U.S. patent application number 11/430136 was filed with the patent office on 2007-07-05 for method, apparatus and system of spatial division multiple access communication in a wireless local area network.
Invention is credited to Nir Shapira.
Application Number | 20070153760 11/430136 |
Document ID | / |
Family ID | 38224293 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070153760 |
Kind Code |
A1 |
Shapira; Nir |
July 5, 2007 |
Method, apparatus and system of spatial division multiple access
communication in a wireless local area network
Abstract
Some demonstrative embodiments of the invention include a
method, apparatus, and system of performing simultaneous downlink
transmission over a wireless medium to a plurality of wireless
stations, using Spatial Division Multiple Access (SDMA) in a
wireless local area network (WLAN). In one demonstrative embodiment
of the invention, the method may include synchronizing transmission
of first and second data frames to be received by first and second
wireless stations, respectively, such that the transmissions of the
first and second data frames end substantially simultaneously.
Other embodiments are described and claimed.
Inventors: |
Shapira; Nir; (Raanana,
IL) |
Correspondence
Address: |
PEARL COHEN ZEDEK LATZER, LLP
1500 BROADWAY 12TH FLOOR
NEW YORK
NY
10036
US
|
Family ID: |
38224293 |
Appl. No.: |
11/430136 |
Filed: |
May 9, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11319526 |
Dec 29, 2005 |
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11430136 |
May 9, 2006 |
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Current U.S.
Class: |
370/350 ;
455/502 |
Current CPC
Class: |
H04W 56/0005 20130101;
H04B 7/2681 20130101; H04W 72/04 20130101; H04W 36/18 20130101;
H04W 92/10 20130101 |
Class at
Publication: |
370/350 ;
455/502 |
International
Class: |
H04J 3/06 20060101
H04J003/06 |
Claims
1. A method of simultaneously transmitting data to two or more
wireless stations, the method comprising: synchronizing
transmission of first and second data frames to be received by
first and second wireless stations, respectively, such that the
transmissions of said first and second data frames end
substantially simultaneously.
2. The method of claim 1, wherein synchronizing comprises beginning
the transmission of a shorter of the first and second data frames
after beginning the transmission of a longer of the first and
second data frames.
3. The method of claim 1, wherein synchronizing comprises beginning
the transmissions of said first and second frames substantially
simultaneously, and transmitting a shorter of the first and second
data frames at a transmission rate higher than a transmission rate
of a longer of the first and second data frames.
4. The method of claim 1 further comprising synchronizing
transmission of at least a third data frame to be received by at
least a third, respective, wireless station, such that the
transmissions of said first, second, and at least third data frames
end substantially simultaneously.
5. The method of claim 1, wherein the first and second wireless
stations operate in accordance with a standard relating to an IEEE
802.11 standard.
6. A method of simultaneous transmission of data to a plurality of
wireless stations, the method comprising: transmitting at least
first and second blocks of one or more data frames to at least
first and second wireless stations, respectively, wherein at least
part of said first block and at least part of said second block are
transmitted substantially simultaneously; and transmitting first
and second block-acknowledgement request frames to said first and
second wireless stations, respectively, after transmitting said
first and second data blocks.
7. The method of claim 6 comprising beginning the transmissions of
said first and second blocks substantially simultaneously.
8. The method of claim 6 comprising transmitting said at least
first and second block-acknowledgement request frames substantially
simultaneously.
9. The method of claim 6 comprising sequentially transmitting said
at least first and second block-acknowledgement request frames.
10. The method of claim 6 comprising transmitting said first
block-acknowledgement request frame and at least part of said
second data block substantially simultaneously.
11. The method of claim 6, wherein transmitting said at least first
and second data blocks comprises transmitting at least first,
second and third blocks of data frames to at least first, second
and third wireless stations, respectively, and wherein at least
part of said first block and at least part of said third block are
transmitted substantially simultaneously.
12. The method of claim 6 comprising synchronizing transmissions of
one or more frames of said first block with one or more frames of
said second block, respectively, such that that transmissions of
the one or more frames of said first block begin substantially
simultaneously with transmissions of the one or more frames of said
second bloc, respectively.
13. An apparatus of simultaneously transmitting data to two or more
wireless remote stations in two or more channels comprising: a
controller to synchronize transmission of first and second data
frames to be received by first and second wireless stations,
respectively, such that the transmissions of said first and second
data frames end substantially simultaneously.
14. The apparatus of claim 13, wherein said controller begins
transmission of a shorter of the first and second data frames after
beginning the transmission of a longer of the first and second data
frames.
15. The apparatus of claim 13, wherein said controller begins the
transmissions of said first and second frames substantially
simultaneously, and transmits a shorter of the first and second
data frames at a transmission rate higher than a transmission rate
of a longer of the first and second data frames.
16. An apparatus of simultaneous transmission of data to a
plurality of wireless stations, the apparatus comprising: a
controller to transmit at least first and second blocks of one or
more data frames to at least first and second wireless stations,
respectively, wherein at least part of said first block and at
least part of said second block are transmitted substantially
simultaneously; and to transmit first and second
block-acknowledgement request frames to said first and second
wireless stations, respectively, after transmitting said first and
second data blocks.
17. The apparatus of claim 16, wherein said controller begins the
transmissions of said first and second blocks substantially
simultaneously.
18. The apparatus of claim 16, wherein said controller transmits
said at least first and second block-acknowledgement request frames
substantially simultaneously.
19. The apparatus of claim 16, wherein said controller sequentially
transmits said at least first and second block-acknowledgement
request frames.
20. The apparatus of claim 16, wherein said controller transmits
said first block-acknowledgement request frame and at least part of
said second data block substantially simultaneously.
21. The apparatus of claim 16, wherein said controller synchronizes
transmissions of one or more frames of said first block with one or
more frames of said second block, respectively, such that that
transmissions of the one or more frames of said first block begin
substantially simultaneously with transmissions of the one or more
frames of said second bloc, respectively.
22. A system of simultaneously transmitting data, the system
comprising: first and second wireless stations; and an access point
to synchronize transmission of first and second data frames to be
received by said first and second wireless stations, respectively,
such that the transmissions of said first and second data frames
end substantially simultaneously.
23. The system of claim 22, wherein said access point begins
transmission of a shorter of the first and second data frames after
beginning the transmission of a longer of the first and second data
frames.
24. The system of claim 22, wherein said access point begins the
transmissions of said first and second frames substantially
simultaneously, and transmits a shorter of the first and second
data frames at a transmission rate higher than a transmission rate
of a longer of the first and second data frames.
25. The system of claim 22, wherein the first and second wireless
stations operate in accordance with a standard relating to an IEEE
802.11 standard.
26. A system of simultaneous transmission of data, the system
comprising: at least first and second wireless stations; and an
access point to transmit at least first and second blocks of one or
more data frames to said at least first and second wireless
stations, respectively, wherein at least part of said first block
and at least part of said second block are transmitted
substantially simultaneously; and to transmit first and second
block-acknowledgement request frames to said first and second
wireless stations, respectively, after transmitting said first and
second data blocks.
27. The system of claim 26, wherein said access point begins the
transmissions of said first and second blocks substantially
simultaneously.
28. The apparatus of claim 26, wherein said access point transmits
said at least first and second block-acknowledgement request frames
substantially simultaneously.
29. The system of claim 26, wherein said access point sequentially
transmits said at least first and second block-acknowledgement
request frames.
30. The system of claim 26, wherein the first and second wireless
stations operate in accordance with a standard relating to an IEEE
802.1 1 standard.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation In Part Application of
U.S. patent application Ser. No. 11/319,526, filed Dec. 29, 2005,
the entire disclosure of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of wireless
communication. In particular, embodiments of the invention relate
to a method, apparatus and system for use of Spatial Division
Multiple Access (SDMA) in a wireless local area network (WLAN).
BACKGROUND OF THE INVENTION
[0003] In a wireless local area network (WLAN), a single central
base station, e.g., an access point (AP) may communicate with
multiple mobile stations (STA) over a wireless communication link
in what may be referred to as point to multi-point communication.
For example, the AP may utilize a time domain duplexing (TDD)
channel access scheme, in which transmissions to the multiple
stations may be multiplexed in different time slots in the same
frequency band, or a frequency domain duplexing (FDD) channel
access scheme, in which transmissions to the multiple stations may
occur simultaneously, but in different frequency bands. Thus,
although an AP in a WLAN may potentially communicate with multiple
users, in many cases, for example, in TDD and/or FDD systems, the
communication is point to point at any single instance of time and
frequency.
[0004] Spatial division multiple access (SDMA) is a method of
multiplexing several signal streams, each one targeted to a
different destination, simultaneously, by utilizing multiple
antennas. An SDMA channel access method may enable the use of the
same frequency at the same time to communicate with several
stations located in different places. For example, an SDMA AP
having multiple antennas may use a beamforming technique to
transmit to several remote stations simultaneously. Each transmit
antenna may transmit the intended signal multiplied by a certain
weight, and by dynamically controlling the weights of each antenna
the transmission may be directed to a desired location. Under
certain assumptions, it can be shown that data transmissions to N
users can be multiplexed together using N antennas, for a total
capacity increase by a factor N compared with simple legacy
networks that allow access to the wireless medium for only a single
user at a time.
