USRE35402E

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Publication number: USRE35402E
Application number: US08/277,142
Authority: US
Inventors: Donald L. Schilling
Original assignee: InterDigital Technology Corp
Current assignee: InterDigital Technology Corp
Priority date: 1992-01-13
Filing date: 1994-07-05
Publication date: 1996-12-17
Anticipated expiration: 2012-01-13
Also published as: AU3467793A; US5260967A; US5410568A; WO1993014590A1

Abstract

A spread-spectrum transmitter and receiver using code division multiple access with time division multiple access technology for spread-spectrum communications. At a base station, a spread-spectrum transmitter includes a multiplexer for time multiplexing a synchronization-code signal and a plurality of data signals, which may be encoded as a plurality-encoded data signals, to generate a time-multiplexed signal. A chip code generator generates a chip-code signal which is modulo added with the time-multiplexed signal by an EXCLUSIVE-OR gate to generate a spread-spectrum-time-multiplexed signal. A transmitter transmits the spread-spectrum-time-multiplexed signal over a communications channel. A spread-spectrum receiver at the base station processes a plurality of spread-spectrum signals, received from a plurality of remote units in as time division sequence of spread-spectrum signals, using a despreader circuit to generate a time-division signal. A demultiplexer demultiplexes the time-division signal as a plurality of data signals or as a plurality of privacy-encoded data signals. A remote unit, which may be a handset, includes a despreader circuit for despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal. A synchronization-code matched filter detects the synchronization-code signal embedded in the time-multiplexed signal and thereby generates a timing signal. A controller uses the timing signal to generate a control signal. The control signal controls a transmit-receive switch for switching an antenna between the despreader circuit to the transmitter at the remote unit.

Description

.Iadd.This patent is a reissue of U.S. Pat. Ser. No. 5,260,967 entitled, CDMA/TDMA SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD, having Ser. No. 07/819,906, filing date Jan. 13, 1992, and issue date of Nov. 9, 1993. .Iaddend.

.Iadd.RELATED PATENTS

This patent is a reissue of U.S. Pat. Ser. No. 5,260,967 entitled, CDMA/TDMA SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD, having Ser. No. 07/819,906, filing date Jan. 13, 1992, and issue date of Nov. 9, 1993. .Iaddend.

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 f₁ and the plurality of remote units may communicate to the base station at a second carrier frequency f₂. 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 tint 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 remote-data signal with a remote-privacy-code signal as a remote-privacy-encoded data signal. The remote-privacy-code signal is defined herein to include any signal that can be used to encrypt or add privacy to the remote-data signal. The remote-spreading means spread-spectrum processes the remote-privacy-encoded data signal with a second chip-code signal as a spread-spectrum signal. The remote-transmitter means at the remote unit converts the spread-spectrum signal to a form suitable for sending over the communications channel. The second chip-code signal, which is generated at each of the plurality of remote units, may be the same as the first chip-code signal generated at the base station.

The present invention also includes, at a base station, the method for spread-spectrum processing a plurality of data signals on a first carrier frequency by generating a synchronization-code signal, encoding the plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded data signals, time multiplexing the synchronization-code signal and the plurality of privacy-encoded data signals, respectively, to generate a time-multiplexed signal, spread-spectrum processing the time-multiplexed signal with a first 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 the communications channel.

Also, at the base station, the method includes receiving a spread-spectrum-time-division signal by 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 timing signal, and decoding, using the timing signal, the plurality of remote-privacy-encoded signals as a plurality of remote-data signals.

Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate particular embodiments of the invention, and together with the description, serve to explain the principles of the invention.

FIG. 1 is a block diagram of a base station transmitter;

FIG. 2 is a block diagram of a base station receiver;

FIG. 3 is a block diagram of a remote unit transmitter and receiver;

FIG. 4 is a block diagram of a correlator for despreading spread-spectrum signals;

FIG. 5 is a timing diagram of multiplexed data from the base station of FIGS. 1 and 2;

FIG. 6 is a timing diagram of a packet of data transmitted from a first remote unit relative to the multiplexed data transmitted from the base station of FIGS. 1 and 2;

FIG. 7 is a block diagram of a base station transmitter;

FIG. 8 is a timing diagram of data transmitted between the base station of FIG. 7 and a plurality of remote units;

FIG. 9 is a block diagram of a base station transmitter; and

FIG. 10 is a timing diagram of data transmitted between the base station of FIG. 9 and a plurality of remote units.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals indicate to like elements throughout the several views.

In the exemplary arrangement shown in FIG. 1, a spread-spectrum transmitter which can be used for spread-spectrum processing a plurality of data signals on a first carrier frequency f₁, is shown comprising base-synchronization means, a plurality of base-privacy means, multiplexer means, base-spreading means, and base-transmitter means. The base-synchronization means is coupled to the multiplexer means. The plurality of base-privacy means is coupled between a plurality of inputs having the plurality of data signals, and the multiplexer means. The plurality of data signals includes the first data signal, d₁ (t), second data signal d₂ (t), through N^th data signal, d_N (t). Each data signal is assumed to be sent to a respective remote unit. The base-spreading means is coupled between the multiplexer means and the base-transmitter means.

The base-synchronization means is shown as asynchronization code generator 81 and thesynchronization buffer 82. Thesynchronization buffer 82 is coupled to thesynchronization code generator 81. Also shown is aclock 57 which is coupled through afirst divider 55 and asecond divider 54 to the synchronization-code generator 81.

The plurality of base-privacy means is embodied as a plurality of privacy-code generators, shown as first privacy-code generator 52, second privacy-code generator 62, through N^th privacy-code generator 72, coupled to a plurality of EXCLUSIVE-OR gates, shown as first EXCLUSIVE-OR gate 51, second EXCLUSIVE-OR gate 61, through N^th EXCLUSIVE-OR gate 71, respectively. More particularly, the first EXCLUSIVE-OR gate 51 is coupled to the first privacy-code generator 52 and to a first input having the first input signal, d₁ (t) The second EXCLUSIVE-OR gate is coupled to the second input which provides the second data signal, d₂ (t), and to second privacy-code generator 62. The N^th EXCLUSIVE-OR gate is coupled to the N^th input which provides the N^th data signal, d_N (t), and to the N^th privacy-code generator 72.

The multiplexer means is embodied as a plurality of buffers, shown asfirst buffer 53,second buffer 63 through N^th buffer 73 and asynchronization buffer 82, and amultiplexer 75. Themultiplexer 75 is coupled to thefirst buffer 53,second buffer 63, through N^th buffer 73, and to thesynchronization buffer 82. Thefirst buffer 53 is coupled to the first EXCLUSIVE-OR gate 51. Thesecond buffer 63 is coupled to the second EXCLUSIVE-OR gate 61. The N^th buffer 73 is coupled to the N^th EXCLUSIVE-OR gate 71. Thesynchronization buffer 82 is coupled to the synchronization-code generator 81. Themultiplexer 75 has its timing coupled to the output of thefirst divider 55. Thefirst divider 55 divides the time of the clock signal fromclock 57.

The base-spreading means is embodied as an EXCLUSIVE-OR gate 76,lowpass filter 77, and a chip-code generator 56. The EXCLUSIVE-OR gate 76 is coupled to themultiplexer 75, to thelowpass filter 77, and to the chip-code generator 56. The chip-code generator 56 also is coupled to theclock 57. The output of the EXCLUSIVE-OR gate 76 is coupled to alowpass filter 77.

Theclock 57 provides an appropriate clock signal for operation of the spread-spectrum transmitter and receiver at the base station. The clock signals are time divided byfirst divider 55 andsecond divider 54. The divided clock signal outputted fromsecond divider 54 is used for timing of the plurality of privacy-code generators, and the synchronization code generator. The divided clock signal outputted fromfirst divider 55 is used for timing of themultiplexer 75. For the embodiment of FIG. 1, thefirst divider 55 divides the timing of the clock signal by 15, and thesecond divider 54 divides the timing from thefirst divider 55 by 100.

