Abstract= This contribution considers the estimation and compensation of residual multiple carrier frequency offsets (CFOs) and sampling frequency offsets (SFOs) in the uplink of a multiuser OFDM system. Based on the obseirvation of a useirdependent phase rotation, an approximate maximum-likelihood (ML) estimator is proposed in order to estimate the CFOs and SFOs to track the phase in a post-synchronization step. Biterror-ratio (BER) simulations show that the system performance degradation due to (small) frequency mismatches is negligible if the proposed method is applied.Besides its advantages, OFDM (Orthogonal Frequency Division Multiplexing) is known to have some major drawbacks like highly increased sensitivity to CFOs caused by oscillator mismatches as well as motion-induced Doppler shifts and SFOs. Synchronization in frequency must therefore be counteracted to avoid severe error-rate degradations. Furthermore, OFDM is even more vulnerable to synchronization mismatches in multiuser systems bearing in mind that users hold different oscillator frequencies and, in addition, move independeniitly (i.e. exhibit differenit Doppler shifts). The effect of synchronization impairments in a multiuser scenario on the signal-to-interference power ratio has been studied e.g. in [2].In the downlink, the multi-parameter estimation problem reduces to independent single problems, which can be solved with conventional techniques (e.g. [4]). In contrast, in the uplink of a multiuser OFDM system the problem of jointly estimating (and also compensating) multiple frequency offsets (FOs) turns out to be much more challenging.Only a few contributions have addressed the uplink synchronization problem in multiuser OFDM. In [5], signals from different users are separated by filter banks at the base station thus avoiding multiple access interference (MAI). This strategy assumes that adjacent subcarriers are allocated to users which disables the exploitation of frequency diversity. In [3], arbitrary subcarrier allocations are allowed. However, the parameter estimation for one user assumes that all the other users are already perfectly synchronized to the base stations. Moreover, SFOs have not been considered in these approaches.In this contribution, we consider the joint estimation of multiple CFOs and SFOs in the uplink of OFDM/SDMA (Space Di ision Multiple Access) sy stems undr the assumption that a pre-synchronization of CFOs has already been carried out (e.g. in the downlink at the user side). Thus, only residual CFOs and SFOs that cause a user-dependent phase rotation in time and frequency need to be estimated and compensated. The approach is an extension of [1] The paper is organized as follows: In section II, an uplink OFDM/SDMA system model including CFOs and SFOs is introduced. Based on the assumption of small FOs, an approximate ML estimator of the users' FOs is proposed in section III. Section IV presents the receiver design used for the simulations. In section V, numerical results are shown in order to verify the mathematical ...
This paper introduces new joint estimation methods of multiple carrier frequency offsets (CFOs) and channel impulse responses (CIR) in application to multiuser orthogonal frequency division multiplexing (OFDM) systems. The estimators are derived from the optimal maximum-likelihood (ML) principle. Complexity reductions are achieved by exploiting the correlation properties of the training sequence (TS). The grid search algorithm is converted into a polynomial root finding procedure which leads to low-complexity closed-form estimators for moderate CFOs. Furthermore, iterative estimators for larger CFOs are proposed. Numerical results confirm that the performance degradation due to the approximations compared to the Cramer-Rao bound (CRB) is small and may be negligible in practice.
Terahertz time-domain spectroscopy (THz-TDS) systems based on ultra-high repetition rate mode-locked laser diodes (MLLDs) and semiconductor photomixers show great potential in terms of a wide bandwidth, fast acquisition speed, compactness, and robustness. They come at a much lower total cost than systems using femtosecond fiber lasers. However, to date, there is no adequate mathematical description of THz-TDS using a MLLD. In this paper, we provide a simple formula based on a system-theoretical model that accurately describes the detected terahertz spectrum as a function of the optical amplitude and phase spectrum of the MLLD and the transfer function of the terahertz system. Furthermore, we give a simple yet exact relationship between the optical intensity autocorrelation and the detected terahertz spectrum. We theoretically analyze these results for typical optical spectra of MLLDs to quantify the effect of pulse chirp on the terahertz spectrum. Finally, we confirm the validity of the model with comprehensive experimental results using a single-section and a two-section MLLD in a conventional THz-TDS system.
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