HK1106596B - Physical layer header scrambling in satellite broadcast systems - Google Patents

Description

Physical layer header scrambling in satellite broadcast systems

Cross Reference to Related Applications

This application claims the earlier filing date of U.S. provisional application No.60/561,418 entitled "Co-channel Interference differentiation for DVB-S2", filed 4/12/2004, as 35u.s.c. 119(e), the entire contents of which are hereby incorporated by reference.

Technical Field

The present invention relates to communication systems, and more particularly, to a method and apparatus for minimizing signal interference.

Background

Fig. 1A and 1B illustrate a typical satellite-based broadcasting system in the related art.

Fig. 1A shows a communication system, and in particular, a television broadcast system 20 that transmits and receives audio, video, and data signals via satellite. Although the present invention is described in the context of a satellite-based television broadcast system, the techniques described herein are equally applicable to other methods of program content delivery, such as terrestrial radio systems, cable-based systems, and the internet. Furthermore, although the present invention is described primarily with respect to television content (i.e., audio and video content), the present invention may be practiced using a variety of program content materials, including: video content, audio and video related content (e.g., television viewer channels), or data content.

The television broadcasting system 20 includes: a transmitting station 26, an uplink dish 30, at least one satellite 32, and receiver stations 34A-34C (collectively referred to as receiver stations 34). The transmitting station 26 includes a plurality of inputs 22 to receive various signals such as: analog television signals, digital television signals, video band signals, original programming signals, and computer generated signals containing HTML content. In addition, input 22 receives signals from a digital video server having a hard disk or other digital storage medium. The transmitting station 26 also includes a plurality of timing inputs 24 that provide electronic schedule information regarding the timing and content of various television channels, such as that found in television schedules contained in newspapers and television guides. A transmitting station 26 converts data from timing input 24 into program guide data. Program guide data may also be manually entered at the site of the transmitting station 26. The program guide data includes a plurality of "objects". The program guide data object includes data for forming an electronic program guide that is ultimately displayed on the user's television.

The transmitting station 26 receives and processes the various input signals received on input 22 and timing input 24, converts the received signals to a standard form, combines the standard signals into a single output data stream 28, and continuously sends the output data stream 28 to the uplink dish 30. The output data stream 28 is a digital data stream that is typically compressed using MPEG2 encoding, although other compression schemes may be used.

The digital data in the output data stream 28 is divided into a plurality of packets, each such packet being marked with a Service Channel Identification (SCID) number. The SCIDs are then used by receiver 64 (shown in fig. 1B) to identify packets corresponding to each television channel. Error correction data is also included in the output data stream 28.

The output data stream 28 is a multiplexed signal modulated by the transmitting station 26 using standard frequency and polarization modulation techniques. The output data stream 28 preferably comprises 16 frequency bands, each frequency band being left or right polarized. Alternatively, vertical polarization and horizontal polarization may be used.

The uplink dish 30 continuously receives the output data stream 28 from the transmitting station 26, amplifies the received signal and transmits a signal 31 to at least one satellite 32 although a single uplink dish and satellite is shown in fig. 1, it is preferred that multiple dishes and satellites be used to provide additional bandwidth to help ensure continuous transfer of the signal.

The satellites 32 rotate in geosynchronous orbits about the earth. The satellites 32 each include a plurality of transponders that receive the signal 31 transmitted by the uplink dish 30, amplify the received signal 31, frequency shift the received signal 31 to a lower frequency band, and then transmit an amplified frequency-shifted signal 33 back to the receiver station 34.

Receiver station 34 receives and processes signals 33 transmitted by satellite 32. The receiver station 34 will be described in further detail below with respect to fig. 1B.

Fig. 1B is a block diagram of one of the receiver stations 34 that receives and decodes audio, video, and data signals. Typically, receiver station 34 is a "set-top box," also known as an Integrated Receiver Decoder (IRD), which typically resides in a home or multi-dwelling unit, to receive satellite broadcast television signals. The receiver dish 60 may be an outdoor unit (ODU), which is typically a smaller dish mounted on a home or multi-dwelling unit. However, the receiver dish 60 can also be a larger, ground-mounted dish, if desired.

The receiver station 34 includes: receiver dish 60, alternate content source 62, receiver 64, monitor 66, recording device 68, remote control 86, and access card 88. The receiver 64 includes: tuner 70/demodulator/Forward Error Correction (FEC) decoder 71, digital-to-analog (D/a) converter 72, CPU 74, clock 76, memory 78, logic 80, interface 82, Infrared (IR) receiver 84, and access card interface 90. The receiver dish 60 receives the signal 33 transmitted by the satellite 32, amplifies the signal 33 and passes the signal 33 to the tuner 70. The tuner 70 and the demodulator/FEC decoder 71 operate under the control of a CPU 74.

The CPU 74 operates under the control of an operating system stored in the memory 78 or in a secondary memory within the CPU 74. The functions performed by the CPU 74 are controlled by one or more control programs or applications stored in memory 78. The operating system and application programs include the following instructions: when read and executed by the CPU 74, causes the receiver 64 to perform the functions and steps necessary to implement and/or use the present invention, typically by accessing and processing data stored in the memory 78. The instructions to implement such an application are tangibly embodied in a computer-readable medium, such as the memory 78 or access card 88. The CPU 74 may also communicate with other devices through the interface 82 or receiver dish 60 to accept commands or instructions to be stored in the memory 78, thereby making a computer program product or article of manufacture according to the invention. Also, the terms "article of manufacture," "program storage device," and "computer program product" as used herein are intended to encompass any application accessible by the CPU 74 from any computer-readable device or media.

Memory 78 and access card 88 store various parameters for receiver 64, such as a list of channels that receiver 64 is authorized to process and display, zip codes and area numbers for areas in which receiver 64 is used, model names and numbers for receiver 64, a serial number for access card 88, a name, address and telephone number of the owner of receiver 64, and a name of the manufacturer of receiver 64.

The access card 88 is removable from the receiver 64 (as shown in fig. 1B). When inserted into the receiver 64, the access card 88 is coupled to an access card interface 90, which access card interface 90 communicates with a customer service center (not shown) via the interface 82. The access card 88 receives access authorization information from the customer service center based on the user's specific account information. In addition, access card 88 communicates with the customer service center regarding billing and customization of services.

