Many mobile operators around the world have HSPA
technology to thank for their mobile broadband success. But as subscription
rates continue to rise dramatically, fed by huge smartphone sales and the
demand for bandwidth-hungry data, mobile operators face increasing challenges
to keep their customer bases satisfied. HSPA evolution provides a
cost-efficient answer.

 

 HSPA drives mobile broadband

The rapid growth of mobile broadband traffic in recent
years has been driven and facilitated by the twin landmark appearances of, and
developments in, new devices and HSPA technology. HSPA has a large footprint
across many markets, providing wide-area coverage for a variety of terminals,
including popular smartphones.

 

Several commercial LTE networks have also been deployed
recently, aimed at meeting the longer-term needs of mobile broadband consumers.
HSPA networks have a larger ecosystem, and consequently the majority of growth
in mobile broadband traffic in coming years is likely to occur in these
networks. one thing is certain – there will be no slowdown in the pace of
mobile broadband traffic growth. Ericsson estimates that mobile broadband
subscriptions will almost top 5 billion by 2016 – and by that time, it is
expected that more than 75 percent of these subscriptions will use HSPA
networks.

 

As such, HSPA technology must continually evolve so that
it can handle this extensive growth and the corresponding consumer demand for
higher data rates and better coverage. Additionally, operators facing a lack of
spectrum, or experiencing faster-than-expected traffic growth, must improve
spectral efficiency.the large increase in smartphone-generated traffic in
networks places even more requirements on HSPA networks – and these
requirements must be accommodated.

 

The beauty of HSPA evolution is that previous investments
in infrastructure are protected while the network is being upgraded. While
today’s data demands had not been foreseen when operators invested in HSPA
technology, even just a few years ago, the capability of the technology is such
that upgrades, rather than new network rollouts, will provide a response to the
data challenges that operators will experience in coming years. Accordingly, HSPA
evolution is highly appealing to operators simply on the basis of its
cost-effectiveness.

 

The natural progression of HSPA is to evolve the
technology to meet the IMT-A requirements established ITU [1]. these
requirements apply to systems with capabilities beyond those of iMt2000
systems. When the improvements become part of the standard, the evolved HSPA
technology will perform on a par with other 4G technologies. this evolved HSPA
technology is clearly capable of meeting operator demands for increased
capacity and end-user demands for higher data rates.

 

Though carefully selected, the IMT-A scenarios have
been simplified to some extent so that the results can be more easily evaluated
and compared. therefore, it is necessary to expand the analysis beyond the IMT-A requirements by considering the actual user experience in real-world
scenarios.

 

The new traffic patterns originating from smartphones are
among the developments that are not fully covered in the IMT-A evaluation
scenarios. the number of smartphones using mobile networks has grown at a
massive pace in recent years. Such growth presents obvious challenges for HSPA
networks rolled out in the recent past, so it is vital that the continued
evolution of HSPA takes smartphone traffic into account.

 

This paper outlines the route towards IMT-A compliance.
it refers to some of the major advances made possible by earlier 3GPP HSPA
releases and how the continuous development of those achievements, and new
functionality, are influencing the development of the next release – 3GPP
Release 11 (R11). An analysis of HSPA Release 10 (R10) indicates that many of
the IMT-A requirements have already been fulfilled. this paper also outlines
the changes that have been made to the standard to improve handling in response
to growth in smartphone use. it highlights how HSPA should be developed to meet
the needs of smartphone users, while preserving network resources and terminal
battery life. the significance of the smartphone challenge is specifically
addressed in section 3 of this paper.

HSPA on a par with 4G

ITU has developed a process for determining whether or
not mobile systems are IMT-A capable to qualify as IMT-A capable, a
system must fulfill a specific set of requirements. For some of these
requirements, a simple assessment against the standard is sufficient to
determine whether a system is IMT-A capable. in this paper, such requirements
are referred to as capabilities. other performance requirements must be evaluated
through the use of simulation scenarios that have been carefully specified by ITU. if a technology has these capabilities and fulfills these performance
requirements, ITU can classify the technology as IMT-A capable. two wireless
technologies are currently classified as IMT-A capable: LTE R10 and IEE 802.16m.

Capabilities of IMT-A Systems

 

Support for higher bandwidths: one of the IMT-A
capability requirements is that a system must support downlink transmission
bandwidths of up to 40MHz.

