Dealing with interference in GPS-enabled apps

A handset is an ideal product to be GPS-enabled. The integration of a GPS receiver into a handset can create a simultaneous-GPS (S-GPS) application where the GPS receiver is used together with other wireless communication systems from various frequency bands. Consumers expect a handset with GPS capability to be reliable in receiving and amplifying the signal from the satellites because any error in the reception would cause inaccurate information on the location. Unfortunately, the quality of the GPS signals are often times compromised by interfering RF signals.

The integration of a GPS receiver on the same board with other wireless mobile communication transmitters exposes the receiver to the intrasystem interference, which may degrade the GPS receiver's sensitivity and linearity. While the transmitter is in transmit mode, part of the transmitting signal will leak to the GPS receiver path. Consequently, the receiver would experience a high total input power that may saturate the receiver's back end. This would generate a nonlinear signal at the receiver's back end and create errors to the receiving signal.

To avoid this phenomenon, the out-of-band transmitting signal needs to be blocked from going into the GPS receiver path. Thus, the GPS receiver path is required to have a good rejection on the out-of-band transmitting signal (interferer). By having a good rejection to the interferer, it will prevent the GPS chipset from being overloaded by the strong interfering power, and the chipset is able to provide a linear amplification to the received signal.

Sensitivity, linearity
Typically, the designer will put filters at both sides of the GPS LNA. A filter in front of the LNA helps to reject the out-of-band signal and prevent the LNA from being saturated. This filter should have a very low insertion loss. Putting a high insertion loss filter before the LNA should be avoided because this will increase the system's noise figure. According to the Friis equation, the total noise figure is dominated by the noise figure or loss of the first stage. A second filter at the back of the LNA can be used to further improve the out-of-band rejection to prevent the later stage from being overloaded.

However, a front filter with insertion loss as low as 0.5dB in front of the LNA will still degrade the cascaded noise figure even though the LNA has an exceptional good noise figure of 0.8dB. The cascaded noise figure is dominated by the first stage just when the gain is adequately high. The negative gain of the first stage filter causes the cascaded noise figure to degrade to 1.35dB. Besides, this solution involves three components (filter-LNA-filter).

The solution explained in the previous section can be simplified to an LNA-filter solution by using an LNA with very good linearity as the first stage and a very good out-of-band rejection filter as the second stage. This section explains an LNA-filter module that is suitable to be used at the front end of a GPS receiver. The module is an integration of a low noise high linearity Enhancement Pseudomorphic HEMT (E-pHEMT) LNA and a low insertion loss superior-out-of-band rejection thin-film bulk acoustic resonator (FBAR) filter. This combination will create a front-end with excellent noise figure while maintaining the linearity.

Higly linear LNAs
E-pHEMT is Avago Technologies' proprietary technology that can produce highly linear LNAs. FBAR is a resonator technology developed by Avago that can produce small-sized filters. With the integration of an FBAR filter, the LNA module offers sufficient rejection to the cellular and personal communications services (PCS) bands, helping the receiver's performance in concurrent or S-GPS operation.

An LNA-filter module with high linearity enables it to handle higher input power without compressing the received signal. Ultimately, the filter in front of the LNA module can be omitted as long as there is enough isolation between the GPS path and the PCS or cellular paths. Without a front filter, the system's noise figure is now dominated by the LNA, where the noise figure can be as low as 0.8dB. This implementation would greatly improve the sensitivity of the receiver.

By using a dual-antenna solution, the GPS signal will have a separate path or chain from the PCS/cellular signal to meet the 40dB isolation.

Integrating the LNA with the filter also causes the module's input impedance to look more concentrated (small impedance spreading on Smith chart) due to the filter's narrow bandwidth. This will make the impedance matching between the antenna and the input LNA module much easier compared with a discrete LNA without a post-filter. A single IC solution also ensures a more reliable and consistent receiver performance.

Blocking the PCS signal

The LNA module effectively blocks the PCS signal from leaking into the GPS chipset. The interferer power is as low as -57dBm while having 40dB isolation between GPS path and PCS path.

With proper arrangement and design, the single LNA module solution is able to replace the filter-LNA-filter solution, which has higher noise figure and more complicated architecture.

- Chew Ean Tan
Application Engineer
Wireless Semiconductor Division
Avago Technologies Ltd