IEEE 6th CAS Symp. on Emerging Technologies: Mobile and Wireless CO". Shanghai, China, May 3 I-June 2,2004
A Rough Timing Recovery Method for OFDM Systems Xiang CHEN, Shidong ZHOU, Yan YAO State Key Lab on Microwave & Digital Communications Tsinghua University, Beijing, CHINA, I00084
[email protected]
Abstract-The orthogonal frequency division multiplex technique has recently been attracting considerable interest especially for next generation mobile systems. In this paper, a rough timing recovery method based on a special synchronization frame structure is proposed. Simulation results indicate that the proposed algorithm exhibits a moderate implementntional complexity and high robustness against both channel noise and time selective fading. Keywords- OFDM, timing recovery, pseudo noise sequence (PN sequence), synchronization.
I INTRODUCTION Orthogonal Frequency Division Multiplex (OFDM) signaling is proved to be an effective way to combat the negative effects of multipath and fading by dividing the frequency selective fading channel into a number of flat fading subchannels corresponding to the OFDM subcarrier frequencies [I]. Compared with single carrier systems, channel equalization of OFDM systems is less complex, but the time and fkequency synchronization has to be more exact especially under mobile multipath environment. If the timing recovery is not exact enough, the excursion of subcarrier and intercarrier interference will be induced, and the performance of the channel estimation will degrade. Until now, many methods have been proposed and discussed to remove the effects of timing offset and frequency offset [2]-[SI. In [2], a system structure of OFDM for frequency and block synchronization has been proposed, but the searching process and algorithm haven't been presented. In [3], a scheme using a path delay estimation method and delay-locked loop (DLL) has been presented to do the sampling clock synchronization. A more exact synchronization can be obtained by evaluation of the cyclic prefix (CP) of the OFDM blocks in [SI, but this is only suitable in time-invariant or slow time-variant channel. Under multipath fast fading environment, the CP has been destroyed by intersymbol interference (ISI), and then it is difficult to obtain very exact timing synchronization [SI.
In this paper we focus on acquiring the rough timing as
quickly as possible, and approach the problem from a viewpoint to employ much simpler frame structure and processing. The method proposed in this paper uses many mature techniques such as PN sequence correlation receiver, and it can provide several scales of rough timing quickly over both AWGN channel and fast fading channel. This paper is organized as follows. Section II describes the OFDM system model and the special synchronization frame sttucture. We assume that the proposed method is applied to the downlink of a wireless OFDM system. Section 111 presents the new rough timing recovery algorithm based on PN sequences in time domain and provides a rough timing-tracking scheme. Simulation results in different channel environments are reported in Section IV. Conclusions are drawn in Section V. II OFDM SYSTEM A.
OFDM System Model
Fig.1 shows the baseband block diagram of OFDM system based on a widehand wireless application. The OFDM signal is generated by inverse Fast Fourier Transform (IFFT). Then CP is added to each OFDM symbol in order to avoid ISI. An OFDM frame is formatted by adding time domain pilot-PN sequences. After passing through the channel, received signal at a receiver is filtered by low pass filter (LPF). The detected signal is corrected timing offset in synchronization. In Fig 1, the timing recovery includes two processes: rough timing recovery and exact timing recovery. In this paper we firstly focus on the first stage. B.
Synchmnization Frame Structure
Fig.2 shows the special synchronization frame format used in this paper. The data transmission frame structure has four hierarchies: super-frame, frame, timeslot and symbol. A super-frame includes 256 frames named from frame-0 to frame-255. Each frame consists of ten timeslots. The duration of each timeslot is IOms, so there are totally 10,240 samples with the sample ikequency 10.24MHz. Each timeslot
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consists of two domains: time domain pilot sequence and OFDM symbol domain. There are eight OFDM symbols in the symbol domain, and each symbol of period T=1240 samples has a useful data part of period T.=1024 samples and the cyclic prefix which consists of the last 216 samples of useful data part. Certainly, in Fig.1, the exact timing recovely and channel estimation can be obtained by the frequency domain pilots in the useful data part. A time domain pilot sequence has two parts: zero-power protection segment (64 samples) and a PN ,sequence (256
samples), which totally occupies only 3.125% time domain resource in each timeslot. The PN sequence prior to each timeslot is employed for rough timing recovery by a simple processing in this paper. Especially, the selection and mapping scheme of PN sequences will be presented in order to obtain several scales of rough timing in next section. Taking one and another, this frame structure is simple and the additional overhead for rough timing synchronization is very low.
