多天線-正交分頻多工存取無線網路之資源配置

Abstract

摘要 在傳收兩端同時使用多根天線，配合預先編碼技術(precoding)，我們可以在每個子載波上面獲得多個不同的空間通道(spatial channel)。另一方面，正交分頻多工存取(OFDMA)將一個寬頻帶切成許多窄頻子通道，以便平行傳送的訊號只受到非頻率選擇性衰退(frequency nonselective fading)，不但可以簡化接收複雜度，且因子通道會隨著時間以及使用者的位置而變化，傳送端可利用這些變化所造成的分集(diversity)，視各子通道之增益適當地調整其傳輸功率與調變訊號之階數，進而大大提高其頻寬使用效率。因此多天線-正交分頻多工存取系統可以分配的資源就包含了空間、頻率以及使用者這三個範疇。

在本論文中我們將會探討如何有效的配置空間、頻率以及使用者這三個維度的資源將用戶的傳輸功率與平均的位元錯誤率降到最低。我們首先提出了兩種以奇異值分解(singular value decomposition)為基礎的預先編碼技術。我們首先利用奇異值向量的線性組合來合成預先編碼向量以建立多個正交通道。我們提出的第二個預先編碼技術則把通道矩陣的秩(rank)限制移除，以增加頻譜使用效率。雖然如此一來將會讓空間通道間產生干擾，而讓資源配置的問題因需滿足某些訊號干擾比之要求而更形複雜，但透過適當的設計，仍可將用戶間的干擾控制在一定的範圍下。針對這兩個預先編碼技術我們分別提出不同的動態資源分配演算法讓用戶的總傳輸功率降到最低。

除此之外，我們還針對實用的碼書(codebook)預先編碼技術之資源配置進行探討，我們提出了動態的子載波分配及傳送功率調整等兩種基本方式及其組合來讓用戶的平均位元錯誤率降到最低。

Abstract By using multiple antennas at both the transmit and receive sides, one obtains multiple eigenmodes (eigenchannels) on the same carrier through beamforming (precoding). With each eigenchannel represents an equivalent SISO channel, array gain is obtained by using only the strongest eigenmode but the capacity is maximized by allocate the transmit power across subchannels according to the water-filling result. The orthogonal frequency division multiplexing (OFDM) scheme divide a wideband channel into parallel narrowband subchannels so that signals propagate through each subchannel suffer from frequency nonselective fading. The OFDM-based multiple access (OFDMA) scheme has been shown to be capable of achieving the maximum spectral efficiency with extremely high probability. A key ingredient of an OFDMA system is that it can exploit the diversity offered by the time-varying and user(location)-dependent nature of the subchannels via proper scheduling and power/subchannel allocation. Hence a MIMO-OFDMA system is expected to offer high capacity through an efficient use of the spectral, spatial and user domain resources. The main design issue we try to solve in this thesis is the following. Given the users’ rate requirements of a MIMO-OFDMA system, how to apportion the transmission resources in space, frequency, and user domains so that the total transmit power and each user’s average bit error rate are minimized. We first consider two orthogonal precoding schemes based on singular value decomposition (SVD). In the first design we construct orthogonal eigenchannels by performing linear operations on the channel matrix’s singular vectors under the channel rank constraint. To improve spectral efficiency, we then remove the rank constraint on the number of users allowed on a subcarrier. Although the resulting co-channel interference may cause performance degradation, it is more than compensated for by the increased capacity through a proper RA plan that ensure the associated signal-to-interference ratios are within the tolerable limits. The proposed RA algorithms for both precoding scenarios are designed to minimize the total transmit power while satisfying the users’ QoS constraints. Finally, we examine the resource allocation (RA) issue for MIMO systems with limited feedback. More specifically, we consider the codebook based precoding scheme and suggest subcarrier assignment scheme based on the Lagrange multiplier method. For a given subcarrier assignment, we then present a power allocation method which minimizes the average bit error rate performance.