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The required flexibility of the software-defined radio front end may currently be met with better overall performance by employing tunable narrowband circuits rather than pursuing a truly wideband approach. A key component of narrowband transceivers is appropriate filtering to reduce spurious spectral content in the transmitter and limit out-of-band interference in the receiver. In this paper, recent advances in flexible, frequency-selective, circuit components applicable to reconfigurable SDR front ends are reviewed. The paper contains discussion regarding the filtering requirements in the SDR context and the use of intelligent, adaptive control to provide environment-aware frequency discrimination. Wide tuning-range frequency-selective circuit elements are surveyed including bandpass and bandstop filters and narrowband tunable antennas. The suitability of these elements to the mobile wireless SDR environment is discussed.

Orthogonal frequency-division multiplex (OFDM) offers a low-complexity solution to equalization in multipath channels but does so by increasing the symbol period. This places a limit on the mobility of such systems since time variations in the channel during the symbol period introduce intercarrier interference (ICI), hence, degrading performance. Solutions to reduce ICI in the literature require a high degree of processing. Increasing terminal mobility also places greater requirements on synchronization processing to track the rapidly changing channel. This paper uses multiple antennas at the receiver so that the channel response can be decomposed into a number of more slowly varying channels. Independent synchronization processing and correction can be applied to each of the derived channels before combining the signals prior to the fast Fourier transform (FFT) process. By individually processing the channels, the effective channel is compressed in the time and frequency domains, improving system performance. Perfect tracking of the multipath clusters is initially assumed to show the potential benefits, followed by operation with an idealized tracking algorithm. Operation with more realistic processing algorithms using fixed sectored elements improving the bit error rate (BER) is investigated. Finally, the benefits are then demonstrated with real measured channels from an urban environment. Index Terms--Antenna arrays, digital communications, Digital Video Broadcasting (DVB), orthogonal frequency-division multiplex (OFDM), synchronization.

Electrical Balance Duplexers (EBDs) provide transmit-receive isolation to implement a form of self-interference cancellation to facilitate simultaneous transmission and reception from a single antenna in systems like In-Band Full-duplex (IBFD) transceivers. EBD works by coupling transmitter, receiver, antenna, and balancing impedance using a hybrid coupler. In recent pieces of literature, antenna impedance variations are considered the main factor limiting the EBD isolation bandwidth, while the EBD balancing impedance needs to be equal as much as possible to the antenna impedance to achieve high isolation. But hybrid couplers are also not ideal elements, and their S-parameters are not stable in the frequency domain. In this work, five broadband RF devices (two antennas, two couplers, and a 50-Ohm RF-load) are used to form four EBD set-ups. One of the antennas, designed by the authors, has more impedance stability in the frequency domain than the other one, which is a commercial antenna. Also, one of the couplers, designed by the authors, has more S-parameter stability in Ultra-Wideband (UWB) frequency domain than the other one, which is a commercial UWB coupler. The implemented EBDs show that when both antenna and coupler have strong S-parameter stabilities in the frequency domain, wider isolation bandwidth (UWB 1.5–3.5 GHz range) and higher EBD isolation are obtained. [ABSTRACT FROM AUTHOR]

An adaptive frequency-domain equalization (FDE) algorithm for implementation in single-carrier (SC) multiple-input multiple-output (MIMO) systems is presented. A novel method of reducing the overhead required to train the proposed equalizer is outlined.

This paper describes a discretely tunable filter topology based on lumped-distributed coupled transmission lines, particularly suitable for microelectromechanical systems switching devices. This topology is capable of simultaneous wide-band center frequency and bandwidth tuning, limited only by the electrical size of the transmission lines and the placement density of the switching devices. Low fractional bandwidths can be achieved without the need for large coupled-line spacings due to the antiphase relationship of the lumped capacitive and distributed electromagnetic coupling coefficients. The positions of the additional poles of attenuation due to the lumped capacitive coupling can be selected either above or below band leading to the choice of a narrow bandwidth design having good high-side performance or a design with compromised upper stopband performance, but with no bandwidth tuning limitations. The interaction between a pair of lumped-distributed coupled transmission lines is analyzed and the resulting model is used to develop a filter synthesis procedure. The synthesis procedure and filter performance are validated through theoretical and experimental comparisons using a filter with low-side attenuation poles. Index Terms--Discrete tuned, filter synthesis, lumped-distributed coupled transmission lines, microelectromechanical systems (MEMS), microwave bandpass filters, tunable filters.

