CODE REUSE MULTIPLE ACCESS FOR SATELLITE RETURN

LINK

CROSS-REFERENCES TO RELATED APPLICATIONS [0001] The present application claims priority to U.S. Patent Application Serial No. 11/538,431 filed October 3, 2006.

[0002] U.S. Patent Application Serial No. 11/538,431 filed October 3, 2006 is a continuation-in-part patent application. However, the present application does not claim priority to earlier filed patent applications in this family. Prior U.S. applications include: • U.S. Patent Application Serial No. 11/431,228 filed May 9, 2006, a continuation application currently pending;

• U.S. Patent Application Serial No. 09/531,996 filed May 20, 2000, now U.S. Patent No 7,065,125; and

• U.S. Provisional Application Serial No. 60/148,925 filed August 13, 1999. BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to communication techniques and in particular to a spread spectrum communication technique via a satellite communication network.

[0004] A spread spectrum communication system is a system in which the transmitted signal is "spread" over a wide range of frequencies. Typically the bandwidth is much wider than the minimum bandwidth needed to transmit the information being sent. The underlying premise of this technique is that, in channels (typically wireless) with narrowband noise, an increase in the transmitted signal bandwidth produces a corresponding increase in the probability that the received information will be correct. [0005] Though inefficient in its use of bandwidth, an advantage of spread spectrum is its resistance to interference. Another advantage is that the technique can be combined with existing systems having narrower operating bandwidths that fall within the spread spectrum bandwidth. The presence of a spread spectrum signal only slightly increases the noise floor that the narrow band receivers see.

[0006] Various spread spectrum techniques are known: In a frequency hopping system, the carrier frequency of the transmitter changes from among a pre-selected set of carrier frequencies in accordance with a pseudo-random code sequence. The frequencies selected from the list are dictated by the codes in the sequence. In a time hopping system, the period and duty cycle of the pulsed radio frequency ("RF") carrier are varied in a pseudo-random manner in accordance with the pseudo-random code sequence. In a pulsed frequency modulated ("FM") system, the RF carrier is modulated with a fixed period and fixed duty cycle sequence. During the transmission of each pulse, the carrier frequency is frequency modulated. Hybrid systems incorporate aspects of two or more other systems.

[0007] Direct sequence ("DS") spread spectrum is a well known technique for transmitting digital data. The name direct sequence derives from the fact that the data sequence is directly multiplied by a high rate spreading sequence before it is transmitted over the channel. The spreading sequence is a sequence that transitions much faster than the data sequence. Instead of being called bits as in the data sequence, the individual states of the spreading sequence are called chips. The ratio of the chip rate to the data rate is commonly known as the spreading gain (ox processing gain), since it is the ratio by which the bandwidth of the data sequence is increased once it is multiplied by the spreading sequence. [0008] Refer to Fig. 1 for an illustration of the operation of direction sequence signaling. The figure shows a time domain representation of a case in which the chip rate of spreading sequence signal C is four times the rate of the data sequence D; i.e.

— = 4 x — . The resulting spread signal S is the product of the two sequences. When

¹ C ¹ D the data bit in D is in a first logic state, signal S follows the spreading sequence C and when the data bit in D is in second logic state, signal S is the compliment of C. The spreading sequence signal shown in Fig. 1 does not repeat. In practice though, many conventional direct sequence spread spectrum techniques employ a spreading code that repeats once per data bit. Code representation 100 is the notational convention used in the disclosure of the present invention to represent a spreading sequence in explaining the operation of the invention. The top half of representation 100 contains an identifier for the code, in this case "Code C." The bottom half of representation 100 indicates the various chips in the spreading sequence and identifies them by number.

[0009] These spreading sequences are generally referred to as codes, as in Code Division Multiple Access ("CDMA"). CDMA generally refers to a technique by which users are allocated different spreading codes to enable them to use the same channel without interfering with each other. Another technique, dubbed Spread ALOHA CDMA ("S A/CDMA"), uses a single maximal length code (or a small number of maximal length codes for different service classes) that repeats once per data symbol and relies on arrival time to separate the different incoming signals. A maximal length code is a code of length 2 ^M -1 , where M is an integer, and which has certain desirable autocorrelation properties. For more information on maximal length codes, refer to Pateros, Charles N., "An Adaptive Correlator Receiver for Spread Spectrum Communication," Ph. D. Thesis, Rensselaer Polytechnic Institute, Troy, N. Y., 1993.

[0010] Various code spreading strategies are known: In U.S. Pat. No. 5,084,900 to Taylor, the technique described uses a slotted Aloha CDMA system, where each packet uses the same code for the first transmission. When a collision occurs on the first packet, the packet is retransmitted using randomly selected codes from a known pool of codes. No particular code type or relationship of code length to symbol interval is specified.

[0011] Consumer broadband satellite services are gaining traction in North America with the start up of star network services using Ka band satellites. While such first generation satellite systems may provide multi-gigabit per second (Gbps) per satellite overall capacity, the design of such systems inherently limits the number of customers that may be adequately served. Moreover, the fact that the capacity is split across numerous coverage areas further limits the bandwidth to each subscriber.

[0012] While existing designs have a number of capacity limitations, the demand for such broadband services continues to grow. The past few years have seen strong advances in communications and processing technology. This technology, in conjunction with selected innovative system and component design, may be harnessed to produce a novel satellite communications system to address this demand.

[0013] In U.S. Pat. No. 5,450,395, Hostetter et al. describe a single code system where data bit length codes are used and each user always transmits a data 'one'. The data is encoded in the time position of each broadcast. The technique requires accurate time synchronization among all of the users.

[0014] In U.S. Pat. No. 5,537,397, Abramson describes a spread ALOHA CDMA technique in which multiple users employ the same spreading code. The code is a maximal length code that repeats once per symbol. The users are time aligned at the chip level, but asynchronous at the data bit level. A subtractive multi-user receiver for this application is also described. In U.S. Pat. No. 5,745,485, Abramson extends his '397 patent to allow multiple codes to support different traffic types on the same channel.

[0015] Dankberg, et al. describe, in U.S. Pat. No. 5,596,439, a technique for self- interference cancellation for a relay channel. The technique is referred to as Paired Carrier Multiple Access ("PCMA"), since it allows a pair of channels (one in each direction) to share the same relay channel.

[0016] In U.S. Pat. No. 5,761 ,196, Ayerst, et al. describe a CDMA system employing re-use of spreading sequences. A central controller distributes seeds for maximal length codes as needed by users.

[0017] What is needed is a communication method which improves upon the multiple access collision performance of the prior art communication techniques. What is needed is a system which can realize improved performance over prior art CDMA communication systems without further complication to the system.

SUMMARY OF THE INVENTION

[0018] The invention is a communication technique that employs direct sequence spread spectrum signaling for the access channel. Called Code Reuse Multiple Access ("CRMA"), the method is a novel and non-obvious extension of the Code Division Multiple Access ("CDMA") and Spread ALOHA CDMA ("SA/CDMA") multiple access techniques. The invention realizes an improvement in collision performance as compared to conventional multi-access techniques. [0019] In accordance with the invention, a small number of spreading codes relative to the number of simultaneous transmitters are provided. In one embodiment, only a single spreading code is used. In one embodiment, the length of the spreading code is longer than the length of a data symbol. In another embodiment, the length of the spreading code is such that the code does not repeat during modulation of the data sequence. To minimize the acquisition implementation complexity of the system, shortened codes are used in the preamble portion of a data burst. In one embodiment, the preamble is spread by using a code that repeats one or more times.

