EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: fonograph on August 26, 2018, 03:00:58 am
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Is there some limit where no matter how much power we got,the bitrate cant be increased any further for fixed bandwidth channel?
I read that wireless communication bitrate can be either increased by increasing channel bandwidth or power.This makes me wonder,is there any limit to increasing bitrate with power? Increasing bitrate with bandwidth is easy to understand,infinite bandwidth = infinite bitrate,but what about the power method?
Like... imagine a 1 Hz channel,how could you ever send 1 Gb/s through such narrow channel even if you had all the energy of entire visible universe at your disposal? What kind of modulation would be needed to achieve that? When I think in terms of OFDM,bunch of sub carriers with phase shift keying,lets say binary and gaussian filtered so its GBPSK... then at 1000 symbol rate per sub carriers,it would take 1 million carriers to get that data throughput... how the hell can there ever be million carriers in 1 Hz changing thousand times a second? The inter-symbol interference would be gigantic!
I dont see how trowing bunch of power on it would solve that issue... or am I wrong? Did some math nerd come up with some borderline magic way to make it work? Or is there some limit for given bandwidth that you cant cross no matter how much power you got... how is it?
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It is not about the power, the power only affects the distance of reception. If your transmitter (amplifier) and receiver have huge (infinite) linearity and dynamic range, you can represent your signal in a number of discrete steps changing the value once a second. You can transfer one byte per second with 256 levels while maintaining 1 Hz bandwidth.
OFDM is just a way to split the signal into fewer narrow bands where transmitter is more linear, since it is hard to design linear wide band amplifiers.
Basically all the things we have to invent for communications are due to limited linearity and dynamic range of the amplifiers.
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Shannon showed that the maximum capacity of a channel is given by:
$$C = B \log_2\Bigg(1+\frac{S}{N}\Bigg) $$
Where \$C\$ is the capacity in bit/s, \$B\$ is the bandwidth in Hertz, \$S\$ is the signal power, and \$N\$ is the noise power. In most case we are limited by thermal noise, and in that case we can only increase SNR by increasing signal power. This is possible with better and more efficient antennas/filters, or starting out with more power in the first place.
The thing is that if our signal power is much, much higher than the noise power, we might be limited by the noise power that is actually transmitted by the transmitter. This noise can be due to the noise and distortion in the mixers and amplifiers, phase noise in the synthesizers, etc.
So getting higher signal power will help, but at some point it gets harder an harder. In addition, the SNR part is in the \$\log_2 \$ term, in other words, you get diminishing returns. Usually, it is easier to just increase the bandwidth instead - which is (one of the reasons) why people are so interested in the 60 GHz and other millimeter wave bands for communications.