Abstract:
An embodiment of the present invention provides an apparatus, comprising a subscriber station (SS) operable to communicate with a base station (BS) and at least one additional subscriber station (SS) in a wireless metropolitan area network, wherein the at least one additional SS attempts to overhear a first message from the SS and piggy back a second message for the overheard message from the SS&#39;s uplink data to the BS with its uplink data.

Description:
BACKGROUND 
     In wireless metropolitan area networks (WMANs) such as WiMAX, traffics in the uplink are usually short messages. For example, the short message may be an acknowledgement of a received packet, or a mouse click on a website, or a bandwidth request to send an email, or a beamforming matrix feedback due to a sudden channel variation, etc. These short messages are generated in a non-periodic or unpredictable fashion and thus it is hard for the base station to allocate resources (i.e. subchannels) for their transmission. In the current WiMAX systems, for example, either base station periodically polls the subscribers or the subscribers have to contend for the uplink resources to send the messages. 
     Both the polling and contention are inefficient and consume a significant system overhead. From channel coding perspective, the protection of short message is less efficient than that of the long message because the powerful channel codes require large block sizes. It is better to aggregate short messages and to encode them together. 
     Besides the increased overhead, the latency of the contention is unbounded, which is undesirable for delay sensitive applications. Although increasing the polling rate reduces the latency, the efficiency is also reduced because most of the polled subscribers do not have messages to send. 
     Thus, a strong need exists for improvements in existing schemes for short messages used in WMANS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1  provides an illustration of a collision at the base station and overhearing by a nearby subscriber of an embodiment of the invention; 
         FIG. 2  illustrates the usage of open subchannels according to an embodiment of the invention; 
         FIG. 3  illustrates the allocation of open subchannels for delay sensitive and delay insensitive short messages according to an embodiment of the present invention; and 
         FIG. 4  is a graphic illustrating interference successive cancellation at the base station according to an embodiment of the present invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the preset invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, units and/or circuits have not been described in detail so as not to obscure the invention. 
     Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulate and/or transform data represented as physical (e.g., electronic) quantities within the computer&#39;s registers and/or memories into other data similarly represented as physical quantities within the computer&#39;s registers and/or memories or other information storage medium that may store instructions to perform operations and/or processes. 
     Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. For example, “a plurality of stations” may include two or more stations. 
     In WMANs such as WiMAX, the subscriber station sends the base station lots of spontaneous short messages. These messages are generated in a non-periodic or unpredictable fashion and thus it is hard for the base station to allocate time/frequency resources for the transmission of these messages. Examples of the message include mouse clicks during web surfing and uplink bandwidth requests. Either base station periodically polls the subscribers or the subscribers have to contend for the uplink resources to send the messages. Both the polling and contention are inefficient and consume a significant system overhead, whose efficiency is typically 30%. Besides the overhead, the latency of the contention is unbounded, which is undesirable for delay sensitive applications. An embodiment of the present invention, increases the throughput of short messages by opportunistic message forwarding, which exploits peer-to-peer connectivity among co-located subscribers with good (potentially LOS) links to each other and spatial isolation among subscribers far from each other. In WMANs, the subscribers far apart are isolated by large path loss because of the low antenna mounting and high surrounding buildings. However, the adjacent subscribers e.g. on the same floor can listen each other. Therefore, although the short messages simultaneously sent by the subscribers collide at the base station that has high antenna mounting, each message may still be received by the sender&#39;s neighboring subscribers. 
     If one of the neighbors happens to be allocated uplink transmission resource, it can squeeze the overheard short message into its uplink data and sends both to the base station. The aggregation of the short message and the original uplink data may obtain a better channel coding protection for the short message and lower packet error rates due to the increased block size. In addition, the system throughput is increased by contention overhead reduction and by augmented spatial reuse during the overhearing. If it is hard to squeeze the whole overheard short message into the allocated uplink resource block, the overhearing subscriber may send a bandwidth request using the allocated uplink resource block to ask for additional resource block. The additional resource block may be used for some part of the uplink data or the overheard short message. For example, the overhearing subscriber may cut some of its uplink data to accommodate the overheard short message in the originally allocated resource block and send the remaining data in the subsequently allocated resource block. Since the piggyback bandwidth request is contention free, the system throughput is improved. 
     Looking now at  FIG. 1 , generally as  100 , is a WMAN such as WiMAX, wherein the base station (BS)  110  has high antenna mounting and thus can receive signals from all subscribers. On the other hand, the subscribers usually have low antenna mounting and thus they can not hear each other if they are far apart. This is because there are multiple high obstacles such as large buildings  120  and  150  on the propagation path between two subscribers and thus the path loss between two subscribers is much higher than that between a subscriber and the BS. However, the nearby subscribers, e.g., those on the same floor or in the same street, can still listen to each other (could potentially have line-of-sight connectivity). 
