Digitized Speech Transmission at VHF Using Existing FM Mobile Radios

76 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. VT-31, NO. 2 , MAY 1982

Digitized Speech Transmission at VHF Using Existing FM Mobile Radios

PREDRAG M. PETROVIC

Absrracr-An extensive study of digitized speech transmission over existing VHF FM mobile radio sets is presented. Objective criteria expressed in terms of bit error rate (BER) are determined for the performance evaluation of analog FM radios when transmitting digital voice. These criteria are used to define the technical characteristics such as the receiver sensitivity, co-channel rejection, and adjacent channel selectivity in the case of digital FM transmission. The concept of performance measurements of existing sets in the digital mode of operation is suggested. The results of performance measurements reported here include the following: 1) radiated FM spectra, 2 ) bit error rate curves, 3) receiver sensitivity, 4) BER performance in the presence of analog or digital FM interference on the same or adjacent channel, 5 ) signal plus noise plus distortion to noise plus distortion ratio (SINAD) performance in the presence of co-channel or adjacent channel interference for both types of interfering signals, 6 ) co-channel rejection and adjacent channel selectivity for various combinations of the wanted and interfering signals, 7) receiver selectivity for both modes of operation, and 8) adjacent-signal selectivity for various combinations of the desired and undesired signals. Optimum values of design parameters of digital voice radio system are deduced from the obtained results. Finally, the implications of digitized speech transmission on the technical characteristics, operating range, channel reuse distance, and adjacent channel interference performance of analog FM radios are discussed.

F I. INTRODUCTION

OR THE PURPOSE of providing not only a highly secure communication capability but also a high speed digital

data transmission compatibility, digital voice transmission techniques in the VHF land mobile radio channels have been recently studied [ 11 - [ 121 , There are two possible approaches to the implementation of digital speech transmission in mobile radio. The first approach is to develop new radio sets using digital modulation methods. On the other hand, a more immediate line of investigation relates to digitized speech transmission over existing standard VHF FM mobile radio sets. This second possibility, i.e.. to exploit the properties of existing sets, will be considered in this paper.

At the present time there are essentially no data available as to the performance of analog FM radios when transmitting digitized voice. Consequently, there is also no published work on the performance comparison of analog speech and digital speech transmission at VHF with existing sets. The purpose of this paper is to fill the aforesaid void and to present an extensive study of digitized speech transmission via analog radios for land mobile service. The experimental and theoret- ical work reported here arose from a requirement to provide means for 1) the optimization of the parameters affecting

Manuscript received September 11, 1981; revised January 6 , 1982. The author is with the Research and Development Institute, Elek-

tronska Industrija, Batajnicki Put 23, 11080 Beograd, Yugoslavia.

digitized speech transmission quality, and 2) the estimation of the influence of digitized speech transmission on the technical characteristics, operating range, and channel reuse distance of existing mobile sets. The results obtained from this study are important in establishing frequency plans for VHF FM radios processing digital speech and guidelines for coexistence of analog and digital speech transmission with existing sets.

This paper has the following outline. In Section I1 some relevant characteristics of an analog FM radio are given. In Section I11 spectral shaping of digitized speech is discussed, and the modified duobinary technique is chosen for this purpose. Then the consideration of speech encoding for mobile radio follows. Further, a basic configuration of digital speech transmission in mobile radio is presented. In Section VI the performance criteria of VHF FM radios transmitting digital speech are determined. Next, the radio equipment technical characteristics of interest for compatibility investigations of analog-digital coexistence are selected, and the concept of measurements of these characteristics is suggested. The results of performance measurements of VHF FM radios transmitting digitized voice are presented in Section VIII. These results include 1) radiated FM spectra, 2 ) measured bit error rate curves, 3) receiver sensitivity? 4) error rate performance in the presence of analog FM or digital FM interference on the same or adjacent channel, 5) signal plus noise plus distortion to noise plus distortion ratio (SINAD) measurements in the presence of analog FM or digital FM interference on the same or adjacent channel, 6) co-channel rejection and adjacent channel selectivity for various combinations of the wanted and interfering signals, 7) receiver selectivity curve, and 8) adjacent signal selectivity versus frequency separation between the wanted and interfering carrier for various combinations of the desired and undesired signals. All these measurements were carried out in the static laboratory environment. Section IX summarizes optimal values of some parameters affecting digitized speech transmission quality which are deduced from the obtained results. Finally, analog and digital speech transmission with existing FM radios are compared and the implications of digitized speech transmission on the technical characteristics, operating range: and channel reuse distance of the analog sets are briefly discussed. In Section XI we present conclusions and a summary.

11. ANALOG FM RADIO CHARACTERISTICS In this section a current FM mobile radio set will be con-

sidered. The FM modulator consists of an integrator, a phase modulator, and a few frequency multiplication stages. The

0018-9545/82/0500-0076S00.75 0 1982 IEEE

PETROVIC: DIGITIZED SPEECH TRANSMISSION 7 7

h , 1

- 2 5 E -30 8 - 3 5

20 50 100 200 500 1000200(1 500010000

f r e q u e n c y ( H z )

Fig. 1. Amplitude characteristic of mobile radio channel.

2 ,

1 2 3 L 5 6 7 8 9 1 0

f r e q u e n c y ( k H z ) Fig. 2. Group-delay characteristic of mobile radio channel.

receiver is of the usual limiter-discriminator type. The radio set operates in the 160 MHz band, while the channel separation is 25 kHz with a peak and root mean square (rmsj frequency deviation of 5 and 3 kHz, respectively.

In the case of digitized speech transmission, the input signal bypasses the audio part of the set and drives the FM modulator directly. The channel from the input of the FM modulator in the transmitter to the output of the discriminator in the receiver can be described as a linear channel with a d e f i e d amplitude and group-delay characteristic. The amplitude- frequency response of the mobile radio channel is depicted in Fig. 1. This characteristic gives an indication of the available bandwidth of the channel. The 3 dB bandwidth is in the range from 200 Hz to 6 kHz. The upper frequency is determined by the intermediate frequency (IF) filters in the mobile receiver. Very low audio frequencies and direct current (dcj cannot be transmitted due to the properties of the phase modulator. The group-delay characteristic is shown in Fig. 2. I t can be seen from this figure that the group-delay characteristic is rather flat over the available bandwidth.

111. SPECTRAL SHAPIhG OF DIGITIZED SPEECH

To enable digitized speech to be transmitted over the existing set, spectral shaping of the modulating binary signal is needed in order to adapt it to the characteristics of the mobile radio channel. Since there is no transmission of dc and very low frequencies, we have to choose a spectral shaping of digitized speech signal that does not need these very low frequencies and dc for undistorted transmission. In addition to this requirement, it is necessary to make the radiated FM spectrum as narrow as possible for minimum adjacent channel radiation. This means that the spectrum of the digital signal applied to the FM modulator should have steep rolloff at higher frequencies, apart from removal of the dc content and suppression of the lower frequencies. Finally, a maximum possible bit rate must be achieved in the available bandwidth in order to ensure a better intelligibility of digitized speech.

