FLUX DENSITY REDUCTION IN OCDMA SATELLITE COMMUNICATION SYSTEM

Abstract: A method for reducing the power of spectra density in each beam and removing Pseudo-Noise (PN), P2, from the incoming signal without affecting beam-to-beam frequency isolation in a spread spectrum OCDMA (Orthogonal Code Division Multiple Access) satellite communication system in which one or more base stations communicate with a plurality of subscriber terminals located in different beams of a multi-beam satellite relay system. A base station transmits a signal for each beam which is comprised of a set of orthogonal spaced subcarriers, each of which is modulated by a set of orthogonal functions which are overlaid with a PN sequence forming a coded spreading sequence for an information signal. Each of orthogonal function of the set carries data for a single user in the beam.

Description: FLUX DENSITY REDUCTION IN OCDMA SATELLITE COMMUNICATION SYSTEM REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of our copending parent application Serial No. 08/989,466 filed December 12, 1997. BACKGROUND AND BRIEF DESCRIPTION OF PRIOR ART A number of consortiums have been formed to develop satellite based Personal Communications Systems (PCS) with global coverage. These systems include Globalstar (Globalstar System Application before the FCC by Loral Cellular Systems, Corp., June 3, 1991), Odyssey (Application of TRW Inc. before the FCC to Construct a New Communications Satellite System "Odyssey," May 31, 1991), Ellipso (filed November, 1990), and ECCO (filed by Constellation Communications Inc. in June, 1991 ), among others. All of these systems employ multibeam antennas which provide isolation between geographical areas, thus allowing spectrum reuse. The intent of these systems is that a subscriber can place telephone calls directly through the satellite network from almost anywhere on the Earth, using a portable handset much like the present cellular telephones. These systems also include mobile and fixed user terminals. All of the systems mentioned are required to use spread spectrum CDMA techniques for bandsharing purposes. In 1990, Gilhousen et al published the paper "Increased Capacity Using CDMA for Mobile Satellite Communication" (IEEE Journal on Selected Areas in Communications, May 1990) which showed that system capacity is increased in a multibeam satelhte system by using CDMA with a frequency reuse factor of one (i.e. the total frequency band is reused in each beam) compared to orthogonal systems such as FDMA or TDMA, which would typically employ a frequency reuse of 1 :3 or 1 : 5 , depending on the amount of isolation between beams. The use of "marginal isolation" to provide full frequency reuse in quasi-orthogonal (nonsynchronous) CDMA system is suggested in Patent No.4,901 ,307 by Gilhousen, Jacobs and Weaver (February 13, 1990). The Globalstar application discloses a signal which is essentially the same as the IS-95 terrestrial cellular standard. This system uses Orthogonal CDMA (OCDMA) on the forward (base-to-mobile) links and nonsynchronous CDMA on the return links. However, no distinction is made between the OCDMA forward link and the nonorthogonal CDMA return link in frequency reuse. Full frequency reuse is employed in each beam on both forward and return links, and this is assumed to be the best choice. The basic signal format for OCDMA is disclosed by M. J.E. Golay in IDA Report 108, page 110 (1965). The system described is orthogonal in both the forward and return link directions. OCDMA forward and return links are being considered for satellite PCS systems. Related patents which disclose improvements include U.S. Patent No. 5,375,140 to Natali, and titled "Doubly Orthogonal Code and Frequency Division Multiple Access Communication System," and U.S. Patent No. 5,668,795 to Magill et al and titled "Modulation System for Spread Spectrum CDMA Communication, incorporated herein by reference. The capacity of a synchronous OCDMA system can be increased, under certain conditions, by employing a frequency reuse factor other than one and taking advantage of the orthogonal properties of the multiple access codes within a beam as disclosed in our patent application Serial No.08/989,466 filed December 12, 1997 and entitled "Increased Capacity in an OCDMA System by Frequency Isolation". For example, the total capacity of the OCDMA system may be increased by employing 1 :3 frequency reuse rather than full frequency reuse in every beam. In this case, frequency isolation is more important than additional spectrum to maximize capacity, even though full frequency reuse is possible. Typically, for 1 :3 frequency reuse, the available spectrum would be split into 3 subbands, which are assigned to the beams in a 3-frequency reuse pattern. Unfortunately, for the same forward link power, the power flux density