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| United States Patent |
6,282,233
|
|
Yoshida
|
August 28, 2001
|
Multi-user receiving apparatus and CDMA communication system
Abstract
A multi-user receiving apparatus for inputting a CDMA reception signal,
performing an interference canceling process in parallel for each user
signal on an m-th stage of M stages (where m is any integer of
1.ltoreq.m.ltoreq.M; and M is any integer of M .gtoreq.2), and outputting
demodulated signals on the M-th stage is disclosed, that comprises a
plurality of IEUs (interference estimating units) disposed corresponding
to the (M-1) stages and the number of user signals, and a plurality of
subtracting units disposed corresponding to the (M-1) stages, wherein each
of the IEUs Inputs an interference cancellation residual signal obtained
in an (m-1)-th stage interference canceling process and a signal of which
a symbol replica corresponding to the same user signal on the (m-1)-th
stage is weighted with a first weighting coefficient, generates an m-th
stage symbol replica, outputs the m-th stage symbol replica to the
(m+1)-th stage, and outputs a spread signal that is the difference between
the m-th stage symbol replica and the (m-1)-th stage symbol replica
weighted with the first weighting coefficient, and wherein each of the
subtracting units subtracts signals of which the spread signals that are
output from the IEUs on the m-th stage are weighted with a second
weighting coefficient from a signal of which the (m-1)-th stage
interference cancellation residual signal is delayed by a predetermined
value and outputs the resultant signal to the (m+1)-th stage.
| Inventors:
|
Yoshida; Shousei (Tokyo, JP)
|
| Assignee:
|
NEC Corporation (Tokyo, JP)
|
| Appl. No.:
|
287563 |
| Filed:
|
April 6, 1999 |
Foreign Application Priority Data
| Apr 07, 1998[JP] | 10-094623 |
| Current U.S. Class: |
375/148 |
| Intern'l Class: |
H04J 013/04 |
| Field of Search: |
375/144,148,349
370/335,342,479
|
References Cited [Referenced By]
U.S. Patent Documents
| 5966403 | Oct., 1999 | Pon | 375/208.
|
| 6192066 | Feb., 2001 | Asanuma | 375/130.
|
| 6208683 | Mar., 2001 | Mizuguchi et al. | 375/140.
|
| Foreign Patent Documents |
| 9-270736 | Oct., 1997 | JP.
| |
| 9-270766 | Oct., 1997 | JP.
| |
| 10-51353 | Feb., 1998 | JP.
| |
Primary Examiner: Le; Amanda T.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas, PLLC
Claims
What is claimed is:
1. A multi-user receiving apparatus for inputting a CDMA (Code Division
Multiple Access) reception signal, performing an interference canceling
process in parallel for each user signal on an m-th stage of M stages
(where m is any integer of 1.ltoreq.m.ltoreq.M; and M is any integer that
is 2 or larger), and outputting demodulated signals on the M-th stage,
comprising:
a plurality of IEUs (interference estimating units) disposed corresponding
to the (M) stages and the number of user signals; and
a plurality of subtracting units disposed corresponding to the (M-1)
stages,
wherein each of the IEUs inputs an interference cancellation residual
signal obtained in an (m-1)-th stage interference canceling process and a
signal of which a symbol replica corresponding to the same user signal on
the (m-1)-th stage is weighted with a first weighting coefficient,
generates an m-th stage symbol replica, outputs the m-th stage symbol
replica to the (m+1)-th stage, and outputs a spread signal that is the
difference between the m-th stage symbol replica and the (m-1)-th stage
symbol replica weighted with the first weighting coefficient, and
wherein each of the subtracting units subtracts signals of which the spread
signals that are output from the IEUs on the m-th stage are weighted with
a second weighting coefficient from a signal of which the (m-1)-th stage
interference cancellation residual signal is delayed by a predetermined
value and outputs the resultant signal to the (m+1)-th stage.
2. The multi-user receiving apparatus as set forth in claim 1,
wherein each of the IEUs has a plurality of path processing portions
corresponding to a plurality of channels as a multi-path, each of the path
processing portions having:
despreading means for inputting the (m-1)-th stage interference
cancellation residual signal and despreading the interference cancellation
residual signal;
a first adding unit for adding an output signal of the despreading means
and a signal of which the (m-1)-th stage symbol replica is weighted with
the first weighting coefficient;
a detecting unit for demodulating an output signal of the first adding unit
with the channel estimation value corresponding to each path;
a second adding unit for adding the output signals of the detecting units
corresponding to the individual paths;
a decision unit for determining a symbol of an output signal of the second
adding unit;
a multiplying unit for multiplying an output signal of a decision unit by
the channel estimation value corresponding to each path and generating the
m-th stage symbol replica corresponding to each path;
a subtracting unit for subtracting a signal of which the (m-1)-th stage
symbol replica is weighted with the first weighting coefficient from an
output signal of the multiplying unit;
spreading means for spreading an output signal of the subtracting circuit
corresponding to each path; and
a third adding unit for adding output signals of the spreading means
corresponding to the individual paths.
3. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 2,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
4. The multi-user receiving apparatus as set forth in claim 1,
wherein a first stage inputs a reception signal as the interference
cancellation residual signal obtained in the (m-1)-th stage interference
canceling process,
wherein zero is used as the (m-1)-th stage symbol replica of the same user
signal, and
wherein the M-th stage outputs demodulated signals rather than performing
the interference canceling process and outputting the m-th stage symbol
replicas and the spread signals.
5. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 4,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
6. The multi-user receiving apparatus as set forth in claim 1,
wherein the first weighting coefficient and the second weight coefficient
are real numbers that are 1 or smaller.
7. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient on the m-th stage are [1-(1-.alpha.).sup.m-1, .alpha.],
respectively (where .alpha. is a real number that is 1 or smaller).
8. The multi-user receiving apparatus as set forth in claim 7,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
9. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient are 1 and .alpha., respectively (where .alpha. is a real
number that is 1 or smaller).
10. The multi-user receiving apparatus as set forth in claim 9,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
11. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient are .alpha. and .alpha., respectively (where .alpha. is a real
number that is 1 or smaller).
