Heterogeneous Networks: Challenges and Solutions in mmWave Communications

HETEROGENEOUS NETWORKS
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Abstract - This paper is a review of five academic
papers on mmWave communication. The papers
reviewed are “10 Gb/s HetSNets with Millimeter-
Wave Communications: Access and Networking –
Challenges and Protocols,” “Random Access in
Millimeter-Wave Beamforming Cellular
Networks: Issues and Approaches,” “Low-
Latency Heterogeneous Networks with
Millimeter-Wave Communications,”
“Safeguarding 5G wireless communication
networks using physical layer security,” and
“Coverage and capacity in mmWave cellular
systems.”
Heterogeneous and small cell networks
(HetsNets) lead to an increase in efficiency of
spectrum and throughput through hierarchical
deployments. Attaining the capacity requirements of
future 5G unwired networks, the most feasible
technique that has been proposed is the mmWave
communications with unprecedented spectral
resources. Despite the better understanding of the
mmWave physical layer, there are still challenges in
implementing it in HetSNets effectively and
efficiently from the perspective of networking and
access. The 3GPP backwards-compatible frame
structure, with a basis on time-division duplex that
enhances backhaul links as well as high capacity
access. Further discusses the kinds of networking
challenges associated with the multihop characteristic
of the mmWave backhauling mesh.
In the recent past, the mmWave
communication has been given much attention thanks
to its enabling relationship with the 5G technology.
This is because there has been development in
mmWave beam forming technology as well as
campaigns in channel measurements championed by
the industry together with scholars. Thus, this article
contains an analysis on basic issues of RACH with
regards to mmWave communications and give the
possible solutions to tackle the challenges and give
future directions on the same.
In this section of the article, a review on
HetNets end-to-end latency with mmWave
communications has been given. Generally, there is a
challenge in formulating and optimizing the delay
problem with buffers in mmWave cellular
technology, owing to the fact that conventional
techniques of graph-based network optimization do
not apply when queues are put into consideration.
Thus, this paper has attempted to form an adaptive
low-latency strategy that employs the use of
cooperative networking in reducing end-to-end
latency. In the end, there is a demonstration that
Heterogeneous Networks 1

proper cooperative networking is significant in
lowering end-to-end latency.
Index terms
Beam scheduling, BS, Cell densification, HetSNets,
FDD, LOS, MeNB, mmWave
communications, MS, NLOS, OFDM, QoE, QoS,
RACH, Random access preamble, RAR, RAT, RLF,
SenBs, Spectral efficiency, TDD, UEs, Zadoff-Chu,
3GPP LTE network
INTRODUCTION
Various quality of service requirements for
wireless communication traffic load has been on the
upward trend exponentially as a result of the
widespread use of applications of mobile internet by
smart terminals. This trend is expected to continue,
requiring the new and emergent wireless network
designs to take care of these requirements. The
emerging 5G network technology is expected to
come with superior features that will enable users to
experience less of the communication challenges
associated with the current 4G and 3G technologies.
These features may include, but not limited to
features such as achieving a factor of 10 to 100
higher user data rates, and enhancing the overarching
vision of tactile internet that will need to have an
end-to-end latency which is lower than 5ms so as to
offer a response time which is ultra-fast.
Similarly, to cope with the ever-growing
mobile traffic, an enhancement on radio access
technology is key, by increasing cell density,
increasing spectral efficiency and an increased
bandwidth frequency. Cell densification has a
challenge in handling a large volume of interference
between cells due to the fact that cell deployment
usually occurs in an unplanned way, and the
interference management signaling is limited by
network non-ideal backhaul links. Thus, with respect
to these challenges, dealing with the traffic demand
will need a straightforward solution like increasing
the communication bandwidth. Owing to this, the
mmWave band has been deemed as the most
promising cellular network band.
Latency is considered to be one of the key
players in the future of mobile communication,
especially in QoS. However, low-latency is an uphill
task in 5G heterogeneous networks thanks to the
following reasons:
● The use of buffers in 5G for handling the
unexpected heavy traffic, while the queuing
delay seriously malfunctions the QoS in 5G
● Performing networking optimizations for
lesser latency is made difficult by the
diversity of HetNets architecture and/or
RATs
Deployment of mmWave communications in
HetsNets
Basically HetsNets comprises of a multiple
layer of radio access nodes in a 3GPP LTE network,
for instance, a macrocell eNB (MeNB) and several
multiple small cell eNBs (SenBS) like femto, relay
and pico eNBS. Such networks operate by having
each of their SeNB combining its backhaul data with
data obtained from the other network nodes then
sending to the MeNB. Separating SeNBs with short
distance ranges of 100-200m enables the mitigation
of severe propagation losses. Similarly, mmWave
radio communication can be used to provide
coverage within the small cells, thereby limiting the
interference level undergone on the sub 3-GHz
frequency bands applied in traditional cellphone
communication [1].
There are various deployment scenarios for
mmWave communication in HetsNet used for user
access links and backhaul as discussed briefly below.
Heterogeneous Networks 2

Figure 1. Illustration of typical deployment scenarios
with both mmWave and microwave systems.
Scenario 1 (Baseline) - a wired backhaul such as an
optical fibre is used to connect a MeNB to the SeNB
that it donates. This is a traditional approach where
either the MeNB or SeNB serve the user equipment
UEs on the microwave band. This scenario requires
that interference between the SeNBs and the MeNB
must be avoided maximally by carefully designing
interference coordination schemes.
Scenario 2 - here the UEs communicate to the MeNB
via a microwave band, while the UEs served by the
SeNBs operate on an mmWave radio. A wired
backhaul is used to connect the MeNB to the SeNBs.
Despite this scenario not requiring an interference
coordination scheme, the UES must support dual
bands in order to have a smooth handover between
the SeNBs and the MeNB.
Scenario 3 - here, the mmWave radio comes in
handy for backhaul transmission between the SeNBs
and the MeNB. Its implementation needs a single-
hop for backhaul transmission using mmWave radio.
This scenario achieves a quick deployment of SeNBs
due to the fact the UEs are not changed as the
network facilities get an upgrade. As in the case of
scenario 1, advanced interference coordination must
be performed in this scenario [2].
Scenario 4 - In scenario 4, single-hop wireless
backhaul adopts the use of mmWave communication
for the SeNBs. In addition, the UEs are served in a
small cell by the SeNBS via mmWave radio that
thanks to its tremendous bandwidth can raise
significantly the capacity of the network.
Scenario 5 - Improving network capacity can also be
achieved by increasing the reusability of
geographic spectrum, leading to a dense small cell
deployment. Dense SeNBs can be effectively
connected with the MeNb using the multihop
wireless backhaul. This scenario is where the SeNBs
cooperate amongst themselves and via an mmWave
radio communicate with their donor MeNB.
In conclusion, scenario 1 is the baseline, while
scenario 2, 3 and 4 act as a subset to scenario 5.
Frame structure
Heterogeneous Networks 3