U.S. patent application number 14/629511 was filed with the patent office on 2016-08-25 for aggregation mechanism for moving vehicles.
The applicant listed for this patent is Cisco Technology, Inc.. Invention is credited to Matt Silverman, Paul Stager, Evgeny YANKEVICH.
Application Number | 20160249398 14/629511 |
Document ID | / |
Family ID | 56693906 |
Filed Date | 2016-08-25 |
United States Patent
Application |
20160249398 |
Kind Code |
A1 |
YANKEVICH; Evgeny ; et
al. |
August 25, 2016 |
AGGREGATION MECHANISM FOR MOVING VEHICLES
Abstract
In one embodiment, a method implemented on an access point of a
wireless communication system includes: determining an estimate for
a relative velocity between a mobile wireless client device and the
access point, the mobile wireless client device communicating
wirelessly with the access point over a channel; determining a
channel coherence time for the channel using said estimated
relative velocity; and determining a maximum aggregated frame size
based on the determined channel coherence time.
Inventors: |
YANKEVICH; Evgeny;
(Beachwood, OH) ; Stager; Paul; (Akron, OH)
; Silverman; Matt; (Shaker Heights, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cisco Technology, Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
56693906 |
Appl. No.: |
14/629511 |
Filed: |
February 24, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 28/06 20130101;
H04W 4/023 20130101; H04W 76/15 20180201; H04W 4/027 20130101; H04L
27/2602 20130101; H04L 27/2646 20130101; H04L 25/0222 20130101;
H04L 1/0007 20130101 |
International
Class: |
H04W 76/02 20060101
H04W076/02; H04W 72/04 20060101 H04W072/04; H04W 4/02 20060101
H04W004/02 |
Claims
1. A method implemented on an access point of a wireless
communication system, said method comprising: determining an
estimate for a relative velocity between a mobile wireless client
device and said access point, said mobile wireless client device
communicating wirelessly with said access point over a channel;
determining a channel coherence time for said channel using said
estimated relative velocity; and determining a maximum aggregated
frame size based on said determined channel coherence time.
2. The method of claim 1, further comprising adjusting a packet
aggregation process according to said determined maximum aggregated
frame size for a next transmission between said mobile wireless
client device and said access point.
3. The method of claim 1, wherein said determining an estimate for
a relative velocity is based at least on a signal strength
indication received from said mobile wireless client device.
4. The method of claim 3, further comprising: providing estimates
of parameters relevant to said mobile wireless client device using
said received signal strength indication, said parameters
comprising: an instant velocity of said mobile wireless client
device; an angle between said mobile wireless client device and
said access point; a distance between said mobile wireless device
and said access point; and a position of said mobile wireless
client device relative to said access point; and determining an
estimate of a relative velocity between said mobile wireless client
device and said access point using said estimates.
5. The method of claim 1, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregated
frame size based on said determined channel coherence time for a
particular modulation and coding scheme, channel bandwidth and
number of spatial streams used by said mobile wireless client
device to communicate wirelessly with said access point.
6. The method of claim 5, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregate
medium access control protocol data unit for a short guard interval
using said determined channel coherence time.
7. The method of claim 5, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregate
medium access control protocol data unit for a long guard interval
using said determined channel coherence time.
8. The method of claim 5, further comprising: determining a maximum
physical layer conversion protocol data unit frame size using said
determined channel coherence time; and determining a maximum
aggregate medium access control protocol data unit for a short
guard interval using said determined maximum physical layer
conversion protocol data unit frame size.
9. The method of claim 5, further comprising: determining a maximum
physical layer conversion protocol data unit frame size using said
determined channel coherence time; and determining a maximum
aggregate medium access control protocol data unit for a long guard
interval using said determined maximum physical layer conversion
protocol data unit frame size.
10. The method of claim 1, further comprising: computing a
plurality of channel coherence times, each of said computed channel
coherence times corresponding to a particular relative velocity
between a mobile wireless client device and said access point;
storing said plurality of computed channel coherence times at said
access point; and wherein said determining a channel coherence time
for said channel comprises retrieving a stored channel coherence
time corresponding to said estimated relative velocity.
11. The method of claim 10, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregated
frame size based on said retrieved channel coherence time for a
particular modulation and coding scheme, channel bandwidth and
number of spatial streams used by said mobile wireless client
device to communicate wirelessly with said access point.
12. The method of claim 11, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregate
medium access control protocol data unit for a short guard interval
using said retrieved channel coherence time.
