Gateways play a major role in communications over two-hop wireless or hybrid systems. For instance, indoor-outdoor communication networks, satellite communications, and hybrid underwater-wireless communications. It can extend the coverage of cellular systems, and connect the isolated users in rural areas. This thesis considers mainly four different themes. Firstly, the design and performance analysis of a single gateway equipped with a single antenna was investigated over different wireless channel models. The gateway uses random access protocol to connect a random number of independent non-collaborative users. New mathematical expressions were derived for the overall spectral efficiency. Afterwards, the results were extended to include collaborative users in a space division multiple access (SDMA) scenario, where the linear zero-forcing (ZF) beamformer was employed at the gateway. New analytical expressions were derived for the overall spectral efficiency (SE) in both collaborative and non-collaborative SDMA scenarios. In addition to that, different types of fading and shadowing models were considered. Thereafter, a new model for cooperative multiple gateways was introduced and analysed in details for two different scenarios, namely, single and multiple antenna gateway. In this aspect, it was assumed that the getaways are arbitrarily distributed within the service area. Finally, a comprehensive analytical framework for hybrid underwater-wireless communication where the gateway acts as a convert-and-forward relay was developed. New analytical results were derived for the spectral efficiency over different wireless/underwater channel models. The accuracy of the new mathematical results was confirmed by Monte Carlo simulation. The results in our thesis showed that the gateway can greatly enhance the communication systems performance. Moreover, enhancement in spectral efficiency and coverage area extension can be achieved by integrating linear precoder with gateway systems.