U.S. patent application number 13/947327 was filed with the patent office on 2015-01-22 for method to control the effects of out-of-cell interference in a wireless cellular system using over-the-air feedback control.
This patent application is currently assigned to Alcatel Lucent. The applicant listed for this patent is Alcatel Lucent. Invention is credited to Shirish Nagaraj, Subramanian Vasudevan.
Application Number | 20150023318 13/947327 |
Document ID | / |
Family ID | 38566865 |
Filed Date | 2015-01-22 |
United States Patent
Application |
20150023318 |
Kind Code |
A1 |
Nagaraj; Shirish ; et
al. |
January 22, 2015 |
Method To Control The Effects Of Out-Of-Cell Interference In A
Wireless Cellular System Using Over-The-Air Feedback Control
Abstract
A mobile terminal is controlled via over-the-air feedback so as
to enable its data transmissions to be independently and
successfully decoded at each of the base stations in its active set
absent a transmit power limitation or data retransmission limit.
Using the decoded data, the channel is re-estimated and the
waveform received from the mobile terminal is reconstructed and
subtracted from the total interference at each base station in the
active set where decoding has been successful. As a result,
transmissions from other mobile terminals, which have yet to be
successfully decoded at such a base station, will experience a
higher signal-to-noise ratio and thus an increased likelihood of
being successfully decoded.
Inventors: |
Nagaraj; Shirish; (Hoffman
Estates, IL) ; Vasudevan; Subramanian; (Morristown,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alcatel Lucent |
Paris |
|
FR |
|
|
Assignee: |
Alcatel Lucent
Paris
FR
|
Family ID: |
38566865 |
Appl. No.: |
13/947327 |
Filed: |
July 22, 2013 |
Current U.S.
Class: |
370/331 |
Current CPC
Class: |
H04W 52/28 20130101;
H04W 52/48 20130101; H04W 36/24 20130101; H04W 52/56 20130101; H04W
52/286 20130101; H04W 52/40 20130101; H04W 52/362 20130101; H04W
52/36 20130101; H04W 52/367 20130101 |
Class at
Publication: |
370/331 |
International
Class: |
H04W 52/48 20060101
H04W052/48; H04W 36/24 20060101 H04W036/24 |
Claims
1-20. (canceled)
21. A mobile terminal, comprising: a receiver configured to receive
commands from a plurality of base stations; and a transmitter
configured to: send, when the a mobile terminal is in handoff
status, data packets to the plurality of base stations in the
mobile terminal's active set of base stations, the active set
including a serving base station and at least one non-serving base
station; and increase the transmit power of the transmitter, in
response to the receiver receiving a command to increase the
transmit power from any of the base stations, unless the transmit
power is at its maximum power or the transmit power exceeds a power
targeted by the serving base station by a predetermined amount.
22. The mobile terminal of claim 21 wherein the transmit power is
increased by a predetermined up-step.
23. The mobile terminal of claim 21 wherein: in the event that: the
transmit power is at its maximum power, or the transmit power
exceeds a power targeted by the serving base station by a
predetermined amount, and the mobile terminal receives a command to
increase its transmit power from any of the base stations in the
active set, the transmitter is configured to increase or decrease
the transmit power only in response to commands received from the
serving base station.
24. The mobile terminal of claim 21 wherein the transmitter is
configured to increase the transmit power only if the receiver
determines the received command is determined to be reliable.
25. The mobile terminal of claim 24 wherein the receiver determines
that the command to increase the transmit power is reliable on the
condition that a power of the received command to increase the
power is greater than a predetermined threshold.
26. A mobile terminal, comprising: a receiver configured to receive
commands from a plurality of base stations; and a transmitter
configured to: send, when the mobile terminal is in handoff status,
data packets to a plurality of base stations in an active set of
base stations, the active set including a serving base station and
at least one non-serving base station; and increase a transmit
power in response to the receiver receiving a negative
acknowledgment (NACK) of a successful decoding of a given packet
received from any of the base stations at the end of that packet's
retransmission limit, unless the transmit power is at its maximum
power or the transmit power exceeds a power targeted by the serving
base station by a predetermined amount.
