Space-time coded cooperative transmission has been proposed as a means to prolong the battery lifetime of conventional single-antenna energy-constrained wireless sensor nodes. However, newly proposed smart-antenna sensor prototypes enable the existence of more than one antenna within a single-radio transceiver architecture. Exploiting such capability, this paper proposes simple non-cooperative space-time techniques for single-RF switched-antenna systems that reduce significantly both the transmission and the circuit energy consumption. The energy savings of the suggested schemes are shown not only against conventional SISO systems, but also against cooperative diversity schemes in Rayleigh and log-normal shadowed Rayleigh fading channels with distance-based path loss.

Despite the well known benefits of Multiple Input - Multiple Output (MIMO) systems, their remarkable implementation complexity limits the wide application to modern transceiver architectures. In order to simplify the inclusion of MIMO technology in mobile devices, this paper studies the capabilities of parasitic antenna structures on supporting MIMO transmission. In contrast to conventional antenna arrays the proposed MIMO architecture utilizes only a single radio - frequency chain, which reduces dramatically the implementation complexity and cost. The results show that the proposed MIMO architecture is able to achieve even better performance as compared to more complex MIMO systems with typical antenna arrays.

A novel orthogonal space time block code (OSTBC) that is capable of extracting a diversity order of two via a single RF frontend is proposed in this paper. The scheme is implemented using a practical antenna system of two transmit antennas connected to a common ground. An orthogonal block of two real (non-complex) signals is transmitted over a duration of two symbol periods by exciting one antenna every symbol period while exciting the other parasitically by the field of the active one. A switched-antenna system similar to the one reported in [10] can be used for simultaneously driving and switching the antenna elements. Besides the orthogonality of the signals, the array far-field is a linear mixture of two eigenpatterns onto which the BPSK signals are mapped, hence independent fading among the signals is almost always guaranteed.

This paper proposes a new system architecture for continuous monitoring wireless sensor networks, utilizing recent developments in the areas of cooperative beam-forming, radio frequency integrated circuit technology, and spread-spectrum techniques. In the proposed system, nodes are activated only when they are triggered to send their own data to the fusion center directly, without burdening the network with MAC layer collisions and energy-consuming routing algorithms, thus guaranteeing long lifetime cycles. The nodes consist of a simple active reflector circuit which is synchronized by an incident wave transmitted by a distant base station. Since the nodes can be deployed in large numbers, the network acts as a single antenna array with high directive gain pointing towards the direction of the fusion center. At the receiver, data from each node can be de-multiplexed with the use of a frequency shift keying direct sequence spread spectrum modulation scheme that enables multiple access without destroying the beamforming characteristics of the network.

In this paper, the problem of wireless communications in human tissue is addressed for biosensor network applications. Today's offered solutions involve the usage of RF communication systems, which have shown to provide adequate performance to ensure proper network functionality. Yet they do not ensure that human tissue is not harmfully affected. In order to overcome this issue, we propose the use of ultrasonic waves for wireless communications in biosensor networks, motivated by the fact that, in contrast to electromagnetic waves, ultrasound has been used for almost a century now for medical purposes without any reported side effects. A simple propagation model is used to evaluate ultrasonic waves as a communication medium, study the feasibility of our proposal, the effective bit rates that may be achieved, the power efficiency of the scheme, as well as other system issues.

Multiple input-multiple output (MIMO) systems have drawn remarkable interest in recent years, since they are known to boost the spectral efficiency of wireless communication systems compared to single antenna architectures. However, the implementation cost and circuit complexity of such systems conflicts the guaranteed high performance gain. Recent research efforts focus on the investigation of cheaper and low complexity transceivers, introducing a novel beamspace MIMO (BS-MIMO) architecture. BS-MIMO systems have shown equivalent performance with the corresponding conventional systems, while they have been also evaluated in closed loop environments. In this paper we extend the closed loop BS MIMO approach presenting advanced limited feedback techniques which outperform predecessor feedback schemes.

A universal scheme for encoding multiple symbol streams using a single driven element (and consequently a single radio frequency (RF) frontend) surrounded by parasitic elements (PE) loaded with variable reactive loads, is proposed in this paper. The proposed scheme is based on creating a MIMO system by expanding the far-field of a compact parasitic array into an orthogonal set of angular functions (basis). Independent information streams are encoded by means of angular variations of the far-field in the wavevector domain, rather than spatial variations as usually happens in conventional MIMO systems. The array can spatially multiplex the input streams by creating all the desired linear combinations (for a given modulation scheme) of the basis functions. The desired combinations are obtained by projecting the ratio of the symbols to be spatially multiplexed on the ratio of the basis functions' weights (complex coefficients), which is a function of the currents induced on the PE within the antenna domain, and controlled by the independent reactive loadings.

