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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, thisalso brings rigo
2、rous requirement for receiver design. To solve the I/Q mismatch issue in the802.16e 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 short per
3、iod 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 wireless pro
4、tocol intended for establishing metropolitan area networks that supply broadband data and voice services. 802.16e adopts advancedtechniques including SOFDMA（Scalable orthogonal frequency division multiple access）、MIMO、Advanced Modulation（QPSK，16QAM，64QAM）, which enables the tranceiver to support pea
5、k 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 known as Zero I
6、F 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 phase and amplitude
7、 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 transmit through IFFT
8、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 of signals. More
9、over, 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 mismatch model is p
10、resented 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/Q mismatch model
11、ing 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 independentgain error K , and phase error into quadrature signal generation process as show
12、n in Figure. 1. So, = (1)21 = r1 (t ) ( passband )2 cos(ot + 1 )2 sin(ot + 2 )i(t ) q(t ) Figure 1: I/Q mismatch modelThe complex envelop of the signal,r1 (t) at the receiver input, is given byr1 (t) = I (t) + jQ(t )(2)Where I (t ) , Q(t ) are the respective base band in-phase and quadrature signal
13、components. After downconversion and low pass filtering, we obtaini1 = K cos K sin I (3) q1 sin( + ) cos( + ) Q Where i1 , Supposedq1 are the base-band signals effected by I/Q mismatch.I (t ) , Q(t ) are independent and a complex Gaussian random process with zero mean.11. We can reach that the corre
14、lation ofI (t ) , Q(t ) equals to :CorrI ,Q (0) = E(I (t )Q(t ) = 0(4)From (3) and (4)sin = 1 1E(i1 q1 )K =E(i 2 ) / E (q 2 )(5)(6)1KE(q 2 )22Supposed that the signal ofi1 ,q1 are temporal stationary signals, so we can obtain that:K = (i1 (n) / (q1 (n)(7)nn1sin = 1 i1 (n) q1 (n) n (8) 1In whichi1 (n
15、)q1 (n)Kq2 (n)nare the discrete signal ofi1 ,q1 at the time n.K andsin are theestimation of K and sin . Then thei1 (n)q1 (n)are recovered as :I = i1 / K(9)Q= sec (i1 sin + q1 )(10)WhereI , Q are the recovery signals in two orthogonal channels.3. Performance Simulation resultIn the 802.16e system, th
16、e proposed module is implemented as in Figure 2Figure 2: Simulation implement diagramThe 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
17、can converge to acceptable accuracy in one symbol period.In the simulation, we compare the algorithm of estimating K factor with two commonly usedalgorithms in Power normalization. They are:IK = i1 (n) (11) q1 (n)K II= i1 (n + 1) i1 (n)(12) q1 (n + 1) q1 (n)The simulation results are listed in the F
18、igure 37.Figure 3 : Detecting length of the 3 schemesIn Figure 3, 3 schemes are compared in AWGN whenK = 6.9dBand = 10 . The x 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 con
19、vergence speed and can finish estimation in two symbol periods.In Figure 4 and 5, MSE of estimations forK = 6.9dB and = 10under 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
20、model, proposed by 5.In theextreme situation when gain and phase error are very large, the proposed scheme can still do a goodcorrection to I/Q mismatch.Figure 4: K estimation in AWGNFigure 5: sin estimation in AWGNFigure 6: K estimation with K ranging from -30dB to 30dB in AWGNFigure 7:K estimation
21、 with K ranging from -30dB to 30dB in SUIFigure 8: sin estimation with ranging from -25 degree to 25 degree in AWGNFigure 9: sin estimation with ranging from -25 degree to 25 degree in SUI4. ConclusionI/Q mismatch is a key effect in Zero IF 802.16e receiver. In this paper, a novel IQ imbalance compe
22、nsation scheme in time domain is proposed. It has been shown that very good compensation of IQ mismatch is achievable even for very high gain and phase errors.Reference1 Mobile WiMAX Part I: A Technical Overview and Performance Evaluation. Wimax Forum, Aug 20062 I. Held, 0. Klein, A. Chen and V. Ma,
23、 Low complexity digital I/Q imbalance correction in OFDM WLANreceivers, 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
24、on Wireless Communications, vol. 4, pp.872-877, May 2005.4 A schuchert,R. Hasholzner and P . Antonine,”A novel I/Q imbalance compensation scheme for reception ofOFDM signals”, IEEE transactions on Consumer electrionics, vol 47,pp 313318 Aug 20015 IEEE Standard for Local and metropolitan area network
25、s Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, IEEE, Feb. 20066 Ingolf Held, Oliver Klein, Albert Chen, Vincent Ma, “Low Complexity Dig
26、ital IQ Imbalance Correction inOFDM WLAN Receivers”, IEEE, 2004Author Brief Introduction：Tian Feng：pursing master degree in electrical engineering in BUPT (Beijing)Since 2005 she has started study and research on telecommunication in the Network and Switching national key lab in BUPT. In 2006, she worked as an assistance in Conexant Beijing design center. And in 2007, she worked as the scientist assistance in Broadcom Beijng Research Center. Her research focuses on Wireless broadband transmission and tranceiver optimization.
