CDMA/TDMA spread-spectrum communications system and method

Claims

I claim:

1. A method for spread-spectrum processing a plurality of data signals on a carrier frequency f.sub.o, comprising the steps of:

generating a plurality of privacy-code signals;

modulo adding each of the respective plurality of privacy-code signals to each of the plurality of data signals to generate a plurality of privacy-encoded data signals, respectively;

generating a synchronization-code signal;

time multiplexing the synchronization-code signal and the plurality of privacy-encoded-data signals, respectively, to generate a time-multiplexed signal;

generating a chip-code signal;

spread-spectrum processing the time-multiplexed signal with the chip-code signal as a spread spectrum-time-multiplexed signal; and

transmitting the spread-spectrum-time-multiplexed signal on the carrier frequency over a communications channel.

2. The method as set forth in claim 1 wherein the step of transmitting includes the steps of:

generating a carrier frequency;

shifting the spread-spectrum-time-multiplexed signal to the carrier frequency;

amplifying the spread-spectrum-time-multiplexed signal at the carrier frequency; and

radiating the amplified spread-spectrum-time-multiplexed signal at the carrier frequency over the communications channel.

3. A method for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising the steps of:

generating a synchronization-code signal;

time multiplexing the synchronization-code signal and the plurality of data signals, respectively, to generate the time-multiplexed signal;

spread-spectrum processing the time-multiplexed signal with a chip-code signal to generate a spread-spectrum-time-multiplexed signal; and

converting the spread-spectrum-time-multiplexed signal to a form suitable for sending over a communications channel.

4. The method as set forth in claim 3 wherein the step of spread-spectrum processing includes the steps of:

generating a chip-code signal; and

spread-spectrum processing the time-multiplexed signal with the chip-code signal as the spread-spectrum-time-multiplexed signal.

5. The method as set forth in claim 3 wherein the step of converting includes the steps of:

generating a carrier frequency;

shifting the spread-spectrum-time-multiplexed signal to the carrier frequency;

amplifying the spread-spectrum-tim-multiplexed signal at the carrier frequency; and

radiating the amplified spread-spectrum-time-multiplexed signal at the carrier frequency over a communications channel.

6. A method for spread-spectrum processing a spread-spectrum-time-division signal, wherein the spread-spectrum-time-division signal includes a plurality of spread-spectrum signals received sequentially in time from a plurality of remote units, comprising the steps of:

despreading the spread-spectrum-time-division signal as a time-division signal;

demultiplexing the time-division signal as a plurality of remote-privacy-encoded-data signals;

generating a synchronization-code signal;

generating a timing signal;

a generating a plurality of remote-privacy-code signals; and

decoding the plurality of remote-privacy-encoded-data signals, respectively, as a plurality of remote-data signals.

7. The spread-spectrum receiver as set forth in claim 6 wherein the steps of despreading includes the steps of:

generating a chip-code signal; and

despreading using the chip-code signal the received spread-spectrum-time-division signal as the time-division signal.

8. A method for spread-spectrum processing a spread-spectrum-time-division signal, wherein the spread-spectrum-time-division signal includes a plurality of spread-spectrum signals received sequentially in time from a plurality of remote units, comprising:

despreading the spread-spectrum-time-division signal as a time-division signal; and

demultiplexing the time-division signal as a plurality of remote-data signals.

9. The method as set forth in claim 8 wherein the steps of despreading includes the steps of:

generating a chip-code signal; and

despreading, using the chip-code signal, the received spread-spectrum-time-division signal as the time-division signal.

10. A method for spread-spectrum processing a spread-spectrum-time-division signal, wherein the spread-spectrum-time-division signal includes a plurality of spread-spectrum signal received sequentially in time from a plurality of remote units, comprising the steps of:

despreading the spread-spectrum-time-division signal as a time-division signal;

demultiplexing the time-division signal as a plurality of remote-privacy-encoded data signals; and

decoding the plurality of remote-privacy-encoded signals as a plurality of remote-data signals, respectively.

