It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

Underwater wireless information transfer is of great interest to the military, industry, and the scientific community, as it plays an important role in tactical surveillance, pollution monitoring, oil control and maintenance, offshore explorations, climate change monitoring, and oceanography research. In order to facilitate all these activities, there is an increase in the number of unmanned vehicles or devices deployed underwater, which require high bandwidth and high capacity for information transfer underwater. Although tremendous progress has been made in the field of acoustic communication underwater, however, it is limited by bandwidth. All this has led to the proliferation of underwater optical wireless communication (UOWC), as it provides higher data rates than the traditional acoustic communication systems with significantly lower power consumption and simpler computational complexities for short-range wireless links. UOWC has many potential applications ranging from deep oceans to coastal waters. However, the biggest challenge for underwater wireless communication originates from the fundamental characteristics of ocean or sea water; addressing these challenges requires a thorough understanding of complex physio-chemical biological systems. In this paper, the main focus is to understand the feasibility and the reliability of high data rate underwater optical links due to various propagation phenomena that impact the performance of the system. This paper provides an exhaustive overview of recent advances in UOWC. Channel characterization, modulation schemes, coding techniques, and various sources of noise which are specific to UOWC are discussed. This paper not only provides exhaustive research in underwater optical communication but also aims to provide the development of new ideas that would help in the growth of future underwater communication. A hybrid approach to an acousto-optic communication system is presented that complements the existing acoustic system, resulting in high data rates, low latency, and an energy-efficient system.

Underwater Optical Wireless Communication

This chapter contains sections titled: Introduction Overview of Multicarrier CDMA Systems Channel Model Performance of MC-CDMA System Performance of Overlapping Multicarrier DS-CDMA Systems Performance of Multicarrier DS-CDMA-I Systems Performance of AMC DS-CDMA Systems Performance of SFH/MC DS-CDMA Systems Chapter Summary and Conclusion 
]]>

Overview of Multicarrier CDMA

We consider the problem of estimating sparse communication channels in the MIMO context. In small to medium bandwidth communications, as in the current standards for OFDM and CDMA communication systems (with bandwidth up to 20 MHz), such channels are individually sparse and at the same time share a common support set. Since the underlying physical channels are inherently continuous-time, we propose a parametric sparse estimation technique based on finite rate of innovation (FRI) principles. Parametric estimation is especially relevant to MIMO communications as it allows for a robust estimation and concise description of the channels. The core of the algorithm is a generalization of conventional spectral estimation methods to multiple input signals with common support. We show the application of our technique for channel estimation in OFDM (uniformly/contiguous DFT pilots) and CDMA downlink (Walsh-Hadamard coded schemes). In the presence of additive white Gaussian noise, theoretical lower bounds on the estimation of sparse common support (SCS) channel parameters in Rayleigh fading conditions are derived. Finally, an analytical spatial channel model is derived, and simulations on this model in the OFDM setting show the symbol error rate (SER) is reduced by a factor 2 (0 dB of SNR) to 5 (high SNR) compared to standard non-parametric methods - e.g. lowpass interpolation.

Estimation of Sparse MIMO Channels with Common Support

/pdf/ad-hoc-networks-technologies-and-protocols-mahkhtf0hu.pdf

Ad Hoc Networks: Technologies and Protocols

Space-time trellis codes (STTCs) have been designed for quasi-static Rayleigh channels. When these codes are directly used for additive white Gaussian noise (AWGN) channels or Ricean channels with a high K factor, their frame error-rate (FER) performance is no longer optimal. We propose open-loop (no feedback) preprocessing methods for improving the error-rate performance of these codes when used in channels with a high K. For K=0 (Rayleigh channels), these open-loop schemes generally do not alter the FER performance of the STTCs. The preprocessing operation at the transmitter is either to rotate the constellation points in one of the transmit antennas relative to the other by some angle, or suitably divide (unequally) the total power between the two transmit antennas. The angle of rotation and power division is optimized by maximizing the d/sub free/ of the STTCs. Simulation results show that for AWGN or Ricean fading channels with a large K, the FER performance can be significantly improved by doing either rotation or power division at the transmitter. While the FER performance is nearly unchanged for quasi-static Rayleigh channels, the improvement in performance is between 1 to 3 dB for Ricean or AWGN channels.