Taking all these requirements into consideration, we have chosen the modified duobinary code, also known as the class 4 partial response signaling scheme or the second-order bipolar code for spectral shaping of digitized speech in mobile radio [ 131. The shape of the amplitude spectrum for the modified duobinary code is a half-cycle sinusoid. This code requires a bandwidth of 1/2 T Hz in order to transmit l / T bits/s. Also, it offers suppression of low frequencies and a rather steep spectral rolloff at higher frequencies up to 1/2 T Hz. Using the modified duobinary code a bit rate of 12 kbitsjs can be achieved in the mobile radio channel bandwidth of approximately 6 kHz.

IV. SPEECH ENCODIKG FOR MOBILE RADIO

Several factors affect the choice of a speech encoding procedure for mobile radio. These factors are the transmission capacity of a mobile radio channel, the problem of severe bit error bursts, synchronization: and coder complexity [14] ,

As was found in the previous section? the transmission rate of digitally encoded speech should not exceed 12 kbits/s. In addition, average bit error rates under fading and shadowing conditions as high as five percent are possible at times with error rates even reaching 50 percent in occasional bursts. Finally, coders should be simple and inexpensive with low technical expenditure for synchronization. As far as the speech quality is concerned? it will be discussed briefly in this section and in greater detail in Section VI.

In mobile radio communications the speech intelligibility is of primary interest, with the flawless reproduction or fidel- ity of speech provided by the link being of secondary interest. The word intelligibility scores of 97-98 percent are obtained for an analog FM land mobile radio under strong signal conditions in the laboratory environment. Although the results of word intelligibility for adaptive delta modulation (ADM) at 12 kbits/s vary from 85 to 95 percent depending on the particular variant employed, ADM is still the most suitable method for speech encoding in mobile radio owing to its hlgh resistance to channel errors, low complexity and cost. and simple synchronization.

Careful attention must be paid to the design of an ADM coder since its tolerance to channel errors and the intelligibility of ADM encoded speech directly affect the receiver sensitivity, and thus the communications range in the case of digitized speech transmission via existing VHF mobile radios.

[151.

V. BASIC CONFIGURATION OF DIGITIZED SPEECH TRANShlISSION IN MOBILE RADIO

The basic configuration of digitized speech transmission in mobile radio is shown in Fig. 3. At the transmitting side the analog voice is fed through a low-pass voice-band filter into an adaptive delta modulator. The ADM encoded speech is then enciphered and applied to the modified duobinary coder. Finally, the resulting modified duobinary signal is integrated before an application to the phase modulator. The cascaded integrator and phase modulator together constitute a frequency modulator. Transmission is modified duobinary FM. At the receiving side a frequency discriminator converts the

7 8 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. ~ ~ - 3 1 , NO. 2 , MAY 1982

r - - - - - - - - - - a u d i o

I modtfied ' I

amplifier ADM +scrambler 7 duobinary

transmit and +-

arnplifler modulator coder c ~ d e r +

f i I t e r ~ I 4 t 4 Lstgnd_ord_ +M- mokle-s&

~~

c l o c k TRANSMITTING SIDE

ornpllfier oud io

A D M descramb-

f i l t e r and +

d e c o d e r l e r --c --c regenerator -

r-------- mlaers

ampllfier

kta_nndaad- F-M mobile set

RECEIVING SIDE

Fig. 3. Basic configuration of digitized speech transmission in mobile radio.

modified duobinary FM to the baseband modified duobinary code. The FM discriminator is followed by the regenerator, a descrambler, an ADM decoder, and a low-pass voice-band filter. A highly effective and robust digital phase-locked loop based on the so-called early-late type of the synchronizer has been designed for clock recovery. This method of synchronization was suggested by Bennett [ 161 .

All the additional circuitry such as low-pass voice-band filters, an ADM codec, a digital scrambler, the modified duobinary codex, the regenerator, and the bit synchronizer can be mounted in a separate unit or incorporated in an existing mobile set using large scale integration (LSI) and thick-film hybrid technology.

VI. PERFORMANCE CRITERIA OF VHF FM RADIOS TRANSMITTING DIGITAL SPEECH

The aim of our investigations was to gather data on the performance of existing FM radios transmitting digital speech and to compare this performance with that one measured in the case of analog speech transmission. For this purpose, it is necessary first to establish the performance criteria of these sets when processing digitized voice.

In the case of analog FM transmission, the performance criteria of existing mobile radio equipment are usually expressed in terms of SINAD. A convenient measure of the threshold of performance for narrow-band receivers is a specified value of SINAD, the conventionally accepted value being 12 dB [ 171 - [ 191. This defines the reference receiver sensitivity, i.e., the minimum usable RF signal level at the receiver input in the absence of interference. When a co-channel or adjacent channel interfering signal is superimposed on a wanted signal, a degradation of 12 dB SINAD to 6 dB is taken as the criterion to defme the maximum permissible level of an interfering signal [ 181, [20] -[22].

In the case of digital transmission, the performance criteria of VHF FM radios will be expressed in terms of the bit error rate (BER). First, we have to determine the BER threshold that defines the minimum usable received R F signal level. I t must be emphasized that methods of the subjective assessment of voice quality are to be used in order to ensure that the figure proposed for the BER threshold and the SINAD of 12 dB lead to the same subjective speech quality. For this purpose, word intelligibility tests were performed on the standard

100, 1

? 12 dB receiver

SI NAD sensitivity

60- 0.1 0.2 0.3 0.5 0.0 1 2

RF input signal level ( y V )

Fig. 4 . Word intelligibility versus RF input signal level for VHF FM mobile radio.

mobile radio set operating in the conventional analog speech transmission mode. The word intelligibhty was measured with modified rhyme tests [23]. The results of these tests are shown in Fig. 4. It can be seen from Fig. 4 that an intelligibility score of nearly 85 percent corresponds to the 12 dB SINAD receiver sensitivity. Using the connection between speech transmission quality and word intelligibility [24], [25] an 85 percent articulation score can be estimated to provide the minimum grade of satisfactory performance. The next step was to measure the word intelligibility versus BER performance of the ADM coder operating at bit rates of interest for mobile radio. The tests were made on the continu- ously variable slope delta (CVSD) coder at bit rates of 9.6 and 12 kbits/s and the obtained results are displayed in Fig. 5 . It can be deduced from these results that an 85 percent word articulation score is obtained at 12 kbits/s with the BER of approximately 8 X lop2. The CVSD coder operating at 9.6 kbits/s with the BER of 5 X provides only slightly lower level of intelligibility. This means that the BER of 8 X lop2 defines the minimum usable received RF signal level in the case of 12 kbits/s ADM encoded speech transmission over existing analog FM radios. The BER threshold at 9.6 kbits/s is 5 X It should be noted that the choice of an ADM coder affects the value of the BER threshold. The choice of the CVSD coder for speech encoding in mobile radio is the trade-off among reproduced speech quality, channel error

