Showing papers on "Spectral efficiency published in 2008"

Spatial Modulation

Abstract: Spatial modulation (SM) is a recently developed transmission technique that uses multiple antennas. The basic idea is to map a block of information bits to two information carrying units: 1) a symbol that was chosen from a constellation diagram and 2) a unique transmit antenna number that was chosen from a set of transmit antennas. The use of the transmit antenna number as an information-bearing unit increases the overall spectral efficiency by the base-two logarithm of the number of transmit antennas. At the receiver, a maximum receive ratio combining algorithm is used to retrieve the transmitted block of information bits. Here, we apply SM to orthogonal frequency division multiplexing (OFDM) transmission. We develop an analytical approach for symbol error ratio (SER) analysis of the SM algorithm in independent identically distributed (i.i.d.) Rayleigh channels. The analytical and simulation results closely match. The performance and the receiver complexity of the SM-OFDM technique are compared to those of the vertical Bell Labs layered space-time (V-BLAST-OFDM) and Alamouti-OFDM algorithms. V-BLAST uses minimum mean square error (MMSE) detection with ordered successive interference cancellation. The combined effect of spatial correlation, mutual antenna coupling, and Rician fading on both coded and uncoded systems are presented. It is shown that, for the same spectral efficiency, SM results in a reduction of around 90% in receiver complexity as compared to V-BLAST and nearly the same receiver complexity as Alamouti. In addition, we show that SM achieves better performance in all studied channel conditions, as compared with other techniques. It is also shown to efficiently work for any configuration of transmit and receive antennas, even for the case of fewer receive antennas than transmit antennas.

Bit-Interleaved Coded Modulation

Abstract: The principle of coding in the signal space follows directly from Shannon's analysis of waveform Gaussian channels subject to an input constraint. The early design of communication systems focused separately on modulation, namely signal design and detection, and error correcting codes, which deal with errors introduced at the demodulator of the underlying waveform channel. The correct perspective of signal-space coding, although never out of sight of information theorists, was brought back into the focus of coding theorists and system designers by Imai's and Ungerbock's pioneering works on coded modulation. More recently, powerful families of binary codes with a good tradeoff between performance and decoding complexity have been (re-)discovered. Bit-Interleaved Coded Modulation (BICM) is a pragmatic approach combining the best out of both worlds: it takes advantage of the signal-space coding perspective, whilst allowing for the use of powerful families of binary codes with virtually any modulation format. BICM avoids the need for the complicated and somewhat less flexible design typical of coded modulation. As a matter of fact, most of today's systems that achieve high spectral efficiency such as DSL, Wireless LANs, WiMax and evolutions thereof, as well as systems based on low spectral efficiency orthogonal modulation, feature BICM, making BICM the de-facto general coding technique for waveform channels. The theoretical characterization of BICM is at the basis of efficient coding design techniques and also of improved BICM decoders, e.g., those based on the belief propagation iterative algorithm and approximations thereof. In this text, we review the theoretical foundations of BICM under the unified framework of error exponents for mismatched decoding. This framework allows an accurate analysis without any particular assumptions on the length of the interleaver or independence between the multiple bits in a symbol. We further consider the sensitivity of the BICM capacity with respect to the signal-to-noise ratio (SNR), and obtain a wideband regime (or low-SNR regime) characterization. We review efficient tools for the error probability analysis of BICM that go beyond the standard approach of considering infinite interleaving and take into consideration the dependency of the coded bit observations introduced by the modulation. We also present bounds that improve upon the union bound in the region beyond the cutoff rate, and are essential to characterize the performance of modern randomlike codes used in concatenation with BICM. Finally, we turn our attention to BICM with iterative decoding, we review extrinsic information transfer charts, the area theorem and code design via curve fitting. We conclude with an overview of some applications of BICM beyond the classical coherent Gaussian channel.

