Patent Grant
7940790
In many applications, a communication medium is shared among a number of nodes. The nodes compete with one another for access to the shared communication medium. At any given moment, there may be more than one of the nodes that wish to transmit data over the shared communication medium. A system is typically put in place to facilitate access to the shared communication medium by the various nodes. Various categories of such multiple access systems have been developed.
One category of multiple access systems utilizes contention protocols. Examples of these contention protocols include the ALOHA protocol and the slotted ALOHA protocol, which are known in the art. Here, each node is allowed to freely transmit its data over the shared communication medium at any time or any slotted time. In a system employing a hub, each node sends its transmission to the hub, which then broadcasts the transmission to all nodes. In a system without a hub, each node directly broadcasts its transmission to all nodes. In either case, every node listens to the channel for its own transmission and attempts to receive it. If a node is unsuccessful in receiving its own transmission, the node can assume that its transmission was involved in a collision with another transmission, and the node simply re-transmits its data after waiting a random amount of time. In this manner, collisions are allowed to occur but are resolved by the nodes.
Another category of multiple access systems utilizes carrier sense protocols. Examples include persistent carrier sense multiple access (persistent CSMA) and non-persistent carrier sense multiple access (non-persistent CSMA) protocols, which are known in the art. Generally speaking, these protocols require each node to listen to the shared communication medium before transmitting. Only if the shared communication medium is available is the node allowed to transmit its data. In persistent CSMA, when a node senses that the shared communication medium is not available, the node continually listens to the shared communication medium and attempts to transmit as soon as the medium becomes available. In non-persistent CSMA, when a node senses that the shared communication medium is not available, the node waits an amount of time before attempting to listen to the shared communication channel for an opportunity to transmit. Even though a node listens first before transmitting, there still exists a probability for collisions. This is because when the medium is available, two or more nodes can detect the availability and decide that they are going to transmit data. Various techniques have been developed to handle such collisions.
Yet another category of multiple access systems utilizes contention free protocols. Here, each node can reserve the shared communication medium in order to transmit data. The node can transmit data without colliding with transmissions from other nodes. This is because the shared communication medium is reserved, for a particular time duration for example, for the node's transmission and not for any other transmission. A significant advantage of contention free protocols is that the communication medium is not taken up by unsuccessful transmissions that collide with one another and the resulting re-transmission attempts. This can lead to a more efficient use of the shared communication medium, especially as the number of nodes and number of data transmissions increase.
However, contention free protocols require a reservation process that allows nodes to reserve use of the shared communication medium. Making such reservations also requires communications, which are also susceptible to collisions. Collisions within the reservation process can lead to collisions during subsequent data transmission. Thus, to take full advantage of the benefits of contention free access, more efficient systems for reservation of the shared communication medium are needed.
The present invention relates to methods and apparatuses for communicating using a shared communication medium. A request is sent from a first node in a plurality of nodes over the shared communication medium. The shared communication medium is organized to include a request signal space and a scheduled transmission signal space. The request signal space includes a plurality of request segments each having a different location within the request signal space. The scheduled transmission signal space includes a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space. The request occupies more than one request segment from the plurality of request segments. Each of the more than one request segments is randomly selected by the first node. An assignment is received for assigning the request to a transmission opportunity in the scheduled transmission signal space. The assignment is made in response to detecting at least one of the more than one request segments. From the first node, a data transmission is sent using the assigned transmission opportunity in the scheduled transmission signal space.
The request signal space may comprise a plurality of request intervals with the plurality of request segments organized within the plurality of request intervals. Each of the more than one request segments is selected from a different one of the plurality of request intervals.
The plurality of request intervals may be organized based on an expected loading of the shared communication medium.
The plurality of request intervals and the plurality of request segments may be organized based on a collision probability of the request.
The request signal space may be organized based on code division multiplexing, where each of the more than one request segments is randomly selected by selecting a code word.
