By the turn of the Millenium, the first popular standards for wireless LAN (IEEE 802.11a and b, and 802.11g in 2003) were widely available, and aimed in the first instance at delivering connectivity "on the road" in airports, hotels, Internet cafes, and shopping malls. Their goal was to provide web browsing, email and, for business users, access to the office network through Virtual Private Network (VPN) applications.
Later, wireless LAN moved firmly into the home and home office environment, with professionals wanting to spend at least part of their work week at home rather than commute to the office, and with their families downloading ever-more music, TV and film. Now, at home, in the office, on the campus, or wherever we work or play, we've got multiple devices that need to be connected together: computers, printers, games consoles, media servers, scanners and more. We want access to all our stored material — data, pictures, whatever — from devices as small as a smartphone or as large as the screen in an auditorium — and to be able to share it with friends and colleagues instantly. We want speeds that match a wired gigabit LAN connection, without the cables.
802.11n was introduced in 2009, improving the maximum single-channel data rate from the 54 Mb/s of 802.11g to over 100 Mb/s, and introducing 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.
Beyond what we do today, there are other proposed usage models, summarized in Table 1, that require higher data throughput to support today's "unwired" home and office.
Category |
Usage Model |
Wireless Display |
|
Distribution of HDTV |
|
Rapid upload/download |
|
Backhaul |
|
Outdoor campus / auditorium |
|
Manufacturing floor |
|
Table 1: Proposed new WLAN usage models
To cater for these, two new IEEE project groups aimed at providing "Very High Throughput" (VHT) have been set up. Working Group TGac aims to specify 802.11ac as an extension of 802.11n, providing a minimum of 500 Mbit/s single link and 1 Gbit/s overall throughput, running in the 5 GHz band. Working Group TGad in partnership with the Wireless Gigabit Alliance (WiGig) have jointly proposed 802.11ad, providing up to 7 Gbps throughput using approximately 2 GHz of spectrum at 60 GHz over a short range. (60 GHz transmission suffers from large attenuation through physical barriers.) Bearing in mind the huge number of existing client devices, 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 and ad to be compatible at the Medium Access Control (MAC) or Data Link layer, and differ only in physical layer characteristics. Devices could then have three radios: 2.4 GHz for general use which may suffer from interference, 5 GHz for more robust and higher speed applications, and 60 GHz for ultra-high-speed within a room — and support session switching amongst them. Both new standards are currently in draft form. 802.11ad is scheduled to be finalized by the end of 2012, while 802.11ac is scheduled to be finalized by the end of 2013. However, devices complying with draft versions of the standards will appear before this.
Figure 1: IEEE 802.11 Standards Evolution
The 802.11 standards specify either direct sequence spread spectrum (DSSS) techniques or orthogonal frequency division multiplexing (OFDM) schemes. IEEE 802.11b operates in the 2.4 GHz band and uses DSSS techniques to spread the energy in a single carrier over a wider spectrum. Two coding schemes are used in 802.11b to spread the spectrum to a single carrier. Complementary code keying (CCK) is mandatory, while packet binary convolution coding (PBCC) is optional. CCK is used to increase the original 802.11 peak data rate to 11Mbs using DQPSK modulation. PBCC makes use of forward error correction to improve the link performance when noise is the limitation.
The 802.11g standard is an extension of the 802.11b that adds 802.11a OFDM transmission modes to the 802.11b standard, providing the benefits of 802.11a throughout but in the 2.4 GHz band.
802.11a operates in the 5GHz band and, like 802.11g, uses multiple carrier OFDM as its transmission scheme. For 802.11a there are 52 carriers in all, 48 of which are used to carry data and 4 that are used as pilots.
All the legacy WLAN standards described here use fixed modulation formats for the preamble. Varying data rates are achieved by changing the modulation for the data transmission portion of the packet. Similar modulation formats, such as BPSK, are often used in the early part of the burst. The burst area contains important information such as frequency and burst length, and is less prone to bit errors. 802.11b and 802.11g use (D)BPSK and (D)QPSK for data transmission. 802.11a and 802.11g OFDM modes map data symbols using BPSK and QPSK for lower data rates and quadrature amplitude modulation (QAM) schemes for faster bit rates.
Most recently the push is to increase data rates to >100Mbps through the use of the MIMO smart antenna technology (802.11n). 802.11n utilizes OFDM but changes the packet structure to support legacy 802.11 versions.
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 802.11ac, and highlights the mandatory extensions from 802.11n. 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. The theoretical 802.11ac maximum data rate is 6.93 Gb/s, using 160 MHz bandwidth, 8 spatial streams, modulation and coding scheme 9 (MCS9) with 256QAM 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.
