DESCRIPTION
WIRELESS BASE STATION DEVICE USING COLLABORATIVE HARQ
COMMUNICATION SYSTEM, WIRELESS TERMINAL DEVICE, WIRELESS
COMMUNICATION SYSTEM, AND WIRELESS COMMUNICATION METHOD
Technical Field
The present invention relates to collaborative
transmission system technology using a distributed antenna.
Packet communication technology includes, for example, E-UTRA
(Evolved Universal Terrestrial Radio Access) communication
technology which has been studies as a next generation mobile
telephone communication standard.
Background Art
Relating to the spread-spectrum code division multiple
access, widely studied is the soft handoff technology for
preventing the communications from being interrupted by being
transmitted and received the same signals simultaneously,
between two base stations when a mobile terminal moves from one
cell to an adjacent cell. As the prior art relating to a
collaborative transmission, for example, a system described in
the patent document 1, the following non-patent document 1, etc.
is disclosed. In the prior art, a collaborative transmission
system for successfully increasing the link capacity is
disclosed.
Based on a similar concept, a collaborative
transmission system using a distributed antenna arranged in a
different base station is proposed in relation to the
multi-input and multi-output (MIMO) technology corresponding
to macroscopic fading. As the prior art obtained by combining
the MIMO technology and the collaborative transmission
technology, for example, the systems described in the following
non-patent documents .2 through 6 are proposed. These systems,
aim at attaining both a macroscopic diversity effect and a MIMO
effect.
The discussions of the macroscopic diversity with a
collaborative transmission have been made in a planning project
of a new mobile telephone communication standard such the LTE
(Long Term Evolution) etc. for which a standardizing operation
is performed by a standardizing organization 3GPP (3rd
Generation Partnership Project), for example. These
discussions are disclosed by, for example, the following
non-patent document 7. However, since it has been hard to
distribute data of a high layer to different base stations, the
collaborative transmission has not been realized, but a system
of distributing data only to one base station has been used for
simple implementation.
Recently, the LTE advanced standard as a next generation .
standard of the LTE has been developed as the fourth generation
system (4G). In the standard, especially at a system
performance request relating to the frequency efficiency for
downlink (DL) and uplink (UL), a rather positive target is set.
A practical discussion of the problem above has been disclosed
in, for example, the following non-patent document 8.
To attain the above-mentioned target, some
corporations have presented useful propositions about a beam
forming transmission, intra-cell interference control, and
relay control. In the propositions, the point of the discussion
relating to the collaborative transmission has been taken up
again to reconsider the possibility of the implementation. To
be concrete, it is disclosed in, for example, the following
non-patent document 9 or 10. In the LTE advanced, the target
of the throughput of a user at the edge of a cell is set as
approximately 1.4 times as high as that in the release 8 of the
LTE communication standard. By taking this into account, the
collaborative transmission system is expected as an important
candidate in the LTE advanced technology.
Before adopting the collaborative transmission
technology in the next generation communication standard such
as the LTE advanced etc., there are a number of points to be
discussed. It is, for example, a search of data and control
channel, transmission timing, user packet scheduling, hybrid
automatic repeat request (HARQ) process, etc. between eNodes-B
through the X2 interface. The most important search among them
is that relating to the HARQ.
In the LTE communication standard etc., the packet
communication technology is required to enable the high-speed
communications at a mobile terminal. In the packet
communication, a reception device receives communication
information while detecting an error based on the error
correction code added to a communication packet by transmission
device. Then, the reception device returns to the transmission
device an ACK (acknowledgement) or a NAK (negative
acknowledgement) about the reception status of the
communication packet. The transmission device retransmits
transmission information when the reception device returns a
NAK or when no transmission status confirmation can be received
before a certain period has passed after a packet is
transmitted.
In the HARQ technology adopted in the LTE etc., for
example, the retransmission pattern is determined on the
transmission device side after considering that the data whose
decoding has failed by the reception device is not discarded
but decoded by a combination with retransmission data in the
process of a layer 1 protocol hierarchical level of the LTE
etc. On the reception device side, the data whose reception
has failed is not discarded, but decoded by a combination with
retransmission data. Thus, retransmission control is realized
with high efficiency and high accuracy.
Therefore, in the next generation packet communication
system, it is an important problem to determine how the HARQ
is to be realized in the collaborative transmission system to
realize a collaborative transmission system with a high
diversity effect.
However, in the prior art disclosed as Patent Document
1 or non-patent documents 1 through 10, no practical technology
for realizing the HARQ in the collaborative transmission has
not been disclosed.
In addition, the system described in the following
patent document 2 is disclosed as prior art obtained by
combining the HARQ and the MIMO technology. Patent Document
2 refers to a practical system for realizing the HARQ in the
packet transmission using a MIMO multiple transmission antenna.
However, the MIMO is based on that a plurality of
antennas are accommodated in one base station while the
collaborative transmission is based on that the antennas of a
plurality of base stations arranged in a distributed manner
perform a collaborative transmission in the downlink direction
toward a mobile terminal. To realize a collaborative
transmission including a HARQ between the base stations
arranged in the distributed manner, it is necessary to solve
the problems, which is not necessary in the MIMO, of the
communication system for user data and channel data, timing,
etc. among the base stations. Especially, the combination of
a new data packet and a retransmission data packet in the HARQ
with the collaborative transmission is not disclosed by the
above-mentioned prior art, which remains as an unsolved
problem.
Patent Document 1: National Publication of International
Patent Application No.2008-503974
Patent Document 2: National Publication of International
Patent Application No. 2008-517484
Non-patent Document 1: A. J. Viterbi, A. M. Viterbi, K. S.
Gilhousen, and E. Zehavi, "Soft handoff extends CDMA cell
coverage and increases reverse link capacity", IEEE J. Sel.
Areas Commun., vol. 12, pp. 1281-1288, October, 1994.
Non-patent Document 2: W. Roh and A. Paulraj, "MIMO channel
capacity for the distributed antenna systems", in IEEE VTC 02,
vol. 3, pp. 1520-1524, Sept. 2002.
Non-patent Document 3: Z. Ni and D. Li, "Impact of fading
correlation and power allocation on capacity of distributed
MIMO", IEEE Emerging technologies: Frontiers of Mobile and
Wireless Communication, 2004, Volume 2, May 31-June 2, 2004
Page(s): 697-700 vol. 2.
Non-patent Document 4: Syed A. Jafar, and S. Shamai, "Degrees
of freedom region for the MIMO X Channel", IEEE Transactions
on Information Theory, Vol. 54, No. 1, pp. 151-170, January
2008.
Non-patent Document 5: D. Wang, X. You, J. Wang, Y. Wang, and
X. Hou, "Spectral Efficiency of Distributed MIMO Cellular
Systems in a composite Fading Channel", IEEE International
conference on, Communications, 2008. ICC '08, pp. 1259-1264,
May 19-23, 2008.
Non-patent Document 6: 0. Simeone, 0. Somekh, ; H. V. Poor,
andS. Shamai, "Distributed MIMO in multi-cell wireless systems
via finite-capacity links", Communications, Control and Signal
Processing, 2008. ISCCSP2008. 3rd International Symposium on,
pp. 203-206, March 12-14, 2008.
Non-patent Document 7: 3GPP TR 25.814 v7.0.0. Physical layer
aspects for evolved UTRA, release-7, June 2006.
Non-patent Document 8: 3GPP TR 36.913 V7.0.0., Requirements
for Further Advancements for E-UTRA, release-8, V8.0.0, June
2008.
Non-patent Document 9: 3GPP TSG RAN WG1 Meeting #53bis Warsaw,
Poland, "Collaborative MIMO for LTE-A downlink", June 30-July
4, 2008, Rl-082501.
Non-patent Document 10: 3GPP TSG RAN WG1 Meeting #53bis Warsaw,
Poland, "Network MIMO Precoding", June 30-July 4, 2008,
R1-082497
Disclosure of Invention
The problem of the present invention is to realize an
appropriate and efficient HARQ process in the collaborative
transmission system.
The aspect described below is based on the wireless
communication system in which the first wireless base station
device and the second wireless base station device perform a
collaborative transmission process to allow the wireless
terminal device not to discard a packet on which decoding has
failed but to combine the packet with a retransmitted packet
and decode the resultant packet while controlling the
retransmission of a packet according to the transmission status
information returned from the wireless terminal device, the
wireless base station device or the wireless terminal device
which belong to the wireless communication system, or the
wireless communication method for realizing the process.
