1-Scheduling in HSDPA

8/13/2019 1-Scheduling in HSDPA

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OUT LINE: 1-Why HSDPA????

2-Architecture.

3-MAC-hs sub-layer. 4-Scheduling in HSDPA.

5-Hyprid Arq and soft combining in HSDPA.

6-Data flow of HSDPA.


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1-Shared channel transmissions

In 3G the transmission is through dedicated channels.

HSDPA introduced shared channel transmission,where there is no dedicated channel or assigned codes,but there are a set of codes of SF 16 are timemultiplexed between users through different TTIs.

Shared channel transmission is simplified throughintroduction of HS-DSCH.

The idea is that the node B performs 3G dedicatedchannel transmissions and the remaining power isassigned to HSDPA users, which maximizes the powerefficiency of NB as it is always sending with max.power.


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-Same as the WCDMA but there is extra MAC sub-layer inserted in the node B to increase its functions

which will decrease the signaling and will tend tofaster adaptation to rapid variation.

-Minimization of the architectural changes is desirableas it simplifies introduction of HSDPA in already

deployed networks and also secures operation inenvironments where not all cells have been upgradedwith HSDPA functionality.


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-Each UE using HSDPA will receive HS-DSCHtransmission from one cell, the serving cell.

-Mobility from a cell supporting HSDPA to a cell that isnot supporting HSDPA is easily handled.

-Uninterrupted service to the user can be provided, albeitat a lower data rate, by using channel switching in theRNC and switch the user to a dedicated channel in thenon-HSDPA cell.

Similarly, a user equipped with an HSDPA-capableterminal may be switched from a dedicated channel toHSDPA when the user enters a cell with HSDPA support.


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RNCfunctions:RNC will :


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Admission control Decide if a new user can join this cell or no.

based on the Node B power.

-Done by measuring power of Node B.


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Congestion control:

There are another signaling processes beside previousones as the streaming services.

MAC-d priority queue must b taken into account.


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How measurements are done:


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3-MAC-hs sub-layerAs mentioned before, the MAC-hs is a new sub-layer

located in the Node B and responsible for:

1) the HS-DSCH scheduling,2)rate control

3) hybrid-ARQ protocol operation.

So the functions of the Node B has increased and now,

it is the Scheduler.


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-scheduling determines system performance.

-In HSDPA the scheduler (node B) decides which user ,HS-DSCH should be transmitted and at what data rate.

-efficient scheduling strategies require at least:

information about the instantaneous channelconditions at the UE.

information about the buffer status and priorities ofthe data flows.


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-Every UE send channel condition indicator (CQI) tonode at regular intervals.

-This CQI is calculated at the UE based on the SNR of

the pilot signal.-CQI is expressed as recommended transport-blocksized taking into account the receiver performance.

-So terminal with more advanced receiver will reportlarge CQI and will receive data at higher rate than theone with less advanced receiver at same channelconditions.


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Details of HSDPA scheduling: In addition to to channel quality, scheduler should

take into account buffer status and perriority

queues. Example :-streaming services.

-RRC signaling (handover between 2cells).

-UEs for which there is no data awaitingtransmission should not be scheduled.


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5-Hyprid-ARQ and soft combining

Hybrid-ARQ retransmissions are thereforesignificantly less costly in terms of delay compared toRLC retransmissions.

Due to: 1. There is no need for signaling between the

Node B and the RNC for the hybrid-ARQretransmission. Consequently, any Iub/Iur delays areavoided for retransmissions.

2. The RLC protocol is typically configured with

relatively infrequent status reports of erroneous datablocks (once per several TTIs) to reduce the signalingload, while the HSDPA hybrid-ARQ protocol allows forfrequent status reports (once per TTI), thus reducingthe roundtrip time.


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In HSDPA, the hybrid ARQ operates per transportblock or, equivalently, per TTI that is, whenever theHS-DSCH CRC indicates an error, a retransmissionrepresenting the same information as the original

transport block is requested.As there is a single transport block per TTI, the

content of the whole TTI is retransmitted in case of anerror. This reduces the amount of uplink signaling as a

singleACK/NAKbit per TTI is sufficient.


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Having multiple transport blocks per TTI with thepossibility for individual retransmissions were quite

small.


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Incremental redundancy is the basic scheme for softcombining, that is, retransmissions may consist of adifferent set of coded bits than the originaltransmission.

The rate matcher uses puncturing (or repetition) tomatch the number of code bits to the number ofphysical channel bits available.

Node B decides whether to use incremental

redundancy or Chase combining by selecting theappropriate puncturing pattern for the retransmission.


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The UE receives the coded bits and attempts to decode

them. In case the decoding attempts fails, the UEbuffers the received soft bits and requests aretransmission by sending a NAK.

Once the retransmission occurs, the UE combines thebuffered soft bits with the received soft bits from theretransmission and tries to decode the combination.


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6-Data flow


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operation The PDCP performs (optional) IP header compression.

The output from the PDCP is fed to the RLC protocolentity.

After possible concatenation, the RLC SDUs aresegmented into smaller blocks of typically 40 bytes.

An RLC PDU is comprised of a data segment and the

RLC header.


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If logical-channel multiplexing is performed in MAC-d, a 4-bit header is added to form a MAC-d PDU.

In MAC-hs, a number of MAC-d PDUs, possibly of

variable size, are assembled and a MAC-hs header isattached to form one transport block, subsequentlycoded and transmitted by the physical layer.


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1-Scheduling in HSDPA

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