Showing papers on "Spectral efficiency published in 2010"

Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

Abstract: A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.

Generalised spatial modulation

Abstract: In this paper, a generalised technique for spatial modulation (SM) is presented. Generalised spatial modulation (GSM) overcomes in a novel fashion the constraint in SM that the number of transmit antennas has to be a power of two. In GSM, a block of information bits is mapped to a constellation symbol and a spatial symbol. The spatial symbol is a combination of transmit antennas activated at each instance. The actual combination of active transmit antennas depends on the random incoming data stream. This is unlike SM where only a single transmit antenna is activated at each instance. GSM increases the overall spectral efficiency by base-two logarithm of the number of antenna combinations. This reduces the number of transmit antennas needed for the same spectral efficiency. The performance of GSM is analysed in this paper, and an upper bound on the bit-error-ratio (BER) performance is derived. In addition, an algorithm to optimise the antenna combination selection is proposed. Finally, the performance of GSM is validated through Monte Carlo simulations. The results are compared with traditional SM. It is shown that for the same spectral efficiency GSM performs nearly the same as SM, but with a significant reduction in the number of transmit antennas.

Elastic Bandwidth Allocation in Flexible OFDM-Based Optical Networks

Abstract: Orthogonal Frequency Division Multiplexing (OFDM) has recently been proposed as a modulation technique for optical networks, because of its good spectral efficiency, flexibility, and tolerance to impairments. We consider the planning problem of an OFDM optical network, where connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers and choosing an appropriate modulation level taking into account the transmission distance. Using algorithms developed in our previous works, we evaluate the spectrum utilization gains that can be obtained by utilizing the elastic bandwidth allocation of OFDM, when compared to a traditional WDM network.

LTE-Advanced: Heterogeneous networks

Abstract: Long-Term Evolution (LTE) allows operators to use new and wider spectrum and complements 3G networks with higher data rates, lower latency and a flat IP-based architecture. To further improve broadband user experience in a ubiquitous and cost effective manner, 3GPP has been working on various aspects in the framework of LTE Advanced. Since radio link performance is approaching theoretical limits with 3G enhancements and LTE, the next performance leap in wireless networks will come from the network topology. LTE Advanced is about improving spectral efficiency per unit area. Using a mix of macro, pico, femto and relay base-stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband experience to users anywhere in the network. This paper discusses the need for an alternative deployment model or topology using heterogeneous networks. To enhance the performance of these networks, advanced techniques are described which are needed to manage and control interference and deliver the full benefits of such networks. Range extension allows more user terminals to benefit directly from low-power base-stations such as picos, femtos, and relays. Adaptive inter-cell interference coordination provides smart resource allocation amongst interfering cells and improves inter-cell fairness in a heterogeneous network. In addition, the performance gains with heterogeneous networks using an example macro/pico network are shown.

Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM

Abstract: We discuss the generation, wavelength-division-multiplexed (WDM) long-haul transmission, and coherent detection of 112-Gb/s polarization-division-multiplexed (PDM) 16-ary quadrature amplitude modulation (16-QAM) at a line rate of 14 Gbaud and spectral efficiencies beyond 4 b/s/Hz. We describe the (off-line) digital signal processing and blind filter adaptation algorithms used in our intradyne receiver and characterize its performance using both simulated and measured 16-QAM waveforms. We measure a required optical signal-to-noise ratio of 20.2 dB (0.1-nm reference bandwidth; 10-3 bit-error ratio), 3.2-dB off the theoretical limit. We study the effects of finite analog-to-digital converter resolution, laser frequency offset, laser phase noise, and narrowband optical filtering. Our experiments on a 25-GHz WDM grid (4.1-b/s/Hz spectral efficiency) reveal a 1-dB penalty after 7 passes though reconfigurable optical add/drop multiplexers (ROADMs) and an achievable transmission reach of 1022 km of uncompensated standard single-mode fiber. At a spectral efficiency of 6.2 b/s/Hz (16.67-GHz WDM channel spacing) a transmission reach of 630 km is attained.

