In addition to producing a dual-mode IMT2000DS/IMT2000 TC handset, the designer
may be required to integrate positioning capability. There are at least eight technology
options for providing location information: cell ID (with accuracy dependent on network
density), time difference of arrival, angle of arrival, enhanced observed time difference
(handset-based measurement), two satellite options (GPS and GLONASS, or
Global Navigation Satellite System), a possible third satellite option (Galileo), and
assisted GPS (network measurements plus GPS).
GPS receives a signal from any of the 24 satellites (typically 3 or 4) providing global
coverage either at 1.5 GHz or at 1.5 GHz and 1.1 GHz, for higher accuracy. The RF carrier
carries a 50 bps navigational message from each satellite giving the current time
(every 6 seconds), where the satellite is in its orbit (every 30 seconds) and information
on all satellite positions (every 12.5 minutes). The 50 bps data stream is spread with a
1.023 Mcps PN code.
The huge spreading ratio (1,023,000,000 bps divided by 50) means that the GPS
receiver can work with a very low received signal level—typically 70 nV into 50 ohms,
compared to a handset receiving at 1 μV. In other words, the GPS signal is 143 times
smaller. The received signal energy is typically -130 dBm. The noise floor of the
receiver is between -112 dBm and -114 dBm (i.e., the received signal is 16 to 18 dB
below the noise floor).
Although the GPS signal is at a much lower level, GPS and IMT2000DS do share a
similar signal-to-noise ratio, which means that similar receiver processing can be used
to recover both signals. The practical problem tends to be the low signal amplitudes
and high gains needed with GPS, which can result in the GPS receiver becoming
desensitized by the locally generated IMT2000 signal. The solution is to provide very
good shielding, to be very careful on where the GPS antenna is placed in relation to the
IMT2000 antenna, or to not receive when the handset is transmitting.
If the GPS receiver is only allowed to work when the cellular handset is idle, significant
attention has to be paid to reducing acquisition time.
An additional option is to use assisted GPS (A-GPS). In A-GPS, because the network
knows where the handset is physically and knows the time, it can tell the handset
which PN codes to use (which correlate with the satellites known to be visible overhead).
This reduces acquisition time to 100 ms or less for the three satellites needed for
latitude, longitude, and altitude, or the four satellites needed for longitude, latitude,
and altitude. 90
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