structure similar to the receiver structure for Alamouti's space-time coding [1]. We also show that it is not always required to have more complex receiver structure at the RN in order to achieve better spectral efficiencies.

Turbo codes are presently ubiquitous in the context of mobile wireless communications among other application domains. A decoder for such codes is typically the most power intensive component in the baseband processing chain of a wireless receiver. The iterative nature of these decoders represents a dynamic workload. This brief presents a dynamic power management policy for these decoders. An algorithm is proposed to tune a power manageable decoder according to a prediction of the workload involved within the decoding task. By reclaiming the timing slack left when operating the decoder at a high power mode, the proposed algorithm continuously looks for opportunities to switch to a lower power mode that guarantees the task completion. We apply this technique to an long term evolution Turbo decoder and explore the feasibility of a VLSI implementation on a CMOS technology of 65 nm. Energy savings of up to 54% were achieved with a relatively low loss in error-correction performance.

In this paper we focus on the cell-edge users whose performance is severely limited by the interfering signals of diverse rates and strengths. In contrast to the suboptimal single-user detection, we propose an interference suppression strategy based on a low complexity matched filter (MF) based receiver. This proposed receiver exploits the structure of dominant interference in the detection process, instead of assuming it to be Gaussian and merging it in noise. This receiver is also characterized by the reduction of one complex dimension in the detection process thereby making it low complexity receiver structure. For comparison purposes, we also include the analysis of MMSE receiver and show that while MMSE detection loses one diversity order in the presence of one interferer, the proposed receiver recuperates the lost order of diversity. We further show that MMSE detection suffers from a coding loss as the interference gets stronger while the proposed receiver exhibits a coding gain as either the interference gets stronger or its modulation order decreases. Based on these results, we further propose a novel fractional frequency reuse (FFR) scheme for cellular systems.

In this paper, we assess the random coding error exponents (EEs) corresponding to decode-and-forward (DF), compress-and-forward (CF) and quantize-and-forward (QF) relaying strategies for a parallel relay network (PRN), consisting of two sources, two relay stations (RSs) and single destination where the RSs access to the destination via orthogonal, error-free, limited-capacity backhaul links. Among these relaying strategies, the DF and QF studied in this paper differ from their well-known conventional versions in certain aspects. In the DF relaying, each RS applies maximum-likelihood (ML) detection and sends the message corresponding to the detected signal along with a reliability information to the destination which finalize the decision on the transmitted message. In QF relaying, as opposed to the Gaussian codebook and vector quantization (VQ) theoretical model used for deriving bounds, we consider a simple and practical relaying strategy consisting of finite-alphabet constellations (i.e., M-QAM) at the sources and symbol-by-symbol uniform scalar quantizers (uSQs) at the RSs. We also show, through numerical analysis, that the proposed QF relaying can provide better EEs than the others when the modulation constellation sizes selected by the sources match to the network conditions, i.e., operating signal-to-noise ratio (SNR), and the backhaul capacity is sufficient. This behavior is due to the structure inherent in the considered modulation alphabets, which Gaussian signaling lacks.

In the context of multiuser relay-aided multi-way communications, we identify and meet the optimal degrees of freedom (DOF) for different multiuser uplink-downlink settings of practical importance. Under the imposed constraint of using simple linear techniques, the proposed solutions draw from interference-neutralization (IN) methods which linearly manipulate signals in time and space, and manage to reduce the effect of multiuser interference and of the half-duplex constraint. Focus is placed on asymmetric settings where the connectivity, size and rate of the uplink and downlink groups may vary.

This paper considers a communication network comprised of two nodes, which have no mutual direct communication links, communicating two-way with the aid of a common relay node (RN), also known as separated two-way relay (TWR) channel. We first recall a cut-set outer bound for the set of rates in the context of this network topology assuming full-duplex transmission capabilities. Then, we derive a new achievable rate region based on hash-and-forward (HF) relaying where the RN does not attempt to decode but instead hashes its received signal, and show that under certain channel conditions it coincides with Shannon's inner-bound for the two-way channel [1]. Moreover, for binary adder TWR channel with additive noise at the nodes and the RN we provide a detailed capacity achieving coding scheme based on structure codes.

This paper focuses on the performance of LTE in rural areas which is based on a channel measurement campaign conducted with the Eurecom OpenAirInterface LTE testbed at 800MHz. This testbed is based on LTE release 8 PHY layer and implements transmission modes 1 (single antenna - SISO), 2 (transmit diversity), and 6 (single-user MIMO - closed loop rank-1 precoding) in real time. In addition to the throughput recorded from the real modem, the raw channel estimates were stored and used for extrapolating the performance to transmission mode 5 (multi-user MIMO). This extrapolation is done by means of a mutual-information based link-quality model that abstracts the performance of multi-user (MU) MIMO for an interference aware receiver proposed by Ghaffar et al and then the results are compared with the performance of abstraction to transmission mode 2 and 6. The superior performance of MU MIMO mode (with interference aware receiver) over other transmission modes is illustrated and it is shown that if the channel admits then MU MIMO is the preferred option.

