U.S. patent application number 11/073835 was filed with the patent office on 2005-07-07 for system topologies for optimum capacity transmission over wireless local area networks.
Invention is credited to Terry, John.
Application Number | 20050147075 11/073835 |
Document ID | / |
Family ID | 33101459 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050147075 |
Kind Code |
A1 |
Terry, John |
July 7, 2005 |
System topologies for optimum capacity transmission over wireless
local area networks
Abstract
An inventive method provides optimum topology for a
multi-antenna system dedicated to higher throughput/capacity by
bundling the Point Coordination Function (PCF) operation in
infrastructure mode of the current and/or enhanced IEEE MAC with
PHY specifications that employ some form of coherent weighting
based on CSI at the transmitter in conjunction with the
corresponding optimum receiver detection based on CSI.
Specifically, CSI is measured from a control message, so data
messages and control messages are separated. In the contention
period of IEEE 802.11, the RTS/CTS exchange is used for CSI and the
data message is sent following the CTS message. In the contention
free period, a poll by the PC is separated from a data frame, which
gives the polled station the first opportunity to send a data
message. This change in topology results in various changes to the
frame exchange format in the CFP for various scenarios of data and
control messages to be exchanged.
Inventors: |
Terry, John; (Garland,
TX) |
Correspondence
Address: |
HARRINGTON & SMITH, LLP
4 RESEARCH DRIVE
SHELTON
CT
06484-6212
US
|
Family ID: |
33101459 |
Appl. No.: |
11/073835 |
Filed: |
March 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11073835 |
Mar 7, 2005 |
|
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10753206 |
Jan 6, 2004 |
|
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60460553 |
Apr 4, 2003 |
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Current U.S.
Class: |
370/338 ;
370/206 |
Current CPC
Class: |
H04W 74/08 20130101;
H04W 74/06 20130101; H04W 24/00 20130101; H04W 84/12 20130101; H04W
74/02 20130101; H04W 40/02 20130101; H04L 1/1621 20130101; H04W
92/18 20130101; H04W 28/06 20130101 |
Class at
Publication: |
370/338 ;
370/206 |
International
Class: |
H04J 011/00; H04Q
007/24; H04Q 007/20 |
Claims
1-24. (canceled)
25. In a wireless local area network wherein a first network entity
transmits to a second network entity a packet having a guard
interval preceding one of a data signal and a training sequence,
the improvement comprising: the first network entity measuring
channel state information CSI for the channel between the first and
second network entity; the first network entity selecting a length
of the guard interval based on the CSI; and the first network
entity sending the packet with the guard interval of length
selected based on CSI.
26. In the wireless local area network of claim 25, wherein the
first network entity encodes the packet using a capacity enhancing
code, the improvement comprising: the first network appending to a
tail end of the packet an iterative decoding signal extension.
27. In the wireless local area network of claim 26, wherein the
capacity enhancing code is at least one of a low density parity
check code and a turbo code.
28. A method for transmitting a packet in a wireless local area
network WLAN, comprising: determining, at a first network entity,
channel state information CSI for a channel between the first and a
second network entity in a WLAN; selecting, at the first network
entity, a guard interval length from among at least two lengths
based on the determined CSI; and sending a packet with the guard
interval of the selected length from the first network entity over
the WLAN.
29. The method of claim 28, further comprising: encoding the packet
with a capacity enhancing code, and including in the packet an
iterative decoding signal extension disposed at a tail end of said
packet.
30. The method of claim 29, wherein the capacity enhancing code
comprises one of a low density parity check code and a turbo
code.
31. The method of claim 28, wherein determining CSI comprises
measuring CSI from a poll message.
32. The method of claim 28 in two iterations, wherein for a first
iteration a first packet comprises a cyclic prefix of a first
length and a guard interval of the second length, and for a second
iteration a second packet comprises a cyclic prefix of a length
different from the first length and a guard interval of a length
different from the second length, wherein the first and second
iterations of the method are for the same first network entity and
the same channel.
33. The method of claim 28, wherein selecting a guard interval
length comprises selecting from among lengths that are integer
multiples of 0.4 microseconds.
34. The method of claim 28, wherein selecting a guard interval
length comprises selecting from among a first and second length,
wherein the second length is twice the first length.
35. A network entity for communicating over a wireless local area
network WLAN comprising: a receiver for receiving a message from an
entity of a wireless local area network WLAN; a memory for storing
at least two different guard interval lengths, each associated with
a different channel state information CSI; a processor, coupled to
the receiver and the memory, for determining CSI from the received
message, and for selecting from the memory one guard interval
length associated with the determined CSI; a transmitter having an
input coupled to an output of the processor, and an output for
coupling to an antenna, said transmitter output for outputting a
packet comprising a guard interval of the selected length.
36. The network entity of claim 35, wherein the transmitter
comprises an encoder for encoding the packet with a capacity
enhancing code, further wherein the packet further comprises an
iterative decoding signal extension disposed at a tail end of said
packet.
37. The network entity of claim 36, wherein the capacity enhancing
code comprises one of a low density parity check code and a turbo
code.
38. The network entity of claim 35, wherein the message is a poll
message and determining CSI comprises measuring CSI from the poll
message.
39. The network entity of claim 35, wherein the processor varies a
cyclic prefix of a packet to be transmitted according to the
selected guard interval length for that packet.
40. The network entity of claim 35 wherein the at least two
different lengths are each multiples of 0.4 microseconds.
41. The network entity of claim 35 wherein the at least two
different lengths comprise a first and second length, wherein the
second length is twice the first length.
42. The network entity of claim 35, wherein the network entity
comprises a mobile station.
43. A device for communicating over a wireless local area network
WLAN comprising: a receiver for receiving a message from a node of
a WLAN; means for determining channel state information CSI from
the received message; means for determining a guard interval length
from at least two lengths based on the determined CSI; and means
for transmitting a packet comprising a guard interval of the
selected length.
