Joint Application of Spread Spectrum and OFDM Modulation for Microwave Radio Communication Used for Unmanned Aerial Vehicle

The wireless communication system used by an Unmanned Aerial Vehicle has special requirements. Two separate data streams are used, one for telemetry and control of the vehicle, and one for payload. The first stream is of relatively low data rate, but must be very robust. The second stream requires high data rate, but a best effort approach is acceptable. Real-time remote control usage mandates low response time (turn around latency) of the system, which demands small frames and frequent direction changes over the half duplex channel. These two different requirements must be accounted for in the design of the modulation system. In this paper our solution based on the joint application of a chirp spread spectrum modulation with a correlator based demodulator for the telemetry and control data stream and a high data rate orthogonal frequency division multiplexing (OFDM) modulation for the payload is presented. The OFDM communication occupies the same bandwidth as the spread spectrum modulation, but - unlike the Chirp signal - utilizes its full bandwidth. By using a correlation detector for all timing recovery tasks and keeping the same timing for the OFDM part of the frame, all control loops normally needed for this task (symbol synchronization PLL, COSTAS loop) can be eliminated. This results in the whole signal processing path being a finite impulse response system which aids us in a highly pipelineable design implementation and makes it possible to create a high speed implementation with a low-end field programmable gate array (FPGA) circuit.