U.S. patent application number 10/117128 was filed with the patent office on 2002-11-21 for instantaneous joint transmit power control and link adaptation for rts/cts based channel access.
Invention is credited to Larsson, Peter.
Application Number | 20020172186 10/117128 |
Document ID | / |
Family ID | 26814949 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020172186 |
Kind Code |
A1 |
Larsson, Peter |
November 21, 2002 |
Instantaneous joint transmit power control and link adaptation for
RTS/CTS based channel access
Abstract
A method for closed loop link adjustment based on a Request To
Send-Clear To Send (RTS-CTS) channel access scheme includes the
following steps. Designating a station as an originating station.
Transmitting a RTS frame with predetermined transmit power from an
originating station, prior to an intended DATA transmission,
sounding the channel such that reception characteristics can be
evaluated at a designated receiving station. Transmitting, in
response to the originating station, a CTS frame with a
predetermined transmit power from the receiving station with
directives of link adjustments. Transmitting a DATA frame from the
originating station to the receiving station frame complying with
link adjustment directives to the extent of the originating
stations capabilities. And, transmitting an acknowledge (ACK) frame
in response to the originating stations from the receiving station
indicating result of DATA frame reception.
Inventors: |
Larsson, Peter; (Solna,
SE) |
Correspondence
Address: |
Ronald L. Grudziecki, Esq.
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
26814949 |
Appl. No.: |
10/117128 |
Filed: |
April 8, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60282191 |
Apr 9, 2001 |
|
|
|
Current U.S.
Class: |
370/349 |
Current CPC
Class: |
H04W 52/54 20130101;
H04W 52/46 20130101; H04W 28/18 20130101; H04W 74/0816 20130101;
H04W 52/243 20130101; H04W 52/16 20130101; H04W 52/08 20130101;
H04W 28/26 20130101; H04W 52/50 20130101; H04W 52/10 20130101 |
Class at
Publication: |
370/349 |
International
Class: |
H04J 003/24 |
Claims
1. A method for closed loop link adjustment based on a Request To
Send-Clear To Send (RTS-CTS) channel access scheme comprising the
steps of: designating a station as an originating station;
transmitting a RTS frame with predetermined transmit power from an
originating station, prior to an intended DATA transmission,
sounding the channel such that reception characteristics can be
evaluated at a designated receiving station; transmitting, in
response to the originating station, a CTS frame with a
predetermined transmit power from the receiving station, the CTS
frame comprising link adjustment directives; transmitting a DATA
frame from the originating station to the receiving station, based
on the link adjustment directives and the originating station's
capabilities; transmitting an acknowledge (ACK) frame in response
to the originating stations from the receiving station indicating a
result of DATA frame reception at the receiving station.
2. The method of claim 1, wherein the link adjustment indicates a
change in transmit power.
3. The method of claim 2, wherein the link adjustment is relative
to the transmit power used for the RTS frame.
4. The method of claim 1, wherein the link adjustment comprises
link adaptation of forward error correction and signal
constellations.
5. The method of claim 1, wherein the link adjustment comprises a
change in transmit power and link adaptation of forward error
correction and signal constellations.
6. The method of claim 1, comprising: the originating station
evaluating reception characteristics of the CTS frame; the
originating station conveys link adjustment information in a
successive DATA frame; and the receiving station transmits a
corresponding ACK frame in accordance with the link adjustment
information in the successive DATA frame.
7. The method of claim 1, wherein link adjustment is performed
continuously on successive and consecutive ACK and DATA frames when
multiple fragments of DATA are transmitted.
8. A method for open loop group transmit power control in a
wireless system, comprising: transmitting a frame conveying
transmit power information for the frame to any proximate station;
a proximate station receiving the frame and determining the path
gain based on measured signal strength of the received frame and on
the transmit power information conveyed in the received frame;
selecting path gains originating from a group of stations;
determining a transmit power required to reach any of the selected
path gains; selecting the minimum of a) the highest transmit power
and b) allowed transmit power, wherein the allowed transmit power
is determined by regulatory requirements and station transmit power
capabilities; and assigning the selected transmit power to Clear To
Send (CTS) messages.
9. The method of claim 8, wherein the wireless system comprises an
infrastructure-less system or Independent Basic Service Set (IBSS)
and the frame is a 802.11 BEACON frame.
10. The method of claim 8, wherein the frame comprises an
indication of a minimum required receive power.
11. The method of claim 10, wherein the minimum required receive
power is set relative to an indicated transmit power level.
12. The method of claim 10, wherein the required transmit power is
determined based on the minimum required receive power.
13. The method of claim 8, wherein the RTS transmit power setting
is determined based on input parameters related only to the
destination station.
14. The method of claim 8, comprising assigning the selected
transmit power to Request to Send (RTS) messages.
15. A method for open loop group transmit power control in a
wireless system, comprising the steps of: selecting, by a first
station, at least one station within a group; transmitting a
transmit power information request from a first station to the at
least one selected stations; transmitting a transmit power response
in a frame, the frame comprising transmit power information for the
frame, to any proximate station from the at least one selected
stations in orderly manner preventing collisions; receiving the
frame and determining the path gain based on measured signal
strength of the received frame and the transmit power information
in the received frame; selecting path gains originating from the
group; determining required transmit power to reach any of the
selected path gains; selecting the minimum of a) the highest
transmit power and b) allowed transmit power, wherein the allowed
transmit power is determined by regulatory requirements and
stations transmit power capabilities; and assigning the selected
transmit power to Clear To Send (CTS) messages.
16. The method of claim 15, wherein the wireless system is an
infrastructure system or a Basic Service Set (BSS) and wherein the
transmit power request is conveyed as an information element in an
IEEE 802.11 management frame.
17. The method of claim 16, wherein the management frame is a
beacon, probe request or generic management frame.
18. The method of claim 15, wherein the transmit power response is
conveyed as an information element in an IEEE 802.11 management
frame.
19. The method of claim 18, wherein the management frame is a probe
request, probe response or generic management frame.
20. The method of claim 17, wherein the RTS transmit power setting
is determined based on input parameters related only to the
destination station.
21. The method of claim 15, comprising assigning the selected
transmit power to Request To Send (RTS) messages.
22. A method of tiered transmit power comprising: determining a
sequence of frames that must be exchanged for successful
communication; assigning different transmit power levels to ones of
the frames having different topological objectives or distance
objectives.
23. The method of claim 22, wherein a transmit power setting for a
frame that should be received by as many stations as possible is
limited by regulatory requirements and station transmit power
capabilities.
24. The method of claim 22, wherein the frame that should be
received by as many stations as possible is a beacon frame.
25. The method in claim 22, wherein frames destined for a set of
multiple stations are transmitted with power that is a) sufficient
to reach all stations within the set and b) equal to or less than a
maximum power specified by regulatory requirements.
26. The method of claim 25, wherein the frames destined for a set
of multiple stations are multicast data frames.
27. The method in claim 25, wherein a frame that is destined for a
single station but should be overheard by multiple stations in a
set is transmitted with a power that is a) sufficient to reach all
stations within the set and b) equal to or less than the a maximum
power specified by regulatory requirements.
28. The method in claim 27, wherein a frame destined for a single
station is transmitted with a power that is a) sufficient to reach
the intended station and b) equal to or less than the a maximum
power specified by regulatory requirements.
29. A method for interference mitigation based on open loop
transmit power control enabling tighter medium reuse, comprising
the steps of: conveying transmit power control information for
every transmitted frame in the frame by any station transmitting;
receiving the transmitted frames and determining a path gain based
on measured signal strength of the received frames and transmit
power information conveyed by the received frames; determining a
maximum instantaneously allowed transmit power based on all
overheard frames; and performing at least one of a) reducing
transmit power, and b) reducing transmit power in combination with
adjusting link rate to ensure that the maximum instantaneously
allowed transmit power is not exceeded during any transmission
attempt.
30. The method in claim 29, wherein Request To Send (RTS) and Clear
To Send (CTS) frames convey transmit power information.
31. The method of claim 29, comprising: determining a maximum
allowed receive power; and determining the maximum allowed transmit
power based on the determined maximum allowed receive power.
32. The method of claim 29, wherein a station defers access and
enters a back off mode when the maximum instantaneous allowed
transmit power has to be exceeded.
Description
[0001] This application claims priority from U.S. Provisional
Application No. 60/282,191, filed on Apr. 9, 2001 in the English
language, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to the field of wireless
communications, in particular to transmit power control and link
adaptation techniques and mechanisms.
