NXP GPS LNA User Manual

Global Position System
Low Noise Amplifier

GPS, LNA, Sensitivity, Jamming,
Cohabitation, TTFF

This White Paper explains why an external low noise amplifier results in a better performance.
Next generation mobile handsets will be equipped with GSM, WLAN, Bluetooth and GPS.
Integrating all these functionality onto one printed circuit board gives housing problems or
cohabitation challenges. Some of them can be solved easily at low cost but others cannot.
In this paper the GPS and its cohabitation with GSM and WCDMA will be explained.

1 GPS Integration Challenge

Currently people are using Personal Navigation Devices (PND)
to finding a location around the globe. Likewise for wireless
LAN, wherever you are, you want to have the Internet available.
Downside of this multi functionality is that you have to carry
many devices with you. For this reason mobile handset makers
start to integrate the GPS WLAN etc. into one device.

Referring to GPS, the received signal strength is very weak.
The satellites have an orbit altitude of 20200 km (90 degrees
elevation) from earth. It’s transmit power is 44.8 Watt at

1575.43 MHz and the antenna gain is 12 dBi. Assume that the
PND device has an antenna gain of 4 dBi. Then the received
signal power is -120 dBm using the free space loss model.
Including additional losses (atmospherically, antenna) the
received power is -125 dBm. While the noise power in the

system bandwidth (2.046 MHz) is -110 dBm! But due to the
modulation scheme, which is direct spread-spectrum there is
a processing gain of 43 dB. In fact signals can be recovered
theoretically till -110-43=-153 dBm. In order to improve quality
of services the received power strength requirement will be
several dB above the theoretical level. For instance, during
acquisition the received power required is -135 dBm and for
tracking -147 dBm.

For cohabitation it is important to cope with the low signals
level at the GPS frequency. Unfortunately, this integration is not
straightforward. Putting different wireless functionality into one
housing is called cohabitation. Compare this with interpersonal
relationship you have to put in effort in to make it happen.

Figure 1 depicts an example of GSM and GPS cohabitation.
Suppose that the GSM transmitter transmits at 1800 MHz at
a power level of +36 dBm (4 Watt). The GPS receiver receives
GPS signals at 1575.42 MHz (L1 frequency) at a power level of

-125 dBm (about 0.1 fWatt). The isolation between the GSM
output and the GPS’s receiver input is approx. 15 dB. Leaving
21 dBm of GSM power at the GPS receive input.

Block diagram of GSM and GPS integrated on one PCB

Fig. 1

Filtering will help to reduce the GSM power level at the GPS
receiver’s input, see Fig 2, but will increase the noise level,
which reduces the GPS receiver sensitivity. High attenuation
filters have high insertion losses. Next to this the wideband
noise emission from the power amplifier of the GSM cannot be
filtered out, because it is present directly in the GPS band.

Finally, the GPS antenna might be separated far away from the
GPS receiver IC. Long PCB tracks results in an increased noise
level, which reduces the GPS receiver sensitivity as well. Note that
integrated GPS antennas have often low antenna gains (-6 dBi).

Cohabitation gives four problems to solve:

} Reduced GPS receiver sensitivity due to high filter losses to

attenuate out of band jamming signals (GSM, DCS, WCDMA)
to avoid overdriving the Low Noise Amplifier (LNA) and
improve out of band O-IIP3

} Cross modulation that corrupts GPS reception

} Wideband noise power generate by jamming PA

} Antenna GPS receiver separation

GSM transmits at class 3 (+36 dBm). At the GPS antenna input the received GSM power is 21
dBm. The band pass filter attenuates the GSM signal with 40 dB. Leaving –19 dBm at the input
of the GPS front-end IC

External LNA with BPF filters and GPS front end receiver

Fig. 2

2 GPS and GSM-1800 Cohabitation

GSM-1800 uses 1710–1785 MHz to send information from
the mobile station to the base transceiver station (uplink) and
1805–1880 MHz for the other direction (downlink), providing 374
channels (channel numbers 512 to 885). Duplex spacing is 95 MHz.

What will the end user experience when using an external LNA?
Improved sensitivity results in a shorter time to first fix (TTFF).
TTFF definition is the specification detailing the time required
for a GPS receiver to acquire satellite signals and navigation
data, and calculate a position solution. The latter is called a fix.
Long TTFF results in an increased frustration level of the users.
This long TTFF can be improved by using an external LNA.

Resume, GPS receivers suffer a lot from receiver desensitization
if a neighbor jamming signal(s) is (are) present. This white
paper concentrates mostly on problem one and two. Problem
three can be solved by applying GPS blanking during Tx burst.
Problem four is solved by placing the LNA as close as possible
to the GPS receive antenna or use an active antenna with a high
gain LNA.

GSM-1800 is also called DCS (Digital Cellular Service) in the United
Kingdom, while being called PCS in Hong Kong (not to mix up with
GSM-1900 which is commonly called PCS in the rest of the world.)

According to the 3GPP TS 45.005 V8.3.0 (2008-11) there are
three power classes defined for GSM-1800.
These power classes are:

Class 1: 1 Watt or (+30 dBm),

Class 2: 0.25 Watt or (+24 dBm) and

Class 3: 4 Watt or (+36 dBm).

In case of GSM-1800 cohabitation, proper filtering and
proper amplifier design can solve problems 1 and 2. The NXP
BGU7005 LNA has special properties to improve the GPS
receiver sensitivity.