Abstract-In this paper, we propose a robust STBC transmission scheme to combat timing synchronization errors over frequency-selective multiple-access channels. We start by deriving the equivalent channel model in the presence of timing synchronization errors. Based on this correlated equivalent channel model, the statistical channel power gain profile is analyzed and shown to exhibit unequal channel power gains on different subcarriers. Furthermore, a robust statistical bit loading algorithm is proposed to optimize the BER performance in scenarios where link adaptation based on instantaneous channel information is infeasible or undesirable. Simulation results show that our proposed scheme is robust to timing synchronization errors, providing around 7.5dB SNR advantage at the BER of 10 −4 over the conventional scheme in various channel environments.
Abstract-In-phase and Quadrature-phase (I/Q) imbalance is a major performance-limiting impairment for directconversion Orthogonal Frequency Division Multiplexing (OFDM) transceivers. I/Q imbalance degrades the Signal-to-Noise-Ratio in an OFDM system by causing Inter-Channel-Interference (ICI) between image subcarriers. Doppler spread due to mobility also causes ICI but mainly between adjacent subcarriers. In this work, we investigate the combined effects of mobility and I/Q imbalance in a Multiple-Input-Multiple-Output-OFDM system with the Alamouti Space-Frequency Block-Coding scheme and design an effective compensation technique. Furthermore, we propose a novel pilot allocation and channel estimation scheme, and exploit the problem structure to reduce complexity.
1 Abstract-In cellular networks, a mobile station (MS) may be located in the coverage hole or out of the coverage of the base station (BS). In such a scenario, one or more relay stations (RSs) can be used to facilitate the transmission. Here, a thresholdbased opportunistic cooperative ARQ transmission scheme is proposed. Based on this scheme, the transmission between the BS and the MS can be separated into two parts: firstly, the transmission between BS and RSs (BS-RSs link) and secondly, the transmission between RSs and MS (RSs-MS link). The proposed scheme is different from the conventional ACK/NACK used for unicast transmission, in which two new types of ARQ messages are introduced for multicast transmission, namely, the relayassociated ACK/NACK (i.e., R-ACK/R-NACK) and the cooperative ACK/NACK (i.e., C-ACK/C-NACK), for BS-RSs link and RSs-MS link, respectively. A pre-defined threshold is applied to evaluate the reliability of the BS-RSs link. If the number of reliable RSs is larger than the threshold value, the reliable RSs will transmit the packet to the MS in a cooperative manner. Since reliable transmission exists in the BS-RSs link, the BS will release the packet and further transmission of this packet occurs only between reliable RSs and the MS. Therefore, the proposed scheme can reduce the latency of cellular networks and the processing burden on the BS as well as achieve higher throughput as shown in the performance analysis and simulation results.
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