Parallel wireless communication apparatus, method, and system

An access point in a wireless network communicates with multiple mobile stations simultaneously using spatial-division multiple access. The access point transmits to the mobile stations frames that end at different times within a predefined window of time. When the predefined window in time is a short interframe space (SIFS) in an IEEE 802.11 compatible network, the access point will be done transmitting the frames in time to receive acknowledgements from the multiple mobile stations.

FIELD

The present invention relates generally to computer networks, and more specifically to wireless networks.

BACKGROUND

Wireless networks typically include mobile stations and access points. An access point may communicate with many different mobile stations, but typically communicates with only one mobile station at a time.

DESCRIPTION OF EMBODIMENTS

FIG. 1shows a diagram of a wireless network. Wireless network100includes access point (AP)102and mobile stations (STA)110,120, and130. In some embodiments, wireless network100is a wireless local area network (WLAN). For example, one or more of mobile stations110,120, and130, or access point102may operate in compliance with a wireless network standard such as ANSI/IEEE Std. 802.11, 1999 Edition, although this is not a limitation of the present invention. As used herein, the term “802.11” refers to any past, present, or future IEEE 802.11 standard, including, but not limited to, the 1999 edition.

As explained below, in some embodiments, mobile stations110,120, and130operate in compliance with an 802.11 standard, and access point102is capable of maintaining simultaneous 802.11 compliant communications with multiple mobile stations. Mobile stations110,120, and130may be any type of mobile station capable of communicating in network100. For example, the mobile stations may be computers, personal digital assistants, wireless-capable cellular phones, or the like.

Access point102communicates with mobile station110(also referred to as “STA1”) using signal112. Access point102communicates with mobile station120(also referred to as “STA2”) using signal122, and access point102communicates with mobile station130(also referred to as “STA3”) using signal132. When access point102sends signals to one or more mobile stations, this is referred to as the “downlink,” and when access point102receives signals from one or more mobile stations, this is referred to as the “uplink.” In various embodiments of the present invention, access point102may communicate simultaneously with multiple mobile stations on the downlink and may communicate simultaneously with multiple mobile stations on the uplink.

Access point102includes antennas104. Access point102may be any type of access point having multiple antennas capable of communicating using spatial-division multiple access (SDMA). Spatial-division multiple access is a technique that allows multiple simultaneous independent transmissions from one wireless device that has multiple antennas to other wireless devices that may or may not have multiple antennas. For example, in some embodiments, access point102utilizes SDMA on the downlink to transmit to two or more of mobile stations110,120, or130simultaneously. Also for example, in some embodiments, access point102utilizes SDMA on the uplink to receive from two or more of mobile stations110,120, or130simultaneously. As used herein, the terms “parallel stations,” “parallel group,” or “parallel STAs” refer to a group of mobile stations that communicate simultaneously with access point102.

In some embodiments of the present invention, a medium access control layer (MAC) in access point102controls the timing and contents of SDMA transmissions to parallel STAs. For example, a MAC in access point102may coordinate the timing of frames sent to multiple stations such that the frames end near in time to each other. When the multiple mobile stations respond, the MAC receives the responses, even if they overlap in time. Various embodiments of SDMA transmissions are described below with reference toFIGS. 4-9.

Spatial-division multiple access increases both user density and network throughput of wireless systems by utilizing spatial channels in the environment. For example, multiple spatial channels may be formed by a combination of the signal path(s) and the antenna patterns between an AP and multiple STAs.

In some embodiments, access point102may use zero-forcing beamformers for both downlink and uplink of signals to achieve SDMA. The zero-forcing beamformer is a known technique for SDMA interference cancellation using known channel state information. In some embodiments, channel state information is gathered by access point102during a prior uplink packet reception. Various embodiments of gathering state information, also referred to as “estimating spatial channels,” is described below with reference toFIGS. 2 and 3.

