HK1070763B - System and method for increasing feeder link capacity in a satellite communications system - Google Patents

Description

System and method for increasing feeder link capacity in a satellite communications system

The application is a divisional application of Chinese patent application entitled "System and method for increasing feeder link Capacity in satellite communication System" of the invention having application number 00818034.2 at 27/10/2000.

Background

I. Field of the invention

The present invention relates generally to satellite communications, and more particularly to increasing the feeder link capacity of terrestrial satellite communications by transmitting a QPSK signal formed from two BPSK signals.

Related Art

Today, competitive satellite transit systems provide mobile and fixed satellite-based voice and data services that can reach the furthest parts of the world at competitive prices. A user specially equipped with a communication device communicates directly with a satellite orbiting overhead, for example, rather than communicating with a terminal base station in a conventional cellular network. The satellite (in some cases, more than one satellite) receives, amplifies, and forwards these signals to one or more earth stations, which we refer to herein as gateways or hubs. The gateway then passes this information to an existing communication network (e.g., Public Switched Telephone Network (PSTN), Public Land Mobile Network (PLMN), another gateway) or within the same gateway to other signal recipients. Similarly, call requests originating from an existing network go through the gateway all the way to the satellite and then down to the destination subscriber. A network of geostationary satellites can provide coverage over a large area of the ground.

Satellite communication systems provide economical long-distance transmission (cost is not necessarily a function of distance), broad service coverage, relief of artificial constraints or natural barriers such as geopolitical boundaries, the ability to cover undeveloped areas without adding expensive terrestrial telephone infrastructure, and new service capabilities such as positioning.

As these satellite communication systems get used, even more users want to utilize such systems and capabilities, which becomes an increasingly important issue. Existing systems use one or more channelization techniques to increase capacity, such as Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), spatial channelization (sometimes referred to as frequency reuse), and polarization multiplexing schemes.

Description of satellite communication systems using Code Division Multiple Access (CDMA) spread spectrum signals is disclosed in U.S. patent No. 4901307, published 2/13/1990, entitled spread spectrum multiple access communication system using satellites or terrestrial repeaters, and U.S. patent No. 5691974, published 11/25/1997, entitled method and apparatus for using full spectrum to transfer power in a spread spectrum communication system that tracks individual receiver phase time and energy, both assigned to the assignee of this invention and incorporated herein by reference.

In the forward link, information is communicated from the gateway to the user terminal across one or more beams formed by using beamforming antennas. Each beam covers a particular geographic area within the satellite footprint. These beams typically comprise a number of so-called sub-beams (also referred to as Frequency Division Multiple Access (FDMA) channels or CDMA channels) covering a common geographical area, each occupying a different frequency band. More particularly, in conventional spread spectrum communication systems, one or more preselected Pseudorandom Noise (PN) code sequences are used to modulate or "spread" a user information signal over a predetermined frequency spectrum band prior to modulation on a carrier signal transmitted as a communication signal. In addition, the information signal typically uses both PN and PN_ITransmission on in-phase channel of spreading codes, in turn usingPN_QThe code is transmitted on a quadrature phase channel. PN spreading is a method of spread spectrum transmission well known in the art that produces a communication signal having a bandwidth that is much greater than the bandwidth of the data signal. On the forward link, the PN spreading code is typically shared by all communication signals in a given sub-beam.

In conventional CDMA spread spectrum communication systems, "channelization" codes are used to distinguish different user terminals within a satellite sub-beam over a forward link. The channelization codes form orthogonal channels in the sub-beams of the communication signal transmission. That is, each user terminal has its own orthogonal channel provided over the previous connection using a unique channelisation orthogonal code. Walsh functions are typically used to perform channel codes, also referred to as Walsh codes or Walsh sequences, to establish what are referred to as Walsh channels. Typical orthogonal code lengths are 64-bit code chips for terrestrial systems and 128-bit code chips for satellite systems.