In this paper, we first verify a previously proposed Kronecker-structure-based narrow-band model for nonline-of-sight (NLoS) indoor multiple-input-multiple-output (MIMO) radio channels based on 5.2-GHz indoor MIMO channel measurements. It is observed that, for the narrow-band case, the measured channel coefficients are complex Gaussian distributed and, consequently, we focus on a statistical description using the first- and second-order moments of MIMO radio channels. It is shown that the MIMO channel covariance matrix can be well approximated by the Kronecker product of the covariance matrices, seen from the transmitter and receiver, respectively. A narrow-band model for NLoS indoor MIMO channels is thus verified by these results. As for the wide-band case, it is observed that the average power-delay profile of each element of the channel impulse response matrix fits the exponential decay curve and that the Kronecker structure of the second-order moments can be extended to each channel tap. A wide-band MIMO channel model is then proposed, combining a simple COST 259 single-input--single-output channel model and the Kronecker structure. Monte Carlo simulations are used to generate indoor MIMO channel realizations according to the models discussed. The results are compared with the measured data using the channel capacity and good agreement is found. Index Terms--Antenna arrays, channel capacity, channel modeling, measured channel data, multipath channels, multiple-input-multiple-output (MIMO) channels.

Smart antennas are widely recognized as an enabling technology for addressing the demand of future wireless network capacity when employed in place of traditional fixed-coverage sector antennas. Furthermore, switched-beam antenna systems offer a robust implementation against multipath propagation effects and reduced complexity that is inherent with fully adaptive implementations. This paper introduces a simple closed-form expression for evaluating the capacity increase of W-CDMA cellular networks employing switched-beam antennas. The expression incorporates the effect of practical antenna patterns and the impact of multipath scattering on code orthogonality, as well as that of pilot signal power. The results show that a reduction in downlink interference of approximately 6 dB can be achieved by installing an eight-beam antenna system in a 120[degrees] sector configuration when representative values of these parameters are taken into account. Index Terms--Antenna arrays, beamforming, code-division multiple-access (CDMA), digital cellular systems, smart antennas, switched-beam antennas, third-generation wireless systems.

This paper introduces a novel metric for determining the spatial decorrelation between the up- and down-link wireless bearers in frequency division duplex (FDD) networks. This metric has direct relevance to smart or adaptive antenna array base-station deployments in cellular networks, which are known to offer capacity enhancement when compared to fixed coverage solutions. In particular, the results presented were obtained from field trial measurement campaigns for both urban and rural scenarios, with the observations having a direct impact on the choice of down-link beamforming architecture in FDD applications. Further, it is shown that significant spatial decorrelation can occur in urban deployments for bearer separations as small as 5 MHz. Results are presented in terms of both instantaneous characteristics as well as time averaged estimates, thus facilitating the appraisal of smart antenna solutions in both packet and circuit switched networks. Index Terms--Adaptive arrays, frequency division duplex (FDD) spatial propagation, smart antennas.

A 3-dB compact hybrid coupler is presented in this paper in an ultra-wideband frequency range from 1.5 GHz to 3.2 GHz with 90° phase deference between the two output ports. The proposed coupler is formed by two notched elliptically shaped microstrip lines and four phase shifters, which are broadside coupled through an elliptically shaped slot. A combination of impedance matching technique and structural modification then has been employed to increase coupler efficiency. The design is demonstrated assuming a 0.51-mm-thick Rogers RO4350B substrate. Results of simulation and measurements show that the designed device exhibits a coupling of 3 ± 1 dB across the aimed bandwidth. This ultra-wideband coupling is accompanied by smooth isolation in the order of better than 25 dB and return loss in the order of better than 17 dB. The manufactured device including microstrip ports and phase shifters occupy an area of 35mm x 30mm x 1:1mm (0:27λ x 0:23λ x 0:009λ) which makes it a compact suitable device for UHF applications and measurements, specifically measuring and determining isolation in in-band full duplex transceivers, because of its smooth isolation versus frequency and ultra-wideband bandwidth. [ABSTRACT FROM AUTHOR]

