One of the key requirements of fifth generation (5G) systems is having a connection to mobile networks without interruption at anytime and anywhere, which is also known as seamless connectivity. Nowadays, fourth generation (4G) systems, Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A), are mature enough to provide connectivity to most terrestrial mobile users. However, for airborne mobile users, there is no connection that exists without interruption. According to the regulations, mobile connectivity for aircraft passengers can only be established when the altitude of the aircraft is above 3000 m. Along with demands to have mobile connectivity during a flight and the seamless connectivity requirement of 5G systems, there is a notable interest in providing in-flight wireless services during all phases of a flight. In this thesis, many issues related to the deployment and operation of the onboard systems have been investigated. A measurement and modelling procedure to investigate radio frequency (RF) propagation inside an aircraft is proposed in this thesis. Unlike in existing studies for in-cabin channel characterization, the proposed procedure takes into account the deployment of a multi-cell onboard system. The proposed model is verified through another set of measurements where reference signal received power (RSRP) levels inside the aircraft are measured. The results show that the proposed model closely matches the in-cabin RSRP measurements. Moreover, in order to enforce the distance between a user and an interfering resource, cell sectorization is employed in the multi-cell onboard system deployment. The proposed propagation model is used to find an optimum antenna orientation that minimizes the interference level among the neighbouring evolved nodeBs (eNBs). Once the optimum antenna deployment is obtained, comprehensive downlink performance evaluations of the multi-cell, multi-user onboard LTE-A system is carried out. Techniques that are proposed for LTE-A systems, namely enhanced inter-cell interference coordination (eICIC) and carrier aggregation (CA), are employed in the system analysis. Different numbers of eNBs, antenna mounting positions and scheduling policies are examined. A scheduling algorithm that provides a good tradeoff between fairness and system throughput is proposed. The results show that the downlink performance of the proposed onboard LTE-A system achieves not only 75% of the theoretical limits of the overall system throughput but also fair user data rate performance, irrespective of a passenger's seat location. In order to provide the seamless connectivity requirement of 5G systems, compatibility between the proposed onboard system deployment and the already deployed terrestrial networks is investigated. Simulation based analyses are carried out to investigate power leakage from the onboard systems while the aircraft is in the parked position on the apron. According to the regulations, the onboard system should not increase the noise level of the already deployed terrestrial system by 1 dB. Results show that the proposed onboard communication system can be operated while the aircraft is in the parked position on the apron without exceeding the 1 dB increase in the noise level of the already deployed terrestrial 4G network. Furthermore, handover parameters are obtained for different transmission power levels of both the terrestrial and onboard systems to make the transition from one system to another without interruption while a passenger boards or leaves the aircraft. Simulation and measurement based analyses show that when the RSRP level of the terrestrial system is below -65 dBm around the aircraft, a boarding passenger can be smoothly handed over to the onboard system and vice versa. Moreover, in order to trigger the handover process without interfering with the data transmission, a broadcast control channel (BCCH) power boosting feature is proposed for the in-cabin eNBs. Results show that employing the BCCH power boosting feature helps to trigger the handover process as soon as the passengers step on board the aircraft.