U.S. patent application number 11/329979 was filed with the patent office on 2007-02-22 for rate adaptation using semi-open loop technique.
Invention is credited to Won-Joon Choi, Jeffrey M. Gilbert, Huanchun Ye, Ning Zhang.
Application Number | 20070041322 11/329979 |
Document ID | / |
Family ID | 36678228 |
Filed Date | 2007-02-22 |
United States Patent
Application |
20070041322 |
Kind Code |
A1 |
Choi; Won-Joon ; et
al. |
February 22, 2007 |
Rate adaptation using semi-open loop technique
Abstract
In a semi-open loop rate adaptation scheme for a multiple-input
multiple-output (MIMO) system, a transmitter can advantageously use
one or more quality metrics of an uplink as well as knowledge of
device characteristics of both ends to perform fast and accurate
rate adaptation.
Inventors: |
Choi; Won-Joon; (Santa
Clara, CA) ; Zhang; Ning; (Santa Clara, CA) ;
Ye; Huanchun; (Santa Clara, CA) ; Gilbert; Jeffrey
M.; (Santa Clara, CA) |
Correspondence
Address: |
BEVER HOFFMAN & HARMS, LLP
2099 GATEWAY PLACE
SUITE 320
SAN JOSE
CA
95110
US
|
Family ID: |
36678228 |
Appl. No.: |
11/329979 |
Filed: |
January 10, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60643459 |
Jan 12, 2005 |
|
|
|
Current U.S.
Class: |
370/235 ;
370/252 |
Current CPC
Class: |
H04B 7/0413 20130101;
H04L 1/0016 20130101; H04L 1/1607 20130101; H04L 1/20 20130101;
G01R 29/26 20130101; H04W 52/24 20130101; H04L 1/0002 20130101;
H04L 47/10 20130101; H04L 25/0204 20130101; H04L 1/0009 20130101;
H04L 47/263 20130101; H04L 1/0003 20130101 |
Class at
Publication: |
370/235 ;
370/252 |
International
Class: |
H04J 1/16 20060101
H04J001/16 |
Claims
1. A method for adapting a rate in a multiple-input multiple-output
(MIMO) system, the system including a first node and a second node
in which transmissions from the first node to the second node are
on a downlink channel and transmissions from the second node to the
first node are on an uplink channel, each node including multiple
transmitters and receivers, the method for the first node
comprising: estimating the uplink channel using a packet sent by
the second node to the first node; transposing the uplink channel
to provide an estimated downlink channel; and using transmitter and
receiver characteristics from the first and second nodes and the
estimated downlink channel to accurately adapt the rate, wherein
the receiver characteristics include a sensitivity of the second
node.
2. The method of claim 1, wherein the sensitivity of the second
node includes a post-detection signal to noise ratio (SNR).
3. The method of claim 1, wherein the transmitter characteristics
include an output power per data rate.
4. The method of claim 1, wherein the transmitter characteristics
include an error vector magnitude (EVM) per output power.
5. The method of claim 1, wherein using the transmitter and
receiver characteristics includes: computing a post-detection
signal to noise ratio (SNR) of the second node based on the
estimated downlink channel, noise floor information from the second
node, and a receiver structure of the second node.
6. The method of claim 5, wherein using the transmitter and
receiver characteristics further includes: adjusting the
post-detection SNR with a transmit output power of the second node
for a received data rate of the packet.
7. The method of claim 6, wherein using the transmitter and
receiver characteristics further includes: after the adjusting,
computing an estimated post-detection SNR for each rate at the
second node using the transmitter power per rate of the first node,
thereby building a sensitivity table for the second node.
8. The method of claim 7, wherein using the transmitter and
receiver characteristics further includes: adjusting the estimated
post-detection SNR for each rate at the second node with a
transmitter EVM per power of the first and second nodes, if the
transmitter EVM is not negligible.
9. The method of claim 7, wherein using the transmitter and
receiver characteristics further includes: choosing an optimized
rate by using the sensitivity table for the second node.
10. The method of claim 9, wherein using the sensitivity table
includes choosing a highest rate whose estimated post-detection SNR
is larger than a threshold SNR.
