Spectrum Coexistence Mechanisms for Mobile Networks in Unlicensed Frequency Bands

Mobile network operators have historically experienced increasing traffic loads at a steady pace, which has always strained the available network capacity and claimed constantly for new methods to increase the network capacity. A key solution proposed to increase the available spectrum is the exploitation of the unlicensed spectrum in the 5 GHz bands, predominantly occupied by Wi-Fi technology. However, an uncontrolled deployment of mobile networks in unlicensed bands could potentially lead to a resource starvation prob lem for Wi-Fi networks and therefore degrade their performance significantly. To address this issue, the 3rd Generation Partnership Project (3GPP) standardised the Long Term Evolution Unlicensed (LTE-U) and Licensed Assisted Access (LAA) technologies. The main philosophy of these technologies is to allow mobile operators to benefit from the vast amount of available spectrum in unlicensed bands without degrading the performance of Wi-Fi networks, thus enabling a fair coexistence. However, the proposed coexistence mechanisms have been proven to provide very limited guarantees of fairness, if any at all. This thesis proposes several improvements to the 3GPP coexistence mechanisms to en able a truly fair coexistence between mobile and Wi-Fi networks in unlicensed bands. In particular, various methods are proposed to adjust the transmission duty cycle in LTE-U and to adapt/select both the waiting and transmission times for LAA. The main novelty of this work is that the proposed methods exploit the knowledge of the existing Wi-Fi activity statistics to tune the operating parameters of the coexistence protocol (duty cycle, contention window size and its adaptation, transmission opportunity times, etc.), optimise the fairness of spectrum coexistence and the performance of mobile networks. This research shows that, by means of a smart exploitation of the knowledge of the Wi-Fi activity statistics, it is possible to guarantee a truly fair coexistence between mobile and Wi-Fi systems in unlicensed bands. Compared to the 3GPP coexistence mechanisms, the proposed methods can attain a significantly better throughput performance for the mobile network while guaranteeing a fair coexistence with the Wi-Fi network. In some cases, the proposed methods are able not only to avoid degradation to the Wi-Fi network but even improve its performance (compared to a coexistence scenario between Wi-Fi networks only) as a result of the smart coexistence mechanisms proposed in this thesis. The proposed methods are evaluated for the 4G LTE standard but are similarly applicable to other more recent mobile technologies such as the Fifth Generation New Radio in Unlicensed bands (5G NR-U)

LTE/Wi-Fi Coexistence in Unlicensed Bands Based on Dynamic Transmission Opportunity

With the rapid proliferation of smart devices, the demand for more licensed spectrum bands arises. Due to the scarcity of the licensed spectrum, the 3rd Generation Partnership (3GPP) has recently deployed Long Term Evolution (LTE) networks using the Licensed Assisted Access (LAA) scheme over unlicensed bands. On the other hand, the Wi-Fi technology is the main technology that operates over these unlicensed bands. Thus, the major concern is to attain a fair coexistence mechanism between these coexisting technologies (i.e., LTE and Wi- Fi). In this paper, we focus on the downlink scenario under different traffic loads to study the effect of the maximum Transmission Opportunity (TxOP) period for LTE-LAA in the performance of LTE-LAA/Wi-Fi coexistence. A dynamic TxOP period method is proposed to provide better fairness and higher total aggregated throughputs for the coexisting networks based on the Hybrid Automatic Repeat Request (HARQ) reports. The novelty of this work is that the existing HARQ reports are exploited to update the TxOP period for LAA in a dynamic manner. We show that the TxOP period plays a key role in the coexistence between LTE-LAA and Wi-Fi networks over unlicensed bands. The simulation results show that the proposed dynamic TxOP method improves the fairness and achieves higher total aggregated throughputs for both coexisting networks as compared to the static TxOP period used by the standard Category 4 LBT (Cat 4 LBT) method defined by 3GPP

Performance Analysis of Dual-Hop AF Cognitive Relay Networks with Best Selection and Interference Constraints

In this paper, a dual-hop underlay cognitive relay network (CRN) with a best-relay selection (BRS) scheme under spectrum-sharing constraints from the primary user (PU) is investigated. The system model in this work consists of one PU, one secondary user (SU) and a few SU relays. Both users exchange the information using a half-duplex mode through amplify-and-forward (AF) SU relays. Moreover, all channels are modelled using the Nakagami-m fading model. In this work, the outage probabilities (OPs) are derived for the proposed system model under several scenarios to investigate the network performance under interference power constraint Ip at the PU receiver. In addition, the impacts of the number of relays and the channel fading severity parameters are investigated as well. Furthermore, the system performance is investigated for several PU locations. The various numerical results are verified using a Monte Carlo simulation. Overall, the obtained results show that several factors such as the number of relays, channel fading severity parameters and PU location have a major impact on the outage performance of the SU. The simulation and analytical results are perfectly matched, confirming the accuracy of the analytical derivations