 

Following 3GPP Release 8 (R8), HSPA has facilitated
multi-carrier operation, which enables node-B to schedule data simultaneously
on multiple carriers.this functionality obviously results in an increase in
peak rates. But more interestingly, it also results in an increase in spectral efficiency.
Recent evolutions have continued to capitalize on that breakthrough. As of R10,
HSPA supports multi-carrier operation on up to four carriers in the downlink (which
can be spread across one or two frequency bands) and up to two carriers in the
uplink. 3GPP is currently specifying an 8-carrier HSDPA operation as part of
the R11 requirements.

 

The performance of an 8-carrier HSDPA system is depicted
in Figure 1. While the 8-carrier solution, of course, outperforms the 4-carrier
solution, a single 8-carrier system also provides higher capacity than a pair
of 4-carrier solutions.

 

Peak spectral efficiency: ITU has also established a set
of uplink and downlink requirements for peak spectral efficiency – defined as
the peak rate divided by the bandwidth used. The IMT-A requirements are listed
in table 1, along with proven values for HSPA R10 and estimated values for HSPA
R11.

 

The IMT-A requirements were met in LTE R10 with features
such as DL 4×4 MIMo, UL 2×2 MIMo and UL 64QAM. table 1 also shows how the
inclusion of similar features in HSPA R11 would clearly exceed the IMT-A peak
spectral efficiency requirements through the resulting predicted values.

 

Features introduced to increase peak spectral efficiency
will also improve performance in certain scenarios. Such benefits are clearly
shown in the comparison of the performance of DL 4×4 MiMo with that of HSDPA
R10, outlined in Figure 2.

 

Latency: latency is a huge influencing factor for mobile broadband
subscribers when it comes to customer loyalty, so it is not surprising that the IMT-A system requirements are tough in terms of low-latency provision, both for
the control plane and user plane.

 

Control-plane latency is measured as the time it takes to
establish a user-plane connection from an idle state. user-plane latency is the
one-way transit time between a packet being available in the terminal and the
same packet being available in the base station.

 

HSPA R10 fulfills the IMT-A latency requirements. From an
idle state such as CELL_PCH, a user-plane connection can be set up in less than
100ms, thereby fulfilling the requirement for control-plane latency. Also,
assuming that the terminal is in an active state, the transit time for a packet
is significantly less than 10ms, thereby fulfilling the requirement for
user-plane latency.

 

Handover interruption time: Another important
characteristic of a cellular system is the interrupt that occurs during a
handover. Because HSPA R10 implements soft handover in the uplink and
synchronized handovers in the downlink, there are essentially no interruptions
during a handover. HSPA R10 therefore fulfills the IMT-A requirements for handover
interruption.

 

Performance Requirements

 

In addition to bandwidth and peak spectral efficiency
requirements, ITU has formulated performance criteria that must be met by all IMT-A capable systems. the systems must perform at a high level in terms of:

 

–  Average and cell-edge spectral efficiency in the uplink
and downlink

–  Voice over IP (VoIP) capacity

 

–  Mobility traffic channel rate

 

Cell-edge spectral efficiency is defined as the fifth
percentile of the user bit rates, divided by the bandwidth. An assessment of
the performance requirements was conducted in four test-case scenarios, which
differed in terms of deployment and mobility:

 

–  Indoor hotspot

 

–  Urban micro

 

–  Urban macro

 

–  Rural macro.

 

HSPA R10 performance for average and cell-edge
spectral efficiency was evaluated for each of the four scenarios.the results
for average spectral efficiency are outlined in Figure 3, while Figure 4
displays the results for cell-edge spectral efficiency. in both cases, HSPA R10
performance is compared with the IMT-A requirements.

 

The results clearly show that HSPA R10 fulfills the IMT-A
requirements.

Powerful linear receivers and the incorporation of eight
receive antennas per cell were key factors enabling the uplink performance to exceed
the IMT-A requirements. the requirements would have been more difficult to
fulfill with fewer receive antennas. State-of-the-art radio resource management
functionality, including link and rank adaptation, was used to boost downlink performance.

 

Thanks to the strength and adaptability of HSPA
technology, these kinds of evolutionary measures made it possible to deliver performance
results beyond the IMT-A requirements.

