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Implementing 802.11ah – The Sub-1 GHz Wi-Fi Standard

Capstone Research Paper

April 22, 2016 Ashwin Barve

Jaimin Shah

Sahil Shah

Anirudh Menon

Interdisciplinary Telecom Program

University of Colorado Boulder

Dr. Ken Baker, Faculty Advisor

University of Colorado Boulder

Bruce Montgomery, Industry Advisor

Inovonics Wireless Corp.

Abstract— This paper describes research conducted on the new IEEE standard 802.11ah. The new sub-1 GHz standard is targeted at machine-to-machine and long distance Wi-Fi communications. A complex network requires a ubiquitous and cost-efficient wireless protocol to cover these large markets. The 802.11ah standard was developed to meet this industry requirement. Currently, researchers are unable to experiment with this new member of the 802.11 family because there are no commercial devices available in the market that support the 802.11ah protocol. This research paper describes the development of a prototype for the 802.11ah standard by making use of a B200 USRP and a 900 MHz antenna. The features that are investigated with this prototype include extended range, modulation technique, and power efficiency.

Keywords: Wi-Fi, IEEE 802.11ah, Software Defined Radios (SDR), long-range, sub-1 GHz, machine-to-machine, GNU radio.

I.   INTRODUCTION Task Group ah, assigned to develop the new IEEE 802.11ah standard is expected to complete its work in 2016. This new standard provides a number of features such as extended coverage range, less energy consumption, and increased data rates. Further, this will act as an alternative to 802.15 Wireless Personal Area Network

(WPAN) standards, thereby enabling Machine-to-Machine and Internet of Things (IoT) wireless access networks [1].

Features such as extended range were achieved by introducing changes to the physical and the MAC layers in the standard. The capacity of an 802.11ah Access Point (AP) is inversely proportional to the speed of the data transfer, since the wireless medium gets shared by the total number of devices.

In outdoor environments, interference has a major impact on the performance of WLANs [2]. Thus, in this paper, a special focus is made on the extended range results obtained in the outdoor environment. The legacy standards like IEEE 802.11ac and Zigbee that are currently used for short and long range data transfer have some limitations with respect to the efficiency and range of the transfer. As the 802.11ac signals have a higher channel bandwidth, the signal strength is drastically reduced and is unable to travel longer distances. Additionally, 802.11ac enabled devices face battery problems since they are not very power efficient. The 802.11ah standard tackles many of these problems making the data transfer more efficient in terms of energy and power reduction.

This paper describes the use of GNU radio to build the prototype for 802.11ah standard. It provides signal processing blocks that can be used with external RF hardware to create Software Defined Radios (SDR), thereby supporting wireless communications research.

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II. RESEARCH QUESTION AND SUB PROBLEMS

A. Research Question

‘Can standard software-defined radios support the IEEE 802.11ah protocol in an environment with interference from other IEEE 802.11 and proprietary protocols?’

B. Problem Setting

With the advent of Internet of Things, the industry needs a machine-to-machine communication protocol that can transmit over long distances. To this end, the IEEE 802.11ah standard serves as a potential solution. To completely understand where the standard is placed among its competition, the first step is to implement and develop a prototype that works in a real world environment. These results will help make an informed decision on the direction and future of IEEE 802.11ah research in machine-to-machine communication.

C. Research Sub Problems

1.  Select appropriate hardware and software    Comparing this standard with the existing long distance communication technology requires implementation using the appropriate hardware. However, there are no commercial off-the-shelf 802.11ah devices available in the market yet. To decide on appropriate electronics for implementation, the set of underlying communication modules must be understood thoroughly. Figure 1 provides the basic architecture of a SDR device.

Figure 1: Software Defined Radio Architecture [1]

An analysis of all hardware supporting sub-1 GHz protocols resulted in shortlisting E210 and B200 as potential hardware.

To complement the shortlisted hardware, another study was conducted for choosing the appropriate SDR software platform. The most popular software platform for radio engineers was found to be GNU radio, serving as a general purpose front-end for the radio.

2.  Modify existing modules, code and testing

The GNU radio software platform is built in layers with each layer supporting a particular function using a different programming language. For the 802.11ah elements to work, changes must reflect the adherence of all layers to the parameters of the standard. Making changes in all layers can be tedious and can complicate the implementation.

