Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-05-17
2004-01-20
Le, Amanda T. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000
Reexamination Certificate
active
06680968
ABSTRACT:
BACKGROUND
1. Field
The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for pilot signal acquisition.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, “cdma2000 High Rate Packet Data Air Interface Specification”, and incorporated herein by reference. Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading of transmitted data, including transmitted pilot signals. CDMA receivers commonly employ RAKE receivers, described in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from neighboring base stations, and two or more multipath demodulators (fingers) for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled “MULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS”, filed Sep. 30, 1994, and co-pending U.S. patent application Ser. No. 09/283,010, entitled “PROGRAMMABLE MATCHED FILTER SEARCHER”, filed Mar. 31, 1999, both assigned to the assignee of the present invention and incorporated herein by reference.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of the base station. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. For example, in IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique time offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips. A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. It is also possible to differentiate base stations by using unique PN sequences for each rather than offsets of the same PN sequence. In this case, a finger would adjust its PN generator to produce the appropriate PN sequence for the base station to which it is assigned.
In searcher design, an increased sampling rate of the incoming signal translates to finer time resolution and hence a better result in terms of PN space search accuracy is achieved. However, those better results typically come with a trade-off of increased computation time or increased complexity, or both. An established practice is to provide samples of the incoming received signal to the searcher at a resolution of twice the chip rate. As such, when performing search calculations to determine the location of a pilot, there is always uncertainty of one half chip in the alignment between the PN sequence being generated at the receiver and the PN sequence embedded in the received signal.
The effect of this misalignment in pilot acquisition is that energy calculations for a given hypothesis offset being tested may actually under-report the true energy at that offset. For example, if there is a quarter-chip discrepancy between timing of the received PN sequence and the PN sequence generated for correlation with it, there will still be energy detected, but it will be substantially less than the actual energy available for receiving a signal at that offset. As a result, a valid pilot may not register enough energy to exceed a programmed threshold and therefore may be ignored. This may lead to selection of sub-optimal multipath signals depending on the relative size of the offset error. In the system context, the under-reported energy translates into increased mean time of search acquisition and as a result data rate and capacity suffer. To compensate for these ill effects, the system may need to be over-engineered. There is therefore a need in the art for improved pilot energy calculation techniques to more accurately detect pilot signals for increased acquisition performance.
SUMMARY
Embodiments disclosed herein address the need for increased pilot detection accuracy. In one aspect, pilot energy calculations corresponding to PN offsets surrounding the PN offset of a local pilot energy maximum are combined with that local pilot energy maximum resulting in a compensated, more accurate local pilot energy maximum. In another aspect, the combination of nearby energy calculations is combined with the local pilot energy maximum via a function with pre-calculated compensation factors. The pre-calculated compensation factors can be determined to minimize mean-squared error in the resultant compensated local pilot energy maximum. These factors can be calculated based on a matched filter used in conjunction with the disclosed embodiments. These aspects have the benefit of increasing the accuracy of pilot searching, which translates to increased acquisition speed, increased data rate, decreased power, and improved overall system capacity. The techniques described herein apply equally to both access points and access terminals. Various other aspects of the invention are also presented.
The disclosed method and apparatus provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.
REFERENCES:
patent: 5101390 (1992-03-01), Kuwabara
patent: 6201828 (2001-03-01), El-Tarhuni et al.
patent: 6445728 (2002-09-01), Byun
patent: 2002/0106007 (2002-08-01), Lomp et al.
patent: 19609324 (1997-09-01), None
Black Peter J.
Hinderling Jurg
Baker Kent D.
Le Amanda T.
Qualcomm Incorporated
Rouse Thomas R.
Wadsworth Philip R.
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