U.S. patent application number 09/939231 was filed with the patent office on 2002-05-16 for multibeam wireless communications method and system including an interference avoidance scheme in which the area of each transmitted beam is divided into a plurality of sub-areas.
This patent application is currently assigned to Nortel Networks Limited. Invention is credited to Matyas, Robert, Senarath, Nimal G., Strawczynski, Leo L..
Application Number | 20020058514 09/939231 |
Document ID | / |
Family ID | 24852567 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020058514 |
Kind Code |
A1 |
Senarath, Nimal G. ; et
al. |
May 16, 2002 |
Multibeam wireless communications method and system including an
interference avoidance scheme in which the area of each transmitted
beam is divided into a plurality of sub-areas
Abstract
A method for reducing interference in a wireless system, and a
system for performing the method. The wireless system should
include at least two, and preferably four, beam formers and a
plurality of mobile units. The method includes the steps of
transmitting beams B1, B2, B3 and B4 into first, second, third and
fourth beam areas, respectively. At least two sub-areas are defined
within each of the first, second, third and fourth beam areas based
upon the degree of overlap with adjacent beam areas, whereby each
of the beam areas includes at least one overlapping sub-area and at
least one non-overlapping sub-area. The method further includes
coding signals of the beams B1, B2, B3 and B4 for receipt by a
particular mobile unit based upon which one of the sub-areas that
the particular mobile unit is located within. If the invention is
practiced with a TDM scheme, at least three time periods are
utilized, wherein during the first time period (T1), simultaneous
transmissions are made for receipt by mobile units located within
sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and G1.sub.4; during a
second time period (T2), transmissions are made for receipt by
mobile units located within sub-areas G2.sub.1 and G2.sub.4; and
during a third time period (T3), transmissions are made for receipt
by mobile units located within sub-areas G2.sub.2 and G2.sub.3. If
the invention is practiced with an FDM scheme, the group of
frequencies assigned to each cell is divided such that half of the
frequencies serve mobile units located within sub-areas G1.sub.1,
G1.sub.2, G1.sub.3 and G1.sub.4, and the other half of the
frequencies serve mobile units located within sub-areas G2.sub.1,
G2.sub.2, G2.sub.3 and G2.sub.4.
Inventors: |
Senarath, Nimal G.; (Nepean,
CA) ; Matyas, Robert; (Nepean, CA) ;
Strawczynski, Leo L.; (Ottawa, CA) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Nortel Networks Limited
|
Family ID: |
24852567 |
Appl. No.: |
09/939231 |
Filed: |
August 24, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09939231 |
Aug 24, 2001 |
|
|
|
09710085 |
Nov 10, 2000 |
|
|
|
Current U.S.
Class: |
455/450 ;
455/63.4 |
Current CPC
Class: |
H04W 16/28 20130101;
H04W 16/24 20130101 |
Class at
Publication: |
455/450 ;
455/562; 455/63 |
International
Class: |
H04Q 007/20 |
Claims
What is claimed is:
1. A wireless communications system comprising: at least four beam
formers arranged within a cellular communications network, said
beam formers including a first beam former for transmitting a first
beam (B1) into a first area and a second beam former for
transmitting a second beam (B2) into a second beam area, where said
second beam area is adjacent said first beam area, and a third beam
former for transmitting a third beam (B3) into a third beam area
and a fourth beam former for transmitting a fourth beam (B4) into a
fourth beam area, where said fourth beam area is adjacent said
third beam area; a mobile switching center for controlling signals
transmitted from said at least four beam formers, including sending
coded signals along said beams B1, B2, B3 and B4 such that: each of
said first, second, third and fourth beam areas are effectively
divided into at least two sub-areas such that said first beam area
includes sub-areas G1.sub.1 and G2.sub.1, said second beam area
includes sub-areas G1.sub.2 and G2.sub.2, said third beam area
includes sub-areas G1.sub.3 and G2.sub.3, and said fourth beam area
includes sub-areas G1.sub.4 and G2.sub.4; and wherein during a
first time period (T1), simultaneous transmissions are made for
receipt by mobile units located within sub-areas G1.sub.1,
G1.sub.2, G1.sub.3 and G1.sub.4; during a second time period (T2),
transmissions are made for receipt by mobile units located within
sub-areas G2.sub.1 and G2.sub.4; and during a third time period
(T3), transmissions are made for receipt by mobile units located
within sub-areas G2.sub.2 and G2.sub.3.
2. The wireless communications system according to claim 1, wherein
said sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and G1.sub.4 are areas
with little or no interference from adjacent beams and said
sub-areas G2.sub.1, G2.sub.2, G2.sub.3 and G2.sub.4 are areas with
greater interference from adjacent beams.
3. The wireless communications system according to claim 1,
wherein: said sub-area G1.sub.1 begins near an apex of said first
area and extends generally down a center of said first area, and
said sub-area G2.sub.1 is located outside of said sub-area
G1.sub.1; and said sub-area G1.sub.2 begins near an apex of said
second area and extends generally down a center of said second
area, and said sub-area G2.sub.2 is located outside of said
sub-area G1.sub.2.
4. The wireless communications system according to claim 1 wherein
said first and second areas are divided into sub-areas G1.sub.1,
G2.sub.1, G1.sub.2, and G2.sub.2 based upon the
carrier-to-interference ratio (C/I) of signals being received
within each sub-area.
5. The wireless communications system according to claim 1, wherein
said beams B1, B2, B3 and B4 are each rotated by half of the
average beamwidth of all of the beams, thereby creating new
sub-areas RG1.sub.1 and RG2.sub.1 in said first beam area, new
sub-areas RG1.sub.2 and RG2.sub.2 in said second beam area, new
sub-areas RG1.sub.3 and RG2.sub.3 in said third beam area and new
sub-areas RG1.sub.4 and RG2.sub.4 in said fourth beam area, so that
each mobile now has the option of selecting from either the rotated
beams or the original beams, giving rise to more directed beams for
the mobiles, thereby increasing both coverage and overall
throughput.
6. The wireless communications system according to claim 1, wherein
said beams B1, B2, B3 and B4 are each rotated by a portion of their
beamwidth that is approximately equal to 1/nth of the average
beamwidth, where n is the total number of rotated positions for
each beam, thereby creating new sub-areas, and further wherein said
new sub-areas are served by time periods other than said first,
second and third time periods.
7. The wireless communications system according to claim 5,
wherein: during a fourth time period (T4), simultaneous
transmissions are made for receipt by mobile units located within
said sub-areas RG1.sub.1, RG1.sub.2, RG1.sub.3 and RG1.sub.4;
during a fifth time period (T5), transmissions are made for receipt
by mobile units located within said sub-areas RG2.sub.1 and
RG2.sub.4; and during a sixth time period (T6), transmissions are
made for receipt by mobile units located within said sub-areas
RG2.sub.2 and RG2.sub.3.
8. The wireless communications system according to claim 7, wherein
each mobile unit is assigned to a beam and a rotation position
based on the following criteria, wherein, for a given mobile, the
best rates from all the beams that can be supported in said time
slots T1, T2, T3, T4, T5 and T6 are, respectively, r1, r2, r3, r4,
r5 and r6, and further wherein R1=max(r1, r4) and R2=max(r2, r3,
r5, r6): if 2R1>R2 and r1>r4, then mobile unit is served in
said sub-area G1.sub.1, G1.sub.2, G1.sub.3 or G1.sub.4; if
2R1.gtoreq.R2 and r1<r4, then mobile unit is served in said
sub-area RG1.sub.1, RG1.sub.2, RG1.sub.3 or RG1.sub.4; if 2R1<R2
and max(r2, r3)>max(r5, r6) and r2.gtoreq.r3, then mobile unit
is served in said sub-area G2.sub.1 or G2.sub.4; if 2R1<R2 and
max(r2, r3)>max(r5, r6) and r2<r3, then mobile unit is served
in said sub-area G2.sub.2 or G2.sub.3; if 2R1<R2 and max(r2,
r3).ltoreq.max(r5, r6) and r5.gtoreq.r6, then mobile unit is served
in said sub-area RG2.sub.1 or RG2.sub.4; and if 2R1<R2 and
max(r2, r3).ltoreq.max(r5, r6) and r5<r6, then mobile unit is
served in said sub-area RG2.sub.2 or RG2.sub.3.