from the satellite has now tripled in the one-third of the band used. This is disadvantageous since, in many cases, the system capacity becomes limited by the FCC/CTI limitation on maximum power flux density of the satellite forward link signal incident on the Earth's surface. It also changes the band sharing scenario, since the signal now takes up only 1/3 of the available spectrum in each beam, but is potentially 5 dB stronger. OBJECTS OF THE INVENTION The object of this invention is to allow the capacity increase in a synchronous OCDMA system gained by employing a frequency reuse factor other than one, as disclosed in our U. S. patent application Serial No. 08/989,466 for "Increased Capacity in an OCDMA System by Frequency Isolation", without increasing the power flux density of the satellite forward link signal. Another object of this invention is to allow the capacity increase in a synchronous OCDMA system gained by employing a frequency reuse factor other than one, and still allow the signal power to be spread uniformly across the available spectrum for band sharing purposes. FEATURES OF THE INVENTION A. A spread spectrum CDMA communication system in which one or more base stations communicate with a multiplicity of subscriber terminals located in different beams of a multi- beam satellite relay system, the base station(s) transmits a signal for each beam which is comprised of a set of orthogonally spaced subcarriers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set carriers data for a single user in said beam, a source of selected carrier signals and means to modulate said information signals onto said subcarriers to form a beam signal and said coded spreading sequence on said beam signal. The available frequency spectrum is divided into nonoverlapping or orthogonal frequency segments which are assigned to different beams in a 1 :N ra frequency reuse pattern. Each subscriber terminal has a receiver with means to coherently demodulate the base station signal. The improvement comprising: • A PN spreading code, P2, with chipping rate N-jζR- is applied to all of the beam signals simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density in each beam. • A means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation. B. The above system in which the subcarriers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing ^l^ and > „ distinct sets of subcarrier frequencies are formed to be assigned in the 1 :N ra beam frequency reuse pattern. C. The above system in which the data intended for each user is voice data. D A spread spectrum CDMA satellite communication system, in which more than one satellite illuminates the same region on the earth, and one or more base stations communicate with a multiplicity of subscriber terminals located in different beams of the multi-beam satellite relay system, and the base station(s) transmits a signal for each beam which is comprised of a set of orthogonally spaced subcarriers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set carries data for a single user in said beam, a source of selected carrier signals and means to modulate said information signals onto said subcarriers to form a beam signal and said coded spreading sequence on said beam signal. The available frequency spectrum is divided into non- overlapping or orthogonal frequency segments which are assigned to different beams in a 1 :N m frequency reuse pattern. Each subscriber terminal has a receiver with means to coherently demodulate the base station signal. The improvement comprising: • A PN spreading code, P3, with a chipping rate NjζR c is applied to all of the beam signals simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density in each beam and reduce interference from neighboring satellites. • A means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation. E. The above system in which the subcarriers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing N m %R. and ^ distinct sets of subcarrier frequencies are formed to be assigned in the 1 :n m beam frequency reuse pattern. The above system in which the data intended for each user is voice data G. A spread spectrum CDMA satellite communication system, in which another satellite communication system illuminates the same region on the earth, and one or more base stations communicate with a multiplicity of subscπber terminals located m different beams of the multi- beam satellite relay system, and the base statιon(s) transmits a signal for each beam which is compπsed of a set of orthogonally spaced subcamers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set carπes data for a single user in said beam, a source of selected carrier signals and means to modulate said information signals onto said subcamers to form a beam signal and said coded spreading sequence on said beam signal The available