Optimal Diversity Research Articles

MNAFSA: A novel approach for optimization in dynamic environments with global changes

Artificial fish swarm algorithm (AFSA) is one of the state-of-the-art swarm intelligence algorithms that is widely used for optimization purposes in static environments. However, numerous real-world problems are dynamic and uncertain, which could not be solved using static approaches. The contribution of this paper is twofold. First, a novel AFSA algorithm, so called NAFSA, has been proposed in order to eliminate weak points of standard AFSA and increase convergence speed of the algorithm. Second, a multi-swarm algorithm based on NAFSA (mNAFSA) was presented to conquer particular challenges of dynamic environment by proposing several novel mechanisms including particularly modified multi-swarm mechanism for finding and covering potential optimum peaks and diversity increase mechanism which is applied after detecting an environment change. The proposed approaches have been evaluated on moving peak benchmark, which is the most prominent benchmark in this domain. This benchmark involves several parameters in order to simulate different configurations of dynamic environments. Extensive experiments show that the proposed algorithm significantly outperforms previous algorithms in most of the tested dynamic environments modeled by moving peaks benchmark.

The grass-free lawn: Management and species choice for optimum ground cover and plant diversity

In Britain, managed grass lawns provide the most traditional and widespread of garden and landscape practices in use today. Grass lawns are coming under increasing challenge as they tend to support a low level of biodiversity and can require substantial additional inputs to maintain. Here we apply a novel approach to the traditional monocultural lawnscape by replacing grasses entirely with clonal perennial forbs. We monitored changes in plant coverage and species composition over a two year period and here we report the results of a study comparing plant origin (native, non-native and mixed) and mowing regime. This allows us to assess the viability of this construct as an alternative to traditional grass lawns. Grass-free lawns provided a similar level of plant cover to grass lawns. Both the mowing regime and the combination of species used affected this outcome, with native plant species seen to have the highest survival rates, and mowing at 4cm to produce the greatest amount of ground coverage and plant species diversity within grass-free lawns. Grass-free lawns required over 50 percent less mowing than a traditionally managed grass lawn. Observations suggest that plant forms that exhibited: (a) a relatively fast growth rate, (b) a relatively large individual leaf area, and (c) an average leaf height substantially above the cut to be applied, were unsuitable for use in grass-free lawns. With an equivalent level of ground coverage to grass lawns, increased plant diversity and a reduced need for mowing, the grass-free lawn can be seen as a species diverse, lower input and potentially highly ornamental alternative to the traditional lawn format.

A Novel Method of Beamforming to Improve the Space Diversity

At present, the systems of mobile communications demand major capacity in the services, therefore optimum space diversity is needed. In this paper, we propose a ne w method of beam-forming to improve the space diversity and achieve frequency reuse. Our proposal consists in solving a system of linear equations from an 8-elements linear antenna array that increases the size of the side lobes. This method i s compared with the algorithm of Dolph-Chebyshev. Our method shows competitive results in increasing the space diversi ty.

Diversity Transmission of Synchronization Sequences in MIMO Systems

We address diversity schemes for synchronization signal transmission in MIMO systems. We provide both analytical and numerical evaluation of different diversity schemes in different scenarios in terms of signal-to-noise ratio, spatial correlation and maximum Doppler frequency. We first derive an expression for probability of detection in presence of transmit, receive and time diversity. We establish statistical properties of optimum spatial diversity transmission schemes and analytically characterize also the performance of more practical diversity schemes. Analytical results are verified with simulations. Finally we provide extensive results on both optimum schemes as well as the more practical open-loop schemes in different diversity scenarios. The results of the paper can be used to facilitate practical design and implementation of synchronization in multi-antenna wireless systems.

On the Optimum Diversity–Multiplexing Tradeoff of the Two-User Gaussian Interference Channel With Rayleigh Fading

In this paper, the optimum tradeoff between diversity and multiplexing gains in a two-user quasi-static Rayleigh fading interference channel (IC) is studied. The diversity and multiplexing gains are two basic performance measures in wireless networks which characterize the transmission reliability and the data rate, respectively. It would be of interest to investigate the optimal tradeoff between these two measures. First, we develop a coding scheme for the two-user quasi-static Rayleigh fading Gaussian IC with interference level α := log INR/log SNR ≥ 1. Then, for this coding scheme the achievable diversity-multiplexing tradeoff (DMT) is characterized. Our achievable DMT coincides with its outer bound. In the low and high rate regions (to be defined later), the proposed coding scheme is a one-level Gaussian code, independent of the channel state information (CSI). In the middle rate region (to be defined later), the proposed coding scheme, depending on the partial CSI, can be a one-level or a two-level Gaussian code. We show that the relevant partial CSI can be represented by only one bit determined by the absolute value of the channel gain. In the middle rate region, we assume that the single-bit partial CSI for all the four channel gains of the Gaussian IC are available at both transmitters.

