Lets take a look at the technology that literally commanding all and any form of wireless communications OFDM without requiring any prior electronics knowledge.
Orthogonal Frequency Division Multiplexing is the most sought modulation technique in today's cutting edge wireless either it be 802.11 (WLAN), 802.16(WiMax), 3GPP 36 series (LTE)..
Single Carrier Modulation:
In the traditional communications we make use of a single Tx antenna and a single Rx Antenna. All the incoming data is modulated using a single carrier and sent OTA, at the Rx side its is demodulated and data after removing the carrier is given out.
Multi-Carrier Modulation:
FDM:
But in order to gain high throughput a new scheme is introduced. Where in we divide the channel in to different sub channels and assign a carrier to each sub-channel for modulation known as sub-carrier and finally multiplex all the carriers+data and send using the single Tx Antenna.
But in order to avoid interference (inter-symbol interference) we use a guard spacing between the sub channels causing in bandwidth wastage.
OFDM:
In order to save the wastage and use it effectively we choose the sub-channels and sub-carriers such that they are orthogonal to each other meaning they don't interfere with each other. So we can skip the guard spacing between the sub-channels and save bandwidth.
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| FDM Vs. OFDM |
Truly speaking it should be termed as FDMO, as you will see why just in a moment. lets understand each word in detail in the acronym.
Frequency Division (FD):
Instead of modulating all data with one carrier, divide the bandwidth in to equally separated sub-channels and assign a carrier to each sub-channel called sub-carrier.
Multiplexing (M):
As we have multiple sub-carriers but a single Tx antenna we need to map all the sub-carriers rather we need to multiplex all of them and send OTA. We make use Parallel to Serial converter for this.
Orthogonality(O):
We all know from the high school maths that orthogonality means "product of slopes = -1" meaning one signal will not overlap with the other.
In a similar context in the time domain a sub channel is represents as a rectangle which in a frequency domain converts to a "sinc" wave, now if we multiplex the "sinc" waves of all sub carriers sub channels it will look something like the below.
Now at the Rx side we can decode this wave because of the orthogonality, see the "arrows" they represent the frequency at which impact of all other sub carriers is zero, so we get to Rx only that particular sub-carrier we are interested in.
*2: By using an IFFT for modulation we implicitly chose the spacing of the subcarriers in such a way that at the frequency where we evaluate the received signal (indicated as arrows) all other signals are zero.
Advantages:
- Saves bandwidth
- Carries more no of data than a conventional carrier.
Disadvantages:
- Highly depends on the orthogonality achieved, need a good sync on Tx and Rx and also no multipath effect.
- High PAPR: Peak to Average Power Ratio.
The reason for high PAPR is that, as we use many sub-carriers with same bandwidth, the sum of peak power of all the sub-carriers varies instantaneously.
OFDM exhibits peaks whose power strongly exceeds the mean power: the signal is said to have a high PAPR. This prevents use of high-efficiency amplification devices (High Power Amplifiers, HPA), which can cause nonlinearities reuslting in in-band distortion and increases Out-Of-Band Radiation (OOBR), inter Symbol Interference and a high Bit-Error-Rate (BER).
Mitigations
1.
a) Pilot: Instead of using all the sub-carriers/channels for data, use some selected sub carriers to send with zero-data so that the Rx can tune itself with the Tx. These are kind of training subcarriers.
b) Cyclic Prefix (CP): In order to overcome the MultiPath effect, OFDM implements a clever technique of CP where in we first need to find the "Doppler Spread" of the multipath channel and then copy that amount of time from the data signal from the last to the first. So that even if that signal undergoes the multipath fading, we still have the data intact only the copied part is lost.
Doppler Spread: The amount of delay induced by the multipath channel to the 1st signal causing to overlap with the 2nd singal leading to infamous inter symbol interference.
2. There are many methods but below are famous ones
- Clipping and Filtering
- Peak Windowing
References:
Note: All the pictures here are taken from the references and some additional resources. The author do not claim any copy right.
