Orthogonal code division multiple access on return link of satellite links

Orthogonal Code Division Multiple Access on the Reverse Link of the Satellite Link

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 60/391,438 filed on June 24, 2002, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION I. Field of the Invention This application generally relates to wireless communication systems, and more particularly to the improvement of the bandwidth utilization rate of the reverse link in a satellite communication system by an access terminal using an orthogonal code division multiple access method in the reverse link Method and device.

II. Background Various satellite communication systems have been developed over the years. An early system structure is called Time Division Multiple Access (TDMA), which is characterized by allocating multiple time slots in a communication channel to each of multiple terminals, and interacting with the terminals that occur in the specially allocated time slots. Communication. An improved system structure is called Code Division Multiple Access (CDMA). CDMA-based systems generally work in the CDMA mode in the forward link (FL) direction, and work in the TDMA mode in the reverse link (RL) direction.

There are two main modes of CDMA, asynchronous CDMA and synchronous orthogonal CDMA. In the asynchronous mode of CDMA operation, the signals from different terminals are not synchronized and therefore cause interference with each other. In the synchronous mode of operation, the transmission signals from different terminals are timed, such as arriving synchronously at the receiver. In the synchronization mode of CDMA, if orthogonal coding is used to distinguish different terminal transmissions, there is no cross interference between different signals received from the terminal. In this case, higher bandwidth efficiency is achieved due to reduced interference. On the forward link, since all signals originate from the same location, the gateway, the orthogonality between different codes is maintained. However, on the reverse link, since there is no synchronization mechanism, signals from different terminals arrive asynchronously at the gateway due to their different propagation delays. What is needed is a method and apparatus for operating an orthogonal CDMA-based reverse link for a satellite communication system.

Summary of the invention

A method for operating an access terminal is disclosed. The method sends a reference signal to a gateway; and receives a message that allocates a time slot, a coding channel for transmission, a transmission time adjustment instruction, A transmit power command, and a data rate command, the transmit power command based at least in part on the location of the access terminal relative to the center of the beam.

In other embodiments, a message may be sent to the gateway indicating the amount of data to be sent from the access terminal; and the transmit power command is based at least in part on the amount of data to be sent from the access terminal. Sending the time adjustment instruction is based at least in part on the position of the access terminal relative to the center of the beam. Moreover, during the time slot specified by the time slot allocation, the message modulated by Walsh coding can be sent to the gateway using the reverse link. The transmission is based on the transmission power specified by the transmission power command and specified by the data rate command. Data rate.

In another embodiment, before transmitting at least a part of the amount of data to be transmitted, the transmission time is adjusted to achieve coding synchronization, and the signal is coded and modulated according to the coding channel allocated to the access terminal, and the transmission power command specifies The power level is transmitted to transmit at least a portion of the amount of data to be transmitted.

In a further aspect, the degradation of the channel condition is detected, for example due to the fading of the electron flow, and the transmission power is increased to supplement the degradation of the channel condition.

In a further embodiment, a method of operating a communication system is disclosed, including receiving a message from an access terminal at a gateway, determining a channel condition between the gateway and the access terminal, and determining the reception of the received message at the gateway Signal strength. Then the timing offset of the received message is determined at the gateway, and the time slot allocation, coding channel allocation, transmission time adjustment command, transmission power command and data rate command are sent to the access terminal.

This can be done or repeated for multiple access terminals; where the time slot allocation sent to each of the multiple access terminals is the same. Therefore, multiple coded and modulated messages can be received during the time slot specified by the time slot allocation, and they can be expanded in parallel.

A receiver is disclosed, which has a downconverter coupled with an A/D converter, the receiver has a plurality of despreaders, each despreader has an input coupled to the A/D converter, each The despreader also has an output terminal coupled with a plurality of data demodulators, and each data demodulator has an input terminal coupled to a corresponding despreader output terminal.

In a further embodiment, each of the plurality of despreaders includes a Walsh despreader, and each of the plurality of Walsh despreaders is coupled to a Walsh encoding source. The Walsh encoding source can be coupled to a bus, and each of the plurality of Walsh despreaders is also coupled to the bus. Each of the plurality of Walsh despreaders includes at least one stored Walsh code.