PETROVIC: DIGITIZED SPEECH TRANSMISSION 79

2 loo’

‘5 80

- 1 2 k b i t l s 9.6 k b i t l s

5 0 0 2 0.5 1 2 5 10 20

b i t e r r o r r a t e ( O h )

Fig. 5 . \Vord intelligibility versus bit error rate for CVSD coder.

resistance. and coder complexity. This choice is supported by the fact that at the present time there are commercially available chips employing the CVSD algorithm. For that reason. it seems reasonable to adopt the BER values of 5 X and 8 X lo-’ as the most suitable ones to define the reference receiver sensitivity at 9.6 and 12 kbitsis, respectively.

Our next task was to determine the criterion which would define the maximum permissible level of an interfering signal at the receiver input in the case of digital transmission. Tllis criterion should correspond to the lower limit of mininlally adequate speech quality or threshold of intelligbility. Using again the connection between speech transmission quality and word intelligibility [24] we adopted a 75 percent intelligibility score to be the minin~um one acceptable for mobile radio. This intelligibility score agrees quite well with the threshold of intelligibility suggested for tactical communications in 1261 . I t should be mentioned for the sake of illustration that with trained radio operators. scores as low as 70 percent have been deemed acceptable in certain applications [ 2 7 ] . It can be found from Fig. 5 that the ADhl coder operating at 12 kbits/s with the BER of 13 X lo-’ provides a 75 percent word intelligibility score. Tllis level of intelligibility is obtained at 9.6 kbitsis with the BER of 10 X lo-’, Kow? it is possible to define the maximum permissible level of an interfering signal a t the receiver input in the case of digital Fhl transmission. The BER increase due to interference from 8 X lo-’ to 13 X lo-’ is taken as the criterion at 12 kbitsls. At the bit rate of 9.6 kbits, s the increase of the BER from 5 to 10 percent is assumed as the measure of the maximum allowable interference.

In this way the objective criteria are determined for the performance evaluation of existing Fhl radios transmitting digital speech. These criteria correspond with the nature of digital transmission and can be simply measured. Consequently: the definitions based on the signal-to-noise ratio at the receiver audio output are avoided ana. in addition. there is no need for the repetitive use of subjecrive evaluations.

V11. COKCEPT OF hlEASUREhlESTS

The performance comparison of analog and digital speech transn~ission a t VHF with existing sets will include consideration of the radiated FR.1 spectra. receiver sensitivity. co-channel interference rejection, and adjacent channel selectivity. All these characteristics except transmitter sideband spectrum have to be defined for digital transmission in accordance with the performance criteria derived in the previous section.

The receiver sensitivity is measured as a digital FM input signal level which provides the BER of 8 X lo-’ at 12 kbits/s. Alternatively a RF input signal level producing the BER of 5 X is used for determination of the receiver sensitivity at 9.6 kbits/s. As can be seen from this defmition: the value of frequency deviation is not specified for receiver sensitivity measurements. In the case of digital Fhl transmission the modulation index iz (defined as peak-to-peak deviation 2Afdivided by the bit rate 1/T) is fixed and the choice of optimum value for h will be discussed in Section 1X.

The signal-to-interference protection at 12 kbits/s is defined to be the ratio of wanted to interfering signal level to increase a 8 X lo-’ BER to 13 X lo-’. The interference protection level at 9.6 kbits/s is the value of the wanted-to- interfering signal ratio to increase the BER from 5 X to 10 X lo-’. These definitions are applied to co-channel re-

jection and adjacent channel selectivity measurements in the case of digital FM transmission.

The BER performance and receiver sensitivity are measured with the experimental system shown in Fig. 6. A pseudorandom binary sequence (for example CCITT 5 11 pseudorandom pattern) is generated at the bit rate of 9.6 or 12 kbits/s, and modulates the FM transmitter through a modified duobinary coder. The synthesized RF signal generator with an external FM modulation capability is used as the FM transmitter in these measurements. The FM receiver is a double conversion type one. where the first and second intermediate frequency (IF) frequencies are 10.7 MHz and 455 kHz, respectively. The demodulated signal at the discriminator output is further regenerated and fed into the error rate counter for the BER measurements.

It should be mentioned that the receiver sensitivity and SIKAD performance in the analog mode of operation are measured using the RF signal generator internally modulated with a 1 kHz tone at 3 kHz deviation (standard test rnodula- tionj and a distortion meter.

Co-channel rejection and adjacent channel selectivity for various combinations of the wanted and interfering signals are measured with the test apparatus depicted in Fig. 7. There are two types of measurements related to the co-channel and adjacent channel interference protection. First, a mobile receiver is provided with an analog FM wanted signal at a level giving a SIKAD of 12 dB. To produce an analog FM desired signal RF signal generator no. 1 is internally modulated at standard test modulation. To t h s standard test modulated signal is added an interfering signal on the same or adjacent channel. Depending on the type of the interfering signal, i.e., analog Fhl or digital FM, RF signal generator no. 2 is internally modulated with 400 Hz at 3 kHz deviation or externally modulated with the modified duobinary signal, respectively. The level of the interference is adjusted until 12 dB SINAD is reduced to 6 dB. This can be done with the interfering transmitter at various frequency separations from the wanted signal. The described experimental arrangement is also used to measure the SINAD performance in the presence of analog or digital FM interference.

The second type of measurements related to the situation when a mobile receiver is provided with a digital FM wanted


t ransmtt c lock n

I 1 a u d i o

pseudorandom o u t p u t

g e n e r a t o r m o d l f i e d R F a n d

g e n e r a t o r c o d e r B E R - s i g n a l - d u o b l n a r y - m o b l l e

r e c e i v e r - r e g e n e r a t o r

c o u n t e r 1 , .~ ~ I - ext rac ted c lock 1 r e g e n e r a t e d b i t s t r e a m

Fig. 6. Experimental test system for BER and receiver sensitivity measurements.

