In this paper, we investigate both the bit error rate (BER) and outage performance of free-space optical (FSO) links over strong turbulence combined with various pointing error conditions. Considering atmospheric turbulence and pointing errors as main factors that deteriorate the quality of an optical link, we obtain a unified finite series approximation of the composite probability density function, which embraces generalized pointing error models. This approximation leads to new unified formulas for the BER and outage capacity of an FSO link, which account for the two possible detection mechanisms of intensity modulation/direct detection and heterodyne detection. Selected simulation results confirm that the newly derived approximations can give precise predictions of both the average BER and the outage capacity of FSO communication that are generally applicable to all environments.

In this paper, we deal with the performance analysis of a cumulative distribution function (CDF)-based multiuser scheduling scheme for downlink non-orthogonal multiple access (NOMA). With this scheme, the multiple users having relatively better channel gain are selected. We first statistically analyze the probability of user selection in terms of the weight of each user determining the time fraction. We confirm that equal weight ensures fair resource allocation. Under this equal weight condition for fairness, we derive the distribution of signal-to-interference-plus-noise ratio (SINR) when the specific user is selected. Simulation results show that with this scheme, the time resources can be allocated more fairly compared to the proportional fair scheduling scheme, especially under NOMA conditions. Further, by adjusting the weight of desired user, the average rate and selection probability can be controlled.

In this letter, we address the beamforming in full-duplex self-energy recycling relay system, where an amplify-and-forward relay delivers the source message to the destination in a single time slot while recycling a portion of its RF energy through energy harvesting. The signal received at the realy is divided into two circuits, one for the full-duplex relaying and the other for the energy harvesting. The beamforming vectors and the power division ratio at the relay are optimized to maximize the harvested energy while maintaining a certain reception quality threshold at the destination. Simulation results show various advantages and trade-offs of the proposed system.

Order statistics find applications in various areas including communications and signal processing. In this paper, we introduce new results of the joint statistics of partial products of ordered random variables (RVs) based on a Mellin-transform-based unified analytical framework. With the proposed approach, we can systematically derive the joint statistics of any partial products of ordered statistics, in terms of the Mellin transform and the probability density function (PDF). Our Mellin-transform-based approach can apply when all the K-ordered RVs are involved even for more complicated cases, when only the Ks (Ks <; K) best RVs are also considered. In addition, the closed-form expressions for the exponential RV special case are presented. As an application example, these results can apply to the performance analysis of various wireless communication systems over fading channels.

In this paper, we propose a threshold-based-multiple optical signal selection scheme (TMOS) for free-space optical systems based on wavelength division multiplexing. With the proposed TMOS, we can obtain higher spectral efficiency while reducing the potential increase in complexity of implementation caused by applying a selection-based beam selection scheme without a considerable performance loss. Here, to accurately characterize the performance of the proposed TMOS, we statistically analyze the characteristics with heterodyne detection technique over independent and identically distributed Log-normal turbulence conditions taking into considerations the impact of pointing error. Specifically, we derive exact closed-form expressions for the average bit error rate, and the average spectral efficiency by adopting an adaptive modulation. Some selected results shows that the average spectral efficiency can be increased with TMOS while the system requirement is satisfied.

The outage probability of two‐way relaying network with orthogonal space–time block coded (STBC) transmission is analysed. Two sources equipped with different numbers of antennas send orthogonal STBC coded signals with different parameters. The channel gain between two antenna elements is assumed to be Rayleigh faded. The amplify‐and‐forward (AaF) relay with a single antenna repeats the received signal after power scaling. Exact expressions for the outage probabilities are provided, where the authors integrate out the outage probability over one of the three different regions on the plane defined by the inverses of channel gains. According to the orthogonal STBC code rates and the source and relay powers, one integration region is selected among the three. The closed‐form expression is verified via a set of comparisons with the simulation results. The authors also show that the maximum diversity is attained when the orthogonal STBC is applied at both sides of the two‐way channel.

A closed-form expression of the capture probability of generalized selection combining (GSC) RAKE receivers was introduced in [1]. The idea behind this new performance metric is to quantify how the remaining set of uncombined paths affects the overall performance both in terms of loss in power and increase in interference levels. In this previous work, the assumption was made that the fading is both independent and identically distributed from path to path. However, the average strength of each path is different in reality. In order to derive a closed-form expression of the capture probability over independent and non-identically distributed (i.n.d.) fading channels, we need to derive the joint statistics of ordered non-identical exponential variates. With this motivation in mind, we first provide in this paper some new order statistics results in terms of both moment generating function (MGF) and probability density function (PDF) expressions under an i.n.d. assumption and then derive a new exact closed-form expression for the capture probability GSC RAKE receivers in this more realistic scenario.

