HetNet: The future of mobile networking
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By Hu Guojie
As mobile demand for data exceeds all expectations, heterogeneous network
architecture with multiple frequency bands, radio access technologies, and base
stations of varying coverage, is the only way forward.
Nobody in the mobile telco industry needs to be reminded of the scary statistics
regarding data demand, especially in the hotspots, which is driving operators to
increase their base station density and improve spectral efficiency through
multiple-input multiple-output (MIMO), dual carrier (DC), and various LTE
technologies. However, base station deployment is either hitting the saturation point
or becoming untenable in major urban areas, so Wi-Fi, micro base stations, and other
supplements are filling in the gaps, making for a heterogeneous network (HetNet)
architecture that has sprung into being largely by necessity.
Key HetNet technologies
One key challenge in HetNet is seamless micro base station introduction into a live
network, as it can have a potentially adverse effect on key performance indicators
(KPIs) such as drop rates that stem from macro-micro base station interference, so
coordination is needed here. Micro base station deployment is necessary for macro
base station offload in scenarios with numerous hotspots, but deployment
requirements and costs for the former can be reduced by using solutions for flexible
site backhaul and integrated power supply, feeders, and surge protection. With a large
number of micro base stations in place, macro-micro base station O&M needs to be
both consistent and easy, if costs are to be held in check.
Precise hotspot identification
A micro base station is only effective for macro base station offload when deployed in
an actual hotspot. Operators can generate network traffic maps by collecting traffic
geo-information, related location data, and grid maps of live user equipment (UE) on
the network. Considering the coverage area of a micro base station, the recommended
precision for a generated map of traffic is a 50m x 50m grid. Operators can evaluate
micro base station effectiveness by comparing traffic maps generated before and after
deployment, which can enable further optimization down the road.
Integrated micro base stations
Site acquisition for large and unsightly equipment is becoming cost-prohibitive and
unpopular with property owners; this will increasingly push micro base station
deployment onto poles or walls, making a simple and clean installation process a must.
To achieve this, transmission, power supply, and surge protection could be integrated,
along with everything else, into an unobtrusive form factor (spherical or rectangular)
that does not exceed 8kg, so that a single person can effectively install it.
Flexible base station backhaul
Transmission is a significant challenge for micro base station deployment. Since most
micro base stations are grafted, so to speak, onto whatever infrastructure is available,
most sites have no pre-existing transmission support; flexibility is what is needed here.
A last-mile solution for micro base station requires both fixed and wireless backhaul
(preferably the former). Fiber is the primary medium for fixed base station backhaul
via point-to-point (P2P) service or a passive optical network (xPON) with optical
network units (ONUs) deployed either indoors or out.
Wireless backhaul is more flexible but less reliable. Typical radio backhaul solutions
utilize 60GHz microwave, LTE TDD, E-Band microwave, or Wi-Fi, with each having
its own advantages.
Unlicensed 60GHz microwave enjoys cost advantages for micro BS deployment
scenarios that are short-distance and high-bandwidth, while an LTE TDD solution
would support non-line-of-sight (NLOS) point-to-multipoint (P2MP) backhaul.Wi-Fi,
on the other hand, is good for low-cost data services.
SON features
To meet mobile broadband network requirements five years from now, micro base
station deployment will invariably exceed the number of macro base stations. The
easy deployment and maintenance that SON enables will prove essential to reducing
O&M costs over the long term.
A self-organizing micro base station can automatically detect the surrounding radio
environment conditions and automatically plan and configure radio parameters such
as frequency, scrambling code, and transmission power. A traditional base station
cannot do this, which is why a micro base station with SON features costs 15% less in
terms of network planning man hours.
What’s more, a micro base station can automatically detect changes in the
surrounding radio environment; when a new neighbor is deployed, it can
automatically optimize network parameters for scrambling codes, neighboring cells,
transmission power, and handover. On a traditional network, network optimization is
a crucial part of network maintenance; when the former is automatic, manpower costs
are reduced 10 to 30%.
Macro-micro base station coordination
One of the key advantages of HetNet architecture is that it allows gradual and flexible
capacity expansion based on need as opposed to forecast. When hotspots are sporadic,
only a few micro base stations are needed, and they can use the same frequencies as
their macro counterparts. However, to reduce interference between the two, a
coordination solution is needed.
One such solution that can improve capacity and user experience at the same time is
Huawei’s Cloud BB (Baseband) architecture. When the number of traffic hotspots
increases and more micro base stations are deployed, engineers can flexibly allocate
carriers among micro base stations to maximize capacity.
Simulations have shown that network capacity and cell-edge user throughput can be
increased by either a “1 macro + 3 micro” solution or a “3 micro RRU” solution
(Figure 1).
