U.S. patent application number 11/450091 was filed with the patent office on 2007-01-11 for directional antenna physical layer steering for wlan.
Invention is credited to John E. Hoffmann, Kevin P. Johnson, George Rodney JR. Nelson, John A. Regnier.
Application Number | 20070008219 11/450091 |
Document ID | / |
Family ID | 32073344 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070008219 |
Kind Code |
A1 |
Hoffmann; John E. ; et
al. |
January 11, 2007 |
Directional antenna physical layer steering for WLAN
Abstract
A technique for steering a directional antenna such as may be
used in a Wireless Local Area Network (WLAN) device. The technique
detects signal parameters during reception of short sync pulses in
the very beginning portion of a Packet Protocol Data Unit (PPDU)
frame. As a result, the antenna can be steered to an optimum
direction for reception prior to receiving other portions of a
preamble that may be needed to acquire carrier signal phase and
frequency.
Inventors: |
Hoffmann; John E.;
(Indialantic, FL) ; Nelson; George Rodney JR.;
(Merritt Island, FL) ; Regnier; John A.; (Palm
Bay, FL) ; Johnson; Kevin P.; (Palm Bay, FL) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Family ID: |
32073344 |
Appl. No.: |
11/450091 |
Filed: |
June 9, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10675583 |
Sep 30, 2003 |
7061427 |
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11450091 |
Jun 9, 2006 |
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60414947 |
Sep 30, 2002 |
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60415847 |
Oct 3, 2002 |
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Current U.S.
Class: |
342/367 |
Current CPC
Class: |
H01Q 1/2291 20130101;
H01Q 1/1257 20130101; H01Q 21/005 20130101 |
Class at
Publication: |
342/367 |
International
Class: |
H04B 7/00 20060101
H04B007/00 |
Claims
1. A method for controlling a directional angle of a steerable
antenna array, the method comprising the steps of: (a) configuring
the array in an omnidirectional mode; (b) determining a metric from
a synchronization pulse contained in a radio signal received by the
array; (c) steering the array to a candidate angle based on the
determined metric; (d) repeating steps (b) and (c) for at least one
additional candidate angle; and (e) selecting a candidate angle
based on the determined metrics.
2. A method as defined in claim 1 wherein the radio signal
comprises short synchronization pulses and long synchronization
pulses and the synchronization pulse from which the metric is
determined is a short synchronization pulse.
3. A method as defined in claim 1 further comprising: correlating a
portion of the synchronization pulse against an expected version of
that portion to provide a measure of how well the pulse has been
received.
4. A method as defined in claim 3 wherein the portion of the
synchronization pulse is a first half of the pulse.
5. A method as defined in claim 1 further comprising: swapping real
and imaginary samples of a portion of the synchronization pulse to
provide a response of the antenna array to the radio signal.
6. A method as defined in claim 5 wherein the metric is determined
from the provided response.
7. A method as defined in claim 5 wherein the portion of the
synchronization pulse is a second half of the pulse.
8. A method as defined in claim 1 wherein the metric is determined
from a portion of the synchronization pulse.
9. A method as defined in claim 1 wherein steps (b) and (c) are
performed over a first portion of the synchronization pulse and
step (d) is performed over a second portion of the synchronization
pulse.
10. A method as defined in claim 9 wherein the first portion is a
first half of the synchronization pulse and the second portion is a
second half of the synchronization pulse.
11. An apparatus comprising: a steerable antenna array configured
to receive a radio signal; and a processor configured to: (a)
configure the array in an omnidirectional mode, (b) determine a
metric from a synchronization pulse contained in a radio signal
received by the array, (c) steer the array to a candidate angle
based on the determined metric, (d) repeat (b) and (c) for at least
one additional candidate angle, and (e) select a candidate angle
based on the determined metrics.
12. An apparatus as defined in claim 11 wherein the radio signal
comprises short synchronization pulses and long synchronization
pulses and the synchronization pulse from which the metric is
determined is a short synchronization pulse.
