U.S. patent application number 16/915764 was filed with the patent office on 2020-10-15 for pilot symbol patterns for transmission through a plurality of antennas.
The applicant listed for this patent is Apple Inc.. Invention is credited to Ming Jia, Jianglei Ma, Wen Tong, Hua Xu, Dongsheng Yu, Hang Zhang, Peiying Zhu.
Application Number | 20200328923 16/915764 |
Document ID | / |
Family ID | 1000004929103 |
Filed Date | 2020-10-15 |
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United States Patent
Application |
20200328923 |
Kind Code |
A1 |
Ma; Jianglei ; et
al. |
October 15, 2020 |
Pilot Symbol Patterns for Transmission through a Plurality of
Antennas
Abstract
A method and apparatus for improving channel estimation within
an OFDM communication system. Channel estimation in OFDM is usually
performed with the aid of pilot symbols. The pilot symbols are
typically spaced in time and frequency. The set of frequencies and
times at which pilot symbols are inserted is referred to as a pilot
pattern. In some cases, the pilot pattern is a diagonal-shaped
lattice, either regular or irregular. The method first interpolates
in the direction of larger coherence (time or frequency). Using
these measurements, the density of pilot symbols in the direction
of faster change will be increased thereby improving channel
estimation without increasing overhead. As such, the results of the
first interpolating step can then be used to assist the
interpolation in the dimension of smaller coherence (time or
frequency).
Inventors: |
Ma; Jianglei; (Kanata,
CA) ; Jia; Ming; (Ottawa, CA) ; Tong; Wen;
(Ottawa, CA) ; Zhu; Peiying; (Kanata, CA) ;
Zhang; Hang; (Nepean, CA) ; Xu; Hua; (Nepean,
CA) ; Yu; Dongsheng; (Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
1000004929103 |
Appl. No.: |
16/915764 |
Filed: |
June 29, 2020 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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16666650 |
Oct 29, 2019 |
10700905 |
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16915764 |
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16433273 |
Jun 6, 2019 |
10476719 |
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16666650 |
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16255702 |
Jan 23, 2019 |
10348542 |
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16433273 |
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16110205 |
Aug 23, 2018 |
10200226 |
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16255702 |
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15891750 |
Feb 8, 2018 |
10075314 |
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16110205 |
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15642956 |
Jul 6, 2017 |
9929889 |
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15891750 |
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15226101 |
Aug 2, 2016 |
9705720 |
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15642956 |
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15046934 |
Feb 18, 2016 |
9432232 |
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15226101 |
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13944022 |
Jul 17, 2013 |
9270510 |
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15046934 |
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13665982 |
Nov 1, 2012 |
8842756 |
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13944022 |
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12064566 |
Sep 4, 2008 |
8331465 |
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PCT/CA2006/001380 |
Aug 22, 2006 |
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13665982 |
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60710527 |
Aug 23, 2005 |
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60722744 |
Sep 30, 2005 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 27/2611 20130101;
H04L 27/2647 20130101; H04B 7/08 20130101; H04L 27/2626 20130101;
H04L 5/005 20130101; H04L 27/2602 20130101; H04L 1/0618 20130101;
H04L 5/0007 20130101; H04L 25/02 20130101; H04L 5/0048 20130101;
H04L 1/0009 20130101; H04L 5/0051 20130101; H04L 25/0232 20130101;
H04L 1/0003 20130101; H04B 7/06 20130101; H04L 27/2613 20130101;
H04L 5/0023 20130101; H04L 25/0226 20130101; H04W 52/325
20130101 |
International
Class: |
H04L 27/26 20060101
H04L027/26; H04L 5/00 20060101 H04L005/00; H04W 52/32 20060101
H04W052/32; H04L 25/02 20060101 H04L025/02; H04B 7/06 20060101
H04B007/06; H04B 7/08 20060101 H04B007/08 |
Claims
1. A base station comprising: a plurality of transmit antennas;
digital circuitry, wherein the digital circuitry is configured to:
transmit, via the plurality of transmit antennas, a data traffic
transmission on a downlink time frequency resource to a user
equipment device, wherein the time frequency resource includes a
plurality of orthogonal frequency division multiplexing (OFDM)
symbol durations in time and a plurality of subcarriers in
frequency; transmit, via the plurality of transmit antennas, a
first set of time division multiplexed (TDM) pilots in an initial
OFDM symbol of the time frequency resource; and transmit, via the
plurality of transmit antennas, a second set of TDM pilots in a
second OFDM symbol of the time frequency resource, wherein the
second OFDM symbol is 7 OFDM symbols after the initial OFDM
symbol.
2. The base station of claim 1, wherein the initial OFDM symbol and
the second OFDM symbol are not utilized to transmit data
traffic.
3. The base station of claim 1, wherein the digital circuitry is
further configured to interleave TDM pilots from different antennas
on each of the initial OFDM symbol and the second OFDM symbol.
4. The base station of claim 1, wherein the initial OFDM symbol of
the time frequency resource is a first symbol of a slot.
5. The base station of claim 1, wherein the first set of TDM pilots
and the second set of TDM pilots are used in a beamforming
zone.
6. The base station of claim 1, wherein each intervening subcarrier
between the initial OFDM symbol and the second OFDM symbol carries
one or both of control information and data information.
7. A base station comprising: a plurality of transmit antennas;
digital circuitry, wherein the digital circuitry is configured to:
transmit, via the plurality of transmit antennas, a data traffic
transmission on a downlink time frequency resource to a user
equipment device, wherein time frequency resource includes a
plurality of orthogonal frequency division multiplexing (OFDM)
symbol durations in time and a plurality of subcarriers in
frequency; transmit, via the plurality of transmit antennas, a
first set of time division multiplexed (TDM) pilots in an initial
OFDM symbol of the time frequency resource, wherein the first set
of TDM pilots are used for demodulation of the data traffic, and
wherein no pilots in the time frequency resource other than the
first set of TDM pilots are used for demodulation of the data
traffic.
8. The base station of claim 7, wherein the initial OFDM symbol is
not utilized to transmit data traffic.
9. The base station of claim 7, wherein the digital circuitry is
further configured to interleave TDM pilots from different antennas
on the initial OFDM symbol.
10. The base station of claim 7, wherein the initial OFDM symbol of
the time frequency resource is a first symbol of a slot.
11. The base station of claim 7, wherein the first set of TDM
pilots is used in a beamforming zone.
