System and method for generating signal waveforms in a CDMA cellular telephone system

Claims

We claim:

1. In a direct sequence spread spectrum modulator, a signal orthogonalizer comprising:

pilot channel signal generator means for generating a pilot signal according to a first orthogonal function; and

communication channel signal generator means for receiving an input information signal, generating an orthogonal function signal according to a second orthogonal function, combining said orthogonal function signal with said input information signal to provide a resultant communication channel signal.

2. The signal orthogonalizer of claim 1 wherein said communication channel signal generator means is further for receiving at least one additional input information signal, generating for each additional input information signal an additional orthogonal function signal each according to a respective additional orthogonal function, combining each additional orthogonal function signal with a respective one of said additional input information signals to provide corresponding resultant additional communication channel signals.

3. The signal orthogonalizer of claim 2 wherein said first, second and each additional orthogonal function are Walsh functions.

4. In the modulator of claim 2 wherein each of said input information signal and additional input information signals are error correction encoded.

5. The signal orthogonalizer of claim 1 wherein said first and second orthogonal functions are Walsh functions.

6. A communication system modulation system comprising:

pilot channel signal generator means for generating as a pilot signal a first orthogonal function signal according to a first orthogonal function selected from a set of Walsh functions;

communication channel signal generator means for receiving an input information signal, generating a second orthogonal function signal according to a second orthogonal function selected from said set of Walsh functions wherein said second orthogonal function is different from said first orthogonal function, combining said second orthogonal function signal with said input information signal and providing a resultant communication signal; and

spreading means for receiving said pilot signal and said communication signal, generating a pseudorandom noise (PN) signal of a predetermined PN code, combining said PN signal with each of said pilot signal and said communication signal to produce corresponding PN spread pilot and communication signals.

7. The modulation system of claim 6 wherein:

said communication channel signal generator means is further for receiving at least one additional input information signal, generating for each additional input information signal an additional orthogonal function signal each according to an additional orthogonal function selected from said set of Walsh functions wherein each additional orthogonal function is different from said first and second orthogonal functions and each other additional orthogonal function, combining each additional orthogonal function signal with a respective one of said additional input information signals to provide corresponding resultant additional communication signals; and

said spreading means is further for receiving each additional communication signal, combining said PN signal with each additional communication signal to produce additional communication signals.

8. The modulation system of claim 7 further comprising modulation means for modulating said pilot, communication and additional communication signals upon a carrier signal for transmission.

9. A communication system modulation system comprising:

a pseudorandom noise (PN) generator having an output wherein said PN generator generates and provides at said PN generator output a PN signal of a predetermined PN code;

a pilot channel signal generator having an output wherein said pilot signal generator provides at said pilot channel signal generator output a pilot signal according to a first Walsh function from a set of Walsh functions;

at least one system channel signal generator each having an input and an output, wherein each system channel signal generator receives at said respective system channel signal generator input a respective system information signal and provides at said respective system channel signal generator output a respective system channel signal representative of each respective system information signal orthogonalized according to a respectively different Walsh function from a first subset of Walsh functions of said set of Walsh functions exclusive of said first Walsh function;

at least one user channel signal generator each having an input and an output, wherein each user channel signal generator receives at said respective user channel signal generator input a respective user information signal and provides at said respective user channel signal generator output a user channel signal representative of each respective user information signal orthogonalized according to a respectively different Walsh function selected from a second subset of Walsh functions of said set of Walsh functions exclusive of said first Walsh function and said first subset of Walsh functions; and

a combining circuit having a plurality of inputs and an output, each combining circuit input coupled to a respective one of said PN generator output, said pilot channel signal generator output, each of said system channel signal generator outputs and each of said user channel signal generator outputs, wherein said combining circuit combines said PN signal with said pilot channel signal, each of said system channel signals and each of said user channel signals and provides corresponding PN spread signals at said combining circuit output.

10. The modulation system of claim 9 wherein each user channel signal generator comprises:

an encoder circuit having an input and an output, each encoder circuit input receiving said respective user information signal;

a Walsh function generator having an output; and

a modulo-2 adder having a pair of inputs respectively coupled to said encoder circuit output and said Walsh function generator output, and an output coupled to a respective combining circuit input.