13. An apparatus as defined in claim 11 wherein the processor is
further configured to: correlate a portion of the synchronization
pulse against an expected version of that portion to provide a
measure of how well the pulse has been received.
14. An apparatus as defined in claim 11 wherein the processor is
further configured to: swap real and imaginary samples of a portion
of the synchronization pulse to provide a response of the antenna
array to the radio signal.
15. An apparatus as defined in claim 14 wherein the metric is
determined from the provided response.
16. An apparatus as defined in claim 11 wherein (b) and (c) are
performed over a first portion of the synchronization pulse and (d)
is performed over a second portion of the synchronization
pulse.
17. An apparatus as defined in claim 16 wherein the first portion
is a first half of the synchronization pulse and the second portion
is a second half of the synchronization pulse.
18. An apparatus comprising: (a) means for configuring the array in
an omnidirectional mode; (b) means for determining a metric from a
synchronization pulse contained in a radio signal received by the
array; (c) means for steering the array to a candidate angle based
on the determined metric; (d) means for repeating steps determining
a metric and steering the array for at least one additional
candidate angle; and (e) means for selecting a candidate angle
based on the determined metrics.
19. An apparatus as defined in claim 18 wherein the radio signal
comprises short synchronization pulses and long synchronization
pulses and the synchronization pulse from which the metric is
determined is a short synchronization pulse.
20. An apparatus as defined in claim 18 further comprising: means
for correlating a portion of the synchronization pulse against an
expected version of that portion to provide a measure of how well
the pulse has been received.
21. An apparatus as defined in claim 18 further comprising: means
for swapping real and imaginary samples of a portion of the
synchronization pulse to provide a response of the antenna array to
the radio signal.
22. An apparatus as defined in claim 21 wherein the metric is
determined from the provided response.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/675,583, filed on Sep. 30, 2003, which claims the benefit of
U.S. Provisional Application No. 60/414,947 filed Sep. 30, 2002 and
U.S. Provisional Application No. 60/415,847 filed Oct. 3, 2002. The
entire teachings of the above applications are incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] Wireless Local Area Network (WLAN) equipment continues to be
used as a solution for many different data connectivity
applications. WLANs are now viewed as an ideal solution for
providing access to wireless equipped personal computers within
home networks, mobile access to laptop computers and personal
digital assistants (PDAs), as well as providing robust and
convenient access in business applications.
[0003] Indeed, at the present time many laptop computers are
shipped from the factory with WLAN interface cards. Certain
microprocessor manufacturers, such as Intel, have also announced
intentions to incorporate WLAN capability directly into processor
chip platforms. These and other initiatives will continue to drive
the integration of WLAN equipment into personal computers of all
types.
[0004] It is already the case that in many cities, WLAN access
equipment operating in accordance with the IEEE 802.11a, 802.11b,
and 802.11g standards is in wide use. In these cities one can now
find "hot spots" that provide network connectivity. Unfortunately,
having tens, if not hundreds, of closely spaced wireless networks
using the same radio spectrum means that interference becomes a
problem. That is, although the 802.11 standards provide for robust
signaling in the form of spread spectrum radio frequency
modulation, and using orthogonal frequency division multiplexing
over modulated subcarriers, crowding of the radio spectrum still
increases noise and therefore decreases performance for all
users.
[0005] It is recognized that directional antenna arrays can be used
to steer radio frequency energy between a transmitter and receiver.
This greatly reduces the amount of interference that would
otherwise be created for concurrent users of the spectrum. The use
of such arrays in wireless subscriber equipment has been described
in U.S. Pat. No. 6,100,843 entitled "Adaptive Antenna for Use in
Same Frequency Networks"; U.S. Pat. No. 6,400,317 entitled "Methods
and Apparatus for Antenna Control in a Communications Network"; and
in U.S. Pat. No. 6,473,036 entitled "Method Apparatus for Adapting
Antenna Array to Reduce Adaptation Time While Increasing Array
Performance". Each of these patents is assigned to Tantivity
Communications, Inc., the assignee of the present application.
