U.S. patent application number 12/554811 was filed with the patent office on 2009-12-31 for signal generation using phase-shift based pre-coding.
Invention is credited to Jae-Won Chang, Jin-Young Chun, Bin-Chul Ihm, Jin-Hyuk Jung, Moon-Il Lee.
Application Number | 20090323863 12/554811 |
Document ID | / |
Family ID | 39091685 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090323863 |
Kind Code |
A1 |
Lee; Moon-Il ; et
al. |
December 31, 2009 |
SIGNAL GENERATION USING PHASE-SHIFT BASED PRE-CODING
Abstract
A phase-shift based pre-coding scheme used in a transmitting
side and a receiving side that has less complexity than those of a
space-time coding scheme, that can support various spatial
multiplexing rates while maintaining the advantages of the
phase-shift diversity scheme, that has less channel sensitivity
than that of the pre-coding scheme, and that only requires a low
capacity codebook is provided.
Inventors: |
Lee; Moon-Il; (Gyeonggi-Do,
KR) ; Ihm; Bin-Chul; (Gyeonggi-Do, KR) ; Chun;
Jin-Young; (Seoul, KR) ; Chang; Jae-Won;
(Gyeonggi-Do, KR) ; Jung; Jin-Hyuk; (Gyeonggi-Do,
KR) |
Correspondence
Address: |
LEE, HONG, DEGERMAN, KANG & WAIMEY
660 S. FIGUEROA STREET, Suite 2300
LOS ANGELES
CA
90017
US
|
Family ID: |
39091685 |
Appl. No.: |
12/554811 |
Filed: |
September 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11754873 |
May 29, 2007 |
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12554811 |
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60803340 |
May 26, 2006 |
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Current U.S.
Class: |
375/308 |
Current CPC
Class: |
H04L 27/26362 20210101;
H04L 27/2634 20130101; H04B 7/0682 20130101; H04B 7/0456 20130101;
H04B 7/0465 20130101; H04B 7/0671 20130101; H04B 7/0619
20130101 |
Class at
Publication: |
375/308 |
International
Class: |
H04L 27/20 20060101
H04L027/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2006 |
KR |
10-2006-65303 |
Oct 2, 2006 |
KR |
10-2006-97216 |
Claims
1-28. (canceled)
29. A method for transmitting a signal by preceding the signal in a
multiple antenna system using multiple subcarriers, the method
comprising: selecting a specific preceding matrix from a codebook,
wherein a column vector of a first preceding matrix within the
codebook for a first multiplexing rate is included as a column
vector of a second preceding matrix within the codebook for a
second multiplexing rate, the second multiplexing rate being
greater than the first multiplexing rate; preceding the signal
using the specific preceding matrix; and transmitting the precoded
signal to a receiver.
30. The method of claim 29, wherein preceding matrixes within the
codebook are a unitary matrix.
31. The method of claim 29, wherein the specific preceding matrix
is selected based on feedback information received from the
receiver.
32. The method of claim 29, wherein selecting the specific
preceding matrix comprises determining a diagonal matrix for phase
shift.
33. The method of claim 29, wherein the specific preceding matrix
is selected based on a multiplexing rate.
34. The method of claim 33, wherein a number of column vectors of
the specific preceding matrix is determined based on the
multiplexing rate.
35. A method for receiving a precoded signal in a multiple antenna
system, the method comprising: receiving the signal from a
transmitter; selecting a specific precoding matrix from a codebook,
wherein a column vector of a first precoding matrix within the
codebook for a first multiplexing rate is included as a column
vector of a second precoding matrix within the codebook for a
second multiplexing rate, the second multiplexing rate being
greater than the first multiplexing rate; and performing a function
opposite to a preceding on the received signal using the specific
precoding matrix.
36. The method of claim 35, wherein preceding matrixes within the
codebook are a unitary matrix.
37. An apparatus for transmitting a signal by preceding the signal
in a multiple antenna system using multiple subcarriers, the
apparatus comprising: a memory unit storing a codebook, wherein a
column vector of a first precoding matrix within the codebook for a
first multiplexing rate is included as a column vector of a second
preceding matrix within the codebook for a second multiplexing
rate, the second multiplexing rate being greater than the first
multiplexing rate; a precoder selecting a specific precoding matrix
from the codebook stored in the memory unit and precoding the
signal using the specific preceding matrix; and an antenna for
transmitting the precoded signal to a receiver.
38. The apparatus of claim 37, wherein preceding matrixes within
the codebook are a unitary matrix.
39. The apparatus of claim 37, wherein the precoder selects the
specific preceding matrix based on feedback information received
from the receiver.
40. The apparatus of claim 37, wherein the precoder determines a
diagonal matrix for a phase shift.
41. The apparatus of claim 37, wherein the precoder selects the
specific preceding matrix based on a multiplexing rate.
42. The apparatus of claim 41, wherein a number of column vectors
of the specific precoding matrix is determined based on the
multiplexing rate.
43. An apparatus for receiving a precoded signal in a multiple
antenna system, the apparatus comprising: one or more antennas for
receiving the signal from a transmitter; and a precoder selecting a
specific precoding matrix from a codebook and performing a function
opposite to a preceding on the received signal using the specific
preceding matrix, wherein a column vector of a first precoding
matrix within the codebook for a first multiplexing rate is
included as a column vector of a second preceding matrix within the
codebook for a second multiplexing rate, the second multiplexing
rate being greater than the first multiplexing rate.
44. The apparatus of claim 43, wherein precoding matrixes within
the codebook are a unitary matrix.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] Pursuant to 35 U.S.C. .sctn. 119, this application claims
the benefit of earlier filing date and right of priority to
Provisional Application No. 60/803,340, filed on May 26, 2006,
Korea Application No. 10-2006-65303, filed Jul. 12, 2006, Korean
Application No. 10-2006-97216, filed Oct. 2, 2006, the contents of
which are hereby incorporated by reference herein in their
entirety.
BACKGROUND
[0002] This disclosure relates to signal generation using
phase-shift based pre-coding.
[0003] Certain multi-carrier based wireless access techniques do
not adequately support mobile communication systems with various
types of antenna structures.
[0004] The present inventors recognized certain shortcomings
related to certain multi-carrier based multiple antenna
transmitting and/or receiving techniques. Based upon such
recognition, the following features have been conceived.