In this paper the problem of wireless communications in human tissue is addressed for biosensor network applications. Todaypsilas offered solutions involve the usage of RF communication systems, which have shown to provide adequate performance to ensure proper network functionality. Yet, they do not ensure that human tissue is not harmfully affected. In order to overcome this issue, we propose the use of ultrasonic waves for wireless communications in biosensor networks, motivated by the fact that, in contrast to electromagnetic waves, ultrasound has been used for almost a century now for medical purposes without any reported side-effects. A simple propagation model is used to evaluate ultrasonic waves as a communication medium, study the feasibility of our proposal, the effective bit rates that may be achieved, the power efficiency of the scheme, as well as other system issues.

Beamforming capabilities of electronically steerable passive array radiator (ESPAR) antennas have attracted considerable attention in recent literature. In this paper we consider a different beamforming approach, presenting an efficient optimization algorithm which enables such antennas to create specific transmit radiation patterns in real time, rather than patterns aiming at signal to interference ratio maximization. Moreover, we investigate the proposed algorithm over a novel multiple input - multiple output architecture, showing that even cost and size sensitive mobile handsets may efficiently operate in closed loop communication environments.

A framework for implementing spatial multiplexing using an electronically steerable parasitic array radiator (ESPAR) antenna is presented. While traditional multi-element arrays (MEA) show great potential for meeting the increasing demand for higher data rates, they come at the expense of computationally demanding signal processing, larger size, and higher consumption of DC power, making traditional MEA impractical at userspsila hand-held devices. In this work, we introduce a novel methodology for using ESPAR antennas as a compact, cheap and less power-hungry MEA, for spatially multiplexing BPSK symbols, while still providing comparable spectral efficiency and performance as traditional MEA. The main idea is to exploit the strong mutual coupling among the ESPAR antenna elements at the transmitter side to create a linear combination of orthogonal patterns (beams), onto which the symbols are directly mapped, instead of the traditional approach of sending the symbols toward different locations within the antenna domain. On the other side, the receiver (base-station (BS) or access point (AP)) is equipped with a traditional uniform linear array (ULA), and decodes the received signal using the Vertical Bell labs layered space time (VBLAST) algorithm.

Recently, a novel beamspace multiple input multiple output (BS-MIMO) transmission technique has appeared, which increases the spectral efficiency of open-loop communication systems while using compact antenna structures with a single radio-frequency (RF) front-end at the transmitter. In this paper, we extend the aforementioned architecture proposing an efficient limited feedback technique for capacity optimization, considering electronically steerable parasitic array radiator (ESPAR) antennas at the transmitter. The performance of the resulting closed-loop BS-MIMO is evaluated against equivalent traditional MIMO systems that require a much larger number of active antenna elements to operate. The results are very promising, paving the way for integrating the proposed system in cost and size sensitive wireless handheld devices such as mobile terminals and mobile personal digital assistants.

In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

In this paper the use of electronically steerable passive array radiator (ESPAR) antennas is introduced for achieving increased spectral efficiency characteristics in multiple-input, multiple-output (MIMO) systems using a single active element, and compact antennas. The proposed ESPAR antenna is capable of mapping phase-shift-keying (PSK) modulated symbols to be transmitted onto orthogonal basis functions on the wavevector domain of the multi-element antenna (MEA), instead of the traditional approach of sending different symbol streams in different locations on the antenna domain. In this way, different symbols are transmitted simultaneously towards different angles of departure at the transmitter side. We show that the proposed system, called beamspace-MIMO (BS-MIMO) can achieve performance characteristics comparable to traditional MIMO systems while using a single radio-frequency (RF) front-end and ESPAR antenna arrays of total length equal to λ/8, for popular PSK modulation schemes such as binary-PSK (BPSK), quaternary-PSK (QPSK), as well as for their offset-PSK and differential-PSK variations.

We describe HyMAC, a new hybrid MAC layer protocol for wireless sensor networks that is, to the best of our knowledge, the first effort to combine the strengths of both TDMA and FDMA schemes in these constrained networks. While allowing the network to operate in an energy-efficient collision-free manner, HyMAC takes advantage of the multiple frequencies provided in the radio component of recent sensor node hardware platforms such as MICAZ, TELOS and FireFly. We show how HyMAC outperforms protocols such as B-MAC, RT-Link and MMSN achieving high throughput and small bounded end- to-end delay suitable for newer types of sensor network applications such as real-time voice streaming.

In this work we present a two-bounce geometrically-based stochastic channel model suitable for ad-hoc networks equipped with directional antennas. It extends existing geometric models with the addition of an interferer. We consider the use of directional antennas both at the receiver and transmitter of the communication link, adding information regarding both the angle of departure from the transmitter and the angle of arrival at the receiver. The performance of a system using directional antennas is evaluated in terms of signal to interference ratio (SIR) improvement in typical indoor and outdoor scenarios in both LOS and NLOS cases. We present results showing that such a system may achieve an average 24 dB SIR improvement with standard deviation less than 2.5dB.