Performance Evaluation of Flexible TDD Switching in 3GPP LTE System ∗ Dept.
Yufeng Wang∗ , Kimmo Valkealahti† , Kodo Shu‡ , Ravi Sankar∗ and Salvatore Morgera∗
of Electrical Engineering, University of South Florida, Tampa, USA Technologies Laboratory, Nokia Research Center, Helsinki, Finland ‡ Radio Systems Laboratory, Nokia Research Center, Beijing, China Email:
[email protected], {ext-kimmo.valkealahti, kodo.shu}@nokia.com, {sankar, sdmorgera}@eng.usf.edu † Radio
Abstract—LTE TDD has received increasing attention in the next-generation wireless networks, due to its ability of dynamic allocations for uplink and downlink spectrums. In this paper, we evaluate the system performance with a simple flexible TDD switching scheme by system level simulation. The scheme considers switching among three different TDD configuration structures, based on the empty/non-empty buffer conditions of uplink and downlink data traffic, in order to allocate radio resources more efficiently. The system performance is evaluated with a system level simulator, under the practical parameter settings in 3GPP LTE system. The numerical results show that there are certain gains in terms of throughput for both uplink and downlink by the proposed flexible TDD switching scheme, compare with fixed TDD.
by adjusting the transmission durations correspondingly [4]. LTE TDD is able to adjust its DL and UL configurations for different demands of UL or DL transmission, while it is not supported in LTE FDD. In future-generation systems, a large amount of traffic belongs to data traffic and Internet service, resulting in time-varying traffic demands on UL and DL [5]. Hence, LTE TDD is more efficient than LTE FDD in terms of spectrum usage and it is desirable to use TDD to dynamically allocate radio resources with respect to different traffic demands [5]. There are seven different UL and DL configurations in LTE TDD, providing different subframe numbers of UL and DL [6]. In flexible TDD switching, the distribution of subframes between the transmission directions can be adapted to the data traffic and is done either symmetrically with same number of DL and UL subframes or asymmetrically with different subframe numbers [3]. Fixed TDD uses one of the configurations, while flexible TDD considers switching different configurations, based on different demands of UL/DL data traffic or different applications’ requirements. In this paper, we investigate the LTE TDD switching scheme among three TDD configurations, depending on the different demands of UL and DL data transmission. Specifically, when there is more demand of DL traffic, the system will use the configuration which has more DL subframes; when there is more demand of UL traffic, it will switch to the configuration with more UL subframes; when the demand is the same for both UL and DL, it will use the configuration with same UL and DL subframes. We show that with flexible TDD switching, the system performance can be improved, compared with fixed TDD system. The rest of the paper is organized as follows. Section II outlines the system model. Section III presents the proposed flexible TDD switching algorithm. Numerical examples are shown in Section IV, in which the system performance improvements can be observed, compared with fixed TDD, in terms of system throughput. Finally, Section V concludes the paper.
I. I NTRODUCTION Ever increasing demand of higher data rate, more reliable and cost efficient communication networks necessitates an extensive research into new techniques, algorithms and network architectures. One such significant contribution is the notion of “Long Term Evolution (LTE)”, which offers important benefits to operators and end users, such as capacity improvement, lower cost, simplicity and wide range of terminals. LTE is a standard for wireless communication of high-speed data and it is maintained as a project of the 3rd Generation Partnership Project (3GPP) [1]. LTE supports both frequency division duplexing (FDD) and time division duplexing (TDD) as radio access technologies. In FDD, the transmission in uplink (UL) and downlink (DL) is simultaneous in different frequency bands. In TDD, the UL and DL data traffic are transmitted under the same frequency band, but within different time domain. Thus, FDD uses a paired frequency bands [2], while TDD uses an unpaired frequency bands [3]. In general, FDD represents higher device and infrastructure volumes, however, TDD can be a good complement, which is able to be used in spectrum center gaps [4]. For LTE TDD, since the hardware is the same as LTE FDD, except for the radio unit, the operators can benefit from the economies of scale that come with broadly supported FDD products [4]. For LTE TDD, one important feature is the asymmetry of UL and DL data rates, due to the use of different DL/UL configurations. The data rates for UL and DL can vary dynamically to satisfy different applications’ requirements. The DL data rate can be up to nine times higher than UL rate,
II. S YSTEM M ODEL We consider using the flexible TDD switching algorithm for homogeneous deployments in 3GPP LTE systems, in which
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only base stations (eNB) and user equipments (UE) are deployed in the network. The network infrastructure is assumed to be a cellular network with 3-sectored eNBs deployed in the middle of each site. Each site comprises three cells (sectors). The UEs are randomly distributed in the network, thus, different cells may have different number of UEs. For traffic model, we consider fixed-size files of 1 megabyte are generated in UL and DL for each UE and each file comprises of 667 packets with a packet size of 1500 bytes; the interarrival rate of files has negative exponential distribution with mean value of 10 seconds for DL and 20 seconds for UL. Note that we restrict ourselves to the case with more DL traffic than UL traffic, thus we assume inter-arrival time in DL is less than in UL.