Electrical Balance Duplexers (EBDs) in modern communication systems provide Transmit (TX)-Receive (RX) isolation to implement a form of self-interference (SI) cancellation to facilitates simultaneous transmission and reception from single antenna. EBD works by coupling transmitter, receiver, antenna, and balancing impedance using a hybrid junction where the balancing impedance needs to be equal to the antenna impedance to achieve a high isolation. Variations in antenna impedance with respect to frequency significantly reduces the isolation bandwidth and is dominant factor in limiting the isolation. A method based on Sample Standard Deviation is firstly introduced to quantify impedance stability in the frequency domain. Then a frequency independent antenna with a core structure of equiangular Archimedean spiral is designed to achieve impedance stability in frequency domain. The antenna impedance at an ultra-wideband (UWB) frequency range of 1.5GHz to 4GHz is more smoothened in some design steps using electromagnetic absorbers, capacitive impedance tuning and modification techniques whilst this electrically small antenna also has circular polarization, electromagnetic compatibility, and suitable radiation efficiency. In comparison with the literature, using this antenna in the EBD stage along with a simpler balancing impedance has resulted to about six times wider 20 dB EBD isolation bandwidth. [ABSTRACT FROM AUTHOR]

Featured Application: This manuscript presents development of a pyrolysis model for a polyisocyanurate foam and demonstrates how this model can be coupled with an empirical model of flame heat feedback to predict the dynamics of flame spread on this widely used building product. Polyisocyanurate (PIR) foam is a robust thermal insulation material utilized widely in the modern construction. In this work, the flammability of one representative example of this material was studied systematically using experiments and modeling. The thermal decomposition of this material was analyzed through thermogravimetric analysis, differential scanning calorimetry, and microscale combustion calorimetry. The thermal transport properties of the pyrolyzing foam were evaluated using Controlled Atmosphere Pyrolysis Apparatus II experiments. Cone calorimetry tests were also carried out on the foam samples to quantify the contribution of the blowing agent (contained within the foam) to its flammability, which was found to be significant. A complete pyrolysis property set was developed and was shown to accurately predict the results of all aforementioned measurements. The foam was also subjected to full-scale flame spread tests, similar to the Single Burning Item test. A previously developed modeling approach based on a coupling between detailed pyrolysis simulations and a spatially-resolved relationship between the total heat release rate and heat feedback from the flame, derived from the experiments on a different material in the same experimental setup, was found to successfully predict the evolution of the heat release rate measured in the full-scale tests on the PIR foam. [ABSTRACT FROM AUTHOR]

Since conjugate (perfect) matching of complex loads and sources over a wide frequency band is not possible, imperfect matching is necessary. In this brief, formulas for the coverage of tunable matching networks for imperfect matching have been derived for the first time. It has been found that the coverage area in this case expands beyond the perfect matching area with more circles required to define the coverage. Analytical expressions for the centers and radii of these circles have been derived for the first time. The theoretical analysis has been provided for the T and $\Pi$ networks and verified by circuit simulation and measured data. [ABSTRACT FROM AUTHOR]

This paper presents a novel, tunable, frequency-division duplexing radio frequency (RF) front end that combines passive and active self-interference (SI) cancellation. An electrical-balance duplexer is used to passively cancel transmitter noise in the receive band, and an active canceller is employed to suppress SI in the transmit (Tx)-band. Subsystem specifications are developed, and a system-level analysis of noise and SI powers in this novel architecture is provided, thereby illustrating its operation. A proof-of-concept demonstrator, built from a software-defined radio and discrete RF components, has been characterized across a range of duplex configurations in the 700–950-MHz range and also at 1900 (Long-Term Evolution (LTE) band 3) and 2600 MHz (LTE band 7). The prototype achieves an impressive 6.0–7.4-dB noise figure in the presence of a +27-dBm LTE uplink Tx blocker for duplex separations of 47.5 MHz and above. The duplexer has also been tested against reference sensitivity test cases defined in the 3GPP LTE specification, demonstrating specification compliant sensitivity in LTE bands 28 (700 MHz), 3, and 7. [ABSTRACT FROM AUTHOR]

