An Improved SCH Detection in Cell Search of 3GPP LTE System

Transcription

1 Vol.94 (Networking and Communication 25), pp An Improved SCH Detection in Cell Search of 3GPP LTE System Youngkwon Ryu, Jeong Gon Kim 2 ERICSSON-LG R&D Center, Anyang, Kyunggi, Korea youngkwon.ryu@ericsson.com 2 Dep. of Electronic Engineering, Korea Polytechnic University, Shiheung, Kyunggi, Korea Corresponding Author, jgkim@kpu.ac.kr Abstract. Long Term Evolution (LTE) system of the third generation partnership project (3GPP) has primary and secondary synchronization channels to help the terminal search cell. Primary synchronization channel (P- SCH) is used to acquire slot timing and cell group. Secondary synchronization channel (S-SCH) is used to obtain frame boundary and cell identity (Cell ID). Cell search plays important role to find a cell and handover to neighbor cell smoothly. This paper proposes interference cancelling (IC) technique in cell search. Canceling the pre-detected synchronous channel from the received signal results in the improved search performance for the next target cell. Keywords: LTE, cell search, interference cancellation Introduction The user equipment (UE) must perform the cell search process before it starts to communicate with network. During this cell search operation, UE acquires Cell ID and timing information Timing synchronization across the cells is mandatory in TDD mode. But even in FDD mode, most cells need to be deployed with timing alignment for many beneficial reasons. Cell synchronization may cause the hidden cell problem, which means that the cell to be detected may be hidden behind the serving or stronger cell []. Cell search algorithms have been proposed in many papers [2-5]. Most of them focus mainly on the complexity reduction. Cell search with IC generally increase search performance at the cost of increased latency and complexity. Searching for neighbor cell which is done regularly to help handover, has the delay tolerant property and it suggests that there is a room for adopting IC technique in cell search. 2 System with IC Overall initial cell searcher system using proposed IC is shown in Figure. For the first cell detection, there is no regenerated signal but for the second cell detection, the ISSN: ASTL Copyright 25 SERSC

2 Vol.94 (Networking and Communication 25) regenerated signal from the first is removed from the original received time domain signal before it is detected. Since interference from the pre-known cells is removed, detection performance for the next cell is improved. S-SCH detector AWGN Pst PNoise FIR r ( n ) rˆ ( n ) + - P-SCH detector Channel Est. Cell Decision Cell ID P2nd L hˆ( l l ) sˆ( l n ) Channel Est. Timing Signal Regenerator h ˆ ( l ) Fig.. Cell searcher system with IC Finite impulse response (FIR) filter is introduced to reduce noise by extracting SCH sequence mapped to the central.25mhz bandwidth as shown in [4] Time domain correlation is used for P-SCH detection and the frequency domain correlation is used for S-SCH detection. And the coherent detection is adopted for S- SCH detection as it has much performance enhancement over non-coherent one as shown in [5]. P-SCH detector detects P-SCH signal at the sample position n that satisfies max N k p j N * k ( k ) rˆ ( k n ) rˆ ( k n ) 2 2 where p j ( k ) is P-SCH time domain symbol pattern from FIR output at sample index k for root index j indicating physical-layer cell identity D Thr N () (2) ID, N is total sample length of cyclic prefix (CP) and P-SCH symbol, and D is the detection Thr threshold. Note that in case of non-ic, the detector determines any peak larger than the pre-defined threshold as the valid cells enabling to detect multiple cells at once. S-SCH detector selects m and m that satisfy arg max 6 m, m l s ( l ) S ( l ) (2) m, m, j where s ( l ) is S-SCH sequence pattern at the l th S-SCH sub-carrier with m, m, j m and m values and physical-layer cell identity j. m and m values determine frame boundary and Cell ID along with j. The IC applied signal at sample position n can be written as 32 Copyright 25 SERSC

