Large-scale Fading Research Articles

Measurements and Analysis of Large-Scale Fading Characteristics in Curved Subway Tunnels at 920 MHz, 2400 MHz, and 5705 MHz

Wave propagation characteristics in curved tunnels are of importance for designing reliable communications in subway systems. This paper presents the extensive propagation measurements conducted in two typical types of subway tunnels—traditional arched “Type I” tunnel and modern arched “Type II” tunnel—with 300- and 500-m radii of curvature with different configurations—horizontal and vertical polarizations at 920, 2400, and 5705 MHz, respectively. Based on the measurements, statistical metrics of propagation loss and shadow fading (path-loss exponent, shadow fading distribution, autocorrelation, and crosscorrelation) in all the measurement cases are extracted. Then, the large-scale fading characteristics in the curved subway tunnels are compared with the cases of road and railway tunnels, the other main rail traffic scenarios, and some “typical” scenarios to give a comprehensive insight into the propagation in various scenarios where the intelligent transportation systems are deployed. Moreover, for each of the large-scale fading parameters, extensive analysis and discussions are made to reflect the physical laws behind the observations. The quantitative results and findings are useful to realize intelligent transportation systems in the subway system.

Moments of Macrodiversity SC Receiver with Two Microdiversity EGC Receivers over Gamma Shadowed Rician Multipath Fading Channel

SC macrodiversity system with dual EGC microdiversity receivers operating over shadowed multipath fading environment is considered. Received signal experiences correlated Gamma long term fading and Rician short term fading. SC macrodiversity receiver reduces Gamma large scale fading effects, while EGC microdiversity receivers mitigate short term fading effects on system performance. Closed form expressions for arbitrary order output moments of macrodiversity structure output signal is evaluated. Numerical results, obtained for arbitrary order output moments are presented graphically to show Gamma long term fading severity effect, correlation coefficient effects and Rician factor effect.

Medium Access Probability of Cognitive Radio Network at 1900 MHz and 2100 MHz

Conventionally fading analysis of wireless LAN or MAN means small scale fading i.e. wide fluctuation of received signal with small variation of time and distance. In analysis of fading channel we consider the received signal in power, voltage or SNR as a random variable then statistical probability density function (pdf) like Rayleigh, Rician or Nakagami-m is used to get the probability of different phenomena. Most of the pdf is governed by two parameters: mean and variance of the random variable. In recent literature the mean value is taken constant but in this paper we consider the mean value as a slowly varying random variable and depend on the parameters of large scale fading. In this paper the concept of large and small scale fading is combined, in analysis of performance of cognitive radio network in context of medium access probability specially at 1900MHz and 2100MHz.

On the Secrecy Enhancement With Low-Complexity Large-Scale Transmit Selection in MIMO Generalized Composite Fading

This letter proposes a low-complexity antenna selection scheme based on large-scale fading to enhance the secrecy performance in independent and non-identical generalized-K fading channels. New analytical results for the secrecy outage probability and the probability of non-zero secrecy capacity are derived when the receiving terminals employ the maximum ratio combining scheme. In addition, simple and insightful approximations are obtained for the secrecy outage probability in the high signal to noise ratio regime, which shed light on the impact of key system parameters on the secrecy performance. Simulation results are provided to validate the analytical expressions.

Downlink Rate Analysis for Virtual-Cell Based Large-Scale Distributed Antenna Systems

