A Joint PHY/MAC Architecture for Low-Radiated Power TH-UWB Wireless Ad-Hoc Networks

Due to environmental concerns and strict constraints on interference imposed on other networks, the radiated power of emerging pervasive wireless networks needs to be strictly limited, yet without sacrificing acceptable data rates. Pulsed Time-Hopping Ultra-Wide Band (TH-UWB) is a radio technology that has the potential to satisfy this requirement. Although TH-UWB is a multi-user radio technology, non-zero cross-correlation between time-hopping sequences, time-asynchronicity between sources and a multipath channel environment make it sensitive to strong interferers and near-far scenarios. While most protocols manage interference and multiple-access through power control or mutual exclusion (CSMA/CA or TDMA), we base our design on rate control, a relatively unexplored dimension for multiple-access and interference management. We further take advantage of the nature of pulsed TH-UWB to propose an interference mitigation scheme that reduces the impact of strong interferers. A source is always allowed to send and continuously adapts its channel code (hence its rate) to the interference experienced at the destination. In contrast to power control or exclusion, our MAC layer is local to sender and receiver and does not need coordination among neighbors not involved in the transmission. We show by simulation that we achieve a significant increase in network throughput

Performance Evaluation of Impulse Radio UWB Networks using Common or Private Acquisition Preambles

For impulse-radio ultra-wide band (IR-UWB) networks without global synchronization, the first step for correct packet reception is packet detection and timing acquisition: Before recovering the payload of the packet, the destination must detect that the packet is on the medium and determine when exactly the payload begins. Packet detection and timing acquisition rely on the presence of an acquisition preamble at the beginning of each packet. How this preamble is chosen is a network design issue and it may have quite an impact on the network performance. A simple design choice of the network is to use a common acquisition preamble for the whole network. A second design choice is to use an acquisition preamble that is private to each destination. The throughput with the latter choice is likely to be much higher, albeit at the cost of learning the private acquisition preamble of a destination. In this paper, we evaluate how using a common or private acquisition preambles affects the network throughput. Our analysis is based on analytical modeling and simulations. Using our analytical model, we show that a private acquisition preamble yields a tremendous increase in throughput compared to a common acquisition preamble. The throughput difference grows with the number of concurrent transmitters and interferers. This result is confirmed by simulations. Furthermore, additional simulations on multi-hop topologies with TCP flows demonstrate that a network using private acquisition preambles has a stable throughput. On the contrary, using a common acquisition preamble exhibits the presence of a compounding effect similar to the exposed terminal issue in IEEE 802.11 networks: the throughput is severely degraded and complete flow starvation may occur

Effect of Interfering Users on the Modulation Order and Code Rate for UWB Impulse-Radio Bit-Interleaved Coded M-ary PPM

We consider the impact of multi-user interference on a bit-interleaved coded-modulation system with M-ary PPM (BIC M-ary PPM) in an impulse-radio ultra-wideband physical layer. In a realistic scenario such as an ad hoc network, the interference is inherently variable. This justifies the need for a physical layer that can optimally adapt its transmission parameters to the interference level. We use puncturing on the channel code so that we can not only change the modulation order $M$ but also the channel code rate. We study by simulation how the optimal combination of modulation order $M$ and channel code rate behaves with various degrees of interference. The results show that BIC M-ary PPM can be successfully adapted to various levels of interference conditions. It also shows the benefit of both rate and modulation adaptation, especially in the presence of multi-user interference

Concurrent and Parallel Transmissions are Optimal for Low Data-Rate IR- UWB Networks

The Internet of Things, emerging pervasive and sensor networks are low data-rate wireless networks with, a priori, no specific topology and no fixed infrastructure. Their primary requirements are twofold: First, low power consumption and, due to environmental concerns, low emitted power. Second, robustness to poor propagation environments and multi-user interference. Impulse-radio ultra-wide band (IR-UWB) physical layers have the potential to satisfy these requirements. Because the features of IR-UWB physical layers differ from narrow-band physical layers, the design rules of IR-UWB networks are likely to be different than for narrow-band wireless networks. Indeed, to optimally use the resources available, it is crucial for the network layers to take into account and take advantage of the underlying physical layer. Therefore, we are interested in the design of IR-UWB networks in a low data-rate, self-organized, and multi-hop context. We concentrate on the medium access control (MAC) layer and the physical layer. In the case of low data-rate IR-UWB networks, the optimal design is to allow for parallel and concurrent transmissions at the MAC layer. Interference is managed with rate adaptation, no power control and an interference mitigation scheme at the physical layer. A protocol that implements the optimal design and allows for parallel transmissions outperforms protocols that use exclusion or power control

Conditional Bit Error Rate for an Impulse Radio UWB Channel with Interfering Users

We consider a multi-user impulse radio UWB physical layer in a multipath environment. We propose a fast and efficient method to compute the conditional bit error rate (BER), given some realizations of the channels from source/interferer to destination, and of delay differences. Our motivation is packet level simulation of large scale or dense impulse radio UWB networks. The conditional BER is used in a packet level simulator with block fading channel assumption to sample packet transmission error events. However, due to the timescale difference between physical layer events and network events, a pulse-level simulation of the BER in a realistic multipath channel environment is infeasible. Our solution is based on a novel combination of large deviation and importance sampling