Complexity-performance trade-off of algorithms for combined lattice reduction and QR decomposition

Over the last years, numerous equalization schemes for multiple-input/multiple-output channels have been studied in the literature. New low-complexity approaches based on lattice basis reduction are of special interest, since they achieve the optimum diversity behavior. Although the per-symbol equalization complexity of these schemes is very low, the initial calculation of the required matrices may impose an enormous burden in arithmetic complexity. In this paper, we give a tutorial overview and assess algorithms, which, given the channel matrix, result in the feedforward, feedback, and unimodular matrix required in lattice-reduction-aided decision-feedback equalization or precoding. To this end, via a unified exposition of the Lenstra–Lenstra–Lovász (LLL) algorithm, the LLL with deep insertions, and the reversed Siegel approach similarities and differences of these approaches are enlightened. A modification of the LLL swapping criterion, better matched to the equalization setting, is discussed. It is shown that using lattice-reduction-aided equalization/precoding better performance can be achieved at lower complexity compared to classical equalization or precoding approaches.

Diversity Analysis for Linear Equalizers over ISI Channels

It has been shown in the literature that, with zero-padding prefix (ZPP), the optimum diversity gain of frequency selective channels can be obtained by uncoded signals and zero forcing (ZF) equalizers. In this paper, we derive two important results about linear equalizers over frequency selective channels: 1) With cyclic prefix and any rate-1 unitary precoding, which includes the uncoded/coded-OFDM/SCFDMA systems as special cases, linear equalizers can only obtain order-1 diversity; and 2) Although linear equalizers can achieve the optimum diversity order with zero-padding prefix and uncoded signals, the required SNR (signal-to-noise ratio) is an increasing function of the symbol-length. The second result implies that we may not be able to take advantage of the diversity gain in practice due to the high SNR requirements. Special cases are also investigated to gain some physical insights about the effects of different prefixes on the performance of linear equalizers. The results in this paper can be utilized in the design of up/down-link LTE systems with linear equalizers.

Lattice-reduction-aided MMSE equalization and the successive estimation of correlated data

Over the last years, novel low-complexity approaches to the equalization of MIMO channels have gained much attention. Thereby, methods based on lattice basis reduction are of special interest, as they achieve the optimum diversity order. In this paper, a tutorial overview on LRA equalization optimized according to the MMSE criterion is given. It is proven that applying the zero-forcing BLAST algorithm to a suitably augmented channel matrix – the inverse of the square root of the correlation matrix of the data symbols times the noise variance forms its lower part – results in the optimum solution. This fact is already widely used but lacks a formal proof. It turns out that it is more important to take the correlations of the data correctly into account than what type of lattice reduction actually is used.

Fixed-complexity sphere encoder for multi-user MIMO systems

In this paper, we propose a fixed-complexity sphere encoder (FSE) for multi-user multi-input multi-output (MU-MIMO) systems. The proposed FSE accomplishes a scalable tradeoff between performance and complexity. Also, because it has a parallel tree-search structure, the proposed encoder can be easily pipelined, leading to a tremendous reduction in the preceding latency. The complexity of the proposed encoder is also analyzed, and we propose two techniques that reduce it. Simulation and analytical results demonstrate that in a 4 × 4 MU-MIMO system, the proposed FSE requires only 11.5% of the computational complexity needed by the conventional QR decomposition with M-algorithm encoder (QRDM-E). Also, the encoding throughput of the proposed encoder is 7.5 times that of the QRDM-E with tolerable degradation in the BER performance, while achieving the optimum diversity order.

利用Cramér-Rao理论研究相位差法的最佳离焦量

利用Cramér-Rao理论研究相位差法的最佳离焦量

Performance analysis of a noise-normalized FFH/MFSK receiver over Rayleigh fading channels with partial-band noise jamming

Fast frequency hopping can reduce the degradation caused by interference. The performance of noise-normalized fast frequency-hopped M-ary orthogonal frequency-shift-keying (FFH/MFSK) receivers over frequency nonselective Rayleigh fading channels with partial-band noise jamming is analyzed. Instead of numerical evaluation, a closed-form error probability expression is given based on the theory of complex variables. The simulation results validate the analytical results. It is shown that there is an optimum diversity order and the system performance gets better with an increase in the modulation order.

