Pilot symbol transmission for time-varying fading channels: an information-theoretic optimization

We consider the optimal design of pilot-symbolassisted modulation (PSAM) in time-varying flat-fading channels (FFC). The FFC is modeled as an autoregressive Gauss-Markov random process, whose realization is unknown at the transmitter or at the receiver. Our measure of optimality for channel estimation is the rate of information transfer through the channel. The parameters that are available for this optimization are the power ratio and the fraction of time that are allocated to pilot transmission. Our approach is different from (and builds upon) a recent study of PSAM for the Gauss-Markov FFC, where the aim was to minimize the maximum steady-state minimum mean square error (MMSE) of channel estimation for equal power allocated to pilot and data symbols and for a fixed pilot insertion ratio. To this end, we examine a lower bound on the capacity and find the optimal pilot transmission parameters that maximize this bound. Our analysis shows that this capacity lower bound is more sensitive to the pilot power allocation ratio in relatively slow fading channels (with the normalized fading rate fDT8~0.01).We observe that for such slow fading rates, optimal power allocation and optimal pilot spacing are more sensitive to the operating SNR. Another finding is that equal power allocation strategy is suboptimal by at most 1 dB for the slow fading rate of fDT8~0.0

A Simple Recursively Computable Lower Bound on the Noncoherent Capacity of Highly Underspread Fading Channels

Real-world wireless communication channels are typically highly underspread: their coherence time is much greater than their delay spread. In such situations it is common to assume that, with sufficiently high bandwidth, the capacity without Channel State Information (CSI) at the receiver (termed the noncoherent channel capacity) is approximately equal to the capacity with perfect CSI at the receiver (termed the coherent channel capacity). In this paper, we propose a lower bound on the noncoherent capacity of highly underspread fading channels, which assumes only that the delay spread and coherence time are known. Furthermore our lower bound can be calculated recursively, with each increment corresponding to a step increase in bandwidth. These properties, we contend, make our lower bound an excellent candidate as a simple method to verify that the noncoherent capacity is indeed approximately equal to the coherent capacity for typical wireless communication applications. We precede the derivation of the aforementioned lower bound on the information capacity with a rigorous justification of the mathematical representation of the channel. Furthermore, we also provide a numerical example for an actual wireless communication channel and demonstrate that our lower bound does indeed approximately equal the coherent channel capacity.The work of T. H. Loh was supported by the 2013 - 2017 Electromagnetics and Time Metrology Programme of the National Measurement Office, an Executive Agency of the U.K. Department for Business, Innovation and Skills, under Projects EMT13018This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/TWC.2016.253167