The clock 76 provides the current local time to the CPU 74. Preferably, the interface 82 is coupled to a telephone jack 83 at the site of the receiving station 34. The interface 82 allows the receiver 64 to communicate with the transmitting station 26 shown in fig. 1A via a telephone jack 83. The interface 82 may also be used to transmit data to and receive data from a network, such as the internet.

The signal transmitted from the receiver dish 60 to the tuner 70 is a plurality of modulated Radio Frequency (RF) signals. The desired RF signal is then downconverted to baseband by tuner 70, which also produces in-phase and quadrature (I and Q) signals. The two signals are then passed to a demodulator/FEC Application Specific Integrated Circuit (ASIC) 71. Demodulator ASIC 71 then demodulates the I and Q signals and the FEC decoder correctly identifies each transmitted symbol. The symbols of a received Quadrature Phase Shift Keying (QPSK) or 8PSK signal carry 2 or 3 data bits, respectively. The corrected symbols are converted into data bits, which are then combined into payload data bytes, and finally form a data packet. A data packet may carry 130 data bytes or 188 data bytes (187 data bytes and 1 sync byte).

In addition to the digital satellite signals received by the receiver dish 60, preferably, other sources of television content are also used. For example, the alternate content source 62 provides additional television content to the monitor 66. An alternate content source 62 is coupled to the tuner 70. Alternate content source 62 may be an antenna for receiving off-air signals (i.e., National Television Systems Committee (NTSC) signals), a cable for receiving Advanced Television Standards Committee (ATSC) signals, or other content source. Although only one alternate content source 62 is shown, multiple sources may be used.

Initially, when data enters the receiver 64, the CPU 74 looks for initialization data, which is commonly referred to in the industry as a boot object. The guide object identification may find the SCIDs of all other program guide objects. The boot object is always sent with the same SCID, so the CPU 74 knows that it must look for packets marked with that SCID. Information from the guide object is used by the CPU 74 to identify packets of program guide data and send them to the memory 78.

The remote control 86 emits an Infrared (IR) signal 85, which signal 85 is received by an infrared receiver 84 in the receiver 64. Other types of data entry devices may alternatively be used, such as, by way of example and not limitation, an Ultra High Frequency (UHF) remote control, a keyboard on the receiver 64, a remote keyboard, and a remote mouse. When a user requests display of a program guide by pressing a "guide" button on remote control 86, a guide request signal is received by IR receiver 84 and sent to logic circuitry 80. Logic circuitry 80 informs CPU 74 of the guidance request. In response to the guide request, the CPU 74 causes the memory 78 to transfer the program guide digital image to the D/a converter 72. The D/a converter 72 converts the program guide digital image to a standard analog television signal which is then transmitted to the monitor 66. The monitor 66 then displays the TV video and audio signals. Alternatively, the monitor 66 may be a digital television, in which case digital-to-analog conversion in the receiver 64 is not necessary.

The user interacts with the electronic program guide using remote control 86. Examples of user interactions include: select a particular channel or request additional guide information. When the user selects a channel using the remote control 86, the IR receiver 84 relays the user's selection to the logic circuit 80, which then passes the selection to the memory 78, which memory 78 is accessible by the CPU 74. The CPU 74 performs an MPEG2 decoding step on the audio, video and other packets received from the FEC decoder 71 and outputs the audio and video signals of the selected channel to the D/a converter 72. The D/a converter 72 converts the digital signal into an analog signal, and outputs the analog signal to the monitor 66.

Such a communication system 20, shown here by way of example as a television broadcast system 20, may require high quality transmission due to digital technology. When packets and other data are transmitted from the uplink dish 30 to the receiver 64, the symbols and bits in the packets intended for other receiver stations 34 are transmitted from the satellite 32 down to the receiver 64, typically on the same frequency, because the transmission frequency is subject to the limitations of the satellite 32 and the available transmission frequency is subject to the governing grant for transmission at a particular frequency within the spectrum.

In addition, the data frames are encoded in such a way that they interfere with each other, and the receiver 64 cannot derive which packets it should decode and present them on the monitor 66. Other system operators, satellites 32 operating in adjacent orbital slots, or other spot transmission beams in other spot beam satellite broadcast systems 20.

As more data is transmitted by the communication system 20 (i.e., programming of more channels on the satellite broadcast programming system that can be viewed on the monitor 66), the interference between data packets will increase and, as a result, the quality of signal reception will deteriorate.

To achieve optimal use of the available spectrum and to deliver programming for a large number of different channels, rf transmissions using the same frequency may be directed to different geographical regions. However, in areas near different service areas, it is possible that the receiving station may detect the desired transmission, as well as other co-frequency transmissions. The unwanted transmissions become interference and severely degrade the overall performance of the intended channel receiver.

Traditionally, the negative effects of co-channel interference have been minimized by redesigning the frequency allocations allocated to the various transponders or satellites 32. However, beyond a certain level, this does not alleviate the problem any more.

It can thus be seen that there is a need in the art to minimize interference in broadcast systems.

Disclosure of Invention

To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for minimizing co-channel interference in a communication system. A method according to the invention comprises: scrambling a first header of a first signal using a first scrambling code; scrambling a second header of the second signal using a second scrambling code; and transmitting the first signal with the scrambled first header and the second signal with the scrambled second header through different frequency channels of the communication system.

Optional additional elements of the method include: the first signal further comprises a first pilot block, which is also scrambled using a first scrambling code; the first signal further comprises a first payload portion, the second signal further comprises a second payload portion, the first payload portion is scrambled using a third scrambling code, the second payload portion is scrambled using a fourth scrambling code, the third scrambling code and the fourth scrambling code are gold codes, the first scrambling code and the third scrambling code are paired, the first scrambling code and the second scrambling code are selected from a limited number of codes, the number of the limited number of codes is determined based on the number of interfering channels within the communication system, and information related to the first scrambling code and the second scrambling code is transmitted to a receiver within the communication system.

An apparatus according to the invention comprises: a scrambler to scramble at least a portion of a first header of a first signal using a first scrambling code and to scramble at least a portion of a second header of a second signal using a second scrambling code; and a transmitter for transmitting the first signal and the second signal over different frequency channels of the communication system.

Optional additional elements of the apparatus include: the first signal further comprises a first pilot block, which is also scrambled using a first scrambling code; the first signal further comprises a first payload portion and the second signal further comprises a second payload portion, the first payload portion is scrambled using a third scrambling code, the second payload portion is scrambled using a fourth scrambling code, the third and fourth scrambling codes are gold codes, the first and third scrambling codes are paired, the first and second scrambling codes are selected from a limited number of codes, the number of the limited number of codes is determined based on the number of interfering channels within the communication system, and the transmitter transmits information related to the first and second scrambling codes to a receiver within the communication system.