Responding to the unprecedented challenges imposed by the 5G technologies, mobile operators have given significant attention to Heterogeneous Cloud Radio Access Networks (H-CRAN) due to their beneficial features of performing optimization, cost effectiveness, and improving spectral and energy efficiency performance. H-CRAN inherits the attractive benefits of Heterogeneous Networks (HetNet) and the cloud computing by facilitating interference mitigation, scalability, and radio resource control. Consequently, H-CRAN is proposed in this article as a cost-effective potential solution to alleviate intertier interference and improve cooperative processing gains in HetNets by employing cloud computing. H-CRAN can provide efficient resource sharing at the spectrum, network, and infrastructure levels. Therefore, this article proposes H-CRAN cooperative interference mitigation method that enhances the time sharing among Radio Remote Heads (RRH) users. The study proposes an enhanced Almost Blank Subframe (ABSF) technique to increase the SINR and throughput of the small-cell (low power base station) and macrocell users. Simulation results show that the proposed Dynamic Programming-Diverse Almost Blank Subframe (ABSF) Pattern (DP-DAP) scheme improved the macro- and small-cell users up to 56% and 35%, respectively, as compared to other state-of-the-art ABSF schemes. [ABSTRACT FROM AUTHOR]

When a digital TV system is operating far in excess of the operational threshold, there is no discernible advantage to either the user or broadcaster due to the hard failure characteristics of digital TV. In a digital network, this threshold can vary due to changes in the propagation medium and user receiving equipment, necessitating a transmit power margin sufficient to cover all propagation and equipment variations. Hence if the system could be adaptively tuned to operate at just above the threshold, then significant energy savings could be made. A spatially adaptive system was evaluated through hardware-in-the-loop analysis; here a channel emulator was used to physically model array gain and fading conditions, and feedback nodes were constructed allowing broadcast powers to be manipulated based on evaluating user feedback. The effects of such a system on the U.K. Digital terrestrial television (DTT) network were evaluated and the advantages highlighted potential power savings of up to 2.1 dB being obtainable. This equates to an electricity reduction of between 33 million and 58 million kWh per year, saving between £3.5 million and £6.1 million ($4.4 million and $7.6 million) and reducing carbon emissions by between 13.7 million and 24 million CO2e. This paper thus illustrates the potential energy savings for a DTT network, using the U.K. as a worked example. [ABSTRACT FROM PUBLISHER]

Matching networks for dual-band power amplifiers typically rely on complex, non-general techniques, which either use switches or result in large and lossy matching networks. In this work, mathematical optimization is employed to design the matching networks for multi-band power amplifiers. The theory of continuous modes is utilized together with accurate models for the device package to define the required impedance terminations theoretically thus allowing mathematical optimization to be used for the design. This technique depends on neither the network architecture nor the number of frequency bands. Therefore, simple and compact multi-band matching networks can be achieved. As proof of concept, a triple-band amplifier at 0.8, 1.8, and 2.4 GHz has been designed using the proposed method. The fabricated amplifier demonstrates maximum power added efficiencies of 70%, 60%, and 58% and output powers of 40 dBm, 41 dBm and 40 dBm for the three frequency bands, respectively. The presented design approach is highly suitable for the next generation of wireless systems. [ABSTRACT FROM AUTHOR]