[0020] Further in accordance with the invention, the coding technique can be combined with a Paired Carrier Multiple Access ("PCMA") system. This combination provides a reverse channel without having to allocate an additional operating frequency range or having to provide a separate link. [0021] Still further in accordance with the invention, a multi-user receiver structure efficiently processes a multitude of received signals by centralizing the header acquisition process. A header acquisition component tuned to a spreading code acquires received data bursts. A pool of demodulation components is provided. An individual user data transmission is then fed to a selected one of the demodulation components to process the data portion of the received data burst.

[0022] The present invention presents many advantages over prior art systems. The invention exhibits improved multiple access collision performance as compared to SA/CDMA systems. The improved performance is achieved without adding to the complexity of standard CDMA systems. Moreover, the invention further reduces system complexity by eliminating some of the limitations of SA/CDMA, since the spreading codes of the present invention are not required to be maximal length codes of data symbol length.

CRMA OVER A SATELLITE RETURN LINK

[0023] According to a particular aspect of the the invention, a plurality of demodulators serve to demodulate a composite signal of a single return sub-channel that is received from an upstream receiver in a satellite ground station receiver system incorporating the invention. The composite signal is a composite of individual upstream signals generated by subscriber terminals transmitting randomly in time and relayed through a satellite.

[0024] In a specific embodiment, in order to synchronously generate the source upstream chip encoded direct sequence spread spectrum signal of a CRMA random access transmission at any subscriber terminal, the spreading chip clock of the spread spectrum signal for the transmitter of the subscriber terminal is derived directly from a received high-speed downstream channel signal by deriving a clock from the pre- demodulated high-speed signal, for example by occasionally capturing a symbol, (where the data-stream is typically faster than the subscriber terminal can fully demodulate).

[0025] The invention will be better understood by reference to the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Fig. 1 is a diagram demonstrating the operation of direct sequence spread spectrum signaling.

[0027] Fig. 2a is a diagram illustrating the code reuse multiple access ("CRMA") channel.

[0028] Fig. 2b is an alternate embodiment illustrating the CRMA technique in conjunction with a paired channel multiple access ("PCMA") system.

[0029] Fig. 3 is a diagram illustrating the operation of a standard multiple access channel. [0030] Fig. 4 is a diagram illustrating the operation of a Spread ALOH A/Code Division Multiple Access ("S A/CDMA ^" ) channel.

[0031] Fig. 5 is a diagram illustrating the operation of a CRMA channel according to the invention.

[0032] Fig. 6 is a diagram detailing a collision event in a SA/CDMA channel. [0033] Fig. 7 is a diagram detailing the collision avoidance properties of CRMA.

[0034] Fig. 8 is a diagram illustrating a spreading code that does not repeat during the burst preamble.

[0035] Fig. 9 is a diagram illustrating the process of splitting a preamble spreading code into two shorter codes that repeat during the burst preamble. [0036] Fig. 10 is a diagram illustrating the process of splitting a preamble code into four shorter codes that repeat during the preamble.

[0037] Fig. 1 1 is a schematic representation of a multi-user receiver in accordance with the present invention.

[0038] Fig. 12 is a schematic representation showing the basic components of a transmitting station in accordance with the invention.

[0039] Fig. 13 is a block diagram of a Satellite Modem Termination System (SMTS) into which the present invention is incorporated.

[0040] Fig. 14 is a block diagram of a portion of a receiver for obtaining a clock signal.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0041] Fig. 2a shows a typical environment in which the present invention operates. As shown in the figure, a large number of users 1 , 2, ... N broadcast to a common multi-user receiver 200. The receiver demodulates all of the signals Si, S ₂ , ... S _N received on the single multiple access channel 210, it being understood that each signal S _x occupies the same frequency range as the other signals. A property of this environment is that there is no synchronized coordination among the users and no time synchronization between each user 1 , 2 ... N and the receiver 200. Each user transmits independently of the other and asynchronousl y, in bursts based on its offered traffic. Consequently, there will be inevitable periods of time when two or more users are transmitting their signals to the receiver 200 simultaneously.

[0042] Refer now to the traffic model of Fig. 3 which illustrates a typical pattern of transmission activity on a standard multiple access channel. The signals from the various users are active when that user is transmitting data. In the figure, User 2 is the first to transmit a burst 302. User 1 then transmits a burst 304 during the last half of the transmission of User 2 ^" s burst 302, and so on. In a standard channel, there is no way to separate the transmissions at the receiver, so any transmissions that overlap in time will be lost. The scenario depicted in Fig. 3 shows an especially congested traffic pattern. Bursts 302 and 304 overlap and so receiver 200 will not be able to separate the overlapping transmissions. Burst 306 overlaps initially with burst 308 from User 1 and then with burst 310 from User 2. The occurrence of two or more transmissions arriving such that they cannot be resolved is referred to as a "multiple access collision, ^" or simply collision. In the scenario shown in Fig. 3, only the single transmission 320 received during the time period t ₃ is received free of interference; it is the only transmission that is received successfully.

[0043] In Code Division Multiple Access ("CDMA ^" '), each user is provided with a unique spreading code so that the bursts from different users can be more easily resolved. This requires a large number of codes. More significantly, the technique requires that each user have a corresponding receiver at the base station tied to that spreading sequence. This translates to a base station that is complex and costly to implement and maintain, since each user-assigned spreading code must have its own dedicated receiver hardware/firmware at the base station.

[0044] As discussed in U.S. Pat. No. 5,537,397 ("SA/CDMA"), the multiple access capability of a spread spectrum channel is not derived by the use of different spreading codes by each transmitter, but by the nature of the direct sequence spread spectrum signal itself. For minimum complexity, therefore, a single code system is ideal. U.S. Pat. No. 5,745,485 argues that different codes for different traffic types may be worth the extra complexity in the receiver in order to simplify the total system complexity. It should be noted here that the SA/CDMA systems as described in the referenced patents all use spreading sequences that have a period equal to the symbol period of the transmitted data. That is, the spreading sequence repeats in the same amount of time spanned by a data bit. As well, SA/CDMA utilizes maximal length spreading sequences, which have desirable properties, but are not required for multiple access.

[0045] Consider the prior art techniques wherein the spreading code repeats once per data symbol. Each time the code repeats, there is a potential collision with another user who starts transmitting within one chip period of this time. A collision, therefore, is the event when two transmitters use the same spreading sequence and each begins transmitting its data burst within a time span of 1 chip period of the other. The resulting interference of the collision makes both signals unusable to the receiver, so the data sent by both transmitters is lost. As U.S. Pat. No. 5,537,397 shows, the throughput of such a system is a function of the spreading gain, the number of spreading sequence chips per data symbol.