BRIEF DESCRIPTION
[0005] A phase-shift based pre-coding scheme used in a transmitting
side and a receiving side that has less complexity than those of a
space-time coding scheme, that can support various spatial
multiplexing rates while maintaining the advantages of the
phase-shift diversity scheme, that has less channel sensitivity
than that of the pre-coding scheme, and that only requires a low
capacity codebook has been conceived and provided herein. In
particular, the matrix used for performing phase-shift based
pre-coding can be more easily expanded and implemented according to
any changes in the number of antennas being employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 shows an exemplary structure of a Multiple-Input
Multiple-Output (MIMO) system using Orthogonal Frequency Division
Multiplexing (OFDM).
[0007] FIG. 2 shows an exemplary structure of a transmitting side
for a multiple antenna system using the cyclic delay diversity
method.
[0008] FIG. 3 shows an exemplary structure of a transmitting side
for a multiple antenna system using the phase-shift diversity
method.
[0009] FIG. 4 is a graph showing examples of two types of
phase-shift diversity methods.
[0010] FIG. 5 shows an exemplary structure of a transmitting side
for a multiple antenna system using a pre-coding method.
[0011] FIG. 6 shows the exemplary procedures in performing a
phase-shift diversity method in a system having 4 antennas with a
spatial multiplexing rate of 2.
[0012] FIG. 7 shows an example of how a phase-shift based
pre-coding method is applied to the system of FIG. 6.
[0013] FIG. 8 shows an exemplary pre-coding matrix used in the
phase-shift based pre-coding method for the system of FIG. 7.
[0014] FIG. 9 shows an exemplary block diagram of a transceiving
apparatus the supports the phase-shift based pre-coding method.
[0015] FIG. 10 shows an exemplary block diagram of a SCW OFDM
transmitting unit within the radio communication unit of FIG.
9.
[0016] FIG. 11 shows an exemplary block diagram of a MCW OFDM
transmitting unit within the radio communication unit of FIG.
9.
[0017] FIG. 12 is a graph showing a comparison in performance
differences when the phase-shift pre-coding (PSP) method of the
present invention disclosure and the spatial multiplexing (SM)
method of the background art are respectively applied to a ML
(Minimum Likelihood) receiver and a MMSE (Minimum Mean Squared
Error) receiver.
[0018] FIGS. 13 and 14 are graphs showing a comparison of the
performance differences per coding rate for the phase-shift based
pre-coding method of the present disclosure and for the background
art spatial multiplexing method being applied to a MMSE (Minimum
Mean Squared Error) receiver for a PedA (ITU Pedestrian A) fading
channel environment and a TU (Typical Urban) fading channel
environment.
[0019] FIGS. 15 through 17 are graphs showing a comparison of the
performance differences when the present disclosure phase-shift
based pre-coding method and the background art spatial multiplexing
method are applied to a system employing SCW (Single CodeWord) and
MCW (Multi CodeWord) in a PedA (ITU Pedestrian A) fading channel
environment and a TU (Typical Urban) fading channel
environment.
[0020] FIG. 18 is a graph showing the performance differences in
the cases where the spatial diversity method+cyclic delay diversity
method is applied, and the present disclosure phase-shift based
pre-coding method+cyclic delay diversity method is applied to a MCS
(Modulation and Coding Set) in a flat fading channel
environment.
[0021] FIG. 19 shows an exemplary overview structure of downlink
baseband signal generation according to the present disclosure.
DETAILED DESCRIPTION
[0022] Information communication services have become more popular
and with the introduction of various multimedia services and high
quality services, there is an increased demand for enhanced
wireless (radio) communication services. In order to actively meet
such demands, the capacity and data transmission reliability of the
communication system should be increased. To increase communication
capacity in a wireless (radio) communication environment, one
method would be to find newly usable bandwidth and another would be
to improve the efficiency of given resources. As some examples of
the latter method, multiple antenna transmitting/receiving
(transceiving) techniques are recently gaining attention and being
actively developed, whereby a plurality of antennas are provided at
the transceiver in order to obtain diversity gain by additionally
securing spatial domain for resource utilization, or increasing
transmission capacity by transmitting data in parallel via each
antenna.
[0023] Among such multiple antenna transceving techniques, an
example would be Multiple-Input Multiple-Output (MIMO) system based
on Orthogonal Frequency Division Multiplexing (OFDM), the general
structure of which will now be explained with reference to FIG.
1.
[0024] At the transmitting side (or transmitter), a channel encoder
101 serves the purpose of reducing the effects due to the channel
or noise by attaching repetitive bits to the transmission data
bits. A mapper 103 changes the data bit information into data
symbol information. A serial-to-parallel converter 105 changes
serial inputs into parallel outputs for the purpose of MIMO
processing on the data symbols. In the receiving side (or
receiver), a multiple antenna decoder 109, a parallel-to-serial
converter 111, a demapper 113, and a channel decoder 115 perform
the opposite operations as those of the multiple antenna encoder
107, the serial-to-parallel converter 105, the mapper 103, and the
channel encoder 10 of the transmitting side described above.
[0025] In a multiple antenna OFDM system, various techniques are
necessary to increase transmission reliability. For example,
space-time code (STC) techniques, cyclic delay diversity (CDD)
techniques, antenna selection (AS) techniques, antenna hopping (AH)
techniques, spatial multiplexing (SM) techniques, beam-forming (BF)
techniques, pre-coding techniques, and the like may be employed.
Among these techniques, some will be explained in more detail
hereafter.
[0026] The space-time code (STC) technique, for a multiple antenna
environment, relates to continuously (sequentially) transmitting
the same signal, but in case of repetitive transmissions,
transmitting through different antennas is performed, in order to
obtain spatial diversity gain. The following matrix represents the
most basic space-time code that is used in a system with two
transmit antennas.
1 2 [ S 1 - S 2 * S 2 S 1 * ] ##EQU00001##
[0027] In the above matrix, the rows represent antennas and the
columns represent time slots.
[0028] Such space-time code technique has some shortcoming. For
example, respectively different forms of space-time codes are
required according to how the antenna structure changes, the
transmitting side and receiving side have increased complexity
because data symbols are repeatedly transmitted through a plurality
of time slots in order to obtain spatial diversity, and has
respectively lower performance compared to that of other
closed-loop systems because data is transmitted without using
feedback information. Table 1 below shows the need for respectively
different space-time codes according to antenna structures.