The base-transmitter means is embodied as anoscillator 74 which generates a first carrier frequency, f₁, aproduct device 78, and apower amplifier 79. Theproduct device 78 is coupled between theoscillator 74, and thelowpass filter 77 andpower amplifier 79. The output of thepower amplifier 79 typically is coupled through anisolator 99 which may be embodied as a circulator, to anantenna 98. Theisolator 99 also is coupled to a receiver at the base station.

The plurality of data signals, d₁ (t), d₂ (t), . . . , d_N (t), are encoded by the plurality of EXCLUSIVE-OR

gates

51, 61, 71 with the plurality of privacy-code signals, g₁ (t), g₂ (t), . . . , g_N (t), which are generated by the plurality of privacy-

code generators

52, 62, 72, respectively. The plurality of EXCLUSIVE-OR

gates

51, 61, 71 modulo-2 add each of the respective privacy-code signals g₁ (t), g₂ (t), . . . , g_N (t), to each of the plurality of data signals, d₁ (t), d₂ (t), . . . , d₂ (t), respectively, to generate a plurality of privacy-encoded-data signals. The plurality of privacy-encoded-data signals are stored in the plurality of

buffers

53, 63, 73.

Each of the privacy-code signals may be an encrypting signal or a signal which provides privacy to particular data signal, as is well known in the art. Other types of encoding may be used, such as encryption with the Data Encryption Standard.

The synchronization-code generator 81 generates a synchronization-code signal. The synchronization-code signal is stored in thesynchronization buffer 82.

Themultiplexer 75 from thesynchronization buffer 82 time multiplexes the synchronization-code signal and from the plurality of

buffers

53, 63, 73, the plurality of privacy-encoded-data signals, respectively, to generate a time-multiplexed signal. Typically, the synchronization-code signal is initially time multiplexed in a first time slot, and then sequentially, the first privacy-encoded-data signal, second privacy-encoded-data signal, through N^th privacy-encoded-data signal are time multiplexed in subsequent time slots. A frame of data is defined herein as the time-multiplexed synchronization-code signal and the plurality of privacy-encoded signals. Accordingly, a time-multiplexed signal includes a frame of data. The synchronization-code signal may be a sequence of 1-bits and 0-bits used for identifying the beginning of each frame of data.

Based on the timing from the synchronization-code signal, a particular privacy-encode data signal can be determined. Preferably, each of the privacy-encoded-data signals and the synchronization-code signal have the same number of bits for a time slot, although a different length sequence of bits could be used for the synchronization-code signal.

The chip-code generator 56 generates a first chip-code signal using a first chip codeword. The time-multiplexed signal outputted from themultiplexer 75, and the first chip-code signal. C_B (t), from the chip-code generator 56, are spread-spectrum processed by the EXCLUSIVE-OR gate 76 to generate a spread-spectrum-time-multiplexed signal. The spread-spectrum processing typically is modulo-2 adding the time-multiplexer signal with the chip-code signal using the EXCLUSIVE-OR gate 76. Thelowpass filter 77 filters the spread-spectrum-time-multiplexed signal to remove out-of-band energy. The filtered spread-spectrum-time-multiplexed signal is then frequency translated, by operation of theproduct device 78 and the first carrier signal supplied by thefirst oscillator 74, to the first carrier frequency, f₁. Thus, after amplification bypower amplifier 79, the spread-spectrum-time-multiplexed signal is transmitted at the first carrier frequency, f₁, byantenna 98 over a communications channel.

As an example, assume the synchronization-code signal and 100 privacy-encoded data signals are multiplexed by themultiplexer 75 at multiplex rate, f_M. In this example, assume that each data signal is sampled at the rate of 16 kilobit per second (kb/s) and compressed by thefactor 101. The resulting data rate, d_M, in the time-multiplexed signal is 101×16 kb/s, which is approximately 1.6 Mb/s. The time-multiplexed signal is then Spread by EXCLUSIVE-OR gate 76 using a short first chip-code signal generated from a 15 chip codeword by chip-code generator 56. Each remote unit uses the same first chip-code signal, C_B (t).

As illustratively shown in FIG. 2, a spread-spectrum receiver for spread-spectrum processing a spread-spectrum-time-division signal is shown comprising theantenna 98, theisolator 99, abandpass filter 101, a low noise amplifier (LNA) 102, amixer 103,local oscillator 104, automatic-gain-control (AGC)circuit 105, base-despreader means, base-demultiplexer means, base-synchronization means, and base-decoder means. Thebandpass filter 101 is coupled through theoscillator 99 to theantenna 98. Theisolator 99 also coupled to the transmitter of FIG. 1. Thelow noise amplifier 102 is coupled between themixer 103 and thebandpass filter 101, and themixer 103 is coupled to thelocal oscillator 104 which generates a first local signal. TheAGC circuit 105 is coupled between themixer 103 and the base-despreader means. The base-demultiplexer means is coupled to the base-despreader means, to the base-synchronization means and to the base-decoder means.

In FIG. 2, the base-despreader means is embodied asdespreader circuitry 110, the base-demultiplexer means is embodied as thedemultiplexer 115, the base-synchronization means is embodied as the synchronization-code generator 81 and the timing andsynchronization circuit 120, and the base-decoder means is embodied as a privacy-code generator 121 and an EXCLUSIVE-OR gate 122. Thedespreader circuitry 110 may be a matched filter, a correlator, or any other circuit which can be used for despreading a received spread-spectrum signal having spread-spectrum modulation. A matched filter may be implemented using a tapped-delay-line filter, such as a surface-acoustic-wave device. A typical correlator which can be used for despreading a spread-spectrum signal is depicted in FIG. 4, and includes amixer 410, chip-code generator 415 andfilter 420.

Thedemultiplexer 115 is coupled to thedespreader circuitry 110. The synchronization-code generator 81 is coupled to the timing andsynchronization circuit 120, and the timing andsynchronization circuit 120 is coupled to thedemultiplexer 115 and to the privacy-code generator 121. The privacy-code generator is coupled to the EXCLUSIVE-OR gate 122. For this particular embodiment, the base-decoder means, and accordingly the privacy-code generator 121, are shown for decoding a particular remote privacy-encoded data signal from the plurality of remote-privacy-encoded data signals received by the spread-spectrum receiver. Normally, a plurality of base-decoder means would be employed for decoding a plurality of remote-privacy-encoded-data signals.

A spread-spectrum-time-division signal received byantenna 98 passed throughoscillator 99 and is filtered bybandpass filter 101 and amplified bylow noise amplifier 102. Themixer 103, using the first local signal from thelocal oscillator 104, shifts the received spread-spectrum-time-division signal to the appropriate intermediate frequency or to baseband frequency for processing by thedespreader circuitry 110. The appropriate frequency depends on the particular embodiment of thedespreader circuitry 110. TheAGC circuit 105 normalizes the power of the received spread-spectrum-time-division signal.

The spread-spectrum-time-division signal is defined herein to include a plurality of spread-spectrum signals received sequentially in time from the plurality of remote units. Each remote unit transmits a spread-spectrum signal for a short duration in time. The timing for transmitting from a particular remote unit is at the remote unit's time slot. The remote unit's time slot is with respect to when the remote unit receives the spread-spectrum-time-multiplexed signal from the base station, and decodes the synchronization-code signal. A time slot for a particular remote unit may be fixed, or variable. For a variable time slot, a protocol signal from the base station may set the location of the time slot.

The remote unit generates a control signal from the decoded synchronization-code signal. The control signal sets the time for when each remote unit transmits its own spread-spectrum signal. Each remote unit has its own time slot, which is different from the time slots of the other remote units, for transmitting a spread-spectrum signal. The time slots may be set by the base station by a protocol when a remote unit initiates communication with the base station. A synchronization-code signal is not received, however, at the base station. The timing for defining a frame of a received plurality of spread-spectrum signals from the plurality of remote units is provided at the base station by the synchronization-code generator 81.