9. A wireless communications system comprising: at least four beam
formers arranged within a cellular communications network, said
beam formers including a first beam former for transmitting a first
beam (B1) into a first area and a second beam former for
transmitting a second beam (B2) into a second beam area, where said
second beam area is adjacent said first beam area, and a third beam
former for transmitting a third beam (B3) into a third beam area
and a fourth beam former for transmitting a fourth beam (B4) into a
fourth beam area, where said fourth beam area is adjacent said
third beam area; a mobile switching center for controlling signals
transmitted from said at least four beam formers, including sending
coded signals along said beams B1, B2, B3 and B4 such that: each of
said first, second, third and fourth beam areas are effectively
divided into at least two sub-areas such that said first beam area
includes sub-areas G1.sub.1 and G2.sub.1, said second beam area
includes sub-areas G1.sub.2 and G2.sub.2, said third beam area
includes sub-areas G1.sub.3 and G2.sub.3, and said fourth beam area
includes sub-areas G1.sub.4 and G2.sub.4; and wherein a group of
frequencies are assigned to all of said beam areas within a single
cell; further wherein said assigned group of frequencies is divided
such that half of said assigned group of frequencies serve mobile
units located within sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and
G1.sub.4, and the other half of said assigned group of frequencies
serve mobile units located within sub-areas G2.sub.1, G2.sub.2,
G2.sub.3 and G2.sub.4.
10. The wireless communications system according to claim 9,
wherein: the group of frequencies assigned to sub-areas G2.sub.1,
G2.sub.2, G2.sub.3 and G2.sub.4 is again divided in half, with one
sub-group of this group being assigned to sub-areas G2.sub.1 and
G2.sub.4 and the other sub-group being assigned to sub-areas
G2.sub.2 and G2.sub.3.
11. The wireless communications system according to claim 9, said
sub-area G1.sub.1 begins near an apex of said first area and
extends generally down a center of said first area, and said
sub-area G2.sub.1 is located outside of said sub-area G1.sub.1; and
said sub-area G1.sub.2 begins near an apex of said second area and
extends generally down a center of said second area, and said
sub-area G2.sub.2 is located outside of said sub-area G1.sub.2.
12. The wireless communications system according to claim 9,
wherein said beams B1, B2, B3 and B4 are each rotated by half of
the average beamwidth of all of the beams, thereby creating new
sub-areas RG1.sub.1 and RG2.sub.1 in said first beam area, new
sub-areas RG1.sub.2 and RG2.sub.2 in said second beam area, new
sub-areas RG1.sub.3 and RG2.sub.3 in said third beam area and new
sub-areas RG1.sub.4 and RG2.sub.4 in said fourth beam area, so that
each mobile now has the option of selecting from either the rotated
beams or the original beams, giving rise to more directed beams for
the mobiles, thereby increasing both coverage and overall
throughput; and further wherein each of said new sub-areas
RG1.sub.1, RG2.sub.1, RG1.sub.2, RG2.sub.2, RG1.sub.3, RG2.sub.3,
RG1.sub.4 and RG2.sub.4 are served by different frequencies than
said sub-areas G1.sub.1, G2.sub.1, G1.sub.2, G2.sub.2, G1.sub.3,
G2.sub.3, G1.sub.4, and G2.sub.4.
13. A method for reducing interference in a wireless system
including at least two beam formers and a plurality of mobile
units, the method comprising the steps of: transmitting a first
beam (B1) from a first beam former into a first area, defining two
sub-areas within said first area as sub-area G1.sub.1 and sub-area
G2.sub.1; transmitting a second beam (B2) from a second beam former
into a second area, defining two sub-areas within said second area
as sub-area G1.sub.2 and sub-area G2.sub.2; coding signals of said
beams B1 and B2 for receipt by a particular mobile unit based upon
whether the particular mobile unit is located within said sub-area
G1.sub.1, said sub-area G2.sub.1, said sub-area G1.sub.2 or said
sub-area G2.sub.2, such that: during a first time period (T1),
making simultaneous transmissions from both said first and second
beam formers for receipt by mobile units located, respectively,
within said sub-area G1.sub.1, or within said sub-area G1.sub.2;
during a second time period (T2), making transmissions from said
first beam former for receipt by mobile units located within said
sub-area G2.sub.1; and during a third time period (T3), making
transmissions from said second beam former for receipt by mobile
units located within said sub-area G2.sub.2.
14. The method according to claim 13, wherein: said first area is
adjacent to said second area; said sub-area G1.sub.1 begins near an
apex of said first area and extends generally down a center of said
first area, and said sub-area G2.sub.1 is located outside of said
sub-area G1.sub.1; and said sub-area G1.sub.2 begins near an apex
of said second area and extends generally down a center of said
second area, and said sub-area G2.sub.2 is located outside of said
sub-area G1.sub.2.
15. The method according to claim 14, wherein said sub-areas
G1.sub.1 and G1.sub.2 are each generally teardrop-shaped.
16. The method according to claim 13, wherein said first and second
areas are divided into said sub-areas G1.sub.1, G2.sub.1, G1.sub.2,
and G2.sub.2 based upon the carrier-to-interference ratio (C/I) of
signals being received within each sub-area.
17. The method according to claim 13, wherein a mobile unit is
assigned to one of said sub-areas G1.sub.1, G2.sub.1, G1.sub.2, and
G2.sub.2 according to the following process: measuring the
carrier-to-interference ratio (C/I) for a mobile unit during a 4/4
cycle to define a first rate; measuring the carrier-to-interference
ratio (C/I) for a mobile unit during a 2/4 cycle to define a second
rate; and comparing said first rate to said second rate, and if
said second rate is larger than twice said first rate, assigning
said mobile unit to said sub-area G2.sub.1 for said beam B1, or to
said sub-area G2.sub.2 for said beam B2, otherwise said mobile unit
is assigned to said sub-area G1.sub.1 for said beam B1, or to said
sub-area G1.sub.2 for said beam B2.
18. The method according to claim 13, further comprising:
transmitting a third beam (B3) from a third beam former into a
third area, defining two sub-areas within said third area as
sub-area G1.sub.3 and sub-area G2.sub.3; transmitting a fourth beam
(B4) from a fourth beam former into a fourth area, defining two
sub-areas within said fourth area as sub-area G1.sub.2 and sub-area
G2.sub.2; coding signals of said beams B3 and B4, such that: during
said period T1, making simultaneous transmissions from said third
and fourth beam formers for receipt by mobile units located,
respectively, within said sub-area G1.sub.3 or within said sub-area
G1.sub.4; and during said period T2, making transmissions from said
fourth beam former for receipt by mobile units located within
sub-area G2.sub.4; and during said period T3, making transmissions
from said third beam former for receipt by mobile units located
within sub-area G2.sub.3.
19. The method according to claim 13, wherein said time period T1
is longer than both said time period T2 and said time period
T3.
20. The method according to claim 19, wherein said time period T2
is approximately equal in duration to said time period T3.
21. The method according to claim 13, wherein said time periods T1,
T2 and T3 are determined according to the formula
T1/(T2+T3)=N1/N2=X, where N1 is the number of mobile units assigned
to said sub-area G1.sub.1 for said beam B1 or to said sub-area
G1.sub.2 for said beam B2, N2 is the number of mobile units
assigned to said sub-area G2.sub.1, for said beam B1 or to said
sub-area G2.sub.2 for said beam B2, and X is a predetermined
constant.