     As shown in  FIG. 1 , subscriber station  1  (SS 1 )  140  and subscriber station  3  (SS 3 )  130  have short messages to send the BS  110 . For example, and not by way of limitation, they resort to the contention mechanism for uplink bandwidth. They send the BS CDMA ranging codes to request bandwidth. In the conventional scheme, if the arrivals of the codes collide and the BS  110  can not detect both, SS 1   140  and SS 3   130  have to try again later. Embodiments of the present invention remedy this problem by providing a scheme based on overhearing and opportunistic forwarding, which exploits peer-to-peer connectivity among co-located subscribers with good (potentially LOS) links to each other and spatial isolation among subscribers far from each other. Accordingly, the neighbor subscriber of SS 1 , i.e. subscriber SS 2   160 , can help SS 1   140  in transmitting the short message belonging to SS 1   140 . In this context, we assume that SS 2   160  has already been allocated uplink resources by the BS for its data transmission. Using its high-quality peer-to peer connectivity to SS 1   140 , SS 2   160  attempts to overhear the CDMA code of SS 1   140 . If the interference from SS 3   130  is blocked by the surrounding buildings as shown in  FIG. 1 , the overhearing attempt to SS 2   160  succeeds. Then, SS 2  will send its uplink data and the overheard CDMA code index for SS 1 &#39;s  140  bandwidth request together using a larger channel codeblock for better error protection (if the BS  110  does not acknowledge the reception of the CDMA code sent by SS 1   140 ). The piggyback of the overheard short message cause negligible bandwidth to SS 2   160 , because the size of the short message is about the same as the length of the normal zero padding that is used to fill a FEC block. Besides, since higher modulation order and spatial multiplexing may be applied by SS 2   160 , the piggyback overhead is further reduced. Furthermore, if SS 2   160  has to cut its data to accommodate the short message, then SS 2   160  can piggyback a bandwidth request for the remaining data. The piggyback request for additional bandwidth is used in the existing system and it is much more efficient than the contention based request. Similarly, the co-located subscriber SS 4   120  can help SS 3   130  to submit the CDMA code. During the overhearing, a distributed multiple-input multiple-output (MIMO) system is formed by the antennas from three receivers, i.e., the BS  110 , SS 2   160 , and SS 4   120 , and the antennas from the two senders, i.e., SS 1   140  and SS 3   130 . Since there are more receive antennas than transmit antennas, the two simultaneously transmitted codes can be decoded in theory. The scheme presented under the CDMA code-based bandwidth request example discussed above can be generalized to other types of short message transmissions such as a service request generated by a mouse click on a website. Embodiments herein provide an efficient scheme for short messages next. The most popular mode of WiMAX is the OFDMA mode. In one embodiment, the overhearing SS may overhear a long message sent by a sending SS. Instead of forwarding the long message, the overhearing SS may convert the long overheard message into a short message to help the sending SS. For example, the overhearing SS may piggy back an uplink bandwidth request for the sending SS. In this way, the BS knows that the sending SS has data to send and allocates resource for the long message. 
     Turning now to  FIG. 2 , shown generally as  200 , in the uplink subframe  205 , the BS  110  allocates subchannels to scheduled subscribers for them to send their data respectively. The subchannel is a unit of time-frequency transmission resource. An embodiment of the present invention provides the allocation of some subchannels in the uplink subframe  205  for direct transmission of unscheduled short messages. These subchannels may be referred to herein as open subchannels  210  and  215 . The usage of the open subchannels is illustrated in  FIG. 2  at  200 , where the station labels refer to  FIG. 1 . Short messages are directly sent to the open subchannels by the subscribers who do not have allocated uplink resource. The subscriber provides channel training symbols only for the demodulation of the open subchannel. In one embodiment, the open subchannel can randomly selected by each subscriber. In another embodiment, the selection of the open subchannel may contain some information. For example, the subscriber may compute from part of the short message to generate an index and use the index to select an open subchannel. The short message can be a beamforming matrix feedback due to sudden channel variation. The BS  110  and a set of distributed overhearing stations (e.g. SS 2   160  and SS 4   120 ) jointly receive the sent short messages. The overhearing station must have already been allocated subchannels in the same subframe latter than the open subchannels as shown in (a) of  FIG. 2 . 
     Since uplink allocation is broadcasted by the BS  110  one frame ahead in the current WiMAX, the overhearing station knows whether it should overhear or not. Moreover, it is desirable to allocate the open subchannels  210 ,  215  at the beginning of the uplink subframe  205  to increase the number of potential overhearing stations. The overheard messages are piggybacked by the overhearing stations with their data on allocated subchannels (allocated subchannel for SS 4  at  220  and allocated subchannel for SS 2  at  225 ) latter in the same uplink subframe  205  as shown in (a) of  FIG. 2 . 