Optimal Linear Cooperation for Spectrum Sensing in Cognitive Radio Networks

Abstract: Cognitive radio technology has been proposed to improve spectrum efficiency by having the cognitive radios act as secondary users to opportunistically access under-utilized frequency bands. Spectrum sensing, as a key enabling functionality in cognitive radio networks, needs to reliably detect signals from licensed primary radios to avoid harmful interference. However, due to the effects of channel fading/shadowing, individual cognitive radios may not be able to reliably detect the existence of a primary radio. In this paper, we propose an optimal linear cooperation framework for spectrum sensing in order to accurately detect the weak primary signal. Within this framework, spectrum sensing is based on the linear combination of local statistics from individual cognitive radios. Our objective is to minimize the interference to the primary radio while meeting the requirement of opportunistic spectrum utilization. We formulate the sensing problem as a nonlinear optimization problem. By exploiting the inherent structures in the problem formulation, we develop efficient algorithms to solve for the optimal solutions. To further reduce the computational complexity and obtain solutions for more general cases, we finally propose a heuristic approach, where we instead optimize a modified deflection coefficient that characterizes the probability distribution function of the global test statistics at the fusion center. Simulation results illustrate significant cooperative gain achieved by the proposed strategies. The insights obtained in this paper are useful for the design of optimal spectrum sensing in cognitive radio networks.

Coherent detection in optical fiber systems

Abstract: The drive for higher performance in optical fiber systems has renewed interest in coherent detection. We review detection methods, including noncoherent, differentially coherent, and coherent detection, as well as a hybrid method. We compare modulation methods encoding information in various degrees of freedom (DOF). Polarization-multiplexed quadrature-amplitude modulation maximizes spectral efficiency and power efficiency, by utilizing all four available DOF, the two field quadratures in the two polarizations. Dual-polarization homodyne or heterodyne downconversion are linear processes that can fully recover the received signal field in these four DOF. When downconverted signals are sampled at the Nyquist rate, compensation of transmission impairments can be performed using digital signal processing (DSP). Linear impairments, including chromatic dispersion and polarization-mode dispersion, can be compensated quasi-exactly using finite impulse response filters. Some nonlinear impairments, such as intra-channel four-wave mixing and nonlinear phase noise, can be compensated partially. Carrier phase recovery can be performed using feedforward methods, even when phase-locked loops may fail due to delay constraints. DSP-based compensation enables a receiver to adapt to time-varying impairments, and facilitates use of advanced forward-error-correction codes. We discuss both single- and multi-carrier system implementations. For a given modulation format, using coherent detection, they offer fundamentally the same spectral efficiency and power efficiency, but may differ in practice, because of different impairments and implementation details. With anticipated advances in analog-to-digital converters and integrated circuit technology, DSP-based coherent receivers at bit rates up to 100 Gbit/s should become practical within the next few years.

Optimal spectrum sensing framework for cognitive radio networks

Abstract: Spectrum sensing is the key enabling technology for cognitive radio networks. The main objective of spectrum sensing is to provide more spectrum access opportunities to cognitive radio users without interfering with the operations of the licensed network. Hence, recent research has been focused on the interference avoidance problem. Moreover, current radio frequency (RF) front-ends cannot perform sensing and transmission at the same time, which inevitably decreases their transmission opportunities, leading to the so-called sensing efficiency problem. In this paper, in order to solve both the interference avoidance and the spectrum efficiency problem, an optimal spectrum sensing framework is developed. More specifically, first a theoretical framework is developed to optimize the sensing parameters in such a way as to maximize the sensing efficiency subject to interference avoidance constraints. Second, in order to exploit multiple spectrum bands, spectrum selection and scheduling methods are proposed where the best spectrum bands for sensing are selected to maximize the sensing capacity. Finally, an adaptive and cooperative spectrum sensing method is proposed where the sensing parameters are optimized adaptively to the number of cooperating users. Simulation results show that the proposed sensing framework can achieve maximum sensing efficiency and opportunities in multi-user/multi-spectrum environments, satisfying interference constraints.

