This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
Efficiency and Dependability of Direct Mobile-to-Mobile Data Transfer for UMTS Downlink in Multi-Service Networks Larissa Popova, Thomas Herpel and Wolfgang Koch Lehrstuhl f¨ur Mobilkommunikation, Universit¨at Erlangen-N¨urnberg, Germany Email:
[email protected] {popova, koch}@LNT.de
Abstract— This paper is a sequel of previous work, in which we proposed a concept based on direct mobile-to-mobile (m2m) data exchange on uplink channels for distribution of large popular content in dynamic wireless environment. We showed the feasibility and performance benefit of m2m file sharing applications in terms of thereby released overall downlink capacity in UTRA-FDD systems, where multiple m2m user groups coexist in the network. A pure m2m traffic scenario was investigated and the wireless interference only among the neighboring groups was taken into account. The present paper extends the basic model by introducing a hybrid case of traffic. We consider an UMTS system scenario with coexisting conventional UMTS users and m2m file sharing participants and analyze the cross-traffic impact in order to find out the degree of m2m data transfer performance degradation due to increased uplink interference. The results indicate a slight performance degradation of m2m file sharing applications, in terms of relative losses of m2m link quality (number of corrupted data) caused by additional interference from cross-traffic. Nevertheless, no dramatic decrease in service probability and overall throughput for m2m applications in an UMTS network was observed. Furthermore, results show that by a well-designed m2m routing policy the ability to interconnect wireless devices on the partly unused uplink carrier frequencies (due to typical asymmetric uplink/downlink traffic load) might be a profitable goal and offers a promising alternative for distribution of popular content.
I. I NTRODUCTION Due to the tremendous growth of the user population with their demand for high data rate multimedia services relatively limited transmission capacity of radio interface quickly becomes saturated, which leads to degradation or even loss of the service. Typically, the downlink is the potential bottleneck, while free resources may be available in the uplink. The current architecture of cellular systems has shown its limitations to cope with the challenging problem of the spectral resources utilization in very heterogeneous wireless environment of the third generation of radio networks. The above considerations identify the necessity of employing new paradigms in
the architecture of cellular communication systems, as well as in policies in handling of user service requests. Nowdays, mobile networks are widely used for so-called background services such as music, digital camera images or movie files downloads, which are non-real time demanding and therefore delay insensitive. To overcome the capacity limitations of cellular networks and to prevent exhaustion of the downlink capacity we developed in our previous study1 a new cooperative concept which was realized on the basis of the benefits derived from the above mentioned properties of specified service type. In our concept the users that are interested in downloading a popular file form a mobile cooperative community and using the fact that traffic load of multimedia services is asymmetrically distributed between uplink and downlink, contribute their own normally only partly used uplink capacity for providing the packets of the content to other users in multicast mode on the uplink carrier frequencies. Our concept is denoted as m2m (mobile-to-mobile). By using such a cooperative technique a major part of nonreal time multimedia traffic is shifted away from the downlink, making the released downlink capacity available for other (e.g. real-time demanding) services. The contribution of this work is to analyze the efficiency and dependability of the proposed m2m concept for cellular radio networks like UMTS by consideration of mixed traffic scenarios. We built on our concept with m2m file sharing participants only, and introduced dynamic populations of conventional UMTS speech users joining and leaving the system. Obviously, the speech traffic in the uplink is a potential source of interference to m2m users, which operate on the same uplink frequency. We 1 The basic concept and first performance results are summarized in a manuscript submitted for the 6th International Conference of Networking and is currently under review. However, the understanding of the present paper does not require its knowledge.
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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
investigate the speech traffic impact in terms of the degree of performance degradation of m2m data transfer. Currently, there are only a few proposals for a mobile peer-topeer file sharing architecture. In [6] the authors are motivated by the potential UMTS capacity improvement of embedding WLAN (Wireless Local Area Network) systems in UMTS for more efficient routing of information between users of the system. The effort of this research is focused on the basic applicability and enhanced routing of the information in the proposed hybrid network. The feasibility of the eDonkey Internet protocol in a GPRS environment was investigated in [3], [5] and extended to UMTS radio networks in [4]. The main focus lies on resource mediation and control by using different strategies for data caching in the wired part of the network. No direct data transmission among the users is intended and no cross-traffic is considered. The concept proposed in this work differs from other wireless peer-to-peer solutions in that respect that our performance results were evaluated taking into account the specifics of UTRAN, e.g. wireless interference, realistic propagation model and realistic traffic scenario. The rest of the paper is organized as follows. In Section II we revisit some basic characteristics and assumptions of the m2m model and summarize the previous results. In Section III the mechanism of m2m file sharing in a presence of other services in the system is introduced. The impact of the speech traffic on the performance of our m2m solution is evaluated in Section IV. Finally, Section V concludes the paper.
