Osama Alrabadi

MIMO Transmission Using a Single RF Source: Theory and Antenna Design

An approach for transmitting multiple signals using a single switched parasitic antenna (SPA) has been recently reported. The idea there is to map the signals to be transmitted onto a set of basis functions that serve as “virtual antennas” in the beamspace (i.e., wavevector) domain. In this paper, we generalize the derivation of the antenna pattern basis functions regarding a three-element SPA of arbitrary radiating elements, within a symmetric array topology, for multiplexing signals in the wavevector domain (using different beampatterns) rather than in the hardware antenna domain with multiple feeding ports. A fully operational antenna system example is modeled, optimized regarding its return loss and the power imbalance between the basis functions, and finally realized. The measurements of the SPA show good agreement with the simulated target values, revealing an accurate design approach to be adopted as a fast SPA prototyping methodology. The SPA has been successfully employed for multiplexing two binary phase-shift-keying (BPSK) datastreams over-the-air, thus paving the way for practically compact and highly efficient MIMO transceiver designs.

Beamforming via Large and Dense Antenna Arrays Above a Clutter

The paper sheds light on the beamforming (BF) performance of large (potentially unconstrained in size) as well as dense (but physically constrained in size) antenna arrays when equipped with arbitrarily many elements. Two operational modes are investigated: Single-layer BF and multi-layer BF. In the first mode, a realistic BF criterion namely the average BF gain is revisited and employed to understand the far-field and the near-field effects on the BF performance of large-scale antennas above a clutter. The diminishing throughput returns in a single-layer BF mode versus the number of antennas necessitate multi-layering. In the multi-layer BF mode, the RF coverage is divided into a number of directive non-overlapping sector-beams in a deterministic manner within a multi-user multi-input multi-output (MIMO) system. The optimal number of layers that maximizes the users sum-rate given a constrained antenna array is found as a compromise between the multiplexing gain (associated with the number of sector-beams) and the inter-beam interference, represented by the side lobe level (SLL).

MIMO transmission and reception techniques using three-element ESPAR antennas

A novel scheme for spatially multiplexing two BPSK signals using a 3-element ESPAR (electronically steerable parasitic array radiator) antenna was reported in. In this paper we first optimize the set of loads controlling the parasitic elements within the transmission mode by maximizing the outage capacity. We also propose different reception techniques for spatially demultiplexing real and complex signals (using the same set of loads). The proposed transmission and reception schemes are evaluated in an indoor peer-to-peer mobile ad-hoc channel via simulations, where each ad-hoc node is equipped with a 3-element ESPAR terminal.

Spatial Multiplexing with a Single Radio: Proof-of-Concept Experiments in an Indoor Environment with a 2.6-GHz Prototype

This letter validates a previously reported concept regarding the capability of transmitting multiple signals using one RF chain and a compact switched parasitic array (SPA). The experiments were conducted in an indoor environment using a 2.6 GHz prototype made of a single active printed dipole coupled to two passive ones. To the best of our knowledge, this is the first over-the-air experiment of spatial multiplexing with a single RF frontend yet to be demonstrated.

Increasing throughput in cellular networks with higher-order sectorization

We study the impact of higher-order sectorization (up to 12 sectors per cell site) in cellular networks with low angle spread and a high density of users. Under ideal sector patterns (with no intersector interference), the mean throughput per site scales directly with S, the number of sectors per site. Fixing the number of antennas per site to be 12, higher-order sectorization with S = 12 and single-antenna transmission per sector is shown to achieve higher average throughput compared to a conventionally sectorized system with S = 3 and capacity-achieving multiuser MIMO transmission with M = 4 antennas per sector. A circular antenna array architecture is proposed for generating S = 12 sectors using fixed beamforming. Beams generated using an array prototype were measured in an anechoic chamber were shown to achieve a similar response as the one used in simulations, and the wind load improvement is a factor of 8 compared to the S = 3, M = 4 configuration.

Spatial multiplexing by decomposing the far-field of a compact ESPAR antenna

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.