Electrical balance duplexers (EBDs) present a potential alternative to the fixed-frequency duplexing filters used for frequency division duplexing in cellular handset radio frequency front ends. However, the transmit-to-receive (Tx–Rx) isolation can be affected by interaction between the antenna and the environment, and therefore, the EBDs balancing impedance must adaptively track time-domain antenna impedance variation. A rail scenario presents a potentially demanding use case for an EBD, as fast moving trains in the vicinity of the antenna may cause dynamically changing reflections, which can be received as self-interference. In this paper, measured dynamic antenna reflection coefficients at 745 and 1900 MHz from train mounted antennas are included in the EBD circuit simulations in order to investigate the resulting variation in Tx–Rx isolation, and determine requirements for balancing impedance adaptation. This paper also presents the results from rail-based field trials of a hardware prototype EBD, which implements real-time antenna impedance tracking. Results show that the rail scenario does result in variation in Tx–Rx isolation, but that rebalancing the EBD at the intervals of 5 ms was sufficient to maintain >50 dB isolation for ~95% of the time. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) is a promising multiple access technique for fifth-generation mobile networks. In this letter, a novel multi-tier NOMA (TNOMA) scheme is proposed for a coordinated multi-point (CoMP) network. This TNOMA scheme aims to improve the coverage of high data rate services in a CoMP network, especially for the case when access points are not able to serve all of the users in the network. To bench-mark the TNOMA scheme against one-tier NOMA-CoMP, the effective sum-rate performances are evaluated. The results show that the performance difference between one-tier only NOMA-CoMP and multi-tier NOMA-CoMP is influenced by the transmission-nodes-selection threshold value ( \epsilon k ) and the target data rate. [ABSTRACT FROM PUBLISHER]

Tunable matching networks (MNs) are essential components for agile radio frequency systems. To optimally design such networks, the total area they cover on the Smith chart needs to be determined. In this paper, the coverage areas of typical MNs have been determined analytically for the first time. It has been found that the coverage area is encompassed by up to five arcs. Analytical expressions for the centers and radii for these arcs have been derived. The theoretical analysis is provided for four typical MNs and verified by circuit simulation and measured data. Moreover, a dynamically load-modulated power amplifier has been designed using the presented theoretical techniques, which demonstrates a measured improvement in the power added efficiency of up to 5% in the frequency range of (0.8–0.9) GHz. [ABSTRACT FROM AUTHOR]

Brominated polymeric flame retardants (FR) have been developed that can provide effective flame retardant performance in polymer systems that previously relied on hexabromocyclododecane (HBCD). This polymeric FR material exhibits a superior environmental profile to that of HBCD and other lower molecular weight FR molecules. The brominated polymer flame retardant is based on a copolymer of styrene and butadiene where the polybutadiene portion is brominated on the 1,2 and 1,4 isomer units to give a brominated polybutadiene. Bromination is best carried out with a quaternary ammonium tribromide reagent which provides a selective bromination of just the polybutadiene portion of the copolymer. A styrene-butadiene-styrene (SBS) triblock architecture is utilized where the presence of the styrene enhances the thermal stability and gives the copolymer to better compatibility when blended with polystyrene. The resulting brominated polybutadiene-polystyrene (BrPBPS) polymeric flame retardant contains similar aliphatic bromine as in HBCD but with a higher molecular weight structure. BrPBPS provides similar flame retardant activity as HBCD in polystyrene blends where both release HBr to provide gas phase activity. Both also provide enhanced polymer degradation as another major pathway for condensed FR activity. [ABSTRACT FROM AUTHOR]

In this letter, an opportunistic NOMA (ONOMA) scheme is proposed in a coordinated multipoint (CoMP) system. Compared with the conventional joint-transmission (JT) NOMA in CoMP, the main purpose of the ONOMA scheme is to advance the capability of the CoMP system and the effectiveness of the successive-interference-cancellation (SIC) decoding process. The relationship between the topology of the ONOMA cells and the sum-rate of CoMP network is analyzed. Meanwhile, the outage probability of ONOMA system is derived and evaluated in an ideal case and nonideal case, respectively. In the numerical results, the sum-rate and the outage performance of the proposed ONOMA CoMP are compared with the JT-NOMA CoMP and it is shown that the proposed ONOMA scheme outperforms the conventional JT-NOMA scheme in CoMP system. [ABSTRACT FROM PUBLISHER]

Electrical balance duplexing enables simultaneous transmit and receive from a single antenna; however, the transmit-to-receive isolation depends on the ability of the balancing algorithm to determine the correct balancing impedance. A novel balancing algorithm based on in situ characterization of the duplexer self-interference channel is proposed. The algorithm requires no a priori knowledge of the antenna impedance or hybrid junction characteristics and automatically compensates for circuit imperfections. A novel balancing network implementation that uses active signal injection is also proposed. A hardware prototype implementing the proposed balancing algorithm and combining passive and active balancing techniques has achieved 81.5-dB isolation over an 80-MHz bandwidth. [ABSTRACT FROM PUBLISHER]

