Joint Radio Resource Management and QoS Implications of Software Downloading for SDR Terminals Nicolas Motte, Robert Rümmler 2, David Grandblaise, Lucas Elicegui, Didier Bourse, Eiko Seidel 3 - Motorola Labs, Paris, France, 2 - Kings College London, UK, 3 - Panasonic European Laboratories GmbH, Germany ABSTRACT This paper presents research on advanced spectrum sharing techniques lead in TRUST and focuses on Radio Access Technologies (RATs) joint radio resource management (JRRM). This paper provides details on cellular FDD/TDD JRRM and addresses the different sources of traffic (regular asymmetric and download) to be considered in an SDR context. The TRUST research on impact of software downloads on UMTS system resources is introduced and some of the future research topics to be investigated in the TRUST follow-up project SCOUT are presented. A. Spectrum sharing I. JOINT RADIO RESOURCE MANAGEMENT The terminal re-configurability capability provided by Software Defined Radio (SDR) technology introduces flexibility in spectrum management when different heterogeneous radio systems exist in a same geographical area. The total flexible spectrum management approach would consist in providing the service required by the user with the most appropriate available Radio Access Technology (RAT) on the most appropriate frequency band in this radio composite environment. However, a first step towards a flexible scheme is illustrated in Figure. Also, to keep as simple as possible this sharing spectrum figure, only two existing RATs belonging to the same operator in a same geographical area are considered. However, this solution is obviously valid for more than two RATs and for different operators. In this fixed spectrum allocation approach, each technology has its own (and fixed) allocated spectrum but each user has the capability to operate on any RAT thanks to re-configurable terminals. This introduces a degree of flexibility enabling performing load balancing between the different RATs. RAT # RAT #2 Fixed frontier Figure : A first step towards a Flexible Spectrum Management So, given the SDR technology and its capability to enable inter operability / co operation between heterogeneous radio systems RAT and RAT2, a vertical handover management between those systems is needed. Accordingly, this raises new challenges in term of Radio Resource Management (RRM) to design inter System (RATs) handover procedures. The design of this vertical handover while optimizing resource utilization requires a joint basis of radio resource management between the RATs. In TRUST, a smart mechanism has been proposed to manage RAT and RAT2 on common basis. A high level description of this scheme, namely Joint RRM Module is depicted in Figure 2. Individual User s Indicators Monitoring and Assessment If Trouble Information Request Joint RRM Module 2 3 Overall System and User Indicators Monitoring and Assessment Indicators Transmission Figure 2: Joint RRM Scheme Notification of Decision 4 User Systems This scheme aims at optimizing the use of radio resources, resulting in both capacity and QoS improvements when RAT and RAT2 are operated in the same geographical area and when users have reconfigurable terminals. The use of this Joint RRM scheme is motivated by the fact that one user experiencing bad radio conditions or blocking and accordingly has no solution to have a successful call in his current RAT, can be allowed to handover in the
other RAT. Of course, such an inter system handover can only be initiated if it does not disturb the new RAT in a given proportion. To enable such an inter system handover, this scheme considers indicators based on both conditions experimented by the user as well as the overall system behaviors. For instance, this vertical handover could be triggered by the following criteria: The unbalanced traffic load between RATs, The user profile: Depending on the user s service requirements, scheduling between users can be introduced to switch between RATs, The radio conditions experimented by the user, The RATs availability on the geographical area where the user operates. B. Scenario As a starting point in the TRUST research, the performance of this joint RRM scheme has been assessed for a vertical handover between UMTS FDD and TDD modes as depicted in Figure 3. Figure 3: Vertical Handover between UMTS FDD and TDD Modes in DL The objective of this simple scenario is to assess capacity and QoS gains when the UMTS FDD and TDD modes of a given operator can inter-operate in the downlink. This scenario focuses in assessing performance of inter RATs call Admission Control schemes, and inter RATs Channel Allocation schemes between both RATs. The scenario illustrates the flexible spectrum allocation scheme presented in A where a given frequency band is dedicated to FDD DL and another one is dedicated to TDD DL. It is assumed that terminals display dual FDD TDD modes capabilities, these two RATs being available in the operating Manhattan environment. A first sub scenario (Figure 3) considers only speech service for both modes. It is assumed an unbalanced traffic. The