Performance Study of MIMO-OFDM Platform in Narrow-band Sub-1 GHz Wireless LANs Stefan Aust NEC Communication Systems, Ltd., 1753 Shimonumabe, Nakahara-ku, Kawasaki, Kanagawa 211-8666, Japan Email: stefanaust@ieee.org R. Venkatesha Prasad, Ignas G.M.M. Niemegeers EEMCS, Delft University of Technology, P.O. Box 5031, 2600 GA Delft, The Netherlands Email: rvprasad@ieee.org; I.G.M.M.Niemegeers@tudelft.nl Abstract—Using multiple antennas at the transceivers has become a necessity in high data rate wireless communication systems. Multiple-input multiple-output (MIMO) orthogonal fre- quency division multiplexing (OFDM) has been adopted as a mandatory modulation scheme in the upcoming IEEE 802.11ah standard. IEEE 802.11ah will specify carrier frequencies for Wireless Local Area Networks (WLANs) operating in the sub- 1 GHz band. This is challenging for MIMO-OFDM in sub-1 GHz because of limited bandwidth. Thus building a prototype will provide necessary information to understand this new scenario. We build a prototype in order to test and validate modifications to physical layer (PHY) and media access control (MAC) for narrow-band data transmissions. We present here, in detail, the steps to build a real-time MIMO-OFDM testing platform that is useful for evaluating narrow-band sub-1 GHz transmission characteristics. We analyzed potential MIMO-OFDM implemen- tations and conducted extensive measurements on our platform in order to verify the required system enhancements. This paper gives a hands-on account of designing and testing a sub-1 GHz WLAN platform. Index Terms—WLAN, IEEE 802.11ah, sub-1 GHz, MIMO- OFDM, narrow-band, platform. I. I NTRODUCTION The excellent propagation characteristics of frequencies below 1 GHz, such as longer range and better propagation performance, motivates the design of sub-1 GHz wireless communication systems. Studies have shown that sub-1 GHz frequencies have higher reach, e.g., behind vehicles and build- ings, where 2.4 GHz WLAN lead to many gray zones [1]. There are already frequencies assigned in different countries allowing the license-free use in the sub-1 GHz ISM (industrial, scientific, medical) radio band, e.g., USA, China, Europe, and Japan. For instance, in Japan the 915-930 MHz band is available for short-range wireless sensors and RFID units [2]. Such sensors can provide a coverage range of up to 70 m line- of-sight (LOS) in indoor locations and up to several hundreds of meter in outdoor environments. Moreover, due to simple modulation schemes, these modules are very power efficient and provide long battery life. Besides the use of sub-1 GHz frequencies for wireless sensors, the IEEE 802.11 work group has started a new standardization project to utilize sub-1 GHz frequencies for WLANs [3]. The IEEE 802.11ah Task Group aims to standardize a long-range WLAN system that will allow up to 1 km range. The modulation scheme proposed will have to support multi-antenna systems and MAC functions for longer range, robustness and power efficiency. In order to study the performance of such new WLANs operating at carrier frequencies below 1 GHz, we propose a MIMO platform. We specifically aim at carrier frequencies in the 900 MHz ISM band by using highly flexible Software Defined Radio (SDR) and open-source software. There is no standardized MIMO-OFDM based hardware available that uses carrier frequencies in the sub-1 GHz band. Here, off-the- shelf solutions are not possible. Therefore, we propose a low- cost solution that allows a simple operation of MIMO-OFDM functions using SDR. A platform based on MIMO-OFDM features both in software and hardware is desirable to validate the MIMO performance. A flexible protocol stack would allow us to operate on new standards, such as IEEE 802.11ah. We propose to use an open-source software to design a modular and flexible WLAN platform. The platform should: • be independent from the carrier frequency f c ; • be easy to reconfigure for new protocol stacks, such as IEEE 802.11ah, IEEE 802.11ac, etc.; and, • allow MIMO-OFDM operation in narrow-band wireless systems, e.g., 1 MHz. With the help of our platform we evaluate PHY and MAC building blocks, such as narrow-band channelization and preamble design. Specifically, the contribution of this paper can be summarized as follows: 1) we explicate on the issues in applying MIMO-OFDM to narrow-band sub-1 GHz; 2) we propose a MIMO-OFDM platform and use software defined radio; and, 3) we validate our MIMO-OFDM platform and discuss performance results. This paper is organized as follows: In Section II motivation for sub-1 GHz WLANs are outlined. We then present the sub-1GHz WLAN building blocks in Section III. The evaluation results of the proposed platform are pre- sented in Section IV and in addition we discuss the strengths and weaknesses of potential MIMO implementations as a basis for our proposed platform. We conclude in Section V. 978-1-61284-824-2/2013 - Copyright is with IFIP 9th International Workshop on Wireless Network Measurements 2013, May 13, 2013 89