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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