WO1998053563A2

WO1998053563A2 - Method and system for communicating digital data by simultaneously code division multiple accessing a plurality of channels

Info

Publication number: WO1998053563A2
Application number: PCT/US1998/010000
Authority: WO
Inventors: David C. Robb
Original assignee: Spectraplex, Inc.
Priority date: 1997-05-22
Filing date: 1998-05-15
Publication date: 1998-11-26
Also published as: AU7574598A; WO1998053563A3

Abstract

A communication system includes a transmitter that transmits data using code division multiple access on a virtual channel that comprises a plurality of physical channels. Specifically, data is processed by spread spectrum techniques by applying a plurality of spread spectrum codes to the data. Each of the spread data signals are modulated onto a corresponding carrier and the modulated carriers are simultaneously transmitted over a virtual channel. Each of the plurality of virtual channels comprises a portion of a plurality of physical channels, each said portion being different for each of the plurality of virtual channels. A receiver receiving a signal demodulates the received signal by a plurality of carrier signals to generate for each physical channel having data thereon a received data sequence. Each data sequence is combined with a spread spectrum code corresponding to the physical channel to generate a recovered data signal. The plurality of recovered data signals are correlated together and an estimate of the originally transmitted data signal is generated.

Description

TITLE OF THE INVENTION:

METHOD AND SYSTEM FOR COMMUNICATING DIGITAL

DATA BY SIMULTANEOUSLY CODE DIVISION MULTIPLE

ACCESSING A PLURALITY OF CHANNELS

FIELD OF THE INVENTION

The present invention relates to electronic communications, and more particularly to electronic communications using simultaneous code division multiple access over a plurality of channels.

BACKGROUND OF THE INVENTION With the growing utilization of available channels for communication, there has been considerable interest in making maximum use of the limited resources available, especially in electromagnetic wave communication as well as telephony. In present radio communication methods, particular frequencies known as channels, are allocated for particular uses. Each channel is of some finite bandwidth to accommodate modulation spread and inaccuracies in frequency setting. As a consequence, with present methods, the number of channels a particular portion of the electromagnetic spectrum can provide is simply the frequency span available divided by the bandwidth associated with each channel. Under this scheme, the number of communication channels available is severely limited. This is particularly significant because allocation of the limited number of available radio channels is a present problem that is becoming more difficult each day as the number of potential users grows. Further, most current methods do not allow simultaneous use of a particular frequency by different users, so that the efficiency of the channel is determined by the percent of time the channel is actually used for communication, and is not sitting idle. Various methods such as those described below have been applied in attempts to increase the utilization, or spectrum efficiency, of particular frequency ranges. Capacity expansion and spectrum efficiency increases have been effected by Frequency Division Multiple Access, Time Division Multiple Access, Code Division Multiple Access, bandwidth reduction, spatial division as in cellular telephony, and a variety of others. Each method has had a measure of success, as well as some significant limitations and problems.

Frequency Division Multiple Access is the method in most common use, and the one most familiar to users. AM broadcast radio, television, and other services rely on this method to distinguish between and select stations. In this method, each user or service is assigned a particular frequency of operation and an associated range of frequencies about that center frequency.

The range of frequencies accommodates tuning error and modulation spread of the main, or carrier, frequency. Two or more services cannot share the same frequency within a common geographic area because of mutual interference. Also, since each service is limited to a single frequency band, multipath and other forms of natural interference can significantly affect the signals and reduce the communication effectiveness. "Ghosting" in television is a familiar example of multipath interference.

Time Division Multiple Access is commonly used in telephony, and offers the advantage of expanding the communication capacity of a limited number of channels, be they wires, optical fibers, radio frequency bands, or others. This method operates by sampling a number of signals at a high rate, combining in sequence each of the samples, and then reconstructing the original signals from the respective samples at the receiving end. The number of signals that can be accommodated is determined by the ratio of the total period between successive samples of a given signal, to the duration of the individual samples. The disadvantages are numerous, and include a severe limitation on the bandwidth of the information signals being carried. This limitation arises from the fact that the information signals must be sampled as part of the process of time division multiplexing. Consequently, the signal bandwidths must be less than the Nyquist frequency, one-half of the sampling frequency, in order to avoid creation of aliased signal components in the reconstructed signal. Further, the complexity of such systems is high, and close synchronization between transmitter and receiver must be maintained in order to properly reconstruct the numerous sampled information signals. The method is very useful in telephony where many of the variables can be closely controlled, but is prohibitively difficult to implement in many other applications.