     If the BS  110  receives the short message directly, then there is no need for the overhearing station to forward the message. The BS  110  may broadcast the acknowledgement of each short message received by itself in the next frame as shown in (b) of  FIG. 2 . Uplink subframe  230  may include open subchannel  1   245  and open subchannel  2   250 . Downlink subframe with DL MAP is shown at  235  and Uplink subframe  240  may include allocated subchannel for SS 4   265  and allocated subchannel for SS 2   260 . The BS  110  may broadcast the connection ID (CID) of the short message the BS  110  directly received, or the BS  110  may broadcast the open subchannel index where the short message is received correctly. The overhearing station may decide whether to piggyback according to the BS&#39;s  110  broadcast. This conditional forwarding increases the delay and may not be desirable as compared to the scheme in (a) of  FIG. 2 . Further, the BS  110  can adjust the number of open subchannels in each frame so that the mean number of simultaneous transmissions on the same open subchannel varies and the throughput is maximized. If too many overhearing stations forward the message for the same subscriber station, the BS  110  may ask some of the overhearing stations to stop the forwarding for the subscriber station. The BS  110  or the operator may give the overhearing station an incentive in terms of higher scheduling priority or fee deduction. 
     The techniques in (a) and (b) of  FIG. 2  can be jointly applied as shown in  FIG. 3 . If  FIG. 3  uplink subframe may include open subchannel  1   320 , open subchannel  2   325 , scheduled subchannel  0   330  and scheduled subchannel  1   335 . Downlink subframe  310  may include DL MAP  340 . Uplink subframe  315  may include scheduled subchannel  2   345 . Short messages are categorized into delay sensitive and delay insensitive classes. Since decoding of the overheard message takes time, the BS  110  needs to reserve time for the decoding. In  FIG. 3 , open subchannels  320  and  325  for delay sensitive short messages are allocated at the beginning of the uplink subframe  305  so that the decoding time for the overhearing station can be maximized. Between the open subchannels  320  and  325  for the delay sensitive messages and the prescheduled subchannels  330  and  335  for the overhearing stations, open subchannels for delay insensitive messages and prescheduled subchannels for nonoverhearing stations may be allocated. The overhearing station may make uses to the gap to decode the overheard delay sensitive message first and then the delay insensitive one. If the decoding of the delay sensitive message is successful, the decoded message is squeezed into the prescheduled subchannel of the overhearing station and sent with the overhearer&#39;s uplink data. The overhearing station may continue to decode the delay insensitive message in the current uplink subframe  305  and decide whether to continue decode and forward the message based on the broadcasted acknowledgements of the open subchannels in the next downlink subframe  310 . The overhearing station should not forward the messages that were already directly received by the BS  110 . If the overheard message is not received by the BS  110 , the overhearing station may forward it out in the next uplink subframe  315 . 
     The main difference between the proposed technique and the conventional relay technique is that the proposed technique is opportunistic while the conventional relay is static. In the conventional relay, message forwarding is conducted for each packet constantly and requires lots of system overheads that set up the relay route and manage the retransmission and the bandwidth request of the each hop. In contrast, the proposed opportunistic technique skips the overheads, because the message forwarding occurs only if everything happens to be set for free. Namely, the overhearing to get the message is free because it doesn&#39;t cause interference, and the overhearing is conducted only if the overhearing station has already granted the uplink bandwidth latter in the subframe. 
     Furthermore, WiMAX network is interference limited. Strong interference from other co-channel transmissions can intermittently and randomly corrupt the reception of a given link. The conventional relay requests retransmissions for the corrupted packet, which causes overhead and directly fight with the interference by paying more transmission power and bandwidth. In contrast, in an embodiment of the present invention the opportunistic overhearing doesn&#39;t fight with the interference and only makes use of the good intervals of the channel. Finally, in an embodiment of the present invention, the base station may dynamically use spatial division multiple access (SDMA) to receive data from the different subsets of overhearing stations for high throughput and this dynamic is hard to schedule for the conventional relay. In sum, the gain of the proposed techniques is obtained from the overhead reduction, the increased spatial reuse of the multiple transmissions, and slightly the reduced path loss. 
     Multiuser detection and successive interference cancellation may be conducted in embodiments of the present invention. As shown in  FIG. 4 , generally as  400 , three subscribers, i.e., SS 1   420 , SS 3   440 , and SS 5   410 , collide at the BS  405 , the BS  405  can not decode the three collided short messages but it can buffer the received signal. Buildings causing interference between subscribers are shown at  415 ,  430  and  435 . If SS 2   425  and SS 4   445  forward the overheard short messages of SS 1   420  and SS 3   440  to the BS  405  latter, the BS  405  can estimate the channel responses from SS 1   420  and SS 3   440  using the forwarded messages and then regenerate the received signal components from SS 1   420  and SS 3   440 . The regenerated signals are subtracted from the buffered, received signal and only the signal from SS 5   410  remains. The BS  405  can then decode SS 5 &#39;s  410  message. 
     In the case that the overhearing is not successful, the overhearing station may still be able to provide useful information. For example, the overhearing station may forward the reliability information of each overheard bit, which is called soft bit. The forwarded reliability information can be aggregated at the BS  405  and the BS  405  uses all the information including its own to decode messages. However, the soft bit may consume more forwarding bandwidth than the hard decision bit and reduces the efficiency. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.