Cooperative Multicell Precoding: Rate Region Characterization and Distributed Strategies With Instantaneous and Statistical CSI

Abstract: Base station cooperation is an attractive way of increasing the spectral efficiency in multiantenna communication. By serving each terminal through several base stations in a given area, intercell interference can be coordinated and higher performance achieved, especially for terminals at cell edges. Most previous work in the area has assumed that base stations have common knowledge of both data dedicated to all terminals and full or partial channel state information (CSI) of all links. Herein, we analyze the case of distributed cooperation where each base station has only local CSI, either instantaneous or statistical. In the case of instantaneous CSI, the beamforming vectors that can attain the outer boundary of the achievable rate region are characterized for an arbitrary number of multiantenna transmitters and single-antenna receivers. This characterization only requires local CSI and justifies distributed precoding design based on a novel virtual signal-to-interference noise ratio (SINR) framework, which can handle an arbitrary SNR and achieves the optimal multiplexing gain. The local power allocation between terminals is solved heuristically. Conceptually, analogous results for the achievable rate region characterization and precoding design are derived in the case of local statistical CSI. The benefits of distributed cooperative transmission are illustrated numerically, and it is shown that most of the performance with centralized cooperation can be obtained using only local CSI.

Performance Limits of Nyquist-WDM and CO-OFDM in High-Speed PM-QPSK Systems

Abstract: We compare by simulation the performance of two techniques for generating polarization-multiplexed quadrature phase-shift keying with a high spectral efficiency (SE). The first is based on coherent optical orthogonal frequency-division multiplexing (CO-OFDM). The second, which we call Nyquist wavelength-division multiplexing (N-WDM), is based on the use of optical pulses having an “almost” rectangular spectrum, with bandwidth ideally equal to the Baud-rate. We show that the two techniques have the same sensitivity and SE under idealized assumptions. However, we found that CO-OFDM requires a much larger receiver bandwidth and proportionally faster speed of the analog-to-digital converters. We also tested CO-OFDM and N-WDM over long-haul nonlinear links and found N-WDM to outperform CO-OFDM in this case, too.

Compressive Estimation of Doubly Selective Channels in Multicarrier Systems: Leakage Effects and Sparsity-Enhancing Processing

Abstract: We consider the application of compressed sensing (CS) to the estimation of doubly selective channels within pulse-shaping multicarrier systems (which include orthogonal frequency-division multiplexing (OFDM) systems as a special case). By exploiting sparsity in the delay-Doppler domain, CS-based channel estimation allows for an increase in spectral efficiency through a reduction of the number of pilot symbols. For combating leakage effects that limit the delay-Doppler sparsity, we propose a sparsity-enhancing basis expansion and a method for optimizing the basis with or without prior statistical information about the channel. We also present an alternative CS-based channel estimator for (potentially) strongly time-frequency dispersive channels, which is capable of estimating the ?off-diagonal? channel coefficients characterizing intersymbol and intercarrier interference (ISI/ICI). For this estimator, we propose a basis construction combining Fourier (exponential) and prolate spheroidal sequences. Simulation results assess the performance gains achieved by the proposed sparsity-enhancing processing techniques and by explicit estimation of ISI/ICI channel coefficients.

Cooperative MIMO channel models: A survey

Abstract: Cooperative multiple-input multiple-output technology allows a wireless network to coordinate among distributed antennas and achieve considerable performance gains similar to those provided by conventional MIMO systems. It promises significant improvements in spectral efficiency and network coverage and is a major candidate technology in various standard proposals for the fourth-generation wireless communication systems. For the design and accurate performance assessment of cooperative MIMO systems, realistic cooperative MIMO channel models are indispensable. This article provides an overview of the state of the art in cooperative MIMO channel modeling. We show that although the existing standardized point-to-point MIMO channel models can be applied to a certain extent to model cooperative MIMO channels, many new challenges remain in cooperative MIMO channel modeling, such as how to model mobile-to-mobile channels, and how to characterize the heterogeneity and correlation of multiple links at the system level appropriately.