A gateway is disclosed. The gateway has an encoding source coupled to at least one encoding modulator and coupled to a plurality of despreaders, and an upconverter is coupled to at least one encoding modulator and further coupled to an antenna. The down converter is coupled to the antenna and further coupled to a plurality of despreaders, and the plurality of demodulators are each coupled to a corresponding one of the despreaders. Each of the multiple despreaders may include a Walsh despreader, and the encoding source is an orthogonal Walsh code source. In short, the embodiments of the present invention provide improved bandwidth efficiency; improved ability to overcome electron flow fading or other channel degradation phenomena; reduced transmission power; or various combinations thereof. By advantageously using orthogonal CDMA in the reverse link of a ground-synchronous satellite-based communication system, embodiments of the present invention allow multiple access terminals to transmit concurrently in a beam, with each access terminal having a unique For time slot/coded channel allocation, the power used by the terminal is the same or lower than the combined power used by a single access terminal using TDMA as its access mode. The advantageous use of orthogonal CDMA in the reverse link allows one or more access terminals to transmit with higher transmission power to overcome effects such as channel degradation caused by electron flow fading, each access terminal being at In a common beam and allocated to a common time slot. The ability to allow higher transmission power for a particular terminal will increase the effective data rate of the terminal by enabling higher-order modulation techniques. In this way, even if the peak power requirement of the terminal power amplifier is not increased, the total transmission power on the reverse link can be effectively increased, thereby increasing the total RL throughput for the same terminal peak transmission power as in the TDMA system.

BRIEF DESCRIPTION OF THE DRAWINGS Through the following detailed description in conjunction with the accompanying drawings, the features, properties and advantages of the present invention will become more apparent. The same elements in the accompanying drawings have the same labels, among which: Figure 1 is used to reverse A block diagram representation of a receiver that receives multiple concurrent CDMA transmissions from corresponding multiple terminals on the link.

Figure 2 is a block diagram representation of a transmitter in a terminal suitable for sending messages on the reverse link of an OCDMA satellite communication system.

Fig. 3 is a flowchart showing an illustrative process of the operation of the gateway according to the present invention.

Fig. 4 is a flowchart showing an illustrative process of terminal operation according to the present invention.

Detailed Description Generally speaking, embodiments of the present invention provide improved bandwidth efficiency, improved ability to overcome electron flow fading or other channel degradation phenomena, reduced transmission power, or various combinations thereof. More specifically, by advantageously using orthogonal CDMA in the reverse link of a terrestrial satellite-based communication system, embodiments of the present invention allow multiple access terminals to transmit concurrently in a beam, with each terminal There is a unique time slot/coded channel allocation, and the power used by the terminal is the same as or lower than the combined power used by a single access terminal using TDMA as its access mode. In other embodiments of the present invention, the advantageous use of orthogonal CDMA in the reverse link allows one or more terminals to transmit with higher transmission power to overcome effects such as channel degradation caused by electron flow fading. All are in a common beam and are assigned to a common time slot. Or, the ability to provide higher transmission power may increase the effective data rate of the terminal by enabling higher-order modulation techniques.

In the following description, various aspects of the present invention will be described. However, it will be obvious to those skilled in the art that the present invention can be implemented with only some or all aspects of the present invention. For the purpose of illustration, specific numbers, materials, and configurations are proposed to fully understand the present invention. However, it will be obvious to those skilled in the art that the present invention can also be implemented with other details. In other cases, well-known features are omitted or simplified so as not to obscure the present invention.

Reference to "an embodiment", "an embodiment" or similar expressions herein means that a specific characteristic, structure, operation, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present invention. As such, the appearances of such phrases or expressions do not necessarily all refer to the same embodiment. Moreover, various specific characteristics, structures, operations, or characteristics may be combined in any suitable manner in one or more embodiments.

The term forward link data rate control as used herein refers to the index of the maximum decodable data rate, which is determined based on one or more metrics of the signal-to-noise ratio on the forward link of the satellite system. The maximum data rate that can be decoded by the incoming terminal.

