White paper Efficient resource utilization improves the customer experience Multiflow, aggregation and multi band load balancing for Long Term HSPA Evolution Executive summary Contents 2. Executive summary 3.Resource utilization in current networks 5.Features enhancing network utilization 5.Multi Band Load Balancing 7.Multi Carrier HSDPA 9. Multiflow 12. HSPA – LTE Carrier Aggregation 13. Further considerations 14. Summary 15. Abbreviations With the growing popularity of smartphones and the increasing use of applications designed to make use of their capabilities, traffic is rising dramatically. As well as application related traffic, with frequent updates to and from applications such as social networking sites and health monitoring functions, smartphones are giving rise to significant signaling loads. Much of this traffic is bursty in nature, leading to imbalances in network utilization. Resource requirements vary greatly over time and between cells and frequency layers. At any one time, many parts of the network have significant free resources, while other parts need to deliver high data speeds. Underused resources are common in a typical network. This is inefficient for network operators, as well as potentially degrading the user experience, it also means communications service providers (CSPs) may not be making efficient use of network investments. An answer to this is provided by features that form part of the latest 3GPP standardization release of Long Term HSPA Evolution, the 3GPP Rel 11, as well as related features from earlier HSPA standardization releases. 3GPP These features take advantage of under-used resources to enhance performance for the user. Present The first of these features is Multi Band Load Balancing (MBLB), which spreads traffic over the different layers, such that more resources are made available for each user and performance is therefore improved. Another feature is Multi Carrier HSDPA, which is extended in Rel 11 to eight carriers. This improves utilization by allowing free resources in the other carriers to be used flexibly. Multiflow is a 3GPP Rel 11 feature candidate, designed to improve cell edge data rates by enabling the transmission of data from multiple cells instead of via a single cell as in HSDPA today. This leads to a doubling of the power available for the wanted signal, increasing the overall user throughput HSPA-LTE carrier aggregation, a feature candidate for future 3GPP releases, enhances traffic steering by enabling fast load balancing between the two radios, ensuring efficient spectrum utilization even when traffic is very bursty. The gain is similar to that of multi carrier HSPA: if the load is low, large efficiency gains can be expected, whereas when loads are high, the gain decreases. These features bring a major improvement to HSPA by using network resources more efficiently, giving larger throughputs for end users and allowing faster response times. New features Future Carrier aggregation MIMO MIMO 4x 2x Multipoint systems Further enhancements to CELL_FACH + HSPA+LTE aggregation Figure 1. Long Term HSPA Evolution components. 2 Efficient resource utilization improves the customer experience 3GPP Release 11+ Long Term HSPA Evolution Resource utilization in current networks HSPA is the leading cellular data service currently in use around the world. Traffic on HSPA networks continues to grow and evolve as users develop new ways of interacting with one another and the information around them, and CSPs seek to differentiate and maximize their revenue. expect always on, landline-like connectivity and which may operate even while the user is not interacting with the phone. Social networking, news, healthcare monitoring, push e-mail and other autonomous apps may give rise to small amounts of update data in both directions. Another factor is interactive usage, which may range from web browsing, for which short, high burst speeds are critical to the user experience, to voice and video, where steady QoS is key. It also covers file down/uploading, in which average burst speeds affect the user experience. Apart from application data, smartphones generate signaling load that must be dealt with effectively by the network. The smartphone segment of the market has experienced very rapid growth within a short time, leading to a wide user base and a rich diversity of applications. Smartphone traffic may be driven by a number of processes that Average usage 12.2% over 48 hour period Average TTI usage over all cells 0.20 0.18 0.16 0.14 0.12 0.10 0.06 0.04 0.02 0 5 10 15 20 25 30 35 40 45 Hours for two days Figure 2. Average TTI usage over all cells in an RNC area versus the hours in a 48 hour period. 