FDD offered traffic is low whereas the TDD offered traffic is high (TDD is in congestion). Here, the vertical handover consists in the following tasks. In the initial phase, if the user cannot be allocated with the required radio resources in his native mode (FDD or TDD), he can be allowed to switch to the other mode if needed. It aims at minimising the generation of blocking in the native mode. For on going calls, a same approach is used. To avoid dropping, a user can switch from on mode to the other one in order to keep a good call quality. For this dropping scheme, two RRM policies are considered. The difference between those two RRM policies (policy and 2) is the time threshold over which the handover between the two modes is triggered. Dropping time threshold is 5s for policy and 3s for policy 2. The second sub scenario considers a mixture of speech and web services. Web service is provided by the TDD mode and speech service is provided by the FDD mode. As sub scenario, a same unbalanced traffic is considered but only one policy for dropping is considered. C. Methodology To assess this performance, an event driven methodology is used. Users arrive in the system according to a Poisson traffic model. Each user is given a profile (in term of service, position, mobility). When a user arrives, given his service, he negotiates radio resources in the access phase. This initial access phase is either a success or a failure depending on the strategy specified above. If he fails, he is blocked and leaves the system. Otherwise, the user call goes on. Inner loop power control and hard handover are processed individually. During this step, either the user ends his call successfully, or he fails due to dropping, bad FER and the adopted RRM strategy. Concerning the link budget, both intra and inter cell is taken into account. Main radio parameters are taken into [2] and [3]. D. Results Results are expressed in term of capacity gain relative to the case where the vertical handover between FDD and TDD modes is not possible. Gains on QoS improvement are also derived. For sub scenario considering single speech service only, both RRM policies and 2 improve the capacity gain in term of satisfied users. At a level of 95% of system satisfaction, the gain is 6.5% for RRM policy and 7.4% for RRM policy 2 as depicted in Figure 4. In other words, for the same initial set of radio resources, more users are admitted and satisfied when FDD and TDD inter operates. Accordingly, QoS is also improved. The decreasing of time dropping threshold from 5s to 3s enables to reduce significantly FER and consequently the link quality of users. This QoS gai n (compared to the case where no vertical handover is performed) is all the more improved that traffic is unbalanced.
For sub scenario 2 considering a speech and web services mixture, the capacity gain is all the more important with a gain of 24%. This is mainly due to the multiplexing statistic between the circuit and bursty traffics. However, the scheme considered here is relatively basic. Higher gains are expected to be achieved through further investigations. Figure 4: Capacity Gain for Sub scenario As a conclusion, results have shown that significant capacity gains are conceivable when TDD and FDD radio resources are jointly managed. This is all the more beneficial when traffics to be offered in both systems are unbalanced. Accordingly, the concept of a joint radio resource management is more than promising and needs further investigations. It provides the expected flexibility for a better spectrum efficiency and ensures a smart user centric communication environment. At least two axis are envisaged in the development of this research: () Integration of other cellular and non-cellular RATs in the joint radio resource management scheme, (2) Integration of the different natures of traffic supported by the reconfigurable SDR terminals (asymmetric regular, download traffic). II. QOS IMPLICATIONS OF SOFTWARE DOWNLOADING Downloading applications, services as well as entire protocol suites to end-user terminals is a key enabler for ubiquitous terminal operation and seamless service provision. Whereas many potential schemes for downloading software to reconfigurable terminals may be envisaged, over-the-air (OTA) software download followed by immediate installation on the end-user terminal promises to offer ever-present network and service availability. The impact of software downloads on the QoS of regular users is a critical issue in evaluating the feasibility of OTA software downloading. Within TRUST the impact of software downloading on overall network capacity and QoS of regular users was investigated by carrying out system level simulations for UMTS FDD. In particular, the trade-off between deploying circuit-switched and packet-switched bearer services for software downloading was carefully evaluated. The results of the simulations are described in detail in [4]. A. Methodology Simulations examined the impact of software downloads on UMTS system resources in a multi-cell environment, focusing on the deployment of two