Energy Efficiency Analysis in Amplify-And-Forward and Decode-And-Forward Cooperative Networks
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In this paper, we have studied the energy efficiency of cooperative networks operating in either the fixed Amplify-and-Forward (AF) or the selective Decode-and-Forward (DF) mode. We consider the optimization of the M-ary quadrature amplitude modulation (MQAM) constellation size to minimize the bit energy consumption under given bit error rate (BER) constraints. In the computation of the energy expenditure, the circuit, transmission, and retransmission energies are taken into account. The link reliabilities and retransmission probabilities are determined through the outage probabilities under the Rayleigh fading assumption. Several interesting observations with practical implications are made. For instance, it is seen that while large constellations are preferred at small transmission distances, constellation size should be decreased as the distance increases. Moreover, the cooperative gain is computed to compare direct transmission and cooperative transmission.Keywords:
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Wireless channels are affected by short-term fading and long-term fading (shadowing). This shadowing must be into account overall when mobility is present in the wireless scenario. Using a compound fading model, it is studied capacity and diversity gain for Rayleigh fading with shadowing. This work provides wide quantitative results for rayleigh fading with shadowing and multiple input and multiple-output (MIMO) distributed system that is not previously reported. Moreover, it is compared the performance with and without shadowing.
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We propose the Optimum Power Allocation (OPA) scheme for Distributed Antenna Systems(DAS) in the time-varying Rayleigh fading channel. Recently, the OPA schemes which uses the Channel State Information (CSI) including a small scale (fast) fading have been proposed. However, the channel is changing vary fast over time due to small scale fading, therefore Bit Error Rate (BER) increases. Because of this reason, we derive the OPA for minimizing BER in DAS, which only uses a large scale fading to CSI and excepts a small scale fading. The simulation results show that the proposed OPA achieves better BER performance than conventional OPA considering a small scale fading in time-varying Rayleigh fading channel, and also has similar performance in Rayleigh flat-fading environment. The BER performance of proposed OPA which derived in Rayleigh fading channel is similar to minimum BER of Ricean fading channel which has small Line-of-Sight (LOS).
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This work models the fast fading characteristics in indoor to indoor and outdoor to indoor channels at 3.8 GHz. Starting from real world measurements, the fast fading characteristics are extracted and properly modeled, taking into account the nodes movement conditions. For static environments, the fast fading is modeled with the well known Ricean distribution. As expected the K-factor depends on the path loss. On the other hand, in mobile environments, the probability density function of the signal envelope is accurately described by the recently developed second-order scattering fading (SOSF) distribution. The SOSF distribution combines the Rayleigh, Ricean and double Rayleigh fading and any linear combinations of these. Our results show that the higher the mobility, the more severe is the fading.
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The paper addresses Rayleigh fading, primarily in the UHF band, that affects mobile systems such as cellular and personal communication systems (PCS). The paper itemizes the fundamental fading manifestations and types of degradation. Two types of fading, large-scale and small-scale, are described. Two manifestations of small-scale fading (signal dispersion and fading rapidity) are examined, and the examination involves two aspects: time and frequency. Two degradation categories are defined for dispersion: frequency selective fading and flat fading. Two degradation categories are defined for fading rapidity: fast and slow. A mathematical model using correlation and power density functions is presented. This model yields a symmetry to help view the Fourier transform and duality relationships that describe the fading phenomena.
