Poor Radio Path

Interference

Modulation – QPSK 16-QAM 64-QAM?

Coding – 1/4 rate 3/4 rate

Figure 8

Modulation and Coding

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.17 © Wray Castle Limited

2.4 Incremental Redundancy (IR)

The first stage of the IR process is to apply a very strong error protection coding to the datagram. HSDPA uses 1/3 rate turbo coding for this.

The resulting data is then punctured in three ways, producing three separate groups of data bits (P1, P2 and P3).

The first P1, is transmitted in the DL to the UE and the UE attempts decode this block of data to recreate the original datagram. As the punctured data (P1) is about the same size as the datagram that is being recreated, there is very little error protection.

If no (or very few) errors have occurred, the UE will be able to successfully recreate the original datagram from P1.

If channel conditions have generated errors in the data, the UE will request a retransmission. The UE retains its copy of P1 in its IR buffer.

When the network receives the request for a retransmission, it will send the second, punctured version of the data (P2).

The UE now has both P1 and P2 and attempts to recreate the original datagram.

This effectively now has half-rate protection, since P1+P2 is about double the size of the original datagram.

If this cannot be decoded successfully the UE repeats the request process and receives P3, providing an effective 1/3 rate protection.

Decode successful NACK send next block

Decode fails

Store first and second block NACK send next block

Decode fails Store first block Redundancy

Puncturing Coding

Data for transmission

ACK Uu

Node B UE

Figure 9

Incremental Redundancy (IR)

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.19 © Wray Castle Limited

2.5 Link Adaptation

The combined effect of modulation and coding changes on the downlink is the ability to change rapidly the downlink configuration to suit the instantaneous radio conditions. In standard UMTS implementations, power control is employed to compensate for different channel conditions. Power control will increase the UE’s power when conditions are poor. This power control attempts to provide a consistent Signal to Interference Ratio (SIR) at the receiver despite radio conditions.

While this mechanism may be appropriate for some traffic, it does not optimize the cell throughput. HSDPA tackles this problem in another way. It attempts to keep the transmitter power levels constant and adjusts the coding rate and modulation used on the channel in order to achieve optimum throughput.

The Node B also seeks to allocate resources to UEs that are not experiencing deep fades at that moment, preferring to wait until the UE (inevitably) comes out of the fade before allocating significant resources, further optimizing throughput.

UE

Higher Data Rate

Lower Data Rate

Transmission Time Interval (TTI) = 2 ms (short)

Figure 10 Link Adaptation

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.21 © Wray Castle Limited

2.6 Cell Power Utilization

One of the direct benefits of HSDPA is the efficient use of the available cell power.

If the cell does not support HSDPA, the total power from the Node B will fluctuate over time depending on the number of UEs connected and the power required for each connection.

HSDPA allows the operator to exploit this unused power within the cell to carry data.

Power-Controlled Dedicated Channel

Common Channels Total Cell

Power

time

Power-Controlled Dedicated Channel

Common Channels

time HS-DSCH

Figure 11

Cell Power Utilization

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.23 © Wray Castle Limited

2.7 Fast Cell Selection

For HSDPA access to work effectively the system must be able to change the configuration of the channel within very short timescales. These timescales should match or be faster than the short-term radio fluctuations which occur within the channel, such as fading.

UEs can monitor a number of cells at the same time and select from this group which should be used for DL transmissions. The UE is best placed to make this decision and can base it on the radio conditions it is experiencing at that moment.

The UE indicates which cell in the active set is currently best and receives its data from that cell.

Node B A

Node B B

Node B C

UE Cell

Selection Information

UE measures pilot channels and power levels to indicate ‘best’ cell

Figure 12 Fast Cell Selection

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.25 © Wray Castle Limited

2.8 Multiple Input Multiple Output (MIMO) Antenna Arrays

Multiple Input Multiple Output (MIMO) antenna arrays offer significant performance improvements over conventional single antenna configurations.

A number of antennas are placed at the Rx and Tx. If there are two at the Tx and a further two at the Rx, there are four possible direct radio paths between the Tx and Rx. Each of these is open to multipath effects, creating more radio paths between the Tx and Rx.

These radio paths can then be constructively combined, thus producing micro diversity gain at the Rx.

For this to work effectively the antennas should be uncorrelated; that is, be mounted so that they are independent of each other.

Tx Rx

Tx Rx

Total of four permutations Each of the four is also subject to multipath effects

= 2

= 2

Total of 16 permutations Each of the 16 is also subject to multipath effects

= 4

= 4

Figure 13

Multiple Input Multiple Output (MIMO)

HSDPA/UPA Principles and Application

RP2500/S1/v2

1.27 © Wray Castle Limited

The main objective of Enhanced UL, or HSUPA as it is more widely known, is to improve the performance of the uplink dedicated transport channels increasing capacity, increasing throughput and reducing channel delay (latency).

This will be achieved by implementing enhanced transport and physical channels, moving some functions from the RNC to the Node B, enhancing the functions within the MAC layer and utilizing HARQ techniques like HSDPA.