When a micro base station is deployed, coordination with the network’s macro base
stations increases overall cell capacity by 80 to 130%. If a micro remote radio unit
(RRU) is added instead, this increase expands to the 90-to-150% range.
Cell-edge user throughput is also significantly increased through “1 macro + 3 micro
RRU” (Figure 2). For 5% of cell-edge users, the solution throughput increases by a
factor of five over “1 macro + 3 micro.”
AAS technology
MIMO is a key technology for radio networks as it improves both spectral efficiency
and single-site capacity, and will be commercially deployed in the near future as
supporting terminals become available. Both it and higher-order MIMO (HO-MIMO),
which multiplexes more than four channels, enable multiple-channel reception and
transmission for base stations with dual-polarized antennas, while the basic
technology itself adaptively selects reception & transmission modes and antenna ports
according to air interface channel quality.
Most operators are greatly interested in commercially deploying micro base stations
with adaptive antenna system (AAS) technology as their expansion potential is
somewhat better, thanks to their smaller radio propagation environment, than it is for
macro base stations.
Multi-antenna array elements that support MIMO can utilize numerous technologies
to expand capacity, including cell-level beamforming (BF), user-level BF, and cell
virtualization. All can be done to enhance spectral utilization.
Cell-level BF enhances site selection flexibility and a 10% increase in both average
cell throughput and cell-edge user throughput to boot for outdoor micro base stations,
with any AAS-enabled micro base station providing hardware support for future SON
solutions, which may potentially improve O&M cost, O&M efficiency, and traffic
offload.
Next-gen indoor solutions
Since 70 to 80% of mobile broadband service traffic is generated indoors, operators
must focus on indoor capacity challenges. For small hotspots, operators should
implement an outdoor micro base station solution or an indoor pico base station
deployment solution to enhance indoor coverage.
For indoor coverage in large buildings, a commonly-adopted solution is the
distributed antenna system (DAS), which improves both network coverage and KPIs,
though implementation is difficult as capacity increases are limited and key
technologies such as MIMO are difficult to adapt to pre-existing architecture. In
addition, a DAS alone cannot manage and monitor equipment indoors, making
network faults hard to locate. This represents a potential drag on both user satisfaction
and total cost of operation (TCO).
Based on the distributed base station concept, a next-generation indoor solution would
simplify deployment by introducing RRUs, which can be configured via software for
flexible capacity expansion. Such a solution can be managed and monitored by
enabling the location and rectification of faults in any RRU from a centralized O&M
center.
Deployment scenarios
Indoor hotspots
Indoor hotspots are categorized by partition (multiple or none) and vary by coverage
size (small, medium or large). Residential users are increasingly relying on mobile
networks for voice and Internet, so a typical small to medium-sized multi-partitioned
indoor hotspot would be a residential building. Small to medium-sized
non-partitioned hotspots, on the hand, include supermarkets, subways, and
medium-sized conference halls, among other areas with low ceilings, users in motion,
and high capacity demands.
Large multi-partitioned indoor hotspots include large office buildings, upscale hotels,
and other places where both user density and demand are high. However, both
coverage and capacity requirements must be considered for this scenario owing to the
presence of elevators and high floors (macro base station vertical coverage is often
poor).
Large non-partitioned indoor hotspots, on the other hand, are typically transit hubs,
where subscriber density is high with peaks generally sporadic.
Outdoor hotspots
Outdoor hotspots fall into three categories – small, independent hotspots (HotDots),
hotspots that follow a street (HotLines), and large hotspot zones (HotZones). In a
HotDot scenario (a coffee shop), demand is high but coverage is small and users are
somewhat stationary. In a HotLine scenario, subscriber density and traffic
requirements are high, with coverage resembling a street map with some overlap with
the surrounding businesses, and this must be considered during deployment. A
HotZone is typically a square or some other public gathering space where user density
and demand are both high, but only at certain and largely predictable times.
Any outdoor hotspot can utilize a micro base station outdoor solution, while a small
or medium-sized multi-partitioned indoor hotspot would be better suited for an
“outdoor coverage + indoor” solution. Small to medium-sized non-partitioned indoor
hotspots are fine for a micro base station indoor solution or a micro BS + DAS
solution, while large multi-partitioned and large non-partitioned indoor hotspots are
suited for a next-generation indoor solution.
Conclusion
Mobile networks of the future will need great capacity and better user experience, and
HetNet is how we get there. Micro base stations must be accurately deployed in traffic
hotspots for macro base station offload, and with proper macro-micro coordination,
KPI impact is minimal. However, any micro base station should have integrated
power supply, feeders, and surge protection, to minimize site requirements and
deployment costs.
An optimized next-generation indoor solution enjoys natural advantages for enabling
flexible base station deployment, smooth capacity evolution, and remote fault location
and rectification. Certain deployed scenarios have been identified and operators must
now start matching them with their own needs.
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