Code Division Multiple Access (CDMA) is a recent technique whereby the information carrying signal is modulated by a unique digital code sequence. At the receiver, the received signal is correlated with the same unique code sequence to obtain the original information signal. In theory, many users could share the same frequency allocation, so long as their digital code sequences were sufficiently different. In practice, the number of users is often reduced below the theoretical number by problems such as near-far interference, among others. In near-far interference, the nearer, stronger signal obscures the farther, weaker signal in the receiver, so that the far signal falls below the detectability limits of the receiver when the gain is sufficiently low to prevent saturation by the stronger near signal. The method is also complex, and considerable effort is required to design and develop systems that can acquire a signal in a reasonable time, and can maintain the synchronization necessary for proper decoding. Further, bandwidth limitations on the information portion of the signal may be restrictive given certain constraints on the bandwidth and operating frequency of the total system.

George Cooper and Clare McGillem discuss Code Division Multiple Access and other versions of what are termed "spread spectrum" systems in: "Modern Communications and Spread Spectrum," McGraw-Hill, 1986, pp. 268-411. The authors discuss the disadvantages of spread spectrum approaches. Among these disadvantages are that the more useful forms require a wide bandwidth high quality channel, that a long acquisition time may be required, and the systems are complex in implementation, in addition to the near-far problem already discussed. Ulrich Rohde and T. T. N. Bucher present similar discussions and analysis in "Communications Receivers," McGraw-Hill, 1988, pp. 462-471. Another approach to the problem of expanding capacity is spatial division, an exemplary model of which is cellular telephony. For some time, radio frequency allocations have been geographically distributed so services using the same frequency are sufficiently separated in space that they do not interfere with each other. Cellular telephony employs that principle on a much more local scale. "Electronic Communications Handbook," McGraw-Hill, 1988, pp. 22.1-22.19, Andrew Inglis, editor, presents a discussion of the principles of cellular telephone systems. By careful control of transmitter power, clever assignment of frequencies, and a marvelously complex switching and control system, several thousand users can be accommodated by only a few hundred frequencies. However, such systems will saturate so that no new users can be accommodated, the technology is expensive, conversations are not private, interference problems are significant, and service areas are limited, to cite only a few of the major problems of spatial division carried to the extreme.

One current approach involves combination of spread spectrum methods with cellular structure. The article "Spread Spectrum Goes Commercial" by Donald Schilling, Raymond Pickholtz, and Laurence Milstein in IEEE Spectrum of August 1990, pp. 40-45, discusses such combination. According to the article, each cell in a current cellular system serves about 55 users at a time. With spread spectrum methods, 150 to 300 users could be accommodated initially, with up to 1 ,000 users later. This is about a twenty fold improvement in capacity, but at a substantial cost, and will probably still fall short of demand in certain high density areas. As noted above, CDMA systems are susceptible to near-far interference. Specifically, CDMA systems operate best in an equal power condition in which a receiver receives all signals from all users at the same power level. If the users are distributed spatially, the transmit power of the users must be adjusted so that when the signal arrives at the receiver it is at the same power level as the power level of all of the other signals. If the power levels are not equal, even if they differ by a small amount, the higher received power signal may dominate the other signals received by the receiver, even though the higher received power signal is not using the code of the receiver, and capture the receiver.

Specifically, the CDMA receiver performs a binary correlation of all the received signals by applying the code signal to the received signal for such correlation. When the intended receive signal is at a distance from the receiver, a high power near signal appears as a large signal with noise ripples that include the intended received signal for the receiver. After correlation with the binary spreading sequence, the correlated data will not match the transmitted data because the near signal has dominated the transmission. Such a near- far interference occurs even with a power difference between the near and far signals of only a factor of 2.

One solution to the near-far interference problem is implementation of power control at the transmitter. The power of the transmitter is adjusted to account for the variations in the mobile environment so that the power received at a receiver is the same power from each user. Such power adjustment may occur as often as 800 times per second. Such a system requires a complicated scheme for measuring the power at the base station of the received signals and looking for changes in the levels of relative users. In response to such measurements, appropriate power adjustments are transmitted back to all users in the system. Such power adjustments require large data time and processing input. In addition, precise measurements of the received signals is required.

SUMMARY OF THE INVENTION The present invention provides a method and communication system in which digital data is spread by a plurality of spread spectrum codes and/or modulated onto one or more RF carriers to form a plurality of physical channels over which the data is simultaneously transmitted. The plurality of physical channels form a virtual channel. Digital data is code spread with a plurality of spreading codes to form a plurality of spread spectrum sequences. Each of the plurality of the spread spectrum sequences is modulated onto a corresponding carrier to obtain one or more modulated carriers, and the plurality of modulated carriers are transmitted simultaneously. The present invention also provides a method that includes receiving the transmitted plurality of modulated carriers and demodulating the plurality of modulated carriers. Each of the demodulated plurality of modulated carriers is combined with a corresponding one of the plurality of spread spectrum sequences to form a plurality of received data signals. Each of the said plurality of received data signals are correlated with each of the others of the plurality of received data signals to form a plurality of correlation signals which are combined to form an estimate of the originally transmitted digital data.

