HSDPA DimensioningChannelization codes allocated
for HS-DSCH transmission
8 codes (example)
Smooth Upgrade
STANDARDIZED
REDUCED DELAY
SPEED
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HSDPA is an improvement of best-effort packet data on the DL in WCDMA in terms of speed, capacity and end user performance.
Higher bit rates up to 14 Mbps and system throughput which is 2-3 times higher than WCDMA Release 99.
End user performance is also improved by means of reduced Round Trip time and higher bit rates.
HSDPA is standardized. The current WCDMA is extended by HSDPA in Release 5 of 3GPP specifications. Mainly, a new DL transport channel is introduced that enhances support for interactive, background and, to some extent, streaming services and some powerful RRM functionality is introduced.
In a WCDMA network, it is possible not to upgrade all cells with HSDPA functionality since we have mobility between HSDPA and non HSDPA cells.
We don’t need a separate carrier for HSDPA. Voice and data on the same.
We have simultaneous use of high speed packet data bearers and conversational bearers.
The basic RAN architecture supports HSDPA. The only need is to have some additional HW in RBS and additional SW for both RBS, RNC and RXI nodes.
All of these make a smooth and cost-efficient upgrade to HSDPA possible.
Slide *
Fast adaptation of transmission parameters to fast variations in radio conditions
Main functionality to support HSDPA
Fast link adaptation
Fast Hybrid ARQ
Fast channel-dependent scheduling
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The main idea in HSDPA is to use the radio resources, mainly power and code, in an optimal way, that is possible by adapting rapidly to the instantaneous radio conditions.
3 main functionalities which makes this possible are Fast Link Adaptation, Fast Hybrid ARQ (Automatic Repeat Request) and Fast Channel-dependent Scheduling. We shall look at them in the next slide.
Slide *
2 ms time basis
Shared Channel Transmission
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What are the new features that enables the high data rates and better system throughput? We shall shortly introduce them in this slide and go into details in the coming slides.
We have Short Transmission Time Interval, shorter than R99. It is only 2 milliseconds and therefore reduces the round trip time. The main benefit from shorter TTI is the reduced delays and therefore improved end-user performance. It also makes RRM functionalities much faster.
Then we have Shared Channel Transmission. One HS dedicated channel shared dynamically by all HSDPA users , mainly in the time domain. That results in system capacity gain.
Fast Scheduling gives priority to the users with favorable radio conditions. This is also something which contributes to system capacity.
Fast link adaptation gives maximum channel utilization because data rate is adapted to radio conditions. Higher order modulation gives higher data rates.
And finally, Fast retransmissions together with soft combining leads to better end-user performance and capacity increase.
Slide *
Fast Link Adaptation
Fast hybrid ARQ
Fast Channel-dependent Scheduling
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Short TTI is the main reason for the reduced air-interface delay which comes from reduced Round Trip Times.
Of course it improves the overall delay and also tracking of channel conditions. Reduced latency is about 75 ms.
The end-user performance is improved in the TCP/IP based services since short TTI improves the interaction with TCP.
Main user benefit of HSDPA for web browsing as an example is the reduced round trip times since TCP slow start has a big impact in end-user performance when downloading many relatively small objects (like web pages).
And also short TTI is actually necessary to benefit from other functionalities such as fast link adaptation, fast scheduling and fast hybrid ARQ. Because they are performed in a 2 ms time frame which makes 500 times per second.
Just as a comparison with R99, it is good to know that the TTI for DCH is between 10-80 ms.
The TTI for DCH 64 PS is for example 20 ms.
Slide *
Network & Technology Consulting
Shared Channel Transmission
A set of radio resources dynamically shared among multiple users, primarily in the time domain
Efficient code utilization
Efficient power utilization
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In HSDPA, a new DL transport channel is introduced called high speed DL shared channel. The idea is that a part of the total downlink code resource is dynamically shared between a set of packet-data users, primarily in the time domain. The codes are allocated to a user only when they are actually to be used for transmission which leads to efficient code and power utilization.
In HSDPA, maximum 15 channelization codes with Spreading Factor (SF)= 16 can be used for this new DL channel. In P4, 5 channelization codes are used enabling user data rates up to 4.32 Mbps (the system is capable of enabling 4.32 Mbps). For UE capabilities, see slide XX.
The main benefit with DL shared channel transmission is to reduce the risk for code-limited capacity. Sharing codes in the code domain, in other words, code multiplexing (in P5), is also possible by employing different subsets of the complete channelization code set for different users.
