Wcdma Lec 6_22nov14

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WCDMA/UMTS Network Advanced Mobile Communication Lec-6 HSDPA Overview

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Lecture on WCDMA Lec 6_22Nov14

Transcript of Wcdma Lec 6_22nov14

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WCDMA/UMTS Network

Advanced Mobile Communication Lec-6

HSDPA Overview

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Offers enhanced data rate for Best Effort Packet Data.

Increases system capacity and spectrum efficiency.

Lowers the latency/Round Trip Time

Thus HSDPA brings benefit to both the Operators and end users.

High Speed Downlink Packet Access

4.8 Mbps5 Codes16 QAM

14.4 Mbps15 Codes16 QAM

21.6 Mbps15 Codes64 QAM

28.8 Mbps15 Codes

MIMO

42.2 Mbps2x15 Codes

64 QAM

2005 2007 2008 2009

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HSDPA employ the following key technologies to achieve high data rates;

• Shared Channel Transmission• Higher Order Modulation Scheme (16 QAM)• Link Adaptation• Radio Channel Dependent Scheduling• Hybrid ARQ with Soft Combining.

HSDPA also utilizes the following enhancements;

• Shorter TTI• Dynamic Power Allocation

In order to further increase the data rates, following technologies have been incorporated into the standard and are already deployed in 3G networks on large scale;

• 64 QAM• Dual Carrier• Multiple Input Multiple Output (MIMO 2x2)

HSDPA MechanismsIn order to support last three technologies more efficiently and with minimum delay, MAC layer functionality is added to Node B. This new entity is called MAC-hs. (In R99 , transport channels are handed by RNC, since MAC layer was not implemented in Node B). MAC-hs reduces the transmission delay for hybrid ARQ and allow up-to-date channel quality estimates for link adaptation and channel dependent scheduling.

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Shorter TTI to reduce the air interface delay by reducing the Round Trip Times

Less probability of an error due to outage of the channel conditions.

More efficient when packet transmission is necessary

Decreased Buffer size

Short TTI is also necessary to benefit from other functionalities such as fast link adaptation, fast scheduling and fast hybrid ARQ

Shorter TTI (2msec)

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• A new DL Transport Channel (HS-DSCH) is introduced.

• Part of total downlink code resource is dynamically shared between a set of packet-data users. The codes are allocated when they are actually be used for transmission. This leads to efficient code and power utilization.

• Maximum 15 channelization with SF=16 can be used for this new channel. Per cell configurable.

• Code sharing is also possible through the use of different subsets of complete channelization code set for different users. With code multiplexing, several users can be scheduled within one TTI.

• Node B allocates the codes to the users every 2 msec.

Shared Channel Transmission

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After serving common and dedicated channel, remaining cell power can be assigned to HS-DSCH. Resulting in more efficient use of cell power.

No fast power control on HS-DSCH. Instead, user data rate is varied according to the instantaneous radio conditions and the available power in the cell.

Dynamic Power Allocation

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• R99 uses QPSK for downlink transmission. To support higher data rates, higher order modulation schemes 16QAM and 64 QAM can be used.

• 16QAM is more bandwidth efficient i.e. can carry more bits per Hz.

• Higher order modulation schemes require greater receive energy per bit (i.e. useful in better radio conditions e.g. close to cell).

• The modulation scheme is part of the Transport Format Resource Combination (TFRC)

Higher Order Modulation Schemes

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Radio link conditions keep on varying for a user. • Path loss and Shadowing• Interference level variations• Fast multipath fading

The goal is ensure sufficient Eb/No for all communication links despite variations in channel conditions.Bit Energy = Power x (1/Data Rate)

Power Control achieves this by adjusting Tx power while keeping data rate constant. This works well for constant data rate services but not a very efficient method for services that do not require a fixed data rate e.g. best effort services. Power control is used in R99.

