Patent Application
20110317633
A wireless local-area networks (WLAN) links devices using a wireless distribution method (such as orthogonal frequency-division multiplexing (OFDM)). A WLAN allows users to move around an area serviced by the WLAN while still maintaining interconnectivity. For this reason WLANS have become increasingly popular with consumers both in the home and in commercial and public areas.
Improvements in communication technology drive the increase of wireless data rate in wireless local area networks (WLANs). For example, the latest ratified Institute of Electrical and Electronics Engineers (IEEE) 802.11 n standard has boosted the physical layer (PHY) data rate to hundreds of megabytes per second (Mbps). The physical layer is the first and lowest layer in the seven-layer Open System Interconnection (OSI) model of computer networking.
The increase in the data rate in the PHY is mainly due to wider channel bandwidth and advanced modulation techniques like Multiple-Input Multiple-Output (MIMO). Future standards like the IEEE 802.11 ac standard and the 802.11 ad standard are already poised to provide even faster PHY speeds on the order of gigabytes per second (Gbps).
However, the throughput efficiency, which is the ratio between the network throughput and the PHY data rate, has degraded rapidly as the PHY data rate has increased. One reason for this is the inadequacy of current IEEE 802.11 medium access control (MAC) data communication protocol. The MAC is a sublayer of the data link layer, which is the second layer of the OSI networking model. For example, given that most internet protocol (IP) packets have a maximal transmit unit (MTU) size around of 1500 bytes, the efficiency ratio in an 802.11 n network at 300 Mbps is around twenty percent. In other words, the 300 Mbps data rate can sustain an actual throughput of only about 60 Mbps.
One reason for such inefficiency is that the current MAC data communication protocol allocates the entire channel as a single unit. Such allocation can become too coarse grained when the channel width increases or PHY data rate increases. Even if a sender has a small amount of data to send the data still has to compete for the whole channel. The time spent resolving this contention for the channel therefore becomes an overhead to the useful channel time. Unfortunately, this overhead cannot be easily reduced due to physical and electronics constraints. As a result, the higher the PHY data rate the lower the throughput efficiency.
Attempts have been made to deal with this lack of WLAN efficiency. One way to improve efficiency is to extend the useful channel time on data transmission by sending larger frames. Indeed, the IEEE 802.11 n standard allows frame aggregation (or sending multiple frames together). However, when the PHY data rate goes up the aggregated frame size needs to go up as well. This means to achieve an efficiency of eighty percent in a 300 Mbps network it would require the frames to be as big as 23 KB. This means longer delays as the sender waits to collect enough frames before actual transmission, which adversely affects transport control protocol (TCP), real-time applications like voice over IP (VoIP), video conferencing, and to web browsing. Indeed, tests have shown that a network throughput of 300 Mbps an 802.11 n network can only increase to around 80 Mbps after frame aggregation is enabled.
One way to improve WLAN efficiency is by reducing the channel width and creating additional channels. Each node in the network may transmit on these small channels simultaneously, thereby amortizing MAC coordination overhead among multiple users. One problem, however, with this idea is that it is difficult to slice a channel band into multiple subchannels without losing useful bandwidth.
One common practice in creating subchannels is to waste both edges of two adjacent subchannels as “guard band” so that the useful transmissions are properly spaced out to avoid interfering with each other. However, these guard bands can add up to significant overhead, especially as the number of subchannels increase. Moreover, no matter the width of a subchannel the guard-band width cannot be easily reduced due to the power mask requirement.
In order to avoid the need for guard bands, some WLANs use the orthogonal frequency division multiplexing (OFDM) wireless distribution method. OFDM is a well understood PHY-layer technology that works best if the frequency and width of subchannels are strategically picked and transmission on each subchannel is synchronized in a way to become “orthogonal” (and thus non-interfering) to one another. Although some cellular networks have proposed to use OFDM in channel multi-access (OFDMA), it requires tight synchronization among user handsets and it does not support random access. It thus remains a technical challenge on how to use OFDM-type channelization for dividing a channel into subchannels among distributed and asynchronous stations in a random-access WLAN, where it is impractical and unnecessary to achieve the same tight synchronization.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
Embodiments of the fine-grained channel access system and method facilitate fine-grained channel access in a high-data rate wide-band wireless local-area network (WLAN). Embodiments of the system and method are designed to enable fine-grained subchannel random access based on orthogonal frequency-division multiplexing (OFDM). Embodiments of the system and method are a cross-layer design that addresses challenges in both the physical layer (PHY) and the media access control layer (MAC) design due to the asynchronous and distributed nature of WLANs.
