Method and apparatus for using Walsh shift keying in a spread spectrum communication system

Claims

What I claim is:

1. A method for modulating data in a spread spectrum communication system in which information is communicated by forming data symbols into digital communication signals, comprising the steps of:

generating N orthogonal functions of length n having a predefined recursive relationship among each other, N being a power of 2;

forming M mutually orthogonal modulation symbols having a length Ln using said N orthogonal functions and respective inverses thereof, where M equals the product of L and N; and

mapping data symbols into said preselected modulation symbols by selecting one of said modulation symbols according to binary values for every log M data symbols.

2. The method of claim 1 wherein M is at least 2 and less than or equal to 64.

3. The method of claim 1 wherein said communication signals being modulated are transmitted to communication system subscribers on a forward communication link.

4. The method of claim 1 wherein said orthogonal functions comprise Walsh functions.

5. The method of claim 1 wherein said mapping step comprises the steps of:

selecting a first orthogonal function for transmission when data symbols in said digital communication signals have one binary value; and

selecting a second orthogonal function for transmission when data symbols in said digital communication signals have a second binary value.

6. The method of claim 1 wherein said forming and mapping steps comprise steps of:

generating first and second n-length orthogonal functions;

generating a first 2n-length code sequence using said first orthogonal function twice, when a pair of data symbols in said digital communication signals have a first value;

generating a second 2n-length code sequence using said first orthogonal function and its inverse, when a pair of data symbols have a second value;

generating a third 2n-length code sequence using said second orthogonal function twice, when a pair of data symbols have a third value; and

generating a fourth 2n-length code sequence using said second orthogonal function and its inverse, when a pair of data symbols have a fourth value.

7. The method of claim 1 wherein preselected first, second, third, and fourth n-length orthogonal functions are used to produce modulation symbols, and said forming and mapping steps comprise generating sixteen 4n-length code sequences in response to binary values of sets of four data symbols, said code sequences comprising:

four sequences in which said first, second, third, and fourth functions are repeated four times, respectively, each in response to one of four values of the data symbols; and

three sets of sequences, each in response to one of twelve values of the data symbols, in which said first, second, third, and fourth functions are repeated two times, respectively, and accompanied by two inversions of said repeated sequences, with the relative position of the inversions in each sequence in each of said sets being shifted from inversions in other sequences so as to maintain substantial orthogonality.

8. The method of claim 1 wherein said step of mapping comprises the step of applying said data symbols to a Fast Hadamard Transformer so as to transform data symbols into preselected modulation symbols.

9. The method of claim 1 wherein said step of mapping comprises the step of applying said data symbols to a modulation symbol storage device so as to transform data symbols into preselected modulation symbols.

10. The method of claim 1 wherein modulated communication signals are transferred from a gateway type base station using at least one satellite based repeater to at least one remote subscriber unit within said communication system.

11. The method of claim 1 wherein said communication system comprises a wireless telephone/data communication system in which remote users are located within a plurality of cells and communicate information signals to at least one gateway, using code division multiple access (CDMA) spread spectrum type communication signals.

12. The method of claim 1 further comprising the steps of:

receiving a plurality of data signals to be transmitted to communication system subscribers over separate user channels; and

encoding each data signal to produce coded data symbols for each user channel.

13. Apparatus for modulating communication signals in a spread spectrum communication system in which information is communicated by forming coded data symbols into digital communication signals, comprising:

means for generating N orthogonal functions of length n having a predefined recursive relationship among each other, N being a power of 2;

means for forming M mutually orthogonal modulation symbols of length Ln, using said N orthogonal functions and respective inverses thereof, where M equals the product of L and N; and

means for mapping data symbols into said modulation symbols, connected to receive data symbols and orthogonal modulation symbols, for selecting one of said modulation symbols according to binary values for every log M data symbols.

14. The apparatus of claim 13 wherein:

said means for generating comprises at least one orthogonal function generator which outputs first and second orthogonal functions, respectively; and

said means for forming comprises selection means connected to receive said data symbols and said first and second functions, which responds to binary values for said data symbols by selecting said first orthogonal function as an output when said symbols have one value and selecting said second orthogonal function as an output when data symbols have a second value.

15. The apparatus of claim 14 comprising first and second orthogonal function generators.

16. The apparatus of claim 13 wherein M is at least 2 and less than or equal to 64.

17. The apparatus of claim 13 further comprising means for transmitting said communication signals being modulated to communication system subscribers on a forward link.

