15-10-2012, 01:12 PM
Long-Term Channel Information-Based CoMP Beamforming in LTE-Advanced Systems
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Abstract
Coordinated multi-point (CoMP) transmission and
reception is a network multiple-input multiple-output (MIMO)
technology considered in 3GPP LTE-Advanced systems. In order
to improve reliability and capacity of the services for the user
equipments (UEs) at the cell edges, CoMP utilizes cooperation
among neighboring enhanced node Bs (eNBs). Accordingly,
backhaul delay for sharing control signals among eNBs should
be carefully handled as UE mobility increases in a fading
channel environment; otherwise, CoMP operations derived from
inaccurate channel state information (CSI) of neighboring eNBs
can severely degrade the system performance. We propose CoMP
beamforming schemes using long-term CSI such as spatial
correlation matrices instead of instantaneous CSI of neighboring
cells. Since long-term CSI varies relatively slowly, the proposed
scheme is inherently robust even when a UE moves at a high
speed and/or the backhaul delay is large. Using a multi-cell
simulator which reects realistic CoMP system environments,
the performance gain of the proposed schemes is evaluated and
discussed.
INTRODUCTION
Coordinated multi-point (CoMP) transmission and reception
is a network multiple-input multiple-output (MIMO) transmission
technology being considered for the emerging 3GPP
LTE-Advanced standard [1], which is a promising 4G cellular
standard being developed by 3GPP standardization group.
CoMP allows the cooperation among neighboring cells to
improve coverage, cell-edge capacity, and/or system efciency.
For downlink CoMP, two different approaches are being
considered, namely, joint processing (JP) and coordinated
scheduling/beamforming (CS/CB), and their conceptual diagrams
of the CoMP architecture are illustrated in Fig. 1 [1].
With JP, signals designated for a single user equipment (UE)
are simultaneously transmitted from multiple neighboring base
stations, referred to as enhanced node Bs (eNBs) in 3GPP.
These eNBs cooperate in order to work as a single transmitter
with geographically separated transmit antennas.
COMP ARCHITECTURE
We consider an LTE system with a number of eNBs, which
are equipped with multiple antennas and are connected through
a wireline backhaul. Note that the LTE standard denes the
interfaces between eNBs, e.g., X2 interface or interface based
on Internet protocol (IP) [1, 6], and this results in notable
delays for exchange of control signals between eNBs in
practice. A distance-based association rule is assumed in this
paper, so that mobile UEs are associated with the closest eNB,
while it can easily be extended to the other association rules
like a received SINR-based association. Due to the mobility,
the channel state of a UE varies over time according to the
fast fading. In typical single-cell MIMO operations, this timevarying
CSI is directly fed back to the associated eNB. In
addition, the CSI also needs to be shared among cooperating
eNBs for CoMP operations.
For CoMP, we basically assume that a UE estimates the
downlink channel from each cooperating eNB to the UE, and
feeds back the CSI to the associated eNB, which is referred to
as the serving eNB for the UE. Then the serving eNB relays the
received CSI to the neighboring eNBs through the backhaul,
and we refer to such neighboring eNBs that cooperate with
the serving eNB as aiding eNBs.
SIMULATION RESULTS
In this section, we evaluate the performance of the CoMP
CB schemes presented in Section III via computer simulations.
We use MATLAB to build a simulator which incorporates
realistic channel and system environments of 3GPP LTEAdvanced
systems. Three eNBs connected through a wired
backhaul are considered, and one UE is associated with each
eNB. UEs are located along with the line connecting the
origin and the serving eNB, as depicted in Fig. 4. Here, the
origin denotes the equidistant point from three eNBs. In all
three cells, the distances between the UE and the origin are
set to the same. The number of antennas of eNBs and UEs
are set to four and one, respectively. We consider orthogonal
frequency division multiplexing (OFDM) channel with 60 data
subcarriers, which are assumed to have a frequency correlation
dened by [10]. For a MIMO spatial channel model (SCM),
a ray-based model considered in IEEE 802.16m evaluation
methodology document (EMD) is employed [11].
CONCLUSION
In this paper, we propose beamforming schemes for CoMP
CS/CB using long-term CSI for 3GPP LTE-Advanced systems.
Using a MATLAB based CoMP simulator implementing
realistic 3GPP LTE-Advanced system environments, the
performance gain of the proposed schemes is demonstrated
and discussed. The utilization of long-term CSI is shown
to make CoMP CS/CB schemes robust to the UE mobility
and backhaul delay. Especially, the proposed SIS-BF scheme,
exploiting the long-term CSI of the aiding cells while utilizing
the instantaneous CSI of the serving cell, is found to have a
remarkable performance gain in many meaningful operational
regions. As a future work, we plan to study scheduling
algorithms for CoMP CS/CB that incorporate SIS-BF schemes.
and develop beamforming schemes for JP utilizing long-term
CSI.