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  • 1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME 51 LTE – INTER-CELL INTERFERENCE MANAGEMENT TO IMPROVE THE QUALITY RECEPTION IN DOWNLINK RADIO RESOURCES LINKS Younes BALBOUL, Fatiha MRABTI, Najiba EL AMRANI EL IDRISSI, Signals, Systems and Components Lab. Faculty of Science and Technology, Fez, Morocco ABSTRACT LTE (long term evolution) target a high bit rate and QoS (Quality of Service) but is hampered by the significant increase of the number of users. The cellular concept enabled to expand the service coverage to an unlimited large area by dividing the service area into multiple cells with a BS deployed in each cell and reusing the given frequency spectrum repeatedly in each cell. This, however, brings in the co-channel interference problem among the neighboring cells and hence the cellular concept can truly achieve its goal only when the inter-cell interference is properly resolved. The inter-cell resource management refers to a collection of operations that intend to achieve a maximized QoS by minimizing the performance degradation caused by the inter-cell interference. In this work, to improve the downlink radio link, we will use, Partial Frequency Reuse with LP- OFDM modulation in the inner-cell and in the cell-edge we will use Partial Frequency Reuse and OFDM modulation supported by the coordination with adjacent cells. Matlab simulation of different scenarios showed an improvement in bit error rate at the receiver. The results are processed according to the Monte Carlo criteria. I. INTRODUCTION The choice of an appropriate modulation, OFDM (Orthogonal frequency division multiplexing), and multiple-access technique (MIMO Multi Input Multi Output) for mobile wireless data communications is very important to achieve good system performance. OFDM has been considered as a promising modulation technique for the future generation wireless systems, such as Wimax (Worldwide Interoperability for Microwave Access) and LTE. Not only for its inherent ability to combat inter-symbol interference (ISI) resulting from frequency selective fading, but also for the flexibility it offers in radio resource allocations, as each subcarrier can be modulated adaptively to exploit frequency-domain diversity and improve achievable data rates on available frequency spectrum. However, in designing practical networks, optimization should be performed in a multi-cell environment considering one of the most performance limiting factors, namely inter cell interference (ICI) [1], [2]. INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) ISSN 0976 – 6464(Print) ISSN 0976 – 6472(Online) Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME: www.iaeme.com/ijecet.asp Journal Impact Factor (2014): 7.2836 (Calculated by GISI) www.jifactor.com IJECET © I A E M E
  • 2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME 52 The market Trends (like a multicast services, streaming …), require some radio access networks able to carry more and more data with stringent requirements in terms of QoS (minimum bit rate, delay, etc.) The solution is to move to the reuse of bandwidth, but this technique is limited by the inter cell interference (ICI) problem. In this paper, the goal is to improve the quality of the down-link radio transmission. To do it we will proceed along two scenarios: • To improve the inner-cell down-link radio transmission we will use the Partial Frequency Reuse and LP-OFDM modulation (Linearly Precoded Orthogonal Frequency Division Multi-plexing). • To improve the cell-edge down-link radio transmission, we will also use the Partial Frequency Reuse and the OFDM modulation with the coordination of adjacent cells. The Matlab simulation of these scenarios showed an improvement in bit error rate at the receiver. The results are processed according to the Monte Carlo criteria. II. THE PROBLEM OF INTER-CELL INTERFERENCE IN DOWN LINK L.T.E One limiting aspect for system throughput performance in cellular networks is the inter-cell interference, especially for cell-edge users. The impact of interference on the achievable data rate for a given user can be expressed analytically. If a user k is experiencing no interference, then its achievable rate in an RB (Resource Block) m of sub frame f can be expressed as: D୩,୬୭ି୧୬୲ୣ୰୤ୣ୰ୣ୬ୡୣ ൌ B M log ቈ1 ൅ Pୱሺm, fሻ|H୩ ୱ ሺm, fሻ|ଶ N଴ ቉ Where M is the number of resources allocated in the band B, and H୩ ୱ ሺm, fሻthe gain of the channel of thecell s that serves the user k, P (m, f) is the transmission power of the cell sand N0 is the noise power. Figure 1. Example of inter-cell interference with two cells If the neighboring cells to the cell s emit, then it is likely that the same resources are used, and hence the achievable rate for user k is reduced to: D୩,୵୧୲୦ ୧୬୲ୣ୰୤ୣ୰ୣ୬ୡୣ ൌ B M log ቈ1 ൅ Pୱሺm, fሻ|H୩ ୱ ሺm, fሻ|ଶ N଴ ൅ ∑ P୧ሺm, fሻ|H୩ ୧ ሺm, fሻ|ଶ ୧ ஷୱ ቉