12. The multi-user receiving apparatus as set forth in claim 11,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
13. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient corresponding to an n-th user signal (where n is any positive
integer) on the m-th stage are [1-(1-.alpha..beta..sub.n1)
(1-.alpha..beta..sub.n2) . . . (1-.alpha..beta..sub.nm-1),
.alpha..beta..sub.nm ], respectively (where .alpha. and .beta..sub.nm are
any real numbers that are 1 or smaller).
14. The multi-user receiving apparatus as set forth in claim 13,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
15. The multi-user receiving apparatus as set forth in claim 13,
wherein .beta..sub.nm is assigned depending on the ratio of signal power to
interference power of each slot (transmission power control interval) of
each user signal or on the signal power of each slot, and
wherein .beta..sub.nm is assigned depending on each slot of each user
signal on each stage.
16. The multi-user receiving apparatus as set forth in claim 13,
wherein .beta..sub.nm is assigned depending on the likelihood of a
determination symbol of each user signal, on the distance from a symbol
determination point, or on a symbol power, and
wherein .beta..sub.nm is assigned depending on each symbol of each user
signal on each stage.
17. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient corresponding to an n-th user signal (where n is any positive
integer) on the m-th stage are 1 and .alpha..beta..sub.nm, respectively
(where .alpha. and .beta..sub.nm are any real numbers that are 1 or
smaller).
18. The multi-user receiving apparatus as set forth in claim 17,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
19. The multi-user receiving apparatus as set forth in claim 17,
wherein .beta..sub.nm is assigned depending on the ratio of signal power to
interference power of each slot (transmission power control interval) of
each user signal or on the signal power of each slot, and
wherein .beta..sub.nm is assigned depending on each slot of each user
signal on each stage.
20. The multi-user receiving apparatus as set forth in claim 17,
wherein .beta..sub.nm is assigned depending on the likelihood of a
determination symbol of each user signal, on the distance from a symbol
determination point, or on a symbol power, and
wherein .beta..sub.nm is assigned depending on each symbol of each user
signal on each stage.
21. The multi-user receiving apparatus as set forth in claim 6,
wherein the first weighting coefficient and the second weighting
coefficient corresponding to an n-th user signal (where n is any positive
integer) on the m-th stage are .alpha..beta..sub.nm-1 and
.alpha..beta..sub.nm, respectively (where .alpha. and .beta..sub.nm are
any real numbers that are 1 or smaller).
22. The multi-user receiving apparatus as set forth in claim 21,
wherein .alpha. is assigned depending on the ratio of signal power to
interference power of each user signal or on the number of user signals,
and
wherein .alpha. is constant for each user signal when the ratio of signal
power to interference power for each user is the same and the number of
user signals does not vary.
23. The multi-user receiving apparatus as set forth in claim 21,
wherein .beta..sub.nm is assigned depending on the ratio of signal power to
interference power of each slot (transmission power control interval) of
each user signal or on the signal power of each slot, and
wherein .beta..sub.nm is assigned depending on each slot of each user
signal on each stage.
24. The multi-user receiving apparatus as set forth in claim 21,
wherein .beta..sub.nm is assigned depending on the likelihood of a
determination symbol of each user signal, on the distance from a symbol
determination point, or on a symbol power, and
wherein .beta..sub.nm is assigned depending on each symbol of each user
signal on each stage.
25. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 6,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
26. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 1,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
27. A CDMA (Code Division Multiple Access) multi-user receiving apparatus
for performing an interference canceling process for all user signals on a
plurality of stages and outputting demodulated signals corresponding to
the user signals, each stage comprising:
a plurality of interference estimating units corresponding to a plurality
of paths, each of the interference estimating units having detecting unit
for inputting an interference cancellation residual signal obtained on the
preceding stage and a signal of which a preceding-stage symbol replica
corresponding to the current user signal is weighted with a first
weighting coefficient and demodulating the resultant signal with a
propagation path estimation value of each path so as to output a current
stage symbol replica and a spread signal that is the difference between
the current stage symbol replica and the preceding stage symbol replica is
spreaded;
a plurality of multiplying units corresponding to the number of paths, each
of the multiplying units generating a signal of which the current stage
symbol replica is weighted by the current stage first weighting
coefficient;
a delaying unit for delaying the preceding stage inference residual signal
for a predetermined time period; and
a subtracting unit for subtracting signals of which the spread signals
generated in the interference estimating portions for all the user signals
are weighted with a second weighting coefficient from an output signal of
the delaying unit and outputting the resultant signal as the next stage
interference cancellation residual signal.
28. The multi-user receiving apparatus as set forth in claim 27,
wherein the first weighting coefficient and the second weight coefficient
are real numbers that are 1 or smaller.
29. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 28,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
30. The multi-user receiving apparatus as set forth in claim 27,
wherein the first weighting coefficient is any real number that is 1 or
smaller and the first weighting coefficient varies on each stage, and
wherein the second weighting coefficient is any real number that is 1 or
smaller and the second weighting coefficient is constant on each stage.
31. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 30,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
32. A CDMA communication system having a multi-user receiving apparatus as
set forth in claim 27,
wherein the multi-user receiving apparatus is used for a base station or a
mobile station.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access)
multi-user receiving apparatus for performing an interference canceling
process in parallel for individual users on a plurality of stages and for
outputting demodulated signals for the individual users on the last stage,
in particular, to a multi-user receiving apparatus with excellent
interference cancellation characteristics in a small hardware scale.
2. Description of the Related Art
CDMA system using direct sequence (DS) spreading process (hereinafter
referred to as DS-CDMA system) has become attractive as a multiple access
system for a mobile communication system having a base station and
portable mobile stations because of the probability of remarkable increase
of the subscriber capacity. In the DS-CDMA system, each user signal is
spreaded in a wide frequency band with particular code and transmitted to
a propagation path. On the receiver side, the code-multiplexed signal is
de-spreaded and a desired signal is detected. When spreaded codes assigned
to individual users correlate, they interfere and the reception
characteristics deteriorate.