Sharing in the code domain is useful for providing efficient support of small payloads when the transmitted data does not require the full set of HS-DSCH codes configured in the cell. Useful when supporting terminals cannot despread the full set of codes.
Number of codes which will be used in each cell is configured (P4) or slowly adapted by RNC according to # of resources needed for packet data services on HS channel and other services such as voice (P5). It’s the RBS which dynamically allocates the codes to the users every 2 ms.
Slide *
Network & Technology Consulting
Fast Channel-dependent Scheduling
Scheduling = which UE to transmit to at a given time instant and at what rate
Formally part of MAC-hs (a new MAC sub-layer in RBS)
Basic idea: transmit at fading peaks
May lead to large variations in data rate between users
Tradeoff: fairness vs cell throughput
high data rate
low data rate
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Fast Scheduling is about to decide to which terminal the shared channel transmission should be directed at any given moment.
It’s called channel-dependent scheduling because it’s dependent on the instantaneous channel condition.
The basic idea is to transmit at the fading peaks of the channel in order to increase the capacity and to use the resources more efficiently.
But this might lead to large variations in data rate of the users.
The trade-off is between the cell throughput and fairness against users. In some cases, there might be a particular user who is perhaps on the cell border which might not be allocated the radio resources because he does not have good enough C/I. Remember that we don’t have SHO for dedicated shared channel.
So there are a number of scheduling algorithms which takes into consideration the trade-of between throughput and fairness. And the next slide will be about them.
Slide *
Round Robin (RR)
Cyclically assign the channel to users without taking channel conditions into account
Simple but poor performance
Proportional Fair (PF)
Assign the channel to the user with the best relative channel quality
High throughput, fair
Max C/I Ratio
Assign the channel to the user with the best channel quality
High system throughput but not fair
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The scheduling strategy has a large impact on the system capacity and the end user performance. The variations in the radio-conditions should be utilized by a good scheduler. We have a couple of different schedulers.
Round Robin scheduler allocates the channel to users sequentially. Quite simple but offers rather poor performance.
Proportional Fair is the one which allocates the channel to the user with relatively best channel quality. It gives rather high throughput and rather fair, whereas Max C/I allocates the channel with absolutely best channel quality. It does not care about fairness at all.
A practical scheduler should operate somewhere between the RR and the max C/I scheduler and exactly where it should operate is dependent on the traffic load and traffic type among the other things. The higher the system load, the more visible the difference between the different scheduling algorithm. But Proportional Fair scheduler is proposed for Interactive/Best Effort traffic and also to avoid that some users get no throughput.
For streaming, traffic priorities can be taken into account. Streaming services before background services can be prioritized (P5+).
Short term variations in the channel such as multipath fading and variations in interference level can be acceptable and go unnoticed for many packet-data applications. Long term variations such as the distance between terminal and the RBS are more restrictive. So the practical scheduler should be fair to long-term variations and should exploit short term variations.
Slide *
Adjust transmission parameters to match instantaneous channel conditions
HS DL Shared Channel: Rate control (no Fast Power control)
Adaptive coding
Adapt on 2 ms TTI basis fast
R99: Power control (no Rate control constant data rate possible)
High data rate
Low data rate
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They can be combined to maximize the instantaneous use of the fading radio channel.
There is no Power control for the DL HS shared channel unlike the dedicated channels in R99.
Instead, fast link adaptation is used to maximize user throughput by adjusting data rate and the transmission power in RBS to the instantaneous radio condition.
And this is done in 2 ms time basis.
The system adjusts the data rate by varying the effective code rate and by changing the modulation scheme. If we compare this to R99 we can say that
With only powered controlled channels as in R99, it is difficult to come up to a maximal use of cell power.
The main idea here is that we can afford not having a constant data rate for some services (best-effort services) and for them we can exploit the total cell power.
On the other hand, we provide constant data rate for the other services through dedicated channels such as voice and video.
We shall talk about higher modulation in the next slide.
Slide *
16QAM is optional in RBS
16QAM is mandatory in the UE, except for the 2 lowest UE categories
16QAM gives approximately double data rates
16QAM is mainly useful at good radio conditions
16QAM typically requires more advanced receivers in the UE
2 bits
4 bits
but mandatory in the UE except for 2 UE categories.
It gives approximately double data rate as QPSK given a fixed code rate.
It is more bandwidth efficient but of course requires better channel conditions.
16 QAM is good in bandwidth limited scenarios but not in power limited scenarios. It’s basically good near to base station and low dispersive environments. A good example is micro and indoor cells.
