chronization in the downlink of 3GPP LTE system is pre- sented. This new algorithm can reduce the MSE (mean square error) of detected fractional carrier ...
An Improved Coarse Synchronization Scheme in 3GPP LTE Downlink OFDM Systems Na Ding, Chen Chen, Wenhua Fan,Yun Chen and Xiaoyang Zeng State Key Lab. of ASIC and System,Department of Microelectronics, Fudan University Shanghai,China Email:{chenyun,xyzeng}@fudan.edu.cn Abstract—In this paper, an improved algorithm of coarse syn-
chronization in the downlink of 3GPP LTE system is presented. This new algorithm can reduce the MSE (mean square error) of detected fractional carrier frequency offset (FCFO) by an order of magnitude and improve the accuracy rate of start point estimation by near 80 percent when SNR equals 0 in contrast with conventional coarse synchronization algorithms, such as ML, MMSE, S&C and MC. The simulation result for the proposed algorithm is presented in comparison with the conventional algorithms. I.
INTRODUCTION
To meet the requirements of high-speed and broadband for mobile wireless communication, the 3GPP(3rd Generation Partner Project) proposed LTE system in 2004, supporting 3GPP's long-term competitiveness of its technology as well. According to Release 8[1] specified by 3GPP, OFDMA(Orthogonal Frequency Division Multiple Access) technology is used in the downlink of LTE system while SCFDMA(Single Carrier Frequency Division Multiple Access ) in uplink. OFDM provides several advantages, such as high spectral efficiency, simple receiver implementation and robustness in a multi-path environment because of subcarriers’ orthogonality. On the other hand, OFDM technology is very sensitive to frequency and time offset caused by the channel and crystal oscillator, resulting in inter subcarriers interface (ISI) which would impair the data. To protect the orthogonality among subcarriers and avoid ISI, precise timing and frequency synchronization is necessary. In a cellular system, an user equipment (UE) must be able to perform initial cell search which is aimed at achieving synchronization and searching for a appropriate base station to set up communication when it powers on. In the past several years, extensive research about synchronization and cell search based on cyclic prefix and synchronization signals in 3GPP LTE system has been conducted. However, few researches focus on coarse synchronization in LTE system, in which fractional carrier frequency offset (FCFO) and timing should be detected. In several literatures, ML(Maximum Like This work is supported by National Science and Technology Major Project of China (No. 2011ZX03003-003-03)
978-1-4673-0219-7/12/$31.00 ©2012 IEEE
-lihood) is proposed to perform coarse synchronization in LTE system[3][4][5][6]. There are many other common-used methods for coarse synchronization like MC, MMSE, S&C and so on. These algorithms perform well in other systems. However, considering the special frame structure in LTE system, common algorithms display some disadvantages because the limited symbols in the CPs will increase the detection error. In this paper, an improved algorithm suitable for LTE system is proposed. In the following sections, some knowledge about the LTE system will be presented, mainly the downlink frame structure and synchronization signals. Section 3 will discuss conventional coarse synchronization schemes in wireless communication. The improved coarse synchronization algorithm will be presented in section 4 and the simulation results in AWGN and TU-6 channel using MATLAB and comparisons will be presented in section 5. The conclusion of this study will be presented in section 6. II.