Thedespreader circuitry 110 despreads the received spread-spectrum-time-division signal as a time-division signal. The time-division signal is defined herein to include a plurality of remote-privacy-encoded data signals, or a plurality of remote-data signals, which are in different time slots based on the time when each remote unit sent the spread-spectrum signal having the remote-data signal or the remote-privacy-encoded data signal. The despreader circuitry removes the chip-code signal which was added to the spread-spectrum signal by each of the plurality of remote units. If each of the plurality of remote units uses the same chip codeword, then only one despreader circuitry is required. The plurality of remote units may be divided into two or more sets of remote units, with each set using it own chip codeword. For each of the chip codewords the despreader circuitry would include corresponding devices, connected in parallel, for despreading each of the corresponding set of spread-spectrum signals. Although some of the spread-spectrum signals would be received with a different spreading sequence, they are still received sequentially in time from the plurality of remote units. Thus, a time-division signal, as defined herein, would be outputted form the despreader circuitry.

The received time-division signal is demultiplexed by thedemultiplexer 115 as a plurality of remote-privacy-encoded data signals, or a plurality or remote-data signals if the spread-spectrum signals sent by the plurality of remote units were not privacy encoded. Essentially, the plurality of remote-privacy-encoded data signals were transmitted by a remote unit, and each remote-privacy-encoded data signal is decoded by decoder circuitry which, for a particular remote-privacy-encoded-data signal, includes the privacy-code generator 121 and EXCLUSIVE-OR gate 122.

The synchronization-code generator 81, which is the same as used by the spread-spectrum transmitter of FIG. 1, generates the synchronization-code signal which triggers the timing andsynchronization circuit 120 to generate a timing signal for clocking thedemultiplexer 115. The timing signal also triggers the privacy-codedgenerator 121 so that the remote-privacy-encoded signal from thedemultiplexer 115 is decoded at the appropriate time.

The present invention also includes a plurality of remote units, with each remote unit receiving the spread-spectrum-time-multiplexer signal which is transmitted from the base station. The spread-spectrum-time-multiplexer signal has a synchronization-code signal and the plurality of privacy-encoded data signals, at the first carrier frequency. Within the plurality of privacy-encoded data signals is a particular privacy-encoded data signal, defined herein as a first privacy-encoded data signal, to be received by the remote unit. The remote unit also spread-spectrum transmits a remote-data signal at a second carrier frequency. Generally, 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.

As illustratively shown in FIG. 3, anantenna 198 is coupled through a transmit-receiveswitch 199 to abandpass filter 151. Thebandpass filter 151 is coupled to amixer 152, which is also coupled to thesecond oscillator 153. Themixer 152 is coupled to an automatic-gain-control (AGC)circuit 154.

The remote-despreader means is embodied asdespreader circuitry 155, the remote-synchronization means is embodied as synchronization-code matchedfilter 157, control means is embodied as thecontroller 161. The remote-decoder means is shown asbuffer 156, privacy-code generator 159, and EXCLUSIVE-0R gate 158. Also illustrated is a digital-to-analog converter 160.

The spread-spectrum transmitter for the remote unit includes remote-privacy means, remote-spreading means, and remote-transmitter means. The remote-privacy means is embodied as a privacy-code generator 132, an EXCLUSIVE-OR gate 133 andbuffer 134. The remote-spreading means is embodied as the chip-code generator 165, an EXCLUSIVE-OR gate 135, andlowpass filter 136. The remote-transmitter means is embodied as anoscillator 137,product device 138, optionally an automatic-power-control (APC)circuit 139, and asecond product device 140 coupled to theoscillator 153. The remote-transmitter means also may include apower amplifier 141 which is coupled through the transmit-receiveswitch 199 to theantenna 198.

Theclock 162 provides appropriate timing synchronization for operation of the spread-spectrum transmitter and receiver of the remote unit. The clock signals are time-divided by firstremote divider 163 and secondremote divider 164. For the embodiment of FIG. 3, by way of example, the first remote-divider 163 divides the timing or the clock signal by 15, and the secondremote divider 164 divides the timing of the signal from the firstremote divider 163 by 100. The timing signals from thesecond divider 164 provide appropriate timing for the privacy-code generator 132, and analog-to-digital converter 131. The output of thefirst divider 163 provides timing for thebuffer 134.

A received spread-spectrum-time-multiplexed signal atantenna 198 passes through the transmit-receiveswitch 199 and is filtered bybandpass filter 151. Themixer 152, in cooperation with theoscillator 153, shifts the received spread-spectrum-time-multiplexed signal to an appropriate intermediate frequency or baseband frequency for processing bydespreader circuitry 155. The appropriate intermediate frequency or baseband frequency is determined on the particular embodiment ofdespreader circuitry 155. TheAGC circuit 154 normalizes the power of the received spread-spectrum-time-multiplexed signal.

Thedespreader circuitry 155 may be embodied as a surface-acoustic-wave device, a tapped-delay-line match filter, or a correlator or any other circuitry which may be used for despreading the received spread-spectrum-time-multiplexed signal, as is well known in the art. As illustratively shown in FIG. 4, the correlator may include a chip-code generator 415, amixer 410 and a filter 42C. The despread, spread-spectrum-time-multiplexed signal is the time-multiplexed signal, which was originally sent by the spread-spectrum transmitter at the base station.

The synchronization-code matchedfilter 157 detects the synchronization-code signal embedded in the time-multiplexed signal, and sends an appropriate timing signal to thecontroller 161. In response to the timing signal from the synchronization-code-matchedfilter 157, thecontroller 161 generates appropriate control signals which actuatebuffer 156, privacy-code generator 159 and digital-to-analog converter 160, so that the appropriate channel from the time-multiplexed signal is demultiplexed and received at the remote unit. As an example, the synchronization-code-matchedfilter 157 may be embodied as a 160 bit stage matched filter, which is matched to a 160 bit codeword used to generate the synchronization-code signal. Thebutter 156 stores only the selected privacy-encoded data signal, or the selected data signal, and not the entire time-multiplexed signal. The selected privacy-encoded data signal, or the selected data signal, is the signal which is desired to be received at the remote unit. When receiving the selected first privacy-encoded data signal, the privacy-code generator 159 generates a first privacy-coded signal which is processed by the EXCLUSIVE-OR gate 158 to generate the first data signal. In a preferred embodiment, analog voice may be converted to a data signal and transmitted to a remote unit. In this embodiment, the digital-to-analog converter 160 converts the first data signal to the analog voice signal as an output signal.

If the analog voice were to be transmitted from the remote unit to a base station, then the analog signal input would be converted by analog-to-digital converter 131 to a remote-data signal, d_Hi (t). When privacy is used, the remote-data signal is encoded with a remote-privacy-code signal by the EXCLUSIVE-OR gate 133 as a remote-privacy-encoded-data signal. The remote-privacy-code-signal is generated by the privacy-codedgenerator 132. The remote-privacy-encoded data signal is stored in thebuffer 134. With an appropriate timing from thefirst divider 163, the remote-privacy-encoded data signal is spread-spectrum processed by EXCLUSIVE-OR gate 135 with a second chip-code signal generated by the chip-code generator 165 using the appropriate chip codeword assigned to the remote unit. Thelowpass filter 136 filters the spread-spectrum signal. The spread-spectrum signal is shifted to a second carrier frequency by operation offirst mixer 138 andoscillator 137, andsecond mixer 140 andoscillator 153. TheAPC circuit 139 regulates the power transmitted by the remote station in response to a level provided byAGC circuit 154. The spread-spectrum signal is amplified bypower amplifier 141 and radiated byantenna 198.