As the systems discussed above increase capacity using channelization techniques, future capacity demands may require much more capacity than these conventional techniques can now provide. For example, the maximum number of subscribers that can be served by each satellite is determined in part by the capacity of the gateway-to-satellite communications link, referred to herein as the feeder link. This has been described as a lack of feeder link bandwidth and can occur regardless of whether channelization or modulation techniques are used. Given that feeder link capacity provides an upper limit on the number of subscribers to a particular satellite service, the bandwidth available for the feeder link must be used in an efficient manner.

Accordingly, there is a need for improved systems and methods that increase feeder link capacity.

Summary of The Invention

It is noted that the preferred embodiment of the present invention is directed to a system and method for satellite communications in which at least one gateway communicates with a satellite over a feeder link and a satellite communicates with terrestrial user terminals over a user link. The feeder link signal is generated by multiplexing a first Binary Phase Shift Keying (BPSK) signal and a second BPSK signal. The feeder link signal is modulated by a Quadrature Phase Shift Keying (QPSK) signal, spread by QPSK. Feeder link signals are transmitted from the gateway to the satellite. The satellite demultiplexes the feeder link signal to recover the first and second BPSK signals. The first and second BPSK signals are then modulated to produce first and second user link signals. The two user link signals. Then modulated by BPSK and spread by QPSK. The user link signal is transmitted from the satellite to one or more user terminals.

The present invention increases feeder link capacity by transmitting QPSK modulated, QPSK spread signals over the feeder link, rather than by transmitting BPSK modulated, QPSK spread signals. The feeder link capacity is approximately doubled because both signals have approximately the same bandwidth, but the QPSK modulated, QPSK spread signal can carry double the information. The increase in capacity comes at the expense of additional system complexity and hardware costs at the gateway and satellite, which are not a great concern with some communication systems having additional capacity limitations.

An advantage of the present invention is that feeder link capacity can be increased without requiring modifications to existing user equipment communicating with BPSK modulated, QPSK spread satellites. This eliminates the need to re-deploy large numbers of user terminals or wireless devices to accommodate new users. QPSK modulated, QPSK spread feeder link signals are demultiplexed in the satellite to form two BPSK modulated, QPSK spread signals, which are then transmitted to the user terminal.

Further features and advantageous aspects of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

According to the present invention there is provided a method of increasing the capacity of a feeder link in a satellite communications system, comprising the steps of: modulating a first binary phase shift keying BPSK signal with an in-phase PN code and a carrier signal to generate a first signal component; modulating said first BPSK signal with a quadrature-phase PN code and a quadrature phase of said carrier signal to produce a second signal component; modulating a second BPSK signal with said in-phase PN code and said quadrature phase of said carrier signal to produce a third signal component; modulating said second BPSK signal with said quadrature-phase PN code and said carrier signal to produce a fourth signal component; adding the first signal component, the second signal component, the third signal component, and the negative of the fourth signal component to produce a feeder-link signal.

According to the present invention there is also provided a multiplexer for increasing the capacity of a feeder link in a satellite communications system, comprising: means for modulating a first binary phase shift keying BPSK signal with an in-phase PN code and a carrier signal to produce a first signal component; means for modulating said first BPSK signal with an orthogonal PN code and the quadrature phase of said carrier signal to produce a second signal component; means for modulating a second BPSK signal with said in-phase PN code and said quadrature phase of said carrier signal to produce a third signal component; means for modulating said second BPSK signal with said quadrature-phase PN code and said carrier signal to produce a fourth signal component; means for adding the first signal component, the second signal component, the third signal component, and the negative of the fourth signal component to produce a feeder link signal.

Brief Description of Drawings

The invention is described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which a portion first appears is indicated by the leftmost digit(s) in the corresponding reference number.

