World’s First TV White Space WiFi Prototype Based on IEEE 802.11af Draft Standard Developed

October 17, 2012

The National Institute of Information and Communications Technology (NICT, President: Dr. Hideo Miyahara) has developed the world’s first WiFi prototype in the TV White Spaces (TVWS) (470 MHz~710 MHz) based on the IEEE 802.11af draft specification. IEEE 802.11af is currently the only task group (TG) under the IEEE 802.11 working group (WG) for WiFi technologies in the TVWS. The developed system is the first prototype that verifies the physical (PHY) and media access control (MAC) layer design of the draft specification, following the worldwide trend of prompting the TVWS for wireless communication systems.

Background

In the past few years, many countries have started to replace the current analog television technology with digital television (DTV). The Federal Communications Commission (FCC) in the United States derived the transition to DTV successfully on June 12, 2009. Consequently, regulators have initiated opening up some of the currently unused broadcast TV spectrum between 54-698 MHz referred to as TVWS to wireless communication systems. The Office of Communications (Ofcom) in the UK and regulators in many other countries are also following the same trend, encouraging organizations around the world to start efforts to prompt research and standardization activities.

IEEE 802.11af TG was formed in 2009 under IEEE 802.11 WG. The target is to define modifications to both the 802.11 PHY and MAC layers to meet the legal requirements for channel access and coexistence in the TVWS. The 802.11af has been closely following various regulations in order to prompt the WiFi technologies in TVWS worldwide. It is widely considered as one of the most promising technologies for the TVWS. In September 2012, the 802.11af released its first stable draft standard (Draft 2.0). NICT is one of the most active contributors and leading parties of the 802.11af.

Achievements

The prototype of the IEEE 802.11af draft standard.

The developed prototype is the world’s first WiFi system in TVWS based on the IEEE 802.11af draft standard. It verifies the PHY and MAC layer design of the draft specification. One of the OFDM PHY modes that take a single 6 MHz TV channel to operate is implemented with transmission power of 20 dBm. The prototype has an interface and works with White Space Data Base (WSDB) developed by NICT and the full MAC specification of the secured protocol is implemented for primary user (licensed TV broadcaster) protections. The prototype also has an interface and works with the Registered Location Secure Server (RLSS) that is defined in the 802.11af draft standard to avoid interference with other white space users (secondary users). NICT has developed the RLSS server. It is approved that the primary users and secondary users operating in the co-channels can be sufficiently protected.

Future prospects

There are many benefits of 802.11af systems compared with other current WiFi technologies. Firstly, it allows for much longer distances to be achieved. Current WiFi systems use frequencies in the ISM bands, where the signals are easily absorbed. Secondly, by operating in the TVWS, the usable spectrum is much broader than that of ISM bands when efficiently aggregated. Therefore, it is widely believed that 802.11af systems offer sufficient advantages to enable a broad market.

With the evolution of regulations regarding the TVWS worldwide, it is expected IEEE 802.11af may adapt to those regulation updates and complete the standard by 2014. We are now working on the next revision to implement the full PHY specification and new features come along with the regulatory updates. We are also looking for the opportunities for technical transfer.

This research was conducted under a contract of R&D for radio resource enhancement, organized by the Ministry of Internal Affairs and Communications, Japan.

Appendix

The prototype system

•The developed access points and stations

　The developed access points integrate 802.11af draft (D2.0) interface^[1-3] together with traditional 802.11a/b/g/n interface operating in 2.4 GHz and 5 GHz in order to support multiband operation. When the traffic in the 2.4 GHz or 5 GHz is congested, it can be offload to TVWS bands. In addition, the access point is equipped with USB, LTE and WiMAX interface as shown in Fig. 1 to support a variety of traffic. Table 1 summarizes the major PHY parameters.

Fig.1: Access point with multiband interface

Item	Value
Frequency	470-710MHz
Channel bandwidth	6MHz
Transmission power	20dBm
MCS	BPSK
MAC	IEEE 802.11af Draft Specification (D 2.0)
Antenna gain	0dBi

Table 1: PHY Parameters of UHF

Fig. 2： Interference mitigation with secured control protocol of 802.11af MAC

The full MAC specification of the 802.11af draft (D2.0) has been implemented. In order to protect the primary services on co-channels, the 802.11af specified secured control protocol for communications between access points and stations, also between access points and local coordination server namely Registered Location Secure Server (RLSS). In addition, the 802.11af defined the interface with the TVWS database to identify available spectrum. Fig. 2 compares the case when the control protocol is adopted with the otherwise case. It is shown that with the effect of the control protocol, the system automatically identifies the channels that are operating primary service. Therefore, vacant channels are chosen by the access points to operate WiFi so that no harmful interference is mutually generated.

＜References＞

1. "802.11af PHY overview" https://mentor.ieee.org/802.11/dcn/12/11-12-0866-02-00af-phy-overview.pptx

2. Zhou Lan, Keiichi Mizutani, Gabriel Villardi, Ha-Nguyen Tran, and Hiroshi Harada "An Overview of Standardization Development of IEEE 802.11af for WiFi in TVWS", SRW2012-11, August 2012

3. Keiichi Mizutani, Ryuhei Funada, Zhou Lan, and Hiroshi Harada，“Multi-channel Wireles LAN with Variable Bandwidth in TV White Space，”IEICE Tech. Report，Vol. 112, No. 192, pp. 169-173, Aug. 2012．

Glossary

TV White Spaces

TV White Spaces (TVWS) are vacant frequencies made available for unlicensed use at locations where spectrum is not being used by licensed services, such as television broadcasting. This spectrum is located in the VHF (54-216 MHz) and UHF (470-698 MHz) bands in the United States (Other countries may have different frequency allocations) and has characteristics that make it highly desirable for wireless communications.

IEEE 802.11af Task Group

IEEE 802.11af Task Group was established in 2009. It is the first project in 802.11 WLAN working group for developing standards of enabling technologies for WiFi in TVWS. The official website of the project is: