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The outage performance of a dual-hop mobile ad hoc network (MANET), which employs amplify-and-forward (AF) relaying in a Rayleigh fading environment in the presence of a Poisson field of interferers at the destination, is investigated. All mobile users are assumed to follow the Poisson arrival model along the highway and can only communicate with each other via a stationary base station or a mobile relay station. Under this MANET model, a single-integral expression for the exact outage probability and a tight lower bound are both derived. The impacts of transmitting power, path-loss exponent, and spatial density of interferers on the performance of dual-hop systems are found. It is revealed that the outage probability of each hop, as a function of distance, transmitting power, etc., has a different impact on the end-to-end outage probability. The outage probability floor is derived when the transmission power at the source/relay becomes large. The optimal power allocation for the dual-hop link is obtained. The behavior of the outage probability during the communication process between the moving nodes is investigated. The analytical results are validated with Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

Broadband mobile communication systems experience fading over a wide frequency band, which induces severe intersymbol interference and drastic channel gain fluctuation, resulting in a time–frequency doubly selective fading channel gain. Most existing works on fading channel modeling assume wide-sense stationarity with respect to (w.r.t.) time and uncorrelated scattering w.r.t. delay. However, due to the time-varying movement of mobile terminals, fading is usually nonstationary w.r.t. time, i.e., the Doppler spectrum is nonstationary. In addition, electromagnetic signals reflected/scattered from the same clustter/object are correlated and may induce nonstationarity w.r.t. delay due to time-varying cluster environments, e.g., wind causes the movement of leaves, resulting in time-varying scattering of leaves. To model nonstationary broadband mobile fading channels, this paper introduces a two-dimensional evolutionary spectrum (2-D ES) approach, which is compatible with the power spectral density of a stationary process. Based on the 2-D ES, we also propose an estimation method to estimate the 2-D ES parameters from a trace (sample path) of a fading process. Furthermore, we show how to set up a simulator to generate a 2-D stationary or nonstationary fading process according to our developed 2-D ES framework. Using the 2-D ES approach, we analyze a class of Nakagami- $m$ channels which is nonstationary in both the time and frequency domains. Two examples are used to show the accuracy of our developed 2-D ES-based channel modeling approach. [ABSTRACT FROM AUTHOR]

Energy harvesting technique is a potential way for relay node energy supply in cooperative networks with respect to deployment flexibility and maintenance charge reduction. However, unlike traditional power source, it is intuitive that the fluctuation of harvested energy flow can degrade the benefit resulted from relay-aided cooperative transmission. To this end, this paper focuses on the performance analysis of an energy harvesting relay-aided cooperative network under slow fading channel from a perspective of outage behavior and attempts deriving the closed-form expression of outage probability of the proposed cooperative protocol. Compared with simple direct transmission protocol, it is proved that a multiplicative gain for improving system performance in terms of minimizing outage probability can be obtained by energy harvesting relay-aided cooperative transmission. In particular, if there are multiple available relay nodes around the environment or the energy-exhausted probability of relay node is small, it is very profitable to employ relay-aided cooperative transmission protocol. [ABSTRACT FROM PUBLISHER]

We consider providing broadband wireless services for passengers in a high-speed train and local users (low-mobility users) in the same OFDMA system. For the train, a two-hop architecture, under which, passengers communicate with base stations (BSs) via a mobile relay (MR) is employed. Then all passengers in the train can be treated as one large user represented by the MR. Since the channel conditions of the MR and local users are different in both large scale-fading and small-scale fading, we allocate the system resources over two time-scales for them. We formulate the resource allocation problem in the downlink OFDMA system as a capacity optimization problem for the MR subject to the sum capacity constraint of local users. We discuss the problem in two cases where perfect intercarrier interference (ICI) cancellation and no ICI cancellation are applied at the MR. The corresponding capacities and optimal power and subcarrier allocation (OPSA) policies are presented for each case. The capacities obtained in these two cases bound the achievable transmission rate of the MR and provide meaningful guidance for the application of ICI cancellation. Finally, the implementation issues of resource allocation policies are discussed to simplify application in practical scenarios. [ABSTRACT FROM PUBLISHER]

Widespread deployment of high-speed railways in recent years has created huge demand for high-mobility broadband wireless communications. To provide broadband wireless access for passengers on the train, a well-acknowledged approach is to apply a two-hop architecture, under which passengers communicate with base stations (BSs) via an access point (AP) installed in the train. We consider the uplink transmission from the AP to the BSs on the ground along the railway. The key problem we discuss is how to match user-data arrival process and time-varying channel service process with a delay constraint at the AP. We first assume the constant-rate data arrival and the wireless channel affected only by large-scale fading in high-speed railway scenarios. We present the optimal power-allocation policy given the delay constraint. Based on the policy, we show that there exists a power–delay tradeoff in our system model, with the delay within a limited range. Besides, we find that the average transmit power and the train velocity have a tradeoff, which is similar to the power–delay tradeoff. Besides, given average transmit power constraint, the delay constraint is inversely proportional to the train velocity. Finally, when small-scale fading is considered, we modify the optimal power-allocation policy and provide a convenient approach to counteract Nakagami- $m$ fading in high signal-to-noise ratio (SNR) regimes. The modified power-allocation policy can be well adapted to different fading statistics in different terrestrial environments experienced by the high-speed train. [ABSTRACT FROM PUBLISHER]

