OFDM Transmitter Experiment

This is a short tutorial on OFDM signalling. I am always fascinated by OFDM (Orthogonal Frequency Domain Multiplexing) signalling method used in modern communication systems. OFDM method is widely used in WLAN (WiFi) and 4G wireless communications, and most probably future 5G systems. OFDM is a method that uses multiple carriers (e.g. sine waves) to transport information or data, and these carriers are harmonically related (e.g. if frequency of Carrier 1 is 2 MHz, then frequency of Carrier 2 will be 4 MHz, Carrier 3 is 6 MHz and so forth), hence the term ‘orthogonal’ because sine waves with such frequencies are orthogonal when we integrate their product. Moreover, each of these harmonically related carriers can be independently modulated by the digital data. I tried to generate my own OFDM signals, using a custom design hardware as shown in the picture. The hardware is actually a training kit we designed to teach basic digital and analog communication principles using a single carrier. So recently we reprogrammed the hardware to also generate multiple carriers to support OFDM signalling. Here we just implement a ‘basic’ OFDM scheme. We input a 1 Mbps (mega bit per second) digital data bit stream, split it into 8 parallel tracks of 0.125 Mbps each, and each of these parallel tracks then modulate a carrier using BPSK (binary phase-shift keying) approach. It is basic because in commercial system, each carrier can be modulated using any digital modulation methods like BPSK, QPSK, 16-QAM, 64-QAM etc. Also, commercial system implements a guard interval between each data bits to further resist the effect of fading, and typically forward error coding (FEC) is employed to encode the bit stream in each parallel track. Nevertheless, this is still an interesting demo, do check out the video where I superimpose the actual time-domain and frequency-domain waveforms side-by-side. An interesting phenomenon prevalent in OFDM signalling is the high PAPR (peak to average power ratio) in the time-domain waveform, you can see that in the video.