Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-08-25
2003-01-21
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000
Reexamination Certificate
active
06510172
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and spread spectrum communication systems using satellites. More specifically, the invention relates to a communication system architecture in which communication signals are transmitted using transmission modules employing connected sets of modulators and spreading elements coupled to respective analog transmitters, to decrease data transfer requirements. The invention further relates to a method of redistributing certain signal modulation functions in a CDMA spread spectrum communication system to decrease data transfer 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. The use of spread spectrum modulation techniques, such as code division multiple access (CDMA), in multiple access communication systems is 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 wireless communication systems in which a number of generally mobile or remote system users or subscribers employ transceivers to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers typically communicate through gateways and satellites, or base stations (also referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals.
Base stations cover cells, while satellites have footprints on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into “sectors” by using directional antennas at the base station. Similarly, a satellite's footprint can be geographically divided into “beams”, through the use of beam forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be assigned multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite communication systems, each CDMA channel can be referred to as a “sub-beam” because there may be several of these channels per “beam”, or occupying the area covered by a beam.
In a typical spread-spectrum communication system, one or more, generally a set or pair of, preselected pseudonoise (PN) code sequences are used to modulate or “spread” user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user communication link, PN spreading codes or binary sequences are used to distinguish between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given CDMA channel or sub-beam.
Orthogonal channelizing codes are used to reduce interference and discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link. That is, each user terminal has its own orthogonal channel provided on the forward link by using a unique “covering” orthogonal code. Walsh functions are generally used to implement 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 addition, some form of signal diversity is used to reduce the deleterious effects of fading and additional problems associated with relative user, or satellite, movement within a communication system. 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. A form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space diversity is provided using multiple signal paths, typically, through different antennas or communication signal beams.
Base stations for terrestrial cellular communication systems typically use six antennas, two per each of three sectors in a sub-divided cell. Some designs plan for using additional antennas and polarization modes, providing additional CDMA channels. Base stations used with satellites, also referred to as gateways or hubs, use an array of transmitters, on the order of 32 or more, connected to one or more antennas to accommodate multiple beams on each carrier frequency. also provide service to multiple satellites, typically on the order of three or four at any given time. In one exemplary system, on the order of six satellites are used in each of eight orbital planes and even more satellites are contemplated for some systems. In addition, the number of communication channels or circuits per sub-beam in a satellite is on the order of 128 channels rather than the 64 typically found in terrestrial cellular systems. These factors greatly increase the amount of data and signal processing that must be accommodated within a system gateway as opposed to typical base stations.
When information, including voice, in the form of digital data is to be transferred to system users or subscribers by a gateway it is first encoded and interleaved as desired, and then “covered” and “spread” using appropriate orthogonal and spreading codes. Each data signal is processed by at least one modulator for each analog signal path over which it is to be transferred, for diversity purposes. The spread encoded data is then transferred to one or more analog transmitters where it is up-converted to an appropriate intermediate frequency and used to modulate a carrier waveform to form a desired communication signal.
Each analog transmitter represents one pre-selected diversity signal path for a signal, and multiple user signals are typically transferred through each analog transmitter, at any time. The signals for each analog transmitter are received from an array or number of modulator elements within the gateway, or base station, which are each assigned to process communications for particular users using particular signal path diversities. The signals from several modulators are combined to form a single output waveform for each analog transmitter. This means that data intended for each analog transmitter must be transferred along common busses or cable assemblies connected to the outputs of all modulators. That is, all modulators and analog transmitters are interconnected or connected using one set of common data busses in order to potentially process multiple path (diversity) signals for any given combination of analog transmitter, antenna, satellite, and user.
For current traffic channel data rates found within communication systems, the gateway busses transferring signals between digital modulators and analog transmitters would have to handle on the order of several gigabits-per-second (Gbps) or more. The ou
Ogrod Gregory D.
Pham Chi
Qualcomm Inc.
Tran Khai
Wadsworth Philip R.
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