HSPA evolution for future mobile-broadband needs

Transcription

1 The communications technology journal since 1924 HSPA evolution for future mobile-broadband needs August 28,

2 Smarter networks 2 HSPA evolution for future mobile-broadband needs As HSPA continues to evolve, addressing the needs of changing user behavior, new techniques develop and become standardized. These techniques provide network operators with the flexibility, capacity and coverage needed to carry voice and data into the future. NIKLAS JOHANSSON, LINDA BRUS, ERIK LARSSON, BILLY HOGAN AND PETER VON WRYCZA Mobile broadband (MBB), providing high-speed internet access from more or less anywhere, is becoming a reality for an increasing proportion of the world s population. There are several factors fuelling the need for high-performance MBB networks, not the least, the growing number of mobile internet connections. As Figure 1 illustrates, global mobile subscriptions (excluding M2M) are predicted to grow to 9.1 billion by the end of Nearly 80 percent of mobile subscriptions will be MBB ones 1, indicating that MBB will be the primary service for most operators in the coming years. Impact of affordable smartphones To a large extent, the rapid growth of MBB can be attributed to the widespread availability of low-cost MBBcapable smartphones, which are BOX A Terms and abbreviations CELL_FACH Cell forward access channel CPC Continuous Packet Connectivity DPCH Dedicated Physical Channel EUL Enhanced Uplink HS-DSCH High-Speed Downlink Shared Channel HSDPA High-speed Downlink Packet Access HSPA High-speed Packet Access HSUPA High-speed Uplink Packet Access low-power node replacing voice-centric feature phones. For less than USD 100, consumers can purchase highly capable WCDMA/ HSPA-enabled smartphones with dualcore processors and dual-band operation that support data rates up to 14.4Mbps. This price-to- sophistication ratio has turned the smartphone into an affordable mass-market product, and has accelerated the increase in smartphone subscriptions estimated to rise from 1.2 billion at the end of 2012 to 4.5 billion by Ericsson ConsumerLab studied a group of people to assess how they perceived network quality and what issues they encountered when using their smartphones. The study identified two key factors that are essential to the perceived value of a smartphone: a fast and reliable connection to the data network, and good coverage 2. These findings highlight an important goal for operators: to provide all network users with high-speed data services and good-quality voice services everywhere. This can be M2M MBB MIMO ROT SRB UL URA_PCH UTRAN WCDMA machine-to-machine mobile broadband multiple-input multiple-output rise-over-thermal Signaling Radio Bearer uplink UTRAN registration area paging channel Universal Terrestrial Radio Access Network Wideband Code Division Multiple Access achieved by securing: capacity to handle growing smartphone traffic cost-efficiently; flexibility to manage the wide range of traffic patterns efficiently; and coverage to ensure good voice and app user experience everywhere. App coverage For smartphone applications, like social networking and video streaming, to function correctly, access to the data network and a network that can deliver a defined minimum level of performance is needed. The relationship between the performance requirements (in terms of data speed and response time) of an application and the actual performance delivered by the network for that user at their location at a given time determines how well the user perceives the performance of the application. The term app coverage denotes the level of network performance needed to provide subscribers with a satisfactory user experience for a given application. In the past, the task of dimensioning networks was simpler, as calculations were based on delivering target levels of voice coverage and providing a minimum data rate. Today s applications, however, have widely varying performance requirements. As a result, dimensioning a network has become a more dynamic process and one that needs to take these varying performance requirements into consideration, for apps that are currently popular with subscribers. Footprint Illustrated in Figure 2, at the end of 2012, 55 percent of the world s

3 3 population was covered by WCDMA/ HSPA, a figure that is set to rise to 85 percent by the end of Today, many developed markets are nearing the 100 percent population coverage mark 3. This widespread deployment, together with support for the broadest range of devices, makes WCDMA/HSPA the primary radio-access technology to handle the bulk of MBB and smartphone traffic for years to come. Since its initial release, the 3GPP WCDMA standard has, and continues to, evolve extensively. Today, WCDMA/ HSPA is a best-in-class voice solution with exceptional voice accessibility and retainability. It offers high call retention as well as being an excellent access technology for MBB, as it delivers high data rates and high cell-edge throughput all of which enable good user experience across the entire network. The continued evolution of WCDMA/ HSPA in Releases 11 and 12 includes several key features that aim to increase network flexibility and capacity to meet growing smartphone traffic and secure voice and app coverage. Evolution of traffic patterns Applications have varying demands and behaviors when it comes to when and how much data they transmit. Some apps transmit a large amount of data continuously for substantial periods of time and some transmit small packets at intervals that can range from a few seconds to minutes or even longer. Applications have varying demands, typically sending lots of data in bursts, interspersed with periods of inactivity when they send little or no data at all. Rapid handling of individual user requests, enabled by high instantaneous data rates, improves overall network performance as control-channel overhead is reduced and capacity for other traffic becomes available sooner. So, if a network can fulfill requests speedily, all users will experience the benefits of reduced latency and faster round-trip times. Web browsing on a smartphone is a classic example of a bursty application, both for uplink and downlink communication. When a smartphone requests the components of a web page from the network (in the uplink) they are FIGURE 1 Mobile and MBB subscriptions ( ) 1 Subscriptions/lines (million) 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 transferred in bursts (in the downlink), and the device acknowledges receipt of the content (in the uplink). As a result, uplink and downlink performance becomes tightly connected and therefore better uplink performance has a positive effect on downlink data rates as well as overall system throughput. For web browsing, the instantaneous downlink speed for mobile users needs Mobile subscriptions Mobile broadband to be much higher on average than the uplink speed. However, the number of services requiring higher data rates in the uplink, such as video calling and cloud synching of smartphone data, is on the rise. As user behavior changes, trafficvolume patterns also change, and measurements show it is becoming more common for uplink levels to be on FIGURE 2 Population coverage by technology ( ) % population coverage >85% >90% ~55% >85% (Source: Ericsson 1 ) ~10% ~60% GSM/EDGE WCDMA/HSPA LTE

