Worst-case Channel Conditions Research Articles

Statistical analysis and performance evaluation of optical array receivers for deep-space optical communications under random tracking errors

Telescope array receiver is a viable architecture for Earth-based reception in a deep-space optical communication system. In this paper, effects of random tracking errors on the performance of optical array receivers for an inter-planetary deep-space optical communications link between Earth and Mars is investigated. The paper has two major parts. In the first part, statistical analysis and mathematical modeling of the impact of tracking errors on general direct-detection optical communication receivers is presented. The analytical results show that tracking errors could severely degrade the performance of optical receivers; hence, these need to be compensated, especially in a deep-space link. In the second part, design and analysis of a closed-loop tracking subsystem for telescope array receivers operating in a deep-space link is presented. An end-to-end simulation platform for communication between Earth and Mars is implemented that incorporates direct-detection array receivers and the proposed tracking subsystems to alleviate the effects of random tracking errors. Extreme channel conditions, i.e., maximum distance, strong background noise and turbulence conditions are modeled to evaluate the performance bounds. Simulations results depict that in worst-case channel conditions, the proposed architecture mitigates the impact of tracking errors to be within reasonable limits. Comparison of achievable data rates show that in the presence of random tracking errors, replacement of a large telescope (10m diameter) with an array of relatively smaller-sized telescopes (i.e., 100, 1m telescopes) results in acceptable performance loss (i.e., <13%). Hence, performance degradation due to tracking errors does not pose a major limitation in employing array architecture in deep-space communication. The presented analysis further strengthens the viability of array architecture compared to a monolithic, large telescope for deep-space communications.

A novel user pairing scheme for functional decode-and-forward multi-way relay network

In this paper, we consider a functional decode and forward (FDF) multi-way relay network (MWRN) where a common user facilitates each user in the network to obtain messages from all other users. We propose a novel user pairing scheme, which is based on the principle of selecting a common user with the best average channel gain. This allows the user with the best channel conditions to contribute to the overall system performance. Assuming lattice code based transmissions, we derive upper bounds on the average common rate and the average sum rate with the proposed pairing scheme. Considering M-ary quadrature amplitude modulation with square constellation as a special case of lattice code transmission, we derive asymptotic average symbol error rate (SER) of the MWRN. We show that in terms of the achievable rates, the proposed pairing scheme outperforms the existing pairing schemes under a wide range of channel scenarios. The proposed pairing scheme also has lower average SER compared to existing schemes. We show that overall, the MWRN performance with the proposed pairing scheme is more robust, compared to existing pairing schemes, especially under worst case channel conditions when majority of users have poor average channel gains.

Cyclic Prefix Length Determination for Orthogonal Frequency Division Multiplexing System over Different Wireless Channel Models Based on the Maximum Excess Delay Spread

Cyclic Prefix (CP) is one of the key OFDM parameters. It is used to completely eliminate both Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) as long as the CP length is greater than the channel delay spread. By eliminating the ISI and ICI, the CP compensates for the effect of the multi-path dispersion; but it consumes a considerable amount of the scarce spectrum and the power. Contemporary OFDM systems usually use a fixed and large CP length to tolerate worst case channel condition. This technique, however, causes a loss in bandwidth efficiency as well as consumes relatively more transmitter energy. Therefore, there is a need to adopt the CP length based on the channel parameters. This paper investigates the effect of varying the CP length on the OFDM system over different wireless channel models, where the variable CP length is estimated based on the Maximum Excess Delay Spread (MEDS) of the channel. According to this method, the estimated CP length optimizes the system capacity and improves the overall system performance.

Process-Variation Tolerant Channel-Adaptive Virtually Zero-Margin Low-Power Wireless Receiver Systems

This paper presents a process-variation tolerant, continuously channel-adaptive wireless front-end architecture and related adaptation algorithms to allow a radio-frequency transceiver to function with minimum power at all channel conditions and manufacturing process corner. Current wireless transceiver front-ends are designed for worst case channel conditions and a limited degree of post manufacture tuning is performed to compensate for process variations. It is shown how the proposed architecture can result in significant power savings over current practice without compromising system-level bit-error rate (while keeping the end-user experience unaffected). In contrast to traditional wireless circuits with limited tunability, such a zero-margin design is achieved by close loop adaptation of the wireless front-end circuits to ensure that they only consume the minimum power and deliver just enough performance (and not any more) for any channel condition. The adaptation methodology is applied to a WLAN receiver design and hardware measurement data for an adaptive receiver is presented showing a > 3 × power improvement under best case channel conditions.