[0005] However, the integration of higher capacity transmission
technology into existing wireless LANs may require operation in
accordance with the existing systems' physical layer (PHY) and
media access control layer (MAC) protocols, e.g., for backwards
compatibility. For example, the MAC protocol may ensure that all
users have an equal opportunity to contend for access to the
medium, provide means for avoiding collisions, e.g., due to
concurrent transmissions by two or more stations, and provide a
method of recovery from collisions.
[0006] The Institute of Electrical and Electronics Engineers (IEEE)
802.11 family of standards ("IEEE-Std 802.11, 1999 Edition (ISO/IEC
8802-11: 1999)" and derivatives thereof) provides one current MAC
protocol for WLAN systems. For example, the IEEE 802.11 MAC may
regulate access to the wireless medium by equal priority for access
contention, e.g., using a collision sense multiple access/collision
avoidance (CSMA/CA) scheme, in which each station implements a
carrier sense mechanism to detect the state of the wireless medium,
and a positive acknowledgement scheme to ensure correct reception
of data frames.
[0007] Backward compatibility of APs with user stations operating
on earlier, slower versions of a transmission standard may reduce
overall throughput. For example, in the IEEE 802.11g standard the
throughput may reach 54 Mbps. However, in a deployment scenario
having legacy stations designed to an earlier standard, e.g., IEEE
802.11b, that may communicate at less than 11 Mbps, the legacy
stations may dominate the usage of the wireless medium to the
detriment of user stations of more recent design. This problem may
be further compounded as new standards such as, e.g., the IEEE
802.11n multiple-input-multiple-output (MIMO) standard which allows
for data rates over 100 Mbps, are deployed.
SUMMARY OF SOME DEMONSTRATIVE EMBODIMENTS OF THE INVENTION
[0008] Some demonstrative embodiments of the invention include a
method, apparatus, and/or system of performing simultaneous
downlink transmission over a wireless medium to a plurality of
wireless stations, using Spatial Division Multiple Access (SDMA) in
a wireless local area network (WLAN).
[0009] According to some demonstrative embodiments of the
invention, the method may include synchronizing transmission of
first and second data frames to be received by first and second
wireless stations, respectively, such that the transmissions of the
first and second data frames end substantially simultaneously.
[0010] According to some demonstrative embodiments of the
invention, the synchronizing may include beginning the transmission
of a shorter of the first and second data frames after beginning
the transmission of a longer of the first and second data
frames.
[0011] According to some demonstrative embodiments of the
invention, the synchronizing may include beginning the
transmissions of the first and second frames substantially
simultaneously, and transmitting a shorter of the first and second
data frames at a transmission rate higher than a transmission rate
of a longer of the first and second data frames.
[0012] According to some demonstrative embodiments of the
invention, the method may also include synchronizing transmission
of at least a third data frame to be received by at least a third,
respective, wireless station, such that the transmissions of the
first, second, and at least third data frames end substantially
simultaneously.
[0013] According to another demonstrative embodiment of the
invention, a method of simultaneous transmission of data to a
plurality of wireless stations may include transmitting at least
first and second blocks of one or more data frames to at least
first and second wireless stations, respectively, wherein at least
part of the first block and at least part of the second block are
transmitted substantially simultaneously; and transmitting first
and second block-acknowledgement request frames to the first and
second wireless stations, respectively, after transmitting the
first and second data blocks.
[0014] According to some demonstrative embodiments of the
invention, the method may include beginning the transmissions of
the first and second blocks substantially simultaneously.
[0015] According to some demonstrative embodiments of the
invention, the method may include transmitting the at least first
and second block-acknowledgement request frames substantially
simultaneously.
[0016] According to some demonstrative embodiments of the
invention, the method may include sequentially transmitting the at
least first and second block-acknowledgement request frames.
[0017] According to some demonstrative embodiments of the
invention, the method may include transmitting the first
block-acknowledgement request firame and at least part of the
second data block substantially simultaneously.
[0018] According to some demonstrative embodiments of the
invention, the method may include transmitting the at least first
and second data blocks comprises transmitting at least first,
second and third blocks of data frames to at least first, second
and third wireless stations, respectively, and wherein at least
part of the first block and at least part of the third block are
transmitted substantially simultaneously.
[0019] According to some demonstrative embodiments of the
invention, the method may include synchronizing transmissions of
one or more frames of the first block with one or more frames of
the second block, respectively, such that that transmissions of the
one or more frames of the first block begin substantially
simultaneously with transmissions of the one or more frames of the
second bloc, respectively.
BRIEF DESCRIPTION OF TIE DRAWINGS
[0020] 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 features and advantages thereof,
may best be understood by reference to the following detailed
description when read with the accompanied drawings in which:
[0021] FIG. 1 is a schematic diagram of wireless communication
system in accordance with some demonstrative embodiments of the
invention;
[0022] FIG. 2 is a schematic flowchart of a method of wireless
transmission in accordance with one demonstrative embodiment of the
invention;
[0023] FIGS. 3A and 3B are schematic timing diagrams showing
start-time and end-time coordination, respectively, which may be
used by methods of simultaneous wireless transmission in accordance
with some demonstrative embodiments of the invention;
[0024] FIGS. 4A and 4B are schematic timing diagrams showing
simultaneous block acknowledgments and sequential block
acknowledgements, respectively, which may be used by methods of
simultaneous wireless transmission in accordance with some
demonstrative embodiments of the invention;
[0025] FIG. 5 is a schematic flowchart of a method of wireless
transmission in accordance with another demonstrative embodiment of
the invention;
[0026] FIG. 6 is a schematic illustration of a frame format for a
transmission mode;
[0027] FIG. 7 is a schematic flowchart of a return acknowledgment
(ACK) frame detection method in accordance with some demonstrative
embodiments of the invention; and
[0028] FIG. 8 is a schematic illustration of a structure of a
preamble signal.
[0029] It will be appreciated that for simplicity and clarity of
illustration, elements shown in the figures have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements may be exaggerated relative to other elements for clarity.
Further, where considered appropriate, reference numerals may be
repeated among the figures to indicate corresponding or analogous
elements.
DETAILED DESCRIPTION OF SOME DEMONSTRATIVE EMBODIMENTS OF THE
INVENTION
[0030] 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 present 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.
[0031] Some portions of the detailed description, which follow, are
presented in terms of algorithms and symbolic representations of
operations on data bits or binary digital signals within a computer
memory. These algorithmic descriptions and representations may be
the techniques used by those skilled in the data processing arts to
convey the substance of their work to others skilled in the
art.
[0032] Unless specifically stated otherwise, as apparent from the
following discussions, it is appreciated that throughout the
specification discussions utilizing terms such as "processing,"
"computing," "calculating," "determining," or the like, refer to
the action and/or processes of a computer or computing system, or
similar electronic computing device, that manipulate and/or
transform data represented as physical, such as electronic,
quantities within the computing system's registers and/or memories
into other data similarly represented as physical quantities within
the computing system's memories, registers or other such
information storage, transmission or display devices. In addition,
the term "plurality" may be used throughout the specification to
describe two or more components, devices, elements, parameters and
the like.
[0033] It should be understood that the present invention may be
used in a variety of applications. Although the present invention
is not limited in this respect, the circuits and techniques
disclosed herein may be used in many apparatuses such as personal
computers, stations of a radio system, wireless communication
system, digital communication system, satellite communication
system, and the like.
[0034] Stations intended to be included within the scope of the
present invention include, by way of example only, wireless local
area network (WLAN) stations, wireless personal area network (WPAN)
stations, two-way radio stations, digital system stations, analog
system stations, cellular radiotelephone stations, and the
like.
[0035] Types of WLAN communication systems intended to be within
the scope of the present invention include, although are not
limited to, systems described by the "IEEE-Std 802.11, 1999 Edition
(ISO/IEC 8802-11: 1999)" standard, and more particularly in
"IEEE-Std 802.11b-1999 Supplement to 802.11-1999, Wireless LAN MAC
and PHY specifications: Higher speed Physical Layer (PHY) extension
in the 2.4 GHz band", "IEEE-Std 802.11a-1999, Higher speed Physical
Layer (PHY) extension in the 5 GHz band", "IEEE-Std 802.11g -2003
Supplement to 802.11-1999, Wireless LAN MAC and PHY specifications:
Further Higher Data Rate Extension in the 2.4 GHz band, Draft 8.2",
"IEEE-Std 802.11e -2005 Specific requirements Part 11: Wireless LAN
Medium Access Control (MAC) and Physical Layer (PHY) specifications
Amendment 8: Medium Access Control (MAC) Quality of Service
Enhancements", and the like.
[0036] Types of WLAN stations intended to be within the scope of
the present invention include, although are not limited to,
stations for receiving and/or transmitting spread spectrum signals
such as, for example, Frequency Hopping Spread Spectrum (FHSS),
Direct Sequence Spread Spectrum (DSSS), Orthogonal
Frequency-Division Multiplexing (OFDM) and the like.