Thecontroller 161 provides the control signal to the transmit-receiveswitch 199 so that upon receiving and decoding the synchronization-code signal, and thereby generating the control signal, the transmit-receiveswitch 199 is directed to thepower amplifier 141 for transmitting the spread-spectrum signal. After transmitting the spread-spectrum signal, the transmit-receiveswitch 199 returns to the position for receiving a spread-spectrum-time-multiplied signal. The transmit-receiveswitch 199 is always on receive except during transmission.

Thus, the spread-spectrum receiver of the remote unit detects its received time slot and the synchronization-code signal in the spread-spectrum-time-multiplexed signal received from the base station.

The present invention also includes, at a base station, the method for spread-spectrum processing a plurality, of data signals on a first carrier frequency by generating a synchronization-code signal, encoding the plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded data signals, respectively, time-multiplexing the synchronization-code signal and the plurality of privacy-encoded data signals, respectively, to generate a 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 the communications channel.

Also, at the base station, the method includes receiving a spread-spectrum-time-division signal by despreading the spread-spectrum-time-division as a time-division signal, demultiplexing the time-division signal as a plurality of remote-privacy-encoded data signals, generating a timing signal and decoding, using the timing signal, the plurality of remote-privacy-encoded signals as a plurality of remote-data signals.

At a remote unit, the method including receiving a spread-spectrum-time-multiplexed signal, which has a synchronization signal and a first privacy-encoded-data signal, at a first carrier frequency, and spread-spectrum transmitting a remote-data signal at a second carrier frequency. The second carrier frequency may be the same as the first carrier frequency. The method includes despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal, and detecting in the time-multiplexed signal, the synchronization-code signal. In response to detecting the synchronization-code signal, a timing signal and a control signal are generated. A first privacy-encoded-data signal is decoded from the time-multiplexed signal as first data signal.

The method, at the remote unit, also includes encoding a remote-data signal with a remote-privacy-code signal as a remote-privacy-encoded-data signal. The method spread-spectrum processes the remote-privacy-encoded-data signal with a second chip-code signal as a spread-spectrum signal, and converting the second spread-spectrum signal to a form suitable for sending over a communications channel.

FIG. 5 illustratively shows, at the output of themultiplexer 75, the time-multiplexed signal as a synchronization-code signal, g₀ (t) concatenated with a plurality of privacy-encoded data signals. The plurality of privacy-encoded-data signals are represented, by way or example, as 100 privacy-encoded data signals. The plurality of privacy-encoded-data signals alternatively may be a plurality of data signals, if privacy encoding is not used. The first privacy-encoded data signal 401, 99^th privacy-encoded data signal 499, and 100^th privacy-encoded data signal 500 are shown. In a particular embodiment, a particular privacy-encoded data signal might be encoded as 160 bits, as illustratively shown in FIG. 5.

FIG. 6 illustrates thetime slots 300 or the time-multiplexed signal, where the synchronization-code signal is at position zero and subsequently followed by 100 slots for privacy-encoded data signals. At a particular remote unit, for example a first remote unit which receives the first channel of the time-multiplexed signal, the remote unit transmits at a short time later a spread-spectrum signal back to the base station. Timing is shown asslot 350 for the remote unit, and thetime slots 300 are shown for the base station.

The spread-spectrum transmitter at the base station alternatively may transmit the plurality of data signals, or the plurality of privacy-encoded-data signals, at the same time, using code division multiple access. A block diagram for such a base station transmitter is shown in FIG. 7 with its corresponding timing sequence illustrated in FIG. 8. An alternative block diagram for a base station transmitter is shown in FIG. 9 with its corresponding timing sequence illustrated in FIG. 10.

Accordingly, a spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency may comprise base-synchronization means, a plurality of base-privacy means, a plurality of base-spreading means, combiner means, multiplexer means, and base-transmitter means. The plurality of base-privacy means is optional. In FIG. 7, the base-synchronization means is embodied as asynchronization code generator 81, and the plurality of base-privacy means is embodied as EXCLUSIVE-OR gate 51 and first privacy-code generator 52. EXCLUSIVE-OR gate 61 and second privacy-code generator 62, through EXCLUSIVE-OR gate 71 and N^th privacy-code generator 72. The first privacy-code generator 51 generates a first privacy-code signal, the second privacy-code generator generates a second privacy-code signal, and the N^th privacy-code generator generates an N^th privacy code signal. EXCLUSIVE-OR gate 51 is coupled to firstprivacy code generator 52 and uses the first privacy-code signal for encoding the first data signal as a first privacy-encoded-data signal. EXCLUSIVE-OR gate 61 is coupled to second privacy-code generator 62 and uses the second privacy-code signal for encoding the second data signal as a second privacy-encoded-data signal. EXCLUSIVE-OR 71 is coupled to N^th privacy-code generator 72 and uses the N^th privacy-code signal for encoding the N^th data signal as an N^th privacy-encoded-data signal.

The plurality of base-spreading means is shown as EXCLUSIVE-OR gate 704 coupled to synchronization-code generator 81 and to synchronization-chip-code generator 701, EXCLUSIVE-OR gate 703 coupled to first message-chip-code generator 702, EXCLUSIVE-OR gate 773 coupled to second message-chip-code generator 772, through EXCLUSIVE-OR gate 783 coupled to N^th message-chip-code generator 782.

The synchronization-chip-code generator 81 uses a synchronization-chip codeword for generating a synchronization-chip-code signal. EXCLUSIVE-OR gate 704 spread-spectrum processes the synchronization-code signal with the synchronization-chip-code signal to generate a spread-spectrum processed-synchronization-code signal. The first message-chip-code generator 702 uses a first message-chip codeword for generating a first message-chip-code signal. EXCLUSIVE-OR gate 703 spread-spectrum processes the first privacy-encoded-data signal, or the first data signal if privacy encoding is not used, with the first chip-code signal to generate a first spread-spectrum signal. The second message-chip-code generator 772 uses a second message-chip codeword for generating a second message-chip-code signal. EXCLUSIVE-OR gate 773 spread-spectrum processes the second privacy-encoded-data signal, or the second data signal if privacy encoding is not used, with the second message-chip-code signal to generate a second spread-spectrum signal. Similarly, the N^th message-chip-code generator 782 uses an N^th message-chip codeword for generating a N^th message-chip-code signal. EXCLUSIVE-OR gate 783 spread-spectrum processes the N^th privacy-encoded-data signal, or the N^th data signal if privacy encoding is not used, with the N^th message-chip-code signal to generate an N^th spread-spectrum signal.

The spread-spectrum processing of each of the plurality of privacy-encoded-data signals or each of the plurality of data signals, is done with proper timing and chip rate to produce a desired spread-spectrum processing gain. Timing for each privacy-code generator and for each message-chip-code generator may be from a common clock signal as illustrated in FIGS. 7 and 9.

The combiner means is shown ascombiner 705. The EXCLUSIVE-OR gate 703 is coupled to the EXCLUSIVE-OR gate 51 and tocombiner 705. The EXCLUSIVE-OR gate 773 is coupled to the EXCLUSIVE-0R gate 61 and to thecombiner 705. The EXCLUSIVE-OR gate 783 is coupled to the EXCLUSIVE-OR gate 71 and to thecombiner 705.

In a preferred embodiment, thecombiner 705 linearly combines the plurality of spread-spectrum signals from the plurality of base-spreading means. Nonlinear combining may equivalently work without serious degradation in performance, however. Thecombiner 705 combines the plurality of spread-spectrum signals as a combined-plurality of spread-spectrum signals.

The multiplexer means is shown asmultiplexer 708,first buffer 706 andsecond buffer 707. EXCLUSIVE-OR gate 704 is coupled to synchronization-code generator 81 and throughsecond buffer 707 to themultiplexer 708. Thecombiner 705 is coupled through thefirst buffer 706 to themultiplexer 708. Themultiplexer 708 time multiplexes the spread-spectrum-processed-synchronization-code signal with the combined-plurality of spread-spectrum signals, as a multiplexed-spread-spectrum signal. Thefirst buffer 706 stores the combined-plurality of spread-spectrum signals until themultiplexer 708 selects thefirst buffer 706. Thesecond buffer 707 stores the spread-spectrum-processed-synchronization-code signal until themultiplexer 708 selects thesecond buffer 707.