Performance analysis of FFH/MFSK receivers with noise‐normalisation combining over Nakagami‐m fading channels with partial‐band jamming

AbstractThe performance of a fast frequency‐hopped M‐ary orthogonal frequency‐shift‐keying (FFH/MFSK) receiver with noise‐normalisation combining over Nakagami‐m fading channels with partial‐band noise jamming and additive white Gaussian noise (AWGN) is analysed. Instead of numerical integration, a closed‐form error probability expression is obtained. The analytical results validate the simulation results. It is shown that there is an optimum diversity order and the corresponding best‐case performance is investigated. Copyright © 2010 John Wiley & Sons, Ltd.

Performance study of fast frequency-hopped/ M -ary frequency-shift keying systems with timing and frequency offsets over Rician-fading channels with both multitone jamming and partial-band noise jamming

In this study, the effects of timing and frequency offsets on bit-error rate (BER) performance of fast frequency-hopped M-ary frequency-shift keying communication systems over Rician-fading channels with both multitone jamming (MTJ) and partial-band noise jamming (PBNJ) are investigated. Analytical BER expressions are derived for both linear-combining and product-combining receivers. Numerical results show that under both MTJ and PBNJ conditions, the BER performance falls between the two extreme cases, in which either MTJ or PBNJ presents. It is found that the BER performance is severely degraded as the timing or frequency offset increases. The product-combining receiver is found to be more sensitive to the timing and frequency offsets than the linear-combining receiver. It is also observed that for the linear-combining receiver, the optimum diversity order increases as the timing and frequency offset increases over both Rician-fading and Rayleigh-fading channels; however, the reverse is true for the product-combining receiver.

Performance Analysis of Optimum Diversity Combining for Partially Coherent Frequency-Selective Fading Channel With Intersymbol Interference

The performance of coherent BPSK and QPSK systems in the presence of frequency-selective fading and carrier phase errors is analyzed. Frequency-selective fading channels are modeled as multipath Rayleigh, Nakagami- m, or Rician two-ray fading models wherein the presence of a delayed ray causes intersymbol interference (ISI). A diversity system with L-branches is used for signal detection using an optimum equal-gain-combining receiver. The effect of ISI on system performance is assessed using a truncated pulse train of the ISI signal. For Rayleigh-faded channels, the simulation result that demonstrates the influence of Doppler spread on system performance was also included. The following two performance criteria are considered: the average bit error probability and the signal-to-noise-ratio (SNR) gain penalty due to ISI.

Performance Analysis of a Maximum-Likelihood FFH/MFSK Receiver with Partial-Band-Noise Jamming over Frequency-Selective Fading Channels

A maximum-likelihood (ML) receiver is proposed for a fast frequency-hopping M-ary frequency-shift-keying system. The effects of frequency-selective Rayleigh-fading channels, partial-band-noise jamming, and additive white Gaussian noise are considered. It is shown that side information on the noise variance, the jamming variance, as well as the power levels of the correlated paths, is required to implement the ML receiver. Analytical bit-error rate expressions are derived for the ML receiver. Our analysis also shows that there exists an optimum diversity order under certain channel conditions.

The influence of habitat structure and flow permanence on macroinvertebrate assemblages in temporary rivers in northwestern Zimbabwe