Accurate information regarding a cognitive radio user's location and environment can enhance the adaptive and spectral awareness capabilities of cognitive radio systems. In this paper, a single-path multiband Time-of-Arrival (TOA) positioning technique for cognitive radio is proposed and the performance evaluated using maximum-likelihood (ML) location estimation for a typical rural scenario where signal line-of-sight (LOS) between a transmitter and receiver is prevalent. The multiband Cramer Rao Lower Bound (CRLB) time-delay and channel fading coefficient estimates are derived followed by an estimation combining technique which involves selecting an overall optimum estimate using all the utilized bands. It is observed that the improvement in positioning accuracy for the multiband system depends primarily on the number of utilized bands as well as the (signal-to-noise ratio) SNR of these bands.

In this paper we look at the diversity of equal gain transmission (EGT) for single user (SU) MIMO and multiuser (MU) MIMO in the context of third generation partnership project long term evolution (3GPP LTE). We look at the low resolution LTE precoders which are based on the principle of EGT. Our analytical results show that EGT has full diversity in SU MIMO while the simulation results show that there is a loss of diversity for EGT in MU MIMO mode.

In this paper we look at the effect of discrete constellation alphabets on linear precoding for the downlink of multiuser (MU) MIMO in the context of LTE. We underline the fundamental difference in the approach of precoding if the alphabets are assumed to be discrete constellations rather than the idealized Gaussian assumption. We show that the problem of finding global optimal linear precoder taking into account discrete inputs is non-convex and we propose a method of finding a near optimal linear precoder. Underlining the viability of MU MIMO for future wireless communications as LTE, we further propose in this paper a precoding strategy based on low resolution LTE precoders which necessitate 2 bits feedback from the users. The proposed strategy encompasses geometrical interference alignment at eNodeB and the use of low complexity MU detectors at the users. On one hand, this strategy relegates the interference seen by each user by a geometric scheduling algorithm while on the other hand, users exploit the structure of this interference in the detection process. Simulation results validate improved performance of the proposed strategy over single user schemes.

We consider a multi-pair two-way relay channel (TWRC) where the single-antenna mobile terminals (MT) on each pair seek to communicate, and can do so, via a common multiple antenna relay station (RS). In the multi-pair TWRC, the main bottleneck on system performance is the interference seen by each MT due to the other communicating MT pairs. In this paper, we try to tackle this problem in the spatial domain by using multiple antennas at the RS. Considering Amplify-and-Forward (AF) and Quantize-and-Forward (QF) relaying strategies, different transmit/receive beamforming schemes at the RS are proposed. We compare our proposed schemes to each other and to the Decode-and-Forward (DF) relaying strategy with achievable sum rate taken as a performance metric and show that in a wide range of signal-to-noise ratio (SNR) our schemes outperform the DF relaying strategy.

Underlining the viability of multiuser (MU) MIMO for future wireless communications as long term evolution (LTE), we propose in this paper a precoding strategy based on the low resolution LTE precoders which necessitates 2 bits feedback from the user equipment (UE). The proposed strategy encompasses geometrical interference alignment at eNodeB (LTE notation for base station) followed by the exploitation of interference structure by the UEs. On one hand, this strategy relegates the interference seen by each UE by a geometric scheduling algorithm while on the other hand, UEs exploit the structure of residual interference in the detection process.

In this paper, we assess the random coding error exponents (EEs) corresponding to decode-and-forward (DF), compress-and-forward (CF) and quantize-and-forward (QF) relaying strategies for a parallel relay network (PRN), consisting of a single source and two relays. Moreover, through numerical analysis we show that the EEs achieved by using QF relaying along with non-Gaussian signaling (coded modulation, M-QAM) at the source and symbol-by-symbol uniform scalar quantizers (uSQs) at the relays is better than that achieved by DF and CF relaying strategies when the system is in the low signal-to-noise ratio (SNR) regime and the backhaul capacity is sufficient. This behavior is due to the structure of coded modulation, as opposed to Gaussian signaling, which leads to better EEs for simple relaying strategies compared to its more complex counterparts.