Another method according to the invention comprises: scrambling a first header of the first signal using a first scrambling code, scrambling a first payload of the first signal using a first gold code; scrambling a second header of the second signal with a second scrambling code, scrambling a second payload of the second signal with a second gold code; and transmitting the first signal with the scrambled first header and the second signal with the scrambled second header through different channels of the communication system.

Optional additional elements of the method include: the first scrambling code and the second scrambling code are selected to minimize co-channel interference between the first signal and the second signal, the first scrambling code and the second scrambling code are used as paired code pairs, and each signal further comprises at least one pilot block, which is scrambled with their respective headers.

Other aspects, features and advantages of the present invention are inherently included in the disclosed systems and methods or will become apparent from the following detailed description and the accompanying drawings. The detailed description and drawings merely illustrate specific implementations and embodiments of the invention, however, the invention is capable of other different embodiments and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

Drawings

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIGS. 1A and 1B illustrate a typical satellite-based broadcast system in the prior art;

fig. 2A is a diagram of a digital broadcasting system capable of minimizing co-channel interference according to an embodiment of the present invention;

FIG. 2B is a diagram of an exemplary transmitter employed in the digital transmission facility of the system of FIG. 2A;

FIG. 3 is a diagram of an exemplary demodulator in the system of FIG. 2A;

FIGS. 4A and 4B are a diagram of a frame structure used in the system of FIG. 2A and a diagram of logic for scrambling a frame header with a different Unique Word (UW) for each frame transmitted over an adjacent common channel, respectively, according to an embodiment of the present invention;

fig. 5 is a diagram of a scrambler to separate co-channel interference in accordance with various embodiments of the present invention;

FIG. 6 is a diagram of an exemplary scrambling sequence generator for use in the scrambler of FIG. 5;

fig. 7 is a diagram illustrating a periodic characteristic of cross-correlation between common channel frames according to an embodiment of the present invention;

FIG. 8 is a flow diagram of a process for generating different physical layer sequences according to an embodiment of the invention;

FIG. 9 is a flow diagram of a process for generating a scrambled physical header according to an embodiment of the present invention;

FIG. 10 is a flow diagram of a process for transmitting scrambling parameters according to an embodiment of the present invention;

FIG. 11 is a diagram illustrating various embodiments of the invention for managing scrambling parameters;

FIG. 12 is a flow diagram for scrambling a received frame based on a number of pre-specified scrambling parameter sets, according to an embodiment of the present invention; and

fig. 13A to 13B are flowcharts showing steps of the present invention.

Detailed Description

An apparatus, method and software for reducing co-channel interference in a digital broadcast and interactive system are described. In the following description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration several embodiments of the invention. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

SUMMARY

In the present invention, the digital data transmitted from the transmitting station 26 via the signal 31, the satellite 32 and the signal 33 contains three main components: a header of a data frame, called a physical layer header or PL header; payload data; and optionally additional inserted symbols, referred to as pilot symbols, which the receiver 64 uses to mitigate the deleterious effects of degradation, primarily the effects of phase noise, in the receiver station 34 the demodulator/FEC decoder 71 can quickly acquire the correct phase at the beginning of each data frame by using the PL header, for many 8PSK and QPSK transmission regimes, pilot symbols are also required to track the phase noise more accurately, however, in a particular example, when the PL headers for the desired signal and the interfering co-frequency signal are aligned in time, the interference is so strong that the demodulator/FEC decoder 71 cannot determine the phase of the carrier frequency associated with the desired signal with the necessary accuracy, meaning that when the demodulator 71 seeks to maintain phase lock on the desired signal, the undesired signal exhibits the same header symbols or pilot symbols, demodulator 71 may be confused by the presence of an undesired signal and therefore unable to track the phase of the desired signal, this confusion in demodulator 71 is known in the art as "dropping" demodulator 71 from the desired signal, if demodulator 71 is pulled 45 degrees away from the optimum constellation point for QPSK transmission, the demodulator will not correctly recognize the symbol, this will cause an error, if it is not corrected quickly, the data error will be recognized as a loss of lock, this will in turn cause microprocessor 74 to instruct demodulator 71 to reacquire the signal, which will cause a loss of data until the desired signal is captured, this loss of data will present incorrect data on monitor 66, which may be seen by the viewer as a service interruption on monitor 66, common channel interference will cause the viewer to see a given monitor 66 fading to a black screen, or a cluttered picture, or hear a distracting sound and fail to view the desired television channel with motion and dialog on a monitor, it is apparent that co-channel interference can have a deleterious effect on the television broadcast system 20.

The present invention provides several factors that will mitigate the effects of such co-channel interference.

The first approach is to provide different start of frame (SOF) sequences and/or scrambling codes to those channels that can be affected by such co-channel interference. The demodulator 71 may then look for a particular SOF when required to tune to one or the other of the data frames and can derive the difference between them. Alternatively, or in combination, the difference in the codes used to scramble such interfering signals may be large enough that the cross-correlation between the two data frames is reduced to some extent, wherein the demodulator 71 may lock on the desired transmission and ignore the deleterious effects of the interfering frequency channels. Furthermore, different scrambling techniques may be used for PL headers on different channels, and/or different scrambling techniques or different scrambling codes may be applied to the payload data, either in combination with or separate from the scrambling of the PL headers, which will reduce or eliminate the lift-off effect.

Another method for reducing the effects of co-channel interference is to sense the time when demodulator 71 is pulled away from tracking the particular phase of a given signal. Such a pull-away or disengagement of phase tracking would indicate the presence of an interfering data frame, and demodulator 71 may then choose not to update the phase tracking with the PL header or pilot symbols.

Another approach of the present invention is to shift the transmission frequency of the modulated rf signal by a small amount, for example 1MHz, so that for a given data frame, the demodulator 71 can search the SOF portion of the PL header in a different frequency space. The number of offsets and, for example, the direction up or down with respect to frequency may be based on the number of independent rf transmissions or downlink beams of the satellite 32 that will be present at the same time and potentially cause co-channel interference. Furthermore, the data frames within the signal may be offset in time, e.g., a data frame starts first, while the interfering data frame is delayed by a certain number of symbols, so that the SOF portion of the PL header will occur at a different time for each data frame. This will allow the demodulator 71 to know which data frame has been received based on the known offset of the data frame and then demodulate the correct signal.

