U.S. patent application number 16/435137 was filed with the patent office on 2019-09-26 for rate adapt algorithm for a wireless connection.
This patent application is currently assigned to CAMBIUM NETWORKS LIMITED. The applicant listed for this patent is Cambium Networks Limited. Invention is credited to Oleksandr Andrusenko, Daniel Sullivan, Sergii Topchii.
Application Number | 20190297521 16/435137 |
Document ID | / |
Family ID | 54150458 |
Filed Date | 2019-09-26 |
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United States Patent
Application |
20190297521 |
Kind Code |
A1 |
Sullivan; Daniel ; et
al. |
September 26, 2019 |
RATE ADAPT ALGORITHM FOR A WIRELESS CONNECTION
Abstract
In a radio device, an automatic repeat request circuit
communicates data messages to remote radio devices at selected data
rates. Responses to the data messages are collected and statistics
developed about success rates of the data messages at the selected
data rates. A current data rate is selected based on the statistics
and a packet is transmitted. The data rate can be adapted using the
statistics about success rates of the data messages.
Inventors: |
Sullivan; Daniel; (Elmhurst,
IL) ; Andrusenko; Oleksandr; (Bila Tserkva, UA)
; Topchii; Sergii; (Kiev, UA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cambium Networks Limited |
Ashburton |
|
GB |
|
|
Assignee: |
CAMBIUM NETWORKS LIMITED
Ashburton
GB
|
Family ID: |
54150458 |
Appl. No.: |
16/435137 |
Filed: |
June 7, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14463382 |
Aug 19, 2014 |
10321350 |
|
|
16435137 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 28/04 20130101;
H04W 28/0242 20130101; H04L 1/0016 20130101; H04L 1/0009 20130101;
H04L 1/0019 20130101; H04L 1/1607 20130101; H04W 24/08 20130101;
H04L 1/0003 20130101 |
International
Class: |
H04W 28/02 20060101
H04W028/02; H04L 1/16 20060101 H04L001/16; H04L 1/00 20060101
H04L001/00; H04W 28/04 20060101 H04W028/04; H04W 24/08 20060101
H04W024/08 |
Claims
1. A method comprising: at a radio device, probing different
respective data rates for radio communication on a link between the
radio device and a remote radio device; determining success
percentages for each respective data rate based on a response to
the probing; estimating a throughput for the link for each
respective data rate; and selecting the respective data rate that
gives maximum throughput for the link.
2. The method of claim 1 wherein probing comprises: selecting a
data rate; communicating data from the radio device to the remote
radio device; detecting an acknowledgement of the communicated data
from the remote radio device; and storing first data indicative of
the detected acknowledgement for the selected data rate; or
detecting lack of an acknowledgement of the communicated data from
the remote radio device; and storing second data indicative of the
detected lack of an acknowledgement for the selected data rate.
3. The method of claim 2 wherein selecting a data rate comprises
selecting a Modulation and Coding Scheme (MCS) index.
4. The method of claim 2 further comprising: estimating minimum
possible packet error rate (PER) on higher data rates, based on
available information about PER on lower data rates.
5. The method of claim 2 further comprising: determining duration
of a change in packet error rate; if the determined duration is
longer than a predetermined threshold, possibly changing the
selected data rate to a more optimal data rate; and if the
determined duration is shorter than the predetermined threshold,
maintaining the selected respective data rate without a change.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of U.S. patent
application Ser. No. 14/463,382, filed Aug. 19, 2014, titled "RATE
ADEPT ALGORITHM FOR A WIRELESS CONNECTION," the contents of which
is incorporated herein by reference in its entirety.
BACKGROUND
[0002] The present application relates generally to wireless
communication. More specifically, the present application relates
to system and method for adapting communication data rate in a
wireless connection.
[0003] Wireless communication systems have been developed to
provide relatively low-cost radio communication to users in a
variety of locations. Such wireless communication systems typically
operate according to one or more wireless standards. The wireless
standard defines the communication flow between radios in a system
including message format and content and timing. Examples of such
wireless standards include cellular telephone standards such as the
Global System for Mobile Communication (GSM) and wireless standards
such as Institute of Electrical and Electronic Engineers (IEEE)
standard 802.11 and its variants. Other examples abound.
[0004] A typical wireless communication system includes one or more
fixed base stations or access points which provide radio
communication to a geographic area surrounding the base station or
access point. One or more subscriber modules in the geographic area
establish radio communication with the base station or access point
to receive communication services. The communication services
received by subscriber modules may include one or both of voice
services and data services. The base station or access point may
have separate network access for internet access and call
management. Some systems, such as cellular systems, provide handoff
of calls between base stations so that the subscriber modules may
be mobile.
[0005] Such communication systems may use licensed bands of
frequencies or unlicensed bands of frequencies. Typically, a
cellular system uses a licensed band. The system operator generally
has exclusive use and control over radio communications for a
specified band of channels for a specified geographic region.
Smaller wireless systems are typically unlicensed. Any radio in the
area may begin communicating.
[0006] Interference may occur and must be accommodated for radios
operating in unlicensed bands. For example, a radio communicating
with another, distant radio may suddenly be exposed to
transmissions from a third, nearby radio. Because of proximity, the
nearby radio may be received as a much stronger signal then the
distant radio, generating errors in received data. Such interferers
must be accommodated for reliable communication.
[0007] Such problems and other problems in radio communication
systems and method are addressed in the present disclosure.
BRIEF SUMMARY
[0008] A radio device and a method for a radio device provide in
one embodiment for transmitting packets from the radio device at a
current data rate. Error data is collected to estimate packet error
rates for the radio device. Weight values are determined for each
data rate and a maximum weight value is identified. A best rate
value is selected as the data rate associated with the maximum
weight value and the best rate value may be used to transmit a
subsequent packet from the radio device.
[0009] In another embodiment, in a radio device, an automatic
repeat request circuit communicates data messages to remote radio
devices at selected data rates. Responses to the data messages are
collected and statistics developed about success rates of the data
messages at the selected data rates. A current data rate is
selected based on the statistics and a packet is transmitted. The
data rate can be adapted using the statistics about success rates
of the data messages.
[0010] In another embodiment, a method for a radio device includes
probing at different data rates for successful communication over a
radio link. Success percentages for each rate are determined and
data stored. An estimate is made of throughput for the link for
each data rate. The data rate that gives maximum data throughput
for the link is selected for subsequent packet transmissions from
the radio device.
[0011] These and other advantages, aspects and novel features of
the present invention, as well as details of an illustrated
embodiment thereof, will be more fully understood from the
following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram of an example radio communication
system;
[0013] FIG. 2 is a more detailed block diagram of two exemplary
radios operating in the radio communication system of FIG. 1;
[0014] FIG. 3 is a graph showing a first function used in
accordance with one embodiment of a rate adapt algorithm;
[0015] FIG. 4 is a graph showing a second function used in
accordance with one embodiment of a rate adapt algorithm;
[0016] FIG. 5 is a flow diagram illustrating an exemplary
embodiment of a method of rate selection in a radio device for use
in the radio communication system of FIG. 1;
[0017] FIG. 6 is a flow diagram illustrating an exemplary
embodiment of a method for calculating the best particular data
rate among all of the available data rates for use in the radio
communication system of FIG. 1; and
[0018] FIG. 7 is a flow diagram illustrating an exemplary
embodiment of an algorithm for finding a higher rate than the
current rate should it be determined that these higher rates have
not been used for too long a period with the rate selection method
of FIG. 5.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0019] A system and method are disclosed for optimizing a rate
adapt algorithm over a wireless link to support an interference
environment in addition to traditional non-interference
environments. The rate adapt algorithm allows two communicating
radios to tailor communication to a changing environment. When
possible, the rate of data exchange is increased under the rate
adapt algorithm. In the presence of interference or other factors,
the rate of data exchanged is adjusted under the rate adapt
algorithm to achieve higher throughput by using higher rates in the
presence of interference. The rate of data exchange is adapted to
the environment on a real-time or near-real time basis.
[0020] Rate adapt algorithms over wireless environments typically
support non-interference environments or are designed based on
these types of environments. A non-interference environment is one
where only two radios are active at a given time on a channel, a
transmitter and a receiver. Multiple access rules control and limit
other radios from accessing the channel and creating
interference.
[0021] One standard for measuring interference on a channel is
Carrier to Interference and Noise Ratio (CINR). CINR defines the
level that a radio frequency (RF) carrier is heard compared to
environmental noise and other RF Interferers added together. The
higher the CINR, the louder the carrier is heard. Higher CINR is
required for higher rate of data exchange. Higher rates of data
exchange are considered more aggressive but are conventionally more
desirable. Higher rates of data exchange enhance user convenience
and more efficiently use scarce resources such as radio channels.
Where one or both of the linked radios is battery powered, a higher
rate of data exchange can reduce current drain and extend battery
life.
[0022] One standard for defining the rate of data exchange is
referred to as the Modulation and Coding Scheme (MCS). MCS defines
how bits are sent over the RF medium. An exemplary MCS that may be
considered are conventionally referred to as MCS1 through MCS15
specified by IEEE 802.11n. In this definition, each MCS level is
defined by the modulation type, the error correction rate, and the
number of streams that are sent for an RF subcarrier. The
modulation types are 64-QAM which sends six bits for each symbol,
16-QAM which sends four bits for each symbol, and QPSK which sends
two bits for each symbol. The error correction rates include 5/6,
3/4, 2/3, and 1/2. For example, then, when using 5/6 error
correction, five data bits are sent and one error correction bit is
sent for every six bits. The number of streams being sent refers to
the number of independent data paths sent over the radio channel.
