Which data rate, deviation and modulation index should I use?
the recommended modulation is 2GFSK, so from now on, let's consider only this type of modulation.
The occupied bandwidth of a 2GFSK signal can be estimated based on the following formula:
The modulation index can be calculated in the following way:
According to the formula for occupied bandwidth, the OBW value is higher with higher data rate and deviation. If one is using lower OBW, the receiver BW can also be set lower, which reduces the noise level in the receiver, and this way improves sensitivity. Hence it seems a good idea to transmit with as low data rate as possible, but applications often require a minimum data rate to send the necessary information in the available time slot. The data rate and the crystal tolerance should be chosen to stay within the allowed channel under extreme conditions as well according to the related regulation standard. Beside this, depending on the crystal accuracy the possibility of frequency offset between the transmitter and the receiver can grow. The right choice for data rate and crystal type depends on several parameters: carrier frequency, carrier power, channel bandwidth, modulation index, and of course the data rate and the crystal type cannot be considered separately. A higher frequency signal introduces higher absolute frequency error with the same type of crystal. A high power application can violate the modulation bandwidth regulation easier, especially for narrow band channels (e.g. 25 kHz). Also, the modulation index determines the occupied bandwidth: higher modulation index introduces higher bandwidth.
To investigate the modulation index, let’s consider 3 typical instances: H = 1, H = 0.5, H = 2. If we choose the desired data rate for our application, the deviation determines the used modulation index. Also, the deviation determines the distance between the transmitted symbols: higher modulation index introduces more distance between symbols. Simultaneously, a higher data rate introduces higher occupied bandwidth, which causes that wider bandwidth is necessary in the receiver, this way a higher thermal noise will appear and the sensitivity will be worse. Thus, by increasing the deviation, the distance between symbols will also grow, which improves sensitivity, but on the other side, the occupied bandwidth will be wider, which causes sensitivity loss. These two effects seems to compensate each other, but there is an optimal choice, at H = 1. This is the value where the receiver sensitivity is at the optimal level, thus this is the recommended modulation index.
To sum up, the recommended modulation index for 2GFSK modulated signals is H =1. The data rate and the crystal type should be chosen based on the requirements of the application to meet the standard regulations. If the data rate is chosen, the H =1 modulation index determines the necessary deviation.