Cooperative communications with relay-selection: when to cooperate and whom to cooperate with?

Abstract: In this paper; we propose a new cooperative communication protocol, which achieves higher bandwidth efficiency while guaranteeing the same diversity order as that of the conventional cooperative schemes. The proposed scheme considers relay selection via the available partial channel state information (CSI) at the source and the relays. In particular, we discuss the multi-node decode-and-forward cooperative scenarios, where arbitrary N relays are available. The source determines when it needs to cooperate with one relay only, and which relay to cooperate with in case of cooperation, i.e., "When to cooperate?" and "Whom to cooperate with?". An optimal relay is the one which has the maximum instantaneous scaled harmonic mean functionof its source-relay and relay-destination channel gains. For the symmetric scenario, we derive an approximate expression of the bandwidth efficiency and obtain an upper bound on the symbol error rate (SER) performance. We show that full diversity is guaranteed and that a significant increase of the bandwidth efficiency is achieved. Moreover, we present the tradeoff between the achievable bandwidth efficiency and the corresponding SER. Finally, the obtained analytical results are verified through computer simulations.

A Low-Complexity Detector for Large MIMO Systems and Multicarrier CDMA Systems

Abstract: We consider large MIMO systems, where by 'large' we mean number of transmit and receive antennas of the order of tens to hundreds. Such large MIMO systems will be of immense interest because of the very high spectral efficiencies possible in such systems. We present a low-complexity detector which achieves uncoded near-exponential diversity performance for hundreds of antennas (i.e., achieves near SISO AWGN performance in a large MIMO fading environment) with an average per-bit complexity of just O(NtNr), where Nt and Nr denote the number of transmit and receive antennas, respectively. With an outer turbo code, the proposed detector achieves good coded bit error performance as well. For example, in a 600 transmit and 600 receive antennas V-BLAST system with a high spectral efficiency of 450 bps/Hz (using BPSK and rate-3/4 turbo code), our simulation results show that the proposed detector performs to within about 7 dB from capacity. This practical feasibility of the proposed high-performance, low-complexity detector could potentially trigger wide interest in the theory and implementation of large MIMO systems. We also illustrate the applicability of the proposed detector in the low-complexity detection of high-rate, non-orthogonal space-time block codes and large multicarrier CDMA (MC-CDMA) systems. In large MC-CDMA systems with hundreds of users, the proposed detector is shown to achieve near single-user performance at an average per-bit complexity linear in number of users, which is quite appealing for its use in practical CDMA systems.

Cellular networks with an overlaid device to device network

Abstract: Spectrum sharing is a novel opportunistic strategy to improve spectral efficiency of wireless networks. Much of the research to quantify such a gain is done under the premise that the spectrum is being used inefficiently by the primary network. Our main result is that even in a spectrally efficient network, device to device users can exploit the network topology to render gains in additional throughput. The focus will be on providing ad-hoc multihop access to a network for device to device users, that are transparent to the primary wireless cellular network, while sharing the primary network's resources.

A compressed sensing technique for OFDM channel estimation in mobile environments: Exploiting channel sparsity for reducing pilots

Abstract: We consider the estimation of doubly selective wireless channels within pulse-shaping multicarrier systems (which include OFDM systems as a special case). A new channel estimation technique using the recent methodology of compressed sensing (CS) is proposed. CS-based channel estimation exploits a channel's delay-Doppler sparsity to reduce the number of pilots and, hence, increase spectral efficiency. Simulation results demonstrate a significant reduction of the number of pilots relative to least-squares channel estimation.