[0004] 2. Background Information
[0005] The IEEE 802.11 is a wireless LAN (Local Area Network)
standard that has been standardized by IEEE (Institute of
Electrical & Electronics Engineers). The IEEE 802.11 wireless
LAN standard is currently undergoing a process of extending the
standard with QoS (Quality of Service) features. The objective is
to enable, for example, computers or multimedia devices to
communicate under QoS constraints. This standard extension goes
under the name IEEE 802.11e and is managed by the so-called task
group e, TGe.
[0006] Recently, the IEEE 802.11 standard was also extended with a
new physical layer allowing higher data rates than the previous
physical layer. Various data rates are enabled through several code
rates and signal constellations. The purpose is to allow link
adaptation depending on the channel quality. The high rate PHY
(physical layer) on the so-called 5 GHz band is called IEEE 802.11a
and is based on OFDM (Orthogonal Frequency Division Multiplexing).
The corresponding so-called 2.4 GHz band PHY is called IEEE 802.11b
and uses single carrier modulation schemes.
[0007] IEEE 802.11 operates either in a DCF (Distributed
Coordination Function) or a PCF (Point Coordination Function) mode.
The former is for distributed operation and the latter for
centralized control from an access point, AP. So far the PCF mode
has not been ratified by implementers as the complexity is consider
to high, instead DCF is used both for the distributed operation as
well as with the AP.
[0008] The origin of IEEE 802.11 access scheme is traced back to
BTMA (Busy Tone Multiple Access) which was the first proposed
method for distributed control of channel access avoiding the well
known hidden terminal problem.
[0009] In MACA (Multiple Access with Collision Avoidance), proposed
by Phil Karn in 1980, the introduction of a Request To Send (RTS)
and Clear To Send (CTS), handshake phase prior data transmission
solved the idea of distributed reservation. This presented a more
feasible basis to build a practical system upon as it did not
divide the frequency band in a channel for data and busy tones, as
in the BTMA scheme. Also the idea of random exponential back off,
that was later used in IEEE 802.11, was introduced in MACA.
[0010] In MACAW (Multiple Access with Collision Avoidance for
Wireless), the basic mechanism of MACA was refined. Among other
things, a link acknowledgment, ACK, scheme was introduced. The
access scheme of IEEE 802.11 is now based to a great extent on
principles developed in MACAW.
[0011] Other ongoing standardization activities in IEEE 802.11
include the so-called TGh (Task Group h, i.e., an IEEE task group
for IEEE 802.11h) that has the objective of designing and including
transmit power control (TPC), as well as distributed frequency
selection (DFS), in IEEE 802.11a. The purpose of power control from
a standardization point of view is primarily to enable IEEE 802.11a
stations, STAs, to conform to European regulatory requirements.
[0012] As background information, the basic access principles for
IEEE 802.11 will now be described. For more detailed information
the reader is referred to the standard IEEE 802.11-1999 (which
replaces IEEE 802.11-1997), the standard IEEE 802.11a-1999 (High
data rate on the 5 GHz Band), and the standard IEEE 802.11b-1999
(High data rate on the 2.4 GHz Band). Good and simple overviews may
also be found in a) "Smart Antenna Systems and Wireless LANs",
authored by Garret T. Okamoto and published by Kluwer academic
publishers (ISBN 0-7923-8335-4), and "IEEE 802.11 Handbook, A
Designers Companion", authored by Bob O'Hara and Al Patrick (ISBN
0-7381-1855-9).
[0013] There are two modes of channel access scheme operation in
the Distributed Co-ordination Function (DCF), one based on CSMA/CA
(Carrier Sense Multiple Access/Collision Avoidance) and one based
on CSMA/CA including RTS-CTS message exchange. A MIB (Management
Information Base) attribute "dot11RTSThreshold" is used to
differentiate the use of the two. MPDUs (MAC Protocol Data Units,
where "MAC" stands for Medium Access Control) shorter than the
threshold is sent without RTS-CTSs, whereas longer MPDUs are sent
with RTS-CTSs. The focus here is the RTS-CTS based CSMA/CA
mechanism that enables mitigation of hidden stations and hence in
general provides a more efficient use of the wireless medium.
[0014] FIGS. 1A-1D show a communication procedure between a station
T and a station R, and related effects on nearby stations E, F, G,
H. In FIG. 1A, station T transmits an RTS (Request to Send) signal
to the station R. The transmit range 102 of the station T
encompasses the stations R, E and F, but not the stations H, G.
Thus the stations R, E and F receive or overhear the RTS signal,
but the stations H, G do not. In a next step shown in FIG. 1B, in
reply to the RTS signal, the station R sends a CTS (Cleared to
Send) reply signal to the station T. As shown in FIG. 1B, the
transmit range 104 of the station R encompasses the station F, H
but not the stations E, G. After receiving the CTS signal, in FIG.
1C the station T transmits a DATA signal to the station R, and then
in FIG. 1D the station R acknowledges receipt of the DATA signal by
sending an ACK signal or message to the station T.
[0015] Since the station H is a hidden station with respect to the
station T, it is informed of the intention of station T to transmit
via the reply CTS message sent by the station R (since station H is
not hidden from the station R, i.e., it is within the transmit
range 104 of the station R). As a consequence, the station H will
not transmit and disturb ongoing reception by the station R.
Stations E and F will in a similar manner defer channel access to
the stations T and R, after overhearing the RTS from the station T
and/or the CTS from the station R. As shown in FIGS. 1A-1D, station
G is hidden from both stations T and R, and therefore will likely
not overhear the RTS or CTS, and therefore it may transmit.
[0016] FIG. 2 illustrates frame formats used in IEEE 802.11, where
the numbers above the boxes indicate the size of the information in
the box. Note, Address 4 in the DATA and MANAGEMENT frame exists
only for DATA frames in a wireless DS (Distribution System), and
does not exist in MANAGEMENT frames.
[0017] FIG. 3 illustrates the frame exchange including RTS and CTS.
When frames are received by stations other than those intended to
receive the frames, a so called NAV (Network Allocation Vector) is
set according to a duration value indicated in a field of the
frame. This provides an additional collision avoidance mechanism to
the physical channel access sensing and is therefore called virtual
channel sensing. As long as either the physical or virtual channel
sense indicates activities on the channel, a station must remain
silent When the channel becomes free, stations start contending for
the channel according to the channel access principles defined in
the IEEE 802.11-1999 standard. In general, the NAV can only be
extended if new frames are received. There exist some special
instances when the NAV can be reset as well, but that is not the
normal operation.
[0018] FIG. 4 illustrates use of RTS-CTS with DATA fragmentation.
Each fragment and ACK then acts as implicit RTS and CTS. Additional
fragments are indicated by a bit (field) in the frame control of
the fragments.
[0019] According to the IEEE 802.11-1999 standard, CTS should be
sent with the same link rate as RTS, and ACK should be sent with
the same link rate as DATA. The original purpose is to enable the
originating or transmitting station (e.g., the station T of FIG. 1)
to calculate the duration value prior to RTS transmission.
[0020] FIG. 5 shows a detailed example of two stations attempting
to access a channel through the RTS-CTS phase. In FIG. 5, each time
slot=9 microseconds, the SIFS (Short Inter-Frame Spaces) time=16
microseconds, a CCA (carrier sense) time<4 microseconds, a min
CW (Contention Window)=15 time slots, a max CW=1023 time slots, an
air propagation time<<1 microsecond (in FIG. 5, it is 0
microseconds), DIFS=SIFS+2 time slots=34 microseconds, RTS=52
microseconds @ 6 megabytes/second (RTS=24 microseconds @ 54
megabytes/second), and CTS=44 microseconds @ 6 megabytes/second
(CTS=24 microseconds @ 54 megabytes/second).
[0021] International Publication No. WO-9501020 A discloses that
each station in a wireless LAN (Local Area Network), using
time-distributed multiple access control, listens to traffic using
the network communications channel, for example, for
spread-spectrum, frequency-hopping transmissions. Each station
constructs its own network allocation vector from the received
transmission contents, indicating when the channel will be in use.
Message transmission uses four-way handshaking with two short
control packets, "Request to send" (RTS) and "Clear to send" (CTS).
The RTS packet includes the data transmission length, enabling the
various receiving stations in the network to reserve and block
their use of the communications channel over the period of time
concerned. The CTS packet repeats this data length, for the benefit
of receiving stations not within range of the source transmission.
This document corresponds to the IEEE 802.11 standard defined in
the IEEE 802.11-1999 standard.
[0022] Some ideas regarding transmission power control in DBTMA
(Dual Busy Tone Multiple Access), are described in S.-L. Wu, Y.-C.
Tseng, and J.-P. Sheu, "Intelligent Medium Access for Mobile Ad Hoc
Networks with Busy Tones and Power Control", Int'l Conf. on
Computer Communications and Networks, 1999, pp. 71-76. DBTMA is an
extension of BTMA with dual busy tones instead of a single busy
tone.
