U.S. patent number RE40,056 [Application Number 10/678,053] was granted by the patent office on 2008-02-12 for methods of controlling communication parameters of wireless systems.
This patent grant is currently assigned to Intel Corporation. Invention is credited to Robert W. Heath, Jr., Arogyaswami J. Paulraj, Peroor K. Sebastian.
United States Patent |
RE40,056 |
Heath, Jr. , et al. |
February 12, 2008 |
Methods of controlling communication parameters of wireless
systems
Abstract
The present invention provides a method for controlling a
communication parameter in a channel through which data is
transmitted between a transmit unit with M transmit antennas and a
receive unit with N receive antennas by selecting from among
proposed mapping schemes an applied mapping scheme according to
which the data is converted into symbols and assigned to transmit
signals TS.sub.p, p=1 . . . M, which are transmitted from the M
transmit antennas. The selection of the mapping scheme is based on
a metric; in one embodiment the metric is a minimum Euclidean
distance d.sub.min,rx of the symbols when received, in another
embodiment the metric is a probability of error P(e) in the symbol
when received. The method can be employed in communication systems
using multi-antenna transmit and receive units of various types
including wireless systems, e.g., cellular communication systems,
using multiple access techniques such as TDMA, FDMA, CDMA and
OFDMA.
Inventors: |
Heath, Jr.; Robert W. (Hayward,
CA), Sebastian; Peroor K. (Mountain View, CA), Paulraj;
Arogyaswami J. (Stanford, CA) |
Assignee: |
Intel Corporation (Santa Clara,
CA)
|
Family
ID: |
34752828 |
Appl.
No.: |
10/678,053 |
Filed: |
October 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09464372 |
Dec 15, 1999 |
6351499 |
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Reissue of: |
09585948 |
Jun 2, 2000 |
06298092 |
Oct 2, 2001 |
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Current U.S.
Class: |
375/267; 375/299;
375/347; 455/101; 455/102; 455/69 |
Current CPC
Class: |
H01Q
1/246 (20130101); H01Q 3/2605 (20130101); H04B
7/0669 (20130101); H04B 7/0697 (20130101); H04B
7/084 (20130101); H04B 7/0857 (20130101); H04L
1/0001 (20130101); H04L 1/0003 (20130101); H04L
1/0009 (20130101); H04L 1/0026 (20130101); H04L
1/06 (20130101); H04L 1/0618 (20130101); H04B
7/0673 (20130101) |
Current International
Class: |
H04B
7/06 (20060101); H04B 7/08 (20060101) |
Field of
Search: |
;375/216,267,299,347
;455/69,101,102,135,136,272 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0951091 |
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Oct 1999 |
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EP |
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WO 98/09381 |
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Mar 1998 |
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WO |
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WO98/09385 |
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Mar 1998 |
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WO |
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Other References
Paulraj, A. Taxonomy of space-time processing for wireless
networks, IEE Proc--Radar Sonar Navig., vol. 145, No. 1, Feb. 1998.
cited by examiner.
|
Primary Examiner: Ha; Dac V.
Attorney, Agent or Firm: Cool, P.C.; Kenneth J. Proksch;
Michael A.
Parent Case Text
RELATED APPLICATIONS
This patent application is a continuation-in-part of patent
application Ser. No. 09/464,372 filed on Dec. 15, 1999, which is
herein incorporated by reference.
Claims
What is claimed is:
1. A method of controlling a communication parameter of a channel
for transmitting data between a transmit unit having a number M of
transmit antennas and a receive unit having a number N of receive
antennas, said method comprising: a) providing proposed mapping
schemes for converting said data into symbols and assigning said
data to transmit signals TS.sub.p, where p=1 . . . M, for
transmission from said M transmit antennas; b) obtaining a
measurement of said channel at said receiver; c) using said
measurement to compute for each of said proposed mapping schemes a
minimum Euclidean distance d.sub.min,vx of said symbols when
received; and d) selecting an applied mapping scheme from said
proposed mapping schemes based on said minimum Euclidean distance
d.sub.min,vx, thereby controlling said communication parameter.
2. The method of claim 1, wherein said proposed mapping schemes
comprise modulating said data in a constellation selected from the
group consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM.
3. The method of claim 1, wherein said proposed mapping schemes
comprise coding said data at predetermined coding rates.
4. The method of claim 1, wherein said proposed mapping schemes
comprise at least one method selected from the group consisting of
diversity coding and spatial multiplexing.
5. The method of claim 4, wherein said at least one method
comprises diversity coding of order k ranging from 1 to M.
6. The method of claim 5, wherein said diversity coding is selected
from the techniques consisting of space-time block coding, transmit
antenna selection, Equal Gain Combining, Maximum Ratio Combining
and delay diversity coding.
7. The method of claim 5, wherein said proposed mapping scheme
comprises a random assignment of said transmit signals TS.sub.p to
a number k of said M antennas.
8. The method of claim 5, wherein said proposed mapping scheme
comprises an assignment of said transmit signals TS.sub.p to a
number k of said M antennas based on a required minimum Euclidean
distance d.sub.min,required.
9. The method of claim 8, wherein said required minimum Euclidean
distance d.sub.min,required is related to a quality parameter of
said data.
10. The method of claim 4, wherein said at least one method
comprises spatial multiplexing of order k ranging from 1 to M.
11. The method of claim 10, wherein said spatial multiplexing
comprises a random assignment of said transmit signals TS.sub.p to
a number k of said M antennas.
12. The method of claim 10, wherein said spatial multiplexing
comprises an assignment of said transmit signals TS.sub.p to a
number k of said M antennas based on a required minimum Euclidean
distance d.sub.min,required.
13. The method of claim 12, wherein said required minimum Euclidean
distance d.sub.min,required is related to a quality parameter of
said data.
14. The method of claim 10, wherein said receive unit is selected
from the group consisting of maximum likelihood receivers, zero
forcing equalizer receivers, successive cancellation receivers and
minimum mean square error receivers.
15. The method of claim 1, wherein a minimum Euclidean distance
d.sub.min,tx of said symbols when transmitted is stored in a
database.
16. The method of claim 15, wherein said database is located in a
unit selected from the group consisting of said transmit unit and
said receive unit.
17. The method of claim 1, wherein said communication parameter is
selected from the group consisting of data capacity, signal
quality, spectral efficiency and throughput.
18. The method of claim 1, further comprising: a) determining a
quality parameter of said data; b) establishing a relation between
said quality parameter and a required minimum Euclidean distance
d.sub.min,required necessary to satisfy said quality parameter.
19. The method of claim 18, wherein said quality parameter is
selected from the group consisting of signal-to-interference noise
ratio, signal-to-noise ratio, power level, level crossing rate,
level crossing duration, bit error rate, symbol error rate, packet
error rate, and error probability.
20. The method of claim 1, wherein said transmit unit and said
receive unit operate in accordance with at least one multiple
access technique selected from the group consisting of TDMA, FDMA,
CDMA, OFDMA.
21. The method of claim 20, wherein said proposed mapping schemes
comprise diversity coding selected from the group consisting of
space-time block coding, transmit antenna selection, Equal Gain
Combining, Maximum Ratio Combining and delay diversity coding.
22. A method of controlling a communication parameter of a channel
for transmitting data between a transmit unit having a number M of
transmit antennas and a receive unit having a number N of receive
antennas, said method comprising: a) providing proposed mapping
schemes for converting said data into symbols and assigning said
data to transmit signals TS.sub.p, where p=1 . . . M, for
transmission from said M transmit antennas; b) obtaining a
measurement of said channel at said receiver; c) using said
measurement to compute for each of said proposed mapping schemes a
probability of error P(e) in said symbols when received; and d)
selecting an applied mapping scheme from said proposed mapping
schemes based on said probability of error P(e), thereby
controlling said communication parameter.
23. The method of claim 22, wherein said proposed mapping schemes
comprise modulating said data in a constellation selected from the
group consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM.
24. The method of claim 22, wherein said proposed mapping schemes
comprise coding said data at predetermined coding rates.
25. The method of claim 22, wherein said proposed mapping schemes
comprise at least one method selected from the group consisting of
diversity coding and spatial multiplexing.
26. The method of claim 25, wherein said at least one method
comprises diversity coding of order k ranging from 1 to M.
27. The method of claim 26, wherein said diversity coding is
selected from the techniques consisting of space-time block coding,
transmit antenna selection, Equal Gain Combining, Maximum Ratio
Combining and delay diversity coding.
28. The method of claim 26, wherein said proposed mapping scheme
comprises a random assignment of said transmit signals TS.sub.p to
a number k of said M antennas.