27. The mobile terminal of claim 26 wherein the transmitter is
configured to increase the transmit power by a predetermined
up-step.
28. The mobile terminal of claim 26 wherein: if the transmit power
is at its maximum power or if the transmit power exceeds a power
targeted by the serving base station by a predetermined amount, and
the receiver receives from any of the base stations in the active
set a command to increase the transmit power, then for a subsequent
data transmission the transmitter is configured to increase the
transmit power only if the receiver receives a NACK from the
serving base station and to decrease the transmit power if the
receiver receives an ACK from the serving base station.
29. The mobile terminal of claim 26 wherein the transmitter is
configured to increase the transmit power only if the receiver
determines a NACK received from any of the base stations in the
active set is reliable.
30. The mobile terminal of claim 26 wherein the receiver determines
the NACK is reliable by comparing a power of the received NACK with
a predetermined threshold.
31. A method for interference management in a wireless
communication system, the method comprising: at a mobile terminal
in handoff status, sending data packets to a plurality of base
stations in the mobile terminal's active set of base stations, the
active set including a serving base station and at least one
non-serving base station; and in response to receiving a command to
increase the transmit power from any of the base stations in the
active set, increasing the mobile terminal's transmit power, unless
the transmit power is at its maximum power or the transmit power
exceeds a power targeted by the serving base station by a
predetermined amount, and
32. The method of claim 31 wherein the mobile terminal's transmit
power is increased by a predetermined up-step.
33. The method of claim 31 wherein if the transmit power is at its
maximum power or if the transmit power exceeds a power targeted by
the serving base station by a predetermined amount, and the mobile
terminal receives a command to increase its transmit power from any
of the base stations in the active set, then controlling the
transmit power up or down only in response to commands received
from the serving base station.
34. The method of claim 31 further comprising determining the
reliability of a received command to increase the transmit power,
and increasing the transmit power only if the received command is
determined to be reliable.
35. The method of claim 34 wherein the command to increase the
transmit power is determined to be reliable if a power of the
received command to increase the power is greater than a
predetermined threshold.
36. A method for interference management in a wireless
communication system comprising: at a mobile terminal in handoff
status, sending data packets to a plurality of base stations in its
active set of base stations, the active set including a serving
base station and at least one non-serving base station; and in
response to receiving a negative acknowledgment (NACK) of a
successful decoding of the packet from any of the base stations in
the active set at the end of a given packet's retransmission limit,
increasing the mobile terminal's transmit power unless the transmit
power is at its maximum power or the transmit power exceeds a power
targeted by the serving base station by a predetermined amount.
37. The method of claim 36 wherein the mobile terminal's transmit
power is increased by a predetermined up-step.
38. The method of claim 36 wherein: if the transmit power is at its
maximum power or if the transmit power exceeds a power targeted by
the serving base station by a predetermined amount, and the mobile
terminal receives a command to increase its transmit power from any
of the base stations in the active set, then for a subsequent data
transmission the mobile terminal increases its transmit power only
if it receives a NACK from the serving base station and decreases
its transmit power if it receives an ACK from the serving base
station.
39. The method of claim 36 further comprising determining the
reliability of a received NACK from any of the base stations in the
active set, and increasing the transmit power only if the received
NACK is determined to be reliable.
40. The method of claim 36 wherein the NACK is determined to be
reliable by comparing a power of the received NACK with a
predetermined threshold.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a Continuation of U.S. application Ser.
No. 11/409,161 filed on Apr. 21, 2006, to Shirish Nagaraj, et al.,
now issued as U.S. Pat. No. 8,493,941, commonly assigned with the
present invention and incorporated herein by reference in its
entirety.
TECHNICAL FIELD
[0002] This invention relates to wireless communications.