[0023] However, power control is not supported in known RTS-CTS
based channel access schemes.
[0024] With respect to DBTMA with TPC, BTMA (Busy Tone Multiple
Access) as such is generally not a viable solution for distributed
channel access as it is extremely unpractical. It is merely used as
a simple system to study in the academic literature. Also, control
messages use maximum Transmit Power (TP), and therefore it is not
possible for control messages to share a channel with data traffic
as that would cause harmful interference peaks for data reception.
Another drawback is that information regarding fixed TP is assumed
known at the receiver. In addition, DBTMA with TPC only attempts to
solve a problem in a specific situation, namely in a distributed
system where stations are neither associated with APs, nor
associated in a group with other stations. Another drawback is that
asymmetries in interference, link gain, or TP capabilities are not
been considered.
[0025] There are also additional problems common to each of general
RTS-CTS, IEEE 802.11 and DBTMA, namely a) link adaptation has not
been considered in the RTS-CTS framework, and b) asymmetries in
terms of link adaptation capabilities have not been considered.
SUMMARY OF THE INVENTION
[0026] Exemplary embodiments of the invention take a more
far-reaching transmit power-control (TPC) approach than outlined
IEEE 802.11 TGh, and have the objective of improving the overall
system performance in IEEE 802.11a as well as other RTS-CTS based
channel access scheme to the greatest feasible extent In doing so,
this will implicitly address QoS goals considered in IEEE 802.11
TGe.
[0027] A further goal of the invention is to address the issue of
link adaptation (LA) in conjunction with RTS-CTS frame
exchange.
[0028] A further goal of the invention is to address the issue of
link adaptation (LA) in a common framework with TPC.
[0029] In accordance with an exemplary embodiment of the invention,
a method for closed loop link adaptation based on a Request To
Send-Clear To Send (RTS-CTS) channel access scheme includes the
following steps. Designating a station as an originating station.
Transmitting a RTS frame with predetermined transmit power from an
originating station, prior to an intended DATA transmission,
sounding the channel such that reception characteristics can be
evaluated at a designated receiving station. Transmitting, in
response to the originating station, a CTS frame with a
predetermined transmit power from the receiving station with
directives of link adaptations. Transmitting a DATA frame from the
originating station to the receiving station frame complying with
link adjustment directives to the extent of the originating
stations capabilities. Transmitting an acknowledge (ACK) frame in
response to the originating stations from the receiving station
indicating result of DATA frame reception.
[0030] In accordance with another exemplary embodiment of the
invention, a method for open loop group transmit power control in
an infrastructureless system (i.e. an IBSS or Independent Basic
Service Set), includes the following steps. Transmitting a frame
conveying transmit power information for the corresponding frame to
any proximate station. Receiving, by one of the proximate stations,
the frame and determining the path gain based on measured signal
strength of the received frame and respective transmit power
information conveyed in the received frame. Selecting path gains
originating from the same group (i.e., IBSS). Determining a
required transmit power to reach nodes associated with any of the
selected path gains. Selecting the minimum of the highest transmit
power and allowed transmit power, wherein the allowed transmit
power is determined by regulatory requirements and stations
transmit power capabilities. Assigning the selected transmit power
to Request To Send (RTS), Clear To Send (CTS) messages and other
frames destined for nodes associated with any of the selected path
gains.
[0031] In accordance with another exemplary embodiment of the
invention, a method for open loop group transmit power control in
an infrastructure system (i.e., a BSS or Basic Service Set),
includes the following steps. Selecting, by an access point (AP),
at least one station within a group. Transmitting a transmit power
information request from an AP to the at least one selected
stations. Transmitting a transmit power response with transmit
power information for the corresponding frame to any proximate
station from the at least one selected stations in an orderly
manner preventing collisions. Receiving the frame with the transmit
power response and determining the path gain based on measured
signal strength of the received frame and respective transmit power
information in the received frame. Selecting path gains originating
from the same group (i.e., BSS). Determining required transmit
power to reach nodes associated with any of the selected path
gains. Selecting the minimum of the highest transmit power and
allowed transmit power, wherein the allowed transmit power is
determined by regulatory requirements and stations transmit power
capabilities. Assigning the selected transmit power to Request To
Send (RTS), Clear To Send (CTS) messages and other frames destined
for nodes associated with any of the selected path gains.
[0032] In accordance with another exemplary embodiment of the
invention, a method of tiered transmit power includes the steps of
determining a sequence of frames that must be exchanged for
successful communication, and assigning different transmit power
levels to those frames wherein the frames have different
topological objectives or distance objectives.
[0033] In accordance with another exemplary embodiment of the
invention, a method for interference mitigation based on open loop
transmit power control enabling tighter medium reuse, includes the
following steps. Conveying transmit power control information for
and in every transmitted frame by any station transmitting.
Receiving frames and determining a path gain based on measured
signal strength of the received frames and respective transmit
power information conveyed by the received frames. Determining the
maximum instantaneously allowed transmit power based on all
overheard frames such that ongoing communication is not noticeably
disturbed. Conditioning transmit power, and if feasible and
necessary, reducing transmit power and other transmit parameters
(e.g., link rate etc), to ensure that the maximum transmit power
condition is not exceeded during any transmission attempt.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Other objects and advantages of the present invention will
become apparent to those skilled in the art from the following
detailed description of preferred embodiments, when read in
conjunction with the accompanying drawings wherein like elements
have been designated with like reference numerals and wherein:
[0035] FIGS. 1A-1D show an RTS-CTS-DATA-ACK message exchange.
[0036] FIG. 2 shows exemplary MAC Frame formats of IEEE 802.11.
[0037] FIG. 3 shows NAV setting together with RTS-CTS.
[0038] FIG. 4 shows NAV setting when fragmentation is employed
together with RTS-CTS.
[0039] FIG. 5 shows two sources or originating stations/nodes
attempting to access the same channel in IEEE 802.11 a.
[0040] FIG. 6 shows an example of a Tiered TPC in an IBSS-like
system in accordance with exemplary embodiments of the
invention.
[0041] FIG. 7 shows an example of Joint TPC and LA on DATA with an
optional extension to ACK in accordance with exemplary embodiments
of the invention.
[0042] FIG. 8 shows TPC information derived from a BEACON in IBSS
in accordance with exemplary embodiments of the invention.
[0043] FIGS. 9A and 9B show IBSS path gain estimates from a BEACON
in accordance with exemplary embodiments of the invention.
[0044] FIG. 10 shows a Request for TP Information issued by an AP
and responded to by an addressed station in accordance with
exemplary embodiments of the invention.
[0045] FIG. 11 shows an example of BSS TP_Request, TP_Reply
exchange that establishes path-gain knowledge in accordance with
exemplary embodiments of the invention.
[0046] FIG. 12 shows exemplary TP_Request and TP_Reply IEs in
accordance with exemplary embodiments of the invention.
[0047] FIG. 13 shows concurrent and adjacent DATA transmissions
enabled by TPC in accordance with exemplary embodiments of the
invention.
[0048] FIG. 14 shows an interference profile at a receiving station
in accordance with exemplary embodiments of the invention.
[0049] FIG. 15 shows exemplary frame formats including Closed Loop
TPC and LA in accordance with exemplary embodiments of the
invention.
[0050] FIG. 16 shows exemplary frame formats including TP
Information fields in accordance with exemplary embodiments of the
invention.
[0051] FIG. 17 shows a frame format including a generic field for
TP and LA information in accordance with exemplary embodiments of
the invention.
[0052] FIG. 18 shows a table describing a TPC policy following a
tiered approach in accordance with exemplary embodiments of the
invention.
[0053] FIG. 19 shows a Transmit Power Information Request Element
format in accordance with exemplary embodiments of the
invention.
[0054] FIG. 20 shows a Transmit Power Information Element format in
accordance with exemplary embodiments of the invention.
[0055] FIG. 21 shows BEACON modifications in accordance with
exemplary embodiments of the invention.
[0056] FIG. 22 shows Probe Request modifications in accordance with
exemplary embodiments of the invention.
[0057] FIG. 23 shows Probe Response modifications in accordance
with exemplary embodiments of the invention.
[0058] FIG. 24 shows a P.sub.TX Request format in accordance with
exemplary embodiments of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] European Regulatory requirements for the "5 GHz band"
defined by ERC (European Radiocommunications Committee), limits the
mean EIRP (Effective Isotropically Radiated Power) to 200 mW and 1
W, in the 5150-5350 MHz (indoor) and the 5470-5725 MHz (indoor and
outdoor) band respectively. Further, DFS (Distributed Frequency
Selection) shall be applied over both bands in conjunction with TPC
(Transmit Power Control), the latter operating both in down and
uplink. IEEE 802.11 devices operating in the ERC area must
therefore comply with stated conditions. As the IEEE 802.11
standard currently does not incorporate the required TPC
mechanisms, it is an objective of exemplary embodiments of the
present invention to present methods with respect to TPC, such that
ERC directives can be fulfilled. In doing so, it is a further
objective of exemplary embodiments of the present invention to
provide TPC methods enabling link and system performance
enhancements.
