Hybrid coherent and frequency-shifted-reference ultrawideband radio

Ultrawideband communications often occur in heterogeneous networks where different receivers have different complexity and energy consumption requirements. In this case it is desirable to have a modulation scheme that works well with coherent receivers as well as simpler receivers, namely transmitted-reference (TR) receivers. In particular, we consider a TR scheme that employs slightly frequency-shifted-reference (FSR) signals D.L. Goeckel, Q. Zhang, Slightly frequency-shifted-reference ultrawideband (UWB) radio, IEEE Trans. Commun. (2007)] and thus avoids one of the main drawbacks of conventional TR schemes, namely the need to implement a delay line. We propose and analyze a modulation scheme that works well with both FSR receivers (where it has at least the same performance as conventional TR modulation), and coherent receivers. Coherent receivers receiving conventional TR modulation suffer a 3 dB penalty, because they cannot make use of the energy invested into the reference pulse. Our proposed scheme avoids this drawback by including a data preprocessor that can be viewed as a nonsystematic rate-$1/2$ convolutional code. These codes give 1.5 dB gain over our previously proposed constraint-length-two systematic codes at a BER of $1\times 10^{-4}$ in 802.15.3a CM4 multipath fading channels. We also develop a sliding-window based scheme to derive the template waveform that is needed for coherent rake receivers. This scheme exploits the data preprocessor structure and flexibly uses the received signal over a certain window. The distortion to the self-derived template waveform is a decreasing function of the window length; in the extreme but unrealistic case of a very long window length in a slowly fading channel, the self-derived template waveform is noiseless (i.e., the ideal template without distortion).