12. A user equipment device (UE) comprising: one or more receive
antennas; and a processor, wherein the processor is configured to
receive transmissions via the one or more receive antennas, wherein
the UE is configured to: receive, via the one or more of receive
antennas, a data traffic transmission on a downlink time frequency
resource to a user equipment device, wherein the time frequency
resource includes a plurality of orthogonal frequency division
multiplexing (OFDM) symbol durations in time and a plurality of
subcarriers in frequency; receive, via the one or more of receive
antennas, a first set of time division multiplexed (TDM) pilots in
an initial OFDM symbol of the time frequency resource; and receive,
via the one or more receive antennas, a second set of TDM pilots in
a second OFDM symbol of the time frequency resource, wherein the
second OFDM symbol is 7 OFDM symbols after the initial OFDM
symbol.
13. The UE of claim 12, wherein the initial OFDM symbol and the
second OFDM symbol are not utilized to receive data traffic.
14. The UE of claim 12, wherein the initial OFDM symbol of the time
frequency resource is a first symbol of a slot.
15. The UE of claim 12, wherein the first set of TDM pilots and the
second set of TDM pilots are used in a beamforming zone.
16. The UE of claim 12, wherein each intervening subcarrier between
the initial OFDM symbol and the second OFDM symbol carries one or
both of control information and data information.
17. The UE of claim 12, wherein the UE is further configured to:
measure attenuation of the first and second sets of time division
multiplexed (TDM) pilots and estimate the attenuations of data
symbols in between the first and second sets of time division
multiplexed (TDM) pilots.
Description
PRIORITY CLAIM INFORMATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 16/666,650, filed on Oct. 29, 2019, titled
"Pilot Symbol Patterns for Transmission through a Plurality of
Antennas", which is a continuation of U.S. patent application Ser.
No. 16/433,273, filed on Jun. 6, 2019 (issued as U.S. Pat. No.
10,476,719 on Sep. 19, 2019), titled "Pilot Symbol Patterns for
Transmission through a Plurality of Antennas", by Jianglei Ma et
al., which is a continuation of
[0002] U.S. patent application Ser. No. 16/255,702, filed on Jan.
23, 2019, (issued as U.S. Pat. No. 10,348,542 on Jul. 9, 2019)
titled "Pilot Symbol Patterns for Transmission through a Plurality
of Antennas", by Jianglei Ma et al., which is a continuation of
[0003] U.S. patent application Ser. No. 16/110,205, filed on Aug.
23, 2018, (issued as U.S. Pat. No. 10,200,226 on Feb. 5, 2019)
titled "Pilot Symbol Patterns for Transmission through a Plurality
of Antennas", by Jianglei Ma et al., which is a continuation of
[0004] U.S. patent application Ser. No. 15/891,750, filed on Feb.
8, 2018, (issued as U.S. Pat. No. 10,075,314 on Sep. 11, 2018)
titled "Pilot Symbol Patterns for Transmission via a Plurality of
Antennas", by Jianglei Ma et al., which is a continuation of
[0005] U.S. patent application Ser. No. 15/642,956, filed on Jul.
6, 2017, (issued as U.S. Pat. No. 9,929,889 on Mar. 27, 2018)
titled "Pilot Symbol Patterns for Transmission via a Plurality of
Antennas", by Jianglei Ma et al., which is a continuation of
[0006] U.S. patent application Ser. No. 15/226,101, filed on Aug.
2, 2016, (issued as U.S. Pat. No. 9,705,720 on Jul. 11, 2017)
titled "Pilot Symbol Patterns for Transmission via a Plurality of
Antennas", by Jianglei Ma et al., which is a continuation of
[0007] U.S. patent application Ser. No. 15/046,934, filed on Feb.
18, 2016, (issued as U.S. Pat. No. 9,432,232 on Aug. 30, 2016)
titled "Pilot Symbol Patterns for Transmit Antennas", by Jianglei
Ma et al., which is a continuation of
[0008] U.S. patent application Ser. No. 13/944,022, filed on Jul.
17, 2013 (issued as U.S. Pat. No. 9,270,510 on Feb. 23, 2016),
titled "Adaptive Two-Dimensional Channel Interpolation", by
Jianglei Ma et al., which is a continuation of
[0009] U.S. patent application Ser. No. 13/665,982, filed on Nov.
1, 2012 (issued as U.S. Pat. No. 8,842,756 on Sep. 23, 2014),
titled "Adaptive Two-Dimensional Channel Interpolation", which is a
continuation of
[0010] U.S. patent application Ser. No. 12/064,566, filed on Sep.
4, 2008 (issued as U.S. Pat. No. 8,331,465 on Dec. 11, 2012), which
is a U.S. National Stage of
[0011] International Application No. PCT/CA2006/001380, filed on
Aug. 22, 2006, which claims the benefit of priority to:
[0012] U.S. Provisional Application No. 60/722,744, filed on Sep.
30, 2005; and U.S. Provisional Application No. 60/710,527, filed on
Aug. 23, 2005.
[0013] All of the above identified applications are incorporated by
reference in their entireties as though fully and completely set
forth herein.
[0014] The claims in the instant application are different than
those of the parent application or other related applications. The
Applicant therefore rescinds any disclaimer of claim scope made in
the parent application or any predecessor application in relation
to the instant application. The Examiner is therefore advised that
any such previous disclaimer and the cited references that it was
made to avoid, may need to be revisited. Further, any disclaimer
made in the instant application should not be read into or against
the parent application or other related applications
BACKGROUND
Field of the Application
[0015] This invention relates to Orthogonal Frequency Division
Multiplexing (OFDM) communication systems, and more particularly to
channel interpolation with the use of pilot symbols.
Background of the Disclosure
[0016] In wireless communication systems that employ OFDM, a
transmitter transmits data to a receiver using many sub-carriers in
parallel. The frequencies of the sub-carriers are orthogonal.
[0017] Channel estimation in OFDM is usually performed with the aid
of known pilot symbols which are sparsely inserted in a stream of
data symbols. The attenuation of the pilot symbols is measured and
the attenuations of the data symbols in between these pilot symbols
are then estimated/interpolated.
[0018] Pilot symbols are overhead, and should be as few in number
as possible in order to maximize the transmission rate of data
symbols. It is desirable that channel estimation in OFDM be as
accurate as possible without sacrificing bandwidth.
SUMMARY
[0019] In one embodiment, there is provided a method comprising
receiving channel estimates for four pilot symbols in a scattered
pilot pattern in time-frequency; calculating the channel response
for the pilot symbols in both a first direction and a second
direction; determining whether the channel changes more slowly in
one direction than the other; and interpolating in the direction of
slower channel change.
