High Speed Packet Access Plus (HSPA+)

1. Higher-order modulation: Packing more data into every signal

One of the most direct ways to increase data speed without using more radio spectrum is to pack more bits of information into each symbol that is transmitted over the air. Early HSPA relied primarily on QPSK (2 bits per symbol) and 16-QAM (4 bits per symbol) for data transmission.

HSPA+ introduced support for more complex modulation schemes on the downlink:

• 64-QAM (Quadrature Amplitude Modulation): This modulation order increases the number of bits carried by each symbol. 64-QAM allows for 6 bits of data to be encoded into a single transmission symbol ($2^6 = 64$). Compared to the 4 bits per symbol of 16-QAM, this represents a 50% increase in peak data rate and spectral efficiency for a given channel bandwidth. The 64 constellation points are closer together, so the signal is more sensitive to noise and interference. The network can select 64-QAM when the UE and cell support it and the radio quality provides the required SINR. Such conditions often occur near the Node B, although distance alone does not determine eligibility.

2. MIMO technology: Multiple antennas and spatial streams

HSPA+ included Multiple-Input Multiple-Output (MIMO) technology. The basic idea of MIMO is to use more than one antenna at the base station (transmitter) and more than one antenna at the mobile device (receiver) to send multiple, independent data streams over the same radio channel at the same time.

In the context of HSPA+, a common implementation was $2 \times 2$MIMO on the downlink. This meant that the Node B used two transmit antennas and the UE required two receive antennas. When the channel and configuration allowed spatial multiplexing, the data could be split into two streams, each transmitted from a different antenna. Reflections and other propagation effects caused the streams to reach the UE's two receive antennas through different paths. Signal processing in the phone could then distinguish, separate, and recombine the streams.

This technique, known as spatial multiplexing, can create two parallel data streams over the same frequency channel. With suitable UE and network support, the theoretical peak rate can approach twice the single-stream value. A connection that might achieve $21 \text{ Mbit/s}$ with a single stream could potentially reach $42 \text{ Mbit/s}$ using $2 \times 2$ MIMO.

3. Multi-carrier operation: Combining WCDMA carriers

Another technique for increasing peak rate was to allow a capable UE to aggregate the capacity of multiple WCDMA frequency channels simultaneously. Each UMTS FDD carrier has a bandwidth of approximately $5 \text{ MHz}$. Multi-carrier HSPA+ allowed the UE and network to establish coordinated connections on more than one carrier.

The most common implementation of this was Dual-Carrier HSDPA (DC-HSDPA). A capable UE could connect to two adjacent $5 \text{ MHz}$ carriers from the same cell site. The network could schedule packets across both carriers as a coordinated aggregate, with a combined occupied bandwidth of about $10 \text{ MHz}$. This arrangement could approximately double the theoretical peak download speed when both carriers and the UE supported the feature. For instance, a network offering $21 \text{ Mbit/s}$ per carrier could offer up to $42 \text{ Mbit/s}$ to a DC-HSDPA capable device. Later releases expanded the concept to more carriers, such as 4C-HSDPA with four carriers.

4. Flatter network architecture and Continuous Packet Connectivity (CPC)

Alongside the radio upgrades, later HSPA evolution included network and signalling refinements for packet-path efficiency and UE battery life.

• Flatter Packet Path: HSPA evolution supported a move towards a flatter, IP-based network infrastructure. In deployments using Direct Tunnel, specified user-plane forwarding functions could bypass the RNC and let the user-plane tunnel terminate closer to the Node B. The variant and tunnel endpoints depended on the release and network configuration. This option reduced processing steps and could lower latency.
• Continuous Packet Connectivity (CPC):Packet connectivity could remain available while the UE radio spent periods in a lower-activity state. CPC introduced enhancements designed to reduce radio power consumption during such periods. These included Discontinuous Reception (DRX) and Discontinuous Transmission (DTX), which allowed the UE's transmitter and receiver to sleep during configured cycles and wake quickly when new data arrived. The mechanisms were configurable for supported UE and network states and could improve battery life during intermittent data traffic.