BACKGROUND OF THE INVENTION
[0003] In a wireless system, mobile terminals transmit and receive
data over bi-directional wireless links from one or more base
stations. The mobile terminal-transmit direction is known as the
uplink and the mobile terminal-receive direction is known as the
downlink. The set of base stations with which a mobile terminal is
communicating is known as the active set of base stations for that
mobile terminal, or that mobile terminal's active set. During
normal conditions when a mobile terminal is within a base station's
coverage area, the active set for that mobile terminal would
generally be a single base station so that the active set includes
only that one base station, which is that mobile terminal's serving
station. When a mobile terminal, however, is within the range of
multiple base stations and as such is in a handoff state, the
active set includes the multiple base stations, which are each
monitoring the signal from that mobile terminal and decoding it
when able to do so. Only one of the base stations, however, is the
serving base station for that mobile terminal and the other base
stations are non-serving base stations. FIG. 1 shows three base
stations 101, 102 and 103 and their respective cell coverage areas
104, 105 and 106. Mobile terminal 107 is shown within the coverage
area 104 of base station 101, which is its serving base station.
Non-serving base stations 102 and 103, however, in addition to
serving base station 101 constitute the active set for mobile
terminal 107.
[0004] In a data system, on the downlink, the mobile terminal 107
receives data from only one base station but has the option of
reselecting the serving base station in order to receive data from
any other base station in its active set depending on from which
base station the mobile terminal receives a signal with the highest
signal-to-noise ratio. On the uplink, the serving and the
non-serving base stations each attempts to demodulate and decode
transmissions from the mobile terminal.
[0005] The capacity of a wireless system refers either to the
number of mobile terminals that can simultaneously transmit or
receive data, or the aggregate date rate of these mobile terminals,
either expressed in mobile terminals/sector, erlangs/sector or data
throughput/sector. The uplink capacity of the system can be
different from its downlink capacity. For symmetric services, such
as voice, (i.e., required throughput/data rate for a mobile
terminal on the uplink is equal to that on the downlink), the
overall system capacity is limited by the lower of uplink and
downlink capacity. In current wireless systems specified by
standards such as CDMA2000 1 x, EV-DO Rev 0 and Rev A, HSDPA/EDCH,
and WiMAX, the uplink has a substantially lower capacity than the
downlink. This imbalance needs to be remedied for full use of
downlink capacity and to maximize the number of mobile terminals
than can operate symmetric services on the system.
[0006] In wireless systems that are based on direct spread or
multi-carrier (optionally with precoding) CDMA, a plurality of
mobile terminals within a sector (and across sectors) re-use a
spreading sequence or a set of frequency tones to communicate with
their respective active sets, while being differentiated by mobile
terminal-specific codes. A mechanism for increasing uplink sector
capacity is to perform successive interference cancellation on
these transmissions at the base station transceiver. FIG. 2
illustrates base station noise rise components. As shown, at a base
station receiver 200, the total rise over thermal noise in a sector
consists of the composite signal 201 from the mobile terminals
within that sector for which that base station is the serving base
station, and the out-of-cell interference 202 caused by mobile
terminals transmitting in adjacent sectors. The latter includes
interference from those mobile terminals in the adjacent sectors
for which base station receiver 200 is within these mobile
terminals' active set but for which base station 200 is
non-serving, plus the interference caused by other transmitting
mobile terminals in other sectors that do not include base station
200 within each such mobile terminal's active set.
[0007] An illustrative method of interference cancellation is
disclosed in U.S. Pat. No. 7,385,944. Using such an interference
cancellation method, if the decoding of any mobile terminal is
successful, its signal is reconstructed and subtracted from the
composite received signal at the base station. FIG. 3 shows a
successive interference cancellation scheme at an exemplary base
station receiver 300 that is the serving base station for four
mobile terminals 301, 302, 303 and 304 within a sector of that base
station receiver 300. The received power at base station receiver
300 from mobile terminals 301, 302, 303, and 304 is respectively
P.sub.--1, P.sub.--2, P.sub.--3, and P.sub.--4. In addition, base
station receiver 300 receives a composite signal power as the
result of out-of-cell interference (IOC) caused by transmissions
from mobile terminals out of the sector. When a particular
transmission from a mobile terminal from within the sector is
successfully decoded by the base station receiver, the transmission
is reconstructed and subtracted from the composite signal at the
base station, after which another received signal is demodulated,
decoded, reconstructed and subtracted from the remaining composite
signal. This process is repeated for each of the remaining signals.
