Showing papers in "IEEE Wireless Communications Letters in 2014"
TL;DR: An analytical approach to optimizing the altitude of LAPs to provide maximum radio coverage on the ground shows that the optimal altitude is a function of the maximum allowed pathloss and of the statistical parameters of the urban environment, as defined by the International Telecommunication Union.
Abstract: Low-altitude aerial platforms (LAPs) have recently gained significant popularity as key enablers for rapid deployable relief networks where coverage is provided by onboard radio heads. These platforms are capable of delivering essential wireless communication for public safety agencies in remote areas or during the aftermath of natural disasters. In this letter, we present an analytical approach to optimizing the altitude of such platforms to provide maximum radio coverage on the ground. Our analysis shows that the optimal altitude is a function of the maximum allowed pathloss and of the statistical parameters of the urban environment, as defined by the International Telecommunication Union. Furthermore, we present a closed-form formula for predicting the probability of the geometrical line of sight between a LAP and a ground receiver.
2,153 citations
TL;DR: The proposed hybrid precoding scheme, named phased-ZF (PZF), essentially applies phase-only control at the RF domain and then performs a low-dimensional baseband ZF precoding based on the effective channel seen from baseband.
Abstract: Massive multiple-input multiple-output (MIMO) is envisioned to offer considerable capacity improvement, but at the cost of high complexity of the hardware. In this paper, we propose a low-complexity hybrid precoding scheme to approach the performance of the traditional baseband zero-forcing (ZF) precoding (referred to as full-complexity ZF), which is considered a virtually optimal linear precoding scheme in massive MIMO systems. The proposed hybrid precoding scheme, named phased-ZF (PZF), essentially applies phase-only control at the RF domain and then performs a low-dimensional baseband ZF precoding based on the effective channel seen from baseband. Heavily quantized RF phase control up to 2 bits of precision is also considered and shown to incur very limited degradation. The proposed scheme is simulated in both ideal Rayleigh fading channels and sparsely scattered millimeter wave (mmWave) channels, both achieving highly desirable performance.
653 citations
TL;DR: In this paper, the power-limited throughput of a communication system utilizing incremental redundancy (INR) hybrid automatic repeat request (HARQ) is studied. But the throughput in the case of variable-length coding is not considered.
Abstract: This letter studies the power-limited throughput of a communication system utilizing incremental redundancy (INR) hybrid automatic repeat request (HARQ). We use some recent results on the achievable rates of finite-length codes to analyze the system performance. With codewords of finite length, we derive closed-form expressions for the outage probabilities of INR HARQ and study the throughput in the cases with variable-length coding. Moreover, we evaluate the effect of feedback delay on the throughput and derive sufficient conditions for the usefulness of the HARQ protocols, in terms of power-limited throughput. The results show that, for a large range of HARQ feedback delays, the throughput is increased by finite-length coding INR HARQ, if the sub-codeword lengths are properly adapted.
207 citations
TL;DR: This work describes the downlink rate distribution at a typical user equipment in a heterogeneous cellular network (HetNet), where shadowing, following any general distribution, impacts cell selection while fading does not, and studies the impact of shadowing on load balancing in terms of the optimal per-tier selection bias needed for rate maximization.
Abstract: Considering both small-scale fading and long-term shadowing, we characterize the downlink rate distribution at a typical user equipment (UE) in a heterogeneous cellular network (HetNet), where shadowing, following any general distribution, impacts cell selection while fading does not. Most prior work either ignores the impact of channel randomness on cell selection or lumps all the sources of randomness into a single variable, with cell selection based on the instantaneous signal strength, which is unrealistic. As an application of the results, we study the impact of shadowing on load balancing in terms of the optimal per-tier selection bias needed for rate maximization.
195 citations
TL;DR: The tradeoff between energy efficiency and spectral efficiency in device-to-device communications underlaying cellular networks with uplink channel reuse and a distributed energy-efficient resource allocation algorithm is proposed by exploiting the properties of the nonlinear fractional programming.
Abstract: In this letter, we investigate the tradeoff between energy efficiency (EE) and spectral efficiency (SE) in device-to-device communications underlaying cellular networks with uplink channel reuse. The resource allocation problem is modeled as a noncooperative game, in which each user equipment is self-interested and wants to maximize its own EE. Given the SE requirement and maximum transmission power constraints, a distributed energy-efficient resource allocation algorithm is proposed by exploiting the properties of the nonlinear fractional programming. The relationships between the EE and SE tradeoff of the proposed algorithm and system parameters are analyzed and verified through computer simulations.
