Global Positioning System Reference
In-Depth Information
5.13
Considerations for Indoor Applications
The generic receiver design described in the previous sections is a continuously
tracking design for outdoor (direct LOS) applications where the C/N 0 may be mod-
estly deteriorated by signal attenuation due to foliage, multipath, antenna gain
roll-off, and so on, or by signal interference due to unwanted in-band RF signals.
The principles described thus far assume that the carrier and code loops are closed
after the search process, and, as long as the C/N 0 remains above the tracking thresh-
old, the receiver will operate satisfactorily. The E-911 requirement for autonomous
cellular phone location has motivated the development of highly specialized GPS
receivers that can obtain range measurements indoors where the C / N 0 is 20 dB or
more lower than the normal situation. These receivers, which are integrated with
cellular phone handsets, maximize the use of aiding from communication networks
to avoid the need to read the satellite navigation message data or any other tracking
mode that requires continuous satellite signal tracking. The objective of these spe-
cialized designs is to obtain a reasonable estimate of the user position by extensive
averaging of the code correlations with several weak GPS SV signals until there is
enough signal processing gain to obtain a set of transmit time measurements from
them. The indoor measurements do not have to be nearly as accurate as is expected
for the continuously tracking GPS receiver designs. In indoor applications, it is often
acceptable for the position determination to only be sufficiently accurate so as to
identify the building location. As a result, indoor GPS applications are probably the
only operational situation where multipath signals are considered to be friendly
signals by the GPS receiver since there is little or no direct GPS signal energy inside
most buildings.
There are numerous variations of network-aided GPS techniques. This section
outlines some basic principles to obtain the high sensitivities needed for indoor
applications. Section 9.4 provides a detailed overview of network-assistance tech-
niques, including industry standards.
One common practice for GPS receivers in cellular handsets is to never close any
tracking loops, but rather to dwell on the GPS signals in a controlled (net-
work-aided) search mode long enough to extract enough crude range measurements
to provide a snapshot position. The rover receivers utilize communications aiding in
order to dwell within the correlation range of the C/A code (about 300m per
correlator) and within the carrier Doppler bin in a signal search mode. To accom-
plish this, they utilize a reference receiver with a clear view of the sky within the cel-
lular network to provide the information needed to replicate their code phase and to
synthesize their Doppler estimate during an extended search dwell. If the aiding is
such that the SV signals remain within the Doppler bin and the correlation window
during the extended search dwell, then sufficient processing gain is achieved to pro-
duce valid transmit time measurements. A number of factors make this practical.
The user dynamics are small inside buildings, and their distance from the relay (GPS
reference) station is not large; therefore, the difference between the user LOS range
and Doppler to the SVs is not much different from that of the reference station.
Extending the correlator range and the Doppler tolerance is part of the innovative
designs of these specialized receivers. Also, the latency of aiding is tolerable to the
user receiver since the aiding is applied open loop to the carrier wipeoff process (i.e.,
 
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