LTE SYSTEM DESCRIPTION
A. Frame Structure
160 CP
2048
144 CP
2048
144 CP
2048
144 CP
2048
144 CP
2048
144 CP
2048
144 CP
2048
Figure 1. Frame structure of FDD type in LTE system
According to Release 8, LTE support six bandwidths and fixed subcarrier spacing of 15 KHz [1]. There are two frame structure types called FDD type and TDD type in a LTE system. While both of them are widely used, a study focusing on FDD is considered in this paper. According to 3GPP LTE specification, downlink transmission of FDD type is organized into radio frame with duration of 10ms. Each radio frame consists of ten sub-frames in which there’re two slots included. Each slot is composed of seven OFDM symbols in the normal CP arrangement, however, there are 6 OFDM
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symbols included in a slot in the extended CP arrangement. This paper focuses on the normal CP arrangement in which length of the first CP is 160 samples and the others’ are 144 samples when bandwidth equals 20MHz. Frame structure of normal CP arrangement is presented in Fig.1. B. Synchronization Signals Two Synchronization signals are defined as PSS(Primary Synchronization Signal) and SSS(Secondary Synchronization Signal) in LTE system. PSS and SSS are mapped on 62 subcarriers located symmetrically around the DC subcarrier and transmitted within the last two OFDM symbols of the first and the tenth slot in one frame. Three primary Synchronization sequences are three different and length-62 Zadoff-Chu sequences in frequency domain, each corresponding to a sector identity of 0,1 and 2. The primary synchronization sequences are orthogonal to each other. The secondary synchronization signal consists of 168 different sequences and each corresponds to a group identity from 0 to 167. Secondary synchronization sequence has the same length of 62 as the primary synchronization sequence.
Considering the special frame structure in LTE downlink systems and the situation that lengths of CP in the system are different and very short, acquiring averaging result over several OFDM symbols are proposed in [3] to ensure the accuracy of the coarse synchronization. Furthermore, conventional coarse synchronization schemes can only figure out the start of one OFDM symbol which makes the follow-up PSS detection complicated because the correlation operations will be performed very slowly with the OFDM symbol received one by one. So the conventional coarse synchronization schemes are high-complexity in hardware. IV.
Hlaing Minn has proposed a coarse algorithm applying for signal which contains a training symbol having L repeated parts by exploiting the periodic nature of the time-domain signal. The Minn algorithm [2] takes several repetitive symbol blocks into consideration. For a training symbol containing L parts of M samples each, the Minn algorithm is illustrated by (1)-(5).
In many literatures PSS and SSS are used to perform the synchronization and cell-search. However, cell-search and synchronization using PSS and SSS are based on symbol synchronization which is coarse synchronization in LTE downlink systems. Only if the coarse synchronization is accurate enough, the subsequent processes can get satisfactory results. III.
IMPROVED COARSE SYNCHRONIZATION SCHEME
L−2
M −1
k =0
m=0
P(d ) = ∑ b(k ) ⋅∑ r * (d + kM + m)
(1)
⋅ r (d + (k + 1) M + m)
CONVENTIONAL COARSE SYNCHRONIZATION SCHEMES M −1 L −1
Because of the multi-path fading in the communication channel and oscillator mismatch between the transceiver and receiver, carrier frequency offset and white Gaussian noise are generated within the received signal. The Objective of synchronization is to recovery OFDM symbol timing and to detect the carrier frequency offset. The whole synchronization procedure for LTE system consists of three steps. The first step conducted in time domain called coarse synchronization is to detect the symbol timing and FCFO and the second one conducted in frequency domain aims at finding out the sector ID and location of PSS based on correlations of received PSS with local primary signals. The last step named secondary synchronization is conducted in frequency domain and it makes use of received SSS to detect frame timing and group ID. Coarse synchronization, as the first step in the whole synchronization procedure, is very important because inaccuracy of slot timing detection and FCFO detection will influence the performance of the next two steps. ML (Maximum Likelihood) is the most common algorithm used in coarse synchronization [3][4][5][6]. This algorithm is based on the correlation operations of CP in an OFDM symbol. Other legacy algorithms include MMSE, MC, S&C[7]. As mentioned above, CPs of seven OFDM symbols in a slot are different and the lengths of CPs are too short to detect the start-point and FCFO accurately. Consequently, the shortage of data for estimation renders these algorithms inaccurate to reach a robust estimation under unfavorable channel condition.
