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Orthogonal frequency division multiplexing (OFDM) technology is presented for use in free-space optical (FSO) communications accompanied by the spatial diversity reception. Using quadrature phase shift keying (QPSK) modulation formats, the OFDM signals show robustness to support high spectral efficiency and compatibility with the spatial diversity reception to improve receiver sensitivity. Compared with the single-carrier QPSK signal, the OFDM-QPSK signal with 64 sub-carriers can reduce the BER from 2.87 × 10−3 to 2.98 × 10−4 at the SNR of 6 dB. Using a two-aperture spatial diversity reception with OFDM, the BER can be reduced from 2.45 × 10−3 of a single receiver to 6.10 × 10−4 under moderate turbulence conditions. Under strong turbulence, the BER of the single receiver is 2.14 × 10−2. It can be improved to 1.16 × 10−3 by using four-aperture receivers, and even 6.87 × 10−4 by using six-aperture receivers. The optimized aperture number should be selected according to channel conditions.

In this paper, average channel capacity of optical wireless communication system is evaluated under the combined effect of geometrical loss, attenuation due to weather conditions and weak atmospheric turbulence using a simple closed form expression. Fading induced due to atmospheric turbulence is modeled by log-normal distribution. Considering the fact that the sum of log-normal random variables can be well approximated by another log-normal random variable, the proposed expression has been utilized to compute the channel capacity for spatial diversity reception employing maximum ratio combining and equal gain combining over uncorrelated turbulence-induced fading conditions. It is shown that spatial diversity is an effective technique to mitigate the impairments caused by various atmospheric conditions such as haze, rain and fog. The quantitative improvement in channel capacity achieved by using diversity techniques is investigated and compared. Accuracy of the results is validated with exact results computed using Monte Carlo simulation.

This study investigates the effect of spatial receive diversity on specific absorption rate (SAR) reduction based on outage probability analysis for wireless capsule endoscope (WCE). The communication performance of WCE depends much on the transmit power, which is strictly regulated in order to satisfy a safety guideline in terms of SAR, whereas WCE requires high communication performance due to its real-time data transmission. For the purpose of SAR reduction for a WCE scenario, the authors pay attention to the expectation that applying spatial diversity reception to WCE systems can not only improve the wireless communication performance but also reduce SAR. To begin with, based on finite-difference time-domain simulations with a numerical human body model, the outage probability is calculated under this implant propagation channel and the required transmit power is derived to secure a permissible outage probability. Then, the local peak SAR is calculated under the required transmit power when the WCE moves through the digestive organs. Finally, the simulation results demonstrate that applying spatial diversity reception can significantly reduce SAR for WCE.

Generalized pollinators visit multiple co‐flowering plant species and may transfer heterospecific pollen grains. Recent studies have indicated that the effect of heterospecific pollen (HP) on reproduction success is variable and depends on the identity of donor and recipient species. However, few studies have documented variation in HP receipt and evaluated the reproductive effects of HP receipt across geographic locations under natural conditions. We investigated the spatial variation of pollen deposition across eight sites and how the pollen receipt related to the seed set of Salvia przewalskii, a subalpine perennial herb in Hengduan Mountain in southwest China. We found that stigmatic pollen loads substantially varied among sites for several metrics, including quantities of conspecific and heterospecific pollen, the proportion of HP, and species composition of HP donors. Five different plant families were the most common HP source at one or two sites, and the proportion of HP ranged from 3.4% to 51.3% across sites. The association of conspecific pollen with seed set was positive and variable among sites, whereas the association of HP receipt and seed set was negative and not significantly different among sites. Our results demonstrate variation in the quantity and fitness effect of pollen receipt across sites, which is a precondition for evolution of local adaptation. Further study of variation in patterns and effects of HP receipt for the same recipient species across natural communities would allow better understanding of the ecological and evolutionary consequences of HP receipt. We documented variations in HP receipt and evaluated its reproductive effects across geographic locations under natural conditions. The association of conspecific pollen with seed production was positive and variable among sites, whereas the association of HP receipt and seed production was negative and not significantly different among sites.

New physical–mathematical models for the calculation of the outage probability of the maximal ratio combining and selection combining dual branch spatial diversity schemes for broadband fixed wireless access networks operating above 10 GHz are presented. At these frequency bands and considering line-of-sight conditions, rain attenuation is the dominant fading mechanism, which should be taken into account in the radio communications system design. The models are based on bivariate inverse Gaussian (IG) distribution and on the adoption of a spatial correlation coefficient for the convergent terrestrial wireless links. IG distribution has been shown that models accurately the rain attenuation fading for both temperate and tropical climatic regions. The models are validated with numerical results and some useful conclusions are drawn.