,extracted clock 1 regenerated bit stream

transrnlt clock - rn a

pseudorandom + b

g e n e r a t o r a n d

m o d i f i e d R F s i g n a l - d u o b i n a r y - g e n e r a t o r - T - p a d - B E R c o d e r N o . 1

r c c e i v e r 6 d B m o b i l e

c o u n t e r a u d i o output

d i s t o r t i o n

Fig. 7 . System block diagram for co-channel rejection and adjacent channel selectivity measurements.

signal at a level giving the BER of 8 percent (5 percent at 9.6 kbits/s). In this case R F signal generator no. 1 is externally modulated with the modified duobinary signal. The level of analog or digital FM interference produced by R F signal generator no. 2 is adjusted until the BER is increased to 13 percent (10 percent at 9.6 kbits/s). This configuration is also used to measure the BER performance in the presence of analog or digital FM interference.

The selectivity of the receiver in both modes of operation is measured when an unmodulated carrier produced by R F signal generator no. 2 is used as the interfering signal at various frequency separations from the wanted signal.

VIII. RESULTS OF MEASUREMENTS

A . Radiated FM Spectra

Measured power spectra of the modified duobinary FM signal for the bit rate of 9.6 kbits/s and several values of modulation index h (0.4, 0.6,0.7, and 0.8) are shown in Fig. 8. The radiated modified duobinary FM spectrum for l /T = 12 kbits/s and h = 0.7 is plotted in Fig. 9. For the sake of comparison, the transmitter output spectra in the analog mode of operation for the usually used modulation frequencies (0.4, 1 and 3 kHz) and an average voice modulation (VOSIM) are depicted in Fig. 10. In the vertical power scale, 0 dB refers in all cases to the power of the unmodulated FM carrier.

B. BER Performance

The SINAD and quieting performance of a typical land mobile FM radio receiver used in BER measurements are shown in Fig. 11. Measured bit error rate curves as a function of RF input signal level at the bit rate of 9.6 kbits/s with the modulation index h as a parameter are given in Fig. 12. Finally, Fig. 13 illustrates results obtained in the measurement

0

- 1 0

- 2 0 m U Y

- 3 0 L

0,

0 3

-60

- 5 0

-60

-7 n

I

1

- 1 v > 0 5 10 15 20 2 5

f r e q u e n c y o f f s e t ( k H z ) Fig. 8 . Power spectra of modified duobinary FM signal measured at

9.6 kbits/s for several values of h.

of BER performance of the mobile receiver at the bit rate of 12 kbits/s for five values of h (0.4, 0.5, 0.6, 0.7, and 0.8). Taking into account only the BER performance, the best performance at the bit rate of 9.6 kbits/s is achieved with a modulation index of h = 1.2. I t can be found from Fig. 13 that the lowest BER at 12 kbits/s is obtained for h = 0.7.

To investigate the effect of carrier frequency stability on the BER performance we have measured the degradation in

PETROVIC: D I G I T I Z E D SPEECH TRANSMISSION

0

- 1 0

- 2 0

m v -30

L aJ

2 -40 0.

- 5 0

-6 0

-70 L 0 5 10 15 20 25

f r e q u e n c y o f f s e t ( k H z )

Fig. 9. Power spectrum of modified duobinary FM signal measured at 12 kbitsls with h = 0.7.

0

- 1 0 Af=3 kHz Af-3 kHz

A f = 5 kHz

-20 - m u - -30 L

a 3 : - 4 0

-5 0

-6 0

-70 I 0 5 10 15 20 25

>

f r e q u e n c y o f f s e t ( k H z )

Fig. 10. Power spectra of analog FM signal for tone and VOSIM modu- lations.

2 6

24

2 2

I 1 . 1 . . I

R F Input s i g n a l leve l ( N V )

Fig. 11. SINAD and quieting performance of a typical land mobile FM radio receiver.

0.1 02 0.3 0 5 0.8 1 2 5 10 20 50

1 o+ i 1 6’

0.3 0.4 0.5 0.6 0.8 1 2 3 4 RF Input signal level (,uV)

Fig. 12. Measured bit error rate curves versus RF input signal level at 9.6 kbits/s with the modulation index h as a parameter.

82 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. VT-31, NO. 2, MAY 1982

o 16’ - I t - hz0 .4 \. ... ‘ ---- h.0.5 \.-*.. .......... h.0.6 \ ...

\

10’J : : : ; : 03 0.4 0.5 0.6 0.8 1 2 3 L 6

R F input s i g n a l lcvcl ( p V )

Fig. 13. Measured bit error rate curves versus RF input signal level a t 12 kbits/s with the modulation index h as a parameter.

TABLE I BER PERFORMANCE DEGRADATION VERSUS CARRIER

FREQUENCY OFFSET

Carrier offset (kHz) +0.5 -0.5 + 1 -1 BER performance degradation (dB) 1.2 2.3 3 3.6

TABLE I1 RECEIVER SENSITIVITY IN THE DIGITAL MODE OF OPERATION AS A FUNCTION OF MODULATION INDEX

Modulation index h 0.4 0.5 0.6 0.7 0.8 1 1.2 Receiver sensitivity (bV) at 9.6 kbits/s 0.93 0.62 0.47 0.43 0.40

Receiver sensitivity (pV) at 12 kbits/s 0.73 0.60 0.45 0.40 0.54

TABLE 111

OPERATION VERSUS 12 dB SINAD SENSITIVITY RECEIVER SENSITIVITY IN THE DIGITAL MODE OF

- 1 2 dB SINAD sensitivity (pV) 0.21 0.24 0.27 0.34 0.53 Receiver sensitivity in the digital mode of operation (pV) 0.37 0.41 0.47 0.58 0.92

error performance as a function of carrier frequency offset for 1/T = 9.6 kbits/s and h = 0.8. The results are given in Table I.

C. Receiver Sensitivity

The receiver sensitivity in the digital mode of operation is measured as a function of modulation index h and the results of measurements for both transmission rates are presented in Table 11. From the receiver sensitivity point of view the best results are obtained with h = 1.2 at 9.6 kbits/s and with h = 0.7 at 12 kbits/s. For the sake of comparison, the measured 12 dB SINAD receiver sensitivity for analog FM transmission is 0.27 pV (Fig. 11 j.

To find how the 12 dB SINAD sensitivity affects the receiver sensitivity in the case of digital FM transmission the sensitivity measurements for both modes of operation are made on five receivers. The results are given in Table 111. It

can be found from the obtained results that the degradation of receiver sensitivity in the digital mode of operation for 1 /T = 9.6 kbits/s and h = 0.8 slightly varies about 4.8 dB. The measured variation is kO.15 dB.