Embodiments of the system and method take an entire wireless channel and divide the whole channel width into proper size subchannels commensurate with PHY data rate and typical frame size. When OFDM is used to eliminate the interchannel guard-bands, one challenge is how to coordinate the transmissions as WLAN is distributed and asynchronous in nature. First, embodiments of the system and method use a new PHY architecture based on OFDM that retains orthogonality among subchannels by solely relying on the coordination mechanisms in existing WLANs (such as carrier-sensing and broadcasting). Second, embodiments of the system and method employ a frequency-domain contention technique that uses physical layer request-to-send/clear-to-send (RTS/CTS) signaling and frequency domain backoff to efficiently coordinate the subchannel access.
More specifically, embodiments of the system and method take a wireless channel and divide the channel into multiple fine-grained subchannels. Then each node on the WLAN contends independently for each of the fine-grained subchannels. A first OFDM technique is used to signal an access point in the WLAN that the node desires one or more of the subchannels. This first OFDM technique uses a first fast Fourier transform (FFT) size.
Each node then simultaneously transmits a multi-tone RTS (M-RTS) signal to the access point by using the first OFDM technique. Sometimes more than one node will desire the same subchannel, and this will cause contention among the nodes. The access point resolves any contention between the nodes using a frequency domain contention technique that includes a frequency domain backoff technique. Once any contention is resolved by the access point, the access point then assigns each node one or more of the subchannels.
A second OFDM technique (which is different from the first OFDM technique used for signaling) is used for data transmission. This second OFDM technique uses a second FFT size for transmitting data on each node's assigned subchannels. In addition, embodiments of the system and method employ two different guard times to handle symbol time misalignment of the asynchronous signaling and the data transmission over the fine-grained subchannels. One of the guard times uses a short cyclic prefix while the other guard time uses a long cyclic prefix (as compared to each other).
It should be noted that alternative embodiments are possible, and that steps and elements discussed herein may be changed, added, or eliminated, depending on the particular embodiment. These alternative embodiments include alternative steps and alternative elements that may be used, and structural changes that may be made, without departing from the scope of the invention.
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
In the following description of embodiments of the fine-grained channel access system and method reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration a specific example whereby embodiments of the fine-grained channel access system and method may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the claimed subject matter.
The client computing device 110 is in wireless communication 115 with a wireless access point (AP) 120, which acts as a receiver in embodiments of the system 100 and method. In turn, the AP 120 typically is in communication with a wide-area network 125 (such as the Internet). The communication parts in embodiments of the fine-grained channel access system 100 agree on spectrum channel allocation and subchannel division schemes, as shown within the AP 120 of
It should be noted that
When the client computing device 110 has frames to transmit, a subchannel request module 145 residing on the client computing device 110 generates subchannel access requests based on the frame size and an output of a frequency domain backoff module 150. The subchannel request module 145 performs carrier sense on the entire wireless channel 135 and when the channel 135 is sensed idle the module 145 sends a multi-tone request to send (M-RTS) signal to the AP 120. This M-RTS carries the subchannel access request using a first fast Fourier transform (FFT) size 155.
The AP 120 receives requests from client nodes (including the client computing device 110) for subchannels access 160. These concurrent requests may contend for each of the subchannels. The AP 120 resolves the contention with a frequency domain contention resolution module 165 that deals with contention between nodes that are requesting the same subchannel. Subchannels are assigned to node by the module 165, and then the AP 120 sends out the subchannel assignments for the nodes 170 to the respective nodes using a multi-tone clear-to-send (M-CTS) 175 using the first FFT size.
The client computing device 110 is one of the nodes that receives one or more subchannel assignments 180 (depending on the number of subchannels requested). Using the one or more subchannels assigned to it, a node (such as the client computing device 110) can send data. Embodiments of the fine-grained channel access system 100 and method facilitate receipt and transmission of data using a data transmission module 185. The data transmission module 185 transmits using a second FFT size 190. The transmitted data are received by a data receiving module 195 located on the AP 120. If the data is received correctly an acknowledgement (ACK) signal is sent from the AP 120 back to the client computing device 110.