Cooperative spectrum sensing (CSS) is a promising technique in cognitive radio networks (CRNs) that utilises multi-user diversity to mitigate channel instability and noise uncertainty. In this study, the relationship between ‘cooperation mechanisms’ and ‘spatial-spectral diversity’ over multiple channels jointly sensing is investigated in the presence of an imperfect reporting channel. The multiple channels are sensed at the receiver built on the filter bank-based multi-carrier system. The multi-channel CSS strategies are modelled by the introduced ‘cooperative ratio’ to balance the requirements on ‘sensing accuracy’, ‘efficiency’ and ‘overhead’, which is quantitatively characterised by the energy consumption. The target of CSS is to maximise the aggregate opportunistic throughput of secondary users (SUs) by jointly considering constraints on sensing overhead and the aggregate interference to primary users (PUs). The optimisation is divided into two sequential sub-optimisation processes, ‘multi-user diversity optimisation’ and ‘multi-channel diversity optimisation’. An approach is developed from generic algorithms to solve the two sub-problems. Numerical results show that the optimal CSS scheme is effective in improving channel utilisation for SUs with low interference to PUs. This study establishes a valuable cooperative model for the design of multi-channel spectrum sensing algorithms in CRNs.

The use of multiple-input, multiple-output (MIMO) technology is proposed to provide unequal error protection (UEP) for layered source coding. Two priority layers are considered: a base (high priority (HP)) layer and an enhancement (low priority (LP)) layer. The MIMO is composed of two transmit antennas and two receive antennas. The HP bit stream is modulated by using four quadrature amplitude modulation (4-QAM) and the LP bit stream is modulated by using 16-QAM. The HP and the LP symbols are then coded by using a spatial–time block coding and multiplexed before transmission. The performance of the proposed system is compared with an equal error protection (EEP) system of 64-QAM and two transmit and two receive antennas. The EEP system uses spatial diversity in its MIMO to improve the decision reliability at the receiver. Both systems provide a similar capacity. The simulation results show a considerable improvement in performance for the UEP system over the EEP one.

Increasing the number of transmit antennas can improve the diversity gain of the system, and accordingly reduce the transmit power dissipation. However, the high circuit power dissipation incurred cannot be ignored. Generalised selection combining, which could provide a certain spatial diversity in the transmit diversity systems, performs a good balance between system performance and practical implementation cost. In this study, the authors propose an energy efficient generalised selection combining (EE-GSC) scheme which obtains improved transmitter energy efficiency (EE) by providing a best tradeoff between the diversity gain and the circuit power dissipation of multiple antennas. Based on the classical results of order statistics, a theoretical analysis of EE-GSC performance is carried out in detail over Rayleigh fading channels. Based on this analysis, the average number of active branches as well as the average power dissipation of the proposed scheme is also derived. Numerical results are also given to further illustrate the EE performance of the proposed scheme.

In this article, a computationally-efficient approach is presented for Bit Error Rate (BER) calculations in multi-user Multiple Input Multiple Output (MIMO) frequency selective fading environments, operating in spatial multiplexing transmission mode. To this end, theoretical expressions for the equivalent Signal to Interference plus Noise Ratio (SINR) per active user and transmission mode, as well as mean BER, are derived. The key idea is that all parameters related to BER calculations can be expressed as sums of identically distributed random variables (RVs). Hence, computational burden can be reduced, since summation formulas take into account the parameters of a standalone RV along with the correlation of an arbitrary pair of RVs. As results indicate, the proposed approach can accurately estimate mean BER in multiuser MIMO orientations with increased reception diversity order, for arbitrary number of transmit/receive antennas, a modulation scheme, and a number of resolvable multi-path components.

Spatial diversity schemes are often used to extract additional performance from wireless communication systems. Incorporating the partial relay selection (PRS) protocol into a distributed switch-and-stay combining (DSSC) cooperative communication network gives the benefit of diversity while simplifying hardware, processing and feedback requirements. Because only a single relay is ever active, the destination employs no combiner; the best relay is chosen based on first-hop relay conditions. However, achieved performance is worse than other distributed protocols, such as distributed selection combining (DSC). In this study, signal space diversity (SSD) is added to the DSSC–PRS system to provide further diversity and error performance gains, at the expense of necessitating a maximum-likelihood detector with increased complexity at the receiver. Analytical results are presented in the form of a lower bound based on the minimum distance lower bound for SSD systems and are verified with simulation. The DSSC–PRS–SSD system shows an improvement of 5 dB at a symbol error rate of 10−4 as well as a clear diversity order improvement. Spectral efficiency of the new system with SSD is slightly decreased at low signal-to-noise ratios, but is still an improvement over other distributed schemes, such as DSC.