The base transmitter means is shown as anoscillator 74, aproduct device 78, apower amplifier 79, and anantenna 98. Anisolator 99 may be inserted between theantenna 98 and thepower amplifier 79 for isolating the base-station receiver from the base-station transmitter. Theproduct device 78 may be a mixer or other device which can raise a spread spectrum signal to a carrier frequency. Theproduct device 78 is coupled to theoscillator 74, themultiplexer 706 and thepower amplifier 79.

The synchronization-code generator 81 generates a synchronization-code signal. The synchronization-chip-code generator 701 generates a chip-code signal, and the EXCLUSIVE-OR gate 704, spread-spectrum processes the synchronization-code signal with the chip-code signal to generator a spread-spectrum-processed-synchronization-code signal.

The plurality of privacy-

code generators

52, 62, 72 generate a plurality of privacy-code signals. The plurality of EXCLUSIVE-OR

gates

51, 61, 71, encode the plurality of data signals, d₁ (t), d₂ (t), . . . , d_N (t), with the plurality of privacy-code signals to generate a plurality of privacy-encoded-data signals, respectively. Thus, by way of example, the first data signal, d₁ (t), is encoded with the first privacy-code signal from the first privacy-code generator 52 by the EXCLUSIVE-OR gate 51 as the first privacy-encoded-data signal. The second data signal, d₂ (t), is encoded with the second privacy-code signal from the second privacy-code generator 62 by the EXCLUSIVE-OR gate 61 as the second privacy-encoded-data signal. The N^th data signal, d_N (t), is encoded by the N^th privacy-code signal from the privacy-code generator 72 by the EXCLUSIVE-OR gate 71 as the N^th privacy-encoded-data signal.

The plurality of EXCLUSIVE-OR

gates

704, 703, 773, 783, spread spectrum process the synchronization-code signal and the plurality of privacy-encode data signals, or data signals if privacy encoding is not used, with a plurality or chip-code signals to generate a plurality or spread-spectrum signals. The plurality of chip-code signals are generated by the synchronization-chip-code generator 701, first message-chip-code generator 702, second message-chip-code generator 772 through N^th message-chip-code generator 782. The plurality of EXCLUSIVE-OR

gates

704, 703, 773, 783 spread-spectrum process the synchronization-code signal and the plurality of privacy-encoded-data signals by modulo-2 adding the plurality of chip-code signals. Accordingly, the synchronization-code signal is modulo-2 added to the synchronization-chip-code signal from synchronization-chip-code generator 701 by EXCLUSIVE-0R gate 704. The first privacy-encoded-data signal is modulo-2 added by the EXCLUSIVE-OR gate 703 to the first chip-code signal from the first message-chip-code generator 702. The second privacy-encoded-data signal is modulo-2 added by the EXCLUSIVE-OR gate 773 to the second chip-code signal from the second message-chip-code generator 772. The N^th privacy-encoded-data signal is modulo-2 added to the N^th chip-code signal from the N^th message-chip-code generator 782 by the EXCLUSIVE-OR gate 783.

Thecombiner 705 combines the plurality of spread-spectrum signals From the plurality of EXCLUSIVE-OR

gates

703, 773, 783 to generate a combined-spread-spectrum signal. Themultiplexer 706 time multiplexes the spread-spectrum-processed-synchronization-code signal with the combined-plurality or spread-spectrum signals.

The multiplexed-combined spread-spectrum signal is converted to a form suitable for sending over a communications channel by function of theoscillator 74 raising the multiplexed-combined-spread-spectrum signal to a carrier frequency withproduct device 78. Thepower amplifier 79 amplifies the multiplexed-combined-spread-spectrum signal at the carrier frequency, which is radiated byantenna 98. The spread-spectrum system timing is shown in FIG. 8. In this case, the spread-spectrum-processed-synchronization-code signal is sent first and then the combined-plurality of spread-spectrum signals is sent as a base CDMA signal. Subsequently, each mobile unit sends its signal to the base station in its selected time slot, as previously discussed.

The present invention of FIG. 7 can be modified as shown in FIG. 9 by having the synchronization-code signal become a generic-chip-code signal which is combined at the same time with the spread-spectrum signals from the plurality of EXCLUSIVE-OR

gates

703, 773, 783. A generic-chip-code signal is used without spread-spectrum processing a data signal, or for spread-spectrum processing a data signal with a relatively low data rate. Accordingly, a spread-spectrum channel which is despread with the generic-chip-code signal can be used to derive a carrier signal, which effectively can be used as a pilot signal. The carrier signal or pilot signal can be used for synchronously despreading the other spread-spectrum channels and provide other timing at a remote unit receiver. In this case, as shown in FIG. 10, the generic-chip-code signal is sent concurrently in time with the plurality of spread-spectrum signals as the combined spread-spectrum signal, over the communication channel. Subsequently, each mobile unit sends its message to the base station, as set forth previously, in a time slot measured from the generic-chip-code code signal. Timing may be derived from a common clock signal and/or the generic-chip-code signal.

In use, a spread-spectrum transmitter of FIG. 1 at a base station encodes a plurality of data signals with a plurality of privacy-code signals to generate a plurality of privacy-encoded data signals, respectively. A synchronization-code signal is generated by the synchronization-code generator, and is time multiplexed with the plurality of privacy-encoded data signals bymultiplexer 75 to generate a time-multiplexed signal. The time-multiplexed signal is spread-spectrum processed by EXCLUSIVE-OR gate 76 using a first chip-code signal generated by a chip-code generator 56, to generate a spread-spectrum-time-multiplexed signal. The spread-spectrum-time-multiplexed signal is transmitted on a first carrier frequency over a communications channel. The synchronization-code signal defines a frame which includes the plurality of privacy-encoded-data signals.

At a remote unit, the spread-spectrum-time-multiplexed signal is received and demultiplexed usingdespreader circuitry 155. A synchronization-code matchedfilter 157 detects the synchronization-code signal embedded in the despread signal, and generates a timing signal. The timing signal actuates thecontroller 161 for generating control signals, which cause thebuffer 156 to store the selected privacy-encoded data signal embedded in the time-multiplexed signal from thedespreader circuitry 155. The privacy-code generator 159 and EXCLUSIVE-OR gate 158 decodes the privacy-encoded data signal to generate the data signal being received from the base station.

At the remote unit, a remote-data signal. d_Hi (t) is encoded by EXCLUSIVE-OR gate 133 by a remote-privacy-code signal from privacy-code generator 132, and the resulting remote-privacy-encoded data signal is stored inbuffer 134. Thebuffer 134 stores the remote-privacy-encoded-data signal until the control signal is sent bycontroller 161 to transmit. The remote-privacy-encoded data signal is then spread-spectrum processed by EXCLUSIVE-OR gate 135 using a second chip-code signal generated by chip-code generator 165. The output of the EXCLUSIVE-OR gate 135 is a spread-spectrum signal. The control signal also actuated the transmit-receiveswitch 199 to the transmit position. The spread-spectrum signal is transmitted at a second carrier frequency over the communications channel to the base station.

Thecontroller 161 generates the control signal which actuates the transmit-receivedswitch 199, to be only in the transmit position while transmitting from the remote unit. Otherwise, the transmit-receiveswitch 199 is in the receive mode. Additionally, the remote unit has its timing based on detection of the synchronization-code signal, and transmits its spread-spectrum signal within a designated time slot. The designated time slot can correspond to the time slot of the privacy-encoded data signal being received by the remote unit from the base station.

