Interference Mitigation in Colocated Wireless Systems

The placement of base station transceivers at close proximity to one another is a major challenge for RF engineers. In a colocated setting, the base station receivers have to receive weak desired signals in the presence of high-power transmit/jamming signals from colocated base station transmitters; resulting in major interference issues. The thesis identifies two major mechanism of interference for the colocated victim receiver. First, the strong jamming signals mix within the victim receiver front-end to produce intermodulation products that may fall on its desired receive channel and cause interference. The strong signals may also saturate the receiver circuits and cause desensitization. Second, large jamming signals from one colocated transmitter can radiate into the antenna system of a second colocated transmitter. The signals enter the second transmitter in the reverse direction and mix in the output stage of its power amplifier to produce intermodulation products. These ‘reverse’ intermodulation products get radiated from the antenna system and may fall on the victim receiver’s desired channel. The thesis proposes a reference antenna based adaptive cancellation system that reduces the jammers before they hit the victim receiver’s front-end circuits, thus, mitigating intermodulation distortions and desensitization. The practicality of the system is analyzed. Practical measurements show an increased third-order intercept point (IP3) performance, implying a higher tolerance to jamming signals. This benefit comes at the expense of reduced sensitivity. The IP3 and noise figure expressions derived for the overall system reveals that the cancellation coupler in the reference path is a compromise between achieving higher IP3s and lower noise figures. Hence,