IEEE 802.11 initiated a group to investigate very high throughput (VHT) technologies. The Wi-Fi Alliance provides the following general usage models: wireless display, distributed high definition TV (HDTV) rapid upload/download backhaul, outdoor campus, auditorium and manufacturing floor. Specific usages include video streaming around a house, rapid sync-and-go and wireless I/O.
Streaming around the home will involve TVs and DVRs with wireless capability and 100+ Mbps aggregate of videos from DVR can be displayed wirelessly on TVs in different rooms. Rapid sync-and-go will allow users to quickly sync movies or pictures between mobile devices, such as a phone and a laptop. With a 1 Gbps radio link, transferring a 1 GB video file will take less than a minute.
802.11ac operates in the 5 GHz band. Its scope includes:
- Single link throughput supporting at least 500 Mbps,
- Multi-station throughput of at least 1 Gbps,
- Exclusion of 2.4 GHz band,
- Backward compatibility and co-existence with legacy 802.11 devices in the 5 GHz band.
How does 802.11ac support VHT?
802.11n supports features such as MIMO, 40 MHz and packet aggregation to significantly increase data rates of 802.11a/b/g. As an evolution of 802.11n, 802.11ac adds 80 MHz, 160 MHz and non-contiguous 160 MHz (80 + 80 MHz) channel bandwidth. 802.11ac enhances throughput with its multi-user capability in the form of downlink multi-user MIMO (DL MU-MIMO).
802.11ac increases the modulation constellation size from 64 QAM to 25 QAM. The number of spatial streams is increased to 8 to support DL MU-MIMO. The packet aggregation size limits are increased to support higher data rates. 802.11ac defines non-overlapping channels to avoid in-band interference.
The Physical Layer
The 802.11ac PHY layer is similar to 802.11n. The preamble consists of the following fields:- Legacy short training field (STF): start of packet detection, AGC setting, initial frequency and timing synchronization.
- Legacy long training field (LTF): channel estimation, fine frequency and timing synchronisation.
- Legacy signal field (L-SIG): Spoofs legacy devices, indicates VHT payload symbol length.
- VHT-SIG-A: Replaces 802.11n HT-SIG, contains VHT PHY single user and some MU parameters.
- VHT-STF: Similar to 802.11n HT-STF; allows readjustment of AGC.
- VHT-LTF: Similar to 802.11n HT-LTF; used for channel estimation.
- VHT-SIG-B: New VHT field; contains additional per-user parameters.
The data field follows the preamble. The first 16 bits is the Service field — in 802.11ac, this includes a CRC for VHT-SIG-B.
The MAC Layer
802.11ac modifies the 802.11n MAC to address co-existence and medium access with wider channels. Minor modifications are made to the 802.11n aggregation mechanism to improve efficiency at gigabit per second data rates.With much wider channels in 802.11ac, it is harder to avoid overlap between neighbouring BSS. It is also harder to choose a primary channel common to all overlapping networks. 802.11ac address this problems by improving co-channel interference using three techniques:
- enhanced secondary channel Clear Channel Assessment (CCA),
- improved dynamic channel width operation,
- a new operating mode notification frame.
The basic requirement for an OFDM-based device is to receive a valid signal at level -82 dBm. It must also detect any signal at level -62 dBM, called Energy Detect (ED).
When 802.11n added the 40 MHz channel and a secondary 20 MHz channel, only ED was required on the secondary channel due to the added complexity of detecting a valid 802.11 signal on the secondary channel.
In 802.11ac, valid signal detect on the secondary channels is added at level -72 dBm or -69 dBm according to bandwidth, to improve CCA performance on the secondary channels. A device is required to detect a valid packet on the secondary channels based on the packet preamble and the middle of the packet.
Referring to the diagram below, AP1 is transmitting to STA1 while STA2 is transmitting to AP2. AP2 occupies overlapping channels of the secondary 40 MHz channel of AP1. STA1 and STA2 might interfere with each other but the interference is not heard by the two APs. To address this problem, bandwidth signalling is added to the RTS and CTS frames.
As shown in the diagram below, AP1 sends an RTS with the bandwidth of the intended transmission (80 MHz comprised of channels 36, 40, 44 and 48). Before STA1 replies with a CTS frame, it senses the medium on all secondary channels for PIFS. If the secondary 40 MHz channel is free, STA1 sends a CTS response with the bandwidth of the clear channels, i.e., 40 MHz comprised of channels 36 and 40. Then, AP1 sends data to STA1 only on the clear channels and STA1 replies with Block Ack (BA) frames that are duplicated over the clear channels.
In 802.11ac DL MU-MIMO, an AP simultaneously transmit data steams to multiple client devices. In the example illustrated in the following diagram, the AP has 6 antennas, a smart phone has one antenna (STA1), a laptop has two antenna (STA2) and a TV set has two antennas (STA3). The AP can simultaneously transmit one data stream to STA1, two data streams to STA2 and two data streams to STA3.
The advantage of DL MU-MIMO is that client devices with limited capability do not degrade the network capacity by occupying too much time on air due to their lower data rates. The network capacity is based on the aggregate of the clients of the simultaneous transmission. 802.11ac defines that the maximum number of users in a transmission is four, the maximum number of spatial streams per users is four and the maximum total number of spatial streams (summed over the users) is eight.
Source
- IEEE 802.11: Wireless LANs
- IEEE 802.11ac Wi-Fi Standard
- 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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