At the base station a plurality of spread-spectrum signals are received and despread. The plurality of spread-spectrum signals have been designated herein as a spread-spectrum-time-division signal, inasmuch as the plurality of spread-spectrum signals are received sequentially in time. At the base station, each spread-spectrum signal is despread as a time-division signal. The time-division signal is demultiplexed bydemultiplexer 115 as a plurality of remote-privacy-encoded data signals. Appropriate circuitry is used to decode the remote-privacy-encoded data signal as the remote-data signal. d_Hi (t).

It will be apparent to those skilled in the art that various modifications may be made to the CDMA/TDMA spread-spectrum system of the instant invention without departing from the scope or spirit of the invention, and it is intended that the present invention cover modifications and variations of the CDMA/TDMA spread-spectrum system provided they come within the scope of the appended claims and their equivalents.

Claims

I claim:

1. A spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency, f₀, comprising:

a plurality of privacy-code generators for generating a plurality of privacy-code signals, respectively;

a plurality of EXCLUSIVE-OR gates coupled to said plurality of privacy-code generators for 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;

a plurality of data buffers coupled to said plurality of EXCLUSIVE-OR gates for storing the plurality of privacy-encoded data signals, respectively;

a synchronization-code generator for generating a synchronization-code signal;

a synchronization buffer coupled to said synchronization-code generator for storing the synchronization-code signal;

a multiplexer coupled to said synchronization buffer and to said plurality of data buffers for time multiplexing the synchronization-code signal and the plurality of privacy-encoded-data signals, respectively, to generate a time-multiplexed signal; p1 a chip-code generator for generating a chip-code signal;

an EXCLUSIVE-OR gate coupled to said multiplexer and to said chip-code generator, for spread-spectrum processing the time-multiplexed signal with the chip-code signal as a spread-spectrum-time-multiplexed signal; and

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

2. A spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising:

base-synchronization means for generating a synchronization-code signal;

base-privacy means for encoding a plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded-data signals, respectively;

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

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

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

3. The spread-spectrum transmitter as set forth in claim 2 wherein said base-privacy means includes:

a plurality of privacy-code generators for generating the plurality of privacy-code signals, respectively; and

a plurality of EXCLUSIVE-OR gates for modulo adding each of the respective plurality of privacy-code signals to each of the plurality of data signals, respectively.

4. The spread-spectrum transmitter as set forth in claim 2 wherein said multiplexer means includes:

a plurality of data buffers for storing the plurality of privacy-encoded-data signals:

a synchronization buffer for storing the synchronization-code signal; and

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

5. The spread-spectrum transmitter as set forth in claim 2 wherein said base-spreading means includes:

a chip-code generator for generating a chip-code signal; and

an EXCLUSIVE-OR gate for spread-spectrum processing the time-multiplexer signal with the chip-code signal to generate the spread-spectrum-time-multiplexer signal.

6. The spread-spectrum transmitter as set forth in claim 2 wherein said base-transmitter means includes:

an oscillator for generating a carrier frequency;

a product device for shifting the spread-spectrum-time-multiplexed signal to the carrier frequency;

a power amplifier for amplifying the spread-spectrum-time-multiplexed signal at the carrier frequency; and

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

7. A spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising:

.Iadd.a plurality of privacy-code generators for generating a plurality of privacy-code signals, respectively;

a plurality of EXCLUSIVE-OR gates for modulo adding each of the respective plurality of privacy-code signals to a respective one of the plurality of data signals to generate a plurality of privacy-encoded data signals; .Iaddend.

synchronization means for generating a synchronization-code signal;

multiplexer means for time multiplexing the synchronization-code signal and the plurality of .Iadd.privacy-encoded .Iaddend.data signals, respectively, to generate a time-multiplexed signal;

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

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

8. The spread-spectrum transmitter as set forth in claim 7 wherein said multiplexer means includes:

a plurality of data buffers .Iadd.coupled to said plurality of .[.EXCLUSIVE.]. .Iadd.ELUSIVE-OR .Iaddend.gates, respectively, for storing the plurality of .Iadd.privacy-encoded .Iaddend.data signals;

a synchronization buffer for storing the synchronization-code signal; and

a multiplexer for time multiplexing the synchronization-code signal and the plurality of .Iadd.privacy-encoded .Iaddend.data signals, respectively, to generate the time-multiplexed signal.

9. The spread-spectrum transmitter as set forth in claim 7 wherein said spreading means includes:

a chip-code generator for generating a chip-code signal; and

an EXCLUSIVE-OR gate for spread-spectrum processing the time-multiplexed signal with the chip-code signal as the spread-spectrum-time-multiplexed signal.

10. The spread-spectrum transmitter as set forth in claim 7 wherein said transmitter means includes:

an oscillator for generating a carrier frequency;

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

11. A spread-spectrum receiver 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:

a despreader circuit for despreading the spread-spectrum-time-division signal as a time-division signal;

a demultiplexer coupled to said despreader circuit for demultiplexing the time-division signal as a plurality of remote-privacy-encoded-data signals;

a synchronization-code generator for generating a synchronization-code signal;

a timing circuit coupled to said synchronization-code generator and responsive to the synchronization-code signal for generating a timing signal; a plurality of privacy-code generators coupled to said timing circuit and responsive to the timing signal for generating a .Iadd.respective .Iaddend.plurality of remote-privacy-code signals; and

a plurality of EXCLUSIVE-OR gates coupled to said demultiplexer and to said plurality of privacy-code generators for decoding the plurality of remote-privacy-encoded-data signals, respectively, as a plurality of remote-data signals.

12. The spread-spectrum receiver as set forth in claim 11 wherein said despreader .[.means.]. .Iadd.circuit .Iaddend.includes a surface-acoustic-wave device for despreading the received spread-spectrum-time-division signal as the time-division signal.

13. The spread-spectrum receiver as set forth in claim 11 wherein said despreader .[.means.]. .Iadd.circuit .Iaddend.includes a tapped-delay-line matched filter for despreading the received spread-spectrum-time-division signal as the time-division signal.

14. The spread-spectrum receiver as set forth in claim 11 wherein said despreader .[.means.]. .Iadd.circuit .Iaddend.includes:

a chip-code generator for generating a chip-code signal;

a mixer coupled to said chip-code generator and responsive to the chip-code signal for despreading the received spread-spectrum-time-division signal as the time-division signal; and

a filter coupled to said mixer for filtering the time-division signal. .[.

15. A spread-spectrum receiver 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:

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

demultiplexer means for demultiplexing the time-division signal as a plurality of remote-data signals..]..[.16. The spread-spectrum receiver as set forth in claim 15 wherein said despreader means includes a surface-acoustic-wave device for despreading the received spread-spectrum-time-division signal as the time-division signal..]..[.17. The spread-spectrum receiver as set forth in claim 15 wherein said despreader means includes a tapped-delay-line matched filter for despreading the received spread-spectrum-time-division signal as the time-division signal..]..[.18. The spread-spectrum receiver as set forth in claim 15 wherein said despreader means includes:

a chip-code generator for generating a chip-code signal;

a filter coupled to said mixer for filtering the time-division signal..]..[.19. The spread-spectrum receiver as set forth in claim 15 wherein said demultiplexer means includes a demultiplexer for demultiplexing the time-division signal as the plurality of data

signals..].20. A spread-spectrum receiver 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:

base-despreader means for despreading the spread-spectrum-time-division signal as a time-division signal;

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

base-synchronization means for generating a timing signal; and

base-decoder means responsive to the timing signal for decoding the plurality of remote-privacy-encoded signals as a plurality of remote-data

signals, respectively. 21. The spread-spectrum receiver as set forth in claim 20 wherein said base-despreader means includes a surface-acoustic-wave device for despreading the received

spread-spectrum-time-division signal as the time-division signal. 22. The spread-spectrum receiver as set forth in claim 20 wherein said base-despreader means includes a tapped-delay-line matched filter for despreading the received spread-spectrum-time-division signal as the

time-division signal. 23. The spread-spectrum receiver as set forth in claim 20 wherein said base-despreader means includes:

a chip-code generator for generating a chip-code signal;

a filter coupled to said mixer for filtering the time-division signal. 24. The spread-spectrum receiver as set forth in claim 20 wherein said base-demultiplexer means includes a demultiplexer for demultiplexing the time-division signal as the plurality of privacy-encoded-data signals.