107 Gb/s coherent optical OFDM transmission over 1000-km SSMF fiber using orthogonal band multiplexing

Abstract: Coherent optical OFDM (CO-OFDM) has emerged as an attractive modulation format for the forthcoming 100 Gb/s Ethernet. However, even the spectral-efficient implementation of CO-OFDM requires digital-to-analog converters (DAC) and analog-to-digital converters (ADC) to operate at the bandwidth which may not be available today or may not be cost-effective. In order to resolve the electronic bandwidth bottleneck associated with DAC/ADC devices, we propose and elucidate the principle of orthogonal-band-multiplexed OFDM (OBM-OFDM) to subdivide the entire OFDM spectrum into multiple orthogonal bands. With this scheme, the DAC/ADCs do not need to operate at extremely high sampling rate. The corresponding mapping to the mixed-signal integrated circuit (IC) design is also revealed. Additionally, we show the proof-of-concept transmission experiment through optical realization of OBM-OFDM. To the best of our knowledge, we present the first experimental demonstration of 107 Gb/s QPSK-encoded CO-OFDM signal transmission over 1000 km standard-single- mode-fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated system employs 2x2 MIMO-OFDM signal processing and achieves high electrical spectral efficiency with direct-conversion at both transmitter and receiver.

Complex Field Network Coding for Multiuser Cooperative Communications

Abstract: Multi-source relay-based cooperative communications can achieve spatial diversity gains, enhance coverage and potentially increase capacity when multiuser detection is used to effect maximum likelihood demodulation. If considered for large networks, traditional relaying entails loss in spectral efficiency that can be mitigated through network coding at the physical layer. These considerations motivate the complex field network coding (CFNC) approach introduced in this paper. Different from network coding over the Galois field, where wireless throughput is limited as the number of sources increases, CFNC always achieves throughput as high as 1/2 symbol per source per channel use. In addition to improved throughput, CFNC- based relaying achieves full diversity gain regardless of the underlying signal-to-noise-ratio (SNR) and the constellation used. Furthermore, the CFNC approach is general enough to allow for transmissions from sources to a common destination as well as simultaneous information exchanges among sources.

A Novel Approach to MIMO Transmission Using a Single RF Front End

Abstract: In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

Channel-aware scheduling algorithms for SC-FDMA in LTE uplink

Abstract: Single-carrier frequency division multiple access (SC-FDMA) has been selected as the uplink access scheme in the UTRA Long Term Evolution (LTE) due to its low peak-to-average power ratio properties compared to orthogonal frequency division multiple access. Nevertheless, in order to achieve such a benefit, it requires a localized allocation of the resource blocks, which naturally imposes a severe constraint on the scheduler design. In this paper, three new channel-aware scheduling algorithms for SC-FDMA are proposed and evaluated in both local and wide area scenarios. Whereas the first maximum expansion (FME) and the recursive maximum expansion (RME) are relative simple solutions to the above-mentioned problem, the minimum area-difference to the envelope (MADE) is a more computational expensive approach, which, on the other hand, performs closer to the optimal combinatorial solution. Simulation results show that adopting a proportional fair metric all the proposed algorithms quickly reach a high level of data-rate fairness. At the same time, they definitely outperform the round-robin scheduling in terms of cell spectral efficiency with gains up to 68.8% in wide area environments.

Multi-Stage Pricing Game for Collusion-Resistant Dynamic Spectrum Allocation

Abstract: In order to fully utilize scarce spectrum resources, dynamic spectrum allocation becomes a promising approach to increase the spectrum efficiency for wireless networks. However, the collusion among selfish network users may seriously deteriorate the efficiency of dynamic spectrum sharing. The network users' behaviors and dynamics need to be taken into consideration for efficient and robust spectrum allocation. In this paper, we model the spectrum allocation in wireless networks with multiple selfish legacy spectrum holders and unlicensed users as multi-stage dynamic games. In order to combat user collusion, we propose a pricing-based collusion-resistant dynamic spectrum allocation approach to optimize overall spectrum efficiency, while not only keeping the participating incentives of the selfish users but also combating possible user collusion. The simulation results show that the proposed scheme achieves high efficiency of spectrum usage even with the presence of severe user collusion.

