Advanced Wireless Local Area Networks in the Unlicensed Sub-1GHz ISM-bands

Abstract

This dissertation addresses the challenges of wireless local area networks (WLANs) that operate in the unlicensed sub-1GHz industrial, scientific, and medical (ISM) band. Frequencies in the 900MHz spectrum enable a wider coverage due to the longer propagation characteristics of the radio waves. To utilize globally available sub-1GHz (S1G) ISM-bands, the IEEE 802.11ah Task Group started to standardize a new WLAN protocol in 2010. The IEEE 802.11ah WLAN protocol enables moderate data rates over a wider coverage area. However, this introduces newer challenges that need to be addressed in such a system. They are, reduction of energy consumption and the network performance in high density WLANs. Additionally, the presence of short-range wireless personal networks (WPANs) in the S1G ISM radio-band is of paramount importance. Coexistence problems between WLANs and WPANs compromise the data transmission performance in both networks due to numerous data retransmissions. However, the fact that no IEEE 802.11ahWLAN currently exists limits the discussion on potential system improvements. Therefore, a novel wireless prototype is proposed in this thesis that enables the testing of S1G WLANs. This dissertation is organized into two parts. The first part outlines the motivation for S1G WLANs (Chapter 1) and introduces the emerging IEEE 802.11ah WLAN protocol amendment (Chapter 2). It includes the fundamentals of the S1G physical layer (PHY) and media access control (MAC) strategies, followed by an introduction of the IEEE 802.11ah WLAN protocol functions. The second part includes the building of such a network and testing it on a testbed. To obtain over-the-air evaluation results of the new S1G WLAN, a novel narrow-band multiple-input multiple-output (MIMO) IEEE 802.11ah WLAN prototype is proposed (Chapter 3). This prototype consists of software-defined radio (SDR) hardware and software components operating at 900MHz. In general, the prototype allows first-hand experiments using carrier frequencies in the license exempt 915 to 930MHz ISM-band in Japan. The motivation is to obtain performance results of data transmission under controlled environmental conditions and to evaluate the findings, including the signal characteristics, upper bound of throughput, and coexistence issues. The prototype utilizes the 802.11ah PHY and MAC scheme along with the limited channel bandwidth of 1MHz. Additionally, a novel SDR-based spectral-time sensor is developed to observe the spectral characteristics of the wireless signals in the 920MHz radio-band in real-time and in batch mode. To increase the wireless coverage range, MIMO modifications are proposed to exploit precoding features (Chapter 4). The proposed modifications include the use of modified Preparata codes, including the Grassmannian and Kerdock manifold. The obtained precoding performance is compared with numerical results. The results indicate that the proposed codebook modifications can provide significant coding gains with reduced computational complexity. Additionally, findings on wireless beamforming in S1G WLANs, which is an important signal transmission technique to mitigate wireless interference in long-range outdoor scenarios, are presented and discussed. Multi-flow scenarios are evaluated, in which observed performance gains suggest that further optimization would be beneficial, e.g., for deviceto- device (D2D) communication systems. Single-input single-output (SISO) performance evaluations indicate the upper throughput boundaries of the S1G WLANs, whereas MIMO evaluation results provide insights on the transmission performance of concurrent flows when beamforming is used. Next, the energy consumption of Wi-Fi modules is evaluated and energy consumption reduction strategies are proposed which are beneficial for the deployment of so called Wi-Fi sensors in the S1G ISM-band. More specifically, to reduce the energy consumption of Wi-Fi sensors, a novel multi-antenna switching algorithm is proposed (Chapter 5). The evaluation results indicate significant energy consumption reduction for multi-antenna Wi-Fi sensors. Of paramount importance are the interference challenges in WLANs when wireless personal body area networks (WPANs) coexist in the same radio-band. Packet collisions between WLANs and WPANs are evaluated with the help of the proposed IEEE 802.11ah WLAN prototype (Chapter 6). The spectral-time observation results of the WPAN-to-WLAN packet collisions are illustrated, and the collision patterns are presented. The problems with management frame collisions in highly dense WLANs are discussed, and a radio resource monitoring and management (RRMM) scheme for long-range S1G WLANs is proposed (Chapter 7). To minimize packet collisions in largely dense WLANs the orthogonal sectorization of the S1G WLANs is proposed and is studied analytically. The results indicate that the proposed sectorization is beneficial to reduce collisions of WLAN management frames. Finally, findings are summarized and a detailed view of potential future research for S1G WLANs is provided (Chapter 8).