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 
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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

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Ad Hoc Networks: Technologies and Protocols

Multiple receive antennas are known to provide receiver diversity but typically require considerable separation between them. We introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. Since the antennas are close, the samples are highly spatially correlated and does not help in performance improvement in a typical wireless system with flat fading. However, this concept is very useful particularly in an OFDM system with frequency selective fading which has an inherent multipath diversity. Under these circumstances, the required space diversity for performance improvement is obtained by the transformation of multipath diversity to useful space diversity in frequency domain inherently by an OFDM system (G. V. Rangaraj et al., June 2003). The minimum separation required between the antennas under such circumstances is derived analytically and we have show that even with a separation of only 0.44/spl lambda/, the required spatial correlation in the frequency domain becomes sufficiently low.

Introducing space sampling for OFDM systems with multipath diversity

This paper presents computationally efficient algorithms for a Digital Signal Processor (DSP) implementation of transmultiplexers, timing and carrier recovery, and detection of data for on-board satellite receiver. The transmultiplexer which generates a time domain sequence of a Frequency Division Multiplexed (FDM) signal is implemented using DFT filter banks. Carrier and timing recovery and data detection are implemented assuming no training sequence. It is very important to design efficient algorithms to perform these tasks so that computational requirements are kept to a minimum thereby saving power. The paper proposes a novel 4th power based algorithm to determine the carrier offset. It is based on the idea that if there are P samples per symbol, then one of them is closer to the ideal Nyquist sampling instant than others, and, has the least ISI. A recursive method to isolate this minimum-ISI sample is proposed. This sample is used to estimate the carrier phase. Since the method identifies the least ISI sequence, it is said to perform both the carrier and timing offset estimation. After compensating for constant frequency offset, timing recovery is performed using a modified Peak Average Energy Criterion (PAEC) algorithm. A Digital Phase Locked Loop (DPLL) is used to track the finer variation in frequency offset. Results of simulations evaluating performance of these algorithms are presented for a number of frequency and timing offset situations. It is seen that the computational load is maximum at "start-up". We have proposed a particular start-up scheme and calculated its peak computational load in Millions of Instructions per second (MIPs).

DSP Algorithms for On-Board Satellite Transmultiplexer and Receiver

Among the linear precoding techniques block diagonalization (BD) based schemes are well-known transmit strategies employed in the downlink of multiuser MIMO (MUMIMO) systems. Employing BD type precoding algorithms at the transmit side suppresses multi-user interference (MUI) by converting a MU-MIMO broadcast channel into multiple single-user MIMO (SU-MIMO) channels and then decomposing the SU-MIMO channels into multiple independent parallel sub-channels. Substantial work has been done in designing linear precoding algorithms. However, the use of these techniques in the implementation of WLAN standards is an open question. In this paper both the traditional as well as the recent techniques for precoding are investigated and schemes proposed for their implementation in the framework of 802.11ac WLAN standard.

Implementation of Block Diagonalization Type Precoding Algorithms for IEEE 802.11ac Systems

We consider a problem of asynchronous communication over a discrete memoryless channel (DMC) A transmitter seeks to communicate $B$ bits of information to a receiver at a random time $V$  For a general distribution of $V$ , we propose an asymptotically error-free variable length coding scheme that adapts the codeword length and distribution based on the arrival probabilities in a slot For a simple class of L-step arrival distributions, we characterize the capacity of the DMC, and also show that the variable length coding scheme achieves higher rates and lower cost of communication in comparison with the fixed length coding scheme studied in prior works Using numerical work, we illustrate our results for a binary symmetric channel in a number of interesting scenarios

Devendra Jalihal

Papers

Introducing space sampling for OFDM systems with multipath diversity

DSP Algorithms for On-Board Satellite Transmultiplexer and Receiver

Implementation of Block Diagonalization Type Precoding Algorithms for IEEE 802.11ac Systems

Variable Length Coding for Asynchronous Communication

Signal detection theory approach to the multiple parallel moving targets problem