Transceiver architectures utilizing various self-interference suppression techniques have enabled simultaneous transmission and reception at the same frequency. This full-duplex wireless offers the potential for a doubling of spectral efficiency; however, the requirement for high transmit-to-receive isolation presents formidable challenges for the designers of full duplex transceivers. Electrical balance in hybrid junctions has been shown to provide high transmit- to-receive isolation over significant bandwidths. Electrical balance duplexers require just one antenna, and can be implemented on-chip, making this an attractive technology for small form factor devices. However, the transmit-toreceive isolation is sensitive to antenna impedance variation in both the frequency domain and time domain, limiting the isolation bandwidth and requiring dynamic adaptation. Various contributions concerning the implementation and performance of electrical balance duplexers are reviewed and compared, and novel measurements and simulations are presented. Results demonstrate the degradation in duplexer isolation due to imperfect system adaptation in user interaction scenarios, and requirements for the duplexer adaptation system are discussed. [ABSTRACT FROM PUBLISHER]

This paper investigates electrical balance (EB) in hybrid junctions as a method of achieving transmitter–receiver isolation in single antenna full duplex wireless systems. A novel technique for maximizing isolation in EB duplexers is presented, and we show that the maximum achievable isolation is proportional to the variance of the antenna reflection coefficient with respect to frequency. Consequently, antenna characteristics can have a significant detrimental impact on the isolation bandwidth. Simulations that include embedded antenna measurements show a mean isolation of 62 dB over a 20-MHz bandwidth at 1.9 GHz but relatively poor performance at wider bandwidths. Furthermore, the operational environment can have a significant impact on isolation performance. We present a novel method of characterizing radio reflections being returned to a single antenna. Results show as little as 39 dB of attenuation in the radio echo for a highly reflective indoor environment at 1.9 GHz and that the mean isolation of an EB duplexer is reduced by 7 dB in this environment. A full duplex architecture exploiting EB is proposed. [ABSTRACT FROM PUBLISHER]

In this paper, we seek to characterize the performance of spatial modulation (SM) and spatial multiplexing (SMX) with an experimental testbed. Two National Instruments (NI) PXIe devices are used for the system testing: one for the transmitter and one for the receiver. The digital signal processing (DSP) that formats the information data in preparation for transmission is described, along with the DSP that recovers the information data. In addition, the hardware limitations of the system are also analyzed. The average bit-error ratio (ABER) of the system is validated through both theoretical analysis and simulation results for SM and SMX under the line-of-sight (LoS) channel conditions. [ABSTRACT FROM PUBLISHER]

Using the damped harmonic oscillator equations as a mathematical template, several novel chaotic oscillators are developed with an emphasis on mathematical simplicity and ease of electronic circuit implementation. These chaotic systems offer an intuitive introduction to chaos theory, enabling comparison of mathematical and computational analyses with experimental results. [ABSTRACT FROM AUTHOR]

Flame retardant producers and end users are continually looking for more sustainable solutions through innovation in their customer offerings. Successful solutions need to satisfy a range of characteristics, from processing to technical and environmental performance, which sometimes conflict. This paper covers a new class of brominated polymeric lame retardants designed and developed by The Dow Chemical Company for use in polystyrene foam to meet existing ire and use requirements and have an improved environmental profile to meet regulatory guidelines. An overview of the environmental, health and safety performance of these new polymeric lame retardants will be presented along with preliminary performance data of their use in polystyrene foam. [ABSTRACT FROM AUTHOR]

Mathematical analysis of Doherty amplifiers have assumed many simplifications. Most notably, the peaking amplifier does not contribute power into the load and the peaking stage has an observed impedance of infinity. This paper will show that these simplifications impair the performance of a single-input Doherty amplifier and that phase tuning for compensation is needed to improve the overall system performance. The dual-input Doherty amplifier is capable of overcoming the limitations of power-dependent phase imbalance and phase compensation lines at the input of the peaking stage; however, the characterization of such an architecture is not straightforward. A new measurement technique is proposed to measure dc current, dc voltage, and output power levels to allow unique characterization of a dual-input Doherty amplifier. Phase compensation lines at the input of the peaking amplifier will be shown to be not required, as long as correct offset lines are calculated for both the carrier and peaking stage and that the \lambda/4 transmission-line length is not necessarily required for active load–pull. Results of a dual-input inverse load modulated Doherty amplifier are presented where the peaking stage delivers 10 dB less of maximum output power than the carrier, while still maintaining Doherty behavior. The peaking stage can therefore be implemented with a smaller device than the carrier. [ABSTRACT FROM PUBLISHER]