Bit interleaved coded modulation (BICM) because of its improved diversity over fast fading channels is an attractive transmission scheme for future wireless systems. For coded BICM systems, receivers need to employ max log MAP demodulators (demappers) that calculate soft-decision metrics i.e. log-likelihood ratios (LLRs) for the decoder. The complexity of the calculation of these LLRs is exponential in the number of bits per symbol and moreover for systems exploiting spatial dimension (MIMO), the complexity further increases exponentially in the number of transmit antennas. In this paper we propose matched filter (MF) based low complexity max log MAP bit metrics for BICM single input single output (SISO) and low dimensional BICM MIMO systems using Gray encoded M-ary quadrature amplitude modulation (QAM) alphabets. For SISO systems, the maximum likelihood (ML) detector needs computation and comparison of minimum distances between the received symbol and M constellation points on the complex plane for the calculation of each LLR. In this paper we show that these LLRs can be computed precisely from the MF output and therefore do not necessitate any minimum distance calculations. For low dimensional BICM MIMO systems, we further propose a MF based bit metric which successfully trims down one complex dimension of the system thereby reducing complexity. Both these metrics substantially reduce the number of calculations needed for each LLR without compromising the performance and MF being an integral part of all receiver structures facilitates their hardware implementation. Simulation results over Rayleigh fading channels verify similar performance of the simplified metrics as those of the original metrics but with a significant reduction in the complexity.

In this paper, we carry out the performance analysis of recently proposed low complexity max log MAP detector for interference suppression under the realistic imperfections of correlation and non Gaussian alphabets. We also incorporate MMSE detector in our analysis taking into account non Gaussian alphabets however intricacy in the analysis of correlated case restricts our analysis to the case of iid fading. Employing moment generating function(MGF)-based approach, we derive upper bounds of coded pairwise error probability (PEP) and study the effect of non Gaussian interference and correlation. The novelty of this contribution is the performance analysis taking into account discrete alphabets as the existing work is based on the unrealistic Gaussian assumption for these alphabets.

In this work, we revisit classical packet radio networks with a modern treatment of physical-layer (PHY) procedures, medium-access (MAC) and geographic channel-driven routing protocols. Our network model assumes that nodes are randomly distributed on the plane according to a homogeneous spatial Poisson process, using which we provide a novel representation of interference statistics resulting from packet collisions. Using this representation, we develop a cross-layer analysis methodology which allows multi-hop routing protocols to be treated using generic information-theoretic models for the underlying PHY/MAC procedures. These models inherently characterize modern procedures such as channel code rate adaptation, incremental redundancy and packet combining/capture. These models further allow for the assessment of the tradeoff between spatial throughput, measured in bit-meters per signal-space dimension, the range of each transmission and the average transmission delay. A generic formulation based on system parameters, such as system bandwidth, propagation models, etc., is given to analyze this tradeoff in an operational setting which can be used to build system simulators for such networks. Finally, from a purely PHY perspective, the results of this work show that coding and incremental retransmission provide a means for reliable communication coupled with a completely decentralized multiple-access strategy.

Performance of future wireless communication systems being interference limited, researchers are focusing on interference alignment, interference mitigation and interference suppression to diminish, manage or exploit these interferers. In this paper, we carry out the performance analysis of the recently proposed low complexity max log MAP detector and linear MMSE detector for interference suppression under the realistic conditions of correlated fading in cellular environment. We assume only receive correlation as base stations (BSs) due to their extended separation are likely to be uncorrelated. However the intricacy of realizing requisite antenna spacing in the mobile station (MS) combined with the lack of scattering would instigate the individual antennas at MS to be correlated. Employing moment generating function (MGF)-based approach, we derive upper bounds of coded pairwise error probability (PEP) and study the degrading effect of correlation on both the detectors.

Future wireless communication systems being characterized by tight frequency reuse, adaptive modulation and coding schemes and diversified data services will be interference limited by interfering signals of diverse rates and strengths. Keeping in view such a scenario, we propose in this paper the application of a proposed low complexity match filter (MF) based detector for spatial interference cancellation in the presence of one strong interferer. We derive an analytical upper bound for the coded pair wise error probability (PEP) for the proposed MF based detector using the moment generating function (MGF) based method and prove that this detector not only recuperates the diversity order lost by MMSE but also exhibits a coding gain as the interference gets stronger. We also study in this paper the effect of non Gaussian interference on coded PEP of MMSE linear detection and demonstrate the deficit of one order of diversity and a coding loss as the interference gets stronger. Our analysis provides insights to explain the relative performance of MMSE and proposed MF based detectors as a function of the strength of interference. Finally we demonstrate the strength of our new analytical PEP upper bounds by simulations.

This paper is based on the idea of exploiting the discrete constellation alphabets in linear precoding for the downlink of multiuser (MU) MIMO. We study the effect of discrete constellation inputs on the sum rate of different linear precoders while confining ourselves to the case of two single antenna users. We show that contrary to the case of Gaussian alphabets where altruistic solutions as interference cancellation (channel inversion - CI) and interference attenuation (regularized channel inversion - RCI) are the recommended strategies for linear precoding, it is beneficial not to attenuate or cancel the MU interference, if it is coming from discrete constellations. This interference belonging to finite sized constellations has a structure which can be effectively exploited in improving the error resilience at the users. Under such a scenario, it is better to use the degrees of freedom available at the transmitter to improve the desired signal strength at the users instead of utilizing them to nullify or attenuate the undesired signals (interferences) at the users. Therefore the egoistic linear solutions as matched filter (MF) based precoding bears the potential of enhanced sum rate and improved performance as compared to the altruistic linear solutions.