Routing and Spectrum Allocation in OFDM-Based Optical Networks with Elastic Bandwidth Allocation

Abstract: Orthogonal Frequency Division Multiplexing (OFDM) has been recently proposed as a modulation technique for optical networks, due to its good spectral efficiency and impairment tolerance. Optical OFDM is much more flexible compared to traditional WDM systems, enabling elastic bandwidth transmissions. We consider the planning problem of an OFDM-based optical network where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers. We introduce the Routing and Spectrum Allocation (RSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, and present various algorithms to solve the RSA. We start by presenting an optimal ILP RSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RSA into two substituent subproblems, namely, (i) routing and (ii) spectrum allocation (R+SA) and solves them sequentially. We also propose a heuristic algorithm that serves connections one-by-one and use it to solve the planning problem by sequentially serving all traffic matrix connections. To feed the sequential algorithm, two ordering policies are proposed; a simulated annealing meta-heuristic is also used to obtain even better orderings. Our results indicate that the proposed sequential heuristic with appropriate ordering yields close to optimal solutions in low running times.

Comparison of Fractional Frequency Reuse Approaches in the OFDMA Cellular Downlink

Abstract: Fractional frequency reuse (FFR) is an interference coordination technique well-suited to OFDMA based wireless networks wherein cells are partitioned into spatial regions with different frequency reuse factors. This work focuses on evaluating the two main types of FFR deployments: Strict FFR and Soft Frequency Reuse (SFR). Relevant metrics are discussed, including outage probability, network throughput, spectral efficiency, and average cell- edge user SINR. In addition to analytical expressions for outage probability, system simulations are used to compare Strict FFR and SFR with universal frequency reuse based on a typical OFDMA deployment and uniformly distributed users. Based on the analysis and numerical results, system design guidelines and a detailed picture of the tradeoffs associated with the FFR systems are presented, showing that Strict FFR provides the greatest overall network throughput and highest cell-edge user SINR, while SFR balances the requirements of interference reduction and resource efficiency.

Trellis Coded Spatial Modulation

Abstract: Trellis coded modulation (TCM) is a well known scheme that reduces power requirements without any bandwidth expansion. In TCM, only certain sequences of successive constellation points are allowed (mapping by set partitioning). The novel idea in this paper is to apply the TCM concept to the antenna constellation points of spatial modulation (SM). The aim is to enhance SM performance in correlated channel conditions. SM considers the multiple transmit antennas as additional constellation points and maps a first part of a block of information bits to the transmit antenna indices. Therefore, spatial multiplexing gains are retained and spectral efficiency is boosted. The second part of the block of information bits is mapped to a complex symbol using conventional digital modulation schemes. At any particular time instant, only one antenna is active. The receiver estimates the transmitted symbol and the active antenna index and uses the two estimates to retrieve the original block of data bits. In this paper, TCM partitions the entire set of transmit antennas into sub-sets such that the spacing between antennas within a particular sub-set is maximized. The scheme is called trellis coded spatial modulation (TCSM). Tight analytical performance bounds over correlated fading channels are proposed in this paper. In addition, the performance and complexity of TCSM is compared to the performance of SM, coded V-BLAST (vertical Bell Labs layered space-time) applying near optimum sphere decoder algorithm, and Alamouti scheme combined with TCM. Also, the performance of all schemes with turbo coded modulation is presented. It is shown that under the same spectral efficiency, TCSM exhibits significant performance enhancements in the presence of realistic channel conditions such as Rician fading and spatial correlation (SC). In addition, the complexity of the proposed scheme is shown to be 80% less than the V-BLAST complexity.

Interference management for 4G cellular standards [WIMAX/LTE UPDATE]

Abstract: 4G cellular standards are targeting aggressive spectrum reuse (frequency reuse 1) to achieve high system capacity and simplify radio network planning. The increase in system capacity comes at the expense of SINR degradation due to increased intercell interference, which severely impacts cell-edge user capacity and overall system throughput. Advanced interference management schemes are critical for achieving the required cell edge spectral efficiency targets and to provide ubiquity of user experience throughout the network. In this article we compare interference management solutions across the two main 4G standards: IEEE 802.16m (WiMAX) and 3GPP-LTE. Specifically, we address radio resource management schemes for interference mitigation, which include power control and adaptive fractional frequency reuse. Additional topics, such as interference management for multitier cellular deployments, heterogeneous architectures, and smart antenna schemes will be addressed in follow-up papers.