13. The method of claim 11, wherein said determining a maximum
aggregated frame size comprises determining a maximum aggregate
medium access control protocol data unit for a long guard interval
using said retrieved channel coherence time.
14. The method of claim 11, wherein: said computing further
comprises computing a plurality of maximum physical layer
conversion protocol data unit frame sizes, each of said maximum
physical layer conversion protocol data unit frame sizes being
computed using a corresponding computed channel coherence time;
said storing further comprises storing said plurality of computed
maximum physical layer conversion protocol data unit frame sizes;
and wherein said determining a channel coherence time for said
channel further comprises retrieving a stored maximum physical
layer conversion protocol data unit frame size corresponding to
said estimated relative velocity.
15. The method of claim 14, wherein said determining a maximum
aggregate medium access control protocol data unit comprises
determining a maximum aggregate medium access control protocol data
unit for a short guard interval using said retrieved maximum
physical layer conversion protocol data unit frame size.
16. The method of claim 14, wherein said determining a maximum
aggregate medium access control protocol data unit comprises
determining a maximum aggregate medium access control protocol data
unit for a long guard interval using said retrieved maximum
physical layer conversion protocol data unit frame size.
17. An access point controller comprising a processor, said
processor being operable to: determine an estimate for a relative
velocity between a mobile wireless client device and said access
point, said mobile wireless client device communicating wirelessly
with said access point over a channel; determine a channel
coherence time for said channel using said estimated relative
velocity; and determine a maximum aggregated frame size based on
said determined channel coherence time.
18. The apparatus of claim 17, wherein said access point controller
further comprises a Kalman filler configured to provide estimates
of parameters relevant to said mobile wireless client device using
said received signal strength indication, said parameters
comprising: an instant velocity of said mobile wireless client
device; a distance between said mobile wireless device and said
access point; an angle between said mobile wireless client device
and said access point; and a position of said mobile wireless
client device relative to said access point.
19. The apparatus of claim 17, wherein said mobile wireless client
device is a workgroup bridge located in a moving vehicle, said
workgroup bridge being operable to serve one or more mobile client
devices without wireless network connectivity located in said
moving vehicle.
20. One or more computer readable tangible storage media encoded
with software comprising computer executable instructions and when
the software is executed operable to: determine an estimate for a
relative velocity between a mobile wireless client device and an
access point, said mobile wireless client device communicating
wirelessly with said access point over a channel; determine a
channel coherence time for said channel using said estimated
relative velocity; and determine a maximum aggregated frame size
based on said determined channel coherence time.
Description
TECHNICAL FIELD
[0001] The present disclosure generally relates to methods and
apparatus to estimate a channel coherence time and a maximum
aggregation frame size in wireless communication systems.
BACKGROUND
[0002] Channel coherence time describes the time duration during
which characteristics of a wireless channel do not vary. Knowledge
of the channel coherence time or the detection of a sudden change
in the channel state may be used to improve the efficiency of
several functions in a wireless system. Indeed, the length of the
aggregated packets is usually less than the channel coherence time
to ensure that data are successfully exchanged (transmitted and/or
received) between wireless client devices and access points.
[0003] Mobility of the wireless client devices is another factor
which reduces the channel coherence lime. Therefore, when a
wireless client device is mobile relative to the access points, a
minimal packet aggregation length suitable for all velocities, or
non-aggregation at all, is used to ensure proper/successful data
exchanges.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] The present invention will be understood and appreciated
more fully from the following detailed description, taken in
conjunction with the drawings in which:
[0005] FIG. 1 is a simplified block diagram illustration of a
wireless client device and an access point of a wireless
communication network, constructed and operative in accordance with
an embodiment of the present invention;
[0006] FIG. 2 is a simplified block diagram illustration of an
access point controller, constructed and operative in accordance
with an embodiment of the present invention; and
[0007] FIG. 3 is a simplified flow chart diagram of a method for
determining a maximum aggregate frame size in accordance with
embodiments of the present invention.
DESCRIPTION OF EXAMPLE EMBODIMENTS
Overview
[0008] In one embodiment, a method implemented on an access point
of a wireless communication system includes: determining an
estimate for a relative velocity between a mobile wireless client
device and the said access point, the mobile wireless client device
communicating wirelessly with the access point over a channel;
determining a channel coherence time for the channel using the
estimated relative velocity; and determining a maximum aggregated
frame size based on the determined channel coherence time.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0009] Reference is now made to FIG. 1, which shows a wireless
communication system or network constructed and operative in
accordance with an embodiment of the present invention. The
wireless communication network 100 comprises a moving vehicle 110
such as, but not limited to, a train moving along a track 120.
