Copyright © 2013 John Wiley & Sons, Ltd. To utilize spectrum resources more efficiently, dynamic spectrum access attempts to allocate the spectrum to users in an intelligent manner. Uncoordinated sharing with cognitive radio (CR) users is a promising approach for dynamic spectrum access. In the uncoordinated sharing model, CR is an enabling technology that allows the unlicensed or secondary users to opportunistically access the licensed spectrum bands (belonging to the so-called primary users), without any modifications or updates for the licensed systems. However, because of the limited resources for making spectrum observations, spectrum sensing for CR is bound to have errors and will degrade the grade-of-service performance of both primary and secondary users. In this paper, we first propose a new partial spectrum sharing policy, which achieves efficient spectrum sharing between two licensed networks. Then, a Markov chain model is devised to analyze the proposed policy considering the effects of sensing errors. We also construct a cross-layer design framework, in which the parameters of spectrum sharing policy at the multiple-access control layer and the spectrum sensing parameters at the physical layer are simultaneously coordinated to maximize the overall throughput of the networks, while satisfying the grade-of-service constraints of the users. Numerical results show that the proposed spectrum sharing policy and the cross-layer design strategy achieve a much higher overall throughput for the two networks.

ABSTRACT In this paper, we propose a heterogeneous‐prioritized spectrum sharing policy for coordinated dynamic spectrum access networks, where a centralized spectrum manager coordinates the access of primary users (PUs) and secondary users (SUs) to the spectrum. Through modeling the access of PUs and multiple classes of SUs as continuous‐time Markov chains, we analyze the overall system performance with consideration of a grade‐of‐service guarantee for both the PUs and the SUs. In addition, two new call admission control (CAC) strategies are devised in our models to enhance the maximum admitted traffic of SUs for the system. Numerical results show that the proposed heterogeneous‐prioritized policy achieves higher maximum admitted traffic for SUs. The trade‐off between the system's serving capability and the fairness among multiple classes of SUs is also studied. Moreover, the proposed CAC strategies can achieve better performance under max‐sum, proportional, and max‐min fairness criteria than the conventional CAC strategies. Copyright © 2011 John Wiley & Sons, Ltd.

This study considers a scenario where two licensed networks share spectrum partially and opportunistically based on sensing of cognitive radios. The authors' main contribution is a discrete-time Markov chain analysis characterising the impact of imperfect sensing and sensing periodicity on the network performance. A cross-layer design framework is also developed, where the parameters in spectrum sensing and spectrum sharing are jointly optimised. Results show that the proposed policy achieves a higher throughput than conventional strategies.

This study explores the performance of multi-tone frequency shift keying (MT-FSK) in ultrawideband (UWB) wireless communications as an alternative to the standard modulation techniques. Its advantage is its ability to provide multiple user access even within the recently tightened regulations on UWB communications. Unlike previous works that discussed MT-FSK from the information theory perspective, here the possibility of practical implementation is explored. The basic system parameters such as tone bandwidth, number of frequencies and duty cycle are studied in terms of propagation characteristics, optimum data rate and system complexity. With optimum parameters, the authors show that a system using MT-FSK is capable of data transmission at a data rate of 250 Mbps with bit-error-rate below 10−5 whereas the receiver complexity is lower than for standard methods utilising the UWB spectrum by means of impulse radio or multi-band orthogonal frequency division multiplexing (OFDM).

It has been 20 years since the word ultrawideband (UWB) has first been used in the open literature. In these 20 years, ideas have been transformed into real products. Yet, some might object that UWB has not yet lived upto the promises of gigabit wireless. This review shows that despite some drawbacks, UWB is not only needed because of the spectrum gridlock but it can still deliver its promises, including gigabit wireless. To do so, the gap between the potential, which is achievable, and the current performance must be closed. Thus, this review identifies some main issues of UWB (range, bit error rate performance, data rate, chip complexity and issues associated by strong narrowband interference). It shows that their reasons are well understood and addressed by erudite research, which includes low complexity chip design, alternative modulation schemes, multiple antenna systems, UWB radio-over-fibre, higher band UWB and interferer detection and suppression techniques. A specific feature of this review is the cross-layer approach. The latest findings from different system layers, for example, wave propagation, are linked to the system performance or complexity.