  • 3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME 53 With ∑ P୧ሺm, fሻ|H୩ ୧ ሺm, fሻ|ଶ ୧ ஷୱ isthe sum of the interference and the index i represent the interfering cells. The loss of throughput due to interference experienced by user k is: D୩,୪୭ୱୱ ൌ D୩,୬୭ି୧୬୲ୣ୰୤ୣ୰ୣ୬ୡୣ െ D୩,୵୧୲୦ ୧୬୲ୣ୰୤ୣ୰ୣ୬ୡୣ D୩,୪୭ୱୱ ൌ B M log ቎ 1 ൅ SNR 1 ൅ ቂ ଵ ୗ୒ୖ ൅ γቃ ିଵ቏ WithSNR ൌ ୔౩ሺ୫,୤ሻ|ୌౡ ౩ ሺ୫,୤ሻ|మ ୒బ is the signal to noise ratio at the receiver input, and γ ൌ ∑ ୔౟ሺ୫,୤ሻ|ୌౡ ౟ ሺ୫,୤ሻ|మ ౟ ಯ౩ ୔౩ሺ୫,୤ሻ|ୌౡ ౩ ሺ୫,୤ሻ|మ ൌ ୔౟౤౪౛౨౜౛౨౛౤ౙ౛ ୔౫౩౛౨ ౡ . Figure 2 shows the rate loss for the user k due to the variation of the coefficient α ൌ 10logଵ଴ሺγሻ at SNR = 0 dB. Figure 2. The rate loss due to inter-cell interference In the LTE broadband (reuse factor is 1), the rate of loss of data at the inner-cell will be greater than 8% and less than 40% and in the cell-edge, it is greater than 40% and 90% (Figure 2). This is unacceptable in a system that must ensure high throughput [3]. The current challenge is to develop a system that reduce the effect of inter-cell interference by the use of a suitable cell pattern, more flexible frequency reuse techniques and Semi- static inter-cell interference coordination allowing improved radio transmission experience. In the next part of this paper we will introduce our contribution which present the main aspects of a new technique for managing inter cell interference. This technique is based on Partial Frequency Reuse, semi-static coordination techniques and the pre-coding process wish implement space diversity and frequency. All these new techniques allow improving the quality of service for all users (whether inner-cell or cell-edge users).
  • 4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. III. IMPROVING RADIO TRANSMISSION To improve radio transmission experience in downlink our work is based on Frequency Reuse (Figure 3) with round robin algorithm scheduling, coupled with two radio transmission techniques namely: LP- physical resources block (PRB) from different adjacent base stations in cell Figure 3. Figure A. Algorithm concept In the proposed algorithm, the treatment of inter areas according to the value of SINR (Signal receivers. 1) the inner-cell area with SINR greater than or equal to 2 - OFDM) is used, International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME TRANSMISSION EXPERIENCE IN DOWNLINK To improve radio transmission experience in downlink our work is based on with round robin algorithm scheduling, coupled with two radio -OFDM in inner-cell and the simultaneous transmission of several physical resources block (PRB) from different adjacent base stations in cell-edge (Figure Figure 3. Partial Frequency Reuse. Figure 4. managing inter-cell interference. he proposed algorithm, the treatment of inter-cell interference for a cell is separate in two areas according to the value of SINR (Signal to Interference plus Noise Ratio) received at the user with SINR greater than or equal to 2dB, the LP-OFDM (Linear Precoding International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – © IAEME To improve radio transmission experience in downlink our work is based on the Partial with round robin algorithm scheduling, coupled with two radio cell and the simultaneous transmission of several (Figure 4). cell interference for a cell is separate in two to Interference plus Noise Ratio) received at the user OFDM (Linear Precoding
  • 5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME 55 2) In the cell-edge area with SINR strictly lower to 2 dB, two or three PRB with OFDM modulation from different adjacent cells are allocated to the user. Figure 5 gives an overview of this new technique. Figure 5. Operating principle B. Inner-cell : LP-OFDM Technique In this paper we use the results of a research that was conducted by IEEE for LP-OFDM modulation technique and uses these results in the context of LTE system. This research proposes an iterative receiver for the LP-OFDM with channel coding. This gives very good results in terms of BER at a very low cost of complexity. In fact, the proposed iterative receiver requires no matrix inversion and its complexity do not increase with linear precoding, and with the modulation order. Inter symbol interference created by linear precoding are effectively removed in the iterative "turbo effect"[4]. Figure 6. The BER performance of the LP-OFDM with iterative receiver and coding rate ½.