4 Smarter networks 4 FIGURE 3 FIGURE 4 Where to improve, densify and add Area traffic density UL ROT Y Improve Densify Add Dense urban Improve Densify Improve Urban Suburban Rural Relationship between maximum interference and peak rate X Rate Legend Y = X = Maximum interference handled by the network Maximum uplink data rate that can be achieved par with downlink levels, and in some cases even outweigh the downlink traffic. Consequently, continuing to develop data rates to secure uplinkheavy services is key to improving overall user performance. High performance networks The standard approach used to create a high-performance network with wide coverage and high capacity is to first improve the macro layer, then densify it by deploying additional macro base stations, and finally add low power nodes (s) in strategic places, such as traffic hotspots, that can offload the macro network. Each step addresses specific performance targets and applies to different population densities, from urban to rural as illustrated in Figure 3. The evolution of WCDMA/HSPA includes a number of features that target macro layer improvement and how deployments where s have been added can be enhanced. Improving the uplink Features in the 3GPP specification have recently achieved substantial improvement of uplink capabilities. Features such as uplink multi-carrier, higherorder modulation with MIMO, EUL in CELL_FACH state, and Continuous Packet Connectivity (CPC) have multiplied the peak rate (up to 34Mbps per carrier in Release 11) and increased the number of simultaneous users a network can support almost fivefold. Given the high uplink capabilities already supported by the standard, the next development (Release 12) will enable and extend the use of these capabilities to as many network users as possible. The maximum allowed uplink interference level in a cell, also known as maximum rise-over-thermal (ROT), is a highly important quantifier in WCDMA networks. This is because the maximum allowed interference level has a direct impact on the peak data rates that the cell can deliver. Typically, macro cells are dimensioned with an average ROT of around 7dB, which enables UL data rates of 5.7Mbps (supported by most commercial smartphones), and secures voice and data coverage for cell-edge users.