In order to maximize the system throughput, future wireless communication systems will employ a very tight frequency reuse. This leads to interference limited systems where the interference is high at the cell edges. The key ingredient to such networks are thus receivers that are able to exploit the structure of this interference instead of nulling or attenuating it. In this paper we apply such a receiver structure to a distributed multiple-input multiple-output (MIMO) scenario to decode two independent data streams from two synchronized base stations. Further we show how the distributed MIMO receiver is implemented in real-time on the OpenAirInterface platform and provide results from field trials and compare them to the simulation results. Applications of the distributed MIMO receiver include single-frequency cellular as well as mesh networks. OpenAirInterface is an experimental open-source real-time hardware and software platform for experimentation in wireless communications and signal processing. Its current implementation provides a full software modem comprising physical and link layer functionalities for cellular and mesh network topologies.

Future wireless communication systems being characterized by tight frequency reuse, adaptive modulation and coding schemes and diversified data services will be interference limited by the interfering signals of diverse rates and strengths. Frequency reuse factor of 1 being proposed for the next generation mobile systems will lead to 1 or 2 dominant interferers. We propose in this paper a low complexity maximum likelihood (ML) demodulator for interference suppression. The proposed demodulator is also applicable to general multi-stream (spatially multiplexed) MIMO systems where it reduces one complex dimension of the system i.e. the complexity of ML detection reduces from O (|chi|eta) to O(|chi|eta-1) where n is number of transmit antennas/spatial streams. This is a fundamental result as space and technology constraints shall be restricting future MIMO systems to low dimensionality. Therefore reduction of one complex dimension in detection shall enable implementation of ML detectors at the receivers. We look at the performance of linear MMSE demodulator and the proposed demodulator in the presence of interferers of diverse strengths and rates and simulation results demonstrate much improved performance of the proposed demodulator for interference suppression.

Future wireless communication systems characterized by tight frequency reuse, adaptive modulation and coding schemes and diversified data services will be interference limited by interfering signals of diverse rates and strengths. We propose in this paper a low complexity algorithm for spatial interference cancellation in the presence of one strong interferer. This algorithm is based on an earlier proposed low complexity max log MAP detector. It encompasses two strategies for interference cancellation which have been termed as partial interference cancellation (PIC) and absolute interference cancellation (AIC). Their corresponding selection in the receiver is dictated by the relative strength and the rate of interfering stream. In the scenario of interfering stream being relatively weak or of higher rate as compared to the desired stream, the mobile station (MS) resorts to PIC while when the interfering stream is relatively stronger or is of lower rate, the MS switches to AIC. Finally we analyze the performance of proposed algorithm by simulations.

In this work we study the capacity of multi-user multiple-input multiple-output (MU-MIMO) downlink channels with codebook-based limited feedback using real measurement data. Several aspects of MU-MIMO channels are evaluated. Firstly, we compare the sum rate of different MU-MIMO precoding schemes in various channel conditions. Secondly, we study the effect of different codebooks on the performance of limited feedback MU-MIMO. Thirdly, we relate the required feedback rate with the achievable rate on the downlink channel. Real multi-user channel measurement data acquired with the Eurecom MIMO OpenAir Sounder (EMOS) is used. To the best of our knowledge, these are the first measurement results giving evidence of how MU-MIMO precoding schemes depend on the precoding scheme, channel characteristics, user separation, and codebook. For example, we show that having a large user separation as well as codebooks adapted to the second order statistics of the channel gives a sum rate close to the theoretical limit. A small user separation due to bad scheduling or a poorly adapted codebook on the other hand can impair the gain brought by MU-MIMO. The tools and the analysis presented in this paper allow the system designer to trade-off downlink rate with feedback rate by carefully choosing the codebook.

The mobile communication systems today, have different radio spectrum, radio access technologies, and protocol stacks depending on the network being utilized. This gives rise to need of a flexible hardware platform that is capable of supporting all the different standards in the entire wireless communication frequency range. We present a generic baseband prototype architecture for SDR applications, subdivided into a high level control module and a digital signal processing engine. The DSP engine is composition of highly configurable processing blocks, each dedicated to specific algorithms based on the analysis of different standards. We also present the internal architecture, simulation results and use cases for different air-interfaces of two processing blocks as case studies.

