With all the focus on LTE, LTE-Advanced and now 5G, it is easy to forget that High Speed Packet Access (HSPA/HSPA+), based on 3G WCDMA, is still the dominant mobile broadband technology worldwide. In February 2014, the Global mobile Suppliers Association (GSA) reported that all WCDMA operators had launched Hspa, with 547 networks providing access to more than 1.5 billion subscribers worldwide. Two thirds of these networks had upgraded to HSPA+ and 159 of them supported Dual-Cell HSPA+, enabling Downlink data rates of up to 42Mbit/s. The number of HSPA/HSPA+ users is continuing to grow strongly and a number of technical innovations will continue to enhance its capabilities over the coming years. Meanwhile, LTE reached a quarter of a billion subscribers in the first half of 2014 and it is likely to be the end of the decade before LTE overtakes the number of HSPA/HSPA+ users.

In its first incarnation (specified in 3GPP Release 99 in 2000) the 3G WCDMA radio interface was capable of delivering a maximum data rate of 2Mbit/s and practical implementations were generally capped at 384kbit/s. However, with the introduction of High Speed Downlink Packet Access (HSDPA) in Release 5 and High Speed Uplink Packet Access (HSUPA) in Release 6, 3GPP began a long line of enhancements to extend the performance of WCDMA way beyond those initial systems.

In principle, deploying all of the features of the latest 3GPP Release 11 standard could enable an HSPA+ network to deliver peak data rates of 336Mbit/s in the downlink and 69Mbit/s in the uplink, and further increases lie ahead. As with any radio technology, the peak data rates are only part of the story, because they indicate performance only in the optimum radio conditions. However, 3GPP has also made strides to improve the performance of HSPA+ in more difficult conditions, such as in the borders between cells.

The following table summarises major steps in the evolution of the HSPA/HSPA+ family of technologies and below the table I discuss some of the techniques used.

New channels and operating principles introduced in 3GPP Releases 5 and 6 were better suited to the efficient delivery of high speed, low latency, bursty data traffic and paved the way for Evolved HSPA (HSPA+) in Release 7 onwards. For downlink data transmission, HSDPA departed from the dedicated channel approach of Release 99. Instead it opted for a shared transmission channel, using fast scheduling to provide mobiles with access to the channel according to their instantaneous data requirements and signal conditions. The system could rapidly adapt the modulation and coding used according to the quality of each radio link, for example using 16-state Quadrature Amplitude Modulation (QAM) to boost the transmission speed when possible. A further development was the introduction of physical layer Hybrid Automatic Repeat Requests (HARQ), which enabled rapid retransmission of data packets received with errors. Furthermore, the mobile could combine the original packets with their retransmissions, so as to improve the probability of successful decoding. Release 6 introduced fast scheduling and physical layer HARQ to the uplink, albeit to a dedicated channel rather than a shared channel, to achieve similar benefits in HSUPA.

Higher Order Modulation enables HSPA and HSPA+ to operate at increased data rates in favourable signal conditions. The Release 99 WCDMA standard relied on QPSK modulation to carry data across the radio interface. QPSK modulation enables a transmitted data symbol to adopt one of four states and therefore is able to carry 2 bits per symbol. 3GPP Release 5 introduced the option to carry 4 bits per symbol on the downlink by using 16-state QAM, thereby doubling the raw data rate of the radio interface when signal conditions permitted. A further 50% improvement was enabled in Release 7 with the introduction of 64-state QAM, which can carry 6 bits per symbol. For the uplink, 16-state QAM was introduced in Release 7 and 64-state QAM was added in Release 11.

Multiple Input Multiple Output (MIMO) antennas can improve HSPA+ performance in a number of ways, by using multiple antennas at base stations and mobile devices. I have discussed the use of Mimo in LTE previously and many of the same principles apply to HSPA+. Various techniques can be applied to increase the maximum data rates experienced by individual users in good signal conditions, to improve the data rates achieved in poor coverage (such as close to the edge of cells) and to extend the overall capacity of a system.

In an urban environment it is common to experience strong signal levels in conjunction with complicated (commonly referred to as high geometry) multipath propagation between a transmitter and receiver. In such a situation it is possible use MIMO antennas and associated signal processing to send more than one data stream with the same spreading code in the same bandwidth, and to extract the individual data streams at the receiver. This is known as Spatial Multiplexing (SM) and enables an increase in the maximum throughput achieved by HSPA+ without an increase in bandwidth. Using two antennas at the transmitter and receiver can achieve double the data rate of single antennas (if the signal conditions are suitable). In less favourable signal conditions, or if one of the parties in the link does not support MIMO, the multiple antennas can be used for beamforming or diversity with a single data stream, which can enhance performance in poor coverage, for example towards the edge of a cell.

MIMO was first introduced in HSPA+ in 3GPP Release 7, enabling the use of up to two transmit and two receive antennas (known as 2×2 MIMO) for downlink transmission. Release 11 added 4×4 MIMO for the downlink and 2×2 MIMO for the uplink.

Dual-/Multi-Cell operation (also referred to as Dual-/Multi-Carrier operation) is similar to LTE Carrier Aggregation, which I have discussed before. WCDMA, HSDPA and HSDPA+ all operate on the basis of 5MHz bandwidth Carriers. However, 3GPP Release 8 introduced downlink Dual-Cell operation, in which two carriers on the same base station could be combined to achieve an effective bandwidth of 10MHz, to achieve higher data rates and lower latencies. This is illustrated in figures 1 and 2 below. In Dual-Cell operation the network is able to schedule transmissions over more than one carrier. The ability to balance traffic between the two carriers provides diversity and trunking efficiencies that improve network capacity and the average data rate available to the user. If the mobile is able to receive data via both carriers simultaneously it can also benefit from an increase in peak data rate.

Initially the HSPA+ Dual-Cell implementation was restricted to two adjacent downlink carriers in a particular frequency band and it could not be used in conjunction with MIMO. However, subsequent developments have extended the scope considerably. 3GPP has introduced more flexibility in the combinations of downlink carriers that can be aggregated, including non-contiguous carriers, both within and between frequency bands. This helps network operators to exploit fragmented spectrum allocations. The number of downlink carriers that can be combined has also been increased, up to four in Release 10 and eight in Release 11. Dual-Cell operation of the uplink was introduced in Release 9.

Forthcoming 3GPP releases will bring further flexibility in the mix and match of carriers and frequency bands, including the possibility of using unpaired TDD spectrum to boost the capability of the downlink, in a technique known as Supplemental Downlink.

Multiflow extends the principle of Dual-/Multi-Cell operation by allowing a network to schedule transmissions for an individual mobile device via two different sectors or base stations, as illustrated in figures 3 and 4 above. By providing two distinct radio paths and exploiting unused resources, Multiflow bolsters throughput and capacity in the border region between neighbouring radio cells. The opportunity to balance the traffic load on adjacent cells can also be used to manage radio or backhaul congestion. If a mobile device is able to receive data from different cells simultaneously, then there are further benefits for peak data rate. 3GPP Release 11 supports downlink Multiflow across two sectors or base stations on up to two carriers (i.e. a maximum of four radio links, as shown in figure 4). Qualcomm demonstrated Multiflow in this configuration at Mobile World Congress (MWC) in February 2014. Multiflow will become increasingly important as the introduction of small cells changes network topology and demands greater flexibility in the utilisation of cells of different sizes.

With HSPA+ developments and deployments continuing at a pace, it will remain an important part of the 3GPP eco-system for a number of years to come.