Wireless local area network access points (APs) are deployed along
the path of the track 120 to provide network connectivity to end
users located in the moving vehicle 110. For the sake of simplicity
of depiction, only one AP 130 is shown in FIG. 1 although those
skilled in the art will appreciate that a plurality of APs 130 are
typically provided at regular intervals along the path of the track
120. The AP 130 is connected to a wired local area network (WLAN)
and is operable to communicate wirelessly over channels with one or
more wireless client devices 111 located in the moving vehicle
110.
[0010] The wireless client device 111 may be any type of electronic
device having wireless connectivity such as, but not limited to, a
laptop computer, a tablet computer, a smartphone, a mobile
computing device or any other handheld device. Alternatively and/or
additionally, the wireless client device 111 may be a workgroup
bridge (WGB) device which operates as a mobile wireless client
device with respect to the trackside AP 130. In such a situation,
the WGB device is connected to a switch to serve client devices
inside the moving vehicle 110 which do not have wireless network
connectivity.
[0011] The AP 130 and the one or more wireless client devices 111
comprises one or more antennas for communicating over channels
using a wireless communication protocol, such as an IEEE 802.11
wireless communication protocol. IEEE 802.11 technology is evolving
from 802.11a/b/g to 802.11n/ac in order to meet the high-throughput
demand of different wireless client devices. To achieve high
throughput between an access point and a wireless client device,
IEEE 802.11n introduced two frame aggregation schemes. One of these
schemes, known as A-MPDU (aggregate Medium Access Control Protocol
Data Unit), amortizes the physical (PHY) protocol overhead over
multiple frames by packing several MPDUs into a single A-MPDU. IEEE
802.11ac uses the same frame aggregation scheme but extends the
maximum frame size. Also, all the frames transmitted use the A-MPDU
scheme. The maximum frame size of a PPDU (Physical layer conversion
Protocol Data Unit typically comprising a PHY header, a preamble
and an A-MPDU) is further dependent on the channel coherence time
for the IEEE 802.11n and ac wireless networking standards.
[0012] The channel coherence time may be defined as the time
interval during which the wireless channel parameters do not vary,
or vary by an insignificant amount. For moving vehicles, such as
the moving vehicle 110, the parameters of the channel are defined
by the Doppler frequency spread which is a function of the relative
velocity between the wireless client device 111 and the AP 130.
[0013] FIG. 1 shows a moving vehicle 110 moving in the direction of
arrow 112 associated with the wireless client device 111. In such a
configuration, the Doppler frequency shift (F.sub.0) of the
transmission due to the movement of the wireless client device 111
relative to the AP 130 may be approximated by the following
equation:
F D = V .lamda. c cos .alpha. = V F c c cos .alpha.
##EQU00001##
[0014] where: [0015] V denotes the instant velocity of the moving
vehicle 110; [0016] c denotes the propagation speed of light in
vacuum; [0017] .lamda..sub.c denotes the wavelength of the carrier
frequency; [0018] .alpha. denotes the angle between the vector of
movement (i.e. arrow 112) and the AP 130; and [0019] F.sub.c
denotes the carrier frequency. Hence, the Doppler frequency shift
reaches its maximum for a angle values of 0 and 180 degrees. Also,
the Doppler frequency shift reaches its minimum for an a angle
value of 90 degrees. For a wireless client device 111 moving at a
velocity of 200 mph and a carrier frequency of 6 GHz, the
approximate maximal Doppler shift value would be 1.78 KHz. In the
worst case scenario in which the transmission reflects from another
moving vehicle coming in the opposite direction, the maximal
Doppler shift value will double to 3.55 KHz. Although there is no
specific definition for the channel coherence time (T.sub.c), is
known to be inversely proportional to the maximum Doppler frequency
spread (F.sub.o.sub.MAX):
[0019] T c .apprxeq. 1 F D MAX ##EQU00002##
To achieve a proper and successful transmission 1 reception, the
duration of the PPDU is typically less than the channel coherence
time. Therefore, the channel coherence time (and consequently the
maximum frame size of the PPDU) is dependent on the position and
velocity of the wireless client device 111 relative to the AP 130.
In order to ensure an optimal quality of service, the AP 130
comprises a controller 140 which is configured to provide estimates
of the maximum frame size of the PPDU as well as the maximum frame
size of the A-MPDU. The AP 130 is then operable to use these
estimates to dynamically aggregate packets that are to be
transmitted to the wireless client device 111 over the channel.