A first packet transmission unit transmits as a first
packet a new data packet or a retransmission data packet
corresponding to a retransmit request from the first wireless
base station device to the wireless terminal device when the
retransmit request is issued to the collaborative transmission
process by the wireless terminal device.
A packet transfer unit transfers the information about
a second packet different from the first packet between the new
data packet and the retransmission data packet from the first
wireless base station device to the second wireless base station
device. The packet transfer unit performs a transfer process
using, for example, an X2 interface regulated between the first
wireless base station device and the second wireless base
station device.
The second packet transmission unit transmits the
second packet according to the information transferred from the
packet transfer unit in synchronization with the transmission
process of the first packet by the first packet transmission
unit from the second wireless base station device to the
wireless terminal device when the retransmit request is issued.
With the above-mentioned configuration, each of the
first wireless base station device and the second wireless base
station device has a retransmission buffer unit, and the first
wireless base station device can be configured to hold the
information about the packet on which a collaborative
transmission process is performed for the wireless terminal
device in the retransmission buffer unit in the first wireless
base station device, and the second wireless base station device
can be configured not to hold the information about the packet
on which the collaborative transmission process is performed
for the wireless terminal device in the retransmission buffer
unit in the second wireless base station device.
With the above-mentioned configuration, the first
packet can be configured as a retransmission data packet, and
the second packet can be configured as a new data packet. In
this instance, the packet transfer unit reads the information
about the retransmission data packet from the retransmission
buffer unit in the first wireless base station device, and
transfers the information to the second wireless base station
device. The packet transfer unit transfers, for example, the
communication control information relating to the second
wireless base station device for communication between the
first wireless base station device and the wireless terminal
device and the information relating to the transmission timing
of the second packet by the second wireless base station device.
With the configurations up to the aspects above, a
control information communication unit for communicating the
control information about the communication by the first
wireless base station device to the wireless terminal device
and the control information about the communication by the
second wireless base station device to the wireless terminal
device between the first wireless base station device and the
wireless terminal device can be further included. For example,
the control information communication unit can perform the
transmission of control information from the first wireless
base station device to the wireless terminal device through a
physical downlink control channel and perform the transmission
of the control information from the wireless terminal device
to the first wireless base station device through a physical
uplink control channel. The physical uplink control channel
in this case includes at least, for example, the individual
channel quality indication information for each of the first
wireless base station device and the second wireless base
station device, and the precoding matrix indication information
and the rank indication information common to the first wireless
base station device and the second wireless base station device.
In addition, the physical downlink control channel includes at
least, for example, the individual modulation and coding scheme
information and the individual precoding information for each
of the first wireless base station device and the second
wireless base station device.
The wireless communication system according to claim
6 or 7 has the characteristics above.
With the configuration described above, the control
information from the wireless terminal device to the first
wireless base station device can be configured to include the
transmission status information (HARQ-ACK/NAK) indicating a
reception result of the packet from the first wireless base
station device and a reception result of the packet from the
second wireless base station device, respectively.
With the configuration above, the first wireless base
station device can be configured to centrally control at least
the assignment of a wireless terminal device, the assignment
of communication resources, and the control of transmission
timing associated with the collaborative transmission process.
The wireless terminal device for performing the
communication by the wireless communication system having the
above-mentioned configuration has the following aspects.
A retransmission data packet reception unit performs a
receiving process on a retransmission data packet when a
retransmit request is issued.
When the retransmission data packet reception unit
successfully performs the receiving process on the
retransmission data packet, a new data packet reception unit
performs a successive interference cancellation process on the
received signal received by the wireless terminal device
through the retransmission data packet on which the receiving
process has been successfully performed, and the receiving
process of a new data packet according to a resultant received
signal is performed.
With the configuration of the aspect of the wireless
terminal device, a collaborative transmission process
determining unit for determining whether or not the
collaborative transmission process is to be performed and
determining the first wireless base station device and the
second wireless base station device for performing the process
when the execution of the collaborative transmission process
is determined can be further included. For example, the
collaborative transmission process determining unit makes a
determination according to the information about the reception
power for the reference signal to be received from each wireless
base station device currently in communication.
Brief Description of Drawings
FIG. 1 is an explanatory view of a network model based
on which the present embodiment is designed;
FIG. 2 is a configuration of an embodiment of the
transmission device;
FIG. 3 is a configuration of an embodiment of the reception
device;
FIG. 4 is an explanatory view of grouping cases in which
two eNodes-B collaboratively operate;
FIG. 5 is an explanatory view of the collaborative
downlink HARQ transmission system for a scenario 2;
FIG. 6 is an explanatory view of the collaborative
downlink HARQ transmission system for a scenario 3;
FIG. 7 is an example of an operation sequence of a
determining process of a serving eNB and a collaborative eNB;
FIG. 8 is an explanatory view of a data channel and a
control channel;
FIG. 9 is an example of a data format of a UCI and a DCI;
FIG. 10 is an example of the transmission timing between
a control channel and a data channel;
FIG. 11 is a graph indicating the a BLER to geometry for
each UE on the initial transmission, retransmission #1, #2, and
#3 in the simulation result;
FIG. 12 is a graph indicating the CDF of the SINR to a
S-eNB and a C-eNB with and without SIC in the simulation result;
FIG. 13 is a graph indicating the probability of a link
gap between a serving eNB and a collaborative eNB;
FIG. 14 is a graph indicating the SINR to link gap between
a serving eNB and a collaborative eNB with and without SIC at
the CDF point of 0.5; and
FIG. 15 is a graph indicating the gain to link gap by the
cancellation between the serving eNB and the collaborative eNB
at the CDF point of 0.5.
Best Mode for Carrying Out the Invention
The best embodiments are described below in detail with
reference to the attached drawings.
First, the system network model is described according
to the embodiments of the present invention.
FIG. 1 is an explanatory view of a network model based
on which the present embodiment is designed.
To hold generalities, a network is configured as a
packet communication system including two wireless base
stations for collaboratively performing a service on a wireless
mobile terminal (UE: User Equipment) such as a mobile telephone
terminal etc. A packet communication system can be realized
as, for example, an E-UTRA (Evolved Universal Terrestrial Radio
Access) system in accordance with the LTE communication
standard on which a standardizing operation is performed by
3GPP.
[0029] In the LTE etc., a base station is referred to as an
eNode-B (evolved Node B) . In the present embodiment, in the
description below, a base station is referred to as an eNode-B
or an eNB for short.
As illustrated in FIG. 1, one of the two wireless base
stations is a serving base station (serving eNode-B,
hereinafter referred to as a "serving eNB" or a "S-eNB" for
short), and the other is referred to as a collaborative base
station (collaborative eNode-B, hereinafter referred to as a
"collaborative eNB" or a "C-eNB" as necessary). The
determination as to which the eNB belongs, a serving eNB or a
collaborative eNB, depends on the long-period power intensity
received by each UE. Therefore, the positioning of the eNB for
each UE can be different. As a reasonable definition, the
long-period power intensity from the serving eNB received by
each UE is higher than that of the collaborative eNB.
FIG. 2 is a configuration of a packet transmission
device according to an embodiment configured in the eNode-B on
the network illustrated in FIG. 1. FIG. 3 is a configuration
of a packet reception device according to an embodiment
configured in the UE illustrated in FIG. 1. The transmission
device in FIG. 2 is provided on the downlink side of the eNode-B,
and the reception device in FIG. 2 is provided on the downlink
side of the UE. The configuration of the
transmission/reception device on the uplink channel side of the
devices has a common configuration, and the detailed
description is omitted here.
The transmission device illustrated in FIG. 2 includes
a new data packet transmission unit 201, a retransmission data
packet transmission unit 202, a channel assignment unit 203,
a modulation unit 204, a wireless processing unit 205, a
transmission control unit 206, an uplink control channel
reception unit 207, and an X2 control channel
transmission/reception unit 208. The new data packet
transmission unit 201 is further configured by a block
generation unit 201-1, a new portion acquisition unit 201-2,
and a new data packet coding unit 201-3. The retransmission
data packet transmission unit 202 is further configured by a
retransmission buffer unit 202-1, a retransmission portion
acquisition unit 202-2, and a retransmission data packet coding
unit 202-3.