Reliable Multimedia Transmission Over Cognitive Radio Networks Using Fountain Codes

Abstract: With the explosive growth of wireless multimedia applications over the wireless Internet in recent years, the demand for radio spectral resources has increased significantly In order to meet the quality of service, delay, and large bandwidth requirements, various techniques such as source and channel coding, distributed streaming, multicast etc have been considered In this paper, we propose a technique for distributed multimedia transmission over the secondary user network, which makes use of opportunistic spectrum access with the help of cognitive radios We use digital fountain codes to distribute the multimedia content over unused spectrum and also to compensate for the loss incurred due to primary user interference Primary user traffic is modelled as a Poisson process We develop the techniques to select appropriate channels and study the trade-offs between link reliability, spectral efficiency and coding overhead Simulation results are presented for the secondary spectrum access model

Smart regenerative relays for link-adaptive cooperative communications

Abstract: Without being necessary to pack multiple antennas per terminal, cooperation among distributed single-antenna nodes offers resilience to shadowing and can, in principle, enhance the performance of wireless communication networks by exploiting the available space diversity. Enabling the latter however, calls for practically implementable protocols to cope with errors at relay nodes so that simple receiver processing can collect the diversity at the destination. To this end, we derive in this paper a class of strategies whereby decoded bits at relay nodes are scaled in power before being forwarded to the destination. The scale is adapted to the signal-to-noise-ratio (SNR) of the source-relay and the intended relay-destination links. With maximum ratio combining (MRC) at the destination, we prove that such link-adaptive regeneration (LAR) strategies effect the maximum possible diversity while requiring simple channel state information that can be pragmatically acquired at the relay. In addition, LAR exhibits robustness to quantization and feedback errors and leads to efficient use of power both at relay as well as destination nodes. Analysis and corroborating simulations demonstrate that LAR relays are attractive across the practical SNR range; they are universally applicable to multibranch and multi-hop uncoded or coded settings regardless of the underlying constellation; and outperform existing alternatives in terms of error performance, complexity and bandwidth efficiency.

Distributed Rule-Regulated Spectrum Sharing

Abstract: Dynamic spectrum access is a promising technique to use spectrum efficiently. Without being restricted to any prefixed spectrum bands, nodes choose operating spectrum on-demand. Such flexibility, however, makes efficient and fair spectrum access in large-scale networks a great challenge. Prior work in this area focused on explicit coordination where nodes communicate with peers to modify local spectrum allocation, and may heavily stress the communication resource. In this paper, we introduce a distributed spectrum management architecture where nodes share spectrum resource fairly by making independent actions following spectrum rules. We present five spectrum rules to regulate node behavior and maximize system fairness and spectrum utilization, and analyze the associated complexity and overhead. We show analytically and experimentally that the proposed rule-based approach achieves similar performance with the explicit coordination approach, while significantly reducing communication cost.

Transmitter Noise Effect on the Performance of a MIMO-OFDM Hardware Implementation Achieving Improved Coverage

Abstract: This paper presents analysis of performance measurements from a MIMO-OFDM IEEE 802.11n hardware implementation at 5.2 GHz using four transmitters and four receivers. Two spatial multiplexing systems are compared; one which uses a zero-forcing (ZF) detector and the other a list sphere detector (LSD). We show that the measured results do not align with standard prediction based on simulation assuming uncorrelated receiver noise. We show that the discrepancy can be explained by the inclusion of transmitter noise into the channel model. This effect is not included in existing MIMO-OFDM channel models. The measured results from our hardware implementation show successful packet transmission at 600 Mb/s with 15 bits/s/Hz spectral efficiency at 73% coverage for ZF and 84% coverage for LSD with an average receiver signal to noise ratio (SNR) of 26 dB.