FIGS. 2 and 3show frame sequences to estimate channel state information of communications channels. In some embodiments, channel state information of all STAs in a parallel group may be obtained before SDMA transmission and reception. Referring now toFIG. 2, an AP sends short frames and forces STAs to send back short frames. As shown inFIG. 2, in some embodiments, an AP sends IEEE 802.11 Null-data frames in turn to STAs and estimates the STAs' channels from the received acknowledgement (ACK) frames. For example, as shown inFIG. 2, the AP sends a Null-data frame210to STA1, which returns ACK frame212; the AP sends Null-data frame220to STA2, which returns ACK frame222; and the AP sends Null-data frame230to STA3, which returns ACK frame232. Embodiments represented byFIG. 2may be employed after channel access using either point coordination function (PCF), or distributed coordination function (DCF) of an 802.11 MAC protocol, but the invention is not limited in this respect. In some embodiments, transmission of Null-data frames may have the side effect of setting the NAV of all STAs in range because the Null-data frames may be transmitted using nominally omni-directional antenna patterns

In some embodiments, frames other than Null-data frames and ACK frames are utilized to estimate spatial channels. For example, in some embodiments, frames such as IEEE 802.11 request-to-send (RTS) and clear-to-send (CTS) frames are utilized to estimate spatial channels. In some embodiments, RTS and CTS frames are sent following channel access using the DCF of an 802.11 MAC protocol. Regardless of the type of frame utilized, the network-allocation-vector (NAV) may be set to the end of the last ACK or CTS to prevent an unintended STA from acquiring the medium during the SDMA training process. Whenever an AP loses channel information of a STA, it can employ various embodiments of spatial channel estimation to update channel information. Many different possible frame types may be utilized to estimate spatial channels without departing from the scope of the present invention.

Referring now toFIG. 3, an AP utilizes a combination of Null-data frames and RTS frames to force STAs to send back short frames. In embodiments represented byFIG. 3, the final Null-data frame may be replaced with an RTS frame (which solicits a CTS response). In some embodiments, this may have the side effect of setting the NAV of all STAs in range because the RTS and CTS may both be transmitted using nominally omni-directional antenna patterns.

FIGS. 4-7show SDMA frame sequences that correspond to parallel communications using the IEEE 802.11 point coordination function (PCF). In some embodiments, frames sent using PCF are sent during a contention-free (CF) period. During the contention-free period, STAs cannot initiate frame exchange sequences because their NAV timer prevents channel access using DCF. They transmit only in two cases: a STA transmits an acknowledgement frame (ACK) after receiving a data (or management) frame directed to it; and a STA transmits after receiving a frame with a contention-free-poll (CF-Poll). In other words, STAs transmit during a contention-free period only if the AP solicits a response. The STAs may include polled, unpolled, pollable, and unpollable STAs.

Referring now toFIG. 4, an AP sends Data frames or Data+CF-Ack frames to parallel STAs using SDMA and then switches to receive mode to receive ACKs from the STAs in parallel using SDMA. For example, the AP sends Data frame402to STA1, Data+CF-Ack frame404to STA2, and Data frame406to STA3. In some embodiments, the multiple frames sent simultaneously on the downlink are coordinated in time such that they all end within a predetermined time window. For example, as shown inFIG. 4, frames402,404, and406all end within a predetermined time window equal to one IEEE 802.11 short interframe spacing (SIFS). In some embodiments, the predetermined time window is greater than or less than one SIFS. For example, in some embodiments, the predetermined time window is substantially equal to a short-interframe spacing (SIFS) period plus 10% of a slot time.

In response to the frames send to the STAs, the STAs transmit ACK frames back to the AP. For example, STA1transmits ACK frame408; STA2transmits ACK frame410; and STA3transmits ACK frame412. Although as shown inFIG. 4STA2may hear STA1's transmission of ACK, STA2will still transmit its ACK since an 802.11 compliant STA will send an ACK (or CF-ACK) regardless of the channel idle/busy status.