5 5 High data rates, such as 11Mbps (available since release 7) and 34Mbps (available since release 11) require ROT levels greater than 10dB and 20dB respectively. Figure 4 illustrates the relationship between ROT and peak data rate. The maximum uplink interference level permissible is determined by a number of factors including the density of the network, the capability of the network to handle interference (for example with advanced techniques such as Interference Suppression), and the capabilities of the devices in the network, including both smartphones and legacy feature phones. The Lean Carrier solution, introduced in Release 12, is an additional capability that helps operators meet the needs of high-data-rate users. This multi-carrier solution is built on the Release 9 HSUPA dual-carrier one that is currently being implemented in commercial smartphones. The dualcarrier solution allows two carriers, primary and secondary, to be assigned to a user. By doing this, the traffic generated by the user can be allocated in a flexible way between the two carriers, while at the same time doubling the maximum peak rate achievable. The Lean Carrier solution optimizes the secondary carrier for fast and flexible handling of multiple high-data-rate users, through more efficient granting and lower cost per bit. The solution is designed to support multiple bursty data users in a cell transmitting at the highest peak rates without causing any uplink interference among themselves or to legacy users. To maximize energy efficiency, the Lean Carrier solution should cost nothing in system or terminal resources on the secondary carrier until the user starts to send data. Lean Carrier can be flexibly deployed according to the needs of the network. For example, the maximum ROT on a user s secondary (lean) carrier can be configured to support any available uplink peak data rate, while the maximum ROT on a user s primary carrier can be configured to secure cell-edge coverage for signaling, random access and legacy (voice) users. Rate adaptation is another technology under study that results in increased network capacity for some common traffic scenarios, such as areas where FIGURE 5 Rate adaptation results in predictable interference levels Received power Baseline: Fixed rate variable power DATA Control subscribers are a mix of high and low-rate users or areas where there are only high-rate users. High uplink data rates require more power. Maintaining a fixed data rate at the desired quality target in an environment where interference levels vary greatly can result in large fluctuations in received power. To avoid such fluctuations, the concept of rate adaptation can be applied. High-rate users are assigned with a fixed received-power budget, and as interference levels change, bit rates are adapted to maintain the desired quality target, while not exceeding the allowed power budget. In short, as illustrated in Figure 5, the bit rate is adapted to received power, and not the power to the rate. Limiting fluctuations in received power for high-rate users is good for overall system capacity because these high-rate users can transmit more efficiently, and other users in the system, including low-rate ones such as voice users, consume less power when power levels are stable and predictable. Maintaining a device in connected mode for as long as possible is another technique that can be used to improve performance of the uplink. Smartphone users want to be able to rapidly access the network Rate adaptation: Fixed received power and variable rate DATA Control Time from a state of inactivity. Maintaining a device in a connected-mode state, such as CELL_FACH or URA_PCH, for as long as possible is one way of achieving this access to the network from these states is much faster than from the IDLE state. In recent releases, connected mode has been made more efficient from a battery and resource point of view through the introduction of features such as CPC, fractional DPCH and SRB on HS-DSCH. As a consequence it is now feasible to maintain inactive devices in these states for longer. As the number of smartphone users increases, networks need flexible mechanisms to maintain high system throughput, even during periods of extremely heavy load. Allowing the network to control the number of concurrently active users, as well as the number of random accesses, is one such mechanism. Improvements that enable high throughput under heavy load, and allow users to benefit from lower latency in connected mode, while enabling service-differentiated admission decisions and control over the number of simultaneous users, have been proposed for Release 12. Expanding voice and app coverage Good coverage is crucial for positive smartphone user experience and customer loyalty 2, which for operators

6 Smarter networks 6 FIGURE 6 FIGURE Release 11 uplink transmit diversity beamforming translates into securing voice coverage and delivering data-service coverage that meets the needs of current and future apps. There are several ways to improve coverage for voice and data. One way System-level gains for scenario described in Box B User throughput gain (percent) is to use lower frequency bands, and refarming the 900MHz spectrum from GSM, for example, provides a considerable coverage improvement when compared to 2GHz bands. Voice coverage can be significantly extended with 1W 5W BOX B The system The scenario shown in Figure 7 is for bursty traffic. Four s have been added to each macro base station in the network, and 50 percent of the users are located in traffic hotspots. The transmission power for the macro base station was 20W, and 1W and 5W s were deployed. s were deployed randomly and no range expansion was used. Gains are given relative to a macro-only deployment. Offloading was 32 percent for 1W s and 41 percent for 5W s, where offloading is a measure of the percentage of traffic served by the. lower-rate speech codecs, whereas, four-way receiver diversity and advanced antennas can improve coverage for both voice and data. Uplink transmit diversity was introduced in Release 11. This feature supports terminals with two antennas to increase the reliability and coverage of uplink transmissions and decrease overall interference in the system. It works by allowing the device to use both antennas for transmission in an efficient way using beamforming. Figure 6 illustrates how the radio transmission becomes focused in a given direction, resulting in a reduction in interference between the device and other nodes, and improving overall system performance. An additional mode within uplink transmit diversity is antenna selection. Here, the antenna with the best radio propagation conditions is chosen for transmission. This is useful, for example, when one antenna is obstructed by the user s hand. Uplink transmit diversity increases the coverage of all uplink traffic for voice calls and data transmissions. With Release 11, multi-flow HSDPA transmissions are supported. This allows two separate nodes to transmit to the same terminal, improving performance for users at the cell edge and resulting in better app coverage. In Release 12, simultaneous app data and voice call transmissions will become more efficient, and the time it takes to switch transmission time interval from 10ms to 2ms is considerably shorter. These improvements increase both voice and app coverage Average Cell edge Enhancing small-cell deployments The addition of small cells through deploying s in a macro network resulting in a heterogeneous network is a strategic way to improve capacity, data rates and coverage in urban areas. Typically, the deployment of s is beneficial in hotspots where data usage is heavy, to bridge coverage holes created by complex radio environments, and in some specific deployments such as in-building solutions. Figure 7 shows the performance gains in a heterogeneous-network deployment (described in Box B). Offloading to small cells not only

