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Challenges of the
Evolving 3G Technology
by Nick Hallam-Baker, Aeroflex
Some challenges arise with the introduction of an enhanced air interface.
The rollout of the world’s first 3.5G cellular networks based upon High-Speed
Downlink Packet Access (HSDPA) technology marks a clear shift toward the support
of packet data services. HSDPA networks offer higher data rates and reduced
packet latencies to provide new services comparable with home broadband
experience. However, HSPDA addresses only half the story, and there are
benefits to be gained by enhancing the uplink.
You no doubt have seen headlines extolling the 14-Mb/s downlink data rate
offered by HSDPA. This is a very important feature of the technology and
provides services comparable to those offered by WiFi hotspots plus the
advantage of improved mobility, security, and ease of use of a cellular network.
HSDPA marks a major step in the evolution of cellular networks to a system
specifically optimized for downlink packet data. HSUPA extends this packet data
enhancement to the uplink.
Evolution of 3G Networks
With the introduction of HSDPA, significant aspects of the management of the air
interface have been moved from the radio network controller (RNC) down into the
base station or Node B in 3G terminology. Moving this functionality closer to
the air interface means that the 3G system can react more quickly to changes in
the quality of the wireless link between the user and the Node B and to the
user’s data requirements. In turn, this efficient and reactive control allows
for higher data throughput and greater cell capacity.
The higher data rates associated with HSDPA are achieved through the use of
advanced channel coding and modulation techniques with the backup of fast
retransmission in case things go wrong. To take advantage of these features, the
Node B receives information from each user about the quality of the downlink
channel.
In times of very good channel quality, the Node B can transmit very high data
rates with little error protection from the channel-coding algorithms. However,
as the channel quality decreases, the effective user data rate is reduced since
the Node B must send more robust information with greater error protection.
Depending upon the environment, the user’s channel quality can change rapidly,
and the Node B must react quickly if it is to maintain maximum efficiency of the
air interface.
Another major advantage of HSDPA was the introduction of a shortened packet
duration or time transmission interval (TTI). Reducing the minimum TTI from 10
ms in a Release 99 system to 2 ms for HSDPA provides several advantages. First,
it allows the Node B to react more quickly to changes in the channel quality
experienced by each user. But perhaps more importantly, it reduces the latency
associated with the transmission of each packet. The packet latency represents
the delay between the data packet transmission and its successful reception and
decode.
For many services, such as voice over IP (VoIP) or interactive gaming, excessive
packet latency rapidly degrades the quality of service. When coupled with hybrid
automatic retransmission request (ARQ), allowing fast retransmission of
erroneous packets, the reduced latency through the network enables the cellular
operator to offer a much wider range of services.
The Need for HSUPA
Given the improvements in efficiency and services provided by HSDPA, the next
step would be to apply similar concepts to the uplink. High-Speed Uplink Packet
Access (HSUPA), sometimes termed Enhanced Uplink, represents the latest 3GPP
Release 6 technology and aims to provide optimized packet data support in the
uplink.
However, some industry commentators have said it is the downlink that is
critical with uplink usage being much less important. Consequently, is there any
need to introduce this technology? While it is true that many broadband services
currently are dominated by downlink data transfer, it is important to recognize
the need for efficient packet data support on uplink.
Services such as multimedia messaging are already popular and likely to grow as
high-quality cameras become standard in most handsets. Additionally, it is
likely that the use of symmetric services such as voice and VoIP will start to
increase, both offering more efficient use of bandwidth than their
circuit-switched counterparts.
Finally, in recent years, TCP/IP has become the ubiquitous transport mechanism
for packet data, and the physical (PHY) and medium access control (MAC) layer
aspects of HSDPA are ideally suited to supporting this higher-level protocol.
However, downlink TCP/IP services require corresponding packet acknowledgements
to be sent on the uplink and, with a high rate downlink, this alone can generate
a significant load on the uplink.
If acknowledgements on the uplink are lost or delayed, TCP/IP also may adapt to
its perceived link quality by reducing the downlink data rate. Mixing and
matching HSDPA downlink with the acknowledgements transmitted on conventional
Release 99 uplink channels present inefficiencies in terms of uplink bandwidth
and latency.
