Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-09-26
2004-12-07
Chin, Stephen (Department: 3634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S142000, C375S150000, C375S137000
Reexamination Certificate
active
06829290
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present embodiments relate to wireless communications systems and are more particularly directed to selecting paths for further processing in such systems.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” More particularly, CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (“WCDMA”). WCDMA includes alternative methods of data transfer, one being time division duplex (“TDD”) and another being frequency division duplex (“FDD”). The present embodiments may be incorporated in either TDD or FDD and, thus, both are further introduced here. TDD data are transmitted in one of various different forms, such as quadrature phase shift keyed (“QPSK”) symbols or other higher-ordered modulation schemes such as quadrature amplitude modulation (“QAM”) or 8 phase shift keying (“PSK”). In any event, the symbols are transmitted in data packets of a predetermined duration or time slot. Within a data frame having 15 of these slots, bi-directional communications are permitted, that is, one or more of the slots may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station. Further, the spreading factor used for TDD is relatively small, whereas FDD may use either a large or small spreading factor. FDD data are comparable in many respects to TDD including the use of 15-slot frames, although FDD permits a different frequency band for uplink communications (i.e., user to base station) versus downlink communications (i.e., base to user station), whereas TDD uses a single frequency in both directions.
Due to various factors including the fact that CDMA communications are along a wireless medium, an originally transmitted communication from a base station to a user station may arrive at the user station at multiple and different times. Each different arriving signal that is based on the same original communication is said to have a diversity with respect to other arriving signals originating from the same transmitted communication. Further, various diversity types may occur in CDMA communications, and the CDMA art strives to ultimately receive and identify the originally transmitted data by exploiting the effects on each signal that are caused by the one or more diversities affecting the signal.
One type of CDMA diversity occurs because a transmitted signal from a base station is reflected by objects such as the ground, mountains, buildings, and other things that it contacts. As a result, a same single transmitted communication may arrive at a receiving user station at numerous different times, and assuming that each such arrival is sufficiently separated in time, then each different arriving signal is said to travel along a different channel and arrive as a different “path.” These multiple signals are referred to in the art as multiple paths or multipaths. Several multipaths may eventually arrive at the user station and the channel traveled by each may cause each path to have a different phase, amplitude, and signal-to-noise ratio (“SNR”). Accordingly, for one communication from one base station to one user station, each multipath is originally a replica of the same originally transmitted data, and each path is said to have time diversity relative to other multipath(s) due to the difference in arrival time which causes different (uncorrelated) fading
oise characteristics for each multipath. Although multipaths carry the same user data to the receiver, they may be separately recognized by the receiver based on the timing of arrival of each multipath. More particularly, CDMA communications are modulated using a spreading code which consists of a series of binary pulses, and this code runs at a higher rate than the symbol data rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. Given the transmission of the CDMA signal using chips, then multipaths separated in time are distinguishable at the receiver because of the low auto-correlations of CDMA codes. Also, given that numerous multipaths may arrive at a receiving user station, the prior art endeavors to select certain of these multipaths and then to perform various processing on those paths in an effort to combine the signals to remove the effects of the diversity and to better recover the originally-transmitted data represented by those signals. However, before this selection process occurs, various acquisition steps are performed by the receiving user station and which are discussed below by way of further introduction.
According to the prior art a receiving user station first processes incoming signals, often using what is referred to as a searcher and in a first acquisition stage. Specifically, each incoming frame includes a so-called synchronization channel against which correlations may be made by the receiving user station for purposes of acquisition, where the synchronization channel includes two codes, namely, a primary synchronization code (“PSC”) and a secondary synchronization code (“SSC”). The PSC is presently a 256 chip Golay code and the same PSC code is transmitted from numerous base stations. Each base station group transmits a unique set of SSC code words. In any event, during the first acquisition stage, the user station continuously samples information in at least one slot and performs a PSC correlation on those samples. For example, this technique may be implemented by applying the received information to a matched filter having the 256 chip Golay code of the PSC as coefficients to the filter, and the results of the correlations may be processed further such as through the use of averaging. Moreover, the number of measured correlations typically depends on the data rate and sample rate. For example, presently a single slot in a frame has a 667 &mgr;sec duration corresponding to a chip rate of 3.84 Mcps (although in the past the chip rate was 4.096 Mcps and provided a 625 &mgr;sec slot duration). Further, such a slot typically includes 2560 chips, and the PSC correlation measurement or sampling is typically twice per chip, thereby giving rise to a total of 5120 sample positions evaluated per slot. In any event, as a result of these measurements, one or more paths within the evaluated time period are found to have relatively Garge PSC correlations, and the position(s) of these path(s) are generally used to identify the timing of incoming frames. Lastly, since the PSC is the same for various base stations, then note that the identified one or more paths may correspond to one or more base stations.
Also according to the prior art, a receiving user station next processes incoming signals to perform a second acquisition stage with respect to the SSCs in the incoming signals. Recalling that the SSC is base-station specific, note that the second acquisition stage is therefore directed to a particular base station. Further, note that the accumulation of several SSCs across a frame or frames are sometimes referred to in
Dabak Anand G.
Gatherer Alan
Hosur Srinath
Schmidl Timothy M.
Brady III Wade James
Chin Stephen
Neerings Ronald O.
Pathak Sudhanshu C.
Telecky , Jr. Frederick J.
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