frequency spectrum is divided into non-overlappmg or orthogonal frequency segments which are assignee to different beams in a 1 N m frequency reuse pattern Each subscπber terminal has a receiver with means to coherently demodulate the base station signal The improvement comprising • A PN spreading code, P2, with chipping rate N^R,, is applied to all of the beam signals simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density m each beam • A means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation H. The above system in which the subcamers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing N- j χl^ and > , distinct sets of subcamer frequencies are formed to be assigned m the 1 N ru beam frequency reuse pattern I. The above system in which the data intended for each user is voice data J. A spread spectrum CDMA satellite communication system, in which another satellite communication system illuminates the same region on the earth, and a multiplicity of subscπber terminals located in different beams of the multi-beam satellite relay system communicate with one or more base stations, and the subscπber terminals transmit a signal which is compπsed of one of a set of orthogonally spaced subcamers, which is modulated by one of a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, the orthogonal function of said set cames data for transmission to the base station m said beam, a source of the selected subcamer signal and means to modulate said information signal onto said subcamer to form a return link signal and said coded spreadmg sequence on said return link signal The available frequency spectrum is divided into non-overlapping or orthogonal frequency segments which are assigned to different beams in a 1 N ra frequency reuse pattern Each base station has a receivers) with means to coherently demodulate the subscπber terminal signals The improvement comprising • A PN spreading code, P2, with chipping rate N ru χR c is applied to the subscriber terminal transmit signal to spread the signal energy over a wider bandwidth in order to reduce the transmit power spectral density. • A means in the base station receiver for correlating the received signal with a synchronized replica of P4 to remove P2 from the incoming signal without affecting beam-to-beam frequency isolation. K. The above system in which the subscarriers in a beam are orthogonally spaced across the available frequency spectrum with spacing N R c and N ru distinct sets of subcarrier frequencies are formed to be assigned in the 1 :N ru beam frequency reuse pattern. L. The above system in which the data intended for each user is voice data. DESCRIPTION OF THE DRAWINGS The above and other objectives, advantages and features of the invention will become now apparent when considered with the following description and accompanying drawings wherein: Figure 1 is a schematic illustration of a satellite based OCDMA communication system incorporating the invention, Figure 2 is a graph illustrating four orthogonal subcarriers spaced at the chipping rate, Figure 3 is a functional block diagram of OCDMA subcarrier generation, Figure 4 is a graph illustrating subcarriers spaced by 3 *Rc across available spectrum, Figure 5 is a graph illustrating three sets of subcarriers assigned to mutually orthogonal slots in different beams, Figure 6 illustrates a typical multibeam antenna pattern with band segments assigned in a 1 :3 frequency reuse pattern, Figure 7 is an illustrating of beam signals with spreading code P 2 applied with chipping rate of three times that of the orthogonal codes, Figure 8a is a graph showing the normalized power spectrum of beam signals before spreading with the PN code P 2 (N ru : =3), and Figure 8b is a graph of normalized power spectrum of beam signal after spreading with PN code P 2 , clocked at three times the chipping rate of the orthogonal codes. DETAILED DESCRIPTION OF THE INVENTION The invention is intended for a satellite communication system in which the satellite transmits an OCDMA signal which provides the Forward Link (FL) to communicate with a multiplicity of user terminals as shown in Figure 1. The satellite signal may be relayed from a base station, or it may be generated on the satellite. The FL OCDMA signal preferably incorporates the improvements of Natali U.S. Patent No. 5,623,487 titled "Doubly Orthogonal Code and Frequency Division Multiple Access Communication System," in which multiple carriers (called "subcarriers") are orthogonally spaced in frequency at the OCDMA code chip rate, Re, as shown in Figure 2. Each subcarrier is modulated with up to Nu orthogonal binary sequences, each carrying different data Typically, each orthogonal function carries the data for a different user, however multiple functions may be assigned to a single high rate user. The orthogonal codes