In multi-user multiple-input multiple-output (MU-MIMO) systems, channel state information at the transmitter (CSIT) allows for multi-user spatial multiplexing and thus increases the system throughput. We assume that CSIT is obtained by means of a finite-rate feedback channel through channel vector quantization (CVQ) at the receiver. In this paper we use real channel measurements to study the effect of CVQ on the sum rate of a MU-MIMO system employing linear precoding. The measurement data has been acquired using Eurecom's MIMO Openair Sounder (EMOS). The EMOS can perform realtime MIMO channel measurements synchronously over multiple users. We consider CVQ using a Fourier codebook, a random codebook and a random codebook exploiting the second order statistics of the channel. For comparison, we also show the capacity of a single-user system using time division multiple access (TDMA) with no CSIT at all. The results show that the Fourier codebook shows very poor performance in the measured channels. Random codebooks - although suboptimal - provide a much better performance in the measured channels.

In this paper, we consider Gaussian parallel relay networks with phase fading where a source node wants to communicate with a destination node with the assistance of two intermediate relay nodes. For this scenario, outer bounds are derived and three achievable schemes are considered. As well as amplify-and-forward (AF) and decode-and-forward (DF) schemes, we also consider a scheme where the relays exploit block quantization and random binning, which we call BQRB relaying. We show that in the broadcast channel limited regime, where received powers at the relay nodes are very small, BQRB outperforms the other schemes with increasing multiple access channel quality. Moreover, it is seen that BQRB achievable rate performance tends to the rate achievable by a point-to-point single-input multiple-output system.

In this paper, we consider single-cell cellular networks assisted with fixed relay station (RS), used by mobile stations (MS) to access the base station (BTS) via a relaying strategy. The RSs are positioned around the BTS, in such a way that wireless channels on the relay link (from RSs to the BTS) are line-of-sight, we analyze the achievable sum-of-rates for up-link communications. We compare two relaying strategies at the RSs, namely amplify-and-forward (AF) and compress-and-forward (CF). It is assumed that mobile signals and relay signals are emitted on orthogonal bands (FDD), with the possibility of having a larger bandwidth (BW) on the relay-to-base links. We predict the system gains bought by relays, in comparison with two other reference systems. One reference is an ideal relay-based system where the relays enjoy noiseless communications to the BTSs, i.e. a so-called distributed antenna system (DAS). The second reference is offered by a conventional cellular systems without relays, but same number of overall infrastructure antennas. In this paper, it is demonstrated the surprising result that with a relay bandwidth just twice that of the mobile's bandwidth, the system capacity approaches that of an ideal distributed antenna system, (while probably being much superior in practice in terms of ease of deployment and cost). The capacity gains of the relay-assisted network over a conventional network are also analyzed.

We address the source-channel coding problem of a sensor observing a slowly time-varying Gaussian source and communicating its information to a receiver through a Gaussian channel. Due to the slowly time-varying characteristic of the source, we consider that the sensor is capable of using many channel dimensions per source symbol. Under an energy constraint per source realisation, we derive a theoretical lower bound on the MSE distortion as well as an analytical upper bound based on a practical coding scheme involving a linear uniform quantizer followed by an orthogonal modulation and a MAP receiver. Other coding schemes coupled with an MMSE estimator are also proposed and their performances are compared. An extension to the case where the sensor has the capability of encoding a sequence of N source components is studied and a general upper bound in that case is obtained.

In this paper we consider channel coding for dual data streams with unequal error protection (UEP) for the objectives of low complexity receiver structures and prioritized handling of data in MIMO broadcast systems. We focus on high spectral efficiency bit interleaved coded modulation (BICM) MIMO OFDM system where two independently coded spatial streams of symbols are simultaneously transmitted by an antenna array using antenna cycling. In some sense, the receiver then views a multiple access channel (MAC) and consequently the reception is based on successive interference cancellation (SIC). The limited adaptability of the proposed system helps gear up to a higher data rate as channel conditions improve without any adjustment at the transmitter. This leads to devising a broadcast strategy incorporating different levels of service. Standard receiver solutions for such schemes employ sub-optimal linear minimum mean square error (MMSE) successive stripping decoders. We propose a novel low complexity near optimal demodulator based on match filter outputs for a dual stream system which exhibits better performance and lower complexity as that of MMSE based demodulator.

In multi-user multiple-input multiple-output (MU-MIMO) systems, spatial multiplexing can be employed to increase the throughput without the need for multiple antennas and expensive signal processing at the user equipments. In theory, MU-MIMO is also more immune to most of propagation limitations plaguing single-user MIMO (SU-MIMO) systems, such as channel rank loss or antenna correlation. However, in this paper we show that this is not always true. We compare the capacity and the correlation of measured MU-MIMO channels for both outdoor and indoor scenarios. The measurement data has been acquired using Eurecompsilas MIMO openair sounder (EMOS). The EMOS can perform real-time MIMO channel measurements synchronously over multiple users. The results show that in most scenarios MU-MIMO provides a higher throughput than SU-MIMO also in the measured channels. However, in outdoor scenarios with a line of sight, the capacity drops significantly when the users are close together, due to high correlation at the transmitter side of the channel. In such a case, the performance of SU-MIMO and MU-MIMO is comparable.

