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 was a major step up. 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 trade-off, however, is that the 64 distinct points on the modulation constellation diagram are much closer together, making the signal far more sensitive to noise and interference. As a result, 64-QAM can only be used by users who have a very strong and clean radio signal, typically those located close to the Node B.

2. MIMO Technology: Using Multiple Antennas to Double the Speed

Perhaps the most significant innovation introduced in HSPA+ was the adoption of 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+, the most common implementation was $2 \times 2$ MIMO on the downlink. This meant the Node B would use two transmit antennas and the UE would need two receive antennas. The data intended for the user would be split into two separate streams. Each stream would be transmitted from a different antenna. The signals would travel through the radio environment, bouncing off buildings and other objects, arriving at the UE's two receive antennas via slightly different paths. Advanced signal processing in the phone could then distinguish these two streams, separate them, and recombine them.

This technique, known as spatial multiplexing, effectively creates two parallel data pipes over the same frequency channel. The theoretical result is a doubling of the peak data rate. A connection that might achieve $21 \text{ Mbit/s}$ with a single antenna could now potentially reach $42 \text{ Mbit/s}$ using $2 \times 2$ MIMO.

3. Multi-Carrier Operation: Bonding Channels Together

Another powerful technique to boost speeds was to allow a single user to aggregate, or combine, the capacity of multiple WCDMA frequency channels simultaneously. Each UMTS carrier has a bandwidth of $5 \text{ MHz}$. Multi-carrier HSPA+ allowed a capable UE to establish connections on more than one of these carriers at once.

The most common implementation of this was Dual-Carrier HSDPA (DC-HSDPA). As the name implies, a user's device could connect to two adjacent $5 \text{ MHz}$ carriers from the same cell site. The network would then schedule data packets for the user across both carriers as if they were one large $10 \text{ MHz}$ channel. This technique provided a straightforward way to double the theoretical peak download speed. For instance, a network that offered a peak of $21 \text{ Mbit/s}$ per carrier could now offer up to $42 \text{ Mbit/s}$ to a DC-HSDPA capable device. Later releases of the standard expanded this concept to combine even more carriers (e.g., 4C-HSDPA to combine four carriers).

4. Flatter Network Architecture and Continuous Packet Connectivity (CPC)

Beyond the major radio interface upgrades, HSPA+ also introduced refinements to the network architecture and signaling to improve efficiency and battery life.

• All-IP Architecture: HSPA+ encouraged a move towards a flatter, all-IP network infrastructure. This meant streamlining the data path, bypassing the RNC for certain user data forwarding functions (a feature sometimes called "Direct Tunnel"), and routing data packets more directly from the SGSN to the Node B. This helped to reduce latency and processing overhead in the network, bringing the 3G architecture one step closer to the flat design of LTE.
• Continuous Packet Connectivity (CPC): The "always-on" nature of packet data was a challenge for the battery life of early smartphones. CPC introduced a set of enhancements designed to reduce the power consumption of the UE';s radio during periods of inactivity. This included features like Discontinuous Reception (DRX) and Discontinuous Transmission (DTX), which allowed the phone';s transmitter and receiver to "sleep" for longer, pre-negotiated cycles while still being able to wake up very quickly to handle new data packets. This significantly improved the battery life of smartphones in typical usage scenarios involving intermittent data traffic.