are overlaid with a longer PN code, P,, which is common to all the users within a beam. Different beams may use different PN codes or time shifted versions of the same code if desired. In order to maintain orthogonality, the orthogonal signals must arrive at the receiver in time synchroni sm (this is a trivial requirement on the FL, since all signals originate at the same base station or at the satellite). The subcarrier signal generation process is shown in block diagram form in Figure 3. The present invention is further intended for a system that incorporates the improvement disclosed in U. S. patent application Serial No.08/989,466 entitled "Increased Capacity in an OCDMA System by Frequency Isolation", to increase system capacity by providing frequency isolation between beams of a multibeam antenna. This is ordinarily implemented by splittingthe available spectrum into N m subbands and assigning the subbands to different beams in a 1 :N ra frequency reuse pattern. However, in the system disclosed here, the subcarriers are spaced across the full available spectrum at frequency intervals of N^R-as shown in Figure 4 for N m =3. Thus, N m sets of disjoint subcarrier frequencies are available in Figure 5, and can be assigned to beams in a 1 :N ra frequency reuse pattern such as shown in Figure 6 for N rl ,=3. Frequency isolation is maintained due to the orthogonal spacing of the subcarriers; however, the signal power is not yet spread uniformly. The FL signal is now modulated with a second PN sequence ? z which has chipping rate equal to N m times the chipping rate of the orthogonal sequences, as shown in Figure 7 This sequence is identical in all beams and acts to spread the signal power uniformly as shown in Figures 8a and 8b The received FL signal is correlated with a synchronized replica of P which effectively removes it from the incommg signals, I e the resultant signals can now be treated as though the PN code P 2 was never applied The adjacent beam signals are isolated from the m-beam signal by virtue of the orthogonal frequency spacmg Thus, the mcorporating of this invention allows uniform spectral spreading of the FL signal, while maintaining frequency isolation between adjacent beams While the invention has been descπbed m relation to preferred embodiments of the invention, it will be appreciated that other embodiments, adaptations and modifications of the invention will be apparent to those skilled in the art

Claims: WHAT IS CLAIMED IS: 1 A spread spectrum CDMA communication system in which one or more receiver equipped base stations communicate with a multiplicity of receiver equipped subscπber terminals located m different beams of a multi-beam satellite relay system, the base statιon(s) transmits a signal for each beam which is compπsed of a set of orthogonally spaced subcamers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set earners data for a single user in said beam, a source of selected earner signals and means to modulate said information signals onto said subcamers to form a beam signal and said coded spreading sequence on said beam signal, and wherein the available frequency spectrum is divided mto nonoverlapping or orthogonal frequency segments which are assigned to different beams in a 1 N m frequency reuse pattern and each subscπber terminal has a receiver with means to coherently demodulate the base station signal, the improvement comprising • means for applying a PN spreading code, P2, with chipping rate N ^χR ^ to all of the beam signals simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density in each beam, and • means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation 2 The system of claim 1 in which the subcarriers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing N -jζR c and N w distinct sets of subcamer frequencies are formed to be assigned in the 1 N ^beam frequency reuse pattern 3 The system of claim 1 in which the data intended for each user is voice data 4 In a spread spectrum CDMA satellite communication system, in which more than one satellite illuminates the same region on the earth, and one or more receiver equipped base stations communicate with a multiplicity of receiver equipped subscπber terminals located m different beams of the multi-beam satellite relay system, and the base statιon(s) transmits a signal for each beam which is compπsed of a set of orthogonally spaced subcamers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set carnes data for a single user in said beam, a source of selected earner signals and means to modulate said information signals onto said subcamers to form a beam signal and said coded spreading sequence on said beam signal, and wherein the available frequency spectrum is divided into non-overlapping or orthogonal frequency segments which are assigned to different beams in