FIG. 1 depicts a satellite communications environment for use with the present invention;

fig. 2A depicts a preferred frequency characteristic diagram for the forward feeder link;

FIG. 2B depicts a preferred frequency characteristic diagram of a user link beam;

FIG. 3A depicts a preferred spatial channelization of a forward user link with an illumination zone divided into beams;

fig. 3B depicts a preferred spatial channelization of a reverse user link with an illumination zone divided into beams;

fig. 4 depicts a flow chart illustrating a preferred method of increasing feeder link capacity;

FIG. 5 depicts a flowchart illustrating QPSK/QPSK feeder link signal generation in more detail;

FIG. 6 depicts a preferred apparatus for performing multiplexing operations;

FIG. 7 depicts a flow diagram illustrating more detail of demultiplexing feeder link signals; and

figure 8 depicts a preferred apparatus for performing the demultiplexing operation.

Detailed description of the embodiments

I. Overview of the Environment

The present invention is directed to a system and method for increasing the capacity of a feeder link in a satellite communications system. The gateway combines (multiplexes) the two BPSKs to form a QPSK signal. The resulting QPSK signal, referred to herein as a feeder link signal, is then transmitted from the gateway to the satellite using the feeder link. The satellite splits (demultiplexes) the feeder link signal back into two BPSK signals, which are transmitted to the user terminal. The feeder link capacity is approximately doubled because the QPSK feeder link signal containing the information of the two BPSK signals has approximately the same bandwidth as a single BPSK signal.

Fig. 1 depicts a satellite communications environment 100 in which the present invention is useful. A satellite 102 orbiting the earth 104 communicates with a user terminal 106 over a user link 110. The satellite 102 also communicates with the gateway 108 over a feeder link 112.

Satellite 102 is preferably a simple, low cost satellite designed to minimize production and launch costs. The satellites 102 are preferably in low earth orbit, which can allow communication with low power user terminals 106 (e.g., wireless devices such as cellular phones). However, those skilled in the art will appreciate that the principles described herein apply to satellites of varying complexity and orbit, as well as to various wireless devices.

The user terminal 106 may represent many different communication devices such as, but not limited to, a radiotelephone, a data transceiver, or a paging or positioning receiver, and may be a hand-held, mobile, or automobile-mounted, or fixed station unit as desired. The typical handheld unit is similar in design to a conventional cellular telephone. In the preferred embodiment, the handheld unit is capable of communicating with a terrestrial cellular network and also capable of communicating with the satellite 102. Typical mobile user terminals include cell phones and full sets of automotive tools. Automotive tools provide a battery power source, high rf power output, and high gain antenna. The fixed station units communicate with the satellites 102 but typically do not communicate with other terrestrial cellular networks. Fixed station units are typically used to provide services to areas that are not terrestrial cellular or wireless network services. They are fixedly mounted with a primary power supply, high radio frequency power and a fixed high gain antenna.

The gateway 108 associates the user terminal 106 with other terrestrial communication networks (e.g., cellular systems, conventional telephone networks, satellite systems) and with other networks through the satellite 102. For example, gateway 108 receives telephone calls or other communication requests (e.g., facsimile and text messages) from a terrestrial switching device (not shown) and transmits the calls to the appropriate subscriber terminal 106 using feeder link 112 and subscriber link 110. In the return direction, the subscriber terminal 106 transmits to the gateway 108 using a subscriber link 110 and a feeder link 112. Gateway 108 then connects a communication connection or calls a terrestrial switching device that can then connect to other desired signal receivers over a standard telephone system. The connection may also be established directly from the gateway to a terrestrial cellular subscriber or another user terminal 106. In a preferred embodiment, gateway 108 establishes feeder link 112 using one or more parabolic antennas 120. For example, gateway 108 supports voice communication, paging, messaging, and data transfer.

Feeder link 112 represents two-way communication between satellite 102 and gateway 108. In the preferred embodiment, the feeder link includes all communications between the user terminals 106 served by the satellite 102 and a terrestrial communications network connected to the gateway 108. Feeder link 112 also includes other communications such as telemetry and control instructions transmitted between gateway 108 and satellite 102 or subscriber terminal 106.