Hence, it is important to choose a base framework to modify, resulting in changes reflecting on all layers. Going through multiple technical papers involving the implementation of radio technology, the code repository of IEEE 802.11a/g/p trans-receiver by Bastian Bloessl was finalized. This serves as a framework to implement the IEEE 802.11ah standard. The decision was based on the ease of working through the layers in GNU radio by means of chooser blocks. Figure 2 depicts a typical modulation chooser block in the implementation.

Figure 2: Chooser block for appropriate encoding

To obtain an eventual result using the working setup, a spectrum analyzer acts as the receiving end. Alternatively, the same setup can be configured with the same receiver end, devices, and chooser block values. This test setup uses Agilent Technologies N9340B to obtain the resulting waveform.

III. LITERATURE REVIEW

Adame with the help of a research team at the University of Pompeu Fabra, Barcelona conducted an analysis on the machine to machine communication techniques using the IEEE 802.11ah protocol. The results were more towards the theoretical side of the protocol and on details on how the standard can be implemented. The advantages are broadly discussed with greater emphasis on the details of the encoding scheme and how it may affect the other parameters [3]. This paper is extremely helpful for the project as it gives an in-depth analysis of how the power management and indication bits work in relation to the

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encoding scheme to provide the advantages promised by the standard.

Aust and Prasad have depicted a research idea that is very relevant to the project as it uses a practical approach for proving the assumptions. The paper written by them describes the use of a Universal Software Radio Peripheral (USRP) and a GNU Radio module for evaluating the performance of the IEEE 802.11ah standard in an environment that is completely wireless with the help of a Software Defined Radio (SDR) for transmission of the radio waves [1]. A similar prototype has been implemented in the project by using the Orthogonal Frequency Division Multiplexing (OFDM) to avoid any interference between nearby channels and to find a plausible solution for the transfer of data.

To create an IEEE 802.11ah prototype it is important to know the difference between the PHY and MAC (Medium access control) layers of the existing IEEE 802.11g/n/ac and the new 802.11ah standard. According to the paper written by Bel and Adame the new MAC layer can support up to 8190 nodes and stations as well as help in conserving the power and energy of the device by increasing the sleep time of the nodes to 98% [4]. This paper provides an in-depth description of the machine-to-machine communications showing the advantages of using the IEEE 802.11ah prototype by enhancing the channel access methods and PHY layer encoding techniques.

Restricted Access Window (RAW) is another important feature that has been described briefly by Raessi, et al. IEEE 802.11ah tends to work better when there are more stations and hence better redundancy. RAW works well with this need as it will block the unwanted stations from trying to access the channel and thus prevent collision problems [5]. The mechanism works by giving each station a time slot for sending the uplink data based on the duration value of the RAW.

Different SDRs can be used with the GNU radio functionality. The different types of SDRs are broadly classified by Ossmann in his paper. The implementation of a software defined radio requires the knowledge of Python programming which makes it scalable and simple to use [6]. The paper dives deep into the details of how to implement SDRs and introduces useful network simulation tools. The essential need for software defined radios in the future of wireless communications also has been explained by Ossmann in the paper.

Yuan, et al. provide an update on the recent developments to the IEEE 802.11ah standard with regards to performance parameters. The paper begins with a description of machine-to-machine communications or machine-type communications in the sub-1GHz standard. The research then proceeds to describe-in-detail the advances of the standard in the recent past [6]. This paper serves as an update on the 802.11ah standards learned in previous papers from the researchers at the University of Pompeu

Fabra and helps interpret the direction in which the standard is going. The advances can be utilized to serve a variety of applications, both small-scale and long-distance.

The research article written by Rondeau describes how GNU Radio helps to bridge the gap between simulation and the real world. The previous and the following references have discussed the use of GNU Radio with software. The author has proved through this tutorial how easy it is to transfer from simulation to real-world operation using GNU Radio [7]. Rondeau has described the process of building a FM Radio Receiver which will be the foundation for the research project.

The tutorial by Tran Minh Trung describes the procedure to make use of the python programming language to create flow graphs and connect signal blocks. One important point which the author has mentioned in his tutorial is that any programming language, high-level or low-level, can be used to program individual modules. However, python programming is most efficient and extremely easy to piece the required modules together. Python acts as a binder to connect all the modules together by extracting all the values required from the blocks and combining them into one script or program to avoid extra efforts [8]. Also, the software for GNU Radio works best for the Ubuntu operating system. Trung has also explained the procedure to implement GNU Radio GUI on the Ubuntu operating system.