One stream means one data path and two streams means two data
paths. Thus, in this example, MCS15 uses 64-QAM 5/6 over two RF
streams sends 10 bits (i.e. 5*2) for every symbol. MCS1 which uses
QPSK 1/2 over one RF streams sends 1 bit (i.e. 1*1) for every
symbol. In this example, MCS15 provides the highest rate of data
exchange and MCS1 provides the lowest rate of data exchange.
[0023] Depending on current channel conditions, then, the receiver
and transmitter negotiate a particular data rate to use. In the
current example using MCS definitions, the receiver and the
transmitter or some other network entity selects a best current MCS
level. As conditions change, the MCS level is changes or adapted to
the changing conditions, even on a frame-by-frame basis. As the
distance between transmitter and receiver grows, for example, the
MCS level may be reduced. As other environmental noise decreases,
for another example, MCS level may be increased. One operational
goal is to increase to the highest MCS level possible while
maintaining reliable communication.
[0024] One measure of reliable communication in a radio channel is
link budget. The link budget defines how much an RF signal degrades
in power between two radios. The power of a radio signal from a
transmitting radio to a receiving radio may be referred to as
Receive Signal Strength Indication (RSSI). RSSI defines the level
that an RF signal is received at. Typically in a 20 MHz channel, RF
signals can be received between -95 dBm (very low, the level of the
inherent noise of the environment) and -40 dBm (very loud, above
which a radio starts to receive errors with the signal). RSSI
pertains both to the intended signal a radio wants to hear and any
interference signals a radio does not want to hear. Typically, high
link budgets can provide higher RSSI and low link budgets can
provide lower RSSI.
[0025] A conventional measurement of errors in a radio channel is
Packet Error Rate (PER). PER defines the percentage of errors
encountered when data is sent over the RF medium at a specific MCS.
One operational goal is to reduce or minimize PER. One method for
achieving this goal is adapting the MCS level to current conditions
including interference.
[0026] In an unlicensed environment, interference can be a
substantial problem. In a licensed environment, the spectrum owner
can control all channel access and substantially eliminate all
interference. Such control is not possible in an unlicensed
environment. Even in relatively uncrowded spaces, with few radios
present, interference is not a substantial problem. However, in
some environments, multiple access points can be established in the
same geographic area to create multiple wireless internet service
provider networks competing for the same unlicensed channels. For
example, these channels are generally in the bands at 2.4 GHz and
5.0 GHz. Other radios on the channel are referred to as interferers
with respect to a particular transmitter and receiver.
[0027] When an interferer becomes active and begins transmitting,
the CINR decreases substantially, causing loss of transmitted
packets. The conventional response to packet loss is retrying
transmission of the packets. This has been successful in that all
packets will usually be received on the fourth or fifth retry, even
in a high-interference environment.
[0028] Conventional rate adapt algorithms do not account for
interferers on the channel. As a result the conventional rate adapt
algorithm often modifies the selection of the MCS rate based on the
current PER and the link budget. The rate adapt algorithm adjusts
to a less aggressive rate MCS with less throughput when PER
increases and adjusts to a more aggressive MCS with greater
throughput when PER decreases. If there is no interference in an
environment, the closer radios get together, the higher the CINR
they experience which implies that a more aggressive MCS can be
expected to send data without having PER increase. MCS adjustment
is done on a stair-step basis, with progressively lower MCS levels
being tried as PER increases and progressively higher MCS levels
being tried as PER decreases.
[0029] However, in interference environments, interferers may be
operating and create RF noise (i.e., interference) such that a
respective transmitter and receiver cannot reliably hear each other
when the interferers are transmitting. These interferers may be
intermittent or constant and may be loud (i.e. have a high RSSI at
the receiver) or soft (i.e. have a low RSSI at the receiver). There
also may be multiple interferers. This can cause CINR to vary for a
receiver.
[0030] Therefore, in interference environments, selection of a
most-appropriate MCS or other data rate may be based on both the
interference present at any given moment and link budget. The
present disclosure creates a rate adapt algorithm that considers
both interference and link budget to choose the optimal MCS such
that overall throughput of the connection between radios is
maximized.
[0031] In accordance with some embodiments, a method for a rate
adapt algorithm probes different data rates or MCS level to learn
what is the best MCS level to use for a particular time. The probes
may be in the form of Automatic Repeat reQuest (ARQ) data messages
or data communications. The method includes determining success
percentages for MCS levels based on the response to the probes. All
MCS levels may be probed or a subset of MCS of the MCS levels may
be probed. The method also includes determining an estimated
throughput for each MCS. Further, the method includes selecting the
MCS level that gives the maximum throughput for a link. The maximum
throughput for current conditions may be referred to as the
goodput.
[0032] The method further includes, during short time periods,
probing to see if packets were received successfully at each
selected data rate or MCS level so the transmitting radio can
dynamically adapt to a changing environment including
interferers.
[0033] The probes may be any data that is sent or received or
combination of data that is sent or received. In some embodiments,
the probes are regular data that is normally sent between a
transmitting radio and a receiving radio over a radio link. For
example, an automatic repeat-request process may be put in place in
which the transmitting radio sends a frame and waits for receipt of
an acknowledgement transmission from the receiving radio. The
acknowledgement transmission may be referred to as an ACK. If the
receiving radio successfully receives the packet, it responds with
an ACK message. Success of receipt may be determined by any error
detection or correction technique. If the receiving radio receives
the packet but with errors, it may responds with a
non-acknowledgement transmission, referred to as a NACK. The
transmitting radio can respond to receipt of the NACK from the
receiving radio by re-transmitting the same packet. Also, if the
transmitting radio does not receive either an acknowledgment
transmission or a non-acknowledgement transmission (no ACK or
NACK), the transmitting radio does not know whether the packet was
not received at all by the receiving radio or if it was received
but a subsequent ACK message was unsuccessfully transmitted by the
receiving radio. The transmitting radio can retransmit the packet.
Either a non-acknowledgement or no acknowledgement can be treated
by the transmitting radio as lack of an acknowledgement of the
communicated frame from the receiving radio.
[0034] The transmitting radio maintains a metric based on
acknowledgment transmissions it receives or lack of acknowledgement
transmissions it receives. This can be done, for example by
counting ACK and NACK messages from the receiving radio and storing
a count or other data indicative of the success of transmission and
receipt over the link to the remote radio. The stored data is
indicative of a percentage of successful packet transmissions.
[0035] Further, this process of probing can be done at different
MCS levels or data rates. In this manner, the transmitting radio
can track success rate based on received ACK messages, NACK
messages or no received message. Statistics can be derived and
stored in the transmitting radio. This can also be done for a set
time period or for varying time periods, so that when interference
is likely or has been detected, more probe data is obtained and
stored. In this manner, the data rate or MCS level for the link can
be selected dynamically and rapidly.
[0036] The technique can be used by radios at both ends of the
link. A radio communication includes two different link sessions.
These may be termed the downlink, from an access point (AP) to a
subscriber module (SM) and an uplink from the SM to the AP. Both
the AP and the SM may perform the method of adapting data rate as
described herein. Each respective rate adapt algorithm performed by
the AP and the SM is independent because each receiver sees
different interference. If the AP and SM are physically close, such
as less than 100 meters, they will likely see the same interferers.
Due to availability of directional antennas, it is possible to see
different interferers on the AP and the SM even if distance between
the AP and the SM is small. Typically, however, the two radios are
not close and will see different interferers. The radio on the link
that sends the data and receives the ACK or NACK (or no response)
decides which rate to select using the rate adapt algorithm.
[0037] Accordingly, the method and system disclosed herein enable
determining what rate for communication of data on a radio link
based on the data received back over the link from the intended
receiver. This rate selection is done even in an uncontrolled,
interference-based environment.
[0038] Referring now to the drawing, FIG. 1 shows a simplified
block diagram of an example radio communication system 100. The
radio communication system 100 is an example system that includes
wireless devices and networks that may implement a combination of
methods described herein to implement a rate adapt algorithm which
dynamically accommodates presence of interfering devices. In FIG.
1, the radio communication system 100 includes an access point 102,
a first subscriber module (SM) 104, a second subscriber module 106,
a first interferer 108 and a second interferer 110. In the
illustrated embodiment, the AP 102 communicates with the first SM
104 and the second SM 106 using a physical layer (PHY) based on
Institute of Electrical and Electronic Engineers (IEEE) 802.11n.
However, the radio communication system 100 is intended to be
exemplary only and is intended for the illustration of certain
operational points to be described below. The devices and
procedures described herein are intended to be used with the widest
variety of networks including network topologies and data
communication technologies. For example, in one embodiment, a radio
system uses a PHY that is based on IEEE 802.11 (i.e. data is
transmitted over 52 subcarriers and symbol time is 4 psec), but the
media access control (MAC) layer used is time division duplex (TDD,
i.e. a frame with downlink and uplink parts).
[0039] The AP 102 provides radio communication to a service area
surrounding the AP 102. In the illustrated embodiment, the AP 102
operates according a Time Division Duplex (TDD) protocol defined as
a frame (e.g. 5 msec in length) with both downlink (DL) and uplink
(UL) sub-frames. These sub-frames are composed of separate timed
opportunities allocated to specific SMs. The devices and techniques
described herein may be extended to standards and protocols
including IEEE 802.11n.