The present invention further relates to methods and apparatuses for communicating using a shared communication medium involving a plurality of nodes including a first node and a second node. At the second node, a request is received from the first node over the shared communication medium. The shared communication medium is organized to include a request signal space and a scheduled transmission signal space. The request signal space includes a plurality of request segments each having a different location within the request signal space. The scheduled transmission signal space includes a plurality of scheduled transmission segments each having a different location within the scheduled transmission signal space. The request occupies more than one request segment from the plurality of request segments. Each of the more than one request segments is randomly selected by the first node. At the second node, an assignment is sent for assigning the request to a transmission opportunity in the scheduled transmission signal space. The assignment is made in response to detecting at least one of the more than one request segments. At the second node, a data transmission is received that uses the assigned transmission opportunity in the scheduled transmission signal space.
The present invention relates to communications conducted over a shared communications medium involving a plurality of nodes. The invention is specifically related to techniques employed for requesting opportunities for scheduled transmissions.
Referring to
Furthermore, embodiments of the present invention may be implemented in different network topologies that involve a shared communication medium. These may include star topologies, mesh topologies, bus topologies, and others.
Scheduler node 102 provides control over access to the shared communication medium by access nodes 104, 106, 108, and 110. In order to transmit data over the shared communication medium, an access node, such as access nodes 104, 106, 108, and 110, first sends a request to scheduler node 102. In response, scheduler node 102 assigns an opportunity for data transmission to the access node. Scheduler node 102 sends an assignment message associated with the assignment to the access nodes. Upon receiving the assignment, the access node that made the request can transmit data in the assigned transmission opportunity. This general scheme of request, assignment, and transmission is used in various embodiments of the invention. However, other embodiments of the invention may involve variations and different operations.
For example,
A network may also operate under a “hybrid mode,” which is described below. Referring back to
Alternatively, a system may contain a mixture of access nodes operating under different assignment modes. Some of the access nodes in the system may operate under a “scheduler mode.” Some of the access nodes in the system may operate under a “no scheduler mode.” Finally, some of the access nodes in the system may operate under a “hybrid mode.”
Symbols
Generally speaking, a basic unit of data transmission is referred to here as a “symbol.” A symbol can be defined to have one out of a number of possible values. For example, a binary symbol may have one of two possible values, such as “0” and “1.” Thus, a sequence of N binary symbols may convey 2N possible messages. More generally speaking, an M-ary symbol may have M possible values. Thus, a sequence of N M-ary symbols may convey MN possible messages.
The concept of symbol and the methods by which a symbol can assume values is quite general. In many applications, a symbol is associated with a defined baseband pulse shape which is up-converted to a carrier frequency with a particular phase relationship to the carrier and with a particular amplitude. The amplitude and/or phase of the symbol is known as the modulation and carries the information of a symbol. The set of permissible modulation points defined in the amplitude and phase plane is known as the modulation constellation. The amount of information that a symbol may convey is related to the number of discrete points of the constellation. 16-QAM is an example of an amplitude-phase constellation which allows transmission of up to 4 bits of information per symbol. In some applications, only the phase is used for modulation. Quadra-phase shift keying (QPSK) is an example of pure phase modulation which allows transmission of up to 2 bits of information per symbol. In other applications, the symbol waveform may be defined such that symbol phase may either not exist or be difficult to receive accurately, in which case pure amplitude modulation can be used. One example of binary amplitude modulation is on-off amplitude-shift keying modulation which allows transmission of up to 1 bit of information per symbol.
Each symbol may occupy a particular portion of the relevant signal space. Specifically, each symbol may be said to occupy a certain amount of “time-bandwidth product.” Here, an amount of time-bandwidth product is a scalar quantity that may be measured in units of Hz-seconds and does not necessarily dictate how the signal is distributed within the signal space. In theory, symbols cannot be strictly limited in both time and frequency. It is customary, however, to define the time-bandwidth product of a signal to be the time-bandwidth product of the region in which the preponderance of signal energy resides. Since precise definitions of time-bandwidth product vary somewhat throughout the literature, figures showing symbol boundaries in time-frequency space should only be considered as approximate representations.