D. BER Performance in the Presence of Co-Channel or Adjacent Channel Interference

Measured BER performance in the presence of co-channel interference for 1 /T = 9.6 kbits/s and h = 0.8 is displayed in Fig. 14. The effect of co-channel interference on error performance for l /T= 12 kbits/s and h = 0.7 is shown in Fig. 15. The dashed curves in both figures present the BER per fom- ance in the absence of interference. The solid curves are results of error rate measurements performed in the presence of co- channel analog or digital FM interference with the signal-to- interference (S/Ij ratios of 10 dB and 20 dB. It can be concluded from the measured data that the influence of co-channel analog FM interference on BER performance degradation is more critical than that one caused by the digital FM undesired signal.

Fig. 16 illustrates the impact of adjacent channel analog or digital FM interference on error performance for 1/T = 9.6 kbits/s and h = 0.8. Fig. 17 shows the results of BER measure-

PETROVIC: DIGITIZED SPEECH TRANSMISSION 8 3

1 o-2-- a, I

2

2 Q,

= 10 n 1 ) no interference

.' - 4

2 ) S/1=10 d B , digital FM \

3 ) S I I = 2 0 dB, analog FM \ Interference \

Interference \ \ 1

> 0.3 0.4 0.5 0.6 0.0 1 2 3 4

R F i n p u t s i g n a l l e v e l ( y V )

Fig. 14. Measured BER performance in the presence of co-channel interference for l / T = 9.6 kbits/s and h = 0.8.

A 10-11

L Y 2 ) S/1=20 d B , dlgltal FM \,

- & c

Interference - 10 3 ) S/I 20 dB, analog F M \ n

.' \

Interference \ \

,o-5 4 ) S/l = l o d B , d l g l t a l F M \

Interference \ \ \

'! 1 0 1 : : : : : D

0.3 0.4 0 5 0.6 0.8 1 2 3 4 6

R F i n p u t s i g n a l l eve l ( g V )

Fig. 15. Measured BER performance in the presence of co-channel interference for 1/7 = 12 kbits/s and h = 0.7.

10 - 4 +

L a, 2 ) SII=-50dB,analog FM ' = 1 0 .. n 3 1 Sl1=-60 dB I analog F M l2 - L interference

interference

1 0 -5.. 4 ) S11=-60 dB, dlgital F M interference

\ 10

- 6 \l D

0.3 0.4 0.5 0 6 0.0 1 2 3 6 R F input s ~ g n a l l e v e l ( p V )

F-ig. 16. Measured BER performance in the presence of adjacent cham ne1 interference for l / T = 9.6 kbits/s and h = 0.8.

lo- ' ,

a - ?

IO- '..

a - 6 : 10 '

n

10- $..

2 ) S/1=-50 dB, analog FM \

3 ) S/I=-50 dB, d lg l ta l FM \ Interference

lnterference \

4 ) S/I=-60 d B , a n a l o g F M \ Interference

Interference \l

\ \

5 ) S/I =-60 d B , d l g l t a l F M \

1 0 1 : : : : : - 6 \

D 0.3 0.4 0.5 0 6 0.8 1 2 3 4 6

RF Input s l g n a l l e v e l ( ~ ' 4 )

Fig. 17. Measured BER performance in the presence of adjacent channel interference for 1/7 = 12 kbits/s and h = 0.7.

ments made in the presence of adjacent channel interference for 1/T = 12 kbits/s and h = 0.7. The signal-to-interference ratios of -50 dB and -60 dB are used in experiments. The degradation in error performance caused by the digital FM interference is more severe than that one produced by the analog FM interfering signal.

E. SINAD Performance in the Presence of Co-Channel or Adjacent Channel Interference

Measured SINAD performance in the presence of co-channel interference is given in Fig. 18. The dashed curve relates to the interference-free reception. The solid curves refer to three types of interfering signals, namely, analog FM, digital FM with 1/T = 9.6 kbits/s, and h = 0.8, and digital FM with l / T = 12 kbits/s and h = 0.7 which are included in measurements at levels 10 dB and 6 dB below the desired signal.

The effect of adjacent channel analog or digital FM interference on SINAD performance is shown in Fig. 19. The signal- to-interference ratios for the same three types of interfering signals as in the previous case vary from -57 dB to -77 dB. These values with the exception of Si1 = -77 dB are selected to be approximately 3 dB below the value of adjacent channel selectivity for the particular combination of the wanted and interfering signal.

Inspecting Figs. 18 and 19 it can be seen that the most severe degradation of SINAD performance in the case of co- channel interference is caused by the analog FM interfering signal, while in the presence of adjacent channel interference the effect of the digital FM unwanted signal with 1/T = 12 kbits/s and h = 0.7 appears to be the most critical one.

F. Co-Channel Rejection

Co-channel rejection of the mobile receiver in the analog mode of operation is 5 dB. The measured values of co-channel rejection for the remaining combinations of the wanted and interfering signals are given in Tables IV, V, and VI. Measure- ments are made for both transmission rates and three values of modulation indices, namely, 0.6, 0.7, and 0.8 at 9.6 kbits/s and 0.5, 0.6, and 0.7 at 12 kbits/s. The modulation


/---- 1

/ /

/ /

3 4

5

1 ) no interference

2 ) SI1 =10 dB analog FM interference

3 ) S l I = 6 dB I digital F M lnterference, l lTs9.6 kbitls,

4 ) S l I = 6 dB, digital FM Interference, 1/ l= 12 kbitls ,

/ 5 ) SI1 = 6 d B , analog FM interference

h=0.8

h.0.7

0 1 > 0.1 0.2 0.3 0.5 1 2 3

R F i n p u t s i g n a l l e v e l O J V )

Fig. 18. Measured SINAD performance in the presence of co-channel interference

2 8

2 6

2 4

22

2 0

m 1 8

- 1 6

a 1 4

y, 1 2

0

0

2

1 0

8

6

4

2

0

A

L

0.1

no interference 0 - - - - -

0

/ /

Sll=-77 dB,analog FM Interference

> 0.2 0.3 0.5 1 2 3

R F input s i g n a l level ( p V ) Fig. 19. Measured SINAD performance in the presence of adjacent

channel interference.

TABLE IV

WANTED AND INTERFERING SIGNALS CO-CHANNEL REJECTION IN THE CASE O F DIGITAL FM

1 /T (kbits/s) 9.6 9.6 9.6 12 12 12

Co-channel rejection (dB) 3 2.4 2 9.2 7.7 6 h l 3 h 2 0.6 0.7 0.8 0.5 0.6 0.7

TABLE V COCHANNEL REJECTION IN THE CASE OF DIGITAL FM

WANTED AND ANALOG FM INTERFERING SIGNALS

1/T (kbits/s) 9.6 9.6 9.6 12 12 12

Co-channel rejection (dB) 12.7 10.1 7.9 10 8.1 7.3 h l 0.6 0.7 0.8 0.5 0.6 0.7

TABLE VI COCHANNEL REJECTION IN THE CASE O F ANALOG FM WANTED

ANI) DIGITAL FM INTERFERING SIGNALS

1/T (kbits/s) 9.6 9.6 9.6 12 12 12

Co-channel rejection (dB) 4 4.4 4.8 3.4 3.8 4.3 h2 0.6 0.7 0.8 0.5 0.6 0.7

indices of the digital FM wanted and interfering signals are designated with hl and h 2 , respectively.