As an interference canceler, a multi-user receiving apparatus that cancels
such interferences using spreaded codes of all users and characteristics
of propagation paths is known. An example of such a multi-user receiving
apparatus has been disclosed by M. K. Varanasi and B. Aashang "Multistage
Detection in Asynchronous Code-Division Multiple-Access Communications",
IEEE Trans, Commun., vol. COM-38, No. 4. pp. 509-519, April 1980. In the
example, the first stage circuit demodulates all user signals, generates
interference replicas of the user signals, and subtracts interference
replicas other than an interface replica for a desired user signal from
the received signal. The next stage circuit demodulates the desired user
signal with the output signal of the first stage circuit. Thus, the
demodulated result of the second stage circuit is improved in comparison
with the demodulated result of the first stage circuit. In the multi-stage
structure, the interference canceling process is repeated a plurality of
times and thereby the interference cancellation characteristics are
improved.
Another related art reference has been disclosed by Fukasawa, Satoh (T),
Kawabe, and Satoh (S) "Structure and Characteristics of Interference
Canceler Based on Estimation of Propagation Path Using Pilot Signal
(translated title)" Journal of The Institute of Electronics, Information
and Communication Engineers, Japan, B-II Vol. J77-B-II No. 11, November
1994. In the related art reference, an interference cancellation residual
signal propagation structure is used to simplify the apparatus. In
addition, a determination symbol for each user signal is treated as a
replica. Thus, the hardware scale is reduced. However, in the detecting
process on each stage, interfered propagation path characteristics
estimated on the first stage are used. Thus, when an estimated error of a
propagation path is large, the interference cancellation characteristics
largely deteriorate.
In a recent year, a modification system of such a related art reference has
been proposed. In the system, a propagation path is estimated on each
stage rather than the first stage so as to suppress the deterioration of
interference cancellation characteristics against a propagation path
estimation error. Such a system has been disclosed by Sawahashi, Miki,
Andoh, and Higuchi "Sequential Channel Estimation Type Serial Canceler
Using Pilot Symbols in DS (Direct Sequence)--CDMA", The Institute of
Electronics, Information and Communication Engineers, Japan, Wireless
Communication System Study Group Technical Report, RCS95-50, July 1995. In
the technical paper, a serial process structure for sequentially
demodulating and canceling interferences of user signals in the order of
higher reception signal levels is used. Another related art reference has
been disclosed by Yoshida and Atokawa "Sequential Propagation Path
Estimation Type CDMA Multi-Stage Interference Canceler Using Symbol
Replica Process" The Institute of Electronics, Information and
Communication Engineers, Japan, Wireless Communication System Study Group
Technical Report, RCS96-171, February, 1997 (Japanese Patent Laid-Open
Publication No. 10-51353). As with the system proposed by Fukasawa et.
al., in the system proposed by Yoshida et. al., although a symbol replica
process is performed in an interference cancellation residual propagation
type structure, symbol replicas for individual user signals are handled so
as to sequentially estimate propagation paths. Thus, the hardware scale
can be reduced and the interference cancellation characteristics can be
improved.
FIG. 8 is a block diagram showing an example of the structure of a CDMA
multi-user receiving apparatus disclosed by Yoshida et. al as Japanese
Patent Laid-Open Publication No. 10-51353. In FIG. 8, the apparatus has a
plurality of IEUs 112-m-n disposed on a plurality of stages. IEUs 112-m-n
disposed on each stage correspond to individual user signals. An IEU
112-m-n that corresponds to the highest hierarchical level user signal
performs an interference canceling process for the lowest hierarchical
level user signal on the preceding (m-1)-th stage. An IEU 112-m-n that
corresponds to other than the highest hierarchical level user signal
performs an interference canceling process for the (n-1)-th hierarchical
level user signal. An IEU 112-m-n inputs an error signal obtained in the
interference canceling process and an interference replica estimated by
the IEU 112-(m-1)-n corresponding to the same hierarchical level user
signal on the preceding stage, re-estimates the current m-th stage
interference replica, outputs the re-estimated interference replica to an
IEU 112-(m+1)-n corresponding to the same hierarchical level user signal
on the next (m+1)-th stage, and outputs the result of the diffusing
process as the difference between an interference replica on the current
m-th stage and an interference replica on the preceding (m-1)-th stage.
IEU 112-M-1, . . . , IEU 112-M-N on the last M-th stage output demodulated
results as demodulated user signals rather than re-estimating interference
replicas on the current M-th stage.
As shown in FIG. 8, the interference canceling process is performed by M
column.times.N line circuits (where M represents the number of stages; and
N represents the number of user signals). Reception levels of individual
user signals are pre-assigned. Each user signal is connected to each stage
in series corresponding to a reception level. A demodulating process and
an interpolation canceling process are performed for user signals in the
order from the highest signal level to the lowest signal level. In this
structure, since the interference canceling process is performed in
series, interference replicas can be sequentially canceled. Thus, although
excellent interference cancellation characteristics are accomplished, the
circuit structure is complicated and a delay in the demodulating process
is large.
FIG. 9 is a block diagram showing another example of the structure of a
CDMA multi-user receiving apparatus. In the receiving apparatus, the delay
of the demodulating process is small. The interference canceling process
is a simple parallel structure apparatus. The receiving apparatus has
multiplying units disposed on the output side of interference estimating
units IEU. Each multiplying unit multiplies an output signal of the
interference estimating unit by a weighting coefficient .alpha. that is 1
or smaller. Thus, the interference cancellation characteristics are
improved.
The CDMA multi-user receiving apparatus shown in FIG. 9 has M stags (where
M is any integer that is two or larger) for demodulating N user signals
(where N is any integer that is 1 or larger). A first stage interference
canceling process circuit 101-1 comprises a delaying unit 103-1,
interference estimating units (IEU) 102-1-1 to 102-1-N, multiplying units
105-1-1 to 105-1-N, and a subtracting unit 104-1. The multiplying units
105-1-1 to 105-1-N multiply output signals of the interference estimating
units 102-1-1 to 102-1-N by a weighting coefficient .alpha., respectively.