The spread-spectrum receiver as set forth in claim 20 wherein said base-synchronization means includes:

a synch-code generator for generating a synchronization code; and

a timing circuit responsive to the synchronization-code signal for

generating a timing signal. 26. The spread-spectrum-receiver as set forth in claim 20 wherein said base-decoder means includes:

a plurality of privacy-code generators responsive to the timing signal for generating a plurality of privacy-code signals.Iadd., respectively.Iaddend.; and

a plurality of EXCLUSIVE-OR gates for .[.the.]. decoding the plurality of privacy-encoded data signals as a plurality of data signals, respectively.

7. A remote unit for receiving a spread-spectrum-time-multiplexed signal, having a synchronization signal and a plurality of privacy-encoded data signals including a first privacy-encoded-data signal, on a first carrier frequency, f₁, and for spread-spectrum transmitting a remote-data signal on a second carrier frequency, f₂, comprising:

an antenna;

a despreader circuit coupled to said antenna for despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

a synchronization-code matched filter coupled to said despreader circuit and responsive to a synchronization-code signal embedded in the time-multiplexed signal for generating a timing signal;

a controller coupled to said synchronization-code matched filter and responsive to the timing signal for generating a control signal;

a first buffer coupled to said despreader circuit and to said controller, said first buffer responsive to the control signal for storing the first privacy-encoded-data signal embedded in the time-multiplexed signal;

a first privacy-code generator coupled to said controller and responsive to the control signal for generating a first privacy-code signal;

a first EXCLUSIVE-OR gate coupled to said first privacy-code generator and to said first buffer for decoding the first privacy-encoded-data signal as a first data signal;

a second privacy-code generator coupled to said controller and responsive to the control signal for generating a remote-privacy-code signal;

a second EXCLUSIVE-OR gate coupled to said second privacy-code generator for encoding the remote-data signal with the remote-privacy-code signal as a remote-privacy-encoded-data signal;

a second buffer for storing the remote-privacy-encoded-data signal;

a second chip-code generator for generating a second chip-code signal:

a third EXCLUSIVE-OR gate coupled to said second buffer and to said second chip-code generator for spread-spectrum processing the .[.remote-privacy-encoded.]. .Iadd.remote-privacy-encoded-data .Iaddend.signal with the second chip-code signal as a spread-spectrum .Iadd.encoded .Iaddend.signal;

a transmitter for transmitting the spread-spectrum encoded signal on the second carrier frequency, f₂ ; and

a transmit-receive switch responsive to the control signal for switching

the antenna from the despreader circuit to the transmitter. 28. The remote unit as set forth in claim 27 wherein said despreader circuit includes a surface-acoustic-wave device for despreading the received spread-spectrum-time-multiplexed signal as the time-multiplexed signal.

The remote unit as set forth in claim 27 wherein said despreader circuit includes a .[.tapped-delay-live.]. .Iadd.tapped-delay-line .Iaddend.matched filter for despreading the received

spread-spectrum-time-multiplexed signal. 30. The remote unit as set forth in claim 27 wherein said despreader circuit includes:

a first chip-code .Iadd.generator for generating a first chip-code .Iaddend.signal;

a mixer coupled to said first chip-code generator and responsive to the first chip-code signal for despreading the received spread-spectrum-time-multiplexed signal as the time-multiplexed signal; and

a filter for filtering the time-multiplexed signal. 31. A remote unit 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:

remote-despreader means for despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

remote-synchronization means responsive to the time-multiplexed signal for detecting the synchronization-code signal and generating a timing signal:

control means responsive to the timing signal for generating a control signal;

remote-decoder means responsive to the control signal for decoding the first privacy-encoded data signal embedded in the time-multiplexed signal as a first data signal;

remote-privacy means responsive to the control signal for encoding the remote-data signal with a remote-privacy-code signal as a remote-privacy-encoded data signal;

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

remote-transmitter means for converting the spread-spectrum signal to a

form suitable for sending over a communications channel. 32. The remote unit as set forth in claim 31 wherein said remote-despreader means includes a surface-acoustic-wave device for despreading the spread-spectrum-time-multiplexer signal as the time-multiplexed signal.

The remote unit as set forth in claim 31 wherein said remote-despreader means includes a tapped-delay-time matched filter for despreading the spread-spectrum-time-multiplexed signal as the

time-multiplexed signal. 34. The remote unit as set forth in claim 31 wherein said remote-despreader means includes:

a filter for filtering the time-multiplexed signal. 35. The remote unit as set forth in claim 31 wherein:

said remote-synchronization means includes a synchronization-matched filter responsive to the synchronization-code signal for generating the timing signal; and

said control means includes a controller responsive to the timing signal

for generating the control signal. 36. The remote unit as set forth in claim 31 wherein said remote-decoder means includes:

a first buffer responsive to the control signal for storing the first privacy-encoded data signal embedded in the time-multiplexed signal;

a first-privacy-code generator responsive to the control signal for generating a privacy-code signal; and

a first EXCLUSIVE-OR gate for decoding the first privacy-encoded data

signal as the first data signal. 37. The remote unit as set forth in claim 31 wherein said remote-privacy means includes:

a second privacy-code generator responsive to the control signal for generating the remote-privacy-code signal; and

a second EXCLUSIVE-OR gate for encoding the remote-data signal with the

remote-privacy-code signal as the remote-privacy-encoded data signal. 38. The remote unit as set forth in claim 31 wherein said remote-spreading means includes:

a second buffer for storing the remote-privacy-encoded data signal;

a second chip-code generator for generating a second chip-code signal; and

a third EXCLUSIVE-OR gate for spread-spectrum processing the second privacy-encoded signal with the second chip-code signal as a

spread-spectrum signal. 39. The remote unit as set forth in claim 31 further including:

an antenna: and

a transmit-receive switch responsive to the control signal for switching the antenna from said .[.despreader.]. .Iadd.remote-despreader .Iaddend.means to said .[.transmitter.]. .Iadd.remote-transmitter

.Iaddend.means. 40. A remote unit 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: remote-despreader means for despreading the spread-spectrum-time-multiplexed signal as a time-multiplexed signal;

remote-synchronization means responsive to the time-multiplexed signal for detecting the synchronization-code signal and generating a control signal;

.Iadd.remote-privacy means responsive to the control signal for encoding the remote-data signal with a remote-privacy code signal as a remote-privacy-encoded-data signal; .Iaddend.

remote-spreading means for spread-spectrum processing the .[.remote-data.]. .Iadd.remote-privacy-encoded-data .Iaddend.signal with a second chip-code signal as a spread-spectrum signal; and

remote-transmitter means for converting the spread-spectrum signal to a

form suitable for sending over a communications channel. 41. The remote unit as set forth in claim 40 wherein said remote-despreader means includes a surface-acoustic-wave device for despreading the spread-spectrum-time-multiplexed signal as the time-multiplexed signal.