Employing feedback in multiple-input-multiple-output (MIMO) systems offers the potential to significantly increase data rates over open-loop schemes. However, the impact the propagation channel has on the behavior and performance of the scheme must be considered and, where appropriate, the signaling designed to suit. In this paper, measured outdoor MIMO propagation data is used to evaluate initially the distributions and statistics of the coherence of the eigenmodes of the channel, for a wide range of mobility scenarios and user devices. Then, the impact on link performance of the differing coherences in time and frequency is evaluated in terms of the feedback delay and spectral spacing of pilots in a practical feedback scheme used in mobile WiMAX. It is found that not only mobility and physical environment, but also the directivity of the antenna array affect the eigen-coherence and also, therefore, the performance of a feedback scheme. Bit-error rate and information capacity are both shown to be sensitive to increased feedback delay and pilot spacing, however, the closed-loop system always outperforms its open-loop counterpart. [ABSTRACT FROM PUBLISHER]

Geometric frequency tuning in planar electromagnetic resonators is common in many applications. It comes, however, at a penalty in the resonance quality, Q0. The literature traces the causes of such penalty often in terms of the shortcomings in the added elements and materials, which were used to achieve the tuning. In this paper, however, it is shown that another underlying source of quality degradation exists at the fundamental geometric level. This source, unlike other added sources of degradation during tuning, will always exist (even before tuning takes place) and will rely on the “modal areas” of the geometric modifications made to host the tuning mechanism. Hence, it forms an upper bound to the performance that can be achieved from a geometically-tuned planar resonator, carries an important insight to resonator design in general, and significantly helps in the understanding of the problem of geometric tuning in particular. We present the electromagnetic theory behind this limit and canonically demonstrate it using practical microwave resonator examples. The theory, finite-element method simulation, and experiment results are presented and good agreement is observed. It is shown that incorporating such understanding into the design process of tunable planar resonators can help optimize their performance against a given set of design requirements. Furthermore, the presented theory provides a useful electromagnetic model as a tool for estimating Q0 for geometrically modified or irregular metal patches and planar resonators in general, to assist analysis, and design at any wavelength or application. The theory also asserts that, under a given mode, a planar resonator will always have its maximum Q0 before introducing any internal subtractive geometric modifications (e.g., cuts, apertures, or slits) to its original shape. [ABSTRACT FROM AUTHOR]

The electromagnetic coupling of antennas with the human body is a well-known, but often neglected, problem in body area network (BAN) communications. The resulting distortion of the antenna's free-space characteristics may, in turn, demand more complex terminals to cope with the different conditions encountered as antennas are mounted on different parts of the body and/or varying separations from the skin. Therefore, in this paper, we propose a mounting method to provide a more stable response and to reduce the antenna's coupling to the body for ultra-wideband (UWB) BAN applications. The proposed mounting method is evaluated on two UWB BAN antennas, characterizing the antennas coupling with the body by analyzing variations in the antennas reflection coefficient between 4–9 GHz, when mounted at different distances from the skin and at different locations on the body. The modified mounting method was found to increase the correlation coefficient between the antenna's reflection coefficient when in free space and mounted using 5 mm of spacer, for the two antennas, from 0.22 and 0.13, to 0.66 and 0.90. The free-space characteristics of the modified antennas are also tested, showing a reduction of up to 4.7 dB of radiated power into the body. [ABSTRACT FROM AUTHOR]