Fig. 2A depicts a preferred frequency characteristic diagram of the forward feeder link 112 (i.e., from the gateway 108 to the satellite 102). Communications between the gateway 108 and the satellite 102 use FDMA, CDMA, and polarization multiplexing schemes to efficiently utilize the available bandwidth. As shown in fig. 2A, the frequency band of feeder link 112 is divided into two or more channels 202 corresponding to subscriber link 110. In the preferred embodiment, the band is divided into eight channels 202(I, K, M, O, H, C, G and D) using the right hand rotated polarization (RHCP) and eight channels 202(L, N, P, J, A, F, E and B) using the left hand rotated polarization (LHCP). In a typical system design, each channel 202 preferably placed between 5091MHz and 5250MHz covers a bandwidth that is 16.5MHz wide and is separated by a bandwidth of 19.38 MHz. The feeder link 112 also includes a command channel 204 that passes command information between the satellite 102 and the gateway 108.

Fig. 2B illustrates a preferred frequency characteristic of an exemplary channel 202. As shown, each channel is formed of two or more (preferably 13) FDMA subchannels 204, which may also be referred to as user link beams or CDMA channels in a CDMA communication system. Each user link beam 204 is further divided into two or more orthogonal code channels (preferably up to 128) implemented with Walsh code sequences. For example, the user link beam 204 may contain 128 signals or signal channels, each determined by a different user terminal 106, where each signal is BPSK modulated according to conventional techniques. Many different kinds of information may be transmitted on each CDMA channel, including but not limited to voice and data.

A similar frequency map is also preferably used at reverse feeder link 112, with the spacing of channels 202 being between 6875MHz and 7075 MHz. Otherwise, the reverse feeder links 112 and the forward feeder links 112 in the description of fig. 2A and 2B are identical.

The user link 110 represents two-way communication between the satellite 102 and the user terminal 106 of the satellite service. In the preferred embodiment, the user link 110 uses spatial channelization to efficiently utilize the available spectrum. Spatial channelization may be used to divide a terrestrial region served by a satellite into two or more service areas, referred to herein as satellite footprint. Spatial channelization is possible through different techniques of generating beams, i.e., directional transmission of electromagnetic energy. Each region is beamformed so that most of the energy transmitted by the beamed coverage covers the ground of the service area. This allows a small set of frequencies to be reused in a known pattern across the illuminated area. CDMA communication systems allow the same set of frequencies to be used in adjacent areas. It will be appreciated that similar techniques may be used to receive signals transmitted by users within a satellite service area. Moreover, the general concepts described herein also apply to other embodiments that use different additional capacity increase techniques.

Fig. 3A depicts spatial channelization of user link 110 divided into beams 302A that traverse 302P, also referred to generally as antenna beam configuration 300A. The outer perimeter of antenna beam structure 300A determines the approximate limits of the geographic area served by satellite 102. Those user terminals 106 within this area are served (at least) by the satellite 102. The antenna beam configuration 300A is preferably applied on the forward (i.e., from the satellite 102 to the user terminal 106) user link 110. In another embodiment, each beam transmits signals with substantially uniform energy across the region identified for it in FIG. 3, including appropriate compensation for the curvature of the earth (i.e., constant flux mode). However, each mode preferably takes into account any uneven distribution of user terminals across each beam, as described in commonly owned co-pending U.S. patent application serial No. 09/378562, filed on 19/8/1999 and entitled "satellite communication system using wide fixed beams and narrow steerable beams," which is incorporated herein by reference.

Fig. 3B illustrates another alternative antenna beam configuration 300B that is preferably used in the reverse user link 110 (i.e., from the user terminal 106 to the satellite 102). Comparing fig. 3A and 3B illustrates that the antenna beam configurations for the forward and reverse user links are preferably different. Other alternative embodiments use the same antenna beam configuration in both.

The satellite 102, which essentially functions as a repeater, receives signals from a gateway, hub, fixed or base station 108 over a feeder link 112 and retransmits them over a subscriber link 110 to a subscriber terminal 106. The satellite 102 demultiplexes the signal on the feeder link 112 into two or more channels 202 and a command channel 204 for use therein. All communications determined for those user terminals 106 within a given beam 302 are transmitted on a single channel 202. The personal user terminals 106 then select their assigned code channels within the sub-beam or CDMA channel in accordance with spread spectrum techniques well known to those skilled in the art. Although not discussed herein, satellite 102 also functions as a repeater that transfers subscriber link 110 to gateway 108 using reverse feeder link 112.