E (d ) = ∑ ∑ r (d + i + kM )
2
(2)
i =0 k = 0
b(k ) = q (k )q (k + 1), k = 0,1,..., L − 2
(3)
q ( k ) denotes the signs of the repeated parts of the training symbol. r represent the received symbol. The timing metric can be expressed as ⎛ L P (d ) ⎞ Λξ ( d ) = ⎜ ⎟ ⎝ L − 1 E (d ) ⎠
2
(4)
The start-point and FCFO can be acquired using (5). d ' = arg max Λξ ( d ), ξ f = d
1 ∠P ( d ') 2π
(5)
The Minn algorithm has been proved effective and accurate. As mentioned above, lengths of CPs in a slot are different in which the longer one is 160 samples and the shorter is 144 samples under the scenario of 20MHz bandwidth. In order to make the work in a receiver simple and easy, the start point of a slot instead of an OFDM symbol is necessary to be found out. To acquire better performance of FCFO detection and accurate start point of a slot using CPs, some improvements are made on Minn algorithm. The improved algorithm is named as slot coarse synchronization which means a whole slot is needed. Equation (6), (7) and (8) show the details of Slot algorithm in LTE system.
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M1 −1
P(d ) = ∑ r*(d + m) ⋅ r(d + N + m) m=0
5 M2 −1
+∑∑ r*(d + M1 + kM2 + (k +1)N + m) k =0 m=0
(6)
⋅ r(d + M1 + kM2 + (k + 2)N + m)
E (d ) =
M 1 −1
∑
V.
r (d + i) ⋅ r * (d + i)
+∑
∑ r (d + M
1
+ kM 2 + ( k + 1) N + i )
k =0 i = 0
SIMULATION RESULTS
The simulation parameters of the LTE standards are shown in Tabel 1. TU-6 is utilized as the model for multipath fading channel.
i=0
5 M 2 −1
Slot coarse synchronization scheme make the subsequent PSS detection easier by indicating slot timing, which means that in conventional synchronization scheme as shown in [3][4][5], PSS detection is performed by correlations between local primary synchronization sequences and received signals. Without knowing the slot timing, the correlation may be conducted 140 times at most. The slot coarse synchronization decreases the number to 20, one seventh of the conventional one.
(7)
⋅ r * ( d + M 1 + kM 2 + ( k + 1) N + i )
TABLE I.
TABLE 1 SIMULATION PARAMETERS OF THE LTE STANDARDS
Parameter
M 1 is equal to 160 which is the length of the longer CP.
FFT size Carrier Frequency Band Width Number of Tx/Rx antenna Channel Mobility CP Length
M 2 is equal to 144 which is the length of the other CPs. ⎛ P(d ) ⎞ Λξ ( d ) = ⎜ ⎟ ⎝ E (d ) ⎠
d ' = arg max Λξ (d ), ξ f = d
2
(8)
1 ∠P (d ') 2π
(9)
d ' indicates the start point of a slot and ξ indicates the fractional carrier frequency offset. The improved algorithm contains correlation operations of seven OFDM symbols in a slot. The longer the length of correlation operation, the more accuracy acquired. Fig.3 shows the values of metrics generated from slot coarse synchronization algorithm and ML algorithm mentioned in [3][4][5][6]. Peaks could be observed, with the highest one indicating the start point of each OFDM slot. The proposed algorithm could suppress the side lobes to a great extent, comparing with the ML algorithm. Thus, a better estimation accuracy could be acquired and the complexity of subsequence process will be lower. f
Figure 2. Comparison of metric value profile between the proposed algorithm and ML algorithm siullated with AWGN channel ,SNR = 0dB and FCFO=0.2. The left one is resulted from proposed algorithm and the right one is resulted from ML algorithm. The X axis is the number of the symbols esitimated and the Y axis is the metric value of each method.