22. The method according to claim 18, further comprising: rotating
beams B1, B2, B3 and B4 by a portion of their respective
beamwidths, thereby creating new sub-areas RG1.sub.1 and RG2.sub.1
in said first beam area, new sub-areas RG1.sub.2 and RG2.sub.2 in
said second beam area, new sub-areas RG1.sub.3 and RG2.sub.3 in
said third beam area and new sub-areas RG1.sub.4 and RG2.sub.4 in
said fourth beam area; and coding signals of said beams B1, B2, B3
and B4 such that: during a fourth time period (T4), simultaneous
transmissions are made for receipt by mobile units located within
said sub-areas RG1.sub.1, RG1.sub.2, RG1.sub.3 and RG1.sub.4;
during a fifth time period (T5), transmissions are made for receipt
by mobile units located within said sub-areas RG2.sub.1 and
RG2.sub.4; and during a sixth time period (T6), transmissions are
made for receipt by mobile units located within said sub-areas
RG2.sub.2 and RG2.sub.3.
23. A method for reducing interference in a wireless system
including at least four beam formers and a plurality of mobile
units, the method comprising the steps of: transmitting a first
beam (B1) from a first beam former into a first area; transmitting
a second beam (B2) from a second beam former into a second area;
transmitting a third beam (B3) from a third beam former into a
third area; transmitting a fourth beam (B4) from a fourth beam
former into a fourth area; defining at least two sub-areas within
each of said first, second, third and fourth beam areas based upon
the degree of overlap with adjacent beam areas, whereby each of
said beam areas includes at least one overlapping sub-area and at
least one non-overlapping sub-area; and coding signals of said
beams B1, B2, B3 and B4 for receipt by a particular mobile unit
based upon which of said sub-areas the particular mobile unit is
located within.
24. The method according to claim 23, wherein said coding is
divided into at least three sequential time periods such that the
method includes the following additional steps: during a first time
period (T1), making simultaneous transmissions from all four of
said beam formers for receipt by mobile units located within said
non-overlapping sub-areas; during a second time period (T2), making
transmissions from said first and fourth beam formers for receipt
by mobile units located within said overlapping sub-areas within
said first and fourth areas; and during a third time period (T3),
making transmissions from said second and third beam formers for
receipt by mobile units located within said overlapping sub-areas
within said second and fourth areas.
25. The method according to claim 23, further comprising the steps
of: defining at least a third sub-area within each of said first,
second, third and fourth beam areas based upon the degree of
overlap with adjacent beam areas, whereby each of said beam areas
includes at least one non-overlapping sub-area and at least two
overlapping sub-areas, further defined as a first overlapping
sub-area and a second overlapping sub-area; comparing the strength
of each beam signal within a particular sub-area to determine
whether a particular mobile unit is located within said
non-overlapping sub-area, said first overlapping sub-area or said
second overlapping sub-area.
26. The method according to claim 25, further comprising the steps
of: determining that a particular mobile unit is located within
said non-overlapping sub-area if the strength of all beam signals
but one are less than a threshold value Y1; determining that a
particular mobile unit is located within said first overlapping
sub-area if the difference between signal strengths from adjacent
beams is less than a threshold value Y2, and the signal strength of
said two adjacent beams combined is greater than a threshold value
Y3; and determining that a particular mobile unit is located within
said second overlapping sub-area if the difference between signal
strengths from adjacent beams is less than said threshold value
Y3.
27. The method according to claim 26, wherein said threshold values
Y1, Y2 and Y3 are all different values from each other.
28. The method according to claim 23, further comprising the steps
of: effectively dividing each of said first, second, third and
fourth beam areas into at least two sub-areas such that said first
beam area includes sub-areas G1.sub.1 and G2.sub.1, said second
beam area includes sub-areas G1.sub.2 and G2.sub.2, said third beam
area includes sub-areas G1.sub.3 and G2.sub.3, and said fourth beam
area includes sub-areas G1.sub.4 and G2.sub.4; and assigning a
group of frequencies to all of said beam areas within a single
cell; dividing said assigned group of frequencies such that half of
said assigned group of frequencies serve mobile units located
within sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and G1.sub.4, and the
other half of said assigned group of frequencies serve mobile units
located within sub-areas G2.sub.1, G2.sub.2, G2.sub.3 and
G2.sub.4.
29. The method according to claim 23, further comprising the steps
of dividing the group of frequencies assigned to sub-areas
G2.sub.1, G2.sub.2, G2.sub.3 and G2.sub.4 in half again, and
assigning one sub-group of this group to sub-areas G2.sub.1 and
G2.sub.4 and assigning the other sub-group to sub-areas G2.sub.2
and G2.sub.3.
30. A beam forming apparatus for use with a wireless communication
system, said beamforming apparatus comprising: means for
transmitting a beam into a first area and for defining two
sub-areas within said first area as sub-area G1 and sub-area G2;
means for coding signals of said beam for receipt by a particular
mobile unit based upon whether the particular mobile unit is
located within said sub-area G1 or said sub-area G2 such that:
during a first time period (T1), making transmissions from said
beam former for receipt by mobile units located within said
sub-area G1, and during a second time period (T2), making
transmissions from said first beam former for receipt by mobile
units located within said sub-area G2.
31. The beam forming apparatus according to claim 30, wherein a
mobile unit is assigned to one of said sub-areas G1 or G2 by:
measuring the carrier-to-interference ratio (C/I) for a mobile unit
during a 4/4 cycle to define a first rate; measuring the
carrier-to-interference ratio (C/I) for a mobile unit during a 2/4
cycle to define a second rate; and comparing said first rate to
said second rate, and if said second rate is larger than twice said
first rate, assigning said mobile unit to said sub-area G2,
otherwise said mobile unit is assigned to said sub-area G1.
32. A system of signals for use in a wireless communications system
including at least a first beam former and a second beam former and
a plurality of mobile units, the signals comprising: signals
transmitted from the first beam former into a first area, where
said first area is divided into at least two sub-areas defined as
sub-area G1.sub.1 and sub-area G2.sub.1; signals transmitted from
the second beam former into a second area, where said second area
is divided into at least two sub-areas defined as sub-area G1.sub.2
and sub-area G2.sub.2; coding said signals from said first and
second beam formers for receipt by a particular mobile unit based
upon whether the particular mobile unit is located within said
sub-area G1.sub.1, said sub-area G2.sub.1, said sub-area G1.sub.2
or said sub-area G2.sub.2, such that: signals transmitted during a
first time period (Ti) are transmitted simultaneously from both
said first and second beam formers for receipt by mobile units
located, respectively, within said sub-area G1.sub.1, or within
said sub-area G1.sub.2; signals transmitted during a second time
period (T2) are transmitted from said first beam former for receipt
by mobile units located within said sub-area G2.sub.1; and signals
transmitted during a third time period (T3) are transmitted from
said second beam former for receipt by mobile units located within
said sub-area G2.sub.2.
33. The system of signals according to claim 32, wherein: said
first area is adjacent to said second area; said sub-area G1.sub.1
begins near an apex of said first area and extends generally down a
center of said first area, and said sub-area G2.sub.1 is located
outside of said sub-area G1.sub.1; and said sub-area G1.sub.2
begins near an apex of said second area and extends generally down
a center of said second area, and said sub-area G2.sub.2 is located
outside of said sub-area G1.sub.2.
34. The system of signals according to claim 32, wherein said beams
B1, B2, B3 and B4 are each rotated by a portion of their respective
beamwidths, thereby creating new sub-areas RG1.sub.1 and RG2.sub.1
in said first beam area, new sub-areas RG1.sub.2 and RG2.sub.2 in
said second beam area, new sub-areas RG1.sub.3 and RG2.sub.3 in
said third beam area and new sub-areas RG1.sub.4 and RG2.sub.4 in
said fourth beam area, said system further comprising: coding
signals of said beams B1, B2, B3 and B4 such that: signals
transmitted during a fourth time period (T4) are simultaneously
transmitted for receipt by mobile units located within said
sub-areas RG1.sub.1, RG1.sub.2, RG1.sub.3 and RG1.sub.4; signals
transmitted during a fifth time period (T5) are transmitted for
receipt by mobile units located within said sub-areas RG2.sub.1 and
RG2.sub.4; and signals transmitted during a sixth time period (T6)
are transmitted for receipt by mobile units located within said
sub-areas RG2.sub.2 and RG2.sub.3.