TABLE-US-00001 TABLE 1 Spatial # of STC Scheme Multiplexing Rate Tx
antennas 1 2 [ S 1 - S 2 * S 2 S 1 ] ##EQU00002## 1 2 1 2 [ S 1 S 2
] ##EQU00003## 2 2 1 2 ( 1 + r 2 ) [ S 1 + jr S 4 r S 2 + S 3 S 2 -
r S 3 jr S 1 + S 4 ] , r = 5 .+-. 1 / 2 ##EQU00004## 2 2 1 2 [ S 1
S 2 S 3 S 4 S 2 * - S 1 * S 4 * - S 3 * S 3 - S 4 - S 1 S 2 S 4 * S
3 * - S 2 * - S 1 * ] ##EQU00005## 1 4 1 2 [ S 1 S 2 0 0 - S 2 * S
1 * 0 0 0 0 S 3 S 4 0 0 - S 4 * S 3 * ] ##EQU00006## 1 4 1 2 [ S 1
- S 2 * S 5 - S 6 * S 2 S 1 * S 6 S 5 * S 3 - S 4 * S 7 - S 8 * S 4
S 3 * S 8 S 7 * ] ##EQU00007## 2 4
[0029] Cyclic Delay Diversity (CDD) is a method in which frequency
diversity gain is obtained at the receiving side, by using the
antennas to respectively transmit signals with different delays or
different magnitudes when transmitting OFDM signals in a system
having multiple transceiving antennas.
[0030] FIG. 2 shows a general structure of a transmitting side of a
multiple antenna system using the cyclic delay diversity
method.
[0031] Upon separating and delivering the OFDM symbols to each
antenna via a serial-to-parallel converter and a multiple antenna
encoder, an Inverse Fast Fourier Transform (IFFT) for changing a
frequency domain signal into a time domain signal and a cyclic
prefix (CP) for minimizing interference between channels are added
and transmitted to the receiving side. Here, the data sequence
delivered to the first antenna is transmitted to the receiving side
as is (i.e., without any changes), while the data sequence
transmitted from other transmit antennas is delayed in cyclic shift
manner when compared to a first antenna.
[0032] Meanwhile, when such cyclic delay diversity scheme is
implemented in the frequency domain, the cyclic delay may be
mathematically expressed as a multiplication of phase
sequences.
[0033] Namely, as can be seen in FIG. 3, a particular phase
sequence (e.g., phase sequence 1.about.phase sequence M) that has
been set differently for each antenna is multiplied to each data
sequence in the frequency domain, and upon performing IFFT (Inverse
Fast Fourier Transform) processing, such can be transmitted to the
receiving side. This is referred to as a phase-shift diversity
scheme.
[0034] By using the phase-shift diversity method, the flat fading
channel may be changed to a frequency selectivity channel, and
frequency diversity gain or frequency scheduling gain may be
obtained according to a cyclic delay sample.
[0035] Namely, as can be seen in FIG. 4, in the phase-shift
diversity scheme, when generating a phase sequence by using a
relatively large value cyclic delay sample, because the phase
variation period is shortened (decreased), frequency selectivity is
increased and as a result, the channel codes may exploit frequency
diversity gain. Such is typically used in so-called open-loop
systems.
[0036] Also, when a small value cyclic delay is used, the phase
variation period is lengthened (increased), and by using this in a
closed-loop system, resources are allocated to the most
satisfactory channel region such that frequency scheduling gain can
be obtained. Namely, as can be seen in FIG. 4, in the phase-shift
diversity scheme, when a relatively small value cyclic delay is
used for generating a phase sequence, certain sub-carrier regions
of the flat fading channel result in an increase in channel
magnitude, while other sub-carrier regions result in a decrease in
channel magnitude. In such case, for an OFDMA system that
accommodates multiple users, when a signal is transmitted through a
sub-carrier having an increased channel magnitude per user, the
signal-to-noise ratio can be increased.
[0037] However, despite some benefits of the above-described cyclic
delay diversity scheme or phase-shift diversity scheme, because the
spatial multiplexing rate is 1, the data transmission rate cannot
be increased as desired.
[0038] The pre-coding scheme may include a codebook based
pre-coding method used when there is a finite (or limited) amount
of feedback information in a closed-loop system and may include a
method of performing feedback upon quantization of channel
information. Here, codebook based pre-coding refers to obtaining
signal-to-noise ratio (SNR) gain by feeding back, to the
transmitting side, an index of a pre-coding matrix that is already
known by both the transmitting side and the receiving side.
[0039] FIG. 5 depicts an exemplary structure of a transceiving side
of a multiple antenna system using codebook based pre-coding. Here,
the transmitting side and the receiving side respectively have a
finite pre-coding matrix (P.sub.1.about.P.sub.L), and at the
receiving side, channel information is used to feed back an optimal
pre-coding matrix index (I), while at the transmitting side, a
pre-coding matrix corresponding to the fed back index is applied to
transmission data (x.sub.1.about.x.sub.Mt).
[0040] Such codebook based pre-coding scheme is beneficial in that
effective data transmission is possible due to feedback of the
index. However, because a stable channel is necessary, such
codebook based pre-coding may not be fully appropriate for a mobile
environment with severe channel changes. Also, some loss in the
uplink transmission rate may occur due to the feedback overhead for
the preceding matrix index. Additionally, because a codebook is
needed in both the transmitting side and the receiving side,
increased memory usage may be required.
[0041] The present inventors recognized at least the
above-described issues in certain data transmission, reception, and
processing techniques for multiple antenna systems. Based upon such
recognition, the following features have been conceived to address
and/or to solve such issues.
[0042] This disclosure relates to and claims priority benefit of
U.S. Provisional Application No. 60/803,340 (filed May, 26, 2006),
Korean Patent Application number 10-2006-0065303 (filed Jul. 12,
2006), and Korean Patent Application number 10-2006-0097216 (filed
Oct. 2, 2006), which contents are specifically incorporated herein
by reference.
[0043] The present disclosure relates to a phase-shift based
pre-coding method of a multiple antenna system using a plurality of
sub-carriers, as well as a generalized phase-shift based pre-coding
method and a transceiver device supporting the same.
