There is an enormous amount of unlicensed spectrum worldwide at 60 GHz. IEEE 802.11ad was introduced to address the developing 60 GHz market. To distinguish it from other standards, IEEE 802.11ad includes two specific requirements:
- Enable fast session transfer between 802.11 PHYs,
- Maintain the 802.11 user experience — Fast session transfer provides seamless rate fallback and rate rise between VHT in the 60 GHZ and 2.4/5 GHz PHYs for multi-band devices. It also enables combo 60 GHz + 2.5/5 GHz devices to deliver expected WLAN coverage. However, it is not mandatory for devices to have multi-band capability.
VHT in the 60 GHz maintains the 802.11 user experience by maintaining the network architecture of the 802.11 system, e.g., infrastructure basic service set, extended service set, access point and station.
802.11ad reuses and maintains backward compatibility to 802.11 management plane, e.g., association, authentication, security, measurement, capability exchange and the management information base (MIB). It also supports an operation mode that enable a throughput of at least 1 Gbps.
The Physical Layer
802.11ad has three packet structures:- Control PHY is designed for low SNR operation prior to beamforming.
- Single Carrier (SC) PHY enables low power / low complexity transceivers.
- Low Power SC PHY is a derivative of the SC PHY. It provides additional support for further reduction in implementation processing power with simpler coding and shorter symbol structure.
To ensure interoperability between all 802.11ad devices, the Control PHY and SC PHY are mandatory for all devices. Each PHY frame structure is composed of a short training filed (STF), channel estimation field (CE), header, data and beamforming training fields (optional).
802.11ad introduces an architecture called personal basic service set (PBSS) that retains the existing infrastructure and is independent of the BSS network architecture. One station takes the role of the PBSS Control Point (PCP). Only the PCP transmits beacon frames.
The diagram above shows the beacon interval (BI) structure, which consists of four phases:
- Beacon transmission interval (BTI), where new station discovery occurs. During BTI, an AP / PCP performs one or more beacon transmission in different directions.
- Association beamforming training (A-BFT), where beamforming occurs between the AP / PCP and the station (STA).
- Announcement time (AT) conveys control and management information between AP / PCP and the STA, such as association, schedule etc. Alternatively, an SP can be allocated dynamically to a STA.
- Data transmission time (DTT) contains the contention-based access periods (CBAPs) and service periods (SPs). The prescribed stations access the channel during SPs based on negotiation with the AP / PCP.
The MAC Layer
The Clear-to-Send (CTS) and Contention-Free End (CF-E) frames are not valid frames for 802.11ad. 802.11ad defines Directional Band CTS (DBandCTS), DBand Denial-to-Send (DBandDTS) and DBanddCF-End frames.The frame format of DBandCTS is similar to CTS with an additional field, named Transmitter Address (TA) field, which contains the address information of both the transmitter and receiver to STAs that did not receive the RTS frame. Instead of maintaining one overall Network Allocation Vector (NAV) counter, the STA maintains one NAV timer for each source and destination pair.
Upon receiving a RTS frame addressed to itself, if at the start of the RTS reception a STA has a non-zero NAV timer, the STA does not return the DBandCTS frame. Instead, it transmits a DBandDTS frame to the transmitter of the RTS frame. The source STA of a SP can transmit a DBandCF-End frame to the destination STA of the SP and to the PCP /AP to truncate a SP.
Channel access during DTT is coordinated using a schedule by the AP / PCP, which sends the schedule to other stations. Access during SPs is reserved for specific stations (as announced in the schedule) or granted by AP / PCP. With scheduled access, 90% MAC efficiency can be achieved.
802.11ad's fast session transfer (FST) enables seamless integration of 60 GHz with 802.11a/b/g/n/ac for multi-band devices. FST allows the transfer of communication from any band / channel to any other band / channel. It supports both simultaneous and non-simultaneous operation. It also supports both transparent (the MAC address is the same on both band / channels) and non-transparent (MAC addresses are different) implementations.
Source:
- IEEE 802.11: Wireless LANs
- E. Perahia, M.X. Gong, "Gigabit Wireless LAN: An Overview of IEEE 802.11ac and 802.11ad", ACM SIGMOBILE Mobile Computing and Communications Review, Volume 15 Issue 3, July 2011
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