18. The apparatus of claim 13 wherein said orthogonal functions comprise Walsh functions.

19. The apparatus of claim 13 wherein said mapping means comprises means for selecting a first orthogonal function for transmission when data symbols in said digital communication signals have one binary value, and for selecting a second orthogonal function for transmission when data symbols in said digital communication signals have a second binary value.

20. The apparatus of claim 13 wherein said forming and mapping means comprise:

at least one orthogonal function generator which outputs first and second n-length orthogonal functions, respectively; and

selection means connected to receive said data symbols and said first and second functions, and respond to binary values for said data symbols by selecting:

a first 2n-length code sequence for output, comprising said first orthogonal function used twice, when a pair of data symbols in said digital communication signals have a first value;

a second 2n-length code sequence for output, comprising said first orthogonal function and its inverse, when a pair of data symbols have a second value;

a third 2n-length code sequence for output, comprising said second orthogonal function used twice, when a pair of data symbols have a third value; and

a fourth 2n-length code sequence for output, comprising said second orthogonal function and its inverse, when a pair of data symbols have a fourth value.

21. The apparatus of claim 20 comprising first and second orthogonal function generators.

22. The apparatus of claim 13 wherein said mapping means comprises a Fast Hadamard Transformer which is configured to transform data symbols into preselected modulation symbols.

23. The apparatus of claim 13 wherein said mapping means comprises a modulation symbol storage device which is configured to receive data symbols and output preselected modulation symbols.

24. The apparatus of claim 13 further comprising means for transferring said modulated communication signals from a gateway type base station using at least one satellite based repeater to at least one remote subscriber unit within said communication system.

25. A method for demodulating communication signals in a spread spectrum communication system in which information is communicated by orthogonally encoded communication signals, comprising the steps of:

receiving spread spectrum communication signals having a common carrier frequency modulated using M mutually orthogonal modulation symbols having a length Ln formed by using a preselected number of n-length orthogonal functions and respective inverses thereof, where M equals the product of L and said preselected number;

inputting said signals into at least two sets of N correlators, and correlating said signals with said preselected number of n-length orthogonal functions, in parallel;

applying correlated output signals to corresponding demodulators for each set of correlators, and demodulating said correlated signals into M energy values in each demodulator representing each of said M mutually orthogonal modulation symbols respectively;

combining the resulting M energy values from each demodulator into a single set of M energy values; and

mapping said single set of energy values into energy metric data using a dual maximum metric generation process.

26. The method of claim 25 wherein M is at least 2 and less than or equal to 64.

27. The method of claim 25 wherein said communication signals being demodulated are received by communication system subscribers on a forward communication link.

28. The method of claim 25 wherein said orthogonal functions comprise Walsh functions.

29. The method of claim 25 wherein said preselected number of orthogonal functions is at least 2 and less than or equal to 4.

30. The method of claim 25 wherein modulated communication signals are transferred from a gateway type base station using at least one satellite based repeater to at least one remote subscriber unit within said communication system.

31. The method of claim 25 wherein said communication system comprises a wireless telephone/data communication system in which remote users are located within a plurality of cells and communicate information signals to at least one gateway, using code division multiple access (CDMA) spread spectrum type communication signals.

32. The method of claim 25 further comprising the steps of:

inputting said signals to at least one coherent demodulator, and demodulating said correlated signals into at least one amplitude value;

combining any resulting amplitude values from each coherent demodulator into a single amplitude value; and

combining said single amplitude value and an output of said dual maximum metric generation process into composite metric values for data symbols.

33. Apparatus for demodulating communication signals in a spread spectrum communication system in which information is communicated by orthogonally encoded communication signals, comprising:

means for receiving spread spectrum communication signals having a common carrier frequency modulated using M mutually orthogonal modulation symbols having a length Ln using a preselected number N of n-length orthogonal functions and respective inverses thereof, where M is the product of L and said preselected number;

at least two sets of N correlators connected to receive said spread spectrum signals and correlate said signals with said preselected number of n-length orthogonal functions, in parallel;

a plurality of demodulators each connected to receive outputs of one corresponding set of correlators so as to demodulate said correlated signals into M energy output values in each demodulator representing each of said M mutually orthogonal modulation symbols respectively;

means for combining the resulting M energy values from each demodulator into a single set of M energy values; and

means for mapping said energy values into energy metric values using a dual maximum metric generation process.