Value 2048 2.0GHz 20MHz 1/1 AWGN, TU-6 30km/hr Normal
Five algorithms consisting of ML,MMSE,MC,S&C and slot method which represents the improved Minn algorithm are simulated using MATLAB. Success rate of start point detection in AWGN channel is shown in Fig.3(a) with varying SNR from -8dB to 9dB. Simulation results show that the proposed algorithm has a significant advantage over the others. SR(Success Rate of start-point-detection) of improved Minn algorithm is almost 100% when SNR is larger than 0 while SRs of the others are around 20%~30% when SNR equals 0. Fig.3(c) shows the MSE of FCFO-detection in AWGN channel with SNR varying from -8dB to 9dB and Fig.3(d) is part of Fig.3(c) with SNR varying from 0dB to 9dB. Fig.3(c) indicates that the MSE of slot coarse synchronization scheme is about two orders of magnitude better than the others. The performances of the improved Minn algorithm and the others are almost the same when the SNR is large than 2dB shown in Fig.s(d). From Fig.3(b) and Fig.3(d), it is obvious that the gap of MSE increases between the proposed algorithm and the other four algorithms as the SNR decreases. In Fig.3(b), MSE of MMSE method is not included because the MSE value is much larger than the others. It is not necessary to be compared with others. The MSE for the proposed algorithm alters slowly with SNR, accounting for the stability of estimation. Comparing the results in Fig.3(b) and Fig.3(d), the multipath and delay makes the performance of five algorithms worse but the slot synchronization algorithm is better than the other four methods in both of AWGN and TU-6 channels. Slot synchronization algorithm does indeed improve the performance of coarse synchronization in 3GPP LTE system.
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(a)
(b)
(c)
(d)
Figure 3. Simulation results under conditions given in Tabel. 1. Fractional Carrier Frequency Offset is 0.2 times sub-carrier spacing. The label of Minn represents the improved Minn algorithm.(a) Successful Rate (SR) of start-point-detection in AWGN channel with SNR varying from -8dB to9dB. (b) MSE(Mean Square Error) of FCFO detection in TU-6 channel with mobility rate of 30Km/h and SNR varying from 0dB to 9dB. (c) MSE (Mean Square Error) of FCFO detection in AWGN channel with SNR varying from -8dB to 9dB. (d) MSE (Mean Square Error) of FCFO detection in AWGN channel with SNR varying from 0dB to 9dB.
VI.
CONCLUSION
In this paper, an improved coarse synchronization method detecting fractional carrier frequency offset and slot timing is proposed, which is named Slot synchronization algorithm. Comparing to the conventional coarse synchronization algorithms, a new idea of detecting the start point of a slot is proposed and decrease the complexity of hardware. Meanwhile, the proposed algorithm is able to detect the FCFO more accurately and improve the detecting success rate of the slot timing. The simulation results show the comparisons between four conventional methods and the improved one in four figures, which indicate that the proposed scheme would provide a better success rate for start point estimation and better MSE for FCFO estimation. REFERENCES [1] 3GPP TS 36.211, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal
[2]
[3]
[4]
[5]
[6]
[7]
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Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8), V8.5.0 (2008-12) Hlaing Minn,Vijay K. Bhargava,and Khaled Ben Letaief,“A Robust Timing and Frequency Synchronization for OFDM System”, IEEE Transactions on Wireless Communications,Vol.2,No.4(2003) Konstantinos Manolakis,David Manuel Guti´ errez Est´ evez1,Volker Jungnickel,Wen Xu,Christian Drewes, “A Closed Concept for Synchronization and Cell Search in 3GPP LTE Systems”, WCNC ,IEEE,pp.1-6(2009) Pei-Yun Tsai and Hsiang-Wei Chang, “A New Cell Search Scheme in 3GPP Long Term Evoltion Dowmlink OFDMA Systems”, WCSP,pp.15(2009) Qi Wang,Christian Mehlführer,Markus Rupp, “Carrier Frequency Synchronization in the Downlink of 3GPP LTE”,Personal Indoor and Mobile Radio Communications (PIMRC),2010 IEEE 21st International Symposium on,pp.939-944(2010) Wen Xu,Manolakis, K.,”Robust Synchronization for 3GPP LTE System”,GLOBECOM 2010,2010 IEEE Global Telecommunications Conference,pp.1-5(2010) Stefan H. Muller-Weinfurtner,”On the Optimality of Metrics for Coarse Frame Synchronization in OFDM: A Comparison”,Personal Indoor ans Mobile Radio Communications 9th IEEE(1998)