Despite substantial rate gains achieved by coordinated transmission from a massive amount of geographically distributed antennas, the resulting computational cost and channel measurement overhead could be unaffordable for a large-scale distributed antenna system (DAS). A scalable signal processing framework is therefore highly desirable, which, as recently demonstrated in \cite{Dai_TWireless}, could be established based on the concept of virtual cell. In a virtual-cell based DAS, each user chooses a few closest base-station (BS) antennas to form its virtual cell, that is, its own serving BS antenna set. In this paper, we focus on a downlink DAS with a large number of users and BS antennas uniformly distributed in a certain area, and aim to study the effect of the virtual cell size on the average user rate. Specifically, by assuming that maximum ratio transmission (MRT) is adopted in each user's virtual cell, the achievable ergodic rate of each user is derived as an explicit function of the large-scale fading coefficients from all the users to their virtual cells, and an upper-bound of the average user rate is established, based on which a rule of thumb is developed for determining the optimal virtual cell size to maximize the average user rate. The analysis is further extended to consider multiple users grouped together and jointly served by their virtual cells using zero-forcing beamforming (ZFBF). In contrast to the no-grouping case where a small virtual cell size is preferred, it is shown that by grouping users with overlapped virtual cells, the average user rate can be significantly improved by increasing the virtual cell size, though at the cost of a higher signal processing complexity.

Optimal Precoder Design Maximizing the Worst-Case Average Received SNR for Massive Distributed MIMO Systems

In this letter, we consider a distributed multiple-input multiple-output (D-MIMO) system, where the channel is flat fading and may be correlated, and experiences both small and large-scale fading. We assume that the full knowledge of channel state information (CSI) is available at the receiver and only the first-and second-order statistics of the channel are available at the transmitter. For such a system employing the linear zero-forcing (ZF) receiver, an efficient linear precoding technique is proposed to optimize the worst-case performance of average signal-to-noise ratio (SNR) at the receiver and a simple closed-form optimal precoder is derived. In addition, under some realistic assumptions, some significant asymptotic properties are established for the massive D-MIMO system designed in this letter. Computer simulations verify our theoretic analysis and show that our presented optimal system attains significant performance gains over the currently available equal power-loading system.

Approximate ergodic capacity of multiuser massive multiple input multiple output in a Rayleigh fading uplink channel with variance profile

The effect of large-scale fading variations over massive multiple input multiple output (MIMO) antenna array on sum-capacity of massive MIMO system has not been thoroughly investigated in the literature. This study considers a multiuser massive MIMO system with a Rayleigh fading channel that takes into account large-scale fading variations over base station antenna array. Two scenarios are investigated: distributed antenna base station and linear array base station. In both scenarios, the authors obtain an approximate closed-form expressions for uplink ergodic sum-capacity for both low and high signal-to-noise ratio (SNR) regimes. It is shown that the proposed closed-form expressions are accurate in a wide range of practical SNRs; for example, the approximation error is below 10% for SNRs lower than −10 dB and higher than 17 dB for low-SNR and high-SNR closed-form expressions, respectively. It is also shown that the proposed closed-form expressions are not only valid for massive MIMO scenario but also for a system with a few number of base station antennas. The numerical results show that the proposed closed-form expressions have much less computational complexity than previous works.

Selection of the Best Two-Hop AF Wireless Link under Multiple Antenna Schemes over a Fading Channel

In evaluating the performance of a dual-hop wireless link, the effects of large and small scale fading has to be considered. To overcome this fading effect, several schemes, such as multiple-input multiple-output (MIMO) with orthogonal space time block codes (OSTBC), different combining schemes at the relay and receiving end, and orthogonal frequency division multiplexing (OFDM) are used in both the transmitting and the relay links. In this paper, we first make compare the performance of a two-hop wireless link under a different combination of space diversity in the first and second hop of the amplify-and-forward (AF) case. Our second task in this paper is to incorporate the weak signal of a direct link and then by applying the channel model of two random variables (one for a direct link and another for a relayed link) we get very impressive result at a low signal-to-noise ratio (SNR) that is comparable with other models at a higher SNR. Our third task is to bring other three schemes under a two-hop wireless link: use of transmit antenna selection (TAS) on both link with weak direct link, distributed Alamouti scheme in two-hop link and single relay antenna with OFDM subcarrier. Finally, all of the schemes mentioned above are compared to select the best possible model. The main finding of the paper is as follows: the use of MIMO on both hops but application TAS on both links with weak direct link and the full rate OFDM with the sub-carrier for an individual link provide a better result as compared to other models.