The subtracting unit 104-1 subtracts output signals of the multiplying
units 105-1-1 to 105-1-N from an output signal of the delaying unit 103-1.
An interference estimating unit (IEU) 102-m-n on the m-th stage (where m is
any integer of 1.ltoreq.m.ltoreq.M) for the n-th user signal (where n is
any integer of 1.ltoreq.n.ltoreq.N) inputs an interference cancellation
residual signal (an output signal of a subtracting unit 104-(m-1))
obtained in the interference canceling process on the (m-1)-th stage and a
symbol replica (a replica that is output from an IEU 102-(m-1)-n)
corresponding to the same user signal on the (m-1)-th stage, generates an
m-th stage symbol replica, outputs the generated symbol replica to the
(m+1)-th stage, and outputs a spreaded signal that is the difference
between the m-th stage symbol replica and an (m-1)-th stage symbol
replica.
A multiplying unit 105-m-n multiplies the output signal of the interference
estimating unit 102-m-n by the weighting coefficient .alpha.. A
subtracting unit 104-m subtracts the output signals of the multiplying
units 105-m-n for all user signals from a signal of which the (m-1)-th
stage interference cancellation residual signal is delayed by a delaying
unit 103-m for the IEU process, updates the interference cancellation
residual signal, and outputs the resultant signal to the (m+1)-th stage.
FIG. 2 is a block diagram showing the structure of the interference
estimating unit (IEU) 102-m-n. The IEU 102-m-n has a plurality of path
processing portions (#1 to #K) corresponding to a plurality of propagation
paths as a multi-path. An inversely spreading means 11 inputs an (m-1)-th
stage interference cancellation residual signal (an output signal of a
subtracting unit 104-(m-1)). The despreading means 11 performs an
despreading process for a signal corresponding to the each path. A first
adding unit 12 adds an output signal of the despreading means 11 and an
(m-1)-th stage symbol replica (a replica that is output from an IEU
102-(m-1)-n). A detecting unit 13 inputs an output signal of the first
adding unit 12. A propagation path estimating means 20 detects a
propagation path estimation value corresponding to the each path. A
complex conjugate means 21 outputs a complex conjugate value to a
multiplying unit 22. The multiplying unit 22 multiplies the complex
conjugate value by the output signal of the first adding unit 12 so as to
demodulate the signal corresponding to the each path. A second adding unit
14 adds output signals of the detecting units 13 corresponding to the
individual paths (#1 to #K). A decision unit 15 determines a symbol of an
output signal of the second adding unit 14. A multiplying unit 16
multiplies an output signal of the decision unit 15 by the propagation
path estimation value that is output from the propagation path estimating
means 20 corresponding to the each path of the paths (#1 to #K) and
generates a symbol replica corresponding to the each path. A subtracting
unit 17 subtracts an (m-1)-th stage symbol replica from an m-th stage
symbol replica. A spreading means 18 spreads an output signal of the
subtracting unit 17 corresponding to the current(each) path. A third
adding unit 19 adds outputs signals of the spreading means 18
corresponding to the individual paths #1 to #K.
On the first stage, a reception signal as an interference cancellation
residual signal obtained in an (m-1)-th stage interference canceling
process is used. In addition, on the first stage, zero as a symbol replica
corresponding to the same user signal on the (m-1)-th stage is used. On
the M-th stage, the interference canceling process is not performed. In
addition, spread signals as the difference between m-th stage replicas and
(m-1)-th stage replicas are output. Instead, demodulated signals are
output.
Generally, the characteristics of a parallel process structure interference
canceler are inferior to the characteristics of a serial process structure
interference canceler. This is because in the serial structure, the
interference canceling process can be performed between user signals on
each stage. On the other hand, in the parallel process, the interference
canceling process can be sequentially performed.
When the interference canceling process is performed for user signals in
the order of the largest signal levels, the characteristics are further
improved. In the interference canceler shown in FIG. 9, an output signal
of each IEU 102-M-N is multiplies by a real number .alpha. that is 1 or
smaller so as to improve the characteristics of the parallel process. The
real number .alpha. alleviates the interference canceling process. Thus,
all interferences are not canceled on the first stage. Instead, the
interferences are gradually canceled on a plurality of stages. In other
words, the interference canceling process is alleviated on the first stage
that has a large propagation path estimation error and a large determined
symbol error. Thus, the interference cancellation error is suppressed. The
interference canceling performance is shared with downstream stages that
have a small propagation path estimation error and a small determined
symbol error. Consequently, the interference cancellation characteristics
can be improved.
In the above-described multi-user receiving apparatuses, with a parallel
structure, characteristics of a serial interference canceler can be
accomplished. However, the characteristics of the conventional multi-user
receiving apparatus are not sufficient in comparison with those in the
serial structure interference canceler.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a multi-user receiving
apparatus that has interference cancellation characteristics similar to
those of a serial structure interference canceler and that has a small
hardware scale suitable for a base station and a mobile station of a
DS-CDMA communication system.
The present invention is a multi-user receiving apparatus for inputting a
CDMA (Code Division Multiple Access) reception signal, performing an
interference canceling process in parallel for each user signal on an m-th
stage of M stages (where m is any integer of 1.ltoreq.m.ltoreq.M, and M is
any integer that is 2 or larger), and outputting demodulated signals on
the M-th stage, comprising a plurality of IEUs (interference estimating
units) disposed corresponding to the (M-1) stages and the number of user
signals, and a plurality of subtracting units disposed corresponding to
the (M-1) stages, wherein each of the IEUs inputs an interference
cancellation residual signal obtained in an (m-1)-th stage interference
canceling process and a signal of which a symbol replica corresponding to
the same user signal on the (m-1)-th stage is weighted with a first
weighting coefficient, generates an m-th stage symbol replica, outputs the
m-th stage symbol replica to the (m+1)-th stage, and outputs a spread
signal that is the difference between the m-th stage symbol replica and
the (m-1)-th stage symbol replica weighted with the first weighting
coefficient, and wherein each of the subtracting units subtracts signals
of which the spread signals that are output from the IEUs on the m-th
stage are weighted with a second weighting coefficient from a signal of
which the (m-1)-th stage interference cancellation residual signal is
delayed by a predetermined value and outputs the resultant signal to the
(m+1)-th stage.