0.9 Cumulative distribution A key characteristic of the traffic growth is that traffic has become bursty, with periods of activity in which high burst speeds are critical to user experience, interspersed with periods of inactivity. Radio resource requirements vary greatly over time and between cells and frequency layers. At any one time, many parts of the network have significant unused resources, while other parts need to deliver high data speeds. From the figures, the following can be seen: 1 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 The coming years are also expected to witness a significant expansion in the amount of machine-to-machine (M2M) communications within networks, which will bring new types of traffic profile and QoS requirements. An example of this can be seen in Figures 2 and 3, where the average Transmission Time Interval (TTI) usage over all cells in a Radio Network Controller (RNC) area of a mature 3G network is shown, both against the hours in a 48 hour period and as a cumulative distribution function (cdf) over the different cells. The TTI usage is a measure of the network load in a cell. 0.08 0 In many markets, tablets and PC dongles have seen significant uptake, generating large amounts of data when users are active. Traffic patterns may involve web browsing, video streaming and file up/download, with requirements similar to smartphones 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 TTI usage Figure 3. Cumulative distribution of average TTI usage during busy hour per cell. 0.9 1 •The average load over 48 hours is 12.2%. •During the busiest hour of the day, 20% of the cell capacity is used on average, or 6.8% of the overall daily traffic. •19% of the cells have a load of less than 1%, where the median TTI load over the different cells is 9%. •5% of the cells have an average load during busy hour of more than 77%. Efficient resource utilization improves the customer experience 3 Resource utilization in current networks Due to the bursty nature of the data traffic and delays in state transfers between idle and connected modes, the number of users connected to a cell is much larger than the number of users with actual data reception or transmission. An example of this can be seen in Figure 4. Sampled over a 24 hour period and across all cells in one RNC area, it compares the cumulative distribution of the average number of connected users per hour with the average number of connected users with data in the buffers. The median for the number of connected users is around 3.6, while only in 5% of the time and cells is there more than one user with data present. With packet traffic, two key aspects of performance are user equipment (UE) burst throughput and packet call capacity. Packet call capacity is the maximum packet call load that, when offered to a cell, can be served to the users. Packet call capacity is typically restricted by the slowest burst throughputs, so improving these not only makes it fairer for users but also improves packet call capacity. In recent years, research and standardization has focused on maximizing link spectral efficiency through features such as Higher order Modulations, MIMO, Continuous Packet Connectivity (which also aims to improve user equipment battery life) and on managing or mitigating interference via technologies such as interference cancelling receivers in the downlink and uplink interference cancellation. Progress on these features has enabled good link efficiency and interference management. However, improving the ability of the network to focus resources instantly where they are needed by using a more liquid capacity has great potential for enabling improved user experiences and higher packet call capacities. The rest of this paper focuses on these features. 1 0.9 Cumulative distribution 0.8 0.7 0.6 Number of active users Number of active users with data 0.5 0.4 0.3 0.2 0.1 0 0.01 0.1 1 10 100 Number of users Figure 4. Cumulative distribution of the number of connected users and number of users with data in the buffers over a 24 hour period in a mature 3G network. 4 Efficient resource utilization improves the customer experience Features enhancing network utilization As we saw in the previous section, underused resources are common in a typical network. In this section, we introduce four features that use these free resources to enhance performance for the user. Multi Band Load Balancing Multi Band Load Balancing (MBLB) is applicable when separate bands are used for HSPA, such as the 900 and 2100 MHz band. The feature spreads the traffic over the different layers, such that more resources are made available for each user and performance is improved. This is relevant for today’s mature HSPA networks today, since, as shown in the previous section, traffic is distributed quite unequally over the different cells (see Figure 2). There are several benefits, as illustrated in Figure 5. multi-band HSPA networks. A user can be redirected to another layer under different circumstances: •Maximize coverage from the low frequency layer. •Balance the network load, i.e. maximize the user throughputs. •Avoid frequent handovers by, for instance using different settings for fast moving mobiles. •Matching device and network capability, such as MIMO, Dual Carrier (DC), and operating band capability. •Matching services to network capability, such as speech service. •During the setup of a call •When there is no active data transmission and reception •During transition to the Cell_DCH state •When entering a new cell with different preferred layer priorities The MBLB feature uses several mechanisms to manage the load and customer experience in multi-layer and Maximum coverage from low frequency band Balance the network load 900 900 2100 2100 Avoid frequent handovers Match UE and network capability 900 2100 Micro Only far away calls go to low band Several criteria are taken into account in the layer selection decision, including capabilities and speed, the service used, the load and channel quality in the source and target cells and the signal strength of the target cell. The actual change of layer can then be applied via handover, radio bearer re-configuration, or redirection. Direct load to least