1,721,039 research outputs found
Iterative receiver for hybrid asymmetrically clipped optical OFDM
Get PDFThis paper proposes an iterative receiver to enhance the performance of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) in optical wireless communication systems. In HACO-OFDM scheme, asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) signals are transmitted simultaneously, which is more spectrally efficient compared with ACO-OFDM and PAM-DMT. However, the existing HACO-OFDM receiver directly recovers the signals in the frequency domain, which could not eliminate the interference thoroughly between ACO-OFDM and PAM-DMT signals and limits its performance. In our proposed receiver, the ACO-OFDM and PAM-DMT signals are detected in the frequency domain and regenerated in the time domain. After that, they are subtracted from the received signals iteratively. Thus, ACO-OFDM and PAM-DMT signals can be distinguished. By taking advantage of the signal symmetry properties of ACO-OFDM and PAM-DMT in the time domain, pairwise clipping is utilized to further reduce the effect of noise and estimation error, resulting in improved performance. In addition, unequal power allocation is proposed to guarantee that ACO-OFDM and PAM-DMT signals have similar performance in HACO-OFDM systems. Simulation results show that the proposed method provides significant signal-to-noise ratio gain over the conventional receiver for both equal and unequal power allocations at the cost of slightly increased complexity
Asymmetrical hybrid optical OFDM for visible light communications with dimming control
Get PDFThis letter proposes an asymmetrical hybrid optical orthogonal frequency division multiplexing (AHO-OFDM) scheme for dimmable visible light communication systems. In the proposed scheme, either asymmetrically clipped optical OFDM (ACO-OFDM) or pulse-amplitude-modulated discrete multitone (PAM-DMT) signal is inverted and then both the signals are combined for transmission, where pulsewidth modulation is no longer required for dimming control. The power of ACO-OFDM and PAM-DMT signals is adjusted so that the amplitude of the combined AHO-OFDM signal is asymmetrical, which could utilize all the available subcarriers as well as the entire dynamic range of light-emitting diodes with various dimming levels. Simulation results show that the proposed scheme could achieve a wide dimming range with a small throughput fluctuation
Multiuser MIMO-OFDM for visible light communications
No full textVisible light communication (VLC) is emerging as a promising technique to provide ubiquitous wireless connection. In this paper, a multiuser VLC system utilizing multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is investigated. Since the distances of the multiple transmitter-receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain. For each subcarrier in OFDM, the corresponding precoding matrix is calculated in the frequency domain to eliminate multiuser interference. Phase information in the frequency domain is first considered, where complex, instead of real, channel matrices are used for precoding, which reduces the channel correlation and achieves better performance. Moreover, minimum dc bias, unified dc bias, and asymmetrically clipped optical OFDM-based schemes are proposed to generate real-valued nonnegative signals for intensity modulation, and their performances are validated via simulations with zero forcing and minimum mean-squared error (MMSE) precoding techniques
Compact user-specific reconfigurable intelligent surfaces for uplink transmission
No full textLarge-scale antenna arrays employed by the base station (BS) constitute an essential next-generation communications technique. However, due to the constraints of size, cost, and power consumption, it is usually considered unrealistic to use a large-scale antenna array at the user side. Inspired by the emerging technique of reconfigurable intelligent surfaces (RIS), we firstly propose the concept of user-specific RIS (USRIS) for facilitating the employment of a large-scale antenna array at the user side in a cost- and energy-efficient way. In contrast to the existing employments of RIS, which belong to the family of base-station-specific RISs (BSS-RISs), the US-RIS concept by definition facilitates the employment of RIS at the user side for the first time. This is achieved by conceiving a multilayer structure to realize a compact form-factor. Furthermore, our theoretical results demonstrate that, in contrast to the existing single-layer structure, where only the phase of the signal reflected from RIS can be adjusted, the amplitude of the signal penetrating multi-layer US-RIS can also be partially controlled, which brings about a new degree of freedom (DoF) for beamformer design that can be beneficially exploited for performance enhancement. In addition, based on the proposed multi-layer US-RIS, we formulate the signal-to-noise ratio (SNR) maximization problem of US-RIS-aided communications. Due to the non-convexity of the problem introduced by this multi-layer structure, we propose a multi-layer transmit beamformer design relying on an iterative algorithm for finding the optimal solution by alternately updating each variable. Finally, our simulation results verify the superiority of the proposed multi-layer US-RIS as a compact realization of a large-scale antenna array at the user side for uplink transmission
An optimal scaling scheme for DCO-OFDM based visible light communications
No full textDC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) is widely used in visible light communication (VLC) systems to provide high data rate transmission. As intensity modulation with direct detection (IM/DD) is employed to modulate the OFDM signal, scale up the amplitude of the signal can increase the effective transmitted electrical power whereas more signals are likely to be clipped due to the limited dynamic range of LEDs, resulting in severe clipping distortion. Thus, it is crucial to scale the signal to find a tradeoff between the effective electrical power and the clipping distortion. In this paper, an optimal scaling scheme is proposed to maximize the received signal-to-noise-plus-distortion ratio (SNDR) with the constraint of the radiated optical power in a practical scenario where DC bias is fixed for a desired dimming level. Simulation results show that the system with the optimal scaling factor outperforms that with fixed scaling factor under different equivalent noise power in terms of the bit error ratio (BER) performance