29. The method of claim 26, wherein said proposed mapping scheme
comprises an assignment of said transmit signals TS.sub.p to a
number k of said M antennas based on a required probability of
error P(e).sub.req.
30. The method of claim 25, wherein said at least one method
comprises spatial multiplexing of order k ranging from 1 to M.
31. The method of claim 30, wherein said spatial multiplexing
comprises a random assignment of said transmit signals TS.sub.p to
a number k of said M antennas.
32. The method of claim 30, wherein said spatial multiplexing
comprises an assignment of said transmit signals TS.sub.p to a
number k of said M antennas based on a required probability of
error P(e).sub.req.
33. The method of claim 30, wherein said receive unit is selected
from the group consisting of maximum likelihood receivers, zero
forcing equalizer receivers, successive cancellation receivers and
minimum mean square error receivers.
34. The method of claim 22, wherein a minimum Euclidean distance
d.sub.min,tx of said symbols when transmitted is stored in a
database.
35. The method of claim 34, wherein said database is located in a
unit selected from the group consisting of said transmit unit and
said receive unit.
36. The method of claim 22, wherein said communication parameter is
selected from the group consisting of data capacity, signal
quality, spectral efficiency and throughput.
37. The method of claim 22, wherein said transmit unit and said
receive unit operate in accordance with at least one multiple
access technique selected from the group consisting of TDMA, FDMA,
CDMA, OFDMA.
38. The method of claim 37, wherein said proposed mapping schemes
comprise diversity coding selected from the group consisting of
space-time block coding, transmit antenna selection, Equal Gain
Combining, Maximum Ratio Combining and delay diversity coding.
39. A communication system with a controlled communication
parameter of a channel for transmitting data between a transmit
unit having a number M of transmit antennas and a receive unit
having a number N of receive antennas, said transmit unit having a
mapping circuit comprising: a) a conversion unit for converting
said data into symbols; b) an assigning unit for assigning said
data to transmit signals TS.sub.p, where p=1 . . . M, for
transmission from said M transmit antennas, said converting and
said assigning being in accordance with proposed mapping schemes;
said receive unit comprising: a) a channel estimator for obtaining
a measurement of said channel; b) a computing block for computing
for each of said proposed mapping schemes a minimum Euclidean
distance d.sub.min,rx of said symbols when received; and c) a
selection block for selecting an applied mapping scheme from said
proposed mapping schemes based on said minimum Euclidean distance
d.sub.min,vx, thereby controlling said communication parameter.
40. The communication system of claim 39, wherein said assigning
unit comprises a diversity coding block and a spatial multiplexing
block.
41. The communication system of claim 40, wherein said diversity
coding block comprises at least one block selected from the group
consisting of a space-time coding block, a transmit antenna
selection block, Equal Gain Channel coding block, Maximum Ratio
Channel coding block and delay diversity coding block.
42. The communication system of claim 40, wherein said receive unit
is selected from the group consisting of maximum likelihood
receivers, zero forcing equalizer receivers, successive
cancellation receivers and minimum mean square error receivers.
43. The communication system of claim 39, further comprising a
database for storing a minimum Euclidean distance d.sub.min,tx for
said symbols when transmitted.
44. The communication system of claim 39, wherein said computing
block is selected from the group consisting of blocks for computing
data capacity, signal quality, spectral efficiency and
throughput.
45. The communication system of claim 44, further comprising a
quality parameter computation block for determining a quality
parameter of said data, said quality parameter being selected from
the group consisting of signal-to-interference noise ratio,
signal-to-noise ratio, power level, level crossing rate, level
crossing duration, bit error rate, symbol error rate, packet error
rate, and error probability.
46. The communication system of claim 45, further comprising an
assessment block for establishing a correlation between said
quality parameter and a required minimum Euclidean distance
d.sub.min,required.
47. The communication system of claim 39, said communication system
operating in accordance with at least one multiple access technique
selected from the group consisting of TDMA, FDMA, CDMA, OFDMA.
48. A communication system with a controlled communication
parameter of a channel for transmitting data between a transmit
unit having a number M of transmit antennas and a receive unit
having a number N of receive antennas, said transmit unit having a
mapping circuit comprising: a) a conversion unit for converting
said data into symbols; b) an assigning unit for assigning said
data to transmit signals TS.sub.p, where p=1 . . . M, for
transmission from said M transmit antennas, said converting and
said assigning being in accordance with proposed mapping schemes;
said receive unit comprising: a) a channel estimator for obtaining
a measurement of said channel; b) a computing block for computing
for each of said proposed mapping schemes a probability of error
P(e) of said symbols when received; and c) a selection block for
selecting an applied mapping scheme from said proposed mapping
schemes based on said probability of error P(e), thereby
controlling said communication parameter.
49. The communication system of claim 48, wherein said assigning
unit comprises a diversity coding block and a spatial multiplexing
block.
50. The communication system of claim 49, wherein said receive unit
is selected from the group consisting of maximum likelihood
receivers, zero forcing equalizer receivers, successive
cancellation receivers and minimum mean square error receivers.
51. The communication system of claim 48, wherein said diversity
coding block comprises at least one block selected from the group
consisting of a space-time coding block, a transmit antenna
selection block, Equal Gain Channel coding block, Maximum Ratio
Channel coding block and delay diversity coding block.
52. The communication system of claim 48, further comprising a
database for storing a required probability of error
P(e).sub.req.
53. The communication system of claim 48, wherein said computing
block is selected from the group consisting of blocks for computing
data capacity, signal quality, spectral efficiency and
throughput.
54. The communication system of claim 53, further comprising a
quality parameter computation block for determining a quality
parameter of said data, said quality parameter being selected from
the group consisting of signal-to-interference noise ratio,
signal-to-noise ratio, power level, level crossing rate, level
crossing duration, bit error rate, symbol error rate, packet error
rate, and error probability.
55. The communication system of claim 48, said communication system
operating in accordance with at least one multiple access technique
selected from the group consisting of TDMA, FDMA, CDMA, OFDMA.
.Iadd.56. A wireless communication device comprising: a channel
estimator, responsive to one or more antennas, to receive a
plurality of signals associated with a communication channel, and
to obtain a measurement of the communication channel; a processing
element, responsive to the channel estimator, to compute for a
plurality of proposed mapping schemes a minimum Euclidean distance
between symbols of the received signal(s) based, at least in part,
on the channel measurement; and a selection block, responsive to at
the processing element, to select a mapping scheme from the
proposed mapping schemes based, at least in part, on the computed
Euclidean distance with the 0ommunieationohannel, wherein the
selected mapping scheme denotes how content at a remote
communications device is to be applied to one or more antenna(s)
associated with the remote communications device, and wherein the
wireless communication device is able to provide an indication of
the selected mapping scheme to the remote communications
device..Iaddend.
.Iadd.57. A wireless communication device according to claim 56,
wherein the performance parameter is one or more of a measure of
receive signal strength, a measure of interference, a
signal-to-noise ratio (SNR), a signal-to-interference and noise
ratio (SINR), a bit-error rate (BER), a packet-error rate
(PER)..Iaddend.
.Iadd.58. A wireless communication device according to claim 56,
wherein the channel estimator obtains a measurement of the channel
coefficients matrix H characterizing the communication
channel..Iaddend.
.Iadd.59. A wireless communication device according to claim 58,
wherein selection block selects an applied mapping scheme for use
by a remote transmitter of the communication channel from a
plurality of potential mapping schemes based, at least in part, on
the measurement of the channel coefficient matrix H..Iaddend.
.Iadd.60. A wireless communication device according to claim 58,
further comprising: a local transmitter, responsive to the
selection block, to provide the indication of the selected mapping
scheme to the remote communication device for application to
subsequent transmission via the communication channel..Iaddend.
.Iadd.61. A wireless communication device according to claim 56,
wherein the proposed mapping schemes include one or more of
modulating said data in a constellation selected from the group
consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM..Iaddend.
.Iadd.62. A wireless communication device according to claim 56,
wherein the wireless communication device is a wireless station in
a wireless network including one or more antenna(e) through which
downlink signals are received from remote wireless access
point..Iaddend.
.Iadd.63. A wireless communication device according to claim 56,
further comprising: a memory system to store content including
executable content; and one or more processor element(s), coupled
with the memory system, to selectively access and execute at least
a subset of the stored content to implement one or more of the
channel estimator, computing block and selection
block..Iaddend.
