###### Abstract  
Visible light communication and positioning (VLCP) is a promising candidate for constructing a multi-functional wireless network with large-scale connectivity and centimeter-level positioning. However, there is still a lack of effective methods to offer simultaneous visible light communication (VLC) and visible light positioning (VLP) functions for multiple users. Thus, we propose a multi-user VLCP system based on a dual-domain multiplexing (DDM) scheme, where both the time and code resources are multiplexed to transmit VLCP signals for multiple users simultaneously. In the proposed system, the data of different users are distinguished by using code division multiplexing technology, while the VLCP signals transmitted from different LEDs are separated by adopting time division multiplexing technology. The performances, including bit-error rate and positioning error, are evaluated through both simulation and experimentation to verify the feasibility of the proposed multi-user VLCP system. In the experiment, a VLCP system with four LED transmitters was able to simultaneously support low-speed VLC with free error and accurate VLP with a 2 cm precision for eight users. This offers an effective solution to support a large number of users with simultaneous VLC and VLP functions in the future multi-functional wireless network. visible light communication and positioning; visible light communication; visible light positioning; code division multiplexing; time division multiplexing +
Footnote †: journal: _Photonics_  
The customarily used multiplexing technology includes time-division multiplexing (TDM), frequency-division multiplexing (FDM), and code-division multiplexing (CDM). In the TDM-based VLCP system, the total time resource is divided into several time slots and assigned for VLC signals and VLP signals, respectively [15; 16]. However, this will reduce the transmission efficiency of VLC and also increase the time delay for both VLC and VLP, especially when the VLCP system simultaneously supports a large number of users. In the FDM-based VLCP system, the total frequency resource is divided into two sub-bands for VLC and VLP, respectively [17; 18]. However, the VLP signals will occupy the limited bandwidth of commercial LEDs (usually a few MHz), which reduce the capacity of VLC. In addition, the TDM-based solutions and FDM-based solutions require additional guard time slots and bands to avoid interference between VLC and VLP signals, respectively. This reduces the time and spectral utilization. As for the CDM-based VLCP system, a set of spreading codes is divided and assigned to LEDs to distinguish different VLCP signals, then the correlation values between the spreading codes and the VLCP signals are used to recover the user data for VLC and calculate receiver signal strength (RSS) for VLP [19; 20]. However, it is hard for the CDM-based solution to support a large number of users because each LED can only support the VLC function for one user, which means the number of LEDs limits the user number. Therefore, new approaches are required to support multiple users for VLCP systems. In this paper, we propose a multi-user VLCP system based on the dual-domain multiplexing (DDM) scheme, which multiplexes the code and time resources to transmit different VLCP signals for multiple users simultaneously. Specifically, each user is assigned a unique spreading code to encode the user data, and the LED transmitters transmit the VLCP signals in turn. In this way, the data of different users are distinguished by CDM, while the VLCP signals transmitted from different LEDs are separated by TDM. For achieving simultaneous VLC and VLP functions, on the receiver side, the spreading codes are used to decode the received signal to obtain the correlation values. Then, the correlation value can be used to recover the user data for VLC and to calculate the RSS for VLP.