The network of satellites 102 is preferably configured to have partially overlapping service areas. As is known, this network can serve user terminals over a large geographical area.

In the preferred embodiment, the subchannel signals or user-link beams transmitted on user-link 110 take the form of:

X*PN_I*cos(ω_ct)+X*PN_Q*)sin(ω_ct)

where X is the sum of the binary data modulated Walsh codes assigned to those user terminals 106 of a particular user link beam 204. PN (pseudo-noise)_IIs an in-phase pseudo-random noise (PN) spreading sequence, PN_QIs a quadrature phase spreading sequence, and ω_cIs at itThe center frequency of the user link beam 204 on which the signal is transmitted. These signals are described as being BPSK modulated, QPSK spread (BPSK/QPSK): BPSK modulation is because X represents the same information signal X, which modulates both the in-phase and quadrature-phase components; QPSK spreading is due to the fact that the signal uses two different spreading sequences (PN)_IAnd PN_Q). This is again different from true QPSK modulation, which has only one signal (X) exciting both in-phase and quadrature-phase components.

The user terminal 106 retrieves the user link beam signal, spreads the in-phase and quadrature-phase components with spreading sequences, respectively, and then selects their assigned code channels. This BPSK/QPSK signal is preferably transmitted over the user link 110. As mentioned above, it is desirable to increase additional user capacity in newer or next generation satellite communication systems. This may require adding more beams on the user link and corresponding sub-channels on the feeder link. However, to accommodate existing systems, there is an adjusted frequency configuration and yet further need or desire to remain within the same allocated feeder bandwidth. Therefore, a need exists for a technique that allows additional beams to be transmitted over feeder links within the existing bandwidth. Even when additional bandwidth is available, certain constraints may exist that unacceptably limit the number of beams that may be utilized with existing methods.

As will be described below, according to the present invention, QPSK modulated/QPSK spread signals (QPSK/QPSK signals) are substituted for transmitting BPSK/QPSK signals over the feeder link 112. As a result, twice as many subscriber link signals can be transmitted over feeder link 112 without increasing the required bandwidth. Thus, the capacity of the feeder link 112 is doubled.

Summary of the invention

Fig. 4 depicts a flow chart illustrating a preferred method of increasing the capacity of feeder link 112 in accordance with the present invention. In step 402, two BPSK signals are multiplexed together to produce a QPSK/QPSK feeder link signal. This multiplexing is preferably performed by the gateway 108, although it may be implemented anywhere else desired. In step 404, QPSK/QPSK feeder link signals are transmitted from the gateway 108 to the satellite 102 over the feeder link 112.

In step 406, the satellite 102 demultiplexes the QPSK/QPSK signal back into the two original BPSK signals. In step 408, the two BPSK signals are modulated to form a BPSK/QPSK user link signal. In step 410, user link signals are transmitted from the satellite 102 to the user terminal 106 over the user link 110. Transmitting QPSK/QPSK signals instead of BPSK/QPSK signals can more effectively double the feeder link capacity because they all require approximately the same bandwidth, while QPSK/QPSK carries twice the information. With existing communication system designs, the increase in capacity would incur the overhead of additional hardware on the satellite 102 that demultiplexes and remodulates the QPSK/QPSK feeder link signals.

The following sections describe these steps in detail.

Generating QPSK/QPSK feeder link signals

Figure 5 depicts a flowchart illustrating in more detail the QPSK/QPSK feeder link signal generation in step 402. These operations are described in conjunction with fig. 6, and fig. 6 depicts a preferred device for performing the multiplexing operation, multiplexer 600. The multiplexer 600 includes eight modulators 602A, 602B, 602C, 602D, 604A, 604B, 604C, and 604D and two adders 608A and 608B. However, the functions of adders 608A and 608B may be performed by a single logical addition element, although this is generally inconvenient to implement. Multiplexer 600 receives two BPSK signal inputs, shown as X and Y in fig. 6, and outputs a QPSK/QPSK feeder link signal 612. Multiplexer 600 is preferably located in gateway 108, but one skilled in the art will appreciate that these operations are performed the same anywhere as long as a communication path is established with gateway 100.