34. The apparatus of claim 33 further comprising:

at least one coherent demodulator connected to receive said spread spectrum signals and demodulate said signals into at least one amplitude value;

an amplitude combiner connected to receive an output of said coherent demodulator and combine resulting amplitude values from each coherent demodulator into a single amplitude value; and

an energy combiner connected to receive said single amplitude value and an output of said dual maximum metric generation process and combine them into composite metric values for data symbols.

35. The apparatus of claim 34 comprising at least two coherent demodulators.

36. The apparatus of claim 33 wherein said preselected number of functions is 64 or less.

37. The apparatus of claim 33 wherein M is at least 2 and less than or equal to 64.

38. The apparatus of claim 33 wherein said orthogonal functions comprise Walsh functions.

39. A spread spectrum communication system, comprising:

a plurality of gateway type base stations each including at least one communication signal transmitter that transmits signals comprising data symbols to active system users, comprising:

a plurality of function generating means each for providing at least one of a plurality of orthogonal functions of a plurality of orthogonal functions of length n having a predefined recursive relationship among each other;

means for selecting N of said orthogonal functions for each active system user, N being a power of 2;

means for forming M mutually orthogonal modulation symbols of length Ln, for each active system user using said N selected orthogonal functions and respective inverses thereof, where M is the product of L and N;

means for mapping data symbols into said modulation symbols for each active system user, connected to receive data symbols and orthogonal modulation symbols for each active system user, and for selecting one of said modulation symbols according to binary values for every log M data symbols;

a plurality of spreading means each connected to said means for mapping for receiving modulation symbols for respective users and for producing a spread spectrum data signal; and

combination means for combining modulation symbols for substantially all active users receiving signals over a common carrier frequency into a communication signal;

a plurality of mobile communication units, each including a mobile receiver, comprising:

means for selecting and receiving a spread spectrum communication signal from at least one gateway; and

demodulation means connected to the means for selecting and receiving, for providing modulation symbols for respective users by demodulating the received spread spectrum communication signal.

40. The system of claim 39, wherein said mobile receivers further comprise:

at least two sets of N correlators connected to receive said spread spectrum communication signals and correlate said signals with said preselected number of n-length orthogonal functions, in parallel;

a plurality of demodulators each connected to receive outputs of one corresponding set of correlators so as to demodulate said correlated signals into M energy output values in each demodulator representing each of said M mutually orthogonal modulation symbols respectively;

means for combining the resulting M energy values from each demodulator into a single set of M energy values; and

means for mapping said energy values into energy metric values using a dual maximum metric generation process.

41. A method of generating a spread spectrum communication signal, comprising the steps of:

generating a plurality of orthogonal functions of length n, each being generated according to a respective function of a plurality of orthogonal functions;

receiving a plurality of system subscriber data signals comprising data symbols to be transmitted to active system subscribers over separate user channels;

forming M mutually orthogonal modulation symbols for each channel having a length Ln using N of said plurality of orthogonal functions and respective inverses thereof, where M equals the product of L and N;

mapping data symbols for each channel into said preselected modulation symbols for that channel by selecting one of said modulation symbols according to binary values for every log M data symbols; and

combining streams of said modulation symbols for all channels after said mapping step into a serial data stream spread spectrum data signal.

42. The method of claim 41 wherein said communication system comprises a wireless telephone/data communication system in which remote users are located within a plurality of cells and communicate information signals to at least one gateway, using code division multiple access (CDMA) spread spectrum type communication signals.

43. The method of claim 41 wherein M is at least 2 and less than or equal to 64.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and satellite repeater type spread spectrum communication systems. More particularly, the invention relates to a method and apparatus for using multiple orthogonal codes to generate spread spectrum communication signals. The invention further relates to a method of using shift keying of multiple Walsh function code sequences for signal modulation in code division spread spectrum type communication systems to provide system users with improved energy metrics for non-coherent signal demodulation.

II. Description of the Related Art

A variety of multiple access communication systems have been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ASCII), the basics of which are well known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over the other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in a multiple access communication system is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," is assigned to the assignee of the present invention, and is incorporated herein by reference.