Fig. 1.
B. Model Characteristics and Assumptions •
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II. R EVISITING BASIC M2M MODEL CHARACTERISTICS AND ASSUMPTIONS
In this Section we shortly summarize our previous research and present the obtained results. A. M2M Concept To enable the wireless peer-to-peer technique we revised the idea of a mesh cooperative architecture for the fixed-line Internet [2] and appropriately extended it for the UTRA-FDD (Frequency Division Duplex)2 . To illustrate the main concept of the m2m technique, consider the example in Figure 1. Mobile terminals (MTs) voluntarily participate in file sharing via direct mobile-to-mobile data transfer with the purpose to reconstruct the original popular content, which is distributed in the network (see description in Section I). We now assert general assumptions and radio interface restrictions for our analysis. 2 Although we focus on the FDD mode of WCDMA the principle can be applied to other systems as well.
M2M Concept.
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We simulate a population of users with Poisson distributed service requests, that are interested in downloading a popular file. Each MT has a given constant speed during a simulation session. All users are assumed to be pedestrians. Initially, the original file is available in the core network only. In a cellular system, in contrast to Internet peer-to-peer applications, the number of users who can cooperate with each other is limited by the transmit power of the MT and its coverage range, which will be typically less than a cell. Therefore, in case of wireless cooperative community formation, mobile communities are location and radio propagation dependent. Thus, to cooperate with each other, m2m users must be organized into groups, which satisfy the following conditions: 10lgPT X − Lij − Λij ≥ −112 dBm | PT X = PT X,min , ∀i, j ∈ group, where Lij is the pathloss between MTs i and j and Λij is the random variable describing the shadowing process. PT X,min = −44 dBm is the minimum transmit power allowed in UMTS according to the 3GPP specifications [1] which ensures minimum interference to other links. Appropriately modified radio propagation model for low antenna heights for both, transmitter and receiver was used. The simplest way to inform a new MT about all other MTs, requesting the same content in its coverage range is to transmit ”Hello” packets by all MTs, periodically (similar to the concept in [6]). But this procedure puts considerable load on signalling channels. Thus, it would be more efficient if upon arrival each new m2m user contacts a Node B that provides information about all other m2m users already in the system in the range of tens of meters, to determine the potential members of the group. In case no appropriate group for a new user is found it forms a group with stand-alone user only. Only MT, assigned to a multiple member group sends
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
a ”Hello” packet to get an appropriate information about the pathloss to any other MTs from its group. In such a way we can reduce the signalling information between stand-alone MTs and prolong the limited battery life of MTs. • The size of the groups is limited. Each MT can be a member of only one group at the same time. • Since all users in the network are mobile throughout the file download time, the groups need to be periodically updated (every 100 radio frames)3 and reshaped in order to check positions of MTs and their radio propagation characteristics on the one hand, and to track and authorize new m2m users in the group in case they fulfil the above mentioned” joingroup” criteria, on the other hand. • In order to reduce the transmission of identical packets on the network links and in turn to increase the efficiency of usage of scarce and asymmetrically loaded uplink/downlink bandwidth the m2m data transfer is performed in multicast mode on the uplink carrier frequency, whereas receivers in the group switch to listen on the uplink. • Each user has information about the packets it has received and the packets which are already received by the other members of its group. • The limitation of battery capacity of the handsets results in a lower online time compared to the fixed-line desktop PCs. Thus, in order to maximize the efficiency of using multicast in each group the file sharing policy proposed in this work is based on a local ”most-utile-block” scheme. It should be understood as follows. The packets, which ”sender” candidate could share with the members of his group should be useful for maximum possible number of users. • Identification of the sender is done using a unique scrambling code. • To support the intra-group transfer only a signalling channel on Node B is used. • Users, which have not found any useful packets within a specified time interval, try to connect to the Node B for packet delivery. • Without loss of generality we assume, that the mobile terminals are able to receive in both uplink and downlink. C. Revisiting results: Comparison of M2M file sharing with conventional UMTS data transmission Extensive simulations have been carried out indicating significant performance advantages of using m2m mode in UMTS network for distribution of popular non-real time contents compared to the conventional downlink unicast mode. For the simulations we assume that there is mobile specific content type of 500 Kbyte size to be distributed with the m2m strategy. The most important parameters used in our simulations are summarized by Table I. 3 In this work we show the results based on the above mentioned ”group-update scheme”. Alternative group update policies are currently under consideration.