 

VoiP capacity and mobility traffic channel rate are also
significant considerations in the continued evolution of HSPA technology. in terms
of IMT-A requirements for VoiP capacity and traffic channel rate, performance
evaluation has yet to be conducted.

 

Fulfilling the IMT-A requirements is important. ITU has
put significant time and effort into quantifying and defining the requirements
that future wireless systems should meet. Systems that fulfill the capabilities
and achieve the target performance are therefore well-placed to handle
increasing demands on user bit rates and system capacity.

 

However, current mobile systems face additional
challenges not foreseen by ITU when it stipulated the requirements. in
particular, the large increase in data traffic from smartphones has placed new
demands on wireless systems, due to new traffic patterns and user behavior. the
ability to meet such demands will be essential in the evolution of wireless
systems, particularly HSPA technology.

 

Smartphones and Performance

 

Fulfilling the IMT-A requirements is important. ITU has
put significant time and effort into quantifying and defining the requirements
that future wireless systems should meet. Systems that fulfill the capabilities
and achieve the target performance are therefore well-placed to handle
increasing demands on user bit rates and system capacity.

 

However, current mobile systems face additional
challenges not foreseen by ITU when it stipulated the requirements. in
particular, the large increase in data traffic from smartphones has placed new demands
on wireless systems, due to new traffic patterns and user behavior. the ability
to meet such demands will be essential in the evolution of wireless systems, particularly
HSPA technology.

 

HSPA Radio Resource control (RRC) states

 

One factor that currently limits end-user experience in
an HSPA system is the RRC state machine. this state machine was included in the
original release of the WCdMA standard (R99), and remained unchanged until HSPA
Release 7 (R7). it was designed to boost performance while simultaneously
limiting resource consumption in both the network and the terminal. Figure 5
outlines the RRC states that can be assigned to HSPA user equipment (ue).

 

Due to the limited bit rates and spectral efficiency of
other states, data transmission takes place almost exclusively in the CELL DCH
state. When the data transmission has ended, the terminal remains in the same
state for an average time of about half a second before it is moved to a more
resource-efficient state.

 

3GPP R7 was a landmark in terms of the speed at which
terminals could be moved between states. Prior to 3GPP R7, transition to
CELL DCH could take 2s from the idle state and about 500ms from other states. the
consequence was a delay in down-switches to avoid associated delays in
up-switches, which led to high consumption of network resources and a high rate
of battery usage. 3GPP R7 and R8 features specifically addressed this
situation.

 

Signaling channel transmission rates were increased to
speed up the state changes. As a result, the transition to CELL DCH takes less
than 1s from the idle state and about 200ms from the other states. The new
features significantly increased user data rates in CELL FACH as well as making
it possible, for the first time, to transmit user data during state change.

 

Another improvement was the fast dormancy feature
introduced in 3GPP R8, which enables the UE to indicate that it has finalized
its transmission. This extra information enables the network to determine the appropriate
state into which the UE should be moved.

 

A further improvement meant that terminal power
consumption in CELL FACH could be reduced by introducing discontinuous
reception (dRX).

 

CELL FACH: The Smartphone State

 

While the improvements in releases 7 and 8 were
significant, the major growth and impact of smartphone traffic presents major
challenges for the RRC state machine.

 

Smartphone traffic patterns are difficult to predict, are
intermittent in nature and are irregularly spread over relatively long time
intervals compared with classic interactive traffic. this has a strong impact
on UE state switching, as it represents a challenge to established models of resource
allocation dimensioning, user-handling and quality-of-service provisioning. the
ideal answer would enable Smartphones to be kept in the same state for the
longest possible period of time. it should be possible for this state to
efficiently serve non-critical traffic by maintaining a low consumption of
network and battery resources. A switch to other states should only occur when
the highest data rates are required or when total inactivity is detected. this would
also reduce the amount of signaling traffic.

 

No current RRC state has all of the properties required
to achieve this. With the exception of CELL FACH and CELL DCH, all RRC states
are limited or disabled in terms of transmission and reception of data. Introducing
behavioral changes in this respect would effectively mean a change in the very
definition of these states and would require massive efforts and new agreements
for standardization.