By utilizing SDMA, the AP is able to transmit information in different spatial channels to parallel STAs simultaneously. Further, the AP is able to receive information from parallel STAs simultaneously using separate spatial channels. As shown inFIG. 4, the parallel transmissions are ended prior to an expected start time of the first response. In general, the parallel transmissions all end in a window in time having a width that is related to the short interframe spacing (SIFS).

Referring now toFIG. 5, the AP sends multiple frames of the following IEEE 802.11 types: Data, Data+CF-ACK, ACK, or CF-ACK frames. Further, in some embodiments, the AP may add a single CF-Poll in the frames to ask for data from one STA. If the CF-ACK is added, one of the frames, i.e., Data+CF-ACK, Data+CF-ACK+CF-Poll, CF-ACK+CF-Poll may be sent in the parallel group of transmissions. The downlink sequences shown inFIGS. 4 and 5generally do not cause retransmissions in the uplinks. In contrast, the downlink sequence inFIG. 6, described below, may cause retransmission in the uplink.

Referring now toFIG. 6, the AP sends Data frames along with multiple CF-Polls in the parallel group of transmissions to ask for uplink traffic. The timing relationships are the same as that described above with reference toFIG. 4. If the termination time instances of the uplink frames from multiple STAs that require acknowledgments are within a time window, (e.g., one SIFS period plus 10% of a slot time), the AP doesn't cause retransmission in the uplink. In some embodiments, only those uplink frames requesting an acknowledgement matter to the time window. If the termination time instances spread over the time window, then the AP still receives the acknowledgements from the STAs, but it may not acknowledge the received uplink Data frames in time. If the AP sends out the acknowledgements late, it may cause retransmission in the uplink depending on the ACKTimeout implementation on the STAs. The specific example shown in theFIG. 6does not cause uplink retransmission.

Referring now toFIG. 7, an example is shown where efficiency is improved by sending data in the guard gap, (e.g., one SIFS period). In this example, the AP knows that STA1has data to send and that STA2hasn't data to send. This information may be retrieved from the more-data fields of frames previously received. The AP predicts that the uplink transmission of STA1should be longer than the one of STA2. So, the AP sends more data to STA2than to STA1to exploit the guard gap between transmission and reception of STA1's data.

The frame sequences shown inFIGS. 8 and 9correspond to parallel communications using distributed coordination function (DCF). The AP sets the network-allocation-vector (NAV) of all STAs in the vicinity to prevent unintended STAs from interfering with the beamformed signals. The NAV can be set by transmission of RTS or CTS frames using nominally omni-directional radiation antennas.FIG. 8shows the AP setting the NAV using a broadcast CTS frame, andFIG. 9shows the AP setting the NAV using a unicast RTS frame addressed to one of the STAs in its parallel group. As in the previous figures, the termination times of the parallel frames are coordinated to end in a time window. In some embodiments, the time window is related to a short interframe spacing (SIFS), and in some embodiments, the time window is substantially equal to a short-interframe spacing (SIFS) period plus 10% of a slot time.

FIG. 10shows a system diagram in accordance with various embodiments of the present invention. Electronic system1000includes antennas1010, radio interface1020, physical layer (PHY)1030, media access control (MAC) mechanism1040, Ethernet interface1050, processor1060, and memory1070. In some embodiments, electronic system I000may be an access point that can communicate in parallel with multiple 802.11 compliant mobile stations. For example, electronic system1000may be utilized in network100as access point102. Also for example electronic system1000may be an access point capable of communicating with mobile stations using frame sequences shown in the previous figures.

In some embodiments, electronic system1000may represent a system that includes an access point as well as other circuits. For example, in some embodiments, electronic system1000may be a computer, such as a personal computer, a workstation, or the like, that includes an access point as a peripheral or as an integrated unit. Further, electronic system1000may include a series of access points that are coupled together in a network.

