Programmable matched filter searcher for multiple pilot...

Details Programmable matched filter searcher for multiple pilot... Programmable matched filter searcher for multiple pilot...

1999-12-21

2002-11-12

Ghayour, Mohammad H. (Department: 2734)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S143000, C375S147000, C375S343000

Reexamination Certificate

active

06480529

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for detecting one or more pilot signals with a programmable matched filter searcher.
II. Description of the Related Art
Pseudorandom noise (PN) sequences are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1
provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units
10
a-d
conduct wireless communication by establishing one or more RF interfaces with one or more base stations
12
a-d
using CDMA modulated RF signals. Each RF interface between a base station
12
and a subscriber unit
10
is comprised of a forward link signal transmitted from the base station
12
, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO)
14
and public switch telephone network (PSTN)
16
. The links between base stations
12
, MTSO
14
and PSTN
16
are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
Each subscriber unit
10
communicates with one or more base stations
12
by utilizing a rake receiver. A RAKE receiver is 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 pilot from neighboring base stations, and two or more 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, assigned to the assignee of the present invention and incorporated herein by reference.
Inherent in the design of direct sequence spread spectrum communication systems is the requirement that a receiver must align its PN sequences to those of the base station. 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 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, fingers would adjust their PN generators to produce the appropriate PN sequence for the base station to which it is assigned.
Subscriber units locate base stations by utilizing searchers. A fast, flexible, and hardware efficient matched filter searcher is described in co-pending U.S. patent application Ser. No. 09/283,010 (hereinafter the '010 application), entitled “PROGRAMMABLE MATCHED FILTER SEARCHER”, filed Mar. 31,1999, assigned to the assignee of the present invention and incorporated herein by reference. This searcher adds flexibility to the parallel computation features of a matched filter, allowing a variable number of coherent accumulations and a variable number of non-coherent accumulations to be performed at high speed for a wide range of search hypotheses in a resource efficient manner. Many of the features of this searcher are applicable to the present invention as well, and will be described in greater detail below.
The FCC has mandated that by October 2001, carriers must provide the location of a cell phone user making an emergency 911 call to within 125 meters. In addition to providing mandated location services, wireless carriers are interested in providing revenue-generating location-based services such as roadside assistance, traffic updates, yellow page directory assistance, and the like.
A variety of approaches can be taken to solve this problem, among them are solutions based on the Global Positioning System (GPS). The Global Positioning System comprises a constellation of 24 satellites. Each satellite contains a clock that is kept synchronized to GPS time by monitoring ground stations. GPS receivers on the ground can use signals received from several GPS satellites to determine position and time.
Each GPS satellite transmits two microwave carriers: a 1575.42 MHz L
1
carrier which carries the signals used for Standard Positioning Service (SPS), and a 1227.60 MHz L
2
carrier which carries signals needed for Precise Positioning Service (PPS). PPS is used by governmental agencies and allows a higher degree of accuracy in positioning.
The L
1
carrier is modulated by the Coarse Acquisition (C/A) code, a 1023-chip pseudorandom code transmitted at 1.023 Mcps that is used for civil position location services. Each GPS satellite has its own C/A code that repeats every 1 ms. The code used for PPS is a 10.23 MHz code that is 267 days in length.
Each GPS satellite has a different C/A code that belongs to a family of codes called Gold codes. Gold codes are used because the cross-correlation between them is small. Each GPS satellite generates a unique C/A code sequence. A GPS receiver reproduces the C/A sequence for a particular satellite and correlates it with the received signal over all possible offsets. When correlation is found, the start time of the code is referred to as the time of arrival (TOA) at the receiver. This TOA is a measure of the range to the satellite, with an offset due to any mismatch between the receiver clock and GPS time. The TOA is also referred to as the pseudorange. Once the pseudoranges from each of 4 satellites have been obtained, a position fix can be computed by solving for the intersection of 4 spheres. Using 4 satellites allows the receiver clock uncertainty to be cancelled out.
GPS position location can be done based solely on signals received from GPS satellites, as just described, but can also be accomplished using a hybrid scheme. Such hybrid schemes are often useful when additional information is available to reduce the complexity of the position location task. One example is a wireless network, where the base station can provide information to limit the required search windows or can provide accurate time corresponding to GPS time. One such system is described in co-pending U.S. patent application Ser. No. 09/187,

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