The present invention provides a communication system that includes a plurality of transmit code division multiple access (CDMA) circuits, each of which combines the digital data with a spread spectrum sequence corresponding to the transmit CDMA circuit to generate a plurality of spread spectrum signals. A modulator coupled to each of the plurality of transmit

CDMA circuits modulates each of the plurality of spread spectrum signals onto a corresponding one of a plurality of carriers to generate a plurality of modulated data signals. A transmitter coupled to the modulator receives the modulated data signals and simultaneously transmits the plurality of modulated data signals.

The present invention also includes a communication system that includes a receiver that receives and demodulates the simultaneously transmitted plurality of data signals to form a plurality of received data signals. A plurality of received CDMA circuits coupled to the receiver generate a plurality of recovered data signals in response to the plurality of received data signals and to the plurality of spread spectrum sequences. A correlator coupled to the plurality of received CDMA circuits forms a plurality of correlation signals, each of which corresponds to the correlation between one of the plurality of recovered data signals and another of the plurality of recovered data signals and generates an estimate of the digital data from said plurality of correlation signals. The present invention advantageously provides improved data transmission over a fixed carrier bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram illustrating a code division multiple access

(CDMA) communication system in accordance with the present invention. Figure 2 is a block diagram illustrating a transmitter of the CDMA communication system of Figure 1 in accordance with one embodiment of the present invention. Figure 3 is a block diagram illustrating a receiver of the CDMA communication system of Figure 1 in accordance with one embodiment of the present invention.

Figure 4a is a data flow diagram of an illustrative encoding of data for transmission in accordance with the present invention. Figure 4b is a data flow diagram of an illustrative decoding of received data in accordance with the present invention.

Figure 5 is a block diagram illustrating a transmitter of the CDMA communication system of Figure 1 in accordance with a second embodiment of the present invention. Figure 6 is a block diagram illustrating a receiver of the CDMA communication system of Figure 1 in accordance with a second embodiment of the present invention.

Figure 7 is a block diagram illustrating a transmitter of the CDMA communication system of Figure 1 in accordance with a third embodiment of the present invention.

Figure 8 is a block diagram illustrating a receiver of the CDMA communication system of Figure 1 in accordance with a third embodiment of the present invention.

Figure 9 is a block diagram illustrating a transmitter of the CDMA communication system of Figure 1 in accordance with a fourth embodiment of the present invention. Figure 10 is a block diagram illustrating a receiver of the CDMA communication system of Figure 1 in accordance with a fourth embodiment of the present invention.

Figure 1 1 is an example of assignments of physical channels and spread spectrum codes to virtual channels in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a block diagram illustrating a code division multiple access (CDMA) communication system 100 in accordance with the present invention.

The CDMA communication system 100 comprises a plurality of transmitters 102, a plurality of receivers 104, and a communication medium 106. For simplicity and clarity, only two transmitters 102-1 and 102-2 and only two receivers 104-1 and 104-2 are shown. Each transmitter 102 provides an information signal through the communication medium 106 to the plurality of receivers 104.

The information signal may be encoded by a plurality of codes, such as spread spectrum codes, modulated onto a plurality of RF channels, or a combination of both. In one embodiment of the present invention, the information signal is a CDMA signal simultaneously transmitted over a plurality of RF channels. A plurality of spread spectrum codes are applied thereto to form a plurality of different CDMA data signals. Each CDMA data signal is modulated onto a corresponding carrier signal and all modulated carriers are simultaneously transmitted, which accordingly simultaneously communicates the data signal over multiple channels.

In each of the receivers 104, the received data for each channel is demodulated and a spread spectrum code corresponding to the channel is applied to the received data to generate a recovered data signal which includes noise induced by processing of the transmitter, the transmission through the transmission medium 106 and the processing by the receiver 104. The recovered data signals are correlated with each other to form correlated signals which are then combined to form an estimate of the original transmitted data. The information signal may be communicated as a radio frequency signal, a laser beam, a laser beam and an optical fiber, a current in a wire, a sound wave or the like. In accordance with one embodiment of the present invention, the information signal may include a mix of several different carriers.

Figure 2 is a block diagram illustrating the transmitter 102 in accordance with the present invention. The transmitter 102 comprises a digital data source 202, a summation unit 204, a plurality of spread spectrum mixers 206-1 through 206-n, a radio frequency (RF) modulator 208, and a communication medium interface 210.

The digital data source 202 applies the digital data to selected ones of the plurality of spread spectrum mixers 206-1 through 206-n. The digital data signal may be, for example, voice data. Spread spectrum codes 212-1 through 212-n are applied to the same selected ones of the plurality of spread spectrum mixers 206-1 through 206-n. In response to these applied signals and codes, the plurality of spread spectrum mixers 206-1 through 206-n form a plurality of spread data signals which are applied to the summation unit 204 which combines these signals and applies these signals to the radio frequency modulator 208. In response to an RF carrier signal 214 and the combined spread data signal, the RF modulator 208 modulates the spread data signals to form a combined modulated carrier signal. The combined modulated carrier signal is applied to the communication medium interface 210 for simultaneous communication of the data on the RF carrier over the communication medium 106 (Figure 1). The communication medium interface 210 may couple, for example, to an antenna, a cable, a fiber optic system, or the like.