35. The system of signals according to claim 32, wherein each
mobile unit is assigned to a beam and a rotation position based on
the following criteria, wherein, for a given mobile, the best rates
from all the beams that can be supported in said time slots T1, T2,
T3, T4, T5 and T6 are, respectively, r1, r2, r3, r4, r5 and r6, and
further wherein R1=max(r1, r4) and R2=max(r2, r3, r5, r6): if
2R1.gtoreq.R2 and r1.gtoreq.r4, then mobile unit is served in said
sub-area G1.sub.1, G1.sub.2, G1.sub.3or G1.sub.4; if 2R1.gtoreq.R2
and r1<r4, then mobile unit is served in said sub-area
RG1.sub.1, RG1.sub.2, RG1.sub.3 or RG1.sub.4; if 2R1<R2 and
max(r2, r3)>max(r5, r6) and r2.gtoreq.r3, then mobile unit is
served in said sub-area G2.sub.1 or G2.sub.4; if 2R1<R2 and
max(r2, r3)>max(r5, r6) and r2<r3, then mobile unit is served
in said sub-area G2.sub.2 or G2.sub.3; if 2R1<R2 and max(r2,
r3).ltoreq.max(r5, r6) and r5.gtoreq.r6, then mobile unit is served
in said sub-area RG2.sub.1 or RG2.sub.4; and if 2R1<R2 and
max(r2, r3).ltoreq.max(r5, r6) and r5<r6, then mobile unit is
served in said sub-area RG2.sub.2 or RG2.sub.3.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
wireless communications, and more particularly to multibeam
wireless communications systems and methods in which interference
from adjacent beams is minimized, without unduly reducing capacity,
by dividing each beam area into at least two sub-areas.
BACKGROUND OF THE INVENTION
[0002] One of the many current uses of wireless communication
principles is within a cellular network, such as the cellular
networks employed by the increasingly popular cellular telephone
systems. In such systems, the geographical area is divided into a
plurality of adjoining cells, such as cells 12 of a network 10 of
FIG. 1. Mobile units (such as cellular telephones) move about the
geographical area encompassed by the cellular array, and
information is transmitted to/from the mobile units from/to a base
transmitter station (BTS).
[0003] One type of cellular arrangement common in North America is
known as the center excitation arrangement, whereby a BTS is
situated within the center of each cell. FIG. 2 schematically
depicts one cell 12 of a center excitation arrangement, whereby BTS
14 transmits a downlink radiation beam into each of the three
sectors 16, 18, and 20. In the FIG. 2 example, each sector 16, 18,
and 20 is covered by a beam with a 120.degree. azimuth angle, so
that full 360.degree. coverage is provided by the three beams of
BTS 14. It should be noted that the sectors may be divided
differently, such as by having six beams each having a 60.degree.
azimuth angle, twelve beams each having a 30.degree. azimuth angle,
etc., so long as the full 360.degree. of coverage is provided by
the combination of beams. It should also be noted that multiple
beams may be used in each sector. Although the intention is to
cover only the area specified by the azimuth angle of the beam,
practically, the signal spreads over a larger area, giving rise to
interference (which will be discussed in more detail below).
[0004] There is also a second type of excitation arrangement, known
as edge excitation, which is commonly used in Europe. In such an
arrangement (not shown in the figures), the BTS is situated at the
intersection of three cells, and beams are directed towards the
center of each cell. In contrast, in the center excitation
arrangement discussed above, the BTS is situated at the center of a
cell, and the beams are directed outwardly from the BTS.
[0005] There is a need in cellular systems (both edge excitation
and center excitation systems) to provide more capacity to transmit
information over the beams to the mobile units. Theoretically,
capacity gains can be realized by increasing the number of beams,
since each beam can carry a certain amount of information. Thus, in
theory, a system using four beams per sector will have a greater
capacity than one with three beams per sector.
[0006] However, the present inventors have realized that, in
practice, some of the expected capacity gains are often diminished
by interference received from adjacent beams. This is the case
because beams are not transmitted along an exact azimuth angle, so
there will be some overlap between adjacent beams. For example,
referring to FIG. 2, since the exact angle of 120.degree. cannot be
created, there will be some overlap between the beam of sector 16
and the beam of sector 18 around line 22. Similar beam overlap
occurs around line 24 between the beam of sector 18 and the beam of
sector 20, as well as around line 26 between the respective beams
of sectors 16 and 20. Such overlaps cause interference that
diminishes the capacity of the system below the capacity that would
otherwise be expected.
[0007] For example, the present inventors' simulation results
showed a slight loss of capacity when increasing the number of
beams from three per sector to four per sector (i.e., when changed
from nine beams per cell to twelve beams per cell). Although one
would expect an increase in cell capacity due to the increased
number of simultaneous beams in the cell, the loss due to increased
beam interference was larger than the gain obtained from increasing
the number of beams. Thus, it is desirable to find a way to
increase capacity, without increasing interference.
BRIEF SUMMARY OF THE INVENTION
[0008] The present invention provides a method for reducing
interference in a wireless system, and a system for performing the
method. Although the proposed scheme can be employed in systems
with any number of beams, the performance gain will be lower with a
smaller number of beams. In the sample embodiments discussed below,
four beam formers are used per sector, as well as a plurality of
mobile units. The method includes the steps of transmitting beams
B1, B2, B3 and B4 into first, second, third and fourth beam areas,
respectively. At least two sub-areas are defined within each of the
first, second, third and fourth beam areas based upon the degree of
overlap with adjacent beam areas, whereby each of the beam areas
includes at least one overlapping sub-area and at least one
non-overlapping sub-area. It should be noted that the term
"overlapping areas" refers to areas receiving excessive
interference from other beams, and that a geographical relationship
may or may not exist.
[0009] The method further includes coding signals of the beams B1,
B2, B3 and B4 for receipt by a particular mobile unit based upon
which one of the sub-areas that the particular mobile unit is
located within.
[0010] If the invention is practiced with a TDM scheme (time
division multiplex), at least three time periods are utilized,
wherein during the first time period (T1), simultaneous
transmissions are made for receipt by mobile units located within
sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and G1.sub.4; during a
second time period (T2), transmissions are made for receipt by
mobile units located within sub-areas G2.sub.1 and G2.sub.4; and
during a third time period (T3), transmissions are made for receipt
by mobile units located within sub-areas G2.sub.2 and G2.sub.3.
[0011] If the invention is practiced with an FDM scheme (frequency
division multiplex), the group of frequencies assigned to each cell
is divided such that half of the frequencies (F1) serve mobile
units located within sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and
G1.sub.4, and the other half of the frequencies (F2) serve mobile
units located within sub-areas G2.sub.1, G2.sub.2, G2.sub.3 and
G2.sub.4. The F2 set of frequencies is further divided into two
groups, F2.sub.1 and F2.sub.2, with F2.sub.1 serving G2.sub.1 and
G2.sub.3 and F2.sub.2 serving G2.sub.2 and G2.sub.4.
[0012] Another extension of the present invention is called a
"Rotation Beam Arrangement." Under the TDM version of this
implementation, we introduce two more mobile areas for each beam
and an additional three time slots for transmission. All the beams
will be rotated by half of the average beam coverage angle, and the
rotated G1/G2 areas, which will be called RG1.sub.1, RG1.sub.2,
RG1.sub.3, RG1.sub.4, RG2.sub.1, RG2.sub.2, RG2.sub.3 and
RG2.sub.4, are defined similar to the original beam areas G1.sub.1,
G1.sub.2, G1.sub.3, G1.sub.4, G2.sub.1, G2.sub.2, G2.sub.3 and
G2.sub.4. Now a mobile will be assigned to one of these eight areas
according to the best C/I (carrier to interference ratio), and
transmissions to those mobiles will be done during the
corresponding time slot, as explained below.