This gives another option to compensate for varying radio conditions i.e. data rate adjustment. This is also referred to as (fast) link adaptation. This is achieved by;• Adjusting channel coding rate (R = 1/3 to 1)• Adjusting Modulation Scheme (QPSK, 16QAM and 64QAM)• The link adaptation is done on 2 msec TTI basis => fast.

Fast Link Adaptation

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In a conventional ARQ scheme, received data blocks that cannot be correctly decoded are discarded and retransmitted data blocks separately decoded.

In case of hybrid ARQ with soft combining, received data blocks that cannot be correctly decoded are not discarded. Instead the corresponding received signal is buffered and soft combined with later received retransmissions of the same set of information bits. Decoding is then applied to the combined signal.

The use of hybrid ARQ with soft combining increases the probability for correct decoding of retransmissions, compared to conventional ARQ.

Fast Hybrid ARQ with Soft Combining

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Fast Channel Dependent Scheduling Scheduling is about deciding which UE to transmit at a given time and what at rate;

• Formally part of MAC-hs (in Node B)• Might include QoS and priorities per service and user.

The basic idea is to transmit at the fading peaks of the channel in order to increase the capacity and to use resources more efficiently.

Following three scheduling algorithms are implemented;

• Round Robin: Radio resources allocated to users on sequential basis.• Proportional Fair: Schedules all users in the cell but prioritizes users with better channel quality, ensuring that

nonetheless all users receive a guaranteed minimum throughput.• Maximum CQI: Users with the best instantaneous channel conditions are allocated as much as possible.

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High Speed Downlink Shared Channel (HS-DSCH): Transport channel that carried user data

High Speed Physical Downlink Shared Channel (HS-PDSCH): Physical Downlink Channel that carries the user data and layer 2 overhead bits over the air interface.

High Speed Shared Control Channel (HS-SCCH): Physical DL channel that carries control information necessary to decode the HS-PDSCH.

High Speed Dedicated Physical Control Channel (HS-DPCCH): Physical Uplink Channel for sending ACK/NACK reports and Channel Quality reports.

HSDPA Transport and Physical Channels

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HS-DSCH: High Speed Downlink Shared Channel HS-DSCH is a Transport channel used to carry user data.

HS-DSCH is never in soft handover.

HS-DSCH is mapped to one or several HS-PDSCH (SF=16) which are simultaneously received by the UE i.e. HS-DSCH uses a common channelization code resource dynamically shared among several users.

The dynamic allocation of channelization codes is done on 2msec TTI basis.

There is at most one HS-DSCH per UE and there is at most one HS-DSCH transport block of dynamic size.

HS-DSCH is always associated with an UL/DL DPCH called Associated DCH (A-DCH). Downlink A-DCH is used for Layer 3 Control signaling.

Downlink signaling information necessary to operate HS-DSCH is carried by shared channel HS-SCCH. Uplink related signaling information is carried by HS-DPCCH.

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HS-DSCH: High Speed Downlink Shared Channel

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Downlink Physical Channel with SF = 16.

15 consecutive OVSF codes can be used for the set of HS-DSCH. Thus, up to 15 HS-PDSCHs can be used to carry HS-DSCH.

HS-PDSCH is not power controlled.

It carries user data and layer 2 overhead bits mapped from the transport channel: HS-DSCH.

HS-PDSCH: High Speed Physical Downlink Shared Channel

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HS-SCCH: High Speed Shared Control Channel Downlink channel used to carry Physical Layer Control Channel.

If no data is carried by HS-DSCH then no HS-SCCH is transmitted.

1 HS-SCCH can carry signaling for 4 UEs. This imply that 4 users can be code division multiplexed in one TTI. HS-SCCH spans 3 slots for each TTI and uses SF = 128. However, channelization code is not fixed.

HS-SCCH consists of 2 parts;

Slot 1: Contains Modulation Type and Channelization code. Scrambled with UE ID. The first part is needed prior to HS-PDSCH demodulation. UE ID is needed to determine if an HS-SCCH or which HS-SCCH contains control information for the UE.