 

The high resource and power consumption experienced by
the terminal and the network while in CELL DCH state rules out its deployment
to handle Smartphone traffic, even with the introduction of Continuous Packet Connectivity
(CPC). In fact, terminals that are kept in the CELL DCH state are a major
source of uplink interference, even after their transmissions have ended. in
terms of the CELL FACH state, the 3GPP R7 and R8 evolutions improved latency
and provided smoother and faster transition between states for mobile broadband-capable
Terminals. The fundamental capabilities of this state make it highly attractive
for handling Smartphone traffic. With further improvements to facilitate better
control of network resources, increase downlink spectrum efficiency, improve
uplink coverage, decrease the terminal power consumption and further reduce transmission
latency in the state, it becomes possible to substantially extend the period of
time that Smartphones can be kept in the CELLFACH state. Frequent packet
traffic can be transmitted efficiently without having to transit to CELL DCH
Because terminals in the CELL_FACH state generate substantially less uplink
interference, the overall uplink interference is also significantly reduced.

 

In terms of improvements, the introduction of fast
channel-state information feedback in the CELL FACH state would increase
downlink spectrum efficiency. it would also contribute to optimal network
resource allocation and boost downlink transmission performance in the
CELL FACH state to similar levels as those achieved in the CELL DCH state.
However, the highest user bit rates will still only be supported in the CELL DCH
state, using features such as MIMo.

 

One limitation of enhanced dedicated Channel (e-dCH) in
the standardized R8 CELL FACH state is the lack of support for concurrent use
of 2ms and 10ms transmission time intervals (TTIs). in reality, this means that
in most cells, for coverage reasons, the application of E-DCH in the CELL FACH
state requires the use of 10ms TTI. This means that subscribers using the
shorter-latency cell will be denied the benefits of the 2ms TTI. This would mean that
the only subscribers needing to use the 10ms TTI would be those in locations
with poor coverage, while the remaining subscribers could benefit from the
advantages of the 2ms TTI.

 

E DCH in the CELL DCH state provides the valuable option
of using per-hybrid automatic repeat request (HARQ) grants for users of 2ms
enhanced uplink (eul). A grant then becomes valid only in a subset of the HARQ
processes, making it possible to introduce uplink TDM. this capability should
also be introduced in the CELL FACH state to improve radio interface control
and the handling of small packets. Furthermore, although E-DCH should be the
preferred option to carry uplink traffic in the CELL FACH state, the handling of
extremely small packets is conducted more efficiently by the R99 PRACH.
Accordingly, such a fallback option should also be introduced in the standard.

 

As already outlined, UE battery consumption in the CELL_FACH
state is still too high. More aggressive DRX schemes must therefore be
introduced to make it possible for the UE to remain in the CELL FACH state for
longer time periods. Since downlink transmissions can only be initiated when
the ue receiver is switched on, the introduction of longer DRX cycles comes at
the price of increasing latency for network-originating transmissions. Further
latency enhancements may also be required to counteract the impact of longerDRX cycles.

 

The above-mentioned improvements to the CELL FACH state
are all part of ongoing R11 work.

 

 Conclusion

 

The HSPA ecosystem is undoubtedly the main facilitator of
the massive growth in mobile broadband traffic. A large number of
well-functioning HSPA networks now provide wide-area coverage in many markets,
while a great variety of HSPA devices are also available, including many
popular Smartphones.

 

There is no doubt that mobile broadband traffic growth
will continue to accelerate and, accordingly, that operators must upgrade their
HSPA networks and provide the service capabilities demanded by their customers.

 

Such upgrades will only be possible through the evolution
of HSPA technology. the latest stage in HSPA evolution relates to the need to
improve the standard so that it meets the IMT-A requirements that were not
fulfilled by R10. this primarily concerns support for higher bandwidths and
higher peak spectral efficiency.

 

Additional improvements relate to developing the RRC
state machine. Such improvements will allow networks to handle Smartphone
traffic more efficiently and improve the subscriber experience by reducing
network resource consumption and terminal battery consumption.

 

Significantly, the proposed evolution of the standard
enables existing networks to be upgraded in a cost-efficient manner. With
continued evolution of the standard, HSPA technology will be well placed to
handle future capacity-related requirements and subscriber demand. Ericsson is
committed to driving this evolution to the benefit of the industry.

 

 

By Ericsson
[email protected]