[0060] Exemplary embodiments of the present invention can be
applied in both infrastructure-based 802.11 WLANs with an AP
(Access Point), or Infrastructure BSS (Basic Service Set), as well
as ad hoc-oriented 802.11 networks, or independent BSS (IBSS). DCF
(Distributed Coordination Function) has often been the preferred
mode of operation as well as the fundamental channel access mode of
802.11. Against that background, a TPC scheme taking the DCF mode
as a starting point is consistent with exemplary embodiments of the
invention. Exemplary embodiments of the present invention can be
extended or implemented to support the (E)PCF (Point Coordination
Function) or the HCF (Hybrid Coordination Function) mode.
[0061] Exemplary embodiments of the present invention enable not
just compliance with ERC requirements, but also enable significant
enhancement of system performance in terms of throughput, delay and
prolonged battery life. Exemplary embodiments of the present
invention also provide mechanisms and procedures to implicitly
enhance experienced QoS (Quality of Service) as well as reduce the
need for overlap BSS handling. In accordance with exemplary
embodiments of the invention, TPC for IEEE 802.11 is proposed with
some modification in the current 802.11 MAC specification that may
be incorporated as part of the changes within the 802.11e
framework.
[0062] In accordance with exemplary embodiments of the invention,
methods, protocols and frame structures are disclosed that enable
both joint and independent TPC and LA (Link Adaptation) in
conjunction with a RTS/CTS based channel access scheme. In
accordance with exemplary embodiments of the invention, mechanisms
are provided to differentiate TPC depending on topological goals.
In an exemplary embodiment of the invention, Group based TPC
mechanisms for frames like RTS and CTS are provided. Note that the
term "group" is synonymous with the collection of all stations in a
BSS or in an IBSS, but can also be interpreted in other groupings
not specified by or explicitly defined in IEEE 802.11. Exemplary
embodiments of the present invention also provide a TPC mechanism
for interference mitigation, such that stations belonging to other
Groups (BSSs or IBSSs, where "BSS" stands for Basic Service Set and
"IBSS" stands for Independent Basic Service Set) are not interfered
with. At the same time this enables reuse of the channel provided a
suitable TP (Transmit Power) level is selected. Interference
mitigation mechanisms may alternatively be employed within a group,
e.g., an infinite large and dispersed group. In accordance with
exemplary embodiments of the present invention, as a basis for
those mechanisms, both closed loop as well as open loop TPC are
applied.
[0063] In reducing generated interference and minimizing power
consumption, it is vital to apply the most aggressive and precise
TPC scheme to the bulk traffic of the network, most likely
consisting of DATA (and ACK) frames. Next to DATA frames, the RTS
and CTS frames may, depending on the adot11Threshold value, be
relatively prevalent and hence considered as important contributors
to undesired interference and power consumption. As RTS and CTS
frames in general are shorter than DATA frames, their supplement to
the overall average interference picture will accordingly also be
lower. Frames occurring merely occasionally, such as Beacons, have
even lesser impact on the average interference situation. In
addition, of diminishing the radiated average interference level,
the issue of minimizing peak interference and associated variations
is also of interest. Different traffic conditions may alter
assumptions above, but the given statements are believed to be true
in most if not all relevant scenarios. Those issues together with
the objectives set forth earlier motivate the following two items.
First, a so-called TPC policy to be defined, giving very rough
guidelines on TP algorithmic goals. Second, a TPC mechanism to be
defined, designed to support the TPC policies.
[0064] One aspect of the invention is to enable a tiered TPC policy
in a CTS/RTS based channel access system. The motivation for this
is that different topological and communication range aspects need
to be met depending on frame type. Note that those frames, as the
one in IEEE 802.11, have a timely and logical relation to each
other. In accordance with exemplary embodiments of the invention,
the TPC policy follows a tiered approach defining three levels.
Frames with different topological destination objectives are
divided among those three Tier-classes. FIG. 18 shows the three
major TPC tiers.
[0065] As Tier 1 frames are sent with high transmit power, this
class also adopts a policy of being constrained in time. The reason
is to minimize random interference peaks within and towards
neighboring (I)BSS. This may be achieved by confining Tier 1
traffic around Beacon transmit occasions, i.e. sent regularly
around TBTT (Target Beacon Transmission Time).
[0066] In Tier 1, Beacon TPC, the BEACON frame as defined in the
IEEE 802.11-1999 standard, and other conceivable frames/messages
with similar topological destination purposes, must generally reach
as far as possible. However, those must also conform to regulatory
requirements in terms of used TP. As in the IEEE 802.11-1999
standard, such messages are often scheduled at regular time
intervals and having priorities above other traffic and may
therefore transmit at fixed but the highest permitted TP.
[0067] In Tier 2, relating to RTS, CTS and TPC, two major
embodiments or cases exist. In the first embodiment, Fixed TPC,
RTS/CTS are sent with the highest possible TP, but are limited by
regulatory TP requirements. Hence the TP setting is identical to
the Beacon information. The purpose of this embodiment is to be
able to inform distant stations of ongoing DATA transmissions such
that they can select DATA transmit parameters that will mitigate
generation of disturbing interference. Although not supported by
802.11 per se, it should be noted that RTS/CTS frames could, in
principle, be sent in a special control channel separated from data
transmissions, hence avoiding interfering with said data
transmissions.
[0068] In the second embodiment, Group TPC, the RTS and CTS are
sent with a sufficiently high TP so as to reach members or stations
within the same group/BSS, but preferably with a sufficiently low
TP so as to a) not reach members within another group, and b) stay
within regulatory TP requirements and limits. In this second
embodiment one purpose is to reduce the interference impact on
other stations' DATA transmissions due to RTS/CTS message exchange,
when RTS, CTS messages are sent in the same channel and potentially
concurrently with DATA transmissions.
[0069] It is worthwhile to further clarify reasons for performing
or enabling group oriented TPC, because the conventional, known
RTS-CTS scheme does not inherently solve this. FIG. 5 shows a case
where two IEEE 802.11 a stations belonging to the same group
attempt to get control over the medium by sending RTSs and CTSs,
with link rates of 6 Mbps. Due to unfortunate time relations
between the duration of the RTS start to the CTS end, in relation
to 802.11a's timeslot (TS) structure, the virtual carrier sense for
source 1 (assumed hidden to source 2) will only work roughly 12 TSs
after source 2 accessed the medium first. However, the physical
carrier sense of source 1 would need roughly 8 TSs to detect the
CTS from destination 2. As a result, while virtual carrier sense
mitigates hidden terminals for the duration of data reception when
the channel has been reserved, the traditional carrier sense
function is still required in the RTS-CTS phase for IEEE 802.11(a).
A further consequence is that it is vital that stations belonging
to the same group, and hence sharing the channel, employ transmit
powers such that all stations within the group can be reached. For
very large contention windows and higher link rates (e.g. 54 Mbps),
the impact of this curious blocking effect may be lowered. However,
in IEEE 802.11, the reason to use the larger contention windows
occurs only when traffic intensity is high, but then it remains
quite likely that two STAs will still access the channel within
e.g., 8 TSs.
[0070] In accordance with an exemplary embodiment of the invention,
each RTS-CTS phase is housed within a single time slot, so that the
TPC can also rely on the relaying function through the CTS
frame.
[0071] In a variant of the Tier 2 second embodiment, group based
TPC is performed for the CTS message, while applying a lowest
possible TP for the RTS frame. This primarily targets the case
where the RTS-CTS phase can be housed within a single TS, i.e. not
a IEEE 802.11a system. The motivation for this is that DATA
reception is more vulnerable than the reception of ACKs, due to the
potentially longer interference exposure time for the DATA
frame.
[0072] In Tier 3, with respect to DATA, ACK TPC+LA, for DATA
transmission an Instantaneous Closed Loop TPC and Instantaneous
Closed Loop LA is deployed via feedback information conveyed in the
CTS messages. In addition to this, DATA TPC also conforms to
regulatory aspects as well as the RTS/CTS TPC setting. TPC and LA
(Link Adaptation) for ACK generally follows the parameters for
DATA. A special case is fragmented transmission when DATA and ACK
headers acts as implicit RTS and CTS messages. Then the DATA, ACK
and TPC may optionally become equivalent to RTS, CTS and TPC.
