DPS and JT
DPS and JT
DPS and JT
RESEARCH
Open Access
2012 Maa ttanen et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
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coordination
coordination
Page 2 of 18
Coordination area
Joint transmission
Figure 2 Illustration of joint transmission where the user is
served simultaneously from two points.
coordination
Coordination area
Heterogeneous scenario
Figure 1 Illustration of a heterogenous network scenario with
three base-stations, each one connected by an interface to three
low-power nodes. Transmission is coordinated within sectors of one
base station as well as within its corresponding three low power
nodes.
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Coordinated beamforming,
Coordinated scheduling
Figure 4 Illustration of coordinated beamforming and
coordinated scheduling where the network coordinates beams
and scheduling to avoid interference (red arrow) to a the user.
Page 3 of 18
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2 System model
In this article, we consider the physical layer of LTEAdvanced downlink for FDD operation where the
transmission scheme is orthogonal frequency division
multiplexing (OFDM). In LTE-Advanced, the physical
resource blocks (PRB) are dened as groups of 12
consecutive subcarriers in frequency while the subframe/transmit time interval (TTI) duration is 1 ms which
consists of 14 OFDM symbols. Thus, the minimum timefrequency resource allocation is 12 subcarriers over 14
OFDM symbols. More details on bandwidths and subcarrier spacings, for example, can be found in [1,18]. As inter
symbol interference may be removed using a cyclic prex that is longer than the length of the channel impulse
response, we can consider the received signal per subcarrier in frequency domain. To simplify notation, we omit
the frequency and time domain indexing, and the signal
model reects subcarrier level spatial samples within one
multicarrier symbol, unless otherwise stated.
2.1 Signal model
Page 4 of 18
antennas and each user has Nr receive antennas. Stating the matrix dimensions of the variables beneath the
symbols, the signal yk received by the user k can be written
as
yk
Nr 1
Hk,i
Nr Nt
Wi
Nt rk
xi
rk 1
j =i
Hk,j
Nr Nt
Wj
Nt rj
xj
rj 1
nk ,
Nr 1
(1)
where Hk,i is the Nr Nt MIMO channel between the
serving base station i and user k, and nk denotes the
scaled noise vector whose entries are i.i.d. complex Gaus2
sian variables with zero mean and variance P , where 2
is the variance of additive white Gaussian noise and P is
the transmitted signal power. The precoding matrix Wi
applied for the transmission has rk columns, and rk is
the transmission rank for user k. The transmitted signal
xi is of length rk 1. Assuming spatially uncorrelated
and equal-variance transmit signal elements, we have
is conE(xi xH
i ) = Irk and the total transmission power
trolled by precoding matrix by requiring Tr WH
i Wi = 1.
Each element of xi , or each column of Wi , corresponds to
a transmission layer for user k. The matrices Hk,j , where
index j {1, . . . , M}, j = i, are the MIMO channels
between interfering transmission points and user k. The
interfering transmission points are transmitting rj layers,
where each signal vector xj is precoded by the precoding
matrix Wj , where index j {1, . . . , M}, j = i.
If the transmission points cooperate, the interference
conditions change. For example, a UE may be scheduled
to receive data from two points while the third point is
muted. Alternatively, a UE may be scheduled to receive
data only from one point, but one or more points coordinate scheduling or mute to reduce the interference. A
general signal model for the hybrid CoMP, where M is
the total number of interfering points and N M points
cooperate for user k, reads
yk =
L
Hk,l Wl xl +
n Hk,n Wn xn
n=NL+1
l=1
N
M
(2)
Hk,m Wm xm + nk .
m=MN+1
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The typical operation in LTE/LTE-Advanced is a singlecell operation which means that there is no cooperation
between the eNBs. A UE selects the serving cell on the
basis of received signal quality. In Release 10 LTE, dierent RSs are dened for channel estimation, namely CSI
reference symbols (CSI-RS) and demodulation reference
symbols (DM-RS). After cell selection, the eNB congures
the CSI-RS and DM-RS congurations for the UE. From
the CSI-RS conguration, the UE k measures the MIMO
channel Hk,i and calculates the CSI feedback. The DM-RS
is transmitted for demodulation purposes and enables the
UE to measure the eective channel Hk,i Wi .