170 citations
TL;DR: This letter investigates the joint multicast transmit beamforming and receive power splitting problem for minimizing the transmit power of the BS subject to signal-to-noise ratio (SNR) and energy harvesting constraints at each receiver.
Abstract: This letter considers simultaneous wireless information and power transfer (SWIPT) in multiple-input single-output (MISO) multicasting systems where each receiver is equipped with a power splitting device and can receive both information and energy from the base station (BS) continuously at the same time. We investigate the joint multicast transmit beamforming and receive power splitting problem for minimizing the transmit power of the BS subject to signal-to-noise ratio (SNR) and energy harvesting constraints at each receiver. Both scenarios of perfect and imperfect channel state information (CSI) at the BS are studied. Due to non convexity of the problems, we use semidefinite relaxation (SDR) technique to solve the problems. Interestingly, we show that the SDR is in fact tight in certain scenarios.
170 citations
TL;DR: The problem of secure transmission in two-hop amplify-and-forward untrusted relay networks and the ergodic secrecy capacity (ESC) is considered and compact expressions for the ESC in the high signal-to-noise ratio regime are presented.
Abstract: We consider the problem of secure transmission in two-hop amplify-and-forward untrusted relay networks. We analyze the ergodic secrecy capacity (ESC) and present compact expressions for the ESC in the high signal-to-noise ratio regime. We also examine the impact of large scale antenna arrays at either the source or the destination. For large antenna arrays at the source, we confirm that the ESC is solely determined by the channel between the relay and the destination. For very large antenna arrays at the destination, we confirm that the ESC is solely determined by the channel between the source and the relay.
155 citations
TL;DR: The single nearest and N-nearest RRH association strategies are presented, and the corresponding impact on the ergodic capacity of C-RANs is analyzed, where RRHs are distributed according to a stationary point process.
Abstract: Characterizing user to remote radio head (RRH) association strategies in cloud radio access networks (C-RANs) is critical for performance optimization. In this letter, the single nearest and N-nearest RRH association strategies are presented, and the corresponding impact on the ergodic capacity of C-RANs is analyzed, where RRHs are distributed according to a stationary point process. Closed-form expressions for the ergodic capacity of the proposed RRH association strategies are derived. Simulation results demonstrate that the derived uplink closed-form capacity expressions are accurate. Furthermore, the analysis and simulation results show that the ergodic capacity gain is not linear with either the RRH density or the number of antennas per RRH. The ergodic capacity gain from the RRH density is larger than that from the number of antennas per RRH, which indicates that the association number of the RRH should not be bigger than 4 to balance the performance gain and the implementation cost.
121 citations
TL;DR: An iterative technique is devised, the iterative Hungarian method (IHM), which can offer near-optimal performance with polynomial complexity in relay-aided device-to-device (D2D) communications underlying cellular networks.
Abstract: The joint relay selection and related subchannel and power allocation problem is investigated for relay-aided device-to-device (D2D) communications underlying cellular networks. We show the optimal power allocation problem can be solved in a closed-form. Considering that the associated relay selection and subchannel assignment problem is NP-complete, we devise an iterative technique, the iterative Hungarian method (IHM). Interestingly, numerical results show that the proposed technique can offer near-optimal performance with polynomial complexity.
116 citations
TL;DR: Comparisons between the half-duplex and FD systems are carried out, where analytical and simulation results reveal that the FD system is superior in high source power regions with either fixed or large power constraints at the relay.
Abstract: This paper investigates the optimal power allocation scheme and corresponding capacity limit of a full-duplex (FD) dual-hop amplify-and-forward relay system under residual self-interference whose variance is proportional to the λ'th power of the transmitted power (0≤λ≤1). At first, the related optimization problems are shown to be quasiconcave under both per-node and sum power constraints. Given the non-linearity of the derivative, bisection is then proposed to obtain the optimal power strategies. The capacity and optimal schemes are then analyzed in different high power regions. Specifically, we apply the dominant balance method to show that full power at the relay is suboptimal when its power constraint approaches a large value. Following a similar approach, we then show that the multiplexing gain of the FD scheme with the optimal allocation is 1/[1+λ]. Comparisons between the half-duplex and FD systems are finally carried out, where analytical and simulation results reveal that the FD system is superior in high source power regions with either fixed or large power constraints at the relay.