Another approach of the present invention is to use a different shift keying pattern within each data frame. Typically, QPSK transmission mode will be more resistant to co-channel interference effects than 8PSK transmission mode.

System diagram

Fig. 2A is a digital broadcast system 100 capable of minimizing co-channel interference in accordance with an embodiment of the present invention, the digital communication system 100 includes a digital transmission facility 101 that generates signal waveforms that are broadcast to one or more receivers 105 via a communication channel 103 according to an embodiment of the present invention, the communication system 100 is a satellite communication system that supports, for example, audio and video broadcasts, as well as interactive services, such communication systems are shown in fig. 1A and 1B and described above. Such as Electronic Program Guides (EPGs), high speed internet access, interactive advertising, telephony, and email services, which may also include television services such as pay-per-view, TV commerce, video-on-demand, near video-on-demand, and audio-on-demand services, in this case, the receiver 105 is a satellite receiver.

In broadcast applications, the continuous mode receiver 105 is widely used. With respect to synchronization (e.g., carrier phase and carrier frequency), codes that perform better in low signal-to-noise ratio (SNR) environments collide with these receivers 105. A physical layer header and/or pilot symbols may be used for such synchronization. Therefore, an important consideration regarding system performance is the consideration of co-channel interference for the physical layer header and/or pilot symbols. This interference can degrade receiver performance because the physical layer header and/or pilot are used to acquire and/or track carrier phase and carrier frequency.

Many digital broadcast systems 100 require the use of additional training symbols in addition to the normal overhead bits in the frame structure for their synchronization process. This increase in overhead is particularly desirable when the signal-to-noise ratio (SNR) is low; this is typical when high performance codes are used in conjunction with high order modulation. Traditionally, continuous mode receivers employ a feedback control loop to acquire and track carrier frequency and phase. These approaches, which are purely based on feedback control loops, are subject to strong Radio Frequency (RF) phase noise and thermal noise, resulting in high cycle slip rates and error floor with respect to overall receiver performance. Thus, these approaches, in addition to having a limited acquisition range and requiring a longer acquisition time, also burden the increased overhead in training symbols for a particular performance goal. Furthermore, these conventional synchronization techniques depend on a particular modulation scheme, thereby hindering flexibility in the use of the modulation scheme.

In system 100, receiver 105 achieves carrier synchronization by examining the preamble, header, and/or unique scrambling code or Unique Word (UW) embedded in the broadcast data frame structure (as shown in fig. 4A), thereby reducing the use of overhead specified specifically for training purposes. The receiver 105 will be described more fully below with respect to fig. 3.

In this discrete communication system 100, a sending facility 101 generates a discrete set of possible messages representing media content (e.g., audio, video, textual information, data, etc.); each possible message has a corresponding signal waveform. These signal waveforms are attenuated, or otherwise altered, via the communication channel 103. To combat noise in the broadcast channel 103, the transmitting facility 101 employs forward error correction codes, such as Low Density Parity Check (LDPC) codes or concatenation of different FEC codes.

LDPC or other FEC codes, or other codes generated by the transmitting facility 101, facilitate high-speed execution without incurring any performance penalty. These constructed LDPC codes output from the transmission facility 101 avoid assigning a small number of check nodes to bit nodes that are already vulnerable to channel errors due to the modulation scheme (e.g., 8 PSK). Such LDPC codes have parallelizable decoding processes (unlike turbo codes) that advantageously involve simple operations such as addition, comparison, and table lookup. Furthermore, well-designed LDPC codes do not show any indication of the error floor, e.g., even if the signal-to-noise ratio increases, the error aspect is not reduced. If an error floor is present, other codes such as Bose/Chaudhuri/Hocquenghem (BCH) codes may be used to effectively suppress such an error floor.

According to an embodiment of the invention, the transmitting facility 101 generates a parity check matrix-based LDPC code (which facilitates efficient memory access during decoding) using a relatively simple encoding technique as explained below in fig. 2 to communicate with the satellite receiver 105.

Transmitter function

FIG. 2B is a diagram of an exemplary transmitter employed in the digital transmission facility of the system 100 of FIG. 2A. the transmitter 200 in the transmission facility 101 is equipped with an LDPC/BCH encoder 203 that accepts input from an information source 201 and outputs a higher redundancy coded stream suitable for error correction processing at the receiver 105. the information source 201 produces k signals from a discrete alphabet X.

The encoder 203 generates a signal from the alphabet Y using a simple encoding technique that utilizes only a parity check matrix by imposing a structure on the parity check matrix and outputs it to the modulator 205. In particular, a limit is imposed on the parity check matrix by constraining a specific portion of the matrix to a triangular matrix. This limitation results in negligible performance loss and therefore is an attractive compromise.

The scrambler 209 scrambles the FEC encoded symbols according to the present invention to minimize co-channel interference, as will be more fully described below.

The modulator 205 maps the scrambled messages from the scrambler 209 into signal waveforms that are sent to the transmit antenna 207, which transmit antenna 207 transmits these waveforms over the communication channel 103. The transmit signal from the transmit antenna 207 is propagated to a demodulator, as discussed below. In the case of a satellite communication system, a signal transmitted from the antenna 207 is relayed via a satellite.

Demodulator

Fig. 3 is a diagram of an exemplary demodulator/FEC decoder 71 in the system of fig. 1. demodulator/FEC decoder 71 includes a demodulator 301, a carrier synchronization module/descrambler 302 and an LDPC/BCH decoder 307 and supports reception of signals from transmitter 200 via antenna 303. According to an embodiment of the present invention, demodulator 301 provides filtering and symbol timing synchronization to the LDPC encoded signal received from antenna 303, and carrier synchronization module 302 provides frequency and phase acquisition and tracking, and descrambling to the signal output from demodulator 301. After demodulation, the signal is forwarded to LDPC decoder 307, which attempts to reconstruct the original source message by generating message X'.

As for the receiving end, if both the desired carrier and the interfering carrier use the same modulation and coding configuration (or pattern), when the frame headers (as shown in fig. 4A) are exactly aligned in time and their relative frequency offsets are small, the interference may cause significant errors in the phase estimation of the demodulator, and as a result, the demodulator may periodically generate errors. This occurs when the frequency of the signal and the symbol clock are close enough, although they may drift relative to each other.