We consider a wireless sensor network deployed in an area to measure the realization of a finite multi-dimensional, slowly time-varying physical random field. Each sensor observes one noisy realization of the field, maps it linearly into a signal with a signature and sends it across a white Gaussian multiple access channel, under a constraint on the total energy given to all the sensors per field realization. The receiver or the 'collector node' receives all the signals and tries to construct an estimate of the field within a certain mean distortion based on the MSE fidelity criterion. We derive, under the total energy constraint, a lower-bound on the distortion, an achievable one, and another lower-bound under a TDMA transmission scheme. In the case of the non-existence of the observation noise, we find the asymptotic decreasing behavior of the achievable distortion as a function of the number of sensors. Moreover, we derive a lower-bound on the distortion over all possible encoding techniques, assuming a free collaboration and information exchange between the sensors. We compare these bounds for a particular example with another bound on the achievable distortion [1].

We consider a wireless communication system formed by a single cell with one base station and K user terminals. User channels are characterized by frequency-selective fading due to small-scale effects, modeled as a set of M parallel block-fading channels, and a frequency-flat distance-dependent path loss. We compare delay-limited systems with variable-rate systems under fairness constraints, in terms of the achieved system spectral efficiency C (bit/s/Hz) versus Eb/N0. The considered delay-limited systems impose "hard-fairness": every user transmits at its desired rate on all blocks, independently of its fading conditions. The variable-rate system imposes "proportional fairness" via the popular Proportional Fair Scheduling (PFS) algorithm, currently implemented in 3G wireless for data (delay-tolerant) applications. We find simple iterative resource allocation algorithms that converge to the optimal delay-limited throughput for orthogonal (frequency-division multiple access (FDMA)/time-division multiple access (TDMA)) and optimal (superposition/interference cancellation) signaling. In the limit of large K and finite M we find closed-form expressions for C as a function of Eb/N0. We show that in this limit, the optimal allocation policy consists of letting each user transmit on its best subchannel only. Also, we find a simple closed-form expression for the throughput of PFS in a cellular environment, that holds for any K and M. Finally, we obtain closed-form expressions for C versus Eb/N0in the low and high spectral efficiency regimes. The conclusions of our analysis in terms of system design guidelines are as follows: a) if hard fairness is a requirement, orthogonal access incurs a large throughput penalty with respect to the optimal (superposition coding) strategy, especially in the regime of high spectral efficiency; b) for high spectral efficiency, PFS does not provide any significant gain and may even perform worse than the optimal delay-limited system, despite the fact that the imposed fairness constraint is laxer; c) for low to moderate spectral efficiency, the stricter hard-fairness constraint incurs in a large throughput penalty with respect to PFS

In this correspondence, we present an efficient protocol for the delay-limited fading ARQ single relay half-duplex channel. The source is using an Automatic Retransmission reQuest (ARQ) retransmission protocol to send data to the relay and the destination. When the relay is able to decode, both the relay and the source send the same data to the destination providing additional gains. The proposed protocol exploits two kinds of diversity: 1) space diversity available through the cooperative (relay) terminal, which retransmits the source's signals and 2) ARQ diversity obtained by leveraging the retransmission delay to enhance the reliability. The performance characterization is in terms of the achievable diversity, multiplexing gain and delay tradeoff for a high signal-to-noise ratio (SNR) regime. Finally, we show the benefits of power control on the diversity by controlling the source's power level over the retransmission rounds.

In this paper, the problem of resource allocation in multiuser single-antenna wideband OFDM(A) systems is considered from a cross-layer point of view. The main motivation is to show advantages of such systems with respect to narrowband systems. Despite the maximum normalized average throughput is not increased with respect to that in a narrowband system, a more efficient use of resources is possible by considering frequency as an additional resource to be allocated. It is shown that when the bandwidth is considerably larger than the coherence bandwidth of the channel and the channel is varying slowly with respect to the scheduling period, average delay can significally be improved. In that case, the average delay is only proportional to the scheduling period and not to the channel coherence time (as it is the case in narrowband systems). Furthermore, different reasource allocation policies are analyzed to show average delay improvements when buffer occupancy information is used.