[0040] In the example of FIG. 1, the AP 102 operates as a base
station for radio devices within a cell or service area surrounding
the AP 102. In this example, the access point 102 provides radio
communication service to one or more subscriber modules including
SM 104 and SM 106 near the access point. As used herein, radio
communication generally refers to the communication of data by the
transmission and reception of radio frequency signals
representative of the data. Each subscriber module, SM 104 and SM
106 communicates with the access point 102 using a radio
communication protocol such as TDD. The radio communication
protocol defines frequency allocation, timing, frame structure and
other characteristics of the transmission and reception of
information between radio devices including the access point 102
and other radio devices including SM 102 and SM 104.
[0041] In the example of FIG. 1, the access point 102 is in radio
communication with two radio devices, including a first subscriber
module, SM 104, and a second subscriber module, SM 106. The
subscriber modules SM 104 and SM 106 may be considered clients of
the access point 102 and may be referred to as wireless client
devices. An access point such as AP 102 and its corresponding
wireless client devices SM 104 and SM 106 may be referred to as a
wireless network. While FIG. 1 shows two subscriber modules in
radio communication with the access point 102, this is intended to
be exemplary only. Any number of subscriber modules may communicate
with the AP 102. Also, the subscriber modules illustrated in FIG. 1
may selectively leave the wireless network and re-enter the network
and other subscriber modules may enter the wireless network as
well.
[0042] Each of SM 104 and SM 106 may include a radio communication
circuit in combination with any other suitable device or equipment.
Exemplary devices that may include a radio circuit and form a
subscriber module or wireless client device include a mobile phone,
a smart phone, a personal digital assistant (PDA), a laptop
computer, a tablet computer, a personal computer and any other data
processing device, or a fixed radio device located in a home,
business or other area requiring data communications access. In the
example of FIG. 1, SM 104 is embodied as a mobile phone and SM 106
is embodied as a laptop computer. For each of SM 104 and SM 106, a
radio circuit provides data communication between the data
processing device and the access point 102. The radio circuit may
be a hardware or software module or component or group of
components and may be a permanent part of the subscriber module or
may be removable or detachable from the subscriber module.
[0043] The access point 102 may, in turn, provide data
communication between respective subscriber modules among the SM
104 and the SM 106, or between a respective subscriber module and
another network such as the internet. The arrangement of access
point 102 and subscriber modules SM 104 and 106 is referred to as a
point to multi-point (PMP) arrangement or scheme. The network may
be any network or combination of networks and may include directly
or indirectly the Internet or networks in communication with the
Internet.
[0044] FIG. 2 is a block diagram of a representative access point
202 and representative subscriber module 204. The access point 202
may be of form AP 102 of FIG. 1. Similarly, the subscriber module
204 may be representative of one of SM 104 or SM 106 in FIG. 1.
However, the embodiments shown are intended to be exemplary
only.
[0045] The access point 202 includes a host processor 206, a
network interface 328 and an antenna 216. In other embodiments, the
access point 202 may include more or fewer or alternative elements
relative to those shown in FIG. 2.
[0046] The host processor 206 controls operation of the access
point 202. The host processor 206 may include one or more circuits,
modules, interfaces or code for implementing control functions. For
example, the host processor 206 may include a microprocessor and
memory. The memory may store data and instructions for controlling
the microprocessor and other components of the access point 202.
The microprocessor in turn may operate in response to the stored
data and instructions to control operation of the access point.
[0047] The network interface 208 controls data communication
between the access point 322 and other devices, including the
subscriber module 204. The network interface 208 controls wireless
communication using the antenna 216. In this regard, the network
interface 208 may implement one or more radio circuits to transmit
and receive radio communications by means of the antenna 216. The
network interface 208 implements a physical layer (PHY) 218 in
accordance with the Open Systems Interconnection (OSI) model of
computer networking and a transmitter and receiver circuit (TX/RX)
220. In some embodiments the network interface 208 may be
implemented in a single commercial semiconductor device or chipset.
Examples of such chipsets include Avastar 88W8764 and Atheros
AR9002U UB94.
[0048] The transmitter portion of TX/RX 220 is adapted to transmit
a radio signal representation of the downlink frame of a TDD frame.
The transmitter portion of TX/RX 220 may apply the appropriate
modulation and coding schemes (MCS) as specified by the 802.11
standard to the downlink frame before transmission. The transmitter
portion may also include mixers, frequency synthesizers, digital to
analog convertors, power amplifier etc.
[0049] The receiver portion of TX/RX 220 is configured to receive
radio signal representations of uplink frames of TDD frames. The
receiver decodes the radio signals to recover the uplink data. The
receiver includes amplifiers, automatic gain control circuits, IF
amplifiers, demodulators etc. The receiver may also measure
characteristics of the received radio signals including the receive
signal strength (RSSI), the signal to noise ratio (SNR) of a
received radio signal and the carrier interference and noise ratio
(CINR). Further, the network interface 208 implements a time
division duplex (TDD) module (MAC) 222. The TDD module 222 may
control the operation of the PHY 218 including configuring the
appropriate MCS.
[0050] In one embodiment, the network interface 208 implements the
IEEE 802.11n protocol, including the 802.11n PHY layer. The network
interface 208 may also or instead implement other data
communication protocols for both wireless and wire line
communication. For example, the network interface 208 may control
communication to other wire line network elements such as a
wireline Ethernet network. In this regard, the network interface
may implement protocols such as Ethernet or internet protocol (IP)
for communication with other network elements.
[0051] The network interface 208 may include data processing
circuits such as one or more processors, circuits, interfaces,
modules and memory for implementing network control and
communication. The network interface 208 may be programmed with
data and instructions to perform the functions described herein.
Moreover, the network interface 208 may include analog circuitry
such as amplifiers, oscillators and filters for data communication
with the antenna 216.
[0052] The antenna 216 may be any suitable device or combination of
devices for transmission and reception of signals. In one example,
the antenna 216 is a multiple-input, multiple-output (MIMO) antenna
array for data communication. In one particular embodiment, the
antenna 216 is configured for communication at frequencies such as
2.4 GHz, and 5 GHz.
[0053] The subscriber module 204 includes a host processor 224, a
network interface 226 and an antenna 228. In other embodiments, the
client device 204 may include other components providing other
functionality.
[0054] The host processor 224 controls operation of the client
device 204. The host processor 224 may include one or more
circuits, modules, interfaces or code for implementing control
functions. For example, the host processor 224 may include a
microprocessor and memory. The memory may store data and
instructions for controlling the microprocessor. The microprocessor
in turn may operate in response to the stored data and instructions
to control operation of the client device 204.
[0055] The network interface 226 includes circuits that control
data communication between the client device 204 and other devices,
including the access point 202. The network interface 226 includes
circuits that control wireless communication using the antenna 228.
In this regard, the network interface 226 may implement one or more
radio circuits to transmit and receive radio communications by
means of the antenna 228. In the illustrated embodiment, the
network interface 226 implements a physical layer (PHY) 230 as well
as a transmitter and receiver circuit (TX/RX) 232. Further, the
network interface 226 implements a time division duplex (TDD)
module 234. The network interface 226 forms a radio circuit for
radio communication with a remote access point or other radio
device.
[0056] In one embodiment, the network interface 226 implements a
derivative of the IEEE 802.11n protocol, including the 802.11n PHY
layer 230 and a TDD module 234. In this regard, the device 204
forms a subscriber module or SM. The network interface 226 may also
or instead implement other data communication protocols for both
wireless and wire line communication. For example, the network
interface 226 may control communication to other components of the
client device 204.
[0057] The network interface 226 may include data processing
circuits such as one or more processors, circuits, interfaces,
modules and memory for implementing network control and
communication. The network interface 226 may be programmed using
data and instructions to perform the functions described herein.
Moreover, the network interface 226 may include analog circuitry
such as amplifiers, oscillators and filters for communication with
the antenna 228.
[0058] The antenna 228 may be any suitable device or combination of
devices for transmission and reception of signals. In one example,
the antenna 228 is a multiple-input, multiple-output (MIMO) antenna
array for data communication. In one particular embodiment, the
antenna 228 is configured for communication at frequencies such as
2.4 GHz and 5 GHz.
[0059] The subscriber module 204 may implement a timing circuit
that maintains timing and synchronization information for the
subscriber module 204. In one example, the client device receives
timing or synchronization information periodically from the access
point 202. This information may come in the form of a beacon signal
or data frame timing element sent with each data frame transmitted
by the access point 202. By synchronizing the timing information
transmitted in the beacons and data frames to the periodic
synchronization event, the local times of the subscriber modules
may be synchronized to the periodic synchronization event. The
subscriber modules may accordingly schedule the start of their
respective portions of uplink frames at appropriate times during
the uplink frame.
[0060] In operation, the access point 202 and subscriber module 204
are in selective wireless data communication. The access point 202
operates as a base station and provides data communication in a
service area adjacent to the access point 202 to client devices or
subscriber modules such as the subscriber module 204. Data
communication is conducted according to a TDD protocol. The access
point 202 operates as a host or server to client devices such as
the subscriber module 204 in the service area and establishes a
communication network for the subscriber modules in the service
area.