[0006] However, WLAN signaling has special considerations in that
communication is expected to be on a peer-to-peer basis with
extremely short packet lengths. It has heretofore been thought
quite difficult to require WLAN subscriber equipment to steer an
antenna array, to one of many possible candidate angles, during
such very short intervals.
SUMMARY OF THE INVENTION
[0007] The present invention is a technique for implementing an
antenna steering at the physical layer of a Wireless Local Area
Network (WLAN) device. Implementing the antenna steering decision
at the physical layer eliminates involving higher communication
layers, which would otherwise require modification of standardized
communication processing software, such as the Media Access Control
(MAC) or Link layers.
[0008] In one embodiment, the invention provides techniques for
signal detection during short sync symbol reception in the very
beginning of a preamble portion of a WLAN frame. Specifically, in
the context of an 802.11a or 802.11g Packet Protocol Data Unit
(PPDU) frame (packet), this may be concluded within only a few
initial training sequence symbols of the Physical Layer Convergent
Procedure (PLCP) preamble portion. Operating very quickly during
these so-called short sync pulses, the antenna will be steered to
an optimum direction prior to receiving other portions of the
preamble. This permits the radio receiver equipment to use the
remainder of the preamble to acquire carrier phase lock and
frequency synchronization, in just about the same manner as if no
directional antennal were present. The remaining preamble portions
can thus be processed according to standard WLAN frame
processing.
[0009] One specific technique employed is to set an antenna array
to an omni-directional mode prior to reception of the first short
sync pulse. This permits Automatic Gain Control (AGC) circuitry in
the receiver to track for an initial short sync pulse. During
reception of the next one or two short sync pulses, a signal metric
such as a correlation is used to evaluate the observed response
against an expected response. The expected response can either be a
stored response that is the optimum expected for a short sync.
Alternatively, the expected response can be a stored version of a
measured response received with an omni setting during the initial
short sync pulse.
[0010] In accordance with certain other aspects of the invention,
correlations can be performed over a first and second half of a
short sync pulse by swapping real and imaginary samples. This
provides twice as many candidate angles to be tested for each
subsequent short sync pulse.
[0011] With either of these two techniques, by the time of arrival
of the fourth short sync pulse, the antenna array has been steered
to a candidate direction. This provides at least five to six
additional short sync pulses that may be used by the receiver to
acquire frequency and phase lock.
[0012] A third technique involves the use of finite impulse
response comb filtering. This may be performed through the use of
inverse Fast Fourier Transforms. The process here is to implement
an ideal comb type filter response for both signal and noise and
then convolve it with the received short sync signal. An
approximate estimate of a signal to noise ratio can be derived as a
ratio of observed signal and noise filter responses. The candidate
angle exhibiting the strongest signal to noise ratio is then
selected to be used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0014] FIG. 1 is a block diagram of a typical wireless local area
network (WLAN) receiver showing the location of implementation of
an antenna steering algorithm according to the present
invention.
[0015] FIG. 2 is a high level diagram of a Packet Protocol Data
Unit (PPDU) used in an 802.11a or 802.11g network.
[0016] FIG. 3 is a more detailed view of the preamble portion of
the header.
[0017] FIG. 4 is a time domain representation of the real and
imaginary portions of a PLCP preamble or "short sync" pulse.
[0018] FIG. 5 is a more detailed view of the short sync pulse
showing the real and imaginary parts, as well as a magnitude
portion.
[0019] FIG. 6 is a frequency domain plot of the magnitude of the
short sync pulse.
[0020] FIG. 7 is a three-dimensional view showing the frequency to
main amplitude and phase response of the short sync pulse in the
frequency domain.
[0021] FIG. 8 is another representation of the preamble portion of
a PPDU.
[0022] FIG. 9 is a time domain plot of a long sync pulse portion of
the Physical Layer Convergent Procedure (PLCP) preamble.
