Specifications

to the extreme adjacent channel transmitted power (ACP) attenuation requirements, and inherent ro-

bustness against channel nonlinearities. Relatively simple implementation of non-coherent demodulators

and synchronization algorithms also significantly contributes to the efficient channel usage, especially

in packet-based switching networks. The systems thus maintain good power efficiency while the

spectral efficiency reaches compromising values not exceeding 1 bit/s/Hz.

3.1.1. Digital modulation for narrowband channel

The prime classification of the digital modulation techniques into a nonlinear (or exponential) and linear

modulation class is based on the way how the modulated signal has been generated. The complex

modulation envelope of the linearly modulated signal such as M-PSK, M-QAM etc. can be described

by a linear superposition of the properly filtered modulation impulses weighted by the information

symbols. In case of the nonlinear modulation techniques, this general rule is valid only for the modulation

signal which modulates the phase of the fundamental carrier signal. Thus the modulation process itself

is nonlinear, exponential. The M-CPFSK in this case is recognized as a general class of nonlinear or

exponential digital modulation with a continuous phase change.

3.1.2. Adjacent channel power and spectrum efficiency

The adjacent channel power or adjacent channel interference (ACI) is that part of the total output power

of a transmitter under defined conditions of modulation, which falls within a specified pass-band centred

on the nominal frequency of either of the adjacent channels. This power is the sum of the mean power

produced by the modulation, hum and noise of the transmitter. Adjacent channel power is usually ref-

erenced to the unmodulated carrier power [1]:

For a channel separation of 25 kHz, the adjacent channel power shall not exceed a value of 60 dB

below the transmitter power without the need to be below -37 dBm.

It is interesting to note that, until 07/2007, the standard strictly demanded the adjacent channel power

ratio of -70 dB.

The ACP parameter is particularly important in LMR systems, since it influences the density of the radio

channels that can be used in a given area. Its value originated in the use of the traditional analog fre-

quency modulated (FM) radio systems. Ironically, it was one of the main limitations for why those systems

were – for many years – not able to utilize spectrally more efficient modulation schemes. The problem

in this case is that all the advanced multi-level modulation techniques such as M-PSK, M-QAM, OFDM,

CDMA or FBMCM have one negative property and that is a non-constant modulation envelope.

Fig. 3.1: Modulated signal spectrums. (left) 2CPFSK with R=10.4 kBaud, modulation index h~0.6.

(right) 2CPFSK with R=17.3 kBaud, modulation index h~0.2. 30 dB attenuator used in series.

Data speed and Modulations