Just as a simple example, a signal spanning a bandwidth of 1 Hz and lasting a duration of 1 second may have a time-bandwidth product of 1 Hz-second. A signal spanning a bandwidth of 0.5 Hz and lasting a duration of 2 seconds may also have a time-bandwidth product of 1 Hz-second. Similarly, a signal spanning a bandwidth of 0.1 Hz and lasting a duration of 10 seconds may also have a time-bandwidth product of 1 Hz-second. These examples do not assume any multiplexing of the signal space, which is discussed separately below. Also, the particular values used in these and other examples described herein are for illustrative purpose only. Different values may be used in actual systems.
The measurement of a symbol in terms of an amount of time-bandwidth product is also applicable when different signal space multiplexing techniques are employed. Such techniques may include time-division multiplexing, frequency-division multiplexing, wavelet-division multiplexing, code-division multiplexing, and/or others. In each of the following four examples, a symbol occupies a time-bandwidth product of 1 Hz-second, even though different signal space multiplexing techniques are used.
In a first example,
In a second example,
In a third example,
In a fourth example, a code division multiplexing scheme is applied to a frequency channel having a bandwidth of 32 Hz over a duration of 1 second. For this example, it is assumed that there are 32 different orthogonal code words, each comprising a unique 32-chip binary pattern. Each code word represents a unique “code channel.” To send a symbol on a particular code channel, the symbol value is used to modulate the code word associated with the code channel, and the resulting signal is sent. In the case of binary phase shift keying (BPSK) symbols, for instance, a symbol having a value of “1” may be sent by simply sending the code word, and a symbol having a value of “0” may be sent by sending the inverted version (180-degree phase shift) of the code word. The 32 symbols sent using 32 different “code channels” are non-interfering, and as a group, they occupy a common 32 Hz by 1 second portion of the time-frequency space. In this example, each symbol has an effective time-bandwidth product of 1 Hz-second.
Symbol-Level Request
Referring back to
The use of a symbol-level request allows for highly efficient utilization of the available signal space. Because of its compact size, a symbol-level request may not have sufficient capacity to carry a significant data payload. However information may be conveyed in the choice of the location within the request signal space in which the symbol-level request is transmitted. Thus, the existence of a symbol-level request in the request signal space, as well as the location where the symbol-level request exists in the request signal space, can convey important information that is used to facilitate the assignment of transmission opportunities within the shared communication medium.
Request Signal Space and Scheduled Transmission Signal Space
The shared communication medium utilized by access nodes 104, 106, 108, and 110 may be organized into a request signal space and a scheduled transmission signal space. Just as an example, the shared communication medium may be implemented as a satellite “return-link” that allows signals to be sent from access nodes 104, 106, 108, and 110 to scheduler node 102.
The request signal space may be used by access nodes 104, 106, 108, and 110 to send requests—e.g., symbol-level requests—to request opportunities for the scheduled transmission of data. Specifically, the request signal space may be organized into a plurality of request segments. Each request segment generally refers to a portion of request signal space that may be used for sending a request.
The scheduled transmission signal space may be used by access nodes 104, 106, 108, and 110 to transmit data once requests for transmission have been granted. The scheduled transmission signal space may be organized into a plurality of scheduled transmission segments. Each scheduled transmission segment generally refers to a portion of the scheduled transmission signal space that may be used for sending a data transmission.
The request signal space, as well as the scheduled transmission signal space, may be organized based on various multiplexing techniques. Thus, the plurality of request segments in the request signal space may represent allotments defined based on one or more types of multiplexing techniques applied to the request signal space. As mentioned previously, these may include time division multiplexing, frequency division multiplexing, wavelet division multiplexing, code division multiplexing, and/or other multiplexing techniques. Similarly, the plurality of scheduled transmission segments in the scheduled transmission space may represent allotments defined based on one or more types of multiplexing techniques applied to the scheduled transmission signal space.