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The effect of different degrees of both slow propagation fading and antenna fluctuations on the performance of communication systems in the presence of interference was analyzed. The performance of the system was evaluated from the aspect of the probability of error which in turn is related to the required system margin. The results are plotted for a large variety of fading conditions for both the desired and undesired signals and antenna fluctuations. Some typical results are given below: Propagation Fading with No Antenna Fluctuations [1] For greater than 85 percentage satisfactory operation, the fading of the additive interfering signal has negligible effect on the probability of error when the desired signal experiences "strong" or Rayleigh distributed type of fading. The system margin for 99 per cent of satisfactory operation when the desired signal is Rayleigh fading and whether or not the undesired signal is fading is 20 db. [2] When the fading in the desired path is normally distributed, the effect of a normal fading interference signal is to increase the system margin compared to a nonfading interfering signal by 1 db and 1.4 db for 90 and 99 per cent satisfactory operation, respectively. The system margin for 99 per cent satisfactory operation is 5.2 db and 6.6 db when the undesired signal is not fading and is normally fading, respectively. [3] When there is no fading in the desired signal path and the undesired signal is fading, the system margin for 99 per cent satisfactory operation is 5 db for normal fading and 6.8 db for Rayleigh fading. [4] The system margin for 99 per cent of satisfactory operation when the desired signal is normal fading and the undesired signal is Rayleigh fading is 15 db. The system margin must be increased to 20 db when the desired signal is Rayleigh fading and the undesired signal is normal fading. Propagation Fading and Antenna Fluctuations [1] For a good UHF airborne antenna, the approximate variance of 0.1 is calculated from the vertical and horizontal patterns. The system margin for 99 per cent satisfactory operation is 6.4 db for no propagation fading in the desired or undesired paths. When there is normal fading in both the transmission paths, the system margin for 99 per cent satisfactory operation increases to 8 db or only an increase of 1.6 db over the case for no propagation fading. [2] Increasing the antenna variance to 0.3 causes an increase in the system margin for 99 per cent satisfactory operation and normally distributed fading in both paths to 9.6 db or an increase of 1.6 db due to the increased antenna variance. [3] The percentage of satisfactory operation as a function of system margin for antenna variances equal to 0 and 0.1 for a Rayleigh fading desired signal and a normally fading undesired signal shows that above a 10-db system margin, the curves are indistinguishable. A 20-db system margin is needed for 99 per cent of satisfactory operation. [4] The effect of the antenna variance is very significant when the desired signal is normal fading and the undesired signal is Rayleigh fading. For 99 per cent of satisfactory operation, the system margin must be increased from 15 db to 26 db when the antenna variance is increased from 0 to 0.10.
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Minimum duration outages have been used to characterize time-dependent performance in analysis of quantities such as average duration of an outage, the probability of outage, and the frequency of outage, in lognormal shadow fading (Mandayam et al., 1996) and Rayleigh fading (Lai and Mandayam, 1997), respectively. In this paper, we compare and contrast the effect of minimum duration outages on fade margin selection in channels subject to lognormal shadow fading and Rayleigh fading. A comparative analysis of relevant minimum durations for lognormal shadow fading and Rayleigh fading reveals the widely different time-scales active in outage considerations for each type of fading. Further, it is observed that lognormal shadow fading impacts outage on a time-scale that is much larger than that due to Rayleigh fading. The distinct time-scales revealed by the analysis show that the time-scales determined by the application govern the relative importance of the type of fading.
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The radio channels between nodes of an indoor peer-to-peer network show specific fast fading characteristics. Depending on the mobility and on the scattering properties of the environment, different kinds of fading distributions can occur: Ricean fading between static nodes, but also Rayleigh or even double-Rayleigh fading between mobile nodes. We investigate fast fading in indoor peer-to-peer networks based on radio channel measurements. It turns out that the fading statistics change over time. While the predominant fading mechanism is a combination of Rayleigh and double-Rayleigh fading, Ricean fading also occasionally occurs. On top of that, indoors, the statistics of the fast fading change over time even for small-motions of the nodes, since the propagation environment is inhomogeneous. We comprehensively model these effects using a hidden Markov model, parameterized from our measurements. The model is validated, revealing a convincing fit between the model and the measurements.
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Several fading models, including the general fading model, the Rayleigh fading model, and the simple three-path fading model are simulated to study the effect of fading in radio communication links. These models are implemented in hardware using computer control. The effect of fading is demonstrated using speech as the test signal. For the Rayleigh fading model, it is found that, when the automatic gain control (AGC) of the receiver is off, the received signal fades as in real-life transmission. When the AGC is on, the effect of fading is not noticeable most of the times but the noise level goes up when fading occurs, as expected. When the fading is severe, it is found that AGC cannot totally compensate for the fading; this is also expected. Other models are tested in a similar manner and yield expected results.< >
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The recently proposed fluctuating two-ray (FTR) model is gaining momentum as a reference fading model in scenarios where two dominant specular waves are present. Despite the numerous research works devoted to the performance analysis under FTR fading, little attention has been paid to effectively understanding the interplay between the fading model parameters and the fading severity. According to a new scale defined in this work, which measures the hyper-Rayleigh character of a fading channel in terms of the amount of fading, the outage probability and the average capacity, we see that the FTR fading model exhibits a full hyper-Rayleigh behavior. However, the two-wave with diffuse power fading model from which the former is derived has only strong hyper-Rayleigh behavior, which constitutes an interesting new insight. We also identify that the random fluctuations in the dominant specular waves are ultimately responsible for the full hyper-Rayleigh behavior of this class of fading channels.
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