Effective Interference Cancellation Scheme for Device-to-Device Communication Underlaying Cellular Networks

Abstract: It is expected that Device-to-Device (D2D) communication is allowed to underlay future cellular networks such as IMT-Advanced for spectrum efficiency. However, by reusing the uplink spectrums with the cellular system, the interference to D2D users has to be addressed to maximize the overall system performance. In this paper, a novel method to deal with the resource allocation and interference avoidance issues by utilizing the network peculiarity of a hybrid network to share the uplink resource is proposed and the implementation details are described in a real cellular system. Simulation results prove that satisfying performance can be achieved by using the proposed mechanism.

Closed-form expressions for nonlinear transmission performance of densely spaced coherent optical OFDM systems.

Abstract: There has been a trend of migration to high spectral efficiency transmission in optical fiber communications for which the frequency guard band between neighboring wavelength channels continues to shrink. In this paper, we derive closed-form analytical expressions for nonlinear system performance of densely spaced coherent optical OFDM (CO-OFDM) systems. The closed-form solutions include the results for the achievable Q factor, optimum launch power density, nonlinear threshold of launch power density, and information spectral efficiency limit. These analytical results clearly identify their dependence on system parameters including fiber dispersion, number of spans, dispersion compensation ratio, and overall bandwidth. The closed-form solution is further substantiated by numerical simulations using distributed nonlinear Schrodinger equation.

Cooperative distributed antenna systems for mobile communications [Coordinated and Distributed MIMO]

Abstract: Cooperative distributed antenna systems have drawn considerable attention in recent years due to their potential to enhance both the coverage and the spectral efficiency of mobile communication systems In this article the conceptual description of a cooperative DAS for mobile communications is presented together with key techniques for DASs, including distributed multiple- input multiple-output for single and multiple users, handover, and transmit power allocation Furthermore, theoretical and simulation results on the spectral or transmit power efficiency of cooperative DASs are given and compared with those on conventional collocated antenna systems It is shown that a cooperative DAS can provide very promising performance enhancements in capacity and transmit power efficiency

Generalised spatial modulation with multiple active transmit antennas

Abstract: We propose a new generalised spatial modulation (GSM) technique, which can be considered as a generalisation of the recently proposed spatial modulation (SM) technique. SM can be seen as a special case of GSM with only one active transmit antenna. In contrast to SM, GSM uses the indices of multiple transmit antennas to map information bits, and is thus able to achieve substantially increased spectral efficiency. Furthermore, selecting multiple active transmit antennas enables GSM to harvest significant transmit diversity gains in comparison to SM, because all the active antennas transmit the same information. On the other hand, inter-channel interference (ICI) is completely avoided by transmitting the same symbols through these active antennas. We present theoretical analysis using order statistics for the symbol error rate (SER) performance of GSM. The analytical results are in close agreement with our simulation results. The bit error rate performance of GSM and SM is simulated and compared, which demonstrates the superiority of GSM. Moreover, GSM systems with configurations of different transmit and receive antennas are studied. Our results suggest that using a less number of transmit antennas with a higher modulation order will lead to better BER performance.

Design and experimental evaluation of multi-user beamforming in wireless LANs

Abstract: Multi-User MIMO promises to increase the spectral efficiency of next generation wireless systems and is currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such systems in practice. In this paper, we present the design and implementation of the first multi-user beamforming system and experimental framework for wireless LANs. Using extensive measurements in an indoor environment, we evaluate the impact of receiver separation distance, outdated channel information due to mobility and environmental variation, and the potential for increasing spatial reuse. For the measured indoor environment, our results reveal that two receivers achieve close to maximum performance with a minimum separation distance of a quarter of a wavelength. We also show that the required channel information update rate is dependent on environmental variation and user mobility as well as a per-link SNR requirement. Assuming that a link can tolerate an SNR decrease of 3 dB, the required channel update rate is equal to 100 and 10 ms for non-mobile receivers and mobile receivers with a pedestrian speed of 3 mph respectively. Our results also show that spatial reuse can be increased by efficiently eliminating interference at any desired location; however, this may come at the expense of a significant drop in the quality of the served users.

