Ahmad Helmi Azhar

Bio: Ahmad Helmi Azhar is an academic researcher from University of Oxford. The author has contributed to research in topics: Visible light communication & Orthogonal frequency-division multiplexing. The author has an hindex of 4, co-authored 4 publications receiving 576 citations.

A Gigabit/s Indoor Wireless Transmission Using MIMO-OFDM Visible-Light Communications

Abstract: This letter reports an experimental demonstration of indoor wireless visible-light communication transmission at 1 Gb/s. The system consists of a four-channel multiple-input multiple-output link that uses white LED sources, each transmitting signals at 250 Mb/s using orthogonal frequency division multiplexing modulation. A nine-channel imaging diversity receiver is used to detect the signals, and an average bit error rate of 10-3 is achieved at the room illumination level of ~1000 lux at 1-m range.

Demonstration of high-speed data transmission using MIMO-OFDM visible light communications

Abstract: This letter reports an experimental demonstration of high-speed indoor optical wireless Optical multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) system with an imaging receiver. The system consists of a 2x1 array of white LEDs that transmit data to a 9 channel imaging receiver that uses a 3x3 photodetector array. A total transmission rate of 220Mbit/s at a BER of 10-3 is achieved over a range of 1m. An overview of the design specifications, optical design and experimental setup are reported in this letter, together with results and discussion of how improvements to the system may be achieved. © 2010 IEEE.

Demonstration of high-speed data transmission using MIMO-OFDM visible light communications

Abstract: This letter reports an experimental demonstration of high-speed indoor optical wireless Optical multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) system with an imaging receiver. The system consists of a 2×1 array of white LEDs that transmit data to a 9 channel imaging receiver that uses a 3×3 photodetector array. A total transmission rate of 220Mbit/s at a BER of 10−3 is achieved over a range of 1m. An overview of the design specifications, optical design and experimental setup are reported in this letter, together with results and discussion of how improvements to the system may be achieved.

Experimental comparisons of optical OFDM approaches in visible light communications

Abstract: This paper presents an experimental comparison of optical orthogonal frequency division multiplexing (OFDM) approaches in visible light communications (VLC). We demonstrate that OFDM schemes with asymmetric signals suffer from baseline wander in practical transmission experiments, which degrades the overall bit-rate performance. Several schemes that combat this effect are reported and the performances of these schemes are measured. However, it is demonstrated that a DCO-OFDM scheme achieves higher bitrate at target BER in a VLC system that is bandwidth-limited.

LED Based Indoor Visible Light Communications: State of the Art

Abstract: Visible Light Communication (VLC) is an emerging field in Optical Wireless Communication (OWC) which utilizes the superior modulation bandwidth of Light Emitting Diodes (LEDs) to transmit data. In modern day communication systems, the most popular frequency band is Radio Frequency (RF) mainly due to little interference and good coverage. However, the rapidly dwindling RF spectrum along with increasing wireless network traffic has substantiated the need for greater bandwidth and spectral relief. By combining illumination and communication, VLC provides ubiquitous communication while addressing the shortfalls and limitations of RF communication. This paper provides a comprehensive survey on VLC with an emphasis on challenges faced in indoor applications over the period 1979–2014. VLC is compared with infrared (IR) and RF systems and the necessity for using this beneficial technology in communication systems is justified. The advantages of LEDs compared to traditional lighting technologies are discussed and comparison is done between different types of LEDs currently available. Modulation schemes and dimming techniques for indoor VLC are discussed in detail. Methods needed to improve VLC system performance such as filtering, equalization, compensation, and beamforming are also presented. The recent progress made by various research groups in this field is discussed along with the possible applications of this technology. Finally, the limitations of VLC as well as the probable future directions are presented.

A 3-Gb/s Single-LED OFDM-Based Wireless VLC Link Using a Gallium Nitride $\mu{\rm LED}$

Abstract: This letter presents a visible light communication (VLC) system based on a single 50- μm gallium nitride light emitting diode (LED). A device of this size exhibits a 3-dB modulation bandwidth of at least 60 MHz - significantly higher than commercially available white lighting LEDs. Orthogonal frequency division multiplexing is employed as a modulation scheme. This enables the limited modulation bandwidth of the device to be fully used. Pre- and postequalization techniques, as well as adaptive data loading, are successfully applied to achieve a demonstration of wireless communication at speeds exceeding 3 Gb/s. To date, this is the fastest wireless VLC system using a single LED.

A Gigabit/s Indoor Wireless Transmission Using MIMO-OFDM Visible-Light Communications

Abstract: This letter reports an experimental demonstration of indoor wireless visible-light communication transmission at 1 Gb/s. The system consists of a four-channel multiple-input multiple-output link that uses white LED sources, each transmitting signals at 250 Mb/s using orthogonal frequency division multiplexing modulation. A nine-channel imaging diversity receiver is used to detect the signals, and an average bit error rate of 10-3 is achieved at the room illumination level of ~1000 lux at 1-m range.

High-speed visible light communication systems

Abstract: This article presents recent achievements and trends in high-speed indoor visible light communication (VLC) research. We address potential applications and future visions for the VLC technology, where transport of information is “piggybacked” on the original lighting function of LED-based lamps. To mature this technology and transfer it into practice, our recent research is focused on real-time implementation and trials. For the first time, a bidirectional real-time VLC prototype achieving data rates of up to 500 Mb/s is presented. This system paves the way for future real world applications. Finally, we discuss the remaining technical challenges as well as the research outlook in the field of high-speed VLC systems.