This letter investigates the outage performance of overhearing amplify-and-forward (AF) cooperative relaying system, where multiple sources transmit information symbols to a destination through multiple helping overhearing AF relays with space-time network coding (STNC) employed. First, by introducing the overhearing among the relays of STNC, a new transmission protocol, i.e., the STNC-based cooperative relaying with overhearing AF relays (STNC-OHAF), is presented. Then, the instantaneous end-to-end SNR expression of STNC-OHAF is analyzed. Based on this, an explicit expression of the outage probability for STNC-OHAF over independent but not necessarily identically distributed (i.n.i.d) Rayleigh fading channels is theoretically derived and the ability of STNC-OHAF to achieve full diversity order is proved. Numerical results validate our theoretical analysis and show that by introducing overhearing among relays, the outage performance of the STNC-based AF relaying system can be greatly improved. Besides, it is also shown that there is a trade-off between the normalized sum efficient system transmission rate and the number of sources. [ABSTRACT FROM PUBLISHER]

Applying network coding (NC) in the downlink of two-way relay networks can significantly increase the throughput. However, the fading nature of wireless channels usually causes great asymmetry between two downlink channels, which can degrade the performance of NC. Therefore, it is very necessary to perform an opportunistic scheme that selects between broadcast and unicast at the relay node based on channel side information. This paper focuses on the fundamental limit of the sum rate in the downlink. To solve the maximization problem of the sum rate in the downlink, we first introduce an auxiliary variable, known as marginal capacity income of transmit power, and then derive the corresponding power allocation in closed-form expression and present an opportunistic switching selection strategy between broadcast/unicast. Numerical results also show that our proposed scheme achieves a larger downlink sum throughput compared with other existing known schemes. [ABSTRACT FROM PUBLISHER]

In this paper, we analyze the outage probability of space–time network coding (STNC) in a distributed cooperative relaying system, where multiple sources transmit their information to a common destination with the help of multiple decode-and-forward (DF) relays in timedivision multiple-access (TDMA) mode. We first derive an exact closed-form expression of the outage probability for STNC with an arbitrary number of relays for independent but not necessarily identically distributed (i.n.i.d.) Rayleigh fading channels. With the developed result, we discuss the impact of the transmit SNR, the outage threshold, the number of relays, the nonorthogonal codes, and the relay position on the system performance. [ABSTRACT FROM PUBLISHER]

High-speed railways (HSRs) have been widely deployed all over the world in recent years. Different from traditional cellular communications, the high mobility of HSR communication makes it essential to implement power allocation (PA) along time. In the HSR case, the transmission rate greatly depends on the distance between the base station (BS) and the train. As a result, the train receives a time-varying data rate service when passing by a BS. It is clear that the most efficient PA will spend all the power when the train is nearest the BS, which will cause great unfairness along time. On the other hand, channel inversion allocation achieves the best fairness in terms of constant rate transmission. However, its power efficiency is much lower. Therefore, power efficiency and fairness along time are two incompatible objects. For the HSR cellular system considered in this paper, a tradeoff between the two is achieved by proposing a temporal proportional-fair PA scheme. In addition, a near-optimal closed-form solution and one algorithm finding \epsilon-optimal allocation are presented. [ABSTRACT FROM PUBLISHER]

ABSTRACT Secure transmission of information over wireless channels in the presence of an eavesdropper has attracted much attention recently. Previous work assumes that a transmitter has perfect knowledge of the channel side information (CSI) of the legitimate channel and the eavesdropper's channel. To loosen this strong requirement, this paper considers the case where a transmitter has perfect knowledge of the CSI of the legitimate channel and only the average channel gain of the eavesdropper's channel. In this case, some information may be leaked to the eavesdropper because the eavesdropper's channel may have a higher gain than the legitimate channel. Hence, we propose, for the first time, to study leakage-probability-constrained secrecy capacity of a fading channel, that is, secrecy capacity under the constraint on leakage probability. We also propose an optimal power allocation strategy that maximizes leakage-probability-constrained secrecy capacity under average power constraint. Moreover, we study the impact of the bias of the average gain estimate of the eavesdropper's channel on leakage-probability-constrained secrecy capacity. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In the paper, we investigate the information spread problem in a joint vehicle-to-infrastructure (V2I) and vehicle-to-vehicle (V2V) communication system. A scene is considered where more information centers (or base stations) are allocated along the road so that the information centers are able to broadcast timely messages to vehicles within the range of the broadcast signal of each base station, which we shall refer to as broadcast zone . The seamless information spread is used to guarantee that messages are correctly received by each vehicle, regardless of whether it pulls into broadcast zones or not. We first derive the maximum throughput of the V2I downlink system for both additive white Gaussian noise (AWGN) channels and Rayleigh fading channels with Doppler effects. A measurement-based algorithm to estimate the throughput is also proposed. We then discuss the maximum achievable amount of information that can be relayed forward along a vehicular stream. A network coding technique will then be proposed to cancel the interference caused by relay signals to vehicles that are receiving messages from the corresponding information center. These theoretical results will give more insight into the vehicular communication system design. [ABSTRACT FROM PUBLISHER]