Frequency tracking techniques for maximum likelihood equalizers for high speed wireless mobile networking in a doubly-faded outdoor channel

This paper presents frequency tracking techniques based on per-survivor channel tracking and phase-locked loops to combat the performance degradation in maximum likelihood sequence estimation due to Doppler shifts and residual carrier offsets. The targeted application is in high bit rate mobile wireless networking in the outdoor environment with mobility of up to 100 km/hr. and achieving a bit rate of 2 Mbps in the 2.4 GHz ISM band. With a low cost frequency reference of 2.5 ppm and a maximum Doppler of 200 Hz, the residual frequency offset felt at the receiver can be as much as 1200 Hz, giving rise to the need for frequency tracking, especially for large data packets. The techniques we propose eliminates the stability and narrow capture range problems exhibited by traditional phase-locked loops when applied to the maximum likelihood equalizer due to the large delay introduced in the feedback loop and shows a factor of 10-100 improvement in the BER at an E_b /N₀ of 20 dB