The UE feedback consists of a wideband RI and a wideband or subband PMI and CQI. The CQI may be seen
as indicative of the post-processing SINR, i.e., the SINR
per stream after receiver processing. It is possible to have
less independently modulated and coded data streams Ns
than there are transmitted layers rk . In this case, one data
stream is transmitted on several layers. In LTE, the maximum number of independently modulated and coded data
streams Ns is two. This means that when the number
of transmission layers, or equally the transmission rank,
is higher than two, a so-called layer to codeword mapping procedure is applied [1]. In this context, a codeword
means a block of channel coded bits.
For the estimated MIMO channel, the UE selects a
(r )
precoding matrix Fk k of size Nt rk from a predened
codebook and feeds back the index, PMI, as a recommendation for the serving eNB for the precoder Wi . Note that
with these deliberately separate notations of Fk and Wi ,
we intend to point out that the precoder selection done by
the UE is only a recommendation towards eNB. For single
stream single-user transmission, the optimal choice for a
precoding vector fk for user k is known to be [19,20]
fk
Page 5 of 18
3 CoMP in LTE-Advanced
(3)
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The agreed CoMP work item targets specication of intraand inter-cell DL CoMP schemes operating in homogeneous and HetNet deployments [30]. Four main scenarios
have been studied so far
intra-site scenario where multiple co-located sectors
of the same eNB site are cooperating (Scenario 1),
illustrated in Figure 5,
inter-site scenario with high-power RRHs where
multiple non-co-located points having the same
transmit power are cooperating (Scenario 2),
illustrated in Figure 6,
low-power RRHs within the coverage of the
high-power macro cell, each operating its own cell ID
(Scenario 3), illustrated in Figure 1, and
low-power RRHs within the coverage of the
high-power macro cell, each operating with the same
cell ID (Scenario 4). In [31], Scenario 4 is discussed in
Page 6 of 18
coordination
coordination
coordination
Intra-sitecoordination
Figure 5 Illustration of intrasite coordination where
transmission is coordinated within sectors of one base station.
coordination
Inter-site coordination
Figure 6 Illustration of intersite coordination where all three
base stations are connected by ber and controlled by one
scheduling unit.
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that can be congured to a UE as two separate transmission points or as one virtual 4Tx transmission point.
In addition, a term CSI-RS resource is dened as a CSIRS conguration and an interference assumption, which
provides a CQI assumption.
For selecting the points forming the CoMP measurement set, an eNB can monitor the uplink signal received
powers, for example through sounding RSs. As multiple transmission points are connected to a centralized
CoMP scheduler that receives the sounding RSs, a classication can be made of the link qualities for the points
involved in a CoMP cluster. After this, the best two or
three points that are reliable for CoMP transmission are
selected. The reliability of a point is dened such that the
link power is within an X dB power window (usually of
56 dB) from the serving point link power. Alternatively,
the UEs may compute and report the received power
value of the CSI-RS, that is receiver power for the CSIRS transmission from points in CoMP cluster. The eNB
then selects the best points which are the most suitable for
CoMP transmission.
Page 7 of 18
Point 2
DPS
DPS
JT
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Point 2
Point 3
DPS
DPS
DPS
JT
JT
JT
JT
mean two CQIs for that scheme instead of one. In addition, CQI may be per subband. Hence, the rank utilization,
feedback frequency granularity, and the number of points
for which CSI feedback is computed are all factorizing the
overall feedback overhead that needs to be sent from the
receiver to the transmitter. In the following section, we
conduct further analysis of these topics.