Cognitive radio (CR) is an enabling technology that allows the unlicensed users to opportunistically access the licensed spectrum bands, without any modifications or updates for the licensed systems. In this paper, we first propose a new spectrum sharing policy, which achieves efficient spectrum sharing between two licensed networks. Then, a Markov chain model is developed to analyze the proposed spectrum sharing policy in the cases with and without spectrum handoff. Numerical results show that the proposed policy achieves a higher overall throughput of the two networks. Extensive simulations verify the analytical model.

This paper considers a joint antenna selection method in amplify-and-forward (AF) relay networks where the source, relay and destination terminals are all equipped with multiple antennas. The fact that the system's full diversity can be maintained by antenna selection at each terminal makes it a promising solution to reduce the hardware complexity of multiple-input multiple-output (MIMO) terminals while realizing the diversity benefits of MIMO in relay networks. Since the exhaustive search for antenna subset selection is computationally prohibitive, we devise a low-complexity near-optimal joint antenna selection algorithm based on a constrained cross entropy optimization (CCEO) method to maximize the achievable rate and the convergence is guaranteed. Simulation results reveal both the effectiveness and the efficiency of the proposed algorithm and the significant performance improvement over other benchmark selection techniques. Finally, it is illustrated that the proposed CCEO algorithm can always achieve near-optimal results regardless of the number of selected antennas, outage probabilities and the signal-to-noise ratios (SNRs) at the terminals.

This study attempts to exploit the potential of link directionality to increase the achievable capacities of ad hoc networks. When an IEEE 802.11 ad hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2C. However, most of the dual-channel 802.11 protocols proposed in the literature appear only to be able to achieve less than 60% of the 2C targeted capacity. The authors thus propose a link-directionality-based dual-channel medium access control protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. The main idea is to assign channels according to link directionality to allow a link to transmit simultaneously within the carrier-sensing region of another link provided that these transmissions do not interfere with each other. Simulations show that our proposed scheme can achieve more than 85% of our targeted capacities, 0.85×2C=1.7C, in large-scale random topologies. In lattice and irregular topologies, the throughput is boosted up to 2.83C and 2.13C, respectively. An approach for capacity analysis is also introduced to determine the throughput improvements that can be achieved by our proposed protocol. We believe using link directionality for channel allocations is a key step that yields significant potential for multiplying the capacity of ad hoc networks.

In this letter, we propose a novel receive antenna selection algorithm based on cross entropy optimization to maximize the capacity over spatially correlated channels in multiple-input multiple-output (MIMO) wireless systems. The performance of the proposed algorithm is investigated and compared with the existing schemes. Simulation results show that our low complexity algorithm can achieve near-optimal results that converge to within 99% of the optimal results obtained by exhaustive search. In addition, the proposed algorithm achieves near-optimal results irrespective of the mutual relationship between the number of transmit and receive antennas, the statistical properties of the channel and the operating signal-to-noise ratio.

Cyclic delay diversity (CDD) is an attractive diversity scheme due to its low complexity and compatibility to the existing standard orthogonal frequency division multiplexing systems. This paper presents an novel receiver antenna selection criterion for transmit CDD systems to obtain the maximal diversity gain. The performance of the proposed system is very close to the corresponding orthogonal space-time block coding diversity schemes with optimum receiver antenna selection, but CDD has the advantages of much lower complexity and no data rate reduction. Moreover, in order to make the proposed system simpler for implementation, a low complexity antenna selection algorithm has been presented.

The application of coherent detection to indoor optical free-space communications is considered here. Analytical expressions are derived for a wide line-of-sight (W-LOS) optical link which determine the coverage area and the required photo-detection area given different binary digital transmission schemes and a given bit rate. The coverage area is maximised for systems using both metal–semiconductor–metal (MSM) and PIN photo-detectors. Results show that MSM photo-detectors, due to their inherently lower capacitance per unit photo-detection area, result in a greater coverage area than their PIN counterparts. To demonstrate the viability of implementing coherent detection indoors, a 200 Mb/s LOS optical link which uses coherent detection is also reported.