[0013] T1: G1.sub.1, G1.sub.2, G1.sub.3, G1.sub.4
[0014] T2: G2.sub.1 and G2.sub.4
[0015] T3: G2.sub.2 and G2.sub.3
[0016] T4: RG1.sub.1, RG1.sub.2, RG1.sub.3, RG1.sub.4
[0017] T5: RG2.sub.1 and RG2.sub.4
[0018] T6: RG2.sub.2 and RG2.sub.3
[0019] As explained below, under this rotated beam arrangement,
more mobiles will be assigned to G1 or inner beam areas (rotated or
original) since most of the mobiles in the original G2 area would
now be covered by the rotated G1 positions. This increases the
proportion of time system transmit with a reuse factor of 1, thus
providing a higher throughput. Moreover, this "Rotation Beam
Arrangement" scheme does not require additional antennas.
[0020] Although the "Rotation Beam Arrangement" scheme is described
using two rotated positions, a system can be designed with n
rotated positions by rotating the beams by 1/n th of beamwidth each
time. Depending on the degree of overlap among adjacent beams,
there may be an optimum number of rotated positions. One of
ordinary skill in the art should be able to extend this invention
to different numbers of rotated positions.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0021] FIG. 1 is a schematic drawing of a cell cluster of a
standard cellular network;
[0022] FIG. 2 is a schematic representation of a cell with a center
excitation arrangement;
[0023] FIG. 3 is a schematic drawing of a basic cell array of the
present invention;
[0024] FIG. 4 is a schematic of a set of beam areas and sub-areas
of the first embodiment of the present invention;
[0025] FIG. 5 shows the schematic of FIG. 4 with the beams
rotated;
[0026] FIG. 6 is a time chart for the first embodiment;
[0027] FIG. 7 is a schematic of a set of beam areas and sub-areas
of the second embodiment of the present invention;
[0028] FIG. 8 is a depiction of a scheme for use with the G2
sub-areas with the second embodiment;
[0029] FIG. 9 is a variation on FIG. 8; and
[0030] FIG. 10 is another variation on FIG. 8.
DETAILED DESCRIPTION OF THE INVENTION
[0031] There will now be described by way of example the best mode
contemplated by the inventors for carrying out the present
invention. In the following description, numerous specific details
will be set forth in order to provide a thorough understanding of
the present invention. It should be apparent to those of ordinary
skill in the art that the present invention may be practiced
without using these specific details. In other instances,
well-known methods and structures have not been described in detail
so as not to unnecessarily obscure the present invention.
[0032] Referring to FIG. 3, one example of the basic cell array 100
of the present invention will be described. FIG. 3 shows a
plurality of cells 110 that are each divided into three 120.degree.
sectors (112, 114, 116), as known to those of ordinary skill in the
art. For the purpose of illustration only, the present invention
will be described using three 120.degree. sectors that each include
four downlink radiation beam patterns per sector. However, it
should be noted that each cell may be sectored into other divisions
(such as 30.degree. sectors, 60.degree. sectors, etc.), as well as
having a lesser or a greater number of beams. It should also be
noted that the invention will be described primarily in association
with the time division multiplexing (TDM) mode of operation.
However, one of ordinary skill in the art should be able to apply
the concepts of the present invention to other modes of operation,
such as the frequency division multiplexing (FDM) mode. One
possible example of such an application has been explained in the
Background Section above.
[0033] In this example, each sector is served by four beams, with
each beam covering a different beam area. These beam coverage areas
are numbered, respectively, as beam areas 118, 120, 122, and 124.
For the sake of simplicity, only one cell is shown to be divided
into the full set of twelve beam areas, and one adjacent cell is
shown to be partially divided into two beam areas (120, 122).
However, it should be noted that all of the cells are divided into
three sectors with four beams per sector for a total of twelve beam
areas. Each of the beams may be formed by any conventional
beamforming apparatus, such as by directional antennas that produce
directional radiation beams.
[0034] While developing the present invention, the present
inventors considered a previous proposal.sup.1 based on a reuse
concept in which half of the beams transmit at any one time,
whereby interference between adjacent beams is avoided. For
example, in a 2/4 reuse scheme, two of the four beams in each four
beam sector transmit at a time. Thus, referring to FIG. 3, the
beams transmitting to areas 118 and 122 transmit during a first
time period, and the beams transmitting to areas 120 and 124
transmit during a second time period. Such an alternating
transmission sequence eliminates interference between adjacent
beams with areas overlapping each other (both within a single cell
and across adjacent cells) because adjacent beams do not transmit
at the same time, and therefore the overlap is eliminated. The
capacity of this 2/4 scheme was calculated to be 32.7 Mbps in a
cell capacity simulation with adaptive modulation and coding, as
well as with fast cell selection with a DVB-T code set, and a cell
capacity per 5 MHz. These simulations, which were conducted under
the same conditions as the simulation discussed in the Background
Section above, reveal that the 2/4 reuse scheme has a higher
capacity than either the 3/3 scheme or the 4/4 scheme. However,
even higher capacities are desirable. .sup.1Wen Tong, Leo L.
Strawczynski, Shalini Periyalwar and Claude Roger, "Multibeam
Antenna System for High Speed Data," DOI Number: 11964RO, Ref:
016896/0045.
[0035] One drawback of the 2/4 scheme is that, since each beam is
being transmitted only during half of the full time period, there
is no information being transmitted by that beam during the other
half of the time (i.e., when it is in the off state). Thus,
potential information transfer capability is being wasted.
Accordingly, one important aspect of the preferred embodiment of
the present invention relates to a method of reducing this wasted
potential by dividing the geographical area covered by each beam
into sub-areas.
[0036] Referring now to FIGS. 3 and 4, a first preferred embodiment
of the present invention will be described. By way of example only,
the description will relate to a TDM system having a beam array
configured with a re-use factor of 1 (for a reuse factor of n, the
beam array is divided into n beam clusters). However, it is
contemplated that the concepts of the present invention can be
applied to arrays with other re-use factors, as well as to other
types of cellular systems, such as an FDM system.
[0037] FIG. 3 shows that each cell 110 is divided into three
sectors (112, 114, 116), and that each sector is served by four
beams (with coverage areas 118, 120, 122, and 124), as with the 2/4
scheme described above. Once again, a different number of sectors,
as well as a different number of beams per sector, may be utilized
if desired.
[0038] FIG. 4 shows an enlargement of two beam areas (from the
total of twelve beam areas) in each of two adjacent cells, where
the beam areas have been further divided into sub-areas. Beam areas
120 and 122 are from one cell, and beam areas 124 and 118 are from
an adjacent cell. As can be seen in this figure, beam area 122 is
adjacent to area 120 of the same cell, as well as being adjacent to
beam area 124 of the adjacent cell. Beam area 118 is served by beam
B1, beam area 124 is served by beam B2, area 120 by B3 and area 122
by B4.
[0039] An important feature of the present invention is that the
mobile receiving units located within each of the beam areas (118,
120, 122, and 124) are divided into two sub-areas G1 and G2, with
regard to the downlink communications assigned to the particular
mobile units. Because of the non-uniform geographic distribution of
signal levels and interference, G1 and G2 may not be rigid areas
with distinguished geographical locations. However, in general, as
can be seen from FIG. 4, sub-area G1.sub.1 is the region located in
the center of radiation beam pattern 118 of beam B1, and sub-area
G2.sub.1 is the region located outside of area G1 .sub.1, but still
within beam pattern 118. Similarly, sub-area G1.sub.2 is located in
the center of pattern 124 of beam B2, and sub-area G2.sub.2 is
located outside of G1.sub.2. Sub-areas G1.sub.3 and G2.sub.3 of
beam B3 and sub-areas G1.sub.4 and G2.sub.4 of beam B4 are also
similarly configured.