[0020] In some embodiments, the method of further comprises
interpolating in the direction of faster channel change.
[0021] In some embodiments, the step of interpolating in the
direction of faster channel change is performed using the result
from the step of interpolating in the direction of slower channel
change.
[0022] In some embodiments, the channel changes are calculated by
performing an inner products operation.
[0023] In some embodiments, the first direction is a time direction
and the second direction is a frequency direction.
[0024] In some embodiments, the first direction is a frequency
direction and the second direction is a time direction.
[0025] In some embodiments, the scattered pilot pattern is a
regular diamond lattice.
[0026] In some embodiments, the scattered pilot pattern is an
irregular diamond lattice.
[0027] In some embodiments, the scattered pilot pattern is kite
shaped.
[0028] In another embodiment, there is provided an OFDM receiver
comprising: one or more receive antennas; the OFDM transmitter
being adapted to receive channel estimates for four pilot symbols
in a scattered pilot pattern in time-frequency, calculate channel
changes for the pilot symbols in a first direction and a second
direction, and interpolate in the direction of slower channel
change.
[0029] In yet another embodiment, there is provided a method of
interpolation using a set of four pilot symbols in a scattered
pilot pattern in time-frequency wherein the set of four pilot
symbols comprise first and second pilot symbols on a common
sub-carrier frequency, spaced in time, and third and fourth pilot
symbols transmitted on different sub-carriers on a common OFDM
symbol period, the method comprising: determining a first channel
change between the first and second pilot symbols; determining a
second channel change between the third and fourth pilot symbols;
determining which of the first and second channel change is slower;
if the first channel change is slower, interpolating using the
first and second pilot symbols to generate a channel estimate for
the common sub-carrier frequency at the common OFDM symbol period,
and then using the channel estimate in subsequent interpolations to
determine other channel estimates; and if the second channel change
is slower, interpolating using the third and fourth pilot symbols
to generate a channel estimate for the common sub-carrier frequency
at the common OFDM symbol period, and then using the channel
estimate in subsequent interpolations to determine other channel
estimates.
[0030] In yet another embodiment, a method of inserting pilot
symbols into OFDM sub-frames for transmission by a plurality of
transmitting antenna, the OFDM sub-frames having a time domain and
a frequency domain, each OFDM sub-frame comprising a plurality of
OFDM symbols, the method comprising: for each sub-frame, defining a
set of at least two OFDM symbols none of which are consecutive that
are to contain pilot symbols; at each antenna, inserting pilot
symbols in each of the set of at least two OFDM symbols in a
scattered pattern that does not interfere with the scattered
pattern inserted by any other antenna.
[0031] Other aspects and features of the present invention will
become apparent to those ordinarily skilled in the art upon review
of the following description of specific embodiments of the
invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Preferred embodiments of the invention will now be described
with reference to the attached drawings in which:
[0033] FIG. 1 is a diagram of a single antenna perfect diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0034] FIG. 2 is a flowchart of a method of performing adaptive
interpolation in accordance with one embodiment of the present
invention;
[0035] FIG. 3 presents simulation results for one example of
adaptive interpolation;
[0036] FIG. 4A is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can used in accordance with
an embodiment of the present invention;
[0037] FIG. 4B is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can used in accordance with
an embodiment of the present invention;
[0038] FIG. 5A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0039] FIG. 5B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0040] FIG. 5C is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0041] FIG. 5D is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0042] FIG. 6 is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0043] FIG. 7A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0044] FIG. 7B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0045] FIG. 7C is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0046] FIG. 7D is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0047] FIG. 8 is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0048] FIG. 9A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0049] FIG. 9B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0050] FIG. 9C is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0051] FIG. 9D is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0052] FIG. 10A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0053] FIG. 10B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0054] FIG. 11A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0055] FIG. 11B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0056] FIG. 12A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0057] FIG. 12B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention;
[0058] FIG. 13 is a block diagram of a cellular communication
system;
[0059] FIG. 14 is a block diagram of an example base station that
might be used to implement some embodiments of the present
invention;
[0060] FIG. 15 is a block diagram of an example wireless terminal
that might be used to implement some embodiments of the present
invention;
[0061] FIG. 16 is a block diagram of a logical breakdown of an
example OFDM transmitter architecture that might be used to
implement some embodiments of the present invention;
[0062] FIG. 17 is a block diagram of a logical breakdown of an
example OFDM receiver architecture that might be used to implement
some embodiments of the present invention;
[0063] FIG. 18 is a block diagram of one embodiment of the present
invention;
[0064] FIG. 19 illustrates a time division multiplexing (TDM) Pilot
Pattern for use in a single antenna scenario, according to some
embodiments; and
[0065] FIG. 20 illustrates a time division multiplexing (TDM) Pilot
Pattern for use in a four antenna scenario, according to some
embodiments.
DETAILED DESCRIPTION
[0066] Channel estimation in OFDM is usually performed with the aid
of pilot symbols. More particularly, at an OFDM transmitter, known
pilot symbols are periodically transmitted along with data symbols.
The pilot symbols are typically spaced in time and frequency.
[0067] The variations in phase and amplitude resulting from
propagation across an OFDM channel are referred to as the channel
response. The channel response is usually frequency and time
dependent. If an OFDM receiver can determine the channel response,
the received signal can be corrected to compensate for the channel
degradation. The determination of the channel response is called
channel estimation. The transmission of known pilot symbols along
with data symbols allows the receiver to carry out channel
estimation.
[0068] When a receiver receives an OFDM signal, the receiver
compares the received value of the pilot symbols with the known
transmitted value of the pilot symbols to estimate the channel
response.
[0069] Since the channel response can vary with time and with
frequency, the pilot symbols are scattered amongst the data symbols
to provide a range of channel responses over time and frequency.
The set of frequencies and times at which pilot symbols are
inserted is referred to as a pilot pattern. In some cases, the
pilot pattern is a diagonal-shaped lattice, either regular or
irregular.
[0070] FIG. 1 is an example pilot pattern which can be used in
accordance with one embodiment of the present invention. Pilot and
data symbols are spread over an OFDM sub-frame in a time direction
120 and a frequency direction 122. Most symbols within the OFDM
sub-frame are data symbols 124. Pilot symbols 126 are inserted in a
diamond lattice pattern. In the illustrated example, the diamond
lattice pattern in which each encoded pilot symbols are inserted
within the OFDM sub-frame is a perfect diamond lattice pattern as
illustrated by pilot symbols h.sub.1, h.sub.2, h.sub.3 and
h.sub.4.