.Iadd.64. A wireless communication device comprising: a channel
estimator, responsive to one or more antennas, to receive a
plurality of signals associated with a communication channel, and
to obtain a measurement of the communication channel; a processing
element, responsive to the channel estimator, to compute for a
plurality of proposed mapping schemes a probability of error of the
received signal(s); and a selection block, responsive to the
processing element, to select a mapping scheme from the proposed
mapping schemes based, at least in part, on the computed
probability of error, wherein the selected mapping scheme denotes
how content at a remote communications device is to be applied to
one or more antenna(s) associated with the remote communications
device, and wherein the wireless communication device is able to
provide an indication of the selected mapping scheme to the remote
communications device..Iaddend.
.Iadd.65. A wireless communication device according to claim 64,
wherein the performance metric is one or more of a measure of
receive signal strength, a measure of interference, a
signal-to-noise ratio (SNR), a signal-to-interference and noise
ratio (SINK), a bit-error rate (BER), a packet-error rate
(PER)..Iaddend.
.Iadd.66. A wireless communication device according to claim 64,
wherein the channel estimator obtains a measurement of the channel
coefficients matrix H characterizing the communication
channel..Iaddend.
.Iadd.67. A wireless communication device according to claim 64,
further comprising: a local transmitter, responsive to the
selection block, to communicate the select applied mapping scheme
to a remote transmitter of the communication channel for
application to subsequent via the communication
channel..Iaddend.
.Iadd.68. A wireless communication device according to claim 64,
wherein the proposed mapping schemes include one or more of
modulating said data in a constellation selected from the group
consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM..Iaddend.
.Iadd.69. A wireless communication device according to claim 64,
further comprising: a memory system to store content including
executable content; and one or more processor element(s), coupled
with the memory system, to selectively access and execute at least
a subset of the stored content to implement one or more of the
channel estimator, computing block and selection
block..Iaddend.
.Iadd.70. A wireless communication device comprising: a conversion
unit, to receive data for wireless transmission to a remote device
and to convert the received data into symbols; an assignment unit,
responsive to the conversion unit, to assign the symbols to
transmit signals TS.sub.p of the communication channel, where p=I .
. . M, for transmission from M transmit antennas; and a receive
element, coupled with the conversion unit and the assignment unit,
to receive an indication of a selected mapping scheme from a
plurality of possible mapping schemes from a remote communication
unit, wherein the conversion and assignment are performed in
accordance with the select mapping scheme..Iaddend.
.Iadd.71. A wireless communication device according to claim 70,
wherein the indication of the selected mapping scheme is received
from a remote wireless communication device and is selected based,
at least in part, on a minimum Euclidean distance of symbols in the
received signals TS.sub.p..Iaddend.
.Iadd.72. A wireless communication device according to claim 70,
wherein the indication of the selected mapping scheme is received
from a remote wireless communication device and is selected based,
at least in part, on a probability of error of said symbols in the
received signals TS.sub.p..Iaddend.
.Iadd.73. A wireless communication device comprising: two or more
omnidirectional antenna(e) through which signals associated with a
communication channel are received; a channel estimator, responsive
to at least a subset of the antennas, to obtain a measurement of
the received communication channel; a processing element,
responsive to the channel estimator, to compute for a plurality of
proposed mapping schemes a minimum Euclidean distance between
symbols of the received signal(s) based, at least in part, on the
channel measurement; and a selection block, responsive to estimator
and the processing element, to select a mapping scheme from the
proposed mapping schemes based, at least in part, on the computed
minimum Euclidean distance to, wherein the selected mapping scheme
denotes how content at a remote communications device is to be
applied to one or more antenna(s) associated with the remote
communications device, and wherein the wireless communication
device is able to provide an indication of the selected mapping
scheme to the remote communications device..Iaddend.
.Iadd.74. A communication device according to claim 73, comprising:
a local transmitter, responsive to the selection block, to provide
the indication of the selected mapping scheme to the remote
communication device for application to subsequent transmission via
the communication channel..Iaddend.
.Iadd.75. A wireless communication device according to claim 74,
wherein the proposed mapping schemes include one or more of
modulating said data in a constellation selected from the group
consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM..Iaddend.
.Iadd.76. A wireless communication device comprising: two or more
omnidirectional antenna(e) through which signals associated with a
communication channel are received; a channel estimator, responsive
to at least a subset of the antennas, to obtain a measurement of
the received communication channel; a processing element,
responsive to the channel estimator, to compute for a plurality of
proposed mapping schemes a probability of error of the symbols of
the received signal(s); and a selection block, responsive to and
the processing element, to select a mapping scheme from the
proposed mapping schemes based, at least in part, on the computed
probability of error, wherein the selected mapping scheme denotes
how content at a remote communications device is to be applied to
one or more antenna(s) associated with the remote communications
device, and wherein the wireless communication device is able to
provide an indication of the selected mapping scheme to the remote
communications device..Iaddend.
.Iadd.77. A communications device according to claim 76, further
comprising: a local transmitter, responsive to the selection block,
to communicate the select applied mapping scheme to a remote
transmitter of the communication channel for application to
subsequent transmission via the communication channel..Iaddend.
.Iadd.78. A wireless communication device according to claim 76,
wherein the proposed mapping schemes include one or more of
modulating said data in a constellation selected from the group
consisting of PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM..Iaddend.
Description
FIELD OF THE INVENTION
The present invention relates generally to wireless communication
systems and methods, and more particularly to controlling a
communication parameter between transmit and receive units with
multiple antennas.
BACKGROUND OF THE INVENTION
Wireless communication systems serving stationary and mobile
wireless subscribers are rapidly gaining popularity. Numerous
system layouts and communications protocols have been developed to
provide coverage in such wireless communication systems.
The wireless communications channels between the transmit and
receive devices are inherently variable and thus their quality
fluctuates. Hence, their quality parameters also vary in time.
Under good conditions wireless channels exhibit good communication
parameters, e.g., large data capacity, high signal quality, high
spectral efficiency and throughput. At these times significant
amounts of data can be transmitted via the channel reliably.
However, as the channel changes in time, the communication
parameters also change. Under altered conditions former data rates,
coding techniques and data formats may no longer be feasible. For
example, when the channel performance is degraded the transmitted
data may experience excessive corruption yielding unacceptable
communication parameters. For instance, transmitted data can
exhibit excessive bit-error rates or packet error rates. The
degradation of the channel can be due to a multitude of factors
such as general noise in the channel, multi-path fading, loss of
line-of-sight path, excessive Co-Channel Interference (CCI) and
other factors.
By reducing CCI the carrier-to-interference (C/I) ratio can be
improved and the spectral efficiency increased. Specifically,
improved C/I ratio yields higher per link bit rates, enables more
aggressive frequency re-use structures and increases the coverage
of the system.
It is also known in the communication art that transmit units and
receive units equipped with antenna arrays, rather than single
antennas, can improve receiver performance. Antenna arrays can both
reduce multipath fading of the desired signal and suppress
interfering signals or CCI. Such arrays can consequently increase
both the range and capacity of wireless systems. This is true for
wireless cellular telephone and other mobile systems as well as
Fixed Wireless Access (FWA) systems.
In mobile systems, a variety of factors cause signal degradation
and corruption. These include interference from other cellular
users within or near a given cell. Another source of signal
degradation is multipath fading, in which the received amplitude
and phase of a signal varies over time. The fading rate can reach
as much as 200 Hz for a mobile user traveling at 60 mph at PCS
frequencies of about 1.9 GHz. In such environments, the problem is
to cleanly extract the signal of the user being tracked from the
collection of received noise, CCI, and desired is signal portions
summed at the antennas of the array.
In FWA systems, e.g., where the receiver remains stationary, signal
fading rate is less than in mobile systems. In this case, the
channel coherence time or the time during which the channel
estimate remains stable is longer since the receiver does not move.
Still, over time, channel coherence will be lost in FWA systems as
well.
Antenna arrays enable the system designer to increase the total
received signal power, which makes the extraction of the desired
signal easier. Signal recovery techniques using adaptive antenna
arrays are described in detail, e.g., in the handbook of Theodore
S. Rappaport, Smart Antennas, Adaptive Arrays, Algorithms, &
Wireless Position Location; and Paulraj, A. J et al., "Space-Time
Processing for Wireless Communications", IEEE Signal Processing
Magazine, Nov. 1997, pp. 49-83.
Prior art wireless systems have employed adaptive modulation of the
transmitted signals with the use of feedback from the receiver as
well as adaptive coding and receiver feedback to adapt data
transmission to changing channel conditions. However, effective
maximization of channel capacity with multiple transmit and receive
antennas is not possible only with adaptive modulation and/or
coding.
