The IEEE 802.11ac is a suggested enhancement of the IEEE 802.11 specifications for wireless Local Area Network. It operates on 5GHz band and props up backwards compatibility with other 802.11 technologies on the same band such as 802.11n. The main purpose is to provide a high-throughput within Basic Service Set (BSS). The IEEE 802.11ac has some fundamental improvements in both physical and MAC layer; such as (MUâ€MIMO), and has an advanced digital communication concepts to the 802.11technology, such as space division multiplexing. These ameliorations will lead to get a maximum multi-station throughput of at least 1Gbps and a maximum single link throughput of at least 500 Mbps.
Introduction:
Nowadays, 802.11 Wi-Fi can be considered the best network access technology; therefore, it replaces 802.3 Ethernet at the edge of most Local Area Networks. It has been crucial to business adoption of "Wi-Fi Certified" products to keep up with the advancing enhancements in wireless LAN reliability, availability, speed and etc. Hence, they have to adapt client devices; such as laptops and smartphones, and infrastructure elements like access points and controllers to these improvements.
As the years have passed, IEEE 802.11b (11 Mbps), 802.11a/g (54 Mbps) and 802.11n (600 Mbps) WLANs have been migrated respectively increasing both data rates and network capacity
Today, Wi-Fi IEEE 802.11n are commonly used is a large number of enterprises providing dual-band 11n radios which operate three MIMO antennas to carry 450 Mbps. However, A lot of organizations are facing huge difficulties due to huge demand for high-throughput applications like wireless backhaul as the client growth and diversity go up.
To achieve this high speed, the IEEE has been introducing 802.11ac which is a new enhanced technology for Very High Throughput operating on 5GHz. Wi-Fi Certified draft 11ac are expected to emerge on December 2013, providing a maximum data rates above 1 Gbps. As it has been announced, IEEE 802, 11ac will be commonly used over the world by the late of 2014 or early 2015.
802.11ac is designed and optimized to coexist efficiently with existing 802.11a/n devices, with strong carrier sense, a single new preamble that appears to be a valid 802.11a preamble to 802.11a/n devices, and extensions to request-to-send/clear-to-send (RTS/CTS) to help avoid collisions with users operating on slightly different channels.
This report tends to give complete knowledge of "ac" adds to 802.11 to illustrates the most fundamental advantages of 11ac. It also discusses 11ac features that enterprises should look for. Finally, guidance is provided to know how to get the most from 11ac while protecting your company's past WLAN investments.
IEEE 802.11ac:
IEEE 802.11ac is the latest IEEE Wi-Fi standard operating only in the 5 GHz band. The main target of 802.11ac working group, which was formed in 2009, is to achieve a high-throughput much better than 802.11n. IEEE802.11ac provides a maximum multi-station throughput of at least 1Gbps and a maximum single link throughput of at least 500 Mbps. Therefore, IEEE802.11ac is known as "Gigabit Wi-Fi" and "5G Wi-Fi," and it its products expected to be available in 2013.
Gigabit Wi-Fi props up backwards compatibility with other 802.11 technologies on the same band such as 802.11n; therefore, dual-band n/ac devices are backward compatible with 802.11n. An 802.11ac access point uses beam-forming, which directs the majority of signal energy you transmit from a group of transducers (like audio speakers or radio antennae) in a chosen angular direction. The technology also supports multiuser MIMO (MU-MIMO) for up to four simultaneous data streams. 802.11ac will come to market in phases as flowing:
Phase 1
When: First half of 2012
What: Nominally 1.3 Gbps - 256-QAM, 80 MHz channels, 3 spatial streams
Phase 2
When: Late 2013, early 2014
What: Nominally 3+ Gbps - MU-MIMO, 256-QAM, 80 and 160 MHz channels, 3+ spatial streams
The next diagram shows the generations of wireless LAN standards.
Enhancements of IEEE 802.11ac:
The IEEE 802.11ac is an emerging very high throughput (VHT) WLAN standard that may achieve the PHY data rates of close to 7 Gbps for the 5 GHz band.
The notable enhancement of IEEE 802.11ac is at the physical layer. They include
Improved modulation:
IEEE802.11ac uses Orthogonal Frequencyâ€Division Multiplexing (OFDM) modulation to modulate bits for transmission over the wireless medium. It optionally allows the use of 256 QAM in addition to the mandatory Quadrature Phase Shift Keying (QPSK), Binary PSK (BPSK), 16 QAM and 64 QAM modulations. The main advantage of using 256 QAM is to increases the number of bits per subâ€carrier from 6 to 8, resulting in a 33% increase in PHY rate. The 256 QAM can only be used in high signalâ€toâ€noise ratio (SNR) scenarios for very favorable channel conditions. The support of 256 QAM will increase the maximum PHY rate that can be supported by the system, but will have no effect in typical scenarios and will not lead to any reach increase for the service. Also, supporting 256 QAM requires transmitter and receiver to be designed such that the inherent SNR (transmit and receive Error Vector Magnitude, or EVM) of the system is able to accommodate the higher constellation.
