Design Article
802.11ac Wireless LAN: what’s new and the impact on design and test
Mirin Lew
11/9/2011 5:17 PM EST
The first popular standards for wireless LAN (IEEE 802.11a and b), and later 802.11g, were designed primarily to connect a laptop PC in the home and office, and later to allow connectivity “on the road” in airports, hotels, Internet cafes, and shopping malls. Their main function was to provide a link to a wired broadband connection for Web browsing and email.
Since the speed of the broadband connection was the limiting factor, a relatively low-speed wireless connection was sufficient. 802.11b provided up to 11 Mb/s at 2.4 GHz, and data rates increased to 54 Mb/s with 802.11a at 5 GHz and 802.11g at 2.4 GHz, all in unlicensed spectrum bands. However, new usage models with the need for higher throughput were recognized: data sharing amongst connected devices in the home or small office and wireless printing as examples.
A study project was set up which produced 802.11n in 2009. It improved the maximum single-channel data rate to over 100 Mb/s, and introduced MIMO (multiple input, multiple output or spatial streaming), where up to 4 separate physical transmit and receive antennas carry independent data that is aggregated in the modulation/demodulation process.
Today, there are further usage models, summarized in Table 1, that require even higher data throughput to support today’s “unwired office”.
Click on image to enlarge.
To cater for these, a new IEEE working group (TGac) aims to specify 802.11ac to deliver “Very High Throughput” (VHT) as an extension of 802.11n, providing a minimum of 500 Mb/s single link and 1 Gb/s overall throughput, running in the 5 GHz band. Bearing in mind the huge number of existing client devices – laptops, netbooks, tablets and smartphones - backward compatibility with existing standards using the same frequency range is a “must”. The goal is for all the 802.11 series of standards to be backward compatible, and for 802.11ac to be compatible at the Medium Access Control (MAC) or Data Link layer, and differ only in physical layer characteristics (see figure 1). 802.11ac is scheduled to be finalized by the end of 2013, however devices complying with draft versions of the standards may appear before this.
Click on image to enlarge.
Technical differences from 802.11n
The 802.11ac physical layer is an extension of the existing 802.11n standard, and maintains backward compatibility with it. Table 2 shows the physical layer features of 802.11n, and Table 3 shows how this is extended for 802.11ac. The theoretical maximum data rate for 802.11n is 600 Mb/s using 40 MHz bandwidth with 4 spatial streams, though most consumer devices are limited to 2 streams.
Click on image to enlarge.
Next: MIMO re-visited
Since the speed of the broadband connection was the limiting factor, a relatively low-speed wireless connection was sufficient. 802.11b provided up to 11 Mb/s at 2.4 GHz, and data rates increased to 54 Mb/s with 802.11a at 5 GHz and 802.11g at 2.4 GHz, all in unlicensed spectrum bands. However, new usage models with the need for higher throughput were recognized: data sharing amongst connected devices in the home or small office and wireless printing as examples.
A study project was set up which produced 802.11n in 2009. It improved the maximum single-channel data rate to over 100 Mb/s, and introduced MIMO (multiple input, multiple output or spatial streaming), where up to 4 separate physical transmit and receive antennas carry independent data that is aggregated in the modulation/demodulation process.
Today, there are further usage models, summarized in Table 1, that require even higher data throughput to support today’s “unwired office”.
Click on image to enlarge.
Table 1: New WLAN usage models.
To cater for these, a new IEEE working group (TGac) aims to specify 802.11ac to deliver “Very High Throughput” (VHT) as an extension of 802.11n, providing a minimum of 500 Mb/s single link and 1 Gb/s overall throughput, running in the 5 GHz band. Bearing in mind the huge number of existing client devices – laptops, netbooks, tablets and smartphones - backward compatibility with existing standards using the same frequency range is a “must”. The goal is for all the 802.11 series of standards to be backward compatible, and for 802.11ac to be compatible at the Medium Access Control (MAC) or Data Link layer, and differ only in physical layer characteristics (see figure 1). 802.11ac is scheduled to be finalized by the end of 2013, however devices complying with draft versions of the standards may appear before this.
Click on image to enlarge.
Fig 1: OSI 7-layer model from Computer Desktop Encyclopedia, 2011 The Computer Language Company Inc.