Bandwidth partitioning in decentralized wireless networks

Abstract: This paper addresses the following question, which is of interest in the design of a multiuser decentralized network. Given a total system bandwidth of W Hz and a fixed data rate constraint of R bps for each transmission, how many frequency slots N of size W/N should the band be partitioned into in order to maximize the number of simultaneous links in the network? Dividing the available spectrum results in two competing effects. On the positive side, a larger N allows for more parallel, non- interfering communications to take place in the same area. On the negative side, a larger N increases the SINR requirement for each link because the same information rate must be achieved over less bandwidth. Exploring this tradeoff and determining the optimum value of N in terms of the system parameters is the focus of the paper. Using stochastic geometry, the optimal SINR threshold - which directly corresponds to the optimal spectral efficiency - is derived for both the low SNR (power-limited) and high SNR (interference-limited) regimes. This leads to the optimum choice of the number of frequency bands N in terms of the path loss exponent, power and noise spectral density, desired rate, and total bandwidth.

On 3G LTE Terminal Implementation - Standard, Algorithms, Complexities and Challenges

Abstract: Currently, 3GPP standardizes an evolved UTRAN (E-UTRAN) within the Release 8 Long Term Evolution (LTE) project. Targets include higher spectral efficiency, lower latency, higher peak data rate when compared to previous 3GPP air interfaces. The air interface of E-UTRAN is based on OFDMA and MIMO in downlink and on SCFDMA in uplink Main challenges for a terminal implementation include efficient realization of the inner receiver, especially for channel estimation and equalisation, and the outer receiver including a turbo decoder which needs to handle data rates of up to 75 Mbps per spatial MIMO stream. We show that the inner receiver can nicely and straightforwardly be parallelized due to frequency domain processing. In addition to the computational complexity of even a simple linear equaliser, one of the challenges is an efficient implementation considering necessary flexibility for different MIMO modes, power consumption and silicon area. This paper will briefly overview the current LTE standard, highlight a functional data flow through the single entities of an LTE terminal and elaborate more on possible first implementation details, including sample algorithms and first complexity estimates.

On the Performance Analysis of Network-Coded Cooperation in Wireless Networks

Abstract: In this letter, a network-coded cooperation scheme with dynamic coding mechanism (DC-NCC) is proposed. In DC-NCC, the relay dynamically adapts forming the network-coded data based on the observed instantaneous source-to-relay channel quality, and then forwards the network-coded data towards corresponding destinations. Under the assumption (denoted as A) that each destination can reliably overhear the data from other sources, the diversity-multiplexing tradeoff of DC-NCC is proved to outperform that of conventional cooperation (CC), This verifies that DC-NCC outperforms CC in bandwidth efficiency. Moreover, DC-NCC offers reduced system outage probability and single-pair outage probability compared with CC, and achieves the same full diversity order as CC at high signal-to-noise ratio (SNR). Numerical results also show that with dynamic coding, DC-NCC avoids unnecessary error propagation which can be caused by coding those erroneous data into network-coded data. Finally the performance of DC-NCC is discussed in case the assumption A is removed. For a wireless network composed of N source-destination (s-d) pairs and a single relay node, although there is a certain outage probability increase for each s-d pair, DC-NCC still achieves a diversity-multiplexing tradeoff superior to CC.

2 dB Better Than CP-OFDM with OFDM/OQAM for Preamble-Based Channel Estimation

Abstract: OFDM/OQAM is a special type of multi-carrier modulation that can be considered as an alternative to conventional OFDM with cyclic prefix (CP) for transmission over multi-path fading channels. Indeed, as it requires no CP, it has the advantage of a theoretically higher spectral efficiency. Furthermore, efficient pulse shaping can also be easily implemented with OFDM/OQAM. However, the classical channel estimation methods used for OFDM cannot be directly applied to OFDM/OQAM. In this paper we present an analysis of this problem and we introduce a new preamble-based channel estimation method. The performance results are obtained either by considering an IEEE802.22 channel model or regarding to the channel delay spread variation of a two-tap channel. The proposed OFDM/OQAM channel estimation method is evaluated, in both scenarios, using different pulse shaping and taking conventional CP-OFDM as reference.