[0046] For the purpose of the invention, a unique notation was developed to facilitate the explanation and understanding of the features of the invention. Referring to Fig. 4, for example, each data burst is represented by a series of vertical lines. Each vertical line represents the beginning of the spreading code as it is being applied to (modulated onto) the data sequence comprising the data burst. Hence, each vertical line represents the modulation of one or more data symbols (data bits, where the transmitted information is binary data) by the spreading code. The number of data symbols modulated depends on the bit rate, the chip rate, and the number of chips comprising the spreading code. For example, given a symbol (bit) rate of five symbols (bits) per second and a 200-chip spreading sequence at a chip rate of 40 chips per second, 25 symbols (bits) are modulated in one cycle of the spreading sequence. Thus for a 100 symbol data sequence, the spreading sequence will repeat four times during modulation of the data sequence; the

data sequence spans a total time of 20 seconds and the spreading sequence spans a time period of five seconds.

[0047] Refer again to the traffic model shown Fig. 3 and to the diagrams of Figs. 4 and 6. As mentioned before, multiple access collisions can occur every time the spreading sequence repeats. Fig. 4 shows the traffic pattern depicted in Fig. 3 in an SA/CDMA channel, illustrating the operation of collision avoidance. Recall that under SA/CDMA, the spreading code repeats at a rate equal to the data rate, i.e., the code repeats for every data bit..

[0048] Consider the traffic pattern of Fig. 3. At time t _x a data burst from user N (signal S _N - _A ) is transmitted. At time t _x +], a data burst from user 2 (signal S _2-A ) is transmitted. A collision between signal S ₂ -A and signal S _N - _A occurs, since their spreading codes align within one chip interval at the time indicated by 402.

[0049] The reason for the occurrence of this collision is explained with reference to Fig. 6. Code A is used for the multiple access on the channel. User 2 -A, transmitting signal S ₂ - _A , begins his broadcast at the time indicated by 402. This occurs one data bit after User N-A, who broadcasts signal S _N - _A - The first chip of the respective spreading codes aligns. Consequently, both transmissions are lost during the overlap because they cannot be resolved by the receiver into separate transmissions.

[0050] It has been discovered that the multiple access contention performance of the system can be improved by allowing the spreading code sequence length to be longer than the data symbol length. In one embodiment of the invention built upon this discovery, the spreading code sequence is longer than the length (i.e., number of bits) of the data sequence to be transmitted in a single data burst. It was discovered that the number of potential collisions decreases dramatically by this technique. In addition, the complexity needed to provide for chip and bit timing coordination is removed. Another advantage of the invention is that different users can transmit with different data rates simultaneously.

[0051] This collision reduction property is clearly demonstrated in Fig. 5 using the unique notational convention described above. Fig. 5 is also based on the traffic model of Fig. 3. According to one embodiment of the invention, a long code is used which does not repeat during the length of the data bursts. Consequently, each burst in Fig. 5 is represented by only one line. There is only one possible collision event per burst, greatly

decreasing the collision probability. As can be seen in Fig. 5, for the traffic pattern of

Fig. 3 no collisions occur.

[0052] Referring to Fig. 7, the figure details the same event that caused a collision for the SA/CDMA system in Fig. 6. Code B, which does not repeat during the length of the burst, is used for multiple access in this CRMA channel. Here, at the time indicated by reference numeral 703, the two broadcasts S ₂ - _B and S _N - _B are separated by exactly one data bit time, as in Fig. 6. However, since Code B does not repeat for the duration of the data burst, there is no collision.

[0053] In accordance with an embodiment of the invention, the length of the spreading code is longer than the length of a data symbol. Length refers to a span of time. Hence, the length of the spreading code refers to the length of time spanned by the code. For a spreading code, this depends on the number of chips in the code and the chip rate of the code. Similarly, the length of a data symbol is simply the inverse of the data rate. For example, a 10 bits/second data rate translates to a data symbol whose time length is 100 milliseconds. In addition, as will be discussed shortly, one can speak of the length of a spreading code in terms of its spreading gain g which is the ratio of the chip rate to the data rate.

[0054] In an embodiment of the invention, spreading codes are selected that are much longer than a data symbol. Thus, the spreading code can have a length that exceeds the length of the longest data sequence. For example, a data sequence of 100 bits at 10 bits/second data rate would have a length of 10 seconds. In accordance with an embodiment of the invention, the spreading code would span period of time greater than 10 seconds long. Expressed in terms of spreading gain g, a spreading code having a 10 second period would comprise (100 x g) chips. In this case, there would be only one possible contention event per burst. Only if two transmitters started their bursts within one chip time of each other would a contention collision occur. Note that the collision probability is only a function of the chip period (not the spreading gain) and the burst transmission rate.

[0055] Generation of long spreading codes is a straightforward process. Well understood pseudo-random number generation techniques are known for producing spreading sequences (codes). Similarly, it is a simple matter to provide logic which breaks up the long spreading codes into segments of lengths appropriate for the

modulation of a data sequence in preparation for transmission, and for the subsequent demodulation of a received data burst.

[0056] It is noted the invention is advantageous in that a transmitter can receive data for transmission at a rate different from another transmitter. Consequently, the spreading gain of one transmitter can be different from the spreading gain of another transmitter.

[0057] Referring for a moment to Fig. 12, an embodiment of a transmitter according to the invention includes a data source to provide data sequences for transmission. The data sequence feeds into a combining component 1202 which produces the spread sequence for transmission. The combining component preferably includes a digital signal processor ("DSP") 1212 in communication with a memory 1214. DSP firmware is provided to perform the spreading operations. Among other things, memory 1214 contains the spreading code(s) to be combined with an incoming data sequence by DSP 1212. As will be explained further below, memory 1214 also contains the spreading code(s) used to spread the data preamble for the preamble portion of transmitted data burst. Combining component 1202 feeds the spread signal to a conventional transmission component 1204 for subsequent transmission.

[0058] An impact that must be considered are the ramifications of using the long code scheme of the present invention in connection with the burst acquisition complexity. At the beginning of each broadcast burst, a known data preamble is transmitted to allow the receiver to acquire the transmitted signal. Typically this involves correlating the incoming signal with a stored copy of the spread header. For the case of a spreading sequence that is only the length of the data bit, the correlator need only be as long as the data bit. For longer codes, the correlator has to be as long as the shorter of the spreading code length or the header length. Complexity in the correlator component is an issue because the multiply and add operations of the correlator must run at the chip rate, rather than at the much slower data symbol rate. Once the signal is acquired, the downstream demodulation of the data portion of the data burst occurs at the data symbol rate. Hence, the data portion of a burst can be spread using much longer codes without degrading system performance during demodulation at the receiving end. [0059] To address the complexity of the acquisition component, two strategies are provided in accordance with the invention. First, we can minimize the effective length of

the spreading sequence in connection with the preamble portion of the data burst. Two techniques for accomplishing this are provided by the invention.

[0060] Fig. 8 illustrates the first technique. Where the spreading code, Code C, is longer than the preamble, the code is truncated to a length to fit the period of time spanned by the data sequence comprising the preamble. Note that though the entire length of the spreading code may be longer than the preamble, it can be considered to be the length of the preamble for acquisition purposes. If this code requires a preamble detector that is too complex to implement, then shorter codes can be used that repeat during the preamble. This is the second technique, namely, repeating segments of the spreading sequence during the preamble. It is noted that this second technique has the effect of increasing the collision probability, thus presenting a design trade-off decision to the system designer. However, another aspect of the invention permits shorter codes to be used, while keeping the collision probability the same. The discussion now turns to additional embodiments of the invention. [0061] In the embodiment shown in Fig. 9, two codes are employed, Code D and Code E. One half of the transmitters use Code D to spread its preamble data sequence and the half of the transmitters use Code E. Each code is half the length of the preamble and repeats once during modulation of the preamble. Each time the code repeats, a multiple access collision can occur among the transmitters using Code D; likewise with the Code E transmitters. However, since only half of the channel users will be using either Code D or Code E, the collision probability is the same as if all users were using a single, long code. Note that all users could use the same code, Code D. This would result in a simpler system that had a larger probability of collision compared to a system employing Code C.

