However, the picture is quite different for users with lower SNR, and especially for those who are mobile, where a device is much less likely to be able to take advantage of higher-order modulation schemes such as 64QAM for a large percentage of the time. At pedestrian speeds, data rates of 4 to 9 Mbits/s were most common, depending on the received signal and cell loading (Fig. 1). Rates fell to 3 to 4 Mbits/s at vehicular speeds (Fig. 2).

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It is also worth noting that the oft-quoted maximum data rates for HSPA+ refer to the physical layer, whereas users naturally experience data performance at the application layer. Application-layer data rates are always less than those at the physical layer. The difference depends on protocol stack implementations and the application actually in use.

Device receiver performance also plays a big role in data performance. A device uses its channel quality indicator (CQI) to report SNR to the network. The network scheduler then uses this information to determine the number of HS-PDSCH (downlink channel) codes, the transport block size (TBS), and the modulation scheme to be assigned to the device.

Compared to devices with better receivers, devices with poorer receiver designs will likely exhibit higher error rates when the scheduler assigns a given set of parameters. This not only directly affects the data throughput experienced by users, it also wastes network resources from an operator’s perspective.

Figure 2. During testing, Category 14 chipsets and devices saw pedestrian rates drop to 3 to 4 Mbits/s at vehicular speeds.
Select figure to enlarge.

Optimizing the network scheduling algorithms to provide the best balance of user experience and network capacity is a significant challenge for operators and their infrastructure vendors. More high-end devices are now taking advantage of the benefits of receive diversity—for example, the use of more than one antenna on a device to reduce the effects of multipath effects on the received signal. Device receiver performance looks set to improve further as more advanced Type 2i and Type 3i interference-aware receiver designs come to market.

Another important point about HSPA+ is that networks that support it also support the lower-numbered categories such as Category 9 and Category 10 (14 Mbits/s). This means that even users in areas too noisy for 64QAM modulation can still benefit from higher maximum data rates than those on networks that only support 7.2 Mbits/s.

So what’s ahead for HSPA+? Release 8 added dual-carrier 64QAM, aggregating two adjacent 5-MHz channels to double data to 42 Mbits/s. This is likely to be the next popular evolution path for operators, since hardware upgrades are expected to be minimal and some chipset vendors are already adding support for it. If MIMO is employed as well, maximum rates with Release 8 increase to 84 Mbits/s. With Release 9 extending multi-carrier techniques beyond 10-MHz deployments to bring even faster data rates (though still not 4G), there is plenty of evolution left yet in HSPA+.