Mitigation of Inter-Femtocell Interference with Adaptive Fractional Frequency Reuse

Abstract: In this paper, we consider the use of fractional frequency reuse (FFR) to mitigate inter-femtocell interference in multi-femtocell environments. The use of universal frequency reuse can be said optimum in terms of the ergodic system spectral efficiency. However, it may cause inter-femtocell interference near the cell edge, making it difficult to serve the whole cell coverage. To alleviate this problem, we adjust the frequency reuse factor with the aid of femtocell location information. The proposed scheme can provide reasonably high ergodic system spectral efficiency, while assuring a desired performance near the cell boundary. Simulation results show that the proposed scheme is quite effective in indoor environments such as residential or office buildings.

Distributed Optimal Relay Selection in Wireless Cooperative Networks With Finite-State Markov Channels

Abstract: Relay selection is crucial in improving the performance of wireless cooperative networks. Most previous works for relay selection use the current observed channel conditions to make the relay-selection decision for the subsequent frame. However, this memoryless channel assumption is often not realistic given the time-varying nature of some mobile environments. In this paper, we consider finite-state Markov channels in the relay-selection problem. Moreover, we also incorporate adaptive modulation and coding, as well as residual relay energy in the relay-selection process. The objectives of the proposed scheme are to increase spectral efficiency, mitigate error propagation, and maximize the network lifetime. The formulation of the proposed relay-selection scheme is based on recent advances in stochastic control algorithms. The obtained relay-selection policy has an indexability property that dramatically reduces the computation and implementation complexity. In addition, there is no need for a centralized control point in the network, and relays can freely join and leave from the set of potential relays. Simulation results are presented to show the effectiveness of the proposed scheme.

Experimental demonstration of a record high 11.25Gb/s real-time optical OFDM transceiver supporting 25km SMF end-to-end transmission in simple IMDD systems.

Abstract: The fastest ever 11.25Gb/s real-time FPGA-based optical orthogonal frequency division multiplexing (OOFDM) transceivers utilizing 64-QAM encoding/decoding and significantly improved variable power loading are experimentally demonstrated, for the first time, incorporating advanced functionalities of on-line performance monitoring, live system parameter optimization and channel estimation. Real-time end-to-end transmission of an 11.25Gb/s 64-QAM-encoded OOFDM signal with a high electrical spectral efficiency of 5.625bit/s/Hz over 25km of standard and MetroCor single-mode fibres is successfully achieved with respective power penalties of 0.3dB and -0.2dB at a BER of 1.0 x 10(-3) in a directly modulated DFB laser-based intensity modulation and direct detection system without in-line optical amplification and chromatic dispersion compensation. The impacts of variable power loading as well as electrical and optical components on the transmission performance of the demonstrated transceivers are experimentally explored in detail. In addition, numerical simulations also show that variable power loading is an extremely effective means of escalating system performance to its maximum potential.

1-Tb/s Single-Channel Coherent Optical OFDM Transmission With Orthogonal-Band Multiplexing and Subwavelength Bandwidth Access

Abstract: A 1-Tb/s single-channel coherent optical OFDM (CO-OFDM) signal consisting of continuous 4104 spectrally-overlapped subcarriers is generated using a recirculating frequency shifter (RFS). Theoretical and experimental analysis of the RFS is performed to study its effectiveness in extending OFDM bandwidth. In particular, the RFS produces a 320.6-GHz wide frequency comb from a single laser with superior flatness and tone-to-noise ratio (TNR). The 1-Tb/s CO-OFDM signal is consisted of 36 uncorrelated orthogonal bands achieved by adjusting the delay of the RFS to an integer number of OFDM symbol periods. The 1-Tb/s CO-OFDM signal with a spectral efficiency of 3.3 bit/s/Hz is successfully received after transmission over 600-km SSMF fiber without either Raman amplification or dispersion compensation.