distinct traffic classes, namely Low Constrained Data (LCD) and Unconstrained Delay Data (UDD), both for a 64kbits/sec data channel. In the following both traffic classes are referred to as LCD64 and UDD64 respectively. According to [3], a subscriber making use of LCD data services can be considered satisfied if the following constraints are fulfilled: The user has sufficiently good quality more than a certain fraction of time within the session. A threshold percentage of 98% was used for the simulations. The user does not get dropped. A call is dropped if the instantaneous BER is larger than a given BER Threshold for a given dropping time-out. A dropping time-out of 5s was used throughout. For UDD traffic a user is defined as satisfied if the active session throughput, S, of the session is equal or greater than 0% of the data rate, R. The reason for such a loose requirement is that there is no guaranteed bit rate or tight delay requirement for the UDD traffic service. While the LCD traffic service is based on the BER that corresponds to an SIR target, the UDD service uses different BLERs according to channel conditions and interference situations. For each BLER there is a different likelihood that a packet is received successfully or not. Only if a packet is received correctly the packet is considered in the overall statistics. A simple retransmission technique is applied and the overall active session period will increase depending on the number of retransmissions needed. Simulations were first carried out for a system loaded with speech users without any software downloads taking place. Subsequently simulations were performed within a system with only software downloads taking place. This was done in order to obtain a reference point with respect to the number of satisfied users the system can support for each service. As a next step a mixed service environment was simulated in a system with only a limited number of users performing software downloads. This allowed for an in-depth analysis of the impact resulting from software downloads on system resources as well as on the QoS of regular users to take place. B. Results For LCD64 software downloads, in which a given data rate is guaranteed, the impact on system resources was found to be high. This is true for a small number of users performing a software download (e.g. for 0% of the total users). Due to the interference generated by each new user, the performance is strongly degraded as the system gets saturated. Even a single new user
performing a software download was seen to severely limit the QoS received by others users in a way that is not deemed to be acceptable. The system is not able to sustain a large number of users requiring a very fast software download by means of a fixed data rate such as the LCD64 circuit-switched bearer. This is due to the fact that fixed data rate services do not allow for radio resources to be handled efficiently within the network. Figure 5 shows the system capacity for different numbers of speech users with a variable number of users performing a software download via the 64kbps data channel. 27.68 active speech-users per sector were simulated. It can be observed that a software download with a reasonable data rate of 64kbps will limit system capacity significantly. The number of speech users is reduced to a third, while there are only about 7 (approx. 20%) users performing a software download. Thus only a small percentage of users performing a software download will lead to a reduction in system capacity. On the other hand the impact of software downloads on system resources are less severe for a packet-switched software download with a variable data rate such as with the UDD64 traffic service. For the same number of download users almost twice as many speech users can be supported compared to the fixed data rate download service. The download process can overcome a temporary decrease of reception quality, resulting in more retransmissions and lower throughput. This makes it easier to keep the system stable during high load situations. Figure 6 confirms that there is less of an impact on the QoS of non-software download users and that for the same number of download users almost twice as many speech users can be supported compared to the equivalent scenario with LCD64. Therefore a network operator performing several simultaneous software downloads should deploy packet-switched instead of circuit-switched bearers in order to not unduly load the network and at the same time efficiently handle the traffic resulting from the software downloads. Ratio of satisfier lcd64-users per sector 0.98 0.96 0.94 0.92 0.9 0.88 0.86 0.84 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 Active lcd64-users per sector Site spacing: 2.8 km Site spacing: 6 km Threshold 98% Figure 5: Ratio of satisfied LCD64-users in a loaded system Ratio of satisfied users Figure 6: Ratio of satisfied UDD64 users in a system with different percentages of satisfied speech users Figure 7 shows how the average download time per module increases as a function of the system load. For a