As can be seen from these results the lowest resistance to co-channel interference is obtained for the combination of digital FM wanted and analog FM interfering signals.

G. Adjacent Channel Selectivity

Adjacent channel selectivity of the mobile receiver in the analog mode of operation is -77 dB. The measured values of adjacent channel selectivity for the remaining combinations of the wanted and interfering signals are presented in Tables VII, VIII, and IX. The results are given in the same way as in the case of co-channel rejection.

It should be noted that the lowest rejection of adjacent channel interference results in the case of digital FM wanted and interfering signals at 12 kbits/s.

H. Receiver Selectivity

Receiver selectivity curves for three types of wanted signals, i.e., analog FM, digital FM with 1/T = 9.6 kbits/s and h = 0.8 and digital FM with 1/T = 12 kbits/s and h = 0.7 are shown in Fig. 20.

I. Adjacent Signal Selectivity

The results related t o the measurements of adjacent-signal selectivity as a function of frequency separation from the desired carrier for digital FM wanted signal with 1 /T = 9.6 kbits/s and h = 0.8 are presented in Fig. 21. Both types of interference (analog and digital FM) are included in experiments. Fig. 22 shows the adjacent-signal selectivity for digital FM wanted signal with 1 /T = 12 kbits/s and h = 0.7. Finally, the adjacent-signal selectivity curves for analog FM wanted signal and three types of interfering signals, namely, analog FM, digital FM with l /T = 9.6 kbits/s and h = 0.8 and digital FM with 1/T = 12 kbits/s and h = 0.7 are plotted in Fig. 23. It can be generally concluded that at frequencies up to 12 kHz

PETKOVIC: DIGITIZED SPEECH TRANSMISSION 8 5

TABLE VI1 ADJACENT CHANNEL SELECTIVITY IN THE CASE OF DIGITAL

F\f WANTED AND INTERFERING SIGNALS

l /T(kbi t s / s ) 9.6 9.6 9.6 12 12 12

Adjacent channel selectivit), (dB) -68.4 -68.2 -68 -64.5 -62.5 -60

11 1 , 2 0.6 0.7 0.8 0.5 0.6 0.7

TABLE VI11 ADJACENT CHANNEL SELECTIVITY IN THE CASE O F DIGITAL

FM WANTED AND ANALOG FM INTERFERING SIGNALS

l /T (kb i t s i s) 9.6 9.6 9.6 12 12 12

Adjacent channel selectivity (dB) -67 -69.5 -71.7 -68.1 -69.5 -70.6

'71 0.6 0.7 0.8 0.5 0.6 0.7

TABLE IX ADJACENT CHANNEL SELECTIVITY IN THE CASE O F ANALOG

FM LVANTED AND DIGITAL FM INTERFERING SIGNALS

l!T (kbiws) 9.6 9.6 9.6 12 12 12

Adjacent channel selectivity (dB) -71.6 -69.3 -65.5 -68.4 -63.6 -60

h2 0.6 0.7 0.8 0.5 0.6 0.7

c d B

0 s -80 2 a L

- 7 0 0 5

-60 I! L

-50 0- z

t - 4 0 L m

t L -30

- 0

m 0 - _

I ........ d l g l t a l FM

---- d l g l t a l FM 1/T-9.6 kbit/s

1/7-12 kblt/s ,

, h.0.8

h=O 7

f r e q u e n c y s e p a r a t i o n ( k H z )

Fig. 20. Receiver selectivity in the case of analog and digital FM transmission.

' 9 , m -701 N I

8 - 6 O i {'/ /--

I

0 . - I

* V .- - 50 e

ss e20 0 5 10 15 20 25

f requency s e p a r a t i o n ( k H z )

Fig. 21. Adjacent signal selectivity for digital FM wanted signal with 1 /T = 9.6 kbits/s and h = 0.8.

7 d B Q

P x -80

e -70 -. 0

m N

'p -60 X

m * e -!jO

.- -40 E e 0

0 '= -30 e - g -20 .- u)

'E P -10

t l

.- E O

g *10

.c

I 0 I e

e

+20

interference

interference

f r e q u e n c y s e p a r a t i o n ( k H z )

Fig. 22. Adjacent signal selectivity for digital FM wanted signal with 1 / T = 12 kbits/s and h = 0.7.

8 6 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. VT-31, NO. 2 , M A Y 1982

(111.96 kblt /s, h.08) lnierference

L ........ d ~ g ~ t a l FM 4 4 1 0 - (1/7=12 kblt I s , h=O 7 ) 0 Interference

P .20 0 5 10 15 20 2 5 - >

f r e q u e n c y s e p a r a t t o n ( k H z )

Fig. 23. Adjacent signal selectivity for analog F!d wanted signal,

away from the wanted carrier the effect of analog FM interference is more serious and at frequency separation above 12 kHz the degradation of adjacent signal selectivity due to digital FM interference is more prominent. This is quite in agreement with conclusions taken on BER and SINAD performance degradations owing to co-channel or adjacent channel interference (Sections IV-D and IV-E).

IX. OPTIMUM VALUES OF DESIGN PARAMETERS

The optimum values of design parameters of digital voice radio system will be given in this section using the results obtained in the measurements. The parameters considered are the transmission rate 1/T, modulation index h , 3 dB cutoff frequency f, of the low-pass filter in the modified duobinary signal generator before the FM modulator and 3 dB cutoff frequency f,' of the low pass filter in the regenerator after the FM discriminator.

Digitized speech transmission at 12 kbits/s enables better intelligibility of ADM encoded speech. On the other hand, the transmission at 9.6 kbits/s offers advantages with respect to adjacent channel power and adjacent channel selectivity. The final decision on the bit rate depends on the user requirements for the particular application. It should be mentioned that the BER measurements at 14.4 kbits/s were also carried out. Unfortunately, the BER below 2 X l op3 could not be achieved under strong signal conditions with the modulation index as low as h = 0.25.

The choice of the optimum value of modulation index h is the trade-off among the opposite requirements related to the receiver sensitivity and co-channel rejection on one side and adjacent channel power and adjacent channel selectivity on the other. In selection of the optimum value for h it seems reasonable to give the small preference to the requirements related to the receiver sensitivity and co-channel rejection. As a conse-

quence, h = 0.8 at 9.6 kbits/s and h = 0.7 at 12 kbits/s are chosen as the optimum values of the modulation index.

