1、An efficient compensation scheme for I/Q mismatch in802.16e receiverFeng TianTelecommunication school，Beijing University of Post &Telecommunication，Beijing（100876）E -mail ：memofcatAbstract802.16e is the evolving protocol providing high mobility and high data transmission. However, this also brin

2、gs rigorous requirement for receiver design. To solve the I/Q mismatch issue in the 802.16e receiver, an efficient I/Q mismatch compensation scheme is proposed. This scheme is of low complexity and is implemented in time domain, and compared to the other schemes it can compensate the mismatch in a s

3、hort period without any pre-knowledge of the transmitted signals. The good performance and fast convergence of this scheme will make it helpful to implement a low-cost receiver in 802.16e.Keywords ：802.16e ，ZIF receiver，I/Q mismatch1. IntroductionIEEE 802.16e, commonly known as mobile WiMAX, is a wi

4、reless protocol intended for establishing metropolitan area networks that supply broadband data and voice services. 802.16e adopts advanced techniques including SOFDMA（Scalable orthogonal frequency division multiple access）、MIMO 、Advanced Modulation（QPSK ，16QAM ，64QAM ）, which enables the tranceiver

5、 to support peak DL data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel in1. These techniques also brings rigorous requirement for receiver design. Currently, a popular architecture to implement a low cost receiver is the direct down conversion or

6、known as Zero IF architecture. In such systems, I/Q mismatching is an issue hard to avoid. This will cause severe degradation to SOFDMA and High-order quadrature amplitude modulation such as 16 QAM, 64 QAM. When down conversion in Receiver, the un-ideality of the analog circuit will result in a phas

7、e and amplitude mismatch between I and Q signals coming from local oscillator, i.e. I/Q mismatch. Several methods are proposed in 2 4. Most of these schemes are implemented in frequency domain, using adaptive algorithm and estimate and compensate the mismatch after IFFT, so the mismatch will transmi

8、t through IFFT causing a mirror image of the OFDM symbol added to the original symbol 5.This will increase the complexity and accuracy of the compensation algorithm. In this paper, a time domain compensation scheme is proposed. Its implemented exactly after ADC and doesnt require any pre-knowledge o

9、f signals. Moreover, the scheme is able to converge to an acceptable accuracy in fast speed, i.e. in two symbol periods. Such a easily-implemented scheme is very practical in designing a low-cost 802.16e receiver.This following of this paper is organized as below: In the second section, the I/Q mism

10、atch model is presented and a compensation scheme is described in detail. In the third section, some simulation results are given comparing to the common algorithm of amplitude normalization under AWGN and SUI channel. In the fourth section, this scheme is concluded and future work is inspired.2. I/

11、Q mismatch modeling and proposed schemeIn practice, RF splitting junction, quadrature signal generators, mixers and filters are the main contributors to I/Q mismatches. Its reasonable to map the mismatch factor as: frequency independent gain errorK , and phase error into quadrature signal generation

12、 process as shown in Figure. 1. So,= =21 (1Figure 1: I/Q mismatch modelThe complex envelop of the signal,r 1(t at the receiver input, is given by(2r 1(t =I (t +jQ (t conversion and low pass filtering, we obtainWhere I (t , Q (t are the respective base band in-phase and quadrature signal components.

13、After downK sin I i 1K cos =q sin(+ cos(+ Q 1Where i 1, Supposed(3q 1are the base-band signals effected by I/Q mismatch.I (t , Q (t are independent and a complex Gaussian random process with zero mean.I (t , Q (t equals to :. We can reach that the correlation ofCorr I , Q (0=E (I (t Q (t =0From (3 a

14、nd (4 (4K =(6(51E (i q sin =1121K E (q 1Supposed that the signal ofi 1, q 1 are temporal stationary signals, so we can obtain that:(7 K =i (n q (n 1sin =1K q (n 11n21n(8In which i 1(n estimation ofq 1(n are the discrete signal of i 1, q 1 at the time n. K and sin are theK and sin . Then the i 1(n q

15、1(n are recovered as :(9I =i 1/K''Q =sec (i 1sin +q 1Where''(10I '' , Q '' are the recovery signals in two orthogonal channels.3. Performance Simulation resultIn the 802.16e system, the proposed module is implemented as in Figure 2 Figure 2: Simulation implement diagr

16、amThe compensation module is inserted after Coarse Timing and Packet detection. In the 802.16e, two long preamble symbols are adopted with 1024 or 2048 samples in each. We can see in Figure 3 that the proposed scheme can converge to acceptable accuracy in one symbol period.In the simulation, we comp

17、are the algorithm of estimating K factor with two commonly usedalgorithms in Power normalization. They are:K I =i (n q (n 11111(11K IIi (n +1 i (n =q (n +1 q (n 1(12The simulation results are listed in the Figure 37. Figure 3 : Detecting length of the 3 schemesIn Figure 3, 3 schemes are compared in

18、AWGN when K =6.9dB and =10°. Thex coordinateaxis indicate the symbol length used in detection K and the Mean square error in the numerical simulations. We can see that the proposed scheme has the fastest convergence speed and can finish estimation in two symbol periods.In Figure 4 and 5, MSE of

19、 estimations for K =6.9dB and=10° under different SNRs are given.Figure 6 and 7 display the performance in AWGN when K ranges in -30dB , 30dB and in°°25, 25.Figure 8 and 9 display the performance in SUI channel model, proposed by 5.In the extreme situation when gain and phase error ar

20、e very large, the proposed scheme can still do a good correction to I/Q mismatch. Figure 4: K estimation in AWGN Figure 5:sin estimation inAWGN Figure 6: K estimation with K ranging from -30dB to 30dB in AWGN Figure 7:K estimation with K ranging from -30dB to 30dB in SUI Figure 8: sin estimation wit

21、h ranging from -25 degree to 25 degree in AWGN Figure 9: sin estimation with ranging from -25 degree to 25 degree in SUI 4. Conclusion I/Q mismatch is a key effect in Zero IF 802.16e receiver. In this paper, a novel IQ imbalance compensation scheme in time domain is proposed. It has been shown that

22、very good compensation of IQ mismatch is achievable even for very high gain and phase errors. -6- Reference 1 Mobile WiMAX Part I: A Technical Overview and Performance Evaluation. Wimax Forum, Aug 2006 2 I. Held, 0. Klein, A. Chen and V. Ma, "Low complexity digital I/Q imbalance correction in O

23、FDM WLAN receivers, IEEE Vehicular Technology Conf, vol. 2, pp. 1172-1176, May 2004. 3 J. Tubbax, B. Come, L. VanderPerre, S. Donnay, M. Engels, H. and DeMan M. Moonen, "Compensation of IQ Imbalance and Phase Noise in OFDM Systems," IEEE Transactions on Wireless Communications, vol. 4, pp.

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