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Top1. Introduction
Modern wireless communication systems are required to provide robust connection in rapidly changing channel environments. An optimal capacity is desired in regular stationary channels; while in mobile channels, the interference mitigation is more important to secure good link quality. In a wireless system, the interference may come from multiple sources, for example, the near band or even inband blocker, the frequency selective fading effects and the Doppler effects, to name a few. The interference can significantly degrade quality of the received signal and jeopardize the achievable data rate. The blocker impact can be alleviated by a linear receiver with large dynamic range, and the Doppler effects can be mitigated by excellent timing tracking, both of which have drawn a lot of attention in the past several decades. Many coding techniques, such as space-time (ST), space-frequency (SF) and space-time-frequency (STF) techniques (Bauch, 2003; Eilert, 2008; Idris, 2008) have been studied in regular stationary channels. The ST coding scheme in orthogonal frequency division multiplexing (OFDM) blocks provides the spatial and temporal diversities offered in the multi-antenna system while SF coding within a single OFDM block provides spatial and frequency diversities. With the cyclic prefix (CP), the OFDM scheme transforms the frequency-selective fading channel into a set of parallel frequency flat channels.
However, OFDM systems are known to be sensitive to the frequency error, such as the carrier frequency offset (CFO), sampling frequency offset (SFO) and the Doppler effects, which break orthogonality among the subcarriers, resulting in inter-carrier interference (ICI). Consequently, signal to noise and interference ratio (SNIR) is degraded substantially. In the literature, many ICI mitigation schemes have been proposed, such as ICI self-cancelation (SC) (Zhao & Haggman, 2001), extended Kalman filter (Shi, 2010), OFDM symbol time compression in one half duration (El-Bakry, 2017), parallel cancellation (Yeh & Wang, 2004; Yeh & Wang, 2007; Yeh & Yao, 2012), and conjugate cancellation (CC) (Wang et al., 2013; Yeh, 2015; Yeh et al., 2007). The CC employs the two-path transmission technique architecture to transmit one OFDM symbol in two parallel OFDM blocks. Although the redundancy causes a reduction in bandwidth efficiency, it supports a larger signal alphabet size to regain the bandwidth efficiency as that of the regular ST scheme due to its excellent ICI suppression.
The analysis of ST-, and CC-OFDM systems was available in (Yeh, 2015; Yeh et al., 2007). In this paper, we study the architecture and precoder integration of the 2x2 ST-OFDM and CC-OFDM to form 2x2 STCC-OFDM systems. Our contributions are given as follows. First, we present the 2x2 STCC-OFDM systems. We demonstrate that the precoded 2x2 STCC-OFDM systems provide a BER improvement over that of the precoded 2x2 ST structures in frequency selective fading channels. Second, we analyze the dynamic switching between the ST-OFDM and the STCC-OFDM scheme to maximize the system throughput while suppressing the interference. Third, the peak-to-average power ratio (PAPR) of all precoded STCC-OFDM systems is provided. Although only 2x2 architectures are studied in detail, all shown OFDM architectures can be extended to the other multi-input multi-output (MIMO) configurations. They are backward compatible with the existing OFDM systems. Moreover, they may serve as baseband building blocks in the OFDM transceivers and can be adaptively switched between 2x2 ST or STCC architectures as needed. This adaptability to enhance the reliability (i.e. BER improvement) of the communication system is important, specifically, in bad weather conditions or disaster response such as hurricanes, etc.