Confinement-Higgs transition in a disordered gauge theory and the accuracy threshold for quantum memory

We study the +/- J random-plaquette Z_2 gauge model (RPGM) in three spatial dimensions, a three-dimensional analog of the two-dimensional +/- J random-bond Ising model (RBIM). The model is a pure Z_2 gauge theory in which randomly chosen plaquettes (occuring with concentration p) have couplings with the ``wrong sign'' so that magnetic flux is energetically favored on these plaquettes. Excitations of the model are one-dimensional ``flux tubes'' that terminate at ``magnetic monopoles.'' Electric confinement can be driven by thermal fluctuations of the flux tubes, by the quenched background of magnetic monopoles, or by a combination of the two. Like the RBIM, the RPGM has enhanced symmetry along a ``Nishimori line'' in the p-T plane (where T is the temperature). The critical concentration p_c of wrong-sign plaquettes at the confinement-Higgs phase transition along the Nishimori line can be identified with the accuracy threshold for robust storage of quantum information using topological error-correcting codes: if qubit phase errors, qubit bit-flip errors, and errors in the measurement of local check operators all occur at rates below p_c, then encoded quantum information can be protected perfectly from damage in the limit of a large code block. Numerically, we measure p_{c0}, the critical concentration along the T=0 axis (a lower bound on p_c), finding p_{c0}=.0293 +/- .0002. We also measure the critical concentration of antiferromagnetic bonds in the two-dimensional RBIM on the T=0 axis, finding p_{c0}=.1031 +/-.0001. Our value of p_{c0} is incompatible with the value of p_c=.1093 +/-.0002 found in earlier numerical studies of the RBIM, in disagreement with the conjecture that the phase boundary of the RBIM is vertical (parallel to the T axis) below the Nishimori line.Comment: 16 pages, 11 figures, REVTeX, improved numerics and an additional autho

Managing Impulsive Interference in Impulse Radio UWB Networks

Wireless sensor networks are ideally built on low-cost, low-complexity nodes that have a low power consumption to guarantee a long network lifetime. These are all properties that can potentially be achieved with impulse radio ultra-wide band (IR-UWB). In addition, IR-UWB has a fine timing resolution resulting in accurate ranging and localization possible. For all these reasons, IR-UWB is an extremely interesting physical layer technology for wireless sensor networks. In this article, we consider the management of impulsive interference in IR-UWB networks. Impulsive interference is due to uncoordinated concurrent transmissions. It occurs, for instance, when several independent piconets operate in close vicinity and is also present in some MAC layer proposals that allow concurrent transmissions. If not properly addressed, impulsive interference can severely affect the throughput and energy consumption of an IR-UWB network; as such, it already needs to be taken into account in the design phase. First, we show that impulsive interference is a serious concern for IR-UWB networks. Second, we present techniques at the physical layer and at the link layer to cope with and combat such interference efficiently. Finally, we present DCC-MAC as an example of an interference-aware design

Synchronization for Impulse-Radio UWB With Energy-Detection and Multi-User Interference: Algorithms and Application to IEEE 802.15.4a

Energy-detection (ED) receivers can take advantage of the ranging and multipath resistance capabilities of impulse-radio ultra-wideband (IR-UWB) physical layers at a much lower complexity than coherent receivers. However, ED receivers are extremely vulnerable to multi-user interference (MUI). Therefore, the design of IR-UWB ED architectures must take MUI into account. In this paper, we present the design and evaluation of two complementary algorithms for reliable and robust synchronization of IR-UWB ED receivers in the presence of MUI: 1) power-independent detection and preamble code interference cancellation (PICNIC) and 2) detection of start-frame-delimiter through sequential ratio tests (DESSERT). PICNIC addresses packet detection and timing acquisition while DESSERT focuses on start-frame-delimiter (SFD) detection. Both algorithms are evaluated with the IEEE 802.15.4a IR-UWB physical layer, standardized for low data-rate networks. The performance evaluation with extensive simulations show that our algorithms outperform nonrobust synchronization algorithms by up to two orders of magnitude in the presence of MUI

Clock-Offset Tracking Software Algorithms For IR-UWB Energy-Detection Receivers

We present a clock-offset tracking algorithm for impulse-radio ultra-wide band (IR-UWB) energy-detection receivers. There is a complexity versus performance trade-off for the design of IR-UWB energy-detection receivers: Extremely low-complexity energy-detection receivers are built with a large, constant integration duration; they are robust to clock drifts but are sensitive to noise enhancement effects and cannot adapt to channel variations. More sophisticated energy-detection receivers use a shorter integration duration and combine several weighted outputs of the energy collector; they are robust to noise enhancement effects, can adapt to channel variations and offer a much better performance than non-adaptive receivers. However, they become sensitive to clock offsets. Hence, there is a need for low-complexity clock-offset tracking solutions to support adaptive energy-detection receivers. Our solution is constructed around the Radon transform, an image processing tool traditionally used to detect line features in images. Our solution is fully compatible with the IEEE 802.15.4a standard, does not increase the hardware complexity of the receiver and reduces the performance loss due to clock offset to less than 0.5 dB