As explained in detail below, a new transmission opportunity appears only when the entire channel is idle. At that time each station tries to contend for different subchannels after the channel is idle for a certain amount of time. This is based on a distributed coordination function interframe space (DIFS).
Embodiments of the system 100 and method then use a first orthogonal frequency-division multiplexing (OFDM) technique using a first fast Fourier transform (FFT) size to signal the access point (AP) 120 (box 220). This is done at the physical layer of the OSI model. Each node transmits a specially-designed OFDM symbol (called Multi-tone RTS or M-RTS signal) to the AP 120 by using the first OFDM technique (box 230). The M-RTS signal is discussed in detail below. This M-RTS signal has embedded therein each node's contention information in a set of subcarriers for each subchannel that the node intends to access.
All M-RTS signals are resolved at the AP 120. In addition, the AP 120 resolves any subchannel contention between nodes (box 240). Any contention between the nodes is resolved using a frequency domain contention technique that includes a frequency domain backoff technique. The AP 120 then broadcasts the contention results in a corresponding multi-tone clear-to-send (M-CTS) OFDM signaling symbol. Then only the nodes assigned subchannels will use them for data transmissions. Each node is assigned one or more subchannels (box 250). Note that a node may contend for multiple subchannels based on its instantaneous traffic demands. The AP 120 will then generate an acknowledgement on each subchannel where a data frame has been successfully decoded.
Embodiments of the fine-grained channel access system 100 and method also use a second OFDM technique having a second FFT size for data transmission on the plurality of fine-grained subchannels (box 260). In addition, two different guard times are used in order to avoid inter-subchannel interference among asynchronous signaling and data transmissions over the subchannels (box 270) from different senders. These two different guard times are discussed in detail below. Embodiments of the system 100 and method then are able to transmit and receive data over the plurality of fine-grained subchannels (box 280).
At time T3 each of the nodes will transmit a modified RTS signal simultaneously. This RTS signal is a specially-designed OFDM symbol and is called a Multi-tone RTS (M-RTS) 320. Each node embeds its contention information in a set of subcarriers for each of the four subchannels that it intends to access.
At time T4 there is a first short interframe space (SIFS) 330 delay. All M-RTS signals are resolved at the AP 120, and at time T5 the AP 120 will broadcast the contention results in a corresponding M-CTS 340 OFDM signaling symbol. At time T6 there is a second SIFS 350 delay. Then at time T7 only the nodes assigned subchannels will use those assigned subchannels for data transmissions. Note that a node may contend for multiple subchannels based on its instantaneous traffic demands. At time T8 there is another there is a third SIFS 360 delay. Then at time T9 the AP 120 will generate an acknowledgement (ACK) on each subchannel where a data frame has been successfully decoded.
Downlink transmissions follow similar steps, but the AP 120 will initiate an M-RTS signal and receiving stations may return M-CTS. However, since embodiments of the fine-grained channel access system 100 and method do not use random time backoff, it needs to separate uplink and downlink transmissions. Otherwise, collisions would happen under bi-directional traffic. Embodiments of the system 100 and method do so by assigning different DIFS times to uplink and downlink transmissions, as described in detail below.
Embodiments of the fine-grained channel access system 100 and method are a cross-layer design that enables fine-grained channel access in high-rate wide-band WLAN. Embodiments of the system 100 and method are based on OFDM and divide a wide-band channel into a set of orthogonal subchannels where different nodes can contend for each subchannel individually. For the sake of clarity, this document first assumes a scenario that includes a single access point and multiple mobile stations or nodes. It is straightforward to extend embodiments of the fine-grained channel access system 100 and method to the multi-access point access point case. Also, while in the following discussion a 20-MHz channel is primarily used as an example to explain the operation of embodiments of the fine-grained channel access system 100 and method, it should be noted that embodiments of the system 100 and method are of a scalable design that works for arbitrarily wide channel bands.
The AP 120 contains a subchannel division configuration 130.
Embodiments of the configuration 130 then divide the entire wireless channel 135 into a plurality of fine-grained subchannels that are equal in number to the number of subchannels determined earlier (box 420). Moreover, embodiments of the configuration 130 allow each node in the wireless network to contend for different subchannels independently (box 430). The output of the configuration 130 is a plurality of fine-grained subchannels (box 440).