Frame structure

Fig. 4A is a diagram of an exemplary frame structure used in the system of the present invention. As an example, the LDPC encoded frame 400 shown may support, for example, satellite broadcast and interactive services. Frame 400 includes a physical layer header (denoted as a "PL header") 401 and occupies a slot, and frame 400 also includes other slots 403 for data or other payload. Further, according to an embodiment of the invention, frame 400 employs pilot block 405 after every 16 slots to aid in carrier phase and frequency synchronization. Note that pilot block 405 is optional. Although shown after 16 slots 403, a pilot block (or pilot sequence) 405, which may represent a scrambled block, may be inserted anywhere along the frame 400.

In an exemplary embodiment, the pilot insertion process inserts pilot blocks every 1440 symbols. In this case, the pilot block includes 36 pilot symbols. For example, in physical layer frame 400, as such, after PL header 401, a first pilot block is inserted after 1440 payload symbols, a second pilot block is inserted after 2880 payload symbols, and so on. If the pilot block position coincides with the start of the next PL header 401, no pilot block 405 is inserted.

According to embodiments of the invention, carrier synchronization module 302 (fig. 3) employs PL header 401 and/or pilot block 405 for carrier frequency and phase synchronization. The PL header 401 and/or pilot block 405 may be used for carrier synchronization, i.e., for operations to aid in frequency acquisition and tracking, and may be used for phase tracking loops. Thus, the PL header 401 and pilot block 405 are considered to be "training" or "pilot" symbols and constitute, individually or collectively, a training block.

Each PL header 401 typically includes: a start of frame (SOF) portion comprising 26 symbols; and a physical layer signaling code field (PLS code) field including 64 symbols. Typically, the SOF portion is the same for all PL headers 401 of all signals transmitted without further scrambling.

For QPSK, 8PSK, and other modulations, the pilot sequence 405 is a 36 symbol long segment (each symbol is: () (ii) a I.e. 36 symbols (PSK). In frame 400, a preamble may be inserted after 1440 data symbolsA frequency sequence 405. In this case, the PL header 401 may have 64 possible formats depending on the modulation, coding, and pilot configuration.

When the PL headers 401 of the interfering and desired carriers (i.e., co-channel) are aligned in time, the coherent effects from the interfering PL headers 401 can introduce significant phase errors, causing unacceptable degradation in performance. Likewise, if both co-channels use pilot symbols (both use the same gold code for pilot block 405), pilot block 405 will be scrambled in exactly the same way, so that the coherent impact of the pilot block in the interfering carrier (or co-channel) remains a problem.

To mitigate the effects of co-channel interference, frame 400 is scrambled in a pilot pattern. Typically, in this mode, the non-header portion 407 is scrambled with a gold code sequence that is unique to the transmitter. However, in broadcast mode, the entire frame 400 including the pilot block 405 is scrambled using a common code; for example, all receivers 105 are provided with the same gold sequence. The scrambling process will be further explained with respect to fig. 4B, fig. 5, fig. 6, fig. 8, and fig. 9. As used herein, the scrambled pilot sequence is also denoted as the "pilot segment" of the frame 400.

I and Q exchange

Another method that may be used in accordance with the present invention is to swap the in-phase (I) and quadrature-phase (Q) portions of a signal while keeping the common channel phase intact. This phase exchange will break the phase coherence in the common channel data frame 400, which minimizes or prevents interference between two data frames 400 in the common channel.

Applying different scrambling codes to PL headers

As shown in fig. 4B, to reduce the effects of co-channel interference, each co-channel may employ several different Unique Word (UW) patterns of the same length as the PL header 401 to scramble the PL header 401. For example, for the desired carrier and the interfering carrier (i.e., co-channel), exclusive-or (via XOR logic 409) of the different UW patterns 411, 413 with the PL header 401 may be performed. In this way, the power associated with the PL header 401 of the interfering carrier is no longer coherently added to the PL header 401 of the desired carrier.

Although the frame 400 is described with respect to a structure that supports satellite broadcast and interactive services (and is compatible with the Digital Video Broadcast (DVB) -S2 standard), it should be recognized that the carrier synchronization techniques of the present invention are applicable to other frame structures.

Further, a single PL header 401 may be scrambled before appending the PL header 401 to the frame 400, and the single PL header 401 may be scrambled without scrambling the other PL headers 401. The present invention contemplates selecting the scrambling code (or seed to generate the scrambling code) based on anticipated co-channel interference between the two data frames 400, or, alternatively, selecting not to employ the scrambling code. The PL header may be scrambled again as part of the scrambling of the data frame 400, as shown in fig. 5, or it may be encrypted using an encryption scheme.

The codes 411 and 413 used to scramble the PL header 401 may be gold codes, other seed generated codes, or other encoding schemes as described herein. The number of satellites 32 or the amount of expected co-channel interference in the communication system 100.

Channel scrambling thereof

Fig. 5 is a diagram of a sequence scrambler for separating co-channel interference in accordance with an embodiment of the present invention. According to an embodiment of the invention, the scrambling code is a complex sequence that can be constructed from gold codes. That is, the scrambler 209 generates a scrambling sequence rn (i). Table 1 defines how the scrambling sequence rn (i) is scrambled using the scrambler 209 for the frame according to the scrambler sequence generator of fig. 6. In particular, table 1 shows the mapping of input symbols to output symbols based on the output of the scrambler 209.

Rn(i)	Input (i)	Output (i)
			0	I+jQ	I+jQ
1	I+jQ	-Q+jI
			2	I+jQ	-I-jQ
3	I+jQ	Q-jI

TABLE 1

The use of different seeds for either of these two m-sequence generators will produce different gold sequences. By using different seeds for different services, mutual interference may be reduced.

In broadcast mode, the physical layer header 401 of 90 symbols may remain constant for a particular physical channel. The gold sequence is reset at the beginning of each frame and therefore the scrambled pilot is also periodic with a period equal to the frame length. Because the data carrying the information in the frame varies and occurs randomly, co-channel interference is random and degrades the operating signal-to-noise ratio. Without the use of this scheme, the carrier and phase estimates would be skewed for the receiver due to the time-invariant nature of the original physical layer header 401 and pilot block 405, depending on the pilot and physical layer headers used for such acquisition and tracking. This will also degrade performance in addition to the signal-to-noise ratio degradation performance associated with random data.