This will make the RF design of a system that supports 256 QAM more challenging. Unlike 802.11n, 802.11ac does not support the use of unequal modulation (UEQM). This means that all streams in a multiâ€stream transmission have to be modulated with the same constellation size. UEQM, by contrast, enables the system to modulate weaker streams with lower modulations, which allows for more fineâ€grained optimization of the data rate to a particular channel environment. This may be important for higher numbers of streams, especially in combination with beam forming.
Increased bandwidth
The most notable feature of 802.11ac is the extended bandwidth of the wireless channels. 802.11ac warrant support of 20, 40 and 80 MHz channels. Moreover, it is optional to use contiguous 160 MHz channels or nonâ€contiguous 80+80 MHz channels. It is a very efficient way to double the channel bandwidth (from 40 to 80 MHz); hence, increase performance in a great way. Alternatively, an 80 MHz system can use a lower number of antennas to provide the same performance as a 40 MHz system. However, this approach should be weighed against other spectrally efficient techniques that provide performance increase. In addition, in most realistic scenarios the performance is not only a function of the PHY rate, but will also be affected by interference from other networks in close proximity. Different bandwidth levels will be affected differently in an interference scenario. Also, reducing the number of antennas eliminates diversity and reduces the robustness of the transmission. These aspects will be discussed further below. ((Increased channel bandwidth: The previous versions of 802.11 standards have typically used 20 MHz channels, although 802.11n used up to 40 MHz wide channels. The 802.11ac standard uses channel bandwidths up to 80 MHz. To achieve this it is necessary to adapt automatic radio tuning capabilities so that higher-bandwidth channels are only used where necessary to conserve spectrum))
Required antenna diversity
Increasing the bandwidth enhances the performance of a single stream. If the target is to improve the PHY rate or the maximum throughput of a system regardless of QoS considerations, this may be all that is needed. One has to recognize, however, that transmission of highâ€quality content such as video has more requirements than just increasing the maximum ideal PHY rate. To ensure stable delivery of video, the number of antennas should be higher than the number of spatial streams. Diversity is a critical part of stable data delivery with QoS (see e.g. [1]). Therefore, even 80 MHz systems will have to be built using multiple antennas if they are going to be used in applications that require stable and reliable transmission of data (such as video). This narrows the cost and power advantage between a (singleâ€stream) 80 MHz bandwidth system and a (twoâ€stream) 40 MHz system.
Error correction coding
The advances in chip manufacturing technology have enabled designers to take advantage of additional levels of processing power when compared to previous implementations of the 802.11 standards. This has enabled the use more sensitive coding techniques that depend on finer distinctions in the received signal. IN addition to this more aggressive error correction codes that use fewer check bits for the same amount of data have been utilised within the 802.11ac format.
Increased Number of Streams
802.11ac allows support for up to 8 spatial streams - up from a maximum of 4 streams in 802.11n. Support for more than one spatial stream is optional, however. It is not clear whether a realâ€world, singleâ€user MIMO channel can realistically support that many streams. The increased number of streams may be most useful in combination with MUâ€MIMO.
MUâ€MIMO
MUâ€MIMO was added to 802.11ac to address the multiâ€STA throughput requirement. In MUâ€MIMO, the Access Point (AP) - or possibly another STA - transmits independent data streams to several STAs at the same time. Through preprocessing of the data streams at the transmitter (similar to what happens in beamforming), the interference from streams that are not intended for a particular STA is eliminated at the receiver of each STA. Therefore, in theory, each STA receives its data free of interference from the
transmissions that are simultaneously directed towards other STAs. In MUâ€MIMO, the spatial degrees of freedom are used to create independent transmissions to different STAs, while in singleâ€user MIMO, these spatial degrees of freedom are used to increase the throughput from AP to STA.
The complexity of MUâ€MIMO falls mostly on the AP (or transmitting STA), where the preprocessing happens. The receiving STAs only need the capability to report channel information to the AP so it can calculate the preprocessing matrices. The required channel information from the receiving STA is very similar to what is required for explicit feedback beamforming. As such, the complexity for the STA is no more than the complexity already involved in supporting explicit feedback beamforming as a receiver.