It has been found from measured BER curves and measured baseband power spectra that the optimum value of the filter cutoff frequency f, is 0.6 X 1/T [13]. The same value is selected for the filter cutoff frequency f,'. Using only the filtering properties of the mobile set (without the low-pass filter in the regenerator) the BER performance degradation of about 5 dB can be expected.

X. PERFORMANCE COMPARISON OF ANALOG AND DIGITAL SPEECH TRANSMISSION AT VHF

WITH EXISTING SETS

As we found the optimum values of design parameters of digital voice radio system in the previous section it is now possible to make the comparison of analog and digital speech transmission at VHF with existing sets. The performance comparison will include consideration of the radiated FM spectra, receiver sensitivity, co-channel rejection and adjacent channel selectivity. Also the implications of digitized speech transmission on the operating range and channel reuse distance of existing mobile sets will be considered. Finally, adjacent channel interference effects in the case of analog and digital FM transmission wdl be briefly discussed.

A . Radiated FM Spectra

Since the necessary bandwidth of 16 kHz is specified by CCIR for class F 3 of emissions [28] it would be interesting to compare spectrum components 8 and 17 kHz away from the carrier in the case of analog and digital FM transmission. This comparison is made in Table X for various modulating signals.

B. Receiver Sensitivity

The values of the receiver sensitivity in the digital mode of operation at bit rates of 9.6 and 12 kbits/s are 0.47 and 0.40 pV, respectively. For the sake of comparison, the 12 dB SINAD receiver sensitivity is 0.27 pV. The degradation of the receiver sensitivity in the case of digitized speech transmission is 4.8 dB at 9.6 kbits/s. The degradation at 12 kbits/s is 3.4 dB.

C. Co-Channel Rejection

The worst-case values of co-channel rejection in the case of digital FM transmission for the bit rates of 9.6 and 12 kbits/s are 7.9 and 7.3 dB, respectively (Table Vj. In the analog mode of operation the co-channel rejection is 5 dB.

D. Adjacent Channel Selectivity

The worst-case values of adjacent channel selectivity in the digital mode of operation at bit rates of 9.6 and 12 kbits/s are -65.5 and -60 dB, respectively (Tables VI1 and IX). The adjacent channel selectivity in the case of analog FM transmission is -77 dB.

E. Operating Range

Using the results of the receiver sensitivity measurements under static nonfading conditions (Section X-B) and assuming

PETROVIC: DIGITIZED SPEECH TRANSMISSION 87

TABLE X SPECTRUM COMPONENTS 8 AND 17 kHz AWAY FROM CARRIER FOR ANALOG AND DIGITAL FM

Type of Modulation Level of FM Spectrum Relative to Carrier (dB)

8 kHz away from carrier 17 kHz away from carrier

VOSIhl 1 kHz tone. A f = 3 kHz

3 kHz tone. Af = 5 kHz l / T = 9.6 kbitsis, 11 = 0.8 1/T = 1 2 kbits/s, h = 0.7

-45 -33 -20 -24 - 2 2

-80 -75 -66 -64 - 5 2

an inverse fourth-power dependence of received power on range, it can be found that the operating range in the digital mode of operation at 9.6 kbits/s is 0.76 Ro. Here Ro is the operating range in the case of analog FM transmission. For the bit rate of 12 kbits/s the operating range is 0.82 R o .

However, this simple model of radio propagation ignores the very substantial received signal fluctuations due to fading. Using the measured BER performance at 9.6 kbits/s and known statistics of fades the performance degradation of 3.5 dB can be predicted under fading conditions at a 5 X BER. In the same way the degradation of 7 dB owing to fading is anticipated at a SINAD of 12 dB in the case of analog FM transmission. This means that the analog and digital voice radio systems will perform about the same in a given opera- tional environment, This expectation is confirmed by the results of field tests.

F. Channel Reuse Distance To gain insight into the implications of digitized speech

transmission on channel reuse distance of existing sets a model of co-channel and adjacent channel interference in land mobile radio suggested by Gosling [29] is used. This model is devel- oped assuming inverse fourth-power dependence of mean received signal power on range and uncorrelated Rayleigh fading for wanted and interfering signals.

Assuming the co-channel protection ratio values of 5 dB and 8 dB for analog and digital FM transmission, respectively, and a probability of interference-free reception of 0.9, the increase in channel reuse distance of existing sets due to digitized speech transmission will be in the ratio 1.13 to 1.

G. Adjacerzt Channel Interference Effects

In regard to adjacent channel interference there is a high risk zone around any transmitter, within which the probability of this type of interference is unacceptably high. Applying the same model as in the previous case and assuming the adjacent channel protection ratio values of -75> -65. and -60 dB for analog FM, digital FM with 1/T = 9.6 kbits/s and digital FM with l / T = 12 kbits/s, respectively, and a probability of interference-free reception of 0.99. the radius R, of high risk zone for all three cases of interest is calculated. The values of R , are 0124, 0114, and DilO for analog FM and digital FM at bit rates of 9.6 and 12 kbitsis, respectively, where D is the distance between wanted and interfering transmitters.

XI. CONCLUSION

A detailed analysis of digitized speech transmission over existing VHF FM mobile radio sets is presented in this paper.

Objective criteria expressed in terms of bit error rate are determined for the performance evaluation of analog FM radios when transmitting digital voice. These criteria correspond with the nature of digital transmission and can be simply measured. In this way the definitions based on the signal-to-noise ratio at the receiver audio output are avoided and, in addition, there is no need for the repetitive use of subjective evaluations. The aforesaid criteria are used to define the selected technical characteristics such as the receiver sensitivity, co-channel rejection and adjacent channel selectivity in the case of digital FM transmission. Further, the concept of performance measurements of existing sets in the digital mode of operation is suggested. The results of extensive performance measurements are presented and the optimum values of design parameters of digital voice radio system are deduced from the obtained results. The values of modulation index h of 0.8 and 0.7 are chosen as the optimum ones at bit rates of 9.6 and 12 kbits/s, respectively.

Finally, the implications of digitized speech transmission on the technical characteristics, operating range, channel reuse distance, and adjacent channel interference performance of existing sets are discussed. The degradation of the receiver sensitivity in the case of digitized speech transmission is found to be 4.8 dB at 9.6 kbits/s and 3.4 dB at 12 kbits/s. The resistance to co-channel interference is 2-3 dB lower in the d i a - tal mode of operation. The values of adjacent channel selectivity in the case of digital FM transmission are -65 dB and -60 dB at bit rates of 9.6 and 12 kbits/s> respectively. Using the results of laboratory measurements it can be predicted the reduction in operating range of 24 and 18 percent at transmission rates of 9.6 and 12 kbits/s, respectively. However, it seems reasonable to expect that the analog and digital voice radio systems will perform about the same in a given opera- tional environment due to the fact that the higher performance degradation in the analog mode of operation is anticipated under fading conditions. It has been found that the increase in channel reuse distance of existing sets due to digitized speech transmission is in the ratio 1.13 to 1. The radius of high risk adjacent channel interference zone around any transmitter in the digital mode of operation will be 1.7 and 2.4 times larger at bit rates of 9.6 and 12 kbits/s, respectively.