HSUPA Features
Many of the features and enhancements of HSUPA can be traced directly back to
HSDPA. Additionally, new challenges have been addressed to balance the need for
efficient management of the uplink air interface against costly control
signaling and implementation complexity.
Like HSDPA, HSUPA offers enhanced data rates, fast packet retransmission
mechanisms, and reduced packet latencies. The uplink data rate for HSUPA is
increased up to a theoretical maximum of 5.76 Mb/s.
One of the techniques used to achieve this is adaptive channel coding, which
adjusts the amount of error correction according to load and channel conditions.
Hybrid ARQ (HARQ) packet transmission techniques and the 2-ms TTI also are
copied from HSDPA.
The support of the high data rates and the reduced TTI present a number of
complexities and challenges to both handset and infrastructure designers. From
the handset perspective, the support of the high data rates necessitates many
considerations within the radio, baseband, and protocol implementations. More
powerful DSPs, faster ASICs, and increased memory are required to handle the
larger amounts of user data and faster data processing.
The reduced TTI requires that the handset must react more quickly to
retransmission requests and control signaling from the network. The high data
rates also may require improvements in the quality of the components in the RF
transmitter stage to ensure that the encoded data is not corrupted by its own
radio.
Implementation of these features will take time and can add significant cost to
the manufactured price of a handset. For this reason, a range of six handset
capability classes has been defined supporting different data rates and
different TTIs. The lower classes accommodate lower data rates, starting from
700 kb/s, and some classes only handle a 10-ms TTI. The first commercial
handsets will likely conform to these lower capability classes with the higher
rates being rolled out as the technology evolves.
Infrastructure manufacturers face similar pressures when updating their Node Bs
to support HSUPA with their requirements for higher quality radio receiver
architectures and improved baseband processing. Additionally, cellular operators
are likely to pressure their infrastructure suppliers to provide support for the
high data rates from the initial release to minimize their own rollout costs and
timescales.
Control and Scheduling
Perhaps the biggest challenge associated with HSUPA is not the physical transfer
of data over the air interface but its management. The air interface of an HSUPA
cell must be carefully controlled to ensure that each user receives the uplink
bandwidth required while preventing cell overload due to too many users trying
to send data at the same time. This is the task of the scheduling algorithms.
Like HSDPA, the scheduling algorithms for HSUPA are located in the Node B,
enabling quick responses to changing channel conditions and user data
requirements. In itself, this presents an interesting problem.
For HSDPA, the Node B controls the downlink resources and possesses all of the
information required to share those resources among all users. For HSUPA, the
situation is more complex since information about uplink data bandwidth
requirements resides with each user and little information is available to
indicate the uplink channel quality for each user.
To add to the challenge, an inferior HSUPA scheduling algorithm could result in
each user generating excessive interference in the cell. This would cause an
overload and mean that no one succeeds in getting data over the air interface.
While each Node B manufacturer must define and optimize its own HSUPA scheduling
algorithms, the mechanism of communicating the control information between
network and handset has been defined by the 3GPP standards. The challenge was
how to coordinate multiple users, each with their own specific data
requirements, while minimizing the control signaling and associated delays. An
example of HSUPA uplink and downlink signaling is provided in Figure 1.
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Figure 1. Uplink and Downlink Signaling for HSUPA |
First, the Node B needs to know whether each user has any data that it wishes to
transmit. Two methods are defined by the 3GPP standards to address this problem.
Each user may periodically inform the network about the status of its uplink
data buffers. This Scheduling Information (SI) includes details about the amount
of data waiting to be transmitted and its corresponding priority.
While the SI contains many details, it is complex to encode and presents a
comparatively large overhead in signaling data. Consequently, a second method
provides just a single bit of information that allows the handset to indicate
whether the Node B has allocated sufficient resources for it to transmit its
data. Thoughtfully termed the Happy Bit, this information is transmitted every 2
ms and allows the Node B to fine tune its uplink resource allocations.
