Scott Nelson Carney

Narrowband FM cellular system

The voice and data transmission parameters of a spectrally-efficient analog frequency division multiple access (FDMA) cellular system with 12.5 kHz channel assignments are described. Low-speed 100 bit/s Manchester data is transmitted during voice activity and provides channel control. In addition, 200 bit/s baseband data patterns support channel supervision. The use of this subaudible signaling method has yielded improved system operation. Eliminating the high-frequency supervisory audio tone has reduced adjacent-channel interference and has allowed increased voice deviation. Using the low-speed data for control messages does not require interruption of the voice activity. The handoff and disconnect protocols have improved falsing protection over current systems.<<ETX>>

Data transmission performance of 18 kbps non-coherent GMSK in the land mobile environment

Data is presented showing the BER performance of 18 kbps GMSK using non-coherent demodulation. Multilevel decision data detection is used for data recovery in the radio designed for use at 851-870 MHz with a 25 kHz channel separation. A 0.2 Gaussian FIR and a 7-pole analog premodulation filter are compared for frequency response, transmit eye pattern, and modulated spectrum. Performance is compared for two receivers with different IF selectivities provided by crystal filters having 8-poles (10.7 kHz bandwidth) and 10-poles (13.6 kHz bandwidth). Received eye patterns are shown to compare distortion from the receiver bandpass filters. Static BER data is compared along with sensitivity degradation from frequency offset. Receiver susceptibility to adjacent-channel interference is also presented.

Design and Test of a Spectrally Efficient Land Mobile Communications System Using LPC Speech

A communication system was built and tested to operate in the land mobile VHF band (150-174 MHz) at a channel separation of only 6 kHz. The audio source was digitally encoded at 2.4 kbits/s using linear predictive coding (LPC). The speech data stream was transmitted by frequency shift keying (FSK) which allowed the use of class-C transmitters and discriminator detection in the receiver. Baseband filtering of the NRZ data resulted in a narrow transmitter spectrum. The receiver had a 3 dB bandwidth of 2.4 kHz which allowed data transmission with minimal intersymbol interference and frequency offset degradation. A 58 percent eye opening was found. Bit error rate (BER) performance was measured with simulated Rayleigh fading at typical 150 MHz rates. Additional tests included capture, ignition noise susceptibility, adjacent channel protection, degradation from frequency offset, and bit error effects upon speech quality. A field test was conducted to compare the speech quality of the digital radio to that of a conventional 5 kHz deviation FM mobile radio.

Co-channel interference comparison for conventional land mobile frequency modulation and 2400 BPS LPC/FSK system

The bit error rate (BER) performance of a Narrowband Frequency Shift Keyed (FSK) data transmission system is examined under co-channel interference conditions. Bit error rate data is presented as a function of carrier-to-interference (C/I) ratio for both static and simulated Rayleigh fading signals. Gaussian baseband premodulation data filtering is used in the FSK transmitter. The receiver has a 2.4 kHz bandwidth and discriminator detection. Methods of testing are described. Voice intelligibility of the Linear Predictive Coded (LPC) Speech is compared to that of a conventional 5 kHz deviation FM receiver for various C/I ratios. An equivalent audio tone-to-interference ratio (T/I) was determined for the Narrowband LPC/FSK system by comparing BER data and C/I measurements to FM audio T/I and C/I data. Rayleigh fading C/I performance is also compared.