Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-10-09
2002-12-10
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06493376
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to spread spectrum communication systems, such as wireless data or telephone systems, and satellite communication systems. More particularly, the invention relates to a method and apparatus for generating, identifying, and acquiring spread spectrum communication signals using layered or overlayed PN spreading and identifier codes having differing periods or chip rates.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users, such as code division multiple access (CDMA) spread spectrum techniques. CDMA techniques in multiple access communication systems are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters”, and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, under the title “Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,” which are both assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which a large number of generally mobile or remote system users or subscribers use transceivers to communicate with other system users or desired signal recipients, such as through a connected public telephone switching network. The system users communicate through gateways and satellites, or terrestrial base stations (also referred to as cell-sites or cells) using CDMA spread spectrum communication signals.
In a typical spread-spectrum communication system, one or more sets or pairs of preselected pseudorandom noise (PN) code sequences are used to modulate or ‘spread’ user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the underlying data signal. In the base station- or gateway-to-user communication link, also referred to as the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or between signals of different beams, satellites, or gateways, as well as between multipath signals.
These codes are typically shared by all communication signals within a given cell or beam, and time shifted or offset between adjacent beams or cells to create different spreading codes. The time offsets provide unique beam identifiers which are useful for beam-to-beam handoff and for determining signal timing relative to basic communication system timing.
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between signals intended for different users within a cell or between user signals transmitted within a satellite beam, or sub-beam, on a forward link. That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique ‘covering’ or ‘channelizing’ orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. In this arrangement, each Walsh function of 64 or 128 chips is typically referred to as a Walsh symbol.
PN code based modulation techniques used in CDMA signal processing allow spectrally similar communication signals to be quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished from each other, provided path length differential causes relative propagation delays in excess of the PN code chip period. If a PN chip rate of say approximately 1.22 MHz is used, a spread spectrum communication system can distinguish or discriminate between signals or signal paths differing by more than one microsecond in path delay or time of arrival.
Wideband CDMA techniques permit problems such as multipath fading to be more readily overcome and provide a relatively high signal gain. However, some form of signal diversity is also generally provided to further reduce the deleterious effects of fading and additional problems associated with acquiring and demodulating signals in the presence of relative user and satellite or source movement within a communication system. Such movement along with large distances causes substantial dynamic changes in path lengths. Generally, three types of diversity are used in spread spectrum communication systems, including time, frequency, and space diversity. Time diversity is obtainable using error correction coding or simple repetition and time interleaving of signal components, and a form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Space diversity is provided using multiple signal paths, typically, through different antennas or communication signal beams.
Typical CDMA spread spectrum communication systems contemplate the use of coherent modulation and demodulation techniques for forward link user terminal communications. In communication systems using this approach, a ‘pilot’ signal (or other known signal) can be used as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station or gateway throughout a given region of coverage. A single pilot is typically transmitted by each gateway or base station for each frequency used, typically referred to as a CDMA channel, an FDM channel, or as a sub-beam in some systems. This pilot is shared by all users using that CDMA channel, from a common source. Generally, sectors each have their own distinct pilot signals while satellite systems transfer a pilot within each satellite beam, or frequency or sub-beam, which originates with gateways using the satellite. This provides signals that can be readily distinguished from each other, also distinguishing between beams and cells while providing simplified signal acquisition.
Pilot signals are employed by user terminals to obtain initial system synchronization, and provide robust time, frequency, and phase tracking of transmitted signals and a channel gain reference. Phase information obtained from a pilot signal is used as a phase reference for coherent demodulation of communication system or user information signals. Since pilot signals do not generally involve data modulation, they essentially consist of the PN spreading codes which are modulated onto a carrier frequency. Sometimes, the PN spreading codes are referred to as pilot code sequences. The PN spreading codes are generally time shifted with respect to each other to achieve distinguishable pilot signals.
Pilot signals are generally used to gauge relative signal or beam strength for received communication signals. In many systems, pilot signals are also generally transmitted at a higher power level than typical traffic or other data signals to provide a greater signal-to-noise ratio and interference margin. This higher power level also enables an initial acquisition search for a pilot signal to be accomplished at high speed while providing for very accurate tracking of the pilot carrier phase using relatively wide bandwidth, and lower cost, phase tracking circuits.
As part of the process of establishing a communication link, the user terminal employs a receiver referred to as a ‘searcher receiver’, or simply ‘searcher’, to synchronize with the pilot phase and PN spreading code timing in the presence of unknown carrier frequency offsets. Several techniques and devices have been used to provide this searcher function. One such technique is disclosed in U.S. Pat. No. 5,109,390 entitled “Diversity
Boodakian Margo
Butler Brian
Harms Brian
Skinner Gordon
Boodakian Margo
Choi Jae-Hee
Corrielus Jean
Ogrod Gregory D.
Qualcomm Inc.
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