With the introduction of G-Rakes or dual antennas in UEs, higher modulation will be chosen more often by link adaptation.
So in the future, higher modulation will be utilized more by means of advanced receivers in the terminals.
Slide *
Rapid retransmissions of erroneous data
Hybrid ARQ protocol terminated in Node B
short RTT (typical example: 12 ms)
Soft combining in UE of multiple transmission attempts
reduced error rates for retransmissions
P1,1
P1,1
NACK
P1,2
P1,2
ACK
P2,1
P2,1
NACK
P2,2
P2,2
ACK
P3,1
ACK
P1,1
P2,1
P3,1
Fast hybrid Automatic Repeat ReQuest allows UEs to rapidly request retransmissions of erroneously received transport blocks.
The UE attempts to decode each transport block it receives, reporting to RBS its success or failure 5 ms after the reception of the transport block.
The hybrid ARQ mechanism in RBS can rapidly respond to retransmissions requests.
This leads to shorter Round Trip Times.
The UE employs soft combining, that is it combines soft information from previous transmission attempts with the current transmission to increase the probability of soft combining.
This reduces error rates for retransmissions.
This functionality is mainly sort of fine tuning the effective code rate and compensating for errors made by link adaptation mechanism.
Slide *
HS-DSCH must share the transmission power with all other channels
Dynamic power allocation
Best power utilization
Dedicated channels (power controlled)
Total available cell power
We can briefly describe power allocation in HSDPA here.
HS-DSCH is dynamically allocated the remaining power after power has been allocated to other channels.
By other channels I mean Control channels and Dedicated Channels for R99 traffic and Control Channels for HSDPA.
Dynamic power allocation is very efficient because it allows for full use of the overall available cell power.
Typical power allocation for HS-DSCH can be in the range 30% to 80% of the overall base station power. In the higher case, the cell basically carries only HS-DSCH traffic, in addition to necessary control channels.
Slide *
new functionality in Node B!
New HW and SW in Node B
SW upgrade in RNC
R99: scheduling, TF selection, link layer (ARQ)
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An important objective of the HSDPA design is to retain the functional split between layers and nodes (as in R99). A minimum change in the architecture of RAN makes a smooth upgrade to HSDPA possible.
Once again, the key features are rapid adaptation of varying radio conditions and fast transmissions of data. Therefore the corresponding functionality should be placed close to the air interface.
That is the reason why we have Fast Link adaptation, scheduling and hybrid ARQ in RBS.
We can say that the introduction of HSDPA mainly affects RBS, in particular, through the addition of a new medium access control sub layer called MAC-hs.
Scheduling has been moved from RNC to RBS which improves Iub. This makes SHO impossible for HSDPA since scheduling is handled by a single RBS. But instead of SHO gain we have a scheduling gain.
RBS 3000 can be upgraded by modifying the baseband hardware and by remotely loading the new software. A large part of the RBS software is related to MAC-hs functionality and the scheduler.
The users who leave HSDPA due to coverage problems can be switched to DCH channels by RNC (P5). Or from DCH to HSDPA switching is also done by RNC. RNC is already HW prepared but needs additional software. The same for RXI nodes.
Impact of CN from HSDPA should be minor. Ericsson SGSN already supports subscriber-based quality of service differentiation.
Slide *
L1 peak rates (Mbps)
QPSK / 16 QAM
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12 UE categories have been defined to accommodate low end and high end implementations of HSDPA with peak data rates from 0.9 to 14 Mbps.
The peak rates and user throughput are mainly dependent on the modulation and the maximum number of codes in the system that the HS channel can use. The ”user data throughput” is the throughput for Layer 1 excluding MAC/RLC headers. This is the ”useful” data rate for an user. The system (in P4) is capable of transmitting 4.32 Mbps (user data rate). But it’s the UEs which limit the capacity. Higher end-user data rates will be possible in future by means of UEs having advanced receivers such as G-RAKE.
Self-interference coming from multipath propagation low C/I probability of QAM is low. Solution: G-RAKE to suppress self-interference and easily corporate into the current RAKE implementation. higher end-user data rates
Category: number of supported codes, minimum inter TTI interval, number of soft values in terminal’s hybrid ARQ buffer, L1 peak rate, modulation scheme.
Rapid retransmissions attempt to decode HS-DSCH bits in 5 ms and transmit ACK/NACK on the uplink HS-DPCCH
Main design challenges: the amount of buffering required in the digital baseband and the need for HS baseband processing.
