NSN White paper February Nokia Solutions and Networks Smart Scheduler

Transcription

1 NSN White paper February 2014 Nokia Solutions and Networks Smart Scheduler

2 CONTENTS 1. Introduction 3 2. Smart Scheduler Features and Benefits 4 3. Smart Scheduler wit Explicit Multi-Cell Coordination 3.1 Distributed RAN with X2+ and non-ideal backhaul 3.2 Distributed RAN with slow centralized scheduling and non-ideal backhaul Centralized RAN (C-RAN) Enhanced Inter-Cell Interference Control (eicic) with co-channel small cells 4. Further Evolution of LTE Scheduling Summary Abbreviations Page 2

3 1. Introduction As of January 2014, Long Term Evolution (LTE) has been successfully deployed by more than 250 operators, with more than 200 million customers enjoying high mobile broadband data rates. LTE in FDD and TDD mode (TD-LTE) is designed for a so-called frequency reuse of one where all the cells use the same frequency. Reuse of one provides the highest network efficiency and enables high data rates close to the base station. The challenge with reuse of one is the high inter-cell interference when the terminal (User Equipment UE) is located between two cells. The data rate over the cell area is illustrated in Figure 1. Boosting the cell edge performance is the main motivation of Smart Scheduler. Smart Scheduler can also enhance the average data rates and system capacity by considering signal fading and interference in packet scheduling decisions. Smart Scheduler algorithms, benefits, impact on the network architecture and further evolution are discussed in this white paper. If not otherwise explicitly stated, all statements are valid for both LTE (in FDD mode) as well as for TD-LTE. Frequency f1 Frequency f1 Data rate Cell A High data rate close to BTS UE Low data rate at cell edge Cell B Fig. 1. Frequency reuse of one creates high inter-cell interference Page 3

4 2. Smart Scheduler features and benefits LTE radio technology is highly standardized by 3GPP but only with regard to the interfaces the network algorithms including link adaptation, power control and packet scheduling are not standardized. Therefore, there can be differences in network performance due to the different algorithms being used by different vendors. The most relevant features and benefits are described in this section. Packet scheduling can use different input information for resource allocation and for interference coordination: Channel Quality Information (CQI) from UE to BTS for downlink scheduling. Sounding Reference Signal (SRS) measurements and interference measurements in the frequency domain for uplink scheduling. Load and other information exchange over the X2 interface between base stations. X2 interface in Release 8 allows some exchange of information between the base stations, but further extensions will be discussed in 3GPP and can also be added proprietarily. Quality of Service (QoS) parameters from the packet core network These different input information options are illustrated in Figure 2. QoS Gateway QoS Coordination over X2+ Channel quality information (CQI) Cell A UE Cell B Fig. 2. Input information for coordinating the resource usage Page 4

5 Smart Scheduler can utilize the different input values to optimize packet scheduling and link adaptation. LTE allows considerable freedom to define allocations in the time, frequency and power domains. A number of different features are required for the different use cases. The same features are utilized both in Frequency Division Duplex (FDD) and Time Division Duplex (TDD) based LTE. The Smart Scheduler utilizes the following main features: Frequency Selective Scheduling (FSS) improves performance in the case of frequency selective fading and fractional inter-cell interference. FSS consists of Channel Aware Scheduling (CAS) and Interference Aware Scheduling (IAS). The field measurements show +30% gains for the cell edge data rates. Interference shaping further improves the efficiency of the intercell interference avoidance by FSS. When the cell loading is low, the number and set of physical resource blocks is adapted only slowly according to traffic fluctuations. This approach makes it more efficient for the adjacent highly loaded cells to avoid intercell interference based on UE CQI reporting. Studies show gains exceeding 100%. QoS differentiation improves cell edge performance by allocating more resources for users in weak channel conditions. QoS can be utilized to maintain the data rate, for example for video streaming services. Further flexibility is obtained by using operator specific QoS Class Identifier (QCI) values. The minimum guaranteed cell edge data rate can be obtained also by Nominal Bit Rate (NBR) which works even without guaranteed bit rate QoS classes. Cell edge prioritization has only a minor impact on the cell aggregate throughput capacity, in typical case 30% cell edge throughput improvement can be obtained at the cost of 5% cell throughput capacity. The capacity measured in number of satisfied subscribers is still higher. Interference aware uplink power control considers the adjacent cells when allocating the uplink transmission power. The feature minimizes inter-cell interference and helps to boost uplink data rates. Intra-frequency load balancing helps when the load in the adjacent cells is not balanced. The idea is to modify handover parameters based on the information exchange of the X2 interface. If there are double the users in the adjacent cell, the intra-frequency load balancing can improve the cell edge data rate by 30%. Page 5