The scrambler 209 employs a different scrambling sequence (n in fig. 6) to further separate the co-channel interference. One scrambling sequence for the physical layer header and one scrambling sequence for the pilot. Different pilots are specified according to different seeds of the gold sequence from the n values.

Thus, the present invention contemplates separately scrambling several combinations of PL headers 401, pilot blocks 405, and payloads 403 for co-channel interference mitigation. Depending on the complexity of the system, the PL header 401 and pilot block 405 (if present) for a given channel may be scrambled using a different code than the co-channel without scrambling the payload 403. Basically, the symbols of all non-payloads 403 present in one frequency channel 400 are scrambled using one code, and the symbols of all non-payloads 403 present in another frequency channel 400 are scrambled using a different code.

Further, the PL headers 401 and pilot blocks 405 (if present) for two different channels may be scrambled using different scrambling codes, and the payloads 403 for the channels may be scrambled using other codes. For example, a first scrambling sequence may be applied to a first PL header 401 and a second scrambling sequence may be applied to a second PL header 401. A third scrambling sequence (typically a gold code) is applied to the first payload 403 and a fourth scrambling sequence (also typically a gold code) is applied to the second payload.

It is also contemplated within the scope of the invention that there may be systems that use paired code pairs for the PL header 401 and payload 403, therefore, a given scrambling code for the PL header 401 is always used along with the scrambling code used to scramble the payload 403 of the PL header 401.

It is also contemplated within the scope of the invention that each payload 403 signal within system 20 receives a unique scrambling code. Further, each PL header 401 may receive a unique scrambling code, which may be paired with the scrambling code used for the payload 403, if desired.

Although described as a single scrambling sequence for a given frequency channel 400, the present invention also contemplates that the scrambling sequence may be changed or rotated after a given number of frames are transmitted. The scrambling sequences for the PL header 401, payload 403, or both may be rotated on a random or periodic basis as desired without departing from the scope of the invention.

Golden sequence generator diagram

Fig. 6 is a diagram of an exemplary scrambling sequence generator used in the scrambler of fig. 5. Although a gold sequence generator is shown in fig. 6, other sequence generators may be used within the present invention without departing from the scope of the present invention. By using different sequences for the common channels, i.e., different initialization seeds for each common channel, interference may be mitigated. In this example, the preferred polynomial employed by the golden sequence generator 700 is 1+ X⁷+X¹⁸And 1+ Y⁵+Y⁷+Y¹⁰+Y¹⁸. For example, to support n common channels, in an exemplary embodiment of the present invention, the seed may be programmed into the m-sequence generator 701. Initializing a polynomial based on a given seed for the common channel. According to an embodiment of the invention, a search algorithm is used to generateSeed, the search algorithm minimizes the worst cross-correlation between each pair of common channel pilot segments.

Generating different PL sequences

Fig. 8 is a flow diagram of a process for generating different physical layer sequences according to an embodiment of the invention. In step 801, different initialization seeds are assigned to respective co-channels. Next, via step 803, a gold sequence is generated based on the seed. Then, a scrambling sequence is constructed from the gold sequence for each different service, as in step 805. In step 807, the physical layer sequence is output by the scrambler 209.

The present invention may use a different initialization seed for each channel, and thus any pilot signal 405 in each signal will contain different symbols, which greatly reduces the cross-correlation between two interfering co-channels. Once the pilot symbols 405 are distinguishable, the demodulator 71 may track one data frame 400 based on almost the entire pilot symbols 405, which minimizes interference between the data frames 400.

Fig. 9 is a flow diagram of a process for generating a scrambled physical header according to an embodiment of the present invention. The transmitter 200 (of fig. 2A) receives input symbols associated with a physical header or pilot sequence, as in step 901. In step 903, the transmitter maps the input symbols according to the scrambling sequence generated by the scrambler 209. Then, via step 905, an output symbol is generated. Thereafter, the transmitter outputs the frame with the scrambled physical and/or scrambled pilot sequence (step 907).

Fig. 10 is a flowchart of a process for transmitting a scrambling parameter according to an embodiment of the present invention. As described above, for the pilot pattern, different gold sequences are employed for different services to reduce co-channel interference. Furthermore, using a different UW pattern of the same length as the head 401 may minimize the correlation additivity of the head 401. Thus, the receiver needs the appropriate UW to descramble the PL header 401 and also the appropriate gold sequence to descramble the payload data and pilot block.

In step 1001, a transmitter (e.g., transmitter 200) transmits scrambling parameters for each supported carrier (co-channel) to the receiver 64, typically by embedding the scrambling parameters in an Advanced Program Guide (APG) portion of the payload 403. this is accomplished by having the advanced program guide portion of the payload 403 available on at least one transponder from the satellite 32. typically, the APG portion of the payload 403 available on each transponder from the satellite 32 can direct the receiver 64 to receive the APG on a particular transponder at startup if such direction to the receiver 64 is necessary. furthermore, the transmitter 200 can use other methods for transmitting a scrambling code, such as through a telephone line that interacts with the receiver 64 via the interface 82. according to one embodiment of the invention, the scrambling parameters include a scrambling code and an index for the scrambling sequence number of each carrier or channel Scramble and payload data 403 (along with pilot block 405, if present) is a frame scrambled by a default gold sequence such as sequence No.0, step 1003, the receiver 65 initially tunes to the carrier to obtain the scrambling parameters and stores the set of scrambling parameters for all carriers to be received (via step 1005), step 1007, when the receiver switches to another carrier, the particular scrambling parameter for that carrier is retrieved, via step 1009, specifically, the stored index is retrieved to find the correct UW and the stored gold sequence number, step 1011, the frame received over the particular carrier is properly descrambled.

FIG. 11 is a diagram illustrating various embodiments of the invention for managing scrambling parameters. In this example, the satellite system 20 includes a transmitting station 26 that stores scrambling parameters 1100 in an external memory, i.e., database 1102, for all carriers used in the system 20. Two methods may be used to transmit the scrambling parameters to the receiver stations 34A-34C via the satellite 32.

In a first approach, the receiver 34 stores all sets of scrambling parameters corresponding to the carriers assigned to the receiver 34. In this manner, the transmitting station 26 need only indicate a particular entry associated with the correct set of scrambling parameters to be used by the receiver 34 for a particular carrier. The update command merely indicates the indices for these UW and gold sequence numbers in the database 1102 of the receiver 34.