Terminals or user terminals are sometimes also referred to as access terminals, subscriber units, mobile units, mobile stations, or simply as "users", "mobile stations" or "subscribers" in some communication systems according to preferences. These terms are well known in the art.

In the illustrative satellite communication system, the bandwidth available for communication between the access terminal and the gateway is divided into four channels. These four channels are referred to herein as a heartbeat channel, a deterministic reserved channel, a statistical multiplexing channel, and a random access channel. Then pay special attention to how the access terminal uses these channels and describe the functions of these channels.

For the random access channel, the access terminals send their initial packets on the random access channel to activate the physical layer. This initial grouping will include bandwidth requests for deterministic reserved channels and/or statistical multiplexed reserved channels on the reverse link. The random access channel can also be used by the access terminal to send short packets to avoid the delay associated with requesting to reserve bandwidth on the channel. The random access channel is an asynchronous CDMA channel.

The deterministic reserved channel is divided into multiple time slots. In various embodiments, the deterministic reserved channel is also divided into coded channels. The access terminal obtains a slot/code channel reservation on the deterministic reservation channel by explicitly making a request for bandwidth. By sending a channel allocation message to the access terminal on the forward link (FL), the gateway can grant this request. In this way, a unique time slot/coded channel allocation will be provided to each of the many access terminals.

The power channel is divided into short time slots/coded channels. Each access terminal is assigned a time slot/coded channel on the power channel. The power channel provides a communication path that helps determine the channel adjustment between the gateway and each access terminal. First, the power channel provides a feedback channel through which access terminals can send their channel adjustments to the gateway. Secondly, the power channel also provides a reference signal through which the gateway can measure the signal-to-noise ratio received on the reverse link. By measuring the received signal-to-noise ratio on the reverse link, the gateway can determine the maximum data rate that the access terminal can send on the reverse link with a given transmit power. Furthermore, the access terminals measure the received signal-to-noise ratio on the forward link and determine the maximum data rate at which they can decode correctly.

The access terminal sends the index of the maximum decodable data rate to the gateway on the power channel, also known as FL-DRC (forward link data rate control). The gateway sends data to the access terminal at the data rate specified in the message sent by the access channel on the power channel. Therefore, the power channel provides feedback to the gateway regarding channel adjustments currently available to the access terminal. The data sequence sent on the power channel time slot/code channel is also used by the gateway to estimate the time offset between the access terminal and the gateway. As described below, this time offset estimation is used to adjust the transmission time required for the access terminal to achieve code synchronization between reverse link transmissions from different access terminals. The gateway can also use the signal on the power channel to estimate the received signal strength (RSS) on the reverse link so that each access terminal can provide a received signal strength indicator (RSSI). The transmission power and data rate of the access terminal is determined based at least in part on RSSI information.

The bandwidth on the STATMUX channel is divided into multiple orthogonal coding channels. Each coded channel is allocated to an active terminal within a specific duration. A variety of ways can be used to specify the length of time the coded channel is allocated to a specific terminal. In an embodiment, the terminal maintains the coded channel until the coded channel has been idle for a time exceeding the preselected time interval, at which time the coded channel is released to the gateway so as to be allocated to another terminal.

One type of code suitable for generating reserved channels for statistical multiplexing is the Walsh code. At least one unique Walsh code is assigned to each of multiple access terminals that are allowed to transmit substantially simultaneously. That is, each active access terminal is assigned a Walsh code on the reserved channel of statistical multiplexing. Maintain statistically multiplexed reserved channel coding channel allocations for access terminals that "actively" exchange data with the gateway. In other words, each active access terminal is allocated a specific amount of bandwidth on the statistical multiplexing reserved channel of the reverse link. Note that some access terminals may not fully utilize their allocated bandwidth on the statistically multiplexed reserved channel. Therefore, some bandwidth on the statistical multiplexing reserved channel is wasted. However, as described below, according to the present invention, the statistical characteristics of the bandwidth utilization rate on the statistical multiplexing reserved channel can be used to improve the efficiency of the statistical multiplexing reserved channel.