[0071] A two dimensional interpolator is used to estimate the
channel response at point h which is between four points of known
channel response, i.e. pilot symbols h.sub.1, h.sub.2, h.sub.3 and
h.sub.4. Point h can then be used as an additional point from which
the receiver can carry out channel estimation. The use of point h
would, of course, not add any overhead to the OFDM signal.
[0072] The channel interpolation scheme is adaptive, i.e. it is a
scheme which can adapt to varying conditions in the
h(i,j)=w.sub.1(i,j)h.sub.1+w.sub.2(i,j)h.sub.2+w.sub.3(i,j)h.sub.3+w.sub-
.4(i,j)h.sub.4
channel. The following formula presents a particular example of
adaptive two-dimensional (time direction and frequency direction)
interpolator to calculate point h:
where w.sub.1(i,j)+w.sub.2(i,j)+w.sub.3(i,j)+w.sub.4(i,j)=1.
[0073] In this case, the two dimensional channel interpolation can
be viewed as the sum of two one-dimensional interpolations.
[0074] The weights w.sub.k(i,j) may be adapted to coherence time
and frequency of the channel. In some embodiments, if the channel
coherence is less in the time direction than it is in the frequency
direction, then h would be calculated using the following
formula:
h(i,j)=w.sub.1(i,j)h.sub.1+w.sub.2(i,j)h.sub.2+w.sub.3(i,j)h.sub.3+w.sub-
.4(i,j)h.sub.4
where
w.sub.3(i,j)=0,
w.sub.4(i,j)=0, and
w.sub.1(i,j)+w.sub.2(i,j)=1.
[0075] Alternatively, if the channel coherence is greater in the
time direction than it is in the frequency direction, then h would
be calculated using the following formula:
h(i,j)=w.sub.1(i,j)h.sub.1+w.sub.2(i,j)h.sub.2+w.sub.3(i,j)h.sub.3+w.sub-
.4(i,j)h.sub.4
where
w.sub.1(i,j)=0,
w.sub.2(i,j)=0, and
w.sub.3(i,j)+w.sub.4(i,j)=1.
[0076] In another embodiment, the weights in both directions (time
and frequency) are adaptively changed according to the channel
coherence in the time and frequency directions as follows:
h(i,j)=c.sub.timew.sub.1(i,j)h.sub.1+c.sub.timew.sub.2(i,j)h.sub.2+c.sub-
.freqw.sub.3(i,j)h.sub.3+c.sub.freqw.sub.4(i,j)h.sub.4
c.sub.time+c.sub.freq=1
w.sub.1(i,j)+w.sub.2(i,j)+w.sub.3(i,j)w.sub.4(i,j)=1
[0077] According to one embodiment, the sequence of interpolation
is adapted to the coherence of the channel.
[0078] One way to achieve adaptive interpolation is to divide the
interpolation into two one-dimensional steps as shown in the
flowchart illustrated in FIG. 2: [0079] i. at step 210, calculate
the channel changes in both time and frequency directions and
determine in which direction the channel changes faster; [0080] ii.
at step 220, perform one-dimensional interpolation in the direction
with slower channel change to calculate h; and [0081] iii. at step
230, using h, perform one-dimensional interpolation in the
direction with faster channel change.
[0082] The method of adaptive interpolation set out above takes
advantage of the fact that interpolated results from the direction
of larger coherence time/frequency is more reliable, and hence is
interpolated first. The calculation of h will effectively increase
the density of pilot symbols in the direction of faster change
thereby improving channel estimation without increasing overhead.
As such, the results of the first interpolating step can then be
used to assist the interpolation in the dimension of smaller
coherence time/frequency.
[0083] In general, there are at least three ways to evaluate the
channel change between two pilots, including: [0084] i. Euclidean
distance. One problem with Euclidean distance, however, is that it
is not sensitive to phase change; [0085] ii. Phase change. One
problem with phase change, however, is computation complexity; and
[0086] iii. Amplitude change. One problem with amplitude change,
however, is that it is insensitive to phase change.
[0087] In light of these drawbacks a way to measure channel change
so as to take both amplitude change and phase change into account,
while at the same time keeping the computation complexity to a
minimum, is desirable. According to an embodiment of the invention,
therefore, a way of using the inner products of the two pilot
assisted channel estimates as a measurement of channel change is
shown below.
.LAMBDA..sub.time=h.sub.3,h.sub.4=|h.sub.3.parallel.h.sub.4|
cos(.theta..sub.3,4)
.LAMBDA..sub.freq=h.sub.1,h.sub.2=|h.sub.1.parallel.h.sub.2|
cos(.theta..sub.1,2)
[0088] .LAMBDA..sub.time denotes channel change in the time
direction.
[0089] .LAMBDA..sub.freq denotes channel change in the frequency
direction.
[0090] The term "<h.sub.nh.sub.m>" denotes the inner product
of h.sub.n and h.sub.m.
[0091] The term "|h.sub.n|" denotes the magnitude of the vector
h.sub.n. If h.sub.n=a+bi then |h.sub.n|=sqr(a.sup.2+b.sup.2).
[0092] The term "cos(.theta.1,2)" denotes the cosine of the
difference in angle between h.sub.n and h.sub.m:
cos(.theta.n,m)=cos(.theta.n-.theta.m). If h.sub.n=a+bi then
.theta.n can be calculated as .theta.n=tan.sup.-1(b/a).
[0093] The vector h.sub.n can be represented as
h.sub.1=|h.sub.1|e.sup.i.theta.n, or as h.sub.n=a+bi, where
a=|h.sub.n| cos(.theta..sub.n), and b=|h.sub.n|
sin(.theta..sub.n).
[0094] When the amplitude changes linearly between the two channel
estimates, the maximum .LAMBDA. is achieved when
|h.sub.1|=|h.sub.2| in frequency and |h.sub.3|=|h.sub.4| in
time.
[0095] Hence the more the channel changes, the smaller the
.LAMBDA., regardless whether this change is in amplitude or phase.
The inner product is able to solve phase ambiguity. When
|.theta.|>.pi./2 (which rarely occurs), cos(.theta.) becomes
negative, and hence smaller. An inner product may then be computed,
which requires two real multiplications and one real addition, and
the result is therefore a real number.
[0096] Referring again to FIG. 1, what follows is an example of the
adaptive interpolation method.