Study on Adaptive Modulation Used in Fading Channels

Most of traditional modulation and coding techniques do not adapt to fading or nonlinear conditions. These non-adaptive methods require a fix-link margin to maintain acceptable performance when the channel quality is poor. Thus, these systems are effectively designed for the worst-case channel conditions, resulting in insufficient utilization of the full channel capacity. Adapting to signal fading allows the channel to be used more efficiently since the power rate can be adjusted to take advantage of favorable channel conditions. In this paper, we present a simplified mathematical model to calculate the probability of SER for any given constellation of M-QAM. This model plays an important role in designing spectrally efficient adaptive modulation over nonlinear mobile channels. Instantaneous spectral efficiency and its histogram distribution are found for linear and nonlinear channel cases.

Adaptive noise-predictive maximum-likelihood (NPML) data detection for magnetic tape storage systems

Advanced data detection will be one of the key enablers to achieving the very high areal recording densities of future tape storage systems. Departing from the partial-response maximum-likelihood (PRML)-based read channel design traditionally used in tape systems, this paper describes noise-predictive maximum-likelihood (NPML) detection, which is a technique that has been known for many years in the hard-disk-drive industry but has been introduced for the first time in the tape storage industry in IBM tape drives. In the NPML read channel design, the readback signals are conditioned prior to data detection so that their noise components are statistically decorrelated and reduced in power. This paper describes the basic principles of NPML detection and its application to tape systems in the form of a 16-state detector. It is argued that, because of the inherent variability of the recording channel characteristics in tape drives, fully adaptive NPML detection needs to be realized in order to optimize detection performance. Actual readback waveforms of data recorded on metal particulate as well as barium-ferrite particulate tape media are used to illustrate the error rate performance achieved by 16-state and 32-state NPML detectors. It is shown that, under realistic worst-case channel conditions, a 16-state NPML detector could offer an improvement in error rate after an error-correcting code of approximately two orders of magnitude.

Adaptive modulation and combining for bandwidth and power efficient communication over fading channels

Traditional fading mitigation techniques are designed relative to the worst-case channel conditions, resulting in a poor utilization of the spectrum and the available power a good percentage of the...

Video signal transmission for IS-95 environment

A novel compression scheme for video sequences that is robust to fading errors in a spread spectrum environment is presented. A 3D coder has been designed in which the sequence was decomposed into spatio-temporal sub-bands and encoded using vector quantisation. Channel simulation shows good image reconstruction with a PSNR of 27 dB under the worst-case channel conditions of 18 dB.

Adaptive radio for multimedia wireless links

The quality of wireless links suffers from time-varying channel degradations such as interference, flat-fading, and frequency-selective fading. Current radios are limited in their ability to adapt to these channel variations because they are designed with fixed values for most system parameters such as frame length, error control, and processing gain. The values for these parameters are usually a compromise between the requirements for worst-case channel conditions and the need for low implementation cost. Therefore, in benign channel conditions these commercial radios can consume more battery energy than needed to maintain a desired link quality, while in a severely degraded channel they can consume energy without providing any quality-of-service (QoS). While techniques for adapting radio parameters to channel variations have been studied to improve link performance, in this paper they are applied to minimize battery energy. Specifically, an adaptive radio is being designed that adapts the frame length, error control, processing gain, and equalization to different channel conditions, while minimizing battery energy consumption. Experimental measurements and simulation results are presented to illustrate the adaptive radio's energy savings.

Implementation of high speed data sets with microprogrammable data processors

Modern high speed data sets (or Modems) are usually required to perform a great deal of digital signal processing to maintain acceptable error rates for worst-case channel conditions. Previously, this digital processing task was implemented with dedicated SSI/MSI TTL circuits. With the advent of high speed microprogrammable units (ALU's, sequencers, etc.), it has become possible to replace a random logic design with a functionally equivalent microprogrammed design. This results in better density, lower power requirements, and greater versatility and flexibility. The microprogrammed system described is designed to provide accurate emulation of an existing data set while maintaining the advantages of microprogrammed architecture versus random logic.