The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07)
FREQUENCY OFFSET ESTIMATION FOR HIGH SPEED USERS IN E-UTRA UPLINK Hyunsoo Cheon Telecommunication R&D Center, Samsung Electronics, Co. Ltd., Suwon, Korea
[email protected] A BSTRACT
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Figure 1: E-UTRA Uplink Frame Structure ej2πf1t FFT
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1-4244-1144-0/07/$25.00 c 2007 IEEE
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I NTRODUCTION
Orthogonal frequency division multiple access (OFDMA) becomes a basic multiple access technology of forthcoming broadband wireless communication systems, since it provides the channel dependent scheduling possibility and rich robustness to frequency selective fading due to local scattering. As an alternative, single-carrier frequency division multiple access (SC-FDMA), which has low peak-to-average power ratio (PAPR) property, has been also adopted as an uplink multiple access scheme to evolved universal terrestrial radio access (E-UTRA) [1]. Since the modulation and demodulation processes in those multiple access schemes utilize the fast Fourier transform (FFT) for frequency domain equalization (FDE), cyclic prefix is transmitted for avoiding inter-symbolinterference (ISI). However, these schemes are sensitive to frequency offset, which is caused by the imperfection of the local oscillator (LO) and Doppler shift, and requires accurate frequency offset estimator and compensator to suppress intercarrier-interference (ICI), especially in high mobility scenarios. In E-UTRA, the generic frame structure is shown in figure 1, and the pilot symbol is transmitted with 0.5 msec interval. Based on the pilot symbol, the frequency range that can be es Hz, so that it can cover maximum timated is Doppler shift caused by high speed UEs over 350 km/h (approximately 800 Hz), even when the carrier frequency is within Band VII (2.5 GHz). However, considering high speed train (HST) scenario in [3], frequency offset caused by Doppler shift in open area can be twice as much as one can expect in general case, i.e., 1.6 kHz. Thus, frequency offset estimator, which can cover the frequency range over 1 kHz, is required to maintain the link quality in the high mobility environment. Considering the multi-user environment, the frequency synchronization scheme proposed in [5] is the application of the cyclic-prefix assisted scheme in [6] to multi-user case. In this
Additive Noise & Interference e
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E-UTRA (Enhanced-Universal Terrestrial Radio Access)is a forth-coming broadband wireless communication system, which provides high data rate access up to 100 Mbps with 20 MHz bandwidth, and prefers OFDMA (orthogonal frequency multiple access) in downlink and SC-FDMA (single carrierfrequency division multiple access) in uplink. In this paper, we propose frequency offset estimation scheme to support high mobility users traveling over 300km/h ground speed in E-UTRA uplink. The proposed scheme jointly exploits pilot symbols and cyclic prefix for frequency offset estimation, and provides low estimation error.
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Figure 2: E-UTRA Uplink (SC-FDMA) Model scheme, the receiver requires multiple bandpass filters to discriminate multi-user signals for timing and frequency synchronization. However, when the allocated resource is subcarrierwise distributed in OFDMA case, it is very difficult to discriminate multi-user signal using bandpass filter bank. In this paper, we propose joint frequency offset estimation scheme for FFT-based frequency division multiple access, which exploits the cyclic prefix and the reference signal (pilot symbol) for accuracy and range. This paper is organized as follows. We first describe the EUTRA uplink and the frequency offset problem in the next section. In section 3, the proposed scheme is presented. Section 4 provides simulation results, and conclusions are provided in the final section. II.
P ROBLEM F ORMULATION
In this paper, the frequency offset problem in E-UTRA uplink is considered. The uplink multiple access scheme is SCFDMA, whose baseband signal model is shown in figure 2. The user data bit-stream is fed to discrete Fourier transform (DFT) after channel coding and rate-matching. The DFT output is assigned to allocated sub-bands, and modulated by -point inverse DFT (IDFT) with sampling interval . Assuming that sub-carriers are allocated to user and
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07)
the cyclic prefix length is , SC-FDMA signal is given by
Time-Domain Approach Correlator Bank
¾
for otherwise
(2)
where is a sum of all additive noise including thermal noise and other cell interference, and and are transmit power and arbitrary initial phase for user , respectively. Note that this paper is concentrated on the frequency offset estimation, so that the effect of multipath fading is ignored. The receiver FFT output is written as
´ µ
(3)
where denotes symbol index. In E-UTRA uplink frame structure shown in figure 1, the two received pilot symbols in the same subframe is correlated as follows.