11. The method as set forth in claim 10 wherein the steps of despreading includes the steps of:

generating a chip-code signal; and

despreading, using the chip-code signal, the received spread-spectrum-time-division signal as the time-division signal.

12. The method as set forth in claim 11 wherein steps of synchronization includes the steps of:

generating a synchronization code; and

generating, using the synchronization-code signal a timing signal.

13. The method as set forth in claim 10 wherein the step of decoding includes the steps of:

generating, using the timing signal a plurality of privacy-coded signals; and

generating a synchronization code; and

generating, using the timing signal, the synchronization-code signal.

14. The method as set forth in claim 10 wherein the steps of decoding includes the steps of:

generating, using the timing signal, a plurality of privacy-code signals; and

decoding the plurality of privacy-encoded data signals as a plurality of data signals, respectively.

15. A method for receiving a spread-spectrum-time-multiplexed signal, having a synchronization signal and a plurality a privacy-encoded data signals including a first privacy-encoded-data signal, on a first carrier frequency, f.sub.1, and for spread-spectrum transmitting a remote-data signal on a second carrier frequency, f.sub.2, comprising the steps of:

despreading a spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

generating, using a synchronization-code signal embedded in the time-multiplexed signal, a timing signal;

generating, in response to the control signal, a first privacy-code signal;

decoding the first privacy-encoded-data signal as a first data signal;

generating, in response to the control signal, a remote-privacy-code signal;

encoding the remote-data signal with the remote-privacy-code signal as a remote-privacy-encoded-data signal;

generating a second chip-code signal;

spread-spectrum processing the remote-privacy-encoded signal with the second chip-code signal as a spread-spectrum signal; and

transmitting the spread-spectrum signal on the second carrier frequency, f.sub.2.

16. The method as set forth in claim 15 wherein step of despreading includes:

despreading, using the first chip-code signal, the received spread-spectrum-time-multiplexed signal as the time-multiplexed signal; and

filtering the time-multiplexed signal.

17. A method for receiving a spread-spectrum-time-multiplexed signal, having a synchronization-code signal and a plurality of privacy-encoded data signals including a first privacy-encoded data signal, at a first carrier frequency, and for spread-spectrum transmitting a remote-data signal at a second carrier frequency, comprising the steps of:

despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

detecting in the time-multiplexed signal the synchronization-code signal and generating a timing signal;

decoding, using the timing signal, the first privacy-encoded data signal embedded in the time-multiplexed signal as a first data signal;

encoding, in response to the control signal, the remote-data signal with a remote-privacy-code signal as a remote-privacy-encoded data signal;

spread-spectrum processing the remote-privacy-encoded data signal with a second chip-code signal to generate a spread-spectrum signal; and

converting the spread-spectrum signal to a form suitable for sending over a communications channel.

18. The method as set forth in claim 17 wherein the step of despreading includes the steps of:

generating a first chip-code signal;

despreading, using the first chip-code signal, the received spread-spectrum-time-multiplexed signal as the time-multiplexed signal; and

filtering the time-multiplexed signal.

19. The method as set forth in claim 17 wherein the step of decoding includes the steps of:

storing, in response to the control signal, the first privacy-encoded data signal embedded in the time-multiplexed signal;

generating, in response to the control signal, a privacy-code signal; and

decoding the first privacy-encoded data signal as the first data signal.

20. The method as set forth in claim 17 wherein the step of encoding includes the steps of:

generating, in response to the control signal, the remote-privacy-code signal; and

encoding the remote-data signal with the remote-privacy-code signal as the remote-privacy-encoded data signal.

21. The method as set forth claim 17 wherein steps of spread-spectrum processing includes the steps of:

storing the remote-privacy-encoded data signal;

generating a second chip-code signal; and

spread-spectrum processing the second privacy-encoded signal with the second chip-code signal as a spread-spectrum signal.