As such, each request segment may have a different “location” within the request signal space. For example, if a request signal space is organized according to a time division multiplexing technique, each request segment may comprise a different time slot in the request signal space. Here, each particular request segment is said to correspond to a different location (in time) in the request signal space. The same concept can be applied to a request signal space organized according to a frequency division multiplexing technique. In such a case, each request segment may comprise a particular frequency sub-channel and be said to correspond to a different location (in frequency) in the request signal space. The same concept can be applied to a request signal space organized according to a code division multiplexing technique. In such a case, each request segment may comprise a particular code word and be said to correspond to a different location (in code space) in the request signal space. Similarly, the concept can be applied to a request signal space organized according to a combination of different multiplexing techniques, such as a combination of time division multiplexing and frequency division multiplexing techniques. In this particular example, each request segment may comprise a particular time slot in a particular frequency sub-channel and be said to correspond to a different location (in time and frequency) in the request signal space.
Also, the separation between the request signal space and the scheduled transmission signal space may be based on different multiplexing techniques. In one embodiment, time division multiplexing is employed. For example, the request signal space and the scheduled transmission signal space may be defined over different time slots and a common frequency range. In another embodiment, frequency division multiplexing is employed. For example, the request signal space and the scheduled transmission signal space may be defined over a common time duration and different frequency ranges. In yet another embodiment, code division multiplexing is employed. For example, the request signal space and the scheduled transmission signal space may be defined over a common time duration and a common frequency range, but using different code words. Other embodiments of the invention may involve different combinations and/or variations.
Feedback Signal Space
A feedback signal space may be utilized for sending the assignment messages from scheduler node 102 to access nodes 104, 106, 108, and 110. In some embodiments of the invention, the feedback space is not a part of the shared communication medium. Continuing with a satellite system example, the feedback signal space may be implemented as a satellite “forward-link” that allows signals to be sent from scheduler node 102 to access nodes 104, 106, 108, and 110. This satellite “forward-link” may be separate from the “return-link” mentioned previously.
The present invention broadly covers different combinations of multiplexing techniques as applied to the request signal space and/or the scheduled transmission signal space. In the figures discussed below, a number of examples of such multiplexing combinations are presented. The various combinations of multiplexing techniques described below are presented for illustrative purposes and are not intended to restrict the scope of the invention.
In the figures below, only a representative portion of the relevant signal space is shown. For example, if four frames of signals are shown, it should be understood that more frames may be used even though they are not explicitly illustrated. Also, the particular proportions of the various signal space designs are provided as mere examples.
Time Division Multiplexing Request Signal Space and Scheduled Transmission Signal Space Partitioning with Time Division Multiplexing Request Segments and Scheduled Transmission Segments
In this particular embodiment, the shared communication medium 600 is organized as one continuous sequence of TDM time slots. For example, the shared communication medium 600 may comprise a particular frequency channel. Each TDM time slot occupies the entire bandwidth of the frequency channel, but only for a specific time duration. Here, the TDM time slots are shown as being organized into “frames,” such as Frame 0, Frame 1, Frame 2, and Frame 3. For ease of illustration,
For example,
In the particular grid representation shown in
In
When an access node, such as access nodes 104, 106, 108, and 110 of
In one example, the request signals shown in
Scheduler node 102 shown in
The entire request and assignment process takes place in an anonymous manner with respect to the identity of the access nodes. Thus, a symbol-level request sent from an access node does not explicitly identify the access node. For example, assume that access node 104 sent the symbol-level request in “REQ. 0:7” (Slot 7 of the request signal space of Frame 0). This symbol-level request is merely a symbol transmitted at a particular location within the request symbol space. The symbol-level request does not explicitly identify the access node 104. Similarly, the corresponding assignment message “REQ. 0:7, SCH. 13” broadcast from scheduler node 102 does not explicitly identify access node 104 as the intended recipient of the assignment message. Instead, the assignment message merely announces that the symbol-level request sent in the “REQ. 0:7” slot has been assigned to the scheduled transmission segment “SCH. 13.” All of the access nodes 104, 106, 108, and 110 receive the broadcast assignment message. However, only access node 104 accepts the assignment and proceeds to send a data transmission in the scheduled transmission segment identified by the assignment. This is possible because each access node keeps track of the location(s) of the symbol-level request(s) it has sent. Access node 104 recognizes the request “REQ. 0:7” identified in the assignment as one of its own and thus accepts the assignment. The other access nodes 106, 108, and 110 do not recognize the request “REQ. 0:7” identified in the assignment as one of their own and thus do not accept the assignment.