(4)
where superscript denotes complex conjugate, and subscript RS means reference signal. Since the time interval between two pilot symbols is 0.5 ms, the frequency offset can be obtained as
User signal Select
where is the initial position of -th SC-FDMA symbol in time-domain. Imposing timing offset and frequency offset , the received signal at the serving Node B can be written as
User signal Select
FFT
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(5)
where is an integer. Using only the reference signal, the frequency range, which can be estimated, is Hz, and the effective range becomes narrower due to estimation error. Considering frequency bands defined in 3GPP, it can cover that maximum doppler shift caused by high speed UEs over 350 km/h, even when the carrier frequency is within Band VII (2.57 GHz). However, considering high speed train (HST) scenario [3], frequency offset caused by Doppler shift in open area can be twice as much as one can expect in general case, i.e., 1.7 kHz (approx.).
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Figure 3: Proposed Frequency Offset Estimation Schemes III.
P ROPOSED F REQUENCY O FFSET E STIMATOR
In this section, we proposed a multi-user frequency offset estimation scheme for high speed UEs in E-UTRA. The proposed scheme utilizes the cyclic prefix and two pilot symbols, and provides the joint estimate of the frequency offset in (orthogonal/signal-carrier) frequency division multiple access. Generally, the frequency offset is measured from the phase difference between two repeated patterns. Thus, the frequency offset estimation using cyclic prefix in multi-user environment first requires the separation of each user’s signal from the received signal. In [5], the bandpass filtering is recommended for user signal separation. However, bandpass filtering for each user requires large amount of complexity even for consecutively allocated user signals, and it is very difficult to apply for user signal distributed deterministically or pseudo-randomly within given frequency band. As an alternative, we propose two FFT-based approaches as depicted in figure 3. The proposed approaches are time-domain approach and frequency-domain approach according to signal space where the phase difference is measured, and both approaches are equivalent in terms of duality. In the time-domain approach, the cyclic prefix is reconstructed by partial FFT of user signal in frequency domain, , and the phase difference is obtained by correlating the received cyclic prefix and reconstructed one. In the frequencydomain approach, the receiver estimate the phase difference in frequency domain by correlating and IDFT results of the received cyclic prefix. A. Time-Domain Approach The cyclic prefix for user is reconstructed from the selected FFT output by partial DFT, which can be written as
where is the cyclic prefix sample length. We use the cyclic prefix of all the SC-FDMA symbols in the
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07)
same sub-frame, i.e.
Scenario
A
where subscript denotes cyclic prefix. Since sub-carrier spacing is 15 kHz in generic frame structure, we have
C
Tunnel with leaky cable Tunnel for multi-antennas
Frequency Offset Estimation
As previously stated, the cyclic prefix assisted frequency offset estimator has wide estimation range, i.e., kHz. Although we assume that all the symbols in a subframe is used for frequency offset estimation and its total energy amounts nearly the same as in pilot-symbol assisted case, the cyclic prefix assisted scheme still shows large estimation error due to its estimation range. Thus, jointly combining both estimates, i.e. and , the frequency offset estimate can be obtained as follows.
where
IV.
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A. HST Scenario For simulating the high speed train scenario, we adopt and modify the simulation assumptions in [2] and [4] for band VII, and the propagation models are described in table 1. Since the kHz maximum doppler shift in scenario B is within range, scenario A and C are considered for simulation. Given maximum doppler shift , the maximum doppler shift is
(6)
(7)
S IMULATION R ESULTS
In this section, the computer simulation is conducted for performance evaluation of the proposed scheme.
where is the carrier frequency in Hz, is the velocity of the train, and where is the speed of light, both in m/s. In HST scenario, Doppler shift is written by
where
0
Figure 4: Doppler Shift Trajectory in scenario A and C
Thus, we can find the same and as obtained in the time-domain approach using the following.
