John W. Nieto

An adaptive link assignment algorithm for dynamically changing topologies

An adaptive link assignment algorithm for the distributed optimization of dynamically changing network topologies is presented. The algorithm is responsible for determining the network connectivity by controlling the selection of links to be established and disconnected. This algorithm is designed to recover from predictable link outages as well as massive unpredictable failures. To minimize computational time complexity as well as to improve transient response. Some known graph-theoretic algorithms are utilized. >

An Investigation of Constant-Envelope Variations of OFDM Waveforms for Use on HF Multipath Fading Channels

Orthogonal Frequency Division Multiplexing (OFDM) has become a very popular technique for digital data transmission on multipath fading channels due to its low computational complexity and simple equalization process. However, the multipath component of these types of channels causes a phenomenon known as frequency selective fading. This type of fading can severely degrade or completely eliminate the signal energy of many of the OFDM tones producing an irreducible error rate, even when no noise is present. Consequently, most OFDM systems operating in multipath fading environments utilize some form of forward error correction (FEC) and block interleaving. OFDM waveforms which utilize FEC are usually referred to as coded OFDM (COFDM). One of the main drawbacks of OFDM and COFDM waveforms is the very large peak power to average power ratio (PAR) which requires the use of very linear power amplifiers (PA) and a large power back-off into the PA. In recent years there has been much interest in creating constant-envelope variations of OFDM and COFDM waveforms in order to overcome the PAR drawback. This paper will investigate constant-envelope (CE) variants of OFDM and COFDM waveforms for use on HF multipath fading channels.

An investigation of OFDM-CDMA waveforms using different modulation schemes on HF multipath/fading channels

One of the key limitations of Orthogonal Frequency Division Multiplexing (OFDM) waveforms when used for data transmission on multipath fading channels is frequency selective fading. This type of fading can cancel out or severely degrade the signal strength of many of the OFDM tones, producing an irreducible error rate. In the early 1990’s, researchers combined some of the characteristics of Code Division Multiple Access (CDMA) and Spread Spectrum (SS) with OFDM in order to create a more robust modulation scheme which could survive frequency selective fading and thus OFDM-CDMA was born. This paper will investigate the performance of OFDM-CDMA waveforms using different modulation schemes on HF multipath fading channels. In addition, several Multi-User Detection (MUD) schemes will be applied in the demodulation process to determine their benefits.

Performance comparison of uncoded and coded OFDM and OFDM-CDMA waveforms on HF multipath/fading channels

Orthogonal Frequency Division Multiplexing (OFDM) is a very popular technique used for data transmission on multipath fading channels. The multipath component of these types of channels causes a phenomenon known as frequency selective fading. This type of fading can severely degrade or completely eliminate the signal energy of many of the OFDM tones producing an irreducible error rate, even when no noise is present. In the early 1990s, researchers combined some of the characteristics of Code Division Multiple Access (CDMA) and Spread Spectrum (SS) with OFDM in order to create a more robust modulation scheme capable of surviving frequency selective fading without the need for forward error correction (FEC) techniques and thus OFDM-CDMA was born. This paper will investigate the performance of uncoded and coded OFDM and OFDM-CDMA waveforms on various HF multipath/fading channels.

Waveform design considerations for transmission of digital voice over HF

The development of digital voice (DV) technology has had a significant impact on high frequency (HF) voice communications. The secure features of DV in combination with the benefits of modern digital signal processing (i. e. equalization, narrow-band interference suppression, FEC, interleaving, etc) have provided users with some great advantages when communicating over challenging HF links. A new, higher quality, DV algorithm (vocoder) called mixed excitation linear prediction enhanced (MELPe) was recently developed and standardized. This new vocoder operates at data rates of 2400 bits per second (bps) and 1200 bps with a goal of providing higher quality DV in applications where limited bandwidth is available to users. However, the MELPe development focused only on the vocoder algorithm and no work was done in the specific area of physical-layer waveforms that would be used to convey the DV bits over wireless links (i.e. in HF, independent of the actual waveform which would be used to carry the data over-the-air). This paper considers how to best combine MELPe with current standardized HF waveforms and also investigates if new waveforms needs to be developed to carry MELPe over HF links.