In
Frequency Division Multiplexing Request Signal Space and Scheduled Transmission Signal Space Partitioning with Frequency Division Multiplexing/Time Division Multiplexing Request Segments and Scheduled Transmission Segments
This example demonstrates that the request signal space and the scheduled transmission signal space may have very different symbol structures. In the request signal space, a symbol is transmitted over a narrow channel over a longer time duration. These are labeled as Request Slots 0, 1, 2, 3, etc. in the figure. By contrast, in the schedule transmission signal space, a symbol is transmitted over one of the two wide channels over a shorter time duration. These are labeled as Data Transmission Indices 0, 1, 2, 3, . . . , 63 in the figure. Despite this difference in symbol structures, a symbol transmitted in the request signal space may have the same time-bandwidth product as a symbol transmitted in the scheduled transmission signal space. Thus,
Also shown in
Time Division Multiplexing Request Signal Space and Scheduled Transmission Signal Space Partitioning with Code Division Multiplexing Request Segments
The structure shown in
For ease of illustration, the numerous symbols are not individually shown in this figure. Instead, boxes representing multiple symbols are shown. In the transmission signal space, each short box represents 8 scheduled transmission symbols. In the request signal space, each long box represents a 32-chip CDM request interval. Although the signal segments representing individual chips of any particular CDM code may be similar in design to the signal segments representing the scheduled transmission symbols, the chips of any particular code are linked in a particular code pattern (e.g., a 32-chip pattern), whereas the scheduled transmission symbols may be individually modulated. As shown,
More specifically, in this example each 456-symbol frame supports 1 scheduled transmission segment and 32 request segments. The 1 scheduled transmission segment comprises the first 424 symbols of the frame. The 32 request segments comprise the 32 possible code words that may be transmitted in the remaining portion of the frame. In other words, the remaining portion of the frame is code division multiplexed and organized as a 32-chip request interval.
Here, a 32-chip Walsh CDMA code is shown. In this code space there exist 32 different possible code sequences, or code words (“M”), each having a length of 32 chips (“L”). Indices 0 through 31 are used to identify the 32 different code words.
One or more of the access nodes 104, 106, 108, and 110 can send one or more requests (each in the form of one of the 32 possible code words) in a particular request interval. This is illustrated in
Scheduler node 102 detects reservation requests by correlation over the request interval against all reservation request codes. Scheduler node 102 broadcasts assignment messages in feedback signal space (not shown). The assignment messages are broadcast to access nodes 104, 106, 108, and 110 to inform the access nodes of the assignments made, so that each access node may correctly send data in the assigned scheduled transmission segment(s).
The entire request and assignment process takes place in an anonymous manner with respect to the identity of the access nodes. Thus, a symbol-level request sent from an access node does not explicitly identify the access node. For example, assume that access node 104 sends the symbol-level request comprising Code Word 13. This symbol-level request is merely a signal transmitted at a particular code location within the request symbol space. The symbol-level request does not explicitly identify access node 104.
Similarly, the corresponding assignment message would not explicitly identify access node 104 as the intended recipient of the assignment message. Instead, the assignment message merely announces that the symbol-level request corresponding to Code Word 13 in Frame 0 has been assigned to a particular scheduled transmission segment. All of the access nodes 104, 106, 108, and 110 receive the broadcast assignment message. However, only access node 104 accepts the assignment and proceeds to send a data transmission in the scheduled transmission segment identified by the assignment. This is possible because each access node keeps track of the location(s) in code space of the symbol-level request(s) it has sent in each frame. Access node 104 recognizes the request identified in the assignment as one of its own and thus accepts the assignment. The other access nodes 106, 108, and 110 do not recognize the request identified in the assignment as one of their own and thus do not accept the assignment.