In step 502, a first BPSK signal, shown as X in FIG. 6, is spread by an in-phase PN spreading code (PN)_I) And carrier signal modulation produces a first signal component 606A. Modulator 602A passes PN_IModulating X, typically by multiplicationAnd modulator 604A passes cos (ω)_ct) modulates the output of modulator 602A. The final first signal component may be denoted X × PN_I*cos(ω_ct)。

In step 504, X is quadrature-phase PN spreading code (PN)_Q) And quadrature-phase carrier signal modulation produces a second signal component 606B. Modulator 602B passes PN_QModulate X and modulator 604B passes sin (ω)_ct) modulates the output of modulator 602B. The final second signal component may be denoted X × PN_Q*sin(ω_ct)。

In step 506, the second BPSK signal, shown as Y in FIG. 6, is PN encoded_IAnd quadrature-phase carrier signal modulation produces a third signal component 606C. Modulator 602C through PN_IModulate Y, while modulator 604C passes sin (ω)_ct) modulates the output of modulator 602C. The final third signal component may be denoted Y × PN_Q*sin(ω_ct)。

In step 508, Y is homoPN_QAnd carrier signal modulation produces a fourth signal component. Modulator 602D through PN_QModulate Y and modulator 604D passes cos (ω)_ct) modulates the output of modulator 602D. The final fourth signal component may be denoted Y × PN_Q*cos(ω_ct)。

Next, the program adds the first, second and third signal components (606A, 606B, 606C) together and subtracts or transforms the sign plus the fourth signal component (606D). This may be considered in step 509 as performing this summing operation with a single summing element, such as the functional combination elements 608A and 608B shown in fig. 6. However, in the preferred embodiment, the summing process using the respective summing elements 608A and 608B shown in FIG. 6 is subdivided into smaller steps. This is presented as an example implementation technique and fewer more or less complex elements may be used to perform the desired signal summing or combining operations, as will be apparent to those skilled in the art.

In step 50, the second and third signal components 606B and 606C are summed to form a fifth signal component. Adder 608A performs this summation operation. The difference between the sum of the first (606A) and fifth signal components and the fourth signal component (606D) is calculated in step 512 and the result is the feeder link signal. As shown in fig. 6, adder 608A performs this difference operation to sum the first (606A), fourth (606D), and fifth signal components, with the fourth signal component being a negative input.

It will be further appreciated by those skilled in the art that other groupings of two or three signals and more or fewer summing or combining logic elements, or other orders and arrangements of summed signals, may be used to achieve the desired summing and combining of signals without departing from the teachings of the present invention.

The final feeder link signal 612 can be expressed as:

(X*PN_I-Y*PN_Q)cos(ω_ct)+(X*PN_Q+Y*PN_I)sin(ω_ct)

where X is PN_I-Y*PN_QIs referred to as the in-phase component of the feeder-link signal 612, and X PN_Q+Y*PN_IReferred to as the quadrature-phase component of feeder link signal 612.

In the preferred embodiment, X represents the sum of the binary data modulated orthogonal Walsh codes assigned to those user terminals 106 of a single user link beam 204. However, in alternative embodiments, X may represent any arbitrary signal input. For example, X may represent a single signal or a combination of two or more signals. Furthermore, X is not limited to digital modulation, but may represent an analog modulated or unmodulated signal.

A skilled artisan will appreciate that multiplexer 600 may be implemented in hardware, software, or a combination of both. For example, modulator 602 or 604(602A, 602B, 602C, 602D, 604A, 604B, 604C, and 604D) may represent a hardware multiplier or an equivalent software program that multiplies two signals. Similarly, the adder 608(608A, 608B) may represent a hardware adder or equivalently a software program that adds two or more signals. Those skilled in the art will also appreciate that many types of multiplexers may have different combinations of modulators, adders and other functional units that combine to produce feeder link signal 612. The components of multiplexer 600 selected as shown in fig. 6 are primarily for ease of explanation.