The traditional tradeo between feedback load versus performance relates to the tradeo between network centric
and UE centric CoMP. The UE centric CoMP refers to
the operation where the UE selects the coordination set
and the preferable CoMP scheme based on channel and
interference measurements and sends the corresponding
feedback. The advantages are that because the UE has the
instantaneous knowledge on the downlink channel and
interference conditions, it may deduce the best CoMP
feedback for these conditions. Thus, feedback savings are
possible in principle because, for example, a UE could
send feedback only when the channel conditions are good
and only for specic CoMP schemes. From the network
perspective, the richer the feedback the scheduler entity
has, the better the expected network performance is. If
the network may receive information from every active
UE and it has, for example, information about the number of served UEs and achieved transmissions rates, it can
more eciently evaluate which CoMP schemes should
be applied. This could be benecial in enabling a exible balance between transmission methods to the users.
Thus, receiving feedback for multiple CoMP transmission hypothesis from one UE would be benecial. When
considering network centric CoMP, which is the commonly supported method, higher layer signaling should
be considered as well. This means that the CoMP operation can be designed either transparent to the UE meaning that the UE always feeds back certain CQIs based
on CSI-RS resources congured for it, or the UE may
be congured by higher protocol layers to calculate a
scheme-specic feedback.
Page 8 of 18
|gH
k
2
|gH
k Hk,i wi |
N
+ |gH
k
n=NL+1
n Hk,n Wn |2
M
m=MN+1
Hk,m Wm xm |2 + 2
(4)
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Page 9 of 18
Parameter
Scenario
UE speed
1 km/h
Tx point # of antennas
2, X-pol 45 deg
UE # of antennas
Measurement set
Scheduler
HARQ
2 retransmissions
CSI Estimation
Ideal
Receiver
MMSE
Codebook
3GPP 2 Tx [1]
Feedback
OLLA step-up/down
No delay, 19 /1 dB
cn = an ejn ,
the strongest point. Special care needs to be taken when
thinking about fallback/single-cell performance, because
the single-cell operation is performed also in CoMP eligible cells. A fallback point means that the serving point
and the corresponding feedback should be Release 10 specic. Release 10-specic CQI refers to the case where no
muting or other cooperation form is applied, that is n =
1, n. The importance of always feeding back the fallback
CQI is evaluated and illustrated in the results section.
(5)
2
1.9
1.8
1.7
CQIDPB
+CQIDPB
1
2
1.6
CQIDPS
+CQIDPS
1
2
CQIJT,aggr.
CQIDPB
+CQIDPS
1
2
1.5
1.4
1.3
1.2
1.1
10
15
20
TTI per user drop
25
30
Figure 7 Extended link performance of non-coherent JT with several dierent CQI feedback hypotheses as a function of a scheduled link
duration. OLLA mechanism corrects CQI mismatch at the transmitter.
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Page 10 of 18
2.5
2
CQIDPB
+CQIDPB
1
2
CQIDPS
+CQIDPS
1
2
CQIJT,aggr.
CQIDPB
+CQIDPS
1
2
1.5
10
15
20
TTI per user drop
25
30
Figure 8 Extended link performance of JT transmission with QPSK combiner and dierent CQI feedback hypotheses as a function of a
scheduled link duration. OLLA mechanism corrects CQI mismatch at the transmitter.
N
n=1
M
cn he
k,n xk +
he
k,m xm + nk ,
(6)
m=MN+1
where he
k,n = Hk,n wn is the precoded channel between
the kth user and nth transmission point. For the two
transmission points case, i.e., N = 2, optimal amplitude combiners an can be selected as in [35]. In practice, however, the power pooling between transmission
points is not possible, because total transmission power
at the transmission point cannot be exceeded due to system specications and regulatory issues. If the resources
at both transmission points have been scheduled to a
single user, it is from a user perspective always worth
0.35
no phase shift, 6PRB scheduled
cyclic BPSK shift per PRB, 6PRB scheduled
0.3
0.25
0.2
0.15
0.1
0.05
0
5
5
10
(CQI1+CQI2)/CQIJT [dB]
15
Figure 9 Cumulative density function of CQI mismatch with and without BPSK cyclical phase shift.
