Yahia R. Ramadan

Power allocation for device-to-device communication underlaying massive MIMO multicasting networks

Device-to-device communication (D2D) is an emerging technology aiming at enhancing the performance of next generation wireless communication systems. D2D communication enables two mobile stations to communicate directly without traversing the Base Station (BS). Smart techniques should be applied to manage the interference between the BS and the D2D communicating devices. In this paper, we investigate the problem of BS precoder design and Power Allocation (PA) to deploy D2D communication in a single-cell network. We assume that the BS uses a very large number of antennas, referred to as Massive Multiple-Input-Multiple-Output (MIMO). We propose solution algorithms to maximize the sum of the achievable data rates of the D2D pairs, or their minimum, while maintaining some Quality of Service (QoS) constraints on the Cellular User Equipments (CUEs) which communicate with the BS. Simulation results show that the proposed solutions are superior to the conventional power allocation schemes.

A New Shared Spectrum Access Paradigm between Cellular Wireless Communications and Radio Astronomy

This paper proposes a new paradigm of shared spectrum access between cellular wireless communications (CWC) and radio astronomy systems (RAS). Traditionally, most spectrum chunks are separately allocated to CWC and RAS, with a few shared spectrum without guaranteed spectrum access for RAS. Furthermore, RAS sites are generally set up at remote locations, protected by radio quiet zones. In view of growths in spectrum demands from both systems, in population, and in applications and service deployments, the existing paradigm of geographical and spectral isolation between CWC and RAS will become inefficient. This paper develops a three phase spectrum access which enables geographical and spectral coexistence between CWC and RAS. Additionally, traffic statistics based improved spectrum access is developed. Furthermore, a built-in fine tuning mechanism is presented for addressing mismatches between design and practical environments as well as for facilitating service evolutions. Performance evaluation results demonstrate advantages of the proposed paradigm.

Minimum outage RF beamforming for millimeter wave MISO-OFDM systems

We consider the design of the radio-frequency (RF) precoder for minimum outage probability transmission for millimeter wave (mmWave) MISO-OFDM systems under the assumption of partial channel knowledge at the transmitter and full channel knowledge at the receiver. We formulate the problem as minimizing the cumulative distribution function (CDF) of an L-stage hypo-exponentially distributed random variable, where L is the number of propagation paths. To solve the optimization problem of the RF precoder, we propose an iterative gradient ascent algorithm. Simulation results show that the proposed algorithm converges in a small number of iterations and achieves lower outage probability, with full diversity order, than the conventional eigenvector beamforming. Moreover, the proposed scheme is more robust against human blockage than the conventional eigenvector beamforming and the maximum capacity scheme with full channel knowledge at the transmitter.

A New Paradigm for Spectrum Sharing Between Cellular Wireless Communications and Radio Astronomy Systems

This paper proposes a new paradigm for spectrum sharing between cellular wireless communications (CWC) and radio astronomy systems (RAS). In contrast to the existing paradigm of geographical and spectral isolation between CWC and RAS, this paper develops a three phase spectrum access, which enables geographical and spectral coexistence between CWC and RAS. Shared spectrum access zone (SSAZ) is created around the RAS site and CWC cells within the SSAZ follow the three phase spectrum access scheme while those outside the SSAZ have full spectrum access. In addition, system characteristics-based improved spectrum sharing is developed. Furthermore, a built-in fine tuning mechanism is presented for addressing mismatches between design and practical environments as well as for facilitating service evolutions. Performance evaluation results demonstrate that the proposed paradigm offers 1) certain guaranteed spectrum access to RAS, which is impossible in the existing paradigm, 2) capability to handle higher peak and mean traffics to CWC under spectrum restructuring of both CWC and RAS bands, and 3) overall improved spectrum utilization.

Hybrid Analog–Digital Precoding Design for Secrecy mmWave MISO-OFDM Systems

Millimeter-wave large-scale antenna systems typically apply hybrid analog–digital precoders to reduce hardware complexity and power consumption. In this paper, we design hybrid precoders for physical-layer security under two types of channel knowledge. With full channel knowledge at transmitter, we provide sufficient conditions on the minimum number of RF chains needed to realize the performance of the fully digital precoding. Then, we design the hybrid precoder to maximize the secrecy rate. By maximizing the average projection between the fully digital precoder and the hybrid precoder, we propose a low-complexity closed-form hybrid precoder. We extend the conventional projected maximum ratio transmission scheme to realize the hybrid precoder. Moreover, we propose an iterative hybrid precoder design to maximize the secrecy rate. With partial channel knowledge at transmitter, we derive a secrecy outage probability upper-bound. The secrecy throughput maximization is converted into a sequence of secrecy outage probability minimization problems. Then, the hybrid precoder is designed to minimize the secrecy outage probability by an iterative hybrid precoder design. Performance results show the proposed hybrid precoders achieve performance close to that of the fully digital precoding at low and moderate signal-to-noise ratios (SNRs), and sometimes at high SNRs depending on the system parameters.