[0020] Reference is now made to FIG. 2, which is a simplified block
diagram illustration of a controller constructed and operative in
accordance with an embodiment of the present invention. The
controller 140 typically comprises an input/output (I/O)
communication interface 141, a processor 142, a mobility estimator
143 and a memory 144.
[0021] The I/O communication interface 141 may be any suitable
communication interface providing a two-way coupling enabling the
AP 130 and the controller 140 to exchange data. The I/O
communication interface 141 is further operable to pass data
received from the AP 130 to the processor 142, the mobility
estimator 143 or to the memory 144 for later use.
[0022] The processor 142 is a microcontroller or a microprocessor,
for example, configured to execute software instructions stored in
memory 144.
[0023] The mobility estimator component 143 may include a filer,
such as for example, but not limited to, a Kalman filter which is
configured to provide estimates of different parameters relevant to
the wireless client device 111 based at least on a received signal
strength indication (RSSI). Non-limiting examples of these
parameters include, but are not limited to: instant velocity of the
wireless client device 111; distance between the wireless client
device 111 and the AP 130; angle between the vector of movement
(i.e. arrow 112) associated with the wireless client device 111 and
the AP 130; and coordinates of the wireless client device 111 in
the coordinate system of the AP 130. These estimates may then be
passed to the processor 142 for immediate processing and/or stored
in memory 144 to be processed at a later time.
[0024] The memory 144 may comprise read only memory (ROM), random
access memory (RAM), magnetic disk storage media devices, optical
storage media devices, flash memory devices, electrical, optical,
or other physical/tangible (e.g. non-transitory) memory storage
devices. Thus, in general, memory 144 may comprise one or more
computer readable storage media (e.g. memory device) encoded with
software comprising computer executable instructions and when the
software is executed (by the processor 142) it is operable to
perform the operations described hereinbelow. For example, memory
144 stores or is encoded with instructions for: [0025] determining
a relative velocity between the wireless client device 111 and the
AP 130; [0026] determining a channel coherence time; [0027]
determining a maximum frame size of the PPDU and/or the A-MPDU; and
[0028] controlling the packet aggregation process.
[0029] The memory 144 is further operable to store information/data
including, but not limited to, AP neighbor lists and neighbor
tables, aggregation length tables, regulatory domain channel
definitions. Dynamic Frequency Selection (DFS) channel definitions,
MAC addresses, Internet Protocol (IP) addresses, Modulation and
Coding Schemes (MCS) index values, etc.
[0030] Reference is now made to FIG. 3, which is a flow chart
diagram of a method for estimating an A-MPDU frame size operative
in accordance with an embodiment of the present invention.
[0031] The process starts at step 310 where the mobility estimator
143 provides estimates of different parameters relevant to the
wireless client device 111 based at least on a received signal
strength indication (RSSI). Typically, the mobility estimator 143
receives a RSSI relevant to the transmission received from the
wireless client device 111 as input. In response, the mobility
estimator 143 which may include a liter such as, but not limited
to, a Kalman filter, returns estimates of at least one or more of
the following parameters; the instant velocity of the wireless
client device 111; the distance between the wireless client device
111 and the AP 130; the angle between the vector of movement (i.e.
arrow 112) associated with the wireless client device 111 and the
AP 130; and the coordinates of the wireless client device 111 in
the coordinate system of the AP 130. The x and y coordinates of the
wireless client device 111 are typically returned by the Kalman
filter. Alternatively, the x-coordinate may be returned by the
Kalman filter while the y-coordinate, corresponding to the distance
between the AP 130 and the path 120 may already be known by or
provided to the AP 130. Based on these estimated parameters, the
mobility estimator 143 is further configured to determine the
relative velocity between the wireless client device 111 and the AP
130. Alternatively, the processor 142 is configured to execute a
relevant set of instructions stored in memory 144, which when
executed return the relative velocity using the estimates provided
by the mobility estimator 143. This is achieved using the following
mathematical equations:
cos .alpha. .ident. x c D ##EQU00003##
[0032] where: x.sub.c denotes the estimated x-coordinate of the
wireless client device 111 within the coordinate system of the AP
130; and D is the estimated distance between the wireless client
device 111 and the AP 130; and
V c = V cos .alpha. .ident. V x c D ##EQU00004##
[0033] where: V.sub.c is the relative velocity; and V is the
estimated instant velocity of the wireless client device 111.
