3 Advanced aspects of optical modulations in achieving high bit rates and immunity to signal degradation
3.7 OFDM – multicarrier modulation – parameters, benefits and limits
Parameters
- Available bandwidth B is split into N sub-channels.
- The width of each sub-channel is then Δf = B/N.
- Modulation rate is Δf = 1/T , where T is duration of one symbol.
Main benefits
- Spectral efficiency gives as the ratio of transmission rate and bandwidth
- By increasing the number of sub-channels, while keeping the same overall transmission rate, one can potentially reduce the modulation speed in particular sub-channels and extend the symbol duration.
- It allows reducing Inter-Symbol Interference (ISI)
Disadvantages
- It is necessary to maintain and monitor spacing between sub-channels
- In case the spacing between adjacent channels is not constant – this case is known as jitter – the principle of orthogonal channels is affected and as a result, Inter-symbol Interference (ISI) and Inter-Channel Interference (ICI) are observed.
- The above can lead to increased bit error rate (BER).