The reception device illustrated in FIG. 3 includes a
wireless processing unit 301, a retransmission data packet
reception unit 302, a new data packet reception unit 303, a
reception control unit 304, and an uplink control channel
transmission unit 305. The retransmission data packet
reception unit 302 is further configured by a retransmission
data packet demodulation unit 302-1, a retransmission buffer
unit 302-2, a retransmission portion combination unit 302-3,
a retransmission data packet decoding unit 302-4, and a output
distribution unit 302-5. The new data packet reception unit
303 is further configured by a retransmission data packet
re-coding unit 303-1, a retransmission data packet
re-modulation unit 303-2, a canceller unit 303-3, a new data
packet demodulation unit 303-4, and a new data packet decoding
unit 303-5.
Described below in detail are the operations of the
embodiments of the transmission device and the reception device
with the above-mentioned configurations.
A very unique and important behavior for the HARQ can be
the block error rate of normally 1% or less when a retransmission
data packet is decoded after the HARQ combining process
performed by the retransmission portion combination unit 305-3
illustrated in FIG. 2. In the embodiment illustrated in FIG.
2, in the successive interference cancellation process (SIC)
performed by the canceller unit 303-3, a decoded retransmission
data packet is positively used, thereby realizing an effective
SIC process. That is, in the embodiment illustrated in FIG.
2, a retransmission packet is first detected in the UE, and then
other packets (new or retransmission packets) are detected.
Next, in the present embodiment, one new packet and one
retransmission packet are delivered in complete
synchronization toward one UE from two collaboratively
operating eNodes-B which implement a transmission device of a
downlink system illustrated in FIG. 1.
FIG. 4 is an explanatory view of grouping cases in which
two eNodes-B collaboratively operate. In this example, a
collaborative transmission is grouped into four types of
scenarios. Each scenario refers to a different channel
resource assignment, and a different control channel design.
For simplicity, the explanation here refers to the case of one
UE only, but the scenario for a plurality of UEs is described
later.
In the scenario 1 illustrated in FIG. 4 (a) , it is assumed
that only a new data packet is delivered to a UE positioned at
the cell edge from the serving eNB. To realize a macroscopic
transmission collaboratively, some new data packets are
transferred from the serving eNB to the collaborative eNB
through the X2 interface. Then, the new data packets are
delivered simultaneously to a corresponding UE from both
eNodes-B. On the UE side, the receiving process is performed
while suppressing the interference from each other.
In the scenario 2 illustrated in FIG. 4 (b) it is assumed
that two types of transmission packet are delivered to the UE
positioned at the cell edge. One is a retransmission data
packet, and another packet is a new data packet. The
retransmission data packet is delivered from a serving eNB to
a UE simultaneously when the new data packet transferred from
the serving eNB through an X2 interface is delivered from a
collaborative eNB to a UE. In the UE, as described later, the
new data packet reception unit 303 illustrated in FIG. 3
performs the receiving process while suppressing the
interference from each other in the SIC process.
In the scenario 3 illustrated in FIG. 4(c), as in the
scenario 2, the two types of transmission packets, that is, the
retransmission data packet and the new data packet, are
delivered. In the scenario 3, unlike the scenario 2, a new data
packet is delivered from the serving eNB to the UE
simultaneously when a retransmission data packet is delivered
from the collaborative eNB to the UE. In this case, the
retransmission data packet is transferred from the serving eNB
to the collaborative eNB. In the UE, as described later, the
new data packet reception unit 303 illustrated in FIG. 3
performs the receiving process while suppressing the
interference from each other in the SIC process.
In the scenario 4 illustrated in FIG. 4 (d), it is assumed
that only the retransmission data packet is delivered from the
serving eNB to the UE at the cell edge. To collaboratively
realize a macroscopic transmission, some retransmission data
packets are transferred from the serving eNB to the
collaborative eNB through the X2 interface. Then, the
retransmission data packets are simultaneously delivered to the
corresponding UE from both eNBs. The UE performs the receiving
process while suppressing the interference from each other.
It is considered that the scenario 2 illustrated in FIG.
4(b) and the scenario 3 illustrated in FIG. 4(c) are better
transmission systems for providing the highest diversity gain
by a macroscopic transmission analysis and a cancellation gain,
by the SIC process because since the BLER (block error rate)
for the retransmission data packet after a HARQ combination is
sufficiently low, the retransmission data packet can be first
extracted, and then the new data packet can be extracted by the
SIC process, thereby acquiring a better result. Therefore, it
is preferable that one new data packet and one retransmission
data packet can be constantly acquired as a rule of the
collaborative transmission, and they can be transmitted
simultaneously from both the serving eNB and the collaborative
eNB. According to the system level simulation result described
later, it is certain that if an UE moves at the speed of 3 km/h,
the probability of a retransmission is 8 - 10 %. However, if
it moves at the speed of 30 km/h, the probability of a
retransmission increases up to 70 - 80 %. Therefore, when there
are terminal groups coexisting and moving at different speeds,
the probability of retransmissions can be estimated as 30 - 40 %.
It means the possibility of the collaborative HARQ transmission
between the new data packet and the retransmission data packet
is 23 - 29 %. It is considered that the probability that the
scenario 1 illustrated in FIG. 4(a) as a normal collaborative
transmission without a retransmission is approximately 70 %.
However, since the scenario 4 illustrated in FIG. 4 (d) indicates
a low occurrence probability of a HARQ packet, it does not occur
in a practical system. Therefore, the probability that the
scenario 4 is adopted is nearly zero.
By the search above, the description below is
concentrated on the cases of the scenario 2 illustrated in FIG.
4 (b) and the scenario 3 illustrated in FIG. 4 (c) as an operation
of the transmission device of the eNode-B downlink system
illustrated in FIG. 2. One of these scenarios is selected and
designed during the implementation. A more preferable
scenario between them is described later.
FIG. 5 is an explanatory view of the collaborative
downlink HARQ transmission system for the scenario 2.
First, in FIG. 5(b) , if a new data packet received at the
UE (for example, a new data packet #0) enters an erroneous state,
the data is retransmitted from the serving eNB simultaneously
with the new packet (for example, a new data packet #12)
delivered from the collaborative eNB (C-eNB) to the synchronous
transmission timing determined by the serving eNB (S-eNB). A
similar process occurs with a retransmission packet #4 (or #11)
transmitted with the new data packet #17 (or #15).
FIG. 5(a) is a block diagram of the configuration of
the process of the transmission device for the scenario 2. When
the transmission device in FIG. 2 is implemented as a downlink
system on the serving eNB side, a retransmission buffer unit
504 on the serving eNB side in FIG. 5(a) corresponds to the
retransmission buffer unit 202-1 illustrated in FIG. 2. A first
packet transmission unit 501 on the serving eNB side corresponds
to the portion excluding the retransmission buffer unit 202-1
in the retransmission data packet transmission unit 202
illustrated in FIG. 2. Furthermore, an RF 503 on the serving
eNB side corresponds to the portion configured by the channel
assignment unit 203, the modulation unit 204, and the wireless
processing unit 205 illustrated in FIG. 2. On the other hand,
when the transmission device is implemented as a downlink system
on the collaborative eNB side, the second packet transfer unit
503 on the collaborative eNB side in FIG. 5(a) corresponds to
the new data packet transmission unit 201 in FIG. 2. An RF 505
on the collaborative eNB side corresponds to the portion
configured by the channel assignment unit 203, the modulation
unit 204, and the wireless processing unit 205 in FIG. 2.
Furthermore, a packet transfer unit 502 for transferring a new
data packet from the serving eNB to the collaborative eNB
corresponds to an X2 control channel transmission/reception
unit 108 illustrated in FIG. 2.
As understood from the process configuration described
above, when the serving eNB and the collaborative-eNB each
having a transmission device of a downlink system illustrated
in FIG. 2 operate according to the scenario 2, the first packet
transmission unit 501 performs an operation of transmitting a
retransmission data packet 507 in the transmission device on
the serving eNB side. On the other hand, in the transmission
device on the collaborative eNB side, the second packet transfer
unit 503 performs the operation of transmitting a new data
packet 508 corresponding to the information transferred from
the serving eNB by the packet transfer unit 502.
FIG. 6 is an explanatory view of the collaborative
downlink HARQ transmission system for the scenario 3.
First, in FIG. 6(b), when the new data packet (for example,
a new data packet #0) received by the UE enters an erroneous
state, the data is transferred through the X2 interface along
a corresponding control channel to the collaborative eNB. Then,
it is retransmitted from the collaborative eNB simultaneously
with a new packet (for example, a new data packet #4) delivered
from the serving eNB to the synchronous transmission timing
determined by the serving eNB. A similar process is generated
with a retransmission packet #5 (or #14) transmitted with a new
data packet #9 (or #7).
FIG. 6(a) is a block diagram of the process
configuration of the transmission device for the scenario 3.