[0073] The tiered TPC policy can be summarized as:
P.sub.TX(DATA, ACK).ltoreq.P.sub.TX(RTS,
CTS).ltoreq.P.sub.TX(Beacon).ltor- eq.P.sub.TX(Regulatory
requirements) (1)
[0074] An illustrative but simplified example of an IBSS-like
system is shown in FIG. 6, which illustrates how different TP
settings for different frames result in different transmit ranges.
The second embodiment of Tier 2 (i.e., Case 2 of RTS-CTS TPC) is
utilized. Note, in an IBSS, all stations send beacons occasionally
and regularly, but merely two beacon ranges are shown here for
simplicity. As specifically shown in FIG. 6, the ring 102 is the
DATA transmission range for the station T, and the ring 104 is the
DATA transmission range for the station R. A ring 212 is the RTS
transmission range for the station T, and the ring 214 is the RTS
transmission range for the station R. As can be seen, the rings
212, 214 are larger than the rings 102, 104. Larger yet are the
rings 216, 218, which are the BEACON transmission ranges of the
stations F, H respectively.
[0075] With respect to Tier 3, each node or station receiving an
RTS message assesses preferably the instantaneous carrier to
interference ratio, CIR, and other channel parameters of choice.
Subsequently, a desired reduction or increase of transmit power
used for the RTS frame is determined. A relative transmit power
adjustment request P.sub.TX.sub..sub.--.sub.Request is then
conveyed in the CTS message back to the originating station. The
originating station adjusts the transmit power level accordingly
for the subsequent DATA frame transmission. The same procedure is
repeated for the ACK, i.e., the originating station conveys a
corresponding transmit power adjustment request targeted for the
ACK. Note that both RTS and CTS are sent with TPC Tier 2 related
mechanisms described further below. The details of selecting the
transmit power are implementation specific, but the mechanism as
such inherently enables a very precise adjustment with respect to
instantaneous experienced CIR at the receiver. An example of the
flexibility given is that the implementation specific algorithm can
always respond (for example, to an RTS) with a
P.sub.TX.sub..sub.--.sub.Request, thereby forcing the sender of
DATA to use full permitted domain transmit power. While this is
indeed possible, it generally results in a poor-performing system,
as spatial reuse is reduced and power consumption increased. Note
that in sending short Data or Management frames that do not exploit
the RTS-CTS scheme, information derived from the open loop group
TPC can be used. This is of particular interest when data or
management frames are sent through multicast or broadcast to a
group of receivers.
[0076] For Tier 3 closed loop DATA TPC, DATA LA and joint TPC and
LA for DATA, consider the following example. Assuming that a
station T has a data packet to send to a station R and that the
employed channel access scheme with back off and similar features
has already acted, the station T sends a RTS message with a TP of
P.sub.TX(RTS) to station R. This transmission can be conditioned so
that it does not harm any ongoing communication.
[0077] Station R receives the RTS frame and determines the received
power P.sub.TX(RTS) and other optional link characteristics that
can provide additional guidance to the LA and the TPC algorithms.
Optional link characteristics may include channel state information
or simply delay spread information.
[0078] The station R also experiences and measures interference
I.sub.RX at the same time from some other station(s) and other
interference characteristics that can provide additional guidance
to the LA and the TPC algorithms.
[0079] In a successive step, P.sub.RX(RTS) with other optional link
characteristics, interference I.sub.RX with other interference
characteristics and knowledge about noise is used by the station R
to determine settings for the DATA transmission from station T to
station R.
[0080] In one preferred step, link, interference and noise
knowledge are used by the station R to determine changes in TP
relative the TP employed for the RTS message, for example closed
loop with signaling of relative change.
[0081] In an alternative but also preferred step, link,
interference and noise knowledge are used by the station R to
determine settings of the LA based on the TP employed for the RTS
message.
[0082] In a combined step, both LA and TP settings are determined
by the station R. In a successive step, determined link control
information is transferred in the CTS frame to the station T. The
CTS transmission can be conditioned so that it does not harm any
ongoing communication. In a successive step, the station T uses the
settings indicated in the link control information received in the
CTS message when sending DATA. This transmission can be conditioned
so that it does not harm any ongoing communication.
[0083] In an additional step of the invention, the procedure is
repeated for successive and consecutive frame exchanges.
[0084] Accordingly, the DATA frame carries link control information
for the ACK frame that adjusts its TP relative the CTS message.
Although not originally permitted in IEEE 802.11, the ACK rate can
be adapted as determined by the originating station T in future
systems.
[0085] In addition, when IEEE 802.11 employs fragmentation, DATA
and ACK act as RTS and CTS frames and will then convey link control
information where the power control also relates to the last sent
frame. Optionally, the RTS and CTS messages are sent with duration
field indications covering the expected duration. This transmission
can be conditioned so that it does not harm any ongoing
communication.
[0086] FIG. 7 shows the principle of closed loop joint TPC and LA
based on RTS-CTS frame exchange. Option 1 allows an adaptation of
TP and LA for the ACK. Note that according to the IEEE 802.11-1999
standard, DATA and ACK should use the same LA scheme. However,
future extensions or the development of similar systems are not
excluded. As shown in FIG. 7, in a first step 702 the originating
station T sets RTS power to P.sub.TX(RTS), and then in step 704
sends the RTS to the station R. In step 704, the station R measures
P.sub.RX(RTS) and I.sub.RX, and determines P.sub.TX(DATA) and
LA(DATA) and then conveys the determined information in the CTS in
step 708 to the station T. In step 710 the station T makes
adjustments according to the determined P.sub.TX(DATA) and LA(DATA)
received in the CTS. Optionally, the station T also measures
P.sub.RX(CTS) and I.sub.RX, determines P.sub.TX(ACK) and LA(ACK),
and then adds this determined information into the DATA. Then in
step 712, the station T transmits the DATA to the station R. In
step 714, the station R makes appropriate adjustments if the
received data included P.sub.TX(ACK) and LA(ACK), and then in step
716 the station R sends an ACK to the station T.
[0087] With respect to conditions for not harming other
communication, at least two exemplary options exist. A first option
follows the legacy channel access principle for IEEE 802.11, i.e.,
when the physical or the virtual carrier sense indicates that the
channel is occupied, access is deferred. A drawback of this state
of the art technique is that it does not attempt to reuse the
channel even when it would be possible as TPC is employed.
[0088] A second alternative option in accordance with exemplary
embodiments of the invention, exploits overheard information of TP
indications and maximum allowed receive power. Such information may
be included in, and derived from, the header information of
primarily RTS and CTS frames, but also DATA, ACK and possibly other
frames. This scheme is described in greater detail further below,
and is also referred to as Interference Mitigation.
[0089] For Tier 1, BEACON TPC, the allowed transmit power level
allows domain specific settings. Each station within an (I)BSS uses
the allowed domain transmit power when sending a frame containing
any of the appropriate IEs defined in FIGS. 19-23. If the transmit
power capability is lower than the domain transmit power level, the
former will be used. For IEEE 802.11 the setting of Beacon TP is
determined by IEEE 802.11 management information base, MIB of the
node initiating a (I)BSS. For the IBSS, the TPC setting for the
BEACON is distributed as an Information Element, IE, conveyed in
the BEACON itself. The frame format and procedure for this are
described further below.
[0090] With respect to Tier 2 (e.g., RTS, CTS TPC), the aim is to
determine a transmit power setting such that all stations or nodes
within an (I)BSS will have a sufficient CIR to be able to receive
frames. This mechanism is also useful for TPC setting for broadcast
and multicast traffic within the (I)BSS, but it is primarily aimed
towards RTS and CTS frames.
[0091] While the RTS-CTS frame exchange efficiently prevents hidden
stations from accessing the channel, further enhanced by the
virtual carrier sense, the RTS-CTS frames themselves need to be
protected with classical physical carrier sense. As a result, it is
vital to ensure that all stations within the same (I)BSS transmit
with sufficient power so as to reach each other. However, from the
viewpoint of interference and power consumption, it is preferred to
send with the least possible transmit power. The Group oriented TPC
proposed here intend to strike a balance between those two somewhat
conflicting goals.