In U.S. Pat. Nos. 5,592,490 to Barratt et al., 5,828,658 to
Ottersten et al., and 5,642,353 Roy III, teach about spectrally
efficient high capacity wireless communication systems using
multiple antennas at the transmitter; here a Base Transceiver
Station (BTS) for Space Division Multiple Access (SDMA). In these
systems the users or receive units have to be sufficiently
separated in space and the BTS uses its transmit antennas to form a
beam directed towards each receive unit. The transmitter needs to
know the channel state information such as "spatial signatures"
prior to transmission in order to form the beams correctly. In this
case spatial multiplexing means that data streams are transmitted
simultaneously to multiple users who are sufficiently spatially
separated.
The disadvantage of the beam-forming method taught by Barratt et
al., Ottersten et al., and Roy III is that the users have to be
spatially well separated and that their spatial signatures have to
be known. Also, the channel information has to be available to the
transmit unit ahead of time and the varying channel conditions are
not effectively taken into account. Finally, the beams formed
transmit only one stream of data to each user and thus do not take
full advantage of times when a particular channel may exhibit very
good communication parameters and have a higher data capacity for
transmitting more data or better signal-to-noise ratio enabling
transmission of data formatted with a less robust coding
scheme.
U.S. Pat. No. 5,687,194 to Paneth et al. describes a Time Division
Multiple Access (TDMA) communication system using multiple antennas
for diversity. The proposed system exploits the concept of adaptive
transmit power and modulation. The power and modulation levels are
selected according to a signal quality indicator fed back to the
transmitter.
Addressing the same problems as Paneth et al., U.S. Pat. No.
5,914,946 to Avidor et al. teaches a system with adaptive antenna
beams. The beams are adjusted dynamically as the channel changes.
Specifically, the beams are adjusted as a function of a received
signal indicator in order to maximize signal quality and reduce the
system interference.
The prior art also teaches using multiple antennas to improve
reception by transmitting the same information, i.e., the same data
stream from all antennas. Alternatively, the prior art also teaches
that transmission capacity can be increased by transmitting a
different data stream from each antenna. These two approaches are
commonly referred to as antenna diversity schemes and spatial
multiplexing schemes.
Adaptive modulation and/or coding in multiple antenna systems
involve mapping of data converted into appropriate symbols to the
antennas of the transmit antenna array for transmission. Prior art
systems do not teach rules suitable for determining such mappings
under varying channel conditions. Specifically, the prior art fails
to teach efficient methods and rules for mapping data signals to
antennas in systems using multiple transmit antennas and multiple
receive antennas in order to control one or more communications
parameters under varying channel conditions. Development of methods
and rules for selecting appropriate mapping schemes from the many
possible choices would represent a significant advance in the
art.
SUMMARY
The present invention provides a metric for selecting appropriate
mapping schemes for transmitting data while controlling a
communication parameter in a channel between a wireless transmit
and receive unit, both using multiple antennas. The method of the
invention teaches how to select mapping schemes based on the metric
which takes into account a quality parameter of received signals or
received data.
The method of the invention calls for controlling a communication
parameter in a channel through which data is transmitted between a
transmit unit with M transmit antennas and a receive unit with N
receive antennas. The method calls for providing proposed mapping
schemes according to which the data or bit stream is converted into
symbols and assigned to transmit signals TS.sub.p, p=1. . . M,
which are transmitted from the M transmit antennas. A measurement
of the channel at the receiver, e.g., a determination of the
channel coefficients matrix H, is used to compute a minimum
Euclidean distance d.sub.min,rx of the symbols when received in
each of the proposed mapping schemes. This computation can be
performed based on H and the proposed mapping schemes only. In this
embodiment the minimum Euclidean distance d.sub.min,rx is used as a
metric for selecting from the proposed mapping schemes an applied
mapping scheme to be employed for transmission of the data. The
selection of the mapping scheme based on the minimum Euclidean
distance metric d.sub.min,rx allows one to control the
communication parameter.
The data can be converted into symbols in accordance with any
suitable modulation technique. For example, the data can be
converted into symbols modulated in constellations selected from
among PSK, QAM, GMSK, FSK, PAM, PPM, CAP, CPM or any other
modulation scheme associating data with a constellation. The
mapping scheme can involve coding the data at certain coding rates.
Furthermore, the mapping scheme can include at least one method
selected from among diversity coding and spatial multiplexing.
When the mapping scheme includes diversity coding a k-th order
diversity coding, where k ranges from 1 to M, can be used. The
diversity coding can be selected from techniques consisting of
space-time block coding, transmit antenna selection, Equal Gain
Combining (EGC), Maximum Ratio Combining (MRC) and delay diversity
coding or any other antenna diversity scheme. Alternatively, the
diversity coding can include a random assignment of the transmit
signals TS.sub.p to k of the M transmit antennas. In accordance
with yet another approach, the assignment of the transmit signals
TS.sub.p to k of the M transmit antennas can be based on a required
minimum Euclidean distance d.sub.min,required. The required minimum
Euclidean distance can be determined based on its relation to one
or more quality parameters that the transmitted data has to
maintain. For example, the quality parameter can be
signal-to-interference noise ratio, signal-to-noise ratio, power
level, level crossing rate, level crossing duration, bit error
rate, symbol error rate, packet error rate, and error
probability.
When the mapping scheme includes spatial multiplexing a k-th order
spatial multiplexing (where k ranges from 1 to M) can be used.
Spatial multiplexing can involve random assignment of the transmit
signals TS.sub.p to k of the M transmit antennas. Alternatively,
the assignment of the transmit signals TS.sub.p to k of the M
transmit antennas can be based on the required minimum Euclidean
distance d.sub.min,required necessary to maintain one or more of
the quality parameters.
It is convenient to store a minimum Euclidean distance d.sub.min,tr
of the symbols when transmitted in a database. The database can
reside in the transmit unit or in the receive unit (or it can be
available in both).
Among others, the communication parameter to be controlled can
include data capacity, signal quality, spectral efficiency or
throughput.
It is advantageous to establish a relation between the quality
parameters and the required minimum Euclidean distances
d.sub.min,required necessary to satisfy the quality parameters,
i.e., maintain the quality parameter above a specified threshold.
The relations between the minimum Euclidean distances
d.sub.min,required for all possible mapping schemes and the
corresponding quality parameters are also conveniently stored in a
database.
The method of the invention can be employed in communication
systems such as wireless systems, e.g., cellular communication
systems, using multiple access techniques selected from among TDMA,
FDMA, CDMA and OFDMA. In conjunction with these techniques the
mapping schemes can include diversity coding selected from
techniques including space-time block coding, transmit antenna
selection, Equal Gain Combining (EGC), Maximum Ratio Combining
(MRC) and delay diversity coding or any other antenna diversity
scheme.
In another embodiment of the invention the metric used is the
probability of error, P(e). In this case the measurement of the
channel is used to compute for each of the proposed mapping schemes
a probability of error P(e) in the symbol when received. The
applied mapping scheme is then selected from the proposed mapping
schemes based on the probability of error P(e) to control the
communication parameter.
The proposed mapping schemes in this embodiment can include random
or determined assignment of transmit signals TS.sub.p to k of the M
transmit antennas as discussed above both in case of diversity
coding and spatial multiplexing. In particular, the assignment can
be based on a required probability of error P(e).sub.req.
The invention further encompasses a communication system which uses
the minimum Euclidean distance d.sub.min,rx of said symbols when
received, to select the applied mapping scheme from among the
proposed mapping schemes. The invention also includes a
communication system which uses the probability of error to select
the appropriate applied mapping scheme.
A detailed description of the invention and the preferred and
alternative embodiments is presented below in reference to the
attached drawing figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a simplified diagram illustrating a communication system
in which the method of the invention is applied.
FIG. 2 is a simplified block diagram illustrating the transmit and
receive units according to the invention.
FIG. 3 is a block diagram of an exemplary transmit unit in
accordance with the invention.
FIG. 4 is a block diagram of an exemplary receive unit in
accordance with the invention.
FIG. 5A is a detailed block diagram illustrating a selection block
and related components involved in selecting an applied mapping
scheme from proposed mapping schemes based on a minimum Euclidean
distance metric.
FIG. 5B is a detailed block diagram illustrating a selection block
and related components involved in selecting an applied mapping
scheme from proposed mapping schemes based on a probability of
error metric.
FIG. 6 is a block diagram of another transmit unit in accordance
with the invention.
DETAILED DESCRIPTION
The method and wireless systems of the invention will be best
understood after first considering the high-level diagrams of FIGS.
1 and 2. FIG. 1 illustrates a portion of a wireless communication
system 10, e.g., a cellular wireless system. For explanation
purposes, the downlink communication will be considered where a
transmit unit 12 is a Base Transceiver Station (BTS) and a receive
unit 14 is a mobile or stationary wireless user device. Exemplary
user devices include mobile receive units 14A, 14B, 14C which are
portable telephones and car phones and a stationary receive unit
14D, which can be a wireless modem unit used at a residence or any
other fixed wireless unit. Of course, the same method can be used
in uplink communication from wireless units 14 to BTS 12.