When the transmission device in FIG. 2 is implemented as a
downlink system on the serving eNB side, a retransmission buffer
unit 604 on the serving eNB side in FIG. 6(a) corresponds to
the retransmission buffer unit 202-1 in FIG. 2. A first packet
transfer unit 601 on the serving eNB side corresponds to the
new data packet transmission unit 201 in FIG. 2. Furthermore,
an RF 605 on the serving eNB side corresponds to the portion
configured by the channel assignment unit 203, the modulation
unit 204, and the wireless processing unit 205. On the other
hand, when the transmission device in FIG. 2 is implemented as
a downlink system on the collaborative eNB side, the second
packet transfer unit 603 on the collaborative eNB side in FIG.
6(a) corresponds to the portion excluding the retransmission
buffer unit 202-1 in the retransmission data packet
transmission unit 202 in FIG. 2. In addition, an RF 605 on the
collaborative eNB side corresponds to the portion configured
by the channel assignment unit 203, the modulation unit 204,
and the wireless processing unit 205 in FIG. 2. Furthermore,
a packet transfer unit 602 for transferring a retransmission
data packet from the retransmission buffer unit 604 in the
serving eNB to the collaborative eNB corresponds to the X2
control channel transmission/reception unit 108 in FIG. 2.
As understood from the process configuration described
above, when the serving eNB and the collaborative eNB each
having a transmission device of a downlink system illustrated
in FIG. 2 operate according to the scenario 3, the first packet
transmission unit 601 performs an operation of transmitting a
new data packet 607 in the transmission device on the serving
eNB side. On the other hand, in the transmission device on the
collaborative eNB side, the second packet transfer unit 603
performs the operation of transmitting a retransmission data
packet 608 corresponding to the information transferred from
the retransmission buffer unit 604 in the serving eNB by the
packet transfer unit 502.
With respect to the entire complexity, the scenario 2
is more preferable than the scenario 3 because, according to
the scenario 2, the collaborative eNB receives a new block
transferred from the serving eNB through the X2 interface, and
can deliver a new data packet generated based on the received
block without considering whether or not the packet has been
correctly received on the UE side as described later in the
explanation of the control channel. As described later, the
serving eNB is totally responsible including the control
channel access for the receiving process and the HARQ. This
simplifies the design of the collaborative eNB. However, it
is obvious that the configuration of the scenario 3 can be
adopted.
Described below is a further detailed operation of the
transmission device in FIG. 2 with the process of the scenarios
2 and 3 above.
In FIG. 2, the block generation unit 201-1 generates a
block of a predetermined size from an information bit to be
transmitted. The size of a block generated by the block
generation unit 201-1 is equal to the amount of information bit
which can be stored in one packet. That is, a normal packet
to be transmitted by a transmission device includes information
bits corresponding to one block.
The retransmission buffer unit 202-1 temporarily holds
for a retransmission a block of the information bits generated
by the block generation unit 201-1. The retransmission buffer
unit 202-1 can sequentially discard the block which has been
correctly decoded by the reception device and is not to be
retransmitted.
The transmission control unit 206 controls the new
portion acquisition unit 201-2 and the retransmission portion
acquisition unit 202-2 according to the control signal received
by the uplink control channel reception unit 207 from the UE
side through a control channel.
Practically, when the transmission device in FIG. 2
operates as a serving eNB for a certain UE according to the
scenario 1 (refer to FIG. 4(a)), and if a transmission of a
retransmission data packet does not be instructed by the UE side,
then the following operation is performed. That is, the
transmission control unit 206 first instructs the new portion
acquisition unit 201-2 to acquire a new block generated by the
block generation unit 201-1 and corresponding to the UE to be
processed, and output it to the new data packet coding unit 201-3
for a transmission. The transmission control unit 206
instructs the retransmission portion acquisition unit 202-2 to
stop the operation. Furthermore, the transmission control
unit 206 instructs the new portion acquisition unit 201-2 to
output the new block also to the X2 control channel
transmission/reception unit 208, and transfer it also to the
collaborative eNB corresponding to the UE to be processed.
On the other hand, when the transmission device in FIG.
2 operates as a collaborative eNB for a certain UE according
to the scenario 1, and if the UE side does not instruct the
serving eNB corresponding to the UE to transmit a retransmission
data packet, then the following operation is performed. That
is, the transmission control unit 206 instructs the new portion
acquisition unit 201-2 to acquire a new block received by the
X2 control channel transmission/reception unit 208 and
transferred from the serving eNB corresponding to the UE to be
processed, and output it to the new data packet coding unit 201-3
for a transmission.
Next, when the transmission device in FIG. 2 operates
as a certain serving eNB for a UE according to the scenario 2
(refer to FIG. 4 (b)), and if the number of received NAKs received
for the certain UE by the uplink control channel reception unit
207 has reached a predetermined number, the following process
is performed. That is, the transmission control unit 206
instructs the retransmission portion acquisition unit 202-2 to
acquire a transmitted block (retransmission block)
corresponding to the NAK held in the retransmission buffer unit
202, and output it to the retransmission data packet coding unit
202-3 for a retransmission. In addition, the transmission
control unit 206 instructs the new portion acquisition unit
201-2 to acquire a new block generated by the block generation
unit 201-1 and corresponding to the UE to be processed, and
output it not to the new data packet coding unit 201-3 but to
the X2 control channel transmission/reception unit 208 to
transfer it to the collaborative eNB corresponding to the UE
to be processed.
On the other hand, when the transmission device in FIG.
2 operates as a collaborative eNB for a certain UE according
to the scenario 2, and if the number of received NAKs received
by the uplink control channel reception unit 207 in the serving
eNB corresponding to the certain UE has reached a predetermined
number, then the following process is performed. That is, the
transmission control unit 206 instructs the new portion
acquisition unit 201-2 to acquire a new block received by the
X2 control channel transmission/reception unit 208 and
transferred from the serving eNB corresponding to the UE to be
processed, and output it to the new data packet coding unit 201-3
for a transmission.
When the transmission device in FIG. 2 operates as a
serving eNB for a certain UE according to the scenario 3 (FIG.
4 (c) ) , and if the number of received NAKs received by the uplink
control channel reception unit 207 for the UE has reached a
predetermined number, then the following process is performed.

That is, the transmission control unit 206 instructs the
retransmission portion acquisition unit 202-2 to acquire a
transmitted block (retransmission block) corresponding to the
NAK held in the retransmission buffer unit 202 to output it not
to the retransmission data packet coding unit 202-3 but to the
X2 control channel transmission/reception unit 208 and transfer
it to the collaborative eNB corresponding to the UE to be
processed. The transmission control unit 206 instructs the new
portion acquisition unit 201-2 to acquire a new block generated
by the block generation unit 201-1 and corresponding to the UE
to be processed, and output it to the new data packet coding
unit 201-3 for a retransmission.
On the other hand, when the transmission device in FIG.
2 operates as a collaborative eNB for a certain UE according
to the scenario 3, and if the number of received NAKs received
by the uplink control channel reception unit 207 in the serving
eNB corresponding to the certain UE has reached a predetermined
number, then the following process is performed. That is, the
transmission control unit 206 instructs the retransmission
portion acquisition unit 202-2 to acquire a retransmission
block received by the X2 control channel transmission/reception
unit 208 and transferred from the serving eNB corresponding to
the UE to be processed, and output it to the retransmission data
packet coding unit 202-3 for a transmission.
An ACK and a NAK are control signals stored with user
data, transferred from a certain UE to be processed, and
received by the uplink control channel reception unit 207 in
the transmission device operating as a serving eNB for the
certain UE as uplink control information (UCI) described later.
These ACK and NAK indicate whether or not a reception error of
a packet has occurred in the UE, and is returned from the UE
to the corresponding serving eNB for each received packet.
In the transmission device in FIG. 2, when a new block
is input from the new portion acquisition unit 201-2, the new
data packet coding unit 303-1 in the new data packet
transmission unit 201 generates a new packet in which the new
block is included in an information bit section and a
corresponding parity bit is included in a parity bit section.
When a retransmission block is input from the
retransmission portion acquisition unit 202-2, the
retransmission data packet coding unit 202-3 in the
retransmission data packet transmission unit 202 generates a
retransmission packet in which the retransmission block is
included in an information bit section and a corresponding
parity bit is included in a parity bit section.
The channel assignment unit 203 assigns the new packet
generated by the new data packet coding unit 201-3 or the
retransmission packet generated by the retransmission data
packet coding unit 202-3 to a communication channel
corresponding to the UE to be processed, and outputs the
resultant frame data to the modulation unit 204.
