Analysis and management of intra- and inter-system interference in wireless communications systems
Date
2018
Authors
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Publisher
University of Delaware
Abstract
The tremendous successes of wireless communications, including cellular systems, wireless local area networks (WLAN), satellite communications and underwater acoustics (UWA), have had a great impact on almost every aspect of human life, such as social networks, entertainment, machine-to-machine communications, smart cities and ocean resource explorations. As a result of these flourishing wireless networks, we have been confronted by an explosion of connected devices and data-hungry applications, and, future wireless systems are going to face exponentially increasing data traffic demands.
Heterogeneous networks, the integration of various technologies/standards, are a promising solution to provide ubiquitous coverage, and improve network performance (such as spectrum efficiency and energy efficiency). The third generation partnership project (3GPP) has standardised licensed-assisted access (LAA) in Release 13/14 and proposed new radio (NR) based unlicensed access in Release 15, in which cellular systems are allowed to access unlicensed spectrum. However, a major challenge to realizing the potential advantages of spectrum sharing is the inter-system interference, i.e., cellular systems have to coexist with other radio access technologies, especially WLAN, which already operate in the unlicensed medium.
In addition, instead of sharing the spectrum for transmission and reception (either in separate time slots or different frequency bands), in-band full-duplex (IBFD) has been proposed to enable wireless terminals to transmit and receive simultaneously over the same frequency band, with the advantage of doubling the spectral efficiency, in addition to the benefits obtained using other technologies. The major drawback of an IBFD system is the high level of intra-system interference created by its own transmissions while trying to receive a distant and useful signal. In general, to achieve the two-fold spectrum efficiency gain, multiple interference cancellation schemes, includes antenna cancellation, analog cancellation, and digital cancellation, are required to suppress the severe interference.
In this dissertation, we investigate and analyze intra- and inter-system interference under three scenarios: IBFD cooperative relaying, IBFD underwater acoustic transmission, and coexistent WLAN/LAA networks.
For IBFD cooperative relaying, we evaluate three different types of interference: self-loop interference, cross-talk interference and possible direct-link interference (from the source to the destination). In particular, we first investigate the spectral efficiency of IBFD relaying when the self-loop interference and the cross-talk interference cannot be completely suppressed due to imperfect channel estimation. Then, we analyze the impact of the interference from the direct link by deriving closed-form expressions for the outage probability of half-duplex (HD) and IBFD relaying over Rayleigh fading channels. We show that the "signal" from the direct link might cause a severe error floor in the system performance, even when self-loop and cross-talk interference can be completely suppressed.
We next focus on deploying IBFD transmissions to extremely bandwidth-limited UWA systems, which is more challenging due to the much harsher UWA propagation environment. By analyzing the challenges in implementing IBFD UWA systems, we propose an acoustic-specific design that includes both analog and digital cancellation, and present its performance in the presence of imperfect channel state information (CSI), ambient noise, and quantization noise from the analog-to-digital converter. In addition, for IBFD UWA cooperative transmissions, we employ orthogonal frequency division multiplexing (OFDM) techniques to overcome the impact of the residual self-loop interference, and propose a delay-diversity scheme with a Viterbi detector to utilize the long-delay reflected interference (interference due to reflections from sea surface or seafloor).
Finally, we investigate the inter-system interference in coexistent WLAN/LAA systems. We first analyze the coexistence challenges of Wi-Fi (i.e., WLAN) and LAA posed by frequent collisions, which are caused by the use of different sensing/detection methods. To improve the coexistence performance of Wi-Fi and LAA in downlink transmissions, a distributed adaptive algorithm is proposed to adjust LAA's Energy Detection (LAA-ED) thresholds. Further, we extend the adaptive energy detection algorithm to the multi-carrier case, and propose a simple, but efficient, carrier-selection algorithm based on LAA-ED thresholds. Via simulations, the proposed adaptive energy detection and carrier selection algorithms are shown to improve the overall system performance as well as achieve better fairness among Wi-Fi and LAA networks.