loaded layers, ensuring that micro layer also gets traffic 2100 2100 2100 2100 Micro High speed UE goes to umbrella layer, avoid macros Direct UEs according to service or HSPA capability (DC, MIMO) Figure 5. Example of Multi Band Load Balancing features and the improvements they bring. Efficient resource utilization improves the customer experience 5 As an example, Figure 6 shows the performance in terms of user throughput of the redirection scheme at the transition to Cell_DCH. The layer selection in this example takes into account information on channel quality and load in the serving and target cells: at the transition to Cell_DCH, a UE (Rel 6 or later) can report the best intra/inter-frequency cells (target cells). The RNC may then enforce a redirect to a target cell if it has sufficient channel quality and whose load is lower than the serving cell, thus optimizing the customer experience. The performance plot shows that the redirection mechanism offers no significant benefit in terms of UE throughput when the mobility settings for idle and connected mode are optimized. However, redirects provide a large gain when non-optimal mobility settings are adopted. The optimum settings are challenging to identify in real networks with inconsistent load, cell size, antenna orientations and tilting. Therefore, the redirect scheme could be a simple way to boost network performance. Average user throughput 1200 User throughput (Mbps) 1000 800 600 400 200 0 Optimal settings Suboptimal settings Figure 6. Average UE throughput with and without MBLB redirection (redirect). 6 Efficient resource utilization improves the customer experience Suboptimal settings with MBLB redirection Multi Carrier HSDPA Dual Carrier (DC) HSDPA is a 3GPP release 8 feature commercially deployed in a large number of markets. However, the disadvantage of the feature is that it limits the aggregation to two 5 MHz radio carriers within the same band. This is changed in Rel 9, which introduces DC for carriers in different bands. Rel 10 extends the functionality to aggregation over four carriers, with Rel 11 extending it still further to eight carriers. This leads to a peak data rate of 672 Mbps when combined with 4x4 MIMO. The benefits of aggregating multiple carriers are significant for the end user, since a diversity gain can be achieved from scheduling on the best carrier(s) and especially due to the fact that free resources in the other carriers can be used flexibly. As described in the first section, free resources are often available. The gains can be seen in Figures 7 and 8. These show the cumulative distribution of the average user throughput and the mean packet call delay for the macro cells scenario, with an average cell load of 1 Mbps consisting of bursty traffic. 1 0.9 0.8 Cumulative probability 0.7 0.6 0.5 0.4 0.3 1 carrier available from 8 carrier bandwidth 0.2 4 carriers available from 8 carrier bandwidth All carriers available in 8 carrier bandwidth 0.1 0 0 10 20 30 40 50 60 70 80 90 100 User data throughput (Mbps) Figure 7. Cumulative distribution of the average data throughput (Mbps) for 1, 4 and 8 carriers at low offered load (1 Mbps). Efficient resource utilization improves the customer experience 7 0.18 Mean data connection delay (s) The gains depend significantly on the load in the system. If the load is high, then there will be fewer free resources on the other carriers, which results in lower gains. Multi carrier HSPA also gives a capacity gain, which can be seen in Figure 9, which shows the mean cell throughput per carrier as a function of the offered load per carrier. It can be seen that with an offered load per carrier of around 2 Mbps, the system with a single carrier starts to become saturated, whereas with a larger number of carriers, the offered load can still be served. Using multicarrier aggregation increases the total packet call capacity of the network, in addition to the gains in individual user throughput. 0.16 0.14 1 carrier available from 8 carrier bandwidth 0.12 4 carriers available from 8 carrier bandwidth All carriers available in 8 carrier bandwidth 0.1 0.08 0.06 0.04 0.02 0 Scheme Figure 8. Mean data connection delay (s) for 1, 4 and 8 carriers at low offered load (1 Mbps) with data connections of 1 Mbit. Mean packet call throughput (Mbps) 60 50 Single carrier Quad carrier 40 Oct carrier 30 20 10 0 0 1 2 3 Offered load per carrier (Mbps) Figure 9. Mean normalized cell throughput (Mbps) for 1, 4 and 8 carriers as a function of the offered load. 8 Efficient resource utilization improves the customer experience 4 5 Multiflow Another feature enabling a better use of resources in cellular systems is Multiflow. This is a 3GPP Rel 11 feature candidate, designed to improve cell edge data rates by enabling the transmission of data from multiple cells to a UE at the common cell edge, instead of transmitting the data via a single cell as in HSDPA today. This is illustrated in Figure 10 for dual cell operation. Each of the data flows in Multiflow can be scheduled independently. This leads to a doubling of the power available for the desired signal at the UE, which is used to increase the overall user throughput. For Rel 11, Multiflow is considered for up to four different flows over two different frequencies, one can send data from up to four different cells to a UE. Multiflow can be done among cells of the same site (intra-site Multiflow) or between sites (inter-site Multiflow). In the latter case, the