11. The modulation system of claim 9 wherein each system channel signal generator comprises:

an encoder circuit having an input and an output, each encoder circuit input receiving said respective system information signal;

a Walsh function generator having an output; and

a modulo-2 adder having a pair of inputs respectively coupled to said encoder circuit output and said Walsh function generator output, and an output coupled to a respective combining circuit input.

12. In a direct sequence spread spectrum communications modulator in which a plurality of input signals to be transmitted are bandwidth spread according to a predetermined pseudorandom noise spreading code, a method for orthogonalizing said input signals comprising the steps of:

generating a plurality of orthogonal function signals; and

modulating each of said channel signals with a different one of said orthogonal function signals.

13. The method of claim 12 further comprising the step of providing a selected one of said orthogonal function signals as a pilot channel signal for bandwidth spreading according to said predetermined pseudorandom noise spreading code.

14. The method of claim 13 wherein said step of modulating each of said input signals comprises the steps of:

receiving an input information signal

combining each input information signal with a respective one of said orthogonal function signals

to provide a respective orthogonalized information signal for bandwidth spreading according to said predetermined pseudorandom noise spreading code.

15. The method of claim 13 wherein said orthogonal functions are Walsh functions.

16. The method of claim 12 wherein said step of modulating each of said input signals comprise the steps of:

receiving an input information signal

combining each input information signal with a respective one of said orthogonal function signals

to provide a respective orthogonalized information signal for bandwidth spreading according to said predetermined pseudorandom noise spreading code.

17. The method of claim 12 wherein said orthogonal functions are Walsh functions.

18. A modulation method for direct sequence spread spectrum communications comprising the steps of:

generating a plurality of orthogonal function signals; and

receiving at least one input information signal;

modulo-2 adding each of said information signals with a different one of said orthogonal function signals to provide orthogonalized information signals;

generating a predetermined pseudorandom noise spreading code; and

modulo-2 adding each of said orthogonalized information signals with said pseudorandom noise spreading code to provide bandwidth spread orthogonalized information signals.

19. The method of claim 18 further comprising the step of modulo-2 adding a selected one of said orthogonal function signals with said pseudorandom noise spreading code to provide a pilot signal.

20. The method of claim 19 further comprising the step of summing said bandwidth spread orthogonalized information signals and said pilot signal.

21. The method of claim 18 further comprising the step of summing said bandwidth spread orthogonalized information signals.

22. In a communication system in which a plurality of remote user stations communicate, via a radio link with a base station, with other user stations, said base station having a base station transceiver for communicating user station information signals to intended recipient remote user stations and for receiving remote user station communicated information signals for transfer to intended recipient user stations, said base station transceiver comprising:

base station transmission means for receiving at least one user station information signal, orthogonalizing each received user station information signal according to a respective predetermined orthogonal function, bandwidth spreading and transmitting said orthogonalized user station information signals as a base station communication signal; and

base station reception means for receiving and extracting from each remote user station transmitted remote user station communication signal a corresponding remote user station information signal for transfer to intended recipient user stations.

23. In the communication system of claim 22 each remote user station has a remote user station transceiver for communicating remote user station information signals to said base station for transfer to an intended recipient user station and for receiving and extracting from said base station communication signal respective user station information signals intended for said recipient remote user station, said remote user station transceiver comprising:

remote user station transmission means for receiving a remote user station information signal, converting said remote user station information signal into respective groups of orthogonal function signals, spread spectrum modulating and transmitting said orthogonal function signals as a remote user station communication signal; and

remote user station reception means for receiving and extracting from said base station communication signal said user station information signal intended for said remote user station.

24. The communication system of claim 23 wherein said base station reception means comprises first demodulation means for despreading each received remote user station communication signal, transform processing each despread remote user station communication signal, and providing corresponding outputs of remote user station information signals.