Advantageously, the signals from the mobile terminals that are
decoded later in the demodulation and decoding process do not "see"
the interference from transmissions from the mobile terminals that
were decoded earlier in the sequence. FIG. 3 shows a successive
calculation of the signal-to-noise ratios (Snr.sub.--1-Snr.sub.--4)
of the four mobile terminals 301-304, respectively. Starting with
mobile terminal 301, Snr.sub.--1 is calculated as
P.sub.--1/(P.sub.--2+P.sub.--3+P.sub.--4+IOC+N), where N is the
measurable thermal noise. The contribution from each is
successively subtracted off from the received composite signal at
the base station receiver, so that, for the last mobile terminal
304, Snr.sub.--4 is calculated as P.sub.--4/(IOC+N). Since the
mobile terminals that are decoded later see a higher
signal-to-noise ratio, they are capable of supporting either a
higher rate of transmission and/or increased reliability.
[0008] In the above-described scenario, it is not possible for the
base station receiver to successfully decode the transmissions of
all the mobile terminals that have this base station sector in
their active set. As a result, most of the out-of-cell interference
received by a base station receiver cannot be deducted. Thus, as
noted above, the signal-to-noise ratio for station 304 is still
limited by this out-of-cell interference IOC.
[0009] Typically, the power control rule followed by mobile
terminals is to either (i) follow power control commands from the
serving sector in its active set, or (ii) follow a rule known as
the or-of-the downs, whereby the mobile terminal lowers its power
if any of the base stations in the active set instructs it to do so
via a down power control command. While an or-of-the-downs power
control ensures successful reception of the mobile terminal's
transmission at [[at]] least one base station (presumably the one
with the best uplink connection from the mobile terminal), it also
ensures that the mobile terminal's transmission is not received
with adequate signal-to-noise ratio to be successfully decodable at
all of the base stations in the active set. Thus, this undecodable
interference limits the capacity gain from a system employing
successive interference cancellation. Even as mobile terminals
within a sector transmit with ever increasing powers, in order to
increase their signal-to-noise ratios at the base station receiver
(and hence achieve higher data rates), their interference to
adjacent sectors grows proportionately, thereby limiting the rates
that can be achieved by mobile terminals in those sectors. In turn,
the interference from the mobile terminals in adjacent sectors
marginalizes the gains for the mobile terminals with the sector
under consideration that increased their power in the first
place.
[0010] A methodology is thus desired that enables a base station
receiver to reconstruct and subtract the out-of-cell interference
from the composite received signal so that the signal-to-noise
ratio can be improved for all in sector mobile terminals.
SUMMARY OF THE INVENTION
[0011] In an embodiment, a device, e.g. a mobile terminal, includes
a receiver and a transmitter. The receiver is configured to receive
commands from a plurality of base stations. The transmitter is
configured to send, when the a mobile terminal is in handoff
status, data packets to the plurality of base stations in the
mobile terminal's active set of base stations, wherein the active
set including a serving base station and at least one non-serving
base station. The transmitter is further configured to increase the
transmit power of the transmitter, in response to the receiver
receiving a command to increase the transmit power from any of the
base stations, unless the transmit power is at its maximum power or
the transmit power exceeds a power targeted by the serving base
station by a predetermined amount.
[0012] Another embodiment is a method, e.g. for interference
management in a wireless communication system. The method includes
sending, at a mobile terminal in handoff status, data packets to a
plurality of base stations in the mobile terminal's active set of
base stations, wherein the active set includes a serving base
station and at least one non-serving base station. The method
further includes increasing the mobile terminal's transmit power,
in response to receiving a command to increase the transmit power
from any of the base stations in the active set, unless the
transmit power is at its maximum power or the transmit power
exceeds a power targeted by the serving base station by a
predetermined amount.