Spectral Efficiency of Distributed MIMO Cellular Systems in a Composite Fading Channel

Abstract: In this paper, accurate approximations of the spectral efficiency are presented for co-located multiple-input multiple-output (C-MIMO) and distributed MIMO (D-MIMO) cellular systems in the composite channel model, which includes path loss, shadow fading, and multipath fading. Firstly, conditioned on the desired user position, the analytical approximations of the mean and variance of the mutual information are derived for C-MIMO and D-MIMO at high SNR. Because the exact distribution of the mutual information in a composite Rayleigh- lognormal environment is difficult to analyze, we apply Gaussian approximation to the distribution of the mutual information. Then, the growth in ergodic capacity and outage capacity of a D-MIMO channel with the number of antennas as well as the variance of the shadow fading is well understood. Assuming that the users are randomly distributed in the cell, the closed-form expressions for the mean spectral efficiency and mean outage spectral efficiency are derived. Finally, the numerical results are presented which validate the analytical results.

MMSE subcarrier equalization for filter bank based multicarrier systems

Abstract: Filter bank based multicarrier systems (FBMC) offer a number of benefits over conventional orthogonal frequency division multiplexing (OFDM) with cyclic prefix (CP). One benefit is the improved spectral efficiency by not using a redundant CP and by having much better control of out-of-band emission. Another advantage is the ease of accommodating multiple users in an FDMA fashion especially in the uplink, i. e. the multiple access channel (MAC). On the other hand, more elaborate equalization concepts are needed compared to the single-tap per-subcarrier equalizer sufficient in the OFDM with CP case. In this contribution we will present an efficient linear minimum mean square error (MMSE) equalization concept. Since FBMC systems employ offset-quadrature-amplitude-modulation (OQAM), a fractionally spaced per-subcarrier equalizer will be derived, which takes into account the inter-subcarrier interference due to frequency selective multipath fading. Still this is handled with decoupled per-subcarrier equalizers. Simulation results will show that OQAMFBMCs allow for savings in transmit power which have to be paid with a practicable amount of computational overhead.

Duplex schemes in multiple antenna two-hop relaying

Abstract: A novel scheme for two-hop relaying defined as space division duplex (SDD) relaying is proposed. In SDD relaying, multiple antenna beamforming techniques are applied at the intermediate relay station (RS) in order to separate downlink and uplink signals of a bi-directional two-hop communication between two nodes, namely, S1 and S2. For conventional amplify-and-forward two-hop relaying, there appears a loss in spectral efficiency due to the fact that the RS cannot receive and transmit simultaneously on the same channel resource. In SDD relaying, this loss in spectral efficiency is circumvented by giving up the strict separation of downlink and uplink signals by either time division duplex or frequency division duplex. Two novel concepts for the derivation of the linear beamforming filters at the RS are proposed; they can be designed either by a three-step or a one-step concept. In SDD relaying, receive signals at S1 are interfered by transmit signals of S1, and receive signals at S2 are interfered by transmit signals of S2. An efficient method in order to combat this kind of interference is proposed in this paper. Furthermore, it is shown how the overall spectral efficiency of SDD relaying can be improved if the channels from S1 and S2 to the RS have different qualities.

Spectrum allocation in two-tier networks

Abstract: Two-tier networks, comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays), offer an economically viable way to improve cellular system capacity. The capacity-limiting factor in such networks is interference. The cross-tier interference between macrocells and femtocells can suffocate the capacity due to the near-far problem, so in practice hotspots should use a different frequency channel than the potentially nearby high-power macrocell users. Centralized or coordinated frequency planning, which is difficult and inefficient even in conventional cellular networks, is all but impossible in a two-tier network. This paper proposes and analyzes an optimum decentralized spectrum allocation policy for two-tier networks that employ frequency division multiple access (including OFDMA). The proposed allocation is optimal in terms of area spectral efficiency (ASE), and is subjected to a sensible quality of service (QoS) requirement, which guarantees that both macrocell and femtocell users attain at least a prescribed data rate. Results show the dependence of this allocation on the QoS requirement, hotspot density and the co-channel interference from the macrocell and surrounding femtocells. Design interpretations are provided.