[0037] Devices, systems, and methods incorporating aspects of
embodiments of the invention are also suitable for computer
communication network applications, for example, intranet and
Internet applications. Embodiments of the invention may be
implemented in conjunction with hardware and/or software adapted to
interact with a computer communication network, for example, a
local area network (LAN), a wide area network (WAN), or a global
communication network, for example, the Internet.
[0038] Part of the discussion herein may relate, for demonstrative
purposes, to transmitting a frame, e.g., a physical layer (PHY)
protocol data unit (PPDU) or a media access control (MAC) service
data unit (MSDU). However, embodiments of the invention are not
limited in this regard, and may, include, for example, transmitting
a signal, a packet, a block, a data portion, a data sequence, a
data signal, a data packet, a preamble, a signal field, a content,
an item, a message, or the like.
[0039] FIG. 1 schematically illustrates a block diagram of a
wireless communication system 100 in accordance with some
demonstrative embodiments of the invention. It will be appreciated
by those skilled in the art that the simplified components
schematically illustrated in FIG. 1 are intended for demonstration
purposes only, and that other components may be required for
operation of the wireless devices. Those of skill in the art will
further note that the connection between components in a wireless
device need not necessarily be exactly as depicted in the schematic
diagram.
[0040] Wireless communication system 100 may include, for example,
one or more wireless Access Points (APs), e.g., an AP 110 having N
transmit antennas 112, suitable, e.g., for spatial division
multiple access (SDMA) transmission. System 100 may also include
one or more stations (STAs), e.g., stations 120, 130, and 140
having one or more radio frequency antennas 122, 132, and 142,
respectively, to receive transmissions from AP 110. Antennas 112,
122, 132, and 142 may include, for example, a dipole antenna,
omnidirectional antenna, semi-omnidirectional antenna, and/or any
other type of antenna suitable for transmission and/or reception of
radio frequency signals.
[0041] According to some demonstrative embodiments of the
invention, AP 110 may communicate with one or more of stations 120,
130, and 140 via one or more wireless communication links, e.g., a
downlink 190 and/or an uplink (not shown). For example, downlink
190 may include one or more wireless channels, e.g., spatial
channels 191, 192, 193 and/or 194 corresponding to the plurality of
antennas 112. AP 110 may transmit to one or more of STA 120, 130,
and/or 140 via the multiple antennas 112 using an SDMA transmission
scheme, e.g., as explained in detail below with reference to FIGS.
2, 3, 4 and/or 5. Stations 120, 130, and 140 may be adapted to SDMA
operation or may operate according to legacy standards, e.g., IEEE
802.11.
[0042] It will be appreciated that although FIG. 1 schematically
illustrates three stations for demonstrative purposes, system 100
may include more than three stations. Although embodiments of the
invention are not limited in this respect, AP 110 may communicate
with a large number, denoted U, of remote stations, wherein U may
be much larger, for example, than the N transmit antennas. For
example, in accordance with some embodiments of the invention, AP
110 may divide the set of U stations into subsets, e.g., equal to
or smaller than the number of antennas N, for simultaneous
transmission. Thus, AP 110 may use the N antennas 112 for forming a
plurality of orthogonal beams, e.g., such that the power directed
toward the intended destination stations in the subset is
maximized, while interference generated to other stations may be
minimized, e.g., using a beamforming technique. Although
embodiments of the invention are not limited in this respect, AP
110 may select the subset of destination stations according to a
predefined criterion such as, e.g., maximizing the overall sum rate
of transmissions to the subset members, or maximizing the quality
of service (QoS) for the subset members.
[0043] According to some demonstrative embodiments of the
invention, AP 110 may generate the set of spatial channels, e.g., K
spatial channels, to be transmitted, using antennas 112, to the set
of destination stations, e.g., K destination stations including one
or more of stations 120, 130 and 140, by applying a precoding
matrix to a set of inputs including a set of transmissions, e.g., K
transmissions, intended to the set of destination stations,
respectively. The preceding matrix may include, for example, a set
of beamforrning vectors, e.g., K beamforming vectors, which may be
based, for example, on channel state information of the set of
destination stations, respectively. Each beamforrning vector may
be, for example, of size N, resulting in a precoding matrix,
denoted W, that may be, for example, of size K.times.N. In some
embodiments, the precoding matrix W may include one or more
additional vectors orthogonal to the beamforming vectors, which may
supplement the matrix W to be an orthogonal N.times.N matrix. The
preceding matrix W may be defined for example, for each frequency
bin, e.g., in OFDM operation.
[0044] According to some demonstrative embodiments of the
invention, AP 110 may include a SDMA preprocessor 180 to process
and prepare data intended for transmission to one or more
respective users, as described below. For example, preprocessor 180
may include a subset selector 182 to select a subset of user
stations, allocate data to be transmitted to the selected subset,
and to compute beamforming vectors for transmission, as described
below. Although embodiments of the invention are not limited in
this respect, preprocessor 180 may include high-bandwidth inputs,
e.g., for receiving channel estimates; and/or high-bandwidth
outputs, e.g., for providing parameters necessary for transmission,
e.g., the vectors of the precoding matrix. Preprocessor 180 may be
implemented using any suitable combination of memory, hardwired
logic, and/or general-purpose or special-purpose processors, as is
known in the art. In accordance with different demonstrative
embodiments of the invention, preprocessor 180 may be implemented
as a separate entity or as subsystem of either a Media Access
Controller (MAC) 160, and/or a Physical Layer (PHY) 170.
[0045] In some embodiments, the SDMA transmission process may be
controlled by MAC 160 or other suitable entity. Although the
invention is not limited in this respect, MAC 160 may perform
functions of the data link layer of the seven-layer Open Systems
Interconnect (OSI) model of network communication protocols, as
known in the art. MAC 160 may receive, for example, user data from
higher network layers, e.g., data intended for stations 120 and
140, as shown in FIG. 1.
[0046] According to some demonstrative embodiments of the
invention, the following components of AP 110 may perform functions
associated with processing and preparing data for SDMA
transmission: SDMA queues 150, MAC 160, PHY 170, and/or SDMA
preprocessor 180. Alternatively, AP 110 may include any other
suitable components for performing these functions.
[0047] SDMA queues 150 may include, for example, a set of queues
that may store incoming data, e.g., from network interface 102,
prior to SDMA transmission. In accordance with some embodiments of
the present invention, AP 110 may implement one queue per user per
priority and per traffic type, as compared to legacy WLAN
standards, e.g., IEEE 802.11, that implement a single queue per
priority level. For example, a system that supports U users and P
priority levels may include UP queues in SDMA queues 150.
Maintaining these U P queues may enable the subset selection
mechanism to associate packets destined to orthogonal stations.
[0048] In some embodiments, MAC 160 may include a superset of
pre-existing single-user-at-a-time MAC systems that operate in
accordance with a known WLAN standard, e.g., IEEE 802.11.
Alternatively, MAC 160 may be specifically adapted for SDMA
operation while retaining backward compatibility with a known WLAN
standard, e.g., the IEEE 802.11 standard. For a set of N
transmissions, e.g. using N antennas 112, MAC 160 may perform the
MAC functions for N packets, e.g., simultaneously. In addition, MAC
160 may control the sequence of events involved in the SDMA
transmission, e.g., as described below.
[0049] In some embodiments, PHY 170 may include, for example, N
instances of pre-existing PHY units that may operate in accordance
with a current WLAN standard, e.g., the IEEE 802.11 standard, along
with a module that may perform the SDMA beamforming as the physical
layer of a modem (not shown in FIG. 1). Alternatively, PHY 170 may
be specifically adapted for SDMA operation while retaining backward
compatibility with a current WLAN standard, e.g., the IEEE 802.11
standard.
[0050] According to some demonstrative embodiments of the
invention, subset selector 182 may determine the data subsets for
SDMA transmission to up to U users. In determining the data
subsets, subset selector 182 may interact with, for example, SDMA
queues 150, MAC 160, and/or PHY 170. Although the invention is not
limited in this respect, subset selector 182 may partition the
frames in SDMA queues 150 into SDMA subsets, e.g., according to the
queue status and the spatial channel characteristics of the remote
station. In alternate embodiments, subset selector 182 may
partition the frames according to any other suitable criteria.
Subset selector 182 may pass the information regarding subset
members to MAC 150 or other suitable entity for sequencing. Subset
selector 182 may also compute the beamforming vectors to be used by
PHY 170 for frame transmission. It is to be understood that these
computations may be performed by other modules in preprocessor 180
or elsewhere in AP 110 without departing from the scope of the
invention.
[0051] Although the invention is not limited in this respect,
according to some demonstrative embodiments of the invention, AP
110 may transmit to one or more of stations 120, 130 and 140
downlink transmissions of high-priority traffic, e.g., video
traffic, during one or more transmission cycles. For example, AP
110 may divide a transmission cycle into a first time interval
("the high-priority interval"), having a period T.sub.VD; and a
second time interval ("the other traffic interval"), having a
period T.sub.OT.
