Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-09-03
2001-08-14
Hsu, Alpus H. (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000
Reexamination Certificate
active
06275483
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to CDMA spread spectrum cellular communication systems in which an unmodulated (pilot) spread spectrum signal, such as a pseudonoise (PN) signal, is transmitted by each base station with a base station specific PN code offset and utilized by a user station to identify a number of existing base stations and provide a strength indication for the signal from each base station as it is received at the user station. In particular, the present invention relates to a novel method and implementation to achieve fast identification of pilot paths originating from any transmitting base station and having any specified signal-to-noise ratio (SNR).
BACKGROUND OF THE INVENTION
Spread spectrum communications have been established as one of the more viable alternatives for cellular systems. In such systems, the particularly desirable property of low cross-correlations of PN spread spectrum codes for signals separated in time by an amount equal to or larger than one code element (chip), has given rise to the use of the same PN code in the same frequency band by different base stations in order to maximize capacity and facilitate system engineering. A PN code is a binary pseudorandom code and each of its elements is called a chip. Several well known sets of PN codes exist, such as M sequences and Gold codes. Because of the pseudonoise code properties, the code autocorrelation results a peak only when the offset between the received and the local PN codes is less than a chip. In that case, the received and local codes become effectively synchronized.
Signal separation among different cells can be achieved by having the different base stations use different offsets of the same PN code. The user station can simultaneously demodulate signals that are strong enough to be discriminated from background noise and are separated by more than one chip in time. Therefore, the user station, which is typically mobile, can simultaneously maintain communication with more than one base station, to enable smooth transitions between base stations. This concept is known in the literature as soft hand-off. To facilitate soft hand-off, the user station may identify received pilot multipaths from several base stations that use the same PN code but have an appropriately different PN code offset, measure the combined SNR of multipaths corresponding to a pilot signal transmitted by each of the base stations, and report the pilot PN code offsets and the SNR measurements to the base station or stations with which it currently communicates.
Since cell sites can have different sizes and the user station can be at any point within a particular cell, the propagation delay of the pilot transmitted by a base station can be on the order of a few tens to a few hundred chips. Once a user station has established communications with at least one base station, that base station communicates to the user station certain predetermined parameters relating to information about the cellular system. Among those parameters is timing information, in terms of PN code offsets from the base station's pilot offset, for PN codes of pilots corresponding to other base stations. Those base stations, and the corresponding pilots, are categorized into sets depending on the likelihood for the base station to participate in soft handoff.
For each pilot, the propagation delay range establishes a PN code offset uncertainty area around the PN code offset with which the pilot was originally transmitted by the corresponding base station. The user station needs to examine the entirety of such PN code offset uncertainty area, determine each offset at which a path with an SNR above a specified level exists, accurately measure the combined SNR for the possibly more than one paths originating from the pilot signal that is transmitted by each base station, and report the results to the base station or stations currently supporting the communication.
The user station needs to maintain pilot signals in sets, called soft hand-off sets. The set of each pilot from a particular base station is primarily determined by the corresponding SNR. There are usually several pilot signals in each set and each of them needs to have its strength and relative offset determined within a set-dependent time period. Pilot sets that are more important for reliable communications need to be updated more frequently. For proper performance, excessive delays between successive SNR measurements must be avoided. Thus, there is a need to minimize the time required to reliably identify the offsets corresponding to usable pilot signals. Only for those offsets are subsequent time consuming SNR measurements performed. Another reason for minimizing the time required for identification of usable pilot paths is to limit power consumption at the user station.
Finally, in addition to soft handoff, the user station needs to identify offsets corresponding to paths with the largest SNRs in order to assign those paths to available demodulators, also known as Rake fingers, and maximize the total SNR in order to optimize the communication quality. Although paths corresponding to the information signal need to be identified for that purpose, if the transmission of signals from the base station is synchronous, identifying a pilot path is equivalent to identifying an information signal path. This goal also requires continuous search of a window for path identification and SNR measurements.
SUMMARY OF THE INVENTION
The present invention operates in code division multiple access (CDMA) cellular-based communication systems in which a user station can demodulate multiple signals, not necessarily coming from the same base station. In accordance with the present invention, a method is provided to rapidly identify at the user station spread spectrum code offsets at which actual signal paths exist. In such systems a plurality of base stations transmit data unmodulated pilot signals, wherein each of the pilot signals comprises a radio frequency signal modulated by a predetermined spread spectrum code at a predetermined rate. The base stations modulate their respective radio frequency signals with the same spread spectrum code but each of the base stations have a different, predetermined or random offset of the spread spectrum code from other of the base stations in a given vicinity, or, alternatively, the base stations modulate their respective radio frequency signals with different spread spectrum codes that are known by the user station. The inventive method, for the communications unit to identify the arrival time of each path from a specific base station by selecting specific code offsets with which to demodulate the signals of different paths originating from the specific base station, comprises the following steps. First, a radio frequency signal modulated by a predetermined spread spectrum code is received. The radio frequency signal is demodulated to produce a demodulated signal. A generated signal encoded with the spread spectrum code at a rate substantially the same as the predetermined rate is provided. A predetermined set of code offsets within which to search for the signals of different paths of the base stations is also provided. A new subset of code offsets is selected by processing the demodulated signal with the generated signal at a first predetermined offset of the set of code offsets to produce a decision value related to a degree of correlation between the demodulated signal and the generated signal, determining an SNR measurement at the predetermined offset using the decision value, repeating the steps of processing, and determining for the remaining the offsets in the set of code offsets for a predetermined number of iterations, and continuing further processing for a subset of code offsets which belong to the set of code offsets and which are of a predetermined number and have the largest SNR values. The step of selecting a new subset of code offsets is repeated for a predetermined number of iterations.
In some embodiments the us
Lee Yuan Kang
Papasakellariou Aristides
Brady III W. James
Hsu Alpus H.
Moore J. Dennis
Nguyen Brian
Telecky , Jr. Frederick J.
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