A soft-hard combination-based cooperative spectrum sensing scheme for cognitive radio networks.

In this paper we propose a soft-hard combination scheme, called SHC scheme, for cooperative spectrum sensing in cognitive radio networks. The SHC scheme deploys a cluster based network in which Likelihood Ratio Test (LRT)-based soft combination is applied at each cluster, and weighted decision fusion rule-based hard combination is utilized at the fusion center. The novelties of the SHC scheme are as follows: the structure of the SHC scheme reduces the complexity of cooperative detection which is an inherent limitation of soft combination schemes. By using the LRT, we can detect primary signals in a low signal-to-noise ratio regime (around an average of −15 dB). In addition, the computational complexity of the LRT is reduced since we derive the closed-form expression of the probability density function of LRT value. The SHC scheme also takes into account the different effects of large scale fading on different users in the wide area network. The simulation results show that the SHC scheme not only provides the better sensing performance compared to the conventional hard combination schemes, but also reduces sensing overhead in terms of reporting time compared to the conventional soft combination scheme using the LRT.

CHARACTERIZATION OF AN OFF-BODY CHANNEL AT 2.45 GHZ IN AN UNDERGROUND MINE ENVIRONMENT

Underground mines are challenging environments for off-body wireless communication, since the signal propagation is majorly affected by small scale and large scale fading. The use of multiple antennas at the transmitter and the receiver sides is a known technique to combat fading and enhance capacity. In this paper, the channel parameters of a 2 × 2 Multiple-Input Multiple- Output (MIMO) off-body system are investigated in an underground gold mine and compared to the Single-Input Single-Output (SISO) system parameters. Measurement campaigns were conducted using monopole antennas at a center frequency of 2.45 GHz for both Line Of Sight (LOS) and None Line of Sight (NLOS) scenarios. The measured frequency responses are converted into impulse responses through an Inverse Fourier Transform (IFT). The results show that for a constant transmitted power, the path loss exponents at NLOS are smaller than their counterpart values at LOS. The channel capacity values decrease as the propagation distance increases and when the link is obstructed at NLOS. The RMS delay spread is generally increasing with distance for both LOS and NLOS situations. When a fixed Signal-to-Noise Ratio (SNR) is assumed, MIMO topologies improved the SISO capacity by roughly 8 bps/Hz. The channel characterization results demonstrate that the MIMO configurations provided a remarkable improvement in terms of capacity, coherence bandwidth, and time delay spread compared to the SISO topologies.

Sum Rate Analysis of MU-MISO Systems with ZF Beamforming over Composite Fading Channels

The performance of multiuser multiple-input single-output (MU-MISO) systems is not only affected by small-scale multipath fading but also by large-scale fading (i.e., shadowing) and path loss. In this paper, we concentrate on the sum rate distribution of MU-MISO systems employing linear zero-forcing beamforming, accounting for both multipath fading and shadowing effects, as well as spatial correlation at the transmit and receiver sides. In particular, we consider the classical spatially correlated lognormal model and propose closed-form bounds on the distribution of the achievable sum rates in MU-MISO systems. With the help of these bounds, we derive a relationship between the interuser distance and sum rate corresponding to 10% of the cumulative distribution function under different environmental conditions. A practical conclusion from our results based on the considered system is that the effect of spatially correlated shadowing can be considered to be independent when the interuser distance is approximately five times the shadowing correlation distance. Furthermore, a detailed analysis of the effects of composite channel attenuation consisting of multipath fading and shadowing is also provided.

Experimental Characterization and Correlation Analysis of Indoor Channels at 15 GHz

The indoor radio channels at 15 GHz are investigated based on measurements. The large- and small-scale fading behaviors as well as the delay dispersion characteristics are discussed. It is found that the large-scale fading, RiceanK-factor, and delay spread can be described by log-normal distributions. Furthermore, both autocorrelation and cross correlation properties of the above parameters are analyzed and modeled. These parameters characterize fading and delay behaviors as well as their mutual dependency and can be used as empirical values for future wireless system design and simulation in 15 GHz short-range indoor channels.