A dynamic decode-&-forward (DDF) is redesigned to meet the crucial requirements for type-2 user equipment (UE) relays which are being considered for next-generation cellular standards (e.g., LTE-Advanced). Toward this, the proposed modified DDF (M-DDF) realizes a fast jump-in relaying and sequential decoding where a subframe decoding is employed to yield a combination of energy/information/mixed combining, in conjunction with the random codeset for encoding and re-encoding at source and multiple UE relays, depending on their channel conditions. These additional features existing in the M-DDF circumvent the need of exchanging control messages between multiple relays and end user near cell boundary, which is not allowed for type-2 UE relays. Through comparison among the DDF with single and multiple orthogonal channels and M-DDF, the M-DDF bridges the gap between former relaying protocols with application to type-2 UE relays, offering reliable rate capacity with less channel usage and further improvement, if the early termination of decoding is made available.

In conventional multiuser parallel scheduling schemes, every scheduled user is interfering with every other scheduled user, which limits the capacity and performance of multiuser systems, and the level of interference becomes substantial as the number of scheduled users increases. Based on the above observations, we investigate the trade-off between the system throughput and the number of scheduled users through the exact analysis of the total average sum rate capacity and the average spectral efficiency. Our analytical results can help the system designer to carefully select the appropriate number of scheduled users to maximize the overall throughput while maintaining an acceptable quality of service under certain channel conditions.

Spread spectrum receivers with generalized selection combining (GSC) RAKE reception were proposed and have been studied as alternatives to the classical two fundamental schemes: maximal ratio combining and selection combining because the number of diversity paths increases with the transmission bandwidth. Previous work on performance analyses of GSC RAKE receivers based on the signal to noise ratio focused on the development of methodologies to derive exact closed-form expressions for various performance measures. However, some open problems related to the performance evaluation of GSC RAKE receivers still remain to be solved such as the exact performance analysis of the capture probability and an exact assessment of the impact of self-interference on GSC RAKE receivers. The major difficulty in these problems is to derive some joint statistics of ordered exponential variates. With this motivation in mind, we capitalize in this paper on some new order statistics results to derive exact closed-form expressions for the capture probability and outage probability of GSC RAKE receivers subject to self-interference over independent and identically distributed Rayleigh fading channels, and compare it to that of partial RAKE receivers.

We propose adaptive threshold based relay selection scheme for type-2 (user equipment) relay that requires only minimum 1-bit feedback, for which each relay reports to the source if its R-D (relay-destination) instantaneous channel gain is above threshold (i.e., available). A source then selects the best relay among those that yields the highest S-R (source-relay) instantaneous gain, provided threshold is adjusted according to a target signal-to-noise ratio (SNR). Exact and upper bound on the symbol-error rate (SER) are derived for M-PSK signaling to show that the proposed scheme provides the SER performance close to that of an optimal scheme . Further, it is shown that the spectral efficiency can be improved without degrading the link quality in terms of the SER, via adaptive selection between relay (S-R-D) link and direct (S-D) link based on their link SNRs.

For third generation systems and ultrawideband systems, RAKE receivers have been introduced due to the advantage of RAKE receivers which is their ability to combine different replicas of the transmitted signal arriving at different delays in a rich multipath environment. In principle, RAKE receivers combine all resolvable paths which gives the best performance in a rich diversity environment. However, this is usually costly in terms of hardware required as the number of RAKE fingers increases. Therefore, generalized selection combining (GSC) RAKE reception was proposed and has been studied by many researcher as an alternative to the classical two fundamental diversity schemes: maximal ratio combining and selection combining. Previous work on performance analyses of GSC RAKE receivers based on the signal to noise ratio focused on the development of methodologies to derive exact closed-form expressions for various performance measures. However, the remaining set of uncombined paths affect the overall performance both in terms of loss in power. Therefore, to have a full understanding of the performance of GSC RAKE receivers, we introduce in this paper the notion of capture probability, which is defined as the ratio of the captured power (essentially combined paths power) to that of the total available power. The major difficulty in these problems is to derive some joint statistics of ordered exponential variates. With this motivation in mind, we capitalize in this paper on some new order statistics results to derive exact closed-form expressions for the capture probability over independent and identically distributed Rayleigh fading channels.

