Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1999-12-28
2003-02-25
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S347000, C375S349000
Reexamination Certificate
active
06526090
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present embodiments relate to spread spectrum communications systems and, more particularly, to the assignment of spread spectrum signals to demodulation elements in a communications receiver.
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”) cellular communications. 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”, or a sector of a cell, as that term is used in the art. Due to movement of the user station and the fixed location of other cells with corresponding base stations, the user station may communicate with one or more than one base station at a time in the same frequency band, and this operation is typically referred to as soft handoff. As a result and in a simple example, this dual base station, user station communication is achieved by two different “paths” of forward/reverse communication, that is, a first path from the user station to the first base station and a second path from the user station to the second base station and a first path from the first base station to the user station and a second path from the second base station to the user station.
In the actual reality of CDMA communications, and due to the wireless medium, a same transmitted communication from a base station to a user station may arrive at the user station at multiple and different times, where each different arriving signal is said to travel along a channel and arrive as a different “path.” This is because the transmitted signal from the base station is reflected by objects such as the ground, mountains, buildings, and other things which it contacts. These multiple signals are referred to in the art as multiple paths or multipaths. Thus, several multipaths may eventually arrive at the user station but 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 between one base station and one user station, each multipath is a replica of the same user information, and each path is said have time diversity relative to other mulitpath(s) due to the difference in arrival time which causes different (uncorrelated) fading
oise characteristics for each multipath. However, as detailed later, other types of signals with the same information also may arrive at the receiver at different times and, thus, to distinguish these other signals from multipath signals then for purposes of the remainder of this document the time diversity among different multipaths is referred to as mulitpath diversity.
Although multipaths carry the same user information to the receiver they may be separately recognized by the receiver based on the timing of arrival of each multipath. More particularly and as known in the art, 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 use of transmission of the CDMA signal using chips, then multipaths separated in time by more than one of these chips are distinguishable at the receiver because of the low auto-correlations of CDMA codes as known in the art.
By way of further introduction, note that other types of signal diversities are known in the art, where one such diversity is referred to as base station diversity. More particularly, recall from above that a user station may over a common time period receive the same user information from two different base stations. Thus, the information received from one base station is said to have base station diversity with respect to the information received from another base station. Again, so long as these base station diverse signals are adequately separated in time or use different codes, then they are distinguishable from one another at the receiver.
In the prior art, after identifying received paths the receiving station sets forth to demodulate certain ones of the paths. More particularly, it is known in the art to include a demodulation circuit in a CDMA receiver, where such a circuit commonly includes more than one demodulation element. For example, one such type of demodulation circuit is a Rake combiner, which is given its name in the art to suggest the notion of a yard rake having “fingers” and where each finger corresponds to a different demodulation element. In contemporary systems, such a demodulation circuit may include on the order of four or six different demodulation elements, where each element is capable of concurrently demodulating a received path that is assigned to the element. Accordingly, in the prior art as different multipath or base station diversity signals are received, the receiver selects which ones to assign to its available demodulation elements. Thus, this requires a determination if any such demodulation elements are not currently in use, of if they are in use, whether there should be a reassignment whereby a newly-received path pre-empts that use to take the place of a path that was earlier assigned to a demodulation element and is currently being demodulated by that element.
The objective of the process of selecting certain paths for demodulation is typically to optimize a performance parameter such as the frame error rate (FER) or the symbol error rate (SER), or to enhance the reliability of the communication link by reducing the probability of an outage. As an example, a method for demodulation element assignment in a CDMA communications system is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS”, issued Feb. 6, 1996 (the '165 patent). The '165 patent provides different demodulation assignment methods based on whether the receiver is the user station or the base station, each of which is separately discussed below.
In the '165 patent, the demodulation element assignment method for the user station emphasizes base station diversity. More particularly, the '165 patent approach requires that the user station always has a demodulation element assigned to at least one path originating from the signal transmitted by each different base station. Thus, if the user station receives a path from a new base station (i.e., one which is not currently being demodulated by the user station), then the user station unconditionally assigns a demodulation element to the received path. This assumes that the user station communicates with more than one base station at that particular time instant, that is, that the user station is in soft handoff. If the user station is not in soft handoff or if it has a demodulation element assigned to each base station participating in soft handoff, the remaining unassigned demodulation elements are assigned to paths having the largest SNR (or signal-to-interference ratio (SIR)). Also, as long as every different base station communicating with the user station has at least one demodulation element assigned, all but the strongest path (i.e., largest SNR or SIR) from each base station may be dismissed and the demodulation elements be reassigned to paths with the largest SNRs.
Having introduced soft handoff above, a few other prior art observations relating to soft handoff are now noted. The assignment of a first path from a new base station to a demodula
Brady III Wade James
Neerings Ronald O.
Pham Chi
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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