Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-09-14
2001-06-19
Maung, Nay (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S457000, C370S335000
Reexamination Certificate
active
06249680
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to methods for passively determining the location of a radio transmitter. More particularly, it relates to methods for determining the location of cellular telephones, especially in severe multipath urban environments, and applications of such methods to CDMA cellular telephone networks.
BACKGROUND OF THE INVENTION
Desirability of Location Information
A long standing problem in radio communications is accurately determining the location of a mobile radio transmitter. Precise location information in a cellular telephone network is desirable for various reasons that have been recognized in the prior art. These applications of location information have yet to be realized in urban environments, however, because no practical method of providing accurate location information has yet been developed.
Multipath as the Primary Problem in Location Finding
Multipath is the greatest source of error in prior art methods of location finding. As shown in
FIG. 1
, multipath is typically caused by the reflection of signals from objects in the environment, such as buildings, hills, and other structures. A signal transmitted from a cellular phone
30
, for example, is reflected from structures
32
,
34
, and
36
, resulting in three multipath signals arriving at a base station
38
in addition to a direct path signal. The multipath signals not only have different directions from the direct signal, but different delays as well. As a result, multipath is a problem for location-finding systems based on direction finding, and those based on time-of-arrival measurements. For example signals from phone
30
arrive at base station
38
from different directions and with different delays, with the result that three additional cellular phones
40
,
42
, and
44
appear to be transmitting similar signals from different directions and at different distances. In some cases, signals from a phone
50
can arrive at base station
38
from nearly opposite directions, one from actual phone
50
and another from an apparent phone
52
. In urban environments, often no direct path exists and the base station receives only multipath signals. For example, phone
46
has no direct path signal to base
38
. Consequently, it appears from base
38
that a unique signal is originating from an apparent phone
48
which has a very different location from actual phone
46
. Clearly, multipath significantly complicates the communication of signals, and, in particular, complicates the problem of accurately determining the true location of a transmitter. Since a large proportion of cellular phone usage is in urban environments which have severe multipath, it is especially important to solve this problem. All prior art methods, however, have failed to provide consistent and accurate location information in multipath environments.
Prior Art Location Finding Techniques
As illustrated in
FIG. 2
, a common prior art approach to determining the position of a mobile phone
54
involves measuring temporal information, e.g. time of arrival (TOA) or time difference of arrival (TDOA), at three or more synchronized base stations
56
,
58
, and
60
. By communicating this temporal information between the base stations over a communication line
62
, the transmitter location can be determined. In more sophisticated time-based approaches, handshaking protocols are used in conjunction with time delay measurements.
These methods have the disadvantage that signals from cellular phone
54
must be received by several base stations
56
,
58
, and
60
at the same time. In many circumstances, however, the phone
54
is only within the range of one or two base stations. Additionally, expensive high accuracy clocks are required at the base stations and expensive high bandwidth communication lines
62
are required between the base stations in order to allow accurate temporal correlation of their received signals from phone
54
. More significantly, this approach encounters serious difficulties in urban environments since there is, in most cases, no direct path between the mobile and the base station. Consequently, the multipath can introduce large temporal delays. Although spread spectrum techniques can reduce the effects of multipath to some extent, they cannot provide high accuracy in severe multipath environments or cases where there is no direct path signal. Consequently, these methods fail to determine positions accurately and consistently in many cases.
Although the prior art does disclose several techniques for location finding that attempt to mitigate multipath effects, they all fail in the presence of severe multipath and when there is no direct path signal. These techniques are all characterized by an attempt to cope with multipath by circumvention or discrimination of multipath signals from direct path signals. In cases of severe multipath, however, there often is no direct path signal at all. In such cases, these approaches fail. Moreover, averaging techniques are based on assumptions about the distribution of multipath that are not generally valid, especially in severe multipath environments. Even in cases where such assumptions do hold, these averaging techniques do not yield accurate position information.
As illustrated in
FIG. 3
, another prior art approach determining the location of a phone
68
makes use of antenna arrays
64
and
66
for direction finding. For example, U.S. Pat. No. 5,515,378 to Roy, III et al. discloses a method and apparatus for estimating positions and velocities of mobiles from antenna array measurements of their signals. When an estimate of location is made based only on the directional information from a single base station, such an estimate has a very poor accuracy. To obtain more accurate location estimation, the DOA parameters must be supplemented by TOA measurements and/or parameter measurements sent over a communication line
72
from other base stations. Even in this case, however, the estimates are still not sufficient to accurately determine a correct location since a direct path may not exist at all, as in severe multipath environments. For example, since no direct path exists from phone
68
to either base station
64
or
66
, phone
68
will appear to be located at the location of a false phone
70
.
U.S. Pat. No. 4,799,062 to Sanderford, Jr. et al. proposes an approach to location finding using a differential multipath signal technique. They point out that when the positions of two mobiles are close to each other, their multipath signals should be nearly the same. Consequently, if a reference signal from a known transmitter location near the mobile were subtracted from the mobile's signal, the multipath effects should cancel and the differential position between the two could be determined. The patent, however, does not disclose in detail how such a method might be implemented. Moreover, in severe multipath environments the approach of Sanderford fails. Since the multipath components of the signal can change significantly over distances on the order of 30 meters or less, the differential position will be accurate only in cases where the phone is already within sight of the mobile, therefore defeating the purpose of the technique. Even in cases of less severe multipath, the technique is unattractive to implement due to the need for a reference signal from a nearby transmitter having a known location.
LeBlanc et al. in U.S. Pat. No. 5,602,903 discloses a method for determining the location of a mobile by making RF measurements, such as signal strength, error rate, and signal quality, then comparing these measurements with a database of similar measurements made when the receiver was at a known location. The disclosed method, however, suffers from the disadvantage that it requires the measurement of information at multiple base stations to determine location. Because the RF measurements that are measured by the method are only weakly correlated with position, measurements from a single base station are not sufficient to determine
Bar Abraham
Hilsenrath Oliver
Wax Mati
Lumen IPS, Inc
Maung Nay
Trinh Sonny
U.S. Wireless Corporation
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