[0040] The different sub-areas G1 and G2 are chosen based upon the
overlap of one beam area with an adjacent beam, which depends on
both terrain characteristics and beam pattern. Sub-areas G1.sub.1,
G1.sub.2, G1.sub.3 and G1.sub.4 are the non-overlapping regions,
and sub-areas G2.sub.1, G2.sub.2, G2.sub.3, and G2.sub.4 are the
overlapping regions. Thus, for example, sub-area G1.sub.1 is the
region of beam area 118 (from beam B1) that does not overlap with
adjacent beam area 124 (from beam B2) and beam area 120 (from beam
B3), so there will be negligible interference from adjacent beams
B2 and B3. On the other hand, sub-area G2.sub.1 (of beam area 118)
is a region that does include a slight overlap with adjacent beam
areas 124 and 120, so some interference from these adjacent beams
may result.
[0041] In order to avoid interference from adjacent beams (when
operating in the TDM mode), the present invention utilizes a scheme
whereby the transmissions to the mobile units that are located in
sub-areas G1 are separated in time from the transmissions to the
mobile units located in sub-areas G2. Referring now to FIG. 6,
which is a chart showing the different time periods for
transmission to the different sub-areas by each beam, a preferred
embodiment of the interference avoidance scheme of the present
invention will be explained. In this figure, the shaded areas
represent time periods where transmissions to mobile units within a
particular sub-area are being made. The location of a particular
mobile, i.e., which sub-area it is positioned in, may be determined
by any of the methods known in the art, such as by reviewing the
carrier to interference ratio (C/I) of signals received by the
mobile unit, by pilot measurements, etc. The location of the border
between sub-area G1 and sub-area G2 may be decided upon when the
system is first set-up by running a simulation, or it may be
changed dynamically based upon the loading distributions. One
example of an mobile optimum assignment methodology is described
below.
[0042] In the preferred embodiment, an optimum methodology to
assign a mobile unit to sub-area G1 or to sub-area G2 area is based
on the C/I measurement that the mobile unit experiences. The mobile
unit measures C/I during a 4/4 cycle (CI4), as well as during a 2/4
(CI2) cycle. Depending on the code/modulation levels available in
the system for dynamic rate changes, let us assume that these two
C/I values will correspond to rates R4 and R2, respectively, for
the 4/4 cycle and the 2/4 cycle (i.e., the mobile unit will receive
the R4 rate if it is assigned to the G1 sub-area and the mobile
unit will use the R2 rate if it is assigned to G2 sub-area).
[0043] It is advisable to assign the mobile unit to the G2 sub-area
only if its R2 value is larger than twice the R4 value because,
during the transmission to a G2 mobile unit, only half of the beams
can be used, effectively reducing the contribution to capacity by a
factor two. Otherwise (if the R2 value is equal to or less than
twice the R4 value), the mobile unit should be assigned to the G1
sub-area.
[0044] In a similar way, if we choose three reuse schemes, 4/4, 2/4
and 1/3, the assignment of mobiles to a corresponding sub-area (G1,
G2 or G3, such as shown in FIG. 6) can be done according to the
following rule. Let the rate that can be supported for a given
mobile unit by each scheme be R1, R2 and R3, respectively, as
described above. Then, compare R1, R2/2 and R3/3, and assign the
mobile unit to G1, G2 or G3, respectively, depending on whether R1,
R2/2 or R3/3 is the largest.
[0045] While still referring to FIG. 6, as well as to FIG. 4, the
operation of the particular beam formers during each time period
will be described next. First, during time period T1, all four
beams, B1, B2, B3, B4, make simultaneous transmissions, carrying
information signals intended specifically for the mobile units that
are located within the beam's particular sub-area G1. Thus, during
time T1, beam B1 only transmits information intended for receipt by
mobile units located within sub-area G1.sub.1; beam B2 only
transmits information intended for receipt by mobile units located
within sub-area G1.sub.2; beam B3 only transmits information
intended for receipt by mobile units located within sub-area
G1.sub.3; and beam B4 only transmits information intended for
receipt by mobile units located within sub-area G1.sub.4. Since the
mobiles for the G1 areas are selected such that there is enough
`open space` between sub-areas G1.sub.1, G1.sub.2, G1.sub.3, and
G1.sub.4, the signals do not overlap each other, and no
interference is created. One selection methodology is discussed in
more detail below.
[0046] In time period T2, only beams B1 and B4 transmit, and not
beams B2 and B3. Moreover, beam B1 is configured to only transmit
information intended for mobile units located within sub-area
G2.sub.1, and beam B4 only transmits information intended for units
located within sub-area G2.sub.4. As can be seen in FIG. 4, there
is essentially no overlap between sub-areas G2.sub.1 and G2.sub.4,
so only a slight amount of interference is possible with the
transmissions made during time period T2.
[0047] Time period T3 is similar to time period T2, except the
other group of beams now transmit information intended for mobiles
located with their associated G2 sub-areas. Thus, beam B2 only
transmits information intended for mobile units located within
sub-area G2.sub.2, and beam B3 only transmits information intended
for units located within sub-area G2.sub.3. In the T3 time period,
as with the T2 time period, interference from adjacent beam signals
is reduced because sub-areas G2.sub.2 and G2.sub.3 do not overlap
each other. The T4, T5 and T6 time periods are essentially the same
as time periods T1, T2 and T3, respectively, except that during
time periods T4, T5 and T6, all of the beams are rotated by half of
the average beamwidth of all of the beams in order to increase the
number of users in the G1 beam areas (the inner beam areas). FIG. 5
shows one example of how the beams may be rotated, where the dashed
lines represent the rotated sub-areas. Thus, RG1.sub.1 is rotated
sub-area G.sub.1, RG1.sub.2 is rotated sub-area G1.sub.2, etc.
Although not shown in the drawings (for the sake of simplicity),
the G2 sub-areas will also be rotated to correspond to the G1
sub-areas. In this example, each beam is rotated by half of the
average beamwidth, since there are two positions (a rotated
position and an original position). However, there may be other
numbers of rotated positions (n), in which case the beams are
rotated by 1/n th of a beamwidth into each new position. Since the
G1 areas use a reuse factor of 1, the overall throughput increases
as a result. In addition, this provides more uniform coverage to
users, thus increasing the fairness of the system.
[0048] In the preferred embodiment, time periods T1, T2, T3, T4, T5
and T6 are selected so that they are proportional to the number of
users assigned to these time slots, so that there is a fair
allocation of users. Under the assumption that there is a uniform
geographical distribution of the users, T1=T4 and T2=T3=T5=T6.
These time periods are preferably an integer multiple of the
minimum time period that can be allocated to a single user in a
system. For example, in a proposed 1.times.EV scheme, this time
interval is 1.67 msec (where 1.times.EV stands for the enhanced
standard for cdma2000). Of course, it is contemplated that other
time ranges, as well as other ratios of T1, T2 and/or T3 may also
be utilized.
[0049] T1, T2, T3, G1, and G2 are selected according to the
following formula if the goal is to allocate equal resources to
each mobile (note that equal resource allocation does not mean
equal throughput for individual mobiles):
T1/(T2+T3)=N1/N2=X,
[0050] where N1 and N2 are the number of mobiles assigned to G1 and
G2, respectively, and there is an optimum value of X for a given
beam arrangement which maximizes the overall system throughput.
[0051] When the geographical distribution of the mobiles is not
uniform, different beams will have different number of mobiles in
the G1 and G2 areas, and the ratio between the overall duty cycles
T1/(T2+T3) needs to be chosen by averaging out the ratio N1/N2 over
a long period of time, for example, over more than 100 time slots.
