Moving to IEEE 802.11ac wireless standard


THE industry is abuzz with excitement about the next incarnation of the incredibly successful Wi-Fi wireless standard. The next version due for release from the Institute of Electrical and Electronics Engineers (IEEE) 802 standards committee is dubbed IEEE 802.11ac.

IEEE 802.11ac reached draft 1.4 in November 2011, but is only slated for completion in December 2013. The Wi-Fi Alliance has already started the VHT5G technical task group, which is working on the interoperability test plan, with certification launch planned for early 2013.
Wireless standards
Osama Aboul-Magd, Chair of IEEE 802.11ac claims the major parts of the draft, including the PHY Clause, have been stable for a number of months with only minor changes during review process. “Draft 2.0 is now in a ballot that will close mid-February 2012,” he says.

However, as with most other preceding wireless standards, progress towards official adoption hasn’t stopped manufacturers launching products under a draft version of the specification.

“Some chip vendors have already announced their plans for … chips based on Draft 1.0 which was not approved by the working group,” Aboul-Magd told Electronics News.

Perhaps this is why, in its report titled “Wi-Fi Chipset Evolution: From 802.11n to 802.11ac and 802.11ad”, analyst ABI Research claims the transition to IEEE 802.11ac standard wireless will occur rapidly, with devices utilising the new standard emerging in 2012, with market dominance by 2014.

Given the speed with which previous IEEE 802.11g has given way to the n standard, ABI Research expects the same rapid transition will occur between n and ac. This is due largely to competition between Wi-Fi chipset vendors, who quickly introduce products supporting the standards in order to increase their market share during transition.

That’s great for the consumer, but a challenge for engineers who have to get to grips with the nuances of the new technology before they can employ it in their next product. So what are the differences in IEEE 802.11ac compared to older versions, and what does that mean for the designer?

Need for speed

Wireless is an increasingly popular method of providing connectivity to home automation devices, as well as portable products like smart phones and tablets with multimedia capabilities. But with the data-sucking bandwidth demands of applications like file transfers, video streaming, wireless display and cellular network offload, extra bandwidth is sorely needed.

Broadcom wirelessBroadcom was one of the first chipset vendors to announce IEEE 802.11ac chips, launching them at January’s CES 2012. The company is marketing these chips as “fifth generation Wi-Fi” (5G Wi-Fi).

According to Michael Hurlston, senior vice president and general manager of Broadcom’s wireless local area network (WLAN) line of business, many devices now have Wi-Fi Direct capabilities. This technology is a peer-to-peer communications protocol allowing communications between nodes within a WLAN, without having to go through an access point.

“With peer-to-peer connections, our customers are pushing us very hard for faster communications for media transfer,” Hurlston explains. “Once we get past the wide area network (WAN) bottleneck and inside the home, the use cases are primarily driven by these in-home data transfer type applications.”

The evolution of cellular WAN speeds from 3G and LTE (or 4G) is also driving the need for faster mobile data offloading, promoting greater wireless speeds.

Theoretically, IEEE 802.11n provides a maximum data transfer rate of 600 Mb/s using 40 MHz bandwidth with four spatial streams. In comparison, IEEE 802.11ac has a top speed of 6.93 Gb/s, using 160 MHz bandwidth, eight spatial streams, MCS9 with 256 QAM modulation, and short guard interval.

In the real world, Hurlston says real-world users are likely to see a speed boost of three to four times the throughput when using IEEE 802.11ac compared to IEEE 802.11n.

Addressing range and noise concerns

To accelerate transfers, the new standard allows for greater bandwidth via mandatory support for 20, 40 and 80 MHz wide channels. IEEE 802.11n has mandatory support for 20 or 40 MHz only.

There are also the options to use a contiguous 160 MHz channel, and a non-contiguous 80 + 80 MHz mode that splits the 160 MHz channel into two in order to avoid reserved regions.

Another change is the exclusive use of the 5 GHz spectrum, in contrast with IEEE 802.11n which uses the traditional 2.4 GHz, with 5GHz as an option. The 5 GHz spectrum is (currently) less crowded, meaning potentially less interference from other devices.

To ensure backward compatibility, most vendors will release 802.11n/802.11ac dual-band chipsets.

Despite some advantages, the use of 5 GHz means a degree of loss in propagation range. But Rohit Gaikwad, director of Systems Design Engineering at Broadcom, says several techniques can be used to improve the range.

“The first is to use higher transmit power. The second is to reduce noise with external low noise amplifiers. The third is low density parity check (LDPC), which gives another 3dB plus improvement over binary convolution codes (BCC) [for error correction]. And the last is beamforming,” Gaikwad explained.

LDPC provides significant improvements over BCC, especially when dealing with channels subject to periodic interference. In situations where BCC will simply fail, resulting in high error flow, LDPC can squeeze some performance out of the channels.

Broadcom’s chips will support both BCC and LDPC, and also apply LDPC to the 11n part of the chips.

Beamforming, while present as an option in IEEE 802.11n, was not often implemented. It is also an optional part of IEEE 802.11ac, but is expected to be more widely used.

