14-02-2013, 09:44 AM
High Speed Downlink Packet Access: WCDMA Evolution
[attachment=50790]
ABSTRACT
This article gives an overview of the high speed downlink
packet access (HSDPA) concept; a new feature which is coming
to the Release 5 specifications of the 3GPP
WCDMA/UTRA-FDD standard. To support an evolution towards
more sophisticated network and multimedia services,
themain target of HSDPA is to increase user peak data rates,
quality of service, and to generally improve spectral efficiency
for downlink asymmetrical and bursty packet data services.
This is accomplished by introducing a fast and complex channel
control mechanism based on a short and fixed packet
transmission time interval (TTI), adaptive modulation and
coding (AMC), and fast physical layer (L1) hybrid ARQ.To facilitate
fast schedulingwith a per-TTI resolution in coherence
with the instantaneous air interface load, theHSDPA-related
MAC functionality is moved to the Node-B. The HSDPA concept
facilitates peak data rates exceeding 2 Mbps (theoretically
up to and exceeding 10 Mbps), and the cell throughput
gain over previous UTRA-FDD releases has been evaluated to
be in the order of 50-100% or even more, highly dependent on
factors such as the radio environment and the service provision
strategy of the network operator.
Introduction
Data services are anticipated to have an enourmous rate of
growth over the next years (the so-called data tornado) and
will likely become the dominating source of traffic load in 3G
mobile cellular networks. Example applications to supplement
speech services include multiplayer games, instant
messaging, online shopping, face-to-face videoconferences,
movies, music, as well as personal/public database access.
As more sophisticated services evolve, a major challenge of
cellular systems design is to achieve a high system capacity
and simultaneously facilitate a mixture of diverse services
with very different quality of service (QoS) requirements.
Various traffic classes exhibit very different traffic symmetry
and bandwidth requirements. For example, two-way
speech services (conversational class) require strict adherence
to channel symmetry and very tight latency, while
Internet download services (background class) are often
asymmetrical and are tolerant to latency. The streaming
class, on the other hand, typically exhibits tight latency requirements
with most of the traffic carried in the downlink
direction.
Concept Description
The fundamental characteristics of the HS-DSCH and the
DSCH are compared in Table 1. On the HS-DSCH, two fundamentalCDMAfeatures,
namely variable spreading factor
(VSF) and fast power control, have been deactivated and replaced
by AMC, short packet size, multi-code operation, and
fast L1 hybridARQ(HARQ). While being more complicated,
the replacement of fast power control with fastAMCyields a
power efficiency gain due to an elimination of the inherent
power control overhead. Specifically, the spreading factor
(SF) has been fixed to 16, which gives a good data rate resolution
with reasonable complexity. In order to increase the
link adaptation rate and efficiency of the AMC, the packet
duration has been reduced from normally 10 or 20 ms down
to a fixed duration of 2 ms. To achieve low delays in the link
control, the MAC functionality for the HS-DSCH has been
moved from the RNC to the Node-B.
Modulation and coding options in HSDPA
To substitute the functionality of fast power control and
VSF, the modulation, coding, and multi-code part of HSDPA
must cover a wide dynamic range corresponding to the
channel quality variations experienced at theUE(including
fast as well as distance-dependent variations). The means of
adaptation are the code rate, the modulation scheme, the
number of multi-codes employed, as well as the transmit
power per code. The HS-DSCH encoding scheme is based on
the Release 99 rate-1/3 Turbo encoder but adds rate matching
with puncturing and repetition to obtain a high resolution
on the effective code rate (approximately from 1/6 to
1/1). To facilitate very high peak data rates, the HSDPAconcept
adds 16QAM on top of the existing QPSK scheme available
in Release 99. The combination of 16QAM and e.g.
rate-¾ channel encoding enables a peak data rate of 712
kbps per code (SF=16). Higher robustness is available with
a QPSK rate-¼ scheme but at the penalty of having only a
119 kbps data rate per code.