11. A first node in a multiple-input multiple-output (MIMO) system,
the system including a second node in which transmissions from the
first node to the second node are on a downlink channel and
transmissions from the second node to the first node are on an
uplink channel, each node including multiple transmitters and
receivers, the first node comprising software with
computer-implementable instructions, the first node including:
instructions for estimating the uplink channel using a packet sent
by the second node to the first node; instructions for transposing
the uplink channel to provide an estimated downlink channel; and
instructions for using transmitter and receiver characteristics
from the first and second nodes and the estimated downlink channel
to accurately adapt a rate, wherein the receiver characteristics
include a sensitivity of the second node.
12. The first node of claim 1, wherein the instructions for using
the transmitter and receiver characteristics include: instructions
for computing a post-detection signal to noise ratio (SNR) of the
second node based on the estimated downlink channel, noise floor
information from the second node, and a receiver structure of the
second node.
13. The method of claim 12, wherein the instructions for using the
transmitter and receiver characteristics further include:
instructions for adjusting the post-detection SNR with a transmit
output power of the second node for a received data rate of the
packet.
14. The first node of claim 13, wherein the instructions for using
the transmitter and receiver characteristics further include:
instructions for computing an estimated post-detection SNR for each
rate at the second node using the transmitter power per rate of the
first node, thereby building a sensitivity table for the second
node.
15. The first node of claim 14, wherein the instructions for using
the transmitter and receiver characteristics further include:
instructions for adjusting the estimated post-detection SNR for
each rate at the second node with a transmitter EVM per power of
the first and second nodes, if the transmitter EVM is not
negligible.
16. The first node of claim 14, wherein the instructions for using
the transmitter and receiver characteristics further include:
instructions for choosing an optimized rate by using the
sensitivity table for the second node.
17. The first node of claim 16, wherein the instructions for using
the sensitivity table include instructions for choosing a highest
rate whose estimated post-detection SNR is larger than a threshold
SNR.
18. A node in a multiple-input multiple-output (MIMO) system, the
node comprising: a first table that indicates a post-detection SNR
for rates at another node in the MIMO system.
19. The node of claim 18, further including a second table that
indicates transmitter output power per rate at the node.
20. The node of claim 19, further including a third table that
indicates a transmitter EVM per power of the node and the other
node.
21. The node of claim 20, further including software with
computer-implementable instructions for accessing at least the
first and second tables.
22. The node of claim 20, further including software with
computer-implementable instructions for accessing the first,
second, and third tables.
Description
RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional Patent
Application 60/643,459, entitled "Rate Adaptation Using Semi-Open
Loop Techniques" filed Jan. 12, 2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to rate adaptation in a
wireless environment and in particular to using a semi-open loop
technique to achieve an optimized rate.
[0004] 2. Related Art
[0005] Because the condition of a channel in a wireless environment
varies over time, rate adaptation can be advantageously used to
achieve optimized throughput in a system with multiple PHY (i.e.
physical device) rates. Rate adaptation is especially important in
a multiple-input multiple-output (MIMO) system because the number
of streams introduces yet another dimension to the channel
condition. In general, there are two categories of rate adaptation
techniques: a closed-loop rate adaptation and an open-loop rate
adaptation.
[0006] In the closed-loop rate adaptation, the intended receiver
estimates some function of its receive signal (e.g. the channel
state information (CSI)), and sends it back to the transmitter. The
transmitter determines the optimized rate for its next transmission
based on the feedback from the receiver. Unfortunately, this
closed-loop rate adaptation has significant system overhead
associated with determining the appropriate feedback.
[0007] In the open-loop rate adaptation, the transmitter uses trial
and error to determine an optimized rate. Thus, the open-loop rate
adaptation scheme does not incur any feedback overhead. However,
because the transmitter receives no feedback from the receiver, the
rate is typically slow to change and can result in errors as
incorrect rates are selected.
[0008] Therefore, a need arises for a fast and accurate rate
adaptation technique that minimizes system overhead.
SUMMARY OF THE INVENTION
[0009] A method for quickly and accurately adapting a rate in a
multiple-input multiple-output (MIMO) system while minimizing
system overhead is described. This system can include first and
second nodes in which transmissions from the first node to the
second node are on a "downlink channel" and transmissions from the
second node to the first node are on an "uplink channel". Each node
in the MIMO system can include multiple transmitters and
receivers.