44. The device of claim 43, wherein the means for determining a
guard interval length comprise a processor couple to a memory,
wherein said memory is for storing at least two guard interval
lengths and at least two values of CSI, each value of CSI
associated with one of the guard interval lengths.
45. The device of claim 43, wherein the means for transmitting
comprises a transmitter coupled to at least one antenna.
46. The device of claim 43 wherein the received message comprises a
poll message, and wherein the means for determining CSI from the
received message comprises a processor coupled to the receiver for
measuring CSI from the poll message.
Description
PRIORITY STATEMENT
[0001] The present invention claims priority from co-pending
Provisional U.S. Patent Application No. 60/460,553, filed with the
U.S. Patent Office on Apr. 4, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates broadly to Wireless Local Area
Networks (WLANs) and specifically to a topology for multi-channel
wireless time division duplex (TDD) systems so that channel state
information (CSI) may be acquired and used to optimize data
throughput.
BACKGROUND
[0003] Highly functional computers have been interconnected with
one another in what is termed a local area network (LAN) to enable
users of individual computers within a predefined set to share
files with one another. Traditional hardwired LANs are being
superceded by wireless LANs (WLANs) as WLANs realize increased
capacity. Data protocols for WLANs are generally organized into
layers or levels of the communication system, each layer
facilitating interoperability between various entities within the
network.
[0004] The Institute of Electrical and Electronic Engineers (IEEE)
standard for WLANs, IEEE 802.11, provides protocols for a physical
(PHY) layer and a Medium Access Control (MAC) layer, shown in block
diagram form at FIG. 1A. The following discussion relates to that
802.11 standard in its current form, though it is evolving. The PHY
layer 21 provides protocol for the hardware of WLANs termed
stations or nodes. A station may be mobile station, wireless
enabled laptop or desktop personal computer, and the like. The PHY
layer concerns transmission of data between those stations, and
there are currently four different types of PHY layers: direct
sequence spread spectrum (DSSS) 22, frequency-hopping spread
spectrum (FHSS) 23, infrared (IR) pulse modulation 24, and
orthogonal frequency-division multiplexing (OFDM).
[0005] The MAC layer 25 is a set of protocols that maintain order
in the use of the shared bandwidth or medium, and the 802.11
standard specifies two modes of communication: a compulsory
Distributed Coordination Function (DCF) 26, and an optional Point
Coordination Function (PCF) 27. A Basic Service Set (BSS) 31 is
shown in FIG. 1B, and is defined as a group of stations 32 that are
under the control of a single coordination function, which in
802.11 is termed a Point Coordinator PC that may also be an Access
Point (AP) 33. A BSS is roughly analogous to a group of mobile
telephone users within a cell of a single base station, with the
base station as the AP 33. Conceptually, every station in a BSS can
communicate with every other station in that BSS, though
degradations to the transmission medium due to multipath fading or
interference from nearby BSSs can result in `hidden` stations. The
802.11 standard provides for two types of networks: ad hoc and
infrastructure. Individual stations in the ad-hoc network are
deliberately grouped as a BSS, but any station in the BSS may
communicate directly with any other station in the BSS without
channeling all traffic through the centralized Access Point (AP). A
good example of an ad hoc network is a meeting where employees
bring laptop computers together to communicate and share files. One
of the stations serve as a Point Coordinator to coordinate
transmissions and avoid collisions, but the PC in an ad hoc network
does not act as an AP 33 that may link the BSS 31 to other BSSs or
networks. Conversely, the infrastructure network uses one or more
fixed network APs 33 by which wireless stations can communicate
beyond the BSS 31. These network APs are sometime used to bridge
the BSS to other BSSs to form an extended service set (ESS) and/or
to wired networks such as the internet or a conventional intranet
as shown in FIG. 2A. If AP service areas overlap, handoffs can
occur for roaming stations that move between APs similar to
cellular networks commonly used for mobile telephony. In the MAC
layer, the DCF operates in both ad hoc networks and infrastructure
networks. However, since PCF requires an AP 33, PCF may operate
only in infrastructure networks.
[0006] Avoiding collisions (simultaneous transmissions) between
stations in a BSS is complicated by the fact that while a wireless
station is transmitting, it cannot monitor the transmission medium
(the channel or channels) for other traffic that may interfere with
its own transmissions. For example, one problem arising from the
inability to listen while transmitting in WLANs is termed a "hidden
node". Assume stations A, B and C in a BSS are disposed as in FIG.
1B, with B physically located between A and C. If stations A and C
cannot communicate directly with one another due to distance,
multipath fading, or some other reason, stations A and C are hidden
from one another. Absent some collision control scheme, station A
may listen to the channel, sense it is clear, and transmit a packet
to station B. Whether or not station C is transmitting to B is
unknown to A, except through coordination by the PC. Simultaneous
transmissions from stations A and C to station B would result in
collision and lost transmissions, since all stations in a BSS 31
communicate over the same channel.
[0007] DCF seeks to minimize collisions by prioritizing stations
waiting to transmit based on a time delay basis. In DCF, each
station 32 with a data message to transmit contends for the next
available slot on the BSS channel during what is termed a
contention period CP 29. Time delays for various stations have a
random component, but procedures ensure a waiting station moves up
in priority the longer it waits. Details of the DCF prioritization
protocol are described in detail below. Once a station sends its
data message, which is included in a MAC Service Data Unit (MDSU),
it must contend with all other waiting stations for another
available slot. PCF is provided to avoid the situation where
time-sensitive data from one station cannot be assembled into one
MDSU, which is constrained to a maximum length. For example,
station A may wish to send an audio or video clip that spans three
MDSU's to station B, but contending for a separate transmission
slot for each of the MDSUs would potentially result in the clip
being undecipherable. While a relatively large buffer in the
receiving station may store and re-assemble the separately received
clip portions after a not insignificant delay, that option is
generally not seen as viable in the long term due to the dual
constraints of low power consumption and small physical size of
wireless stations. When implemented, PCF takes priority over DCF in
that a contention free period (CFP) 28 is established whereby
station A may send its data messages without contending for a time
slot. During the CFP 28, other stations stand by and await either a
poll by the PC during the CFP 28 or a contention period (CP) 29 in
which the various stations contend for a slot as in DCF above.