Note that Orthogonal CDMA is an access method. It is known and used for the forward link of satellite communication systems, but not for the reverse link. However, a method and apparatus for using orthogonal CDMA in the reverse link are disclosed in U.S. Patent Provisional Application No. 60/391,437 filed on June 24, 2002, which is entitled "Orthogonal CDMA In Return Direction (Reverse Orthogonal CDMA)", incorporated herein by reference and assigned to the assignee of this application. Generally, the transmission of the access terminal on the reverse link of the relatively stationary satellite system is synchronized by the control signaling sent by the gateway to the access terminal. The gateway uses the transmission from the access terminal on the reverse link to determine the time offset that each access terminal must introduce before orthogonal CDMA transmission, so that the signals from all access terminals arrive at the gateway synchronously with Walsh codes.

In one embodiment, the gateway uses the data sequence on the power channel to estimate the time offset between the received sequence and the expected reference time. In addition to timing synchronization (ie, code phase synchronization), the frequency offset between the transmitter of the access terminal and the receiver of the gateway must also be small enough so that the phase change of the signal in an orthogonal coding period is negligible. In a method for achieving frequency synchronization between an access terminal and a gateway, the access terminal adjusts the frequency of its oscillator to lock to the frequency of the signal received from the gateway.

Note that the aforementioned synchronization mechanism is designed to synchronize the arrival of signals at the Walsh code boundary, where those signals are initiated by different access terminals. Once synchronization between different access terminals is achieved on the reverse link, Walsh codes can be used to multiplex data from multiple users. Each access terminal is assigned a specific Walsh code on the reverse link. Since the data transmission time of different access terminals on the reverse link is adjusted so that the Walsh codes arrive at the gateway synchronously, the orthogonality between different access terminal codes can be maintained at the receiver on the reverse link.

Note that compared with TDMA, greater bandwidth efficiency can be obtained by using orthogonal CDMA. In an embodiment, as described below, the reverse link bandwidth efficiency of the deterministic reserved channel can be improved by using OCDMA compared with the TDMA method.

First, consider an illustrative example of reservation-based multiple access technology, in which each access terminal is allocated a dedicated time slot on the reverse link in a TDMA manner. Access terminals send their data in specific time slots allocated to them by the gateway. The gateway can use the forward link control channel to send the reserved vector to the access terminal. During the reverse link time slot, the gateway can determine the channel conditions seen by the access terminal and make an RSSI estimate based at least in part on one or more reference signals sent by the access terminal to the gateway. This RSSI is used to generate rate and power control information for the access terminal. Then, the access terminal will transmit with the maximum power and the maximum data rate at which the gateway can correctly decode the data received from the access terminal. The data rate that an access terminal can transmit depends at least in part on the location of the access terminal in a given beam. In a more specific illustrative example, the gateway measures the signal strength received from each access terminal on the power channel, and based at least in part on this measurement, determines that the access terminal can operate in the reverse chain at a given power level. The maximum rate of transmission on the road. Next, the gateway sends information called reverse link data rate control (reverse link-DRC) to the access terminal through the forward link. Access terminals transmit at the data rate specified by the reverse link-DRC sent to them by the gateway.

The access terminal at the center of the beam will transmit at the highest prescribed rate because the access terminal at the center of the beam has the highest antenna gain. However, the access terminal at the edge of the beam has a lower antenna gain (4dB lower in this illustrative example), and the signal power it receives at the gateway is lower. Therefore, access terminals at the edge of the beam are required to transmit their data at a lower rate. In other words, the bandwidth efficiency of an access terminal depends on its position in the beam. The access terminal at the center of the beam has the highest bandwidth efficiency, and the access terminal at the edge of the beam has the lowest bandwidth efficiency. In order to provide equal data rates to all access terminals under these conditions, more time must be allocated to the access terminals far from the beam center, especially for the access terminals located at the edge of the beam (that is, the farthest from the beam center). Unfortunately, allocating more time to those access terminals that are far from the center of the beam reduces the overall throughput of the system. For example, an access terminal at the center of the beam will transmit with a bandwidth efficiency of 1.5 bits/sec/Hz, while an access terminal at the edge of the beam will transmit with a bandwidth efficiency of about 0.75 bits/sec/Hz, because the gateway receives the signal The power is about 4dB lower than the signal power of the access terminal in the center of the beam. According to the present invention, by using OCDMA, the bandwidth efficiency of the access terminal at the edge of the beam can be improved, so that it generally matches the bandwidth efficiency of the access terminal at the center of the beam, that is, 1.5 bits/sec/Hz.

