Multiplex communications – Communication over free space
Reexamination Certificate
1998-09-09
2002-02-12
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
C370S252000, C370S328000, C370S335000, C370S342000, C455S422100, C455S452200, C455S522000
Reexamination Certificate
active
06347080
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to detection of data rates in communication signals. More specifically, the present invention relates to a system and method for detecting when zero-rate data frames in communication signals are received at a communications receiver.
II. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
A variety of multiple access communications systems and techniques have been developed for transferring information among a large number of system users. These techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) spread-spectrum techniques, the basics of which are well known in the art. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, entitled “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 incorporated herein by reference.
The above-mentioned patent documents disclose multiple access communications systems in which a large number of generally mobile or remote system users employ user terminals or mobile stations to communicate with other system users or users of other connected systems, such as a public telephone switching network. The user terminals communicate through gateways and satellites using CDMA spread-spectrum type communications signals.
Communications satellites form beams which illuminate “spots” produced by projecting satellite communications signals onto the Earth's surface. A typical satellite beam pattern comprises a number of beams arranged in a predetermined coverage pattern. Typically, a number of CDMA channels (also referred to as sub-beams), each occupying a different carrier frequency, are transmitted in a beam.
In a typical spread-spectrum communications system, a set of preselected pseudo-random noise (PN) code sequences is used to spread information signals over a predetermined spectral band prior to modulation onto a carrier frequency for transmission. 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 underlying data signal being transmitted. In a forward communications link (that is, in a communications link originating at a gateway and terminating at a user terminal), different PN spreading codes or binary sequences are used to distinguish signals transmitted by one or more gateways over different beams. These PN codes are shared by all communications signals within a given sub-beam.
In a typical CDMA spread-spectrum system, different channelizing codes are used to discriminate between signals intended for different user terminals within a sub-beam on the forward link. That is, a unique orthogonal code channel is provided for each user terminal on the forward link by using a unique “channelizing” orthogonal code. Walsh functions, or equivalent codes, are generally used to implement the channelizing codes.
Typical CDMA spread-spectrum communications systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation. In mobile communications systems using this approach, a “pilot” carrier signal (hereinafter referred to as a “pilot signal”) is used as a phase reference for demodulation. When this approach is used on the forward link, a pilot signal, which typically contains no data modulation, is transmitted by a gateway throughout a coverage region. A single pilot signal is typically transmitted by each gateway for each beam on each frequency (sub-beam) used. These pilot signals are shared by all user terminals receiving signals from the gateway.
In the forward link, pilot signals are used by user terminals to obtain initial system synchronization and as a time, frequency, and phase reference for demodulating other signals transmitted by the gateway. Phase information obtained from a pilot signal is used as a carrier phase reference for coherent demodulation of other system signals or traffic signals. Since all forward link signals on a sub-beam are transmitted synchronously, this technique allows all traffic signals on a sub-beam to share a common pilot signal as a phase reference, providing for a less costly and more efficient signal acquisition or tracking mechanism.
When a user terminal is not involved in a two-way communications session (that is, the user terminal is not receiving and transmitting traffic signals simultaneously), the gateway can convey information to that particular user terminal using a signal channel known as a paging channel. For example, when a call has been placed to a particular mobile station or user terminal, phone, the gateway alerts the mobile station by means of a message on the paging channel. Paging channel messages are also used to distribute information about traffic channel assignments, access channel assignments, system configuration, and the like.
A user terminal can respond to a paging channel message by sending an access channel message over the reverse link (that is, the communications link originating at the user terminal and terminating at the gateway). The access channel is also used by a user terminal when it originates a call, that is requests that a communication link be established.
In a preferred embodiment, data on the forward and reverse links are transmitted at a variable rate. During conversations as well as data transmissions, there are often pauses and periods of intermittent activity that generate little information to be transmitted. On a duplex communication link, such as voice telephone links, often one party is speaking and the other is listening. For voice telephone links, therefore, one of the two links is idle almost half of the time, and can be transmitted using a lower data rate. Additionally, during pauses or periods of intermittent activity, data on forward and reverse links are transmitted at a lower rate. This lower data rate can approach or be equal to zero, that is, no data, in some situations.
Variable rate data transmission results in many benefits in a multiple access system. Detection of the transmitted signal by the receiver depends on signal energy. By decreasing the rate of data transmission, signal energy can be maintained with a lower average transmission power. This is very useful for power limited applications, such as in mobile transceivers. In such cases, reducing the average transmitter output power increases the time before power sources such as batteries must be recharged or replaced, and overall battery life. Additionally, lower power reduced rate transmission reduces the interference and background noise seen by other receivers on the sub-beam. Since transmitters are power controlled to maintain a constant signal energy to noise power ratio at the receiver, lower background noise power results in reduced signal energy requirements. Lower background noise power, therefore, allows other transmitters on the sub-beam to transmit at lower power. Reduced interference may also allow desired additional user capacity in a communication system.
What is needed, therefore, is a system and method that allow the transmission and detection of zero-rate data transmission on the forward and reverse communication links.
SUMMARY OF THE INVENTION
The present invention sati
Jou Yu-Cheun
Nicolas Julien
Silberger Amnon
Chin Wellington
Ogrod Gregory D.
Phan M.
Qualcomm Inc.
Wadsworth Philip R.
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