J.K. Townsend

The effects of timing jitter and tracking on the performance of impulse radio

Impulse radio (IR) is a promising ultra-wideband technique for tactical military communications. A key feature of time-hopping IR are the very narrow pulses used to convey information. Analysis of such time-hopping schemes under a variety of assumptions have been reported in the literature. However, none of these studies to date consider the effects of timing jitter and tracking on time-hopping in a ultra-wideband (UWB) setting. We consider the effects of timing jitter and tracking on the performance of binary and 4-ary UWB communications. We find that the performance of IR is very sensitive to timing jitter and tracking, at least in part due to the very narrow pulses. We also find that in the presence of timing jitter and tracking, orthogonal 4-ary pulse position modulation (PPM) out performs binary offset PPM at all jitter levels in thermal and pulse noise. Simulation results are presented that quantify the sensitivity of binary and 4-ary IR to timing jitter and tracking error.

A novel impulse radio network for tactical military wireless communications

Two of the major concerns in tactical military wireless communication networks are covertness and throughput. Impulse radio is an ultra-wideband code division multiple access (UWB-CDMA) technique being considered as the physical layer for future networks. Impulse radio exhibits low power spectral density and relatively high immunity to fading but suffers from relatively long acquisition times. In traditional packet radio networks (PRNs), the physical layer link is terminated while scheduling half-duplex transmissions or in the absence of data packets. To re-establish physical layer links in an impulse radio system, signaling packets containing exceptionally long acquisition headers transmitted at higher powers are required. Hence, to improve covertness in an impulse radio based network, we propose a MAC layer scheme we call sustained link networks (SLN) where the physical layer links are maintained continuously. We develop a full-duplexing scheme which takes advantage of the low duty cycle nature of impulse radio to maintain physical layer links capable of supporting bi-directional data transfer. During the periods between data bursts, the physical layer links are maintained by transmitting supplementary data at low bit rates and low power levels. We quantify the performance of an impulse radio receiver implementing the full-duplex scheme developed. We also present performance comparisons of an SLN based on impulse radio with traditional PRN variations implemented using impulse radio.

An algorithmic approach to the optimization of importance sampling parameters in digital communication system simulation

Importance sampling is recognized as a potentially powerful method for reducing simulation runtimes when estimating the bit error rate (BER) of communications systems using Monte Carlo simulation. Analytically, minimizing the variance of the importance sampling (IS) estimator with respect to the biasing parameters has typically yielded solutions for systems for which the BER could be found analytically. A technique for finding an asymptotically optimal set of biasing parameter values, in the sense that as the resolution of the search and the number of runs used both approach infinity, the algorithm converges to the true optimum, is proposed. The algorithm determines the amount of biasing that minimizes a statistical measure of the variance of the BER estimate and exploits a theoretically justifiable relationship, for small sample sizes, between the BER estimate and the amount of biasing. The translation biasing scheme is considered, although the algorithm is applicable to other parametric IS techniques. Only mild assumptions are required of the noise distribution and system. Experimentally, improvement factors ranging from two to eight orders of magnitude are obtained for a number of distributions for both linear and nonlinear systems with memory. >

Performance of local power control in peer-to-peer impulse radio networks with bursty traffic

We present and investigate the performance of an impulse radio system with bursty traffic in a peer-to-peer network, with a combination of a bit rate variation scheme and a local power control algorithm. The peer-to-peer architecture considered is motivated by survivability, covertness, and the need for rapid deployment. Because the performance of a peer-to-peer architecture is topology dependent, we develop a model for generating random configurations of peer-to-peer links, and use the model to analyze the performance of the peer-to-peer network. Power control and bit rate variation techniques are developed to minimize the fraction of time that the aggregate transmitted power exceeds a given threshold. This fraction is further reduced by making initial power assignments based on local information available at connection set-up and by augmenting the closed loop power control scheme with an open loop power level adjustment. The amount of open loop power adjustment made depends on the change in processing gain due to bit rate variation. Simulation results are presented that show the performance of the peer-to-peer system with power control and bit rate variation schemes for different topologies.