[0052] According to some demonstrative embodiments of the
invention, AP 110 may perform one or more operations of a SDMA
transmission method, e.g., as described below with reference to
FIGS. 2, 3A, 3B, 4A, 4B, and/or 5, during the high-priority
interval, e.g., to perform downlink transmission of high priority
traffic including, for example, Quality of Service (QoS)
constrained streams, e.g., including video streams. Although the
invention is not limited in this respect, it will be appreciated by
those of ordinary skill in the art that the term "high-priority"
traffic as used herein may include any stream having a particular
set of QoS constraints. The high-priority traffic may include, for
example, a transmission stream carrying a particular kind of
traffic characterized by a set of QoS parameters, e.g., PER target,
delay constraint, jitter constraint, bandwidth, and the like.
[0053] According to some demonstrative embodiments of the
invention, during the "other traffic" time interval AP 110 may
perform any suitable uplink and/or downlink transmission
operations, which may include, for example, sporadic uplink
traffic, delayed block ACK, downlink broadcast, and/or allowing for
the operation of neighboring Basic Service Sets (BSSs). For
example, AP 110 may operate within the "other traffic" interval at
a mode ("the normal mode of operation") in accordance with any
suitable communication standard, e.g., the 802.11 standard.
[0054] According to some demonstrative embodiments of the
invention, AP 110 may divide a period, denoted T.sub.cyc, the
transmission cycle, e.g., as follows: T.sub.cyc=T.sub.VD+T.sub.OT
(Equation 1)
[0055] According to some demonstrative embodiments of the
invention, a span of the T.sub.cyc period may be, for example, in
the order of 10 milliseconds (ms); the T.sub.OT period may be, for
example, at least Ims, e.g., depending on uplink rates. A short
T.sub.cyc period may result, for example, in using a relatively
long period for "other traffic" transmissions. Although the
invention is not limited in this respect, voice over IP VoIP
transmissions for uplink and/or downlink may be passed, for
example, during the T.sub.OT period, while VoIP frame delay may be
critical for adequate voice quality. Furthermore, delayed block ACK
reply may be expected during the T.sub.OT period, e.g., as
described below. Thus, a long T.sub.cyc period may result in long
video frame delays and an increase in system delay.
[0056] Reference is also made to FIG. 2, which schematically
illustrates a method 200 of wireless transmission in accordance
with one demonstrative embodiment of the invention. Although not
limited in this respect, one or more operations of the method of
FIG. 2 may be performed by a suitable AP, e.g. AP 110, in a WLAN to
transmit network data to selected users.
[0057] As indicated at block 210, the method may include performing
a coarse selection of a subset of stations from which the relevant
candidate stations for the SDMA transmission may be selected. The
initial subset selection may reduce the burden on SDMA preprocessor
180 of performing exact fine subset selection on a large set of
candidate stations, which may require a complex algorithm, and may
also reduce the overhead involved in sending learning frames to a
large group of candidate stations in order to obtain channel state
information. Although embodiments of the invention are not limited
in this respect, SDMA preprocessor 180 may reduce the candidate
group size from a maximum of U user stations, e.g., to a number
which is closer to N (the number of antennas 112), for example, by
using simple, non-computationally intensive operations. For
example, a non-limiting list of such operations may include ranking
according to priority, signal strength, or past subset information
which has not completely aged, and/or may include other suitably
simple operations. For example, subset selector 182 may use
information from past subset selection decisions, e.g., to predict
that including certain stations in a subset may result in a poor
sum rate, and thereby avoid that selection. Stations may be
selected to be substantially orthogonal, for example, orthogonality
may be checked based on the cross-correlation between the spatial
signatures of the candidate stations.
[0058] As indicated at block 220, the method may include reserving
the wireless medium for a duration sufficient for completing the
simultaneous downlink transmission to the wireless stations of the
selected set of stations. For example, the method may include
silencing the wireless medium before downlink transmission. For
example, silencing the medium may include sending a clear to
send-to-self (CTS-to-self) broadcast frame to indicate that the AP
plans to reserve the medium for the time needed to complete the
SDMA downlink cycle. Although the invention is not limited in this
respect, the reservation time span may be taken from the estimate
generated in the coarse subset selection. In alternate embodiments,
the reservation time span may be computed by other suitable
methods. For example, the desired time span may be set in the
duration field of the CTS-to-self frame. All stations that receive
this frame may be required to refrain from transmission for the
period of time set in the duration field, e.g., as defined by the
802.11 standard. Optionally, in some embodiments the reservation
time span may be determined according to a fairness criterion, for
example, to allow other stations into the medium, e.g., for uplink
traffic to take place. In such embodiments, the reservation time
span may be sufficiently short so as to not disrupt or delay
sensitive traffic of other stations, e.g., VoIP packets or frames.
For example, to enable VoIP traffic by other stations, the
reservation time may be in the order of 10 ms, although embodiments
of the invention are not limited in this regard. After the medium
is relinquished for the use of other stations, a new reservation
cycle may begin.
[0059] As indicated at block 230, the method may include performing
a channel query for a group of stations, for example, those
selected in coarse subset selection, e.g., to obtain updated
channel estimates and/or spatial signatures. In certain TDD
systems, the downlink channel may be assumed to be identical to the
uplink channel under the channel reciprocity assumption,and an
implicit channel estimate method may be used. In such cases, the
downlink channel estimate of each station may be obtained, for
example, by sending a short packet to each station that may elicit
another packet as a reply from the respective stations, and the
uplink channel states may then be estimated from the stations'
respective reply signals. In other cases, for example, where the
reciprocity assumption does not hold, explicit channel estimates
may be obtained by, for example, by having the remote station
return the downlink channel state as a response to an explicit
downlink query request.
[0060] According to some non-limiting embodiments of the invention,
the channel query may be obtained, e.g., in an 802.11 WLAN system,
by sending a Null-Data frame that does not carry actual data to the
stations, and each station may respond with an acknowledgement
(ACK) frame from which the uplink channel state may be estimated.
Alternatively, a Block ACK Request frame (BAR) may be used as a
query frame, to which each station may return a Block ACK (BA)
frame from which the uplink channel state may be obtained. The
downlink channel state may be obtained implicitly under the
reciprocity assumption. Although the invention is not limited in
this respect, channel queries may be performed sequentially for
each station that was selected by the coarse subset selection
mechanism and/or, in some alternative embodiments, only for those
stations for which the most recent channel estimate may be deemed
outdated.
[0061] As indicated at block 240, method 200 may also include
performing a fine subset selection, e.g., after the channel states
have been updated. A set of M fmal stations, where M.ltoreq.N, to
be included in the subsequently transmitted SDMA subset may be
chosen according to a suitable optimization metric such as, but not
limited to, e.g., maximum sum-rate or the maximum of the minimum
rate of the M users. For example, the fme subset selection may
include enumerating all possible subsets (for U.ltoreq.N, there are
2.sup.u-1 ways to arrange the U stations in subsets, excluding the
trivial subset of zero stations), calculating the achievable
sum-rate for each possible subset under the given power constraint
of the system, and/or choosing the subset of M stations having the
maximum sum-rate. It will be appreciated that, for the U subsets of
size one, the sum rate may be computed based on the rate possible
for standard WLAN transmission; whereas for larger subsets of two
or more stations, the sum-rate computation may involve a more
complicated calculation of channel matrix inversion. In some
embodiments of the invention, the set of beamforming vectors may be
determined as part of the fme subset selection.
[0062] Optionally, in other embodiments of the invention, the fine
subset selection and beamforming vector computation may be
performed incrementally, e.g., such that the algorithm may run in
parallel to the channel queries. For these embodiments, the initial
computation may be performed after the first station channel query.
The calculation may be updated incrementally, e.g., each time
another channel estimate is obtained, until a final channel
estimate may be completed. At this point, only the last step of the
subset selection algorithm may remain to be performed. This
incremental computation may enable SDMA preprocessor 180 (FIG. 1)
to perform the calculation over a long time span, thereby possibly
reducing hardware and software resource requirements. In some
embodiments, the incremental computation may be used to select a
preliminary subset based on information regarding a reduced number
of stations, schedule downlink transmission for the preliminary
subset, and continue selection of a primary subset, e.g., to be
scheduled immediately after transmission to the first subset, based
on the remaining stations' channel state information. Thus, the
incremental computation method may prevent transmission delay while
waiting for the completion of the final subset selection
computations.
[0063] As indicated at block 250, method 200 may include performing
an SDMA downlink transmission, e.g., beginning after completion of
the fine subset selection and beamforming vector computation.
Alternatively, method 200 may begin SDMA transmission for a
preliminary subset while still calculating the final subset
selection, and continue SDMA transmission for a primary subset when
all calculations are complete, e.g., as described below with
reference to FIG. 5. In some embodiments, e.g., where N frames are
transmitted simultaneously to N stations and an immediate
(simultaneous) ACK policy is used, the overall time span of the
preliminary SDMA subset transmission may equal that of the longest
duration frame in the subset.
[0064] Although the invention is not limited in this respect,
according to some demonstrative embodiments of the invention, the
downlink SDMA may be performed at an Equal Frames Span (EFS) or an
Unconstrained Frames Span (UFS). In the UFS mode, the simultaneous
frames for subset members may be unsynchronized. For example, at
the start of the subset transmission the frames may be transmitted
simultaneously. Since each frame in UFS mode can have a different
rate, the transmission of different frames may end at
unsynchronized times. For each station, the next frame may start a
SIFS period after its previous frame. When a UFS subset is
scheduled for a particular span, the number of frames transmitted
to each station during the period may vary, and may depend, for
example, on the frame's length and rate. The end of a UFS subset
may have some inefficiencies since the subset span may not be an
integer multiple of each station's frames span. In the EFS mode,
frames may be constrained to start together, e.g., as described
below.