BTS 12 has an antenna array 16 consisting of a number of transmit
antennas 18A, 18B, . . . , 18M. Receive units 14 are equipped with
antenna arrays 20 of N receive antennas (for details see FIGS. 2, 3
and 4). BTS 12 sends transmit signals TS to all receive units 14
via channels 22A and 22B. For simplicity, only channels 22A, 22B
between BTS 12 and receive units 14A, 14B are indicated, although
BTS 12 transmits TS signals to all units shown. In this particular
case receive units 14A, 14B are both located within one cell 24.
However, under suitable channel conditions BTS 12 can transmit TS
signals to units outside cell 24, as is known in the art.
The time variation of channels 22A, 22B causes transmitted signals
TS to experience fluctuating levels of attenuation, interference,
multi-path fading and other deleterious effects. Therefore,
communication parameters of channels 22A, 22B such as data
capacity, signal quality, spectral efficiency or throughput undergo
temporal changes. Thus, channels 22A, 22B can not at all times
support efficient propagation of high data rate signals TS or
signals which are not formatted with a robust coding algorithm.
In accordance with the invention, antenna array 16 at BTS 12 can be
used for spatial multiplexing, transmit diversity or a combination
of the two to reduce interference, increase array gain and achieve
other advantageous effects. Antenna arrays 20 at receive units 14
can be used for spatial multiplexing, receive diversity or a
combination of the two. All of these methods improve the capacity,
signal quality, range and coverage of channels 22A, 22B. The method
of the invention finds an optimum choice or combination of these
techniques chosen adaptively with changing conditions of channels
22A, 22B. The method of the invention implements an adaptive and
optimal selection of spatial multiplexing, diversity as well as
rate of coding and bit-loading over transmit antenna array 16 to
antenna array 20.
Specifically, the method of the invention addresses these varying
channel conditions by adaptively controlling one or more
communication parameters based on a metric. FIG. 2 illustrates the
fundamental blocks of transmit unit 12 and one receive unit 14
necessary to employ the method. Transmit unit 12 has a control unit
26 connected to a data processing block 28 for receiving data 30 to
be converted and mapped in the form of transmit signals TS in
accordance with a number of proposed mapping schemes to transmit
antennas 18A, 18B, . . . , 18M for transmission therefrom. An
up-conversion and RF amplification block 32 supplies the transmit
signals TS to antennas 18A, 18B, . . . , 18M.
On the other side of the link, receiving unit 14 has N receive
antennas 34A, 34B, . . . , 34N in its array 20 for receiving
receive signals RS. An RF amplification and down-conversion block
36 processes receive signals RS and passes them to data processing
block 38. Data processing block 38 includes a channel measurement
or estimation unit (see FIG. 4) which obtains a measurement of the
channel coefficients matrix H characterizing channel 22.
A metric-based processing unit 40 uses matrix H and knowledge of
the proposed mapping schemes to select an applied mapping scheme
which should be used by transmit unit 12. In particular, given a
communication parameter which is to be controlled, e.g., maximized
or kept within a prescribed range, unit 40 makes a decision about
which of the proposed mapping schemes should be selected as the
applied mapping scheme under prevailing conditions of channel 22.
This selection is fed back as indicated by dashed line 42 to
transmit unit 12. In case channel 22 is a time-division duplexed
(TDD) channel, which is reciprocal between the receive and transmit
units, no separate feedback is required. In response, unit 26
employs the applied mapping scheme in processing data 30. This
ensures that a selected communication parameter or parameters are
controlled.
An exemplary embodiment of a transmit unit 50 for practicing the
method of the invention is shown in FIG. 3. Data 52, in this case
in the form of a binary stream, has to be transmitted. Before
transmission, data 52 may be interleaved and pre-coded by
interleaver and pre-coder 54 indicated in dashed lines. The purpose
of interleaving and pre-coding is to render the data more robust
against errors. Both of these techniques are well-known in the
art.
Data 52 is delivered to a conversion unit, more specifically a
coding and modulation block 56. Block 56 converts data 52 into
symbols at a chosen modulation rate and coding rate. For example,
data 52 can be converted into symbols through modulation in a
constellation selected from among PSK, QAM, GMSK, FSK, PAM, PPM,
CAP, CPM or other suitable constellations. In this embodiment, data
52 is modulated in accordance with 4QAM, represented by a
constellation 58 with four points (the axes Q and I stand for
quadrature and in-phase). In particular, data 52 is 4QAM modulated
at a certain modulation rate and coding rate. The transmission rate
or throughput of data 52 will vary depending on the modulation and
coding rates.
Table 1, below, illustrates some typical modulation and coding
rates with the corresponding constellations which can be used in
the proposed mapping schemes. The entries are conveniently indexed
by a mapping index.
TABLE-US-00001 TABLE 1 Modulation Output Mapping Rate Coding
Throughput Constel- Index (bits/symbol) Rate (bits/s/Hz)
d.sub.min,tx lation 1 1 1 1 4 BPSK 2 1 1/2TCM 1 7.2 4 PAM 3 2 1 2 2
4 QAM 4 2 2/3TCM 2 4.3 8 PSK 5 3 1 3 0.58 8 PSK 6 3 3/4TCM 3 1.32
16 PSK 7 4 1 4 0.4 16 QAM 8 4 4/5TCM 4 0.8 32 QAM
In this table minimum Euclidean distances d.sub.min,tx are listed
with symbol energies E.sub.s normalized to equal 1. The
abbreviation TCM stands for Trellis Coded Modulation, which is
well-known in the art and involves the simultaneous application of
coding and modulation. The mapping index column can be used to more
conveniently identify the proposed constellations, modulation and
coding rates which are to be used as part of the proposed mapping
schemes.
Tables analogous to Table 1 for other constellations can be easily
derived. Specifically, similar tables can be produced for
constellations GMSK, PPM, CAP, CPM and others. It should be noted
that modulation and coding are well-known in the art.
The next to last column of Table 1 indicates a minimum Euclidean
distance d.sub.min,tx in the constellation, where the subscript tx
indicates the transmit side. This is the shortest distance between
any two points in the constellation. The minimum Euclidean distance
between two points in 4QAM constellation 58 is indicated by a solid
line. A longer distance dt, is also indicated in a dashed line. The
code used increases this minimum Euclidean distance d.sub.min,tx as
is clear from in Table 1. The minimum Euclidean distances for any
other can be calculated or obtained from standard tables. For more
information on the derivation of these distances see Stephen B.
Wicker, Error Control Systems for Digital Communication and
Storage, Prentice Hall, 1995, Chapter 14.
Once coded and modulated in symbols, data 52 passes to a switch 60.
Depending on its setting, switch 60 routs data 52 either to a
spatial multiplexing block 62 or to a diversity coding block 64.
Both blocks 62 and 64 have a number k of outputs, where k.ltoreq.,
to permit order k spatial multiplexing or order k diversity coding.
A switching unit 68 is connected to blocks 62 and 64 for switching
the k order spatially multiplexed or k order diversity coded
signals to its M outputs. The M outputs lead to the corresponding M
transmit antennas 72 via an up-conversion and RF amplification
stage 70 having individual digital-to-analog converters and
up-conversion/RF amplification blocks 74.
Together, switch 60, blocks 62, 64 and switching unit 68 act as an
assigning unit 76 for assigning data 52 to transmit signals
TS.sub.p, where p=1. . . M, for transmission from the M transmit
antennas 72. It should be noted that for spatial multiplexing of
order k or diversity coding of order k, where k<M, not all
antennas 72 may be assigned transmit signals TS.sub.p. The criteria
for selecting which of antennas 72 will be transmitting transmit
signals TS.sub.p will be discussed below.
Thus, data 52 undergoes conversion into symbols and assignment to
transmit signals TS.sub.p which are transmitted from antennas 72.
This conversion and assignment of data 52 represent a mapping
scheme. Specifically, all the possible combinations of conversions
and assignments represents possible or proposed mapping schemes
which can be used by transmitter 50 to transmit data 52 from its M
antennas 72 over channel 22.
Transmit unit 50 also has a controller 66 connected to coding and
modulation unit 56 and to switch 60. A database 78 of proposed
mapping schemes is connected to controller 66. Database 78
conveniently contains tables, e.g., two tables: one for diversity
coding and one for spatial multiplexing, or one integrated table or
look-up table for both diversity coding and spatial multiplexing.