The client computing device 110 (as well as other nodes) contains a subchannel request module 145.
Otherwise, embodiments of the module 145 generate a short orthogonal frequency-division multiplexing (OFDM) symbol for signaling by applying a smaller fast Fourier transform (FFT) that is a first FFT size (box 530). This first FFT size is smaller as compared to the second FFT size, which is described below.
Embodiments of the module 145 then embed in the short OFDM symbol the node's contention information (box 540). This embedded information is embedded in a set of subcarriers that the node intends to access to generate a M-RTS signal. Moreover, a M-CTS signal is generated from the short OFDM symbol (box 550).
Embodiments of the module 145 attach a short cyclic prefix to every M-CTS signal that acts as a first guard time (box 560). A long cyclic prefix is attached to each M-RTS signal to act as a second guard time (box 570). Embodiments of the module 145 transmit the M-RTS signal in the physical layer (PHY) using a first OFDM technique having a first FFT size (box 580). In addition, the M-CTS signal is transmitted in the physical layer as necessary (box 590). The operation of the signaling module 145 now will be discussed in further detail.
This section presents an analysis of the symbol timing misalignment problem in a carrier sense multiple access (CSMA)-based WLAN. Based on this analysis, the next section describes the PHY structure of embodiments of the fine-grained channel access system 100 and method.
In a WLAN, transmissions from distributed nodes are coordinated based on carrier-sensing and overhearing broad-cast frames (such as RTS/CTS). Unlike conventional MACs that use these mechanisms to avoid simultaneous transmissions, embodiments of the fine-grained channel access system 100 and method exploit simultaneous transmissions to enable concurrent access from different nodes but in orthogonal subchannels.
In embodiments of the fine-grained channel access system 100 and method, carrier-sensing coordinates the transmissions of M-RTS. After they sense an idle channel for a fixed DIFS time, two nodes A and B may transmit their M-RTS symbols simultaneously. However, since there is frequently a delay to sense the channel and for a signal to propagate from one node to another, these two M-RTS symbols cannot be transmitted and received at exactly the same time.
In a worst case scenario of the symbol timing difference of two such M-RTS symbols received by the AP, assume that node A senses the channel idle for tdifs seconds, and then node A transmits M-RTS first. It will take at most tprop for the signal to arrive at node B. However, it may take node B at least tcca time to reliably sense the busy channel. If the signal arrives at node B after node B starts a sensing slot, then node B may not be able to assess a busy channel. Note that is does depend on the way in which the node implements the carrier-sensing and threshold setting, but remember the worst case is being considered.
Continuing with the worst-case scenario, after the sensing slot, node B still declares an idle channel and sends its M-RTS as well. The radio signal of node B's M-RTS is actually emitted after tRxTx. And it will take at most another tprop for the M-RTS to arrive at the AP. The total misaligned time is characterized by the following equation, where the bound is tight at the worst case:
t
err
rts
≦t
cca
+t
RxTx+2×tprop. (1)
Depending on the values of the parameters, it is possible that terr its can be as large as lips.
If coordination is performed by overhearing a broadcast frame (such as M-CTS or DATA), the timing misalignment can be shorter because the two senders can be synchronized by a reference broadcast. In a worst-case scenario of symbol timing misalignment of data frames after receiving a M-CTS broadcast, assume that the jitter for a local timer can be neglected since the waiting time is usually very small (about a few microseconds). In this case, the timing misalignment is bounded by twice the propagation time. This is shown in the following equation, where terr is about 2 μs in a WLAN environment:
t
err≦2×tprop. (2)
Based on the analysis in last subsection, embodiments of the fine-grained channel access system 100 and method need to provide a guard time long enough to handle the symbol timing misalignment in a WLAN based on carrier sensing and broadcasting. This guard time can be as large as 11 μs if coordination is done with carrier sensing, or 2 μs if transmissions are synchronized by a preceding broadcast. In addition, an additional 800 ns guard time can be included for typical spreading delay in indoor environments. Embodiments of the fine-grained channel access system 100 and method include two guard-time sizes. Some embodiments include a long cyclic prefix (CP) of 11.8 μs and a short CP of 2.8 μs. The long CP is attached to M-RTS only, while a short CP is attached to every M-CTS, DATA, and ACK OFDM symbols, and therefore is the major overhead of concern.