[0061] Referring again to FIG. 1, first interferer 108 and second
interferer 110 represent interferer radio devices that may create
radio frequency (RF) interference with communication between the AP
102 and SM 104 or between the AP 102 and SM 106. The radio
communication system 100 operates in an uncontrolled,
interference-based environment using unlicensed frequencies for
radio communication. For example, when radio communication system
is embodied as an unlicensed system, communication is on bands in
the range of 5.0 GHz and 2.4 GHz. Since these bands are unlicensed,
any radio may make use of them and thus will create interference
for other radios present in the area, such as the AP 102 and SM 104
and SM 106. That is, interfering radios such as the first
interferer 108 and the second interferer 110 may appear or
disappear on a radio channel with no warning by turning on a
transmitter to communicate data. This may occur at the same time
the AP 102 is communicating to SM 104 or SM 106 or both, or at a
time when the SM 104 or SM 106 is communicating to the AP 102. In
the event of such a simultaneous communication, the intended
receiver, either the SM 104 or SM 106 or the AP 102, may receive
none or only a portion of the transmission due to interference from
the first interferer 108 or the second interferer 110.
[0062] The respective radio devices in the radio communication
system 100 can control aspects of their communication to optimize
data exchange for given circumstances. For example, the AP 102 may
assign time slots in a time division, multiple access arrangement
so that each subscriber module has a dedicated time period on a
frequency. In another example, each transmitting radio may select a
data rate of communication and inform its intended receiver of the
selected data rate. Higher data rates may be considered more
aggressive and will encode each symbol with more bits per symbol.
In some systems such as 802.11n, the transmitter may select a
Modulation and Coding Scheme (MCS) level. The MCS level defines how
bits are sent over the RF medium. The MCS to be considered are MCS1
through MCS15 used by 802.11n. MCS is comprised of the modulation
type, the error correction rate, and the number of streams that are
sent for an RF subcarrier. Generally, MCS 15 is considered to be a
higher data rate than MCS levels with a lower MCS index such as MCS
9 or MCS 1.
[0063] A given radio in the radio communication network 100,
including the SM 104, will tailor its ratio of carrier to
interference and noise (CINR) ratio to ensure reliable receipt of
its communications. The transmit level of the carrier may be
selected given detected levels of noise and interference. One
exemplary goal is to select a CINR of 30 dB for communication using
MCS 15 and to select a CINR of 9 dB for communication using MCS
1.
[0064] The drawing of FIG. 1 is not intended to be to scale.
However, it is intended generally illustrate that first SM 104 is
relatively more distant from AP 102 than second SM 106. Also, it is
intended to illustrate that the first interferer 108 and the second
interferer 110 are relatively closer to the AP 102 than are the
first SM 104 and the second SM 106.
[0065] For the distant SM 104, there are no nearby interferers. The
constraint on successful communication on the downlink is distance
from the AP 102. A transmitted carrier wave from the SM 104 is
attenuated as it travels to the AP and, for reliable receipt, the
SM 104 must select an MCS level suitable for receipt at the
distance from the AP 102. For the uplink, interferers close to the
AP cause interference similarly for all SMs transmitting to that
AP.
[0066] A useful measure of channel quality is the ratio of carrier
to interference and noise (CINR), measured in dB. A received signal
strength has a base level of approximately -95 dB for a 20 MHz
channel. If no interference is present, CINR is 30 dB if the signal
is received at -65 dB. Thus, CINR declines in the presence of
interference and also declines if the received signal strength
decreases, either because of distance or fading on the channel.
[0067] A given radio in the radio communication network 100,
including the SM 104, will tailor its MCS to ensure reliable
receipt of its communications. The MCS of the carrier will be
selected given detected levels of noise and interference as
inferred from the packet error rate (PER). As an example, the
distant SM 104, with generally no interferer nearby, will have a
fairly constant CINR of 14 and be able to use MCS 10 on the
downlink with some selectable variation to MCS 11 and MCS 9.
[0068] The nearby SM 106 has several interferers nearby, including
first interferer 108 and second interferer 110. If no interferer is
active, the nearby SM 106 may have CINR of 30 dB. However, activity
by either first interferer 108 or second interferer 110 may cause
the CINR of the nearby SM 106 to decrease to 20 dB or even 10 dB.
The AP for the downlink must select a MCS level or data rate based
on the presence or absence of interference that will give the
overall best throughput for a time period.
[0069] Throughput is variable and does not vary regularly with MCS
level. This is especially true given the time varying and
unexpected nature of interference on a channel. The interference
will not be constant but will be present for some percentage of
time, such as 10 percent or even 90 percent. When the interferer is
absent, a fast, aggressive MCS level may be the best choice. For
example, given a recurring interference, it may be conventional to
shift to a lower MCS level with a less aggressive bits-per-symbol
encoding. However, in fact, a more aggressive MCS level with higher
data rate and higher bits-per-symbol encoding may successfully
complete packet transmission before an interferer returns.
[0070] Conventional rate adapt algorithms use link budget as a
basis for selecting MCS levels. However, the presence of
interference may not be handled well by such conventional
algorithms. For example, the presence of an interferer may cause a
large number of transmission errors to occur. The conventional
algorithm will respond by selecting a lower MCS level. As noted,
though, throughput may instead be maximized by instead selecting a
more aggressive, higher MCS level.
[0071] In accordance with the embodiments disclosed herein, each
radio on a link operates to optimize MCS level selection and hence
throughput of a rate adapt algorithm where the radios are
functioning in a variety of interference environments. The rate
adapt algorithm sends test packets out at various MCS levels to
understand what the packet error rate (PER) is for all MCS levels
used. This allows the rate adapt algorithm to determine for a given
communication path between two radios what the best throughput is
that can be obtained for all of the MCS level choices.
[0072] A few examples help describe the problem solved.
[0073] Scenario 1: Traditional link budget constrained link. If a
link is limited due to link budget such as when the radios on the
link are so far apart that only a conservative MCS will succeed,
this algorithm will try a more aggressive MCS level, but PER will
be high. It will determine the highest MCS level that will succeed
with a low PER and this will maximize throughput by maximizing MCS
level.
[0074] Scenario 2: The CINR for a link is high, but there are
interferers that are always present that raise the noise floor all
of the time. Just like in scenario 1, the rate adapt algorithm will
determine the highest MCS level that will succeed with a low PER
and this will maximize throughput by maximizing MCS level.
[0075] Scenario 3: The link budget for a link is high, but there is
an interferer that occurs only a percentage such as 10% of the
time. When the interferer is there, CINR is low because the radios
on the link cannot hear each other due to the interferer and the
radios cannot communicate with any MCS level. When the interferer
is not there, CINR is high and communication can occur with the
most aggressive MCS. In this scenario, the most aggressive MCS is
the right one to choose to maximize throughput. Choosing a less
aggressive MCS will cause the data to take longer to be sent over
the radio link and increase the PER due to increasing the chances
of having the interferer occur. Also, the lower MCS will have less
throughput even without PER.
[0076] An infinite number of scenarios combining interferers and
link budget can occur that need to optimize MCS throughput (i.e.
measured in bits per symbol) multiplied by (1-PER) to determine
what the throughput is for each MCS and hence select the correct
MCS level that maximizes throughput.
[0077] There is a rate weight for each MCS level and it is an
indicator of quality of the MCS. Embodiments of the rate adapt
algorithm select an MCS level to use for transmission by choosing
the MCS level with the highest rate weight. The rate weight is
affected by throughput, success of packets sent, and time since the
MCS level was last sent. This allows the right MCS level to be
determined for any type of environment such as only link
budget-constrained, only interferer-constrained (one or more
interferers each with low or high RSSI and varying percentages of
when they are present), or some combination of link budget and
interferers.
[0078] Exemplary rate adapt algorithms described herein use an
aging mechanism to increase the rate weight for those MCS levels
which were not used for an extended time. This is done because RF
and environmental conditions may change over time. For example, an
interferer may start or stop or an object might move into or out of
the RF path and hence change the link budget.
[0079] An Automatic Repeat-request (ARQ) mechanism is used in which
packets sent over a wireless medium can be lost and ARQ retries
lost packets to make sure they are received. To prevent packet loss
over the ARQ mechanism which causes a significant latency increase
to data sent, embodiments of the rate adapt algorithm have a
mechanism to force a more robust MCS rate (i.e. choose the MCS with
the lowest PER) in case the number of undelivered packets starts to
grow. This allows loss-sensitive and latency-sensitive protocols,
such as Transmission Control Protocol (TCP), to work better in high
interference conditions.
[0080] Also, embodiments of the rate adapt algorithm contain
special processing for cases when there was no traffic for a long
time. This processing reduces a "slow start" effect when data
transmission starts at an MCS level other than optimal MCS
level.
[0081] Using the embodiments described herein and extensions
thereof, optimal throughput can be obtained for any link between
two radios no matter the link budget or interferers present. This
allows more data to be sent which is important to Internet Service
Providers (ISPs) and their end customers. This increases the
efficiency of use of radio resources including channels and
circuits that process the radio signals.
[0082] The description of embodiments of the rate adapt algorithm
follows. This algorithm may be implemented by a data processing
device such as the network interface 208 of the access point 202 of
FIG. 2 or the network interface 226 of the subscriber module 226 of
FIG. 2. A microprocessor or other data processor can implement the
algorithm in conjunction with code and data stored in a memory
device. The algorithm may be performed as needed and results
updated periodically or on any basis.
