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Yi-Jen Chiu,Szu-Lin Su,Shao-Ping Hung 대한전자공학회 2007 ITC-CSCC :International Technical Conference on Ci Vol.2007 No.7
Direct conversion receiver is widely utilized in today’s communication system. However, it usually faces a common problem of front-end distortions such as IQ imbalance and frequency offset. When treated separately, effective algorithms exist for estimating and compensating for IQ imbalance as well as frequency offset. With both effects present, such algorithms do not lead to useful estimates of the related parameters. OFDM is sensitive to nonidealities in the receiver front-end. This leads either to stringent front-end specifications and, thus, an expensive device, or large performance degradations. A good summary, Guanbin [1] discusses the transmitter calibration briefly and proposes an estimation technique for calculating the imbalance parameters. For the IEEE 802.11a OFDM standard, Tubbax observe that additional effects from OFDM channel estimation and correction can affect the IQ correction circuits and proposes a smoothing procedure to compensate. The above procedure is not effective when the CFO is too small. This paper extends the previous results mentioned above by developing a simple and adaptive least-square (LS) algorithm to estimate and compensate for IQ imbalance assuming the IQ imbalance compensation is not affected by the CFO, then the CFO is estimated using any of several conventional algorithms. With the additional requirement of a known periodic training sequence (see Su and Chiu ) this process extends the results of to practical cases when CFO is small. Our compensation scheme eliminates the IQ imbalance based on one OFDM symbol and performs well in the presence of CFO. The compensation scheme has fast convergence and small residual degradations. The solution is implemented with an adaptive filter to adjust the parameters of estimation and is computationally relatively inexpensive. With the proposed method as shown in Fig. 1, we can estimate the effect of IQ imbalance without knowing CFO exactly by means of an repetitive training sequence as depicted in Fig.2. Two set of I/Q mismatch parameters, (εr=1.5㏈, ?φr=1.5˚) and (εr=3㏈, ?φr=3˚), are used in the simulations. Fig. 3 shows the performance comparison between the systems with and without I/Q imbalance compensation. It shows that the proposed algorithm can give a satisfactory performance for CFO > 5㎑. To reduce the drawback of high sensitivity under low CFO channel, we modify the adaptive scheme and the fine IQ estimation method to solve the problem of performance degradation when CFO is small. Therefore, our IQ imbalance estimation/compensation scheme potentially leads to low-cost and low-complexity receivers. The simulation results show that the modified scheme can achieve much better performance as shown in Fig. 4.
Adaptive MMSE Rake-Equalizer Receiver Design with Channel Estimation for DS-UWB System
Yi-Jen Chiu,Szu-Lin Su 대한전자공학회 2007 ITC-CSCC :International Technical Conference on Ci Vol.2007 No.7
The UWB systems have a fine path resolution by transmitting information with ultra short pulses. The Rake receiver is known to be a technique that can effectively combine paths with different delays and obtain the path diversity gain. However, the UWB multipath channel is spread over dozens of symbols in the case of ultra high-speed communications of several hundreds Mbps, which results in a strong frequency selective channel. Consequently, the Rake receiver needs a large number of fingers and the computational complexity of the Rake receiver becomes high. The conventional Rake receiver employs the weight vector to perform the maximal ratio combining (MRC) which maximizes the output signal-to-noise ratio (SNR) when the interference is modeled as additive white Gaussian noise. But for its complexity and cost to remain low, the number of fingers that can be afforded is too small to capture the ample energy provided by the UWB channel, which entails a large number of paths (often>50). In this paper, we introduce a robust receiver design incorporating a channel estimation scheme for DS-UWB over a realistic indoor multipath channel in Fig. 1. The proposed receiver reduces intense multi-path destruction and severe ISI by using a combined adaptive Rake and equalizer structure referred to as the adaptive MMSE Rake-equalizer receiver in Fig. 2. Relevant receiver parameters are estimated using the MMSE algorithm. This has motivated studies of multipath combining receivers that process only a subset of the available Lp resolved multipath components. We obtain more robust signal detection at the receiver side by the adaptive channel estimation in order to extract more accurate channel state information. The channel characteristics are first estimated using the LMS adaptive algorithm on the training sequence of the preamble. The proposed receiver is able to employ the energy of a few paths and obtain better performance by the proposed receiver design. Simulation results show that channel estimation is necessary and that increasing the number of Rake fingers is effective in improving system performance. We also show that DS-UWB with adaptive MMSE Rake-equalizer has better BER performance than DS-UWB with MRC Rake receiver, especially when the number of fingers is large. In particular it is shown that the performance of an adaptive MMSE Rake-equalizer receiver of number of Rake fingers equal to 5 (L=5) outperforms an MRC Rake receiver of number of Rake fingers equal to 10 (L=10) over CM4 channels. Finally, the LMS algorithm does give a satisfactory performance with fast convergence in Fig. 3 and Fig. 4. The proposed adaptive MMSE Rake-equalizer receiver is able to employ the energy of a few paths and obtain better performance by the suitable receiver design. It shows that the adaptive algorithm gives a satisfactory performance.