Propagation measurements using a large array are used to study the angle of arrival (AOA) across the ultrawideband (UWB) frequency range of 3.1 to 10.6 GHz. A two-dimensional Unitary ESPRIT algorithm is employed to give a high resolution estimation of AOA including both the azimuth and elevation angles of multipath components. The frequency dependence of AOA is investigated over the UWB frequency band. The multipath rays form clusters in both angular and temporal domains. Within a cluster the azimuth and elevation AOAs are determined to follow Laplacian and Gaussian distributions respectively. In the indoor environment considered, a typical cluster extends over an angular sector of approximately 14 degrees in azimuth and 9 degrees in elevation, with up to 5 clusters observed. We note that these propagation characteristics will allow UWB systems to utilise smart antennas or MIMO structures to improve overall throughput.

Abstract—A low complexity user scheduling algorithm based on a novel adaptive Markov chain Monte Carlo (AMCMC) method is proposed to achieve the maximal sum capacity in an uplink multiple-input multiple-output (MIMO) multiuser system. Compared with the existing scheduling algorithms, our algorithm is not only more efficient but also converges to within 99 % of the optimal capacity obtained by exhaustive search. We demonstrate the convergence of the proposed scheduling algorithm and study the tradeoff between its complexity and performance. Index Terms—Adaptive Markov chain Monte Carlo (AM-CMC), multiple-input multiple-output (MIMO), multiuser selec-tion, scheduling, sum capacity. I.

This paper investigates the impact of channel bandwidth on fading in wireless channels using indoor measurements. The variation of channel energy over a local region is examined for narrowband, wideband and ultrawideband (UWB) channels, and the corresponding fade depth is evaluated. The relation between bandwidth and fade depth is then captured with a simple dual-slope model. The effect of antenna polarization and line-of- sight blockage is also investigated. We observe that the fade depth initially falls rapidly with bandwidth, reaching 4 dB at 1 GHz, but further reduction in fading with bandwidth is much slower.

Polarized single-input multiple-output (SIMO) arrays can improve system robustness and rate while offering a compact design. We investigate the error performance of Impulse Radio ultrawideband (UWB) systems with multiple- antenna arrays employing polarization orthogonality. Based on indoor channel measurements, we evaluate the improvement in the link reliability due to dual- or triple-polar antenna arrays. Using a two-dimensional combining receiver, it is demonstrated that polarization diversity can be very effective in reducing the overall system complexity, making Impulse Radio implementation practically feasible.

In this paper, we clarify inter-link interference in wireless ad-hoc networks by using link-directional interference graphs (I-graph). Most of the interference graphs in the literature simply model the DATA and ACK traffic of a link by a single vertex. They fail to capture the link-directionalities. In fact, some instances of directional traffic can actually transmit simultaneously but are prohibited by the interference graphs. Thus, in our link-directional interference graph, a link is represented by two vertices, one for DATA traffic and the other for ACK traffic. We then apply a colouring algorithm in the I-graphs. The colouring results provide insights in order to boost network capacities in TDMA or FDMA ad-hoc networks. We show that the network capacities can be improved by 100% in a triangular topology and 33% in a lattice topology. Simulations also show that a distributed dual channel protocol assigning channels according to link-directionalities can boost the capacities by 70% in large-scale random networks. We believe this is a first paper in the literature to take into account link-directionalities in interference graphs.

Multi-carrier code division multiple access (MC-CDMA) was proposed to improve capacities of wireless networks. Instead of increasing the data transmission rates of wireless links, the authors propose to assign independent data streams obtained by MC-CDMA for transmitting uplink and downlink traffic separately in order to eliminate the hidden-node and exposed-node problems between cochannel BSSs in WLANs. Simulations show that our scheme can improve network throughputs by 106% in an exposed-node scenario and by 113% in a hidden-node scenario. The authors also consider the hardware and MAC requirements of our scheme under different settings of physical carrier sensing range. The authors believe this is a first paper in the literature to assign MC-CDMA generated data streams in WLANs so as to mitigate the hidden-node and exposed-node problems.

A simple and efficient virtual-source ray-tracing technique for the simulation of indoor wideband radio and optical propagation channels is proposed. The parametric deterministic model considers the room geometry, transceiver locations, material properties and probe signal types. It is applied to the indoor ultrawideband channel in the FCC-allocated 3.1–10.6 GHz band, and a range of novel results are presented to illustrate several possible applications. The channel small-scale fading statistics and spatial variability are examined by synthesising a densely sampled aperture. Multiple-antenna array systems are simulated to evaluate multiple-input multiple-output performance. The multipath angular characteristics are analysed from the simulated azimuth-delay profile. The simulation results closely match previous channel measurement studies and statistical models, validating the proposed technique. It is shown that specular reflection is dominant, and power convergence is achieved with three reflections in a typical indoor environment. Thus, it is demonstrated that despite its simplicity, the model yields reliable and accurate results, and can therefore be a useful tool for indoor wireless network planning and performance prediction.