In operation, system1000sends and receives signals using antennas1010, and the signals are processed by the various elements shown inFIG. 10. Antennas1010may be an antenna array or any type of antenna structure that supports SDMA.

Radio interface1020is coupled to antennas1010to interact with a wireless network. Radio interface1020may include circuitry to support the transmission and reception of radio frequency (RF) signals. For example, in some embodiments, radio interface1020includes an RF receiver to receive signals and perform “front end” processing such as low noise amplification (LNA), filtering, frequency conversion or the like. Further, in some embodiments, radio interface1020includes beamforming circuitry to support SDMA processing. Also for example, in some embodiments, radio interface1020includes circuits to support frequency up-conversion, and an RF transmitter. The invention is not limited by the contents or function of radio interface1020.

Physical layer (PHY)1030may be any suitable physical layer implementation. For example, PHY1030may be a circuit block that implements a physical layer that complies with an IEEE 802.11 standard or other standard. Examples include, but are not limited to, direct sequence spread spectrum (DSSS), frequency hopping spread spectrum (FHSS), and orthogonal frequency division multiplexing (OFDM).

Media access control (MAC) mechanism1040may be any suitable media access control layer implementation. For example, MAC1040may be implemented in software, or hardware or any combination thereof. In some embodiments, a portion of MAC1040may be implemented in hardware, and a portion may be implemented in software that is executed by processor1060. Further, MAC1040may include a processor separate from processor1060. MAC1040may implement any of the parallel communications embodiments of the present invention. For example, MAC1040may provide frames and their coordinated timing to achieve parallel communications using SDMA.

Processor1060may perform method embodiments of the present invention, such as method1100(FIG. 11). Processor1060represents any type of processor, including but not limited to, a microprocessor, a digital signal processor, a microcontroller, or the like.

Memory1070represents an article that includes a machine readable medium. For example, memory1070represents a random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), read only memory (ROM), flash memory, or any other type of article that includes a medium readable by processor1060. Memory1070may store instructions for performing the execution of the various method embodiments of the present invention.

Ethernet interface1050may provide communications between electronic system1000and other systems. For example, in some embodiments, electronic system1000may be an access point that utilizes Ethernet interface1050to communicate with a wired network or to communicate with other access points. Some embodiments of the present invention do not include Ethernet interface1050. For example, in some embodiments, electronic system1000may be a network interface card (NIC) that communicates with a computer or network using a bus or other type of port.

FIG. 11shows a flowchart in accordance with various embodiments of the present invention. In some embodiments, method1100may be used to communicate with parallel mobile stations using SDMA. In some embodiments, method1100, or portions thereof, is performed by an access point, a processor, or an electronic system, embodiments of which are shown in the various figures. Method1100is not limited by the particular type of apparatus, software element, or system performing the method. The various actions in method1100may be performed in the order presented, or may be performed in a different order. Further, in some embodiments, some actions listed inFIG. 11are omitted from method1100.

Method1100is shown beginning at block1110in which an access point estimates spatial channels for a plurality of stations. In some embodiments, this corresponds to sending frames to each of the plurality of stations in turn, and receiving frames back. For example, referring now back toFIG. 2, null-data frames may be sent, and ACK frames may be received. Also for example, referring now back toFIG. 3, an RTS frame may be sent and a CTS frame may be received. In some embodiments, each spatial channel is estimated from the received frames.

At1120, frames are transmitted to each of the plurality of stations wherein the frames are timed to end within a predefined window. In some embodiments, the frames have a format compatible with an IEEE 802.11 standard. The frames may be sent using point coordination function (PCF) or distributed coordination function (DCF). Any type of suitable frame may be transmitted, including but not limited to, RTS, CTS, polling frames, and non-polling frames. The length of the predefined window may be related to the length of a short interframe space (SIFS) or other space., For example, in some embodiments, the predefined window may be substantially equal to a SIFS plus 10% of a slot time.