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Full detail has been published in CAMSAP 2009 conference.
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Tadilo Endeshaw, Batu Chalise and Luc Vandendorpe
Université catholique de Louvain (Belgium)
12-Jun-14 1 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Presentation Outline Motivation of establishing MSE duality
MSE duality under imperfect CSI
As an application example of MSE duality under imperfect CSI, we examine
Robust sum MSE minimization problem Proposed duality based iterative solution (alternating
optimization)
Simulation results
Conclusions
12-Jun-14 2 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
12-Jun-14 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09) 3
K21 B,,B,BB
T
T
K
T
2
T
1 n,,n,nn
0,1~d NC
1S
kkd C
Motivation of MSE duality Consider the following downlink system model
)(W K21 W,,W,W blkdiag kk SM
kW CkSN
kB C
Motivation of MSE duality cont’d For the above downlink system model, the
instantaneous mean square error (MSE) between and is given by
Assume we are interested to solve the following problem
12-Jun-14 4 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
k
^
d
kd
Direct treatment of the above problem has
Complicated mathematical structure.
Difficult to examine.
Now, let us also see the following uplink system model
12-Jun-14 5 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Motivation of MSE duality cont’d
K21 T,,T,TT
)(V K21 V,,V,V blkdiag
K21 H,,H,HH
12-Jun-14 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09) 6
Motivation of MSE duality cont’d
The above uplink problem has - Simple mathematical structure. - Global optimal solution.
For any given , if we can get proper scaling factors , such that
(or any other combination)
we conclude that, global optimal solution of the
downlink problem is guaranteed.
12-Jun-14 7 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Motivation of MSE duality cont’d
Such approach of solving the downlink problem is called duality based approach
Thus, duality based approach of solving the downlink problem has two benefits
Simple mathematical structure.
Exploit the hidden convexity of the downlink problem.
Existing work on duality based approach for solving the downlink problem assume that perfect CSI is available at the BS and MSs.
12-Jun-14 8 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Motivation of MSE duality cont’d
MSE duality under imperfect CSI
In this work we establish three kinds of MSE dualities. Namely:
Sum MSE duality
User wise MSE duality and
Symbol wise MSE duality
when imperfect CSI is available at the BS and MSs.
Then, as an application example we examine the robust sum MSE minimization problem
Utilize Bayesian robust design approach
12-Jun-14 9 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Channel modeling
Considering antenna correlation at the BS, we model the Rayleigh fading channel as
When MMSE channel estimation is employed at the MSs,
can be expressed as where is the estimated channel and
- We establish MSE duality for any
- Then, we solve the following robust design problem.
where is the kth user AMSE.
12-Jun-14 10 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
AMSE transfer from uplink to downlink Sum AMSE transfer: The sum AMSE of the uplink and
downlink channels are given by
If we choose , with
we can achieve
12-Jun-14 11 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
AMSE transfer from uplink to downlink Like the above transformations, we can also transfer
the kth user and lth symbol AMSEs from downlink channel to uplink channel.
By similar approach, we can transfer the AMSE from downlink to uplink channel.
12-Jun-14 12 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Application example
Application example cont’d
12-Jun-14 13 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
For convenience, consider the problem in the following 2 cases
Case 1: When
Case 2: For any
Case 1: In such a case, the robust sum MSE
minimization problem in the uplink channel can be formulated as a semi-definite programming (SDP) problem for which
Global optimum is guaranteed.
Consequently, global optimum of the original downlink problem is guaranteed by using our sum AMSE transfer.
12-Jun-14 14 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Case 2: The robust problem cannot be formulated as an SDP problem. Thus, the solution method discussed for Case 1 cannot be applied here. Hence, we propose the alternating optimization technique. To do this we decompose the precoders and decoders as
Thus, the new equivalent uplink and downlink system models become
Application example cont’d
12-Jun-14 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09) 15
Application example cont’d
By collecting the powers and filter matrices as
where are the filters for the lth symbol with , the AMSE of the lth symbol in the uplink channel can be written as
12-Jun-14 16 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Application example cont’d
where
12-Jun-14 17 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
and f(i) is the smallest k, s.t, . For fixed
, the power allocation part of the
robust sum MSE minimization problem is expressed as
Application example cont’d
12-Jun-14 18 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Since is a Posynomial, the above optimization problem is a Geometric programming (GP), for which
- Global optimal solution is guaranteed.
- Solved with a worst-case Polynomial time complexity.
Application example cont’d
12-Jun-14 19 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Thus, our alternating optimization is performed as
follows
Uplink channel
- First we get optimal Q by solving the GP problem.
- With optimal Q of the GP, are updated by
MMSE receiver
Downlink channel
- Now, we first ensure the same performance as the
uplink channel by using the sum AMSE transfer (i.e., uplink
to downlink channel). This is achieved by choosing
and
Application example cont’d
12-Jun-14 20 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
- With the optimal , are
updated by MMSE receiver.
Uplink channel
- First we ensure
- With the optimal , Update by
MMSE receiver.
Application example cont’d
Simulation result (For case I)
Comparison of GM and Alg I, K=2, N=4 and =2
K : # of users N : # of BS antennas : user k’s # of antennas Alg I: The proposed alternating optimization
12-Jun-14 21 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
kM
kM
We model Rc as
12-Jun-14 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09) 22
Simulation result (For case II)
Comparison of robust and naive designs K=2, N=4 and = 2 kM
Conclusions In this work we establish 3 types of
MSE duality under imperfect CSI.
As an application example robust sum MSE minimization
Our robust design has better performance than the non-robust/naive design.
Large antenna correlation factor further increases the sum AMSE of the downlink system.
12-Jun-14 23 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)
Thank You!
12-Jun-14 24 Tadilo Endeshaw, Université catholique de Louvain, Belgium (CAMSAP 09)