The Third Generation Partnership Project (3GPP) develops High Speed Packet Access (HSPA) to make broadband a reality everywhere (Kumar, Sengupta and Liu 366). HSPA+, also referred to as HSPA Evolution, is a natural evolution of HSPA and provides a next-generational performance as a result of cost effective additional upgrades that leverage existing investments thus enabling operators to offer broadband at lower prices without sacrificing voice services (Kumar, Sengupta and Liu 366). The aim of this paper is to explain the working of HSPA and how subsequent 3GPP releases have caused a natural evolution to HSPA+ offering up to 84 Mbps throughput on the downlink.
HSPA Evolution (HSPA+) is a natural evolution of the HSPA mobile broadband standard by 3GPP. HSPA+ offers a cost effective upgrade of existing mobile broadband systems that use the Wideband Code Division Multiple Access (WCDMA) such as UMTS. HSPA+ demonstrates low latencies and high throughputs without sacrificing device power while also utilizing low frequency ranges of 850 MHz and 900 MHz which make it suitable for deployment as an upgrade to existing 2G networks. Key drivers for the adoption of HSPA+ has been the need to provide a cost effective high capacity network that is robust against attenuation when delivering high capacity for data without sacrificing voice. Commercial deployment of HSPA+ depends on the availability of devices with the technological capacity to utilize the increased capacity provided by HSPA+. ST-Ericsson is an industry leader in the development of platforms and devices that utilize HSPA+ and are at the forefront of assisting carriers deploy robust technologies for their networks (“ST- Ericsson”; Para. 4-6).
Optimal design of wireless broadband systems ensures that they meet the extraordinary rations of efficiency and capacity. The future demand for mobile broadband demands that operators have high capacity networks with lower costs per bit and the roadmap developed by 3GPP offers that possibility (Kumar, Sengupta and Liu 366). HSPA is a broadband standardization that is realized through gradual increases of the functionality of 3G UMTS and subsequent standardization of the newly added functionalities (Bøhagen and Binningsbø 52).
HSPA comprises of individual parts put into the 3GPP standard to provide an increment in peak throughput and the system’s capacity (Bøhagen and Binningsbø). Both HSPA and UMTS use a Wideband Code Division Multiple Access (WCDMA) based air interface that is a modification of the ‘spread spectrum’ technology in both uplink and downlink communication (Bøhagen and Binningsbø 52). Individual user signal is scrambled having a user-specific channelization code that spreads to cover the whole 5 MHz of bandwidth allocated to the technology, to make robust in view of the narrowband interference and channel frequency selectivity (Bøhagen and Binningsbø 52).
Architecture of HSPA and 3GPP Releases
In order to facilitate the increase of speed and reliability of the system, HSPA embraces key structural difference from its precursor, UMTS system, such that instead of using Radio Network Controller (RNC) to Mobility Management and Radio Resource Management, the HSPA uses the nodeB and thus reduces radio packet length to 2 ms from a previous 80 ms realized in 3G UMTS (Bøhagen and Binningsbø). Reduction of the radio packet length facilitates a significant lowering of latency, additionally; nodeB reacts faster to swift changes in capacity compared to RNC (Bøhagen and Binningsbø 52). Installation of multiple antennas at the transmitter as well as on the receiver, multiple-input multiple-output technology (MIMO), may be used increase the efficiency of wireless systems (Bøhagen and Binningsbø 50). Spatial multiplexing is now an assumed industry standard that has seen implementation among the major wireless standards because in ideal cases it should deliver a linear increase in throughput as the number of antennas is increased (Bøhagen and Binningsbø). Figure 1 shows the architecture of HSPA.
Channelization codes for HSPA are based on orthogonal variable spreading factor (OVSF) that creates a hierarchical tree mesh where the level occupied in the hierarchical order depends on the spreading factor of the code (Bøhagen and Binningsbø 53). A service assigns to a code and the bit rate needed by the service determines the occupancy of the code in the tree (Bøhagen and Binningsbø 53). HSPDA has 15 available channelization codes and each channel uses one code, a user might be assigned might use one or more codes up to a maximum of 15 codes. In theoretical terms, peak throughput of 3.6 is achievable with five codes, and the increase in the number of codes occupied by and individual is proportional to the throughput obtained (Bøhagen and Binningsbø 53). Ten codes give a throughput of 7.2 Mbps and 15 codes give a throughput of 14.4 Mbps when using the 16QAM modulation (Bøhagen and Binningsbø 53).