Slot 2 and Slot 3: Contains transport block size and HARQ parameters (ARQ process number, redundancy version and new data indicator)

HS-PDSCH is transmitted with a delay of 2 slots in relation to its associated HS-SCCH.

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HS-DPCCH: High Speed Downlink Physical Control Channel Uplink Physical channel for HSDPA. It spans 3 time slots of 2 msec with a fixed SF = 256.

1 HS-DPCCH per user in the cell.

HS-DPCCH consists of 2 parts;

• Slot 1: Contains ACK/NACK information. Channel coded using (10,1) repetition code.

• Slot 2 and 3: Carry the CQI information i.e. information reflecting the instantaneous down-link radio channel conditions to assist the node B in transport format selection (fast link adaptation) and the scheduling. This is based on channel quality measurements based on CPICH. Channel coding is done using (20,5) repetition code.

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A-DCH: Associated Dedicated Channel

1 R99 DCH Downlink/Uplink is associated with each HSDPA user.

A-DCH Downlink

• DCH channel used to transmit power control information for the UL associated DCH and other potentially needed Layer 3 signalling (e.g. bearer reconfigurations, RRC Measurement Control Messages etc.)

A-DCH Uplink

• DCH channel used to send UL user data (64, 128, 384kbps), RLC and TCP ACKs, HTTP requests etc.

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Overall Timing Relation Node B transmits control information on HS-SCCH relating to how to decode and which UE shall decode

the next transport block on HS-PDSCH.

After transport block is received on HS-PDSCH, the UE will process the data and measure quality on CPICH over 5 msec (7.5 slots).

After data is processed, UE sends ACK/NACK and CQI on HS-DPCCH. ACK/NACK is based on CRC of the transport block and CQI is based on measurements on the CPICH.

When ACK/NACK and CQI report is received at the Node B, it takes 2.5 sec (2.5 slots) to process the data and make a scheduling decision based on report received on HS-SCCH.

This gives RTT of 12 msec per HARQ process.

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Hybrid ARQ Processes

One Hybrid ARQ (HARQ) entity per user.

Each HARQ entity consist of up to several HARQ processes.

• There can be 8 HARQ processes active for a UE. (If MIMO is used then there will be 2x6 HARQ processes)

• Multiple HARQ processes allow continuous transmission to a single user.

• Separate re-ordering function needed to support in-order delivery.

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HSDPA Deployment Strategies HSDPA can be introduced to the network with shared or with dedicated carrier

• Carrier shared between HSDPA and R99

• Operator definable or dynamic resource sharing between HSDPA and R99

– Preferred in low loaded/pre-launch networks– Can cause performance degradation to R99 users. Increased

interference due to HSDPA power will reduce R99 coverage (lower CPICH Ec/No).

• Dedicated HSDPA carrier

• HSDPA UE directed to HSDPA carrier based on 3GPP Release or UE capability and QoS request

– Preferred in loaded networks– Can provide high HSDPA throughput

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HSDPA introduces 2 new measurements;

• Periodic Measurements - used in Serving HS-DSCH Cell Selection Procedure

• Event 1d HS - used to evaluate and trigger HS-DSCH Cell Change when UE is on a PS Interactive 64/HS or 384/HS channel.

These are complementary measurements to normal 1a, 1b, 1c and 1d measurements, which are used for soft/softer HO on A-DCH.

Event triggered measurements and mobility on A-DCH will be an input to take HSDPA mobility decisions.

Measurement Handling in HSDPA

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Before the PS Interactive Radio Bearer can be setup on an HS-DSCH channel, a suitable serving HS-DSCH cell shall be selected.

To make that accurate estimation, which is the best serving cell, the RNC needs to have fresh information, which is achieved by forcing the UE to send 2 periodic measurement reports.