[0044] Hereafter, a phase-shift based pre-coding method will be
explained with respect to a 2-antenna system and a 4-antenna
system, and a method of forming a generalized phase-shift based
pre-coding matrix to be extendedly used for a system with an
N.sub.t number of (physical or virtual) antennas will be
described.
[0045] Phase-Shift Based Pre-Coding Method
[0046] A phase-shift based pre-coding matrix (P) proposed herein
may be generally expressed in the following manner:
P N i .times. R k = ( w 1 , 1 k w 1 , 2 k w 1 , R k w 2 , 1 k w 2 ,
2 k w 2 , R k w N i , 1 k w N i , 2 k w N i , R k ) [ Equation 1 ]
##EQU00008##
[0047] Here, w.sub.i,j.sup.k, i=1, . . . , N.sub.t, j=1, . . . , R
refers to multiple complex weight values determined by sub-carrier
index k, N.sub.t refers to the number of (physical or virtual)
transmit antennas, and R refers to a spatial multiplexing rate. If
the phase-shift based pre-coding matrix (P) is used in case of a
physical antenna scheme, the N.sub.t may be number of antenna port.
Such sub-carrier index k can be replaced by resource index
including sub-band index. However, if P is used in case of a
virtual antenna scheme, the N.sub.t may be a spatial multiplexing
rate (i.e., R). Here, the multiple complex weight values may be
different according to the OFDM symbol being multiplied to the
antenna and according to the corresponding sub-carrier index. The
multiple complex weight value may be determined according to at
least one of a channel condition and whether feedback information
exists or not.
[0048] The pre-coding matrix (P) of Equation 1 may be a unitary
matrix for reducing the loss in channel capacity of a multiple
antenna system. Here, to consider the conditions for forming a
unitary matrix, the channel capacity of a multiple antenna system
may be expressed by the following mathematical equation:
C u ( H ) = log 2 ( det ( I N r + SNR N HH H ) ) [ Equation 2 ]
##EQU00009##
[0049] Here, H refers to a multiple antenna channel matrix having a
size of N.sub.r.times.N.sub.t, and N.sub.r indicates the total
number of receiving antennas. If a phase-shift based pre-coding
matrix (P) is applied to the above Equation 2, the following may be
obtained:
C precoding = log 2 ( det ( I N r + SNR N HPP H H H ) ) [ Equation
3 ] ##EQU00010##
[0050] As shown in Equation 3, to minimize channel loss, PP.sup.H
needs to be an identity matrix, thus the phase-shift based
pre-coding matrix (P) should be a unitary matrix such as the
following:
PP.sup.H=I.sub.N.sub.t [Equation 4]
[0051] In order for the phase-shift based pre-coding matrix (P) to
be a unitary matrix, two conditions should be satisfied. Namely, a
power constraint and an orthogonality constraint should be
simultaneously satisfied. Here, the power constraint refers to
making the size of each column of the matrix to equal one (1), and
the orthogonality constraint refers to making orthogonal
characteristics between each column of the matrix to be satisfied.
The above matters may be expressed mathematically in the following
manner:
w 1 , 1 k 2 + w 2 , 1 k 2 + + w N t , 1 k 2 = 1 , w 1 , 2 k 2 + w 2
, 2 k 2 + + w N t , 2 k 2 = 1 , w 1 , R k 2 + w 2 , R k 2 + + w N t
, R k 2 = 1. [ Equation 5 ] w 1 , 1 k * w 1 , 2 k + w 2 , 1 k * w 2
, 2 k + + w N t , 1 k * w N t , 2 k = 0 w 1 , 1 k * w 1 , 3 k + w 2
, 1 k * w 2 , 3 k + + w N t , 1 k * w N t , 3 k = 0 w 1 , 1 k * w 1
, R k + w 2 , 1 k * w 2 , R k + + w N t , 1 k * w N t , R k = 0 [
Equation 6 ] ##EQU00011##
[0052] As an exemplary embodiment, a generalized equation for a
2.times.2 phase-shift based pre-coding matrix will be provided, and
a mathematical expression for satisfying the above-described two
conditions will be examined. Equation 7 shows a generalized
expression of a phase-shift based pre-coding matrix having a
spatial diversity rate of 2 and having two transmit antennas.
P 2 .times. 2 k = ( .alpha. 1 j k .theta. 1 .beta. 1 j k .theta. 2
.beta. 2 j k .theta. 3 .alpha. 2 j k .theta. 4 ) [ Equation 7 ]
##EQU00012##
[0053] Here, .alpha..sub.i, .beta..sub.i (i=1, 2) are real numbers,
.theta..sub.j (j=1, 2, 3, 4) refers to phase values, and k is a
sub-carrier index of the OFDM signal. In Equation 7, a relationship
between a phase values .theta..sub.i (i=1, . . . , 4) of a
frequency domain and a cyclic delay value .tau..sub.i (i=1, . . . ,
4) of a time domain is expressed as follows:
.theta..sub.i=-2.pi./N.sub.fft.tau..sub.i
where, N.sub.fft denotes the number of subcarriers of the OFDM
signal.
[0054] It can be understood that by those skilled in the art that
the description thus far may be modified and altered in various
ways without departing from the technical scope of the present
disclosure. Accordingly, the technical scope should not be limited
to that described in the detailed description but should be
embraced by the scope of the claims. In order to implement such
pre-coding matrix as a unitary matrix, the power constraint of
Equation 8 and the orthogonality constraint of Equation 9 should be
satisfied.
|.alpha..sub.1e.sup.jk.theta..sup.1|.sup.2+|.beta..sub.2e.sup.jk.theta..-
sup.3|.sup.2=1,
|.alpha..sub.2e.sup.jk.theta..sup.4|.sup.2+|.beta..sub.1e.sup.jk.theta..s-
up.2|.sup.2=1, [Equation 8]
(.alpha..sub.1e.sup.jk.theta..sup.1)*.beta..sub.1e.sup.jk.theta..sup.2+(-
.beta..sub.2e.sup.jk.theta..sup.3)*.alpha..sub.2e.sup.jk.theta..sup.4=0,
[Equation 9]
[0055] Here, the superscript (*) denotes a conjugate complex
number. An example of a 2.times.2 phase-shift based pre-coding
matrix that satisfies the above Equations 7 through 9 may be
expressed as follows:
P 2 .times. 2 k = 1 2 ( 1 j k .theta. 2 j k .theta. 3 1 ) [
Equation 10 ] ##EQU00013##
[0056] Here, .theta..sub.2 and .theta..sub.3 have the relationship
as shown in Equation 11 due to the orthogonality limitations.