114 citations
TL;DR: In this paper, a simple yet powerful and versatile analytical framework for approximating the SIR distribution in the downlink of cellular systems is introduced, which is based on the mean interference-to-signal ratio and yields the horizontal gap (SIR gain).
Abstract: We introduce a simple yet powerful and versatile analytical framework for approximating the SIR distribution in the downlink of cellular systems. It is based on the mean interference-to-signal ratio and yields the horizontal gap (SIR gain) between the SIR distribution in question and a reference SIR distribution. As applications, we determine the SIR gain for base station silencing, cooperation, and lattice deployment over a baseline architecture that is based on a Poisson deployment of base stations and strongest-base station association. The applications demonstrate that the proposed approach unifies several recent results and provides a convenient framework for the analysis and comparison of future network architectures and transmission schemes, including amorphous networks where a user is served by multiple base stations and, consequently, (hard) cell association becomes less relevant.
TL;DR: An optimal resource allocation scheme is proposed that jointly selects the training duration, the training signal power, and the data signal power in order to maximize the sum spectral efficiency for a given total energy budget spent in a coherence interval.
Abstract: We consider the uplink of multicell multiple-input-multiple-output systems, where the base stations (BSs), which are equipped with massive arrays, simultaneously serve several terminals in the same frequency band. We assume that the BS estimates the channel from uplink training and then uses the maximum ratio combining technique to detect the signals transmitted from all terminals in its own cell. We propose an optimal resource allocation scheme that jointly selects the training duration, the training signal power, and the data signal power in order to maximize the sum spectral efficiency for a given total energy budget spent in a coherence interval. Numerical results verify the benefits of the optimal resource allocation scheme. Furthermore, we show that more training signal power should be used at low signal-to-noise ratios (SNRs) and vice versa at high SNRs. Interestingly, for the entire SNR regime, the optimal training duration is equal to the number of terminals.
TL;DR: This letter presents the improvement of a multipath-assisted tracking approach using information about the relevance of deterministic multipath components in an environment using information fed to a tracking filter as an observation noise model.
Abstract: In a radio propagation channel, deterministic reflections carry important position-related information. With the help of prior knowledge such as a floor plan, this information can be exploited for indoor localization. This letter presents the improvement of a multipath-assisted tracking approach using information about the relevance of deterministic multipath components in an environment. This information is fed to a tracking filter as an observation noise model. It is estimated from a few training signals between anchors and an agent at known positions. Tracking results are presented for measurements in a partial non-line-of-sight environment. At a bandwidth of 2 GHz, an accuracy of 4 cm can be achieved for over 90% of the positions if additional channel information is available. Otherwise, this accuracy is only possible for about 45% of the positions. The covariance of the estimation matches closely to the corresponding Cramer–Rao lower bound.
TL;DR: Closed-form analytical expressions of secrecy outage probability and the probability of nonzero secrecy capacity have been derived and the validity of these analysis models is verified by simulation results.
Abstract: An underlay cognitive radio unit over Nakagami- m fading channel, which consists of a source S, a secondary user (SU), and an eavesdropper who wants to eavesdrop the information between S and SU, is considered. The transmit power of S is simultaneously adjusted according to the channel state information of S-PU link and a given threshold interference power that the primary user can tolerate. Closed-form analytical expressions of secrecy outage probability and the probability of nonzero secrecy capacity have been derived. The validity of our analysis models is verified by simulation results.
TL;DR: It is shown that for a wide range of fading distributions, no channel state information is needed at the transmitters, provided that the receiver has access to some statistical channel knowledge and is equipped with multiple antennas.
Abstract: This letter studies a multiple-access transmission scheme that exploits interference for an efficient function computation in sensor networks. The central question is how much channel knowledge is generally needed and how the channel estimation effort can significantly be reduced. It is first shown that the channel magnitude at the transmitters is sufficient to achieve the same performance as with full channel state information. It is further shown that for a wide range of fading distributions, no channel state information is needed at the transmitters, provided that the receiver has access to some statistical channel knowledge and is equipped with multiple antennas.
TL;DR: This letter proposes a unified differential spatial modulation (DSM) architecture, where a flexible rate-diversity tradeoff is achieved, while enabling a simple single-RF transmitter structure along with non-coherent detection that dispenses with channel estimation at the receiver.