[0010] In this method, the first node can estimate the uplink
channel using a packet sent by the second node to the first node.
This uplink channel can be transposed to provide an estimated
downlink channel. The first node can use transmitter and receiver
characteristics from both the first and second nodes and the
estimated downlink channel to accurately adapt the rate. Notably,
the receiver characteristics can include the sensitivity of the
second node.
[0011] In one embodiment, using the transmitter and receiver
characteristics can include computing a post-detection signal to
noise ratio (SNR) of the second node based on the estimated
downlink channel, noise floor information from the second node, and
a receiver structure of the second node. This post-detection SNR
can be adjusted with a transmit output power of the second node for
a received data rate of the packet. After the adjusting, an
estimated post-detection SNR for each rate at the second node can
be computed using the transmitter power per rate of the first node,
thereby building a sensitivity table for the second node. If the
transmitter EVM is not negligible, then the estimated
post-detection SNR for each rate at the second node can be adjusted
with a transmitter EVM per power of the first and second nodes.
[0012] The first node can use the sensitivity table to choose the
optimized rate. In one embodiment, using the sensitivity table can
include choosing the highest rate whose estimated post-detection
SNR is larger than a threshold SNR.
[0013] A node that can quickly and accurately adapting its rate in
a MIMO system is also described. This node includes various tables
that can be accessed by software with computer-implementable
instructions. Specifically, the node can include a table that
indicates the post-detection SNR for rates at another node in the
MIMO system. The node can further include a table that indicates
transmitter output power per rate at the node as well as a table
that indicates a transmitter EVM per power of the node and the
other node. Notably, the node can further include software with
computer-implementable instructions for accessing the
above-described tables and performing the above-described
steps.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 illustrates a simplified multiple-input
multiple-output (MIMO) system.
[0015] FIG. 2 illustrates one technique that can be used to obtain
the transmit power information per data rate.
[0016] FIG. 3 illustrates a technique for accessing and using a
transmitter EVM versus transmitter power table.
[0017] FIG. 4 illustrates an exemplary technique that can
accurately evaluate the downlink quality of a channel in a MIMO
system.
[0018] FIG. 5 illustrates a node including various tables that can
be accessed by software with computer-implementable
instructions.
DETAILED DESCRIPTION OF THE FIGURES
[0019] In a semi-open loop rate adaptation scheme for a
multiple-input multiple-output (MIMO) system, a transmitter can
advantageously use one or more quality metrics of an uplink as well
as knowledge of transmitter/receiver characteristics of both nodes
to perform fast and accurate rate adaptation. FIG. 1 illustrates a
simplified MIMO system 100 in which the semi-open loop rate
adaptation technique can be used. In MIMO system 100, each
transceiver includes a plurality of transmitters (Txs) and
receivers (Rxs). For example, a first transceiver, referenced as
node 105, can include transmitters 101A and 101B as well as
receivers 102A and 102B. A second transceiver, referenced as node
106, can include transmitters 103A and 103B as well as receivers
104A and 104B. Note that each transmitter/receiver pair, e.g.
transmitter 101A/receiver 102A, shares an antenna.
[0020] MIMO system 100 can divide a data stream into multiple
unique streams. Node 105 can modulate each of these multiple
streams and then simultaneously transmit each stream through a
different antenna in the same frequency channel. By leveraging
multipath, i.e. reflections of the signals, each MIMO receive chain
of node 106 can be a linear combination of the multiple transmitted
data streams. Node 106 can separate these data streams using MIMO
algorithms that rely on estimates of the channels between node 105
and 106.
[0021] For purposes of understanding the semi-loop rate adaptation
technique, a transmission from node 105 to node 106 is referenced
herein as a "downlink" whereas a transmission from node 106 to node
105 is referenced as an "uplink". Note that the terms downlink and
uplink merely describe the signal flow direction in a physical
channel. Notably, the physical channels between node 105 and node
106 are reciprocal (i.e. exhibit the same characteristics) as long
as both downlink and uplink channels use the same frequency. In a
mathematical notation, channel reciprocity is represented by
H.sub.D=H.sub.U.sup.T, where H.sub.D is the downlink channel (i.e.
from node 105 to node 106), and H.sub.U is the uplink channel (i.e.
from node 106 to node 105).
