HSPA Evolution

HSPA Evolution

Some of the new features that builds on and enhances the performance of Rel5 HSDPA and Rel 6 HSUPA include:

MIMO

Multiple Input Multiple Output (MIMO) may in a most generalized way be viewed as the use of multiple antennas at both the transmitter and the receiver arranged to operate in one of the 2 following ways:

• As diversity gain for increasing the carrier-to-interference-ratio which is typically used in bad channel conditions or at the cell edge borders for increased coverage. Pre-coding is used so that the data streams from the two antennas can be combined coherently.
• Transmission in multiple layers or streams for increasing the maximum achievable data rate(spatial multiplexing) which requires high carrier-to-interference-ratio and is therefore typically used near the nodeB and in good channel conditions. Pre-coding is used in this case to create two orthogonal data streams that can carry separate flows of information.  

Higher Order Modulation

• Release 7 supports 64QAM in the downlink and 16QAM in the uplink
• Higher Order Modulation can in some cases be combined with MIMO for maximum bit rates.

Continuous Packet Connectivity (CPC)

In order to reduce delay after a period of no data transfer, it is advantageous to let the terminal keep the HS-DSCH and the E-DCH that were assigned when the UE first entered state CELL-DCH. However this comes with a cost in the form of increased interference in the uplink and reduced battery life for the UE. In earlier releases this situation has been targeted by different power-saving states (FACH: CELL_PCH: URA_PCH: IDLE). With Rel7 CPC, it is possible to keep the UE in state CELL_DCH efficiently during periods of low activity. CPC consist of three main features:

• Discontinous transmission (DTX)
• Discontionous reception (DRX)
• HS-SCCH less operation

After a certain period of inactivity the UE goes into a preconfigured cycle where it is only allowed to transmit/receive during certain intervals. DTX/DRX should be used in combination and corresponding cycles should be matched to each other.

• HARQ is not part of CPC which means that the UE will transmit/receive the ACK/NACK’s regardless of the DTX/DRX cycles.
• DTX/DRX cycles are configured and activated by RRC signalling

HS-SCCH less operation

• The serving HS-DSCH cell can activate or deactivate HS-SCCH less mode for a UE by sending a HS-SCCH less order.

• It is possible to transmit user data on HS-DSCH without the accompanying HS-SCCH control channel however this will require blind decoding of the HS-DSCH.
• The complexity of the blind decoding is reduced by using one out of four predefined formats for the HS-DSCH.
• All formats are limited to QPSK modulation and at most two channelization codes.
• The HS-PDSCH CRC is 24-bits long and is UE specific. Its generation follows the same procedure as the CRC for the second part of the HS-SCCH.
• HARQ is limited to two retransmissions with a predefined redundancy version.
• HS-SCCH less operation is mainly targeted at relative low bitrate services (such as VOIP) where the signaling overhead becomes proportionally large.

Enhanced CELL_FACH operation

To further reduce latency due to state changes by the UE, the following additions has been made to state CELL_FACH

• In release 7 it is possible to use the HS-DSCH in state CELL_FACH during the transition to state CELL_DCH
• In release 8 it is also possible to use E-DCH in state CELL_FACH
• The HS-DSCH in state CELL_FACH is used to enable reception of BCCH, CCCH and DCCH logical channels.
• Using HS-DSCH’s higher transmission capabilities decreases the transmission times for signaling and system information broadcast.
• In release 10, the HS-DSCH reception is also supported in CELL_PCH and URA_PCH state in a similar way as for CELL_FACH.

Layer 2 protocol enhancements

Flexible RLC PDU size:

• Larger PDU sizes for keeping the relative overhead of header information small.
• Smaller PDU sizes for keeping the padding overhead small when user date content is small.

DUAL Cell HSDPA operation

• Dual Cell operation is characterized as simultaneous reception of up to two HS-DSCH transport channels.
• Certain categories of UE’s may be configured for DUAL Cell operation with or without MIMO simultaneously.
• Each of these HS-DSCH transport channels has its own associated UL and DL signaling and own HARQ entity.
• The maximum number of HS-SCCH’s for a UE in Dual Cell operation is 6.
• Certain categories of UE’s may be configured for simultaneous reception of two transport channels  carried over separate frequency bands.
• In Rel10, Four Carrier HSDPA operation with or without simultaneous MIMO has been defined.