The available capacity for HSPA is subject to the requirements of circuit switched (CS) voice calls that carry precedence over mobile broadband traffic. To achieve the robustness and advanced user experience offered to users of mobile broadband the following technologies are involved; Hybrid-ARQ referring to Hybrid Automatic Repeat Request that is used together with Adaptive Modulation and Coding (AMC) to make quick responses to swift changes in the radio environment. Since packets are sent to the radio interface every 2 ms, transmit and receive process happen simultaneously so that there is no waiting of decoding, detection and retransmission of data packet that has been wrongly decoded (Dahlman 16-18). In the release 7 of HSPA+, 64QAM became available for the downlink theoretically providing a peak throughput of 21 Mbps within the 5 MHz carrier. After Release 7 by 3GPP, HSPA uses a standardized ‘Dial-code word MIMO’, which introduces spatial multiplexing. 64QAM is now usable in combination with MIMO and delivers a 6 bit transmission per symbol that increases the peal rate by a half to 42 Mbps (Kumar, Sengupta and Liu 373). Table 1, below summarizes modulations and throughput peaks for given 3GPP releases:
The available 5 MHz bandwidth constraints user, peak throughput in HSPA is increased by the concept of ‘dual carrier’, which allows a single user to receive data from two carriers concurrently so that their theoretical throughput doubles. The dual carrier concept allows further optimization of the system by presenting nodeB with an opportunity to utilize chanced scheduling over the dual 5 MHz blocks. The dual carrier concept when expanded to include four carriers results to HSPA terminals using a bandwidth of 20 MHz thus quadrupling theoretical peak throughput. Release 8 of 3GPP introduces a dual-carrier operation in the downlink of adjacent carriers that doubles the peak rate for HSPA Evolution from 21 Mbps to 42 Mbps without using MIMO, such that rate for users is doubled in typical bursty traffic, therefore also doubling user throughput hence a substantial increase in cell capacity (Kumar, Sengupta and Liu 373).
3GPP has initiated definitions for Release 9 that moves the HSPA evolution further by focusing on multicarrier enhancements. In this release, a combination of MIMO and multicarrier in 10 MHz will allow for achievement of 84 Mbps throughput for users and increase uplink rates to 23 Mbps within the limit of the device transmitting power. A combination of dual-carrier operation and MIMO having 64QAM downlink doubles the peak rate to 84 Mbps and translates to a greater user capacity in support of a large number of users or a high data rate for a given number of users. Additionally, combination of up to four downlink carriers, MIMO and 64QAM will likely push peak throughput beyond 100 Mbps as shown in Figure 2
Drivers for the adoption of HSPA+
There are several factors that influence operator choice for a particular technology; these are also closely linked with development trends that aim to harmonize bands across national borders so facilitate smooth roaming transition (Bøhagen and Binningsbø 62). Movement of mobile traffic from voice domination to data domination has greatly influenced the choice of carriers to deploy high capacity cost efficient networks. Secondly, improved user experience brought by low latencies and increased throughput make great business sense as a strategy to attract and retain customers. Thirdly, competition and market positioning determines the extent of influence that a carrier faces which influences their decision to deploy the next generation technology transition (Bøhagen and Binningsbø 62).
A key driver for the adoption of HSPA+ has been frequency bands and development trends. Attenuation exposure of a signal increases with increasing frequency just like the lower the frequency the higher their reach. When considering capacity higher frequencies of 2.1 GHz are considered for deploying HSPA, however low frequency deployment at bands of 850 MHz and 900 MHz is practicable and delivers a wide coverage (Bøhagen and Binningsbø 62). In most user devices that deliver a rich multimedia experience through the fast growing HSPA+ , the underlying technology is developed by ST-Ericsson, a company that is the industry leader for HSPA platform development today (“ST- Ericsson”; para.1-4). The company has been able to deliver devices that are usable on various aspects of mobile computing such as modems, phones and PC-cards that have a low power consumption, low thermal heat and small in size compared to the competition (“ST- Ericsson”; Para. 4-6). Development of appropriate devices to use existing networks is an essential driving factor in the adoption and development of HSPA+ networks by carriers.
The industry evolves HSPA through 3GPP releases and after Release 6 HSPA assumed the name HSPA+. HSPA+ in Release 7 provides up to 42 Mbps peak data rates in the downlink (Kumar, Sengupta and Liu 376). Combinations of MIMO and Multicarrier provide robustness against attenuation and in Release 9 promise up to 84 Mbps downlink throughput (Kumar, Sengupta and Liu 375-376).
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