[0065] Reference is now made to FIGS. 3A and 3B, which
schematically illustrate timing of data frames for simultaneous
wireless transmission in accordance with some demonstrative
embodiments of the invention.
[0066] Referring to FIG. 3A, two data streams 310 and 320,
respectively, may be transmitted simultaneously by an AP to two
user stations such as, e.g., stations 120 and 140, respectively,
with an identical transmission start time 301. The data frames of
each data stream, frame 311 and frame 321, may have differing time
spans and consequently may end at different times 302 and 303,
respectively.
[0067] According to some embodiments of the invention, e.g., for
802.11 WLAN systems, receiving stations may be required to respond
to a correctly received packet by transmitting an ACK frame. The
ACK frame may be required to be returned after a pre-defined (e.g.,
constant or fixed) time interval which may be referred to as the
Short Inter-Frame Space (SIFS). As illustrated in FIG. 3A, the AP,
e.g., AP 110 (FIG. 1), may begin the transmission of frame 311 to a
first station, e.g., station 120 (FIG. 1), and the transmission of
frame 321 to a second station, e.g., station 140 (FIG. 1),
substantially simultaneously, e.g., at time 301. The first station
may begin transmitting an ACK frame 312 at time 308, e.g.,
following the predefined SIFS from frame 311. The second station
may begin transmitting an ACK frame 322 at time 309, e.g.,
following the predefined SIFS from frame 321. Due to the shorter
time span of frame 311 relative to the time span of frame 321, the
first station may reply with ACK 312 prior to the completion of the
transmission of frame 321 by the AP, thereby introducing a
collision. For example, AP 110 (FIG. 1) may be in transmit mode at
time 308 and not in receive mode, and may thus not detect ACK frame
312 from station 120 (FIG. 1). In addition, the transmission of ACK
frame 312 may interfere with the proper reception of data frame 321
by station 140 (FIG. 1), which may result in corruption of the data
frame due to interference. If station 140 (FIG. 1) receives a
corrupted frame 321, station 140 (FIG. 1) may not send ACK frame
322 at time 309, a situation which may be referred to as the
"return ACK problem".
[0068] Referring to FIG. 3B, an SDMA WLAN transmission system in
accordance with some demonstrative embodiments of-the present
invention, e.g., system 100 of FIG. 1, may solve the return ACK
problem by using a staggered transmission scheme. According to some
embodiments of the invention, MAC 160 (FIG. 1) may schedule frame
transmission times of each subset, for example, such that two or
more, e.g., all, the frames in the subset end substantially at the
same time. This timing may ensure that tvo or more, e.g., all, ACK
frames may be sent after all the frames have been transmitted. To
achieve that, the frames in the subset may be sent staggered in
time. For example, two data streams 330 and 340 may be transmitted
substantially simultaneously to two user stations, e.g., stations
120 and 140 (FIG. 1), wherein frame 339 may have the same time span
as frame 311 and frame 341 may have the same time span as frame
321. However, in this scenario the transmission start times of data
streams 330 and 340, times 301 and 304 respectively, may
differ.
[0069] As indicated in FIG. 3B, the AP may stagger the start times
for the transmission of frames 331 and 341, e.g., times 304 and 301
respectively. By offsetting the transmission start time 304 of the
shorter frame, i.e., frame 331, by a "stagger time" 350 equal to
the difference in time span of the two frames, two ACK frames 332
and 342 may start simultaneously at time 306 and/or return
substantially simultaneously at time 307, e.g., after a short
inter-frame space (SIFS) interval. It will be appreciated, however,
that stagger time 350 may represent unused transmission time. Thus,
in accordance with demonstrative embodiments of the invention, the
transmission time spans of the frames of the chosen subset may be,
for example, as close to equal as possible, so as to utilize the
wireless medium efficiently.
[0070] Although embodiments of the invention are not limited in
this respect, SDMA system 100 (FIG. 1) may employ a method to
equalize the time spans of the N transmissions, for example, by
controlling the transmit rate for each station. For example,
according to one embodiment of the invention, the transmit rate to
a station may be controlled, e.g., by decreasing or increasing the
power allocation to the station so as to decrease or increase the
corresponding transmit rate, respectively. According to another
embodiment of the invention, each frame may be transmitted at a
particular modulation and coding scheme (MCS) appropriate to the
signal to noise ratio (SNR) experienced by the corresponding
station intended to receive the frame. The time span of the frame
transmission may be calculated as the ratio of the packet length,
e.g., in bits, and the transmit rate, e.g., in bits per second, of
the MCS. Another method of substantially equalizing the
transmission time span may include, for example, fragmenting a
relatively longer frame into smaller fragments such that the
fragments' duration equals the duration of a shorter frame
transmitted to another station in the subset. It will be
appreciated that the terms "longer" and "shorter" frames, as used
herein, may refer to the time span required for transmitting a
frame, which may depend, for example, on the packet size, and/or on
the transmit rate for the frames.
[0071] Although embodiments of the invention are not limited in
this respect, a time span equalization algorithm may be
incorporated as part of the fine subset selection algorithm of the
invention. Additionally or alternatively, the time spans may be
partially equalized in the coarse subset selection, for example, by
trying to match user stations that have similar length packets in
their respective queues and also have similar receive signal
strengths (RSSs). It will be appreciated that the RSS may be a good
predictor of the SNR, and the ultimate transmission rate.
[0072] Reference is now made to FIG. 4A, which schematically
illustrates a simultaneous block acknowledgment mechanism that may
be used by methods of simultaneous wireless transmission in
accordance with some embodiments of the invention. Although
embodiments of the invention are not limited in this respect, an
ACK response may not be mandatory after each received frame. For
example, the IEEE 802.11e quality of service (QoS) extension of the
IEEE 802.11 standard defines a mechanism for block acknowledgement
(block ACK, referred to herein as "BA"), in which the AP may
transmit a block of frames followed by a request for
acknowledgement of the transmitted block, and, if the block of
frames is successfully received, the station may send a BA frame in
confirmation. For embodiments of the invention where a block ACK
may be defined, there may be no need for the staggering scheme
described above as the wireless medium may be used in a more
efficient manner.
[0073] As illustrated in FIG. 4A, the AP may transmit
simultaneously in SDMA to multiple user station, e.g., three
stations 120, 130 and 140 (FIG. 1), and may transmit using one or
more data streams and/or channels, for example, two data streams
410 and 420. In accordance with embodiments of the invention, the
AP may send up to N simultaneous transmissions via up to N spatial
channels or streams, where N is the number of transmit antennas. In
the present example, two spatial streams are illustrated for
clarity of demonstration, but it is understood that embodiments of
the invention may include more than two channels or streams. In
addition, it will be appreciated that at each particular instance
in time, the two spatial streams may be used to substantially
simultaneously transmit to two stations. However, SDMA transmission
to more than two stations using two channels may be accomplished by
substantially simultaneous transmission to a first SDMA subset
including two stations, e.g., stations 120 and 130 (FIG. 1),
followed by substantially simultaneous transmission to a second
subset including two stations, e.g., stations 120 and 140 (FIG.
1).
[0074] As shown in FIG. 4A, the AP may transmit one or more frames
to each of stations 120, 130, and 140 (FIG. 1), for example: three
frames 411, 412, and 413 to station 120 (FIG. 1), two frames 421
and 422 to station 130, and one frame 423 to station 140 (FIG. 1).
It will be appreciated that, since an immediate ACK response may
not be expected after each frame, aligning the frames to the
different user stations may no longer be required. Two or more
successive frames of a transmitted block may be separated by a
predefined SIFS period, e.g., shown in FIG. 4A as the difference
between times 402 and 403. At the end of the block ACK period,
e.g., at time 407, the AP may substantially simultaneously send BA
request frames 415 and 425 to those stations whose blocks may be
complete, e.g., stations 120 and 130 (FIG. 1), respectively. In
addition, a BA request frame may be sent to another station, e.g.,
station 140 (FIG. 1), after, for example, a later block. It will be
appreciated that sending the BA requests substantially
simultaneously may ensure that the returned BA frames are
transmitted and received substantially simultaneously. It will be
further appreciated that any unused stagger time, e.g., the time
difference between times 406 and 407, may be negligible when using
a block ACK mechanism for a transmission block of multiple frames,
for example, as compared to a single frame transmission block when
BA may not be supported. Additionally or alternatively, the AP may
align the simultaneously transmitted frames, e.g., using a time
span equalization algorithm as described above with reference to
FIG. 3, to align the frames' respective SIFS times to reduce
interference.