[0097] Assume:
h.sub.1=0.4423-1.0968i
h.sub.2=-0.0051-0.1484i
h.sub.3=0.1258-0.3413i
h.sub.4=0.3958-0.5883i
[0098] The central point, known from a simulation, has the value of
h=0.2859-0.4224i.
[0099] The inner product is then calculated as follows:
h.sub.1h.sub.2=0.1605
h.sub.3h.sub.4=0.2506
[0100] where h.sub.1h.sub.2=denotes the inner product of h.sub.1
and h.sub.2.
[0101] If h.sub.1=a.sub.1+ib.sub.1 and h.sub.2=a.sub.2+ib.sub.2
then the inner product can be calculated as
h.sub.1h.sub.2=a.sub.1a.sub.2+b.sub.1b.sub.2.
[0102] Alternatively, h.sub.1h.sub.2=|h.sub.1.parallel.h.sub.2|
cos(.theta..sub.2-.theta..sub.1).
[0103] Since h.sub.1h.sub.2<h.sub.3h.sub.4, the channel changes
faster in the h.sub.1/h.sub.2 direction.
[0104] h is then estimated in both the frequency and time
directions:
{tilde over (h)}.sub.h1,h2=0.5(h.sub.1+h.sub.2)=0.2186-0.6226i
{tilde over
(h)}.sub.h3,h4=0.5(h.sub.3.+-.h.sub.4)=0.2608-0.4648i
[0105] Compared with the known h, obviously {tilde over
(h)}.sub.h3,h4 provides a better estimate {tilde over
(h)}.sub.h1,h2; hence {tilde over (h)}.sub.h3,h4 can be used to
improve the channel interpolation quality in the h.sub.1/h.sub.2
direction.
[0106] Note that there is no requirement that h be the middle point
equidistant from h.sub.1, h.sub.2, h.sub.3 and h.sub.4.
[0107] In the example above, the interpolation sequence was
determined to be: [0108] i. interpolate between the two pilots in
the time direction first to calculate h, and [0109] ii. use h
and/or one or both of the two pilots to interpolate in the
frequency direction.
[0110] Of course, if the initial calculation used to determine
which channel direction changes faster determines that the
h.sub.3/h.sub.4 direction changes faster, then the interpolation
sequence will be: [0111] i. interpolate between the two pilots in
the frequency direction first to calculate h, and [0112] ii. use h
and/or one or both of the two pilots to interpolate in the time
direction.
[0113] Once h is calculated, any one of a number of conventional
channel estimation techniques can be used. Such channel estimation
techniques typically consist of two steps. First, the attenuations
at the pilot positions are measured. This measurement is calculated
using the formula:
H ( n , k ) .ident. Y ( n , k ) X ( n , k ) ##EQU00001##
where X(n,k) is the known pilot symbol, and Y(n,k) is the received
pilot symbol.
[0114] These measurements are then used to estimate (interpolate)
the attenuations of the data symbols in the second step. Persons
skilled in the art will appreciate that such channel estimation
techniques include, but are not limited to, linear interpolation,
second order interpolation, maximum likelihood (least square in
time domain), linear minimum square error and others.
[0115] In another embodiment, a "majority vote" is used to
determine the interpolation sequence for all the "diamonds" across
the frequency domain. This means that there are several
calculations performed along the frequency direction for the
channel change. Some results will indicate there is more change in
time, while other results indicate there is more change in
frequency. The "majority vote" option means the choice whether to
interpolate first in the time direction or the frequency direction
is arrived at by assessing the majority of the results. For
example, if the majority of the results indicate that the channel
changes faster in the time direction, then interpolation is first
performed in the frequency direction, and then in the time
direction. If the majority of the results indicate that the channel
changes faster in the frequency direction, then interpolation is
first performed in the time direction, and is then performed in the
frequency direction.
[0116] In accordance with an embodiment of the invention, FIG. 3
presents simulation results for the adaptive interpolation method
described above. The results show the benefit of adaptive
interpolation when channel changes slower in the time direction
when UE speed is low, and slower in the frequency direction when UE
speed is h1gh. The curve of "ideal channel" is of the case with
clean known channel, i.e. with no interpolation loss and additive
noise. As shown this approach recoups most of the interpolation
loss. The results were obtained with the majority vote option
described above.
[0117] It is not necessary that there be a regular diamond shaped
pilot pattern in order to use the adaptive interpolation method
described above. For example, an irregular diamond shaped pilot
pattern can be used in accordance with other embodiments of the
present invention, such as the scattered pilot patterns shown in
FIGS. 4A to 11. In FIGS. 4A to 11, the number of OFDM symbols per
Transmission Time Interval (TTI) is odd instead of even. In some
embodiments, the scattered pilot patterns can be generated by more
than one antenna such as is shown in FIGS. 5, 7, 9, 10, 11 and
12.
[0118] In general, the adaptive interpolation method works with all
"staggered" pilot patterns which describes all shapes other than a
square, which does not work. A perfect diamond shape, which is the
most favorable shape, is a special case of a staggered pilot
pattern. Another example of a pattern which would work is a "kite"
pattern where the pilots are spread further apart in one direction
than the other.
[0119] More generally, in FIGS. 4A to 11, in each sub-frame, pilots
are transmitted by part of the sub-carriers in at least two
non-contiguous OFDM symbols by at least one transmit antenna. The
pilot sub-carriers in the first OFDM symbol and the second OFDM
symbol are staggered in the frequency domain. In FIGS. 5A, 5D, 7A,
7D, 9A and 9D, pilot symbols from all transmit antennas are
transmitted through the same non-contiguous OFDM symbols. This
arrangement will save the terminal power since only two OFDM
symbols are coded to obtain the channel information.
[0120] FIG. 4A is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can used in accordance with
an embodiment of the present invention. The overhead associated
with this pilot pattern is 1/28 per antenna. Pilot and data symbols
are spread over an OFDM sub-frame in a time direction 420 and a
frequency direction 422. Most symbols within the OFDM sub-frame are
data symbols 424. Pilot symbols 426 are inserted in an irregular
diamond lattice pattern. In this embodiment, an OFDM sub-frame
comprises eight sub-carriers 428 and seven OFDM symbols 430.
[0121] As with the scattered pilot pattern in FIG. 1, there is
first performed a calculation of the channel changes in both the
time direction and the frequency direction and a comparison is made
as to which direction the channel changes faster. One-dimensional
interpolation is then performed in the direction with slower
channel change. One-dimensional interpolation is then performed in
the direction with faster channel change.