data is split in the RNC and directed to each of the different base stations, taking the throughput and load from that cell into account. In the intra-site case, the data is split in the MAC layer and the base station can perform joint scheduling in order to further optimize resource usage (similar to DC HSDPA). Both of these cases are illustrated in Figure 11. Scheduling of the Multiflow streams can be done in different ways. A common requirement for the scheduler is to minimize the effect on the non Multiflow terminals. This can be done by differentiating scheduling for the serving cell and the cell that is assisting in Multiflow transmission. More precisely, the traffic in each cell is prioritized in such a way that traffic belonging to UEs that use the cell as a serving cell is prioritized over the UEs that use it as an assisting cell. This means the benefit from Multiflow will only be seen when the neighboring cell has unused resources. As outlined previously, in current networks there is a large Multiflow potential, as typically, many TTIs are available where there is no user scheduled. Current HSDPA Interference HSDPA Multi Point Signal Signal Signal Signal Data stream 1 Data stream 2 Data stream 1 Data stream 1 Figure 10. Multiflow transmission and conventional HSDPA. RNC Base station Base station Inter-site multi flow Inter-site multi flow Base station RNC Figure 11. Intra-site and inter-site Multiflow. Efficient resource utilization improves the customer experience 9 Other scheduling methods are also possible, based, for example, on the UE throughput, load, service type, or QoS. Multiflowdoes not require coordination of the packet schedulers taking part in the Multiflow transmission, thus simplifying the concept and enabling inter-site deployment. Uncoordinated transmission, however, may lead to situations where a UE receives two flows simultaneously from two base stations. To spatially separate and successfully decode the flows, the terminal must have a minimum of two receive antennas and interferenceaware receiver chains. Figure 12 shows the cumulative distribution of the throughput experienced by the user with and without Multiflow (including both intrasite and inter-site Multiflow UEs). At the low values of the cumulative distribution, users at the cell edge gain particular benefit from Multiflow, since they are the most likely to receive transmissions from multiple cells with adequate signal quality. 1.0 Cumulative distribution 0.9 Reference all UEs 0.8 Multiflow all UEs 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 14 16 18 User experienced throughput (Mbps) Figure 12. Cumulative distribution of throughputs experienced by users with and without inter-site multiflow. Total offered load is 400 kbps/cell. 10 Efficient resource utilization improves the customer experience 20 Several variations of Multiflow are considered in 3GPP, depending on the number of carriers in use in the network and on the amount of simultaneous RX chains that the UE can handle. In a network in which only one carrier frequency is used, the UE will be required to receive up to two links simultaneously. Hence this variant is called Single Frequency Dual Cell (SF-DC) aggregation. In a dual carrier network, the UE can best take advantage of a neighboring cell’s carriers if it has a receiver with four RX chains; hence this variant is labeled Multiflow Dual Frequency Quad Cell (DF-4C) aggregation. The combination of Multiflow and multiple beams can be used to further 11 User experienced throughput (Mbps) The Multiflow gain depends on the offered load, see Figure 13. At low load, the gains are considerable, whereas they disappear at high load. This is because at high load, the assisting cells do not have free resources and thus will never schedule to the Multiflow user. All UEs ref All UEs mflow 5-% tile ref 5-% tile mflow 10 9 8 7 6 5 4 3 2 1 0 0 0.5 1 1.5 2 2.5 3 3.5 Offered load (Mbps) Figure 13. Mean user throughput versus offered load per cell. optimize the system. As an example, Figure 14 shows the case where vertical sectorization is used in combination with Multiflow. This way, during high load, one can utilize the capacity increase due to vertical sectorization, whereas during low load, users at the cell edges benefit from Multiflow. Potential Multiflow areas Figure 14. Combination of vertical sectorization and multipoint. Efficient resource utilization improves the customer experience 11 B: Femto terminals seeing DL macro interference, esp. under cell range extension A: Macro terminal Many pico cells seeing strong downlink interference from small cell D: Many small cell terminals creating uplink interference to macro cell C: Macro terminal creating strong uplink interference to small cell HSPA – LTE Carrier Aggregation Macro cell HSPA-LTE carrier aggregation is a feature under consideration in 3GPP for future releases beyond 3GPP Rel 11. The idea is that one UE can simultaneously use resources from both LTE and HSPA, thus increasing the peak data rate and cell edge data rates of both systems. Even before Rel 11, it is possible to aggregate over several carriers in both LTE and HSPA, LTE with traffic being steered between the two systems by inter-system handovers, as illustrated in Figure 15. HSPA-LTE carrier aggregation enhances traffic steering by enabling fast load balancing between the two radios, ensuring efficient spectrum utilization even under the most bursty traffic conditions. The gain mechanisms are very similar to that of multi-carrier HSPA: if the load is low, large gains can be expected, whereas when loads are high, the gain decreases. LTE carrier aggregation Handover between HSPA and LTE HSPA Figure 15. HSPA + LTE aggregation. 