[0062] To further lower the correlation overhead in the acquisition circuitry, this process is further extended in Fig. 10. Here, four codes, Code F, Code G, Code H and Code I, are utilized by four groups of transmitters. Each code repeats four times during modulation of the preamble, so the four codes are each allocated to one fourth of the users. Hence by splitting the preamble spreading code in this fashion, per the invention, the complexity in the acquisition circuitry is greatly reduced without affecting the collision performance. Here, the use of one, two or three codes would simplify the acquisition system while again increasing the collision probability compared to a system using a longer code.

[0063] The spreading code used for spreading the preamble data sequence can be simply a portion of the spreading code used to spread the data sequence of the data portion of the burst. More generally, the spreading code for the preamble does not have to be derived from the spreading code for the data portion. In an embodiment of the invention, more than one spreading code is used for spreading preambles, while an identical longer code is used to spread the data sequence of the data burst. In yet another embodiment of the invention, more than one spreading code is used for spreading preambles, while two or more identical longer codes are used to spread the data sequences of data bursts from multiple transmitters. [0064] The preamble is always shorter than the burst, and is often a very small fraction of the burst. Thus, the multiple access collision performance of the system may be adequate even if there is only one spreading code used for the burst preambles, and even if that code repeats many times during the preamble interval. The performance enhancement comes about by the use of the longer code used for the data portion of the data bursts.

[0065] In still yet another embodiment, one may consider the bandwidth of the single carrier multiple access channel. If the channel bandwidth far exceeds the bandwidth required of the highest spreading chip rate, then the channel may be divided into multiple narrower sub-channels. The sub-channels may be of equal or unequal bandwidth, depending on the bandwidth requirements of the different spreading codes. Thus, where lower complexity transmitters are desired, this embodiment realizes greater utilization of a given multiple access carrier channel by allowing for lower chip rates.

[0066] Fig. 1 1 , which is of particular interest for a satellite uplink channel, shows a receiver 1 100 configured in accordance with the invention. The receiver represents the second strategy to address the acquisition complexity issue. The embodiment of receiver 1 100 shown in Fig. 1 1 uses a preamble detector module (typically implemented as a correlator) that hands off the demodulation to a pool of demodulators.

[0067] Receiver 1 100 includes an antenna 1 102 for receiving a beam bearing a signal. The antenna feeds the signal to an analog-to-digital (a/d) front end 1 102. The a/d front end is coupled to a preamble detection component 1 106. A demodulation controller component 1 108 selectively activates a pool of demodulation units 1 1 10. The demodulation units feed into a data handling system 1 1 12.

[0068] In operation, an incoming signal is received by an antenna 1 1 10 and is processed by a/d front end 1 102. The front-end digitizes the signal to produce a stream of samples. The samples are placed on a digital sample bus 1134. The sample bus makes the samples available to preamble detection component 1 106 and to the demodulation units in demodulator pool 1 1 10.

|0069] The preamble detection component receives the sample stream, and using the same data preamble and preamble spreading code as in the transmitters, determines the timing, phase and frequency offsets of transmission preambles in the received signal. This detection data is placed on a control bus 1 136. [0070] In an alternate embodiment, transmitters are divided into two or more groups. Each group having a preamble spreading code (or codes) assigned to it where each transmitter in a group uses its assigned spreading code to spread the preamble. At the receiver end, additional preamble detectors 1 120 are provided, one for each spreading code that is used. The output of a/d front end 1 104 feeds into each of the additional preamble detection components 1 120.

[0071] When a burst is detected by preamble detection component 1 106, demodulation controller 1 108 is activated by appropriate signaling over control line 1 132. The demodulation controller selects an idle demodulator from pool 1 1 10. The selected demodulator picks up the corresponding detection data from sample bus 1 134 and utilizes it to process the data portion of the burst. After demodulation, the demodulator forwards the demodulated data to data handling system 1 1 12 over signal bus 1 138. Once the burst is over, the demodulator informs demodulation controller 1108. The demodulator is placed back into the pool of available demodulators 1 1 10. The number of demodulators in pool 1 1 10 is a function of the maximum number of simultaneous transmitters that are allowed on the channel. This number is typically determined by the number of simultaneous transmissions that can occur on the channel without compromising the ability of the receiver to detect all the transmitters.

[0072] Referring to Fig. 2b, in another embodiment of the present invention, the inventive code reuse scheme is combined with a PCMA technique. This communication technique is explained, for example, in U.S. Pat. No. 5,596,439, which is fully incorporated herein for all purposes. In this embodiment, a base station 220 is the hub to which a number of users are connected. The forward channel is a high powered signal

222 that is broadcast from the hub to all the users 230. The reverse channel 232 (backchannel), from the users back to the hub, is a CRMA channel configured according to the present invention. By using PCMA, hub 220 can broadcast data (as well as control messages) to users 230 in the same frequency band as the multiple access reverse channels. By employing PCMA, the hub can remove its own transmitted signal from its received signal and then detect all the CRMA users accordingly as disclosed above.

[0073] The unique combination of a PCMA technique and the inventive code reuse technique permits data exchange in both directions between a base station and its associated transmitters. Conventionally, the back-channel is provided by a landline connection or by a wireless method that uses a different frequency range. An advantage of this unique combination is that the back-channel is obtained practically for free. The back-channel can contain controlling information sent to the hub. The hub can be an Internet service provider (ISP) where the downlink is the PCMA channel and user inputs are transmitted by the CRMA channel of the invention. [0074] With reference to FIG. 13, an embodiment of a SMTS 310 is shown in block diagram form. Baseband processing is done for the inbound and outbound links 135, 140 by a number of geographically separated gateways 1 15. Each SMTS 310 is generally divided into two sections, specifically, the downstream portion 305 to send information to the satellite 105 and the upstream portion 315 to receive information from the satellite 105.

[0075] The downstream portion 305 takes information from the switching fabric 416 through a number of downstream (DS) blades 412. Those DS blades 412 are divided among a number of downstream generators 408. This embodiment includes four downstream generators 408, with one for each of the downstream channels 800. For example, this embodiment uses four separate 500 MHz spectrum ranges having different frequencies and/or polarizations. A four-color modulator 436 has a modulator for each respective DS generator 408. The modulated signals are coupled to the transmitter portion 1000 of the transceiver 305 at an intermediate frequency. Each of the four downstream generators 408 in this embodiment has J DS blades 412. [0076] The upstream portion 315 of the SMTS 310 receives and processes information from the satellite 105 in the baseband intermediate frequency. After the receiver portion 1 100 of the transceiver 305 produces all the sub-channels 912 for the four separate

baseband upstream signals, each sub-channel 912 is coupled to a different demodulator

428. Some embodiments could include a switch before the demodulators 428 to allow any return link sub-channel 912 to go to any demodulator 428 to allow dynamic reassignment between the four return channels 908. A number of demodulators are dedicated to an upstream (US) blade 424.