[0122] FIG. 4B is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can used in accordance with
an embodiment of the present invention. Though similar to FIG. 4A,
in this case one of the pilots in each diamond lattice is offset by
one OFDM symbol position. Thus, the adaptive interpolation method
does not require that the scattered pilots line up in either or
both of the time direction and the frequency direction. In the case
of staggered pilot patterns where the pilots do not line up in
either the time direction, the frequency direction, or both, it is
more accurate to refer to the "h.sub.1/h.sub.2 direction" and the
"h.sub.3/h.sub.4 direction" rather than the time direction and the
frequency direction.
[0123] FIG. 5A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention. The overhead
associated with this scattered pilot pattern is 1/28 per antenna.
In this embodiment, an OFDM frame comprises eight sub-carriers 528
and seven OFDM symbols 530.
[0124] Pilot and data symbols are spread over an OFDM frame in a
time direction 420 and a frequency direction 522. Most symbols
within the OFDM frame are data symbols 524. Pilot symbols 526 are
inserted in an irregular diamond lattice pattern.
[0125] As with the scattered pilot pattern in FIG. 1, there is
first performed a calculation of the channel changes in both the
time direction and the frequency direction and a comparison is made
as to which direction the channel changes faster. One-dimensional
interpolation is then performed in the direction with slower
channel change. Using these measurements, one-dimensional
interpolation is then performed in the direction with faster
channel change.
[0126] FIGS. 5B, 5C and 5D are three other examples of scattered
pilot patterns which can be generated according to this
embodiment.
[0127] FIG. 6 is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0128] FIG. 7A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0129] FIG. 7B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0130] FIG. 7C is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0131] FIG. 7D is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0132] FIG. 8 is a diagram of a single antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0133] FIG. 9A is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0134] FIG. 9B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0135] FIG. 9C is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0136] FIG. 9D is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0137] FIG. 10A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0138] FIG. 10B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0139] FIG. 11A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0140] FIG. 11B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0141] FIG. 12A is a diagram of a one antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0142] FIG. 12B is a diagram of a four antenna irregular diamond
lattice scattered pilot pattern which can be used in accordance
with an embodiment of the present invention.
[0143] For the purposes of providing context for embodiments of the
invention for use in a communication system, FIGS. 13-17 will now
be described. As will be described in more detail below, the method
of the present invention can, in one embodiment, be implemented
through means of the channel estimation logic of a conventional
OFDM receiver (see channel estimation 96 in FIG. 17).
[0144] FIG. 13 shows a base station controller (BSC) 10 which
controls wireless communications within multiple cells 12, which
cells are served by corresponding base stations (BS) 14. In
general, each base station 14 facilitates communications using OFDM
with mobile and/or wireless terminals 16, which are within the cell
12 associated with the corresponding base station 14. The movement
of the mobile terminals 16 in relation to the base stations 14
results in significant fluctuation in channel conditions. As
illustrated, the base stations 14 and mobile terminals 16 may
include multiple antennas to provide spatial diversity for
communications.
[0145] A h1gh level overview of the mobile terminals 16 and base
stations 14 upon which aspects of the present invention may be
implemented is provided prior to delving into the structural and
functional details of the preferred embodiments. With reference to
FIG. 14, a base station 14 is illustrated. The base station 14
generally includes a control system 20, a baseband processor 22,
transmit circuitry 24, receive circuitry 26, multiple antennas 28,
and a network interface 30. The receive circuitry 26 receives radio
frequency signals bearing information from one or more remote
transmitters provided by mobile terminals 16 (illustrated in FIG.
13). A low noise amplifier and a filter (not shown) may cooperate
to amplify and remove broadband interference from the signal for
processing. Downconversion and digitization circuitry (not shown)
will then downconvert the filtered, received signal to an
intermediate or baseband frequency signal, which is then digitized
into one or more digital streams.
[0146] The baseband processor 22 processes the digitized received
signal to extract the information or data bits conveyed in the
received signal. This processing typically comprises demodulation,
decoding, and error correction operations. As such, the baseband
processor 22 is generally implemented in one or more digital signal
processors (DSPs) or application-specific integrated circuits
(ASICs). The received information is then sent across a wireless
network via the network interface 30 or transmitted to another
mobile terminal 16 serviced by the base station 14.
[0147] On the transmit side, the baseband processor 22 receives
digitized data, which may represent voice, data, or control
information, from the network interface 30 under the control of
control system 20, and encodes the data for transmission. The
encoded data is output to the transmit circuitry 24, where it is
modulated by a carrier signal having a desired transmit frequency
or frequencies. A power amplifier (not shown) will amplify the
modulated carrier signal to a level appropriate for transmission,
and deliver the modulated carrier signal to the antennas 28 through
a matching network (not shown). Various modulation and processing
techniques available to those skilled in the art are used for
signal transmission between the base station and the mobile
terminal.
[0148] With reference to FIG. 15, a mobile terminal 16 configured
according to one embodiment of the present invention is
illustrated. Similarly to the base station 14, the mobile terminal
16 will include a control system 32, a baseband processor 34,
transmit circuitry 36, receive circuitry 38, multiple antennas 40,
and user interface circuitry 42. The receive circuitry 38 receives
radio frequency signals bearing information from one or more base
stations 14. A low noise amplifier and a filter (not shown) may
cooperate to amplify and remove broadband interference from the
signal for processing. Downconversion and digitization circuitry
(not shown) will then downconvert the filtered, received signal to
an intermediate or baseband frequency signal, which is then
digitized into one or more digital streams.
[0149] The baseband processor 34 processes the digitized received
signal to extract the information or data bits conveyed in the
received signal. This processing typically comprises demodulation,
decoding, and error correction operations. The baseband processor
34 is generally implemented in one or more digital signal
processors (DSPs) and application specific integrated circuits
(ASICs).
[0150] For transmission, the baseband processor 34 receives
digitized data, which may represent voice, data, or control
information, from the control system 32, which it encodes for
transmission. The encoded data is output to the transmit circuitry
36, where it is used by a modulator to modulate a carrier signal
that is at a desired transmit frequency or frequencies. A power
amplifier (not shown) will amplify the modulated carrier signal to
a level appropriate for transmission, and deliver the modulated
carrier signal to the antennas 40 through a matching network (not
shown). Various modulation and processing techniques available to
those skilled in the art are used for signal transmission between
the mobile terminal and the base station.