Performance enhancement of space-time trellis codes when encountering AWGN and Ricean channels

Turbo codes and Low Density Parity Check (LDPC) codes have been shown to be practical codes that can approach Shannon capacity in several communication systems. In terms of performance and implementation complexity, LDPC codes and turbo codes are highly comparable, especially at coding rates around 1/2. In many recent wireless standards such as 3GPP LTE and WiMax, both turbo and LDPC codes have been recommended at the encoder. However, the decoder for turbo codes involves trellises and the BCJR algorithm, while the decoder for LDPC codes uses sparse graphs and the message passing algorithm. Therefore, in several implementations, a designer is forced to implement either the turbo decoder or the LDPC decoder. The main idea behind this work is to enable the implementation of both decoders using a common architecture. We view the constituent convolutional code in a turbo code as a block code, and construct a sparse parity check matrix for it. Then, the sparse matrix and the associated bipartite graph are used for decoding the convolutional code by soft message passing algorithms. Simulation results show a manageable degradation in performance with a reduction in complexity.

http://www.ncc.org.in/download.php?f=NCC2010/A1_6.pdf

Common architecture for decoding turbo and LDPC codes

Spatial correlation is introduced when antennas are not well separated, which typically leads to performance degradation in space diversity systems for flat fading wireless channels. However, in a frequency-selective environment with orthogonal frequency division multiplexing (OFDM), multipath diversity can help in overcoming this performance degradation. This is due to transformation of a highly spatially correlated channel impulse response to less spatially correlated channel frequency response inherently by an OFDM system in the presence of multipath diversity . In this letter, we numerically evaluate the minimum antenna separation required for a target spatial correlation in the frequency domain in situations with low multipath diversity. Performance results for two highly spatially correlated receive antennas with such spacing are provided for low multipath diversity channels and it is found to be within 1 dB (at BER=10/sup -3/) of spatially uncorrelated reception for QPSK modulation.

On OFDM systems with spatially correlated antennas in low multipath diversity situations

This paper addresses the issue of peak-to-average power ratio (PAPR) reduction in multiple-input and single-output systems with a number of power allocation schemes. We propose a constellation-based power allocation scheme that is capable of achieving a lower PAPR when compared to the optimal power allocation scheme. Although this method of power allocation improves the PAPR performance, there is a slight degradation in bit error rate (BER) performance. However, the degradation in BER performance can be compensated through the use of co-ordinate interleaving technique. We also propose a constrained-optimal power allocation scheme with which it is possible to control the PAPR

Power Allocation in MISO Channel With Co-Ordinate Interleaving Technique

Open-loop (no feedback) pre-processing methods for improving the error-rate performance of space-time trellis codes in additive white Gaussian noise (AWGN) and Ricean channels which nearly preserve the performance in Rayleigh channels are proposed. The rotation based pre-processing preserves the Rayleigh channel performance, whereas there is a small degradation in performance in the power division based pre-processing. This degradation depends on the trellis complexity, and is more pronounced for the lower state trellis codes. In this letter, we propose a one-bit feedback method, that improves the performance of the power division method in Rayleigh channels. Simulation results for a 2/spl times/1 system show that this improvement is significant for the higher state trellis codes. The one-bit feedback is used to determine which of the two channels has a higher gain, and the higher power is assigned to the corresponding transmitter.

Devendra Jalihal

Papers

Performance enhancement of space-time trellis codes when encountering AWGN and Ricean channels

Common architecture for decoding turbo and LDPC codes

On OFDM systems with spatially correlated antennas in low multipath diversity situations

Power Allocation in MISO Channel With Co-Ordinate Interleaving Technique

Pre-processed space-time trellis codes with one-bit feedback