25. The communication system of claim 24 wherein said remote user station reception means comprises second demodulation means for despreading said base station communication signal, deorthogonalizing said despread base station communication signal according a predetermined one of said orthogonal functions, and providing a corresponding output of said user station information signal intended for said remote user station.

26. The communication system of claim 23 wherein said remote user station reception means comprises demodulation means for despreading said base station communication signal, deorthogonalizing said despread base station communication signal according a predetermined one of said orthogonal functions, and providing a corresponding output of said user station information signal intended for said remote user station.

27. A communication system in which a plurality of remote user stations communicate, via a radio link with a base station, with other user stations, said base station having a base station transceiver for communicating user station information signals to intended recipient remote user stations and for receiving remote user station communicated information signals destined for intended recipient user stations, said base station transceiver comprising:

base station transmitter means for generating a plurality of orthogonal function signals wherein one of said orthogonal functions signals is a pilot signal, receiving at least one user station information signal each intended for a recipient remote user station, combining each received user station information signal with a respective another one of said plurality of orthogonal function signals to produce corresponding resultant base station intermediate signals, generating a base station first pseudorandom noise (PN) signal of a first predetermined PN code, combining said base station first PN signal with each of said pilot and base station intermediate signals to produce corresponding base station PN spread pilot and base station intermediate signals, modulating said base station PN spread pilot and base station intermediate signals upon a base station carrier signal and transmitting said modulated base station carrier signal as a base station communication signal; and

base station receiver means for receiving and extracting from each remote user station transmitted remote user station communication signal a corresponding remote user station information signal.

28. The communication system of claim 27 further comprising at least one remote user station each having a remote user station transceiver for communicating to said base station remote user station information signals destined for an intended recipient user station and for receiving and extracting from said base station communication signal a respective user station information signal intended for each recipient remote user stations, said remote user station transceiver comprising:

remote user station transmitter means for receiving a remote user station information signal, converting sequential portions of said remote user station information signal into respective orthogonal function signal portions according to a value of said respective remote user station information signal portion to produce a remote user station intermediate signal, generating a remote user station first pseudorandom noise (PN) signal of a predetermined remote user station PN code, combining said orthogonal function signal portions with said remote user station first PN signal to produce a PN spread remote user station intermediate signal, modulating said remote user station PN spread remote user station intermediate signal upon a remote user station carrier signal and transmitting said modulated remote user station carrier signal as a remote user station communication signal; and

remote user station receiver means for, receiving and demodulating said base station communication signal, generating a receiver orthogonal function signal identical to a predetermined one of said orthogonal function signals, generating a remote user station second pseudorandom noise (PN) signal of said first predetermined PN code, combining said receiver orthogonal function signal with said receiver second PN signal so as to provide a correlation signal, correlating said demodulated base station communication signal with said correlation signal to produce said user station information signal intended for said remote user station.

29. The communication system of claim 28 wherein said remote user station receiver means is further for extracting timing information from said base station PN spread pilot signal in said base station communication signal, said timing information for use in generating said remote user station first PN signal.

30. The communication system of claim 28 wherein said base station receiver means is further for demodulating received remote user station communication signals, generating for each received remote user station communication signal a base station second pseudorandom noise (PN) signal of a corresponding predetermined remote user station PN code, correlating each demodulated received remote user station communication signal with a corresponding base station second pseudorandom noise (PN) signal, transform processing each correlated remote user station communication signal, and providing corresponding outputs of said remote user station information signals.

31. The communication system of claim 30 further comprising controller means coupled to said base station for receiving user station information signals from user stations of a first user station network destined for remote user stations of a second user station network, transferring said user station information signals to said base station, receiving from said base station said remote user station information signals, and transferring said remote user station information signals to said intended recipient user stations of said first user station network.

32. The communication system of claim 31 further comprising at least one additional base station each having a base station transceiver for receiving selected ones of said user station information signals from said controller means, communicating said received user station information signals to intended recipient remote user stations, receiving remote user station information signals from said remote user stations, and transferring said received remote user station information signals to said controller means.