y
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cxacltbniigreco"~ Fig.1 Basfband block diagram of OFDM system
pattem in one frame, we can know the number of the timeslot where the PN sequence is located, and the expression of the h e synchronization state machine can be obtained quickly. In section 111-C, we can see that this method helps to improve the performance of rough timing recovery- and to ;educe the probability of losing
~ 0 ( 0 ( 1 ( 0 ~ 1 ~ 1 ) 0 ( 0 ( 1 ~ 1 Fig3 PN sequence mapping pancm (Note: 0 denotes the first kind of PN spqucncc;
1240 r a m p k
Fig.2 Synchmniralion frame format
111 Rough Timing Recovery Method
A. The Selection and Mapping Scheme of PN Sequences
Fig.4 Block diagram of "PN Addition"
B. In Fig.2, there is a PN sequence in each timeslot for rough timing synchronization, so the number of PN sequences is ten in one frame. In order to obtain rough timing on different scales including frame, timeslot and symbol, two different kinds of PN sequences are chosen in our scheme. These two kinds of PN sequences are distributed to each timeslot's head in one frame with a certain pattem (Fig.3). At the transmitter, "PN addition" block (in Fig.1) finishes the selection of PN sequence by this pattern just as Figd shows. According to the PN sequence's mapping
Rough 7iming Recovery Algorithm
The proposed rough timing recovery method is based on the results of the two PN sequences' correlation calculation. The correlator outputs the sum of the products of the. received signals and the local PN sequence. According to the autocorrelation of the PN sequence, if the received PN sequence is synchronous to the local one, the correlator will output a very high peak, otherwise the output amplitude will be very low. In order to reduce the effects by multipath fading and channel noise, the output of correlator should be normalized by
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the signal power, by which the threshold of hard decision is set. For example, we can set half of the peak value as the threshold. After hard decision of normalized outputs, the distributed pattern of the two PN sequences in each timeslot can be ascertained. With the result above, the initial position of the path with strongest power can he recovered, which is also called the timing of main path to the receiver. Certainly, because of the time vm’ant fading, the most powerful correlation result is perhaps caused by other paths to the receiver, that is unreliable. Then we consider the PN sequence pattem as a linear code (just as Fig.4 shows). Comparing the result of hard decision to the known PN sequence pattern, we can obtain the two codes’ hamming distance. The position where the code distance is smallest (the perfect one should be zero) is optimum for rough timing recovery. According to Fig.5, the concrete steps of the proposed algorithm are described as follows:
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Stepl: The correlation is calculated between local PN sequences and received signals. The outputs are temp1
7
and temp2 in Fig.5; S t e p 2 Power measurement is performed to acquire the average power during the duration of each timeslot; Step3: Temp1 and temp2 are then respectively divided by the average power of the time and are normalized; the outputs are ttl and tt2 in Fig.5; Step4: Set the threshold of hard decision by the detective peak; then perform hard decision to ttl and tt2, the PN sequence pattem in one frame can be obtained, just as HDI and HD2 in Fig.5; Step5 Consider the PN sequence pattern as a linear code, and calculate the code distance between local PN pattern and HDI (or HD2) respectively, then plus the two code distances; Searching the position where the sum of code distances is the smallest, we then get the optimum position for rough timing recovery; Step6: Output the results of Step 5 to a synchronization state machine (defined in Section 111-C), through which whether the OFDM system is roughly synchronous can be concluded.