[0092] As indicated earlier, a number of cases need to be
differentiated. A first case is Fixed TPC. In a system where the
control frames RTS and CTS do not share the channel with DATA
transmissions, the need for precise and tight TPC of RTS, CTS
messages is lower than if the channel is shared. The channel can be
considered not to be shared when RTS, CTS are separated e.g., in
time (as for example in a TDD/TDMA structure, where "TDD" stands
for Time Division Duplex and "TDMA" stands for Time Division
Multiple Access), in code (as for example in DS-CDMA, which stands
for Direct Sequence Code Division Multiple Access), or in frequency
(as for example in FDD, where "FDD" stands for Frequency Division
Duplex). A drawback with the frequency division is that the channel
cannot be considered to be reciprocal, and hence the channel gain
may differ for RTS, CTS and DATA channels. An additional method to
mitigate interference influence from RTS, CTS messages when sharing
the channel with DATA is to use a strong burst error correcting
code for DATA. As an example, an Reed-Solomon (RS) code of length N
RS-symbols with N-K RS-redundant symbols, may correct up to
floor((N-K)/2) unknown RS-symbols or floor((N-K)) known erroneous
RS-symbols. Two exemplary options for the ACK exist. Either it
shares the channel with DATA using the same TPC scheme, or it
shares the channel with RTS, CTS using the same TPC scheme.
[0093] Consequently, the TPC regulates the TP preferably to the
permitted level according to regulatory requirements and attainable
by the equipment itself. Note that BSS- and IBSS-like systems do
not need to be differentiated from TPC point of view here.
[0094] A second case is Group TPC. In order to handle both BSS- and
IBSS-like systems two methods are deployed.
[0095] The procedure for IBSS Group TPC is based on conveying
transmit power level information, P.sub.TX, as an information
element, IE, in the regular IBSS Beacon. Hence, P.sub.TX merely
represents the transmit power employed for the frame in which the
IE itself is transferred within. The intent of using the Beacon is
because it complies well with both power sleep mode operation as
well as the Tier 1 objectives. In addition to the transmit power
level information, a minimum required receive power level,
P.sub.RX.sub..sub.--.sub.min is sent in the same IE.
[0096] Each station receiving a Beacon with the IE determines path
gain and subsequently required transmit power. Each station also
assess that the Beacon originates from a station within the same
IBSS. Over the time, as the IBSS Beacon transmit time is somewhat
randomized, Beacons from all stations within the same IBSS and
within range will be received. Based on the collated information,
the maximum required transmit power is selected among the stations.
Old transmit power updates lose validity over time as new updates
are not overheard.
[0097] In an IEEE 802.11 IBSS, each STA (station) will attempt to
transmit a BEACON frame at the TBTT plus a random small delay. A
STA overhearing another BEACON refrain from transmitting. As the
BEACON is transmitted with relatively high power, all STAs within
the IBSS have sufficient SNR (Signal to Noise Ratio) to correctly
decode the message except collisions occur where decoding may
fail.
[0098] An additional IE (apart from already existing in the IEEE
802.11-1999 standard) indicates the TP level, P.sub.TX(BEACON),
that is used when sending the BEACON. The IE is incorporated into
the BEACON frame itself, as shown for example in FIG. 21. As
P.sub.TX(BEACON) and the received signal strength P.sub.RX(BEACON)
derived from the BEACON frame is known, the path gain can be
calculated. This is repeated for all received BEACONS. The minimum
path gain from any STA belonging to the same IBSS is subsequently
extracted and used to calculate the TP for the RTS and CTS message.
Alternatively the TPC Group procedure described below is used when
STAs require different minimum receive power.
[0099] An advantage of exploiting the BEACON is that Power Save
enabled STAs wake up and listen for the BEACON.
[0100] FIG. 8 shows a station sending one BEACON that is received
by a number of other STAs within the same IBSS. As shown in FIG. 8,
a BEACON sending STA (station) first sets P.sub.TX(BEACON) to a
maximum allowed level, and indicates P.sub.TX(BEACON) in the
BEACON. Optionally, the BEACON sending STA (station) also
determines P.sub.RX.sub..sub.--.sub.min- , and also indicates
P.sub.RX.sub..sub.--.sub.min in the BEACON. Next, the station sends
the BEACON to other STAs (stations), and each of the other stations
measures P.sub.RX(BEACON) and then determines path gain and
required transmit power.
[0101] When P.sub.RX.sub..sub.--.sub.min is indicated in the
BEACON, a more precise determination of required transmit power can
be accomplished, as an estimated value of
P.sub.RX.sub..sub.--.sub.min is required if not indicated in the
BEACON.
[0102] An alternative view of the effect of receiving the BEACON
with TPC information conveyed therein is shown in FIGS. 9A-B. The
ring 902 indicates a BEACON transmit range of the station or node
C, and G.sub.CA, G.sub.CB, G.sub.CE, G.sub.CF, and G.sub.CG
respectively represent the path gain from the node C to each of the
nodes or stations A, B, E, F and G. First the station C sends a
BEACON as shown in FIG. 9A, and subsequently other station will
send BEACON At a later instance, each STA will have knowledge about
average path gain to each STA, from which they received BEACON(s),
within the same IBSS, and possibly also other IBSSs. The
illustration in FIG. 9B indicates the path gain knowledge acquired
by station B, with path gains G.sub.AB, G.sub.CB, G.sub.FB,
G.sub.GB between the station B and the stations A, C, F and G
respectively. As station moves, the weight of old path gain
information is assumed to decrease.
[0103] With respect to BSS-like systems in Group TPC, the procedure
for BSS group TPC is somewhat similar to the procedure for IBSS,
but the channel probing sequence is directed by the AP. A transmit
power information request directed towards a selected STA is issued
by the AP. This request is sent via an IE, e.g., carried in a Probe
request or other suitable frame, e.g., just immediately after the
Beacon. Subsequently, a Probe response or other suitable frame is
sent back from the addressed STA with another IE indicating the
used transmit power information P.sub.TX and preferably also a
minimum required receive power level, P.sub.RX.sub..sub.--.sub.min.
The Probe request and Probe response (or alternative suitable
frames) employs the Tier 1 TPC setting rule. Each STA receiving the
Probe response (or alternative suitable frames) with the IE,
determines path gain and subsequently required transmit power. Each
STA also assess that the frame originates from a STA within the
same BSS. Over the time, frames with the desired IE from all STAs
within the same BSS and within range are received. For each
individual STA, the maximum required transmits power is then
selected among the STAs with respect taken to changing channel gain
over time.
[0104] The polling sequence of STAs belonging to a BSS is an
implementation specific issue and not defined in the standard. Note
that the scheme allows those implementing the scheme to adjust
P.sub.RX.sub..sub.--.sub.min and manage the algorithmic dynamics in
any desirable manner. Note also, that by regulating
P.sub.RX.sub..sub.--.sub.- min, stations will try to adaptively
compensate desired receive power in the presence of an adjacent
interfering BSSs. Hence if maximum domain transmit power is the
optimum, the system will tune transmit power parameters
accordingly. In contrast, other situations will conserve the
resources instead. Moreover, as Tier 1 information shall not
interfere with Tier 2 traffic, due to timely division, interference
measurement guiding the setting of P.sub.RX.sub..sub.--.sub.min
should preferably exclude Tier 1 related interference.
[0105] Specifically, in an IEEE 802.11 BSS the AP sends the BEACON,
whereas non-AP STAs does not send any BEACON, and as a result, the
IBSS solution does not work. However, just prior, during, or after
the BEACON, the AP performs a TP_Request of non-AP STAs. It
requests one or more STAs that belongs to the BSS to send a
TP_Response with corresponding TP setting conveyed therein. The
TP_Response is preferably sent with the same TP setting as the
BEACON is using. It should be noted that STAs within a BSS may use
any transmit power indication in the BEACON to determine required
transmit power towards the AP.
[0106] Various options on how this can be implemented is
envisioned, but not limited to the exemplary embodiments given
here. A specific TP_Request message is defined as one IE. Another
IE is used for TP_Response, indicating the used TP level for the
same message it is conveyed in. The TP_Request IE can e.g., be
included in a BEACON, PROBE_REQUEST, or a so-called
GENERIC_MANAGEMENT_FRAME that is currently under development in the
standardization of IEEE 802.11 enhancements. The TP_Response IE can
e.g., be included in a PROBE_REQUEST, PROBE_RESPONSE or a so-called
GENERIC_MANAGEMENT_FRAME.
[0107] The TP polling scheme of the AP can for example be
accomplished in a round robin fashion or targeted in particular
towards STAs expected to be at coverage boarder.
[0108] Note that if the group of STAs defined by the BSS or IBSS
extends over a large range there is a possibility that TP will be
set to the same level as the BEACON TP level for the RTS-CTS
frames.
[0109] In an optional embodiment, the IE includes not just the used
TP P.sub.TX(FRAME), but also a measure of the minimum required
receive power P.sub.RX.sub..sub.--.sub.min. A known lowest link
rate is assumed when defining P.sub.RX.sub..sub.--.sub.min.