Continuous aperture phased MIMO: Basic theory and applications

Abstract: Given the proliferation of wireless communication devices, the need for increased power and bandwidth efficiency in emerging technologies is getting ever more pronounced. Two technological trends offer new opportunities for addressing these challenges: mm-wave systems (60–100GHz) that afford large bandwidths, and multi-antenna (MIMO) transceivers that exploit the spatial dimension. In particular, there has been significant recent interest in mm-wave communication systems for high-rate (1–100 Gb/s) communication over line-of-sight (LoS) channels. Two competing designs dominate the state-of-the-art: i) traditional systems that employ continuous aperture “dish” antennas and offer high power efficiency but no spatial multiplexing gain, and ii) MIMO systems that use discrete antenna arrays for a higher multiplexing gain but suffer from power efficiency. In this paper, we propose a new communication architecture - continuous aperture phased MIMO - that combines the advantages of both designs and promises very significant capacity gains, and commensurate gains in power and bandwidth efficiency, compared to the state-of-the-art. CAP-MIMO is based on a hybrid analog-digital transceiver architecture that employs a novel antenna array structure - a high-resolution discrete lens array - to enable a continuous aperture phased-MIMO operation. We present the basic theory behind CAP-MIMO and the potential capacity/power gains afforded by it. We also highlight potential applications of CAP-MIMO in mm-wave communications.

Extended Active Interference Cancellation for Sidelobe Suppression in Cognitive Radio OFDM Systems With Cyclic Prefix

Abstract: The sidelobe of noncontiguous orthogonal frequency-division multiplexing (OFDM) signals is required to be deeply suppressed in the licensed user's band in cognitive radio (CR) systems. To this end, we propose a novel method of adding extended active interference cancellation (EAIC) signals to suppress sidelobes and to shape the spectrum of the CR-OFDM signal with a cyclic prefix (CP). For simplicity, we called the proposed scheme EAIC-CP. The key idea of the proposed EAIC-CP is to employ some cancellation signals consisting of tones spaced closer than the interval of OFDM subcarriers to cancel the sidelobes of OFDM signals. Moreover, we derive the optimal cancellation signals to minimize the total sidelobe power subject to a self-interference constraint. Numerical results show that, when the guard bandwidth is equal to one OFDM subcarrier interval, the EAIC-CP scheme offers more than a 45.0-dB sidelobe suppression with unnoticeable signal-to-noise ratio (SNR) loss at a symbol error rate (SER) from 10-2 to 10-3 for 64 quadratic-amplitude modulation (64QAM). Moreover, the EAIC-CP scheme can achieve high spectrum efficiency with low implementation complexity.

Distributed Relay-Assignment Protocols for Coverage Expansion in Cooperative Wireless Networks

Abstract: One important application of cooperative communications is to extend coverage area in wireless networks without increasing infrastructure. However, a crucial challenge in implementing cooperation protocols is how to select relay-source pairs. In this paper, we address this problem based on the knowledge of the users' spatial distribution which determines the channel statistics. We consider two scenarios at the destination node, when the receiver uses MRC and when no-MRC is used. First, we characterize the optimal relay location to minimize the outage probability. Then, we propose and analyze the performance of two schemes: a distributed nearest neighbor relay assignment in which users can act as relays, and an infrastructure-based relay-assignment protocol in which fixed relay nodes are deployed in the network to help the users forward their data. The outage probabilities of these two schemes are derived. We also derive universal lower bounds on the performance of relay-assignment protocols to serve as a benchmark for our proposed protocols. Numerical results reveal significant gains when applying the proposed simple distributed algorithms over direct transmission in terms of coverage area, transmit power, and spectral efficiency. At 1 percent outage probability, more than 200 percent increase in coverage area can be achieved, 7 dBW savings in the transmitted power, and the system can operate at 2 b/s/Hz higher spectral efficiency.

A Unified Treatment of Optimum Pilot Overhead in Multipath Fading Channels

Abstract: The optimization of the pilot overhead in single-user wireless fading channels is investigated, and the dependence of this overhead on various system parameters of interest (e.g., fading rate, signal-to-noise ratio) is quantified. The achievable pilot-based spectral efficiency is expanded with respect to the fading rate about the no-fading point, which leads to an accurate order expansion for the pilot overhead. This expansion identifies that the pilot overhead, as well as the spectral efficiency penalty with respect to a reference system with genie-aided CSI (channel state information) at the receiver, depend on the square root of the normalized Doppler frequency. It is also shown that the widely-used block fading model is a special case of more accurate continuous fading models in terms of the achievable pilot-based spectral efficiency. Furthermore, it is established that the overhead optimization for multiantenna systems is effectively the same as for single-antenna systems with the normalized Doppler frequency multiplied by the number of transmit antennas.

