The new 802.11ac standard from IEEE, is currently penetrating wireless network markets with its new features and technology. Watson (2013), Public Marketing of Meru Networks, claims the 802.11ac standard will be able to provide a maximum client data rate per radio of 1.3 Gbps. How does it manage to achieve those higher bandwidths compared to the last standard? One part of this technology that makes it possible is the complex modulation technique that 802.11ac employs which is 256-QAM. This modulation technique is a combination of phase-shift keying and amplitude-shift keying. QAM itself stands for quadrature amplitude modulation. In order to understand how 256-QAM could give higher bandwidth we need to know how QAM works.
The QAM works by modifying either amplitude, phase or both. For example in 4-QAM (commonly known as QPSK) the only attribute which is modified is the signal phase. Thus, it gives four combinations which are 00, 01, 10, 11 or we can say each wave can carry 2 bits.
4-QAM Constellation Map
As we can see from the figure above, it has the same amplitude (one circle) but four different phases. Meanwhile in 16-QAM, it modifies both amplitude and phase that will result in 16 combinations. this means that each wave can carry 4 bits (24 = 16). By increasing the complexity of phase-amplitude combinations, it will also increase the number of bit it carries. Since 802.11ac employ 256-QAM, it means that each wave could carry 8 bits (28 = 256) which is faster than 16-QAM.
16-QAM Constellation Map
This means that the higher the modulation the better the carrier signal is since it can carry more bits. Ian (n.d.) explained that the more complex the modulation degree is the less resilient the signal will be to noise and interference. This is true because in higher levels of modulation, signal is packed and the boundary between one amplitude or phase with another is very small and little interference might cause the receiver to misinterpret the signal. However this problem can be easily overcome by increasing the power of signal which means increasing the amplitude so it will enlarge the boundary between amplitude and phase which in turn will make the signal more resilient. However this solution also brings a disadvantage; since we increase the power of the signal it means that we will need a lot more power for transmitting the signal. By doing that it will drain the battery power quickly if it were to be used in mobile devices such as smart phones or laptops.
Ian (n.d.) explained another method to address the problem by using dynamic adaptive modulation, which is provided in most recent wireless devices. This type of system will examine the surrounding wireless environment to decide the type of modulation it must use to send the data. If it is possible to use the highest modulation, it will. If there are too many noises and interference the transmitter will then use a lower modulation that is more resilient to send the data.
In summary, 802.11ac is the network standard that will bring wireless networks to the next level, but in its development it has faced many challenges since it uses higher level modulation 256-QAM. It has to be able to maintain the signal resilience against noise and interference as well as maintaining power usage as minimally as possible. There are also other features of 802.11ac which is not covered in this writing that can address problem described above. The resource is a good start to gain more information about other 802.11ac features.
Poole, Ian. (n.d.). Comparison of 8-QAM, 16-QAM, 32-QAM, 64-QAM 128-QAM, 256-QAM, etc. Retrieved from http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/8qam-16qam-32qam-64qam-128qam-256qam.php
The New 802.11ac Standard: 256-QAM Explained [Video file]. Retrieved from http://www.youtube.com/watch?v=NbrRGBRk5fM
Watson, Richard. (2013, July). Understanding the IEEE 802.11ac Wi-Fi standard: preparing for the next gen of WLAN. Retrieved from http://www.merunetworks.com/collateral/white-papers/wp-ieee-802-11ac-understanding-en