US20160249398A1 - Aggregation mechanism for moving vehicles - Google Patents

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.

Description

TECHNICAL FIELD
• 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
• 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.
• 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
• The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:
• 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;
• 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
• 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
• 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
• 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.
• 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.
• 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.
• 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.
• 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 (F₀) 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:
• $F_D=\frac{V}{{\lambda }_c}·\cos \alpha =\frac{V·F_c}{c}·\cos \alpha$
• where:
• • V denotes the instant velocity of the moving vehicle 110;
  • c denotes the propagation speed of light in vacuum;
  • λ_c denotes the wavelength of the carrier frequency;
  • α denotes the angle between the vector of movement (i.e. arrow 112) and the AP 130; and
  • F_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_c), is known to be inversely proportional to the maximum Doppler frequency spread (F_{o MAX} ):
• $T_c\approx \frac{1}{F_{D_{\mathrm{MAX}}}}$
• 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.
• 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.
• 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.
• The processor 142 is a microcontroller or a microprocessor, for example, configured to execute software instructions stored in memory 144.
• 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.
• 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:
• • determining a relative velocity between the wireless client device 111 and the AP 130;
  • determining a channel coherence time;
  • determining a maximum frame size of the PPDU and/or the A-MPDU; and
  • controlling the packet aggregation process.
• 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.
• 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.
• 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:
• $\cos \alpha \equiv \frac{x_c}{D}$
• where: x_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
• $V_c=V·\cos \alpha \equiv V·\frac{x_c}{D}$
• where: V_c is the relative velocity; and V is the estimated instant velocity of the wireless client device 111.
• 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_c is inversely proportional to the maximum Doppler frequency spread. Therefore, the channel coherence time T_c may be obtained using the relative velocity V_c determined in step 310. The following table (Table 1) gives some examples of determined channel coherence times for different determined relative velocities:

• TABLE 1
  Aggregation Length Table

  		Channel
  Index	Relative velocity	coherence time	Number of
  Vc/10 (km/h)	Vc (km/h)	Tc (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

• 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.
• The process then moves to step 340 in which the maximum A-MPDU frame size is determined for the channel coherence time T_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_len) and a constellation of other parameters including, but not limited to, the number of uncoded bits per OFDM symbols (N_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 2
  802.11n MCS parameters

  		Number of uncoded
  		bits per OFDM
  	Preamble length	symbols NDBPS

  	MCS	Prlen (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 3
  802.11ac MCS parameters

  		Number of uncoded bits
  	Preamble length	per OFDM symbols NDBPS

  	MCS	Prlen (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

• 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:
• • for long guard interval and using the channel coherence time T_c determined in step 320:
• $L=\left[\frac{T_c-{\mathrm{Pr}}_{\mathrm{len}}}{{\mathrm{LGI}\_T}_{\mathrm{OFDM}}}\right]·\frac{N_{\mathrm{DPBS}}\left(\mathrm{BW},N_{\mathrm{SS}}\right)}{8}$
• • for short guard interval and using the channel coherence time T_c determined in step 320:
• $L=\left[\frac{T_c-{\mathrm{Pr}}_{\mathrm{len}}}{{S\mathrm{GI}\_T}_{\mathrm{OFDM}}}\right]·\frac{N_{\mathrm{DPBS}}\left(\mathrm{BW},N_{\mathrm{SS}}\right)}{8}$
• • for long guard interval and using the maximum PPDU frame size determined in step 330
• $L=\left[\frac{N_{\mathrm{OFDM}}-{\mathrm{Pr}}_{\mathrm{len},S}}{{\mathrm{LGI}\_T}_{\mathrm{OFDM}}}\right]·\frac{N_{\mathrm{DPBS}}\left(\mathrm{BW},N_{\mathrm{SS}}\right)}{8}$
• • for short guard interval and using the maximum PPDU frame size determined in step 330:
• $L=\left[\frac{N_{\mathrm{OFDM}}-{\mathrm{Pr}}_{\mathrm{len},S}}{{\mathrm{SGI}\_T}_{\mathrm{OFDM}}}\right]·\frac{N_{\mathrm{DPBS}}\left(\mathrm{BW},N_{\mathrm{SS}}\right)}{8}$
• where:
• • T_c denotes the channel coherence time in microseconds;
  • Pr_len denotes the PHY preamble length in microseconds;
  • Pr_len,s denotes the PHY preamble length in OFDM symbols;
  • N_OFDM denotes the maximum PPDU frame size in OFDM symbols;
  • N_DBPS(BW, N_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);
  • LGI_T_OFDM denotes the duration of the OFDM symbol including long guard interval (which is usually equals to 4 microseconds);
  • SGI_T_OFDM denotes the duration of the OFDM symbol including short guard interval (which is usually equals to 3.6 microseconds);
  • N_ss denotes the number of spatial streams; and
  • BW denotes the signal bandwidth.
• 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.
• 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_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.
• 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).
• 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.
• 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.
• 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:

Claims (20)

What is claimed is:

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.

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