[0023] FIG. 10 is a plot of magnitude in the frequency domain for
the long sync pulse.
[0024] FIG. 11 is a frequency domain amplitude and phase diagram
for the long sync pulse.
[0025] FIG. 12 is a high level structured English description of
one embodiment of the physical layer steering algorithm.
[0026] FIG. 13 is a structured English description of a second
embodiment.
[0027] FIG. 14 is a structured English description of a third
embodiment of the steering algorithm.
DETAILED DESCRIPTION OF THE INVENTION
[0028] A description of preferred embodiments of the invention
follows.
[0029] The present invention is implemented as an antenna steering
algorithm typically in the base band physical layer signal
processor of a Wireless Local Area Network (WLAN) receiver.
Specifically, the invention involves various techniques to try
candidate antenna settings in response to receiving one or more
very short duration synchronization pulses that typically make up
an initial portion of a preamble. A metric is used to evaluate the
candidate responses, and an antenna setting is then stabilized for
reception of the remaining portions of the preamble as well as the
traffic portion of a protocol data unit (frame). The invention thus
does not require modification of higher layer processing components
such as the Media Access Control (MAC) layer to perform antenna
optimization for each received packet.
[0030] FIG. 1 illustrates a block diagram of a Wireless Local Area
Network (WLAN) transceiver which includes a directional antenna
110, antenna controller 120, band select filter 130, Radio
Frequency/Intermediate Frequency (RF/IF) circuitry 140, associated
amplifiers 132, 133 and switches 131, channel select filter 145,
associated switches 142, 148, Intermediate Frequency/Base Band
(IF/BB) circuits 160, Base Band processor 170, and Media Access
Control (MAC) layer processor 180.
[0031] The band select 130, RF/IF 140 and IF/BB 160 operate in
conjunction with the base band processor 170, in accordance with
known techniques, to implement the physical layer (PHY) of the WLAN
protocol. For example, these components may implement a physical
layer such as specified by the Institute for Electrical and
Electronic Engineers' (IEEE) 802.11a Standard. This standard
specifically provides for a physical layer that implements wireless
data transmission in an unlicensed radio band at 5.15 through 5.825
GigaHertz (GHz). Using spread spectrum signaling, in particular
orthogonal frequency division multiplexing, payload data rates from
6 through 54 Megabits per second (Mbps) can be provided. Modulation
schemes that are implemented in 802.11a include binary phase shift
keying, quadrative phase shift keying 16 QAM and 64 QAM, with
convolutional coding of one-half, two-thirds, or three-quarter
rates.
[0032] What is important to note here is that the equipment 100
includes a directional antenna array 110 that may be steered to a
number of different azimuthol angles. Through the use of the
steerable array 110, it is possible to increase the selectivity of
the base band processor 120 thereby improving the performance (that
is rejection of unwanted signals and noise) of the equipment 100.
An antenna controller 120 forms part of the physical layer
processor in order to permit setting the array 110 at one of N
angles. The steering algorithm 175 implemented in the base band
processor 170 selects candidate angles to try during an initial
processing phase. The candidate angles are evaluated by the
steering algorithm 175 with the antenna controller setting the
array 110 in a fixed condition for reception of the remainder of
the Packet Protocol Data Unit (PPDU) frame. The invention thus
accomplishes this without making modifications to the MAC layer 180
or higher level layers with the communication protocol that would
be implemented by an associated computer host (not shown).
[0033] Before describing in detail how a steering algorithm 175 is
implemented, it is important to understand the format of a PPDU
frame. The format of one such frame is shown in FIG. 2. Here the
PPDU frame 200 is seen to include a Physical Layer Convergent
Procedure (PLCP) preamble portion 210, a signal portion 220, and a
data portion 230. The PLCP preamble 210 consists of twelve
Orthogonal Frequency Division Multiplex (OFDM) symbols; these
symbols will be described in much greater detail below. The signal
portion 220 consists of one symbol as shown in the more detailed
view of the PLCP header 240. These include a number of bits coded
as Binary Phase Shift Keyed (BPSK) at a half rate including a rate
field 242, a reserved bit 243, a length bit 244, a parity bit 245,
a tail bit section 246 and service bit section 247. A data portion
230 more particularly includes the Protocol Service Data Unit
(PSDU) fields 250 that include the actual payload data, a tail
portion 252 and pad bits 254.