~~~
Fig.5 Block diagram ofthe pmposcd rough timing recovery
detective result is matching to local PN pattern, the output is 1, which means this frame is synchronous. So the state transform depends on the detective results. Only when three continuous outputs are all 1 (or 0), the system returns to the synchronous state from the tracking state (or contrarily). IV SIMULATION RESULTS Fig.6 Rough timing synchronization state machine
C. Rough liming Tracking Method In order to keep the roughly synchronous state as long
as possible, a synchronization state machine is designed, and an algorithm for rough timing tracking is proposed as Fig.6. There are two kinds of states in this method: .synchronous state and tracking state. Each kind of state has three states: Sb SI and S2 belong to synchronous state; To, T,and T2 belong to tracking state. In Fig.6, when the detective result of PN pattern in one frame is not matching to local PN pattern, the output is 0, which means this frame loses the synchronous state; when the
To show the performance of the proposed method, we consider a 10.24M symbolk transmission over three channel models. The first one is AWGN, and the other two are both multipath fading channel defined in ITU-R M. 1225: channel-vehicle-A and channel-vehicle-B. The carrier frequency is 3.2GHz and the speed of motion is 5km/h, IZOkm/h or 2 5 0 k d . The two PN sequences we choose are both the first 256 samples of the downlink scrambling code in WCDMA system. In our simulations, the synchronization time is fixed to be the duration of one frame. Then the ‘correct synchronization is considered when the rough timing position is at the first sample of frame head over AWGN channel or is located in the area where the multipaths
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distribute over multipath channels. All simulations are
performed over 1000 times under the same environment.
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Fig.7 Synch. Probability over AWGN
Fig.9 Speh. Probabilik over Channel-Vehicle-B
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Fig.8 Synch. Robability over Channel-Vehicle-A
Fig.10 lmpmved Performance over Channel-Vehicle-A
the proposed rough timing recovery method by construction in practice, and consummate the whole timing synchronization scheme including rough and exact timing based on joint time-frequency pilots.
The performances over different channels are shown in Fig.7-9. In Fig.7, the rough synchronization probability over AWGN channel is nearly 100% when SNR is over -15dB. Even over multipath fading channels just as in Fig.8 and 9, the performances are also excellent. In order ACKNOWLEDGEMENT to improve the performances over multipath channels This work is supported by China’s 863 Beyond 3G when the SNR is low, the power of PN sequence ( P P ~ ) Project “FuTUW-Future Technologies for Universal can be doubled to the power of signals (PDAri\).Fig.10 Radio Environment, N o . 2001AA123012. shows the improved performance.
REFERENCES V CONCLUSIONS “Daa transmission by frequency division multiplexing using the discrete Fourier transform,” IEEE Trans. Communications.. voI.19, pp.628-634, 1971. [2] GSantclla, “A frequency and Symbol synchronization system for OFDM signal: architecm and simulation results,” IEEE Trans. Yehicvlor Technology, ~01.49,pp.254-275,2000. [I] S.B.Wcinstein and P.M.Ebert,
A novel method of rough timing recovery has been proposed for OFDM transmission systems. This method can provide rough timing quickly with several scales from frame timing to symbol timing. Computer simulation results show that by tbe proposed method rough timing can be obtained with much shorter capturing time and much higher synchronization probability under both AWGN and fast multipath fading environments. Moreover, because it is tolerant to fast multipath fading environment, we can apply the proposed method to the high-mobility wireless communication systems such as next generation mobile systems. In future study, we need to verify the hardware size of
[SI Y.Baoguo, L.Khaled Ben and S.C.Roger, “Timing recovery for OFDM Transmission,” IEEE Joumol. Selected Amas in Communications.,~01.18,pp227872291.2wO. [4] H.Minn and V.KBhqava, “On timing offset estimation for OFDM sysDms,’*IEEE Commun. Lelters. ,vo1.4, pp.242-244.2000. [5] Y.H.Kim, I.Song and H.GKim "Performance analysis of B coded OFDM system in time-varying multipath rayleigh fading channels,” IEEE Trans. Vehicular Technologv.,~01.48,pp.1610-1615, 1999.
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