fully loaded system, where still almost all users are experiencing satisfactory QoS, the average download time is more than doubled. Predicting the actual time of a download is difficult as it depends on a variety of parameters. Average time DL per module [s] 0.98 0.96 0.94 0.92 0.9 0.88 0.86 0.84 5 6 7 8 9 0 2 3 4 5 6 7 8 9 20 Active udd64-users per sector 7 5 3 9 7 5 3 55.38 active speech 46.5 active speech 36.93 active speech 27.69 active speech 20.3 active speech Threshold 98% 7 8 9 20 2 22 23 24 25 26 27 Active users per sector Site spacing: 2.8 km Site spacing: 6 km Figure 7: Average download time for module with UDD64 software download We summarise that the impact of a software download using an LCD or circuit-switched bearer service on system resources and QoS of regular users is strong. The impact on system resources can be mitigated if variable rate software downloads or packet-switched bearers (in accordance with the UDD traffic class) are deployed. Therefore software downloading should preferably be packet-switched in order to maximise system resources. III. OBJECTIVES OF FUTURE RESEARCH This paper first presents a Spectrum Sharing technique, the Joint Radio Resource Management Module, which takes full advantage of the flexibility provided by reconfigurable terminals in an UMTS context. The gain brought by this RRM rule is high enough to purchase the research in this field. Further research on joint radio resource management algorithms considering asymmetric traffic scenarios is envisaged in the TRUST follow-up project SCOUT. These asymmetric scenarios will be defined in collaboration with operators. Then, radio resource management algorithms will be developed and evaluated in order to maximize the capacity of the cellular system and the QoS offered to users.
Challenge: Optimize the Software Download without degrading the QoS of regular users t The objectives are: first, maximize the asymmetry performance for cellular systems, second, design Software Download traffic models corresponding to identified scenarios, and develop then specific management rules, in order to optimize the system s QoS, and if needed, refine the software download models. IV. CONCLUSION Current Visited Area New RAT Area Figure 8: Potential Scenari of Software Downloading The re-configurable capability introduces also a new type of traffic in cellular networks: Software Download Traffic. First, the modeling of this traffic has to be refined depending on specific scenarios (mass upgrade, download of a new air interface ). Figure 8 represents a potential scenario of investigation, in which a user will have to manage a SW download, changing of RAT coverage. The challenge in this case is to optimize the download of the new air interface in the time interval t minimizing the impact on other users. Therefore the impact of Software Download Traffic on regular one has to be evaluated for different scenarios. To minimize its possible effects (reduced capacity, degradation of the overall system s QoS ), Spectrum Management schemes will be designed. Depending on the scenario, the use of broadcasting and multicasting in support of cellular download will also be investigated as well as the use of shared transport channels. The Figure 9 depicts the structure of this future research. This paper has presented research lead in TRUST on joint radio resource management (JRRM) applied to cellular technologies and on the impact of software downloads on UMTS system resources. JRRM provides the expected flexibility for a better spectrum efficiency and ensures a smart user centric communication environment. The main developments to be envisaged are the integration of new RATs to be jointly managed and the integration different natures of traffic supported by reconfigurable SDR terminals (asymmetric regular, download traffic). ACKNOWLEDGEMENTS This work has been performed in the framework of the IST project IST-999-2070 TRUST, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues from Siemens AG, Siemens RMRL, France Télécom Research & Development, Centre Suisse d Electronique et de Microtechnique S.A., King s College London, Motorola Ltd., Panasonic European Laboratories GmbH, Telefonica Investigacion Y Desarrollo S.A. Unipersonal, Toshiba Research Europe Ltd., TTI Norte S.L., University of Bristol, University of Southampton. REFERENCES Software Download Traffic models Impact of asymmetric regular traffic on spectrum management Design spectrum management schemes Asymmetric Regular Traffic Refinement of the Traffic Model Impact of Software Download on regular traffic Design spectrum management schemes Software Download Traffic [] Access Networks RF System Scenarios (Release 999). 3GPP 3G TR 25.942 3.0.0 (200-03), Technical Specification Group Radio [2] UMTS 30.03 version 3.2.0, Selection Procedures for the Choice of Radio Transmission Technologies of the UMTS. [3] T. Farnham et. al., Report on Assessment of Novel Solutions on System Aspects of Reconfigurable Terminals and Recommendations for Standardisation, D4.3 IST-999-2070 TRUST, October 200 Figure 9: Future Research Framework