Abstract: This letter proposes a unified differential spatial modulation (DSM) architecture, where a flexible rate-diversity tradeoff is achieved, while enabling a simple single-RF transmitter structure along with non-coherent detection that dispenses with channel estimation at the receiver. In our proposed scheme, by assigning a set of sparse complex-valued antenna-index matrices, only one transmit antenna element is activated during each symbol interval and then a phase-shift keying (PSK) symbol is transmitted from the activated antenna element. The explicit benefit of the proposed scheme's universal DSM framework is ability to strike a balance between previous DSM schemes, such as the symbol-based and the block-based DSM schemes. Moreover, to attain a useful attainable diversity gain, we further extend the proposed DSM scheme in a manner that permits flexible planning of the number of symbols employed per antenna-index block.
TL;DR: A case study involving both monostatic and multistatic ultra-wideband SRs for indoor environments is presented by jointly considering the wireless medium, ranging technique, and tracking algorithm.
Abstract: Sensor radars (SRs) are important for a variety of applications requiring passive tracking of moving targets. The accuracy of passive tracking is severely degraded by wireless propagation impairments such as multipath, clutter, and non line-of-sight conditions, especially in indoor environments. These impairments can be alleviated by exploiting the multiple sensing and smart processing of radar signals. In this letter, we aim to illustrate the dependence of sensor topologies, waveform processing methods, and tracking algorithm parameters on SR performance. A case study involving both monostatic and multistatic ultra-wideband SRs for indoor environments is presented by jointly considering the wireless medium, ranging technique, and tracking algorithm.
TL;DR: This letter proposes a multiple-RF antenna switching operation that allows for a longer pulse shape than that enabled by a single-RF SM scheme, and demonstrates that upon increasing the number of transmit antenna elements, the SM scheme is capable of outperforming the classic MIMO systems.
Abstract: In this letter, we investigate the effects of antenna switching on band-limited spatial modulation (SM), where the employment of an SM-specific practical time-limited shaping filter is taken into account. More specifically, we evaluate the SM scheme's performance penalty imposed on a bandwidth efficiency, which arises due to the time-limited pulse transmissions. Furthermore, to combat this limitation while maintaining lower RF chains than transmit antenna elements, we propose a multiple-RF antenna switching operation that allows us to exploit a longer pulse shape than that enabled by a single-RF SM scheme. It is demonstrated in our simulations that upon increasing the number of transmit antenna elements, the SM scheme is capable of outperforming the classic MIMO systems, under the assumptions of a low number of RF chains and realistic pulse shaping.
TL;DR: The analytical results herein confirm the significant potential advantages of the C-RAN architecture in the presence of full-duplex base stations, as long as sufficient fronthaul capacity is available and appropriate mobile station scheduling, or successive interference cancellation at the mobile stations, is implemented.
Abstract: The conventional design of cellular systems prescribes the separation of uplink and downlink transmissions via time-division or frequency-division duplex. Recent advances in analog and digital domain self-interference interference cancellation challenge the need for this arrangement and open up the possibility to operate base stations, particularly low-power ones, in a full-duplex mode. As a means to cope with the resulting downlink-to-uplink interference among base stations, this letter investigates the impact of the Cloud Radio Access Network (C-RAN) architecture. The analysis follows an information theoretic approach based on the classical Wyner model. The analytical results herein confirm the significant potential advantages of the C-RAN architecture in the presence of full-duplex base stations, as long as sufficient fronthaul capacity is available and appropriate mobile station scheduling, or successive interference cancellation at the mobile stations, is implemented.
TL;DR: A "Feller-paradox" like relationship is established between the area of the association cell containing origin and that of a typical association cell and the resulting association cells in stationary association.
Abstract: Characterizing user to access point (AP) association strategies in heterogeneous cellular networks (HetNets) is critical for their performance analysis, as it directly influences the load across the network. In this letter, we introduce and analyze a class of association strategies, which we term stationary association, and the resulting association cells. For random HetNets, where APs are distributed according to a stationary point process, the area of the resulting association cells are shown to be the marks of the corresponding point process. Addressing the need of quantifying the load experienced by a typical user, a "Feller-paradox" like relationship is established between the area of the association cell containing origin and that of a typical association cell. For the specific case of Poisson point process and max power/\SINR association, the mean association area of each tier is derived and shown to increase with channel gain variance and decrease in the path loss exponents of the corresponding tier.