[0034] FIG. 3 is a more detailed view of the PLCP preamble portion
and in particular, a training sequence that occurs in a beginning
portion. The PLCP preamble 120 includes short and long training
sequences consisting of a number of samples that permit a receiver
to perform signal detection, automatic gain control, diversity
selection, course frequency adjustment, and timing synchronization
as well as fine frequency and in timing offset estimation. The rate
field 245 and message length field 244 permit decoding of the
remainder of the frame by indicating its encoding data rate and
length in terms of symbols. The PSDU field 250 is the
convolutionally encoded and scrambled payload data. The tail bits
252 are bits required for the convolutional decoder decoding
process to converge to a known zero state and the pad bits 254
extend the message to fit evenly into a fixed integer number of
OFDM symbols.
[0035] FIG. 3 also shows the format of the PLCP preamble 210. Here
can be seen the short synchronization (short sync) section 212 and
long sync section 214. The short sync section 212 consists of ten
short sync symbols, t.sub.1 t.sub.2 . . . t.sub.10, each having a
duration of 800 nanoseconds (providing an aggregate duration of 8
microseconds (.mu.s)). According to the IEEE 802.11a specification,
signal detection, automatic gain control, and diversity selection
is expected to be performed by approximately the occurrence of the
seventh short sync symbol t.sub.7. Course frequency offset
estimation and timing synchronization then proceeds on the
remaining three to four symbols at the end of the short sync
sequence.
[0036] A double guard band GI2 is provided prior to the inclusion
of two long sync symbols T.sub.1 and T.sub.2. The entire duration
of the long sync portion of the preamble 214 is 8.0 microseconds as
was in the case of the short sync symbol section. What is important
to note here is that there is not a particularly long amount of
time available to steer an antenna array at the beginning of the
PLCP preamble. For example, by time t.sub.7 or by at least by the
time t.sub.8, it is expected that the receiver will already be
performing course frequency offset estimation. Thus, if an antenna
array is to be steered such that it is optimized for each received
PPDU frame, the steering must be completed, and the antenna may not
be further steered or "spinning" after approximately t.sub.6.
Otherwise, the receiver will be prone to not properly obtaining
course frequency and timing synchronization, never mind not being
able to perform fine frequency and timing offset synchronization
needed to properly decode the data symbols occurring later in the
frame.
[0037] FIG. 4 is a diagram illustrating the real and imaginary
portions of a short sync portion of the PLCP preamble. The short
sync pulses 212 each consist of a known burst of energy in both the
real and imaginary data planes. (The X-axis here is based on sample
number and not specifically the time duration.) It should be noted
that time duration of 8 microseconds corresponds to receipt of
approximately 160 samples at a 20 MHz complex sample rate.
[0038] FIG. 5 is a more detailed view of a single PLCP short sync
pulse in the time domain. Shown here are sixteen (16) samples taken
across the symbol duration of 800 nanoseconds (that is, at a rate
of 50 nanoseconds per complex sample or 20 MegaHertz). The dashed
part going across the top of the page represents the complex
magnitude of the PLCP short sync pulse. The plot 510 in the heavier
shaded line represents the real portion of that same short sync
pulse; the lighter weight line 520 indicates the imaginary portion
of the short sync pulse.