[0022] With channel reciprocity, node 105 can estimate the uplink
channel from the packets sent by node 106, and transpose it to
obtain the downlink channel, as long as the uplink and downlink
packet use the same number of streams. For example, if an ACK
(acknowledgment) packet is used as the uplink packet, then the ACK
packet needs to be sent using the same number of streams as the
downlink packet. (Note that an ACK packet may be sent using a data
rate lower than that used to transmit a data packet. Additionally,
the ACK packet may or may not be sent using the same power that is
typically used for this lower rate.)
[0023] Notably, while the physical channel is reciprocal, the radio
frequency (RF) circuits in nodes 105 and 106 may not be.
Specifically, the optimized rate for the downlink from node 105 to
node 106 should be a function of transmitter 101A/101B, the channel
from node 105 to node 106, and receiver 104A/104B. In contrast, the
optimized rate of the uplink measured at node 105 should be a
function of transmitter 103A/103B, the channel from node 106 to
node 105, and receiver 102A/102B.
[0024] Therefore, in accordance with one aspect of the invention,
node 105 can use the transmitter and receiver characteristics of
both nodes 105 and 106 to estimate the uplink quality and then
compute the equivalent downlink quality. Nodes 105 and 106 can
exchange these transmitter and receiver characteristics initially
and/or periodically.
Transmitter Characteristics
[0025] In one embodiment, the transmitter characteristics can
include the transmitter output power per data rate and the
transmitter EVM (error vector magnitude) per transmitter output
power. With respect to transmitter output power, the power
amplifiers of transmitters 101A/101B (node 105) and 103A/103B (node
106) may be asymmetrical, thereby resulting in different transmit
powers delivered by each node. Moreover, to add complexity to this
asymmetry, the transmit power of a power amplifier can vary per
rate and the tolerance of power amplifier non-linearity can depend
on the data rate as well as power amplifier implementation
specifics. Therefore, to accurately capture the equivalent downlink
quality by estimating the uplink quality, node 105 should know the
transmit power information per data rate for node 106.
[0026] FIG. 2 illustrates one technique 200 that can be used to
obtain the transmit power information per data rate. In step 201,
an initial table of transmitter power per data rate can be
accessed. In one embodiment, this table can include the worst-case
output power vs. rate characteristics. These characteristics can be
determined through lab bench testing, for example. Therefore, in
one embodiment, this information can be created in step 201. In
another embodiment, a vendor can provide this information, thereby
allowing immediate use of the table.
[0027] In step 202, this table can be slowly adapted, if necessary,
based on receiver RSSI (receiver signal strength indicator)
measurements. For example, in one embodiment, the transmit power
for the highest rate can be reduced if an ACK RSSI is more than
enough to improve a transmit EVM.
[0028] In step 203, the transmit power information per data rate
tables at the two nodes can be exchanged. That is, the
downlink/uplink designation shown in FIG. 1 is from the perspective
of node 105. An opposite relationship can be defined from the
perspective of node 106. Thus, steps 201, 202, and 203 can be
performed at each node in the wireless network. In one embodiment,
the transmit power per data rate tables can be exchanged at an
initial link setup. In another embodiment, these tables can be
updated periodically during operation of the wireless network.
Table 1 indicates exemplary transmit powers for various data rates
(referenced as MCS0-MCS7). TABLE-US-00001 TABLE 1 Transmit Power
Per Data Rate Transmit Power Data Rate (MCS) (dBm) MCS0 20 MCS1 20
MCS2 20 MCS3 18 MCS4 18 MCS5 17 MCS6 15 MCS7 14
[0029] With respect to transmitter EVM per transmitter output
power, the transmitter EVM generally depends on the transmit power
due to power amplifier non-linearity. Because transmitter EVM per
transmit power is determined by the characteristics of the power
amplifier and each node can use different power amplifiers,
transmitter EVM information per transmit power can also be
exchanged in one embodiment of the invention.
[0030] FIG. 3 illustrates a technique 300 for accessing and using a
transmitter EVM versus transmitter power table. In step 301, a
transmitter EVM vs. transmitter power table can be accessed. In one
embodiment, the transmitter EVM vs. transmitter power table can be
created during manufacturing.
