Analysis of Interference from Secondary System in TV White Space

Abstract: The demand of bandwidth has increased in recent years with the advent of new technologies in wireless communication systems along with the rise of traffic flow. This has resulted the radio spectrum into a scarce entity. However, recent measurements suggest that some part of the licensed spectrum is not efficiently utilized due to static spectrum allocation. TV broadcast systems operating in licensed UHF band of radio spectrum is one of the examples. So to increase the spectral efficiency, the TV band which is the licensed spectrum, can be shared with the secondary users that are the license-exempt ones with cognitive radio system. After the ruling of FCC that allows unlicensed devices to operate in the ‘unused’ portions of TV bands known as TV Whitespace, other countries have also shown similar interests and there are significant works going on in this field. Among these works, there are some that have proposed analytical method for finding the permissible power required for the secondary users in TV Whitespace. Such analytical methods are meant to provide permissible power levels to secondary users such that there is no harmful interference from such license-exempt ones to the primary system. These recent works considers either co-channel interference constraint or the adjacent channel interference constraint only while devising the analytical method. Such analytical approach considers only one interference constraint and assumes the other one to be negligible. In other words these works have assumed one of the interference constraints as the dominating one and at the same time the other interference to be negligible. There is no verification of a dominating interference in such cases and the fact if such assumed interference constraint is the one that causes the damage to the primary system. In this research project, specific scenarios constituting of several parameters are used to determine the dominating interference from the secondary users in TVWS. Along with it, this project also finds if such dominating interference causes the damage in the TV reception and where if any. Since such finding could be dependent on the scenario of secondary system, the interference behavior is also analyzed by varying the height of secondary user transmitter and its density. Finally there is also a comparison between two power allocation methods employed for the secondary users to look into the fairness in allocating the permissible power. The dominance is evaluated in terms of the ratio of co-channel and adjacent channel interference being greater than a specific reasonable value. The damage is evaluated in terms of violation of minimum threshold requirement for acceptable and smooth reception of primary system receivers. Here the results indicate that such dominance changes along the radial distance of the primary system’s coverage where the co-channel interference is found to be dominant in the boundary and the region closer to the boundary. And adjacent channel interference is found to be dominant in the region closer to the primary transmitter. The dominance is not obvious for the region in between these two dominating areas. The result shows that considering co-channel interference constraint only ensures the protection requirement but the permissible power with such constraint is much lower than the permissible power obtained with adjacent-channel interference constraint only. The variation in height of secondary users indicates that with the decrease in secondary user height the adjacent channel interference dominating region increases and with the increase in secondary user height, the co-channel interference region increases. However there is no change in interference behavior with the change in secondary user density. Also the constant power allocation method is seen to be the fair one among the two power allocation methods used.