Improving 3G/4G receiver sensitivity (TIS) using antenna impedance matching networks

Abstract

This paper explores new applications for antenna impedance matching networks. It is seen that a tunable matching network can improve the cell phone system noise figure and total integrated sensitivity (TIS) by at -1 dB at 700 MHz -3 GHz. The reason is that a mismatched low-noise amplifier has a higher noise figure and a lower gain than a noise-matched LNA, and therefore, a tunable matching network is important for noise matching in receiver applications. This would lead to shorter download times and more efficient 3G and 4G mobile networks.Index Terms -Tunable circuits and devices, cellular phones, RLC circuits, radio-frequency micro-electro-mechanical systems.Detailed diagram of the receive path, from the antenna port to the transceiver. drops from 12.4 dB to 10-6 dB and the NF increases from dB to 1.9-4.5 dB at a VSWR ofS-lO, respectively. The reason is that the LNA noise emanating from the gate is reflected back to the amplifier by the antenna mismatch and this increases the LNA noise figure. These values degrade the system performance, even for a VSWR of 5. Note that the LNA is followed by a mixer and baseband amplifiers with a noise figure of 6-7 dB, and therefore, a reduction in the LNA gain (and increase in the LNA noise) can have a significant impact on the transceiver performance. In fact, assuming a mixer+baseband NF of 7 dB, the transceiver chip NF degrades from 1.7 dB for a 50 Q source impedance to 2.7 dB for a VSWR of 5 (Fig. 4).Fortunately, the effect of the antenna mismatch is not as drastic as shown above due to the presence of a 2.4 dB switch+filter loss (0.8+1.6 dB) between the antenna and the LN A, which results in a system NF (defmed at the antenna port) of 4.1 dB for a 50 Q antenna impedance. The antenna impedance is also attenuated by 4.8 dB on the Smith chart, and a VSWR of 10 becomes 2.8 at the amplifier input port.This results in an LNA gain of 10.5 and a NF of 1.7 which yields a system NF of 5.1 dB at the antenna port. The system NF is 1 dB larger than a matched antenna and degrades the link margin at the edge of the coverage area, or in poor signal conditions. Therefore, the use of an impedance matching network in the Rx mode to improve the antenna impedance to