The table or tables contain modulation rates, coding rates,
throughputs, and minimum Euclidean distances for mapping schemes
employing diversity coding and for mapping schemes employing
spatial multiplexing. The tables or table can also include a
mapping index column, as does table 1, to simplify the
identification of the coding and modulation rates to be used in the
proposed mapping schemes. In an integrated table the mapping index
can serve as a mapping scheme index to identify all mapping
parameters, i.e., whether diversity coding or spatial multiplexing
is employed and at what coding rate, modulation rate, throughput
and associated minimum Euclidean distance. The convenience of using
one mapping scheme index resides in the fact that feed back of
mapping scheme index to transmit unit 50 does not require much
bandwidth.
Specifically, transmit unit 50 receives feedback denoted Rx from
receive unit 90 (see FIG. 4) via a feedback extractor 80. Feedback
extractor 80 detects the mapping scheme index and forwards it to
controller 66. Controller 66 looks up the corresponding mapping
scheme which is to be applied in database 78. In cases where
channel parameters, e.g., channel coefficients matrix H, have to be
known to employ the applied mapping scheme (e.g., when the
diversity coding technique is Maximum Ratio Combining), receive
unit 90 may also send the channel parameters to feedback extractor
80. Extractor 80 delivers the channel parameters to controller 66
as well as diversity coding block 64 and spatial multiplexing block
62. In the event of using a time-division duplexed (TDD) channel
22, the feedback information, i.e., the channel parameters are
obtained during the reverse transmission from the receive unit or
remote subscriber unit, as is known in the art, and no dedicated
feedback extractor 80 is required.
FIG. 4 illustrates receive unit 90 for receiving receive signals RS
from transmit unit 50 through channel 22 with N receive antennas
92. Receive unit 90 has an RF amplification and down-conversion
stage 94 having individual RF amplification/down-conversion/ and
analog-to-digital converter blocks 96 associated with each of the N
receive antennas 72. The N outputs of stage 94 are connected to a
block 98 which performs receive processing, signal detection and
decoding functions. The N outputs of stage 94 are also connected to
a channel estimator 100. Channel estimator 100 obtains a
measurement of channel 22; in particular, it determines the channel
coefficients matrix H representing the action of channel 22 on
transmit signals TS.sub.p.
Estimator 100 is connected to block 98 to provide block 98 with
matrix H for recovery of data 52. Specifically, block 98 uses
matrix H to process the received signals RS prior to reversing the
operations performed on data 52 at transmit unit 50. The output of
block 98 yields the reconstructed data stream. A deinterleaver and
decoder unit 102 is placed in the data stream if a corresponding
interleaver and coder 54 was employed in transmitter 50 to recover
original data 52.
Channel estimator 100 is also connected to a channel parameters
computation block 104. Block 104 computes the prevailing parameters
of channel 22. In particular, block 104 can compute channel
parameters such as SINR, Frobenius norms, singular values,
condition of channel coefficients matrix H and other channel
parameters. The actual computational circuits for computing these
parameters are known to a person skilled in the art.
Block 104 is further connected to a selection block 106. Block 106
analyzes received constellation 108 which corresponds to
transmitted constellation 58 after being subjected to the action of
the channel 22, i.e., after channel coefficients matrix H is
applied. Block 106 selects from the proposed mapping schemes an
applied mapping scheme which is to be used in mapping data 52 to
transmit antennas 72 of transmit unit 50.
In another embodiment, minimum Euclidean distance d.sub.min,rx
computed for symbols received is used as the metric for controlling
the communication parameter. FIG. 5A illustrates a detailed block
diagram showing selection block 106 and related components involved
in selecting the applied mapping scheme based on minimum Euclidean
distance d.sub.min,rx. Block 106 contains a database 108 of the
minimum Euclidean distances d.sup.2.sub.min,tx; here these are the
distance values squared, for all constellations 58 in the proposed
mapping schemes on the transmit side. The distance information in
database 108 is associated with the respective proposed mapping
schemes and can be ordered in tables for diversity coding and
spatial multiplexing with the associated constellations, modulation
rates and coding rates in a similar form as in database 78
discussed above. In fact, like database 78, database 108 may
contain a copy of an integrated table or look-up table as discussed
above. For mathematical reasons, it is convenient to work with the
square values of the minimum Euclidean distances and the
embodiments described herein shall take advantage of this fact.
Database 108 is connected to a computing block 110. Computing block
110 computes a minimum Euclidean distance d.sub.min,rx for received
symbols based on matrix H of channel 22. Due to the action of
channel 22 minimum Euclidean distance d.sup.2.sub.min,tx for the
symbols transmitted from transmit unit 50 will have changed in the
received symbols. In other words,
d.sup.2.sub.min,tx.noteq.d.sup.2.sub.min,rx because of the action
of channel coefficients matrix H. The actual change in the minimum
Euclidean distance between the transmitted and received
constellations will depend not only on the constellation,
modulation rate and coding rate but also on the assignment of data
52 to transmit signals TS.sub.p for transmission from transmit
antennas 72. In other words, the minimum distance depends on the
entire proposed mapping scheme. Therefore, computing block 110 has
a sub-block 112 for computing d.sup.2.sub.min,Diversity for
received symbols which are diversity coded and sub-block 114 for
computing d.sup.2.sub.min,SM for received signals which are
spatially multiplexed. Both sub-blocks 112, 114 obtain the value of
d.sup.2.sub.min,sx from database 108.
The diversity coding methods can include techniques such as
space-time block coding, transmit antenna selection, Equal Gain
Combining, Maximum Ratio Combining and delay diversity coding. All
of these coding methods are described in the prior art.
Alternatively, a random assignment of transmit signals TS.sub.p to
k of transmit antennas 72 can be made. This is especially useful
when transmit unit 50 is initially turned on, since no stable
information about channel 22 may be available at that time. The
order of the diversity coding methods is k, where
2.ltoreq.k.ltoreq.M. Let us designate the throughput at order k=M
diversity coding to be r bits/s/Hz. When k<M--fewer than all M
transmit antennas 72 are being used for diversity--the throughput
remains at r bits/s/Hz. Sub-block 112 uses the channel coefficients
matrix H and d.sup.2.sub.min,tx from database 108 to compute
d.sup.2.sub.min,Diversity to evaluate diversity coding methods
listed above. The mathematics involved in these computations will
be addressed below. Sub-block 112 then selects from among the
d.sup.2.sub.min,Diversity values the largest one for each data rate
r. This is the best selection since it ensures the lowest
probability of data corruption or error.
Spatial multiplexing methods are known in the art. Spatial
multiplexing in the present invention can involve a prescribed or a
random assignment of transmit signals TS.sub.p to k of transmit
antennas 72. Random transmit antenna assignment is especially
useful when transmit unit 50 is initially turned on, since no
stable information about channel 22 may be available at that time.
The order of spatial multiplexing is k, where 2.ltoreq.k.ltoreq.M.
Let us designate the throughput per antenna 72 at k-th order
spatial multiplexing to be r/k bits/s/Hz. When k<M then M-k
transmit antennas 72 are used for diversity.
Sub-block 114 uses the channel coefficients matrix H and
d.sup.2.sub.min,tx from database 108 to compute d.sup.2.sub.min,Sm
for the spatial multiplexing methods. The mathematics involved in
these computations will be addressed below. Sub-block 114 then
selects from among the d.sup.2.sub.min,SM values the largest one
for each data rate r. This is the best selection since it ensures
the lowest probability of data corruption or error.
Computing block 110 is in communication with a decision making
circuit 116. Both sub-blocks 112, 114 deliver their choices of the
largest d.sup.2.sub.min,Diversity and d.sup.2.sub.min,SM for each
data rate r respectively to decision making circuit 116.
In accordance with another embodiment and as indicated in FIG. 5A,
decision making block 116 is also connected to a block 122 whose
function is to determine a required minimum Euclidean distance
d.sup.2.sub.min,required. Block 122 is in communication with
communication parameters computation block 104 and with a data
quality parameter block 124.
Block 124 informs block 122 of a quality parameter, e.g.,
acceptable bit error rate (BER) or other threshold, which has to be
observed. In fact, the quality parameter can be any of the
following: signal-to-interference noise ratio, signal-to-noise
ratio, power level, level crossing rate, level crossing duration,
bit error rate, symbol error rate, packet error rate, and error
probability. The quality parameter selected can be dictated by the
type of service, e.g., fixed rate service, between transmit unit 50
and receive unit 90, or by other requirements placed on data 52 or
any other aspect of the communication link. As an example, a fixed
BER is chosen as the quality parameter in this embodiment. The BER
is translated into a corresponding probability of error P(e) and
supplied to block 122; here P(e) is specifically the probability of
symbol error.
