Yan Wu

Complexity and Performance Comparison of Filter Bank Multicarrier and OFDM in Uplink of Multicarrier Multiple Access Networks

We compare filter bank multicarrier (FBMC) and orthogonal frequency-division multiplexing (OFDM) in the uplink of a multiple access network. Our study reveals that the high sensitivity of OFDM to carrier frequency offset (CFO) among different users and the need for interference cancellation methods to reduce this sensitivity leads to very complex and yet not very high performance systems. In FBMC-based networks, on the other hand, near-perfect performance is achieved without any need for interference cancellation, thanks to the excellent frequency localized filters used in the realization of FBMC systems.

Sensitivity analysis of offset QAM multicarrier systems to residual carrier frequency and timing offsets

This paper presents a sensitivity analysis of filter bank multicarrier (FBMC) systems to carrier frequency offset (CFO) and timing offset. Although the emphasis of the paper is on the class of FBMC systems that are based on offset quadrature amplitude modulation (OQAM), the developed results are found applicable to cosine modulated-based FBMC systems as well. We assume that coarse CFO and timing offset estimations and compensations have already been performed so that the residual CFO and timing offset are small. A simple closed-form approximation to the signal-to-interference ratio (SIR) is derived. This approximation is compared numerically with the exact SIR and confirmed to be accurate for a variety of FBMC systems with different prototype filters. The derived SIR is characterized by a pair of coefficients that are only dependent on the underlying prototype filter, thus, allows the designer to pick from the choices of the prototype filter the one that has minimum sensitivity to CFO and/or timing offset.

Carrier frequency offset estimation for multiuser MIMO OFDM uplink using CAZAC sequences : performance and sequence optimization

This paper studies carrier frequency offset (CFO) estimation in the uplink of multi-user multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Conventional maximum likelihood estimator requires computational complexity that increases exponentially with the number of users. To reduce the complexity, we propose a sub-optimal estimation algorithm using constant amplitude zero autocorrelation (CAZAC) training sequences. The complexity of the proposed algorithm increases only linearly with the number of users. In this algorithm, the different CFOs from different users destroy the orthogonality among training sequences and introduce multiple access interference (MAI), which causes an irreducible error floor in the CFO estimation. To reduce the effect of the MAI, we find the CAZAC sequence that maximizes the signal to interference ratio (SIR). The optimal training sequence is dependent on the CFOs of all users, which are unknown. To solve this problem, we propose a new cost function which closely approximates the SIR-based cost function for small CFO values and is independent of the actual CFOs. Computer simulations show that the error floor in the CFO estimation can be significantly reduced by using the optimal sequences found with the new cost function compared to a randomly chosen CAZAC sequence.

Near Maximum Likelihood Synchronization for Filter Bank Multicarrier Systems

Filter bank multicarrier (FBMC) systems have recently been proposed as superior alternative to the conventional orthogonal frequency division multiplexing, for certain applications. In this letter, we suggest a new preamble for FBMC systems and for it develop a near maximum likelihood joint channel, carrier frequency, and timing phase estimation method. The superior performance of our design over those already in the literature is shown through computer simulations.

Effect of Carrier Frequency Offset on Offset QAM Multicarrier Filter Bank Systems over Frequency-Selective Channels

This paper presents an analysis of the effect of carrier frequency offset (CFO) on offset QAM (OQAM) multicarrier filter bank (MCFB) systems, also known as staggered modulated multitone (SMT), over frequency-selective channels. This effect may be quantified by signal-to-interference ratio (SIR). We drive an accurate expression for the interference power and the desired signal power. Then SIR of SMT systems is calculated. Next, we drive an approximated SIR when the maximum delay of the channel is small in comparison with symbol spacing. The approximated SIR show that the SIR over frequency-selective channels converges to that over additive white Gaussian noise (AWGN) channels. Numerical results show that with increasing number of subcarriers, both of accurate and approximated forms of SIR as a function of CFO over frequency-selective channels converge to that over AWGN channels. Also, we compare SMT and orthogonal frequency division multiplexing (OFDM). It is shown that OFDM has better SIR in small CFO over frequency-selective channels. But with large number of subcarriers or high CFO, SMT outperforms OFDM.