k.theta..sub.3=-k.theta..sub.2+.pi. [Equation 11]
[0057] The pre-coding matrix may be stored in the form of a
so-called codebook (or some equivalent type of preceding scheme,
etc.) within a memory (or other type of storage device) of the
transmitting side and the receiving side. Such codebook may include
various types of pre-coding matrices formed by using a finite
number of respectively different .theta..sub.2 values. Also, the
.theta..sub.2 value may be appropriately set according to the
channel environment, transmission rank, system bandwidth and
whether or not feedback information exists, and by setting the
.theta..sub.2 value to be relatively small (e.g., 2 cyclic delay
samples) if feedback information is used or by setting the
.theta..sub.2 value is set to be relatively high (e.g.,
N.sub.fft/N.sub.t cyclic delay samples) if feedback information is
not used, high frequency diversity gain may be obtained.
[0058] Even when a phase-shift based pre-coding matrix such as that
of Equation 7 is formed, situations where the spatial multiplexing
rate should be set to be small compared to the actual number of
antennas according to the channel environment may occur. In such
case, in the phase-shift based pre-coding matrix formed in the
above manner, a particular number of columns corresponding to the
current spatial multiplexing rate (i.e., the spatial multiplexing
rate that was made smaller) may be selected to re-generate a new
phase-shift based pre-coding matrix. Namely, a new pre-coding
matrix applied to the corresponding system need not always be
formed whenever the spatial multiplexing rate changes. Instead, the
initially formed phase-shift based pre-coding matrix may be
employed as is (i.e., without any changes), but one or more
particular columns of the corresponding matrix may be selected to
re-form the pre-coding matrix.
[0059] As one such example, the pre-coding matrix of the above
Equation 10 assumes that the multiple antenna system has 2 transmit
antennas with a spatial multiplexing rate of 2, but there may be
some situations where the spatial multiplexing rate may actually be
reduced to 1 for some particular reason. In such case, pre-coding
may be performed by selecting a particular column from the matrix
of Equation 10 above, and if the second column is selected, the
phase-shift based pre-coding matrix is the same as that shown in
Equation 12 below, and this becomes the same as the cyclic delay
diversity scheme for two transmit antennas of the conventional
art.
P 2 .times. 1 k = 1 2 ( j k .theta. 2 1 ) [ Equation 12 ]
##EQU00014##
[0060] Here, this example assumes a system having 2 transmit
antennas, but such can also be expanded for applicability to
systems with 4 (or more) antennas. Namely, after generating a
phase-shift based pre-coding matrix for the case of 4 transmit
antennas, pre-coding may be performed upon selecting one or more
particular columns according to the changes in the spatial
multiplexing rate.
[0061] As an example, FIG. 6 shows a case where the related art
spatial multiplexing and cyclic delay diversity are applied to a
multiple antenna system having 4 transmit antennas and with a
spatial multiplexing rate of 2, while FIG. 7 shows a case where the
phase-shift based pre-coding matrix of Equation 10 is applied to
such a multiple antenna system.
[0062] According to FIG. 6, a 1.sup.st sequence (S.sub.1) and a
2.sup.nd sequence (S.sub.2) are delivered to the 1.sup.st antenna
and the 3.sup.rd antenna, and a 1.sup.st sequence
(s.sub.1e.sup.jk.crclbar.1) and 2.sup.nd sequence
(s.sub.2e.sup.jk.crclbar.1) that have been phase-shifted by using
phase sequence of e.sup.jk.crclbar.1 are delivered to the 2.sup.nd
antenna and the 4.sup.th antenna. Accordingly, it can be understood
that the overall spatial multiplexing rate is 2.
[0063] On the other hand, according to FIG. 7,
s.sub.1+s.sub.2e.sup.jk.crclbar.2 is delivered to the 1.sup.st
antenna,
s.sub.1e.sup.jk.crclbar.1+s.sub.2e.sup.jk(.crclbar.1+.crclbar.2) is
delivered to the 2.sup.nd antenna,
s.sub.1e.sup.jk.crclbar.3+s.sub.2 is delivered to the 3.sup.rd
antenna, and
s.sub.1e.sup.jk(.crclbar.1+.crclbar.3)+s.sub.2e.sup.jk.crclbar.1 is
delivered to the 4.sup.th antenna. Thus, when compared to the
system of FIG. 6, the advantage of the pre-coding method can be
obtained, and because cyclic delay (or phase-shift) can be
performed for 4 antennas by employing a uniform pre-coding matrix,
the advantage of the cyclic delay diversity scheme can also be
obtained.
[0064] As one of example, the above-described phase-shift based
pre-coding matrices per different spatial multiplexing rates with
respect to a 2-antenna system and a 4-antenna system may be defined
as follows.
TABLE-US-00002 TABLE 2 2-antenna system 4-antenna system Spatial
multiplexing rate 1 Spatial multiplexing rate 2 Spatial
multiplexing rate 1 Spatial multiplexing rate 2 1 2 ( 1 e j .theta.
1 k ) ##EQU00015## 1 2 ( 1 - e - j .theta. 1 k e j .theta. 1 k 1 )
##EQU00016## 1 4 ( 1 e j .theta. 1 k e j .theta. 2 k e j .theta. 3
k ) ##EQU00017## 1 4 ( 1 - e - j .theta. 1 k e j .theta. 1 k 1 e j
.theta. 2 k - e - j .theta. 3 k e j .theta. 3 k - e - j .theta. 2 k
) ##EQU00018##
[0065] Here, .theta..sub.j (j=1, 2, 3) refers to a phase angle
according to the cyclic delay value, and k refers to an OFDM
sub-carrier index. In Table 2 above, each pre-coding matrix of the
four situations may be obtained by using a particular portion of
the pre-coding matrix with respect to a multiple antenna system
having 4 antennas with a spatial multiplexing rate of 2 (as can be
seen in FIG. 8). Accordingly, each pre-coding matrix for such four
situations need not be separately provided in a codebook, thus the
memory resources in the transmitting side and the receiving side
may be conserved.