Block 104 provides block 122 with the channel parameters, e.g.,
channel coefficients matrix H. The channel parameters are included
in the derivation of d.sup.2.sub.min,required. In the present
embodiment, d.sup.2.sub.min,required is derived directly from the
required P(e) using an established relationship:
.function..ltoreq..times..function..times..times. ##EQU00001##
where N.sub.e is the number of nearest neighbors in the
constellation and can be found for each proposed mapping scheme
based on the channel coefficients matrix H, Q(x)=1/2erfc(x/ {square
root over (2)}), where erfc is the complementary error function,
E.sub.s is the symbol energy and N.sub.o is the noise variance.
When other quality parameters are used, d.sup.2.sub.min,required
can be derived from other relationships involving different
parameters from among those delivered from block 124 and from block
104. In any event, the relation between the quality parameter and
d.sup.2.sub.min,required necessary to satisfy the quality parameter
should be established.
The value of d.sup.2.sub.min,required is supplied to decision
making circuit 116 and the choice between d.sup.2.sub.min,Diversity
and d.sup.2.sub.min,SM is made such that the value which exceeds
d.sup.2.sub.min,required and which supports the maximum data rate r
is selected. For example, when both values comply with data rate r
and are larger than d.sup.2.sub.min,required then the larger of the
two is chosen. If neither d.sup.2.sub.min,Diversity or
d.sup.2.sub.min,SM is satisfactory, then additional proposed
mapping schemes are evaluated by sub-blocks 112, 114 until either
one produces a value of d.sup.2.sub.min which exceeds
d.sup.2.sub.min,required and then this value is chosen.
It should also be noted, that when either diversity coding or
spatial multiplexing is employed in the proposed mapping schemes,
the assignment of transmit signals TS.sub.p to k of the M antennas
72 can be made based on d.sup.2.sub.min,required. This assignment
can be made by choosing the subset k of M transmit antennas 72
which provides the maximum data rate r for the given
d.sup.2.sub.min,required.
During operation receive unit 90 repeats the computation of
d.sub.min as channel 22 changes. In the case of a moving receiver
90, e.g., a cellular telephone, this recalculation should be
performed more frequently, since the channel coherence time is
short. In case of a stationary receiver 90, e.g., a wireless modem,
the coherence time is longer and d.sub.min can be recomputed at
longer intervals.
This selection is delivered to a feedback 118, which passes the
choice on to a transmitter 120 of the receive unit 90. Transmitter
120 sends the choice of the applied mapping scheme characterized by
the largest d.sup.2.sub.min at the desired data rate r back to
transmit unit 50. Conveniently, transmitter 120 can send the
mapping scheme index identifying enabling transmit unit 50 to
locate and retrieve the applied mapping scheme from database
78.
The applied mapping scheme includes the modulation rate and coding
rate, as well as a choice of the diversity method or spatial
multiplexing method which yielded that largest d.sup.2.sub.min
value picked at decision making block 116. Advantageously, feedback
118 is also connected to channel parameters computation block 104,
as shown, to additionally transmit back to transmit unit 50 the
parameters of channel 22, e.g., the channel coefficients matrix H,
determined at receiver 90.
FIG. 5B illustrates an alternative embodiment of the invention in
which a selection block 150 relies on the probability or error P(e)
as a metric to select the applied mapping scheme from the proposed
mapping schemes. Analogous blocks in this embodiment retain the
reference numbers from FIG. 5A. In particular a computing block 152
has two sub-blocks 154, 156 for computing the probability of error
for diversity coding P(e).sub.Diversity and probability of error
for spatial multiplexing P(e).sub.SM for each data rate r.
Sub-blocks 154, 156 are connected to a decision making circuit 158.
Of the P(e).sub.Diversity and P(e).sub.SM values circuit 158
chooses the one which is the lowest from the proposed mapping
schemes and supports the highest data rate r. This choice is fed
back via transmitter 120 to transmit unit 50 as in the
above-described embodiment.
Preferably, a block 160 provides the d.sup.2.sub.min,required value
based on a quality parameter, e.g., a desired BER in the case of
fixed BER service, to a block 162 for computing the required
probability of error P(e).sub.required. Once again, the
relationship: .function..ltoreq..times..function..times..times.
##EQU00002## can be used in this computation. Block 162 used
d.sup.2.sub.min,required as well as channel parameters from
communication parameters computation block 104 to compute
P(e)required. This computed value of P(e).sub.required is then
supplied to decision making circuit 158 to select the suitable
value from among the P(e).sub.Diversity and P(e).sub.SM values for
the proposed mapping schemes. In this case the lowest value of P(e)
is selected.
It should be noted that in the event transmit unit 50 receives
feedback of channel information, whether using TDD or simple
feedback, it could make the selection of applied mapping scheme on
its own. In other words, transmit unit 50 can select the mapping
scheme index and apply the corresponding mapping scheme from
database 78. This alternative approach would be convenient when
receive unit 90 does not have sufficient resources or power to
evaluate the proposed mapping schemes. Of course, transmit unit 50
would then contain all the corresponding computation and
decision-making blocks contained in receive unit 90 as described
above.
FIG. 6 illustrates another embodiment of a transmit unit 200.
Corresponding parts have been labeled with the same reference
numbers as in FIG. 3. In this case, data 52 to be transmitted is
first delivered to a switch 202. Depending on the setting of switch
202 data 52 is passed either to a coding and modulation block 204
and spatial multiplexing block 206 or to a space-time coding block
208. In this embodiment space-time coding block 208 assumes all the
functions of coding, modulating and applying a diversity technique
to data 52. Meanwhile, blocks 204 and 206 implement coding,
modulation and spatial multiplexing respectively.
Both blocks 206 and 208 have a number k of outputs, where
k.ltoreq.M, to permit order k spatial multiplexing or order k
diversity coding respectively. Switching unit 68 is connected to
blocks 206 and 208 for switching the k order spatially multiplexed
or k order diversity coded signals to its M outputs. The M outputs
lead to the corresponding M transmit antennas 72 via up-conversion
and RF amplification stage 70 having individual digital-to-analog
converters and up-conversion/RF amplification blocks 74.
Together, switch 202, blocks 204, 206, 208 and switching unit 68
act as an assigning unit 210 for assigning data 52 to transmit
signals TS.sub.p, where p=1. . . M, for transmission from the M
transmit antennas 72. As in transmit unit 50, the criteria for
selecting the applied mapping scheme will dictate the setting of
switch 202 and operation of blocks 206, 208, 68. In other words,
the applied mapping scheme will be used to set all parameters of
assigning unit 210. As before, this function is achieved with the
aid of feedback denoted Rx from receive unit 90 (see FIG. 4) via a
feedback extractor 80. Feedback extractor 80 detects the mapping
scheme index and forwards it to controller 66. Controller 66 looks
up the corresponding mapping scheme which is to be applied in
database 78. In cases where channel parameters, e.g., channel
coefficients matrix H, have to be known to employ the applied
mapping scheme receive unit 90 may also send the channel
parameters. Extractor 80 delivers the channel parameters to
controller 66 as well as spatial multiplexing block 206 and
space-time coding block 208. Once again, in the event of using a
time-division duplexed (TDD) channel 22, the feedback information,
i.e., the channel parameters are obtained during the reverse
transmission from the receive unit or remote subscriber unit, as is
known in the art, and no dedicated feedback extractor 80 is
required.
The above embodiments will provide a person of average skill in the
art with the necessary information to use the two metrics, minimum
Euclidean distance and probability of error for making the
appropriate selection of applied mapping scheme in communications
systems with various multi-antenna transmit and receive units. In
addition, the below examples suggest some specific implementations
to further clarify the details to a person of average skill in the
art. The transmit diversity coding in these examples includes
space-time block coding, selection of k transmit antennas, equal
gain combining and maximum ratio combining. The spatial
multiplexing in these examples includes spatial multiplexing using
a maximum likelihood (ML) receiver, spatial multiplexing with a
linear receiver such as a zero-forcing equalizer (ZFE) receiver and
minimum mean square error (MMSE) receiver and spatial multiplexing
with successive canceling receiver.
At a data transfer rate r and minimum Euclidean distance
d.sup.2.sub.min,rx of the transmitted constellation, the minimum
Euclidean distance for space-time block coding (stbc) is
d.sup.2.sub.min,stbc on the receive end and is expressed in terms
of channel coefficients matrix H. H is an M.sub.r.times.M.sub.t
matrix where M.sub.r is the number of receive antennas 92 and
M.sub.t is the number of transmit antennas 72 known to the receiver
with the Frobenius norm defined as: .times. .times..times.
