OFDMA (orthogonal frequency-division multiple access), a technology in Wi-Fi 6
What is OFDMA?
Beginning to understand OFDMA starts with understanding FDM. FDM is Frequency division multiplexing which means sending one or more signals on the same channel with each signal in unique range of frequencies in the given bandwidth. FDM allows multiple users to share one single link or channel without interference. It is efficiently able to divide channel between different users. When implemented in this way, different signals travel separately and simultaneously without interference through the same channel and can help optimize the usage of channel for sending more than one signals.
The next evolution in the process of frequency multiplexing can be OFDM. OFDM is Orthogonal Frequency Division Multiplexing. The main difference here is OFDM allows overlapping of subchannels unlike the predecessor where fixed bandwidths are allocated. This is possible by making the signals orthogonal to each other. Orthogonal means signals are multiplexed in a way that only one signal is at its peak at any given point even though there are no guard bands between different channels. The successor in this line is OFDMA or orthogonal frequency-division multiple access. It is a technology utilized in Wi-fi 6 and it establishes independently modulating subcarriers within frequencies. Allowing back and forth transmission between multiple clients.
Difference between OFDM and OFDMA
While both OFDM and OFDMA have similar function, the main difference between OFDM and OFDMA lies in the number of users. Both OFDM and OFDMA allows multiple subcarriers in given bandwidth. However, in case of OFDM, the entire bandwidth is allocated to a single user. OFDMA on other hand as the name suggests has ‘multiple access’ or simply speaking distribution between multiple users.
Characteristics of OFDMA
These are defined industry norms or standards prescribed by IEEE in context with the Wi-fi standards congruent with OFDMA. OFDMA simply speaking makes the application of these standards possible in the Wi-fi version. It is a IEEE 802.11ax standard.
Backward compatible: which means the earlier wi-fi standards and channels will work.
Better network performance: Optimized orthogonal subchannel division allows for better efficiency of channel.
Connect more devices simultaneously: Efficient usage allows for a broader availability of devices on the hotspots.
OFDMAs are especially useful in dense device environments with higher SLAs This is primarily done by increasing the amount of throughput, optimizing the spectral efficiency, and reducing the power consumption of the overall devices of the network. 802.11ax APs can utilize this mechanism to achieve 4x increase in network throughput as compared to the earlier version of OFDM. This can also be seen in a different way where we can say that older versions of Wi-fi are client-centric. While OFDMA is AP-centric where AP stands for Access Point. OFDMA thus divides channel into subchannels called Resource Units (RU). It can determine the allocation by doing dynamic allocation of resources as per requirement of the channel and the number of users.
The difference between OFDM and OFDMA is mainly that of single-user and multiple-user throughout respectively. The division of channel with the help of IFFT or inverse fast Fourier transform mathematically speaking. Statistically speaking, OFDMA introduces longer symbol time of 12.8 microseconds as compared to 3.2 microsecond of OFDM and in relation will have subcarrier spacing change from 312.5 KHz to 78.125 KHz.
Types of Subcarriers:
There are three main types of subcarriers in OFDMA:
Data Subcarriers: These subcarriers will use the same modulation and coding schemes (MCSs) as 802.11ac and two new MCSs with the addition of 1024-QAM.
Pilot Subcarriers: The pilot subcarriers do not carry modulated data; however, they are used for synchronization purposes between the receiver and transmitter.
Unused Subcarriers: The remaining unused subcarriers are mainly used as guard carriers or null subcarriers against interference from adjacent channels or sub-channels.
The other benefits of OFDMA are that it provides multi-user diversity. OFDMA allows different users to transmit over different parts of the frequency spectrum known as traffic channel. Hence different users see different channel qualities. Hence deep faded channel for a user may still be useful for the others. It also simplifies receiver as it eliminates intra-cell interference which avoids multi-user detection of CDMA type.
There are also a few disadvantages of OFDMA most of which concern the complexity. The permutation and de-permutation rules of subcarriers for allocation and deallocation to subchannels are complex. This makes transmitter and receiver algorithms complex for data processing/extraction unlike OFDM. OFDMA has higher PAPR (Peak to Average Power Ratio). Hence large amplitude variations lead to increase of in-band noise. Moreover, it increases BER (Bit Error Rate) when OFDMA signal passes through the RF amplifier having nonlinearities. Co-channel interference is more complex compared to CDMA technique.