This paper presents a number of distributed resource allocation schemes for fixed relay stations in a network based on orthogonal frequency-division multiple access (OFDMA). Results show that the spectrum efficiency and user fairness can be enhanced by cognitive radio techniques at the relay nodes. Iterative waterfilling and iterative multilevel waterfilling are exploited in the power allocation, and results show that optimal power allocation is achieved for the applications under study. These iterative methods are then combined with a power reallocation algorithm to further enhance spectrum utilization. A novel iterative barrier-constrained waterfilling algorithm is proposed to address the data rate limitations imposed by poor base-station-to-relay channels. The proposed technique offers fast convergence, and complexity is reduced by a distributed implementation across the network. Finally, a novel distributed subcarrier and power allocation algorithm is proposed. Simulation results indicate that, although multiuser fairness is imperfect, capacity gains of up to 20% can be achieved compared with the traditional equal power allocation method. When multiuser fairness constraints are applied, a near-perfect fairness index is achieved; however, the peak capacity is reduced by approximately 10%. [ABSTRACT FROM PUBLISHER]

Distributing the construction of a multiple-input multiple-output (MIMO) system among more than one mobile user reduces hardware and processing complexity at the terminal end, and may result in improved channel propagation conditions, particularly those related to spatial correlation. In this paper, measured outdoor propagation data is used to study a 4\,\times\,4 virtual MIMO system formed from a two-user 4\,\times\,4 classical system using only two of the antennas at each constituent user. The capacity, K-factor, and spatial correlation are evaluated for 226 possible pairs of users whose channels were measured while standing and walking. The proportions of pairings that result in a capacity increase and K-factor and correlation reductions over both, only one and neither of the constituents are determined. It is found that correlation reduction appears to be a stronger sign of capacity improvement than is K-factor reduction. When choosing a partner for a given single user, results show that it is easier to improve these three parameters as the user's value of them becomes poorer, so thresholds applicable to this data set are found which specify values of capacity, K, and correlation beyond which at least 50% of possible pairings improve matters. [ABSTRACT FROM AUTHOR]

Code orthogonality is a useful indicator in determining the performance of the synchronous downlink of third-generation Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access systems. In this paper, previous work on quantifying code orthogonality for single-antenna [or single-input single-output (SISO)] links is extended to fixed beamforming and transmit diversity schemes. Code orthogonality estimates from real measured channel data are extracted for typical outdoor radio environments. The results indicate that the spatial conditioning of fixed beamforming radio channels generally improves code orthogonality. In contrast, full code-rate transmit diversity schemes degrade code orthogonality. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a new class of training sequences that are optimal for least squares (LS) channel estimation in systems employing transmit diversity and single-carrier (SC) modulation with a cyclic prefix (CP) extension. The sequences have a constant envelope in the time domain and are orthogonal in the frequency domain. Transmission of these sequences facilitates optimal (in the LS sense) estimation of the channel impulse response at the receiver while precluding the peak-to-average power ratio problem that is inherent in other CP-based architectures such as orthogonal frequency division multiplexing. [ABSTRACT FROM AUTHOR]

In this paper, we first verify a previously proposed Kronecker-structure-based narrow-band model for nonline-of-sight (NLoS) indoor multiple-input-multiple-output (MIMO) radio channels based on 5.2-GHz indoor MIMO channel measurements. It is observed that, for the narrow-band case, the measured channel coefficients are complex Gaussian distributed and, consequently, we focus on a statistical description using the first- and second-order moments of MIMO radio channels. It is shown that the MIMO channel covariance matrix can be well approximated by the Kronecker product of the covariance matrices, seen from the transmitter and receiver, respectively. A narrow-band model for NLOS indoor MIMO channels is thus verified by these results. As for the wide-band case, it is observed that the average power-delay profile of each element of the channel impulse response matrix fits the exponential decay curve and that the Kronecker structure of the second-order moments can be extended to each channel tap. A wide-band MIMO channel model is then proposed, combining a simple COST 259 single-input- single-output channel model and the Kronecker structure. Monte Carlo simulations are used to generate indoor MIMO channel realizations according to the models discussed. The results are compared with the measured data using the channel capacity and good agreement is found. [ABSTRACT FROM AUTHOR]

Presents a study that assessed a wideband indoor channel model that incorporates both the clustering of multipath components (MPC) and the correlation between the spatial and temporal domains in relation to wireless local area networks. Description of measurement setup and environments; Data analysis and processing techniques used for extracting MPC parameters and for identifying clusters; Comparison of the proposed model and existing models.