Each of the IEUs has a plurality of path processing portions corresponding
to a plurality of propagation paths as a multi-path, each of the path
processing portions having despreading means for inputting the (m-1)-th
stage interference cancellation residual signal corresponding to the
current path and despreading the interference cancellation residual
signal, a first adding unit for adding an output signal of the despreading
means and a signal of which the (m-1)-th stage symbol replica is weighted
with the first weighting coefficient, a detecting unit for demodulating an
output signal of the first adding unit with the propagation path
estimation value corresponding to the current path, a second adding unit
for adding the output signals of the detecting units corresponding to the
individual paths, a decision unit for deciding a symbol of an output
signal of the second adding unit, a multiplying unit for multiplying an
output signal of the decision unit by the propagation path estimation
value corresponding to the current path and generating the m-th stage
symbol replica corresponding to the current path, a subtracting unit for
subtracting a signal of which the (m-1)-th symbol replica is weighted with
the first weighting coefficient from an output signal of the multiplying
unit, spreading means for spreading an output signal of the subtracting
circuit corresponding to the current path, and a third adding unit for
adding output signals of the spreading means corresponding to the
individual paths.
A first stage inputs a reception signal as the interference cancellation
residual signal obtained in the (m-1)-th stage interference canceling
process, wherein zero is used as the (m-1)-th stage symbol replica of the
same user signal, and wherein the M-th stage outputs demodulated signals
rather than performing the interference canceling process and outputting
the m-th stage symbol replicas and the spread signals.
The first weighting coefficient and the second weight coefficient are real
numbers that are 1 or smaller.
The first weighting coefficient and the second weighting coefficient on the
m-th stage are [1-(1-.alpha.).sup.m-1, .alpha.], respectively (where
.alpha. is a real number that is 1 or smaller).
The first weighting coefficient and the second weighting coefficient are 1
and .alpha., respectively (where .alpha. is a real number that is 1 or
smaller).
The first weighting coefficient and the second weighting coefficient are
.alpha. and .alpha., respectively (where .alpha. is a real number that is
1 or smaller).
The first weighting coefficient and the second weighting coefficient
corresponding to an n-th user signal (where n is any positive integer) on
the m-th stage are [1-(1-.alpha..beta..sub.n1) (1-.alpha..beta..sub.n2) .
. . (1-.alpha..beta..sub.nm-1), .alpha..beta..sub.nm ], respectively
(where .alpha. and .beta..sub.nm are any real numbers that are 1 or
smaller).
The first weighting coefficient and the second weighting coefficient
corresponding to an n-th user signal (where n is any positive integer) on
the m-th stage are 1 and .alpha..beta..sub.nm, respectively (where .alpha.
and .beta..sub.nm are any real numbers that are 1 or smaller).
The first weighting coefficient and the second weighting coefficient
corresponding to an n-th user signal (where n is any positive integer) on
the m-th stage are .alpha..beta..sub.nm-1 and .alpha..beta..sub.nm,
respectively (where .alpha. and .beta..sub.nm are any real numbers that
are 1 or smaller).
.alpha. is assigned depending on the ratio of signal power to interference
power of each user signal or on the number of user signals. .alpha. is
constant for each user signal when the ratio of signal power to
interference power for each user is the same and the number of user
signals does not vary.
.beta..sub.nm is assigned depending on the ratio of signal power to
interference power of each slot (transmission power control interval) of
each user signal or on the signal power of each slot. .beta..sub.nm is
assigned depending on each slot of each user signal on each stage.
.beta..sub.nm is assigned depending on the likelihood of a determination
symbol of each user signal, on the distance from a symbol determination
point, or on a symbol power. .beta..sub.nm is assigned depending on each
symbol of each user signal on each stage.
The present invention is a CDMA (Code Division Multiple Access) multi-user
receiving apparatus for performing an interference canceling process for
all user signals on a plurality of stages and outputting demodulated
signals corresponding to the user signals, each stage comprising a
plurality of interference estimating units corresponding to a plurality of
paths, each of the interference estimating units having detecting unit for
inputting an interference cancellation residual signal obtained on the
preceding stage and a signal of which a preceding-stage symbol replica
corresponding to the current user signal is weighted with a first
weighting coefficient and demodulating the resultant signal with a
propagation path estimation value of the current path so as to output a
current stage symbol replica and a spread signal that is the difference
between the current stage symbol replica and the preceding stage symbol
replica is spreaded, a plurality of multiplying units corresponding to the
number of paths, each of the multiplying units generating a signal of
which the current stage symbol replica is weighted by the current stage
first weighting coefficient, a delaying unit for delaying the preceding
stage inference residual signal for a predetermined time period, and a
subtracting unit for subtracting signals of which the spread signals
generated in the interference estimating portions for all the user signals
are weighted with a second weighting coefficient from an output signal of
the delaying unit and outputting the resultant signal as the next stage
interference cancellation residual signal.
In a CDMA communication system according to the present invention, the
multi-user receiving apparatus is used for a base station or a mobile
station.
The above-described individual means and units allow the interference
cancellation characteristics of the parallel process structure multi-user
receiving apparatus to improve.
These and other objects, features and advantages of the present invention
will become more apparent in light of the following detailed description
of a best mode embodiment thereof, as illustrated in the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram showing the structure of a multi-user receiving
apparatus according to a first embodiment of the present invention;
FIG. 2 is a block diagram showing the structure of an interference
estimating unit (IEU) according to the present invention;
FIG. 3 is a block diagram showing the structure of a multi-user receiving
apparatus according to another embodiment of the present invention;
FIGS. 4A to 4C are graphs showing chronological variation of a received
signal power of each user signal according to the present invention;
FIG. 5 is a graph showing an example of a .beta..sub.nm determining method
according to the present invention;
FIG. 6 is a graph showing another example of the .beta..sub.nm determining
method according to the present invention;
FIG. 7 is a graph showing a further example of the .beta..sub.nm
determining method according to the present invention;
FIG. 8 is a block diagram showing an example of the structure of a
conventional serial structure multi-user receiving apparatus; and
FIG. 9 is a block diagram showing an example of the structure of a
conventional multi-user receiving apparatus.