To amortize the short CP overhead a longer OFDM symbol for data is desirable. A longer OFDM symbol is achieved by applying a larger FFT size N. Although in theory one can choose any large FFT size N, there are a few practical considerations. First, a large N requires more computational power to calculate the transform as the complexity of FFT is O(Nlog(N)). Second, with large N the adjacent subcarriers are spaced very closely. Therefore, it is more sensitive to the error in frequency setting of distributed nodes. Distributed nodes typically have some small frequency offset (a few hundred Hz). Thus, the adjacent subcarriers ideally have enough separation to accommodate such an offset. In practice, subcarriers are usually separated by at least 15 KHz.
In some embodiments of the fine-grained channel access system 100 and method, the FFT size of the DATA OFDM symbol is selected to be 256 in a 20 MHz channel. Moreover, in some embodiments its FFT period is 12.8 μs, and this yields a cyclic-prefix overhead of 18%, which is comparable to the CP overhead of the IEEE 802.11 standard (which is 20%). ACK symbols have the same structure as DATA symbols. M-RTS and M-CTS have a larger FFT size (such as 512-point) because they are signaling symbols that use a low modulation rate, and embodiments of the system 100 and method need slightly more subcarriers to embed control information.
In principle, embodiments of the fine-grained channel access system 100 and method can allocate each subcarrier independently to give maximal flexibility. However, in practice it is difficult for a node to use only a single subcarrier to transmit data for two practical reasons. First, using only one subcarrier means frequency selective fading. If that subcarrier encounters deep fading, then all data will be lost. Thus, it is desirable for a wireless PHY to code across multiple subcarriers to achieve spectrum diversity gain. Second, although a preamble may be used for channel estimation and compensation, the wireless channel may change during the period of data transmission. For high modulation rates, it is desirable for the receiver to track the changes in the wireless channel. This tracking is typically done by adding an additional training subcarrier (or pilot) along with other data subcarriers.
Therefore, embodiments of the fine-grained channel access system 100 and method group a set of subcarriers which are not necessarily contiguous) into a subchannel and uses it as the basis for channel access. Each subchannel contains sixteen data subcarriers and one pilot subcarrier. Thus, in some embodiments of the system 100 and method a single subchannel is 1.33 MHz wide with 6% pilot overhead (which is comparable to the IEED 802.11 standard (7%).
With different modulation modes, the PHY data rate of a single subchannel can range from 512 Kbps to 20.25 Mbps. The lowest data rate is achieved with BPSK, ½ coding on each data subcarrier; while the highest data rate is achieved by applying 64QAM, ⅚ coding and four spatial multiplexing streams on four antennas. With a 1.33 MHz subchannel, a 20 MHz 802.11 channel contains 14 orthogonal subchannels. The remainder of the spectrum is used as guard-bands separating adjacent wide-band channels.
It is also straightforward for embodiments of the system 100 and method to support wider band channels, such as 40-100 MHz or wider. For example, to support a 40 MHz channel, embodiments of the system 100 and method double the FFT size for all OFDM symbols. Since the sampling rate is also doubled the symbol period does not change. When using a 40 MHz channel, the guard-bands that would be ideal to separate two 20 MHz channels can be saved. Thus, embodiments of the system 100 and method can have 29 orthogonal subchannels with 40 MHz. Even though embodiments of the fine-grained channel access system 100 and method have slightly lower data rates than the IEEE 802.11 n standard, the effective throughputs are actually dramatically higher due to diminished overheads.
Some embodiments of the fine-grained channel access system 100 and method perform subcarrier allocation by having MRTS and M-CTS symbols use a 512-point FFT size in a 20 MHz channel. Wider channels use a proportionally longer FFT point size, and reduce the number and therefore overhead of guard-bands. For example, with a 40 MHz channel, some embodiments of the system 100 and method uses a 1024-point FFT size with nearly half the number of guard-bands. In order to explain this format each band will be explained by using a 20 MHz channel as an example.
III.B.3.i. Tag Band
The tag band contains a predefined binary sequence to identify the type of this OFDM symbol. The tag band is interleaved on selected subcarriers whose indexes are a multiple of four. Thus, the tag band creates a repeating pattern in time-domain samples. The receiver can use this pattern to detect the symbol using correlation. In a 20 MHz channel, the tag band contains 32 subcarriers. For M-RTS, the tag sequence starts with zero, {0, 1, 0, 1, . . . , 0, 1}, while for M-CTS the sequence becomes {1, 0, 1, . . . , 0, 1, 0}.