With a method defined to indicate the amount of data each user wishes to send,
the next question is how does the Node B inform the user of its bandwidth
allocation? The solution is for the Node B to allocate each user with a
transmission grant that indicates the proportion of his transmit power that may
be used to send HSUPA data. The grant effectively equates to a maximum data rate
at which the user is permitted to transmit until a new grant is issued.
Two mechanisms are provided for signaling this grant: the Absolute Grant and the
Relative Grant. As the names suggest, the Absolute Grant specifies a precise
value. The Relative Grant simply indicates a single step offset.
Since the Absolute Grant contains more information and a larger downlink
signaling overhead, it is likely that it would be used relatively infrequently.
Instead, the Relative Grant, requiring a much lower signaling overhead, could be
used for fine adjustments to the operation of each user. Further reductions in
downlink signaling overhead can be achieved by sending the same grant
information to multiple users by sharing the Absolute or Relative Grant
channels.
Soft Handover
A final twist to the scheduling management is the capability of an HSUPA network
to support soft handover. Soft handover, a technique widely associated with CDMA
systems, is the process where a user receives data from and transmits data to
multiple base stations. This provides particular benefits at the cell edges
where signal quality might be poor.
For an HSUPA user, the importance of soft handover has two aspects: power
control and resource allocation. First, as a user transmits data, he generates
noise-like interference to other users within nearby cells. If a user is near
the cell edge, he will have to transmit at a higher power to compensate for
being farther away from the Node B, causing greater interference. This effect
can be offset by soft handover where the uplink data is received and combined by
multiple base stations, reducing handset transmit power and the resulting
interference.
The second consideration applies to resource allocation. For example, an HSUPA
user in a lightly loaded cell may be given a grant that allows it to transmit at
high data rates, maximizing the benefit to that user. However, if the user then
moves toward a heavily loaded cell, his transmissions could result in excessive
interference. The support of soft handover in HSUPA provides a mechanism for the
new cell to directly manage the interference from this new user.
The HSUPA network handles the uplink data rate from each user by means of
allocating grants. By controlling the data rates, the network manages the
interference generated by each user.
Figure 2 illustrates the network management in a typical soft handover scenario.
At any instant in time, each user has a master serving cell. The serving cell is
responsible for managing the uplink data requests from each of its users and
allocating appropriate grants. In a soft handover scenario, other non-serving
cells may be configured to decode the uplink HSUPA data.
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Figure 2. Soft Handover Scenario |
In the example in Figure 2, the serving cell is lightly loaded, and the user has
been issued a large grant. However, one of the non-serving cells is supporting a
higher traffic load. Therefore, the non-serving cell is able to signal a
Relative Grant to users to request that they reduce their transmit power.
This mechanism is a key feature of HSUPA since it allows different Node Bs to
manage the uplink air interface without involving the RNC. This, in turn, means
that the network can react much more quickly to changes in user data
requirements and interference scenarios to maintain optimal resource allocation.
HSUPA—Slideware or Reality?
HSUPA represents a significant enhancement in the evolution of cellular
technology. In conjunction with HSDPA, it paves the way for mobile, easy-to-use,
and secure broadband services that we currently tend to associate with fixed
wireline or short-range wireless systems. What is more, the efficient use of the
air interface will provide improved quality and a wider range of enhanced
services.
With network trials planned for early 2007 and rollout commencing later that
year, HSUPA is a reality. The leading network manufacturers already are at the
complex stage of testing and validating HSUPA Node B implementations.
In the future, cellular systems will continue to evolve, and the 3GPP standards
groups already are working on the long-term evolution of this technology. By
considering upgrades and enhancements to the network architecture, new
air-interface modulation schemes, and advanced multiple RF antenna techniques,
cellular capability will continue to increase to meet the expected growth in
mobile data services.
About the Author
Nick Hallam-Baker is the product manager for the Wireless TM500 Test Mobile
product line at Aeroflex. e-mail:
Nick.Hallam-Baker@aeroflex.com.
Aeroflex, Cambridge Technology Centre, Melbourn, Hertfordshire SG8 6DP, 44
(0)1763 262277 |