Orthogonal code synchronization system and method for spread spectrum CDMA communications

Abstract

A mobile terminal first transmits to a base station a signal in a reverse-link direction wherein a timing used for said signal is set according to a transmission received in a forward-link direction from the base station. At the base station, a synchronization sequence in the transmitted signal is detected and the timing of the transmitted signal is evaluating. The base station then sends timing adjustment information in the form of synchronization commands to the mobile terminal based on said evaluation. These synchronization commands indicate timing adjustments in one-eighth of a chip increments.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 09 / 653,057, filed Sep. 1, 2000; which is a continuation of U.S. patent application Ser. No. 09 / 280,327, filed Mar. 29, 1999 now U.S. Pat. No. 6,141,332; which is a continuation of Ser. No. 08 / 807,228 filed Feb. 28, 1997 now U.S. Pat. No. 5,943,331, which are incorporated by reference as if fully set forth.TECHNICAL FIELD [0002] This invention relates to spread-spectrum communications and more particularly to a system and method using orthogonal codes and knowledge of the distance between a mobile terminal and a base station to adjust and align the phase of an information channel to achieve orthogonality at the base station. BACKGROUND ART [0003] Referring to FIG. 1, message data, d(t), are processed by spread-spectrum modulator 51, using a message-chip-code signal, g1(t), to generate a spread-spectrum data signal. The spread-spectrum data signal is processed by trans...

AI Technical Summary

Benefits of technology

[0017] In a CDMA system, there is a better way to derive a clean local carrier, at the receiver, than deriving it from the information channel. In a CDMA system it is possible to send the same RF carrier but with a different PN code superimposed on it. This signal has no unknown information on it; it is a completely predetermined signal known on both ends of the link. Since this signal has a different code than the user information channel code, it is completely resolvable from the user information channel. Therefore, the two signals can occupy the same spectrum at the same time and only cause minor interference to each other. This signal is called a pilot channel and can be filtered with a narrow filter at the receiver which allows it to be a very stable reference. The user information channel phase is then compared to this clean reference to determine what changes are made to reflect the information on the user information channel. On the forward link, the same pilot channel is used as the reference for many mobile user stations. As a result, the power of the pilot channel can be made several times greater than the power of an individual user information channel and still have a small impact on the total power transmitted by the base station. This power factor, combined with the fact that all the signals have the same originating point and the same timing source, makes it easy to use orthogonal codes on this forward transmission link. All the mobile users receive the same composite CDMA forward transmission signal and use the same pilot channel to extract their assigned user information channel from the composite CDMA signal.

Problems solved by technology

In a fading channel, such as the ionosphere or any channel containing multipath, or more generally, any channel in which the received signal's amplitude fluctuates with time, synchronous demodulation is not practical since the phase of the incoming signal typically is not the same as the phase of the reference.

Such a system is complex and suffers degradation of about 6 dB at error rates of 10−2.

The prior art does not provide a system and method for synchronously communicating, using spread-spectrum modulation, with a base station and in combination using range to the mobile terminal to achieve orthogonality at the base station.

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