Demultiplexing feeder link signals

Returning to fig. 4, in step 404, feeder link signals 612 are transmitted from the gateway 108 to the satellite 102 in accordance with conventional satellite communications techniques. As described above, feeder link signal 612 is preferably transmitted on a particular user link beam 204 of feeder link 112.

In step 406, the feeder link signal 612 is demultiplexed in the satellite 102, producing two BPSK signals (X and Y in fig. 6) that are combined in the gateway 108 to form the feeder link signal 612. FIG. 7 depicts a flowchart illustrating step 406 in more detail. These operations are described in conjunction with fig. 8, which depicts a preferred device transponder 800 that performs demultiplexing operations. The transponder 800, which preferably is located in the satellite 102, includes a demultiplexer 802 and a user link modulator 804.

In step 702, feeder link signal 612 is demodulated into in-phase and quadrature components. As shown in fig. 8, the demultiplexer 802 includes six demodulators 812A, 812B, 814A, 814B, 814C, and 814D, two summers 816A and 816B, a Voltage Controlled Oscillator (VCO)806, and a 90-degree phase shifter 808. Demultiplexer 802 receives feeder link signal 612 from a receive antenna (not shown) on satellite 102 and passes it to demodulators 812A and 812B. The skilled artisan will appreciate that additional modulation stages are often required between the antenna and the transponder 800, such as converting signals from one RF frequency to another or from RF down to IF frequencies depending on the assigned link frequency. These additional stages are not further described and discussed herein because their design and implementation is more in the conventional art.

VCO806 generates carrier signal cos (ω)_ct) for connecting to a feeder chainThe carrier signals of the path signals 612 are in phase. Thus, demultiplexer 802 preferably employs coherent demodulation. Maintaining coherency can be ensured using conventional techniques. The output of the phase shifter 808 is the quadrature phase carrier signal sin (ω)_ct)。

Demodulator 812A demodulates feeder-link signal 612 with the carrier signal to produce an in-phase component X × PN of feeder-link signal 612_I-Y*PN_Q. Demodulator 812B demodulates feeder link signal 612 with the quadrature-phase carrier signal to produce quadrature-phase component X × PN of feeder link signal 612_Q+Y*PN_I。

In step 704, the first BPSK signal X is formed by summing the PN and the binary phase of the signal X_IThe multiplied in-phase component sum and PN_QThe multiplied quadrature phase components are added. Demodulator 814A combines the in-phase component with the PN_IMultiplying to generate XxPN_I ²-Y*PN_IPN_Q. Demodulator 814B combines the quadrature phase component with the PN_QMultiplying to generate XxPN_Q ²+Y*PN_IPN_Q. Notably, the satellite 102 must know the spreading code PN_IAnd PN_QThe spreading code must be synchronized with the symbols 606 of the feeder link signal. Adder 816A adds the output of demodulator 814A and the output of demodulator 814B to produce 2X. (Recall that)

In step 706, the second BPSK signal Y is generated by obtaining the AND PN_QThe multiplied in-phase component sum and PN_IThe difference between the multiplied quadrature phase components. Demodulator 814C combines the in-phase component with the PN_QMultiplying to generate XxPN_IPN_Q-Y*PN_Q ². Demodulator 814D combines the quadrature phase component with PN_IMultiplying to generate XxPN_IPN_Q+Y*PN_I ². Adder 816B adds the output of demodulator 814C to the negative or opposite sign output of demodulator 814DTaken up, 2 by Y is produced.

Thus, demultiplexer 802 outputs two outputs 2X and 2Y.