20
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Page 11 of 18
2.1
1.9
1.8
OLPA, no CPS
1.7
no OLPA, no CPS
no OLPA, BPSK CPS per PRB
1.6
1.5
1.4
10
15
TTI per drop
20
25
30
e j2
e
e
Re{he
k,1 hk,2 e } = |hk,1 hk,2 |.
0.35
0.3
6PRB allocated
24PRB allocated
0.25
0.2
0.15
0.1
0.05
0
5
5
(CQI1+CQI2)/CQIJT [dB]
10
Figure 11 Cumulative density function of CQI mismatch with 6/24PRB scheduled bandwidth.
15
(7)
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Value
Cellular layout
Trac model
Full buer
Deployment scenarios
Carrier frequency
2.00 GHz
Antenna conguration
Number of UEs
Transmission schemes
SU-MIMO with JT
SU-MIMO with DPS
UE receiver
3GPP option 1
UE Feedback
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2 TX-points
Reference symbol
overhead
Scheduler algorithm
PF
Interference modelling
OLLA
HARQ
Channel gain 1
H
e
2Re{he
k,1 c2 hk,2 }
constructive/destructive addition
2
|he
k,2 |
Channel gain 2
(9)
.
2
|he
k,1 | +
SINRk
DPB
= SINRDPB
k,1 + SINRk,2 + SINR ,
(10)
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Page 13 of 18
CQIk
10
, CQIRel
k,1
DPS
CQIDPS
k,1 , CQIk,2
DPB
CQIk,1 , CQIDPB
k,2
DPS
CQIDPB
,
CQI
k,1
k,2
Rel 10
DPB
CQIDPB
k,1 , CQIk,1 , CQIk,2
Primary point
CQI1
SJT, aggr.
S1
Iout +N+S2 , Iout +N
S1
Iout +N+S2
S1
Iout +N
S1
Iout +N+S2
S1
S1
Iout +N , Iout +N+S2
mismatch on the link performance, extended link simulations have been carried out under various CQI feedback
hypotheses. The main simulation assumptions are summarized in Table 3. The simulation procedure is as follows:
Four RRHs are dropped into every sector of the hexagonal macro network. The users are dropped non-uniformly
(Conguration 4b) into the middle site until a user satisfying the CoMP threshold is found. Network generation
and user dropping are according to Scenario 3/4 in [18].
The found CoMP user is scheduled in JT CoMP mode and
its feedback is computed. Finally, a pre-dened number of
TTIs is simulated while OLLA is employed.
Figure 7 shows the performance of the estimated CQI
for several settings of muting hypothesis. In the case that
the CQIDPB are fed back, performance suers only minor
degradation. A similar investigation has been run with
a QPSK combiner. Figure 8 shows that with the QPSK
combiner, the CQI mismatch can be kept even smaller
and the performance of CQIJT,aggr. can already be reached
within 20 iterations of OLLA algorithm. The CQI mismatch with CQIDPB feedback can be minimized by the
following approaches
1. Adapting the phase combiner (BPSK) with
outer-loop-phase-adaptation (OLPA);
2. Cyclical phase shift at the time of transmission,
random/cyclical phase of the combiner.
3. Scheduling of suciently large bandwidth, where the
SINR averages out due to frequency selective
channel.
While the rst approach always aims to keep the CQI
mismatch positive, the two other approaches aim at setting E(SINR ) = 0.
Figure 9 shows the impact of BPSK cyclical phase shift
per PRB on the CQI mismatch. A single frequency chunk
of six PRBs has been scheduled in a round-robin manner.
It can be seen signicant that the cyclical phase shift eciently averages out the above-mentioned CQI mismatch.
While the LTE standard allows the phase shift per PRB,
it might negatively impact the reliability of the dedicated
channel estimation.
