正交分頻多工存取多天線系統之通訊資源分配與小型基地台配置最佳化

Abstract

蜂巢式無線行動通訊系統之涵蓋區劃分與基站(base station)之位置分佈、發射功率與高度及地形、地物有密切的關連。提高發射功率與高度往往不是擴大基站涵蓋區的最有效方法。我們要研究的解決方案是以每一主要基站（以下簡稱主站）配以多個中繼站的分層式網路架構。主站即是現有大細胞(macro cell)之基站而中繼站則服務原有涵蓋區往外延伸之用戶，也可視為小細胞(small cell)之基站。主站與其中繼站間之連接可透過光纖或高指向性微波鏈路。我們的分析只考慮後者，主要是取其佈建簡易。中繼站在此扮演雙重角色：一方面負責其涵蓋區用戶行動裝置(UE)與主站間（或其他中繼站）之通訊，另一方面則作為主站與下一個中繼站間之中繼。對這類的多重跳接式(multi-hop)網路架構中繼站位置之選擇、頻段與功率之分配是決定網路容量的最重要因素，另一關鍵因素則是傳輸方式。我們考慮的是多天線的正交分頻多工存取(MIMO-OFDMA)，這是一個能夠有效提高系統容量的空中介面，不但可有效對抗頻率選擇性衰減(frequency selective fading)也能提供了較有彈性的資源分配。

本文的主題即是在探討在固定系統總功率限制下，如何針對上述多重跳接蜂巢網路，找到最佳系統加權總容量(weighted sum rate)的資源分配(resource allocation)方式。這些無線通訊資源除了系統功率外還包含了時空-頻寬上的自由度以及中繼站群之設置。

為了簡化這個的資源分配問題，我們假設每一基站所服務的用戶數都是固定的，且UE是均勻分佈(uniform distribution)在涵蓋區內。因此我們只需考慮一個以主站為圓心的扇形服務區。在此區內之用戶由主站及兩個中繼站以至多三重跳接 (three hops)的方式來服務，且每站的服務範圍彼此不相重疊。

在分析系統效能時，我們將系統中的通訊鏈路分成兩類：可直視 (line-of-sight, LOS)鏈路與非可直視(Non-Line-of-Sight, N-LOS)鏈路，並假設基站間之鏈路屬於前者，而基站與用戶間之鏈路則屬後者。此外，由於中繼站用戶間之鏈路需經多重跳接來傳收，所以分配資源時要考慮到時間選擇性衰減(time-selective fading)所產生的通道老化 (channel aging) 效應。亦即，我們需假設基站所獲得的通道狀態資訊(channel state information)是有誤差的。

我們使用了距離平方法則(squared distance criterion)推導了一個近似最佳(near-optimal)的中繼站佈署規則，運用拉格朗日法(Lagrange method)以及對偶分解(dual decomposition) 來處理資源分配所產生的對偶問題(dual problem)並得到了最佳的資源分配解。我們同時推導出最佳資源分配的必要條件且設計出所對應的演算法。接著我們藉由簡化功率分配而發展一低複雜度的鏈結式(link-based)資源分配法，並利用其載波及功率分配，而得到改良的混合式(hybrid)資源分配法。

電腦模擬證明我們所提出的中繼站部署法確實很接近實際的最佳解。而在高訊號雜訊干擾比(SINR)的環境中我們所提出的鏈結式資源分配法與預設式資源分配法的效能與最佳資源分配法的效能間僅有微小落差。這三種資源分配法都可以相當有效地降低通道老化效應。最後我們證實適當的頻帶複用(frequency reuse)的架構，可控制同頻干擾(co-channel interference)且因此可增進系統頻譜使用效率。

We consider a hierarchical (two-tier) cellular system which consists of macro and small cells (SCs). A macro cell base station (BS) is connected to multiple SC BSs via multi-hop dedicated microwave or millimeter links to extend its coverage. The SC BSs act as decode-and-forward relays to forward the macro BS originated message to either an user equipment (UE) or another SC BS. The SC BSs are placed to have disjoint coverage areas with macro and other SC BSs.

The multiple-input-multiple-output (MIMO) orthogonal frequency division multiple access (OFDMA) scheme is assumed in our study for the promised high capacity and its robustness against frequency-selective fading and flexibility in resource allocation (RA).

For such a hierarchical MIMO-OFDMA cellular system, the radio resources include conventional time-frequency-space degrees of freedom and SC BSs (relays). The main focus of this thesis can thus be regarded as a generalized down link RA problem and the RA goal is maximizing the average weighted sum rate of the system with a total power constraint.

To simply the generalized RA problem, we assume that each BS has to serve a fixed number of UEs which are uniformly distributed and consider a three-hop cell with a circular coverage area served by a combination of a macro BS and several SC BS's, each covers a disjoint area.

In analyzing the system capacity, we classify the communication links in the system into line-of-sight (LoS) and non-line-of-sight (NLoS) links and assume that the links between BS's belong to the former category while links between an UE and a BS fall in the latter category. Since RA is to be performed by a macro BS and signal transmission may involve multiple hops, we take into account the channel aging effect caused by a link's time selectivity. In other words, we use the predicted channel state information (CSI) for RA and consider the imperfect CSI when evaluating the system capacity.

We apply a squared distance criterion to find the near-optimal SC/relay placements. Using Lagrange method and dual decomposition, we obtain the optimal RA solution by solving the dual problem of our concerned optimization problem. The necessary conditions for the optimal RA are derived. We then propose a link-based RA with a closed-form general power allocation (PA) formula. Finally, we derive a hybrid RA including link-based subcarrier assignment and optimal PA.

Numerical results demonstrate that our SC placement solutions yield performance very close to that achievable by the optimal placement solutions. In the high SINR region, the proposed RA algorithms suffer only minor performance loss with respect to those of the optimal RA solution. All three RA algorithms do compensate well for the performance degradation caused by channel aging effect. Finally, we show that the frequency reuse plan improves the spectrum efficiency performance with negligible co-channel interference.