應用於MIMO-OFDM時間同步之研究

Abstract

近幾年在無線技術發展下，新一代無線通訊系統中，正交分頻多工 (Ortho- gonal Frequency Division Multiplexing, OFDM)已經逐漸變成主流核心，在一個多路徑衰減的室內通道中，正交分頻多工被證明能有效的對抗由多路徑衰減所帶來的不理想因素。 本論文主要提出一個可以應用在IEEE所制定的無線區域網路標準IEEE 802.11n 上之時間同步(Timing Synchronization)。其中IEEE 802.11n 所使用的方式即為多輸入多輸出正交分頻多工(Multi-Input-Multi-Output Orthogonal Frequency Division Multiplexing), MIMO-OFDM)。 本論文所提出之同步架構分為兩部份，一部分為封包同步(Packet synchronization)，另一部份則為時間同步(Timing synchronization)。以上兩者主要皆為利用安置在每個封包內前端格式固定之Preamble，在低訊號雜訊比和多路徑衰減的通道時可以快速，正確的達到同步。其中封包同步使用兩個L-STS (Legacy Short Training Sequences)，而時間同步則只使用ㄧ個L-STS。

Due to the explosive growth demand for wireless communications, the next-generation wireless communication systems are expected to provide ubiquitous, high-quality, high-speed, reliable, and spectrally-efficient. However, to achieve this objective, several technical challenges have to be overcome attempt to provide high-quality service in this dynamic environment [1]. Orthogonal frequency division multiplexing (OFDM), one of the multi-carrier modulation schemes, turns out to be a strong candidate for the future wideband wireless systems because of its high spectral efficiency and simplicity in equalization. However, OFDM also has its drawbacks. The notable issues of OFDM system are more sensitive to synchronization errors than single carrier system [2], [3]. Most OFDM synchronization methods have one or some of the following limitations or drawbacks: have a limited range of operation, address only one task, have a large estimation variance, lack robust sync detection capability, and require extra overheads [4]. In this work, we introduce a timing synchronization algorithm for 4*4 MIMO-OFDM systems. The synchronization scheme contains two partitions: One is packet synchronization, including packet detection and symbol boundary detection. By sharing resources, packet detection and symbol boundary detection detect simultaneously. It uses the first two Legacy Short Training Sequences (L-STS). The other is timing synchronization, particularly using only one L-STS (3rd L-STS), both coarse and fine timing synchronization use a half L-STS.