Slide *
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Now we can start looking at the channel structure in HSDPA.
A-DCH mobility including intra and inter-RNC. A-DCH has intra-RNC mobility means that HSDPA user can move to another HSDPA enabled cell.
No channel switching!
from cell_DCH just to idle state, i.e. no cell_FACH for HS users
No inter-frequency handover! UEs are moved to HSDPA frequency layer during RAB assignment in a multi-carrier network.
HS users are prioritised over DCH packet users by admission and congestion control.
Slide *
Associated Dedicated Channel – A-DCH
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HS-DSCH is used for data transmission and it is never in soft handover.
HS-DSCH mobility (w/o loss off PDP context) supported by means of cell selection, i.e. fall back to idle mode from cell_DCH and then a HSDPA enabled cell is chosen and an HSDPA connection is established. This happens when HSDPA user goes to another cell over Iur or leaves his HSDPA enabled cell for an non-HSDPA cell.
An HSDPA user can fall back to DCH due to coverage when leaving an HSDPA cell and enter into an non-HSDPA cell or due to the lack of free CE (This is just a P5 plan).
HS-SCCH is used for control signaling needed for HS-DSCH and it is never in soft handover either.
A-DCH UL/DL: A set of dedicated UL and DL channels for uplink traffic and DL services that are not carried on HS-DSCH. One A-DCH channel pair is set up for every HSDPA user, active in the cell.
A-DCH has SHO and Softer HO. A-DCH has intra-RNC mobility that means HSDPA user can move to another HSDPA enabled cell. A-DCH has also inter-RNC mobility.
Slide *
Control signalling to mobiles scheduled in a 2 ms interval
UE identity for which the HS-SCCH is intended (and HS-DSCH)
Informs the UE about:
HS-DSCH code set
Modulation scheme (QPSK/16QAM)
HS-DSCH transport format (number of transport blocks per TTI and number of bits per transport block)
Hybrid ARQ information
Never in soft handover
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The main task for HS-SCCH is to tell which UE that will receive data on HS-DSCH.
It carries control information from MAC-hs such as
identity of the terminal,
HS dedicated shared channel code set, modulation scheme and transport format selected by link adaptation mechanism,
One channel is shared by all users in P4.
It is possible to have more than one HS-SCCH per cell if code multiplexing is used (not in P4). (In release 5 at most 4 HS-SCCHs can be monitored.)
Never in SHO
A-DCH DL
A-DCH UL
3.4 kbps SRB (control signalling: RRC & NAS)
High-Speed Dedicated Physical Control Channel (HS-DPCCH)
ACK/NACK for H-ARQ
Can be in
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A-DCH (Associated Dedicated CH) in DL consists of 3.4 kbps SRB and can be in soft/softer handover. It carries
Release 99 signalling to support mobility between HS and non-HS cells.
It will be able to carry Voice/video (multi-RAB) in P5.
A-DCH UL consists of 384 (or 64) kbps data and + 3.4 kbps SRB + HS-DPCCH and can be in soft/softer handover except HS-DPCCH that only can be in softer since it is terminated in the RBS. It carries
Release 99 signalling
UL data transmission
ACK/NACK for Hybrid ARQ
CQI for scheduling
For UL, 384 kbps is chosen primarily. UL 64 kbps can be chosen due to coverage or lack of free CEs.
HS-DPCCH (UL) SF=256.
No need for new sites, no need for new spectrum/carrier
No need for RBS configuration
End user data rate is adapted to radio conditions
We can have the same cell range as in R99
HSDPA cell border throughput better than DCH (R99)
More power gives most gain in improving the coverage
HSDPA gives ~3 times more downlink capacity than DCH
More power gives considerable capacity improvement
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No need for new sites, no need for new spectrum, new carriers
No need for RBS configuration
End user data rate is adapted to radio conditions
We can have the same cell range as in R99
Better utilization of HSDPA with the use of high power class RBSs.
Why better coverage ?
Better power utilization
In HSDPA the bit rate change is done in smaller steps than for DCH possible to use all available power for traffic
Possible to use 100% of RBS power instead of 75%
Slide *
HS-DPCCH High-Speed Dedicated Physical Control Channel
HS-DSCH High-Speed Downlink Shared Channel
MAC-hs Medium Access Control - High Speed
NAS Non-Access Stratum
PS Packet Switched
R99 Release 99 of WCDMA specification
RA Rural Area
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Filiz.Gulkan@Ericsson.com