Spatial Carrierless Amplitude and Phase Modulation Technique for Visible Light Communication Systems

Abstract

Spatial carrierless amplitude and phase modulation (S-CAP) technique is developed in this paper as a physical layer solution to improve the spectral efficiency of the conventional CAP scheme while preserving its low complexity transceiver design. The S-CAP technique is proposed and investigated for systems employing the visible light communication (VLC) technology. An analytical expression for the joint detection of the spatial and signal bits for the user equipment (UE) experiencing line-of-sight propagation (LOS) is derived and validated via simulation. The effect of multipath propagation and user mobility on the bit-error-rate (BER) performance of the proposed S-CAP are also investigated. It is found that the (BER) performance of S-CAP in LOS is dictated by the minimum of the channel gains h min , the signal constellation points (SCP) and the channel gain dissimilarity, ∆|h|. The power factor imbalance (PFI) and multiple photodetectors (PDs) are then introduced to improve performance and mitigate channel impairments. The use of PFI and PDs in LOS result in signal-to-noise (SNR) gain of 33.5 dB and 43 dB, respectively. The proposed scheme is thus a novel implementation of CAP in a multiple-input multiple-output (MIMO) system and demonstrates its potential as a suitable physical layer solution for VLC technology.

Enhanced Subband Index Carrierless Amplitude and Phase Modulation in Visible Light Communications

“…where T is log 2 (M) for m-CAP and it is as defined in (6) and (7) for eSI-CAP and SI-CAP, respectively.…”

“…Using multiple LEDs, each with a 4 MHz 3 dB bandwidth, a MIMO CAP technique is demonstrated to achieve a data rate of 249 Mb/s at a BER of 3.2 × 10 −3 in [6]. Similarly, spatial modulation techniques which carry additional information on the indices of LEDs have also been implemented to increase the achievable throughput of the conventional CAP in [7]. Furthermore, subband-indexed CAP (SI-CAP) has been proposed in [8] to improve the spectral efficiency of the multiple band version of CAP (m-CAP).…”

Carrierless amplitude and phase modulation in wireless visible light communication systems

“…The optimum values of the equalizer tap coefficients depend on the channel response and therefore need to be optimized through a training process using appropriate algorithms such as the least mean squares (LMS) and recursive least squares (RLS) algorithms [73]. Equalization at the transmitter has benefits in terms of the signal-to-noise ratio (SNR) of the link as it amplifies only the useful signal but requires a priori knowledge of the channel response, which in the case of indoor VLC systems is strongly location dependent [29,[74][75][76]. As a result, a bi-directional link and a suitable communication process are required to achieve this.…”

“…CAP signals exhibit a small horizontal eye-opening which is a direct result of the use of the orthogonal pulse shaping functions. Figure 5a shows the BER performance for CAP-4 transmission and different timing offsets τ , while figure 5b compares the BER performance between CAP transmission of different constellation orders M for no (τ = 0) and a small (τ = 0.02 T) timing offset [76]. The plots indicate that small timing offsets can lead to significant performance degradation, with the effect becoming more severe for higher constellation orders.…”

“…Figure 7a shows the spectral efficiency improvement η as a function of the number of LEDs N t for different QAM constellation orders M. The implementation of the scheme is relatively simple and the fact that a single LED is active at any time removes inter-carrier or inter-band interference (IBI) at the receiver. However, it has been shown that the power efficiency of the scheme is lower than that of the equivalent CAP scheme, due to the requirements for distinct channel gains for the different LED sources [76]. Ways to mitigate this issue include the application of the PFI technique in a similar manner to MIMO to differentiate the channel gains or the use of multiple photodiodes (PDs) at different locations to increase the probability of correct symbol recovery.…”