OFDM in WLAN: 802.11n: Similar to 802.11a and 802.11g with minor changes

In the previous article we have discussed about OFDM in 802.11 a/g, now let's take a look at how OFDM works in 11n. 

OFDM has little to do with the drastic 11n rate boost, MIMO is the key player there. Lets leave the MIMO part aside, its a candidate for next article. As far as OFDM is concerned it will work per antenna, but the basic concept per antenna is the same as 11g with few changes.

20 MHz:  This is same as 802.11 a/g but 

a) With an extra 4 sub carriers.

In 802.11a/g we use 48 sub-carriers for data and 4 sub-carriers for pilot. But in 11n we make use of an extra 4 sub-carriers from the reserved for data sub-carriers.So this results in increased throughput.

b) Short Guard Interval (Optional)

we have short guard interval introduced in 11nIn the previous article we have take the guard interval as 1/4 of symbol time, now in 11n they have still decreased the guard interval to 1/8 of the symbol time. So it comes down to,                          3.2us *1/8=0.4us 

Lets take an example to understand:

a) Full Guard Interval

20MHz/64=0.3125; 1/0.3125 =3.2us + 1/4 * 3.2 us=4 us

b) Short guard Interval

20MHz/64=0.3125; 1/0.3125 =3.2us + 1/8 * 3.2 us=3.6 us

Note: Important point to note here is that while doing the calculations for the FFT period we have considered the maximum carriers 64 not the used carriers, so the above calculation of 3.2us holds good in spite of increased sub carriers.

40 MHz: 

Its same as 20MHz but the process repeated in both primary and secondary channels.

So as derived above the total no of sub-carriers in  a 40MHz channel is 52*2 (primary and secondary) + 4 (extra).

The reason for these extra 4 data sub carriers is that we don't need that many (8) pilot sub-carriers combined for both the channels, so we converted some of them to data sub carriers.

Data Sub-Carriers = 52 *2 = 104

Pilot sub-carriers  = 4 + 4  = 8

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Total Sub-carriers = 112 with 8 pilot sub-carriers.

Now out of that if remove 2 sub-carriers from 8 and convert them to 2 and add 2 more (why?) 

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Data Sub-carriers = 104+2 (from pilot)+2 (extra)=108

Pilot sub-carriers = 4-1 + 4-1=6

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Total sub-carriers = 108+6 =114

I am not sure of the exact rationale behind adding some number of sub carriers  but its safe to attribute that to the evolution, as the IEEE std progresses they are trying to make use of all the reserved sub-carriers in that process.

With the same logic in the previous article we can similarly derive the rate calculations for the above 20MHz and 40Mhz as well. The above tables from the IEEE 802.11-2012 will help you.