[0031] Note that this transmitter EVM vs. transmitter power table
can include a temperature variation lookup. To use this temperature
variation lookup, a temperature sensor can be positioned close to
the power amplifier. The temperature difference between the sensor
temperature and the room temperature (or, alternatively, the
temperature at which the manufacturing calibration was done) can be
used to lookup the EVM difference.
[0032] In one embodiment, the information in the transmitter EVM
vs. transmitter power table can include an initial table based on
the calibration temperature, a temperature correction table, and a
current temperature. In one embodiment, part-to-part temperature
variations can be calibrated during manufacturing, and an average
temperature characteristic can be used for all parts. In this
manner, only one temperature correction table, based on average
temperature characteristics, need be generated.
[0033] In another embodiment, step 301 can include a continuous
calibration during operation of the device. For example, if
feedback from the receiver node is supported, then an EVM can be
measured at the receiver node any time a packet is transmitted at
any output power level. In one embodiment, to build a complete
transmitter EVM vs. transmitter power table, the transmissions can
cover all the possible output power levels being used within a
given time window (during which the temperature change is
negligible).
[0034] In step 302, the tables can be exchanged at an initial link
setup between the nodes. In one embodiment, the transmitter EVM vs.
transmitter power table can be updated periodically during
operation of the wireless network.
[0035] Note that the above-described transmitter output power per
data rate table and the transmitter EVM per transmitter output
power table can be combined into a single transmitter EVM per data
rate table. Table 2 indicates EVMs for various data rates
(referenced as MCS0-MCS7). TABLE-US-00002 TABLE 2 EVM Per Data Rate
Transmit power Data Rate (MCS) (dBm) MCS0 -5 MCS1 -10 MCS2 -13 MCS3
-16 MCS4 -19 MCS5 -22 MCS6 -25 MCS7 -27
Receiver Characteristics
[0036] In accordance with one aspect of the invention, the receiver
sensitivity, which can be defined as performance per rate, can also
be exchanged. Note that the receiver architecture can determine the
ease of defining the sensitivity for MIMO systems. In one
embodiment, the SNR per stream can be defined after-an equalizer in
the receiver chain, which is sometimes called "post-detection SNR",
which advantageously measures the effect of the equalizer.
[0037] The post-detection SNR per stream can be calculated from the
channel and the noise floor with a priori knowledge of the MIMO
receiver. For example, if a linear receiver including an MMSE
(minimum mean square error) detector is used, the post-detection
SNR per stream can be derived as follows.
[0038] In the downlink transmission from node 105 to node 106, the
error covariance matrix of the linear MMSE receiver at node 106 can
be defined by the equation:
R.sub.e=.delta..sup.2(H.sub.D*H.sub.D+.delta..sup.2I).sup.-1
[0039] where .delta..sup.2 is the noise variance at a receiver of
node 106.
[0040] The post-detection SNR for a stream i can then be computed
using the equation: SNR i = 1 r e , i - 1 ##EQU1##
[0041] where r.sub.e,i is the i.sup.th diagonal element. (Note that
R.sub.e is an N.times.N matrix where N is the number of streams and
the diagonal elements of the matrix are elements (1,1), (2,2), . .
. (N,N) of R.sub.e.)
[0042] The receiver sensitivity per rate table can be defined as
the post-detection SNR per rate for a given PER (packet error
rate). This table can be obtained through lab bench testing or
updated periodically based on packet error statistics. In one
embodiment, the receiver sensitivity per rate table can be divided
into two parts: (1) post-detection SNR to SNR at the decision (i.e.
the demodulator) device, and (2) the SNR at the decision device per
rate for a given PER. A simple form of the first mapping could be a
linear function with clipping (i.e. y=min(x,y_max), where y_max is
the maximum SNR achievable in the system given the implementation
loss). The second mapping can be obtained by simulations and/or lab
bench testing, and will be updated periodically based on packet
error statistics.
[0043] Note that the SNR at the decision device can be important
because the post-detection SNR may not represent the full effects
of circuit impairments (e.g. dynamic range, phase noise, etc.). The
SNR at the decision device can be measured either by computing EVM
with pilots (known signals) or by computing EVM with the data.