The selection of the spread spectrum codes applied to the plurality of spread spectrum mixers 206 is determined in accordance with conventional code division multiple access techniques which are well known in the art. The selection of ones of the plurality of spread spectrum mixers 206-1 through 206-n to which the digitized voice data is to be applied may be in accordance with the techniques described in U.S. Patent No. 5,548,819, the subject matter of which is incorporated herein by reference in its entirety. Specifically, the spread spectrum mixers 206-1 through 206-n correspond to a plurality of physical channels. The selection of these physical channels determines the physical channels over which the data is to be communicated. Sets of physical channels form virtual channels. Each virtual channel is unique. The selection of said ones of the spread spectrum mixers 206 is determined by the selected virtual channel. In particular, data from one transmitter 102 is to be communicated over a virtual channel. Physical and virtual channels are described below in conjunction with Figure 11.

In one embodiment of the present invention, the transmitted data of different virtual channels is not synchronized. Within each virtual channel, the data on each physical channel is synchronized.

For each virtual channel, the bits of each of the transmitted signals from the spread spectrum mixers 206-1 through 206-n, are preferably slightly shifted in time relative to the bit positioned in the other virtual channels. In such an embodiment, the data may be provided to shift registers (not shown) and clocked out at different transmit periods for each of the bits. Accordingly, when the data is received at one receiver, data from other virtual channels is transitioning during the sample bit time and when integrated over the bit time averages to zero while data from the desired channel remains constant during the sample bit time. In one embodiment of the present invention, a number of n virtual channels are transmitted at one time, the bit interval is divided into n times and, for each of the n times, data from one of the virtual channels is transmitted.

Figure 3 is a block diagram illustrating the receiver 104 in accordance with the present invention. The receiver 104 comprises a communication medium interface 302, a radio frequency (RF) modulator 304, a plurality of receive spread spectrum mixers 306-1 through 306-n, a correlator 308, and a data processor 310. The communication medium interface 302 receives data signals from the communication medium 106 and provides the received data signals to the RF modulator 304. In response to an applied RF frequency signal 314, the RF modulator 304 down converts the received digital data from -l ithe modulated carrier to baseband and then applies the baseband data to selected spread spectrum mixers 306-1 through 306-n.

The selection of the mixers 306-14 through 306-n is determined by the physical channels of the received data. Spread spectrum codes 312-1 through 312-n are applied to selected spread spectrum mixers 306-1 through 306-n in accordance with the spread spectrum codes 212-1 through 212-n of the channels as applied by the transmitter 102 of Figure 2. The mixed spread spectrum signals are provided to the correlator 308 which calculates correlation between the signals from the mixers 306 and generates a correlation signal which is applied to the data processor 310 for processing.

The correlation performed by the correlator 308 may be the covariance described in U.S. Patent No. 5,548,819. Such correlation correlates each of the next spread spectrum signals with each other to determine the correlation between each of the mixed spread spectrum signals. Figure 4a is a data flow diagram of an illustrative encoding of data for transmission in accordance with the present invention. In this illustrative example, a data sequence "010" from the analog to digital converter 204 is spread into a data sequence "001100" according to an exemplary processing rate of two times. The data sequence "001100" represents the data to be communicated over a virtual channel. For illustrative purposes, the virtual channel comprises the first four physical channels that are processed by the spread spectrum mixers 206-1 through 206-4. Each spread spectrum mixer 206-1 through 206-4 modifies the input data by a spread spectrum code. In an illustrative example, the modification of the data is performed by adding the data to the spread spectrum code. For example, the spread spectrum mixer

206-1 has a spread spectrum code "111010" applied thereto which is added to the data sequence "001100" to generate data sequence "110110" which is transmitted over the first physical channel by the RF modulator 208. Likewise, the spread spectrum mixer 206-2 applies the spread spectrum code "010001 " to the data to generate the data sequence "011101" which is transmitted over the second physical channel. Likewise, the spread spectrum mixer 206-3 adds the spread spectrum code "101100" to the data to generate the data sequence "100000" which is transmitted over the third physical channel. Likewise, the spread spectrum mixer 206-4 adds the spread spectrum code "000101 " to the data to generate the data sequence "001000" which is transmitted over the fourth physical channel. Figure 4b is a data flow diagram of an illustrative decoding of received data in accordance with the present invention. The illustrative data sequence shown in Figure 4b is the analog voltage representative of the received digital signal as output from the RF modulator 304. The first physical channel has a data sequence of .9 volts, .4 volts, .2 volts, .8 volts, .8 volts and .1 volts. In the illustrative example of Figure 4b, a logic one has a voltage in the range of .5 to