[0066] Referring to Table 2, it should be noted that when forming
an appropriate phase-shift based pre-coding matrix according to a
changed spatial multiplexing rate, a new column that satisfies the
orthogonal constraint to the other columns may be added.
[0067] Generalized Phase-Shift Based Pre-Coding Method
[0068] Thus far, the procedures of forming a phase-shift based
pre-coding matrix when there are 4 transmit antennas with a spatial
multiplexing rate of 2 was explained, but the phase-shift based
pre-coding method of the present disclosure may be expanded to a
system having N.sub.t antennas (here, N.sub.t being a natural
number of 2 or greater) with a spatial multiplexing rate of R
(here, R being a natural number of 1 or greater). Such may be
obtained by using the same method described previously, and can be
generalized as the following Equation 13.
P N i .times. R k = ( j .theta. 1 k 0 0 0 j .theta. 2 k 0 0 0 0 0 j
.theta. N t k ) ( U N t .times. R ) [ Equation 13 ]
##EQU00019##
[0069] Here, at the right-side of the equal (=) symbol, the
phase-shift diagonal matrix is combined with unitary matrix (U)
used for a particular purpose that satisfies the following
condition:
U.sub.N.sub.t.sub..times.R.times.U.sub.N.sub.t.sub..times.R=I.sub.R.times-
.R. By multiplying the phase-shift diagonal matrix and some unitary
matrix, the phase-shift based preceding that satisfies both the
power constraint and the orthogonal constraint may be obtained. In
addition, the phase-shift based precoding matrix can be generated
by multiplying one or more phase-shift diagonal matrices and/or one
or more unitary matrices that can be obtained from feedback
information or downlink channel state information. In equation 13,
the phase-shift diagonal matrix can be implemented by time domain
cyclic delay method if the k indicates sub-carrier index.
[0070] As an example of the unitary matrix (U), a specific
pre-coding matrix for obtaining signal-to-noise ratio (SNR) gain
may be used, and in particular, if Walsh codes are used for such
pre-coding matrix, the phase-shift based pre-coding matrix
generating equation may be as follows:
P 4 .times. 4 k = 1 4 ( j .theta. 1 k 0 0 0 0 j .theta. 2 k 0 0 0 0
j .theta. 3 k 0 0 0 0 j .theta. 4 k ) ( 1 1 1 1 1 - 1 1 - 1 1 1 - 1
- 1 1 - 1 - 1 1 ) [ Equation 14 ] ##EQU00020##
[0071] In Equation 14, it is assumed that the system has 4
(physical or virtual) transmit antennas with a spatial multiplexing
rate of 4. Here, by appropriately re-forming the unitary matrix
(U), a particular transmit antenna may be selected (i.e., antenna
selection) and/or the adjusting of spatial multiplexing rate (i.e.,
rate tuning) may be possible.
[0072] The following Equation 15 shows an example of how the
unitary matrix (U) may be re-formed in order to group 2 antennas of
a system having 4 antennas.
P 4 .times. 4 k = 1 4 ( j .theta. 1 k 0 0 0 0 j .theta. 2 k 0 0 0 0
j .theta. 3 k 0 0 0 0 j .theta. 4 k ) ( 0 0 1 1 0 0 1 - 1 1 1 0 0 1
- 1 0 0 ) [ Equation 15 ] ##EQU00021##
[0073] Also, the following Table 3 shows an exemplary method for
re-forming the unitary matrix (U) to be appropriate for the
corresponding multiplexing rate if the spatial multiplexing rate
changes according to time, channel environment, and the like.
TABLE-US-00003 TABLE 3 ##STR00001##
[0074] Here, Table 3 shows some examples where column 1, columns
1.about.2, and columns 1.about.4 of the unitary is/are selected
according to the multiplexing rate, but not meant to be limited to
such. For example, if the multiplexing rate is 1, one of the
1.sup.st through 4.sup.th columns may be selected, if the
multiplexing rate is 2, two particular columns (e.g., one pair
among the (1,2), (2,3), (3,4), (1,3), . . . , (2,4) pairs of
columns) may be selected, and if the multiplexing rate is 4, all
columns may be selected.
[0075] Alternatively, the unitary matrix (U) may also be provided
in codebook format in the transmitting side and the receiving side.
In such case, the transmitting side receives index information of
the codebook as feedback from the receiving side, then the
appropriate unitary matrix (i.e., unitary preceding matrix in a
codebook) corresponding to the index is selected from its codebook,
and then the above Equation 13 is used to form a phase-shift based
pre-coding matrix.
[0076] Transceiving Apparatus Supporting a Phase-Shift Based
Pre-Coding Method
[0077] FIG. 9 is a block diagram of an exemplary structure for a
transceiving apparatus that supports the phase-shift based
pre-coding method of the exemplary embodiments of the present
invention. This exemplary embodiment of the transceiving apparatus
assumes that the unitary matrix (U) for forming the phase-shift
based pre-coding matrix is provided in codebook format, but is not
meant to be limited to such, as described above.
[0078] The transceiving apparatus may be comprised of an input unit
(901) used to select a desired function or receiving information, a
display unit (903) used to show various information in using the
transceiving apparatus, a memory unit (905) used to store various
programs needed for operating the transceiving apparatus and data
to be transmitted to the receiving side, a radio (wireless)
communication unit (907) used to receive signals and transmit data
to the receiving side, a voice processing unit (909) used to
convert and amplify digital voice signals into analog voice signals
for outputting through a speaker (SP) and to amplify the voice
signals from a microphone (MIC) for converting into digital
signals, and a control unit (911) used to control the overall
operations of the transceiving apparatus.
[0079] The radio communication unit (907) will be explained in more
detail as follows. For reference, FIG. 10 is a block diagram
showing an exemplary structure of a SCW (Single Codeword) OFDM
transmitter unit that is included in the radio communication unit
(907), and FIG. 11 shows an exemplary structure of a MCW (Multiple
Codeword) OFDM transmitter unit. Also, various receiver units that
correspond to each transmitter unit also exist and perform the
opposite functions as those of the transmitter units, but their
detailed explanations will be omitted merely for the sake of
brevity.