Filterbank based multi carrier transmission (FBMC) — evolving OFDM: FBMC in the context of WiMAX

Abstract: This paper gives an overview of some outcomes produced in the FP7 project PHYDYAS — PHYsical layer for DYnamic AccesS and cognitive radio. The main concern of the project is to analyze and propose a new physical layer succeeding OFDM, i.e. a multi carrier system applying filter banks omitting the severe out-of-band leakage of OFDM. The new physical layer was investigated within two different scenarios. On the one side a cognitive radio scenario exploiting the stronger frequency containment for improved dynamic spectrum access. On the other side a cellular basestation ruled network harvesting the increased spectral efficiency. Within this paper we concentrate on the latter. We compare a conventional OFDM based system to its modified version applying the filter bank in terms of complexity, performance and compatibility. Additionally we propose a smooth evolution path ensuring legacy support.

Energy efficient cooperative LEACH protocol for wireless sensor networks

Abstract: We develop a low complexity cooperative diversity protocol for low energy adaptive clustering hierarchy (LEACH) based wireless sensor networks. A cross layer approach is used to obtain spatial diversity in the physical layer. In this paper, a simple modification in clustering algorithm of the LEACH protocol is proposed to exploit virtual multiple-input multiple-output (MIMO) based user cooperation. In lieu of selecting a single cluster-head at network layer, we proposed M cluster-heads in each cluster to obtain a diversity order of M in long distance communication. Due to the broadcast nature of wireless transmission, cluster-heads are able to receive data from sensor nodes at the same time. This fact ensures the synchronization required to implement a virtual MIMO based space time block code (STBC) in cluster-head to sink node transmission. An analytical method to evaluate the energy consumption based on BER curve is presented. Analysis and simulation results show that proposed cooperative LEACH protocol can save a huge amount of energy over LEACH protocol with same data rate, bit error rate, delay and bandwidth requirements. Moreover, this proposal can achieve higher order diversity with improved spectral efficiency compared to other virtual MIMO based protocols.

A geometric approach to improve spectrum efficiency for cognitive relay networks

Abstract: Cognitive radio (CR) enables dynamic spectrum access (DSA). In CR networks (CRNs), unlicensed cognitive users (CUs) can share the radio spectrum with the licensed primary users (PUs) if the incurred signal-to-noise-and-interference ratio (SNIR) to the PUs is above a predetermined threshold. In this paper, we investigate the use of relaying to enhance the spectrum utilization of an idealized two-dimensional geometric network in Rayleigh fading channels. To do so, the CUs should be made transparent to the PUs while achieving acceptable reliability themselves. Based on these conditions, we first derive the geometric condition under which CUs could transmit over a single hop with the PUs being active. If the destination is beyond the reach of the source in a single hop, multihop relaying will be used for shorter-distance less-power transmission. Then, we propose and analyze two multihop routing strategies, termed nearest-neighbor routing (NNR) and farthest-neighbor routing (FNR). In multihop relaying, we also study the benefits of concurrency, in which more than one CUs are allowed to transmit at the same time to improve the end-to-end (e2e) channel utilization. Finally, we compare the e2e performance of CRNs with and without relaying.

Adaptive joint scheduling of spectrum sensing and data transmission in cognitive radio networks

Abstract: We consider a cognitive radio (CR) network that makes opportunistic use of a set of channels licensed to a primary network. During operation, the CR network is required to carry out spectrum sensing to detect active primary users, thereby avoiding interfering with them. However, spectrum sensing may cause negative effect on the performance of the CR network, as all CR communications has to be postponed during channel sensing. This paper focuses on adaptively scheduling spectrum sensing and data transmission so that negative impacts to the performance of the CR network are minimized. We first consider the case when CR nodes always have data to transmit and experience time-varying channels. Based on knowledge of channel conditions, the sensing periods are adaptively scheduled to maximize the spectrum efficiency of the CR operation. We show how optimal sensing/transmission scheduling policies can be obtained and prove some important structural properties of such optimal policies. We then consider the case when CR nodes experience both stochastic data arrival and time-varying channels. By treating each sensing period as a 'virtual sensing packet'', we convert the problem of joint spectrum-sensing/datatransmission scheduling into a standard queueing model. Based on that, an efficient scheduling algorithm that takes into account channel and queue conditions of the CR network is proposed.