[0077] The US blades 424 serve to recover the information received from the satellite 105 before providing it to the switching fabric 416. The US scheduler 430 on each US blade 424 serves to schedule future use of the return channel 900 for each subscriber terminal 130. Future needs for the subscriber terminals 130 of a particular return channel 900 can be assessed and bandwidth/latency adjusted accordingly in cooperation with the resource manager and load balancer (RM/LB) block 420.

|0078] The RM/LB block 420 assigns traffic among the US and DS blades. By communication with other RM/LB blocks 420 in other SMTSes 310, each RM/LB block 420 can reassign subscriber terminals 130 and channels 800, 900 to other gateways 1 15. This reassignment can take place for any number of reasons, for example, lack of resources and/or loading concerns. In this embodiment, the decisions are done in a distributed fashion among the RM/LB blocks 420, but other embodiments could have decisions made by one master MR/LB block or at some other central decision-making authority. Reassignment of subscriber terminals 130 could use overlapping service spot beams 205, for example.

[0079] Referring to Figure 14, a symbol from a downstream signal receiving antenna is applied to a sampling-type symbol edge detector 1410 according to the invention, from which a useable clock signal is extracted. The symbol detector 1410 samples the high speed broadband symbol-bearing signal to detect a periodic edge of the symbol. Sampling allows the detector to extract a clock without having to demodulate the transmitted sequence embedded in the downstream symbol(s).

[0080] The extracted clock signal is then processed in a clock cleaner 1412, from which various suitable clock signals can be propagated. In one embodiment, the full-rate symbol clock from the downstream signal is synthesized and applied as a direct sequence spread spectrum chip clock. Analog clock pulse signals may for example be applied to the RF transmitter stage 1204, whereas a digital sequence clock may for example be applied to a digital modulator, such as combiner 1202. Other types of clocks, such as

harmonically related lower speed clocks and harmonically related higher speed clocks, may also be generated at the clock cleaner 1412.

[0081] The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art in light of this disclosure. Therefore, it is not intended that this invention be limited, except as indicated by the appended claims.

APPENDIX A

VERY HIGH-SPEED BROADBAND SATELLITE COMMUNICATION The following information is given as background in order to understand the environment of high-speed satellite communication, particularly as employed to service subscribers accessing high speed networks.

FIG. IA is a block diagram of an exemplary satellite communications system 100 configured according to various embodiments of the invention. The satellite communications system 100 includes a network 120, such as the Internet, interfaced with a gateway 1 15 that is configured to communicate with one or more subscriber terminals 130, via a satellite 105. A gateway 1 15 is sometimes referred to as a hub or ground station. Subscriber terminals 130 are sometimes called modems, satellite modems or user terminals. As noted above, although the communications system 100 is illustrated as a geostationary satellite 105 based communication system, it should be noted that various embodiments described herein are not limited to use in geostationary satellite based systems, for example some embodiments could be low earth orbit (LEO) satellite based systems.

The network 120 may be any type of network and can include, for example, the Internet, an IP network, an intranet, a wide-area network ("WAN"), a local-area network ("LAN ^" '), a virtual private network, the Public Switched Telephone Network ("PSTN"), and/or any other type of network supporting data communication between devices described herein, in different embodiments. A network 120 may include both wired and wireless connections, including optical links. Many other examples are possible and apparent to those skilled in the art in light of this disclosure. As illustrated in a number of embodiments, the network may connect the gateway 1 15 with other gateways (not pictured), which are also in communication with the satellite 105.

The gateway 1 15 provides an interface between the network 120 and the satellite 105. The gateway 1 15 may be configured to receive data and information directed to one or more subscriber terminals 130, and can format the data and information for delivery to the respective destination device via the satellite 105. Similarly, the gateway 1 15 may be configured to receive signals from the satellite 105 (e.g., from one or more subscriber

terminals) directed to a destination in the network 120, and can format the received signals for transmission along the network 120.

A device (not shown) connected to the network 120 may communicate with one or more subscriber terminals, and through the gateway 1 15. Data and information, for example IP datagrams, may be sent from a device in the network 120 to the gateway 1 15. The gateway 1 15 may format a Medium Access Control (MAC) frame in accordance with a physical layer definition for transmission to the satellite 130. A variety of physical layer transmission modulation and coding techniques may be used with certain embodiments of the invention, including those defined with the DVB-S2 and WiMAX standards. The link 135 from the gateway 1 15 to the satellite 105 may be referred to hereinafter as the downstream uplink 135.

The gateway 1 15 may use an antenna 1 10 to transmit the signal to the satellite 105. In one embodiment, the antenna 1 10 comprises a parabolic reflector with high directivity in the direction of the satellite and low directivity in other directions. The antenna 1 10 may comprise a variety of alternative configurations and include operating features such as high isolation between orthogonal polarizations, high efficiency in the operational frequency bands, and low noise.

In one embodiment, a geostationary satellite 105 is configured to receive the signals from the location of antenna 1 10 and within the frequency band and specific polarization transmitted. The satellite 105 may, for example, use a reflector antenna, lens antenna, array antenna, active antenna, or other mechanism known in the art for reception of such signals. The satellite 105 may process the signals received from the gateway 1 15 and forward the signal from the gateway 1 15 containing the MAC frame to one or more subscriber terminals 130. In one embodiment, the satellite 105 operates in a multi-beam mode, transmitting a number of narrow beams each directed at a different region of the earth, allowing for frequency re-use. With such a multibeam satellite 105, there may be any number of different signal switching configurations on the satellite, allowing signals from a single gateway 1 15 to be switched between different spot beams. In one embodiment, the satellite 105 may be configured as a "bent pipe" satellite, wherein the satellite may frequency convert the received carrier signals before retransmitting these signals to their destination, but otherwise perform little or no other processing on the contents of the signals. A variety of physical layer transmission modulation and coding techniques may be used by the satellite 105 in accordance with certain embodiments of the invention, including those defined with the DVB-S2 and WiMAX

standards. For other embodiments a number of configurations are possible (e.g., using LEO satellites, or using a mesh network instead of a star network), as evident to those skilled in the art.

The service signals transmitted from the satellite 105 may be received by one or more subscriber terminals 130, via the respective subscriber antenna 125. In one embodiment, the antenna 125 and terminal 130 together comprise a very small aperture terminal (VSAT), with the antenna 125 measuring approximately 0.6 meters in diameter and having approximately 2 watts of power. In other embodiments, a variety of other types of antennas 125 may be used at the subscriber terminal 130 to receive the signal from the satellite 105. The link 150 from the satellite 105 to the subscriber terminals 130 may be referred to hereinafter as the downstream downlink 150. Each of the subscriber terminals 130 may comprise a single user terminal or, alternatively, comprise a hub or router (not pictured) that is coupled to multiple user terminals. Each subscriber terminal 130 may be connected to consumer premises equipment (CPE) 160 comprising, for example computers, local area networks, Internet appliances, wireless networks, etc.

In one embodiment, a Multi-Frequency Time-Division Multiple Access (MF-TDMA) scheme is used for upstream links 140, 145, allowing efficient streaming of traffic while maintaining flexibility in allocating capacity among each of the subscriber terminals 130. In this embodiment, a number of frequency channels are allocated which may be fixed, or which may be allocated in a more dynamic fashion. A Time Division Multiple Access (TDMA) scheme is also employed in each frequency channel. In this scheme, each frequency channel may be divided into several timeslots that can be assigned to a connection (i.e., a subscriber terminal 130). In other embodiments, one or more of the upstream links 140, 145 may be configured with other schemes, such as Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Code Division Multiple Access (CDMA), or any number of hybrid or other schemes known in the art.

