Vanlin, S and Tamma, Bheemarjuna Reddy
(2016)
On Improving Data Rates of Users in LTE
HetNets.
PhD thesis, Indian institute of technology Hyderabad.
Abstract
The proliferation of smartphones and tablets has led to huge demand for data
services over cellular networks. Cisco VNI mobile forecast (2014-2019) tells that although only 3.9% of mobile connections were Long Term Evolution (LTE) based they
accounted for 40% of the mobile traffic and this will rise to 51% by 2019, by which
the mobile data usage will grow 11 fold to over 15 Exabytes per month. Reports by
Cisco and Huawei tell that 70% of the traffic is generated in indoor environments
such as homes, enterprise buildings and hotspots. Hence, it is very important for
mobile operators to improve coverage and capacity of indoor environments. Indoor
data demand is partly met by intensifying the deployment of Macro Base Stations
(MBSs/eNodeBs) in LTE cellular networks. Owing to many obstacles in the communication path between MBS and users inside the building, radio signals attenuate at a
faster rate as the distance increases. Thus, Indoor User Equipments (IUEs) receive
still low signal strength ( i.e., Signal-to-Noise Ratio, SNR) compared to Outdoor
User Equipments (OUEs). To address this problem, one can deploy a large number
of Low Power Nodes (LPNs) a.k.a. small cells (e.g., Picos and Femtos) under an
umbrella MBS coverage and thereby form an LTE Heterogeneous Network (HetNet).
Small cells are mainly being deployed in homes, enterprise buildings and hotspots
like shopping malls and airports to improve indoor coverage and data rates. This is
a win-win situation as telecom operators also benefit by reduction in their CAPEX
and OPEX.
Though the deployment of Femtocells improves indoor data rates, the resulting
LTE HetNet may face a host of problems like co-tier and cross-tier interference (due
to frequency reuse one in LTE) and frequent handovers (due to short coverage areas of
Femtocells). Deployment of Femtos inside a building can lead to signal leakage at the
edges/corners of the buildings. This causes cross-tier interference and degrades the
performance of OUEs in High Interference Zone (HIZone) around the building area,
which are connected to one of the MBSs in the LTE HetNet. Arbitrary placement of
Femtos can lead to high co-channel cross-tier interference among Femtos and Macro
BSs and coverage holes inside buildings. If Femtos are placed without power control,
this leads to high power consumption and high inter-cell interference in large scale deployments. Our goal is to address these problems by developing efficient architecture,
Femto placement and power control schemes in LTE HetNets.
Random or unplanned placement of the Femtos leads to poor SNR and hence
affects achievable data rates of IUEs. Hence, placement of Femtos is important for
the cellular operators to perform planned deployment of minimum number of Femtos
with no coverage holes and guarantee a good signal quality with no co-tier interference. Once the placement of Femtos is done optimally in enterprise environments,
operators need to ensure that traffic load is evenly distributed among neighboring
Femtos for improving Quality of Service (QoS) of IUEs by efficiently utilizing the
network resources. In traditional cellular networks, the uplink access and downlink
access of UEs are coupled to the same (Femto) cell. Suppose a Femto is fully loaded
when compared to its neighboring Femtos, the traditional offloading or load balancing algorithms will try offloading some of the UEs for both their uplink and downlink
access from the loaded cell to one of less loaded neighboring cells (i.e., target cell)
provided that these UEs could get connected to the chosen target cell. This type of
offloading is a forced handover to reduce traffic imbalance and trigger for handover is
not based on better signal strength from the target cell. But, the offloaded UEs are
connected for both their uplink and downlink access to the same target cell. Since
UEs are most likely separated by walls and floors from their connected cells in enterprise environments, these offloaded UEs now have to transmit with higher transmit
power in the uplink and thereby affects their battery lives. In order to reduce the
battery drain for the offloaded UEs while maintaining their QoS, we employ the Decoupled Uplink and Downlink (DUD) access method in such a way that, the uplink
of UE is connected to the closest Femto while the downlink is connected to a less
loaded neighboring Femto.
To maximize the utilization of the limited operating spectrum and provide higher
data rate for IUEs, operators can configure Femtos in open access mode with frequency reuse one (i.e., all Femtos and MBSs operates on a same frequency) in LTE
HetNets. However, this leads to high co-tier interference and cross-tier interference.
Another problem in enterprise buildings having Femtos is frequent handovers, that
happens when IUEs move from one room/floor to another room/floor inside the
building. This leads to degradation of network performance in terms of increased
signaling overhead and low throughputs. In order to reduce this kind of unnecessary
handovers in enterprise buildings, Femtos should be placed optimally with handover
constraints. Hence, we obtain the optimal coordinates from the OptHO model by
adding handover constraints to the Minimize Number of Femtos (MinNF) model
which guarantees threshold Signal-to-Interference plus Noise Ratio (SINR) of -2 dB
for all IUEs inside the building. Such optimized deployment of Femtos reduces the
number of handovers while guaranteeing good SINR to all IUEs.