For the OCDMA reverse link according to the present invention, L users are each assigned a unique Walsh code. Each user sends a data modulation symbol during each Walsh code interval. In other words, each access terminal repeats a data modulation symbol L times within its assigned Walsh code period, resulting in a processing gain L. Let (Eb/Nt)TDMA denote the energy per bit measured in a TDMA-based system for a given coding and modulation scheme. Therefore, if each access terminal in the OCDMA system transmits with its maximum available power, the Eb/Nt received on the OCDMA channel of an access terminal on the reverse link is determined by (Eb/Nt)OCDMA Expressed as follows: (Eb/Nt)OCDMA=L(Eb/Nt)TDMA In other words, for the same transmission power at the access terminal, due to the processing gain of OCDMA, the achievable Eb/Nt on the OCDMA channel is the TDMA channel L times that can be achieved. Therefore, in the case of OCDMA, higher-order modulation can be used, thereby achieving higher bandwidth efficiency than the TDMA method. Note that in OCDMA, each access terminal effectively has 1/L of the bandwidth of one access terminal in a TDMA system. That is, for the same modulation/coding selection, the data rate on an OCDMA channel is L times lower than that on a TDMA channel. However, it can be seen from the above discussion that, since a higher order modulation/coding scheme can be used for the OCDMA access terminal for the same transmission power, the bandwidth efficiency of OCDMA is higher.

Note that if 1/L of the maximum available transmission power is transmitted in the case of OCDMA, the Eb/Nt achieved by the OCDMA access terminal will be the same as that achieved by the TDMA method. In practice, the transmit power of an OCDMA access terminal with a code length of L can be selected: the transmit power is made to be between the maximum available transmit power and 1/L of the maximum available transmit power. According to the described embodiment, by selecting the transmit power, the bandwidth efficiency of the access terminal is selected accordingly. For example, an access terminal in the center of the beam will transmit at 1/L of the maximum available transmit power. Since the Eb/Nt achieved by them has not changed compared with the TDMA system, they still have the highest bandwidth efficiency (in this illustrative embodiment). The middle is 1.5 bits/sec/Hz) to send data. The access terminal at the edge of the beam will select a transmit power between the maximum available transmit power and 1/L of the maximum available transmit power.

In an embodiment, the transmit power level is selected so that the Eb/Nt achieved by the access terminal at the edge of the beam is still high enough to support the maximum modulation/coding rate and the same bandwidth efficiency as the access terminal at the center of the beam. Therefore, all access terminals that use orthogonal Walsh codes to transmit in parallel will transmit with the maximum available modulation/coding (ie, maximum bandwidth efficiency), which will increase the overall beam bandwidth efficiency. Note that the access terminal in the center of the beam will transmit at a power higher than 1/L of its maximum available transmit power, and increase its modulation/coding rate above the rate feasible in the case of TDMA. Therefore, on the reserved channel, the bandwidth efficiency of the OCDMA system is higher than that of the TDMA system.