Rate Adaptation
[0044] FIG. 4 illustrates an exemplary technique 400 that node 105
(FIG. 1) can use to evaluate the link quality from node 105 to node
106 (i.e. the downlink quality) by estimating the link quality from
node 106 to node 105 (i.e. the uplink quality). In technique 400,
while relying on channel reciprocity, node 105 can calibrate the
differences in Tx/Rx characteristics between node 105 and node 106
to assess a more accurate downlink quality.
[0045] In step 401, node 105 can estimate the uplink channel using
channel estimation (i.e. CSI) based on the preamble (i.e. training
fields). In step 402, node 105 can transpose the estimated uplink
channel (i.e. by making row elements into column elements and vice
versa) to get the downlink channel. In step 403, node 105 can
compute the post-detection SNR of node 106 based on the downlink
channel, the noise floor information of node 106 (as measure by
node 106 and provided to node 105), and the receiver structure of
node 106 (e.g. like the type of channel equalizer: MMSE equalizer
or ZF equalizer, or another type of structure). In step 404, node
105 can adjust the computed post-detection SNR with the transmitter
output power of node 106 for the received data rate. In step 405,
node 105 can compute the post-detection SNR for each rate at node
106 with the transmitter power per rate table of node 105, thereby
building a sensitivity table for node 106. In step 406 (in one
embodiment, an optional step), node 105 can adjust the estimated
post-detection SNR for each rate at node 106 with the transmitter
EVM per power tables of node 105 and node 106, if necessary (e.g.
when the transmitter EVM is not negligible (e.g. if the EVM is more
than 10 dB below the SNR). In step 407, node 105 can choose the
optimized rate by referring to the post-detection SNR per rate
table of node 106. In one embodiment, the optimized rate is the
highest rate whose estimated post-detection SNR is larger than the
required (i.e. the minimum SNR to get to less than 10% PER).
[0046] FIG. 5 illustrates a node 500 including various tables that
can be accessed by software with computer-implementable
instructions. Specifically, node 500 can include a table 501 that
indicates the post-detection SNR for rates at node 106 (FIG. 1).
This table is also called a sensitivity table herein. Node 500 can
further include a table 502, which indicates transmitter output
power per rate at node 105, as well as a table 503, which indicates
a transmitter EVM per power of nodes 105 and 106. Tables 501, 502,
503 can be stored using any standard memory devices or structures.
Notably, node 500 can further include software 504 with
computer-implementable instructions (residing on a
computer-readable medium) for accessing tables 501, 502, and 503
and performing technique 400 (FIG. 4).
Miscellaneous Probing
[0047] In accordance with one aspect of the invention, probing can
be advantageously used to determine the optimized number of streams
for the MIMO system, the guard intervals to be used for the packets
forming those streams, and the bandwidth (i.e. 20/40 MHz) to be
used.
[0048] The choice of the number of streams can significantly affect
the success of rate adaptation. Notably, conventional channel
estimation can readily determine that reducing the number of
streams is appropriate. However, determining whether increasing the
number of stream is appropriate can be difficult using standard
techniques. In one embodiment, additional channel estimation can be
performed using probes to determine if increasing the number of
streams is appropriate. For example, to obtain more channel
information, a device can periodically probe for a larger number of
streams. As described above, if the uplink packet (e.g. an ACK
packet) always uses the same number of streams as the downlink
packet (e.g. a data packet), then the reverse channel can be
advantageously estimated.
[0049] Orthogonal frequency division multiplexing (OFDM) can
advantageously reduce multipath distortion in a MIMO system.
Specifically, the densely packed subcarriers in the MIMO system are
orthogonal to ensure non-interference even under multipath
conditions. An OFDM symbol includes a fast Fourier transform (FFT)
interval (from which the data is extracted) preceded by a guard
interval. The guard interval can advantageously serve as a
repository for echoes from the previous symbol, thereby preventing
such echoes from adversely affecting the subsequent FFT interval.
In one embodiment, the guard interval can be 800 ns in duration,
which is commensurate with the longest indoor multipath. In another
embodiment, the guard interval can be 400 ns in duration, which is
commensurate with the longest indoor multipath of a home or small
office environment. In yet another embodiment, the guard interval
can be 1600 ns in duration, which is commensurate with the longest
outdoor multipath. As used herein, the terms "half guard interval"
and "full guard interval" refer to the 400 ns and 800 ns
durations.