Massive MIMO Channel-Aware Decision Fusion

In this paper, we provide a study of channel-aware decision fusion (DF) over a “virtual” multiple-input multiple-output (MIMO) channel in the large-array regime at the DF center (DFC). The considered scenario takes into account channel estimation and inhomogeneous large-scale fading between the sensors and the DFC. The aim is the development of (widely) linear fusion rules, as opposed to the unsuitable optimum log-likelihood ratio (LLR). The proposed rules can effectively benefit from performance improvement via a large array, differently from existing suboptimal alternatives. Performance evaluation, along with theoretical achievable performance and complexity analysis, is presented. Simulation results are provided to confirm the findings. Analogies and differences with uplink communication in a multiuser (massive) MIMO scenario are underlined.

Experimental Characterization of Radio Channel in Ruins Environment

This letter addresses the fading statistics of a radio propagation channel in a ruins environment, which is of vital importance for a pedestrian localization system based on cellphone signals. The channel of ruins is investigated in eight typical rescue scenarios at two common mobile communication bands (900 MHz and 1.8 GHz). To characterize the channel, an empirical path loss model is parameterized through experiments. Based on extensive measurements, large-scale fading characteristics at different polarizations and various antenna heights are examined and analyzed. Furthermore, the properties of small-scale fading including coherence bandwidth, decorrelation distance, and distance-dependent features are studied in this work. The results reported here can be applied to simulate the ruins channel and design the rescue system.

Large-Scale Fading Characteristics of Indoor Channel at 45-GHz Band

The channel measurement and modeling of large- scale fading characteristics of 45-GHz band in three typical indoor environments are investigated. An automatic measurement system owning a sufficient dynamic range is designed to overcome the prominent attenuation in 45-GHz band. After conducting extensive measurements, large-scale fading characteristics including path-loss exponent, cross-polarization discrimination ratio, and standard deviation of shadow fading are obtained and compared in different scenario setups. Goodness-of-fit test is applied to validate whether the assumed log-normal distribution can describe the shadow fading characteristics.

Large-scale fading behavior for a cellular network with uniform spatial distribution

Large-scale fading (LSF) between interacting nodes is a fundamental element in radio communications, responsible for weakening the propagation, and thus worsening the service quality. Given the importance of channel-losses in general, and the inevitability of random spatial geometry in real-life wireless networks, it was then natural to merge these two paradigms together in order to obtain an improved stochastical model for the LSF indicator. Therefore, in exact closed-form notation, we generically derived the LSF distribution between a prepositioned reference base-station and an arbitrary node for a multi-cellular random network model. In fact, we provided an explicit and definitive formulation that considered at once: the lattice profile, the users' random geometry, the effect of the far-field phenomenon, the path-loss behavior, and the stochastic impact of channel scatters. The veracity and accuracy of the theoretical analysis were also confirmed through Monte Carlo simulations.

多基站多用户MIMO系统下行链路基于准最大化和速率预编码和功率分配的双层交替迭代结构

In multi-cell multiuser MIMO systems, the precoder of maximizing sum-rate (MSR) is optimal in linear and sum rate senses. However, there is no closed-form solution for MSR. In this paper, an alternating iterative quasi-MSR precoder of approaching MSR is developed by transforming the MSR problem into the one maximizing the product of fractional quadratic functions. In addition, a low-complexity alternating grid search power allocation (AGSPA) method is designed based on MSR principle in the case of single receive antenna per user. Then, an alternating iterative structure (AIS) between the proposed quasi-MSR and the low-complexity AGSPA is proposed. From simulations, the proposed AIS performs much better than the four common precoders maximizing signal-to-leakage-and-noise ratio, block diagonalization, minimum mean square error, and rate maximization in terms of sum rate in the presence of large-scale fading and antenna correlation.