Additional details of PCF are provided below.
[0008] Historically, the development of WLAN systems, and wireless
systems in general, have taken two paths, one focused on
specifications for the PHY layer and the other for the MAC layer.
For example, the IEEE 802.11(e) task group is developing MAC layer
enhancement to improve MAC layer throughput regardless of physical
layer throughput. The IEEE 802.11(g) task group has developed a
physical layer specification that facilitate data rates of 20+
megabits per second (Mbps) in the 2.4 GHz. Range, but must keep MAC
layer changes to a minimal. Though both working groups operate
concurrently, in practice there appears little interaction between
the two groups. Advantages that may be gained by a more holistic
approach are never recognized by the groups' single-layer
focus.
[0009] Recently, the IEEE has approved a High Throughput Study
Group (HTSG) for 802.11, whose charter is to provide higher
throughput than enabled by current IEEE 802.11 standards. The High
Throughout Task Group (HTTG) will develop the actual standards,
which appears to be the first time that modifications to the MAC
and physical layers will be developed coherently since the division
of those layers. A recent study showed that the current IEEE MAC
and physical layers is limited to a throughput of 0.2 Mbps per 1000
byte packet per operational mode. Existing 54 Mbps modes therefore
have approximately 28 Mbps throughput for a 1000 byte packet.
Maintaining the same ratios, then a 108 Mbps data rate yields a
throughput of 56 Mbps for a 1000 byte packet.
[0010] It is well-known that optimum capacity is achieved when
Channel State Information (CSI) is known and used at both the
transmitter and receiver, and that MIMO systems (multiple
input/receive antennas and/or multiple output/transmit antennas)
provide a substantial increase in capacity as compared to more
traditional systems employing a single antenna on all transceivers.
For example, knowing CSI enables a transmitter to parse data among
different channels in a manner that takes advantage of the entire
channel capacity on each channel, rather than allowing the
time-sensitive bandwidth to be not fully used. Some communication
standards such as Code Division Multiple Access (CDMA) reserve a
feedback channel to provide CSI to the transmitter. Unfortunately,
CSI via a feedback channel is imperfect due to feedback delays and
changing channel characteristics. Regardless, the 802.11 standard
does not entail a feedback channel, there are no physical layer
specifications in 802.11 that are based on CSI, and some
researchers believe the lack of CSI in the standard prohibits the
adoption of a feedback channel in future versions of 802.11.
[0011] Thus, there is a need in the art to provide an optimum
throughput/capacity topology for multi-antenna wireless systems
that imposes changes that are backwards compatible with current
WLAN stations.
SUMMARY OF THE INVENTION
[0012] Fortunately, there are resolutions to this problem that are
embodied in the present invention. As mention above, there are no
physical layer specifications in the IEEE 802.11 standard that are
based on CSI at the transmitter. Operation of the Contention Free
Period (CFP) is described in the IEEE 802.11(e) draft standard,
herein incorporated by reference. Depending on the physical layer
standard 802.11(a), 802.11(b) or 802.11(g), the CFP modulation is
derived from one of their operational modes.
[0013] A system according to an embodiment of this invention
provide the optimum topology for a multi-antenna system dedicated
to higher throughput/capacity by bundling the Point Coordination
Function (PCF) operation in infrastructure mode of the current
and/or enhanced IEEE MAC with PHY specifications that employ some
form of coherent weighting based on CSI at the transmitter in
conjunction with the corresponding optimum receiver detection based
on CSI.
[0014] In one embodiment of the present invention is a method of
communicating over multiple sub-channels of a WLAN. The method
includes sending a control message that is not combined with a data
message from a first network entity to a second network entity. The
control message may be, for example, a CTS message during the CP or
a poll during the CFP, but in any case the control message is to
facilitate sequencing of wireless transmissions among at least two
entities in a wireless network. In the inventive method, the
control message is received at the second network entity, which
uses it to obtain channel state information CSI. The CSI is used to
determine the capacities of at least a first and second sub-channel
of the wireless network, and to determine which has the greater
capacity. A data message to be sent is divided into at least a
first and second data message segment, wherein the relative sizes
of the segments are based on the relative capacities of the
sub-channels. The division itself is preferably via an eigenmode or
water-filling known in the art to exploit varying capacities of
sub-channels. When the first sub-channel is determined to have the
greater capacity, the first data message segment will then define a
greater size than the second data message segment. Further in the
method and in response to receiving the control message, the second
network entity sends the first data message segment over the first
sub-channel, and the second data message segment over the second
sub-channel of the wireless network. In this manner, CSI is
obtained and used to send the segmented data message, though not
necessarily the control messages.
[0015] In a particular embodiment, the first network entity is a
point coordinator PC of a wireless network basic service set BSS
operating during a contention free period CFP, the control message
is a poll of the second network entity, and the PC may respond with
an ACK message combined with a data message for the first network
entity. Preferably, where the PC sends a poll of a third network
entity during the same CFP as the poll of the second network
entity, and the PC fails to receive a response from the third
network entity within a first time period such as a SIFS, the PC
then polls a fourth network entity within a second time period such
as a PIFS that is no greater than twice the first time period.
Where the PC receives from a network entity an ACK message combined
with a data message, the PC may respond with an ACK message
combined with a separate control message that signals an end of a
contention free period. In the 802.11 standard, for example, such a
message from the PC would be a combined ACK and CFP-End
message.
