無線區域網路第一層與第二層核心技術之設計與實現

Abstract

對於一個支持IEEE 802.11的接收器來說，最近以來的一個挑戰就是讓接收器架構越簡潔越好，譬如在無循環前綴單載波分組傳輸(non-cyclic prefix single-carrier block transmission, non-CP SCBT))、單輸入單輸出(single-input single-output, SISO)與多輸入多輸出(multi-input multi-output , MIMO)正交頻分複用(orthogonal frequency division multiplexing , OFDM)間進行有效率的硬體共享。基於頻率域類比數位轉換器(frequency-domain analog-to-digital conversion, FD-ADC)技術，本論文提出了一個多模接收器在頻率域上去處理所有的數位訊號，為了要在頻率域回復符號時序(symbol timing)，本論文提出了一個採用了符號速率循序並用匹配濾波器(matched filter)結果去搜尋的頻率域符號同步器(FD symbol synchronizer)，由模擬與實作結果顯示這個提出的頻率域符號同步器在低訊雜比下仍然很強健並且在VLSI實作上有很低的複雜度。而為了要讓等化器(equalizer)盡量簡潔，在無循環前綴單載波分組傳輸上，另外也提出了一個採用了單FFT架構以及球面解碼(sphere decoding)演算法的單載波頻率域等化器(SC-FDE)，因此IEEE 802.11b的等化可共用MIMO-OFDM收發機中的硬體元件。 除此之外，我們還設計了一個事前修剪的技術去更進一步降低使用空間多工多輸入多輸出傳輸中信號偵測的複雜度，這個事前修剪的技術利用zero forcing (ZF)的偵測結果及Nq-QAM星座圖上多層次結構的特性去減少傳統K-best演算法的搜尋空間，因此這方法很適合同時擁有K-best及ZF偵測器的接收器。 除了上述的實體層問題外，因為無線高速網際網路(Internet)的存取的增加讓資料由存取網路(access network)轉傳到網際網路的高速無線後置網路(wireless backhaul network)的需求變的必要，而實務上更高的傳輸率要更高的基地台密度，因此使得在高速無線後置網路的佈署中，使用基礎網路的架構變的不符成本效益，在這情況下，IEEE 802.11s 無線網狀網路(wireless mesh network, WMN)提供一個吸引人的方法來快速且低成本的佈署，在本論文中研發了IEEE 802.11s 無線網狀網路並實際佈署了一個3x3的格狀拓樸網狀網路在實驗室及一個跨三層樓的建築物中，考量到無線網狀功能的可攜性，網狀網路的開發是在一個現成的商用無線晶片中的純軟體延伸，其中使用模組化軟體設計及不需要高成本硬體更動，為了要加強傳輸廣播類(broadcast-type)網狀網路控制封包的可信度，數種廣播策略在實驗室中進行路由重建率、可接受的延遲及通道使用率等評量，對於網狀網路的佈署上，我們的觀察指出RTS/CTS可以增加網路吞吐量達到87.5%，另外比起使用IEEE 802.11b/g，用802.11n傳輸可在多重資料流(multi-stream)或多點跳躍(multi-hop)的通訊上能達到更好的公平性(fairness)，在本論文中總結的網狀網路的實驗觀察希望能提供給要佈署小型或中型室內IEEE 802.11s無線網狀網路的人一些導引。

Recently, one of the major challenge for a IEEE 802.11 compatible receiver is to make the receiver architecture as compact as possible, i.e., efficient hardware sharing between non-cyclic prefix single-carrier block transmission (non-CP SCBT), single-input single-output (SISO) and multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Based on frequency-domain analog-to-digital conversion (FD-ADC) technology, this dissertation presents a multi-mode receiver to handle all digital signals in frequency domain. A frequency-domain (FD) symbol synchronizer adopting a symbol-rate sequential search with simple matched filter detection is presented to recover symbol timing over the frequency domain. Simulation and implementation results show that the proposed FD symbol synchronizer is robust at low single-to-noise (SNR) and low complexity for VLSI implementations. To make equalizer as compact as possible, a single-carrier frequency-domain equalization (SC-FDE) for non-CP SCBT is proposed with single-FFT architecture and sphere decoding algorithm. Thus, the equalization of IEEE 802.11b can reuse the hardware components in the MIMO-OFDM modem. Moreover, a pre-pruning scheme is designed to further reduce the complexity of MIMO detection module for MIMO transmission using spatial multiplexing. The pre-pruning scheme reduces the search space of conventional K-best algorithm by using the zero forcing (ZF) detection result and the property of multilevel structure in Nq-QAM constellation. Hence, it is very attractive for the receivers equipping with both K-best and ZF detectors. In spite the issues mentioned above in physical layer, a high rate wireless backhaul network transporting data between the access network and the wired Internet becomes essential due to the increasing of wireless high-speed Internet access. The infrastructure network becomes cost ineffective in the deployment of a high-rate wireless backhaul network due to the higher data rates requires much higher cell densities to realize in practice. Under this situation, IEEE 802.11s wireless mesh network (WMN) can provide an attractive approach for the fast and low cost deployment. This dissertation develops an IEEE 802.11s WMN and then deploys a testbed with 3-by-3 grid topology in both laboratory and field crossing three floors of the building. For the portability of mesh functions, the mesh development is a pure software extension for commercial off-the-shelf WLAN chipsets with modularized software design and without costly hardware modifications. To improve the transmission reliability of broadcast-type mesh control frames, several broadcasting strategies are evaluated based on the routing construction ratio, acceptable latency, and channel utilization in the laboratory testbed. For the WMN deployment, our observations indicate that RTS/CTS can improve throughput by up to 87.5%. Moreover, compared with the IEEE 802.11b/g, 802.11n achieves better fairness for multi-stream or multi-hop communications. The experimental observations of WMN deployment summarized in this dissertation are expected to provide guidance for the small or medium scale indoor IEEE 802.11s WMN.