V. user link modulation

Returning again to fig. 4, in step 408, the two BPSK signals are modulated to produce two BPSK modulated, QPSK spread user link signals. As shown in fig. 8, user-link modulator 804 includes eight demodulators 822A, 822B, 822C, 822D, 824A, 824B, 824C, 824D, two adders 826A, 826B, a voltage controlled oscillator 818, and a 90-degree phase shifter 820.

A first BPSK/QPSK signal 832 is generated by the operation of demodulators 822A, 822B, 824A and 824B and adder 826A. PN for demodulator 822A_QDemodulates the output 2X of adder 816A and then multiplies its output by sin (ω) in demodulator 824A_ct) generating 2X PN_Qsin(ω_ct). Frequency omega_cIs the center frequency (as described above) when the appropriate user link beam is transmitted within a particular beam on the user link 110. PN for demodulator 822B_IThe output of summer 816A is demodulated and then multiplied by cos (ω) in demodulator 824B_ct) generating 2X PN_Icos(ω_ct). Adder 826A adds the output of demodulator 824A to the output of demodulator 824B to produce a first BPSK/QPSK user link signal 832, given the form:

2*X*PN_Icos(ω_ct)+2*X*PN_Q sin(ω_ct)

notably, user link signal 832 is transmitted on user link 110 in its own form as described above.

A second BPSK/QPSK signal 834 is generated by the operation of demodulators 822C, 822D, 824C and 824D and summer 826B. PN for demodulator 822A_QDemodulates the output 2 x Y of adder 816B and then multiplies its output by cos (ω) in demodulator 824C_ct) generating 2 x Y PN_Q cos(ω_ct). Demodulator 822D through PN_IOf demodulation summer 816BOutputs and multiplies the output by sin (ω) in demodulator 824D_ct) generating 2 x Y PN_Isin(ω_ct). Adder 826B adds the output of demodulator 824C to the output of demodulator 824D to generate a second BPSK/QPSK user link signal 834, which is given by:

2*Y*PN_Qcos(ω_ct)+2*Y*PN_I sin(ω_ct)

notably, the user link signal 814 is transmitted over the user link 110 in its appropriate form as described above.

Returning to fig. 4, in step 410 user link signals 832 and 834 are transmitted over user link 110 to user terminal 106. This transmission is accomplished in accordance with the above-described and conventional satellite-to-terrestrial communication techniques.

Conclusion VI

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims (4)

1. A method of increasing feeder link capacity in a satellite communications system, comprising the steps of:

modulating a first binary phase shift keying BPSK signal with an in-phase PN code and a carrier signal to generate a first signal component;

modulating said first BPSK signal with a quadrature-phase PN code and a quadrature phase of said carrier signal to produce a second signal component;

modulating a second BPSK signal with said in-phase PN code and said quadrature phase of said carrier signal to produce a third signal component;

modulating said second BPSK signal with said quadrature-phase PN code and said carrier signal to produce a fourth signal component;

adding the first signal component, the second signal component, the third signal component, and the negative of the fourth signal component to produce a feeder-link signal.

2. The method of claim 1 wherein said step of adding comprises the steps of:

adding said second and third signal components to produce a fifth signal component; and

adding the first signal component, the fifth signal component, and the negative of the fourth signal component to produce the feeder-link signal.

3. A multiplexer for increasing feeder link capacity in a satellite communications system, comprising:

means for modulating a first binary phase shift keying BPSK signal with an in-phase PN code and a carrier signal to produce a first signal component;

means for modulating said first BPSK signal with an orthogonal PN code and the quadrature phase of said carrier signal to produce a second signal component;

means for modulating a second BPSK signal with said in-phase PN code and said quadrature phase of said carrier signal to produce a third signal component;

means for modulating said second BPSK signal with said quadrature-phase PN code and said carrier signal to produce a fourth signal component;

means for adding the first signal component, the second signal component, the third signal component, and the negative of the fourth signal component to produce a feeder link signal.

4. The multiplexer of claim 3 wherein said means for adding comprises:

means for adding said second and third signal components to produce a fifth signal component; and

means for adding the first signal component, the fifth signal component, and the negative of the fourth signal component to produce the feeder-link signal.