Key HSPA+ Throughtput Feature
13th September 2011
There are two main throughput enhancements to HSPA+, these include MIMO (Multiple Input Multiple Output) and DC-HSDPA (Dual Cell HSDPA).
In Release 7, HS-PDSCH channels are able to operate in either MIMO or non-MIMO Mode. Whilst in non-MIMO mode the HS-PDSCH’s are able to be coded using either the STTD (Space Time Transmit Diversity) or D-TxAA (Double Transmit Antenna Array) methods.
The use of multiple antennas at both the transmitter and receiver enable multiple streams to be used. These parallel streams can share the SINR (Signal to Noise and Interference Ratio) and therefore avoid the saturation point issue.
The most common method for achieving this is identified as MIMO (Multiple Input Multiple Output). This employs an antenna configuration which uses two antennas at the transmitter and two antennas at the receiver, resulting in a 2x2 MIMO channel. Note that this indicates 2 input channels (transmit) and 2 output channels (receive).
To share the SINR a method called Spatial Multiplexing is used. This takes two blocks of data which are encoded and modulated. These generate two symbol streams, one from each antenna. The diagram illustrates the basic concept of the process.
In this case 2x2 MIMO can be modelled as two parallel streams, each having some specific SINR. Note that the data blocks don’t need to be of the same size since they are precoded differently. In addition, the system uses feedback in the form of PCI (Precoder Control Information) and multiple CQI (Channel Quality Indicator). The PCI is chosen to give the best performance and the CQI indicates the feedback for each of the streams - enabling one stream to be modified independently.
MIMO Transmit Power and Throughput
Obviously, having two transmit antennas requires power to be transmitted from both. However, the total transmitted power is the same. This is achieved by reducing each transmitted stream by 3dB. In addition, assuming that the two streams have the same amount of noise, the SINR value will be half of the SINR value for a single stream transmission (assuming the same total power from one antenna).
The throughput of a SM (Spatial Multiplexing) MIMO system is typically dependent on the number of antennas. The diagram illustrates the comparison between 1x2, 2x2 and 4x4. Notice that the capacity effectively doubles with the doubling of antennas. It is also worth noting that the effective range is also reduced!
Spacial Multiplexing versus Transmit Diversity
The diagram illustrates a general comparison between Spatial Multiplexing and Transmit Diversity. Note that when the SINR is high (typically closer to the Base Station) the system can benefit from Spatial Multiplexing. However, when the SINR drops, at some point it is best to switch to a Transmit Diversity method. This point is sometimes referred to as the AMS (Adaptive MIMO Switch) point.
Release 8 MIMO Improvements
In pre-Release 8 it is not possible to operate MIMO and 64QAM together. However, in Release 8 this is available if the SINR is sufficient and category 19/20 mobiles are used. This enables the theoretical rate of 21.6Mbps x 2 = 43.2Mbps. In reality, due to the maximum Transport Block Size, this figure is about 42.2Mbps.
Dual Cell Operation (Release 8)
DC-HSDPA (Dual Cell HSDPA), is a Release 8 enhancement to HSPA. It provides a method to aggregate two adjacent carriers in the downlink. In so doing, it is suitable for service providers with 10MHz or more of FDD paired spectrum. The diagram illustrates the concept of Dual Carrier operation.
The main reason behind DC-HSDPA, i.e. multi-carrier, is to improve resource utilization and therefore increase spectrum efficiency. This is achieved by having joint resource allocation, as well as load balancing across both carriers.
Dual Cell verses MIMO
The inclusion of MIMO or Dual Cell operation is to improve the user experience, providing a consistent service across the cell. This is especially an issue at the cell edges. The diagram illustrates a general comparison of single carrier and dual carrier operation. Notice that when the cell is loaded (high number of users), both the first and second carrier will be loaded. However, the inclusion of dual cell means that the system can benefit from frequency domain scheduling and carrier load balancing.
It is also worth noting that when the cell has low loading, the data rate can be doubled.
Even though MIMO and Dual Cell HSDPA both quote the same peak rate of 42.2Mbps (category 20 and 24 respectively) there are other considerations:
- Spectral Efficiency - Obviously spectral efficiency varies depending on various deployment options. However it is broadly agreed under loaded conditions Dual Cell operation provides up to a 20% increase. In contrast, D-TxAA MIMO is quoted as up to 10%. However, the Dual Cell option does require an additional 5MHz carrier!
- Data Rate Improvement - Dual Cell provides a gain which is apparent all over the cell. In contrast, a MIMO system has improved gain when closer to the Node B.
- Node B Requirements - Dual Carrier can operate with a single PA (Power Amplifier) per sector. In contrast MIMO requires two power amplifiers.
- Mobile Requirements - In order to deploy MIMO, the mobile must also have at least two antennas. In contrast, Dual cell can operate via one antenna.