Temporary rivers within the Nyaodza-Gachegache subcatchment in northwestern Zimbabwe were investigated to examine the role of flow permanence and habitat structure on macroinvertebrate community composition. Macroinvertebrate communities of intermittent and ephemeral rivers displayed significant differences in the number of taxa, macroinvertebrate abundance, Shannon and Simpson diversity indices and in size class structure. Intermittent sites were characterised by higher numbers of taxa, diversity and Ephemeroptera and Trichoptera richness compared to ephemeral sites. The fauna of ephemeral sites was dominated by a single taxon (Afrobaetodes) (Ephemeroptera, Baetidae) whilst larger sized taxa (e.g. Elassoneuria (Ephemeroptera, Oligoneuriidae), Dicentroptilum (Ephemeroptera, Baetidae), Aethaloptera (Trichoptera, Hydropsychidae), Pseudagrion (Odonata, Coenagrionidae) and Tholymis (Odonata, Libellulidae) were exclusively restricted to intermittent sites. Clear differences were observed between sand, gravel, cobble and vegetation habitats. Vegetation and cobbles supported distinct communities, with some taxa exclusively restricted either to vegetation (e.g. Pseudagrion, Leptocerina (Trichoptera, Leptoceridae), Cloeon (Ephemeroptera, Baetidae), Afronurus (Ephemeroptera, Heptageniidae) and Povilla (Ephemeroptera, Polymitarcidae) or cobble (e.g. Aethaloptera and Dicentroptilum) habitats. In terms of ensuring optimum diversity within the subcatchment, we consider conservation of critical habitats (cobbles and vegetation) and maintenance of natural flows as the appropriate management actions.

Assessing indicators for water diversion based on improving water quality of shallow lakes

Based on the goal of water diversion for shallow lakes, taking the requirement of ecological hydraulics and economic factor into consideration, an attempt to establish an assessing indicator system for water diversion scheme of shallow lakes is put forward in this paper. The indicators are consisted of lake quality-improving indicators, hydraulic indicators and economic indicator. By the case of Lake Xuanwu, a 2-D water-quality model is adopted to simulate different schemes of water diversion for Lake Xuanwu with different flow flux, diversion mode, flow distribution of inlets, and flow distribution of outlets. Detailed assessing indicators of statistic results are obtained, and then the optimum water diversion scheme is assessed. The method can be applied to the assessment of water diversion on similar shallow lakes.

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

We present an optimum diversity receiver called blind ratio combining (BRC), which minimizes the average symbol error probability or maximizes the average output signal-to-noise ratio, where the channels' time delays and the random phases are known while the fading amplitudes are unknown. In contrast to previous works where efforts were made to find a posteriori probabilities at the receiver, the BRC simply calculates the optimum weights, which depend on the channel's statistics, avoiding continuous channel estimation, and thus, it significantly reduces the system's complexity. In nonidentical multipath fading channels with power delay profile (PDP), the BRC receiver performs between maximal ratio combining (MRC) and equal gain combining (EGC), and attains to keep its performance comparable—and in some cases superior—to that of generalized selection combining (GSC), while for large values of the decay factor it approaches MRC. Moreover, in the important practical case of exponential PDP—common in RAKE receivers modeling and adopted for the Universal Mobile Telecommunications System (UMTS) spatial channel modeling by the European Telecommunications Standards Institute-Third Generation Partnership Project (ETSI-3GPP)—the optimum weights can be accurately approximated by simple elementary functions. Furthermore, it is proved that the utilization of these weights ensures an error performance improvement over EGC for arbitrary PDPs. The proposed BRC receiver can be efficiently applied in wireless wideband communication systems, where a large number of diversity branches exists, due to the strong multipath effects.

Performance Analysis of Optimum and Suboptimum Selection Diversity Schemes on Rayleigh Fading Channels With Imperfect Channel Estimates

In this paper, we present a comparative analysis on the effects of channel estimation errors on the performance of optimum and suboptimum selection diversity (SD) receivers on Rayleigh-fading channels. By modeling the estimation errors as independent complex Gaussian random variables, we derive simple closed-form expressions for the average probability of error for both optimum and suboptimum SD schemes with noisy channel estimates. With dual diversity and imperfect estimates, we establish a connection between optimum SD and maximal-ratio combining (MRC), and between suboptimum SD and equal-gain combining diversity schemes. Interestingly, we show that the optimum SD receiver structure and the resulting performance for differential binary coherent phase-shift keying (DBPSK) signaling can be obtained, in a straightforward way, as a special case of the performance of the optimum SD scheme with binary PSK signaling and channel estimation errors. For a fixed average power and bit duration, in conjunction with pilot-assisted minimum mean-square error channel estimation, we show that the optimum coherent SD scheme coincides with that of the optimum noncoherent SD scheme with binary frequency-shift keying (BFSK) signaling, whereas the coherent MRC scheme coincides with the optimum noncoherent receiver (i.e., the square-law combiner) for BFSK. The optimum number of diversity channels, under an energy-sharing mode of operation, is also studied. Finally, we formulate the problem of optimal pilot placement, consider channel estimation with a practical pilot-symbol-assisted modulation technique, and present some numerical results illustrating the comparative performances of various SD receivers