[0083] On each scheduling frame n, the rate adapt algorithm should
make a decision of which rate to use for scheduling and for
transmitting packets.
[0084] Making the decision, the rate adapt algorithm should
estimate a weight q.sub.k(n) for each available rate k, and then it
should select the rate with the best estimated weight:
k ( n ) = arg max k q k ( n ) ##EQU00001##
[0085] Stage 1. Goodput selector over PER estimates.
q1.sub.k(n)=(1-PERest.sub.k(n))r.sub.k
[0086] In this equation, r.sub.k is PHY rate of rate k, and
PERest.sub.k(n) is a formula of estimation (aging) of PER based on
historical sliding average of PER and its age.
PER k ( n ) = PER k ( m ) K old + InstantPER K new ##EQU00002## K
new = K amount K age + K amount ##EQU00002.2## K old = 1 - K new
##EQU00002.3## K amount = sent AmountFactor ##EQU00002.4## K age =
AgeFactor Age k ( n ) - K shift ##EQU00002.5## InstantPER = 1 -
delivered sent ##EQU00002.6## Age k ( n ) = min ( n - m , 1 )
##EQU00002.7##
[0087] where
[0088] n is number of current scheduling frame,
[0089] m is number of scheduling frame when previous ACK/NACK
message was received for rate k,
[0090] sent is number of packets sent between scheduling frames m
and n on rate r.sub.k,
[0091] delivered is number of packets ACKed in latest ACK/NAME
message for rate r.sub.k.
[0092] AgeFactor, AmountFactor and K.sub.shift are configurable
parameters with default values as follows:
[0093] AgeFactor=10, AmountFactor=20, K.sub.shift=0.03
[0094] Any suitable default values may be used, depending on
particular details of an implementation of the rate adapt
algorithm.
[0095] PEREst.sub.k(n) is calculated as PER.sub.k(n) as if there
was sent=1, delivered=1.
[0096] Stage 2. PER History Aging.
[0097] The goal of this stage is to take into account PER history
aging. That is, the longer the radio has not used rate k, the
higher value the radio would have for Age.sub.k(n), and the higher
should be the rate weight. The effect is to tend to select rates
with high age values. In other words, this stage defines how often
the radio should probe other rates and which rates should be
probed. This stage replaces estimation for PER from stage 1,
described above.
q2.sub.k(n)=f.sub.A(Age.sub.k(n),PER.sub.k(n))r.sub.k
[0098] If the history for rate r.sub.k was updated recently, then
the radio can use the PER for this rate without any adjustments. If
the history for rate r.sub.k wasn't updated for a sufficiently long
time, then the radio may make a preference for this rate, just to
update PER history. So f.sub.A(0,p) should be equal to (1-p), while
f.sub.A(.infin.,p) should be equal to 1.
[0099] One formula for other values of age is the following:
f A ( a , p ) = 1 - p c 4 a 4 + c 4 , ##EQU00003##
[0100] where c is a configurable parameter for age threshold.
[0101] FIG. 3 shows a graph of the function f.sub.A(a,p) for p=0.75
and c=10. Any suitable value for p and c may be used. In one
embodiment, a value of 1000 is used for c.
[0102] Stage 4. Number of Retries.
[0103] The goal of this stage is to consider the number of ARQ
retries the radio has made following transmission of a packet that
is not received. The more retries occurring, and the higher is a
maximum retry number, the more cautious the radio should be to not
select rates with high loss probability. This is done so as to not
increase latency and to not allow loss to leak through ARQ
retries.
[0104] Accordingly, a retry index R(n) is introduced to determine
how many retries there have been on frame n. Then calculate:
q4.sub.k(n)=f.sub.R(R(n),Age.sub.k(n),PER.sub.k(n))r.sub.k
[0105] In some embodiments, the retry index may be normalized and
R(n)=0 when there are no retries at all and to R(n)=1 when there
are many packets with retry number equal to max_retry, the maximum
possible retry number. Any suitable value may be selected as the
maximum possible retry number.
[0106] Boundary Cases:
[0107] R(n)=0. In this case the radio can select any rate, so
f.sub.R should be equal to 1-PER.sub.k (n);
[0108] R(n)=1. In this case the radio should not select rates with
high PER, so f.sub.R should be close to 0 if PER is relatively high
and close to 1 if PER is relatively low.
[0109] In one exemplary embodiment,
f R ( R , a , p ) = { f A , if { 0 .ltoreq. R .ltoreq. r 1 } p , if
{ r 1 < R .ltoreq. 1 } and { ( 1 - f A ) N ( 1 - R ) .ltoreq. p
thr } p c 4 , if { r 1 < R .ltoreq. 1 } and { ( 1 - f A ) N ( 1
- R ) > p thr } ##EQU00004##
[0110] where
[0111] a=Age.sub.k(n) is an age of rate k,
R = R ( n ) = CurrentRetryNumber N ##EQU00005##
is a retry index,
[0112] p=PRR.sub.k(n)=1-PER.sub.k(n) is a Packet Receive Rate (PRR)
on rate k. Note that this is not the same `p` variable that was
used above in stage2,
[0113] f.sub.A=f.sub.A (a, 1-p) is a result of stage2 calculation
for specified PRR,
[0114] r.sub.1 is a retry index threshold, a proposed value is
r.sub.1= 2/7,
[0115] N=7 is a number of ARQ retries,
[0116] p.sub.thr is PER threshold (expected PER after 7th retry), a
proposed value is p.sub.thr=0.0001=0.01%,
[0117] c.sub.4 is bad retry adjustment, proposed value is
c.sub.4=0.01.
[0118] FIG. 4 shows a graph for p=0.5,
age=1100(f.sub.A.apprxeq.0.8), r.sub.1= 2/7, p.sub.thr=0.01=1%
[0119] Stage 6. Low-Volume Data Traffic.
[0120] Stage 2 (aging) should work fine in cases when there is some
constant probe data flow. But if there is no probe data for a long
time, then all rates will be affected by aging, and when traffic at
last will appear, the f.sub.A formula will return the same or
nearly the same value for all rates. In this case, q.sub.k(n) will
try to select MCS level 15. But if the channel is bad enough,
packets will be lost, retransmitted again and lost again on the
same MCS level 15, then retransmitted on MCS level 14, lost again
and so on until the number of retries will exceed maximum value and
the packet will be dropped.
[0121] This case may be especially important in a very simple
scenario: ping in idle state. In a ping operation, a transmitter
sends a message asking the receiver to respond. In the best case,
latency for a ping will be very big; in a bad case, the ping will
be lost; and in a very bad case, the ping won't be sent because ARP
will be lost. Further, the next ping (or ARP) will be sent only 1
second later. This is a long enough duration so that the same
scenario will be repeated for the next ping.
[0122] Therefore, special handling is required for this case. One
exemplary solution is to modify the age calculation. That is, the
age calculation should consider not only the amount of time that
has elapsed, but also the number of transmissions that have
occurred. The following may be used
Age 6 k ( n ) = min ( age k ( n ) , sent k ( n ) c 1 c 2 ) ,
##EQU00006##
[0123] where
[0124] age.sub.k(n)--time (number of scheduled frames) passed since
last data transmission on rate k,
[0125] sent.sub.k(n)--number of data transmissions on any rate
since last transmission on rate k,
[0126] c.sub.1--age threshold, with an exemplary value of 1000
(scheduled frames),
[0127] c.sub.2--transmission threshold, with an exemplary value of
20 (data transmissions).
[0128] Therefore, the formula derived above for Stage 2 should be
modified as follows:
q2.sub.k(n)=f.sub.A(Age6.sub.k(n),PER.sub.k(n))r.sub.k
[0129] Stage 7, Part 1: A Large Number of Bad Possible Data Rates
(Reduce Number of Probes).
[0130] One possible case involves a large number of the possible
MCS levels that are unsatisfactory. The processing described above
for Stage 2 introduces aging as a way to ensure probes are
periodically performed on such bad rates. But if the number of
unacceptable MCS levels rates is large enough, an undue amount of
time would be spent probing, which is inefficient and to be
avoided. Stage 7 of some embodiments of the rate adapt algorithm
introduces a process for resolving such a problem.
[0131] Stage 7 operates to estimate unknown packet error rate (PER)
on a higher rate using known PER value on a lower rate. If
estimated PER is high, probing should be avoided. In the case of
high estimated PER, reducing the number of probes improves
performance. If estimated PER is low, probing may be done as usual.
PER does not affect the number of probes in this case.
[0132] If there is non-zero PER on any rate, it can be caused
either by insufficient RSSI or by an interferer. In the case of
insufficient RSSI, all higher MCS levels will have a higher PER
value. In the case of interference causing non-zero PER, it is
possible to calculate a minimum possible PER on higher rates as
follows. Interferer duration in an exemplary IEEE 802.11n system
can vary from 40 .mu.sec (corresponding to the preamble duration in
a packet plus 1 symbol) to the full length of corrupted packet. The
shorter the interferer duration; the lower will be PER on a higher
MCS level. Accordingly, with the minimum duration of interferer,
the loss on higher MCS level will be at least
PEREst m ( n ) = PER k ( n ) r k r m . ##EQU00007##
[0133] This value is not the same PEREst as was defined above in
connection with Stage 1. This value is useful to decide if there is
value in changing the MCS level rate from m.sub.1 to m.sub.2:
[0134] if
(1-PEREst.sub.m.sub.2(n))r.sub.m.sub.2<(1-PER.sub.m.sub.1(n))-
r.sub.m.sub.1,
[0135] or, after substitution,
[0136] if
r.sub.m.sub.2PER.sub.k(n)r.sub.k<r.sub.m.sub.1PER.sub.m.sub.1-
(n)r.sub.m.sub.1, then the radio should not select rate m.sub.2.