6 Multi-cell scheduling can reduce the power levels (muting or related variants) in adjacent cells to minimize the interference. The multicell scheduling coordinates resource allocation between multiple cells in time and in frequency, using a selection of users and power levels in multiple cells to combine the benefits of frequencyselective scheduling and spectral efficiency gain due to reduced interference. The coordination happens between the sectors of one base station, or over the X2 interface between the base stations. Multi-cell scheduling can improve cell edge performance by 20%. Multi-cell scheduling requires inter base station time synchronization. TD-LTE base stations need to be synchronized while the synchronization of LTE FDD base stations is not mandatory and is typically not used by operators for FDD deployments. Note also that the reference signals are overlapping in adjacent cells in a synchronized network. Therefore, UEs should preferably support cancellation of common reference signals for better performance. The Smart Scheduler use cases, features and gains are shown in Figure 3 and Figure 4. Figure 4 shows the gains of the individual scheduling functionalities when used jointly. More gain can be obtained in HetNet scenarios with eicic. Use case Feature Fractional inter-cell interference Unbalanced loading between cells Multi-cell scheduling Intra- and inter-frequency load balancing Lower gain Fig. 3. Smart Scheduler use cases and solutions HetNet eicic Minimum cell edge rate required QoS differentation and nominal bit rate Fractional inter-cell interference Frequency selective fading FSS including Interference Aware Scheduling (IAS) and Channel Aware Scheduling (CAS) Baseline scheduler Highest gain 120% 100% 80% Multi-cell scheduling Intra-frequency load balancing Nominal bit rate and QoS Frequency selective scheduling Fig. 4. Smart Scheduler downlink data rate gains with non-ideal backhaul 60% 40% 20% 0% Cell edge Average Page 6

7 Let s now analyze Frequency Selective Scheduling (FSS) the most important part of the Smart Scheduler. The multipath propagation in the mobile environment makes the fading frequency selective. The typical coherence bandwidth of the macro cell channel is 1-2 MHz, therefore, there are faded and non-faded frequencies within one LTE carrier. LTE radio uses Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. Therefore, FSS allows use of those parts of the carrier (called Physical Resource Blocks) not faded for the transmission. The concept is illustrated in Figure 5. Information about channel fading can be obtained from UE CQI reports in downlink and from Sounding Reference Symbols (SRS) in uplink. Resource block Carrier bandwith Transmit on those resource blocks that are not faded Frequency Fig. 5. Frequency Selective Scheduling to minimize fading impact Page 7

8 FSS can also be applied to avoid inter-cell interference. An example is shown in Figure 6 where the interfering cell is partially loaded. The UE is connected to the target cell but receives strong interference from the adjacent interfering cell. The UE reports sub-banded CQI values in the frequency domain to the target cell. Low CQI values are reported on those sub-bands where the interfering cell has on-going transmission while high CQI values are reported in other sub-bands. The target cell with FSS tends to allocate those downlink physical resources blocks to the UE where the interference is lowest. The other resource blocks in the target cell can be allocated to other UEs that do not receive interference from the adjacent cell. Benefits of FSS include: Effective inter-cell interference coordination without the need for explicit inter-bts coordination Utilization of UE CQI reports for interference mitigation and without the need for coordination signaling between the base stations Improved cell edge data rates as well as total cell capacity. Fractional load in adjacent cell UE reports subband CQI Frequency selective scheduling Frequency Interfering cell CQI 1 (low) CQI 2 (high) CQI 3 (high) CQI 4 (low) CQI 5 (low) CQI 6 (high) CQI 7 (high) CQI 8 (high) UE A Target cell No transmission Transmission in adjacent cell Transmission to UE A Transmission to other UEs Fig. 6. Frequency Selective Scheduling (FSS) to minimize inter-cell interference Page 8

9 As part of the Smart Scheduler concept, the underlying link adaptation function is critical for the success of features such as FSS. The quality of reporting from each active terminal is always monitored and compensation is constantly conducted in order to improve the value of the scheduler decisions. With such methods, NSN has in numerous commercial LTE networks shown the practical value of FSS. An example field measurement result with 10 MHz bandwidth is shown in Figure 7. Cell edge throughput Cell capacity Mbps Mbps FSS off FSS on 5 0 FSS off FSS on Fig. 7. Field measurements with FSS in downlink Interference shaping further improves the efficiency of the inter-cell interference avoidance by FSS. Interference shaping is illustrated in Figure 8. When the cell loading is low, the number and set of physical resource blocks is adapted only slowly according to traffic fluctuations. This approach makes it more efficient for the adjacent high loaded cells to avoid inter-cell interference based on UE CQI reporting. The studies show major gains for the cell edge data rates in those cases where the loading is unbalanced between the cells: the gains can exceed 100%. High gains can be achieved in the distributed solution with cleverer scheduling without any fast signaling over the X2 connection and without any centralized network element. Low loaded cell: Only slow changes in frequency domains High loaded cell: Robust inter-cell interference avoidance based on CQI reports Fig. 8. Interference shaping for more efficient interference avoidance Page 9