DESCRIPTION OF PREFERRED EMBODIMENTS
Next, with reference to the accompanying drawings, embodiments of the
present invention will be described.
[First Embodiment]
FIG. 1 is a block diagram showing the structure of a multi-user receiving
apparatus according to a first embodiment of the present invention. The
multi-user receiving apparatus comprises M receiving units 1-1, . . . ,
and 1-M on M stages (where M is any integer that is 2 or larger). A
receiving unit 1-m (where 1.ltoreq.m.ltoreq.M) on each stage comprises N
interference estimating units (referred to as IEU) 2-m-n, a delaying unit
3-m, an adding unit 4-m, a first multiplying unit 6-m-n, and a second
multiplying unit 5-m-n. The N IEUs 2-m-n receive N user signals (where N
is any integer that is 1 or larger).
The n interference estimating units (IEU) 2-m-n for n user signals (where n
is any integer of 1.ltoreq.n.ltoreq.N) on the m-th stage (where m is any
integer of 1.ltoreq.m.ltoreq.M) input an interference cancellation
residual signal (an output signal of an adding unit 4-(m-1)) obtained in
the interference canceling process on the (m-1)-th stage and signals of
which symbol replicas corresponding to the same user signals on the
(m-1)-th stage are weighted with a first weight coefficient by first
multiplying units 6-m-n, generates m-th stage symbol replicas, output the
generated symbol replicas to the (m+1)-th stage, and output spread signals
that are the difference between the m-th stage symbol replicas and
(m-1)-th stage symbol replicas.
The second multiplying units 5-m-n multiply the output signals of the IEUs
2-m-n by a second weighting coefficient. The subtracting unit 4m subtracts
the output signals of the multiplying units 5-m-n for all the user signals
from a signal of which the (m-1)-th stage interference cancellation
residual signal is delayed for the process period of the IEUs by the
delaying unit 3-m, updates the interference cancellation residual signal,
and outputs the resultant signal to the (m+1)-th stage. The structure of
the interference estimating unit (IEU) 2-m-n is the same as that of the
conventional IEU shown in FIG. 2.
In FIG. 2, each IEU 2-m-m has a plurality of path processing portions
corresponding to a plurality of paths as a multi-path. An despreading
means 11 corresponding to each path inputs an (m-1)-th stage interference
cancellation residual signal and inversely spreads the interference
cancellation residual signal. A first adding unit 12 adds an output signal
of the despreading means 11 and an (m-1)-th stage symbol replica
corresponding to the current path. A detecting unit 13 inputs an output
signal of the first adding unit 12. A complex conjugate means 21 converts
a propagation path estimation value received from a propagation path
estimating means 20 into a complex conjugate corresponding to the current
path. A multiplying unit 22 multiplies the complex conjugate by the output
signal of the first adding unit so as to demodulate the signal
corresponding to the current path.
A second adding unit 14 adds output signals of the detecting units 13
corresponding to the individual paths. Thus, a path diversity effect can
be obtained.
A decision unit 15 determines a symbol of the output signal of the second
adding unit 14. In QPSK modulation, the decision unit 15 determines Ich
level and Qch level of an orthogonal signal as (1/2, 1/2), (-1/2, 1/2),
(-1/2, -1/2), and (1/2, -1/2) corresponding to the first quadrant, the
second quadrant, the third quadrant, and the fourth quadrant,
respectively.
Next, a multiplying unit 16 multiplies the output signal of the decision
unit 15 by a propagation path estimation value corresponding to the
current path and generates an m-th stage symbol replica.
A subtracting unit 17 subtracts the (m-1)-th stage symbol replica from the
m-th stage symbol replica. A spreading means 18 spreads the output signal
of the subtracting unit 17.
Finally, a third adding unit 19 adds the output signals of the spreading
means 18 corresponding to the individual paths. On the first stage, a
signal received from an antenna is used as an interference cancellation
residual signal obtained in the interference canceling process on the
(m-1)-th stage. In addition, zero is used as a symbol replica for the same
user signal on the (m-1)-th stage. On the M-th stage, the interference
canceling process is not performed. In addition, spread signals that are
the difference between the m-th stage replicas and the (m-1)th stage
replicas are not output. Instead, demodulated signals corresponding to the
individual paths are output.
In the first embodiment, the second multiplying units 5-m-n are disposed on
the output side of the IEUs 2-m-n. Alternatively, the second multiplying
units 5-m-n may be disposed on the output side of the subtracting units 17
or the spreading means 18 of the IEUs 2-m-n.
As described above, unlike with the related art reference, in the first
embodiment, the first multiplying units 6-m-n that weight a first
weighting coefficient are disposed. Thus, since the weighting process can
be optimally performed, the interference cancellation characteristics can
be further improved. The weighting process is performed so as to gradually
cancel interferences on a plurality of stages. In other words, the
interference canceling process is alleviated on the first stage that has a
large propagation path estimation error and a large determined symbol
error. Thus, the interference cancellation error is suppressed. The
interference canceling performance is shared with' downstream stages that
have a small propagation path estimation error and a small determined
symbol error. Consequently, the interference cancellation characteristics
can be improved.
Next, the first and second weighting effects will be described. Assuming
that a propagation path and a symbol are ideally estimated, an
interference canceling process for a particular user signal (first user
signal) will be described. Assuming that the received signal level of the
first user signal is 1, since the symbol replica level is defined as a
propagation path estimation value, the symbol replica level R1 on the
first stage is 1 (namely, R1=1). Assuming that the weighting coefficient
of the second multiplying unit 5-1-1 is .alpha., the signal component e1
of the first user contained in the first stage interference cancellation
residual signal that is output from the subtracting unit 4-1 is expressed
as follows:
e1=1-.alpha.R1=1-.alpha. (1)
Assuming that the weighting coefficient of the first multiplying unit 6-2-1
on the second stage is .alpha., the symbol replica level R2 on the second
stage is expressed as follows:
R2=e1+.alpha.R1=1
Thus, the signal level of the first user signal is completely restored.
