Baud rate/Capacity/Spectral Efficiency

Modern optical communications would often use the term baud rate , capacity, modulation and spectral efficiency . Let's take a look at the relationship and meaning:-

By Baud rate we mean maximum number of symbols which can be transmitted per unit of time .

By Modulation we mean encoding the bits into symbols using a carrier wave . So when we say we use 8qam as a modulation we mean to say we encode 6bits/symbol .We use M-ary modulation so 2^3 equates to 8 and since we use dual polarization we effectively have 6bits/symbol

By spectral efficiency we mean bits/sec/Hz , so lets say if we transmit 10gbps in 50 Ghz spacing the spectral efficiency would be 0.2 bits/sec/Hz , if we transmit 10gbps in 100Ghz spacing the spectral efficiency 0.1bits/sec/Hz , so as we can see the spectral efficiency is reduced .

Now finally the relation Capacity = Modulation*Baud rate

So let's cay we want to calculate the capacity which can be transmitted if we have QPSK modulation with 64Gbaud

As per formula QPSK has 4bits/symbol which equates to 4*64 which equates to 256GBPS, considering 20 to 25 percent FEC (obviously depends on the type of FEC being used) effectively we can transmit 200GBPS