[0110] FIG. 10 shows an exemplary case where an AP issues an
TP_Request IE in a BEACON. As shown in FIG. 10, in step 1002 a
BEACON sending AP (Access Point) selects one or optionally more
stations (STAs), and indicates a TP_Request IE in the BEACON. Then
in step 1004, the AP sends the BEACON to the selected, addressed
station(s). In step 1006 the addressed station responds to the
request by a) setting P.sub.TX(FRAME) to a maximum allowed level,
b) indicating the P.sub.TX(FRAME) in an IE TP_Response, c)
optionally determining P.sub.RX.sub..sub.--.sub.min and indicating
the determined P.sub.RX.sub..sub.--.sub.min in the IE TP_Response.
In the next step 1008, the addressed station issues the TP_Response
IE in any suitable frame type. If multiple stations were addressed,
they respond in an orderly fashion according to address sequence.
Each frame is divided a SIFS (Short Inter-Frame Space) apart. In
steps 1010, 1012, the other stations receiving the frame(s)
containing the TP_Response IE(s) measure the P.sub.RX(FRAME), and
determine path gain and required transmit power (and optionally
include explicit P.sub.RX.sub..sub.--.sub.min information in the
transmit power determination if P.sub.RX.sub..sub.--.sub.min is
received in the TP_Response IE).
[0111] An alternative view of the TP_Request and TP_Response
exchange is shown in FIGS. 1A-B with the calculated path gain
indicated. In FIG. 11A, station C is an AP and has a transmit range
indicated by the ring 1102, and path gains G.sub.CA, G.sub.CB,
G.sub.CE, G.sub.CF and G.sub.CG from the station C to each of the
stations A, B, E, F and G respectively. FIG. 11A also shows a TP
request sent from the station C (AP) to the station G.
[0112] FIG. 11B shows a similar situation, but from the vantage
point of the station G. The ring 1104 indicates the transmit range
of the station G, and the path gains G.sub.GA, G.sub.GB, G.sub.GC,
G.sub.GE and G.sub.GF from the station G to each of the stations A,
B, C (AP), E, and F respectively are shown. Also shown is a TP
Response from the station G to the station C (AP).
[0113] Some general aspects pertaining to Case 2 (Group TPC) of
Tier 2 are now discussed. In particular, FIG. 12 shows the content
of the TP_Request and TP_Reply IE, and their place in a management
frame body of arbitrary type. The Management Frame Body 1204
includes multiple Fix fields, and also multiple IEs. Each IE has a
format 1206, including an Element ID of 1 octet or byte, a Length
field of one octet, and an Information field having a length
indicated in the Length field. Note, TP_Request is only used in BSS
operation. The table 1202 shown in FIG. 12 describes an exemplary
TP_Request format (corresponding to the Element ID x in the table),
and describes an exemplary TP_Reply IE format (corresponding to the
Element ID y in the table). FIG. 20 also shows an exemplary
Transmit Power IE (Information Element) format, and FIG. 19 shows
an exemplary Transmit Power Information Request Element format.
[0114] FIG. 21 shows how a Management frame subtype BEACON can be
modified to include three new IEs, in accordance with exemplary
embodiments of the invention. In particular, the 11.sup.th IE in
the frame can include Domain Information, the 12.sup.th IE in the
frame can be a Transmit Power Information Request Element, and the
13.sup.th IE in the frame can be a Transmit Power Information
Element. It should be noted that Transmit Power Information Request
Element could also be included in other frames, such as management
frames. Also note that Transmit Power Information Element could
optionally also be included in the BEACON for a BSS.
[0115] FIG. 22 shows how a probe request can be modified in
accordance with exemplary embodiments of the invention, to include
a Transmit Power Information Request Element. FIG. 23 shows how a
probe response can be modified to include a Transmit Power
Information Element.
[0116] In an exemplary embodiment of the invention, the policy of
group TPC may be employed only towards the CTS frame, whereas the
RTS frame employs a TP setting with respect to the intended
receiver. The schemes depicted earlier for RTS, CTS TPC to acquire
group TP level knowledge are hence used merely for the CTS frame.
The RTS TP level is determined with an independent algorithm but
limited upwards by the TP setting for the CTS frame. Any overheard
messages carrying TP information (as described further above, for
example) and sent by the intended receiver, may be used as inputs
to determine the TP level for the RTS frame.
[0117] An exemplary embodiment of the TPC group algorithm includes
the following steps: Monitor the channel for messages carrying an
IE indicating the TP of the corresponding frame. Next, determine if
the IE was sent by a STA k belonging to the same (I)BSS (group) and
if so, determine the required TP. If the IE includes interference
information, this is also considered when determining the TP
P.sub.TX(RTS).sub.k. The TP is preferably determined for the lowest
data rate, requiring the least TP and hence minimize generated peak
interference. Next, set P.sub.TX(RTS)=max(P.sub.TX(RTS), . . .
P.sub.TX(RTS).sub.k, . . . P.sub.TX(RTS).sub.K), where k indexes
stations (STAs) within the same (I)BSS (group). The same TP is used
for a CTS message, for example P.sub.TX(CTS)=P.sub.TX(RTS).
[0118] In accordance with exemplary embodiments of the invention, a
procedure is provided to increase the spatial reuse through Open
Loop Interference Mitigation Control as outlined below. With this
procedure, a station or node can determine maximum permitted TP and
can transmit frames without disturbing (to any noticeable degree)
ongoing communication which would not be permitted under the
current channel access rules in the IEEE 802.11-1999 standard.
[0119] Specifically, TP indications and maximum allowed receive
power P.sub.RX.sub..sub.--.sub.MAX are included in, and derived
from, the header information of primarily RTS and CTS frames, but
also DATA and ACK frames. The maximum allowed receive power
P.sub.RX.sub..sub.--.sub.MAX is related to the experienced
interference and noise level. It is most important to include and
detect the information in the CTS frame header as DATA reception is
in general more vulnerable for interference compared to e.g., ACK,
due to in general longer frames and possibly also higher required
CIR resulting from higher link rates. It should be noted that
P.sub.RX.sub..sub.--.sub.max can optionally be determined from the
P.sub.RX.sub..sub.--.sub.min by asserting a level for
P.sub.RX.sub..sub.--.sub.max that is sufficiently smaller than
P.sub.RX.sub..sub.--.sub.min.
[0120] FIG. 13 shows two station pairs, (T1, R1) and (T2, R2)
communicating with each other. Path gains G.sub.11, G.sub.12,
G.sub.21, G.sub.22 indicating the path gain between T1 and R1, T1
and R2, T2 and R1, and T2 and R2, are shown. The ring 1302
indicates the transmission range of the station or node T1, and the
ring 1304 indicates the transmission range of the station or node
T2. Under the traditional IEEE 802.11 rule the station T2 and the
station R2 would not normally be able to transmit as station T1 and
station R1 are already using the medium. However if the TP for the
station pairs can fulfil the conditions 1 C 1 I 1 = P 1 G 11 P 2 G
21 min and ( 2 ) C 2 I 2 = P 2 G 22 P 1 G 12 min ( 3 )
[0121] where C/I is the interference ratio, P is the transmit
power, G is the channel gain and .gamma..sub.min is the minimum
required C/I ratio for likely reception, then it can be possible to
house or permit multiple and "overlapping" transmissions.
[0122] Assuming in FIG. 13 that the station T2 has acquired path
gain knowledge and maximum allowed receive power from the station
R2 (through overhearing earlier CTS with duration indication, for
example) it may send a frame (e.g. RTS or DATA) provided that
following condition is fulfilled, 2 P 2 P 1 G 11 min G 21 = P RX
max min G 21 ( 4 )
[0123] The frame is however only likely to be received at station
R2 provided 3 P 2 min P 1 G 12 G 22 ( 5 )
[0124] It is important to note that both station T2 and station R2
must ensure that neither of them is interfering with any of station
T1 or station R1. A failure of response from station R2 may be due
to interference from either station T1 or station R1. In such case
the transmission is deferred until the channel becomes free
according to traditional rules defined in the IEEE 802.11-1999
standard.
[0125] As IEEE 802.11 utilizes a shared channel between RTS, CTS
and DATA, this means that RTS, CTS messages preferably are TP
controlled. As a consequence of this, there is no guarantee that
P.sub.TX(RTS), P.sub.TX(CTS), P.sub.RX.sub..sub.--.sub.max and
Duration can be detected.
[0126] In a non-IEEE 802.11 system with a channel where RTS, CTS
messages does not directly impact DATA reception success, then
P.sub.TX(RTS), P.sub.TX(CTS), P.sub.RX.sub..sub.--.sub.max and
Duration information can be distributed more widely thanks to that
RTS, CTS messages can use a less aggressive TPC.