[0016] Further according to another aspect of the present
invention, when the method is executed during a contention free
period CFP, and the first network entity is a point coordinator PC
and the control message is a first poll of the second network
entity, there exists an instance where a polled station does not
respond to its poll. To avoid confusion with the terms above,
assume an initial poll of an initial network entity or station
occurs prior to the poll of the second network entity or station.
Prior to sending a control message without a data portion from the
PC to the second network entity, the method preferably also
includes sending from the PC an initial poll without a data message
to an initial network entity. Upon the PC failing to receive a
response to the initial poll from the initial network entity within
a first time period such as a SIFS, the PC then preferably sends,
within a second time period such as a PIFS that is greater than the
first time period, either a data message to the initial network
entity or the first poll of the second network entity as described
above.
[0017] The present invention may also be adapted for
station-to-station data communications during the CFP. Where the
method as summarized above is executed during a CFP, the data
message in its various segments is sent over the sub-channels from
the second network entity to a third network entity that is not a
point controller PC. In that instance, the method further includes
the third network entity sending to the second network entity an
ACK message within a first time period, in response to receiving
the data message segments. The PC may then send, within a period of
time following the ACK message from the third entity that is less
than twice the first time period, either a poll to a network
entity, or a data message to the second network entity that is
divided into data message segments based on CSI that is measured
from at least one data message segment sent from the second network
entity to the third network entity. If the PC is to allow the
second and third stations to exchange multiple data messages
between them, the PC will wait a PIFS before transmitting. If the
PC is to allow only one cohesive data message from the second to
the third entity, it need wait only one SIFS after the ACK message
from the third to the second entity, or one PIFS following the data
message from the second to the third entity.
[0018] In the above method, at least one of the network entities is
preferably a mobile station such as a mobile phone. The term mobile
station as used herein includes any portable electronic device that
has a telephonic capability, such as cellular phones, portable
communicators, PDAs with telephonic capability, and further
includes the various accessories to the above that expand the
capabilities or functionality of the mobile station with which they
are coupled.
[0019] According to another embodiment of the present invention is
a method of communicating data over a wireless network according to
an IEEE 802.11 standard as it exists as of the priority date of
this application. In this embodiment, the improvement to the 802.11
standard includes separating by at least one Short InterFrame Space
SIFS a poll and a data message sent by a point controller PC while
in a contention free period CFP. This allows data messages sent
from the PC to be transmitted with the benefit of knowing CSI, with
at least one possible exception noted below.
[0020] Preferably, CSI is also obtained during the contention
period CP during a Request-to-Send/Clear-to-Send RTS/CTS exchange.
In that instance, CSI is used to determine relative capacities of
at least a first and second sub-channel to parse a data message
from a station sending the RTS to a station sending the CTS.
Specifically, a data message from the RTS-sending station is parsed
into at least a first data message segment defining a first size
and a second data message segment defining a smaller second size.
The relative segment sizes are based on relative capacities of a
first and second sub-channel as determined by the measured CSI. The
larger first data message segment is sent over the higher capacity
first sub-channel and the smaller second data message segment is
sent over the lower capacity second sub-channel. Parsing of the
overall data message is based on relative sub-channel capacity as
determined by the measured CSI, such as by eigenmode or
water-filling techniques known in the art.
[0021] Considering again the CFP, this embodiment of the present
invention preferably restricts the PC to sending only one of five
possible messages: a poll; a data message parsed according to
measured CSI and transmitted among at least two sub-channels; a
data message so parsed and transmitted combined with an ACK
message; a CFP-End message; and a CFP-End message combined with an
ACK message. Conversely, 802.11 currently allows a data message to
be combined with a poll message, and does not provide that an ACK
can be combined with a CFP-End message since there appears no
opportunity for the latter to ever need be combined as the standard
currently exists. Preferably, the PC can combine a data message
only with an ACK message, else the data message may not be combined
with any other message.
[0022] Preferably, the PC is allowed to send a data message without
valid measured CSI to a station only upon non-receipt of a response
from that same network entity to its poll within one SIFS. Most
preferably, the PC can only send a data message with either valid
measured CSI or estimated CSI.
[0023] Where the PC and the polled station each have a data message
to send, one difference of the present invention as compared to the
802.11 standard is that the polled station is preferably allowed to
send its data message first. Preferably, between the time the PC
polls the station and the time the PC may next transmit, the polled
station may send a data message to another station (that is not the
PC) without using measured valid CSI for the channel between the
polled station and the another station. In this instance, the
another station is allowed an opportunity (one SIFS) to send an ACK
message to the polled station prior to the time the PC is next
allowed to transmit.
[0024] Another aspect of the present invention is a network entity
for communicating over a wireless local area network, such as a
mobile station, a point controller, an access point, or any other
entity on the WLAN. The network entity includes a receiver for
receiving over at least two sub-channels a control message from an
entity of a wireless local area network. The control message is
preferably a CTS message or a poll. The mobile station further has
a processor for determining a capacity of a first sub-channel and a
capacity of a second sub-channel based on channel state information
CSI measured from the control message. It further includes means
for parsing a data message into at least first and second segments
based on the relative determined capacities of the first and second
sub-channels. To best exploit the multi-channel capability in both
transmit and receive functions, the mobile station has a first and
second antenna having inputs coupled to an output of the means for
parsing. The first antenna is for transmitting at least the first
segment over the first sub-channel and the second antenna for
transmitting at least the second segment over the second
sub-channel. In certain embodiments, there may be a crossfeed
between antennas with differential weighting for each data message
segment so that each segment is actually transmitted over each
sub-channel, and increased capacity is realized by the differential
weights assigned to each segment.