One drawback of MUâ€MIMO is that the amount of time that the medium is occupied is determined by the slowest link among all APâ€STA pairs (or, more generally, the link that requires the most time to finalize its transmission). No new data can be sent to any of the STAs until all transmissions to STAs in the MUâ€group have ended. If there is too much difference in either the amount of data or throughput going to various
STAs, this may lead to inefficient use of the wireless medium. At this point, MUâ€MIMO is a wellâ€studied concept, but practical considerations will likely defer implementation of this feature to later generations of 802.11ac products. Additional work may be needed to guarantee the efficient use of MUâ€MIMO
((MIMO and MU-MIMO: In order to achieve the required spectral usage figures to attain the data throughput within the available space, the spectral usage figure of 7.5 bps/Hz is required. To achieve this, MIMO is required, and in the case of IEEE 802.11ac Wi-Fi, a form known as Multi-User MIMO, or MU MIMO is implemented. MU-MIMO enables the simultaneous transmission of different data frames to different clients. The use of MU-MIMO requires that equipment is able to utilise the spatial awareness of the different remote users. It also needs sophisticated queuing systems that can take advantage of opportunities to transmit to multiple clients when conditions are right.))
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.
WifiMimo
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.
2.3.2 Differences Between 802.11ac and 802.11n
802.11ac has avoided the battles of 802.11n and instead has focused on extending the tremendous advances made in 802.11n to deliver the next generation of speed and robustness.
For instance, 802.11n pioneered aggregation through the selective use of A-MPDU, A-MSDU, and A-MPDU of A-MSDU (see Appendix). 802.11ac actually requires every 802.11ac transmission to be sent as an A-MPDU aggregate. This is due in part because of the intrinsic efficiency of A-MPDU and for some other reasons too (see section 2.3.5).
In a further example, 802.11ac extends the 802.11n channel access mechanism: virtual carrier sense and backoff occur on a single 20 MHz primary channel; then CCA is used for the remaining 20 MHz subchannels immediately before transmitting on them.
Given the power of A-MPDU and the 802.11n channel access mechanism, 802.11ac actually didn't need to innovate much in the MAC. Indeed, extensions to the RTS/CTS mechanism are the only new mandatory MAC feature.
802.11n does include many options with reduced value. 802.11ac takes a very pragmatic approach to them. If a "useless" option is used and affects a third-party device, then typically 802.11ac forbids an 802.11ac device (operating in 802.11ac mode) from using the option. If a "useless" option has not been used in 802.11n products or only affects the devices that activate the option, then the feature is not updated for 802.11ac but is instead "left to die."
For instance, there is no 802.11ac version of the "802.11n greenfield" preamble format. 802.11ac only defines one preamble format, which, to legacy 802.11a/11n devices, will look safely like an 802.11a preamble followed by a payload with a bad CRC. This means that legacy devices don't try to transmit over the top of the 802.11ac transmission, nor do they attempt to send a bad payload up the stack.
802.11n introduced a "reduced interframe spacing," which reduces overheads between consecutive transmissions, but experience has shown that A-MDPU solves much the same problem, but even more efficiently. 802.11ac devices operating in 802.11ac mode are not permitted to transmit RIFS (as of Draft 3.0).
802.11n features that are not updated for 11ac (or explicitly forbidden for 802.11ac devices operating in 802.11ac mode) include all the 802.11n sounding options, including extension LTFs, the calibration procedure, antenna selection, PCO, LSIG TXOP protection, unequal modulation, 4Ã-3 and 3Ã-2 STBC modes, MCS32, and dual CTS protection. If you don't know these terms, then no problem, because almost certainly you'll never need to know them.
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Conclusion
802.11ac has the potential to provide the next generation in highâ€throughput wireless systems. To fully
realize this potential, 802.11ac systems will have to go beyond a minimal implementation that simply
exploits the wider bandwidth channels available to this technology. Any new system will be measured
against currently available 802.11n systems that already implement MIMO processing with spaceâ€division
multiplexing, LDPC, STBC, beamforming, multiple streams and a variety of other PHY, MAC and coexistence enhancements. Firstâ€generation 802.11ac systems must be evaluated in light of this comparison. As a minimum, such systems would have to match the feature set that is already provided by currentâ€generation 802.11n. Preferably, any nextâ€generation system would include some truly nextâ€generation features (such as MUâ€MIMO) in addition to the channel bandwidth increases that are readily available in this new technology. The bandwidth increase of 11ac is currently a concern in situations with limited bandwidth resources. Frequency is a scarce resource that needs to be used as efficiently as possible. Exploiting channel diversity by using a higher number of spatial streams allows more efficient spectrum use than simply doubling the bandwidth of the transmission. Channel and antenna diversity, therefore, remain important requirements, even for systems that are capable of wider bandwidth. It is believed that a 4x4 system with a maximum number of spatial streams and MUâ€MIMO will be required, at a minimum, in order for 11ac to fully realize its potential. Such a system would provide higher bandwidth in sparsely populated networks, while providing QoS, good performance and coexistence in denser network environments.
