A New Software-Defined Vehicular Radio Communications System Based on DS-CDMA Technology Using Orthogonal Walsh-Hadamard Codes

This paper presents a new software-defined vehicular radio communication system based on the DS-CDMA technology using orthogonal Walsh-Hadamard codes. The proposed radio communication system can simultaneously receive an audio stereo signal and a data signal with DS-CDMA modulation within standard FM radio broadcasting (87.5 – 108 MHz). This study also investigated the transmission/reception of the same data signal with frequency modulation in different ISM radio bands. The data transmission rate is significantly increased to 32 kbit/s using DS-CDMA technology, in which the input data signal is divided into 16 parallel sequences that are spread with orthogonal Walsh-Hadamard sequences. This reduces multi-access interference, jamming, and their contribution to the system self-noise. To perform symbol synchronization at reception, the Walsh-Hadamard codes of length 16 were reshaped by inserting synchronization pulses at the transmitter and, correspondingly by appending padding zeros at the receiver. The paper presents a new software-defined radio receiver capable to receive at the same time the data signal with DS-CDMA modulation, as well as the audio stereo signal. The proposed radio receiver includes a carrier synchronization loop operating at the baseband and a novel symbol timing synchronization loop based on synchronization pulses. The proposed radio communication system was implemented in MATLAB/Simulink and simulated under the conditions of channel impairments, considering the carrier frequency offset and timing offset on AWGN, Rayleigh, and Rician communication channels. To validate the simulation results, the bit error rate, depending on the signal-to-noise ratio of the received signal, was plotted for different communication channel models. The experimental results were obtained under laboratory conditions using USRP-type devices.