[0034] Then, at step 320, the determined relative velocity is used
to determine a channel coherence time. As explained above in
relation to FIG. 1, the channel coherence time T.sub.c is inversely
proportional to the maximum Doppler frequency spread. Therefore,
the channel coherence time T.sub.c may be obtained using the
relative velocity V.sub.c determined in step 310. The following
table (Table 1) gives some examples of determined channel coherence
times for different determined relative velocities:
TABLE-US-00001 TABLE 1 Aggregation Length Table Channel Index
Relative velocity coherence time Number of V.sub.c/10 (km/h)
V.sub.c (km/h) T.sub.c (microseconds) OFDM symbols 1 10 18520 4629
2 20 9346 2336 3 30 6252 1562 4 40 4694 1173 5 50 3760 940 6 60
3126 781 7 70 2680 670 8 80 2348 587 9 90 2088 522 10 100 1880 470
11 110 1708 426 12 120 1564 391 13 130 1446 361 14 140 1344 335 15
150 1254 313 16 160 1174 293 17 170 1104 276 18 180 1044 261 19 190
990 247 20 200 940 235 21 210 894 223 22 220 854 213 23 230 816 204
24 240 784 196 25 250 752 188 26 260 724 180 27 270 696 174 28 280
672 168 29 290 648 162 30 300 628 156 31 310 608 151 32 320 588 147
33 330 570 142
[0035] At step 330, the maximum frame size of the PPDU in
Orthogonal Frequency Division Multiplexing (OFDM) symbols may be
determined. Determining the number of OFDM symbols per coherence
time may be easily performed since the duration of each OFDM symbol
is 4 microseconds. The results of this determination for each
determined coherence channel time is illustrated in the last column
of Table 1. Those skilled in the art will appreciate that step 330
may be performed in addition or alternatively to step 320.
[0036] The process then moves to step 340 in which the maximum
A-MPDU frame size is determined for the channel coherence time
T.sub.c determined in step 320 and/or for the maximum PPDU frame
size determined in step 330. This determination is typically
dependent on the MCS, the signal bandwidth (BW) and the number of
spatial streams (NSS) used by the wireless client device 111 to
communicate with the AP 130. The MCS typically defines the PPDU
preamble length (Pr.sub.len) and a constellation of other
parameters including, but not limited to, the number of uncoded
bits per OFDM symbols (N.sub.DBPS). The preamble length further
depends on the number of spatial streams that will be used for the
next signal transmission. The following tables (Table 2 and Table
3), provided for illustration purposes only, shows these different
parameters for each MCS for the 802.11n and 802.11ac wireless
communication protocols:
TABLE-US-00002 TABLE 2 802.11n MCS parameters Number of uncoded
bits per OFDM Preamble length symbols N.sub.DBPS MCS Pr.sub.len
(microseconds) 20 MHz 40 MHz 0 36 26 54 1 36 52 108 2 36 78 162 3
36 104 216 4 36 156 324 5 36 208 432 6 36 234 486 7 36 260 540 8 40
52 108 9 40 104 216 10 40 156 324 11 40 208 432 12 40 312 648 13 40
416 864 14 40 468 972 15 40 520 1080 16 44 78 162 17 44 156 324 18
44 234 486 19 44 312 648 20 44 468 972 21 44 624 1296 22 44 702
1458 23 44 780 1620
TABLE-US-00003 TABLE 3 802.11ac MCS parameters Number of uncoded
bits Preamble length per OFDM symbols N.sub.DBPS MCS Pr.sub.len
(microseconds) 20 MHz 40 MHz 80 MHz 1 SS 0 36 26 54 117 1 36 52 108
234 2 36 78 162 351 3 36 104 216 468 4 36 156 324 702 5 36 208 432
936 6 36 234 486 1053 7 36 260 540 1170 8 36 312 648 1404 9 36 X
720 1560 2 SSs 0 40 52 108 234 1 40 104 216 468 2 40 156 324 702 3
40 208 432 936 4 40 312 648 1404 5 40 416 864 1872 6 40 468 972
2106 7 40 520 1080 2340 8 40 524 1296 2808 9 40 X 1440 3120 3 SSs 0
44 78 162 351 1 44 156 324 702 2 44 234 486 1053 3 44 312 648 1404
4 44 468 972 2106 5 44 624 1296 2808 6 44 702 1458 X 7 44 780 1620
3510 8 44 936 1944 4212 9 44 1040 2160 4680
[0037] Determination of the maximum A-MPDU frame size (in octets)
is therefore computed for a particular MCS, a signal bandwidth and
a specific number of spatial streams using the following
mathematical equations: [0038] for long guard interval and using
the channel coherence time T.sub.c determined in step 320:
[0038] L = [ T c - Pr len LGI _ T OFDM ] N DPBS ( BW , N SS ) 8
##EQU00005## [0039] for short guard interval and using the channel
coherence time T.sub.c determined in step 320:
[0039] L = [ T c - Pr len S GI _ T OFDM ] N DPBS ( BW , N SS ) 8
##EQU00006## [0040] for long guard interval and using the maximum
PPDU frame size determined in step 330
[0040] L = [ N OFDM - Pr len , S LGI _ T OFDM ] N DPBS ( BW , N SS
) 8 ##EQU00007## [0041] for short guard interval and using the
maximum PPDU frame size determined in step 330:
[0041] L = [ N OFDM - Pr len , S SGI _ T OFDM ] N DPBS ( BW , N SS
) 8 ##EQU00008##
[0042] where: [0043] T.sub.c denotes the channel coherence time in
microseconds; [0044] Pr.sub.len denotes the PHY preamble length in
microseconds; [0045] Pr.sub.len,s denotes the PHY preamble length
in OFDM symbols; [0046] N.sub.OFDM denotes the maximum PPDU frame
size in OFDM symbols; [0047] N.sub.DBPS(BW, N.sub.ss) denotes the
number of uncoded data bit per OFDM symbol. This parameter is a
function of the signal bandwidth, the number of spatial streams and
further depends on the supported standard (e.g. 802.11n or
802.11ac); [0048] LGI_T.sub.OFDM denotes the duration of the OFDM
symbol including long guard interval (which is usually equals to 4
microseconds); [0049] SGI_T.sub.OFDM denotes the duration of the
OFDM symbol including short guard interval (which is usually equals
to 3.6 microseconds); [0050] N.sub.ss denotes the number of spatial
streams; and [0051] BW denotes the signal bandwidth.
[0052] Finally, the process moves to step 350 where the controller
140 is configured to adjust the packet aggregation process for the
next transmission according to the maximum A-MPDU frame size
determined in step 340. Once done, the process goes back to step
310 thereby enabling a dynamic control and adjustment of the
maximum A-MDPU frame size during communication between the wireless
client device 111 and the AP 130.
[0053] Although FIG. 3 is described as a real time sequential
process, the calculations for the channel coherence time and/or the
maximum PPDU frame size may be performed in advance and stored in
memory 144 in any suitable form. These calculations may be
performed for a plurality of different relative velocities and
signal characteristics. In this example embodiment of the present
invention, step 310 remains unchanged and provides as output the
determined relative velocity between the wireless client device 111
and the AP 130. Then, the determined relative velocity may be used
as an initial entry index to retrieve the channel coherence time
T.sub.c (step 320) and/or the maximum PPDU frame size (step 330) by
table look-up. Subsequently, the maximum A-MPDU frame size may be
computed for a particular MCS, signal bandwidth and number of
spatial streams used by the mobile wireless client device 111 to
communicate wirelessly with the AP 130. Finally, step 350 remains
unchanged and the controller 140 is configured to adjust the packet
aggregation process for the next transmission according to the
maximum A-MPDU frame size retrieved in step 340. Once done, the
process goes back to step 310 thereby enabling a dynamic control
and adjustment of the maximum A-MDPU frame size during
communication between the wireless client device 111 and the AP
130.
[0054] As explained above in relation to FIG. 1, the moving vehicle
110 may comprise a plurality of wireless cent device 111.
Therefore, the determination of the maximum A-MPDU frame size may
be performed globally (i.e. for a WGB 111 serving a plurality of
client devices 111) or individually (i.e. on a per wireless client
device basis).
[0055] Although the above embodiments have been described in the
context of the 802.11 communication protocol, those skilled in the
art will realize that other implementations are possible.
[0056] It is appreciated that various features of the invention
which are, for clarity, described in the contexts of separate
embodiments may also be provided in combination in a single
embodiment. Conversely, various features of the invention which
are, for brevity, described in the context of a single embodiment
may also be provided separately or in any suitable
subcombination.
[0057] It will be appreciated by persons skilled in the art that
the present invention is not limited by what has been particularly
shown and described hereinabove. Rather the scope of the invention
is defined by the appended claims and equivalents thereof:
* * * * *