[0039] What can be noted from this diagram is that symmetry exists
between samples 1 through 8 and samples 9 through 16. Specifically,
the first portion of the real part (i.e., samples 1 through 8)
corresponds to the second portion of the imaginary part (samples 9
through 16). Likewise, the second portion of the real part (samples
9 through 16) corresponds to the first portion of the imaginary
part, (samples 1 through 8). This symmetry is indicative of several
techniques that may be used to shorten processing needed to
probably detect a short sync pulse. Specifically, as long as one
can track at least one half of a short sync pulse, then it should
be possible to properly detect it, since the second half is
redundant, in a sense. This characteristic of a short sync pulse
can be further exploited in a manner that can be described in
greater detail below in connection with the steering algorithm.
[0040] FIG. 6 is a diagram illustrating the frequency domain
magnitude response of a short sync pulse over 64 samples. As can be
seen, the frequency content exists in twelve fixed "expected" bins.
There is no expected energy in the remaining 52 bins. This
particular response will be used in connection with one aspect of
the steering algorithm to determine a metric as an approximation of
a signal to noise ratio given an observed actual short sync
detected pulse.
[0041] FIG. 7 is a frequency domain amplitude and phase plot for
the short sync preamble pulse showing the relative phases of the 12
energy bins that comprise the pulse.
[0042] FIG. 8 is included here as a reminder of the format of the
long sync pulses T.sub.1, T.sub.2. These pulses occur during the
long sync portion 242, and are used primarily for phase estimation
and fine frequency acquisition processing. The long sync pulse is
formatted in the time domain as shown in FIG. 9. The frequency
domain response shown in FIG. 10. A sample plot showing the complex
real and imaginary frequency domain characteristic of the long sync
pulse is shown in FIG. 11. This plot is included to show that the
frequency domain magnitude response of the long sync pulse is such
that energy occurs in each frequency bin, at least with the 64
samples that would be available. It would thus be difficult to
generate an estimated signal to noise ratio or other metrics from
such a pulse.
[0043] It is important to also note here that at the time of
reception of the long sync pulse, a receiver is expected to be
performing a fine tuning operation. At this point it is also
probably too late to therefore be changing the antenna directional
settings.
[0044] Thus what is needed is a technique for steering the antenna
on the short sync pulses 212 only. In general, these algorithms
must be performed as quickly as possible, as the time available is
only a few microseconds. Furthermore, the algorithm must work in
synchronization with signal acquisition processing, such that a
result is obtained prior to any long sync or fine frequency
estimation processing required for each packet. It should also be
understood that these algorithms operate with antennas that can be
steered with extremely small latency time, less than one
microsecond, or approximately the duration of one short sync
pulse.
[0045] A first steering algorithm 175 shown in FIG. 12 proceeds as
follows. In a first step 1200, the array 110 is configured for an
omnidirectional receiving mode. This preferably completes prior to
reception of even the first short sync pulse. In the next step
1210, the Automatic Gain Control (AGC) circuitry of the receiver is
allowed to track for the duration of the first short sync pulse
(t.sub.1). In the case of 802.11a, this will be for a duration of
800 nanoseconds (ns). At step 1212 the AGC is locked and the set
amount is dropped off by six decibels.
[0046] In the next step 1230, a metric is determined. This can, in
one embodiment, be a correlation performed over the first half of
the short sync pulse, i.e., the first 400 nanoseconds of pulse
t.sub.2 (FIG. 3), but other metrics are possible. The correlation
is performed such that the detected t.sub.2 pulse is compared
against an ideal expected version. The correlation thus provides a
measure of how well the short sync pulse has been received at the
candidate angle. A second correlation is then performed over the
second half of the short sync pulse in state 1240.
[0047] In state 1242 the real and imaginary samples are swapped
during this second correlation step. This then gives a baseline for
an omnidirectional response.
[0048] In state 1250 the array 110 is steered for a first candidate
angle out of a number of candidate angles. The number of candidate
angles depends upon the configuration of the antenna array; in one
embodiment there are four candidate angles. From state 1260, the
correlation steps 1230, 1240 and 1242 are repeated for each of the
four candidate angles, with correlation results being stored for
each candidate angle. The candidate angle that provided the best
correlation result is then selected as the angle to be used for the
remainder of short sync and the remainder of PPDU processing. This
angle is selected in state 1270, and in state 1280 the candidate
antenna direction is set. The steering algorithm of FIG. 12 can
thus be completed in as little as six short sync pulses. This
permits additional receiver processing, such as frequency
estimation, to operate on the four or so remaining short sync
pulses T.sub.7 through T.sub.10 after the antenna has reached a
stable setting.