Furthermore, we assume the flexible TDD switching scheme is coordinated among the three cells in one site, i.e., three cells use the same frame configuration based on the joint UL and DL buffer conditions. Otherwise, DL transmission in one cell could obstruct simultaneous UL transmission in another cell within the same site. Note that having co-sited cells and coordinated TDD switching may reduce the gain from the proposed scheme, while in the scenarios with non co-sited cells the gains could be higher. TABLE I F LEXIBLE TDD S WITCHING S CHEME .
DL buffer empty DL buffer not empty
UL buffer empty Configuration 1 Configuration 2
UL buffer not empty Configuration 0 Configuration 1
IV. N UMERICAL R ESULTS
Fig. 1.
In this section, we present the numerical results of flexible TDD switching scheme in 3GPP LTE system. We first describe our simulation environment and parameter settings in our system level simulator, then we present the numerical examples, focusing on the throughput comparison between flexible and fixed TDD schemes.
Three configurations for flexible TDD switching scheme.
III. F LEXIBLE TDD S WITCHING S CHEME
A. Simulation environment and parameter settings
We consider three different TDD configurations as shown in Fig. 1. There are 10 subframes in each of the configuration structures, in which 8 subframes are allocated to UL and DL accordingly.1 Configuration 1 has same number of subframes allocated for both UL and DL (4 subframes each), while configuration 0 has asymmetry transmissions, allocating 6 subframes for UL and 2 subframes for DL, configuration 2 also has asymmetry transmissions which allocates 2 subframes for UL and 6 subframes for DL. Table I shows the flexible TDD switching scheme that switches among the three configurations, depending on the buffer condition of UL and DL data traffic. Specifically, • when the buffer in DL is empty and UL is not empty, configuration 0 is used, indicating higher number of subframes for UL will be allocated. • when the buffer in UL is empty and DL is not empty, configuration 2 is used, indicating higher number of subframes for DL will be allocated. • when the buffer in both DL and UL are empty or not empty, configuration 1 is used, indicating same number of subframes for both DL and UL will be allocated. Note that in this work, we only consider a simplified scheme based on empty or non-empty DL/UL buffer condition to evaluate the performance of flexible TDD. In a practical scenario, a configurable limit of the buffer load and a more sophisticated scheme based on DL/UL traffic ratios could be used for switching between the TDD configurations.
The network layout is a 19-site and 3-sectored hexagon system with cell wrap-around as shown in Fig. 2 [7]. We assume the traffic arrival rate for DL is twice than UL, based on the fact that there are more data traffic demands in DL than UL in practice. Other parameter settings are shown in Table II according to the LTE system requirements [9]. We use a system level simulator for performance analysis and we focus on the throughput of DL and UL with different number of users in the network. The flexible TDD switching scheme uses three TDD configurations according to the buffer conditions in UL and DL as described in Section III, while fixed TDD only use configuration 1 for allocating UL and DL transmissions. TABLE II S IMULATION PARAMETERS . Parameter Simulation case Cellular layout Total eNB TX power Inter-site distance (ISD) Distance-dependent Path loss Shadowing standard deviation Carrier frequency Bandwidth Minimum distance between UE and eNB Thermal noise density eNB antenna gain UE antenna gain UE noise figure eNB noise figure
1 The subframes 1 and 6 are special subframes, which contain downlink pilot time slot(DwPTS), guard period (GP) and uplink pilot time slot (UpPTS), and are not used for transmission [8].
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Value 3GPP LTE TDD Hexagonal grid, 19 sites, 3 sectors per site, wrap around 46 dBm 500 m 128.1 + 37.6 × log 10(R), R in km 8 dB 2 GHz 10 MHz 35m -174 dBm/Hz 15 dBi 0 dBi 9 dB 5 dB
1
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Fixed, 57−user Flex, 57−user Fixed, 285−user Flex, 285−user Fixed, 570−user Flex, 570−user
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(a) DL throughput of flexible and fixed TDD, Fig. 3.