Here,
[0137] m.sub.1 is the current MCS rate index,
0.ltoreq.m.sub.1.ltoreq.15
[0138] m.sub.2 is the MCS rate we are estimating,
0.ltoreq.m.sub.2.ltoreq.15
[0139] k is the MCS rate with non-zero PER such that
m.sub.1<k<m.sub.2.
[0140] This procedure is modified to conform to the rest of the
rate adapt algorithm and to take aging into account. Accordingly,
let k.sub.0 be a number of current rate, and let k be a number of
rate for which we want to calculate q.sub.k(n) such that
k>k.sub.0. Then for each rate ki between k.sub.0 and k (i.e.
k.sub.0<ki<k), we can calculate
g.sub.ki(n)=f.sub.A(Age.sub.ki(n),PER.sub.ki(n))r.sub.ki
[0141] and then we can estimate
q k ( n ) .ltoreq. qest k ( n ) = ( 1 - PEREst k ( n ) ) r k == ( 1
- ( 1 - f A ( Age ki ( n ) , PER ki ( n ) ) ) r ki r k ) r k == r k
- r ki + f A ( Age ki ( n ) , PER ki ( n ) ) r ki .
##EQU00008##
[0142] Therefore,
q.sub.k(n).ltoreq.r.sub.k-r.sub.ki+f.sub.A(Age.sub.ki(n),PER.sub.ki(n))r-
.sub.ki
[0143] Two more simplifications may be made. First, the limitation
on k.sub.0(current rate) can be removed. Second, a case for ki=k
can be incorporated:
q.sub.k(n)=min.sub.ki[r.sub.k-r.sub.ki+f.sub.A(Age.sub.ki(n),PER.sub.ki(-
n))r.sub.ki],
[0144] where ki.ltoreq.k. Or, using relations from Stage 4,
q7.sub.k(n)=min.sub.ki[r.sub.k-r.sub.ki+q4.sub.ki(n)], where
1.ltoreq.ki.ltoreq.k.
[0145] Implementing this relation in a data processing system
requires an inner loop (from i to k) for each k. Complexity of this
formula is O(K.sup.2), where K is a number of available rates.
Accordingly, optimization is desirable.
[0146] Rewriting,
q7.sub.k(n)-r.sub.k=min.sub.ki[q4.sub.ki(n)-r.sub.ki],
[0147] where 1.ltoreq.ki.ltoreq.k, but
min.sub.ki[q4.sub.ki(n)-r.sub.ki]=min[q4.sub.k(n)-r.sub.k,min.sub.ki[q4.-
sub.ki(n)-r.sub.ki]],
[0148] where 1.ltoreq.ki.ltoreq.k-1, can be optimized as
follows
q7.sub.1(n)=q4.sub.1(n),
q7.sub.k(n)-r.sub.k=min[q4.sub.k(n)-r.sub.k,q7.sub.k-1(n)-r.sub.k-1]
for any k>1, or
q7.sub.1(n)=q4.sub.1(n),
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)+r.sub.k-r.sub.k-1] for
any k>1.
[0149] Stage 7, Part 1 (Continued). Modification for Dual-Stream
Rates.
[0150] As noted above, in some applications more than a single
stream of data may be communicated between radios. However, the
relation that is described above is suitable only in cases when the
radio uses the same-stream rates. More specifically, the above
relation is usable in cases when rates with higher MCS index have
higher number of bits per symbol, i.e. when (1) all rates have
ascended ordering and (2) the difference between highest
single-stream rate and lowest dual-stream rate is at least 8. For
example, this relation will work with the set of rates or MCS
levels MCS1, MCS9, MCS10, but will not work with the set of MCS
levels MCS2, MCS3, MCS9, MCS10. The failure occurs because the
algorithm tries to estimate minimum loss on MCS9 using loss on MCS2
and MCS3. This is incorrect.
[0151] If it is desired to use all single-stream rates and all
dual-stream rates in the same algorithm, the algorithm should be
modified to take into account dependency between single-stream and
dual-stream rates. For example, MCS11 is dependent on MCS10, MCS9,
MCS3, MCS2, MCS1, but is not dependent on MCS4, MCS5, MCS6 and
MCS7. To consider these dependencies, the relation above can be
modified in the following way.
q7.sub.k(n)=q4.sub.k(n) for k=1,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)r.sub.kr.sub.k-1]
for any 2<k<7,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-8(n)+r.sub.k-r.sub.k-8]f
or k=9,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)+r.sub.k-r.sub.k-1,q7.sub.k-8(n-
)+r.sub.k-r.sub.k-8]
[0152] for any 10<k<15.
[0153] This relation is not defined for k=0, k=8. If it is desired
to extend the algorithm for these values, the limits for k may be
modified as follows:
q7.sub.k(n)=q4.sub.k(n)
for k=0,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)+r.sub.k-r.sub.k-1]
for any 1<k<7,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-8(n)+r.sub.k-r.sub.k-8]
for k=8,
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)+r.sub.k(q7.sub.k-8(n)+r.sub.k--
r.sub.k-8]
[0154] for any 9<k<15.
[0155] Stage 7, Part 2: Aging Speed (Lower Rates should be Aged
Faster than Higher Rates).
[0156] As proposed by Stage 2, q2.sub.k(n) values are adjusted by
introducing aging. But the value q2.sub.k(n) still depends on
r.sub.k value, so higher rates or MCS levels will be aged faster
than lower rates. This is not desired, since in the same case when
there a lot of unacceptable MCS levels, MCS15 will be aged faster
than any other rate. As a result, the algorithm will select MCS15,
send some probes, return back, a bit later select MCS14, send some
more probes, and so on. Instead, what is desired is to start probes
from lowest rate, and to not repeat probes on higher rates if
possible, as described in Stage 7, part 1.
[0157] Accordingly, it should be ensured that, under the same
conditions, aging for lower rates will be faster than aging for
higher rates. Therefore, aging coefficients are introduced to
adjust the age value for each rate.
[0158] The relation is unchanged:
f A ( a , p ) = 1 - p c 4 a 4 + c 4 ##EQU00009##
[0159] but parameter c becomes variable:
c k = c 1 r k r 1 , ##EQU00010##
[0160] where c.sub.1 is an initial rate threshold. One exemplary
value as a default for this threshold is 100.
[0161] Stage 8, Part 1: Too Old Age and Min_Stepping Mechanism.
[0162] It is possible that no data was sent from the radio during
an extended period of time. In this case, aging for several rates
can produce similar results for different rates or MCS levels.
However, the algorithm must select only one rate or MCS level and
send a probe on the selected rate. But as described above in
connection with Stage7, part 2, it is preferable to select lower
rates for probing before higher rates.
[0163] Consider the following corner case. During some period of
time, let PER.sub.1=0%, PER.sub.2=100%, PER.sub.3=100%,
PER.sub.4=100%, PER.sub.5=50%. If the radio selects MCS5 and
experiences a high number of retries, Stage 4 would periodically
operate to reduce the rate. It may occur that, with given PER
values, the radio cannot select MCS2, MCS3 nor MCS4 due to high
historical PER on those rates. As a result, the radio would select
MCS1. But if the radio operates on MCS1 and experiences a low
number of retries, q.sub.5 would be definitely higher than q.sub.2,
regardless of whether MCS2 was aged or not. Probably q.sub.4 could
be higher than q.sub.5, so the transmitting radio may select MCS4
instead of MCS5 in case if it was aged, but the radio could not
select MCS2. As a result, every time the radio needed to reduce
rate from MCS5, the radio would select MCS1, and would never try
MCS2. But it may be that channel conditions have already changed,
and MCS2 has become acceptable. Accordingly, it is desired to probe
on MCS2. The algorithm should not probe when the number of retries
is high. The radio shouldn't probe during jump down. Accordingly,
in some embodiments, probing is done by forcing a probe on MCS2
instead of selecting a higher MCS level.
[0164] Therefore, a "min_stepping" mechanism is introduced to force
probes instead of jumps to higher rates. The mechanism is described
as follows. If a new rate k(n), selected by
arg max k q 7 k ( n ) , ##EQU00011##
is higher than the current rate k(n-1), then the radio should check
all rates or MCS levels between k(n) and k(n-1). If at least one of
those rates has a too-old age, then the algorithm should select
that rate instead of k(n).
[0165] To strictly define this stage, it is necessary to define
what a "too-old age" is. Some embodiments may define it in the
following way:
if Age.sub.k(n)>TOO_OLD_AGE,set Age.sub.k(n)=TOO_OLD_AGE
[0166] One possible value for TOO_OLD_AGE is 2000.
[0167] Also, it is necessary to define what is meant by "higher"
and "between" in the proposal above. To do this, the algorithm may
sort all rates by bitrate in order to assign a temporary index for
each rate based on sorting order, and to consider a rate with
higher temporary index to be higher than a rate with lower
temporary index.