3 Vol.94 (Networking and Communication 25) r ( n ) rˆ ( n ) L l hˆ ( l ) sˆ( l n ), r ( n ), for n Start otherwise n n End (3) ˆ l where r ( n ) is FIR output, h ( ) is estimated channel impulse response, L is the length of channel impulse response, s ˆ ( n ) is a sequence composed of S-SCH and P- SCH to be cancelled, and L n and n are start sample of SCH of the cell to be Start End removed, respectively. h ˆ( l ) sˆ( l n ) in (3) represents the regenerated signal. l 3 Simulation Results The parameters used in simulation are summarized in Table. Table. Simulation Parameters Parameters Value Bandwidth MHz Number of cell 2 (Cell ID st=5, Cell ID 2nd=39) Number of Rx antenna 2 Fading ETU 3km/h Center frequency 2GHz Accumulation frame Power difference between the first and second cell, P st P 2nd 2 cases with 3dB and 5dB Sample offset between st and 2 nd cell 3 False alarm rate for detecting P-SCH. The detection performances of both P-SCH and S-SCH detectors using IC technique compared with non-ic are shown in Figure 3. The P-SCH detector with IC can adopt lower threshold than the other while meeting false alarm rate because it finds peak from interference removed signal, which results in performance gain. The reason of slight poor P-SCH detection performance with IC at low SNR for the second cell detection in 3dB power offset case is that the regenerated signal is not so accurate enough to remove the first cell correctly. But IC gets good cancellation advantage in rather high SNR. S-SCH detection performances for the first cell are similar for both detectors as both detectors select physical-layer cell identity group that has the largest correlation value in the same way. But detection performance for the second cell is improved greatly with IC. As the dynamic range of S-SCH is higher than P-SCH, the effect of the imperfect cancellation of the first cell is negligible. Since Cell ID is determined by the detection results from both P-SCH and S-SCH, S-SCH would dominate the overall cell search performance. Copyright 25 SERSC 33

4 Vol.94 (Networking and Communication 25) P-SCH Missing Prob st cell 2nd cell with 3dB offset 2nd cell with 5dB offset st, IC, -3dB st, non-ic, -3dB st, IC, -5dB st, non-ic, -5dB 2nd, IC, -3dB 2nd, non-ic, -3dB 2nd, IC, -5dB 2nd, non-ic, -5dB SNR P st /P noise (a) P-SCH detection performance.9.8 st cell 2nd cell with 5dB offset S-SCH Missing Prob nd cell with 3dB offset st, IC, -3dB.3 st, non-ic, -3dB st, IC, -5dB.2 st, non-ic, -5dB 2nd, IC, -3dB 2nd, non-ic, -3dB. 2nd, IC, -5dB 2nd, non-ic, -5dB SNR P st /P noise (b) S-SCH detection performance Fig. 2. Performance of cell searcher for P-SCH and S-SCH 4 Conclusion In this paper, cell search algorithm using IC technique is introduced. It is shown from the simulation that IC can improve cell search performance especially for the second 34 Copyright 25 SERSC

5 Vol.94 (Networking and Communication 25) cell which is usually hidden in synchronous network. Synchronous network is mandatory in TDD mode and is very likely even for the FDD system for many beneficial reasons. So the proposed technique can be used widely to improve cell search performance in both systems. Small cells are expected to be used more and more to offer better coverage [6]. This will make SCH pollution severer. Proposed technique is advantageous in this environment. References. Bengt Lindoff, Tobias Ryden and David Astely, A Robust Cell Search Algorithm for 3GPP LTE, European Wireless Conference 29, pp , May Konstantinos Manolakis, David Manuel Gutierrz Estevez and Volker Jungnickel, A Closed Concept for Synchronization and Cell Search in 3GPP LTE systems, Wireless Communication and Networking Conference 29, pp. -6, Apr Juan I. Mazarico, Voronique Capdevielle, Afef Feki and Vinod Jumar, Detection of Synchronization signals in Reuse- LTE networks, Wireless Days (WD) 29 2nd IFIP, pp. -5, Dec Hendra Setiawan and Hiroshi Ochi, A low complexity physical-layer identity detection for 3GPP Long Term Evolution, ICACT, vol., pp. 8-3, Feb Jung-In Kim, Jung-Su Han, Hee-Jin Roh and Hyung-Jin Choi, SSS detection method for initial cell search in 3GPP LTE FDD/TDD dual mode receiver, 9th ISCIT, pp , Sep Okino, K., Nakayama, T., Yamazaki, C., Sato, H., and Kusano, Y. Pico Cell Range Expansion with Interference Mitigation toward LTE-Advanced Heterogeneous Networks Communications Workshops (ICC), 2 IEEE International Conference on, pp. -5, 5-9 June 2. Copyright 25 SERSC 35