In this way, unfair allocation of time slots between the G1 mobiles
and the G2 mobiles can be minimized. On the other hand, if desired,
the system can provide an unfair allocation to increase the
capacity by increasing the duty cycle for the G1 mobiles, i.e., by
choosing T1/(T2+T3)>average (N1/N2). Also, if we assign the G2
mobiles double the time slots allocated to the G1 mobiles, to
account for 50% active time, the capacity improvement will be
decreased.
[0052] In the preferred embodiment, the selection of G1 or G2 is
done based on the following C/I measurements. For both rotated and
non-rotated positions, C/I is measured using pilots included in
corresponding time slots. The data rate that can be supported by
each beam can be found based on the C/I measurements using the code
set performance tables usually available for the modulation and
coding sets that are being used. Assume, for a given mobile, the
best rates (from all the beams) that can be supported in the time
slots T1, T2, T3, T4, T5 and T6 are r1, r2, r3, r4, r5 and r6,
respectively. T1, T2, T3 are dedicated for the non-rotated beam
position, and T4, T5 and T6 are dedicated for the rotated beam
position. T1 and T4 use a reuse of 1 (i.e., belong to G1 mobiles)
while T2, T3, T5 and T6 use a reuse of 2 (G2 mobiles--alternating
transmissions). The following decision rules can be used to assign
the mobiles to each beam and time slot:
[0053] Let R1=max(r1, r4), R2=max(r2, r3, r5, r6) (i.e., R1 is the
best rate for the mobile if it is allocated to a G1 area, R2 is the
best rate for the mobile if it is allocated to a G2 area).
[0054] Then,
[0055] If 2R1.gtoreq.R2:
[0056] The mobile is assigned to a 4/4 scheme or a G1 area;
[0057] if r1.gtoreq.r4,
[0058] the mobile is served in the original (non-rotated) beam
position,
[0059] else
[0060] the mobile is served in the rotated position.
[0061] endif
[0062] Else:
[0063] The mobile is assigned to a 2/4 scheme or a G2 area;
[0064] If max(r2, r3)>max(r5, r6),
[0065] the mobile is served in the original (non-rotated) beam
position with a 2/4 scheme and the time slot T2 or T3 (or the
corresponding beams) is selected based on whether r2>r3 or
not.
[0066] else
[0067] the mobile is served in the rotated position and the time
slot T5 or T6 (or the corresponding beams) is selected based on
whether r5>r6 or not.
[0068] endif
[0069] endif
[0070] Instantaneous imbalances of loading in each beam/beam
position can easily be addressed by modifying the above equations
to take into account the loading situation of the beams.
[0071] In addition, if a multi-user detection (MUD) scheme is
applied to the present invention, there should be a greater
increase in capacity than that found in a 2/4 scheme (which rose
from 32.7 to 55.6 Mbps when a MUD scheme was applied). This is the
case because of the lower levels of interference present in the 2/4
scheme.
[0072] In accordance with another aspect of the present invention,
the static interference avoidance technique described above for use
with a TDM scheme can also be applied with an FDM scheme. Such a
system will be briefly explained while referring back to FIG. 4.
However, the beam rotation aspect of the invention will not be
described for the FDM scheme since it should be apparent to those
of ordinary skill in the art that beam rotation can be applied to
the FDM scheme in a similar manner to that described above for the
TDM scheme.
[0073] When the present invention is applied with a FDM scheme, the
frequencies being transmitted within each cell are divided into two
groups--one group for the mobile units in the G1 sub-areas and a
second group for the mobile units in the G2 sub-areas, and this
second group is again divided in half, with one sub-group of
frequencies being allocated to the G2.sub.1and G2.sub.4sub-areas
and the other sub-group being allocated to the G2.sub.2 and
G2.sub.3 sub-areas. Thus, half of the frequencies allocated to the
cell are transmitted for receipt by mobile units located within
sub-areas G1.sub.1, G1.sub.2, G1.sub.3 and G1.sub.4; one quarter of
the frequencies are transmitted for receipt by mobile units located
within sub-areas G2.sub.1 and G2.sub.4; and the final quarter of
the frequencies are transmitted for receipt by mobile units located
within sub-areas G2.sub.2 and G2.sub.3. In the FDM scheme, all of
the frequencies are being transmitted at all times, unlike the TDM
scheme in which the G2 sub-areas are only served for a half or
other designated portion of the total time.
[0074] As a further modification, the present invention can also be
applied to a scheme that is somewhat of a hybrid of the FDM and the
TDM schemes. In such a hybrid scheme, half of the frequencies are
allocated to the G1 sub-areas for transmission at all times (like a
pure FDM scheme). The other half of the frequencies are allocated
to all of the G2 sub-areas (and are not divided in half again, as
in the pure FDM scheme). The half of the frequencies allocated to
G2 sub-areas are alternately transmitted for receipt by either the
mobile units located within sub-areas G2.sub.1 and G2.sub.4, or for
receipt by the mobile units located within sub-areas G2.sub.2 and
G2.sub.3. Accordingly, with this hybrid TDM/FDM scheme, there are
essentially only two primary time periods (compared with the three
primary time periods with TDM), a first time period where mobile
units within sub-areas G1.sub.1, G1.sub.2, G1.sub.3, and G1.sub.4
are served, as well as those in sub-areas G2.sub.1 and G2.sub.4;
and a second time period where mobile units within sub-areas
G1.sub.1, G1.sub.2, G1.sub.3, and G1.sub.4 are again served, as
well as those in sub-areas G2.sub.2 and G2.sub.3.
[0075] FIG. 7 shows a second embodiment of the present invention,
wherein this embodiment includes a third sub-area G3, in addition
to the two sub-areas G1 and G2 described above. For this
embodiment, the primary discussion will relate to the present
interference avoidance technique as utilized with an FDM scheme,
with a brief section discussing its utilization with a TDM
scheme.
[0076] In this embodiment, the three sub-areas G1, G2 and G3 are
divided in the following manner. The G1 sub-areas are those
sub-areas where there is one primary beam signal (such as the B1
signal for sub-area G1.sub.1), and all of the other signals in that
sub-area are of a lower power than a certain threshold power level
Y1 (dB). The value of Y1 (and Y2, which is mentioned below), for
example, can be between 1 dB and 10 dB, depending on the
code/modulation levels available. Y1 (and Y2) are preferably pilot
power levels, since it is difficult to do comparisons with C/I
values. Thus, the G1 sub-areas are the centers of each of the
respective beams, and they are those areas of the highest
power.
[0077] The G2 sub-areas are those sub-areas where the adjacent
beams from the same cell site are relatively strong, but the beams
from the adjacent cells are relatively weak. In the G2 sub-areas,
the difference between the power levels from one beam to an
adjacent beam (from the same cell) is less than a certain threshold
power level Y2 (dB), and the power of both of these two beams
should be higher than the power of the beams from the adjacent
cells, at least by a certain threshold, Y3, where Y2 and Y3 are
preferably different from the threshold value Y1 mentioned above.
The G3 sub-areas are the sub-areas where the adjacent beams from
different cells are relatively strong. In the G3 sub-areas, the
difference between the power levels from one beam to a beam from
the adjacent cell is less than the threshold Y3 (dB).
[0078] In FDM operation, the frequencies allotted to a particular
cell are divided into three groups to serve three areas, G1, G2,
and G3. The mobiles in the G1 sub-areas are always served with
their group of assigned frequencies, and simultaneous transmissions
from all of the beams are permitted at all times without any
restriction from the other transmissions in the G2 and G3
sub-areas.
[0079] The mobiles in the G3 sub-areas are served by a 2/4 pattern
with a reuse factor of two. More particularly, half of the G3
frequency spectrum (i.e., one quarter of the cell's full spectrum)
is simultaneously transmitted for receipt by mobile units in the
G3.sub.1 and the G3.sub.4 sub-areas, while the other half of the G3
spectrum is also simultaneously being transmitted for receipt in
the G3.sub.2 and G3.sub.3 sub-areas.