[0075] Referring to FIG. 4B, which schematically illustrates a
sequential block acknowledgment mechanism that may be used by
methods of simultaneous wireless transmission in accordance with
some embodiments of the invention, the AP may request BA frames
sequentially instead of simultaneously. FIG. 4B schematically
illustrates an exemplary scenario in which the AP may transmit
substantially simultaneously in SDMA to multiple user stations
supporting block ACK, e.g., stations 120 and 130 (FIG. 1), using
one or more data streams and/or channels, e.g., data streams 430
and 440, respectively, but with sequential BA frame requests. For
example, as illustrated in FIG. 4B, a first BA frame request 435
may be sent to station 120 (FIG. 1) at time 408, e.g., after a
frame 432 of block 431 is transmitted to station 120 (FIG. 1).
However, the AP may delay transmitting BA frame request 445 to
station 130 (FIG. 1), for example, by a minimum of the time span
needed to receive the BA 485 from station 120 (FIG. 1), e.g., until
a time 409. Depending on the block acknowledgement policy in use,
station 120 (FIG. 1) may transmit block ACK 485 either immediately
after receiving BA request 435 and a SIFS time (as shown), or the
station may delay transmitting BA 485 to a later time. It will be
appreciated that this sequential BA frame request transmission may
result in the returned BA frames not overlapping, which may ease
detection by the AP at the cost of extending the time needed to
complete the entire transaction
[0076] Referring back to FIG. 2, as indicated at block 260, the AP
may be required to detect the returning ACK frames from all
stations to which it transmitted. Thus, a robust ACK presence
detection scheme may be required to maintain the integrity of the
MAC 160's protocol. According to some demonstrative embodiments of
the invention, complete detection of the simultaneous ACK frame
contents, including the decoding of the check sum bits in the frame
trailer, may require uplink SDMA by AP 110 (FIG. 1). As is known in
the art, uplink SDMA may be accomplished by applying receive
beamforrning techniques on each uplink signal.
[0077] As indicated at decision block 270, if the outgoing queues
contain additional data fragments intended for the user subset
selected in block 240 and/or if one or more return ACK signals were
not detected, transmission method 200 may return to block 250. The
downlink transmission method may repeat the SDMA downlink
transmission and return ACK detection for any remaining and/or
unsuccessfully transmitted fragments or frames, e.g., until all
data for the intended user subset is successfully transmitted.
[0078] As indicated at decision block 280, if the outgoing SDMA
queues 150 contain additional data frames for a different set of
remote stations, method 200 may calculate a new coarse subset of
users as indicated at block 285. Method 200 may perform uplink
queries to the newly calculated subset of stations, select a new
final subset, broadcast the additional data frames, and detect
return ACKs, e.g., as described above. To enable channel learning
from the uplink queries, method 200 may optionally re-silence the
medium, e.g., by sending a CTS-to self frame to reserve a time span
as indicated at block 220. Although embodiments of the invention
are not limited in this respect, the medium may be silenced
whenever the reserved time span expires. As indicated at block 290,
the downlink transmission cycle may end when all pending data
frames are successfully transmitted.
[0079] Reference is now made to FIG. 5, which schematically
illustrates a method of wireless transmission in accordance with
another demonstrative embodiment of the invention. Although not
limited in this respect, one or more operations of the method of
FIG. 5 may be performed by an AP, e.g. AP 110 (FIG. 1), in a WLAN
to transmit network data to selected users, e.g., during the
T.sub.VD period.
[0080] As indicated at block 502, the method may include reserving
the wireless medium for a duration corresponding to the T.sub.VD
period. For example, silencing the medium may include sending a
clear to send-to-self (CTS-to-self) broadcast frame to indicate
that the AP plans to reserve the medium, e.g., as described above
with reference to FIG. 2. For example, a Network Allocation Vector
(NAV) value in the CTS-to-self frame may include a duration value
corresponding to the T.sub.VD period, e.g., in order to ensure that
all stations able to receive the CTS-to-self frame will refrain
from transmitting during the T.sub.VD period. The CTS-to-self frame
may be sent at a relatively low rate, e.g., at the minimal possible
rate, for example, in order to enable far stations to receive the
CTS-to-self frame. For example, the CTS-to-self may be transmitted
a rate of 6 Mega-bits-per-second (MBPS) OFDM, e.g., if a 5 GHz band
is used; or a rate of 1-2 MBPS CCK (11b mode), e.g., if a 2.4 GHZ
band is used.
[0081] According to some demonstrative embodiments of the
invention, the T.sub.VD period may be terminated, e.g., by AP 110
(FIG. 1), for example, if queues 150 (FIG. 1) are empty, by sending
a CF-end frame. This may result in clearing the NAV values by one
or more stations receiving the CF-end frame, thus switching to the
T.sub.OT period.
[0082] According to some demonstrative embodiments of the
invention, the T.sub.VD period may be extended, e.g., by AP 110
(FIG. 1), for example, if the T.sub.VD period is to end and queues
150 (FIG. 1) include frames which are intended to be transmitted
during the T.sub.VD period, an additional CTS-to-self frame may be
sent before the NAV value expires.
[0083] According to some demonstrative embodiments of the
invention, the duration field of the MAC header for downlink
frames, e.g., including learning frames and/or SDMA downlink
frames, which may be transmitted, e.g., by AP 110 (FIG. 1), during
the T.sub.VD period, may include a value indicating the time left
until the end of the T.sub.VD period. Although the invention is not
limited in this respect, in some demonstrative embodiments of the
invention, a Length field in a Physical Layer Convergence Procedure
(PLCP) Signal symbol, for one or more frames transmitted during the
T.sub.VD period, e.g., each frame transmitted during the T.sub.VD
period including the CTS-to-self frame, may include the length of
the transmitted frame.
[0084] According to some demonstrative embodiments of the
invention, the T.sub.VD period may be divided into SDMA sub-cycles.
Although the invention is not limited in this respect, each
sub-cycle may include, for example, a learning period, and a
downlink SDMA transmission succeeding the learning period. The
learning period may include, for example, sending downlink probing
frames, e.g., Null-Data frames or Block ACK frames, e.g., as
described below. The downlink SDMA transmission may include
simultaneously transmitting WLAN downlink frames to a chosen SDMA
set of stations ("the SDMA Subset"), which may be selected, for
example, based on channel state information received during the
leaming period.
[0085] As indicated at block 504, the method may include performing
a coarse selection of the SDMA subset of stations from which the
relevant candidate stations for the SDMA transmission may be
selected. Any coarse subset selection method and/or algorithm may
be implemented, e.g., as described above with reference to FIG.
2.
[0086] As indicated at block 506, the method may also include
performing one or more learning operations during the learning
period, to obtain, for example, updated channel estimates and/or
spatial signatures. For example, AP 110 (FIG. 1) may sequentially
send downlink probe frames to one or more stations to be probed,
e.g., to determine the channel estimates of the one or more
stations. The uplink channel of a station may be estimated from an
uplink frame, e.g., an ACK frame, received in response to the
downlink probe transmitted to the station by AP 110 (FIG. 1). The
downlink probe frame may include any suitable probe frame. For
example, the downlink frame may include a Null Data frame, e.g., if
a delayed block ACK scheme is used, or when block ACK is not
supported. Alternatively, the probe frame may include a block ACK
request frame, e.g., if an immediate block ACK is supported.
According to some demonstrative embodiments of the invention, the
downlink probe frames may be transmitted in an isotropic manner,
e.g., without beamforming. Although the invention is not limited in
this respect, the downlink probe frames may be transmitted downlink
probe rate such that the probe frame error rate is smaller than a
predefined error rate, e.g., 10.sup.-2. The probe frames may be
transmitted at a relatively slow rate, e.g., since the probe frames
may be relatively short.
[0087] According to some demonstrative embodiments of the
invention, the method may also include determining whether a probe
frame is to be retransmitted, e.g., if a response to the probe
frame has not been received, as indicated at block 508. According
to some demonstrative embodiments of the invention, a probe frame
transmitted to a station may act as a Block ACK Request to the
station, e.g., if an immediate Block ACK is supported. A Block ACK
frame may be received after a SIFS period, e.g., in response to the
Block ACK Request. A retransmission period following the learning
period may include frames that were not acknowledged in the Block
ACK frame.
[0088] According to some demonstrative embodiments of the
invention, the probe frames may be retransmitted a predefined
number of retransmissions until an ACK is received, as indicated at
block 510. The retransmission number may be a configurable system
parameter. A probe frame that has reached its retransmission limit
may be dropped from the current subset. A renewed attempt to probe
this station may be performed at a succeeding learning period. The
retransmitted probe frame may be transmitted at a lower rate
compared to the transmission rate of the first transmission of the
probe frame.
[0089] According to some demonstrative embodiments of the
invention, the probe frames may be ordered according to predefined
priority policy. The policy may include, for example, initially
scheduling probe frames intended for stations for which
retransmission of frames is planned, e.g., since retransmission may
have the maximum priority. Other probe frames may be ordered, for
example, according to the QoS weights.
[0090] As indicated at block 512, according to some demonstrative
embodiments of the invention, the method may include performing the
learning operations if the channel estimates have aged, e.g.,
beyond a predefined aging time period. The aging time period may
depend, for example, on the subset size, e.g., a subset including
more stations will age faster than a subset with few stations.