[0013] In any embodiment of the aforementioned device or method,
the transmit power may be increased by a predetermined up-step. In
any embodiment, the transmitter may be configured to increase or
decrease the transmit power only in response to commands received
from the serving base station, on the condition that: the transmit
power is at its maximum power; the mobile terminal receives a
command to increase its transmit power from any of the base
stations in the active set, and the transmit power exceeds a power
targeted by the serving base station by a predetermined amount.
[0014] In any embodiment the transmitter may be configured to
increase the transmit power only if the receiver determines the
received command is determined to be reliable. In such embodiments
the receiver may determine that the command to increase the
transmit power is reliable on the condition that a power of the
received command to increase the power is greater than a
predetermined threshold.
[0015] In another embodiment a device, e.g. a mobile terminal,
includes a receiver and a transmitter. The receiver is configured
to receive commands from a plurality of base stations. The
transmitter is configured to send, when the mobile terminal is in
handoff status, data packets to a plurality of base stations in an
active set of base stations, the active set including a serving
base station and at least one non-serving base station. The
receiver is further configured to increase a transmit power in
response to the receiver receiving a negative acknowledgment (NACK)
of a successful decoding of a given packet received from any of the
base stations at the end of that packet's retransmission limit,
unless the transmit power is at its maximum power or the transmit
power exceeds a power targeted by the serving base station by a
predetermined amount.
[0016] Yet another embodiment is a method, e.g. for interference
management in a wireless communication system. The method includes
sending, at a mobile terminal in handoff status, data packets to a
plurality of base stations in its active set of base stations,
wherein the active set includes a serving base station and at least
one non-serving base station. The method further includes
increasing the mobile terminal's transmit power, in response to
receiving a negative acknowledgment (NACK) of a successful decoding
of the packet from any of the base stations in the active set at
the end of a given packet's retransmission limit, unless the
transmit power is at its maximum power or the transmit power
exceeds a power targeted by the serving base station by a
predetermined amount.
[0017] In any embodiment of the device or method described
immediately above, the transmitter may be configured to increase
the transmit power by a predetermined up-step. In any such
embodiment, if the transmit power is at its maximum power or if the
transmit power exceeds a power targeted by the serving base station
by a predetermined amount, and the receiver receives from any of
the base stations in the active set a command to increase the
transmit power, then for a subsequent data transmission the
transmitter is configured to increase the transmit power only if
the receiver receives a NACK from the serving base station and to
decrease the transmit power if the receiver receives an ACK from
the serving base station. In any embodiment of the device or method
described immediately above, the transmitter may be configured to
increase the transmit power only if the receiver determines a NACK
received from any of the base stations in the active set is
reliable. In any embodiment of the device or method described
immediately above the receiver may determine the NACK is reliable
by comparing a power of the received NACK with a predetermined
threshold.
BRIEF DESCRIPTION OF THE DRAWING
[0018] The present invention will be better understood from reading
the following description of non-limiting embodiments, with
reference to the attached drawings, wherein below:
[0019] FIG. 1 shows a prior art arrangement of a mobile terminal
and the serving base station and non-serving base stations in its
active set;
[0020] FIG. 2 illustrates base station noise rise components;
[0021] FIG. 3 shows a prior art successive interference
cancellation scheme at an exemplary base station receiver that is
the serving base station for four mobile terminals within a sector
of that base station receiver;
[0022] FIG. 4 shows the steps at a mobile terminal in accordance
with a first embodiment that employs an or-of-the ups rule in
controlling its transmit power during handoff;
[0023] FIG. 5 shows the steps at a mobile terminal in accordance
with a second embodiment that employs an or-of-the-NACKS rule in
controlling its transmit power during handoff; and
[0024] FIG. 6 shows the steps at a mobile terminal in accordance
with a third embodiment that employs an or-of-the-NACKS rule in
controlling its retransmissions of data during handoff.