a 1 N m frequency reuse pattern and each subscπber terminal has a receiver with means to coherently demodulate the base station signal, the improvement composing • means for applying a PN spreading code, P3, with a chipping rate N ru χR c simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density in each beam and reduce interference from neighboring satellites, and • means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation 5 The system of claim 4 m which the subcarriers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing N -^R c and N ru distinct sets of subcamer frequencies are formed to be assigned in the 1 N ra beam frequency reuse pattern 6 The system of claim 4 in which the data intended for each user is voice data 7 A spread spectrum CDMA satellite communication system, in which another satellite communication system illuminates the same region on the earth, and one or more receiver equipped base stations communicate with a multiplicity of receiver equipped subscπber terminals located in different beams of the multi-beam satellite relay system, and the base statιon(s) transmits a signal for each beam which is compπsed of a set of orthogonally spaced subcamers, each of which is modulated by a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, each orthogonal function of said set carries data for a single user in said beam, a source of selected carrier signals and means to modulate said information signals onto said subcarriers to form a beam signal and said coded spreading sequence on said beam signal, and wherein the available frequency spectrum is divided into non-overlapping or orthogonal frequency segments which are assignee to different beams in a 1 ^ frequency reuse pattern, and each subscriber terminal has a receiver with means to coherently demodulate the base station signal, the improvement comprising: • means for applying a PN spreading code, P2, with chipping rate N n ^ . to all of the beam signals simultaneously to spread the signal energy over a wider bandwidth in order to reduce the power spectral density in each beam. • a means in the receiver for correlating the received signal with a synchronized replica ofP2 to remove P2 from the incoming signal without affecting beam- to-beam frequency isolation. 8. The system of claim 7 in which the subcarriers of the beam signal are orthogonally spaced across the available frequency spectrum with spacing N ,_χR c and N ru distinct sets of subcarrier frequencies are formed to be assigned in the 1 :N m beam frequency reuse pattern. 9. The system of claim 7 in which the data intended for each user is voice data. 10 In a spread spectrum CDMA satellite communication system, in which another satellite communication system illuminates the same region on the earth, and amultiplicity of receiver equipped subscπber terminals located in different beams of the multi-beam satellite relay system communicate with one or more receiver equipped base stations, and the subscπber terminals transmit a signal which is compπsed of one of a set of orthogonally spaced subcamers, which is modulated by one of a set of orthogonal, or nearly orthogonal, functions which are overlaid with a pseudo-noise (PN) sequence forming a coded spreading sequence for an information signal, the orthogonal function of said set carries data for transmission to the base station in said beam, a source of the selected subcamer signal and means to modulate said information signal onto said subcamer to form a return link signal and said coded spreading sequence on said return link signal, wherein the available frequency spectrum is divided into non-overlappmg or orthogonal frequency segments which are assigned to different beams in a 1 N ra frequency reuse pattern, each base station has a receιver(s) with means to coherently demodulate the subscriber terminal signal, the improvement comprising • a PN spreading code, P2, with chipping rate N ru χR. is applied to the subscπber terminal transmit signal to spread the signal energy over a wider bandwidth in order to reduce the transmit power spectral density, and • a means in the base station receiver for coπelatmg the received signal with a synchronized replica of P4 to remove P2 from the incoming signal without affecting beam-to-beam frequency isolation 11 The system of claim 10 m which the subscamers in a beam are orthogonally spaced across the available frequency spectrum with spacing N -j(R 0 and N m distinct sets of subcamer frequencies are formed to be assigned in the 1 N m beam frequency reuse pattern 12 The system of claim 10 m which the data mtended for each user is voice data

Inventors: NATALI FRANCIS D, OHLSON JOHN, NATALI, FRANCIS, D., OHLSON, JOHN
Applicants: STANFORD TELECOMM INC, STANFORD TELECOMMUNICATIONS, INC.
Application year: 1999
Main classification: H04Q7
IPC codes: H04B7, H04Q7