The U.S. Pat. No. 4,901,307 patent discloses a multiple access communication system technique in which a large number of generally mobile or remote system users each employ a transceiver to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers communicate through satellite repeaters and gateways or terrestrial base stations (also sometimes referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals. Such systems allow the transfer of various types of data and voice communication signals between system users, and others connected to the communication system.

Communication systems using spread spectrum type signals and modulation techniques, such as disclosed in U.S. Pat. No. 4,901,307, provide increased system user capacity over other techniques because of the manner in which the full frequency spectrum is used concurrently among system users within a region, and `reused` many times across different regions serviced by the system. The use of CDMA results in a higher efficiency in utilizing a given frequency spectrum than achieved using other multiple access techniques. In addition, the use of wide band CDMA techniques permits such problems as multipath fading to be more readily overcome, especially for terrestrial repeaters.

Pseudonoise (PN) modulation techniques used in wide band CDMA signal processing provide a relatively high signal gain which allows spectrally similar communication channels or signals to be more quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished, provided any path length difference causes relative propagation delays in excess of the PN chip duration, that is, the inverse of the bandwidth. If a PN chip rate of say approximately 1 MHz is used, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to system data rate, can be employed to discriminate between signal paths differing by more than one microsecond in path delay or time of arrival. This differential corresponds to a path length differential of approximately 1,000 feet. A typical urban environment provides differential path delays in excess of one microsecond, and some areas upwards of 10-20 microseconds in delay.

The ability to discriminate between multipath signals greatly reduces the severity of multipath fading, although it does not typically totally eliminate it because of occasional paths with delay differentials of less than a PN chip period. The existence of low delay paths is more especially true for satellite repeaters or directed communication links where multipath reflections from buildings and other terrestrial surfaces is greatly reduced. Therefore, it is desirable to provide some form of signal diversity as one approach to reducing the deleterious effects of fading and additional problems associated with relative user, or repeater, movement.

Generally, three types of diversity are produced or used in spread spectrum type communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using data repetition, time interleaving of data or signal components, and error coding. A form of frequency diversity is inherently provided by CDMA in which the signal energy is spread over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.

Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a mobile user through two or more base stations, for terrestrial-based repeater systems; or two or more satellite beams or individual satellites, for space-based repeater systems. That is, in the satellite communication environment or for indoor wireless communication systems, path diversity may be obtained by deliberately transmitting or receiving using multiple antennas. Furthermore, path diversity may be obtained by exploiting a natural multipath environment by allowing a signal arriving over different paths, each with a different propagation delay, to be received and processed separately for each path.

If two or more signal reception paths are available with sufficient delay differential, say greater than one microsecond, two or more receivers may be employed to separately receive these signals. Since these signals typically exhibit independent fading and other propagation characteristics, the signals can be separately processed by the receivers and the outputs combined with a diversity combiner to provide the final output information or data, and overcome problems otherwise existent in a single path. Therefore, a loss in performance only occurs when the signals arriving at both receivers experience fading or interference in the same manner and at the same time. In order to exploit the existence of multipath signals, it is necessary to utilize a waveform that permits path diversity combining operations to be performed.

Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Mar. 31, 1992, and U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Apr. 28, 1992, both assigned to the assignee of the present invention, and incorporated herein by reference.

The CDMA techniques disclosed in U.S. Pat. No. 4,901,307 contemplate the use of coherent modulation and demodulation for both communication directions or links in user-satellite communications. In communication systems using this approach, a pilot carrier signal is used as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. The phase information obtained from tracking the pilot signal carrier is then used as a carrier phase reference for coherent demodulation of other system or user information signals. This technique allows many user signal carriers to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism. In satellite repeater systems, the return link generally does not require a pilot signal for phase reference for gateway receivers. In a terrestrial wireless or cellular environment, the severity of multipath fading and resulting phase disruption of the communication channel, generally precludes use of coherent demodulation techniques for the user-to-base station link, where a pilot signal is not typically used. However, the present invention allows the use of both noncoherent modulation and demodulation techniques as desired.

While terrestrial based repeaters and base stations have been predominantly employed, future systems will place more heavy emphasis on the use of satellite based repeaters for broader geographic coverage to reach a larger number of `remote` users and to achieve truly `global` communication service. Unfortunately, in the satellite environment, several factors sometimes have a negative impact on the usefulness of traditional signal diversity and frequency and phase tracking techniques.