The remote unit as set forth in claim 40 wherein said remote-despreader means includes a tapped-delay-time matched filter for despreading the spread-spectrum-time-multiplexed signal as the

time-multiplexed signal. 43. The remote unit as set forth in claim 40 wherein said remote-despreader means includes:

a filter for filtering the time-multiplexed signal. 44. The remote unit as set forth in claim 40 wherein:

said remote-synchronization means includes a synchronization-matched filter responsive to the synchronization-code signal for generating .[.and.]. .Iadd.a .Iaddend.timing signal; and

further including control .Iadd.means, responsive to the .Iaddend.timing

signal.Iadd., .Iaddend.for generating the control signal. 45. The remote unit as set forth in claim 40 wherein said remote-spreading means includes:

a second buffer for storing the .[.remote-data.]. .Iadd.remote-privacy-encoded-data .Iaddend.signal;

a second chip-code generator for generating .[.a.]. .Iadd.the .Iaddend.second chip-code signal; and

a third EXCLUSIVE-OR gate for spread-spectrum processing the .[.remote-data.]. .Iadd.remote-privacy-encoded data .Iaddend.signal with

the second chip-code signal to generate a spread-spectrum signal. 46. The remote unit as set forth in claim 40 further including:

an antenna; and

.Iaddend.means. 47. A method for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising the steps of:

generating a synchronization-code signal;

encoding a plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded data signals, respectively;

time multiplexing the synchronization-code signal and the plurality of privacy-encoded data signals, respectively, to generate a 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. .[.48. 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 a time-multiplexed signal;

converting the spread-spectrum-time-multiplexed signal to a form suitable for sending over a communications channel..]..[.49. A method for receiving a spread-spectrum-time-multiplexed signal, comprising the steps of:

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

demultiplexing the time-multiplexed signal as a plurality of data

signals..].50. A method for receiving a spread-spectrum-time-division signal, comprising the steps of:

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

demultiplexing the time-division signal as a plurality of remote-privacy-encoded remote-data .[.signal.]. .Iadd.signals.Iaddend.;

generating a timing signal; and

decoding in response to the timing signal, the plurality of remote-privacy-encoded data signals as a plurality of remote-data signals.

1. A method of receiving a spread-spectrum-time-multiplexed signal, having a .[.synchronization.]. .Iadd.synchronization-code .Iaddend.signal and a first privacy-encoded data signal, at a first carrier frequency, and for .[.a.]. spread-spectrum transmitting a remote-data signal at a second carrier frequency, .Iadd.comprising .Iaddend.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;

generating, in response to the timing signal, a control signal;

decoding .[.a.]. .Iadd.the .Iaddend.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 as a spread-spectrum signal; and

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

communications channel. 52. A spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising:

base-synchronization means for generating a synchronization-code signal.Iadd.; .Iaddend.

a plurality of base-spreading means for spread-spectrum processing the synchronization-code signal and the plurality of data signals .[.and.]. with a plurality of chip-code signals .Iadd., respectively, .Iaddend.to generate a spread-spectrum-processed-synchronization-code signal and a plurality of spread-spectrum signals.[., respectively.].;

combiner means for combining the plurality of spread-spectrum signals.[., respectively,.]. to generate a combined-spread-spectrum signal;

multiplexer means for time multiplexing the spread-spectrum-processed-synchronization-code signal and the combined-spread-spectrum signal as a multiplexed-spread-spectrum signal; and

base-transmitter means for converting the .[.time-multiplexed spread-spectrum.]. .Iadd.multiplexed-spread-spectrum .Iaddend.spread-spectrum signal to a form suitable for sending over a

communications channel. 53. The spread-spectrum transmitter as set forth in claim .[.2.]. .Iadd.45.Iaddend., further including:

a plurality of base-privacy means for encoding the plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded-data signals, respectively; and

wherein said plurality of base-spreading means spread-spectrum processes the plurality of privacy-encoded-data signals with the plurality of chip-code signals to generate the plurality of spread-spectrum signals,

respectively. 54. The spread-spectrum transmitter as set forth in claim 53 wherein each of said .Iadd.plurality of .Iaddend.base-privacy means includes:

a privacy-code generator for generating .[.the.]. .Iadd.a .Iaddend.privacy-code signal: and

an EXCLUSIVE-OR gate for modulo-2 adding the .[.respective.]. privacy-code signal to .Iadd.a respective one of .Iaddend.the .Iadd.plurality of

.Iaddend.data .[.signal, respectively.]. .Iadd.signals.Iaddend.. 55. The spread-spectrum transmitter as set forth in claim .[.52.]. .Iadd.53 .Iaddend.wherein each of said .Iadd.plurality of .Iaddend.base-spreading means includes:

a chip-code generator for generating a chip-code signal: and

an EXCLUSIVE-OR gate for spread-spectrum processing .Iadd.a respective one of .Iaddend.the .Iadd.plurality of .Iaddend.privacy-encoded-data .[.signal.]. .Iadd.signals .Iaddend.with the chip-code signal to generate .Iadd.a respective one of .Iaddend.the .Iadd.plurality of

.Iaddend.spread-spectrum .[.signal.]. .Iadd.signals.Iaddend.. 56. The spread-spectrum transmitter as set forth in claim 52 wherein said multiplexer means includes:

a first data buffer for storing the combined-spread-spectrum signal:

a second buffer for storing the spread-spectrum-processed-synchronization-code signal; and

a multiplexer for time multiplexing the spread-spectrum-processed-synchronization-code signal and the combined-spread-spectrum signal, respectively, to generate the

multiplexed-spread-spectrum signal. 57. The spread-spectrum transmitter as set forth in claim 52 wherein said base-transmitter means includes:

an oscillator for generating a carrier frequency;

a product device for shifting the combined-spread-spectrum signal to the carrier frequency;

a power amplifier for amplifying the multiplexed-spread-spectrum signal at the carrier frequency; and

an antenna for radiating the amplified multiplexed-spread-spectrum signal

at the carrier frequency over the communications channel. 58. A spread-spectrum transmitter for spread-spectrum processing a plurality of data signals on a carrier frequency, comprising:

base-genetic means for generating a generic-chip-code signal;

a plurality of base-spreading means for spread-spectrum processing the plurality of data signals with a plurality of chip-code signals to generate a plurality of spread-spectrum signals;

combiner means for combining the generic-chip-code signal and the plurality of spread-spectrum signals, respectively, to generate a combined-spread-spectrum signal; .[.and.].

base-transmitter means for converting the combined-spread-spectrum signal to a form suitable for sending over a communications channel.Iadd.;

a plurality of base-privacy means for encoding a plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded data signals, respectively; and

respectively.Iaddend.. .[.59. The spread-spectrum transmitter as set forth in claim 58, further including:

a plurality of base-privacy means for encoding a plurality of data signals with a plurality of privacy-code signals as a plurality of privacy-encoded-data signals, respectively; and

respectively..].60. The spread-spectrum transmitter as set forth in claim .[.59.]. .Iadd.58 .Iaddend.wherein each of said .Iadd.plurality of .Iaddend.base-privacy means includes:

a privacy-code generator for generating .Iadd.a respective one of .Iaddend.the .Iadd.plurality of .Iaddend.privacy-code .[.signal, respectively.]. .Iadd.signals.Iaddend.; and

an EXCLUSIVE-OR gate for modulo-2 adding the respective privacy-code signal to .Iadd.a respective one of .Iaddend.the .Iadd.plurality of .Iaddend.data

.[.signal, respectively.]. .Iadd.signals.Iaddend.. 61. The spread-spectrum transmitter as set forth in claim 58 wherein each of said .Iadd.plurality of .Iaddend.base-spreading means includes:

a chip-code generator for generating a respective chip-code signal; and

an EXCLUSIVE-OR gate for spread-spectrum processing .Iadd.a respective one of .Iaddend.the .Iadd.plurality of .Iaddend.privacy-encoded-data .[.signal.]. .Iadd.signals .Iaddend.with the .Iadd.respective .Iaddend.chip-code signal to generate .Iadd.a respective one of .Iaddend.the .Iadd.plurality of .Iaddend.spread-spectrum .[.signal,

respectively.]. .Iadd.signals.Iaddend.. 62. The spread-spectrum transmitter as set forth in claim 58 wherein .[.each of.]. said base-transmitter means includes:

an oscillator for generating a carrier frequency;

a power amplifier for amplifying the combined-spread-spectrum signal at the carrier frequency: and

an antenna for radiating the amplified combined-spread-spectrum signal at the carrier frequency over the communications channel.