From the above research study, it can be concluded that as there are no off the shelf devices available, a proper 802.11ah prototype needs to be formed using the GNU radio and the USRP modules. The sole aim of the research conducted into this new standard is to enhance the existing physical and MAC layers, so as to provide better services at a much lower cost. For this, a simple simulation of a transmitter and receiver with the needed parameters can be made as a baseline. Using this setup, it becomes very efficient to simulate and find out how the 802.11ah protocol will actually work in the real world using the specification parameters.

For the design of the prototype, the knowledge of how the modulation occurs for a Radio frequency to be generated is required. Every modulation scheme requires the combination of the sine and cosine signals in a particular manner to create a signal with a specific angle and amplitude. The sin and cos signals are selected because they are quadrature to each other. Depending on the phase angle and amplitude the signal is placed on the coordinate axis and further signals can be consequently added based on the number of in-phase and quadrature levels used. For each Quadrature Amplitude Modulation (QAM) technique, each point refers to the angle-amplitude combination. The throughput increases proportionally with the number of I/Q levels of the modulation scheme. The only downside to the technique is that the noise level will increase along with the number of signal points.

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The new IEEE 802.11ah standard has better features in terms of maximum data rate, coverage range, and modulation scheme as compared to the traditional IEEE 802.11 protocols. The difference between 802.11ah, 802.11n, and 802.11ac can be shown as below:

Features 802.11ah 802.11n 802.11ac

Frequency 900 MHz 2.4,5 GHz 5 GHz

Modulation scheme

BPSK OFDM OFDM

Maximum Data Rate

40 Mbps 600 Mbps 6.93 Gbps

Coverage Range

1 Km 70m to 250m

80m with 3 antennas

Bandwidth 1,2,4,8,16 MHz

20 and 40 MHz

20,40,80, 160 MHz

Sampling rate

1,2,4,8,16 MHz

20 and 40 MHz

20,40,80 and 160 MHz

Table 1: Difference between different IEEE 802.11 standards

Overall, all these papers have been extremely helpful in contributing to this project. The understanding of the protocol and the selection of the hardware for implementation of the prototype was made easier by the results of the survey. The functioning of the SDR should complement the GNU functionality properly. The understanding on how this is possible is explained by these papers.

IV. PROCEDURE An important task that had to be accomplished was to decide which Software Defined Radio was to be used for the prototype. There are two types of radios that are predominantly used for IEEE prototypes. The SDR types are the E310 and B200 series by Ettus research [9]. The B200 series was selected for the project due to its ease of use as it can be used as a plug and play device that can transmit the waves given by the GNU radio module without extra configurations. On the other hand, the E310 series has its own processor and Linux machine due to which extra processing is required internally to work concurrently with the GNU radio module. Figure 3 shows USRP B200. As can be seen in the figure, the device has five ports out of which three can be used for transmission or reception while remaining two can only be used only for transmission.

To build our prototype, we have used GNU Radio, USRP B200, a spectrum analyzer and a 900 MHz antenna. GNU

Radio is useful to implement software radios as it provides a free software development toolkit. The toolkit is used to perform signal processing and also to process various GNU Radio blocks. Using GNU Radio, the software radios can be implemented using extremely low-cost RF hardware equipment like USRPs. The applications running on GNU radio are written in python programming language, while the signal processing path, which is extremely critical from the performance point of view, is written in C++.

Figure 3: USRP B200

Bastian Bloessl, a German scholar, has put up his work on 802.11a/g/p Transceiver in the form of a repository on Github [10]. His repository includes the GNU radio blocks which he used for his experiment, the underlying python code for each block, the XML codes for each block, and the transceiver flow graphs in the form of “.grc” files. The output modules include different types of chooser blocks used for choosing the encoding scheme, the bandwidth required, and the sampling rate.

Implementation of the 802.11a/g/p Transceiver code on GNU Radio Companion was the first step in the procedure of building the 802.11ah prototype. By understanding the underlying XML and python codes of each block in the transceiver code, it was evident that the python code takes the various parameters that are included in the xml code and generates the modules on GNU Radio Companion. The next step was to understand the PHY and MAC layers of 802.11ah and modify the parameters in the 802.11a/g/p code correspondingly. 802.11ah uses the frequency band 902-926 MHz in USA. Additionally, it supports channel widths of 1, 2, 4, 8, and 16 MHz. However, the standard works best with 2 MHz wide channels. The parameters in the XML code of the chooser blocks were modified according to the 802.11ah parameters. Thus, the bandwidth was changed to 902 MHz, the channel width was changed to 2 MHz, and the encoding scheme was changed to 256 QAM modulation.