22. A method for receiving a spread-spectrum-time-multiplexed signal, having a synchronization-code signal and a plurality of data signals including a first data signal, at a first carrier frequency, and for spread-spectrum transmitting a remote-data signal at a second carrier frequency, comprising the steps of:

despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

detecting in time-multiplexed signal the synchronization-code signal and generating a control signal;

spread-spectrum processing the remote-data signal with a second chip-code signal as a spread-spectrum signal; and

converting the spread-spectrum signal to a form suitable for sending over a communications channel.

23. The method as set forth in claim 22 wherein said remote-despreader means includes the steps of:

generating a first chip-code signal;

despreading using the first chip-code signal the received spread-spectrum-time-multiplexed signal as the time-multiplexed signal; and

filtering the time-multiplexed signal.

24. The method as set forth in claim 22 wherein the step of spread-spectrum processing includes the steps of:

storing the remote-data signal;

generating a second chip-code signal; and

spread-spectrum processing the remote-data signal with the second chip-code signal to generate a spread-spectrum signal.

BACKGROUND OF THE INVENTION

The present invention relates to spread-spectrum communications, and more particularly to a direct sequence, code division multiple access, time division multiple access, spread-spectrum system.

DESCRIPTION OF THE RELEVANT ART

Spread-spectrum modulation has been, and still is used extensively in military communications systems both to permit communications which are not detectable by enemy jamming systems, and to resist jamming by an enemy desiring to disrupt communications. Signals which are not detectable by enemy intercept systems are called low probability of intercept signals.

For commercial applications using spread-spectrum modulation, full duplex operation is desirable, allowing a base station and a remote unit to communicate with each other simultaneously. One approach for full duplex is to assign a pair of chip codewords unique to the base station and the remote unit, and have them communicate simultaneously in time and at the same carrier frequency with each other. A problem with this approach is that the spread-spectrum signal radiated at the remote unit raises the noise level at the input to the spread-spectrum receiver at the remote unit.

Another approach for full duplex operation, which alleviates the problem of having the spread-spectrum transmitter raising the noise power level at the input to the spread-spectrum receiver at the remote unit or base station, is to assign different carrier frequencies for transmitting and receiving. Thus, the base station may communicate to a plurality of remote units at a first carrier frequency f.sub.1 and the plurality of remote units may communicate to the base station at a second carrier frequency f.sub.2. Using two carrier frequencies requires filters at each remote unit and at the base station to prevent leakage of signal energy from the respective spread-spectrum transmitter to the spread-spectrum receiver at each remote unit and at the base station. Additionally, different and long chip codewords are required with multiple remote units, which require complicated acquisition and tracking circuits. Using filters and long chip codewords adds to increased circuit complexity and cost.

OBJECTS OF THE INVENTION

A general object of the invention is a mobile cellular communications system, which allows full duplex operation without the need for complex filters for separating transmitter and receiver frequencies and complex tracking and acquisition circuits.

Another object of the invention is an inexpensive personal communications network, mobile cellular communications system.

An additional object of the invention is a spread-spectrum communications system which has little or no interference between users.

A still further object of the invention is a spread-spectrum system in which the system performance is thermal noise power limited rather than interference limited.

SUMMARY OF THE INVENTION

The present invention provides a system and method which transmit a code division multiple access (CDMA), time division multiple access (TDMA), spread-spectrum communications signal between a base station and a plurality of remote units. The base station time multiplexes a synchronization-code signal and a first plurality of data signals, or a first plurality of privacy-encoded data signals, to generate a time-multiplexed signal, and then spread-spectrum processes the time-multiplexed signal with a first chip-code signal to generate a spread-spectrum-time-multiplexed signal. The spread-spectrum-time-multiplexed signal is defined herein to include a time-multiplexed signal which is spread-spectrum processed with a chip-code signal.

Each remote unit receives and despreads the spread-spectrum-time-multiplexed signal, and demultiplexes a respective data signal being sent to the remote unit embedded in the time-multiplexed signal as a function of the synchronization-code signal. At a specified time, the remote unit sends a remote-data signal, or a remote-privacy-encoded data signal, to the base station by spread-spectrum processing the remote-data signal, or remote-privacy-encoded data signal, and transmitting the spread-spectrum signal over the communications channel. Since the remote unit is not transmitting and receiving at the same time, a transmit-receive switch can be employed to switch an antenna between the despreader circuitry and the transmitter at the remote unit. A remote-data signal is defined herein as a data signal being sent from a remote unit to the base station. A remote-privacy-encoded-data signal is defined herein as an encoded remote-data signal being sent from the remote unit to the base station.

