Ian D. Holland

The Gaussian free space optical MIMO channel with Q-ary pulse position modulation

The main drawback in communicating via the free space optical (FSO) channel is the detrimental effect the atmosphere has on a propagating laser beam. Atmospheric turbulence causes random fluctuations in the irradiance of the received optical laser beam, commonly referred to as scintillation. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of scintillation. In particular, the FSO multiple-input multiple-output (MIMO) channel with Q-ary pulse position modulation (QPPM) and transmit repetition under the assumption of non-ideal photodetection is analyzed in terms of its uncoded bit error rate (BER) and ergodic channel capacity. This analysis is based, in part, on the use of irradiance fluctuation samples that were obtained from a laser range experiment that was conducted in the presence of moderate turbulence conditions. Expressions are derived for the log-likelihood ratio (LLR) of a received bit, uncoded BER and ergodic capacity. Using these results it is shown that large gains are possible with the use of MIMO combined with strong coding techniques.

Multiple-input multiple-output options for 60 GHz line-of-sight channels

The 60 GHz band is an attractive candidate for wireless indoor communications as it offers a large amount of license free spectrum. Non-rich scattering situations, which commonly arise at 60 GHz due to the particular propagation conditions, result in relatively high spatial correlation. Spatial multiplexing, a widely considered concept for multiple-input multiple-output (MIMO) communications, can offer large capacity gains if the spatial correlation is low. In this paper, we demonstrate that spatial multiplexing (SM) in conjunction with polarisation diversity is a viable option for 60 GHz line-of- sight (LoS) channels. The performance of the proposed scheme, which uses two antenna elements with orthogonal polarisation at each end of the transmission link, is assessed in terms of capacity. It is then compared to an optimal MIMO beamforming scheme that uses two parallel and identical antennas per link end. Furthermore, the impact of antenna misalignment is investigated.

Performance of an adaptive QAM scheme over correlated Rayleigh fading with non-zero delay

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These schemes aim to utilise channel capacity better than fixed transmission schemes, by adapting signal transmission parameters, such as modulation constellation and transmit power. Adaptive modulation schemes, which adapt the modulation constellation, have gained considerable favour for exploiting time-varying channel conditions without increasing the level of cochannel interference. However, the traditional adaptive modulation schemes are designed assuming zero delay between the channel estimation and the modulation mode adaptation. In the case of non-zero delay, the transmitter would update the modulation mode based on outdated channel state information from the receiver. We investigate an adaptive quadrature amplitude modulation (AQAM) scheme, which, by way of retransmissions, allows a targeted reliability level to be met, even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated. Closed form expressions for the average number of bits per symbol (BPS) throughput and the average bit error rate (BER) of the proposed scheme are derived.

Analysis of an adaptive QAM scheme with non-zero delay for Rayleigh fading channels

Link adaptation techniques have recently been proposed as a spectrally efficient method of obtaining high quality service for mobile communication systems. These techniques adapt signal transmission parameters, e.g. modulation constellation and transmit power, according to time-varying channel conditions, in order to better utilise channel capacity compared to fixed transmission schemes. Adaptive modulation schemes, which adapt modulation constellation, exploit time-varying channel conditions without increasing the level of co-channel interference, and have gained considerable favour for this reason. However, one drawback is the effect of estimation delay, whereby the transmitter updates modulation mode based on outdated channel state information from the receiver. In this paper, we propose an adaptive quadrature amplitude modulation (AQAM) scheme, which by way of retransmissions, allows a targeted reliability level to be met even in the presence of non-zero delay. The effect of non-zero delay on performance is then investigated, resulting in a closed form expression for the average number of bits per symbol (BPS) throughput.

Soft-combining schemes for JPEG image transmission over wireless channels

The evolution of digital mobile communications along with the increase of integrated circuit complexity has resulted in frequent use of error control coding to protect information against transmission errors. Soft decision decoding methods, such as the maximum a posteriori (MAP) decoding algorithm, process soft information and aim at minimizing bit error rate. This paper investigates application of a trellis-based MAP decoding approach to JPEG image transmission in wireless channels. When used with a soft-combining technique, it is shown to improve the performance significantly. This facilitates transmission of highly error-sensitive JPEG images in mobile radio systems, even when the ratio of average bit energy to noise power spectral density is quite low.

A high-frequency divider in 0.18 µm SiGe BiCMOS technology

High speed frequency dividers are critical parts of frequency synthesisers in wireless systems. These dividers allow the output frequency from a voltage controlled oscillator to be compared with a much lower external reference frequency that is commonly used in these synthesisers. Common trade-offs in high frequency dividers are speed of division, power consumption, real estate area, and output signal dynamic range. In this paper we demonstrate the design of a high frequency, low power divider in 0.18 µm SiGe BiCMOS technology. Three dividers are presented, which are a regenerative divider, a master-slave divider, and a combination of regenerative and master-slave dividers to perform a divide-by-8 chain. The dividers are used as part of a 60 GHz frequency synthesizer. The simulation results are in agreement with measured performance of the regenerative divider. At 48 GHz the divider consumes 18 mW from a 1.8 V supply voltage. The master-slave divider operates up to 36 GHz from a very low supply voltage, 1.8 V. The divide-by-8 operates successfully from 40 GHz to 50 GHz.