Collisions Between Requests
A collision occurs when two or more symbol-level requests sent from different access nodes, such as access nodes 104, 106, 108, and 110, collide with one another. Such collisions may occur if the two or more symbol-level requests are sent in the same request segment. If scheduler node 102 detects such a collision, it may simply not send an assignment. When the access node senses that a symbol-level request it has sent has not been responded to (e.g., no assignment message is received from the scheduler node 102 in response to the symbol-level request), the access node may retransmit the symbol-level request. The retransmission technique may also take into account considerations such as retransmission count, latency, and quality of service (QOS). For example, the access node may keep track of a retransmission count, such as the number of times a particular request has been retransmitted. If the retransmission count exceeds a maximum threshold, the request may be discarded so that no further retransmission is attempted. Also, the access node may keep track of a measure of latency, such as the amount of time that has elapsed since the access node began attempting to request a scheduled transmission segment for a particular message. If the latency exceeds a maximum threshold, the request may be discarded so that no further retransmission is attempted. Furthermore, the access node may take into account a measure of QOS for a request. If the request is associated with a higher QOS, the request may be given a higher priority in retransmission
Request collisions not detected by scheduler node 102 as a collision may need to be handled differently. Here, two or more symbol-level requests collide. However, scheduler node 102 fails to recognize that a collision has occurred. Instead, scheduler node 102 treats the collision as a valid symbol-level request. Thus, scheduler node 102 sends an assignment message associating the symbol-level request to a scheduled transmission segment. Each access node that sent a symbol-level request involved in the collision accepts this assignment, and in response sends its own data transmission. As a result, a collision of multiple data transmissions can occur in the scheduled transmission segment specified in the assignment.
The system may handle such a scenario in a number of ways. For example, the system may require acknowledgment of receipt of a data transmission. Here, when an access node, such as access nodes 104, 106, 108, and 110, sends a data transmission in an assigned scheduled transmission segment, the access node expects the intended recipient to respond by sending an acknowledgement (ACK) message to confirm successful receipt of the data transmission. If the data transmission is not successfully received, as indicated by the lack of an ACK message from the intended recipient, the access node re-sends the data transmission. The process of re-sending the data transmission may be similar to that of the original transmission. That is, the access node may first send a symbol-level request, then receive an assignment associating the symbol-level request to a particular scheduled transmission segment, and finally send the data transmission in the assigned scheduled transmission segment.
Alternatively, the scheduler node 102 may assign follow up request segments to resolve collisions. Just as an example, assume two access nodes 104 and 106 each send an initial request in the same initial request segment. This causes a collision. Even if scheduler node 102 can properly decode the initial request, it remains ambiguous as to which access node should be deemed the originator of the request. If scheduler node 102 does not resolve the collision and instead simply sends an assignment message associating the initial request with a scheduled transmission segment, both access nodes 104 and 106 may accept the assignment and attempt to send a data transmission in the scheduled transmission segment. The likely result is that the signals will be jumbled and neither data transmission will be properly received.
Thus, instead of just sending an assignment associating the initial request segment with a scheduled transmission segment, scheduler node 102 may send an assignment associating the initial request segment with one or more follow up request segments. These follow up request segments provide an opportunity for access nodes involved in a collision to send secondary requests that may avoid collision with one another.
Continuing with the above example of a collision involving access nodes 104 and 106, scheduler node 102 may detect the possible collision and send an assignment that associates the initial request with 32 follow up request segments. Access node 104 responds by randomly selecting 1 out of the 32 follow up request segments and sending a secondary request in its randomly selected request segment (e.g., request segment 8). Access node 106 also responds by randomly selecting 1 out of the 32 follow up request segments and sending a secondary request in its randomly selected request segment (e.g., request segment 13).
There is a high probability (31/32) that access nodes 104 and 106 would randomly select different ones of the 32 possible follow up request segments to send their respective secondary requests. If this scenario occurs (more likely), the collision has been resolved. That is, now access nodes 104 and 106 have sent secondary requests in distinct request segments. Scheduler node 102 can simply process the secondary requests separately by assigning a scheduled transmission segment to the secondary request sent from access node 104, and separately assigning another scheduled transmission segment to the secondary request sent from access node 106.