Figure 10 shows the average throughputs as a function of simulated TTIs per user drop. Again a single
Cooperating point
CQI2
Remarks
Optimal for JT
S2
Iout +N+S1
S2
Iout +N
S2
Iout +N
S2
Iout +N+S2
Rel 10 CQIs
No correct fallback
Correct fallback
Feedback load increased
10
SU-MIMO: CQIRel
k,1
JT, aggr.
JT: CQIk
10
, CQIRel
k,1
DPS
JT: CQIDPS
k,1 , CQIk,2
DPB
JT: CQIDPB
k,1 , CQIk,2
DPB
JT: CQIk,1 , CQIDPS
k,2
Rel 10
DPB
JT: CQIDPB
,
CQI
k,1
k,1 , CQIk,2
Average
(bps/Hz/point)
Coverage
(bps/Hz/UE)
1.848 (0%)
0.0367 (0%)
1.830 (1.0%)
0.0406 (10.6%)
1.828 (1.1%)
0.0390 (6.3%)
1.820 (1.5%)
0.0336 (8.4%)
1.819 (1.6%)
0.0396 (7.9%)
1.817 (1.7%)
0.0389 (6.0%)
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Coverage
(bps/Hz/UE)
10
SU-MIMO: CQIRel
k,1
2.387 (0%)
0.0627 (0%)
JT, aggr.
10
, CQIRel
JT: CQIk
k,1
DPS
DPS
JT: CQIk,1 , CQIk,2
DPB
JT: CQIDPB
k,1 , CQIk,2
DPB
JT: CQIk,1 , CQIDPS
k,2
Rel 10
DPB
JT: CQIDPB
k,1 , CQIk,1 , CQIk,2
2.386 (0.0%)
0.0712 (13.6%)
2.378 (0.4%)
0.0651 (3.8%)
2.364 (1.0%)
0.0606 (3.3%)
2.371 (0.7%)
0.0682 (8.8%)
2.368 (0.8%)
0.0676 (7.8%)
Page 14 of 18
CDF
0.6
0.5
0.4
0.3
0.2
aggregated+fallback CQIs
2x muted+fallback CQIs
muted and nonmuted CQIs
0.1
0
4
2
OLLA Offset [dB]
Figure 12 Cumulative density function of OLLA oset with dierent CQI feedback hypothesis.
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Coverage
(bps/Hz/UE)
10
SU-MIMO: CQIRel
k,1
1.848 (0%)
0.0367 (0%)
JT, aggr.
Non-coherent JT: CQIk
JT, aggr.
JT with 1bit combiner: CQIk
JT, aggr.
JT with 2bit combiner: CQIk
JT, aggr.
JT with 4bit combiner: CQIk
1.830 (1.0%)
0.0406 (10.6%)
1.848 (0%)
0.0428 (16.6%)
1.856 (0.4%)
0.0433 (18.0%)
1.858 (0.5%)
0.0438 (19.3%)
Coverage
(bps/Hz/UE)
2.387 (0%)
0.0627 (0%)
2.386 (0.0%)
0.0712 (13.6%)
JT, aggr.
2.415 (1.2%)
0.0737 (17.5%)
JT, aggr.
2.428 (1.7%)
0.0739 (17.9%)
JT, aggr.
JT with 4bit combiner: CQIk
2.431 (1.8%)
0.0749 (19.5%)
JT, aggr.
Page 15 of 18
In addition to JT CoMP, other CoMP schemes are important in the LTE-Advanced evolution. In Tables 10 and
11, the performance of DPS CoMP and JT CoMP is
shown with dierent handover margins (HO). The handover margin biases the transmit point selection in the
simulation modeling, i.e., any of the potential serving
points providing the strongest links within the margin
according to the UEs measurements,may become the
Table 10 DPS and JT CoMP network performance in
HetNet Scenario 3 Conguration 1 with dierent handover
margins
10
SU-MIMO: CQIRel
k,1 , HO=0dB
JT, aggr.
