Timing synchronization and receiver design for ultra-wideband communications.

摘要

Pulsed ultra-wideband (UWB) technology has potential low-power applications in short-range indoor wireless networking, covering low-rate from 1kbps to high-rate up to 1Gbps. There are mainly two tasks in UWB coherent receivers: one is timing synchronization and the other one is data demodulation. In a typical UWB dense multipath environment, a major challenge to accomplishing those two tasks successfully is sufficient energy capture from a large amount of multipath echoes of ultra-short pulses at low implementation complexity.;In the absence of channel knowledge, we first develop an efficient sampling strategy for correlation-based receivers to accomplish adequate energy capture, using a noisy correlation template constructed directly from the received waveform. Merging our sampling operation based on noisy template (NT) with low-complexity timing acquisition schemes, we derive several low-complexity synchronizers, including enhanced cyclostationarity-based blind synchronizers, as well as data-aided maximum likelihood timing offset estimators, all operating at low frame or symbol rates. Both analysis and simulations confirm evident improvement in timing accuracy when using our noisy template.;Building on the concept of NT proposed in synchronization phase, a unified receiver structure based on NT is developed for peer-to-peer links. After timing is acquired, a cyclic shifted version of the original NT is obtained to act as the template at the data demodulation phase. The NT receiver design enables sufficient energy capture with full multipath diversity, and achieves asymptotically optimal detection performance with robustness to mis-timing. To alleviate the noise effect, a decision directed (DD) scheme is presented to lower the noise variance of the template. The detection error performance of the NT receiver is analyzed and compared with that of RAKE receivers, under realistic channel and timing estimation errors. Insights on the design tradeoffs of NT versus RAKE reception are provided, using unifying metrics that capture the relative importance of various performance-critical factors of individual receivers in the UWB regime. Both analysis and simulations confirm that the NT receiver outperforms the RAKE with a limited number of fingers under practical operating conditions.;Next, we move on to the case of multiple access UWB ad-hoc networking, wherein more than one node may request for simultaneous medium access thus rendering the synchronization task even more demanding. Building on the noted concept of NT that proves to be efficient in the single-user case, we introduce a new weighted average noisy template (WANT) by employing user-specific training symbols. The user-specific WANTs are suited for receiver signal processing in multiple access scenarios. They lead to the design of a multiple access timing synchronizer that is resilient to both multiple access interference and noise, without any channel knowledge. To keep low complexity without degrading accuracy performance, the optimal joint timing estimator is finalized in the form of simpler non-interacting estimators, namely one for each active user.;Finally, we consider the effect of inter-symbol interference (ISI) in high-rate applications. A multiple symbol differential detector (MSDD) is derived from the GLRT rule in order to mitigate ISI. The energy contained in ISI components is collected by estimating the extra symbol-long segments due to ISI other than the interested segment. Although no NT is explicitly used, sufficient energy capture still maintains via averaging multiple symbol-long segments weighted by the data candidates. The resulting MSDD receiver is capable of mitigating ISI effectively for high-rate transmissions, without using the complicated accurate tap-by-tap channel estimations.