7 7 provides increased capacity for handling smartphone traffic, it also results in enhanced app coverage. To maximize spectrum usage, the traditional macro base stations and s share the same frequency, either with separate or shared cell identities. These deployments, illustrated in Figure 8, are referred to as separate cell and combined cell. It is possible to operate both separate and combined-cell deployments based on functionality already implemented in the 3GPP standard, and such deployments have been shown to provide substantial performance benefits over macro-only deployments. Today, combined cells tend to be deployed in specific scenarios, such as railroad, highway and in-building environments. Separate-cell deployments, on the other hand, are more generic and provide a capacity increase in more common scenarios. In 3GPP Release 12, small-cell range expansion techniques and control channel improvements are being introduced to enable further offloading of the macro network. Mobility performance enhancements for users moving at high speeds through small cell deployments are also being investigated by 3GPP. When a macro cell in a combinedcell deployment is complemented with additional s close to users, the data rate and network capacity is improved. By allowing the network to reuse the same spreading codes in different parts of the combined cell, the cell s capacity can be further increased a technique being studied in Release 12. And as there is no fundamental uplink/ downlink imbalance in a combined cell, mobility signaling is robust, signaling load is reduced, and network management is simplified. In summary, heterogeneous networks are essential for handling growing smartphone traffic because they support flexible deployment strategies, increase the capacity of a given HSPA network, and extend voice and app coverage. The improvements standardized in Release 12 will further enhance these properties. Conclusions WCDMA/HSPA will be the main FIGURE 8 deployment scenarios Macro RNC References s deployed as separate cells on the same carrier technology providing MBB for many years to come. Operators want WCDMA/HSPA networks that can guarantee excellent user experience throughout the whole network coverage area for all types of current and future mobile devices. The prerequisites for networks are: capacity to handle growing smartphone traffic cost-efficiently; flexibility to manage the wide range of traffic patterns efficiently; and coverage to ensure good voice and app user experience everywhere. HSPA evolution, through the capabilities already available in 3GPP and those under study in 3GPP Release 12, aims Macro RNC s deployed as part of a combined cell on the same carrier to fulfill these prerequisites. There are several ways to improve voice and app coverage. Enhancements to the uplink improve the ability to quickly and efficiently serve bursty traffic improving user experience and increasing smartphone capacity. Small-cell improvements will increase network capacity for smartphone traffic and further improve voice and app coverage. With all of these enhancements, WCDMA/HSPA, already the dominant MBB and best-in-class voice technology, has a strong evolution path to meet the future demands presented by the growth of MBB and highly capable smartphones globally. 1. Ericsson Mobility Report, June 2013, available at: 2. Ericsson ConsumerLab report, January 2013, Smartphone usage experience the importance of network quality and its impact on user satisfaction, available at: 3. International Communications Market Report 2011, Ofcom, available at: stakeholders.ofcom.org.uk/binaries/research/cmr/cmr11/icmr/icmr2011.pdf

8 Smarter networks 8 Linda Brus joined Ericsson in Since then, she has been working with system simulations, performance evaluations, and developing algorithms for WCDMA RAN. Today, she is a system engineer in the Technical Management group in the Product Development Unit WCDMA and Multi- Standard RAN, working with concept development for the RAN product and HSPA evolution. She holds a Ph.D. in electrical engineering, specializing in automatic control (2008) from Uppsala University, Sweden. Niklas Johansson is a senior researcher at Ericsson Research. He joined Ericsson after receiving his M.Sc. in engineering physics and B.Sc. in business studies from Uppsala University in Since joining Ericsson, he has been involved in developing advanced receiver algorithms and multi-antenna transmission concepts. Currently, he is project manager for the Ericsson Research project that is developing concepts and features for 3GPP Release 12. Billy Hogan joined Ericsson in 1995 and works in the Technical Management group in the Product Development Unit WCDMA and Multi-Standard RAN. He is a senior specialist in the area of enhanced uplink for HSPA. He works with the system design and performance of EUL features and algorithms in the RAN product, and with the strategic evolution of EUL to meet future needs. He is currently team leader of the EUL Enhancements team for 3GPP release 12. He holds a B.E. in electronic engineering from the National University of Ireland, Galway, and an M.Eng in electronic engineering from Dublin City University, Ireland. Peter von Wrycza is a senior researcher at Ericsson Research, where he works with the development and standardization of HSPA. He received an M.Sc. (summa cum laude) in electrical engineering from the Royal Institute of Technology (KTH), Stockholm, Sweden, in 2005, and was an electrical engineering graduate student at Stanford University, Stanford, CA, in In 2010, he received a Ph.D. in telecommunications from KTH. Erik Larsson joined Ericsson in Since then has held various positions at Ericsson Research, working with baseband algorithm design and concept development for HSPA. Today, he is a system engineer in the Technical Management group in the Product Development Unit WCDMA and Multi-Standard RAN and works with concept development and standardization of HSPA. He holds an M.Sc. in engineering physics (1999) and a Ph.D. in signal processing (2004), both from Uppsala University, Sweden. Telefonaktiebolaget LM Ericsson SE Stockholm, Sweden Phone: Fax: Uen ISSN Ericsson AB 2013