Traffic and environmental settings Traf. load (max. num. of m2m users/cell) Maximum m2m group size Antenna type Cell radius User profile Radio interface and algorithmic settings File size Required gross data rate Maximum user data rate with 1/2-rate coding Size of logical packets for m2m data Receiver sensitivity Transmission power in m2m mode Eb /N0 target Inner loop power control for m2m sender Simulation step size Number of steps Group update period
10 (low), 30 (med.), 50 (high) 7 omni-directional 50 m Pedestrian 500 KByte 60 kb/s 30 kb/s 225 bit (coded) -112 dBm -44 dBm 3 dB OFF 1 radio frame (0.01 sec) 40.000 - 120.000 100 radio frames (1 sec)
TABLE I M AIN SIMULATION PARAMETERS .
We consider four performance measures: • Overall downlink throughput gain: data volume reduction in the downlink. • Service probability gain: increase in the number of served users (%). • Download time gain: download time reduction. We define the download time as the time window, in which the user receives the complete file. The criterion for the download time gain is the 90% quantile of finished downloads in the system. • Relative losses of link quality: number of corrupted data in (%). We sum up the main performance results in Table II. In the Load Data volume in DL in conv. mode [MB/cell] Data volume in DL in m2m mode [MB/cell] Released DL capacity [%] Time m2m [sec]
low 5.73 1.33 76.70 +189.3
medium 16.51 2.93 82.23 -20.1
high 34.44 4.67 86.44 -142.3
TABLE II OVERALL DOWNLINK THROUGHPUT GAIN .
Table II the relative download time reduction for complete file download in m2m network mode is compared with conventional UMTS data transmission (+: m2m slower, -: m2m faster). The rows ”Data volume in DL in conv. mode [MB/cell]” and ”Data volume in DL in m2m [MB/cell]” show how many Mbyte of data had to be sent via the downlink channels in order to distribute the data file of 500 Kbyte size to the users within one cell. All analysis are focused on the technological characteristics of mobile peer-to-peer applications, other resulting problems
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
and challenges such as a fair scheduling, security and rights management are beyond the scope of this work.
the performance of the proposed m2m technique is affected only moderately.
III. R ESTRICTED G ROUP O RGANIZATION P OLICY For the results in Table II it was assumed that the m2m groups can consist of users from two or more neighboring cells. Nevertheless further investigations on the proposed m2m concept such as the actual implementation of the file sharing protocol or determination of the Node B/RNC responsibilities in supporting the m2m data transfer might yield the necessity for a strictly cellbased group organization policy. Figure 2 shows the effect of the restriction in the group organization policy on the performance of the proposed m2m technique in a low and high loaded UMTS system.
Probability of completed downloads
Comparison of download times (medium load)
Groups restr. to one cell 2.49 1.38 28.9
Groups not restr. 1.94 1.41 33.8
Data volume in DL [MB] Users per one UL-chan. Service prob. gain, %
Groups restr. to one cell 7.1 2.66 62.35
Groups not rest. 5.31 3.19 68.27
TABLE III I MPACT OF THE RESTRICTION IN THE GROUP ORGANIZATION POLICY ON DOWNLINK THROUGHPUT, SERVICE PROBABILITY GAIN AND NUMBER OF MULTICAST RECEIVERS PER GROUP FOR LOW TRAFFIC LOAD ( TOP ) AND HIGH TRAFFIC LOAD ( BOTTOM ).
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Intuitively, with an increased number of m2m users in the network more larger groups could be formed across the cell border and the probability of potential group members being rejected increases, yielding comparatively smaller and less efficient m2m communities.
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Fig. 2. Impact of group restriction policy on file download time for low traffic load (top) and high traffic load (bottom).
The restriction of the grouping to members of the same radio cell in high loaded system leads to an increase of file download time in order of 20% compared to the more relaxed group organization policy and puts some additional load to the downlink resources, whereas the loss in terms of service probability is not so pronounced (see Table III). In turn, in case of low traffic load
IV. A LGORITHM In order to study the efficiency and reliability of the m2m concept in more realistic network scenarios, we extended our model for more service classes by introducing the population of speech users and analyze the cross-traffic impact in order to find out the degree of m2m data transfer performance degradation due to increased uplink interference. The following flow chart visualizes the mechanism of m2m file sharing with cross-traffic in presence. V. N UMERICAL RESULTS In this section we present some numerical results, demonstrate efficiency and dependability of the proposed m2m concept in a cellular system with a UMTS radio interface by taking into account impact of cross-traffic on the m2m performance gain. Particularly in hotspot environment users increasingly demand ubiquitous data availability. Thus, the main focus of our analysis lies in the optimization of data availability to users in hotspots (e.g. airports, railway stations) by using the m2m file sharing technique. We simulate a network area populated by MTs of two service classes. We consider the mixed traffic scenarios, where the m2m file sharing participants coexist with the UMTS speech users. In this scenario we assume that service requests for speech users are served in conventional mode, where discontinuous transmission is organized by providing individual links from the Node B to each user. The network architecture is assumed to support two
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
Fig. 3.
