Actual Channel Impulse Response Research Articles

IEEE 802.15.22.3 Spectrum Characterization and Occupancy Sensing Application Testbed

Dynamic Spectrum Access (DSA) is becoming a key enabler for wireless communication technologies. Many DSA concepts—such as the Citizens Broadband Radio Service and TV White Space—rely on spectrum databases to identify available radio channels. However, propagation models and other assumptions used by these databases are usually poor for the given scenario, typically leading to over-conservative calculations in terms of permitted secondary spectrum access, geographical bounds or transmit powers in order to mitigate perceived interference risk. Spectrum sensing allows better understanding of the actual propagation and actual channel impulse response, allowing spectrum databases to tune their propagation models to increase the availability of spectrum for secondary access as assessed by such spectrum databases, or to increase the availability of local access licenses as assessed by the telecommunications regulator. A critical limitation on the uptake of ubiquitous sensing is that measurement systems are either proprietary or do not have commercial use-cases; hence little economy of scale exists to drive deployment. For this reason, the IEEE 802.15.22.3 Task Group was formed to standardize a Spectrum Characterization and Occupancy Sensing (SCOS) system capable of addressing a wide range of sensing tasks and be easily and flexibly deployed across a variety of hardware options. This paper provides a high-level description of this system and a first testbed implementing the standard in a laboratory-controlled environment. Preliminary performance assessments are also reported, where it is shown that the system is able to detect users with a power level down to −80 dBm with a probability of detection equal to 1 and controlled false alarm rates.

Coherent equalization for underwater acoustic communications: A performance evaluation of adaptive MMSE and time-reversal techniques

A simple analytical expression is presented for evaluating and comparing the performance of channel-estimate-based coherent equalizers. A channel-estimate-based equalizer is one whose filter coefficients are calculated as a function of an explicit estimate of the acoustic channel impulse response. This is in contrast to a direct adaptation equalizer whose coefficients are calculated directly as the solution to a data estimation error minimization problem. The equalizer performance expression is different from other commonly used expressions because it is formulated from the viewpoint of the estimate of the channel impulse response rather than the actual channel impulse response. The expression allows the dependence of the performance of coherent equalizers on channel complexity and the quality of the channel estimate to be separately evaluated. The expression is used to evaluate and compare the performance of adaptive MMSE and time-reversal equalizers. [Work supported by ONR Ocean Acoustics.]

A decision feedback equalizer with time-reversal structure

This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered. >