III.B.3.ii. Contention Band and Resolution Band
In M-RTS, sixteen subcarriers are allocated to each subchannel for contention. So, a total of 224 subcarriers are allocated for a contention band for all 14 subchannels arbitrated using an M-RTS symbol. Contention results are embedded in the resolution band in M-CTS.
The resolution band occupies more subcarriers compared to the contention band in the M-RTS. There are 24 subcarriers for each subchannel. However, since BAM is less robust compared to phase-shift keying modulation (such as BPSK), BAM's reliability can be improved by replicating on multiple subcarriers and using soft-decoding. It can be shown that by replicating twice, BAM can achieve the same performance of BPSK. In some embodiments of the fine-grained channel access system 100 and method, four-time replication is used when high reliability is required.
With four-time replication only six bits are allowed to carry the resolution results. Currently, six bits can represent the winner number (1-16) and two other special states. If these 6 bits are all zero it means the subchannel is not used and no node has contended for this subchannel. Alternately, if the bits are all ones, then the subchannel is reserved by the receiver. In this case no transmission is allowed on this subchannel during the contention period.
III.B.3.iii. Receiver Band
The receiver band contains the hash vector for intended receivers. In some embodiments of the fine-grained channel access system 100 and method, the hash vector contains 40 bits and each bit again is replicated four times for reliability. So the total number of subcarriers used is 160.
III.B.3.iv. Network Allocation Vector (NAV) Band
The network allocation vector (NAV) band encodes the expected number of OFDM symbols in the following data frame. Some embodiments of the fine-grained channel access system 100 and method contains 64 subcarriers. Due four times replication, the actual bits that can be modulated are 16. There are several ways to encode the data symbol number in the NAV band. One technique is to use each single bit to present a certain number of OFDM symbols. Thus, sixteen bits can present numbers between 3 to 48, with a step of 3 OFDM symbols. In a M-CTS, only the highest bit is echoed back and all nodes that overhear the M-CTS defer their contention for a corresponding period.
Embodiments of the fine-grained channel access system 100 and method include the data transmission module 185.
Embodiments of the module 185 then make a determination as to whether the entire channel has been idle for more than the DIFS time (box 620). If not, then embodiments of the module 185 wait a moment and check again (box 630). If so, then embodiments of the module 185 generate a long OFDM symbol for data transmission by applying a larger FFT size that is a second FFT size (box 640). The second FFT size is larger as compared to the first FFT size, described above.
Embodiments of the module 185 then attach a short cyclic prefix to every data symbol to act as the first guard time (box 650). Moreover, the short cyclic prefix is attached to every acknowledgement symbol to also act as the first guard time (box 660). Embodiments of the module 185 than transmit data using a second OFMD technique having the second FFT size (box 670).
Embodiments of the fine-grained channel access system 100 and method use the M-RTS/M-CTS signal exchange to avoid collisions in each subchannel. M-RTS/M-CTS are specially-designed OFDM symbols. M-RTS/M-CTS use simple Binary Amplitude Modulation (BAM) to modulate a single bit on each subchannel. Specifically, BAM maps a binary “0” to zero amplitude in a subcarrier, and it uses a random complex on the unit circle (eiθ) to represent binary “1”. This modulation is also known as On-Off Keying. BAM can be easily detected at the receiver by comparing the energy on a subcarrier against a threshold, without the need to recover the symbol phase.
The AP 120 receives multiple M-RTS signals from nodes (box 715). The AP 120 then detects the multiple signals that are superposed on different subcarriers of the contention band (box 720). The AP 120 may detect multiple ones on different subcarriers of the contention band. The AP 120 then selects a winning node for a subchannel based on predefined rules (box 725). In other words, embodiments of the module 165 can arbitrate a winning node by selecting a subcarrier based on some predefined rules (such as the one with highest frequency).
The AP 120 then sends a M-CTS signal containing an identification of the winning subcarrier for each subchannel as a subchannel allocation broadcast (box 730). After receiving the M-CTS signal, each node compares the subchannel allocation broadcast by the AP 120 to the subcarrier it has transmitted for the subchannel (box 735). A determination then is made as to whether the winning subcarrier and the randomly-selected subcarrier transmitted by the node match each other (box 740).