33. The communication system of claim 32 wherein said controller means is further coupled to each of said additional base stations for receiving user station information signals from said user stations of said first user station network destined for remote user stations of said second user station network, transferring said user station information signals to at least one of said base station and said additional base stations, receiving from at least one of said base station and said additional base stations said remote user station information signals, transferring said remote user station information signals destined for user stations of said first user station network to said intended recipient user stations of said first user station network.

34. A communication system in which a plurality of remote user stations communicate with other user stations, via a radio link with a base station, said communication system comprising:

a base station having base station transmission means for communicating user station information signals to intended recipient remote user stations, said base station transmission means comprising:

(a) pilot channel signal generator means for generating as a pilot signal an orthogonal function signal according to a first Walsh function selected from a set of Walsh functions;

(b) at least one communication channel signal generator means each for receiving a respective user station information signal each intended for a different one of said remote user stations, generating an additional Walsh function signal according to an additional Walsh function selected from said set of Walsh functions wherein each additional Walsh function is different from said first Walsh function, combining each additional Walsh function signal with said respective user station information signal to produce a respective communication channel signal;

(c) base station spreading means for receiving said pilot signal and each communication channel signal, generating a base station pseudorandom noise (PN) signal of a first predetermined PN code, combining said PN signal with each of said pilot channel and communication channel signals to produce corresponding base station PN spread pilot and communication signals; and

(d) base station transmission means for receiving and modulating said base station PN spread pilot and communication channel signals upon a base station carrier signal and transmitting said modulated base station carrier signal as a base station communication signal;

said base station having base station reception means for receiving and extracting from each remote user station transmitted remote user station communication signal a corresponding remote user station information signal destined for intended recipient user stations;

at least one remote user station each having remote user station transmission means for communicating remote user station information signals to said base station, said remote user station transmission means comprising:

(a) orthogonal function encoder means for, receiving an input remote user station information signal, converting sequential portions of said input signal into respective Walsh function signal portions wherein each Walsh function signal portion is according to a Walsh function selected from a plurality of Walsh functions according to a value of said respective portions of said input remote user station information signal, and providing an output of said Walsh functions functions function signal portions;

(b) remote user station spreading means for, receiving each of said Walsh function signal portions, generating a remote user station pseudorandom noise (PN) signal of a second predetermined PN code, combining said Walsh function signal portions with said remote user station PN signal so as to produce a remote user station PN spread signal; and

(c) remote user station transmission means for receiving and modulating said remote user station PN spread signal upon a remote user station carrier signal and transmitting said modulated remote user station carrier signal as a remote user station communication signal; and

said at least one remote user station each having remote user station reception means for receiving and extracting from said base station communication signal said user station information signal intended for each respective remote user station.

35. The communication system of claim 34 wherein said base station reception means comprises:

base station receiver means for removing said remote user station carrier signal from each received remote user station communication signal to provide respective remote user station PN spread signals; and

base station demodulation means for despreading each remote user station PN spread signal, transform processing each despread remote user station PN spread signals, and providing corresponding outputs of remote user station information signals.

36. The communication system of claim 35 wherein each remote user station reception means comprises:

remote user station receiver means for removing said base station carrier signal from said received base station communication signal to provide said base station PN spread pilot and communication channel signals; and

remote user station demodulation means for despreading said base station PN spread pilot and communication channel signals, deorthogonalizing said despread pilot and communication channel signals respectively according to said first and a predetermined one of said Walsh functions, and providing a corresponding output of said user station information signal intended for said remote user station.

37. The communication system of claim 34 wherein each remote user station reception means comprises:

remote user station receiver means for removing said base station carrier signal from said received base station communication signal to provide said base station PN spread pilot and communication channel signals; and

remote user station demodulation means for despreading said base station PN spread pilot and communication channel signals, deorthogonalizing said despread pilot and communication channel signals respectively according to said first and a predetermined one of said Walsh functions, and providing a corresponding output of said user station information signal intended for said remote user station.

38. The communication system of claim 34 further comprising controller means coupled to said base station for receiving said user station information signals from user stations of a first user station network intended for remote user stations of a second user station network, coupling said user station information signals to said base station, receiving from said base station said remote user station information signals, and transferring said remote user station information signals to said intended recipient user stations of said first user station network.