1.0 volts, and a logic 0 has a voltage in the range between 0 and .5 volts. Illustrative errors introduced in the received signal, such as caused by near-far interference, are indicated in a dashed square around the sampled data voltage. For example, in the first data sequence, the second data value .4 volts is indicative of a logic 0 when, in fact, the data should be a logic one. The data of the first physical channel is applied to the spread spectrum mixer 306-1 and mixed with the spread spectrum code "111010" which also corresponds to the spread spectrum code applied by the transmitter 102 for the first physical channel. The resultant sequence of .1, .6, .8, .8, .2, .1 volts is applied to the correlator 308. Likewise, for the second through fourth physical channels, the received data is applied by the respective spread spectrum mixers 306-2 through 306-4 to generate corresponding data sequences.

As a simplified illustrative example, the correlator 308 generates a correlation by adding the analog values for each data bit and averages them and then demodulates them at the 2 to 1 processing rate, by adding pairs of successive bits to generate an average. The average is then converted to a logic value based on the conversion described above wherein a logic zero is a signal between 0 and .5 volts and a logic one is a signal between .5 and 1.0 volts. As shown in Figure 4b, the correlator 308 generates the data sequence 010 which corresponds to the data transmitted by the transmitter 102. It should be noted that the received data sequence has an illustrative bit error rate of one out of six bits for the first and third physical channels and a two out of six bit error rate for the second and fourth physical channels.

Figure 5 is a block diagram illustrating a transmitter 102 in accordance with a second embodiment of the present invention. The transmitter 102 includes a digital data source 502, a summation unit 504, a spread spectrum mixer 506, a plurality of radio frequency (RF) modulators 508, and a communication medium interface 510.

The digital data source 502 provides a digital data signal to a first input of the spread spectrum mixer 506. A spread spectrum code 512 is applied to another input of the spread spectrum mixer 506. In response to the digital data and the spread spectrum code 512, the spread spectrum mixer 506 forms a spread data signal which is applied to each of selected ones of the plurality of radio frequency modulators 508-1 through 508-n. Each selected radio frequency modulator 508 modulates the mixed data signal onto a corresponding one of a plurality of RF carriers 514-1 through 514-n, which are applied to the summation unit 504, which forms a combined modulated carrier signal. The combined modulated carrier signal is applied to the communication medium interface 510 for simultaneous communication of the data on the plurality of carriers over the communication medium 106 (Figure 1).

The selection of ones of the plurality of radio frequency modulators 508-1 through 508-n to which the spread data signal is to be modulated onto may be in accordance with the techniques described in U.S. Patent No. 5,548,819. Specifically, each of the radio frequency modulators 508 corresponds to a respective physical channel. The selection of said ones of the radio frequency modulator 508 is determined by those of the plurality of physical channels that define the virtual channel.

Figure 6 is a block diagram illustrating a receiver 104 in accordance with a second embodiment of the present invention. The receiver 104 comprises a communication medium interface 602, a plurality of radio frequency modulators 604, a received spread spectrum mixer 606, a correlator 608, and a data processor 610. The communication medium interface 602 receives data signals from the communication medium 106 and provides received data signals to each of the RF modulators 604 which selectively down converts, in response to a corresponding one of a plurality of RF carriers 614-1 through 614-n, the digital data from the modulated carrier. The digital data may be at baseband or alternatively at an IF frequency. The digital data is applied to the correlator 608 which performs a covariance function such as described in U.S. Patent No. 5,548,819. The correlated data from the correlator 608 is applied to a spread spectrum mixer 606 which applies a spread spectrum code 612 as applied by the transmitter 102 of Figure 5. The resulting despread signal is provided to the data processor 610 for processing.

Figure 7 is a block diagram illustrating a transmitter 102 in accordance with a third embodiment of the present invention. The transmitter 102 includes a digital data source 702, a plurality of spread spectrum mixers 706, a radio frequency modulator 708, a summation unit 709, and a communication medium interface 710.

A digital data source 702 provides a digital data signal to the plurality of spread spectrum mixers 706-1 through 706-n. One of a plurality of spread spectrum codes 712-1 through 712-n is applied to a respective input of one of the spread spectrum mixers 706-1 through 706-n, respectively. In response to the digital data and the spread spectrum code 712, the spread spectrum mixers

706 form a plurality of spread data signals which are applied to the summation unit 704 which applies the summed signal to the plurality of radio frequency modulators 708-1 through 708-n. The radio frequency modulators 708-1 through 708-n modulate the mixed data signal onto a respective corresponding one of a plurality of RF carriers 714-1 through 714-n to form a plurality of modulated carrier signals which are applied to the summation unit 709. The summation unit 709 applies the added modulated carrier signals to the communication medium interface 710 for simultaneous communication of the data which has been spread by both a plurality of spread spectrum codes 712 and modulated onto a plurality of RF carriers 714. The selection of the ones of the plurality of spread spectrum mixers 706-1 through 706-n and of ones of the plurality of RF modulators 708-1 through 708-n to which the data is spread may be in accordance with the techniques described in U.S. Patent No. 5,548,819. Specifically, a virtual channel in this system comprises selected ones of the plurality of spread spectrum codes 712 and RF carriers 714. The selection of said ones of the plurality of the spread spectrum codes 712 and RF carriers 714 is determined by the physical channels corresponding to the virtual channel.