[0080] In the SCW OFDM transmitter unit, a channel encoder (101)
adds redundancy bits to prevent the transmit data from being
distorted at (over) the channel, and channel encoding is performed
by using error correcting code codes (such as, turbo codes, LDPC
codes, and the like). Thereafter, an interleaver (1020) performs
interleaving through code bit parsing for minimizing losses due to
instantaneous noise during the data transmission procedure, and a
mapper (1030) converts the interleaved data bits into OFDM symbols.
Such symbol mapping may be performed through phase modulation
techniques (such as, QPSK, etc.) and amplitude modulation
techniques (such as, 16QAM, 8QAM, 4QAM, etc.). Thereafter, the OFDM
symbols are processed through the pre-coder (1040) of the present
invention disclosure, then processed through a sub-channel
modulator (not shown) and an Inverse Fast Fourier Transform (IFFT)
unit (1050) are included into a carrier of the time domain. Upon
processing through as filter unit (not shown) and an analog
converter (1060), transmission via a radio channel is performed.
Meanwhile, at the MCW OFDM transmitter unit, the only difference is
that the OFDM symbols are processed through a channel encoder (111)
and an interleaver (1120) in a parallel manner for each channel,
otherwise, the remaining structural element (1130.about.1160) may
be the same (or similar).
[0081] The pre-coding matrix forming module (1041, 1141) determines
a reference column corresponding to a first sub-carrier in a
particular pre-coding matrix, and the remaining columns are
determined by phase-shifting the reference column using a phase
angle that is increased by a certain (consistent) amount. Here, a
unitary matrix having a size of (number of transmit
antennas).times.(spatial multiplexing rate) is employed to perform
pre-coding, and such unitary matrix is provided for each index of
each sub-carrier, whereby the unitary matrix with respect to the
first index is phase-shifted to obtain the unitary matrix of each
remaining index.
[0082] Namely, the pre-coding matrix forming module (1041, 1141)
selects a certain 1.sup.st pre-coding matrix from a codebook
previously stored in the memory unit (905). A 2.sup.nd pre-coding
matrix with respect to a sub-carrier or a sub-band of the 2.sup.nd
index is formed by applying a phase-shift of a certain size to the
1.sup.st pre-coding matrix. Here, the size of the shifted phase may
be variously set according to the current channel condition and/or
whether or not feedback information from the receiving side exists.
A 3.sup.rd pre-coding matrix with respect to a sub-carrier or a
sub-band of the 3.sup.rd index is formed by performing a
phase-shift on the 2.sup.nd pre-coding matrix. Namely, the forming
procedure of the 2.sup.nd pre-coding matrix is repeated during the
formation procedure of the 3.sup.rd pre-coding matrix through the
last pre-coding matrix.
[0083] The pre-coding matrix re-forming module (1042, 1142) selects
a particular number of columns (in each pre-coding matrix formed by
the pre-coding matrix forming module (1041, 1141)) that corresponds
to the given spatial multiplexing rate and deletes the remaining
columns in order to re-form a pre-coding matrix. Here, a pre-coding
matrix comprised of the above-described selected columns may be
newly formed. In selecting the particular column(s) of the
pre-coding matrix, one or more random columns may be selected or
one or more particular columns may be selected according to
pre-defined rules.
[0084] The pre-coding module (1043, 1143) performs pre-coding by
applying OFDM symbols for the corresponding sub-carrier into each
pre-coding matrix determined in the above manner.
[0085] Generalized Phase-Shift Based Pre-Coding Method
[0086] Hereafter, the pre-coding matrix determining module (1041,
1141), the pre-coding matrix re-forming module (1042, 1142), and
the pre-coding module (1043, 1143) according to another exemplary
embodiment will be explained.
[0087] The pre-coding matrix determining module (1041, 1141)
selects a particular unitary matrix (U) by referring to the unitary
matrix index that was fed back from the receiving side or by using
a pre-defined matrix, and the selected unitary matrix (U) is
applied to the above Equation 13 to determine a phase-shift based
pre-coding matrix (P). Here, the phase-shift value of the former
matrix of Equation 13 should be previously set.
[0088] There may be situations where the spatial multiplexing rate
needs to be adjusted due to changes in channel environment or where
data transmission needs to be performed by selecting a particular
antenna among multiple transmit antennas due to various reasons. In
such case, when a change in spatial multiplexing rate and/or a
change in the number of antennas is informed from the control unit
(911), the pre-coding re-forming module (1042, 1142) searches for a
unitary matrix (U) that is appropriate for the corresponding
situation or a previously selected unitary matrix (U) is re-formed
to be appropriate for the corresponding situation. In the former
case, there is the advantage that the desired phase-shift based
pre-coding matrix can be quickly obtained because a separate
re-forming procedure is not necessary, but there is the
disadvantage that memory usage increases because a codebook that is
be used for various situations needs to be provided. Also, in the
latter case, processing load is created due to the re-forming
procedures, but the codebook capacity can be reduced. The unitary
matrix re-forming procedure according to spatial multiplexing rate
changes or changes in the number of transmit antennas was explained
previously with respect to Equation 14 and Table 3.
[0089] The pre-coding module (1043, 1143) performs pre-coding by
applying the OFDM symbols (with respect to the corresponding
sub-carrier or sub-band) to the phase-shift based pre-coding matrix
determined in the above manner.
[0090] The control unit (911) informs the pre-coding matrix
re-forming module (1042, 1142) about various information (such as,
the changed spatial multiplexing rate, the changed total number of
antennas to be used, etc.) that is used for re-forming the
pre-coding matrix.
[0091] The transceiving apparatus according to the present
invention disclosure may be used in so-called Personal Digital
Assistants (PDAs), cellular phones, Personal Communication Service
(PCS) phones, GSM phones, WCDMA phones, Mobile Broadband System
(MBS) phones, and the like.
[0092] By applying both the phase-shift based pre-coding method
described herein and the spatial multiplexing method of the
background art to a multiple antenna OFDM open loop system that
does not employ feedback information, the differences in
performance of these two methods will be explained with reference
to some experimental test results. Table 4 shows the parameters
that were applied to the system for this experimental test.