As shown in the figure 2, the 802.11a/g/p transmitter code contains various chooser blocks that are analogous to the Macros used in C programming language. The changes

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made in the parameters in the chooser block will reflect in the code running on the back-end. This implies that the parameters will take the new values as configured in the chooser blocks. The chooser blocks are used to serve various functions like encoding scheme, the frequency used, sampling rate, etc. The encoding block includes eight choices which correspond to the eight different modulation schemes supported by the 802.11a/g/p standards. BPSK 1/2, BPSK 3/4, QPSK 1/2, QPSK 3/4, 16QAM 1/2, 16QAM 3/4, 64QAM 2/3, and 64QAM 3/4 are the various modulation techniques supported by the 802.11a/g/p standard. 802.11ah supports a few additional modulation techniques like 64QAM 5/6, 256QAM 3/4, and 256QAM 5/6. These additional modulation techniques were added to the encoding function in the XML code which is running behind the various blocks displayed in the GNU Radio companion as shown in figure 4. The XML code files are part of the repository posted by Bastian Bloessl on Github.

</param> <key>id</key>

<value>encoding</value> </param> <param> <key>label</key> <value>Encoding</value> </param> <param> <key>labels</key>

<value>["BPSK 1/2", "BPSK 3/4", "QPSK 1/2", "QPSK 3/4", "16QAM 1/2", "16QAM 3/4", "64QAM 2/3", "64QAM 3/4"]</value> </param>

Figure 4: XML code showing Encoding Function

Another block whose parameters were modified is the frequency block. Bloessl’s transceiver code includes 2.4 GHz and 5 GHz frequency options with a spacing of 5MHz between each pair of frequencies for 2.4 GHz, which represents the channel width used for the 802.11a/g/p standards, and 1 MHz spacing between the 5 GHz frequencies. This provides frequency options like 2.412 GHz, 2.417 GHz, 2.422 GHz, 2.427 GHz, 5.170 GHz, 5.180 GHz, 5.190 GHz, 5.200 GHz, etc. The basic channel width for the 802.11ah standard is 1 MHz. However, in order to increase the data throughput capability, two adjacent channels can be bonded together to form a 2 MHz channel. The spectrum available in USA for implementing 802.11ah is 902-926 MHz. Thus, the frequency options provided in the modified 802.11ah GNU radio code are 902 MHz, 904 MHz, 906 MHz, 908 MHz, etc. These frequency values were changed in the XML code in the frequency function. Figure 5 shows the frequency chooser module as displayed in GNU radio companion.

Figure 5: Frequency Chooser module for 802.11ah

According to the Nyquist theorem, the sampling rate should be at least twice the bandwidth of the bandlimited channel. In the case of 802.11ah, 2 MHz channel width is preferred because of the enhanced throughput capability offered by it. Thus, ideally the sampling rate should be at least twice the value of the desired channel width. However, this is not the case with 802.11ah. In order to retain the phase information most of the current modulation schemes use the concept of I/Q sample generation in which two samples are generated for every incoming signal. The generation of I/Q samples eliminates the need to keep the sampling rate twice of the channel width. Hence, for a 2 MHz channel, a sampling rate equal to the value of the channel width is sufficient to overcome the aliasing effect. The sampling rate options available for 802.11ah can be seen in figure 6.

Figure 6: Sample rate

The successful implementation of the modified 802.11 ah Transceiver code on GNU Radio Companion was followed by the transmission of the I/Q samples to the USRP, B210. After research on different USRPs suitable to our prototype, the choice was narrowed down to B210 and E310. E310 contains a power PC which performs the sampling of the input digital signal and produces the I/Q samples to be handed over to the antenna. For GNU radio to function along with E310, we need to cross compile the

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code which added unnecessary complexity. Compared to E310, B210 is very simple to work with. It eliminates the complexity of E310 and shifts the processing work to the PC running GNU Radio.

The 802.11ah protocol aims at increasing the range of the communication distance while reducing the data rate or channel capacity at the same time. This trade-off is considered the most important decision taken for designing this protocol. For these parameters to change accordingly the channel bandwidth needs to be decided with care. According to Shannon’s capacity equation, bandwidth is directly proportional to the capacity of the channel. The 802.11ah protocol has a default channel bandwidth of 2 MHz to ensure that the capacity is less but also take into consideration that the signal to noise calculation will be greater due to less interference and noise. Thus, the 802.11ah protocol ensures data communication for long distances but at a slower data rate.