[0050] Because determining the appropriate guard interval is based
on the operating environment (i.e. the delay spread of the channel)
rather than fading, a rate table of various rates and their
associated guard intervals can be developed over time. That is, the
choice of guard interval can be different for each data rate
because different data rates will have different sensitivities to
multipath. Note that this rate table will depend on the
environment, although the delay spread is assumed to be unchanged
during the period. For example, in contrast to an outdoor
environment, an indoor environment is relatively static.
[0051] In one embodiment, the guard interval choice can be
determined by either measuring the channel flatness (e.g. how
correlated is the channel from one bin to another bin. If the delay
spread is small, then the channel variation is small. For example,
a "0" delay spread channel is flat in the frequency domain. On the
other hand, if the delay spread is large, then the channel varies
significantly from bin to bin) directly (e.g. using channel
estimations) and using this measurement as an index to determine
which rates should use full guard intervals or reduced guard
intervals. In another embodiment, packets can be sent with both
full and reduced guard intervals. At this point, the EVMs
associated with those packets and then the EVM difference between
those two packets can be measured. The EVM difference can be used
to determine if the impact to the EVM is sufficient to preclude the
use of the most effective rates.
[0052] In one embodiment, the receiver that receives the packets
can make this determination and provide feedback in a closed loop
manner to the transmitter. In another embodiment, reciprocity can
be used such that the uplink packets always use the same guard
interval setting. In this case, the transmitter can estimate the
flatness or EVM of the uplink packets when the downlink packets are
sent using a different guard interval.
[0053] According to the IEEE 802.11 family of standards, which
governs wireless communications, each frequency band includes a
predetermined number of frequency channels. For example, the 2.4
GHz frequency band includes 14 channels, wherein each channel when
occupied has a 22 MHz bandwidth and the center frequencies of
adjacent channels are 5 MHz apart. In contrast, 5 GHz frequency
band includes 12 channels, wherein each channel when occupied has a
20 MHz bandwidth and the center frequencies of adjacent channels
are 20 MHz apart.
[0054] Notably, using a wider channel could advantageously increase
capacity, i.e. the transfer rate. Specifically, a 40 MHz channel
always has greater capacity than a 20 MHz channel, and increasingly
so as the signal to noise ratio (SNR) increases. In one embodiment,
the 20/40 MHz decision can be separate from the rate adaptation
determination. There are three modes of operations: 40 MHz, mixed
40 MHz/20 MHz, and 20 MHz.
[0055] If no or minimal interference is present on the extension
channel, then the device can operate in the 40 MHz mode. In this
case, the receiver can perform dynamic 20/40 MHz detection on a
packet-by-packet basis. In this mode, the transmitter can transmit
40 MHz packets unless certain conditions exist (e.g. the MAC times
out due to 40 MHz CCA busy or multi-rate retry to send failing 40
MHz packets at 20 MHz). Note that 6 Mbps and 20 MHz is currently
the last rate in the rate table.
[0056] If weak interference exists on the extension channel, then
the device can operate in the mixed 20/40 MHz mode. In this case,
the receiver can perform dynamic 20/40 MHz detection on a
packet-by-packet basis. Note that although the transmitter can
transmit 20 MHz packets, the carrier frequency can be set as if for
40 MHz transmission.
[0057] If heavy interference exists on the extension channel, then
the device can operate solely in the 20 MHz mode. In this case, the
carrier frequency can be set at the middle of the 20 MHz band, the
receiver only detects 20 MHz packets, and the transmitter transmits
only 20 MHz packets.
[0058] Switching between modes can be based on the long term
sensing of extension channel activity. In one embodiment, switching
between modes can be limited to be between the 40 MHz mode and the
40/20 MHz mixed mode or, alternatively, between the 40/20 MHz mixed
mode and the 20 MHz mode.
[0059] Although illustrative embodiments of the invention have been
described in detail herein with reference to the accompanying
figures, it is to be understood that the invention is not limited
to those precise embodiments. They are not intended to be
exhaustive or to limit the invention to the precise forms
disclosed. As such, many modifications and variations will be
apparent. For example, although a MIMO system is discussed in
detail herein, semi-open technique 400 can be readily suited for
any time division duplex (TDD) system. Accordingly, it is intended
that the scope of the invention be defined by the following Claims
and their equivalents.
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