Shining a Light into the Darkness: How Cooperative Relay Communication Mitigates Correlated Shadow Fading Tingting Lu, Pei Liu, Shivendra Panwar NYU Polytechnic School of Engineering Brooklyn, New York, 11201 Email: tl984@nyu.edu, peiliu@gmail.com, panwar@catt.poly.edu Abstract—In a cellular network, connections between the Base Station (BS) and Mobile Stations (MS) may fail when the channel is in a deep fade. Shadow fading is large-scale fading which can cause significant received power loss for a wide area. This will lead to lost connections and/or packet loss which is harmful to mobile users, especially to those who are using real-time applications such as video conferencing. Cooperative communication is an efficient way to reduce outage and provide better Quality of Service (QoS) support for delay sensitive applications. A third station, which is often referred as a relay, can be used to forward signals between the BS and the MS. This paper focuses on a study of the performance of relay deployments under correlated shadow fading. We consider the downlink direction in a single cell deployment, for which the shadowing effect is modeled as an angle and distance based correlated shadowing. The received signal-to-noise ratio (SNR) is then calculated by assuming jointly Gaussian shadow fading at the MS. Simulation results show channel variations over time with fixed user speed under different relay deployments. These results demonstrate that a modest number of relays can improve the performance of real-time applications significantly. I. I NTRODUCTION In a cellular communication system, the connection between the base station and a mobile station may be dropped when the mobile enters a deeply shadowed area. Shadow fading due to buildings, mountains or even trees significantly reduces the power of the received signal. In most cases, shadow fading is assumed to be temporally and spatially independent [1]. In [2] and [3], the effects of correlated shadowing in connectivity is demonstrated, which indicates that reliable connectivity will be much more difficult to maintain than indicated by independent fading shadow models. In a cellular system, for a downlink, the transmitter is a Base Station (BS) and the receiver is a Mobile Station (MS). For real-time applications (e.g. video conferencing), which require high bandwidth and are delay sensitive, the session may get dropped. In general, within a speed limit, the faster the MS moves, the more frequently the channel condition changes and the higher the connection loss frequency. The focus of this paper is to study channel variations due to correlated shadow fading, and provide a solution to reduce the frequency and duration of dropped connections by employing relays when the MS is moving. Cooperative communication has been proven to be an effi- cient way to mitigate fast fading and increase the robustness of Base Station Relay Mobile User Fig. 1. Cooperative Communication Example data connections [4, 5]. However, cooperative communication can also efficiently maintain the connection whenever the channel condition experiences a sudden deterioration due to shadow fading by switching to different relays. Figure 1 is an example of cooperative communication where relays and BS are placed on the top of buildings. In this case, when the MS moves to the area behind a tall building, with a high probability the signal transmitted from the BS will be obstructed by the building, and consequently the MS will encounter deep shadow fading. The channel between BS and MS will degrade and data rate will drop suddenly. In the worst case scenario, the connection with the BS may be totally lost. To combat this effect and enhance the signal received by the MS in this case, relays can be deployed on the top of tall buildings to relay the signals from the BS to MS to maintain good channel conditions between the BS and MS. Cooperative communication has been a topic of research for several years. Madan et al. [6] studied multi-user spatial diversity in a shadow-fading environment. Other work [7–9], studied relay selection and cooperative relaying over different fading channels in different systems. Patwari et al. [10] studied relay placement in realistic deployments and confirmed that Rayleigh fading alone is not an appropriate assumption for evaluating network performance in a real deployment. In [11, 12], the authors analyzed outage probability and its duration with cooperative relaying. In an 4G-LTE network, which is strongly resilient to multipath fading, shadow fading becomes the most important fading factor [1]. Given the presence of relays, the channel variation experienced by an 978-1-4799-8088-8/15/$31.00 ©2015 IEEE