Achievable Sum-Rate Maximizing AF Relay Beamforming Scheme in Two-Way Relay Channels

Abstract: This paper deals with design of a amplify-and- forward (AF) relay beamforming matrix in a two-way relay protocol when a relay node has multiple antennas. Two-way relaying is a promising protocol since it provides enhanced spectral efficiency compared to one-way relaying protocols. In this paper we derive the achievable sum-rate upper bound of AF beamforming scheme and propose the achievable sum-rate maximizing relay beamforming scheme when the destination and the relay node have perfect knowledge of the channel state information (CSI) for forward and backward channels. The solution of the sum-rate maximizing scheme is obtained by using the proposed algorithm, general power iterative algorithm, which solves a global optimization problem efficiently. Comparing with one-way relaying protocol, the improved sum-rate performance of the proposed scheme is verified by numerical simulation.

Learning sparse doubly-selective channels

Abstract: Coherent data communication over doubly-selective channels requires that the channel response be known at the receiver. Training-based schemes, which involve probing of the channel with known signaling waveforms and processing of the corresponding channel output to estimate the channel parameters, are commonly employed to learn the channel response in practice. Conventional training-based methods, often comprising of linear least squares channel estimators, are known to be optimal under the assumption of rich multipath channels. Numerous measurement campaigns have shown, however, that physical multipath channels tend to exhibit a sparse structure at high signal space dimension (time-bandwidth product), and can be characterized with significantly fewer parameters compared to the maximum number dictated by the delay-Doppler spread of the channel. In this paper, it is established that traditional training-based channel learning techniques are ill-suited to fully exploiting the inherent low-dimensionality of sparse channels. In contrast, key ideas from the emerging theory of compressed sensing are leveraged to propose sparse channel learning methods for both single-carrier and multicarrier probing waveforms that employ reconstruction algorithms based on convex/linear programming. In particular, it is shown that the performance of the proposed schemes come within a logarithmic factor of that of an ideal channel estimator, leading to significant reductions in the training energy and the loss in spectral efficiency associated with conventional training-based methods.

Channel estimation with scattered pilots in OFDM/OQAM

Abstract: OFDM/OQAM is a multi-carrier modulation scheme that can be considered as an alternative to conventional OFDM with cyclic prefix (CP) for transmission over multi-path fading channels. As it requires no CP, it has the advantage of a theoretically higher spectral efficiency. Furthermore, efficient pulse shaping can also be easily implemented. However, the classical channel estimation methods used for OFDM cannot be directly applied to OFDM/OQAM. In this paper, we present an analysis of this problem and we introduce a channel estimation method using scattered pilots. The performance results are evaluated for a slow varying multi-path channel model. For both scenarios, the results of OFDM/OQAM, using 2 different pulse shaping, are compared to those of CP-OFDM.

Specific Emitter Identification for Cognitive Radio with Application to IEEE 802.11

Abstract: Cognitive radio (CR) is believed to be an enabling technology for increasing spectrum efficiency. A CR collects spectrum usage information from not only its own spectrum sensing module, but also from peer CRs. The heavy dependence on spectrum knowledge from other CRs requires identification of malicious CR devices that could generate spoofed information. In addition, it also needs to track the users associated with problematic CR devices which unintentionally violate spectrum usage etiquette. The specific emitter identification (SEI) concept is applied to identification of such non-cooperative CR devices. In this paper, second-order cyclic features of OFDM signals are proposed as a means of increasing CR network security and stability through SEI. For this exploratory work, IEEE 802.11a/g signals from different WLAN cards are measured and classified using hidden Markov Models (HMMs).