DETAILED DESCRIPTION
[0025] With reference to the flowchart in FIG. 4, the methodology
employed at a mobile terminal operating in accordance with an
or-of-the-ups rule is shown. At step 401, a determination is made
whether the mobile terminal is in handoff (i.e., whether or not the
active set is greater than one). In addition, but not shown, an
optional additional determination may be made of whether the mobile
terminal's average path losses to the base stations in its active
set differ from each other by more than a predetermined amount. If
it is not in handoff, at step 402, the mobile terminal follows
power control commands received from its one serving base station.
If it is in handoff (and if the optional determination of average
path losses indicates that such average path losses differ from
each other by less than the predetermined amount), then, at step
403, the mobile terminal monitors power control commands from the
base stations in its active set and accumulates power control
commands (the sequence of +1 s and -1 s) from the serving base
station to determine the power targeted by the base station.
Specifically, by accumulating these +1 s and -1 s, the power level
at which the serving base station would like the mobile terminal to
operate can be determined. At step 404, a determination is made
whether any of the power commands received by any of the base
stations in the active set is an "up" (an or-of-the ups rule). If
not, indicating that the received power level at all of that base
stations in the active set is at or above the target (and in turn
that the transmission is likely to have been successful at all of
the base stations), then, at step 405, the mobile terminal
decreases its transmit power by a predetermined down-step. If, at
step 404, any of the power commands received from any of the base
stations in the active set is an "up" (indicating that transmission
in unlikely to have been successful at the base station(s) from
which the up is received), then the mobile terminal transmit power
may be increased. Before increasing its transmit power, however, a
determination is made, at step 406, whether the mobile terminal is
already at its maximum transmit power, or whether the mobile
terminal transmit power exceeds the power targeted by the serving
base station by more than a predetermined amount. If either is the
case, then, at step 407, the mobile terminal reverts to an
or-of-the-downs power control rule (i.e., the mobile terminal
lowers its power if any base station in the active set instructs it
to do so). Alternatively, but not shown, the mobile terminal
reverts to following power control from only the serving base
station. If, at step 406, the mobile terminal is not at its maximum
transmit power and does not exceed the power targeted by the
serving base station by the predetermined amount, then, at step
408, the mobile terminal increases its transmit power by a
predetermined up-step.
[0026] With reference to the flowchart in FIG. 5, the methodology
employed at a mobile terminal operating in accordance with an
or-of-the-NACKS rule is shown. At step 501, a determination is made
whether the mobile terminal is in handoff. As in the first
embodiment, an optional addition determination may be made of
whether the mobile terminal's average path losses to the base
stations in its active set differ from each other by more than a
predetermined amount. If it is not in handoff, at step 502, the
mobile terminal follows acknowledgment feedback (ACKS/NACKS) from
its one serving base station. If it is in handoff, (and if the
optional determination of average path losses indicates that such
average path losses differ from each other by less than the
predetermined amount), then, at step 503, at the end of the packet
transmission limit (i.e., after the mobile terminal has transmitted
a data packet its maximum allowed number of times), the mobile
terminal monitors acknowledgments from all base stations in the
active set. In addition, the transmit power of the mobile terminal
that is required for a successful reception at the serving base
station is tracked. At step 504, a determination is made whether at
the end of the packet transmission limit any of the acknowledgments
is a NACK, indicating an unsuccessful reception (an or-of-the-NACKS
determination) at the base station(s) from which the NACK(s) is
received. It should be noted that in certain systems a NACK is
indicated by base station silence, i.e., the absence of the base
station transmitting an ACK. If none of the responses is a NACK,
then the packet has been successfully received at each of the base
stations in the active set and, at step 505, the mobile terminal
power is decreased by a down-step for a next packet transmission.