10 3. Smart Scheduler with explicit multi-cell coordination Further performance improvements can be obtained by coordinating resource allocation in adjacent base stations. The network architecture options for supporting multi-cell scheduling are shown in Figure 9. a) Distributed RAN with X2+ and non-ideal backhaul enb#1 Coordinated scheduling (inter-enb) Fast local scheduling X2+ enb#1 Coordinated scheduling (inter-enb) Fast local scheduling b) Distributed RAN with slow centralized scheduling and non-ideal backhaul X3 Coordinated scheduling enb#1 Fast local scheduling... enb#n Fast local scheduling c) Centralized RAN with fast centralized scheduling and dark fiber connection Super-eNB (baseband pool) Common packet scheduling Direct fiber with multi-gbps... Fig. 9. Network architecture options for explicit multi-cell scheduling 3.1 Distributed RAN with X2 and non-ideal backhaul Today s LTE architecture (99% of deployments) is shown in Figure 9a using non-ideal backhaul with microwave radio, IP connected fiber or copper based transport. The multi-cell scheduling needs to coordinate the resource usage in adjacent base stations over non-ideal backhaul while still fully utilizing FSS gains in fast scheduling. The coordination between cells of different base stations will utilize the X2 interface. Each scheduler that requests coordination from its neighboring base stations to aid a user at the cell edge can still take into account FSS gains for that user, thus FSS gains can be fully preserved while adding the gains from multi-cell coordination. The evolution from fully distributed architecture to multi-cell coordination over X2 is a straightforward software upgrade no new network elements or interfaces are needed. Note that fast local coordination can be implemented between the cells in one base station without any interbase station coordination. Page 10

11 3.2 Distributed RAN with slow centralized scheduling and non-ideal backhaul Another architecture alternative is shown in Figure 9b with a new centralized network element for coordinating the distributed schedulers. A new interface between base stations and the centralized scheduler is required. Involving an additional interface and information exchange to an additional entity has a negative impact on the responsiveness of this architecture. The distributed base stations still run the fast scheduling while the centralized element can only set scheduling limitations to minimize the interference. The performance gain of the centralized element is similar to the coordination over the X2 interface. Coordinated multi-cell scheduling and muting over non-ideal backhaul was studied in 3GPP under the title Enhanced Coordinated Multipoint (ecomp) during The conclusion taken in December 2013 was that the gains for inter-site macro-macro scenario are below 5% in the best case over intra-site and less if the backhaul latency increases (several tens of ms). The further focus of ecomp will be in the HetNet scenarios between macro cells and small cells. 3.3 Centralized RAN (C-RAN) The final multi-cell architecture shown in Figure 9c is centralized scheduling in the baseband pool. This is the architecture for a network with ideal transport. The baseband pool requires a low latency direct dark fiber connection between the RF heads and the baseband pool. The baseband pool is also referred to as Centralized Radio Access Network (C-RAN). C-RAN is like a super-sized base station. C-RAN enables the most advanced multi-cell coordination because all the functionalities are in the same location: link adaptation, power control, fast FSS and multi-cell coordination. C-RAN architecture also enables Joint Transmission and Joint Reception Coordinated Multipoint (CoMP) between different sites while intra-site CoMP can be implemented also in the distributed RAN architecture. CoMP functionality is defined in 3GPP Release 11 but uplink CoMP can be implemented also with legacy Release 8 UEs while the downlink CoMP requires Release 11 UEs. Uplink CoMP gives more gain, while downlink CoMP gains are limited. An excellent use case for C-RAN is to boost capacity in stadiums and other mass event locations. These events tend to be uplink limited because many people want to send pictures from the event. The UE transmission is received by a single cell in the traditional solution while the same UE transmission can be received by multiple cells and combined in the baseband module. The inter-cell interference turns into a constructive signal. The solution is illustrated in Figure 10. The installation of fiber between baseband modules and RF is relatively simple in these event areas. Page 11

12 NSN Flexi Multiradio base station enables CoMP by providing fast interconnections between the baseband modules. NSN C-RAN has been validated in commercial networks in large stadiums and the practical gains exceed 100%. Fig. 10. Centralized RAN for boosting mass event capacity 3.4 Enhanced Inter-Cell Interference Control (eicic) with co-channel small cells Small cells are an attractive solution for boosting hot spot capacity and coverage. The interference management needs to be considered when the small cells are deployed on the same frequency as the macro cells. 3GPP Release 10 brings a solution for managing the interference in the time domain. The solution is called enhanced Inter-Cell Interference Coordination (eicic) and is shown in Figure 11. The macro cell leaves some empty sub-frames called Almost Blank Subframes (ABS). During these sub-frames, the small cell can serve UEs that would otherwise receive too much co-channel interference from the macro cell. Page 12

