Implementation and Performance Evaluation of Cooperative Wireless Communications with Beamforming and Software-Defined Radio Techniques

Software-defined radio (SDR) and transmit beamforming are two key techniques for next-generation wireless communications. In order to blaze a path to introduce these high demand advanced techniques to future entry-level communication engineers, an educational module was developed with well-defined objectives, learning outcomes, and assessment rubrics. This module is developed with the insights of benefits and challenges obtained from a Research Experience for Undergraduate project. Through this module, the students will not only gain valuable knowledge of the state-of-art beamforming technique, SDR concepts, and the universal software radio peripheral (USRP) platform, but also improve their creative thinking ability, hands-on and programming skills. Additional benefits include increased students’ interests in communication engineering, higher retention rate and more minority students pursuing graduate degrees. Background and motivation With the significant growth in the number of users using various types of portable devices on diverse real and non-real time, high and low data rate applications, future wireless communication systems are expected to operate under the strict constraint of limited spectrum to provide ubiquitous communications in a heterogeneous environment composed of sophisticated digital communication systems, infrastructures, and services 1 . To meet the future wireless communications requirements, software-defined radio (SDR) and transmit beamforming have been proposed to be the key techniques 2 that future graduating communication engineers should be capable of designing and implementing. SDR is a flexible and cost efficient platform where some or all of the radio’s operating functions (physical layer processing) are implemented through modifiable software or firmware operating on a computer, embedded system, or programmable processing devices such as field programmable gate arrays (FPGA), digital signal processors (DSP), general purpose processors (GPP), or programmable System on Chip (SoC) 3 . SDR can cope with the broad range of wireless standards, frequency bands, and user requirements by changing its software implemented functionalities on-the-fly 4 . Transmit beamforming, a promising multiple antenna technique for high frequency and power efficiency by steering its antennas’ transmissions towards the direction of the intended receiver, enables increased coverage range, increased data rate, or decreased net transmit power for a fixed and desired received power 5 . However, in many scenarios, such as cellular phone, nodes in a wireless sensor network, and Internet-of-Things (IoT) applications, a transmitter may only be equipped with a single omni-directional antenna, and hence, it may not be able to implement beamforming on its own. Cooperative beamforming is a practical implementation of transmit beamforming for size and/or cost constrained devices. In cooperative transmit beamforming, a number of distributed transmit devices, each equipped with single antenna, cooperatively organize themselves into a virtual antenna array and focus their transmissions in the direction of the intended receiver, such that, after propagation, the signals combine constructively at the desired receiver 6 . Despite the compelling needs of SDR and beamforming expertise in the wireless industry, few schools are offering undergraduate courses on these high demand advanced topics. Typical undergraduate communication systems course mainly focuses on the theories of basic analog and digital modulation techniques. The students learn from equations and block diagrams and practice with theory-based homework questions and a few computer simulations through Matlab. In recent years, several efforts have been taken to integrate hands-on projects and experiential experiences of advanced topics, such as SDR, into undergraduate Electrical Engineering education. Mao et al offered SDR based senior design projects and SDR-related experiments for analog and digital modulated systems 7, 8 . Blass et al presented a student project that implemented a global positioning system repeater using SDR 9 . Bonior et al used SDR as an enabler to encourage undergraduate students to consider graduate level studies 10 . Jiang and Mao attempted to implement SDR based courses in minority institution 11 . Wu et al developed an affordable, evolvable, and expandable laboratory suite to allow different institutions to offer laboratories in communications and networking courses 12 . However, to the best of our knowledge, there is no existing work that introduces cooperative transmit beamforming, the key technique in next-generation communication systems, with SDR to undergraduate electrical engineering students. To bridge the gap between the undergraduate communication systems education and the industrial demands of entry-level electrical engineers with SDR and beamforming expertise, an educational module has been developed for Communication Systems course. This module is developed based on a Research Experience for Undergraduate (REU) project that focused on implementation and performance evaluation of cooperative wireless communications with beamforming and SDR technique. Since it has been reported extensively in the literature of engineering education that the undergraduate students will benefit from the involvement of hands-on and research activities 13 , the active and creative pedagogy is used in the development. It is expected that when hands-on and research experiences are incorporated into conventional lecture and/or laboratory courses, students will be motivated to learn because students usually react favorably to having curricular content that is not presented in textbook 14 . The rest of the paper is organized in the following manner: First, the theoretical background of cooperative beamforming and the software defined radio platform is introduced. Then a REU project that implemented and evaluated a cooperative wireless communication system with SDR and beamforming techniques is described. After that, a course module with hands-on project for communication systems course that integrates hands-on and research activities is explained. The learning outcome and assessment rubrics are also presented in this Section. Finally, conclusions are drawn. Cooperative transmit beamforming Beamforming with antenna arrays is a well-studied multiple input multiple output (MIMO) technique. It provides space-division multiple access which enables significant increase in communication rate 15 . Cooperative transmit beamforming is a practical implementation of beamforming technique for next generation wireless networks. It applies distributed transmission technique in which randomly distributed nodes in a network cluster form a “virtual antenna array” and calibrate their transmissions to a faraway destination. Under this scheme the individually transmitted signals add up coherently at the intended receiver to enhance communication range and/or power efficiency. The destination receives highly reliable data without each node exceeding its power, size, and/or cost constraint. Figure 1 shows the system model of a cooperative transmit beamformer. Figure 1. System model of a cooperative transmit beamformer In order to obtain the large potential benefit offered by cooperative transmit beamforming, the key challenge is that the transmitted signals from each transmitter must be precisely synchronized so that they can be aligned in phase at the intended receiver. However, a distinguishing feature for cooperative transmit beamformer is that each transmitting device has its own local oscillator (LO) and there is no regular and precisely known location of the transmitting and receiving devices. The carrier frequency of each transmitter is typically generated by multiplying the frequency of the LO up to a fixed nominal frequency. But due to manufacturing tolerances and temperature variations, even when two oscillators are set to the same nominal frequency, the carrier frequencies would in general have a non-zero frequency offset with respect to each other and exhibit variations on the order of 10–100 parts per million (ppm) with respect to the nominal. Moreover, all oscillators undergo frequency drifts over time. If uncorrected, these frequency variations among transmitters are catastrophic for transmit beamforming since the phases of the signals may drift out of alignment over the duration of the transmission and may even result in destructive combination at the destination. Furthermore, when there is no precise location information of the transmitting and receiving devices, it is impossible to determine the phases that the cooperative transmitters must employ in order to direct energy towards the destination 16 . Decentralized, feedback-based synchronization architecture could be used to tackle all of the preceding uncertainties 16 . In this scheme, each transmitter adapts its frequency and phase independently based on the feedback packet broadcasted by the receiver. In each feedback packet, only one bit information is included to indicate the change in the received signal strength. After receiving the feedback packet, each transmitter conducts two independent and concurrent processes to synchronize its frequency to the receiver’s frequency and steer the beam. In frequency synchronization process, an extended Kalman filter is applied on the preamble and header symbols of the feedback packet to track the LO’s frequency offset between the transmitter and the receiver. In beam steering process, the one bit information in the payload of feedback packet is used to adjust the phase relationship between the transmitters so that the transmitted signals add up coherently at the intended receiver. The randomized ascent algorithm could be used for phase adjustment as follows: 17 , each transmit device adds a random phase perturbation to its current phase before each transmission