Feature |
Mandatory |
Optional |
Channel bandwidth |
20 MHz, 40 MHz, 80 MHz |
160 MHz, 80+80 MHz |
FFT size |
64, 128, 256 |
512 |
Data subcarriers / pilots |
52 / 4, 108 / 6, 234 / 8 |
468 / 16 |
Modulation types |
BPSK, QPSK, 16QAM, 64QAM |
256QAM |
MCS supported |
0 to 7 |
8 and 9 |
Spatial streams and MIMO |
1 |
2 to 8 |
Operating mode / PPDU format |
Very high throughput / VHT |
Table 2: IEEE 802.11ac key specifications, with mandatory additions to 802.11n highlighted
The new wider channel bandwidths are shown in Figure 1. 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.
Figure 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.
The 2.4- and 5-GHz wireless bands are today heavily congested and fundamentally lack the capacity to deliver the multi-gigabit data rates required for emerging consumer applications.
Data capacity is ultimately tied to modulation bandwidth, so the extreme gigabit data-rates required for uncompressed high-definition multimedia transmissions must be accommodated, including known futures such as 2048x1080 and 4096x2160 (Digital Cinema) or 3D. Such data rates demand large spectrum allocation, and simplicity of high-volume manufacturing demands that this spectrum bandwidth should be a small percentage of the transmission frequency. The global unlicensed band that already exists at around 60 GHz, where multi-GHz modulation bandwidths are practical, meets the requirement.
The unlicensed frequency allocations at around 60 GHz in each region do not match exactly, but there is substantial overlap; at least 3.5GHz of contiguous spectrum is available in all regions that have allocated spectrum.
Figure 3: 60GHz Band Channel Plan and Frequency Allocations by Region
The ITU-R recommended channelization comprises four channels, each 2.16 GHz wide, centered on 58.32 GHz, 60.48 GHz, 62.64 GHz and 64.80 GHz respectively. As Figure 2 illustrates, not all channels are available in all countries. Channel 2, which is globally available, is therefore the default channel for equipment operating in this frequency band. In November 2011 this channelization and the corresponding spectrum mask for the occupying signal were approved by ITU-R WP 5A for global standardization.
The 802.11ad specification defines a backwards-compatible extension to the IEEE 802.11-2007 specification that extends the MAC and physical layer (PHY) definitions as necessary to support short-range (1m - 10m) wireless interchange of data between devices over an ad-hoc network at data rates up to ~7 Gbps in the 60 GHz unlicensed band. It also supports session switching between the 2.4 GHz, 5 GHz and 60 GHz bands.
Multiple-antenna configurations using beam-steering are an optional feature of the specifcations. Beam-steering can be employed to circumnavigate minor obstacles like people moving around a room or a piece of furniture blocking line-of-sight transmission, but longer free-space distances (e.g. > 10m) and more substantial obstructions (e.g. walls, doors, etc.) will prevent transmission.
The ad-hoc network is established and managed through bi-directional protocol exchanges using a low data-rate control channel Modulation and Coding Scheme 0 (MCS 0) while bulk data transfer takes place over an appropriate higher-rate mode (MCS1..31). The higher rate modes employ beam-steering techniques for Near-Line-Of Sight (NLOS) operation.
802.11ad uses RF burst transmissions that start with a synchronization preamble (common to all modes) followed by header and payload data. The preamble is always single-carrier modulation, the header and data may use single-carrier (SC) or OFDM modulation depending on the target bit rate.
In December 2011 specification P802.11ad D5.0 was approved to proceed to its first Sponsor Ballot with the expectation that it will complete balloting and pass to the IEEE review committee by July 2012 and the Standards Board by December 2012.
Agilent has the industry's most extensive offering of WLAN technology instruments, tools, and systems for R&D, certification and manufacturing including the latest WLAN technology, 802.11ac and 802.11ad. And Keysight's expert test engineers continue to evolve our test products to keep pace with emerging technologies such as MIMO (multiple input, multiple output) in the 802.11n standard and the new 802.11ac.
| Press Release: | Agilent Technologies Introduces Signal Processing Libraries for 802.11ac WLAN and 60-GHz 802.11ad |
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| Photos: | Keysight's 160-MHz Signal Analyzer and Signal Generation Software for 802.11ac WLAN Signals |
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| Keysight Documents: | ||
| Contacts: | Janet Smith, Americas Sarah Calnan, Europe Iris Ng, Asia |
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