DESIGN AND EXPERIMENTAL EVALUATION OF A PILOT TONE AIDED CARRIER RECOVERY SYSTEM FOR WIRELESS PERSON

Experimental tests performed on a prototype circuit for pilot-tone aided carrier recovery demonstrate the circuit's ability to provide a feasible, reliable and economic solution for reference signal recovery. The potential applications benefits of reference signal recovery include improved bit error rate performance for wireless modems. SUMMARY An attractive digital modulation for PCS is quadrature phase shift keying (QPSK). Efficient demodulation of QPSK signals requires a carrier recovery (CR) circuit. In conventional CR methods, a phase locked loop (PLL) is employed. In fast multipath fading channels, which characterize wireless personal communications, the performance of a PLL degrades drastically, leading to poor bit error rate (BER) performance and irreducible BER floors. Pilot aided modulation techniques have been shown to be effective in reducing the detrimental effects of fast multipath fading, path loss, delay spread and Doppler shifts. These schemes transmit an unmodulated pilot tone together with the data signal. Since the pilot suffers the same impairments as the signal, it can be used to extract a reference signal which is coherent in frequency and phase with the received signal. The reference can be employed to remove the random phase variations from the received signal and normalize the random amplitude variations. Improved BER performance and lower BER floors become feasible, out-performing noncoherent detection. Additionally, pilot-aided schemes are faster than PLL methods in synchronizing bursty signals. The drawbacks are that the pilots take up additional power andor bandwidth, are sensitive to frequency shifts, and cause some envelope fluctuation. Additionally, if the pilots are located at the ends of the data spectrum, they are susceptible to adjacent channel interference. However, under most circumstances these shortcomings are more than offset by the benefits. We chose the frequency of lOMHz for the carrier. In the transmitter, two tones are added to the modulated signal: One tone at 13MHz, a second at 7MHz. The picture below shows the measured power spectral density of the signal at the transmitter output. Fig. 1. The spectra at the transmitter output. It is comprised of the data spectra, centered at 10MHz; the tone at 13MHz, shown on the right; and the tone at 7MHz, shown on the left. 342 0-7803-2140-5195 $3.00 " 1995 IEEE In the receiver, a band-pass filter (BPFJ passes only the tone at 13MHz. Similarly, BPF2 passes only the tone at 7MHz. Next, the signals at the outputs of these filters are mixed. The resulting signal is comprised of one tone at 20MHz, and a second tone at 6MHz. This signal is filtered by BPF3, which blocks the tone at 6MHz. The resulting signal is connected to a frequency-divide-by-2 circuit, which is followed by a smoothing filter. The resultant signal is in frequency and phase coherence with the received signal, and is shown in Figs. 2 and 3 below. Fig. 2. Time domain display of the recovered reference signal. Frequency of signal is lOMHz, amplitude 1 . 5 5 rms. Fig. 3. Frequency domain display of the recovered reference signal. Center 10.OMHz. The main limitation of the proposed carrier recovery scheme is the relatively large bandwidth of the filters. Numerous authors have shown that in a practical modem, this large bandwidth can allow too much phase noise. The excess phase error can degrade the BER performance of the modem. But if the bandwidth is too small, the system may not be able to track down a frequency-shifted pilot tone (the tone can be shifted due to Doppler spread, frequency-selective fading, temperature effects, component wear, etc.) In practice, one rule of thumb is to design the CR system such that it's bandwidth is between one and two orders of magnitude smaller than the data bandwidth. The system presented here could be improved by using more selective filters and low-noise components. Additionally, differential encodinddecoding operations may be necessary to resolve the 180" phase ambiguity which may be caused by the frequency divider. A seemingly plausible concern is that this CR proposal is wasteful of transmitter power and channel bandwidth. However, since one can choose to invest an arbitrarily small amount of power in the two transmitted tones, and then amplify the received tones as needed, the power spent in the pilots can have negligible effects on the overall power efficiency of the modem. Furthermore, the tones can be placed close to the edges of the band, so that the amount of extra bandwidth they require is negligible.