Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-11-05
2003-03-11
Ton, Dang (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S318000
Reexamination Certificate
active
06532222
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to signal demodulation in wireless communication systems. More particularly, this invention relates to apparatus and methods for improving the assignment of parallel demodulators to multipaths of wireless signals.
Wireless communication systems typically include pluralities of stationary transceivers (transmitter/receivers), known as base stations, and pluralities of mobile transceivers, known as mobile stations. Such systems control user access to typically shared communication channel capacity by using multiple access communication protocols. One such protocol is CDMA (code division multiple access).
In CDMA wireless systems, each mobile station communicating with a base station is assigned a unique code. This unique code is used to encode data transmitted from the base station to the mobile station assigned that code. Encoded data is transmitted via a modulated signal, which is a signal that has one or more of its wave characteristics (e.g., frequency, amplitude, or phase) modified to represent the encoded data. Upon receipt of a modulated signal, the encoded data is retrieved from the signal by a process known as signal demodulation. The encoded data is then decoded to extract the transmitted user information.
CDMA is used in both cellular and PCS (personal communications service) mobile communication networks. These types of networks divide geographic areas into cells. Each cell has a base station that communicates with mobile stations in that cell. A PCS network typically has smaller cells and operates at lower power and higher frequencies than common cellular networks. CDMA wireless systems typically adhere to either EIA/TIA IS-95, a digital CDMA standard for U.S. cellular radio systems published by the Electronic Industries Association/Telecommunications Industry Association, or ANSI J-STD-008, a similar standard for PCS networks published by the American National Standards Institute.
As mobile stations move from cell to cell, they are “handed-off” from one base station to another. In CDMA wireless systems, such hand-offs are “soft.” A soft handoff is one in which a mobile station first establishes communication with one or more base stations from cells that the mobile station may be entering before terminating communication with the base station from the cell that the mobile station is exiting. A mobile station in soft handoff can therefore be communicating with several base stations simultaneously. Such base stations are known as “active” base stations and are in the “active set” of that mobile station.
Each base station in soft handoff with a mobile station transmits the same information substantially simultaneously to that mobile station. Transmissions from the mobile station are separately received and demodulated by each those base stations. The demodulated transmissions that meet quality criteria are then combined and passed on to a mobile switching center. A mobile switching center switches transmitted data between wireless and wire-based networks (e.g., a public switched telephone network).
CDMA wireless systems are characterized by multipath signal propagation. Multipath signal propagation is a phenomenon that occurs when a base station transmits a signal to a mobile station and the mobile station receives multiple copies of that signal.
These multiple copies are typically created by signal reflection, and travel to the mobile station via different paths through the air. These copies, commonly referred to as multipath components, or simply multipaths, are separated from each other by small time intervals. These time intervals measured from a common reference are commonly known as “phases” of the transmitted signal. Thus a signal spread over several multipaths can be received by a mobile station at several phases, each phase corresponding to a multipath.
A mobile station can receive multipaths of a signal from either a single base station or several base stations. For example, assume a mobile station is in soft handoff with three base stations. If a signal transmitted from each base station results in two multipaths from the first base station, three multipaths from the second base station, and two multipaths from the third base station, the mobile station can receive the transmitted signal via seven multipaths. Multipaths associated with a particular base station can be distinguished from multipaths of other base stations by the spreading sequence (i.e., code) used to modulate the signals transmitted by that base station.
Signals transmitted by base stations in CDMA wireless systems based on the IS-95 or ANSI J-STD-008 standard typically include several communication channels, such as a pilot channel, other overhead channels (e.g., synch and paging channels), and traffic channels. Traffic channels carry information transmitted by users. Each base station in the active set of a mobile station assigns a particular traffic channel to a mobile station. That traffic channel is modulated by a unique “Walsh” code. The pilot channel, on the other hand, is modulated by a Walsh code known by all mobile stations. The pilot channel provides a phase reference that enables mobile stations to perform coherent demodulation of signals transmitted to them over their assigned traffic channel.
Mobile stations in CDMA wireless systems typically include CDMA terminals. A CDMA terminal receives, demodulates, combines, and decodes traffic channel signals received via different multipaths of a wireless signal from one or more active base stations. Note that such traffic channel signals can be combined because the same user information is carried by each multipath.
A CDMA terminal typically includes a “rake” receiver, which has a plurality of parallel demodulators (known as “fingers”). Each demodulator can be independently tuned (i.e., assigned) to a particular phase to demodulate traffic channel signals received at that phase. The CDMA terminal combines the demodulated signals from the assigned demodulators, and then decodes the combined signal to extract the transmitted information, which may be, for example, a voice communication. By combining separately demodulated signals in this manner, each carrying the same user information, the signal-to-noise ratio is enhanced, usually resulting in high quality wireless communication.
However, the exact phases at which multipaths appear at mobile stations are generally not known by the mobile stations. The presence of a multipath at a particular phase can be determined by measuring the power received at that phase. Furthermore, to improve the signal-to-noise ratio, and thus improve communication quality, demodulators should preferably be assigned to phases at which the strongest multipaths appear. Demodulators assigned to weak or non-existent multipaths contribute primarily only noise to the process of combining demodulated traffic channel signals. The strength of a multipath is indicated by the power level of the pilot channel signal contained within that multipath. CDMA terminals therefore continually measure power at various phases of received pilot signals to first determine whether multipaths are present and then to preferably identify the strongest multipaths.
To conduct initial power measurements, rake receivers typically have a search demodulator, commonly known as a “search finger,” in addition to the parallel demodulators used for traffic channel demodulation. The search demodulator typically follows a schedule to measure pilot channel power at various phases within search windows of several base stations. A search window is a period of time encompassing the probable phases at which multipaths from a particular base station may appear. A relatively high value of measured power at a particular phase within the search window of a base station generally indicates the presence of a multipath from that base station at that phase. Demodulator assignment logic within CDMA terminals analyzes the various power measurements to determine the pilot signal phases at which to assign demodulator
Balachandran Krishna
Cao Zhendong
Chang Kirk Kuokuang
Dinc Abdulkadir
Rege Kiran Meghashyam
Fish & Neave
Ton Dang
LandOfFree
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