Stochastic gradient optimization of importance sampling for the efficient simulation of digital communication systems

Importance sampling (IS) techniques offer the potential for large speed-up factors for bit error rate (BER) estimation using Monte Carlo (MC) simulation. To obtain these speed-up factors, the IS parameters specifying the simulation probability density function (PDF) must be carefully chosen. With the increased complexity in communication systems, analytical optimization of the IS parameters can be virtually impossible. We present a new IS optimization algorithm based on stochastic gradient techniques. The formulation of the stochastic gradient descent (SGD) algorithm is more general and system-independent than other existing IS methodologies, and its applicability is not restricted to a specific PDF or biasing scheme. The effectiveness of the SGD algorithm is demonstrated by two examples of communication systems where the IS techniques have not been applied before. The first example is a communication system with diversity combining, slow nonselective Rayleigh fading channel, and noncoherent envelope detection. The second example is a binary baseband communication system with a static linear channel and a recursive least square (RLS) linear equalizer in the presence of additive white Gaussian noise (AWGN).

Simulation of rare events in communications networks

Computer simulation is an important tool in the analysis and design of communications networks. In spite of the advances in computational power, using simulation to obtain rare event probabilities such as cell/packet loss or delay in networks still requires prohibitively long execution times. We provide an overview of importance sampling techniques and how they can be used to provide orders of magnitude speedup for many network problems.

Importance sampling methodologies for simulation of communication systems with time-varying channels and adaptive equalizers

Two importance sampling (IS) methodologies for Monte Carlo simulation of communication links characterized by time-varying channels and adaptive equalizers are presented. One methodology is denoted as the twin system (TS) method. A key feature of the TS method is that biased noise samples are input to the adaptive equalizer, but the equalizer is only allowed to adapt to these samples for a time interval equal to the memory of the system. In addition to the TS technique, the IA method, a statistically biased, but simpler, technique for using IS with adaptive equalizers that is based on the independence assumption between equalizer input and equalizer taps is presented. Experimental results show run-time speedup factors of two to seven orders of magnitude for a static linear channel with memory, and of two to almost five orders of magnitude for a slowly-varying random linear channel with memory for both the IA and TS methods. >

Evaluation of the covertness of time-hopping impulse radio using a multi-radiometer detection

Covert operation is an important requirement for military communications systems. Impulse radio is a promising ultra-wideband technique for tactical military communications. Using a simple, sub-optimal radiometer detection system, impulse radio has been shown previously to exhibit low probability of detection (LPD) compared to alternative systems. In this paper we quantitatively evaluate the covertness of impulse radio using a more complex radiometer detection scheme ideally suited for detection of time-hopping impulse radio signals. The more complex detection system that we consider utilizes multiple, wideband radiometers with outputs that are ORed and compared to a threshold. This multi-radiometer detection system is used to quantify covertness for single and multiple user configurations. We also evaluate covertness for cases where the detector incorporates varying amounts of prior knowledge about the impulse radio signals. The average covertness of impulse radio for a specified number of users is determined and then compared to DS-CDMA schemes with equivalent number of users.

Threshold discrimination and blanking for large near-far power ratios in UWB networks

A simple chip-discrimination technique is presented for use with ultra-wideband (UWB) impulse radio (IR) that improves performance for large near-far interference power ratios. A typical spread-spectrum IR that employs a matched-filter sum for bit decisions is susceptible to small numbers of large power pulses that can dominate the bit decision-threshold statistics. This letter describes a technique for chip discrimination prior to the spreading summation, that can greatly reduce the effects of large near-far power ratios among interferers. The technique exploits the very narrow pulsewidth and resulting low-duty-cycle characteristic only achievable with ultra-wide bandwidth. A statistical model is developed that predicts bit-error performance for binary offset pulse position modulation as a function of near-far density and power for varying discrimination thresholds. An analytic solution for perfect chip blanking is developed, and is in good agreement with chip discrimination for large near-far power ratios. We find that even a small number of very near interferers can greatly reduce the performance of a system without blanking or discrimination. Results show substantial improvement using this method for near interferers with near-far power ratios greater than 20 dB.

Chip discrimination for large near far power ratios in UWB networks

A simple chip discrimination technique is presented for use with ultra wide band (UWB), impulse radio (IR) that improves performance for large near/far interference ratios. A typical spread-spectrum IR that employs a matched filter sum for bit decisions is susceptible to small numbers of large power pulses that can dominate the bit decision threshold statistics. This paper describes a technique for chip discrimination prior to the spreading summation that can greatly reduce the effects of large near/far interferers. The technique exploits the unique time domain characteristics that only UWB systems can provide. A statistical model is developed that predicts bit error performance for binary offset pulse position modulation (PPM) as a function of near/far density and power for varying discrimination thresholds. We find that even a small number of very near interferers can greatly reduce the performance of a system without blanking or discrimination. Results show substantial improvement using this method for near interferers with near/far power ratios greater than 20 dB.