If there is no match, then node will repeat its request for a subchannel at a later time (box 745). In there is a match, then the node will transmit data symbols on that subchannel corresponding to the winning subcarrier box (box 750). This transmission is performed after a short interframe space (SIFS) delay, as specified in the IEEE 802.11 standard.
There are a few issues need to be considered in the frequency domain contention. First, the issue of what happens if two nodes transmit on the same contention subcarrier. It is very possible for two nodes to pick up the same random number and transmit on the same contention subcarrier. Thus, their transmitted energy is additive at the AP side. If the AP picks this subcarrier as the winner, a collision will happen since both nodes will consider themselves the winner and send data symbols on the same subchannel simultaneously. It is also possible (although the possibility is small) that two nodes' signals are destructive and cancel each other, so the AP may not be able to detect the transmissions. In this case the nodes involved will conclude that they were not allocated the subchannels they requested.
Second is the issue of how large is K? Clearly, a large K will have fewer collisions in a subchannel. However, a larger K also means more subcarriers are used and result in a larger signaling overhead. This is because more subcarriers need a larger FFT size and therefore a longer FFT period. Some embodiments of the fine-grained channel access system 100 and method set K=16, which is the initial contention window size in the IEEE 802.11 standard.
Third is the issue of who is responsible to return M-CTS. Any node that overhears the M-RTS transmissions can arbitrate the subchannel access. Simultaneous transmissions of M-CTS from different nodes are also allowed since, in a single broadcast domain, these M-CTS symbols are likely to have the same contention results and their energy is additive on each subcarrier. But it is still reasonable that only the potential receivers return M-CTS for M-RTS based on the following considerations: (1) the irrelevant nodes can be set to power-save mode, because it is unfair for them to spend power responding to M-RTS signals not for them; (2) it may unnecessarily create more exposed terminals if non-receiver nodes reply with M-CTS and block transmissions that could happen without interference.
One issue is how to specify a receiver in M-RTS. This is a non-trivial matter, since multiple nodes may transmit M-RTS simultaneously to different receivers and the receiver information may be mixed. Embodiments of the fine-grained channel access system 100 and method resolve this issue by using a membership vector of m subcarriers to present receiver information in M-RTS. Before transmitting M-RTS a node will hash the receiver's ID into a value between 0-(m−1). Then, the corresponding subcarrier will carry a “1” bit. Any node receiving M-RTS will check if the subcarrier corresponding to its ID has been set. If true, it returns a M-CTS. It is possible that multiple nodes have hash value collisions. But with a reasonably-sized m the number of station collisions remains small (2-3 in a typical WLAN setting), where one AP may be associated with several tens of stations. In some embodiments of the system 100 and method m is equal to 40 bits.
In embodiments of the fine-grained channel access system 100 and method, nodes will choose how many subchannels to request via contention based on their traffic requirements. If the traffic requirements are light, a node may choose fewer subchannels, while a heavily-loaded node may contend for as many as all subchannels for most efficient communication. However, if there are many nodes in a network contending for many subchannels, the collision avoidance mechanism provided by M-RTS/C-RTS may not be sufficient to represent all contenders since the number of subcarriers in the contention band is limited. One way to scale the ability to avoid collisions is to use multiple M-RTS symbols. But doing so will introduce more signaling overhead.
Instead, embodiments of the system 100 and method use a novel frequency domain backoff technique to scale in a heavily-contented dense network. The basic idea is to control the maximal number of subchannels that one node can access based on the observed collision level. More specifically,
Operation of embodiments of the frequency domain backoff module 150 begins by define a state variable, Cmax, which controls the maximal subchannel the node can access in the next transmission opportunity (box 800). It should be noted that each node maintains a state variable. Thus, when the channel is idle for DIFS, a node may pick up to n subchannels to contend for, where n=min(Cmax, Iqueue). In this case Iqueue is the number of fragments in the node's local sending queue.
Embodiments of the module 150 update the state variable, Cmax, by using one of two techniques (box 810). This update technique used is based on the contention situation on the channel. There can be multiple update strategies. In a first embodiment of the module 150 a reset-to-max technique is used to update the state variable (box 820). This first embodiment emulates the behavior of the binary exponential backoff (BEB) used in the IEEE 802.11 standard. In particular, when a collision is detected on a subchannel that a node has transmitted on, embodiments of the module 153 will reduce Cmax by half (box 830). Once all transmissions are successful embodiments of the module 150 reset the state variable, Cmax, to the total number of subchannels (box 840).