A subscriber terminal, for example 130-a, may transmit data and information to a network 120 destination via the satellite 105. The subscriber terminal 130 transmits the signals via the upstream uplink 145-a to the satellite 105 using the antenna 125-a. A subscriber terminal 130 may transmit the signals according to a variety of physical layer transmission modulation and coding techniques, including those defined with the DVB-S2 and WiMAX standards. In various embodiments, the physical layer techniques may be the same for each of the links

135, 140, 145, 150, or may be different. The link from the satellite 105 to the gateway 1 15 may be referred to hereinafter as the upstream downlink 140.

Turning to FIG. IB, a block diagram is shown illustrating an alternative embodiment of a satellite communication system 100. This communication system 100 may, for example, comprise the system 100 of FIG. 1 A, but is in this instance described with greater particularity. In this embodiment, the gateway 1 15 includes a Satellite Modem Termination System (SMTS), which is based at least in part on the Data-Over-Cable Service Interface Standard (DOCSIS). The SMTS in this embodiment includes a bank of modulators and demodulators for transmitting signals to and receiving signals from subscriber terminals 130. The SMTS in the gateway 1 15 perfoπns the real-time scheduling of the signal traffic through the satellite 105, and provides the interfaces for the connection to the network 120.

In this embodiment, the subscriber terminals 135 use portions of DOCSIS-based modem circuitry, as well. Therefore, DOCSIS-based resource management, protocols, and schedulers may be used by the SMTS for efficient provisioning of messages. DOCSIS-based components may be modified, in various embodiments, to be adapted for use therein. Thus, certain embodiments may utilize certain parts of the DOCSIS specifications, while customizing others.

While a satellite communications system 100 applicable to various embodiments of the invention is broadly set forth above, a particular embodiment of such a system 100 will now be described. In this particular example, approximately 2 gigahertz (GHz) of bandwidth is to be used, comprising four 500 megahertz (MHz) bands of contiguous spectrum. Employment of dual-circular polarization results in usable frequency comprising eight 500 MHz non- overlapping bands with 4 GHz of total usable bandwidth. This particular embodiment employs a multi-beam satellite 105 with physical separation between the gateways 1 15 and subscriber spot beams, and configured to permit reuse of the frequency on the various links 135, 140, 145, 150. A single Traveling Wave Tube Amplifier (TWTA) is used for each service link spot beam on the downstream downlink, and each TWTA is operated at full saturation for maximum efficiency. A single wideband carrier signal, for example using one of the 500 MHz bands of frequency in its entirety, fills the entire bandwidth of the TWTA, thus allowing a minimum number of space hardware elements. Spotbeam size and TWTA power may be optimized to achieve maximum flux density on the earth's surface of -1 18 decibel-watts per meter squared per megahertz (dbW/m ² /MHz). Thus, using approximately 2

bits per second per hertz (bits/s/Hz), there is approximately 1 Gbps of available bandwidth per spot beam.

With reference to FIG. 12A, an embodiment of a forward link distribution system 1200 is shown. The gateway 1 15 is shown coupled to an antenna 1 10, which generates four downstream signals. A single carrier with 500 MHz of spectrum is used for each of the four downstream uplinks 135. In this embodiment, a total of two-frequencies and two polarizations allow four separate downstream uplinks 135 while using only 1 GHz of the spectrum. For example, link A 135-A could be Freq IU (27.5-28.0 GHz) with left-hand polarization, link B 135-B could be Freq IU (27.5-28.0) GHz with right-hand polarization, link C could be Freq 2U (29.5-30 GHz) with left-hand polarization, and link D could be Freq 2U (29.5-30 GHz) with left-hand polarization.

The satellite 105 is functionally depicted as four "bent pipe ^" connections between a feeder and service link. Carrier signals can be changed through the satellite 105 "bent pipe ^" connections along with the orientation of polarization. The satellite 105 converts each downstream uplink 135 signal into a downstream downlink signal 150.

In this embodiment, there are four downstream downlinks 150 that each provides a service link for four spot beams 205. The downstream downlink 150 may change frequency in the bent pipe as is the case in this embodiment. For example, downstream uplink A 135-A changes from a first frequency (i.e., Freq IU) to a second frequency (i.e., Freq ID) through the satellite 105. Other embodiments may also change polarization between the uplink and downlink for a given downstream channel. Some embodiments may use the same polarization and/or frequency for both the uplink and downlink for a given downstream channel.

Referring next to FIG. 12B, an embodiment of a return link distribution system is shown. This embodiment shows four upstream uplinks 145 from four sets of subscriber terminals 125. A "bent pipe ^" satellite 105 takes the upstream uplinks 145, optionally changes carrier frequency and/or polarization (not shown), and then redirects them as upstream downlinks 140 to a spot beam for a gateway 1 15. In this embodiment, the carrier frequency changes between the uplink 145 and the downlink 140, but the polarization remains the same. Because the feeder spot beams to the gateway 1 15 is not in the coverage area of the service beams, the same frequency pairs may be reused for both service links and feeder links.

Turning to FIGs. 2A and 2B, examples of a multi-beam system 200 configured according to various embodiments of the invention are shown. The multi-beam system 200 may, for example, be implemented in the network 100 described in FIGs. I A and I B. Shown are the coverage of a number of feeder and service spot beam regions 225, 205. In this embodiment, a satellite 215 reuses frequency bands by isolating antenna directivity to certain regions of a country (e.g., United States, Canada or Brazil). As shown in FIG. 2A, there is complete geographic exclusivity between the feeder and service spot beams 205, 225. But that is not the case for FIG. 2B where there may in some instances be service spot beam overlap (e.g., 205-c, 205-d, 205-e), while there is no overlap in other areas. However, with overlap, there are certain interference issues that may inhibit frequency band re-use in the overlapping regions. A four color pattern allows avoiding interference even where there is some overlap between neighboring service beams 205.

In this embodiment, the gateway terminals 210 are also shown along with their feeder beams 225. As shown in FIG. 2B, the gateway terminals 210 may be located in a region covered by a service spotbeam (e.g., the first, second and fourth gateways 210-1 , 210-2, 210-4).

However, a gateway may also be located outside of a region covered by a service spotbeam (e.g., the third gateway 210-3). By locating gateway terminals 210 outside of the service spotbeam regions (e.g., the third gateway 210-3), geographic separation is achieved to allow for re-use of the allocated frequencies.

There are often spare gateway terminals 210 in a given feeder spot beam 225. The spare gateway terminal 210-5 can substitute for the primary gateway terminal 210-4 should the primary gateway terminal 210-4 fail to function properly. Additionally, the spare can be used when the primary is impaired by weather.

Referring next to FIG. 8, an embodiment of a downstream channel 800 is shown. The downstream channel 800 includes a series of superframes 804 in succession, where each superframe 804 may have the same size or may vary in size. This embodiment divides a superframe 804 into a number of virtual channels 808(1 -n). The virtual channels 808(1 -n) in each superframe 804 can be the same size or different sizes. The size of the virtual channels 808(1 -n) can change between different superframes 804. Different coding can be optionally used for the various virtual channels 808 (1 -n). In some embodiments, the virtual channels are as short as one symbol in duration.