Technical differences from 802.11n
The 802.11ac physical layer is an extension of the existing 802.11n standard, and maintains backward compatibility with it. Table 2 shows the physical layer features of 802.11n, and Table 3 shows how this is extended for 802.11ac. The theoretical maximum data rate for 802.11n is 600 Mb/s using 40 MHz bandwidth with 4 spatial streams, though most consumer devices are limited to 2 streams.
Click on image to enlarge.
Table 2: IEEE 802.11n key specifications.
Click on image to enlarge.
Click on image to enlarge.
Click on image to enlarge.
Click on image to enlarge.
The theoretical 802.11ac maximum data rate is 6.93 Gb/s, using 160 MHz bandwidth, 8 spatial streams, MCS9 with 256 QAM modulation, and short guard interval. A more practical maximum data rate for consumer devices might be 1.56 Gb/s which would require an 80 MHz channel with 4 spatial streams, MCS9, and normal guard interval.
The new wider mandatory channel bandwidths are shown in figure 2. While 160 MHz and 80+80 MHz modes are both included as optional features in the 802.11ac standard, it is likely that first devices will have a maximum of 80 MHz bandwidth, and no more than the maximum 4 spatial streams specified in 802.11n.
Click on image to enlarge.
Fig 2: IEEE 802.11ac frequency allocation for Europe/Japan/Global regions.
For 20 and 40 MHz channels, the number of subcarriers and pilots and their positions are the same as in 802.11n. New values are defined in 802.11ac for 80 MHz channels, and a 160 or 80+80 MHz channel is defined in the same way as two 80 MHz channels.
Within the frame structure, the preamble and training fields make it possible for the receiver to auto-detect the physical layer standard being used. 802.11n and 802.11ac preamble frames are shown in figure 3.
Within the frame structure, the preamble and training fields make it possible for the receiver to auto-detect the physical layer standard being used. 802.11n and 802.11ac preamble frames are shown in figure 3.
Click on image to enlarge.
Fig 3: Comparison of 802.11n and 802.11ac frame formats.
The first 4 fields in both preambles are intended to be received by non-HT and non-VHT stations for backwards compatibility. The initial Legacy Short and Long Training Fields (L-STF and L‑LTF) and signal field (L-SIG) are similar to the same fields in 802.11a/b/g, while the difference in the 4th field (symbols 6 and 7) identifies the frame as either 802.11n or 802.11ac.
Examining the VHT preamble in more detail, for channels wider than 20 MHz, the legacy fields are duplicated over each 20 MHz sub-band with appropriate phase rotation. Subcarriers are rotated by 90 or 180 degrees in certain sub-bands in order to reduce the peak-to-average power ratio (PAPR). To signal VHT transmission and enable auto-detection, the first symbol of the VHT-SIG-A is BPSK, while the second symbol is BPSK with 90 degrees rotation (QBPSK). This differs from the HT-SIG for 802.11n where both symbols use QBPSK modulation. The VHT-SIG-A field contains the information required to interpret VHT packets — bandwidth, number of streams, guard interval, coding, MCS and beamforming.
Examining the VHT preamble in more detail, for channels wider than 20 MHz, the legacy fields are duplicated over each 20 MHz sub-band with appropriate phase rotation. Subcarriers are rotated by 90 or 180 degrees in certain sub-bands in order to reduce the peak-to-average power ratio (PAPR). To signal VHT transmission and enable auto-detection, the first symbol of the VHT-SIG-A is BPSK, while the second symbol is BPSK with 90 degrees rotation (QBPSK). This differs from the HT-SIG for 802.11n where both symbols use QBPSK modulation. The VHT-SIG-A field contains the information required to interpret VHT packets — bandwidth, number of streams, guard interval, coding, MCS and beamforming.
The remaining fields in the preamble are intended only for VHT devices. The VHT-STF is used to improve automatic gain control estimation in Multiple Input Multiple Output (MIMO) transmission. Next there are the long training sequences that provide a means for the receiver to estimate the MIMO channel between the transmit and receive antennas. There may be 1, 2, 4, 6 or 8 VHT-LTFs depending on the total number of space-time streams. The mapping matrix for 1, 2 or 4 VHT-LTFs is the same as in 802.11n, with new ones added for 6 or 8 VHTLTFs. The VHT-SIG-B field describes the length of the data and the modulation and coding scheme (MCS) for single or multi-user modes.
Next: MIMO re-visited
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