As an example of how to make the bandwidth efficiency of OCDMA higher than that of a TDMA system, consider a system where the Eb/Nt at the center of the beam is X dB and at the edge of the beam it is X-3 dB. Assume that the data rate of the access terminal at the center of the beam is R, and the data rate of the access terminal at the edge of the beam is R/2. For the same level of service scheduler, where each access terminal has enough time for all access terminals to achieve the same average data rate, the average throughput of the two access terminals will be R/1.5. So the bandwidth efficiency of the TDMA system is (R/1.5)/W. Now consider an OCDMA system in which two access terminals at the center of the beam and the edge of the beam each obtain an orthogonal code of length 2. Therefore, if the access terminal at the beam edge and the access terminal in the TDMA system transmit at the same power, the Eb/Nt achieved for the OCDMA access terminal at the beam edge will be X dB due to the processing gain. If the access terminal in the beam center transmits with half the power of the access terminal in the TDMA system, the Eb/Nt of the access terminal in the beam center will also be X dB. Therefore, in the OCDMA system, both access terminals can transmit with the same bandwidth efficiency as the access terminal in the beam center of the TDMA system. Therefore, the bandwidth efficiency in the OCDMA system in this example is R/W, which is 1.5 times the bandwidth efficiency of the TDMA system. Note that in this example, the peak rate of the OCDMA access terminal is lower than the peak rate of the TDMA system. By allowing the access terminal in the center of the beam to also transmit at the maximum available rate, the bandwidth efficiency of the above-mentioned OCDMA example can be further improved. In this case, the achieved Eb/Nt will be X+3 dB. Therefore, the access terminal will transmit at a rate greater than 2 (e.g., up to 2R). Note that if an orthogonal code with a length greater than 2 is used, the processing gain of each OCDMA access terminal will be higher, resulting in a higher realized Eb/Nt for the OCDMA access terminal. In this case, a higher order signal set and coding rate can be used, which in turn allows for higher bandwidth efficiency in the OCDMA system according to the present invention.

Note that there are also restrictions on the transmission power of the OCDMA terminal and the size of the modulated signal set used. Because higher-order modulation is used, due to the higher peak-to-average ratio of the modulated signal set, more back-offs are required in the transmit power of the amplifier. In one embodiment, the total transmission power from all OCDMA terminals on the uplink is limited to a threshold that does not exceed the allowable interference level to adjacent satellites.

Another advantage of CDMA is its interference averaging capability. The data rate that the access terminal can send on the reverse link depends on the Eb/Nt that will be received from the access terminal at the gateway. In order to determine the data rate used by the access terminal to transmit, the gateway predicts the Eb/Nt received from the access terminal and selects the highest rate that can be correctly decoded under this Eb/Nt. The gateway notifies the access terminal of the determined reverse link data rate. The received Eb/Nt is of course dependent on the co-channel interference from access terminals transmitting simultaneously in the co-channel beam. Therefore, in order to make an accurate prediction of the received Eb/Nt, it is necessary to be able to make an accurate prediction of co-channel interference. However, in TDMA systems, the variation of co-channel interference is usually very large because there are a small amount of TDMA co-channel interference signals. Therefore, in order to ensure that the packet can be decoded correctly, a margin must be included in the Eb/Nt prediction to compensate for changes in co-channel interference. In order to provide this margin, a backoff in the achievable data rate is required, and a corresponding reduction in throughput derived from it. In a CDMA system, due to the statistical multiplexing of a large number of access terminals, the interference from different access terminals will be averaged. Therefore, the change in interference will be smaller in a CDMA system. As a result, the required backoff is lower than the data rate in a CDMA system, which in turn leads to higher capacity.

Since the access terminal sends a reservation request message to the gateway and the time required for the gateway to grant the request, allocating reservations to the access terminal for reverse link transmission increases the delay before data transmission. This delay is especially undesirable when sending short packets. In order to avoid this increased delay, short messages may be sent on the random access channel without previous reservations. A commonly used random access channel is the Aloha scheme based on TDMA. The disadvantage of the TDMA-based Aloha scheme is its low spectral efficiency. Another random access scheme is Aloha based on CDMA, which has higher bandwidth efficiency and lower delay than the Aloha scheme based on TDMA. As described below, OCDMA can be designed to improve reserved bandwidth efficiency based on the random access part of the reverse link. Next, a statistical multiplexing reserved channel will be described, which can replace the random access channel to send short packets.