[0168] If two rates have the same bitrate (for example, MCS3 having
104 bits per symbol, and MCS9 also having 104 bits per symbol), the
algorithm may consider the single-stream rate to be higher than the
double-stream rate since the preamble for double-stream rates is
longer than preamble for single-stream rates, so that in general,
double-stream rates are a bit more expensive than single-stream
rates with the same bitrate.
[0169] The following shows the result of sorting:
[0170] 0, 8, 1, 2, 9, 3, 10, 4, 11, 5, 6, 7, 12, 13, 14, 15
[0171] In one example, if the current rate is MCS9, the new
selected rate is MCS14. However, MCS4 and MCS10 both have age 2000.
In this example, the algorithm should select MCS10 because it has a
lesser index in the table above than MCS4.
[0172] Stage 8, Part 2: Limitation for Probe Frequency.
[0173] As defined by Stage 6, each rate has its own aging formula,
so it is theoretically possible that several rates become aged
simultaneously or with a small delay. In such a case, the algorithm
tries to make several probes simultaneously. If all aged rates are
bad, the algorithm increases the number of re-transmits. This has
the effect of increasing total latency.
[0174] To avoid the increase in latency, the algorithm introduces
one more limitation. The algorithm ensures that the min_stepping
mechanism doesn't operate too frequently. To do this, the algorithm
disables the min_stepping mechanism for a time period defined by
min_step_limit scheduling frames since last min_step. An example
value for min_step_limit is 10 scheduling frames.
[0175] Stage 9: Using PER on First Retry
[0176] In some circumstances, it is possible for the observed
packet error rate to increase suddenly for a short period of time.
In this case, the transmitting radio implementing the present
method will try to select another rate. The transmitting radio will
probably reduce the rate. The transmitting radio will not return
back to the current rate until the aging mechanism has decided to
probe on the previous rate. However, aging time can be much longer
than the interference presence time, with the result that
throughput can be degraded.
[0177] To avoid throughput degradation, a decision to reduce rate
should be delayed for a while, to first determine if the observed
PER change is short or long in duration. If the observed PER rate
change is a relatively short duration, there is no need to reduce
rate. However, if the observed PER change is of a relatively long
duration, the rate may be reduced. Stage 9 of the rate adapt
algorithm introduces a process for resolving such a problem. Stage
9 operates to have different PER values for each possible number of
retries:
PERretr.sub.k(n,r)=PERretr.sub.k(m,r)K.sub.old+InstantPERretr(r)K.sub.ne-
w,
[0178] where
InstantPERretr ( r ) = 1 - i = 0 r acked ( i ) + i = 0 r - 1 nacked
( i ) sent , ##EQU00012##
acked(i) is a number of packets delivered exactly after i
unsuccessful attempts to deliver a packet, and nacked(i) is a
number of packets undelivered exactly after i unsuccessful attempts
to deliver a packet. Using the definitions provided above, the
formula for InstantPERretr can be simplified:
InstantPERretr ( r ) = i = r N nacked ( i ) sent , ##EQU00013##
[0179] where N is maximum allowed number of retries.
[0180] With the given formula for PERretr.sub.k(n,r), we can
redefine PER.sub.k(n) as follows:
PER k ( n ) = { PERretr k ( n , 0 ) , if { prev_rate .noteq. k } or
{ k ( m ) < prev_rate } PERretr k ( n , 0 ) , if { R ( n ) >
1 } PERretr k ( n , 1 ) , otherwise , ##EQU00014##
[0181] where k(m) is an MCS index of the rate that was selected for
transmission during the m'th scheduling cycle, prev_rate is the MCS
index selected for previous transmission, m is scheduling cycle
number such that n-m.ltoreq.stage9_limit, R(n) is retry index on
scheduling cycle n, stage9_limit is any suitable value, for example
10.
[0182] In other words, the algorithm can use
PER.sub.k(n)=PERretr.sub.k(n,0) when (1) the rate for which the
algorithm calculates PER is not the same rate as was selected for
previous transmission, or (2) the rate was increased recently, or
(3) the retry index is greater than 1. Otherwise, we use
PER.sub.k(n)=PERretr.sub.k(n,1).
[0183] Alternatively, the algorithm can make a preference for the
rate which was successfully used recently.
[0184] Note that, for dual-stream rates (or for non-MCS rates), the
condition k(m)<prev_rate may be rewritten in more complicated
form, to compare not MCS indexes but rates order for any given
order. For example, the order of MCS indexes defined above in Stage
8 can be used: 0, 8, 1, 2, 9, 3, 10, 4, 11, 5, 6, 7, 12, 13, 14,
15. Using this order, we assume a special comparison when an item
with lower index is considered less than an item with higher index,
i.e. 0<8<1<2<9 and so on.
[0185] FIG. 5 is a flow diagram illustrating a method 500 of rate
selection in a radio device. The method 500 may be implemented in
any suitable radio device. Typical examples include radios
operating in an IEEE 802.11n WiFi system. Such typical radios
include an access point such as AP 102 and subscriber modules such
as SM 104 and SM 106 illustrated in FIG. 1. The method 500 includes
in an exemplary embodiment a series of processing steps to be
performed by the radio device. The processing steps may be
performed by one or more processors executing data and instructions
stored in a memory, such as the network interface 208 of AP 202 and
the network interface of client device 204 illustrated in FIG. 2.
The method steps may rely on associated data collection and
processing activities such as determination of received signal
strength indication (RSSI), carrier to interference and noise ratio
(CINR) and packet error rate (PER). Such associated data collection
may be done by receiving relevant data and information from a
remote radio or network device, or by measuring signal levels or
tracking data at the radio device itself. For example, the radio
device may transmit probes and receive ACK and NACK messages from a
remote radio. The ACK and NACK messages are processed to develop
statistics. Such statistics may be indicative of the PER.
[0186] The method begins at block 502. The method may begin in
response to any suitable condition or variation. For example, the
method may begin when the radio device is turned on and becomes
active on the network. Alternatively, the method may begin when the
radio device seeks to transmit a packet and needs to select a rate
or MCS level to use for the transmission. Still further, the method
may be processed continually as information such as packet error
rate, RSSI and CINR are updated.
[0187] At block 504, the method determines a current best rate as a
maximum value of the variable q7k, described above. In one
embodiment, q7k may be derived as
q7.sub.k(n)=min[q4.sub.k(n),q7.sub.k-1(n)+r.sub.k-r.sub.k-1] for
any k>1. FIG. 6, discussed below, illustrates a simplified
example of calculation of q7k calculation. The q7k values may be
stored in an array or matrix for easy manipulation by a processor,
with a rate value of k as an index. In one embodiment, the rates k
correspond to MCS levels for IEEE 802.11n. Other rates may be used
as the method is extended to other applications and other
technologies.
[0188] At block 506, a logical test is performed. It is determined
if, first, the value for best_rate established in block 504 exceeds
the current rate being used. For example, the current rate may be
MCS9, which may be read from a stored data location, and the
best_rate determined at block 504 may be determined to be MCS10. A
second logical test is performed at block 506 to determine if the
last probe age exceeds a minimum step limit.
[0189] If both logical tests of block 506 pass, control proceeds to
block 508. There, the value for the best_rate is set to the result
of the function min_stepping with the current rate and the
best_rate determined at block 504 as arguments. The minimum
stepping operation is described above to force probe transmissions
on rates that might otherwise be avoided based on aged PER data.
Control then proceeds to block 510.
[0190] If one or both of the logical tests of block 506 fail,
control proceeds directly from block 506 to block 510. At block
510, the selected rate to use for a next packet transmission is the
value of the variable best_rate. The method ends at block 512 and
the selected rate is returned for further processing.
[0191] FIG. 6 is a flow diagram illustrating an exemplary
embodiment of a method 600 for calculating the variable q7k. As
noted above, this value corresponds to an indication of quality of
a particular rate or MCS level specified by the MCS index k. The
method begins at block 602. The method may begin operation due to
any suitable initiating event, such as receipt of new or updated
information about RSSI or CINR or occurrence of an event such as
the need to transmit a packet. The method may be implemented as a
set of instructions and data that may be accessed by a processor of
a radio device operating, for example, in an IEEE 802.11n radio
communication network. However, the techniques illustrated and
described in conjunction with the drawings may be extended to other
devices and systems as well.
[0192] At block 604, the method begins by retrieving stored
historical data about packet error rates experienced by the radio
device. In one aspect of operation, the radio device routinely
determines a packet error rate for each data rate k being used. As
noted, the data rate k may refer to the MCS index used in an
802.11n system. Thus, the stored PER data may include an array of
data stored by timestamp or other indication of the age of the data
and stored by rate k. The PER data may be determined using any
suitable technique. In one exemplary technique, the radio device
sends probe transmissions and determines if the probe transmissions
were reliably received. In one particular embodiment, an Automatic
Repeat-request (ARQ) mechanism is used in which packets sent by the
radio device over a wireless medium can be monitored for receipt.
The intended receiver responds with an ACK transmission, a NACK
transmission or no transmission at all, providing an indication of
success of receipt and therefore an indication of packet error
rate.
[0193] At block 606, the method retrieves stored historical data
about age of PER data for each respective rate k. The data defines
the time since PER data for each rate or MCS index was updated. The
data may be stored and retrieved in any suitable manner.
[0194] At block 608, a logical determination is made about the age
of the PER data for each rate k. In a first logical test, it is
determined if the age data retrieved at block 606 are considered
too old. The age value for each rate k is compared with a threshold
value labelled TOO_OLD_AGE. Any value may be used for this
threshold value. An example value is 2000.