Figure 8 is a block diagram illustrating a receiver 102 in accordance with a third embodiment of the present invention. The receiver 102 comprises a communication medium interface 802, a plurality of radio frequency modulators 804-1 through 804-n, a first correlator 805, a plurality of spread spectrum mixers 806-1 through 806-n, a second correlator 809 and a data processor 810. The communication medium interface 802 receives data signals from the communication medium 106 (Figure 1) and provides received communication signals to each of the plurality of RF modulators 804-1 through 804-n. Each of the RF modulators 804 selectively down converts the digital data from the modulated carrier in response to a respective one of a plurality of RF carriers 814-1 through 814-n. The down converted data may be at baseband or alternatively at an IF frequency. The down converted digital data is applied to the first correlator 805 which correlates the data. In one embodiment of the present invention, the correlation is the covariance function described in U.S. Patent No. 5,548,819. The correlated data from the correlator 805 is applied to the plurality of spread spectrum mixers 806-1 through 806-n. Each spread spectrum mixer 806 applies a corresponding one of a plurality of spread spectrum codes 812-1 through 812-n which corresponds to the spread spectrum codes provided by the transmitter 102 of

Figure 7. The resultant despread signals from each of the plurality of spread spectrum mixers 806 are applied to the second correlator 809 which correlates the data. In one embodiment of the present invention, the correlation is the covariance function described in U.S. Patent No. 5,548,819. The resulting despread correlated data is provided to the data processor 810.

Figure 9 is a block diagram illustrating a transmitter 102 in accordance with a fourth embodiment of the present invention. The transmitter 102 includes a digital data source 902, a plurality of spread spectrum mixers 906-1 through 906-n, a plurality of radio frequency modulators 908-1 through 908-n, a summer 509, and a communication medium interface 910.

The digital data source 902 provides a digital data signal to each of the plurality of spread spectrum mixers 906-1 through 906-n. Spread spectrum codes 912-1 through 912-n are applied to respective ones of the plurality of spread spectrum mixers 906-1 through 906-n. In response to the digital signal and the applied spread spectrum code 912, each of the spread spectrum mixers 906 forms a spread data signal which is applied to some of the plurality of radio frequency modulators 908-1 through 908-n. The connections between the spread spectrum mixers 906 and the radio frequency modulators 908-1 through 908-n are selected in accordance with the allocation of the physical channels to the corresponding virtual channel that is to be transmitted. In this embodiment of the present invention, each virtual channel comprises some of the spread spectrum codes and some of the RF carriers. Each radio frequency modulator 908-1 through 908-n modulates the mixed data signal onto a corresponding one of a plurality of carriers 914-1 through 914-n to form a combined modulated carrier signal. The combined modulated carrier signal is applied to the summer 909 which adds the carrier signals together for simultaneous communication of the data by the communication medium interface 910 to the communication medium 106 (Figure 1). The selection of the spread spectrum codes 912 and the radio frequency carriers 914, according to one embodiment of the present invention, is in accordance with the techniques described in U.S. Patent No. 5,548,819. Figure 10 is a block diagram illustrating a receiver 104 in accordance with a fourth embodiment of the present invention. The receiver 104 comprises a communication medium interface 1002, a plurality of radio frequency modulators 1004-1 through 1004-n, a plurality of first correlators 1005, a plurality of spread spectrum mixers 1006-1 through 1006-n, a second correlator 1009 and a data processor 1010. The communication medium interface 1002 receives data signals from the communication medium 106 (Figure 1) and provides received communication signals to each of the plurality of RF modulators 1004-1 through 1004-n. Each of the RF modulators 1004 selectively down converts the digital data from the modulated carrier in response to a respective one of a plurality of RF carriers 1014-1 through 1014-n. The down converted data may be at baseband or alternatively at an IF frequency. Each of the plurality of RF modulators 1004 applies the down converted digital data to selected ones of the plurality of first correlators 1005, each of which correlates such data. In one embodiment of the present invention, the correlation is the covariance function described in U.S. Patent No. 5,548,819. The correlated data from each of the plurality of correlators 1005 is applied to a respective one of the plurality of spread spectrum mixers 1006-1 through 1006-n. Each spread spectrum mixer 1006 receives a corresponding one of a plurality of spread spectrum codes 1012-1 through 1012-n which corresponds to the spread spectrum codes provided by the transmitter 102 of Figure 9. The resultant despread signals from each of the plurality of spread spectrum mixers 1006 are applied to the second correlator 1009 which correlates the data. In one embodiment of the present invention, the correlation is the covariance function described in U.S. Patent No. 5,548,819. The resulting despread correlated data is provided to the data processor 1010. In one embodiment of the present invention, the receiver does not include the first correlators 1005, and the RF modulators 1004 apply the down converted signal directly to the spread spectrum mixers 1006.