In an illustrative embodiment consistent with the present invention, the gateway allocates a coded channel to each "active" access terminal on a statistically multiplexed reserved channel. As described above, the coded channel is allocated to an active terminal within a specified duration. The duration may be predetermined or set to expire when the coding channel has been inactive for more than a certain time interval. OCDMA is effectively configured to send short messages without requiring the access terminal to make an explicit reservation request for reverse link bandwidth from the gateway. If the access terminal has a short packet to send (for example, an acknowledgement), it will use the Walsh code assigned to it to send the packet, and it will send it at its predetermined power level and data rate. However, the inefficient source in this scheme causes a certain number of coding channels to be kept idle. Since the access channel may not have any data to be sent in certain time periods, these idle coding channels exist. However, if an access terminal that does transmit is allowed to transmit at a higher power level and higher bandwidth efficiency, statistical multiplexing can be used to improve bandwidth efficiency.

In an example of how the statistical multiplexing reserved channel operates, assume that there are a total of 2500 access terminals, and at a given point in time, 10% of the access terminals are active. Thus, each active access terminal can be assigned a Walsh code of length 256, for example. The modulation/coding scheme (and data rate) used by the access terminal to send its data will depend on the amount of data it needs to send. If it has very little data to send, it will choose a low data rate modulation/coding scheme and therefore send its data at a relatively low power level. Other access terminals with more data to transmit will use higher order modulation/coding schemes and will transmit with higher power. In other words, each access terminal will adjust its data rate and power level according to its data rate requirements to achieve a specific delay.

Assume that x% of access terminals will use the time slots they are allocated. Therefore, since multiple access terminals are allowed to transmit using orthogonal codes during each time slot, the actual amount of power generated by all access terminals in a beam on the reverse link will be the most viable transmission power. About x%. If the number of assigned orthogonal codes is L, each terminal will transmit between 1/L of its maximum available power and the maximum available power. Ideally, one would expect the terminals to transmit at their maximum available transmit power. However, the total transmit power on the uplink from all transmitting terminals must not exceed a specific upper limit so as not to exceed the level of interference to adjacent satellites. Another consideration that limits the transmit power from each active terminal is the power available at the satellite on the downlink. It must be ensured that the total transmit power from all active terminals does not exceed the transmit power limit available in the satellite on the downlink of the reverse link. If the average percentage of the coded channel used is known, the bandwidth can be effectively used by allowing access terminals using its coded channel to transmit with higher power and higher modulation/coding.

For example, if half of the access terminals are expected to use their assigned Walsh codes on average, only half of the power will be generated on the uplink of the reverse link. Therefore, each access terminal will be allowed to transmit at twice its nominal transmit power and with higher bandwidth efficiency. In this case, only half of the access terminals using their Walsh codes will not result in a loss of bandwidth efficiency, because by allowing access terminals to output at higher power and transmit at higher modulation/coding rates, Improve the bandwidth efficiency of the access terminal. This results in compensating the statistical multiplexing gain of the unused coding channel.

Note that when the number of Walsh codes is large, such as greater than eight, the access terminal is allowed to transmit at a higher power, which can further improve the bandwidth efficiency of the OCDMA-based statistical multiplexing reserved channel. Assume that eight Walsh codes are allocated. For the same bandwidth efficiency, each access terminal in the OCDMA system will need L times the time to transmit its data, but it will also transmit at a data rate of 1/L. However, in this case, each access terminal will transmit at a power output level that is 9 dB lower than the power output level required by a comparable TDMA access terminal. As mentioned above, if each OCDMA access terminal is allowed to transmit at their maximum power, the Eb/Nt achieved for each access terminal on the reverse link will be 9dB higher, which in turn allows higher Modulation/coding scheme and much higher bandwidth efficiency. Of course, there will also be restrictions on the transmission power of OCDMA access terminals. Such restrictions will be determined by factors that are not limited to the following: such as adjacent satellite interference, the available downlink power in the satellite on the reverse link, and Cumulative noise/intermodulation interference generated by all access terminals on the uplink.