39. The communication system of claim 38 wherein said controller means is further for receiving from said base station as one of said user station information signals a first remote user station information signal of one remote user station of said second user station network intended for another remote user station of said second user station network, coupling said one user station information signal to said base station for communication to said another remote user station, receiving from said base station as another one of said user station information signals a second remote user station information signal from said another remote user station intended for said one remote user station, and coupling said another user station information signal to said base station for communication to said one remote user station.

40. A communication system modulator comprising:

means for generating a plurality of orthogonal function signals; and

means for combining a plurality of input information signals with a different one of said orthogonal function signals to provide a plurality of orthogonalized information signals;

means for generating a common pseudorandom noise (PN) signal;

means for combining each orthogonalized information signals with said common PN signal to provide respective bandwidth spread orthogonalized information signals.

41. The modulator of claim 40 further comprising:

means for generating a plurality of unique pseudorandom noise (PN) signals; and

wherein said means for combining each orthogonal function signal with a respective input information signal is further for combining a respective unique PN signal with a respective one of said orthogonalized information signals to scramble each of said orthogonalized information signals.

42. The modulator of claim 41 wherein said means for combining each orthogonalized information signals with said common PN signal is for combining another orthogonal function signal of said plurality of orthogonal function signals with said common PN signal to provide a bandwidth spread pilot signal.

43. The modulator of claim 40 wherein said orthogonal functions are Walsh functions.

44. The modulator of claim 40 wherein said means for combining each orthogonalized information signals with said common PN signal is for combining another orthogonal function signal of said plurality of orthogonal function signals with said common PN signal to provide a bandwidth spread pilot signal.

45. The modulator of claim 40 further comprising error correction means for error correction encoding each of said input information signals.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to cellular telephone systems. More specifically, the present invention relates to a novel and improved system and method for communicating information, in a mobile cellular telephone system or satellite mobile telephone system, using spread spectrum communication signals.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. However the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference.

In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also referred to as cell-sites stations, cell-sites or for short, cells) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.

The satellite channel typically experiences fading that is characterized as Rician. Accordingly the received signal consists of a direct component summed with a multiple reflected component having Rayleigh fading statistics. The power ratio between the direct and reflected component is typically on the order of 6-10 dB, depending upon the characteristics of the mobile unit antenna and the environment about the mobile unit.

Contrasting with the satellite channel, the terrestrial channel experiences signal fading that typically consists of the Rayleigh faded component without a direct component. Thus, the terrestrial channel presents a more severe fading environment than the satellite channel in which Rician fading is the dominant fading characteristic.

The Rayleigh fading characteristic in the terrestrial channel signal is caused by the signal being reflected from many different features of the physical environment. As a result, a signal arrives at a mobile unit receiver from many directions with different transmission delays. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile telephone systems, significant phase differences in signals traveling on different paths may occur. The possibility for destructive summation of the signals may result, with on occasion deep fades occurring.

Terrestrial channel fading is a very strong function of the physical position of the mobile unit. A small change in position of the mobile unit changes the physical delays of all the signal propagation paths, which further results in a different phase for each path. Thus, the motion of the mobile unit through the environment can result in a quite rapid fading process. For example, in the 850 MHz cellular radio frequency band, this fading can typically be as fast as one fade per second per mile per hour of vehicle speed. Fading this severe can be extremely disruptive to signals in the terrestrial channel resulting in poor communication quality. Additional transmitter power can be used to overcome the problem of fading. However, such power increases effect both the user, in excessive power consumption, and the system by increased interference.

The CDMA modulation techniques disclosed in U.S. Pat. No. 4,901,307 offer many advantages over narrow band modulation techniques used in communication systems employing satellite or terrestrial repeaters. The terrestrial channel poses special problems to any communication system particularly with respect to multipath signals. The use of CDMA techniques permit the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of multipath, e.g. fading, while also exploiting the advantages thereof.