As explained in fig. 12, the second approach employs a caching mechanism for pre-selected or pre-specified scrambling parameter entries. Likewise, the receiver 34 includes a memory 78 to store the pre-specified sets of parameters.

Fig. 12 is a flow diagram of descrambling a received frame based on a pre-specified scrambling parameter set, according to an embodiment of the present invention. Using this method, k sets of scrambling parameters corresponding to the carriers to be used by the receiver 34 are pre-selected or pre-specified, as in step 1201. In other words, only k pre-selected UWs and k gold serial numbers are stored in the table. The value of k may be configured according to the size of the memory 78. As a result, the transmitting station 26 need only send 2 logs for each carrier₂k bits. Furthermore, if a fixed association between UW and gold sequence number is preserved, the number of bits sent can be further reduced-log of one time per carrier is sent₂Number of k bits. Thus, the receiver 34 stores only k sets of scrambling parameters in the memory 78, per step 1203.

Using this "caching" concept, receiver 34 need not be instructed by transmitter station 26 as to a particular set of scrambling parameters. At this point, if the receiver 34 determines that the transmitting station 26 has indicated such an indication, per step 1205, the receiver 34 retrieves the appropriate scrambling parameters from the memory 78 and descrambles the frame received over the particular carrier, per step 1207.

Alternatively, receiver 34 may determine a valid entry in the scrambling parameter table in memory 78 itself, as in step 1209, assuming k is small enough not to unduly burden the processing power of receiver 34. When the receiver is first tuned to a particular carrier, the receiver 34 may perform a search process to step through all possible sets of k pre-selected UW and gold sequence numbers stored in the memory 78 without having to receive these parameters via the default carrier. Once a valid or correct set of UW and gold sequence numbers is found for a particular carrier after the search, the information may be stored in memory 78 for that carrier, per step 1211. The information may then be used to descramble the frame (step 1213). Thus, a valid set of scrambling parameters is used at a later time without having to search again when needed.

Although there are k internal sets of UW and golden sequence numbers stored in the memory 78 of the receiver 34, each of the sets may be replaced by a new UW and golden sequence number under remote command of the transmitting station 26.

The processes of fig. 8 through 10, and fig. 12 advantageously provide reduced co-channel interference, thereby enhancing receiver performance. As explained in fig. 13, these processes may be implemented as software and/or hardware.

Alternate shift keying scheme

Another approach of the present invention is to use a different shift keying pattern within each data frame 400. Typically, QPSK transmission mode will be more resistant to the PL header 401 interference effects than 8PSK transmission mode. Also, some data frames 400 may be transmitted in the first PSK mode and other frames 400 may be transmitted in the second PSK mode, which may reduce the number of bits/symbols within the data frames 400 that may structurally create interference. In addition, each time slot 403, pilot block 405, or PL header 401 may be transmitted in a different PSK or ASK pattern to further reduce the structural interference and, therefore, reduce or eliminate co-channel interference.

Sensing phase tracking detachment

Another method according to the present invention for reducing the effects of co-channel interference is to sense when the demodulator 71, or typically the carrier synchronization module 302 within the demodulator 71, is suddenly or abnormally pulled away from tracking the particular phase of a given encoded frame 400. This pulling away or "drop-out" of phase tracking will indicate the presence of an interfering data frame, and carrier synchronization module 302 may then choose not to update the phase tracking from PL header 401 or pilot symbols 405. Although the phase of a given signal or encoded frame 400 may change slowly, the carrier synchronization module 402 may use reference phase tracking to keep track of the phase of a given symbol, if desired.

Also, the present invention may use the carrier synchronization module 302 to determine the presence of interfering code frames 400, and the present invention may choose to update the carrier synchronization module 302 phase tracking information, or ignore the phase tracking information, to allow the carrier synchronization module 302 to track the carrier frequency of a given code frame 400 that has been captured. The carrier synchronization module 302 may use a statistical model or other method to determine how to track the phase of the desired encoded frame 400 rather than following the phase tracking information generated by the presence of the undesired interfering encoded frame 400.

Changes in SOF sequences

The invention also contemplates: for those encoded frames 400 that may be affected by such co-channel interference, the interfering encoded frames 400 may have different start of frame (SOF) sequences and/or scrambling codes. Typically, the SOF is the first 26 bits of a 90-bit PL header 401, but the SOF may be a larger or smaller number of bits. Furthermore, although changes in SOF sequences are described, these techniques may be applied to any portion of the PL header 401, if desired. The demodulator 71 can then look for a different SOF in the PL header 401 when required to tune to one or another data frame 400 and can continue to lock on to the desired signal without becoming disengaged due to co-channel interference.

Further, a different SOF sequence may be selected from a set of a limited number of SOF sequences, which may be stored in the receiver 64 so that the receiver 64 may detect or find a particular SOF sequence in the PL header 401 when needed.

Transmission frame timing offset

As shown in fig. 7, there may be an offset of two frames 601, 605 in time. The data frames 400 may be offset according to time as shown in fig. 7, e.g., one data frame 400 starts first and the interfering data frame 400 delays by a certain portion of a symbol or the entire number of symbols, so that for each data frame the SOF portions of the PL headers 401 will occur at different times and not structurally interfere with each other. This would allow the tuner 70 or demodulator 71 to know which data frames 400 have been received based on the known time and/or frequency offset for the data frames, or by processing the strongest signal which is the presumed desired signal, and then demodulate the correct data frames 400. The data frame 400 may be offset by any length longer than one symbol interval.

Transmission frequency offset

Another approach of the present invention is to shift the transmission frequency of the data frames 601, 606 by a small amount, e.g., 1MHz, so that for a given data frame 400, the demodulator 71 can search for the SOF portion of the PL header 401 in different frequency spaces. The number of offsets and, for example, the direction up or down with respect to frequency may be based on the number of data frames 400 or downlink beams of the satellite 32 that will be present at the same time and potentially cause co-channel interference.

Flow chart

Fig. 13A and 13B are flow charts illustrating steps of the present invention.

Block 1300 represents scrambling a first header of a first signal with a first scrambling code.

Block 1302 represents scrambling a second header of a second signal with a second scrambling code.

Block 1304 represents transmitting a first signal with a scrambled first header and a second signal with a scrambled second header over different channels of a communication system.

Alternatively, the present invention may be performed as follows.

Block 1306 represents scrambling a first header of the first signal with a first scrambling code.

Block 1308 represents scrambling a first payload of the first signal with a first gold code.