[0151] In OFDM modulation, the transmission band is divided into
multiple, orthogonal carrier waves. Each carrier wave is modulated
according to the digital data to be transmitted. Because OFDM
divides the transmission band into multiple carriers, the bandwidth
per carrier decreases and the modulation time per carrier
increases. Since the multiple carriers are transmitted in parallel,
the transmission rate for the digital data, or symbols, on any
given carrier is lower than when a single carrier is used.
[0152] OFDM modulation utilizes the performance of an Inverse Fast
Fourier Transform (IFFT) on the information to be transmitted. For
demodulation, the performance of a Fast Fourier Transform (FFT) on
the received signal recovers the transmitted information. In
practice, the IFFT and FFT are provided by digital signal
processing carrying out an Inverse Discrete Fourier Transform
(IDFT) and Discrete Fourier Transform (DFT), respectively.
Accordingly, the characterizing feature of OFDM modulation is that
orthogonal carrier waves are generated for multiple bands within a
transmission channel. The modulated signals are digital signals
having a relatively low transmission rate and capable of staying
within their respective bands. The individual carrier waves are not
modulated directly by the digital signals. Instead, all carrier
waves are modulated at once by IFFT processing.
[0153] In operation, OFDM is preferably used for at least down-link
transmission from the base stations 14 to the mobile terminals 16.
Each base station 14 is equipped with "n" transmit antennas 28, and
each mobile terminal 16 is equipped with "m" receive antennas 40.
Notably, the respective antennas can be used for reception and
transmission using appropriate duplexers or switches and are so
labeled only for clarity.
[0154] With reference to FIG. 16, a logical OFDM transmission
architecture will be described. Initially, the base station
controller 10 will send data to be transmitted to various mobile
terminals 16 to the base station 14. The base station 14 may use
the channel quality indicators (CQIs) associated with the mobile
terminals to schedule the data for transmission as well as select
appropriate coding and modulation for transmitting the scheduled
data. The CQIs may be directly from the mobile terminals 16 or
determined at the base station 14 based on information provided by
the mobile terminals 16. In either case, the CQI for each mobile
terminal 16 is a function of the degree to which the channel
amplitude (or response) varies across the OFDM frequency band.
[0155] Scheduled data 44, which is a stream of bits, is scrambled
in a manner reducing the peak-to-average power ratio associated
with the data using data scrambling logic 46. A cyclic redundancy
check (CRC) for the scrambled data is determined and appended to
the scrambled data using CRC adding logic 48. Next, channel coding
is performed using channel encoder logic 50 to effectively add
redundancy to the data to facilitate recovery and error correction
at the mobile terminal 16. Again, the channel coding for a
particular mobile terminal 16 is based on the CQI. In some
implementations, the channel encoder logic 50 uses known Turbo
encoding techniques. The encoded data is then processed by rate
matching logic 52 to compensate for the data expansion associated
with encoding.
[0156] Bit interleaver logic 54 systematically reorders the bits in
the encoded data to minimize the loss of consecutive data bits. The
resultant data bits are systematically mapped into corresponding
symbols depending on the chosen baseband modulation by mapping
logic 56. Preferably, Quadrature Amplitude Modulation (QAM) or
Quadrature Phase Shift Key (QPSK) modulation is used. The degree of
modulation is preferably chosen based on the CQI for the particular
mobile terminal. The symbols may be systematically reordered to
further bolster the immunity of the transmitted signal to periodic
data loss caused by frequency selective fading using symbol
interleaver logic 58.
[0157] At this point, groups of bits have been mapped into symbols
representing locations in an amplitude and phase constellation.
When spatial diversity is desired, blocks of symbols are then
processed by space-time block code (STC) encoder logic 60, which
modifies the symbols in a fashion making the transmitted signals
more resistant to interference and more readily decoded at a mobile
terminal 16. The STC encoder logic 60 will process the incoming
symbols and provide "n" outputs corresponding to the number of
transmit antennas 28 for the base station 14. The control system 20
and/or baseband processor 22 as described above with respect to
FIG. 14 will provide a mapping control signal to control STC
encoding. At this point, assume the symbols for the "n" outputs are
representative of the data to be transmitted and capable of being
recovered by the mobile terminal 16.
[0158] For the present example, assume the base station 14 has two
antennas 28 (n=2) and the STC encoder logic 60 provides two output
streams of symbols. Accordingly, each of the symbol streams output
by the STC encoder logic 60 is sent to a corresponding IFFT
processor 62, illustrated separately for ease of understanding.
Those skilled in the art will recognize that one or more processors
may be used to provide such digital signal processing, alone or in
combination with other processing described herein. The IFFT
processors 62 will preferably operate on the respective symbols to
provide an inverse Fourier Transform. The output of the IFFT
processors 62 provides symbols in the time domain. The time domain
symbols are grouped into frames, which are associated with a prefix
by prefix insertion logic 64. Each of the resultant signals is
up-converted in the digital domain to an intermediate frequency and
converted to an analog signal via the corresponding digital
up-conversion (DUC) and digital-to-analog (D/A) conversion
circuitry 66. The resultant (analog) signals are then
simultaneously modulated at the desired RF frequency, amplified,
and transmitted via the RF circuitry 68 and antennas 28. Notably,
pilot signals known by the intended mobile terminal 16 are
scattered among the sub-carriers. The mobile terminal 16, which is
discussed in detail below, will use the pilot signals for channel
estimation.
[0159] Reference is now made to FIG. 17 to illustrate reception of
the transmitted signals by a mobile terminal 16. Upon arrival of
the transmitted signals at each of the antennas 40 of the mobile
terminal 16, the respective signals are demodulated and amplified
by corresponding RF circuitry 70. For the sake of conciseness and
clarity, only one of the two receive paths is described and
illustrated in detail. Analog-to-digital (A/D) converter and
down-conversion circuitry 72 digitizes and downconverts the analog
signal for digital processing. The resultant digitized signal may
be used by automatic gain control circuitry (AGC) 74 to control the
gain of the amplifiers in the RF circuitry 70 based on the received
signal level.
[0160] Initially, the digitized signal is provided to
synchronization logic 76, which includes coarse synchronization
logic 78, which buffers several OFDM symbols and calculates an
auto-correlation between the two successive OFDM symbols. A
resultant time index corresponding to the maximum of the
correlation result determines a fine synchronization search window,
which is used by fine synchronization logic 80 to determine a
precise framing starting position based on the headers. The output
of the fine synchronization logic 80 facilitates frame acquisition
by frame alignment logic 84. Proper framing alignment is important
so that subsequent FFT processing provides an accurate conversion
from the time domain to the frequency domain. The fine
synchronization algorithm is based on the correlation between the
received pilot signals carried by the headers and a local copy of
the known pilot data. Once frame alignment acquisition occurs, the
prefix of the OFDM symbol is removed with prefix removal logic 86
and resultant samples are sent to frequency offset correction logic
88, which compensates for the system frequency offset caused by the
unmatched local oscillators in the transmitter and the receiver.