[0025] These and other features, aspects, and advantages of
embodiments of the present invention will become apparent with
reference to the following description in conjunction with the
accompanying drawings. It is to be understood, however, that the
drawings are designed solely for the purposes of illustration and
not as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The present invention is better understood in light of the
following drawings.
[0027] FIG. 1A is a prior art block diagram showing MAC and PHY
layer structures in 802.11.
[0028] FIG. 1B is a prior art block diagram showing BSS's connected
to a wired network by a Distributions System.
[0029] FIG. 2A is a prior art timing diagram showing a CFP overlain
on a regular system implementing pure DCF.
[0030] FIG. 2B is similar to FIG. 2A but reflecting changes
according to the present invention.
[0031] FIG. 2C is similar to FIG. 2B but showing a different
exchange of data packets.
[0032] FIG. 2D is similar to FIG. 2B but showing yet another
different exchange of data packets.
[0033] FIG. 3 is a timing diagram showing a RTS/CTS Frame Exchange
during the contention period.
[0034] FIG. 4 is a prior art block diagram showing a fragmentation
in IEEE 802.11 MAC.
[0035] FIG. 5 is a prior art block diagram showing a IEEE 802.11
data frame format.
[0036] FIG. 6 is a prior art block diagram showing an ACK
frame.
[0037] FIG. 7 is a prior art block diagram showing a PS-Poll
Control Frame.
[0038] FIG. 8 is a prior art block diagram showing a RTS Control
Frame.
[0039] FIG. 9 is a prior art block diagram showing a CTS and ACK
Control Frame.
[0040] FIG. 10 is a graph of 2.times.2 Capacity curves m Rayleigh
channels.
[0041] FIG. 11 is a PDSU for Optimum Topology according to the
present invention
DETAILED DESCRIPTION
[0042] In the 802.11 standard, a Point Controller (PC) coordinates
prioritization during the contention free period CFP 28. The PC is
functionally within the Access Point (AP) 33 of a BSS 31 and is
usually physically collocated with it, so the term AP 33 is used
herein to indicate either or both the AP 33 and PC. A station 32
may serve as the AP 33 and the CP. FIG. 2A is a prior art timing
diagram showing transmissions sent (above the line designated 34)
and received (below the line 34) by the PC according to the 802.11
standard. The time period illustrated in divided into the
contention free period 28 and the contention period 29, which
together comprise a CFP Repetition interval 35 sometimes referred
to as a superframe. The CFP repetition intervals 35 continue so
that, when PCF 27 is in use, the CFPs 28 and CPs 29 alternate. The
CFP is described with reference to FIG. 2A, and the CP is described
below in conjunction with the distributed coordination function DCF
26. Prioritization of transmissions by the various stations 32 in a
BSS 31 is therefore via PCF 27 during a contention free period 28,
and via DCF 26 during the contention periods 29.
[0043] A superframe 35 begins with a beacon frame 36 transmitted by
the PC, regardless of whether PCF is active or not. The beacon
frame 36 is a management frame that provides timing and protocol
related parameters to the stations. Each beacon frame 36 also
announces when the next beacon frame will be transmitted, so that
all stations 32 are aware of superframe lengths. To enable PCF 27
to take priority over DCF 26, the PC transmits the beacon frame 36
after a PCF Interframe Space (PIFS) 37 (about 25 .mu.s) following
the end of the last superframe 35. Because the PIFS 37 is shorter
than a DCF Interframe Space (DIFS, about 34 .mu.s) that the DCF 26
must wait following the end of a superframe 35, PCF 27 can take
priority. A Short Interframe Spacing (SIFS) 38 spans about 16 .mu.s
and is the amount of time a station 32 is allowed to reply to the
PC. Each station 32 within the BSS 31 resets a Network Allocation
Vector (NAV) 41 based on the beacon frame 36. In FIG. 2A, the NAV
41 informs the station 32 to set the beginning of the next CP 29 at
the maximum span, and not to transmit during the intervening CFP 28
except under two circumstances: in response to being polled by the
PC, or to send an ACK message within one SIFS after receiving a
data message.
[0044] After the beacon frame 36, the PC delays one SIFS 38 and may
send any of the following: a data-only frame, a data+poll frame 42,
a poll-only frame, or a CFP-end frame. The PC maintains a list of
stations for which it has data, and typically polls those stations
first in order to piggyback that data with its poll of the station.
Referring to FIG. 2A, the PC polls a first station and piggybacks
data with that poll in a data+polling frame 42 (both data and poll
are directed to the first station). Upon receiving the data, the
first station responds with an acknowledgement (ACK), but itself
piggybacks data (U1) on its ACK in a data+ACK frame 43. The first
station is allowed a SIFS 38 to respond to the AP's poll, but may
send its data (U1) to any station or to the PC. [If it is sent to a
station other than the PC, that station has one SIFS to send its
ACK, without piggybacking data, back to the first station.]
[0045] After receiving the data+ACK frame 43 from the first station
(U1+ACK), the PC waits one SIFS and polls another station (or the
same station). In the event the previous first station sent its
data (U1) to the PC, the PC will piggyback an ACK for that first
station in the data+poll it sends to a second station in a
data+poll+ACK frame 44 (D2+ACK+Poll, data and poll directed to the
second station, ACK directed to first station). In FIG. 2A, the
second station does not respond within one SIFS, so after waiting a
total of one PIFS, the PC sends a poll with data (D3) to a third
station in another data+poll frame 42 (D3+Poll, data and poll to
third station). The third station responds within a SIFS with data
(U3) and an ACK in its data+ACK frame 43. When the PC has no more
stations to poll, or when the CFP as determined by the beacon frame
36 nears its end, the PC transmits a CFP-End frame 45 to signal all
stations 32 that the CFP 28 is ended.