Concurrent Communication and Sensing in Cognitive Radio Devices: Challenges and an Enabling Solution

Cognitive radios (CRs) need to continuously monitor the availability of unoccupied spectrum. Prior work on spectrum sensing mainly focused on time-slotted schemes where sensing and communication take place on different time periods in the same frequency. This however leads to a) limited CR throughput as data transmissions need to be interrupted for the sensing task, and b) unreliable detection performance since sensing happens in specific confined time durations. The paper describes the basic design challenges and hardware requirements that restrain CRs from simultaneously and continuously sensing the spectrum while transmitting in the same frequency band. The paper then describes a novel approach based on spatial filtering that promises to empower CRs with concurrent transmission and sensing capabilities. The idea is to equip the CR with redundant transmit antennas for forming an adaptive spatial filter that selectively nulls the transmit signal in the sensing direction. By doing so, a wideband isolation level of ~ 60 dB is obtained by the antenna system. Finally, by following the spatial filtering stage with active power cancellation in the radio-frequency stage and in the baseband stage, a total isolation in excess of a 100 dB required for enabling concurrent communication and sensing can be obtained.

Spatial spectrum sensing for wireless handheld terminals: design challenges and novel solutions based on tunable parasitic antennas [Dynamic Spectrum Management]

This article addresses the major design challenges of antenna systems intended for cognitive transceivers under compactness constraints. The first part of the article provides an overview of the design challenges with regard to sensing over different frequencies and angular directions, referred to as spatial spectrum sensing. The second part describes a novel approach based on parasitic antenna theory aiming at making spatial spectrum sensing feasible for portable lightweight terminals. The idea is to replace the wideband antenna required for sensing over a large bandwidth by a tunable narrowband antenna for both sensing and communication purposes. The flexibility of the proposed antenna system lies in its capability to scan both the frequency and spatial resource dimensions simultaneously via a single RF chain within a miniaturized antenna system. This is done by properly tuning a set of reactive loads connected to a group of parasitic elements closely coupled to the driven (active) element. By doing so, the operational frequency subband leaps to another subband (frequency tuning). Moreover, at every subband, circular permutations of the reactive loads rotate the narrowband beam pattern to different angular positions, giving the cognitive transceiver the capability of sensing over various segments of the space.

Breaking the Transmitter–Receiver Isolation Barrier in Mobile Handsets With Spatial Duplexing

In full-duplex radio communication systems like e-UTRAN, CDMA-2000, the radio transmitter (Tx) is active at the same time as the radio receiver (Rx). The Tx and the Rx will be using separate dedicated frequency bands and the Tx-Rx isolation is ensured by duplex filters. However, agile duplexers required for multiband operation are almost nonexistent while dedicating a bank of narrowband filters is bulky and incurs considerable switching losses. In this paper, we propose an approach that dramatically reduces the complexity of the RF frontend, first by replacing the duplex filter with a spatial filter and second, by codesigning the filtering antennas and the RF frontend. The spatial filter is synthesized by equipping the Tx with redundant antennas. By properly weighting the Tx antennas, the Tx signal is selectively attenuated in the Rx direction. The spatial filter can be tuned to different frequency bands as long as the antennas are made tunable. Moreover, the spatial filter may directly benefit from the balanced architecture of the power amplifiers (PAs) thus reducing the total system complexity and insertion loss. Finally, simulation and initial measurement results are provided in a challenging low-frequency band, serving as a proof-of-concept.

Aerial modulation for high order PSK transmission schemes

The performance of PSK schemes degrades as the modulation order increases and becomes somehow unreliable over wireless fading channels. In this work, we improve the performance of high order PSK transmission schemes over Rayleigh fading channels by mapping the neighboring symbols of a PSK signal constellation onto a set of weakly-correlated field patterns. We implement the scheme using a compact two element ESPAR (electronically steerable parasitic array radiator) antenna with an inter-element spacing of λ/16. An appreciable gain regarding the system performance is obtained using the proposed scheme.