1Mbps: Economical and practical
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By Li Yuesheng
A bit rate of 1Mpbs achieves an optimal balance between cost and delivery of a
pleasing mobile broadband (MBB) service experience anytime, anywhere, but the
“anytime-anywhere” part is easier said than done. Comprehensive measures are
available to achieve it, through better technology, optimal deployment, or both.
Data services are replacing voice services as the cornerstones of telco profit. Legacy
KPIs such as dropped call rate and handover success rate no longer effectively assess
the user experience, as many data services are self-healing, making effective
assessment and delivery of a good MBB service experience real challenges.
Defining optimal user experience
MBB services can be divided into ten categories – WAP/web browsing, streaming
video, VoIP, social networking, instant messaging (IM), cloud applications, email, file
transfer, gaming, and machine-to-machine (M2M) communications. User experience
for each typically falls into three intuitive levels – bad, good, and excellent; each has a
certain bandwidth range correlated with it, depending on the service.
According to Huawei statistics (Figure 1), 300Kbps is satisfactory for 50% of all
service categories (IM, VoIP, microblog, WAP browsing, and 360p video), making it
adequate for basic service bandwidth, while 1Mbps can satisfy for 90% of all service
categories, including gaming, web browsing, and standard-definition (SD) video,
making it suitable as the mainstream standard bandwidth. But, how do we deliver it
anytime, anywhere?
Balancing user experience and network scale
Reasonable inter-site distance
A good user experience requires a live network to deliver a certain data rate anytime,
anywhere. Network capacity is key to high-speed access anytime, especially during
busy hours, while sound network coverage is needed to ensure sufficient data speed
anywhere, notably at cell edges. And finally, network quality is crucial to promising
data rates that users can actually enjoy. In other words, user experience is affected by
network capacity, coverage, and quality, and a satisfactory user experience can be
defined as delivery of the expected data rate to 95% of areas 90% of the time.
UMTS is a soft capacity (self-interfering) technology, and operators need to consider
this when promising users a certain data rate anytime, anywhere. During off hours,
network load and interference are light, so the data rate is high anywhere within
coverage, but during peak times, network load is heavy with interference severe,
leading to low data rates anywhere within coverage.
Network simulations show a 1Mbps cell-edge data rate to be a clear line of
demarcation for an HSPA cell that is at 90% of its capacity limit, as the theoretical bit
rate limit a signal can reach is only 1.1Mbps, regardless of the inter-site distance,
thanks to capacity limits and inter-site interference.
Cell-edge data rates of 1Mbps have been obtained in real-world commercial
circumstances during busy hours. With appropriate inter-site distance,
“anytime-anywhere” 1Mbps data rates can be ensured with the reliability mentioned
earlier.
Table 1 shows the typical inter-site distance for UMTS (three sectors, inter-cell
distance at 1.5 times the cell radius) that delivers a field-verified 1Mbps cell-edge data
rate.
Increasing the inter-site distance can lead to a significant drop off in data rate. If
inter-cell distance increases 1.67 or 2.25 fold, respectively, the data rate at the cell
edges will decrease to 600Kbps or 300Kbps, in any environment.
Network scale is inversely proportional to the square of the inter-site distance. Based
on the relationship between inter-site distance and cell-edge data rate, the network
scale corresponding to a cell-edge data rate of 300Kbps or 600Kbps will be 0.2 or
0.36 times that for the cell-edge data rate of 1Mbps, respectively.
Improved user experience requires greater network scale
For the previous 1Mbps example involving 90% cell load, the maximum cell-edge
data rate is 1.1Mbps, due to inter-cell interference, but if the cell load is reduced to 70%
(high load), 50% (medium load), or 30% (low load), the cell-edge data rate can be
enhanced to 1.2Mbps, 1.5Mbps or 2Mbps, respectively.
However, a lower cell load will lead to reduced cell throughput, meaning enlarged
network coverage at the cost of capacity (cell throughput is negatively correlated with
network coverage). In a 2100MHz network, if the cell-edge data rate increases from
1Mbps to 1.2Mbps or 1.5Mbps, the cell load must be lowered from 90% to 70% or
50%, with the cell throughput range decreased from 2.7-to-3.2Mbps to
2.4-to-2.9Mbps or 1.8-to-2.2Mbps, respectively. In this way, network scale is
enlarged 1.1 or 1.5 fold, respectively, to ensure an equivalent overall network
capacity.