12 Multi carrier reception of LTE HSPA + LTE aggregation HSPA carrier aggregation Efficient resource utilization improves the customer experience Pico cell Multi carrier reception of HSPA Simultaneous reception of HSPA and LTE Further considerations The previous section described different features which help boost the customer experience by improving radio utilization. These features focus primarily on the downlink but also lead to uplink improvements: •Multi Band Load Balancing improves the uplink performance, since, when directing the UE to another layer, both downlink and uplink are considered in cell and layer selection. Benefits are similar to those in the downlink. •Multiple carrier HSPA is also supported for the uplink from Rel 9, however the number of carriers is limited to two. A different number of carriers is supported in the downlink and uplink because downlink traffic volumes exceed uplink volumes, and because the UE will often become limited by transmit power as the number of carriers increases. •Multiflow is a pure downlink feature. The uplink signal will typically be in soft or softer handover when multipoint is being used in the downlink. In addition to the features mentioned in the previous section, Long Term HSPA Evolution brings further improvements: •Further enhancements to Cell_ FACH, while maintaining the good performance of Cell_PCH and Cell_DCH. This is mainly focused on traffic from smartphones. •Uplink Closed Loop Transmit Diversity, enhancing the uplink to support TX diversity. At a later phase, uplink MIMO may be added to the specification, enhancing the uplink peak data rate. •Downlink 4x4 MIMO, enhancing spectral efficiency and peak data rate in the downlink. Efficient resource utilization improves the customer experience 13 B: Femto terminals seeing DL macro interference, esp. under cell range extension Summary A: Macro terminal Many pico cells seeing strong downlink interference from small cell D: Many small cell terminals creating uplink interference to macro cell Traffic in today’s networks is bursty, alternating between periods of activity in which high burst speeds are critical to user experience, and periods of Macro cell inactivity. This results in a significant amount of free resources in today’s mature HSPA networks. A number of features are being introduced to improve the customer experience by increasing the utilization of these network resources. An overview of the different features and their benefits is given in Figure 16. These features bring a major improvement to HSPA by simply using network resources more efficiently C: Macro terminal creating leading to the end strong user uplink seeing larger interference to small cell throughputs and faster response times. The benefits of these features are hard to quantify because they are often inter-dependent and also vary according to the actual network scenario. However, some possible benefits include: •MBLB: Optimum performance can be achieved with a minimal amount of tuning needed, leading to lower operational costs •Multi carrier HSPA: With eight carriers, an increase in user throughput of up eight times that of a single carrier could be expected As well as the features dealt with in this white paper, other features beyond its scope are being developed and will be introduced simultaneously, maintaining the rapid evolution of HSPA. Multi band load balancing (MBLB) - Improves the user performance - Utilizes free downlink and uplink resources in other bands/carriers - Operates on a per second level - Supported for all UEs Multi carrier HSPA - Improves peak rates and user throughput - Utilizes free downlink and uplink resources in other co-located carriers/cells - Operates on a per TTI level - Supported for Rel 8+ UEs (2 carriers for Rel 8 up to 8 carriers for 3GPP Rel 11) - Improves cell edge user throughputs - Utilizes free downlink resources in other cells (intra- and intersite) - OPrates on a per TTI level - Candidate for 3GPP Rel 11 - Requires UE support Multiflow - Improves the user performance - Utilizes free downlink and uplink resources in other systems - Requires UE support HSPA - LTE aggregation Figure 16. Feature overview. LTE 14 •Multiflow can lead to a gain at the cell edge of up to 50% •HSPA-LTE carrier aggregation can achieve significant peak Pico cell data rate gains although the amount depends on spectrum allocations and load. LTE carrier aggregation Efficient resource utilization improves the customer experience Multi carrier of LTE Abbreviations 3GPP Cell_DCH Cell_FACH Cell_PCH CSP DC DF-4C HSDPA HSPA LTE M2M MBLB MIMO QoS RNC SF-DC TTI UE Third Generation Partnership Project Cell Dedicated Channel Cell Forward Access Channel Cell Paging Channel Communications service provider Dual Carrier Dual Frequency Quad Cell High Speed Downlink Packet Access High Speed Packet Access Long Term Evolution Machine-to-machine Multi Band Load Balancing Multiple-Input Multiple-Output Quality of Service Radio Network Controller Single Frequency Dual Cell Transmission Time Interval User Equipment Efficient resource utilization improves the customer experience 15 Nokia Siemens Networks Corporation P.O. 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