Figure 11 is an example of assignments of physical channels and spread spectrum codes to virtual channels in accordance with the present invention. The allocation chart of Figure 11 is illustrative of the allocation of RF physical channels and spread spectrum codes to virtual channel sets.

Although it is specifically not a restriction of the present invention that no more than half of the physical channels assigned to a given virtual channel may be used in common with another of the virtual channels, this is the case in the example of Figure 11 since this results in an increased signal to noise ratio. In the example of Figure 11 , it can be seen that the physical channels include 1 through 12 RF channels and 6 spread spectrum codes 13 through 18. As described above in the embodiments of Figures 2-3 and 5-10, the number of RF channels and spread spectrum codes may be varied. In the example of Figure 1 1, it can be seen that each of the virtual channels is modulated by six of the physical channels. This results in two of the physical channels (those numbered 13 and 14 in the example of Figure 11) being used four times, while the remainder of the physical channels are each used seven times.

In order to allocate physical channels to virtual channels, the assigned virtual channels are multiplexed onto the appropriate physical channels. In one embodiment of the present invention, such multiplexing is done by analog summation of the several virtual channels assigned of the physical channels followed by appropriate adjustment of the gain for each physical channel such that the amplitude will be appropriate to the carrier means.

The RF channels correspond to the RF carriers and corresponding RF modulators described above and the spread spectrum codes are applied to the spread spectrum mixers as described above for the various transmitters. The selection of the RF channels and the spread spectrum codes may be determined, for example, by the available spectrum bandwidth. It should be noted that alternative physical channels are within the scope of the invention. For example, a plurality of code channels, or any other means not now known or to be developed in the future for dividing a transmission means into a plurality of distinct carrier means or channels, may be substituted for the example of the physical channels discussed herein in relation to the described embodiments of the present invention.

The above channel allocations have been described in conjunction with the transmitters. However, the receivers also use the physical channels and virtual channels described herein. The receivers separate the data into the various physical channels by, for example, well known filtering methods and modulating with the appropriate RF carriers to form the physical channels. Likewise, the down converted data is mixed with the spread spectrum codes that correspond to the known transmitters. The present invention provides a communication system that allows a plurality of transmitters to each transmit data on a different virtual channel. The virtual channels allow concurrent transmission of data over physical channels. Each receiver processes the received signal by modulating such signal to generate a received data signal for each physical channel. Spread spectrum codes are applied to the received data signals and the resultant signals are cross correlated to generate an estimate of the originally transmitted data. This arrangement of the virtual channels provides a gain equivalent that can be used to offset power differences between transmitters. This allows the power adjustments of the transmitter to be less frequent.

Claims

WHAT IS CLAIMED IS:

1. A method for communicating digital data comprising: code spreading digital data with a number n spread spectrum codes to generate a number n spread data sequences; modulating each of the number n spread data sequences onto selected ones of a number m corresponding carriers to generate a number m modulated carriers, the sum of the number n and the number m being greater than or equal to three; and transmitting simultaneously the modulated carriers.

2. The method of claim 1 further comprising: receiving said transmitted modulated carriers; demodulating said received plurality of modulated carriers; combining each of the demodulated modulated carriers with a corresponding spread spectrum code to form a plurality of received data signals; correlating each of the said plurality of received data signals with each of the others of said plurality of received data signals to form a plurality of correlation signals; and combining said plurality of correlation signals to form an estimate of the digital data.

3. A method for processing digital data comprising: demodulating a plurality of received modulated carriers; combining each of the demodulated received modulated carriers with a corresponding spread spectrum code to form a plurality of received data signals; correlating each of the said plurality of received data signals with each of the others of said plurality of received data signals to form a plurality of correlation signals; and combining said plurality of correlation signals to form an estimate of the digital data.

4. A communication system for communicating digital data comprising: a plurality of code spreading transmit circuits, each code spreading circuit for combining the digital data with a spread spectrum code corresponding to the code spreading transmit circuit to generate a plurality of spread spectrum signals; and a modulator having a plurality of inputs, each input coupled to a corresponding one of the plurality of code spreading transmit circuits for modulating each of the plurality of spread spectrum signals onto a corresponding one of a plurality of carriers to generate a plurality of modulated data signals for simultaneous transmission of said plurality of modulated data signals.