We consider the uplink and the downlink of a multiuser wireless system with one base station and K user terminals (single-cell case). Each user is affected by a position-dependent path loss, fixed in time, and by a slowly time-varying frequency-selective fading channel modeled as M parallel block-fading channels. We study the system throughput (sum rate) versus Eb/N0under hard fairness and proportional fairness constraints. We obtain closed-form expressions for the throughput in the limit of a large number of users, as well as for the asymptotics in the low and high SNR regions

We consider the downlink of a multiuser multi-cell system. Each cell is equipped with multiple antennas transmitting over M parallel channels. We study the benefits of Distributed Antenna arrays on the performance of such systems and the gain on fairness between users offered by this technique,compared to co-located antenna systems. We show that the Macro-Diversity introduced by distributed antennas combined with the Max-Min allocation algorithm, not only considerably increases the system Spectral Efficiency but also offers a remarkable enhancement on the Minimum Allocated Rate and thus the fairness between users.

In this paper, we present an efficient protocol for the delay-limited fading automatic retransmission request single relay channel. The source is using an ARQ retransmission protocol to send data to the relay and the destination. When the relay is able to decode, both the relay and the source send the same data to the destination providing additional gains. The proposed protocol exploits two kinds of diversity: (i) space diversity available through the cooperative (relay) terminal, which retransmits the source's signals, (ii) ARQ diversity obtained by leveraging the retransmission delay to enhance the reliability. The performance characterization is in terms of the achievable diversity, multiplexing gain and delay tradeoff for a high signal-to-noise ratio (snr) regime

We consider the uplink and the downlink of a multiuser wireless system with one base station and K user terminals. We model wideband transmission by considering M parallel subchannels, each of which is affected by fading. The fading processes in each subchannel are slowly time-varying with respect to the coding block length. Hence, in order to maintain given rate requirements for each user and each channel state, power control is used. We study the delay-limited achievable sum rate (throughput) versus the system E/sub b//N/sub O/, under orthogonal and optimal signaling. We show that for both the orthogonal and the optimal schemes, in the limit of large K and finite M, the optimal allocation strategy consists of allocating each user to its best subchannel only. Hence, we are able to quantify the multiuser diversity gain by comparing the case K /spl rarr/ /spl infin/ with the single-user delay-limited case. Finally, we show that the limits of optimal signaling can be approached by relatively simple convolutional codes and iterative joint multiuser decoding with appropriate power control. The proposed scheme can be regarded as a practical version of the optimal successive decoding approach, that mitigates the error propagation due to the suboptimality of the user channel codes.

We consider the problem of resource allocation in multiuser single-antenna wideband OFDM(A) systems. The key advantage of such systems with respect to narrow band systems is the possibility of considering frequency as an additional resource to be allocated. Although the maximum normalized average throughput is not increased with respect to that in a narrowband system, a more efficient use of resources is possible especially if the bandwidth is considerably larger than the coherence bandwidth of the channel and the channel is varying slowly with respect to the scheduling updates. This is mainly because randomness in the system is increased by the wideband resources. The work presented in this paper analyzes the effects of bandwidth on the delay characteristics. To this end, the relationship between ergodic information rates, stability and delay in multiuser communications systems is studied and candidate resource allocation policies are presented and simulated

The paper aims at characterizing the second order statistics of indoor ultra-wideband (UWB) channels using channel sounding techniques. We present measurement results for different scenarios conducted in a laboratory setting at Institut Eurecom. These are based on a eigendecomposition of the channel autocovariance matrix, which allows for determining the growth in the number of significant degrees of freedom of the channel process as a function of the signaling bandwidth as well as the statistical correlation between different propagation paths. We show empirical eigenvalue distributions as a function of the signal bandwidth for both line-of-sight and non line-of-sight situations. Furthermore, we give examples where paths from different propagation clusters (possibly arising from reflection or diffraction) show strong statistical dependence.

This work investigates the performance of combined orthogonal channel and antenna allocation algorithms in multiple-antenna multi-channel systems. In (I. Toufik et al, IEEE VTC2004) a max-min allocation algorithm is proposed for an N-user system with N parallel sub-channels. Here, we extend this algorithm to the multiple-antenna systems and compare its performance in two different transmission scenarios (spatial multiplexing and space time coding). The techniques are applicable, for instance, in MIMO systems using OFDMA systems with dynamic sub-carrier allocation. We show that multiuser diversity, and thus an increase of aggregate data rates with the size of the user population, can still be successfully achieved even under a hard fairness constraint. Moreover, multiple-antennas permit spatial multiplexing. The techniques considered here do not require phase information in the channel allocation process, which, from a practical point-of-view is particularly important for time-division duplex systems exploiting channel reciprocity.

We establish lower bounds on the capacity of wireless ad hoc networks with two types of non-uniform traffic patterns. We first focus on the impact of traffic patterns where local communication predominates and show the improvement in terms of per user-capacity over ad hoc networks with unbounded average communication distances. We then study the capacity of hybrid wireless networks, where long-distance relaying is performed by a fixed overlay network of base stations. We investigate the scaling of capacity versus the number of nodes and the density of base stations in the area of the network. The throughput capacity results under these two scenarios hold with probability one as the number of nodes goes to infinity.