If, however, at step 504, any of the base station responses is a
NACK, then the mobile terminal power may be increased. Before
increasing its transmit power, however, a determination is made, at
step 506, whether the mobile terminal is already at its maximum
transmit power, or whether the mobile terminal transmit power
exceeds the power targeted by the serving base by more than a
predetermined amount. If either is the case, then, at step 507, the
mobile terminal reverts to an or-of-the-ACKS power control rule for
subsequent packet transmissions (i.e., the mobile terminal
increases its transmit power only if doesn't receive an ACK from at
least one base station in its active set). Alternatively, but not
shown, the mobile terminal reverts to following power control based
on the ACK/NACK received from only the serving base station. If, at
step 506, the mobile terminal is not at its maximum transmit power
and does not exceed the power targeted by the serving base station
by the predetermined amount, then, at step 508, the mobile terminal
increases its transmit power by a predetermined up-step.
[0027] With reference to the flowchart in FIG. 6, another
methodology employed at a mobile terminal operating in accordance
with an or-of-the-NACKS rule is shown. At step 601, a determination
is made whether the mobile is in handoff. As in the previously
discussed embodiments, in addition, but not shown, an additional
optional determination may be made of whether the mobile terminal's
average path losses to the base stations in its active set differ
from each other by more than a predetermined amount. If it is not
in handoff, at step 602, the mobile terminal follows acknowledgment
feedback from its one serving base station. If it is in handoff,
(and if the optional determination of average path losses indicates
that such average path losses differ from each other by less than
the predetermined amount), then, at step 603, the mobile terminal
monitors acknowledgments from all the base stations in its active
set. At step 604, a determination is made whether any of the base
station responses to a packet transmission is a NACK. If not, at
step 605, the mobile terminal prepares to transmit the next data
packet. If, however, at step 604, it does receive a NACK from one
or more base stations in its active set, the mobile terminal may
retransmit the packet. If, at step 606, the retransmission limit
has not been reached for this packet, then, at step 607, the mobile
terminal retransmits the data packet. If, however, at step 606 the
retransmission limit has been reached for this packet, then, at
step 608, retransmission for subsequent packets is performed based
on an or-of-the-ACKS rule wherein the packet is not retransmitted
if an ACK is received from any base station in the active set.
Alternatively, but not shown, if the retransmission limit has been
reached, retransmission for subsequent packets may be based on the
ACK/NACK acknowledgments received only from the serving base
station.
[0028] In each of the embodiments described above, the mobile
terminal may perform a test to determine the reliability of the
ACKS or NACKS, or power control bits that it receives from the base
stations in its active set in order to determine whether or not to
raise its transmit power or to retransmit a packet. Specifically,
the received power of the received ACKS/NACKS or power control bits
can be compared against a threshold. In those systems in which ACKS
and NACKS are both positively transmitted, then if the power is
greater than the threshold, then the received ACKS/NACKS or power
control bits can be assumed to be reliable and used to make the
appropriate determination of increasing/decreasing the transmit
power or retransmitting the packet. On the other hand, if the power
is below the threshold, a decision to increase the power or
retransmit the packet is made only on the basis of those ACKS/NACKS
or power control bits that are deemed to be reliable. For those
systems in which a NACK is indicated by base station silence a NACK
would be deemed to be reliable if its power was below a
threshold.
[0029] Upon a successful decoding of a mobile station's data
transmission the received signal from the mobile terminal is
reconstructed and subtracted off from the composite received signal
prior to decoding other mobile terminal's transmissions with a
concomitant improved signal-to-noise ratio for these other mobile
terminals' transmissions. For mobile terminals that have adopted
the fallback position for power control or ACK/NACK feedback, the
amount of interference presented to the non-serving base stations
is likely to be decreased and the penalty due to not being able to
successfully decode those transmissions is reduced.
[0030] As previously discussed, the described methodology can be
used in any system where out-of-cell interference limits the
signal-to-noise ratio for one or more in-cell mobile terminals.
Examples of such system include a system employing successive
interference cancellation or an OFDMA system that allows only one
in-cell transmission over a given set of frequency tones.
[0031] The above-described embodiment is illustrative of the
principles of the present invention. Those skilled in the art can
devise other embodiments without departing from the spirit and
scope of the present invention.
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