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Using the duality, we can find the equivalent approach in frequency-domain. After partial FFT of the received cyclic prefix, we have
Static 1000m with Doppler Single tap Infinity Rice fading Static 300m with Doppler
BS-track Rician Train Maximum Doppler distance, factor, speed Dmin K (km/h) frequency (Hz)
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Frequency-Domain Approach
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where is an integer. Assuming that the frequency synchronization is acquired during initial cell search and it is maintained using downlink reference signal, so that can be said to be 0. B.
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Table 1: Propagation Models for Uplink in modified HST scenario
is derived as
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where is the initial distance of the train from the Node-B crossing, and is the distance of the Node-B from the railway track, both in meters, and is time in second. Based on given scenario, the doppler shift trajectory can be expressed as figure 4. B. Single-user Case In E-UTRA uplink, the cyclic prefix assisted scheme can exploit almost the same SNR as in the pilot assisted case, so that
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07)
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Figure 5: Estimation Error in Single-user Case (
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the performance of two schemes is also comparable in terms of the normalized estimation error. The simulation results in single-user case are shown in figure 5. The figure 5 (a) shows the frequency offset estimation error vs. received symbol energy over noise PSD when the frequency error is 0 and 1600 Hz. Since the integer multiple of 2 kHz is determined by the cyclic prefix assisted approach, the proposed scheme provides inaccurate estimate due to large estimation error of the cyclic prefix assisted scheme at low SNR. This property can also be observed in figure 5 (b). The figure 5 (c) shows the estimation error curves based on HST scenario. As shown in the figure, the residual frequency error in RS only schemes is out of range which can be covered by channel estimator, and the consequent performance degradation is hard to compensate with uplink power control. C.
Multi-user Case
For multi-user simulation, we assume that two users, victim ( ) and aggressor ( ), are consecutively allocated
Figure 6: Estimation Error Curves in Multi-user Case (AWGN Channel, and =10)
in the frequency domain, and their bandwidth is 1.8 MHz, i.e., ( and = 10). Note that the victim when there is no aggressor is set to 5 dB. Figure 6 shows estimation error curves in the case that there exists the aggressor in the adjacent frequency band. In the figure 6 (a), it is shown that the effect of the aggressor’s interference is mainly dependent on the aggressor-to-victim power ratio (AVPR) and aggressor’s frequency offset. Especially, 6 (b) shows that the proposed scheme provides good estimates even when the imposed frequency offset is out of the range where the pilot assisted scheme can cover. As stated in the previous section, the pilot assisted scheme is less sensitive to the effective SNR than the cyclic prefix assisted scheme. Thus, the proposed scheme provides good estimates for larger frequency offset range.
The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07)
V.
C ONCLUSIONS
In this paper, the frequency offset estimation scheme has been provided for high mobility environment in E-UTRA. The proposed scheme is basically pilot assisted, and exploits the cyclic prefix for estimation range extension. It is an unbiased estimator, and provides accurate estimate even when large doppler shift exists. The simulation results shows low mean square estimation error with wide estimation range up to 1/2 carrier spacing, which covers worst case Doppler shift in high speed train scenario over 2 GHz frequency bands. R EFERENCES [1] 3rd Generation Partnership Project, TS36.211 Physical Channels and Modulation, 2007. [2] R4-060627, “Simulation Results and Testing Conditions for High-Speed Train,” Ericsson, 3GPP RAN4 #39, Shanghai, China, May 2006. [3] R4-061161, “Proposed Scenarios for High Speed Train Requirements,” Ericsson, 3GPP RAN4 #41, Riga, Latvia, Nov. 2006. [4] R4-070066, “Way forward on high speed train,” NTT DoCoMo, 3GPP RAN4 #42, St.Louis, USA, Feb. 2007. [5] J.-J. van de Beek, P. O. B¨orjesson, M.-L. Boucheret, , D. Landstr¨om, J. M. ¨ ¨ Arenas, P. Odling, C. Ostberg, M. Wahlqvist, and S. K. Wilson, “A Time and Frequency Synchronization Scheme for Multiuser OFDM,” IEEE J. Select. Areas Commun., vol. 17, no. 11, pp. 63–66, Nov. 1999. [6] J.-J. van de Beek, M. Sandell, and P. O. B¨orjesson, “ML Estimation of Time and Frequency Offset in OFDM Systems,” IEEE Trans. Signal Processing, vol. 45, no. 7, pp. 1800–6, July 1997.
