Also if we have a Power constraint then FSK modulations schemes look
promising since they require lower Eb/No to obtain the same average BER.
Let us take a deeper look into the BW-limited region. Recall we have previously discussed the fact
that 16-QAM had better SNR (or Eb/No) performance requirements than 16-PSK. This is clearly
shown in the plot, where notice their normalized capacity of (bps/Hz) are the same for both modulation
schemes. Now let??™s discuss the move from BPSK to QPSK; here we notice they both require ~6.8 dB
of Eb/No to maintain the target BER. However, we notice our normalized capacity doubled from 1 to
2 bps/Hz. Also we discussed the BER of QPSK was identical to that of antipodal signaling. Besides
the receiver complexity, the advantages far outweigh the disadvantages for this particular choice of
modulation schemes.
Let??™s turn our attention into the Power limited region. This shows us that the FSK family of modulation
schemes is spectrally inefficient. Recall we present the orthogonality principle in the FSK discussion;
we see as we move from 2 level to 4 level FSK, our normalized capacity remains at 0.5 bps/Hz.
The reason is we doubled our data rate when going from BFSK to 4-level FSK; however, we also doubled
our occupied BW, thus keeping the ratio of the two the same.
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