For existing UMTS networks, operators can split three sectors into multiples (perhaps
six) or use small cells to enhance network capacity when inter-site distance
adjustment is difficult. In other words, coverage (cell-edge data rate) can be improved
at the cost of capacity.
Finding the balance for user experience and network scale
Figure 2 illustrates how a cell-edge data rate of 300Kbps, 600Kbps, 1Mbps, or
1.5Mbps can achieve a good user experience for 50%, 70%, 90%, and 95% of service
categories, respectively, as network scale increases 0.20, 0.36, 1.00, and 1.50 fold,
also respectively, when compared with a 1Mbps cell-edge data rate.
As the figure shows, a 1Mbps anytime-anywhere data rate is the best balance point
between user experience and network resource consumption, and operators can act
accordingly, based on their current stage of network development. In the developing
stages, operators can add more base stations and expand coverage, while for a more
mature network, operators can add more carriers/sectors and use advanced
technologies such as dual-carrier-HSDPA, four-carrier-HSPA, MIMO, and LTE.
Deployment for optimal user experience
Dense deployment for GSM/UMTS
Using the same or adjacent frequency bands, UMTS and GSM base stations can be
co-sited 1:1 (Figure 3) to ensure 1Mbps anytime, anywhere. For instance, when both
GSM and UMTS use 900MHz (or UMTS2100MHz with GSM1800MHz), UMTS (at
1Mbps anytime, anywhere) has the same inter-site distance as GSM.
However, for operators who run GSM900/850MHz with UMTS2100/1900MHz, four
times as many UMTS sites are needed as GSM to deliver 1Mbps anytime, anywhere,
as GSM900/850MHz has twice the coverage that UMTS2100/1900MHz does.
Introduction of UMTS900/850MHz, therefore, becomes necessary to expand network
coverage and enhance user experience; operators who do not have
UMTS900/850MHz or four times as many UMTS sites as they do GSM must build
more.
Capacity expansion is key
Timely network expansion is key to ensuring 1Mbps anytime, anywhere; each cell has
a capacity limit for a certain number of users. If traffic exceeds the limit, operators
need to scale their networks; otherwise, the cell-edge data rate and user experience
will deteriorate.
The maximum user capacity for a cell is calculated based on traffic models and cell
throughput; both are greatly affected by ever-changing user behavior and the radio
environment in general. The maximum user number supported by a cell greatly
fluctuates and involves collection of core network data, making it unhelpful as a
network expansion reference.
There are many ways to expand network capacity; operators typically expand their
networks based on cell load, as this measure reflects how radio resources are
consumed by users, services, and the radio environment. In normal cases, user
experience can be guaranteed when cell load is not congested.
As cell load fluctuates, an instant peak load of 90% on a live network does not reflect
the real cell load and cannot be used as a prerequisite for capacity expansion; instead,
it should be based on the average cell load. Huawei suggests using an average cell
load of 70% as the threshold for capacity expansion. In general, operators can expand
network capacity by adding carriers, splitting sectors, adding more sites, or upgrading
the network.
HetNet is also needed
On most networks, about 20% sites serve 80% of users, far exceeding macro site
capacity limit and degrading user experience. In addition, a large amount of traffic is
generated indoors, yet macro sites lag behind there in terms of comprehensive and
in-depth network coverage.
Even with a reasonable inter-site distance of macro sites, operators cannot guarantee a
good user experience throughout the entire network, making a heterogeneous network
(HetNet) architecture a must for enhancing network capacity/user experience in
hotspots and eliminating poor coverage areas in general.
As the optimal balance point between user experience and network resource
consumption, 1Mbps anytime anywhere, requires reasonable inter-site distance and
timely network capacity expansion.
Operators can flexibly deploy more sites and carriers, upgrade to HSPA+, split three
sectors into six (or more), utilize UMTS900MHz, or introduce HetNet, and most will
have to do more than one.
Figure 1 Bandwidth requirements and user experience (For details, see pdf)
Inter-cell distance (km)
Frequency
Dense urban
Urban
Suburban
Rural
band
area
area
area
area
2100MHz
0.45
0.72
2.63
7.64
900MHz
0.92
1.50
5.49
14.01
Table 1 Typical inter-site distance delivering 1Mbps cell-edge data rate
Figure 2 Cell-edge data rate, network scale, and user experience(For details, see
pdf)
Figure 3 Inter-site distance for GSM/UMTS co-deployment(For details, see pdf)
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