Of course, there is a low probability (1/32) that access nodes 104 and 106 would happen to select the same one out of the 32 possible follow up segments to send their respective secondary requests. If this scenario occurs (less likely), the collision remains unresolved. Here, scheduler node 102 may repeat the procedure over again in order to resolve the collision. That is, scheduler node 102 may send yet another assignment to assign another set of 32 follow up request segments. Access nodes 104 and 106 would again separately make random selections out of the 32 follow up request segments, and so on. This process may repeat until the collision is resolved or until a termination condition is reached.
While the example discussed above involves two access nodes 104 and 106, the approach is applicable in collision situations involving more than two nodes. Also, the probability of collision resolution in each iteration can be modified by changing the number of follow up request segments assigned (e.g., to a number greater than or less than 32).
In one embodiment of the invention, the probability of collision is reduced by reducing the probability of two or more access nodes sending an initial request in the same request segment. By reducing the probability of collision during an initial request, fewer collisions occur and thus fewer resources are required to resolve collisions. For example, the probability of collision can be reduced by increasing the number of request segments in the request signal space. By increasing the number of request segments, the probability that more than one access node will send an initial request in the same initial request segment is reduced.
Multiple Request Intervals
In another embodiment of the invention, the probability of collision is reduced by organizing the request signal space into multiple request intervals. Here, each request includes a request segment from each request interval. For example,
The structure shown in
For ease of illustration, the numerous symbols are not individually shown in this figure. Instead, boxes representing multiple symbols are shown. In the transmission signal space, each short box represents 8 scheduled transmission symbols. In the request signal space, each long box represents a 32-chip CDM request interval. Thus,
More specifically, in this example each 456-symbol frame supports 1 scheduled transmission segment and 4 request intervals each having 32 request segments. The 1 scheduled transmission segment comprises the first 328 symbols of the frame. The 4 request intervals comprise the remaining 128 symbols of the frame. Each request interval comprises 32 request segments, or 32 possible code words. In other words, each request interval is code division multiplexed and organized as a 32-chip request interval.
Again, a 32-chip Walsh CDMA code is shown as an example. In Request Interval 0 there exists M=32 code words, each having a length of L=32 chips. Indices 0 through 31 are used to identify the 32 different possible code words.
One or more of the access nodes 104, 106, 108, and 110 can send one or more requests. Here, each request includes one of the 32 possible code words in each of the 4 request intervals. Thus, each request comprises a request sequence of 4 code words. Each code word is independently and randomly chosen. This is illustrated in
In one example, the request signals shown in Frame 0 of
Node 106 may send a request occupying Slot 13 of Request Interval 0, Slot 5 of Request Interval 1, Slot 11 of Request Interval 2, and Slot 1 of Request Interval 3.
Node 108 may send a request occupying Slot 18 of Request Interval 0, Slot 9 of Request Interval 1, Slot 22 of Request Interval 2, and Slot 27 of Request Interval 3.
Finally, node 110 may send a request occupying Slot 22 of Request Interval 0, Slot 16 of Request Interval 1, Slot 22 of Request Interval 2, and Slot 30 of Request Interval 3.
Scheduler node 102 detects reservation requests by correlation over each request interval against all reservation request codes. Scheduler node 102 selects the request interval with the most occupied request segments to use for assignment of scheduled transmission segments. In
In response, scheduler node 102 broadcasts assignment messages based on Request Interval 0 in the feedback signal space (not shown). The assignment messages are broadcast to access nodes 104, 106, 108, and 110 to inform the access nodes of the assignments made, so that each access node may correctly send data in the assigned scheduled transmission segments.
The entire request and assignment process takes place in an anonymous manner with respect to the identity of the access nodes. Thus, a request sent from an access node does not explicitly identify the access node. Instead, requests are merely signals transmitted at a particular code location within the request signal space. The request does not explicitly identify access nodes 104, 106, 108, or 110.