Spectral and code efficiency
Before reaching the pole capacity, a synchronous WCDMA
system may be capacity limited due to either a power shortage
or a code shortage. One of the major benefits of the
HSDPA concept is the ability to make a tradeoff among
power and code efficiency to accommodate the current state
of the cell. This aspect is illustrated in Figure 3, where the
five example TFRCs are plotted in a diagram showing both
their power efficiency (measured as allowed noise power to
user bit energy ratio for a BLER of 10%, e.g. I0/Ei) and their
respective code efficiency (measured as supported data rate
per code). If the Node-B has relatively more power resources
than code resources available (code limited), the link adaptation
algorithm will optimize for a more code efficient
TFRCwhile a more robust TFRCwith more multi-codes will
be used when the Node-B is mainly power limited.
Performance
The performance of theHS-DSCHdepends on a large number
of aspects, such as (i) channel conditions including othercell
interference and time dispersion, (ii)UEdemodulation performance
and capability, (iii) nature and accuracy of RRM algorithms,
and (iv) hardware imperfections. The throughput performance
for a single link employing link adaptation is shown
for different channel profiles and average Ior/Ioc values in Figure
6 versus the code power allocation. In the estimation of the
UE channel quality (Es/No) at the Node-B some errormust be
expected. In these simulations, a lognormally distributed error
with a standard deviation of 1 dB and a 2 ms AMC delay
have been assumed. In general, theHARQ mechanismmakes
the LA very robust towards channel estimation errors and
scheduling delays. With fast L1 HARQ, the degradation in
throughput due to channel estimation errors is approximately
halved compared to an AMC system without HARQ [11].
Continued Evolution
HSDPA provides a significant cell capacity gain for packet
data traffic in WCDMAand is thus an important part of the
continuous 3G evolution. Since the HSDPA concept offers
improved code efficiency and dynamic range in user data
rates, it can utilize improvements in detector performance
foreseen in the future. Hence, it may be viewed as an
enabler for more advanced communication techniques, including
equalizers, multi-user or multi-code interference
cancellation, as well as advanced multiple input multiple
output (MIMO) techniques. The HSDPA concept can be introduced
gradually in the network with incremental introduction
of advanced packet scheduling and link enhancement
strategies. The performance and cost/complexity
issues of further improvements will be considered within future
3GPP standardization framework to further evolve the
WCDMA concept.
[attachment=50790]
ABSTRACT
This article gives an overview of the high speed downlink
packet access (HSDPA) concept; a new feature which is coming
to the Release 5 specifications of the 3GPP
WCDMA/UTRA-FDD standard. To support an evolution towards
more sophisticated network and multimedia services,
themain target of HSDPA is to increase user peak data rates,
quality of service, and to generally improve spectral efficiency
for downlink asymmetrical and bursty packet data services.
This is accomplished by introducing a fast and complex channel
control mechanism based on a short and fixed packet
transmission time interval (TTI), adaptive modulation and
coding (AMC), and fast physical layer (L1) hybrid ARQ.To facilitate
fast schedulingwith a per-TTI resolution in coherence
with the instantaneous air interface load, theHSDPA-related
MAC functionality is moved to the Node-B. The HSDPA concept
facilitates peak data rates exceeding 2 Mbps (theoretically
up to and exceeding 10 Mbps), and the cell throughput
gain over previous UTRA-FDD releases has been evaluated to
be in the order of 50-100% or even more, highly dependent on
factors such as the radio environment and the service provision
strategy of the network operator.
Introduction
Data services are anticipated to have an enourmous rate of
growth over the next years (the so-called data tornado) and
will likely become the dominating source of traffic load in 3G
mobile cellular networks. Example applications to supplement
speech services include multiplayer games, instant
messaging, online shopping, face-to-face videoconferences,
movies, music, as well as personal/public database access.
As more sophisticated services evolve, a major challenge of
cellular systems design is to achieve a high system capacity
and simultaneously facilitate a mixture of diverse services
with very different quality of service (QoS) requirements.
Various traffic classes exhibit very different traffic symmetry
and bandwidth requirements. For example, two-way
speech services (conversational class) require strict adherence
to channel symmetry and very tight latency, while
Internet download services (background class) are often
asymmetrical and are tolerant to latency. The streaming
class, on the other hand, typically exhibits tight latency requirements
with most of the traffic carried in the downlink
direction.