13 The benefit of eicic comes when several small cells can benefit from macro cell empty subframes. eicic performance is further boosted in Release 11 by using UE interference cancellation for the minimization of inter-cell interference, which is known as further enhanced ICIC (feicic). Optimized eicic requires that the number of ABS frames and the handover parameters are adjusted dynamically according to the instantaneous traffic conditions and UE locations. The semi-static solution is a slow approach for modifying the feicic parameters over several seconds. The fast feicic adaptation uses quick adaptation for the number of ABS sub-frames to reallocate resources between macro cells and small cells depending on the instantaneous requirements. NSN s unique algorithm is based on the fast adaptation of ABSblanking and cell range extension for maximum benefit from small cell deployments. The throughput gains are shown in Figure 12: dynamic eicic can nearly double the user throughputs in heterogeneous networks. = Sub-frame with normal transmission = Almost blank sub-frame (ABS) Macro Sub-frame (1 ms) Pico Pico cell can serve also such UEs that receive stronger macro cell signal Pico cell can reuse same frequency as macro when UE is closer to pico Fig. 11. Time domain interference management with enhanced ICIC Semi-static felcic 70% Fast felcic 90% Baseline w/o felcic: 0% Fig. 12. Throughput gain from enhanced ICIC Page 13

14 4. Further evolution of LTE scheduling 3GPP is working with Inter-site carrier aggregation in Release 12. The feature allows the UE to receive data simultaneously from the macro cell and from the small cell. The two cells do not need any fiber backhaul, although wireless backhaul with some delay is fine. The X2 interface is used between the macro cell and small cell for scheduler coordination. The macro cell and the small cell can share the same frequency or the small cell can use a dedicated frequency. The feature is illustrated in Figure 11. Inter-site carrier aggregation uses Dual Connectivity where the UE has simultaneous radio connection to both macro and to small cell. That brings benefits in terms of reliable mobility. 3GPP is also working on a solution where UEs can cancel the inter-cell interference by obtaining assistance information from the network. This feature is called Network Assisted Interference Cancellation and Suppression (NAICS) and it is part of Release 12. If UEs can cancel interference, it may be more efficient to use all resources in cochannel cells instead of muting resources. The multi-cell scheduling and muting algorithms need to be designed in such a flexible way that they can benefit from the future advanced UE capabilities. Inter-site Carrier Aggregation and Dual Connectivity Fig. 13. Inter-site carrier aggregation in Release 12 Page 14

15 5. Summary While LTE has been highly standardized by 3GPP, the network algorithms including packet scheduling are not standardized. The packet scheduling in LTE has the freedom to control the resource allocation in the time and in the frequency domain. Smart Scheduler can push cell edge data rates by more than 100% in the presence of inter-cell interference compared to baseline wideband scheduling, and improve the cell capacity by more than +20%. The main component of Smart Scheduler is frequency selective scheduling that avoids the fading and interference in the frequency domain combined with Quality of Service differentiation and intra-frequency load balancing. NSN s innovation Interference Shaping increases the cell edge throughput further by up to 100% when the cell loading is unbalanced. Additional cell edge gains can be obtained by multi-cell scheduling. Multi-cell scheduling is a simple software upgrade to distributed base stations. Scheduling information is shared between base stations over the X2 interface. The detailed standardization of multi-cell coordination is considered in 3GPP Release 12. The most advanced multi-cell coordination can be obtained with baseband pooling in Centralized RAN. The baseband pool deployment assumes direct fiber connection between baseband and RF sites. Centralized RAN provides the biggest benefits in uplink capacity, which is most useful in high capacity events. The efficiency of small cell deployment can be boosted by using dynamic eicic configuration to manage the interference between macro cells and small cells. 6. Abbreviations 3GPP BTS CoMP CQI C-RAN ecomp eicic FDD FSS LTE OFDMA QoS RRH SC-FDMA SRS UE Third Generation Partnership Project Base Station Coordinated Multipoint Channel Quality Information Centralized Radio Access Network Enhanced CoMP Enhanced Inter-Cell Interference Coordination Frequency Division Duplex Frequency Selective Scheduling Long Term Evolution Orthogonal Frequency Division M Quality of Service Remote Radio Head Single Carrier Frequency Division Multiple Access Sounding Reference Symbols User Equipment Page 15

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