It should be mentioned that the same procedure for determination of performance criteria and definition of selected technical characteristics as well as the concept of performance measurements can be applied to the future mobile sets employing spectrally efficient digital modulation techniques such as Gaussian filtered minimum-shift keying [9] and tamed frequency modulation [ lo] . In addition, the work


reported in this paper is of interest for CCIR Study Pro- gramme BP8 [30], w h c h relates to digitized speech transmission in the land mobile service.

ACKNOWLEDGMENT

The author would like to thank Dr. Z. Stojanovii and Professor I. Stojanovid, Department of Electrical Engineering, Beograd, for useful comments concerning this paper. He also wishes to acknowledge several helpful discussions with Mr. M. Damjanovic‘, Ei-FEU Pionir, Beograd.

131

141

151

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P. van der Wurf, R. J . Sluyter, and W. A. M. Snijders, “Transmission of digitized speech and data via existing mobile sets for radio telephony,” Conf. Proc. World Telecommun. Forum, pp. 1.4.7.1-1.4.7.5, Geneva, Oct. 6 8 , 1975.

IM. R. Andrewes, “Digital modulation techniques for mobile communications,” presented at the Inst. Elec. Eng. Colloquium on Modulation Techniques for Mobile Services, Dec. 13, 1976, London, Digest No. 1976157, pp. 5/1-5/2. D. Veray, “Transmissions numeriques sur voies radio,” Revue Technique-Thomson CSF, vol. 8, Dec. 1976. R . E . Bailey, “A digitally encoded voice encryption system,” presented at the IEEE Veh. Technol. Conf., March, 16-18 1977, Orlando, FL. H. M. Sachs, “Digital voice considerations for the land-mobile radio services,” presented at the 27th IEEE Veh. Technol. Conf., pp. 207-219, Mar. 16-18, 1977, FL. K. Hirade and K. Murota, “A study of modulation for digital mobile telephony,” presented at the 29th IEEE Veh. Technol. Conf., pp. 13-19, 27-30, Mar. 1979, Chicago. D. Muilwijk, “Tamed frequency modulation-a bandwidth-saving digital modulation method, suited for mobile radio,” Philips Telecomm. Rev., vol. 37, pp. 35-49, Mar. 1979. J . Noordanus, “New digital phase modulation methods to establish digital voice transmission in mobile radio networks, with optimum spectrum efficiency,” Conf. Proc. of the World Telecommun. Forum, pp. 2.3.3.1-2.3.3.7, 23-26, Sept. 1979, Geneva. R. H. Coe, D. J. McQuade, and L. Weiss, “Digital voice protection system and method,” U.S.A. Patent 4 167 700, Sept. 11, 1979. P. Petrovi;, “Spectral shaping of digitized speech for mobile radio applications,” Electron. Lett . , vol. 16, pp. 363-365, May 9, 1980. -, “Adaptive delta modulator for mobile radio communications,” in Proc. IEEE Int. Conf. Commun., pp. 4 .4 .14 .4 .5 , June 4-7, 1978, Toronto, Canada. - . “An analvsis of digitized soeech transmission over existing

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VHF FM mobile radio sets,” presented at the Inst. Elec. Eng. Int. Conf. Commun. Equip. and Syst., Apr. 16-18, 1980, Birmingham, Publ. No. 184, pp. 192-196. W. R. Bennett and J. R. Davey, Dura Transmission. New York: McGraw-Hill, 1965. CCIR Recommendation 331-4, XIV Plenary Assembly, 1978, Kyoto. “IEEE test procedure for frequency-modulated mobile communications receivers,” IEEE Trans. Veh. Technol., vol. VT-18, pp. 86-99, Aug. 1969. EIA Standard RS-204-A. CCIR Recommendation 332-4, XIV Plenary Assembly, 1978, Kyoto. R. T. Buesing, “Modulation methods and channel separation in the land mobile service,” IEEE Trans. Veh. Technol., vol. VT-19, pp. 187-206, May 1970. J . R. Glasser, “Adjacent channel interference in FM communications systems,” presented at the 18th IEEE Veh. Technol. Conf., Dec. 1967, pp. 87-97. A. S. House, C. E. Williams, M. H. L. Hecker, and K. D. Kryter, “Articulation-testing methods: consonantal differentiation with a closed-response set,” J . Acousr. SOC. A m . , vol. 37, pp. 158-166, Jan. 1965. N . B . Pokrovskiy, Raschet i izmereniye razborchivosti rechi. Moscow: Svyazizdat, 1962. M. A. Sapozhkov, The Speech Signal in Cybernetics and Com- munications. Moscow: State Communications and Radio Publishing House, 1963. P. Benedik, M. Rebolj and M. Zornada, “Comparison of SSB: FM modulation technique in tactical military communications in 20-80 MHz,” presented at the Inst. Elec. Eng. Int. Conf. Commun. Equip. Syst., April 1976, Birmingham, pp. 340-343. A. J . Leitich, “Voice scrambler design for mobile radio,” in Proc. Int . Conf. Cybern. Soc., pp. 51-55, 19-21 Sept. 1977, Washington DC . CCIR Recommendation 478-2 (Mod I), Conclusions of the Interim Meeting of Study Group 8 (Mobile services), Nov. 2 6 D e c . 19, 1980, Geneva. W. Gosling, “A simple mathematical model of co-channel and adjacent channel interference in land mobile radio,” Radio Electron. E n g . , vol. 48, pp. 619-622, Dec. 1978. Study Programme BP/8, Conclusions of the Interim Meeting of Study Group 8 (Mobile Services), Nov. 26-Dec. 19, 1980.

Predrag M. Petrovic‘ was born in Beograd, Yugoslavia, on December 22, 1947. He received the Diploma Engineer and M.Sc. degrees in electrical engineering from the Faculty of Electrical Engineering, University of Beograd, Beograd, Yugoslavia, in 1971 and 1978, respectively.

Since 1972 he has been with Research and Development Institute of Elektronska Industrija, Beograd, Yugoslavia. His current research in- terests include digitized speech transmission

techniaues for mobile radio and methods for low bit rate sueech encodine.

Digitized Speech Transmission at VHF Using Existing FM Mobile Radios

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