# Visible-Light CDMA Communications Using Inverted Spread Sequences
###### Abstract  
Visible-light communication (VLC) using light-emitting diodes (LEDs) is gaining attention in the wireless communication field. LEDs can be used as data transmitters without losing their main functionality as lighting devices. In some VLC applications, such as traffic signs and road signals in intelligent transportation systems, high brightness is required to help people recognize the signs and signals conveyed by the light sources. In this paper, the use of inverted modified prime sequence codes (MPSCs) is shown to be efficient for increasing brightness in an optical code-division multiple access (CDMA) system for VLC, while the original MPSCs, namely non-inverted codes, provide much lower brightness. The average light intensity of a system using an inverted MPSC is several times the intensity of a system using an original MPSC, without losing the capabilities of channel multiplexing and multi-user interference canceling. Average light intensity and normalized fluctuation are investigated for the optical CDMA systems with the original and inverted MPSCs. The results show that the systems with the inverted MPSCs provide higher average light intensity and lower normalized fluctuation than the systems with the original MPSCs do. Moreover, the bit error rates of the systems with the inverted MPSCs are evaluated by computer simulation and compared with those of the systems with the original MPSCs. visible-light communication; optical wireless communication; code-division multiple access; modified prime sequence code +
Footnote †: journal:  
0000-0001-6500-0001-6500]T.M.M.M.M.M  
CDMA systems. Modified prime sequence codes (MPSCs) [16] and generalized MPSCs (GMPSCs) [17; 18] have been developed for synchronous optical CDMA systems. The density of a code is defined as the ratio of marks in its codeword, which determines the average light intensity of the light source. Optical signature codes usually have a low density, which means that the average light intensity of each light source is low even if the light source has the potential for high brightness. For example, PSCs, MPSCs, and GMPSCs constructed from a finite field GF(\(q\)) have the same density of \(1/q\). The density of many optical signature codes is much lower than \(1/2\), because low density leads to low cross-correlation in unipolar codes. In some applications, the light sources for optical CDMA also serve as road or railway traffic signs and signals in ITSs. High brightness is required in such cases to help people recognize the signs and signals. Therefore, we set a goal of attaining an average light intensity more than half of the potential which is the light intensity that non-modulated light sources inherently possess. To attain this goal, we adopted the inverted codes of MPSCs and GMPSCs, referred to as _inverted MPSCs_ hereafter, as introduced in this paper. MPSCs and GMPSCs have an excellent property of multi-user interference (MUI) cancellation in synchronous optical CDMA, when they are used with a proper cancellation technique. Three major MUI cancellation techniques can remove MUI completely. They are the equal-weight orthogonal (EWO) scheme [19], [PERSON]'s scheme [20], and [PERSON]'s scheme [21]. All these schemes have a common advantage, which is error-free performance under an ideal link where the noise can be considered negligible. This feature stems from the special correlation properties of MPSCs and those of GMPSCs. This paper proposes a new optical CDMA scheme that is suitable for VLC requiring high brightness. The feature of the proposed scheme is to adopt an inverted MPSC as a signature code in combination with MUI cancellation techniques. The density of inverted codes is \((q-1)/q\), which is much higher than that of the original codes. The EWO scheme and [PERSON]'s scheme with an inverted code provide a MUI cancellation property that works as well as that of conventional schemes with a non-inverted code. However, when [PERSON]'s scheme is combined with an inverted code, MUI cannot be canceled.