Preferably, the synchronization logic 76 includes frequency offset
and clock estimation logic 82, which is based on the headers to
help estimate such effects on the transmitted signal and provide
those estimations to the correction logic 88 to properly process
OFDM symbols.
[0161] At this point, the OFDM symbols in the time domain are ready
for conversion to the frequency domain using FFT processing logic
90. The results are frequency domain symbols, which are sent to
processing logic 92. The processing logic 92 extracts the scattered
pilot signal using scattered pilot extraction logic 94, determines
a channel estimate based on the extracted pilot signal using
channel estimation logic 96, and provides channel responses for all
sub-carriers using channel reconstruction logic 98. In order to
determine a channel response for each of the sub-carriers, the
pilot signal is essentially multiple pilot symbols that are
scattered among the data symbols throughout the OFDM sub-carriers
in a known pattern in both time and frequency. Examples of
scattering of pilot symbols among available sub-carriers over a
given time and frequency plot in an OFDM environment are found in
PCT Patent Application No. PCT/CA2005/000387 filed Mar. 15, 2005
assigned to the same assignee of the present application.
Continuing with FIG. 17, the processing logic compares the received
pilot symbols with the pilot symbols that are expected in certain
sub-carriers at certain times to determine a channel response for
the sub-carriers in which pilot symbols were transmitted. The
results are interpolated to estimate a channel response for most,
if not all, of the remaining sub-carriers for which pilot symbols
were not provided. The actual and interpolated channel responses
are used to estimate an overall channel response, which includes
the channel responses for most, if not all, of the sub-carriers in
the OFDM channel.
[0162] The frequency domain symbols and channel reconstruction
information, which are derived from the channel responses for each
receive path are provided to an STC decoder 100, which provides STC
decoding on both received paths to recover the transmitted symbols.
The channel reconstruction information provides equalization
information to the STC decoder 100 sufficient to remove the effects
of the transmission channel when processing the respective
frequency domain symbols.
[0163] The recovered symbols are placed back in order using symbol
de-interleaver logic 102, which corresponds to the symbol
interleaver logic 58 of the transmitter. The de-interleaved symbols
are then demodulated or de-mapped to a corresponding bitstream
using de-mapping logic 104. The bits are then de-interleaved using
bit de-interleaver logic 106, which corresponds to the bit
interleaver logic 54 of the transmitter architecture. The
de-interleaved bits are then processed by rate de-matching logic
108 and presented to channel decoder logic 110 to recover the
initially scrambled data and the CRC checksum. Accordingly, CRC
logic 112 removes the CRC checksum, checks the scrambled data in
traditional fashion, and provides it to the de-scrambling logic 114
for de-scrambling using the known base station de-scrambling code
to recover the originally transmitted data 116.
[0164] In parallel to recovering the data 116, a CQI, or at least
information sufficient to create a CQI at the base station 14, is
determined and transmitted to the base station 14. As noted above,
the CQI may be a function of the carrier-to-interference ratio
(CR), as well as the degree to which the channel response varies
across the various sub-carriers in the OFDM frequency band. The
channel gain for each sub-carrier in the OFDM frequency band being
used to transmit information is compared relative to one another to
determine the degree to which the channel gain varies across the
OFDM frequency band. Although numerous techniques are available to
measure the degree of variation, one technique is to calculate the
standard deviation of the channel gain for each sub-carrier
throughout the OFDM frequency band being used to transmit data.
[0165] FIG. 18 is a block diagram of one embodiment of the present
invention. In this embodiment, the present invention is shown being
implemented within channel estimation logic 96 of FIG. 17 with the
conventional aspects of channel estimation logic 96 being shown in
dotted outline for ease of reference. Persons skilled in the art
will appreciate that the present invention could be implemented as
a separate logical component as well.
[0166] Shown is time direction channel calculator 127 which
performs the calculation of channel change in the time direction.
Frequency direction channel calculator 129 performs the calculation
of channel change in the frequency direction. As explained above,
the preferred calculation is the inner product of the two pilot
assisted channel estimates being compared. Though time direction
channel calculator 127 is shown as being illustrated to the right
of frequency direction channel calculator 129, this does not mean
that the time direction channel calculation is necessarily to be
performed first or that the calculations cannot be performed
simultaneously. Either calculation can be performed first, or both
can be performed simultaneously. Channel direction comparator 131
compares the results of the calculations performed by both
direction channel calculator 127 and frequency direction channel
calculator 129 for the purpose of comparing and ascertaining which
channel direction, time or frequency, changes slower. Channel
direction selector 133 selects which of the two directions changes
slower. Block 135 is utilized to interpolate, first in the
direction of slower change, and then in the direction of faster
change, in accordance with conventional means.
[0167] In operation, time direction channel calculator 127 receives
two pilot assisted channel estimates and performs the calculation
of channel change in the time direction. Frequency direction
channel calculator 129 performs the calculation of channel change
in the frequency direction though these two calculations can be
performed in different order or simultaneously. Channel direction
comparator 131 compares the results of the calculations performed
by both direction channel calculator 127 and frequency direction
channel calculator 129 and compares which channel direction, time
or frequency, changes slower. Channel direction selector 133
selects the direction of slower change and interpolation is then
performed by block 135 in that direction first, and then in the
direction of faster change in accordance with conventional
means.
[0168] FIGS. 13 to 18 each provide a specific example of a
communication system or elements of a communication system that
could be used to implement embodiments of the invention. It is to
be understood that embodiments of the invention can be implemented
with communications systems having architectures that are different
than the specific example, but that operate in a manner consistent
with the implementation of the embodiments as described herein.
[0169] In accordance with an embodiment of the invention FIG. 19
presents a time division multiplexing (TDM) Pilot Pattern for use
in a single antenna scenario. According to this embodiment the
overhead associated therewith is: 1/28 per Antenna.
[0170] In accordance with an embodiment of the invention FIG. 20
presents a TDM Pilot Pattern for use in a four antenna scenario.
According to this embodiment the overhead associated therewith is:
1/28 per Antenna.
[0171] Numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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