Carrier Frequency Offset Estimation for Multi-User MIMO OFDM Uplink Using CAZAC Sequences

In the uplink of multi-user multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, there are multiple carrier frequency offsets (CFOs) from the multiple users. In this paper, we study algorithms to estimate these multiple CFO values. We first derive the maximum likelihood (ML) estimator and show that the complexity of the ML estimator increases exponentially with the number of users so that the estimator is not suitable for practical implementations. To reduce the complexity, we propose a suboptimal algorithm using constant amplitude zero autocorrelation (CAZAC) sequences. The complexity of the proposed method increases only linearly with the number of users. Using computer simulations, we compare the performance using CAZAC training sequences with that using the m sequence and the short training field (STF) of the IEEE 802.11n systems. The results show that in the low to medium SNR regions, the performance using CAZAC sequences is very close to the single-user Cramer-Rao bound. For high SNR regions, an error floor exists due to multiple access interference (MAI). The error floor using CAZAC sequences is more than 10 times smaller compared to the error floor using the other two sequences.

Suppression of Constant Modulus Interference in Multimode Transceivers by Closed-Loop Tuning of a Nonlinear Circuit

In multimode transceivers, the transmitter for one communication standard may induce a large interferer in the receiver for another standard. To linearly suppress this interferer, which can be several orders of magnitude larger than the desired received signal, the receiver should have a very large linear dynamic range, resulting in excessive power consumption. Many of potential interferers have a constant modulus modulation. Baier and Friederichs introduced a tuneable nonlinear circuit which can suppress a constant modulus interferer without excessive power consumption. Since an open- loop tuning method is used, the worst- case interference suppression was strongly limited by inaccuracy of circuit components. To alleviate this limitation, we propose a closed- loop tuning method that exploits the locally available interference as side information. Our analysis shows that the proposed method can strongly suppress the interferer while a symbol error rate performance close to that of an exactly linear receiver is achieved. Simulation results for a practical scenario confirm this analysis, and promise much smaller power consumption than for linear interference suppression approaches.

Suppression of constant modulus interference in multimode transceivers using an adaptive nonlinear circuit

In multimode transceivers, the transmitter for one communication standard may induce a large interference in the receiver for another standard. Using linear techniques to suppress this interference requires a receiver with a very large dynamic range, leading to an excessive power consumption. A much more power efficient approach suppresses the interference using an adaptive Nonlinear Interference Suppressor (NIS). In previous works an ideal model was used to study the receiver performance afforded by the NIS. In this paper we present experimental results of a transceiver testbed that uses an implementation of the NIS, fabricated in 140 nm CMOS technology. Main imperfections that limit the NIS performance are identified and simple models are presented that explain the experimental results. Even with these imperfections, a substantial interference suppression is achieved.

On the Optimality of the Null Subcarrier Placement for Blind Carrier Offset Estimation in OFDM Systems

Liu and Tureli proposed a blind carrier frequency offset (CFO) estimation method for orthogonal frequency-division multiplexing (OFDM) systems, making use of null subcarriers. The optimal subcarrier placement that minimizes the Cramer-Rao bound (CRB) of the CFO estimation was reported by Ghogho. In this paper, we study the optimality of the null subcarrier placement from another perspective. We first show that the SNR of the CFO estimation using null subcarriers is a function of the null subcarrier placement. We then formulate the CFO-SNR optimization for the null subcarrier placement as a convex optimization problem for small CFO values and derive the optimal placement when the number of subcarriers is a multiple of the number of null subcarriers. In addition, we show that the SNR-optimal null subcarrier placement also minimizes the theoretical mean square error in the high SNR region. When the number of subcarriers is not a multiple of the number of null subcarriers, we propose a heuristic method for the null subcarrier placement that still achieves good performance in the CFO estimation. We also discuss the optimality of the null subcarrier placement in practical OFDM systems, where guard bands are required at both ends of the spectrum.

Low-Complexity Carrier Frequency Offset Estimation for Multiuser Offset QAM Filter Bank Multicarrier Systems Uplink

In this paper, we study carrier frequency offset (CFO) estimation in the uplink of multiuser offset QAM filter bank multicarrier (FBMC) communication systems. A low-complexity frequency-domain CFO estimator using periodical training sequences is proposed. We derive the theoretical mean square error (MSE) for the proposed estimator and computer simulations show that the derived MSE matches the simulated MSE closely. Compared with state-of-the- art time-domain estimators, the proposed estimator achieves better performance with a much lower computational complexity.