Which modulation (OOK, 2FSK, 2GFSK, 4FSK, 4GFSK) should I use?
Let’s compare the above listed modulation types in pairs.
1. OOK vs. FSK
OOK is a digital amplitude modulation type, which represents 2 different states: the presence or absence of the carrier based on the symbol to be transmitted. Contrarily, for FSK modulation the amplitude of the transmitted signal is fixed, but the frequency can be different based on the given symbol.
The consecutive on-off switching of the PA for OOK modulation causes that the bandwidth increases compared to FSK modulation using the same data rate, this way FSK is a more spectrally efficient modulation. The wider bandwidth makes it necessary to apply a wider filter in the receiver, which increases the noise level and this way the sensitivity will get worse. If the occupied bandwidth is the same for OOK and FSK modulation, the sensitivity is similar (but in this case OOK modulation has lower data rate to have the same bandwidth as FSK). Beside these, OOK modulation is more sensitive for fading, since the information is carried only by the amplitude. Along with this, FSK modulation is more sensitive for the frequency offsets between the transmitter and the receiver.
To sum up, FSK modulation is more spectrally efficient so has better sensitivity and is less sensitive for fading. OOK modulation is less sensitive for the frequency inaccuracy, and thus is commonly used in applications where the frequency accuracy can not be guaranteed.
The above figure shows the spectrum of an OOK modulated signal with 40 kbps data rate and a 2FSK modulated signal with 40 kbps data rate and 20 kHz deviation.
2. FSK vs. GFSK
FSK modulation creates high level spurious contents (at integer multiples of the symbol rate) as well as relatively high side lobes on the transmitter side, which can cause regulation standard violations. A Gaussian filter can be applied to the symbols before creating the frequency modulated signal to suppress these spurs and side lobes by smoothing the baseband signal. This way the bandwidth can be slightly reduced, but the distance between symbols will decrease which causes a slightly worse receiver sensitivity (~0.5 dB) for GFSK modulated signals. Still, GFSK is a generally used modulation type, since it reduces spurious contents on the transmitter side significantly and the loss in sensitivity is negligible.
The above figure shows the spectrum of a 2FSK and a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation.
3. 2(G)FSK vs. 4(G)FSK
A typical application of 4(G)FSK modulation is transmitting with the same data rate but occupying only half the bandwidth of the 2(G)FSK signal. For example, 2GFSK, 40kbps data rate, 20kHz deviation (OBW = 80 kHz) and 4GFSK, 20ksps symbol rate (= 40 kbps data rate), 10 kHz outer deviation (= 10/3 kHz inner deviation) (OBW = 40 kHz). The smaller bandwidth introduces lower noise level in the receiver, this results sensitivity improvement. On the other hand, the deviation on inner symbols will be lower, which causes sensitivity loss. The two effects compensate each other, but the latter one has a stronger effect on sensitivity, so eventually in this case 4(G)FSK will have slightly (~2 dB) worse sensitivity than 2(G)FSK.
To have the same occupied bandwidth for 4(G)FSK as 2(G)FSK, we need to use higher data rate. For example, 2GFSK, 100kbps data rate, 50kHz deviation (OBW = 200 kHz) and 4GFSK, 100ksps symbol rate (= 200 kbps data rate), 50 kHz outer deviation (= 50/3 kHz inner deviation) (OBW = 200 kHz). This way, the occupied bandwidth will be the same, but the (inner) deviation will be less for 4(G)FSK, eventually this results sensitivity degradation (~5 dB).
For more details on sensitivity difference between 2(G)FSK and 4(G)FSK modulation, refer to http://community.silabs.com/t5/Wireless-Knowledge-Base/4GFSK-vs-2GFSK-sensitivity-on-Si446x/ta-p/144255.
To sum up, 4(G)FSK is generally used to transmit with the same data rate in half the bandwidth of 2(G)FSK, or to occupy the same bandwidth using a higher data rate. In both cases 4(G)FSK will have worse sensitivity.
The above figure shows the spectrum of a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation and a 4GFSK modulated signal with 20 ksps symbol rate and 10 kHz outer deviation.
Based on the above explained reasons, our recommendation is 2GFSK modulation, since it is more spectrally efficient than OOK or 2FSK, the Gaussian filter suppresses the spurious contents and side lobes on the transmitter side significantly while the receiver sensitivity is just slightly worse than 2FSK and a few dB better than 4GFSK.
The appearance of the spurs depends on the modulation index. In the last spectrum plot, the modulation index for the 4GFSK signal is 1/3 which is quite close to 0.5, the modulation index of MSK. MSK has continuous phase transitions, this way these spurs don't appear. On the other hand, the lower modulation index and lower deviation on inner symbols cause sensitivity degradation compared to 2GFSK.