Constant envelope waveforms for use on HF multipath fading channels

Modern single-carrier HF waveforms require tight digital and analog filters in order to meet current HF bandwidth allocations. Unfortunately, this filtering process produces a large variation in the peak-power of the waveform relative to the average-power (about 3-6 dB depending on the symbol modulation). The effect of this peak power to average power ratio (PAPR) is that an average power back-off is required at the input to a power-amplifier (PA) (when using a peak-power limited PA) to avoid operating in the non-linear region of the PA. Similarly, orthogonal frequency division multiplexing (OFDM) waveforms suffer from an even larger PAPR, typically 6-14 dB. An alternative to these waveforms is to use constant-amplitude (or envelope) waveforms such as continuous phase modulation (CPM) or constant envelope OFDM (CE-OFDM). This paper will investigate the potential of modifying the low data rate waveforms in US MIL-STD-188-110B (110B) with constant-amplitude variants in an effort to improve on-air performance by maximizing the average transmit power. These new constant envelope waveforms will be compared to the standardized 110B single-carrier HF waveforms and to new candidate OFDM waveforms on several HF channels in order to understand the performance trade-offs offered by the new waveforms.

Effects of code rate and channel estimation on OFDM and OFDM-CDMA waveforms

Orthogonal Frequency Division Multiplexing (OFDM) is a widely used technique for data transmission on multipath fading channels. The multipath component of these types of channels causes a phenomenon known as frequency selective fading. This type of fading can severely degrade or completely eliminate the signal energy of many of the OFDM tones producing an irreducible error rate, even when no noise is present. In the early 1990s, researchers combined some of the characteristics of Code Division Multiple Access (CDMA) and Spread Spectrum (SS) with OFDM in order to create a more robust modulation scheme capable of surviving frequency selective fading without the need for forward error correction (FEC) techniques and thus OFDM-CDMA was born. This paper will investigate how the code rate and channel estimation affect the performance of coded OFDM and OFDM-CDMA waveforms on various HF multipath/fading channels.

Understanding the effects of power amplifiers on bandwidth efficient single-carrier and multi-carrier waveforms

Power amplifiers (PAs) convert low-power radio frequency (RF) signals into high-power RF signals. Most wireless communication systems employ power amplifiers in order to increase the operating range of the system. However, this conversion process can have some undesired effects on the underlying physical layer waveforms that are used for communicating. This paper will investigate these undesired effects on two types of waveforms: single-carrier and multi-carrier. Of main interest will be the effects caused by PAs to waveform out-of-band emissions, average and peak transmit power and received signal-to-noise ratio (SNR)

Waveform and RF power amplifier interdependencies in battery-powered tactical radio applications

This paper will examine the subtle interdependencies of physical layer waveforms and RF power amplifiers when applied to a battery-powered tactical radio system. Traditional physical layer analysis usually compares the signal-to-noise ratio (SNR) required to achieve a desired bit error rate (BER) but does not consider how the peak-power to average-power ratio (PAPR) of the waveform will affect the design and performance of the power amplifier. This paper will address both the BER performance of physical layer waveforms and the class/efficiency/power constraints of power amplifiers for use in a battery-powered tactical environment.

Evaluation of a novel Constant Envelope Spread-Spectrum Modulation technique

Constant envelope, spread spectrum modulation is highly desirable for low-power, battery-operated systems - but only if the cost of any increase in receiver complexity or reduction in bit error rate performance is offset by a realizable reduction in either the transmit power or DC power requirements for the system. This paper defines and analyzes a novel approach to a constant envelope spread spectrum modulation type and provides an analysis of the complexity and performance of the modulation scheme. The hybrid continuous phase modulation (CPM) waveform is a constant-envelope modulation that provides an inherent phase and frequency diversity which has similar properties to the standard spread-spectrum m-PSK DSSS modulation. The performance in AWGN, multipath, and a severe interference environment is provided and compared to a simple example of random PN spread-spectrum BPSK modulation and a gold-code spread MSK modulation - both modulation schemes with a similar receiver complexity as the hybrid CPM modulation scheme.