The modulation unit 204 modulates the frame data output
from the channel assignment unit 203, and outputs the data to
the wireless processing unit 205.
The wireless processing unit 205 performs a predetermined
wireless transmitting process on the frame data after the
modulation, and transmits the resultant data through an antenna
not illustrated in the attached drawings.
Described next is the detailed operation of the
reception device illustrated in FIG. 3 and implemented in the
downlink system in the UE.
As illustrated in FIG. 3, the reception device is provided
with the retransmission data packet reception unit 302 and the
new data packet reception unit 303.
In FIG. 3, the reception control unit 304 can recognize
whether a received packet is a new data packet or a
retransmission data packet according to the new data indication
information (refer to FIG. 9(b)) included in the downlink
control information (DCI) transmitted from the serving eNB with
the received packet through a physical downlink control channel
as described later. The recognition is similar to the
identification between the scenario 1 and the scenario 2, or
between the scenario 1 and the scenario 3. The reception
control unit 304 performs the identifying process based on the
output of the retransmission data packet demodulation unit
302-1 which constantly performs the demodulating process.
By the identification, when the reception device
operates according to the scenario 1 (FIG. 4 (a)) described above,
the retransmission data packet reception unit 302, the
retransmission data packet re-coding unit 303-1, the
retransmission data packet re-modulation unit 303-2, and the
canceller unit 303-3 in the new data packet reception unit 303
do not operate, and the received signal received by the wireless
processing unit 301 through an antenna passes through the
canceller unit 303-3 in the new data packet reception unit 303
and enters the new data packet demodulation unit 303-4.
The new data packet demodulation unit 303-4 demodulates
the received packet from each communication channel configuring
the received signal input from the wireless processing unit 301,
and outputs the received packet to the new data packet decoding
unit 303-5.
The new data packet decoding unit 303-5 decodes the
input new data packet, and outputs resultant new information
bits to the processing unit at the subsequent stage but not
illustrated in the attached drawings.
On the other hand, in the identifying process by the
reception control unit 304, when the reception device
illustrated in FIG. 3 operates as the scenario 2 (FIG. 4(b))
or the scenario 3 (FIG. 4 (c)), both retransmission data packet
reception unit 302 and new data packet reception unit 303
operate under the control of the reception control unit 304.
Described first is the operation of the retransmission
data packet reception unit 302.
The retransmission data packet demodulation unit 302-1
demodulates the received packet from each communication channel
configuring the received signal input from the wireless
processing unit 301, and outputs the received packet to the
retransmission portion combination unit 302-3. The
retransmission data packet demodulation unit 302-1 performs a
demodulating process regardless of whether the received packet
is a retransmission data packet or a new data packet to enable
the identifying process by the reception control unit 304.
With the timing of processing on a retransmission packet
indicated by the reception control unit 304, the retransmission
portion combination unit 302-3 combines the retransmission data
packet input from the retransmission data packet demodulation
unit 302-1 with the past data packet held in the retransmission
buffer unit 302-2 after a first reception failure. Then, the
retransmission portion combination unit 302-3 outputs the
combination result to the retransmission data packet decoding
unit 302-4. The reception control unit 304 receives
retransmission sequence information and other control
information as a part of downlink control information (DCI)
transmitted with a received packet from the serving eNB through
the physical downlink control channel, and notifies the
retransmission portion combination unit 302-3 of these pieces
of control information. The retransmission portion
combination unit 302-3 performs the process of combining
retransmission packets in the HARQ. system according to the
control information.
The retransmission data packet decoding unit 302-4
decodes the input retransmission data packet, and outputs the
resultant reconstructed information bits to the output
distribution unit 302-5.
When the information bits are successfully reconstructed,
the output distribution unit 302-5 outputs them to the
processing unit at the subsequent stage but not illustrated in
the attached drawings. Simultaneously, the output
distribution unit 302-5 outputs the reconstructed information
bits to the retransmission data packet re-coding unit 303-1 in
the new data packet reception unit 303.
Described next is the operation of the new data packet
reception unit 303.
When the reconstructed information bits are input from
the output distribution unit 302-5, the retransmission data
packet re-coding unit 303-1 and the retransmission data packet
re-modulation unit 303-2 are operated, and a replica of a
successfully received retransmission data packet is generated.
The canceller unit 303-3 performs a cancelling process
on the interference signal components in the retransmission
data packet received from the serving eNB (in the case of the
scenario 2) or the collaborative eNB (in the case of the scenario
3) for the received signal input from the wireless processing
unit 301 as a successive interference cancellation process.
Thus, the canceller unit 303-3 appropriately extracts only the
received signal components of the new data packet received from
the collaborative eNB (in the case of the scenario 2) or the
serving eNB (in the case of the scenario 3), and outputs the
result to the new data packet demodulation unit 303-4.
the new data packet demodulation unit 303-4 demodulates
the received packet from each communication channel configuring
the received signal from which the interference components
input from the canceller unit 303-3 are removed, and outputs
the received packet to the new data packet decoding unit 303-5.
The new data packet decoding unit 303-5 decodes the
input new data packet, and outputs the resultant new information
bits to the processing unit at the subsequent stage but not
illustrated in the attached drawings.
If the reconstructing process on the retransmission data
packet fails in the retransmission data packet reception unit
302, and no input is performed from the output distribution unit
302-5 to the retransmission data packet re-coding unit 303-1,
then the input from the retransmission data packet
re-modulation unit 303-2 to the canceller unit 303-3 is set to
zero. Thus, the operation of the canceller unit 303-3 becomes
invalid equivalently. As a result, the new data packet
demodulation unit 303-4 and the new data packet decoding unit
303-5 extract a new data packet without the cancelling process.
In FIG. 3, the reception control unit 304 correctly
recognizes the physical downlink control channel from the
serving eNode-B described later according to, for example, the
reference signal (RS) in the received signal. As an RS group
between the serving eNB and the collaborative eNB, a signal
group in which signals have the same patterns but different
phase shifts, for example, those orthogonal to each other, can
be used to easily identify the channel between the serving eNB
and the collaborative eNB.
As an example of a variation of a system of processing
the above-mentioned reception device, the following
interactive system capable of improving the system performance
can also be applied.
• First, a retransmission data packet is extracted, and if it
is correctly received, a new data packet is extracted in the
SIC process by a canceller unit.
• If the retransmission data packet is not successfully received,
a new data packet is extracted. If the new data packet is
correctly received, the retransmission data packet is extracted
again in the SIC process by the canceller unit.
Thus, in the present embodiment, a retransmission data
packet and a new data packet are assigned to the serving eNB
and the collaborative eNB (in the case of the scenario 2) or
inversely (in the case of the scenario 3) to perform a
collaborative transmission, thereby successfully and
simultaneously transmitting a retransmission data packet and
a new data packet corresponding to the same UE using the same
channel resources. Thus, in the collaborative transmission
system according to the present embodiment, channels can also
be effectively used.
The assignment of channel resources and the user
scheduling for a collaborative transmission are centrally
controlled by the transmission control unit 206 (FIG. 2) in the
serving eNB. As an important parameter for determining whether
or not a collaborative transmission is to be performed, a link
gap ?ue or, in place of it, a reference signal receiving power
(RSRP) difference used as a term in the LTE is used. The
parameter is defined as a difference of logarithm received
signal powers between the serving eNB and the collaborative eNB
in the UE. If the link gap Aue is smaller than the link gap
target ? as another parameter, the collaborative transmission
is performed Otherwise, a normal transmission is preferable.
Using these parameters, a band width for a collaborative
transmission can be easily controlled.
The reception control unit 304 in the reception device
(FIG. 3) of the UE sequentially detects the RSRP deffrence of
each received RS during communications, and notifies the
serving eNB side of the result through the uplink control
channel transmission unit 305. As a result, the uplink control
channel reception unit 207 in the current serving eNB (FIG. 2)
receives it, and the transmission control unit 206 (FIG. 2)
determines whether or not the collaborative transmission is to
be continued, determines a new serving eNB, etc.
Described above is the collaborative HARQ transmitting
process relating to one UE, but each UE can identify the
execution status of the collaborative transmission according
to an RS signal group and identify the serving eNB and the
collaborative eNB as described above. Thus, each eNode-B can
control whether it functions as a serving eNB or a collaborative
eNB for each UE, and can perform the same process as the process
mentioned above.
FIG. 7 is an example of an operation sequence of a
determining process of a serving eNB and a collaborative eNB.