[0046] One drawback with the prior art, at least in certain
circumstances, is that the polling frames and the data frames from
the PC may be combined into a single frame (data+poll 42 or
data+ACK+poll 44). At the time of that combined frame transmission,
the PC does not know the channel state between it and the intended
station. While channel state may not change significantly over a
single CFP repetition interval 35 when used in a wired network,
channel states change much more rapidly in WLANs. To increase
capacity over a fixed bandwidth, multiple sub-channels are
preferably used such as in a single input/multiple output (SIMO)
communication system, a multiple input/single output (MISO) system,
or most preferably a multiple input/multiple output (MIMO) system.
Any of these are referred to hereafter as a MIMO system unless
otherwise stipulated. The multiple sub-channels of a wireless MIMO
system are each subject to rapid changes due to fading, multipath,
etc., so wireless MIMO systems need to know the state of the
different sub-channels to send different data portions over the
strongest channels, or to partition the data to be sent into sizes
that maximize the respective capacities of the various sub-channels
as those sub-channels exist at the time of transmission. When the
PC polls a station, it has not yet received any feedback from that
station by which to measure the true channel. Since the
sub-channels change rapidly, it is highly unlikely that the
coherence interval (the interval over which the measured state of
the channel does not change significantly) spans an entire CFP
repetition interval 35. Said another way, any measurements of the
channel made in one CFP 28 are unlikely to be valid estimates of
the channel during the next CFP 28. Sending a data message combined
with a poll necessarily implies sending the data either regardless
of channel quality or with invalid (i.e., outside the coherence
interval) estimates of the channel. Either option is a waste of
bandwidth as compared to maximum capacity theory. Among other
aspects, the present invention modifies the specific frame exchange
of FIG. 2A to enable entities transmitting data frames to do so
with knowledge of the channel, termed in the art as channel state
information or CSI.
[0047] FIG. 2B is similar to FIG. 2A but shows the same substantive
exchange of data frames depicted in FIG. 2A (one data frame from
the AP to each of a first, second, and third station, and one data
frame from the first and third stations to the AP) accomplished
according to the present invention. For each of FIGS. 2B-2D, only
the CFP 28 is shown and the interval between frames is one SIFS
unless otherwise noted. At the start of the CFP 28, the PC
transmits a beacon frame 36 as described. The PC next transmits a
polling-only frame 46 (P1) that is directed to the first station.
The first station has a data frame for the PC, and has the
opportunity to measure actual CSI between it and the PC in the
polling frame 46. The first station uses that CSI to send a data
only frame 47 back to the PC within one SIFS of the end of the
polling frame 46. The PC receives the data only frame 47
(designated U1) and uses it to measure the channel between it and
the first station. Using that CSI, the PC then sends its data for
the first station combined with an acknowledgement that it (the PC)
received the data frame from the first station in a data+ACK frame
43. This obligates the first station to reply with an ACK only
frame 48 that it received the data correctly. After a SIFS, the PC
then polls the second station (P2) in a polling-only frame 46. The
second station does not respond within a SIFS, so after a total
delay of one PIFS, the PC polls a third station. The exchange
between the PC and the third station is similar to that described
between the PC and the first station for FIG. 2B.
[0048] On first glance, it appears the exchange of frames of FIG.
2B introduces an inefficiency as compared to that of FIG. 2A, due
to an increased number of frames and interframe spacings. However,
the poll only 46 and ACK only 48 frames are quite short, whereas
any frame that includes data 42, 43, 44, 47 may be substantially
longer. In the present invention as embodied in FIG. 2B, the poll
only frames 46 may be sent without valid CSI and all frames that
include data are transmitted to maximize the available capacity of
the channel. Preferably, all frames carrying data are sent with
valid CSI by use of the present invention, though FIG. 2C notes an
exception. While additional MAC overhead may be increased as
compared to the method of 802.11, throughput is increased due to
the larger size of frames with data as compared to those without.
Various frame sizes and throughputs are detailed below with
reference to FIGS. 5-10.
[0049] FIG. 2C is an illustration of frame exchange for the
instance where the AP has data for the first and third station, and
only the third station has data for the PC. The beacon 36 and
polling only 46 (P1) frames are as described with reference to FIG.
2B. Since the first station of FIG. 2C has no data for the PC, it
does not respond to the poll within a SIFS and the PC is allowed to
transmit again after a PIFS 37. In one embodiment of the invention,
the PC sends a data-only frame 27 (D1) to the first station without
having had an opportunity to measure CSI (since the first station
did not respond to the poll within a SIFS). The first station sends
an ACK only frame 48, and the remainder of FIG. 2C is similar to
FIG. 2B except the portion beginning with the frame designated
ACK+U3. Rather than sending an ACK only frame 48 as in FIG. 2B, the
third station has data for the PC, which it sends with an
ACKnowledgement in a data+ACK frame 43. Assuming there are no
further stations for the PC to poll, it responds to this last
transmission from the third station with an ACK+end frame 49,
wherein the ACK is directed to the third station and the CF-END
portion is directed to all stations 32 of the BSS 31.
[0050] As an alternative to the scenario described for FIG. 2C
wherein the PC sends a data only frame 47 to the first station
without benefit of CSI, the first station (or any station being
polled but not having data to transmit to the PC) may be obliged to
reply with an ACK only frame 48 in order that the PC may measure
the channel. Since the PC may also not have data for the station
responding to a poll with an ACK only frame 48, there is a
potential to waste bandwidth that in the cumulative becomes
non-negligible. This wasting aspect may be minimized by including
within the poll frame information that indicates whether or not the
PC has data to send to the polled station, which may be as little
as a single bit (e.g., 0 indicates no data, 1 indicates data). The
polled station may disregard that information if it has data to
send to the PC (as in FIG. 2B), allow a SIFS to expire without
responding if the information indicates there is data (as in the
exchange depicted in FIG. 2C between the PC and the first station),
or respond with an ACK only frame 48 if the information indicates
there is data coming from the PC (as in the exchange depicted in
FIG. 2D between the PC and the second station).