In this work we investigate the achievable rates of ultra-wideband (UWB) systems using a m-ary pulse position modulation (PPM) with non-coherent receivers in multipath fading environments. We derive a random coding bound on the achievable information rates and highlight the influence of system parameters (bandwidth, delay spread). We also investigate the effect of the use of hard decisions prior to channel decoding and characterize its impact on system performance.

This paper investigates the performance of narrowband, slowly fading, and delay-limited multiple-antenna systems where channel state information (CSI) is available at the transmission end. This situation can arise in time-division duplex (TDD) based two-way systems where channel state estimation can be performed using the signal received from the opposite link. Power control methods which attempt to keep the transmission rate constant at the expense of randomizing the transmit power are considered. It is shown that significant savings in average transmit power (sometimes on the order of tens of decibels) can be expected compared to systems which keep the total transmit power constant. Several practical channel coding examples using are illustrated and their bit and frame error rate performance are discussed.

This work considers the achievable performance for coded systems adapted to a multipath block-fading channel model. This is a particularly useful model for analyzing mobile-radio systems which employ techniques such as slow frequency-hopping under stringent time-delay or bandwidth constraints for slowly time-varying channels. In such systems, coded information is transmitted over a small number of fading channels in order to achieve diversity. Bounds on the achievable performance due to coding are derived using information-theoretic techniques. It is shown that high diversity can be achieved using relatively simple codes as long as very high spectral efficiency is not required. Examples of simple block codes and carefully chosen trellis codes are given which yield, in some cases, performances approaching the information-theoretic bounds.

We study the system capacity of cellular systems with time-division multiple access, slow time-frequency hopping (F-TDMA), and conventional single-user processing at the receivers. System capacity is formally defined as the maximum of the product of the number of users per cell times the user spectral efficiency for a given maximum outage probability. We adopt an information-theoretic definition of outage as the event that the mutual information of the block-interference channel resulting from a finite number of signal bursts spanned by the transmission of a user code word falls below the actual code rate, because of fading, shadowing, and interference. Starting from this definition, we develop a general framework which naturally takes into account many different aspects of F-TDMA cellular systems like channel reuse, channel utilization, waveform design, time-frequency hopping, voice activity exploitation, handoff, and power control strategies. Most importantly, our analysis does not rely on the choice of a particular coding scheme and can be applied to a very large class of systems in order to find guidelines for capacity-maximizing system design. A numerical example based on a typical urban mobile environment shows that there is a considerable capacity gap between actual F-TDMA systems and the limits predicted by our analysis. However, this gap can be filled by carefully designed (practical) systems, which make use of conventional single-user processing and simple coded modulation schemes.

This work considers the achievable diversity for coded systems appropriately characterized by a block-fading channel model. We are primarily interested in cases where the number of uncorrelated fading channel realizations (blocks) F, is small, so that ideal interleaving assumptions do not hold. This is usually the case in mobile radio systems which employ coded slow frequency-hopping such as the GSM system and its derivatives, We show that the diversity order is limited to a value less than or equal to F which depends on the code rate and the size of the signaling constellation. We report on the results of code searches for rate 1/n convolutional codes for simple AM constellations, which show the minimum complexity needed to achieve maximum diversity. We also present computer simulations of some codes in order to determine the effect of code complexity on the frame and bit error-rate performance.

The work considers coded MPSK systems with non-coherent detection over correlated Rayleigh fading channels. The authors extend the results of Knopp and Lieb (1994), in which a block coded-modulation technique for non-coherent detection on AWGN channels was developed, by considering the performance of this technique in a fading environment. The performance of the maximum-likelihood decoder is studied for various fade rates using union bounding techniques and the exact expression for the pairwise error event probability. It is shown that significant performance improvements can be obtained over differentially-coherent detection with the use of little or no symbol interleaving. Finally, the author address the performance of a reduced-complexity/sub-optimal decoding strategy with the aid of computer simulations.

This work considers coded M-ary phase-shift keying (MPSK) schemes with noncoherent detection. A class of block codes called module-phase codes is described. The algebraic framework used for describing these codes relies on elements from module theory which are discussed along with a method for constructing such codes for noncoherent detection. It is shown that differential encoding may be viewed as a specific code from a particular class of module-phase codes. Two classes of codes that achieve significant coding gain with respect to coherent detection of uncoded MPSK are presented. In the first class of module-phase codes, the coding gain is achieved at the expense of bandwidth expansion. In the second class, however, the coding gain is achieved at the expense of signal constellation expansion without expanding bandwidth. Finally, an integrated demodulation/decoding technique based on a modification of information set decoding is presented. It Is shown that this reduced-complexity, suboptimal decoding strategy performs nearly as well as maximum-likelihood decoding.>