With reference to FIG. 9, an embodiment of an upstream channel 900 is shown. This embodiment uses MF-TDMA, but other embodiments can use CDMA, OFDM, or other access schemes. The upstream channel 900 has 500 MHz of total bandwidth in one embodiment. The total bandwidth is divided into m frequency sub-channels , which may differ in bandwidth, modulation, coding, etc. and may also vary in time based on system needs.

In this embodiment, each subscriber terminal 130 is given a two-dimensional (2D) map to use for its upstream traffic. The 2D map has a number of entries where each indicates a frequency sub-channel 912 and time segment 908(1-5). For example, one subscriber terminal 130 is allocated sub-channel m 912-m, time segment one 908-1 ; sub-channel two 912-2, time segment two 908-2; sub-channel two 912-2, time segment three 908-3; etc. The 2D map is dynamically adjusted for each subscriber terminal 130 according to anticipated need by a scheduler in the SMTS.

Referring to FIG. 13, an embodiment of a channel diagram is shown. Only the channels for a single feeder spot beam 225 and a single service spot beam 205 are shown, but embodiments include many of each spot beam 225, 205 (e.g., various embodiments could have 60, 80, 100, 120, etc. of each type of spot beam 225, 205). The forward channel 800 includes n virtual channels 808 traveling from the gateway antenna 1 10 to the service spot beam 205. Each subscriber terminal 130 may be allocated one or more of the virtual channels 808. m MF- TDMA channels 912 make up the return channel 900 between the subscriber terminal (ST) antennas 125 and the feeder spot beam 225.

Referring next to FIG. 3, an embodiment of a ground system 300 of gateways 1 15 is shown in block diagram form. One embodiment could have fifteen active gateways 1 15 (and possibly spares) to generate sixty service spot beams, for example. The ground system 300 includes a number of gateways 1 15 respectively coupled to antennas 1 10. All the gateways 1 15 are coupled to a network 120 such as the Internet. The network is used to gather information for the subscriber terminals. Additionally, each SMTS communicates with other SMTS and the Internet using the network 120 or other means not shown.

Each gateway 1 15 includes a transceiver 305, a SMTS 310 and a router 325. The transceiver 305 includes both a transmitter and a receiver. In this embodiment, the transmitter takes a baseband signal and upconverts and amplifies the baseband signal for transmission of the

downstream uplinks 135 with the antenna 1 10. The receiver downconverts and tunes the upstream downlinks 140 along with other processing as explained below. The SMTS 310 processes signals to allow the subscriber terminals to request and receive information and schedules bandwidth for the forward and return channels 800, 900. Additionally, the SMTS 310 provides configuration information and receives status from the subscriber terminals 130. Any requested or returned information is forwarded via the router 325.

With reference to FIG. 11, an embodiment of gateway receiver 1 100 is shown. This embodiment of the receiver 1 100 processes four return channels 900 from four different service spot beams 205. The return channels 900 may be divided among four pathways using antenna polarization and/or filtering 1 104. Each return channel is coupled to a low-noise amplifier (LNA) 1108. Down conversion 11 12 mixes down the signal into its intermediate frequency. Each of the upstream sub-channels 912 is separated from the signal by a number of tuners 1 1 16. Further processing is performed in the SMTS 310.

Referring next to FIG. 10, an embodiment of a gateway transmitter 1000 is shown. The downstream channels 800 are received at their intermediate frequencies from the SMTS 310. With separate pathways, each downstream channel 800 is up-converted 1004 using two different carrier frequencies. A power amplifier 1008 increases the amplitude of the forward channel 900 before coupling to the antenna 1 10. The antenna 110 polarizes the separate signals to keep the four forward channels 800 distinct as they are passed to the satellite 105.

With reference to FIG. 4, an embodiment of a SMTS 310 is shown in block diagram form. Baseband processing is done for the inbound and outbound links 135, 140 by a number of geographically separated gateways 1 15. Each SMTS 310 is generally divided into two sections, specifically, the downstream portion 305 to send information to the satellite 105 and the upstream portion 315 to receive information from the satellite 105.

The downstream portion 305 takes information from the switching fabric 416 through a number of downstream (DS) blades 412. The DS blades 412 are divided among a number of downstream generators 408. This embodiment includes four downstream generators 408, with one for each of the downstream channels 800. For example, this embodiment uses four separate 500 MHz spectrum ranges having different frequencies and/or polarizations. A four- color modulator 436 has a modulator for each respective DS generator 408. The modulated signals are coupled to the transmitter portion 1000 of the transceiver 305 at an intermediate

frequency. Each of the four downstream generators 408 in this embodiment has J virtual DS blades 412.

The upstream portion 315 of the SMTS 310 receives and processes information from the satellite 105 in the baseband intermediate frequency. After the receiver portion 1 100 of the transceiver 305 produces all the sub-channels 912 for the four separate baseband upstream signals, each sub-channel 912 is coupled to a different demodulator 428. Some embodiments could include a switch before the demodulators 428 to allow any return link sub-channel 912 to go to any demodulator 428 to allow dynamic reassignment between the four return channels 908. A number of demodulators are dedicated to an upstream (US) blade 424.

The US blades 424 serve to recover the information received from the satellite 105 before providing it to the switching fabric 416. The US scheduler 430 on each US blade 424 serves to schedule use of the return channel 900 for each subscriber terminal 130. Future needs for the subscriber terminals 130 of a particular return channel 900 can be assessed and bandwidth/latency adjusted accordingly in cooperation with the Resource Manager and Load Balancer (RM/LB) block 420.

The RM/LB block 420 assigns traffic among the US and DS blades. By communication with other RM/LB blocks 420 in other SMTSes 310, each RM/LB block 420 can reassign subscriber terminals 130 and channels 800, 900 to other gateways 115. This reassignment can take place for any number of reasons, for example, lack of resources and/or loading concerns. In this embodiment, the decisions are done in a distributed fashion among the RM/LB blocks 420, but other embodiments could have decisions made by one master MR/LB block or at some other central decision-making authority. Reassignment of subscriber terminals 130 could use overlapping service spot beams 205, for example.

Referring next to FIG. 5, an embodiment of a satellite 105 is shown in block diagram form. The satellite 105 in this embodiment communicates with fifteen gateways 1 15 and all STs

130 using sixty feeder and service spot beams 225, 205. Other embodiments could use more or less gateways/spot beams. Buss power 512 is supplied using a power source such as chemical fuel, nuclear fuel and/or solar energy. A satellite controller 516 is used to maintain attitude and otherwise control the satellite 105. Software updates to the satellite 105 can be uploaded from the gateway 1 15 and performed by the satellite controller 516.

Information passes in two directions through the satellite 105. A downstream translator 508 receives information from the fifteen gateways 1 15 for relay to subscriber terminals 130 using sixty service spot beams 205. An upstream translator 504 receives information from the subscriber terminals 130 occupying the sixty spot beam areas and relays that information to the fifteen gateways 1 15. This embodiment of the satellite can switch carrier frequencies in the downstream or upstream processors 508, 504 in a "bent-pipe" configuration, but other embodiments could do baseband switching between the various forward and return channels 800, 900. The frequencies and polarization for each spot beam 225, 205 could be programmable or preconfϊgured.