[0051] FIG. 2D depicts frame exchange for additional scenarios. The
beacon 36 and exchange between the PC and the first station are as
in FIG. 2C. Upon polling a second station with a polling only frame
46 (P2), the second station responds with a data frame to another
station 51 rather than to the PC. This station-to-station data
frame 51 is sent without the benefit of valid measured CSI, since
there is no prior communication, within the coherence interval,
from the recipient of the station-to-station data frame 51 by which
to measure the channel. The recipient station then responds with an
ACK only frame 48 directed back to the sending station. Though the
data in frame 51 was directed toward another station, the PC still
listens and uses it to measure the channel between it and the
second station. Following the ACK only frame 48 directed back to
the second station, the PC may send a data only frame 47 to the
second station without drawing a direct response from it. The PC
may wait a PIFS, to allow the second station an opportunity to send
additional station-to-station data frames 51. The second station
sends an ACK only frame 48 back to the PC, which then polls a third
station with a polling only frame 46. The third station in the
scenario of FIG. 2D has no data to transmit, so the PC waits a PIFS
37 and transmits a CF-END frame 45 to transition into the
contention period 29.
[0052] In any of the above instances, any of the PC or stations may
have more than one frame with data to send. Due to the potential
size of the data frames and the speed with which the channel may
vary over time (the length of the coherence interval), it may be
necessary in one instance that the sender re-acquire CSI from the
last transmission of the intended recipient, and in another
instance it may have negligible effect on data throughput that the
sender re-use the originally measured CSI. So long as the frames in
question are sent within the coherence interval established when
CSI is measured, then CSI is considered valid whether or not is was
measured based on a frame received immediately preceding the next
frame to be sent.
[0053] The above description pertains to the CFP 28 wherein the PC
controls which station in an infrastructure network may next
transmit. Following is a description as to how the present
invention may be used within the contention period 29 following the
CFP 28. Since the CFP 28 exists only while in the point
coordination function 27, operation within the CP 29 is within the
base DCF 26 layer of MAC 25 and is detailed at FIG. 3.
[0054] DCF lies directly on the PHY layer 21 and is based on
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
protocol, because wireless stations cannot listen for collisions
while transmitting. As known in DCF, when a station has a frame
with data to be transmitted, it first listens to ensure no other
station is transmitting over the prescribed channel and transmits
only if the channel is clear for a set period of idle time, termed
a DCF-interframe space (DIFS) 38 that is longer than a PIFS. If the
channel is busy, the station instead chooses a random "backoff
factor" which determines a delay period 58 wait until it is allowed
to transmit its data. During periods in which the channel is clear,
the transmitting station decrements its backoff counter to shorten
the delay period 58 so a delayed station gradually gains a higher
priority to transmit. When the backoff counter reaches zero and the
channel is clear for the duration of a DIFS 38, the station may
transmit its frame with data. Since the probability that two
stations will choose the same backoff factor is small, collisions
between data frames from different stations are minimized.
[0055] When a particular station's backoff counter reaches zero and
it senses the channel is clear for an entire DIFS 38, that station,
termed the source 52 or transmitting station, first sends out a
short ready-to-send (RTS) frame 53 containing information on the
length of the frame with data to be transmitted. If the intended
destination 54 to which the RTS 53 is directed hears it, the
receiving station 54 responds with a short clear-to-send (CTS)
frame 55. Only after this exchange does the source 52 send its data
frame 47 during the CP 29. When the destination 54 receives the
transmitted data frame 47 successfully (as determined in 802.11 by
a cyclic redundancy check CRC), the receiving station (or PC)
transmits an acknowledgment (ACK) frame 48. This back-and-forth
exchange is necessary to avoid the "hidden node" problem previously
explained. If the receiving station 54 has a data frame 47 to send,
it must contend for a transmit slot as above and cannot piggyback
data onto its ACK frame 48. During this process, other stations 56
defer transmission access 57 until they sense the channel is clear
for a DIFS plus their backoff factor.
[0056] The present invention exploits the RTS/CTS interchange to
provide valid CSI to at least the source 54 for use in transmitting
the data frame 47. The benefits of the destination 54 using CSI
obtained from the RTS/CTS exchange for use in transmitting the ACK
only frame 48 are relatively minor as that frame is small. Since
each station is at differing times both a source 52 and a
destination 54, the means to implement the present invention are
already in place and can be used for the ACK only frame 48, even if
its practical effect is merely to send an unparsed ACK frame 48
over the most robust of the available sub-channels.
[0057] There is another opportunity within the 802.11 standard by
which a station may obtain valid CSI for the channel over which it
intends to transmit. A listening station, such as the other station
56 of FIG. 3 that is not a source 52 or destination 54 of a
particular exchange, may transmit a CTS message to itself in
accordance with the standard to obtain CSI. That CSI may then be
used, within the coherence interval in which it is valid, to
reserve the channel and preserve a clear channel access CCA
mechanism.
[0058] FIG. 4 is a prior art block diagram of a MAC Service Data
Unit (MSDU) 58, the term used to represent units of transmission in
the MAC layer 25 of the 802.11 standard. As noted above, different
messages may be "piggybacked", and the different fragments 59 of
the MDSU 58 represent those different messages, which may each be
independently addressed. Each fragment includes a leading MAC
header 61, a trailer 62 that includes a cyclic redundancy check
CRC, and a frame body 53 between them. A single MDSU 58 may include
more than one frames or fragments 59 (as in data+ACK frame,
ACK+poll frame, etc.), or only one frame or fragment 59 (as in the
poll only frame, data only frame, etc.)