At the base station, a plurality of spread-spectrum signals are received from the plurality of remote units as a spread-spectrum-time-division signal, which is despread as a time-division signal. The time-division signal accordingly is demultiplexed as a plurality of remote-data signals, or a plurality of remote-privacy-encoded data signals. The spread-spectrum-time-division signal is defined herein to include a plurality of spread-spectrum signals received sequentially in time from a plurality of remote units. The time-division signal is defined herein to include a plurality of remote-data signals, or a plurality of remote-privacy-encoded data signals, which are in different time slots due to the timing of when they are sent from each of the plurality of remote units.

More particularly, a spread-spectrum transmitter at the base station includes base-synchronization means, a plurality of base-privacy means, multiplexer means, base-spreading means, and base-transmitter means. The synchronization means generates a synchronization-code signal, and the plurality of base-privacy means encodes a plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded data signals, respectively. The encoding with a privacy-code signal may be an encryption type of privacy, or a less secure type of privacy. The multiplexer means time multiplexes the synchronization-code signal and the plurality of privacy-encoded data signals, respectively, to generate a time-multiplexed signal. The base-spreading means spread-spectrum processes the time-multiplexed signal with a first chip-code signal to generate a spread-spectrum-time-multiplexed signal. The time-multiplexed signal is converted by base-transmitter means to a form suitable for sending over the communications channel.

The plurality of base-privacy means is optional, and accordingly, the plurality of data signals need not be encoded with the plurality of privacy-code signals as a plurality of privacy-encoded data signals. If the plurality of privacy means were not used, then the multiplexer means time multiplexes the plurality of data signals to generate a time-multiplexed signal.

The spread-spectrum receiver at the base station processes a spread-spectrum-time-division signal using base-despreader means, base-demultiplexer means, base-synchronization means, and optionally, a plurality of base-decoder means. The base-despreader means despreads a received spread-spectrum-time-division signal as a time-division signal. The base-demultiplexer means demultiplexes the time-division signal as a plurality of remote-data signals or, in the event the plurality of data signals were encoded, as a plurality of remote-privacy-encoded data signals. The base-synchronization means, which is the same as used by, and operates cooperatively with, the spread-spectrum transmitter at the base station, generates the synchronization-code signal and a timing signal. In response to the timing signal, each of the base-decoder means decodes each of the remote-privacy-encoded signals as a remote-data signals.

At a remote unit, a spread-spectrum-time-multiplexed signal is received at the first carrier frequency. As set forth previously, the spread-spectrum-time-multiplexed signal includes a synchronization-code signal and a plurality of privacy-encoded data signals. The privacy-encoded-data signal to be received by the remote unit is defined herein to be the first privacy-encoded-data signal. A remote-data signal is spread-spectrum transmitted from the remote unit at a second carrier frequency.

The remote unit includes remote-despreader means, remote-synchronization means, control means, remote-decoder means, remote-privacy means, remote-spreading means, and remote-transmitter means. The remote-despreader means despreads the spread-spectrum-time-multiplexed signal received from the base station as a time-multiplexed signal. The remote-synchronization means detects the synchronization-code signal embedded in the time-multiplexed signal, and generates a timing signal. The control means generates a control signal in response to the synchronization-code signal. The control signal has the proper timing, relative to the synchronization-code signal, for operating the remote unit. The proper timing includes having the appropriate data signal or privacy-encoded data signal embedded in the time-multiplexed signal stored in a buffer. The remote-decoder means, using timing from the control signal, decodes a first privacy-encoded data signal embedded in the time-multiplexed signal as a first data signal.

In response to the control signal, the remote-privacy means encodes the remot