HSDPA continued [Under LTE > 3G till
Rel 6 > HSDPA]
» HSDPA overall description - 25.308, Rel 6 «
» Phy channels and their mapping mapping on transport channels - 25.211, Rel 6 «
Concept
As discussed in earlier article, concept of HSDPA is to use time-shared channelisation codes among UEs for downlink transmissions. This is possible as CDMA codes are independent of radio conditions i.e. radio response for two different CDMA codes remain same.
An example is shown below.
Physical channels and their mapping onto transport channels
The physical shared channels are called HS-PDSCHs (High Speed - Physical Downlink Shared CHannels); this map to a transport channel called HS-DSCH (High Speed-Downlink Shared CHannel). (Similar to GPRS) We will need another common channel carrying allocation information (like UE identity - H-RNTI, channelisation codes, transport format etc.). The physical channel carrying this information is called HS-SCCH (HS-Shared Control CHannel). The transport channel remain same i.e. HS-DSCH.
Here are few parameters of HS-PDSCH:
TTI - 10ms
Spreading factor - 16
Modulation - QPSK, 16QAM
Coding
- Turbo coding with 24 bit CRC
This takes care of downlink; in uplink, we will need channel to send retransmission and quality related information. An additional dedicated (per UE) physical channel, HS-DPCCH (HS-Dedicated Physical Control CHannel for HS-DSCH) is used for this purpose. This being dedicated, transport channel remain same as R99 one i.e. DCH.
Find below Rel 5 channel mapping diagram (reproduced from related UMTS article).
Retransmission technique
HARQ with Incremental Redundancy has been specified for HSDPA downlink transmission. HS-DPCCH carry this information.
Channel quality information
Quality information is known as Channel Quality Indicator (CQI). CQI is calculated by UE based on signal-to-noise ratio of the common pilot (CPICH). CQI is not exactly signal-to-noise ratio but rather estimated transport block size ! The estimation is done by UE. This mean UE based on its capability can actually request (appropriate amount of) transport blocks.
Power control
The uplink power control commands are sent over R99 DPCH channel. Dedicated channel allow for fast power control. Downlink power control is interesting as the channels are shared ! Overall radio conditions for scheduled UEs and power available after being allocated to dedicated channels determine downlink power control.
Impact on RAN (protcol stack)
The main idea of HSDPA is to gain from varying instantaneous radio conditions for UEs in as speedy way as possible. This mean the functionality is to be located to close to radio access (i.e. in base station/Node B rather than in RNC) or close to PHY layer (i.e. in MAC).
A new sublayer has been introduced in MAC called MAC-hs for this purpose. This layer is located in Node B and it is in charge of HARQ, Flow control, Scheduling etc.. MAC-hs being in Node B would also mean soft handover not possible for inter-Node B scenario.
Lot of details are missing in above description of HSDPA, but it gives fair idea of the principals involved in HSDPA enhancement.
References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, UMTS by Sanchez and Thioune, and Release 5 and 6 documents at 3gpp.
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