[0091] According to some demonstrative embodiments of the
invention, the downlink transmission may be optionally divided into
two consecutive transmissions to two respective subsets of stations
during two respective time periods, denoted S1 and S2,
respectively. As indicated at block 514, the method may include
performing the selection of a preliminary subset. The preliminary
subset may be determined, for example, based on channel knowledge
of part of the stations, for example, learned during part, e.g.,
the beginning, of the learning period. The method may also include
performing a downlink SDMA transmission to the preliminary subset,
e.g., during the period S1, as indicated at block 516.
[0092] As indicated at block 518, the method may also include
performing a selection of a main subset. The main subset may be
determined, for example, based on full channel state knowledge
received from substantially most or all of the stations, e.g., at
the end of the learning period, e.g., as described above with
reference to FIG. 2. As indicated at block 520, the method may also
include performing a downlink SDMA transmission to the main subset,
e.g., during the period S2.
[0093] According to some demonstrative embodiments of the
invention, the processing of the channel information gathered
during the learning period may consumes a time period, denoted
T.sub.SDMA.sub.--.sub.calc. Since the decision on the main subset
may not be made until the end of the T.sub.SDMA.sub.--calc period,
the selection of the preliminary subset may enable performing the
SDMA downlink transmission to the preliminary subset, e.g.,
substantially right after the learning period, based on the channel
state of stations probed during part of the learning period, e.g.,
at the start of the learning period. The transmission time period
SI for the transmission to preliminary subset may be scheduled
immediately after the learning period, and the time period for the
transmission to the main subset S2 may succeed the period S1.
Although the invention is not limited in this respect, the
preliminary subset may have a size of, for example, one frame. The
time period S1 may be forced, for example, to be a size one subset,
e.g., plain beamforming. It may be assumed that the
T.sub.SDMA.sub.--calc period is shorter than the period S1, such
that the main subset may be determined before the period S1 has
ended.
[0094] Although the invention is not limited in this respect,
according to some demonstrative embodiments of the invention, the
downlink SDMA transmission may be performed at the EFS or the UFS,
e.g., as described above with reference to FIGS. 2, 3A and/or
3B.
[0095] As indicated at block 522, the method may include performing
one or more additional SDMA downlink transmissions, e.g., SDMA
sub-cycles, for example, if there is enough time remaining within
the reserved period T.sub.VD .
[0096] As indicated at block 524, the method may include
determining whether an immediate Block ACK or a delayed Block ACK
scheme is implemented. As indicated at block 528, the method may
include switching to the normal mode of operation, e.g., if the
immediate Block ACK scheme is implemented.
[0097] If the delayed Block ACK scheme is implemented, the Block
ACK frames may be returned during the T.sub.OT period. Accordingly,
the method may include transmitting Block ACK request frames, e.g.,
substantially at the end of the T.sub.VD period, before the
beginning of the T.sub.OT period. The delayed Block ACK Request may
be acknowledged by a standard ACK frame, e.g., immediately. The
Block ACK Request may have a system configurable number of
retransmissions. When the retransmission counter reaches the
retransmission threshold, the next Block ACK request may be
scheduled at the end of the next T.sub.VD period.
[0098] According to some demonstrative embodiments of the
invention, the Block ACK request may request for an acknowledgement
of all frames that were transmitted after the last accepted Block
ACK reply, e.g., using a sequence number of the first frame for
which an acknowledgment is required.
[0099] According to some demonstrative embodiments of the
invention, a subset, e.g., the main subset or the preliminary
subset, may be defined by the set of preceding matrices, or
beamforming vectors, which may be associated with the subset. The
rate of each frame in the subset may be changed during the lifetime
of the subset (i.e. during the time the beamforming vectors stay
fixed), e.g., in order to take into account effects of channel
aging. For the EFS transmission scheme the power allocation between
stations can also be changed. The precoding matrices may change,
for example, during the life time of the subset (e.g. to enable
channel prediction). Since the precoding matrices cannot change
during the transmission of a frame, changing the preceding matrices
may be more convenient in EFS mode, where all frames end together.
In a subset that immediately follows a block ACK, the preliminary
subset downlink transmission may be replaced by a retransmission
period, e.g., if retransmission is required. Retransmitted frames
may be sent one after the other in beamforming mode, e.g., using a
subset of size one, as described above with reference to block
510.
[0100] In some other embodiments of the invention, uplink channel
impairments may preclude SDMA detection of return ACK signals. For
example, as the remote stations simultaneously transmitting ACK
signals may not be synchronized with one another, the different
uplink signals may undergo different timing and frequency offsets.
These differences may cause inter-user interference for existing
receive beamforming techniques.
[0101] Although embodiments of the invention are not limited in
this respect, the AP may be able to detect the presence of an ACK
frame without actually decoding the frame. Thus, some embodiments
may take advantage of the lack of transmitted data in the ACK frame
to reduce the detection requirements on the SDMA AP. A scheme for
such detection may include, for example, spatial demultiplexing
and/or correlation techniques that use knowledge of certain return
ACK signal parameters such as, for example, gain frequency offset
and/or spatial signature, e.g., as obtained in the channel query of
block 230. Examples of detection schemes in accordance with
demonstrative embodiments of the invention are described below with
reference to FIGS. 6 and 7.
[0102] Reference is now made to FIG. 6, which schematically
illustrates a frame format 600 for an Orthogonal Frequency Division
Multiplexing (OFDM) transmission mode. Frame format 600 may begin
with a PLCP preamble signal 601, which may include a Short Preamble
(SP) and a Long Preamble (LP), as known in the art. Preamble signal
601 may be followed by a Signal symbol 602, and data carrying
symbols 603. Although the invention is not limited in this respect,
the duration of each data carrying symbol may be, for example, 4
.mu.Sec, and may be composed of a 3.2 .mu.Sec data symbol and a 0.8
.mu.Sec Guard Interval (GI) which may be referred to as a Cyclic
Prefix (CP). As is known in the art, the CP may precede the data
symbol and may be a copy of a last portion, e.g., the last 0.8
.mu.Secs of the corresponding data symbol.
[0103] Reference is now made to FIG. 7, which is a schematic
flowchart of a return ACK frame detection method 700 in accordance
with some demonstrative embodiments of the invention. Although not
limited in this respect, the return ACK detection method 700 may be
performed by a suitable AP in a WLAN, e.g., SDMA AP 100 (FIG. 1),
to detect return ACK frames from a set of K remote stations to
which AP 110 (FIG. 1) may have transmitted data frames.
[0104] As indicated at block 710, return ACK detection method 700
may include setting gain values at an AP, e.g., AP 110, for return
ACK detection. The end of a downlink transmission to a set of
remote stations, e.g., SDMA cycle 200 of FIG. 2, may indicate the
start of a SIFS period generated by each station. For some
demonstrative embodiments of the invention, an AP may preset
appropriate gain values during the SIFS period prior to the start
of the ACK signals. For example, the AP may estimate the expected
receive power from each station of the set during an uplink query
in the downlink transmission cycle, e.g., uplink query 230 of FIG.
2, and preset appropriate gain values based on the sum of these
expected receive powers. Presetting the gain values may also
eliminate the need to activate an Automatic Gain Control (AGC)
circuit in an AP for reception. As is known in the art, an AGC
circuit may take a certain time period, e.g., at least 4 .mu.Secs,
to converge. Thus, reliance on the AGC may prevent using a
beginning part of the ACK frame preamble signal, e.g., the SP, for
detection purposes.
[0105] As indicated at block 720, return ACK detection method 700
may include setting Fast Fourier Transform (FFT) window locations
for detecting the preamble signals of a number of return ACK
frames, e.g., K return ACK frames. According to some demonstrative
embodiments, e.g., when using an OFDM modulation scheme having a
3.2 .mu.Sec data symbol, the FFT window may start substantially
immediately after the CP in order to allow demodulation of the
entire OFDM data symbol. In other demonstrative embodiments, e.g.,
where the CP may be a cyclic extension of the data symbol, the FFT
window may start at any point during the CP, and OFDM demodulation
may still be viable. It will be appreciated that any time shift in
starting the FFT window may translate to a recoverable phase shift
in the frequency domain after the FFT.
[0106] Although embodiments of the invention are not limited in
this respect, a MAC protocol, e.g., as defined in the IEEE 802.11
specification, may allow some tolerance in SIFS generation timing
by remote stations. For example, this tolerance may be about
.+-.10% of a slot time: a system having a slot time of 20 .mu.Secs
may have an uncertainty in the start of the preamble of about .+-.2
.mu.Secs. For such a system, the preambles being received at the AP
may have a relative timing offset of up to 4 .mu.Secs. In addition,
the FFT window size for demodulation may be specified by an
operational mode or modulation scheme of an existing standard, for
example, 3.2 .mu.Secs for OFDM according to IEEE 802.11. Thus, due
to these constraints, in some embodiments it may not be possible to
find a single FFT window start time for uplink SDMA that will be
valid for the incoming frames of all K signals.
[0107] Reference is now made to FIG. 8, which schematically
illustrates a structure of a preamble signal 800, e.g., preamble
signal 601 of FIG. 6. For some embodiments of the invention, the
duration of preamble signal 800 may be, for example 16 .mu.Secs.