Figure 1 is a block diagram representation of a receiver for receiving multiple concurrent CDMA transmissions from corresponding multiple terminals on the reverse link. The receiver of FIG. 1 includes an antenna 102 coupled to a down converter 104. The down converter obtains the RF signal and reduces the frequency. Various techniques for down-conversion are known, but will not be detailed here. The output of the down converter is coupled to an analog-to-digital (A/D) converter 106, which converts the analog signal into a corresponding signal in the digital domain. The output of the analog-to-digital converter 106 is coupled to each of the plurality of despreaders 108a, 108b, 108n. There is no specific limit to the number of despreaders. However, in some embodiments, the number of despreaders is equal to the number of coded modulated (ie CDMA) signals that the gateway may receive from the access terminal in any particular time slot . The encoding source 110 is also coupled to each despreader 108a, 108b, 108n. The encoding source 110 provides the despreader with the encoding required for despreading the incoming CDMA signal. The output terminal of each despreader 108a, 108b, 108n is respectively coupled to an input terminal and a data demodulator 112a, 112b, 112n.

According to the present invention, this structure is used here because multiple access terminals are sending OCDMA signals to the gateway in parallel.

Figure 2 is a block diagram representation of an in-terminal transmitter suitable for sending messages in the reverse link of an OCDMA satellite communication system. In previous systems, the access terminal received the CDMA signal from the gateway, but used TDMA to communicate in the reverse link. The transmitter shown in FIG. 2 is not only suitable for transmitting CDMA signals, but also adjusts its transmission power as described above. The transmitter includes a data modulator 202 that modulates a baseband signal and a code modulator 204. The code modulator 204 further modulates the signal to be transmitted according to the code channel allocated to the specific access terminal. In this illustrative example, the output of code modulator 204 is upconverted by a series of mixers 206. Any suitable up-conversion device can be used. The final transmitter circuit 208 determines the transmission power according to the control signal received from the power control unit 212. The power control unit 212 is coupled to receive control information from the power control parameter storage unit 214. Transmitter power control is a function of the transmission power command received by the access terminal from the gateway, and the access terminal itself determines the effect of channel degradation. By measuring the signal strength on the forward link, the terminal estimates any change in signal strength on the forward link, such as fading due to electron flow. Whenever the signal strength on the forward link changes, the terminal estimates the corresponding signal strength change on the reverse link, and adjusts the data rate and/or transmit power on the reverse link accordingly. In one embodiment, the terminal uses a calibration table to determine the reverse link signal strength change based on the forward link signal change.

Figure 3 is a flowchart showing an illustrative process of gateway operation according to an embodiment. The illustrative process includes receiving a message from the access terminal at the gateway in step 302, and determining the channel condition between the gateway and the access terminal therefrom. The access terminal will use any appropriate means of contacting the gateway for this initial message. The method further includes: determining the received signal strength of the message received in step 302 at the gateway in step 304. An operation is also performed for determining the timing offset of the received message at the gateway in step 306. Determining this offset is one aspect of establishing OCDMA in the reverse link. The illustrative method also includes sending a time slot allocation, a coded channel allocation, a transmission time adjustment command, a transmission power command, and a data rate command to the access terminal in step 308.

Figure 4 is a flowchart according to one embodiment of an illustrative process showing terminal operation. The illustrative process includes sending a reference signal from the access terminal to the gateway in step 402. Subsequently, in accordance with the method of FIG. 4, the following operations are performed: in step 404, a message is received at the access terminal, a time slot is allocated, a coding channel for transmission, a transmission time adjustment command, a transmission power command, and a Data rate command. The time slot allocation informs the access terminal about the time it wants to send, and the coding channel command informs the access terminal about how to encode and send data corresponding to OCDMA. Sending time adjustment commands allows the access terminal to advance or delay its transmission time in order to maintain the required degree of code synchronization for OCDMA in the reverse link. The transmit power and data rate commands inform the access terminal about how it should operate in order to achieve the desired effective data rate.

Conclusion The method and apparatus according to the present invention use orthogonal CDMA in the reverse link of a satellite communication system to advantageously provide a larger transmit power margin to compensate for channel degradation effects such as electron flow fading. Similarly, by allowing higher bandwidth efficiency, the embodiments of the present invention allow access terminals far away from the beam center to transmit at higher power levels, thereby allowing higher order modulation schemes, which in turn allows the beam center Multiple terminals have the same average data rate regardless of their position relative to the beam center.

It should be understood that the present invention is not limited to the above-mentioned embodiments, but includes any and all embodiments within the scope of the claims.