Outer Bounds of Spatial Degrees of Freedom for
MIMO Interference Channel without CSIT and with
only Local CSIT
Shixin Peng, Desheng Wang and Yingzhuang Liu
Department of Electronics and Information Engineering
Huazhong University of Science & Technology
Wuhan, China 430074
Email: psx6050@gmail.com, dswang@mail.hust.edu.cn, liuyz@mail.hust.edu.cn
Abstract—This paper discusses characteristics of spatial de-
grees of freedom for K-user symmetric MIMO interference chan-
nel. We assume that the interference channel is equipped with M
t
antennas at each transmitter and N
r
antennas at each receiver
and there is no channel extension over time and frequency. Two
outer bounds are obtained for total spatial degrees of freedom of
the interference channel with no channel state information(CSI)
and only local CSI available at the transmitters respectively.
Index Terms—Channel state information (CSI), degrees of free-
dom (DoFs), interference channel (IC), multiple-input-multiple-
output (MIMO), isotropic fading.
I. INTRODUCTION
The characterization of capacity of Gaussian interference
channel has been explored from an information theoretic
perspective for more than thirty years. However, it is still
challenging to understand information theoretic capacity of
Gaussian interference channel. Instead, the degrees of free-
dom (DoF) as an alternative to approximate the capacity of
interference channel in the high SNR regime is widely used
to represent the available interference-free signal dimensions
in communication link and can be defined as
DoF = lim
SNR→∞
C
Σ
(SNR)
log SNR
. (1)
where C
Σ
(SNR) is the ergodic sum capacity at signal-to-
noise rate. Reference [8] showed the exact number of spatial
DoF for a two user nondegenerate (full rank channel matrices)
MIMO Gaussian interference channel with M
1
, M
2
antennas
at transmitters 1, 2 and N
1
, N
2
antennas at the corresponding
receivers, and perfect channel knowledge at all transmitters
and receivers, was min{M
1
+ M
2
,N
1
+ N
2
, max(M
1
,N
2
),
max(M
2
,N
1
)}. For the K-user interference channel, in the
case of independently faded parallel channels (i.e. time or
frequency selective), it was shown in [1] that up to
K
2
DoF
was achievable, which means each user can achieve one half
of the total DoF free from interference no matter how many
of other users shared the wireless medium. In [2], authors
characterized DoF of the cellular network when the channel
coefficients are time- or frequency-varying. However, these
exciting results depend critically on the assumption that global
channel state information (CSI) is available at the transmit-
ters, which implies excessive training and an overwhelming
feedback overhead in practice. In addition, the conclusion in
[1] is built on the condition that the number of independently
faded parallel channel, i.e. the channel extensions (over time or
frequency), is unbounded. A realistic communication always
tries to avoid great amount of feedback load and channel
extension due to the limitation of available wireless channel
resource. Therefore, The requirements with global channel
state information at the transmitters (CSIT) and infinite chan-
nel extension restrain further application of the interference
alignment scheme.
Recently, some related work has been developed to consider
the DoF of MIMO interference channel in absence of CSIT. In
[6], authors provided a DoF region outer bound for a 2-user
MIMO interference channel on the assumption of no CSIT,
which was shown to be tight for all possible combination of
the number of antennas at each node except for one special
class of channels where the DoF region remain unknown. They
argued that the loss of DoF due to lack of CSIT depended on
the relative magnitudes of the number of antenna at each node.
The complete DoF region of 2-user interference channel in
absence of CSIT was determined in [3], [5]. However, for K-
user (K>2) MIMO interference channel, the DoF of network
in absence of CSIT is still an open problem.
In this paper, we firstly provide an outer bound in theorem
3.1 for total spatial DoF of the K-user symmetric MIMO
isotropic fading interference channel when each transmitter
don’t know any CSI. The proof of the outer bound is based on
isotropic fading characteristic of interference channel. Then we
develop a MIMO multiple-access channel(MAC) outer bound
on the total spatial DoF for the K-user symmetric MIMO
interference channel by utilizing the concept of genie message
sets and degraded received signal when only local CSI is
available at the transmitters. In the proof of the second outer
bound, a degraded received signal with degraded noise will
firstly be assumed to assure that the corresponding receiver
can correctly decode its intended signal with sufficiently high
probability. Then we suppose that genies provide some of re-
ceivers with side information containing the entire codewords
978-1-4673-6349-5/13/$31.00 ©2013 IEEE
IEEE 2013 Tencon - Spring
308