TL;DR: In this letter, a novel transmit-receive antenna pair selection (TRAPS) scheme is proposed for bidirectional full-duplex communications between two nodes, where each node is equipped with two antennas, used for either transmission or reception.
Abstract: In this letter, a novel transmit-receive antenna pair selection (TRAPS) scheme is proposed for bidirectional full-duplex (FD) communications between two nodes, where each node is equipped with two antennas, used for either transmission or reception. Specifically, one transmit and receive antenna combination is selected based on two system performance criteria: 1) maximum sum-rate (Max-SR), and 2) minimum symbol-error-rate (Min-SER). For the Max-SR approach, we obtain a sum-rate upper bound. And for the Min-SER, a SER lower bound is derived, by introducing a simple near-optimal Min-Max criterion. Finally, we compare the proposed TRAPS approaches with the traditional FD communication system in terms of average sum-rate and SER, respectively. Simulation results verify our analysis, and show a significant performance improvement.
TL;DR: The problem of maximizing the weighted sum-rate maximization across multiple clusters is tackled by deriving an iterative algorithm that converges to a locally optimal point of the problem and significantly outperforms the existing approach based on intra-cluster optimization.
Abstract: This work studies the joint design of precoding and fronthaul compression for the downlink of multi-cluster cloud radio access networks (C-RANs). In these systems, each cluster of radio units (RUs) is managed by a different control unit (CU) through fronthaul links of limited capacity. Each CU performs baseband processing, including channel coding and linear precoding, for all the connected RUs, and then compresses the precoded signals for transmission to the RUs over the corresponding fronthaul links. In a recent work, it was shown that joint, rather than conventional separate, compression of the signals of different RUs leads to significant performance gains. However, the approach therein was limited to the intra-cluster optimization of precoding and joint compression, hence leading to significant performance loss in the presence of mutually interfering clusters. This work proposes instead to perform the inter-cluster optimization of precoding and joint compression. Specifically, the problem of maximizing the weighted sum-rate maximization across multiple clusters is tackled by deriving an iterative algorithm that converges to a locally optimal point of the problem. From numerical results, it is confirmed that the proposed scheme significantly outperforms the existing approach based on intra-cluster optimization.
TL;DR: It is proved that correlation between channels works in favor at medium to high signal-to-noise ratio (SNR), however it degrades the performance at low SNR.
Abstract: This letter investigates the secrecy performance of multiple-input single-output (MISO) wiretap channels when the eavesdropper channel is correlated with the main one. Assuming that the transmitter employs a transmit antenna selection (TAS) scheme, we derive a tight closed-form expression for the secrecy outage probability and analytically evaluate the secrecy performance. An asymptotic outage expression is also derived, from which a diversity analysis is carried out. We prove that correlation between channels works in favor at medium to high signal-to-noise ratio (SNR), however it degrades the performance at low SNR.
TL;DR: This paper proposes two channel estimation schemes that completely remove pilot contamination in time-division duplex (TDD) multicell cellular systems, and the exact closed-form expression for average mean square error (MSE) of the proposed estimators is derived.
Abstract: This paper addresses the problem of channel estimation in time-division duplex (TDD) multicell cellular systems, where the performance of such systems is usually bounded by a bottleneck due to pilot contamination. We propose two channel estimation schemes that completely remove pilot contamination. The exact closed-form expression for average mean square error (MSE) of the proposed estimators is derived. More importantly, our proposed estimators do not need to know the second-order statistics of either desired user channels or interfering user channels. Finally, simulated results confirm gains over existing channel estimation schemes.
TL;DR: The analysis shows that Multi Channel RSS Average (MCRA) can indeed reduce the component of the RSS variability due to self-interference produced by strong and quasi-static multipath replicas of the received signal, while it is ineffective against fading components caused by obstacles blockage.
Abstract: This letter shows that the accuracy of Radio Signal Strength (RSS)-based ranging can be increased by averaging RSS samples collected on different RF channels. Starting from a multi-cluster propagation model, the analysis shows that Multi Channel RSS Average (MCRA) can indeed reduce the component of the RSS variability due to self-interference produced by strong and quasi-static multipath replicas of the received signal, while it is ineffective against fading components caused by obstacles blockage. The theory is backed up by a set of experimental results obtained in different scenarios that confirm the capability of MCRA to effectively reduce RSS variability and increase RSS-based ranging accuracy.