The signal can be received using an SDR or a spectrum analyzer. A spectrum analyzer may have some issues as the noise figure is generally high. If this issue is resolved, then the analyzer can be a great tool to use as a receiver. Few other things to be addressed are selecting the appropriate resolution bandwidth and span frequency. Setting these parameters to an accurate value will lead to better results. Also, the antenna used for receiving by the spectrum analyzer should be a 900 MHz antenna for a precise result.

V.  RESULTS This section depicts the results obtained on a spectrum analyzer after receiving the 802.11ah radio signals. The setup for this test included a laptop running GNU radio companion, USRP B200, a N9340B spectrum analyzer by Agilent Technologies, and a 900 MHz antenna. GNU radio generates I/Q samples using the parameters provided by the code and transfers them to the USRP for further processing. The 900 MHz antenna is connected to the transmitting port of the USRP, which in turn is connected to the laptop.

Figure 7: Spectrum analyzer output for 802.11ah

Figure 7 shows the peak obtained from the assigned frequency of 902 MHz with a 2 MHz channel bandwidth.

The results confirm the reception of the IEEE 802.11ah protocol in a college campus setup. The on-campus wireless lab was chosen as the environment for the test, since it has access points working on different IEEE 802.11 protocols trying to interfere with the output signal. In general, the lab has IEEE 802.11a, ac and n protocols functioning at the same time during the school session.

This specific test environment helps reach the inference that the IEEE 802.11ah protocol can co-exist with other IEEE 802.11 protocols. Additionally, this inference proves that the protocol can compete against other sub-1 GHz protocols like LoRaWAN and Z-Wave in the commercial adoption for long-distance machine-to-machine communication.

VI.  CONCLUSION The goal of this paper is to implement a working prototype of the IEEE 802.11ah standard using existing hardware and software. This prototype will serve as a platform for companies that are currently using proprietary protocols for long distance communications to implement an alternative solution. Using this research further studies and implementations can be made to test performance parameters such as extended range, efficiency, and power saving capabilities.

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VII. REFERENCES [1] S. Aust, and R. V. Prasad, “Advances in Wireless M2M and IoT: Rapid SDR prototyping of IEEE 802.11ah”, [Online]. Available: http://www.ieeelcn.org/lcn39demos/Aust.pdf

[2] S. Aust, V. Prasad, and I. Niemegeers, “Outdoor Long-Range WLANs: A Lesson for IEEE 802.11ah,” IEEE Communications Surveys & Tutorials, vol. 17, no. 3, pp. 1761–1775.

[3] Adame, T.; Bel, A.; Bellalta, B.; Barcelo, J.; Oliver, M., “The IEEE 802.11ah: The WiFi Approach for M2M Communications”, IEEE Wireless Communications - Volume 21, 2014.

[4] A. Bel, T. Adame, B. Bellalta, J. Barcelo, J. Gonzalez, and M. Oliver, “CAS-based Channel Access Protocol for IEEE 802.11ah WLANs”, NeTS Research Group, May 2013.

[5] O. Raessi, T. Levanen, and M. Valkama, “Performance evaluation of IEEE 802.11ah and its restricted access window mechanism,” Communications Workshops (ICC), 2014 IEEE International Conference, pp. 460 – 466, Jun. 2014.

[6] Z. Yuan, W. Haiguang, Z. Shoukang, L. Zhongding, “Advances in IEEE802.11ah Standardization for Machine Type Communication in Sub-1GHz WLAN”, IEEE International Conference on Communications, 2013.

[7] T. Rondeau, “Tutorial: Using GNU Radio with Hardware”, GNU Radio, 22-July-2015, [Online]. Available: https://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_Hardware_Considerations

[8] T. M. Trung, “GNU Radio and USRP – A Quick Tutorial”, ICE1332 Information and Communication University, May-2008, [Online]. Available: https://mobicomclass.files.wordpress.com/2008/07/gnu-radio-usrp-quick-tutorial.pdf

[9] Ettus Research, "USRP B200/B210 Bus Series", [Online] Available: https://www.ettus.com/content/files/b200-b210_spec_sheet.pdf

[10] B. Bloessl, “IEEE 802.11 a/g/p Transceiver”, GitHub, Oct-2014, Available: https://github.com/bastibl/gr-ieee802-11.git