(b) UL throughput of flexible and fixed TDD.
Throughput comparison with different user numbers.
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Fig. 2.
Fig. 4. Configuration usage with different user numbers of flexible TDD switching.
A 19-site and 3-sectored hexagon system layout.
B. Throughput
TDD. Similarly, the gain is larger when user number is relatively small. This is due to the fact that when user number is small, the supported transmission rate is large and there is more chance to complete transmitting the data from DL or UL buffers, so that the chance for using flexible TDD increases. However, when the user number is large, the chance that there are data traffic in the buffer of both UL and DL increases, so both flexible and fixed TDD perform with the same configuration 1 and accordingly the gain becomes less3 . From the comparisons in the figure, clearly that the flexible TDD switching performs better than fixed TDD in terms of
The throughput comparison between flexible TDD switching scheme and fixed TDD scheme is shown in Fig. 3. Presented are DL and UL throughput with different number of users in the network. Fig. 3(a) plots the DL throughput of flexible and fixed TDD schemes, with user numbers varying from 57, 285 to 5702 . The depicted throughputs are active session throughputs per UE. The total number of transferred bits is divided by the time that the transmission buffer is nonempty. The curves show that flexible TDD achieves better throughput than fixed TDD, while the gain becomes less as increasing user numbers. The throughput in UL is shown in Fig. 3(b), where a similar gain is obtained by using flexible
3 The principle of the proposed TDD switching scheme is to utilize the unused resources from one link to the other link, which occurs more in a low loaded system. In high loaded system, such flexibility becomes less due to our scheme based on empty/non-empty buffer condition. However, with more advanced schemes based on DL/UL traffic ratios, there is potential gain regardless of the traffic loading level.
2 The numbers of users are selected based on our 19-site, 3-sectored network layout, i.e., 57, 285 and 570 users correspond to 1, 5, 10 users per sector, respectively.
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system throughput. C. Frame Usage Fig. 4 plots the configuration usage by flexible TDD switching scheme. With 57 users, we can observe that configuration 0 is used over 10% and configuration 2 is used over 30%, indicating the asymmetry of UL and DL allocations based on the buffer conditions accordingly. Thus, the flexible TDD switching scheme allocates the radio resources more efficiently. Similarly for the case with 285 and 570 users, the only difference is that with more users, the chance of using the same configuration 1 increases, because the time for the buffer in both UL and DL are not empty increases. Since fixed TDD only uses configuration 1 for both UL and DL transmission, configurations 0 and 2 will be simply 0%, which is not presented here. V. C ONCLUSION In this paper, we evaluate the performance of a simple flexible TDD switching scheme with system level simulation. We consider three TDD configurations for flexible TDD switching. The TDD switching scheme is based on the empty/nonempty buffer conditions of UL and DL traffic, in order to more efficiently allocate the radio resources accordingly. The evaluation results indicate that using flexible TDD switching, the system performance can be improved in terms of both UL and DL throughput, compared with the fixed TDD. ACKNOWLEDGMENT Part of this work has been conducted as a research project at Nokia Research Center (NRC) in Beijing, China, during the summer 2011. The help from Dr. Cassio B. Ribeiro, Dr. Haipeng Lei and the support from NRC are gratefully acknowledged. R EFERENCES [1] “The 3rd Generation Partnership Project (3GPP),” [Available]: http:// www.3gpp.org/LTE [2] “3GPP. TS 25.104. ,” Base Station (BS) radio transmission and reception (FDD) [3] “3GPP. TS 25.105. ,” Base Station (BS) radio transmission and reception (TDD) [4] “LTE - an introduction,” Ericsson. 2009. [Available]: http://www. ericsson.com/res/docs/whitepapers/lte overview.pdf. [5] P. Chan, E. Lo, R. Wang, E. Au, V. Lau, R. Cheng, W. Mow, R. Murch and K. Letaief, “The Evolution Path of 4G Networks: FDD or TDD?,” IEEE Commun. Mag., vol. 44, no. 12, pp. 42-50, 2006. [6] R. Ratasuk, A. Ghosh, W. Xiao, R. Love, R. Nory, and B. Classon, “TDD design for UMTS Long Term Evolution,” IEEE PIMRC, Cannes, France, Sep. 2008 [7] R4-113570, CATT, “Interference study with system simulation for LTE TDD eIMTA,” 2011. [8] 3GPP LTE for TDD Spectrum in the Americas, Nov. 2009 [9] 3GPP Release 8, “Technical Specifications and Technical Reports for a UTRAN-based 3GPP system”
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