[0059] FIG. 5 shows a more detailed view of a data only frame 47
that may be one of the fragments 59 of an MDSU 58. The number of
octets dedicated to each portion of the frame 47 is listed directly
below the block. Each of FIGS. 5-9 are known in the art and
consistent with the 802.11 standard, and are presented hereto
demonstrate quantitative gains in using the present invention as
compared to the current 02.11 standard. In the data only frame 47
of FIG. 5, the various portions of the header 61 use thirty octets,
the trailer 62 uses four octets, and the body 63 carrying the
substantive data may extend to 2312 octets, depending upon the
amount of data to be sent. By comparison, FIG. 6 represents an ACK
only frame 48 with a sixteen octet header 61, a four octet trailer
62, and a four octet body 63. FIG. 7 represents a poll only frame
48 with a sixteen octet header 61, a four octet trailer 62, and a
zero octet body 63. FIG. 8 represents a RTS Control Frame 53 having
the same relative sizes as those of the poll only frame 48 of FIG.
7 but with different header fields. FIG. 9 represents a CTS Control
Frame 55 having a ten octet header 61, a four octet trailer 62, and
a zero octet body 63. Using these relative frame sizes, one can
calculate the data throughputs for various scenarios to compare a
wireless network using the topology of the present invention to the
topology currently stipulated in the 802.11 standard. Those
calculations as concerning the present invention are presented
below.
[0060] The minimum criteria for optimum transmission topology for
wireless time division duplex TDD networks are:
[0061] 1) valid CSI is present at the transmitter,
[0062] 2) eigen-mode transmission is performed, and
[0063] 3) the frame/packet is received by the intended recipient
within a period less than the coherent time of the channel.
[0064] To achieve the capacities possible with the present
invention, the transmitter should employ some weighting mechanism
to assign frames, packets, fragments, or whatever division of the
entire package to be transmitted to various sub-channels based on
the measured quality of those sub-channels. Eigen-mode or
waterfilling is one technique known in the art to do so, described
mathematically below. For ad hoc networks and infrastructure
networks during the contention period, the RTS/CTS exchange may be
used. During the contention free period, the revised frame exchange
described above may be employed to achieve valid CSI. In either
case, the coherent weighting is done at the PHY layer 21, so the
present invention modifies both the MAC and PHY layers.
1TABLE 1 Half Duplex Frame Efficiency for 1500 byte packets using
Optimum Topology Configurations @ MAC SAP 12 24 54 6 Mbps Mbps Mbps
Mbps 100 Mbps 200 Mbps CFP-Poll 95.35% 93.05% 88.75% 79.6% 68.7%
52.93% CP- 93.8% 90.9% 85.6% 74.74% 62.55% 46.2% RTS/CTS
[0065] Frame Efficiency as used in Table 1 is the time required to
transmit the information portion of packet divided by the total on
air time for packet. Thus, the overall capacity is found by
multiplying the frame efficiency by the capacity/throughput, which
are shown in Table 2 below:
2TABLE 2 802.11 Capacity Requirements in bps using Optimum Topology
Configurations @ MAC SAP 54 100 200 6 Mbps 12 Mbps 24 Mbps Mbps
Mbps Mbps CFP-Poll 0.52 1.07 2.25 5.65 12.13 31.5 CP-RTS/CTS 0.533
1.10 2.34 6.02 13.32 36.1
[0066] The capacity requirements are computed as raw data rate/12
Msymbols/sec/Frame efficiency to yield the target
throughput/capacity at the MAC SAP layer. The theoretical best
performance for these capacity requirements can be read from FIG.
10 for a 2.times.2 configuration (2 input antennas, 2 output
antennas) in Rayleigh fading, or computed using the formula below
for any arbitrary MIMO configurations 1 C = log 2 [ det ( I M + N
HH .dagger. ) ] bps/ Hz
[0067] Eigen-mode transmission as noted above is described as
follows. Let the singular value decomposition of H be H=U.SIGMA.V
where U and V are unitary matrices and .SIGMA. be a diagonal matrix
With positive real values on the diagonal elements representing the
singular values of the channel. If the transmitted vector r is
pre-multiplied by V in the transmitter and received vector is post
multiplied by U.sup.H in the receiver, i.e., Vr U.sup.H=V (Hx+n)
U.sup.H=.SIGMA.x+m, where m=Vn*U.sup.H and there is no noise
amplification and remains spatially white.
[0068] Because a single MAC layer must interface with disparate PHY
layers, the 802.11 standard uses an additional protocol layer
termed the Physical Layer Convergence Protocol (PLCP) disposed
between them that is defined differently for each transmission
method. The PLCP adds a preamble and a header (each of various
sizes) to a PLCP Service Data Unit (PSDU), which is the portion of
the complete transmission frame (PPDU or PLCP Protocol Data Unit at
the PHY layer) that carries the actual data to be transmitted
between stations or between the point controller PC and a station.
FIG. 11 is a block diagram showing a PSDU 65 for optimum topology
according to the present invention, with times and numbers of bits
tailored for compatibility with the 802.11 standard as it currently
is written. The present invention enables the length of a guard
interval 66a, 66b to be selectable (to vary) based on the CSI. For
certain channels, the delay spread of the channel is shorter than
other time and hence not necessary to keep a fixed cyclic prefix
(CP) overhead. Further, if capacity achieving codings are used,
such as low density parity check codes (LDPC) or Turbo codes, then
additional time is allocated at the end of the packet for iterative
decoding, which is not currently available in current IEEE 802.11
standard or its amendments. This additional time is represented in
the PSDU 65 of FIG. 11 as an iterative decoding signal extension
67.
[0069] While there has been illustrated and described what is at
present considered to be a preferred embodiment of the claimed
invention, it will be appreciated that numerous changes and
modifications are likely to occur to those skilled in the art. It
is intended in the appended claims to cover all those changes and
modifications that fall within the spirit and scope of the claimed
invention.
* * * * *