FENG et al.: POWER SCALING OF FD TWO-WAY MASSIVE MIMO RELAY SYSTEMS WITH CORRELATED ANTENNAS AND MRC/MRT PROCESSING 4741
y
S
i
(n) =
√
P
S
√
M
u
H
RS
i
FH
S
¯
i
R
s
¯
i
(n − δ)
1
+
√
P
S
√
M
u
H
RS
i
F
2K
j=i,
¯
i
H
S
j
R
s
j
(n − δ)
2
+
j∈
i
/i
√
P
S
√
M
u
H
S
j
S
i
s
j
(n)
3
+ H
RS
i
FH
RR
x(n − δ)
4
+
√
P
S
√
M
u
H
S
i
S
i
s
i
(n)
5
+H
RS
i
Fn
R
(n − δ)
6
+n
S
i
(n)
7
. (5)
length of the training sequences scales linearly with the
number of antennas at the users, but not with the number
of massive antennas at the relay. When the training sequences
are sufficient long, the accuracy of channel estimation is very
high and channel estimation errors can be ignored. Thus here
we assume both the users and the relay have the perfect cross
CSI, i.e., H
S
i
R
and H
RS
i
.
After the self-interference cancellation, the received signal
at the user node S
i
becomes (5), shown at the top of this page.
In (5), the first term
1
is the desired received signal,
the second term
2
is inter-pair interference, the third term
3
is intra-group interference, the fourth term
4
is the propagated
residual self-loop interference from the relay node, the fifth
term
5
is the residual self-loop interference at the user node
S
i
, the sixth term
6
is the propagated noise from the relay,
and the last term
7
is the noise at the user node S
i
.
B. Channel Model
Due to insufficient antenna spacing and scattering, spatial
correlation among antennas generally occurs and is considered
in this paper. Following the Kronecker model [20], [30],
the channels can be generally expressed as
H
AB
= R
1
2
B
W
AB
T
1
2
A
, A, B = R, S
i
, S
j
. (6)
In (6), W
AB
consists of independent and identically distrib-
uted (i.i.d.) Gaussian random entries with zero mean. When
A = B and neither A nor B equals to R, W
AB
is associated
with the channel from the user A to the user B and the variance
of its elements is μ
2
.WhenA = B and one of A and B equals
to R, W
AB
is associated with the cross channel between users
and the relay and the variance of its elements is γ
2
, while
when A = B, W
AB
is corresponding to the self-loop channel
at the node and the variance of its elements is α
2
.Onthe
other hand, R
B
and T
A
are deterministic nonnegative definite
matrices, characterizing the receiving antenna correlation at
the node B and the transmitting antenna correlation at the
node A, respectively. Without loss of generality, we assume
the correlation matrices R
B
and T
A
are normalized such
that tr(R
S
i
) = N
u
,tr(T
S
i
) = M
u
,fori = 1, 2, ···, 2K ,
and tr(R
R
) = N
R
,tr(T
R
) = M
R
. Moreover, the relay’s
antenna correlation matrices R
R
and T
R
have uniformly
bounded spectral norms, i.e., lim sup
N
R
R
R
≤R < ∞
and lim sup
M
R
T
R
≤T < ∞ [31]. These assumptions
are practical and have been widely adopted in [32] and [33].
Notice that the Kronecker model in (6) is valid for MIMO
channels with arbitrary number of antennas when there is a
physical separation between the transmitting antennas and the
receiving antennas as shown in [20], [30], and [34]. Since we
assume different sets of antennas are used for transmission
and reception respectively at each node, i.e., the transmitting
antennas are physically separated from the receiving antennas
at each node, the Kronecker model is also applicable to the
self-loop channel [20].
C. Amplifying Matrix F
In the considered system, the relay amplifies the received
signal and forwards it to all users simultaneously. One popular
AF scheme with low complexity, i.e., maximum ratio combin-
ing and maximum ratio transmission (MRC/MRT), is adopted
here. Notice that the amplifying matrix F can also be designed
based on other criteria, e.g., zero-forcing and minimum mean
square error, etc.. The analysis of other schemes can be easily
extended from our work. With the CSI corresponding to
the links between the users and the relay, H
RS
i
and H
S
i
R
,
i = 1, 2, ···, 2K , the MRC/MRT amplifying matrix is
designed as
F = β
F, (7)
where β is the amplification factor which is set to satisfy the
relay’s transmission power constraint tr(E(x(n)x
†
(n))) = P
R
and
F is given as [15]
F = H
H
H
†
BC
PH
H
H
†
MAC
. (8)
In (8), H
H
H
BC
=
H
T
RS
1
H
T
RS
2
... H
T
RS
2K
T
∈ C
(2KN
u
)×M
R
denotes the broadcast channel (BC) response from the relay
to all users, while H
H
H
MAC
=
H
S
1
R
H
S
2
R
... H
S
2K
R
∈
C
N
R
× (2KM
u
)
denotes the multiple access channel (MAC)
response from users to the relay node, P is a block permutation
matrix as P = diag(P
1
, P
2
,...,P
K
),inwhichP
k
has the form
of
P
k
=
0
N
u
,M
u
P
2k−1
P
2k
0
N
u
,M
u
(9)
where
P
2k−1
,
P
2k
∈ C
N
u
×M
u
are general permutation matri-
ces with binary elements and characterize the information
exchange between the kth pair of users (2k − 1, 2k).
With (1), (2), and the knowledge of cross CSI (i.e., H
RS
i
and
H
S
i
R
), the relay’s power constraint tr(E(x(n)x
†
(n))) = P
R
can