Similarly, the corresponding assignment message would not explicitly identify the intended recipient of the assignment message. Instead, the assignment message merely announces that, in Frame 0, the symbol-level requests in Request Interval 0 corresponding to Code Word 8, Code Word 13, Code Word 18, and Code Word 22 have been assigned to particular scheduled transmission segments. For example, as discussed above, scheduler node 102 selects Request Interval 0 to use for the assignment of scheduled transmission segments because scheduler node 102 detected the most occupied request segments in this request interval. The corresponding assignment messages include the pair of data “REQ. 0:0:8, SCH. 13” (assuming Transmission Segment 13 is the next available scheduled transmission segment). This indicates that the request sent in the request segment known as “REQ. 0:0:8” (the request segment located at Frame 0, Request Interval 0, Slot 8) has been assigned the scheduled transmission segment known as “SCH. 13” (the scheduled transmission segment located in Frame 13). The remaining assignment messages follow a similar format. The second assignment message includes the pair of data “REQ. 0:0:13, SCH. 14.” The third assignment message includes the pair of data “REQ. 0:0:18, SCH. 15.” The fourth assignment message includes the pair of data “REQ. 0:0:22, SCH. 16.”
Each of the access nodes 104, 106, 108, and 110 receive the broadcast assignment message. However, in the above example, access node 104 only accepts the assignment message “REQ. 0:0:8, SCH. 13,” access node 106 only accepts the assignment message “REQ. 0:0:13, SCH. 14,” access node 108 only accepts the assignment message “REQ. 0:0:18, SCH. 15,” and access node 110 only accepts the assignment message “REQ. 0:0:22, SCH. 16.” This is possible because each access node keeps track of the locations in code space of the symbol-level request(s) it has sent in each frame. Each access node recognizes the request identified in the assignment that corresponds to the request it sent. The access nodes do not recognize the requests identified in the assignment that were sent by other access nodes, and thus do not accept the other assignments.
Other embodiments of the invention may use different request and/or scheduling modes than those described above. For example, in some embodiments, request and assignment messages may not identify the frame in which the request originated. Instead, the timing synchronization of the system may allow the frame to be determined by scheduler node 102 as well as by access nodes 104, 106, 108, and 110. Similarly, in some embodiments, assignment messages may not identify the scheduled transmission segment associated with each request. Again, the timing synchronization of the system may allow for information on scheduled transmissions to be kept in a local transmission queue that can be updated by scheduler node 102 and accessed by access nodes 104, 106, 108, and 110.
While the example shown in
Determining the Number of Request Intervals and Request Segments
In implementing a system with multiple reservation request intervals, as illustrated in
An important factor affecting this balance is the system loading, or the expected number of concurrent requests. In the example of
Table 1 shows collision performance at a fixed value of 64 chips per unit loading, according to an embodiment of the invention. This table shows the minimum probability of collision from each of the plots in
Table 2 shows collision performance at a fixed value of 96 chips per unit loading, according to an embodiment of the invention. This table shows the minimum probability of collision from each of the plots in
Table 3 shows collision performance at a fixed value of 128 chips per unit loading, according to an embodiment of the invention. This table shows the minimum probability of collision from each of the plots in
Table 4 shows collision performance at a fixed value of 256 chips per unit loading, according to an embodiment of the invention. This table shows the minimum probability of collision from each of the plots in
The data presented in the preceding tables shows that there is a relationship between request sequence length, the number of code words in a request sequence, and system loading. In each table, the minimum probability of collision occurs at a request sequence length of twice the fixed value of chips per unit loading. This data can be used to select the optimum request sequence length based on expected system loading and required collision probability.
While the present invention has been described in terms of specific embodiments, it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described specific embodiments. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, substitutions, and other modifications may be made without departing from the broader spirit and scope of the invention as set forth in the claims.
The present application claims benefit of priority under 35 U.S.C. 119(e) of U.S. provisional Application No. 60/943,155, filed on Jun. 11, 2007, entitled “Linked Orthogonal Reservation Request Codes,” Client Reference No. VS-0267-US, the content of which is incorporated herein by reference in its entirety.
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