A UE determines, for example, the eNode-Bl as a serving eNB and
the eNode-BO as a collaborative eNB according to an RS signal
group in the state in which communications with the eNode-BO
and the eNode-Bl are performed using, for example, control
signals 0 and 1 (S1 in FIG. 7) . Thus, the UE performs
communications with the eNode-B1 using, for example, a random
access channel RACH. Upon receipt of a notification of a data
channel and a control channel from the eNode-B1 (S2 in FIG. 7),
the UE notifies the eNode-B1 as a serving eNB of the information
relating to the eNode-BO as a collaborative eNB using the
control channel (S3 in FIG. 7). As a result, a notification
is issued from the eNode-Bl to the eNode-BO using the X2
interface, and the eNode-BO notifies the UE of the data channel
and the control channel (S4 in FIG. 7) . Thus, the UE can receive
a collaborative transmission from the eNode-Bl and the eNode-BO.
In this case, it receives a packet of collaborative transmission
data and control information from the eNode-Bl as a serving eNB,
and receives only the packet of collaborative transmission data
from the eNode-BO as a collaborative eNB.
Described next is the control channel communicated
between a control channel designing eNode-B and the UE.
In the configuration of the present embodiment, an
important control signal is communicated through a link between
the serving eNB and the UE. That is, the link between the
serving eNB and the UE is configured so that it has a more
important function that the link between the collaborative eNB
and the UE.
In designing a control channel, three channels are
regarded. They are a physical uplink control channel (PUCCH) ,
a physical downlink control channel (PDCCH), and an X2 control
channel (X2CCH).
In addition, a control channel is designed according
to the above-mentioned scenario 2 (FIG. 4(b)) because the
scenario can provide better system performance and lower
complexity for both the control channel and the data channel.
The selection is confirmed in evaluating the system level
simulation described later.
FIG. 8 is an explanatory view of a data channel and a
control channel and their communication directions. The
restrictions on the two types of channels are described below.
• A new data packet can be transmitted on the two links, that
is, from the serving eNB to the UE and from the collaborative
eNB to the UE.
• A retransmission packet can be transmitted only on the link
from the serving eNB to the UE.
• The PUCCH indicated as a C1 is transmitted on the link from
the UE to the serving eNB.
• The PDCCH indicated as a C2 is transmitted on the link from
the serving eNB to the UE.
• Only a new data packet and a control signal relating to the
packet are delivered from the serving eNB to the collaborative
eNB using the X2 interface. The control channel in the X2
interface is indicated as C3.
By the above-mentioned design of the control channel
for the collaborative transmission, the amount of control
channel can be exceedingly reduced, and the system latency can
be considerably shortened by the HARQ process in a single
direction. Described below in more detail is the design of each
of the three channels.
First described is the design of the PUCCH.
In the design described below, the PUCCH corresponds to
the uplink control information (UCI) including the following
two periodic signals. One includes a channel quality
indication (CQI), a precoding matrix indication (PMI), and a
rank indication (RI), and expressed by CQI/PMI/RI. The other
includes a HARQ-ACK/NAK. A PUCCH is transmitted only on the
link from the UE to the serving eNB. In FIG. 8, it is indicated
by C1. The PUCCH is terminated by the uplink control channel
transmission unit 305 (FIG. 3) in the UE and the uplink control
channel reception unit 207 (FIG. 2) in the eNode-B operating
as a serving eNB. Each active UE separates the serving eNB and
the collaborative eNB by, for example, a high layer control
signal.
Each UE observes a channel response according to the
reference signal (RS) from the serving eNB as well as the
collaborative eNB. As described above, the phases of the RS
of both NBs are set so that they can be orthogonal to each other.
The uplink control channel transmission unit 305 (FIG. 3) in
the UE notifies the uplink control channel reception unit 207
(FIG. 2) in the serving eNB corresponding to the UE of a
periodical UCI. The CQI/PMI/RI included in the UCI corresponds
to the quality of both links, that is, the link from the serving
eNB to the UE and the link from the collaborative eNB to the
UE. Then, the UCI is only transmitted to the corresponding
serving eNB for the following two reasons.
• Generally, the quality of the link from the serving eNB to
the UE is better than the that from the collaborative eNB to
the UE, which ensures the performance for the UL control
channel.
• It exceedingly reduces the amount of control channel, and
simplifies the control channel design.
FIG. 9(a) illustrates a data format of an example
of a UCI for both links. The format includes individual CQI
for the respective links. It also includes corresponding PMI
and RI. The field information corresponding to the PMI and RI
is the same for both links.
The ACK or NAK (HARQ-ACK/NAK) included in the UCI for
the HARQ process is the information about whether or riot a
reception error of a packet has occurred in the UE. The
retransmission data packet decoding unit 302-4 and the new data
packet decoding unit 303-5 in the reception device illustrated
in FIG. 3 notifies the uplink control channel transmission unit
305 that it is necessary to retransmit a packet being processed
when an error rate is equal to or higher than a predetermined
threshold and the number of repetitions of a decoding process
reaches a predetermined number in each decoding process. Thus,
the uplink control channel transmission unit 305 transmits, to
the serving eNB corresponding to the UE to which the unit belongs,
a NAK for each received packet for which a retransmission is
specified. In the case other than the above-mentioned
condition, when the retransmission data packet decoding unit
302-4 and the new data packet decoding unit 303-5 successfully
receive each received packet, the uplink control channel
transmission unit 305 transmits an ACK for each received packet
which has successfully received to the serving eNB
corresponding to the UE including the unit.
The HARQ-ACK/NAK included in the UCI is received by the
uplink control channel reception unit 207 (FIG. 2) in the
serving eNB, and the information is passed to the transmission
control unit 206. The transmission control unit 206 performs
the retransmitting process on the HARQ as described above. In
this case, it is preferable that the retransmitting process is
performed only to the UE from the serving eNB as described in
the scenario 2 for the following reasons.
The transmission latency in the HARQ process for a
transmission packet can be reduced.
• The control channels including the PDCCH and the X2CCH can
be simplified.
• The complexity for the collaborative eNB can be reduced
because a transmitted new packet is not left in the
retransmission buffer unit 302-2 (FIG. 2) arranged in the
collaborative eNB. The collaborative eNB is only to transmit
a new packet after the control channel (X2CCH) from the X2
interface.
The field of the HARQ-ACK/NAK on the PUCCH is designed
to include the ACK/NAK signal (2 bits) corresponding to both
of the serving eNB and collaborative eNB for the transmission
data packet corresponding to both of the serving eNB and
collaborative eNB.
Described next is the design of the PDCCH.
In the design, the PDCCH is transmitted only from the
serving eNB to de destination UE so that it can be indicated
as a C2 in FIG. 8. In this case, the PDCCH is terminated by
the transmission control unit 206 (FIG. 2) in the eNode-B
operating as a serving eNB and the reception control unit 304
(FIG. 3) in the UE.
That is, each UE decodes only the PDCCH from the serving
eNB corresponding to the UE for the following two reasons.
• The quality of the link from the serving eNB to the UE is better
than that from the collaborative eNB to the UE. This ensures
the performance for the control channel.
Transmitting the PDCCH from only one link considerably
moderates the load of the control channel.
The downlink control information (DCI) transmitted
through the PDCCH can indicate whether or not a collaborative
transmission is currently being performed. For the purpose,
a new bit is introduced to the DCI. As another expression, a
PCI includes a bit identifying whether a transmission packet
is a new data packet or a retransmission data packet, that is,
whether it is the scenario 1 or the scenario 2, or whether it
is the scenario 1 or the scenario 3. It is used to indicate
the reception device to perform or not to perform the HARQ
processing. The information can be attained by using the new
data indication information (FIG. 9(b) described later) already
prescribed and existing in the LTE standard.
[0097] Furthermore, the DCI includes the following
information
• In addition to the modulation and coding scheme (MCS) for the
serving eNB in the format 1, format 1A, and format 1C, 5 bits
of additional MCS for the collaborative eNB is required.
• Additional MCS (5 bits) and precoding information in the
format 2
The DCI for both links including the above-mentioned
information is collectively encoded using the CRC specifying
the UE. FIG. 9(b) is an example of the DCI using the format
2. In FIG. 9(b), the "RB assigning header" and the "RB
assignment" are control information relating to the assignment
of a resource block. The "new data indication information" is
the information specifying whether a transmission packet is a
new data packet or a retransmission data packet. A "redundant
version" is the control information about a HARQ. The "MCS-1"
and the "MCS-2" are the MCSs respectively for a serving eNB and
a collaborative eNB. The precoding information 1 and the
precoding information 2 are the precoding information
respectively for the serving eNB and the collaborative eNB.