Practical Coarse Relay Site Planning: Performance Analysis Over Composite Fading/Shadowing Channels

Relay deployments promise to alleviate the limitations of conventional macrocell networks, such as poor indoor penetration and coverage holes in a cost-efficient way. In this context, the capacity of the wireless relay link between a relay node (RN) and its serving base station has a crucial impact on the end-to-end performance. The deployment flexibility of RNs, which mainly stems from the wireless relay link, compact physical characteristics, and low-power consumption, can be exploited by relay site planning (RSP) to overcome the limitations of the relay link and, thus, enhance the system performance. To this end, RSP is carried out via selecting an RN deployment location from a discrete set of alternatives considering the signal-to-interference-plus-noise ratio (SINR) on the relay link as the selection criterion. In practice, the so-called coarse RSP takes into account only large-scale fading due to shadowing. Nevertheless, as RNs are stationary, the wireless channels pertaining to relay deployments are subject to simultaneous impairments by both shadowing and multi-path fading, i.e., composite fading/shadowing. In this paper, we present the performance of coarse RSP that can be used for planning and dimensioning of two-hop cellular relay networks in composite fading/shadowing environments, where co-channel interference is also present. The relay link is modeled by Nakagami-lognormal distribution while the access link between a mobile terminal and its serving RN is modeled by Rician-lognormal distribution. Further, we provide an accurate analytical framework through closed-form expressions for relay link SINR, link rates, and end-to-end rate. Results show that coarse RSP can still yield high performance improvements in terms of both SINR and rate considering composite fading/shadowing channels. Moreover, coarse RSP is shown to effectively decrease the amount of fading on the relay link and, thus, mitigate the detrimental effects of composite fading/shadowing.

Measurement and Analytical Study of the Correlation Properties of Subchannel Fading for Noncontiguous Carrier Aggregation

The new multichannel/multicarrier technologies can potentially support much higher data rates in mobile multiple- access environments, such as carrier aggregation (CA) defined in the fourth-generation Long-Term Evolution Advanced enhance- ment. The correlation properties of noncontiguous subchannels are critical for the performance of CA, including cell coverage, frequency diversity, and channel state estimation. This paper has studied the correlation of the large-scale fading (LSF) and small-scale fading (SSF) of arbitrarily separated subchannels by realistic channel measurement and analytical modeling. We first obtain the subchannel correlation from the ultrawideband (UWB) channel measurement. This new approach avoids probing multiple subchannels simultaneously with channel sounders, which would be prohibitively complicated. The cross correlation of two distinct subchannels and the autocorrelation of a single subchannel are evaluated. Second, a new propagation model for the Nakagami-m fading in a multipath environment is proposed. The frequency- domain level crossing rate (FD-LCR) is defined and derived based on the propagation model. Then, a Markov chain for the frequency fading of the wideband channel is established, which can generate the correlated gain/power of subchannels in a wide bandwidth. Therefore, this model can be used as a fast simulator of CA over correlated subchannels. Finally, the cross correlation coefficients (CCCs) between two Nakagami-m fading subchannels obtained from the measurement and the proposed Markov model are compared with the analytical results in the literature. This paper provides important insights into the fading correlation of the multicarrier channels and can be applied to the design and simulation of indoor/outdoor multicarrier communications. Index Terms—Carrier aggregation (CA), channel measurement, channel modeling, CLEAN, fading, ultrawideband (UWB).

Study of Transmitted Power on Large Scale Fading Models in Ouagadougou, Burkina Faso

In modern cellular networks, electromagnetic prediction tools are mostly used in the planning and optimization of the capacity and coverage. This paper presents a comparative study of large scale fading models using GSM received signal in mobile cellular installation during the harmattan season in Ouagadougou, Burkina Faso, West-Africa. The study area is a typical suburban area. The land morphology is a savannah. The received power signal is carried out with help of Nemo Outdoor tools during a wide over-air test measurement. The large scale model of Hata Okumura is considered and compared to the data fit. The results have indicated that the theoretical Hata model may give a little agreement in the study area.