RF beamforming for secrecy millimeter wave MISO-OFDM systems

Due to the tiny wavelength of millimeter waves (mmWave), tens of antennas can be packed into a small area in mmWave transceivers. However, implementing a radio-frequency (RF) chain for each antenna is impractical due to the high cost and power of mixed-signal devices. To reduce the cost and get benefit from the antennas, an analog RF beamformer is implemented using variable gain amplifiers and analog phase shifters. In this paper, we design the RF precoder for physical layer security. We consider two degrees of channel knowledge at the transmitter. For full channel knowledge at the transmitter, the RF precoder is optimized to maximize the secrecy rate. Two solution algorithms, with different computational complexity, are proposed based on semidefinite relaxation and gradient ascent. For partial channel knowledge at the transmitter, the RF precoder is designed to maximize the secrecy rate under secrecy outage constraint. The problem is safely approximated, and solved by a gradient ascent algorithm within bisection search. Numerical results show that the proposed algorithms converge quickly, and outperform the conventional secrecy schemes.

Spectrum sharing between WiFi and radio astronomy

The proliferation of wireless local area network also known as WiFi system has enabled easy wireless information access for consumers. However, it also causes radio frequency interference (RFI) to passive wireless systems such as radio astronomy systems (RAS), making almost impossible to get useful scientific observations around the WiFi bands. This paper proposes a new paradigm for the coexistence between WiFi and RAS. The proposed approach creates a coexistence access zone (CAZ) around the RAS site within which WiFi and RAS follow a pre-determined time-division spectrum access. Two modified WiFi medium access control (MAC) protocols are developed to embed the time-division coexistence access. Furthermore, traffic statistics based improved spectrum access is developed. Performance evaluation results show that at the cost of slight WiFi throughput reduction, RAS achieves substantial RFI-free spectrum access which were infeasible in the existing paradigm.

Robust RF Beamforming for Millimeter Wave MIMO–OFDM Systems

In conventional multiple-input multiple-output systems, each antenna is connected to a radio-frequency (RF) chain. Due to the tiny wavelength of millimeter waves (mmWave), tens of antennas can be packed into a small area in mmWave transceivers. However, implementing an RF chain for each antenna is impractical due to the high cost and power of mixed-signal devices. In order to reduce the cost and get benefit from the antennas, an analog RF beamformer is implemented using variable gain amplifiers and analog phase shifters. In this paper, we propose a novel spatial diversity scheme for mmWave RF beamforming. The transmitter is assumed to have partial knowledge of the channel to the receiver. We formulate the spatial diversity problem as maximizing the geometric mean of the projections of the RF precoder on the transmit steering vectors and the geometric mean of the projections of the RF combiner on the receive steering vectors. To solve the optimization problem of the RF beamformers, we propose two solution algorithms. The first algorithm is based on semidefinite relaxation (SDR). Due to the high computational complexity of the SDR algorithm, we propose a simpler second algorithm, which is gradient ascent algorithm. Simulation results show that the proposed spatial diversity scheme outperforms the conventional spatial diversity schemes in case of no blockage and in case of blockage to any propagation path regardless of the number of propagation paths.

Artificial Noise Aided Hybrid Precoding Design for Secure mmWave MISO Systems With Partial Channel Knowledge

In this letter, we propose a hybrid analog–digital precoder design to enhance the physical layer security of millimeter-wave (mmWave) multiple-input single-output (MISO) systems with partial channel knowledge. We derive a closed-form expression for the average signal-to-interference-and-noise ratio (SINR) of the eavesdropper (Eve) as a function of the hybrid precoder. Using the average SINRs of Eves, we derive a secrecy rate lower bound. Then, we propose a low-complexity artificial-noise-aided (AN-aided) hybrid precoder design to maximize the secrecy rate lower bound. Numerical results show that the proposed AN-aided hybrid precoder achieves comparable performance to that of the fully digital precoder, with much lower hardware complexity. Moreover, the proposed AN-aided hybrid precoder outperforms the existing hybrid precoder design. Effect of finite-resolution phase shifters on the proposed precoder is also investigated.

Reliable RF beamforming for millimeter wave MIMO-OFDM systems

In this paper, we propose a novel spatial diversity scheme for the radio-frequency (RF) beamforming in millimeter wave (mmWave) MIMO-OFDM systems under the assumption of partial channel knowledge at the transmitter and full channel knowledge at the receiver. We formulate the spatial diversity problem as maximizing the geometric mean of the projections of the RF precoder on the transmit steering vectors and the geometric mean of the projections of the RF combiner on the receive steering vectors. To solve the optimization problem of the RF beamformers, we propose a simple gradient ascent algorithm. Simulation results show that our proposed spatial diversity scheme outperforms the other spatial diversity schemes in literature in case of no blockage and in case of blockage to any propagation path regardless of the number of propagation paths.