Comparison of adaptive beamforming and orthogonal STBC with outdated feedback

We compare the achievable performance of adaptive beamforming (A-BF) and adaptive orthogonal space time block coding (A-OSTBC) with outdated channel feedback. We extend our single user setup to multiuser diversity systems employing adaptive modulation, and illustrate the impact of feedback delay on the multiuser diversity gain with either A-OSTBC or A-BF. Using closed-form expressions for spectral efficiency and average BER of a multiuser diversity system derived in this paper, we prove that the A-BF scheme outperforms the A-OSTBC scheme with no feedback delay. However, when the feedback delay is large, the A-OSTBC scheme achieves better performance due to the reduced diversity advantage of A-BF. We observe that more transmit antennas bring higher spectral efficiency for BF. With small feedback delay, this becomes inverted using OSTBC, due to the effect of channel-hardening. Interestingly, however, we show that A-OSTBC with multiple users enjoys improved spectral efficiency when the number of transmit antennas is increased and the feedback delay is significant

The adaptive value of polyembryony.

From an evolutionary standpoint, the production of offspring is the single most important aspect of an animal's life. Offspring carry an individual's genes into the next generation and it is the differential representation of genes in a population that drives evolutionary change. There are a variety of ways in which animals create offspring, ranging from cases where parents make identical copies of themselves by budding or parthenogenesis, to the standard case in vertebrates where gametes from a male and female fuse in sexual reproduction to produce the next generation. In this article we describe an usual variant of sexual reproduction, polyembryony.

Design optimisation of ring elements for broadband reflectarray antennas

Plane wave scattering from a flat surface consisting of two periodic arrays of ring elements printed on a grounded dielectric sheet is investigated. It is shown that the reflection phase variation as a function of ring diameter is controlled by the difference in the centre resonant frequency of the two arrays. Simulated and measured results at X-band demonstrate that this parameter can be used to reduce the gradient and improve the linearity of the reflection phase versus ring size slope. These are necessary conditions for the re-radiating elements to maximise the bandwidth of a microstrip reflectarray antenna. The scattering properties of a conventional dual resonant multilayer structure and an array of concentric rings printed on a metal backed dielectric substrate are compared and the trade-off in performance is discussed.

Generic SoC QR Array Processor for Adaptive Beamforming

A generic architecture for implementing a QR array processor in silicon is presented. This improves on previous research by considerably simplifying the derivation of timing schedules for a QR system implemented as a folded linear array, where account has to be taken of processor cell latency and timing at the detailed circuit level. The architecture and scheduling derived have been used to create a generator for the rapid design of System-on-a-Chip (SoC) cores for QR decomposition. This is demonstrated through the design of a single-chip architecture for implementing an adaptive beamformer for radar applications. Published as IEEE Trans Circuits and Systems Part II, Analog and Digital Signal Processing, April 2003 NOT Express Briefs. Parts 1 and II of Journal reorganised since then into Regular Papers and Express briefs

Equalisation with adaptive time lag

In an adaptive equaliser, the time lag is an important parameter that significantly influences the performance. Only with the optimum time lag that corresponds to the best minimum-mean-square-error (MMSE) performance, can there be best use of the available resources. Many designs, however, choose the time lag either based on preassumption of the channel or simply based on average experience. The relation between the MMSE performance and the time lag is investigated using a new interpretation of the MMSE equaliser, and then a novel adaptive time lag algorithm is proposed based on gradient search. The proposed algorithm can converge to the optimum time lag in the mean and is verified by the numerical simulations provided.