Performance Enhanced Broadband Communication Systems via Sequence Design
Fung, Carrson C.
|關鍵字:||多天線;空間相關度;通道估測;峰值因數;MIMO;spatial correlation;superimposed training sequence;channel estimation;PAPR|
|摘要:||在寬頻通訊系統中，多輸入多輸出技術已經被證明可以提供高速以及增進頻帶效率的重要技術。寬頻通訊系統標準例如IEEE 802.16e、3GPP LTE和LTE Advanced都採用多天線技術。而這些先進的通訊系統十分依賴適當設計後的訊號，來獲得正確的通道狀態並且執行精確的同步。但因傳送這些非資料訊號，例如像保護位元和指標，使得無線通訊系統會蒙受頻率效率的降低。為了達到多輸入多輸出系統所保證的資料傳輸率增益，準確的空間相關訊息是非常重要的。此外，使用正交分頻多工技術雖然有許多益處像是簡單的等化器設計，但高的峰值因數往往會發生，因而降低了發射機的功率效率。當多輸入多輸出系統和正交分頻多工系統結合時，由於越來越多的傳送端，高峰質因數更容易發生並嚴重地對系統造成影響。
MIMO technology has proven to be the key enabler of high-speed, bandwidth efficient broadband communication systems such as IEEE 802.16e, 3GPP LTE and LTE-Advanced. Similar to many traditional systems, these advanced communication systems rely heavily on proper signaling in order to obtain correct channel state information and perform precise synchronization. Unfortunately, traditional signaling methods can incur a loss of spectral efficiency due to transmission of overhead data such as preamble, guard bits and pilots. Moreover, accurate spatial correlation information is crucial in achieving the theoretical capacity gain promised by MIMO. Furthermore, with the use of OFDM, high PAPR is often incurred, thus lowering the power efficiency at the transmitter. The problem is worsened when OFDM is combined with MIMO as more RF chains are required for transmission. In this thesis, a new signaling scheme is proposed for spatially correlated MIMO channels which exploits affine precoding to produce robust superimposed training sequence such that CSI can be accurately obtained even when uncertainty in the spatial correlation matrix exists. The sequence is algebraically added to the data such that there is no loss of spectral efficiency. The proposed scheme does not require accurate knowledge about the spatial correlation matrix and it is shown to outperform previously proposed robust correlated MIMO channel estimators such as relaxed MMSE (RMMSE) and least-squares-RMMSE (LS-RMMSE). A solution for the sequence can be obtained easily by using a projection on convex sets based iterative algorithm which is guaranteed to converge as long as the training sequence matrix is initialized to have full rank. Furthermore, it is shown that the proposed scheme is asymptotically identical to the RMMSE based schemes when the MIMO channel is spatially uncorrelated. A power allocation scheme is also proposed that can maximize the detection performance. Next, a joint superimposed sequence design is proposed to jointly perform robust channel estimation and lower the PAPR of MIMO-OFDM systems. A per-tone affine precoding technique is proposed to reduce the PAPR such that no side information is required to be transmitted for the removal of the sequence at the receiver. This is in contrast to previous known techniques which incurs a large amount of transmission overhead, or can dramatically increase the BER. Furthermore, some of these techniques are based on heuristics that cannot optimally lower the PAPR. Even though redundant information has to be sent, this can be as small as 1 symbol/subcarrier. Furthermore, the proposed design allows the designer to easily trade off between BER and PAPR reduction performance. Simulation results have shown that the proposed scheme outperforms the tone reservation scheme not only in PAPR reduction but also in transmit efficiency.