The growing interest for high data rate wireless communications over the last few decades gave rise to the emergence of a number of wideband wireless systems. The resulting scarcity of frequency spectrum has been forcing wireless system designers to develop methods that will push the spectral elciency to its limit. One such method is to have multiple systems utilize the same spectrum by allowing some unavoidable interference to occur between them. The idea of co-channel systems is tested in the industrial, scientific and medical (ISM) bands and it is found to be a very beneficial approach. Therefore, it can be foreseen that co-channel systems might be a potential solution to the growing spectral crowding problem. Besides the systems that are designed to be co-channel, it is sometimes also possible to encounter that multiple systems occupy the same band undesirably. This kind of unintentional co-channel system scenarios might occur especially due to the dense re-use of available frequency bands. Another reason for unwanted co-channel usage might be the coexistence of third generation (3G) and fourth generation (4G) systems. Since 4G systems will probably be targeting to use the same frequency bands as their 3G counterparts, and since the transition from 3G to 4G will take some time, unintentional co-channel scenarios might be observed between the 3G and 4G systems. This dissertation consists of baseband receiver algorithms for OFDMA-based systems that target at handling the potential co-channel interference (CCI) in various co-channel system scenarios. Three CCI avoidance and two CCI cancellation algorithms are proposed that can be applied to intentional and unintentional co-channel systems. Femtocells, which have recently been introduced as a new class of personal-use base stations that can coexist with macrocell networks in a shared spectrum manner, might constitute an appropriate example for both types of co-channel systems. Therefore, they are considered to be one of the co-existing systems in most of the algorithms presented.