TABLE-US-00004 TABLE 4 Parameter Parameter value # of sub-carriers
512 # of guard carriers 106 (left), 105 (right) # of pilots 28
(perfect channel estimation) MIMO scheme Spatial Multiplexing (SM)
and phase-shift pre- coding (PSP) MIMO receiver MMSE (Minimum Mean
Squared Error), ML (Minimum Likelihood) Bandwidth 7.68 MHz Carrier
frequency 2 GHz Channel model Flat, Ped-A (ITU Pedestrian A), TU
(Typical Urban), Mobility: 30 km/h, 250 km/h # of OFDM symbols 8
(localized) per frame MCS (Modulation and QPSK (R = 1/4, R = 1/3. R
= 1/2, R = 3/4) Coding Set) Channel coding 3GPP Turbo
(Max-long-MAP), 8 Iterations # of transmit antennas 2 # of receive
antennas 2 Antenna correlation (0%, 0%)
[0093] FIG. 12 is a graph showing a comparison in performance
differences when the phase-shift-based pre-coding (PSP) method of
the present invention disclosure and the spatial multiplexing (SM)
method of the background art are respectively applied to a ML
(Minimum Likelihood) receiver and a MMSE (Minimum Mean Squared
Error) receiver.
[0094] As depicted, in a system that applies the PSP method, a
larger gain can generally be obtained when compared to the
background art spatial multiplexing method. More specifically, in
the ML receiver, the PSP method results in slightly more gain when
compared to the SM method, but in the MMSE receiver, it can be seen
that a larger gain may be obtained with the PSP method as the
signal-to-noise ration (SNR) increases.
[0095] FIGS. 13 and 14 are graphs showing a comparison of the
performance differences per coding rate for the phase-shift based
pre-coding method of the present invention disclosure and for the
background art spatial multiplexing method being applied to a MMSE
(Minimum Mean Squared Error) receiver for a Ped-A (ITU Pedestrian
A) fading channel environment and a TU (Typical Urban) fading
channel environment.
[0096] As depicted, it can be seen that on the Ped-A (ITU
Pedestrian A) fading channel environment and the TU (Typical Urban)
fading channel environment, the PSP method can obtain a large gain
by increasing frequency selectivity while decreasing coding rate
(R=1/3, R=1/4).
[0097] FIGS. 15 through 17 are graphs showing a comparison of the
performance differences when the present invention disclosure
phase-shift based pre-coding method and the background art spatial
multiplexing method are applied to a system employing SCW (Single
Codeword) and MCW (Multiple Codeword) in a Ped-A (ITU Pedestrian A)
fading channel environment and a TU (Typical Urban) fading channel
environment.
[0098] In general, when the spatial multiplexing method is applied
to SCW, higher performance compared to that of MCW is achieved,
because the channel code can additionally obtain spatial diversity
gain and can obtain coding gain due to an increase in codeword
length, but has the drawback that reception requires a high degree
of complexity. As depicted, in a system with the spatial
multiplexing method applied thereto, there is a large difference in
performance between SCW and MCW. However, if the present invention
disclosure phase-shift based pre-coding method is applied, a larger
gain compared to that of the SCW of a system with the spatial
multiplexing method applied thereto can be obtained. Namely, as
depicted, a much larger gain is generated when phase-shift based
pre-coding is applied in comparison to when the background art
spatial multiplexing method is applied to MCW, and although a
respectively smaller gain is generated compared to when SCW is
applied, but it is clear that an improvement in performance is
achieved.
[0099] FIG. 18 is a graph showing the performance differences in
the cases where the spatial diversity method+cyclic delay diversity
method is applied, and the present invention disclosure phase-shift
based pre-coding method+cyclic delay diversity method is applied to
a MCS (Modulation and Coding Set) in a flat fading channel
environment.
[0100] As depicted, for all coding rates (R=1/2, 1/3, 1/4), it can
be seen that superior performance is achieved when the present
invention disclosure phase-shift based pre-coding method+cyclic
delay diversity method is applied, compared to when the background
art spatial diversity method+cyclic delay diversity method is
applied.
[0101] As for some exemplary effects of the present invention
disclosure that employs the phase-shift based pre-coding method,
when compared to the space-time coding method, the degree of
complexity of the transceiver is relatively low, the advantages of
the phase-shift diversity scheme is maintained while supporting
various spatial multiplexing rates. Compared to the pre-coding
method, relatively less channel sensitivity and the need for a
relatively small capacity codebook can be expected. Furthermore, by
using a generalized phase-shift based pre-coding matrix,
phase-shift based pre-coding can be easily expanded and applied
regardless of the number of transmit antennas in the system.
[0102] FIG. 19 shows an exemplary overview structure related to
downlink baseband signal generation according to the present
disclosure.
[0103] As for industrial applicability, the features and aspects
described herein are related to and can be implemented for various
types of radio communication techniques. Some non-limiting examples
may include broadband wireless air interface techniques,
Multiple-Input Multiple-Output (MIMO) techniques, so-called 3.5G or
4G systems designed to provide higher data rates and IP-based data
services, etc. and/or various radio communication standards, such
as, but not limited to, WCDMA, 3GPP, 3GPP2, OFDM, OFDMA, HSDPA,
UMTS, OMA, IEEE 802.11n, IEEE 802.16, etc.
[0104] As such, at least some of the features described herein are
applicable to such standards that have been developed or that are
continuing to evolve. Also, at least some of the features described
herein may be implemented in various types of devices (e.g., mobile
phones, wireless communication terminals, user equipment (UE),
radio communication protocol entities, etc.) in terms of hardware,
software, or some appropriate combination thereof.
[0105] Any reference in this specification to "one embodiment," "an
embodiment," "example embodiment," etc., means that a particular
feature, structure, or characteristic described in connection with
the embodiment is included in at least one embodiment of the
invention. The appearances of such phrases in various places in the
specification are not necessarily all referring to the same
embodiment. Further, when a particular feature, structure, or
characteristic is described in connection with any embodiment, it
is submitted that it is within the purview of one skilled in the
art to affect such feature, structure, or characteristic in
connection with other ones of the embodiments.
[0106] Although embodiments have been described with reference to a
number of illustrative embodiments thereof, it should be understood
that numerous other modifications and embodiments can be devised by
those skilled in the art that will fall within the scope of the
principles of this disclosure. More particularly, various
variations and modifications are possible in the component parts
and/or arrangements of the subject combination arrangement within
the scope of the disclosure, the drawings and the appended claims.
In addition to variations and modifications in the component parts
and/or arrangements, alternative uses will also be apparent to
those skilled in the art.
* * * * *