[0127] In case it is determined that e.g., an RTS frame requiring
P.sub.TX(RTS) can be sent without disturbing ongoing communication
with good margin, employed LA(RTS) may be increased in the extent
that P.sub.TX(RTS) can be allowed to increase.
[0128] A procedure is now provided for determining I.sub.RX. This
procedure can be used, for example, in the Tier 3 Closed Loop DATA
TPC, DATA LA and Joint TPC and LA for DATA scheme described further
above. The determined value of I.sub.RX can also be used when
determined P.sub.RX.sub..sub.--.sub.min that can be subsequently
transmitted in at least one of the following: a) tier 1 frames; b)
tier 2 frames; or c) tier 3 frames. In receiving an RTS message,
the receiving station R determines preferably the instantaneous
carrier to Interference ratio CIR. As one preferred option, the
interference is not just determined based on measured RSSI
(Received Signal Strength Indicator), but is also determined based
on DURATION information that has been derived from overheard
traffic between other STAs. In this manner, the expected I.sub.RX
at the start of the DATA reception can be determined. FIG. 14 shows
an interference profile at a receiving station R, and timing of the
RTS and DATA signals from a Source, relative to the CTS signal from
the destination ( the receiving station R). As shown in FIG. 14, a
period DIFS occurs before the RTS signal is sent, and a SIFS
separates the RTS from the CTS in time, and a SIFS also separates
the CTS from the DATA in time. The interference profile at the
destination indicates that interference is measured on overheard
frames with duration information. In this example, the interference
increases before the DIFS period begins, and then decreases to a
lower level after the CTS, before the DATA transmission. Note that
I.sub.RX may additionally also be filtered to better reflect a long
time average interference level.
[0129] In accordance with exemplary embodiments of the invention, a
procedure is also provided to determine minimum required receive
power. The minimum required receive power,
P.sub.RX.sub..sub.--.sub.min, is used for group TPC of RTS and CTS
frames such that they can reach every intended station, even
stations that experience different interference or have different
noise floors. This information is normally distributed in IE
elements, for example those defined in FIGS. 12 and 19-23. However,
when frames (e.g. CTS) also include P.sub.RX.sub..sub.--max as
discussed in further above, that information will provide
additional input to determine P.sub.RX.sub..sub.--.sub.min through
the relation 4 P RX min = P RX max Constant ( 6 )
[0130] where the constant typically is the required carrier to
interference ratio .gamma..sub.min. The reciprocal procedure may
also be used, i.e., given P.sub.RX.sub..sub.--.sub.min in a frame,
P.sub.RX.sub..sub.--.sub.max can be determined.
[0131] With respect to multihop networking, a multihop network
employing a RTS, CTS based channel access scheme can utilize the
proposed methods and achieve additional benefits over the already
discussed. In some envisioned multihop networks, the path gain is
used as the cost in calculating the shortest path to the
destination. With this metric, the shortest path is the path with
the minimum required TP as well as generating minimum interference.
In determining the shortest path cost, the path gain to neighboring
STAs need to be acquired. If all frames, including RTS, CTS but
also e.g., BEACONS the latter being sent at high TP, carry such TP
information, then the load and intensity of messages probing path
gains to neighbors can potentially be reduced. Another issue is
that more precise link gain knowledge is useful is in so called
topology control. Topology control is a well-known technique for
maintaining sufficient and sensible connectivity in a multihop
network when TPC is employed.
[0132] With respect to Asymmetrical link capabilities, due to the
closed loop approach for DATA and ACK TPC, exemplary embodiments of
the invention support the case having asymmetrical link(s). This
may be due to a number of reasons, including for example the
following reasons. Communication in each direction takes place over
a non-reciprocal but short term stationary channel, e.g. FDD
(Frequency Division Duplex). Stations have different TP and LA
capabilities. The interference situation is different at the two
communicating STAs. The symmetrical cases are automatically
handled, as they are degenerated cases of the more asymmetric
cases.
[0133] Asymmetries in noise and interference are also supported for
the RTS, CTS TPC since P.sub.RX.sub..sub.--.sub.min can be
included.
[0134] With respect to Frame Structure, a number of different
embodiments are available for the frames depending to which extent
mechanisms as defined in this disclosure are exploited. Sizes of
proposed frame elements are only exemplary and may differ in
reality. Exemplary frame structures as defined in the IEEE
802.11-1999 standard are used, but other frame formats with similar
function are conceivable. For example, TPC and LA information may
not just be signaled in OSI Layer 2 (MAC) frames, but also in e.g.,
OSI Layer 1(PHY) or OSI Layer 3 (Network) frames.
[0135] In a first scenario, the frame format is as depicted in FIG.
15. This scenario addresses the closed loop TPC and LA for, DATA,
an optional successive ACK and optional support of multiple
fragments of DATA. In the RTS frame shown in FIG. 15, the fields
are the same as those defined in the IEEE 802.11-1999 standard. In
each of the CTS, DATA and MANAGEMENT, and ACK frames, a new field
of one octet or byte is provided, for example between the RA and
FCS (Frame Check Sequence) fields in the CTS frame. This new field
is mandatory in the CTS frame, but is optional in the other frames.
The new field can include a) Closed Loop (CL) TPC, or b) CL LA, or
c) CL joint TPC and LA. For example, the field can include a
P.sub.TX Request. FIG. 24 shows an exemplary format of the P.sub.TX
Request, including a reserved section of bits B0-B1 and a data
section of bits B2-B7 including CL-TPC info in 1 dB steps. In the
DATA and MANAGEMENT frame shown in FIG. 15, the new field is used
if the ACK frame(s) is(are) adjusted. In the ACK frame, the new
field is used if the successive DATA frame(s) is(are) adjusted.
[0136] In a second scenario addressing the open loop TPC for
interference mitigation, the frame format is as depicted in FIG.
16. In each of the RTS, CTS, DATA and MANAGEMENT, and ACK frames, a
new field of one octet or byte is provided between the Transmit
Address (TA) and FCS fields, and the Receive Address (RA) and FCS
fields respectively. The new field (relative the IEEE 802.11-1999
standard) is a P.sub.TX-P.sub.RX.sub..sub.- --.sub.max field that
includes only P.sub.TX, or P.sub.TX and
P.sub.RX.sub..sub.--.sub.max combined. This new field may be
mandatory in the RTS and CTS frames, but is optional in the DATA
and MANAGEMENT frame and the ACK frame. In the DATA and MANAGEMENT
frame and the ACK frame, the new field is used at least if
successive DATA fragments will be sent.
[0137] The generic frame format defined in the IEEE 802.11-1999
standard is depicted in FIG. 17. A generic field of length X for
any type of combination of TPC, LA, TP information and receive
power threshold is included therein.
[0138] In summary, exemplary embodiments of the present invention
convey numerous advantages. For example, the proposed mechanisms,
protocols and frame structures allow advanced and precise RRM
(Radio Resource Management) management through TPC and LA under
topologies like IBSS, BSS and entirely distributed networks. In
addition, both the TPC and the LA mechanism are to large extent
instantaneous thanks to conveying TPC and LA information in RTS and
CTS frames, (and optionally in DATA and ACK frames). As the bulk
interference comes (or should come) from data transmission, a very
tight instantaneous TPC and LA reduces the generated interference
to a bare minimum. In addition, as the bulk energy consumption
comes (or should come) from data transmission, a very tight
instantaneous TPC and LA reduces the power consumption to a bare
minimum. The invention supports asymmetrical links. The invention
supports group based TPC for RTS and CTS frames and hence reduces
the generated interference as well as power consumption related to
those messages to a bare minimum. The invention reuse the Beacon
and Target Beacon Transmission Time, TBTT for measuring path gains,
thereby complying well with power saving objectives as well as
being power consumption efficient. An increased spatial reuse is
attained through conditioning the channel access to being allowed
as long as ongoing communication is not disturbed noticeable. A
tiered TPC approach with few occasional high TP transmissions
guiding many regular low power TP transmissions reduces the
generated interference, reduces the power consumption and to a bare
minimum whereas system capacity is potentially enhanced. Multihop
based networks can take additional advantage of the distributed TP
information in e.g. RTS, CTS frames and thereby reducing the load
an intensity of probing frames to neighbors used for determining
average path gain to neighbors that may be used in shortest path
metric or for topology control.
[0139] It will be appreciated by those skilled in the art that the
present invention can be embodied in other specific forms without
departing from the spirit or essential characteristics thereof, and
that the invention is not limited to the specific embodiments
described herein. The presently disclosed embodiments are therefore
considered in all respects to be illustrative and not restrictive.
The scope of the invention is indicated by the appended claims
rather than the foregoing description, and all changes that come
within the meaning and range and equivalents thereof are intended
to be embraced therein.
* * * * *