With reference to FIG. 6A, an embodiment of an upstream translator 504 is shown in block diagram form. A Receiver and Downconverter (Rx/DC) block 616 receives all the return link information for the area defined by a spot beam 205 as an analog signal before conversion to an intermediate frequency (IF). There is a Rx/DC block 616 for each service spot beam area 205. An IF switch 612 routes a particular baseband signal from a Rx/DC block 616 to a particular upstream downlink channel. The upstream downlink channel is filled using an Upconverter and Traveling Wave Tube Amplifier (UC/TWTA) block 620. The frequency and/or polarization can be changed through this process such that each upstream channel passes through the satellite 105 in a bent pipe fashion.

Each gateway 1 15 has four dedicated UC/TWTA blocks 620 in the upstream translator 504. Two of the four dedicated UC/TWTA blocks 620 operate at a first frequency range and two operate at a second frequency range in this embodiment. Additionally, two use right-hand polarization and two use left-hand polarization. Between the two polarizations and two frequencies, the satellite 105 can communicate with each gateway 1 15 with four separate upstream downlink channels.

Referring next to FIG. 6B, an embodiment of a downstream translator 508 is shown as a block diagram. Each gateway 1 15 has four downstream uplink channels to the satellite 105 by use of two frequency ranges and two polarizations. A Rx/DC block 636 takes the analog signal and converts the signal to an intermediate frequency. There is a Rx/DC block 636 for all sixty downstream uplink channels from the fifteen gateways 1 15. The IF switch 612 connects a particular channel 800 from a gateway 1 15 to a particular service spot beam 205. Each IF signal from the switch 628 is modulated and amplified with a UC/TWTA block 632. An antenna broadcasts the signal using a spot beam to subscriber terminals 130 that occupy

the area of the spot beam. Just as with the upstream translator 504, the downstream translator 508 can change carrier frequency and polarization of a particular downstream channel in a bent-pipe fashion.

FIG. 7 comprises a block diagram illustrating a set of subscriber equipment 700 which may be located at a subscriber location for the reception and transmission of communication signals. Components of this set of subscriber equipment 700 may, for example, comprise the antenna 125, associated subscriber terminal 130 and any consumer premises equipment (CPE) 160, which may be a computer, a network, etc.

An antenna 125 may receive signals from a satellite 105. The antenna 125 may comprise a VSAT antenna, or any of a variety other antenna types (e.g., other parabolic antennas, microstrip antennas, or helical antennas). In some embodiments, the antenna 125 may be configured to dynamically modify its configuration to better receive signals at certain frequency ranges or from certain locations. From the antenna 125, the signals are forwarded (perhaps after some form of processing) to the subscriber terminal 130. The subscriber terminal 130 may include a radio frequency (RF) frontend 705, a controller 715, a virtual channel filter 702, a modulator 725, a demodulator 710, a filter 706, a downstream protocol converter 718, an upstream protocol converter 722, a receive (Rx) buffer 712, and a transmit (Tx) buffer 716.

In this embodiment, the RF frontend 705 has both transmit and receive functions. The receive function includes amplification of the received signals (e.g., with a low noise amplifier (LNA)). This amplified signal is then downconverted (e.g., using a mixer to combine it with a signal from a local oscillator (LO)). This downconverted signal may be amplified again with the RF frontend 705, before processing of the superframe 804 with the virtual channel filter 702. A subset of each superframe 804 is culled from the downstream channel 800 by the virtual channel filter 702, for example, one or more virtual channels 808 are filtered off for further processing.

A variety of modulation and coding techniques may be used at the subscriber terminal 130 for signals received from and transmitted to a satellite. In this embodiment, modulation techniques include BPSK, QPSK, 8PSK, 16APSK, 32PSK. In other embodiments, additional modulation techniques may include ASK, FSK, MFSK, and QAM, as well as a variety of analog techniques. The demodulator 710 may demodulate the down-converted signals,

forwarding the demodulated virtual channel 808 to a filter 706 to strip out the data intended for the particular subscriber terminal 130 from other information in the virtual channel 808.

Once the information destined for the particular subscriber terminal 130 is isolated, a downstream protocol converter 718 translates the protocol used for the satellite link into one that the DOCSIS MAC block 726 uses. Alternative embodiments could use a WiMAX MAC block or a combination DOCSIS/WiMAX block. A Rx buffer 712 is used to convert the high-speed received burst into a lower-speed stream that the DOCSIS MAC block 726 can process. The DOCSIS MAC block 726 is a circuit that receives a DOCSIS stream and manages it for the CPE 160. Tasks such as provisioning, bandwidth management, access control, quality of service, etc. are managed by the DOCSIS MAC block 726. The CPE can often interface with the DOCSIS MAC block 726 using Ethernet, WiFi, USB and/or other standard interfaces. In some embodiments, a WiMax block 726 could be used instead of a DOCSIS MAC block 726 to allow use of the WiMax protocol.

It is also worth noting that while a downstream protocol converter 718 and upstream protocol converter 722 may be used to convert received packets to DOCSIS or WiMax compatible frames for processing by a MAC block 726, these converters will not be necessary in many embodiments. For example, in embodiments where DOCSIS or WiMax based components are not used, the protocol used for the satellite link may also be compatible with the MAC block 726 without such conversions, and the converters 718, 722 may therefore be excluded. Various functions of the subscriber terminal 130 are managed by the controller 715. The controller 715 may oversee a variety of decoding, interleaving, decryption, and unscrambling techniques, as known in the art. The controller may also manage the functions applicable to the signals and exchange of processed data with one or more CPEs 160. The CPE 160 may comprise one or more user terminals, such as personal computers, laptops, or any other computing devices as known in the art.

The controller 715, along with the other components of the subscriber terminal 130, may be implemented in one or more Application Specific Integrated Circuits (ASICs), or a general purpose processor adapted to perform the applicable functions. Alternatively, the functions of the subscriber terminal 130 may be performed by one or more other processing units (or cores), on one or more integrated circuits. In other embodiments, other types of integrated circuits may be used (e.g., Structured/Platform ASICs, Field Programmable Gate Arrays (FPGAs) and other Semi-Custom ICs), which may be programmed in any manner known in

the art. The controller may be programmed to access memory unit (not shown). It may fetch instructions and other data from the memory unit, or write data to the memory-unit.

As noted above, data may also be transmitted from the CPE 160 through the subscriber terminal 130 and up to a satellite 105 in various communication signals. The CPE 160, therefore, may transmit data to DOCSIS MAC block 726 for conversion to the DOCSIS protocol before that protocol is translated with an upstream protocol converter 722. The slow-rate data waits in the Tx buffer 716 until it is burst over the satellite link.

The processed data is then transmitted from the Tx buffer 716 to the modulator 725, where it is modulated using one of the techniques described above. In some embodiments, adaptive or variable coding and modulation techniques may be used in these transmissions. Specifically, different modulation and coding combinations, or "modcodes," may be used for different packets, depending on the signal quality metrics from the antenna 125 to the satellite 105. Other factors, such as network and satellite congestion issues, may be factored into the determination, as well. Signal quality information may be received from the satellite or other sources, and various decisions regarding modcode applicability may be made locally at the controller, or remotely. The RF frontend 705 may then amplify and upconvert the modulated signals for transmission through the antenna 125 to the satellite.