Accordingly, the preamble may include a Short Preamble (SP) 820
with a duration of, e.g., 8 .mu.Sec, and a Long Preamble (LP) 840
with a duration of, e.g., 8 .mu.Sec. The SP may include a number of
repetitions of the t.sub.i signal 821, for example, 10 repetitions
wherein each repetition is, e.g., 0.8 .mu.Secs long. The LP may
include a number of repetitions of the T.sub.i signal 841, for
example, 2.5 repetitions wherein each repetition is, e.g., 3.2
.mu.Secs long. In accordance with the frame format in use, preamble
signal 800 may be followed by a signal field 850 and one or more
data symbols 860, as known in the art.
[0108] Referring back to FIG. 7, According to some demonstrative
embodiments of the invention, setting the FFT window locations may
include setting separate FFT window start times for the SP and for
the LP of the return ACK frames, e.g., when there is no valid
single FFT window for the frames of all K signals. For example,
there may be a valid FFT window for individual demodulation of the
SP and the LP when their respective durations are longer than a
maximal time offset. For example, if both the short and long
preambles have durations of 8 .mu.Secs and the maximal time offset
is 4 .mu.Secs, there may be a span of at least 4 .mu.Secs for a
valid FFT window for each preamble signal.
[0109] Although embodiments of the invention are not limited in
this respect, the SP FFT window location may be specified as a
percentage of a slot time, e.g., about 10%, plus an additional time
period, e.g., the CP time period, after the expected start time of
the received preambles. The extended slot time may ensure that all
received ACK signals overlap in time, while the additional CP delay
may ensure orthogonality of the different bins, and may thereby
enable proper demodulation. For some demonstrative embodiments that
operate partially or completely in accordance with IEEE 802.11
standards, the SP FFT window location may be, for example, between
1 and 2 .mu.Secs plus a CP time after the expected start time of
the preambles, depending on the specific 802.11 mode. Although
embodiments of the invention are not limited in this respect, the
LP FFT window location may be set to commence at a certain time
delay after the SP window location. For example, a delay equal to
the duration of the SP. For example, for embodiments that operate
partially or completely in accordance with IEEE 802.11 standards,
the LP FFT window location may be 8 .mu.Secs after the SP window
location.
[0110] In some embodiments, return ACK detection method 700 may
include performing the FFT in the selected windows on signals
received via the one or more antennas of AP 110, as indicated at
block 730. For a set of N antennas, a total of N Fast Fourier
Transforms may be performed for each preamble signal, to produce a
FFT output vector of size N. It will be appreciated that in
embodiments where more than one FFT window is set for each frame,
e.g., a SP window and a LP window, method 700 may include
performing 2N transformations to produce the output vector of size
N.
[0111] As indicated at block 740, method 700 may include performing
SDMA decoding to obtain a vector that may represent the incoming
preamble signal from each of, e.g., K stations. According to some
demonstrative embodiments of the invention, the output of the N
FFTs in each of the output frequency bins may be a vector of size
N. To demultiplex the K preamble signals, method 700 may include
applying a precoding matrix W to the FFT output vector in each
frequency bin. For example, preceding matrix W may be an N.times.K
matrix including, for example, a set of K beamforming vectors of
size N, e.g., as generated for transmission to K stations through a
set of N spatial channels, where N is the number of transmit
antennas. Precoding matrix W may be generated for downlink
transmission by the SDMA preprocessor, as explained above with
reference to FIG. 1. In accordance with embodiments of the
invention, applying W to the FFT output vector may result in a
vector of size K, e.g., y.sub.k, which may represent the preamble
signal for each of a set of K receiving stations.
[0112] As indicated at block 750, method 700 may include detecting
the presence of a return ACK preamble signal for each station. It
will be appreciated that preamble vector y.sub.k may include some
noise distortion, e.g., acquired while passing through the
effective channel for each station. Thus, if a station sent a
return ACK signal, the corresponding portion of the preamble vector
may include both signal and noise; whereas if a station did not
send a return ACK signal, the corresponding portion of the preamble
vector may include only noise. Although embodiments of the
invention are not limited in this respect, testing for each
station's ACK signal presence may include discerning between two
hypotheses: H.sub.1: Y.sub.k=S.sub.kh.sub.k+n.sub.k H.sub.0:
y.sub.k=n.sub.k (Equation 2) where hypothesis H.sub.1 assumes that
an ACK signal has been sent and hypothesis H.sub.0 assumes that an
ACK signal has not been sent (i.e., that a received signal may
include only noise). According to hypothesis H.sub.1, a received
signal in the k-th frequency bin, Y.sub.k, may be a corresponding
preamble value, e.g., s.sub.k, multiplied by a channel coefficient,
e.g., h.sub.k, plus a noise signal, e.g., n.sub.k. According to
hypothesis H.sub.0, a received signal in the k-th frequency bin,
e.g., y.sub.k, may be a noise-only signal, e.g., n.sub.k.
[0113] According to some embodiments of the invention, a
s.sub.kh.sub.k signal may be known from receipt of an appropriate
preamble signal during a previous uplink query. As is known in the
art, for a known signal, e.g., s.sub.kh.sub.k, an optimal detector
may be a correlator, e.g., X, which may be calculate according to
the following equation: .chi. = y , s h = k .times. y k ( s k h k )
* ( Equation .times. .times. 3 ) ##EQU1## Accordingly, detecting
ACK signal presence may include comparing the correlator output to
a threshold value, e.g.,.tau. to determine if an ACK signal has
been sent. For example, the following set of equations may be used
to test the hypotheses H.sub.1 and H.sub.0:
X>.tau..fwdarw.H.sub.1 X>.tau..fwdarw.H.sub.0 (Equation
4)
[0114] In some embodiments, detecting ACK signal presence may
optionally include one or more fine-tuning techniques. For example,
the threshold value r may be tuned to provide a good balance
between the probability for misdetection and the probability for
false alarm. In some embodiments, for example, the correlation
outputs (e.g., the energy detection values) from different sections
of preamble signals (e.g. SP and LP signals), may be averaged to
provide a more robust detection process. Additionally or
alternatively, in some embodiments a received signal such as
y.sub.k may be phase adjusted to correct for a time shift in the
FFT window prior to correlation, as is known in the art.
Accordingly, the preamble signals received during a previous uplink
query may be stored without frequency offset compensation, e.g., to
enable detection of signals coming from different remote stations
having different frequency offsets.
[0115] In some embodiments, e.g., in a sufficiently high SNR
environment, detecting ACK signal presence may include using a
simple energy detector. It will be appreciated that, because the
noise distortion component of the received signal may be relatively
small in a high SNR environment, an energy detection method may be
sufficient for detecting ACK presence. For example, the detector
may apply the following equation: .chi. = y , y = k .times. y k 2 (
Equation .times. .times. 5 ) ##EQU2##
[0116] Some embodiments of the invention may be implemented by
software, by hardware, or by any combination of software and/or
hardware as may be suitable for specific applications or in
accordance with specific design requirements. Embodiments of the
invention may include units and/or sub-units, which may be separate
of each other or combined together, in whole or in part, and may be
implemented using specific, multi-purpose or general processors or
controllers, or devices as are known in the art. Some embodiments
of the invention may include buffers, registers, stacks, storage
units and/or memory units, for temporary or long-term storage of
data or in order to facilitate the operation of a specific
embodiment.
[0117] Some embodiments of the invention may be implemented, for
example, using a machine-readable medium or article which may store
an instruction or a set of instructions that, if executed by a
machine, for example, by AP 110 of FIG. 1, or by other suitable
machines, cause the machine to perform a method and/or operations
in accordance with embodiments of the invention. Such machine may
include, for example, any suitable processing platform, computing
platform, computing device, processing device, computing system,
processing system, computer, processor, or the like, and may be
implemented using any suitable combination of hardware and/or
software. The machine-readable medium or article may include, for
example, any suitable type of memory unit, memory device, memory
article, memory medium, storage device, storage article, storage
medium and/or storage unit, for example, memory, removable or
non-removable media, erasable or non-erasable media, writeable or
re-writeable media, digital or analog media, hard disk, floppy
disk, Compact Disk Read Only Memory (CD-ROM), Compact Disk
Recordable (CD-R), Compact Disk Re-Writeable (CD-RW), optical disk,
magnetic media, various types of Digital Versatile Disks (DVDs), a
tape, a cassette, or the like. The instructions may include any
suitable type of code, for example, source code, compiled code,
interpreted code, executable code, static code, dynamic code, or
the like, and may be implemented using any suitable high-level,
low-level, object-oriented, visual, compiled and/or interpreted
programming language, e.g., C, C++, Java, BASIC, Pascal, Fortran,
Cobol, assembly language, machine code, or the like.
[0118] While the invention has been described with respect to a
limited number of embodiments, it will be appreciated that many
variations, modifications and other applications of the invention
may be made. Embodiments of the present invention may include other
apparatuses for performing the operations herein. Such apparatuses
may integrate the elements discussed, or may comprise alternative
components to carry out the same purpose. It will be appreciated by
persons skilled in the art that the appended claims are intended to
cover all such modifications and changes as fall within the true
spirit of the invention.
* * * * *