[0195] Further at block 608, a logical determination is made about
the minimum stepping procedure. As noted above, to eliminate issues
of latency, embodiments of the method ensure that the minimum
stepping procedure is not performed too frequently. Thus, a value
for the age of the last probe for any rate, labelled
last_probe_age, is compared with the threshold value labelled
min_step_limit. Any suitable value, such as 10 scheduling frames
may be used for the min_step_limit.
[0196] If both logical determinations of block 608 are met, control
proceeds to block 610. This will be true for a given rate k if its
PER data matches the too old age threshold and if the last probe on
any rate was sent more recently than the minimum step limit. If
these conditions are both true, at block 610 the value for q7k is
set to a value of 0 or other value which ensures the rate will not
be selected.
[0197] If either logical determination of block 608 fails, control
proceeds to block 612. At block 612, the function f.sub.A is
evaluated to determine weighting given to the rate k based on aging
of the PER data for rate k. An exemplary relation for f.sub.A is
specified above. Other relations may be used as well.
[0198] At block 614, the function f.sub.R is determined for rate k.
The function f.sub.R uses a retry index to determine how many
retries have been made when sending a given frame. An exemplary
relation for f.sub.R is specified above.
[0199] At block 616, the value q4.sub.k is determined. This term
represents an indication of weight of a particular rate or MCS
level specified by the MCS index k, taking into account aging and
retry data for the rate k. An exemplary relation for the
determination of the value q4.sub.k is provided above.
[0200] Subsequently, using the value q4.sub.k a value q7.sub.k is
determined at block 618. This may be determined using the formula
shown in FIG. 6. Thus, the value of q7.sub.k is selected based on
the value of rates evaluated over rate k, the rate being evaluated,
and rate i which represents all rates between the current rate
k.sub.0 and rate k, and q4.sub.k as determined in block 616. The
value of q7.sub.k is returned and the method 600 of FIG. 6 ends at
block 620.
[0201] FIG. 7 is a flow diagram illustrating an exemplary
embodiment of a method 700 for determining a minimum stepping
procedure for use with the rate selection method of FIG. 5. The
minimum stepping procedure may be useful in a case where PER data
is aged and may no longer be reliable. Under certain conditions,
the stored PER data would prompt the rate adapt algorithm to select
a higher rate. Since the PER data is aged, it may be desirable to
select rates having aged data and probe to determine current PER
information. However, probing for lower data rates may be preferred
to probing at higher data, as discussed above. The minimum stepping
procedure operates to force probes at lower rates rather than
selecting higher rates.
[0202] The method 700 begins at block 702. The method 700 may be
implemented as a set of instructions and data that may be accessed
by a processor of a radio device operating, for example, in an IEEE
802.11n radio communication network. However, the techniques
illustrated and described in conjunction with the drawings may be
extended to other devices and systems as well. The method may begin
due to any appropriate situation. One example might be the aging of
packet error rate data stored at a radio device along with the
determined need to probe for new PER data.
[0203] At block 704, a variable temp_rate is set equal to the
current data rate, which may be the current MCS index value. At
block 706, the variable temp_rate is used to retrieve stored
historical data about age of PER data for the current rate. The
stored data defines the time since last updating of PER data for
the rate or MCS index defined by the value temp_rate. The data may
be stored and retrieved in any suitable manner.
[0204] At block 708, a logical determination is made about the age
of the PER data for the age data retrieved at block 706. It is
determined if the age data retrieved at block 706 are considered
too old. The age value for the current rate is compared with a
threshold value labelled TOO_OLD_AGE. Any value may be used for
this threshold value. An example value is 2000.
[0205] If the logical determination of block 708 concludes that the
age value is other than the threshold value TOO_OLD_AGE, at block
710, it is determined if the current value of the variable
temp_rate is set equal to the value of the variable best rate. If
not, control proceeds to block 714 where the variable temp_rate is
set to the result of procedure NextRate using the current value of
temp_rate as the argument. Procedure NextRate requires an ordered
list or rates and operates to select the next rate from the list.
In systems with both single-stream and dual-stream MCS rates, it
may use a data array described above in connection with
single-stream rates and double-stream rates. In one example,
NextRate(4)=11, NextRate(7)=12, NextRate(12)=13. In systems with
only single-stream rates, the procedure may just select the next
MCS index. For example, NextRate(4)=5, NextRate(6)=7. In systems
with some another set of rates, such as non-802.11n systems, it
should use its own list of rates to determine what a NextRate is.
Control then returns to block 706 where age data for the new value
of temp_rate is retrieved.
[0206] If, at block 708, it was concluded that the retrieved age
data was set equal to the threshold value TOO_OLD_AGE, at block
716, the value of variable best_rate is set equal to the current
value of temp_rate. This value is returned and the procedure ends
at block 720. As explained above, if a new rate k(n), selected
by
arg max k q 7 k ( n ) , ##EQU00015##
is higher than the current rate k(n-1), then the radio device
should check all rates or MCS levels between the selected new rate
k(n) and the current k(n-1), where n identifies the frame. If at
least one of those intermediate rates has a too-old age, then the
algorithm should select that rate instead of k(n) as the best rate
to use.
[0207] On the other hand, if at block 710, if it was determined
that the current value of the variable temp_rate is already equal
to the value of the variable best_rate, the value best_rate is
returned and the procedure ends at block 720.
[0208] From the foregoing, it can be seen that the presently
disclosed embodiments provide method and apparatus for an improved
rate scan algorithm in a radio device. The method for a rate adapt
algorithm probes different data rates or MCS level to learn what is
the best MCS level to use for a particular time. The method
includes determining success percentages for MCS levels based on
the response to the probes. All MCS levels may be probed or a
subset of the MCS levels may be probed. The method also includes
determining an estimated throughput for each MCS. Further, the
method includes selecting the MCS level that gives the maximum
throughput for a link.
[0209] The disclosed method and apparatus allow the right MCS to be
determined for any type of environment in which the radio device
may operate, such as only link budget-constrained, only
interferer-constrained, or some combination of link budget and
interferers. An aging mechanism is employed to increase the rate
weight for those MCS levels which have not been used for a long
time.
[0210] The disclosed rate adapt algorithm may employ a mechanism to
force a more robust MCS rate (i.e. the MCS rate with the lowest
PER) in case the number of undelivered packets starts to grow. This
allows loss-sensitive and latency-sensitive protocols, (such as
Transmission Control Protocol TCP), to work more reliably in high
interference conditions. Also, the disclosed rate adapt algorithm
contains special processing for cases when there was no traffic for
a long time.
[0211] The methods, devices, processing, and logic described above
may be implemented in many different ways and in many different
combinations of hardware and software. For example, all or parts of
the implementations may be circuitry that includes an instruction
processor, such as a Central Processing Unit (CPU),
microcontroller, or a microprocessor; an Application Specific
Integrated Circuit (ASIC), Programmable Logic Device (PLD), or
Field Programmable Gate Array (FPGA); or circuitry that includes
discrete logic or other circuit components, including analog
circuit components, digital circuit components or both; or any
combination thereof. The circuitry may include discrete
interconnected hardware components and/or may be combined on a
single integrated circuit die, distributed among multiple
integrated circuit dies, or implemented in a Multiple Chip Module
(MCM) of multiple integrated circuit dies in a common package, as
examples.
[0212] The circuitry may further include or access instructions for
execution by the circuitry. The instructions may be stored in a
tangible storage medium that is other than a transitory signal,
such as a flash memory, a Random Access Memory (RAM), a Read Only
Memory (ROM), an Erasable Programmable Read Only Memory (EPROM); or
on a magnetic or optical disc, such as a Compact Disc Read Only
Memory (CDROM), Hard Disk Drive (HDD), or other magnetic or optical
disk; or in or on another machine-readable medium. A product, such
as a computer program product, may include a storage medium and
instructions stored in or on the medium, and the instructions when
executed by the circuitry in a device may cause the device to
implement any of the processing described above or illustrated in
the drawings.
[0213] The implementations may be distributed as circuitry among
multiple system components, such as among multiple processors and
memories, optionally including multiple distributed processing
systems. Parameters, databases, and other data structures may be
separately stored and managed, may be incorporated into a single
memory or database, may be logically and physically organized in
many different ways, and may be implemented in many different ways,
including as data structures such as linked lists, hash tables,
arrays, records, objects, or implicit storage mechanisms. Programs
may be parts (e.g., subroutines) of a single program, separate
programs, distributed across several memories and processors, or
implemented in many different ways, such as in a library, such as a
shared library (e.g., a Dynamic Link Library (DLL)). The DLL, for
example, may store instructions that perform any of the processing
described above or illustrated in the drawings, when executed by
the circuitry.
[0214] Various implementations have been specifically described.
However, many other implementations are also possible. Further,
various aspects of the disclosed method and apparatus have been
described as stages with an assigned number. It is to be noted that
these descriptive aspects are included solely for discussion and no
further conclusion is to be drawn. The operations associated with
each stage or other descriptive aspect may be performed in any
suitable order and no other ordering is intended or implied. It is
therefore intended that the foregoing detailed description be
regarded as illustrative rather than limiting, and that it be
understood that it is the following claims, including all
equivalents, that are intended to define the spirit and scope of
this invention.
* * * * *