[0049] Because of the in-phase and quadrative symmetry of each
short sync pulse, it is possible to perform a correlation over a
second half of a short sync pulse, using a different candidate
angle than used for the first half. However, this assumes that the
antenna array can be steered to a new candidate angle in about 30
to 200 nanoseconds. It also assumes that the correlation can be
completed in such a timeframe. When this is possible, the algorithm
can determine a correlation value for two different candidate
angles for every short sync pulse. Determination of which
embodiment is best for a particular implementation depends upon the
availability of high speed correlation hardware and fast switching
antenna components.
[0050] A second technique used for antenna steering algorithm 175
is described in FIG. 13. This process is similar to that shown in
FIG. 12. From state 1300, the system sets the antenna in
omnidirectional mode for reception of a first short sync pulse ti.
In state 1310, rather than correlate against an optimized expected
short sync response, an actual first half and second half short
sync response are stored in states 1310 and 1315. These references
are stored for use in later calculation of the correlation of four
possible angles. The actual response will contain multipath
distortion information, which can be potentially beneficial over a
technique that uses only ideal responses. Otherwise the process
here proceeds after state 1315 as in FIG. 12, to perform an AGC
track and correlate over first and second half portions of a short
sync pulse (if desired) for each of the four candidate angles. The
best candidate angle is selected in state 1370, and the final
antenna angle set in state 1380.
[0051] Yet another process shown in FIG. 14 may be used to
determine a candidate antenna setting. This approach is to
precompute a ideal response as a comb filter. This, in turn, allows
calculation of an estimated signal to noise ratio rather than a
simple best amplitude response that is used in the processes of
FIGS. 12 and 13.
[0052] In step 1400, this process performs a Fast Fourier Transform
(FFT) of an ideal short sync pulse. The result would typically look
like the response that was seen in FIG. 6 above. At state 1410, the
inverse of FFT of this ideal pulse is taken to provide an ideal
time domain energy or "signal" response. Specifically, all bins
with no expected energy, i.e., the 52 bins that are not expected to
have any energy, are set to zero and the IFFT is run.
[0053] In state 1420 the other bins of "non-interest", that is the
bins having no expected energy level, are taken from the short sync
response for FFT. A "mirror" of this response is then developed
with, for example, magnitude "one" values placed in the 52 bins
where noise is expected and magnitude "zero" in the bins where
energy is expected. The inverse FFT of this "noise filter" is then
taken in state 1430 to provide a "noise" time domain response.
[0054] In state 1440 the received waveform is correlated against
both of these time domain sequences, i.e., for both the "signal"
and "noise" filter responses. An expected "pseudo signal to noise"
ratio is developed in state 1450. This can be calculated as a ratio
of a peak of the "signal" correlation divided by the peak of the
"noise" correlation at each bin location.
[0055] Specifically, each of the short sync pulses received for a
candidate angle are fed to be convolved with both the signal and
noise filters. Taking a ratio of these two responses provides a
quasi-estimate of the signal to noise ratio to be used as the
metric to measure how well each antenna angle should be expected to
perform.
[0056] The FFTs and inverse FFTs could be taken over 64 samples, as
suggested by FIG. 6. However, it should be understood that a
shorter FFT size or sample set of 32 samples could be used and
still obtain measurable results. That is, if digital signal
processor timing constraints allow only half as many samples for
the filters, at least an energy sample and at least one noise
sample for each expected peak value is available in the frequency
domain. Shorter sample amounts would not be possible, at least for
802.11a, given that the twelve energy levels would not map in an
integral fashion in anything less that 32 bins.
[0057] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
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