A multiple-input and multiple-output (MIMO) technology, beamforming utilises multiple antennas and information from client devices to adjust the phase of its transmissions in order to compensate for phase shift. The result is increased signal strength to the client and an improvement in range.

Improving efficiency with multiuser-MIMO

Another new option for the IEEE 802.11ac standard is multiuser-MIMO, which is a new multi-antenna technique used on the downlink only. It is an attempt to improve the overall system efficiency.
Multiuser-MIMO uses multiple antennas to transmit data simultaneously to multiple users. Where a transmitter has four antennas, for example, two may be used in a 2 x 2 MIMO configuration to transmit to one user, while the other two antennas are used for transmissions to two other users in 1 x 1 configurations.

The standard allows for up to four users in multiuser-MIMO and up to four streams per user, but the total number of streams is limited to a maximum of eight.

Gaikwad says that various complexities can arise with multiuser-MIMO, since the co-located antennas become disparate.

“You have to make sure that each client receives its data flawlessly at the highest throughput, while seeing the other clients’ data in its noise or in its null vector space,” he explained. “It involves quite a bit of hand shaking, which means learning the channel, which is part of beam forming, but here you are making sure that you have very specific protocols to learn each person’s channel and signal to them appropriately.”

Additionally, the users may be at different locations with disparate signal-to-noise ratio (SNR) requirements, so while one user may handle quadrature amplitude modulation (QAM), another may require binary phase shift keying (BPSK). There are also complications with ensuring collision-free communication to ensure clients receive their data frames in order, even if the packet lengths are different.

Testing times

Mirin Lew, an applications specialist for the Microwave and Communications Division of Agilent Technologies, says using wider bandwidth channels could also pose challenges for manufacturers during test.

“The generation of 80 MHz bandwidth signal can be a challenge for many of the signal generators that are currently used today for WLAN testing, because the maximum sample rates can’t support the usual minimum two times oversampling that we want to use for our waveforms,” says Lew.

If oversampling is not used, aliasing artefacts will be visible in the band edges. However, Lew points out that these can be filtered by instrumentation software.

“To generate signals that are 160 MHz or even wider, use a wideband arbitrary waveform generator to create the analogue IQ signal, and then apply that to the external IQ inputs in the RF signal generator for upconversion,” Lew explains.

Complex constellations

IEEE 802.11ac uses higher modulation and coding schemes (MCS), stepping from a modulation of 64QAM for IEEE 802.11n to 256QAM, so more bits can be transmitted per symbol, improving spectral efficiency. (QAM is a modulation scheme that modifies the amplitudes of two carrier waves in order to convey two data streams.)

Lew explains that while this provides a speed boost, it also means IEEE 802.11ac is more sensitive to noise and other signal corruption.
“Use of 256QAM modulation requires better Error Vector Magnitude [EVM] performance in the transmitter and receiver because the constellation points are much closer together,” he says. “You can’t have a lot of noise or error in the signal if you’re to accurately figure out which constellation point you are on.” (Constellation points are marks on a constellation diagram that is a 2-D representation of a signal subject to a modulation scheme such as QAM.)

The IEEE 802.11ac specifications call for -32 dB EVM for 256QAM, compared to -28 dB for 64QAM.

“To get to those levels, you need to have a good linearity and phase noise in your system,” Lew explains.

EVM errors can come from various sources, including imperfections in the IQ modulator like gain and balance, or skew. They can also arise from phase noise or non-linearity in the power amplifier.

While some phase noise can be removed by the phase tracking in the WLAN receiver, the receiver cannot properly track fast phase changes.

Agilent Technologies has two tools that the company claims help improve EVM performance. The SystemVue W1917 WLAN Baseband Verification Library can simulate the effects of these errors on the system, and assist in optimising the design, while the 89600 VSA software can help identify the causes of EVM in the signal being measured.

Challenges ahead

New features and changes to the upcoming IEEE 802.11ac wireless standard promise to deliver on demands for greater throughput, better range and improved efficiency.

But with the advantages introduced by the new standard come several complications, which means engineers will face new challenges when creating devices incorporating the technology.

Adding to the complications is the fact that the standard has not yet been finalised. However, chip makers like Broadcom have added programmability hooks in their chip software, so they are confident that any upcoming changes will be small enough to deal with by software upgrades.

According to IEEE 802.11ac chair Osama Aboul-Magd, the new standard is an evolution of IEEE 802.11n, on which manufacturers have already cut their teeth. The hope is that the experience from the current standard will translate to the development of the new devices.

The move to 80 MHz wide channels, while boosting speed, will require the ability to generate and test wider waveforms. 256QAM, while improving spectral efficiency, means more sensitive board designs to cater for a lower tolerance for noise. Multiuser-MIMO, while helping to increase system efficiency, will also require complex work to ensure proper communication between the access point and clients.

For these reasons, Aboul-Magd expects the new features to be prioritised and introduced to the market in phases, with the mandatory features in the first products phase and optional features emerging in later phases.

The nature of the tech industry is such that with great improvements come great challenges, but there is no doubt that the expertise of engineers and careful design work will see the upcoming transition yielding great benefits for both vendors and consumers alike.