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• Mobile data traffic volume keeps growing.
• Available radio spectrum is limited.
Coordinated MIMO (Co-MIMO) increases the system-level spectral efficiency (SE).
• Performance can be further increased by more antennas per cooperating node (e.g. RRH).
• However, cost and power constraints limit the number of elements on conventional arrays.
(E.g. each antenna is typically fed by its own radio frequency (RF) module.)
Load-controlled parasitic antenna arrays (LC-PAA) offer an attractive alternative!
Introduction
Coordinated MIMO and Cloud RAN / RRHs
Transmission Protocol
• Learning phase: For each beam combination, each BS transmits a pilot signal. Then, the UTs
estimate the direct and cross channels and report to their BS the channel estimates.
• Beam selection phase: Based on this information, the BSs select jointly the optimum beam
combination, in terms of the achieved sum-rate (SR) throughput.
• Precoding / transmission phase: The BSs transmit precoded symbols over the selected
beams.
Degrees of Freedom of LC-PAAs
• We consider single-fed LC-PAAs with M elements (i.e., 1 active and M − 1 passive
antennas), were each parasitic element terminates to a load with purely imaginary impedance.
• The degrees-of-freedom (DoF) provided by this antenna system are given by:
DoF =
M − 1
2
+ 1
Sum-Rate Capacity Bounds of LC-PAAs
• Low-SNR: C ≈ DoF ×SNR×log
2
e.
• High-SNR: C ≈ K ×log
2
DoF ×SNR .
A THENS INFORMATION TECHNOLOGY (AIT)
K ONSTANTINOS NTOUGIAS, DIMITRIOS NTAIKOS, CONSTANTINOS B. P APADIAS
R OBUST MULTI-CELL PRECODING OVER L OAD-C ONTROLLED
BEAMS WITH SINGLE-FED ANTENNA ARRAYS
Using load-controlled antennas for Cooperative MIMO
• LC-PAAs use less RF chains than antennas.
‒ One or a few active elements are surrounded by closely-spaced passive elements.
‒ Parasitic elements are terminated via tunable loads.
‒ By adjusting load impedances, we control the mutual coupling between the antennas.
‒ Thus, we can shape and steer radiation beam patterns as desired.
• Have been shown to achieve beamforming (BF), transmit diversity (Tx Div), open-loop /
closed-loop MIMO (OL-/CL-MIMO), and multi-user (MU) precoding via load control.
• However, attaining arbitrary solutions for the loads is not always possible.
‒ For instance, the spatial multiplexing (SM) solutions depend on the input signal.
‒ Also, the desired solutions may require loads that result in system instability.
‒ Finally, channel-based multi-user precoding with LC-PAAs is virtually unexplored.
Our divide-and-conquer approach: “First form the beams, than precode over them.”
• Yet load circuit / tuning limitations discourages the shaping of arbitrary / optimal beams.
Beam switching: “Switch between different sets of fixed loads (predetermined beams).”
Figure 1: A beam generated by a single-RF printed Yagi array with 7 parasitic elements.
System Setup
• We assume a single cooperation cluster comprising K cells (RRHs) with one active user each.
• Each RRH is equipped with a single-fed LC-PAA of 1 active and M−1 passive elements.
• Each user utilizes a terminal with a single, omni-directional antenna.
• At each timeslot, each RRH can generate 1 out of L predetermined beams.
‒ K beams selected out of L
possible combinations in total.
• CSI is shared between the BSs and single-user decoding is used at the user terminals (UT).
Figure 2: System setup for K = 2, L = 4 and M = 5.
• Coordinated MIMO relies on the cooperation between neighboring nodes.
‒ The cooperating base stations (BS) exchange control information and channel state
information (CSI) and /or user data to control the inter-cell interference (ICI).
‒ The various flavors of Co-MIMO differ in terms of their performance vs. backhaul
capacity and delay requirements tradeoff.
• The cloud radio access network (Cloud-RAN) architecture facilitates the application of Co-
MIMO.
‒ Centralized pools of virtual base-band units (BBU) are connected to remote radio heads
(RRH) that are located at the cell sites through optical transport systems.
Beam Selection Criterion
• BS
computes for each beam combination l =1,…, L
the channel vector h
∈ℂ
×1
which
is comprised of the direct channel UT
-BS
, h
, and the cross channels UT
-BS
, h
,
k,m = 1, … , K, m ≠ k.
• Then, the BSs exchange these channel vectors and form the composite channel matrix H
∈
ℂ
×
whose columns are h
†
.
• Next, the performance metric T
= tr H
H
†
−1
is calculated.
• Finally, the beam combination l that results in the minimum value of T
is selected.
System and Signal Models
• System model:
y = Hx + n = HWP
12
s + n
• Multi-cell zero-forcing (ZF) precoding: Low complexity and good performance at high
signal-to-noise-ratio (SNR):
h
†
w
2
=0⇒ F = H
+
= H
†
HH
†
−1
, W =
F :,k
F :,k
.
Power Allocation
• SR throughput maximization problem:
max
>0
R =
=1
R
=
=1
log
2
1 + SINR
=
=1
log
2
1 + h
†
w
2
p
s.t. p
≤ P (per-BS power constraints)
• Water-filling solution:
p
= v
−
1
h
+
Water level
Noise power
Effective channel after precoding
Performance Evaluation
Figure 3: Numerical simulation results for various beamwidths over a channel with 5 scatterers.
Conclusion
Precoding over switched LC-PAAs is a highly promising approach for Co-MIMO networks with
small access points (such as RRHs) that need to employ more antennas than RF chains.
[1] D. Ntaikos, B. Gizas, C. B. Papadias, L. Roullet, F. Taburet, “Over -the-air demonstration for RRH-based LTE access with the use of
parasitic antenna arrays: Results from the FP7 project HARP ,” Eur. Conf. on Netw. and Comm. (EuCNC), Paris, France, June 29 – July
2, 2015.
[2] K. Ntougias, D. Ntaikos, C. B. Papadias, “Robust Low-Complexity Arbitrary User- and Symbol-Level Multi-Cell Precoding with
Single-Fed Load-Controlled Parasitic Antenna Arrays,” 23
rd
IEEE Int. Conf. on Telecomm. (ICT), Thessaloniki, Greece, May 16 – 18,
2016.
[3] K. Ntougias, D. Ntaikos, C. B. Papadias, “Coordinated MIMO with Single-Fed Load-Controlled Parasitic Antenna Arrays”, 17
th
IEEE Int. Workshop on Sig. Proc. Adv. in Wireless Comm. (SPAWC), Edinburgh, UK, July 3 – 6, 2016.
{kontou, dint, cpap}@ait.gr
Coordinated MIMO and Cloud RAN / RRHs
Objectives
• In previous works, we have studied various multi-cell user- and symbol-level precoding
schemes and low-feedback techniques and compared their performance with the one
accomplished in equivalent setups that utilize conventional antennas (see [1-3]).
• Here we are interested in the analysis of the approach, especially in asymptotic regimes.
References