Pilot Designs for Consistent Frequency-Offset Estimation in OFDM Systems

This paper presents pilot designs for consistent frequency-offset estimation of orthogonal frequency-division multiplexing systems in frequency-selective fading channels. We describe two design approaches, namely, consistency in the probabilistic sense and absolute consistency. Existing preambles and pilot designs in the literature do not guarantee the absolute consistency. We derive general criteria for both approaches, present sufficient conditions on the pilot structures over the maximum carrier frequency offset (CFO) estimation range (half of the sampling rate), and derive simple pilot designs satisfying these conditions. We also extend the sufficient conditions to any arbitrary but fixed CFO estimation range, and present some generalized design patterns. Furthermore, the CFO estimation performances of distinct consistent pilot designs can be quite different at moderate or low signal-to-noise ratio (SNR) due to different statistics of outliers which also yields a link failure. We develop efficient pilot-design criteria that provide both consistency and robustness against outliers at moderate-to-low SNR. Our consistent pilot designs facilitate flexible and economical implementation, while our robust pilot designs enable wireless links with less outage and better resilience

I. Introduction

Carrier frequency offset (CFO) estimation plays a crucial role in establishing a reliable wireless communication link. Preamble or pilot-aided CFO estimation is adopted in all current wireless communication systems (e.g., 2G, 3G, wireless PAN/LANIMAN, etc.) to achieve a reliable estimation performance with low latency and complexity. The maximum absolute value of the CFO which can be handled by an estimator is one-half of the sampling rate. In most wireless systems and existing works in the literature (e.g., [1] [3] [4] [5] [6] [7] [8] [9]), handling such a large CFO is not necessary, and hence, the preambles are designed for a smaller estimation range. An important issue that most of the previous works overlooked is the CFO estimation consistency, defined as the unambiguousness of the estimation metric trajectory within the considered estimation range in the absence of noise, regardless of the channel impulse response (CIR). For consumer-related applications, inconsistency of the estimation under certain channel conditions may be tolerable, but consistency will still be a desirable feature if the associated cost is affordable. For emergency and other critical wireless systems (e.g., flight control, space shuttle control, military combat scenarios, etc.), the consistency should not be compromised. As an example, consider a scenario where a person is in an emergency and trying to make a 911 call. Suppose the corresponding channel response is not changing and happens to yield an inconsistent estimate. Then, a communication link will not be established due to a complete loss in frequency synchronization. This example highlights the importance of estimation consistency for emergency scenarios.