JT: CQIk
HO=0dB
DPS
,
CQI
DPS: CQIDPS
k,1
k,2 , HO=0dB
Rel 10
SU-MIMO: CQIk,1 , HO=3dB
JT, aggr.
JT: CQIk
HO=3dB
DPS
DPS: CQIk,1 , CQIDPS
k,2 , HO=3dB
Average
(bps/Hz/point)
Coverage
(bps/Hz/UE)
1.848 (0%)
0.0367 (0%)
1.830 (1.0%)
0.0406 (10.6%)
1.821 (1.5%)
0.0426 (16.1%)
1.830 (1.0%)
0.0292 (20.4%)
1.812 (1.9%)
0.0355 (3.3%)
1.814 (1.8%)
0.0374 (1.9%)
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10
SU-MIMO: CQIRel
k,1 , HO=0dB
JT, aggr.
JT: CQIk
HO = 0 dB
DPS
DPS: CQIDPS
k,1 , CQIk,2 , HO = 0 dB
Rel 10
SU-MIMO: CQIk,1 , HO = 3 dB
JT, aggr.
JT: CQIk
HO=3dB
DPS
,
CQI
DPS: CQIDPS
k,1
k,2 , HO=3dB
Average
(bps/Hz/point)
Coverage
(bps/Hz/UE)
2.387 (0%)
0.0627 (0%)
2.386 (0.0%)
0.0712 (13.6%)
2.369 (0.6%)
0.0684 (9.1%)
2.375 (0.5%)
0.0508 (19.0%)
2.376 (0.5%)
0.0641 (2.2%)
2.360 (1.1%)
0.0641 (2.2%)
Page 16 of 18
1
Single Cell
DPS
JT
0.9
0.8
0.7
CDF
0.6
0.5
0.4
0.3
0.2
0.1
0
10
0
5
10
Subcarrier C/I [dB]
15
20
Figure 13 Cumulative density function of SINR with single-cell (point) transmission and two dierent multi-point schemes.
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6 Conclusions
In this article, we have addressed the problem of the
feedback design and studied the associated link-level performance and the realistic system-level performance of
CoMP in LTE-Advanced. We have studied practical niterate CSI feedback and CoMP feedback design, namely
PMI and CQI feedback, for dierent CoMP modes, and
also evaluated the associated performance with both linklevel and system-level simulations. The realistic systemlevel evaluations of LTE-Advanced CoMP were performed
for dierent CoMP modes and for dierent practical
deployment scenarios. These simulation results indicate
that CoMP can provide considerable cell-edge gains over
the baseline Release 10 system with realistic UE feedback.
The results that are obtained and reported in this study
also indicate that the nature of the deployment scenario
has a clear impact on the relative performance of JT and
DPS type CoMP schemes. Relatively simple DPS schemes
can outperform JT schemes in heterogeneous networks
when the user distribution is not uniform but concentrated around the coverage area of the RRHs. When studying the CoMP schemes under biased handover conditions,
it was observed that the DPS CoMP scheme can clearly
aid in the mobility management of real networks. This is a
very important practical benet, in addition to improved
cell edge performance, in cellular mobile radio systems.
Competing interests
The authors declare that, they have no competing interests.
Author details
1 Renesas Mobile Europe Ltd., Porkkalankatu 24, 00180 Helsinki, Finland.
2 Department of Communications Engineering, Tampere University of
Technology, Korkeakoulunkatu 1, FI-33720 Tampere, Finland. 3 Department of
Signal Processing and Acoustics, Aalto University, P.O. Box 13000, FI-00076
Aalto, Finland.
Received: 22 June 2012 Accepted: 16 October 2012
Published: 27 November 2012
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Cite this article as: Maa ttanen et al.: System-level performance of LTEAdvanced with joint transmission and dynamic point selection schemes.
EURASIP Journal on Advances in Signal Processing 2012 2012:247.