This paper experimentally investigates the performance of multiple-input multiple-output (MIMO) systems in indoor ultra wideband channels. The improvement in robustness and information rate due to spatial and polar antenna arrays is evaluated. The subchannel correlation, power gains of supported eigenmodes, and branch power ratios are analyzed. The polar arrays are found to experience lower correlation than spatial arrays. SNR gains of up to 3–5 dB are reported with 1$\,\times\,$2 and 1$\,\times\,$3 spatial arrays, respectively; the latter is shown to double the coverage range. The mutual information capacity is found to scale almost linearly with the MIMO array size, with very low variance. It is confirmed that the device compactness achieved by the polar array comes with only a small penalty in the achievable capacity and SNR gain compared to the spatial array. The multiple-antenna UWB techniques explored in this paper offer the potential for high data rate and robust communications.

This paper experimentally investigates the performance of multiple-input multiple-output (MIMO) systems in indoor ultrawideband (UWB) channels. The improvement in robustness and information rate due to spatial and polar antenna arrays is evaluated. The subchannel correlation, power gains of supported eigenmodes, and branch power ratios are analyzed. The polar arrays are found to experience lower correlation than that of spatial arrays. SNR gains of up to 3 and 5 dB are reported with 1times2 and 1times3 spatial arrays, respectively; the latter is shown to double the coverage range. The mutual information capacity is found to scale almost linearly with the MIMO array size, with very low variance. It is confirmed that the device compactness achieved by the polar array comes with only a small penalty in the achievable capacity and SNR gain compared to the spatial array. The multiple-antenna UWB techniques explored in this paper offer the potential for high-data-rate, robust communications.

The variation of channel fading and dispersion statistics between the ultrawideband (UWB) channel subbands is characterized experimentally. Within the FCC-allocated UWB frequency range (3.1-10.6 GHz), the multipath characteristics in various subbands are found to be highly dissimilar. It is demonstrated that the higher subbands suffer greater attenuation, and the rms delay spread varies significantly with the center frequency. The impact of this variation on multiband-OFDM UWB system performance is evaluated, and a large performance variation across the band is reported

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2 - C. However, most of the multichannel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% of the 2 ldr C targeted capacity. Simulations show that our proposed scheme can achieve more than 106% of our targeted capacities, 1.06*2 ldr C = 2.12 ldr C. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link on different channels, a key step that yields significant potential formultiplyingthe network capacities of ad-hoc networks.

The frequency-dependent pathloss characteristics of an indoor radio channel are investigated over the FCC-allocated ultrawideband (UWB) frequency range (3.1-10.6 GHz). Complex channel transfer functions are measured under a variety of propagation conditions. The dispersion index is introduced as a measure of the frequency-dependence pathloss and is estimated for each measured channel response. Its first order statistics exhibit significant deviation from the nominal value. The small-scale spatial analysis of the dispersion index reveals a periodic structure for vertically polarized channels. Fractional calculus is used to demonstrate the impact of this dispersion on system performance, and the consequent signal waveform distortion is evaluated.

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel 802.11 protocol. Simulations show that the proposed scheme can achieve more than 78% of our targeted capacities. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link in different channels. This yields a large potential for multiplying the network capacities of ad-hoc networks by using only two channels.

When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be2middotC. However, most or the multi-channel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% or the 2middotC targeted capacity. In our paper (2006), we proposed a link-directionality-based dual channel MAC protocol (DCP) to boost the network capacities up to 78% of our targeted capacities, 78%*2middotC = 1.56middotC. However, DCP still failed to reach the 2middotC capacity target due to the overheads incurred by the protocol. In this paper, we implement a power exchange algorithm on top of the DCP in an attempt to double the capacities of networks using the single-channel 802.11 protocol. This algorithm incurs relatively small overheads and can further release the protocol constraints imposed by DCP. Simulations show that the proposed scheme (DCPwPEA) can achieve more than 132% of our targeted capacities, 132%* 2middotC = 2.64middotC. We believe this protocol can be extended to allow multi-link simultaneous transmissions which can multiply the network capacities of ad-hoc networks by using only two channels

The accurate detection of the direct path in a dense multipath environment is vital for reliable performance of time-based and angle-of-arrival (AOA) based location estimation techniques. This paper investigates the effect of measurement bandwidth and antenna directivity on the detection of the direct path in a typical indoor scattering environment. It is experimentally shown that the ranging error diminishes with increase in measurement bandwidth and the standard deviation of the root mean square ranging error (RMSRE) is reduced by using directional transmit antennas. The results are obtained in a line-of-sight (LOS) propagation environment and are based on the time-of-arrival (TOA) based ranging technique.