.times..function..times..times..lamda. ##EQU00003## where
.lamda..sub.k.sup.2 are the squared singular values. This allows us
to write: .times..times. .times..times.
.times..times..function..times..times..lamda..times. ##EQU00004##
Clearly, performance is sensitive only to the power in H averaged
by the number of transmit antennas 72, i.e.,
.parallel.H.parallel..sub.F.sup.2|M.sub.t. The computations can be
carried out, e.g., in computing block 110, and more specifically in
sub-block 114, after it is supplied with the channel H from
communication parameters computation block 104 and
d.sup.2.sub.min,sx from database 108.
In selection diversity one (k=1) of the M transmit antennas 72 can
be chosen to maximize a quality parameter such as received SNR. In
this case, when data is transmitted at rate r and the minimum
Euclidean distance d.sup.2.sub.min,sel of the constellation has
another expression at the receive end. Let h.sub.k be the k-th
column of H. Then the minimum Euclidean distance
d.sup.2.sub.min,sel in can be written as: .times..times.
##EQU00005## It should be noted that
d.sup.2.sub.min,sel.gtoreq.d.sup.2.sub.min,stbc since the maximum
norm of one column is always greater than the average of the norms
of all the columns. Using formalisms known in the art a more direct
relationship can be written as:
.times..gtoreq..lamda..times..times..gtoreq..function..times..times..lamd-
a..times..times. ##EQU00006## and also
d.sup.2.sub.min,sel.ltoreq.M.sub.t.lamda..sub.max.sup.2(H)d.sup.2.sub.min-
,tx'
From the above it is clear that selection diversity is always
better than space-time block coding for a given channel. Typically,
antenna selection should be employed when at least some partial
knowledge of H is available while space-time block coding can be
used at system start-up when little or no knowledge of H is
available.
For generalized transmit equal gain combining, one finds an optimal
transmit vector which maximizes a quality parameter, e.g., SNR,
under the constraint that the vector consists purely of phase
coefficients. This vector, w, with its components corresponding to
transmit signals, can be defined as: w=[1e.sup.i.phi.1. . .
e.sup.i.phi.N-1]/ {square root over (M.sub.r)} The solution is
found by solving for .phi..sub.1, . . . .phi..sub.N-1 such that
w'H'Hw is maximized. This can be done by optimization techniques
well-known in the art. It is useful to recognize that:
d.sup.2.sub.min,ege.ltoreq.d.sup.2.sub.min,mrc=d.sub.,min,tx.lamda.-
.sub.max.sup.2(H), where mrc stands for maximum ratio combining as
described below. In practice, one can set these two minimum
Euclidean distances as approximately equal
(d.sup.2.sub.min,erc.apprxeq.d.sup.2.sub.min,mrc) and therefore use
techniques developed for maximum ratio combining.
For generalized transmit maximum ratio combining, one can find an
optimal transmit vector which maximizes a quality parameter, e.g.,
SNR. It should be noted that this is usually the best of such
linear techniques. This vector, once again denoted w, is normalized
such that .parallel.w.parallel..sup.2=1 and is found by maximizing
E.sub.sw'H'Hw/N.sub.o subject to this normalization condition. The
solution, found through linear algebra, is w=w.sub.max the correct
singular vector corresponding to the maximum singular value. Given
this one can write d.sup.2.sub.min,mrc as:
d.sup.2.sub.min,mrc=d.sup.2.sub.min,txw'H'Hw=d.sup.2.sub.min,tx.lamda..su-
b.max.sup.2(H). It should be noted that
d.sup.2.sub.min,mrc.gtoreq.d.sup.2.sub.min,ege.gtoreq.d.sup.2hd
min,sel'
When employing spatial multiplexing the computations can be carried
out, e.g., by sub-block 112 in computing block 110. In spatial
multiplexing the type of receive unit 90 is important.
In the first example receive unit 90 is of the ML type. Let s and s
be the transmitted and hypothesized (received) vectors,
respectively, both of dimensions M.sub.t.times.1. The coefficients
in these vectors come from the selected QAM constellation (with |A|
points) which is assumed the same for each of transmit antennas 72.
The average power of the per-antenna constellation is taken to be
one. Let d.sup.2.sub.min,s denote the minimum distance of this per
antenna constellation. Let S denote the set of all |A|.sup.M'
possible s vectors. Then we can write the minimum Euclidean
distance d.sup.2.sub.min,sm-m1 the received constellation as:
.times. .times..di-elect cons..times..function. ##EQU00007## Using
well-known mathematical techniques bounds and approximations can be
used to simplify this expression. For example, the upper bound on
d.sup.2.sub.min,sm-m1 can be defined by denoting E as the space of
error vectors e where E={s-s.noteq.0|s, s.epsilon.S}, and as the
space of error vectors with some of the error vectors removed
therefrom as follows: .ltoreq..times. .times..di-elect
cons..times..times. ##EQU00008## where e is an element of .
Alternatively, a lower bound on d.sup.2.sub.min,sm-m1 can be
defined as follows: .gtoreq..lamda..function..times.
##EQU00009##
The upper bound is optimistic, meaning that it will tend to predict
a minimum Euclidean distance which may be greater than it actually
is in practice. The lower bound is pessimistic, meaning that it
will tend to predict a minimum Euclidean distance which may be
smaller than in practice. A person of average skill in the art will
appreciate that, depending on the required reliability of the
communication system either bound can be used. Alternatively, the
two bounds can be averaged or used together in some other manner to
yield the minimum Euclidean distance in spatial multiplexing with
ML receive unit 90.
In another example receive unit 90 is a successive receiver, which
estimates a single data stream, subtracts that stream out,
estimates the next data stream, subtracts it out and so on. The
performance of this type of receiver is computed based on the
following algorithm: 1) start with M.sub.t data streams and let
H.sub.i=H; 2) find G, which is a ZF/MMSE inverse of H; 3) let
g.sub.i be the row of G with the minimum norm, i.e.,
.parallel.g.sub.i.parallel..sup.2.ltoreq..parallel.g.sub.j.parallel..sup.-
2 for all j.noteq.i; 4) apply g.sub.i to H to estimate the i-th
stream of data; 5) subtract out the i-th stream of data and remove
the i-th column of H to form a new channel coefficients matrix
H.sub.i-1; 6) repeat the above steps using the new (reduced)
channel coefficients matrix H.sub.i-1.
Let {g.sub.i}.sub.i-1.sup.1.sup.1 be the sequence of linear
equalizers with results from the above recursion. Then we can
estimate the performance of receive unit 90 (assuming no feedback
errors) as follows: .times. .times..times..times..times..times.
##EQU00010## The performance is essentially determined by linear
receiver g.sub.i which has the highest norm. When receive unit 90
is a ZF receiver, then the above expression simplifies to: .times.
.times..times..times. ##EQU00011##
In yet another example, receive unit 90 is a linear receiver which
first separates all the data streams using a linear equalizer (not
shown) and then detects each stream independently. Let G be a
linear receiver. For example, in the ZF case G=H.sup.+ or in the
MMSE case G=[HH'+I/SNR[.sup.-1H'. Let g.sub.i be the i-th column of
G. Then the minimum Euclidean distance of the receiver can be
written as: .times. .times..times..times..times. ##EQU00012##
Once again, when receive unit 90 is a ZF receiver this equation can
be rewritten as: .times. .times..times..times. ##EQU00013## where
the performance depends on the largest magnitude of g.sub.i. Using
a well-known property from linear algebra, namely max
.parallel..sub.i.parallel..sup.2.ltoreq.1/.lamda..sub.min.sup.2(H)
the above equation can used to derive a lower bound as follows:
.gtoreq..lamda..function..times. ##EQU00014##
In this case, the performance will be influenced by the minimum
singular value of H.
When the selection between diversity coding and spatial
multiplexing is performed based on the minimum Euclidean distance
metric as described above it is advantageous to observe the
following procedure. Once the estimate of H is available and fixed
transmission rate r is given the mode of operation yielding the
best performance is selected by: 1) computing
d.sup.2.sub.min,Diversity for the desired diversity coding; 2)
computing d.sup.2.sub.min,SM for the desired spatial multiplexing;
3) choosing diversity coding if
d.sup.2.sub.min,Diversity.gtoreq.d.sup.2.sub.min,SM otherwise
choosing spatial multiplexing.
Communication systems employing the metrics of the invention to
select applied mapped schemes from proposed mapping schemes can be
based on any multiple access technique including TDMA, FDMA, CDMA
and OFDMA.
It will be clear to one skilled in the art that the above
embodiment may be altered in many ways without departing from the
scope of the invention. Accordingly, the scope of the invention
should be determined by the following claims and their legal
equivalents.
* * * * *