[0080] For serving the mobile units in the G2 sub-areas, any one of
the following three schemes may be utilized. The first scheme is
depicted in FIG. 8, which is a schematic of a full cell with a
basic 2/4 reuse pattern for the G2 sub-areas. More specifically,
with this first scheme, the frequencies assigned to the G2
sub-areas are divided in half, with one half designated as G2A and
the other half designated as G2B. Thus, in this example that
includes three 120.degree. sectors with four beams per sector, half
of the G2 spectrum is simultaneously used twelve times within each
cell. Thus, the efficiency of G2 spectrum usage is 0.5 since the
reuse factor is 2. Accordingly, if the equivalent throughput in the
spectrum allocated to the G2 sub-areas is designated as "g2", then
the aggregate throughput per cell equals 12.times.0.5.times.g2,
which can be reduced to 6.times.g2.
[0081] The aggregate throughput per cell for the G2 sub-areas can
be increased to 8.times.g2 by using the second scheme, which will
be termed the intelligent compact reuse scheme for the G2
sub-areas. FIG. 9 is a schematic of a full cell under this second
scheme. Once again, the frequency spectrum assigned to the G2
sub-areas is divided in half (G2A and G2B). However, under this
scheme, some of the beams have both halves of the G2 spectrum
assigned to them (i.e, both G2A and G2B), and some only have half
of the G2 spectrum assigned to them (either G2A or G2B).
[0082] In the intelligent compact reuse scheme operation, one of
the four beams in each sector is assigned both halves of the G2
frequency spectrum (G2A and G2B), with the G2 sub-area on one side
of the G1 sub-area being assigned the G2A frequencies and the G2
sub-area on the other side of the G1 sub-area being assigned the
G2B frequencies. Referring back to FIG. 7, and taking beam area 122
as an example, the sub-area G2.sub.4 that is below the G1.sub.4
sub-area may be assigned the G2A spectrum, and the sub-area
G2.sub.4 that is above the G1.sub.4 sub-area may be assigned the
G2B spectrum. These assignments are loosely represented in FIG. 9
by showing that in beam area 122.sub.X (where subscript "X"
represents that these four beams are in one 120.degree. sector,
subscript "Y" represents a second sector, and subscript "Z" the
third sector), G2A is shown near the right of this section, and G2A
is shown near the left.
[0083] Continuing to the left from the beam area 122.sub.X with
both G2A and G2B included therein, the left side of the G2 sub-area
of beam area 120.sub.X has been assigned the G2A spectrum of
frequencies. By assigning the G2A spectrum here, there will be
negligible interference from overlaps with the G2 sub-area of beam
area 122.sub.X, since the far right side of the G2 sub-area of area
122.sub.X is the G2A spectrum, and the far left side of the G2
sub-area of area 120.sub.X is the G2B spectrum. Still continuing to
the left, the right side of the G2 sub-area of area 118.sub.X is
assigned the G2B spectrum so as not to interfere with the G2A
spectrum of the G2 sub-area of area 120.sub.X. The next area, beam
area 124.sub.Y (which is actually in the next sector), is similar
to area 122.sub.X in that it includes the G2A spectrum on one side
of the G1 sub-area and the G2B spectrum on the other side of the G1
sub-area. In the remainder of the areas, as indicated in FIG. 9, it
is shown that the G2A spectrum is never directly adjacent to the
G2B spectrum.
[0084] In order to avoid unfair service being allocated among the
G2 sub-areas due to asymmetric allocation of the frequencies as
described above, the present invention may optionally include a
feature in which we propose to rotate the frequency allocation to
beams in successive time slots (although this is similar to TDM,
the transmissions are separated primarily based on frequencies).
For example, the G2B frequencies allocated to beam 124.sub.X will
be used for 118.sub.Z in the second time slot, the G2A frequencies
in 118.sup.Z will be used for 120.sub.Z, the G2A and G2B
frequencies of 120.sub.Z will be used in 122.sub.Z, and so on. The
capacity calculations will not be affected by this rotation of
frequency allocation. It should be noted that after three time
slots, the same reuse pattern will be repeated. Since this rotation
is used only for inner G2 mobiles, there will be no impact upon the
mobiles in the G1 and G3 sub-areas.
[0085] In the intelligent compact reuse scheme just described, the
efficiency of the usage of the G2 sub-areas is increased by a
factor of 4/3 over that of the 2/4 reuse pattern described while
referring to FIG. 8. With intelligent compact reuse, half of the G2
spectrum is simultaneously used sixteen times within each cell (for
this example that includes three 120.degree. sectors with four
beams per sector). Accordingly, if the equivalent throughput in the
spectrum allocated to the G2 sub-areas is once again designated as
"g2" then the aggregate throughput per cell equals
16.times.0.5.times.g2, which can be reduced to 8.times.g2 (which is
an increase over the 6.times.g2 aggregate throughput of the 2/4
scheme of FIG. 8).
[0086] The third reuse scheme for the G2 sub-areas is depicted in
FIG. 10, which shows a reuse pattern combined with a softer handoff
scheme. With this scheme, as with the schemes of FIGS. 8 and 9, the
G2 frequency spectrum is divided in half into frequency groups G2A
and G2B. However in this case, one frequency group is assigned to
the mobile units located within one G2 sub-areas of one beam and
the adjacent G2 sub-area on the adjacent beam. For example,
referring back to FIG. 7, the G2A frequency group may be assigned
to both the lower sub-area G2.sub.4 and to the upper sub-area
G2.sub.3, which is adjacent to the lower sub-area G2.sub.4. On the
other hand, the upper sub-area G2.sub.4, as well as the lower
sub-area G2.sub.3, will both be assigned the G2B frequency group.
Thus, as shown in FIG. 10, frequency group G2A alternates with
frequency group G2B at the interfaces between each beam area. Since
the same frequency group is used across a dividing line between
beam areas, there is a softer handoff between adjacent beams since
a particular mobile will be simultaneously receiving signals from
two adjacent beams of the same frequency.
[0087] In the scheme of FIG. 10, the efficiency of G2 usage is 0.5
because the reuse factor is two, which is the same as the 2/4
pattern of FIG. 8. As also similar to the 2/4 pattern, half of the
G2 spectrum is simultaneously used twelve times within each cell.
However, the aggregate throughput per cell of the FIG. 10 scheme is
higher than that of the FIG. 8 scheme due to a gain from the softer
handoff. More specifically, the aggregate throughput per cell for
this scheme equals
12.times.0.5.times.g2.times.k=6.times.g2.times.k, where k is the
softer handoff gain from the mobile unit receiving simultaneous
transmissions from two different beams (where this gain, k, can be
as high as 2). Accordingly, the aggregate throughput per cell for
the FIG. 10 scheme is expected to be higher than that of the FIG. 8
scheme.
[0088] Although it will not be fully described herein, the second
embodiment of the present invention (shown in FIGS. 7-10) can also
be employed with a TDM scheme, instead of with the FDM scheme
discussed above, and each of the three variations of the G2 reuse
schemes described above can be applied to the TDM arrangement.
[0089] It is also contemplated that the areas can be divided into
more than the three sub-areas described above, and that similar
reuse groups can be identified for these sub-areas. For example,
the G3 area discussed above can be subdivided into three areas,
G3A, G3B and G3C, where the G3A area is the area in the middle of
the G3 area, and the mobile in this area will have two strong beams
(from two different cells), with all of the other beams being
relatively weak. The G3B mobiles can see three relatively strong
beams, with all other beams being relatively weak. Similarly, the
G3C mobiles can see four or more strong beams. The reuse factor of
these areas should be higher as the number of interferers are
large. On the other hand, these mobiles can benefit more from the
soft handoff described above, and such a design should be
relatively straightforward.
[0090] While particular embodiments of the present interference
avoidance techniques have been shown and described, it will be
appreciated by those skilled in the art that changes and
modifications may be made thereto without departing from the
invention in its broader aspects and as set forth in the following
claims.
* * * * *