Design of a 2.4 GHz radio transceiver for high-speed wireless data networks
The number of applications and services of wireless systems is growing, increasing the performance requirements of the existing transceivers. Although more and more functions of the traditionally analog architectures are now implemented in the digital domain, the main transceiver components are still analog. A wideband transceiver design is proposed and its ability to achieve transmission bit rates of up to 100 MBPS is investigated. The proposed architecture employs PSK or QAM modulation schemes with the addition of a continuous-wave pilot tone that is used for coherent signal demodulation. A single-IF architecture is implemented, with the addition of necessary circuitry to generate the pilot tone at the transmitter and perform coherent demodulation at the receiver. A spectral gap is created at the center of the transmitted signal and the pilot tone is added by taking advantage of the modulating mixer input bias. At the receiver side a SAW filter isolates the pilot, which is then used to perform the demodulation process.
A prototype is designed and a wireless test-bed is implemented operating in the 2.4 GHz ISM band with an intermediate frequency (IF) bandwidth of 30 MHz and a maximum transmitted power of 20 dBm. Its performance is investigated with bit error rate (BER) measurements employing BPSK and QPSK signals and transmission rates up to 10 MBPS. The measured minimum power that is required at the receiver input to achieve a BER of 10−6 is −77 dBm for BPSK signals and −70 dBm for QPSK signals. The effect of various propagating environments is studied with indoor and outdoor BER measurements and using directive and broadbeam antennas. Simple models employing geometrical optics theory are used to model the propagation losses and verify the measured data.
The phase noise of the recovered pilot tone signal is identified as the limiting factor in the transceiver performance. The dependence of the probability of error of the demodulation process to the phase noise is investigated for BPSK, QPSK and 16QAM signals and the limitations and requirements to achieve a transmission rate of 100 MBPS are explored.