In a CDMA cellular telephone system, the same frequency band can be used for communication in all cells. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore the high speed pseudonoise (PN) modulation allows many different propagation paths to be separated, provided the difference in path delays exceed the PN chip duration, i.e. 1/bandwidth. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to system data rate, can be employed against paths that differ by more than one microsecond in path delay from the desired path. A one microsecond path delay differential corresponds to differential path distance of approximately 1,000 feet. The urban environment typically provides differential path delays in excess of one microsecond, and up to 10-20 microseconds are reported in some areas.

In narrow band modulation systems such as the analog FM modulation employed by conventional telephone systems, the existence of multiple paths results in severe multipath fading. With wide band CDMA modulation, however, the different paths may be discriminated against in the demodulation process. This discrimination greatly reduces the severity of multipath fading. Multipath fading is not totally eliminated in using CDMA discrimination techniques because there will occasionally exist paths with delayed differentials of less than the PN chip duration for the particular system. Signals having path delays on this order cannot be discriminated against in the demodulator, resulting in some degree of fading.

It is therefore desirable that some form of diversity be provided which would permit a system to reduce fading. Diversity is one approach for mitigating the deleterious effects of fading. Three major types of diversity exist: time diversity, frequency diversity and space diversity.

Time diversity can best be obtained by the use of repetition, time interleaving, and error detection and coding which is a form of repetition. The present invention employes each of these techniques as a form of time diversity.

CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy 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 from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in copending U.S. patent application entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", Ser. No. 07/433,030, filed Nov. 7, 1989, now U.S. Pat. No. 5,101,501 issued Mar. 31, 1992 and copending U.S. Pat. application entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", Ser. No. 07/432,552, also filed Nov. 7, 1989, now U.S. Pat. No. 5,109,390 issued Apr. 28, 1992, both assigned to the assignee of the present invention.

The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in copending U.S. patent application entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031, filed Nov. 7, 1989, now U.S. Pat. No. 5,056,109 issued Oct. 8, 1991, also assigned to the assignee of the present invention.

The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplated the use of coherent modulation and demodulation for both directions of the link in mobile-satellite communications. Accordingly, disclosed therein is the use of a pilot carrier signal as a coherent phase reference for the satellite-to-mobile link and the cell-to-mobile link. In the terrestrial cellular environment, however, the severity of multipath fading, with the resulting phase disruption of the channel, precludes usage of coherent demodulation technique for the mobile-to-cell link. The present invention provides a means for overcoming the adverse effects of multipath in the mobile-to-cell link by using noncoherent modulation and demodulation techniques.

The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 further contemplated the use of relatively long PN sequences with each user channel being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence for all time shifts other than zero both have a zero average value which allows the different user signals to be discriminated upon reception.

However, such PN signals are not orthogonal. Although the cross-correlations average to zero, for a short time interval such as an information bit time the cross-correlation follows a binomial distribution. As such, the signals interfere with each other much the same as if they were wide bandwidth Gaussian noise at the same power spectral density. Thus the other user signals, or mutual interference noise, ultimately limits the achievable capacity.

The existence of multipath can provide path diversity to a wideband PN CDMA system. If two or more paths are available with greater than one microsecond differential path delay, two or more PN receivers can be employed to separately receive these signals. Since these signals will typically exhibit independence in multipath fading, i.e., they usually do not fade together, the outputs of the two receivers can be diversity combined. Therefore a loss in performance only occurs when both receivers experience fades at the same time. Hence, one aspect of the present invention is the provision of two or more PN receivers in combination with a diversity combiner. In order to exploit the existence of multipath signals, to overcome fading, it is necessary to utilize a waveform that permits path diversity combining operations to be performed.

It is therefore an object of the present invention to provide for the generation of PN sequences which are orthogonal so as to reduce mutual interference, thereby permitting greater user capacity, and support path diversity thereby overcoming fading.

SUMMARY OF THE INVENTION

The implementation of spread spectrum communication techniques, particularly CDMA techniques, in the mobile cellular telephone environment therefore provides features which vastly enhance system reliability and