When the RRC message ‘Initial Direct Transfer’ is sent towards the PS domain the UE is ordered to report CPICH RSCP and Ec/No for the cells in the active set. This is achieved by sending a new ‘Measurement Control’ message, which will force the UE to send 2 reports for all cells in the Active Set.

The best cell is the cell in the active set that has the best quality in terms of RSCP or Ec/No. HSDPA connection is setup in the best cell.

HSDPA Cell Selection

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Directed RRC setup (Name of Nokia feature. A similar functionality is achieved by different vendors through their respective implementations)

• Is the feature that is used to move only R5 UEs to the HSDPA layer.• DRRC setup works when UE starts call from idle state. • All idle mobiles are forced to camp on f1

Access stratum release indicator and establishment cause reported in RRC connection setup request

Any UE reporting Rel’5 is directed to HSDPA layer, others to Rel’99 layer

Directed RRC setup for HSDPA layer

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The HS-DSCH Cell Change procedures will try to keep the best cell in the active set as the HS-DSCH serving cell.

• Serving HS-DSCH Cell Change - triggered by change of "Best Cell" within the Active Set. Event 1d hs.

• Serving HS-DSCH Cell Change - triggered by removal of the serving HS-DSCH cell from the Active Set. Event 1b or 1c.

• A serving HSDSCH Cell Change can only be performed to an intra-RNC cell in the active set that supports HSDPA.

HS-DSCH Mobility: Serving HS-DSCH Cell Change

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HS-DSCH Mobility: Channel Switching Coverage triggered downswitch to DCH

• The connection quality monitoring using events 2d, 2f, 6a and 6b is used to detect bad quality in UL or DL when using HS-DSCH. Compressed mode and IF or IRAT Handover is not supported for HSDPA, and instead a downswitch to DCH will be made.

Throughput Triggered downswitch to DCH

• Based on inactivity. Transition to Cell_FACH

Throughput Triggered Upswitch

• This is triggered by high channel utilization in the UL or DL as indicated by a measured channel throughput that is close to the maximum capacity of the current transport channel. The UE will be switched to a state with a higher rate in the link that triggered the switch. The HS-DSCH is the preferred channel for the DL regardless of whether the trigger is from the UL or the DL.

• HS-DSCH Cell Selection can also be regarded as a throughput based upswitch. It can be triggered at RAB establishment of a PS interactive connection, by an upswitch attempt from FACH, or by a throughput-based activity trigger for an already established PS interactive connection on DCH.

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UE Categories

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MIMO: Multiple Input Multiple Output An optional feature in WCDMA UTRAN and UE in

3GPP Rel 7. The feature is enabled on RNC or cell level.

It supports precoder based 2x2 SU MIMO. This feature allows for transmission of one or two data streams of HS-DSCH from 2 antenna arrays and reception on 2 antenna arrays in the downlink.

Existing Dual polarized antenna can be used. For each antenna port separate power amplifier is required.

The weighting of streams and distribution over the two antennas is called precoding. The choice of precoder is based on the feedback from the UE, (however, it is not binding).

This feedback is known as Precoding Control Info (PCI) and is sent by UE along with CQI measurements. PCI is based on pilot channel measurements from the two antenna branches.

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An optional feature in WCDMA UTRAN and UE in 3GPP Rel 8. The feature is enabled on RNC or cell level. This enables one UE to simultaneously use the HS-DSCHs of two cells.

The frequency of serving HS-DSCH and the secondary HS-DSCH are adjacent in order to enable wideband (10 MHz) RF front ends.

Multi Carrier is supported using all HS-DSCH modulation schemes, including 64-QAM.

MIMO and MC are not supported simultaneously

In connection to one UE. However, same cell may simultaneous support UEs in MC mode and other UEs in MIMO mode.

For a PS Interactive RAB, MC will take precedence over MIMO.

Multi-Carrier (MC)