In second embodiment of the module 150 an additive increase/multiplicative decrease (AIMD) strategy is used to update the state variable (box 850). In this second embodiment, embodiments of the module 150 detect after transmission that p % of subchannels have collisions (box 860). Embodiments of the module 150 then decrease the state variable, Cmax, by p %, which is a multiplicative decrease (box 870). This reaction is reasonable since p reflects an estimation on the contention level in all subchannels. And when all accesses to subchannels succeed embodiments of the module 150 increases the state variable, Cmax, by one, which is an additive increase (box 880). It is straightforward to prove that both strategies converge if all contending nodes are within a single broadcasting domain.
This section discusses how embodiments of the fine-grained channel access system 100 and method schedule downlink traffic. Embodiments of the system 100 and method separate the uplink and downlink traffic by assigning different DIFS times to the AP and stations. The one with short DIFS has the priority to access the channel by sending M-RTS earlier.
To ensure fairness between uplink and downlink traffic, embodiments of the system 100 and method use a dynamic DIFS assignment strategy. In particular, a fixed DIFS time is assigned to all mobile stations and the AP has two different DIFS time settings. One is shorter than DIFS of mobile stations, and the other one is longer. The AP chooses a DIFS time based on the following rules. First, once an AP accesses the channel with a short DIFS, it will use the long DIFS for its next access. In addition, if an AP receives M-RTS from stations, it will use a short DIFS for its next access.
Embodiments of the fine-grained channel access system 100 and method are designed to operate in a computing environment. The following discussion is intended to provide a brief, general description of a suitable computing environment in which embodiments of the fine-grained channel access system 100 and method may be implemented.
Embodiments of the fine-grained channel access system 100 and method are operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well known computing systems, environments, and/or configurations that may be suitable for use with embodiments of the fine-grained channel access system 100 and method include, but are not limited to, personal computers, server computers, hand-held (including smartphones), laptop or mobile computer or communications devices such as cell phones and PDA's, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like.
Embodiments of the fine-grained channel access system 100 and method may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Embodiments of the fine-grained channel access system 100 and method may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. With reference to
Components of the computer 910 may include, but are not limited to, a processing unit 920 (such as a central processing unit, CPU), a system memory 930, and a system bus 921 that couples various system components including the system memory to the processing unit 920. The system bus 921 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus.
The computer 910 typically includes a variety of computer readable media. Computer readable media can be any available media that can be accessed by the computer 910 and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data.
Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer 910. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer readable media.
The system memory 930 includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) 931 and random access memory (RAM) 932. A basic input/output system 933 (BIOS), containing the basic routines that help to transfer information between elements within the computer 910, such as during start-up, is typically stored in ROM 931. RAM 932 typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit 920. By way of example, and not limitation,
The computer 910 may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,
Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive 941 is typically connected to the system bus 921 through a non-removable memory interface such as interface 940, and magnetic disk drive 951 and optical disk drive 955 are typically connected to the system bus 921 by a removable memory interface, such as interface 950.
The drives and their associated computer storage media discussed above and illustrated in
Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, radio receiver, or a television or broadcast video receiver, or the like. These and other input devices are often connected to the processing unit 920 through a user input interface 960 that is coupled to the system bus 921, but may be connected by other interface and bus structures, such as, for example, a parallel port, game port or a universal serial bus (USB). A monitor 991 or other type of display device is also connected to the system bus 921 via an interface, such as a video interface 990. In addition to the monitor, computers may also include other peripheral output devices such as speakers 997 and printer 996, which may be connected through an output peripheral interface 995.
The computer 910 may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer 980. The remote computer 980 may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer 910, although only a memory storage device 981 has been illustrated in
When used in a LAN networking environment, the computer 910 is connected to the LAN 971 through a network interface or adapter 970. When used in a WAN networking environment, the computer 910 typically includes a modem 972 or other means for establishing communications over the WAN 973, such as the Internet. The modem 972, which may be internal or external, may be connected to the system bus 921 via the user input interface 960, or other appropriate mechanism. In a networked environment, program modules depicted relative to the computer 910, or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,
The foregoing Detailed Description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.