Block 1310 represents scrambling a second header of a second signal with a second scrambling code.

Block 1312 represents scrambling a second payload of the second signal with a second gold code.

Block 1314 represents transmitting the first signal with the scrambled first header and the second signal with the scrambled second header over different channels of the communication system.

Conclusion

In summary, the present invention includes a method and apparatus for minimizing co-channel interference in a communication system. A method according to the invention comprises: scrambling a first header of a first signal using a first scrambling code; scrambling a second header of the second signal using a second scrambling code; and transmitting the first signal with the scrambled first header and the second signal with the scrambled second header through different frequency channels of the communication system.

Optional additional elements of the method include: the first signal further comprises a first pilot block, which is also scrambled using a first scrambling code; the first signal further comprises a first payload portion, the second signal further comprises a second payload portion, the first payload portion is scrambled using a third scrambling code, the second payload portion is scrambled using a fourth scrambling code, the third scrambling code and the fourth scrambling code are gold codes, the first scrambling code and the third scrambling code are paired, the first scrambling code and the second scrambling code are selected from a limited number of codes, the number of the limited number of codes is determined based on the number of interfering channels within the communication system, and information related to the first scrambling code and the second scrambling code is transmitted to a receiver within the communication system.

An apparatus according to the invention comprises: a scrambler to scramble at least a portion of a first header of a first signal using a first scrambling code and to scramble at least a portion of a second header of a second signal using a second scrambling code; and a transmitter for transmitting the first signal and the second signal over different frequency channels of the communication system.

Optional additional elements of the apparatus include: the first signal further comprises a first pilot block, which is also scrambled using a first scrambling code; the first signal further comprises a first payload portion and the second signal further comprises a second payload portion, the first payload portion is scrambled using a third scrambling code, the second payload portion is scrambled using a fourth scrambling code, the third and fourth scrambling codes are gold codes, the first and third scrambling codes are paired, the first and second scrambling codes are selected from a limited number of codes, the number of the limited number of codes is determined based on the number of interfering channels within the communication system, and the transmitter transmits information related to the first and second scrambling codes to a receiver within the communication system.

Another method according to the invention comprises: scrambling a first header of the first signal using a first scrambling code, scrambling a first payload of the first signal using a first gold code; scrambling a second header of the second signal with a second scrambling code, scrambling a second payload of the second signal with a second gold code; and transmitting the first signal with the scrambled first header and the second signal with the scrambled second header through different channels of the communication system.

Optional additional elements of the method include: the first scrambling code and the second scrambling code are selected to minimize co-channel interference between the first signal and the second signal, the first scrambling code and the second scrambling code are used as paired code pairs, and each signal comprises at least one pilot block that is scrambled with their respective headers.

It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto and their equivalents. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended and their equivalents.

Claims (20)

1. A method for minimizing co-channel interference in a communication system having at least a first signal and a second signal, each signal being transmitted on a different frequency channel within the communication system, the method comprising:

scrambling a first header of a first signal using a first scrambling code;

scrambling a second header of a second signal with a second scrambling code, wherein the second scrambling code is different from the first scrambling code; and

a first signal having a scrambled first header and a second signal having a scrambled second header are simultaneously transmitted from a single transmitter over different frequency channels of a communication system.

2. The method of claim 1, wherein the first signal further comprises a first pilot block, and the pilot block is also scrambled using a first scrambling code.

3. The method of claim 2, wherein the first signal further comprises a first payload portion and the second signal further comprises a second payload portion, the first payload portion being scrambled using a third scrambling code and the second payload portion being scrambled using a fourth scrambling code.

4. The method of claim 3, wherein the third scrambling code and the fourth scrambling code are gold codes.

5. The method of claim 4, wherein the first scrambling code and the third scrambling code are paired pairs.

6. The method of claim 5, wherein the first scrambling code and the second scrambling code are selected from a limited number of codes.

7. The method of claim 6, wherein the limited number of codes is determined based on a number of interfering frequency channels within the communication system.

8. The method of claim 1, further comprising the steps of:

information relating to the primary scrambling code and the secondary scrambling code is transmitted to a receiver within the communication system.

9. An apparatus for minimizing co-channel interference in a communication system having at least a first signal and a second signal, each signal being transmitted on a different frequency channel within the communication system, the apparatus comprising:

a scrambler to scramble at least a portion of a first header of a first signal using a first scrambling code and to scramble at least a portion of a second header of a second signal using a second scrambling code, wherein the second scrambling code is different from the first scrambling code; and

a transmitter for simultaneously transmitting the first signal and the second signal over different frequency channels of the communication system.

10. The device of claim 9, wherein the first signal further comprises a first pilot block, the pilot block also being scrambled using a first scrambling code.

11. The apparatus of claim 10, wherein the first signal further comprises a first payload portion and the second signal further comprises a second payload portion, the first payload portion being scrambled using a third scrambling code and the second payload portion being scrambled using a fourth scrambling code.

12. The apparatus of claim 11, wherein the third scrambling code and the fourth scrambling code are gold codes.

13. The apparatus of claim 12, wherein the first scrambling code and the third scrambling code are paired pairs.

14. The apparatus of claim 13, wherein the first scrambling code and the second scrambling code are selected from a limited number of codes.

15. The apparatus of claim 14, the limited number of codes is determined based on a number of interfering frequency channels within the communication system.

16. The apparatus of claim 9, wherein the transmitter transmits information related to the first scrambling code and the second scrambling code to a receiver within the communication system.

17. A method for minimizing co-channel interference in a satellite-based communication system having at least a first signal and a second signal, each signal comprising at least a header and a payload, each signal being transmitted on a different channel within said communication system, said method comprising:

scrambling a first header of a first signal using a first scrambling code;

scrambling a first payload of the first signal using a first gold code;

scrambling a second header of a second signal with a second scrambling code, wherein the second scrambling code is different from the first scrambling code;

scrambling a second payload of the second signal using a second gold code; and

a first signal with a scrambled first header and a second signal with a scrambled second header are transmitted simultaneously from a single transmitter over different frequency channels of a communication system.

18. The method of claim 17, wherein the first scrambling code and the second scrambling code are selected to minimize co-channel interference between the first signal and the second signal.

19. The method of claim 18, wherein the first scrambling code and the first gold code are used as a paired code pair.

20. The method of claim 19, wherein each signal further comprises at least one pilot block, the pilot blocks being scrambled with their respective headers.