TL;DR: In this paper, the authors considered a wireless M2M communication scenario with a massive number of MTCs and showed how to dimension the pool of resources available to each MTC to guarantee the desired reliability of the report delivery within a given deadline.
Abstract: This letter considers a wireless M2M communication scenario with a massive number of M2M devices. Each device needs to send its reports within a given deadline and with certain reliability, e.g., 99.99%. A pool of resources available to all M2M devices is periodically available for transmission. The number of transmissions required by an M2M device within the pool is random due to two reasons-random number of arrived reports since the last reporting opportunity and requests for retransmission due to random channel errors. We show how to dimension the pool of M2M-dedicated resources in order to guarantee the desired reliability of the report delivery within the deadline. The fact that the pool of resources is used by a massive number of devices allows basing the dimensioning on the central limit theorem. The results are interpreted in the context of LTE, but they are applicable to any M2M communication system.
TL;DR: This letter addresses the issue of energy-efficient link selection and data transmission scheduling for delay-tolerant and data-intensive applications in MCC and proposes a scalable approximate dynamic programming (ADP) algorithm that does not require the statistics of exogenous stochastic information.
Abstract: Mobile cloud computing (MCC) converges mobile computing and Cloud computing for augmenting resource-poor mobile devices to run ??heavier?? applications. With the increasing ubiquity of mobile devices, MCC exhibits vast application potential in various areas. Energy-efficient data transmission is a key issue in MCC due to energy-poverty of mobile devices. In this letter, we address the issue of energy-efficient link selection and data transmission scheduling for delay-tolerant and data-intensive applications in MCC. We first formulate the problem as a discrete-time stochastic dynamic program (SDP) that aims to optimize both system throughput and energy consumption. To solve the formulated SDP, we then propose a scalable approximate dynamic programming (ADP) algorithm that does not require the statistics of exogenous stochastic information (e.g., data arrival). Simulation studies show that the proposed ADP algorithm can reduce the average energy consumed for delivering a packet by a maximum of over 40 percent compared to alternative minimum-delay and SALSA policies.
TL;DR: By manipulating both binary and non-binary resilient functions, a novel approach for generating security codes is introduced and their threshold probabilities are derived, which provides the security condition proof for the proposed security codes.
Abstract: By manipulating both binary and non-binary resilient functions, a novel approach for generating security codes is introduced and their threshold probabilities are derived, which provides the security condition proof for the proposed security codes. In particular, by taking advantage of matrix general inverse algorithms the encoding method of the proposed security codes is derived and is practically implementable due to low complexity. Experiments are conducted to examine the proposed security codes and the security system over Binary Symmetric Channel (BSC).
TL;DR: A non-stationary hidden Markov model (NS-HMM), in which the time-varying property of PU behavior is realized, and the results show that the NS-H MM outperforms existing HMM-based approaches.
Abstract: One of the critical challenges for secondary use of licensed spectrum is the accurate modeling of primary users' (PUs') stochastic behavior. However, the conventional hidden Markov models (HMMs) assume stationary state transition probability and fail to adequately describe PUs' dwell time distributions. In this letter, we propose a non-stationary hidden Markov model (NS-HMM), in which the time-varying property of PU behavior is realized. A variant of the Baum-Welch algorithm is developed to estimate the parameters of an NS-HMM. Finally, the performance of the proposed model is evaluated through experiments using real spectrum measurement data. The results show that the NS-HMM outperforms existing HMM-based approaches.
TL;DR: A simple tight closed-form approximation for ASEP over log-normal fading channel is derived that facilitates the computation of average symbol error rate for a broader class of coherent digital modulation schemes.
Abstract: The average symbol error probability(ASEP) is an important measure to evaluate the performance of different modulation techniques over fading channels. The phenomenon of both shadowing and slow fading in wireless communication system is generally characterized by log-normal probability distribution. Approximating log-normal integrals by constructing Taylor series expansion of second order, a simple tight closed-form approximation for ASEP over log-normal fading channel is derived. This expression facilitates the computation of average symbol error rate for a broader class of coherent digital modulation schemes. The efficiency of the expression is numerically examined and comparison of the results is carried out in relation to the existing approximate expressions.