Order statistics find applications in various areas of communications and signal processing. In this paper, we introduce an unified analytical framework to determine the joint statistics of partial sums of ordered random variables (RVs). With the proposed approach, we can systematically derive the joint statistics of any partial sums of ordered statistics, in terms of the moment generating function (MGF) and the probability density function (PDF). Our MGF-based approach applies not only when all theKordered RVs are involved but also when only theKs(KsK) best RVs are considered. In addition, we present the closed-form expressions for the exponential RV special case. These results apply to the performance analysis of various wireless communication systems over fading channels.

In this paper, we propose two new threshold based parallel multiuser scheduling schemes, namely (i) an on-off based scheduling (OOBS) scheme and (ii) a switched based scheduling (SBS) scheme. The objective is to reduce the complexity of implementation without a considerable performance loss in comparison with conventional selection based scheduling. While the OOBS scheme schedules all the users with an SNR above a fixed preselected signal to noise ratio (SNR) threshold, the SBS scheme schedules only the Ksusers with an acceptable SNR if there are enough acceptable users. Otherwise, the scheduler selects the best Ksusers. We analyze statistical characteristics and performances of these proposed schemes. Numerical results show that the proposed schemes can offer some decrease in the average feedback load without an important degradation in average bit error rate and average spectral efficiency.

In this paper, we propose threshold-based power allocation algorithms for a recently proposed down-link parallel multiuser scheduling (SBS) scheme. The proposed algorithms re-allocate the extracted excess transmit power from some acceptable users to those unacceptable users among scheduled users. After the power allocation process, more unacceptable users will enjoy acceptable link reliability as the number of effective acceptable users with an acceptable receive SNR level among the scheduled users is increased without any additional down-link transmit power. Some selected numerical results, show that the proposed power allocation algorithms offer a certain improvement in average spectral efficiency (ASE) and an increase in the average number of effective acceptable users. Although the average bit error rate (BER) performance degrades, especially when the average SNR is close to the SNR threshold, this average BER performance still meets the average BER requirement.

Abstract We introduce in this paper a new adaptive power‐controlled diversity combining scheme that reduces the average transmitted power of the mobile units (MUs) while meeting a certain minimum required quality of service. The key idea is (i) to collect and combine all the available diversity paths at the base station (BS) and then (ii) to request the MU to increase or decrease its transmitted power just to track the required target signal‐to‐noise ratio (SNR). Four power control variants accounting for practical implementation constraints including discrete power levels and transmitter gain saturation are proposed and studied. Some selected numerical results show that the proposed scheme offers considerable savings in the transmitted power levels over a wide SNR range but amplifier saturation leads to a violation of the target BER requirement in the low average SNR range. Additional numerical examples show that the power control variants that take into account practical implementation constraints conserve the main features of the ideal continuous power algorithm. Copyright © 2007 John Wiley & Sons, Ltd.

In this paper, we propose two new threshold based parallel multiuser scheduling schemes, namely (i) an on-off based scheduling (OOBS) scheme and (ii) a switched based scheduling (SBS) scheme. The objective is to reduce the complexity of implementation without a considerable performance loss in comparison with selection based scheduling. While the OOBS scheme schedules all the users with an SNR above a fixed preselected signal to noise ratio (SNR) threshold, the SBS scheme schedules only the Ks users with an acceptable SNR if there are enough acceptable users. Otherwise, the scheduler selects the best Ks users. We analyze statistical characteristics and performances of these proposed schemes. Numerical results show that the proposed schemes can offer some decrease in the average number of feedback load without an important degradation in average bit error rate and average spectral efficiency.

We introduce in this paper a new adaptive power-controlled diversity combining scheme that reduces the average transmitted power of the mobile units while meeting a certain minimum required quality of service. The key idea is (i) to collect and combine all the available diversity paths at the base station and then (ii) to request the mobile unit to increase or decrease its transmitted power just to track the required target signal-to-noise ratio (SNR). Four power control variants accounting for practical implementation constraints including discrete power levels and transmitter gain saturation are proposed and studied. Some selected numerical results, show that the proposed scheme offers considerable savings in the transmitted power levels over a wide SNR range but amplifier saturation leads to a violation of the target BER requirement in the low average SNR range. Additional numerical examples, show that the power control variants that take into account practical implementation constraints conserve the main features of the ideal continuous power algorithm