Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-12-22
2002-10-15
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S260000
Reexamination Certificate
active
06466606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communications. More particularly, the invention concerns a method and apparatus for acquiring a base station's transmission signal in a multi-carrier spread spectrum communication network.
2. Description of the Background Art
The use of code division multiple access (CDMA) modulation is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention and incorporated by reference herein.
Path diversity is obtained in CDMA systems by providing multiple signal paths through simultaneous links between a remote station and two or more cell-sites. Furthermore, path diversity may be obtained by exploiting this multi-path environment through spread spectrum processing, thereby allowing signals on the same frequency arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31,1992, entitled “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein. Further, by CDMA's inherent nature of being a wideband signal, it offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Fading can have deleterious effects on signals but can be controlled to a certain extent by controlling transmitter power. A system for cell-site and remote station power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE, TELEPHONE SYSTEM,” Ser. No. 07/433,031, filed Nov. 7, 1989, now U.S. Pat. No. 5,056,109 assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention and also incorporated by reference herein. In general, the fading effects for related carriers in a multi-carrier communication network are approximately the same.
In the patents mentioned above, a pilot signal is used to acquire a base station's transmission signal. Acquisition means that the remote station detects and verifies the existence of such a signal. Detection means that a signal is present and the remote station detects it. The remote station is able to lock onto the pilot signal more easily than other signals because it is unmodulated by data and is usually transmitted at a higher power level than other signals. A pilot signal is an unmodulated, direct-sequence spread spectrum signal transmitted by a base station or remote station. Generally, a pilot signal provides a phase reference for coherent demodulation of a traffic channel, but may also be used for signal strength comparisons between base stations. Using a pilot signal enables a station to acquire a carrier provided within the traffic channel from a local base station communication network in a timely manner. The remote station gets synchronization information and relative signal power information regarding the carrier from the received pilot signal. The remote station may receive signals from multiple base stations in the network and more than one multipath from each base station. Multipath refers to the possible multiple signals arriving at a receiver antenna at different times. Signals that are in phase will add, and signals that are out of phase will cancel one another.
Scanning across an entire code domain in a uniform manner introduces intolerable delays, so remote stations usually scan pilot signals selectively. For example, a remote station may search around pilot signal offsets in active, candidate, and remaining lists. Offset refers to the different time offset for a signal. A search window, typically between 4 and 130 chips, is specified for each pilot signal offset. The remote station detects a signal by looking at the pilot signal energy on the corresponding time offset.
Further, a traffic channel between a remote station and a base station may comprise one or multiple carriers. A carrier is the underlying frequency or frequencies that are used to carry information. They are modulated through one or more modulation techniques to impose information on a signal. For example, a multi-carrier forward traffic channel, also referred to herein as a forward link (FL), may define a mode of operation used with a spreading rate S where S>1, and that uses X adjacent direct-spread radio frequency (RF) carriers. Interleaved data may be demultiplexed onto each of the X adjacent carriers. For example,
FIG. 1
shows a diagram used to illustrate three (X=3) frequency bands
102
,
104
, and
106
of width 1.25 MHz, common to some CDMA communication networks. From each frequency band, a carrier is selected to form a multi-carrier (MC) forward link
108
that facilitates communication between one or more base stations and a remote station.
Regardless of whether a traffic channel provides a single carrier or multi-carriers, a search of pilot signals to find a phase reference for coherent demodulation of a candidate traffic channel is currently conducted the same way. The mobile station generates a local version of a pilot signal pseudonoise (PN) sequence with a guessed time offset (called a hypothesis), correlates this local PN sequence with the received signal for a number (N) of PN chips, and noncoherently combines the energy of M such coherently integrated energies. The result is compared to a first threshold (T
1
). This is the first dwell in a multiple-dwell process. If the accumulated energy is below T
1
, the mobile station considers that there is no signal present on that time hypothesis and moves to the next hypothesis. This is also referred to as an early dump. Otherwise, the mobile station performs verification on that time hypothesis by using a similar procedure, typically with a different (usually larger) value of N and M. If the accumulated energy obtained in the verification is below a threshold of T
2
, then the time hypothesis is discarded. Otherwise, the mobile station performs further verification. If the accumulated energy surpasses the threshold in the final verification, then acquisition is declared on that time hypothesis and a demodulator is assigned to demodulate the signal at that time hypothesis.
FIG. 2
illustrates such a method
200
.
The method
220
starts in task
202
and yields the results discussed below with reference to
FIG. 3. A
window is searched or “swept” in task
204
. If a hypothesis contained in the window does not exceed a first detection threshold (FTV) as shown in task
206
, and if all hypotheses in the window have not been searched in task
208
for the specified parameters, then the search is repeated using a larger window in task
210
. If all hypotheses for a selected set of parameters have been searched in task
208
, then hypotheses for a next set of selected parameters are used in task
212
and the sweep is repeated in task
Baker Kent D.
Beladi S. Hossain
Corrielus Jean
Qualcomm Incorporated
Wadsworth Philip
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