Orthogonal netted radar transmitter signals require a very low aperiodic autocorrelation peak sidelobe level (PSL), low aperiodic cross-correlation, and a good resilience to small Doppler shifts. A new set of polyphase sequences is presented with good correlation properties as well as resilience to Doppler shifts. These sequences are built using numerical optimization based on correlation properties. A structural constraint is imposed on the optimized polyphase sequences, which maintains Doppler tolerance. Cross entropy (CE) technique is used to optimize the sequences. Correlation and Doppler results are compared with best-known sequences on various merit factors and shown to be superior

This paper presents results of a tri-polarized multiple-input multiple-output (MIMO) channel measurement campaign in typical room-room and floor-floor micro-cellular environments. By implementing the MIMO channels in the polarization rather than conventional spatial domain, we are able to co-locate the antenna elements at both ends of the communications link. This results in a compact system that would be a good candidate for future designs of mobile communications systems including handsets. Benchmarking against the spatial technique, demonstrates that a polarized MIMO system can offer approximately the same 10% outage capacity. In terms of ergodic capacity, the tri-polar outperforms the spatial technique by up to 2 b/s/Hz.

Hybrid systems using evolution to optimize neural network design or training are usually limited in scope and effectiveness. A system is presented that permits the widest variety of networks to be evolved using a two-stage GA approach. Networks generated for a benchmark machine learning task compare favourably with alternative methods.

To maximise the assignment of function of the proteins encoded by a genome and to aid the search for novel drug targets, there is an emerging need for sensitive methods of predicting protein function on a genome-wide basis. GeneAtlas is an automated, high-throughput pipeline for the prediction of protein structure and function using sequence similarity detection, homology modelling and fold recognition methods. GeneAtlas is described in detail here. To test GeneAtlas, a 'virtual' genome was used, a subset of PDB structures from the SCOP database, in which the functional relationships are known. GeneAtlas detects additional relationships by building 3D models in comparison with the sequence searching method PSI-BLAST. Functionally related proteins with sequence identity below the twilight zone can be recognised correctly.

The performance of a three-branch orthogonal polarization scheme consisting of one vertically polarized and two orthogonal horizontally polarized antennas at the receiver with a horizontally and vertically polarized transmitter is investigated. The paper shows that such a scheme provides diversity improvement of about 2dB more than the dual-branch system. Also the signals received at the three branches are highly uncorrelated. This scheme can make significant improvements in the link budget of a mobile network.

A novel approach to the evaluation of bit error rate performance for directive optical wireless links, which takes into account shot noise, fluorescent lighting and intersymbol interference (ISI) is presented. The evaluation technique is based on nonclassical Gauss quadrature rules, with the fluorescent light and the ISI from high pass filtering being modeled mathematically. As an application of this method the BER performance of NRZ-OOK and Manchester modulation has been evaluated and numerical results are presented. It is shown that Manchester modulation is quite robust in combating fluorescent lighting interference and its performance is not critical on the selection of the HPF cutoff frequency.

This paper presents techniques for obtaining direction of arrival (DOA) information from both narrowband and wideband radio channel measurements, with the use of spatial diversity. The multipaths arriving at the receiver are presented in a photographic form, which may be directly compared with the surroundings. The use of wideband channel information allows time-gated measurements to be made of the impinging signals. This technique is ideally suited to the validation of radio cellular propagation tools.

Currently, there is considerable interest in infrared as an alternative medium to radio for indoor wireless communications, particularly for computer networking applications. This paper describes a novel IR-LAN link topology that employs an optical spatial-angular mapping technique which minimizes link losses and facilitates high link rates, whilst maintaining a useful coverage area. Blocking probability effects and overall system performance are examined in terms of aggregate transmission rates.