In LTE HetNets, even though planned deployment of Femtos in open access mode
boosts the IUEs performance, the power leakage from indoor Femtos create interferix
ence to the OUEs in the HIZone in the buildings surrounding areas. We propose
an efficient placement and power control SON (Self organizing Network) algorithm
which optimally places Femtos and dynamically adjusts the transmit power of Femtos
based on the occupancy of Macro connected OUEs in the HIZone. To do this, we
use the same MinNF model to place the Femtos optimally and solve Optimal Femto
Power (OptFP) allocation problem (Mixed Integer Linear Programming (MILP))
which guarantees threshold SINR of -4 dB for IUEs with the Macro users SINR
degradation as lesser than 2 dB. In the OptFP model, Femto’s transmit power is
tuned dynamically according to the occupancy of OUEs in the HIZone. But the
presence of even a single OUE in the HIZone decreases SINR of numerous IUEs,
which is not fair to IUEs. In order to address this issue, we propose two solutions
a) On improving SINR in LTE HetNets with D2D relays and b) A novel resource
allocation and power control mechanism for Hybrid Access Femtos in LTE HetNets,
which we describe in the following two paragraphs.
To guarantee certain minimum SINR and fairness to both IUEs and OUEs in
HIZone, we consider a system model by applying the concept of Device-to-Device
(D2D) communication wherein free/idle IUEs connected to Femto act like UE-relays
(i.e., UE-like BS, forwarding downlink data plane traffic for some of the HIZone
users connected to MBS). We formulate a Mixed-Integer Linear Programming (MILP)
optimization model which efficiently establishes D2D pairs between free/idle celledge IUEs and HIZone users by guaranteeing certain SINRT h for both IUEs and
HIZone users. As D2D MILP model takes more computation time, it is not usable
in real-world scenarios for establishing D2D pairs on the fly. Hence, we propose a
two-step D2D heuristic algorithm for establishing D2D pairs.
In above works, we assume that Femtos are configured in open access mode. But
Hybrid Access Femtocells (HAFs) are favored by the operators because they ensure
the paid Subscribed Group (SG) users certain QoS and then try to maximize the system capacity by serving near-by Non Subscribed Group (NSG) users in a best-effort
manner. To reap in the benefits of HAFs, the operators need to employ effective
resource sharing and scheduling mechanisms to contain co-tier and cross-tier interference arising out of reuse one in the HetNet system. Towards this, we address various
challenges in terms of deployment and operation of HAFs in indoor environments. We
propose an Optimal Placement of hybrid access Femtos (OPF) model which ensures
a certain SINRT h inside the building and a certain SINRT h in the HIZone of the
building. Unlike in previous optimization models, in this model, users in HIZone are
connected to HAF s deployed inside the building. Also we propose a decentralized
Dynamic Bandwidth Allocation (BWA) mechanism which divides the available HAF
bandwidth between the two sets of user groups: SG and NSG. In order to mitigate
co-tier and cross-tier interference, we then propose a dynamic Optimal Power Control
(OPC) mechanism which adjusts the transmit powers of HAFs whenever the users in
the HIZone cannot be served by the HAFs. In such a case, HIZone users connect
to an MBS instead. Since the OPC problem is hard to solve in polynomial time,
we also present a Sub-Optimal Power Control (SOPC) mechanism. To maintain fair
resource allocation between SG and NSG users, we propose an Enhanced Priority
(EP) scheduling mechanism which employs two schedulers which are based on the
Proportional Fair (PF) and the Priority Set (PS) scheduling mechanisms.
In above works, placement of Femtos is optimized to reduce co-channel co-tier
interference among neighboring Femtos and transmit power of Femtos is optimized
to reduce cross-tier interference between MBSs and Femtos. But, for arbitrary deployed Femtos, Inter Cell Interference Coordination (ICIC) techniques could be employed to address co-tier interference problem among Femtos which are connected with
each other over X2 interface. Hence, in this work, we propose an ICIC technique,
Variable Radius (VR) algorithm which dynamically increases or decreases the cell
edge/non-cell edge regions of Femtos and efficiently allocates radio resources among
cell edge/non-cell edge regions of Femtos so that the interference between neighboring Femtos can be avoided. We implement the proposed VR algorithm on top of PF
scheduler in NS-3 simulator and find that it significantly improves average network
throughput when compared to existing techniques in the literature.
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