[0098] The PDCCH including the DCI is stored together with a
user data packet in a subframe regulated in the data format in,
for example, the E-UTRA communication system, and then
transmitted.
Described next is the design of an X2 control channel.
Am X2 control channel (X2CCH) is delivered with a data
packet corresponding to the control channel through the X2
interface indicated by C3 in FIG. 8. Practically, the X2CCH
is terminated by the X2 control channel transmission/reception
unit 208 in the transmission device illustrated in FIG. 2 of
the serving eNB and the collaborative eNB. The X2CCH is
realized on the cable link using, for example, optical fiber.
The X2CCH includes the following information.
• Resource assignment header: 1 bit
• Resource block assignment
• Modulation and coding scheme: 5 bits
• Precoding information
• Transmission timing for subframe
Described next is the timing control between the X2CCH
and the PDCCH.
The transmission timing control is one of the most
important problem for a collaborative transmission. It is
determined by the serving eNB, and is instructed by the
collaborative eNB through the X2 interface. The transmission
timing is determined by considering the latency of the X2
interface.
FIG. 10 is an example of the transmission timing between
a control channel and a data channel. In FIG. 10, the data and
the corresponding X2CCH are transferred to the collaborative
eNB prior to the relating transmission ("PDCCH" and "Data from
S-eNB") from the serving eNB to the UE with the timing t2. The
transmission timing t1 of the data from the collaborative eNB
("Data from C-eNB") is determined by the serving eNB based on
the maximum latency T of the X2 interface. By the synchronous
network between the serving eNB and the collaborative eNB, the
data from the serving eNB and the data from the collaborative
eNB are delivered with predetermined timing tl and t2. It
guarantees the reception of both data with the simultaneous
timing t3.
Including the above-mentioned timing control, the
collaborative transmission for each UE is centrally controlled
by the serving eNB. The control includes the scheduling of the
UE and data, and the transmission timing control.
A system level simulation has been performed to evaluate
the performance of the above-mentioned collaborative HARQ
transmission system according to the present embodiment.
In the system level simulation, a system loaded with the
transmission device (FIG. 2} and the reception device (FIG. 3)
according to the present embodiment is implemented in the cell
network formed by 7 clusters. Each cluster is configured by
19 hexagonal cells, and each cell includes 3 sectors. The
bore-sight point of the antenna of the sector is directed at
the vertex of the hexagon. A surrounding inclusive network
structure is adopted to generate an accurate model of the
generation of interference from an external cell, the cluster
to be observed is arranged at the center, and six copies are
symmetrically arranged at the sides of the central cluster.
Tables 1 and 2 respectively illustrate the simulation case
grouping and condition assumption.
[Table 1]
MINIMAL SET OF UTRA AND EUTRA SIMULATIONS
SIMULATION• . CF ISD BW PLoss SPEED CHANNEL

First, by evaluating the BLER (block error rate) of the
HARQ system according to the present embodiment, a full system
level simulation without a collaborative transmission is
performed.
FIG. 11, in (a), (b), and (c), illustrates the BLER for
each UE as the function of the geometry about the initial
transmission and the retransmission #1, #2, and #3 respectively
in the cases 1, 2, and 3.
Table 3 is a summary of the average BLER of the entire
UE for the initial transmission and the retransmission #1, #2,
and #3 in the cases 1, 2, and 3. The BLER for the initial
transmission for the cases 1 and 3 is about 9%, and that for
the case 2 is 78%. However, after the first retransmission,
the BLER for the cases 1 and 3 is 0.1% or less, and that for
the case 2 is 25%. Thus, when the reception device for
performing an appropriate SIC process according to the present
embodiment is introduced, it can be expected that the system
performance for the collaborative transmission can be improved.
[Table 3]
AVERAGE BLER FOR INITIAL TRANSMISSION, RETRANSMISSION #1, #2,
AND #3 IN CASES 1, 2, AND 3

Described next is the SINR gain from a reception device
for performing a SIC process according to the present
embodiment.
As described above, the link gap target A is an important
parameter having an influence on the collaborative transmission.
In the system level simulation, the parameter is used to control
the band width between the collaborative eNBs. The motive of
performing the system level simulation is to clarify the gain
attained by the scenario 2 with respect to the scenario 3. First,
the CDF (cumulative density function) of the reception SINR
(signal-to-interference and noise power ratio) in the
collaborative transmission user for various set values of the
link gap target A, or 1dB, 10dB, and 19dB is plotted. Thus,
the SINR at the CDF point of 0.5 can be illustrated. This
enables the merit of the SINR from the scenario 2 to be correctly
indicated.
The explanatory legends of the plot graphics are defined
as follows.
• Serving link, No-SIC: SNR (signal-to-noise ratio) or SNR
gain received by a UE from the serving eNB for a serving link)
when there is no SIC cancelling process of the interference from
the collaborative eNB (or the collaborative link). It
corresponds to the scenario 3.
• Collab link, No-SIC: SNR or SNR gain received by a UE from
the collaborative eNB (or a collaborative link) when there is
no SIC cancelling process of the interference from the serving
eNB (or the serving link). It corresponds to the scenario 2.
• Serving link, SIC: SNR or SNR gain received by a UE from the
serving eNB (or a serving link) when there is a SIC cancelling
process of the interference from the collaborative eNB (or the
collaborative link). It corresponds to the scenario 3.
• Collab link, SIC: SNR or SNR gain received by a UE from the
collaborative eNB (or a collaborative link) when there is a SIC
cancelling process of the interference from the serving eNB (or
the serving link). It corresponds to the scenario 2.
FIG. 12, in (a), (b), and (c), illustrates the CDF of
the SINR received by the UE in each case of the reception from
the serving eNB and the collaborative eNB, in each case of with
and without the SIC, and in each case of with each set value
of A, or 1db, 10dB, and 19dB. As the link gap target increases,
the link quality between the serving eNB and the UE becomes
better. In addition, the SIC process by the canceller unit
303-3 (FIG. 3) operates in a better condition with respect to
the link between the collaborative eNB and the UE.
FIG. 13 is a graph indicating the probability of a UE
falling into a link gap target A and determined as a cell edge
user. For the UE, a collaborative transmission is performed.
When the link gap target A indicates a reasonable value about,
for example, 8dB, the rate of the cell edge user is about 60 %,
which is sufficiently large value, and requires a collaborative
transmission.
FIG. 14 is a graph indicating the SINR of the UE as a
function of the value of ? as a function of the link gap target
A when the CDF value is 50 %. FIG. 15 is a result of calculating
the SINR gain of the UE for the two links with and without SIC
in addition to the conditions of FIG. 14.
By comparing the link (link 1) from the collaborative
eNB to the UE with the link (link 2) from the serving eNB to
the UE, some observation results are obtained as follows.
• When a retransmission data packet is delivered from the
serving eNB, the SINR gain for the link 1 in the SIC process
is about 2 through 2.5 dB.
• When the retransmission data packet is delivered from the
collaborative eNB, the SINR gain for the link 2 in the SIC process
is about 1.5 through 1.75 dB.
• When the value of ? increases, the SINR gain of the link 1
becomes larger, and the SINR gain of the link 2 becomes smaller.
Thus, it is preferable that the value of A is not too small or
large. In addition, a small value of A causes a too small
possibility of a collaborative transmission, and a large value
of A causes a too large possibility of a collaborative
transmission. An appropriate value of A is between 8 dB and
10 dB. As a conclusion based on the study of the SINR gain by
the SIC, the retransmission data packet is to be delivered
constantly from the serving eNB.
The present application has proposed the collaborative
transmission system for the HARQ process to again a high SINR
gain using the reception device for performing the SIC process.
The present application realizes the SIC process more
easily by using the unique behavior of the HARQ constantly
indicating a low BLER after the combination of HARQs.
To attain high SINR gain by the SIC process, it is
preferable that a retransmission data packet is eventually
delivered on the link constantly from the serving eNB to the
UE and a new data packet is delivered on the link from the
collaborative eNB to the UE during the delivery. However, it
is obvious that an inverse process can be used.
Relating to a control channel, three channels, that is,
a physical uplink control channel (PUCCH), a physical downlink
control channel (PDCCH), and a X2 control channel (X2CCH), are
regarded by considering the feasibility and the facility. The
design of the control channels can exceedingly reduce the amount
of control channel, and considerably shorten the system
latency.