5. The communication system of claim 4 further comprising: a receiver for receiving and demodulating said simultaneously transmitted plurality of data signals to form a plurality of received data signals; a plurality of code spreading receive circuits coupled to said receiver for generating a plurality of recovered data signals in response to the plurality of received data signals and to the plurality of spread spectrum codes; and a correlator coupled to the plurality of code spreading receive circuits to form a plurality of correlation signals, each correlator signal corresponding to the correlation between one of the plurality of received data signals and another one of the plurality of recovered data signals and to generate an estimate of the digital data from said plurality of correlation signals.

6. A transmitter for communicating digital data comprising: a code spreading transmit circuit having a first input for receiving a digital data signal, having a second output for receiving a spread spectrum code, and having an output for providing a spread spectrum signal in response to the digital data and the spread spectrum code; a plurality of modulators, each modulator having an input coupled to the output of the code spreading transmit circuit and having an output for generating a modulated data signal having a carrier in response to the spread spectrum signal; and a summation unit having a plurality of inputs, each input coupled to a corresponding one of the plurality of modulators and having an output for providing a combined signal in response to the modulated data signals for simultaneous transmission of the plurality of modulated data signals.

7. A transmitter for communicating digital data comprising: a plurality of code spreading transmit circuits, each code spreading transmit circuit having a first input for receiving a digital data signal, having a second output for receiving a corresponding one of a plurality of spread spectrum codes, and having an output for providing a spread spectrum signal in response to the digital data and the corresponding spread spectrum code; a first summation unit having a plurality of inputs, each input coupled to a corresponding one of the plurality of code spreading transmit circuits and having an output for providing a combined spread spectrum signal in response to the plurality of spread spectrum signals; a plurality of modulators, each modulator having an input coupled to the output of the code spreading transmit circuit and having an output for generating a modulated data signal having a carrier in response to the combined spread spectrum signal; and a second summation unit having a plurality of inputs, each input coupled to a corresponding one of the plurality of modulators and having an output for providing a combined signal in response to the modulated data signals for simultaneous transmission of the plurality of modulated data signals.

8. A receiver comprising: a demodulator for demodulating a plurality of simultaneously transmitted received data signals to form a demodulated data signal; a plurality of code spreading receive circuits, each receive circuit coupled to said demodulator for generating a recovered data signal in response to the demodulated data signal and to a corresponding one of a plurality of spread spectrum codes; and a correlator coupled to the plurality of code spreading receive circuits to form a plurality of correlation signals, each correlation signal corresponding to the correlation between one of the plurality of recovered data signals and another one of the plurality of recovered data signals and to generate an estimate of the digital data from said plurality of correlation signals.

9. A receiver comprising: a plurality of demodulators, each demodulator for demodulating a received simultaneously transmitted signal to form a demodulated data signal; a correlator coupled to the plurality of demodulators to form a plurality of correlation signals, each correlation signal corresponding to the correlation between one of the plurality of demodulated data signals and another one of the plurality of demodulated data signals; and a code spreading receive circuit coupled to the correlator for generating an estimate of the digital data from said plurality of correlation signals in response to the correlation signal and to a spread spectrum code.

10. A receiver comprising: a plurality of demodulators for demodulating a received simultaneously transmitted signal to form a plurality of demodulated data signals; a first correlator coupled to the plurality of demodulators to form a plurality of first correlation signals, each first correlation signal corresponding to the correlation between one of the plurality of demodulated data signals and another one of the plurality of demodulated data signals and to generate a first estimate of the digital data from said plurality of first correlation signals; and a plurality of code spreading receive circuits coupled to said first correlator, each code spreading receive circuit for generating a spread spectrum signal in response to the plurality of first estimates and to a corresponding one of a plurality of spread spectrum codes; a second correlator coupled to the plurality of receive CDMA circuits to form a plurality of second correlation signals, each second correlation signal corresponding to the correlation between one of the plurality of spread spectrum signals and another one of the plurality of spread spectrum signals and to generate a second estimate of the digital data from said plurality of second correlation signals.

11. A transmitter for communicating digital data comprising: a number n of code spreading transmit circuits, each code spreading transmit circuit having a first input for receiving a digital data signal, having a second output for receiving a corresponding one of a plurality of data codes, and having an output for providing spread data in response to the digital data and the corresponding spread spectrum code; a number m of modulators, each modulator having a first input for receiving a corresponding one of at least one carrier signal, having an input coupled to the output of selected ones of the code spreading transmit circuits and having an output for generating a modulated data signal in response to the spread data; and a summation unit having a plurality of inputs, each input coupled to a corresponding one of the plurality of modulators and having an output for providing a combined signal in response to the modulated data signals for simultaneous transmission of the plurality of modulated data signals; wherein the sum of the number n and the number m is greater than or equal to three.