Specifications

Fig. 3.3: Modulated signal spectrums. (left) π/4-DQPSK with R=17.3 kBaud, (right) 16-DEQAM with

R=17.3 kBaud.

As for the linear modulation techniques, the differentially encoded formats π/4-DQPSK, D8PSK and

16-DEQAM have been selected and tested mainly due to their low modulation envelope variations and

inherent robustness against negative effects of signal propagation through the narrowband radio

channel.

The 2CPFSK and 4CPFSK have been selected from the nonlinear modulation class. There is one

particular parameter of high importance essentially influencing the characteristics of these modulation

formats and that is a modulation index. It expresses the relation between the modulation rate and the

maximum frequency deviation according to simple rule (1.1)

2∆f

h = ————— , (1.1)

R(M−1)

where R is the modulation rate, M is the number of modulation states and Δf is the maximum frequency

deviation representing the outermost symbol frequency position. The selection of the modulation index

in most practical applications of narrowband LMR has been driven by compromising requirements

between the modulation rate, receiver sensitivity and adjacent channel power level. Its value usually

converges to 1/M with a well known example of MSK, particularly GMSK where M=2, thus h=0.5 as

the lowest value needed to maintain an orthogonal signaling. In order to compare the modulation

formats at the same spectrum efficiency we also measured the properties of 2CPFSK and 4CPFSK

modulations with very low modulation index resulting in use of high symbol rate of 17.3 kBaud.

The examples of transmitted signal spectrums can be seen in Figure. 3.1 to Figure. 3.3. It is interesting

to note the degradation of the signal spectrum with increased symbol rate in case of 2CPFSK and

4CPFSK that implicitly points out that the assigned bandwidth is not used effectively. It can be seen

that the significant amount of the signal power is concentrated within the close vicinity of the carrier

frequency and thus it results in poor ratio between the occupied signal bandwidth and the noise band-

width of radio receiver (Table 3.1).

Data speed and Modulations