Concept Description
The fundamental characteristics of the HS-DSCH and the
DSCH are compared in Table 1. On the HS-DSCH, two fundamentalCDMAfeatures,
namely variable spreading factor
(VSF) and fast power control, have been deactivated and replaced
by AMC, short packet size, multi-code operation, and
fast L1 hybridARQ(HARQ). While being more complicated,
the replacement of fast power control with fastAMCyields a
power efficiency gain due to an elimination of the inherent
power control overhead. Specifically, the spreading factor
(SF) has been fixed to 16, which gives a good data rate resolution
with reasonable complexity. In order to increase the
link adaptation rate and efficiency of the AMC, the packet
duration has been reduced from normally 10 or 20 ms down
to a fixed duration of 2 ms. To achieve low delays in the link
control, the MAC functionality for the HS-DSCH has been
moved from the RNC to the Node-B.
Modulation and coding options in HSDPA
To substitute the functionality of fast power control and
VSF, the modulation, coding, and multi-code part of HSDPA
must cover a wide dynamic range corresponding to the
channel quality variations experienced at theUE(including
fast as well as distance-dependent variations). The means of
adaptation are the code rate, the modulation scheme, the
number of multi-codes employed, as well as the transmit
power per code. The HS-DSCH encoding scheme is based on
the Release 99 rate-1/3 Turbo encoder but adds rate matching
with puncturing and repetition to obtain a high resolution
on the effective code rate (approximately from 1/6 to
1/1). To facilitate very high peak data rates, the HSDPAconcept
adds 16QAM on top of the existing QPSK scheme available
in Release 99. The combination of 16QAM and e.g.
rate-¾ channel encoding enables a peak data rate of 712
kbps per code (SF=16). Higher robustness is available with
a QPSK rate-¼ scheme but at the penalty of having only a
119 kbps data rate per code.
Spectral and code efficiency
Before reaching the pole capacity, a synchronous WCDMA
system may be capacity limited due to either a power shortage
or a code shortage. One of the major benefits of the
HSDPA concept is the ability to make a tradeoff among
power and code efficiency to accommodate the current state
of the cell. This aspect is illustrated in Figure 3, where the
five example TFRCs are plotted in a diagram showing both
their power efficiency (measured as allowed noise power to
user bit energy ratio for a BLER of 10%, e.g. I0/Ei) and their
respective code efficiency (measured as supported data rate
per code). If the Node-B has relatively more power resources
than code resources available (code limited), the link adaptation
algorithm will optimize for a more code efficient
TFRCwhile a more robust TFRCwith more multi-codes will
be used when the Node-B is mainly power limited.
Performance
The performance of theHS-DSCHdepends on a large number
of aspects, such as (i) channel conditions including othercell
interference and time dispersion, (ii)UEdemodulation performance
and capability, (iii) nature and accuracy of RRM algorithms,
and (iv) hardware imperfections. The throughput performance
for a single link employing link adaptation is shown
for different channel profiles and average Ior/Ioc values in Figure
6 versus the code power allocation. In the estimation of the
UE channel quality (Es/No) at the Node-B some errormust be
expected. In these simulations, a lognormally distributed error
with a standard deviation of 1 dB and a 2 ms AMC delay
have been assumed. In general, theHARQ mechanismmakes
the LA very robust towards channel estimation errors and
scheduling delays. With fast L1 HARQ, the degradation in
throughput due to channel estimation errors is approximately
halved compared to an AMC system without HARQ [11].
Continued Evolution
HSDPA provides a significant cell capacity gain for packet
data traffic in WCDMAand is thus an important part of the
continuous 3G evolution. Since the HSDPA concept offers
improved code efficiency and dynamic range in user data
rates, it can utilize improvements in detector performance
foreseen in the future. Hence, it may be viewed as an
enabler for more advanced communication techniques, including
equalizers, multi-user or multi-code interference
cancellation, as well as advanced multiple input multiple
output (MIMO) techniques. The HSDPA concept can be introduced
gradually in the network with incremental introduction
of advanced packet scheduling and link enhancement
strategies. The performance and cost/complexity
issues of further improvements will be considered within future
3GPP standardization framework to further evolve the
WCDMA concept.