Assuming that the weighting coefficient of the second multiplying unit
5-2-1 is .alpha., the signal component e2 of the first user signal
contained in the second stage interference cancellation residual signal is
expressed as follows:
e2=e1-.alpha.(R2-.alpha.R1)=(1-.alpha.).sup.2 (2)
Likewise, when the weighting coefficient of the first multiplying unit
6-m-n is 1-e.sub.m-1 =1-(1-.alpha.).sup.m-1, the m-th stage symbol replica
level Rm becomes 1 (namely, Rm=1). When the weighting coefficient of the
second multiplying unit 5-m-n is always .alpha., the signal component em
of the first user signal contained in the m-th stage interference
cancellation residual signal is expressed as follows:
e.sub.m =(1-.alpha.).sup.m (3)
When .alpha. is 1 or smaller, if m is sufficiently large, e.sub.m converges
to zero. Thus, the interferences can be completely canceled.
To suppress an interference cancellation error, the interference canceling
process should be gradually performed. Thus, .alpha. should be small.
However, since the number of stages M of the interference canceler is
limited, .alpha. is optimized depending on M. It is clear that the first
and second weighting coefficients on the m-th stage are
[1-(1-.alpha.).sup.m-1, .alpha.]. However, to simplify the weighting
processes, as the first and second weighting coefficients, (1, .alpha.) or
(.alpha., .alpha.) are used. However, in such cases, the characteristics
relatively deteriorate.
Although different .alpha. values may be used on each stage, it is known
that optimum values of .alpha. on individual stages are almost equal.
However, the effect is small although the weighting processes are
complicated. In such a case, when .alpha. on the m-th stage is expressed
as .alpha.m, the first and second weighting coefficients are expressed as
[1-(1-.alpha..sub.1) . . . (1-.alpha..sub.m-1), .alpha..sub.m ]. The
present invention is also applied to such a case.
Next, an .alpha. determining method will be described. A major cause of the
deterioration of the parallel process structure interference canceler is a
propagation path estimation error on the first stage. The propagation path
estimating error depends on the ratio of signal power to interference
power of each user signal. Thus, according to the present invention, the
value of .alpha. is assigned so that it is proportional to the ratio of
signal power to interference power of each user signal.
In the CDMA system, to solve the problem of transmission distance, the
transmission power is controlled. In this case, the ratio of signal power
to interference power of each user signal is equally controlled. The ratio
is reversely proportional to the number of user signals. Thus, according
to the present invention, the value of .alpha. is assigned so that it is
reversely proportional to the number of user signals. Thus, when the ratio
of signal power to interference power of each user signal is equal and the
number of user signals does not vary, the value of .alpha. is constant for
the individual user signals.
[Second Embodiment]
FIG. 3 is a block diagram showing the structure of a multi-user receiving
apparatus according to a second embodiment of the present invention. The
structure of the multi-user receiving apparatus according to the second
embodiment is almost the same as the structure of the multi-user receiving
apparatus according to the first embodiment except that a first weighting
coefficient of first multiplying units 36-m-n and a second weighting
coefficient of second multiplying units 35-m-n of the second embodiment
are different from those according to the first embodiment.
The first and second weighting coefficients of the first and second
multiplying units are [1-(1-.alpha..beta..sub.n1)(1-.alpha..beta..sub.n2)
. . . (1-.alpha..beta..sub.nm-1), .alpha..beta..sub.nm ].
In this case, the value of .alpha. is the same as that of the first
embodiment shown in FIG. 1. The value of .beta..sub.nm depends on each
stage and each user signal. Thus, the value of .beta..sub.nm is an
adaptive value that chronologically varies. The first weighting
coefficient is a product of a constant weighting coefficient .alpha. and
an adaptive weighting coefficient .beta..sub.mn. Thus, the weighting
process can be flexibly performed. In the weighting method, the signal
component e.sub.m of the first user signal contained in the m-th stage
interference cancellation residual signal is expressed as follows:
e.sub.m =(1-.alpha..beta..sub.n1) (1-.alpha..beta..sub.n2) . . .
(1-.alpha..beta..sub.nm) (4)
When .alpha. and .beta..sub.nm are real numbers that are 1 or smaller, they
converge to 0. The weighting method according to the second embodiment is
different from that according to the first embodiment in that
.beta..sub.nm varies on each stage and for each user signal. Thus, the
interference canceling process for each user signal can be independently
controlled.
In other words, when a user signal has a large interference cancellation
error due to a propagation path estimation error of an IEU 32-m-n and a
symbol error, the interference canceling process is gradually performed so
as to suppress the characteristics from deteriorating. The interferences
are canceled on downstream stages.
To simplify the first weighting process, (1, .alpha..beta..sub.nm) and
(.alpha..beta..sub.nm -1,.alpha..beta..sub.nm) can be used.
Next, a .beta..sub.nm determining method will be described. When the
transmission power is ideally controlled, the ratio of signal power to
interference power for each user signal is equally controlled. However,
actually, a transmission power control error exists. FIGS. 4A to 4C are
graphs showing received signal powers of individual user signals in the
case that a transmission power control error exists. Since the
transmission power is controlled slot by slot, the received signal power
varies slot by slot. Since the transmission power control error depends on
each user signal, the ratio of signal power to interference power at each
slot varies for each user signal. FIGS. 4A to 4C show the state that the
received signal power of each user signal #1 to #N varies against an
average signal power slot by slot. Thus, according to the present
invention, the value of .beta..sub.nm is assigned so that it is
proportional to the ratio of signal power to interference power of each
slot.
As a real method, each slot signal power of each user signal is measured.
