IEEE 802.11af Tutorial: White-Fi Basics

White-Fi, based on the IEEE 802.11af standard, marks a significant step forward in wireless communication. It utilizes TV white spaces (unused portions of the UHF and VHF spectrum) for data transmission. This technology extends the range of Wi-Fi beyond conventional frequency bands, offering improved coverage in rural and less densely populated areas where TV spectrum is underutilized.

This 802.11af tutorial covers White-Fi basics, 802.11af features, the physical layer, and the benefits of White-Fi technology.

Introduction

The frequency spectrum is allocated for two types of services: licensed and unlicensed. Day by day, the use of wireless spectrum is increasing due to advancements in wireless technologies and standards. The IEEE 802.11af standard was developed for spectrum sharing. This standard enables unlicensed White Space Devices (WSDs) and other licensed services to share the spectrum. The standard also includes rules to comply with international regulatory domains.

Unused frequency spectrum is referred to as “white space,” and devices designed to use this through spectrum sharing are called White Space Devices (WSDs). TV white space exists in the broadcast VHF/UHF television bands. Operating below 1GHz, this signal frequency experiences less material obstruction compared to other bands, making it ideal for non-line-of-sight communication and coverage.

Due to the lower frequency, the TVWS frequency band used in IEEE 802.11af will have less path loss compared to other 2.4 GHz and 5.7 GHz bands. This technology is also referred to as White-Fi.

The technology White-Fi uses the unused frequencies in the TV signal spectrum. Microsoft has pioneered a project to provide last-mile connectivity in countries including India, Kenya, Singapore, the US, and the UK. As we know, the 200-600 MHz frequency is used for TV channels to carry data. In India, 93% of this spectrum is not utilized. White-Fi will utilize this vacant spectrum to provide connectivity.

The unused spectrum spaces are commonly referred to as White Space, hence the name White-Fi. Many technology companies are exploring the use of this unused portion to provide last-mile internet access to users.

802.11af Features

Here are the key features of IEEE 802.11af White-Fi:

• Operates in TV bands 470 to 790 MHz (Europe) and 54 to 698 MHz (USA)
• Single Channel Bandwidth (W) supported are 6, 7, 8 MHz. Other possible supported combinations are 2W, W+W, 4W, 2W+2W.
• A single spatial stream is mandatory in WSDs.
• MIMO with 4x STBC and 4x Multiuser diversity

802.11af Physical Layer

IEEE 802.11af defines the physical layer for WSD devices to use and operate in TVWS frequency bands. This physical layer replaces HT and VHT specifications previously developed for WLAN 802.11a OFDM and 802.11ac variants.

As mentioned, the PHY supports channel bandwidth combinations as outlined above. It supports 144 OFDM subcarriers for 6 and 8 MHz bandwidth channels, and 168 subcarriers for 7 MHz bandwidth channels.

In a single channel bandwidth, subcarriers are arranged as follows:

• Data Subcarriers: -58 to -2, 2 to 58
• DC carrier: Index 0
• Pilot carriers: Total 6 at indices -53, -25, -11, +11, +25, +53

Key Benefits of White-Fi

Here are the advantages of White-Fi:

1. Longer Range: Due to the lower frequencies used, signals can travel further and penetrate obstacles better than traditional Wi-Fi operating at 2.4 GHz or 5 GHz.
2. Spectrum Efficiency: By leveraging dynamic spectrum access and cognitive radio technology, White-Fi enables efficient use of unused spectrum, reducing interference and optimizing bandwidth usage.
3. Affordable Connectivity: White-Fi has the potential to provide affordable internet access in underserved areas, bridging the digital divide and supporting applications like remote education, telemedicine, and smart agriculture.

Conclusion

However, challenges such as spectrum regulation, coexistence with licensed users, and infrastructure costs must be addressed to realize its full potential. Despite these hurdles, White-Fi’s ability to provide broad, cost-effective wireless coverage makes it an attractive solution for expanding internet access globally, particularly in rural and hard-to-reach regions.