Algorithm of m2m concept.
alternative modes of serving user requests (m2m network mode and conventional UMTS mode). The m2m algorithm is implemented as explained in Section IV. The transmission in conventional mode complies with 3GPP specifications for UTRA-FDD. Since the speech users operate on the same uplink frequency as the m2m users, the signals they generate in uplink is a potential source of the m2m signal disturbance, which can lead to further performance degradation of m2m data transfer. A. Simulation Scenarios In this paper we outline several scenarios to demonstrate the reliability of the proposed m2m concept and its robustness to wireless interference. We analyze here the following traffic scenarios: Scenario 1: The number of speech users per cell is kept constant (approx. 3 speech user per cell), while varying the load of m2m users (low, medium, high load, see Table I). Scenario 2: The amount of speech traffic varies proportional to the m2m traffic load, with the mean number of speech users being 20% of the mean number of m2m users in a cell. For low load, this results in 2, for medium load in 6 and for high load in 10 active speech service sessions per radio cell. Scenario 3: The same intention as the setting with 20% speech users load, increasing the intensity to 40%, i.e. 4 (low load), 12 (medium load) and 20 (high load) active speech users per radio
cell. The main subject of varying the speech traffic load is to investigate the influence of the different service mix on the performance of m2m file sharing. We list now the assumptions and parameter settings we employed in our simulations: • The MTs are distributed randomly over the cell area. The radius of the cell is 50 m (hotspot scenario). • The size of the groups is restricted to 7 members. • Initially, no packet is available among m2m users. • The MTs depart from the system immediately after finishing their download. • The size of a logical packet is equal to one UMTS radio frame.4 • The characteristics of the wireless channel in each group can vary from slot to slot (fast fading). • Information about the quality of the multicast signal within the group is obtained based on the ratio of the average received power of the useful signal to that of all relevant interfering signals (C/I) on a slot-by-slot basis. • If the packet is incorrect after detection, we declare a packet loss.5 • The simulation time is 400 - 1200 sec and we collect data framewise. B. Impact of cross-traffic The performance of m2m file sharing is evaluated by comparing the distribution of download times for a complete file download for two uplink interference scenarios. One m2m scenario involves only inter-group interference while the second scenario consists of both m2m inter-group interference and interference from speech traffic which operates on the same uplink frequency as the m2m users. Figure 4 illustrates the influence of the speech traffic interference on the performance of m2m concept in terms of download time increment. Intuitively, the higher the speech traffic load the higher the interference in uplink. From the graphics one can see that the download time increases up to about 25%. It results in decreased service probability for m2m users on one hand and in high retransmission probability (because of amount of corrupted blocks) on the other hand. Relative losses of link quality (number of corrupted data in %) for different group sizes and traffic scenarios are shown in Table IV. Another parameter that is of interest is the released downlink resources. Table V shows how many Mbyte of data had to be sent via the downlink channels in order to support the m2m data exchange and timeout requests. Again there is a tendency, that the higher the cross traffic load and thus the level of interference, 4 Depending on the coding scheme and spreading factor this leads to corresponding packet sizes. 5 Even if a third of the frame (1 frame=15 slots) is corrupted, caused by an unacceptable C/I level, transmitted data is still assumed to be recoverable (due to channel coding).
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
Probability of completed downloads
Comparison of download times (medium m2m load, Scenario 1) 1
demonstrated overall downlink throughput gain up to 85%, obtained by using m2m technique, (see Table V). Furthermore, Load Data vol. m2m mode w/o cr.-traf.[MB/cell] Scenario 2 [MB/cell] Scenario 3 [MB/cell] Data volume in DL in conv. mode [MB/cell]
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medium 2.63 3.43 3.69 16.51
high 5.31 6.33 7.23 34.44
TABLE V
without crosstraffic with crosstraffic 0 0
low 1.94 1.98 2.09 5.73
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DATA VOLUME IN DOWNLINK IN M 2 M MODE FOR DIFFERENT CROSS TRAFFIC INTENSITIES ( FOR USERS WITHIN ONE CELL ).
Probability of completed downloads
Comparison of download times (high m2m load, Scenario 1) 1
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Fig. 4. Impact of additional uplink interference from speech traffic on the file download time.
Load low medium high
inter-group interf. 2.07 5.20 8.89
Scenario 1 7.14 8.76 13.23
Scenario 2 4.21 13.71 16.22
the number of MTs that can be supported by such a system is three time higher than in case of conventional UMTS network. Table VI shows the relative gain in the number of MTs, that can be supported by m2m concept for different traffic scenarios. Although, in this work the comparisons of m2m file sharing with UMTS downlink unicast have been done under quite unfair traffic load conditions (in case of conventional mode we investigated pure data traffic without any cross-traffic at presence), it is evident that the performance gap between two alternative network modes of serving user requests becomes even bigger while introducing a hybrid case of traffic for conventional UMTS mode, too. Since the number of users that can be admitted in the system is bounded by the uplink capacity, the network will serve high prioritized speech calls at the expense of a degradation of service probability for non-real time data traffic.
Scenario 3 9.29 18.29 22.46
TABLE IV I MPACT OF UPLINK INTERFERENCE FOR DIFFERENT TRAFFIC SCENARIOS ( ERRONEOUS DATA [%]).
the more data is corrupted. All this coerce m2m users to request packets of the desired file from Node B more frequently, which in turn impairs the efficiency of m2m concept. Nevertheless, the direct comparison of m2m performance results with those of the conventional UMTS data transmission
Scenario 1 (%) 30.06 52.23 66.53
Scenario 2(%) 33.02 47.15 64.16
Scenario 3(%) 29.23 44.56 61.19
TABLE VI S ERVICE PROBABILITY GAIN IN M 2 M MODE FOR DIFFERENT TRAFFIC SCENARIOS .
VI. C ONCLUSIONS AND PERSPECTIVES In this paper, a concept based on direct mobile-to-mobile data exchange on uplink channels is presented. The aim is to explore the performance benefits of m2m file sharing applications in UMTS networks in terms of thereby released overall downlink capacity, which can be used to provide better Quality of Service (QoS) for real-time services. We simulate the mobile cellular environment where the users of different service classes coexist in the network and analyze the negative impact of uplink interference on the performance of m2m data transfer. To obtain a meaningful statistical characterization of the performance of m2m technique, in terms of its efficiency and dependability, we have conducted a substantial number of simulations with the different parameter settings.
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.
The following conclusions can be drawn from the numerical results: • The proposed data exchange policy is optimal in terms of dynamic group building. Restricting the groups to one radio cell yields decreasing – but still satisfying – system performance. • Depending on the traffic intensity, the impact of wireless interference, either m2m inter-group or from cross traffic, leads to different grades of degradation of m2m performance. • Nevertheless, simulation results demonstrate the performance benefits of using m2m mode in UMTS network compared to the conventional unicast mode: - Overall downlink throughput gain, in terms of released downlink resources achieved by using the m2m technique is up to 85%. - The benefit of using multicast in terms of the relative gain in the number of MTs, that can be simultaneously handled in m2m mode is substantial. The increase in service probability up to 67% was observed. These conclusions demonstrated that the ability to interconnect wireless devices on the unoccupied uplink carrier frequencies might be a promising alternative for distribution of popular content in cellular radio networks like UMTS and motivate further investigations and analysis of the proposed m2m concept. R EFERENCES [1] UE Radio Transmission and Reception. 3GPP TS 25.101. [2] B. Cohen. Incentives Build Robustness in BitTorrent. In Workshop on Economics of Peer-to-Peer Systems, Berkeley, CA, USA, May 2003. [3] T. Hossfeld, K. Tutschku, and F. Andersen. Mapping of File-Sharing onto Mobile Environments: Feasibility and Performance of eDonkey with GPRS. In Proc. of IEEE WCNC, New Orleans, USA, March 2005. [4] T. Hossfeld, K. Tutschku, and F. Andersen. Mapping of Filesharing onto Mobile Environments: Enhancement by UMTS. In Proc. of IEEE Pervasive Computing and Communications (PerCom), Kauai Island, Hawaii, March 2005. [5] T. Hossfeld, K. Tutschku, F. Andersen, H. de Meer, and J. Oberender. Simulative Performance Evaluation of a Mobile Peer-to-Peer File-Sharing System. In Proc. of IEEE Next Generation Internet Networks (NGI), Rome, Italy, April 2005. [6] D. Tacconi, C. Saraydar, and S. Tekinay. Ad Hoc Enhanced Routing in UMTS for Increased Packet Delivery Rates. In Proc. of IEEE WCNC, New Orleans, USA, March 2005.
