To fulfill the 30 dB isolation between the LTE antenna and Global Positioning System (GPS) Low-Noise Amplifier (LNA) input, a filter may be needed in the GPS LNA input if the antenna isolation between the LTE and GPS antenna is less than 30 dB. The minimum attenuation requirement for the filter depends on the isolation between the LTE and GPS antennas and relaxes as the antenna isolation increases. Coupling between closely spaced antennas may be very strong even though they are targeted to operate at different frequencies.
A low Noise Factor (NF) and high gain of LNA ensure high GPS sensitivity. A minimum of 15 dB LNA gain and a maximum of 1 dB NF gain for LNA only are reasonable targets. For a combined LNA and filter module, the NF target is 1.5 dB.
To reach as low as possible total NF, the losses before the LNA input should be minimized. In layout design, this means placing the LNA as close as possible to the GPS antenna and minimizing the length of the transmission line between the antenna and the LNA.