Multi-user sum-rate optimization for visible light communications with lighting constraints
No full textIn visible light communication (VLC) systems, white light emitting diodes (LEDs) are used as illumination sources and transmitters simultaneously. Compared to the phosphor-converted LEDs, multi-chip LEDs have higher modulation bandwidth. Consequently, the multi-chip based VLC systems have great potential for high data rate transmission. Since each chip of the multi-chip LEDs can be modulated independently, parallel communication channels are viable for information transmission. In this paper, in order to maximize the multi-user sum-rate for the multi-chip based multi-input single-output VLC systems, an electrical and optical power allocation scheme is proposed in consideration of the luminance, chromaticity, amplitude and bit error rate constraints. From the perspective of human color vision, the chromaticity constraint is defined within a MacAdam ellipse. As a result, the degree of freedom can be achieved by relaxing the chromaticity constraint from a fixed color point to an elliptic region. Numerical results demonstrate that with the increase of the total luminous flux, the maximum sum-rates present an open-down parabolic tendency due to the limited dynamic range of LEDs. Higher data rate can be achieved under higher correlated color temperature (CCT) for the variation of light components. In addition, the simulation results indicate that the shapes of the chromaticity constrained region (either ellipse or quadrangle) have little impact on the multi-user sum-rate at the same CCT
Improved receiver design for layered ACO-OFDM in optical wireless communications
Get PDFLayered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM) is recently proposed for intensity-modulated directed-detected optical wireless communications, which achieves higher spectral efficiency compared with the conventional ACO-OFDM, since different layers of ACO-OFDM signals are combined to utilize more subcarriers. In this letter, an improved receiver is proposed for LACO-OFDM, which distinguishes different layers of ACO-OFDM signals in the time domain. After that, the structure of ACO-OFDM signals in each layer is exploited to further reduce the noise and inter-layer interference, resulting in the improved performance. Simulation results show that the proposed receiver for LACO-OFDM achieves significant gain over its conventional counterpart
Coded MIMO with asymmetric constellation sizes
No full textAn asymmetric constellation scheme for coded multiple-input–multiple-output (MIMO) transmission is proposed, which applies different constellation mappings to different transmit streams and carefully selects the coding rates for different transmit streams. An improved power allocation is derived to naturally incorporate with the coding rate selection and to further enhance the achievable performance. The proposed scheme provides more flexible choices of data rate selection, and by employing fixed-complexity sphere decoding (FSD) detection, it achieves better performance with reduced detection complexity in comparison with the conventional MIMO using the FSD-based detection with the same constellation set for all stream
Channel feedback codebook design for millimeter-wave massive MIMO systems relying on lens antenna array
No full textThe recently proposed millimeter-wave (mmWave) massive MIMO system relying on a lens antenna array (LAA) significantly reduces the number of radio frequency (RF) chains using beam selection. A high data rate can be achieved based on the reduced-dimensional equivalent channel after beam selection. In frequency division duplexing (FDD) systems, the equivalent channel has to be signaled back to the base station (BS) via a feedback channel based on a codebook. However, no dedicated codebook has been proposed for LAA-aided mm-Wave systems. To fill this gap, in this paper, we propose a reduced-dimensional subspace codebook (RDSC) for LAA-aided mmWave massive MIMO systems. Specifically, under the recently proposed concept of angle coherence time, we first generate the large-dimensional vectors in the channel subspace, which is determined by the angles-of-departure (AoDs) of the dominant paths. Then, based on these vectors in the channel subspace, we create the RDSC by considering both the lens and the beam selector. Finally, the equivalent channel is quantized using the proposed RDSC and fed back to the BS. Finally, we carry out mathematical performance analysis of the proposed RDSC and show that its feedback overhead is rendered proportional to the relatively small number of dominant paths per user. The analytical results are verified by our simulations.</p
Multi-user MIMO-OFDM for indoor visible light communication systems
No full textIn this paper, we investigate an indoor multiuser visible light communication system employing multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM). For each subcarrier in OFDM, the corresponding precoding matrix is calculated in the frequency domain to eliminate multi-user interference. The distances of the multiple transmitter-receiver links are different, which results in various temporal delays and phase differences in the frequency domain. Phase information is firstly considered, whereby complex instead of real channel matrices are used for precoding, which reduces the channel correlation and achieves better performance. Moreover, two DC bias and scaling factor calculation schemes are proposed, and their performances are compared with zero forcing and minimum mean-squared error (MMSE) precoding techniques
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