Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-07-27
2004-02-24
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C375S354000
Reexamination Certificate
active
06697417
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to wireless communications systems and, in particular, to system and method for accurately estimating the earliest arrival of CDMA radio signals, either in the forward or reverse links.
2. Description of Related Art and General Background
Efforts are underway to augment wireless communications systems by adding the capability to locate the position of a particular mobile station (MS). The Federal Communications Commission (FCC) has promulgated a regulation directed to this capability (Docket No. 94-102, third report and order adopted Sep. 15, 1999, released Oct. 6, 1999). This regulation requires wireless carriers adopting hand-held position location solutions to locate the position of a mobile station making an emergency 911 call to within 50 meters for 67% of calls (and to within 150 meters for 95% of calls) by October 2001.
In satisfying this requirement, one approach to determining the position of a MS may be to use the available information at the base stations (BSs) and MSs of a wireless communication system, operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique that is defined in the Telecommunications Industry Association/Electronics Industries Association Interim Standard-95 (TIA/EIA IS-95), entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”, published in July 1993 and herein incorporated by reference. Wireless communication systems employing this technology assign a unique code to each different communication signal and apply pseudonoise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. As long as the receiving apparatus in a CDMA system has the correct code, it can successfully detect and select its signal of interest from the other signals concurrently transmitted over the same bandwidth.
FIG. 1
(Prior Art) illustrates a simplified block diagram of CDMA wireless communication system
100
. System
100
allows MS
110
, typically comprising mobile terminal equipment (TE2 device
102
) and a wireless communication device (MT2 device
104
) to communicate with an Interworking Function (IWF)
108
. The IWF
108
serves as a gateway between the wireless network and other networks, such as the Public Switched Telephone Network (PSTN) and wireline packet data networks providing Internet- or Intranet-based access. MS
110
communicates with BS
106
, which is associated with a geographic cell or sector, via the wireless interface U
m
on the reverse link transmission path. BS
106
is configured to process the communication signals from MS
110
. BS
106
may also include, or be associated with, position processing capabilities (e.g., Position Determination Entity (PDE) server mechanisms).
On the forward link transmission path, BS
106
communicates with MS
110
via the wireless interface U
m
. During forward link transmissions, each BS
106
is capable of transmitting information-bearing signals as well as control signals, such as pilot signals. Pilot signals have a plurality of uses, one of them is to identify the BS
106
best suited to accommodate reverse link transmissions. As such, pilot signals are instrumental in determining which BS
106
to “hand-off” the reverse link transmission to in order to seamlessly maintain communications as the MS
110
travels across different cells or sectors of cells. Pilot signals also provide a time and coherent phase reference to enable MS
110
to obtain initial system synchronization and facilitate coherent demodulation on the forward link. All pilot signals are subjected to the same PN spreading code but with a different code phase offsets to enable MS
110
to distinguish between different pilot signals coming from different sectors or base stations. Each BS
106
may transmit up to 6 different pilot signals with 6 different PN offsets. Use of the same pilot signal code allows MS
110
to find system timing synchronization by conducting a search through all pilot signal code phases of the same code.
As is well known, signal transmissions traveling across air interface U
m
may be subject to multipath propagation. As such, MS
110
may first receive a direct (i.e., line-of-sight (LOS)) signal corresponding to the forward link signal transmitted by BS
106
, followed by time-delayed and attenuated versions of the same signal due to multipath. There may be situations where the first LOS signal is not received and only the multipath components are present. MS
110
may determine the time of arrival (TOA) and energy of all received pilot signals to identify the earliest useable received pilot signal.
To determine the TOA of the received pilot signals, MS
110
may count and store the number of chips (or fractions thereof) of PN code sequences (i.e., PN chips) that lapse from a reference while the signals were received. MS
110
may then identify the earliest received pilot signal by detecting which pilot signal was received after the smallest number of lapsed PN chips. The reference (or zero arrival time) may in general be an arbitrary mark: because of this, isolated TOA measurements cannot be used directly in position determination algorithms. There is the need of at least two TOA measurements corresponding to pilots coming from different geographical points to overcome this arbitrary error. For instance, by subtracting said two measurements, we get a measurement proportional to the difference between the radial distances of the mobile to the two origins: the common error induced by the ambiguity in the zero timing falls out in the subtraction.
To compensate for the effects of multipath propagation, CDMA systems, such as system
100
, employ rake receivers, which process and combine the direct and multipath versions of the forward link pilot signal to generate a better received signal.
FIG. 2
(Prior Art) depicts a high-level functional block diagram of a MS
110
receiver
200
, including a rake receiver demodulator
225
for coherently demodulating the forward link signals received by MS
110
. As indicated in
FIG. 2
, the radio-frequency/digital converter modulo
205
downconverts and digitizes the received signal from the antenna/producing digital samples. The digital samples are supplied to a rake receiver demodulator
225
, which includes a searcher
215
.
Searcher
215
is configured to search for signals by sweeping across the samples that are likely to contain multipath signal peaks in steps of one or half-PN chip increments. Searcher
215
then assigns finger correlators
210
A-C to the stronger multipath signals. Each finger correlator
210
A-C locks onto their assigned multipath signal, coherently demodulates the signal, and continues to track the signal until the signal fades away or the finger correlator
210
A-C is reassigned by searcher
215
. The demodulated outputs of finger correlators
210
A-C are then combined by combiner
220
to form a stronger received signal.
Given the ability to detect the TOA of forward link signals, CDMA systems may, at least in theory, exploit these capabilities to extract MS
110
location information. As noted above, MS
110
is capable of determining the TOA of the received multipath components.
As noted above, the promulgated FCC regulation requires the location of a MS to within 50 meters for 67% of calls. A limitation of current CDMA systems is their inability to estimate TOAs with the necessary resolution to comply with the location requirements. For example, counting lapsed PN sequences to within a tolerance of a PN chip to determine the earliest received pilot signal, is of no consequence in establishing a communications link with the closest BS. However, given the fact that a PN chip corresponds to approximately 800 ns., which translates into a radial distance of 240 meters, such a tolerance clearly fails to comply with the location requirements.
Furthermore, since the LOS signal may not
Fernandez-Corbaton Ivan
Levanon Nadav
Brown Charles
Fan Chieh M.
Kordich Donald
Qualcomm Inc
Wadsworth Philip
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