  • 6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. LP-OFDM with iterative receiver provides better results than the OFDM for SNR more than 2 dB, which justifies the use of this technique for the inner C. Cell-edge : multi-cell coordination The technique used in cell Indeed, for the downlink access, a user at the cell adjacent cells. These resources have the same modulation symbol 4 Modulation), the same channel coding and carry the same information. The receiver combines these resources using the MRC (Maximum Ratio Combining therefore the best channel profile. This will allow to the user to extract the best signal from these resources. The Figure 8 presents the Matlab results simulation of the transmission scenario 7. Figure 7. Physical allocation resource scenario for a user at the cell Figure 8. Simulation of BER versus SNR for 2 and 3 RB with use of MRC at the receiver. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME OFDM with iterative receiver provides better results than the OFDM for SNR more than 2 this technique for the inner-cell where the SNR meets this condition. cell coordination The technique used in cell-edge is based on collaboration between adjacent e Indeed, for the downlink access, a user at the cell-edge receives 2 or 3 different PRB from different adjacent cells. These resources have the same modulation symbol 4-QAM (Quadrature Amplitude Modulation), the same channel coding and carry the same information. The receiver combines these um Ratio Combining): extract the carrier of maximum power and therefore the best channel profile. This will allow to the user to extract the best signal from these presents the Matlab results simulation of the transmission scenario hysical allocation resource scenario for a user at the cell-edge. Simulation of BER versus SNR for 2 and 3 RB with use of MRC at the receiver. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – © IAEME OFDM with iterative receiver provides better results than the OFDM for SNR more than 2 cell where the SNR meets this condition. edge is based on collaboration between adjacent e-Node B. 2 or 3 different PRB from different QAM (Quadrature Amplitude Modulation), the same channel coding and carry the same information. The receiver combines these extract the carrier of maximum power and therefore the best channel profile. This will allow to the user to extract the best signal from these presents the Matlab results simulation of the transmission scenario in Figure edge. Simulation of BER versus SNR for 2 and 3 RB with use of MRC at the receiver.
  • 7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 3, March (2014), pp. 51-57 © IAEME 57 In this simulation, it is clear that combining multiple physical resources, can significantly improve the transmission conditions for the users at the cell-edge with a gain of 3 dB with 2 PRB and a gain greater than 6 dB with three PRB allocations. IV. CONCLUSION A variety of resource scheduling algorithms may be applied by the eNode-B depending on the optimization criteria required. In this paper we focused on the benefits of applying our technique in the LTE systems, in terms of Quality of service enhancement. Our results reveal that the Partial Frequency Reuse scheme with LP-OFDM in inner-cell provides a gain greater than 2 dB for SNR (Signal to Noise Ratio) higher than 3 dB and inter-cell coordination at cell-edge, can significantly improve the transmission conditions with a gain of 3 dB with 2 PRB (physical resource block) and a gain greater than 6 dB with 3 PRB allocations. An overall scheme that implement the algorithm suggested in a system which simulates realistic inter-cell interference complies with the 3GPP specifications and compare it with the algorithms used in LTE will be the subject of our future research. REFERENCES [1] « Soft Frequency Reuse Scheme for UTRAN LTE » 3GPP TSG RAN WG1 Meeting 41, Athens, Greece, 9 – 13 Mai, 2005. [2] Abdul Basit SYED,“Dimensioning of LTE Network. Description of Models and Tools, Coverage and Capacity Estimation of 3GPP Long Term Evolution radio interface » à l’Université de Technologie de Helsinki, Février 2009 à Espoo, Finland. [3] Stefania SESIA, Issam TOUFIK et Matthew BAKER, “LTE – The UMTS Long Term Evolution: From Theory to Practice”, John Wiley & Sons, Ltd, Second edition 2011. P.-J. Bouvet, M. Hélard, , et V. Le Nir: « Low complexity iterative receiver for Linear Precoded OFDM » Cesson-Sévigné, France. Wireless And Mobile Computing, Networking And Communications, IEEE International Conference 2005. [4] Asish B. Mathews and Dr. Pavan Kumar Yadav, “Suppression of Nonlinearity Induced Distortions In Radio Over Fiber Links” International journal of Electronics and Communication Engineering &Technology (IJECET), Volume 4, Issue 4, 2013, pp. 51 - 60, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472. [5] V. Bapuji, R. Naveen Kumar, Dr. A. Govardhan, Prof. S.S.V.N. Sarma, “Maximizing Lifespan of Mobile Ad Hoc Networks With QOS Provision Routing Protocol” International journal of Computer Engineering & Technology (IJCET), Volume 3, Issue 2, 2012, pp. 150 - 156, ISSN Print: 0976 – 6367, ISSN Online: 0976 – 6375.