From the very advent of the radio transmission, it was evident that a radio device should not only use its occupied channel bandwidth effectively, but, in addition, should also avoid any unnecessary interference with other systems. Since then the frequency spectrum had been proving its importance and has become a scarce resource nowadays.
The narrowband radio devices under consideration are specified mostly by the European standard ETSI EN 300 113 . Such radio equipments have to face challenging environmental and radio conditions all over the world. The dynamic range in the vicinity of 100 dB, very strict adjacent channel transmitted power attenuation requirements, high data sensitivity, adjacent channel selectivity, high level of radio blocking or desensitization and high co-channel rejection , are its most important radio characteristics to mention. It is no wonder that for such high dynamic range demands, super heterodyne transceiver architectures with a majority of analog components are still widely used. But yet the radio transceiver has to be small in dimensions, consumes low power and maintains all its parameters over the wide industrial temperature range and over extensive period of time for reasonable price. At the same time, it should provide enough flexibility to accommodate different channel bandwidths, digital modulation formats, data rates, and techniques, to combat negative effects of radio channel. From this point of view, the software defined radio (SDR) concept is, indisputably, a prospective alternative and has not been widely used by these systems. The rapid expansion of the digital signal processing, together with the advancements in signal analog-to-digital converters technology have, in recent years, made such projects economically feasible.
Today’s LMR systems, being subject to , use mostly exponential constant envelope modulations GMSK, 2-CPFSK and 4-CPFSK. The application of the continuous phase modulations is mainly due to the extreme adjacent channel transmitted power (ACP) attenuation requirements, and inherent robustness 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.
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.
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 referenced to the unmodulated carrier power :
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 frequency 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. 1.1: 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.
In the systems, where the transmitter power efficiency is of high importance, the transmitter nonlinearity also creates an important issue. Generally speaking, the higher the transmitter nonlinearity, the higher the transmitter efficiency can be reached. Unfortunately, the device with a nonlinear transfer function also tends to distort the spectrum of the transmitted signal, especially if the modulated signal exhibits the non-constant modulation envelope. In contrast, it is also true that only the non-constant envelope modulation can withstand a strict band limitation by means of modulation filtering – characterized by the roll-off parameter α in the following text. In other words, if the signal has a constant modulation envelope, it has an unlimited spectrum, and, if it has a band limited spectrum, it experiences the amplitude variations, which after passing through the nonlinear power amplifier, would be suppressed, but would also regenerate the side-lobes of the modulated signal spectrum. The phenomenon is known as the spectral re-growth, and it depends mainly on the three transmitter characteristics. Those are peak to average power ratio (PAPR) of the digital modulation scheme in use, transmitter nonlinearity and the efficiency of the power amplifier linearization or pre-distortion technique and all have to be considered when selecting the digital modulation technique for the system, where both power and spectrum are the key issues.
In light of these facts one can arrive at the conclusion that setting up the limit at −60 dB1 rather than −70 dB was a reasonable step, while the initial limit has been left to be beyond the state of the present linearization technology for equipments production which in turn hindered the use of spectrally more efficient modulation techniques.
In this section, the measurement results concerning the overall narrowband transmitter power efficiency are presented. It is no ambition however, to provide exact power efficiency analysis of the particular high power amplifier̶ with the selected linearization circuit proceeded. It is rather to give the example of the practically achievable overall transmitter power efficiencies and to show the differences related to selected digital modulation formats of each of the linear/nonlinear class.
1 The standard  specifying the conformity testing for TETRA-like devices allows -55 dBc in normal or -50 dBc in extreme temperature conditions, assuming channel separation of 25 kHz.
Fig. 1.3: 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 bandwidth of radio receiver (Table 3.1).
Tab. 1.1: Measurement results of the transmitter parameters for selected modes of operation.
|Modulation Format||Symbol Rate||Modul. Parameter||Pout||ACI |
|Occupied Bandwidth @ 99.9%||PIN||ηTX||Spectrum plot|
|2CPFSK||10.4||h=0.6, α=0.28||40||-62||-61||19.8||35||29||Fig. 3.1|
|17.3||h=0.2, α=0.28||40||-62||-61||16.6||35||29||Fig. 3.1|
|4CPFSK||10.4||h=0.3, α=0.28||40||-61||-60||19.6||35||29||Fig. 3.2|
|17.3||h=0.1, α=0.28||40||-61||-60||17.2||35||29||Fig. 3.2|
|Measurement uncertainty ±2 dB|
The measurement values of achievable output power Pout, amount of adjacent channel interference ACI and overall transmitter power efficiency ηTX are collectively given for all the modulation formats in Table 3.1. It can be seen that the ACI limit (-60 dBc) is maintained for all of these settings; however, there are two penalties in case of linear modulation schemes that typically have to be paid for higher spectrum efficiency. Firstly, it is the lower output power level achievable. For this specific transmitter architecture it is in particular 35 dBm @ π/4-DQPSK, D8PSK and 33 dBm @ 16-DEQAM. Secondly, it is the lower value of the overall transmitter power efficiency reached. Comparing to exponential modes of system operation the efficiency of linear operational modes has decreased to 14% and 10%. Despite this negative trend, the achieved values of output power exceeding 3 W, and 2 W respectively, are considered practically applicable for next generation of narrowband LMR devices and as it will be shown in the next section they enable the system to use its occupied bandwidth with even higher communication efficiency.