An average signal power of all user signals is obtained. Corresponding to
the ratio of each slot signal power of each user signal to average power
of all user signals, .beta..sub.nm is determined. In other words, the
following relations are satisfied:
.beta..sub.nm =SQRT (P.sub.nm /((P.sub.1m +P.sub.2m +. . . +P.sub.Nm)/N)),
.beta..sub.nm =1(.beta..sub.nm.gtoreq.1)
where P.sub.nm is each slot signal power of a n-th user signal on an m-th
stage. In this case, .beta..sub.nm is calculated with SQRT. Alternatively,
any nonlinear scale can be used.
When the number of user signals is sufficiently large, the average power of
all the user signals is almost equal to the average signal power of one
user signal. Thus, each slot signal power of each user may be normalized
with the average signal power. In other words, the following relations are
satisfied:
.beta..sub.nm =SQRT(P.sub.nm /A(P.sub.nm)), .beta..sub.nm
=1(.beta..sub.nm.gtoreq.1)
where A(P.sub.nm) is the average signal power of the n-th user signal on
the m-th stage. Since .beta..sub.nm depends on each slot of each user
signal, the deterioration of characteristics due to fluctuation of signal
powers of individual user signals corresponding to a transmission power
control error can be suppressed.
Next, another .beta..sub.nm determining method will be described. In the
method, an interference cancellation error due to a determination symbol
error can be suppressed. In a nonlinear interference canceler
corresponding to the symbol determining method, a determination symbol
error results in a large interference cancellation error. For example,
when a symbol error takes place in BPSK, a symbol interference that is 6
dB higher than the original symbol occurs rather than canceling
interference.
Thus, when a symbol determination error takes place, if the interference
canceling process is not performed, characteristics improve. However, when
a signal is demodulated, a symbol determination error cannot be detected.
Thus, according to the present invention, the likelihood of a determination
symbol is estimated with a pre-determined reception symbol that has been
normalized with an average signal power. When the likelihood is small and
thereby the probability of occurrence of a determination symbol error is
high, a small value is assigned to .beta..sub.nm .
FIG. 5 is a graph showing an example of a .beta..sub.nm determining method
corresponding to the likelihood. In FIG. 5, coordinates of Ich level and
Qch level of an orthogonal signal are illustrated. Thus, in FIG. 5, the
relation between a reception symbol and coefficient .beta. is represented.
In the method, the likelihood is calculated with the reception symbol and
the ratio of signal power to interference power.
Likelihood=(probability of which determination symbol is received as
reception symbol)/(sum of probability of which each modulation symbol in
QPSK is received as reception symbol)
In QPSK, the likelihood at the center of the coordinate is 0.25. The
likelihood at the symbol determination point is close to 1. The likelihood
is low in the vicinity of the center of the coordinates and on I and Q
axes. In FIG. 5, an upper right area of .beta..sub.nm =1 for a 50% symbol
is delimited with a dashed line. The area can be freely set.
As another method, phase likelihood is used.
Phase likelihood=.vertline.(phase of determination symbol)-(phase of
reception symbol).vertline.
In QPSK, the phase likelihood ranges from 0 to 45.degree.. The phase
likelihood should be converted into .beta..sub.nm. For simplicity, when
the phase likelihood is 0.degree., .beta..sub.nm is 1; when the phase
likelihood is 45.degree., .beta..sub.nm is 0.
Such methods are not practical because complicated calculations are
required for obtaining likelihood. As a simple method for determining
.beta..sub.nm, as IQ coordinates shown in FIG. 6, the distance from a
symbol determination point is used. When a reception symbol is present at
the symbol determination point, .beta..sub.nm is assigned so that it is
reversely proportional to the distance to the symbol determination point.
For example, when the reception symbol is present at the symbol
determination point, .beta..sub.nm =1. When the reception symbol is
present at the center of the coordinates, .beta..sub.nm =0. In an upper
right area delimited with a dashed line, .beta..sub.nm =1. Since these
characteristics are similar to a likelihood curve, the characteristics can
be effectively used.
Next, with reference to FIG. 7, another .beta..sub.nm determining method
that uses a symbol power will be described on IQ coordinates. In the
method, the following relations are satisfied:
.beta..sub.nm =SQRT(P.sub.nm /A(P.sub.nm)), .beta..sub.nm
=1(.beta..sub.nm.gtoreq.1)
where P.sub.nm is the symbol power at particular time of the n-th user on
the m-th stage; and A(P.sub.nm) is the average signal power. In this case,
.beta..sub.nm is calculated with SQRT. Alternatively, any nonlinear scale
can be used. Thus, when .beta..sub.nm varies on each stage, for each user
signal, for each symbol, the deterioration of characteristics due to a
determination symbol error can be suppressed.
The above-described methods are only examples of the present invention. In
other words, there will be many variations of the above-described methods.
FIGS. 1 and 2 show the structures of which interferences of all user
signals are canceled in parallel. However, the present invention can be
applied to a serial-parallel hybrid structure interference canceler of
which a parallel process is partly performed and a serial process is
performed for a parallel process user signal. In this case, the
above-described effects can be obtained. Such a structure is included in
the scope of the present invention.
It should be noted that the above-described embodiments may be applied to a
short code spread modulation of which a spread code period is equal to a
symbol period or to a long code spread modulation of which a spread code
period is longer than a symbol period.
As described above, according to the present invention, a multi-user
receiving apparatus with a parallel process structure that is simple and
that is a small delay of demodulating process is provided. In addition, a
symbol replica generated in an interference estimating unit is weighted
with a first weighting coefficient. A interference cancellation residual
signal is weighted with a second weighting coefficient. The weighting
coefficients are varied for each user signal. Thus, the interference
cancellation characteristics can be improved as with the serial structure.
In the multi-user receiving apparatus according to the present invention,
the characteristics of the interference canceler with parallel process
structure can be further improved as with a serial structure interference
canceler.
Although the present invention has been shown and described with respect to
a best mode embodiment thereof, it should be understood by those skilled
in the art that the foregoing and various other changes, omissions, and
additions in the form and detail thereof may be made therein without
departing from the spirit and scope of the present invention.
* * * * *
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