The above-mentioned collaborative transmission system
can also be applied to an intra-eNode-B in which a collaborative
transmission occurs between two transmission points in the same
eNode-B.
We claim :
1. A wireless communication system in which a first
wireless base station device and a second wireless base
station device perform a collaborative transmission process
for realizing the process to allow the wireless terminal
device not to discard a packet on which decoding has failed
but to combine the packet with a retransmitted packet and
decode a resultant packet while controlling the
retransmission of the packet according to transmission
status information returned from the wireless terminal
device, comprising:
a first packet transmission unit to transmit as a
first packet a new data packet or a retransmission data
packet corresponding to a retransmit request from the first
wireless base station device to the wireless terminal
device when the retransmit request is issued to the
collaborative transmission process by the wireless terminal
device;
a packet transfer unit to transfer information about a
second packet different from the first packet between the
new data packet and the retransmission data packet from the
first wireless base station device to the second wireless
base station device; and
a second packet transmission unit to transmit the
second packet according to information transferred from the
packet transfer unit in synchronization with a transmission
process of the first packet by the first packet
transmission unit from the second wireless base station
device to the wireless terminal device when the retransmit
request is issued.
2. The wireless communication system according to claim
1, wherein:
each of the first wireless base station device and the
second wireless base station device has a retransmission
buffer unit; and
the first wireless base station device holds
information about a packet on which a collaborative
transmission process is performed for the wireless terminal
device in the retransmission buffer unit in the first
wireless base station device; and
the second wireless base station device does not hold
the information about a packet on which a collaborative
transmission process is performed for the wireless terminal
device in the retransmission buffer unit in the second
wireless base station device.
3. The wireless communication system according to claim 1
or 2, wherein
the first packet is the retransmission data packet,
and the second packet is the new data packet.
4. The wireless communication system according to claim
3, wherein
the packet transfer unit reads information about the
retransmission data packet from the retransmission buffer
unit in the first wireless base station device, and
transfers the information to the second wireless base
station device.
5. The wireless communication system according to any of
claims 1 through 4, further comprising
a control information communication unit to
communicate between the first wireless base station device
and the wireless terminal device control information about
a communication to the wireless terminal device by the
first wireless base station device and control information
about a communication to the wireless terminal device by
the second wireless base station device.
6. The wireless communication system according to claim
5, wherein
the control information communication unit performs:
a transmission of the control information from the
first wireless base station device to the wireless terminal
device through a physical: downlink control channel; and
a reception of the control information from the
wireless terminal device to the first wireless base station
device through a physical uplink control channel.
7. The wireless communication system according to claim
6, wherein
the physical uplink control channel includes at least
individual channel quality indication information for each
of the first wireless base station device and the second
wireless base station device, and precoding matrix
indication information and rank indication information
common to the first wireless base station device and the
second wireless base station device.
8. The wireless communication system according to claim 6
or 7, wherein
the physical downlink control channel includes at
least individual modulation and coding scheme information
and individual precoding information for each of the first
wireless base station device and the second wireless base
station device.
9. The wireless communication system according to any of
claims 1 through 8, wherein
the control information from the wireless terminal
device to the first wireless base station device includes
transmission status information indicating a reception
result of a packet from the first wireless base station
device and a reception result of a packet from the second
wireless base station device, respectively.
10. The wireless communication system according to any of
claims 1 through 9, wherein
the packet transfer unit transfers the communication
control information relating to the second wireless base
station device for communication from the first wireless
base station device to the wireless terminal device and
information relating to a transmission timing of the second
packet by the second wireless base station device.
11. The wireless communication system according to any of
claims 1 through 10, wherein
the first wireless base station device centrally
controls at least assignment of the wireless terminal
device, assignment of communication resources, and control
of transmission timing relating to the collaborative
transmission process.
12. A wireless terminal device which performs a
communication in a wireless communication system according
to any of claims 1 through 11, comprising:
a retransmission data packet reception unit to perform
a receiving process on the retransmission data packet when
the retransmit request is issued; and
a new data packet reception unit to perform, when the
retransmission data packet reception unit successfully
performs a receiving process on the retransmission data
packet, a successive interference cancellation process on a
received signal received by the wireless terminal device
through a retransmission data packet on which the receiving
process has been successfully performed, and perform the
receiving process on the new data packet according to a
resultant received signal.
13. The wireless terminal device according to claim 12,
further comprising
a collaborative transmission process determining unit
to determine whether or not the collaborative transmission
process is to be performed and determine the first wireless
base station device and the second wireless base station
device performing the process when the execution of the
collaborative transmission process is determined.
14. The wireless terminal device according to any of claim
12 or 13, wherein
the collaborative transmission process determining
unit makes the determination according to information about
reception power for a reference signal to be received from
each wireless base station device currently in
communication.
15. The base station device which performs a communication
in the wireless communication system according to any of
claims 1 through 11, comprising:
a first packet transmission unit to transmit to the
wireless terminal device as a first packet the new data
packet or a retransmission data packet corresponding to a
retransmit request when the base station device operates as
the first wireless base station device and when the
retransmit request to the collaborative transmission
process is issued in the wireless terminal device;
a packet transfer unit to transfer information
relating to a second packet different from the first packet
between the new data packet and the retransmission data
packet to the second wireless base station device when the
base station device operates as the first wireless base
station device, and when the retransmit request is issued;
and
a second packet transmission unit to transmit the
second packet according to information transferred from the
first wireless base station device by the packet transfer
unit to the wireless terminal device in synchronization
with the first packet by the first packet transmission unit
in the first wireless base station device when the base
station device operates as the second wireless base station
device, and when the retransmit request is issued.
16. A wireless communicating method in which a first
wireless base station device and a second wireless base
station device perform a collaborative transmission process
for realizing the process to allow the wireless terminal
device not to discard a packet on which decoding has failed
but to combine the packet with a retransmitted packet and
decode a resultant packet while controlling the
retransmission of the packet according to transmission
status information returned from the wireless terminal
device, comprising:
transmitting as a first packet a novel data packet or
a retransmission data packet corresponding to a retransmit
request from the first wireless base station device to the
wireless terminal device when the retransmit request is
issued to the collaborative transmission process by the
wireless terminal device;
transferring information about a second packet
different from the first packet between the novel data
packet and the retransmission data packet from the first
wireless base station device to the second wireless base
station device; and
transmitting the second packet according to
information transferred in the packet transferring step in
synchronization with a transmission process of the first
packet in the first packet transmitting step from the
second wireless base station device to the wireless
terminal device when the retransmit request is issued.
17. A wireless communication system in which a plurality
of wireless base station devices perform, a collaborative
transmission process on a wireless terminal device,
comprising:
the wireless terminal device including:
a control channel reception unit to receive a
control channel only from a first wireless base station
device; and
a data reception unit to receive data
collaboratively transmitted by at least the first wireless
base station device and the second wireless base station
device based on the received control channel.
18. The wireless communication system according to claim
17, wherein
the first wireless base station device is a serving
base station device for the wireless terminal device.
19. A wireless communication terminal device which
receives data from a plurality of wireless base station
devices in a collaborative transmission, comprising:
a control channel reception unit to receive a control
channel only from a first wireless base station device; and
a data reception unit to receive data collaboratively
transmitted by at least a first wireless base station
device and a second wireless base station device based on
the received control channel.
20. The wireless terminal device according to claim 10,
wherein
the first wireless base station device is a serving
base station device for the wireless terminal device.
21. A wireless communication method in which a plurality
of wireless base station devices perform, a collaborative
transmission process on a wireless terminal device,
comprising:
receiving a control channel only from a first wireless
base station device by the wireless terminal device; and
receiving by the wireless terminal device data
collaboratively transmitted by at least the first wireless
base station device and the second wireless base station
device based on the received control channel.
22. The wireless communication method according to claim
21, wherein
the first wireless base station device is a serving
base station device for the wireless terminal device.

In a transmission device on a serving eNB side, a
first packet transmission unit performs an operation of
transmitting a retransmission data packet. On the other
hand, in a transmission device on a collaborative eNB side,
a second packet transmission unit performs an operation of
transmitting a new data packet corresponding to information
transferred from the serving eNB by the packet transfer
unit. The control information about a communication to a
UE by the serving eNB and the collaborative eNB is
communicated by using only a PUCCH from the UE to the
serving eNB and a PDCCH from the serving eNB to the UE.
The serving eNB and the collaborative eNB perform
communications of a new data packet and communication
control information etc. through an X2 interface.