What’s New in Wi-Fi 6: Long OFDM Symbol

 
What’s New in Wi-Fi 6: Long OFDM Symbol
What’s New in Wi-Fi 6: Long OFDM Symbol
2019-11-29 08:35:05 - last edited 2019-12-06 01:43:02
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What’s New in Wi-Fi 6: Long OFDM Symbol

What is an OFDM Symbol?

OFDM is a technique for transmitting large amounts of digital data over a radio wave. This technology works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to a receiver.

Sounds a bit technical? Here’s an analogy:

Imagine If you’re a company shipping products via freight. You might only have two options: hiring a big truck or a bunch of smaller ones. Both methods can carry the same amount of product (or data in the case of OFDM). But let’s say there’s an accident, the goods in the larger truck are more likely to knock against each other and suffer damage than those in smaller, individual trucks.

When it comes to Wi-Fi, these three smaller trucks would be signals called sub-carriers in an OFDM system. The sub-carriers transmit signals orthogonally, which minimizes intersymbol interference.

OFDM was first implemented in 802.11 standards. The diagram below shows the OFDM symbol in 11g/n/ac. 

 

The red portion represents the preamble and the yellow portion is the “useful” symbol time. OFDM works by transmitting data for 3.2 µs, waiting during the guard interval and repeating the process again. If the guard interval is 0.8 µs, it means that OFDM has a 20% overhead rate and 20% airtime is wasted. So, why do we need the guard interval? Can’t it be 0.1 µs to prolong the time for data transmission?

Nope.

The signal directly transmitted to the receiver will get reflected off walls, ceilings, or other obstacles and cause the multipath effect.

What’s the multipath effect?

Multipath means that a single signal from an RF transmitter can travel multiple paths to arrive at a receiving antenna. On its way, the RF signal encounters objects that reflect, refract, diffract, or otherwise interfere with the signal. When an RF signal is reflected off an object, multiple waveforms are created.

When the different waveforms combine, they cause distortion of the desired waveform and affect the decoding capability of the receiver. So how much multipath can be tolerated is dictated by how wide the red portion (guard interval) is. If there’s no guard interval, or it’s too short, the reflected radio waves will interfere with new signals and affect the quality of signal transmission.

Picture data transmission as a train. You need to space out the departure of two trains (i.e., transmissions) to avoid possible crashes (or in the case of OFDM: server damage to the data). If the interval is too short, there’s a huge possibility that the second train may rear-end the first one. If symbols collide with each other, the data carried by symbols will be lost or damaged.

Long OFDM symbol—The Latest Evolution of Wi-Fi

Increased Speed

Wi-Fi 6 keeps the same channel bandwidth as Wi-Fi 5 (11ac), such as 20 GHz, 40 GHz, and 80 GHz, but the size of the subcarriers has been divided by 4—going from 312.5 KHz to 78.125 KHz wide. In the time domain, this translates to a 4× longer OFDM symbol.

This increase—from 3.2 µs to 12.8 µs—means the proportion of a symbol’s payload is much larger. This increases the frequency domain efficiency and capacity.

 

802.11ac (Wi-Fi 5)

802.11ax (Wi-Fi 6)

Number of Data Subcarriers

234

980

Symbol Duration (μs)

3.6 (3.2 symbol + 0.4 GI)

13.6 (12.8 symbol + 0.8 GI)

Valid Data Subcarriers Rate

65

72.05882

Speed Improvement

 

1.108597

 

Expanded Wi-Fi Coverage

Meanwhile, the GI (Guard Interval) increases from 0.4 µs to 0.8 µs—or even 1.6 µs. This better reduces the multipath effect and boosts Wi-Fi coverage.

For simplicity’s sake: If you stand on top of a hill and shout out “I love you!”, the person on another hilltop wouldn’t hear you clearly. The echoes reflecting off the hillsides interfere with the original shout, making it harder for human ears to identify.

But if you do it slowly—shouting out words only when the other person hears the previous word clearly, maybe they’ll hear “love” twice – the original sound and an echo. This would ensure the accurate delivery of the whole sentence.

This analogy makes it easy to figure out the reason behind the change of GI. The signals coming from afar are more likely to get reflected off of buildings, trees or hills. The multipath effect accumulates as the reflected signals increase.

So how can we possibly reduce the multipath effect and ensure the transmission quality of distant Wi-Fi signals? We can double the guard interval to make it much more tolerant to multipath reflections.

Feel it in our TP-Link Wi-Fi 6 Products

All of these features can be found in TP-Link’s newly launched Wi-Fi 6 router series: Archer AX11000, Archer AX6000, Archer AX50, and Archer AX10—all sporting ultra connectivity and high efficiency. The long OFDM symbol increases wireless network rates and provides robust connectivity. For indoor environments, our Wi-Fi 6 routers enable short GI (0.4 µs) by default and increase the speed by almost 10%. Also, the short GI button can be switched off if the multipath effect is severely affecting signal transmissions.

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